Aspen Plus Modelling of LTMAG

Modelling and Simulation of the

Low Tar Methane Alkali Dual Cyclone Gasifier Including Fuel Pre-treatment and Gas Applications

Kristina Jonsson

Master of Science Thesis in Energy Engineering Umearing Institute of Technology

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Sammanfattning Paring grund av vaumlxthuseffekten och oumlkat oljepris har foumlrnyelsebara energikaumlllor daumlribland biobraumlnslen blivit mer och mer intressanta baringde ur ekonomisk och miljoumlmaumlssig synvinkel Nya braumlnslen maringste undersoumlkas och utvecklas men ocksaring produkter foumlr slutanvaumlndning av dessa braumlnslen saringsom fordonsbraumlnslen braumlnnare och vaumlrmeverk Foumlr att biobraumlnslen och andra foumlrnyelsebara energikaumlllor ska raumlcka till i vaumlrlden aumlr det viktigt att de anvaumlnds effektivt Foumlrgasning av biomassa aumlr en energiomvandlingsprocess som verkningsgradsmaumlssigt anses vara bland de baumlttre Vid foumlrgasing tillsaumltts en mindre maumlngd syre aumln vid foumlrbraumlnning detta sker i form av luft syrgas eller aringnga Den oumlnskade slutprodukten vid foumlrgasning ndash biosyntesgas ndash bestaringr fraumlmst av kolmonoxid (CO) och vaumltgas (H2) Biosyntesgasen har ett flertal anvaumlndningsomraringden saringsom direkt foumlrbraumlnning foumlr el- och vaumlrmeproduktion industriell ersaumlttning av gasol och olja alternativt produktion av drivmedel saringsom metanol etanol syntetisk diesel dimetyleter (DME) eller foumlrnyelsebara kemikalier Dagens biomassafoumlrgasare aumlr dock lite foumlr dyra Utmaningarna foumlr att ta fram kostnadseffektiv teknik ligger i tre olika delar av processen

1 Inmatning av ett homogent laumlttmatat pulver skulle ge betydligt billigare processer 2 Oumlkad foumlrgasningseffektivitet med mindre metan och tjaumlror skulle vara positivt baringde

direkt processekonomiskt och indirekt foumlr enklare reningsteknik 3 Minskad andel foumlroreningar i syntesgasen skulle vara foumlrdelaktigt foumlr hela systemet

Exempel paring orenheter i gasen aumlr tjaumlror och andra kolvaumlten som saumlnker verkningsgraden samt kan saumltta igen i senare delar av systemet Aumlven houmlga metanhalter bidrar till minskad verkningsgrad eftersom metan aumlr inert i drivmedelssyntessteget Dessa kolvaumlten maringste oftast renas bort vilket aumlr kostsamt Andra orenheter som alkalifoumlreningar kan orsaka korrosion i gasturbiner och avaktivera katalysatorerna i drivmedels produktionen Dessa aumlmnen kan aumlven orsaka korrosion belaumlggningar och sandbaumlddsagglomereing i foumlrgasaren Genom att initialt rosta (eng torrefy) biomassa kan ett matningsbart pulver framstaumlllas energieffektivt och daumlrmed kan ocksaring betydligt enklare och mindre investeringstung foumlrgasningsteknik (entrained flow-teknik) anvaumlndas Rostning innebaumlr att braumlnslet upphettas till 250-350degC och resultatet blir ett vaumlldigt torrt och hydrofobt braumlnsle som aumlr laumltt att mala Det malda braumlnslet aumlr sedan laumltt att mata in i tex en foumlrgasare En ny typ av foumlrgasare LTMAG (Low Tar Methane and Alkali Gasifier) vilken ocksaring aumlr under utveckling vid Energiteknik och Termisk Processkemi (ETPC) Umearing Universitet aumlr utformad foumlr att ge saring laringga halter av tjaumlror metan och alkali som moumljligt Alkalikomponenterna koncentreras till ofoumlrgasad koks som sedan foumlrgasas och foumlrbraumlnns i ett andra steg och daumlrmed blandas de aldrig med biosyntesgasenTjaumlror och metan krackas i en houmlgtemperaturzon som vaumlrms av det andra foumlrgasningssteget Den producerade gasen blir saringledes relativt ren fraringn dessa foumlroreningar Denna rapport beskriver utvecklingen av en simuleringsmodell foumlr ett BTL-system (Biomass to Liquid) Modellen inneharingller foumlrbehandling av braumlnslet inklusive rostning foumlrgasning samt foumlrbraumlnning av gasen och aumlven produktion av FT-diesel fraringn gasen Modellen aumlr gjord i processmodelleringsprogrammet Aspen Plus Foumlrgasaren modellerades med reaktorer inbyggda i Aspen Plus som simulerar jaumlmvikt Gassammansaumlttningen fraringn modellen

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verifierades med ett annat program Fact Sage Vattenkylningen av gasen foumlre vaumlrmevaumlxlarna houmljer aumlven H2CO-kvoten i gasen vilket aumlr noumldvaumlndigt innan drivmedelssyntes Det effektiva vaumlrmevaumlrdet paring den producerade gasen fraringn modellen beraumlknades till 80 MJNm3 och foumlrgasningsverkningsgraden bestaumlmdes till 67 naumlr braumlnslet bestod av en blandning av 70 trauml och 30 torv och temperaturen i foumlrgasaren var 750degC och i kolvaumltekrackningen ca 1100degC Dessa processparametrar gav en total systemverkningsgrad foumlr produktion av gas och aringnga paring 93 (baserat paring effektiva vaumlrmevaumlrdet) Vid produktion av FT-diesel fraringn gasen producerades ca 190 lton torrefierat braumlnsle och 130 l naftaton torrefierat braumlnsle Verkningsgraden foumlr produktion av FT-diesel fraringn trauml var 27 baserat paring det effektiva vaumlrmevaumlrdet Modellen aumlr saringledes utvecklad utvaumlrderad och klar att anvaumlnda foumlr systemoptimering

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Abstract Due to the green house effect and the increasing oil price renewable energy including biofuels has become more and more interesting both from economic and environmental point of view Research and development of new fuels and applications for these fuels eg vehicle fuels heating plants and pellet burners is necessary For maximum use of the limited global renewable energy sources it is also important that the utilisation is energy efficient Gasification of biomass is an interesting technique with a relatively high efficiency producing the renewable gas called biosyngas The gasification process can be described as combustion but with a limited supply of oxygen The oxygen may be added as pure oxygen air steam or combinations The principal components in the biosyngas are H2 and CO A number of products can be produced from the biosyngas eg electricity and heat ndash from gas combustion ndash replacement of oil and fossil gas products in industries hydrogen gas synthetic diesel dimethyl ether (DME) methanol ethanol and renewable chemicals via synthesis However the existing gasifiers are too expensive The three main challenges to develop a cost efficient technique are

1 Feeding of a homogenous easy feedable powder would give considerably cheaper processes

2 Enhanced gasification efficiency with less methane and tars would improve process economy and indirect enable a simpler cleaning technology

3 A reduced amount of pollutants in the biosyngas would also be advantageous for the whole system

The pollutants in the gas are eg tars and other hydrocarbons that lower the efficiency and may plug downstream equipment A high amount of methane in the gas also reduces the efficiency since it is inert in the fuel synthesis processes The hydrocarbons are usually cleaned from the gas which is quite expensive Alkali compounds in the gas can cause corrosion in gas turbines and are poisonous to the catalysts in synthetic diesel production These compounds may also cause corrosion fouling and agglomeration in the gasifier By torrefaction of biomass an easily feedable and highly reactive fuel is produced in an energy efficient way Torrefaction means that the fuel is heated to 250-350degC The product is a very dry and hydrophobic fuel which is easy to grind and to feed into eg a gasifier Thanks to the torrefaction process it is possible to use a simpler and less expensive gasifier technique (entrained flow) A new gasifier concept is also being developed at the department of Energy Technology and Thermal Process Chemistry Umearing University This Low tar methane alkali dual cyclone gasifier (LTMAG) is designed to minimize the amounts of tars methane and alkali produced in the gasifier The LTMAG retains the alkali metals and a fraction of the coke which are gasified this gas is burned in a second step and thus never mixed with the biosyngas The tars and methane are decomposed in a second high temperature zone heated by the later combustion stage The syngas produced in the LTMAG is thus relatively clean from all the compounds above

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This report describes the development of a simulation model for the total BTL (Biomass to Liquid) system using the soft ware Aspen Plus The model also includes pre-treatment of the fuel gasification combustion and production of synthetic (Fischer Tropsch) diesel from the gas The gasification was modelled by blocks in Aspen Plus assuming equilibrium and the gas composition from the model was verified by another software Fact Sage The quench both cools the gas before the heat exchanger and increase the H2CO ratio which is necessary prior to synthesis The lower heating value of the gas was calculated from the model to 80 MJNm3 and the gasification efficiency was 67 (LHV) assuming a fuel mixture of 70 wood and 30 peat and a temperature in the gasifier at 750degC and in the cracking 1100degC The system efficiency with the same fuel and temperatures for the system including gas and steam production was 93 (LHV) The amount of FT diesel produced per tonne torrefied fuel was about 190 litres and 130 litres of naphtha and the FT diesel efficiency was 27 Thus the model is developed evaluated and ready to be used for system optimisation

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Confidential information Details concerning the LTMAG are confidential and that is why some parts in this report are concealed in grey I hope that the report is readable and interesting even without this information

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Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

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Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

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Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

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1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

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European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

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The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

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2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

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The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

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shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

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arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

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283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

2

Sammanfattning Paring grund av vaumlxthuseffekten och oumlkat oljepris har foumlrnyelsebara energikaumlllor daumlribland biobraumlnslen blivit mer och mer intressanta baringde ur ekonomisk och miljoumlmaumlssig synvinkel Nya braumlnslen maringste undersoumlkas och utvecklas men ocksaring produkter foumlr slutanvaumlndning av dessa braumlnslen saringsom fordonsbraumlnslen braumlnnare och vaumlrmeverk Foumlr att biobraumlnslen och andra foumlrnyelsebara energikaumlllor ska raumlcka till i vaumlrlden aumlr det viktigt att de anvaumlnds effektivt Foumlrgasning av biomassa aumlr en energiomvandlingsprocess som verkningsgradsmaumlssigt anses vara bland de baumlttre Vid foumlrgasing tillsaumltts en mindre maumlngd syre aumln vid foumlrbraumlnning detta sker i form av luft syrgas eller aringnga Den oumlnskade slutprodukten vid foumlrgasning ndash biosyntesgas ndash bestaringr fraumlmst av kolmonoxid (CO) och vaumltgas (H2) Biosyntesgasen har ett flertal anvaumlndningsomraringden saringsom direkt foumlrbraumlnning foumlr el- och vaumlrmeproduktion industriell ersaumlttning av gasol och olja alternativt produktion av drivmedel saringsom metanol etanol syntetisk diesel dimetyleter (DME) eller foumlrnyelsebara kemikalier Dagens biomassafoumlrgasare aumlr dock lite foumlr dyra Utmaningarna foumlr att ta fram kostnadseffektiv teknik ligger i tre olika delar av processen

1 Inmatning av ett homogent laumlttmatat pulver skulle ge betydligt billigare processer 2 Oumlkad foumlrgasningseffektivitet med mindre metan och tjaumlror skulle vara positivt baringde

direkt processekonomiskt och indirekt foumlr enklare reningsteknik 3 Minskad andel foumlroreningar i syntesgasen skulle vara foumlrdelaktigt foumlr hela systemet

Exempel paring orenheter i gasen aumlr tjaumlror och andra kolvaumlten som saumlnker verkningsgraden samt kan saumltta igen i senare delar av systemet Aumlven houmlga metanhalter bidrar till minskad verkningsgrad eftersom metan aumlr inert i drivmedelssyntessteget Dessa kolvaumlten maringste oftast renas bort vilket aumlr kostsamt Andra orenheter som alkalifoumlreningar kan orsaka korrosion i gasturbiner och avaktivera katalysatorerna i drivmedels produktionen Dessa aumlmnen kan aumlven orsaka korrosion belaumlggningar och sandbaumlddsagglomereing i foumlrgasaren Genom att initialt rosta (eng torrefy) biomassa kan ett matningsbart pulver framstaumlllas energieffektivt och daumlrmed kan ocksaring betydligt enklare och mindre investeringstung foumlrgasningsteknik (entrained flow-teknik) anvaumlndas Rostning innebaumlr att braumlnslet upphettas till 250-350degC och resultatet blir ett vaumlldigt torrt och hydrofobt braumlnsle som aumlr laumltt att mala Det malda braumlnslet aumlr sedan laumltt att mata in i tex en foumlrgasare En ny typ av foumlrgasare LTMAG (Low Tar Methane and Alkali Gasifier) vilken ocksaring aumlr under utveckling vid Energiteknik och Termisk Processkemi (ETPC) Umearing Universitet aumlr utformad foumlr att ge saring laringga halter av tjaumlror metan och alkali som moumljligt Alkalikomponenterna koncentreras till ofoumlrgasad koks som sedan foumlrgasas och foumlrbraumlnns i ett andra steg och daumlrmed blandas de aldrig med biosyntesgasenTjaumlror och metan krackas i en houmlgtemperaturzon som vaumlrms av det andra foumlrgasningssteget Den producerade gasen blir saringledes relativt ren fraringn dessa foumlroreningar Denna rapport beskriver utvecklingen av en simuleringsmodell foumlr ett BTL-system (Biomass to Liquid) Modellen inneharingller foumlrbehandling av braumlnslet inklusive rostning foumlrgasning samt foumlrbraumlnning av gasen och aumlven produktion av FT-diesel fraringn gasen Modellen aumlr gjord i processmodelleringsprogrammet Aspen Plus Foumlrgasaren modellerades med reaktorer inbyggda i Aspen Plus som simulerar jaumlmvikt Gassammansaumlttningen fraringn modellen

3

verifierades med ett annat program Fact Sage Vattenkylningen av gasen foumlre vaumlrmevaumlxlarna houmljer aumlven H2CO-kvoten i gasen vilket aumlr noumldvaumlndigt innan drivmedelssyntes Det effektiva vaumlrmevaumlrdet paring den producerade gasen fraringn modellen beraumlknades till 80 MJNm3 och foumlrgasningsverkningsgraden bestaumlmdes till 67 naumlr braumlnslet bestod av en blandning av 70 trauml och 30 torv och temperaturen i foumlrgasaren var 750degC och i kolvaumltekrackningen ca 1100degC Dessa processparametrar gav en total systemverkningsgrad foumlr produktion av gas och aringnga paring 93 (baserat paring effektiva vaumlrmevaumlrdet) Vid produktion av FT-diesel fraringn gasen producerades ca 190 lton torrefierat braumlnsle och 130 l naftaton torrefierat braumlnsle Verkningsgraden foumlr produktion av FT-diesel fraringn trauml var 27 baserat paring det effektiva vaumlrmevaumlrdet Modellen aumlr saringledes utvecklad utvaumlrderad och klar att anvaumlnda foumlr systemoptimering

4

Abstract Due to the green house effect and the increasing oil price renewable energy including biofuels has become more and more interesting both from economic and environmental point of view Research and development of new fuels and applications for these fuels eg vehicle fuels heating plants and pellet burners is necessary For maximum use of the limited global renewable energy sources it is also important that the utilisation is energy efficient Gasification of biomass is an interesting technique with a relatively high efficiency producing the renewable gas called biosyngas The gasification process can be described as combustion but with a limited supply of oxygen The oxygen may be added as pure oxygen air steam or combinations The principal components in the biosyngas are H2 and CO A number of products can be produced from the biosyngas eg electricity and heat ndash from gas combustion ndash replacement of oil and fossil gas products in industries hydrogen gas synthetic diesel dimethyl ether (DME) methanol ethanol and renewable chemicals via synthesis However the existing gasifiers are too expensive The three main challenges to develop a cost efficient technique are

1 Feeding of a homogenous easy feedable powder would give considerably cheaper processes

2 Enhanced gasification efficiency with less methane and tars would improve process economy and indirect enable a simpler cleaning technology

3 A reduced amount of pollutants in the biosyngas would also be advantageous for the whole system

The pollutants in the gas are eg tars and other hydrocarbons that lower the efficiency and may plug downstream equipment A high amount of methane in the gas also reduces the efficiency since it is inert in the fuel synthesis processes The hydrocarbons are usually cleaned from the gas which is quite expensive Alkali compounds in the gas can cause corrosion in gas turbines and are poisonous to the catalysts in synthetic diesel production These compounds may also cause corrosion fouling and agglomeration in the gasifier By torrefaction of biomass an easily feedable and highly reactive fuel is produced in an energy efficient way Torrefaction means that the fuel is heated to 250-350degC The product is a very dry and hydrophobic fuel which is easy to grind and to feed into eg a gasifier Thanks to the torrefaction process it is possible to use a simpler and less expensive gasifier technique (entrained flow) A new gasifier concept is also being developed at the department of Energy Technology and Thermal Process Chemistry Umearing University This Low tar methane alkali dual cyclone gasifier (LTMAG) is designed to minimize the amounts of tars methane and alkali produced in the gasifier The LTMAG retains the alkali metals and a fraction of the coke which are gasified this gas is burned in a second step and thus never mixed with the biosyngas The tars and methane are decomposed in a second high temperature zone heated by the later combustion stage The syngas produced in the LTMAG is thus relatively clean from all the compounds above

5

This report describes the development of a simulation model for the total BTL (Biomass to Liquid) system using the soft ware Aspen Plus The model also includes pre-treatment of the fuel gasification combustion and production of synthetic (Fischer Tropsch) diesel from the gas The gasification was modelled by blocks in Aspen Plus assuming equilibrium and the gas composition from the model was verified by another software Fact Sage The quench both cools the gas before the heat exchanger and increase the H2CO ratio which is necessary prior to synthesis The lower heating value of the gas was calculated from the model to 80 MJNm3 and the gasification efficiency was 67 (LHV) assuming a fuel mixture of 70 wood and 30 peat and a temperature in the gasifier at 750degC and in the cracking 1100degC The system efficiency with the same fuel and temperatures for the system including gas and steam production was 93 (LHV) The amount of FT diesel produced per tonne torrefied fuel was about 190 litres and 130 litres of naphtha and the FT diesel efficiency was 27 Thus the model is developed evaluated and ready to be used for system optimisation

6

Confidential information Details concerning the LTMAG are confidential and that is why some parts in this report are concealed in grey I hope that the report is readable and interesting even without this information

7

Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

3

verifierades med ett annat program Fact Sage Vattenkylningen av gasen foumlre vaumlrmevaumlxlarna houmljer aumlven H2CO-kvoten i gasen vilket aumlr noumldvaumlndigt innan drivmedelssyntes Det effektiva vaumlrmevaumlrdet paring den producerade gasen fraringn modellen beraumlknades till 80 MJNm3 och foumlrgasningsverkningsgraden bestaumlmdes till 67 naumlr braumlnslet bestod av en blandning av 70 trauml och 30 torv och temperaturen i foumlrgasaren var 750degC och i kolvaumltekrackningen ca 1100degC Dessa processparametrar gav en total systemverkningsgrad foumlr produktion av gas och aringnga paring 93 (baserat paring effektiva vaumlrmevaumlrdet) Vid produktion av FT-diesel fraringn gasen producerades ca 190 lton torrefierat braumlnsle och 130 l naftaton torrefierat braumlnsle Verkningsgraden foumlr produktion av FT-diesel fraringn trauml var 27 baserat paring det effektiva vaumlrmevaumlrdet Modellen aumlr saringledes utvecklad utvaumlrderad och klar att anvaumlnda foumlr systemoptimering

4

Abstract Due to the green house effect and the increasing oil price renewable energy including biofuels has become more and more interesting both from economic and environmental point of view Research and development of new fuels and applications for these fuels eg vehicle fuels heating plants and pellet burners is necessary For maximum use of the limited global renewable energy sources it is also important that the utilisation is energy efficient Gasification of biomass is an interesting technique with a relatively high efficiency producing the renewable gas called biosyngas The gasification process can be described as combustion but with a limited supply of oxygen The oxygen may be added as pure oxygen air steam or combinations The principal components in the biosyngas are H2 and CO A number of products can be produced from the biosyngas eg electricity and heat ndash from gas combustion ndash replacement of oil and fossil gas products in industries hydrogen gas synthetic diesel dimethyl ether (DME) methanol ethanol and renewable chemicals via synthesis However the existing gasifiers are too expensive The three main challenges to develop a cost efficient technique are

1 Feeding of a homogenous easy feedable powder would give considerably cheaper processes

2 Enhanced gasification efficiency with less methane and tars would improve process economy and indirect enable a simpler cleaning technology

3 A reduced amount of pollutants in the biosyngas would also be advantageous for the whole system

The pollutants in the gas are eg tars and other hydrocarbons that lower the efficiency and may plug downstream equipment A high amount of methane in the gas also reduces the efficiency since it is inert in the fuel synthesis processes The hydrocarbons are usually cleaned from the gas which is quite expensive Alkali compounds in the gas can cause corrosion in gas turbines and are poisonous to the catalysts in synthetic diesel production These compounds may also cause corrosion fouling and agglomeration in the gasifier By torrefaction of biomass an easily feedable and highly reactive fuel is produced in an energy efficient way Torrefaction means that the fuel is heated to 250-350degC The product is a very dry and hydrophobic fuel which is easy to grind and to feed into eg a gasifier Thanks to the torrefaction process it is possible to use a simpler and less expensive gasifier technique (entrained flow) A new gasifier concept is also being developed at the department of Energy Technology and Thermal Process Chemistry Umearing University This Low tar methane alkali dual cyclone gasifier (LTMAG) is designed to minimize the amounts of tars methane and alkali produced in the gasifier The LTMAG retains the alkali metals and a fraction of the coke which are gasified this gas is burned in a second step and thus never mixed with the biosyngas The tars and methane are decomposed in a second high temperature zone heated by the later combustion stage The syngas produced in the LTMAG is thus relatively clean from all the compounds above

5

This report describes the development of a simulation model for the total BTL (Biomass to Liquid) system using the soft ware Aspen Plus The model also includes pre-treatment of the fuel gasification combustion and production of synthetic (Fischer Tropsch) diesel from the gas The gasification was modelled by blocks in Aspen Plus assuming equilibrium and the gas composition from the model was verified by another software Fact Sage The quench both cools the gas before the heat exchanger and increase the H2CO ratio which is necessary prior to synthesis The lower heating value of the gas was calculated from the model to 80 MJNm3 and the gasification efficiency was 67 (LHV) assuming a fuel mixture of 70 wood and 30 peat and a temperature in the gasifier at 750degC and in the cracking 1100degC The system efficiency with the same fuel and temperatures for the system including gas and steam production was 93 (LHV) The amount of FT diesel produced per tonne torrefied fuel was about 190 litres and 130 litres of naphtha and the FT diesel efficiency was 27 Thus the model is developed evaluated and ready to be used for system optimisation

6

Confidential information Details concerning the LTMAG are confidential and that is why some parts in this report are concealed in grey I hope that the report is readable and interesting even without this information

7

Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

4

Abstract Due to the green house effect and the increasing oil price renewable energy including biofuels has become more and more interesting both from economic and environmental point of view Research and development of new fuels and applications for these fuels eg vehicle fuels heating plants and pellet burners is necessary For maximum use of the limited global renewable energy sources it is also important that the utilisation is energy efficient Gasification of biomass is an interesting technique with a relatively high efficiency producing the renewable gas called biosyngas The gasification process can be described as combustion but with a limited supply of oxygen The oxygen may be added as pure oxygen air steam or combinations The principal components in the biosyngas are H2 and CO A number of products can be produced from the biosyngas eg electricity and heat ndash from gas combustion ndash replacement of oil and fossil gas products in industries hydrogen gas synthetic diesel dimethyl ether (DME) methanol ethanol and renewable chemicals via synthesis However the existing gasifiers are too expensive The three main challenges to develop a cost efficient technique are

1 Feeding of a homogenous easy feedable powder would give considerably cheaper processes

2 Enhanced gasification efficiency with less methane and tars would improve process economy and indirect enable a simpler cleaning technology

3 A reduced amount of pollutants in the biosyngas would also be advantageous for the whole system

The pollutants in the gas are eg tars and other hydrocarbons that lower the efficiency and may plug downstream equipment A high amount of methane in the gas also reduces the efficiency since it is inert in the fuel synthesis processes The hydrocarbons are usually cleaned from the gas which is quite expensive Alkali compounds in the gas can cause corrosion in gas turbines and are poisonous to the catalysts in synthetic diesel production These compounds may also cause corrosion fouling and agglomeration in the gasifier By torrefaction of biomass an easily feedable and highly reactive fuel is produced in an energy efficient way Torrefaction means that the fuel is heated to 250-350degC The product is a very dry and hydrophobic fuel which is easy to grind and to feed into eg a gasifier Thanks to the torrefaction process it is possible to use a simpler and less expensive gasifier technique (entrained flow) A new gasifier concept is also being developed at the department of Energy Technology and Thermal Process Chemistry Umearing University This Low tar methane alkali dual cyclone gasifier (LTMAG) is designed to minimize the amounts of tars methane and alkali produced in the gasifier The LTMAG retains the alkali metals and a fraction of the coke which are gasified this gas is burned in a second step and thus never mixed with the biosyngas The tars and methane are decomposed in a second high temperature zone heated by the later combustion stage The syngas produced in the LTMAG is thus relatively clean from all the compounds above

5

This report describes the development of a simulation model for the total BTL (Biomass to Liquid) system using the soft ware Aspen Plus The model also includes pre-treatment of the fuel gasification combustion and production of synthetic (Fischer Tropsch) diesel from the gas The gasification was modelled by blocks in Aspen Plus assuming equilibrium and the gas composition from the model was verified by another software Fact Sage The quench both cools the gas before the heat exchanger and increase the H2CO ratio which is necessary prior to synthesis The lower heating value of the gas was calculated from the model to 80 MJNm3 and the gasification efficiency was 67 (LHV) assuming a fuel mixture of 70 wood and 30 peat and a temperature in the gasifier at 750degC and in the cracking 1100degC The system efficiency with the same fuel and temperatures for the system including gas and steam production was 93 (LHV) The amount of FT diesel produced per tonne torrefied fuel was about 190 litres and 130 litres of naphtha and the FT diesel efficiency was 27 Thus the model is developed evaluated and ready to be used for system optimisation

6

Confidential information Details concerning the LTMAG are confidential and that is why some parts in this report are concealed in grey I hope that the report is readable and interesting even without this information

7

Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

5

This report describes the development of a simulation model for the total BTL (Biomass to Liquid) system using the soft ware Aspen Plus The model also includes pre-treatment of the fuel gasification combustion and production of synthetic (Fischer Tropsch) diesel from the gas The gasification was modelled by blocks in Aspen Plus assuming equilibrium and the gas composition from the model was verified by another software Fact Sage The quench both cools the gas before the heat exchanger and increase the H2CO ratio which is necessary prior to synthesis The lower heating value of the gas was calculated from the model to 80 MJNm3 and the gasification efficiency was 67 (LHV) assuming a fuel mixture of 70 wood and 30 peat and a temperature in the gasifier at 750degC and in the cracking 1100degC The system efficiency with the same fuel and temperatures for the system including gas and steam production was 93 (LHV) The amount of FT diesel produced per tonne torrefied fuel was about 190 litres and 130 litres of naphtha and the FT diesel efficiency was 27 Thus the model is developed evaluated and ready to be used for system optimisation

6

Confidential information Details concerning the LTMAG are confidential and that is why some parts in this report are concealed in grey I hope that the report is readable and interesting even without this information

7

Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

6

Confidential information Details concerning the LTMAG are confidential and that is why some parts in this report are concealed in grey I hope that the report is readable and interesting even without this information

7

Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

7

Acknowledgements I would like to thank my supervisors Ingemar Olofsson at ETPC and Lars Baumlckstroumlm at the department of applied physics and electronics for all their help support and good ideas I would also like to acknowledge everyone at ETPC and especially Anders Nordin and Kristoffer Persson for all support and guidance I am very grateful to Rainer Backman for always taking time and having comprehensible answers to my questions Tommy Hedlund at Volvo Lastvagnar has been very helpful in explaining the integration in the factory to me A very important person for me during this work has been Helen Magnusson Thanks for good company in the computer lab It has been very good to discuss the modelling with you Last but definitely not least I am very grateful to Anders Wingren at Etek who has been my private Aspen Support I could not have done this work without your help

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

8

Table of contents

SAMMANFATTNING 2

ABSTRACT 4

CONFIDENTIAL INFORMATION 6

ACKNOWLEDGEMENTS 7

TABLE OF CONTENTS 8

ABBREVIATIONS 10

1 INTRODUCTION 11

11 BACKGROUND 11 12 OBJECTIVE 12

2 THEORY 14

21 TORREFACTION 14 22 EQUIVALENCE RATIO 14 23 GASIFICATION 14 24 THE CHEMICAL EQUILIBRIUM ASSUMPTION 15 25 HEATING VALUE 16 26 THEORETICAL EFFICIENCIES 16 27 PROBLEMS DILUTION AND IMPURITIES IN THE GASIFICATION SYSTEM 17 28 GASIFICATION TECHNIQUES 17

281 Movingfixed bed gasifiers 17 282 Fluidised bed gasifiers (FBG) 18 283 Entrained flow gasifiers 19 284 Indirect gasifiers 19 285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG) 20

29 QUENCH 21 210 CLEANING OF SYNGAS 21

3 THE MODELLED SYSTEM 23

31 TORREFACTION AS PRE-TREATMENT 23 32 GASIFICATION IN LTMAG 23 33 THE CLEANING SYSTEM 23 34 APPLICATIONS 23

341 Gas combustion 23 342 Electricity production 24 343 Methanol synthesis 25 344 Synthesis of Fischer-Tropsch (FT) Diesel 25

35 ASPEN PLUS 27

4 THE ASPEN PLUS MODEL 28

41 PRE-TREATMENT 29 42 GASIFICATION 30 43 GAS CLEANING 32 44 BURNER 32 45 FT SYNTHESIS 32 46 THE INPUT FILE 33

5 RESULTS AND DISCUSSION 34

51 MODEL LIMITATIONS 35

6 CONCLUSIONS 37

7 FUTURE WORK 38

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

9

8 REFERENCES 39

APPENDIX 1 41

APPENDIX 2 43

APPENDIX 3 44

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

10

Abbreviations ASF Anderson-Shulz-Flory BFBG Bubbling Fluidised Bed Gasifier BTG Biomass-To-Gas BTL Biomass-To-Liquids CFBG Circulating Fluidised Bed Gasifier CHP Combined Heat and Power DME Dimethyl Ether EU European Union FB Fluidised Bed FT Fischer Tropsch IGCC Integrated Gasification Combined Cycle LHV Lower Heating Value LPG Liquefied Petroleum Gas LTMAG Low Tar Methane and Alkali Gasifier PAH Poly Aromatic Hydrocarbons PIG Products of Incomplete Gasification PSD Particle Size Distribution RTO Regenerative Thermal Oxidiser VOC Volatile Organic Compounds WGS Water-Gas shift

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

11

1 Introduction

11 Background During the last 150 to 200 years people in the developing countries have been using more and more energy from fossil fuels such as coal oil and natural gas and also become more or less dependent of these sources of energy However it has become more and more obvious that the use of fossil fuels have three major drawbacks

bull Addition of fossil carbon dioxide and other greenhouse gases like methane to the atmosphere which increases the greenhouse effect and in turn increases the global mean temperature

bull The fossil fuels are not formed in nature at the same rate as we consume them bull The oil price will go up when the supply goes down and due to the limited number of

countries with oil reserves the oil price is sensitive to political stability and even the weather eg storms in these countries

The last statement is mainly concerning oil since coal and natural gas are found in more countries and is believed to last throughout a longer period It is no doubt that eg carbon and hydrogen is added to the atmosphere but there has been a debate about whether this causes heating and if it does what will be the consequence of the heating However during the last years researchers governments organisations etc have agreed that we have changed our climate There have been more numbers of severe tropical storms glaciers are shrinking and the weather has become more and more extreme It is falling even less rain in dry areas and more in the wet There have been warnings for a long time that the oil resources of the world are decreasing Hallock et al [2] has studied 42 different scenarios for the peaking of the worldrsquos oil production Their peak varies between 2004 and 2037 depending on the scenario They have also studied prognoses from five other sources where the production starts to decline between 2004 and 2067 Hirsch [3] has put projections from 12 different sources together and their estimates of the peak of oil production vary from 2006 to 2025 (except for one source that does not se any peak) It is very difficult to know what will happen in the future The oil peak depends on many different factors eg oil demand oil production new techniques etcetera Although the estimates are uncertain there are some facts that all point in the same direction

bull No really large (thus also more economic and long lived) oil reservoir has been discovered since 1968 even though the techniques for finding oil have been improved [3]

bull The volume of discovered oil was largest in the 1960s and has been declining since then [2]

bull Even if very large amounts of oil is discovered (161011 m3 in Hallocks example) the peak can only be delayed by up to 25 years [2]

bull US China and Romania used to be net producers but are now net consumers [2] There have been some political efforts to decrease the emissions of CO2 from fossil fuels One of them is the Kyoto protocol that came into effect in February 2005 when countries causing more than 55 of the CO2 emissions in the world had signed the agreement The objective of the protocol is to decrease the total CO2 emissions by 5 compared to the emissions in 1990 For the European Union the emissions should be decreased by 8 percent by 2012 The

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

12

European Union has also introduced a greenhouse gas emission trading scheme to reduce the emissions of greenhouse gases The goal for the EU is that by 2010 12 of the energy should come from renewable fuel Since the transport sector consumes 30 of the energy in EU it is important that the use of fossil fuels for vehicles decrease The aim is that 575 of the motor fuels are renewable by 2010 and possibly up to 20 by 2020 In part due to the fast development in China and India the increasing demands of oil has caused a rise in the crude oil (and other raw material) price The oil price is also dependant on the political stability in the producing countries Even hurricanes can cause a raise in the world oil price According to this fact it should be in the interest of at least countries without oil to reduce the oil dependence Today there are already several suitable renewable motor fuels that can replace the fossil motor fuels eg methanol dimethyl ether (DME) ethanol synthetic diesel and hydrogen all of which can be produced by gasification of biomass Biomass gasification (further described in section 23) is often more efficient than both combustion of biomass to produce electricity [4] and fermentation of sugars to ethanol In the fermentation process the total energy efficiency is 25-30 [5] and for combined processes with ethanol power and heat production the energy efficiency is about 75 [6] About 80 of the energy from the gasified biomass is present in the produced syngas and for FT diesel processes the total energy efficiency (from biomass via atmospheric gasification to FT liquids) is 33-40 [7] The energy efficiency from biomass to Methanol or DME is about 55 [5]These liquid fuels can also be produced by swapping biomass with black liquor gasification attaining efficiencies from biomass to fuel of 66 for methanol 67 for DME and 43 for FT diesel (including both diesel and naphtha from FT-process gives an efficiency from biomass of 65) [8] In combination with heat and power production the efficiency would increase even further This is why gasification is such an interesting technique Although ethanol methanol and DME eventually can be efficiently produced via a gasification process the production of the easily introduced FT-diesel is of most immediate interest for the goals set up for Biofuel Region This report will focus on the process of a new type of biomass gasifier (LTMAG) and the building of an ASPEN Plus simulation model containing

bull Pre-treatment of the fuel (wood and peat) bull Gasification bull Gas cleaning bull Applications for the produced gas

12 Objective The objective of the project has been to develop a useful model of the total system of fuel pretreatment biomass gasifier gas cleaning and heat recovery from the gas as well as the FT synthesis of motor fuel from the gas The results from this model can then be used for system optimization and determining the influence of different process parameters The software used to build the model is Aspen Plus When the model is set in Aspen Plus the input parameters are easy to change and thus a first evaluation of the process can be performed without the need of expensive experimental setups and experiments These simulations are not as accurate as experiments but can give a good first idea of how process parameters change It may also be possible to find optimal working interval or get other knowledge of how the pilot plant should be designed

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

13

The model should also give a good idea of how the pre-treatment works if the torrefaction is self-supporting with energy and how much energy that is needed for drying The gas composition gasification efficiency and the amount of FT diesel and naphtha produced should also be given from the simulations Furthermore the model is to be used in the forthcoming evaluation and development of a new cost efficient BTL-system

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

14

2 Theory

21 Torrefaction Drying and grinding the biomass fuel to a feedable powdered fuel can be problematic and expensive and grinded material are also difficult to store since the particles are hydrophilic and become sticky with unwanted packing as result Torrefaction is a pre-treatment method that makes the wood easier to grind and also may result in a material that is easily feedable These effects are caused by decomposition of the hemicellulose and depolymerisation of the cellulose Other advantages with torrefied biomass is that the resulting powder has a higher heating value and energy density than conventional powder In addition it is hydrophobic [9] Torrefaction of woody biomass means that raw fuel is heated in absence of oxygen at atmospheric pressure to a temperature of 200 to 350degC for 10-30 minutes The amount of volatiles in the fuel is decreased by 5-20 and the moisture content is reduced Studies have shown that the energy needed for grinding can be reduced by more than 50 after torrefaction [9] If torrefaction is used in combination with powder production the cost of the fuel can be reduced significantly compared to conventional powder production A powdered fuel has good contact with the gasifying medium and therefore is more efficiently gasified ie the most cost efficient way to increase reactivity

22 Equivalence Ratio The stoichiometric amount of oxygen for complete combustion is calculated from these reactions C + O2 rarr CO2

H2 + frac12 O2 rarr H2O This means that for each carbon atom in the fuel one molecule of O2 has to be added and consequently half a molecule of O2 per hydrogen molecule The λ is defined as the ratio between the actual oxygen supply and the stoichiometric amount of oxygen equation (1)

2

2

tricstoichiomeO

oxygenO

n

n=λ (1)

23 Gasification There are three types of thermal conversion processes combustion gasification and pyrolysis Combustion is defined as thermal conversion with an equivalence ratio above 10 [10] ie more air than stoichiometrically needed When the equivalence ratio is between 025 and 04 it is gasification and with less than 02 the process is called pyrolysis Gasification is not as widespread as combustion but the process is interesting since the gasification products syngases or biosyngases if biomass is gasified can be utilised for different purposes than heat from combustion The gases can be burned in a gas turbine producing power and heat but the gases can also be reformed to methanol DME hydrogen and synthetic diesel and used as fuel in combustion engines and fuel cells The gasification process is also more efficient than combustion since the exergy losses due to heat emission are smaller [11]

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

15

The fuel used in a gasifier can be coal biomass fuels or even waste fuels The most widely used fuel is coal but there are massive developments regarding both biomass and wastes since CO2 from fossil fuels contribute to the increased green house effect Other advantages with biomass are the low amount of ash and sulphur compared to fossil fuels Biomass is also generally more reactive than coal which means that the gasification temperature can be lower with biomass but a lower temperature may also lead to a higher amount of produced tars [4] The higher reactivity also means that pressurised gasification has more advantages if coal-fuelled than if biomass-fuelled since the relative improvement of the performance is larger for coal [4] In the gasification process the fuel is gasified at temperatures of 750 to 1300degC in the presence of a gasifying medium The three main gasifying media are air pure oxygen and steam or mixtures of the three Other possible media are hydrogen that can form CH4 with carbon or carbon dioxide with carbon monoxide as product according to

C + CO2 rarr 2CO Air and oxygen utilizes direct gasification with the release of heat from partly oxidising the fuel and therefore supplying the endothermic gasification reactions with energy Steam on the other hand utilizes indirect gasification where the heat for the gasification process is supplied from an external heat source and the water molecule is split into hydrogen and oxygen The released oxygen will in turn react with the fuel producing the synthesis gases and some heat to the gasification process Air is of course the cheapest medium but the produced gas is diluted with quite high amounts of nitrogen Oxygen is expensive to produce but the gas will get a higher heating value since only a low amount of inert nitrogen is present Steam production is also quite expensive but the steam generation cost can be reduced if excess heat can be used The resulting synthesis gas may however contain more methane than gasification with air or oxygen at the same temperature and pressure The methane is inert in fuel catalysts which will reduce the overall motor fuel plant efficiency On the other hand the biosyngas will have the highest heating value (thanks to methane) which makes it suitable for eg electricity production in gas turbines The raw gas produced in a gasifier consists mainly of carbon monoxide (CO) and hydrogen gas (H2) but there are also a certain amount of undesired components such as nitrogen (N2) if air is used as oxidizing medium methane (CH4) steam (H2O) carbon dioxide (CO2) tars and other impurities eg alkali soot chlorine compounds sulphur compounds and nitrogen compounds There are various definitions of tar used in literature but according to the international standard for tar and particle measurement in biomass producer gas [12] it is defined as all organic compounds with more than six carbon atoms C6+ The syngas composition may be controlled by temperature pressure residence time reactivity fuel composition and additives [13]

24 The chemical equilibrium assumption The equilibrium process model is based on attainment of chemical equilibrium which means that perfect mixing and infinite residence time is assumed This is of course not fully the case in a real reactor but previous work on comparing equilibrium and experimental results have

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

16

shown almost total agreements for most major gases For this it is important to have a sufficiently high temperature turbulent flow and long residence time Several experimental tests have however found super equilibrium concentrations for some products of incomplete gasification (PIG) like methane and tar compounds

25 Heating Value To determine the heating value of the biosyngas Hess Law is used to calculate the enthalpy of formation

)()()( THvTHTH oEiEi

oC

oCf isumminus=∆

Where f = formation C = compound Ei = element and v = the fraction of the element The oxidation reactions of the gas components are

H2 + frac12 O2 rarr H2O CO + frac12 O2 rarr CO2

CH4 + 2 O2 rarr CO2 +2 H2O The formation enthalpies for the substances are presented in Table 1 Table 1 Formation enthalpies values from Barin [14] Compound Enthalpy of formation (kJmole) H2 0 O2 0 H2O(g) -2416 CO -1106 CO2 -3938 When the values from Table 1 are inserted into Hess Law the enthalpies of formation are determined to molkJH o

COf 2283)15298(2 =∆ and molkJH o

OHf 6241)15298(2 =∆ The

heating value for the dry gas is calculated as

OHo

OHfCOo

COfsyngas vHvHLHV2222 sdot∆+sdot∆=

26 Theoretical efficiencies The theoretical thermal efficiency of the gasification process can be calculated with the following formula

)(

)()( 33

fuelkgMJLHV

fuelkgNmVNmMJLHV

fuel

gasdrygasdryth

sdot=η

The efficiency of the process from fuel to burned gas is determined from the heat emitted in the burner and the energy in the fuel as

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

17

)()(

)(

skgmkgMJLHV

sMJP

fuelfuel

burnerburn

bullsdot

=η

The efficiency of the FT-process from fuel to FT diesel is calculated by the following equation

)()(

)()(

skgmkgMJLHV

skgmkgMJLHV

fuelfuel

dieselFTdieselFTFT bull

minus

bull

minus

sdot

sdot=η

27 Problems Dilution and Impurities in the Gasification System Challenges with biomass gasification are so far the impurities in the biosynthesis gas and the problems they bring in the gasification process and later catalyst steps Such impurities are soot tars alkali salts hydrocarbons sulphur and halogen compounds Unwanted dilution of the biosyngases will reduce the heating value and if the dilution gas is a hydrocarbon the gasification efficiency will be lowered If air is used as gasifying medium the gas contains a quite large amount of N2 which is (almost) inert in both fuel synthesis and gas burning By dilution with nitrogen the partial pressure of the desired gases (and thus the heating value) becomes much lower than desired Methane is inert in both FT and methanol synthesis but if the gas is burned a high amount of methane increase the heating value of the gas The alkali compounds originate from the fuel and can cause corrosion agglomeration and fouling because of the low ash melting point and volatility Alkali metals are also a problem in the produced gas causing vibrations and corrosion due to deposits in the gas turbine [15] or poisoning FT catalyst [16] Tars condense at different temperatures inside the gasifier or in downstream equipment and they form aerosols The aerosols are difficult to remove from the gases [1] Intelligent fuel mixtures can reduce the ash related problems in the gasifier and gas cleaning removes ash particles from the syngas Peat is for example a relatively cheap and efficient additive used to reduce fouling slagging and agglomeration problems The gasifier design affects the amount of tars produced and thus a good design could reduce tar problems The tars could also be cracked thermally or catalytically or removed from the gas in the gas cleaning The last alternative is the least desirable since energy from the tars are lost The gas cleaning should remove other undesired products

28 Gasification techniques Many different techniques for gasification are suggested They can be divided into four main groups Movingfixed bed fluidised bed (FB) entrained flow and indirect gasifiers [1] But there are also a number of techniques that does not fit into any of the above mentioned groups

281 Movingfixed bed gasifiers This type of gasifiers has either a moving or a fixed grate The technique is old well tested and quite simple and robust However problems to keep an isothermal operating temperature

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

18

arise due to the formation of cold air tunnels in the fixed bed gasifiers which limits the up scaling The moving grate gasifiers have got rid of this problem in some extent When the gasifying medium is introduced at the bottom the fuel at the top and the syngas is moving upwards in the reactor the gasifier is an updraft gasifier see Figure 1a In the downdraft gasifier the fuel is fed from the top and after drying and pyrolysis the gasifying medium is introduced Figure 1b The syngas moves downwards There are also crossdraft gasifiers in which the fuel is fed from the top the gasifying medium from the side and the syngas is leaving the gasifier at the opposite side Figure 1c In updraft and crossdraft gasifiers the produced syngas contains a high amount of tars but the tars produced in the downdraft gasifier are cracked in the oxidation The gases produced in downdraft and crossdraft gasifiers has quite low heating value and a lot of exergy is lost in these reactors due to the large amount of heat produced [1]

282 Fluidised bed gasifiers (FBG) The bed consists of a mixture of inert bed material usually sand and fuel and it is fluidised (ie it acts like a fluid) by the gasifying medium The main advantages with FBG are the very good mixing and good kinetics [4] but also efficient heat exchange The bed can be circulating (CFBG) see Figure 2a or bubbling (BFBG) see Figure 2b In the latter the gasifying medium has a higher velocity and thus the bed material follows the gases to a cyclone where it is brought back to the bottom of the gasifier There are less alkali and up-scaling problems with FBGs When combusting or burning biomass in FBG there can be agglomeration problems due to silicate-rich ashes with low melting point the bed particles becomes messy and sticky with agglomerates as a result To avoid agglomeration the gasification temperature is kept sufficiently low and the bed material is exchanged continuously at appropriate rates However the low gasification temperature causes higher amounts of tars and other drawbacks with FBGs may be high amounts of particles and soot in the syngas

a b c Figure 1 Moving fixed bed gasifiers a) updraft b) downdraft and c) crossdraft [1]

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

19

283 Entrained flow gasifiers This type of gasifier works like a powder burner ie the fuel is a gas liquid powder or slurry it is mixed with the gasifying medium and sprayed into the reaction chamber 3 The temperature and pressure of the process are relatively high above 1200 degC and often pressurised But the high temperature also results in a large amount of produced heat [1] which reduces the BTL-efficiency (Biomass-To-Liquids) This type of gasifier often uses a mixture of oxygen and steam to keep the operating temperature sufficiently high and therefore often produces a low tar containing syngas

284 Indirect gasifiers In this type of gasifier steam (in different amounts) is used as gasifying medium and the endothermal gasification process is heated by an external heat source or by partial combustion see Figure 4 Partial combustion is the most common way to keep the gasification temperature sufficiently high ie a mixture of airoxygensteam is used as gasifying medium Since steam is the main gasifying agent the produced gases are not diluted by nitrogen On the other hand the syngases commonly contain higher amounts of methane compared to syngases from other types of gasifiers This high methane content raises the heating value and is an advantage if the gas is combusted but methane is undesired if the gas is to be further catalyzed to eg liquid fuels The investment cost of indirect gasifiers are usually higher than other types of gasifiers due to either the complicated construction of the gasifier or the oxygen plant needed if oxygen is the oxidizing medium [1]

a b Figure 2 Fluidised bed gasifiers a) bubbling and b) circulating [1]

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

20

285 Low Tar Methane Alkali dual cyclone Gasifier (LTMAG)

All of the above presented gasifier techniques suffer from one or several problems which increases the cost for the final product eg cleaning and processing of raw product gas costly technique or expensive pre-treatment of the fuel The research group at ETPC Umearing University has developed a new type of biomass gasifier concept called LTMAG based on improvement of the cyclone gasifier technique developed at Lulearing University of technology The cyclone separates particles from gases and thus the produced syngas is cleaner than gas from other gasifiers The LTMAG is designed to minimize the tars methane and alkali salts in the synthesis gas in a cost-efficient way as described below The fuel is wood powder and peat torrefied (see 21) powderised and mixed with superheated steam and oxygenair thereafter entering the inner cyclone where the first gasification step takes place see Fel Hittar inte referenskaumllla This gasification takes place at approximately 750degC and low equivalence ratio in order to retain the alkali metals and get a residual amount of coke at the bottom of the cyclone The major advantage with the relatively low reaction temperature is that the main parts of the alkali compounds are concentrated to the coke instead of the product gas At the bottom of the cyclone the coke is gasified producing a product gas

consisting mainly of CO and small amounts of CO2 The gas from this gasification step is partly burned in the primary outer combustion zone to heat first inner pyrolysis step via forced convection and radiation If the energy supplied from the primary combustion zone is insufficient there is an option to add

Figure 4 Indirect gasifier [1]

Figure 3 Entrained flow gasifier [1]

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

21

oxygen with the fuel and steam injection and thereby get an exothermal combustion and thus internal heating in the inner primary cyclone gasifier

The produced raw product gases from the inner primary cyclone gasifiers escapes at the top containing quite high amounts of methane and tars since steam is the gasifying medium and because of the low gasification temperature [17] The raw gas then enters a cracking zone that is heated from the outside by a secondary combustor fuelled by the residual gas from the primary outer combustor in order to thermally crack both methane and tars in the raw product gas The temperature in the secondary combustion zone has to be about 1150-1200degC in order for the temperature in the cracking zone to reach a minimum of 1100degC At this temperature the amount of methane and tar could potentially be decreased significantly Yu et al [18] has shown a fast decrease in tar amount of gas when the temperature was increased from 700 to 900 degC and Olofsson [19] has showed very low amounts of tars at 950degC (165 mgNm3) and 1000 degC (95 mgNm3) The high temperatures put the materials to a severe test The excess heat from the flue gases is used to preheat the oxygen and combustion air to 500degC and the excess heat in the syngases is used to generate superheated steam at 500degC

29 Quench The hot gases biosyngases needs to be cooled before entering a heat exchanger In a water-quench water at 20degC is sprayed into the gas cooling the gas and changing the H2CO ratio of the gas against more H2 which is necessary before fuel synthesis

210 Cleaning of syngas The gas produced in a gasifier always contains more or less

contaminants To reach the demands of catalysis process or gas tubine the gas has to be cleaned from parts of these contaminants The gas cleaning could be cold (under 200degC) partially hot (260-540degC) or hot (above 550degC) For gasifying systems at low pressure the cold gas cleaning is the most suitable It is a relatively cheap method since no heat resistant materials or pressure vessels are needed Another reason to use this method for low pressure gases is that if the gases are further processed to fuel or burned in a gas turbine they need to be cooled before the compressor and thus the gases needs cooling even if partial hot or hot gas cleaning were used After gas cooling in the cold gas cleaning there is usually a COS hydrolysis step where COS is converted into H2S that is easier to remove from the gas [16] A wet scrubber removes particulates tars and chars finally sulphur is removed from the gas [1] with eg a ZnO filter

Figure 5 The LTMAG with permission from Ingemar Olofsson

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

22

For syngas produced at higher pressure the partial hot or hot gas cleaning are suitable methods In the partial hot gas cleaning the gases are partially cooled before passing the filter removing particulates and char Then the gases are cleaned from gaseous halogens After halogen removal the gases are cooled further and in the final cleaning step cleaned from H2S [1] Even in hot gas cleaning the gases are cooled before the first cleaning step which is sulphur removal A filter is used for particulate and char removal and then the gases pass a cleaning step where gaseous halogens trace metals and alkali metals are eliminated from the gases [1]

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

23

3 The modelled system The model is simulating a Biomass-To-Liquids (BTL) system and a Biomass-To-Gas (BTG) system where the gas is burned in a gas burner Before entering the gasifier the fuel is torrefied and grinded For the BTL system the biosyngas is also cleaned to reach the demands for the methanol DME or in this case the FT diesel process

31 Torrefaction as pre-treatment The fuel used in the LTMAG is woody biomass possibly with peat as additive with a moisture content of about 40-50 Before entering the gasifier the fuel is dried torrefied and grinded The fuel is dried using excess heat from torrefaction and from a combined power and heating plant at about 90degC The moisture content is reduced to 10 after drying The torrefaction temperature is 270degC in the model and the time is not specified After torrefaction the moisture content has decreased to 3 The amount of volatiles in torrefied wood is lower than in untreated wood Steam and volatiles are removed from the fuel during torrefaction in the simulations Cooling is necessary after the torrefaction otherwise the fuel self-ignites when brought in contact with air again The heat from burning volatiles and cooling the fuel is heating fuel drying and torrefaction After cooling the fuel (now at 120degC) enters the mill where it is grinded to powder with a maximum diameter of 1 mm

32 Gasification in LTMAG In the present report the fuel of interest is a mixture of torrefied wood and peat Powdered fuel is injected into the gasifier where it is gasified according to previous description (section 285) The gasifying medium is steam and oxygen

33 The cleaning system Even though the LTMAG produces clean syngas compared to other gasifiers some gas cleaning is needed For gasifying systems at low pressure (such as LTMAG) the cold gas cleaning is the most suitable The gas is cooled down to 100degC before cleaning The gas cleaning is modelled in Aspen Plus by using a predefined block that divides the produced gas into two streams one with the undesired products and one with the clean gas Thus the energy required for cleaning is not included in the model

34 Applications The produced syngas can be used for many different applications It can be burned in a burner or gas turbine but it can also be synthesised to a motor fuel such as methanol DME hydrogen gas FT diesel or ethanol In the present report both combustion in a burner and synthesis to FT diesel will be investigated

341 Gas combustion In the Umearing EnergiVolvo Lastvagnar project the aim is to exchange fossil gas with renewable biosyngas to become the first CO2-free Volvo factory The factory in Umearing is producing truck cabins for Volvo Trucks Co Today liquefied petroleum gas (LPG that mainly consists of propane and butane) is used to destroy VOC from curing of the paint layer in ovens and concentrated solvents from abatement process There are totally five ldquofurnacesrdquo for drying and curing operating at 90-100degC these are heated by high-temperature hot water from biomass and waste derived district heating instead of LPG For the destruction of VOC the goal is to use biosyngas from a gasifier The exhaust air from the paint processes with

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

24

aromatic solvents pass two concentrators where the volatile organic compounds (VOC) are adsorbed and the purified air is exhausted to the atmosphere The concentrated VOC are then combusted in the Regenerative Thermal Oxidiser (RTO) at about 820degC where biosyngas from a gasifier will enter the furnace mixed with combustion air Hot air regenerates the concentrators and the air will be heated from 130degC to180degC by a combination of LPG and biosyngas burning In the factory there is also two more furnaces for both drying and destruction of VOC where the temperature must exceed 150-200degC and burning of syngas could be used for this purpose using a burner adjusted for both biosyngas and LPG The destruction of VOC requires temperatures of 700 degC in the After Burning Chamber When the gas is burned for destruction or heating there is not the same restricted requirements of impurities as in case of gas turbine or liquid fuel synthesis If using burners on the other hand they must fulfil certain restrictions on heating value of the gas The heating value of the biosyngas must not be below 10-11 MJNm3 The heating value of the LPG used today is about 92 MJNm3

342 Electricity production A gas turbine can also be used for power production from the syngas generated from a typical LTMAG The gas turbine consists of a compressor where air and the syngas are compressed The pressurised air and fuel gas are burned in a combustion chamber after which the hot gases pass a turbine connected to a generator The efficiency for a gas turbine is about 30 which is lower than the efficiency for condensing steam power plants (35-45) [20] Gas turbines are suitable reserve power stations since production costs are low and the start up is fast A higher efficiency is obtained in an integrated gasification combined cycle (IGCC) see Figure 6 The temperature of the flue gas from the gas turbine is quite high and thus this heat can be utilised to generate steam for a steam turbine After the steam turbine steam is condensed and heat for eg district heating is produced The IGCC system could produce twice as much electricity per unit of biomass as a conventional steam turbine [21] Gas turbines requires a clean fuel ie free from particulates and volatile ash components since damage the turbine Biomass contains relatively high amounts of volatile alkali metals which cause corrosion in the gas turbine [22] Table 2 shows the gas demands for gas turbines

Figure 6 An IGCC-system producing power in both gas and steam turbine

and heat for district heating

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

25

Table 2 Demands for gas in gas turbine [1]

Component Demands Gas Turbine Particles lt2ppmW Tars Below dew point Halogens lt1 ppmW Na + K lt001-003 ppmW S-components lt20ppmW N-components lt55 mgm3 Heavy metals 1 ppmW

343 Methanol synthesis The syngas fed into the methanol synthesis must be clean and should have a stoichiometric syngas ratio (H2-CO2)(CO-CO2) above 2 The reactions of the methanol synthesis are

CO + 2H2 rarr CH3OH CO + H2O rarr H2 + CO2

CO2 +3H2 rarr CH3OH + H2O Since the LTMAG in the present work is working at atmospheric pressure the preferred methanol synthesis is at low temperature of about 220-275degC and low pressure of about 50-100 bar over a CuZnOAl2O3-catalyst The process could also be held at high temperature about 350degC and high pressure 250-350 As mentioned the synthesis gas need to be clean from dust and condensable products Below some additional levels of impurities are mentioned

bull Sulphur deactivates the catalyst and the maximum sulphur concentration of the syngas is set to 01 ppm [1]

bull Heavy metals and alkali metals decreases the activity of the catalyst bull Chlorine causes permanent activity decrease of the CuZnOAl2O3-catalyst HCl

reacts with copper producing copper chlorides that sinters and thusreduces the catalyst surface

bull Arsenic poison the catalyst bull Nickel forms methane which decrease the catalyst activity Nickel carbonyls

decompose the catalyst [1] bull Iron forms methane paraffins and waxes which causes lower carbon to methnol

conversion Iron carbonyls decompose the catalyst [1] bull Steam deactivates the catalyst and a high partial pressure of steam change the

equilibrium of the methanol reaction to the left ndash less methanol is produced [1] bull Oil deactivates the catalyst

The total energy efficiency of the production of methanol from biomass via gasification is about 55 [5]

344 Synthesis of Fischer-Tropsch (FT) Diesel FT synthesis is a process where CO and H2 are converted to liquid hydrocarbons over a catalyst according to the reaction

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

26

CO + 2H2 rarr -CH2- + H2O

where the H2CO ratio is near 2 Hydrocarbon chains containing more than one hundred carbon atoms can be produced in the FT-synthesis but according to the Anderson-Shulz-Flory equation (3) the probability for such long chains is very low From the produced hydrocarbons different chemicals and FT diesel ie chains with 12 to 20 carbon atoms (C12 to C20) can be derived The LHV of FT diesel is about 41 MJkg The cetane number of FT diesel is about 70 compared to 45 for fossil diesel A high cetane number facilitates complete combustion and low emissions [8] A regular diesel engine can use a blend of fossil and FT diesel or FT diesel with additives as fuel FT diesel has been produced in large scale from coal in South Africa since late seventies The FT-process can be performed at high temperature (300-350degC) or low temperature (200-250degC) The pressure is in the range 10-40 bars for both processes At high temperature the catalyst is based on iron (Fe) and the yield is mainly chains with up to C11 The optimal H2CO ratio is 17 since the Fe catalyst also promotes the water-gas shift (WGS) reaction (2) which enhances the actual H2CO ratio

222 HCOOHCO +rarr+ (2)

At the lower temperatures iron or cobalt (Co) catalyst is used and in case of Co catalyst the H2CO ratio should be about 215 since no WGS reaction occurs The cobalt catalyst is more expensive but the conversion to hydrocarbons is higher [23] The produced hydrocarbons are mainly waxes (gt C20) these are easy to crack into diesel and therefore the low temperature FT-process is preferred when FT diesel is the desired product [1] To avoid poisoning of the catalyst it is important that the incoming gas is cleaned from impurities Acceptable levels for some impurities are listed in Table 3 Table 3 Requirements for gas into FT synthesis [16 24]

Impurity Removal level According to Boerrigter

Removal level according to Stergaršek

H2S + COS + CS2 lt1ppmV 10 ppb NH3 + HCN lt1ppmV 20 ppb HCl + HBr +HF lt10ppbV Alkaline metals lt10ppbV 10 ppb Solids (soot dust ash) Essentially completely Organic compounds (tars) Below dew point 0 The weight distribution of chains can be predicted by the Anderson-Shulz-Flory model

12)1( minusminus= nn nW αα (3)

where nW is the weight percent of a product and n is the number of carbon atoms in it α is

the probability of chain growth α depends on catalyst temperature partial pressure of the

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

27

syngas and the FT process [25] To reach maximum diesel yield α should be near 1 At such high α the products are mainly heavy hydrocarbons ie waxes which can be cracked into diesel chains The total CO conversion after recirculation of syngases is 85-90 [26] The most promising FT reactor at low temperature is the slurry phase reactor In this type the gases are bubbling through a slurry with catalysts Advantages with this technique are that there is a very good contact between reactants and catalyst and thus the amount of catalyst can be minimized The reactor temperature can also be kept constant and finally the investment cost is lower Large amounts of heat is emitted in the exothermal reaction and thus it is important with an efficient cooling of the reactor The production of FT diesel per tonne of wood (10 moisture) is about 100 liters but with technology improvements the yield can reach 210 ltonne [27]

35 Aspen Plus Aspen is an abbreviation for Advanced System for Process Engineering Aspen Plus is a program that enables development of processes models to simulate heat and mass flows The program uses reaction kinetics mass and heat balances chemical and phase equilibrium to determine the steady-state values of the parameters of interest There are a number of predefined so called blocks that can be used to simulate parts in a process eg a pump a turbine or a gasifier A particular type of block called reactors includes for instance Gibbs reactors stoichiometric reactors (defined by reactions) and yield reactors (defined by yield) The blocks can be connected with material or heat streams in order to create a flow sheet over the entire process All components present in any part of the process must be specified before the program can make any calculations Many components can be found in the databanks included in the program but the user can also define the components Aspen Plus can find a pre-determined value of one parameter by iterating another When the model is set there is a sensitivity analysis tool and tools for presenting and construct plots etcetera in Aspen Plus

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

28

4 The Aspen Plus model The Solids template in Aspen Plus defines the global settings The global units are SI units except for temperatures that are specified in Celsius and pressure in bar The stream class MIXCIPSD (Mixed Conventional Inert solid Particle Size Distribution) was chosen since the modelled streams contains vapour liquid and conventional solid phases The PSD is needed to be able to grind the fuel and the size distribution is 0-50 mm In order for Aspen Plus to be able to split the product streams substreams were defined the ldquoMIXEDrdquo sub stream contained the vapor and liquid phases the fuel component and the solid carbon was in ldquoCIPSDrdquo The fuel is defined as a new component in Aspen Plus the values used for definition are shown in Table 4 The formula for this component was calculated from the composition of torrefied wood [28] The standard solid Gibbs free energy of formation (DGSFRM) at 25degC and 1 atm is defined as 0 kJkmole since the value do not affect the equilibrium calculation An iterating process and a known heating value of the fuel were used to find the value of the standard solid heat of formation (DHSFRM) The solid heat capacity (CPSPO1) and the volume per kmole (VSPOLY) values are temperature independent mean values of heat capacities and densities for wood found in literature All components added in the input fuel stream are listed in Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood Table 4 Values used in fuel definition in Aspen Plus

Property Abbreviation value Enthalpy of formation DHSFRM -27696099 kJkmole Gibbs energy of formation DGSFRM 0 kJkmole Heat capacity CPSPO1-1 2050 kJkmoleK Molar volume VSPOLY 333 m3kmole Formula C497H556O206N018

Table 5 The fuel was added as both chemical compounds and as the ldquoFuelrdquo component defined in Aspen Plus having the composition of torrefied wood

Fuel mixture Components Wood Fuel C H O N Peat C H O N Si Al Ca Fe K Mg Na S Cl Moisture H2O The Base method was set to ideal which means that when calculating enthalpy Gibbs free energy volume thermal conductivity etc gases are assumed to behave like ideal gases also Raoultrsquos and Henryrsquos laws are used When these basic definitions were set the flow sheet building started The flowsheet was expanded with one block at the time No new block was added until the model was able to create results This helped to detect faults and also gave a better survey of the task All outlet streams were cooled to 20degC to simplify the calculation of losses Figure 7 shows an overview of the entire model with some data included Appendix 1 includes a summary of all input data in the model that is possible to change when simulating

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

29

41 Pre-treatment Figure 8 shows the flow sheet of the pre-treatment The fuel input stream consists of the fuel component carbon hydrogen oxygen nitrogen water and the components of peat Heat from torrefaction and excess heat from a combined heat and power (CHP) plant is used for drying the fuel The amount of excess heat is calculated from approximated values of fuel drying A multiplier block decrease the energy input to the dryer so that the temperature reaches 90degC The emission of water is simulated by a separator Part of the added C N2 O2 and water is separated from the fuel in the torrefaction A heat exchanger-block and a separator are used to model the torrefaction The fuel is heated in a heat exchanger block in absence of air The temperature in the block reaches 270degC by using heat from combustion of volatiles and from cooling of torrefied products If this heat not is enough high pressure steam (40 bar 400degC) from a CHP plant could be added In the separator block the splitfraction of the components in both substreams are assigned into the product streams The moisture content is reduced to 3 after the separator and the volatiles emitted in the torrefaction are burned using the inbuilt RGibbs reactor This reactor has two of the parameters pressure temperature and heat duty as required input (the third is calculated by Aspen Plus) The reactor also requires inlet and outlet material streams The given information is used to minimize the Gibbs free energy of the reactants in order to calculate the properties of the outlet stream eg composition enthalpy and volume The heat produced in the reactor can be transferred to another block by connecting them with a heat stream The heat from this reactor is heating the flue gases which in turn heats the torrefaction process After the separator the fuel is cooled to 120degC in order not to self-ignite when brought into air Four mills one primary and three secondary are grinding the fuel into powder with a maximum particle size of 1 mm The bond work index was varied to reach the power consumption from experiments of grinding torrefied wood [9]

Figure 7 Overview of the modelled system with production of FT diesel

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

30

Figure 8 Simplified flow sheet for the pre-treatment The wet fuel is dried to 10 moisture in the drier The dry fuel enters the torrefaction where it is heated causing volatilisation of mainly hydrocarbons These hydrocarbons are burned in a reactor producing heat to the torrefaction After torrefaction the fuel is grinded

42 Gasification Figure 9 shows the flow sheet for the gasifier The gasifying combusting tar cracking and water quench steps of the system are modelled with Rgibbs reactors (read more about Rgibbs reactors in 41) Heat exchanger blocks HEATX are used to preheat the air with outgoing flue gases and to superheat the gasifying medium with syngases Steam (40 bar 400degC) is also produced from cooling syngases and flue gases An FSplit block divides the preheated air into primary secondary and tertiary streams this block requires the same properties and composition of all outlet streams

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

31

In the first step of the process where gasification takes place not all of the carbon can be gasified since some coke is needed to heat the primary gasification reaction from outside This amount of coke is achieved by a very limited supply of steam and possibly oxygen it is also possible to define part of the carbon as inert in the Gibbs reactor The products from an RGibbs reactor are at equilibrium (infinite time in the reactor perfect mixing etc) and products from a real reactor usually are not But because of the long residence time in the LTMAG the simulations should give quite good idea of the final gas composition At equilibrium and atmospheric pressure no methane is produced therefore the pressure in the inner cyclone was set to 3 bars to get an increased amount of methane and a gas composition that is closer to reality Methane works both as methane and as a surrogate for tars in the model since no tars are produced at equilibrium The reactions in the outer cyclone are meant to heat the gasification and cracking process The heat from the second combustion step is inserted to the cracker However the heat from the first combustion step is not added to the first gasification step since it was so difficult to have all the blocks converging in this case Instead the user can check that the heat from the first combustion is slightly larger than the heat needed to reach the desired temperature in the first gasification step The syngas leaving the tar cracker is about 1100degC which is higher than a normal heat exchanger can withstand To lower the gas temperature water at 20degC is sprayed into the gas in a water quench modeled with a RGibbs reactor Thus the gas temperature is lowered and the H2CO ratio of the gas is shifted against more H2

Figure 9 The model of the gasifier is shown above For the process description see section 285

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

32

43 Gas cleaning Since cold gas cleaning is the preferred method when low pressure gasifiers are used the gas is cooled to 100degC producing steam A separator simulates the gas cleaning All undesired products are assigned into one stream and the clean gas into another The scrubber block in Aspen Plus is not used since it requires solid components with Particle Size Distribution (PSD) as input

44 Burner The combustion of the syngas in a burner was modelled by an RGibbs block in order to get the enthalpy of combustion

45 FT synthesis To get an idea of how much FT- diesel that could be produced from the syngas a simplified FT reactor is modelled see Figure 10 All the process parameters are the same as stated by Deneau etal [29] and thus α is assumed to be the same The reactor is a slurry phase reactor with a cobalt catalyst

The gas is cooled to 180degC before entering water-gas-shift The shift reactor is modelled by an Rstoic block and it is used to change the H2CO ratio of the gas to 21 The reaction defined in this block is the WGS-reaction equation (3) If the amount of water in the gas is enough no water is added into the shift reactor The gas is cooled by a heater block compressed to 25 bars and cooled again to 330degC The FT-reactor is modelled with an RStoich block where α = 09 Using the ASF-distribution equation (3) reactions and conversion factors for hydrocarbons paraffines ie CnH2n+2 are defined and included in Appendix2 In a real reactor there are also unsaturated hydrocarbons formed but this is complex to model The RStoic block also needs temperature and pressure as input A separator divides the products from the FT-reactor into five streams

1 Low hydrocarbons (C1-C4) and non hydrocarbons 2 Hydrogen gas

Figure 10 A simplified flowsheet for the FT-synthesis The H2CO ratio is adjusted in a WGS-reactor then the gas is compressed to the FT-reaction pressure before entering that reactor The products from the FT-reactor are mainly naphtha diesel and waxes A pump elevates the pressure of the waxes before the wax-cracking where waxes are decomposed to naphtha and diesel

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

33

3 Naphtha (C5-C12) 4 Diesel (C12-C20) 5 Wax (C20-C30)

Hydrocarbons over C30 are not included in any Aspen database and instead of adding them as user-defined components the probability for C31-C34 is included in the conversion factor for C30 in order to get a total conversion of CO to (almost) 90 In the model a total conversion of the waxes is assumed since 90 of the CO was converted in the FT reaction Before cracking the waxes the stream pass a pump that raise the pressure to 30 bars The hydrocracking is modelled by a RStoic block where the temperature is 330degC and the cracking reactions are defined as in Appendix 3 giving 85 diesel and 15 naphtha as in the Shell Middle Distillate Synthesis [30] The hydrogen gas remaining after the FT synthesis is used in the hydrocracking of the waxes

46 The input file An input-file was created in Excel to simplify the use of the model The user specifies the total amount of fuel and the moisture content of the fuel The input-file calculates the amount of each fuel component and also the amount of steam oxygen and air into the model Table 6 shows the input and returned values from the file Table 6 Input to the input file and values returned from the file

Required input Returned values Fuel in (kgs) Moisture content in fuel Percent wood in the fuel

Amounts of each component into process (kgs) Split-fraction in drying and torrefaction Energy to drying

Gasifying medium-to-fuel ratio Percent steam in gasifying medium

Water and O2 into process (kgs)

Total λ for the combustion Coke to coke gasification (kgh) value from Aspen model

Amount of air into process (kgs) Fraction of air into each combustion step

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

34

5 Results and discussion The developed Aspen Plus model is now capable of simulating pre-treatment gasification gas cleaning gas burning and FT-synthesis Thus the model is a suitable simulation design and optimization tool for the entire BTL system or BTG system but it is also possible to simulate just one part of the system The torrefaction could be self-supporting on energy which is desired since it could be of interest to perform the torrefaction before transporting the fuel to the plant minimizing transport costs In this case eg a small burner could provide heat for drying the fuel before torrefaction However in the present work excess heat from a CHP plant is used for drying Since Aspen Plus calculates the equilibrium amounts and in a real gasifier the reactions will not go all the way to equilibrium there are no tars produced in the simulations The actual pressure in the system will be atmospheric but in order to simulate enhanced levels of hydrocarbons the pressure in the inner cyclone was defined to 3 bar in the simulation The temperature in the hydrocarbon cracking reached 1100degC which is necessary for an efficient decomposition of hydrocarbons The reduction of methane in the model is shown in Figure 11 The composition of the gas produced in the model was compared to the composition of the gas from simulations with the software Fact Sage The result from the comparison is shown in Figure 11 and the gas compositions are in accordance The Aspen Plus model is thus an efficient tool for estimating the gas composition even though the exact equilibrium calculations including many components eg alkali should be made with Fact Sage

The water quench lowers the gas temperature and also raises the H2CO ratio Figure 12 shows the change in both temperature and H2CO ratio at different gasifier pressures with 1 to 20 moles of water added per kg torrefied fuel

Figure 11 Comparison of gas composition from Aspen Plus and Fact Sage values after first part in the cyclone and after the cracking zone Logarithmic scale

Gas from Aspen and Fact Sage

0001

001

01

1

10

100

H2 CO H2O CO2 CH4 N2

Gas component

mo

lek

g t

orr

efie

d f

uel

Cycl A+

Cycl FS

Crack A+

Crack FS

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

35

Water Quench

500

600

700

800

900

1000

1100

1200

1300

1 5 10 15 20

Mole H2O kg torrefied wood

Tem

per

ature

(C)

1

14

18

22

26

3

H2C

O ratio

Temp(C) 1 barTemp (C) 10 barTemp (C) 20 barTemp (C) 30 barH2CO 1 barH2CO 10 barH2CO 20 barH2CO 30 bar

Figure 12 Temperature and H2CO ratio after the water quench at different gasifier pressures and amounts of added water The gas temperature before the quench was about 1100degdegdegdegC The excess heat from the process was used to produce superheated steam at 40 bar 400degC which is consistent with the steam data for a CHP plant This steam is a high quality energy carrier which could be used to produce eg electricity The gasification efficiency for the process is determined to 67 (LHV basis) and the LHV for the biosyngas is 80 MJNm3 The total system efficiency is 48 without steam generation and increased to 93 with steam generation both values on LHV basis The model including FT synthesis gave a FT diesel production of 100 l per tonne wet wood or 190 l diesel per tonne torrefied wood into the process The FT diesel efficiency was 27 based on LHV The naphtha production was 70 l naphtha per tonne wet wood and 130 l naphtha per tonne torrefied wood ETPC has a tool for simulating the gasification with Fact Sage and Excel but to run one such simulation takes several hours with the Aspen Plus model it is possible to simulate not just the gasification but the whole process in a few minutes

51 Model limitations The gases produced in the gasifier contain impurities The cleaning blocks in Aspen Plus requires solid impurities Since the impurities in the gases are in gaseous form a simplified simulation of the cleaning was performed where neither the energy requirement nor the level of cleaning were considered The bed material is not included in the model and thus neither the heating of it

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

36

Since all components appearing in the model must be listed in Aspen Plus and the fuel contains several chemical compounds with many possible products in the gasification the product specified was limited to the most numerous from Fact Sage calculations Aspen Plus is not designed to use many components and it might be difficult to run the simulations if too many components are included Due to this fact the pre-treatment and gasification model is separated from the FT diesel model and the fuel is only as the fuel component in the later model since the FT-process produces many different hydrocarbons The heat transfer in the model is without losses It is modeled using streams with the total reaction heat (called heat duty in Aspen) To get a better model of the heat transfer radiation and convection should be modeled by Fortran code But there were no time for this in the present work The temperature in the first gasification step should be determined by the temperature in the first combustion step but this made the model almost impossible to use since the temperature in gasification affects the amount of coke and the temperature in the combustion is due to amount of coke Thus the model did not converge when connected like that The FT-liquids produced are not of the same composition as from real FT-synthesis The amount of FT diesel produced can give a good idea of how much that is possible to produce from the introduced fuel but the cetane number and other properties should not be determined from this composition

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

37

6 Conclusions The main purpose of this project was to develop a simulation model including a BTG and a BTL system The model is fuel flexible it is easy to change the composition moisture content or amount of fuel in to the model The input file does all calculations of the amount of fuel components steam oxygen air and other needed inputs so the user do not need to calculate them each time the input is changed The pre-treatment model shows that the torrefaction is self-supported on energy The model also determines the amount of energy needed for drying and grinding The water quench is an efficient gas cooler which also shifts the H2CO ratio towards the desired value for FT synthesis The model calculates the gas composition and also the temperature in the cracker and the gas composition gasification efficiency and heating value of the gas are reasonable It is possible to produce not just biosyngas but also superheated steam which is a carrier of high quality energy and could be used to produce electricity This enhances the total system efficiency from 48 to 93 The amount of FT diesel produced - 190 l diesel per tonne torrefied wood - is reasonable since the production should be between 100-210 liters per tonne of wood (10 moisture) according to Boerrigter [27] The model is developed evaluated and ready to be used for system optimisation To sum up the model is easy to use and gives realistic results but unfortunately there has not been time yet to do all desired simulations

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

38

7 Future work Unfortunately there has not been time to utilize the model and do all simulations of interest with it but hopefully these will be done in near future Some interesting things to investigate are

bull How the heat transfer in the gasifier would affect the gasifier if included in the model bull There are no losses in the Aspen blocks but to get more accurate results it is possible

to add blocks called manipulators and define the losses in these bull To change the model so that the production of methane and tars are simulated without

raising the pressure bull To compare the system efficiency with wet dry and torrefied biomass bull A sensitivity analysis could give indications on which variables that are most

important to interesting process parameters

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

39

8 References 1 Olofsson I A Nordin and U Soumlderlind Initial Review and Evaluation of Process

Technologies and Systems Suitable for Cost-Efficient Medium-Scale Gasification for Biomass to Liquid Fuels 2005 Energy Technology amp Thermal Process Chemistry University of Umearing Umearing p 90

2 Hallock J et al Forecasting the limits to the availability and diversity of global conventional oil supply (vol 29 pg 1673 2004) ENERGY 2005 30(10) p 2017-2018

3 Hirsch R Peaking of world oil production impacts mitigation amp risk management 2005 United States Department of Energy National Energy Technology Laboratory p 91

4 Bridgwater A The technical and economic feasibility of biomass gasification for power generation FUEL 1995 74(5) p 631-653

5 Torisson T 2005 p Professor Department of Heat and Power Engineering Lund Institute of Technology Lund University Efficiencies for ethanol methanol and DME processes Personal communication

6 Fransson G et al Svagsyrahydrolys av cellulosa i pilot- och fullskala 2000 p 53 7 Tijmensen M et al Exploration of the possibilities for production of Fischer

Tropsch liquids and power via biomass gasification BIOMASS amp BIOENERGY 2002 23(2) p 129-152

8 Ekbom T N Berglin and S Loumlgdberg Black Liquor Gasification with Motor Fuel Production - BLGMF II 2005

9 Bergmann P Boersma A et al Torrefaction for entrained-flow gasification of biomass 2004 ECN p 5

10 Zevenhoven R and P Kilpinen Control of pollutants in flue gases and fuel gases 2 ed Energy Engineering and Environmental Protection Publications 2002 Espoo

11 Prins M and K Ptasinski Energy and exergy analyses of the oxidation and gasification of carbon ENERGY 2005 30(7) p 982-1002

12 Neeft JPA et al Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases E Commission et al Editors 2000

13 Prins MJ KJ Ptasinski and JFJJ G Thermodynamics of gas-char reaction first and second law analysis Chemical engineering Science 2003 58 p 1003-1011

14 Barin I O Knacke and O Kubaschewski Thermochemical properties of inorganic substances 1977 BerlinHeidelberg Springer Verlag

15 Fredriksson C Cyclone Gasification of Wood Powder in Division of Energy Engineering 1996 Lulearing University of technology Lulearing

16 Boerrigter H et al Gas Cleaning for Integrated Biomass Gasification (BG) and Fischer-Tropsch (FT) Systems 2004 ECN

17 Gil J et al Biomass gasification in fluidized bed at pilot scale with steam-oxygen mixtures Product distribution for very different operating conditions ENERGY amp FUELS 1997 11(6) p 1109-1118

18 Yu Q et al Temperature impact on the formation of tar from biomass pyrolysis in a free-fall reactor JOURNAL OF ANALYTICAL AND APPLIED PYROLYSIS 1997 40-1 p 481-489

19 Olofsson I Low-Tar-Formation using High-Temperature Flash-Gasification of Intelligent Biomass Fuel Mixtures in Energy Technology and Thermal Process Chemistry 2005 Umearing University Umearing p 38

20 Omstaumlllning av energisystemet 1995

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

40

21 Larson ED and WH Robert A review of biomass integrated- gasifiergas turbine combined cycle technology and its application in sugarcane industries with an analysis for Cuba Energy for Sustainable Development 2001 1

22 Salman H Evaluation of a Cyclone Gasifier Design to be Used for Biomass Fuelled Gas Turbines in Department of Mechanical Engineering 2001 Lulearing University of Technology Lulearing

23 Dry ME The Fischer-Tropsch process 1950-2000 Catalysis Today 2002 71 p 227-241

24 Stergaršek A and J Stefan Cleaning of syngas derived from waste and biomass gasificationpyrolysis for storage or direct use for electricity production 2004 To be presented at the workshop Production and Purification of Fuel from Waste and Biomass

25 Hamelinck C et al Production of FT transportation fuels from biomass technical options process analysis and optimisation and development potential ENERGY 2004 29(11) p 1743-1771

26 Choi G et al DesignEconomics of a Once-Through Natural Gas Fischer-Tropsch Plant With Power Co-Production in Coal Liquefaction amp Solid Fuels Contractors Review Conference 1997

27 Boerrigter H Green Diesel Production with Fischer-Tropsch Synthesis 2002 p Presented at the Business Meeting Bio-Energy Platform Bio-Energie 13 September 2002

28 Lipinsky E J Arcate and T Reed Enhanced wood fuels via torrefaction ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 2002 223 p U587-U587

29 Deneau E et al A feasibility study of a Fischer-Tropsch plant for production of CO2-neutral synthetic diesel from gasified black liquor Revised version 2005 KTH Chemical Technology p 88

30 Sage P and M Payne Coal Liquefaction - a Technology Status Review 1999 AEA Technology Environment

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

41

Appendix 1 Using the model The following parameters can be changed in the model with pre-treatment gasification and gas cleaning by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash options Particle Size

Distribution Coldair Temperature pressure flow composition flash options Water Temperature pressure flow flash options Water2 Temperature pressure flow flash options O2 Temperature pressure flow flash options Extheat2 Heat duty Block Parameters Dry Pressure flash options Dryer Splitfraction Multiply Multiplication factor H2Ocool Pressure temperature flash options Torr Temperature difference Flucool Pressure temperature flash options Flucool2 Pressure temperature flash options Torrsep Splitfraction Volburn Temperature pressure Volheat Pressure Mixheat Pressure Torrcool Pressure temperature flash options Mill1-4 Max particle diameter operating mode bond work index Combine Pressure Cyclone Temperature pressure products solid prod stream Cracker Pressure products Quench Pressure products Steampro Temperature Pump 1-2 Pressure efficiency Compress Type pressure Mix Pressure Cleaning Splitfraction Coolclea Pressure temperature flash options Gascool2 Pressure temperature flash options Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Steamgen Temperature Airheat Temperature Airsplit Splitfraction Fluecool Temperature pressure

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

42

The following parameters can be changed in the model with gasification and Fischer Tropsch synthesis by entering the specified block or stream Stream Parameters Fuel Temperature pressure flow composition flash

options Particle Size Distribution Coldair Temperature pressure flow composition flash

options WaterO2 Temperature pressure flow composition flash

options Block Parameters Cyclone Temperature pressure products solid prod stream Cracker Temperature pressure products Steampro Temperature Cool1-8 Pressure temperature Shift Temperature pressure reaction Compr1-2 Pressure FT Temperature pressure reactions FTsplit Splitfraction Pump Discharge pressure Waxcrack Temperature pressure reactions Crackspl Splitfraction Cracker Temperature pressure products Cokegasi Temperature pressure Comb1 Temperature pressure Comb2 Temperature pressure Airheat Temperature Airsplit Splitfraction

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

43

Appendix 2 The Anderson-Schulz-Flory distribution was used to determine the products from the FT synthesis

12)1( minusminus= nn nW αα

Wn = percent by weight of hydrocarbon with n carbon atoms α = 09 probability for chain growth The FT-reaction is defined as

OnHHCHnnCO nn 2222)12( +rarr++ +

n Wn 1 001 2 0018 3 00243 4 00292 5 00328 6 00354 7 00372 8 00383 9 00387 10 00387 11 00384 12 00377 13 00367 14 00356 15 00343 16 00329 17 00315 18 00300 19 00285 20 00270 21 00255 22 00241 23 00226 24 00213 25 00199 26 00187 27 00174 28 00163 29 00152 30 00613 sum 08776 n=31 to n=34 is included in n=30 to reach a carbon conversion of 90

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+

44

Appendix 3 The following reactions took place in the waxcracking to give 15 naphtha and 85 diesel

321526230

3215301426029

301425828

3014281325627

281325426

2813261225225

261225024

381812524823

361712524622

2411221024421

2

2

2

2

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHHC

HCHCHHC

HCHCHHC

HCHCHHC

rarr++rarr+

rarr++rarr+

rarr++rarr+

rarr++rarr++rarr++rarr+