Paper_INFUB1_JBM_V4

7/27/2019 Paper_INFUB1_JBM_V4

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Combustion and l ife-cycle evaluation of torrefied wood for decentralized heat and power production

J.-B. Michel 1,4

,C. Mahmed1

, J. Ropp1

, J. Richard2

, M. Sattler 3

, M. Schmid3

1School of Business and Engineering Vaud, University of Applied Sciences Western Switzerland, 1401

Yverdon-les-Bains, Switzerland2HEPIA, University of Applied Sciences Western Switzerland, 1201 Geneva, Switzerland

3Centre of Appropriate Technology and Social Ecology, Laboratories for Sustainable Energy Systems,

Langenbruck, Switzerland4Corresponding author Ph: +41265577594, Fax:+41265577579,e-mail: [email protected]

ABSTRACT

Torrefied wood pellets are produced from torrefied chips by thermo-chemical pre-treatment of biomass at200-320°C in the absence of oxygen during about 15-30 minutes. Overall, the torrefaction processefficiency has been reported to be 90-95% % as compared to 84% for pelletisation. Torrefaction improvesthe biomass: 30% higher calorific value and 50% higher energy density resulting in much lower handlingand transport costs. The fuel becomes hydrophobic making long term outdoor storage possible. Thepurpose of this project was to compare the combustion and emission characteristics of torrefied vs.normal wood pellets. With no modification to the feeding and the burner parameters, the ignition andcombustion characteristics of torrefied pellets are found very similar to those of normal pellets. Particulateemissions per energy output were found very close and directly related to the ash content in thefeedstock. Using the Taguchi approach, it was possible to establish a model of the boiler performance asa function of the input parameters. Further testing confirmed the validity of the model showing optimum

performance with a defined value of primary and secondary air flow rates which minimized particulateemissions for both the normal pellets and the torrefied pellets.

Keywords: biomass, torrefaction, combustion, pellets, testing, Life-Cycle-Analysis, Life-Cycle-Impact

N.B. In this paper, all figures of thermal energy content of the fuels are given on the Low Calorific Value(LCV) basis.

1 INTRODUCTION

1.1 Purpose of the work

Torrefied wood pellets are an attractive fuel for co-combustion in coal-fired power stations (Maciejewska

et al., 2006). Except for start-up, the process is autothermal (it generates its own energy due to mildpyrolysis reactions) and the energy of the off-gases, which represent about 10% of the input energy, isrecovered. Overall, the process efficiency has been reported to be 90-95% % as compared to 84% for pelletisation in one given set of operating conditions (Uslu, 2008). The purpose of this R&D project is tocompare the combustion and emission characteristics of torrefied wood pellets with those of normal woodpellets.

1.2 Approach

About 1 ton of torrefied pellets have been prepared by ECN on their 100 kg/h pilot facility, using poplar asthe feedstock. Combustion tests have been carried-out on a 50 kW pellet boiler of the company Hoval withnormal pellets and with torrefied pellets. Input and output measurements have been made during start-up



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and during stabilized operation. Flue gas concentrations of O2, CO and NO were measured continuously.Total particulate emissions (TPM) were sampled using a disk filter following the proposed ISO/DIN 13336

standards. A scanning mobility particle sizer (SMPS) was used to determine the size distribution and thetotal number concentrations of particles. The analytical set-up is shown in the appendix. The design of experiment method from Taguchi was used to reduce the number of tests to a minimum while exploringthe complete space of variables with a 9*4 test matrix.

2 SCIENTIFIC INNOVATION AND RELEVANCE

Although there are a large number of publications regarding the torrefaction process itself, this is the firstcomprehensive study on the combustion properties for domestic heating applications and on a completelife cycle analysis including the combustion part. The results are relevant for cogeneration applications.

3 BIOMASS TORREFACTION REVIEW

Torrefied wood was used during the early years of steel production as a reducing agent in blast-furnacesand was afterwards replaced by charcoal and coke (Annales des Mines, 1857). The process is rather simple and involves anaerobic heating of dried biomass chips as shown in Figure 1.

Several reactor types are used depending on the proprietary process. The ECN BO2 process uses avertical moving bed countercurrent with recirculated flue-gas. The temperature is about 240°C with aresidence time of about 20 minutes. Topell use a cyclone type swirling flow (entrained flow) andtemperatures up to 350 °C with a much lower residence time (about 90 seconds) and fast quenching of the torrefied chips. Airless technology (Airless web-site) use a rotary drum reactor, a technology that hasevolved from the ceramic drying technology. The drying and torrefaction technology operates by creatingsuperheated steam generated solely from the moisture contained in the biomass.

Drying to about 20% moisture

Anaerobic heating between 240-320°C

Autothermal process

Flue gas recycling andpostcombustion

Raw biomass chips

Torrefied

Biomasspellets

Mass yield ~70%Energy yield ~90%10% left is partly recoveredLCV increase by ~30%

grinding

pelletisation

Figure 1 – Simplified process description

The work of Prins (2005) demonstrated that the mass yield during torrefaction is typically contains 70%while the energy yield is about 90% of the original energy content, that is 1 kg of chips (dry basis) will lose30% weight and 10% of their initial energy content. Torrefaction gases are produced from thedecomposition of hemicelluloses and are mainly composed of CO, CO2 and acetic acid. No moisture is leftfollowing torrefaction but the torrefied biomass may uptake 6% of moisture from the ambient air.



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In 1985, Pechiney built a 10000 t/y production plant, to use torrefied wood instead of charcoal in electricfurnaces (Peguret, 1986).

This new type of fuel is very promising because it alleviates a lot of the disadvantages of normal biomasspellets:

• The volumetric energy density is 50% higher than with normal pellets resulting in the same reduction

of handling and transport cost per energy output.

• Grinding energy is reduced by 90% and overall, the process efficiency has been reported to be 90-

94% as compared to 84% for pelletisation (Uslu et al., 2008). See Figure 2.

• Torrefied biomass is hydrophobic and therefore not subject to swelling and degradation allowing

outdoor storage and in the long term.

• Its greater calorific value should be beneficial for combustion.

Figure 2 – Process efficiency comparison. Normal pellets (top), torrefied pellets (bottom) after Uslu et al.(2008)

Several large scale production plants are planned or in construction in Europe and elsewhere, for the co-combustion of torrefied wood in coal-fired power stations:

• Energy Center of the Netherlands, BO2 process (Kiel, J et al., 2008). They now work in association

with Vattenfall.

• Atmosclear (Switzerland) large projects planned from 130 to 270 kt/y (Atmosclear web site)

• Integro Earth Fuels, Wyssmont process, USA, 84 kt/y Roxborrow, NC (Integro Earth Fuels web site)

• Topell, NL , Polow Torbed reactor technology, planned 60 ktons/y in Arnhem (NL) together with RWE

(Maaskant, E, 2009)

• 4Energy Invest (B), 38 kt/y in Ambleve (B) and Stramproy (4Energy Invest web site)

Drying Grinding Pelletisation

Drying Grinding PelletisationTorrefaction

η= 90 – 94%

η = 84 %

Energy content

100 (LCV base)

Raw chips, 57% moisture

Energy

Energy

12



4

• Essent trading (RWE) and Stramproy : 90 kt/y in Steenwijk (NL) (Essent trading web site)

A special session was devoted to torrefaction during the 18th

biomass conference and exhibition (Lyon,France) where the status of some of these projects were presented. In Spain, the 500 kg/h pilot plant builtby CENER was presented (Celaya et. al, 2010)

However, there seems to be no project so far directly targeted to domestic heating and cogeneration.

4 COST ANALYSES

Several economic comparisons have shown the benefits of using of torrefied pellets instead of normalpellets. The table below provides a comparison of the cost of pellets for power generation with biomassfrom Canada and from South-Africa shipped to Europe.

Hamelink (2005) reported that feedstock costs contribute around 20–65% of the total delivery costwhereas pre-treatment and transport contribute 20–25% and 25–40%, respectively, depending on the

location of the biomass resources.

According to Uslu (2008) TOP pellets can be delivered at costs as low as 3.3 €/GJ (73.5 €/ton) with abiomass cost of 10 €/ton as compared to 3.9 €/GJ (66.3 €/ton) for normal pellets. This is mainly due tohigher energy density compared to conventional pellets, which lowers both the road and sea transportcosts. This is also in agreement with the work of Peng et al. (2008) for pellets processed in South-Africawith the ECN process and transported to Europe. The comparison with pellets produced in Vancouver andprocessed in Europe after Herold, (2009) is presented in Table 1.

Similarly Kiel (2007) reported delivery costs for sawdust pellets supplied to North-West Europe: 4.7 €/GJfor torrefied and 5.9€/GJ for normal pellets which confirms the economic advantage of torrefied pellets.

Table 1 – Pellet costs from various sources

Cost itemSource 2 (Herold, 2009)

Vancouver B Europe

Source 1 (Peng, 2008)

S-AfricaB Europe

Sawdust case

Production capacity(ktons/y)

40 80 56

Product Pellets PelletsTorrefied pellets

(ECN)

Costs in €/ton product

Raw material 23.6 11 15

Production 70 41 45

Transport 62.6 54 42



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Margin 23.9

Total (€/ton ) 180.1 106 102

(€/GJ) 11.2 6.61 4.99

5 MAIN COMBUSTION AND LIFE-CYCLE ANALYSIS RESULTS

5.1 Test set-up

Combustion tests were carried-out on a 50 kW pellet boiler of the company Hoval shown in Figure 3.

Figure 3 - Schematic of the 50 kW Hoval Biolyt®

boiler and photograph of the sampling system

A forced draught burner is used on this boiler (and not a grid or a drum), allowing a rather accurate controlof primary and secondary combustion. A scanning mobility particle sizer (SMPS) was applied to determinethe size distribution and the total number concentrations of particles in the range from 0.01-0.400 µm.Exhaust gas is taken with a probe, which is also fed with particle free air. The resulting dilution factor isadjusted by the flow rate of the diluting air and the total flow. To prevent condensation of water onto theparticle surface, the dilution factor is chosen high enough, to achieve a dew point below ambienttemperature. (Wieser and Gaegauf, 2000).

The design of experiment method from Taguchi was used to reduce the number of tests to a minimum

while exploring the complete space of variables. A first test campaign was carried out in order to identifymost relevant parameters and their levels. This resulted in a matrix of 4 variables and 3 levels as shown inTable 2 with a total of 9 test cases.



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Table 2 Test case parameters

Factors Level 1 Level 2 Level 3

Pellet type

C1 (classical,swiss forest mix)

T (torrefied)

Poplar

T (torrefiedpoplar)

Secondary air fansetting

35% 45% 60%

Primary air fansetting

35% 40% 45%

Feeding screw

(65% = 100%load)

30% 50% 65%

In the first and second test campaign the so-called classical pellets were commercial Swiss pellets andunfortunately not poplar for comparison with the ECN pellets. The third and last test campaign was carriedout with poplar pellets made specifically for this purpose, referred to later as “C2” pellets.

Consequently, all the results were fitted with a model and the results are discussed in the next section.

5.2 Combustion test results

The combustion behavior of the torrefied pellets was found very similar to that of the normal pellets:

• The warm-up period was slightly reduced

• The mass flow of the torrefied pellets had to be reduced by about 10% to achieve the same

energy input

• The optimum settings of primary & secondary air flows in terms of emissions were identical

Measured Model

Measured Model



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Figure 4 Raw test results and comparison with the model data

The raw results obtained with the various test campaigns are displayed in Figure 4. The points referred toas “model” are the calculated values from a curve fitting model (using a second order polynomial). Thisapproach is necessary to show the separate influence of the various parameters, which otherwise is notpossible with the raw results.

A second measurement campaign was carried out, with the objective of finding the best air settings interms of CO and particulate emissions. Surprisingly, the optimum settings were the same for C and Tpellets with the following values:

• Primary air : 45% ;

• Secondary air : 55%.

Measured

Model



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Figure 5 Comparison of the emission characteristics (interpolated model data)

The comparison of the flue-gas emissions of torrefied (T) and classical (C1 & C2) pellets is given in

Figure 5 with these settings. It shows that torrefied pellets can potentially produce less CO than classicalpellets and at the same time make it possible to reduce the excess air, thereby increasing the thermalefficiency. Particulate emissions were found to depend strongly on the fuel ash content. In this case, theso-called C2 pellets are from poplar with a higher ash content than the poplar used for torrefied pelletswhich explains their higher particulate emissions. The particulate size distributions were also very similar.

NOx emissions are found to be similar in this case, but one could expect lower NOx emissions dependingon the amount of fuel nitrogen that has been released during torrefaction.

5.3 Life-Cycle Impact Assessment summary

The comparison of impacts of the two biomass fuels was performed using the Impact 2002+ life cycleimpact assessment (LCIA) method. The functional unit was the MJ of heat produced by the boiler. Results

are summarized in the following table showing an overall gain of 50% mainly due to the improvement of the overall process efficiency.

Indicator Unit 1MJ-Heat-T 1MJ-Heat-C2 Gain T

Human Health [DALY] (Disability Adjusted Life Years [year] 2.54316E-08 4.66562E-08 45%

EcosystemQuality

[PDF] (Potentially Damage Fraction of species)

0.006238811 0.006907042 10%

110

112

114

116

118

120

20.0 30.0 40.0 50.0 60.0

m g / N m ³ a t 1 3 % O 2

Pin kW

NO = f(P) T C2

0

500

1000

1500

20.0 30.0 40.0 50.0 60.0

m g / N m ³ a t 1 3 % O 2

Pin kW

CO = f(P) T C2

110

112

114

116

118

120

20.0 30.0 40.0 50.0 60.0

m g / N m ³ a t 1 3 % O 2

Pin kW

NO = f(P) T C2

0

500

1000

1500

20.0 30.0 40.0 50.0 60.0

m g / N m ³ a t 1 3 % O 2

Pin kW

CO = f(P) T C2



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Climate Change [kg CO2] 0.004685857 0.013145215 64%

Resources [MJ primary] (overall process) 0.083996764 0.254237773 67%

Single Score [Points normalisés] 5.06726E-06 1.00833E-05 50%

6 EXAMPLE OF APPLICATION: REGIONAL ENERGY CENTRE

The project was part of a fully integrated waste-to-energy (Kompogas) refinery plant of a regional scale.The core of the plant is an anaerobic digester for green waste (20000 t/year wet basis, with 30% drymatter). The digester produces biogas which is fed to the gas line after a methanisation process. Thegreen waste going to process is split into a non lignocellulosic and lignocellulosic fraction. The later isburnt in a solid fuel burning combined heat and power plant (CHP). The CHP uses an externally fired gasturbine (EFGT) or hot air turbine. This type of use of an EFGT will be first of its kind worldwide,representing an economically efficient solution for solid fuel CHP in small and medium scale applications.

This type of fermenting plant produces a non fermentable cellulosic waste. In the case of this plant thewaste output was 5000 t/a (wet, 30 to 40% DM) which contain a significant amount of energy. One of theoptions considered for heat recovery was to process it into a valuable, storable fuel via torrefaction. Thewaste heat from the BCC would supply the drying step with heat. Initial simulations have demonstratedthe economic feasibility.

The project was submitted in December 2008 to the European Commission (ENERGY.2008.8.2.1: Highefficiency poly-generation - renewable energies for applications in industry, Secondary topic:ENERGY.2008.2.2.2: High-efficiency medium-to-large scale electricity generation from biomass andwaste). The material (and associated income) flow of the system is shown in Figure 6 as it is now and asit should have been after the project.

Today, the Kompogas plant in Rümlang, standing for a standard solution is producing 400 kW th of “Naturgas” (biogas upgraded to grid demands, SNG), 145 kW th neighborhood industrial heat demand (allyear). Some 3000 t/a compost and water and 2000 t/a screened waste wood (25% of input) are resultingtoo. The produced gas, power and sold heat represent only 54% “total efficiency” regarding the heat valueof the input. However the rest of the energy is mostly conserved in the compost and screened waste wood

– but those products do not represent an economic value today. The total income with the gas, power andheat production does not exceed € 400000. — per year (50 €/t input). The wet waste has to betransported away to further low grade utilisation.

RENEC: with the biomass combined cycle, the output of gas, power and heat is increased by 45% to anincome of 580000 €/a (72.5 €/t) and additionally, a heat stream of more than 200 kW th is delivered to thetorrefaction plant, which process additional 3250 t/a waste wood into 1250 t/a dry, stable, merchantablepellet fuel, with a market value of € 275000. — (84.5 €/t input). The average of the two values is 76.6 €/t,an increase of 53%.

Why 5000 t/a of waste wood? As mentioned earlier, the standard size of a Kompogas-plant is 20000 t/a(wet green waste input) instead of only 8000 t/a in the case of the demo-plant. Therefore, the typical 25%of screened waste wood will increase from 2000 t/a to 5000 t/a at a standard size Kompogas plant. Thedrying heat demand for the maximum 3250 t/a very wet waste wood will never exceed the heat supply bythe BCC (which itself consumes the left-over of 1750 t/a to be burned).

The concept of the Regional Energy Centre has been first mentioned by CATSE in a Technology-Screening study (Schmid et al., 2007).

Therefore, the project was going well beyond the state of the art with the first 1:1 scale application of anexternally fired gas turbine to generate power and heat from biomass and with an increase of overall



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efficiency compared to the existing plant by 45%. In the same time, the output of valuable products, suchas gas, power, and storable, dense pellet-fuel increase by over 50% from 50 to 76.6 €/t of wet plant input.

This could be achieved using four innovative technologies.

• Autothermal torrefaction of the biowastes by firing the reactor with the off-gas of the torrefactionitself.

• The externally fired gas turbine, additionally increasing in its cycle efficiency:

o by after-firing it with biogas from the anaerobic digestion process (Kompogas-Plant) and

o using fogging to increase the efficiency of the turbocompressor.

• Low temperature Combined Cycle by heat recovery to generate power using ORC-Technology.

A full RENEC (20000 t/a) represents polygeneration of heat, power, SNG and pellet fuel from one sitefrom regional green wastes, collected from a region of approximately 80000 persons (38000 households)

– supplying power for 1500 households (if they are not equipped with an electric water boiler), pellet fuelfor 1000 households and SNG to run 1500 cars. Meaning, RENEC could contribute to up to 3.5% of theenergy supply of the region built into.

Fermenter

LCVBurner

110 kWe

132 kWe Strom Gas engine

ηe = 37%

1250 t/a, dry, stable pellet fuel

835 kWth, 440000 CHF/a = 135 CHF/t

Rekuperated

Band Dryer 275 kWth @ 70°C

120 kWth @ 115 °C Exhaust

Thermo-Oil

Boiler

ηtot = 62%

Wood1750 t/a, 40% DMHu = 1.83 kWh/kg

Waste Heat

(radiation to heat

the fac.) 60 kWth

EFGT

Wood-CHP

ηe = 22%

ηtot = 86%

400

kW

320 kWth

@ 290°C

100 kWe Power

155 kWth Heat @ 40 °C

Waste Heat

(radiation) 10 kWth

ORC

heat CHP

ηe = 16%

ηtot = 95%

33 kWe Strom

Regional Energy Center I Greenwaste to power, grid gas, dry fuel

and heat - total

82% Efficiency Sketch for Kompogas AG from

CATSE (Ökozentrum Langenbruck),9.9.2008 /ms

200kWth @ 170°C

Wood3250 t/a, 40% DMHu = 1.83 kWh/kg

730

kW

Torefaction

ηtot = 90%

Pelletisation

ηtot = 100%

265 kWe

Power 530000 CHF/a

@25 Rp./kWh

=105 CHF/t

KOMPOGAS

KOMPOGAS

Naturgas (>95% CH4) PSA

Methanisation

ηtot = 92%Green-Matter 8000 t/a30% TS

Kompost

BG 160 m3/t, 50% CH4 = 400 kW

1280 kWth x 8000 h/a

Offgas

recirculation8%

400 kW Hu

320000 CHF/a

@10 Rp/kWh=105 CHF/t

BG 160 m3/t, 50% CH4 = 400kW

Kompogas

Fermenter

Heating

65 kWth

District Heating and

Industrial Heat,

130 kWth @ 90°C

Room Heating (winter), Painting-

Bay-Heating (all year), washing

water (all year),

Wet waste wood 3100 t/a40% TS

ParticleFilter

50 kWthBiogas

Foging

Screening

Figure 6 - RENEC I as it should look after the project. All installations mentioned above the dashed lineare the existing 8000 t/a Kompogas-plant (SOTA) near Zurich.



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• 4Energy Invest web site: www.4energyinvest.com (accessed 22.03.10)

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