Energy Conversion and Management 51 (2010) 969–982

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Energy Conversion and Management

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Overview of recent advances in thermo-chemical conversion of biomass

Linghong Zhang a, Chunbao (Charles) Xu b, Pascale Champagne a,*

a Department of Civil Engineering, Queen’s University, Kingston, ON, Canada K7L 3N6b Department of Chemical Engineering, Lakehead University, Thunder Bay, ON, Canada P7B 5E1

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 March 2009Received in revised form 5 November 2009Accepted 27 November 2009Available online 13 January 2010

Keywords:BiomassBioenergyCo-firingPyrolysisGasificationLiquefaction

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.11.038

* Corresponding author. Tel.: +1 613 533 3053; faxE-mail address: champagne@civil.queensu.ca (P. C

Energy from biomass, bioenergy, is a perspective source to replace fossil fuels in the future, as it is abun-dant, clean, and carbon dioxide neutral. Biomass can be combusted directly to generate heat and electric-ity, and by means of thermo-chemical and bio-chemical processes it can be converted into bio-fuels in theforms of solid (e.g., charcoal), liquid (e.g., bio-oils, methanol and ethanol), and gas (e.g., methane andhydrogen), which can be used further for heat and power generation. This paper provides an overviewof the principles, reactions, and applications of four fundamental thermo-chemical processes (combus-tion, pyrolysis, gasification, and liquefaction) for bioenergy production, as well as recent developmentsin these technologies. Some advanced thermo-chemical processes, including co-firing/co-combustionof biomass with coal or natural gas, fast pyrolysis, plasma gasification and supercritical water gasification,are introduced. The advantages and disadvantages, potential for future applications and challenges ofthese processes are discussed. The co-firing of biomass and coal is the easiest and most economicalapproach for the generation of bioenergy on a large-sale. Fast pyrolysis has attracted attention as it isto date the only industrially available technology for the production of bio-oils. Plasma techniques,due to their high destruction and reduction efficiencies for any form of waste, have great applicationpotential for hazardous waste treatment. Supercritical water gasification is a promising approach forhydrogen generation from biomass feedstocks, especially those with high moisture contents.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Bioenergy is a renewable and clean energy source that is de-rived from biomass. It has been attracting great attention thesedays due to the declining fossil fuel reserves and the ever-increas-ing greenhouse effects produced through fossil fuel utilization. Bio-mass refers to all organic materials that stem from green plants asa result of photosynthesis. It is a stored source of solar energy inthe form of chemical energy, which can be released when thechemical bonds between adjacent oxygen, carbon, and hydrogenmolecules are broken by various biological and thermo-chemicalprocesses. Fossil fuels, including primarily coal, oil and naturalgas, also originated from ‘‘ancient” biomass that has been trans-formed through microbial anaerobic degradation and metamor-phic geological changes over millions of years [1,2]. Fossil fuelsare considered to be non-renewable sources of energy consideringthe rate of their formation (millions of years) and consumption. Inaddition, burning fossil fuels releases net carbon dioxide (CO2) tothe atmosphere. By contrast, biomass is a renewable resourceand considered to be CO2 neutral as the CO2 released during com-bustion or other conversion processes will be re-captured by the

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: +1 613 533 2128.hampagne).

regrowth of the biomass through photosynthesis. In addition, thelower emission of environmentally detrimental gases, such as sul-phur dioxide (SO2) and nitrogen oxides (NOx), during the combus-tion of biomass also plays a positive role in reducing global acidrain formation [1,3–8].

Biomass includes a wide range of organic materials, which aregenerally composed of cellulose, hemicellulose, lignin, lipids, pro-teins, simple sugars and starches. Among these compounds, cellu-lose, hemicellulose, and lignin are the three main constituents(Table 1) [9,10]. Biomass also contains inorganic constituents anda fraction of water [5]. As for the elementary composition, carbon(51 wt.%) and oxygen (42 wt.%) together contribute to over 90% ofthe dry weight of a typical biomass. In addition, there are traceamounts of hydrogen (5 wt.%), nitrogen (0.9 wt.%) and chlorine(0.01–2 wt.%) [11].

Wood, energy crops, as well as agricultural and forest residues,which are the main renewable energy sources, are typical exam-ples of biomass. Moreover, food processing wastes, sewage sludge,and the organic components of municipal solid waste (MSW) andpulping by-products (e.g., black liquor) can also be considered bio-mass [3,4,12].

In comparison to fossil fuels, biomass has lower heating valueson a similar weight basis. Specifically, the heating value of biomassis in the range of 15–19 GJ/t, where heating values for agriculture

Table 1Typical levels of cellulose, hemicellulose, and lignin in biomass [9,10].

Component Percent dryweight (%)

Description

Cellulose 40–60 A high-molecular-weight (106 or more) linearchain of glucose linked by b-glycosidiclinkage. This chain is stable and resistant tochemical attack

Hemicellulose 20–40 Consists of short, highly branched chains ofsugars (five-carbon sugars such as D-xyloseand L-arabinose, and six-carbon sugars suchas D-galactose, D-glucose, and D-mannose)and uronic acid. Lower molecular weightthan cellulose. Relatively easy to behydrolyzed into basic sugars

Lignin 10–25 A biopolymer rich in three-dimensional,highly branched polyphenolic constituentsthat provide structural integrity to plants.Amorphous with no exact structure. Moredifficult to be dehydrated than cellulose andhemicellulose

Fig. 2. Development of net power generating capacity from municipal waste andsolid biomass in European Union countries [16].

970 L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982

residues and woody materials are 15–17 GJ/t and 18–19 GJ/t,respectively, compared to 20–30 GJ/t for coals. In addition, the bulkdensity, also known as energy density, is only 10–40% of that ofmost fossil fuels [13]. However, in comparison to fossil fuels, bio-mass has much higher volatile matter content (80% in biomassvs. 20% in fossil fuels) [11], therefore, biomass has a high ignitionstability and can be easily processed thermo-chemically into otherhigher-value fuels, such as methanol (C2H5OH) and hydrogen (H2).

Fig. 1 [4] illustrates biomass from industry, agriculture, forestry,and waste sources, as well as their potential final bioenergy appli-cations. It can be seen that various types of waste biomass, such asfood residuals, agricultural crops, animal wastes, and municipal so-lid wastes have the potential to be eventually converted into en-ergy and other bioproducts, which can be applied for powergeneration, transportation, as well as the production ofbiomaterials.

As a renewable energy source, biomass has been extensivelyutilized in many regions to date. It currently contributes to 14%of the world’s primary energy demand and is considered as thefourth largest energy source [14]. In Canada, which is rich in fossilfuels, approximately 4.7% of the national primary energy for 2006was derived from the conversion of renewable biomass and waste.This fraction is projected to increase to 6–9% over the next 20 years[15].

The usage of biomass as a source of energy has been further en-hanced in recent years in Europe as well. Fig. 2 depicts the netpower generating capacity for 1990, 1995, and 2000 in European

Fig. 1. Biomass resources conver

Union countries. It can be seen that the net power generation fromboth municipal waste and solid biomass has been steadily increas-ing from 1990 to 2000. Furthermore, the International EnergyAgency (IEA) data indicates that the electricity generation from so-lid biomass in the European Union had been growing at an averagerate of 2.5% per year over the last decade [16].

Bioenergy can be converted from biomass via two main types ofprocesses: thermo-chemical and bio-chemical/biological processes[6]. Generally, thermo-chemical processes have higher efficienciesthan bio-chemical/biological processes in terms of the lower reac-tion time required (a few seconds or minutes for thermo-chemicalprocesses vs. several days, weeks or even longer for bio-chemical/biological processes) [17] and the superior ability to destroy mostof the organic compounds. For example, lignin materials are typi-cally considered to be non-fermentable and thus cannot be com-pletely decomposed via biological approaches, whereas they aredecomposable via thermo-chemical approaches [5,18,19].

Thermo-chemical conversion processes mainly include directcombustion, pyrolysis, gasification, and liquefaction [17,20]. Asshown in Fig. 3, the stored energy within biomass could be re-leased directly as heat via combustion/co-firing, or could be trans-formed into solid (e.g., charcoal), liquid (e.g., bio-oils), or gaseous(e.g., synthetic gas and short for syngas) fuels via pyrolysis, lique-faction, or gasification with various utilization purposes. In thisarticle, the authors will provide an overview of the princi-ples, applications and recent developments of the four above-mentioned fundamental thermo-chemical biomass conversionapproaches.

ted to bioenergy carriers [4].

Fig. 3. Thermo-chemical processes for bioenergy production and the correspondingproducts [20].

Fig. 4. Relationship between heating value and moisture content of biomass fuel[24].

L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982 971

2. Combustion

Combustion is the most widely used process for biomass con-version. It contributes to over 97% of bioenergy production in theworld. In some less-developed countries, combustion of traditionalbiomass plays an important role in people’s daily lives as it is themain source of energy available for cooking and heating. Regardedas a proven low cost, but highly reliable technology, combustion isrelatively well understood and commercially available [21–23].There are three main stages that occur during biomass combus-tion: drying, pyrolysis and reduction, and combustion of volatilegases and solid char [23]. The combustion of volatiles gases con-tributes to more than 70% of the overall heat generation. It takesplace above the fuel bed and is generally evident by the presenceof yellow flames. Char is combusted in the fuel bed and is notedby the presence of small blue flames [23,24]. The combustion ofbiomass on a large scale is still considered to be a complex processwith technical challenges associated with the biomass fuel charac-teristics, types of combustors, and the challenges of co-firingprocesses.

2.1. Distinct characteristics of biomass fuels

In comparison to fossil fuels, biomass fuels have relatively lowheating values. This can be explained by two of their distinct char-acteristics: high moisture and high oxygen contents [5,21].

The high moisture content is one of the most significantly dis-advantageous features of using biomass as a fuel. Although thecombustion reactions are exothermic, the evaporation of water isendothermic. To maintain a self-supporting combustion process,the moisture content (on wet basis) of biomass fuels cannot behigher than 65% [5]. In addition, the heating value of the fuel isnegatively correlated with the relative amount of water even whenthe moisture content is within the maximum acceptable limit[5,21,24].

Fig. 4 exhibits the negative linear relationship between themoisture content and the heating value. As the moisture contentincreases, both the higher heating value (HHV) and lower heatingvalue (LHV) decrease. HHV and LHV are used to describe the heatproduction of a unit quantity of fuel during its complete combus-tion. In determining the HHV and LHV values of a fuel, the liquidand vapour phases of water are selected as the reference states,respectively. As HHV incorporates the heat of the condensationof water vapour during the combustion, it is not surprising to ob-serve that the curve of the HHV is always above that of the LHV[24,25].

Another important feature of biomass fuel is its elevated oxygencontent. Typically, the oxygen content of biomass is as high as35 wt.%, approximately ten times higher than that of a high-rankcoal, which is below 4 wt.% [21].

Fouling and corrosion of the combustor are typical issues asso-ciated with biomass combustion. These are considered to be detri-mental because of the resulting reduction in heat transfer in thecombustor. Fouling is commonly associated with the presence ofalkali metals and some other elements (such as silicon, sulphur,chlorine, calcium and iron) in the biomass ash. With a series ofcomplex chemical reactions, these elements are deposited in theforms of chlorides, silicates or sulphates on the wall of the combus-tor or the surface of the heat transfer elements [5,21,26,27]. Gener-ally, herbaceous biomass, such as straws and grass withcomparatively higher contents of alkali, sulphur, chlorine, etc.,has a higher potential for the occurrence of ash deposition and cor-rosion in comparison to woody biomass [21,28].

2.2. Biomass combustion systems

Fixed-bed, fluidized-bed, and entrained flow reactors are thethree typical combustion systems, with increasing carrier gasvelocity within the reactor [28]. A higher gas velocity translatesto an intensive mixing of the feedstock, which enhances the com-bustion efficiency and the heat exchange rate. Hence, the entrainedflow systems would be expected to exhibit the best performanceamong these three types of combustion systems [24].

Fixed-bed systems have been widely used for biomass combus-tion for a number of years. The simplest fixed-bed system is com-posed of one combustion room with a grate. Generally, as soon asthe new biomass feed is added into the furnace, it is pyrolyzed intovolatile gases and chars. Primary and secondary air supplies areprovided under and above the grate for the combustion of charsand volatile gases, respectively. The heat generated through thecombustion of chars is responsible for providing enough heat forthe pyrolysis of newly added biomass. Because of the high contentof volatile matter in biomass fuels, a greater secondary air supply isrequired than the primary air supply; this is one of the major dif-ferences from the process of coal combustion. A fixed-bed biomasscombustion system is typically operated at around 850–1400 �C[23,24]. Typical examples of fixed-bed systems are manual-fedsystems, spreader-stoker systems, underscrew systems, through-screw systems, static grates, and inclined grates [24]. Recent devel-opments have been made to enhance the combustion efficiency.One example is the cyclonic combustion system, which may beviewed as a modified fixed-bed system, suitable for the combus-tion of agricultural residues and particulate wood wastes at a highefficiency [24].

Compared with fixed-bed systems, fluidized systems have high-er combustion efficiency and they are more suitable for large scale

Fig. 5. Schematic view of an entrained flow reactor [30].

Table 2Summary of the feedstocks used in previous co-firing studies.

Types of biomass feed Coal or natural gas References

Wheat straw, sewage sludge, wood chip and WPOS(woody matter from olive stones)

Federal and Bellambi coals [32]

Foot cake (waste from the olive oil industry) Coal (lignite and anthracite) [33]Paper mill sludge Coal [34]Sludge and hog fuel Sub-bituminous coal [35]Gasified sugarcane residues Natural gas [36]Cellulose biomass and non-hazardous

waste (institutional-household waste withplastics and food-related paper components)

Natural gas [37]

972 L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982

operations [23]. Fluidized-bed systems employ silica sand, lime-stone, dolomite, or other non-combustible materials for the bedmaterial. The typical operating temperature is 700–1000 �C, whichis lower than that of fixed-bed systems. The bed materials act asthe heat transfer media which are fluidized by the air flow comingfrom the bottom. The biomass which is intermixed with the mov-ing medium has a high combustion efficiency. Depending on theblowing air velocity, fluidized-bed systems can be further dividedinto bubbling fluidized-bed (BFB) and circulating fluidized-bed(CFB) systems [24]. As an example, for a BFB combustor in a pilotplant, the average bed temperature was maintained at775 ± 75 �C, the mean fluidization velocity was set at 1.2 m/s, andthe injected air was divided into primary air (taking 60% of the to-tal airflow) and secondary air [29]. Due to the high mixing inten-sity created by the upward-flowing air at a high velocity, CFBsystems behave more efficiently than BFB systems. In a CFB com-bustor, fuel particles and the bed materials are separated fromthe fast flowing gas stream in the cyclone and then re-enter intothe reactor. Circulating fluidized-bed systems exhibit severaladvantages, such as the adaptation to various fuels with differentproperties, sizes, shapes, and moisture (up to 60%) and ash con-

tents (up to 50%). In addition, the CFB units can achieve high heattransfer and reaction rates with a compact construction [24].

An example of an entrained flow reactor is illustrated in Fig. 5[30]. The fuel particles are transported into an externally heatedSiC tube pneumatically through an insulated and water-cooledinjector. Prior to the injection, the feeding stream, composed ofair and fuel particles, has to pass through an agitation chamberfor ‘‘disaggregation and filtering of pulses in the feeding”. The feed-ing fuel is ignited by a natural gas/air burner at the reactorentrance.

2.3. Co-firing

Co-firing biomass and coal (directly by burning biomass andcoal or indirectly by gasifying biomass first to produce clean fuelgas that is then burnt with coal in a generation boiler) has beenproven to be a cost-effective technology to achieve the goal ofincreasing use of biomass-to-energy processes for power genera-tion, thereby significantly reducing greenhouse gas emissions. Sig-nificant improvements have been achieved in some traditionalcoal-fired power plants. In comparison with other thermo-chemi-cal processes for biomass utilization and bioenergy production(including sole biomass-based combustion), co-firing is mostcost-effective because of the few modifications that are requiredto upgrade the original coal-based power plants [31]. IEA has re-ported that more than 150 coal-fired power plants (50–700MWe)in the world, to date, have been exposed to the co-firing of coalswith woody biomass or waste materials [4]. Various types of bio-mass, such as woody and herbaceous materials, agricultural resid-uals, and energy crops can be easily co-fired with different types ofcoal in percentage fractions as high as 15% (Table 2). Biomass co-firing with coal could reduce the occurrence of fouling and corro-sion, when compared to using biomass alone, due to the dilutionand the consumption of alkali metals via interactions with sulphuror silica in the coal [38]. In addition to coal, biomass can also be co-fired with natural gas [5,37]. This approach is particularly applica-ble to biomass with high moisture content (>60%) that cannot beburned individually in a combustor [5]. Moreover, co-firing of nat-ural gas with fuel gas derived from low-heating-valued biomassmaterials (sometime referred to as the indirect co-firing approach)can be another promising option for bioenergy utilization[36,39,40].

Depending on the manner biomass is mixed with coal, co-firingcan generally be classified into three categories. In the first cate-gory, biomass is simply blended with coal and then introduced intothe boiler. Due to the inferior properties of biomass (e.g., highermoisture contents, low bulk densities, etc.), co-firing processesfor this group normally are limited to low co-firing ratios. In thesecond category, the biomass feedstock has to be processed sepa-rately and injected into the boiler through dedicated lines.Although the biomass introduction is separate from that of coal,the feedstocks are combusted simultaneously. A common chal-lenge for categories one and two is the changed fly ash composi-

Table 3Typical product yields (dry wood basis) of pyrolysis compared with those ofgasification [3].

Mode Conditions Liquid Char Gas

Fast Moderate temperature (around500oC), short hot vapor residencetime � 1 second

75% 12% 13%

Intermediate Moderate temperature (around500oC), moderate hot vaporresidence time � 10-20 seconds

50% 20% 30%

Slow(carbonization)

Low temperature (around400oC), very long solidsresidence time

30% 35% 35%

Gasification High temperature, (around800oC), long solids and vaporresidence time

5% 10% 85%

L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982 973

tion caused by the addition of biomass. Fly ashes from co-firingprocesses containing biomass are currently unaccepted for cementmanufacture due to the strict interpretation of ASTM C618 [41]. Inthe last category, also called indirect co-firing, biomass is gasifiedbefore the subsequent co-firing process. This method is particu-larly suitable for co-combustion with natural gas and for the utili-zation of low-grade biomass and wastes. More importantly, theindirect co-firing approach is useful for ash management as it sep-arates ash from biomass and coals, alleviating concerns associatedwith the utilization of the biomass-based fly ash for cement indus-tries [42].

Although co-firing biomass with coal may not be as efficient asthe coal-based combustion process for power generation [32], it ispromising due to the following distinct merits over the traditionalcoal-based processes. From an environmental point of view, co-fir-ing reduces the emission of CO2 and other toxic gases, such as SOx

and NOx [23,32–34,38,43–46]. The reduction in sulphur emissionsis not only due to the lower sulphur content in biomass, but alsodue to the retention of sulphur by alkali/alkaline earth compoundspresent in the biomass [38,45]. The decreased NOx emissions ob-tained in co-firing [34] can be attributed to the high moisture con-tent in biomass, which lowers the combustion temperature andconsequently results in lower NOx emissions. However, Armestoet al. [33] observed an increased amount of N2O when coal wasco-fired with foot coke, a waste of high moisture content fromthe olive oil industry, in a bubbling fluidized combustor. From aneconomic point of view, the co-combustion of biomass and coalcould stimulate the production of perennial crops, the greatest po-tential biomass supply [38], support economic development andincrease employment in areas where there are rich woody or agri-cultural biomass resources.

Table 4Summary of previous research on biomass fast pyrolysis for bio-oil production.

Feedstock Reactor Reactiontemperatu

Corncob,wheat straw,oregano stalk

Fluidized-bed 500

Hybrid poplar Fluidized-bed 500corn stover

Cotton straw and stalk Fixed-bed 550

Sunflower (Helianthus annus L.) – pressed bagasse Fixed-bed 550

Linseed (Linum usitatissimum L.) Fixed-bed 550

Rape seed (Brassica napus L.) Fixed-bed 550

Rice husk Fixed-bed >500

Olive cake Fixed-bed 550

a Sweep gas flow rate (cm3/min).

3. Pyrolysis

Pyrolysis is a thermal decomposition process that takes place inthe absence of oxygen to convert biomass into solid charcoal, li-quid (bio-oil), and gases at elevated temperatures. Pyrolysis is con-sidered to be an industrially realized process for biomassconversion [3,47–49]. Based on thermal gravity analysis (TGA)testing of biomass, there are three stages for a typical pyrolysisprocess [47]. The first stage, pre-pyrolysis, occurs between 120and 200 �C with a slight observed weight loss, when some internalrearrangements, such as bond breakage, the appearance of free-radicals, and the formation of carbonyl groups take place, with acorresponding release of small amounts of water (H2O), carbonmonoxide (CO), and CO2. The second-stage is the main pyrolysisprocess, during which solid decomposition occurs, accompaniedby a significant weight loss from the initially fed biomass. The laststage is the continuous char devolatilization, caused by the furthercleavage of C–H and C–O bonds.

Depending on the reaction temperature and residence time,pyrolysis can be divided into fast pyrolysis, intermediate pyrolysis,and slow pyrolysis. Table 3 lists the reaction conditions and theproduct yields of various pyrolysis processes, in comparison withthe gasification process. Typically, fast pyrolysis has an extremelyshort residence time (�1 s); the reaction temperature is approxi-mately 100 �C higher than that of slow pyrolysis (�500 �C vs.�400 �C). Short reaction times combined with an elevated temper-ature generally results in a higher yield of liquid product. In con-trast, slow pyrolysis with comparatively lower reactiontemperatures and longer residence times would produce similaramounts of liquid, solid char, and gas products [3]. Combining allthe factors mentioned above, it may be generally concluded thatin order to maximize the charcoal yield, low temperature andlow heating rates are necessary. If liquid is the desired product, acombination of moderate temperature, short gas residence time,and high heating rate is essential [49].

3.1. Fast pyrolysis

Fast pyrolysis is a pyrolysis process with high heating rate (ashigh as hundreds of �C/min) and short residence time. It particu-larly favours the formation of liquid products, but inhibits the for-mation of solid chars (Table 4) [47,50,56,58]. The liquid products(bio-oils) are composed of an aqueous phase which contains sev-eral light organo-oxygen compounds of low molecular weight,and a non-aqueous phase (tar) which includes a variety of insolu-ble aromatic organic compounds of high molecular weight. Bio-oil,the major product from fast pyrolysis, is a potential liquid fuel that

re (�C)Heating rate(�C/min)

Vapour residencetime (s)

Bio-oilyield (%)

References

– 1–2 41 [50]3539

– �0.4 66 [51]58

550 200a 39.51 [52]

5 50a 52.85 [53]

300 100a 57.7 [54]

300 100a 68 [55]

>200 500–1500a >40 [56]

300 100a 39.4 [57]

974 L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982

can be easily stored and transported. The physical properties ofbio-oil derived from wood pyrolysis are shown in Table 5, in com-parison with the properties of a typical heavy fuel oil [59]. Com-pared with petroleum heavy fuel oil, bio-oil has a high content ofwater (15–30 wt.%), a low (<3) and hence corrosive pH, a muchhigher content of oxygen (35–40 wt.%), as well as a lower heatingvalue (HHV of 16–19 MJ/kg). To date, a number of bio-oils havebeen tested to be successfully utilized in turbines and boilers. Inaddition, bio-oils can also be used as a feedstock for chemical pro-duction, and be upgraded to high-quality fuels.

Several factors need to be considered to maximize liquid yieldin a fast pyrolysis process. These may include: a finer particlesize (smaller than 1 mm); a carefully controlled temperature,for which it has been reported that optimal temperatures shouldbe in the range of 450 �C and 550 �C to obtain the highest bio-oilyield [47,50,52,54,55,60]; a higher heating rate (>200 �C/s); ashorter hot vapour residence time (<4 s with a typical value of2 s); and rapid cooling of the vapours [3,47,56,58].

The most commonly used reactors for fast pyrolysis are bub-bling fluidized-bed [10], circulating fluidized-bed, ablative, en-trained flow [3], rotating cone, and vacuum reactors [10]. Themajor characteristics of the first four categories of reactors arelisted in Table 6. As the mobility of the bed increases from ablativeto entrained flow, the solid bulk density decreases, the primary

Table 5Typical properties of wood pyrolysis bio-oil and of heavy fuel oil [59].

Physical property Bio-oil Heavy fuel oil

Moisture content (wt.%) 15–30 0.1pH 2.5 –Specific gravity 1.2 0.94

Elemental composition (wt.%)C 54–58 85H 5.5–7.0 11O 35–40 1.0N 0–0.2 0.3Ash 0–0.2 0.1HHV (MJ/kg) 16–19 40Viscosity (at 50 �C, cP) 40–100 180Solids (wt.%) 0.2–1 1Distillation residue (wt.%) Up to 50 1

Table 6Summary of characteristics of some common pyrolysis systems [10,58].

Reactor type Ablative Bubbling fluid bed (BFB)

Carrier gas No YesHeating method � Reactor wall/disc � Heated recycle gas

� Hot inert gas� Partial gasification� Fire tubes

Primary heat transfermethod

� Solid–solid � Solid–solid� Gas–solid

Modes of heat transfer(suggested)

� 95% conduction� 4% convection� 1% radiation

� 90% conduction� 9% convection� 1% radiation

Main features � Accepts largesize feedstock

� Very high mechanical charabrasion from biomass

� Compact design� Heat supply problematical� Particulate transport gas

not always required

� High heat transfer rates� Heat supply to fluidizing gas

or to bed directly� Limited char abrasion� Very good solids mixing� Particle size limit < 2mm in

smallest dimension� Simple reactor configuration� Residence time of solids and v

controlled by the fluidizing ga

method of heat transfer changes from solid–solid to gas–solid,and the heat transfer modes vary from predominantly conductionto predominantly convection [58]. A comprehensive description ofthe above-mentioned reactors can be found elsewhere [10].

In addition to bio-oil production, fast pyrolysis has also been re-ported to generate hydrogen gas at higher temperatures (700–1000 �C) [59,61–68]. Two types of reactions were found to bemainly responsible for converting methane (CH4), other hydrocar-bon vapours (C2–C5), simple aromatics, etc. into H2; steam reform-ing (Reaction (1)) and water–gas shift (Reaction (2)) reactions[6,62,69]. Steam reforming reactions convert hydrocarbons intoCO and H2. The CO then reacts with H2O to form H2 and CO2

through water–gas shift reactions [69]:

CnHmOk þ ðn� kÞH2O ¼ ðnþm=2� kÞH2 þ nCO ð1ÞnCOþ nH2O() nH2 þ nCO2 ð2Þ

When combining the two reactions together, the hydrogen pro-duction equation can be summarized as Reaction (3) [62,65]:

CnHmOk þ ð2n� kÞH2O ¼ ð2nþm=2� kÞH2 þ nCO2 ð3Þ

The potential hydrogen yield is positively correlated with thehigh steam-to-carbon (S/C) ratio [62] and the presence of proper cat-alysts, such as the co-precipitated Ni-Al catalysts and the ceria-zir-conia supported Rh or Pt catalysts [66,68], in the systems.

3.2. Conventional moderate and slow pyrolysis

A conventional moderate or slow pyrolysis process, with a rel-atively long vapour residence time and low heating rate, has beenused to produce charcoal for thousands of years [10,50]. The prod-uct, charcoal, can be utilized in a wide range of areas, from domes-tic cooking and heating to metallurgical or chemical use as the rawmaterial for the production of chemicals, activated carbon, fire-works, absorbents, soil conditioners, and pharmaceuticals [70].As reported by Mok et al. [71], a higher yield of charcoal can be ob-tained from biomass feedstocks with higher lignin contents andlower hemicellulose contents. In contrast to fast pyrolysis, slowpyrolysis does not necessarily require a fine feedstock particle size(smaller than 1 mm). A conventional pyrolysis process in a rotary-

Circulating fluid bed (CFB) Entrained flow

Yes Yes� In-bed gasification of char to heat

sand

� Solid–solid� Gas–solid

� Gas–solid

� 80% conduction� 19% convection� 1% radiation

� 4% conduction� 95% convection� 1% radiation

aporss flow rate

� High heat transfer rate� High char abrasion from biomass and

char erosion leading to high charin product

� Char/solid heat carrierseparation required

� Solid recycle required� Increased complexity of system� Maximum particle size up to 6 mm� Possible liquid cracking by hot solids� Possible catalytic activity from hot char� Greater reactor wear possible

� Low heat transfer rates� Particle size limit < 2mm� Limited gas/solid mixing

Table 7Comparison of gasification and combustion [7].

Features Gasification Combustion

Purpose Creation of valuable,environmental friendly,usable products from wasteor lower value material

Generation of heat ordestruction of waste

Process Type Thermal and chemicalconversion using no orlimited oxygen

Complete combustion usingexcess oxygen (air)

Raw GasComposition(before gas

cleanup)

H2, CO, H2S, NH3, andparticulates

CO2, H2O, SO2, NOx, andparticulates

Gas cleanup Syngas cleanup atatmosphereic to highpressures depending on thegasifier design

Flue gas cleanup atatmospheric pressure

Treated syngas used forchemical, fuels, or powergeneration

Treated flue gas isdischarged to atmosphere

Recovers sulphur species inthe fuel as sulphur orsulphuric acid

Any sulphur in the fuel isconverted to SO2 that mustbe removed using flue gasprimarily consists of CO2

and H2O

Solidbyproducts/products

Char or slag Bottom and fly ashes

Ash/charor slaghandling

Low temperature processesproduce a char that can besold as fuel

Bottom ash and fly ash arecollected, treated, anddisposed as hazardouswaste in most cases or canbe sold as a material formaking concrete [71]

High temperature processesproduce a slag, a non-leachable, non-hazardousmaterial suitable for use asconstruction materialsFine particulates arerecycled to gasifier. In somecases fine particulates maybe processed to recovervaluable metals

Pressure Atmospheric to high Atmospheric

Table 8Heating values and applications of four types of synthetic gases [7].

Type ofsynthetic gas

Typical heatingvalues (MJ/m3)

Applications in industry

Low-heating-value gas

3.5–10 Gas turbine fuel, boiler fuel, and fuel forsmelting

Mediumheating-value gas

10–20 Gas turbine fuel, hydrogen production,fuel cell feed, chemical and fuel synthesis,and substitute natural gas withmethanation process

High heating-value gas

20–35 Gas turbine fuel, SNG and hydrogenproduction, fuel cell feed, and chemicaland fuel synthesis

Substitutenatural gas(SNG)

>35 Substitute for natural gas, hydrogen andchemical production, fuel cell feed

L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982 975

kiln reactor or a moving bed reactor can be used for raw materialsthat are not available as powders or fine particles [47].

4. Gasification

Biomass gasification is a process that converts carbonaceous bio-mass into combustible gases (e.g. H2, CO, CO2, and CH4) with specificheating values in the presence of a partial oxygen (O2) supply (typi-cally 35% of the O2 demand for complete combustion) or suitable oxi-dants such as steam and CO2. When air or oxygen is employed,gasification is similar to combustion, but it is considered a partialcombustion process. A detailed comparison between biomass gasifi-cation and combustion has been provided by Rezaiyan and Cherem-isinoff [7] and is summarized in Table 7. In general, combustionfocuses on heat generation, whereas the purpose of gasification isto create valuable gaseous products that can be used directly forcombustion, or be stored for other applications. In addition, gasifica-tion is considered to be more environmentally friendly because ofthe lower emissions of toxic gases into the atmosphere and the moreversatile usage of the solid byproducts [7].

Gasification can be viewed as a special form of pyrolysis, takingplace at higher temperatures to achieve higher gas yields. Biomassgasification offers several advantages, such as reduced CO2 emis-sions, compact equipment requirements with a relatively smallfootprint, accurate combustion control, and high thermal efficiency[7,48]. The product of gasification, syngas, is a gaseous form of bio-energy. In terms of the specific heating values, synthetic gases canbe classified into four groups. The typical ranges of the heating val-ues and the industrial applications of each type of syngas are listedin Table 8. It can be seen that synthetic gas applications range fromsteam or heat generation as a fuel gas, for hydrogen production, asa substitute for natural gas production, as a fuel cell feed, and forthe synthesis of some chemical compounds [7].

Gasification technology has been utilized commercially in sev-eral regions of the world. For instance, in the 1990s, China built morethan 70 biomass gasification systems for household cooking. Each ofthe system has an average gas delivery of 200–400 m3/h to serve800–1600 families. While in India, gasification has been selectedas a perspective method for electricity generation. A report pub-lished in 1999 indicated that the Indian Ministry of Non-conven-tional Energy Source had launched a power plant with a 31 MWetotal capacity. In addition, two larger projects for more than200 MWe were either commissioned or under consideration [72].

4.1. Stages of a typical gasification process

Pyrolysis (Reaction (4)) is the first stage of biomass gasification.During this stage, biomass feedstocks are decomposed into tars andvolatile hydrocarbon gases containing certain quantities of hydrogenbefore the commencement of the gasification reactions. Thereafter, aseries of reactions take place in the gasifier shown as Reactions (2)(presented in Section 3.1) and Reactions (5)–(13) [7,73,74]:

CnHmOp ! CO2 þH2Oþ CH4 þ COþH2 þ ðC2 � C5Þ ð4ÞCþO2 ! CO2 ð5Þ

Cþ 12

O2 ! CO ð6Þ

H2 þ12

O2 ! H2O ð7ÞCþH2O! COþH2 ð8ÞCþ 2H2O! CO2 þ 2H2 ð9ÞCþ CO2 ! 2CO ð10ÞCþ 2H2 ! CH4 ð11ÞCOþ 3H2 () CH4 þH2O ð12Þ

CþH2O! 12

CH4 þ12

CO2 ð13Þ

Reactions (5)–(7) are oxidation reactions that occur in the pres-ence of oxygen. Since Reactions (5) and (6) are exothermic, enoughheat is generated to dry the feedstock, to break up the chemicalbonds (pyrolysis of biomass), and to maintain a high temperaturefor driving the gasification reactions. Among these reactions, Reac-tion (5) has the greatest energy release. In contrast, the heat gen-eration capacity of Reaction (6) is only 65% of that of Reaction

Table 9Advantages and disadvantages of selected types of gasifiers [76].

Advantages Disadvantages

Fixed/moving bed, updraftSimple, inexpensive process Large tar productionExit gas temperature about 250 �C Potential channellingOperates satisfactorily under pressure Potential bridgingHigh carbon conversion efficiency Small feed sizeLow dust levels in gas Potential clinkeringHigh thermal efficiency

Fixed/moving bed, downdraftSimple process Minimum feed sizeOnly traces of tar in product gas Limited ash content allowable in feed

Limits to scale up capacityPotential for bridging and clinkering

Fluidized-bedFlexible feed rate and composition Operating temperature limited by ash clinkeringHigh ash fuels acceptableAble to pressurize High product gas temperatureHigh CH4 in product gas High tar and fines content in gasHigh volumetric capacity Possibility of high C content in fly ashEasy temperature control

Circulating fluidized-bedFlexible process Corrosion and attrition problemsUp to 850 �C operating temperature Poor operational control using biomass

Entrained bedVery low in tar and CO2 Low in CH4

Flexible to feedstock Extreme feedstock size reduction requiredExit gas temperature Complex operational control

Carbon loss with ashAsh slagging

Fig. 6. Configuration of the updraft (counter-current) gasifier [24].

976 L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982

(5). Reactions (8) and (9) are the main gasification reactions; theyare called water–gas reactions. Reaction (12) is the methanationreaction, it proceeds slowly at low-temperatures and in the ab-sence of any catalysts. In addition, the water–gas shift reaction(Reaction (2)), as mentioned in Section 3.1, is also of great impor-tance since it plays a significant role for hydrogen generation. BothReactions (2) and (12) take place in either direction depending onthe specific temperature, pressure, and the reactant concentrationsin the system. From these reactions it can be seen that the syngas isa mixture that is mainly composed of CO, H2, CO2, CH4, and H2O va-pour [7,73,75].

4.2. Gasifiers

Gasifiers are the reactors in which gasification reactions takeplace. Based on the types of reactions, a typical air-blown gasifiercan be divided into four process zones – the drying zone, wherewater present in biomass is evaporated; the pyrolysis zone, inwhich biomass is pyrolyzed into medium-energy calorific volatilegases, liquid, and char; the combustion zone, a region where com-bustion reactions take place with limited amounts of air or oxygenprovided; and the reduction zone, in which CO and H2 are pro-duced [73]. Similar to combustors, various types of gasifiers havebeen developed, such as fixed-bed gasifiers, fluidized gasifiers,and entrained flow gasifiers, whose main advantages and disad-vantages are listed in Table 9.

4.2.1. Fixed-bed gasifiersFixed-bed gasifiers generally produce low-heating-valued syn-

gas. They are suitable for small or medium-scale thermal applica-tions. Since there is no mixing within the reactor, uniformreaction temperatures are difficult to achieve [7]. Fixed-bed gasifi-ers include updraft (counter-current), downdraft (co-current),cross-flow, and open-core gasifiers [7,24,73,75].

4.2.1.1. Updraft (counter-current) gasifiers. The updraft gasifier isthe simplest type of gasifier. As shown in Fig. 6, the biomass isfed at the top while the air is injected at the bottom. Biomassand air move in a counter-current direction. During its downwardmovement, biomass is firstly dried when it goes through a ‘‘dryingzone”. Then in the ‘‘distillation zone” (also called pyrolizationzone), biomass undergoes decomposition and is converted intovolatile gases and solid char. The gases and char will be furtherconverted into CO and H2 as they go pass the ‘‘reduction zone”.Since some of the char settles down in the bottom of the reactor,heat is generated through its combustion in the ‘‘hearth zone”and is transported upward by the up-flowing gas to maintain thepyrolysis and drying processes. In addition, CO2 and H2O vapourare also produced from char combustion. Updraft gasifiers can ac-cept biomass with relatively high moisture content (up to 60%).However, the resulting product gas has a high tar content becausethe tar, newly formed during pyrolysis, does not have the opportu-nity to pass through the combustion zone. Nowadays, most of theupdraft power gasifiers have been decommissioned because ofenvironmental issues, such as the water pollution from tarry resi-dues [7,24,48].

4.2.1.2. Downdraft (co-current) gasifiers. The downdraft gasifier iscurrently one of the most widely used fixed-bed gasification sys-tems. Different from the updraft gasifier, air in the downdraft gas-ifier is introduced into the reactor from the middle part. Thisdesign leads to the reversed order of the hearth zone and thereduction zone. In this gasifier, the injected air and biomass moveco-currently. The drying and distillation zones are heated primarilyby the heat radiated from the hearth zone where some char isburned. After passing through the oxidation zone in which air is

introduced, the remaining char, CO2, and H2O are eventually con-verted into CO and H2. In comparison to the updraft gasifier, theproduced tar from pyrolysis passes through a hot reaction zoneand can be destroyed via thermal cracking, consequently, reducinga significant amount of tar in the product gas [7,24,48].

4.2.1.3. Cross-flow gasifiers. In a cross-flow gasifier, biomass isadded at the top of the reactor and moves downwards. Air is intro-duced from one side of the reactor and the gas products are re-leased from the other side of the reactor on the same horizontallevel. The combustion zone is located in the area of air injection,and the drying and pyrolysis zones are above the vessel [75].

4.2.1.4. Open-core gasifiers. Open-core gasifiers are generally em-ployed to gasify biomass with low bulk density and high ash con-tent. An example of this kind of biomass is rice husk. Instead of thenarrow throat characteristic of other gasifiers, the open-core gas-

Fig. 7. Configuration of the entrained flow gasifier [48].

L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982 977

ifier has a wide mouth for biomass injection to prevent fuel flowinhibition caused by bridging. In addition, the rotating grates andthe water basin at the bottom of the gasifier are specifically de-signed to remove the ash produced during the gasification process[24].

4.2.2. Fluidized-bed gasifiersAs described previously for the combustion and pyrolysis pro-

cesses, fluidized-bed reactors are widely employed as gasifiers.Fluidized-bed gasifiers can also be further classified into bubblingfluidized gasifiers and circulating fluidized gasifiers. In a bubblingfluidized gasifier, air is injected from the bottom of a grate, abovewhich the moving bed is mixed with the biomass feed. The bedtemperature is maintained at 700–900 �C. Biomass is pyrolyzedand cracked through contact with the hot bed material. In a circu-lating fluidized gasifier, the hot bed material is circulated betweenthe reactor and a cyclone separator. During this circulation, bedmaterials and char go back to the reactor, while the ash is sepa-rated and removed from the system.

The major advantage of fluidized-bed gasification over fixed-bed gasification is the uniform distribution of temperature withinthe reactor. In addition, fluidized-bed gasifiers can be sized effec-tively for middle or large scale facilities [7,48].

4.2.3. Entrained flow gasifiersIn an entrained flow gasifier, as shown in Fig. 7, the feed and air

move co-currently and the reactions occur in a dense cloud of veryfine particles at high pressures, varying between 19.7 and69.1 atm, and very high temperatures >1000 �C. This type of gas-ifier has an elevated throughput of syngas. However, due to thehigh operating temperature and pressure, gas cooling is requiredbefore use, which can reduce the overall thermal efficiency of thesystem if the heat recovered in cooling is not re-used [48].

4.3. Tar elimination

The presence of tars in the syngas is generally considered unde-sirable as it is not only an indicator of a low gasification efficiency,but also brings in additional operational difficulty for syngas clean-up, which if not operate properly, may foul or plug the pipes andtubes and result in subsequent operational problems [7,48,77].Tar elimination approaches can be classified into two categories:primary methodologies, for which treatments takes place insidethe gasifier, and secondary methodologies, for which the hot gasclean up is conducted outside the gasifier. Primary methodologiesare considered to be more economically practical and have at-tracted much more attention [74]. They include the addition of cat-alysts, the control of some experimental parameters, and theinnovative design of gasifiers.

Catalysts play an important role in increasing the reaction rateat low-temperature conditions inside the gasifier. They also facili-tate the tar conversion into valuable combustible gases via steamreforming, dry reforming, thermal cracking, hydroreforming andhydrocracking, or water–gas reactions [77]. Catalysts can be uti-lized either as bed materials or as additives to the feedstocks[78]. El-Rub et al. [77] divided gasification catalysts into syntheticcatalysts and minerals (Fig. 8). Synthetic catalysts are produced bychemical methods at a relatively high cost, examples are char, alka-li metal-based catalysts (e.g. Li, Na, K, Rb, Cs, and Fr), transitionalmetal-based catalysts (e.g., Ni, Pt, Ru, and Rh), etc. Compared tosynthetic catalysts, minerals are formed naturally and are thusmore cost-effective. The typical examples of minerals are calcinedrocks, olivine, clay minerals, and iron oxides [77].

Gasifier configuration is another key factor that affects tar for-mation, especially when it is combined with catalysts. Cao et al.[74] introduced an innovative fluidized-bed gasifier in which a

freeboard region with partial circulating fuel gas is located abovethe fluidized hot sand bed. The feedstock and primary air streamare injected into the gasifier from the top and the bottom, respec-tively. This particular design produces a tar-free fuel gas with im-proved heating value. In addition, Brandt and Larsen [79] observedthe reduced tar content in the gas products by using a two-stagegasifier that is composed of a pyrolysis unit and a gasification unitwith a charcoal bed. Nunes et al. [80] also noted a reduction in tarformation when the product gas was passed through a second-stage bed packed with char in a two-stage fixed-bed reactor withdowndraft gasifiers.

4.4. Plasma gasification

Plasma gasification is a gasification process that decomposesbiomass into basic components, such as H2, CO, and CO2 in an oxy-gen-starved environment at an extremely high temperature. Plas-ma is regarded as the 4th state of matter, it is an ionized gasproduced by electric discharges. A Plasma torch is a tubular devicethat has two electrodes to produce an arc. It is an independent heatsource that is not affected by the feed characteristics nor the air/oxygen/steam supply. When electricity is fed, an arc is created,and the electricity is converted into heat through the resistanceof the plasma. A plasma torch can heat the biomass feedstock toa temperature of 3000 �C or higher (up to 15,000 �C). Under suchextremely elevated temperature, the injected biomass stream canbe gasified within a few milliseconds without any intermediatereactions. In addition to the conversion of complex organic com-pounds into simple molecules (H2, CO, and CO2), other productsincluding molten metals, vitrified inorganic compounds [7,81,82]are also formed. The plasma technique has high destruction andreduction efficiencies. Any form of wastes, e.g., liquid or solid, fineparticles or bulk items, dry or wet, can be processed efficiently. Inaddition, it is a clean technique with little environmental impact.Plasma technique has great application potential for treating awide range of hazardous wastes. During the plasma gasificationprocess, the toxicity of the waste can be significantly reduced,and some of the mineral compounds are converted into vitrifiedslag that can be utilized in road construction or landscape design[7,83,84].

The configuration of a shaft-type plasma gasifier reactor isshown in Fig. 9 [84], and the same type of reactor was used byHlína et al. [85] for the treatment of waste streams. The plasmagenerator, in this case, was mounted on one of the side flanges,and the plasma flow was distributed through a series of side holesthat were uniformly arranged over the shaft circumference. Wastewas filled from the top and distributed on a fire grate at the bottomof the gasifier. Gas escaping from the bottom of the reactor was

Fig. 8. Classification and types of catalysts used for tar elimination [77].

Fig. 9. Configuration of a shaft-type plasma gasifier reactor. (1) plasma generator,(2) bin with waste, (3) cover, (4) charging batch, (5) fire grate, (6) bath with waterfor quenching the slag, (7) fire grate rotation drive, (8) gas duct, (9) temperaturesensors and (10) gas sampling [84].

978 L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982

cooled by spraying water from a nozzle, burned with air in anafterburner and then cleaned prior to being released into the atmo-sphere; while slag was collected in a bath [84].

Although the main application of plasma gasification is cur-rently to treat non-biomass solid wastes [82–84,86,87], plasmagasification has been considered as a potential thermo-chemicalapproach for syngas production owing to its high H2 and CO yieldsand extremely low tar generation [85,88].

4.5. Supercritical water gasification

Water exists in three states under normal conditions: solid, li-quid, and gas. When the pressure and temperature are increasedto or above their critical points (22.1 MPa and 374 �C), water goesinto supercritical state, where the gas and liquid phases are misci-ble [89,90]. Supercritical water (SCW) has found many applicationsin recent years due to its unique properties, as shown in Fig. 10. Forinstance, when the pressure is fixed at 25.3 MPa, water density de-creases as temperature increases; the most significant drop occursat the critical temperature [90]. As the density becomes lower,water molecules separate further from each other, hydrogen bondsare broken, and water loses its distinct properties as a liquid due tothe loss of order between different water molecules [90,91]. Conse-quently, the dielectric constant decreases correspondingly. Fig. 10billustrates the significant decrease in the dielectric constant withincreased temperature; the constant at the critical point is only1/10 of that under room temperature. The dielectric constant is ameasure of the polarity of the liquid, it is also a reflection of thesolubility of polar molecules in a fluid. Thus, the lower dielectricconstant of SCW translates into an enhanced solubility of organiccompounds (Fig. 10c), but reduced solubility of inorganic com-pounds (Fig. 10d) [90,91]. Supercritical water has the unique abil-ity to dissolve materials that are normally insoluble in eitherambient liquid water or steam and has complete miscibility withthe liquid/vapour products from the processes, providing a sin-gle-phase environment for reactions that would otherwise occur

in a multiphase system under conventional conditions. The advan-tages of a single supercritical phase reaction medium are apparentin that the inter-phase mass transport processes that could hinderreaction rates are eliminated.

Supercritical water has been utilized as an ideal gasificationmedium for biomass primarily because of its strong solubility fororganic compounds, as well as its high reactivity. As water movesinto the supercritical region and if the pressure is maintained at arelatively low value (while still above its critical pressure, i.e., 22.1MPa), free-radical mechanisms would replace ionic mechanisms”in the system and thus the formation of tar can be minimized[92,93]. Moreover, it has been reported by Feng et al. [94] thatwater is a strong oxidant at temperatures greater than 600 �C. Be-cause of this, oxygen atoms present in water can be transferred tocarbon atoms in biomass to form CO2 and CO, while the hydrogenatoms in both the water and biomass are set free to form H2. Assuch, a portion of the hydrogen gas produced can originate fromwater rather than the biomass [91]. Compared to conventional gas-ification processes, supercritical water gasification presents a high-er gasification efficiency and hydrogen yield [95], with a lower tarformation [96]. In addition, as wet biomass can be gasified directly,the expensive and energy-intensive drying process can be elimi-nated [97,98]. Moreover, due to the high pressure of the reaction,

Fig. 10. Properties of water at 25.3 MPa [90,91].

L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982 979

the reactor can be compact, and the hydrogen gas product canbe pressured, which is convenient for storage and transportation[98].

A few supercritical water gasification studies have been con-ducted to date, covering a wide range of biomass. Some model

Table 10Summary of some work done by previous researchers on direct liquefaction of biomass.

Feedstock Solvent

Woody materials: naCunninghamia lanceolata;Fraxinus mandshurica;Pinus massoniana Lamb.;Populus tomentosa Carr.

Crop residues ethylene carbCorn stover ethylene glycrice straw polyethyleneWheat straw

Pine wood(Pinus desiflora) ethanol and sPoplar wood(Populus alba � glandulosa) amount of ph

Sawdusts of white birch ethylene glyc(Betura Platyphylla Sukatchev var. japonica Hara) ethylene carbJapanese cedar propylene car(Cryptomeria japonica D. Don)Japanese cypress(Chamaecyparis obtusa Endl)

Wood meal of birch phenol(Betula papyrifera Marsh)Wood meal of aspen(Populus tremuloides Michx)Thermomechanical pulpKraft pulpCottonJute fibreKenaf plant meal

compounds have been investigated, such as cellulose [99–102],starch [99], glucose [99,103–105], and lignin [100,102,104]. Inaddition, a few waste biomass feedstocks, e.g., cassava waste[99], corn silage [106], fruit shells [97], sawdust [101,102], ricestraw [102], corn and clover grass [107], and sewage sludge [96]have been investigated.

Similar to the conventional gasification process, the addition ofa small quantity of catalyst to a biomass supercritical water gasifi-cation process can enhance gasification efficiency and hydrogenyield, especially at low reaction temperatures. Examples of thesecatalysts include Ru/TiO2 [100], Ru/C, Pd/C, CeO2 particles, nano-CeO2, nano-(CeZr)xO2 [101], carbon (spruce wood charcoal, macad-amia shell charcoal, coal activated carbon, and coconut shell acti-vated carbon) [108], and potassium hydroxide [109].

5. Direct liquefaction

Direct liquefaction is a low-temperature and high pressure ther-mo-chemical process during which biomass is broken down intofragments of small molecules in water or another suitable solvent.These light fragments, which are unstable and reactive, can thenre-polymerize into oily compounds with various ranges of molec-ular weights [12,49,110]. Direct liquefaction has some similaritywith pyrolysis in terms of the target products (liquid products).However, they are different in terms of operational conditions.Specifically, direct liquefaction requires lower reaction tempera-tures but higher pressures than pyrolysis (5–20 MPa for liquefac-tion vs. 0.1–0.5 MPa for pyrolysis). In addition, drying of thefeedstock is not a necessary step for direct liquefaction, but it iscrucial for pyrolysis. Moreover, catalysts are always essential forliquefaction, whereas they are not as critical for pyrolysis. Com-pared with pyrolysis, liquefaction technology is more challengingas it requires more complex and expensive reactors and fuel feed-ing systems [49].

Catalyst References

no catalyst or [110]K2CO3

onate sulphuric acid [111]olglycol

mall sulphuric acid [112]enol sulphonic acids

ol sulphuric acid [113]onatebonate

alkalies or salt [114]

980 L. Zhang et al. / Energy Conversion and Management 51 (2010) 969–982

Lignocellulosic biomass materials are the most widely usedtypes of biomass for bio-oil production through liquefaction, assummarized in Table 10. Lignocellulosic materials are rich in hy-droxyl groups, thus they can be converted into intermediates byliquefaction for the production of biopolymers, such as epoxy res-ins, polyurethane foams, adhesives for plywood, etc. [111,114].

At the beginning of the liquefaction process, biomass undergoesdepolymerization and is decomposed into monomer units. Thesemonomer units, however, may be re-polymerized or condensedinto solid chars, which are undesirable. A solvent is generallyadded to slow down the higher order solid-state reactions, thusreducing the detrimental condensation reactions [115]. Generally,the biomass liquefaction yield is positively correlated with thedielectric constant of the solvent [111]. For example, Liang et al.[111] observed a much higher liquefaction yield and reaction ratefrom corn stover using ethylene carbonate as the solvent ratherthan ethylene glycol for which the dielectric constants for ethylenecarbonate and ethylene glycol are 90.5 and 38.4, respectively, at40 �C. Phenol, with a dielectric constant of 15 at 40 �C, is also a veryeffective solvent for lignin liquefaction as it prevents condensationreactions. In addition, phenol can dissolve cellulose in the presenceof some catalysts, e.g., zinc chloride [115]. However, phenol recov-ery from the liquefied material can be challenging. Hence, to solvethis problem, a mixture composed of a small amount of phenol andsome lower alcohols, which can be easily recovered, can be em-ployed [11,112]. Furthermore, as summarized by Yamada andOno [113], several organic solvents can convert a large fraction ofwoody materials into soluble products. Examples of these solventsinclude dioxane, MDSO, DMF, acetone, and methyl alcohol.

The use of catalysts is a critical factor in biomass liquefaction asit can reduce the required reaction temperature, enhance reactionkinetics, and improve the yield of desired products [114]. Alkalisand acids are the two typical groups of catalysts employed in bio-mass liquefaction (Table 10). Specifically, alkaline catalysts are ableto enhance the yield of heavy oils and decrease the formation ofresidues [110], while acid catalysts (e.g. sulphonic acids and sul-phuric acid), although they are capable of decreasing the tempera-ture and time required for the liquefaction of lignocellulosicbiomass by enhancing the hydrolysis of cellulosic components, alsohave the potential to condense lignin materials, consequentlyincreasing the amount of insoluble residue (char) [112].

6. Concluding remarks

Bioenergy is an abundant, clean, and renewable energy sourcederived from biomass. It can be released via direct combustion orco-firing/co-combustion with coal or natural gas to generate heatand electricity, or can be converted into bio-fuels (e.g., syngas,bio-oils, and charcoal) through other thermo-chemical processes,such as pyrolysis, gasification, and direct liquefaction. The co-firingof biomass and coal is the easiest and most economical approachfor the generation of bioenergy on a large-sale because of thefew modifications that are required to upgrade the original coal-based power plants. Fast pyrolysis has attracted a great deal ofattention as it is, to date, the only industrially realized technologyfor the production of bio-oils. Plasma gasification is mainly appliedfor hazardous waste treatment because of the high destruction andreduction efficiencies. However, it also has great potential for pro-ducing syngas owing to its high yields of H2 and CO, but extremelylow yield of tar. Supercritical water gasification is a promising ap-proach for hydrogen generation from biomass feedstocks, particu-larly those with high moisture contents, such as sewage sludge.Since water can be considered an effective and ‘‘green” solvent, di-rect liquefaction is also able to process wet biomass feedstocks di-rectly. In contrast to supercritical water gasification, direct

liquefaction focuses more on the production of valuable liquidproducts at comparatively lower temperatures.

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