biorefineries: current activities and future developments

Energy Conversion and Management 50 (2009) 2782–2801

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Energy Conversion and Management

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Biorefineries: Current activities and future developments

Ayhan Demirbas *

Sila Science, Trabzon, Turkey

a r t i c l e i n f o

Article history:Received 29 November 2008Accepted 29 June 2009Available online 30 July 2009

Keywords:BiomassRefuelFractionationBiorefiningPyrolysisGasification

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.06.035

* Tel.: +90 462 230 7831; fax: +90 462 248 8508.E-mail address: [email protected]

a b s t r a c t

This paper reviews the current refuel valorization facilities as well as the future importance of biorefin-eries. A biorefinery is a facility that integrates biomass conversion processes and equipment to producefuels, power, and chemicals from biomass. Biorefineries combine the necessary technologies of the bio-renewable raw materials with those of chemical intermediates and final products. Char production bypyrolysis, bio-oil production by pyrolysis, gaseous fuels from biomass, Fischer–Tropsch liquids from bio-mass, hydrothermal liquefaction of biomass, supercritical liquefaction, and biochemical processes of bio-mass are studied and concluded in this review. Upgraded bio-oil from biomass pyrolysis can be used invehicle engines as fuel.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The need of energy is increasing continuously, because of in-creases in industrialization and population. The growth of world’senergy demand raises urgent problems. The larger part of petro-leum and natural gas reserves is located within a small group ofcountries. For example, the Middle East countries have 63% ofthe global reserves and are the dominant supplier of petroleum.This energy system is unsustainable because of equity issues aswell as environmental, economic, and geopolitical concerns thathave far reaching implications. Interestingly, the renewable energyresources are more evenly distributed than fossil or nuclear re-sources. Also the energy flows from renewable resources are morethan three orders of magnitude higher than current global energyneed.

Today’s energy system is unsustainable because of equity issuesas well as environmental, economic, and geopolitical concerns thathave implications far into the future. Bioenergy is one of the mostimportant components to mitigate greenhouse gas emissions andsubstitute of fossil fuels [1–3]. Renewable energy is one of the mostefficient ways to achieve sustainable development.

Plants use photosynthesis to convert solar energy into chemicalenergy. It is stored in the form of oils, carbohydrates, proteins, etc.This plant energy is converted to biofuels. Hence biofuels are pri-marily a form of solar energy. For biofuels to succeed at replacinglarge quantities of petroleum fuel, the feedstock availability needsto be as high as possible. There is an urgent need to design inte-

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grated biorefineries that are capable of producing transportationfuels and chemicals.

In recent years, recovery of the liquid transportation biofuelsfrom biorenewable feedstocks has became a promising methodfor the future. The biggest difference between biorenewable andpetroleum feedstocks is oxygen content. Biorenewables have oxy-gen levels from 10% to 44% while petroleum has essentially none-making the chemical properties of biorenewables very differentfrom petroleum [4–11]. For example, biorenewable products areoften more polar and some easily entrain water and can thereforebe acidic.

There are two global transportation fuels. These are gasolineand diesel fuel. The main transportation fuels that can be obtainedfrom biomass using different processes are sugar ethanol, cellu-losic ethanol, grain ethanol, biodiesel, pyrolysis liquids, green die-sel, green gasoline, butanol, methanol, syngas liquids, biohydrogen,algae diesel, algae jet fuel, and hydrocarbons. Renewable liquidbiofuels for transportation have recently attracted huge attentionin different countries all over the world because of its renewability,sustainability, common availability, regional development, ruralmanufacturing jobs, reduction of greenhouse gas emissions, andits biodegradability [12,8,13–15]. Table 1 shows the availabilityof modern transportation fuels. Transportation fuels and petro-leum fuelled, biorenewable fuelled (ReFueled), and biorenewableelectricity (ReElectrity) powered vehicles are given in Fig. 1.

The term biofuel or biorenewable fuel (refuel) is referred to assolid, liquid or gaseous fuels that are predominantly producedfrom biomass [16–23]. Liquid biofuels being considered world overfall into the following categories: (a) Bioalcohols [24–27], (b) Veg-etable oils [28–31] and biodiesels [32–45]; and (c) Biocrude and

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Table 1Availability of modern transportation fuels.

Fuel type Availability

Current Future

Gasoline Excellent Moderate–poorBioethanol Moderate ExcellentBiodiesel Moderate ExcellentCompressed natural gas (CNG) Excellent ModerateHydrogen for fuel cells Poor Excellent

A. Demirbas / Energy Conversion and Management 50 (2009) 2782–2801 2783

synthetic oils [46–63,16,64,22,65,21,66,28,67,68]. Biofuels areimportant because they replace petroleum fuels. It is expected thatthe demand for biofuels will rise in the future. Biofuels are substi-tute fuel sources to petroleum; however some still include a smallamount of petroleum in the mixture [69,70,29]. Biofuels are gener-ally considered as offering many priorities, including sustainability,reduction of greenhouse gas emissions, regional development, so-cial structure and agriculture, security of supply [71–75]. Biofuelsamong other sources of renewable energy is drawing interest asalternative to fossil diesel. With an increasing number of govern-ments now supporting this cause in the form of mandates andother policy initiatives the biofuel industry is poised to grow at aphenomenal rate.

Policy drivers for biorenewable liquid biofuels have attracted inrural development and economic opportunities for developingcountries [76]. The European Union is on the third rank of biofuelproduction world wide, behind Brazil and the United States. In Eur-ope, Germany is the largest, and France the second largest pro-ducer of biofuels [56].

The term ‘‘modern biomass” is generally used to describe thetraditional biomass use through the efficient and clean combustiontechnologies and sustained supply of biomass resources, environ-mentally sound and competitive fuels, heat and electricity usingmodern conversion technologies. Biomass, as an energy source,has two striking characteristics. Firstly, biomass is the only renew-able organic resource also one of the most abundant resources.Secondly, biomass fixes carbon dioxide in the atmosphere by pho-tosynthesis. Direct combustion and co-firing with coal for electric-ity production from biomass has been found to be a promisingmethod in the nearest future. Biomass thermochemical conversiontechnologies such as pyrolysis and gasification are certainly not themost important options at present; combustion is responsible forover 97% of the world’s bio-energy production. Ethanol and fatty

Transportation Fuels

Biorenewable Fuels (ReFuels) Petroleum Fuels

Gasoline Diesel fuel Bioethanol Biohydrogen Biodiesel

Otto Engine vehicle

Diesel Engine vehicle

Fuel cell

Hybrid electric vehicle

Renewable electricity (ReElectricity)

Bio-oil

Fig. 1. Transportation fuels and petroleum fuelled, biorenewable fuelled (ReFu-eled), and biorenewable electricity (ReElectrity) powered vehicles.

acid (m)ethylester (biodiesel) as well as diesel produced from bio-mass by Fischer–Tropsch synthesis (FTS) are modern biomass-based transportation fuels. Liquid transportation fuels can be eco-nomically produced by Biomass Integrated Gasification-Fischer–Tropsch (BIG-FT) processes. Modern biomass produced in a sus-tainable way excludes traditional uses of biomass as fuel-woodand includes electricity generation and heat production, as wellas transportation fuels, from agricultural and forest residues andsolid waste. On the other hand, ‘‘traditional biomass’’ is producedin an unsustainable way and it is used as a non-commercialsource—usually with very low efficiencies for cooking in manycountries [77].

A biorefinery is a facility that integrates biomass conversionprocesses and equipment to produce fuels, power, and value-addedchemicals from biomass. The biorefinery concept is analogous totoday’s crude oil refinery, which produce multiple fuels and prod-ucts from petroleum. Biorefinery term refers to the conversion ofbiomass feedstock into a host of valuable chemicals and energywith minimal waste and emissions. Fig. 2 shows a schematic dia-gram of biorefinery concept.

2. Fractionation of plant biomass

Biorefinery includes fractionation for separation of primaryrefinery products. The fractionation refers to the conversion ofwood or plant biomass sample into its constituent components(cellluose, hemicelluloses and lignin). Table 2 shows the typicalstructural component analysis of some plant biomass samples.Fig. 3 shows main components in plant biomass. Fractionation pro-cesses include steam explosion, aqueous separation and hot watersystems. Main commercial products of biomass fractionation in-clude levulinic acid, xylitol and alcohols. Fig. 4 shows the fraction-ation of wood and chemicals from wood. Fig. 5 shows theseparation of plant biomass into cellulose product and ligninstream.

Main fractionation chemicals from wood or plant biomasssample ingredients are:

1. Dissociation of cell components ? Lignin fragment +Oligosaccharides + Cellulose.

2. Hydrolysis of cellulose (Saccharification) ? Glucose.3. Conversion of glucose (Fermentation) ? Ethanol + Lactic acid.4. Chemical degradation of cellulose ? Levulinic acid + Xylitol.5. Chemical degradation of lignin ? Phenolic products.

Biomass Conversion Systems

Thermochemical conversion processes Biochemical conversion processes

Pyrolysis Liquefaction Gasification

Bio-oil Biochar Biosyngas

Residues Sugar feedstocks

Animal food Fermentation

Conditioned gas

ReFuels, chemicals and materials

Combined heat and power Biorefining

Fig. 2. Schematic diagram of biorefinery concept.

Table 2Typical component analysis of some plant biomass samples.

Biomass sample Cellulose (%) Hemicelluloses (%) Lignin (%) References

Apricot stone 22.4 20.8 51.4 [78]Beech wood 44.2 33.5 21.8 [79]Birchwood 40.0 25.7 15.7 [80]Hazelnut shell 25.2 28.2 42.1 [79]Legume straw 28.1 34.1 34.0 [78]Orchard grass 32.0 40.0 4.7 [78]Pine sawdust 43.8 25.2 26.4 [81]Rice straw 34.0 27.2 14.2 [80]Spruce wood 43.0 29.4 27.6 [79]Tea waste 31.2 22.8 40.3 [79]Tobacco stalk 21.3 32.9 30.2 [78]

Plant Biomass Components

Structural compounds (Macromolecular substances)

Low-molecular-weight substances

Holocellulose (Polysaccarides) Lignin Organic matter Inorganic matter

Extractives Ash Cellulose Hemicelluloses

Fig. 3. Main components in plant biomass.

Hard wood

Crushing

Steam explosion

Warm water washing

Aqueous phase (Hemicelluloses) Solid phases (Cellulose + Lignin)

Concentrated sulfuric acid extraction

Aqueous phase (Cellulose hydrolysis products)

Solid phase (Lignin)

Enzymatic fermentation NaOH liquefaction

Ethanol

Enzymatic fermentation

Ethanol

Polymerization

Adhesive

Fig. 4. Fractionation of wood and chemicals from wood.

Fig. 5. Separation of plant biomass into cellulose product and lignin stream.

Biomass

Gasification Electricity and Heat Torrefaction

Chemicals-Solvents -Acids

Biosyngas Products-Hydrogen -Carbon monoxide -Carbon dioxide -Methane -Acetylene -Ethylene -Benzene, toluene, xylene -Light tars -Heavy tars -Ammonia -Water

Cryongenic distillation

Transportation fuels-Fischer-Tropsch diesel -Hydrogen -Methane

Gaseous fuels-Methane -SNG

CO2 removal

Tar distillation

Heavy tars Light tars Solvents Fertiliser

Fig. 6. Gasification-based thermochemical biorefinery.

2784 A. Demirbas / Energy Conversion and Management 50 (2009) 2782–2801

Biorefinery contains to extract valuable chemicals and polymersfrom biomass. The main technologies to produce chemicals frombiomass are: (a) biomass refining or pretreatment, (b) thermo-chemical conversion (gasification, pyrolysis, hydrothermal upgrad-ing), (c) fermentation and bioconversion, and (d) productseparation and upgrading.

Biorefineries will use biomass as a feedstock to produce a rangeof chemicals similar to those currently produced from crude oil inan oil refinery. Over half of the oil produced is used to make trans-port fuels. In the near future, these are likely to be partly replacedwith ethanol or other liquid chemicals produced from sugars andpolysaccharides.

There are four main biorefineries: biosyngas-based refinery,pyrolysis-based refinery, hydrothermal upgrading-based refinery,and fermentation-based refinery. Biosyngas is a multifunctionalintermediate for the production of materials, chemicals, transpor-tation fuels, power and/or heat from biomass. Fig. 6 shows the gas-ification-based thermochemical biorefinery.

3. Thermochemical processes

Thermochemical biomass conversion does include a number ofpossible roots to produce from the initial biorenewable feedstockuseful fuels and chemicals. Thermochemical conversion processesinclude three sub-categories: pyrolysis, gasification, and liquefac-tion. Biorenewable feedstocks can be used as a solid fuel, or con-verted into liquid or gaseous forms for the production of electricpower, heat, chemicals, or gaseous and liquid fuels [48–52].Fig. 7 shows the biomass thermal conversion processes. A varietyof biomass resources can be used to convert to liquid, solid and

Biomass Thermal Conversion Processes

Excess air Partial air No air

Combustion Gasification Pyrolysis and Hydrothermal liquefaction

Heat Fuel gases and syngas Liquids

Fig. 7. Biomass thermal conversion processes.

Table 3Comparison of liquefaction and pyrolysis.

Process Temperature (K) Pressure (MPa) Drying

Liquefaction 525–600 5–20 UnnecessaryPyrolysis 650–800 0.1–0.5 Necessary


gaseous fuels with the help of some physical, thermochemical, bio-chemical and biological conversion processes. Main biomass con-version processes are direct liquefaction, indirect liquefaction,physical extraction, thermochemical conversion, biochemical con-version, and electrochemical conversion [82–84]. Fig. 8 shows thetypes and classification of biomass conversion processes. The con-version of biomass materials has a precise objective to transform acarbonaceous solid material which is originally difficult to handle,bulky and of low energy concentration, into the fuels having phys-ico-chemical characteristics which permit economic storage andtransferability through pumping systems.

Pyrolysis is the fundamental chemical reaction process that isthe precursor of both the gasification and combustion of solid fuels,and is simply defined as the chemical changes occurring when heatis applied to a material in the absence of oxygen. Gasification ofbiomass for use in internal combustion engines for power genera-tion provides an important alternate renewable energy resource.Gasification is partial combustion of biomass to produce gas andchar at the first stage and subsequent reduction of the productgases, chiefly CO2 and H2O, by the charcoal into CO and H2. Theprocess also generates some methane and other higher hydrocar-bons depending on the design and operating conditions of thereactor.

Processes relating to liquefaction of biomass are based on theearly research of Appel et al. [85]. These workers reported that avariety of biomass such as agricultural and civic wastes could beconverted, partially, into a heavy oil-like product by reaction withwater and carbon monoxide/hydrogen in the presence of sodiumcarbonate.

The pyrolysis and direct liquefaction with water processes aresometimes confused with each other, and a simplified comparisonof the two follows. Both are thermochemical processes in whichfeedstock organic compounds are converted into liquid products.In the case of liquefaction, feedstock macro-molecule compoundsare decomposed into fragments of light molecules in the presenceof a suitable catalyst [58]. At the same time, these fragments,which are unstable and reactive, repolymerize into oily compoundshaving appropriate molecular weights [86]. With pyrolysis, on theother hand, a catalyst is usually unnecessary, and the light decom-posed fragments are converted to oily compounds through homo-

Fig. 8. Classification of bioma

geneous reactions in the gas phase. The differences in operatingconditions for liquefaction and pyrolysis are shown in Table 3.

3.1. Pyrolysis process

Table 4 shows the pyrolysis routes and their variants. Conven-tional pyrolysis is defined as the pyrolysis, which occurs under aslow heating rate. This condition permits the production of solid,liquid, and gaseous pyrolysis products in significant portions[68]. Conventional slow pyrolysis has been applied for thousandsof years and has been mainly used for the production of charcoal.The heating rate in conventional pyrolysis is typically much slowerthan that used in fast pyrolysis. A feedstock can be held at constanttemperature or slowly heated. Vapors can be continuously re-moved as they are formed [87]. Slow pyrolysis of biomass is asso-ciated with high charcoal continent, but the fast pyrolysis isassociated with tar, at low temperature (675–775 K), and/or gas,at high temperature. At present, the preferred technology is fastor flash pyrolysis at high temperatures with very short residencetimes [51].

Fast pyrolysis (more accurately defined as thermolysis) is a pro-cess in which a material, such as biomass, is rapidly heated to hightemperatures in the absence of oxygen [51]. Flash pyrolysis of bio-mass is the thermochemical process that converts small dried bio-mass particles into a liquid fuel (bio-oil or biocrude) for almost75%, and char and non-condensable gases by heating the biomassto 775 K in the absence of oxygen. Char in the vapor phase cata-lyzes secondary cracking. Table 5 shows the range of the mainoperating parameters for pyrolysis processes. The biomass pyroly-sis is attractive because solid biomass and wastes can be readilyconverted into liquid products. These liquids, as crude bio-oil orslurry of char of water or oil, have advantages in transport, storage,combustion, retrofitting and flexibility in production andmarketing.

Rapid heating and rapid quenching produced the intermediatepyrolysis liquid products, which condense before further reactionsbreak down higher-molecular-weight species into gaseous prod-ucts. High reaction rates minimize char formation. At higher fastpyrolysis temperatures, the major product is gas. If the purposeis to maximize the yield of liquid products resulting from biomasspyrolysis, a low temperature, high heating rate, short gas residencetime process would be required. For a high char production, a lowtemperature, low heating rate process would be chosen. If the pur-pose were to maximize the yield of fuel gas resulting from pyroly-sis, a high temperature, low heating rate, long gas residence timeprocess would be preferred [51]. Table 6 shows char, liquid and

ss conversion processes.

Table 4Pyrolysis routes and their variants.

Method Residence time Temperature (K) Heating rate Products

Carbonation Days 675 Very low CharcoalConventional 5–30 min 875 Low Char, oil, gasSlow 20–200 900 High Oil, char, gasFast 0.5–5 s 925 Very high Bio-oilFlash-liquida <1 s <925 High Bio-oilFlash-gasb <1 s <925 High Chemicals, gasHydro-pyrolysisc <10 s <775 High Bio-oilMethano-pyrolysisd <10 s >975 High ChemicalsUltra pyrolysise <0.5 s 1275 Very high Chemicals, gasVacuum pyrolysis 2–30 s 675 Medium Bio-oil

a Flash-liquid: liquid obtained from flash pyrolysis accomplished in a time of <1 s.b Flash-gas: gaseous material obtained from flash pyrolysis within a time of <1 s.c Hydropyrolysis: pyrolysis with water.d Methanopyrolysis: pyrolysis with methanol.e Ultra pyrolysis: pyrolysis with very high degradation rate.

Table 5Range of the main operating parameters for pyrolysis processes.

Conventional pyrolysis Fast pyrolysis Flash pyrolysis

Pyrolysis temperature (K) 550–900 850–1250 1050–1300Heating rate (K/s) 0.01–1 10–200 >1000Particle size (mm) 5–50 <1 <0.2Solid residence time (s) 300–3600 0.5–10 <0.5

Table 6Char, liquid and gaseous products from plant biomass by pyrolysis and gasification.

Thermal degradation Residence time (s) Upper temperature (K) Char Liquid Gas

Conventional 1800 470 85–91 7–12 2–5pyrolysis 1200 500 58–65 17–24 8–14

900 550 44–49 26–30 16–22600 600 36–42 27–31 23–29600 650 32–38 28–33 27–34600 850 27–33 20–26 36–41450 950 25–31 12–17 48–54

Slow pyrolysis 200 600 32–38 28–32 25–29180 650 30–35 29–34 27–32120 700 29–33 30–35 32–36

90 750 26–32 27–34 33–3760 850 24–30 26–32 35–4330 950 22–28 23–29 40–48

Fast pyrolysis 5 650 29–34 46–53 11–155 700 22–27 53–59 12–164 750 17–23 58–64 13–183 800 14–19 65–72 14–202 850 11–17 68–76 15–211 950 9–13 64–71 17–24

Gasification 1800 1250 7–11 4–7 82–89


gaseous products from plant biomass by pyrolysis and gasification.It is believed that as the pyrolysis reaction progresses the carbonresidue (semi-char) becomes less reactive and forms stable chem-ical structures, and consequently the activation energy increases asthe conversion level of biomass increases. Pyrolysis liquids areformed by rapidly and simultaneously depolymerizing and frag-menting cellulose, hemicellulose, and lignin with a rapid increasein temperature. Rapid quenching traps many products that wouldfurther react (depolymerize, decompose, degrade, cleave, crack orcondensate with other molecules) if the residence time at hightemperature was extended [87].

Design variables required for fast pyrolysis include the follow-ing: feed drying, particle size, pretreatment, reactor configuration,heat supply, heat transfer, heating rates, pyrolysis temperature, va-por residence time, secondary cracking, char separation, ash sepa-ration, and liquid collection [87].

3.1.1. Char production by pyrolysisIn reference to wood, pyrolysis processes are alternately re-

ferred to as carbonization, wood distillation, or destructive distilla-tion processes. Char or charcoal is produced by slow heating wood(carbonization) in airtight ovens or retorts, in chambers with vari-ous gases, or in kilns supplied with limited and controlled amountsof air. As generally accepted, carbonization refers to processes inwhich the char is the principal product of interest (wood distilla-tion, the liquid; and destructive distillation, both char and liquid).At the usual carbonization temperature of about 675 K, char repre-sents the largest component in wood decomposition products.Typical char from wood contains approximately 80% carbon, 1–3% ash, and 12–15% volatile components. The char yield decreasedgradually from 42.6% to 30.7% for the hazelnut shell and from35.6% to 22.7% for the beech wood with an increase of temperaturefrom 550 to 1150 K whilst the char yield from the lignin content


decreased sharply from 42.5% to 21.7% until at 850 K during thecarbonization procedures. Fig. 9 shows the yield of char fromhazelnut shell pyrolysis [83,88]. The charcoal yield decreases asthe temperature increases. The production of the liquid fractionhas a maximum at temperature between 650 and 750 K. The igni-tion temperature of charcoal increases as the carbonization tem-perature increase [83].

The flow diagram for sustainable char production is presentedin Fig. 10. Char is manufactured in kilns and retorts. Kilns and re-torts generally can be classified as either batch or continuous mul-tiple char manufacturing systems. The continuous multiplesystems are more commonly used than are batch systems. Underthe sustainable charcoal production, the aim is to minimize mate-rial and energy losses at all stages. It is well known that the pres-ence of alkaline cations in biomass affect the mechanism ofthermal decomposition during fast pyrolysis causing primarilyfragmentation of the monomers making up the natural polymerchains rather than the predominant depolymerization that occursin their absence. Wood extractives consist of vegetable oils andvaluable chemicals. The vegetable can be converted to biodieselby transesterification with methanol. The need for increased sup-plies of charcoal produced from improved and efficient pyrolyticprocesses is urgent. Recovery of acetic acid and methanol byprod-ucts from pyrolysis was initially responsible for stimulating thecharcoal industry. It was shown that hot water washing alonewas able to remove a major amount of the alkaline cations (potas-sium and calcium mainly) from wood [89]. The wood is then con-verted into char using improved and efficient kilns after which

Chipping

Hot Water Washing Mineral Matter Disposal

Agricultural Fertilizer Solvent Extraction Wood Extractives

Vegetable Oils

Biodiesel

Pyrolysis Processing

Bio-oil Fuel Charcoal

Gaseous Fuel

Packing and Transport Marketing

WOOD

Fig. 10. Flow diagram for sustainable char production from wood.

30

50

70

90

450 600 750 900 1050Temperature, K

Yie

ld o

f ch

ar, w

t%

Fig. 9. Yield of char from hazelnut shell pyrolysis.

proper handling is ensured during packaging, storage and trans-portation to minimize waste. There are various environmentaland socio-economic benefits associated with each stage in theprocess.

Main parts of sustainable char production are managed produc-tion including additional cost in terms of labor, time and money,feedstock costs, management plans or improved kilns and stoves.The sustainable char production and marketing is better for theenvironment in the medium. The energy system commonly con-sists of energy resources and production, security, conversion,use, distribution, and consumption.

The elemental composition of char, and its properties, dependson final carbonization temperatures. At increased temperatures,the carbon content increases dramatically. The yield, water absor-bency, and hydrogen content decreases rapidly as the carboniza-tion temperature increases. The yield is so low at highertemperatures so the production of charcoal at these temperaturesis only of theoretical interest [88].

Char is very important for developing nations, as well as for us.It is important to learn methods of maximizing its potential, sincesuch a large amount of raw materials is needed for its production.Char is a premium fuel that is widely used in many developingcountries to meet household as well as a variety of other needs.Char can be readily produced from wood with no capital invest-ment in equipment through the use of charcoal piles, earth kilns,or pit kilns. As the names of these processes suggest, hardwoodis carefully stacked in a mound or pit around a central air channel,then covered with dirt, humus, moss, clay, or sod [83].

The briquetting of char improves and provides more efficientuse of biomass-based energy resources such as wood and agricul-tural wastes. The char briquettes that are sold on the commercialmarket are typically made from a binder and filler. The char iscrushed into fines and passed through a variety of screens to makesure the particle size is small enough. A binder, typically starch, isadded to the fines, as well as water. Starch is preferred over otheralternatives (wax and wood pitch) because of its economical priceand availability. As the material flows to the mixer, meteredamounts of about 5% of binder (potato or corn starch) with waterare added. Char compromises 75% of the briquette mixture, whilewater and starch compose 20% and 5%, respectively [90]. Fig. 11 de-picts the flow diagram for char briquette production. The press forbriquetting must be well designed, strongly built and capable of

Wood Lumber Storage Wood Pretreatment Kiln

Lump Charcoal Storage Crusher Screen

Ground Charcoal Storage Starch Binder Storage

Charcoal Feeder Starch Feeder

Mixer

Press Briquetting

Water Adding

Drying

Briquette Storage

Fig. 11. Flow diagram for char briquette production.


agglomerating the mixture of charcoal and binder sufficiently for itto be handled through the drying process.

Manufacturing of briquettes from raw material may be eitheran integral part of a charcoal producing facility, or an independentoperation, with charcoal being received as raw material. Briquettesare a processed biomass fuel that can be burned as an alternativeto wood or charcoal for heat energy. Often they are used forcooking.

Wood has very low sulfur content and so does charcoal. Contin-uous production of charcoal is more amenable to emission controlthan batch production because emission composition and flow rateare relatively constant. Briquette improves also health by provid-ing a cleaner burning fuel. If coaling temperatures are too low,excessive amounts of volatiles will remain in the charcoal andcause heavy smoke when it burns.

3.1.2. Bio-oil production by pyrolysisThe term bio-oil is used mainly to refer to liquid fuels. Biomass

could be converted to liquid, char, and gaseous products via car-bonization process at different temperatures. The chemical compo-sition and yields of the char, gas, condensed liquid, and tar weredetermined as function of the carbonization temperature [83].There are several reasons for bio-oils to be considered as relevanttechnologies by both developing and industrialized countries. They

Table 7Characterization of chemistry and products of biomass pyrolysis and carbonization.

Type Feature and process

General effectsColor changes from brown to black

Flexibility and mechanical strength are lost

Size reducedWeight reduced

ProcessesDehydration

Also known as char forming reactionsProduces volatiles products and char

Pyrolysis of holocellulose DepolymerizationProduces tar

Effect of temperature

At low temperatures dehydration predominatesAt 630 K depolymerization with production of levoglucos

between 550 and 675 K products formed are independent oConventional (carbonization)

At 375–450 K endothermAt 675 K exothermMaximum rate occurring between 625 and 725 K

Pyrolysis Fast and flash pyrolysisof lignin High temperature of 750 K

Rapid heating rateFinely ground feed material

Less than 10% MCRapid cooling and condensation of gases

Yields in 80% range char and gas used for fuel

include energy security reasons, environmental concerns, foreignexchange savings, and socio-economic issues related to the ruralsector. Bio-oils are liquid fuels made from biomass materials, suchas agricultural crops, municipal wastes and agricultural and for-estry byproducts via biochemical or thermochemical processes.

Bio-oils are dark brown, free-flowing organic liquids that arecomprised of highly oxygenated compounds. The synonyms forbio-oil include pyrolysis oils, pyrolysis liquids, biocrude oil, woodliquids, wood oil, liquid smoke, wood distillates, pyroligneous acid,and liquid wood. Bio-oils contain many reactive species, whichcontribute to unusual attributes. Chemically, bio-oil is a complexmixture of water, guaiacols, catecols, syringols, vanillins, furan-carboxaldehydes, isoeugenol, pyrones, acetic acid, formic acid,and other carboxylic acids. It also contains other major groups ofcompounds, including hydroxyaldehydes, hydroxyketones, sugars,carboxylic acids, and phenolics [87].

The pyrolysis of biomass is a thermal treatment that results inthe production of charcoal, liquid, and gaseous products. Amongthe liquid products, methanol is one of the most valuable products.The liquid fraction of the pyrolysis products consists of two phases:an aqueous phase containing a wide variety of organo–oxygencompounds of low molecular weight and a non-aqueous phasecontaining insoluble organics of high molecular weight. This phaseis called tar and is the product of greatest interest. The ratios of

Products and their characterizations

Volatile productsReadily escape during pyrolysis process

59 compounds produced of which 37 have been identified CO,CO2, H2O, acetal, furfural, aldehydes, ketonesTarLevoglucosan is principal component

CharsAs heating continues there is a 80% loss of weight and

remaining cellulose is converted to charProlonged heating or exposure to higher temperature (900 K)reduces char formation to 9%Char

an dominatesf temperature

Approximately 55%

Distillates (20%)Methanol – methoxyl groups, acetic acidAcetone

Tar (15%)Phenolic compounds and carboxylic acidGases

CO, methane, CO2, ethaneBio-oil

Will not mix with hydrocarbon liquidsCannot be distilledSubstitute for fuel oil and diesel in boilers, furnaces, engines,

turbines, etc.Phenols

Utilizes a solvent extraction process to recover phenolics andneutrals

18–20% of wood wt.Secondary processing of phenol formaldehyde resinsAdhesivesInjected molded plastics

Other chemicalsExtraction processChemical for stabilizing the brightness regression of

thermochemical pulp (TMP) when exposed to lightFood flavorings, resins, fertilizers, etc.

Table 8Typical properties and characteristics of wood-derived bio-oil.

Property Characteristics

Appearance From almost black or dark red brown to dark green, depending on the initial feedstock and the mode of fast pyrolysis

Miscibility Varying quantities of water exist, ranging from �15 wt.% to an upper limit of �30–50 wt.% water, depending on production and collectionPyrolysis liquids can tolerate the addition of some water before phase separation occursBio-oil cannot be dissolved in waterMiscible with polar solvents such as methanol, acetone, etc., but totally immiscible with petroleum-derived fuels

Density Bio-oil density is �1.2 kg/L, compared to �0.85 kg/L for light fuel oil

Viscosity Viscosity of bio-oil varies from as low as 25 cSt to as high as 1000 cSt (measured at 313 K) depending on the feedstock, the water content of the oil,the amount of light ends that have collected, the pyrolysis process used, and the extent to which the oil has been agedIt cannot be completely vaporized after initial condensation from the vapor phase at 373 K or more, it rapidly reacts and eventually produces a solidresidue from �50 wt.% of the original liquid

Distillation It is chemically unstable, and the instability increases with heatingIt is always preferable to store the liquid at or below room temperature; changes do occur at room temperature, but much more slowly and they canbe accommodated in a commercial application

Ageing of pyrolysisliquid

Causes unusual time-dependent behaviorProperties such as viscosity increases, volatility decreases, phase separation, and deposition of gums change with time


acetic acid, methanol, and acetone of the aqueous phase were high-er than those of the non-aqueous phase.

Bio-oil has higher energy density than biomass, and can be ob-tained by quick heating of dried biomass in a fluidized bed fol-lowed by cooling. The byproducts char and gases can becombusted to heat the reactor. For utilization of biomass in the re-mote location, it is more economical to convert into bio-oil andthen the transport the bio-oil. Upgraded bio-oil can be used invehicle engines – either totally or partially in a blend.

Biomass is dried and then converted to oily product as knownbio-oil by very quick exposure to heated particles in a fluidizedbed. The char and gases produced are combusted to supply heatto the reactor, while the product oils are cooled and condensed.The bio-oil is then shipped by truck from these locations to thehydrogen production facility. It is more economical to producebio-oil at remote locations and then ship the oil, since the energydensity of bio-oil is higher than biomass. For this analysis, it wasassumed that the bio-oil would be produced at several smallerplants which are closer to the sources of biomass, such that lowercost feedstocks can be obtained.

Typical properties and characteristics of wood-derived bio-oilare presented in Table 8 [87]. Bio-oil has many special featuresand characteristics. These require consideration before any appli-cation, storage, transport, upgrading or utilization is attempted.

The liquid yield from the hardwood was reduced to 6.2% at1150 K. The condensed liquid yield of the hardwood showed amaximum peak (40.9%) at 850 K and then decreased to 6.2% athigher temperatures. The tar yield from all biomass samples wasabout at same level. Table 7 shows the characterization of chemis-try and products from pyrolysis and carbonization processes ofbiomass [83].

Table 9 shows the fuel properties of diesel fuel and biomasspyrolysis oil. The kinematic viscosity of pyrolysis oil varies fromas low as 11 cSt to as high as 115 mm2/s (measured at 313 K)depending on nature of the feedstock, temperature of pyrolysisprocess, thermal degradation degree and catalytic cracking, thewater content of the pyrolysis oil, the amount of light ends thathave collected, and the pyrolysis process used. The pyrolysis oilshave water contents of typically 15–30 wt.% of the oil mass, whichcannot be removed by conventional methods like distillation.Phase separation may partially occur above certain water contents.The water content of pyrolysis oils contributes to their low energydensity, lowers the flame temperature of the oils, leads to ignitiondifficulties, and, when preheating the oil, can lead to prematureevaporation of the oil and resultant injection difficulties. The high-

er heating value (HHV) of pyrolysis oils is below 26 MJ/kg (com-pared to 42–45 MJ/kg for compared to values of 42–45 MJ/kg forconventional petroleum fuel oils). In contrast to petroleum oils,which are nonpolar and in which water is insoluble, biomass oilsare highly polar and can readily absorb over 35% water [91].

The pyrolysis oil (bio-oil) from wood is typically a liquid, almostblack through dark red brown. The density of the liquid is about1200 kg/m3, which is higher than that of fuel oil and significantlyhigher than that of the original biomass. The bio-oils have watercontents of typically 14–33 wt.%, which can not be removed byconventional methods like distillation. Phase separation may occurabove certain water contents. The higher heating value (HHV) isbelow 27 MJ/kg (compared to 43–46 MJ/kg for conventional fueloils).

The bio-oil formed at 725 K contained high concentrations ofcompounds such as acetic acid, 1-hydroxy-2-butanone, 1-hydro-xy-2-propanone, methanol, 2,6-dimethoxyphenol, 4-methyl-2,6-dimetoxyphenol and 2-cyclopenten-1-one, etc. A significant char-acteristic of the bio-oils was the high percentage of alkylatedcompounds especially methyl derivatives. As the temperature in-creased, some of these compounds were transformed via hydroly-sis [92]. The formation of unsaturated compounds from biomassmaterials generally involves a variety of reaction pathways suchas dehydration, cyclisation, Diels–Alder cycloaddition reactionsand ring rearrangement. For example, 2,5-hexandione can under-go cyclisation under hydrothermal conditions to produce 3-methyl-2-cyclopenten-1-one with very high selectivity of up to81% [93].

The mechanism of pyrolysis reactions of biomass was exten-sively discussed in an earlier study [94]. Water is formed by dehy-dration. In the pyrolysis reactions, methanol arises from thebreakdown of methyl esters and/or ethers from decomposition ofpectin-like plant materials. Methanol also arises from methoxylgroups of uronic acid [95]. Acetic acid is formed in the thermaldecomposition of all three main components of wood. When theyield of acetic acid originating from the cellulose, hemicelluloses,and lignin is taken into account, the total is considerably less thanthe yield from the wood itself. Acetic acid comes from the elimina-tion of acetyl groups originally linked to the xylose unit.

Furfural is formed by dehydration of the xylose unit. Quantita-tively, 1-hydroxy-2-propanone and 1-hydroxy-2-butanone presenthigh concentrations in the liquid products. These two alcohols arepartly esterified by acetic acid. In conventional slow pyrolysis,these two products are not found in so great a quantity becauseof their low stability [96].

Table 9Fuel properties of diesel, biodiesel and biomass pyrolysis oil.

Property Test method ASTM D975 (diesel) Pyrolysis oil (bio-oil)

Flash point D93 325 K min –Water and sediment D2709 0.05 max vol.% 0.01–0.04Kinematic viscosity (at 313 K) D445 1.3–4.1 mm2/s 25–1000Sulfated ash D874 – –Ash D482 0.01 max wt.% 0.05–0.01 wt.%Sulfur D5453 0.05 max wt.% –Sulfur D2622/129 – 0.001–0.02 wt.%Copper strip D130 No 3 max –

CorrosionCetane number D613 40 min –Aromaticity D1319 – –Carbon residue D4530 – 0.001–0.02 wt.%Carbon residue D524 0.35 max mass% –Distillation temperature D1160 555 K min –(90% volume recycle) �611 K max


If wood is completely pyrolyzed, resulting products are aboutwhat would be expected by pyrolyzing the three major compo-nents separately. The hemicelluloses would break down first, attemperatures of 470 to 530 K. Cellulose follows in the temperaturerange 510 to 620 K, with lignin being the last component to pyro-lyze at temperatures of 550 to 770 K. A wide spectrum of organicsubstances was contained in the pyrolytic liquid fractions givenin the literature [96]. Degradation of xylan yields eight main prod-ucts: water, methanol, formic, acetic and propionic acids, 1-hydro-xy-2-propanone, 1-hydroxy-2-butanone and 2-furfuraldeyde. Themethoxy phenol concentration decreased with increasing temper-ature, while phenols and alkylated phenols increased. The forma-tion of both methoxy phenol and acetic acid was possibly as aresult of the Diels–Alder cycloaddition of a conjugated diene andunsaturated furanone or butyrolactone.

Timell [97] described the chemical structure of the xylan as the4-methyl-3-acetylglucoronoxylan. It has been reported that thefirst runs in the pyrolysis of the pyroligneous acid consist of about50% methanol, 18% acetone, 7% esters, 6% aldehydes, 0.5% ethylalcohol, 18.5% water, and small amounts of furfural [94]. Pyroligne-ous acids disappear in high-temperature pyrolysis.

The composition of the water soluble products was not ascer-tained but it has been reported to be composed of hydrolysis andoxidation products of glucose such as acetic acid, acetone, simple

Table 10Chemical and physical properties of biomass bio-oils and fuel oils.

Pine Oak P

Elemental (wt.% dry)C 56.3 55.6 5H 6.5 5.0 6O (by different) 36.9 39.2 3N 0.3 0.1 0S – 0.0 0

Proximate (wt.%)Ash 0.05 0.05 <Water 18.5 16.1 2Volatiles 66.0 69.8 6Fixed C (by different) 15.5 14.1 5

Viscosity (mm2/s) at 313 K 44 115 1

Density (kg/m3) at 313 K 1210 1230 –

Pyrolysis yield (wt.%)Dry oil 58.9 55.3 4Water 13.4 10.4 1Char 19.6 12.2 1Gases 13.9 12.4 2

Source: Ref. [91].

alcohols, aldehydes, sugars, etc. [98]. Pyroligneous acids disappearin high-temperature pyrolysis. Levoglucosan is also sensitive toheat and decomposes to acetic acid, acetone, phenols, and water.Methanol arises from the methoxyl groups of aronic acid [94]. Ta-ble 10 shows the chemical and physical properties of biomass bio-oils and fuel oils.

Fig. 12 shows the fractionation of biomass pyrolysis products.The liquid from pyrolysis is often called oil, but is more like tar.This also can be degraded to liquid hydrocarbon fuels. The oil frac-tion consisted of two phases: an aqueous phase containing a widevariety of organo–oxygen compounds of low molecular weight anda non-aqueous tarry phase containing insoluble organics (mainlyaromatics) of high molecular weight. The crude pyrolysis liquid isa thick black tarry fluid. The tar is a viscous black fluid that is abyproduct of the pyrolysis of biomass and is used in pitch, var-nishes, cements, preservatives, and medicines as disinfectantsand antiseptics [99].

The gas product from pyrolysis usually has a medium heatingvalue (MHV) fuel gas around 15–22 MJ/Nm3 or a lower heating va-lue (LHV) fuel gas of around 4–8 MJ/Nm3 from partial gasificationdepending on feed and processing parameters.

Table 11 shows the gas chromatographic analysis of bio-oilfrom beech wood pyrolysis (wt.% dry basis). The bio-oil formedat 725 K contained high concentrations of compounds such as

oplar Hardwood No. 2 oil No. 6 oil

9.3 58.8 87.3 87.7.6 9.7 12.0 10.33.8 31.2 0.0 1.2.2 0.2 <0.01 0.5.00 0.04 0.1 0.8

0.001 0.07 <0.001 0.077.2 18.1 0.0 2.37.4 – 99.8 94.1.4 – 0.1 3.6

1 58 2.6 –

1170 860 950

1.0 – – –4.6 – – –6.4 – – –1.7 – – –


acetic acid, 1-hydroxy-2-butanone, 1-hydroxy-2-propanone,methanol, 2,6-dimethoxyphenol, 4-methyl-2,6-dimetoxyphenol

BIOMASS PYROLYSIS PRODUCTS

CHAR BIO-OIL HHV FUEL GAS LHV FUEL GAS

Slurry Fuel

Active Carbon

Chemicals Electricity

Hydrocarbons Ammonia Electricity Methanol

Ammonia Electricity

Fig. 12. Fractionation of biomass pyrolysis products.

Table 11Gas chromatographic analysis of bio-oil from beech wood pyrolysis (wt.% dry basis).

Compound Reaction temperature (K)

625 675 725 775 825 875

Acetic acid 16.8 16.5 15.9 12.6 8.42 5.30Methyl acetate 0.47 0.35 0.21 0.16 0.14 0.111-Hydroxy-2-propanone 6.32 6.84 7.26 7.66 8.21 8.46Methanol 4.16 4.63 5.08 5.34 5.63 5.821-Hydroxy-2-butanone 3.40 3.62 3.82 3.88 3.96 4.111-Hydroxy-2-propane acetate 1.06 0.97 0.88 0.83 0.78 0.75Levoglucosan 2.59 2.10 1.62 1.30 1.09 0.381-Hydroxy-2-butanone acetate 0.97 0.78 0.62 0.54 0.48 0.45Formic acid 1.18 1.04 0.84 0.72 0.60 0.48Guaiacol 0.74 0.78 0.82 0.86 0.89 0.93Crotonic acid 0.96 0.74 0.62 0.41 0.30 0.18Butyrolactone 0.74 0.68 0.66 0.67 0.62 0.63Propionic acid 0.96 0.81 0.60 0.49 0.41 0.34Acetone 0.62 0.78 0.93 1.08 1.22 1.282,3-Butanedione 0.46 0.50 0.56 0.56 0.58 0.612,3-Pentanedione 0.34 0.42 0.50 0.53 0.59 0.64Valeric acid 0.72 0.62 0.55 0.46 0.38 0.30Isovaleric acid 0.68 0.59 0.51 0.42 0.35 0.26Furfural 2.52 2.26 2.09 1.84 1.72 1.585-Methyl-furfural 0.65 0.51 0.42 0.44 0.40 0.36Butyric acid 0.56 0.50 0.46 0.39 0.31 0.23Isobutyric acid 0.49 0.44 0.38 0.30 0.25 0.18Valerolactone 0.51 0.45 0.38 0.32 0.34 0.35Propanone 0.41 0.35 0.28 0.25 0.26 0.212-Butanone 0.18 0.17 0.32 0.38 0.45 0.43Crotonolactone 0.12 0.19 0.29 0.36 0.40 0.44Acrylic acid 0.44 0.39 0.33 0.25 0.19 0.152-Cyclopenten-1-one 1.48 1.65 1.86 1.96 2.05 2.132-Methyl-2-cyclopenten-1-one 0.40 0.31 0.24 0.17 0.13 0.142-Methyl-cyclopentenone 0.20 0.18 0.17 0.22 0.25 0.29Cyclopentenone 0.10 0.14 0.16 0.23 0.27 0.31Methyl-2-furancarboxaldehyde 0.73 0.65 0.58 0.50 0.44 0.38Phenol 0.24 0.30 0.36 0.43 0.54 0.662,6-Dimethoxyphenol 2.28 2.09 1.98 1.88 1.81 1.76Dimethyl phenol 0.08 0.13 0.18 0.42 0.64 0.90Methyl phenol 0.32 0.38 0.44 0.50 0.66 0.874-Methyl-2,6-dimetoxyphenol 2.24 2.05 1.84 1.74 1.69 1.58

Source: Ref. [91].

Table 12Overview of conversion routes of plant materials to biofuels.

Plant material Conversion route Primarily product Tre

Ligno-cellulosic biomass Flash pyrolysis Bio-oil HyGasification Syngas Wa

CatHydrolysis Sugar FerHydrothermal liquefaction Bio-oil HyAnaerobic digestion Biogas Pur

Sugar and starch crops Milling and hydrolysis Sugar Fer

Oil plants Pressing or extraction Vegetable oil EstPyr

and 2-Cyclopenten-1-one, etc. A significant characteristic of thebio-oils was the high percentage of alkylated compounds espe-cially methyl derivatives.

The influence of temperature on the compounds existing in li-quid products obtained from biomass samples via pyrolysis wereexamined in relation to the yield and composition of the productbio-oils. The product liquids were analyzed by a gas chromatogra-phy mass spectrometry combined system. The bio-oils were com-posed of a range of cyclopentanone, methoxyphenol, acetic acid,methanol, acetone, furfural, phenol, formic acid, levoglucosan, gua-iocol and their alkylated phenol derivatives. Thermal depolymer-ization and decomposition of biomass structural components,such as cellulose, hemicelluloses, lignin form liquids and gas prod-ucts as well as a solid residue of charcoal. The structural compo-nents of the biomass samples mainly affect on pyrolyticdegradation products. A reaction mechanism is proposed whichdescribes a possible reaction route for the formation of the charac-teristic compounds found in the oils. The supercritical waterextraction and liquefaction partial reactions also occur during thepyrolysis. Acetic acid is formed in the thermal decomposition ofall three main components of biomass. In the pyrolysis reactionsof biomass: water is formed by dehydration; acetic acid comesfrom the elimination of acetyl groups originally linked to the xy-lose unit; furfural is formed by dehydration of the xylose unit; for-mic acid proceeds from carboxylic groups of uronic acid; andmethanol arises from methoxyl groups of uronic acid [91].

Table 12 shows an overview of conversion routes of plant mate-rials to biofuels. Chemical groups can be obtained from biomassbio-oil by fast pyrplysis are acids, aldehydes, alcohols, sugars, es-ters, ketones, phenolics, oxygenates, hydrocarbons, and steroids.Chemicals from biomass bio-oil by fast pyrolysis are given in Table16.

3.2. Gasification process

Gasification is a form of pyrolysis, carried out at high tempera-tures in order to optimize the gas production. The resulting gas,known as producer gas, is a mixture of carbon monoxide, hydrogenand methane, together with carbon dioxide and nitrogen.

Most biomass gasification systems utilize air or oxygen in par-tial oxidation or combustion processes. These processes suffer fromlow thermal efficiencies and low calorific gas because of the energyrequired to evaporate the moisture typically inherent in the bio-mass and the oxidation of a portion of the feedstock to produce thisenergy. Fig. 13 shows the fractionation of biomass gasificationproducts. The biomass fuels are suitable for the highly efficientpower generation cycles based on gasification and pyrolysis pro-cesses. Yield a product gas from thermal decomposition composedof CO, CO2, H2O, H2, CH4, other gaseous hydrocarbons (CHs), tars,char, inorganic constituents, and ash. Gas composition of productfrom the biomass gasification depends heavily on the gasificationprocess, the gasifying agent, and the feedstock composition. The

atment Products

drotreating and refining CxHx, diesel fuel, chemicals, oxygenates, hydrogenter gas shift + separation Hydrogenalyzed synthesis Methanol, dimethyl ether, FT diesel, CxHx, SNG (CH4)mentation Bioethanoldrotreating and refining CxHx, diesel fuel, chemicalsification SNG (CH4)

mentation Bioethanol

erification Biodieselolysis Bio-oil, diesel fuel, gasoline

Biomass Gasification Gas cleaning IC Engine Power

Ash

Air, Steam, Oxygen Exhaust gas

Condensate

Fig. 14. System for power production by means of biomass gasification.

Table 13Fixed-bed and fluidized bed gasifiers and reactor types using in gasification processes.

Gasifier Reactor type

Fixed-bed Downdraft, updraft, co-current, c-current, cross current, othersFluidized bed Single reactor, fast fluid bed, circulating bed

Table 14Composition of gaseous products from various biomass fuels by different gasificationmethods (% by volume).

H2 CO2 O2 CH4 CO N2

10–19 10–15 0.4–1.5 1–7 15–30 43–60

BIOMASS GASIFICATION PRODUCTS

MHV FUEL GAS LHV FUEL GAS

CHEMICALS COGENERATION CHEMICALS STEAM BOILER

Methanol

Hydrocarbons Syngas (H2/CO)

Heat Electricity Heat Electricity

Ammonia Syngas (H2/CO)

Gasoline Diesel Fuel Methanol Methanol Fuel Alcohol

Ammonia

Fig. 13. Fractionation of biomass gasification products.


relative amount of CO, CO2, H2O, H2, and (CHs) depends on the stoi-chiometry of the gasification process. If air is used as the gasifyingagent, then roughly half of the product gas is N2. The air/fuel ratioin a gasification process generally ranges from 0.2 to 0.35 and ifsteam is the gasifying agent, the steam/biomass ratio is around 1.The actual amount of CO, CO2, H2O, H2, tars, and (CHs) dependson the partial oxidation of the volatile products, as shown:

CnHm þ ðn=2þm=4ÞO2 ¡ nCOþ ðm=2ÞH2O ð1Þ

The major thermochemical reactions include the following:Steam and methane:

CH4 þH2O ¡ COþ 3H2 ð2Þ

Water gas shift:

COþH2O ¡ CO2 þH2 ð3Þ

Carbon char to methane:

Cþ 2H2 ¡ CH4 ð4Þ

Carbon char oxides (Boudouard reaction):

Cþ CO2 ¡ 2CO ð5Þ

The main steps in the gasification process are:

Step 1. Biomass is delivered to a metering bin from which it isconveyed with recycled syngas or steam, without airor oxygen into the gasifier.

Step 2. The material is reformed into a hot syngas that containsthe inorganic (ash) fraction of the biomass and a smallamount of unreformed carbon.

Step 3. The sensible heat in the hot syngas is recovered to pro-duce heat for the reforming process.

Step 4. The cool syngas passes through a filter and the particu-late in the syngas is removed as a dry, innocuous waste.The clean syngas is then available for combustion inengines, turbines, or standard natural gas burners withminor modifications.

For electricity generation, two most competitive technologiesare direct combustion and gasification. Typical plant sizes at pres-ent range from 0.1 to 50 MW. Co-generation applications are veryefficient and economical. Fluidized bed combustion (FBC) is effi-cient and flexible in accepting varied types of fuels. Gasifiers firstconvert solid biomass into gaseous fuels which is then usedthrough a steam cycle or directly through gas turbine/engine. Gasturbines are commercially available in sizes ranging from 20 to50 MW [100].

Commercial gasifiers are available in a range of size and types,and run on a variety of fuels, including wood, charcoal, coconutshells, and rice husks. Power output is determined by the economicsupply of biomass, which is limited to 80 MW in most regions[101]. Fig. 14 shows the system for power production by meansof biomass gasification. The gasification system of biomass infixed-bed reactors provides the possibility of combined heat andpower production in the power range of 100 kWe up to 5 MWe.A system for power production by means of fixed-bed gasificationof biomass consists of the main unit gasifier, gas cleaning systemand engine.

Gasifiers are used to convert biomass into a combustible gas.The combustible gas is then used to drive a high efficiency, com-bined cycle gas turbine. Combustible gas energy conversion de-vices that are discussed are reciprocating engines, turbines, microturbines, fuel cells, and anaerobic digesters. The resulting combus-tible gas can be burnt to provide energy for cooking and spaceheating, or create electricity to power other equipment. Sincemany of the parasites and disease producing organisms in thewaste are killed by the relatively high temperature in the digestertanks, the digested material can also be used as fertilizer or fishfeed [99].

Various gasification technologies include gasifiers where thebiomass is introduced at the top of the reactor and the gasifyingmedium is either directed co-currently (downdraft) or counter-currently up through the packed bed (updraft). Other gasifier de-signs incorporate circulating or bubbling fluidized beds. Tar yieldscan range from 0.1% (downdraft) to 20% (updraft) or greater in theproduct gases. Table 13 shows the fixed-bed and fluidized bed gas-ifiers and reactor types using in gasification processes.

Commercial gasifier are available in a range of size and types,and run on a variety of fuels, including wood, charcoal, coconutshells and rice husks. Power output is determined by the economicsupply of biomass, which is limited to 80 MW in most regions. Theproducer gas is affected by various gasification processes from var-ious biomass feedstocks. Table 14 shows composition of gaseousproducts from various biomass fuels by different gasificationmethods.

3.2.1. Fischer–Tropsch synthesis (FTS)The FTS was established in 1923 by German scientists Franz

Fischer and Hans Tropsch. The main aim of FTS is synthesis of

Table 15Typical composition of the syngas.

Gaseous product % by volume

Hydrogen 29–40Carbon monoxide 21–32Methane 10–15Carbon dioxide 15–20Ethylene 0.4–1.2Water vapor 4–8Nitrogen 0.6–1.2

Table 16Chemicals from biomass bio-oil by fast pyrolysis.

Chemical Minimum (wt.%) Maximum (wt.%)

Levoglucosan 2.9 30.5Hydroxyacetaldehyde 2.5 17.5Acetic acid 6.5 17.0Formic acid 1.0 9.0Acetaldehyde 0.5 8.5Furfuryl alcohol 0.7 5.51-Hydroxy-2-propanone 1.5 5.3Catechol 0.5 5.0Methanol 1.2 4.5Methyl glyoxal 0.6 4.0Ethanol 0.5 3.5Cellobiosan 0.4 3.31,6-Anhydroglucofuranose 0.7 3.2Furfural 1.5 3.0Fructose 0.7 2.9Glyoxal 0.6 2.8Formaldehyde 0.4 2.44-Methyl-2,6-dimetoxyphenol 0.5 2.3Phenol 0.2 2.1Propionic acid 0.3 2.0Acetone 0.4 2.0Methylcyclopentene-ol-one 0.3 1.9Methyl formate 0.2 1.9Hydroquinone 0.3 1.9Acetol 0.2 1.72-Cyclopenten-1-one 0.3 1.5Syringaldehyde 0.1 1.51-Hydroxy-2-butanone 0.3 1.33-Ethylphenol 0.2 1.3Guaiacol 0.2 1.1


long-chain hydrocarbons from CO and H2 gas mixture. The FTS isdescribed by the set of equations [102]:

nCOþ ðnþm=2ÞH2 ! CnHm þ nH2O ð6Þ

In the FTS one mole of CO reacts with two moles of H2 in thepresence cobalt (Co) based catalyst to afford a hydrocarbon chainextension (–CH2–). The reaction of synthesis is exothermic(DH = –165 kJ/mol):

COþ 2H2 ! �CH2 �þH2O DH ¼ �165 kJ=mol ð7Þ

The –CH2– is a building stone for longer hydrocarbons. A maincharacteristic regarding the performance of the FTS is the liquidselectivity of the process [103].

The FTS is a well established process for the production of syn-fuels. The process is used commercially in South Africa by Sasoland Mossgas and in Malaysia by Shell. The FTS can be operatedat low temperatures (LTFT) to produce a syncrude with a large frac-tion of heavy, waxy hydrocarbons or it can be operated at highertemperatures (HTFT) to produce a light syncrude and olefins. WithHTFT the primary products can be refined to environmentallyfriendly gasoline and diesel, solvents and olefins. With LTFT, theheavy hydrocarbons can be refined to waxes or if hydrocrackedand/or isomerized, to produce excellent diesel, base stock for lubeoils and a naphtha that is ideal feedstock for cracking to light ole-fins. Biomass Integrated Gasification-Fischer–Tropsch (BIG-FT)process is favorable as a production route.

The FTS has been widely investigated for more than 70 years,and Fe and Co are typical catalysts. Cobalt-based catalysts are pre-ferred because their productivity are better than Fe thanks for theirhigh activity, selectivity for linear hydrocarbons, low activity forthe competing water–gas-shift reaction. The FTS product composi-tion strongly influenced by catalyst composition: product from co-balt catalyst higher in paraffins and product from iron catalysthigher in olefins and oxygenates [104].

The variety of composition of FT products with hundreds ofindividual compounds shows a remarkable degree of order withregard to class and size of the molecules. Starting from the conceptof FTS as an ideal polymerization reaction it is easily realized thatthe main primary products, olefins, can undergo secondary reac-tions and thereby modify the product distribution. This generallyleads to chain length dependencies of certain olefin reaction possi-bilities, which are again suited to serve as a characteristic featurefor the kind of olefin conversion.

While gasification processes vary considerably, typically gasifi-ers operate from 975 K and higher and from atmospheric pressureto five atmospheres or higher. The process is generally optimizedto produce fuel or feedstock gases. Gasification processes also pro-duce a solid residue as a char, ash, or slag. The product fuel gases,including hydrogen, can be used in internal and external combus-tion engines, fuel cells, and other prime movers for heat andmechanical or electrical power. Gasification products can be usedto produce methanol, FT liquids, and other fuel liquids and chem-icals. The typical composition of the syngas is given in Table 15.

Catalytic steam reforming of hydrocarbons has been extensivelystudied, especially in the context of methane reforming to makesyngas (H2:CO = 2:1) for methanol and FT liquid synthesis. Metha-nol is produced in large quantities by the conversion of natural gasor petroleum into a synthesis gas. Biomass has also been used as afeedstock for producing synthesis gas used in production of bothmethanol and FT liquids.

In all types of gasification, biomass is thermochemically con-verted to a low or medium-energy content gas. The higher heatingvalue of syngas produced from biomass in the gasifier is 10–13 typ-ically MJ/Nm3. Air-blown biomass gasification results in approxi-mately 5 MJ/Nm3 and oxygen-blown 15 MJ/Nm3 of gas and is

considered a low to medium-energy content gas compared to nat-ural gas (35 MJ/Nm3).

The process of synfuels from biomass will lower the energy cost,improve the waste management and reduce harmful emissions.This triple assault on plant operating challenges is a proprietarytechnology that gasifies biomass by reacting it with steam at hightemperatures to form a clean burning syngas. The molecules in thebiomass (primarily carbon, hydrogen and oxygen) and the mole-cules in the steam (hydrogen and oxygen) reorganize to form thissyngas.

Reforming the light hydrocarbons and tars formed during bio-mass gasification also produces hydrogen. In essence, the systemembodies a fast, continuous process for pyrolizing or thermallydecomposing biomass and steam reforming the resulting constitu-ents. The entire process occurs in a reducing environment biomassgasifiers. Steam reforming and so-called dry or CO2 reforming oc-cur according to the following reactions and are usually promotedby the use of catalysts.

CnHm þ nH2O ¡ nCOþ ðnþm=2ÞH2 ð8ÞCnHm þ nCO2 ¡ ð2nÞCOþ ðm=2ÞH2 ð9Þ

The high-temperature FT technology applied by Sasol in theSynthol process in South Africa at the Secunda petrochemical siteis the largest commercial scale application of FT technology. The

Bio-syngas Gas cleaning

Product upgrade

Product refining

Gasification of biomass

Gas conditioning FTS Main product:

Diesel fuel By products:

Gasoline Kerosene

Specialities

Fig. 15. Production of diesel fuel from bio-syngas by Fisher–Tropsh synthesis (FTS).


most recent version of this technology is the Sasol Advanced Syn-thol (SAS) process. Although the FTS was initially envisioned as ameans to make transportation fuels, there has been a growing real-ization that the profitability of commercial operations can be im-proved by the production of chemicals or chemical feedstock. FTSyields a complex mixture of saturated or unsaturated hydrocar-bons (C1–C40+), C1–C16+ oxygenates as well as water and CO2. Lab-oratory studies are often carried out in differential conditions(conversions of approximately 3%) and therefore very accurateproduct analysis is necessary. Hydrocarbons obtained from a SAStype reactor are: for C5–C10 and C11–C14 hydrocarbons; paraffins13% and 15%, olefins 70% and 60%, aromatics 5% and 15% and oxy-genates 12% and 10%, respectively. From the component break-down of the main liquid cuts it is clear that there is considerablescope for producing chemical products in addition to hydrocarbonfuels [102,105,106].

The FTS in supercritical phase can be developed and its reactionbehavior and mass transfer phenomenon were analyzed by bothexperimental and simulation methods. The same reaction appara-tus and catalysts can be used for both gas phase reaction and liquidphase FT reactions to correctly compare characteristic features ofdiffusion dynamics and the reaction itself, in various reactionphases. Efficient transportation of reactants and products to the in-side of catalysts bed and pellet, quick heat transfer and in situproduct extraction from catalysts by supercritical fluid can beaccomplished.

There has been an increasing interest in the effect of water oncobalt FT catalysts in recent years. Water is produced in largeamounts over cobalt catalysts since one water molecule is pro-duced for each C-atom added to a growing hydrocarbon chainand due to the low water–gas-shift activity of cobalt. The presenceof water during FTS may affect the synthesis rate reversibly as re-ported for titania-supported catalysts, the deactivation rate as re-ported for alumina-supported catalysts and water also has asignificant effect on the selectivity for cobalt catalysts on differentsupports. The effect on the rate and the deactivation appears to de-pend on the catalyst system studied while the main trends in theeffect on selectivity appear to be more consistent for different sup-ported cobalt systems. There are, however, also some differences inthe selectivity effects observed. The present study deals mainlywith the effect of water on the selectivity of alumina-supported co-balt catalysts but some data on the activity change will also be re-ported. The results will be compared with results for othersupported cobalt systems reported in the literature [106].

The activity and selectivity of supported Co FTS catalysts de-pends on both the number of Co surface atoms and on their densitywithin support particles, as well as on transport limitations that re-strict access to these sites. Catalyst preparation variables availableto modify these properties include cobalt precursor type and load-ing level, support composition and structure, pretreatment proce-dures, and the presence of promoters or additives. Secondaryreactions can strongly influence product selectivity. For example,the presence of acid sites can lead to the useful formation ofbranched paraffins directly during the FTS step. However, productwater not only oxidizes Co sites making them inactive for addi-tional turnovers, but it can inhibit secondary isomerization reac-tions on any acid sites intentionally placed in FTS reactors [107].

Iron catalysts used commercially by Sasol in the Fischer–Trop-sch synthesis for the past five decades [108] have several advanta-ges: (1) lower cost relative to cobalt and ruthenium catalysts, (2)high water–gas-shift activity allowing utilization of syngas feedsof relatively low hydrogen content such as those produced by gas-ification of coal and biomass, (3) relatively high activity for produc-tion of liquid and waxy hydrocarbons readily refined to gasolineand diesel fuels, and (4) high selectivity for olefinic C2–C6 hydro-carbons used as chemical feedstocks. The typical catalyst used in

fixed-bed reactors is an unsupported Fe/Cu/K catalyst preparedby precipitation [109]. While having the previously-mentionedadvantages, this catalyst: (1) deactivates irreversibly over a periodof months to a few years by sintering, oxidation, formation of inac-tive surface carbons, and transformation of active carbide phases toinactive carbide phases and (2) undergoes attrition at unacceptablyhigh rates in the otherwise highly efficient, economical slurry bub-ble-column reactor.

It is well known that addition of alkali to iron causes an increaseof both the 1-alkene selectivity and the average carbon number ofproduced hydrocarbons. While the promoter effects on iron hasbeen thoroughly studied only few and on a first glance contradic-tive results are available for cobalt catalysts. In order to completeexperimental data the carbon number distributions are analyzedfor products obtained in a fixed-bed reactor under steady statecondition. Precipitated iron and cobalt catalysts with and withoutK2CO3 were used [107,108].

Fig. 15 shows the production of diesel fuel from bio-syngas byFTS. The design of a biomass gasifier integrated with a FTS reactormust be aimed at achieving a high yield of liquid hydrocarbons. Forthe gasifier, it is important to avoid methane formation as much aspossible, and convert all carbon in the biomass to mainly carbonmonoxide and carbon dioxide [110].

The FTS based gas to liquids technology (GTL) includes the threeprocessing steps namely syngas generation, syngas conversion andhydroprocessing. In order to make the GTL technology more cost-effective, the focus must be on reducing both the capital and theoperating costs of such a plant [111]. For some time now the pricehas been up to $60 per barrel. It has been estimated that the FTprocess should be viable at crude oil prices of about $20 per barrel[112]. The current commercial applications of the FT process aregeared at the production of the valuable linear alpha olefins andof fuels such as LPG, gasoline, kerosene and diesel. Since the FT pro-cess produces predominantly linear hydrocarbons the productionof high quality diesel fuel is currently of considerable interest.The most expensive section of an FT complex is the production ofpurified syngas and so its composition should match the overallusage ratio of the FT reactions, which in turn depends on the prod-uct selectivity [108].

The Al2O3/SiO2 ratio has significant influences on iron-basedcatalyst activity and selectivity in the process of FTS. Product selec-tivities also change significantly with different Al2O3/SiO2 ratios.The selectivity of low molecular weight hydrocarbons increasesand the olefin to paraffin ratio in the products shows a monotonicdecrease with increasing Al2O3/SiO2 ratio. Recently, Jun et al. [113]studied FTS over Al2O3 and SiO2 supported iron-based catalystsfrom biomass-derived syngas. They found that Al2O3 as a structuralpromoter facilitated the better dispersion of copper and potassiumand gave much higher FTS activity.

Recently, therehasbeensomeinterestintheuseofFTSforbiomassconversion to synthetic hydrocarbons. Biomass can be converted tobio-syngas by non-catalytic, catalytic and steam gasification pro-cesses. The bio-syngas consists mainly of H2, CO, CO2 and CH4. TheFTS has been carried out using CO/CO2/H2/Ar (11/32/52/5 vol.%) mix-tureasa model for bio-syngasonco-precipitatedFe/Cu/K,Fe/Cu/Si/K,


andFe/Cu/Al/Kcatalystsinafixed-bedreactor.Someperformancesofthe catalysts that depended on the syngas composition are also pre-sented [113]. The kinetic model predicting the product distributionis taken from Wang et al. [109], for an industrial Fe–Cu–K catalyst.

To produce bio-syngas from a biomass fuel the following proce-dures are necessary: (a) Gasification of the fuel, (b) Cleaning of theproduct gas, (c) Usage of the synthesis gas to produce chemicals,and (d) Usage of the synthesis gas as energy carrier in fuel cells.Fig. 16 shows the green diesel and other products from biomassvia Fisher-Tropsch synthesis.

3.2.2. Gaseous fuels from biomassMain biorenewable gaseous fuels are biogas, landfill gas, gas-

eous fuels from pyrolysis and gasification of biomass, gaseous fuelsfrom Fischer–Tropsch synthesis and biohydrogen [114–123]. Thereare a number of processes for converting of biomass into gaseousfuels such as methane or hydrogen. One pathway uses plant andanimal wastes in a fermentation process leading to biogas fromwhich the desired fuels can be isolated. This technology is estab-lished and in widespread use for waste treatment. Anaerobic diges-tion of bio-wastes occurs in the absence of air, the resulting gascalled as biogas is a mixture consisting mainly of methane and car-bon dioxide. Biogas is a valuable fuel which is produced in digest-ers filled with the feedstock like dung or sewage. The digestion isallowed to continue for a period of from ten days to a few weeks.A second pathway uses algae and bacteria that have been geneti-cally modified to produce hydrogen directly instead of the conven-tional biological energy carriers. Finally, high-temperaturegasification supplies a crude gas, which may be transformed intohydrogen by a second reaction step. This pathway may offer thehighest overall efficiency.

Hydrogen can be produced from biomass via two thermochem-ical processes: (1) gasification followed by reforming of the syngas,and (2) fast pyrolysis followed by reforming of the carbohydratefraction of the bio-oil. In each process, water–gas-shift is used toconvert the reformed gas into hydrogen, and pressure swingadsorption is used to purify the product. Gasification technologiesprovide the opportunity to convert biorenewable feedstocks intoclean fuel gases or synthesis gases. The synthesis gas includesmainly hydrogen and carbon monoxide (H2 + CO) which is also

BIOMASS

GASIFICATION WITH PARTIAL OXIDATION

GAS CLEANING

GAS CONDITIONING –Reforming –Water-Gas Shift –CO2 Removal –Recycle

FISHER–TROPSCH SYNTHESIS

PRODUCT UPGRADING

GREEN DIESEL LIGHT PRODUCTS –Gasoline –Kerosene –LPG –Methane –Ethane

HEAVY PRODUCTS –Light wax –Heavy wax

POWER –Electricity –Heat

Fig. 16. Green diesel and other products from biomass via Fisher–Tropschsynthesis.

called as syngas. Biosyngas is a gas rich in CO and H2 obtainedby gasification of biomass.

Hydrogen can be produced from biomass by pyrolysis, gassifica-tion, steam gasification, steam reforming of bio-oils, and enzymaticdecomposition of sugars. Hydrogen is produced from pyroligneousoils produced from the pyrolysis of lignocellulosic biomass. Theyield of hydrogen that can be produced from biomass is relativelylow, 16–18% based on dry biomass weight [124].

The strategy is based on producing hydrogen from biomasspyrolysis using a co-product strategy to reduce the cost ofhydrogen and concluded that only this strategy could competewith the cost of the commercial hydrocarbon-based technologies[125]. This strategy will demonstrate how hydrogen and biofuelare economically feasible and can foster the development of rur-al areas when practiced on a larger scale. The process of biomassto activated carbon is an alternative route to hydrogen with avaluable co-product that is practiced commercially. The yieldof hydrogen that can be produced from biomass is relativelylow, 12–14% based on the biomass weight [126]. In the proposedsecond process, fast pyrolysis of biomass to generate bio-oil andcatalytic steam reforming of the bio-oil to hydrogen and carbondioxide. Table 16 shows the chemicals from biomass bio-oil byfast pyrolysis.

3.3. Liquefaction process

In the liquefaction process, biomass is converted to liquefiedproducts through a complex sequence of physical structure andchemical changes. In the liquefaction, biomass is decomposed intosmall molecules. These small molecules are unstable and reactive,and can repolymerize into oily compounds with a wide range ofmolecular weight distribution.

Liquefaction of biomass is accomplished by natural, direct andindirect thermal, extraction, and fermentation methods. Moderndevelopment of the liquefaction process can be traced to the earlywork at the Bureau of Mines as an extension of coal liquefaction re-search [127]. In the case of liquefaction, feedstock macro-moleculecompounds are decomposed into fragments of light molecules inthe presence of a suitable catalyst. At the same time, these frag-ments, which are unstable and reactive, repolymerize into oilycompounds having appropriate molecular weights [128].

Direct liquefaction of wood by catalyst was carried out in thepresence of K2CO3 [129]. Indirect liquefaction involves successiveproduction of an intermediate, such as synthesis gas or ethylene,and its chemical conversion to liquid fuels.

Aqueous liquefaction of lignocellulosic materials involves disag-gregation of the wood ultrastructure followed by partial depoly-merization of the constitutive families (hemicelluloses, celluloseand lignin). Solubilization of the depolymerized material is thenpossible [130]. The heavy oil obtained from the liquefaction pro-cess was a viscous tarry lump, which sometimes caused troublesin handling. For this purpose, some organic solvents were addedto the reaction system. Among the organic solvents tested, propa-nol, butanol, acetone, methyl ethyl ketone and ethyl acetate werefound to be effective on the formation of heavy oil having low vis-cosity [94].

The changes during liquefaction process involve all kinds ofprocesses such as solvolysis, depolymerization, decarboxylation,hydrogenolysis, and hydrogenation. Solvolysis results in micellar-like substructures of the biomass. The depolymerization of bio-mass leads to smaller molecules. It also leads to new molecularrearrangements through dehydration and decarboxylation. Whenhydrogen is present, hydrogenolysis and hydrogenation of func-tional groups, such as hydroxyl groups, carboxyl groups, and ketogroups also occur. Fig. 17 shows the procedures for separation ofaqueous liquefaction products.

Biomass

Aqueous liquefaction

Gaseous products

Liquefaction products

Filtration

Water solubles Water insolubles

Evaporation

Water soluble products

Acetone extraction

Filtration

Acetone solubles Acetone insolubles

Evaporation Drying

Acetone soluble products Acetone insoluble products

Fig. 17. Procedures for separation of aqueous liquefaction products.

Table 17Representatives for the products from the HTU process.

Product Component Weight fraction (%)

Biocrude Polycarbonates 47.5Methyl-n-propyl ether 2.5

Gas Carbon dioxide 25.0

Organic compounds Methanol 5.0Ethanol 3.5

Water Water 16.5

FEEDSTOCK

Pretreatment

Pumping

Preheating

HTU REACTOR

Product separation Gas

Air

Catalytic combustion

Flue gas

BIOCRUDE

Process water

Anaerobic digestion

Biogas

Combustion

Heat Power

Light biocrude Heavy biocrude

Upgrading by hydrodeoxygenation Coal co-combustion

Electricity UPGRADED PRODUCTS: – Premium diesel fuel – Kerosene – Feedstock refinery

Fig. 18. Block scheme of commercial HTU plant.


3.3.1. Hydrothermal liquefactionThe hydrothermal liquefaction (HTL) or direct liquefaction is a

promising technology to treat waste streams from various sourcesand produce valuable bio-products such as biocrudes. A majorproblem with commercializing the HTL processes for biomass con-version today is that it remains uneconomical when compared tothe costs of diesel or gasoline production. High transportation costsof large quantities of biomass increase production costs, and poorconversion efficiency coupled with a lack of understanding com-plex reaction mechanisms inhibits growth of the processcommercially.

In the HTU process, biomass is reacted in liquid water at ele-vated temperature and pressure. The phase equilibria in the HTUprocess are very complicated due to the presence of water, super-critical carbon dioxide, alcohols as well as the so-called biocrude.The biocrude is a mixture with a wide molecular weight distribu-tion and consists of various kinds of molecules. Biocrude contains10–13% oxygen. The biocrude is upgraded by catalytic hydrodeox-ygenation in a central facility. Preliminary process studies on theconversion of various biomass types into liquid fuels have indi-cated that HTU is more attractive than pyrolysis or gasification.In HTU the biomass, typically in a 25% slurry in water, is treatedat temperatures of 575–625 K and 12–18 MPa pressures in thepresence of liquid water for 5–20 min to yield a mixture of liquidbiocrude, gas (mainly CO2) and water. Subsequent processingmay be able to upgrade the biocrude to useable biofuel. A largeproportion of the oxygen is removed as carbon dioxide [131].

Biomass, such as wood, with a lower energy density is con-verted to biocrude with a higher energy density, organic com-pounds including mainly alcohols and acids, gases mainlyincluding CO2. Water is also a byproduct. In the products, CO2,the main component of the gas product, can be used to representall gas produced, and methanol and ethanol represent organiccompounds. In Table 17, the weight fraction of each componentis assigned on the basis of the data of the vacuum flash of biocrudeand the data of a pilot plant [132]. Fig. 18 shows the block schemeof commercial HTU plant. The feedstocks, reaction conditions, andthe products for the HTU process are given in Table 18.

Preliminary process studies on the conversion of various bio-mass types into liquid fuels have indicated that HTU is more attrac-tive than pyrolysis or gasification. In HTU the biomass, typically in

a 25% slurry in water, is treated at temperatures of 575–625 K and12–18 MPa pressures in the presence of liquid water for 5–20 minto yield a mixture of liquid biocrude, gas (mainly CO2) and water.Subsequent processing may be able to upgrade the biocrude touseable biofuel. A large proportion of the oxygen is removed as car-bon dioxide [131].

One of the first HTL studies was conducted by Kranich [133]using municipal waste materials (MSW) as a source to produceoil. Three different types of materials from a MSW plant were used:primary sewage sludge, settled digester sludge, and digester efflu-ent. Using a magnetically stirred batch autoclave with a hydrogen-feed system, slurry feed device, a pressure and temperature recor-der, and a wet-test meter for measuring gas product, Kranich pro-cessed the waste sources. The feedstock was first dried thenpowdered. The wastes were also separated into different oil andwater slurries and processed separately. Temperatures rangedfrom 570 to 720 K with pressures up to 14 MPa. Retention timesalso varied between 20 and 90 min. Hydrogen was used as thereducing gas with initial pressures up to 8.3 MPa. Three types ofcatalyst were studied: sodium carbonate, nickel carbonate, and so-dium molybdate. The slurry feedstock was injected into the reactorthrough a pressurized injector and the oil product was extracted bypentane and toluene. Results showed that organic conversion ratesvaried from 45% to 99% and oil production rates were reportedfrom 35.0% to 63.3%. Gas products were found to contain H2, CO2,

Table 18Feedstocks, reaction conditions, and the products for the HTU process.

Biomass feedstocks Wood and forest wastesAgricultural and domestic residuesMunicipal solid wastesOrganic industrial residuesSewage sludge

Reaction conditions Temperature: 300–350 �CPressure: 12–18 MPaResistance time: 5–20 minMedium: liquid water

Main chemical reactions DepolymerizationDecarboxylationDehydrationOxygen removed as CO2 and H2OHydrodeoxygenationHydrogenation

Products (wt.% on feedstock) Biocrude: 45Water soluble organics: 10Gas (>90% CO2): 25Process water: 20

Thermal efficiency 70–90%


and C1–C4 hydrocarbons. The experimental results showed no sig-nificant differences between the applications of the three differentcatalysts. Kranich recommended that the water slurry system wasnot feasible for scale-up and considerations of a commercial scaleprocess were confined to only the oil slurry system. It was also con-cluded that no further development work on hydroliquefaction ofsewage sludge to oil was necessary. Kranich’s recommendationdid not hold, mainly due to increases in crude oil prices and theneed to find new technologies for energy procurement, and thusmany studies on liquefaction of sewage sludge have since beenconducted. Research has indicated that liquefaction is a feasiblemethod for the treatment of sewage sludge wastes and has a highoil producing potential [134–136]. Today HTL research is still beingconducted with sewage sludge; however, focus has shifted to in-clude many varieties of biomass materials.

Several technologies have been developed to convert biomassinto a liquid biofuel with a higher heating value, such as gasifica-tion, fast pyrolysis and the HTU. In the HTU, the biomass is treatedduring 5–20 min with water under subcritical conditions (575–625 K, 10–18 MPa) to give a heavy organic liquid (biocrude) witha heating value of 30–35 MJ/kg. During this process, the oxygencontent of the organic material is reduced from about 40% to be-tween 10% and 15%. The removed oxygen ends up in CO2, H2Oand CO. After 1.6 s at 595 K and 25 MPa, 47% conversion of cellu-lose in water was obtained yielding hydrolysis products (cellobi-ose, glucose, etc., 44%) and decomposition products of glucose(erythrose, 1,6-anhydroglucose, 5-hydroxymethylfurfural, 3%).Furthermore, it has been shown that cellobiose decomposes viahydrolysis to glucose and via pyrolysis to glycosylerythrose andglycosylglycolaldehyde, which are further hydrolyzed into glucose,erythrose and glycolaldehyde. Hydrolysis refers to splitting up ofthe organic particles into smaller organic fragments in water.Hydrothermal decomposition also acts on the large organic mole-cules reducing them into smaller fragments, some of which dis-solve in water.

In the HTU process, biomass chips are pressurized and digestedat 475–525 K with recycle water from the process. Subsequentlythe digested mass is pressurized to 12–18 MPa and reacted in li-quid water at 575–675 K for 5–15 min. Under these conditionsdecarboxylation and depolymerisation take place and a biocrudeis formed, which separates from the water phase. Part of the pro-cess water is recycled. Obviously, the process is very simple withhigh efficiency.

Hydrothermal reaction involves applying heat under pressureto achieve reaction in an aqueous medium. The treatment of organ-ic wastes by SCW reaction undergoes in a homogeneous phase thatinterface mass transfer limitations are avoided and reaction effi-ciencies of 99.9% can be achieved at residence times lower than1 min. Because of the distinctive characteristics of water describedabove, hydrothermal reaction is an effective method for the treat-ment of organic wastes. The reaction can be performed under sub-or supercritical conditions. It can also be classified into two broadcategories: (a) oxidative, i.e. involving the use of oxidants, and (b)non-oxidative, i.e. excluding the use of oxidants.

3.3.2. Supercritical liquefaction and gasificationSupercritical fluid extraction (SFE) has potential as an effica-

cious means of recovering secondary compounds from the ligno-cellulosic biomass, such as waxy lipids, terpenes and phenolics.Supercritical fluids are less viscous and diffuse more quickly thanliquids and can therefore extract plant products more effectivelyand faster than conventional organic solvents [137].

Recently, the supercritical fluid treatment has been consideredto be an attractive alternative in science and technology as chem-ical reaction field. The supercritical water can be realized from theionic reaction field to the radical reaction field. Therefore, thesupercritical water is expected as a solvent for converting biomassinto the valuable substances.

The reforming in supercritical water (SCW) offers severaladvantages over the conventional technologies because of the unu-sual properties of supercritical water. The density of supercriticalwater is higher than that of steam which results in a high space–time yield. The higher thermal conductivity and specific heat ofsupercritical water is beneficial for carrying out the endothermicreforming reactions. In the supercritical region’ the dielectric con-stant of water is much lower. Further, the number of hydrogenbonds is much smaller and their strength is considerably weaker.As a result’ SCW behaves as an organic solvent and exhibitsextraordinary solubility toward organic compounds containinglarge nonpolar groups and most permanent gases. Another advan-tage of SCW reforming is that the H2 is produced at a high pressure’which can be stored directly, thus avoiding the large energy expen-ditures associated with its compression. The SCWG process be-comes economical as the compression work is reduced owing tothe low compressibility of liquid feed when compared to that ofgaseous H2. Fig. 19 shows the schematic setup of the system forsupercritical water liquefaction of biomass.

In supercritical water gasification, the reaction generally takesplace at the temperature over 875 K and a pressure higher thanthe critical point of water. With temperature higher than 875 K,water becomes a strong oxidant, and oxygen in water can be trans-ferred to the carbon atoms of the biomass. As a result of the highdensity, carbon is preferentially oxidized into CO2 but also lowconcentrations of CO are formed. The hydrogen atoms of waterand of the biomass are set free and form H2. The gas product con-sists of hydrogen, CO2, CH4 and CO.

A problem of general nature in SCGW is the required heat ex-change between the reactor outlet and inlet streams. To achievean acceptable thermal efficiency, it is crucial for the process thatthe heat of the inlet stream is utilized as far as possible to pre-heatthe feedstock stream (mainly water) to reaction conditions. At thesame time, heating of the biomass slurry in the inlet tube of a reac-tor is likely to cause fouling/plugging problems because the ther-mal decomposition (>525 K) starts already far below the desiredreaction temperature (>875 K) [138,139]. In addition to being ahigh mass transfer effect, supercritical water also participates inreforming reaction. The molecules in the supercritical fluid havehigh kinetic energy like the gas and high density like the liquid.Therefore, it is expected that the chemical reactivity can be high

Fig. 19. Schematic setup of the system for supercritical water liquefaction ofbiomass.

20

30

40

50

60

610 630 650 670 690 710Temperature, K

Liq

uid

prod

uct,

wt%

Fig. 20. Plot for the yield of liquid products from liquefaction of corn stover at suband supercritical temperatures. Liquefaction time: 70 min. Water-to-solid ratio:5:1.


in it. In addition, ionic product and dielectric constant of supercrit-ical water, which are important parameters for chemical reactions,can be continuously controlled by regulating pressure and temper-ature. Pressure has a negligible effect on hydrogen yield above thecritical pressure of water. As the temperature is increased from 875to 1075 K the H2 yield increases from 53% to 73% by volume,respectively. Only a small amount of hydrogen is formed at lowtemperatures, indicating that direct reformation reaction of etha-nol as a model compound in SCW is favored at high temperatures(>975 K). With an increase in the temperature, the hydrogen andcarbon dioxide yields increase, while the methane yield decreases.The water excess leads to a preference for the formation of hydro-gen and carbon dioxide instead of carbon monoxide. The formedintermediate carbon monoxide reacts with water to hydrogenand carbon dioxide. The low carbon monoxide yield indicates thatthe water–gas-shift reaction approaches completion [138,139].

The capillaries (1 mm ID and 150 mm length tubular reactors)are heated rapidly (within 5 s) in a fluidized sand bed to the de-sired reaction temperature. Experimentation with the batch capil-lary method has revealed that, especially at low temperatures andhigh feed concentrations, char formation occurs. A fluidized bedreactor might be a good alternative to solve the problems relatedto this char and ash formation.

Cellulose and sawdust were gasified in supercritical water toproduce hydrogen-rich gas, and Ru/C, Pd/C, CeO2 particles, nano-CeO2, and nano-(CeZr)xO2 were selected as catalysts. The experi-mental results showed that the catalytic activities were Ru/C > Pd/C > nano-(CeZr)xO2 > nano-CeO2 > CeO2 particle in turn.The 10 wt.% cellulose or sawdust with CMC can be gasified nearcompletely with Ru/C catalyst to produce 2–4 g hydrogen yieldand 11–15 g potential hydrogen yield per 100 g feedstock at thecondition of 773 K, 27 MPa, 20 min residence time in supercriticalwater [140].

Catalysts for low-temperature gasification include combina-tions of stable metals, such as ruthenium or nickel bimetallicsand stable supports, such as certain titania, zirconia, or carbon.Without catalyst the gasification is limited [141]. Sodium carbon-ate is effective in increasing the gasification efficiency of cellulose[142]. Likewise, homogeneous, alkali catalysts have been employedfor high-temperature supercritical water gasification.

Hydrogen is a sustainable, non-polluting source of energy thatcan be used in mobile and stationary applications. In order to eval-uate hydrogen production by SCWG of various types of biomass,extensive experimental investigations have been conducted in re-cent years. The supercritical water reforming of biomass materials

has been found to be an effective technique for the production ofhydrogen with short residence times. Hydrogen production by bio-mass SCW is a promising technology for utilizing high moisturecontent biomass.

The product gases mainly consisted of hydrogen and carbondioxide, with a small amount of methane and carbon monoxidein SCW reforming. Hydrogen yields approaching the stoichiometriclimit were obtained under catalytic supercritical water conditions.Hydrogen production is influenced by temperature, residence time,and biomass concentration. The high yield of hydrogen was ob-tained at high reactor temperature, low residence time, and lowbiomass concentration. The gas product from biomass gasificationin catalytic supercritical water contains about 69% H2 and 30% CO2

in mole fraction. Others like CH4 and CO exist in the gas productwith fewer amounts. Since a drying process is an energy-intensiveoperation and the produced H2 will be stored under high pressure,supercritical water gasification is a promising technology for gasi-fying biomass with high moisture content.

Fig. 20 shows the plot for the yield of bio-oils from liquefactionof corn stover at sub- and supercritical temperatures. The yields ofbio-oils increase increasing temperature. The yield of bio-oils shar-ply increases between 650 K and 680 K and then it reaches a pla-teau. This temperature range (650–680 K) is in the supercriticalzone. At temperatures higher than 690 K the yield of bio-oils areslightly lower. The maximum yield of bio-oil is obtained at reactiontemperature 680 K. The highest bio-oil yield of 55.7% is obtained atreaction temperature 680 K and the yield is 53.3% at 720 K [27].

4. Biochemical processes

Biochemical conversion proceeds at lower temperatures andlower reaction rates and can offer high selectivity for products.Higher moisture feedstocks are generally good candidates for bio-chemical processes. Ethanol production is a biochemical conver-sion technology used to produce energy from biomass. Manyhighly efficient biochemical conversion technologies have devel-oped in nature to break down the molecules of which biomass iscomposed. For ethanol production, biochemical conversionresearchers have focused on a process model of dilute acid hydro-lysis of hemicelluloses followed by enzymatic hydrolysis of cellu-lose. Biodiesel production is a biochemical conversion technologyused to produce energy from oilseed crops [23–27,143–148].

Ethanol is the most widely used liquid biofuel. It is an alcoholand is fermented from sugars, starches or from cellulosic biomass.Cellulosic materials can be used to produce bioethanol. Bioethanolrepresents an important, renewable liquid fuel for motor vehicles.

Biomass

Acid Pretreatment

Enzymatic hydrolysis

Cellulase enzyme

Lignin

C5 sugars

C6 sugars Fermentation

Fermentation

Distillation

Ethanol

Stillage

Fig. 21. Enzymatic hydrolysis process.


Production of bioethanol from biomass is one way to reduce boththe consumption of crude oil and environmental pollution. Conver-sion technologies for producing ethanol from cellulosic biomass re-sources such as forest materials, agricultural residues and urbanwastes are under development and have not yet been demon-strated commercially. In order to produce bioethanol from cellu-losic biomass, a pretreatment process is used to reduce thesample size, break down the hemicelluloses to sugars, and openup the structure of the cellulose component. The cellulose portionis hydrolyzed by acids or enzymes into glucose sugar that is fer-mented to bioethanol. The sugars from the hemicelluloses are alsofermented to bioethanol. The use of bioethanol as a motor fuel hasas long a history as the car itself. It began with the use of ethanol inthe internal combustion engine [149].

Fermentation feedstocks require pretreatment by chemical,physical, or biological means to open up the structure of biomassand reduce the complex carbohydrates to simple sugars. This setof pretreatments is often referred to as hydrolysis. The resultingsugars can then be fermented by the yeast and bacteria employedin the process. Feedstocks high in starch and sugar are most easilyhydrolyzed. Cellulosic feedstocks, including the major fraction oforganics in MSW, are more difficult to hydrolyze, requiring moreextensive pretreatment.

Hydrolysis is often used to pretreat lignocellulosic feedstocks tobreak down the cellulose and hemicelluloses from the lignocellu-lose and break down the compounds into simple sugars. Hydrolysiscan be catalyzed by use of acids (either strong or weak), enzymes,and/or hydrothermal means, the latter including hot water andsupercritical methods. Fig. 21 shows the block diagram of the enzy-matic hydrolysis process.

Fermentation is generally used industrially to convert sub-strates such as glucose to ethanol for use in beverage, fuel, andchemical applications and to other chemicals (e.g., lactic acid usedin producing renewable plastics) and products (e.g., enzymes fordetergents). Strictly speaking, fermentation is an enzymaticallycontrolled anaerobic process although the term is sometimes moreloosely applied to include aerobic processing as well.

5. Conclusions

Biorefinery term refers to the conversion of biomass feedstockinto value-added chemicals and fuels with minimal waste andemissions. The biorefinery concept is analogous to today’s crudeoil refinery. Thermochemical and biochemical conversion productsfrom biomass are upgraded before ultimate refining processes.Biorefinery includes fractionation for separation of primary refin-ery products. The main goals of biorefineries are to integrate pro-duction of higher value chemicals and commodities, as well asfuels and energy, and to optimize use of resources, maximize prof-itability, maximize benefits and minimize wastes.

The benefits of an integrated biorefinery are numerous becauseof the diversification in feedstocks and products. There are cur-rently several different levels of integration in biorefineries whichadds to their sustainability, both economically and environmen-tally. Economic and production advantages increase with the levelof integration in the biorefinery.

Biorefineries can be classified into the following categoriesaccordingly their functions:

� Fast pyrolysis-based biorefineries.� Gasification based biorefineries.� Sugar based biorefineries.� Green biorefinery.� Energy crops biorefinery.� Oilseed biorefinery.� Forest based and lignocellulosic biorefinery.

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