biomass for energy

Journal of the Science of Food and Agriculture J Sci Food Agric 86:1755–1768 (2006)

ReviewBiomass for energyTony Bridgwater∗Bio-Energy Research Group, Aston University, Birmingham B4 7ET, UK

Abstract: Bio-energy is now accepted as having the potential to provide the major part of the projected renewableenergy provisions of the future as biofuels in the form of gas, liquid or solid fuels or electricity and heat.There are three main routes to providing these biofuels – thermal conversion, biological conversion and physicalconversion – all of which employ a range of chemical reactor configurations and designs. This review focuses onthermochemical conversion processes for their higher efficiencies, lower costs and greater versatility in providinga wide range of energy, fuel and chemical options. The technologies of gasification and fast pyrolysis are described,particularly the reactors that have been developed to provide the necessary conditions to optimise performance.The primary products that can be derived as gas, liquid and solid fuels are characterised, as well as the secondaryproducts of electricity and/or heat, liquid fuels and a considerable number of chemicals. 2006 Society of Chemical Industry

Keywords: biomass; bioenergy; pyrolysis; gasification; biofuels

INTRODUCTIONRenewable energy is of growing importance inresponding to concerns over the environment andthe security of energy supplies. Biomass is unique inproviding the only renewable source of fixed carbon,which is an essential ingredient in meeting many ofour fuel and consumer goods requirements. Wood andannual crops and agricultural and forestry residues aresome of the main renewable energy resources available.The biodegradable components of municipal solidwaste (MSW) and commercial and industrial wastesare also significant bio-energy resources, althoughparticularly in the case of MSW, they may requireextensive processing before conversion. Biomass isconsidered the renewable energy source with thehighest potential to contribute to the energy needs ofmodern society for both the developed and developingeconomies worldwide.1,2 In addition, energy frombiomass based on short rotation forestry and otherenergy crops can contribute significantly towardsthe objectives of the Kyoto Protocol in reducinggreenhouse gas emissions and alleviating problemsrelated to climate change.3

Biomass fuels and residues can be converted toenergy via the thermal, biological and mechanical orphysical processes summarised in Fig. 1.4 Thermalprocessing currently attracts the most interest inEurope and Canada while ethanol production is thefocus of attention in the USA for security of supplyreasons. Gasification has traditionally received themost RD&D support as it offers potentially higherefficiencies compared with combustion. Fast pyrolysisis still at a relatively early stage of development butoffers the benefits of a liquid fuel with concomitant

advantages of easy storage and transport as well ascomparable higher power generation efficiencies at thesmaller scales of operation that are likely to be achievedfrom bio-energy systems compared to fossil-fuelledsystems. Combustion systems are widely availableat domestic, small industrial and utility scales;biological conversion processes (fermentation anddigestion) and mechanical processing (e.g. vegetableoils) are well established and are all commerciallyoffered with performance guarantees. This reviewtherefore focuses on the thermal conversion processesof gasification and pyrolysis as they offer highconversion efficiencies, potentially competitive costs,and considerable flexibility in scale of operation andrange of products.

The key difference between thermal and biologicalconversion is that biological conversion gives singleor specific products such as ethanol or biogas(which contains up to 60% methane) and is a slowprocess, typically taking hours, days, weeks (anaerobicfermentation and farm digestion) or years (landfill gasby digestion) for reactions to be completed. Thermalconversion gives multiple and often complex products,with catalysts often used to improve the productquality or spectrum, and takes place in very shortreaction times of typically seconds or minutes. Table 1summarises some of the main products that can beobtained by processing biomass.

A commercial process for realisation of energy andfuel products from biomass comprises a biomassproduction system and five main stages in theconversion plant:4

1 Production of biomass as a short rotation coppice,such as willow; forest residues; annually harvested

∗ Correspondence to: Tony Bridgwater, Bio-Energy Research Group, Aston University, Birmingham B4 7ET, UKE-mail: [email protected](Received 20 December 2005; revised version received 22 March 2006; accepted 23 June 2006)Published online 31 July 2006; DOI: 10.1002/jsfa.2605

2006 Society of Chemical Industry. J Sci Food Agric 0022–5142/2006/$30.00

T. Bridgwater

Combustion

Electricity

Transportfuels etc

Heat

ChemicalsPyrolysis

Gasification

Fermentation

Digestion

Mechanical

Ethanol

Bio-gas

Rape oil

Heat

Thermalconversion

Biologicalconversion

Mechanicalconversion

Product Market

Fuel gas

Bio-oil

Figure 1. Conversion processes, products and applications.

Table 1. Major products from biomass conversion4

ProductBiological

processingPhysical

processingThermal

processing

FuelsAdditives

√ √ √Alcohols

√ √Charcoal

√Diesel-type fuels

√Fischer–Tropschliquids

√

Fuel oil√

Gas√ √

Gasoline√

Hydrogen√ √

ChemicalsAcetone

√Activated carbon

√Butanol

√Ethanol

√ √Fertilisers

√ √Fine chemicals

√ √ √Food additives

√ √Hydrogen

√ √Methane

√ √Methanol

√Resins

√

crops, such as miscanthus; and agricultural residues,such as straw. This includes harvesting, in-fieldprocessing such as chipping, and transport to theconversion plant

2 Feed reception, storage, handling and pre-treatmentto prepare the biomass for the subsequent conver-sion process

3 Conversion of solid biomass to a more usable formof energy by means such as gasification or pyrolysis

4 Primary product refining or clean-up

5 Conversion of the primary product to a marketableend-product such as electricity, heat, liquid biofuelsor chemicals

BIOMASS RESOURCESBiomass is a diffuse resource, arising over verylarge areas, and thus requiring large areas of landwith substantial logistical problems in collection andtransport as well as high costs. Typically, a sustainablecrop of 10 dry t ha−1 y−1 of woody biomass can beproduced in northern Europe, rising to perhaps 15 ormaybe 20 dry t ha\miknus;1 y−1 for energy cropsin southern Europe. Thus an area of 1 km2 or100 ha will produce 1000 dry t y−1, enough for a poweroutput of 150 kWe at low conversion efficiencies, or300 kWe at high conversion efficiencies. It is thereforedifficult to visualise power generation plants basedon indigenous biomass production much bigger thanaround 30–40 MWe anywhere in Europe and eventhese will require a planted area of around 100 km2.

A further complication with almost all forms ofbiomass is their seasonality: forestry and coppicedcrops can only be harvested during the winter months,and energy crops and agricultural residues are evenmore seasonal, typically growing for a only few monthsa year. Extensive provision for storage thus has to bemade. One solution to this problem is a multi-fuelsystem and increasing efforts are under way to developprocesses that can handle a number of different fuels,either mixed or separately.5 The current view is thateven these plants are limited in size, with a typical plantsize of 5–15 MWe likely to dominate the market in theshort term. However, in locations where extensiveindustrial operations are located close to managedforests, it is technically feasible and economicallyattractive to install large-scale bio-energy combinedheat and power (CHP) plants and burn the process

1756 J Sci Food Agric 86:1755–1768 (2006)DOI: 10.1002/jsfa

Biomass for energy

residues to provide heat for the local industry. Biomassand agricultural wastes are very similar in their arisings,with most European industries individually producingcomparable quantities of material, although overallregional and national totals may be substantial.

THERMAL CONVERSION PROCESSESThere are three main thermal processes – pyrolysis,gasification and combustion – available for convertingbiomass to a more useful energy form. Figure 2 sum-marises their products and applications. Combustionis already a well-established commercial technologywith applications in most industrialised and devel-oping countries and development is concentrated onresolving environmental problems. Combustion is wellestablished and widely practised with many examplesof dedicated plant and co-firing applications.6

GasificationFuel gas can be produced from biomass and relatedmaterials either by partial oxidation to give a mixtureof carbon monoxide, carbon dioxide, hydrogen andmethane with nitrogen if air is used as the oxidant, orby steam or pyrolytic gasification. Table 2 summarisesthe main products in each case.

The process of gasification is a sequence ofinterconnected reactions: the first step, drying, is arelatively fast process. The second step, pyrolysis, isalso relatively fast but it is a complex process thatgives rise to the tars that cause so many problemsin gasification processes. Pyrolysis occurs when asolid fuel is heated to 300–500 ◦C in the absenceof an oxidising agent, giving a solid char, condensablehydrocarbons or tar and gases. The relative yieldsof char, liquid and gas mainly depend on the rate ofheating and the final temperature, and this is discussed

later in the section on fast pyrolysis. In gasification bypartial oxidation, both the gas and liquid and solidproducts of pyrolysis then react with the oxidisingagent – usually air, although oxygen can be used – togive the permanent gases CO, CO2, H2, and lesserquantities of hydrocarbon gases. In steam or pyrolyticgasification, the char is burned in a secondary reactorto reheat the hot sand which provides the heat for thegasification.4

The composition of the gas from char gasificationand partial oxidation of the other pyrolysis productsis influenced by many factors, including feed com-position, water content, reaction temperature and theextent of oxidation of the pyrolysis products. However,the overall composition is essentially the equilibriumcomposition of the C–H–O system at the temperatureof gasification. The alkali metals in the biomass act aneffective catalyst to promote the gasification reactions.

Status of biomass gasification technologyA number of gasifier configurations have beendeveloped. A recent survey of gasifier manufacturersfound that 75% of gasifiers offered commercially were

Effi

cien

cy (

%)

50

40

30

20

10

00.1 1 10 100 1000

Output (MWe)

Fluid bedUpdraft

Down-draft

CFBEntrained flow

Figure 3. Relationship between technology, scale and efficiency forelectricity production.

TurbineGasification

Pyrolysis

Combustion Heat

Fuel gas

Bio-oil

ElectricityCHP

Storage

Boiler

Engine

Chemicals

Char Charcoal

Heat

Primaryproduct

Conversion MarketConversion

Storage

Figure 2. Thermal biomass conversion processes.

J Sci Food Agric 86:1755–1768 (2006) 1757DOI: 10.1002/jsfa

T. Bridgwater

Table 2. Modes of thermal gasification4

Method Comments

Partial oxidation with air The main products are CO, CO2, H2, CH4, N2 and tar, giving a low heating value gas of∼5 MJ m−3. Utilisation problems can arise in combustion, particularly in gas turbines.

Partial oxidation with oxygen The main products are CO, CO2, H2, CH4 and tar (no N2), giving a medium heating value gas of∼10–12 MJ m−3. The cost of providing and using oxygen is compensated by a better qualityfuel gas. The trade-off is finely balanced.

Steam (pyrolytic) gasification The main products are CO, CO2, H2, CH4 and tar giving a medium heating value gas of∼15–20 MJ m−3. The process has two stages: the primary reactor produces gas and char, andthe sand and char is passed to a second reactor where the char is burned with air to reheat thesand, which is then re-circulated to the first reactor to provide the heat for reaction. The gasheating value is maximised due to a higher methane and higher hydrocarbon gas content, but atthe expense of lower overall efficiency due to loss of carbon in the second reactor.

Pressure Pressurised gasifiers operate under pressures of typically 15–50 bar. Both capital and operatingcosts are significantly higher for pressurised operation, although these are to some extentbalanced by savings from reduced vessel and piping sizes, the avoidance of a gas compressorfor the gas turbine and higher efficiencies. Liquid feeding as bio-oil or slurry has significantoperational and economic advantages over solid biomass feeding. Pressurised operation is oftencarried out with oxygen.

Oxygen Use of oxygen, usually with pressure operation, gives higher reaction temperatures and hencelower tar levels; smaller and hence lower cost equipment from the absence of nitrogen; andhigher quality gas for both power generation and liquid fuel synthesis. There is, however, asignificant energy and financial cost associated with the use and supply of oxygen, from both itsprocurement and the additional measures needed to mitigate hazards in handling and use.

downdraft, 20% were fluid beds (including circulatingfluid beds), 2.5% were updraft and 2.5% wereother types.7 The range of gasifier technologies andtheir advantages and disadvantages are summarisedbelow4,7 and in Fig. 3.

Atmospheric downdraft gasifiers are attractive forsmall-scale applications up to about 1.5 MWth asthere is a large market in both developed anddeveloping economies. While some configurations stillhave problems with the effective removal of tar andparticulates, considerable progress has been madeand performance guarantees are increasingly offered.8

Biomass Engineering is one company that has madeconsiderable progress in this area with successfuloperation of engines and microturbines.9

Atmospheric updraft gasifiers seem to have littlemarket attractiveness for power applications. Whilethis may be due to the high tar levels in the fuelgas, recent developments in tar cracking have shownthat very low levels can be achieved from dedicatedthermal/catalytic cracking reactors downstream of thegasifier. Another possible reason is that the uppersize of a single unit is around 2.5 MWe so largerplant capacities require multiple units. There aretypically used for heat applications to maximise energyefficiency.10

Atmospheric bubbling fluidised bed gasifiers haveproven to be reliable with a variety of feedstocks atpilot scale and commercial applications in the smallto medium scale, up to about 25 MWth. They arelimited in their capacity size range as they have notbeen significantly scaled up and the gasifier diameteris significantly larger than that of circulating fluid bedsfor the same feedstock capacity. On the other hand,they are more economic for small to medium range

capacities. Their market attractiveness and technologystrength are thus relatively high, although there arefew operational examples.

Atmospheric circulating fluidised bed gasifiers haveproved very reliable with a variety of feedstocks andare relatively easy to scale up from a few MWth

to 100 MWth. Even for capacities above 100 MWth,there is confidence that the industry would be able toprovide reliable gasifiers. These gasifiers appear to bethe preferred system for large-scale applications andthese systems therefore have high market attractivenessand are technically well proven. Examples include theTPS (Termiska Processer AB) atmospheric process,11

and the Varnamo pressurised system.12

The most successful biomass gasifier currentlyoperating for power generation is the Gussing gasifierin Austria which is an indirect gasification systembased on high temperature pyrolysis to generate amedium heating value gas which is burned in an engineto generate 2 MWe.13 This has completed more than15 000 h of operation at the time of writing and has anavailability in excess of 90%.

Pressurised fluidised bed systems, either circulatingor bubbling, are considered of more limited marketattractiveness in the short term because of theirmore complex installation and the additional costsof construction of pressurised vessels. However,pressurised systems have an advantage in integratedcombined cycle applications as the need to compressthe fuel gas prior to utilisation in the combustionchamber of the gas turbine is avoided. Pressurisedsystems are often used with oxygen as the oxidantto improve the gas quality. There is, however, asignificant energy and financial cost associated with theuse and supply of oxygen, from both its procurement


Biomass for energy

and the additional measures needed to mitigatehazards in handling and use.

Very large scale gasification systems currentlybeing considered for processing millions of tonnesof biomass a year would probably be pressurisedoxygen blown systems. There are logistical problemsof biomass supply that require resolution, but areessential for economic synthesis of transport fuels.These technologies would also be applied to pyrolysisliquid gasification, when liquefaction of biomasswould reduce the handling and transport costs ofsolid biomass and simply the gasification process.Consideration is also being given to co-gasificationof biomass with coal and extensive trials have alreadydemonstrated the feasibility of this approach.

Fuel gas qualityThe fuel gas quality requirements, for turbines andliquid fuel synthesis in particular, are very high;Table 3 gives some suggested figures for commongasifiers.14 Tar is a particular problem and remainsthe most significant technical barrier. There are twobasic ways of destroying tars, both of which have beenand continue to be extensively studied:15

• Catalytic cracking using, for example, dolomite ornickel

• Thermal cracking, for example by partial oxidationor direct contact

The gas is very costly to store or transport so it has to beused immediately. Hot gas efficiencies for the gasifier(total energy in the raw product gas as a fraction of

the energy in the feed) can be as high as 95–97% forclose-coupled turbine and boiler applications, and upto 85% for cold gas efficiencies. In power generationusing combined cycle operation, efficiencies of up to50% for the largest installations have been proposed,reducing to 35% for smaller applications.

Gas clean-upGases formed by gasification will be contaminatedby some or all of the constituents listed in Table 4.The level of contamination will vary depending on thegasification process and the feedstock. Gas cleaningmust be applied to prevent erosion, corrosion andenvironmental problems in downstream equipment.4

Applications of product gasFigure 4 summarises the range of fuel, electricityand chemical products that can be derived fromthe product gas. Medium heating value gas fromsteam or pyrolytic gasification, or from oxygengasification, is better suited to synthesis of transportfuels and commodity chemicals because of theabsence of diluent nitrogen, which would pass throughunchanged but reduce process efficiency and increasecosts. The exception is ammonia synthesis, where thenitrogen content derived from air gasification can beutilised in the ammonia synthesis process.

Transport fuels and other chemicalsAs biomass is the only renewable source of fixedcarbon, there is considerable interest in the productionof transport fuels and other commodity chemicals

Table 3. Typical product gas characteristics from different gasifiers14

Gas composition, dry, vol% Gas quality

H2 CO CO2 CH4 N2 HHV∗ (MJ N−1 m3) Tars Dust

Fluid bed air-blown 9 14 20 7 50 5.4 Fair PoorUpdraft air-blown 11 24 9 3 53 5.5 Poor GoodDowndraft air-blown 17 21 13 1 48 5.7 Good FairDowndraft oxygen 32 48 15 2 3 10.4 Good GoodTwin fluid bed 31 48 0 21 0 17.4 Fair PoorPyrolysis for comparison 40 20 18 21 1 13.3 Poor Good

∗ HHV: higher heating value.

Table 4. Fuel gas contaminants and their problems14

Contaminant Examples Problems Solution

Tars Refractive aromatics Clogs filters Difficult to burnDeposits internally

Tar cracking thermally or catalytically,or tar removal by scrubbing

Particulates Ash, char, fluidised bedmaterial

Erosion Filtration, scrubbing

Alkali metals Sodium, potassiumcompounds

Hot corrosion Cooling, condensation, filtration,adsorption

Fuel-bound nitrogen Mainly ammonia and HCN NOx formation Scrubbing, Selective catalytic removal(SCR)

Sulfur, chlorine HCl, H2S Corrosion emissions Lime or dolomite, scrubbing,absorption


T. Bridgwater

MHV gas

Turbine

Engine

Boiler

Gasification

LHV gas Electricity

Steam oroxygen

Air

Conversion

Transportfuels etc

Fuel cell

Heat

Synthesis

Chemicals

Co-firing

Ammonia andfertilisers

Figure 4. Applications for biomass gasification systems. MHV, medium heating value, typically 15 MJ/Nm3; LHV, low heating value, typically 5MJ/Nm3.

Gasification,clean-up,

conditioningBio-oilor

slurry

Methanol synthesis+ MTG or MOGD

Solidbiomass

Hydrocarbonsynthesis

Transportfuels

Refining

Chemicals

Electricity

Figure 5. Transport fuels via biomass gasification. MTG, methanol to gasoline; MOGD, methanol to olefins, gasoline and diesel.

through synthesis gas or syngas, as it is usuallyknown. Syngas is a mixture of carbon monoxide(CO) and hydrogen (H2). There are usually othercomponents arising from gasification such as carbondioxide (CO2), methane (CH4), higher hydrocarbonssuch as ethylene and ethane, propane and propylene,and nitrogen from air gasification. Generally these actas diluents, but different generic and specific processeshave different levels of tolerance for each component.There will also be trace contaminants containingsulfur (e.g. H2S), chlorine (e.g. HCl, COCl) andnitrogen (e.g. NH3) in a range of compounds. Theconcentrations of these trace components will usuallyrequire reduction to a few parts per million for mostcatalyst systems used in synthesising alcohols andhydrocarbons, and each catalyst has its own limitationsand tolerances.

Either solid biomass can be gasified, which forEuropean-derived biomass will tend to limit the size ofplant to the availability of biomass unless considerable

biomass is imported, or bio-oil from fast pyrolysis canbe gasified which has a lower overall efficiency butenables the necessary economies of scale to be realisedin the downstream synthesis of transport fuels andchemicals. Gasification of bio-oil is also potentiallylower cost as it is easier to feed a liquid into apressurised gasifier than solid biomass.

Syngas provides the raw material for the produc-tion of virtually every fuel and chemical in use today,including conventional and unconventional transportfuels, commodity chemicals and speciality chemicals.Some of the possibilities for production of hydrocar-bon transport fuels are shown in Fig. 5, which are ofconsiderable topical significance.

PYROLYSISPyrolysis is thermal decomposition occurring in theabsence of oxygen. It is also always the first step incombustion and gasification, but in these processes

Table 5. Typical product yields (dry wood basis) obtained by different modes of pyrolysis of wood

Mode ConditionsLiquid

(%)Char(%)

Gas(%)

Fast Moderate temperature, around 500 ◦C, Short hot vapour residence time, ∼1 s 75 12 13Intermediate Moderate temperature, around 500 ◦C, Moderate hot vapour residence time ∼10–20 s 50 20 30Slow (carbonisation) Low temperature, around 400 ◦C, very long residence time 30 35 35Gasification High temperature, around 800 ◦C, long residence times 5 10 85


Biomass for energy

it is followed by total or partial oxidation of theprimary products. Lower process temperatures andlonger vapour residence times favour the productionof charcoal. High temperatures and longer residencetimes increase biomass conversion to gas, andmoderate temperatures and short vapour residencetime are optimum for producing liquids. Table 5indicates the product distribution obtained fromdifferent modes of pyrolysis. Fast pyrolysis for liquidsproduction is currently of particular interest becauseliquids can be stored and transported more easily andat lower cost than solid biomass. A number of reviewshave been published to which reference should bemade for detailed information.16–19

Fast pyrolysis occurs in a time of a few secondsor less. Therefore heat and mass transfer processesand phase transition phenomena, as well as chemicalreaction kinetics, play important roles. The criticalissue is to bring the reacting biomass particles tothe optimum process temperature and minimise theirexposure to the intermediate (lower) temperatures thatfavour formation of charcoal. One way this objectivecan be achieved is by using small particles, for examplein the fluidised bed processes that are described later.Another possibility is to transfer heat very fast onlyto the particle surface that contacts the heat sourceas applied in ablative pyrolysis. A critical technicalchallenge in every case is heat transfer to the reactorin commercial systems.

Principles of fast pyrolysisIn fast pyrolysis, biomass decomposes to generatemostly vapours and aerosols and some charcoal. Aftercooling and condensation, a dark brown mobile liquidis formed which has a heating value about half thatof conventional fuel oil. While it is related to thetraditional pyrolysis processes for making charcoal,fast pyrolysis is an advanced process, with carefullycontrolled parameters to give high yields of liquid.The essential features of a fast pyrolysis process forproducing liquids are:

• Very high heating and heat transfer rates at thereaction interface, which usually requires a finelyground biomass feed

• Carefully controlled pyrolysis reaction temperatureof around 500 ◦C and vapour phase temperatureof 400–450 ◦C; the effect of temperature on yields

and product spectrum is discussed in the section onpyrolysis liquid below

• Short hot vapour residence times of typically lessthan 2 s

• Rapid cooling of the pyrolysis vapours to give thebio-oil product

The main product, bio-oil, is obtained in yieldsof up to 75% wt on a dry-feed basis, together withby-product char and gas, which are used within theprocess to provide the process heat requirements sothere are no waste streams other than flue gas and ash.

A fast pyrolysis process includes drying the feed totypically less than 10% water in order to minimise thewater in the product liquid oil (although up to 15%can be acceptable), grinding the feed (to around 2 mmparticle size in the case of fluid bed reactors) to givesufficiently small particles to ensure rapid reaction,pyrolysis reaction, separation of solids (char), andquenching and collection of the liquid product (bio-oil).

Virtually any form of biomass can be considered forfast pyrolysis. While most work has been carried outon wood because of its consistency and comparabilitybetween tests, nearly 100 different biomass typeshave been tested by many laboratories, ranging fromagricultural wastes such as straw, olive pits andnut shells to energy crops such as miscanthus andsorghum, forestry wastes such as bark and solid wastessuch as sewage sludge and leather wastes.

A typical fast pyrolysis process is depicted in Fig. 6showing the necessary preparation steps, alternativereactors and product collection.

ReactorsAt the heart of a fast pyrolysis process is the reactor.Although it probably represents, at most only about10–15% of the total capital cost of an integratedsystem, most research and development has focused onthe reactor, although increasing attention is now beingpaid to control and improvement of liquid qualityand improvement of collection systems. The rest ofthe process consists of biomass reception, storageand handling, biomass drying and grinding, productcollection, storage and, when relevant, upgrading. Thekey aspects of these peripheral steps are described later.A comprehensive survey of fast pyrolysis processes for

BIOMASS BIO-OIL

Cool andcollect

PyrolysisFluid bed

CFBTransported bed

Rotating coneEntrained flow

Ablativeetc Char

Dryingto <10% water

Gas

GrindingTo < ~ 3mm

Charseparation

Figure 6. Conceptual fast pyrolysis process.


T. Bridgwater

liquids production that have been built and tested inthe last 10–15 years has been published.17

Bubbling fluid bedsBubbling fluid beds – usually referred to as just fluidbeds as opposed to circulating fluid beds – havethe advantages of a well understood technologythat is simple in construction and operation, goodtemperature control and very efficient heat transferto biomass particles arising from the high solidsdensity. Fluid-bed pyrolysers give good and consistentperformance with high liquid yields of typically70–75% wt from wood on a dry-feed basis. Smallbiomass particle sizes of less than 2–3 mm are neededto achieve high biomass heating rates, and the rate ofparticle heating is usually the rate-limiting step.

Residence time of solids and vapours is controlledby the fluidising gas flow rate and is higherfor char than for vapours. As char acts as aneffective vapour cracking catalyst at fast pyrolysisreaction temperatures, rapid and effective charseparation/elutriation is important. This is usuallyachieved by ejection and entrainment followed byseparation in one or more cyclones so careful design ofsand and biomass/char hydrodynamics is important.

The earliest pioneering work on fast pyrolysiswas carried out at the University of Waterloo byScott and colleagues20–22 who published extensively.The largest plant currently operating is that byDynamotive in West Lorne, Ontario, Canada, whichhas a demonstration plant that produces 100 t d−1 ofdry biomass feed, with plans for further plants up to400 t/d.23 A 2.5 MWe gas turbine is also provided onsite for generation of power locally and for export tothe grid.

Circulating fluid beds and transported bedCirculating fluid beds (CFBs) have many of thefeatures of bubbling beds described above, exceptthat the residence time of the char is almost thesame as for vapours and gas, and the char is moreattrited due to the higher gas velocities, which canlead to higher char contents in the collected bio-oil. An added advantage is that CFBs are potentiallysuitable for very large throughputs even though thehydrodynamics are more complex – this technology iswidely used at very high throughputs in the petroleumand petrochemical industries. However, heat transferat higher throughputs has not been demonstratedand offers some challenges.4 Heat supply is usuallyfrom recirculation of heated sand from a secondarychar combustor, which can be either a bubbling orcirculating fluid bed. In this respect the process issimilar to a twin fluid-bed gasifier except that thereactor (pyrolyser) temperature is much lower and theclosely integrated char combustion in a second reactorrequires careful control to ensure that the temperatureand heat flux match the process and feed requirements.

A variation on the transported bed is the rotatingcone reactor, invented at the University of Twente24

and implemented by BTG in the Netherlands. In thisconfiguration, the transport is effected by centrifugalforces rather than gas. A 50 t d−1 plant has been builtin Malaysia and was commissioned in summer 2005.

Ablative pyrolysisAblative pyrolysis is substantially different in conceptcompared with other methods of fast pyrolysis.4 In allthe other methods, the rate of reaction is limitedby the rate of heat transfer through the biomassparticles, which is why small particles are required.The mode of reaction in ablative pyrolysis is likemelting butter in a frying pan – the rate of meltingcan be significantly enhanced by pressing the butterdown and moving it over the heated pan surface. Inablative pyrolysis, heat is transferred from the hotreactor wall to ‘melt’ wood that is in contact withit under pressure. The pyrolysis front thus movesunidirectionally through the biomass particle. As thewood is mechanically moved away, the residual oilfilm both provides lubrication for successive biomassparticles and also rapidly evaporates to give pyrolysisvapours for collection in the same way as otherprocesses. The rate of reaction is strongly influenced bypressure, the relative velocity of the wood and the heatexchange surface and the reactor surface temperature.The key features of ablative pyrolysis are therefore asfollows:

• High pressure of particle on hot reactor wall,achieved due to centrifugal force

• High relative motion between particle and reac-tor wall

• Reactor wall temperature less than 600 ◦C

As reaction rates are not limited by heat transferthrough the biomass particles, large particles can beused and, in principle, there is no upper limit to thesize that can be processed. In fact, the process islimited by the rate of heat supply to the reactor ratherthan the rate of heat absorption by the pyrolysingbiomass, as in other reactors. There is no requirementfor inert gas, so the processing equipment is smallerand potentially lower cost. However, the process issurface-area-controlled so scaling is more costly andthe reactor is mechanically driven, and is thus morecomplex. A 50 t d−1 demonstration plant has recentlystarted operating in north Germany25 and a smallresearch unit operates at Aston University.26

Entrained flowEntrained flow fast pyrolysis is, in principle, a simpletechnology, but most developments have not beenas successful as had been hoped, mostly because ofthe poor heat transfer between a hot gas and a solidparticle. High gas flows are required to effect sufficientheat transfer, which requires large plant sizes andentails difficult liquid collection from the low vapourpartial pressure. Liquid yields have usually been lowerthan fluid bed and CFB systems.


Biomass for energy

By-productsCharcoal and gas are by-products, typically containingabout 25 and 5%, respectively, of the energy in the feedmaterial. The pyrolysis process itself requires about15% of the energy in the feed, and of the by-products,only the char has sufficient energy to provide this heat.The heat can be derived by burning the gas and/or thecharcoal byproduct, More advanced configurationscould gasify the char to a (LHV) gas and then burnthe resultant gas more effectively to provide processheat with the advantage that the alkali metals in thechar can be much better controlled and avoid potentialslagging problems from direct char combustion.

Pyrolysis liquid (bio-oil)Crude pyrolysis liquid, or bio-oil, is dark brown andapproximates to biomass in elemental composition. Itis composed of a very complex mixture of oxygenatedhydrocarbons with an appreciable proportion of waterfrom both the original moisture and reaction product.Solid char may also be present. The product spectrumfrom aspen wood and the high dependence ontemperature is shown in Fig. 7.

The liquid is formed by rapidly quenching andthus ‘freezing’ the intermediate products of flashdegradation of hemicellulose, cellulose and lignin.The liquid thus contains many reactive species,which contribute to its unusual attributes. Bio-oil can be considered a micro-emulsion in whichthe continuous phase is an aqueous solution ofholocellulose decomposition products, which stabilisesthe discontinuous phase of pyrolytic lignin macro-molecules through mechanisms such as hydrogenbonding.

Fast pyrolysis liquid has a higher heating value ofabout 16–17 MJ kg−1 as produced with about 25% wtwater that cannot readily be separated. It is composedof a complex mixture of oxygenated compoundsthat provide both the potential and challenge forutilisation. There are some important characteristicsof this liquid that are summarised in Tables 6 and 7and are discussed briefly below, of which the most

0

10

20

30

40

50

60

70

550450 475 500 525 575 600 625 650 675425

Reactor temperature (°C)

Yie

ld o

n d

ry f

eed

(w

t%)

Organics

Char

GasWater

Figure 7. Variation of products from aspen poplar withtemperature.27

Table 6. Typical properties of wood-derived crude bio-oil

Physical property Typical value

Moisture content 25%pH 2.5Specific gravity 1.20Elemental analysis

C 56%H 6.5%O 37.5%N 0.1%Ash 0%

∗HHV as produced 17 MJ kg−1

Viscosity (40 ◦C and 25% water) 50 cPSolids (char) 0.1%Vacuum distillation residue up to 50%

∗ HHV: higher heating value.

Table 7. Characteristics of wood-derived crude bio-oil

• Liquid fuel• Ready substitution for conventional fuels in many stationary

applications such as boilers, engines, turbines• Heating value of 17 MJ kg−1 at 25% wt water, is about

40% that of fuel oil/diesel• Does not mix with hydrocarbon fuels• Not as stable as fossil fuels• Quality needs definition for each application

significant is that it will not mix with any conventionalhydrocarbon-based fuels.

Pyrolysis oil typically is a dark brown, free-flowingliquid. Depending on the initial feedstock and themode of fast pyrolysis, the colour can be almostblack through dark red–brown to dark green, beinginfluenced by the presence of micro-carbon in theliquid and chemical composition. Hot vapour filtrationgives a more translucent red–brown appearance owingto the absence of char. High nitrogen content canimpart a dark green tinge to the liquid.

The liquid has a distinctive odour – an acrid smokysmell due to the low molecular weight aldehydesand acids – which can irritate the eyes on prolongedexposure. The liquid contains several hundreddifferent chemicals in widely varying proportions,ranging from formaldehyde and acetic acid to complexhigh molecular weight phenols, anhydrosugars andother oligosaccharides.

The liquid contains varying quantities of water,which forms a stable single-phase mixture, rangingfrom about 15 wt% to an upper limit of about 30–50wt% water, depending on the feed material, how it wasproduced and subsequently collected. A typical feedmaterial specification is a maximum of 10% moisturein the dried feed material, as both this feed moistureand the water of reaction from pyrolysis, typicallyabout 12% based on dry feed, both report to the liquidproduct. Pyrolysis liquids can tolerate the additionof some water, but there is a limit to the amountof water, which can be added to the liquid beforephase separation occurs, in other words the liquid


T. Bridgwater

cannot be dissolved in water. The addition of waterreduces viscosity, which is useful; reduces heatingvalue, which means that more liquid is required tomeet a given duty; and can improve stability. Theeffect of water is therefore complex and important.It is miscible with polar solvents such as methanoland acetone, but is totally immiscible with petroleum-derived fuels.

The density of the liquid is very high at around1.2 kg L−1, compared with light fuel oil at around0.85 kg L−1. This means that the liquid has about42% of the energy content of fuel oil on a weightbasis, but 61% on a volumetric basis. This hasimplications for the design and specification ofequipment such as pumps and atomisers in boilersand engines.

Pyrolysis liquids cannot be completely vaporisedonce they have been recovered from the vapourphase. If the liquid is heated to 100 ◦C or more totry to remove water or distil off lighter fractions, itrapidly reacts and eventually produces a solid residue

of around 50 wt% of the original liquid and somedistillate containing volatile organic compounds andwater. While bio-oil has been successfully stored forseveral years in normal storage conditions in steeland plastic drums without any deterioration thatwould prevent its use in any of the applicationstested to date, it does change slowly with time, mostnoticeably there is a gradual increase in viscosity.Recent samples that have been distributed for testinghave shown substantial improvements in consistencyand stability.

Applications of bio-oilBio-oil can substitute for fuel oil or diesel in manystatic applications including boilers, furnaces, enginesand turbines for electricity generation.28 Figure 8summarises the possibilities. A range of chemicalsincluding food flavourings, specialities, resins,29 agri-chemicals, fertilisers, and emissions control agents canalso be extracted or derived from bio-oil. At least 400 hoperation has been achieved on a 250 kWe specially

Liquid

Turbine

Engine

Boiler

Fastpyrolysis

Charcoal

Electricity

ExtractionConversion

Transportfuels etc

Charcoalapplications

Upgrading

Chemicals

Gas

Co-firing Heat

Processheat

Pyrolysisheat

Figure 8. Applications for products of fast pyrolysis.

H2

Hydro-treating

Zeolitecracking

Hydrogenseparation

Chemicals

Electricity

Fastpyrolysis

Gasification

Synthesis Transportfuels

RefiningSlurryChar

Liquid

Figure 9. Transport fuels via biomass pyrolysis.


Biomass for energy

GascombustionGasification

Oilcombustion

Charcoal

Pyrolysis

Solid fuelcombustion

(Mixing)Storage

Bio-oil

Fuel gas

StorageBiomass

Fuel gas

Figure 10. Opportunities for co-processing biomass and biofuels in conventional heat and power applications.

modified dual-fuel engine and experience has beengained on a modified 2.5 MWe industrial gas turbine.30

As noted above, upgrading bio-oil to transportationfuels is feasible but not currently economic. A possibleroute is shown in Fig. 9.

Co-firing and co-processingCo-processing of biomass with conventional fuels ispotentially a very attractive option that enables fulleconomies of scale to be realised as well as reducingthe requirements for product quality and clean-up.6

The opportunities are summarised in Fig. 10. Atpresent, co-firing offers the best opportunities formarket penetration of biomass as the overall costs arerelatively low because the power cycle in the coal-firedpower plant is already there.

Almost all co-firing is based on addition of solidbiomass to the coal prior to combustion in theboilers. There are a number of examples where thebiomass is first converted to a fuel gas via gasification,which is directly burned in an existing coal-firedboiler. Trials have also taken place of combustionof fast pyrolysis liquids in both coal fired and gasfired power stations. These latter options have theadvantage that the biomass residual ash is not mixedwith the coal ash, which has an existing market as aconstruction material and can thus be recycled back tothe forest or field. In re-burning applications, (wherethe fuel gas is introduced almost at the top of thecoal boiler), the environmental performance of thepower station is significantly improved, in addition tothe replacement of fossil fuels by renewable biomassfuels.

BIOLOGICAL CONVERSION PROCESSESEthanolIn order to produce ethanol by fermentation, thecellulose and hemicellulose in lignocelluloses needto be hydrolysed to sugars prior to fermentation,and both enzyme and acid hydrolysis are employedfor this purpose.31,32 Carbohydrates such as starchalso require hydrolysis. Only the cellulose in biomassis conventionally converted to ethanol, althoughincreasing success is being obtained with conversion

of hemicellulose, which will improve conversionefficiency and reduce costs. The lignin is a residue thatcan be burned for process heat, particularly for ethanolconcentration, or further processed for production ofrefinery feedstocks or aromatic chemicals. It is thisintegration of energy, fuels and chemicals that isincreasingly considered as a biorefinery, which canbe simply explained as an optimised system of utilisingbiomass in technical, economic, environmental andsocial terms.

The overall efficiencies of ethanol production arerather low, owing to the loss of about half the carbonin holocellulose as carbon dioxide; the loss of thecarbon in lignin, which is not converted; and the needto concentrate the dilute ethanol solution.

Fermentation is particularly suitable for materialswith high moisture content, as drying is not required.Ethanol can be readily converted to ethyl tertiarybutyl ether (ETBE), which can be directly usedas a gasoline additive. Fermentation technology iscommercial and tends to be high cost unless subsidiesare applied and low efficiency unless credits are appliedto the by-products. After a lull in RD&D, there isnow renewed interest in bio-ethanol, with enzymehydrolysis in particular attracting more attention. Asubstantial demonstration plant can be expected soon.Currently there is little RD&D because the technologyis established and viability relies on financial support.

BP and DuPont announced an initiative in June2006 to develop bio-butanol from fermentation ofsuitable biomass feedstocks as an additive for transportfuels, which offers improved properties compared toethanol.33

BiodieselBiodiesel is the ester formed by reacting vegetable oilsor animal fats with methanol or ethanol. Vegetable oilcan be recovered from oil seeds such as rape (canolaor colza), or other crops such as linseed, sunflower,soy etc. The product in its raw form is unsuitable formany applications because it is highly viscous and hasother deleterious properties, so the methyl or ethylester is formed by esterification. However, there havebeen limited attempts to use the cold-pressed oil astransport fuel.


T. Bridgwater

The most common product is rape methyl ester(RME). The raw oil is recovered by pressing, usuallyaccompanied by solvent extraction to improve yields.This raw oil is then subjected to esterification withmethanol or ethanol over a catalyst giving a lowerviscosity and more stable product and glycerine asa by-product. The ester is entirely compatible withdiesel in use and applications and biodiesel is thus anattractive renewable transport fuel.

The yield of vegetable oil per unit land areais, however, very low at around 1–2 t ha−1 y−1 andis accompanied by around 5–8 t ha−1 y−1 of solidresidues such as rape straw, which is not usuallyutilised. The product cost is therefore high. Theconventional conversion technology is semi-batch butsimple and proven, but there have been a numberof recent developments of fully continuous processesthat can handle vegetable oils and waste oils andfats. Animal fats and waste cooking oils can also beprocessed in an analogous way and, while the scopeis limited due to limited raw material availability, thisroute offers significant economic opportunities whilethe waste materials continue to be available at lowcost.

Anaerobic digestionAnaerobic digestion is microbial conversion of organicmaterials to methane and carbon dioxide in theabsence of oxygen. The product gas from farmdigesters is typically around 60% methane, althoughhigher levels have been reported. The typical methanecontent from landfill sites is lower at 50–55%.Digestion is particularly suitable for residues withhigh moisture contents, as drying is not required. Thebiogas can be used for heat with minimal processing, orfor power generation in engines or turbines and (afterupgrading to methane quality by removal of carbondioxide and other components) in fuel cells or asgaseous fuel for transport applications. This requiresincreasingly stringent quality specifications. Landfillsites tend to be very large and typically produce gasover a 20–25 year life. Gas is collected through asystem of wells, which collect and pipe the gas to theuser.

The gas is wet and contains acid components whichrequire careful management to avoid or minimiseproblems in using the gas for heat and/or powerproduction. As landfill gas collection is required inmany countries to comply with legislation to minimisehazards, the additional costs of distributing gas tousers is very small, so landfill gas is an economicallyattractive renewable energy resource. However, aspressure grows to reduce land filling by legislationin the European Union, this resource is expectedto reduce in the long term. On-farm or industrialdigesters using farm wastes, industrial food wastes andhousehold wastes are becoming more widely used forsmaller scale applications. The wastewater industry,in particular, has successfully and effectively useddigestion for in-plant power generation for many years.

Dedicated digesters tend to be costly and inefficientas well making a significant amount of solid residuesthat must be disposed of. Thus small-scale digestion isnot often cost-effective. Although there is widespreadinterest in anaerobic digestion for both waste disposaland energy generation, economic opportunities arelimited to niche applications.

New developments in solid-state fermentation haveallowed the content of the dry solids of the substrateto be as high as 30–35 wt%, compared with only3–10 wt% for traditional liquid phase fermentation.This increases biogas productivity by a factor of about3, and reduces digester volumes. This technologyhas found applications in the processing of thebiodegradable fraction of MSW or source-separatedwaste. In all types of digestion, the carbon dioxidecomponent can be reduced or removed to give higherquality gas, up to natural gas qualities. There is arange of available technologies, including membraneprocesses and conventional scrubbing processes,according to the scale of operation. All are costly, andcareful assessment of the added value of the resultanthigher quality gas is needed. The purified methane caneither be used as transport fuel or added to a naturalgas pipeline.

BIOREFINERYAfter many years of production of chemicals from bio-oil, there is rapidly growing interest in the biorefineryconcept in which fuels and chemicals are optimallyproduced by technical, economic, environmental andsocial criteria.34 Recent examples of a biorefineryinclude utilisation of heavy residues from liquidsmoke production for co-firing in a power stationand production of hydrogen by steam reforming ofthe aqueous residues from recovery of phenolics forresin production. The key feature and objective isoptimum utilisation of products, by-products andwastes as shown in Fig. 11. Some of the alternativesfor achieving this optimum for production of transportfuels and chemicals are shown in Fig. 12.

CONCLUSIONSThere is substantial and growing interest in thermalprocessing of biomass for biofuels, to make bothenergy and chemicals. Gasification and pyrolysisare complementary processes that have differentmarket opportunities and should not be viewedas competitors. In both technologies, there is

BiomassChemicals

CommoditiesFuels

By-products and wastes

Fastpyrolysis

Secondaryprocessing

Tertiaryprocessing

Figure 11. Biorefinery concept.


Biomass for energy

Pyrolysis

Distillation

Gasification Synthesis

Hydro-processing

Biomass

Surpluselectricity

(waste) water

Ethanol

Transport fuelsChemicals

FuelsChemicals

Transport fuelsChemicals

Fermentation

Electrolysis

Lignin

H2

Wet gasification

Power and heat frombyproducts and waste

Figure 12. A biorefinery system with processing options for fuels and chemicals.

still considerable progress to be made in optimalinterfacing of conversion and utilisation of the primaryproducts from conversion, as well as the importantinterface between biomass production and conversionthat has been largely left to market forces.

The main challenges lie first in bringing the thermalconversion technologies closer to the power generationor chemicals production processes, with both sidesof the interface moving to an acceptable middleposition, and second, in fully appreciating that, withrare exceptions, bio-energy systems will always berelatively small and must therefore be technicallyand economically competitive at much smaller scalesof operation than the process and power generationindustries are used to handling.

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