Worldwide Use of Biomass in Power Generation and Combined Heat and Power Schemes

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2002 216: 41Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and EnergyI. M. Arbon

Worldwide use of biomass in power generation and combined heat and power schemes

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Worldwide use of biomass in power generation andcombined heat and power schemes

I M ArbonEngineered Solutions, 15 Newtown, Easton on the Hill, Stamford, Lincolnshire PE9 3NR, UK

Abstract: Biomass is a truly renewable, sustainable source of energy; in its �rewood form, at least, it hasalways been humanity’s primary fuel. Nevertheless, it is only in the very recent past that it has been regardedas a viable substitute in power generation for the fossil fuels that have caused most of the world’senvironmental pollution problems. This paper distinguishes between truly renewable, sustainable sourcesof fuel from agricultural sources, i.e. biomass, and the disposal of domestic, urban and hazardous waste inenergy-from-waste (EfW) plants; although these differences may appear to be marginal, and any EfW plantis of value for power generation, there are particular reasons why the generation of power from genuinebiomass reaps environmental bene�ts.

The bulk of the paper discusses the generation of electric power from a variety of different biomasssubstances, some from purpose-grown ‘energy crops’ but mostly from ‘agricultural residues’. While this ispredominantly through conventionalcombustion systems with steam turbines, more recent experience of bothgasi�cation and pyrolysis, with power generation by other prime movers, such as gas turbines andreciprocating engines is also covered. The concluding section of the paper looks brie�y at the relative bene�tsof combustion, gasi�cation and pyrolysis and what the future is likely to hold for each of these technologies.

Keywords: biomass, renewable energy, gasi�cation, pyrolysis, anaerobic digestion, liquefaction, combinedheat and power (CHP)

1 INTRODUCTION

Biomass is but one form of the currently in vogue ‘renewable’fuels for power generation. The term ‘renewable’ itself hasbeen coined to describe fuels which are renewable in theshort term (and therefore ‘sustainable’), as opposed toso-called fossil fuels (e.g. coal, oil, natural gas, uranium,etc.), stocks of which are rapidly diminishing, and whichrequire millions of years to ‘renew’ themselves.

Interest in renewable energy, based on non-fossil fuels, iscurrently largely focused on (see UK Government’s 1999Consultation Document New and Renewable Energy):

(a) biofuels and biomass (including municipal solid waste(MSW), land�ll gas, agricultural residues, energy crops);

(b) advanced fuel cells (strictly not ‘renewable energy’ butenergy conversion);

(c) solar (passive, active and photovoltaic);(d) water (including hydro, tidal, wave and underwater

current);(e) Wind (onshore, offshore);(f) geothermal.

Renewable and waste fuels are similar and overlap; itcould be argued that a truly renewable fuel is wasted if notutilized and a waste fuel such as MSW is renewable andsustainable while human beings dispose of garbage. Thereare numerous different de�nitions of what constitutes a‘waste’ and what a ‘renewable’ fuel; similarly, there arediffering views on what is meant by ‘biomass’. In this paper,the terms ‘biomass’ and ‘biofuel’ are interpreted as fuels

The MS was received on 5 April 2001 and was accepted after revision forpublication

SPECIAL ISSUE PAPER 41

A01701 # IMechE 2002 Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy

Table 1 De�nition of ‘renewable’ and ‘waste’ fuel sources incommon use

Renewable, sustainable biomass fuelsources Waste fuel sources

Sugar cane waste (bagasse) Sewage digester gasTimbermill waste or sawdust Land®ll gas (LFG)Forestry and arboricultural

residuesMines gasCoke oven gas

Short rotation coppicing (SRC)Straw

Re®nery and processplant ¯are gas/off gas

Rice husks and coffee husks Stripped crude gasPeanut and other nut shells MSWPalm oil and coconut residues Hazardous and chemical wasteMeat and bone meal (MBM) Sewage sludgePoultry litter Hospital and clinical wasteLivestock slurry Vehicle tyres



arising from agricultural sources; ‘waste’ is de�ned as fuelswhich are products of human, urban and industrialprocesses. Table 1 compares solid and gaseous fuels, usedfor power generation, divided in accordance with the abovedescriptions. Energy ef�ciency concepts such as combinedheat and power (CHP) and combined cycle gas turbine(CCGT) schemes can be equally easily integrated whengenerating power from either renewable or waste fuels.

2 METHODS OF CONVERTING BIOMASS TOUSABLE POWER

2.1 Combustion

Despite recent advances made in developing the technologyfor the other methods discussed below, combustion is still,by far, the most common method of converting biomass tousable power. Most of the biomass fuels shown in Table 1can be used directly as fuel in conventional boiler systems.

Other than �rewood, which is humanity’s oldest knownfuel, and which is still used today in many parts of the worldas an inef�cient fuel for space heating and cooking, sugarcane waste (bagasse) was the earliest form of biomass to beconverted to usable power. In a cane sugar factory, eventoday, bagasse is converted to steam through the conven-tional process of combustion.

In the late 19th century it was discovered that althoughbagasse has a low calori�c value (CV) (9.0–12.5 MJ=kg)compared with coal (Table 2), it is plentiful and is anadequate substitute fuel for raising steam. Steam is requiredin the sugar re�ning process (Fig. 1) and it was realized thatsteam could be used as the motive power to drive machineryin a typical cane sugar factory. For years reciprocating steamengines were used for this purpose, �rst mechanicallydriving the crushing mills themselves and then, as technol-ogy developed, driving the shredders and cane knives. From

Cutting & Shredding

Boiler & Steam System

Juice Extraction

Fig. 1 Diagram of main sections of a modern sugar mill. (Reproduced by permission of Fletcher SmithLimited)

Table 2 CVs for different materials used as fuel

Material used as fuel CV (MJ/kg)

Coal 23.0±32.0Fuel oil 40.0±45.0Natural gas 50.0±55.0Plastic 27.0±34.0MSW 8.5±11.0Hospital and clinical waste 17.5±22.5Chemical waste 18.5±23.0Sewage sludge 7.0±13.0 (depending on dryness)Vehicle tyres 32.0±40.0Wood 17.0±20.0Straw 14.0±15.5MBM 20.0±28.0 (depending on fat content)Poultry litter 13.0±14.0

Proc Instn Mech Engrs Vol 216 Part A: J Power and Energy A01701 # IMechE 2002

42 I M ARBON



the 1950s, these engines were gradually replaced by steamturbines which eventually drove electric generators ratherthan the mechanical equipment.

At �rst, bagasse was used as a supplementary fuel,usually in addition to coal, and, although joint �ring isstill common nowadays in many parts of the world, boilergrate technology has progressed to the point that manymodern sugar mills are �red by bagasse alone. The lengthof the growing season is also a factor and coal is often usedas the main fuel during the ‘off-crop’ season. Many othertypes of biomass can also be used as fuels in the combustionprocess, although the boiler=grate design will changedepending on the nature of the biomass fuel. Bagasse, ricehusks, coconut shells and peanut hulls have all been burnedtraditionally on a �xed hearth, usually with a dumping grate,but more recently travelling grates have been used toincrease combustion ef�ciency and to automate the process.However, it should be borne in mind that the high silicacontent in rice husks, which creates rapid fouling andcorrosion, demands that boilers for this waste are verydifferent in design from boilers for other biomasses. Thecyclonic combustor, originally designed for tyres, has beenfound particularly suitable for rice husks, but would be anover-elaborate combustor for other biomass materials.

On the other hand, straw burning uses a speciallydesigned grate under a radiation=convection water tubeboiler, while MBM almost always uses a �uidized bed(Fig. 2), a bubbling bed system which has the boilerintegrated into the top part of the combustion vessel(forced circulation, radiation=convection, water tube type).Circulating �uidized bed systems have a separate waste heatboiler downstream (force circulation convection water tubebundles) and are used for higher CV fuels.

The following fuel parameters have a signi�cant in�uenceon boiler design (see Table 3):

1. Moisture content in�uences the boiler design to thegreatest extent. Conveying and storing of high moisture

fuel is a problem, e.g. bagasse cannot be stored in the fuelbunker of a boiler owing to bunker choking problems.Maintaining the bed temperature is also a major problemin burning high moisture biomass. The requirement forin-bed cooling using coils is avoided in such cases.

2. Ash content. Low ash fuels are dif�cult to burn on atravelling grate owing to a very thin ash layer on the gratecausing overheating. Low ash fuel burning in a �uidizedbed requires a sand bed inventory to be maintained, asash generated is very low. High ash fuels (50–60per cent), however, can be burned very effectively in a�uidized bed.

3. Silica content in ash makes it very erosive and also silicadeposition on boiler tubes creates fouling problems. Asnoted above, rice husk ash has a high silica content andthe boiler design has to take account of this.

4. Alkali salts content in ash reduces the ash softeningtemperature and creates slagging and fouling problems inthe high and low temperature zones of boilers, e.g.superheater and economizers. The tube pitches have tobe selected accordingly. Many biomass fuels have high

Fig. 2 Typical �uidized-bed boiler for modern biomass combustion. (Reproduced by permission of Thermax)

Table 3 Moisture and ash content and CV of biomass fuels

Biomass Moisture (%) Ash (%) CV (MJ/kg)

Bagasse 50 1±2 9.2Bagasse pith 40 2 7.5±8.4Spent bagasse 40 10 12.5Sawdust 35 2 11.3Rice husk 10±15 15±20 12.6±13.8Rice straw 6 16 14.7De-oiled rice bran 16 16 11.3Coffee husk 11±14 2±5 15.0±17.5Peanut shells 10 2±3 16.75Coconut shell 10 1 18.8Coir pith 8 15 16.75Bamboo dust 9±12 7 7.5±14.7Tobacco dust 8 30 11.7Cotton stalk 7 3 18.4Soya straw 8±9 5±6 15.5±15.9


WORLDWIDE USE OF BIOMASS IN POWER GENERATION AND CHP SCHEMES 43



alkali salt content in the ash and the boiler has to bedesigned to take care of this aspect, the major parametersbeing as follows:

(a) �ue gas furnace exit temperature;(b) bed temperature;(c) extent of cooling in bed;(d) boiler tube bank pitches;(e) boiler tube bank cleaning;(f) superheater design to reduce fouling;(g) low temperature corrosion.

5. Metallic salts content in ash (sodium and potassiumsalts) forms a low melting temperature eutectic mixturecausing slagging at the high temperature zone of boilers,i.e. entry to superheater. The bed temperature and �uegas furnace exit temperature have to be designed to belower than the slagging temperature. Co-�ring with coalmay be required to overcome this problem. Rice strawhas a low melting point of about 750 ¯C and requiresspecial design consideration for burning.

6. Particle size. A �uidized bed requires particle sizeswithin the range of 10–20 mm, as �uid bed velocity hasan in�uence on particle size. Bagasse cannot be burned ina �uidized bed owing to particle size; it is burned on atravelling or dumping grate very effectively. Rice husk,bagasse pith, rice straw and coffee husk may all beburned in a �uidized bed with proper co-�ring or alone.

7. Fines content. Some �nes can be burned in the free boardof a �uidized bed. Underfeeding of fuel into the �uidizedbed burns �nes in the bed. A travelling grate limits the�nes in fuels to about 20 per cent whereas in a �uidizedbed with underfeeding the percentage can go higher. Ricehusk can be burned very effectively in a �uidized bedwith both overfeeding and underfeeding.

2.2 Anaerobic digestion

Anaerobic digestion (AD) is a method which is morecommonly used with liquid and semiliquid slurries suchas animal waste; it is also used for obtaining gas fromhuman sewage but is now being applied to a limited degreeto certain wastes and biomass streams. AD utilizes the samebiological processes that occur in a land�ll site but undercontrolled conditions in a digester system. The process(Fig. 3) takes place in the digester tank which is awarmed, sealed, airless container where bacteria fermentorganic material in oxygen-free conditions to producebiogas. In terms of its constituents biogas is very similarto the LFG produced naturally in a land�ll site. The amountof biogas produced is limited by the size of the digester tankso is largely used as a fuel which may be burned in aconventional gas boiler to heat nearby buildings or in areciprocating engine which is used to generate electricity.

Fig. 3 Flowsheet diagram of the AD system. (Reproduced by permission of British BioGen)


44 I M ARBON



2.3 Pyrolysis

Pyrolysis is the thermal degradation of organic waste in theabsence of oxygen to produce a carbonaceous char, oils andcombustible gases. Although pyrolysis is an age-oldtechnology (the common and traditional method of manu-facturing charcoal, for example), its application to biomassand waste materials is a relatively recent development(Fig. 4). An alternative term for pyrolysis is thermolysis,which is technically more accurate for biomass energyprocesses because these systems are usually starved airrather than the total absence of oxygen. Although all theproducts of pyrolysis are useful, the main fuel for powergeneration is the pyrolysis oil. Depending on the process,this oil may be used as liquid fuel for burning in a boiler or asa substitute for diesel fuel in reciprocating engines. Althoughthe future for pyrolysis is extremely promising, there is as yetlittle direct operating experience with this method.

2.4 Gasi�cation

Gasi�cation differs from pyrolysis in that oxygen in theform of air, steam or pure oxygen is reacted at hightemperature with the available carbon in the waste toproduce a gas, ash or slag and a tar product. Although thegasi�cation method is very recent in its application tobiomass and waste materials (Fig. 5), the underlying tech-nology, that of the gasi�cation of coal, is now extremely wellproven. The major bene�t of gasi�cation of biomass is thatthe product gas can be used directly to fuel a gas turbine

generator which itself will form part of a CHP or CCGTsystem, thus signi�cantly improving the overall thermalef�ciency of the plant. The main disadvantage is that thereare many more items of large equipment and the capitalinvestment is correspondingly higher, so the payback periodwill have to be carefully de�ned.

2.5 ‘Gasohol’ production

In the late 20th century the rising price of petroleum (andhence of gasoline) in many countries has led to theincreasing use of gasohol which is typically a mixture of90 per cent unleaded gasoline and 10 per cent alcohol(usually ethanol). Gasohol burns well in gasoline enginesand is a desirable alternative fuel for certain applicationsbecause of the renewability of alcohols such as ethanol andmethanol, which are readily manufactured from renewablebiomass resources. Undiluted methanol and ethanol arealso good fuels for automobile engines because they havehigh octane ratings and low pollution emission, althoughtheir solvent properties can cause problems by dissolvingcertain materials used in modern fuel systems, whereasgasohol may be used in most engines without the solvencyproblem.

Methanol, CH3OH, also known as methyl alcohol, woodalcohol or carbinol, is the simplest of the alcohols. It can beproduced by the destructive distillation (heating in theabsence of air) of hardwood chips or from the synthesis ofhydrogen and carbon monoxide. Methanol is a high octane,

Fig. 4 Diagrammatic representation of the pyrolysis process. (Reproduced by permission of Aston University)





clean burning fuel that is a potentially important substitutefor gasoline in automotive vehicles and stationary engines.

Ethanol, CH3CH2OH, also called ethyl alcohol or grainalcohol, has been produced since prehistoric times, mostlythrough the fermentation of fruit juices; it can also be madeby fermentation from the carbohydrates found in molasses,grains (such as corn, wheat, rye and barley) and otheragricultural products, such as potatoes. Like methanol,ethanol is an excellent engine fuel with a high octanerating and low emissions, and similarly ethanol should beused in a fuel system designed to withstand the alcohol’stendency to dissolve plastic parts.

A similar fuel to gasohol is biodiesel, which is a renew-able diesel fuel substitute that can be made by chemicallycombining any natural oil or fat with methanol or ethanol.Methanol has been the most commonly used alcohol in thecommercial production of biodiesel. In Europe, biodiesel iswidely available both in its neat form (100 per centbiodiesel, also known as B100) and in blends with petro-leum diesel. Most European biodiesel is made from rape-seed oil. In the United States, initial interest in producingand using biodiesel has focused on the use of soybean oil asthe primary feedstock, mainly because the USA is theworld’s largest producer of soybean oil.

2.6 Liquefaction

Another emerging technology, potentially applicable to allbiofuels but most likely to be used for wood, is liquefaction.What nature has done with biomass over millions of yearscan be carried out in a liquefaction plant, in which very wetmaterials that, on a dry basis, would contain about 50 percent oxygen, are converted into a hydrocarbon with 10 percent oxygen or less. In principle, there are three routes for

converting biomass into liquid hydrocarbons, the third ofwhich is actually based on gasi�cation.

Hydrothermal upgrading (HTU) was developed by Shell.In this process biomass has 75 per cent of its oxygenremoved under high pressure and temperature without theuse of hydrogen. Hydrogen is then added to produce a goodquality gasoline. The overall energy ef�ciency of thisprocess is at best 50 per cent.

Flash pyrolysis has a much higher energy ef�ciency atabout 67 per cent. However, in this process a bio-crude isproduced which has the same composition as the drybiomass and the quality of the fuel is therefore lower thanthat produced by HTU. After subsequent catalytic conver-sion into a high quality fuel, the overall conversionef�ciency is lower than that of the HTU process.

The �nal process route is gasi�cation with oxygenfollowed by Fischer–Tropsch or methanol synthesis into aliquid fuel. Fischer–Tropsch synthesis produces an excellentquality jet fuel, but the overall ef�ciency is only 50 per cent,compared with 60 per cent for the methanol route.

3 TYPES OF BIOMASS USED FOR POWERGENERATION

3.1 Sugar cane bagasse

In a cane sugar factory (Fig. 1), the sugar cane is loadedonto a moving table which carries it into the revolving caneknives, which chop it into chips to expose the tissue and toopen the cell structure, thus readying the material foref�cient juice extraction. The cane knives are followed bya shredder, which breaks the chips into shreds. The preparedcane then goes through the crusher, a set of roller mills inwhich the juice is extracted. As the crushed cane proceeds

Fig. 5 Diagrammatic representation of typical gasi�cation process. (Reproduced by permission of ARBREEnergy Limited)


46 I M ARBON



through a series of up to eight four-roll mills, it is forcedagainst a countercurrent of water known as water of macera-tion. Streams of juice extracted from the cane, mixed withthe maceration water, are combined into a mixed juice whichis further processed into sugar. Alternatively, the sugar juicecan be extracted by diffusion. Residual cane �bre, after juiceis removed, is called bagasse; in common with otherbiomass residues, bagasse has proved to be an excellentboiler fuel. Although the following relates speci�cally tosugar mills, the description of steam turbines is fairly typicalof power generation from most biomass residues using thecombustion method.

As noted in section 2.1, by the 1950s modern cane sugarmills were commonly equipped with single-stage impulse-type steam turbines driving the shredder, cane knives andcrushing mills. The predominant layout of steam turbine forthis service became the overhung Curtis wheel (Fig. 6),which proved immensely reliable in service. Americanmanufacturers who dominated the sugar industry in SouthAmerica and South East Asia used the familiar ‘betweenbearings’ layout, previously developed for the petrochemicalindustry; although built to API standards the ‘betweenbearings’ type never proved any more reliable than the‘overhung’ type in sugar mill applications.

The steam turbines, typically running at 6000–8000 r=min, provided mechanical power through a geartrain to the driven equipment at a variety of speeds. Asthe crushing mills would run at 4–6 r=min, the developmentof suitable gear trains to achieve up to a 2000:1 reductionbecame the dominant operating technology. Steam turbineswere simple mechanical devices, speed controlled byconventional mechanical governors. The steam pressurewas fairly low, about 10–15 bar; even today sugar millsystems rarely operate much above 30 bar, except in Indiawhere much higher pressures are becoming commonplace.Steam was expanded in the turbine and exhausted at an

absolute pressure of around 2–3 bar, leaving suf�cientpressure and temperature in the steam to use it further inthe re�ning process; this ‘back-pressure’ type of turbinecame to dominate the sugar industry.

Even with effectively free fuel the operating ef�ciency ofthe steam turbines became important as more sugar millswanted to fuel their boilers using only bagasse (previouslyused as supplementary fuel), thus reducing the amount ofimported coal. In the late 1960s, for example, a Britishmanufacturer, Peter Brotherhood Limited (PBL) received anorder to supply 58 steam turbines for the Cuban sugarindustry. The inlet steam conditions varied between 10.6and 17.5bar all with 2 bar exhaust and the company electedto build a new standard design of multistage turbine, capableof having between three and �ve Rateau stages (Fig. 7).Several hundred of these machines have been supplied forsugar mill applications and even today are occasionallyprovided, for example, for shredder drives.

During the 1970s to 1990s, sugar mill technologyimproved drastically and it became the norm to drive thevarious items of machinery with a.c. electric motors and toutilize the steam raised from the bagasse-�red boilers inmultistage steam turbines in central power stations (exceptin Australia where most machines were driven hydraulicallywith a large electric motor used for the hydraulic powerpack).

It thus became necessary to provide larger multistage,back-pressure steam turbines. As the steam conditions havenot increased greatly the overall enthalpy drop required issimilar to that of the 1950s and therefore the number ofstages has not signi�cantly changed. The �rst, larger, steamturbine was supplied by PBL in 1974, rated at 6 MW,although the inlet steam conditions were only 11 bar, dryand saturated (D&S). Two turbines of this type weresupplied in 1993, each producing 10 MW from steam inletof 44 bar and 410 ¯C, exhausting at 2 bar (Komatipoort,

Fig. 6 Single-stage ‘overhung’ steam turbine. (Reproduced by permission of Peter Brotherhood Limited)





South Africa). The same basic turbine frame size is offeredtoday and one has recently been completed for a sugar millin Zimbabwe, rated at 20 MW (Fig. 8).

As sugar mill technology improved there was littlewastage of bagasse and many factories produced morethan they needed for their own power requirements. Sincemost sugar mills are in remote locations (not necessarilyareas of low population) there is opportunity to providepower for the local community and to become the powerstation for a whole region. This has happened in manylocations, e.g. India and South Africa, and is becomingcommonplace in Australia. In these cases, since no moresteam is required by the sugar re�ning process, there is no

need for back-pressure steam turbines to provide steam atpositive pressure. Therefore condensing steam turbines areused to ensure the maximum enthalpy drop from theavailable steam. Signi�cantly improved power outputs canthus be achieved from the same mass �ow of steam andnowadays sugar mill steam turbines can have power ratingsof 40 or 50 MW.

Another signi�cant development of steam turbine designfor biomass combustion power plants is the arrangement ofsingle-stage turbines in so-called ‘tandem’ or ‘twin’arrangements, or even a combination of the two. At theforefront of this development has been the Germancompany, Kuhnle, Kopp and Kausch (KKK).

Fig. 7 Multistage steam turbine supplied for Cuban sugar industry in 1968. (Reproduced by permission ofPeter Brotherhood Limited)

Fig. 8 A 20 MW steam turbine–generator using sugar cane bagasse. (Reproduced by permission of Peter Brotherhood Limited)


48 I M ARBON



3.2 Timber mill waste

Any timber mill produces quantities of waste wood, eithersawdust or unusable offcuts and bark, which have tradition-ally been burned as waste at the sawmill. Wood has a fairlyhigh CV (Table 2), so can easily be used as fuel to raise steamto generate power on site. Steam generators for this duty aretraditionally similar to industrial coal-�red units, using atravelling grate for the combustion and a natural circulationsingle-drum boiler. Previously the economics of this hadbeen poor since sawmill power requirements are not largeand machinery has been driven by, for example, water power.

If the sawmill waste is considered ‘renewable’ fuel ratherthan ‘waste’ product, the economics become different.Sawmills are usually in remote areas, close to the forest;again, not necessarily in areas of low population density.Their remoteness, however, often precludes grid connectionand diesel generators provide power; delivery of dieselfuel creates logistics problems. The optimum solution isoften to use wood waste from the timber mill as fuel in aboiler to raise the steam to be used in a condensingsteam turbine to generate electrical power ef�ciently.Provided that the boiler plant is close to the forest wherethe wood is harvested, the CO2 emissions from the combus-tion process will be absorbed by the growing trees, creatingwhat is known as a ‘carbon-neutral’ cycle. Timber millwaste generally has a low ash content (<2 per cent), a highsintering temperature, and a low nitrogenand chlorine content(emission precursors).

PBL has supplied several steam turbine–generator sets forthis service and four examples in Table 4 illustrate the powerwhich can be produced from an ef�cient steam cycle usingwood-�red boilers. In each case the company workeddirectly with the sawmill operators to determine the powerrecovery system to meet their needs most closely. In Maine,USA, for example, the dowel mill was isolated and simplyrequired enough power for its domestic needs. Accordingly,a low pressure, low cost boiler was utilized, providing aninexpensive power recovery solution.

The project in Finland, however, wanted to use the waterfrom the condenser in a district heating scheme, so thecondenser pressure had to be relatively high, sacri�cingsome of the available enthalpy drop, but meeting otherheating needs. The project in Ghana used two identicalunits working in parallel.

The steam turbine usually employs the ‘integral conden-ser’ concept, pioneered by PBL for marine applications and

proved to be an ideal, compact package for small land-basedpower stations. In addition to the steam turbine powerrecovery systems described above, there is signi�cant inter-est nowadays in converting wood chips to oil using thepyrolysis process. In several different test installations dieselengines and small gas turbines have been converted to runon pyrolysed oil. Although the experiments have beenlargely successful, it is not known whether there are yetany commercial installations using pyrolysed oil fromtimber mill waste.

More developments have focused on gasi�cation, anumber of which have been demonstrated on a commercialscale (Table 5).

3.3 Forestry and arboricultural residues

In any forest managed for timber harvesting it is notsuf�cient to let nature take its course. Growing trees, forexample, for telegraph poles means concentrating thegrowth on the trunks to ensure that the growing effort isnot dissipated in the branches. Thus side shoots, branchesand twigs will be lopped as they are of no value for timber.Also, not all trees will grow successfully or at the same rateso non-performers are weeded out to give the successfultrees space to grow. It is therefore sometimes economicallyviable to install a small power station using the forestresidues as fuel. Both the boiler and the steam turbinetechnology are similar to those described in Section 3.2.Because forestry and arboricultural residues have generallyweathered less than timber mill waste, they have a lowersintering temperature, and greater nitrogen and chlorinecontent (emission precursors). A 5 MW steam turbine–generator package was supplied in 1988 for ‘embedded’power generation in Newfoundland. This application hasrelatively high steam inlet conditions of 59.6 bar and 480 ¯C,exhausting to an integral condenser at 0.08 bar.

Construction of a recent plant by a consortium in St Paul,Minnesota, USA, breaks the mould in terms of scale, with aplanned start-up date at the end of 2002. This plant willproduce 25 MW of electricity and 73 MW of thermal energy(heating and cooling) to supply approximately 80 per cent ofthe energy needs of the city.

3.4 Short rotation coppicing

This is a variation on the two other wood-fuelled systemsdiscussed above, where the wood crop being harvested is

Table 4 Typical steam turbine generators for timber mills

Maine, USA(1985)

Ghana(1988)

Fiji(1986)

Finland(1994)

Power (kW) 800 2 6 1250 3000 3700Inlet pressure (absolute) (bar) 9.6 21.7 42.3 42.0Inlet temperature (¯C) 177 316 400 480Exhaust pressure (absolute) (bar) 0.086 0.105 0.101 0.65Speed (r/min) 9250 9250 7035 9000





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speci�cally grown as a fuel rather than as timber. In manyrespects this is the power generation equivalent to the use ofenergy crops in countries such as Brazil where the grain isdistilled into alcohols, such as ethanol for fuel in vehicleengines, also known as ‘gasohol’; see section 2.5.

In SRC, fast growing trees are grown in special planta-tions to be harvested every third or fourth year of thegrowing cycle (Fig. 9). The best type of wood for fuel hasa high CV, ideally containing �ammable oils or resins andwhich can be regularly harvested. Ideal trees such as euca-lyptus grow with dif�culty in Europe and North America,where willow and ash are more commonly used. The ipil-ipil tree has been widely used in South East Asia andAustralasia. Typically, the tree crop is harvested every 4years and about 1000 kW of useful power can be producedfrom a 500 ha plantation. The ash residue from the powerstation is used as fertilizer and the CO2 released by burningthe wood is absorbed by the growing trees in a ‘carbon-neutral’ cycle.

In 1981, PBL supplied condensing steam turbine–genera-tor packages for six such power stations in the Philippines,amply proving the ef�cacy of SRC principles; again, the‘integral condenser’ concept was used. In this, as in allwood-�red power stations, the logistics of handling anddrying the fuel must be considered. Table 2 shows wood hasa typical CV of 17–20 MJ=kg, compared with 23–32 MJ=kgfor hard coal and 50–55 MJ=kg for natural gas and may alsorequire large areas for drying and preparation.

An interesting energy crop variant is thistles, which arebeing grown in Burgos and near Huesca, both in Spain, tofuel two 10 MW power plants whose construction iscurrently about to be commenced for Biomasas del PirineoSA. For each plant, 105 000 t=yr of thistles will be grown on6000 ha, and the capital cost will be about £10.5million. Ona smaller scale, a company called TGS Robin operates aunique plant in Nice, France, that generates 6 MW from four

Caterpillar generator sets fuelled by rapeseed methyl ester orsun�ower distillate, which is a form of bio-diesel.

A new variant of SRC technology is currently beingapplied in the UK at the ARBRE Energy project at Eggbor-ough, Yorkshire, which will be fuelled by a dedicated willowplantation of 2000 ha. The plant will use an atmosphericpressure, air-blown, circulating �uidized bed gasi�cationprocess developed by TPS Termiska Processer AB inSweden (see Fig. 5). Following gasi�cation, catalytic tarcracking takes place in a second circulating �uidized bed.The gaseous product is compressed and injected as fuel gasinto a specially converted gas turbine. Overall cycle ef�-ciency is improved by the addition of a steam turbine in aCCGT system which produces a total of 8 MW. Althoughthis provides overall cycle ef�ciency improvements, the lowCVof the gasi�ed wood results in a physically large fuel-gascompressor which is a high parasitic load on the system.The equipment involved in the gasi�cation process isgreater, and therefore very much more expensive, than forthe traditional combustion power plant. It will be interestingto see how the capital versus operating costs are evaluated inpractice.

The ARBRE Energy plant has been partly �nanced by anEU grant, and will attract a higher than pool price rate for theelectricity produced for 15 years under the UK Government’sNon Fossil Fuel Obligation (NFFO) arrangements. Plantstart-up was scheduled for the end of 1999, but technicalproblems have prevented successful operation to date.

3.5 Straw burning

Straw is the residue from the harvest of cereal crops; onlythe ‘ear’ is used for processing as food and the straw stalk iswaste. In the days of manual harvesting, straw had manyuses, e.g. animal feed and bedding, building material.Increasing mechanization, industrialization and urbanizationmeans a huge global oversupply of straw, leading to burningthe straw in the open �eld. Recently this practice hasbecome environmentally unacceptable and most developedcountries (even USA) now legislate against straw burning.

A certain amount of straw can be ploughed back into theland but additional uses must be found; a classic example isthe use of straw in construction board. However, this willabsorb only a small fraction of the over-supply. Since strawhas a reasonably high CV (about 15 MJ=kg), higher thanbagasse for instance, its use as a fuel for power generation isan obvious solution. Both straw incineration and strawpyrolysis have been used in power generation applications.Straw also contains very low levels of chlorine and sulfur,so the boiler can be operated at a higher steam temperaturethan is possible for most waste incinerators. Themain dif�culty is straw’s low density; a kilogram of strawoccupies a relatively large volume, resulting in high trans-portation costs. The low density also has a signi�cant effecton grate design for the boilers, which are consequentlylarger than for conventional fuels.

Fig. 9 Diagrammatic representation of dendro-thermal cyclefor SRC plant. (Reproduced by permission of BalfourBeatty)





One UK power generator, Energy Power ResourcesLimited (EPRL), has already recognized the potential forstraw burning and at the end of 2000 commissioned a36 MW power station at Ely, Cambridgeshire, UK. Theplant will consume 200 000 t=yr of straw and will also beused to burn other baled energy crops such as Miscanthus (awoody grass). The power can be sold at an elevated price of6 p=kW h until 2013 under an NFFO arrangement. The plantcost was £60 million with a design, build and operateturnkey contract being let to FLS Miljø, which used itsproprietary vibrating grate-�ring technology. EPRL has justreceived planning permission for a similar plant in Corby,Northamptonshire, UK.

3.6 Rice and coffee husks

Rice husks, rice straw and coffee husks have very similarCVs to straw (Table 2) but need to be treated very differentlybecause of the high silica content which can cause seriousfouling and corrosion of boiler plant. By collecting rice orcoffee husks from several mills and using them to fuelef�cient suspension burning boilers, a plant can produceelectricity from the resulting steam and good quality ash foruse as a building material or an industrial abrasive. Theutilization of rice and coffee husks is expected to be a majorgrowth area for power generation in third-world and devel-oping countries.

An example of a steam turbine plant operating with ricehusk fuel is that supplied by the British company W H Allen(WHA) to the National Food Authority in the Philippines in1982. This produces more than 2 MW from live steam of14.8 bar and 343 ¯C, condensing at 0.12 bar. The low boileroutput=turbine input steam conditions are due to the silicacontent mentioned above.

In Costa Rica, waste water from coffee mills has beenused effectively to generate biogas to fuel a gas engine.Details of two plants are given in Table 6.

3.7 Peanut and other nut shells

The peanut (Arachis hypogaea) has the peculiar habit ofripening underground, hence its alternative name ‘ground-nut’; nevertheless, it is not a genuine nut. It is native totropical South America but was introduced early into theOld World tropics, since when India, China, West Africa andthe USA have become the largest commercial producers ofpeanuts. The peanut is grown mainly for its edible oil,

except in USA, where some 300 derivative products havebeen developed.

Nutshells have many uses, including the use of blackwalnut shell �our to ‘sand blast’ jet engines and other turbo-machines, which precludes their general use as powergeneration fuel, despite having high CVs. Peanuts havesofter shells, less suitable for other uses than most nutshells,and are sometimes used as fuel. For example, a 1500 kWsteam turbine supplied by WHA to Gambia in 1982 usedpeanut shells as fuel, with live steam of 32 bar and 400 ¯C,condensing at 0.14 bar.

There is a pilot plant in Spain for the gasi�cation ofground almond shells for power generation. The process wasdeveloped by the Institut Catala d’Energia at the Universityof Zaragoza and Energia Natural De Mora SL (Enamora). Itis based on a downdraught moving bed reactor whichproduces an almost tar-free gas, since the process includesoxidation and reduction areas following the gasi�cation. Thepilot unit is owned and operated by Enamora at Morad’Erbe, Catalunya. The plant gasi�es 500 kg=h of almondshells and the biogas is fed to two Volvo engines (whichneed 6–8 per cent diesel to maintain operation) eachgenerating 250 kW of electricity. The overall ef�ciency iscalculated as 21 per cent.

3.8 Palm oil and coconut residues

The African oil palm tree, Elaeis guineensis, has beencultivated as a source of oil in West and Central Africa,its origin, and in Malaysia and Indonesia, as an importantcommercial plant. The outer �eshy portion of the fruit issteamed to destroy the lipolytic enzymes then pressed torecover the palm oil, highly coloured from the presence ofcarotenes. The kernels of the fruit are also pressed inmechanical screw presses to recover palm kernel oil, chemi-cally quite different from oil from the �esh of the fruit.

Palm oil is used in making soaps, candles and lubricantsand in processing tinplate and coating iron plates. Palmkernel oil is used in manufacturing such edible products asmargarine, chocolate confections, and pharmaceuticals. Thecake residue after kernel oil is extracted is a cattle feed.

Palm oil and palm kernel oil are two of the mostimportant commercially exploited vegetable oils; the hugeincrease in demand for vegetable oils over the past twodecades has led to considerable improvements in productionmethods and processing plants and to an above-averagenumber of new mills. The palm oil production process andheating require large quantities of heat; also, mechanicalenergy is needed to drive the various production machines.At present, commercial palm oil mills process more than35 million t=yr worldwide equivalent to some 65 millionbarrels of oil. While the palm oil production process andheating require large quantities of heat and mechanicalenergy to drive the various production machines, thisamounts to less than half of the recoverable energy in theresidues.

Table 6 Plants generating biogas from AD of coffee mill residue

PilasCoffee Mill

NaranjoCoffee Mill

Plant capacity (t/day) 450 800Water consumption (m3/t) 1.0 1.6Ef¯uent treatment capacity (m3/day) 500 1500Biogas production (m3/day) 1000±1500 3000±4500Fuel wood substitute (m3/season) 300 1000Power generation (kW) 100 300


52 I M ARBON



The shell and �bre byproducts, ‘free’ fuel, are generallyused to �re steam raising boilers. Steam is produced atpressures and temperatures higher than those required bythe process so that the steam can be expanded in back-pressure turbines and then passed to the process where thelatent heat in the exhaust steam is utilized in different ways.The steam turbines nowadays are predominantly used todrive electric generators. Power generation requirements forpalm oil plants are relatively low, usually in the range500–1000 kW; single-stage, Curtis wheel, overhung turbinesare almost invariably used in such applications. Typicalsteam conditions are inlet 18 bar and 260 ¯C, exhaust2.5 bar.

Coconut oil plants operate in a similar manner. Thecoconut, the fruit of the coconut palm, Cocos nucifera,yields copra, the dried extracted kernel from which coconutoil, the world’s top ranking vegetable oil, is expressed. Inaddition to the edible kernels and the drink obtained fromgreen nuts, the husk yields coir, a �bre highly resistant tosalt water and used in the manufacture of ropes, mats andbaskets. Because steam is not required for processing, andthe husks can be used for other products, there has been lessemphasis on incineration of waste for power generation.Some power generation plants are being built, however,which burn the coconut residues as described in section 3.2.

AD of the liquid ef�uent (see section 3.11) not onlyreduces the biochemical oxygen demand (BOD) and chemi-cal oxygen demand (COD) content to acceptable levels, butalso generates methane for use as a supplementary fuel inthe boiler. Plants for power generation from palm oil andcoconut residues produce between 1 and 15 MW dependingon whether consolidation of waste from several nearby millscan be organized.

3.9 Meat and bone meal

A fairly new area of interest in this country, awareness ofwhich has been heightened by the bovine spongiformencephalopathy (BSE) crisis and its aftermath, is the safedisposal of the residues of cattle slaughter. Most unusedresidues have high CVs, useful as fuel, although tradition-ally used in other ways. Since the BSE crisis and the fear ofcontamination, incineration is now a legal requirement fortwo categories of cattle:

1. Cattle with BSE diagnosed, or from a herd so diagnosed,are killed and the complete carcasses must be incinerated,or otherwise disposed of, whole. This is carried out as abatch process and energy recovery is not economicallyviable.

2. In order to eliminate BSE, cattle over 30 months old(OTM) are killed and cannot be used for meat (the ‘OTMprogram’). Instead, they are rendered to extract fats forsoap and other cosmetic products. MBM is the productremaining after rendering. Until the recent BSE crisis inEurope, although MBM has a high CV, it was never usedas a fuel because it was much more valuable as an animalfeed. This is now banned, however, and the onlycurrently permissible destruction route for the materialis high temperature incineration.

Because of the OTM programme, some 400 000 t ofMBM is in storage in the UK awaiting destruction, andMBM is being produced at a rate of 175 000 t=yr. Onlyfour plants will be built or converted for MBM incinerationin the UK: one is a converted poultry litter power plantburning the MBM on a grate; the others are �uidized bedincinerators. One of these is being built by a renderingcompany and will generate only enough power for siteuse (turbine generator supplied by KKK); the others will

Fig. 10 Illustration of a typical poultry litter incineration plant. (Reproduced by permission of FibrowattLimited, courtesy of ETSU)





generate more than 9 and 5 MW of electricity respectively, inboth cases with the steam turbine–generator sets beingsupplied by PBL. In each case, the boilers generate steamat around 45 bar and 350 ¯C.

3.10 Poultry litter

In the UK, broiler poultry farms annually produce more than1.5 million tonnes of litter, consisting of a mixture of woodshavings, straw and poultry droppings, an excellent fuel forelectricity generation. (This amount of litter would �ll 250football pitches 2 m deep.) Power generation companies buythe litter from the farming companies and transport it tospecially designed storage facilities at the power station(Fig. 10) where it is tipped into storage pits in a controlledenvironment. To prevent the escape of odours the air fromthe fuel hall is drawn straight into the furnace. Overheadcranes load the fuel into conveyors. The fuel is burned attemperatures in excess of 850 ¯C to ensure the completecombustion of all odours and organic material. The furnace

heats the water in the boiler to produce steam at about 65 barand 450 ¯C to drive a condensing steam turbine.

A company called Fibrowatt Limited operates two suchplants in East Anglia and a third in North Lincolnshire,details of which are given in Table 7. The Thetford plant(Europe’s largest biomass generator) cost £50 million andemploys 30 people on site plus 100–200 local jobs insupport services. The ash is sold as a fertilizer under thename Fibrophos. Plants of 40 MW are planned for Holland,Belgium, Italy and several states of the USA.

There are no waste products from this process, whichoperates in a ‘carbon-neutral’ cycle (Fig. 11). The ash fromthe furnace is conveyed to sealed containers in the ash hall,�y ash is recovered from the �ue gases by the electrostaticprecipitator next to the chimney stack and both types of ashare sold as environmentally friendly, nitrogen-free fertilizer,rich in phosphate and potash. Emissions from the chimneyconsist of steam from the water content of the litter and verylow levels of the normal gases arising from combustion offossil fuels.

Table 7 Poultry litter plants in the UK

Plant name Fibropower Fibrogen Fibrothetford EPRL Elean

Location Eye Glanford Thetford West®eldCounty Suffolk Lincolnshire Norfolk ScotlandPlant start-up 1992 1993 1998 2000Throughput (t/yr) 150000 85 000 (now MBM) 450000 100000Power output (MW) 12.7 13.5 38.5 10.0

Fig. 11 Diagrammatic representation of the ‘carbon-neutral’ cycle. (Reproduced by permission of Fibro-watt Limited)


54 I M ARBON



3.11 Livestock slurry

Liquid organic residues from livestock farming, foodprocessing, beverage production, waste disposal and otherindustries are required to meet increasingly stringentdischarge standards. Where direct discharge to water coursesor treatment in aerobic settling ponds no longer meets thelatest standards, operators are faced with installing new highcapital cost treatment plants. Many forward-looking organi-zations, however, are treating their ef�uents as a potentialenergy source.

Anaerobic digesters produce conditions that encouragethe natural breakdown of organic matter by bacteria in theabsence of air. AD provides an effective method for turningresidues from livestock farming and food processing indus-tries into:

(a) biogas (rich in methane) which can be used to generateheat and=or electricity;

(b) �bre which can be used as a nutrient-rich soilconditioner;

(c) liqor which can be used as liquid fertilizer.

The biogas from a typical AD plant will be suf�cient togenerate a few 100 kW up to 4 MW. In agriculture, small on-farm digesters produce biogas to heat farm buildings.However, an AD project is most likely to be �nanciallyviable if treated as part of an integrated farm waste manage-ment system where feedstocks and AD products are all used.Larger-scale centralized anaerobic digesters are now beingdeveloped, using feedstock from a number of sources. Theprincipal feedstocks for AD in the UK are residues fromlivestock farming, notably cattle and pig slurries (Table 8).

The digestion tank is a warmed, sealed, airless container,where bacteria ferment organic material in oxygen-freeconditions to produce biogas (mainly carbon dioxide andmethane); 30–60 per cent of digestible solids convert tobiogas, which can be used to generate heat and=or electri-city. Biogas may be burned in a conventional gas boiler toheat nearby buildings and the digester and=or to powerassociated machinery or vehicles. If used to generate elec-tricity, an ef�cient CHP system is preferred, where heat isused to maintain the digester temperature and any surplusfor other purposes. Larger-scale CHP plant can supplyhousing or industrial developments, or electricity to the grid.

As fresh feedstock is added to the system, digestate ispumped from the digester to a storage tank, where biogascontinues to be produced; collection and combustion may bean economic and safety requirement. The residual digestate

can be stored then applied to the land without furthertreatment, or separated to produce �ber and liqor. The�ber can be used as a soil conditioner or compost; thenutrient-rich liqor can be used as liquid fertilizer.

Few power generation systems currently exist, althoughthe potential is large. Existing UK facilities use reciprocat-ing engines, although gas turbines are more ef�cient forlarger installations.

4 FIJI—A CASE STUDY IN POWER GENERATIONFROM BIOMASS

The remote Paci�c Islands of Fiji have no indigenous fossilfuels and, with tourism being a major source of revenue tothe islands, a secure environmentally friendly and economicsource of electrical power is vital to the economy of theislands. In addition to tourism, two other major industries inFiji are sugar and forest products.

Fiji has traditionally had two methods of power genera-tion: hydro and diesel. The diesel power station is used whenthe hydro-power dries up each summer. It has been discov-ered that the excess in bagasse during the crushing seasoncoincides with the shortage of hydro power. The Fiji SugarCorporation (FSC) has installed two back-pressure turbo-generator sets, the �rst a 12 MW set at Lautoka, on VitiLevu, the second a 10 MW set at Labasa, on Vanua Levu, totake advantage of this situation and to export the excesspower, thus saving the Fiji economy the cost of importedfuel. A similar third unit is planned for Rarawai.

Steam is raised at 33.1 bar and 355 ¯C and expandeddown to the process absolute pressure of 2.2 bar; themaximum steam �ow possible from the available bagasseis let down through the turbine and any excess steam notrequired by the process is dumped into an atmospheric,water-cooled condenser.

Clearly this is not the most ef�cient scheme possible; itwould have been more ef�cient to raise the steam at 60 barand 500 ¯C and to use a reaction turbine with an extractionat the required process pressure and exhausting to a vacuumcondenser. This solution was considered by FSC butdiscarded for the following reasons:

1. Sugar is a �ckle business with the quantity of canesubject to the vagaries of weather, disease, drought,politics and market price.

2. FSC did not want to enter in to an electricity supplycontract that it may not be able to always ful�l and the

Table 8 Potential for energy recovery from UK livestock population

Resource PopulationPotential CH4 yield(m3/day)

Potential electrical output(MWh/y)

Cattle 12 200000 5 700000 6 200000Pigs 7 900000 800000 900000Poultry 124000000 1 000000 1 100000Total 8 600000 9 400000





Fiji Electricity Authority did not want to be fully depen-dent on a supply that may not be totally secure.

3. As far as the suitability of the equipment for the locationis concerned, Fiji is a long way from anywhere and 60 barsteam requires a high level of water treatment notcurrently available on the island and it was felt that areaction extraction machine was insuf�ciently robust=reliable for a sugar factory.

An impulse back-pressure turbine with a separate dumpcondenser was selected as this gave the necessary �exibilityboth to produce power for export and to concentrate on thecore business of sugar production. As a further considera-tion of �exibility it was decided to design the machine formaximum ef�ciency at 80 per cent of the maximumcontinuous rating (MCR) power.

PBL was selected to supply the steam turbo-generator forthe following reasons:

(a) excellent ef�ciency, superior to that available fromcompetitors from Japan or Germany;

(b) proven track record—a number of similar frames hadbeen in operation in sugar factories in Australia andSudan for almost 25 years;

(c) familiarity with the cane sugar industry—the companyhad supplied over 500 machines to sugar factories over a40 year period.

The cautious approach adopted by FSC has proved to bewell founded as, since installing the two turbo-generatorsets, the Fiji sugar industry has suffered from both a droughtand an excess of rain and the 12 MW machine at Lautokahas suffered from a major water carry-over causing consid-erable damage to the �rst-stage blades.

At Drassa, not far from Lautoka on Viti Levu, TropicWood produces wood chips and other wood products forexport from sustainable pine forests. The power requirementof the sawmill is 3000 kW and on installing a condensingsteam turbine generator set, using steam raised by burningthe wood waste, the mill has become totally independent ofthe local grid.

As well as being economically desirable the aboveschemes are also neutral with regard to ‘greenhouse’ gasessince the CO2 given off by burning both the bagasse and thewood waste is absorbed in the growing of the sugar cane andpine trees.

5 CONCLUSIONS

From at least as early as the 1950s, biomass has beensuccessfully used as a fuel to raise steam for power genera-tion. These early examples generally involved co-�ring ofbagasse in coal-�red boilers but their success has led to mostsugar mill combustion plants nowadays being solely �redwith bagasse. Although bagasse has a relatively low calori�cvalue (Table 3), it has proved to be such a useful fuel insugar mill applications that it has set the pattern for the use

of the other biomass fuels, discussed in this paper, forcombustion systems. Many of these other biomass fuelshave greater CVs but to date by far the most popular form ofconverting biomass for electrical power is by using steamproduced from the combustion of the biomass. The othertechnologies used either are much newer (and hence withvery limited experience) or are for much smaller-scalepower generation schemes.

As already mentioned, the early experience with bagassewas in co-�ring coal-�red boilers and it may be thought thatfurther opportunities will arise in the future for biomass tobe used as a supplementary fuel in fossil fuel �red boilers.Although there will unquestionably be opportunities forsuch co-�ring, there are severe problems which render thisoption, in the view of the author, less of an opportunity thanmay be envisaged at �rst sight. The �rst problem istechnological in that the grate systems used in boilers forcoal are not usually ideal for those used for biomass fuels;indeed, for oil- and gas-�red boilers there is no grate at all,which would make this impractical. The second reason isphilosophical, in that one of the main drivers for thedevelopment of biomass fuels is that they are both renew-able and sustainable and are to be seen as a replacement forfossil fuels.

Turning to conversion technologies other than combus-tion, AD is well known and much experience is beinggained world-wide with the use of AD for liquid andsemiliquid slurries. Little experience has been gained sofar with the use of AD with solid wastes and biomass,although some experience is currently being gained with theuse of AD for MSW. It will be interesting to see howsuccessful these developments are but there will always bean inherent limitation in the size of the equipment requiredfor the AD process compared with the amount of biogasproduced. For this reason it is very unlikely that suf�cientgas will be produced by the AD process to warrant powergeneration by gas turbines, so this will continue to be doneusing small reciprocating engines as the prime mover. Evenmodern reciprocating engines have high maintenancerequirements and the most likely use of AD in the futurewill be to provide power for isolated farms and homesteads.

Pyrolysis, at least in its use with biomass fuels, is still arelatively new and unproven process. Most pyrolysisprocesses produce an oil which can be used as useful fuelin boilers and reciprocating engines. Further development ofthe pyrolysis process can enable the present extremely highcapital cost to be reduced but this is not currently quanti�-able. In the author’s view, it is unlikely that pyrolysis willbecome a commercially competitive process within theforeseeable future.

Gasi�cation of biomass is still very much in its infancy.As far as the UK is concerned, successful commissioning ofthe ARBRE plant will be required before any usefulempirical data can be obtained. In theory, gasi�cationshould provide a much higher overall thermal ef�ciencythan conventional combustion but this is obtained at asigni�cantly higher capital cost. The gasi�cation process


56 I M ARBON



itself is expensive and produces an extremely low CV fuelgas from the biomass. This means that the gas turbine has tobe specially adapted to receive 4–5 times the volume �ow offuel gas to achieve the same mass �ow that would bedelivered by natural gas. Although this problem appears tohave been satisfactorily overcome, there remains the fargreater dif�culty of developing a suitable fuel gas compres-sor for the vast volume �ow required. In the ARBREproject, for example, the fuel gas compressor is so largethat it absorbs 1.6 MW of power in a plant of only 8 MW netoutput, so the parasitic losses are on a scale not previouslyexperienced.

Gasohol production has proved to be technically viable,particularly in South America, and further developments aretaking place with this type of biofuel. The drawbacks ofgasohol are that it is expensive to manufacture and that only10 per cent of the fuel is derived from renewable, sustainablesources. While this is better than nothing, a more promisingsolution would be to use pure alcohol as the fuel but sofar there has been reluctance among automotive enginemanufacturers to re-design engines without the use ofcomponents which are dissolved by the alcohol. In Europeand North America, considerable success has been obtainedwith the use of biodiesel, usually made from rapeseed oil inthe former and soybean oil in the latter. If the costs ofproduction of biodiesel can be reduced, there is no reasonwhy this could not be an excellent substitute for fossil fuels.

There would appear to be good prospects for the threeliquefaction processes discussed in section 2.6; however, allof these processes are very much in the realm of emergenttechnologies and it is too early to have any views on theircommercial viability at this stage.

The main conclusion of this paper is that the developmentof various types of biomass as replacement for fossil fuels isto be encouraged. The UK Government has deemed that thetwo areas of renewable energy where it wants to seedevelopment focused are biomass and offshore wind. It isexpected that many other countries will come to similarconclusions and therefore the prospect for future powergeneration from biomass is extremely good indeed.Nevertheless, unless the other emergent energy conversiontechnologies can be shown to be commercially viable, it is tobe expected that the traditional combustion route will be thebasis of most such plants to be built over the next few years.

ACKNOWLEDGEMENTS

The author wishes to acknowledge, with grateful thanks, thefollowing people without whose help and input this paperwould not have been possible: Paul Darley and RaymondBowell, former colleagues at Peter Brotherhood Limited,Peterborough; Richard Mason, formerly of Allen SteamTurbines, Bedford; Bob Webster at KKK Limited, Well-

ingborough; J. H. Jangada at ME Engineering Limited,Grantham.

REFERENCES

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Documents

Worldwide Use of Biomass in Power Generation and Combined Heat and Power Schemes