handbook of combustion (online) || overview of solid fuels combustion technologies

2Overview of Solid Fuels Combustion TechnologiesDespina Vamvuka

2.1Introduction

Energy drives human life and is crucial for continued human development. It is theconvertible currency of technology. Without energy the whole fabric of society wouldcrumble. Global demand for energy is rapidly increasing with increasing humanpopulation, urbanization, andmodernization. This growth is projected to rise sharplyover the coming years in developing countries. Theworld heavily relies on fossil fuelsto meet its energy requirements – oil, gas, and coal are providing almost 80% of theglobal energy demands. Oil and natural gas prices are continuously rising, due to therapid worldwide increase in their consumption. Coals, covering about 65% of theproven fossil fuel reserves and being widely distributed throughout the world,provide stability in price and availability and will therefore play a major role in theglobal energy system in the coming decades. Furthermore, given the energy crisis,the development and use of renewable energy sources is one of the key challenges inthe shorter and medium term to substitute fossil fuels, to provide commerciallyattractive options for meeting specific energy service needs, and to mitigate green-house gas emissions. Biomass, including all kinds of materials that were directly orindirectly derived not too long ago from contemporary photosynthesis reactions,such as vegetablematter and its derivatives, is a widely dispersed, naturally occurringcarbon resource with great energy potential. It is also considered as a CO2-neutralenergy source. The simple act of burning biomass to obtain heat and often light is oneof the oldest biomass conversion processes known to mankind.

Thus, the study of coal and biomass combustion for power generation or heatingprocesses is of extreme importance, if we are to conserve our sources of energy, whileachieving the required environmental goal efficiently and reliably in a world ofincreasing population and energy needs. This chapter provides a survey of thetechnologies that are either available or are being developed to enable all solid fuels tobe used cleanly and with greater amenity.

Handbook of Combustion Vol. 4: Solid FuelsEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1

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The chapter begins with a discussion of the physical and chemical propertiesof coals, which influence the design and the performance of combustion processes.The conventional methods of combustion – stoker firing, pulverized coal firing, andcyclone firing – are then considered in some detail. However, emphasis is given toemerging clean coal technologies with higher thermodynamic efficiency and inher-ent emission control for reducing oxides of nitrogen and sulfur, or the greenhousegas CO2. The most promising of these technologies include fluidized bed combus-tion, both atmospheric and pressurized, supercritical pulverized-coal combustion,low NOx burners and near-zero emission technologies. For each process, key issues,current status with technical and environmental performance data, as well as futuretechnological developments are discussed.

Following the presentation of coal combustion technologies, this chapter alsofocuses on biomass, which requires specialized combustion techniques, as it differsfrom coal in many important ways, including the organic, inorganic, and energycontent and physical properties. Thus, the influence of the physical and chemicalparameters of biomass fuels on the combustion process isfirstly explained. Currentlyavailable or under development biomass combustion technologies for industrialutilization, such as grate furnaces, underfeed stokers, fluidized bed systems, anddust combustion systems, are described. Improved processes for conversion of virginbiomass and complex waste biomass feedstocks into heat, steam, and electric power,such as large incinerators with heat recovery and modern boiler systems withminimal emissions, are all comprehensively reviewed. Examples and experiencefrom several countries around the world are presented.

Finally, this chapter concludes with an outlook and a summary, highlighting themost promising technologies for the near future and the role of solid fuels inproviding international energy security and sustainable development.

2.2Coal Characteristics Affecting Combustion Processes

The composition and properties of a coal influence all aspects of the combustionprocess, from the milling to the design and performance of the boilers and theenvironmental control systems.

2.2.1Coal Structure and Petrographic Composition

The macropore structure of the coal becomes a major factor in determining charreactivity at the elevated temperatures of combustion, because of limited diffusionalaccess to smaller pores. Thus, chars from low-rank coals are generally more reactivethan chars from higher rank coals, due to their increased porosity. Furthermore, therole of coal structure on char reactivity is related through the variations in active sites,surface area, and pore structure, which result from devolatilization processes [1].

32j 2 Overview of Solid Fuels Combustion Technologies

The petrographic constituents of coal are known to behave differently duringcombustion. Vitrinite-rich particles expand to form cellular structures, whereasfusinite-rich particles show little or no expansion. Expansion is greatest for mediumvolatile coals, but the extent of expansion is influenced by the rate of heating.Vitrinite-rich particles have a higher burn-off rate than fusinite-rich particles [2].

2.2.2Organic Elements and Sulfur Content

The level of organic oxygen in coals affects the degree of reactivity for combustion. Asthe coal is heated, dissociation of this oxygen from the organic matrix occurs, leavingreactive sites for combustion. On the other hand, oxygen functional groups can bindinorganic cations, such as Na, Mg, Ca, and K, which affects the behavior of ash-forming elements during combustion.

The nitrogen content of low-rank coals is not considered a significant factor indeterminingNOx emission levels during combustion, where the flame temperaturesare also low [3].

The sulfur in coal can cause corrosion on the surfaces of the economizers, the airheaters and other ducts of the boiler unit, through sulfuric acid that is generated fromthe organic and inorganic sulfur-bearing compounds via reaction of sulfur trioxidewith water vapor. Even low levels of sulfuric acid in the flue gas (10–50 ppm)will raisethe dew point from awater dew point of 38–49 �C to an acid dew point of 120–175 �C.If the surface temperature of a boiler section is below the dew point, relatively strongsulfuric acid (70–90%) can then condense and seriously corrode these surfaces. Thesulfuric acid can also interact with fly ash in the furnace to form sulfates of sodium,potassium, aluminium and/or iron, which promote corrosion even at temperatureshigher than the acid dew point. The production of sulfur trioxide, and therefore ofsulfuric acid, can be reduced by the presence of water vapor, addition of compoundscapable of removing the oxygen atoms, or injection of fly ash to coat the superheatertubes [4, 5].

2.2.3Moisture and Volatile Matter Contents

The highmoisture content of coals, apart from causing problems in delivery, storage,handling, pulverization, and drying systems, lowers the flue gas temperature duringcombustion and carries sensible heat out with the flue gases. In the case of low-rankcoals, it demands a larger furnace size, tomaintain the same energy output as higherrank coals and therefore increases the CO2 emissions per kWh.

Volatile matter is important for the control of smoke and ignition. Coals with alower content of volatiles burn slowly and the use of auxiliary fuel, or finely groundmaterial, is often needed. Volatiles yield is important in determining flame stabi-lization. Coals that provide a high volatile yield allow use of a smaller burner toachieve the same throughput, when compared to coals of lower volatile yields.

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Furthermore, the high volatile yield coals allow greater flexibility in burner designand operating conditions [3, 6].

2.2.4Calorific Value

Alowcalorificvalue increases toagreatextent thequantityof fuel requiredforburning,for a given steam or electric power production rate. The number and size of themills,the size of the furnace, and all auxiliary equipment must also be larger, to handle thederived throughput and generate the same amount of steam. The increase in coalfeeding rate could wear out the mechanical parts of the system and produce higheramounts of fly ash, thus reducing the efficiency of particulate control equipment.

2.2.5Agglomeration Properties

Some bituminous coals agglomerate during heat up, since they go through asoftening and melting stage. For low rank coals, in comparison to bituminous coals,the same degree of fineness is not required to assure burnout, because particlesurface area will not tend to increase due to agglomeration.

In fluidized beds, agglomeration can take place where low melting point compo-nents or ash particles are present on the particle surface, where there are localized hotspots and the temperature exceeds 900 �C, or a combination of sintered fly ash andfine sorbent [7, 8].

2.2.6Ash Content and Composition

Coal ash and inorganic volatile matter, generated by thermal alteration of mineralmatter, is of great concern for the combustion process in pulverized fuel boilers.Thismaterial not only contributes heavily to stack emissions but also reduces the heattransfer in the furnace, alters the flow of the gases, and deposits on heat transfersurfaces, threatening the integrity of the combustion system by severe corrosion.These deposits reduce the power of the unit and thus increase the cost of the energyproduced. Therefore, large installations must provide for effective countering ofthese hazards.

The extent of ash-related problems depends upon the quantity and association ofinorganic constituents in the coal, the combustion conditions and the systemgeometry [9, 10]. The composition of ash affects its softening temperature, itsviscosity, and ash fouling, fusion characteristics that also determine the mode ofits removal, either as dry ash or as slag.

2.2.6.1 Effect on Ash Softening TemperatureAmong the various constituents of coal ash, the oxides of Al, Si, Ti, Ca, Mg, Na, andK and especially Fe2O3 are responsible for its fusion.


When the ash fusion temperature is lower than the furnace temperature, theretained ash melts. If the design of the system does not allow for the drainage of theash as a slag, then severe clinkering may occur, causing a lot of problems in itsremoval, particularly in the lower parts of the furnacewhere the use of side fans is notpossible.

2.2.6.2 Effect on Slag ViscosityEven if two coal ashes have the same softening temperature they may have widelydifferent flow characteristics. The relevant property of the ash, which representsbetter its fluidity at a stated temperature, is the viscosity. In boiler practice, tapping ofthe slag in the liquid state is readily accomplished at slag viscosities ranging from50–150 poise [3].

For fixed furnace temperatures, the slag viscosity varies with the chemical compo-sitionoftheash.Therelationshipbetweenslagviscosityandashchemicalcompositionmay be correlated in many ways, the best known of which are the equivalent silicapercentage and the base-to-acid-ratio of ash constituents [11, 12]. Slags having similarequivalent silica percentages have a similar viscosity–temperature relationship. Theconstituents of coal ash can be classified as either acidic or basic. The acidicconstituents are silica, alumina, and titania, while the basic constituents are iron,calcium, and alkalis. The viscosity of a coal ash decreases as the base-to-acid ratioincreases to one.

2.2.6.3 Effect on FoulingDuring combustion, the inorganic coal components undergo complex physical andchemical transformations toproduceintermediateashspecies,whichconsistofgases,liquids, and solids. In low-rank coals, mineral grains also interact with organicallyassociated elements. The size and composition of all these intermediate speciesdirectly influence slagging and fouling problems in combustion systems [13, 14].

Fouling is any form of ash deposit that retards heat transfer, or obstructs the flowof gases through the unit. It is classified as [5, 15]:

1) Fused-slag deposits, which develop principally on surfaces exposed to radiantheat transfer, such as on the furnace walls and the first few rows of boiler orsuperheater tubes. These form by impaction of superficially sticky gas-entrainedparticles on solid surfaces and can cause extensive erosion. The severity of thisproblem depends mostly on the size, shape, hardness, and velocity of theparticles, but also appears to vary with the angle of impact.

2) High-temperature bonded deposits, which develop on convection-heating sur-faces in regions of moderately high-gas temperature, principally in the super-heater. These deposits typically consist of a strongly adhering dense core,surrounded by a fly-ash-like layer and they aremainly derived from alkali metals,such as sodium and potassium chloride, sulfate, calcium chloride, fluorapatite[Ca10F2(PO4)6], silica, and sintered ash. The mechanism by which high-temperature deposits build up is evidently connected with the volatility of alkalimetal chlorides and sulfates.

2.2 Coal Characteristics Affecting Combustion Processes j35

3) Low-temperature deposits, which develop on connection heating surfaces of theeconomizer and the air heater. These are formed by condensation of aqueousvapors and entrapment of fly ash and thus are primarily sulfate-based [4]. Fly ashcan cause severe corrosion. Its composition as well as the size and the shape ofthe particles are the main factors that influence this type of corrosion.

High-temperature bonded deposits occur with most low-rank coals, due to theirhigh content in alkali constituents, which volatilize during combustion. The alkalisare also responsible for external corrosion through formation of the alkali irontrisulfates [Na3Fe(SO4)3 and K3Fe(SO4)3], whichmelt below 600 �C [16]. As a result ofthe low fusion temperature ash of low-rank coals, boiler furnacesmust be larger, tubespacing must be more generous, more wall and soot blowers must be provided, andlarger fansmust be available to allow for the added draft loss caused by deposits in theconvective tube banks, compared with furnaces burning bituminous coals.

Organically bound calcium reacts extensively with quartz and clayminerals to giveboth amorphous and crystalline calcium silicates and aluminosilicates. The productsof reaction have lowered melting points; hence their formation may favor enhanceddeposit formation [17]. However, large grains of calcite could �dilute� depositstrength [18].

The silica content of ash, despite its strong influence on the viscosity of the slag, cancause several problems, too. High silica ash is very abrasive and can erode the coalfeeding systems and burners. When high silica content is coupled with high sodiumcontent, massive deposits can form on heat transfer surfaces.

The basic approaches for controlling fouling are: (i) conservative design of furnaceheight and area, to allow ample time for burnout, thereby minimizing the furnaceexit temperature; (ii) installation of an adequate number of sootblowers at spots likelyto be troublesome; (iii) use of fuel additives containing calcium or magnesium toreduce the fluxing ability of any molten ash phases, or containing aluminium to formhigh-melting point materials; (iv) to limit the sodium content of the coal by selectivemining, blending, or ion-exchange; (v) to micronize the coal to produce smaller andweaker deposits; and (vi) to co-fire the coal with other fuels to produce deposits thatcontain primarily silicon, iron, and aluminium and less than 0.8% sodium [4, 19].

2.3Conventional Coal Combustion Technologies

Direct coal combustion is a widely used technology for power production or heatingprocesses.

There are two principal modes of burning coal: in a fuel bed or in suspension. Infuel bed combustion, relatively coarse coal is fed onto a grate and the type of burningis determined by the direction of flow of the fuel and air. This type of firing system isknown as a stoker. Fuel bed appliances can be economically used for heat rates up to�135 tonnes of steam per hour. For higher steam rates recourse is made tosuspension firing.


In suspension firing, fine coal is thoroughly mixed with air in pulverized-coalfurnaces, or in cyclone furnaces. In a pulverized-coal furnace the coal is fullyentrained by the stream of air. In a cyclone furnace the coal is swirled by the streamof air into cylindrical burners and only small particles burn by entrainment.Pulverized-coal furnaces are in widest use among utilities.

2.3.1Stokers

Industrial stokers are the oldest and most common devices used for combustion ofcoal in fuel-beds. They were used to burn coal as early as the 1700s. Recently, stokershave lost a great part of their traditional market to fluidized beds.

In a stoker, the crushed coal (typically 95% less than 32mmand 20–60% less than 6mm) is fed on the grate, throughwhichprimary airflowsupward and through the bedof particles. The raw coal is heated, dried, devolatilized, and burned, leaving ashmaterial at the bottomof the bed. The layer of ash formed on the grate protects it fromexcessive heat. The temperature in the fuel-bed is determined by the rate of burning,which in turn is determined by the relative velocity between the fuel and the gases, therank and size of the coal, as well as the height of the bed. The volatile constituents andthe carbon monoxide produced by partial combustion of the coal char are burnedabove the fuel bed, where secondary air is injected to facilitate the burning.

The power requirements of stokers are low. However, coal losses are considerablein such units. When selecting a stoker, the heat demands, the fluctuations in feed,and the availability of suitable fuels must be taken into account [3].

Stokers have evolved over the years from a simple design to quite sophisticateddevices to burn various fuels, including coal. Depending on the different techniquesfor feeding coal to the grate, they are classified as overfeeds and underfeeds. Theoverfeed group includes spreader, chain, and vibrating grate stokers, while theunderfeed group includes single and multiple retort stokers. Table 2.1 summarizesthe basic characteristics of the various types of stokers.

Table 2.1 Stoker characteristics [20].

Design/operating parameter Spreader Chain and traveling grate Underfeed

Quick response to load change Excellent Fair FairMinimization of carbon loss Fair Fair FairPrevents coal segregation Fair Poor PoorUtilizes wide variety of coals Excellent Poor PoorBurns extremely fine coals Poor Poor PoorPermits smokeless combustion at all loads Poor Good GoodMinimizes fly ash discharge Poor Good GoodMinimizes maintenance Good Good FairMinimizes power consumption Good Good GoodHandles ash easily Excellent Good Fair

2.3 Conventional Coal Combustion Technologies j37

2.3.1.1 Spreader StokersSpreader stokers are themost commonly used type of stokers, because they can burna wide range of coals, from bituminous to lignite, and they can accommodate a widerange of boiler sizes [21]. In these stokers, the coal is thrown and spread over theentire grate surface, by mechanical feeders (Figure 2.1a). Some burning of thesuspended coal fines occurs above the bed, which coupled with a very thin coal bedallows fast response to load changes.

A typical installation consists of feeder-distribution units, air metering grates,forced draft fans for both undergrate and overfire air, dust collecting and re-injectingequipment, and combustion controls to co-ordinate fuel and air supply with loaddemand. Smaller units use dumping grates, while larger units use continuousdischarge grates.

The spreader stoker is usually employed in a capacity range up to 45 tons of steamper hour. It has high availability, simplicity of operation, and high operatingefficiency. However, is has high fly-ash carry-over and combustible heat loss.Reinjection of the unburned fuel to the furnace can increase the efficiency by2–3%. Also, size segregation can be a problem, when fine and coarse coals are notdistributed evenly over the grate, producing a ragged fire and poor efficiency. A coalsize ranging from 0 to 20mm is generally recommended, when boiler load isrelatively constant [3, 22]. Although operation is very sensitive to the size distributionin the feed, a given spreader stokerwill burn almost any kind of coal, provided that the

Figure 2.1 Spreader (a), chain grate (b), vibrating grate (c), and multiple retort underfeed (d)stokers.


appropriate size distribution is used. This makes the spreader stoker an attractiveoption for small installations that might be using coal from several sources.

2.3.1.2 Chain Grate StokersIn a chain-grate stoker, coal is fed from hoppers to a grate, which consists of anendless chain extending into the furnace (Figure 2.1b). The horizontal movement ofthe grate carries the coal into the combustion chamber,where its top surface is ignitedby radiation fromahot refractory arch. Theflame front then travels down through thecoal bed, while the air comes up through it. The bed grows progressively thinner, ascombustion continues, and as the grate turns for the return journey the residual ashis dropped into a container.Heat release rates higher than is permissible with an archcan be achieved if overfire air jets are used to complete the combustion of the volatilegases.

Chain grate stokers are characterized by low fly ash carry-over. They burn mostcoals, but high coking coals can be a problem. Their response time is longer than thatof the spreader stokers [22]. Whilst the top size of the coal should preferably be about30–50mm, a somewhat larger size can be used in the case of low-rank coals withmoisture content below 35%, since they ignite easily and burn freely.

2.3.1.3 Vibrating Grate StokersIn a vibrating grate stoker (Figure 2.1c) the entire structure is supported by severalflexure plates, allowing the grid and its grate to move freely in a vibrating action thatconveys coal from the feeding hopper onto the grate and gradually to the rear of thestoker. Ashes are automatically discharged to a shallow or basement ash pit. The useof high pressure air jets through the front arch provides turbulent gas mixing andpromotes combustion.

The grates are water-cooled and vibration is intermittent. The frequency ofvibration is adjusted by a timing device, which is regulated by the automaticcombustion control system to conform to load variations, synchronizing the fuelfeeding rate with the air supply.

The water-cooled vibrating grate stoker is preferred for its simplicity, low fly ashcarry-over, very low maintenance, suitability for burning multiple-fuels and a widerange of coals from bituminous to lignite. It can also successfully burn coals with ahigh free-swelling index, because the gentle agitation of the grate keeps the bedporous, without the formation of large agglomerates. The stability of air and fueldistribution, in this type of stoker, guarantees a good response of the system to loadfluctuations and operation, without smoke production [3].

2.3.1.4 Underfeed StokersUnderfeed stokers have been widely used in small industrial boilers with outputs ofapproximately 0.5–3MW. The coal is introduced through long retorts below the levelof air tuyeres (Figure 2.1d). Thus, the raw coal is at the bottom, the ash moves awayfrom the retort at the top, and combustion takes place in between. The flamefront tends to move downwards, its speed being matched by the rising flow of coal.


The system hence fulfils the requirement of the flame front, traveling in the oppositedirection to the primary air.

The smallest underfeed stokers use the single and the double retort. In these, thecoal is fed by a screwor a ramand the ash is usually dischargedwith the side-dumpinggrates. These types are suited for steady load operation, due to their ability tomaintainignition over a part of the grate area, then automatically expanding the active burningarea as the controls call for more fuel and air [23]. Larger underfeed stokers usemultiple retorts, inclined at an angle of 25–30� to aid the flow of coal and ash. The ashis discharged either intermittently or continuously.

The size of the coal furnished tounderfeed stokers should be in the range of 30mmto zero. A reduction in the percentage of fines helps to keep the fuel bed porous andextends the range of use to coals with a high free-swelling index.

Underfeed stokers operate with very thick coal beds, causing a high thermal inertiaand a slow response to load changes. Lignites, however, do not agglomerate and tendto burn in a thin layer of independent particles. The air draft, supplied throughtuyeres in the side of the unit, or mechanical agitators, used to keep a thick bed ofbituminous coal moving, are likely to cause the lignite to accumulate in drifts, withthe consequence that burning is uneven across the area of the stoker [18]. Underfeedstokers have trouble burning strongly coking coals, low ash bituminous, and looseash sub-bituminous coals because of the grate overheating. On the other hand, theyhave a clean smokeless combustion and low fly ash carry-over. The smokelesscombustion comes from feeding the coal under the combustion zone. The volatilesescape from the raw coal,flowupward through the combustion zone and burn almostcompletely while passing through the zone [3].

Underfeed stokers are gradually being displaced by the spreader and vibratinggrate types.

There has been little research or development of fixed bed combustion technol-ogies in recent years. In addition to furnace development and fuel improvement forthese systems, the focus has been on reduction of emissions and improvement ofstokers� efficiency. Guidelines for clean and efficient operation of stoker boilers havebeen prepared. Improved overfire air systems and flue gas recirculation systems forcontrolling nitric oxides have been developed [22]. Methods of choosing the optimalrate of travel of the grid and height of the fuel layer for increased thermal efficiencyhave been suggested [24].

2.3.2Pulverized-Coal Furnaces

Suspension firing concerns combustion of air-entrained pulverized coal as it passesthrough a furnace. This technique, which is by far the most commonmethod of coaluse, obviates the need for a supporting grate, eliminating restrictions on equipmentsize and allows satisfactory combustion of virtually any kind of non-caking coal. Inaddition, since it releases substantially more heat per unit volume of combustionspace firing, it is used for steam generation at rates >135 t h�1. Pulverized-coalfurnaces can be built to match steam turbines, which have outputs between 50 and1300 MWe [25].


In most power station boilers, coal must be pulverized, so that 70–80% passes a75 mm screen. This requires the use of high capacity pulverizers, the operating costsof whichmake a significant contribution to the cost of electricity generation. The sizeand the number of themills increase in the case of lower rank coals, due to their lowerheating value. Furthermore, to produce the desired heat input to the furnace, themills must deliver the coal at the proper moisture condition. Excessive moisture canlimit the throughput of the pulverizers.

After the coal is pulverized, it is transported with primary air to a burner into thefurnace, while secondary air is heated and introduced through the burner ports toensure complete combustion. Tomaintain a stable intense flame and avoid flashbackthrough the burner, the coal must be injected at a relatively high velocity, typically15m s�1. Efficient combustion, with no significant loss of unburned carbon,demands some care in matching burner configurations and furnace dimensions,as well as longer residence time than in a fuel-bed, since combustion of the charoccurs in an atmosphere of decreasing oxygen concentration. The residence time inthe furnace is typically 1–2 s, while the combustion temperature is 1300–1700 �C. Alarge portion of the ash leaves the furnace as fly-ash, some of which deposits on thetubes, causing slagging and fouling, and the remaining falls to the bottom of thefurnace and is removed [21, 22, 26].

Most conventional pulverized-coal fired boiler systems use subcritical pressuresteamcycles (<22.1MPa at 540 �C)with superheated and single reheated steam. Thisresults, depending on feedstock, steam conditions, condensing pressure, and plantsize, in thermal efficiencies in the range of 35–38% (based onLHV) [5]. Boiler designsof two kinds are usually used. In the traditional two-pass layout the furnace is toppedby some heat transfer tubing. Then, the flue gas is turned through 180� and passesdownwards through the main heat transfer and economizer sections. The otherdesign uses a tower boiler, where virtually all the heat transfer sections are mountedvertically above the combustion chamber [25].

The main advantages of pulverized-coal combustion are the high reliability,adaptability to all coal ranks, full automation, and excellent capacity for increasingunit size. The main disadvantages are the high energy consumption for pulverizingcoal, high particulate emissions, and high SO2 and NOx emissions.

Most pulverized-coal combustion technologies developed up to 2000, classified asclean coal technologies, are concernedwith controlling and reducing pollution. Someof these technologies, being developed and demonstrated in the USA under the�Clean Coal Technologies Demonstration Program,� are: the LIMB demonstrationproject extension and cool-side demonstration; the 180 MWe demonstration ofadvanced tangentially fired combustion techniques for reduction of NOx emissionsfromcoal-fired boilers; the full scale demonstration of low-NOxCell� burner retrofit;and the micronized coal reburning demonstration for NOx control on a 175 MWe

wall-fired unit [3, 27]. Future developments, including improvements in efficiency,boiler design, materials of construction, and environmental performance, are pre-sented in Section 2.4.

Pulverized coal-fired furnaces are usually classified according to themethod of ashremoval into �dry-bottom� and �wet-bottom� furnaces. The �dry-bottom� furnacesare simpler, more flexible with regard to load range and fuel properties, and more


reliable than �wet-bottom� furnaces. However, they are larger for the same capacity(and thus more costly) and 80–90% of the ash leaves the furnace as fly-ash, whichmust be removed in the electrostatic precipitator. The interest in developing �wet-bottom� units was to avoid this problem of dust disposal as much as possible. Themolten ash flowing from the furnace is quenched by water and reduced to a coarse,granular solid. Ash retention of more than 80% has been achieved with somedesigns. At the same time, higher heat release rates are used, in an effort to reduceequipment size [22, 23].

2.3.2.1 Dry-Bottom FurnacesIn dry-bottom firing, the ideal furnace is so designed that the flames clear thewalls, atmost lightly brushing them. The temperatures developed near thewalls produce littleor no ash melting and the products of combustion are cooled sufficiently beforeleaving the furnace to prevent troublesome ash adherence in the convection banks.This ideal is rarely achieved, and most actual units experience some slagging and/orfouling of heat transfer furnaces. Figure 2.2 shows the most frequently used furnaceand burner configurations. These arrangements cover firing systems suitable for allranks of coal.

In horizontal and opposed horizontal furnace types, coal–airmixtures are blown inon opposite sides of the firebox and impinge on one another near the center of thefurnace. These configurations are usually fired by circular turbulent burners, spaceduniformly across the width of the furnace, in either the front- or rear-wall or both.Each burner has its own relatively independentflame envelope and ignition point andthe firing system may be designed so that an individual burner can be placed inservice, adjusted, or removed independently of the other burners. This type ofcombustion produces high flue gas temperatures and carbon burnout, but has thedisadvantage of producing high NOx levels, too [23].

In tangential and opposite inclined firing furnace types, jet burners project thestreams of coal and air along a line tangential to a small circle, lying in a horizontalplane, at the center of the furnace. The burner turbulence ranges from nil tomoderate, according to the designer�s intent and is replaced by the overall furnaceturbulence. In such furnaces there is a single overall flame envelope. Flame length islong and combustion of coal is rapid. These characteristics give the designer a degreeof control over the peak flame temperature, which can form low levels of NOx

formation, although care must be taken to avoid flame instability at partial loads,when the individual burners tend to assume their own identity. It has also beenargued that such firing arrangements are less vulnerable to unequal proportioning ofair and fuel to the burners, but this can cause imbalance of the superheater, orreheater element temperatures [26, 28].

Single U-flame and double U-flame furnaces are used for firing fuels that are hardto ignite. The fuel with its transport air and a portion of the necessary combustion airis fired vertically downward through a burner arch, located about halfway up thefurnace. The remaining air required for combustion is admitted through the verticalwalls below the arches. Radiation from the rising portion of the coal flames and fromthe burners in the opposite arch (in double U-flame units) helps to assure stable


ignition over a good load range. Experience has shown that the important variablesfor the design of such configurations are the proportioning of furnace volumebetween lower and upper parts, the furnace shape, and the amount and velocity ofcombustion air.

Vertical firing is not frequently used, but it has been applied to units below �150MW.Above this size, an uneconomically large furnace cross sectionwould be neededfor the required burners to be placed in the roof. Although downward firing isinherently stable, it has several disadvantages, such as a tendency toward burneroverheating damage from slag falls and the difficulty of purging combustibles fromthe top of the furnace [3, 21].

2.3.2.2 Wet-Bottom FurnacesEarly wet-bottom furnaces were simply open, single-stage furnaces. The hightemperature necessary for ash melting and retention was produced by locating

Figure 2.2 Dry bottom furnace and burner configurations: (a) horizontal, (b) opposed horizontal,(c) tangential, (d) opposed inclined, (e) single U-flame, (f) double U-flame, and (g) vertical.


burners close together, near the furnace bottom, and using high coal fineness andhighly preheated air.

For coal ash that is difficult to melt and when a load range of 4 : 1 (or greater) isessential, more sophisticated designs of the closed type have been developed, severalof which are shown in Figure 2.3. All these later furnaces are two-stage combustionchambers. In the first stage, the temperature is sustained above the ash flowtemperature, while in the second stage the gases and the entrained ash are cooledbelow the point of troublesome adherence to convention surfaces in boiler, super-heater, and reheater [23].

In favorable circumstances, the collected ashmay be reinjected into the furnace toreduce particulate emissions and combustible loss. The operation of a reinjectionsystem involves additional equipment, auxiliary power, and maintenance costs and

Figure 2.3 Wet bottom, two-stage furnaces.


frequently results in erosion of boiler tubes, fans, and collectors. Thus, many suchinstallations have been abandoned as uneconomical.

Wet-bottom firing is purely an expedient, aimed at easing the problem of ashdisposal. It may also result in a slightly higher boiler efficiency with fuels containingless than 20% ash, because of the lower combustible loss incurred and the lowerexcess air thatmay be tolerated.With higher ash contents, the losses caused by fusionand sensible heat of the slag generally outweigh any gains and a net decrease inefficiency results. Other disadvantages, compared with dry-bottom firing, are lessflexibility of fuel selection, higher incidence of fouling and corrosion, higher levels ofNOx formation, and lower average steam generator availability [3, 21].

2.3.3Cyclone Furnaces

The cyclone furnace was developed in the mid-1940s by Babcock and WilcoxCompany, as a high-temperature, high-turbulence combustion device that operatesseparately from the heat transfer sections of the boiler. Although the furnace wasoriginally developed for low fusion temperature coals, it has been applied success-fully to all coal ranks.

The cyclone furnace can burn relatively coarse coal (�6mm) and slurried fuel. Inaddition, it generates heat releases up to �20000MJ hm�3 of combustion space, ascompared to at best 5800 and 15 600MJ hm�3 in dry- and wet-bottom boilers,respectively, and its size is reduced. Furthermore, owing to the centrifugal forcesand to the presence of molten slag on the walls, the caustic carry-over of fly ash isconsiderably lower in the cyclone furnace than is in the other combustion systems.However, this type of firing favors excessive NOx formation [3, 20, 23].

A cyclone furnace is a water-cooled, refractory-lined horizontal cylinder, inwhich air enters tangentially and imparts a whirling motion. Coal and primary airare introduced at the burner end of the cylinder. The furnace temperature, in the1650 �C range, is sufficient to fuse most coal ash on the refractory walls. The coalparticles are entrained in the high velocity stream and thrown against the furnacewall by centrifugal force, where they are held in the slag layer. Molten slag drains tothe bottom of the furnace and is discharged, while gaseous products flow fromthe discharge and directly into the radiant heat transfer section of the boiler(Figure 2.4).

In cyclone firing, the fuel characteristics are of greatest importance for the designof the furnace. The ash should be of low to moderate fusion temperature and theviscosity of the slag must be sufficiently low (250 P at 1425 �C), so as to permit slagflow at normal furnace operating temperatures. Coals high in sulfur, or having a highratio of iron to calcium plus magnesium, are unsuitable for the cyclone furnace. Inaddition, the moisture content per unit heating value should be limited to thatproducing a calculated adiabatic cyclone temperature above 1870 �C. Fuels, thatcannot produce such a temperature can be assisted by supplementary firing [3, 21].

The use of cyclone furnaces in future applications does not appear likely, due to theexcessive amounts of NOx they generate.


2.4Advanced Clean Coal Technologies

2.4.1Fluidized-Bed Combustion

Fluidized-bed technologies are among the most important recent development incoal combustion. Nowadays, they compete with the stoker boilers in small sizes andwith the pulverized-coal fired boilers in large sizes.

The fluidized-bed consists of a bed of solid particles suspended through theturbulent motion of combustion air distributed from below. The solid particles aremostly inert particles, such as sand, coal ash, or sulfur sorbents, such as limestoneand dolomite. Coal particles make up only around 1% of the bed mass. At lowvelocities, the air passes through the spaces between the solid particles and the bedremains fixed. At high velocities, the air flows through the bed in bubbles and thesystem behaves similarly to an agitated fluid, hence the name �fluidized.� Atvelocities approaching or greater than the free fall velocity of the particles, theparticles are entrained out of the furnace, collected in cyclones, and circulated back tothe bed (Figure 2.5).

The principal advantages of fluidized-bed combustion over the conventionalpulverized coal approach are: (i) high heat transfer rates in the bed, resulting incompact units and thus lower capital and maintenance costs, (ii) increased com-bustion efficiency and heat release rates, up to 3MWm�2 of bed area, (iii) less foulingand corrosion of furnaces, because combustion temperatures are well below thefusion temperatures of the ashes, (iv) combustion at substantially lower (below

Figure 2.4 Operating principle of cyclone furnaces.


1000 �C) and more evenly distributed temperatures, resulting in reduced NOx

emissions, (v) substantial reduction of SOx emissions, due to the use of sulfursorbents in the bed, (vi) easy to handle and usefully employed by-product material,and (vii) fuel flexibility and ability to use low-grade coals, including those with highash, as they operate with a low inventory of combustibles in the bed [22, 29].

However, there are also some disadvantages: (i) commercially proven at smallerscales, compared with pulverized-coal combustion, (ii) relatively large amounts ofsolid residues generated, some of which require special measures for disposal, (iii)higher carbon-in-ash levels than those from pulverized-coal combustion, and (iv)increased N2O formation, due to the lower combustion temperatures [30].

Depending on system pressure, fluidized-bed coal technology consists of twobroad categories of processes: the atmospheric fluidized-bed combustion (AFBC)and the pressurized fluidized-bed combustion (PFBC).

2.4.1.1 AFBC Process

2.4.1.1.1 Process and Key Issues The AFBC process is typically applied to largeindustrial boilers (90 t h�1 of steamor greater) andutility boilers for the production ofsteam for process needs, heatingneeds, and/or electricity generation. The combustorutilizes a bubbling bed or circulating bed configuration.

In the bubblingfluidized bed (BFBC), the crushed coal (particle size 1–40mm) andlimestone are fed to the bed, which is preheated to 430–540 �C, depending on the

Figure 2.5 Coal and gas velocities versus bed expansion for fixed bed, bubbling and circulatingfluidized bed and transport reactors.

2.4 Advanced Clean Coal Technologies j47

particular coal properties (Figure 2.5). The stoichiometric feed rate for limestone is3.1% of the coal feed rate for every 1% of sulfur in the coal [5]. An upward air flow isintroduced into the bed via a distributor plate. When the gas velocity exceeds theminimum fluidizing velocity, the excess gas passes through the bed as bubbles andthe remainder of the gas leaks through the bedmaterial. The bed is then considered tobe bubbling. In practice, BFBCs are operated at gas velocities several times higherthan the minimum fluidizing velocity.

When the coal particles burn, the bed attains its operating temperature, usually800–900 �C, which is maintained uniform due to the high heat transfer rates in thebed and the removal of heat with an in-bed heat exchanger. The low bed temperaturereduces slagging, fouling, and NOx formation, while the limestone after calcinationreacts with SO2 to form calcium sulfate. The residence time in the bed is about 1min,which is usually sufficient for 80–90% burnout.

In the freeboard zone an additional 10–20% of coal burnout is achieved, wheresecondary air is often introduced. However, the small particles are entrained andmust be removed in a cyclone. The unburned carbon and the unreacted limestonecontents in the separated particles, are quite high and justify recycling in the bed. Thefly ash is retained in either an electrostatic precipitator or a fabric bag filter. The fluegases, after exiting the combustor, pass into a convective sectionwhere heat is furtherrecovered and they are cooled and cleaned [3, 22].

For a BFBC, the fluidizing velocity typically ranges from 1 to 3m s�1. The actualvalue reflects a compromise between the capital cost, bed pressure drop, andefficiencies of combustion and sulfur retention. The mean bed particle diametertypically ranges from 0.5 to 1.5mm and depends mainly on the choice of fluidizingvelocity. Fine beds require less heat transfer surface than coarse beds. The bed depthis typically between 0.3 and 1m when fluidized. A minimum bed depth should bemet, to provide sufficient free space in the bed for lateral mixing to occur, thusavoiding temperature gradients and to accommodate the tube bundle. Shallow bedshelp to reduce the fluidizing air fan power consumption; however, they decrease thecombustion and sulfur capture efficiencies. In BFBC, it is desirable to operate at thehighest possible temperatures, while avoiding ash sintering and alkali volatilization,which can cause corrosion. The 800–900 �C bed temperature favors sulfur retention.High excess air levels increase themass flow rate of hot combustion gases emitted tothe atmosphere and thus reduce the boiler efficiency. However, low excess air levelsresult in reduced combustion efficiencies. Depending on other design parameters,the optimum value can vary from 10 to 50% [5, 30, 31].

The circulating fluidized-bed combustion (CFBC) boilers are similar in manyrespects to BFBC boilers. The differences stem from smaller coal and limestoneparticle sizes (mean values 100–300 mm) and higher gas velocities (4–6ms�1) [21].Thus, the boilers are tallerwith a smaller cross section andusually they do not includein-bed heat exchanges, because of potential high velocity erosion. As convective heattransfer coefficients are increased, due to the use of smaller particles, less heattransfer tubing is required in the fluid bed cooler than in the bed of a BFBC [30]. Thefeeding system requires an order of magnitude fewer feed points, because ofenhanced solids mixing, while the cyclone is located before the convection pass to


protect the convection surfaces from erosion, due to high solids loading and velocity,and operates at high temperatures [22].

As schematically shown in Figure 2.5, the bed fills the entire furnacevolume, although most of the mass is still in the lowest third of the bed. A largeportion of the solids is carried out of the furnace, separated in the cyclone andreturned to the furnace. The recycling solids act as a heat carrier to remove the heatgenerated in the combustor, which generally operates at a low excess air level.Combustion takes place both in the furnace and in the cyclone. In some designs, therecirculated solids are fed to an external heat exchanger, where part of the evapo-ration, superheat, or reheat duties may be carried out, prior to return of the solids tothe combustor [32].

Among the advantages of the CFBC are the ability to burn low calorific value fuelscontaining a high proportion of incombustible matter and the high heat transferrates, allowing for better load capabilities. In addition, due to staged combustion,NOx

formation is reduced. Furthermore, due to the use of finer particles, turbulentgas–particle mixing, and a high recycle rate, the desulfurization occurs at fasterreaction rates, greater limestone utilization, and higher overall sulfur capture, so thathigh sulfur retention efficiencies with low Ca/S (<1.5) can be expected [33].

Despite the above attractions, there are some areas of concern with CFBCtechnology. Firstly, the pressure drop is generally greater than with a BFBC, whichresults in increased fan power requirements. Secondly, the large recycle rates requirehigh efficiencies of cyclone, and the high gas velocities combined with highparticulate loadings may lead to system erosion [30, 31].

2.4.1.1.2 Current Status and Experience Though fluidized bed combustion boilersfirst appeared in the 1920s, their use for power generationwas developed in the 1960sand the 1970s, both in USA and in Europe, and there was further rapid growthbetween 1985 and 1995 [25].

Commercial development of BFBC for power generation began in the mid-1970s,when European companies installed the first BFBC boiler burning coal in Scotland.After that several plants were constructed in the USA and Japan. In 2000, the totalinstalled capacity of BFBC power plants larger than 50 MWth worldwide reachedabout 8000 MWth. However, the capacity growth has slowed in recent years, partiallybecause of increased competition from CFBC. Several companies have supplied orcontinue to supply BFBC boilers on a worldwide basis. Themajor suppliers appear tobe Foster Wheeler, Kvaerner, Lurgi, andMitsui Babcock. Two large-scale demonstra-tions on retrofit applications have been carried out: the 160MWeTVAsShawnee plantUnit 10 in the USA, which started up in 1988, and the 350 MWe EPDCs Takeharaplant Unit 2 in Japan, which started up in 1995. For industrial applications, mostinstallations fall in the size range 30–100 MWth, with a few in the range 150–280MWth. The latter have been introduced in Japan, Finland, and Thailand,mostly usingKvaerner technology. However, a maximum size of 300 MWth has also beenclaimed [8, 30, 32, 34].

BFBC plants can have a typical availability exceeding 94% and an efficiency ofaround 30%. Experience has shown that they can achieve NOx emissions of


250–400mgm�3, N2O emissions of 50–200 ppm, SOx emissions of �200mgm�3,and particulate emissions of 20–25mgm�3 [34–36].

The first commercial CFBC unit was started up in Finland in 1979. Following thissuccessful installation, the number of installations has increased rapidly in the lastfew decades and CFBC has by far overtaken BFBC, in terms of installed capacity(Table 2.2). The total installed capacity worldwide is about 65 GWth. Asia (mainlyChina) represents about 52%, North America 26% and Europe 22% of the totalinstalled capacity. The market leaders are Foster Wheeler, Alstrom Power, Kvaerner,and Lurgi-Lentjes-Babcock [8, 30, 32, 37].

To date, CFBC units in operation range in size from a fewMWth to 300 MWe. Thefirst-generation technology reached its peak size with two 300MWe boilers at JacksonEnergy Authority�s Northside plant in Florida, USA. The largest second-generationCFBC units are the 265 MWe units at the Turów plant in Poland. A 460 MWe boiler,which is under construction at Lagisza (Poland), will be the world�s largest CFBCunit. Multiple units have also been adopted. With a subcritical cycle, the plantefficiency is normally between 38 and 40% on a LHV basis, while the availability is inexcess of 90%. CFBC has proved to be able to utilize all types of coal, even those withhigh ash and sulfur contents. The NOx emissions are only around 1/5 of thoseproduced by uncontrolled pulverized-coal combustion (Table 2.3). For most plants,uncontrolledNOx emissions are less than 400mgm�3, while if controlled they can bebelow 130mgm�3 (300 MWe JEA plant designed by Foster Wheeler and funded byDOE, USA [29]). The level of SO2 emissions is generally below 400mg m�3 withoutflue gas scrubbing and particulate emissions of 20–50mgm�3 have been achieved byusing bag filters or electrostatic precipitators [30, 32, 39–44].

2.4.1.1.3 Future Developments A key area for future development of BFBC tech-nology is further increasing fuel flexibility, to extend the range of biomass and wastebeingutilized. BFBCrequiresmuchhigherCa/S ratios for sulfur retention compared

Table 2.2 Steam conditions for large circulating fluidized-bed combustion (CFBC) plants [30].

Plant Plant size (MWe) Superheated steam pressure(bar)/temperature (�C)

Reheated steam(bar)/temperature (�C)

Turów, Poland 3� 235 132/540 25/540Red Hills, USA 2� 250 181/568 37/540Guayama, Puerto Rico 2� 255 174/541 41/541Turów, Poland 3� 260 167/565 39/565Seward, USA 2� 292 174/541 47/541Gilbert, USA 1� 294 174/541 42/541JEA, USA 2� 300 172/538 38/538Baima, China 1� 300 174/540 37/540Sulcis, Italy 1� 340 170/565 36/580Lagisza, Poland 1� 460 275/560 55/580


with CFBC, thus increasing the sorbent cost. Any improvements in sorbent utili-zation efficiency are therefore desirable. As erosion and corrosion of the boiler tubesremain an important issue, work will continue to further improve materials ofconstruction, for example, improved and/or new refractory and alloy coatings. Withregard to the technology size, there is little expectation of further significantincreases. With a size range of 3 to �300 MWth, BFBC will continue to be usedfor industrial boilers and small power units.

For CFBC, improvements that could still be made include reducing carbon losseswith solids discharge and improving fuel flexibility for co-firing with non-coal fuels.Thermal cracking of refractories remains an issue, in CFBC plant, especiallycyclones. Recent designs employing thinner refractories are being used to combatthis [8, 25, 32]. CFBC boilers without external cyclones and with a reduced refractoryinventory are also being offered by somemanufacturers [45]. Several market leadershave been actively developing ultra-large-scale CFBC boilers. Lurgi has recentlylaunched the 500 MWe class of subcritical CFBC boilers on the market. Designs ofbigger capacity (Figure 2.6) and near-zero CO2 CFBC are supercritical units and arediscussed in Sections 2.4.2 and 2.4.3.

CFBCcan be used as part of advanced cycles, based on both combustion and partialgasification of coal. This concept has led to the development of the air blowngasification cycle (ABGC) in the UK. In the ABGC, coal is gasified in an air-blownpressurized fluid bed gasifier to produce fuel gas for a gas turbine and the residualchar is transferred to a CFBC combustor, where it is burned with additional coal toraise steam for a steam turbine. Preliminary studies have indicated that an efficiencyof over 50% is possible [30, 32, 46].

Table 2.3 CFBC versus PC typical emissions [38].

CFBC PCO2 content (%)

7 7 6 6Bituminous coal Lignite Bituminous coal Lignite

NOx (mg m�3)Inherent to system <200 <200 800–1300 500–800Low NOx burners 300–500 <200With SCR <200SO2 (mg m�3)Inherent to system 200–4001a) 200–400b) 2000a) 12 000b)

With FGD <200Ca/S ratio 2.7–1.7 2.5–1.5 1.05 1.05CO (mg m�3) 100–200 20–30 20–50 130–180Cl capture efficiency (%) 20–50 20–50 �90 �90F capture efficiency (%) 90 90 �60 �60

a) Coal sulfur content 1%.b) Coal sulfur content 2.5%.


2.4.1.2 PFBC Process

2.4.1.2.1 Process and Key Issues Pressurized fluidized bed combustion is based oncombustion of coal under pressure, in a deep (3–4m) bubbling fluidized bed at800–900 �C and 1–1.5MPa (Figure 2.7) [32]. In addition to retaining all of theadvantages of AFBC operation, PFBC also offers: (i) a smaller combustion chamberfor a given heat duty, because the reaction rate is increased with operating pressure –thus, PFBC is particularly suitable for retrofit applications; (ii) a combustionefficiency such that a properly designed unit should eliminate the need for solidsrecycle; (iii) increased fuel flexibility – practically all types of fuel, including high ashor high moisture coals, can be burned; (iv) improved sulfur retention with pressure,using dolomite rather than limestone; and (v) NOx emissions that are significantlylower than at atmospheric pressure [3, 8, 45].

Figure 2.6 Scale-up of Foster Wheeler�s circulating fluidized-bed combustion (CFBC) boiler [30].


PFBC has a relatively high heat release per unit of bed area, due to the higherpressure. As a result, adequately high throughput can be obtained at lower fluidizingvelocities of about 1m s�1, compared with 2–3m s�1 for BFBC. The area of heattransfer surface must be increased in proportion to the pressure and the bed isconsequently deeper than in combustion at atmospheric pressure, to submerge theextra tubes. Although deep beds need high fan power, at elevated pressures theadditional pressure drop has less effect on the compression energy requirements.However, the operating temperature is limited by the ash fusion temperature of thecoal and the vapor point of the alkali in the coal [31, 40, 48, 49].

Two basic approaches are being pursued to attain improved thermal efficiency.One uses a combined steam–gas turbine cycle. The heat extracted from the fluid-bed serves to generate steam for driving a steam turbine and the combustion gas isexpanded through a gas turbine. The exhaust gas then passes through aheat exchanger to generate more steam. In this combined cycle, the gas turbineprovides typically 20% of the total power generated, while the steam turbinecontributes the balance (80%) [50]. The other approach uses an inert gas or air ina closed cycle for bed heat extraction. To reduce stack gas temperature, however,steam still must be generated downstream of the combustion gas turbine, as well asin the interstage coolers of the closed cycle turbine. The use of combined gas andsteam cycles provides the PFBC power generation system with a 3–4% efficiencyadvantage over AFBC systems (Figures 2.8 and 2.9) [3]. Through different processvariants, in particular flue gas re-heating, it is possible to effectively raisethe mean process temperature and thereby the efficiency of the plant above 45%.Intense work is in progress to ensure that the exhaust gases exiting the pressurizedboiler are sufficiently clean, so as not to cause erosion or build up on the gas turbineblades.

Figure 2.7 Pressurized bubbling fluidized bed combustion system [47].


If PFBC is of a circulating type, then the heat release level is particularly high (up to40 MWm�2) and capital and operating costs are reduced [30, 35].

2.4.1.2.2 Current Status and Experience PFBCwas developed in the late-1960s in theUK. Today, ABB Carbon is the leading supplier of PFBC technology. Other suppliersinclude Foster Wheeler in Finland, Lurgi-Lentjes-Babcock in Germany, Babcock andWilcox in the USA, and Ishikawajimma Heavy Industries (IHI), Mitsubishi Heavy

Figure 2.8 Coal-fired combined-cycle power plant.

Figure 2.9 Alternate cycle efficiency versus fluidized bed combustor temperature.


Industries (MHI), and Hitachi in Japan. The installed capacity worldwide is about1125 MWe. The first commercial boilers were installed at V€artan in Sweden (135MWe, 1990). Further plants have been built in Spain (Escatrón 79.5MWe, 1991), USA(Tidd 73 MWe, 1991), Japan (Wakamatsu 71 MWe, 1995; Tomatoh-Azuma 85 MWe,1996; Karita 360MWe, 1999; Osaki 250MWe, 2000), andGermany (Cottbus 70MWe,1999). The Karita plant, the largest unit in the world, operates at supercritical steamconditions (Figure 2.10) [8, 25, 45].

PFBC has performed satisfactory with a wide range of fuels. However, there is agreater efficiency penalty associated with the use of high ash or high sulfur coals, dueto the increased pressure. The uncontrolled NOx emissions are generally less than200 ppm [51]. Depending on the Ca/S ratio, sulfur retention efficiencies of 90–98%have been achieved. SO2 emissions vary from 5 to 350 ppm, while particulateemissions are also low (3.5–76mgm�3) (Table 2.4).

All the PFBC demonstration plants have suffered from various problems sincestart-up, such as the failure ofmaterials of construction.Most of these problems havesubsequently been resolved.

PFBC technology remains in the early stages of development. A series of tests withdifferent coals and limestones has shown that the technology can achieve extremelylow levels of emissions [30, 32, 52].

2.4.1.2.3 Future Developments As a relatively new technology, PFBC has consid-erable scope for future technological improvements and developments. There havebeen various R&D activities addressing the following issues: (i) freeboard firing tomaintain a constant flue gas temperature. In this way, the inlet temperature to theexpander section of the gas turbine increases, thus improving the plant�s efficiency.(ii) Fly ash recirculation to improve both sorbent utilization and combustionefficiency. This in turn improves environmental performance, plant efficiency, andeconomics. (iii) Advanced cycle with an enhanced gas turbine inlet temperature. A

Figure 2.10 ABB P200 PFBC system [8].


concept has been developed to feed the vitiated air from the PFBC combustor, afterremoving particulates, to a topping combustor located at the inlet to the gas turbine,fuelled by natural gas. Alternatively, because natural gas represents a higher cost, coalcan be gasified in a fluid bed to provide fuel gas as a topping fuel, resulting in a gasturbine inlet temperature of �1300 �C. (iv) Further demonstration of hot gasfiltration. The filter materials must have a sufficient resistance to high temperaturesunder either oxidizing or reducing conditions [27, 30, 32, 45, 53–57].

For pressurized circulating fluidized bed combustion (PCFBC), gas turbineoperation in a �high dust� environment requires further research. As with advancedPFBC, hot fuel gas and flue gas cleanup remains a critical development area.

2.4.2Supercritical Coal Combustion

2.4.2.1 Process and Key IssuesThe principal interest in using supercritical steamconditions is the potential increasein thermal efficiency. Higher steam conditions allow higher thermal efficiencies,through higher pressure ratio turbines and larger temperature differences betweenthe hottest and coolest parts of the thermodynamic cycle. The choice of steampressure determines whether the boiler is subcritical or supercritical. The latterappertains to pressures above 22.1MPa, when vapor and liquid are not distinguish-able and the water exists as a supercritical fluid. Supercritical conditions requirechanges in furnace tube configurations and design, as well as the use ofdifferent alloys in key areas [57]. Boilers of once-through forced flow are necessary(Figure 2.11). In current plants, double reheat may be used to take full efficiencyadvantage of the high main steam conditions, although the associated capital costincreases may be justifiable only where low condenser pressures are usable at coldseawater sites.

Table 2.4 Emissions achieved at PFBC demonstration plants [30].

Plant V€artan Tidd Escatrón Osaki Karita

NOx withoutcontrol (ppm)

165–191 86–102 120–170 14.4

NOx withcontrol (ppm)

20–33(SNCR)

35–42(SCR)

Sulfur retention (%) 96–98 93 90–95 97.7Ca/S molar ratio 3.3 2.0–2.2 1.7–2.0Ca/S at 90%sulfur retention

2.0 1.8–1.9 1.8

SO2 emissions (ppm) 5–9 350 7.1 7–36Particulates (mg m�3) <30 18 76 �3.5

(two-stagecyclones þbag filters)

5–15 (two-stagecyclonesþ ESP)


Supercritical pulverized-coal combustion plants currently operate at up to 30 MPaand 600 �C with net efficiencies of around 43–45% (LHV), depending on coal typeand plant location. Efficiencies reaching 50–55%, with steam temperature above700 �C and pressure in the region of 30–40 MPa, are possible provided that newmaterials adequate for these conditions are developed. The higher thermal efficiencyoffered by supercritical plants reduces all specific emissions in comparison withsubcritical units, as less fuel needs to be burnt for each mist generated [32, 57, 58].However, the higher surface temperature of the superheater and reheater tubes maycause an increased tendency for coal ash deposition [54, 59].

2.4.2.2 Current Status and ExperienceSupercritical pulverized-coal combustion is a well-proven technology with severaldecades of experience and operation, including some with low value coals. Recentdevelopments in alloy steels have facilitated the flexible operating conditions re-quired for modern plants. Substantial improvements in pollution control technologymean that emissions can be controlled within acceptable limits. Unit sizes to 1000MWe exist. Such units have been built in China, Denmark, Finland, Germany, Japan,The Netherlands, Republic of Korea, and Taiwan.

Capital costs of supercritical plants are 2–3% higher than subcritical plants, butthese costs are offset by lower fuel costs and lower emissions. Developments areongoing, including improvements in the materials of construction and in the designof the boilers and turbines. Spiral mound furnace tube designs, in the zone near thepulverized coal burners, have commonly been adopted to give a longer path for thefluid within the furnace wall tubes, to prevent tube damage arising from overheating.In recent years, rifled tube designs have been available (e.g., the 1000 MWe Misumi

Figure 2.11 Once-through supercritical boiler type.


power station, Japan), that give improved heat transfer and allow vertical tubing to beused throughout the furnace and give greater operating flexibility. Current materialsbased on ferritic/martensitic alloys permit steam temperatures up to around600 �C [60].

There are more than 520 supercritical units in operation worldwide, with about 60in Western Europe. These show an efficiency advantage of up to 3% compared withsubcritical units and have comparable outage rates. The double reheat thermody-namic cycles of the Skaerbaeck and Nordjylland units in Denmark achieve netefficiencies approaching 50% [25, 32, 60–62].

Recently, FosterWheeler designed the 460MWe Lagisza unit in Poland, whichwillbe the world�s first CFBC boiler to incorporate supercritical steam parameters. It willoperate at 27.5 MPa and 560 �C main steam, with 580 �C reheat steam. A net plantefficiency of greater than 43% is expected. Commercial operation started in 2009 [63].In addition, the ABB Carbon P800 module Karita plant has advanced supercriticalsteam conditions of 241 bar, 570/595 �C, and a net thermal efficiency �44%(LHV) [30, 32].

2.4.2.3 Future DevelopmentsPressures to reduce environmental impactswill continue to drive advances influegastreatment technologies for conventional pollutants from supercritical pulverized-coal combustion, while recent heightened interest in developing near-zero emissiontechnologies will accelerate such programs and means of achieving low CO2. In theUK, according to the UK Joint Energy Security of Supply Working Group, there arethree utilities (E.ON, Scottish-Southern Energy, and RWE npower) consideringreplacing existing coal-fired plants that are due to close with supercritical technology,with the option of adding carbon capture capability later [58].

The properties of iron-based alloys will probably be inadequate to allow theefficiency of the supercritical steam cycle to be extended much above 50%. Beyond2015, efficiencies approaching 60% are envisaged using nickel-based �superalloys,�which were developed for use in gas turbines and fast breeder nuclear reactors. TheEuropean Union is supporting the development of a power plant (commissioned for2013) that will rely on the use of superalloys for themost highly stressed components.The main steam pressure would be approximately 37.5 MPa with a steam temper-ature around 700 �C and the expected efficiency is over 55% (LHV). The use of nickelalloys has cost implications that have still to be resolved. In the USA, materials forsteam temperatures up to 870 �C are also being considered [32, 45, 64].

2.4.3In Situ Emissions Control Technologies

2.4.3.1 SOx Control TechnologiesIn-furnace desulfurization is a competitive technology for controlling the SOx

pollutants derived from coal combustion, due to the low capital and operating costs,in contrast to flue gas desulfurization techniques. It is suitable when a moderate(30–70%) SOx removal efficiency is acceptable, especially for old boilers, small


boilers, or retrofit applications. A sorbent injection system is easy to install, operate,andmaintain, eliminating the problems of plugging, scaling, and corrosion found inslurry handling and no wastewater is generated. Hybrid systems may combinefurnace and duct sorbent injection, or introduce a humidification step to reachremoval efficiencies of up to 80–95%. However, these are considered here as post-combustion control technologies [21, 33].

Sorbents include calcium, sodium or magnesium-based compounds, calciumorganic salts, and metal oxides of zinc, iron, and titanium. However, calcium-basedsorbents, such as limestone and calcium hydroxide, are particularly attractive, due totheir low cost and the inertness of the calcium sulfate or calcium sulfide by-product [65].

For pulverized coal furnaces, where temperatures are high, CaSO4 decomposesover 1200 �Cand therefore it is promising to utilize the temperature and burning gasdistribution to produce a-CaSO4 phase and CaS as sulfation products in furnaces.Also, the staged desulfurization process combined with an air-staged combustionpattern, in which sorbents are injected into the primary air field and the upperfurnace to capture SO2 under reducing and oxidizing atmospheres, is promising forpulverized-coal combustion, as is the desulfurization process by flue gas recircula-tion under O2/CO2 conditions. It is valuable to study further these two advancedprocesses, which can give efficiencies of about 80% in furnaces [66].

The development of FBCprovided the opportunity to effectively retain sulfur in thecombustion process, because CaSO4 is stable at the FBC operating temperature of800–900 �C. Generally, the sulfur removal efficiency is increased by increasing therate of sorbent injection. The theoretical addition retains about 80% of the sulfur,while double the theoretical rate retains about 95% of the sulfur. This process has thepotential to generate large quantities of solid by-products, which can present asignificant disposal problem, and hence it is desirable to ensure that the sorbent isused as efficiently as possible. In CFBC, where the gas velocities are higher, thenumber of feed points is smaller, which is an operational convenience. Also, smallersize limestone particles can be used in the feed, which improves the sulfur captureand reduces the Ca/Smole ratio necessary to reach a target value [3, 5, 45, 59]. Resultsfrom PFBC demonstration plants have confirmed that sorbents can perform signif-icantly better under pressurized conditions than at atmospheric pressure. Table 2.4gives the environmental performance of selected PFBC plants.

2.4.3.2 NOx Control TechnologiesThe need to reduce NOx emissions from coal-fired boilers has gained increasedattention in recent years, as more is learnt about the environmental impact of toxicNOx in the form of acid rain, smog, visibility impairment, and climatic warming.

NOx are formed in fuel-lean flames by the attack of an O atom on molecularnitrogen (thermal), in fuel-rich flames via capture of nitrogen by hydrocarbonradicals (prompt), and by the pyrolysis and oxidation of heterocyclic nitrogencompounds in coals (fuel). Factors that influence NOx emissions in coal-fired boilersare coal properties, boiler design, and operation. The relative inertness and insol-ubility of NOx makes flue gas treatment more difficult than for SOx removal.


Consequently, it is often easier to intervene right at the point of fuel combustion toavoid the formation of NOx and their precursors. The principles of the various in situcontrol methods include reducing peak flame temperatures, reducing the residencetime at peak flame temperatures, chemically reducing NOx, oxidizing NOx withsubsequent absorption, removing nitrogen, or a combination of thesemethods. NOx

is reduced chemically by reducing the valence level of nitrogen to zero, after it hasbecome higher. Oxidizing NOx intentionally raises the valence of the nitrogen ion toallow water to absorb to it. This is accomplished by using a catalyst, injectinghydrogen peroxide, creating ozone within the air flow, or injecting ozone into theair flow. Nitrogen is removed from combustion as a reactant by using either lownitrogen content fuels or oxygen instead of air. The simplest and least expensivetechniques that can reduce NOx emissions by 50–80% are: (i) low excess air, (ii) fluegas recirculation, (iii) staged air combustion, (iv) staged fuel combustion, and (v)burner design [3].

Maximum combustion efficiency normally requires 20–30% more air than thestoichiometric amount, depending on the boiler type and the properties of the coalburned. If the excess air is reduced from 25 to 15%, NOx emissions will generally bereduced by approximately 20% [67]. However, as excess air levels decrease, carbonmonoxide, hydrocarbon, and particulate concentrations will increase. The trade-offsin these emissions, nitrogen oxide reductions, and combustion efficiency must bemonitored to achieve the best air-to-fuel ratio.

Recirculation of flue gas dilutes the inlet oxygen concentration and lowers thecombustion zone temperature, thus primarily affecting thermal NOx [5, 68]. Inpulverized-coal boilers, reductions in NOx concentration of up to 30% have beenobtained. Flue gas recirculation has been used commercially for many years.However, the high cost of equipment and the energy penalty attributed to recircu-lation fans make this modification generally unattractive.

In staged air combustion, the amount of air introduced into the burner is less thanstoichiometric and the remainder of the air is added into the boiler through separateports. The advantage is that the reduced amount of oxygen in the burner tends to reactmore with the fuel than with nitrogen in the air and the remainder of the oxygenreacts in an area where the temperature is lower. As a result, there is an overallreduction in NOx formation. This technique is effective in reducing NOx emissionsby 30–70% [36, 69], it is inexpensive, and does not affect boiler efficiency, but can alsoproduce reducing conditions on the tube walls, with the resulting danger of slagging.This can be limited by introducing some air near the walls. In addition, unburnedcarbon levels in the ash may increase and a loss in steam temperature might occur.Nevertheless, the use of staged air combustion has been regarded as the mostsuccessfulmethod forNOx control and overfire air systems have been included in thedesign of new coal-fired boilers, to meet current NOx emission standards.

Staged fuel combustion involves injecting fuel into more than one combustionzone in the boiler. The objective is to inject a sub-stoichiometric part of the fuel withthe bulk of the combustion air in the primary combustion zone. This produces a veryfuel-lean zone, which reduces the formation of thermal NO, by lowering the peaktemperature. However, the potential for formation of fuel NO may be increased. In


the secondary combustion zone, where re-burning fuel is added, the overall air-to-fuel ratio is maintained fuel-rich. Fuel fragments are produced, which react with NOto produce HCN. Subsequently, HCN favors reduction to N2 under the prevailingfuel-rich conditions. Final air addition is then employed, to burn out the unoxidizedhydrocarbons at lower temperatures that do not favor thermal NO formation. Themost effective re-burning fuels are volatile, low-nitrogen containing fuel oils andnatural gas, although coal has been successfully applied in some pilot-scale tests. Theuse of natural gas offers the advantage of additionally reducing SO2, CO2, andparticulate emissions. A 40–70% reduction in NOx emissions can be achieved bystaged fuel combustion, which is a comparatively new technology. Concerns regard-ing this technique are similar to those for other combustion modification processes,such as changes in slagging and fouling characteristics, corrosion of tubes inreducing atmospheres, and higher fan power consumption [3].

Burner design is an alternative approach to reducing formation of both thermaland fuel NOx, bymeans of controllingmixing of the fuel and air. Themajor objective,in burner tune-up, is to alter the fuel and air mixing patterns to provide as muchaerodynamically staged mixing as possible, without additional air injection down-flame. The low NOx configuration, which one hopes to achieve, is a long narrowflame, where the fuel and air mix gradually over the entire flame length. Such flamescan be achieved by reducing the swirl of the secondary air and by changing the angleat which the fuel is injected into the secondary air stream (Figure 2.12) [5, 21]. LowNOx burners are designed to (i) maximize the rate of volatiles evolution and totalvolatile yield from the fuel, with the fuel nitrogen evolving in the reducing part of theflame; (ii) provide an O2-deficient zone where the fuel nitrogen is evolved tominimize its conversion into NOx, but sufficient O2 is available to maintain a stableflame; (iii) optimize the residence time and temperature in the reducing zone tominimize conversion of the fuel nitrogen to NOx; (iv) maximize the char residencetime under fuel-rich conditions to reduce the potential for NOx formation from theN2 remaining in the char after devolatilization; and (v) add sufficient air to completecombustion. The major concern with low-NOx burners is the potential for reducing

Figure 2.12 Schematic diagram of radially stratified flame core (RSFC) burner [70].


combustion efficiency and thereby increasing the unburned carbon level in the flyash. An example of a low-NOx burner is the radially stratified flame core burner,which has been scaled up and commercialized by ABB under license from MIT[59, 71].Over 370unitsworldwide,with a total capacitymore than 125GW,werefittedwith low-NOx burners prior to 1998. Foster Wheeler has been a leading supplier oflow-NOx burners [72]. The number of units installing such burners is continuouslyincreasing. DOE reports that they are currently found onmore than 75% of US coal-fired power capacity [21, 73, 74].

2.4.3.3 Near-Zero CO2 Emissions TechnologiesThe awareness of the increase in greenhouse gas emissions has resulted in thedevelopment of new technologies with lower emissions, which can accommodatecapture and sequestration of carbon dioxide. An approach suited to combustion-based processeswould be oxy-coal combustion (Figure 2.13). The coal would be burntin an oxygen/recycled flue gas mixture containing �35% oxygen instead of air. TheCO2-rich gases from the boiler would be cooled, condensate removed, the recyclestream returned and the balance of CO2 taken off for storage. The cost of the airseparation system might be counterbalanced by the savings in capital cost for a newinstallation, due to the high heat transfer within the boiler, as compared to aconventional pulverized-coal combustion system. Boiler efficiency may also beimproved, but it is not certain what the overall cycle net thermal efficiency wouldbe compared with scrubbing systems and otherwise similar conditions, as oxygenproduction andCO2 compression and liquefaction would still consume considerablequantities of power. Use of an air heater is not necessary in flue gas recycle systemsand changes to heat recovery balances within the boiler economizer would need to becalculated and flows adjusted accordingly tomaintain boiler efficiency [30, 32, 45, 57,59, 76, 77].

The system may be suitable for retrofits of pulverized-coal combustion units, butthere would be more flexibility in cycle design in a new installation. Theoretical

Figure 2.13 Oxy-coal combustion [75].


studies combined with laboratory- and pilot-scale studies have provided an under-standing of the relevant design parameters and operational issues and have indicatedthat oxy-fuel combustion is technically feasible with current technologies, reducingthe risks associated with implementation of new technologies [68, 78–82]. Work isrequired to determine the effects of the unconventional atmosphere on corrosion ofboiler heat transfer surfaces, slagging, and fouling, as well as component develop-ment and demonstration of the system at a commercial scale. In applying oxy-coalcombustion to CFBC, materials issues and fluidization behavior would requireinvestigation and development. Lower cost, less energy-consuming oxygen produc-tion in thermally integrated high-temperature membrane separation equipmentwould improve the efficiency and economics of oxy-coal combustion [32].

The chemical looping combustion concept utilizes a solid oxygen carrier (such asNiO/Ni) to provide oxygen for the combustion of coal [83–85]. The CO2-rich fluegases would be processed into a high purity CO2 end product for various uses orsequestration. The concept avoids the large efficiency penalty associated withcryogenic type air separation units (ASUs). Additionally, the high costs associatedwith both cryogenic-type ASU, or oxygen transport membrane type oxygen supplysystems, are avoided. The trade-off is a more complex boiler process [30, 32, 86].

2.5Biomass Characteristics Affecting Combustion Processes

2.5.1Moisture Content

Themoisture content of green biomass can be quite high and can adversely affect thecombustion process. If the moisture content is excessive, the combustion processmay not be self-sustaining and supplemental fuel must be used, which could defeatthe objective of producing energy by biomass combustion for captive use or market.Similarly to coal combustion, high moisture can cause incomplete combustion, lowoverall thermal efficiencies, excessive emissions (CO2, CO, and so forth), and theformation of products, such as tars, that interfere with operation of the system [87].

Woody biomass fuels containing 10–20% wt moisture are generally preferred forconventional biomass combustion systems, allowing temperatures of 750–1000 �C,without incurring the costs of further biomass drying.

2.5.2Ash Content and Composition

The effects of biomassmineralmatter on plant efficiency and pollutant emissions arethe same as those previously discussed for coal combustion. However, the amountand composition of biomass ash are different [88].

Herbaceous fuels contain silicon and potassium as their principal ash-formingconstituents. They are also commonly high in chlorine relative to other biomass fuels.

2.5 Biomass Characteristics Affecting Combustion Processes j63

Chlorine is shown to be a major factor in deposit formation. Chlorine facilitates themobility of many inorganic compounds, in particular potassium. These propertiesportend potentially severe ash deposition problems at high or moderate combustiontemperatures, due to formation of low melting point alkali silicates or alkalisulfates [89–95].

Many of agricultural by-products also contain high potassium concentrations.Some woods, in contrast, contain far less ash overall. In addition, the ash-formingconstituents contain greater amounts of calciumand less silicon,which forms sulfatedeposits more favorable to sustained furnace operation [96–98].

Furnace-boiler systems for solid biomass fuels are often designed to keep thetemperature in the combustor below about 900 �C, to reduce slagging and formationof molten agglomerates.

Leaching of inorganic constituents prior to combustion has been shown to be anefficient, fast, and low cost way to significantly reduce fireside fouling, by extractinglarge amounts of alkalimetals and chlorine, variable amounts of sulfur, phosphorousand total ash, and other elements [91, 96, 99–101]. Furthermore, chemical additives –such as kaolin, dolomite, calcite, bauxite, emalthite, gibbsite, mullite, ammoniumsulfate, clinochlore, ankerite, aluminium-iron silicates, and oxides of calcium,magnesium, aluminium, and iron – can be used for alkali sorption, or for obstructingreactions with troublesome elements, which lead to eutectic mixtures [102–108].

2.5.3Particle Size

Another factor in biomass combustion is fuel particle size and particle size distri-bution. The furnace design often determines the optimum ranges of these para-meters. But, in general, the smaller the fuel particles themore rapid and complete thecombustion process. Attrition and fragmentation in fluidized beds are important,due to their impact on char burn-off and particle time–temperature history [109]. Incommercial systems, the capital and operating costs of fuel particle size reductionand pre-drying are weighed against their beneficial effects on combustion andfurnace design and costs [110].

2.6Industrial Biomass Combustion Systems

The differences in furnaces suitable for biomass combustion reside mainly in thedesign of the combustion chambers, the operating temperatures, and the heattransfer mechanisms. Considerable advancements have been made in ancillaryhardware design to control the combustion process, to pre-dry the fuel, to removeash, to reduce emissions, and to recover sensible heat from the stack gases, thecondensate, and boiler blowdown.

Industrial combustion systems of a nominal thermal capacity exceeding 100 kWcan be distinguished in fixed-bed, fluidized-bed, and dust systems. The basic


principles of these are described below. Residential and small commercial systemsare reviewed in the Handbook of Biomass Combustion and Co-Firing [111].

2.6.1Fixed Bed Systems

2.6.1.1 Grate Furnaces [111–114]For grate furnaces various technologies are available, such asfixed,moving, traveling,rotating, and vibrating grates, aswell as cigar burners. Careful selection and planningare considered necessary, as all the above-mentioned technologies have certainadvantages and disadvantages, depending of fuel properties.

Grate furnaces are suitable for biomass fuels with highmoisture and ash contentsand varying particle sizes. Awell planned, well-constructed and well-controlled grateguarantees a homogeneous distribution of the fuel, as well as an equal primary airsupply over its various grate areas. Fuel transport over the grate has to be as quitesmooth and homogeneous, so as to prevent, if possible, the creation of �holes� andthe elutriation of fly ash and unburned particles. In addition, a non-homogeneous airsupply may result in slagging, higher fly-ash amounts, and a possible increase ofexcess oxygen, which is needed for complete combustion. Continuously movinggrates, a height control system of the bed of embers, and frequency-controlledprimary air fans for the various grate sections (a reducing atmosphere in the primarycombustion chamber is necessary for lowNOx operation) is the technology needed toachieve these goals. Furthermore, to avoid slagging and to extend the lifetime of thematerials, grate systems can be water-cooled.

Stage combustion should also be obtained, as this is another important aspect ofgrate furnaces; this can be achieved by separating primary and secondary combustionchambers, so as to avoid back-mixing of the secondary air and to separate gasificationand oxidation zones. The better the mixing of flue gas and air, the lower the excessoxygen, and the higher the efficiency. The mixing effect can also be amelioratedby using relatively small channels, or combustion chambers with a vortex orcyclone flow.

The various systems for grate combustion plants based on the flow directions offuel and the flue gas are shown in Figure 2.14: co-current flow systems (flame in the

Figure 2.14 Various arrangements for gas flow in furnaces.

2.6 Industrial Biomass Combustion Systems j65

same direction as the fuel), cross-current flow systems (flue gas removal in themiddle of the furnace), and counter-current flow systems (flame in the oppositedirection of the fuel).

Counter-current combustion is most suitable for fuels with low heating values(such as wet bark, wood chips, or sawdust). Drying and water vapor transport fromthe fuel bed is increased by convection – in addition to the dominating radiant heattransfer to the fuel surface – because the hot flue gas passes over the fresh and wetbiomass fuel entering the furnace. To avoid the formation of strains enriched withunburned gases entering the boiler, as well as avoiding increasing emissions, thissystem requires a good mixing of the gas and secondary air in the combustionchamber.

Co-current combustion is applied for dry fuels, like waste wood or straw, or insystemswhere pre-heated primary air is used. The residence time of unburned gasesreleased from the fuel bed can be increased by this system and NOx reduction canbe improved, due to enhanced contact of the flue gas with the charcoal bed on thebackward grate sections. Higher fly-ash entrainment can occur and should beimpeded by appropriate flow conditions.

Cross-current systems are a combination of co-current and counter-current unitsand are also especially applied in combustion plants with vertical secondary com-bustion chambers. Flue gas recirculation andwater-cooled combustion chambers areused, to achieve adequate temperature control. Combinations of these techniques arealso possible. The mixing of combustible gases and air can be improved by flue gasrecirculation and can be regulated more accurately than water-cooled surfaces.However, it has the disadvantage of increasing the flue gas volume in the furnaceand the boiler section [115]. Water-cooling has the advantage of reducing the flue gasvolume, impeding ash sintering on the furnacewalls, andusually extends the lifetimeof insulation bricks.

Fixed grate systems are only used in small-scale applications. As fuel transport anddistribution among the grate cannot be controlled well, this technology is no longerapplied in modern combustion plants.

In inclined moving grates (Figure 2.15), the fuel is transported along the grate byalternating horizontal forward and backwardmovements. In thisway, both unburnedand burned fuel particles are mixed, the surfaces of the fuel bed are renewed, and amore even distribution of the fuel over the grate surface can be achieved. Usually,the whole grate is divided into several sections, which can move at variousspeeds, depending on the different stages of combustion. The grate bars are madeof heat-resistant steel alloys. What is more, they are equipped with small channels attheir side-walls, for primary air supply. Additionally, they should be as narrowas possible, so that the primary air across the fuel bed is distributed in the mosteffective way.

If themoving frequencies are too high, high concentrations of unburned carbon inthe ash, or insufficient coverage of the grate, will result. Infrared beams situated overthe various grate sections allow for adequate control of the moving frequencies, bychecking the height of the bed. Ash is removed under the grate in dry or wet form.What is common in this case is the fully automatic operation of the whole system.


In moving grate furnaces a wide variety of biofuels can be burned. Primary air forcooling the grate is used in air-cooled units. These furnaces are also suitable for wetbark, sawdust, and wood chips. Water-cooled moving grate systems are recom-mended for dry fuels, or biofuels with low ash-sintering temperatures.

In horizontally moving grates, the horizontal bed is achieved by placing the gratebars in a diagonal position. Advantages of this technology include: (i) the impedimentof uncontrolled fuel movements over the grate because of gravity and (ii) the stokingeffect by the grate movements is increased, thus leading to a very homogeneousdistribution of material on the grate surface and impeding slag formation as a resultof hot spots. An additional advantage is that the overall height can be reduced. Thesesystems should be pre-loaded – so that there is no free space between the bars – toavoid ash and fuel particles falling through the grate bars.

Applications of this technology are a 10 MW Russian boiler at Jõ,geva HeatCompany burning wood chips, a 10 MW Swedish boiler at Viisnurk Ltd. burningwood chips and wood waste, 100 MW boilers in Denmark burning straw, two 93.3MWboilers inHolland at Afval Enrgie Bedrijt burning wastes and a 10MWboiler inAustria at St. Andr€a for the co-firing of bark, wood, and forest residues [116–119].

Travelling grate furnaces are built of grate bars, forming an endless band thatmovesthrough the combustion chamber. Screw conveyors or spreader stokers supplythe fuel onto the grate. The fuel bed itself does not move but is transported throughthe combustion chamber by the grate. The grate is cleaned of ash and dirt at the endof the combustion chamber, while the band turns around. On the way back, the gratebars are cooled by primary air, to avoid overheating and to minimize wear-out. Toachieve complete charcoal burnout the speed of the traveling grate is continuouslyadjustable.

Figure 2.15 Modern grate furnace with infrared control system and section separated primary aircontrol. (1) Drying zone, (2) gasification zone, and (3) charcoal combustion zone [114].


Uniform combustion conditions for wood chips and pellets and low dust emis-sions are the advantages of these systems, due to the stable and almost unmoving bedof embers. What is more, the maintenance or replacement of grate bars is easy.However, the fact that the bed of embers is not stoked results in a longer burn-outtime, as compared to moving grate furnaces. For complete combustion a higherprimary air input is needed (which leads to a lower NOx reduction potential byprimarymeasures). Moreover, non-homogeneous biomass fuels imply the danger ofbridging and uneven distribution among the grate surface, because no mixingoccurs. This disadvantage can be avoided by spreader stokers.

The combination of wet-chemical fuel analysis, in situ flue gasmeasurements, andsimulation tools canmake it possible to define the optimum temperature profile andair staging conditions for test results concerning NOx reduction [120].

Underfeed rotating grate combustion is a new Finnish biomass combustiontechnology. This technology uses conical grate sections, which rotate in oppositedirections. These sections are supplied with primary air from below (Figure 2.16). Asa result, wet and burning fuels are well mixed. In this way, the system becomesadequate for burning very wet fuels, such as bark, sawdust, and wood chips, withmoisture content up to 65%. The combustible gases are then burned out withsecondary air, which takes place in a separate horizontal or vertical combustionchamber. The horizontal version is suitable for generating hot water or steam inboilers, with a nominal capacity between 1 and 10MW. The vertical version is appliedfor hotwater boilers, with a capacity of 1–4MW.The fuel is fed to the grate frombelowwith screw conveyors, which makes it necessary to keep the average particle sizebelow 50mm [111, 121].

Figure 2.16 Underfeed rotating grate: (A) fuel feed, (B) primary combustion chamber, (C)secondary combustion chamber, (D) boiler, (E) flue gas cleaner, (F) ash removal, and (G) stack [121].


Underfeed rotating grate combustion plants can also burn mixtures of solid woodfuels and biological sludge. The system is computer-controlled and allows fullyautomatic operation.

Vibrating grate furnaces consist of a declined finned tube wall placed on springs.Spreaders, screw conveyors, or hydraulic feeders feed the fuel into the combustionchamber. Two or more vibrators transport fuel and ash towards the ash removal.Primary air is fed through the fuel bed frombelow, throughholes located in the ribs ofthe finned tubewalls. Owing to the vibratingmovement of the grate, at short periodicintervals, the formation of larger slag particles is inhibited. For this reason, thistechnique is especially applicable to fuels having sintering and slagging tendencies(e.g., straw, waste wood). The high fly ash emissions caused by the vibrations, thehigher CO emissions due to the periodic disturbances of the fuel bed, and theincomplete burnout of the bottom ash are the disadvantages of vibrating gratefurnaces [111].

Ansaldo Vølund A/S has recently commissioned a wood chips vibrating grateboiler, for heat and power generation. The boiler operates with high steam data,525 �Cand 70–92 bar pressure. At SHEnergi A/S, in Aabenraa, Denmark, a biomassfired boiler plant started commercial operation in 1988. The plant consists of aBenson-type boiler, with a screw stoker/vibration grate combustion system gener-ating 120 t h�1 of steam, which is finally superheated to 542 �C in a separate woodchip fired superheater. The plant is coupled to the 660 MW power plant EV3 and itgenerates 41 MW, at a wet electrical efficiency of 39% [122, 123]. EHN-EnergiaHidroel�ectrica de Navarra has recently installed the first straw-fired plant of 25 MWe

in Sang€uesa, Spain, consuming 160 000 tons of straw per year [124].In Denmark, cigar burners have been developed for straw and cereal bale com-

bustion (Figure 2.17); the fuel is delivered in a continuous process, by a hydraulicpiston, through a feeding tunnel on a water-cooled moving grate. Upon entering thecombustion chamber, the fuel begins to gasify and combustion of the charcoalfollows, while the unburned material is moved over the grate. Grate and furnacetemperature control are very important for straw and cereal combustion, due to theirlow ash sintering and melting points and the high adiabatic temperature ofcombustion, caused by their low moisture content. Therefore, the combustionchambers have to be cooled by water-cooled walls, or by flue gas recirculation, orboth. Furnace temperatures should not exceed 900 �C for normal operation. Fur-thermore, in straw and cereal combustion, very fine and light fly ash particles, as wellas aerosols, are formed from condensed alkali vapors. An automatic heat exchangercleaning system is required to prevent ash deposit formation and corrosion. Systemsfor shredded or cut straw also exist and operate in a similar way to the technologydescribed – only the fuel preparation and feeding are different [111, 122, 126].

2.6.1.2 Underfeed StokersUnderfeed stokers (Figure 2.18) represent an economical and operationally safetechnology, which is suitable for small- and medium-scale systems, up to a nominalboiler capacity of 6 MW. Screw conveyors from below feed the fuel into thecombustion chamber and they transport it upwards, on an inner and outer grate.


In modern combustion plants outer grates are more common. This is because theyallow for more flexible operation and their automatic ash-removing system can beattained easier. Primary air is supplied through the grate, while secondary air isusually supplied at the entrance to the secondary combustion chamber. A new

Figure 2.18 Underfeed stoker for wood chips and sawdust: (1) ash hopper, (2) grate, (3) refractoryand radiation wall, (4) air fans, (5) insulation, (6) fire tube boiler, (7) multicyclone, and (8) flue gasfan [114].

Figure 2.17 Cigar burner for straw and cereal combustion [114, 125].


Austrian development is an underfeed stoker with a rotational post-combustion. Inthis new development, a strong vortex flow is achieved, by a specially designedsecondary air fan equipped with a rotating chain.

Underfeed stokers are suitable for biomass fuels with low ash content (such aswood chips, sawdust, pellets) and small particle sizes (�50mm). Ash-rich biomassfuels like bark, straw, and cereals needmore efficient ash removal systems.Moreover,problems in underfeed stokers can be caused by sintered or melted ash particles,which cover the upper surface of the fuel bed. This happens when the fuel and the airare breaking through the ash-covered surface, thus resulting in unstable combustionconditions. On the other hand, their good partial-load behavior and their simple loadcontrol are the advantages of underfeed stokers. Additionally, because the fuel supplycan be controlled more easily, load changes can be achieved more easily and quicklythan in grate combustion plants [111].

Advanced combustion control techniques of low costmaximize the efficiency withrespect to the emissions of unburnt pollutants. Measurements on a 1 MW under-stoker furnace showed that the efficiency was above 90%, for the whole range of theheat output, and at part load it was improved by up to 5%. CO emissions were below50mgNm�3, which represents a reduction by a factor of five compared to flametemperature control [127].

2.6.2Fluidized Bed Systems

Fluid-bed (FB) combustion systems have been applied since 1960 for combustion ofmunicipal and industrial wastes. Since then, over 300 commercial installations havebeen built worldwide. Regarding technological applications, bubbling fluidized beds(BFBs) and circulating fluidized beds (CFBs) have to be distinguished. Processprinciples have been discussed above. Regarding gaseous and solid emissions, BFBand CFB furnaces normally show lower CO and NOx emissions, due to morehomogeneous and therefore more controllable combustion conditions. Fixed bedfurnaces, in turn, usually emit fewer dust particles and show a better burnout of thefly ash [88].

2.6.2.1 Bubbling Fluidized BedFor plants with a nominal boiler capacity of over 20MW, BFB furnaces start to be ofinterest, since the low excess air quantities necessary increase combustionefficiency and reduce the flue gas volume flow. In contrast to coal-fired BFBfurnaces, the biomass fuel should not be fed onto but into the bed, by inclinedchutes from fuel hoppers, because of the higher reactivity of biomass in com-parison to coal.

The advantage of BFB furnaces is their flexibility concerning particle size andmoisture content of the biomass fuels. Furthermore, it is also possible to usemixtures of different kinds of biomass, or to co-fire them with other fuels. One bigdisadvantage of BFB furnaces, the difficulties they have at partial load operation, issolved in modern furnaces by splitting, or staging, the bed [111, 114].


Industrial-scale fluidized bed reactors are used for the combustion of several typesof biomass in many countries. For example, in Finland over 30 BFB systems areinstalled and the largest boiler is 500 MW. The optimal mixing ratios for commonlyused forest chip qualities, aswell asmixtures of chips and other fuels, the combustionand co-firing properties, emissions, and boiler fouling, are being studied [113, 128]. Ahot-water BFB plant, developed by Motala Energi AB, has been built in Gothenburg,Sweden, giving a thermal output of 27 MW with flue-gas heat recovery. AustrianEnergy�s �ECOFLUID� bubbling fluidized bed has been applied in the Westfieldplant in FIFE/Scotland, Langerbrugge plant for Stora Euso in Belgium, Stendal plantfor ZSG in Germany, and Timelkam plant for Energie AG in Austria, with steamgenerating capacities from9 to 290 t h�1 and a fuel range fromwood, bark, harvestingresidues, sewage sludge, and organic waste resulting from agricultural industry. Theapplied staged combustion concept resulted in a homogeneous temperature profilein the furnace and first pass of the boiler and thus low NOx emission. By usingrefractory lined superheaters, corrosion problems could be minimized, althoughhigh steam parameters could be obtained [129]. In Spain, comparative studies havebeen carried out for different biomass types (forestry, herbaceous, cork sawdust, andso forth) in an atmospheric BFB pilot plant of 1 MW by the CIEMAT Group [130].Combustion parameters were optimized, to obtain a clean process for each type offuel used. Polyaromatic hydrocarbon emissions were found to be higher in herba-ceous than in wood biomass, when particulate emissions were increased andcombustion efficiency decreased. No slagging or ash sintering problems were foundduring the combustion tests, mainly due to the low alkali content in the ash of theconsidered biomasses. The type of fuel feeding affected the temperature distributioninside the combustor and therefore the erosion caused to the internal parts of theequipment. Furthermore, the feasibility of the fluidized bed combustion of energycrop biomasses was demonstrated in this plant, since over 99% efficiency wasobtained in the different experiments [131].

2.6.2.2 Circulating Fluidized BedIn viewof their high specific heat transfer capacity, CFB furnaces start to be of interestfor plants of more than 30 MW, due to their higher combustion efficiencies and thelower flue gas flow produced (boiler and flue gas cleaning units can be designedsmaller).

An example of this technology is the Alholmens kraft plant, in Pietarsaari ofFinland, with an electrical output of 240 MW. The plant produces steam for theadjacent papermill and for a utility generating electricity and heat. AUPM-KymmenePulp and Paper Mill nearby supplies the power plant with wood and bark residues.Since the plant is of giant scale, special attention has been paid to the logistics of fuelprocurement. Table 2.5 gives technical, fuel, and boiler data for this plant.

Another example is the 110 MW CFB boiler at Lenzing AG in Austria. The CFBuses the waste air stream for the combustion of various waste materials, includingwood wastes, RDF, waste packagingmaterial, screenings frommunicipal waste, andsewage sludge. The waste air stream is pre-heated before entering the CFB com-bustor as primary and secondary air, while also being used to pneumatically transport


the solid feed into the combustor (Figure 2.19). Partial capture of the sulfur isachieved by adding limestone to the CFB. The boiler generates 129 t h�1 of steam at80 bar and 500 �C. The steam is used in the Lenzing works to generate electric powerand provide process heating. Through combining the two problems of contaminatedair and solid wastes, the company has gained important benefits through the

Table 2.5 Technical, fuel and boiler data of Alholmens Kraft plant [132].

Technical dataOwner Alholmens Kraft LtdCommissioned 2001Investment cost EUR 170 millionElectricity output 240 MWe

Process steam output 100 MWth

District heat output 60 MWth

Annual electricity production 1300 GWh

Annual heat production 2520 TJ

Fuel dataAnnual fuel consumption 12 600 TJ– Industrial wood and bark residues 35%– Forest residues 10%– Peat 45%– Heavy fuel oil or coal 10%

Boiler dataBoiler supplier Kvaerner Power OyBoiler type Circulating fluidized bed combustionBoiler output 550 MWth

Steam 194 kg s�1, 165 bar, 545 �C

Figure 2.19 Circulating fluidized bed combustor, Lenzing AG, Austria [133].


generation of supplementary steam and reducing the demand for conventional fuelswithin their existing plant [133].

CFB combustion is used in the USA to combust automobile tires, 200 million peryear of which are either disposed of in some form or recycled for retreating or reuse.Emissions of metal oxides, volatile organic compounds, and sulfur oxides from thetires have precluded the use of high ratios of tire fuel in conventional combustors. ACFB system, however, can combust tires with nearly 100% conversion of the carbon,good emission characteristics, and the capability of separating the wire. Carbonmonoxide levels of 25 ppm in theflue gases have been readilymaintainedwith excessair. Sulfur oxide capture with limestone in the fluidized bed and ash recycle can be ashigh as 80%. The sand is dewired and screened to remove any oversized particlesbefore return to the combustor [134].

2.6.3Dust Combustion Systems

In dust combustion systems, fine fuels, like sawdust, are pneumatically injected intothe furnace. An auxiliary burner is used for starting-up; when the combustiontemperature reaches a certain value, this burner is shut down and biomass injectionbegins. Fuel feeding must be controlled very carefully due to the explosion-likegasification of the fine and small biomass particles. The feeding system consists of akey technological unit within the overall system. The particle size and the moisturecontent of the fuel should not exceed 10–20mm and 20% wt, respectively. Fuel/airmixtures are usually injected tangentially into the cylindrical furnace muffle, toestablish a rotational flow (usually a vortex flow). The rotational motion can besupported by the flue gas recirculation in the combustion chamber. Because of thesmall particle sizes used, gasification and char-coal combustion occur simultaneous-ly, so that rapid load changes and efficient load control can be achieved. Owing to thehigh flue gas velocities, the ash is carried with the flue gas and is partly precipitated inthe post-combustion chamber. Air staging and low excess air amounts could lead toreduced NOx emissions. The muffle should be water-cooled, because of the highenergy density at the furnace walls and the high combustion temperature.

Muffle dust furnaces are being used more and more for fine wood wastes,originating from the chipboard industry.

Besidesmuffle furnaces, cyclone burners forwood dust combustion are also in use(Figure 2.20). Depending on the design of the cyclone and the location of fuelinjection, the residence time of the fuel particles in the furnace can be controlledwell [135]. A disadvantage of muffle furnaces and cyclone burners is that insulationbricks wear out quickly, due to thermal stress and erosion. Therefore, other dustcombustion systems are being built without rotational flow, where dust injectiontakes place, as in a fuel oil-or natural gas-fired furnace [111].

Other advanced combustion systems for solid biomass fuels also offer consider-able advantages over conventional designs and are in commercial use or underdevelopment. Some of them are [134, 136]: two-stage systems combining fluidized-bed technology and cyclonic combustion for disposal of waste biomass with heat


recovery; combustion of thickened municipal biosolids dewatered to about 38%solids in a six-hearth incinerator at 700–900 �C; the Whole Tree Energy conceptdeveloped in the USA and patented in 30 countries; and direct-fired gas turbines thatare suitable for small- andmedium-sized industrial and commercial applications, upto 5 MW in capacity.

2.7Outlook

Theoutlookfor themajorcleancoal technologiesdependsonseveral factors, includingprojectedfutureelectricity requirementsandtheattitudesandpoliciesofgovernment,industry, and the general public worldwide. In Europe, it is estimated that by 2020 thecombination of increased electricity demand and the need for power plant replace-mentwill amount toarequirement for300GWeofnewpowercapacity.Russia, amajorcoal-consumingcountry, isprojected toincrease itsconsumptionfromabout250Mttoalmost 400Mtby2030. In theUSA,where there is substantial government andprivateinvestment into RD&D for clean coal technologies, 170GWof new coal-fired capacitywill be required prior to 2030. In China, coal-based power generation is expected toincrease from an estimated 250 to 800 GW, and in India to 161 GW, by 2030. China is

Figure 2.20 Schematic diagram of a two-stage cyclone burner.

2.7 Outlook j75

the world�s largest market for supercritical pulverized-coal combustion. In SouthAfrica,18GWofcoal-firedplantsareprojectedby2015.Australia,whichproduces80%of its electricity fromcoal, is developing thenear-zeroemission technologieswithin itsCoal21 National Action Plan [25, 58]. Generally, CFBC seems to be the preferredtechnology worldwide and several large units are under construction.

In various markets, the average scale of biomass combustion schemes rapidlyincreases, due to improved availability of biomass resources and the economicadvantages of economies of scale of conversion technology. It is also in this field thatcompetitive performance, compared to fossil fuels, is possible, where lower costresidues are available. All over Europe, most notably in Scandinavia, biomassmarkets are developing from purely regional to international markets, with growinginternational trade of biomass and biomass-derived energy carriers. Major marketsfor heat production are in Finland, Sweden,Denmark,Austria,Germany, andFrance,while for electricity production they are in Finland, Denmark, Spain, Germany, andThe Netherlands. Finland is at the cutting edge of the field with development anddeployment of BFBC and CFBC boilers with high fuel flexibility, lower specificinvestment costs, and high efficiency. The largest boiler is 500 MW. Apart fromEurope and the USA, the main growth market is South East Asia, especially withrespect to efficient power generation from biomass wastes and residues.

2.8Summary

Coal, being an economic, reliable, and readily available source of energy, will remainthe fuel of choice for power generation worldwide. Coal is projected to play a majorrole in the global energy system, with more than a 20% share in primary energy andup to 40% in electricity production in 2030. However, due to coal�s pollution andlimitations on CO2 emissions, clean coal technologies with high thermodynamicefficiency must be applied in the new generation of coal-fired power stations.

Stokers, the oldest devices used for combustion of coal in fuel beds, have lost agreat part of their traditional market to advanced clean coal technologies, due to theirlow efficiency and ash clinkering problems.

Pulverized-coal combustion systems are the best-proven technologies and haveevolved over several decades, accounting forwell over 90%ofworld power plants. Theaverage efficiency of larger subcritical plants is in the range 35–36%. Energyconsumption is high and so are the emissions, if uncontrolled. Supercritical unitstake advantage of higher steam temperatures and pressures to achieve higherefficiencies and thus lower specific emissions than subcritical units. Efficiencies ofrecent plants approach 50%.Several data showno increase in specific capital cost, andunit sizes up to 1000 MWe exist. There are more than 520 supercritical plants inoperation worldwide. Long-term programs in progress, to achieve efficiencies over50%, involve materials developments, including the use of superalloys.

Fluidized bed technologies have good fuel flexibility and lower emissions thanconventional combustion systems. In recent decades they have undergone consi-derable development towards improved performance and lower costs. Nowadays,


they compete with small stoker boilers and with large pulverized-coal fired boilers.BFBC is and will be mainly used for small units up to 300 MW, having efficienciesaround 30% and availabilities over 90%. A key area for future development isextending the range of biomass and waste fired, improving the control of heavymetals, and materials of construction. CFBC subcritical units in operation range insize from a few MW to 300 MWe. Plant efficiencies are 38–40% and availabilities90–98%. A 460 MWe boiler with an efficiency of >43%, at Lagisza plant in Poland,will be the world�s largest CFBC unit with a supercritical cycle. Larger supercriticalboilers are under development and research on advanced alloys for heat exchangers iscontinuing. One potential development area might be the use of CFBC as part ofadvanced cycles, based on both combustion and partial gasification of fuel. Coalremains the dominant fuel, but biomass and waste are likely to be increasingly used.PFBC is a relatively new technology. The largest plant is the 360 MWe Karitasupercritical unit, in Japan. Plant efficiencies of up to 44% and very low emissionlevels have been achieved. When used in the circulating mode, it has potential forproviding better performance than other forms of FBC. The use of a toppingcombustor, to increase the gas turbine inlet temperature, and flue gas cleanupremain critical development areas.

In-furnace desulfurization, mainly using calcium-based sorbents, is a competitivetechnology for controlling the SOx pollutants derived from combustion, due to lowoverall costs. Staged processes and flue gas recirculation offer improved efficiencies.

In situ techniques that can reduce NOx emissions achieve efficiencies between 50and 80%. Staged air combustion and low-NOx burners are considered to be the mostsuccessful methods for NOx control.

Technologies for near-zero CO2 emissions are also under investigation, includingoxy-fuel combustion and chemical looping combustion. Flexibility will be greater innew installations.

Combustion of biomass, which is a naturally occurring carbon resource with greatenergy potential and is considered CO2 neutral, is of extreme importance, if we are toconserve our sources of energy while achieving our environmental goals efficiently.More than 95% of all biomass energy utilized today is obtained by direct combustion.Improved processes are available for conversion of virgin biomass and complexwastebiomass feedstocks into heat, steam, and electric power in advanced combustionsystems. Basic concepts include fixed bed, fluidized bed, and dust firing.

Fixed bed furnaces are appropriate for biomass fuels with high moisture content,varying particle sizes, and high ash content. Typical plant capacities range between 20and 50 MWe with related electrical efficiencies in the 25–30% range; however, largerplants up to 100MWalready exist. Moving grate boilers are the preferred technology.

In recent years, the application of fluid-bed combustion allows for efficientproduction of heat and electricity from biomass. On a scale of about 50–100 MWe,electrical efficiencies of 30–40% are possible.

Althoughmajor technological developments have already been achieved, biomassis generally not yet commercially competitive, except for some niche applications.Policy support in the fields of research, development, and demonstration and thecreation of conductive market mechanisms and legislation are essential for a morewidespread introduction of biomass energy systems.

2.8 Summary j77

References

1 Fletcher, T.H. et al. (1992) Chemicalstructure of char in the transition fromdevolatilization to char oxidation.Preprints of Papers- American ChemicalSociety, Division of Fuel Chemistry,37 (2), 677.

2 Cope, R.F. (1992) Effects of Minerals onthe Oxidation Rates of Beulah ZapLignite Char, Ph.D. Dissertation,Department of Chemical Engineering,Brigham Young University, Provo, UT.

3 Vamvuka, D. (2002) Clean Use of Coals.Low Rank Coal Technologies, IonPublications, Athens.

4 Couch, G. (1994) Understanding Slaggingand Fouling During pf Combustion,IEACR/72, IEA Coal Research, London.

5 Williams, A., Pourkashanian, M., andSkorupska, N. (2000) Combustion andGasification of Coal, Taylor & Francis,New York.

6 Johnson, T.R. (1992) Best practicetechnology for low rank coals.Proceedings International Conferenceon Coal, the Environment andDevelopment: Technologies to ReduceGreenhouse Gas Emissions, Paris,OECD/IEA, p. 393.

7 Scott, D.H. (1999) Ash Behaviour DuringCombustion and Gasification, IEA CoalResearch, London.

8 Rousaki, K. and Couch, G. (2000)Advanced Clean Coal Technologies and LowValue Coals, IEA Coal Research, London.

9 Barroso, J., Ballester, J., Ferrer, L.M., andJim�enez, S. (2006) Study of coal ashdeposition in an entrained flow reactor:influence of coal type, blend compositionand operating conditions. Fuel ProcessingTechnology, 87, 737.

10 Harding, N.S. andO�Connor, D.C. (2007)Ash deposition impacts in the powerindustry. Fuel Processing Technology, 88,1082.

11 Nicholls, P. and Reid, W.T. (1940) ASTMTransactions, 62, 141.

12 Vorres, K.S. (1979) Effect of compositiononmelting behavior of coal ash. Journal ofEngineering for Power, 101, 497.

13 Naruse, I., Kamihashira, D., Miyauchi,K.Y., Kato, Y., Yamashita, T., and

Tominaga, H. (2005) Fundamental ashdeposition characteristics in pulverizedcoal reaction under high temperatureconditions. Fuel, 84, 405.

14 Buhre, B.J.P., Hinkley, J.T., Gupta, R.P.,Nelson, P.F., and Wall, T.F. (2006) Fineash formation during combustion ofpulverised coal-coal property impacts.Fuel, 85, 185.

15 Ely, F.G. and Barnhart, D.H. (1963) Coalash-its effect on boiler availability,Chemistry of Coal Utilization,Supplementary Volume (ed. H.H. Lowry),John Wiley and Sons, Inc., New York,London.

16 Shigeta, J.I., Hamao, Y., Aoki, H., andKajigaya, I. (1987) Development of a coalashcorrosivity index forhigh temperaturecorrosion. Journal of EngineeringMaterialsand Technology, 109 (4), 299.

17 Zygarlicke, C.J., Stomberg, A.L.,Folkedahl, B.C., and Strege, J.R. (2006)Alkali influences on sulfur capture for N.Dakota lignite combustion. FuelProcessing Technology, 87, 855.

18 Energy Resources, Co., Inc . (1980) Low-rank coal study, Vol. 3, TechnologyEvaluation, U.S. Dept. Energy Rept.DOE/FC/10066-TI.

19 Vuthaluru, H.B. and Zhang, D.K. (2001)Remediation of ash problems influidised-bed combustors. Fuel, 80 (4),583.

20 Roe, D.E. et al. (1979) Solid Fuels for U.S.Industry, Vol. II, Cameron Engineers.

21 Miller, B.G. (2005) Coal Energy Systems,Elsevier Academic Press, USA.

22 Smoot, L.D. (1993) Fundamentals of CoalCombustion for Clean and Efficient Use,Elsevier, The Netherlands.

23 Ceely, F.J. and Daman, E.L. (1981)Combustion process technology,Chemistry of Coal Utilization, SecondSupplementary Volume (ed. M.A. Elliot),John Wiley & Sons, New York.

24 Rusinowski, H., Szega, M., Szl_:6pte51pt,K, A., and Wilk, R. (2002) Methods ofchoosing the optimal parameters forsolid fuel combustion in stoker-firedboilers. Energy Conversion andManagement, 43, 1363.


25 Fernando, R. (2004) Current Design andConstruction of Coal-Fired Power Plant,IEA Clean Coal Centre, London.

26 Kim, H.P., Kim, S.H., Kim, H.J., andSong, S.H. (2006) The development oftangential coal-fired burner to reduceunburned carbon and enhance flamestability. Proceedings of the 23rd AnnualInternational Pittsburgh CoalConference, Pittsburgh, USA.

27 Committee on the Strategic Assessmentof the, U.S., Dept., of Energy�s CoalProgram (1995) Coal. Energy for theFuture, National Academy Press,Washington, D.C.

28 Yin, C., Rosendahl, L., and Condra, T.J.(2003) Further study of the gastemperature deviation in large-scaletangentially coal-fired boilers. Fuel, 82,1127.

29 U.S. Department of Energy (2008) CleanCoal Technology Programs: ProgramUpdate 2007, DOE/FE-0515,Washington, DC, available at www.netl.doe.gov/technologies/coalpower/cctc/ccpi/pubs/2007_program_update.pdf.

30 Wu, Z. (2006) Developments in FluidisedBed Combustion Technology, IEA CleanCoal Centre, London.

31 Merrick, D. (1984) Fluidised bedcombustion, in Coal Combustion andConversion Technology, MacmillanPublishers Ltd., London.

32 Henderson, C. (2004) Clean CoalTechnologies, IEA Clean Coal Centre,London.

33 The World Bank (1997) A Planner�sGuide for Selecting Clean-CoalTechnologies for Power Plants, WorldBank Technical Paper No 387,Washington, DC.

34 Hulkkonen, S., Fabritius, M., andEnestam, S. (2003) Application of BFBtechnology for biomass fuels: technicaldiscussion and experiences from recentprojects. Proceedings of the 17thInternational Conference on FluidisedBed Combustion, American Society ofMechanical Engineers (ASME)Jacksonville, FL, USA, p. 663.

35 Minchener, A.J., Cross, P.J.I., Smith,T.N., and Fisher, M.J. (2000) FluidisedBed Combustion Systems for PowerGeneration and Other Industrial

Applications, Coal R188, Dept. of Tradeand Industry, London, UK.

36 Tarelho, L.A.C., Matos, M.A.A., andPereira, F.J.M.A. (2005) Axialconcentration profiles and N2O flue gasin a pilot scale bubbling fluidised bedcoal combustor. Fuel ProcessingTechnology, 86, 925.

37 PowerClean (2004) Fossil Fuel PowerGeneration State-of-the Art, PowerCleanR, D&D Thematic Network, Coleraine,UK.

38 Z€olzer, K., Hirschfelder, H., and Holub,M. (1997) Advantages and disadvantagesof fluidized bed combustors in variousareas of application, in The Future ofFluidized Bed Combustion, paper 1.2,VGB-TW-212e VGB-KraftwerkstechnikGmbH, Essen, Germany.

39 Darling, S.L., Hebb, J.L., and Dyr, R.A.(2000) 300 MWe demonstration CFBtakes shape at JEA�s Northside powerplant. Modern Power Systems, 20 (9), 21.

40 Anthony, B. (2003) Atmospheric andPressurized Fluidized Bed Technology,Clean Coal Technology RoadmapWorkshop, Natural Resources Canada,Calgary, Canada, 23.

41 Ven€al€ainen, I. and Psik, R. (2004) 460MWe supercritical CFB boiler design forLagisza power plant. Proceedings of the12th Annual Power-Gen EuropeConference and Exhibition, Barcelona,PennWell corporation, UK, p. 20.

42 Wu, Z. (2005) Clean coal technologies-theway forward for coal. Meeting ofthe London and Home Countries Branchof the,Energy InstituteLondon,UK,p.17.

43 Nowak, W. andMuskala, W. (2001) Cleanand ecological coal combustion in thebinary circulating fluidized bed. Energy,26, 1109.

44 Minchener, A.J. (2003) Fluidized bedcombustion systems forpower generationand other industrial applications. Journalof Power and Energy, 217 (1), 9.

45 Scott, D. and Nilsson, P. (1999)Competitiveness of Future Coal-fired Unitsin Different Countries, IEA Coal Research,London.

46 DTI (2003) The UK Cleaner CoalTechnology Programme: GovernmentPolicy, Dept. of Trade and Industry,London, UK.

References j79

47 Wright, I.G., Stringer, J., and Wheeldon,J.M. (2003) Materials issues in bubblingPFBC systems. Materials at HighTemperatures, 20 (2), 219.

48 Wolf, K.J. (2003) Investigations of theRelease and Sorption of Alkali Metals inPressurized Fluidized Bed Combustionunder Reducing Conditions,Forschungszentrum J€ulich GmbH,Institut f€urWerkstoffe undVerfahren derEnergietechnik 2, Werkstoffstuktur undEigenschaften, Germany.

49 Xiao, J. and Zhang, M. (2005) The PFBC-CCgenerationpowersystemusednaturalgas supplementary combustion.Proceedings of the 18th InternationalConference on Fluidized BedCombustion, Toronto, Canada, AmericanSociety ofMechanical Engineers (ASME),NY, p. 5.

50 Wu,Z. (2003)Understanding FluidisedBedCombustion, IEA Clean Coal Centre,London, UK.

51 Sakuno, S., Shimizu, T.,Misawa,N.,Ueda,H., Sasatsu, H., andGotou, H. (2002) NOx

emission from a 71 MWe pressurizedfluidized bed combustor. Fuel, 81, 373.

52 Ishom, F., Harada, T., Aoyagi, T.,Sakanishi, K., Korai, Y., and Mochida, I.(2004) Problems in PFBC boiler (2):characterization of bed materials foundin a commercial PFBC boiler at differentload levels. Fuel, 83, 1019.

53 Anderson, J. and Anderson, L. (1999)Technical and commercial trends inPFBC.Power-Gen Europe 1999 Conference,Frankfurt, PennWell, UK, p. 8.

54 Sondreal, E.A., Benson,A.S.,Hurley, J.P.,Mann, M.D., Pavlish, J.H., Swanson,M.L., Weber, G.F., and Zygarlicke, C.J.(2001) Review of advances in combustiontechnology and biomass cofiring. FuelProcessing Technology, 71, 7.

55 Termuehlen, H. and Emsperger, W.(2003)Clean and Efficient Coal-fired PowerPlants, ASME Press, New York.

56 NETL (2005) Combustion-Fluidized BedCombustion, National EnergyTechnology Laboratory, Pittsburgh, PA,USA, www.netl.doe.gov/coal/combustion/FBC.

57 Henderson, C. (2005) UnderstandingCoal-fired Power Plant Cycles, IEA CleanCoal Centre, London.

58 Kessels, J., Bakker, S., and Clemens, A.(2007) Clean Coal Technologies for aCarbon-Constrained World, IEA CleanCoal Centre, London.

59 Be�er, J.M. (2000) Combustion technologydevelopments in power generation inresponse to environmental challenges.Progress in Energy and Combustion Science,26, 301.

60 Viswanathan, R., Henry, J.F., Tanzosh, J.,Stanko, G., Shingledecker, J., Vitalis, B.,and Purgert, R. (2005) U.S. program onmaterials technologyforultra-supercriticalcoal power plants. Journal of MaterialsEngineering and Performance, 14 (3), 281.

61 Scheffknecht, G. and Fouilloux, J.P.(2002) Advanced steam power planttechnology for the world market. VGBPower Technology, 82 (7), 41.

62 Armor, A.F., Viswanathan, R., Dalton,S.M., and Annedyck, H. (2003) Ultrasupercritical steam turbines: design andmaterials issues for the next generation.VGB Power Technology, 3 (10), 48.

63 Hotta, A., Kettunen, A., and Utt, J. (2007)Milestones for CFB and out technology-the 460 MWe Lagisza supercritical boilerproject update. Proceedings of the 24thAnnual International Pittsburgh CoalConference, South Africa.

64 Stamatelopoulos, G.N. and Sadlon, E.(2007) Advancement in coal-fired powerplants: higher efficiency and betterenvironmental performance.Proceedings of the 24th AnnualInternational Pittsburgh CoalConference, South Africa.

65 Gupta, H., Thomas, T.J., Park, A.A., Iyer,M.V., Gupta, P., Agnihotri, R., Jadhav,R.A., Walker, H.W., Weavers, L.K.,Butalia, T., and Fan, L. (2007) Pilot-scaledemonstration of the OSCAR process forhigh-temperature multipollutant controlof coal combustion flue gas, usingcarbonated fly ash and mesoporouscalcium carbonate. Industrial &Engineering Chemistry Research, 46,5051.

66 Cheng, J., Zhou, J., Liu, J., Zhou, Z.,Huang, Z., Cao, X., Zhao, X., and Cen, K.(2003) Sulfur removal at hightemperature during coal combustion infurnaces: a review. Progress in Energy andCombustion Science, 29, 381.


67 Wang, X., Luan, T., Cheng, L., and Xiao,K. (2007) Research of boiler combustionregulation for reducing NOx emissionand its effect on boiler efficiency. Journalof Thermal Science, 16 (3), 270.

68 Liu, H., Zailani, R., and Gibbs, B.M.(2005) Pulverized coal combustion in airand in O2/CO2 mixtures with NOx

recycle. Fuel, 84, 2109.69 Ribeirete, A. and Costa,M. (2009) Impact

of the air staging on the performance of apulverized coal fired furnace.Proceedings of theCombustion Institute,Elsevier Inc.

70 Patel, R.L., Thornock, D.E., Borio, R.W.,Miller, B.G., and Scaroni, A.W. (1996)Firing micronized coal with a low NOx

RSFC burner in an industrial boilerdesigned for oil and gas. Proceedings ofthe Thirteen Annual InternationalPittsburgh Coal Conference.

71 LaFlesh, R., Briggs, O., Barlow, D., andWessel, G. (1999) Update on ABB C-E�sRSFC low wall burner technology. ASMEIJPG Conference, San Francisco, CA,USA.

72 Grusha, J., Laux, S., and Slingerland, S.(2002) Operational and performanceupdate on new ultra lowNOx combustionsystems with fuel/air monitoring andcontrol technology. Proceedings of the27th International Technical Conferenceon Coal Utilization and Fuel Systems,(ed. B.A. Sakkestad), V1, p. 399, Coaltechnology association.

73 Wu, Z. (2002) NOx Control for PulverizedCoal Fired Power Stations, IEA CoalResearch, London.

74 DOE (2003) National Energy TechnologyLaboratory Accomplishments FY 2002,Office of Fossil Energy, U.S. Dept. ofEnergy, Washington, D.C., available atwww.netl.doe.gov/publications/others/accomp_rpt/accrpt_toc.html.

75 Makino, K. (2002) Global trends anddevelopments in coal technology forpower generation. First AnnualConference of CCSD, Sydney,Cooperative Research Centre for Coal inSustainable Development (CCSD).

76 McDonald, M. and Palkes, M. (1999) Adesign study for the application of CO2/O2 combustion to an existing 300 MWcoal-fired boiler. Proceedings of

Combustion Canada 99 Conference-Combustion andGlobal ClimateChange,Alberta.

77 Seltzer, A.H., Fan, Z., and Fout, T. (2006)An optimized supercritical oxygen-firedpulverized coal power plant for CO2

capture. Proceedings of the 23rd AnnualInternational Pittsburgh CoalConference, Pittsburgh, USA.

78 Wilkinson, M.B., Simmonds, M., Allam,R.J., and White, V. (2003) Oxy-fuelconversion of heaters and boilers for CO2

capture. 2nd Annual Conference onCarbon Sequestration, Virginia, USA.

79 Buhre, B.J.P., Elliott, L.K., Sheng, C.D.,Gupta, R.P., and Wall, T.F. (2005) Oxy-fuel combustion technology for coal-firedpower generation. Progress in Energy andCombustion Science, 31, 283.

80 Tan, Y., Chui, E., Douglas, M., andThambimuthu, K. (2005) Oxy-fuel coalburner design: from CFD modeling topilot scale testing (eds., E.S. Rubin, D.W.Keith, C.F. Gilboy, M. Wilson, T. Morris,J. Gale, K. Thambimuthu). Proceedingsof the 7th International Conference onGreenhouse Gas Control Technologies 5,Vancouver, p. 1791.

81 Jordal, K., Anheden, M., Yan, J., andStr€omberg, L. (2005) Oxy-fuelCombustion for coal-fired powergeneration with CO2 capture-opportunities and challenges (eds.,E.S. Rubin, D.W. Keith, C.F. Gilboy,M. Wilson, T. Morris, J. Gale,K. Thambimuthu). Proceedings of the7th International Conference onGreenhouse Gas Control Technologies 5,Vancouver, p. 201.

82 Normann, F., Andersson, K., Leckner, B.,and Johnsson, F. (2008) High-temperature reduction of nitrogen oxidesin Oxy-fuel combustion. Fuel, 87, 3579.

83 Shen, L., Zheng, M., Xiao, J., Zhang, H.,and Xiao, R. (2007) Chemical loopingcombustion of coal in interconnectedfluidized beds. Science in China SeriesE-Technological Sciences, 50 (2), 230.

84 Rubel, A., Liu, K., Neathery, J., andTaulbee, D. (2007) Evaluation of oxygencarriers for chemical looping combustionof coal and solid fuels. Proceedings of the24th Annual International PittsburghCoal Conference, South Africa.

References j81

85 Shen, L., Wu, J., and Xiao, J. (2009)Experiments on chemical loopingcombustion of coal with a NiO-basedoxygen carrier. Combustion and Flame,156, 721.

86 Jukkola, G., Liljedahl, G., Nsakala, N.,Morin, J.-X., and Andrus, H. (2005) AnAlstom vision of future CFB technologybased power plant concepts. Proceedingsof the 18th International Conference onFluidized Bed Combustion, Canada,American Society of MechanicalEngineers (ASME), NY, USA, p. 12.

87 Staiger, B., Unterberger, S., Berger, R.,and Hein, K.R.G. (2000) Influence ofbiomass fuel characteristics on thecombustion in grate firing systems (eds.,S. Kyritsis, A.A.M. Beenackers, P. Helm,A. Grassi). Proceedings of the 1st WorldConference on Biomass for Energy andIndustry, Sevilla, p. 801, James & JamesLtd.

88 Vamvuka, D. (2009) Biomass, Bioenergyand the Environment, TziolasPublications, Thessaloniki.

89 Bapat, D.W., Kulkarn, S.V., andBhandarkar, V.P. (1997) Design andoperating experience on fluid bed boilerburning biomass fuels with high alkaliash. Fluidized Bed Combustion, 1, 165.

90 Baxter, L.L., Gale, T., Sinquefield, S., andSclippa, G. (1997) Influence of ashdeposit chemistry and structure onphysical and transport properties,Development in Thermochemical BiomassConversion (eds A.V. Bridgwater andD.G.B. Boocock), Blackie Academic andProfessional, London.

91 Jenkins, B.M., Baxter, L.L., Miles, T.R. Jr,and Miles, T.R. (1998) Combustionproperties of biomass. Fuel ProcessingTechnology, 54 (1–3), 17.

92 Baxter, L.L., Miles, T.R., Miles, T.R. Jr,Jenkins, B.M., Milne, T.A., Dayton, D.,Bryers, R.W., and Oden, L.L. (1998) Thebehavior of inorganic material inbiomass-fired power boilers: field andlaboratory experiences. Fuel ProcessingTechnology, 54 (1–3), 47.

93 Demirbas, A. (2005) Potentialapplications of renewable energysources, biomass combustion problemsin boiler power systems and combustionrelated environmental issues. Progress

in Energy and Combustion Science, 31(2), 171.

94 Obernberger, I., Brunner, T., andB€arnthaler, G. (2006) Chemicalproperties of solid biofuels-significanceand impact. Biomass and Bioenergy, 30,973.

95 Lu, D.Y., Anthony, E., Talbot, R., Winter,F., L€offler, G., and Wartha, C. (2001)Understanding of halogen impacts influidized bed combustion. Energy andFuels, 15, 533.

96 Vamvuka, D. and Zografos, D. (2004)Predicting the behaviour of ash fromagricultural wastes during combustion.Fuel, 83 (14–15), 2051.

97 Vamvuka, D. and Mistakidou, E. (2005)Predicting the effect of inorganicconstituents of wood residues on boilerslagging/fouling and ash disposal (eds.,L. Sjunesson, J.E. Carrasco, P. Helm,A. Grassi). Proceedings of the 14thEuropean Biomass Conference andExhibition, Paris, p. 1328, ETA-Renewable Energies.

98 Vamvuka, D., Topouzi, V., Stratakis, A.,Christou, M., Alexopoulou, E., andPanoutsou, C. (2007) Giant reed as a fuelfor heat and electricity applications inGreece (eds., K. Maniatis, H.P. Grimm,P. Helm, A. Grassi). Proceedings of the15th European Biomass Conference andExhibition, Berlin, p. 777, ETA-Renewable Energies.

99 Davidsson, K.O., Korsgren, J.G.,Pettersson, J.B.C., and J€aglid, U. (2002)The effects of fuel washing techniques onalkali release frombiomass. Fuel, 81, 137.

100 Baker, R.R., Jenkins, B.M., and Williams,R.B. (2002) Fluidized bed combustionof leached rice straw. Energy and Fuels,16 (2), 356.

101 Vamvuka, D. (2009) Comparative fixed/fluidized bed experiments for the thermalbehaviour and environmental impact ofolive kernel ash. Renewable Energy, 34,158.

102 Olofsson, G., Ye, Z., Bjerle, I., andAndersson, A. (2002) Bed agglomerationproblems in fluidized bed biomasscombustion. Industrial & EngineeringChemistry Research, 41, 2888.

103 Tran, Q.T., Steenari, B., Issa, K., andLindqvist,O. (2004)Capture of potassium


and cadmium by kaolin in oxidizing andreducing atmospheres. Energy and Fuels,18, 1870.

104 Aho, M. and Silvennoinen, J. (2004)Preventing chlorine deposition on heattransfer surfaces with aluminium-siliconrich biomass residue and additive. Fuel,83, 1299.

105 Lindstr€om, E., Bostr€om, D., and Öhman,M. (2005) Effect of kaolin and limestoneaddition on slag formation duringcombustion of woody biomass pellets(eds., L. Sjunesson, J.E. Carrasco,P. Helm, A. Grassi). Proceedings of the14th European Biomass Conference,Paris, p. 1319, ETA-Renewable Energies.

106 Vamvuka, D., Zografos, D., and Alevizos,G. (2008) Control methods for mitigatingbiomass ash-related problems influidized beds. Bioresource Technology, 99,3534.

107 Davidsson, K.O., Amand, L.E., Steenari,B.M., Elled, A.L., Eskilsson, D., andLeckner, B. (2008) Countermeasuresagainst alkali-related problems duringcombustion of biomass in a circulatingfluidized bed boiler. Chemical EngineeringScience, 63 (21), 5314.

108 Khan, A.A., de Jong,W., Jansens, P.J., andSpliethoff, H. (2009) Biomasscombustion in fluidized bed boilers:potential problems and remedies. FuelProcessing Technology, 90 (1), 21.

109 Chirone, R., Salatino, P., Scala, F.,Solimene, R., and Urciuolo, M. (2008)Fluidized bed combustion of pelletizedbiomass and waste-derived fuels.Combustion and Flame, 155, 21.

110 dos Santos, F.J. andGoldstein, L. Jr (2008)Experimental aspects of biomass fuels ina bubbling fluidizedbed combustor.Chemical Engineering and Processing,47, 1541.

111 van Loo, S. and Koppejan, J. (2002)Handbook of Biomass Combustion andCo-Firing, IEA Bioenergy, TwenteUniversity Press, Enschede.

112 Schwieger, R. (1980) Power from wood,Power (February), 1.

113 Nussbaumer, T. and Hustad, J.E. (1996)Overview of biomass combustion (eds.,A.V. Bridgwater, D.G.B. Boocock).Proceedings of the Int. Conference onDevelopments in Thermochemical

Biomass Conversion, Canada, p. 1229,Chapman & Hall.

114 Obernberger, I. (1998) Decentralizedbiomass combustion: state of the art andfuture development. Biomass andBioenergy, 14 (1), 33.

115 Yin, C., Rosendahl, L.A., and Kær, S.K.(2008) Grate-firing of biomass for heatand power production. Progress in Energyand Combustion Science, 34 (6), 725.

116 Paist, A. and Kask, Ü. (1998) Boilerconversion technologies to biofuels usedin Estonia (eds., H. Kopetz, T. Weber,W. Palz, P. Chartier, G.L. Ferrero).Proceedings of the 10th EuropeanConference on Biomass for Energy andIndustry, Wurzburg, p. 1498,C.A.R.M.E.N.

117 Schr€ofelbauer, H., Tanschitz, J., andMory, A. (1998) Additional combustion ofbiomass in the coal-fired power stations atS. Andr€a and Zeltmeg/Austria (eds.,H. Kopetz, T. Weber, W. Palz, P. Chartier,G.L. Ferrero). Proceedings of the 10thEuropean Conference on Biomass forEnergy and Industry, Wurzburg, p. 1413,C.A.R.M.E.N.

118 Prander Lans, R., Jensen,A.,Glarborg, P.,Røjel, H., Jensen, P.A., Frandsen, F.,Hansen, J., and Dam-Johansen, K. (2000)Grate firing of straw: activities in theCHEC research centre (eds., S. Kyritsis,A.A.M. Beenackers, P. Helm, A. Grassi).Proceedings of the 1st World Conferenceon Biomass for Energy and Industry,Sevilla, p. 2064, James & James Ltd.

119 van Berlo, M.A.J. and Jonkhoff, E. (2005)Waste is innovation: Amsterdam�s highefficiency waste fired power plant (eds.,L. Sjunesson, J.E. Carrasco, P. Helm,A. Grassi). Proceedings of the 14thEuropean Biomass Conference, Paris,p. 1522, ETA-Renewable Energies.

120 Weissinger, A., Obernberger, I., L€angle,G., and Steurer, A. (1998) NOx reductionby primarymeasures for grate furnaces incombination with in-situ measurementsin the hot primary combustion zone andchemical kinetic simulations (eds.,H. Kopetz, T. Weber, W. Palz, P. Chartier,G.L. Ferrero). Proceedings of the 10thEuropean Conference on Biomass forEnergy and Industry, Wurzburg, p. 1474,C.A.R.M.E.N.

References j83

121 Sermet Oy (2002) Company Brochure,Sermet Oy, Finland.

122 Jørgensen, K., Meier, E.H., and Madsen,O.H. (2000) Modern biomass boilers(eds., S. Kyritsis, A.A.M. Beenackers,P. Helm, A. Grassi). Proceedings of the1st World Conference on Biomass forEnergy and Industry, Sevilla, p. 717,James & James Ltd.

123 Nielsen,C.R., Knudsen, P.,Henriksen,N.,Sander, B., andNielsen, C. (2000) First twoyears of operational and demonstrationprogram experience with the highefficiency biomass boiler at SHenergiA/S,Aabenraa, Denmark (eds., S. Kyritsis,A.A.M. Beenackers, P. Helm, A. Grassi).Proceedings of the 1st World Conferenceon Biomass for Energy and Industry,Sevilla, p. 893, James & James Ltd.

124 Pinto, G.S. andHoiz, C. (2000) The EHN-Sang€uesa 25 MWe straw-fired powerplant (eds., S. Kyritsis, A.A.M.Beenackers, P. Helm, A. Grassi).Proceedings of the 1st World Conferenceon Biomass for Energy and Industry,Sevilla, p. 863, James & James Ltd.

125 Miles, T.R. (1996)Alkali deposits found inbiomass power plants, Research ReportNREL ITP-433-8142 SAND96-8225,National Renewable Energy Laboratory,Oakridge.

126 Nikolaisen, L. (1994) Danish experiencewith straw combustion in district heatingplants. Proceedings of the Comett ExpertWorkshoponBiomassCombustion,Graz.

127 Good, J. and Nussbaumer, T. (1998)Efficiency improvement and emissionreduction by advanced combustioncontrol technique with CO/lamda controland set point optimisation (eds.,H. Kopetz, T. Weber, W. Palz, P. Chartier,G.L. Ferrero). Proceedings of the 10thEuropean Conference on Biomass forEnergy and Industry, Wurzburg, p. 1362,C.A.R.M.E.N.

128 Orjala, M., Ingalsuo, R., Patrikainen, T.,M€akip€aa, M., and H€am€al€ainen, J. (2000)Combusting in wood chips produced bydifferent harvestingmethods in fluidizedbed boilers (eds., S. Kyritsis, A.A.M.Beenackers, P. Helm, A. Grassi).Proceedings of the 1st World Conferenceon Biomass for Energy and Industry,Sevilla, p. 1447, James & James Ltd.

129 Bolh�ar-Nordenkampf, M., Gartnar, F.,Tschanun, I., and Kaiser, S. (2005)Operating experience from FBC plantsusing various biomass fuels (eds.,L. Sjunesson, J.E. Carrasco, P. Helm,A. Grassi). Proceedings of the 14thEuropean Biomass Conference, Paris,p. 1109, ETA-Renewable Energies.

130 Murillo, J.M., Escalada, R., Fern�andez,M., and Carrasco, J. (2000) Feedingsystems influence on the fluidized bedcombustion of cork wastes (eds.,S. Kyritsis, A.A.M. Beenackers, P. Helm,A. Grassi). Proceedings of the 1st WorldConference on Biomass for Energy andIndustry, Sevilla, p. 2072, James & JamesLtd.

131 Martinez, J.M., Varela, M., Escalada, R.,Murillo, J.M., Gonzalez, E., Carrasco, J.,and Manzanares, P. (2000) Combustionassays of brushwood biomass in a BAFBpilot plant (eds., S. Kyritsis, A.A.M.Beenackers, P. Helm, A. Grassi).Proceedings of the 1st World Conferenceon Biomass for Energy and Industry,Sevilla, p. 1987, James & James Ltd.

132 European Bioenergy Networks (2003)Biomass Co-firing-An Efficient Way toReduce Greenhouse Gas Emissions,http://eubionet.vtt.fi.

133 Jeffcoat, I. (2000) Thermal treatment ofbiomass in the circulating fluid bed,Power Generation by Renewables, IMechESeminar Publication Editor, ProfessionalEngineering Publishing, UK.

134 Klass, D.L. (1998) Biomass for RenewableEnergy, Fuels And Chemicals, AcademicPress.

135 Wehinger, F. and K€ob, S. (1998) The KÖB�PYROT� Rotary firing for spare woodwith automatic changing (eds.,H. Kopetz, T. Weber, W. Palz, P. Chartier,G.L. Ferrero). Proceedings of the 10thEuropean Conference on Biomass forEnergy and Industry, Wurzburg, p. 1513,C.A.R.M.E.N.

136 Mc Carroll, R.L. and Partanen, W.E.(1995) Operation of a direct-firedcombustion turbine systemonpulverizedwood. Proceedings of the 2nd BiomassConference of the Americas on Energy,Environment, Agriculture and Industry,Golden, p. 400, National RenewableEnergy Laboratory (NREL).


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