wood and biomass installations

8/11/2019 Wood and Biomass Installations

1/12Steam 41/ Wood and Biomass Installations 30-1

The Babcock & Wilcox Company

Chapter 30

Wood and Biomass Installations

The category of wood and biomass covers a widerange of material that can be used as a source ofchemical energy. Wood encompasses a number ofsources such as bark, wood sticks, sawdust, sanderdust, over- and under-sized wood chips rejected from

the pulping process, whole tree chips, and scrap ship-ping pallets. Biomass is anything that is or recentlywas alive such as straw, vine clippings, leaves, grasses,bamboo, sugar cane (called bagasse after the sugarhas been extracted), palm oil, coffee grounds, and ricehulls from the food processing industry. All of thesematerials can be used as a fuel source to generate steam.

While wood and other biomass fuels were some ofthe first materials used as energy sources, their usedeclined around the early 1900s, when more consis-tent and easily transportable fossil fuels such as coal,oil and natural gas became more available. Neverthe-less, there have always been applications where woodand biomass have been the preferred fuels. There has

therefore been slow but steady progress in the devel-opment of equipment for firing these fuels.Many factors have led to a recent, rapid develop-

ment in this area. Some of these factors include the ris-ing cost of certain fossil fuels, renewable energy options,development of new technology that allows better useof industrial byproducts, and the trend toward cogen-eration in many industries. This includes industries thatproduce both electric power and process steam. Woodand biomass are used in many municipal combined heatand power (CHP) installations, particularly in Europe.

Biomass gasification is a relatively new technologybeing explored to supply fuel gas for power produc-tion by gas engines or gas turbines.

Equipment for chemical and heat recovery in thepaper industry is discussed in Chapter 28. Equipmentfor using municipal solid waste as a fuel is discussedin Chapter 29. Stoker-related information is discussedin Chapter 16.

Steam supply for power production

Cogeneration in industry

Most heating requirements can be met with satu-rated steam at 150 psig (1.03 MPa) or less. Cogenera-tion, or the simultaneous production of electrical or

mechanical energy and heat energy to a process, hasvery high thermal efficiency. Thus, waste heat fromelectricity production becomes part of a usable processrather than being rejected into the atmosphere.

In a cogeneration facility, relatively high pressure

superheated steam passes through a steam turbine orsteam engine where energy is extracted. The exhauststeam is then used as a heat source in a process. Theconversion efficiency, or the amount of heat absorbedin the steam turbine plus the heat absorbed in the pro-cess, versus the amount contained in the steam, ap-proaches 100%. The overall thermal efficiency of a co-generation process is closely approximated by the ther-mal efficiency of the boiler alone.

As well as providing high energy efficiency, thereis a second and often more compelling reason for in-dustries, especially the pulp and paper industry, topractice cogeneration. These industrial plants are of-ten located far from economical and reliable sources

of electricity and conventional fossil fuels in order tobe close to their source of fiber, the forests. Cogenera-tion facilities equipped with wood-fired power boilers,for example, are therefore often justified.

Biomass-fired utilities

Changing economic conditions and environmentalregulations have made utility plants fired by biomassfuels a practical source of electrical energy, in spite oftheir relatively high capital costs. Sometimes the plantis built with a condensing turbine and other times itis built beside or within a plant that can use exhauststeam. Economic conditions favoring such facilitiesinclude the unpredictable and sometimes high cost of

conventional fossil fuels, the relatively low cost of woodwaste and other biomass fuels, and the high cost to trans-port and dispose of biomass in a landfill.

Occasionally an installation is justified with regardto the high cost of emissions control equipment. Mostwood and biomass fuels generate lower nitrogen ox-ides (NOx) and sulfur dioxide (SO2) emissions than con-ventional fossil fuels. In one installation in California,for example, it was found that the amount of NOxpro-duced by incinerating grapevine clippings and otherbiomass could be substantially reduced by burning thematerial in a wood-fired utility installation in a con-trolled manner. This produced environmental benefits


2/1230-2 Steam 41/ Wood and Biomass Installations


while generating electricity that would otherwise beproduced by burning traditional fossil fuels.

The Babcock & Wilcox Company (B&W) operationin Europe completed a significant biomass-fired instal-lation that burns straw in an ultra-supercritical powerplant application. (See Fig. 1.) This straw-fired boilerconsumes 105 MWtof fuel producing 35 MW of elec-tricity and 50 MWtof heat. In this advanced design,carded (loosened by mechanical raking) straw is fed

by screw feeders onto a water-cooled gasification gratewhere a high percentage of the energy content is re-leased by pyrolysis and gasification. The remainingstraw/carbon burns on a water-cooled vibrating grate.

A bag filter removes more than 99% of particulates fromthe flue gas, and the plant operates within strict Euro-pean emissions regulations.

Steam supply to process

Pulp and paper

The pulp and paper industry is the major consumerof biomass fuels because wastes such as bark, sawdust,

shavings, lumber rejects and clarifier sludge are by-products of the pulping, paper-making and lumbermanufacturing processes. There is a great amount ofheat energy available in these products, making themuseful energy sources. (See Figs. 2 and 3.)

The production of pulp and paper requires largequantities of mechanical energy for grinding, chip-ping, cooking and refining. To produce a marketableproduct, the pulp must be dried using steam or heatedsurfaces such as paper machine dryer rolls. These energyneeds are met by using steam in a variety of equipment(steam engines or turbines, steam coil air heaters, dryer rolls,and/or indirect heaters) and by direct steam injection.

These requirements, coupled with the availabilityof the waste products, make boilers fired by wood andwood waste a logical choice for the pulp and paper industry. Most of the developments and improvementsin equipment for these boilers have been driven bythis industrys needs.

Food processing

The food processing industry also has energy needsthat are provided by steam. Mechanical preparationcooking, drying and canning all require a source ofenergy. Many foods leave behind waste byproductsrich in cellulose or other organic (hydrocarbon) mate-rial. Instant coffee production generates coffeegrounds, sugar-making leaves bagasse from sugarcane, coconut preparation discards husks, rice has itshulls removed before packaging, and many types ofnuts are sold roasted with their shells removed.

Many producers have installed boilers that burnsuch biomass materials, usually based very closely onequipment originally designed for the pulp and paper industry. These boilers produce steam that is thenused as an energy source for the plant.Fig. 1 Straw-fired ultra-supercritical boiler (courtesy Energi E2 A/S).

GasOutlet

ScrewFeeders

ConveyorSystem

Water-CooledVibrating Grate

StrawCarding

Rake

Superheater

Fig. 2 Wood-fired two-drum Stirlingpower boiler.




Fuels

Constituents

Wood and most biomass fuels are composed predomi-nantly of cellulose and moisture. The high proportionof moisture is significant because it acts as a heat sinkduring the combustion process. The latent heat of evapo-ration (Hfg) depresses the flame temperature, contribut-

ing to the difficulty of efficiently burning biomass fuels.Cellulose, as well as containing the chemical energyreleased during combustion, contributes fuel-boundoxygen. This oxygen decreases the theoretical air re-quired for combustion and therefore the amount ofnitrogen included in the products of combustion.

Most natural biomass fuels contain little ash. How-ever, some byproducts, such as de-inking sludges, do

contain a great deal of ash, in some cases up to 50% ashon a dry basis. De-inking sludges are particularly diffi-cult to burn because they usually have high moistureand ash contents and low fuel-bound oxygen content.

Burning wood and biomass

The following general guidelines for wood and biom-ass combustion have been developed from experience:

1. Stable combustion can be maintained in most wa-ter-cooled furnaces at fuel moisture contents ashigh as 65% by weight, as-received.

2. The use of preheated combustion air reduces thetime required for fuel drying prior to ignition andis essential to spreader-stoker combustion systems.The design air temperature will typically vary di-rectly with fuel moisture content.

Fig. 3 Multifuel single-drum, 141,000 lb/h (17.8 kg/s) steam flow, bottom-supported boiler with vibrating grate stoker.

Day Bin/HydraulicFuel Silo

Speed ControlledScrew Feeders

Air-SweptSpout

Water-CooledVibrating Grate

IndividualControllable

Primary Air Chambers

Furnace

SteamDrum

Superheater

Economizer

Gas Outlet

Air HeaterAuxiliaryBurners

RadiantSuperheater




3. A high proportion of the combustible content ofwood and biomass fuels burns in the form of vola-tile compounds. A large portion of the combustionair requirement is therefore added above the fuel,as overfire air (OFA).

4. Solid chars produced in the initial stages of com-bustion of these fuels are of a very low density.Conservative selection of furnace size and care-ful placement of the OFA system are used to re-

duce gas velocity and keep char entrainment atan acceptable level. Typical furnace selection cri-teria include a grate heat release rate of 0.47 to 1.1 106Btu/h ft2(1.5 to 3.5 MWt/m2) of grate surfacearea, a furnace liberation rate of 17,000 Btu/h ft3

(176 kW/m3) of furnace volume, and an upward gasvelocity of 20 ft/s (6.1 m/s). This results in furnaceresidence times of approximately three seconds forlarger units to enhance particulate burnout andminimize emissions.

Burning in combination with traditional fuels

Biomass can be burned on a traveling grate with

stoker coal. The biomass is introduced to the furnacethrough a separate conveying system and either aseparate windswept spout below the coal feeder orthrough a combination feeder. (See Chapter 16.)

Biomass can be burned with pulverized coal, oil ornatural gas, using dedicated burners for the latter. Inthis case, the grate is selected for wood firing.

When burning biomass with substantial quantitiesof stoker or pulverized coal, the amount of ash fromthe coal is greater than the amount from the biom-ass. Therefore, the design parameters for the coal(slagging and fouling index) will govern the design.

When burning biomass with heavy fuel oil havinghigh sulfur and vanadium content, the ash that forms

on the convective surfaces can be tenacious and, onceremoved, can be very abrasive. It is preferred to de-sign for low flue gas temperatures and low flue gasvelocities, regardless of the specified contaminants inthe heavy oil and in the biomass, as both flue gas pa-rameters can vary widely over the typical range ofboiler operating conditions. (See Chapter 10.)

Sludge burning

As mentioned above, paper mill sludges, especiallyde-inking sludges, are difficult fuels to burn. Theirhigh moisture and ash content and low fuel-boundoxygen may limit their allowable proportion of thetotal heat input to the furnace.

Specific limits will vary with the sludge compositionand combustion system. Higher sludge inputs for agiven system can normally be achieved by combinedfiring with better quality fuels.

Wood waste mixed with sludge can prove difficultfor the fuel handling system. Frequently, the sludgesegregates from the other fuels (at transfer points onbelt conveyors, for example) and can be fed preferen-tially to one feeder.

In spreader-stoker applications, the wet, densesludge can pile up in one place on the grate veryquickly. Therefore, when a portion of the fuel is sludge,

the boiler operator must continually inspect the grate todetermine whether adjustments are required.

Combustion systems

Many methods have been developed to burn woodand other biomass fuels. The best known and mostsuccessful methods use the following equipment.

Dutch ovenA dutch oven is a refractory-walled cell connected

to a conventional boiler setting. Water cooling is sometimes provided to protect and extend the life of therefractory walls. Wood and other biomass fuels areintroduced through an opening in the roof of the dutchoven and burn in a pile on its floor. OFA is introducedaround the periphery of the cell through rows of holesor nozzles in the refractory walls.

The principal advantage of the dutch oven is thatonly a small portion of the energy released in combustion is absorbed because of its high percentage of refractory surface. Therefore, it is able to burn highmoisture fuels (up to 60% moisture). In addition, as

the fuel is pile-burned, there is a high thermal inventory in the cell that makes the unit less sensitive tointerruptions in the fuel supply.

The dutch oven has distinct disadvantages whencompared to more modern methods. The unit operatesbest when at a steady load and when burning a consistent fuel. It does not respond quickly to load demandThe refractory is subject to damage from the follow-ing sources: spalling and erosion caused by rocks ortramp metal introduced with the fuel, rapid coolingfrom contact with very wet fuels, and overheatingwhen very dry wood is fired.

The dutch oven cell must be shut down regularlyto allow manual removal, or rake out, of the accumu-lated ashes. During this period, either load must bedrastically reduced as is done with multiple cell unitsor auxiliary fuel must be fired as is required for unitsequipped with a single dutch oven.

Pinhole grate

The pinhole grate (Fig. 4) is a water-cooled grateformed by cast iron grate blocks, sometimes referredto as Bailey blocks, that are clamped and bonded tothe spaced floor tubes of a water-cooled furnace. Thegrate blocks have venturi-type air holes to admitundergrate air to the fuel on the grate. This grate isused in conjunction with either mechanical fuel distributors or air-swept fuel spouts. Both produce asemi-suspension mode of burning, wherein the finerportion of the fuel is burned in suspension and theheavier fraction accumulates and burns on the grateThe ash and foreign material that stay on the grate areremoved by raking. Typically, 25% of the air for combustion of wood is introduced as OFA through nozzles in thelower furnace walls and 75% is undergrate air.

This combustion system can follow minor loadswings by varying both the fuel flow and air flow, andis suitable for biomass fuels containing up to approximately 55% moisture. A small amount of refractory isused and maintenance requirements are low.




The main disadvantage of this method is that thegrate requires manual raking of the ashes. Therefore,biomass firing must be stopped on a regular basis.Manual raking also limits the depth of the furnace andtherefore the steam capacity that can be economicallybuilt. Mechanical raking machines have been devel-oped to increase the allowable furnace depth and tospeed the raking process, while providing a certaindegree of protection to the operator from hot gas,flames and hot ash.

Traveling grate

The traveling grate was introduced as an improve-ment over the pinhole grate. (See Fig. 5.) It is a mov-ing grate that allows continuous automatic ash dis-charge and consists of cast iron or ductile iron gratebars attached to chains that are driven by a slowmoving sprocket drive system. The grate bars haveholes in them to admit undergrate air that is also usedto cool the grate bar castings. The split betweenundergrate and overfire air flows depends largelyupon fuel size, volatility and moisture content, whilealso meeting the grate cooling requirements. For coalfiring, 60 to 85% of the air is typically supplied asundergrate with the balance used for OFA. For biomassfiring, 25 to 50% of the air is typically supplied asundergrate. Rows of nozzles in the furnace front walland rear wall are used for OFA. Fuel spreaders andburning mode are identical to those used with the pin-hole grate. The main advantages of this grate are its abil-

ity to follow load swings and the automatic ash dischargethat permits continuous operation on biomass fuels.

The traveling grate was originally developed forspreader-stoker firing of bituminous and subbitumi-nous coals. With coal, the quantity of OFA requiredfor efficient combustion can be as low as 15% of thetotal air. The quantity of ash in coal is also muchhigher than in wood. It is therefore possible to developa relatively large bed of ash on the grate in order to

protect the grate from high temperatures, and to helpdistribute the undergrate air flow. Lower moisture coalcan often be burned without air preheating and rarelyrequires air temperatures in excess of 350F (177C).

The traveling grate, while an improvement over thepinhole grate, must be considered a compromise de-sign for the burning of biomass because the use of pre-heated air and the usually low ash content of biomassreduce the cooling available to the grate. This grate hasmany moving parts that are subjected to the furnaceheat, resulting in higher maintenance costs.

Vibrating grate

The vibrating grate allows intermittent, automatic ash

discharge. The grate consists of cast iron grate bars orbare tube panels attached to a frame that vibrates onan intermittent basis, controlled by an adjustable timer.

Fig. 4 Wood-fired boiler with pinhole grate.

Fig. 5 Wood-fired boiler with traveling grate.




There are two major types that have been used for bio-mass fuels, one water-cooled and the other air-cooled.

The water-cooled vibrating grate (see Fig. 6) is usedin conjunction with the semi-suspension firing mode.Because the grate is water-cooled, high temperatureundergrate air, up to 650F (343C), can be used alongwith very high percentages of OFA and a relativelythin fuel bed. A key advantage of the vibrating grateis the low number of parts that are highly stressed,

moving or in sliding contact. This results in reducedmaintenance requirements. The vibration is intermit-tent and several vibration sequences (vibration fre-quency, duration, and time span between vibratingperiods) are used. A common vibration sequence in-cludes six cycles per second for a short duration ofabout two seconds every two minutes. The vibrationtime and the dwell time can be adjusted to suit the fuelcharacteristics. The vibrating action can help improvefuel distribution on the grate by causing fuel piles to

collapse. This style of biomass combustion system isbeing used very successfully at a number of installations for a wide range of wood fuel moisture contents

The air-cooled vibrating grate can be used for ap-plications similar to those suitable for the water-cooledvibrating grate, but the maximum allowableundergrate air temperature is 550F (288C), even withthe use of stainless steel components. Because it canbe installed with a horizontal grate surface, the air

cooled vibrating grate can be a very effective replacement for a traveling grate, when the traveling gratehas been deemed unacceptable due to high maintenance or repair costs.

Furnace configurations

A variety of furnace configurations can be foundin wood and wood-waste boilers. In the late 1970s, thecontrolled combustion zone or CCZ furnace was de-veloped by B&W specifically for biomass combustion

As shown in Figs. 2 and 6, the CCZ design uses archesin the front and rear walls of the furnace to create alower furnace zone in which combustion of biomass

can be confined. OFA from nozzles located above andbelow the arches penetrates and burns off the vola-tile fuel released from the bed, as well as particlesentrained in the upward flow. This furnace designincorporates a range of OFA systems that had mul-tiple elevations of large ports and increased the OFAcapacity to 40 to 50% of the total combustion air flow

Tighter emission control regulations and advancing computational technology have stimulated the de-velopment and deployment of a straight-wall furnacedesign with fewer, larger OFA ports that can be moreprecisely located and operated. As shown in Fig. 7, thearches and multiple levels of OFA ports for the CCZcan be replaced with straight furnace walls and one

level (sometimes two) of OFA ports. The B&WPrecisionJet OFA system has high velocity ports thatinclude velocity dampers to fine-tune the combustionprocess. Computational models (see Chapter 6) permit the positioning and sizing of the higher velocityOFA ports to better complete the combustion processin a controlled fashion, thereby reducing NOx, CO andparticulate. OFA capacity of up to 60% of the combustion air flow can be used. As discussed in Chapter 16a second option is a horizontal rotary overfire air sys-tem designed to create a double rotating circulation zonein the straight-wall furnace. (See Chapter 16, Fig. 13.)This can also increase turbulence and mixing of air andgaseous combustibles for enhanced performance. With

a straight-wall design and fewer ports, these furnace configurations usually have lower initial capital costs.Process recovery (PR) boilers (see Chapter 28) and

coal-fired power boilers at pulp and paper mills havebeen retrofitted for wood and wood-waste firing ca-pability as mill power and PR needs have evolved. Rising energy prices have also increased the benefit ofhigher internal power generation from waste productfuels. To accommodate the heat release rates discussedearlier, the furnace plan area of the original boilermust typically be enlarged to accommodate the gratearea for the desired heat input. The new enlargedlower furnace is then tapered to connect with the origiFig. 6 Water-cooled vibrating grate unit with CCZ furnace.


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Fuel distributor

The two most common devices for introducing fuelinto the furnace for semi-suspension firing are me-chanical distributors and air-swept spouts. They areboth designed to distribute fuel as evenly as possibleover the grate surface.

The mechanical distributor uses a rotating paddlewheel to distribute the fuel. The speed of the wheel is

varied to suit the fuel characteristics. In some instal-lations, a continually varying speed is used to ensuregood fuel distribution.

The air-swept spout uses high pressure air that iscontinuously varied by a rotary damper to distributethe fuel. Adjustments to the supply air pressure aremade by the operator as the characteristics of the fuelchange. The trajectory of the fuel leaving the spout isaltered by an adjustable ramp at the bottom of the spout.

Burners

Burners are sometimes used to burn all or a portionof the biomass fuel. The burners are somewhat similarin design to a pulverized coal burner. (See Chapter 14.)

Fuels that can be fired in a burner include sander dust,sawdust of less than 35% moisture content, and the finematerial collected from a fuel dryer. Due to the possi-bility of inconsistent fuel flow and quality, a continuouslyoperated auxiliary fuel pilot flame is recommended.

Furnace

A properly designed furnace has two main func-tions. The first is to provide a volume in which the fuelcan be burned completely. The second is to absorbsufficient heat to cool the flue gas to a temperature atwhich the entrained flyash will not foul the convec-tive surfaces. This must be accomplished while match-ing the dimensions of the grate and while providing

sufficient clearance dimensions from auxiliary burn-ers to prevent flame impingement on furnace walls.

Modern boilers are typically of membrane wall con-struction, but in certain limited cases, such as instal-lations in developing countries, reverting to a tubeand tile type of furnace construction may be appro-priate. In such circumstances, the frequent mainte-nance requirement is outweighed by the reduction infirst cost and the ease of operation due to the highertemperatures in the furnace.

Superheater

The sizing of the superheater on a wood-fired boiler

can be complicated by several factors. For a given fuelbeing fired, the setting of the surface depends on the fi-nal steam temperature and on the control range re-quired. The side and back spacing are selected to minimizefouling and erosion potential. (See Chapters 19 and 21.)

However, a wood-fired boiler rarely burns a consis-tent fuel. Variable moisture content and fuel analysesaffect the steam-to-flue gas ratio, and a variety of aux-iliary fuels, e.g., oil, gas or coal, may be available. There-fore, when designing a superheater, the full range of op-erating conditions must be completely understood.

Constituents in the ash can affect superheater de-sign. For example, high levels of chloride are often

found in bark from logs floated in sea water and canrequire the use of high alloy materials such as 310stainless steel to minimize the corrosion rate of the superheater tubes in the high temperature zones.

Boiler bank

Due to the relatively high ratio of flue gas flow tosteam flow and the relatively low pressures and tem-

peratures at which most wood-fired boilers are operateda large amount of saturated (boiling) surface is requiredFurthermore, due to relatively low adiabatic flame temperatures, the amount of heat absorption in the furnaceis usually low compared to fuels such as oil or naturalgas. Therefore, a large portion of the total heating surface in a wood-fired boiler is usually provided as boilerbank.

In some cases, the amount of furnace surface is aug-mented by water-cooled screens in front of superheat-ers to lower flue gas temperatures entering the superheater and to protect it from thermal radiation or fromthe active burning areas in the furnace.

The amount of boiler bank surface is usually very

substantial. This surface can be arranged as cross flowsurface as normally found in a two-drum Stirlingboiler (see Fig. 2), or may be arranged for longitudinal flow as found in a smaller bottom-supportedTowerpakboiler. (See Fig. 9.)

Because wood fuels frequently contain sand orother mineral matter in addition to the ash, flue gasvelocities in the convection pass or boiler bank mustbe kept low, typically below 60 ft/s (18.3 m/s).

Economizer

In most cases, when an economizer is required to reduce the back-end temperature to a specified level, it is

located between the boiler bank and the tubular airheater. The economizer is designed to reduce the flue gastemperature to that required at the air heater gas inlet

Occasionally, the positions of the economizer and airheater need to be reversed. For example, it may benecessary to provide a higher gas temperature to theair heater as part of an installation that includes a fuedryer. The dryer needs the hotter gas to remove moisture from the fuel. This same system may also be required to maintain a specified exit gas temperatureentering the stack even when the dryer (and its thermal load) is out of service for maintenance or repairFor such a system to work effectively under both operating conditions, the economizer needs to be the fi

nal heat trap. In addition, a bypass, preferably on theeconomizer gas side, is needed to permit exit gas temperature control.

In the above example, it should be rememberedthat the economizer gas exit temperature would belower when burning dry fuel from the dryer andwould be higher with the dryer out of service, if nocontrol method was provided.

There are no special mechanical design considerationsfor economizers on wood-fired units, other than to limitthe flue gas velocity. In virtually all cases, a continuousbare tube economizer is used. (See also Chapter 20.)


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feed returned to storage. The surge bins are equipped withvariable speed screw feeders or chain feeders to controlthe rate of biomass fuel fed into the furnace.

These variable speed feeders must be capable ofoperation over a turndown range of four-to-one onautomatic control. They must also be able to operateat very low speeds during startup conditions to builda fuel bed on the grate. The feeder drives must be ofsufficient horsepower to allow the feeder to be started

when the surge bin is full of fuel.Upstream of the surge bins, it is common to have a

large live-bottom storage bin with four to eight hoursof biomass fuel inventory. This is to avoid interrup-tions in the fuel feed to the boiler when there are prob-lems with the outside fuel handling equipment. (SeeChapter 12.)

For many biomass fuels, special fuel preparation equip-ment may be needed to provide effective overall combus-tion. As an example, for the straw-fired unit shown inFig. 1, the straw bale is carded or raked to loosen thestraw before it is fed to the grate by screw feeders.

Ash handling Ash handling systems on biomass-fired boilers can be divided into two main areas, bot-

tom ash and flyash.Bottom ash is the ash that is raked or conveyed off

the grate, plus the ash that falls through the gratebar holes into the undergrate hopper, called a riddlingsor siftings hopper. Bottom ash consists mainly of sandand stones. The ash at the grate discharge is typicallycollected using a submerged drag chain conveyor witha dewatering incline at the discharge end. The siftingscan be collected with drag chain or screw conveyors.

Flyash is the fine ash and unburned carbon that iscollected from all the boiler bank, economizer, airheater and emissions control equipment hoppers. Theash handling equipment can be drag chains, screwconveyors or wet sluicing systems. Because the flyashcontains a high percentage of hot carbon, it is impor-tant that rotary seal valves be used at each hopperdischarge to prevent air infiltration that could createa fire in the hopper. For the same reason, all ash con-veyors should be sealed.

In some instances, the flyash from the boiler bankand air heater hoppers is reinjected into the furnaceto lower the unburned carbon loss and to reduce thequantity of material that must be disposed. However,the high maintenance requirements of these systemshave limited their use.

Air systems

Air systems can be categorized as undergrate orunderfire air and overfire air.Undergrate air is typically low pressure [3 in. wg

(0.75 kPa)] and, depending on the type of grate used,can be anywhere between 40 and 60% of the total airrequired for combustion. The purpose of undergrateair is to help dry the fuel, promote the release of thevolatiles, provide the oxygen necessary for the com-bustion of the devolatilized char resting on the grateand, in the case of an air-cooled grate, cool the gratebars. The pinhole grate and vibrating grate can beprovided with multiple undergrate air compartments

with separate dampers for the operator to bias theundergrate air to the area of the furnace where thefuel is concentrated. Traveling grates can be providedwith only one compartment per drive section.

OFA system capacities are varied and can rangefrom 25 to 60% of the total air. Varying air port ornozzle sizes and air pressures are used to obtain adequate penetration of the air into the rising stream ofvolatiles from the grate. Typically, modern OFA nozzles

are 3 to 6 in. (76.2 to 152.4 mm) in diameter or rectangular with velocity dampers (PrecisionJet) and useair pressures up to 20 in. wg (4.98 kPa). Levels ofnozzles are controlled independently such that the OFAcan be varied with load and fuel characteristics. Wherevery high temperature OFA is used and it constitutesmore than 40% of the total air flow, it is usually eco-nomical and energy efficient to provide a high pressureFD fan, rather than a low pressure FD fan plus a largehigh pressure OFA fan. The OFA ports can be designedto provide a double rotating circulating zone to furtherenhance combustion. (See Chapter 16.)

Emissions control equipment

Dust collector Mechanical dust collectors are usedafter the last heat trap on the boiler to collect the largersize flyash particulate, sometimes as protection for theID fan. They typically consist of multi-cyclonetubesenclosed in a casing structure. The tubes consist ofouter inlet tubes with spin vanes and inner tubes usedwithout recovery vanes. The dust collector efficiencyis in the range of 65 to 75% at an optimum draft lossof 2.5 to 3.0 in. wg (0.62 to 0.75 kPa). Due to the abrasive nature of the flyash, the outer collection tubes andcones are made of high hardness (450 Brinell) abra-sion resistant material. (See Chapter 33.)

Precipitator Electrostatic precipitators are typicallyused after the mechanical collector to reduce the particulate concentration in the flue gas and to meet environ-mental requirements. Due to the high carbon contentin the flyash, it is important to reduce the fire potentialin the precipitator. It is necessary to ensure no trampair enters the precipitator and that the flyash is continu-ously removed from the hoppers. Hopper level detectorsand temperature detectors alert the operator.

Installations can be equipped with fire fighting apparatus such as steam inerting. Other suppliers recommend de-energizing the precipitator if a predetermined oxygen content in the flue gas is exceeded.

Fabric filter or baghouse Due to the fire potentialbaghouse collectors have been rarely used for biomass

fuels to date.Wet scrubbers Wet scrubbers have also been usedto control particulate emissions on biomass-fired boil-ers. Their main disadvantages are the high flue gaspressure drop, which increases the ID fan horsepowerrequirements, and high water consumption. Alsothere is the need for a wet ash collection system and awater separation and clarification system. Wet scrub-bers have given way to electrostatic precipitators asthe preferred means of final flue gas cleanup, providedthere is no need for a scrubber to reduce SO2emissionsfrom auxiliary fuels.




Wet scrubbers with numerous small spray nozzlesin a chamber can be used where low pressure drop andlow water consumption are required. This is particu-larly well suited to retrofit applications where thescrubber can replace the mechanical dust collector forimproved collection efficiency.

Environmental impact

Particulate emissions

About 80 to 95% of the total ash residue producedby a bark- and wood-fired spreader stoker is in theform of gas-borne particulate. This particulate is com-posed of a number of materials, including ash, sandcontaminants introduced during fuel handling, un-burned char from the furnace, and salt fume (usuallypresent only where logs are seawater flumed).

The ash content of wood and bark fuels is low (0.2to 5.3%, dry basis). Therefore, if fuel ash contaminantsare not appreciable, the particulate will usually con-tain high percentages of unburned char.

The particulate loading in the flue gas exiting the

boiler is influenced by factors related to both combus-tion and aerodynamics. Combustion-related factorsaffect particulates by determining the degree of burn-out for the entrained char. They include plan andvolumetric heat release rate (two design parametersthat affect furnace temperature), residence time, andconsequently, char burnout.

The importance of aerodynamic factors is based onthe fact that bark- and wood-fired spreader-stokerunits are designed to operate with some degree of sus-pension burning. Variables that would tend to in-crease the ratio of furnace velocity to mean char par-ticle size would therefore tend to increase flue gasparticulate loading.

Some of these factors include fuel moisture and finescontent, boiler plan area, air staging, and excess air level.

For wood and bark fuels not containing appreciablequantities of sand, particulate loading at the air heaterexit of a modern spreader-stoker unit would typicallybe in the range of 1 to 3 grains/DSCF (2.4 to 7.2 g/Nm3).

Nitrogen oxides

NOxemissions from wood and bark firing are lowcompared with those from traditional fossil fuels. Com-bustion temperatures in wood firing are sufficientlylow, and little thermal NOxis formed from the nitro-gen in the combustion air. Corresponding NOxemis-sions are therefore predominantly a function of thefuel nitrogen content. (See also Chapter 34.)

Conversion of fuel bound nitrogen to NOxis dependent

on a number of operating conditions including excess air,air staging, heat release rate, and fuel sizing and mois-ture content. Empirical studies have also found NOxtovary inversely with fuel moisture content, although themagnitude of this correlation is less significant.

All factors considered, NOxemissions from stoker fir-ing of most wood and bark vary between 0.1 and 0.35lb/106Btu (0.04 to 0.15 g/MJ) heat input, expressedas nitrogen dioxide. Fuel contaminants that may in-

troduce nitrogen compounds (glues and chemicals, forexample) should receive special consideration.

Sulfur dioxide

Wood and bark typically contain 0.0 to 0.1% elemen-tal sulfur on a dry basis. During the combustion processsome of this sulfur can be converted to flue gas SO2, butthe conversion ratio is typically low (10 to 30%). Becausethe quantities of both wood sulfur and flue gas SO2 arenear the low end detection limit of corresponding ana-lytical instruments, correlation of the two is not practical.

Typically, SO2emissions for stoker-fired wood andbark fuels do not exceed 0.03 lb/106Btu (0.01 g/MJ) heatinput. Special consideration should be given to fuels

where sulfur bearing contaminants may be present.

Carbon monoxide

Of all emissions commonly associated with woodand bark firing, carbon monoxide (CO) is usually themost variable. As a gaseous product of incomplete com-bustion, CO is dependent on time, temperature andturbulence considerations.

At normal excess air levels, consistency of both fuelheating value and fuel distribution are considered themost important determinants of CO emissions. Typi-cally, test data showing the highest standard devia-tion in CO correspond to the highest mean CO.

Conditions of high excess air, low excess air, highfuel moisture and reduced load (


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Combined heat and power plant with straw-fired ultra-supercritical boiler (courtesy of Kastrup Luftfoto A/S, Denmark).

wood and biomass installations

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