Fundamentals of Stoker Fired Boiler Design and Operation

Presented at: CIBO Emission Controls Technology Conference

July 15 - 17, 2002

By:Neil Johnson, Detroit Stoker Company and SFT, Inc. Retired

ABSTRACTThe stoker fired community consists of comparably newer units (fifteen years old), units that were installed in the1940s and those in between. Those units were designed with a variety of factors depending on the manufacturerand the boiler design. Except for the units installed after the Clean Air Act, the older units are grand fathered tosome degree from emissions controls. With the exception of biomass fired stokers, to obtain a permit for a newcoal fired stoker today would be exceedingly difficult. It behooves the operators of the existing stokers to maintainand run their units as well as possible. This will extend as long as possible the life of the unit without a majorreplacement which might trigger New Source Review (NSR). This paper will provide design criteria as would beapplied to a new spreader stoker fired unit, some basic emission control techniques and suggestions formaintaining good operating practices.

SPREADER STOKER DESIGN

BackgroundFirst a primer on spreader stoker theory. An understanding of the combustion process for spreader stokers canassist in evaluating operating procedures and changes or additions to the installation which might improveperformance or lower certain emissions. A spreader stoker should release the combustion energy evenly overthe entire grate surface. Then the controlling guideline for design is heat release/ft2 of grate, which whenmultiplied by the grate area results in the maximum input from fuel fed for a given unit. Fuel should be spreadevenly over the grate surface. Some of the energy is released in suspension and some on the grates. Becausethere is a wide range of size and burning characteristics for the many fuels burned on a spreader stoker, theportion of the energy released in suspension varies.

Fuel Types

CoalA wide range of coal types having either high or low fusion temperatures can be burned. Coals having a fusiontemperature down to 2000o can be burned under the right conditions. Free Swelling or Hargrove indexes have

little affect on the burning characteristics of a spreader. TheASTM rankings for bituminous coal, sub-bituminous coal, orlignite fit the spreader combustion process well. In general, all ofthese coal types can be burned on a given unitat the same combustion heat release. There does have to be aconcern for the attributes of each coal type as it relates to boilerfurnace and gas pass design. There are plants that havesubstituted lower grades of coal for cost savings as well assubstituting low sulphur bituminous or sub-bituminous coal tomeet state or local emission requirements. A coal’s volatilematter does affect the combustion process. Volatile content of20% on a dry and ash free basis should be considered aminimum and at that low percent, the grate heat release shouldbe lower.

Coal sizing affects stoker operation. Coals too coarse will notburn at the high rate required for optimum spreader operationand coals too fine can cause operational as well as emissionproblems without proper design and operating procedures. Thetheoretical size is equal proportions of 3/4" x ½", ½" x 1/4", and1/4" x 0. The equal gradation is to allow for the even combustionover the grate surface. This size is not available from a practicalstandpoint and the spreader feeders have the capabilities to

adjust for coal sizing. The American Boiler Manufacturers Association (ABMA) has a curve titled “Distribution ofSizes of Coal - Recommended Limits of Coal sizing for Spreader Stokers” (ABMA Design Guidelines, 1st Ed.)(Figure 1). Coals are being burned successfully having sizing outside of the band on the ABMA curve. It is betterto error on the fine side than the coarse side. As the percent of fines smaller than 16 mesh (0.10") increases sodoes flyash carryover. However, modern precipitators or baghouses can readily handle the carryover fromspreader stokers just as they do for pulverized coal fired boilers and circulating fluid bed fired boilers.

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Refuse FuelsA spreader stoker is an excellent combustor of cellulosewaste such as:$ Wood(Shredded Trees to Sawdust)$ Garbage (Refuse Derived Fuel)$ Bagasse (Sugar Cane Residue)$ Industrial Residue (Paper, Plastics,Wood)$ Furfural Residue$ Peanut Shells$ Shredded TiresMost of these fuels can be burned without auxiliary fuelwith proper attention to fuel moisture, design heat release,combustion air system design, and preheated airtemperature. Co-generation and the emphasis onrenewable fuels has driven increased use of these fuels.The spreader stoker is ideally suited for the combustion ofthese fuels. Size consist of the fuel is important from thestandpoint of efficiency, availability, and low emissions.The curve shown in Figure 2 suggests an appropriate sizerange.

Fuel FeedersTo approach even energy release, it is necessary to havefuel feeder/distributors which will evenly feed the fuel overthe entire grate surface. These feeder/distributors can bemechanical, pneumatic or a combination of both. They

must be placed across the width of the front of the stoker in sufficient quantity to achieve even lateral distributionof the fuel and have the means to longitudinally adjust fuel distribution for various types of fuels and sizing. Theyshould be able to bias the feed rate one feeder to another, and to adjust for segregation of fuel sizing from onefeeder to another. How well the fuel feeder/distributors can adopt to the different characteristics of solid fuelsplays a major part in the ability to operate at lowest possible emissions and highest combustion efficiency.

There are many types of feeders that have been installed through the years. Feeders have been furnished whichare reciprocating, vibrating, drum , or chain conveyor. There are distributors which have overthrowing,overrunning rotors or underthrow rotors. For the distribution of refuse both mechanical and pneumatic types havebeen utilized. This paper will discuss three of these types of feeder/distributors which are the types that the writerwould recommend for a new unit. The chain type feeder both overthrow and underthrow types will be discussedfor coal burning. Air swept distributor spouts are used almost universally for refuse burning. Regardless of thetype of feeder, the results and goals must be the same, that is to distribute the fuel evenly over the entire gratesurface. This then relates to the feeders having the ability to adjust the longitudinal distribution for differences incoal sizing characteristics. Lateral distribution is a function of the feeding width in relation to the grate width, aswell as the ability of the rotor blades of mechanical feeders to splay the fuel in a lateral direction as well aslongitudinally. The absolute minimum feeder width to grate width is 40%. The goal should be to have a feederwidth of at least 50%.

Coal feeders should have a non-segregating distributor interfacing between the coal bunker and the stokerfeeder. A coal scale is recommended between the non-segregating spout and the coal bunker. A coal scaleprovides a method for tracking daily, weekly, or monthly coal usage. All modern coal scale electronics provide forreal time usage in terms of coal rate per hour which is useful for tracking efficiency.

Each coal feeder has the mechanism to regulate coal feed rate within the feeder. Older methods of control wereto connect the feeders mechanically to a pneumatic control system. Present day distributed computer controlsystems can send a 4-20 mA signal to the feeder. With computers there is better control of fuel feed to maximizeboiler efficiency and emission control.

It is not practical to meter refuse at the fuel distributer except for special refuse fuels that will pass through a

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mechanical feeder without problems. The nature of most refuse fuels requires large metering devices of specialdesign to prevent bridging of the fuel and blockages within the feeding device. These feeders do not adapt to theboiler front where the distributor is located. Therefore, the refuse metering device is located above the distributorwith a connecting chute in between. For maximum efficiency, best load following characteristics and lowestemissions, it is recommended that there be a separate metering device for each fuel distributor, and that themetering devices be kept full of fuel at all times. It is also important that the metering device be kept in a verticalplane from the front to prevent lateral maldistribution of the refuse in the furnace.

Chain Type Feeder - Model OTDetroit Stoker Company has been manufacturing chain type coal feeders for years to meter the coal to the rotor.Past models have used mechanical devices such as pawl and ratchet mechanisms to vary the fuel feed to thefurnace. This approach did not provide a truly continuously variable rate of feed. Detroit Stoker Companychanged the design to a fixed gearcase driven by a variable speed motor. The motor can be either DC utilizingan SCR control system or AC utilizing a variable frequencydevice. Either would receive a signal from a distributed controlsystem for precise fuel feed control. The speed controldevices should have the ability to bias one feeder in relation toanother to optimize fuel distribution. The variable speeddevice should have sufficient range to operate the boiler overthe load range and also to allow for variations in fuel quality shouldthere be a change in the fuel source. In addition, a manualgate is provided as shown in Figure 3 which allows anadjustment of the depth of coal on the chain. This permits optimizingthe feeder speed control range for variations in the coal’s heatingvalue from 7,500 BTU lignites up to 13,000 BTU easternbituminous coals. The continuous positive feed of the chain typefeeder with variable speed control devices allows very closefollowing of the signal from today’s sophisticated combustioncontrol systems.

The overthrow rotor which is used to distribute the coal in the furnace from the feeder is driven by a second motorthrough a variable speed drive. This variable speed drive is manually controlled and is used to adjust thelongitudinal distribution of coal over the grates. The functions of the metering drive and the rotor drive are notinterrelated and should be separate drives. The function of the metering drive is to deliver a regulated supply offuel to the furnace in accordance to boiler load while that of the rotor drive is to maintain good distribution of thefuel over the grates.

Chain Type Feeder - Model UTFigure 4 illustrates that the chain type metering device ofthis feeder is the same as that of the feeder describedabove with the exception of the coal depth adjustment.The drives and method of regulating fuel feed are thesame. The difference between the two is the method ofdistributing the coal in the furnace.

Coals available today that are delivered to the stokerhopper can have a wide range of sizing. Western sub-bituminous coals are very friable and tend to break downreadily in shipping and handling. Eastern coals can bepurchased with very controlled sizing as opposed to run ofmine coals which have to be crushed at the power plant.The spreader distributor must be capable of evenlydistributing the coals with this wide range of sizes. Rotor

speed is not enough when the percent of coal smaller than 1/4" is very high. Pneumatic assist has been found tobe helpful in distributing fine coal to the rear of the furnace without excessive rotor speeds. High rotor speedstend to throw the coarser particles of coal onto the rear wall of the furnace. To have the highest energy from thepneumatic assist, it is necessary to have the air impact the coal at the point of highest air velocity and at the sametime at the

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point the rotor imparts the maximum velocity to thecoal. As can be seen from Figure 4, the presence ofthe underthrow rotor permitted designing the air assistnozzles so that they impact on the coal at the bottomof the rotor which is the point of highest velocity. Thecombination of rotor speed and air velocity providesthe boiler operator with maximum flexibility inachieving longitudinal distribution. In addition,experimental work has found that adjusting thetrajectory of the coal as it leaves the feeder canfurther assist in throwing fine coal to the rear of thefurnace, thus the adjustable trajectory plate. Figure 5Illustrates the recommended limits of coal sizing forthe underthrow feeder. By comparing this with thecoal size distribution in Figure 1, it can be seen thatthe underthrow feeder is capable of properly

distributing coal having higher percentages of fines. Thegoal at all times is to allow the operator to optimize fueldistribution in the furnace to achieve even heat release.

Air Swept SpoutBoth mechanical rotor devices and air swept spout typedevices have been utilized to feed refuse fuels into acombustion chamber. Today the air swept spout is usedalmost universally for this purpose. Air swept spouts aresimple, having no moving parts in the fuel stream. They arelower maintenance with higher availability than the mechanicaldistributor. Good distribution of the fuel over the grates can beachieved with the correct location in the furnace, adequatefeeding width (50% of the grate width or more), and goodcontrol of the energy air flow as well as the fuel trajectory intothe furnace. Figure 6 illustrates an air swept spout having anadjustable trajectory plate and air flow control. Full size fuelflow tests have shown that the ability to adjust the trajectoryfrom horizontal to about 10o upwards assists in control of distribution to the rear of stokers having different gratelengths. The distributor should be located low in the furnace, about 3 or 4 feet above the grate, to best controldistribution to the front of the grate.

Air flow control is through the use of a rotating damper as shown in Figure 6. An adjustable fixed blade damperallows control of the minimum air flow when the rotating damper is in the closed position.

Grate Types and Heat ReleaseTo achieve uniform combustion it is necessary to distribute the air uniformly through the grates to release theenergy under optimum combustion conditions. Stratification should be reduced to a minimum so the oxygencontent of the flue gases and the combustion temperatures remain uniform and thus, the velocities rising in thefurnace are also as uniform as possible. A grate design that is highly resistant to air flow is desirable to achieveeven air distribution across the surface and even combustion conditions. Differential pressure across the gratesshould be on the order of 2" to 3" of water.

Grates existing today are probably of the continuous ash discharge type. Intermittent dumping grates areprobably no longer in existence except for small low ash refuse burning applications due to the difficulty inmeeting opacity requirements with intermittent ash dumping. The continuous ash discharge grate types are thetraveling grate and vibrating grate types discharging the ashes off of the front end of the grate. A continuous ash

Figure 6

Figure 5

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discharge grate will have virtually no ash at the rear and the ash bed depth will slowly increase as the gratemoves forward. A desirable depth of ash discharging off of the front of the grates is 4" to 6". The increase in ashdepth from the rear to the front changes the resistance of the fuel bed plus the ash to the air flow. Having a highlyair resistant grate surface will minimize this affect.

Traveling GrateTraveling grate spreader stokers have been in existence since 1938 and are the most popular way to burn coal onstokers for boilers above 50,000 lbs of steam/hr (Figure 7). Inaddition to coal, traveling grate spreader stokers are burning awide variety of waste fuels as discussed previously. Ash isdischarged at the front of the grate for two reasons. First of all,if the ash pit were in the rear, the fuel would be thrown directly intothe ash pit without burning or worse, causing an ash pit fire.Second, the spreader stoker is a size classifier of the fuel and thecoarser fuel is fed to the rear requiring more time to burn. Thespeed of the grate, at a given load, is a function of the poundsof fuel being burned per square feet of grate and the ash contentof the fuel. On a given unit and fuel, the grate speed is a functionof load. The relationship is not exactly linear since as the loadincreases, the rate of flycarbon rising also increases due to theincreased furnace velocities.

Since the function of a spreader stoker is to release equal energyfor each square foot of grate, BTU/SQ FT/HR is the primarydesign criteria. Even though some of the energy is release insuspension, to have a common denominator of comparison, the total BTU input from the fuel is divided by thetotal active air admitting grate area to arrive at a unit heat release. Most units designed to burn bituminous coals,sub-bituminous coals, and lignite can have heat releases up to 750 KBTU/SQ FT/HR. Units exist which run atrates considerably higher. Low volatile bituminous coals as commented on in “Fuel Types for SpreaderStokers” should be designed for a maximum heat release of 600 KBTU/SQ FT/HR to minimize combustible loss.The higher carbon content requires more time to burn out and the lower heat release allows for slower gratespeeds and more time in the furnace. The need for low emissions of NOx and CO also demands a considerationof heat release which will be discussed later. The grate heat release for refuse fuels such as wood or bagassecan be designed for 1,000 KBTU/SQ FT/HR or above depending on fuel moisture conditions and other factorsaffecting good combustion. Burning of refuse fuels will be covered more fully under “Vibrating Grates”.

The ABMA published the “Recommended Design Guideline for Stoker Firing of Bituminous Coals”. Within thisguideline for spreader stokers, allowable input in BTU/FT of WIDTH/HR is tied to the amount of flycarbonreinjection. It was felt that the amount of flycarbon reinjection for a given unit affected the carryover from thefurnace and a greater width would provide more time for burnout of the carbon. This is important to a unit

meeting particulate regulations with a mechanical dust collector.However, the goal of boiler manufacturers was to offer a unit having aminimum width to reduce costs. Because of this, a criteria wasdeveloped for an input per foot of grate width to maintain reasonablewidth to length ratios. This is necessary for good combustion andreduced emissions. A maximum heat release for coal of 14.5MKBTU/FT OF WIDTH is suggested. This input is all right for anyamount of reinjection on a unit equipped with a baghouse orprecipitator.Vibrating GrateAir cooled horizontal vibrating grates have been used to burn coal formany years (Figure 8).Their application has been for small andmedium sized spreader stoker fired boilers with a steaming rate of lessthan 150,000 LBS of ST/HR and for coal driers. Perhaps the term“vibrating” is not quite accurate since they are designed for lowfrequency vibration and the vibration cycle is intermittent. A timingdevice creates dwell time and vibrating time changing with boiler load.Units having more than one module in width are vibrated separately

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Figure 8

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rather than in unison. The vibrating action creates some agitation to thefuel bed and thus, the design heat release is a little more conservativethan for a traveling grate. A maximum of 650 KBTU/SQ FT/HR shouldbe used for bituminous coal.

Refuse such as wood or bagasse is burned successfully on an air cooledvibrating grate stoker. The low ash in wood or bagasse means that thegrate needs to vibrate infrequently. The fuel bed is quiescent withoutslag so the vibrating action readily moves the ash. However, thedevelopment of the water cooled vibrating grate has materially affectedthe wood burning power boiler in the pulp and paper industry as well asin co-generation facilities (Figure 9). Boilers equipped with a watercooled grate have higher availability and lower operating costs. Thegrate surface of the stoker rests on a grid of tubes connected to headersat both ends. This grid and its frame rests on flexing plates which arefastened to a supporting structure. The frequency of vibration and the

timing methods are the same as for the air cooled vibrating grate. Thewater which cools the grate can either be tied to the boiler’s natural circulation or be part of the feed water circuit.In any case, the water must be boiler quality.

The heat release burning refuse fuels such as wood waste, without regard to emissions, is a function of fuelmoisture primarily. Units with fuel having a moisture content from 40% to 55% can be designed at heat releasesup to 1100 KBTU/SQ FT/HR with proper attention to combustion air temperatures. Units with fuels having amoisture content less than 40% have been designed with burning rates of 1250 KBTU/SQ FT/HR. In practice,some units operate at well over design values.

Combustion Air Systems and Temperatures

Coal CombustionSince the goal of combustion on a spreader stoker is to achieve even burning over the entire active grate surface,it is necessary to obtain even air flow through the grates. Careful attention should be paid to the design of theforced draft system supplying the plenum chamber under the grates (Figure 7). Avoid changes in direction orother duct designs which might unbalance the flow of air to the grates. A highly resistant grate which puts most ofthe resistance to air flow across the grates rather than across the ash bed will materially aid the goal of even airdistribution.

When designing for bituminous or sub-bituminous coal, theair temperature can be either ambient or preheated to amaximum air temperature of 350o F. Boilers designed toproduce steam for electrical generation will normally requireboth an economizer and an air heater for maximumefficiency. Boilers designed for process and/or heatingsteam can be designed with just an economizer to achievethe desired flue gas end temperature. If the moisturecontent of the coal exceeds 25%, preheated air isrecommended. Therefore, lignite requires preheated airand, because of the lower combustion temperature with thehigher moisture, 400o F. is permissible.

The overfire air systems for spreader stokers hasundergone major changes over the years. The very old

units had systems designed for 7 ½% to 10% of total air. Later units had systems capable of 15% to 18% of totalair supply. The advent of the Clean Air Act and the subsequent regulation of NOx and CO required methods tocontrol these emissions. Staging has been found to reduce the emissions of NOx. Figure 10 illustrates aspreader stoker equipped with three levels of overfire air for the control of NOx. Tests have shown that staging

Figure 9

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can also assist in lower CO formation. The amount of air that has been used in these three level systems isapproximately 35% of total air. This air must be delivered with sufficient energy to produce turbulence and mixthe burning fuel with oxygen to complete combustion. The temperature of the overfire air can be either ambient orpreheated. The choice should be that of the boiler designer.

It is essential to design the overfire air system with sufficient static pressure to produce the required penetrationinto the combustion chamber for a given nozzle size. Nozzle shape is very important for the most efficientutilization of the fan energy. Tests have shown that some nozzle shapes are much more efficient than others.Later units have been equipped with nozzle sizes up to 3 inches in diameter. Further test work has shown that upto 30 inches static pressure is required to produce the needed energy for penetration and good turbulence.

Refuse combustionThe guidelines for undergrate air flow are no different for refuse firing than for coal firing. It is necessary to

achieve even air flow. Some designs have used zonedundergrate air flow because the longitudinal fueldistribution has been poor. Zoning allows manualcontrol to supply more air where there is a pile of fuelacross the grates. It is better to achieve good fueldistribution as well as air distribution to eliminate thenecessity for manual air adjustments. Virtually allrefuse has a moisture content exceeding 25% andrequires preheated air. Traveling grate spreaderstokers designed to burn refuse fuels having moisturecontents greater than 35% can use preheated airtemperatures up to 550o F. Care should be used inselecting air temperatures for lower moisture fuels toprevent slagging on the grates. A careful examinationof the fuel analysis and fusion temperature should bemade.

The water cooled vibrating grate spreader stoker hasessentially replaced the traveling grate for burning refuse fuels. Since the air flow through the grates has littleaffect on grate temperatures, a higher air temperature can be utilized. When the refuse fuel has a moisturecontent above 35%, the water cooled vibrating grate can utilize air temperatures up to 650o F. However, thesame cautions should be observed for lower moisture fuels.

High quantities of overfire air have been used for spreader stokers burning refuse fuels for many years. The highvolatile proportion in the refuse means a greater proportion of the energy is released in suspension above thegrates with resulting long flame travel. High quantities of overfire air are required to provide turbulence, mixingand oxygen for the complete burn out of the volatile. Increasing the quantity of overfire introduced to the furnaceabove the level of fuel feeding results in lower velocities at the fuel feed level and lower carryover of particulate.Figure 11 illustrates a unit having three levels of overfire air. This configuration has been used with overfiresystems capable of 50% of total air. Since air cooling is less important on a water cooled vibrating grate, higherproportions of overfire air can be used. It is essential to use good design practices in the selection of nozzle sizesand air pressure, as well as the location of the rows of air and the number of nozzles in each row.

Efficiency - Excess Air and Fly Carbon Reinjection

Excess AirThe two controlling factors of efficiency from the combustion system are excess air and carbon loss. To minimizeexcess air, it is necessary to approach the theoretical even release of energy over the furnace plan area or gratesas closely as possible. Operating with low excess air grows in importance by the requirements of low pollutantemissions. Greater attention to having a fuel feeder with the necessary adjustments to provide good distributionand operators that use these tools is essential. Forced draft duct construction and plenum design to provide thebest control of air flow through the grates should be carefully analyzed. Tests have shown that approaching aneven flow of gases rising in the furnace without excursions of velocity results in the best performance. The sealinterface design between the stoker and the boiler with the differential movement is most important. Any

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infiltration of “tramp” air between the stoker and the boiler reduces performance and increases excess air. Closeattention to these matters will allow operation of the unit at 25% excess air or less in the furnace.

Fly Carbon ReinjectionCarbon loss from a spreader stoker is the sum of the bottom ash pit loss plus the loss from discarded fly carbon.A well run spreader stoker should have very low bottom ash pit carbon loss. The fly carbon loss depends on the

amount of reinjection back into the furnace for re-burning.Figures 12, 13, and 14 illustrate the results of testing withand without reinjection from a mechanical dust collector.

For a given stoker/boiler unit burning a given fuel, apercent of the ash will end in the bottom ash pit and apercent will leave the furnace as flyash. For a given unit,the ratio will remain constant. If there is no reinjection, allof the carbon in the flyash caught in the boiler hoppers,mechanical collector hoppers and final flyash collectionsystem will become carbon loss. The combustiblecontent of the flyash caught in the mechanical collector

from a unit burning bituminous coal will be about 60%.By reinjecting the flyash caught in the boiler hoppers andmechanical collector, the part of the ash beingdischarged as flyash will be that in the final collectionsystem. The total amount of ash from the combustion ofthe fuel leaving as flyash remains unchanged. Withcollector reinjection the amount of combustible in thediscarded flyash will be lower and thus, the weight offlyash going to disposal will be lower. Burningbituminous coals will result in a combustible content ofthe flyash being discarded being about 25% or less withmechanical collector reinjection. Spreader stokers havebeen reinjecting for years and now circulating fluid bedfired boilers use the same technique to lower carbonloss.

The reason for the lower combustible in the flyash withcollector reinjection is the fact that smaller sized particles

of flyash have lower concentrations of combustible. Theparticle larger than 30 mesh may have a combustiblecontent of 90% while the particle less than 200 meshmay have a combustible content as low as 5%. Amechanical collector allows little of the particle size largerthan 200 mesh to go to the final cleanup device.Therefore the combustible in the flyash being disposed ofis low. The larger particles have been reinjected into thefurnace for reburning. As the combustible is burned outof the larger particles, they reduce in size until they passthrough the mechanical collector to the final collectiondevice. In this way, the total carbon loss from a spreaderstoker is kept quite low.

Emission ControlThe emissions of sulphur dioxide cannot be controlled in

the combustion process since at least 95% of the sulphur in the coal is converted to SOx. Newer existing plantsregulated under the Clean Air Act have had to install SO2 scrubbers. Older units have had to change to lowsulphur coal. The emissions of nitrogen oxide, carbon monoxide, and hydrocarbons are affected by thecombustion process. Some of the results from the combustion process are predictable.

Excess oxygen and heat release affect nitrogen oxide emissions from spreader stokers. Excess oxygen is the

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most predictable element of the combustion process affecting these emissions. Many tests have been run onsingle units at various excess oxygen levels starting in the 1970s with tests conducted by the ABMA under

contract with the Environmental Protection Agency (EPA) and theDepartment of Energy (DOE) (Bessette, 1981). All of these testshave resulted in graphs of nitrogen oxide emission related toexcess air having essentially the same slope. The same unitswere operated at different loads to simulate changes in grate heatrelease rates. This has shown that nitrogen oxide emissions doincrease with increased heat release rates. That has led to thedesign of spreader stoker fired units having a maximum grate heatrelease of 700 KBTU/SQ FT/HR to minimize nitrogen oxideemissions.

Pershing (1982), in laboratory tests run at the University of Utah,determined that fuel size affects the emissions of nitrogen oxidefrom spreader stokers. He determined that coal particle size less

than 1/10th of an inch produced nitrogen oxides at a higher rate.This is due to the more rapid combustion of the finer particlesproducing higher temperatures. Full size unit testing hasdemonstrated this to be true (Figure 15). A unit operating withcoal falling within the boundaries of the ABMA’s coal size curvefor spreader stokers emitted lower nitrogen oxides over a rangeof excess oxygen values than did the same unit operating with acoal with sizing falling on the fine side outside the ABMA curve.

Furnace temperature from the heat of combustion does affectthe emissions of nitrogen oxide from spreader stoker firing, as itdoes on other types of solid, gaseous, or liquid fuel firing.Combustion air temperature affects furnace temperature andthus, nitrogen oxide emissions. Units with preheated will emit

higher emissions than those utilizing ambient combustion air (Figure 16). If steam conditions permit, it would bewell to design a unit with just an economizer rather than acombination of economizer and air heater.

In the earlier part of the paper, there were comments on overfireair quantities of 30% to 35% of total combustion air for staging ofthe combustion process. Tests have shown that this quantity ofoverfire air, properly located in the furnace, can reduce nitrogenoxide emissions. With a high quantity of overfire air locatedabove the elevation of fuel entry to the furnace, there is very lowexcess oxygen at the grate line. In addition, staging thecombustion process probably lowers the furnace temperature atany given location. For maximum effectiveness, even heatrelease without spikes is most important.

Several years ago, while experimenting with the design of coalfeeders and field testing of a new coal feeder design for the purpose of being able to distribute a wider range ofcoal sizes properly, it was found that the method of fuel feed does affect nitrogen oxide emission. A unit operatedwith “standard” coal feeders adjacent to an identical unit operating with new type coal feeders had 15% to 20%higher nitrogen oxide emissions under all conditions for a 13 day test period (Figure17). Each unit was operatedunder the same conditions as closely as possible for the 13 day test period. Excess oxygen was varied as well asload. It can be seen that the unit with the new type coal feeder had, for the entire 13 day test period, lowernitrogen oxide emissions. The design of the new feeder kept the coal low in the furnace as it was distributed overthe grates. With high percentages of air being used for staging, the oxygen at the grates, and the velocity waslower so less nitrogen oxide was produced.

The emissions of carbon monoxide and Hydrocarbons from a spreader stoker are affected primarily by excessoxygen, heat release rate, and the proper application of overfire air turbulence. Excess oxygen, if it becomes too

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Figure 17

Figure 16

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high, will result in a slight increase in carbon monoxide emissions. At some minimum excess oxygen, which couldbe different from one unit to another, the carbon monoxide increases rapidly. During the 13 day feeder test, bothunits operated with very low carbon monoxide emissions. The level of excess oxygen at which the carbonmonoxide increased was less than 2%. Carbon monoxide emissions, in general, have not been a problem forspreader stokers.

The control of emissions from spreader stokers has been limited to the combustion techniques for minimizing theemissions of nitrogen oxide and carbon monoxide. There are additional in furnace techniques that have beendeveloped. These include:

Natural Gas Reburn TechnologyFlue Gas Recirculation to Reduce Excess OxygenCombination of Gas Reburn and Flue Gas Recirculation

Post combustion systems in use today are Selective Non-Catalytic Reduction (SNCR) and Selective CatalyticReduction (SCR). A combination of best combustion technology, in furnace systems, and post combustionsystems is possible.

OTHER STOKER TYPESOverfeed Mass Burn StokerThe combustion process of the overfeed mass burn stoker is one of progressive burning. The coal is fed out of ahopper by a traveling grate, chain grate, or vibrating grate and conveyed through the furnace from the front end ofthe unit to the rear where the ash is discharged. The depth of fuel being fed to the furnace is manually controlledby a gate at the furnace edge of the coal hopper. Combustion consists of ignition, rapid burning, burnout, andthen ash conveying to the ash pit. The speed of the grate controls the amount of fuel fed from the hopper inaccordance with the load requirements of the unit. Underneath the grate, the plenum chamber is divided into aseries of air zones since different air quantities are required for each phase of the combustion process. Theseare manually set from experience and the type of coal being burned. Since the process is progressive, the heatrelease within the furnace varies with the majority of the energy being released towards the front of the furnace.The progressive burning characteristics require a more conservative heat release from the grates. Maximum heatrelease is 450 KBTU/SQ FT/HR.

All mass burn units today have overfire air quantities of up to about 15% of total air. The overfire air is located inthe front wall of the boiler where most of the burning takes place. The amount of overfire air and its location is notsufficient for staged combustion and thus, any attempts to lower emissions in this manner have beendiscouraged. The only technique for controlling emissions is excess oxygen. The only published work foroperation and emissions from a mass burn stoker is that of the ABMA (Bessette, 1981). Because of the low heatrelease of mass burn stokers, the amount of flyash leaving the furnace is considerably lower than from a spreaderstoker and size of the particle is smaller. The low heat release also results in lower basic nitrogen oxideemissions.

Overfeed mass burn stokers are found on boilers having capacities of less than 150,000 LBS ST/HR. Very fewunits are new enough to have been regulated under the Clean Air Act. Most units are required to control sulphurdioxide emissions and this has been accomplished by burning low sulphur coal.

OPERATIONThere exists today a host of efforts to tighten emission regulations and to include elements previously notregulated. Older grand fathered units may be subject to regulations that they had been exempt from. Newerunits, constructed under New Source Performance Standards (NSPS), may have their permit requirementstightened and new elements added. CIBO has written many documents in response to these proposals pointingout the unreasonableness of the proposed rules and showing the vast difference between utility units andindustrial units when utility testing results were going to be used to apply regulations to industrial units. ABMAand other organizations also have responded to environmental proposals. Some of these proposals do not havegood information on true health affects nor do they consider economic factors.On June 28, 2002 there was a Technology Transfer Openhouse at the Medical College of Ohio. A demonstrationof a Rapid Absorption Process SO2 Reduction system and LoTOx Nox Removal system had been installed withfunds partially provided by the Ohio Coal Development Office/Ohio Department of Development as well asparticipation by the Medical College of Ohio, SFT, Inc., Beaumont Environmental Systems, and BOC Gases(Technical Transfer Paper). The college has three boilers rated at 70,000 LBS ST/HR (two coal fired and one gasfired) and one 40,000 LB ST/HR coal fired boiler. Testing of the two systems showed removal of both sulphur

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dioxide and nitrogen oxide in excess of 90%. It is claimed that other pollutants, including mercury can beremoved by this system. There are other innovative systems being developed and existing facilities shouldresearch what is available. The CIBO and ABMA offices can assist in locating literature on available systems.

Perhaps some readers of this paper generate electricity. CIBO has published an industry type list of non-utilityfacilities with cogeneration. The total number of facilities is 1013, the number of facilities that generate over 25MW is 491, and the number of facilities that send more than 30% of the electrical output to the grid is 268. Theseunits have a big stake in any proposed revision to the regulations.

CIBO is responding to an EPA effort to redefine Maximum Available Control Technology (MACT). CIBO haspointed out the wide diversity of type in the 42,000 boilers and the 15,000 process heaters, and, the little data thatEPA has to support the revisions as well as that some cannot be met. Another area that CIBO is very active in isthe EPA’s efforts to reform the New Source Review (NSR) program.

With all of these forces at work, it behooves the existing plants to operate their units as well as possible toestablish the best baseline data. Non Utility Generators (NUG) are always trying to be as efficient as possibleand operate with minimum costs since selling electricity is their income. Industrial facilities can minimize their fuelcosts, maintenance costs, downtime, and maximize their capacity by good operating habits. In addition, shouldan add on control for emissions be required, the cost of operation and the results from the device will improve withefficient boiler operation.

Existing boilers are controlled by a wide variety of systems. There may be some that are still controlled bypneumatic systems that measure steam flow, air flow and pressure and then modulate the control air pressure tothe controlled device. Newer units as well as older units that have converted are controlled by sophisticateddistributed computer control systems. The device that is controlled may be powered by a pneumatic or electriccontroller. It is essential that the system be checked, maintained and calibrated on a regular basis. Any lossmotion in the linkage to the controlled device should be eliminated. Measuring instruments for temperature,pressure and drafts should be maintained and calibrated on a defined schedule. Oxygen analyzers should becalibrated and the readings recorded as a check on any change in efficiency. Oxygen analyzers are usuallylocated at the boiler or economizer outlets. If a boiler does not have one, either one should be installed or aportable device should be used to record oxygen levels. No matter how well the remainder of the boiler ismaintained, well operating controls and instruments are essential for the continued efficient operation of theboiler.

The coal handling system should be designed to minimize segregation of coal size to the stoker hoppers.Running a sieve analysis on the coal from each spreader stoker feeder or across the width of a mass burn stokerwill indicate the degree of segregation to the stoker. If there is segregation, steps should be taken to minimizethis. Even sizing to each feeder of a spreader stoker or across the width of a mass burn stoker will help inachieving efficient burning of the coal.

A thorough inspection of the stoker should be made at every annual outage. Worn feeder parts that introduce lostmotion should be replaced. Worn parts prevent using the full capabilities of the feeder to distribute the coalevenly. During operation, a regular schedule should be maintained to observe the fire to assure that it is burningevenly over the entire grate surface. An inspection of the fire through the rear doors should be made to see thatthe fire is carried back to the rear wall but not piling at the wall. One should be able to see between the rear walland the flames. Make sure that the depth of ash is that desired and, if not, adjust the bias for the grate speed.Grates, sprockets, bearings, etc. require inspection at the annual outage for wear or warpage should the gratesurface have been overheated. Careful examination of all the seals between the stoker and boiler is a necessity.By keeping the seals in good condition, infiltration of air is kept to a minimum.

The boiler, heat traps, ductwork, particulate collectors, and fans should be checked for leakage of air or flue gas.Infiltration of tramp air can reduce efficiency and increase draft losses through the entire boiler system. This willthen use more induced draft fan horsepower. Infiltration of air can be checked by lighting a torch and see if theflames are drawn into the unit at any location. Another method is to build a water cooled probe for checkingoxygen levels in the furnace against oxygen levels at downstream locations. An increase in oxygen indicatesinfiltration. Also, a smoke bomb can be placed in the unit and then pressurize it to see where the smoke comesout into the building. This method is dirty and smelly. Repair any places that infiltration is found. It will paydividends.

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Draft loss is another indicator of deteriorating efficiency. Slag and soot buildup in the boiler passes, economizeror air heater will increase draft loss. Slag or soot buildup in the dust collector or worn tubes and vanes can alsoincrease draft loss. High draft loss uses more induced draft fan power and, if the increase in draft is high enough,the capacity of the unit will be decreased. One unit has been observed where increased draft loss reduced theunit’s capacity by 20%. Good instrumentation will include draft readings at strategic locations throughout the unitand by checking the recorded records, any deteriorating conditions can be monitored. Speaking of loss ofcapacity due to induced draft fan capacity, regular inspections of the fan blades and scroll for erosion isnecessary. This is especially true if the fan is located upstream of the final particulate cleanup device.

Although the design criteria put forth in this paper may not fit existing units, there are ways to improve the designof any given unit. See if there are ways to improve seals between the stoker and boiler. Look at ways to preventinfiltration of tramp air. Can soot blowing be improved to prevent slag buildup which creates draft loss? How canthe overfire air system be improved to lower nitrogen oxide emissions, decrease particulate carryover, andoperate at lower excess oxygen? Will a new coal feeder for spreader stokers improve the ability to burn a lowercost coal, lower nitrogen oxide emissions, and lower excess air? Any improvement to operation will producelower operating costs and provide an easier transition to possible additional emission control requirements.

REFERENCESAmerican Boiler Manufacturers Association (ABMA), “Recommended Design Guidelines for Stoker FiringBituminous Coals” First Edition.

Bessette, R. D., et al “Emissions and Efficiency Performance of Industrial Coal Stoker Fired Boilers” by ABMADOE/ET 10386-TI (Vol. !), August 1981

Pershing, D. W., et al “Formation and control of NOx Emission from Coal-Fired Spreader-Stoker Boilers” 19th

Symposium (International) on Combustion, Haifa, Israel, 1982.

Goss, W. L. “Technical Transfer Paper Multi-Pollutant Control System”, Medical College of Ohio, June 28, 2002