Stoker Design 4p4d5k

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

ABSTRACT The stoker fired community consists of comparably newer units (fifteen years old), units that were installed in the 1940s and those in between. Those units were designed with a variety of factors depending on the manufacturer and the boiler design. Except for the units installed after the Clean Air Act, the older units are grand fathered to some degree from emissions controls. With the exception of biomass fired stokers, to obtain a permit for a new coal fired stoker today would be exceedingly difficult. It behooves the operators of the existing stokers to maintain and run their units as well as possible. This will extend as long as possible the life of the unit without a major replacement which might trigger New Source Review (NSR). This paper will provide design criteria as would be applied to a new spreader stoker fired unit, some basic emission control techniques and suggestions for maintaining good operating practices.

SPREADER STOKER DESIGN Background First a primer on spreader stoker theory. An understanding of the combustion process for spreader stokers can assist in evaluating operating procedures and changes or additions to the installation which might improve performance or lower certain emissions. A spreader stoker should release the combustion energy evenly over the entire grate surface. Then the controlling guideline for design is heat release/ft2 of grate, which when multiplied by the grate area results in the maximum input from fuel fed for a given unit. Fuel should be spread evenly over the grate surface. Some of the energy is released in suspension and some on the grates. Because there is a wide range of size and burning characteristics for the many fuels burned on a spreader stoker, the portion of the energy released in suspension varies.

Fuel Types Coal A wide range of coal types having either high or low fusion temperatures can be burned. Coals having a fusion temperature 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. The ASTM rankings for bituminous coal, sub-bituminous coal, or lignite fit the spreader combustion process well. In general, all of these coal types can be burned on a given unit at the same combustion heat release. There does have to be a concern for the attributes of each coal type as it relates to boiler furnace and gas design. There are plants that have substituted lower grades of coal for cost savings as well as substituting low sulphur bituminous or sub-bituminous coal to meet state or local emission requirements. A coal’s volatile matter does affect the combustion process. Volatile content of 20% on a dry and ash free basis should be considered a minimum and at that low percent, the grate heat release should be lower. Coal sizing affects stoker operation. Coals too coarse will not burn at the high rate required for optimum spreader operation and coals too fine can cause operational as well as emission problems without proper design and operating procedures. The theoretical size is equal proportions of 3/4" x ½", ½" x 1/4", and 1/4" x 0. The equal gradation is to allow for the even combustion over the grate surface. This size is not available from a practical standpoint and the spreader feeders have the capabilities to adjust for coal sizing. The American Boiler Manufacturers Association (ABMA) has a curve titled “Distribution of Sizes 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 better to error on the fine side than the coarse side. As the percent of fines smaller than 16 mesh (0.10") increases so does flyash carryover. However, modern precipitators or baghouses can readily handle the carryover from spreader stokers just as they do for pulverized coal fired boilers and circulating fluid bed fired boilers.

Refuse Fuels A spreader stoker is an excellent combustor of cellulose waste 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 Tires Most of these fuels can be burned without auxiliary fuel with proper attention to fuel moisture, design heat release, combustion air system design, and preheated air temperature. Co-generation and the emphasis on renewable fuels has driven increased use of these fuels. The spreader stoker is ideally suited for the combustion of these fuels. Size consist of the fuel is important from the standpoint of efficiency, availability, and low emissions. The curve shown in Figure 2 suggests an appropriate size range.

Fuel Feeders To approach even energy release, it is necessary to have fuel feeder/distributors which will evenly feed the fuel over the entire grate surface. These feeder/distributors can be mechanical, 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 distribution of the fuel and have the means to longitudinally adjust fuel distribution for various types of fuels and sizing. They should be able to bias the feed rate one feeder to another, and to adjust for segregation of fuel sizing from one feeder to another. How well the fuel feeder/distributors can adopt to the different characteristics of solid fuels plays 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 which are 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 have been utilized. This paper will discuss three of these types of feeder/distributors which are the types that the writer would recommend for a new unit. The chain type feeder both overthrow and underthrow types will be discussed for coal burning. Air swept distributor spouts are used almost universally for refuse burning. Regardless of the type of feeder, the results and goals must be the same, that is to distribute the fuel evenly over the entire grate surface. This then relates to the feeders having the ability to adjust the longitudinal distribution for differences in coal sizing characteristics. Lateral distribution is a function of the feeding width in relation to the grate width, as well as the ability of the rotor blades of mechanical feeders to splay the fuel in a lateral direction as well as longitudinally. The absolute minimum feeder width to grate width is 40%. The goal should be to have a feeder width of at least 50%. Coal feeders should have a non-segregating distributor interfacing between the coal bunker and the stoker feeder. A coal scale is recommended between the non-segregating spout and the coal bunker. A coal scale provides a method for tracking daily, weekly, or monthly coal usage. All modern coal scale electronics provide for real time usage in 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 were to connect the feeders mechanically to a pneumatic control system. Present day distributed computer control systems can send a 4-20 mA signal to the feeder. With computers there is better control of fuel feed to maximize boiler efficiency and emission control. It is not practical to meter refuse at the fuel distributer except for special refuse fuels that will through a

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

Chain Type Feeder - Model OT Detroit 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 the furnace. This approach did not provide a truly continuously variable rate of feed. Detroit Stoker Company changed the design to a fixed gearcase driven by a variable speed motor. The motor can be either DC utilizing an SCR control system or AC utilizing a variable frequency device. Either would receive a signal from a distributed control system for precise fuel feed control. The speed control devices should have the ability to bias one feeder in relation to another to optimize fuel distribution. The variable speed device should have sufficient range to operate the boiler over the load range and also to allow for variations in fuel quality should there be a change in the fuel source. In addition, a manual gate is provided as shown in Figure 3 which allows an adjustment of the depth of coal on the chain. This permits optimizing the feeder speed control range for variations in the coal’s heating value from 7,500 BTU lignites up to 13,000 BTU eastern bituminous coals. The continuous positive feed of the chain type feeder with variable speed control devices allows very close following of the signal from today’s sophisticated combustion control systems. The overthrow rotor which is used to distribute the coal in the furnace from the feeder is driven by a second motor through a variable speed drive. This variable speed drive is manually controlled and is used to adjust the longitudinal distribution of coal over the grates. The functions of the metering drive and the rotor drive are not interrelated and should be separate drives. The function of the metering drive is to deliver a regulated supply of fuel to the furnace in accordance to boiler load while that of the rotor drive is to maintain good distribution of the fuel over the grates.

Chain Type Feeder - Model UT Figure 4 illustrates that the chain type metering device of this feeder is the same as that of the feeder described above with the exception of the coal depth adjustment. The drives and method of regulating fuel feed are the same. The difference between the two is the method of distributing the coal in the furnace. Coals available today that are delivered to the stoker hopper can have a wide range of sizing. Western subbituminous coals are very friable and tend to break down readily in shipping and handling. Eastern coals can be purchased with very controlled sizing as opposed to run of mine coals which have to be crushed at the power plant. The spreader distributor must be capable of evenly 1 distributing 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 to be helpful in distributing fine coal to the rear of the furnace without excessive rotor speeds. High rotor speeds tend to throw the coarser particles of coal onto the rear wall of the furnace. To have the highest energy from the pneumatic assist, it is necessary to have the air impact the coal at the point of highest air velocity and at the same time at the

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point the rotor imparts the maximum velocity to the coal. As can be seen from Figure 4, the presence of the underthrow rotor permitted deg the air assist nozzles so that they impact on the coal at the bottom of the rotor which is the point of highest velocity. The combination of rotor speed and air velocity provides the boiler operator with maximum flexibility in achieving longitudinal distribution. In addition, experimental work has found that adjusting the trajectory of the coal as it leaves the feeder can further assist in throwing fine coal to the rear of the furnace, thus the adjustable trajectory plate. Figure 5 Illustrates the recommended limits of coal sizing for the underthrow feeder. By comparing this with the coal size distribution in Figure 1, it can be seen that the underthrow feeder is capable of properly

Figure 5

distributing coal having higher percentages of fines. The goal at all times is to allow the operator to optimize fuel distribution in the furnace to achieve even heat release.

Air Swept Spout Both mechanical rotor devices and air swept spout type devices have been utilized to feed refuse fuels into a combustion chamber. Today the air swept spout is used almost universally for this purpose. Air swept spouts are simple, having no moving parts in the fuel stream. They are lower maintenance with higher availability than the mechanical distributor. Good distribution of the fuel over the grates can be achieved with the correct location in the furnace, adequate feeding width (50% of the grate width or more), and good control of the energy air flow as well as the fuel trajectory into the furnace. Figure 6 illustrates an air swept spout having an adjustable trajectory plate and air flow control. Full size fuel flow tests have shown that the ability to adjust the trajectoryFigure 6 from horizontal to about 10o upwards assists in control of distribution to the rear of stokers having different grate lengths. The distributor should be located low in the furnace, about 3 or 4 feet above the grate, to best control distribution 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 damper allows control of the minimum air flow when the rotating damper is in the closed position.

Grate Types and Heat Release To achieve uniform combustion it is necessary to distribute the air uniformly through the grates to release the energy under optimum combustion conditions. Stratification should be reduced to a minimum so the oxygen content of the flue gases and the combustion temperatures remain uniform and thus, the velocities rising in the furnace are also as uniform as possible. A grate design that is highly resistant to air flow is desirable to achieve even air distribution across the surface and even combustion conditions. Differential pressure across the grates should be on the order of 2" to 3" of water. Grates existing today are probably of the continuous ash discharge type. Intermittent dumping grates are probably no longer in existence except for small low ash refuse burning applications due to the difficulty in meeting opacity requirements with intermittent ash dumping. The continuous ash discharge grate types are the traveling grate and vibrating grate types discharging the ashes off of the front end of the grate. A continuous ash

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

Traveling Grate Traveling grate spreader stokers have been in existence since 1938 and are the most popular way to burn coal on stokers for boilers above 50,000 lbs of steam/hr (Figure 7). In addition to coal, traveling grate spreader stokers are burning a wide variety of waste fuels as discussed previously. Ash is discharged 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 into the ash pit without burning or worse, causing an ash pit fire. Second, the spreader stoker is a size classifier of the fuel and the coarser fuel is fed to the rear requiring more time to burn. The speed of the grate, at a given load, is a function of the pounds of fuel being burned per square feet of grate and the ash content of the fuel. On a given unit and fuel, the grate speed is a function of load. The relationship is not exactly linear since as the load increases, the rate of flycarbon rising also increases due to the increased furnace velocities. Since the function of a spreader stoker is to release equal energy for each square foot of grate, BTU/SQ FT/HR is the primary design criteria. Even though2 some of the energy is release in suspension, to have a common denominator of comparison, the total BTU input from the fuel is divided by the total active air itting 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 at rates considerably higher. Low volatile bituminous coals as commented on in “Fuel Types for Spreader Stokers” 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 grate speeds and more time in the furnace. The need for low emissions of NOx and CO also demands a consideration of heat release which will be discussed later. The grate heat release for refuse fuels such as wood or bagasse can be designed for 1,000 KBTU/SQ FT/HR or above depending on fuel moisture conditions and other factors affecting 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 this guideline for spreader stokers, allowable input in BTU/FT of WIDTH/HR is tied to the amount of flycarbon reinjection. It was felt that the amount of flycarbon reinjection for a given unit affected the carryover from the furnace 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. Figure 8 However, the goal of boiler manufacturers was to offer a unit having a minimum width to reduce costs. Because of this, a criteria was developed for an input per foot of grate width to maintain reasonable width to length ratios. This is necessary for good combustion and reduced emissions. A maximum heat release for coal of 14.5 MKBTU/FT OF WIDTH is suggested. This input is all right for any amount of reinjection on a unit equipped with a baghouse or precipitator.

Vibrating Grate Air cooled horizontal vibrating grates have been used to burn coal for many years (Figure 8).Their application has been for small and medium sized spreader stoker fired boilers with a steaming rate of less than 150,000 LBS of ST/HR and for coal driers. Perhaps the term “vibrating” is not quite accurate since they are designed for low frequency vibration and the vibration cycle is intermittent. A timing device 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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rather than in unison. The vibrating action creates some agitation to the fuel bed and thus, the design heat release is a little more conservative than for a traveling grate. A maximum of 650 KBTU/SQ FT/HR should be used for bituminous coal. Refuse such as wood or bagasse is burned successfully on an air cooled vibrating grate stoker. The low ash in wood or bagasse means that the grate needs to vibrate infrequently. The fuel bed is quiescent without slag so the vibrating action readily moves the ash. However, the development of the water cooled vibrating grate has materially affected the wood burning power boiler in the pulp and paper industry as well as in co-generation facilities (Figure 9). Boilers equipped with a water cooled grate have higher availability and lower operating costs. The grate surface of the stoker rests on a grid of tubes connected to headers at both ends. This grid and its frame rests on flexing plates which are fastened to a ing structure. The frequency of vibration and the Figure 9 timing methods are the same as for the air cooled vibrating grate. The water 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 fuel moisture primarily. Units with fuel having a moisture content from 40% to 55% can be designed at heat releases up to 1100 KBTU/SQ FT/HR with proper attention to combustion air temperatures. Units with fuels having a moisture 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 Combustion Since 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 the forced draft system supplying the plenum chamber under the grates (Figure 7). Avoid changes in direction or other duct designs which might unbalance the flow of air to the grates. A highly resistant grate which puts most of the resistance to air flow across the grates rather than across the ash bed will materially aid the goal of even air distribution. When deg for bituminous or sub-bituminous coal, the air temperature can be either ambient or preheated to a maximum air temperature of 350o F. Boilers designed to produce steam for electrical generation will normally require both an economizer and an air heater for maximum efficiency. Boilers designed for process and/or heating steam can be designed with just an economizer to achieve the desired flue gas end temperature. If the moisture content of the coal exceeds 25%, preheated air is recommended. Therefore, lignite requires preheated air and, because of the lower combustion temperature with the higher moisture, 400o F. is permissible. The overfire air systems for spreader stokers has undergone 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 total air supply. The advent of the Clean Air Act and the subsequent regulation of NOx and CO required methods to control these emissions. Staging has been found to reduce the emissions of NOx. Figure 10 illustrates a spreader stoker equipped with three levels of overfire air for the control of NOx. Tests have shown that staging 3

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can also assist in lower CO formation. The amount of air that has been used in these three level systems is approximately 35% of total air. This air must be delivered with sufficient energy to produce turbulence and mix the burning fuel with oxygen to complete combustion. The temperature of the overfire air can be either ambient or preheated. 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 penetration into the combustion chamber for a given nozzle size. Nozzle shape is very important for the most efficient utilization 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 up to 30 inches static pressure is required to produce the needed energy for penetration and good turbulence.

Refuse combustion The 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 zoned undergrate air flow because the longitudinal fuel distribution has been poor. Zoning allows manual control to supply more air where there is a pile of fuel across the grates. It is better to achieve good fuel distribution as well as air distribution to eliminate the necessity for manual air adjustments. Virtually all refuse has a moisture content exceeding 25% and requires preheated air. Traveling grate spreader stokers designed to burn refuse fuels having moisture contents greater than 35% can use preheated air temperatures up to 550o F. Care should be used in selecting air temperatures for lower moisture fuels to prevent slagging on the grates. A careful examination of the fuel analysis and fusion temperature should be made. 4

The water cooled vibrating grate spreader stoker has essentially replaced the traveling grate for burning refuse fuels. Since the air flow through the grates has little affect on grate temperatures, a higher air temperature can be utilized. When the refuse fuel has a moisture content above 35%, the water cooled vibrating grate can utilize air temperatures up to 650o F. However, the same 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 high volatile proportion in the refuse means a greater proportion of the energy is released in suspension above the grates with resulting long flame travel. High quantities of overfire air are required to provide turbulence, mixing and oxygen for the complete burn out of the volatile. Increasing the quantity of overfire introduced to the furnace above 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 overfire systems capable of 50% of total air. Since air cooling is less important on a water cooled vibrating grate, higher proportions of overfire air can be used. It is essential to use good design practices in the selection of nozzle sizes and 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 Air The two controlling factors of efficiency from the combustion system are excess air and carbon loss. To minimize excess air, it is necessary to approach the theoretical even release of energy over the furnace plan area or grates as closely as possible. Operating with low excess air grows in importance by the requirements of low pollutant emissions. Greater attention to having a fuel feeder with the necessary adjustments to provide good distribution and operators that use these tools is essential. Forced draft duct construction and plenum design to provide the best control of air flow through the grates should be carefully analyzed. Tests have shown that approaching an even flow of gases rising in the furnace without excursions of velocity results in the best performance. The seal interface 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. Close attention to these matters will allow operation of the unit at 25% excess air or less in the furnace.

Fly Carbon Reinjection Carbon 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 with and without reinjection from a mechanical dust collector. For a given stoker/boiler unit burning a given fuel, a percent of the ash will end in the bottom ash pit and a percent will leave the furnace as flyash. For a given unit, the ratio will remain constant. If there is no reinjection, all of the carbon in the flyash caught in the boiler hoppers, mechanical collector hoppers and final flyash collection system will become carbon loss. The combustible content of the flyash caught in the mechanical collector

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from a unit burning bituminous coal will be about 60%. By reinjecting the flyash caught in the boiler hoppers and mechanical collector, the part of the ash being discharged as flyash will be that in the final collection system. The total amount of ash from the combustion of the fuel leaving as flyash remains unchanged. With collector reinjection the amount of combustible in the discarded flyash will be lower and thus, the weight of flyash going to disposal will be lower. Burning bituminous coals will result in a combustible content of the flyash being discarded being about 25% or less with mechanical collector reinjection. Spreader stokers have been reinjecting for years and now circulating fluid bed fired boilers use the same technique to lower carbon loss. The reason for the lower combustible in the flyash with collector reinjection is the fact that smaller sized particles 6 of flyash have lower concentrations of combustible. The particle larger than 30 mesh may have a combustible content of 90% while the particle less than 200 mesh may have a combustible content as low as 5%. A mechanical collector allows little of the particle size larger than 200 mesh to go to the final cleanup device. Therefore the combustible in the flyash being disposed of is low. The larger particles have been reinjected into the furnace for reburning. As the combustible is burned out of the larger particles, they reduce in size until they through the mechanical collector to the final collection device. In this way, the total carbon loss from a spreader stoker is kept quite low.

Emission Control The 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 plants regulated under the Clean Air Act have had to install SO2 scrubbers. Older units have had to change to low sulphur coal. The emissions of nitrogen oxide, carbon monoxide, and hydrocarbons are affected by the combustion process. Some of the results from the combustion process are predictable. 7

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 on single 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 the Department of Energy (DOE) (Bessette, 1981). All of these tests have resulted in graphs of nitrogen oxide emission related to excess air having essentially the same slope. The same units were operated at different loads to simulate changes in grate heat release rates. This has shown that nitrogen oxide emissions do increase with increased heat release rates. That has led to the design of spreader stoker fired units having a maximum grate heat release of 700 KBTU/SQ FT/HR to minimize nitrogen oxide emissions.

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Pershing (1982), in laboratory tests run at the University of Utah, determined that fuel size affects the emissions of nitrogen oxide from 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 particles producing higher temperatures. Full size unit testing has demonstrated this to be true (Figure 15). A unit operating with coal falling within the boundaries of the ABMA’s coal size curve for spreader stokers emitted lower nitrogen oxides over a range of excess oxygen values than did the same unit operating with a coal with sizing falling on the fine side outside the ABMA curve.

Furnace temperature from the heat of combustion does affect the emissions of nitrogen oxide from spreader stoker firing, as it does on other types of solid, gaseous, or liquid fuel firing. Combustion air temperature affects furnace temperature and Figure 16 thus, nitrogen oxide emissions. Units with preheated will emit higher emissions than those utilizing ambient combustion air (Figure 16). If steam conditions permit, it would be well to design a unit with just an economizer rather than a combination of economizer and air heater. In the earlier part of the paper, there were comments on overfire air quantities of 30% to 35% of total combustion air for staging of the combustion process. Tests have shown that this quantity of overfire air, properly located in the furnace, can reduce nitrogen oxide emissions. With a high quantity of overfire air located above the elevation of fuel entry to the furnace, there is very low excess oxygen at the grate line. In addition, staging the combustion process probably lowers the furnace temperature at any given location. For maximum effectiveness, even heat release without spikes is most important. Figure 17

Several years ago, while experimenting with the design of coal feeders and field testing of a new coal feeder design for the purpose of being able to distribute a wider range of coal sizes properly, it was found that the method of fuel feed does affect nitrogen oxide emission. A unit operated with “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 operated under the same conditions as closely as possible for the 13 day test period. Excess oxygen was varied as well as load. It can be seen that the unit with the new type coal feeder had, for the entire 13 day test period, lower nitrogen oxide emissions. The design of the new feeder kept the coal low in the furnace as it was distributed over the grates. With high percentages of air being used for staging, the oxygen at the grates, and the velocity was lower so less nitrogen oxide was produced. The emissions of carbon monoxide and Hydrocarbons from a spreader stoker are affected primarily by excess oxygen, heat release rate, and the proper application of overfire air turbulence. Excess oxygen, if it becomes too

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high, will result in a slight increase in carbon monoxide emissions. At some minimum excess oxygen, which could be different from one unit to another, the carbon monoxide increases rapidly. During the 13 day feeder test, both units operated with very low carbon monoxide emissions. The level of excess oxygen at which the carbon monoxide increased was less than 2%. Carbon monoxide emissions, in general, have not been a problem for spreader stokers. The control of emissions from spreader stokers has been limited to the combustion techniques for minimizing the emissions of nitrogen oxide and carbon monoxide. There are additional in furnace techniques that have been developed. These include: Natural Gas Reburn Technology Flue Gas Recirculation to Reduce Excess Oxygen Combination of Gas Reburn and Flue Gas Recirculation Post combustion systems in use today are Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR). A combination of best combustion technology, in furnace systems, and post combustion systems is possible.

OTHER STOKER TYPES Overfeed Mass Burn Stoker The combustion process of the overfeed mass burn stoker is one of progressive burning. The coal is fed out of a hopper by a traveling grate, chain grate, or vibrating grate and conveyed through the furnace from the front end of the unit to the rear where the ash is discharged. The depth of fuel being fed to the furnace is manually controlled by a gate at the furnace edge of the coal hopper. Combustion consists of ignition, rapid burning, burnout, and then ash conveying to the ash pit. The speed of the grate controls the amount of fuel fed from the hopper in accordance with the load requirements of the unit. Underneath the grate, the plenum chamber is divided into a series of air zones since different air quantities are required for each phase of the combustion process. These are manually set from experience and the type of coal being burned. Since the process is progressive, the heat release 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 heat release 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 in the front wall of the boiler where most of the burning takes place. The amount of overfire air and its location is not sufficient for staged combustion and thus, any attempts to lower emissions in this manner have been discouraged. The only technique for controlling emissions is excess oxygen. The only published work for operation and emissions from a mass burn stoker is that of the ABMA (Bessette, 1981). Because of the low heat release of mass burn stokers, the amount of flyash leaving the furnace is considerably lower than from a spreader stoker and size of the particle is smaller. The low heat release also results in lower basic nitrogen oxide emissions. Overfeed mass burn stokers are found on boilers having capacities of less than 150,000 LBS ST/HR. Very few units are new enough to have been regulated under the Clean Air Act. Most units are required to control sulphur dioxide emissions and this has been accomplished by burning low sulphur coal.

OPERATION There exists today a host of efforts to tighten emission regulations and to include elements previously not regulated. Older grand fathered units may be subject to regulations that they had been exempt from. Newer units, constructed under New Source Performance Standards (NSPS), may have their permit requirements tightened and new elements added. CIBO has written many documents in response to these proposals pointing out the unreasonableness of the proposed rules and showing the vast difference between utility units and industrial units when utility testing results were going to be used to apply regulations to industrial units. ABMA and other organizations also have responded to environmental proposals. Some of these proposals do not have good 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 demonstration of a Rapid Absorption Process SO2 Reduction system and LoTOx Nox Removal system had been installed with funds partially provided by the Ohio Coal Development Office/Ohio Department of Development as well as participation 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 gas fired) 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 be removed by this system. There are other innovative systems being developed and existing facilities should research 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-utility facilities with cogeneration. The total number of facilities is 1013, the number of facilities that generate over 25 MW is 491, and the number of facilities that send more than 30% of the electrical output to the grid is 268. These units 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 has pointed out the wide diversity of type in the 42,000 boilers and the 15,000 process heaters, and, the little data that EPA has to the revisions as well as that some cannot be met. Another area that CIBO is very active in is the 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 to establish the best baseline data. Non Utility Generators (NUG) are always trying to be as efficient as possible and operate with minimum costs since selling electricity is their income. Industrial facilities can minimize their fuel costs, maintenance costs, downtime, and maximize their capacity by good operating habits. In addition, should an add on control for emissions be required, the cost of operation and the results from the device will improve with efficient boiler operation. Existing boilers are controlled by a wide variety of systems. There may be some that are still controlled by pneumatic systems that measure steam flow, air flow and pressure and then modulate the control air pressure to the controlled device. Newer units as well as older units that have converted are controlled by sophisticated distributed computer control systems. The device that is controlled may be powered by a pneumatic or electric controller. It is essential that the system be checked, maintained and calibrated on a regular basis. Any loss motion 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 be calibrated and the readings recorded as a check on any change in efficiency. Oxygen analyzers are usually located at the boiler or economizer outlets. If a boiler does not have one, either one should be installed or a portable device should be used to record oxygen levels. No matter how well the remainder of the boiler is maintained, well operating controls and instruments are essential for the continued efficient operation of the boiler. 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 stoker will indicate the degree of segregation to the stoker. If there is segregation, steps should be taken to minimize this. Even sizing to each feeder of a spreader stoker or across the width of a mass burn stoker will help in achieving efficient burning of the coal. A thorough inspection of the stoker should be made at every annual outage. Worn feeder parts that introduce lost motion should be replaced. Worn parts prevent using the full capabilities of the feeder to distribute the coal evenly. During operation, a regular schedule should be maintained to observe the fire to assure that it is burning evenly over the entire grate surface. An inspection of the fire through the rear doors should be made to see that the fire is carried back to the rear wall but not piling at the wall. One should be able to see between the rear wall and 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 grate surface 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 will then use more induced draft fan horsepower. Infiltration of air can be checked by lighting a torch and see if the flames are drawn into the unit at any location. Another method is to build a water cooled probe for checking oxygen levels in the furnace against oxygen levels at downstream locations. An increase in oxygen indicates infiltration. Also, a smoke bomb can be placed in the unit and then pressurize it to see where the smoke comes out into the building. This method is dirty and smelly. Repair any places that infiltration is found. It will pay dividends.

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Draft loss is another indicator of deteriorating efficiency. Slag and soot buildup in the boiler es, economizer or air heater will increase draft loss. Slag or soot buildup in the dust collector or worn tubes and vanes can also increase 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 the unit’s capacity by 20%. Good instrumentation will include draft readings at strategic locations throughout the unit and by checking the recorded records, any deteriorating conditions can be monitored. Speaking of loss of capacity due to induced draft fan capacity, regular inspections of the fan blades and scroll for erosion is necessary. 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 design of any given unit. See if there are ways to improve seals between the stoker and boiler. Look at ways to prevent infiltration of tramp air. Can soot blowing be improved to prevent slag buildup which creates draft loss? How can the overfire air system be improved to lower nitrogen oxide emissions, decrease particulate carryover, and operate at lower excess oxygen? Will a new coal feeder for spreader stokers improve the ability to burn a lower cost coal, lower nitrogen oxide emissions, and lower excess air? Any improvement to operation will produce lower operating costs and provide an easier transition to possible additional emission control requirements.

REFERENCES American Boiler Manufacturers Association (ABMA), “Recommended Design Guidelines for Stoker Firing Bituminous Coals” First Edition. Bessette, R. D., et al “Emissions and Efficiency Performance of Industrial Coal Stoker Fired Boilers” by ABMA DOE/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

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