650750 400425

Reciprocating gate, up to 75; vibrating gate, up to 100 Up to 400 10-300

Source: Adapted from Elliot [14].

are either of the single- or multiple-retort design, and water-cooled furnaces are preferred with underfeed stokers.

A relatively wide range of bituminous coals as well as anthracite can be burned on single- or multiple-retort stokers but typical specifications call for coal that is 3/4 x 1-1/4 in. with less than 50% of the fines passing through a 1/4-inch screen [14]. The free swelling index of the coal should be limited to 5 with single-retort stokers equipped with stationary tuyeres, and up to 7 on single-retort stokers with moving tuyeres as well as on multiple-retort stokers. It is normally recommended that the iron content in the ash be less than 20% as Fe2O3 with an ash fusion temperature above 2400°F and below 15% for coals having a lower ash fusion temperature.

Fuel

Fuel

Fuel Hopper

Distributor Drive

Fuel Hopper

Distributor Drive

Ash Pan

FIGURE 5-10. Working principles of mechanical stokers: (a) underfeed stoker; (b) overfeed stoker (traveling grate stoker); (c) spreader stoker. (From Berkowitz, N., An Introduction to Coal Technology, Academic Press, New York, 1979. With permission.)

Ash Pan

FIGURE 5-10. Working principles of mechanical stokers: (a) underfeed stoker; (b) overfeed stoker (traveling grate stoker); (c) spreader stoker. (From Berkowitz, N., An Introduction to Coal Technology, Academic Press, New York, 1979. With permission.)

Overfeed Stokers Overfeed, or mass-burning stokers, convey coal from the fuel hopper located at the front of the stoker. The depth of the fuel bed conveyed into the furnace is regulated by a vertical, adjustable feed gate across the width of the unit [7,14,21]. The fuel is conveyed into and through the furnace and passes over several combustion air zones. The ash is continuously discharged into a storage hopper at the rear end of the grate. Overfeed mass-burning stokers consist of three designs: chain grate, traveling grate, and water-cooled vibrating grate. Water-cooled furnaces are preferred with all moving-grate stokers to prevent slag formation on the furnace walls.

Chain grates consist of a wide chain with grate bars forming the links. The links are staggered and connected by rods extending across the stoker width. This chain assembly is continuously pulled or pushed through the furnace by an electric or hydraulic drive. The traveling grate has a chain drive (powered electrically or hydraulically) at the side of the grate with crossbars at intervals. Fingers, keys, or clips that form the grate surface are attached to these crossbars in an overlapping fashion to prevent ash from sifting through. The water-cooled, vibrating grate stoker consists of a grate surface mounted on, and in contact with, a grid of watertubes. These tubes are connected to the boiler circulatory system to ensure positive cooling. The structure is supported by a number of flexing plates, allowing the water-cooled grid and grate surface to move freely in a vibratory mode as the fuel bed moves through the furnace.

Chain grate stokers originally were developed for bituminous coal and traveling grate stokers for small sizes of anthracite [21]; however, almost any type of solid fossil fuel can be burned on the three stoker designs, including peat, lignite, subbituminous coal, non-caking bituminous coal, anthracite, and coke breeze [7]. Strongly caking bituminous coals may have a tendency to coke and prevent proper passage of combustion air through the fuel bed in the chain grate and traveling grate designs. In these designs, tempering (i.e., the addition of water or steam) of the fuel bed is done in the fuel hopper to make the bed more porous, although the coal's heating value is decreased. The vibrating action of the grate in the water-cooled vibrating grate design, however, keeps the fuel bed uniform and porous without the addition of water or steam. Coal size ranges for overfeed mass-burning stokers are listed in Table 5-4. These stokers are quite sensitive to segregation of coal sizes or distribution of the coal. If the fuel size is not uniform across the width of the stoker, the fuel bed will not burn uniformly, resulting in unburned carbon being discharged into the ash hopper.

Spreader Stokers Spreader stokers are the most popular of the three types. One reason for this is that they are capable of burning all ranks of coal as well as many waste fuels [21]. In addition, they can accommodate a wide range of boiler sizes. Spreader stokers take fuel from feeders located across the front of the furnace and distribute it uniformly over the grate surface. The objective is to release an equal amount of energy from each square foot of active grate surface [7]. As the coal is spread over the grate, fines in the incoming coal stream burn in suspension while the large pieces fall to the grate, forming a fuel bed; hence, to a limited extent, spreader firing has characteristics similar to pulverized coal combustion. Primary air for combustion is admitted evenly throughout the active grate area, with an overfire air system providing secondary air and turbulence above the grate.

The fuel bed is normally thin, and there is rarely more than a few minutes' worth of coal on the grate. This, coupled with 25 to 50% of the coal being burned in suspension, allows the spreader stoker to respond quickly to load swings. This makes the spreader stoker well suited for industrial applications where process loads fluctuate rapidly. The most common types of grates used today for spreader-stoker firing are the vibrating (or oscillating), traveling, and water-cooled vibrating grates. The water-cooled vibrating grate stoker is designed primarily for refuse burning (although conceivably could be used for coal) and is not discussed here. Stationary, dumping, and reciprocating grates see limited service. Not all of these grates are suited for coal firing.

The intermittent cleaning types of grates are stationary and dumping [14]. The stationary grate is seldom used because of hazards to the operator when removing ash through an open fire door. The dumping grate is seldom used for coal because the cleaning process results in high opacity in the stack. When it is used, it is for capacities of under 50,000 pounds of steam per hour and a heat release rate from the grate of no more than 450,000 Btu/hr-ft2.

The reciprocating grate discharges ash by a slow back-and-forth motion of moving grates alternating with stationary grates, which causes the fuel bed to move forward, dumping the ash into a pit at the front of the boiler. The grate can be used on boilers from 5000 to 75,000 pounds of steam per hour and can accommodate a wide range of bituminous coals or lignite without preparation other than sizing. Because of the stepped nature of the reciprocating grate, it is used only for fuels with sufficient ash quantity to provide an adequate ash depth for insulation on the top of the grates.

The vibrating or oscillating grate is suspended on flexing plates, and an eccentric drive or weights are used to impart a vibrating action to the grate surface, which conveys the ash to the front of the stoker and discharges them into an ash pit [14]. This grate type is well suited for coal.

The traveling grate spreader stoker is the most popular type. The endless grate moves at speeds between 4 and 20 feet per hour, depending on the steam demand, toward the front of the boiler, discharging ash continuously into an ash pit. The return grate then passes underneath in the air chamber. Traveling grate spreader stokers are designed to handle a wide range of coals.

Fluidized-Bed Combustion

Fluidized-bed combustion (FBC) is an emerging technology for the combustion of fossil and other fuels and is attractive because of several inherent advantages it has over conventional combustion systems. These advantages include fuel flexibility, low NOz emissions, and in situ control of SO2 emissions. The fluidized-bed concept was first used around 1940 in the chemical industry to promote catalytic reactions. In the 1950s, the pioneering work on coal-fired fluidized-bed combustion was begun in Great Britain, particularly by the National Coal Board and the Central Electricity Generating Board [2,22]. The U.S. Department of Interior's Office of Coal Research, one of the predecessors of the current Department of Energy (DOE), began studying the fluidized-bed combustion concept in the early 1960s (and still continues sponsoring research into advanced fluidized-bed combustion systems) because it recognized that the fluidized-bed boiler represented a potentially lower cost, more effective, and cleaner way to burn coal [23]. Around 1990, atmospheric fluidized-bed combustion crossed the commercial threshold and every major U.S. boiler manufacturer currently offers fluidized-bed boilers as a standard package. Fluidized-bed coal combustors have been called the "commercial success story of the last decade in the power generation business" and are perhaps the most significant advance in coal-fired boiler technology in a half century.

Fluidized-bed combustion technology is used in both the utility and non-utility sectors and comprises approximately 1% of fossil fuel-fired capacity. Approximately half of the facilities using FBC technologies are utilities or independent power producers. Facilities in the food products and pulp and paper industries, along with educational institutions, make up most of the non-utility FBC facilities [10]. FBC technology accounts for a small proportion of capacity but the technology has increased dramatically over the last 20 years [10]. In 1978, four plants in the United States had four FBC boilers; however, as of December 1996, 84 facilities had 123 FBC boilers, representing 4951 MW of equivalent electrical generating capacity. Because of the fuel flexibility, efficiency, and emissions characteristics of the FBC boilers, this technology is predicted to increase in the future, and additional units are being installed, both commercial units and advanced concepts through cofunded DOE programs (which are discussed in more detail in Chapter 7, Future Power Generation). Figure 5-11 shows the geographic distribution of the FBC facilities [10]. These facilities are distributed throughout the United States; however, Pennsylvania and California account for the largest numbers of plants. Pennsylvania accounts for more than 20% of capacity and California more than 10%. Pennsylvania is a leader in utilizing coal wastes (anthracite culm and bituminous coal gob) in FBC boilers.

In a typical FBC, solid, liquid, or gaseous fuel (or fuels), an inert material such as sand or ash (referred to as bed material), and limestone are kept suspended through the action of combustion air distributed below the

FIGURE 5-11. Number of fluidized-bed combustion facilities by state. (From EPA, Report to Congress: Wastes from Combustion of Fossil Fuels, Vol. 2, Methods, Findings, and Recommendations, U.S. Environmental Protection Agency, U.S. Government Printing Office, Washington, D.C., March 1999, chap. 3.)

combustor floor [14]. The primary functions of the inert material are to disperse the incoming fuel particles throughout the bed, heat the fuel particles quickly to the ignition temperature, act as a flywheel for the combustion process by storing a large amount of thermal energy, and provide sufficient residence time for complete combustion. The FBC concept is attractive because it increases turbulence and permits lower combustion temperatures. Turbulence is promoted by fluidization making the entire mass of solids behave much like a liquid. Improved mixing (and hence enhanced heat transfer to the bed material) permits the generation of heat at a substantially lower and more uniformly distributed temperature than occurs in conventional systems such as stoker-fired units or pulverized coal-fired boilers. The bed temperature in an FBC boiler is typically 1450 to 1650°F. This operating temperature range is well below that at which significant thermally-induced NOz production occurs. Staged combustion can be applied to minimize fuel-bound NOz formation as well. With regard to SO2 emissions, the operating temperature range is where the reactions of SO2 with a suitable sorbent, commonly limestone, are thermodynamically and kinetically balanced [24]. The percent capture for a given sorbent addition rate drops significantly outside the 1450 to 1650° F range. An additional reason why the bed temperature must be kept above 1400° F is that carbon utilization decreases with decreasing temperature, thereby reducing combustion efficiency.

Role of Sorbents in an FBC Process In an FBC system, the sorbent, usually limestone but sometimes dolomite (a double carbonate of calcium and magnesium), undergoes a thermal decomposition commonly known as calcination. When using limestone, the calcination reaction is:

Calcination of limestone is an endothermic reaction that occurs when limestone is heated above 1400°F. Calcination is thought to be necessary before the limestone can absorb and react with gaseous sulfur dioxide.

Capture of the gaseous sulfur dioxide is accomplished via the following equation to produce a solid product, calcium sulfate:

lime + sulfur dioxide + oxygen —> calcium sulfate

The limestone is continuously reacted; therefore, it is necessary to continuously feed limestone with the fuel. The sulfation reaction requires an excess amount of limestone to always be present. The amount of excess limestone that is required is dependent upon a number of factors, such as the amount of sulfur in the fuel, the temperature of the bed, and the physical and chemical characteristics of the limestone.

The primary role of the sorbent in an FBC process is to maintain air quality compliance; however, the sorbent is also important in bed inventory maintenance, which affects the heat-transfer characteristics and affects the quality and handling characteristics of the ash. Depending on the sulfur content of the fuel, the limestone can comprise up to 50% of the bed inventory, with the remaining portion being fuel ash or other inert material. This is especially true of FBC systems firing refuse from bituminous coal cleaning plants that contain high levels of sulfur. When the bed is comprised of a large quantity of calcium (oxide or carbonate), there is the potential for ash disposal concerns as the pH of the ash can become very high.

Comparison of Bubbling and Circulating Fluidized-Bed Combustion Boilers

The principle of FBC systems can be explained by examining Figure 5-12. The fundamental distinguishing feature of all FBC units is the velocity of the air through the unit as illustrated in Figure 5-12 [7,14]. Bubbling beds have lower fluidization velocities, and the concept is to prevent solids from elutriating (i.e., carrying over) from the bed into the convective passes. Circulating fluidized-bed units apply higher velocities to promote solids elutriation. Bubbling fluidized-bed units characteristically operate with a mean particle size between 1000 and 1200 ^m and fluidizing velocities between the minimum fluidizing velocity and the entraining velocity of the fluidized solid particles (i.e., 4 to 12 ft/sec). Under these conditions,

FIGURE 5-12. Fluidizing velocity of air for various bed systems. (Adapted from Power [7] and Elliot [14].)

a defined bed surface separates the high solids-loaded bed and the low solids-loaded freeboard regions. Most bubbling-bed units, however, utilize reinjection of the solids escaping the bed to obtain satisfactory performance. Some bubbling-bed units have the fuel and air distribution configured so that a high degree of internal circulation occurs within the bed [25]. A generalized schematic of a bubbling fluidized-bed boiler is shown in Figure 5-13 [26].

Circulating fluidized-bed (CFB) units operate with a mean particle size between 100 and 300 ^m and fluidizing velocities up to about 30 ft/sec. A generalized schematic of a CFB boiler is given in Figure 5-14 [26]. Because CFBs promote elutriation and the solids are entrained at a high rate by the gas, bed inventory can be maintained only by recirculation of solids separated by the off-gas by a high efficiency process cyclone. Notwithstanding the high gas velocity, the mean solids velocity in the combustor is lowered due to the aggregate behavior of the solids. Clusters of solids are continuously formed, flow downward against the gas stream, are dispersed, are reentrained, and form clusters again. The solids thus flow upward in the combustor at a much lower mean velocity than the gas. The slip velocity between gas and solids is very high with corresponding high heat and mass transfer. This is further illustrated in Figure 5-15, which shows that CFBs achieve higher rates of heat transfer from the solids to the boiler tubes than do bubbling fluidized-bed units [14].

FIGURE 5-13. Generalized schematic diagram of a bubbling fluidized-bed boiler. (From Gaglia, B. N. and A. Hall, Comparison of Bubbling and Circulating Fluidized-Bed Industrial Steam Generation, in Proc. of the International Conference on Fluidized-Bed Combustion, May 3-7, 1987.)
FIGURE 5-14. Generalized schematic diagram of a circulating fluidized-bed boiler. (From Gaglia, B. N. and A. Hall, Comparison of Bubbling and Circulating Fluidized-Bed Industrial Steam Generation, in Proc. of the International Conference on Fluidized-Bed Combustion, May 3-7, 1987.)
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