Fluidized Bed Combustion

Fluidized-bed combustion

(FBC) is a technology that has widespread appeal because of its low emission characteristics, particularly for systems that use sulfur-bearing fuels, such as coal, and its potential for improved heat transfer. Fluidized-bed configurations include bubbling-bed and circulating fluidized bed designs, with atmospheric- or elevated-pressure operation.

In an FBC unit, solid, liquid, or

Fig. 7-23 Basic Commercial/Industrial Packaged Watertube Boiler. Source: Cleaver Brooks

Fig. 7-24 Cutaway View of Integral Furnace Boiler-Membrane Wall Construction. Source: Babcock & Wilcox

Fig. 7-23 Basic Commercial/Industrial Packaged Watertube Boiler. Source: Cleaver Brooks cial/industrial packaged watertube boiler.

Figure 7-24 illustrates a shop-assembled two-drum bottom-supported boiler design. This is a forced draft unit capable of pressures of up to 1,800 psi (124 bar) and temperatures of 1,000°F (538°C). The design features the furnace on one side and boiler bank on the other, separated by a baffle wall. Burners are directed toward the rear

Fig. 7-24 Cutaway View of Integral Furnace Boiler-Membrane Wall Construction. Source: Babcock & Wilcox gaseous fuels, together with inert materials such as sand, silica, or alumina and/or sorbents, such as limestone, are kept suspended through the action of primary air distributed below the combustor floor. Fluidization promotes turbulence, which makes the mass of solids behave more like a liquid. Lower, more uniform distribution of heat results. Depending on the size of the combustion chamber, FBC can release the same amount of heat as a conventional boiler, but at a lower temperature. The lower peak temperatures result in lower rates of formation of NOx emissions.

The FBC boiler includes a specialized firing system that is surrounded by conventional waterwalls. In the bubbling-bed design, shown in Figure 7-26, steam-generating tubes are often set in the bed to achieve the desired heat balance and bed operating temperature. Steaming is controlled by adjusting the firing rate and height, temperature, and other primary bed parameters.

Circulating fluidized-bed designs do not have in-bed tubes, but may use an external heat exchanger and some type of hot-solids (cyclone) separator. The heat exchanger is a refractory-lined box with an immersed tube bundle designed to cool the solids returning from the separator. This compensates for variations in heat-absorption resulting from varying load conditions and fuel properties.

While the largest current and future use of FBC is for solid fuel-burning utility boilers, FBC is increasingly being used for industrial boilers. Growth in the industrial sector is due in part to the need to comply with Clean Air Act Amendment (CAAA) regulations.

Fig. 7-25 Low-Capacity Packaged Watertube Boiler. Source: Cleaver Brooks

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