OCR Text |
Show In this example a solid fuel, such as coal mixed with an S02 sorbent (limestone or dolomite), is fed directly into the fluidized bed (typically by pneumatic conveyors or screw feeders). Combustion of the solid fuel with air occurs nearly instantaneously near the air distributor where more than half of the fuel is burned. Because of vigorous mixing, the bed temperature is nearly uniform with a very localized cooler region immediately adjacent to the fluidized-bed walls, the amount of cooling being dependent on use of a water wall or adiabatic bed design. This uniformity is one of the reasons the fluidized bed is considered one of the best chemical reactors when back-mixing and high rate of heat release are required. A fraction of the larger bed particles are constantly withdrawn from the fluidized bed via gravity flow through a drain pipe. Heat transfer surfaces in the form of water walls and tube bundles are immersed inside the fluidized bed. Overbed heat transfer surfaces can also be provided to recover heat energy from the over-bed space. Treated feed water, free from scale-forming and corrosive matter, is fed into the water wall and/or the over-bed heat transfer tube bundle. The heated water from the water wall and/or over-bed tube bundles is transported to a conventional type of steam drum (not shown in figure 3) where the saturated steam and water are separated. Saturated steam is subsequently fed into the in-bed tube bundle where it is superheated. Because considerable quantities of small particles are entrained by the fluidization gas, some unburned solid fines are typically carried up into the over-bed space. To insure complete combustion of these particles and for maximum thermal energy recovery, secondary over-bed air injection is utilized in this example. The entrained fines are collected in a cyclone train and either reinjected for residual carbon combustion or discarded (or a combination of both). The flue gas effluent from the cyclones goes to the secondary fine particulate recovery system, which could either be a bag filter house, an electrostatic precipitator, or some combination of both. In some cases a final high energy water scrubbing system is also used to control particulate emissions. Emissions Characteristics of Fluidized-Bed Combustion FBC technology is gaining popularity in the United States because of the FBC unit's compact size, its capability to burn a wide variety of fuels (many of which cannot be used by conventional technologies), and its unique potential to directly convert fossil fuel energy to thermal energy in an environmentally acceptable manner. The current sulfur dioxide emissions limit for large combustor units is 1.2 pounds per million Btu; the nitrogen oxides limit is 0.6 pound per million Btu; and the particulate emissions limit is 0.03 pounds per million Btu. It is not unreasonable to assume that these limits will be tightened. 11-7 |