TABLE OF CONTENTS In a typical FBC system, the fuel (or fuels), an inert material such as sand or ash (referred to as bed material), and limestone (when added for sulfur control) are kept suspended through the action of combustion air distributed below the combustor floor. The function of the inert material is to disperse the incoming fuel particles throughout the bed, heat the fuel particles quickly to the ignition temperature, act as a thermal flywheel for the combustion process by storing a large amount of thermal energy, and provide sufficient residence time for combustion. The FBC concept is attractive because it increases turbulence, contains a substantial amount of thermal energy, and permits lower combustion temperatures. Turbulence is promoted by fluidization, making the entire mass of solids behave much as a liquid. Improved mixing permits the generation of heat at a substantially lower and more uniformly distributed temperature than occurs in conventional systems such as stoker-fired boilers or pulverized coal-fired boilers. The turbulent nature of the bed increases the rate of heat transfer on tubes immersed in the bed. The high thermal energy in the bed allows a variety of fuels to be utilized, including low-quality fuels with high mineral matter or moisture contents, as well as a multiple fuels in the same unit. The bed temperature in an FBC boiler is typically 1,450 1,650 F (780 900 C). This temperature is below that at which most inorganic components melt and form slag and agglomerates and is well...
