TABLE OF CONTENTS In fluidized-beds, good mixing is usually achieved, which gives good heat distribution and a uniform temperature distribution. This, in turn, results in effective gas-solids contact giving a high rate of heat transfer from burning fuel to the waterwalls or immersed tubes. In conventional furnaces (stokerfired or pulverized coal-fired), the solids loading in the gas stream are low (i.e., approximately 10 lb per 1,000 lb of gas), and heat transfer from the gas to the waterwalls is primarily by radiation with a lesser contribution from convective heat transfer [14]. In contrast, in a CFB boiler, the gas leaving the furnace contains a high solids concentration, which can be greater than 5000 lb per 1000 lb of gas, and hence, convective heat transfer dominates over radiative heat transfer. For equal temperatures, the heat transfer coefficients in an FBC boiler are considerably higher than those in a conventional furnace [14]. However, because the temperatures are lower in an FBC boiler, the overall heat fluxes between the two systems are similar.What differs between the two systems are the types of heat transfer surfaces employed between the combustion systems. In a conventional system, heat is transferred to the waterwalls (i.e., containment structure) and tubes in the convective pass. In an FBC boiler, three zones of heat transfer must be considered: in-bed, splash zone (interface between the bed and freeboard), and above-bed. In addition, heat transfer areas outside the furnace must also be considered.
In a BFB boiler, heat transfer surfaces...
