7 - CFBC and BFBC of low-rank coals

Abstract

Fluidization is a method of mixing fuel and air in a specific proportion for obtaining combustion. A fluidized bed may be defined as a bed of solid particles behaving as a fluid. It operates on the principle that when an evenly distributed air is passed upward through a finely divided bed of solid particles at low velocity, the particles remain undisturbed; but if the velocity of air flow is steadily increased, a stage is reached when the individual particles are suspended in the air stream. If the air velocity is further increased, the bed becomes highly turbulent and rapid mixing of particles occurs which appear like formation of bubbles in a boiling liquid; the process of combustion as a result is known as fluidized bed combustion (FBC). The velocity of air causing fluidization depends on a number of parameter, for example, the size of fuel particles and density of air fuel mixture. Hence, these parameters are given due consideration, while manipulating with air flow velocity for the desired rate of combustion. FBC is a combustion technology to burn solid fuels. In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone, etc.) through which jets of air are blown to provide the oxygen required for combustion or gasification. The resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical reactions within the bed. FBC plants are capable of burning a variety of low-grade solid fuels, including most types of coal and woody biomass, at high efficiency and without the necessity for expensive fuel preparation (e.g., pulverizing). In addition, for any given thermal duty, FBCs are smaller than the equivalent conventional furnace, so may offer significant advantages over the latter in terms of cost and flexibility.