Reactors with circulating fluidized bed

The circulating fluidized bed (CFB) is viewed as a system in which fine par ticles are transported upwards by a gas stream and then recycled to the bot tom of the column. The flow structure is complex and varies widely dependin g mainly on the gas velocity, vessel geometry and particle size distributio n. Unlike the bubbled bed, the entry, exit and wall configurations have a s trong influence on the flow and mixing patterns of both phases in the colum n. Small particles tend to form relatively large, irregular aggregates or c lusters which disintegrate and form again at appreciable frequencies. Simpl e core-annulus models for circulating fluidized beds assume the upflow of g as and entrained solids in a dilute central core and the downflow of dense clusters in a relatively thin annular zone near the walls. Rapid heat and m ass transfer between gas and particulate solids, uniform temperature in the whole CFB, nearly a plug flow of gas and possible stepwise addition of one or more gaseous components at different levels are among the main features of the CFB reactors. Efficient combustion and pressure gasification of fos sil fuels and other carbon-containing residues, meeting strict environmenta l requirements, are the most rapidly expanding field of CFB applications.