Operating regimes in circulating fluidized bed combustors: fast fluidization or bubbling-entrained bed?

Circulating fluidized bed combustors (or boilers) are one of the most widely used gas–solid convertors around the globe. As it plays an essential role in boiler design, different researchers have suggested different operating regimes for CFB combustors, based on their findings from experiments or practical experiences. Although there seems to be some consensus, a clear expression or conclusion that would be widely accepted in both the industry and academic community has yet to come. By analyzing and comparing reported data, in particular the particle size distributions (PSD) of different bed materials, from a variety of literature, it is reasoned that in commercial boilers the bottom part of the circulating fluidized bed combustor is operated in the bubbling regime even though under high gas velocities, and the upper part of the furnace can be viewed as an entrained bed. Such statement is well supported by analyzing hydrodynamic behaviors including solids volume fraction, solids velocity distribution and so forth, of real industrial boilers or pilot CFB combustor units.