A Circulating Fluidized Bed (CFB) works at higher superficial gas velocities than a BFB to increase the slip velocity and heat transfer to the particles. Due to the high gas velocities, the particles are entrained outside the bed region (called the riser) and cyclones are used downstream to separate the particles from the gas and return them to the bed. During biomass pyrolysis, char and inert (generally sand) particles are present. Depending on the design of the reactor, the char/sand particles mix can be transported to a second reactor where the char is burned. In this case, the hot sand is returned to the riser and the combustion flue gases can be directly fed to the riser as an inert fluidization and heating medium (see Fig. 11.3b).

Ensyn is a well-known company that operates a commercial scale CFB biomass pyrolysis system. Their largest pilot plant can process up to 100 bone-dry tons of biomass per day. Sand is co-fed with biomass in the riser, while the residual char and sand are recovered in a separate BFB combustor where the char is burned. The hot sand is then fed into the riser.