Fluidized beds are widely used in the chemical industry for catalytic cracking and other processes. In fluidized bed reactors, a gas stream (inert gas for pyrolysis) is forced through a bed of powdered material from a distributor plate that supports the bed. At low gas velocities, the bed of particles is non-moving and this is referred to as a fixed bed. As the gas velocity is increased, the drag forces applied on the particles increase until minimum fluidization velocity is reached: the bed is ‘supported’ by the gas and behaves like a fluid. If the gas velocity is increased further, bubbles are formed at the distributor plate and rise through the bed of solids similar (but not identical) to air bubbles in water. Bubbles promote the circulation of solids to ensure a uniform temperature throughout the fluidized bed. A bubbling fluidized bed (BFB) reactor is designed in a way to avoid the entrainment of particles outside the reactor (also called elutriation). The bed zone is narrower to promote the circulation of particles and the formation of bubbles. The gas exits the bed to enter a freeboard zone and a higher diameter disengagement region where the gas velocity is significantly reduced. In the disengagement region, particles that would be entrained in the bed and freeboard zones fall back into the bed by gravity. There is an appreciable amount of established scientific literature on fluidized beds [48,49]. BFB reactors are used for fast pyrolysis by the company Dynamotive, which is operating a pilot-scale unit to convert biomass into bio-oil. The unit has been reported to process at up to 100 tons of biomass per day [50]. Figure 11.3a gives a global view of a possible biomass pyrolysis unit with a BFB.