In fluidized bed gasifiers the reaction space contains a sand-like bed material that is fluidized (the sand is lifted by the gas stream and gets a liquid appearance) or entrained by the oxidant gas (air or oxygen), steam or mixtures thereof being added in the bottom. Autothermal or direct gasifiers use an oxidant, and allothermal or indirect gasifiers, see below, use steam without an oxidant being fed to the gasifier section.
The feedstock for fluidized bed gasifiers can be very broad, but too much of small fuel particles are undesirable. In the case of a stationary (or bubbling) fluidized bed the bed material is kept suspended by the gas in a defined bed volume through which gas in the form of interstitial gas and bubbles pass. Above the bed there is a freeboard section used for disengagement of particles mainly ejected by bubbles erupting on the bed surface.
In a circulating fluidized bed, the gas velocity is higher than for a stationary (bubbling) bed and the bed material is carried up in the gasifier shaft by the gas. Some of this material moves radially to the wall and transported back to the bottom by gravity as part a wall layer sliding down. The remainder of the solid-gas suspension is carried out by the gas to an external primary particulate separator, typically a cyclone, from which it is returned to the bottom of the gasifier by means of a recycle line with a moving bed of solids. The effect is huge net circulation rate between the bottom and the top of the reactor.