The basis for the fluidized bed reactor configurations is the principle of fluidization. By forcing a gas stream (fluidization medium) through a reactor, the fuel together with the inert bed material will behave like a fluid, if the flow velocity is high enough. Air, steam or steam/oxygen mixtures are examples of commonly used fluidization media. Silica sand is the most extensively used bed material, but other bulk solids, especially those exhibiting a catalytic activity, such as olivine sand and dolomite, are also employed.

Fluidized beds provide many features not available in the fixed-bed types, including high rates of heat and mass transfer and good mixing of the solid-phase, which means that reaction rates are high and the temperature is more or less constant in the bed.

Depending on the velocity of the fluidization medium, the fluidized bed gasifiers may be divided into two categories, bubbling fluidized bed (BFB) gasifiers and circulating fluidized bed (CFB) gasifiers.