A fluid bed is a suspension of solid particles in an ascendant current of gas. The gas is introduced at pressure from the bottom of the reactor, whereas the parti­cles enter from above. When the solid remains in suspension, we talk about the fluidization condition that is reached for a determinate speed of the gas in which the fluid bed, made of a solid phase and a gaseous phase, acts as a liquid. The application of the fluid-bed technology to gasification ensures very good mixing between the biomass (reduced into small particles) and the aerating agent, improv­ing the reaction speed. We can also introduce inert fluidizing material (silica sand, alumina, refractory oxides) in the bed with the aim of equalizing the temperature so as to facilitate the heat transfer between the particles and the combustible.

The fluid-bed reactors, in contrast to fixed-bed applications, are characterized by a uniform temperature in the reactor (typically 800-850°C). Using the fluid — bed technology, we obtain a tar content in the producer gas that is intermediate between that obtained from the updraft gasificator and the downdraft gasificator.

The gas exiting from the reactor shows a higher content of solid particles (char, cinder, sand) [2].