Schematic diagram of fixed bed generator.

Equipment in which a gasification process takes place at high temperatures in layers of fuel, e.g. pulverised coal or biomass. The fixed bed generator has a relatively simple design, but the gas produced often contains undesirable impurities. The fuel is fed into the fixed bed generator from the top. Oxygen is fed from the bottom and the resulting gas, syngas, is vented from the top. The fuel gradually descends, with the ash being removed at the bottom. Different types of reactions then take place in the fuel layers: in the uppermost layer the fuel is dried. Below, pyrolysis, the splitting of complex molecules into simpler ones, takes place due to temperatures above 500 °C. Even lower down, gasification occurs as the fuel reacts with carbon dioxide. This produces methane-rich, often tarry syngas. At the lowest part, the residual fuel burns with oxygen, generating heat that heats the upper layers.