In a fixed bed gasifier, gasification reactions occur in a bed layer filled with fixed particles at a roughly constant height. Biofuel enters the bed layer from the upper interface and moves downward by its own gravity, filling the space left by fuel consumption. The gasification medium passes through the gaps between particles and reacts with the solid surface. Compared to the gas flow velocity, the downward movement rate of the fuel layer is very slow, so it is called a fixed bed or a moving bed.

Biofuels undergo several stages in a fixed bed gasifier, including drying, pyrolysis, oxidation, and reduction, in a certain order, and ultimately transform into combustible gases. This reaction method in the bed layer is widely used in industry, such as layer combustion boilers, gas generators, and chemical synthesis reactors. There are mainly two types of fixed bed gasifiers: upward suction and downward suction, depending on the location of the gasifying agent provided and the direction of flow through the fuel layer.