There are two types of biomass gasifiers: fixed bed gasifiers and fluidized bed gasifiers

fixed bed

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.

Fixed bed gasifier is the earliest gasifier that appeared and was once mass-produced and used in automobiles, tractors, and small ships. Due to its simple structure and suitability for distributed biomass resources, gasifiers are still used in small and medium-sized biomass gas production.

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.

fluidized-bed gasifier

Fluidized bed technology was first applied to gas-solid two-phase reaction, and most of its basic theory and practice came from the achievements of the chemical industry. In 1926, German Wenkler applied fluidized bed technology to coal gasification for the first time. Fluidized bed gasification has good mass transfer, heat transfer, and reaction conditions. All fuel particles have the opportunity to react with gasification agents. The fuel has strong adaptability and gasification intensity, making it suitable for large-scale gasification of biomass fuels and gradually developing into one of the mainstream technologies for biomass gasification. Various forms of fluidized bed biomass gasifiers with varying capacities have been developed both domestically and internationally, and demonstration and application projects for biomass gasification power generation and gas steam combined cycle have been established. They are moving towards the high-end direction of producing bio natural gas and synthetic liquid fuels.

Fluidization is a fluidization phenomenon of solid particles under the action of a fluid medium, and it is also a relatively stable state between a fixed bed and a pneumatic conveying bed. Solid materials exhibit fluid like properties in fluidized beds and are easily transported between reactors, forming relatively uniform reaction conditions and good fuel adaptability. Therefore, they are widely used in combustion and gasification engineering.