Fluidized bed gasification of biomass – In bed investigation of gas and tar formation

Abstract

Allothermal steam gasification in fluidized bed reactors is a promising way to convert biomass into a high quality product gas. Due to its low nitrogen content the gas can be used in a variety of processes. Besides the direct production of heat and power in, e.g. internal combustion engines (ICE), the product gas can be converted to clean synthesis gas and second generation biofuels. A major problem in all downstream applications is the high tar content in the product gas. The tar, a mixture of mostly aromatic hydrocarbons, has to be removed prior to downstream processes to avoid blocking of equipment by condensed material. Besides the operational parameters, the gas quality depends on the reactor design.This study investigates the axial formation of tar and main gas components in an allothermal steam-blown bubbling fluidized bed gasifier. Therefore an axial movable sampling probe is used to withdraw gas samples at different heights directly from within the fluidized bed as well as from the freeboard.The gasification agent H2O decreases rapidly over the bed height. The reason is the release of volatile compounds from the injected biomass and formation of dry gas components inside the bed. The main dry gas components CH4, CO, CO2 and H2 increase continuously over the bed height and reach a maximum at the bed surface.The tar concentration of product gas withdrawn from inside the bed is comparably low. No oxygen containing species but only light aromatic and light polyaromatic compounds are measurable in this region. The main fraction of tar is released in the transition zone between fluidized bed and freeboard due to partially degassed fuel particles that float on the surface.