High tar reduction in a two-stage gasifier

The work with a 100-kW(th) two-stage gasifier at the Technical. University of Denmark (DTU) has resulted in a gas with a very low tar content. The reduction in tar has come about by combining the partial oxidation of the pyrolysis gas with the reactions that follow on a charcoal bed in the char gasification unit. Tar is defined as organic contaminants which have retention times equal to or greater than those of phenol on a nonpolar GC column. Two test series were made with wood chips as feedstock, before and after the gasifier was optimized. The effect on tar reduction of a charcoal bed has been investigated by measuring the tar content and composition of the gas after the partial oxidation (above the charcoal bed) and after the passage of the gas through the charcoal. By introducing the correct supply of air, the tar content in the gas after its partial oxidation was about 3000 mg/kg dry wood chips. Furthermore, a conclusive high tar reduction of the aromatic hydrocarbons including PAH was obtained by avoiding bypasses of the charcoal bed. A gas was produced with a tar content as low as 10-40 mg/kg dry wood.