Tar formation during co-gasification of biomass and coal under different gasification condition

•Tar formed in co-gasification of biomass and coal in a decoupled gasifier was studied.•Tar yield was significantly reduced with olivine as catalytic bed materials.•Phenols were totally decomposed at gasifier temperature above 800 °C.•Decomposition of naphthalene was enhanced with increased S/C ratio.

Parametric tests, biomass ratio (BR), gasifier temperature, steam to carbon mass ratio (S/C) and type of bed materials, on tar formation were performed during co-gasification of biomass and coal on an external circulating radial-flow moving bed gasifier (ECRMB). The system is composed of three decoupled reactors, i.e. a gas–solid countercurrent moving bed pyrolyzer, a radial flow moving bed gasifier and a riser-type combustor. Calcined olivine was used as both circulating heat carrier and in-situ tar destruction catalyst. It was found that the operation condition has a large impact on tar composition. Naphthalene was found to be the dominate component in all cases at quantities of 40.7–62.6%. Sulfur contents in the tar were reduced with the increasing BR. Tar compositions shift from phenolic compounds and alkyl-substituted PAHs to non-substituted PAHs with the increase of gasifier temperature. Above gasifier temperature of 800 °C phenols and one-ring aromatics were totally decomposed. Higher S/C enhanced the decomposition of naphthalene. In contrast to silica sand, olivine as bed material showed a higher decomposition effect on the phenols and higher hydrocarbons.