Effects of alkali and alkaline-earth metals and retention time on the generation of tar during coal pyrolysis in a horizontal fixed-bed reactor

To reduce the tar yield, a horizontal fixed bed is proposed to replace a conventional fluidized bed in the pyrolysis stage of two-stage gasification. Effects of alkali and alkaline-earth metals on the generation of tar are studied. Roles of the interparticle and inner-furnace retention time are also investigated. Ultraviolet–visible spectrophotometry was used to determine the aromatic compounds in tars. The results showed that absorbance intensity of the tar solution increased linearly with its concentration within the examined concentration range. The removal of alkali and alkaline-earth metals increased the tar yield of coal. Increase in the absorbance intensity of tar is attributed to the increase in aromatic ring systems. Ion-exchangeable Na plays a dominant role in the retention of tar precursors, mainly polycondensed aromatic rings, by connecting them with the char matrix. Increasing the retention time in interparticle diffusion decreased the yield of tar, especially for polycondensed aromatic rings. Increasing the retention time in inner-furnace diffusion slightly decreased the tar yield. Introduction of a fixed bed could effectively solve the problems of high tar yield from coal and low catalyst contents in char.