A tandem pyrolysis-upgrading strategy in an integrated reactor to improve the quality of coal tar

Abstract

Coal pyrolysis and volatiles upgrading were intelligently coupled into an integrated reactor with two sections that enabled tar quality to be improved. The upper drop tube served as the rapid pyrolysis section to maximize the primary yield of tar. Subsequently, the pyrolysis char together with the released volatiles was introduced into the lower moving bed section, wherein the formed char bed was endowed with a catalytic upgrading function by enhancing volatiles-char interactions. This design succeeded in the consecutive pyrolysis-upgrading process by recombining its pyrolysis products. In the first-step rapid pyrolysis, a much higher yield of tar than the Gray-King assay yield was obtained at 600 °C, but the light fraction (boil pointing < 360 °C) in tar was as low as 43.0%, which was identified as poor quality. After the second-step catalytic upgrading at 600 °C, the resultant tar continued to maintain a relatively higher yield, which was 91.2% of that determined by Gay-King assay, despite being cracked over char. Accordingly, the light fraction in tar reached 57.0%, i.e., a 32.6% increase compared with the drop tube pyrolysis, which enabled a higher yield of light tar. This finding could be attributed to the shift of heavy tar into light tar and gas at the expense of the forming coke over char bed. The experimental results demonstrated that the proposed tandem rapid pyrolysis-volatile upgrading strategy had its unique superiorities in terms of guaranteeing both the quantity and quality of coal pyrolysis tar compared with other pyrolysis processes.