Effect of char on co-pyrolysis of biomass and coal in a free fall reactor

•In-situ volatiles–char re-contact was achieved in a free fall reactor.•Volatiles–char re-contact effectively enhanced tar generation.•Co-pyrolysis and volatiles–char interactions favored acidic component generation.•Volatiles–char re-contact during co-pyrolysis decreased asphaltene content in tar.

Fast pyrolysis and co-pyrolysis of pine sawdust (SD) and sub-bituminous coal (SB) were carried out in a free fall reactor. There is a gas–solid separator at the bottom of the reactor, where a controlled re-contact of the primary volatiles with the nascent char could be achieved. The experiments of the individual fuels with the volatiles–char re-contact resulted in increased tar yield and decreased the light hydrocarbon gas concentrations compared with that without the volatiles–char re-contact. During the co-pyrolysis of SD and SB, synergies were observed to have produced more tar at different biomass blending ratios (BR). Particularly, the re-contact of the volatiles–char effectively enhanced tar generation and suppressed water formation. The volatiles–char interactions promoted the tar generation mainly at the char bed temperature range from 600 °C to 700 °C. At the optimum condition with the BR of 0.50 and char bed temperature of 700 °C, the tar yield increased up to 28.7%. The co-pyrolysis and volatiles–char interactions favored the acidic components generation in the liquid products. Especially, the volatiles–char re-contact during co-pyrolysis improved the quality of tar by decreasing its asphaltene content.

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