Investigation on steam co-gasification of torrefied biomass and coal: Thermal behavior, reactivity, product characteristic and synergy

The steam gasification of rice straw torrefied at 200–300 °C and its co-gasification with coal were examined in this study. The effect of torrefaction temperature on thermal behavior, reactivity, product characteristic, and synergy were investigated. The results showed that torrefaction temperature influenced the pyrolysis and char gasification stages by shifting them to higher temperature ranges. As torrefaction temperature increased, the H2 content increased while the CO and CH4 contents decreased, with H2/CO molar ratio increasing from 1.8 to 2.4. The total gas yield also increased from 0.95 to 1.30 Nm3/kg, especially with a large increase up to 27.4 %. As for the co-gasification, stages of biomass pyrolysis and coal pyrolysis overlapped at torrefaction temperature of 300 °C. The pyrolysis reactivity decreased while the char gasification reactivity increased with the torrefaction temperature increasing. The H2/CO molar ratio decreased slightly from 3.55 to 3.40, while the total gas yield increased from 1.87 to 1.96 Nm3/kg as well as the cold gas efficiency from 75 % to 77 %. The strongest synergistic effect on the reactivity occurred at torrefaction temperature of 250 °C. The increased torrefaction temperature enhanced the synergistic effect on the total gas yield but weakened it on the H2 yield.