Catalytic hydrothermal gasification of aqueous effluents obtained from microwave torrefaction of agricultural residue

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A significant amount of aqueous liquids rich in organic, inorganic contents and leached metals is produced during torrefaction of biomass, which requires additional remediation before disposal. Hydrothermal gasification can produce H2-rich syngas from organic wastes, especially from feedstocks with high-water content. This study presents a novel method of valorizing liquid effluents produced from the torrefaction of oat hull and canola hull via microwave irradiation. Hydrothermal gasification of the liquid effluents (feedstock) was carried out at 375–525 °C for 15–60 min with 1:5 and 1:10 feed-to-water ratios under 22–25 MPa pressure to optimize the operating parameters of the process. Hydrothermal gasification of torrefaction effluents from canola hull and oat hull produced total gas yields of 6.6 mmol/g and 8.7 mmol/g, respectively without the addition of water at optimized conditions (525 °C and 45 min). However, with the addition of water (i.e., 1:10 feed-to-water ratio), the total gas yield increased to 12 mmol/g and 16.1 mmol/g from oat hull and canola hull effluents, respectively. The highest H2 yields (7.8 mmol/g) and carbon gasification efficiency (51%) were obtained at 525 °C in 60 min with a 1:10 feed-to-water ratio. Homogeneous catalysts including KOH, K2CO3 and Na2CO3 were used to improve the gas yields from torrefaction effluents from oat hull at optimal conditions. H2 gas yield increased by 58%, 65% and 66% in presence of K2CO3, Na2CO3 and KOH, respectively. KOH led to maximum H2 yield, hydrogen selectivity and carbon gasification efficiency of 12.8 mmol/g, 89% and 98%, respectively.