A two-stage batch hydrothermal process has been investigated with the aim of enhancing the yields of hydrogen and methane from sawdust. Samples of the sawdust were rapidly treated in subcritical water and with added Na2CO3 (alkaline compound) and Nb2O3 (solid acid) at 280 °C, 8 MPa. Each pre-processing route resulted in a solid recovered product (SRP), an aqueous residue and a small amount of gas composed mainly of CO2. In the second stage, the SRP and the liquid residues were gasified in supercritical water in the presence of Ru/Al2O3 catalyst for reaction times of up to 60 min for the SRP at 500 °C, 30 MPa. Using the catalyst, carbon gasification efficiencies and methane selectivity increased with increasing reaction time. Overall, SRP from the Na2CO3 pre-processing route produced 51% more hydrogen and 61% more methane than the original sawdust under identical reaction conditions. The cumulative yields of methane and hydrogen were 57.1 mol kg−1, 42.5 mol kg−1 and 47.7 mol kg−1, from Na2CO3, Nb2O5 and neutral pre-processing routes, respectively. The combined yield of the two gases from direct SCWG of the original sawdust was 24.6 mol kg−1. The entire process may represent a step-change in future energy production from biomass as the products from the first stage can be used as feedstocks for various other biomass conversion technologies.
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