Co–solvent effects on hydrothermal co–gasification of coal/biomass mixtures for hydrogen production

•Synergetic effects were observed with THF and NMP during co–gasification.•Increasing co–solvent concentrations increased the gasification and hydrogen yields.•The synergetic effect on H2 yield was about 11 mol H2/kg feed for THF.•The hydrogen yields were enhanced with the catalyst combination of K2CO3/CaO. The use of low–rank coals for hydrogen production by gasification can create advantageous options to overcome a potential energy crisis. In this study, the effects of 8 different solvents on gasification and hydrogen yield in coal/biomass co–solvent hydrothermal gasification were investigated. The most effective co–solvent on product conversion and high hydrogen yield was found N–methyl pyrrolidone (NMP) and tetrahydrofuran (THF). Total gas volumes and hydrogen yields obtained from the co–solvent gasification of coal/sorghum blends were higher than the arithmetical sum of the average values of the yields from individual solvent and coal/sorghum blend gasifications. The increase in the hydrogen yield was 11 mol H2/kg feed with the co–solvent THF due to the synergistic effects of THF on hydrogen production. Solvent concentration was effective on both total gas volumes and hydrogen yields. In the catalytic (K2CO3/CaO) runs with a concentration of 20 % (v/v) NMP co–solvent 98.8 ± 0.1 mol H2/kg feed was achieved with a dramatic decrease of CO content in the produced gas mixture. The comparison of the results obtained from subcritical and supercritical water conditions showed that in catalytic processes co–solvent was more effective towards feedstock conversion and almost the same hydrogen yields obtained at lower pressures under the subcritical water conditions.