Hydrogen production by catalytic supercritical water gasification of representative biomass-derived model mixtures

Abstract

Supercritical water gasification enables hydrogen production from wet biomass-derived feedstocks without the need for a drying step. The catalytic supercritical water gasification of three representative multicomponent model mixtures was investigated in a continuous tubular reactor using a commercially available Ni-based catalyst. The mixtures combined model compounds representing organic families found in fruit residues, sewage sludge and dairy effluents. Experiments were conducted at 240 bar and 500–800 °C, using feed concentrations of 5 and 7.5 wt%, a liquid flow rate of 1 L h⁻ 1 and catalyst loads of 10 and 20 g. The results were compared with chemical equilibrium predictions and corresponding non-catalytic experiments performed with the same mixtures and reactor configuration under identical operating conditions. Higher temperatures and lower weight hourly space velocity resulted in higher hydrogen production, a lower liquid organic fraction and gas yields closer to chemical equilibrium. The maximum hydrogen yield reached 10.29 mol H 2 mol⁻ 1 organic feed, while the carbon-to-gas efficiency reached 94.4 % at 800 °C and 5 wt% feed concentration. Compared with the corresponding non-catalytic experiments, the H 2 yield increased from 3.52 to 8.73 mol H 2 mol⁻ 1 organic feed at 800 °C and 7.5 wt% feed concentration using 10 g of catalyst. Thus, the Ni-based catalyst substantially increased hydrogen production and reduced the fraction of organic carbon remaining in the liquid phase. The experimental results also indicate that the hydrothermal reactions occurring upstream of the catalytic bed influence the subsequent conversion of oxygenated, aromatic and nitrogen-containing intermediates and, consequently, the overall gasification performance. The apparent turnover frequencies and Arrhenius parameters were used as reactor-scale indicators to compare the catalytic response rather than as intrinsic kinetic parameters. Overall, the results indicate that the kinetic constraints associated with multicomponent hydrothermal conversion can be mitigated by catalytic supercritical water gasification but not completely overcome.