Subcritical and supercritical water gasification of lignocellulosic biomass impregnated with nickel nanocatalyst for hydrogen production

•Nickel impregnation into pinewood and wheat straw for hydrothermal gasification.•Impacts of temperature, biomass-to-water ratio and residence time on gas yields.•Ni-doped biomass had greater H2 yields with higher carbon gasification efficiency.•Maximum H2 yields at 500 °C, 1:10 biomass-to-water ratio and 45 min at 25 MPa.•Ni-doped wheat straw gave relatively high H2 yields than Ni-doped pinewood at 500 °C.

Advanced concepts in thermochemical biomass conversion are indispensable today for efficient generation of high-quality energy products. One of such strategies used in this study is the impregnation of nickel catalyst into lignocellulosic biomasses (pinewood and wheat straw) for hydrothermal water gasification. The nickel-impregnated feedstocks were gasified in subcritical and supercritical water to study the impacts of temperature (300, 400 and 500 °C), biomass-to-water ratio (1:5 and 1:10) and residence time (15, 30 and 45 min) at 23–25 MPa. The gasification of nickel-impregnated biomasses showed considerably higher total gas yields (9.5–16.2 mmol/g) and hydrogen yields (2.8–5.8 mmol/g) with greater carbon gasification efficiency (19.6–32.6%) when compared to non-catalytic gasification. Furthermore, hydrogen yields from nickel-impregnated biomasses were favored at an optimal temperature of 500 °C, biomass-to-water ratio of 1:10 and residence time of 45 min at 23–25 MPa. The nickel particles doped into the lignocellulosic matrix acted as nanocatalysts by providing active sites for enhancing hydrogen production in supercritical water.

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