Hydrothermal gasification of soybean straw and flax straw for hydrogen-rich syngas production: Experimental and thermodynamic modeling

Abstract

Biofuels produced from lignocellulosic feedstocks are gaining popularity because of the elevating energy demand, increasing greenhouse gas emissions, escalating fuel prices and dwindling fossil fuel resources. Therefore, it has become important to seek alternative energy resources from renewable waste biomass. In this study, agricultural crop residues such as soybean straw and flax straw were gasified in subcritical water (300 °C) and supercritical water (400 and 500 °C) for H2 production. To maximize the non-catalytic process, the impacts of temperature (300–500 °C), biomass-to-water ratio, BTW (1:5 and 1:10), biomass particle size (0.13 mm and 0.8 mm) and residence time (30–60 min) on H2 production were studied at a pressure range of 22–25 MPa. Maximum H2 yield and total gas yields of 6.62 mmol/g and 14.91 mmol/g, respectively were obtained from soybean straw at the highest temperature (500 °C), lower feed concentration (1:10 BTW), smaller particle size biomass (0.13 mm) and longer residence time (45 min). To evaluate the drift in the experimental H2 yield from the theoretical values, thermodynamic modeling using Gibbs free minimization method was performed. The experimental results showed slight deviations from the thermodynamic models due to the temperature gradient and absence of agitation in the tubular batch reactor. However, the KOH catalyst was found to elevate the H2, CO2 and CH4 yields for soybean straw and flax straw. The findings suggest that supercritical water gasification could be an efficient green technology for H2 production from waste biomass.