(699g) Subcritical and Supercritical Water Gasification of Soybean Straw and Flax Straw for Hydrogen Production: Experimental and Thermodynamic Modeling

Over the past few years there has been a tremendous demand for hydrogen (H₂) production for use as transportation fuels, desulfurization, heavy oil upgrading and in fuel cells. H₂ produced from waste biomass is widely considered as a green fuel useful for combined heat and power generation due to its high-energy content. Recently, lignocellulosic biomass has received immense attention as a promising feedstock for biofuel production due to its abundant availability, low cost and the ability to reduce atmospheric CO₂ through photosynthesis. Supercritical water gasification of lignocellulosic biomass is an attractive option for hydrogen production. The process is suitable for biomass with high moisture content because the medium of gasification is water. In addition, the syngas is produced at high pressure, thereby eliminating the need for gas compression. In this study, Soybean straw and Flax straw were characterized and examined as the feedstocks for H₂ production in subcritical (300⁰C) and supercritical water (400⁰C and 500⁰C). To optimize the non-catalytic process, we studied the influence of temperature (300 – 500⁰C), biomass to water ratio, BTW (1:5 and 1:10), biomass particle size (0.03 – 2 mm) and reaction time (30 – 60 min) on H₂ production 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 at highest test temperature (500⁰C), lower feed concentration (1:10 BTW), smallest particle size biomass (< 2 mm) and longest reaction time (45 min). A comparative evaluation of the gasification performance of the two lignocellulosic biomasses reveal that soybean straw exhibited superior H₂ yield (6.62 mmol/g) and total gas yield (14.91 mmol/g). Similarly, the gaseous products from soybean straw showed improved lower heating value (1592 kJ/Nm³). To evaluate the drift of our experimental H₂ yield from the theoretical values, thermodynamic modelling using Gibbs free minimization method (GFMM) was performed. Since supercritical water gasification involves a series of complex reactions with several intermediates formation, the GFMM method was selected as it does not require the identification of intermediate reactions. The experimental results showed good correlation with the thermodynamic models, which will be discussed during the presentation. Our findings suggest that supercritical water gasification of lignocellulosic biomasses could be a useful green technology for effective H₂ production and waste valorization.