Optimising supercritical water gasification of biomass: exploring heating strategy through a quantitative kinetic modelling approach

Abstract

Supercritical water gasification (SCWG) offers a promising method to process wet biomass and realise its full potential as a renewable energy source, as well as to efficiently treat waste biomass streams. To optimise this technology for more energy-efficient operations, this work provides a comprehensive investigation into the impact of heating rate and profile on the SCWG of biomass. Using an upgraded SCWG kinetic model, process simulations were used to explore the potential in enhancing syngas yields and carbon gasification efficiency, and mitigating char formation by changing sub-critical heating rates and heating profiles (e.g., linear, accelerating, decelerating). Reducing sub-critical heating rates from hundreds to a few °C min⁻¹ is found to be beneficial for increasing the yield of H₂ from the SCWG of cellulose and hemicellulose in particular, where the increase in H₂ yield exceeded 10 °C min⁻¹. The dry mass fraction of char produced from lignin SCWG could be reduced from roughly 30 °C min⁻¹ to 20 °C min⁻¹ by increasing the sub-critical heating rate by two orders of magnitude to 690 °C min⁻¹. The effect of sub-critical heating profile was less significant, with the only notable trend being increased lignin-derived char with a decelerating sub-critical heating profile. This work shows the potential improvements that could be made to SCWG by tailoring the sub-critical heating regime in accordance with the feedstock to optimise syngas yields and char formation.