Review of supercritical water gasification with lignocellulosic real biomass as the feedstocks: Process parameters, biomass composition, catalyst development, reactor design and its challenges

Supercritical water gasification (SCWG) is a combined thermal decomposition and hydrolysis process for converting wet biomass feedstock with high water content potentially (80 wt%) to syngas. The process bypasses the need for an energy intensive pre-drying step and also needs relatively shorter residence times (of the order of seconds to minutes) when compared to conventional gasification. The main target of SCWG is to obtain syngas rich in hydrogen whilst minimising char formation. In recent years, SCWG studies have advanced from using model compounds (e.g. glucose and cellulose) towards the use of real biomass and its waste (e.g. sugarcane trash). The use of biomass as a feedstock creates real opportunities for the technology since it is available in some form, regardless of location. This review discusses the findings from SCWG studies that have used real biomass as a feedstock. The effects of reaction temperature, pressure, residence time and feedstock concentration to the hydrogen yields are presented. The relationship between the main components in biomass (cellulose, hemicellulose and lignin) and hydrogen yields are also discussed. Homogeneous and heterogeneous catalysts have been used to enhance SCWG with real biomass feedstock and the benefits of these approaches are also considered. The economic benefits of running the catalytic SCWG at 400 °C compared to non-catalytic operation at 600 °C is evaluated. Reactor configuration and process conditions vary across the literature, and various authors describe the associated challenges (char formation and plugging, corrosion) as well as promising solutions to tackle these key challenges.