Development of Kinetic and Heat Transfer Models for Supercritical Water Gasification of Xylose in a Narrow Tubular Reactor

Thermochemical conversion of renewable, CO₂ neutral biomass feed stocks to hydrogen gas or synthesis gas is a promising technology route for the production of chemicals and fuels that may significantly contribute to the worlds energy supply. Supercritical water (SCW) at temperatures greater than 374ºC and pressures greater than 220 bar is an effective medium for reforming biomass to hydrogen rich gas or synthesis gas. Due to the endothermic nature of biomass reforming reactions, a micron sized tubular reactor designed for intensified heat transfer was used to intrinsically gasify xylose, a model compound for hemicellulose, at various temperatures and residence times. Based on the experimental data, a reaction mechanism and kinetic model were proposed. Heat transfer and kinetic models were used to investigate the effect of tubing diameter on product selectivity and gas yield for the super critical water gasification (SCWG) of xylose. The model predicts that at residence times, < 10 sec, SCWG of xylose greatly benefits from the intensified heat transfer characteristics of very small diameter reactor tubing, resulting in an increase in gasification efficiency and hydrogen yield. Also, high rates of heat transfer decrease the reacting fluid heat up period, minimizing the formation of coke precursors and hard to gasify liquid intermediates. Thus, SCWG of biomass will significantly benefit from intensified heat transfer in flow through reactors.