Simulation and Kinetic Study of Supercritical Water Gasification of Model Biomass

The demand for H₂ is increasing day by day due to many folds. Hydrogen is an important element for many industrial processes like heavy oil upgrading, desulfurization and upgrading of conventional fossil fuel. Moreover, it is becoming more popular as a transportation fuel for combustion of hydrogen does not create any pollution. However, hydrogen is not readily available in nature, although it is abundant around the world. Using current and available technologies for producing hydrogen, which use conventional hydrocarbons, contribute more towards greenhouse gas (GHG) emissions. Contrary to this, hydrogen produced from renewable sources, like biomass, has a great advantage because it contributes towards lower or no net GHG emissions. Among the various available technologies, supercritical water gasification (SCWG) of biomass has a great potential for producing hydrogen. In contrast to the conventional gasification process, SCWG does not require drying of biomass; rather the moisture content and external water is used for the process. In this study supercritical water gasification (SCWG) process of model biomass using the ASPEN Plus software for the production of H₂ is studied. Glucose is used to perform the simulation that represents the cellulose and hemicellulose of real biomass. It is found that temperature has acute effects on product yield, especially hydrogen. Contrary to this, higher concentration favors methane yield, whereas lower concentration favors hydrogen production. However, the pressure did not show any significant effect on product yield. It is also observed that the gasification efficiency increased with the increase in temperature and decreased with the increase in concentration. Besides gasification efficiency (GE) is over 100% of all cases which supports that supercritical water (SCW) takes part in the reaction as an important reactant. In addition to these, with the increase in temperature and concentration, carbon conversion efficiency (CCE) is observed to be increased. Also, two mechanistic kinetic models one based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) and the other based on Eley–Rideal (ER) were developed to describe the reaction mechanism. An ER based model described as the dissociation of adsorbed glucose through Retro-Aldol reaction is found to be the rate determining step with an average absolute deviation 10.6%.