Hydrothermal co-gasification of sorghum biomass and çan lignite in mild conditions: An optimization study for high yield hydrogen production

Abstract

In this study, response surface methodology (RSM) combined with a 3–factor and 3–level Box–Behnken design (BBD) was performed to obtain high yield hydrogen production from hydrothermal co–gasification of sorghum biomass and low rank Çan lignite in a batch type reactor at 500 °C. The individual and the combined effects of the process parameters of coal amount (%) of the coal/biomass mixtures, initial water volume (mL) of the reactor and amount of the coal/biomass mixtures (kg) on system pressure, total gas yield, hydrogen production and product distribution were determined. Water volume directly affected the system pressure and the reaction medium was supercritical water medium above 48.2 mL with a pressure of 22.06 MPa. The highest values of both total gas volume and hydrogen gas volume were reached by gasification of 5.0 g of feedstock. It has been observed that total gas volume and hydrogen volume were directly affected by the water volume in the reactor and the coal ratio of the coal-biomass mixtures. The highest total gas and hydrogen volumes can be achieved under the conditions where the higher levels of water volume of the reactor and lower levels of coal percentage of the coal/biomass mixture were combined. Optimum conditions for maximum hydrogen production with 5.0 g of coal/biomass mixture were determined with numerical optimization as coal percentage of 25.6% and initial water volume of 68.5 mL. By combining the impregnated K2CO3 (3%, (w/w)) and CaO catalysts an excellent hydrogen selectivity was achieved. The hydrogen selectivity was drastically increased from 32.0% to 70.8% by capturing more than 99% of CO2 with a H2/CO2 mol ratio of 88.5.