Effect of CaO on conversion of lignite to hydrogen-rich gas in supercritical water

Conversion of Xiaolongtan lignite to hydrogen-rich gas was performed in supercritical water under conditions of 450–80°C, 23–38 MPa, and 1–30 min in the presence of CaO. The effects of Ca/C molar ratio and reaction conditions on the product distribution and yields of gas products were investigated. The yield of CO2 in gas product decreased and that of hydrogen increased with the increase in Ca/C molar ratio. At a Ca/C molar ratio of 0.42, nearly all CO2 is fixed and the molar fraction of hydrogen in gas product is about 48%. The increase in temperature leads to an increase in the carbon conversion and the yield of hydrogen, and the performance of CaO is significant at higher temperatures. Higher reaction pressure favors the conversion of coal and the formation of H2 and CH4. The effect of residence time is less significant compared with Ca/C molar ratio and temperature. Experiments of char gasification in SCW were also conducted. It shows that the addition of CaO can promote carbon gasification reaction and increase the yields of H2 and CH4.