Upgrading peat to gas and liquid fuels in supercritical water with catalysts

Abstract

Peat was effectively upgraded into gas and liquid fuels by supercritical water treatment at 380–440°C with and without catalyst. Treatment of the peat without using catalyst at a temperature ranging from 350°C to 440°C generally led to 5–10wt% yield of water soluble oil (WSO), about 15wt% yield of heavy oil (HO), 20–27wt% yield of Gas and 20–30wt% yield of Char. As expected, the Gas yield increased and the yields of oils and Char generally decreased at a higher operation temperature. The optimal conditions for a greater HO yield appeared to be a lower residence time and a lower water-to-peat ratio (i.e., a higher initial biomass concentration). Among all the catalysts tested including 0.1M K2CO3, 0.1M FeSO4, 0.1M RuO2, and 0.1M and 1.0M Ca(OH)2, 0.1M Ca(OH)2 was found the most effective for enhancing the production of liquid oils of both HO and WSO. The presence of 1.0M Ca(OH)2 at 410°C produced extremely low CO2 yield, due to the adsorption reaction of CO2 over the Ca(OH)2 catalyst, and a fourfold increase in H2, resulting in a high quality gas product with a HHV of 27.7MJ/kg, as oppose to only 2.81MJ/kg for the gas product without catalyst. Among all the catalysts tested, RuO2 proved to be the best catalyst for upgrading peat to H2-rich gas products, and the formation of H2 attained as high as 2.44mmol/g with 0.1M RuO2 at 410°C, compared with 0.05mmol/g without catalyst. The HOs obtained from the SCW treatments, rich in phenolic compounds and fatty acids, possess much greater caloric values (with a HHV of >33–35MJ/kg) than the dried peat powder used (with a HHV of about 21MJ/kg).