Hydroliquefaction of Big Brown lignite in supercritical fluids

Abstract

Big Brown lignite was liquefied in a fixed bed tube reactor. Three solvents were used in the liquefaction studies, toluene, cyclohexane and methanol. Two co-solvents, tetralin and water were used with toluene. The effects of the solvents and co-solvents were investigated. Supercritical fluid is the fluid at the temperature and pressure above its critical values. Toluene was the main supercritical fluid used in this study. Tetralin and water as co-solvents can contribute hydrogen to stabilize the free radicals produced during liquefaction reaction. The total conversion of Big Brown lignite and yield of liquid were increased. Water is not as good as tetralin in producing hydrogen, but it can increase the polarity of the solvent, which increases the solvency of supercritical fluids. The liquid product was found to consist primarily of saturated hydrocarbons. It illustrated that the free radicals were saturated by hydrogen during liquefaction. Alkylated aromatics and furans are also common chemical species present in the liquid products. The aromatic species were predominantly alkylated phenols, benzenes, indenes, pyridines and naphthalenes. At the supercritical conditions of this study, temperature and flowrate of the solvent were not important to the conversion of Big Brown lignite and yield of liquid, since supercritical fluids have gas-like viscosities with very high solubilities. To get more liquid products, the main point is to produce more free radicals from coal, inhibit the recombination of these radicals, and prevent the decomposition of these radicals to gas. Tetralin and water are good co-solvents for coal hydroliquefaction. Further research on the mechanism of water as a co-solvent in coal hydroliquefaction was recommended.