Experimental study on tar formation during underground coal gasification: Effect of coal rank and gasification pressure on tar yield and chemical composition

Abstract

The yield and composition of tar depending on coal rank and pressure during underground coal gasification (UCG) were studied. Two coals were used in a series of ex-situ UCG experiments: a Welsh semi-anthracite (Six Feet) and a Polish bituminous coal (Wesoła). Four high-pressure gasification trials under two distinct pressure regimes (20 and 40 bar) were conducted. The tar samples were collected directly from the reactor outlet. The following groups of compounds were analysed by use of gas chromatography (GC-MS): light monoaromatic hydrocarbons (BTEX – benzene, toluene, ethylbenzene and xylenes), polycyclic aromatic hydrocarbons (PAHs) and phenols. A series of gasification experiments revealed significant differences in tar yields and composition depending on the coal rank and gasification pressure. Significant decreases in tar contents were observed with the increase in gasification pressure from 20 to 40 bar for both coals. The total yields of the analysed tar components per kg of gasified coal were 2.58 g and 0.41 g for the experiments conducted on the Six Feet samples at 20 bar and 40 bar, respectively. The corresponding values for the Wesoła coal amounted to 5.48 g and 0.95 g. In all experiments, BTEX was a dominant group of tar components, constituting 69–86 % of the total tar yield within the tested range of compounds. The present study further proves that gasification pressure has a significant effect on the chemical composition of the produced UCG tars for both coal samples under study.