Evolution of tar compounds in raw gas from a pilot-scale underground coal gasification (UCG) trial at Wieczorek mine in Poland

This work presents the results of the analyses of tar fractions contained in the process gas during a real underground coal gasification test at the Wieczorek mine in Poland. The samples of tar compounds were collected periodically every two days directly from the process gas during the 60-day gasification test. The coal gasification reagents were as follows: oxygen-enriched air (OEA), air, a mixture of air and carbon dioxide, and air mixed with nitrogen. In the analysed tar samples, the contents of light monoaromatic hydrocarbons (benzene and its derivatives - BTEX), polycyclic aromatic hydrocarbons (PAHs) and phenols were determined. The influence of the gasification parameters on the mass production rates of the individual tar compounds was investigated. It was found that the gasification conditions, especially the concentration of oxygen and the flow of the gasifying agent, had a significant effect on the amount and composition of the tars obtained. During the gasification test, BTEX compounds dominated in tars, and benzene was the main component. The second and third groups of components in tars in terms of mass were PAHs and phenols. On the basis of changes in the concentrations of the produced hydrocarbons and their formation and decomposition temperatures, it was estimated that the temperature inside the reactor was in the range of 750-1000 degrees C. The highest concentrations of BTEX compounds were observed during the main gasification stage and during reactor shut down. These were the results of the intensification of gasification processes in the main stage of the trial and the tar fractionation phenomena inside the output pipeline during the final stage.