Study of properties of tar obtained from underground coal gasification trials

This work presents the results of an experimental study on the properties of tar that originated from underground coal gasification (UCG) trials. Two in situ experiments of UCG were performed using oxygen as the gasification agent under the HUGE and HUGE 2 projects. Trials were conducted using different configurations of the fire channel, gasification time and method of raw gas collection and purification. The obtained tars were analysed over a wide range of parameters that are typical for the assessment of the quality of conventional coke oven tar. The results of the analysis showed that in terms of physical-chemical properties, both tars are highly different from each other, as well as from typical coal tar from a coking plant. One characteristic observation of the UCG-derived tars is a higher content of ash as well as toluene and quinoline insolubles compared with a typical coke oven tar. Both tars are susceptible to thermal destruction and degradation to lighter products, and the characteristics of the coking distillation residue after heating to high temperatures is different from coke oven tar. The tested tars vary in boiling range and fractional composition. The analysis also showed that tars from UCG contain more heterocyclic compounds than does coke oven tar, and the tar from HUGE is heavier than tar from HUGE 2. The content of PAHs in the tar from HUGE is many times higher than in tar from HUGE 2 and at a similar level as coke oven tar; however, the naphthalene content in both UCG tars is many times lower compared to typical coke oven tar. The probable reasons for the differences described in this report include the tar residence time in the sampling point, as well as different extents of tar fractionation phenomena, which resulted from the distinct configurations of the gas transportation and purification systems.