Spatial and temporal evolution of tar during ex-situ underground coal gasification

Abstract

Underground coal gasification is a promising technology for exploiting deep coal seams, while the evolution behaviors of pollutants such as tar in the reaction zone are still not clear. In this work, an ex-situ experimental system of UCG was built, and a series of experiments were carried out with different gasification agents and flow rates to study the concentration and composition of tar samples from different positions and time. Shenmu bituminous coal is cut into specific blocks with drilled holes as initial gasification channel and sampling channels. Tar concentration is calculated according to the weight of tar samples and the corresponding gas flow volume during sampling, while tar composition is analyzed by GC–MS. For the involved conditions, the maximum relative gap of tar concentrations between L = 36 cm and 102 cm is around 296% under 10 L/min oxygen case. The maximum relative gap of PAH percentage in tar between L = 36 cm and 102 cm is around 84% under 10 L/min air case. For oxygen gasification, increasing flow rate from 10 L/min to 15 L/min causes a lower tar concentration at reaction zone and a higher tar concentration at outlet, as well as a lower PAH percentage for all positions. For air gasification, increasing flow rate from 10 L/min to 50 L/min leads to a more uniform distribution of tar concentration and composition. Tar concentration for air gasification is far smaller than that for oxygen gasification. Changing from 10 L/min to 15 L/min for oxygen gasification decreases the carbon emission in gases and increases the percentage of product tar. Changing from 10 L/min to 50 L/min for air gasification decreases the carbon emission in gases but increases the percentage of pollutant tar.