Expansion of three reaction zones during underground coal gasification with free and percolation channels

During underground coal gasification (UCG), the expansion of the three reaction zones (oxidation zone, reduction zone, and dry distillation zone) is closely related to the stability of the gasification process and to the calorific value of the producer gas. The gasification channel structure also greatly affects the expansion process. In this article, the temperature field map and changes in producer gas components were monitored and analyzed in real time in an experimental model of UCG. A free channel and percolation channel were used and compared. The three reaction zones were found to expand during stable gasification, including the area ratios of the three zones and their forward and lateral expansion rates. The differences in the expansion of the three zones between free channel and percolation channel are here explained from the viewpoint of mass transfer. The results showed that, during the gasification of Ulanqab lignite in an oxygen- and CO2-rich environment, the average area ratios of the oxidation zone, reduction zone, and dry distillation zone in free and percolation channels were 1:1.20:1.43 and 1:1.73:2.18, respectively, the corresponding forward expansion rates were 0.077 m/h and 0.019 m/h, and lateral expansion rates were 0.007 m/h and 0.0065 m/h, respectively. Unlike in the free channel, the porous media structure of percolation channel increased the convective mass transfer rate between gasification agent and coal seam, and the stability of the gasification process and the calorific value of the producer gas. This is why the reaction zone expanded further in the percolation channel than in the free channel.