Evolution law of the characteristic field of channel-type underground coal gasification process

The amount of coal resources in abandoned mines in China is about 42 billion tons.The re-mining of coal resources in abandoned mines is difficult,economically unreasonable,and has poor safety issues.Therefore,new recovery technologies must be developed.Underground coal gasification(UCG)technology is a feasible technical route.UCG can rely on the original shafts and roadways of old mines to build underground gasifiers,and use artificially excavated coal roads as gasification channels to form channel gasification.The occurrence and development process of underground coal gasification is essentially the expansion process of the characteristic fields(temperature field,volume fraction field,and stress field)of the gasification reaction zone.The distribution of the characteristic fields of the channel gasification process is studied through underground gasification model tests.The results show that the average expansion speeds of the temperature field along the channel(horizontal)and vertical channel(longitudinal)are 0.089 m/h and 0.012 m/h,respectively,and the average velocity ratio is 7.42 times.Near the injection point,both horizontal and vertical expansion speeds reach the maximum,which are 0.116 m/h and 0.118 m/h respectively.The channel gasification is dominated by wall combustion gasification,when the horizontal expansion speed is more than 11.4 times the vertical expansion speed the reaction process will lose stability.The H₂ and CO volume fraction near the middle of the gasification zone(channel)are higher than the H₂ and CO volume fraction in the coal seam,so the gasification reduction reaction on the channel wall is the main source of H₂ and CO.The overall trend of the CH₄ distribution is that the volume fraction in the coal seam is higher than the volume fraction near the middle(channel),indicating that the CH₄ comes from the pyrolysis of the coal seam.The thermal dispersion effect from the interface between the coal seam and the rock layer to the 20 cm high roof is produced,and the thermal dispersion is obvious in the range of 20-40 cm away from the coal seam,and the dispersion angle is 45°.The stress concentration occurs at each measuring point in the top plate,the maximum concentration of stress is about 0.6 MPa,and the stress concentration factor is 2.0 to 2.5.The stress concentration occurs sequentially along the channel direction,and the time is delayed,it is consistent with the advancing direction of the temperature field.