An optimized numerical solution method for predicting compound dangerous areas of gas emission and coal spontaneous combustion under multi-field coupling in gob

The area in gob with gas emission and coal spontaneous combustion (CSC) presents severe hazards. Based on the analysis of the compound disaster-causing process of gas and CSC, a numerical technique for predicting the compound dangerous area in the gob was proposed in this work. Taking a U-shaped ventilation working face as an example, a mathematical model containing five coupled physical fields of gas concentration field, air flow field, oxygen concentration field, air and solid temperature fields was constructed. This model incorporates the relationship between air flow, temperature, and gas desorption rate. Then a self-developed simulation software was used to solve the multi-field coupled mathematical model, and the distribution characteristics of gas concentration field, solid temperature field and oxygen concentration field in gob were preliminary investigated. The boundaries of the compound danger areas (CDA) in the gob were importantly predicted and discussed before and after extraction in detail. The results show that: by investigating and overlaying the distribution cloud maps of various fields, the compound danger areas of the gob are divided into four levels, namely, CDA-Ⅰ, CDA-Ⅱ, CDA-Ⅲ, and CDA-Ⅳ in descending order of risk. The highest risk CDA-Ⅰ ranges from 14 m along the tendency length (X=39 m, 53 m) and 40 m along the strike length (Y=22 m, 62 m). After taking extraction measures on the return side, the gas concentration in the upper corner reduced to less than 0.8%, and the maximum temperature only increased from 41.26 °C to 44.53 °C. The area of CDA-Ⅰ has increased by 3.6 times. Therefore, while effectively controlling gas, it is also necessary to expand the monitoring range of high-temperature areas. This technology of identifying prevention and control targets for compound hazard in gob can be further promoted to other coal mines with severe spontaneous combustion and high gas concentration.