## 1 Introduction  
A total of 56% of coal mines in China have spontaneous ignition problems [1]. More than 70% of the state-owned key coal mines are high-gas mines [2], and high-gas mines, with a tendency of spontaneous coal ignition, account for about 50% [3]. Due to the double pressure of high gas emissions and easy, spontaneous combustion of coal during mining, the contradiction of safety production in coal mines is becoming more and more prominent. While improving the gas safety of the working face, the risk of natural ignition in the gob will increase; on the contrary, reducing the risk of natural ignition in the working face will easily lead to a gas over-limit, and there will be an imbalance between two types of safety hazards in the treatment of one or the other. At present, the loss of high-quality coal caused by natural-2 combustion has reached more than 4.2 billion tons, resulting in sluggish resource reserves of more than 200 million tons/year, and economic losses of up to billions of yuan. Safe production of coal mines is hindered by high-yield and high-efficiency fully mechanized mining, gas over-limits of fully mechanized caving faces, and spontaneous combustion of goat coal. In Inner Mongolia, Shaanxi, Qinghai, Shanxi, and other regions, there are shallow-buried deep coal seam group mines. Generally, the upper coal seam is mined first and then the lower coal seam. After mining the upper coal seam, the coal rock mass in the gob area collapses, the surface collapses, and the cracks in the gob area develop at the surface. When mining the lower coal seam, the gob of the lower coal seam is connected with the gob of the upper coal seam to form a super-large gob that connects with the surface. In the case of negative-pressure ventilation, when the pressure energy of the goaf is greater than the pressure energy of the working face, the gas in the goaf of the upper coal seam enters the working face through the return air side of the lower coal seam. As a result, gas in the upper corner of the lower coal seam exceeds the limit, which seriously affects the normal production of the mine and threatens the safety of underground workers [4; 5]. The problem of gas leakage in the goaf of the upper coal seam occurs in shallow and deep coal-seam group mining, and the pressure-equalization technology has achieved a good control effect. The technique of pressure equalization is to rationally allocate the mine ventilation system so that the pressure difference between the air leakage sources can reach a certain degree of balance and reduce or eliminate air leakage. On the other hand, a normal pressure drop at both ends of the air leakage channel can be achieved by adjusting the air chamber, damper, etc., so as to reduce the oxygen supply by air leakage and prevent spontaneous coal combustion [6]. The technology of pressure-equalization regulation has been widely used in mines at home and abroad. A scholar summarized the method of pressure equalization to solve the problem of spontaneous coal combustion, which was successfully applied in coal mines. In the early 1960s, Polish scientists proposed unilateral and bilateral connecting pipe pressure to regulate gas chambers, etc., and added measures to equalize the pressure and prevent fires [7]. In the 1980s, a scholar [8] studied the influence of the parameters of the pressure-equalizing system, such as natural wind pressure, local fan working points, and fire wind pressure, on the pressure-equalizing system. In the 1990s, the KJ7 automatic monitoring and regulation system [9; 10] for pressure equalization and fire prevention was jointly developed by several institutions. The development of equal pressure ventilation for fire prevention provides a theoretical basis for positive pressure ventilation to deal with the problems of return airflow and the emissions of toxic and harmful gases in the upper corner of the underground working face, as well as insufficient oxygen concentrations.