TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The oxygen-nitrogen mixture obtained by the air separation comprises nitrogen and oxygen based mixture, where the proportion of oxygen and the nitrogen is determined by the rectification of the oxygen-containing coal bed gas in the rectifying tower. The low-temperature working medium is the mixture of low-temperature gas or liquid or gas-liquid. The methane and oxygen concentration varies at different positions of the rectifying tower, which ensures different ratio of oxygen-nitrogen concentration, different flow, and different gas-liquid ratio of the low-temperature working medium. The rectifying tower is a single-stage rectifying tower, a double-stage rectifying tower or a multi-stage rectifying tower. The rectifying tower comprises an evaporator or a condenser. Preferred Method: When the rectifying tower is the single-stage rectifying tower, the oxygen-containing coal bed gas through the main heat exchanger enters into the evaporator for liquefaction to liquefy the oxygen-containing coal bed gas and then enters into the low temperature working medium single-stage of the rectifying tower for rectifying at the bottom to obtain the methane product, and conveying the methane waste gas from the top of the rectifying tower to other positions through the main heat exchanger. When the rectifying tower is the double-stage rectifying tower, which comprises a first rectifying tower, a second rectifying tower, where the first rectifying tower comprises a first evaporator, and the second rectifying tower comprises a second evaporator, the oxygen-containing coal bed gas through the main heat exchanger enter into the first evaporator and the second evaporator for liquefaction to liquefy the oxygen-containing coal bed gas, and then enters into the first rectifying tower for primary rectification to obtain the methane product at the bottom, and then after the primary rectification, the oxygen-containing coal bed gas is conveyed from the top of the first rectifying tower into the second rectifying tower for rectification, where the oxygen-containing coal bed gas enters into the low-temperature working medium at the bottom of the second rectifying tower to obtain the methane product, where the methane waste gas is conveyed from the top of the second rectifying tower through the main heat exchanger to the other positions. The low-temperature working medium is formed by an air separation device.