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The release of greenhouse gases from underground coal-mine fires is a function of temperature and the concentration of O2. In a laboratory study on spontaneous combustion, samples of coal, coal refuse, and carbonaceous shale were heated at a controlled rate between ambient temperature and 250 °C. In these experiments, the concentration of O2 was not limited and the concentration of CO2 increased with increasing temperature to a maximum of 10%. Carbon monoxide was not detected at temperatures below 100 °C, and the maximum concentration of CO was less than 4%. In field studies, samples of combustion gases were obtained from fires in three abandoned coal mines. These indicated a linear increase in the concentration of CO2 relative to the decreased concentration of O2. At an O2 concentration of 2%, the CO2 concentration approached 15%, and CO was detected only when the O2 concentration was less than 8%. At temperatures over 50 °C, the rate of desorption of CH4 also increased, but the average concentration in the mine atmosphere was 0.20%.

These laboratory experiments and field studies indicate that the rate of gas production is controlled by O2 concentration and temperature, but physical factors, such as overburden fracturing and differences between surface and subsurface temperature and pressure, control the rate of emission to the atmosphere. In coal-mine fires, both chemical and physical factors control the rate and magnitude of contributions to the atmospheric concentration of greenhouse gases.

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