GASES GENERATED IN UNDERGROUND COAL MINE FIRES

During the uncontrolled combustion of coal in underground mine fires, the generation of gaseous compounds is a function of temperature and the concentration of oxygen. The oxidation of the coal produces CO2 and CO with a concurrent decrease in the concentration of O2. In a laboratory study on spontaneous combustion, various carbonaceous samples were heated at a controlled rate between ambient and 250 deg C. The concentration of O2was not limited, and the concentration of CO2 increased with increased temperature to maximum of 10%. In the same study, CO was not detected at temperatures below 100 deg C, and the maximum concentration was less than 4%. The ratio of CO2 to CO decreased with increased temperature. It varied for anthracite and bituminous samples, and asymptotically approached a limiting value of 3 in this study.

Gas samples were obtained from four abandoned mine fire sites. 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%. In the field studies, CO was usually detected only when the O2 concentration was less than 8%, indicating that CO is produced by combustion reaction in an O2 deficient environment. The ratio N2/O2 was used to estimate the degree of O2 deficiency.

At elevated temperatures, methane and other alkane hydrocarbons are desorbed from coal. In laboratory and field studies, it has been shown that as the temperature increases, the concentrations of ethane, propane, butane and pentane increase relative to the concentration of methane. A ratio of higher molecular weight hydrocarbons to total hydrocarbons was used to distinguish combustion and non-combustion areas in abandoned mines.

Laboratory experiments and field studies at abandoned coal mine fires have shown that gas composition is an accurate indicator of the extent and location of combustion zones, the relative concentration of O2, and the generation of combustion products.