Effects of Coal Functional Groups on Adsorption Microheat of Coal Bed Methane

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Abstract

This study measured the adsorption heat for five groups of Chinese coal samples of different ranks at 15 °C and 0.101 MPa using a C80 microcalorimeter. The functional groups of the coal samples were determined by infrared spectroscopy according to quantum chemical theory. The effects of coalification and coal functional groups on the adsorption heat of coal for methane were discussed in terms of energy. As a result, this study has further perfected the adsorption theory of coal bed methane. The results show that the adsorption heat of coal for methane first increases, then decreases with increasing coal rank and reaches a minimum at the fat coal stage. This indicates that coalification has a significant effect on the characteristics of the adsorption heat of coal for methane. These effects can be clearly classified into stages. Coalification influences the adsorption heat for methane by changing the type of coal, the content of oxygen-containing functional groups, and the pore structure. Oxygen-containing functional groups influence the adsorption heat of coal for methane via the adsorption potential of the methane molecules. In the long-flame coal stage, a high content of oxygen-containing functional groups leads to a high adsorption heat of the coal for methane. In the fat coal to coking stages, large numbers of aliphatic series, aliphatic functional groups, and side chains of aromatic condensed nuclei are removed from the coal molecules under the influence of mechanical compression and dehydration. The decrease in oxygen-containing functional groups results in a decrease in the adsorption potential of the coal and a minimum value of its adsorption heat for methane. In the coking coal stage, the adsorption heat for methane changes little because of the weak influences of mechanical compression and dehydration. In the high-rank coal stage (lean coal and anthracite), the internal surface area of the coal increases, with micropores and transition pores caused by the significantly higher degree of aromatization of the coal. This increase in internal surface area improves the adsorption heat of coal for methane to some extent.

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