Abstract
Among the numerous factors that have an impact on coal permeability, coal porosity is one of the main parameters. A change in the mechanical stress applied to coal results in a change of porosity. The main objective of the conducted research was to answer the following question: is a decline in coal permeability a direct effect of a decrease in coal porosity, and does mechanical stress result solely in a porosity change? A study of coal porosity under mechanical stress conditions was conducted using a uniquely constructed measurement stand. The coal samples used were briquettes prepared from a granular coal material (middle-rank coal of type B—meta bituminous, upper carboniferous formation) from the “Zofiówka” coal mine, in Poland. In order to describe coal permeability, the Klinkenberg equation was used, as it takes into consideration the slippage effect, typical of porous media characterized by low permeability. On the basis of the obtained results, it was established that the values of the Klinkenberg permeability coefficient decrease as the mechanical stress and the corresponding reduction in porosity become greater. As the briquette porosity increased, the Klinkenberg slippage effect: (i) disappeared in the case of nitrogen, (ii) and was minor for methane. The briquettes used were characterized by various porosities and showed that mechanical stress results mainly in a change in coal porosity, which, in turn, reduces coal permeability.
Highlights
The phenomena of the sorption and transport of gases occurring in a solid-gas system are closely connected with the porous structure of sorbents
Among the numerous factors that have an impact on coal permeability, coal porosity is one of the main parameters neglected by previous studies
The relationship between the Darcy permeability coefficients k g and the average gas pressure p avg —where the value of k g decreases with rising pressure as a result of, among others, sorption swelling of coal [54,55], as well as the Klinkenberg effect declining along with pressure [56]—is widely known
Summary
The phenomena of the sorption and transport of gases occurring in a solid-gas system are closely connected with the porous structure of sorbents. Bituminous coal is such a sorbent whose pores form a vast and irregular network [1]. The share of macro- and mesopores in coal is small, with these two types of pores playing the major role in the processes of transporting gases [2,3]. The diffusion of gas within ultramicropores requires overcoming a substantial energy barrier, necessary to separate the walls of these pores [6,7]
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