Expansion And Contraction Of Coal At Various Metamorphism Under Influence Of Ch4 And Mixture Ch4 And Co2 Sorption/desorption Process
EXPANSION AND CONTRACTION OF COAL AT VARIOUS METAMORPHISM UNDER INFLUENCE OF CH4 AND MIXTURE CH4 AND CO2 SORPTION/DESORPTION PROCESS
G. Ceglarska-Stefańska, K. Zarębska, S. Hołda
Faculty of Fuels and Energy, University of Mining and Metallurgy, Cracow
It results from laboratory sorption studies carried out in the system bituminous coal – vapours, gases that the sorption/desorption processes are often accompanied by the change in the dimensions of the sorbent. The parallelism of sorption - expansion and desorption - contraction processes is a function of various mechanisms of the placement of sorbate molecules in the non-rigid structure of coal co-polymer. The problems of taking up the gases and vapours by the structure of coal substance and their desorption are interesting and all the time actual not only because of the attempts to describe the inhomogeneous structure of bituminous coals but also because it is closely linked to the phenomena of rock material and gas outbursts during the mining of coal. The working of coal disturbs the quasi-equilibrium state in the rock mass. The effect of this action in gassy mines, among others, could be violent outbursts of crumbled rock and gas from the body of coal into the excavation. The impact of various factors causing the outburst gives rise to it that in spite of numerous studies, there is no unequivocal description of this phenomenon up to date. It is generally assumed that the main factor and the driving force of the outburst is the gas contained in the bed. The amount of gas in coal depends on the type of coal. The possibility of the outburst to occur is also affected by the properties of the coal: its porosity, structure, petrographic composition, dampness, mineral content and the gas sorption and desorption ability.
It results from laboratory studies that the changes in the dimension of coal accompanying the sorption/desorption processes can in situ cause the changes in the stresses within the rock mass and be one of the factors causing the outburst. CO2, CH4, N2, water and water vapour are the main mine gases. The interactions in the system: bituminous coal of various degrees of metamorphism – single gases (CO2 or CH4) were many times described in the literature on the subject, whereas
very few works concern the sorption/desorption of the mixture of CO2 + CH4 (CO2-41.5%; CH4 - 58.5%). Aproject was therefore started to determine the differences in the changes of the dimensions of coals in the sorption/desorption of CO2 + CH4 and to compare them with the results obtained for methane. Thesorption studies were carried out by a volumetric method at 298K in the gas pressure range 0 - 4.0 MPa. The subject of the studies were the coals of various content of volatiles (B-Vdaf = 40.87%; M‑Vdaf = 22.32%) in the form of plates (15´15´5 mm) on to which strain gauges were glued parallel and perpendicular to the direction of stratification. The expansion and the contraction of the samples were calculated on the basis of the changes in the resistance of the gauges. It is generally accepted that the expansion of bituminous coals in the process of CO2 sorption is ca. three times higher than the expansion in the system coal – CH4. It is closely linked to the different mechanism of the placement of the sorbate molecules in the coal structure, and this in turn results from different physico-chemical properties (different kinetic diameters, CO2 – subcritical state, CH4 – supercritical state under the experiment conditions).
Expansion/contraction kinetics curves for the system bituminous coal – CO2+CH4 mixture are different from the expansion/contraction kinetics curves obtained in an experiment carried out under similar conditions for the system coal – methane. The observed disturbances in the coarse of the expansion kinetics curves can be explained by different diffusion rate of CO2 and CH4 into the cross-linked structure of coal co-polymer. In the studied range of gas pressures, the whole spectrum of the surface-bulk phenomena due to the particular properties of bituminous coal occurs. These particular properties are manifested by the discontinuities in elastic areas due, on the one hand, to the porosity of bituminous coal, on the other, to the occurrence of quasi-crystalline units of dimensions dependent on the degree of metamorphosis of the coal. The values of the expansion of the bituminous coal in the process of sorption of the mixture of gases are lower than those obtained for the system coal – CO2, but definitely higher than those for the system coal – CH4.
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