Sorption of methane on lignite from Polish deposits

Abstract

Abstract Coal samples from Miocene lignite deposits (ortholignite—ECE-UN 2002) in Belchatow, Adamow, Konin and Turow (Poland) were analyzed to determine the relationships between coal properties and gas capacity. Investigations presented here addressed the occurrence of methane sorbet in lignite deposits within Poland's largest penetrated lignite deposits (e.g., Belchatow, Adamow, Turow and Konin). Lignite samples collected from surface mines were detritic coal with variable contents of xylites. The highest level of xylites (up to 25 vol.%) was found in carbon samples taken from the Belchatow deposit. Samples from other mines contained no more than 10 vol.% xylites. Petrographic compositions were dominated by huminite group macerals (73–88 vol.%) with atrinite, densinite and texto-ulminite, a porous form of ulminite; inertinite groups were less important components (4–8 vol.%). Isotherms were determined for methane sorption at 298 K with test pressure ranges below 1.2 MPa. At a pressure of 1.0 MPa, the largest gas capacity of approximately 1.7 [dm3 STP/kg] was found in the sample from Belchatow. This result may have been attributable to differences in the porosity of the samples; the porosity of the sample from Belchatow was twice as high as the porosities of the other coal samples. This variation in porosity resulted from the lithologic and maceral composition of the coal sample that contained substantial quantities of porous textinite and texto-ulminite. The thermal sorption equation was used to determine the limiting values of isosteric enthalpy of sorption, which suggested weak interactions between methane and the lignite matrix. The residual gas capacity of the tested samples was also determined. All samples exhibited a high residual methane-bearing capacity, which may not only cause methane to be released from coal at a pressure of 1 bar but also may pose a gas risk during mining operations.