Coalbed gases in excavation fields of mining areas in the Velenje coal basin have been subjected to geochemical and isotopic monitoring since the year 2000, with the aim of obtaining better insights into the origin of coalbed gases. Results from active excavation fields in the mining areas Pesje and Preloge in the year 2013 are presented in this study. Composition and isotopic composition of coalbed gases were determined with methods utilizing mass spectrometry. The chemical and isotopic composition of coalbed gases in the Velenje Basin vary and depend on the composition of the source of coalbed gas before excavation, advancement of the working face, depth of the longwall face, pre-mining activity and newly mined activity. The basic gas components determined in excavation fields are CO2 and methane. The isotopic composition of deuterium in methane has been determined and used to characterize the origin of methane. The isotopic compositions of carbon and hydrogen in methane in the excavation fields show its biogenic origin, while a high Carbon Dioxide Methane Index indicates the bacterial and endogenic origin of CO2.
CO2 concentrations (partial pressure of CO2, pCO2), and isotope compositions of carbon dioxide in air (d13CCO2), temperature (T) and relative humidity (H) have been measured in the atmosphere in the Velenje Basin. Samples were taken monthly in the calendar year 2011 from 9 locations in the area where the largest thermal power plant in Slovenia with the greatest emission of CO2 to the atmosphere (around 4M t/year) is located. Values of pCO2 ranged from 239 ppm to 460 ppm with an average value of 294 ppm, below the average atmospheric CO2 pressure (360 ppm). d13CCO2 ranged from -18.0‰ to -6.4‰, with an average value of -11.7‰. These values are similar to those measured in Wroclaw, Poland. The isotopic composition of dissolved inorganic carbon (d13CDIC) in rivers and lakes from the Velenje basin changes seasonally in the range from -13.5‰ to -7.1‰. The values of d13CDIC indicate the occurrence of biogeochemical processes in the surface waters, dissolution of carbonates and degradation of organic matter being the most important. A concentration and diffusion model was used to calculate the time of equilibration between dissolved inorganic carbon in natural sources (rivers) and atmospheric CO2.
The application of multi-level longwall top coal caving mining method in ultra-thick coal seams generally yields a much higher productivity and is more efficient in comparison to a mechanized single-slice longwall panel. However, the greater productivity achieved by this mining method may further exacerbate the gas emission problems often faced in longwall mining. In order to establish a thorough understanding of gas pressure regimes, and gas emission patterns around a producing multi-level longwall top coal caving face, a suite of in-situ measurements on seam gas pressure, gas composition, and ventilation environment was conducted at Coal Mine Velenje in Slovenia. This paper focuses on the analysis of these field observations which helped develop a conceptual gas emission model for multi-level longwall top coal caving mining of ultra-thick coal seams. It has been found that, at Coal Mine Velenje, the coal zone within 40 m ahead of the face can significantly contribute to the overall district gas emission. In addition, floor coal and roof goaf may both play a major role towards the total gas emitted during mining.