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 mechanised ...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 40m 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.
•The in situ pressure regimes and gas composition at different mining levels of an ultra-thick coal seam were monitored.•Gas pressure and composition response to face advance of the coal seam around a LTCC extraction have been evaluated.•A conceptual gas emission model for a longwall top coal caving panel was developed.
The longwall top coal caving method, which enables the most productive exploitation of thick/ultra-thick coal seams, may result in a distinct geomechanical response of strata and associated gas ...emission patterns around longwall layouts. A two-way sequential coupling of a geomechanical and a reservoir simulator for the modelling of gas emissions around a longwall top coal caving (LTCC) panels was developed building on the understanding established from the analysis of in-situ gas pressure and concentration measurements carried out at Coal Mine Velenje in Slovenia. Model findings have shown that the modelling method implemented can reproduce the dynamic changes of stresses and gas pressure around a LTCC face and predict the total gas emissions and mixed gas concentrations accurately. It was found that, in LTCC panels, although the rate of gas emission from mined coal depends highly on the coal face advance, floor coal and roof goaf act as a constant and steady gas source accounting for a considerable part of the overall gas emission. Research has shown that, at first and/or second mining levels of multi-level LTCC mining, a notable stress relief and pore pressure drop induced by fracturing of the mined and roof coal can be experienced within 40m ahead of the face-line. In the floor coal, on the other hand, the pore pressure change was found to extend to 20m below the mining horizon. Model results have clearly shown the permeability enhancement and gas mobilisation zones around the LTCC panel, which can be the target zones for gas drainage boreholes.
•A two-way sequential coupling approach between a geomechanical and a reservoir simulator was developed.•The model was applied to simulate gas emissions around a longwall top coal caving panel.•Gas emission zones around LTCC panels were successfully identified.
Coal gas outbursts (especially CO2) present a high risk in mining of lignite in the Velenje Coal Mine, located in the Velenje Basin in northern Slovenia. A programme of monitoring geochemical ...parameters was set up to help understand the behaviour of the coalbed gas distribution in advance of the working face using mass spectrometric methods to study its molecular and isotopic compositions and origin. Coalbed gas samples from four different excavation fields (G2/C and K.-130/A from the north and south Preloge mining area and K.-5/A and K.-50/C from the Pesje mining area), which were operational between the years 2010 and 2011 were investigated. The major gas components are CO2 and methane. Temporal changes in the chemical and isotopic composition of free seam gases were observed within boreholes as a function of the advancement of the working face. The study also revealed that at a distance of around 120m from the working face, the influence of coal exploitation by the Velenje Longwall Mining Method causes coalbed gas to migrate. At a distance of 70m the lignite structure is crushed causing desorption of fixed CO2 from the coal. Differences in coalbed gas composition at the longwall panels which underlie the unmined area or under previously mined areas were found. A high CDMI {=CO2/(CO2+CH4)100 (%)} index with values up to 95.6% was typical for areas of pre-mined excavation fields (South Preloge K.-130/A and Pesje area K.-5/A), while in excavation fields with no previous mining activity (North Preloge G2/C and Pesje area K.-50/C) up to 61.9 vol % of CH4 was detected. The concentration measurements and isotopic studies revealed endogenic CO2 (including CO2 originating from dissolution of carbonates) with δ13CCO2 values ranging from −7.0‰ to 5.5‰, microbial methane and CO2 with values ranging from −70.4 to −50‰ and from −11.0 to −7.0‰, respectively. Higher δ13CCH4 values ranging from −50 to −19.8‰ could be attributed to so-called secondary processes influencing the δ13CCH4 value, such as migration due to lignite excavation (escape of isotopically lighter methane). In excavation fields (G2/C and K.-50/C) with no-premining activity higher δ13CCH4 values could also be explained by migration of methane from deeper strata. The δ13CCH4 value also depended on the depth of the excavation field; at shallower levels of the excavation field (K.-5/A) a lower δ13CCH4 value was traced indicating microbial gas, while at deeper levels higher δ13CCH4 values were found.
•Coalbed gas distribution and origin were investigated in Velenje Basin.•CO2 is a major gas component at investigated excavation fields.•Rate of advancement of the working face influences coalbed gas distribution.•Methane was formed through microbial processes, while CO2 is endogenic in origin.•CH4 is accumulated at no pre-mined areas, while CO2 accumulates in pre-mined areas.
Stable isotopes of carbon were used to trace organic and inorganic carbon cycles and biogeochemical processes, especially methanogenesis within different geologic substrates of the Pliocene ...lignite-bearing Velenje Basin in northern Slovenia. Lithotypes of lignite, coalbed gases, calcified woods (xylites), carbonate-rich sediments, and groundwaters were investigated. Carbon isotope (δ13C) values of the different lignite lithotypes ranged from −28.1 to −23.0‰, with the variability likely a function of the original isotopic heterogeneity of the source plant materials and subsequent biogeochemical processes (i.e. gelification, fusinitization, mineralization of organic matter) during the early stage of biomass accumulation and diagenesis. In the lignite seam, CO2 and CH4 were the major gas components with small amounts of N2. The carbon isotope values of CO2 (δ13CCO2) and CH4 (δ13CCH4) were highly variable, ranging from −9.7 to 0.6‰ and −70.5 to −34.2‰, respectively. Carbon dioxide is likely sourced from a mixture of in situ microbial activity and external CO2, while CH4 is dominantly sourced from microbial methanogenesis, with possible addition of thermogenic gas from deeper formations, and the influence of microbial oxidation of methane. Calcified xylites enriched with 13C (δ13C values up to 16.7‰) indicate that microbial methanogenesis was active during formation of the basin. The δ13CDIC values (from −17.4 to −3.2‰) of groundwaters recharging the basin from the Triassic aquifer are consistent with degradation of organic matter and dissolution of dolomite. Groundwaters from the Pliocene sandy and Lithotamnium carbonate aquifers have δ13CDIC values (from −9.1 to 0.2‰) suggestive of degradation of organic matter and enrichment via microbial reduction of CO2.
► In this study we examine carbon cycling in Coal Basin. ► We study stable carbon isotopes in different geological media. ► Majority of methane was generated via microbial methanogenesis. ► Calcite precipitation was driven by microbial methanogenesis via CO2 reduction. ► Highly alkaline waters in the basin provide further evidence of microbial methane.
Underground coal gasification (UCG) is a viable possibility for the exploitation of vast coal deposits that are unreachable by conventional mining and can meet the energy, economic and environmental ...demands of the 21st century. Due to the complexity of the process, and the site-specific coal and seam properties, it is important to acknowledge all the available data and past experiences, in order to conduct a successful UCG operation. Slovenia has huge unmined reserves of coal, and therefore offers the possibility of an alternative use of this domestic primary energy source. According to the available underground coal gasification technology, the energy and economic assessment for the exploitation of coal to generate electricity and heat was made. A new procedure for the estimation of the energy efficiency of the coal gasification process, which is also used to compare the energy analyses for different examples of coal exploitation, was proposed, as well as the technological schemes and plant operating mode in Velenje, and the use of produced synthetic coal gas (syngas). The proposed location for the pilot demonstration experiment in Velenje Coal Mine was reviewed and the viability of the underground coal gasification project in Velenje was determined.
nema
Uncontrolled and excessive gas emissions pose a serious threat to safety in underground coal mining. In a recently completed research project, a suite of monitoring techniques were employed to assess ...the dynamic response of the coal seam being mined to longwall face advance at Coal Mine Velenje in Slovenia. Together with continuous monitoring of gas emissions, two seismic tomography measurement campaigns and a microseismic monitoring programme were implemented at one longwall top coal caving panel. Over 2000 microseismic events were recorded during a period of four months. Over the same period, there also was a recorded episode of relatively high gas emission in the same longwall district. In this paper, a detailed analysis of the processed microseismic data collected during the same monitoring period is presented. Specifically, the analysis includes the spatial distribution of the microseismic events with respect to the longwall face advance, the magnitude of the energy released per week and its temporal evolution. Examination of the spatial distribution of the recorded microseismic events has shown that most of the microseismic activity occurred ahead of the advancing face. Furthermore, the analysis of the gas emission and microseismic monitoring data has suggested that there is a direct correlation between microseismicity and gas emission rate, and that gas emission rate tends to reach a peak when seismic energy increases dramatically. It is believed that localised stress concentration over a relatively strong xylite-rich zone and its eventual failure, which was also identified by the seismic tomography measurements, may have triggered the heightened microseismic activity and the excessive gas emission episode experienced at the longwall panel monitored.
•Over 2000 micro-seismic events were recorded at a longwall top coal caving panel.•The spatial distribution of microseismic events was evaluated against face advance.•An episode of increased gas emission was correlated with mining induced microseismic activities.•A zone with heterogeneous coal structure was successfully detected by seismic monitoring.
The TOPS project takes a radical and holistic approach to coupled UCG-CCS, and thus the site selection criteria for the coupled processes, considering both geological, reservoir and process ...engineering aspects and different end-uses of the produced synthetic gas in order to optimise the whole value chain. In particular, the experimental research carried out utilises a newly constructed high pressure gasification reactor investigating several prospective options of UCG technology implementations. Integrated research addresses field based technology knowledge gaps, such as cavity progression and geomechanics, potential groundwater contamination and subsidence impacts, together with research into process engineering solutions in order to assess the role/impact of site specific factors and selected reagents on the operability of given CO2 emission mitigation options. Ultimately, research aims to minimise the need for on-site CO2 storage capacity as well as maximising the economic yield of UCG through value added end products.
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