Background This document provides a road map for the coal mine methane (CMM) science studies coordinated by the United Nations Environment Programme’s International Methane Emissions Observatory (IMEO). It summarizes and builds upon an IMEO commissioned literature review report (Phase 1) entitled “Coal Mine Methane Emissions: Sources, Mitigation Potential, Monitoring and Emissions Quantification” written by Durucan et al. (2022). After introducing our current understanding of CMM emissions we present a road map for CMM field measurement studies (Phase 2). This road map is set by a series of questions designed to assess our current understanding of global CMM emissions to the atmosphere, provide information to fill knowledge gaps, improve reporting of emissions, and facilitate mitigation of methane emissions from specific CMM sources. We provide further details for one on-going CMM science field measurement study being performed in Poland to clarify the goals of this road map. Finally, the next steps for the CMM science studies road map are noted.
Introduction Coal remains a major fuel in global energy markets and is used primarily by the metallurgical and electrical power generation industries. While global coal production is evenly split between underground and surface mining, there is wide variation of contributions for top producing countries (GEM, 2022). Emissions of methane from coal mining are from the release of gas trapped inside coal and strata surrounding mined coal seams. The vast majority of reported global CMM emissions are from underground coal mines. Coal mining represented 33% (42 Tg CH4 yr−1, range of 29–61) of global fossil-fuel-related methane emissions for the period 2008 to 2017 (Saunois et al., 2020). China leads the world in estimated CMM emissions, followed by Russia, Europe, US, India, Indonesia, and Australia (IEA, 2022). Emissions of methane from coal mining are reported as being comparable in magnitude to the oil and gas energy sectors (Tate, 2022). As part of the Global Methane Initiative the US EPA has promulgated a voluntary Coal Bed Methane Outreach Program targeting CMM emissions. This programme estimates that ~64% of projected 2030 CMM emissions can be avoided with existing mitigation technologies, and that this abatement potential persists through 2050 (US EPA 2022). At the national level guidance is available for effective management of CMM that includes monitoring, reporting, verification, and mitigation (UNECE, 2021).
Coal Mine Methane Emissions Coal mine methane emissions, whether underground or surface, are predicted or estimated today through bottom-up (BU) approaches that may use safety sensors measurements, but typically involve calculation of methane emissions based on emission factors derived from various parameters including the in-situ gas content of coal seam, depth of mining, and production volume. The in-situ methane content of coal is the primary parameter that is moderated by other variables to determine CMM emissions. The gas content of high rank coals (e.g., anthracite) is usually higher than that of low rank coals (e.g., lignite). Current IPCC methane emissions accounting methods use three different reporting levels for factors, moderated into low, medium and high ranges, to estimate CMM emissions for a given country, region, basin, or mine. The highest uncertainty is given by tier 1 emission factors that are global averages. Reported uncertainty is reduced with greater specificity if country, regional, or basin factors can be applied (tier 2). The lowest level of uncertainty is anticipated from tier 3 assessments that use mine specific data. A progression from tier 1 to tier 3 decreases uncertainty on methane emissions estimates reflecting better data granularity and increased precision of methane emissions factor for a given subset of sources. Activity data accuracy is important as this defines absolute emissions for the considered mine, region or country.
Coal Mine Methane Reporting There are significant variations in CMM emission factor values used in different parts of the world, coal basins, and mines, which are reasonable and relate to differences in coal resource characteristics and production methods used. But when considering annual country scale emissions, applied emission factors often have unknown spatial-temporal variability and their extrapolation may not truly represent the emissions being estimated. Although deeper coal seams generally release more methane than shallow seams of the same rank, in a single borehole or a narrow region, significant variability in gas contents is observed, when such data are examined basin wide. Different geological and structural processes, which are regional, coal field and mine specific, that would have taken place over geological time scales, affect the in-situ gas content of coal seams. At the mine level, CMM emissions may vary considerably over time. Average longwall face methane emission rates gradually increase during the productive life of a longwall district. Variability is also seen during a production week. Accounting for CMM emissions is complicated and conducted in different ways around the world. Other variables that influence calculations of emissions include the strength and reservoir properties of the surrounding strata, mining technology and pace of mine development. Longwall mining is the most used method in exploiting deeper coal deposits and is associated with the highest magnitude of emissions per ton of mined coal.
Coal Mine Methane Management and Mitigation Historically, and to the present day, methane management within underground mines is driven by safety concerns, given the possibility of explosive mixtures of methane in coal mine air. Pre-mining drainage of methane, achieved through boreholes that depressurize the coal seam, offers the first mitigation possibility. But in common with techniques that drain methane during mining, unless utilized or destroyed, it is vented to the atmosphere. Regardless of the method used, methane drainage does not recover all the gas from the source seams, nor it is effective against emissions from the mined seam during coal production. The best secondary mitigation possibility is to control ventilation air methane (VAM) that is normally exhausted from the ventilation shafts into the atmosphere and is the single largest source of CMM emissions globally, although it typically only contains 0.1% to 1.0% methane by volume. Technology which can destroy or utilize very low concentrations of VAM air by thermal oxidation requires substantial upfront investment. There is limited commercial scale thermal oxidation of VAM performed around the World. But experience in China, Australia and to a lesser extent the US has demonstrated the benefits of this technology for mitigation of CMM emissions. Given that a negligible amount of global CMM are used or destroyed, there is opportunity for increased mitigation and removal of CMM. As the importance of mitigating atmospheric emissions of methane from coal mining increases, greater scrutiny is needed to refine our understanding of emission fluxes so that the most appropriate mitigation approaches are identified. Currently, CMM emissions accounting methodologies generally rely on BU estimates, on-site emissions monitoring, engineering, and activity data to account for emissions at site level. Given the known uncertainties of BU estimates top-down measurement (TD) campaigns (aerial, satellite, or ground), whether spot or repeat, offer the possibility to verify current inventory estimates. The benefits of comparisons between BU and TD estimates have been widely demonstrated for the oil and gas energy sector, but rarely applied for coal mining. While TD estimates have their own limitations uncertainties are known. Uncertainty assessments of coupled BU and TD approaches allow for reconciliation between different methods, and this offers the possibility of refinement of existing inventory methods. Through the science studies program a CMM emission quantification approach will be designed across mines and countries in a way that