Coal Mine Methane Science Studies Road Map

Background

This document provides a road map for the coal mine methane (CMM) science studies that are 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 split between underground and surface mining, there is a wide geographic diversity of the balance between underground and surface mining, with the former accounting for 95% of production in China and only 10% in India (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 the 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 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 allows consistent comparisons between BU and TD approaches. As an extension of the emission quantification activities, the BU teams in collaboration with IMEO will translate the insights of patterns and differences in emission levels into a research agenda for identifying mitigation opportunities.

           

Science Studies Roadmap

The CMM science studies road map is primarily designed to improve our understanding of the spatial and temporal complexity of emissions estimations, and to develop a robust approach for reconciling BU and TD at the mine-level, i.e., the level at which mitigation must be planned. As a result of the above analyses, potential patterns and differences in emission levels and official calculation/reporting procedures across mines and countries may be identified. These patterns and differences are important for interpreting consistencies and differences in potential future monitoring, reporting, and verification (MRV) programs. By following measurement-based approaches that employ a variety of techniques we intend to both refine BU engineering calculations and TD atmospheric modeling. One of the expected factors requiring careful examination relates to temporal variability of emissions and the ability of MRV methods to capture such behavior. With greater confidence with respect to emissions estimations the potential benefits of mitigation should become clearer. We aim to answer the following questions:

 

1. How accurate are current coal mine methane emission estimates produced by mine operators, national agencies and international bodies?

2. What parameters and methodologies are most appropriate for reporting CMM emissions?

3. How accurate are current estimations of methane drainage from coal mines?

4. Can methane safety sensor networks in mines be supplemented and dual tasked to provide accurate continuous reporting of ventilation air methane to the atmosphere?

5. At the mine level what are the diurnal, weekly, seasonal, and annual variations of CMM emissions?
    
6. How frequently are TD measurements needed for representative, unbiased estimates of annual CMM emissions?

7. What are the best TD atmospheric measurement and modeling techniques for verifying or validating BU estimates?

8. How accurate is satellite-based TD verification of BU estimates using flux quantification methods?

9. How effective are methane destruction systems for VAM?

 

Ongoing Science Studies

The first IMEO CMM science studies project has scientific leadership provided by the German Aerospace Center (DLR) Institute of Atmospheric Physics, the Technical University of Brunswick, the University of Science and Technology in Kraków, and the Technical University of Munich. Three campaigns in the Upper Silesia Basin of Poland in the Springs of 2022 and 2023 and Autumn of 2022 are designed to start to answer to a greater and lesser extent the science questions noted above.

The DLR Institute of Atmospheric Physics is responsible for scientific design of the 60 flight hours of aircraft operations for this on-going project in the Upper Silesia Basin. In contrast to a prior campaign called CoMET 1.0 (carbon dioxide and methane mission) performed in 2017, the IMEO study measures at the mine-level and targets specific mines that have agreed to assist by gathering relevant mine-based activity data for the project. In collaboration with mine operators the Kraków’s University of Science and Technology Department of Mining and Geoengineering will estimate emissions through analysis of methane safety sensor data. These estimates will be compared with normal mine reporting approaches and with other measured flux estimates. In addition, Kraków’s University of Science and Technology Department of Physics and Applied Computer Science are providing ground scoping for aircraft operations with mobile methane measurements. Upon selection of a suitable target ventilation shaft a vehicle housed measurement system can guide the ground location of methane plumes that are accessible by road. This ground truthing assists with locating and observing the mine ventilation shaft plume through the flux measurement approach deployed here by helicopter. The unique and state-of-the-art HeliPOD sonde system, designed and managed by the Technical University of Braunschweig, is an instrument array that can be carried lower, slower, and closer to infrastructure than is possible within fixed wing manned aircraft.

In the Spring of 2023 aerial quantification estimates are complimented by ground-based quantification performed by the Technical University of Munich with two sun viewing FT-IR remote sensing analyzers (EM27) that will be placed upwind and downwind around a given target to enable continuous plume quantification. Kraków’s University of Science and Technology Department of Physics and Applied Computer Science will also perform open path LIDAR (TDLAS) methane measurements of the ventilation shaft air to provide measurements. Additional TD measurements are provided with an airborne GHGSat demonstrator (ESA-project) together with access to GHGSat and other available satellite data. Analysis of the outcomes of all TD (HeliPOD, FT-IR array, and satellites) and BU (Inventory, safety sensors, and shaft measurements) approaches in this project help address CMM science study questions noted earlier.

 

Future Science Studies

Additional science studies are planned for Australia, India, Canada and the US to enhance and expand the findings of CMM emissions from Polish coal mines in Europe’s Upper Silesia Basin. In 2022 a pilot project for estimating emissions from surface coal mining was performed in Australia’s Bowen Basin. Published satellite derived estimates of methane emissions from coal mining in the Bowen Basin are far larger than inventory estimates (Sadavarte et al., 2021). But satellite-based identification and quantification of large methane point sources are rarely coupled with local atmospheric measurements, which are key to increase confidence in the satellite-based emission estimates. Verifying satellite estimates is important as this method is emerging and offers regular observations across the entire world.

This Australian pilot project aims to provide data to help design future verification procedures for satellite CMM emissions using local measurements including airborne and ground-based methods. Due to their large spatial coverage and high spatial resolution TROPOMI satellite observations were initially used to derive methane concentration data. The pilot project already demonstrated that airborne in-situ approaches can quantify methane emissions from open cut coal mines at varying magnitudes of methane emissions and identify individual emission sources within an open cut mine. Future measurements in Australian coal mining regions will further BU and TD reconciliation of individual mines including the validation of satellite derived emission estimates.

 

References

Durucan S, Korre A., and Dr Zhenggang Nie Z., (2022). Coal Mine Methane Emissions: Sources, Mitigation Potential, Monitoring and Emissions Quantification, Imperial College London Consultants, Report 4 August 2022, p1-79. Available upon request.

            IEA (2022), Global Methane Tracker 2022, IEA, Paris, https://www.iea.org/reports/global-methane-tracker-2022.

            GEM, (2022) Global Energy Monitor, https://globalenergymonitor.org/projects/global-coal-mine-tracker/.

            Saunois, M., Stavert, A. R., Poulter, B., Bousquet, P., Canadell, J. G., Jackson, R. B., Raymond, P. A., Dlugokencky, et al., (2020), The Global Methane Budget 2000–2017, Earth Syst. Sci. Data, 12, 1561–1623, https://doi.org/10.5194/essd-12-1561-2020, 2020.

            Sadavarte P., Pandey S., Maasakkers J.D., Lorente A., Borsdorff T., van der Gon H.D., Houweling S, and Ilse Aben I., (2021), Methane Emissions from Super emitting Coal Mines in Australia Quantified Using TROPOMI Satellite Observations, Environ. Sci. Technol., 2021 55 (24), 16573-16580, https://doi 10.1021/acs.est.1c03976.

            Tate R.D. (2022) Bigger than Oil or Gas? Sizing up Coal Mine Methane 2022, Global Energy Monitor, Report March 2022, p1-27, https://globalenergymonitor.org/report/worse-than-oil-or-gas/

            UNECE, (2021), Best Practice Guidance for Effective Management of Coal Mine Methane at National Level: Monitoring, Reporting, Verification and Mitigation, ECE ENERGY SERIES No. 71, p1-87., ISBN: 978-92-1-117287-4, https://unece.org/sustainable-energy/publications/best-practice-guidance-effective-management-coal-….

            US EPA, (2022) Coal Bed Methane Outreach Program, https://www.epa.gov/cmop.