Estimating methane emissions from coal mines

From Global Energy Monitor
This article is part of the Global Coal Mine Tracker, a project of Global Energy Monitor.
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Coal mine methane (CMM) refers to methane (CH4) released from the coal and surrounding rock strata due to mining activities. Three factors can be used to estimate the CH4 emissions from coal mines:[1]

  • Production
  • Gas content at mining depth
  • Emission factor coefficient


Formula

The CH4 emissions for a coal mine can be calculated with the following formula:

Annual CH4 (in million cubic meters) = Production * Gas content at mining depth * Emission factor coefficient[1]

Example for a deep, underground bituminous coal mine, with a depth of 465m:

  • Production: 5 million tonnes per annum
  • Gas content at mining depth: 13.4 m3/tonne
  • Emission factor coefficient: 1.6


Based on these parameters, the annual CH4 for the mine is as follows:
5 * 13.4 * 1.6 = 107.2 million cubic meters per year

Conversions

Methane emissions estimates in million cubic meters can be converted into CO2equivalent (CO2e) based on the global warming potential of methane. The Intergovernmental Panel on Climate Change (IPCC)'s Fifth Assessment Report (2013) provide the global warming potential (GWP) for methane averaged over 20 and 100 year timespans.

  • GWP on a 20 year horizon: 84. [2]
  • GWP on a 100 year horizon: 28. [2]


This means releasing 1 kg of methane is equivalent to releasing 84 kg of CO2 averaged on a 20 year horizon and releasing 1 kg of methane is equivalent to releasing 28 kg of CO2 on a 100 year horizon.[2]

Gas Content

The methane gas content can be calculated using the Langmuir isotherm formula for the appropriate rank coal at the depth of mining.

According to a 2020 study in the Journal of Cleaner Production by Nazar Kholod, Meredyd Evans, Raymond C.Pilcher, Volha Roshchankac, Felicia Ruiz, Michael Cotée, and Ron Collings:[1]

"The equation that predicts the amount of gas that may be contained by coal at a certain pressure, or depth, is known as an adsorption isotherm. Isotherms are commonly expressed by mathematical equations ... used by engineers and scientists involved in designing coal mines and their gas drainage and ventilation systems. These mathematical equations, which are now an industry standard, were developed by Irving Langmuir (1918). The Langmuir equations are generally accepted as the best model for defining gas sorption capacity of coal."

The study defined the Langmuir equation as follows:[1]

EF = VL (d* L) / (PL + d*L)

where

EF is the emission factor

VL is the Langmuir volume coal sample

PL is the Langmuir pressure of that sample

L is the Langmuir constant

d is the mining depth (meters).

Model for Calculating Coal Mine Methane (MC2M)

On average, methane content per ton of coal increases with depth, although the amount varies by coal rank. In their 2020 study in the Journal of Cleaner Production, Kholod et al. developed a Model for Calculating Coal Mine Methane (MC2M) that relied the Langmuir isotherm model to estimate the expected gas content value for a given rank of coal mined at a given depth, based on isotherm testing from many coal basins around the world. The study published the following graph of gas content by coal rank and mining depth, to demonstrate their findings:[1]


Fig. 1. Gas content by coal rank and mining depth

Mine Depth

The depths of coal mines are averaged at the national level in a variety of industry and government reports. When an exact mine depth is unknown, the GEM's Global Coal Mine Tracker uses the most recent and best available estimate.

A sampling of underground coal mine estimates are found in Table 1:

T1: Depth of underground coal mines
Country Depth Year
Australia 500 meters[3] 2014
China 456 meters[4] 2012
Russia 423 meters[5] 2015
Germany 1150 meters[6] 2013
India 300 meters[7] 2014
South Africa 80 meters[8] 2013
United States 346 meters[9] 2017

Emissions Factor Coefficient

The emissions factor coefficient is the average ratio of emissions to gas content. In their 2020 paper, Kholod et al. use a global average emission factor coefficient of 1.7 to project future emissions through 2100. The study based the emission factor coefficient on a central baseline averaged across 6 models. The ratio of emissions to gas content in those models ranged from 1 to 2.3.[1] While the study used 1.7 for projecting future emissions, the study's underlying data documented a current global average of 1.6 for hard coal underground mines and 1.5 for hard coal surface mines.[10]

Using these emission factors, the Model for Calculating Coal Mine Methane (MC2M) found global gas content ranged between 13 and 18 meters per metric ton (m3/t) for underground mines (450 to 1120 meters) and between 3 and 5 m3/t for surface mines (50 to 200 meters). Those figures remained compatible with the Intergovernmental Panel on Climate Change (IPCC) which has suggested using three distinct emission factors depending on mining depth: an emission factor of 10 m3/t for underground mines at depths less than 200 m, 18 m3/t for depths between 200 and 400 m, and 25 m3/t for mines deeper than 400 meters.[1]

IPCC Methodologies

The methodology for estimating fugitive emissions from coal mining and handling, developed by the Intergovernmental Panel on Climate Change (IPCC), is tiered:[11]

  • Tier 1 methodology requires that countries choose from a global average range of emission factors and use country-specific activity data to calculate total emissions. This method is associated with the highest level of uncertainty.[11]
  • Tier 2 methodology uses country- or basin-specific emission factors that represent the average values for the coals being mined.[11]
  • Tier 3 methodology uses direct measurements on a mine-specific basis which, if properly applied, has the lowest level of uncertainty.[11]

Global Coal Mine Tracker

The Global Coal Mine Tracker uses the Kholod et al. Model for Calculating Coal Mine Methane (MC2M) replacing global estimates with actual mine-specific data when available.

Using a combination of global and mine-specific parameters results in a hybrid approach of the IPCC Guidelines for National Greenhouse Gas Inventories.[11] This hybrid approach of the Global Coal Mine Tracker includes:

  • Tier 1 data: Global average emission factor (the coefficients of 1.5 and 1.6).
  • Tier 3 data: Mine specific activity data regarding production and depth.


If mine specific data for depth is unavailable, the Global Coal Mine Tracker relies on

  • Tier 2 data: Country-level or basin-level depth estimates.
  • Tier 3 data: Global-level depth estimates.


Global Coal Mine Tracker, Methane Emissions Estimates
Mine Mine Type Coal Type Production (mtpa) Depth (m) Depth Accuracy Gas Content (m^3/tonne) Emission Factor Annual CH4 (in million cubic meters)
Bengalla Coal Mine Surface Lignite 8.3 270 Exact 3.2 1.5 39.8
Wangjiata Coal Mine Underground Bituminous 8 456 Country Estimate 13.2 1.6 169
Pervomaisky Coal Mine Surface Bituminous 7 50 Global Estimate 3 1.5 13.5

Articles and resources

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Nazar Kholod, Meredydd Evans, Raymond C. Pilcher, Volha Roshchanka, Felicia Ruiz, Michael Coté, and Ron Collings, "Global methane emissions from coal mining to continue growing even with declining coal production," Journal of Cleaner Production, Volume 256, February 2020.
  2. 2.0 2.1 2.2 Intergovernmal Panel on Climate Change, AR5 Fifth Assessment Report, 2013, p. 714.
  3. Commonwealth of Australia, Subsidence from Coal Mining Activities: Background Review, 2014.
  4. X. He and L. Song, Status and Future Tasks of Coal Mining safety in China, Safety Science, 2012
  5. Government of the Russian Federation, Development program of the coal industry by 2030, 2014
  6. S. Prusek,"Review of Support Systems and Methods for Prediction of Gateroards Deformation," in New Techniques and Technologies in Mining: School of Underground Mining (London: Taylor and Francis Group, 2010): 25-35
  7. Government of India, Ministiry of Coal, Coal Directory of India 2013 – 2014, Coal Statistics, 2014
  8. J.N. van der Merwe and M. Mathey Update of coal pillar database for South African coal mining, The Journal of The Southern African Institute of Mining and Metallurgy, 2013: 825-840.
  9. US Longwall Census, Coal Age, 2018.
  10. Raymond Pilcher (methane expert and MC2M study co-author), personal communication, September 2020.
  11. 11.0 11.1 11.2 11.3 11.4 Intergovernmental Panel on Climate Change (IPCC), Chapter 4- Fugitive Emissions, Volume 2 Energy, 2006.

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External resources

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