Estimating methane emissions from coal mines

This article is part of the Global Coal Mine Tracker, a project of Global Energy Monitor.

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Global coal mines Coal mines in China Coal mines in India Coal mines in the United States Coal mines in Australia Coal mines in Russia Coal mines in Indonesia

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Coal mine methane (CMM) refers to methane (CH₄) released from the coal and surrounding rock strata due to mining activities. Global Energy Monitor's Global Coal Mine Tracker uses three factors to estimate the CH₄ emissions from coal mines:

• 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

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

Note: When only capacity data is available for operating coal mines in China, GEM scales output to a 73% utilization rate, which was the national average for mining and washing of coal in the first and second quarters of 2021.

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.
• GWP on a 100 year horizon: 28.

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.

In 2022, the Global Coal Mine Tracker began using the latest conversion factors of the IPCC's Sixth Assessment Report (2022).

• GWP on a 20 year horizon: 82.5.
• GWP on a 100 year horizon: 29.8

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:

"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:

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:

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	2014
China	456 meters	2012
Russia	423 meters	2015
Germany	1150 meters	2013
India	300 meters	2014
South Africa	80 meters	2013
United States	346 meters	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. 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.

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.

IPCC Methodologies

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

• 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.
• Tier 2 methodology uses country- or basin-specific emission factors that represent the average values for the coals being mined.
• Tier 3 methodology uses direct measurements on a mine-specific basis which, if properly applied, has the lowest level of uncertainty.

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. 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

Abandoned Mine Methane

When a coal mine is closed, methane emissions from the site can continue for years unless operators take proactive mitigation measures. Abandoned underground coal mines experience an initial decline in emissions, followed by a period of near-steady release that can persist for decades. This initial decline is much faster for flooded mines than it is for dry mines. Underground coal mines typically emit more methane than surface mines. GEM has not yet applied or developed a methodology for estimating methane emissions for abandoned surface mines.

Just as for Coal Mine Methane, for underground Abandoned Mine Methane (AMM) estimates GEM builds on the MC2M model laid out by Kholod et al. (2020) to calculate an initial gas flow rate. With this figure, we then apply one of two rate equations depending on whether the mine is dry or flooded. Additionally, emissions from flooded mines are near-zero after 8 years, and dry mines often flood eventually. To account for this, GEM sets emissions for all abandoned underground mines to zero after 8 years.

Model for Dry Abandoned Mine Methane

We use the following equation to calculate the dry rate of methane emissions from abandoned underground mines, which is drawn from Kholod et al. (2020). We assume that all methane vents to the atmosphere, and so set the “s” parameter below to 1.The dimensionless parameters Di and b are set to .672 and 2.016746, respectively. Per Kholod et al. (2020):

“q = qi * s (1 + b Di t) ^ (−1/b)

where,

q – gas flow rate at time

qi – initial gas flow rate at time zero (t0) (assumed to be 100%)

s – share of sealed mines, %.

t – elapsed time from t0 (years)

Di – initial decline rate, 1/year

b – the rate of change in the decline rate through time, dimensionless.”

Model for Flooded Abandoned Mine Methane

We use the following equation to calculate the wet rate of methane emissions from abandoned underground mines, which is drawn from Kholod et al. (2020). We assume that all methane vents to the atmosphere, and so set the “s” parameter below to 1. The dimensionless parameter Di is set to 0.304775.  Per Kholod et al. (2020):

Q = qi ℮^(−t Di)

where,

q – gas flow rate at time t

qi – initial gas flow rate at year of abandonment (t0)

t – elapsed time from t0 (years)

℮ – the constant (2.71828), the base of the natural logarithm

Di – decline rate, 1/year.