Underground mining

Fugitive emissions methodology, underground mining. Calculates methane (CH₄) emissions based on the quantity of coal produced. Globally applicable.

Summary

This methodology represents methane (CH₄) emissions associated with the surface mining of coal. The data and calculation methodology are sourced from the IPCC, as published in Volume 2, Chapter 4 - Fugitive Emissions of their 2006 IPCC Guidelines for National Greenhouse Gas Inventories.

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The methodology

Emissions model

Fugitive emissions of greenhouse gases can arise during underground coal mining through several pathways:

• Mining emissions: CH₄ emissions result from the liberation of gases during the breakage of coal and surrounding strata during mining operations

• Post-mining emissions: CH₄ is additionally released from coal during processing and transportation

• Low temperature oxidation: CO₂ is produced by the oxidation of coal carbon (C) on exposure to oxygen

• Uncontrolled oxidation: CO₂ is produced during the uncontrolled burning of coal. This may occur as a consequence of the trapping of heat produced by low temperature oxidation.

Only the first two sources of emissions are considered in this methodology: low temperature oxidation is typically insignificant (especially where rehabilitation practices are employed); uncontrolled oxidation is sometimes significant but extremely difficult to quantify.

The methodology considers the three emissions components in order to calculate net annual emissions:

• CH_4,total (kg) = CH_4,mining + CH_{4,post-mining} - CH_4,recovered

Emissions associated with mining operations: This methodology calculates mining-associated CH₄ emissions based on emissions factors which describe the rate at which CH₄ is released in relation to the quantity of coal produced (in terms of mass, i.e. tonnes). By multiplying these rates (e.g. m³ per tonne) by the annual quantity of coal produced (e.g. tonne per year), an estimate of the annual methane emissions - by volume (i.e. m³ per year) - is made.

Emissions associated with the post-mining phase: This methodology provides two ways of calculating post-mining emissions. (1) As with mining-phase emissions (see above), an emissions factor is available which describes the rate at which post-mining CH₄ is released in relation to the quantity of coal produced. (2) Alternatively, a more accurate post-mining emissions factor can be derived on the basis of the in situ gas content of the coal, since a fixed proportion of this is estimated to be emitted subsequent to extraction. The precise precise fraction of this in situ CH₄ which is released differs depending on whether pre-drainage of mines is practised.

Methane recovery: Where methane recovery/capture is practised, this is deducted from the emissions calculation.

All emissions quantities are calculated on a volumetric basis and converted to mass quantities using the standard value for CH₄ density provided by the IPCC guidelines.

Model data

The extent to which CH₄ is released depends on the depth of the mine. Emissions factors are provided for three depth ranges (<200 m, 200-400 m, >400 m). Each depth range is represented by:

• A mining phase emissions factor (m³ per tonne)

• A post-mining phase emission factor (m³ per tonne)

The methodology also uses several additional values for making the final emissions calculations

• An in situ gas factor which represents the proportion of the in situ gas which is release in the post-mining phase. This factor depends in whether pre-draining of the mine is practised (pre-drainage, 30%; no pre-drainage, 10%)

• A standard value for the density of CH₄ (0.67 tonnes per m³) which enables the conversion of volumetric to mass-based emissions

• The global warming potential of CH₄ which enables the conversion of absolute CH₄ emissions into a CO₂e quantity - i.e. the quantity of CO₂ which would exert the same atmospheric warming effect

Activity data required

Greenhouse gas emissions are directly proportionate to the mass of coal produced annually, which therefore must be provided in order to calculate.

The specification of the in situ gas content of the mined coal is optional but may result in a more accurate estimate of post-mining emissions.

Also optional is the specification of a volume of recovered methane

Calculation and results

Both mining and post-mining CH₄ emissions are calculated and aggregated. Any recovered CH₄ is then subtracted from this total.

Two emissions quantities are ultimately provided, representing: CH₄ and CO₂e emissions. All emissions calculated by this methodology represent those attributable to the specified annual quantity of coal production, minus any CH₄ which is recovered.

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Related methodologies

Other IPCC methodologies which focus on mining-associated fugitive emissions scenarios are available, covering surface mining, abandoned underground mines (tier 1 and tier 2) and methane flaring.

Where recovered methane is burned for energy production, flared, or fed into a gas distribution system, the IPCC methodologies for stationary fuel combustion, methane flaring and oil and gas processing may be of interest.