Choice of method

The fundamental equation for estimating emissions from abandoned underground coal mines is shown in Equation 4.1.9.

Equation 4.1.9

General equation for estimating fugitive emissions from abandoned underground

COAL MINES

CH4 emissions = Emissions from abandoned mines - CH4 emissions recovered

Developing emissions estimates from abandoned underground coal mines requires historical records. Figure 4.1.3 is a decision tree that shows how to determine which Tier to use.

Tier 1 and 2

The two key parameters used to estimate abandoned mine emissions for each mine (or group of mines) are the time (in years) elapsed since the mine was abandoned, relative to the year of the emissions inventory, and emission factors that take into account the mine's gassiness. If applicable and appropriate, methane recovery at specific mines can be incorporated for specific mines in a hybrid Tier 2 - Tier 3 approach (see below).

• Tier 2 incorporates coal-type-specific information and narrower time intervals for abandonment of coal mines.

• Tier 1 includes default values and broader time intervals.

For a Tier 1 approach, the emissions for a given inventory year can be calculated from Equation 4.1.10.

Equation 4.1.10 Tier 1 approach for abandoned underground mines

Methane Emissions = Number of Abandoned Coal Mines remaining unflooded • Fraction of gassy _Coal Mines • Emission Factor • Conversion Factor_

Where units are:

Methane Emissions (Gg year-1) Emission Factor (m3 year-1 )

Note: the Emission Factor has different units here compared with the definitions for underground, surface and post-mining emissions. This is because of the different method for estimating emissions from abandoned mines compared with underground or surface mining.

This equation is applied for each time interval, and emissions from each time interval are added to calculate the total emissions.

conversion Factor:

This is the density of CH4 and converts volume of CH4 to mass of CH4. The density is taken at 20°C and 1 atmosphere pressure and has a value of 0.67^ 10-6 Gg m-3.

Figure 4.1.3 Decision tree for abandoned underground coal mines

Figure 4.1.3 Decision tree for abandoned underground coal mines

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees

Note: See Volume 1 Chapter 4, "Methodological Choice and Key Categories" (noting section 4.1.2 on limited resources) for discussion of key categories and use of decision trees

Tier 3

The Tier 3 approaches (Franklin et al, 2004 and Kershaw, 2005) require mine-specific information such as ventilation emissions from the mine when active, characteristics of the mined coal seam, mine size and depth, and the condition of the abandoned mine (e.g., hydrologic status, flooding or flooded, and whether sealed or vented). Each country may generate its own profiles of abandoned mine emissions as a function of time (also known as emission decline curves) based on known national- or basin-specific coal properties, or it may use more generic curves based on coal rank, or measurements possibly in combination with mathematical modelling methods. If there are any methane recovery projects occurring at abandoned mines, data on these projects are expected to be available. A mine-specific Tier 3 methodology would be appropriate for calculating emissions from a mine that has associated methane recovery projects and could be incorporated as part of a hybrid approach with a national level Tier 2 emissions inventory.

In general, the Tier 3 process for developing a national inventory of abandoned mine methane (AMM) emissions consists of the following steps:

1. Creating a database of gassy abandoned coal mines.

2. Identifying key factors affecting methane emissions: hydrologic (flooding) status, permeability mine condition (whether sealed or vented) and time elapsed since abandonment.

3. Developing mine- or coal basin-specific emission rate decline curves, or equivalent models.

4. Validating mathematical models through a field measurement programme.

5. Calculating a national emissions inventory for each year.

6. Adjusting for emissions reductions due to methane recovery and utilization.

7. Determining the net total emissions.

Hybrid Approaches

A combination of different Tier methodologies may be used to reflect the best data availability for different historical periods. For example, for a given country, emissions from mines abandoned in the distant past may need to be determined using a Tier 1 method. For that same country, it may be possible to determine emissions from mines abandoned more recently using a Tier 2 or 3 method if more accurate data are available.

Fully Flooded Mines

It is good practice to include mines that are known to be fully flooded in databases and other records used for inventory development, but they should be assigned an emission of zero as the emissions from such mines are negligible.

Emissions Reductions through Recovery and Utilization

In some cases, methane from closed or abandoned mines may be recovered and utilised or flared. Methane recovery from abandoned mines generally entails pumping which increases, or "accelerates", the amount of methane recovered above the amount that would have been emitted had pumping not taken place.

Under a mine-specific (Tier 3) approach in which emissions decline curves or models are used to estimate emissions, if emissions reductions are less than the projected emissions that would have occurred at the mine had recovery not taken place for a given year, then the emissions reductions from the recovery and utilization should be subtracted from the projected emissions to provide the net emissions. If the methane recovered and utilized in a given year exceeds the emission that would have occurred had recovery not taken place, then the net emissions from that mine for that year are considered to be zero.

If a Tier 3 method is not used (singly or in combination with Tier 2), the total amount of methane recovered and utilized from abandoned mines should be subtracted from the total emissions inventory for abandoned mines, per Equation 4.1.9, subject to the reported emissions being no less than zero. The Tier 3 method should be used where suitable data are available.

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