Emissions from coal mining
Carbon Capture and Storage (CCS) reduces greenhouse gas emissions resulting from the use of coal. However, there are also greenhouse gases emitted during the mining of coal. The greatest source of greenhouse gas emissions from mining coal arise from so called “fugitive emissions”. All coal seams contain some level of gases, most commonly a mix of carbon dioxide (CO2) and methane (CH4). These gases are released when the coal is mined, and this release is referred to as fugitive emissions.
Coal industry fugitive emissions currently account for around 5% of Australia’s total annual greenhouse gas emissions, which is a relatively small proportion. Nonetheless, the coal industry is actively working to reduce fugitive emissions from mining.
What are fugitive emissions?
Fugitive gas emissions refers to the escape of gases when coal is mined both in open cut and underground operations, and also the gases that escape from the production of oil and gas.
All coal seams and the surrounding strata contain some level of gas due to the processes by which it was formed over geological time. This is commonly referred to as the in-situ gas content. The amount and composition of in-situ gas is highly variable from one seam to another and within a single seam. The major constituents of concern from a GHG emissions perspective are carbon dioxide (CO2) and methane (CH4). Fugitive methane gas emissions account for around 5% of Australia’s total annual greenhouse gas emissions.
The gas needs to be removed from the coal seam before mining the coal to ensure the safety of miners. The COAL21 Fund and the Australian black coal industry’s research program (ACARP) are working on research, development and demonstration programs to assist the coal industry on ways to reduce fugitive emissions.
To date the focus has been on the following technical issues:
- Development of practical and cost effective methods of estimation/measurement of fugitive emissions;
- Development of improved gas drainage technology, including enhanced drainage with a view to application at open cut mines; and
- Development of technology capable of treating the very low and highly variable methane levels in Ventilation Air Methane (VAM)
Underground mines working gassy seams remove seam gas by drilling into the seam from the surface prior to mining (pre-drainage), drilling into prepared blocks underground prior to commencement of operation (in-situ drainage) and in some cases drill into the worked out areas after mining to remove accumulations of gas from the surrounding strata.
Gas is inevitably released as the ground is disturbed during the mining process and management of that gas release is a key part of a mine operation. The approach to gas management differs according to the type of mine operation.
In conjunction with the federal government, ACALET and ACARP are working on improving gas extraction and management from mines. The key emphasis is health and safety of the miners.
Coal mining is an important anthropogenic source of methane emissions. Although agriculture accounts for by far the largest proportion of methane emissions from human activities, emissions from all coal mining related activities - extraction, transport and storage - accounted for around 6% of total global anthropogenic methane emissions in 2010. Fugitive methane emissions from coal mining represented 1.8% of total global greenhouse gas emissions in 2010.
What is Ventilation Air Methane (VAM)?
All coal contains some level of methane (CH4) as a consequence of the geological processes by which it is formed. The amount of gas in coal is highly variable and this too is a consequence of geological processes. This gas is inevitably released when the coal is mined.
Methane in air is explosive in concentrations between 5% and 15% so it is imperative that methane levels in underground mines are kept well below this level, eg less than 1%.
In gassy underground mines the methane is pre-drained prior to mining and there are existing commercial technologies for the abatement of this methane. A significant proportion of the drainage gas from Australian coal mines is already committed for use in one or other of these technologies.
The residual gas after pre-drainage and all gas from non-gassy underground mines is managed during mine operation by maintaining sufficient ventilation air flow to ensure the methane level remains below the designated safety threshold for flammable gases in the working areas of a mine. This safety measure leads to very large volumes of ventilation air with extremely low methane content.
Black coal industry demonstration activities
The removal of methane from ventilation air methane (VAM) is technically very challenging due to its high volumetric flow rate and extremely low methane concentration (deliberately kept below around 1% for safety reasons). Due to the high flow rates, VAM represents more than 60% of underground mine greenhouse gas emissions.
There are a number of proprietary technologies available for mitigating ventilation air methane (VAM) but there have been only a handful of experiments applying these technologies to coal mine VAM. Australia has played a lead role in this research for the past decade or more. Current research, development and demonstration work in Australia includes:
- BHP Billiton at West Cliff Mine demonstration of MEGTEC VOCSIDISERTM technology at its WestVAMP project. The project incorporates the World’s first VAM demonstration power plant
- Bloomfield Group at Bloomfield Colliery, which has a pilot scale development and testing facility for Corkys Ventilation Air Methane – Regenerative After Burner (VAM-RABTM) technology
- Centennial Coal at Mandalong Mine demonstration of Corkys VAM-RABTM technology
- Glencore at Blakefield South Mine demonstration of Corkys VAM-RABTM technology.
A review of the literature will reveal a wide range of other technologies asserting applicability to abatement of ventilation air methane. The black coal industry’s research program ACARP closely monitors such technologies and supports relevant R&D as part of its ongoing strategic research program. However these are either:
- Niche opportunities due to practical limitations
- At a very early stage of development and will require many years to achieve commercial reality or
- Potentially unlikely to be commercially viable due to the high volume ventilation air flows and hence the size and cost of equipment required.