Coal Mine Methane
The methane recovered from working mines can be grouped under the term Coal Mine Methane (CMM). Two key drivers for CMM recovery are mine safety and the opportunity to mitigate significant volumes of methane emissions arising from coal mining activities. There is also strong potential to utilise CMM for energy production.
Methane emissions in working mines arise at two key stages:
(1) Methane is released as a direct result of the physical process of coal extraction. In many modern underground mines, the coal is extracted through longwall mining. Longwall mining, as with other sub-surface techniques, releases methane previously trapped within the coal seam into the air supply of the mine as layers of the coal face are removed, thus creating a potential safety hazard.
(2) Methane emissions arise from the collapse of the surrounding rock strata after a section of the coal seam has been mined and the artificial roof and wall supports are removed as mining progresses to another section. The debris resulting from the collapse is known as gob and also releases methane or ‘gob gas’ into the mine.
Recovery techniques for CMM vary for each of the two stages of emissions.
(1) Methane released from the worked coal face can be diluted and removed by large ventilation systems designed to move vast quantities of air through the mine. These systems dilute methane within the mine to concentrations below the explosive range of 5-15%, with a target for methane concentrations under 1%. The ventilation systems move the diluted methane out of the working areas of the mine into shafts leading to the surface. The methane removed from working mines via this technique is known as Ventilation Air Methane (VAM).
The VAM is released through the ventilation shafts and can then be destroyed or captured for utilisation rather than allowing it to be released directly into the atmosphere, as may have occurred in the past. VAM has the lowest concentration levels of all forms of recoverable methane from coal seams because of its high exposure to air; often displaying levels of 0.05-0.8%.
(2) To pre-empt the release of gob gas from post mining collapse, it is possible for vertical gob wells to be drilled directly into the coal seam’s surrounding strata before mining activities pass through that section. These pre-drilled wells can then remove the gob gas once the collapse takes place, thus avoiding the release of methane directly into the mine. The gob gas can then be destroyed or captured for utilisation via the wells, rather than allowing it to be released directly into the atmosphere. As gob gas is exposed to significantly lower volumes of air than VAM, it displays much higher methane concentration levels - typically between 35-75%.
Destruction & Utilisation
There are two main options available for the end utilisation of CMM.
(1) Power Generation - If projects are seeking to take advantage of the benefits that CMM can provide as an energy source, there are alternatives to simply destroying the gas through flaring systems. Although both VAM and gob gas provide much lower methane concentrations than methane recovered from unmined coal seams, there are power generation technologies available today that can harness the energy production potential of these resources. VAM can not only be used for combustion dilution and cooling purposes in standard gas turbines, but also as a primary fuel in a number of ‘lean-burn’ gas turbine systems. These systems can utilise VAM with methane concentrations as low as 1% (hence the term lean-burn) and therefore can harness the energy potential of high percentages of the VAM recovered from working mines.
VAM’s potential as an energy source can also be harnessed by a number of oxidation systems available on the market today. Methane can be converted to CO2 by the process of oxidation, thus reducing its global warming potential. This process also creates energy which can be used to generate heat or power. Oxidation systems can utilise VAM with methane concentration levels of less than 1%. These systems are often deployed on-site to provide auxiliary heat and power to the mine.
(2) Flaring - Options exist for destroying gas that would otherwise be released directly into the atmosphere. Flaring is an important technology for disposing of the methane safely and efficiently and can help to significantly reduce a major source of GHG emissions. The flared methane is converted to CO2, heat and water. Although flaring still leads to GHG emissions in the form of CO2, because methane’s global warming potential is 23 times greater than that of CO2, flaring actually reduces the overall greenhouse effect. However, the resulting CO2 emissions still clearly present a huge challenge in terms of combating global warming and flaring is therefore not regarded as the most efficient or environmentally friendly of end use options.
Flaring can be performed in either open or enclosed systems, and the technique is similar to that deployed in the oil and gas industries. This method of methane disposal is relatively cheap when compared to the extra costs incurred in developing power generation infrastructure or incorporating recovered methane into a region’s natural gas pipeline network.
Methane emissions from working underground mines make up the majority of emissions from coal mining related activities - around 90% in 2006 according to figures from the US Environmental Protection Agency (US EPA). VAM is widely found to make the greatest contribution to these emissions, with US EPA figures suggesting that over 50% of all global methane emissions from coal mining arise in this form.
At present, there are more than 220 CMM projects worldwide in 14 countries. These projects help to avoid around 3.8 billion cubic metres of methane emissions every year.
Australia has been particularly active in deploying the power generation and oxidation systems currently available. The United States also has vast potential for utilising CMM for energy purposes, but continues to primarily incorporate the gas directly into its pipeline network rather than deploy power generation systems specifically designed for CMM.
Outside of the developed world, China is experiencing significant growth in interest in the recovery and utilisation of CMM due to its high volume of methane emissions from coal mining and the particularly gassy coal seams that are found in the country. A number of projects utilising CMM for energy purposes in China are currently approved or awaiting approval under the Kyoto Protocol’s Clean Development Mechanism (CDM). Of these projects, a number plan to utilise CMM as a fuel within power generation systems. The greatest potential for CMM projects in the developing world lies under the CDM due to the increased profitability that the generation of emissions reduction credits can provide, which acts as an economic driver.
The potential for the development of CMM projects is also high in a number of other countries, including India and Mexico. Mexico in particular is a key area for potential development as some of the world’s gassiest mines are located there.