Methane is the primary constituent of natural gas and is stored within coal as a result of the coalification process whereby plant material is converted to coal. Due to coal mining activities (and subsequent pressure decrease in coal seams), methane is released from the coal and surrounding strata. This leads to the build-up of methane in mines, which potentially creates an explosive hazard. Ventilation and/or degasification systems are used to prevent the build up of methane and to ensure its release to the atmosphere for safety reasons.
Methane is the primary constituent of natural gas and is stored within coal as a result of the coalification process whereby plant material is converted to coal. Due to coal mining activities (and subsequent pressure decrease in coal seams), methane is released from the coal and surrounding strata. This leads to the build-up of methane in mines, which potentially creates an explosive hazard. In order to reduce the pressure within mines, methane can be released to the atmosphere. However, as methane is a powerful greenhouse gas (with a global warming potential of 25), this would create extra greenhouse gas emissions. Coal mines contribute 8% of global anthropogenic methane emissions (IEA, 2008). The largest part of these emissions are caused by China, the United States, India, Australia, Russia, Ukraine and North Korea (see Figure 1).
Methane could be flared instead of releasing it into the atmosphere, which would 'convert' methane into carbon dioxide. Although this would still cause GHG emissions, due to lower global warming potential of CO2 the contribution to building up greenhouse gases in the atmosphere would be smaller. Alternatively, the methane can be used for electricity generation, which is the most universal and economical option for using coal mine gas (WCI, 2006). Two methods of methane recovery can be distinguished:
- Coal mine methane (CMM), which refers to the process whereby methane is released during coal mining activities, and
- Coal bed methane (CBM), which refers to the process whereby methane is recovered from deep unmined coal seams rather than in conjunction with mining activities.
Depending on site-specific characteristics, CMM can be recovered during coal mining through vertical wells and/or horizontal boreholes, but also from abandoned mines, which may continue to emit methane from diffuse vents or ventilation pipes or boreholes. CBM gas can also be extracted by injecting CO2 into a coal seam so that it displaces the methane – this technique is known as enhanced coal bed methane recovery.
Apart from using CMM for electricity generation (using gas engines or turbines), captured methane (at high concentrations) can be sold to natural gas pipeline systems. Moreover, CMM can be used for district heating, as well as boiler fuel or town gas, heat source for mine ventilation air, supplemental fuel for boilers, and for coal drying. Utilisation of CMM, however, depends on the quality of the gas (i.e. methane concentration and presence of contaminants), the availability of end-use options, and project economics. For example, methane concentrations should be as high as 95% for pipeline distribution. Therefore, CBM is preferred for such purposes because it has a high quality methane generation as it is not mixed with ventilation air (as with CMM). Highly concentrated methane can even be used as a vehicle fuel. Medium quality gas (i.e. >25% methane concentration) is suitable for electricity generation in gas engines (internal combustion engines). Given the fluctuating nature of captured methane, a modular design is preferable with a bundled set of smaller capacity generators. This has a number of advantages, such as the possibility to operate these at full capacity, ease of relocation, and the possibility of a staged expansion (WCI, 2006).
Finally, menthane is also released through the ventilation air (ventilation air methane or VAM), but since the methane concentration of VAM is only around 1%, it is not suitable for the power production and heating applications mentioned above. Through oxidation technologies VAM could be captured by heating it to their oxidation temperatures so that it converts into CO2 and water. The recovered heat can subsequently be used by mines for on-site heating purposes, coal processing or even power generation (WCI, 2006).
The Methane to Markets initiative has analysed how the implementation chains for CMM/CBM technologies could be further streamlined in order to enable a widespread application of the technologies. First, it is important that the potential energy value of methane from CMM is recognised in addition to the safety aspects, so that incentives become stronger to capture the methane. Second, the use of CMM/CBM for energy purposes requires a transfer of technology, especially from developed to developing countries. Third, ownership of methane resources have given impetus for disputes in the past and this needs to clarified for each mine. In the UK for example, the authority responsible for methane captured in coal mining activities is different from the owner of the mines. Fourth, financing CMM/CBM projects could be hampered by the uncertainties about the determination of proven methane reserves in mines and coal beds, which could eventually lead to hesitation among banks to finance CMM projects.
CMM technology for recovering and using methane from both active or abandoned coal mines is readily available (WCI, 2006). However, effective technology for increasing the concentration of methane is currently not available. Such technologies would be particularly interesting for China, where approximately 70-80% of CMM has a methane concentration of less than 30% (Su et al., 2006). Although there are some gas separation and upgrading technologies available, research to date has focused instead on the oxidation of low concentration methane, such as in VAM from coal mine ventilation shafts, and has now reached its demonstration phase. It is expected that over the next 20 years, this specific technology will develop further, thereby reducing costs and decreasing the technical applicability concentration level below 0.15% (WCI, 2006). Australia, China, Germany, Poland and the USA recover a considerable amount of methane from coal mines. Germany, the USA and the UK have a considerable number of projects in abandoned mines. Based on available data in the Table below, an estimated total of 50 Mt CO2-eq. are avoided from ‘Methane to Markets’ partner countries annually.
Power production from CMM has developed for more than a decade in such countries as Australia, Germany, Japan, the UK and the USA. Recently, rapid developments have taken place in this field in China, Poland and Ukraine. Based on 2005 data, some fifty CMM power generation projects were operating worldwide at active and abandoned coal mines ranging in size from 150 kWe to 94 MWe, totalling over 300 MWe (WCI, 2006).
In order to determine a mine’s CMM potential, first, data needs to be obtained on the following items:
• Percentage of methane emitted from ventilation air stream;
• Variations in methane concentration and flow rate for ventilation air, pre- and post-drainage gas, if any; and
• Rate of change of methane concentration.
Consequently, successful projects require: a thorough methane resource assessment and gas liberation analysis, assurance of effective integration of mine degasification and utilisation with mining operations; and a ready market for the methane should be available.
The IEA’s Reference Scenario projects global coal demand to grow at an average annual rate of 1.8% between 2004 and 2030; coal’s share in the global energy mix will remain constant at about 25%. Of the total increase in demand, 86% will come from developing countries in Asia, particularly China and India (IEA, 2006). In fact, coal demand by 2030 is expected to be about 19% higher than projected in IEA (2005) because of favourable market circumstances, such as high natural gas prices. CMM is a relatively large and undeveloped resource. China, Russia, Poland and the USA account for over 77% of CMM emissions and thus represent the biggest potential in volumetric terms (WCI, 2006). China's total reserves of CMM, although difficult to quantify precisely, has been estimated at some 35 trillion m3, which is around the same as China’s total natural gas reserves (Methane to Markets, 2006). According to Methane to Markets, Chinese coal mines emit up to 13 billion m3 of methane annually, of which VAM emissions make up about 95% (worldwide VAM emissions have a share of 45% in CMM/CBM emissions). China's share in global CMM emissions is around 40%.
A distinction can be made in CMM applicability in industrialised and developing countries. In countries like the USA, the UK, and the Czech Republic high gas prices and well developed pipeline infrastructures have resulted in significant sales of coal-mine gas to the grid. In developing countries, however, pipeline injection is rarely used because of either insufficient methane concentrations in recovered gas or the inaccessibility or unavailability of pipeline infrastructure (WCI, 2006).
Apart from climate change mitigation and a reduction in other pollutants, the implementation of available cost-effective methane emission reduction opportunities in the coal industry can lead to:
- Improved mine safety – Countries have different regulations for the maximum methane concentrations in mines and accidents still occur frequently, mostly in developing countries. In China, for example, over 6,000 miners lose their lives each year as a result of accidents (IEA, 2006).
- Greater mine productivity – In the 1970s and 1980s the development of deep gassy mines forced the mining industry to employ gas drainage systems to supplement overburdened ventilation systems. It was then concluded that such systems actually improved coal production figures because of less frequent occurrences of peak gas levels in ventilation air, which would stop mining activities temporarily (WCI, 2006).
- Increased revenues - By utilising the methane released as a result of coal mining activities, coal can be mined in a more sustainable manner.
Methane extracted from unmined coal seams (as in CBM) could facilitate the exploitation of coal in areas where it would otherwise not have been exploited. Similarly, it would already extract methane from mines before the coal is actually mined, which would improve the safety situation for miners. In addition, this practice would lead to higher methane concentrations as it is not mixed with ventilation air (Methane to Markets, 2005) and thus has a wider commercial applicability.
Social acceptability of CMM is positively influenced by increased mine safety through properly addressing methane emissions from mining operations, while at the same time increasing productivity (of both coal and methane as higher concentrations can be reaped) through advanced drainage techniques. Previously vented methane can be deployed as an energy resource, for example, in power generation and thus diversify energy use.
As an example, the Jincheng CMM Power Generation CDM project can be explained which aims to capture CMM and use it for on-site power generation. Internationally proven technologies will be deployed to improve mine safety and efficiency during gas drainage and the methane will be diverted to a nearby CBM power plant. Apart from the mitigation of climate change, other tangible benefits can be expected from this project, such as improved worker safety, job creation and overall improved health conditions for miners. By 2008, the project will use at least 166 million m3 of CMM per year to serve about 90,000 households and various commercial and industrial establishments in the area. It is projected that about 410,000 people in Jincheng will directly benefit from the cleaner CMM based energy, reducing indoor and atmospheric pollution. Annualy, almost 3 million tCO2-eq. is projected to be reduced through implementation of this project that has an extraordinary capacity of 120 MW (Su et al., 2006).
As per June 2010, 26 CMM utilization projects were registered as Clean Development Mechanism (CDM) projects by the CDM Executive Board. Four of these projects work on both avoidance and utilization of CMM. All registered CMM projects are located in China and their annual contribution to greenhouse gas emission reductions vary from 45 kt to 3 MtCO2-eq. China’s strategy is to control mine gas outbursts and improve mine safety through an efficient utilisation of mine methane (i.e. using CMM and drainage of methane for safety reasons). China’s National Climate Change Coordination Committee has placed coal methane projects within its top four of priority categories for the CDM.
For the methane emission reduction effect, the GHG accounting methodology "Consolidated methodology for coal bed methane, coal mine methane and ventilation air methane capture and use for power (electrical or motive) and heat and/or destruction through flaring or flameless oxidation --- Version 6" (ACM0008) approved by the CDM Executive Board can be used.
For the calculation of GHG emission reductions achieved by using methane for electricity production, the approved methodology "Consolidated methodology for grid-connected electricity generation from renewable sources --- Version 11" (ACM0002) can be used. This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored.
General information about how to apply CDM methodologies for GHG accounting, as well as how to calculate GHG emission reductions from transportation or industrial use projects, can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html
The following enumeration sums up the major cost items of a new drainage system in a mine:
- The surface extract plant;
- Gas holder(s);
- Mine pipe ranges and fittings;
- Installation of pipes and fittings in shafts and main roadways;
- Drilling machines, drill rods, bits, standpipes and borehole equipment, sealing materials;
- Pumps for wet drilling; and
- On-line drainage network monitoring system.
For CMM projects in general, the associated costs depend on a number of factors, including the depth of mining, distance of the districts to extraction plants, technical lay-out (such as pipe diameters, optimal borehole spacing, etc.), method of monitoring and control, site-specific geochemical and geophysical conditions (i.e. presence of contaminants and gas pollutants, reservoir temperature and pressure or coal bed fracturing), etc. (Su et al., 2006). According to Su et al. (2006), capital costs are expected to be low, especially in case drainage infrastructure is in place. Then, gas drainage facilities, pipeline networks, auxiliary equipment and gas drainage monitoring systems may have to be upgraded or should be installed. According to the IPCC (2001), around 50% of emissions from coal mining could be prevented at costs in the range of USD 0.27-1.09/tCO2-eq. Investment costs of CBM recovery are much higher because no such activities and infrastructure are in place. Calculations have shown that capturing and utilising low concentration methane is not an economically attractive course of action without the CDM (Su et al., 2006).
IEA, 2006. World Energy Outlook 2006, OECD/IEA, Paris, France.
IPCC, 2001. Climate Change Mitigation 2001, Contribution of Working Group III to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, United Kingdom.
Methane to Markets, 2005. Methane to Markets Partnerships Landfill Gas Technical Subcommittee Country specific profile Australia, Australia.
Methane to Markets, 2006. CMM Global Overview, Prepared by the U.S. Environmental Protection Agency Coalbed Methane Outreach Program.
Su, S., Ren, T., Balusu, R., Beath, A., Guo, H. and Mallett, C., 2006. Development of Two Case Studies on Mine Methane Capture and Utilisation in China, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia.
US EPA, 2006. Global Mitigation of Non-CO2 Greenhouse Gases 1990-2020 (EPA Report 430-R-06-003), U.S. EPA, Washington, DC.
WCI, 2006. Coal Mine Methane: An Industry Moving Forward, ECOAL, Vol. 57, April.