Methane, a primary component of natural gas, is an explosive gas that is found in coal seams as a by-product of coal formation. Coal mine operators must ventilate or drain methane gas away from the face of the mine to keep levels well below the explosive range. Coal mine methane (CMM) refers collectively to methane released from coal seams during mining activities. Although it is viewed primarily as a nuisance and a hazard to miners, coal mine methane is an energy resource that can be captured and used to offset electricity needs or to generate revenue. Such productive uses prevent emissions into the atmosphere of methane, a greenhouse gas (GHG) that is over 20 times more potent than carbon dioxide (CO2).
CMM emission sources include the very dilute methane (typically less thane 1%) that escapes from the ventilation shaft as well as the drained methane that is simply emitted to the atmosphere. In 2000, global CMM emissions totalled approximately 30.8 billion m3 (about 440 million metric tonnes of CO2 equivalent). Methane emissions from coal mining activities constitute approximately 8% of all global human-related methane emissions. As shown in Figure 1, the leading emitters of CMM are China, the United States, Russia, Ukraine, North Korea, and Australia.
Figure 1. Global CMM emissions in 2000 (a total of 120 million tonnes of carbon equivalents). Source: US EPA
Globally, only a fraction of CMM is currently recovered and utilized; the percentage harnessed varies widely according to country. Its usage depends on the ability to recover CMM from degasification systems (if any) and on the gas quality (methane concentration and contaminant levels), local infrastructure, and market opportunities. Current uses for CMM include power generation, natural gas pipelines, vehicle fuel, feedstock for industrial processes, boiler fuels, and fuel for home, local district heating, and industrial space heating.
Power generation is the most extensively employed use of CMM around the world. Over 70 CMM power generation projects at both active and abandoned (closed) coal mines are operating in Australia, China, Germany, Poland, Russia, the United Kingdom, Ukraine and the United States. Totalling more than 400 MW, these projects range in output from 150 kW to 94 MW. Some projects generate power for the electricity grid, while many are used to meet on-site electricity demands, or are combined heat and power (CHP) projects in which the heat is captured and used by the mine. Projects currently under development – including the world’s largest CMM project, a 120 MW power generation project in Shanxi Province, China – will increase the total global CMM-based power generation capacity significantly.
This article describes the two technologies most commonly used for generating electricity from drained CMM: internal combustion engines and gas turbines. It also describes an emerging technology that is used to convert dilute ventilation air methane (VAM) into power.
Power generation technologies using drained gas
Many coal mines with gassy coal seams find that they are unable to keep in-mine methane concentrations at a safe level using the ventilation system alone. Therefore, some of the gassiest coal mines install degasification systems to remove methane from the coal seams ahead of or during coal mining. Degasification may take place in advance of mining (‘pre-mine drainage’) either from surface drilled wells or from in-mine boreholes drilled into the coal seam. In general, pre-mine drainage produces very high-quality gas with concentrations of methane that can exceed 90%. Alternatively, gob (or ‘goaf’) wells may be drilled from the surface into the zone where the seam has collapsed following the advance of the longwall. (In the US, ‘gob gas’ refers to the methane released from the rubble zone after the longwall shearers cut the coal. In Europe the term is ‘goaf gas’.)
Gob wells generally produce lower-quality gas due to entrained air and other impurities. The concentration of methane in gob gas varies widely, from less than 25% in some mines in China to 80% in some mines in the United States, depending on how carefully air intrusion is controlled.
The most important factors to be considered in the design of a CMM power generation system include the projected production volume of drained coal mine methane, the average and range of methane concentration, and estimated lifetime for gas production at the mine.
Using drained CMM to generate power is relatively straight-forward. The most important criteria are sufficient on-site space and site accessibility for installation. Typically, both internal combustion engines and turbines can utilize gas that is considered ‘medium’ quality (plant designers may recommend pre-treatment of the gas to maximize the engine or turbine’s lifespan). Depending largely on the regional electricity prices and regulations, the power generated can be sold to the electric grid if there are existing transmission lines nearby, or it can be used on-site to offset the mine’s power use for mine equipment, conveyor belts, ventilation fans and coal preparation.
Based on a US Environmental Protection Agency (EPA) analysis, the choice of commercially available technologies for CMM power generation projects – internal combustion (IC) engines and gas turbines – depends on key project characteristics. IC engines may be generally more suitable for smaller-size installations (3 MW or less) due to lower typical installation costs for this size. In contrast, for larger installations of about 10 MW or more, gas turbines typically have lower installation and maintenance costs.
There is also a critical tradeoff between gas turbines and IC engines in terms of desired efficiency and operating parameters. IC engines generally have higher efficiencies than gas turbines, but turbines can accept lower methane concentrations. This may be a critical factor for mines that produce low-quality CMM. IC engines may be more compatible with alternative uses for dilute methane from ventilation air shafts (such as for combustion air), thereby increasing the engine’s overall efficiency. Compared with gas turbines, IC engines are inherently more modular; they are relatively easy to install in individual units, so that projects can begin at a modest scale and be incrementally expanded. Of course, site-specific information and manufacturer data are required to design a power generation system for a particular location.
Internal combustion engines
Globally, IC engines are the most popular technology choice for electricity generation from CMM. The technology is well understood and is available off-the-shelf. Diesel engines, for example, can readily be converted to run on natural gas. Typically, generation systems at coal mines consist of a group of engines that are each coupled with a power generator. Thus, the number of engines can be readily increased or decreased based on the mine’s operations, providing great flexibility.
IC engines are able to accommodate gas streams with a wide range of methane concentrations and quality that are typical of CMM, especially gob gas that is contaminated with air. Although the size and generation capacity of an individual IC engine may be quite large, CMM-to-power projects typically use IC engines with less than 5 MW of capacity. While specifications for individual engines vary, some IC engines can run on gas with as little as 25% methane, so long as oxygen levels are sufficiently high and carbon dioxide levels sufficiently low. Typically, methane concentrations in the intake air must be controlled to within a narrow range around the target concentration (for example, plus or minus five percentage points) for the engine to work properly. The most sophisticated engines use electronically controlled devices to adjust the fuel/air mixture before injecting it into the engine in order to accommodate the fluctuations that often come with drained CMM.
Examples of IC engine systems
According to US EPA estimates, over 70 IC engine systems around the world run on CMM from active or closed (abandoned) coal mines. Table 1 presents a summary of CMM power generation systems using IC engines. The Appin and Tower Collieries in Australia are home to the world’s largest installation currently in operation (94 Caterpillar engines, each 1 MW). The largest planned CMM power plant in the world – a 120 MW power station – is in development at the Sihe mine in Shanxi Province, China.
The Sihe Mine, Shanxi Province, China – site of the world’s largest planned CMM power generation project (Caterpillar)
IC engines are also used in innovative ways to extract energy from CMM. For example, projects at the Appin and Tower Collieries in Australia have successfully demonstrated the use of air from the ventilation shafts (containing about 1% by volume methane) as combustion air in IC engines. This saves fuel and has the additional benefit of mitigating the methane emissions from the ventilation air. In Poland, the Pniówek Mine is home to a cogeneration system; the heat extracted from IC engine exhaust and hot cooling water is used as an air-conditioning energy source for two absorption chillers.
The CMM power plant being constructed at the Sihe Mine (China Coal Information Institute)
In addition to their use with CMM drained from active mines, IC engines are also widely used at closed (abandoned) mines to generate electricity. Both Germany and the United Kingdom have many abandoned coal mines that are now being tapped for CMM to generate power.
Turbines are widely used in conventional natural gas power generation plants, especially in the United States. However, they are not yet extensively used for generating power from CMM. Turbines are more suitable for larger power installations (10 MW or more), while many CMM power generation projects consist of one or more relatively small units. Currently, about five turbines in the world are used to produce CMM-generated power in the US, the UK, China, and Germany.
In gas turbines, intake air is compressed and heated by the hot product of methane combustion. The resulting hot gas expands as it passes through the turbine, spinning the rotors to generate electricity. Turbines require relatively high flow rates of methane; for example, a 1.5 MW unit requires about 14,000 m3 of pure methane per day. Conventional gas turbines can be adapted to accept methane concentrations of 35% or lower if combustion systems are appropriately adjusted.
Turbines produce high-temperature exhaust, which can be used to dry coal, heat the mine, or generate additional heat through cogeneration. Turbines may be easily configured for combined-cycle operation, in which the turbine exhaust is used in a boiler to generate steam for a steam turbine that drives a generator and produces power.
Examples of gas turbine systems
In the United States, the VP/Buchanan mines operated by CONSOL Energy use two, 44 MW simple-cycle gas turbines to generate electricity that is sold to the electric grid at times of peak demand. These turbines use both CMM and coalbed methane (CBM), making the flow of methane at this source relatively reliable. Because there are only two units, this peak power generation facility is capable of coming on-line relatively quickly. These two turbines constitute nearly all of the CMM power-generating capacity in the US.
In the UK, the Harworth Colliery, operated by UK Coal Mining, uses two combined-cycle gas turbines to produce 7 MW of power. In addition, the exhaust heat is used to produce steam to drive a steam turbine, with a total output that can exceed 12 MW. In operation since 1993, this plant is the oldest CMM power generation project in the UK.
Power generation technology using ventilation air methane
Coal mine ventilation air exhaust streams contain very dilute concentrations of methane, typically below 1%. Yet, their flow rates are so large that ventilation air exhaust constitutes the largest single source of coal mine methane emissions to the atmosphere. The estimated total world ventilation air methane (VAM) emissions in 2000 were 237 million metric tonnes of carbon equivalent (MMTCO2e).1 Historically, mine ventilation systems have been used as the primary mode of removing potentially explosive methane from gassy underground coal mines to maintain safe working conditions underground.
Layout of a large-scale VOCSIDIZER system (MEGTEC Systems, Inc.)
In recent years, there have been exciting new developments in harnessing the dilute methane in coal mine ventilation exhaust systems, although project developers face technological, operational and financial challenges. Several technologies to convert VAM to power are emerging. To date, only one has been demonstrated at coal mines and is being implemented on a commercial scale to generate electricity. The operational challenges of VAM-to-power projects lie in their ability to address the concerns of regulators about the system’s compatibility with mine operations and safety considerations.
Finally, the capital costs for installing VAM power generation systems are relatively high. Thus, developers may seek alternative financing, such as revenue from emission reduction credits. As these hurdles are reduced, projects utilizing VAM for power generation are expected to dramatically increase.
One technology that can be used to recover energy from VAM is oxidation, which captures energy during the conversion of methane to electricity. In general terms, an oxidizing system consists of a gravel or ceramic bed, a fan that blows the ventilation air through the bed, a heating element that initiates oxidation of the methane during start-up, and a heat exchanger that captures excess heat. Two general types of oxidation systems have been proposed for VAM conversion: thermal and catalytic.
To date, the most widely demonstrated VAM conversion technology is a thermal oxidation system (the VOCSIDIZER) produced by MEGTEC Systems. It has been extensively used in industrial applications for reducing emissions of dilute volatile organic compounds (VOC). The VOCSIDIZER has been successfully demonstrated in Australia; the first commercial-scale electricity production project was slated to be operational in Australia by early 2007. A catalytic oxidation technology has been developed and bench-scale tested by Natural Resources Canada, Center for Mineral and Energy Technology (CANMET).
Other potential technologies for recovering useful energy from VAM include lean turbines, which operate on much lower levels of methane than conventional gas turbines (1-2% methane). In addition, Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) has designed and demonstrated a system that co-fires ventilation air and waste coal in a rotary kiln. Exhaust gas from the kiln heats clean air (via an air-to-air heat exchanger), which powers a gas turbine.
Examples of VAM-to-power system
The world’s first commercial VAM-to-power project (‘WestVAMP’) is undergoing commissioning at the West Cliff Colliery in New South Wales, Australia, with operations expected to commence by early 2007. The heat from the unit will be used to drive a 6 MW conventional steam cycle power station being engineered and constructed by Siemens Ltd Power generation group. Using 20% of West Cliff’s available mine ventilation air, the project is expected to abate 1.04 MMTCO2e between 2008 and 2012. Previous projects in Australia demonstrated the oxidation of VAM to produce heat and further showed that steam can be produced from the methane oxidation.
West Cliff Colliery in New South Wales, Australia, is the world’s first commercial project converting dilute ventilation air methane to energy, an emerging technology (MEGTEC Systems, Inc.)
The US Department of Energy (DOE) and US EPA are funding a field demonstration of the same technology in the US. MEGTEC Systems has signed an agreement with CONSOL Energy Inc. to demonstrate the VOCSIDIZER at an abandoned CONSOL coal mine in West Virginia. The project will consume approximately 50,000 m3 per hour of simulated ventilation air (fresh air with the addition of methane from an abandoned coal mine) and will simulate varying VAM concentrations.
In April 2006, researchers from Australia and China announced plans to build the first ever pilot-scale demonstration Ventilation Air Methane Catalytic Turbine (VAMCAT). The low heating value gas turbine will use roughly 1% methane from ventilation air to generate power. A prototype demonstration unit with a power output of 10-30 kW will first be demonstrated at a Chinese mine. Operational performance data and experience gained from this small unit will be used for the design of a second-generation turbine with an output of at least one megawatt.
Ventilation shafts of every coal mine in the world emit vast quantities of methane in a dilute form that has traditionally been ignored as a potential energy source. Even mines with degasification systems often simply vent concentrated methane streams to the atmosphere.
Yet, increasingly, coal mines are recovering these otherwise wasted methane emissions, utilizing the methane to generate power and heat while preventing emissions of greenhouse gases that are the equivalent of millions of tonnes of CO2. Many technologies are available to convert CMM to power and heat, including IC engines and turbines. Exciting innovations such as oxidation systems are beginning to exploit the potential of ventilation air methane.
The current installed capacity of CMM-fuelled power and heat generation projects pales in comparison to global generation of power and heat from conventional fuels such as natural gas and coal. Yet today’s projects represent only a fraction of the global potential for such projects, particularly if more are employed in developing countries or economies in transition, such as China or Russia. With numerous CMM-to-power projects being planned or developed today, and with the active engagement of the international community through the Methane to Markets Partnership, use of this unconventional energy source has a vibrant future.
Pamela M. Franklin is Team Leader, Coalbed Methane Outreach Program, Climate Change Division, US Environmental Protection Agency, Washington DC, US. Olayinka Ogunsola is with the US Department of Energy. Andres Doernberg is with the US Agency for International Development.
The authors are US delegates to the Coal Subcommittee of the Methane to Markets Partnership. For more information on activities in the Methane to Markets Partnership and the upcoming Partnership Expo, see www.methanetomarkets.org.
1. U.S. Environmental Protection Agency. Assessment of the Worldwide Market Potential for Oxidizing Coal Mine Ventilation Air Methane. EPA 430_R-03-002. July 2003. https://epa.gov/cmop/pdfventilation_air_methane.pdf
Methane to Markets Partnership
An international initiative launched in 2004, the Methane to Markets Partnership is promoting the development of methane recovery projects at coal mines, landfills, agricultural operations, and oil and gas systems. The partnership includes 19 countries that represent over 60% of global CMM emissions. Government representatives work together with private industry, financiers, and non-governmental organizations to focus on sector-specific issues. In the coal sector, the partnership has launched pre-feasibility and feasibility studies for CMM power generation projects, supported an in-mine drilling demonstration programme in Ukraine, evaluated opportunities and potential technological and regulatory issues for CMM project development, and conducted numerous workshops and conferences.