With the rise in energy demand has come a growing need for coal, still the prevailing source of energy for power plants around the world. But today, the coal power industry faces new pressures, not only with production, but with its carbon footprint right along the line.
Today, the scrutiny goes all the way back to the initial process of getting the coal out of the ground. Methane (CH4) is given off when organic matter decays, and so coal mines (also landfill sites) are a rich source of the gas. The quantity of gas released from mines increases mainly with coal carbon content and coal depth.
Particularly as the deeper coal deposits were formed, high temperatures and pressures forced methane into micropores within the coal. Huge amounts can be adsorbed, with just a gram of coal estimated to have a surface area of up to 250m² or more.
While coal is being mined, the methane escapes and mine ventilation systems have to drain it to prevent explosions. The low concentrations have made this coal mine methane (CMM) difficult to recover from working mines, but that is now changing.
The richest sources, though, are mines before they are worked (coal bed methane, CBM) and after they are closed (abandoned mine methane, AMM). Coal mining operations normally cause subsidence and fractures in the layers around the worked seams. Old workings therefore become large gas reservoirs and will gradually release the gas to the atmosphere.
CH4 has around 20 times the global warming potential of CO2 and, in 2010, worldwide coal mine methane emissions are expected to total 400 million tonnes of CO2 equivalent and rising. Besides adding to global climate change, simply allowing methane to vent to the air wastes a valuable resource.
Recovering and using the gas is profitable. Recovered methane can fuel vehicles, boilers and district heating systems, dry coal, generate electricity or be injected into natural gas pipelines (methane is the largest constituent of natural gas). It also has industrial uses, for example being a feedstock for producing carbon black, dimethyl ether (DME) and methanol.
Global coal mine methane emissions actually declined between 1990 and 2000 because many deep coal mines closed. This happened in China, the largest methane-emitting country, in the second half of the decade. It happened in Former Soviet Union countries, which also saw restructuring of the energy industries. It happened in the US, where there was increased methane recovery as well as a move to surface mining, which releases much less methane. It also happened in Europe, with further methane reductions aided by EU waste disposal directives.
Emissions are on the rise again now, though, particularly in the developing world as a result of industrialisation and fast-expanding populations and economies. Wherever coal production is planned to increase, methane emissions will also rise unless the gas is recovered.
It is much easier to prevent non-CO2 greenhouse gases emissions than CO2 emissions themselves, and the US in particular has focused its attention on methane. The Methane to Markets Partnership was launched at the end of 2004 in Washington DC, the US, with 14 national governments signing on as partners. From the US, too, the Environmental Protection Agency (EPA) encourages recovery internationally with its voluntary Coalbed Methane Outreach Program (CMOP).
The EPA indicates three major abatement systems: degasification, enhanced degasification and oxidation of VAM.
Recovering the methane
Degasification recovers high-quality CH4 from coal seams. Vertical wells can be drilled up to ten years before mining operations, or horizontal boreholes up to one year or less before. High-quality CH4 also often can be obtained from gob wells (from around mined-out coal seams), with gob gas CH4 concentrations ranging from 50% to over 90%.
The recovered gas can often be injected into a natural gas pipeline, with the US estimating that nearly 60% of mines at first require virtually no purification, although this may be needed later. This is the simplest form of recovery and the upfront costs (for equipment like compressors, gathering lines and dehydrators) and annual costs (mainly drilling costs) are repaid by cost savings from capture and reuse. Where it cannot be injected into pipelines, the methane can be used as a fuel source for gas engines and generating electricity; this method is particularly advanced in Germany and France.
Enhanced degasification recovers methane in the same way, but enriches it using equipment like nitrogen removal units (NRUs) and dehydrators. Upfront costs include an additional $200,000 for the NRU, and higher annual drilling costs because the wells are more closely spaced. The enhanced gas recovery (particularly for medium-quality gas), however, makes another 20% of mines profitable.
Ventilation air methane oxidation removes the gas from working mines using large ventilation fans. Until recently, the low (typically below 1%) methane concentrations in VAM prevented its use. VAM can be burned (CH4 + 2O2 --> CO2 + 2H2O, to heat water – for steam or district heating – or generate electricity. Upfront and annual costs of such thermal or catalytic oxidation depend on concentration and the ventilation air flow rate.
Where are methane resources?
The US EIA (Energy Information Administration) estimates that 60% of the world's recoverable methane reserves are located in China (12%) and the Far East, the Former Soviet Union (23%) and the US (25%).
Without major recovery initiatives, emissions will grow quickly in South and East Asia.
The largest coal consumer and producer is China, producing around one billion metric tons of underground coal a year. The country has a history of CMM recovery dating back to the 1950s, and surface CMM development began in the 1990s. China now reuses around a third of the total methane drainage, but its methane emissions are expected to increase 50% by 2020 and there is great scope for increased recovery.
With a production of 430 million tonnes, India is the world's third-largest coal producer and its production will potentially double by 2010. There is interest in reclamation and in 2007 Coal India Ltd (CIL) invited interested parties with proposals to recover methane from existing and abandoned mines (largely located in the Jharia coalfields).
In Russia, natural gas emissions dominate and are expected to increase. Polish emissions are expected to decline sharply by 2010, largely due to predicted closure of many privatised mines. The Polish economy is still largely coal-based, though with negligible natural gas and oil reserves. Methane recovery and use could increase as mines try to remain profitable.
Elsewhere, emissions have grown strongly in Latin America, but have slowed since the early 2000s in the Middle East, Africa and the OECD countries.
The OECD countries have in fact seen some of the lowest growth rates. This has largely been caused by limited production growth and air quality equipment that has also reduced methane emissions, although natural gas use itself is predicted to grow.
The US has a good recent record of methane reclamation. In 2005 it recovered nearly 80% of all drained CMM.
Active coal mines account for nearly 10% of US anthropogenic methane emissions, and the EPA has identified another 400 promising 'gassy' abandoned mines. These are mainly located in the Appalachian Basins in the East, Black Warrior Basin in the South, the Illinois Basin in the Central US, and several western basins such as the San Juan and Powder River Basins.
Here and elsewhere, recovery methods are improving. Particularly, techniques like enhanced coal bed methane (ECBM) look promising. This injects gas – typically nitrogen or carbon dioxide – into coal seams to improve methane recovery like enhanced oil recovery improves oil recovery. Assuming early tests are successful, the technique could be a good candidate for carbon sequestration, particularly for projects receiving carbon credits.