India ranks seventh in the world in terms of coal resources, and 10th in CBM resources. India has a good resource base for CBM with an estimated resource of 2,000 billion cubic meters (bcm) in 2,000 square kilometres out of which recoverable reserves are about 800 bcm,with a gas production potential of 105 million cubic meters a day over 20 years.
Methane was once regarded by miners as a hazard rather than a resource and many miners died in methane explosions before the introduction of high-capacity ventilation to dilute gasses. However, if methane is not recaptured it is not only lost as a resource but contributes to global warming. Even though the volume of methane contributing to greenhouse gasses is three times smaller than carbon dioxide, its greenhouse potential is 21 times higher. Coal mining is estimated to cause about 9 per cent of global methane emissions.
Methane captured during coal mining could be a significant, ecologically friendly source of energy, producing no particulates and only about half the CO2 associated with coal combustion. Depending on quality methane from mines could be sold to gas companies, used to generate electricity, used to run vehicles, used as feedstock for fertilizer or methanol production, used in blast furnace operators at steelworks; sold to other industrial, domestic or commercial enterprises; or used on-site to dry coal. In the USA today coal bed methane (CBM) represents between two and three per cent of all gas production.
Methane and coal are formed together during coalification, a process in which plant biomass is converted by biological and geological forces into coal. Methane is stored in coal seams and the surrounding strata and released during coal mining. Deeper coal seams contain much larger amounts of methane than shallow seams. Small amounts of methane are also released during the processing, transport, and storage of coal.
Coalbed methane (CBM) is a natural gas formed by geological, or biological, processes in coal seams. CBM consists predominantly of methane. Lower concentrations of higher alkanes and non combustible gases are also often present.
CBM was primarily formed in coal seams as a result of the chemical reactions taking place as the coal was buried at depth. The greater the temperature and duration of burial, the higher the coal maturity (rank) and hence the greater the amount of gas produced. Much more gas was produced during the “coalification” process than is now found in the seams. The lost gas has been emitted at ancient land surfaces, dissipated into the pores of surrounding rocks, removed in solution, and some will have migrated into reservoir structures forming natural gas deposits.
CBM tends to remains firmly locked in the coal at the prevailing pore fluid pressure until released as a result of mining disturbance or by specific gas production activities conducted in boreholes.
If effectively recovered, coal bed methane associated with coal reserves and emitted during coal mining could be a significant potential source of energy.
CBM is the generic name applied to the naturally occurring gas found in coal seams. It is recovered from coal seams as:
Virgin coal bed methane (VCBM) from unmined coal using surface boreholes;Coal Bed Methane (CBM) and Virgin Coal Bed Methane (VCBM) are terms conventionally used for methane drained and captured directly from the coal seams. CBM is generally reserved (in addition to its use as a generic term for all coal seam gas) to describe the gas produced from surface boreholes ahead of mining for coal mine safety and coal production reasons. VCBM is produced by a similar process but completely independently of mining activity. Methane concentrations in VCBM are generally very high, around 99%, and can be used as a replacement for natural gas supplies.
Abandoned mine methane (AMM) from disused coal mines;
When an active coal mine is closed and abandoned, methane continues to be emitted from all the coal seams disturbed by mining, decaying gradually over time unless arrested by flooding due to groundwater recovery. Depending on the methane concentrations, local regulations and the geology it may be possible, or required for public safety reasons to continue draining or venting this Abandoned Mine Methane (AMM). AMM extraction and utilisation schemes aim to recover the gas left behind in unmined coal above and below goaf (worked-out) areas formed by longwall mining methods. The gas can either be transported by pipeline to a nearby user consumer for combustion in boilers or used on-site to generate electricity for local use or sale to the grid. AMM reservoirs consist of groups of coal seams that have been de-stressed, and therefore of enhanced permeability, but only partially degassed by longwall working. Favourable project sites are those where a market for the gas exists, the AMM reservoir is of substantial size and not affected by flooding and the gas can be extracted at reasonably high purity. A number of schemes are in place in countries such as the UK and Germany.
Coal mine methane (CMM) which is captured in working coal mines to allow safe working.- Methane is released as a result of mining activity when a coal seam is mined out and if not controlled to prevent the accumulation of flammable mixtures of methane in air (5-15%) it presents a serious hazard. Gas drainage techniques are used to enable planned coal production rates to be achieved safely by reducing gas emissions into longwall mining districts to a flow that can be satisfactory diluted by the available fresh air. In some instances gas drainage is also needed to reduce the risk of sudden, uncontrolled emissions of gas into working districts. In well managed mines, in favourable geological and mining conditions, the methane concentrations in drained CMM can reach 70% or more. CMM of such quality may be utilised. However, poorly drained mines will only achieve methane concentrations that are much lower, and may be too low for conventional utilization purposes.
Methane capture and its utilisation from coal mines is generally not practiced in India as current levels of coal production in gassy mines are generally achievable using ventilation controls but even where there may be some safety benefit there is some resistance to introducing gas drainage due to a lack of technology, expertise and experience. Additionally, there is the perception that CMM utilisation is not commercially viable.
In addition, very dilute gas mixed with ventilation air, known as ventilation air methane (VAM), is emitted from coal mines.
Ventilation Air Methane (VAM)
Methane released from coal seams into the ventilation air of the active coal mine is called Ventilation Air Methane (VAM). Concentrations of methane in the ventilation air is generally limited by law, for safety reasons, at 0.5 to 2% in different parts of a mine with variations depending on the country.
Concentrations can be controlled by the volume of ventilation air circulated (dilution) or through special drainage (CMM). The concentration of methane in VAM is typically 0.8% or less and is too low for conventional utilisation purposes. However, technologies are being developed to remove the methane, and where additional gas is available to generate electricity using the thermal energy recovered.
