Miners have lived in fear of ‘firedamp’ – methane leaking from coal seams – for centuries and the discovery that it was a highly potent greenhouse gas did nothing to ease these fears. But Janet Wood finds that controlled use of methane can turn the old enemy into a friend – even in fighting climate change.
In April the UK government recognized that using methane gas leaking from current or abandoned coal mine workings to generate electricity helped reduce emissions of greenhouse gas. Although some argued that methane was a fossil fuel, and should be taxed as such, the government exempted electricity generated from coal bed methane from its ‘climate change levy’, which taxes energy use depending on how much greenhouse gas it produces.
The government’s reasoning was sound, because although burning methane does create carbon dioxide (CO2) – a greenhouse gas, methane itself is a far more potent greenhouse gas. Converting it to CO2, and generating electricity in the process, reduces its greenhouse effect by between 17 and 20 times. In Germany, too, methane from abandoned mines now counts as a ‘renewable’ energy resource.
For many mine operators, methane is now seen as a valuable resource
Methane is present in all coals – formed along with the coal as it was converted from trees and other organic materials. Methane is adsorbed onto the surface of coal grains and held there partly by high pressure and low permeability within the coal seams. As the coal is fractured and removed, the methane is desorbed and the gas flows out of the coal.
The desorption effect is wider than the seam being worked: as the coal is removed pressure in the over- and underlying strata decreases and they become fractured, and this brings out the methane. The presence of this explosive gas is a constant danger to miners, but if extracted and piped, it becomes a valuable fuel that can be used in power generation plants.
Methane in proportions of 5-15 per cent is explosive, explains Dr. Sam Holloway, a geologist from the British Geological Survey (BGS); in working mines the aim is to reduce methane to below 1.5 per cent. To help keep to that limit, boreholes are drilled above and below the working seam to desorp methane from the smaller seams surrounding it. The methane is then pumped out.
Methane has been tapped as a local energy source – used to heat water for pit-head baths, for example – for many years, and more extensive applications were already in operation in the UK by the 1970s, as Holloway explains. “The potteries coal field installed a methane drainage system in the 1970s,” he says, “The methane was sold to local industries as a power source. At Harworth colliery methane was used to generate electricity.”
For many mine operators, methane is now seen as a valuable resource in its own right, and that perception has been reinforced by the need to minimize methane emissions to the atmosphere.
An extensive area of interconnected abandoned mine workings are needed for a successful scheme
In the UK mines now use 40 per cent of the methane they produce, according to figures collated by Wardell Armstrong, but the company believes that proportion could be increased to at least 70 per cent. Countries where coal mining is still in the ascendancy have more incentive to use their methane by-products as an energy source. In Australia, the Appin and Tower collieries use half their emissions in 94 reciprocating engines that produce some 94 MWe.
The use of coal mine methane in Australia has been supported by the Greenhouse Gas Abatement programme, which in October 2001 announced it was funding three new projects in the New South Wales coalfields to the tune of A$30 million ($16 million).
BHP Billiton will be offered up to A$6 million towards a A$10.7 million project to install a specialized combustion unit that can burn air containing very low concentrations of methane at the West Cliff colliery. The heat from the unit will be used to drive a conventional steam cycle power station to start up in 2008.
Powercoal will be offered up to A$15 million towards a A$26 million project to link the air intake of Vales Point power station to the mine ventilation systems of Endeavour and Munmorah collieries, incorporating the vented methane into combustion air.
Envirogen will be offered up to A$9 million towards a A$16 million project to install ten generators at Bellambi mine, using gas drained in advance of mining.
Methane from existing coal mines is known as coal mine methane (CMM). But coal mines are abandoned not when there is no coal left, but when removing the coal is no longer economic. Large amounts of coal is still present in the remains of worked areas and in small seams, and methane continues to leak long after the mine is abandoned. If the mine becomes flooded after it is closed the problem can be exacerbated, as methane does not dissolve in water and it is driven upwards as the mine fills.
In recent years companies have explored the potential for removing methane from abandoned mine workings (known as abandoned mine methane, AMM). In a report to the UK government Wardell Armstrong, BGS and the University of Nottingham identified the key features of a successful AMM scheme as:
- An extensive area of interconnected abandoned mine workings
- A large coal volume in unmined seams de-stressed by under and overworkings
- Significant residual methane in unmined seams
- Minimal water ingress with little or no ‘ponding’ in main roadways
- Unfilled shaft or drift from which gas can be extracted
- No connections to shallow outcrop workings so no air in-leakage
- Local market for gas.
Dr. Julian Edwards of Nottingham University, says, “The amount of methane depends very much on the circumstances. You have to look at the number of worked seams – in the UK some of our mines are over 100 years old and have 20 worked seams. You are looking for lots of workings, but with lots of untouched minor seams.”
The characteristics of coals in different areas vary depending on the original organic material, changes over many thousands of years in temperature and pressure, and its history as the coal strata were buckled, lifted or faulted by geological change. This has produced equal variations in the amount of methane in each coal seam and how easily it can be abstracted.
In the UK nearly 50 MW of power is generated from abandoned coal mines and more projects are planned – the major developer, Alkane, saw its shares jump in value when the UK government altered its climate change levy status. Similar projects are under way on mainland Europe and the UK is also working with China in abstracting methane from its abandoned mines.
Using methane to generate electricity that would otherwise offer a hazard to miners and local people or escape to the atmosphere is easily justified. More debate has been roused by companies that extract methane from coal deposits which have not been mined (so-called virgin coal bed methane, VCBM). Here boreholes are drilled into untouched coal seams and the methane is pumped out.
The total amount of methane within the coal seam is determined by the organic properties of the original material and its geological history. But the economic viability of a borehole depends on whether the gas can be extracted.
Once the borehole is drilled, various methods are used to cause fracturing within the coal seam. The fractures may be produced by injecting water, but that requires constant replacement as the water is pumped out along with the gas and fractures can close up.
Developers are now beginning to use sand/water mixtures to produce fractures. The benefit of this approach is that the sand stays in place so fracturing is maintained; there is still work to be done, however, on maintaining the sand/water suspension before and during injection.
Estimates of the amount of methane available for extraction from virgin coal seams place deposits as high as 1×1014 m3. Exploitation has been fastest in the USA, and now nearly seven per cent of the USA’s domestic production of gas is from VCBM, largely in the Powder River basin, Wyoming, and the San Juan basin. There are plans for further development, but these are limited by access rights and lack of a pipeline infrastructure.
Similar concerns have delayed development of VCBM projects in China, Canada and Australia. In the USA developments have run into strong local opposition. Locals are concerned partly about the environmental and practical logistics of drilling upwards of 5000 boreholes each year. They are also concerned about the need for, and production of, water from the coal seams.
As Wardell Armstrong’s Dave Creedy points out, on the face of it using a simple VCBM cycle offers no greenhouse gas abatement benefits, as the gas would otherwise remain in situ in the coal seam. More complex cycles, however, do offer interesting possibilities. Both carbon dioxide and nitrogen are adsorbed onto the coal surface in preference to methane, and both have been used to increase the amount of methane produced from the coal seam.
In the past nitrogen was preferred, as it can be retrieved and therefore offered a more economical method. Now, however, most countries are looking for ways to sequester CO2 so that it is not released to the atmosphere and coal beds are considered a very promising avenue of development for permanent disposal.
As Nottingham University’s Julian Edwards describes it, in a closed cycle the methane would be burned and the resulting CO2 captured. The CO2 would then be piped back into the coal seam where it would displace methane still adsorbed on the coal particles, helping produce more fuel while disposing of the product.
This attractive solution is being pursued by both the EU and the USA. If development is successful it may be that the first CO2-free fossil fuel plant will be one that operates not by digging up the coal, but by leaving it in place.