I have no doubt that in the near future we can indeed harvest up to 80 percent of the methane trapped in coal beds. Furthermore, there is so much of it that it can fulfil the energy needs of the planet for about 200 years. That gives us time to make the change to a hydrogen economy. And, as far as I'm concerned, the best thing of all: if you use the methane on the spot for a local power plant, you can use the CO2 that the plant produces for extracting more methane from the coal bed. It's a closed circle. And completely clean. -Henk Pagnier"
Clean energy from coal bed gas
BY THIJS WESTERBEEK Radio Netherlands
Click to listen to the programme, which was first aired in January 2006 (mp3)
A new method of extracting energy from coal layers that until now were considered too deep to access can help 'bridge the gap' between today's fossil-fuelled economy and the potential hydrogen economy of the future. Better still, this method can be efficient and clean.
Methane production well
Coal contains methane gas, a combustible gas with a high caloric value that can be used in much the same way as natural gas. Many coal layers are simply too deep underground, or too thin to be exploited in the traditional way. Yet, they can hold large deposits of methane. The Dutch research and development institute TNO has recently conducted a large-scale test in Poland of a new way of extracting this valuable gas.
Extracting gas from coal beds is not entirely new; in several parts of the world, it's already common practice. Civil engineer Henk Pagnier explains how the existing methods work:
"First, you can simply pump away the water column from a coal reservoir at a depth of, say, five to six hundred metres. Thus, the coal is depressurised and the gas naturally escapes. It's a rather crude method and it doesn't get all the gas out of the coal, but it is efficient in easily accessible layers. The method is called CBM, Coal Bed Methane."
"Another method is underground, subsurface gasification of coal. That means that you burn or gasify the coal in the subterranean layers themselves and then harvest the production gasses, mostly hydrogen, but also some methane. It is a complex process and fairly difficult to control. The amount of gas produced can be disappointing and the burning coal beds can be dangerous; rocky layers above the coal can crack or even collapse. Therefore, early attempts in the 1980s were abandoned. Still, new research has begun in the United States and the first results seem promising."
CO2 injection well
The new method that Henk Pagnier and his colleagues from TNO have been working on is called ECBM, 'Enhanced Coal Bed Methane'. It takes the old CBM method one step further: the methane gas is not simply released naturally by depressurising the coal, but pushed out by injecting another gas, preferably a gas we want to get rid of anyway, like greenhouse gas CO2. Henk Pagnier says:
"The beauty of the whole thing is that CO2 happens to bind to the coal when injected. Better still, it dissolves the methane gas from the coal in the same chemical process. Instead of the 40 percent of gas which can be extracted from the coal by using the old method, CBM, Enhanced Coal Bed Methane can extract up to 80 percent, and that makes the whole concept economically viable in many more cases."
The purpose of the field experiment in Poland was to prove that the theory could be put into practice. The location was perfect: a depleted CBM production site, where the drill hole for pumping out the water column and harvesting the methane gas could still be used. A second hole was drilled, and CO2 gas pumped in.
The first results were disappointing. Only a minimal amount of methane could be squeezed out, as it were. Dr Pagnier and his colleagues then applied a little trick, inserting a small amount of sand into the CO2. The grains of sand kept open the small cracks that inevitably appear in the coal when pressurised. This hugely enlarged the surface area where CO2 gas could get into direct contact with coal and suddenly the old 'CBM well' did produce promising amounts of Methane.