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Coal clean-up goes underground

A team in Germany has calculated that underground coal gasification combined with an above ground electricity plant and storage of the resulting carbon dioxide back underground can both cut carbon emissions and compete with other energy technologies in the European market. Thomas Kempka and colleagues at the research centres of GFZ Potsdam and RWTH Aachen see the technology as a bridging measure until use of renewable technologies becomes more widespread.

The technique works by drilling two wells into a deep coal seam. The first, an injection well, introduces oxygen and water vapour into the seam, encouraging gasification of the coal at high pressure to produce hydrogen, methane, carbon monoxide and carbon dioxide, which leaves the seam via the production well. The methane and hydrogen can be used as fuel directly, or to create methanol, and also to run a combined cycle electricity plant. Carbon dioxide removed at the surface can then be pumped back down and stored in coal seams where gasification has been completed.

The team reckons the maximum amount of carbon capture achievable in this way is 86%. This would create an energy generation technique 20% cheaper than nuclear electricity generation but with similar emissions. But even a 50% capture rate would bring coal emissions down to those of natural gas.

There has already been considerable research into carbon storage in empty coal seams and into coal gasification, which gives the researchers a head-start, although they reckon it will be 15-20 years before large-scale use of the process. The team has calculated that Europe would have enough coal for this Underground Coal Gasification-Carbon Capture and Storage technique to fulfil all its energy needs for 68 years, although they believe that is important to maintain a mix of sources.

Kempka and colleagues are currently using a 25 square km sample area in Germany that contains 7 coal seams for theoretical studies. They have calculated that to run a 600 MW power plant requires nine square kilometres of coal seam with an average thickness of 1.5 m. This would give a runtime of 20 years, with each seam being used up after about three years and becoming available for carbon dioxide storage. Storage during the first three years, meanwhile, could take place in saline aquifers.

Several aspects of the technique require further investigation, said Kempka at the EGU meeting, including environmental issues such as subsidence, aquifer pollution from compounds released during the gasification process, the safety of the carbon storage, and whether carbon dioxide can dissolve pollutants and transport them elsewhere underground.

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