Underground Coal Gasification
Environmental Factors
Underground Contamination
Absence of coal and ash handling at surface
Production of clean gas requiring reduced processing
High efficiency power generation in gas turbines
No surface gasifier
Trials have shown that small quantities of phenols and benzenes are produced as by-products of the underground gasification process. Most will be carried to the surface with the product gas and removed during gas cleaning. Some will be absorbed by the undisturbed coal and a small proportion will remain in the exhausted cavity and could be a source of potential contamination for the coal and surrounding strata.
How far the contaminants will disperse will depend on the hydrogeology, permeability and geological structure of the coal bed. The hydrogeology of the target area must be well understood and sites for gasification should only be selected where the likelihood of contamination dispersal is minimal. This requires that the proposed UCG site is extensively surveyed at coal seam depth, and monitoring will probably required during and after gasification operations have taken place.
The leakage of gas from the cavity into the adjacent strata is a further factor to consider during site selection, although this is only likely to be a problem with shallow UCG schemes. Low permeability is essential in the target area and operating pressures of the process should be matched as far as possible to the hydrostatic pressure to minimise gas escape.
Surface Contamination
The equipment at surface will include the drilling rigs, wellheads, connecting pipework, and process plant for handling the injection/production gases. A commercial UCG scheme will require a permanent site for the power plant and access, on a temporay basis, to the network of wellheads and connecting pipework above the area of coal under gasification. The use of directional drilling could help to minimises the surface area of land required.
Land restoration programmes should be undertaken as the underlying coal is used up, but the disruption, unlike open cast mining, will have been minimal.
The plant will need to meet all the necessary environmental requirements of process and power plant. Detailed environmental impact assessments will be required.
CO2 Sequestration
In-seam gasification offers the possibility that CO2 produced at surface in the power generation process could be re-injected into the exhausted gasification cavity or adjacent strata. See separate page on UCG and CO2 sequestration.
Importance of Depth
Inseam gasification improves with depth in several ways. Trials have shown that cavity growth and methane production increase with the depth of gasification. The process at depth is more efficient and air emissions will be correspondingly less. In addition, the prospects of watercourse contamination, subsidence, and gas escape will be substantially reduced with a deeper coal seam.
Offshore Coal Fields
The possibility of gasification of the vast reserves of coal that lie offshore would eliminate still further the environmental impact of power generation by UCG. The UK, which has large reserves in the shallow waters of the North Sea, has included a pre-feasibility study of offshore UCG as part of its current programme.
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