Review of Underground Coal Gasification

This report consists of a broad review of underground coal gasification, covering basic principles and techniques, but with an emphasis on environmental issues and a summary of the current status of worldwide activities. Underground coal gasification is a technology that was first envisaged in the late 1800's and has been demonstrated using various techniques at site in the former Soviet Union, the USA, the UK, China and various other countries from the 1930's onwards. Commercial application in the past has been limited to several sites in the former Soviet Union, but in recent years different techniques have been demonstrated by Linc Energy and Carbon Energy in Australia, plus Eskom in South Africa. In most forms, the technology uses drilled wells to access the subject coal seam from the surface and uses air or oxygen from an injection well to gasify the coal and the partially oxidised fuel or synthesis gas is recovered through a production well. Improvements in drilling and geological surveying technologies appear to have made the more reliable construction of gasifiers possible; however, concerns exist about the environmental impacts of the technology. Past trials, particularly in the USA, have shown that operations at poorly selected sites can cause excessive subsidence, groundwater consumption and groundwater contamination. In summary, whether or not UCG can be performed in an environmentally sustainable manner depends very strongly on the site as the performance of the technology is extremely site sensitive. While the technology has benefitted from advances in drilling and geological surveying to the extent that it is likely that site selection and construction can be performed with lower risk, it nevertheless remains necessary to validate that the process can be operated reliably with minimal environmental impact on larger scales. The long history of experimental experience with the technology has meant that many potential issues have been addressed in formulating the site selection criteria, plant designs and operating procedures that are now being used. However, the difficulties in instrumenting underground coal gasification sites need to be addressed so that there can be confidence that the process can be fully managed.