The process of converting hard coal into gas has been around for nearly 200 years. The first practical application of gas derived from gasifying mined coal at the surface was in London's Street lighting system in 1807.
As early as 1812, the Gas Light and Coke Company of London were formed and, by the mid 19th Century, surface coal gasification was a major industry in most of the rapidly developing industrialised nations.
UCG clean coal technology uses a similar process to that used in surface gasification. The main difference between both gasification processes is that in UCG the cavity itself becomes the reactor so that the gasification of the coal takes place underground instead of at the surface.
Here is a link that might be useful:
Underground Coal Gasification