Gasification is the chemical process for converting a solid or liquid fuel into a combustible gas, which can subsequently be used to produce heat, generate power or synthesis a variety of chemicals products such as hydrogen, methanol or synethetic natural gas.

Many hundreds of surface reactors for gasification have been constructed world-wide and about one third operate on coal. The gasification of waste products, as an alternative to waste disposal, is also of growing interest as a commercial option to reduce environmental contamination.

The process of coal gasification in a surface gasifier can be replicated underground by drilling into the hydrocarbon reserves, injecting air or oxygen, and gasifying the in-seam coal seam. The product gases are transported to the surface for processing and utilisation.

The main components of a commercial UCG site for power generation are as follows:

The simplicity of UCG is very attractive but the application of the concept as a large-scale method of coal conversion has proved more difficult.

It is clear from previous tests that the natural permeability of the coal seam to transmit the gases to and from the combustion zone can be unreliable. Methods such as reversed combustion and hydrofracing can be used to improve the flow path. For gasification over long distances in the coal seam, a properly constructed in-seam channel is preferre, before the coal seam is ignited and the gasification cavity is developed. Various methods have been attempted to construct the in-seam channel including:

• Drilling from an outcrop
• Slant drilling from the surface
• Constructing man-made in-seam galleries
• Directional drilling

These methods have all been used in the various trials and commercial projects that have taken place, but until recently no consensus had emerged that well construction for UCG was a reliable or cost effective process.

The technology of directional underground drilling advanced considerably in the 1990's as a result of developments in the oil and gas industries. The same technology is being used regularly for the de-gassing of coal seams in Australia, South Africa and the United States. For the first time, in-seam coal wells can be constructed reliably and accurately, with much less risk of failure than previously encountered.

Furthermore, the option of constructing gasification wells in much deeper coal seams, say over 1000 m, becomes possible, which has advantages in terms of cavity growth, power output and environmental benefits. (including the possibility of CO2 sequestration, see environmental factors)