Underground coal gasification (UCG) is an industrial process, which converts coal into product gas. UCG is an in-situ gasification process carried out in non-mined coal seams using injection of oxidants, and bringing the product gas to surface through production wells drilled from the surface. The product gas could to be used as a chemical feedstock or as fuel for power generation. The technique can be applied to resources that are otherwise unprofitable or technically complicated to extract by traditional mining methods and it also offers an alternative to conventional coal mining methods for some resources.

Process of Underground Coal Gasification

Underground coal gasification converts coal to gas while still in the coal seam (in-situ). Gas is produced and extracted through wells drilled into the unmined coal–seam. Injection wells are used to supply the oxidants (air, oxygen, or steam) to ignite and fuel the underground combustion process. Separate production wells are used to bring the product gas to surface. The high pressure combustion is conducted at temperature of 700–900 °C (1290–1650 °F), but it may reach up to 1,500 °C (2,730 °F). The process decomposes coal and generates carbon dioxide (CO₂), hydrogen (ḥ), carbon monoxide (CO) and small quantities of methane (CH₄) and hydrogen sulfide(H₂S). As the coal face burns and the immediate area is depleted, the oxidants injected are controlled by the operator.

As coal varies considerably in its resistance to flow, depending on its age, composition and geological history, the natural permeability of the coal to transport the gas is generally not adequate. For high pressure break-up of the coal, hydro-fracturing, electric-linkage, and reverse combustion may be used in varying degrees.

Two methods are commercially available. One uses vertical wells and a method of reverse combustion to open internal pathways in the coal. The process was used in the Soviet Union and was later modified by Ergo Exergy. It was tested in Chinchilla site in 1998–2003. Livermore developed another method that creates dedicated inseam boreholes, using drilling and completion technology adapted from oil and gas production. It has a movable injection point known as CRIP (controlled retraction injection point) and generally uses oxygen or enriched air for gasification.

According to the Commonwealth Scientific and Industrial Research Organisation the following coal seam characteristics are most suitable for the underground coal gasification:

• Depth of 100–600 metres (330–2,000 ft)
• Thickness more than 5 metres (16 ft)
• Ash content less than 60%
• Minimal discontinuities
• No nearby aquifers (to avoid polluting supplies of drinking water).