Underground Coal Gasification (UCG) is a process by which coal seams at depths uneconomic for open pit mining can be converted, using drillhole access, to a gas suitable for use as a fuel for power generation or as a chemical feedstock. Its application draws on a wide range of engineering and related disciplines including chemistry, geology, geohydrology, geotechnical engineering (including drilling techniques), and chemical engineering.

The UCG process is initiated by drilling two adjacent boreholes into a coal seam, which is generally at a depth greater than 100 meters, as schematically illustrated in Figure 1. Both vertical and/or deviated drill holes can be utilized to suit the size and shape of the coal seam.

Figure 1.The UGC Process

An oxidant such as air or oxygen mixed with steam is then injected under pressure into one of the boreholes (the injection well) and is ignited at the coal seam. The resulting chemical reactions convert the coal to a gas, which is then extracted through the second borehole (the production well). Expansion of the process is achieved by the addition and linkage of further injection and production wells. The “coal gas” produced (often referred to as “syngas”) has a low calorific value, and is a mixture of hydrogen, carbon monoxide, methane, carbon dioxide and higher hydrocarbons, along with nitrogen if air is used as the oxidant. After preparation, this syngas can be used to fuel a gas turbine, or as the feedstock for other chemical processing plant.

With an appropriate UCG technology, the cost of the gas per unit of energy is much lower than natural gas. When compared to current coal-fired power generation, these factors combine to provide a competitive cost of power at a smaller scale, with lower CO2 emissions, and longer term potential for CO2 sequestration.

There are a number of environmental advantages of using UCG as a fuel source:

• Elimination of the major land disturbance required for open cut mining, and minimization of rehabilitation work after the completion of operations.
• Removal of human safety concerns associated with underground mining.
• Efficient use of coal resource compared to coal seam methane recovery.
• Underground disposal of ash and waste products.
• Reduced carbon dioxide emissions compared to coal-fired power generation when a combined cycle power plant is used.
• Ability to economically fuel power stations of small capacity, giving development flexibility with respect to both size and location.