Underground Coal Gasification (UCG) is a gasification process applied to non-mined coal seams, using injection and production wells drilled from the surface, which enables the coal to be converted in situ into product gas. The process has produced commercial quantities of gas for both chemical processes and power generation.

The UCG process developed, refined and practiced by Ergo Exergy is called the Exergy UCG™ Technology or εUCG™ Technology. It differs from generic UCG in its higher exergy efficiency - hence lower exergy dissipation into the environment.
During the εUCG process, much as in conventional gasification methods, an oxidant reacts with coal of the underground coal seams, and part of released sensible heat is used in coal drying, pyrolysis and the endothermic reactions that reduce the combustion products. The resulting mixture is εUCG gas. The gas composition depends on the coal geology as well as the process parameters. It can be produced using a variety of oxidants, including air and oxygen-rich gaseous blends.

Process parameters - such as operating pressure, outlet temperature and flow - are governed by the coal and rock properties that vary with time and location. Information on the process conditions must be constantly monitored and updated as the underground gasifier develops. Process parameters need to be adjusted accordingly to accommodate the ever-varying conditions of gasification.

The εUCG technology uses a variety of modern drilling methods, including high-precision directional holes, as well as conventional vertical and inclined (or angled) holes. In its arsenal are various methods of well-linking, the capability to inject different oxidants (air, enriched air, O₂/H₂O, CO₂/O₂ and so on), and a great variety of designs of underground gasifiers. It can be applied to coal in a wide range of geological conditions, with the following preferred parameters:

• Coal seam thickness from 0.5 to 30 m.
• Dip from 0^o to 70^o.
• Depth from 30 to 800 m.
• Calorific value (LHV) from 8.0 to 30.0 MJ/kg (which includes low-quality lignite and bituminous coal).
  
  

Unmined and unminable coal deposits, with such obstacles to mining as high fault frequency, volcanic intrusions and other complex depositional and tectonic features, have been often found a part of the εUCG resource base. In every geological setting, a specific εUCG design will be tailor-made to fit the unique conditions of a target coal seam.

Normally, εUCG is applied to relatively deep coal in water-saturated conditions, although it is also possible to gasify unsaturated coal seams that lie above the water table.

εUCG is an industrial technology that operates large-scale gas production facilities consisting of multiple modules or gasifiers.

The specific benefits of operating a large εUCG underground gasifier include the following:

• A practically unlimited supply of coal will be available for gasification; no external coal and water supply is required to sustain the reaction.
• The εUCG process creates an immense underground gas and heat storage capacity, making the gas supply very stable and robust.
• An underground gasifier comprises a number of underground reactors with largely independent outputs. The gas streams from different reactors can be mixed as required, to ensure consistency of overall gas quality. The outputs of reactors can also be varied, in order to optimize coal extraction and overall gas output from the gasifier.
• No ash or slag removal and handling are necessary, since inert material predominantly remains in the underground cavities.
• Ground water influx into the gasifier creates an effective "steam jacket" around the reactor, making the heat loss in situ tolerably small.
• Optimal pressure in the underground gasifier promotes groundwater flow into the cavity, thus confining the chemical process to the boundaries of the gasifier and preventing contamination of the underground environment.
  

Multiple gasifiers may be required to supply fuel to an industrial consumer; the exact number will depend on the size of the fuel supply required and the precise geology of the coal deposit targeted.

The εUCG process is not only a method of coal conversion; it is a method of extracting coal from the underground beds - for all intents and purposes, a mining technique. There are many similarities between εUCG and underground mining: for example, εUCG is concerned with typical mining issues such as coal extraction efficiency, roof stability and groundwater influx. As a coal recovery method, εUCG supplements traditional mining often utilizing coal seams that are impossible or uneconomical to mine using conventional methods. There are εUCG equivalents of conventional underground mining methods including long-wall, short-wall, and bord-and-pillar methods. A successful εUCG operation factors in roof collapse and overburden deformation as necessary technology attributes.

The εUCG is a fossil fuel technology, and as such must address concerns over global warming. It does so in the following ways:

• The raw εUCG gas contains CO₂ in concentrations that vary depending on process conditions and the choice of oxidant. The gas is produced under pressure and at a moderate temperature, and easily lends itself to CO₂ removal by a range of standard methods, with low energy penalty and at a relatively low cost.
• So captured and removed, CO₂ can be permanently stored (or sequestered) in the underground storage zones created by coal extraction in the εUCG operations. The energy penalty and relative cost of CO₂ re-compression and sequestering are comparatively low. Along with that, CO₂ can be injected into deep saline aquifers and deeper coal seams as well as used for enhanced recovery of oil, natural gas and CBM.
• As in conventional IGCC, εUCG gas can be used to generate electricity with a power island efficiency as high as 55%, and with the overall efficiency of UCG-IGCC process reaching 43%. These efficiencies translate into very low rates of greenhouse emissions per unit of net power generated.
• In chemical manufacturing processes such as Fischer-Tropsch syntheses and production of synthetic methane or fertilizers, CO₂ removal is a routine unit operation. Permanent storage of CO₂ in the εUCG-created permeable zones and the other sinks will significantly greenhouse emissions of the overall process, from the initial coal conversion right through to the end product.
  

The εUCG process is designed and tested to prevent or minimize the other, more traditional environmental impacts on air, soil and water (including surface streams and groundwater). The process is conducted in such a way that gasification pressure in the gasifier is always slightly less than the hydrostatic pressure of fluid in the coal seam and surrounding strata. This creates a pressure gradient directed towards the gasifier. As a result, no flow from the gasifier into the surroundings is allowed, thereby preventing both the loss of valuable product and contamination of the underground environment. The thorough characterization of existing aquifers in the vicinity of the underground gasifier and careful monitoring of the hydrostatic pressure in the aquifers during operations, form an integral part of the εUCG groundwater protection strategy.

The εUCG is an environmentally-friendly and exergy-efficient technology for producing competitively-priced gaseous fuel for power generation and chemical processing.