With underground coal gasification (UCG), the actual process takes place underground, generally below 1,200 feet. The underground setting provides both the feedstock source as well as pressures comparable to that in an above-ground gasifier. With most UCG facilities, two wells are drilled on either side of an underground coal seam. One well is used to inject air or oxygen (and sometimes steam) into the coal seam to initiate the gasification reactions. The second well is used to collect the synthesis gas (syngas) that is formed from the gasification reactions and to pipe it to the surface for additional processing and use. A pair of wells can last 8-10 years.
The UCG reactions are managed by controlling the rate of oxygen or air that is injected into the coal seam through the injection well. The process can be halted by stopping the injection of the oxygen or air. After the coal is converted to syngas in a particular location, the remaining cavity (which will contain the left over ash or slag from the coal) may be flooded with saline water and the wells are capped. However, there is also growing interest in using these cavities to store carbon dioxide that could be captured from the above-ground syngas processing (i.e., water shift reaction to produce high hydrogen concentration syngas). For more information on CO2, click here. Once a particular section of a coal seam is exhausted, new wells are drilled to initiate the gasification reaction in a different section of the coal seam.
The syngas that is produced from UCG is the same as that produced by above-ground gasification processes—it can be combusted in a gas turbine to produce electricity or further processed to produce chemicals, transportation fuels, or fertilizers. For more information on power and products from syngas, click here.
Converting Unmineable Coal
UCG can help meet the rising energy demand by converting coal to energy and products, utilizing resources that otherwise would be too deep, of poor quality, or simply not economical to mine. Estimates suggest that UCG could help increase the U.S. recoverable coal reserves by as much as 300%-400% (Accelerating Development of Underground Coal Gasification, Dr. S Julio Friedmann, Lawrence Livermore National Laboratory, 2007). In general, UCG can operate at up to about 80% efficiency—that is, the amount of the syngas energy recovered at the surface is about 80% of the original heating value of the coal feedstock. For more information, visit www.llnl.gov/
Economic Benefits of UCG
There are a number of significant economic benefits associated with UCG that include:
- No need for the coal to be mined
- No need for coal handling
- No need to transport the coal
- No need to prepare the coal to be fed into a reactor
- No need for disposing of ash or slag
- No need for an above ground gasification plant
- Significantly lower capital cost for project development than that of above ground plants
Environmental Benefits of UCG
There are also a number of environmental benefits with UCG:
- Minimal land use
- Significantly reduced use of groundwater or freshwater
- Underground saline water is used
- No environmental impacts traditionally associated with coal mining and handling
- Coal seams being gasified are below the fresh drinking water supplies
- Significantly reduced criteria air pollutants
- All or a substantial portion of the sulfur, mercury, arsenic, tar, ash and particulates found in coal remain underground.
- Any sulfur or metals that reach the surface do so in a chemically reduced state, making them easier to remove
- No landfill disposal required for ash or slag
The carbon dioxide that can be generated during the above-ground processing of the syngas (i.e. ,as part of the water shift reaction used to produce a high hydrogen concentration syngas) provides the opportunity to capture and compress it for underground storage or for transport in a pipeline for enhanced oil recovery (EOR). For more information on CO2, click here.
UCG was first developed more than a century ago and commercially deployed in the former Soviet Union in the 1950s. Today, the most UCG activity is occurring in Australia, China, and South Africa—with China having the largest UCG program worldwide. There are a limited number of projects in Canada and the U.S. In addition, India and New Zealand have begun the initial efforts to explore UCG. Several of these projects involve carbon capture and sequestration.
UCG does face a number of issues, including:
- A number of coal seams may not be suitable for UCG because of geologic or hydrologic conditions
- Project economics may be uncertain until a number of commercial UCG projects are in commercial operation
- Site selection needs to be done properly to avoid potential groundwater contamination and surface subsidence.
These issues can be mitigated through careful project design, site selection, and monitoring. UCG has enormous potential to allow the world to take advantage of coal resources that would otherwise be too expensive or difficult to reach.