Underground Coal Gasification: An Overview of an Emerging Coal Conversion Technology

By Cliff Mallett
Chairman,A Underground Coal Gasification Association
Technical Director, Carbon Energy Limited

Fossil fuels undeniably remain the world’s principal source of energy. They have underpinned the growth of industry and standards of living for the last 300 years. However, finding ways to continue to utilize fossil fuels in a low-carbon and otherwise environmentally-friendly manner is a global priority.

Underground coal gasification (UCG) is one approach to energy production that may allow for emissions and other environmental impacts to be effectively managed. Decarbonization could be achieved by gasifying coal and reforming the syngas product to hydrogen (H2, a clean energy carrier) and safely store the carbon dioxide (CO2).

UCG demonstration rig

UCG demonstration rig

Coal gasification has been carried out for centuries. During the 19th and early 20th centuries numerous towns had their own gas works, responsible for making coal gas (i.e., syngas) from mined coal. The gas was piped to homes and industry. Coal gas, or town gas, is now referred to as syngas and is a mixture of energy gases such as H2, carbon monoxide (CO), as well as methane (CH4).

The development of carrying out gasification underground, UCG, can be attributed to researchers and innovators from around the world. The earliest recorded idea of producing energy by gasifying coal underground came from Sir William Siemens in the late 1800s.1 Working with his brothers, a coal gasifier was invented, which Siemens suggested be placed underground.

The subsequent major step in the development of UCG was in 1910 when patents were granted to an American engineer for UCG methods that closely resemble modern approaches. Then, in 1912, a British chemist, Sir William Ramsay, proposed gasifying coal underground as a way to avoid emissions from burning coal, which were resulting in air quality issues in cities at the time. He believed that this coal-derived syngas would be the fuel of the future.

Ramsay began preparations to trial UCG, but the outbreak of World War I derailed his plans. Interest in UCG was rekindled in the 1930s with the USSR conducting extensive experiments. However, the program was scaled back in the 1960s when the USSR discovered huge natural gas and oil reserves. More recently, momentum has grown yet again as countries including China, the U.S., Canada, Argentina, and Chile have commenced UCG projects.


Most coal-derived energy is obtained when the contained carbon reacts with oxygen (O2), yielding CO2 and releasing energy in the form of heat. If excess O2 is present, combustion occurs with nearly all the carbon converted to CO2. When coal is gasified in an O2-deficient environment, some coal is converted to heat and CO2 and this heat drives the conversion of the remaining coal to syngas. Syngas generated from UCG contains about 80% of the energy that was in the original coal.

To gasify coal underground, O2 or air is pumped down a borehole into a coal seam, the coal is gasified in a cavity created by the conversion of coal to syngas, and the sygnas is extracted through a different (i.e., production) borehole. A number of underground gasifier designs have been demonstrated, the latest being from Australia-based Carbon Energy. In a demonstration project its technology provided consistently high-energy syngas over 20 months and demonstrated the same could be achieved from a single panel of coal for up to 10 years (see article on page 61 for further details).


The primary reason to gasify coal underground is the low cost of energy production. Estimates from UCG companies on the cost of producing UCG syngas range from US$1–3/GJ depending on the coal deposit and on whether air or oxygen is used as the oxidant.2,3 Additional UCG benefits include:

  • It is applicable to very large, deep resources that can consist of low-quality coal not suited for conventional mining (normally conventional mining occurs above 1000 m). The estimated amount of usable coal at such depths could equal or exceed all current mineable coal resources and be a game changer for global energy supply.
  • The energy is produced as syngas, which is readily cleaned using existing processes and transported via pipelines.
  • Multiple uses exist for syngas, such as a fuel for power station gas engines to produce electricity, or chemical feedstock for the production of fertilizers, diesel and gasoline, and methanol derivatives such as olefins and plastics. Syngas can also be readily processed into natural gas.
  • Compared to coalbed methane extraction from the same coal seam, UCG generates over 60 times more energy.
  • UCG offers a small environmental footprint with little surface impact and minimal waste generation.
  • The health and safety issues associated with people working underground can be avoided.
One important benefit of UCG is the small footprint.

One important benefit of UCG is the small footprint.


Early 20th-century UCG trials resulted in significant lessons learned that allowed researchers and technology providers to improve the efficiency and environmental credentials of UCG. One of the major concerns related to UCG has been the ability to avoid affecting groundwater quality. Modern UCG technologies have evolved to ensure destruction of potential contaminants as part of the gasification and decommissioning processes, as well as managing operating pressures to protect groundwater.

A particular observation that evolved from early trials and subsequent research was the “Clean Cavern” concept. This is the process whereby the gasifier is self-cleaned via the steam produced during operation and following decommissioning (during decommissioning while the ground retains heat steam continues to be generated). Another important practice is ensuring that the pressure of the gas in the gasifier is always kept below that of the groundwater surrounding the gasifier cavity. Thus, groundwater is continuously flowing into the gasifier and liquids which could potentially contain chemicals will not be pushed out into the surrounding strata (see Figure 1). The pressure is controlled by the operator using pressure valves at the surface.

FIGURE 1. Operating UCG with a pressure lower than the surrounding area draws groundwater toward the gasifier.

FIGURE 1. Operating UCG with a pressure lower than the surrounding area draws groundwater toward the gasifier.

In addition, the high temperature in the cavity during gasification destroys many of the potentially contaminating organic by-products produced during the process. When operation of a gasifier is stopped, the groundwater pressure in the cavity is reduced to near atmospheric pressure (much lower than the surrounding pressure) to increase the volume of groundwater flowing into the cavity, which increases steam production. A significant percentage of remaining by-products are carried to the surface as vapor via the production well and combusted. This overall approach to UCG has now been successfully implemented at sites in the U.S., Spain, Australia, and South Africa.

Another historic concern related to UCG has been the ability to understand and predict ground subsidence. The UCG process creates a cavity similar to those found at conventional underground coal mines. These cavities are well understood thanks to conventional mining, and thus their behavior can be predicted accurately with modern 3D computer models. Similar to conventional underground coal mining, ground subsidence is predicted before UCG operations commence; if surface subsidence is predicted to significantly affect current or future land use or infrastructure, UCG will not proceed at that particular site.

One of the most rigorous long-term environmental evaluations of UCG pilot sites was carried out by the Queensland Government in Australia from 2008 to 2014. An Independent Scientific Panel appointed by the state government reviewed four years of UCG Pilot Project operations and concluded in the “Independent Scientific Panel Report on the Underground Coal Gasification Pilot Trials” (June 2013) that UCG “could be conducted in a manner that is socially acceptable and environmentally safe when compared to a wide range of resource using activities”.

Decommissioning and rehabilitation of an underground UCG gasifier cavity had not been attempted in the Queensland trials at the time of the ISP evaluations, but in late 2014, independent experts advised the government that Carbon Energy had successfully decommissioned its gasifier, and steam cleaning of the cavity resulted in the cavity posing no environmental or health risks. Groundwater quality will rapidly and naturally be restored to pre-project conditions and no active remediation is required.


Industrial processes require specific, controlled conditions for optimal and safe operation and UCG is no exception. The conditions required for operation of the underground gasifer are established through exploration, prior to construction or operation of a UCG panel. For example, proper UCG site selection is critical—several hydrogeological conditions must be satisfied before proceeding with construction.

First, the coal seam being gasified must be overlain by impermeable strata. The buoyancy of the gas forces it to move upward; thus, the gas will be lost unless the coal seam is capped by strata through which the gas cannot pass, such as shale or clay beds. Second, as coal seams always have some permeability and gas is able to move laterally through coal, the groundwater in the surrounding coal seam must be at a higher pressure than the pressure in the gasifier to prevent the flow of gas away from the gasifier cavity. These primary criteria are illustrated in Figure 2. Other characteristics also must exist at a suitable UCG site—for example adequate groundwater pressure for gasification to occur, coal seams of adequate thickness to maintain gasification temperatures, and appropriate separation from overlying and underlying water-bearing formations.

FIGURE 2. Primary criteria required for a suitable UCG site.

FIGURE 2. Primary criteria required for a suitable UCG site.

Field tests and digital modeling facilitate the development of hydrological models that can be used to predict risks to water supplies. Just as with subsidence modeling, if harmful effects are predicted in the exploration stage, UCG will not proceed.

Similar to other resource production industries, UCG requires appropriate pre-development exploration and investigations to ensure that hydrogeological conditions suit the technology being applied.


Until recently, there have been few new developments in UCG. A commercial UCG plant has been running for many years in Uzbekistan; however detailed information on the operation or output of that plant has not been made public. Developed countries with accessible resources have chosen to access shallower coal deposits using traditional mining methods. Additionally, projects based on traditional approaches to UCG have struggled to produce a consistent, high-quality syngas.

Looking at almost a hundred historical UCG sites worldwide,5 the main difficulties can be categorized as follows:

  • Insufficient knowledge of the site geology
  • Inability to drill boreholes with necessary precision
  • Operating with inappropriate gasification parameters
  • Lack of understanding of the impact of the gasification process on the surrounds of the underground cavity.

More recently, however, there have been major technological innovations which have addressed the issues encountered in previous UCG projects (see Table 2).


These advances facilitate proper site investigation, UCG design performance modeling, and identification of issues with respect to product gas or environmental impacts which demand specification or exclude the site as a UCG prospect. In addition, UCG operators now have access to real-time control of underground processes. This allows interpretation of changes in UCG performance and the design of appropriate responses.

The UCG ignition panel is used to carefully control the process underground.

The UCG ignition panel is used to carefully control the process underground.

Since 2000, long-term UCG pilots in Australia, China, and South Africa utilizing the technologies shown in Table 2 have successfully demonstrated that deep UCG can be low cost and environmentally benign. Results from these trials continue to demonstrate that UCG’s major challenges have been resolved and has led China to incorporate this technology into its Five-Year Plan process for resources and energy.

Recent progress and innovation have made it possible that UCG will be an important technology in the future energy mix. However, progress in nontechnical areas must be made with respect to the interrelated areas of government regulation, community understanding and engagement, and project financing.

Given that the production cost of UCG syngas can be significantly lower than that for production of energy by other means, and its demonstrated environmental credentials, UCG presents an opportunity for high-potential growth investors looking for approaches to generate low-emissions power, synthetic natural gas and other fuels, and chemicals from coal.


Energy demands continue to grow globally, particularly in emerging economies in Asia and Africa. At the same time, there is pressure to minimize the cost and maximize the availability of energy supplies as well as the social imperative to reduce the environmental impact associated with energy.

The adaption and application of new petroleum and mining techniques have demonstrated that consistent supplies of high-quality syngas can be safely produced in commercial-scale UCG projects. Further progress and innovation in the field of UCG has been seen recently and several new commercial UCG projects are nearing commencement. Once the first commercial project is successfully established, I believe there will be an avalanche of follow-on projects, and the industry will become a valuable contributor to global energy production.

The syngas created underground is collected and processed above ground.

The syngas created underground is collected and processed above ground.

A. Dr. Cliff Mallett served as Chairman of the Underground Coal Gasification Association from 2013 to 2015. His tenure at that position concluded near the time of article preparation. Dr. Mallett is also Technical Director at Carbon Energy. Thus, some of the technical innovation discussed in the article is based on his direct involvement with Carbon Energy.


  1. Klimenko, A.Y. (2009). Early ideas in underground coal gasification and their evolution. Energies, 2(2), www.mdpi.com/1996-1073/2/2/456
  2. Carbon Energy. (2012, 26 June). Carbon Energy UCG syngas – low cost source of natural gas. ASX/Media Announcement, www.carbonenergy.com.au/IRM/Company/ShowPage.aspx/PDFs/1561-83497961/LowCostSourceofNaturalGas
  3. Pricewaterhouse Coopers. (2008, May). Industry review and an assessment of the potential of UCG and UCG value added products, www.lincenergy.com/data/media_news_articles/relatedreport-02.pdf
  4. Moran, C., da Costa, J., & Cuff, C. (2013, June). Independent Scientific Panel report on underground coal gasification pilot trials, Independent Scientific Panel to the Queensland Government, www.fraw.org.uk/files/extreme/derm_2013.pdf
  5. UCG Association. (2015). Worldwide UCG projects and developments, www.ucgassociation.org/index.php/ucg-technology/worldwide-ucg-projects-developments (accessed April 2015)

The author can be reached at Cliff@carbonenergy.com.au


The content in Cornerstone does not necessarily reflect the views of the World Coal Association or its members.
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