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Underground coal gasification (UCG) is an industrial process, which enables coal to be converted 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 not economical to extract and also offers an alternative to conventional coal mining methods for some resources. Compared to the traditional coal mining and gasification, the UCG has less environmental and social impact.

Contents

History

The history of UCG is traced back to 1868, when Sir William Siemens suggested the underground gasification of waste and slack coal in the mine.[1] Russian chemist Dmitri Mendeleyev further developed Siemens' idea over the next couple of decades.[2] The first experimental work on UCG was planned to start in 1912 in Durham, the United Kingdom, under the leadership of Nobel Prize winner Sir William Ramsay. However, he was unable to commence the UCG field work before the beginning of the World War I. Subsequently, all efforts in UCG development in Western Europe were discontinued until the end of the World War II.

Ramsay's work did not go unnoticed: In 1913, Russian exile Vladimir Lenin was influenced to write in the newspaperPravda, an article about a "Great Victory of Technology" which promised to liberate workers from the hazardous work in the nation's mines. Almost 20 years later, Stalin supported Soviet engineers in a research and development program targeting development of commercial-scale UCG plants. Thousands of people and significant resources took part in the development of the technology. The estimated cost of replicating these efforts in the West was as much as US$10 billion (at 1976 value)[3]. The first trials in 1937 failed and many top scientists were put on trial with a number being executed. By 1939 the Soviets had successfully begun operating a UCG plant in the Ukraine which was later shut down by German occupation. After the war, the Soviets restarted the UCG program which eventually culminated in the operation of fourteen industrial-scale UCG plants by the end of the 1960s. However, activity subsequently declined due to the discovery of extensive natural gas resources. As a result only one site is still in operation today at Angren in the territory of Uzbekistan.[4]

At about the same time (between the years 1944 and 1959) the shortage in energy and the diffusion of the results of the UCG experiments in the USSR during the period 1934–1940 provoked new interest in UCG in Western Europe. The first research work was directed to the development of UCG in thin seams at shallow depth. The stream method was tested at Bois-la-Dame, Belgium in 1948, and in Djerada, Morocco in 1949. The boreholes method was tested at Newman Spinney and Bayton, United Kingdom, in 1949–1950. A few years later, a first attempt was made to develop a commercial pilot plant—the P5 Trial— at Newman Spinney in 1958–1959. During the 1960s all European work was stopped, due to an abundance of energy and low oil prices. In the USA, a UCG program, built upon Russian experience, was initiated in 1972 and an extensive field testing program was begun, supported by the US Department of Energy and a number of large Oil and Gas companies. A prominent role in the program was played by Lawrence Livermore National Laboratory. The work culminated in the Rocky Mountain 1 trial (1986-1988) that demonstrated exceptional environmental performance. In 1989, the European Working Group on UCG recommended that a series of trials should be undertaken to evaluate the commercial feasibility of UCG. The trial was undertaken by Spain, the UK and Belgium, and was supported by the European Commission. The largest ongoing program is being conducted by China, which includes 16 UCG trials.[5]

The successful demonstration in 1999-2003 near the town of Chinchilla, some 350 km west of Brisbane, in Queensland, Australia has resulted in a surge of interest in the technology. The demonstration involved the gasification of 35,000 tonnes of coal, and resulted in successful environmental performance as per independent audit reports.

Criteria for underground coal gasification

Underground coal gasification projects have specific requirements regarding the coal seam:

  • The seam lies underground at a depth of between 30 and 800 metres (as demonstrated by Ergo Exergy's technology at Chinchilla);
  • The seam thickness is more than 5 metres;
  • The ash content of the coal is less than 60%;
  • The seam has minimal discontinuities; &
  • There are no aquifers nearby (to avoid polluting supplies of drinking water).[6]

Process

The basic underground coal gasification process consists of one production well drilled into the unmined coal-seam for injection of the oxidants, and another] production well to bring the product gas to surface (See Diagram). The coal seam is ignited via the first well and burns at temperatures as high as 1,500 K (1,230 °C), generating carbon dioxide (CO2), hydrogen (H2), carbon monoxide (CO) and small quantities of methane (CH4) and hydrogen sulphide(H2S) at high pressure. As the coal face burns and the immediate area is depleted, the oxidants injected are controlled by the operator, ultimately with the objective of guiding the burn along the seam.

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 satisfactory. For high pressure break-up of the coal, a hydrofraccing, an electric-linkage, and a reverse combustion may be used with varying degrees.[7]

There are two different commercially available underground coal gasification methods. One of the methods uses vertical wells and a method of reverse combustion to open up the internal pathways in the coal. The process was used in the Soviet Union and later it was tested in Chinchilla by using air and water as the injected gases.[7]

Another method that was largely developed in the USA creates dedicated inseam boreholes, using drilling and completion technology adapted from oil and gas production. It has a moveable injection point known as CRIP (controlled retraction injection point) and generally uses oxygen or enriched air for gasification.[7]

Economics

Underground coal gasification allow access to more coal resources than economically recoverable by traditional technologies. By some estimates it will increase economically recoverable reserves by 600 billion tonnes.[8] The Lawrence Livermore National Laboratory estimates that using UCG could increase recoverable coal reserves in the USA by 300%. According to Linc Energy, the capital and operating costs of the underground coal gasification are lower than in traditional mining.[4].

UCG product gas is optimally used to fire combined cycle gas turbine (CCGT) power plants, with some studies suggesting power island efficiencies of up to 55%, with a combined UCG / CCGT process efficiency rate of up to 43%. CCGT power plants using UCG product gas instead of natural gas can achieve higher outputs compare to pulverized-coal-fired power stations (and associated upstream processes) results in a large decrease in GHG emissions.

UCG product gas can also be used for:

  • Synthesis of liquid fuels at a predicted cost equivalent to US$17/bbl;
  • Manufacture of chemicals such as ammonia and fertilizers;
  • Enhanced oil recovery (EOR).

In the roles listed above, UCG product gas replaces the use of natural gas and can provide substantial cost savings. Additional cost savings can be made over traditional coal mining and required coal transport, whereby the UCG process: produces syngas which can piped directly to the end-user, reducing need for rail / road infrastructure; and; lowers the cost of environmental cleanup due to solid waste being confined underground. With regards to new environmental markets, for example an emissions trading scheme,companies whose usual operations involve traditional coal mining and burning may see value in the reduced greenhouse gas emissions (GHG) generated using UCG as an alternative. The expected cost savings could be substantial given the projected high cost of carbon abatement for coal industry companies who are impacted by regulatory schemes e.g. the Australian Government's proposed Carbon Pollution Reduction Scheme.

In 2008, Canadian company Laurus Energy advanced their progress in combining UCG and Carbon capture and storage(CCS) to decrease costly externalities and further increase the technology's environmental credentials.

Waste2Tricity has announced a joint venture with Thornton New Energy Ltd for ultra low carbon emission electricity from coal using UCG combined with low cost fuel cells from AFC Energy PLC: "In a bid to bring to market an ultra low carbon emission technology to convert coal into electricity, Thornton New Energy Ltd and Waste2Tricity Ltd have signed a memorandum of understanding. Also allowing for the capture of carbon dioxide as part of the process, the proposed joint venture is the UK’s first commercial application to generate clean electricity from coal, combining new generation AFC Energy fuel cells with UCG and other proven technologies. The gasification of coal underground generates a fuel with a low emissions profile and the potential for complete carbon capture and storage (CCS) at low energy and financial costs."

"Thornton New Energy, a subsidiary of BCG Energy Ltd, was in January 2009 awarded the first UK licence to carry out UCG and develop deep, previously un-mineable coal reserves under the Firth of Forth, Scotland. Waste2Tricity has exclusive rights for the application of AFC Energy fuel cells with any gasification technology within the UK, including energy from waste."

Environmental and social impacts

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Environmental impacts

UCG offers benefits to the environment over traditional coal mining and coal gasification methods. Most notably UCG eliminates the need for mining. This results in a number of immediate benefits including the elimination of solid waste discharge and reduction in sulphur dioxide (SO2) and nitrogen oxide (NOx).[9] The reduction of solid waste is a major advantage of UCG over traditional coal mining, where large quantities of coal ash, oxides, waste rock and radioactive waste are common discharges. In the case of UCG, this waste is either avoided or contained underground. Due to the absence of coal mining, Appalachian mountaintops are not stripped bare and remain largely preserved, and, there is no need for tailing and ash dams.[2] For comparison, the ash content of UCG syngas is estimated to be approximately 10 mg/m³ compared to smoke from burning where ash content may be up to 70 mg/m³. The containment of ash widens the appeal of UCG technology to nations that have abundant low-quality – high ash content coal reserves and that rely on coal for their energy needs.[10]]]

Subsidence is an issue that is common with all forms of extractive industry. The potential for substantial subsidence is due to removal of the complete seam. While it may leave the ash behind in the cavity, the depth of the void left after UCG would be significantly more than other methods of coal extraction. Subsidence is calculated using a formula that includes depth from the surface, thickness and width of the seam removed, type of formation eg: rock, soil type etc.[11]

When coal is combusted underground, NOx and SO2 atmospheric GHG emissions are lowered, therefore creating the added advantage in stemming acid rain occurrence. Combining UCG with CCS technology allows to compress and reinject some of the CO2 on-site and into the highly permeable depleted gasifer created during the burning process, i.e. where the coal used to be.[2] Also, contaminants such as ammonia and hydrogen sulfide can be removed from UCG product gas used to fire e.g. CCGT power stations at a relatively low cost.

The impact of UCG on ground-water systems has been highlighted by some critics as an environmental concern. Organic and often toxic materials (such as phenol) remain in the underground chamber after gasification and therefore are likely to leech into the ground water, should inappropriate site selection occur. Phenol leechate is regarded as the most significant environmental hazard due to its high water solubility and high reactiveness to gasification. However, research has shown that the persistence of such substances in the water is short and that underground water has been shown to recover within two years.[9] One such example where Lawrence Livermore National University conducted a burn at Hoe Creek, Wyoming (USA) highlights the possible negative environmental impacts of UCG. In this case the operating pressure in the burning cavity was greater than the surrounding rock, resulting in contaminants (including the carcinogen benzene) being pushed away from the cavity and out into the potable groundwater. Since this incident scientists have significantly advanced their knowledge of contaminants produced during the UCG process, and as a result a number of steps have been developed to guard against ground water contamination. Since 1999, the ongoing successful containment of water pollution has been demonstrated at Linc Energy's Chinchilla operation.

UCG does not require an external water source to operate, exhibiting a major environmental advantage over water-intensive coal mining operations and pulverised-coal-fired energy production methods.

Social impacts

Due to the absence of mining in UCG, the risk of injury or death to humans is significantly reduced given that workers no longer need to enter a mine. Impact on the environment is reduced because local communities do not face the detrimental impacts (e.g. air pollution and large scale land degradation) that traditional mining brings.

References

  1. ^ Siemens W., Transactions of Chemical Society, 21, 279, 1868, (citation)
  2. ^ a b c Krupp, F, Horn, M (2008). Earth: The Sequel. Environmental Defense Fund, New york.
  3. ^ Lamb, George H., Underground Coal Gasification, 1977, Energy Technology Review No. 14, Noyes Data Corporation, ISBN 0-8155-0670-8 (citation)
  4. ^ a b "UCG". Linc Energy. http://www.lincenergy.com.au/ucg.php. Retrieved 2007-11-24.  
  5. ^ "Underground Coal Gasification. Current Developments (1990 to date)". CG Engineering Ltd. http://www.coal-ucg.com/current%20developments.html. Retrieved 2007-11-24.  
  6. ^ Andrew Beath (2006-08-18) (PDF). Underground Coal Gasification Resource Utilisation Efficiency. CSIRO Exploration & Mining. http://www.carbonenergy.com.au/uploads/File/carbonenergy/presentations/Innovation&ExcellenceCSIRO%20-%20Aug2006.pdf. Retrieved 2007-11-11.  
  7. ^ a b c "The Basics of UCG". UCG Partnership. http://www.ucgp.com/key-facts/basic-description/. Retrieved 2007-11-11.  
  8. ^ (PDF) Survey of energy resources (21 ed.). World Energy Council (WEC). 2007. p. 7. ISBN 0946121265. http://www.worldenergy.org/documents/ser2007_final_online_version_1.pdf. Retrieved 2007-11-24.  
  9. ^ a b Shu-qin, L., Jun-hua, Y (2002). Environmental Benefits of underground coal gasification. Journal of Environmental Sciences, vol. 12, no. 2, pp.284-288
  10. ^ Katie Walter (2007). "Fire in the Hole". Lawrence Livermore National Library. https://www.llnl.gov/str/April07/Friedmann.html. Retrieved 2008-10-06.  
  11. ^ Subsidence diagram http://www.nswmin.com.au/managing_the_environment/subsidence/longwall_mining_and_subsidence

12. Waste2Tricity and Thornton New Energy UCG deal

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