Underground Coal Gasification
Encyclopedia
Underground coal gasification
(UCG) is an industrial process, which converts coal
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 unprofitable or technically complicated to extract by traditional mining methods, and it also offers an alternative to conventional coal mining
methods for some resources.
in his address to the Chemical Society of London suggested the underground gasification of waste and slack coal in the mine. Russian
chemist
Dmitri Mendeleyev further developed Siemens' idea over the next couple of decades. In 1909–1910, American, Canadian, and British patents were granted to American engineer Anson G. Betts for "a method of using unmined coal". 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
, and the project was abandoned.
who wrote in the newspaper Pravda
an article "Great Victory of Technology" promising to liberate workers from the hazardous work in the mines by underground coal gasification. Between 1928 and 1939, underground tests were conducted in the Soviet Union
by the state-owned organization Podzemgaz. The first test using the chamber method started on 3 March 1933 in the Moscow
coal basin at Krutova mine. This test and several following tests failed. The first successful test was conducted on 24 April 1934 in Lysychansk
, Donetsk Basin by the Donetsk Institute of Coal Chemistry. The first pilot-scale process started 8 February 1935 in Horlivka
, Donetsk Basin. Production gradually increased, and, in 1937–1938, the a local chemical plant began using the produced gas. In 1940, experimental plants were built in Lysychansk
and Tula
. After the World War II
, the Soviet activities culminated in the operation of five industrial-scale UCG plants in the early the 1960s. However, Soviet activities subsequently declined due to the discovery of extensive natural gas
resources. In 1964, the Soviet program was downgraded. only Angren
site in Uzbekistan
and Yuzhno-Abinsk site in Russia continued operations.
. From 1973–1989, an extensive test was carried out. The United States Department of Energy
and several large oil and gas companies conducted several tests. Lawrence Livermore National Laboratory
conducted three tests in 1976–1979 at the Hoe Creek test site in Campbell County, Wyoming. In cooperation with Sandia National Laboratories
and Radian Corporation, Livermore conducted experiments in 1981–1982 at the WIDCO Mine near Centralia, Washington
. In 1979–1981, an underground gasification of steeply dipping seams was demonstrated near Rawlins, Wyoming
. The program culminated in the Rocky Mountain trial in 1986–1988 near Hanna, Wyoming
.
In Europe, the stream method was tested at Bois-la-Dame, Belgium
, in 1948 and in Jerada
, Morocco
, in 1949. The borehole 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 plan, the P5 Trial, at Newman Spinney in 1958–1959. During the 1960s, European work stopped, due to an abundance of energy and low oil prices, but recommenced in the 1980s. Field tests were conducted in 1981 at Bruay-en-Artois and in 1983–1984 at La Haute Deule, France, in 1982–1985 at Thulin, Belgium, and in 1992–1999 the El Tremedal site, Province of Teruel, Spain
. In 1988, the Commission of the European Communities
and six European countries formed a European Working Group.
In New Zealand, a small scale trial was operated in 1994 in the Huntly Coal Basin. In Australia, tests were conducted starting in 1999. China has operated the largest program since the late 1980s, including 16 trials.
, or steam) to ignite and fuel the underground combustion process. Separate production wells are used to bring the product gas to surface. The high pressure combustion
is conducted at temperature of , but it may reach up to 1500 °C (2,732 °F). The process decomposes coal and generates carbon dioxide
, hydrogen
, carbon monoxide
(CO), methane
. In addition, there are small quantities of various contaminants including sulfur oxide
s , mono-nitrogen oxides , and hydrogen sulfide
. As the coal face burns and the immediate area is depleted, the oxidants injected are controlled by the operator.
There are a variety of designs for underground coal gasification, all of which are designed to provide a means of injecting oxidant and possibly steam into the reaction zone, and also to provide a path for production gases to flow in a controlled manner to surface. 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 adequate. For high pressure break-up of the coal, hydro-fracturing
, electric-linkage, and reverse combustion may be used in varying degrees.
The simplest design uses two vertical wells: one injection and one production. Sometimes it is necessary to establish communication between the two wells, and a common method to use reverse combustion to open internal pathways in the coal. Another alternative is to drill a lateral well connecting the two vertical wells. UCG with simple vertical wells was used in the Soviet Union and was later adopted by Ergo Exergy and tested at Linc's Chinchilla site in 1998–2003. It does work, but the method accesses relatively little coal from each well set and is only likely to cost-effective for very shallow coals with low drilling costs.
In the 1980s and 1990s, a method known as CRIP (controlled retraction and injection point) was demonstrated in the United States and Spain
. This method uses a vertical production well and an extended lateral well drilled directionally in the coal. The lateral well is used for injection of oxidant and steam, and the injection point can be changed by retracting the injector. This method accesses a greater quantity of coal from one set of wells. Carbon Energy was the first to adopt a system, which uses a pair of lateral wells in parallel. This system allows a consistent separation distance between the injection and production wells while progressively mining the coal between the two wells. It provides access to the greatest quantity of coal per well set and also allows greater consistency in production gas quality.
A wide variety of coals are amenable to the UCG process. Coal grades from lignite
through to bituminous may be successfully gasified. A great many factors are taken into account in selecting appropriate locations for UCG, including surface conditions, hydrogeology, lithoglogy, coal quantity, and quality. According to Andrew Beath other important criteria includes:
According to Peter Sallans these criteria are:
claim that UCG capital and operating costs are lower than in traditional mining.
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 of up to 43%. CCGT power plants using UCG product gas instead of natural gas
can achieve higher outputs than pulverized-coal-fired power stations (and associated upstream processes, resulting in a large decrease in greenhouse gas
(GHG) emissions.
UCG product gas can also be used for:
In addition, carbon dioxide produced as a by-product of underground coal gasification may be re-directed and used for enhanced oil recovery
.
Underground product gas is an alternative to natural gas and potentially offers cost savings by eliminating mining, transport, and solid waste. The expected cost savings could increase given higher coal prices driven by emissions trading
, taxes, and other emissions reduction policies, e.g. the Australian Government's proposed Carbon Pollution Reduction Scheme
.
and Linc Energy
have both conducted successful pilot projects in Australia. Yerostigaz, a subsidiary of Linc Energy, produces about 1 million m3s (35,314,666.2 cu ft) of syngas
per day in Angren, Uzbekistan
. The produced syngas is used as fuel in the Angren Power Station. In South Africa
, Eskom
(with Ergo Exergy as technology provider) has operated a demonstration plant in preparation for supplying commercial quantities of syngas for commercial production of electricity. ENN has also operated a successful pilot project in China.
In addition, there are companies developing projects in Australia, UK, Hungary, Poland, Canada, US, Chile, China, Indonesia, India, South Africa, Botswana, and other countries.
and nitrogen oxide
emissions. For comparison, the ash content of UCG syngas is estimated to be approximately 10 mg/m³ compared to smoke from traditional coal burning where ash content may be up to 70 mg/m³. However, UCG operations cannot be controlled as precisely as surface gasifiers. Variables include the rate of water influx, the distribution of reactants in the gasification zone, and the growth rate of the cavity. These can only be estimated from temperature measurements, and analyzing product gas quality and quantity.
Subsidence is a common issue with all forms of extractive industry. While UCG leaves the ash behind in the cavity, the depth of the void left after UCG is typically more than other methods of coal extraction.
Underground combustion produces and and lowers emissions, including acid rain
. The process has advantages for geologic carbon storage. Combining UCG with CCS technology allows re-injecting some of the on-site into the highly permeable rock created during the burning process, i.e. where the coal used to be. Contaminants, such as ammonia
and hydrogen sulfide
, can be removed from product gas at a relatively low cost.
Aquifer contamination is a potential environmental concerns. Organic and often toxic materials (such as phenol
) remain in the underground chamber after gasification and, therefore, are likely to leach into ground water, absent appropriate site selection. Phenol leachate
is the most significant environmental hazard due to its high water solubility and high reactiveness to gasification. Livermore conducted a burn at Hoe Creek, Wyoming
, producing operating pressure in the burn cavity greater than the surrounding rock, forcing contaminants (including the carcinogen
benzene
) into potable groundwater. However, some research has shown that the persistence of such substances in the water is short and that ground water recovers within two years.
Coal gasification
Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide , hydrogen , carbon dioxide and water vapour –from coal...
(UCG) is an industrial process, which converts coal
Coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
into product gas. UCG is an in-situ gasification
Gasification
Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures , without combustion, with a controlled amount of oxygen and/or steam...
process carried out in non-mined coal seams using injection of oxidants
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
, 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
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
for power generation. The technique can be applied to resources that are otherwise unprofitable or technically complicated to extract by traditional mining methods, and it also offers an alternative to conventional coal mining
Coal mining
The goal of coal mining is to obtain coal from the ground. Coal is valued for its energy content, and since the 1880s has been widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United States,...
methods for some resources.
History
The earliest recorded mention of the idea of underground coal gasification was in to 1868, when Sir William SiemensCarl Wilhelm Siemens
Carl Wilhelm Siemens was a German born engineer who for most of his life worked in Britain and later became a British subject.-Biography:...
in his address to the Chemical Society of London suggested the underground gasification of waste and slack coal in the mine. Russian
Russians
The Russian people are an East Slavic ethnic group native to Russia, speaking the Russian language and primarily living in Russia and neighboring countries....
chemist
Chemist
A chemist is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties such as density and acidity. Chemists carefully describe the properties they study in terms of quantities, with detail on the level of molecules and their component atoms...
Dmitri Mendeleyev further developed Siemens' idea over the next couple of decades. In 1909–1910, American, Canadian, and British patents were granted to American engineer Anson G. Betts for "a method of using unmined coal". The first experimental work on UCG was planned to start in 1912 in Durham
Durham
Durham is a city in north east England. It is within the County Durham local government district, and is the county town of the larger ceremonial county...
, the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, under the leadership of Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
winner Sir William Ramsay
William Ramsay
Sir William Ramsay was a Scottish chemist who discovered the noble gases and received the Nobel Prize in Chemistry in 1904 "in recognition of his services in the discovery of the inert gaseous elements in air" .-Early years:Ramsay was born in Glasgow on 2...
. However, he was unable to commence the UCG field work before the beginning of the World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
, and the project was abandoned.
Initial tests
In 1913, Ramsay's work was noticed by Russian exile Vladimir LeninVladimir Lenin
Vladimir Ilyich Lenin was a Russian Marxist revolutionary and communist politician who led the October Revolution of 1917. As leader of the Bolsheviks, he headed the Soviet state during its initial years , as it fought to establish control of Russia in the Russian Civil War and worked to create a...
who wrote in the newspaper Pravda
Pravda
Pravda was a leading newspaper of the Soviet Union and an official organ of the Central Committee of the Communist Party between 1912 and 1991....
an article "Great Victory of Technology" promising to liberate workers from the hazardous work in the mines by underground coal gasification. Between 1928 and 1939, underground tests were conducted in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
by the state-owned organization Podzemgaz. The first test using the chamber method started on 3 March 1933 in the Moscow
Moscow
Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent...
coal basin at Krutova mine. This test and several following tests failed. The first successful test was conducted on 24 April 1934 in Lysychansk
Lysychansk
Lysychans'k is a city in the Luhansk Oblast of south-eastern Ukraine. The city is designated as its own separate raion within the oblast, and is located on the high right bank of the Seversky Donets River, approximately 90 km from the oblast capital, Luhansk.Population of 01.05.2010-...
, Donetsk Basin by the Donetsk Institute of Coal Chemistry. The first pilot-scale process started 8 February 1935 in Horlivka
Horlivka
Horlivka is a city in the Donetsk Oblast of eastern Ukraine. As of 2001, the city's population was 292,000. It is a coal mining and chemical industry centre...
, Donetsk Basin. Production gradually increased, and, in 1937–1938, the a local chemical plant began using the produced gas. In 1940, experimental plants were built in Lysychansk
Lysychansk
Lysychans'k is a city in the Luhansk Oblast of south-eastern Ukraine. The city is designated as its own separate raion within the oblast, and is located on the high right bank of the Seversky Donets River, approximately 90 km from the oblast capital, Luhansk.Population of 01.05.2010-...
and Tula
Tula
Tula may refer to:In geography:*Tula, Hidalgo, a town in Mexico, once the capital and sacred city of the Toltec people.*Tula, Tamaulipas, a place in the state of Tamaulipas, Mexico*Tula River in central Mexico...
. After the World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, the Soviet activities culminated in the operation of five industrial-scale UCG plants in the early the 1960s. However, Soviet activities subsequently declined due to the discovery of extensive natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
resources. In 1964, the Soviet program was downgraded. only Angren
Angren
Angren may refer to:*Angren, Uzbekistan, town near Tashkent, Uzbekistan*Isen , fictional river in Middle-earth, also called Angren*Ngamring County, pinyin Angren, county in Tibet...
site in Uzbekistan
Uzbekistan
Uzbekistan , officially the Republic of Uzbekistan is a doubly landlocked country in Central Asia and one of the six independent Turkic states. It shares borders with Kazakhstan to the west and to the north, Kyrgyzstan and Tajikistan to the east, and Afghanistan and Turkmenistan to the south....
and Yuzhno-Abinsk site in Russia continued operations.
Post-war experiments
After World War II, the shortage in energy and the diffusion of the Soviets' results provoked new interest in Western Europe and the United States. In the United States, tests were conducted in 1947–1960 in Gorgas, AlabamaAlabama
Alabama is a state located in the southeastern region of the United States. It is bordered by Tennessee to the north, Georgia to the east, Florida and the Gulf of Mexico to the south, and Mississippi to the west. Alabama ranks 30th in total land area and ranks second in the size of its inland...
. From 1973–1989, an extensive test was carried out. The United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
and several large oil and gas companies conducted several tests. Lawrence Livermore National Laboratory
Lawrence Livermore National Laboratory
The Lawrence Livermore National Laboratory , just outside Livermore, California, is a Federally Funded Research and Development Center founded by the University of California in 1952...
conducted three tests in 1976–1979 at the Hoe Creek test site in Campbell County, Wyoming. In cooperation with Sandia National Laboratories
Sandia National Laboratories
The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....
and Radian Corporation, Livermore conducted experiments in 1981–1982 at the WIDCO Mine near Centralia, Washington
Centralia, Washington
Centralia is a city in Lewis County, Washington, United States. The population was 16,336 at the 2010 census.-History:In pioneer days, Centralia was the halfway stopover point for stagecoaches operating between the Columbia River and Seattle. In 1850, J. G. Cochran came from Missouri with his...
. In 1979–1981, an underground gasification of steeply dipping seams was demonstrated near Rawlins, Wyoming
Rawlins, Wyoming
Rawlins is a city in Carbon County, Wyoming, United States. The population was 8,538 at the 2000 census. It is the county seat of Carbon County...
. The program culminated in the Rocky Mountain trial in 1986–1988 near Hanna, Wyoming
Hanna, Wyoming
Hanna is a town in Carbon County, Wyoming, United States. The population was 873 at the 2000 census.-Geography:Hanna is located at ....
.
In Europe, the stream method was tested at Bois-la-Dame, Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
, in 1948 and in Jerada
Jerada
Jerada is a town in northeastern Morocco, in the Beni Snassen Mountains. It is located at around , close to the border with Algeria. It has approx. 30,000 inhabitants....
, Morocco
Morocco
Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
, in 1949. The borehole method was tested at Newman Spinney and Bayton
Bayton
Bayton is a village and civil parish in the Malvern Hills District of Worcestershire, England. According to the 2001 census it had a population of 443. The village is about 2½ miles south of Cleobury Mortimer. The Village Hall was built following years of hard work by local villagers. There is a...
, United Kingdom, in 1949–1950. A few years later, a first attempt was made to develop a commercial pilot plan, the P5 Trial, at Newman Spinney in 1958–1959. During the 1960s, European work stopped, due to an abundance of energy and low oil prices, but recommenced in the 1980s. Field tests were conducted in 1981 at Bruay-en-Artois and in 1983–1984 at La Haute Deule, France, in 1982–1985 at Thulin, Belgium, and in 1992–1999 the El Tremedal site, Province of Teruel, Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
. In 1988, the Commission of the European Communities
European Commission
The European Commission is the executive body of the European Union. The body is responsible for proposing legislation, implementing decisions, upholding the Union's treaties and the general day-to-day running of the Union....
and six European countries formed a European Working Group.
In New Zealand, a small scale trial was operated in 1994 in the Huntly Coal Basin. In Australia, tests were conducted starting in 1999. China has operated the largest program since the late 1980s, including 16 trials.
Process
Underground coal gasification converts coal to gas while still in the coal seam (in-situ). Gas is produced and extracted through wells drilled into the unmined coal–seam. Injection wells are used to supply the oxidants (air, oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, or steam) to ignite and fuel the underground combustion process. Separate production wells are used to bring the product gas to surface. The high pressure combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
is conducted at temperature of , but it may reach up to 1500 °C (2,732 °F). The process decomposes coal and generates carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
(CO), methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. In addition, there are small quantities of various contaminants including sulfur oxide
Sulfur oxide
Sulfur oxide refers to one or more of the following:* Lower sulfur oxides * Sulfur monoxide * Sulfur dioxide * Sulfur trioxide *Higher sulfur oxides Sulfur oxide (SOx) refers to one or more of the following:* Lower sulfur oxides (SnO, S7O2 and S6O2)* Sulfur monoxide (SO)* Sulfur dioxide (SO2)*...
s , mono-nitrogen oxides , and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
. As the coal face burns and the immediate area is depleted, the oxidants injected are controlled by the operator.
There are a variety of designs for underground coal gasification, all of which are designed to provide a means of injecting oxidant and possibly steam into the reaction zone, and also to provide a path for production gases to flow in a controlled manner to surface. 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 adequate. For high pressure break-up of the coal, hydro-fracturing
Hydraulic fracturing
Considerable controversy surrounds the current implementation of hydraulic fracturing technology in the United States. Hydraulic fracturing, or fracking, is the process of utilizing pressurized water, or some other liquid, to fracture rock layers and release petroleum, natural gas, or other...
, electric-linkage, and reverse combustion may be used in varying degrees.
The simplest design uses two vertical wells: one injection and one production. Sometimes it is necessary to establish communication between the two wells, and a common method to use reverse combustion to open internal pathways in the coal. Another alternative is to drill a lateral well connecting the two vertical wells. UCG with simple vertical wells was used in the Soviet Union and was later adopted by Ergo Exergy and tested at Linc's Chinchilla site in 1998–2003. It does work, but the method accesses relatively little coal from each well set and is only likely to cost-effective for very shallow coals with low drilling costs.
In the 1980s and 1990s, a method known as CRIP (controlled retraction and injection point) was demonstrated in the United States and Spain
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
. This method uses a vertical production well and an extended lateral well drilled directionally in the coal. The lateral well is used for injection of oxidant and steam, and the injection point can be changed by retracting the injector. This method accesses a greater quantity of coal from one set of wells. Carbon Energy was the first to adopt a system, which uses a pair of lateral wells in parallel. This system allows a consistent separation distance between the injection and production wells while progressively mining the coal between the two wells. It provides access to the greatest quantity of coal per well set and also allows greater consistency in production gas quality.
A wide variety of coals are amenable to the UCG process. Coal grades from lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
through to bituminous may be successfully gasified. A great many factors are taken into account in selecting appropriate locations for UCG, including surface conditions, hydrogeology, lithoglogy, coal quantity, and quality. According to Andrew Beath other important criteria includes:
- Depth of 100–600 m (328.1–1,968.5 )
- Thickness more than 5 metres (16.4 ft)
- Ash content less than 60%
- Minimal discontinuities
- Isolation from valued aquiferAquiferAn aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s.
According to Peter Sallans these criteria are:
- Depth of 100–1400 m (328.1–4,593.2 )
- Thickness more than 3 metres (9.8 ft)
- Ash content less than 60%
- Minimal discontinuities
- Isolation from valued aquifers.
Economics
Underground coal gasification allows access to coal resources that are not economically recoverable by other technologies, e.g., that are too deep, low grade, or seams too thin. By some estimates, UCG will increase economically recoverable reserves by 600 billion tonnes. Livermore estimates that UCG could increase recoverable coal reserves in the USA by 300%. Livermore and Linc EnergyLinc Energy
Linc Energy is an Australian energy company specialized on a coal-based synthetic fuel production. It is engaged in development and commercialization of proprietary underground coal gasification technology. Produced gas is used for production of synthetic fuel through gas-to-liquid technology, and...
claim that UCG capital and operating costs are lower than in traditional mining.
UCG product gas is optimally used to fire combined cycle
Combined cycle
In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...
gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
(CCGT) power plants, with some studies suggesting power island efficiencies of up to 55%, with a combined UCG/CCGT process efficiency of up to 43%. CCGT power plants using UCG product gas instead of natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
can achieve higher outputs than pulverized-coal-fired power stations (and associated upstream processes, resulting in a large decrease in greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
(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 ammoniaAmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and fertilizers; - Production of synthetic natural gas;
- Production of hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
for use in hydrogen fuel cells, ect.
In addition, carbon dioxide produced as a by-product of underground coal gasification may be re-directed and used for enhanced oil recovery
Enhanced oil recovery
Enhanced Oil Recovery is a generic term for techniques for increasing the amount of crude oil that can be extracted from an oil field...
.
Underground product gas is an alternative to natural gas and potentially offers cost savings by eliminating mining, transport, and solid waste. The expected cost savings could increase given higher coal prices driven by emissions trading
Emissions trading
Emissions trading is a market-based approach used to control pollution by providing economic incentives for achieving reductions in the emissions of pollutants....
, taxes, and other emissions reduction policies, e.g. the Australian Government's proposed Carbon Pollution Reduction Scheme
Carbon Pollution Reduction Scheme
The Carbon Pollution Reduction Scheme was a proposed cap-and-trade system of emissions trading for anthropogenic greenhouse gases, due to be introduced in Australia in 2010 by the Rudd government, as part of its climate change policy. It marked a major change in the energy policy of Australia...
.
Projects
Cougar EnergyCougar Energy
Cougar Energy Limited is an Australian energy company specialized on underground coal gasification and electric power production by produced syngas...
and Linc Energy
Linc Energy
Linc Energy is an Australian energy company specialized on a coal-based synthetic fuel production. It is engaged in development and commercialization of proprietary underground coal gasification technology. Produced gas is used for production of synthetic fuel through gas-to-liquid technology, and...
have both conducted successful pilot projects in Australia. Yerostigaz, a subsidiary of Linc Energy, produces about 1 million m3s (35,314,666.2 cu ft) of syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
per day in Angren, Uzbekistan
Angren, Uzbekistan
Angren is a city in eastern Uzbekistan east of Tashkent. It was founded in 1946 as a center for the Uzbek SSR's coal industry. It is located on the Angren River....
. The produced syngas is used as fuel in the Angren Power Station. In South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, Eskom
Eskom
Eskom is a South African electricity public utility, established in 1923 as the Electricity Supply Commission by the government of South Africa in terms of the Electricity Act . It was also known by its Afrikaans name Elektrisiteitsvoorsieningskommissie . The two acronyms were combined in 1986 and...
(with Ergo Exergy as technology provider) has operated a demonstration plant in preparation for supplying commercial quantities of syngas for commercial production of electricity. ENN has also operated a successful pilot project in China.
In addition, there are companies developing projects in Australia, UK, Hungary, Poland, Canada, US, Chile, China, Indonesia, India, South Africa, Botswana, and other countries.
Environmental and social impacts
Eliminating mining eliminates mine safety issues. Compared to traditional coal mining and processing, the underground coal gasification eliminates surface damage and solid waste discharge, and reduces sulfur dioxideSulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
and nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
emissions. For comparison, the ash content of UCG syngas is estimated to be approximately 10 mg/m³ compared to smoke from traditional coal burning where ash content may be up to 70 mg/m³. However, UCG operations cannot be controlled as precisely as surface gasifiers. Variables include the rate of water influx, the distribution of reactants in the gasification zone, and the growth rate of the cavity. These can only be estimated from temperature measurements, and analyzing product gas quality and quantity.
Subsidence is a common issue with all forms of extractive industry. While UCG leaves the ash behind in the cavity, the depth of the void left after UCG is typically more than other methods of coal extraction.
Underground combustion produces and and lowers emissions, including acid rain
Acid rain
Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...
. The process has advantages for geologic carbon storage. Combining UCG with CCS technology allows re-injecting some of the on-site into the highly permeable rock created during the burning process, i.e. where the coal used to be. Contaminants, such as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
, can be removed from product gas at a relatively low cost.
Aquifer contamination is a potential environmental concerns. Organic and often toxic materials (such as phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
) remain in the underground chamber after gasification and, therefore, are likely to leach into ground water, absent appropriate site selection. Phenol leachate
Leachate
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....
is the most significant environmental hazard due to its high water solubility and high reactiveness to gasification. Livermore conducted a burn at Hoe Creek, Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
, producing operating pressure in the burn cavity greater than the surrounding rock, forcing contaminants (including the carcinogen
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...
benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
) into potable groundwater. However, some research has shown that the persistence of such substances in the water is short and that ground water recovers within two years.
External links
- Ergo Exergy Tech - global supplier of UCG technology
- UCG Association
- Energy & Environmental Research Centre (EERC) - UCG overview
- CO2SINUS CO2 Storage in in situ Converted Coal Seams - Research Project at the RWTH Aachen University.