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Combustion of oil shale

Oil shale is a fine-grained sedimentary rock, containing significant amounts of kerogen (a solid mixture of organic chemical compounds), from which liquid hydrocarbons can be manufactured. The name oil shale has been described as a promotional misnomer, since the rock is not necessarily a shale and its kerogen is not crude oil; it requires more processing than crude oil, which affects its economic viability as a crude oil substitute.[1][2] Deposits of oil shale are located around the world, including major deposits in the United States. Global deposits are estimated as equivalent to 2.8–3.3 trillion (2.8–3.3 x 1012) U.S. barrels of recoverable oil.[2][3][4][5]

The kerogen in oil shale can be converted to synthetic crude oil through the chemical process of pyrolysis. When heated to a sufficiently high temperature a vapor is driven off which can be distilled (retorted) to yield a petroleum-like shale oil—a form of non-conventional oil—and combustible shale gas (shale gas can also refer to gas occurring naturally in shales). Oil shale can also be burnt directly as a low-grade fuel for power generation and heating purposes, and can be used as a raw material in the chemical and construction materials industries.[6][2]

Oil shale has gained attention as an energy resource as the price of conventional sources of petroleum has risen, and as a way to secure independence from external suppliers of energy.[7][8] The oil shale industry is well-established in Estonia, China, and Brazil, and the United States is taking steps in that direction. At the same time oil shale mining and processing involves a number of environmental issues, such as land use, waste disposal, water use and waste water management, and air pollution.[9][10] The industry has foundered in Australia due to its opposition on these grounds.

Geology

Main article: Oil shale geology
File:OilShaleEstonia.JPG
Outcrop of Ordovician oil shale (kukersite), northern Estonia

Oil shale is an organic-rich sedimentary rock, which belongs to the group of sapropel fuels.[11] It is differentiated from bitumen-impregnated rocks (tar sands and petroleum reservoir rocks), humic coals and carbonaceous shale. While tar sands have been created by biodegradation of oil, the kerogen in oil shales has not yet been naturally transformed into petroleum by heat and pressure.[2][12][13] Coal contains a higher percentage of organic matter than oil shale. In commercial grades of oil shale the ratio of organic matter to mineral matter is about 0.75:5 to 1.5:5. At the same time, the organic matter in oil shale has an atomic ratio of hydrogen to carbon approximately the same as for crude oil and four to five times higher than for coals.[2][11]

Oil shale does not have a definite geological definition nor a specific chemical formula. Oil shales vary considerably in their mineral content, chemical composition, age, type of kerogen, and depositional history.[14] Its organic components are derived from a variety of organisms, such as the remains of algae, spores, pollen, plant cuticles and corky fragments of herbaceous and woody plants, and cellular debris from other aquatic and land plants.[2][15] Some deposits contain significant fossils; Germany's Messel Pit is a Unesco World Heritage Site. The mineral matter in oil shale includes various fine-grained silicates and carbonates.[11][6]

Based on their composition, oil shales are classified as carbonate-rich shales, siliceous shales, and cannel shales.[16] Another classification, assigning kerogen types, is based on the hydrogen, carbon, and oxygen content of oil shales' original organic matter. This classification is known as the van Krevelen diagram.[14] The most used classification of oil shales was developed between 1987 and 1991 by Adrian C. Hutton of the University of Wollongong, adapting petrographic terms from coal terminology. According to this classification, oil shales are designated as terrestrial, lacustrine (lake-bottom-deposited), or marine (ocean bottom-deposited), based on the environment where the initial biomass was deposited. [6][17] Hutton's classification scheme has proven useful in estimating the yield and composition of the extracted oil.[2]

Reserves

Main article: Oil shale reserves
File:OilShaleFossilsEstonia.JPG
Fossils in Ordovician oil shale (kukersite), northern Estonia

Some analysts, along with the United States Geological Survey, draw a distinction between oil shale resources and oil shale reserves. "Resources" may refer to all oil shale deposits, while "reserves" is more narrowly defined as those deposits from which oil can profitably be extracted using existing technologies. Since extraction technologies are still developing, the amount of recoverable kerogen can only be estimated.[6][18] Although oil shale resources occur in many countries, only 33 countries possess deposits of possible economic value.[19][20] Well-explored deposits, which could be classified as reserves, include the Green River deposits in the western United States, the Tertiary deposits in Queensland, Australia, deposits in Sweden and Estonia, the El-Lajjun deposit in Jordan, and deposits in France, Germany, Brazil, China, and Russia. It is expected that these deposits would yield at least 40 liters of shale oil per tonne of shale, using the Fischer assay.[6][14]

Total world resources of oil shale were estimated in 2005 at 411 gigatons, which is enough to yield 2.8 to 3.3 trillion barrels (450 to 520 million tonnes) of shale oil.[2][3][4][5] This is more than world's proved conventional oil reserves, estimated to be 1.317 trillion barrels, as of 1 January 2007.[21] The largest deposits in the world are found in the United States in the Green River basin, which covers portions of Colorado, Utah, and Wyoming; about 70% of this resource is located on federally owned or managed land.[22] Deposits in the United States constitute 62% of world resources; together, the United States, Russia and Brazil account for 86% of the world's resources in terms of shale oil content.[19] These figures are considered tentative, as several deposits have not yet been explored or analyzed.[6][2]

History

Main article: History of the oil shale industry
Production of oil shale (megatons) in Estonia (Estonia deposit), Russia (Leningrad and Kashpir deposits), United Kingdom (Scotland, Lothians), Brazil (Iratí Formation), China (Maoming and Fushun deposits), and Germany (Dotternhausen) from 1880 to 2000[6]

Oil shale has been used as a fuel since prehistoric times, since it generally burns without any processing.[23] It was polished and formed into ornaments by Iron Age Britons.[24] Modern industrial oil shale mining began in 1837 in Autun, France, followed by Scotland, Germany, and several other countries.[25][2] Operations during the 19th century focused on the production of kerosene, lamp oil, and paraffin; these products helped supply the growing demand for lighting that arose during the Industrial Revolution.[26] Fuel oil, lubricating oil and grease, and ammonium sulfate were also produced.[27] The oil shale industry expanded immediately before World War I because of limited access to conventional petroleum resources and the mass production of automobiles and trucks, which generated an increase in gasoline consumption.

Although the Estonian and Chinese oil shale industries continued to grow after World War II, most other countries abandoned their projects due to high processing costs and the availability of cheaper petroleum.[6][2][25][28] Following the 1973 oil crisis, world production of oil shale reached a peak of 46 million tonnes in 1980 before falling to about 16 million tonnes in 2000, due to competition from cheap conventional petroleum.[19][9] The global oil shale industry began to revive in the mid-1990s. In 2003, an oil shale development program was restarted in the United States. A commercial leasing program permitting the extraction of oil shale and tar sand resources on federal lands was introduced in 2005, in accordance with the Energy Policy Act of 2005.[29][30][31]

Industry

Main article: Oil shale industry

As of 2008, oil shale is used industrially in Brazil, China, Estonia and to some extent in Germany, Israel, and Russia. Several additional countries were assessing their reserves or had built experimental production plants, while others had phased out their oil shale industry.[2] Oil shale is used for oil production in Estonia, Brazil, and China; for power generation in Estonia, China, Israel, and Germany; for cement production in Estonia, Germany, and China; and by chemical industries in China, Estonia, and Russia.[28][2][32][33] As of 2005, Estonia alone accounted for about 70% of the world's oil shale production.[32][34]

Romania and Russia have run oil shale-fired power plants, but have shut them down or switched to other fuel sources such as natural gas. Jordan and Egypt are planning to construct oil shale-fired power plants, while Canada and Turkey plan to burn oil shale along with coal for power generation.[2][19][35] Oil shale is used as the main fuel for power generation only in Estonia, where the oil shale-fired Narva Power Plants accounted for 95% of electrical generation in 2005.[36]

Extraction and processing

Main article: Oil shale extraction
Shell's experimental in-situ oil shale facility, Piceance Basin, Colorado

Oil shale is usually mined and then shipped elsewhere, after which it is directly burnt to generate electricity or undergoes further processing. The most-often used methods of surface mining are open pit mining and strip mining. These procedures remove most of the overlying material to expose the oil shale deposits, and are practical when the deposits are close to the surface. Underground mining of oil shale, which removes less of the overlying material, employs the room-and-pillar method.[37]

The extraction of its useful components usually takes place above ground (ex situ processing), although several newer technologies perform this underground (on-site or in situ processing).[38] In either case, after access to the shale is gained, its kerogen is converted to synthetic crude oil and shale gas through the chemical process of pyrolysis. Most conversion technologies involve heating shale in the absence of oxygen to a temperature at which kerogen is decomposed (pyrolysed) into gas, condensable oil, and a solid residue; this usually takes place between 450 °C (842 °F) and 500 °C (932 °F).[18] The process of decomposition begins at relatively low temperatures (300 °C (570 °F)*), but proceeds more rapidly and more completely at higher temperatures.[39]

During the course of in-situ processing, the oil shale is heated underground. These technologies can potentially extract more oil from a given area of land than ex-situ processes, since they can access the material at greater depths than surface mines.[40] Several companies have patented methods for in-situ retorting. However, most of these methods are still in the experimental phase. The methods are usually classified as true in-situ processes (TIS) and modified in-situ processes (MIS). True in-situ processes do not involve mining the oil shale. Modified in-situ processes drill a large shaft to transport workers and equipment to the shale formation, fracture the deposit and crush it, and ignite the rubble.[41]

There are hundreds of patents for oil shale retorting technologies;[42] however, only a few dozen have been tested. As of 2006, only five technologies were in commercial use: Kiviter, Galoter, Fushun, Petrosix, and Alberta Taciuk.[43]

Applications and products

Oil shale can be used as a fuel for thermal power plants, where, like coal, it is burned to drive steam turbines; some of these plants employ the resulting heat for district heating of homes and businesses. Sizable oil shale-fired power plants are located in Estonia, which has an installed capacity of 2,967 megawatts (MW), Israel (12.5 MW), China (12 MW), and Germany (9.9 MW).[19][44]

In addition to its use as a fuel, oil shale may also be used for production of specialty carbon fibers, adsorbent carbons, carbon black, phenols, resins, glues, tanning agents, mastic, road bitumen, cement, bricks, construction and decorative blocks, soil additives, fertilizers, rock wool insulation, glass, and pharmaceutical products.[32] However, oil shale use for production of these items is still small or only in its experimental stages.[6][2] Some oil shales yield sulfur, ammonia, alumina, soda ash, uranium, and nahcolite as shale oil extraction byproducts. Between 1946 and 1952, a marine type of Dictyonema shale was used for uranium production in Sillamäe, Estonia, and between 1950 and 1989 alum shale was used in Sweden for the same purposes.[6] Another of its uses has been as a substitute for natural gas, but as of 2008, producing shale gas as a natural gas substitute is not economically feasible.[45][46]

The oil derived from oil shale is not a direct substitute for crude oil in all applications. It contains higher concentrations of olefins, oxygen, and nitrogen than conventional crude oil, as well as higher viscosities. By comparison with West Texas Intermediate, the benchmark standard for crude oil in the futures contract market, shale oil sulfur content ranges up to 9.5% by weight, where West Texas Intermediate's sulfur content is limited to no more than 0.42%.[47][48] The higher concentrations of these materials means that the oil must undergo considerable upgrading before being used as oil refinery feedstock.[49] Shale oil does not contain the full range of hydrocarbons used in modern gasoline production, and could only be used to produce middle-distillates such as kerosene, jet fuel, and diesel fuel.[4] Worldwide demand for these middle distillates, however, is increasing rapidly.[50]

Economics

Main article: Oil shale economics
Medium-term prices for light-sweet crude oil in US dollars, 2005–2007 (not adjusted for inflation)

During the beginning of the 20th century, the crude oil industry expanded. Since then, the various attempts to develop oil shale deposits have been successful only when the cost of shale oil production in a given region was less than the price of crude oil or its other substitutes.[51]

According to a survey conducted by the RAND Corporation, the cost of producing a barrel of oil at a surface retorting complex in the United States (comprising a mine, retorting plant, upgrading plant, supporting utilities, and spent shale reclamation), would be between US$70–95 (adjusted to 2005 values). This estimate considers varying levels of kerogen quality and extraction efficiency. In order for the operation to be profitable, the price of crude oil would need to remain above these levels. The analysis also discusses the expectation that processing costs would drop after the complex was established. The hypothetical unit would see a cost reduction of 35–70% after its first 500 million barrels were produced. Assuming an increase in output of 25,000 barrels per day during each year after the start of commercial production, the costs would then be expected to decline to $35–48 per barrel within 12 years. After achieving the milestone of 1 billion barrels ([convert: unit mismatch]), its costs would decline further to $30–40 per barrel.[37][32] A comparison of the proposed US oil shale industry to the Alberta tar sands industry has been drawn (the latter enterprise generated over one million barrels of oil per day in late 2007),[52] stating that "the first-generation facility is the hardest, both technically and economically".[53]

Royal Dutch Shell has announced that its in situ extraction technology in Colorado could be competitive at prices over $30 per barrel, while other technologies at full-scale production assert profitability at oil prices even lower than $20 per barrel.[54][55][56][41] To increase the efficiency of oil shale retorting, several co-pyrolysis processes have been proposed and tested.[57][58][59][60][61]

A critical measure of the viability of oil shale as an energy source is the ratio of the energy produced by the shale to the energy used in its mining and processing, a ratio known as "Energy Returned on Energy Invested" (EROEI). A 1984 study estimated the EROEI of the various known oil shale deposits as varying between 0.7–13.3.[62] Royal Dutch Shell has reported an EROEI of three to four on its in situ development, Mahogany Research Project.[54][63][64] An additional economic consideration is the water needed in the oil shale retorting process, which may pose a problem in areas with water scarcity.

Environmental considerations

Main article: Environmental impact of oil shale industry

Oil shale mining involves a number of environmental impacts. These impacts are more pronounced in surface mining than in underground mining. They include acid drainage induced by the sudden rapid exposure and subsequent oxidation of formerly buried materials, the introduction of metals into surface water and groundwater, increased erosion, sulfur-gas emissions, and air pollution caused by the creation of particulates during processing, transport, and support activities.[9][10] It may damage the biological and recreational value of land and the ecosystem in the mining area. In addition, combustion and thermal processing generate waste material, and the atmospheric emissions from its processing and combustion include carbon dioxide, a major greenhouse gas. Environmentalists oppose production and usage of oil shale, as it creates even more greenhouse gases than conventional fossil fuels.[65] Experimental in situ conversion processes and carbon capture and storage technologies may reduce some of these concerns in the future, but at the same time they may cause other problems, including groundwater pollution.[66]

Concerns have been expressed over the oil shale industry's use of water. This is a particularly sensitive issue in the arid regions of the western US.[67] Depending on technology, above-ground retorting uses between one and five barrels of water per barrel of produced shale oil.[37][68][69][70] In situ processing, according to one estimate, uses about one-tenth as much water.[71] A 2007 programmatic environmental impact statement issued by the US Bureau of Land Management stated that surface mining and retort operations produce two to ten gallons of wastewater per tonne of processed oil shale.[68]

Environmental activists, including Greenpeace, have organized strong protests against the industry. As a result, the proposed Stuart Oil Shale Project in Australia was put on hold in 2004.[9][72][73]

See also

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Footnotes

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