In case of the ex-situ method, the oil shale is mined either by underground mining or surface mining from the ground and then transported to a processing facility. At the facility, the oil shale is usually heated to 450 °C to 500 °C (840 °F to 930 °F) at which, the kerogen in the oil shale decomposes to gas, oil vapor and char, a process known as retorting. The gas and oil vapors are separated from the spent shale and cooled, causing the oil to condense. The oil may be used as a fuel oil or upgraded to meet refinery feed specifications by adding hydrogen and removing impurities such as sulfur and nitrogen. The non-condensible retort gas and char may be burnt and the heat energy may be reused for heating the raw shale or generating electricity.
Based on the size of feed oil shale, the ex-situ technologies can be classified into lump shale using technologies and particulate shale using technologies. In general, the lump shale is used in internal hot gas carrier technologies, while the particulate oil shale (less than 10 millimetres (0.4 in)) is used in internal hot solid carrier technologies.[3]
[+] Internal combustion technologies
Internal combustion technologies use heat transferred by flowing gases, which are generated by combustion within the retort. Common characteristics of these technologies are, feed shale consisting of lumps which range from 10 to 100 millimetres (0.4 to 4 in) in diameter, and the retort vapors are diluted with the exhaust generated by the combustion. The main technologies are Kiviter, Union A, Paraho Direct, Superior Direct, and Fushun processes.[6][7] The Kiviter processing takes place in gravitational shaft retorts and it is possible by using only large-particle feed. The process gas combustion products are used as the heat carrier. In the case of kukersite, the yield of oil accounts for 14-17 % of shale and the oil consists of a small amount of low-boiling fractions. Main problems associated with Kiviter process are environmental concerns such as extensive use and pollution of water in the process, as also the waste solid residue which continues to leach toxic substances.[8][9] The Kiviter process is used by Estonian company VKG Oil, a subsidiary of Viru Keemia Grupp.[10] The company operates several retorts, the largest one, having a capacity of processing 40 tonnes per hour of oil shale.
Like the Kiviter, the Fushun-type retort processes oil shale lumps in a vertical shaft kiln. The Fushun Mining Group in Liaoning Province, China operates the largest shale oil plant in the world. In 2003, it employed 80 Fushun-type retortsand as of 2007 it has increased to 180 retorts. Each retort processes about 4 tonnes per hour of shale.[11]
The Paraho Direct is an American version of the lump-processing vertical shaft kiln. This technology is used by Shale Technologies LLC in a pilot plant facility in Rifle, Colorado.[12]
[+] Hot recycled solids technologies
Hot recycled solids technologies use heat, which is transferred by mixing hot solid particles with the oil shale. These technologies usually process oil shale fine particles (less than 10 mm). The heat carrier (usually shale ash) is heated in a separate chamber or vessel, thus the retort vapors are not diluted with combustion exhaust. The main technologies in this category include Alberta Taciuk Process (ATP), Galoter, TOSCO II, Lurgi-Ruhrgas, Chevron STB, LLNL HRS, and Shell Spher processes.[6][7]
In the Galoter process, retorting takes place in a rotary kiln-type retort using fine particles. The spent shale is burnt in a spouted bed and solid shale ash is used as the heat carrier.[9] In case of kukersite, the yield of crude oil accounts for roughly 12 % of shale and the oil consists 15-20 % of low-boiling fractions. The Galoter process is more eco-friendly than the Kiviter process, as the use of water and pollution caused is minimal. However, the burning residue does cause some environmental problems due to organic carbon and calcium sulphide content.[8] The Galoter process is used for oil production by Eesti Energia, an Estonian energy company.[10] The company has two retorts, both processing 125 tonnes per hour of oil shale and plans are underway to build two more.[13] In 2008, another Estonian company, VKG Oil AS, is going to construct a new production line using the Galoter process engineered by Atomenergoproject of St Petersburg.[14]
Alberta Taciuk Processor (ATP) retort
Alberta Taciuk Processor (ATP) retort
Similar to the Galoter process, the Alberta Taciuk processes oil shale fine particles in a rotary kiln-type retort. The unique feature of the Alberta Taciuk process is that drying and pyrolysis of the feed shale and the combustion, recycling and cooling of spent shale, all occur in a single multi-chamber horizontal, rotating vessel.[15][16] The extracted oil consists up to 30 % of low-boiling fractions. The water pollution caused by the process is quite limited.[8] Australian oil companies Southern Pacific Petroleum NL and later Queensland Energy Resources operated a 250 tonnes per hour industrial-scale pilot plant using the Alberta Taciuk Processor. The plant was shut down in 2004. UMATAC Industrial Processes is currently designing a 250 tonnes per hour Alberta Taciuk Processor in China, and is scheduled to start operation in 2008.[17] Estonian VKG Oil is considering construction of a new retort facility using the Alberta Taciuk Processor.[10] Oil shale exploration company LLC has arranged for an exclusive right to license the ATP for research, development and demonstration near Vernal, Utah.[18]
As with the Galoter and Alberta Taciuk process, the TOSCO II also processes oil shale fine particles which are heated with hot recycled solids in a rotary kiln. However, instead of recycling shale ash, the TOSCO II circulates hot ceramic balls between the retort and a heater. The process was tested in a 40 tonnes per hour test facility near Parachute Colorado which was shut down in 1972. The LLNL HRS (hot-recycled-solid) retorting process was developed by the Lawrence Livermore National Laboratory. The technology was used in a 4 tonnes per day pilot plant from 1990 to 1993. A delayed-fall combustor, which is used in this process, gives greater control over the combustion process as compared to a lift pipe combustor. A fluidized-bed mixer is used instead of the screw mixer, which is used in the Lurgi process. The majority of the pyrolysis occurs in a settling-bed unit.[6]
[+] Conduction through a wall technologies
Conduction through a wall technologies use heat, which is transferred by conduction through the retort wall. These technologies normally process fine particles and the retort vapors are not diluted by combustion exhaust. Technologies include Pumpherston, Fischer assay, Hom Tov and Oil-Tech processes.[6][7] Oil-Tech staged electrically heated retort process is developed by Millennium Synfuels, LLC (former Oil Tech Inc.). In this process, the feed oil shale is heated to greater temperatures as it goes further down the retort. The retort-style prototype was reported to have passed a test.[19]
In the Hom Tov process (US Patent 5372708), oil shale fine particles are slurried with waste bitumen and pumped through coils in a heater. Israeli promoters of this process claim that the technology enables the shale to be processed at somewhat lower temperatures with the addition of the catalyzing bitumen. The technology has not been tested in a pilot plant yet.[20] Fischer Assay is a standardized laboratory test that is used to measure the grade of an oil shale sample. A 100 gram sample crushed to 8 mesh (2.38 mm) screen is heated in a small aluminum retort to 500 °C (930 °F) at a rate of 12 °C (54 °F) per minute, and held at that temperature for 40 minutes.[21] The distilled vapors of oil, gas, and water are passed through a condenser and cooled with ice water into a graduated centrifuge tube. The oil yields achieved by other technologies are often reported as a percentage of the Fischer Assay oil yield.
In Red-Leaf Resources EcoShale In-Capsule Process, hot gas generated by natural gas or pyrolysis gas is circulated through an oil shale rubble pile using a set of parallel pipes. The heat is transferred to the shale through the pipe walls rather than being injected directly into the rubble pile, thereby avoiding dilution of the pyrolysis gas with the heating gas. The rubble pile is encapsulated by a low-cost earthen impoundment structure designed to prevent environmental contamination and to provide easy reclamation. Energy efficiency is enhanced by recovering heat from the spent shale by passing cool gas through the heating pipes and then using it to preheat adjacent capsules.[22][23]
A new process from Combustion Processes, Inc., seeks to eliminate carbon dioxide emissions from the shale oil production process. Pyrolysis occurs in a rotating kiln heated by hot gas flowing through an outer annulus. The hot gas is created by burning hydrogen generated in a separate unit by coal gasification followed by carbon dioxide separation. The annular geometry achieves heat transfer to the moving shale through a wall, thereby avoiding dilution of the product gas.
[+] Externally generated hot gas technologies
Externally generated hot gas technologies or indirectly heated technologies use heat, transferred by gases which are heated outside the retort vessel. The main technologies are Petrosix, Union B, Paraho Indirect, and Superior Indirect processes.[6][7] As with the the internal combustion technologies, most of the externally-generated hot gas technologies process oil shale lumps in vertical shaft kilns; however, the retort vapors are not diluted with combustion exhaust. The world’s largest operational surface oil shale pyrolysis reactor is the Petrosix which is located in São Mateus do Sul, Paraná, Brazil. The 11 metres (36 ft) diameter vertical shaft kiln is owned by Petrobras and has being operating since 1992 with high availability. The company operates two retorts, the largest of which processes 260 tonnes per hour of oil shale.[10][24] The largest retort ever built used the Union B technology, developed by Unocal. The Union B processed 400 tonnes per hour of oil shale lumps heated by externally generated hot gas. However, unlike all other vertical shaft kilns, the Union B pumped the oil shale into the bottom of the retort, with the hot gas entering at the top. Unocal operated the retort from 1986 to 1992 near Parachute, Colorado. The Paraho Indirect technology is similar to the Petrosix which is considered a highly reliable technology for use with U.S. oil shale.[10]
[+] Reactive fluids technologies
Reactive fluids technologies are IGT Hytort (high-pressure H2) process, Xtract Technology (supercritical solvent extraction), Donor solvent processes, and Chattanooga fluid bed reactor.[6][25][22] In the IGT Hytort process, developed by the Institute of Gas Technology (IGT), oil shales are processed at controlled heating rates in an atmosphere of hydrogen at high pressure.[26] This technology like other reactive fluid technologies, is more appropriate for oil shales with low hydrogen content, such as the Eastern US Devonian shales, for which only a third of the organic carbon is typically converted to oil during conventional overground retorting. The hydrogen or hydrogen donor react with coke precursors and roughly double the yield of oil, depending on the characteristics of the shale and process.[27]
Chattanooga Corp. has developed an extraction process which uses a fluid bed reactor and an associated hydrogen fired heater. In this process conversion reaction occurs at relatively low temperatures (1,000 °F (540 °C)) through thermal cracking and hydrogenation into hydrocarbon vapors and spent solids. The thermal cracking allows for hydrocarbon vapors to be extracted off the oil shale which is then extracted and scrubbed of solids. The vapor is then cooled, during which condensate drops out of the gas and the remaining hydrogen, light hydrocarbon and acid gases are passed through an amine scrubbing system to remove hydrogen sulfide which is converted to elemental sulfur. The cleaned hydrogen and light hydrocarbon gases are then fed back into the system for compression or into the hydrogen heater which provides the heat for the fluid bed reactor. This nearly-closed-loop allows for an efficient process where nearly all the energy needs are provided by the source material. The demonstration plant in Alberta was able to produce 930 barrels (~130 t) of oil per kilotonne of oil shale at an API gravity ranging between 28 to 30. With hydrotreating, it would be possible to improve this to 38-40 °API. Chattanooga Corp is currently looking at a design to produce a 2,500 barrels (~330 t) per hour facility.[22](wikipedia)
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