Coal liquefaction is the production of liquid fuels from coal using a variety of industrial processes. Specific liquefaction technologies generally fall into two categories: direct (DCL) and indirect liquefaction (ICL) processes. Indirect liquefaction processes generally involve gasification of coal to a mixture of carbon monoxide and hydrogen (syngas) and then using a process such as Fischer–Tropsch process to convert the syngas mixture into liquid hydrocarbons. By contrast, direct liquefaction processes convert coal into liquids directly, without the intermediate step of gasification, by breaking down its organic structure with application of solvents or catalysts in a high pressure and temperature environment. Since liquid hydrocarbons generally have a higher hydrogen-carbon molar ratio than coals, either hydrogenation or carbon-rejection processes must be employed in both ICL and DCL technologies.
As coal liquefaction generally is a high-temperature/high-pressure process, it requires a significant energy consumption and, at industrial scales (thousands of barrels/day), multi-billion dollar capital investments. Thus, coal liquefaction is only economically viable at historically high oil prices, and therefore presents a high investment risk.
One of the main methods of direct conversion of coal to liquids by hydrogenation process is the Bergius process, developed by Friedrich Bergius in 1913. In this process, dry coal is mixed with heavy oil recycled from the process. Catalyst is typically added to the mixture. The reaction occurs at between 400 °C (752 °F) to 500 °C (932 °F) and 20 to 70 MPa hydrogen pressure. The reaction can be summarized as follows:
- n C + (n + 1) H2 → CnH2 n + 2
After World War I several plants based on this technology were built in Germany; these plants were extensively used during World War II to supply Germany with fuel and lubricants. The Kohleoel Process, developed in Germany by Ruhrkohle and VEBA, was used in the demonstration plant with the capacity of 200 ton of lignite per day, built in Bottrop, Germany. This plant operated from 1981 to 1987. In this process, coal is mixed with a recycle solvent and iron catalyst. After preheating and pressurizing, H2 is added. The process takes place in a tubular reactor at the pressure of 300 bar and at the temperature of 470 °C (880 °F). This process was also explored by SASOL in South Africa.
In 1970-1980s, Japanese companies Nippon Kokan, Sumitomo Metal Industries and Mitsubishi Heavy Industries developed the NEDOL process. In this process, coal is mixed with a recycled solvent and a synthetic iron-based catalyst; after preheating, H2 is added. The reaction takes place in a tubular reactor at a temperature between 430 °C (810 °F) and 465 °C (870 °F) at the pressure 150-200 bar. The produced oil has low quality and requires intensive upgrading. H-Coal process, developed by Hydrocarbon Research, Inc., in 1963, mixes pulverized coal with recycled liquids, hydrogen and catalyst in the ebullated bed reactor. Advantages of this process are that dissolution and oil upgrading are taking place in the single reactor, products have high H/C ratio, and a fast reaction time, while the main disadvantages are high gas yield (this is basically a thermal cracking process), high hydrogen consumption, and limitation of oil usage only as a boiler oil because of impurities.
The SRC-I and SRC-II (Solvent Refined Coal) processes were developed by Gulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s. The Nuclear Utility Services Corporation developed hydrogenation process which was patented by Wilburn C. Schroeder in 1976. The process involved dried, pulverized coal mixed with roughly 1wt% molybdenum catalysts. Hydrogenation occurred by use of high temperature and pressure synthesis gas produced in a separate gasifier. The process ultimately yielded a synthetic crude product, Naphtha, a limited amount of C3/C4 gas, light-medium weight liquids (C5-C10) suitable for use as fuels, small amounts of NH3 and significant amounts of CO2. Other single-stage hydrogenation processes are the Exxon Donor Solvent Process, the Imhausen High-pressure Process, and the Conoco Zinc Chloride Process.
There are also a number of two-stage direct liquefaction processes; however, after the 1980s only the Catalytic Two-stage Liquefaction Process, modified from the H-Coal Process; the Liquid Solvent Extraction Process by British Coal; and the Brown Coal Liquefaction Process of Japan have been developed.
Shenhua, a Chinese coal mining company, decided in 2002 to build a direct liquefaction plant in Erdos, Inner Mongolia (Erdos CTL), with barrel capacity of 20 thousand barrels per day (3.2×103 m3/d) of liquid products including diesel oil, liquefied petroleum gas (LPG) and naphtha (petroleum ether). First tests were implemented at the end of 2008. A second and longer test campaign was started in October 2009. In 2011, Shenhua Group reported that the direct liquefaction plant had been in continuous and stable operations since November 2010, and that Shenhua had made 800 million yuan ($125.1 million) in earnings before taxes in the first six months of 2011 on the project.
Chevron Corporation developed a process invented by Joel W. Rosenthal called the Chevron Coal Liquefaction Process (CCLP). It is unique due the close-coupling of the non-catalytic dissolver and the catalytic hydroprocessing unit. The oil produced had properties that were unique when compared to other coal oils; it was lighter and had far fewer heteroatom impurities. The process was scaled-up to the 6 ton per day level, but not proven commercially.
Pyrolysis and carbonization processes
A number of carbonization processes exist. The carbonization conversion occurs through pyrolysis or destructive distillation, and it produces condensable coal tar, oil and water vapor, non-condensable synthetic gas, and a solid residue-char. The coal tar and oil are then further processed by hydrotreating to remove sulfur and nitrogen species, after which they are processed into fuels.
The typical example of carbonization is the Karrick process. In this low-temperature carbonization process, coal is heated at 680 °F (360 °C) to 1,380 °F (750 °C) in the absence of air. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. However, the produced liquids are mostly a by-product and the main product is semi-coke, a solid and smokeless fuel.
The COED Process, developed by FMC Corporation, uses a fluidized bed for processing, in combination with increasing temperature, through four stages of pyrolysis. Heat is transferred by hot gases produced by combustion of part of the produced char. A modification of this process, the COGAS Process, involves the addition of gasification of char. The TOSCOAL Process, an analogue to the TOSCO II oil shale retorting process and Lurgi-Ruhrgas process, which is also used for the shale oil extraction, uses hot recycled solids for the heat transfer.
Liquid yields of pyrolysis and Karrick processes are generally low for practical use for synthetic liquid fuel production. Furthermore, the resulting liquids are of low quality and require further treatment before they can be used as motor fuels. In summary, there is little possibility that this process will yield economically viable volumes of liquid fuel.
Indirect conversion processes
Indirect coal liquefaction (ICL) processes operate in two stages. In the first stage, coal is converted into syngas (a purified mixture of CO and H2 gas). In the second stage, the syngas is converted into light hydrocarbons using one of three main processes: Fischer-Tropsch synthesis, Methanol synthesis with subsequent conversion to gasoline or petrochemicals, and methanation.
In methanol synthesis processes syngas is converted to methanol, which is subsequently polymerized into alkanes over a zeolite catalyst. This process, under the moniker MTG (MTG for "Methanol To Gasoline"), was developed by Mobil in early 1970s, and is being tested at a demonstration plant by Jincheng Anthracite Mining Group (JAMG) in Shanxi, China.
Methanation reaction converts syngas to synthetic natural gas (SNG). The Great Plains Gasification Plant in Beulah, North Dakota is a coal-to-SNG facility producing 160 million cubic feet per day of SNG, and has been in operation since 1984.
The above instances of commercial plants based on indirect coal liquefaction processes, as well as many others not listed here including those in planning stages and under construction, are tabulated in the Gasification Technologies Council's World Gasification Database.
Most coal liquefaction processes are associated with significant CO2 emissions, resulting either from the gasification process or from generation of heat and electricity that serve as energy inputs to the reactors. High water consumption in water-gas shift or methane steam reforming reactions is another adverse environmental effect. On the other hand, synthetic fuels produced by indirect coal liquefaction processes tend to be 'cleaner' than naturally occurring crudes, as heteroatom (e.g. sulfur) compounds are not synthesized or are excluded from the final product.
Pyrolysis of coal produces polycyclic aromatic hydrocarbons, which are known carcinogens. There are also considerations with respect to global warming, especially if coal liquefaction is conducted without carbon capture and storage technologies. Some progress is being made in regards to CO2 emissions control in the context of Shenhua's Erdos CTL plant. Shenhua Group has been conducting a carbon capture and storage demonstration project, which involves injecting CO2 captured from the CTL plant into the saline aquifer of Erdos Basin, at a demonstration rate of 100,000 tonnes per year. As of late October 2013, a total accumulated amount of 154,000 tonnes of CO2 had been injected, and as early as 2012 the injection rate reached or exceeded the design value.
Ultimately, coal liquefaction-derived fuels will be judged relative to targets established for low-greenhouse gas emissions fuels. For example, in the United States, the Renewable Fuel Standard and Low-carbon fuel standard such as enacted in the State of California reflect an increasing demand for low carbon-footprint fuels. Also, legislation in the United States has restricted the military's use of alternative liquid fuels to only those demonstrated to have life-cycle GHG emissions less than or equal to those of their conventional petroleum-based equivalent, as required by Section 526 of the Energy Independence and Security Act (EISA) of 2007.
Research and development of coal liquefaction
The United States military has an active program to promote alternative fuels use, and utilizing vast domestic U.S. coal reserves to produce fuels through coal liquefaction would have obvious economic and security advantages. But with their higher carbon footprint, fuels from coal liquefaction face the significant challenge of reducing life-cycle GHG emissions to competitive levels, which demands continued research and development of liquefaction technology to increase efficiency and reduce emissions. A number of avenues of research & development will need to be pursued, including:
- Carbon capture and storage including enhanced oil recovery and developmental CCS methods to offset emissions from both synthesis and utilization of liquid fuels from coal,
- Coal/biomass/natural gas feedstock blends for coal liquefaction: Utilizing carbon-neutral biomass and hydrogen-rich natural gas as co-feeds in coal liquefaction processes has significant potential for bringing fuel products' life-cycle GHG emissions into competitive ranges,
- Hydrogen from renewables: the hydrogen demand of coal liquefaction processes might be supplied through renewable energy sources including wind, solar, and biomass, significantly reducing the emissions associated with traditional methods of hydrogen synthesis (such as steam methane reforming or char gasification), and
- Process improvements such as intensification of the Fischer-Tropsch process, hybrid liquefaction processes, and more efficient air separation technologies needed for production of oxygen (e.g. ceramic membrane-based oxygen separation).
Recently, the U.S. Department of Energy and the Department of Defense have been collaborating on supporting new research and development in the area of coal liquefaction to produce military-specification liquid fuels, with an emphasis on jet fuel, which would be both cost-effective and in accordance with EISA Section 526.
Coal liquefaction plants and projects
|Project||Developer||Locations||Type||Products||Start of Operations|
|Sasol Synfuels II (West) & Sasol Synfuels III (East)||Sasol (Pty) Ltd.||Secunda, South Africa||CTL||160,000 BPD; primary products gasoline and light olefins (alkenes)||1977(II)/1983(III)|
|Shenhua Direct Coal Liquefaction Plant||Shenhua Group||Erdos, Inner Mongolia, China||CTL (direct liquefaction||20,000 BPD; primary products diesel fuel, liquefied petroleum gas, naphtha||2008|
|Yitai CTL Plant||Yitai Coal Oil Manufacturing Co., Ltd.||Ordos, Zhungeer, China||CTL||160,000 mt/a Fischer-Tropsch liquids||2009|
|Jincheng MTG Plant||Jincheng Anthracite Mining Co., Ltd.||Jincheng, China||CTL||300,000 t/a methanol from MTG process||2009|
|Sasol Synfuels||Sasol (Pty) Ltd.||Secunda, South Africa||CTL||3,960,000 (Nm3/d) syngas capacity; Fischer-Tropsch liquids||2011|
|Shanxi Lu'an CTL Plant||Shanxi Lu'an Co. Ltd.||Lu'an, China||CTL||160,000 mt/a Fischer-Tropsch liquids||2014|
|ICM Coal to Liquids Plant||Developer||Tugrug Nuur, Mongolia||CTL||13,200,000 (Nm3/d) syngas capacity; gasoline||2015|
|Yitai Yili CTL Plant||Yitai Yili Energy Co.||Yili, China||CTL||30,000 BPD Fischer-Tropsch liquids||2015|
|Yitai Ordos CTL Plant Phase II||Yitai||Ordos, Zhungeer-Dalu, China||CTL||46,000 BPD Fischer-Tropsch liquids||2016|
|Yitai Urumqi CTL Plant||Yitai||Guanquanbao, Urunqi, China||CTL||46,000 BPD Fischer-Tropsch liquids||2016|
|Shenhua Ningxia CTL Project||Shenhua Group Corporation Ltd||China, Yinchuan, Ningxia||CTL||4 million tonnes/year of diesel & naphtha||2016|
|Celanese Coal/Ethanol Project||Celanese Corporation – PT Pertamina Joint Venture||Indonesia, Kalimantan or Sumatra||CTL||1.1 million tons of coal/year to produce ethanol||2016|
|Angul CTL plant||Jindal Steel and Power Limited (JSPL)||India, Odisha (Orissa)||CTL||80,000 BPD combination output (> 50% chance of output being methanol, gasoline, diesel) or 5.7 million scm/day of gas||2019|
|Clean Carbon Industries||Clean Carbon Industries||Mozambique, Tete province||Coal waste-to-liquids||65,000 BPD fuel||2020|
|Arckaringa Project||Altona Energy||Australia, South||CTL||30,000 BPD Phase I 45,000 BPD + 840 MW Phase II||TBD|
|Adams Fork Energy - TransGas WV CTL||TransGas Development Systems (TGDS)||Mingo County, West Virginia||CTL||7,500 TPD of coal to 18,000 BPD gasoline and 300 BPD LPG||Operations 2016 or later|
|American Lignite Energy (aka Coal Creek Project)||American Lignite Energy LLC (North American Coal, Headwaters Energy Services)||MacLean County, North Dakota||CTL||11.5 million TPY lignite coal to 32,000 BPD of undefined fuel||Delayed/Cancelled|
|Belwood Coal-to-Liquids Project (Natchez)||Rentech||Natchez, Mississippi||CTL||Petcoke to up to 30,000 BPD ultra-clean diesel||Delayed/Cancelled|
|CleanTech Energy Project||USA Synthetic Fuel Corp. (USASF)||Wyoming||Synthetic crude||30.6 mm bbls/year of synthetic crude (or 182 billion cubic feet per year)||Planning/financing not secured|
|Cook Inlet Coal-to Liquids Project (aka Beluga CTL)||AIDEA and Alaska Natural Resources to Liquids||Cook Inlet, Alaska||CTL||16 million TPY coal to 80,000 BPD of diesel and naphtha; CO2 for EOR; 380 MW electrical generation||Delayed/Cancelled|
|Decatur Gasification Plant||Secure Energy||Decatur, Illinois||CTL||1.5 million TPY of high-sulfur IL coal generating 10,200 barrels per day of high quality gasoline||Delayed/Cancelled|
|East Dubuque Plant||Rentech Energy Midwest Corporation (REMC)||East Dubuque, Illinois||CTL, polygeneration||1,000 tpd ammonia; 2,000 BPD clean fuels and chemicals||Delayed/Cancelled|
|FEDC Healy CTL||Fairbanks Economic Development Corp. (FEDC)||Fairbanks, Alaska||CTL/GTL||4.2-11.4 million TPY Healy-mined coal; ~40k BPD liquid fuels; 110MW||Planning|
|Freedom Energy Diesel CTL||Freedom Energy Diesel LLC||Morristown, Tennessee||GTL||Undetermined||Active|
|Future Fuels Kentucky CTL||Future Fuels, Kentucky River Properties||Perry County, Kentucky||CTL||Not specified. Coal to methanol and other chemicals (over 100 million tons of coal supply)||Active|
|Hunton "Green Refinery" CTL||Hunton Energy||Freeport, Texas||CTL||Bitumen crude oil to 340,000 BPD jet and diesel fuel||Delayed/Cancelled|
|Illinois Clean Fuels Project||American Clean Coal Fuels||Coles County, Illinois||CTL||4.3 million TPY coal/biomass to 400 million GPY diesel and jet fuel||Delayed/Cancelled|
|Lima Energy Project||USA Synthetic Fuel Corp. (USASF)||Lima, Ohio||IGCC/SNG/H2, polygeneration||Three Phases: 1) 2.7 million barrels of oil equivalent (BOE), 2) expand to 5.3 million BOE (3) expand to 8.0 million BOE (47 billion cf/y), 516 MW||Active|
|Many Stars CTL||Australian-American Energy Co. (Terra Nova Minerals or Great Western Energy), Crow Nation||Big Horn County, Montana||CTL||First phase: 8,000 BPD liquids||Active (no new information since 2011)|
|Medicine Bow Fuel and Power Project||DKRW Advanced Fuels||Carbon County, Wyoming||CTL||3 million TPY coal to 11,700 BPD of gasoline||Delayed/Cancelled|
|NABFG Weirton CTL||North American Biofuels Group||Weirton, West Virginia||CTL||Undetermined||Delayed/Cancelled|
|Rentech Energy Midwest Facility||Rentech Energy Midwest Corporation (REMC)||East Dubuque, Illinois||CTL||1,250 BPD diesel||Delayed/Cancelled|
|Rentech/Peabody Joint Development Agreement (JDA)||Rentech/Peabody Coal||Kentucky||CTL||10,000 and 30,000 BPD||Delayed/Cancelled|
|Rentech/Peabody Minemouth||Rentech/Peabody Coal||Montana||CTL||10,000 and 30,000 BPD||Delayed/Cancelled|
|Secure Energy CTL (aka MidAmericaC2L||MidAmericaC2L /Siemens||McCracken County, Kentucky||CTL||10,200 BPD gasoline||Active (no new information since 2011)|
|Tyonek Coal-to-Liquids (formerly Alaska Accelergy CTL Project)||Accelergy, Tyonek Native Corporation (TNC)||Cook Inlet, Alaska||CBTL||Undefined amount of coal/biomass to 60,000 BPD jet fuel/gasoline/diesel and 200-400 MW electricity||Planning|
|US Fuel CTL||US Fuel Corporation||Perry County/Muhlenberg County, Kentucky||CTL||300 tons of coal into 525 BPD liquid fuels including diesel and jet fuel||Active|
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- The SRC-I pilot plant operated at Fort Lewis Wash in the 1970s but was not able to overcome lack of solvent balance problems (continual imports of solvent containing polynuclear aromatics were necessary). A SRC-I demonstration plant was scheduled to be built at Newman, KY but was cancelled in 1981. Based on 1913 work by Bergius it had been noted that certain minerals in coal ash had a mild catalytic activity, and this led to design work on a SRC-II demonstration plant to be built at Morgantown, WV. This too was cancelled in 1981. It appeared based on the work done so far to be desirable to separate the coal-dissolution and catalytic-hydrogenation functions to obtain a greater yield of synthetic crude oil; this was accomplished in a small+scale pilot plant at Wilsonville, AL during 1981-85. The plant also included a critical-solvent deasher to recover a maximum amount of usable liquid product. In a commercial plant, the deasher underflow containing unreacted carbonaceous matter would be gasified to provide hydrogen to drive the process. This program ended in 1985 and the plant was scrapped. Cleaner Coal Technology Programme (October 1999). Technology Status Report 010: Coal Liquefaction (PDF). Department of Trade and Industry. Retrieved 2010-10-23.
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