Methods

The liquefaction processes are classified as direct conversion to liquids processes and indirect conversion to liquids processeses. Direct processes are carbonization

Carbonization

Carbonization or carbonisation is the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis or destructive distillation. It is often used in organic chemistry with reference to the generation of coal gas and coal tar from raw coal...

 and hydrogenation

Hydrogenation

Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

.

Pyrolysis and carbonization processes

There are a number of carbonization processes. The carbonization conversion occurs through pyrolysis

Pyrolysis

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

 or destructive distillation

Destructive distillation

Destructive distillation is the chemical process involving the decomposition of feedstock by heating to a high temperature; the term generally applies to processing of organic material in the absence of air or in the presence of limited amounts of oxygen or other reagents, catalysts, or solvents,...

, and it produces condensable coal tar

Coal tar

Coal tar is a brown or black liquid of extremely high viscosity, which smells of naphthalene and aromatic hydrocarbons. Coal tar is among the by-products when coal iscarbonized to make coke or gasified to make coal gas...

, oil and water vapor, non-condensable synthetic gas, and a solid residue-char

Char

Char is the solid material that remains after light gases and tar coal tar have been driven out or released from a carbonaceous material during the initial stage of combustion, which is known as carbonization, charring, devolatilization or pyrolysis.Further stages of efficient combustion are...

. The condensed coal tar and oil are then further processed by hydrogenation to remove sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 and nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 species, after which they are processed into fuels.

The typical example of carbonization is the Karrick process

Karrick process

The Karrick process is a low-temperature carbonization and pyrolysis process of carbonaceous materials. Although primarily meant for coal carbonization, it also could be used for processing of oil shale, lignite or any carbonaceous materials. These are heated at to in the absence of air to...

. The process was invented by Lewis Cass Karrick

Lewis Karrick

Lewis Cass Karrick was an American petroleum refinery engineer, oil shale and coal technologist, and inventor. He patented several coal and oil shale related inventions, and he also refined and perfected a low-temperature carbonization and pyrolysis process for processing coal and other...

 in the 1920s. The Karrick process is a low-temperature carbonization

Carbonization

Carbonization or carbonisation is the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis or destructive distillation. It is often used in organic chemistry with reference to the generation of coal gas and coal tar from raw coal...

 process, where coal is heated at 680 °F (360 °C) to 1380 °F (748.9 °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

Fluidized bed

A fluidized bed is formed when a quantity of a solid particulate substance is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium...

 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

TOSCO II process

The TOSCO II process is an above ground retorting technology for shale oil extraction, which uses fine particles of oil shale that are heated in a rotating kiln. The particularity of this process is that it use hot ceramic balls for the heat transfer between the retort and a heater...

 and Lurgi-Ruhrgas process

Lurgi-Ruhrgas process

The Lurgi–Ruhrgas process is an above-ground coal liquefaction and shale oil extraction technology. It is classified as a hot recycled solids technology.-History:...

, 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.

Hydrogenation processes

One of the main methods of direct conversion of coal to liquids by hydrogenation process is the Bergius process

Bergius process

The Bergius Process is a method of production of liquid hydrocarbons for use as synthetic fuel by hydrogenation of high-volatile bituminous coal at high temperature and pressure...

. The Bergius process was developed by Friedrich Bergius

Friedrich Bergius

Friedrich Karl Rudolf Bergius was a German chemist known for the Bergius process for producing synthetic fuel from coal, Nobel Prize in Chemistry in recognition of contributions to the invention and development of chemical high-pressure methods...

 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 5000 °C (9,032 °F) and 20 to 70 MPa

Pascal (unit)

The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 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...

 pressure. The reaction can be summarized as follows:

n C + (n + 1) H₂ → C_nH_{2 n + 2}

After 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...

 several plants were built in Germany; these plants were extensively used during 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...

 to supply Germany with fuel and lubricants. The Kohleoel Process, developed in Germany by Ruhrkohle

RAG Aktiengesellschaft

RAG AG, formerly Ruhrkohle AG, is the largest German coal mining corporation. The company headquarters are in Herne in the Ruhr area. The company was founded on 27 November 1968, consolidating several coalmining corporations into the Ruhrkohle AG....

 and VEBA

VEBA

VEBA AG was a German energy company. VEBA was founded in 1929 as a holding company owned by the state of Prussia, and was privatized in 1965. VEBA became a part of E.ON in 2000....

, was used in the demonstration plant with the capacity of 200 ton of 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...

 per day, built in Bottrop

Bottrop

Bottrop is a city in west central Germany, on the Rhine-Herne Canal, in North Rhine-Westphalia. Located in the Ruhr industrial area, Bottrop adjoins Essen, Oberhausen, Gladbeck and Dorsten. The city had been a coal-mining and rail center and contains factories producing coal-tar derivatives,...

, 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 (878 °F). This process was also explored by SASOL

Sasol

Sasol Ltd. is a South African company involved in mining, energy, chemicals and synfuels. In particular, they produce petrol and diesel profitably from coal and natural gas using Fischer-Tropsch process...

 in South Africa.

In 1970-1980s, Japanese companies Nippon Kokan, Sumitomo Metal Industries

Sumitomo Metal Industries

is a company with seat in Osaka, Japan. )It is the third largest integrated steel manufacturer in Japan withthree integrated steelworks and several other manufacturing plants.It is one of the largest manufacturers of Seamless Pipes and Tubes,...

 and Mitsubishi Heavy Industries

Mitsubishi Heavy Industries

, or MHI, is a Japanese company. It is one of the core companies of Mitsubishi Group.-History:In 1870 Yataro Iwasaki, the founder of Mitsubishi took a lease of Government-owned Nagasaki Shipyard. He named it Nagasaki Shipyard & Machinery Works, and started the shipbuilding business on a full scale...

 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 temperature between 430 °C (806 °F) and 465 °C (869 °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 developed by Gulf Oil

Gulf Oil

Gulf Oil was a major global oil company from the 1900s to the 1980s. The eighth-largest American manufacturing company in 1941 and the ninth-largest in 1979, Gulf Oil was one of the so-called Seven Sisters oil companies...

 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

Molybdenum

Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

 catalysts. Hydrogenation occurred by use of high temperature and pressure synthesis gas

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...

 produced in a separate gasifier. The process ultimately yielded a synthetic crude product, Naphtha

Naphtha

Naphtha normally refers to a number of different flammable liquid mixtures of hydrocarbons, i.e., a component of natural gas condensate or a distillation product from petroleum, coal tar or peat boiling in a certain range and containing certain hydrocarbons. It is a broad term covering among the...

, 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 is also a number of two-stage direct liquefaction processes; however, after 1980s only the Catalytic Two-stage Liquefaction Process, modified from the H-Coal Process; the Liquid Solvent Extraction Process by British Coal

British Coal

thumb|right|British Coal company logoThe British Coal Corporation was a nationalised corporation in the United Kingdom responsible for the extraction of coal...

; and the Brown Coal Liquefaction Process of Japan have been developed.

Shenhua

Shenhua Group

Shenhua Group is a state-owned mining and energy company in China. It is the largest coal-producing company in the world. It was founded in October 1995 under the auspices of the State Council of the People's Republic of China.-Activities:...

, a Chinese coal mining company, decided in 2002 to build a direct liquefaction plant in Inner Mongolia, with barrel capacity of 20 koilbbl/d. First tests were implemented at the end of 2008. A second and longer test campaign was started in October 2009.

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.

Indirect conversion processes

The main indirect process is the Fischer-Tropsch process

Fischer-Tropsch process

The Fischer–Tropsch process is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a petroleum substitute, typically from coal, natural gas, or biomass for use as synthetic...

. In this process, coal is first gasified to make 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...

 (a balanced purified mixture of CO and H₂ gas). Next, Fischer-Tropsch catalysts

Catalysis

Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 are used to convert the syngas into light hydrocarbons (like ethane

Ethane

Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....

) which are further processed into gasoline and diesel. This method was used on a large technical scale in Germany between 1934 and 1945 and is currently being used by Sasol in South Africa. In addition to creating gasoline, syngas can also be converted into methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

, which can be used as a fuel, or into a fuel additive

Gasoline additive

Gasoline additives increase gasoline's octane rating or act as corrosion inhibitors or lubricants, thus allowing the use of higher compression ratios for greater efficiency and power, however some carry heavy environmental risks...

.

Syngas may be converted to liquids through conversion of the syngas to methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

 which is subsequently polymerized into alkanes over a zeolite

Zeolite

Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...

 catalyst. This process, named as the Mobil MTG Process, was developed by Mobil

Mobil

Mobil, previously known as the Socony-Vacuum Oil Company, was a major American oil company which merged with Exxon in 1999 to form ExxonMobil. Today Mobil continues as a major brand name within the combined company, as well as still being a gas station sometimes paired with their own store or On...

 in early 1970s.

Carbon dioxide emission

Coal liquefaction methods involve carbon dioxide (CO₂) emissions in the conversion process. Different liquefaction processes have different lifecycle carbon footprints depending on which processes and environmental controls are employed. If coal liquefaction is done without employing either carbon capture and storage

Carbon capture and storage

Carbon capture and storage , alternatively referred to as carbon capture and sequestration, is a technology to prevent large quantities of from being released into the atmosphere from the use of fossil fuel in power generation and other industries. It is often regarded as a means of mitigating...

 technologies or biomass blending, the result is lifecycle greenhouse gas footprints that are generally greater than those released in the extraction and refinement of liquid fuel production from crude oil. If CCS technologies are employed, reductions of 5-12% can be achieved in CTL plants and up to a 75% reduction is achievable when co-gasifying coal with commercially demonstrated levels of biomass (30% biomass by weight) in CBTL plants. For most future synthetic fuel

Synthetic fuel

Synthetic fuel or synfuel is a liquid fuel obtained from coal, natural gas, oil shale, or biomass. It may also refer to fuels derived from other solids such as plastics or rubber waste. It may also refer to gaseous fuels produced in a similar way...

 projects, Carbon dioxide sequestration is proposed to avoid releasing it into the atmosphere. As CO₂ is one of the process streams, sequestration is easier than from flue gases produced in 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...

 of coal with air

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, where CO₂ is diluted by nitrogen and other gases. Sequestration will, however, add to the cost of production. Currently all US and at least one Chinese synthetic fuel projects, are including sequestration in their process designs.