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Coal liquefaction is a process of converting coal into liquid hydrocarbons: liquid fuels and petrochemicals. This process is often known as "Coal to X", where X can be many different hydrocarbon-based products. However, the most common process chain is "Coal to Liquid Fuels" (CTL).[1]

## Historical background

Coal liquefaction originally was developed at the beginning of the 20th century.[2] The best-known CTL process is Fischer–Tropsch synthesis (FT), named after the inventors Franz Fischer and Hans Tropsch from the Kaiser Wilhelm Institute in the 1920s.[3] The FT synthesis is the basis for indirect coal liquefaction (ICL) technology. Friedrich Bergius, also a German chemist, invented direct coal liquefaction (DCL) as a way to convert lignite into synthetic oil in 1913.

Coal liquefaction was an important part of Adolf Hitler's four-year plan of 1936, and became an integral part of German industry during World War II.[4] During the mid-1930s, companies like IG Farben and Ruhrchemie initiated industrial production of synthetic fuels derived from coal. This led to the construction of twelve DCL plants using hydrogenation and nine ICL plants using Fischer–Tropsch synthesis by the end of World War II. In total, CTL provided 92% of Germany's air fuel and over 50% of its petroleum supply in the 1940s.[2] The DCL and ICL plants effectively complemented each other rather than competed. The reason for this is that coal hydrogenation yields high quality gasoline for aviation and motors, while FT synthesis chiefly produced high-quality diesel, lubrication oil, and waxes together with some smaller amounts of lower-quality motor gasoline. The DCL plants were also more developed, as lignite – the only coal available in many parts of Germany – worked better with hydrogenation than with FT synthesis. After the war, Germany had to abandon its synthetic fuel production as it was prohibited by the Potsdam conference in 1945.[4]

South Africa developed its own CTL technology in the 1950s. The South African Coal, Oil and Gas Corporation (Sasol) was founded in 1950 as part of industrialization process that the South African government considered essential for cont

Coal liquefaction originally was developed at the beginning of the 20th century.[2] The best-known CTL process is Fischer–Tropsch synthesis (FT), named after the inventors Franz Fischer and Hans Tropsch from the Kaiser Wilhelm Institute in the 1920s.[3] The FT synthesis is the basis for indirect coal liquefaction (ICL) technology. Friedrich Bergius, also a German chemist, invented direct coal liquefaction (DCL) as a way to convert lignite into synthetic oil in 1913.

Coal liquefaction was an important part of Adolf Hitler's four-year plan of 1936, and became an integral part of German industry during World War II.[4] During the mid-1930s, companies like IG Farben and Ruhrchemie initiated industrial production of synthetic fuels derived from coal. This led to the construction of twelve DCL plants using hydrogenation and nine ICL plants using Fischer–Tropsch synthesis by the end of World War II. In total, CTL provided 92% of Germany's air fuel and over 50% of its petroleum supply in the 1940s.[2] The DCL and ICL plants effectively complemented each other rather than competed. The reason for this is that coal hydrogenation yields high quality gasoline for aviation and motors, while FT synthesis chiefly produced high-quality diesel, lubrication oil, and waxes together with some smaller amounts of lower-quality motor gasoline. The DCL plants were also more developed, as lignite – the only coal available in many parts of Germany – worked better with hydrogenation than with FT synthesis. After the war, Germany had to abandon its synthetic fuel production as it was prohibited by the Potsdam conference in 1945.[4]

South Africa developed its own CTL technology in the 1950s. The South African Coal, Oil and Gas Corporation (Sasol) was founded in 1950 as part of industrialization process that the South African government considered essential for continued economic development and autonomy.[5] However, South Africa had no domestic oil reserves, and this made the country very vulnerable to disruption of supplies coming from outside, albeit for different reasons at different times. Sasol was a successful way to protect the country's balance of payment against the increasing dependence on foreign oil. For years its principal product was synthetic fuel, and this business enjoyed significant government protection in South Africa during the apartheid years for its contribution to domestic energy security.[6] Although it was generally much more expe

Coal liquefaction was an important part of Adolf Hitler's four-year plan of 1936, and became an integral part of German industry during World War II.[4] During the mid-1930s, companies like IG Farben and Ruhrchemie initiated industrial production of synthetic fuels derived from coal. This led to the construction of twelve DCL plants using hydrogenation and nine ICL plants using Fischer–Tropsch synthesis by the end of World War II. In total, CTL provided 92% of Germany's air fuel and over 50% of its petroleum supply in the 1940s.[2] The DCL and ICL plants effectively complemented each other rather than competed. The reason for this is that coal hydrogenation yields high quality gasoline for aviation and motors, while FT synthesis chiefly produced high-quality diesel, lubrication oil, and waxes together with some smaller amounts of lower-quality motor gasoline. The DCL plants were also more developed, as lignite – the only coal available in many parts of Germany – worked better with hydrogenation than with FT synthesis. After the war, Germany had to abandon its synthetic fuel production as it was prohibited by the Potsdam conference in 1945.[4]

South Africa developed its own CTL technology in the 1950s. The South African Coal, Oil and Gas Corporation (Sasol) was founded in 1950 as part of industrialization process that the South African government considered essential for continued economic development and autonomy.[5] However, South Africa had no domestic oil reserves, and this made the country very vulnerable to disruption of supplies coming from outside, albeit for different reasons at different times. Sasol was a successful way to protect the country's balance of payment against the increasing dependence on foreign oil. For years its principal product was synthetic fuel, and this business enjoyed significant government protection in South Africa during the apartheid years for its contribution to domestic energy security.[6] Although it was generally much more expensive to produce oil from coal than from natural petroleum, the political as well as economic importance of achieving as much independence as possible in this sphere was sufficient to overcome any objections. Early attempts to attract private capital, foreign or domestic, were unsuccessful, and it was only with state support that the coal liquefaction could start. CTL continued to play a vital part in South Africa's national economy, providing around 30% of its domestic fuel demand. The democratization of South Africa in the 1990s made Sasol search for products that could prove more competitive in the global marketplace; as of the new millennium the company was focusing primarily on its petrochemical business, as well as on efforts to convert natural gas into crude oil (GTL) using its expertise in Fischer–Tropsch synthesis.

CTL technologies have steadily improved since the Second World War. Technical development has resulted in a variety of systems capable of handling a wide array of coal types. However, only a few enterprises based on generating liquid fuels from coal have been undertaken, most of them based on ICL technology; the most successful one has been Sasol in South Africa. CTL also received new interest in the early 2000s as a possible mitigation option for reducing oil dependence, at a time when rising oil prices and concerns over peak oil made planners rethink existing supply chains for liquid fuels.

Specific liquefaction technologies generally fall into two categories: direct (DCL) and indirect liquefaction (ICL) processes. Direct processes are based on approaches such as carbonization, pyrolysis, and hydrogenation.[7]

Indirect liquefaction processes generally involve gasification of coal to a mixture of carbon monoxide and hydrogen, often known as synthesis gas or simply sy

Indirect liquefaction processes generally involve gasification of coal to a mixture of carbon monoxide and hydrogen, often known as synthesis gas or simply syngas. Using the Fischer–Tropsch process syngas is converted into liquid hydrocarbons.[8]

In contrast, direct liquefaction processes convert coal into liquids directly without having to rely on intermediate steps by breaking down the organic structure of coal with application of hydrogen-donor solvent, often at high pressures and temperatures.[9] 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.

At industrial scales (i.e. thousands of barrels/day) a coal liquefaction plant typically requires multibillion-dollar capital investments.[10]

A number of carbonization processes exist. The carbonization conversion typically occurs through pyrolysis or destructive distillation. It produces condensable coal tar, oil and water vapor, non-condensable synthetic gas, and a solid residue - char.

One 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

One 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, any produced liquids are mostly a by-product and the main product is semi-coke - a solid and smokeless fuel.[2]

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.[11] 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.[11]

Liquid yields of pyrolysis and the Karrick process are generally considered too low for practical use for synthetic liquid fuel production.[12] The resulting coal tars and oils from pyrolysis generally require further treatment before they can be usable as motor fuels; they are processed by hydrotreating to remove sulfur and nitrogen species, after which they are finally processed into liquid fuels.[11]

In summary, the economic viability of this technology is questionable.[10]

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. A 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:[7]