Coal to Liquid (CTL)

CTL is an old technique, developed at the beginning of the 20th century and has recently attracted attention once more. Historically, it helped to fuel the German military during  two world wars. CTL provided 92% of Germany’s air fuel and over 50% of their petroleum supply in the 1940s. South Africa developed CTL-technology in the 1950s during an oil blockade and CTL now plays  a vital part in South Africa's national economy, providing over 30% of their fuel demand.  

The best known CTL-process is Fischer-Tropsch (FT) synthesis, named after the inventors Franz Fischer and Hans Tropsch from the Kaiser Wilhelm Institute in the 1920s. The FT-synthesis is the basis for ICL technology. Friedrich Bergius, also a German. chemist, invented direct coal liquefaction (DCL) as a way to convert lignite into synthetic oil in 1913. Karrick invented a low temperature carbonization process in the USA around the 1930s, as a way to produce smokeless fuel and liquids from oil shale.

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 very small number of commercial enterprises based on generating liquid fuels from coal have been undertaken, most of them based on ICL-technology. The most successful is the South African company Sasol, originally created as a way to protect the country’s balance of payment against the increasing dependence on foreign oil . A new DCL plant has recently become operational in China, possibly marking the beginning of a new era.

BASIC CHEMISTRY OF CTL
The basis for all types of CTL-synthesis is a carbon source combined with a hydrogen source, such as steam. Chemical reactions between carbon and other compounds will eventually fabricate hydrocarbon molecules of the desired length. The original Fischer-Tropsch (FT) process wadm s describe by:

 

Carbon monoxide can be produced by gasification of coal or another carbon rich compound. The necessary reaction energy is applied by adding oxygen or steam. 

The resulting mixture of carbon monoxide and hydrogen is usually called synthesis gas (syngas). It is used to construct hydrocarbon chains  of different lengths using condensation and a suitable catalyst. Generally, the FT-process yields two types of products, described by two different reaction.  

The type of resulting products depends  on the catalysts used and the reactor operating conditions. Olefin-rich products with n in the range 5-10 (naphtha) can be used for making synthetic gasoline and chemicals in high temperature FT-processes. Paraffin-rich products with n in the range of 12-19 are suitable for making synthetic diesel and waxes in low temperature FT-processes.  

The Bergius-process is the basis of DCL. Splitting coal into shorter hydrocarbons, resembling ordinary crude oil is done  by adding hydrogen under high pressure and temperature, thus eliminating the need for a gaseous iddle stage.

CATALYST

Both Bergius- and FT-processes use different catalysts to aid the chemical reactions. Common catalysts are transition metals such as iron, ruthenium or cobalt. Transition metal sulphides, amorphous zeolite and similar compounds have also been utilized. In general, catalysts have a large impact on process efficiency as well as influence over the resulting products. Many catalysts are notoriously sensitive to sulphur-compounds or other substances, which requires special treatment and separation techniques to avoid catalyst poisoning.  

Much research has been made on different catalysts for CTL-processes and in many ways some of the greatest chemical challenges can be found in the right choice of catalysts and the optimization of their performance. Closer discussions on catalysts used in CTL have been done by others.

Source:
Hook, M., Aleklett, K., 2010, "A Review on Coal to Liquid Fuels and Its Coal Consumption," International Journal of Energy Research, 34, Issue 10, October 2010, pp. 848-864.