Coal liquefaction (CTL)

Coal-to-liquid (CTL) technology is a long-established process, originally developed in the first half of the 20th century, that produces diesel or gasoline-type fuels and/or chemical feedstocks from coal. In the era when oil reigned supreme, only very specific geopolitical contexts could justify the industrialization of this sector (see historical box): the production cost of synthetic fuels was much higher than that of petroleum-based fuels due to the high levels of investment required and the then-low price – – of crude oil. Today, growing awareness of the relative scarcity of oil and natural gas, as well as the record-high prices they reached in 2008, has sparked renewed interest in coal liquefaction against a backdrop of supply security and energy diversification. Coal is a leading candidate to replace oil, provided that technologies are implemented to capture and store the large amounts of CO ₂ emitted by the industry.

Chemically speaking, the key challenge of coal liquefaction is to triple the hydrogen content of coal (an H/C atomic ratio of less than 1) to levels compatible with the specifications for petroleum fuels (an H/C ratio of around 2).

Two families of catalytic processes have been developed to address the shortcomings of earlier technologies based on purely thermal processes (coking or pyrolysis) in terms of the quantity and quality of the distillate.

– In the direct route, coal—which has been ground and dissolved in an organic solvent—is fed into a hydroconversion reactor, which converts it directly into a liquid. This liquid, rich in aromatics, is then sent to a deep hydrogenation and/or hydrocracking unit to produce fuels that meet the required specifications.

– In the indirect process, pulverized coal is first converted into a mixture of H ₂ and CO known as synthesis gas (gasification process), and then into a mixture of paraffinic hydrocarbons via the Fischer-Tropsch process, which undergoes a final stage of hydroisomerization and/or hydrocracking.