Coal liquefaction (CTL)

Coal-to-liquid (CTL) liquefaction is an old technology, first developed in the first half of the 20th century, which uses coal to produce diesel or gasoline-type fuels and/or chemical bases. In the era of "king oil", only very specific geopolitical contexts could justify the industrialization of this sector (see historical box): the cost price of synthetic fuels was much higher than that of petroleum fuels, due to the high levels of investment required and the then low price –– of crude oil. Today, awareness of the relative scarcity of oil and natural gas, and the record prices they reached in 2008, has led to renewed interest in coal liquefaction, against a backdrop of security of supply and energy diversification. Coal is an unavoidable candidate to replace oil, subject to the implementation of technologies to capture and store the CO ₂ emitted in large quantities by the sector.

Chemically, the challenge of liquefaction is to triple the hydrogen content of coal (H/C atomic ratio less than 1) to values compatible with petroleum fuel specifications (H/C ratio of the order of 2).

Two families of catalytic processes have been developed to compensate for the inadequacy of previous technologies based on purely thermal processes (coking or pyrolysis) in terms of liquefied product quantity and quality.

–In the direct route, the coal, previously crushed and dissolved in an organic solvent, is sent to 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 to the required specifications.

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