Coal liquefaction

Coal liquefaction is a process of converting coal into liquid hydrocarbons: liquid fuels and petrochemicals. The conversion industry is commonly referred to as "coal conversion" or "Coal To X". "Coal to Liquid Fuels" is commonly called "CTL" or "coal liquefaction", although "liquefaction" is generally used for a non-chemical process of becoming liquid.

Specific liquefaction technologies generally fall into two categories: direct (DCL) and indirect liquefaction (ICL) processes. Indirect liquefaction processes generally involve gasification of coal to a mixture of carbon monoxide and hydrogen (syngas) and then using a process such as Fischer–Tropsch process to convert the syngas mixture into liquid hydrocarbons. By contrast, direct liquefaction processes convert coal into liquids directly, without the intermediate step of gasification, by breaking down its organic structure with application of solvents or catalysts in a high pressure and temperature environment. 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.

As coal liquefaction generally is a high-temperature/high-pressure process, it requires a significant energy consumption and, at industrial scales (thousands of barrels/day), multibillion-dollar capital investments. Thus, coal liquefaction is only economically viable at historically high oil prices, and therefore presents a high investment risk.

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

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. 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:

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