Direct Coal Liquefaction

The direct liquefaction of coal involves partially deconstructing the coal structure thermally under conditions of high temperature and pressure and adding hydrogen either directly or indirectly via a donor-solvent to stabilize the fragments formed. Otherwise these fragments would recombine (repolymerize) to form heavy tar. The technology goes back to Germany in the mid-1920s and was put into large-scale production there in 1939 to produce motor fuels. From the 1970s to the 1990s, U. S. DOE conducted R&D on direct liquefaction of coal and supported construction of two large-scale pilot plants to demonstrate the technology. Plans to build a large-scale plant in the U.S. were canceled due to decreasing crude cost, increasing capital cost projections, concern over technical risk, and increasingly tight specifications on transportation fuels, which would lead to significantly increased refining cost of the coal-derived liquids for the products to meet required fuel specifications.

Air Liquide Drawing
Fig. 3.4 Schematic of a direct coal liquefaction process [1]

Figure 3.4 is a schematic flow diagram illustrating direct liquefaction of coal. Process conditions require temperature between 400°C and 430°C (750-800°F) and pressures between 200 and 250 atm (3,000-3,500 psi). The coal conversion occurs in a two-stage ebulating-bed reactor system with additional hydrogen or donor solvent added between stages. Because coal has a carbon to hydrogen ratio less than 1.0 and transportation fuels have a carbon to hydrogen ratio of about 2.0, large quantities of hydrogen need to be added in the process. This hydrogen is typically provided by the gasification of coal as shown in Fig. 3.4. The resulting "liquid" products from the coal are very heavy, very aromatic, and contain large amounts of sulfur, oxygen, and nitrogen. Thus, they require a lot of upgrading in relatively high-severity refinery-type hydro processing processes.

The Shenhua Group Corp. constructed a first train (one million tonnes per year liquid product) direct-liquefaction plant in Inner Mongolia, which started up in 2009 [7]. Expansion to five million tonnes/year is planned if economics are positive. India is also evaluating direct liquefaction, but has not committed to a plant.

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