Direct Liquefaction Processes

Direct coal liquefaction (DCL) involves contacting coal directly with a catalyst at elevated temperatures and pressures with added hydrogen (H₂), in the presence of a solvent. DCL is a single-step process; indirect coal liquefaction (ICL) consists of two major steps. Therefore, the DCL process is, in principle, the simpler and more efficient of the two processes. It does, however, require an external source of H₂, which may have to be provided by gasifying additional coal feed and/or the heavy residue produced from the DCL reactor. The DCL process produces a product that is highly aromatic and requires substantial upgrading to produce acceptable transportation fuels.

The technology had been demonstrated by the Germans during World War II, but at high cost. Continued development in the United States and other nations, since then, has focused on reducing its costs, via catalysts, reactor design and process efficiency improvements. The U.S. Department of Energy (DOE) had a very active coal liquefaction research program in 1970 into the 1980s in response to the Organization of the Petroleum Exporting Countries (OPEC) oil embargo of 1973, but the funding has been greatly reduced since the 1990s when the DOE development for direct coal liquefaction ended. The DCL technology DOE helped to develop with Hydrocarbon Technologies, Inc., HTI (now part of Headwater, Inc.), was licensed to China in 2002. Shenhua Corporation of China is currently building a 20,000 barrels per day (bpd) DCL plant in Inner Mongolia based on Headwater’s technology.

Many different processes have been developed for DCL, but most are aligned similarly in regards to reaction chemistry and the process concept. Common features are the dissolution of coal into a solvent, followed by the hydrogenation of the coal with H₂ over a catalyst. The process can be very efficient with an overall thermal efficiency in the range of 65%.

Typical Process
The DCL process involves adding hydrogen (hydrogenation) to the coal, breaking down its organic structure into soluble products. The reaction is carried out at elevated temperature and pressure (e.g., 750 to 850°F and 1,000 to 2,500 psia) in the presence of a solvent. The solvent is used to facilitate coal extraction and the addition of hydrogen. The solubilized products, consisting mainly of aromatic compounds, then may be upgraded by conventional petroleum refining techniques such as hydrotreating to meet final liquid product specifications.

Figure 1 shows a typical block flow diagram of a DCL plant showing a hydrotreating unit (HTU) immediately downstream of the direct liquefaction reactor, to upgrade the distillate product as it is being produced.  The DCL processes are generally classified into two main groups: a single-stage versus a two-stage direct liquefaction process.

Figure 1: Simplified DCL Process Scheme

Single-stage Processes
Single stage processes were the first generation DCL technology, developed in the 1960s, and most such programs and facilities have since been superseded or abandoned. A single-stage process attempts to convert coal to liquids in a single reaction stage. Such process may include an integrated online hydrotreating reactor to upgrade the distillates, as shown in Figure 1. Technology developers included:

• H-Coal (HRI, USA)
• Exxon donor solvent (Exxon, USA)
• SRC-1 and II (Gulf Oil, USA)
• Conoco zinc chloride (Conoco, USA)
• Kohleoel (Ruhrkohle, Germany)
• NEDOL (NEDO, Japan)

Two-stage Processes
Most of the two-stage DCL processes were developed in response to the oil embargo in the early 1970s. Two-stage DCL process was developed in recognizing that the conversion process probably proceed in two steps – first, coal dissolution, in which the coal is converted to a soluble form with high molecular weight but with little change in the average composition from the original coal; and a second stage in which the dissolved products are upgraded to lower-boiling liquids with reduced heteroatom content.

Two-stage DCL processes were developed in many different countries, over a period between 1970 to the 1980s, with different levels of success. Processes and technology developers included:

• Catalytic two-stage liquefaction (US DOE and HTI, USA)
• Liquid solvent extraction (British Coal Corporation, UK)
• Brown coal liquefaction (NEDO, Japan)
• Consol synthetic fuel (Consol, USA)
• Lummus (Lummus, USA)
• Chevron coal liquefaction (Chevron, USA)
• Kerr-McGee (Kerr-McGee, USA)
• Mitsubishi Solvolysis (Mitsubishi Heavy Industries, Japan)
• Amoco (Amoco, USA)
• Others

Recent Developments
Very few DCL programs were continued beyond the late 1980s. One exception is the HTI (now Headwater, Inc.,) catalytic two-stage liquefaction process that was funded by DOE. The technology was licensed to Shenhua Corporation of China in 2002 for the construction of a 20,000 bpd plant in Inner Mongolia that started demonstration testing in December 2008.

Figure 2: Shenhua DCL Plant in Inner Mongolia

In April 2009, an Australian company announced plans to pilot test a novel underground coal to liquids process involving injecting water with simulated supercritical properties and an entrained catalyst into deep coal seams. Liquids would then be produced via established oil drilling and pumping methods.

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