Direct Liquefaction Processes
Reason for Discussion of Direct Liquefaction of Coal in Gasifipedia
As the following discussion of Direct Coal Liquefaction (DCL) explains, the fundamental process approach of DCL does not involve gasification at all. Therefore it would seem to be entirely out of scope of Gasifipedia. However, the following considerations require that discussion of DCL be included:
DCL is often compared and contrasted with Indirect Coal Liquefaction (ICL) which includes feedstock gasification as an intrinsic step. Discussion of DCL must be included for the sake of this discussion and completeness when covering coal to liquids technology.
- DCL process cycles often include gasification of solid residuals or additional carbonaceous feedstock to produce hydrogen needed for direct liquefaction reactions.
- Hybrid process approaches involving a combination of direct coal liquefaction and indirect coal liquefaction (in which gasification is an intrinsic step) have been proposed and may be increasingly important in new research and development in coal/biomass to liquids process technology.
Direct coal liquefaction involves contacting coal directly with a catalyst at elevated temperatures and pressures with added hydrogen (H2), in the presence of a solvent to form a raw liquid product which is further refined into product liquid fuels. DCL is termed direct because the coal is transformed into liquid without first being gasified to form syngas (which can then in turn be transformed into liquid products). The latter two-step approach, i.e. the coal to syngas to liquids route is termed indirect coal liquefaction (ICL). 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, biomass and/or the heavy residue produced from the DCL reactor. The DCL process results in a relatively wide hydrocarbon product range consisting of a variety of molecular weights and forms, with aromatics dominating. Accordingly, the product requires substantial upgrading to yield acceptable transportation fuels.
The technology had been demonstrated in Germany 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 Shenhua Corporation of China in 2002, which built a DCL plant in Erdos, Inner Mongolia (see further description below), based on Headwaters™ 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 Configurations
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 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)
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)
Commercial Implementations/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. In 2011, Shenhua Group reported that the direct CTL project in Erdos, Inner Mongolia, with designed fuel production capacity of 1.08 million tonnes per year of liquid products including diesel oil, liquefied petroleum gas (LPG) and naphtha (petroleum ether), had been in continuous and stable operations since November of 2010, and that Shenhua had made 800 million yuan ($125.1 million) in earnings before taxes in the first six months of 2011 on the CTL project. Shenhua noted that it planned to raise fuel production capacity in Erdos to 3 million tonnes a year by adding another direct CTL line in the first phase and to 5 million tonnes a year after a second phase is completed. Additionally, Shenhua noted that it was also planning to start constructing a 56.5 billion yuan, 3 million tonne per year direct CTL project in northwestern Xinjiang in late 2011.1
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.
- An Alternative Route for Coal To Liquid Fuel: Applying the ExxonMobil Methanol to Gasoline (MTG) Process, Samuel Tabak, 2008 Gasification Technologies Converence, October 2008.
- Shenhua Coal to Liquid and Shell Coal Gasification Application and Operation, 2008 Gasification Technologies Conference, October 2008.
- Coal Conversion – Pathway to Alternative Fuels, C. Lowell Miller (DOE) 2007 EIA Energy Outlook Modeling and Data Conference, Washington, DC, March 2007.
- Summary Report of The DOE Direct Liquefaction Process Development Campaign of the late Twentieth Century: Topical Report, DOE Contract DE-AC22-94PC93054, July 2001.
- Technology Status Report: Coal Liquefaction, Department of Trade and Industry, October 1999.