Fuels: Direct Liquefaction Processes
Direct coal liquefaction (DCL) involves reacting coal with hydrogen at elevated temperatures and pressures in the presence of a solvent, possibly with a catalyst, to form a raw liquid product which must be further refined into specification grade liquid fuels. Many different processes have been developed for DCL, but most are aligned similarly in regards to reaction chemistry and the process concept, with common features including dissolution of coal into a solvent and hydrogenation with H2. DCL is termed direct because the coal is transformed directly into a 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, having an overall thermal efficiency in the range of 65%. It does, however, require an external source of H2, which would typically 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. DCL products also contain high concentrations of heteroatoms that must be removed, typically through the use of conventional hydrocracking/hydrotreating processes. To obtain transportation fuels meeting today's requirements for sulfur, nitrogen, and aromaticity, the product requires substantial upgrading, consuming large amounts of hydrogen.
The technology had been demonstrated in Germany during World War II, but at high cost. Since then, continued development in the United States and other nations 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 from the 1970 into the 1990s in response to the Organization of the Petroleum Exporting Countries (OPEC) oil embargo of 1973 and later petroleum market volatility, but the funding has been greatly reduced since the 1990s when the DOE development for direct coal liquefaction ended, in large part because of the challenges of meeting Clean Air Act Amendments of 1990 that imposed strict limitations of aromatic content of fuels and the relative difficulty of removing pollutant species such as sulfur from coal-derived liquids. The DCL technology DOE helped to develop with Hydrocarbon Technologies, Inc., HTI (now part of Headwaters, Inc.), was licensed to Shenhua Corporation of China in 2002. Shenhua developed a DCL plant in Erdos, Inner Mongolia which began commercial operations in 2008 (see further description below). The Shenhua DCL plant is noted to have integrated technologies from DOE, Germany, and Japan with Shenhua’s own innovations.
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 3,000 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, are then 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. Although the simplicity of the fundamental process transformation of coal being liquefied with liquids being subsequently separated and upgraded into final products is depicted, the accompanying process complications of generating and providing hydrogen for both liquefaction and downstream hydrotreating are apparent, along with the need to separate and recycle solvent and deal with byproducts of the conversion. The DCL processes are generally classified into two main groups: single-stage versus two-stage direct liquefaction processes.
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 processes unavoidably included coal liquids hydrotreating to upgrade the liquefaction products, as shown in Figure 1. Technology developers included:
- H-Coal (HRI, USA)
- Exxon donor solvent (Exxon, USA)
- SRC-1 and II (Gulf Oil, USA)
- Kohleoel (Ruhrkohle, Germany)
- NEDOL (NEDO, Japan)
Two-stage DCL processes were developed in recognizing that the coal liquefaction transformations 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 react (primarily with hydrogen) to form lower-boiling liquids with reduced heteroatom content. However, the optimal conditions for these two steps may be different and may make different demands on catalysts, so trying to use a single catalyst at a single set of compromise conditions falls short for both steps. Using separate reactors, one for the dissolution step with a relatively inexpensive catalyst (iron-based) and a downstream reactor for liquids reactions (possibly using a more active, expensive catalyst) provides optimized catalysts and opportunity to use optimal process conditions for either step. This is borne out by the significant increase in liquids yield and product quality demonstrated by the two-stage DCL processes over the earlier single-stage processes.
Two-stage DCL processes were developed in many different countries, over a period from 1980 to the 1990s, with different levels of success. Processes and technology developers included:
- Catalytic Two-stage Liquefaction (DOE, HTI, USA)
- Integrated Two-stage liquefaction (Lummus, USA)
- Brown Coal Liquefaction (NEDO, Japan)
- Liquid Solvent Extraction (British Coal Corporation, UK)
- Supercritical Gas Extraction (British Coal Corporation, UK)
Commercial Implementations/Recent Developments
Very few DCL programs were continued beyond the late 1980s. One exception is the HTI (now Headwaters, Inc.,) catalytic two-stage liquefaction process that was funded by DOE. The technology was licensed to Shenhua Corporation of China in 2002, elements of which were probably utilized in the 24,000 bpd DCL plant in Erdos, Inner Mongolia that started demonstration testing in December 2008. In 2011, Shenhua Group reported that their DCL plant, 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 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.
- Brown Coal Liquefaction Technology
- Catalytic Multistage Liquefaction of Coal at HTI, HRI/HTI, V.R. Pradhan el al, Coal and Gas Conversion Contractors' Review Conference, Contract Report, No. 92PC92147, August, 1995
- Catalytic Multistage Liquefaction of Coal (CMSL) Final Report, Hydrocarbon Technologies, Inc., A.G. Comolli et al., November, 1996
- Chemistry of Coal Liquefaction; Second Supplementary Volume, Martin Elliott, editor, 1981, John Wiley & Sons, Inc., Chapters 27/28/29
- Coal Conversion - Pathway to Alternative Fuels, C. Lowell Miller (DOE) 2007 EIA Energy Outlook Modeling and data Conference, Washington, DC, March 2007
- Coal Liquefaction: A Research and Development Needs Assessment, Department of Energy, February 1989
- Direct Coal Liquefaction: Lessons Learned, R. Malhotra, SRI International, GCEP Advanced Workshop, BYU, Provo, UT, March 2005
- Direct Coal Liquefaction Baseline Design and System Analysis; Executive Summary, Volume 1-7, Bechtel, Amoco, Contract No. 90PC89857, March 1993
- Direct Coal Liquefaction Low Rank Coal Study; Executive Summary, Study, Bechtel, Amoco, Contract No. 90PC89857, February 1995
- Direct Coal Liquefaction Low Rank Coal Study; Executive Summary, Final Report on Design, Capital Cost and Economics for the Low Rank Coal Study, Bechtel, Amoco, Contract No. 90PC89857, February 1995: Volume 1A, Volume 1B
- Direct Coal Liquefaction - Ultra Clean Fuels and a Specific Case Study, Theo L.K. Lee & Jim Lepinski, Headwaters CTL, John Duddy, Axens North America, World CTL 2011
- Direct Liquefaction Proof of Concept Facility, HRI/HTI, A.G. Comolli el al, Technical Progress Report - POC Run 1, Contract No. 92PC92148, August 1995
- Direct Liquefaction Proof of Concept Facility, HRI/HTI, A.G. Comolli el al, Technical Progress Report - POC Run 2, No. 92PC92148, December 1996
Improved Brown Coal Liquefaction (BCL) Process, Sojitz Corp. at CTLtec America's Conference, June 23-24, 2008, Pittsburgh, Pennsylvania
- Overview of Coal Liquefaction, James Lepinski, Headwaters Incorporated, U.S. India Coal Working Group Meeting, Washington DC, November 2005
- Shenhua Coal to Liquid and Shell Coal Gasification Application and Operation, 2008 Gasification Technologies Conference, October 2008
- Shenhua Direct Coal Liquefaction Plant and CTL Development in China, Qingyun Sun, US-China Energy Center, West Virginia University, Unconventional Fuel Forum, May 2008
- Shenhua Direct Coal Liquefaction Project: From R&D to Commercial Demonstration, Wu Xiuzhang, Shenhua Group Co., Ltd, May 26th, 2010
- "Shenhua shows the way to make gas from coal," China Daily; S. Tingting, January 22, 2009
- Summary Report of The DOE Direct Liquefaction Process Development Campaign of the late Twentieth Century: Topical Report, Consol Energy Inc. and Mitretek Systems, DOE Contract DE-AC22-94PC93054, July 2001
- Technology Status Report: Coal Liquefaction, Department of Trade and Industry, October 1999
- Technologies to Reduce or Capture and Store Carbon Dioxide Emissions, The National Coal Council, June 2007
- Ullmann's Encyclopedia of Industrial Chemistry, Coal Liquefaction