Coal liquefaction is the conversion of coal into a synthetic oil in order to supplement natural sources of petroleum. It is an attractive technology because 1) it is well developed and thus could be implemented fairly rapidly and 2) there are relatively large quantities of coal reserves. Coal liquefaction is seen by many as a necessary technology to replace other sources of transportation fuels before production of biofuels or fuel cells can be ramped up to meet the gap left by declining supplies of oil. Hirsch, SAIC, 2/05, p56 found that coal liquefaction and heavy oil refining were potentially the two largest sources of transportation fuels that could be used to mitigate the peaking of conventional oil. The Hirsch report is the most comprehensive and authoritative analysis of mitigating peak oil.
Two methods of producing liquid fuels are direct coal liquefaction (DCL) and indirect coal liquefaction (ICL). In DCL the coal is directly contacted with a catalyst at elevated temperatures in the presence of added hydrogen. The ICL process consists to two major steps: 1) gasification to produce a synthesis gas and 2) conversion of the gas to a liquid by synthesis over a catalyst in a Fischer-Tropsch process. Removal of the sulfur from the coal before passing the gas over the catalyst is necessary to prevent "poisoning" of the catalyst. Removal of the CO2 is also desirable to improve synthesis efficiency. Without mitigation coal liquefaction emits 7-10 times the CO2 of oil production. This deficiency is nearly totally eliminated in the coal gasification projects being demonstrated by DOE. DCL processes are more efficient than ICL processes, 67% vs 55%, but higher quality coal and a more complicated process is required for DCL. Combining coal liquefaction with electricity production leads to a much more efficient process that utilizes some of the heat that would otherwise be wasted. (Technology Status Report 010: Coal Liquefaction, UK Department of Trade and Industry, 1999, Larson and Tingen, Princeton, 2003, Direct Coal Liquefaction, SRI, 3/16/05 )
Work was all but stopped in the US on coal liquefaction by about 1997. In a 2003 study prepared for DOE, Gasification Plant Cost and Performance Optimization, Task 2: Topical Report Coke/Coal Gasification with Liquids Coproduction it was concluded that the overall efficiency is improved, while the cost of electricity is reduced. "Adding hydrocarbon liquids coproduction can improve the return of an IGCC power plant when oil prices are relatively high. This is especially true for a coke coproduction plant because besides providing a refinery with a means of disposing of the low-value byproduct coke, it makes liquids which can be upgraded in the refinery to high-value liquid transportation fuels." Emissions in lb/btu were lower. The study assumed oil at $30/bbl. Whether related to this or not, at about this time, a contract was awarded for building a demonstration plant in Gilberton, PA which will coproduce synthetic fuels and electricity. (more later)
Coal is a much larger energy resource than oil or natural gas with reserves of over one trillion tons, enough to last for 150-200 years at present consumption rates (IEA, 12/10,2003). The R/P (reserves /production per year) is 164 years for coal compared to 66.7 years for natural gas and 40.5 years for oil. The US (27.1%), Russian Federation (17.1%), China (12.6%), India (10.2%), Australia (8.6%) and South Africa (5.4%) have the largest coal reserves. (BP Statistical Review of World Energy, June 2005). While R/P ratios do not indicate an accurate forecast of the length of time a resource may be available, it does give a relative value as to the quantity of the remaining reserves. The actual time a reserve will last at the current production rate is a fraction of these values. Peaking of production of a resource may occur at an R/P of 20-40. R/P values do not take into account the future discoveries, increasing demand for these resources or how easy (expensive) it is to recover the resources. What these ratios do emphasize is the limited quantities of our reserves of fossil fuels and the need to plan for alternate energy sources.
While converting natural gas to liquids (GTL) is a much simpler and less capital intensive process for producing liquid fuels, natural gas is more expensive than coal and is available in lessor quantities - it is believed by many that the peak of natural gas production will occur in less than 30 years, perhaps 15 years. GTL is a much cleaner process - natural gas is a relatively clean feedstock, the contaminants in coal must be removed prior to synthesis, requiring a more complex process. Despite these factors, the cost of oil from coal liquefaction is generally believed to be $30.-$40. per bbl., on the same order as liquids from the GTL process (Silverstein, UtilPoint, 2004). Thus it is believed that coal liquefaction will play a more important role is the supply of transportation fuels.
Coal liquefaction was first developed in Germany in the 1920S's with the development of the Fischer-Tropsch process which remains a core technology in the process. The process was further developed in the 30S's and was used to produce most of Germany's transportation fuels during WWII. South Africa, which like Germany has virtually no internal oil reserves, further developed the process to mitigate against boycotting during apartheid. This led to the development of the Sasol slurry-phase FT process which is now being marketed. They have three operating plants with a total capacity of about 150,000 bbl/day. NEDOL (Japan) has done some large scale development of a process. An extensive review of the various processes is given in the Technology Status Report 010: Coal Liquefaction, UK Department of Trade and Industry, 1999.
The US Department of Energy (DOE) had an active coal liquefaction program in the 1990's (Summary Report, 2001, DOE Clean Coal Technology Compodium). Direct work on coal liquefaction was halted in the late 90's but has been resumed recently, work continued on the most critical steps, gasification and catalyst development, as part of the advanced clean coal program which is aimed at developing clean, efficient ways of generating electricity from coal. In 2003 DOE announced a 5,000 bpd demonstration project using the process shown above, to demonstrate advanced FT fuel production. The project will co-produce 35 MW of electricity, in Gilberton, PA. The plant will incorporate a Shell Global gasifier and FT liquefaction technology provided by Sasol. (http://www.ultracleanfuels.com/articles/spector_01132003.html, Hart's Gas to Liquid Fuels, 2001)
Recently China has shown a strong interest in coal liquefaction and is investing billions of dollars into developing production facilities. Shenhua Group, one of China's largest coal companies is building a US$3.3 billion DCL project. The process was designed by and licensed from Hydrocarbon Technologies, Inc.(HTI). Shenhua has almost completed the construction of the Inner Mongolia coal liquefaction project's infrastructure. Operations of its first production line are expected to commence by 2007. The plant will produce 1 million tons of gasoline and diesel fuel a year. It is expected to process 15 million tons of coal to produce 5 million tons of oil products with four more production lines becoming operational by 2008. Shenhua is planning a second phase to the project with a total investment of 60 billion yuan (US$7.3 billion). (Tian, China Daily, 3/12/04), Coal to Clean Fuel, Fletcher, et al, Sept 2003)
Two areas of the process are receiving most of attention for future development: catalysts and gasifiers. Much work in these areas is proprietary, but some of the work gleaned from the public domain is as follows:
Catalyst cost is a significant portion of the cost of producing synthetic hydrocarbons. More efficient catalysts can reduce the size and thus the capital cost of plants. Work on catalysts is primarily on iron (Fe) for gasoline production and Cobalt (Co) for diesel production. The limiting factor in the process appears to be the ability to contact the catalyst with the coal. Slurry phase reactors have improved this activity, and further improvements are being made by making the catalyst particles very small, on the nanoscale. Schuebert et al of Syntroleum presented a paper, with 86 references, in 2003 that summarized catalyst development for the Cobalt slurry system. The U of KY at its Center for Applied Energy Research (CAER) in conjuction with HTI has done extensive catalyst development for DOE. HTI is supplying its proprietary GelCatTM dispersed nano-scale, iron based catalyst for the Chinese project. The nanocatalytic technology involved in the project has improved the economics of the process by $5 to $10 a barrel according to HTI. Nanocatalysts Sandia Labs has been involved with the development of NiMo catalysts supported on silica-doped hyrdrous titanium oxide for use in the direct liquefaction of coal. These catalysts have been shown to be superior to similar catalysts using alumina supports. This work is a joint project with CAER. Sandia Catalysis Technologies.
Research and development of catalysts and their performances for coal liquefaction has also been active in Japan. A variety of iron based catalysts were extensively examined for use in the liquefaction process. Finer particles were found to reduce significantly the amount of the catalyst to obtain the same oil yield. Usefulness of such catalysts is proved in the continuous operation of a large scale pilot plant. Coal liquefaction catalysts in Japan
Sasol has kept improving its F-T synthesis technology by improving ferric base and cobalt base catalysts.
Gasification has been under development by DOE and several equipment suppliers. DOE has been interested in the environmental aspects of gasification and the process efficiency. In its clean coal program is has been involved in a series of demonstration plants, each one having progressively more sophisticated gasification processes. They are demonstrating the ability to have lower and lower SOx, NOx, mercury and particulate emissions. Using oxygen instead of air in the gasifier produces a more concentrated waste stream from which it is easier to sequester CO2. The efficiency of the gasifiers has gradually improved as experience has been gained from each plant. The major plants in their program, in chronological order are listed below. It is very hard to find any details on the efficiency or the environmental improvements from plant to plant, but there is clear progress from the beginning to the current status. One clear indication is the overall efficiency of the plants.
Great Plains Coal Gasification Plant start ~1980- This was the first coal gasification plant built in response to worries about depleting supplies of natural gas. The plant is still operating, by the Basin Electric Power Cooperative, providing a myriad of by-products, doing further research on by-product production as well as supplying pipeline gas. CO2 from the plant is sold, sent via a 205 mile pipeline, to a Canadian oil company for a study of enhanced oil recovery and CO2 sequestration.
Wabash River Coal Gasification Project, end date 11/95. The Wabash River Coal Gasification Re powering Project was the first full-size commercial integrated gasification-combined cycle (IGCC)plant built in the United States.
Tampa Electric IGCC Project, start date 3/91, end date 3/02. The Polk Power Station is the Nation's first "greenfield" (built as a brand new plant) commercial IGCC power station. The power plant is one of the world's cleanest. The plant's gas cleaning technology removes more than 98 percent of the sulfur in coal, converting it to a commercial product. Nitrogen oxide emissions are reduced by more than 90 percent
Kentucky Pioneer IGCC Project, start 12/94, end 5/05. An IGCC plant using British Gas/Lurgi slagging fixed-bed gasification system coupled with Fuel Cell Engineering's molten carbonate fuel cell (CFC). The coal is gassified in an oxygen-blown, pressurized, slagging, fixed-bed gasifier. The raw product gas is quenched to reduce the temperature and remove tars, oils, ammonia, and particulates. The particulates and condensed tars and oils are recycled to the gasifier.
Gilberton Coal-to-Clean Fuels and Power Project, start 9/03, end 9/09. The Gilberton plant will be the first plant in the US to produce liquid fuels from coal. It will gassify coal wastes to produce a syngas which will be used to produce electric power and steam. A portion of the syngas will be converted into synthetic hydrocarbon liquids via FT synthesis.
Demonstration of a 285MW Coal-Based Transport Gasifier, start 10/05, end 3/15. The plant will demonstrate an air-blown IGCC power plant based on the transport gasifier. The transport gasifier is based on catalytic cracking technology that has been used successfully for over 50 years in the petroleum refining industry. It is cost-effective when handling low rank coals and when using coals with high moisture or high ash content and can be used in both air and oxygen-blown operation.
Mesaba Energy IGCC Project, Techline, start 10/05, end 6/12. This project is to design, construct and operationally demonstrate a next-generation IGCC electric power generating station. The project plant will adopt the E-Gas technology, and will incorporate significant technical advancements and lessons learned since the substantially smaller-scale clean coal demonstration at Wabash River, Indiana. The plant is designed to have significant performance, efficiency and emissions improvements - making the project plant one of the cleanest coal-fueled electric power generating plants in the world when it enters service in 2010
DOE has prepared cost studies on power plants using several types of commercial gasifiers ffto dermine the cost of electricity produced using each type of gasifier. The studies are quite detailed, mostly concerned with costing. Process flow diagrams, with flows, temperatures, pressures and H (total heat content) is given for each stream. Very general descriptions of the environmental controls are given for each system.
- British Gas/Lurgi The BGL process uses an oxygen-blown, moving-bed, slagging gasifier producing raw fuel at 980 F and 395 psia.
- Destec The Destec process uses a two stage oxygen blown, entrained flow gasifier producing raw fuel at 1900 F and 412 psia.
- KRW The KRW gasifier is air blown producing fuel at 1900 F and 400 psia.
- Shell The Shell process uses a dry feed, pressurized, oxygen-blown, entrained flow, slagging gasifier producing raw fuel at 1826 F and 352 psia.
- Texaco This is a pressurized, oxygen blown, slagging system producing raw fuel at 2500 F and 475 psia.
- Transport This gasifier is still in the developmental stage. Both air blown and oxygen-blown circulating bed reactors were studied using both coal and limestone thus reducing or possibly eliminating sulfur cleanup requirements. The gasifier was designed to produce 1657 F, 395 psia steam.