Background
Processes for producing liquid fuels from coal can be grouped into categories of pyrolysis, solvent extraction, catalytic liquefaction and indirect liquefaction. Pyrolysis involves the heating of the coal feedstock to high temperatures to convert the coal into gases, liquids and char. Carbon is removed from the process, increasing the content of hydrogen in the gaseous product, while light and heavy liquids contain less hydrogen than crude oil. Solvent extraction uses a coal-derived liquid that transfers hydrogen to the coal, increasing the yield of liquid hydrocarbons. Catalytic liquefaction adds hydrogen to coal with the aid of a suitable catalyst operating in the liquid phase. Indirect liquefaction first reacts coal with oxygen and steam to produce carbon monoxide and hydrogen. These gases are purified to remove sulfur, nitrogen and ash and are then reacted in the presence of a catalyst to produce liquid products.
These liquefaction products have been used as transportation fuels for over 50 years. This process is an increasingly attractive alternative as conventional, petroleum-derived fuels become less available and more expensive.
Indirect Liquefaction for Transportation Fuels
The Fischer-Tropsch (FT) from coal is well-understood chemistry, discovered in 1923 by the German scientists Hans Fischer and Franz Tropsch. Today it is commercially used by Sasol South Africa, whose facilities produce over 160,000 barrels per day of transportation fuels, including diesel, gasoline and jet fuels. With changing petroleum and energy economics there are several projects under development in the United States. The projected first to be on line will be the Rentech conversion of the Royster-Clark facility in East Dubuque, Illinois. This facility will be on-stream in early 2009, producing about 250,000 gallons of ultra-clean FT diesel fuel per day, some of which could be sold to the Department of Defense for testing in jet engines and ground vehicles.
The remainder will go to transit fleets, agriculture cooperatives and Mississippi River transport. Coal-to- transportation fuels is proven technology with a long history, and the fuels that are produced have favorable characteristics and high value. The fuels are ultra-low sulfur, ultra-low aromatics, high-cetane and biodegradable and are very stable, with a shelf life of over eight years.
History of Commercial Indirect Liquefaction
Germany began the commercialization of indirect liquefaction in the 1930s as a means to produce fuel for the German military in World War II. After WWII the technology was further developed by the U.S. government and Texaco, who together built and operated a plant in Texas until the early 1950s. It was eventually shut down due to the relative economics with petroleum products.
In the 1960s, South Africa was facing restrictions on imported oil due to apartheid, and they turned to a national energy policy that would push for energy independence by using domestic resources of coal. They committed large government resources to build coal gasification and high-temperature Fischer-Tropsch facilities that continue to operate today, producing nearly 200,000 barrels per day. These facilities were provided to Sasol, which develops and operates projects based on their technology and experience throughout the world. In 1981, a small start-up company in Denver called Rentech, Inc., was formed and began research and development of a low-temperature Fischer-Tropsch technology. Rentech, Inc., is leading the deployment of the low-temperature, high-efficiency technology in the United States today and has plans for their first commercial plant to come online in 2009.
In summary, Fisher-Tropsch chemistry has been understood since 1923. The first commercial facilities were built in the 1930s, and Sasol has operated commercially since the early 1960s. Commercial facilities with high- efficiency, low-temperature FT technology are being planned for start up in the United States in 2009.
Review of Coal-to-Liquids Technologies
Coal-to-liquids (CTL) is the process of converting solid coal into liquid fuels and/or chemicals. This section of this report focuses on the conversion of coal-into-liquid transportation fuels. The key to converting coal-into- liquid fuels is hydrogen. Coal typically contains only 5% hydrogen, while distillable liquid fuels typically contain 14% hydrogen. The hydrogen deficit can be made up in two different ways. In the direct route, hydrogen is forced into the coal under high pressure and temperature often in the presence of a catalyst. In the indirect route, coal is gasified with oxygen and steam to produce a synthesis gas (syngas) containing hydrogen and carbon monoxide that is then passed over a catalyst to form hydrocarbons.
There are several additional routes to making transportation fuels from coal. Direct and indirect coal liquefaction can be integrated into a hybrid plant. Direct coal liquefaction can be combined with heavy oil upgrading in a coal and oil co-processing plant. Finally, coal can be partially converted into liquid fuels by mild pyrolysis.
Direct Coal Liquefaction
In the direct coal liquefaction process, pulverized coal is slurried with a recycled oil and heated under high pressure to produce a synthetic crude oil that can be further refined into ultra-clean transportation fuels. The hydrogen required for this process can be produced by gasifying coal and residual carbon or reforming natural gas.
Historical Development
Direct coal liquefaction originated in Germany in 1913, based on work by Friedrich Bergius. It was used extensively by the Germans in World War II to produce high octane aviation fuel. Since that time, tremendous advancements have been made in product yields, purity and ease of product upgrading.
Following the petroleum price and supply disruptions in 1973, the U.S. government began a substantial program to fund the development of alternative fuels, particularly direct coal liquefaction. From 1976 to 2000, the U.S. government invested approximately $3.6 billion (1999 dollars) on improving and scaling up direct coal liquefaction. Early direct liquefaction processes used single-stage reactor configurations. This was replaced by two-stage configurations to achieve higher efficiency of hydrogen utilization. Process equipment and operating conditions were optimized, online hydrotreating and solvent de-ashing were added, and improved catalysts were developed. Pilot and demonstration facilities ranging up to 600 tons per day of coal (1800 bbl/d of fuel oil) were built and operated in the United States.
Following is a partial list of direct coal liquefaction technologies developed during the last half of the twentieth century. Most of these technologies are no longer under development.
A report was issued in July 2001 by the U.S. Department of Energy summarizing the results of its direct coal liquefaction development program. Following are excerpts from the conclusion of that report:
“The DOE direct liquefaction program produced a surprisingly mature technology. The intensive effort between 1976 and 1982 (Phase I), when 90% of the program funds were expended, resulted in a demonstration of the technical feasibility of the major process components. The Phase I processes, however, were deficient in terms of product yield and quality. This stimulated further research and development work between 1983 and 1999 (Phase II). The Phase II work was significantly less costly than earlier demonstration projects, but resulted in substantial improvements in process performance and economics. It now is possible to produce liquids of high quality at high yields that approach the theoretical maximum. At the same time, the cost for a barrel of product dropped by 50% because of process optimization and increased yields. Economics and engineering studies conducted throughout Phase II have reduced the uncertainty, and therefore, the risk associated with commercial deployment of the technology.
“The current technology is well defined in terms of cost and performance. It represents a technically available option for the production of liquid fuels. It can be used domestically in the United States to limit our exposure to oil price increases in the international market or to offset supply reductions. It also can be used by other nations who choose to use domestic coal to meet their transportation fuel needs, thus reducing demands on conventional petroleum sources.
It can be used with coal alone, or to co-process a variety of lower value feedstocks. The results of the DOE program allow direct coal liquefaction to be accurately assessed in context to the costs and risks associated with other options for securing liquid fuel supplies should the need arise.”
“The current technology is well defined in terms of cost and performance. It represents a technically available option for the production of liquid fuels. It can be used domestically in the United States to limit our exposure to oil price increases in the international market or to offset supply reductions. It also can be used by other nations who choose to use domestic coal to meet their transportation fuel needs, thus reducing demands on conventional petroleum sources.
It can be used with coal alone, or to co-process a variety of lower value feedstocks. The results of the DOE program allow direct coal liquefaction to be accurately assessed in context to the costs and risks associated with other options for securing liquid fuel supplies should the need arise.”
Current Commercial Activity
In 1996, the DOE received an inquiry from the Chinese government asking for information on the most advanced direct coal liquefaction available in the United States. The DOE recommended the HTI Coal Process and introduced HTI to the Chinese in December of that year. The Chinese government put direct coal liquefaction into its five-year plan and commissioned Shenhua Group (the largest coal company in China) to develop a direct coal liquefaction project in Inner Mongolia, China.
Shenhua Group studied all of the commercially available direct coal liquefaction technologies from the United States, Japan and Germany and in June 2002 signed a license agreement with HTI to apply HTI’s technology for the first stage of a 50,000 bbl/d project. A process design package was supplied by HTI and engineering was proceeding; however, Shenhua Group wanted to make some modifications to the technology contrary to the advice of HTI. After further negotiation, a new agreement was drafted and signed that allowed Shenhua to use and modify HTI’s technology for the first-stage of the 50,000 bbl/d project. Shenhua paid HTI the full license fee for the technology applied to the first-stage and released HTI from any process performance guarantees.
In October 2004, HTI signed an agreement with Oil India Ltd. (OIL) to conduct testing and a feasibility study for a commercial plant in the Assam state of India. The Assam coal is some of the best coal for direct coal liquefaction because of its high reactivity and yield. Lab-scale tests have been completed and pilot plant testing commenced in late 2005.
In February 2005, HTI signed a memorandum of understanding with the Philippines Department of Energy to evaluate applying direct and/or indirect coal liquefaction in the Philippines. The Philippines’ government has placed high priority on coal liquefaction and desires to make that country the hub for the coal liquefaction industry in Southeast Asia. The first stage of the feasibility study was completed in September 2005.
Process Description
Coal is a solid organic material made up of large, complex molecules containing mostly carbon, plus small amounts of hydrogen, sulfur, nitrogen and oxygen. Raw coal also contains moisture and solid particles of mineral matter (ash). The aim of direct coal liquefaction is to break coal down into smaller component molecules, then to add hydrogen, creating lighter and more stable oil molecules. The process simultaneously removes sulfur, nitrogen and ash, resulting in a clean liquid fuel product.
Typical Direct Coal Liquefaction Process
In a typical direct coal liquefaction process, pulverized coal is dissolved in recycled coal-derived heavy process liquid at about 170 bar and 425°C while hydrogen is added. Most of the coal structure is broken down in the first-stage reactor. Liquefaction is completed in the second-stage reactor, at a slightly higher temperature and lower pressure. A proprietary catalyst is dispersed in the slurry for both stages. A hydrotreater is incorporated in the process to remove sulfur and nitrogen and open up the aromatic structure to achieve higher cetane levels, thereby facilitating the downstream refining process. The bottom-of-the-barrel residue (material boiling above 455°C) is de-ashed and recycled as heavy process liquid. The ash reject, containing residual carbon, can be fed to the gasifier for use in production of hydrogen.
Indirect Coal Liquefaction
Indirect coal liquefaction involves first the gasification of coal to produce synthesis gas, followed by purification to remove CO2 and other contaminants, and then the conversion of the synthesis gas to liquid products using the Fischer-Trospch synthesis process and associated product upgrading.
Historical Development
Indirect coal liquefaction was developed in Germany in 1923 based on work by Dr. Franz Fischer and Dr. Hans Tropsch. During World War II, the technology was used by Germany to produce 17,000 bbl/d of liquid fuels from coal.
After the war, the Fischer-Tropsch synthesis technology was used by HRI (predecessor of HTI) to construct a 7,000 bbl/d gas-to-liquids plant in Brownsville, Texas, in 1949. The plant was operated by Cathage Hydrocol from 1950 to 1953 before shutting down due to declining oil prices. The partial oxidation unit, used to convert the natural gas into synthesis gas to feed the fixed-bed FT reactors at this plant, was the basis for what eventually became the Texaco coal gasification process currently owned by GE Energy.
After the war, the Fischer-Tropsch synthesis technology was used by HRI (predecessor of HTI) to construct a 7,000 bbl/d gas-to-liquids plant in Brownsville, Texas, in 1949. The plant was operated by Cathage Hydrocol from 1950 to 1953 before shutting down due to declining oil prices. The partial oxidation unit, used to convert the natural gas into synthesis gas to feed the fixed-bed FT reactors at this plant, was the basis for what eventually became the Texaco coal gasification process currently owned by GE Energy.
During this same time period (1950–53), Koelbel tested a 1.5 meter diameter slurry-phase FT reactor in Rheinpreussen, Germany. By the mid-1950s, all of the German FT plants were shut down due to declining world oil prices with discovery of abundant oil deposits in the Middle East.
While other countries were shutting down their FT plants, South Africa began commissioning its first indirect coal liquefaction plant. Sasol was established in 1950 with the prime objective to convert low-grade coal into petroleum chemicals and feedstocks. Sasol One was built in Sasolburg and produced its first liquid product in 1955. In 1969 the Natref crude oil refinery was commissioned, and in 1980 and 1982, Sasol Two and Sasol Three respectively began production in Secunda. Today, Sasol produces the equivalent of 150,000 bbl/d of fuels and petrochemicals from coal via the indirect liquefaction process. The process produces in excess of 40% of South Africa’s liquid fuel requirements. Sasol manufactures more than 200 fuel and chemical products in Sasolburg and Secunda in South Africa, as well as at several global locations.
The FT reactors installed in 1995 at Sasol One consisted of five tubular fixed-bed reactors with a capacity of 500 bbl/d each, and three circulating fluidized-bed reactors having a capacity of 2,000 bbl/d each. In 1980/1982, Sasol installed 16 x 6,500 bbl/d circulating fluidized-bed reactors at Secunda. From this engineering effort, it became clear that the circulating fluidized-bed technology had reached its maximum scale-up potential. A new generation 3,500 bbl/d (5-m diameter) fluidized-bed reactor was installed at Sasolburg in 1989. This led to the further scale-up to an 11,000 bbl/d (8-m diameter) reactor in 1995 and the 20,000 bbl/d (10.7-m diameter) reactor in 1998. Between 1995 and 1998, the 16 original circulating-fluidized-bed reactors at Secunda were replaced with 4 x 11,000 bbl/d and 4 x 20,000 bbl/d fluidized-bed reactors. Sasol’s total capital investment for indirect coal liquefaction from 1955 to 2000 exceeded $6 billion.
Interest in gas-to-liquids for monetizing stranded natural gas reserves has influenced most major oil companies to invest billions of dollars (combined) in developing their own FT technology. Following is a list of FT technologies that have reached at least the process development unit (PDU or large pilot-plant scale).
Most of the above companies are focused only on gas-to-liquids (GTL) rather than coal-to-liquids (CTL).
Most of the above companies are focused only on gas-to-liquids (GTL) rather than coal-to-liquids (CTL).
The noticeable exceptions are Sasol, Rentech Incorporated and the Institute of Coal Chemistry, which are active in CTL. Shell is constructing a biomass-to-liquids (BTL) pilot plant in Freiberg, Germany. Iron or cobalt catalyst can be used for indirect coal liquefaction, but iron catalyst offers an advantage in that it can operate with a lower H2/CO ratio typically found in coal-derived syngas.
Current Commercial Activity
Major oil companies are currently spending, or planning to spend, in excess of $25 billion on gas-to-liquids facilities in remote areas such as Qatar, Iran, Nigeria, Bolivia and Australia. Commercial activity on indirect coal liquefaction projects has been less dramatic but is gaining momentum.
In 2004, Sasol reached agreement with the government of China to conduct a feasibility study on two 70,000 bbl/d indirect coal liquefaction projects in China sponsored by Shenhua Group, Luneng Coal Chemicals, Ningxia Coal Group and Sinopec. In July 2004, Yankuang Group started up a 480 bbl/d demo plant. The Institute of Coal Chemistry announced that it is planning to set up a 3,900 bbl/d demo plant in China. And in August 2005, HTI announced signing a license with UK RACE Investment Limited for setting up a 700 bbl/d demo plant in China.
Indirect coal liquefaction projects are also being studied in Australia, Indonesia, India, Pakistan and the Philippines. The United States has several indirect coal liquefaction projects under consideration. Following is a list of those that have been discussed publicly.
The Rentech projects in Illinois and Ohio involve coal-based integrated FT fuel and ammonia production. A preliminary feasibility study has been completed on the Illinois project, and the first phase of front end engineering and design (FEED) has begun. Most of the projects will involve production of some electricity as well as FT diesel and FT naphtha.
Process Description
Indirect coal liquefaction can operate on nearly any coal feedstock as long as the proper gasification and gas cleaning technology are selected. Selection of the proper coal gasification technology is critical because it has perhaps the biggest impact on the overall project cost.
In the gasification process, coal is partially oxidized with oxygen and steam to form carbon monoxide and hydrogen rich syngas. The raw syngas is cooled and cleaned of carbon dioxide and other impurities such as hydrogen sulfide, ammonia, halogens, cyanide and mercury. The H2/CO ratio of the syngas may be adjusted for optimum FT performance. As the clean syngas passes through the FT reactor, it comes in contact with a proprietary catalyst and forms long-chain paraffin hydrocarbons ranging from C1 to C100+ along with some oxygenates such as water and alcohols. The tail gas can be recycled or sent to a gas turbine to generate electricity. The oxygenates and distillable liquids are separated through fractionation. The wax and catalyst are separated through settling and filtration. The wax is sent to a hydrocracker, where it is converted into distillable liquids using a catalyst and hydrogen. The distillable liquids are hydrotreated and separated by fractionation into finished products such as FT diesel and FT naphtha. The oxygenates can be used as feedstock for the gasifier or combusted to produce electricity. The steam generated from cooling the syngas and from cooling the exothermic reactions in the FT reactor can be sent to a steam turbine to generate additional electric power.
Comparison of Direct and Indirect Coal Liquefaction
Figure 2.6 compares typical product characteristics for direct and indirect coal liquefaction products. One of the biggest differences between the two coal liquefaction technologies is that direct coal liquefaction makes high- octane gasoline and low-cetane diesel, while indirect coal liquefaction produces high-cetane diesel and low- octane gasoline. One other difference is that direct coal liquefaction products are denser and therefore tend to have more Btus per gallon than indirect coal liquefaction products.
Hybrid Coal Liquefaction
Hybrid coal liquefaction integrates direct and indirect coal liquefaction into a single plant. This concept takes advantage of the complementary characteristics of the two processes. As mentioned above, direct coal liquefaction makes high-octane gasoline and low-cetane diesel, while indirect coal liquefaction produces high- cetane diesel and low-octane gasoline. Blending the products in an integrated plant allows production of premium quality gasoline and diesel with minimal refining.
Historical Development
The concept of a hybrid DCL/ICL plant has been discussed for many years. The U.S. Department of Energy commissioned MITRE Corporation to study the concept between 1990 and 1991. Initial studies indicated that production costs were slightly lower for a hybrid plant compared to standalone direct or indirect plants. No testing has been done on this concept to date.
Current Commercial Activity
HTI signed two license agreements in August 2005 with UK RACE Investment Limited for two 700 bbl/d plants to be built in China. The first plant will be an indirect coal liquefaction plant, and the second plant will be a direct coal liquefaction plant and will be integrated into the first plant to demonstrate the hybrid concept.
A feasibility study for a 60,000 bbl/d hybrid plant is currently being conducted in the Philippines by HTI in cooperation with private and government entities.
A feasibility study for a 60,000 bbl/d hybrid plant is currently being conducted in the Philippines by HTI in cooperation with private and government entities.
Process Description
The synergy between the direct and indirect processes improves overall thermal efficiency of an integrated hybrid plant. Higher-quality coal can be fed as feedstock to the direct coal liquefaction reactors, and lower- quality coal can be fed to the gasifier to provide syngas for FT synthesis. The hydrogen-rich FT tail gas can be used to provide hydrogen for product upgrading and for direct coal liquefaction.
Blending the raw distillable products prior to refining takes advantage of their complementary characteristics. High-octane naphtha from direct coal liquefaction is blended with low-octane naphtha from indirect coal liquefaction and high-cetane diesel from indirect coal liquefaction is blended with low-cetane diesel from direct coal liquefaction. The blended liquids require less refining to meet premium product specifications than if they were refined separately.
Co-Processing Coal and Heavy Oil
Co-processing of coal and heavy oil is worth considering if there is a low-cost source of heavy oil such as bottom-of-the-barrel resid from a local refinery. The aim of co-processing coal and heavy oil is to simultaneously break down the complex coal and heavy petroleum molecules into smaller distillable molecules, which can be further refined into clean liquid fuel products.
Co-processing can be technically and economically more appealing than direct coal liquefaction because it eliminates the need for recirculating a large stream of internally generated process-derived liquids and lowers the required capital and operating cost. However, co-processing production costs may be higher than direct coal liquefaction production costs if the resid is significantly more expensive than coal on an energy basis.
Historical Development
Co-processing was first tested in 1974 at HRI (now HTI) test facilities in Lawrenceville, New Jersey. Bench- scale tests were conducted on a wide range of materials in the early to mid-1980s. In 1989, tests were run on Ohio coal and Cold Lake resid in the 30 bbl/d process development unit. In the 1990s, co-processing tests were run for customers in Nova Scotia, China, India and Indonesia. A co-processing pilot plant was built in Duliajan, Assam, India, in 1994.
Process Description
In co-processing, a preheated mixture of pulverized coal, catalyst, resid (may also contain a small amount of recycle liquid) and hydrogen is fed into the first of two reactors at a temperature of 435°–460°F and pressure of 170 bars. Most of the coal and resid structure is broken down in the first-stage reactor. Hydrocracking of the intermediate coal and resid products is completed in the second-stage reactor. The distillable products pass through a mild hydrotreater and then further upgraded using conventional refining techniques to produce gasoline, as well as jet and diesel fuels that will meet or exceed existing and planned fuel specifications.
Mild Pyrolysis
Mild pyrolysis is a method of obtaining liquid fuels from coal by heating the coal in an oxygen-free atmosphere, vaporizing the volatile material, and then condensing out the hydrocarbon liquids from the product vapors. This technique is perhaps the oldest method of extracting liquid fuels from coal, but yields and product quality are very low.
Historical Development
At least three mild pyrolysis technologies were developed to pilot-plant scale in the United States in the 1980s. The processes differed mainly in the design of the pyrolyzing reactor. One process, the liquids from coal (LFC) process, was scaled up to a 1,000 stpd demo plant in 1992. The LCF process was developed by SGI International. The demo plant was built in Gillette, Wyoming and owned by Encoal Corporation. Funding was provided by the U.S. Department of Energy Clean-Coal Technology Demonstration Program.
The plant operated up and down for a few years before shutting down. The plant has changed ownership several times since starting up.
Process Description
Mild pyrolysis favors use of high-volatile coals. It consists of heating coal to a temperature in the range of 450°–650°C in an oxygen-free atmosphere, driving off volatile matter from the coal, generating other volatile organic compounds, and condensing out the distillable liquids. Liquid yield is typically less than 20%. The main product is char with a reduced hydrogen, sulfur and nitrogen content.
In a typical mild pyrolysis process, coal is crushed and screened and then heated by a hot gas stream in a rotary-grate dryer. The dried coal is then fed into the main rotary-grate pyrolyzer, where it is heated to about 540°C by a hot recycle gas stream. Upon discharge from the pyrolyzer, the solids are passed to a deactivation step and are then cooled in an indirect rotary drum cooler. The gas from the pyrolyzer is cooled in a quench tower condensing out the distillable liquids. The gases are then recycled to provide fuel for the process. The liquid fuel produced in this process is roughly equivalent to a No. 6 fuel oil.
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