Synthetic fuel
Synthetic fuel or synfuel is a liquid fuel, or sometimes gaseous fuel, obtained from syngas, a mixture of carbon monoxide and hydrogen, in which the syngas was derived from gasification of solid feedstocks such as coal or biomass or by reforming of natural gas.
Common ways for refining synthetic fuels include the Fischer–Tropsch conversion, methanol to gasoline conversion, or direct coal liquefaction.
Classification and principles
There is a range of meanings for the terms 'synthetic fuel' or 'synfuel'.- The most traditional view restricts the input material to coal and the output to liquid hydrocarbons. Some authors additionally allow natural gas as input.
- Newer understandings allow coal, natural gas, or biomass as feedstock. The output can be synthetic crude or synthetic liquid products. Industrial and municipal waste can also be acceptable feedstock.
- Some definitions also allow oil sands and oil shale to be acceptable inputs through synthetic crude.
- The most liberal definition accepts all of the above input materials and allows for any form of output fuel: liquid, gas, or "clean" solid. This is the approach of petrochemist James G. Speight; he also mentions that in the context of substitutes for petroleum-based fuels it has even wider meaning. Depending on the context, methanol, ethanol and hydrogen may also be included in this category.
History
The process of direct conversion of coal to synthetic fuel originally developed in Germany.Friedrich Bergius developed the Bergius process, which received a patent in 1913. Karl Goldschmidt invited Bergius to build an industrial plant at his factory, the Th. Goldschmidt AG, in 1914. Production began in 1919.
Indirect coal conversion was also developed in Germany - by Franz Fischer and Hans Tropsch in 1923. During World War II, Germany used synthetic-oil manufacturing to produce substitute oil products by using the Bergius process, the Fischer–Tropsch process, and other methods.
In 1931 the British Department of Scientific and Industrial Research located in Greenwich, England, set up a small facility where hydrogen gas was combined with coal at extremely high pressures to make a synthetic fuel.
The Bergius process plants became Nazi Germany's primary source of high-grade aviation gasoline, synthetic oil, synthetic rubber, synthetic methanol, synthetic ammonia, and nitric acid. Nearly one third of the Bergius production came from plants in Pölitz and Leuna, with 1/3 more in five other plants.
Synthetic fuel grades included "T.L. Messershmitt Me 262| fuel", "first quality aviation gasoline", "aviation base gasoline", and "gasoline - middle oil"; and "producer gas" and diesel were synthesized for fuel as well. By early 1944 German synthetic-fuel production had reached more than from 25 plants, including 10 in the Ruhr Area. In 1937 the four central Germany lignite coal plants at Böhlen, Leuna, Magdeburg/Rothensee, and Zeitz, along with the Ruhr Area bituminous coal plant at Scholven/Buer, produced of fuel. Four new hydrogenation plants were subsequently erected at Bottrop-Welheim, Gelsenkirchen, Pölitz, and, at 200,000 tons/yr Wesseling. Nordstern and Pölitz/Stettin used bituminous coal, as did the new Blechhammer plants. Heydebreck synthesized food oil, which was tested on concentration camp prisoners. After Allied bombing of Germany's synthetic-fuel production plants, the Geilenberg Special Staff used 350,000 mostly foreign forced-laborers to reconstruct the bombed synthetic-oil plants, and, in an emergency decentralization program, the , to build 7 underground hydrogenation plants with bombing protection. In July 1944 the "Cuckoo" project underground synthetic-oil plant was being "carved out of the Himmelsburg" north of the Mittelwerk, but the plant remained unfinished at the end of World War II. Production of synthetic fuel became even more vital for Nazi Germany when Soviet Red Army forces occupied the Ploiești oilfields in Romania on 24 August 1944, denying Germany access to its most important natural oil source.
Indirect Fischer–Tropsch technologies were brought to the United States after World War II, and a plant was designed by HRI and built in Brownsville, Texas. The plant represented the first commercial use of high-temperature Fischer–Tropsch conversion. It operated from 1950 to 1955, when it was shut down after the price of oil dropped due to enhanced production and huge discoveries in the Middle East.
In 1949 the U.S. Bureau of Mines built and operated a demonstration plant for converting coal to gasoline in Louisiana, Missouri. Direct coal conversion plants were also developed in the US after World War II, including a 3 TPD plant in Lawrenceville, New Jersey, and a 250-600 TPD Plant in Catlettsburg, Kentucky.
In later decades the Republic of South Africa established a state oil company including a large synthetic fuel establishment.
Processes
The numerous processes that can be used to produce synthetic fuels broadly fall into three categories: Indirect, Direct, and Biofuel processes.Indirect conversion
Indirect conversion has the widest deployment worldwide, with global production totaling around, and many additional projects under active development.Indirect conversion broadly refers to a process in which biomass, coal, or natural gas is converted to a mix of hydrogen and carbon monoxide known as syngas either through gasification or steam methane reforming, and that syngas is processed into a liquid transportation fuel using one of a number of different conversion techniques depending on the desired end product.
The primary technologies that produce synthetic fuel from syngas are Fischer–Tropsch synthesis and the Mobil process. In the Fischer–Tropsch process syngas reacts in the presence of a catalyst, transforming into liquid products and potentially waxes.
The process of producing synfuels through indirect conversion is often referred to as coal-to-liquids, gas-to-liquids or biomass-to-liquids, depending on the initial feedstock. At least three projects are combining coal and biomass feedstocks, creating hybrid-feedstock synthetic fuels known as Coal and Biomass To Liquids.
Indirect conversion process technologies can also be used to produce hydrogen, potentially for use in fuel cell vehicles, either as slipstream co-product, or as a primary output.
Direct conversion
Direct conversion refers to processes in which coal or biomass feedstocks are converted directly into intermediate or final products, avoiding the conversion to syngas via gasification. Direct conversion processes can be broadly broken up into two different methods: Pyrolysis and carbonization, and hydrogenation.Hydrogenation processes
One of the main methods of direct conversion of coal to liquids by hydrogenation process is the Bergius process. In this process, coal is liquefied by heating in the presence of hydrogen gas. Dry coal is mixed with heavy oil recycled from the process. Catalysts are typically added to the mixture. The reaction occurs at between to and 20 to 70 MPa hydrogen pressure. The reaction can be summarized as follows:After World War I several plants were built in Germany; these plants were extensively used during World War II to supply Germany with fuel and lubricants.
The Kohleoel Process, developed in Germany by Ruhrkohle and VEBA, was used in the demonstration plant with a capacity of 200 tons of lignite per day, built in Bottrop, Germany. This plant operated from 1981 to 1987. In this process, coal is mixed with a recycled solvent and an iron catalyst. After preheating and pressurizing, H2 is added. The process takes place in a tubular reactor at a pressure of 300 bar and a temperature of. This process has also been explored by SASOL in South Africa.
In the 1970-1980s, the Japanese companies Nippon Kokan, Sumitomo Metal Industries and Mitsubishi Heavy Industries developed the NEDOL process. In this process, a mixture of coal and a recycled solvent is heated in the presence of an iron-based catalyst and H2. The reaction takes place in a tubular reactor at a temperature between and at a pressure of 150-200 bar. The produced oil has low quality and requires intensive upgrading. The H-Coal process, developed by Hydrocarbon Research, Inc., in 1963, mixes pulverized coal with recycled liquids, hydrogen and a catalyst in the ebullated bed reactor. The advantages of this process are that dissolution and oil upgrading take place in a single reactor, the products have a high H:C ratio and a fast reaction time, while the main disadvantages are high gas yield, high hydrogen consumption and the produced oil is only suitable as boiler oil because of impurities.
The SRC-I and SRC-II processes were developed by Gulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s. The Nuclear Utility Services Corporation developed the hydrogenation process which was patented by Wilburn C. Schroeder in 1976. The process involved dried, pulverized coal mixed with roughly 1wt% molybdenum catalysts. Hydrogenation occurred at a high temperature and pressure, with syngas produced in a separate gasifier. The process ultimately yielded a synthetic crude product, Naphtha, a limited amount of C3/C4 gas, light-medium weight liquids suitable for use as fuels, small amounts of NH3 and significant amounts of CO2. Other single-stage hydrogenation processes are the Exxon donor solvent process, the Imhausen High-pressure Process, and the Conoco Zinc Chloride Process.
A number of two-stage direct liquefaction processes have been developed. After the 1980s only the Catalytic Two-stage Liquefaction Process, modified from the H-Coal Process; the Liquid Solvent Extraction Process by British Coal; and the Brown Coal Liquefaction Process of Japan have been developed.
Chevron Corporation developed a process invented by Joel W. Rosenthal called the Chevron Coal Liquefaction Process. It is unique due to the close-coupling of the non-catalytic dissolver and the catalytic hydroprocessing unit. The oil produced had properties that were unique when compared to other coal oils; it was lighter and had far fewer heteroatom impurities. The process was scaled-up to a 6 ton per day level, but not proven commercially.