German scientists, Franz Fischer and Hans Tropsch.
Fischer-Tropsch process diagram from coal to liquid fuel.

The Fischer–Tropsch process (or Fischer–Tropsch Synthesis) is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a petroleum substitute, typically from coal, natural gas, or biomass for use as synthetic lubrication oil and as synthetic fuel.[1] The F-T process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons.

History

The process was developed by two German scientists, Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s. At the time there was a shortage of oil in Germany, forcing research into alternative ways of producing liquid fuels. However, coal was plentiful, and Fischer and Tropsch discovered a way to convert coal to a mixture of gases (syngas) and then to liquid hydrocarbon fuel i.e. gasoline, diesel and kerosene (jet fuel).

The process is expensive, energy-intensive and produces relatively small amounts of fuel compared with traditional crude oil refining, so interest in the technology outside of research waned. However, recently it has been re-investigated as the price of oil rises and concerns about remaining oil reserves increase.

Process chemistry

The Fischer–Tropsch process involves a series of chemical reactions that lead to a variety of hydrocarbons. Useful reactions give alkanes:

(2n+1) H2 + n CO → CnH(2n+2) + n H2O

where 'n' is a positive integer. The formation of methane (n = 1) is generally unwanted. Most of the alkanes produced tend to be straight-chain alkanes, although some branched alkanes are also formed. In addition to alkane formation, competing reactions result in the formation of alkenes, as well as alcohols and other oxygenated hydrocarbons. Usually, only relatively small quantities of these non-alkane products are formed, although catalysts favoring some of these products have been developed.

Other reactions relevant to the F-T process

Several reactions are required to obtain the gaseous reactants required for F-T catalysis. First, reactant gases entering a F-T reactor must first be desulfurized to protect the catalysts that are readily poisoned. The other major class of reactions are employed to adjust the H2/CO ratio:

H2O + CO → H2 + CO2
  • For F-T plants that start with methane, another important reaction is steam reforming, which converts the methane into CO and H2:
H2O + CH4 → CO + 3 H2

Chemical mechanisms

The conversion of CO to alkanes involves net hydrogenation of CO, the hydrogenolysis of C-O bonds, and the formation of C-C bonds. Such reactions are assumed to proceed via initial formation of surface-bound metal carbonyls. The CO ligand is speculated to undergo dissociation, possibly into oxide and carbide ligands.[2] Other potential intermediates are various C-1 fragments including formyl (CHO), hydroxycarbene (HCOH), hydroxymethyl (CH2OH), methyl (CH3), methylene (CH2), methylidyne (CH), and hydroxymethylidyne (COH). Furthermore, and critical to the production of liquid fuels, are reactions that form C-C bonds, such as migratory insertion. Many related stoichiometric reactions have been simulated on discrete metal clusters, but homogeneous F-T catalysts are poorly developed and of no commercial importance.

Process conditions

Generally, the Fischer–Tropsch process is operated in the temperature range of 150–300 °C (302–572 °F). Higher temperatures lead to faster reactions and higher conversion rates but also tend to favor methane production. As a result, the temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favors formation of long-chained alkanes both of which are desirable. Typical pressures range from one to several tens of atmospheres. Even higher pressures would be favorable, but the benefits may not justify the additional costs of high-pressure equipment and higher pressures can leading to catalyst deactivation via coke formation.

A variety of synthesis gas compositions can be used. For cobalt-based catalysts the optimal H2:CO ratio is around 1.8-2.1. Iron-based catalysts promote the water-gas-shift reaction and thus can tolerate significantly lower ratios. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H2:CO ratios (<1).

Product distribution

In general the product distribution of hydrocarbons formed during the Fischer–Tropsch process follows an Anderson-Schulz-Flory distribution,[3] which can be expressed as:

Wn/n = (1-α)2αn-1

Where Wn is the weight fraction of hydrocarbon molecules containing n carbon atoms. α is the chain growth probability or the probability that a molecule will continue reacting to form a longer chain. In general, α is largely determined by the catalyst and the specific process conditions.

Examination of the above equation reveals that methane will always be the largest single product so long as alpha is less than 0.5; however by increasing α close to one, the total amount of methane formed can be minimized compared to the sum of all of the various long-chained products. Increasing α increases the formation of long-chained hydrocarbons. The very long-chained hydrocarbons are waxes, which are solid at room temperature. Therefore, for production of liquid transportation fuels it may be necessary to crack some of the Fischer-Tropsch products. In order to avoid this, some researchers have proposed using zeolites or other catalyst substrates with fixed sized pores that can restrict the formation of hydrocarbons longer than some characteristic size (usually n<10). This way they can drive the reaction so as to minimize methane formation without producing lots of long-chained hydrocarbons. Such efforts have met with only limited success.

Fischer-Tropsch catalysts

A variety of catalysts can be used for the Fischer–Tropsch process, but the most common are the transition metals cobalt, iron, and ruthenium. Nickel can also be used, but tends to favor methane formation ("methanation").

Cobalt-based catalysts are highly active, although iron may be more suitable for low-hydrogen-content synthesis gases such as those derived from coal due to its promotion of the water-gas-shift reaction. In addition to the active metal the catalysts typically contain a number of "promoters," including potassium and copper. Group 1 alkali metals, including potassium, are a poison for cobalt catalysts but are promoters for iron catalysts. Catalysts are supported on high-surface-area binders/supports such as silica, alumina, or zeolites.[4] Cobalt catalysts are more active for Fischer-Tropsch synthesis when the feedstock is natural gas. Natural gas has a high hydrogen to carbon ratio, so the water-gas-shift is not needed for cobalt catalysts. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass.

Unlike the other metals used for this process (Co, Ni, Ru), which remain in the metallic state during synthesis, iron catalysts tend to form a number of phases, including various oxides and carbides during the reaction. Control of these phase transformations can be important in maintaining catalytic activity and preventing breakdown of the catalyst particles.

Fischer-Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. The sensitivity of the catalyst to sulfur is greater for cobalt-based catalysts than for their iron counterparts.

Promotors also have an important influence on activity. Alkali metal oxides and copper are common promotors, but the formulation depends on the primary metal, iron vs cobalt.[5] Alkali oxides on cobalt catalysts generally cause activity to drop severely even with very low alkali loadings. C5+ and CO2 selectivity increase while methane and C2-C4 selectivity decrease. In addition, the olefin to parafin ratio increases.

LTFT and HTFT

High-Temperature Fischer-Tropsch (or HTFT) is operated at temperatures of 330C-350C and uses an iron-based catalyst. This process was used extensively by Sasol in their Coal-to-Liquid plants (CTL). Low-Temperature Fischer-Tropsch (LTFT) is operated at lower temperatures and uses a cobalt based catalyst. This process is best known for being used in the first integrated Gas-to-Liquid (GTL) plant operated and built by Shell in Bintulu, Malaysia.[6]

Gasification

F-T plants associated with coal or related solid feedstocks (sources of carbon) must first convert the solid fuel into gaseous reactants, i.e. CO, H2, and alkanes. This conversion is called gasification. Synthesis gas obtained from coal gasification tends to have a H2/CO ratio of ~0.7 compared to the ideal ratio of ~2. This ratio is adjusted via the water-gas shift reaction. Coal-based Fischer–Tropsch plants can produce varying amounts of CO2, depending upon the energy source of the gasification process. However, most coal-based plants rely on the feed coal to supply all the energy requirements of the F-T process. Ongoing research aims to combine biomass gasification (BG) and Fischer-Tropsch (FT) synthesis to produce renewable transportation fuels (biofuels).[7]

History

Since the invention of the original process by Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s, many refinements and adjustments have been made. The term "Fischer-Tropsch" now applies to a wide variety of similar processes (Fischer-Tropsch synthesis or Fischer-Tropsch chemistry). Fischer and Tropsch filed a number of patents, e.g., US patent no. 1,746,464, applied 1926, published 1930.[8] It was commercialized in Germany in 1936. Being petroleum-poor but coal-rich, in Germany the FT-process was used by Nazi Germany and Japan during World War II to produce ersatz (German: substitute) fuels. F-T production accounted for an estimated 9% of German war production of fuels and 25% of the automobile fuel.[9]

The United States Bureau of Mines, in a program initiated by the Synthetic Liquid Fuels Act, employed seven Operation Paperclip synthetic fuel scientists in a Fischer-Tropsch plant in Louisiana, Missouri in 1946.[10][9]

In Britain, Alfred August Aicher obtained several patents for improvements to the process in the 1930s and 1940s.[11] Aicher's company was named Synthetic Oils Ltd. (Now based in Canada.)

Commercialization

Fluidized bed gasification with FT-pilot in Güssing, Burgenland, Austria

The F-T process has been applied on a large scale in some industrial sectors, although its popularity is hampered by high capital costs, high operation and maintenance costs, the uncertain and volatile price of crude oil, and environmental concerns. In particular, the use of natural gas as a feedstock only becomes practical when using "stranded gas", i.e. sources of natural gas far from major cities which are impractical to exploit with conventional gas pipelines and LNG technology; otherwise, the direct sale of natural gas to consumers would become much more profitable. Several companies are developing the process to enable practical exploitation of so-called stranded gas reserves.

Sasol

The largest scale implementation of F-T technology are in a series of plants operated by Sasol in South Africa, a country with large coal reserves but lacking in oil. Sasol uses coal and now natural gas as feedstocks and produces a variety of synthetic petroleum products, including most of the country's diesel fuel.[12]

Shell Middle Distillate Synthesis

One of the largest implementations of F-T technology is in Bintulu, Malaysia. This Shell facility converts natural gas into low-sulfur diesel fuels and food-grade wax. The scale is 12,000 barrels per day.

Ras Laffan, Qatar

The new LTFT facility scheduled to commission in 2011 at Ras Laffan, Qatar uses cobalt catalysts at 230 °C, converting natural gas to petroleum liquids at a rate of 140,000 barrels/day, with additional production of 120,000 barrels of oil equivalent in natural gas liquids and ethane.

UPM (Finland)

In October 2006, Finnish paper and pulp manufacturer UPM announced its plans to produce biodiesel by Fischer–Tropsch process alongside the manufacturing processes at its European paper and pulp plants, using waste biomass resulted by paper and pulp manufacturing processes as source material.[13]

Rentech (Colorado, USA)

A demonstration scale F-T plant is owned and operated by Rentech Inc in partnership with ClearFuels, a company specializing in biomass gasification. Located in Commerce City, Colorado (U.S.), the facility produces about 10 barrels per day of fuels from natural gas. Commercial scale facilities are planned for Rialto, California, Natchez, Mississippi, Port St. Joe, Florida, and White River, Ontario.[14]

Other

In the US, some coal-producing states have invested in F-T plants. In Pennsylvania, Waste Management and Processors Inc. was funded by the state to implement F-T technology licensed from Shell and Sasol to convert so-called waste coal (leftovers from the mining process) into low-sulfur diesel fuel.[15][16]

Research developments

Choren Industries has built an FT plant in Germany that converts biomass to syngas and fuels using the Shell F-T process.[17][18]

U.S. Air Force certification

Syntroleum, a publicly traded US company (Nasdaq: SYNM) has produced over 400,000 gallons of diesel and jet fuel from the Fischer–Tropsch process using natural gas and coal at its demonstration plant near Tulsa, Oklahoma. Syntroleum is working to commercialize its licensed Fischer-Tropsch technology via coal-to-liquid plants in the US, China, and Germany, as well as gas-to-liquid plants internationally. Using natural gas as a feedstock, the ultra-clean, low sulfur fuel has been tested extensively by the U.S. Department of Energy and the U.S. Department of Transportation. Most recently, Syntroleum has been working with the U.S. Air Force to develop a synthetic jet fuel blend that will help the Air Force to reduce its dependence on imported petroleum. The Air Force, which is the U.S. military's largest user of fuel, began exploring alternative fuel sources in 1999. On December 15, 2006, a B-52 took off from Edwards AFB, California for the first time powered solely by a 50-50 blend of JP-8 and Syntroleum's FT fuel. The seven-hour flight test was considered a success. The goal of the flight test program is to qualify the fuel blend for fleet use on the service's B-52s, and then flight test and qualification on other aircraft. The test program concluded in 2007. This program is part of the Department of Defense Assured Fuel Initiative, an effort to develop secure domestic sources for the military energy needs. The Pentagon hopes to reduce its use of crude oil from foreign producers and obtain about half of its aviation fuel from alternative sources by 2016.[19] With the B-52 now approved to use the FT blend, the C-17 Globemaster III, the B-1B, and eventually every airframe in its inventory to use the fuel by 2011.[19][20]

Carbon dioxide reuse

In 2009, chemists working for the U.S. Navy investigated Fischer-Tropsch for generating fuels, obtaining hydrogen by electrolysis of seawater. When combined with the dissolved carbon dioxide using a cobalt-based catalyst, this study produced mostly methane gas. However, when using an iron-based catalyst, it was possible to reduce the methane produced to 30 per cent with the rest being predominantly short-chain hydrocarbons. Further refining of the hydrocarbons produced applying solid acid catalysts, such as zeolites, can potentially lead to the production of kerosene-based jet fuel.[21]

The abundance of CO2 makes seawater an attractive alternative fuel source. Scientists at the U.S. Naval Research Laboratory stated that, "although the gas forms only a small proportion of air – around 0.04 per cent – ocean water contains about 140 times that concentration".[21] Robert Dorner presented the findings of his work to the American Chemical Society on 16 August 2009, in Washington DC.[22] Of course, such a method requires an energy source - since CO2 is a major product of combustion, converting it back into combustible material is a highly endothermic (energy-absorbing) process. In practice this would probably come from nuclear power, which is in abundant supply aboard nuclear powered ships.

See also

References

  1. US Fuel Supply Statistics Chart
  2. Bruce C. Gates “Extending the Metal Cluster-Metal Surface Analogy” Angewandte Chemie International Edition in English, 2003, Volume 32, pp. 228 – 229. doi:10.1002/anie.199302281
  3. P.L. Spath and D.C. Dayton. "Preliminary Screening — Technical and Economic Assessment of Synthesis Gas to Fuels and Chemicals with Emphasis on the Potential for Biomass-Derived Syngas", NREL/TP510-34929,December, 2003, pp. 95
  4. Andrei Y. Khodakov, Wei Chu, and Pascal Fongarland “Advances in the Development of Novel Cobalt Fischer-Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels” Chemical Review, 2007, volume 107, pp 1692–1744. doi:10.1021/cr050972v
  5. Christine M. Balonek, Andreas H. Lillebø, Shreyas Rane, Erling Rytter, Lanny D. Schmidt, Anders Holmen “Effect of Alkali Metal Impurities on Co–Re Catalysts for Fischer–Tropsch Synthesis from Biomass-Derived Syngas” Catalysis Letters 2010, volume 138, pp 8-13. doi:10.1007/s10562-010-0366-4. <!—this highly specialized reference should be replaced when a broader overview of promotors--Try Kodakov et al... the guy got some decent review papers>
  6. http://www.scribd.com/doc/3825160/Gas-to-Liquids-GTL-Technology page 33-41
  7. Script error
  8. http://www.fischer-tropsch.org/primary_documents/patents/US/us1746464.pdf
  9. 9.0 9.1 Leckel, D., "Diesel Production from Fischer-Tropsch: The Past, the Present, and New Concepts", Energy Fuels, 2009, volume 23, 2342-2358. doi:10.1021/ef900064c
  10. German Synthetic Fuels Scientist
  11. E.g. British patent no. 573,982, applied 1941, published 1945"Improvements in or relating to Methods of Producing Hydrocarbon Oils from Gaseous Mixtures of Hydrogen and Carbon Monoxide" (pdf). January 14, 1941. http://www.fischer-tropsch.org/primary_documents/patents/GB/gb573982.pdf. Retrieved 2008-11-09.
  12. "technologies & processes" Sasol
  13. "UPM-Kymmene says to establish beachhead in biodiesel market", NewsRoom Finland
  14. http://www.rentechinc.com/ (official site)
  15. "Governor Rendell leads with innovative solution to help address PA energy needs", State of Pennsylvania
  16. "Schweitzer wants to convert Otter Creek coal into liquid fuel", Billings Gazette, August 2, 2005, accessed August 13, 2007
  17. Choren official web site
  18. Fairley, Peter. Growing Biofuels - New production methods could transform the niche technology. MIT Technology Review November 23, 2005
  19. 19.0 19.1 Zamorano, Marti (2006-12-22). "B-52 synthetic fuel testing: Center commander pilots first Air Force B-52 flight using solely synthetic fuel blend in all eight engines". Aerotech News and Review.
  20. "C-17 flight uses synthetic fuel blend". 2007-10-25. http://www.af.mil/news/story.asp?id=123073293. Retrieved 2008-02-07.
  21. 21.0 21.1 Kleiner, Kurt (18 August 2009). "How to turn seawater into jet fuel". New Scientist. http://www.newscientist.com/article/dn17632-how-to-turn-seawater-into-jet-fuel.html. Retrieved 2009-08-20.
  22. "FUEL 18 - Catalytic CO2 hydrogenation to feedstock chemicals for jet fuel synthesis.". American Chemical Society. http://oasys2.confex.com/acs/238nm/techprogram/P1260309.HTM. Retrieved 2009-08-20.

External links