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Fischer-Tropsch process
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The Fischer-Tropsch process (or Fischer-Tropsch Synthesis) is a catalyzed chemical reaction in which synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, is converted into liquid hydrocarbons of various forms. The most common catalysts are based on iron and cobalt, although nickel and ruthenium have also been used. The principal purpose of this process is to produce a synthetic petroleum substitute, typically from coal, natural gas or biomass, for use as synthetic lubrication oil or as synthetic fuel.
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Encyclopedia
The Fischer-Tropsch process (or Fischer-Tropsch Synthesis) is a catalyzed chemical reaction in which synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, is converted into liquid hydrocarbons of various forms. The most common catalysts are based on iron and cobalt, although nickel and ruthenium have also been used. The principal purpose of this process is to produce a synthetic petroleum substitute, typically from coal, natural gas or biomass, for use as synthetic lubrication oil or as synthetic fuel. This synthetic fuel runs trucks, cars, and some aircraft engines. (Refer to Sasol.) The use of diesel is increasing in recent years.
Combination of biomass gasification (BG) and Fischer-Tropsch (FT) synthesis is a possible route to produce renewable transportation fuels (biofuels).
Process chemistry
The Fischer-Tropsch process involves a variety of competing chemical reactions, which lead to a series of desirable products and undesirable byproducts. The most important reactions are those resulting in the formation of alkanes. These can be described by chemical equations of the form:
H2 + nCO ? CnH(2n+2) + nH2O
where 'n' is a positive integer. The simplest of these (n=1), results in formation of methane, which is generally considered an unwanted byproduct (particularly when methane is the primary feedstock used to produce the synthesis gas). Process conditions and catalyst composition are usually chosen to favor higher order reactions (n>1) and thus minimize methane formation. Most of the alkanes produced tend to be straight-chained, 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.
Another important reaction is the water gas shift reaction:
- H2O + CO ? H2 + CO2
Although this reaction results in formation of unwanted CO2, it can be used to shift the H2:CO ratio of the incoming Synthesis gas. This is especially important for synthesis gas derived from coal, which tends to have a ratio of ~0.7 compared to the ideal ratio of ~2.
It should be noted that, according to published data on the current commercial implementations of the coal-based Fischer-Tropsch process, these plants can produce as much as 7 tonnes of CO2 per tonne of liquid hydrocarbon products (excluding the reaction water product). This is due in part to the high energy demands required by the gasification process, and in part by the design of the process as implemented.
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 are in the range of one to several tens of atmospheres. Chemically, even higher pressures would be favorable, but the benefits may not justify the additional costs of high-pressure equipment.
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 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, which can be expressed as:
- Wn/n = (1-α)2αn-1
Where Wn is the weight fraction of hydrocarbon molecules containing n carbon atoms. a is the chain growth probability or the probability that a molecule will continue reacting to form a longer chain. In general, a 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, however by increasing a 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 a 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. So far, such efforts have had 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. Cobalt seems to be the most active catalyst, although iron also performs well and can 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, as well as high-surface-area binders/supports such as silica, alumina, or zeolites.
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 chemical phases, including various iron oxides and iron carbides during the reaction. Control of these phase transformations can be important in maintaining catalytic activity and preventing breakdown of the catalyst particles.
The Fischer-Tropsch catalysts are notoriously sensitive to the presence of sulfur-containing compounds among other poisons. The sensitivity of the catalyst to sulfur is higher for cobalt-based catalysts than for their iron counterparts.
Cobalt catalysts are preferred for Fischer-Tropsch synthesis when the feedstock is natural gas due to the higher activity of the cobalt catalyst. 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. While iron catalysts are also susceptible to sulfur poisoning from coal with high sulfur content, the lower cost of iron makes sacrificial catalyst at the front of a reactor bed economical. Also, as mentioned earlier, iron can catalyze the water-gas-shift to increase the hydrogen to carbon ratio to make the reaction more favorably selective.
Synthesis gas production
The initial reactants (synthesis gases) used in the Fischer-Tropsch process are hydrogen gas (H2) and carbon monoxide (CO). These chemicals are usually produced by one of two methods:
- 1. The partial combustion of a hydrocarbon:
CnH(2n+2) + ˝ nO2 ? (n+1)H2 + nCO
When n=1 (methane), the equation becomes 2CH4 + O2 ? 4H2 + 2CO
- 2. The gasification of coal, biomass, or natural gas:
CHx + H2O ? (1+0.5x)H2 + CO
The value of "x" depends on the type of fuel. For example, natural gas has a greater hydrogen content (x=5 to x=3) than coal (x>2).
The energy needed for this endothermic reaction is usually provided by the (exothermic) combustion of oxygen and the hydrocarbon source.
The mixture of carbon monoxide and hydrogen is called synthesis gas or syngas.
The resulting hydrocarbon products are refined to produce the desired synthetic fuel.
The carbon dioxide and carbon monoxide is generated by partial oxidation of coal and wood-based fuels. The utility of the process is primarily in its role in producing fluid hydrocarbons from a solid feedstock, such as coal or solid carbon-containing wastes of various types. Non-oxidative pyrolysis of the solid material produces singas which can be used directly as a fuel without being taken through Fischer-Tropsch transformations. If liquid petroleum-like fuel, lubricant, or wax is required, the Fischer-Tropsch process can be applied.
History
Since the invention of the original process by the German researchers Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s, many refinements and adjustments have been made, and 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 .
The process was invented in petroleum-poor but coal-rich Germany in the 1920s, to produce liquid fuels. It was used by Nazi Germany and Japan during World War II to produce ersatz fuels. By early 1944 production reached more than from 25 plants ~ 6.5 million tons. However, the bombing of German oil facilities during World War II paralyzed much of Germany's synthetic fuel production.
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 chemical plant in Louisiana, Missouri in 1946.
In Britain, Alfred August Aicher obtained several patents for improvements to the process in the 1930s and 1940s, e.g. British patent no. 573,982, applied 1941, published 1945 .
Aicher's company was named Synthetic Oils Ltd. (There is no connection with the Canadian company of the same name.)
Use
Currently, only a handful of companies have commercialised their FT technology.
- Shell in Bintulu, Malaysia, uses natural gas as a feedstock, and produces primarily low-sulfur diesel fuels and food-grade wax.
- Sasol in South Africa uses coal and natural gas as a feedstock, and produces a variety of synthetic petroleum products. Sasol produces most of the country's diesel fuel.
The process was used in South Africa to meet its energy needs during its isolation under Apartheid. This process has received renewed attention in the quest to produce low-sulfur diesel fuel in order to minimize environmental degradation from the use of diesel engines.
A small US-based company, Rentech, is currently focusing on converting nitrogen-fertiliser plants from using a natural gas feedstock to using coal or coke, and producing liquid hydrocarbons as a by-product.
Also Choren Industries has built an FT plant in Germany.
The FT process is an established technology and already 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. There are several companies developing the process to enable practical exploitation of so-called stranded gas reserves.
This technology has been proposed as a way to create transportation fuel from coal if conventional oil were to become more expensive. In Sept. 2005, Pennsylvania governor Edward Rendell announced a venture with Waste Management and Processors Inc. - using technology licensed from Shell and Sasol - to build an FT plant that will convert so-called waste coal (leftovers from the mining process) into low-sulfur diesel fuel at a site outside of Mahanoy City, northwest of Philadelphia. The state of Pennsylvania has committed to buy a significant percentage of the plant's output and, together with the U.S. Dept. of Energy, has offered over $140 million in tax incentives. Other coal-producing states are exploring similar plans. Governor Brian Schweitzer of Montana has proposed developing a plant that would use the FT process to turn his state's coal reserves into fuel in order to help alleviate the United States' dependence on foreign oil.
In Oct. 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.
In August 2007, Louisiana State University announced they had received funding from the US Department of Energy and Conoco Phillips for development of new nanotechnologies for catalysis of coal syngas to ethanol conversion.
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 not 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 US Department of Energy, the Department of Transportation, and 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.
On August 8 2007, Air Force Secretary Michael Wynne certified the B-52H as fully approved to use the FT blend, marking the formal conclusion of the test program.
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. With the B-52 now approved to use the FT blend, the USAF will use the test protocols developed during the program to certify the C-17 Globemaster III and then the B-1B to use the fuel. The Air Force intends to test and certify every airframe in its inventory to use the fuel by 2011.
Demonstration testing of the C-17 burning Fischer-Tropsch fuel was completed on October 22 2007 at Edwards Air Force Base. Testing consisted of a ground test and two flights which demonstrated engine performance throughout the C-17 flight envelope and during some operationally representative maneuvers. Test data is still being reviewed by the 418th FLTS to validate the subjective results of the test. On December 17, 2007 A C-17 Globemaster III using the synthetic fuel blend lifted off shortly before dawn from McChord Air Force Base, Washington, and flew to McGuire Air Force Base, New Jersey, where it was greeted by politicians and by officials from the airline and energy industries. Based on the two successful tests, the Air Force hopes to certify all of its C-17 fleet for the synthetic fuel mixture early in 2008.
CO2 reuse
There are investigations underway to reduce CO2 emissions by using solar power to convert waste CO2 into CO from where the FT process can then convert it to hydrocarbons.
See also
External links
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