The Fischer-Tropsch process often comes up in the discussion of new technologies for producing liquid fuels from solids or gases. The Fischer-Tropsch process produces high value, clean-burning fuels. FT fuels can be used in conventional engines with no modification and have improved combustion which reduces emissions, but may have a lower fuel economy. The resulting fuels are colorless, odorless and low in toxicity. FT fuels have less sulfur, nitrogen oxide, carbon monoxide and particulate matter emissions than petroleum fuels.
The Fischer-Tropsch (FT) process was first developed in Germany in the 1920's. It was used in Germany during WWII and was brought to prominence by Sasol in South Africa to produce oil and gasoline from coal, referred to as coal to liquids (CTL), when they were boycotted during apartheid and where it is still used. Exxon, Rentech, Sasol and Shell offer commercial processes. Shell has a commercial plant in operation converting natural gas to diesel fuel, referred to as gas to liquids (GTL), and several smaller plants are in operation.
The FT process requires a feed stream consisting largely of carbon monoxide and hydrogen. Thus gasification is the first step of coal liquefication or production of Fischer-Tropsch fuels from biomass such as corn stover (corn stalks), wood or switch grass. The feed gas, referred to as syngas, is produced in a gasifier by heating the gas to a temperatures greater than than 700oC. By carefully controlling the oxygen content the hydrocarbons in the feedstock are broken down to carbon monoxide and hydrogen.
The Fischer-Tropsch process converts the feed gas into liquid organic compounds, carbon dioxide and water. The conversion takes place in the presence of a catalyst, usually iron or cobalt. The temperature, pressure and catalyst determine whether a light or heavy syncrude is produced. For example at 330C mostly gasoline and olefins are produced whereas at 180 to 250C mostly diesel and waxes are produced.
Each company has proprietary FT technology, but a common theme is that most use a slurry-phase reactor with a cobalt-based catalyst. Exceptions are Shell and BP, whose processes use a fixed-bed reactor, and Rentech, Inc. (Denver, CO; www.rentechinc.com), which uses an iron-based catalyst. Most companies use autothermal reforming (ATR) rather than steam reforming because it is less expensive, especially when scaled up. "When you double the capacity of a steam reformer, you need double the number of tubes, so you pay about twice the price,"
The partial oxidation route of reforming provides the desired 2:1 ratio and is the preferred route in without consideration of other needs. There are two routes: one uses oxygen and produces a purer syngas without nitrogen; the other uses air creating a more dilute syngas. However, the oxygen route requires an air separation plant that increases the cost of the investment.
Steam reforming is carried out in a fired heater with catalyst-filled tubes that produces a syngas with at least a 5:1 hydrogen to carbon monoxide ratio. To adjust the ratio, hydrogen can be removed by a membrane or pressure swing adsorption system. Helping economics is if the surplus hydrogen is used in a petroleum refinery or for the manufacture of ammonia in an adjoining plant.
There are mainly two types of F-T reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurised boiling water. For a large plant, several reactors in parallel may be used presenting energy savings. The other process uses a slurry reactor in which pre-heated synthesis gas is fed to the bottom of the reactor and distributed into the slurry consisting of liquid wax and catalyst particles. As the gas bubbles upwards through the slurry, it is diffused and converted into more wax by the F-T reaction. The heat generated is removed through the reactor's cooling coils where steam is generated for use in the process.
The resulting organic compounds are a synthetic form of petroleum, analogous to a crude oil, and can be converted into many petroleum products including diesel and gasoline. Alternatively hydrogen can be recovered by further processing, resulting in only carbon dioxide and hydrogen with no hydrocarbons in the product stream. The primary interest at the present time is to produce low sulfur diesel fuel. Production of diesel fuel requires little processing from the FT crude, has low sulfur and aromatic content, high cetane number and and it burns exceptionally clean in a diesel engine. The process is known for high capital cost and high operating and maintenance costs. Recent refinements in the process by the commercial suppliers and government research have improved the process and reduced these costs. It becomes practical when "green" transportation fuels are desired and/or when petroleum fuels become high priced. With current prices of gasoline and diesel, FT fuels are competitive, Hirsch, P43, when produced on a large scale.
Updated to include more details 6/20/05