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Fischer-Tropsch process to produce synthetic liquid hydrocarbons from coal and natural gas:

Fischer-Tropsch (F-T) technology converts coal, natural gas, and low-value refinery products into a high-value, comparatively cleaner burning fuel. The resultant fuel is colorless, odorless, and low in toxicity. In addition, it is virtually interchangeable with conventional diesel fuels and can be blended with diesel at any ratio with little to no modification. Currently, several oil companies are researching large-scale production of Fischer- Tropsch fuels.

For the past 50 years, Fischer-Tropsch fuels have powered all of South Africa’s vehicles—from buses to trucks to taxicabs. The fuel is primarily supplied by Sasol, a world leader in Fischer-Tropsch technologies. Sasol’s South African facility produces more than 150,000 barrels of high quality fuel from domestic low-grade coal daily.

The Fischer-Tropsch process is a catalyzed chemical reaction in which carbon monoxide and hydrogen are converted into liquid hydrocarbons of various forms. Typical catalysts used are based on iron and cobalt. Synthesis gas, a mixture of hydrogen and carbon monoxide, is reacted in the presence of an iron or cobalt catalyst; much heat is evolved, and such products as methane, synthetic gasoline and waxes, and alcohols are made, with water or carbon dioxide produced as a byproduct. The catalyst is usually supported on carbon or silicon dioxide to optimize its activity. An important source of the hydrogen-carbon monoxide gas mixture (known as water gas) is obtained by the gasification of coal (manufacturing process of water gas involves treating white-hot hard coal or coke with a blast of steam; carbon monoxide and hydrogen are formed). 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 is produced in a gasifire by heating the gas to a temperature greater than 700^oC. By carefully controlling the oxygen content the hydrocarbons in the feedstock are broken down to carbon monoxide and hydrogen. The temperature, pressure and catalyst determine whether a light or heavy syncrude is produced.

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.

There are mainly two types of F-T reactors. The vertical fixed tube type has the catalyst in tubes that are cooled externally by pressurized 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 F-T crude, has low sulfur and aromatic content, high cetane number and it burns exceptionally clean in a diesel engine.

Based on available research, there are no significant differences in Fischer- Tropsch fuel’s performance versus petro-diesel fuels. In fact, the higher cetane number of Fischer-Tropsch diesel fuel might result in improved combustion; the cetane number is a primary measure of diesel fuel quality. In addition, many alternative fuels require major changes in vehicle engines, but Fischer-Tropsch fuels require no engine modifications. Fischer-Tropsch fuels, however, are slightly less energy dense than petrodiesel, which might result in lower fuel economy and power. Further investigations of fuel compatibility issues need to take place, as well.