SYNFUELS
For transportation and upgrading reasons there is often a need for converting one fuel into another fuel. On the one hand this may concern the conversion of coal or remote natural gas into a liquid fuel, and on the other hand the conversion of coal into substitute or synthetic natural gas (SNG).
Virtually all modern coal gasification processes were originally developed for the production of synthesis gas for the subsequent production of chemical feedstocks or hydrocarbon liquids via Fischer-Tropsch synthesis. The only place in the world where the process sequence of coal gasification to Fischer-Tropsch is currently practiced is at the Sasol complex in South Africa. For the production of SNG from coal, only one plant is in operation in Beulah, North Dakota. For the conversion of remote natural gas via partial oxidation and Fischer-Tropsch synthesis into hydrocarbon liquids, plants are currently in operation in Malaysia and in South Africa.
This last option is especially attractive when low-cost natural gas is available that cannot be economically transported to markets by pipeline or as liquefied natural gas (LNG). In principle, there are two liquid products that can be produced: methanol and Fischer-Tropsch (FT) liquids. For the production of methanol, the reader is referred to Section 7.1.2, where the production of methanol has been discussed.
Classically, two different FT synthesis process types are available: the ARGE and the Synthol synthesis. In the ARGE process, synthesis gas is converted into straight chain olefins and paraffins over a cobalt containing catalyst at temperatures of about 200°C and pressures of 30-40 bar. The reaction takes place in a large number of parallel fixed-bed reactors that are placed in a pressure vessel containing boiling water for cooling and ensuring an essentially isothermal process.
The product is subsequently hydrogenated in cases where straight paraffins are the desired product. Such products are immanently suitable for the production of solvents and waxes, as the product is completely free from sulfur and nitrogen compounds as well as from aromatics. By adding an acidic function to the hydrogenation catalyst, some iso-paraffins are produced as well that improve the low-temperature characteristics of the premium fuels that can be produced by the ARGE process. Moreover, the boiling range of the products can be controlled within a wide range as the acidic function of the catalyst can be used for hydrocracking the heavier fractions.
Due to the absence of aromatics, the kerosene fraction has a very high smoke point and is a excellent blending component for aviation turbine fuels. For the same reason, the gasoil fraction has a very high cetane number (>70) and is a valuable blending component for automotive diesel fuels. However, a warning is appropriate since the lack of sulphur in unblended FT products can create problems in standard fuel pumps.
In the Synthol process, synthesis gas is converted into an aromatic-rich product over an iron-containing catalyst at temperatures of about 250°C and pressures of 30-40 bar. The reaction takes place in large fluid-bed reactors. The product is rich in aromatics and is used for the production of motor gasoline and as a diesel blending component. This process is being used at the Sasol plant in Secunda and in Mossel Bay, both in South Africa.
In recent years further developments have been made. The Shell Middle Distillate Synthesis (SMDS) process uses a fixed-bed reactor similar to that of ARGE. Sasol has developed its advanced slurry-bed reactor. Exxon, BP, Statoil, and others have demonstration plants in operation or under construction. The effects of various synthesis characteristics on the gas production facility are discussed in what follows.