Page 231 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 217

for low temperature cases. Cobalt is not suitable for high-temperature use due to excessive
methane formation at such temperatures. For once, through maximum diesel production,
cobalt has, despite its high cost, advantages and Sasol has also developed cobalt catalysts
which perform very well in the slurry phase process.
The diesel produced by the slurry phase reactor has a highly paraffinic nature, giving a
cetane number in excess of 70. The aromatic content of the diesel is typically below 3 percent
and it is also sulfur-free and nitrogen-free. This makes it an exceptional diesel as such or it
can be used to sweeten or to upgrade conventional diesels.
The Fischer-Tropsch process is an established technology and already applied on a
large scale, although its popularity is hampered by high capital costs, high operation and
maintenance costs, and the uncertain and volatile price of crude oil. In particular, the use of
natural gas as a feedstock only becomes practical when using stranded gas, that is, sources
of natural gas far from major cities which are impractical to exploit with conventional gas
pipelines and liquiefied natural gas technology; otherwise, the direct sale of natural gas to
consumers would become much more profitable. It is suggested by geologists that supplies
of natural gas will peak 5 to 15 years after oil does, although such predictions are difficult
to make and often highly uncertain. Hence the increasing interest in coal as a source of
synthesis gas.
Under most circumstances the production of synthesis gas by reforming natural gas
will be more economical than from coal gasification, but site specific factors need to be
considered. In fact, any technological advance in this field (such as better energy integration
or the oxygen transfer ceramic membrane reformer concept) will speed up the rate at which
the synfuels technology will become common practice.
There are large coal reserves which may increasingly be used as a fuel source during
oil depletion. Since there are large coal reserves in the world, this technology could be
used as an interim transportation fuel if conventional oil were to become more expensive.
Furthermore, combination of biomass gasification and Fischer-Tropsch synthesis is a very
promising route to produce transportation fuels from renewable or green resources.
Often a higher concentration of some sorts of hydrocarbons is wanted, which might
be achieved by changed reaction conditions. Nevertheless, the product range is wide and
infected with uncertainties, due to lack of knowledge of the details of the process and of
the kinetics of the reaction. Since the different products have quite different characteristics
such as boiling point, physical state at ambient temperature, and thereby different uses and
ways of distribution, often only a few of the carbon chains is wanted. As an example the
LTFT is used when longer carbon chains are wanted, because lower temperature increases
the portion of longer chains.
The yield of diesel is therefore highly dependent on the chain growth probability, which
again is dependent on pressure, temperature, feed gas composition, catalyst type, catalyst
composition, and reactor design. The desire to increase the selectivity of some favorable
products leads to a need of understanding the relation between reaction conditions and
chain growth probability, which in turn request a mathematical expression for the growth
probability in order to make a suitable model of the process.
There have been many attempts to model the product distribution of the Fischer-Tropsch
process. As the knowledge of the process is limited, the modeling of the product distribu-
tions is not accurate. There are deviations between inspected product distributions and
the different models when the conditions are changed. The general consensus seems to be
that the product distribution follows an exponential function, with the probability of chain
growth as an important factor.
In addition to application of the Fischer-Tropsch synthesis to the gasification prod-
ucts from petroleum residua, coal, biomass, wastes, and other carbonaceous feedstocks,
application of the Fischer Tropsch process to the production of liquid fuels from natural
gas is another established technology. Further expansion is planned for 2010, the bulk of