Page 21 - Synthetic Fuels Handbook

FUEL SOURCES 9

conventional transportation fuels, as such requiring neither change in distribution nor
lifestyle changes for consumers.
There are inherent technological advantages with the conversion of coal to liquid prod-
ucts since coal liquefaction can produce clean liquid fuels that can be sold as transportation
fuels such as gasoline and diesel. There are three principal routes by which liquid fuels can
be produced from solid coal: (a) direct conversion to liquids, (b) direct hydrogenation of
coal, and (c) indirect conversion to liquids.
The direct liquefaction of coal by the Bergius process (liquefaction by hydrogenation)
is also available. Several other direct liquefaction processes have been developed (Speight,
1994).
The Bergius process has not been used outside Germany, where such processes were
operated both during World War I and World War II. Several other direct liquefaction pro-
cesses have been developed, among these being the SRC-I and SRC-II (Solvent Refined
Coal) processes developed by Gulf Oil and implemented as pilot plants in the United States
in the 1960s and 1970s (Speight, 1994 and references cited therein).
The direct hydrogenation of coal was explored by the Nuclear Utility Services (NUS)
Corporation in 1976 and involved the thermal conversion of dried, pulverized coal mixed
with roughly 1 percent by weight molybdenum catalyst. The process yielded a limited
amount of propane and butane, a synthetic naphtha (the precursor to gasoline), small
amounts of NH , and significant amounts of carbon dioxide (Lowe et al., 1976).
3
Another process to manufacture liquid hydrocarbons from coal is low temperature car-
bonization (LTC) (Karrick process). Coal is coked at temperatures between 450 and 700°C
compared to 800 to 1000°C for metallurgic coke. The lower temperatures optimize the
production of coal tar that is richer in lighter hydrocarbons than high-temperature coal tar.
The coal tar is then further processed into fuels.
The current process objectives of coal liquefaction by direct methods are mainly
focused on easing operating process severity, minimizing hydrogen requirement, and mak-
ing the product liquid more environmentally acceptable. Due to the recent trends of high
and fluctuating petroleum price in the world market, the relative process economics of
coal liquefaction is also much more favorable. Considering the vast amount of coal reserve
throughout the world and its global distribution of major deposits, this alternative is even
more attractive and also very practical.
Coal can also be converted into liquid fuels by indirect liquefaction which involves
gasification of coal to mixtures of carbon monoxide and hydrogen (synthesis gas) followed
by application of the Fischer-Tropsch process (Chap. 7) in which the synthesis gas is con-
verted to hydrocarbons under catalytic conditions of temperature and pressure.
Fischer-Tropsch synthesis for production of synfuels is an established technology. Fossil
fuels or biomass are converted into syngas via steam reforming, autothermal reforming, or
gasification.
The Fischer-Tropsch process of indirect synthesis of liquid hydrocarbons was used in
Germany for many years and is currently used by Sasol in South Africa. In the process,
coal is gasified to produce synthesis gas (syngas; a balanced purified mixture of carbon
monoxide and hydrogen) and the syngas condensed using Fischer-Tropsch catalysts to
produce low-boiling hydrocarbons which are further processed into gasoline and diesel.
Syngas can also be converted to methanol, which can be used as a fuel, fuel additive, or
further processed into gasoline via the Mobil M-Gas process.
South Africa has established this technology on a large scale during the past 50 years,
and Sasol is operating a coal-to-liquids plant. The product mix consists of 80 percent diesel
and 20 percent naphtha. China has expressed interest in an advanced version of the Sasol
conversion process. A number of other, small-scale projects have already started operation
or are under construction (Coaltrans, 2003).