Page 212 - Synthetic Fuels Handbook

198 CHAPTER SEVEN

When used as an intermediate in the large-scale, industrial synthesis of hydrogen
and ammonia, it is also produced from natural gas (via the steam-reforming reaction)
as follows:
CH + H O → CO + 3H
4 2 2
The synthesis gas produced in large waste-to-energy gasification facilities is used as
fuel to generate electricity.
The manufacture of gas mixtures of carbon monoxide and hydrogen has been an
important part of chemical technology for about a century. Originally, such mixtures were
obtained by the reaction of steam with incandescent coke and were known as water gas.
Used first as a fuel, water gas soon attracted attention as a source of hydrogen and carbon
monoxide for the production of chemicals, at which time it gradually became known as
synthesis gas. Eventually, steam reforming processes, in which steam is reacted with natu-
ral gas (methane) or petroleum naphtha over a nickel catalyst, found wide application for
the production of synthesis gas.
A modified version of steam reforming known as autothermal reforming, which is a
combination of partial oxidation near the reactor inlet with conventional steam reforming
further along the reactor, improves the overall reactor efficiency and increases the flex-
ibility of the process. Partial oxidation processes using oxygen instead of steam also found
wide application for synthesis gas manufacture, with the special feature that they could
utilize low-value feedstocks such as heavy petroleum residua. In recent years, catalytic
partial oxidation employing very short reaction times (milliseconds) at high temperatures
(850–1000°C) is providing still another approach to synthesis gas manufacture (Hickman
and Schmidt, 1993). Nearly complete conversion of methane, with close to 100 percent
selectivity to H and CO, can be obtained with a rhenium monolith under well-controlled
2
conditions. Experiments on the catalytic partial oxidation of n-hexane conducted with added
steam give much higher yields of H than can be obtained in experiments without steam, a
2
result of much interest in obtaining hydrogen-rich streams for fuel cell applications.
The route of coal to synthetic automotive fuels, as practiced by Sasol, is technically
proven and products with favorable environmental characteristics are produced. As is the
case in essentially all coal conversion processes where air or oxygen is used for the utiliza-
tion or partial conversion of the energy in the coal, the carbon dioxide burden is a drawback
as compared to crude oil.
The uses of syngas include use as a chemical feedstock and in gas-to-liquid processes
(Mangone, 2002), which use Fisher-Tropsch chemistry to make liquid fuels as feedstock
for chemical synthesis, as well as being used in the production of fuel additives, including
diethyl ether and methyl tertiary-butyl ether (MTBE), acetic acid, and its anhydride. Syngas
could also make an important contribution to chemical synthesis through conversion to
methanol (Olah et al., 2006). There is also the option in which stranded natural gas is con-
verted to synthesis gas production followed by conversion to liquid fuels.

7.1 GASIFICATION OF COAL

Gasification is the conversion of a solid or liquid into a gas and excludes evaporation because
the process involves chemical change. Thus, gasification is the process by which carbona-
ceous materials, such as coal, petroleum, or biomass, are converted into carbon monoxide
and hydrogen by reacting the raw material at high temperatures with a controlled amount
of oxygen (Chaps. 5 and 8). The resulting gas mixture is called synthesis gas or syngas and
is itself a fuel.