Page 164 - Synthetic Fuels Handbook

150 CHAPTER FIVE

that of low heat-content gas, except that there is virtually no nitrogen. The primary combus-
tible gases in medium heat-content gas are hydrogen and carbon monoxide (Kasem, 1979).
Medium heat-content gas is considerably more versatile than low heat-content gas; like low
heat-content gas, medium heat-content gas may be used directly as a fuel to raise steam,
or used through a combined power cycle to drive a gas turbine, with the hot exhaust gases
employed to raise steam, but medium heat-content gas is especially amenable to synthesize
methane (by methanation), higher hydrocarbons (by Fischer-Tropsch synthesis), methanol,
and a variety of synthetic chemicals.
The reactions used to produce medium heat-content gas are the same as those employed
for low heat-content gas synthesis, the major difference being the application of a nitrogen
barrier (such as the use of pure oxygen) to keep diluent nitrogen out of the system.
In medium heat-content gas, the H /CO ratio varies from 2:3 to 3:1 and the increased
2
heating value correlates with higher methane and hydrogen contents as well as with lower
carbon dioxide contents. Furthermore, the very nature of the gasification process used to
produce the medium heat-content gas has a marked effect upon the ease of subsequent
processing. For example, the carbon-dioxide-acceptor product is quite amenable to use for
methane production because it has (a) the desired H /CO ratio just exceeding 3:1, (b) an
2
initially high methane content, and (c) relatively low water and carbon dioxide contents.
Other gases may require appreciable shift reaction and removal of large quantities of water
and carbon dioxide prior to methanation.
High Heat-Content (High-Btu) Gas. High heat-content gas is essentially pure methane
and often referred to as synthetic natural gas or substitute natural gas (SNG) (Kasem,
1979; cf Speight, 1990). However, to qualify as substitute natural gas, a product must con-
tain at least 95 percent methane; the energy content of synthetic natural gas is 980 to 1080
3
3
Btu/ft (36.5–40.2 MJ/m ).
The commonly accepted approach to the synthesis of high heat-content gas is the cata-
lytic reaction of hydrogen and carbon monoxide:
3H + CO → CH + H O
2 4 2
To avoid catalyst poisoning, the feed gases for this reaction must be quite pure and, therefore,
impurities in the product are rare. The large quantities of water produced are removed by con-
densation and recirculated as very pure water through the gasification system. The hydrogen is
usually present in slight excess to ensure that the toxic carbon monoxide is reacted; this small
quantity of hydrogen will lower the heat content to a small degree.
The carbon monoxide/hydrogen reaction is somewhat inefficient as a means of pro-
ducing methane because the reaction liberates large quantities of heat. In addition, the
methanation catalyst is troublesome and prone to poisoning by sulfur compounds and the
decomposition of metals can destroy the catalyst. Thus, hydrogasification may be employed
to minimize the need for methanation:
[C] coal + 2H → CH 4
2
The product of hydrogasification is far from pure methane and additional methanation
is required after hydrogen sulfide and other impurities are removed.

5.5.3 Physicochemical Aspects

Coal varies widely in chemical composition. The most important constituents are carbon
(C), hydrogen (H), and oxygen (O), with some sulfur and nitrogen, bound together in
complex arrangements. If coal is heated in an inert atmosphere, this intricate molecular