Page 167 - Synthetic Fuels Handbook

FUELS FROM COAL 153

Shift Conversion. The gaseous product from a gasifier generally contains large amounts
of carbon monoxide and hydrogen, plus lesser amounts of other gases. Carbon monoxide
and hydrogen (if they are present in the mole ratio of 1:3) can be reacted in the presence of
a catalyst to produce methane (Cusumano et al., 1978). However, some adjustment to the
ideal (1:3) is usually required and, to accomplish this, all or part of the stream is treated
according to the waste gas shift (shift conversion) reaction. This involves reacting carbon
monoxide with steam to produce a carbon dioxide and hydrogen whereby the desired 1:3
mole ratio of carbon monoxide to hydrogen may be obtained:
CO + H O → CO + H
2 2 2
Methanation. Several exothermic reactions may occur simultaneously within a metha-
nation unit. A variety of metals have been used as catalysts for the methanation reaction; the
most common, and to some extent the most effective methanation catalysts, appear to be
nickel and ruthenium, with nickel being the most widely used (Seglin, 1975; Cusumano
et al., 1978; Tucci and Thompson, 1979; Watson, 1980). The synthesis gas must be desul-
furized before the methanation step since sulfur compounds will rapidly deactivate (poison)
the catalysts (Cusumano et al., 1978). A problem may arise when the concentration of car-
bon monoxide is excessive in the stream to be methanated since large amounts of heat must
be removed from the system to prevent high temperatures and deactivation of the catalyst
by sintering as well as the deposition of carbon (Cusumano et al., 1978). To eliminate this
problem temperatures should be maintained below 400°C (752°F).
Hydrogasification. Not all high heat-content (high-Btu) gasification technologies
depend entirely on catalytic methanation and, in fact, a number of gasification processes
use hydrogasification, that is, the direct addition of hydrogen to coal under pressure to form
methane (Anthony and Howard, 1976):
[C] coal + 2H → CH 4
2
The hydrogen-rich gas for hydrogasification can be manufactured from steam by using
the char that leaves the hydrogasifier. Appreciable quantities of methane are formed directly
in the primary gasifier and the heat released by methane formation is at a sufficiently high
temperature to be used in the steam-carbon reaction to produce hydrogen so that less oxy-
gen is used to produce heat for the steam-carbon reaction. Hence, less heat is lost in the
low-temperature methanation step, thereby leading to higher overall process efficiency.

5.5.4 Gasification Processes
Gasification processes are segregated according to the bed types, which differ in their ability
to accept (and use) caking coals and are generally divided into four categories based on reactor
(bed) configuration: (a) fixed bed, (b) fluidized bed, (c) entrained bed, and (d) molten salt.
In a fixed-bed process the coal is supported by a grate and combustion gases (steam, air,
oxygen, etc.) pass through the supported coal whereupon the hot produced gases exit from
the top of the reactor. Heat is supplied internally or from an outside source, but caking coals
cannot be used in an unmodified fixed-bed reactor.
The fluidized bed system uses finely sized coal particles and the bed exhibits liquid-like charac-
teristics when a gas flows upward through the bed. Gas flowing through the coal produces turbulent
lifting and separation of particles and the result is an expanded bed having greater coal surface area
to promote the chemical reaction, but such systems have a limited ability to handle caking coals.
An entrained-bed system uses finely sized coal particles blown into the gas steam prior
to entry into the reactor and combustion occurs with the coal particles suspended in the gas
phase; the entrained system is suitable for both caking and noncaking coals.