Page 166 - Synthetic Fuels Handbook

152 CHAPTER FIVE

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If a high heat-content (high-Btu) gas [900–1000 Btu/ft (33.5–37.3 MJ/m )] is required, efforts
must be made to increase the methane content of the gas. The reactions which generate methane
are all exothermic and have negative values but the reaction rates are relatively slow and catalysts
may, therefore, be necessary to accelerate the reactions to acceptable commercial rates. Indeed,
the overall reactivity of coal and char may be subject to catalytic effects. It is also possible that
the mineral constituents of coal and char may modify the reactivity by a direct catalytic effect
(Cusumano et al., 1978; Davidson, 1983; Baker and Rodriguez, 1990; Mims, 1991; Martinez-
Alonso and Tascon, 1991).
Process Concepts. While there has been some discussion of the influence of physical
process parameters and the effect of coal type on coal conversion, a note is warranted here
regarding the influence of these various parameters on the gasification of coal.
Most notable effects are those due to coal character, and often to the maceral content. In regard
to the maceral content, differences have been noted between the different maceral groups with
inertinite being the most reactive (Huang et al., 1991). In more general terms of the character of the
coal, gasification technologies generally require some initial processing of the coal feedstock with
the type and degree of pretreatment a function of the process and/or the type of coal. For example,
the Lurgi process will accept lump coal (1 in, 25 mm, to 28 mesh), but it must be noncaking coal
with the fines removed. The caking, agglomerating coals tend to form a plastic mass in the bottom
of a gasifier and subsequently plug up the system thereby markedly reducing process efficiency.
Thus, some attempt to reduce caking tendencies is necessary and can involve preliminary partial
oxidation of the coal thereby destroying the caking properties.
Depending on the type of coal being processed and the analysis of the gas product desired,
pressure also plays a role in product definition. In fact, some (or all) of the following processing steps
will be required: (a) pretreatment of the coal (if caking is a problem); (b) primary gasification of the
coal; (c) secondary gasification of the carbonaceous residue from the primary gasifier; (d) removal
of carbon dioxide, hydrogen sulfide, and other acid gases; (e) shift conversion for adjustment of
the carbon monoxide-to-hydrogen mole ratio to the desired ratio; and (f ) catalytic methanation of
the carbon monoxide/hydrogen mixture to form methane. If high heat-content (high-Btu) gas is
desired, all of these processing steps are required since coal gasifiers do not yield methane in the
concentrations required (Mills, 1969; Graff et al., 1976; Cusumano et al., 1978; Mills, 1982).
Pretreatment. Some coals display caking, or agglomerating, characteristics when heated and
these coals are usually not amenable to treatment by gasification processes employing fluidized
bed or moving-bed reactors; in fact, caked coal is difficult to handle in fixed-bed reactors. The
pretreatment involves a mild oxidation treatment which destroys the caking characteristics of coals
and usually consists low-temperature heating of the coal in the presence of air or oxygen.
Primary Gasification. Primary gasification involves thermal decomposition of the
raw coal via various chemical processes and many schemes involve pressures ranging from
atmospheric to 1000 psi (6.9 MPa). Air or oxygen may be admitted to support combustion
to provide the necessary heat. The product is usually a low heat-content (low-Btu) gas rang-
ing from a carbon monoxide/hydrogen mixture to mixtures containing varying amounts of
carbon monoxide, carbon dioxide, hydrogen, water, methane, hydrogen sulfide, nitrogen,
and typical products of thermal decomposition such as tar (themselves being complex mix-
tures; see Dutcher et al., 1983), hydrocarbon oils, and phenols.
A solid char product may also be produced, and may represent the bulk of the weight
of the original coal. This type of coal being processed determines (to a large extent) the
amount of char produced and the analysis of the gas product.
Secondary Gasification. Secondary gasification usually involves the gasification of
char from the primary gasifier. This is usually done by reacting the hot char with water
vapor to produce carbon monoxide and hydrogen:

[C] + H O → CO + H
char 2 2