Page 174 - Synthetic Fuels Handbook

160 CHAPTER FIVE

are reported to have been successful for processing noncaking coals but are not usually
recommended for caking coals.

Solvent Extraction Processes. Solvent extraction processes are those processes in which
coal is mixed with a solvent (donor solvent) that is capable of providing atomic or molecu-
lar hydrogen to the system at temperatures up to 500°C (932°F) and pressures up to 5000 psi
(34.5 MPa). High-temperature solvent extraction processes of coal have been developed in
three different process configurations: (a) extraction in the absence of hydrogen but using a
recycle solvent that has been hydrogenated in a separate process stage; (b) extraction in the
presence of hydrogen with a recycle solvent that has not been previously hydrogenated; and
(c) extraction in the presence of hydrogen using a hydrogenated recycle solvent. In each of
these concepts, the distillates of process-derived liquids have been used successfully as the
recycle solvent which is recovered continuously in the process.
The overall result is an increase (relative to pyrolysis processes) in the amount of coal
that is converted to lower molecular weight (i.e., soluble) products. More severe conditions
are more effective for sulfur and nitrogen removal to produce a lower-boiling liquid prod-
uct that is more amenable to downstream processing. A more novel aspect of the solvent
extraction process type is the use of tar sand bitumen and/or heavy oil as process solvents
(Moschopedis et al., 1980, 1982; Curtis et al., 1987; Schulman et al., 1988; Curtis and
Hwang, 1992; Rosal et al., 1992).
Catalytic Liquefaction Processes. The final category of direct liquefaction process employs
the concept of catalytic liquefaction in which a suitable catalyst is used to add hydrogen to the
coal. These processes usually require a liquid medium with the catalyst dispersed throughout
or may even employ a fixed-bed reactor. On the other hand, the catalyst may also be dispersed
within the coal whereupon the combined coal-catalyst system can be injected into the reactor.
Many processes of this type have the advantage of eliminating the need for a hydrogen
donor solvent (and the subsequent hydrogenation of the spent solvent) but there is still the
need for an adequate supply of hydrogen. The nature of the process also virtually guaran-
tees that the catalyst will be deactivated by the mineral matter in the coal as well as by coke
lay-down during the process. Furthermore, in order to achieve the direct hydrogenation of
the coal, the catalyst and the coal must be in intimate contact, but if this is not the case,
process inefficiency is the general rule.

Indirect Liquefaction Processes. The other category of coal liquefaction processes
invokes the concept of the indirect liquefaction of coal.
In these processes, the coal is not converted directly into liquid products but involves
a two-stage conversion operation in which coal is first converted (by reaction with steam
and oxygen) to produce a gaseous mixture that is composed primarily of carbon monoxide
and hydrogen (syngas; synthesis gas). The gas stream is subsequently purified (to remove
sulfur, nitrogen, and any particulate matter) after which it is catalytically converted to a
mixture of liquid hydrocarbon products.
The synthesis of hydrocarbons from carbon monoxide and hydrogen (synthesis gas)
(the Fischer-Tropsch synthesis) is a procedure for the indirect liquefaction of coal (Storch
et al., 1951; Batchelder, 1962; Dry, 1976; Anderson, 1984; Jones et al., 1992). This process
is the only coal liquefaction scheme currently in use on a relatively large commercial scale;
South Africa is currently using the Fischer-Tropsch process on a commercial scale in their
SASOL complex.
Thus, coal is converted to gaseous products at temperatures in excess of 800°C (1472°F),
and at moderate pressures, to produce synthesis gas:

[C] coal + H O → CO + H 2
2