Page 175 - Synthetic Fuels Handbook

FUELS FROM COAL 161

The gasification may be attained by means of any one of several processes or even by
gasification of coal in place (underground, or in situ, gasification of coal, Sec. 5.5.5).
In practice, the Fischer-Tropsch reaction is carried out at temperatures of 200 to 350°C
(392–662°F) and at pressures of 75 to 4000 psi (0.5–27.5 MPa). The H /CO ratio is usually
2
2.2:1 or 2.5:1. Since up to three volumes of hydrogen may be required to achieve the next
stage of the liquids production, the synthesis gas must then be converted by means of the
water-gas shift reaction to the desired level of hydrogen:
CO + H O → CO + H
2 2 2
After this, the gaseous mix is purified and converted to a wide variety of hydrocarbons:
nCO + (2n + 1)H → C H 2n+2 + nH O
2
n
2
These reactions result primarily in low- and medium-boiling aliphatic compounds suit-
able for gasoline and diesel fuel.
Reactors. Several types of reactor are available for use in liquefaction processes and any
particular type of reactor can exhibit a marked influence on process performance.
The simplest type of reactor is the noncatalytic reactor which consists, essentially, of a vessel
(or even an open tube) through which the reactants pass. The reactants are usually in the fluid
state but may often contain solids such as would be the case for coal slurry. This particular type
of reactor is usually employed for coal liquefaction in the presence of a solvent.
The second type of noncatalytic reactor is the continuous-flow, stirred-tank reactor,
which has the notable feature of encouraging complete mixing of all of the ingredients,
and if there is added catalyst (suspended in the fluid phase) the reactor may be referred to
as a slurry reactor.
The fixed-bed catalytic reactor contains a bed of catalyst particles through which the
reacting fluid flows; the catalysis of the desired reactions occurs as the fluid flows through
the reactor. The liquid may pass through the reactor in a downward flow or in an upward
flow but the problems that tend to accompany the latter operation (especially with regard
to the heavier, less conventional feedstocks) must be recognized. In the downward-flowing
mode, the reactor may often be referred to as a trickle-bed reactor.
Another type of reactor is the fluidized bed reactor, in which the powdered catalyst
particles are suspended in a stream of up-flowing liquid or gas. A form of this type of reac-
tor is the ebullating-bed reactor. The features of these two types of reactor are the efficient
mixing of the solid particles (the catalyst) and the fluid (the reactant) that occurs throughout
the whole reactor.
The final type of reactor to be described is the entrained-flow reactor in which the solid
particles travel with the reacting fluid through the reactor. Such a reactor has also been
described as a dilute or lean-phase fluidized bed with pneumatic transport of solids.

5.6.3 Products
Liquid products from coal are generally different from those produced by petroleum refining,
particularly as they can contain substantial amounts of phenols. Therefore, there will always be
some question about the place of coal liquids in refining operations. For this reason, there have
been some investigations of the characterization and next-step processing of coal liquids.
As a first step in the characterization of coal liquids, it is generally recognized that some
degree of fractionation is necessary (Whitehurst et al., 1980) followed by one, or more, forms
of chromatography to identify the constituents (Kershaw, 1989; Philp and de las Heras, 1992).
The fractionation of coal liquids is based largely on schemes developed for the characterization