Page 173 - Synthetic Fuels Handbook

FUELS FROM COAL 159

Hydrogen can represent a major cost item of the liquefaction process and, accordingly,
several process options have been designed to limit (or control) the hydrogen consumption
or even to increase the hydrogen-to-carbon atomic ratio without the need for added gas
phase hydrogen (Speight, 1994). Thus, at best, the chemistry of coal liquefaction is only
speculative. Furthermore, various structures have been postulated for the structure of coal
(albeit with varying degrees of uncertainty) but the representation of coal as any one of
these structures is extremely difficult and, hence, projecting a thermal decomposition route
and the accompanying chemistry is even more precarious.
The majority of the coal liquefaction processes involve the addition of a coal-
derived solvent prior to heating the coal to the desired process temperature. This is,
essentially, a means of facilitating the transfer of the coal to a high-pressure region
(usually the reactor) and also to diminish the sticking that might occur by virtue of the
plastic properties of the coal.

5.6.2 Liquefaction Processes

The process options for coal liquefaction can generally be divided into four categories:
(a) pyrolysis, (b) solvent extraction, (c) catalytic liquefaction, and (d) indirect liquefaction.
Pyrolysis Processes. Pyrolysis of coal dates back to the eighteenth century, using tem-
peratures below 700°C in fixed- or moving-bed reactors. The primary product was a low-
volatile smokeless domestic fuel, although the value of the liquid products was also soon
recognized. During the 1920s and 1930s there was a great deal of interest in low-temperature
processes, but interest died in the mid-1940s when gas and oil became readily available
at low prices. With the oil embargo and increased oil prices of the early 1970s, interest
renewed in coal pyrolysis, but in the 1980s interest has again declined along with petroleum
prices (Khan and Kurata, 1985).
The first category of coal liquefaction processes, pyrolysis processes, involves
heating coal to temperatures in excess of 400°C (752°F), which results in the conver-
sion of the coal to gases, liquids, and char. The char is hydrogen deficient thereby
enabling intermolecular or intramolecular hydrogen transfer processes to be opera-
tive, resulting in relatively hydrogen-rich gases and liquids. Unfortunately, the char
produced often amounts to more than 45 percent by weight of the feed coal and,
therefore, such processes have often been considered to be uneconomic or inefficient
use of the carbon in the coal.
In the presence of hydrogen (hydrocarbonization) the composition and relative amounts
of the products formed may vary from the process without hydrogen but the yields are still
very much dependent upon the process parameters such as heating rate, pressure, coal type,
coal (and product) residence time, coal particle size, and reactor configuration. The operat-
ing pressures for pyrolysis processes are usually less than 100 psi [690 kPa; more often
between 5 and 25 psi (34–172 kPa)] but the hydrocarbonization processes require hydrogen
pressures of the order of 300 to 1000 psi (2.1–6.9 MPa). In both categories of process, the
operating temperature can be as high as 600°C (1112°F).
There are three types of pyrolysis reactors that are of interest: (a) a mechanically agi-
tated reactor, (b) an entrained-flow reactor, and (c) a fluidized bed reactor.
The agitated reactor may be quite complex but the entrained-flow reactor has the
advantage of either down- or up-flow operation and can provide short residence times.
In addition, the coal can be heated rapidly, leading to higher yields of liquid (and
gaseous) products that may well exceed the volatile matter content of the coal as deter-
mined by the appropriate test (Kimber and Gray, 1967). The short residence time also
allows a high throughput of coal and the potential for small reactors. Fluidized reactors