Page 159 - Synthetic Fuels Handbook

FUELS FROM COAL 145

The carbon monoxide and hydrogen are produced by the gasification of coal in which a
mixture of gases is produced. In addition to carbon monoxide and hydrogen, methane and
other hydrocarbons are also produced depending on the conditions involved. Gasification
may be accomplished either in situ or in processing plants. In situ gasification is accom-
plished by controlled, incomplete burning of a coal bed underground while adding air and
steam. The gases are withdrawn and may be burned to produce heat, generate electricity, or
are used as synthesis gas in indirect liquefaction or the production of chemicals.
Producing diesel and other fuels from coal can be done through converting coal to syn-
gas, a combination of carbon monoxide, hydrogen, carbon dioxide, and methane. The syn-
gas is reacted through the Fischer-Tropsch synthesis to produce hydrocarbons that can be
refined into liquid fuels. Research into the process of increasing the quantity of high-quality
fuels from coal while reducing the costs could help ease the dependence on ever-increasing
cost but depleting stock of petroleum.
Furthermore, by improving the catalysts used in directly converting coal into liquid
hydrocarbons, without the generation of the intermediate syngas, less power could be
required to produce a product suitable for upgrading in existing petroleum refineries. Such
an approach could reduce energy requirements and improve yields of desired products.
While coal is an abundant natural resource, its combustion or gasification produces both
toxic pollutants and greenhouse gases. By developing adsorbents to capture the pollutants
(mercury, sulfur, arsenic, and other harmful gases), our researchers are striving not only
to reduce the quantity of emitted gases but also to maximize the thermal efficiency of the
cleanup.

5.5 GASEOUS FUELS

The gasification of coal or a derivative (i.e., char produced from coal) is the conversion of
coal (by any one of a variety of processes) to produce gaseous products that are combus-
tible. With the rapid increase in the use of coal from the fifteenth century onward (Nef,
1957; Taylor and Singer, 1957), it is not surprising that the concept of using coal to produce
a flammable gas, especially the use of the water and hot coal (van Heek and Muhlen, 1991),
became commonplace (Elton, 1958). In fact, the production of gas from coal has been a
vastly expanding area of coal technology, leading to numerous research and development
programs. As a result, the characteristics of rank, mineral matter, particle size, and reaction
conditions are all recognized as having a bearing on the outcome of the process; not only in
terms of gas yields but also on gas properties (Massey, 1974; van Heek and Muhlen, 1991).
The products from the gasification of coal may be of low, medium, or high heat-content
(high-Btu) as dictated by the process as well as by the ultimate use for the gas (Fryer
and Speight, 1976; Mahajan and Walker, 1978; Anderson and Tillman, 1979; Cavagnaro,
1980; Bodle and Huebler, 1981; Argonne, 1990; Baker and Rodriguez, 1990; Probstein and
Hicks, 1990; Lahaye and Ehrburger, 1991; Matsukata et al., 1992).
Coal gasification offers one of the most clean and versatile ways to convert the energy
contained in coal into electricity, hydrogen, and other sources of power. Turning coal into
synthetic gas isn’t a new concept; in fact the basic technology dates back to World War II.
Coal gasification plants are cleaner than standard pulverized coal combustion facilities,
producing fewer sulfur and nitrogen byproducts, which contribute to smog and acid rain.
For this reason, gasification appeals as a way to utilize relatively inexpensive and expansive
coal reserves, while reducing the environment impact.
The mounting interest in coal gasification technology reflects a convergence of two
changes in the electricity generation marketplace: (a) the maturity of gasification technology,
and (b) the extremely low emissions from integrated gasification combined cycle (IGCC)