Page 158 - Synthetic Fuels Handbook

144 CHAPTER FIVE

higher levels of waste heat. The emergence of the supercritical turbine concept envisions
running a boiler at extremely high temperature and pressure with projected efficiencies of
46 percent, with further theorized increases in temperature and pressure perhaps resulting
in even higher efficiencies.
A more energy-efficient way of using coal for electricity production would be via solid-
oxide fuel cells or molten-carbonate fuel cells (or any oxygen ion transport based fuel cells
that do not discriminate between fuels, as long as they consume oxygen), which would be
able to get 60 to 85 percent combined efficiency (direct electricity plus waste heat steam
turbine). Currently these fuel cell technologies can only process gaseous fuels, and they are
also sensitive to sulfur poisoning, issues which would first have to be worked out before
large-scale commercial success is possible with coal. As far as gaseous fuels go, one idea
is pulverized coal in a gas carrier, such as nitrogen. Another option is coal gasification
with water, which may lower fuel cell voltage by introducing oxygen to the fuel side of the
electrolyte, but may also greatly simplify carbon sequestration.
The potential for coal to be converted to fuels is dependent upon these properties, not
the least of which is the carbon content (i.e., the chemical composition) and the energy
value (calorific value).
Chemically, coal is a hydrogen-deficient hydrocarbon with an atomic hydrogen-to-
carbon ratio near 0.8, as compared to petroleum hydrocarbons, which have an atomic
hydrogen-to-carbon ratio approximately equal to 2, and methane that has an atomic carbon-
to-hydrogen ratio equal to 4. For this reason, any process used to convert coal to alternative
fuels must add hydrogen.
The chemical composition of the coal is defined in terms of its proximate and ultimate
(elemental) analyses (Speight, 1994). The parameters of proximate analysis are moisture,
volatile matter, ash, and fixed carbon. Elemental or ultimate analysis encompasses the
quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen within the
coal. Additionally, specific physical and mechanical properties of coal and particular car-
bonization properties are also determined.
The calorific value Q of coal is the heat liberated by its complete combustion with oxy-
gen. Q is a complex function of the elemental composition of the coal. Q can be determined
experimentally using calorimeters. Dulong suggests the following approximate formula for
Q when the oxygen content is less than 10 percent:
Q = 337C + 1442(H − O/8) + 93S
C is the mass percent of carbon, H is the mass percent of hydrogen, O is the mass percent
of oxygen, and S is the mass percent of sulfur in the coal. With these constants, Q is given
in kilojoules per kilogram (1 kJ/kg = 2.326 Btu/lb).
The production of fuels from coal in relation to fuels from other energy technologies is
dependent upon the cost of fuels from other sources and, most important, the degree of self
sufficiency required by various level of government. The nature of coal is a major factor
(assuming an ample supply of coal reserves) and the need for desulfurization of the prod-
ucts as well as the various steps leading from the mining of coal to its end use. Nevertheless,
the production of fuels from coal is an old concept having been employed since it was first
discovered that the strange black rock would burn when ignited and produce heat.
Coal can be liquefied by either direct or indirect processes (i.e., by using the gaseous
products obtained by breaking down the chemical structure of coal) to produce liquid prod-
ucts. Four general methods are used for liquefaction: (a) pyrolysis and hydrocarbonization
(coal is heated in the absence of air or in a stream of hydrogen), (b) solvent extraction
(coal hydrocarbons are selectively dissolved and hydrogen is added to produce the desired
liquids), (c) catalytic liquefaction (hydrogenation takes place in the presence of a catalyst),
and (d) indirect liquefaction (carbon monoxide and hydrogen are combined in the presence
of a catalyst).