Environmental Engineering Reference
In-Depth Information
TABLE 3.3
Thermal Efficiencies of Synthetic Fuel Production
Efficiency
a
(%)
Fuel
Product
Coal
Synthesis gas
72-87
Coal
Methane
61-78
Coal
Methanol
51-59
Coal
Hydrogen
62
Oil
Hydrogen
77
Methane
Hydrogen
70-79
Coal, oil, or gas
Hydrogen (electrolytic)
20-30
Oil shale
Oil and gas
56-72
Methanol
Oil and gas
86
Wood
Gas
90
Corn
Ethanol
46
Manure
Gas
90
a
Thermal efficiency is the ratio of the heating value of the synthetic product divided by
the heating value of the parent fuel.
result in a loss of fuel heating value and an increase of carbon emissions per unit of synthetic fuel
heating value.
Table 3.3 summarizes the thermal efficiencies (the ratio of synthetic fuel heating value to
that of the parent fuel) for several synthetic fuel production processes. With but few exceptions,
these efficiencies lie within the range of 60% to 90%. Most conversion processes require high
process temperatures and pressures, need catalytic support to improve the production rate, and
consume mechanical power to provide for the requisite pressurization and heat transfer processing.
The economic and energy costs of synthetic fuel production can only be justified when there are
compensating gains attending the use of synthetic fuels, such as the suitability for use in fuel cells
or convenience of storage and transport.
Synthetic nuclear fuels can be produced in nuclear reactors. Uranium-238, which is not a
fissionable nuclear fuel, can be converted to plutonium-239, which can be used to fuel a nuclear
fission reactor. See Section 6.2 for a description of this process.
3.14.1
The Hydrogen Economy
Hydrogen has been promoted as an environmentally friendly synthetic fuel that can be used in a fuel
cell to generate electrical power at high efficiency while emitting no air pollutants. Two possible
sources of hydrogen fuel are the reforming of methane and the electrolysis of water:
CH
4
+
→
CO
2
+
2H
2
O
4H
2
(3.56)
→
2H
2
+
2H
2
O
O
2
(3.57)
Both of these reactions require additional energy to bring them to completion. The first requires
combustion of additional methane to supply the heat needed for the reforming of methane to
hydrogen. The second cannot be effectuated by heating alone, because there is a great increase in
free energy. Instead, the free energy increase is provided by electric power in an electrolytic cell and
only a small amount of heat is involved. Producing electrolytic hydrogen is very energy inefficient
