Environmental Engineering Reference
In-Depth Information
efficiencies reflect the constraints of the laws of thermodynamics, the limitations of materials, and
the compromises inherent in achieving economical as well as efficient systems. While there is
room for improvement, only modest increases above the values in Table 3.2 can be expected from
extensive development efforts.
3.14
SYNTHETIC FUELS
A synthetic fuel is one that is manufactured from another fuel so as to enhance its usefulness while
retaining as much of the original heating value as possible. Typical examples are oil produced from
coal, oil shale, or tar sands; gas from coal, oil, or biomass; alcohols from natural gas or biomass; and
hydrogen from coal, oil, or natural gas. Some liquid fuels, such as gasoline, are partially synthetic
in that the refining process produces components that are synthesized from petroleum constituents
and added to the natural fractions of petroleum that ordinarily comprise the liquid fuel. Major
advantages of a synthetic fuel, other than its form as liquid, gas, or solid that might enhance its
transportability and convenience of storage, are (a) the removal of base fuel constituents such as
sulfur, nitrogen, and ash that lead to harmful air pollutants and (b) the ability to burn the fuel in
special devices such as gas turbines and fuel cells. A major disadvantage is the cost of synthesizing
the fuel and the loss of its heating value, both of which raise the cost of synthetic fuel heat. This
cost factor has been the major obstacle to widespread production and use of synthetic fuels.
Synthetic fuels are formed in steady flow reactors supplied with fuel and other reactants, usually
at elevated pressures and temperatures. The chemical reactions that generate the synthetic fuels are
aided by the use of catalysts that enhance the reaction rates to economically practical levels. Where
the synthesizing reactions are endothermic, heat must be added to maintain the reactor temperature.
Thus the synthesis proceeds at fixed temperature and pressure.
As an example of a synthetic fuel, consider the production of synthesis gas from coal by the
reaction
C
solid
+
(
H
2
O
)
g
→
(
CO
)
g
+
(
H
2
)
g
(3.55)
where 12 kilograms of solid carbon and 18 kilograms of steam react to form 28 kilograms of carbon
monoxide and 2 kilograms of hydrogen. But the fuel heating value of the carbon is (see Table 3.1)
12 kg
×
32.76 MJ/kg
=
393 MJ, while that of the synthesis gas is 18 kg
×
10.10 MJ/kg
+
2kg
×
141.8 MJ/kg
465 MJ, so that this process would be endothermic in the amount of 2.4 MJ/kg of
products at 25
◦
C. Thus additional coal would have to be burned to provide this amount of heat to
maintain the synthesis. In addition, the second law requires that the free energy of the product gas
not exceed that of the reactant mixture because no mechanical work is invested in the process. As
a consequence, practical synthesis results in a reduction of heating value in the synthesized fuel.
23
Fossil fuels are the final synthesis step in the photosynthetic production of carbohydrates
from carbon dioxide and water in plants, deriving their chemical energy from sunlight. In the
photosynthesis reaction, energetic solar photons provide the large increase in free energy of the
carbohydrate molecules, in contrast to the thermal reforming synthesis of 3.55 where the product
molecules have lower free energy. Any reforming of natural fossil fuels to synthetic form will
=
23
Synthetic fuel may be produced in advanced power plant cycles. See Section 5.3.2.
