Environmental Engineering Reference
In-Depth Information
when the electricity is generated by burning a fossil fuel, because the heating value of the hydrogen
will be less than one-third of the heating value of the fuel burned in the electric power plant.
Proponents of hydrogen as a substitute for fossil fuel in vehicles or power plants point to
its lack of carbon dioxide emissions. But if hydrogen is produced by reforming a fossil fuel,
there is no net reduction in carbon dioxide emissions; in most circumstances there will be an
increase in emissions and costs. If electrolytic hydrogen is produced by electricity, there is no
reduction in carbon dioxide emissions as long as some electricity is produced in fossil fuel plants.
In addition, storing and transporting hydrogen is expensive, so that its production by electricity is
most economically accomplished at the point of use.
On the other hand, hydrogen production from a fossil fuel provides a path for CO
2
recovery
and sequestration (see Section 10.4.4). In this scheme a fossil fuel is converted to a noncarbon fuel,
H
2
, while the CO
2
that is formed in the conversion process, such as equation (3.56) above, can be
recovered and sequestered under ground or in the ocean, preventing its emission into the atmosphere
that would have followed from direct combustion of the fossil fuel. In this manner, 60% to 80% of
the heating value of a fossil fuel may be utilized while reducing or eliminating the emissions of CO
2
.
3.15
CONCLUSION
Nearly 86% of the world's energy is supplied by the combustion of fossil fuel. While the processes
by which the energy of this fuel is made available for human use in the form of heat or mechanical
power are circumscribed by the principles of thermodynamics, the technologies employed are the
consequences of human invention.
In this chapter we have described the implications of the first and second law of thermodynam-
ics for the functioning of selected technologies for producing mechanical power, with particular
attention to the efficiency of conversion of fuel energy to useful work. We found that the fuel
energy that can be made available by burning fuel in air, called the fuel heating value, appears as
the sum of work produced by a heat or combustion engine and heat rejected to the surrounding
environment, as required by the first law of thermodynamics. But only a fraction of the fuel energy
can be converted to work, according to the second law of thermodynamics, with the magnitude of
that fraction depending upon the detailed operation of the technology being used. It is extremely
difficult to convert more than half of the fuel heating value to work, but very easy to convert all of
it to heat alone.
Most mechanical power is produced in steam power plants, where it is converted to electrical
form for distribution to end consumers. Water/steam is circulated within a closed loop, being heated
in a boiler by the combustion of fuel and then powering a steam turbine. The most efficient steam
plants convert about 40% of the fuel energy to mechanical power.
The gas turbine engine, developed initially for aircraft propulsion and utilizing combustion of
fuel in the air steam that flows through the engine, is one prominent form of internal combustion en-
gine. Unlike the steam engine, it does not require the exchange of heat with an external combustion
system. Its performance is limited by the strength of the turbine blades that must endure high com-
bustion temperatures. When used to generate electricity, it's efficiency is only about 30%. By com-
bining with a steam plant, called combined cycle, the efficiency of the combination is about 45%.
The other prominent form of internal combustion engine is the common automobile engine,
either gasoline (spark ignition) or diesel (compression ignition). Here the cyclic nature of the engine
allows much higher combustion temperatures than in the gas turbine, along with higher efficiencies
of 25% to 35%.
