Fossil-Fueled Power Plants - Energy and the Environment - page 88

5

CHAPTER

Fossil-Fueled Power Plants

5.1 INTRODUCTION

In Chapter 2 we saw that fossil-fueled electric power plants worldwide consume 55.5% of the
annual supply of fossil fuel, of which more than 80% is in the form of coal. Thus, fossil-fueled
power plants are major contributors to the anthropogenic emissions of CO 2 and other pollutants,
such as SO 2 ,NO x , products of incomplete combustion, and particulate matter (PM).
Because power plants, for reasons of economy of scale, are usually built in large, centralized
units, typically delivering in the range of 500-1000 MW of electric power, much effort is being
spent on their efficiency improvement and environmental control, since these efforts could result
in significant global reduction of pollutant emissions and conservation of fossil fuel reserves.
Almost all fossil-fueled power plants work on the principle of a heat or combustion engine,
converting fossil fuel chemical energy first into mechanical energy (via steam or gas turbines),
then into electrical energy, as described in Chapters 3 and 4. 1 Most large-scale power plants use
the Rankine steam cycle, in which steam is produced in a boiler heated by a combustion gas; the
steam drives a steam turbine that drives a generator of electricity. 2 Usually, steam turbine plants
provide the base load for a regional grid in combination with nuclear plants. On the other hand,
peak loads are sometimes supplied by gas turbine plants that work on the Brayton cycle. 3 In those
plants, natural gas is burned, and the combustion products directly drive a gas turbine, which in
turn drives a generator.
The best steam cycle power plants can achieve a thermal efficiency above 40%; the U.S. average
is 36% and the worldwide average is about 33%. 4 Gas turbine power plants achieve a thermal
efficiency in the 25-30% range. More advanced power plants exist today that use a combination of
Brayton and Rankine cycles. Combined cycle power plants can achieve an efficiency of about 45%.
The relatively low thermal efficiency of power plants is due to two factors. The first is a consequence
of the Second Law of Thermodynamics, whereby in a heat engine cycle, after performing useful

1 The exception would be a power plant that uses a fuel cell for the conversion of fossil fuel chemical energy
directly into electrical energy. However, very few power plants exist today working on the principle of a fuel
cell. The workings of a fuel cell electricity generator are described in Chapter 3.
2 For a detailed description of the Rankine cycle, see Section 3.10.2.
3 For explanation of base and peak load, see Chapter 4. The Brayton cycle is described in Section 3.10.4.
4 Thermal (or fuel) efficiency is defined as the ratio of electrical energy output to fossil fuel energy input
(see Section 3.13). An equivalent definition is the heat rate . This is the amount of British thermal units or
gigajoules of fuel energy spent per kilowatt hour of electricity produced.

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