Environmental Engineering Reference
Figure 2.12 Proportions (%) of primary energy use in the U.S. transportation sector. (Data from U.S.
Department of Transportation, 1993. Transportation Energy Data Book, Edition 15.)
in the residential sector, this sector could also realize large energy savings, especially in lighting,
space heating, cooling, and ventilation.
Of the total energy used by the transportation sector, about 42% is used for personal automobile
transport, 20% for light trucks (often just used for personal transport), 16% for heavy freight,
9% for air, 7% for water, 4% for pipe, and 2% for rail transport (see Figure 2.12). Considering
that in the United States the transportation fuel is practically all derived from petroleum, that over
50% of petroleum is imported, that automobiles are responsible for about 50% of ground-level
air pollution, and that the transportation sector contributes about one-third of U.S. CO 2 emissions,
the transportation sector could realize significant savings in energy consumption and reduction
in air pollutant and carbon emissions. This could be accomplished by (a) reducing the weight of
automobiles, (b) increased engine fuel efficiency, (c) using fuel cell and battery powered electric
drive cars, (d) using hybrid internal combustion engine-electric powered cars, and (e) increased
use of public transportation. Some of these alternatives are further discussed in Chapter 8.
GLOBAL ENERGY SUPPLY
In this section we shall address only the supply of fossil energy—that is, coal, petroleum, natural gas,
and unconventional sources of fossil energy, such as oil shale, tar sand, geopressurized methane,
and coal seam methane.
Coal is found practically on every continent and subcontinent. It is found buried deeply in the
ground or under the seabed, or close to the surface. Coal characteristics vary widely according to
its biological origin (forests, low growing vegetation, swamps, animals) and geological history (age,
overburden, temperature, pressure). Thus, the chemical and physical characteristics of coal are also
highly variable, such as the content of moisture, minerals (ash), sulfur, nitrogen, and oxygen; heat