Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 92

Sulfur is present in most fossil fuels, usually higher in coal than in crude
oil. Prehistoric plant life is the source of most fossil fuels. Most plants contain
sulfur as a nutrient, and as the plants become fossilized, a fraction of the sulfur
volatilizes (i.e., becomes a vapor) and is released. However, some sulfur remains
in the fossil fuel and can be concentrated because much of the carbonaceous
matter is driven off. Thus, the sulfur in the coal is available to react with
oxygen when the fossil fuel is combusted. In fact, the sulfur content of coal
is an important characteristic in its economic worth; the higher the sulfur
content, the less it is worth. So the lower the content of sulfur and volatile
constituents and the higher the carbon content, the more valuable the coal.
Since combustion is the combination of a substance (fuel) with molecular
oxygen (O 2 ) in the presence of heat [denoted by the above the ar row in

the one-way (i.e., irreversible) reaction], the reaction for complete or efficient
combustion of a hydrocarbon results in the formation of carbon dioxide and
water:

O 2 (CH) x + → CO 2 + H 2 O (B3.1)

Fossil fuels contain other elements which also oxidize. When sulfur is
present, the side reaction forms oxides of sulfur. Thus, sulfur dioxide is formed
as

O 2

+ → SO 2

S (B3.2)

Actually, many other oxidized forms of sulfur can form during combustion, so
air pollution experts refer to them collectively as SO x , a term seen commonly
in air pollution literature.
Similarly, nitrogen compounds also form during combustion, but their
sources are very different from those of sulfur compounds. Recall that the
troposphere , the part of the atmosphere where we live and breathe, is made up
mainly of molecular nitrogen (N 2 ). More than three-fourths of the troposphere
is N 2 , so the atmosphere itself is the source of much of the nitrogen that forms
oxides of nitrogen (NO x ). Because N 2 is relatively nonreactive under most
atmospheric conditions, it seldom enters into chemical reactions, but under
high pressure and at very high temperatures, it will react with O 2 :

O 2

→ 2NO

N 2 + (B3.3)

Where will we find conditions such that N 2 will react this way? Actually,
it is sitting in your driveway or garage. The automobile's internal combustion