Environmental Engineering Reference
In-Depth Information
penalties can be limited to acceptable levels at a cost commensurate with that of other components
of the system.
3.9
COMBUSTION OF FOSSIL FUEL
The source of energy that is utilized in fossil-fueled power systems is the chemical energy that is
released when a fuel is oxidized by burning in air. The most common fossil fuels are hydrocarbons—
that is, mixtures of molecules composed of carbon and hydrogen.
10
Upon their complete combus-
tion, the carbon in the fuel is oxidized to carbon dioxide and the hydrogen to water vapor. The
energy made available in this oxidation is the net amount released when the carbon and hydro-
gen atoms are separated from each other and subsequently combined with oxygen to form carbon
dioxide and water.
Denoting a hydrocarbon fuel molecule as C
n
H
m
, where
n
and
m
denote the number of carbon
and hydrogen atoms in a fuel molecule, the molecular rearrangement accompanying complete
oxidation of the carbon and hydrogen may be represented by the reaction
n
O
2
→
m
2
H
2
O
m
4
C
n
H
m
+
+
n
CO
2
+
(3.23)
For each hydrocarbon molecule,
n
4 diatomic oxygen molecules are required to convert the
carbon and hydrogen to
n
molecules of CO
2
and
m
+
m
/
2 molecules of H
2
O. The ratio of the number
of oxygen molecules to the number of fuel molecules,
n
/
4, is called the
stoichiometric ratio
.
It may be expressed alternatively as a mass ratio by multiplying the number of molecules by their
molecular masses, yielding
+
m
/
oxygen mass
fuel mass
32
n
+
8
m
=
(3.24)
12
n
+
1
.
008
m
This mass ratio lies in the range between 8
/
3
=
2
.
667 (for pure carbon) and 7
.
937 (for pure
hydrogen), being a function of the molar ratio
m
n
only.
Because fossil fuels invariably are burned in air, the stoichiometric proportions are more
usefully expressed in terms of the ratio of air mass to fuel mass by multiplying equation (3.24) by
the ratio of the mass of air to the mass of oxygen in air, which is 4.319:
/
319
32
n
air mass
fuel mass
≡
(
+
8
m
A
/
F
)
st
=
4
.
(3.25)
12
n
+
1
.
008
m
If less air is available than is required for a stoichiometric proportion, then not all of the carbon
or hydrogen will be fully oxidized and some amount of CO, solid C, or H
2
may be present in the
products of combustion. In such “rich” mixtures not all of the available chemical energy is released
in the (incomplete) combustion process. On the other hand, if extra or excess air is available, then
not all of the oxygen available is needed and some will remain unconsumed in the combustion
10
Synthetic fuels made from hydrocarbons may include oxygen-containing components such as alcohols and
carbon monoxide.
