N
ear
i
deal
c
omBuStioN
This is When we burn pure hydro-
gen
in the air. Our atmosphere is 20.9% oxygen
with
the remaining 79.1% nitrogen.
This is nearly as desirable as the example for
ideal
combustion with the only added loss being
the
heat that is carried away from your target
with
the nitrogen. Because nitrogen isn’t part of
the
combustion process, it enters the combustion
chamber at the inlet temperature and leaves
with some of the heat created by the combus-
tion.
If this isn’t recovered at the heat exchanger it is
lost
up the flue.
The main problem with this example is again
the
availability and cost of pure hydrogen.
B
eSt
o
f
t
he
r
eal
w
orld
Natural gas is
a
readily available fuel, and our atmosphere
contains sufficient oxygen. When this is
used as a fuel we get the reaction; shown
in figure 3.
Now the other added outputs are CO2
and hot nitrogen compared to the Ideal
World situation. In addition to this we
have added the output Excess Air.
Excess Air is exactly what the name
implies, air that is in excess of what is
needed to burn all of the fuel. The
reason for this is more related to the
ability to mix all of the fuel and O2 for
complete combustion. Without some
amount of excess air not all of the fuel
would burn completely, and this leads to
the formation of CO instead of CO2.
Other fuels all contain the basic ingre-
dients for combustion, but also may
include other components such as
sulfur, fuel bound nitrogen, soot and
ash and water. These either react with
the oxygen to form other pollutants or
contribute to
additional losses.
Wet Loss
Dry Loss
figure 3
Carbon Monoxide
is formed from incomplete combustion (partial oxidation of carbon in
the fuel). Typical causes are incomplete mixing of fuel and air, low combustion temperatures,
or not enough excess air.
8