TM 5-815-1/AFR 19-6
CHAPTER 11
NITROGEN OXIDES (NOx) CONTROL AND REDUCTION
TECHNIQUES
11-1.
Formation of nitrogen oxides.
tions produce more NOx. The more bulk mixing of fuel
and air in the primary combustion zone, the more tur-
a. Nitrogen oxides (NOx). All fossil fuel burning
bulence is created. Flame color is an index of flame
processes produce NOx. The principle oxides formed
turbulence. Yellow hazy flames have low turbulence,
are nitric oxide (NO) which represents 90-95 percent
whereas, blue flames with good definition are consid-
ered highly turbulent.
which represents most of the remaining nitrogen
c. Burner number. The number of burners and their
oxides.
spacing are important in NOx emission. Interaction
b. NOx formation. Nitrogen oxides are formed pri-
between closely spaced burners, especially in the center
marily in the high temperature zone of a furnace where
of a multiple burner installation, increases flame
sufficient concentrations of nitrogen and oxygen are
temperature at these locations. The tighter spacing
present. Fuel nitrogen and nitrogen contained in the
lowers the ability to radiate to cooling surfaces, and
combustion air both play a role in the formation of
greater is the tendency toward increased NOx emis-
NOx. The largest percentage of NOx formed is a result
sions.
of the high temperature fixation reaction of
d. Excess air. A level of excess air greatly exceeding
atmospheric nitrogen and oxygen in the primary
the theoretical excess air requirement is the major
combustion zone.
c. NOx concentration. The concentration of NOx
Negotiable quantities of thermally formed NOx are
found in stack gas is dependent upon the time, tem-
generated in fluidized bed boilers.
perature, and concentration history of the combustion
e. Combustion temperature. NOx formation is
gas as it moves through the furnace. NOx concentration
dependent upon peak combustion temperature, with
will increase with temperature, the availability of oxy-
higher temperatures producing higher NOx emissions.
gen, and the time the oxygen and nitrogen simul-
f. Firing and quenching rates. A high heat release
taneously are exposed to peak flame temperatures.
rate (firing rate) is associated with higher peak tem-
peratures and increased NOx emissions. A high rate of
11-2.
Factors affecting NOx emissions
thermal quenching, (the efficient removal of the heat
a. Furnace design and firing type. The size and
released in combustion) tends to lower peak tem-
design of boiler furnaces have a major effect on NOx
peratures and contribute to reduced NOx emissions.
emissions. As furnace size and heat release rates
g. Mass transportation and mixing. The con-
increase, NOx emissions increase. This results from a
centration of nitrogen and oxygen in the combustion
lower furnace surface-to-volume ratio which leads to
zone affects NOx formation. Any means of decreasing
a higher furnace temperature and less rapid terminal
the concentration such as dilution by exhaust gases,
quenching of the combustion process. Boilers generate
slow diffusion of fuel and air; or alternate fuel-
different amounts of NOx according to the type of
rich/fuel- lean burner operation will reduce NOx
firming. Units employing less rapid and intense burning
formation. These methods are also effective in
from incomplete mixing of fuel and combustion gases
reducing peak flame temperatures.
generate lower levels of NOx emissions. Tangentially
h. Fuel type. Fuel type affects NOx formation both
fired units generate the least NOx because they operate
through the theoretical flame temperature reached, and
on low levels of excess air, and because bulk misting
through the rate of radiative heat transfer. For most
and burning of the fuel takes place in a large portion of
combustion installations, coal-fired furnaces have the
the furnace. Since the entire furnace acts as a burner;
highest level of NOx emissions and gas-fired
precise proportioning of fuel/air at each of the individ-
installations have the lowest levels of NOx emissions.
ual fuel admission points is not required. A large
i. Fuel nitrogen. The importance of chemically
amount of internal recirculation of bulk gas, coupled
with slower mixing of fuel and air, provides a combus-
temperature level of the combustion processes. Fuel
tion system which is inherently low in NOx production
nitrogen is important at low temperature combustion,
for all fuel types.
but its contribution is nearly negligible as higher flame
b. Burner design and configuration. Burners oper-
temperatures are reached, because atmospheric nitro-
ating under highly turbulent and intense flame condi-
11-1
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