che_march-2024 - 22

Facts At Your Fingertips
NOx Formation in Combustion
Department Editor: Scott Jenkins
O
xides of nitrogen (NOx) are a
family of highly reactive gases
that can be produced naturally,
but largely result from fuel combustion
(industrial combustion and automobile
engines). In the environment,
they are pollutants that react with volatile
organic compounds in complex
ways to produce ground-level ozone,
and they also play a role in producing
smog and acid rain.
NOx formation occurs via different
mechanisms: thermal NOx is based
on temperature and makes up most of
the NOx formed during combustion;
fuel-bound NOx comes from nitrogen
atoms contained fuels; and prompt
NOx is formed when molecular nitrogen
in the air combines with the fuel
in fuel-rich conditions. This one-page
reference outlines these mechanisms
largely responsible for NOx formation
in industrial combustion
Thermally formed NOx
Thermal NOx is formed by oxidation
of N2 in air and requires sufficient temperature
and time to produce NOx.
For fuels that contain no nitrogen in
the parent molecules (for example,
natural gas), this mechanism, also
known as the Zeldovich mechanism,
produces most of the NOx. This may
be approximated by the following integrated
rate expression:
(1)
where the brackets indicate the volume
concentration of the enclosed
species, A and b are constants, t is
the total reaction time (with θ serving
as the dummy variable in the integration),
and T is the absolute temperature.
So NOx from this mechanism
depends on three primary quantities
- temperature, oxygen concentration
and reaction time - and minimizing
any or all of them will reduce
NOx. Since temperature is exponentially
weighted, the peak flame
temperature has an oversized role in
NOx formation.
A rule of thumb is that below ap22
proximately
1,700K, the
residence time in typical gas
turbine combustors is not
long enough to produce significant
thermal NOx. Where
temperatures higher than
1,700K cannot be avoided,
it is necessary to limit residence
time to control NOx
formation, which favors very
short combustor designs.
Fuel-bound mechanism
If a significant number of fuel molecules
contain nitrogen bound in their
structure, the overwhelming share
of NOx will be formed from the fuelbound
mechanism, leading to a rate
equation as shown in Equation (2):
(2)
where κ is a constant. Since the reaction
is fast, and the fuel concentration
is limited by the required stoichiometry,
fuel-bound NOx formation
may be reduced only by reducing the
excess oxygen or switching to lowernitrogen
fuels.
Prompt NOx
The third NOx-formation mechanism
is the Fenimore mechanism,
also called prompt NOx. It is similar
to the fuel-bound mechanism, except
that the nitrogen comes directly
from the combustion air. Since
nitrogen radicals are exceptionally
difficult to pare from molecular nitrogen,
prompt NOx from this mechanism
is usually negligible.
Reducing NOx in burners
Three burner configuration methods
exist for reducing nitrogen oxides in
burners: staged air, staged fuel and
internal fluegas recirculation combined
with staged air or staged fuel.
Staged-air burners. These types of
burners work by introducing 100% of
the fuel into the burner and only part
of the combustion air (primary air),
thus creating a sub-stoichiometric
flame. This flame has a reduced temFIGURE
1. Burners with fuel staging inhibit NOx production
Cleaver-Brooks
perature and therefore inhibits NOx
formation. The flame is completed
with the addition of the secondary air
to complete the combustion process.
This process allows for greater control
at lower burner loads and also accommodates
a wider range of fuels.
Staged-fuel burners. This burner
method introduces 100% of the combustion
air into the burner and splits
the fuel supply into primary and secondary
volumes. The primary fuel
mixes with the combustion air to create
a flame (Figure 1). As with stagedair
burners, peak flame temperature is
lower, and NOx formation is reduced.
Secondary fuel is added to complete
the combustion process. Staged-fuel
burners provide greater NOx reduction,
as the fuel supply has a larger
effect on NOx formation. This method
is more commonly used when a consistent
fuel supply is available.
Internal fluegas-recirculation burners.
This method combines either
staged air or staged fuel with internal
fluegas recirculation to help reduce
NOx formation. The best results are
obtained where internal fluegas recirculation
is used to dilute the fuel gas
in a staged-fuel burner, creating a gas
with a low calorific value.
n
References
1. Stoeger, J., Burner Technologies and Concepts: Meeting
Emissions-Reduction Goals, Chem. Eng., Sept. 2022, pp. 34-39.
2. Vij, A.D., Enclosed Combustion Equipment and Technology,
Chem. Eng., January 2018, pp. 46-49.
3. Al-Hajji, M.H., Burner Inspection and Maintenance, Chem. Eng.,
November 2014, pp. 40-45.
4. Colannino, J., Low-Cost Techniques for NOx Reduction, Chem.
Eng., May 2020, pp. 30-36.
5. Richards, G., Weiland, N. and Strakey, P., Combustion Strategies
for Syngas and High-Hydrogen Fuel, in " Gas Turbine Handbook, "
National Energy Technology Laboratory (NETL), 2006.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2024
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