NOx emissions refer to a group of nitrogen oxides the are commonly found in combustion gases. These harmful emissions are caused by fuel combustion processes occurring in cars and industrial applications. They can cause environmental issues like photochemical smog, acid rain, and ozone accumulations. NOx has also been linked to asthma complications, pulmonary complications, as well as coughing and wheezing. Correctly quantifying these emissions in air pollution control equipment is necessary to maximize the control of NOx gases. This presentation focuses on quantifying thermal NOx. This type of NOx is usually formed in the hottest parts of the burner flame.

1. Quantifying NOx
Emissions
ppmv @ 3% O2 vs. lb/MMBTU

2. What exactly is NOx?
The term NOx refers to a group of nitrogen oxides
including NO, NO2, N2O5, and others commonly
found in combustion gases. Although the ratio of
which compounds are present will vary depending
on how they were formed, for simplicity it is
common to assume all NOx as NO2 during
emissions testing.

3. Background Information

4. Where does NOx come
from?
Nitrogen oxides are naturally occurring compounds as part of the nitrogen
biological cycle
The main culprit for elevated levels of NOx in the atmosphere are fuel
combustion processes occurring in cars and industrial processes
Stationary sources of NOx emissions with more stringent regulations include
nitric acid manufacturing plants, manufacturers of nitrated materials like fertilizer
& explosives, and industrial manufacturers.
EPA uses NO2 as the indicator for the larger group of nitrogen oxides (NOx).

5. Regulatory Guidance
• NOx emissions are regulated by
the EPA and the Clean Air Act
under the 1990 Amendment
• The standard for new nitric acid
manufacturing plants is 3
pounds NOx per ton of nitric
acid produced
• New plants must reduce NOx
emission levels from 1500-3000
ppm to 200 ppm
• Regulations for existing plants
are complicated and vary per
industry

6. How is NOx harmful?
Photochemical smog
Precursor to acid rain due to conversion into HNO3
Tropospheric ozone accumulation
Asthma complications after long-time exposures
Pulmonary complications
Coughing and wheezing

7. What are the types of
NOx?
Chemically Bound NOx
•Combustible nitrogen bearing compounds such as
ammonia and various amines react to the oxidization
process to form NOx.
Thermal NOx
•Nitrogen in the air is directly converted to NOx
through high temperatures.

8. Zeldovich Mechanism
• The Zeldovich mechanism is
a chemical mechanism that
describes the oxidation of
nitrogen and NOx formation.
• Thermal NOx is the
disassociation of N2 & O2 at
elevated temperatures and
subsequent reaction of
nitrogen and oxygen radicals
to form nitric oxide.
𝑁2 + 𝑂𝑥
↔ 𝑁𝑂 + 𝑁𝑥
𝑂2 + 𝑁𝑥
↔ 𝑁𝑂 + 𝑂𝑥

9. What is Thermal NOx?
Thermal NOx is primarily formed in the hottest parts of the
burner flame
Temperatures can easily be well over 3,000°F.
Oxidation at lower combustion temperatures (1,400-1,800°F)
have little to no impact on thermal NOx production.
Burners are responsible for generating NOx on a thermal
oxidizer.

10. Textbook Quantifying
Method

11. Quantifying NOx in
Reference to O2
Regulations often state NOx emissions as a
concentration corrected to a reference oxygen
concentration, typically PPMv @ 3% O2 by volume.
𝑁𝑂𝑥 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝑁𝑂𝑥 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 ∗
% 𝑂2 𝑖𝑛 𝑎𝑖𝑟 − % 𝑂2 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒
% 𝑂2𝑖𝑛 𝑎𝑖𝑟 − % 𝑂2 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑

12. How does this method
work?
Normalizes the emissions from burners, furnaces, boilers, and other devices
where the entire volume of flue gas is a part of the primary combustion process
with no additional gas sources, and the air source is just ambient air.
The burner flue mixing with additional streams causes difficulties with this
method.
The burner is generating thermal NOx, but tests are taken in an exhaust
stream. This is a combination of burner flue gas and process gas which is
not significantly contributing to the total NOx.
This method leads to misrepresentation of the true burner performance.

13. What are the drawback
to this method?
• Even with low actual NOx concentration, correction
factors can create inflated values and misrepresent the
true system performance.
• The burner may be capable of meeting emission
requirements but read out of compliance due to dilution.
• In some cases, undefined results are possible if the
exhaust oxygen concentration is near ambient
conditions.
• Results may be inaccurate when the process gas
composition is not the same as ambient air.
5 𝑝𝑝𝑚𝑣 𝑚𝑒𝑎𝑠𝑢𝑟𝑒 ∗
20.9% − 3%
20.9% − 8%
= 6.9 𝑝𝑝𝑚𝑣 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑
5 𝑝𝑝𝑚𝑣 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 ∗
20.9% − 3%
20.9% − 20.0%
= 99 𝑝𝑝𝑚𝑣 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑
5 𝑝𝑝𝑚𝑣 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 ∗
20.9% − 3%
20.9% − 20.9%
= 𝑢𝑛𝑑𝑒𝑓𝑖𝑛𝑒𝑑

14. Step #1 – Process
Inputs
Process Inputs
Process Gas Flow Rate (SCFM) 10,000
Process Air O2 Concentration (%
Vol.)
20.5217%
Process Air N2 Concentration (%
Vol.)
77.6683%
Process Air H2O Concentration (%
Vol.)
0.8000%
Process Air CO2 Concentration (%
Vol.)
1.0000%
Process Air VOC Concentration (%
Vol.)
0.0100%
Total (% Vol.) 100.0000%
VOC:O2 Molar Ratio 2
• RTO operates at 10,000
SCFM
• The assumed VOC in this
case is methane consuming
at a 2:1 molar ratio as it is
oxidized

15. Step #2 – Burner Inputs
• RTO operates at approximately
1,600°F (assumed)
• To maintain operating
temperature and make up for
heat losses the burner will need
to fire at approximately 1.80
MMBTUH
• Natural gas is used as the fuel
source
• Under these conditions the
burner flue gas will contain
approximately 50 ppmv of NOx
(assumed)
• This data is available from the
manufacturer
Burner Inputs
Burner Firing Rate (BTU/hr) 1,800,000
Burner Excess Air (%) 15%
Fuel Gas:O2 Molar Ratio 2
Fuel Gas Heating Value
(BTU/SCFM)
1,000
Uncorrected NOx Concentration in
Burner Flue Gas (ppmv as NO2)
50

16. Step #3 – Burner NOx
• The required gas flow is calculated
with the firing rate and natural gas
heating value.
• The required oxygen and
combustion air flow can be
determined with the known excess
air rate.
• Total burner flue gas is the sum of
fuel gas and combustion air.
• The total volume and excess
oxygen determines the
concentration of oxygen in the
burner flue gas.
• The NOx concentration is known
from the previous assumption NOx
ppmv @ 3% O2.
Burner NOx
Gas Flow (SCFM) 30
Stociometric O2 Required (SCFM) 60
Excess O2 (SCFM) 9
Total O2 Supplied (SCFM) 69
Air:O2 Ratio 4.785
Total Air Supplied to Burner (SCFM) 330
Total Burner Flue Gas (SCFM) 360
O2 Concentration in Flue Gas (% Vol.) 2.50%
Corrected NOx Concentration in
Burner Flue Gas (ppmv as NO2)
48.6

17. Step #4 – Exhaust NOx
• Total oxygen in the exhaust is
the sum of the process gas and
what is added through the
burner, with the oxygen
consumed by VOC oxidation
subtracted.
• Total exhaust flow is the sum of
process gas and burner flue
gas.
• The new NOx concentration is
the original NOx volume divided
by the new total flow rate.
• With all the information known
the correction can be applied.
Process Exhaust
O2 in Process Inlet (SCFM) 2052
VOC O2 Consumption (SCFM) 2
O2 in Process Exhaust (SCFM) 2059
Total Exhaust Flow (SCFM) 10,360
Exhaust O2 Concentrations (% Vol.) 19.88
%
NOx in Burner Flue Gas (SCFM) 0.018
Uncorrected NOx Concentration in
Oxidizer Exhaust (ppmv as NO2)
1.74
Corrected NOx Concentration in
Oxidizer Exhaust (ppmv as NO2)
30.4

18. What are the results?
Say for example the goal was to achieve 40 ppmv @
3% O2. The burner in this application does not meet
this requirement, but you would not be able to tell from
the stack test results.
However, measurements cannot be made in the
burner flue gas directly but in the process exhaust
where the same correction procedure results in a
misrepresented value of 30.4 ppmv @ 3% O2.
The actual NOx emissions from the burner are 48.6
ppmv @ 3% O2.

19. What does this mean?
There is a risk that the oxidizer is producing more NOx than truly allowed.
Quantifying on a mass basis eliminates this discrepancy and provides a very clear
understanding on the amount of NOx produced.
As the process gas becomes more oxygen deficient the effect becomes more drastic.
The oxygen correction factors are written to discourage dilution of burner flue gases with air,
but in the case of a thermal oxidizer the gas is not always diluted with more oxygen. This is
what skews the results.

20. Practical Real-World
Method

21. Quantifying NOx in
Terms of lb / MMBTU
The preferred method of rating burners is in terms of pounds of
NOx and NO2 per million BTU or lb/MMBTU. This establishes
a reliable mass basis for NOx emissions that avoids the issues
of dilution.

22. How does this method
work? Previous Example: Burner lb NOx/MMBTU
Burner Firing Rate (MMBTU/hr) 1.8
Total Burner Flue Gas (SCFM) 360
Uncorrected NOx Concentration in
Burner Flue Gas (ppmv as NO2)
50
NOx in Burner Flue Gas (SCFM) 0.018
NO2 Molecular Weight 46.0055
NOx in Burner Flue Gas (lb/hr) 0.129
NOx in Burner Flue Gas (lb/MMBTU) 0.071
• It is possible to convert between the
volume basis and the mass basis.
• The previous example describes how to
determine the volume of NOx present in
the flue gas.
• The molecular weight of NO₂ is used to
convert this to a mass flow rate. Dividing
this by the burner firing rate results in a
value for NOx emissions in terms of
lb/MMBTU.
• With the preferred method it becomes
simpler to estimate total NOx emissions
in terms of tons per year if the burner
demand and operating hours are known.
• Uncorrected NOx concentrations in the
exhaust can be directly converted to lb/hr
and divided by the firing rate to verify
burner performance.

23. Step #1 – Burner Firing
Rate
• To demonstrate the advantages of starting with a burner
rated in terms of lb/MMBTU consider the following
example.
• The same RTO from the previous example is used
operating under the same conditions, but this time with
a burner that is rated from the manufacturer at 0.06
lb/MMBTU of NOx.
• These mass values can be directly compared to the
permitted limit for a better representation of the
expected burner performance.
Inputs
Burner Firing Rate (BTU/hr) 1,800,000
Rated NOx Emissions (lb/MMBTUH as 0.06
Assumed Operating Hours per Year (24
Year (24 hours a day; 5 days a week)
week)
6240
Calculated Values
NOx lb/hr 0.108
NOx tons/year 0.337

24. Step #2 – Mass Flow
• Assuming the NOx as NO₂ with a molecular weight of 40.0055, mass flow can be easily converted to a volumetric flow
using the equation below and the findings from the previous slide.
𝑆𝐶𝑀𝐹 =
𝑙𝑏
ℎ𝑟
∗386.79
𝑆𝐶𝐹
𝑙𝑏−𝑚𝑜𝑙
60
𝑚𝑖𝑛
ℎ𝑟
∗𝑀𝑊
𝑙𝑏
𝑙𝑏−𝑚𝑜𝑙
=
0.108
𝑙𝑏
ℎ𝑟
∗386.79
𝑆𝐶𝐹
𝑙𝑏−𝑚𝑜𝑙
60
𝑚𝑖𝑛
ℎ𝑟
∗40.0055
𝑙𝑏
𝑙𝑏−𝑚𝑜𝑙
= 0.015 𝑆𝐶𝐹𝑀 𝑁𝑂𝑥 𝑎𝑠𝑁𝑂2
• With the same assumed oxidizer exhaust flow of 10,360 SCFM the expected NOx concentration is 1.45 ppmv
uncorrected.
NO2 molecular
weight of
40.0055
Mass flow can be
easily converted
to a volumetric
flow
Oxidizer
exhaust flow of
10,360 SCFM
Expected NOx
concentration
is 1.45 ppmv

25. The reverse procedure can be used to easily verify burner
performance during emissions testing.
Measured exhaust concentrations can be converted to mass flow
rates and divided by the known firing rate to easily verify burner
emissions in terms of lb/MMBTU.
The results would be a much better representation of the actual
burner performance.
What does this mean?

26. Methods to Control NOx
Emissions

27. How can NOx
emissions be
controlled?
• Controlling the combustion at the burner:
• Ultra low NOx burners
• Over fire air
• Air staging
• Fuel staging
• Water of steam injection
• Controls external to the combustion zone:
• Selective catalytic reduction (SCR) uses a catalyst to react injected ammonia to
chemically reduce NOx in higher temperature zones
• Selective non-catalytic reduction (SNCR) of NOx by injecting ammonia in lower
temperature zones, with lower NOx removal efficiencies
• A combination of two or more technologies

28. Conifer Standards for
NOx Reduction
• Industry standard low-NOx rated
burner and controls
• Nozzle mix design that can run on
multiple types of available fuel gases
• The supplemental fuel injection (SFI)
system
• Transfers management of combustion
chamber temperature from the burner(s)
to separate fuel line located at the RTO
inlet
• Lower burner(s) demand, results in
smaller “hot spot” around the burner flame
shrinks to reduce NOx emissions