Summary Report for USS 

On

NOx reduction for Kilns #6 and 7

S. Londerville 10/26/11

Background:

Coen Co. has been actively involved in supplying burners for reduced NOx to U.S.S. working 

with U.S.S. consultants (Hatch).

The NOx reduction is difficult due to secondary air preheat of over 2100 deg F at over 300% 

excess air (EA). Both EA and high temperature contribute to high thermal NOx formation. 

Further, high excess air contributes to higher fuel bound nitrogen conversion in the coal fuel. The 

main source of NOx in the system is thermal NOx and partial contribution from fuel bound 

nitrogen in the coal. The kilns are used for the ore induring process requiring high radiant heat 

transfer with pellet exit temperatures of @ 2265 Deg F. Two kilns (line 6 and 7) have been 

retrofitted to reduce NOx by adding additional cold dilution combustion air. The addition of cold 

combustion air will negatively impact kiln efficiency. Each kiln produces about 9400 Long 

tons/day of product. 

The line 7 burner is arranged with coal on the outside part of the burner and cold combustion air 

on the inside. The line 6 burner is arranged with cold combustion air on the outside part of the 

burner and coal on the inside.

In both cases natural gas or oil is introduced in a center fire injectors/atomizers and both units 

have high, over 2100 Deg F secondary air introduced outside of the burner via the hood from the 

cooler. 

Normally for kilns, the fuel is introduced on the axial centerline with a minimal amount of cold 

combustion air for optimal performance. When the secondary air is not excessively high, typical 

NOx reduction techniques used are various forms of staging. Staging is accomplished by

reducing the primary burner air to lengthen the flame, increase luminosity and reduce the 

entrainment of hot secondary air while the flame decreases in temperature, thus limiting peak 

flame temperatures and NOx. In the case of extremely high secondary air, staging cannot be used 

because the peak temperatures and NOx will increase with reduced primary air.

NOx Objectives:

The proposed limit is 180# NO2/hr for all fuels. Natural gas has been able to meet the 180#/hr 

proposed limit with the addition of about 100 % stoichiometric cold combustion air at 200 

MMB. Coal has been quite variable and generally has an average value of 220 to 270 # NO2/hr 

with 240 # NO2 the average at 200 MMB when fired with NG. It is useful to convert the 

proposed limit and achieved limits to help understand what else can be done. The plot below 

compares NOx achieved vs proposed as a function of firing rate. Normally the unit is firing at @ 

200 MMB requiring a NOx emission rate of @0.9 #/MMB to meet 180#/hr.
Fig 1 #NO2/MMB Vs Firing rate

The proposed hourly limit of 180 #/hr will require reduced NOx in # NO2/MMB as a function of 

firing rate. Generally this is not a practical approach as NOx will increase with firing rate. 

Ultimately as the unit efficiency decreases with added primary air, the firing rate must increase 

to maintain production. As a reference point for what to expect as base line we refer the EPA 

document AP-42 (ref5). The NOx emission factor is 1.5 # NO2/ton production.

Using line 6 and 7 normal capacity of 9400 long tons/day this EPA emission factor results in a 

base line emission rate of 646#NO2/hr. The emission factor is rated “D”, meaning “below 

average” and may have variability. This EPA emission factor is equivalent to 3.29# NO2/MMB 

at 200 MMB firing rate. Using the proposed limit and AP-42 base line estimate, line 6 and 7 

would require @ 73% reduction in NOx. Latter in this review we discover the base line on NG 

is actually about 800#/hr or 4# NO2/MMB. Thus, requiring a 78% reduction from baseline. The 

writer would like to review the results to date and provide explanations for these results and 

recommendations forward. But prior to this it may be instructive to review some basic 

background on NOx formation. 

y = -9E-08x3 + 9E-05x2- 0.0285x + 3.8783

R2 = 0.9991

0

0.5

1

1.5

2

2.5

3

100 150 200 250 300 350 400

#

N

O2/M

M

B

MMB Fired

#NO2/MMB vs firing rate

180#/hr target

200#/hr

240#/hr

Poly. (180#/hr target)
Background on NOx Formation, Thermal NOx

Thermal NOx is formed at high temperatures when molecular N2andO2 dissociate and react to 

form NO.

For formation, The Zeldovich equation is used as follows:

 

    eq 2

RT

E

2Ae O N

dt

d NO 















and

 

 

 

0.5

2 eq 0.5

0

eq O

RT

K

O 

One generally accepted practice is to assume O2in equilibrium with O and O2 concentration using 

the Westenburg results for K0see Westenberg, “Turbulence Modeling for CFD.”(1) for O2

equilibrium and Zeldovich constants, A and E as measured by Bowman “Kinetics of Pollution 

Formation and Destruction in Combustion.”(2).

Thermal NOx increases exponentially with temperature as shown in Fig 2 and 3.

Fig2 NOx and CO formation/destruction vs Temperature for premix Nat gas at 10%EA

-5

-4

-3

-2

-1

0

1

2

3

4

5

1400 1900 2400 2900 3400

L

o

g

P

P

M

Deg F

NOx and CO vs time,Temperature at10%EA premixed

CO .01sec

CO .05 sec

CO .1 sec

Nox .01 

sec

Nox .05sec

Nox .1 sec
Fig 3 NOx Emissions as a function of Temperature and Time using ref1 and 2

USS line 6 and 7 generally operate on Coal with NG support. The Secondary air is about 2100 

Deg F. The following chart #4 compares the adiabatic flame temperature of Coal vs natural gas 

vs excess air and combustion air temperature.

Fig 4 Adiabatic Flame temperature vs excess air and air preheat for Coal and NG

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8

#

N

O2/M

M

B

Seconds

#NO2/MMB vs Time and Temperature at 10% EA

3000 Deg F

3200 Deg F

3400 Deg F

2000

2500

3000

3500

4000

4500

5000

0 20 40 60 80 100

Deg F

Excess air

Adiabatic Flame VS Excess air for coal and NG vs 

Air Preheat

NG 80 Deg air

NG 2000 Deg air

Coal 80 Deg air

Coal 2000 Deg air
What is clear is that the thermal component of both Coal and NG have similar adiabatic flame 

temperatures vs excess air. For gaseous fuels we expect similar results of NOx reduction with 

dilution when the flame temperatures are similar. As we will see, solid fuels behave differently

with dilution. The residence time of the high NOx zone in a Kiln flame is on the order of 0.05 to 

0.1 seconds depending on many variables. Given that the NOx target is near .95#/MMB (at 200 

MMB) and by inspection of fig 3 we can determine that a target adiabatic flame temperature 

should be near 3400 Deg F. By inspection of Fig 4, we see that a viable solution is to add 

ambient dilution air to the burner knowing that we cannot change the secondary 2100 Deg F hot 

air. This was the direction taken on both Line 6 and 7 to reduce NOx.

The burners were modified to significantly increase cold primary air to the burner at the expense 

of unit efficiency.

Prompt NOx formation

Another NOx formation mechanism is prompt NOx. This occurs at the flame front and is 

responsible for no more than 20 ppm NOx in most applications. The mechanism can be 

summarized as:

CHx  N2  HCN  CN (not balanced)

HCN  CN+O2  NO  CO  H (not balanced)

Both of these reactions are very fast and do not require high temperature. It would appear that 

one way to reduce NOx from the prompt mechanism would be to dilute the HCN and CN species 

on the fuel side of the combustion zone, or reduce the available oxygen. The use of premix 

combustion is very effective for prompt NOx reduction. The formation of Prompt NOx was not a 

consideration in the NOx reduction program at U.S.S.

Fuel Bound NOx Conversion:

The fuel-bound NOx mechanism is similar to prompt NOx and proceeds through the same HCNCN chemistry. However, the fuel-bound mechanism differs in the following ways.

1. The fuel-bound mechanism requires nitrogen as part of the fuel molecule.

2. At low fuel-nitrogen concentrations, all of the bound nitrogen converts to NOx in gaseous 

fuels.

3. The fuel-bound mechanism can be responsible for hundreds of ppm NOx, depending on the 

amount of nitrogen bound in the fuel. For liquid and solid fuels the conversion ranges from 

80 to 10 % depending on the concentration and excess air in the flame.

The first steps in the chemistry differ in that the intermediates are formed directly from pyrolysis 

of the parent molecule. Ambient nitrogen is unimportant.

CHxNy  HCN  CN (not balanced)
Fuel NOx contribution is a function of % FBN and stoichiometric conditions in the flame zone.

For medium temperature low O2 conditions, Coal FBN conversion is about 25% Ref 3. With 

increasing E.A. this conversion will increase.

For example, Given Coal at HHV=10300 BTU/lbm, 



. 6 

100  

1 

14 

 

1 

1  

 

46 



 

1  

10300  

10 

 

= 01.9



10 

(25%) = .47



10

Evaluation of Line 6 and Line 7 results NG firing:

We will review the results of Natural Gas firing first as it is simpler and clearer to the reader.

A significant amount of data was taken by plant personnel. Figures 5 and 6 is the raw NOx data 

plotted as a function of burner stoichiometry.

Fig 5 raw NOx data from Line 6

y = 823.06x2- 2156.4x + 1551.9

R2 = 0.9723

150.00

200.00

250.00

300.00

350.00

400.00

450.00

500.00

550.00

600.00

0.55 0.75 0.95 1.15

#/hr N

Ox

Stoc air

Line 6 raw data Nox Emmissions (lbs/hr) 185 MMB Avg NG

Line 6 Nox Emmissions 

(lbs/hr)

Poly. (Line 6 Nox Emmissions 

(lbs/hr))
Fig 6 Raw NOx data from line 7

We can clearly see a lot of scatter in the data. This could be real or not. Plotted is a best fit curve 

for both line 6 and 7 indicating significant NOx reduction with added dilution air. We compare 

the best fit and re-plot in figure 7. We see that both lines perform similar with line 6 achieving 

@180 #/hr at 1.1 stoc and line 7 at 1.05 stoc. Figure 8 is a comparison in # NO2/MMB. We 

observe line 7 has slightly better performance at utilization of dilution air to reduce NOx 

although considered insignificant by the writer. When firing NG, Line 7 has a slight advantage to 

reduce thermal NOx. Originally we thought line 6 may have an advantage due to cool air on the 

outside. However with the data scatter and so many variables, it is difficult to draw any concrete 

conclusions.

y = 570.87x2- 1760.5x + 1394

R2 = 0.8611

100.00

120.00

140.00

160.00

180.00

200.00

220.00

240.00

260.00

280.00

300.00

0.90 0.95 1.00 1.05 1.10 1.15 1.20

#

N

O2/hr

Stoc air

LINE 7 Raw data NOX (lbs/hr) 192 MMb avg NG with 5KPPH atm air

LINE 7 NOX (lbs/hr)

Poly. (LINE 7 NOX (lbs/hr))
Fig 7 Comparison of line 6 and 7 NG firing in #/hr

Fig 8 Comparison of line 6 and 7 firing NG in # NO2/MMB

y = 823.06x2- 2156.4x + 1551.9

R2 = 1

y = 570.87x2- 1760.5x + 1394

R2 = 1

100

150

200

250

300

350

400

450

500

0.5 0.7 0.9 1.1

#/hr N

Ox

Stoc air

#/hr NOx vs Stoc air line 6 185 MMB up with air/ line 7 192 

MMB down with air

#/hr NOx line 6

#/hr NOx line 7

Poly. (#/hr NOx line 6)

Poly. (#/hr NOx line 7)

y = 4.2102x2- 11.298x + 8.2707

R2 = 1

y = 2.6851x2- 8.5287x + 6.9029

R2 = 1

0.5

1

1.5

2

2.5

3

0.7 0.8 0.9 1 1.1 1.2

#

N

O2/M

M

B

Stoc air

#NOx/MMB line 6 vs 7 NG only @ 190 MMB

line 6 #/MMB

Line 7#/MMB

Poly. (line 6 #/MMB)

Poly. (Line 7#/MMB)
Figure 9 below is an extension of the NG data to provide some credibility to the reported base 

line data in the EPA document AP-42. We see the AP-42 factor of 658# NO2/hr would 

correspond to a burner stoc air of @ 0.5. Typically we would expect stoc air with NG to be @ 

0.4 corresponding to 800# NO2/hr baseline. But we need to recall the AP-42 NOx factor has a 

“D” rating. So the expected base line is closer to 800# NO2/hr or 4 # NO2/MMB.

Fig 9 Extension of line 6 and 7 data for clarity and support of EPA- AP-42 NOx factor 

Background on Coal Combustion:

The oxidation mechanism for coal involves heat up (fast) to devolitization and final char burnout 

(longer). These mechanisms define the flame volume and the emissions. However, coal fuels 

exhibit an unusual flammability characteristics not found in other liquid or gaseous fuels. The 

rich limit of flammability is almost infinite for solid fuel/air mixtures. This is due to the fact that 

all of the air is present but much of the fuel still exists as un-reacted char or volatiles. The air 

simply does not see all the fuel in the first part of the flame. Also the range of local 

stoichiometric mixtures near and around the solid particle is wide ranged from high excess air to 

near zero when reacting. The situation can be quantified by the measurements of Smoot, Horton 

and Willams, 1976 ref 6 plotted in figure10. The result, coal stability is not tolerant of high 

excess air and has almost no rich limit. The tests indicate coal firing can experience reduced 

flame speeds and possible instabilities at excess air above 20% or so.

A further difference from gaseous fuels is the time required for char oxidation. See Fig 11.

The char particle takes significant time for oxidation.

Based on the fact that the char surface is oxidizing at near stoc conditions and oxidize over 

significant time, coal particles do not react to dilution like gaseous fuels.

0

200

400

600

800

1000

1200

0.2 0.4 0.6 0.8 1 1.2

#

N

O2/hr

Stoc air

Extrapolated Line 6 and 7 NOx NG Data

NOx line6

NOx line 7
Fig 10 Coal flame velocity (related to flammability) VS E.R.= (F/A) 0%EA=1, 100%EA=.5

Figure 11 Sub bituminous Char Burnout Coen code A=60 and E= 17,150

Coal Dust Velocity vs Equivalence Ratio Stoichiometric

(F/A)mass

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12

Equivalence Ratio

cm/sec

cm/sec

Burnout for Subbituminous Coal Char at 15%EA

0

10

20

30

40

50

60

70

80

90

100

0 0.2 0.4 0.6 0.8 1

Seconds

P

erc

e

nt b

urn

o

ut

100 Micron

150 Micron

200 Micron
Evaluation of Line 6 and Line 7 results Coal firing:

Figures 12 and 13 summarize the raw NOx data taken from line 6 and 7.

In fig 12, Line 6, the burner coal nozzle is on the inside and stabilized by NG and a small amount 

of atomizing air near the gas injectors at high swirl. Although NOx appears to decrease with 

increasing air, the amount is minor and only about 270 to 240 #/hr. At stoc levels @.55 we see 

large variations in NOx. The reason is unknown, but the writer speculates that at low air rates 

the atomizing air spinner alternately stabilizes coal or NG with a precessing vortex core. It could 

also be due to measurement issues that the writer is not aware of. Result, over the 50% EA range, 

the NOx indicates a slight decrease of maybe 30#.hr or @12%. Completely insignificant 

compared to NG. The variability +/- @ 20 #/hr could be real or not. 

In fig 13, Line 7 the burner nozzle is on the outer part of the burner and stabilized by hot 

secondary air. NOx again appears to decrease from 250#/hr to 225 #/hr or so. Result, over 50% 

EA range the NOx seems to decrease @25#/hr or @ 10%. Again insignificant compared to NG. 

The variability is about the same as line 6 +/- 20#/hr and could be real or not.

Both units indicate little change with EA. Figures 14 and 15 show NOx plotted as #/MMB.

Line 6 indicates a slight decrease from 1.35 to 1.2# NO2/MMB or maybe 12%. Line 7 indicates 

a minimum at .75 stoc. We conclude that large changes in excess air has little impact on coal 

firing dilution and NOx above .5 stoc. 

Fig 12 Line 6 NO2 #/hr vs Stoc air in burner

y = 2203.6x3- 5496.3x2 + 4356.1x - 828.75

R2 = 0.2333

200.00

220.00

240.00

260.00

280.00

300.00

320.00

340.00

0.50 0.60 0.70 0.80 0.90 1.00

#

N

O2/hr

Stoc air

Line 6 Raw Data NOx Emmissions (lbs/hr)@100MMB Coal 95 MMB NG 

with 3KPPH Atm air

Line 6 Nox Emmissions 

(lbs/hr)

Poly. (Line 6 Nox 

Emmissions (lbs/hr))
Fig 13 Line 7 Raw NO2 #/hr vs Stoc in burner

y = -2578.9x3 + 6096.1x2- 4769.7x + 1467.7

R2 = 0.236

190.00

200.00

210.00

220.00

230.00

240.00

250.00

260.00

270.00

0.50 0.60 0.70 0.80 0.90 1.00

#/hr N

Ox

Stoc air

LINE 7 Raw Data NOX (lbs/hr) @ 130 MMB coal 30 to 50 MMB NG No Atm air

LINE 7 NOX (lbs/hr)

Poly. (LINE 7 NOX (lbs/hr))
Fig 14 Line 6 Plotted in # NO2/MMB

Fig 15 Line 7 Plotted in #NO2/MMB

We will explore the fate of coal fuel bound nitrogen (FBN) to determine the impact on NOx and 

if this explains the slight decrease in NOx with added air and what impact FBN has at all. Figure 

16 shows FBN conversion for coal under medium thermal environments vs stoc air. Figure 17 

shows the #/MMB NOx contribution vs stoc air for the FBN. 

y = 6.7923x3- 17.127x2 + 13.508x -

2.0019

R2 = 0.5304

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

0.50 0.60 0.70 0.80 0.90 1.00

#

N

O

X/M

M

B

Stoc air

# NO2/MMB Coal+NG line6 100MMB coal+95MMB NG

# NO2/MMB

Poly. (# NO2/MMB)

y = 2.6773x2- 4.0041x + 2.7852

R2 = 0.3742

1.10

1.20

1.30

1.40

1.50

1.60

0.50 0.60 0.70 0.80 0.90 1.00

#

N

O2/M

M

B

Stoc air

Line 7# NO2/MMB 130 MMB coal+30 to 50 MMB NG

# NO2/MMB

Poly. (# NO2/MMB)
Fig 16 FBN conversation vs stoc air medium thermal environments

Fig 17 #NO2/MMB vs stoc air from fig 16

Fig 18 Converts the #NO2/MMB to #NO2/hr vs stoc air and coal firing rate. Our intent is to take 

the curve fit data and subtract out the FBN to determine what the thermal NOx is doing.

y = -8.0153x3 + 22.977x2 + 1.4094x + 9.1694

R2 = 0.9969

0

5

10

15

20

25

30

35

40

45

0 0.5 1 1.5 2 2.5

% converstion

Stoc air

FBN Converstion 0.6% FBN Coal No Air Preheat

FBN Converstion

Poly. (FBN 

Converstion)

y = -0.1467x3 + 0.4204x2 + 0.0258x + 

0.1678

R2 = 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5

#

N

O2/M

M

B fro

m FB

N

Stoc air

#NO2/MMB 0.6% FBN Coal No Air Preheat

#NO2/MMB

Poly. (#NO2/MMB)
Fig 18 FBN contribution vs stoc air and firing rate

Figure 19 and 20 are charts corrected for FBN contribution. We plot the average curve fitted 

NOx obtained from Figs 12 and 13. We also plot the FBN contribution from fig 18.

The contribution in line 6 is more since the coal is under medium thermal environments. Line 7 

has the coal on the outside and the FBN conversion is corrected for the hot thermal 

environments. We then subtract the FBN contribution from the total measured NOx. The results 

are plotted over stoc 0.5 to 1.0. We discover that the FBN contribution is small but significant 

relative to the proposed limit of 180#/hr. We further note that the thermal NOx emission at stoc 

=1.0 is about what the NG emission is, @ 180 #/hr. Thus the difference in coal fired NOx vs NG 

fired NOx is directly related to the FBN contribution.

y = -11x3 + 31.533x2 + 1.9342x + 12.584

R2 = 1

y = -14.666x3 + 42.043x2 + 2.5789x + 16.778

R2 = 1

y = -19.066x3 + 54.656x2 + 3.3526x + 21.812

R2 = 1

y = -24.933x3 + 71.474x2 + 4.3842x + 28.523

R2 = 1

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

#

N

O2/hr

Stoc air

FBN Contribution vs Stoc and firing rate 0.6% FBN Coal no APH

75MMB

100MMB

130MMB

170MMB

Poly. (75MMB)

Poly. (100MMB)

Poly. (130MMB)

Poly. (170MMB)
Fig 19 Line 6 estimates of thermal and FBN contribution

Fig 20 Line 7 estimates of thermal and FBN contribution

0

50

100

150

200

250

300

0.5 0.6 0.7 0.8 0.9 1

#

N

O2/hr

Stoc air

Line 6 100MMB Coal 95 MMB NG with 3KPPH atm air

total NOx

FBN contribution

Thermal NOx

0

50

100

150

200

250

300

0.5 0.6 0.7 0.8 0.9 1

#

N

O2/hr

Stoc air

Line7 130 MMB Coal 30 to 50 MMB NG

total NOx

FBN contribution

Thermal NOx
Conclusions:

Both Line 6 and line 7 respond about the same with good NOx reduction on natural gas vs stoc 

air in the burner down to 180 # NO2/hr (@ .9#NO2/MMB) at 200 MMB using @ 100% 

combustion air in the burner.

This will have a significant impact on unit efficiency, but this has not been addressed.

For the same air levels the coal thermal NOx is about the same as NG and the total is higher 

directly due to the FBN conversion as shown in Fig 19 and 20.

Due to stability issues with coal at high excess air it is not recommended to increase air to the 

burner further than stoichiometric knowing the secondary air is 300% EA plus.

This was also observed during testing, coal instability near 100% combustion air in burner and as 

illustrated in fig 10.

Recommended NG Emissions

As discussed, an hourly limit of 180#/hr for NG requires a 73 to 78% reduction at 200 MMB 

firing rate. The reduction requirements will be higher if the load need to increase. See Fig 1.

This limit already requires 100% combustion air to the burner. It is recommended to limit the 

NOx on NG to 1.0#/MMB as opposed to an hourly limit. This will correspond to a 75% 

reduction over baseline and allow for variations in measurements as indicated in figs5 and 6. 

Further this limit will allow load changes as unit efficiency is decreased.

Recommended Coal Emissions

Coal is fired with NG and as shown the FBN contribution is significant. Coal fired results have 

greater variability than NG fired as expected. Adding dilution air increases the FBN contribution.

From figures 12 -15 it recommend that the NOx limit be set at 1.5#NO2/MMB. Anything lower 

will result in non compliance due to variability, limit load and cause flame instabilities. 

 
References:

1. Westenberg, A. E. “Turbulance Modeling for CFD.” Combustion Science and Technology 4 (1971):59-67

2. Bowman, C. T. “Kinetics of Pollution Formation and Destruction in Combustion.” Progress in Energy and 

Combustion Science 1 (1975): 33- 45

3. Application of NOx Control technologys to Power Boilers, A. Rawdon and S. Johnson

4. Coen Internal Documents

5. EPA document AP42 Section 11.23 Feb 1997 Table 11.23-4

6. Smoot L.D., Horton M.D. and Williams, G.A. “Propagation of Laminar Pulverized Coal-Air Flames” 16th

Symposium International on Combustion, The Combustion Institute, Pittsburgh, PA. 375 (1976)