## Summary Report for USS On NOx reduction for Kilns #6 and 7 S. Londerville 10/26/11

**Back ground:**  Coen Co. has been actively involved in supply ing 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 h igh thermal NOx formation. Further, h ig h e xcess air contributes to higher fuel bound nitrogen conversion in th e coal fuel. The main source of NOx in the sy stem 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 L ong tons/day of product. The lin e 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 i ntroduced 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 amoun t of cold combustion air for optimal performance. When the secondary air is not excessively

high, t ypical 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. I n the case of extremely high secondary

air, staging cannot be used because the peak temperatures and NOx w ill 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 generall y has an average value o f 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 firin g rate. Normall y 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 hourl y 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 capacit y of 9400 long tons/day th is 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 propose d 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
- 08x 3 + 9E
- 05x 2

- 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** **O 2** **/ M** **M B** 

**MMB Fired** 



### #NO2/MMB vs firing rate

180#/hr target 200#/hr 240#/hr Poly. (180#/hr target)

### Back **ground on NOx Formation , Thermal NOx** 

Thermal NOx is formed at high temperatures when molecular 2 

### N and 2 

### O dissociate and react to form NO. For formation, The Zeldovich equation is used as follows: 

###       2 eq RT E N O Ae 2 dt NO d 

    

  

### and 

###       5
. 0 eq 2 5
. 0 0 eq O RT K O  

### One generall y accepted practice is to assume 2 O 

### in equilibrium with O and 2 O 

### concentration using the Westenburg results for 0 K 

### see Westenberg, “Turbulence Modeling for CFD.”(1) for 2 O 

### equilibrium and Zeldovich constants, A and E as measured b y Bowman “Kinetics of Pollution Formation and Destruction in Combustion.”(2). Thermal NOx increases exponentially with temperature as shown in Fig 2 and 3.

Fig 2 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 v **s time,T**  **emperature at10% EA** 

**p remixed** 

CO .01sec CO .05 sec CO .1 sec Nox .01 sec Nox .05sec Nox .1 secFig 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 2 1 00 Deg F. The following chart #4 compar es 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** **O 2** **/ M** **M B** 

**Seconds** 

### #NO2/MMB vs Time and T **emper ature**  **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**D e** **g F** 

**Excess air** 

### Adiabatic **Flame VS Ex cess 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 b y 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 L ine 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: C H x  N 2  H C N  C N ( n o t b a l a n c e d ) H C N  C N + O 2  N O  C O  H ( n o t b a l a n c e d ) 

### Both of these reactions are very f ast and do not require high temperature. It w ould 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 oxy gen. The use of premix combustion is very

effective for prompt NOx reduction. The formation of Prompt NOx was not a consideration in the NOx redu ction program at U.S.S. 

### Fuel Bound NOx Conversion:
The fuel
- bound NOx mechanism is similar to prompt NOx and proceeds through the same HCN
CN chemistry . However, the fuel
- bound mechanism differs in the following way s. 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 nitroge n 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 py roly sis of the parent molecule. Ambient nitrogen is unimportant. C H x 

### N y 

###  H C N  C N ( n o t b a l a n c e d )

### Fuel N O x contribution is a function of % FB N and sto ichiometric conditions in the flame zone.

For medium temperature low O2 conditions, Coal FBN conversion is about 25% Ref 3. W ith increasing E.A. this conversion will increase. For example, Given Coal at HHV=10300 BTU/lb m 

### ,

. 6 100 1

14 1

1 46 1 10 300 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 = 823.06x 2

- 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**# /** **h r** **N O** **x**

**Stoc air** 

### Line 6 r **aw data Nox Emmissions**  **(lbs/hr) 185 MMB A vg NG** 

Line 6 Nox Emmissions (lbs/hr) Poly. (Line 6 Nox Emmissions (lbs/hr))Fig 6 Raw NOx data from line 7



### We can clearl y 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 line s 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 b y the writer. When firing NG, Li ne 7 has a slight advantage to reduce thermal NOx. Originall y 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 an y concrete conclusions. y = 570.87x 2

- 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** **O 2** **/ h** **r**

**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 = 823.06x 2

- 2156.4x + 1551.9

R2 = 1

y = 570.87x 2

- 1760.5x + 1394

R2 = 1 100 150 200 250 300 350 400 450 500

0.5 0.7 0.9 1.1**# /** **h r** **N O** **x**

**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 = 4.2102x 2

- 11.298x + 8.2707

R2 = 1

y = 2.6851x 2

- 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** **O 2** **/ 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. Ty pically we would expect stoc air with NG to be @ 0.4 corresponding to 800# NO2/h r 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



### Back **ground 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 exce ss 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 ox idize 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** **O 2** **/ h** **r**

**Stoc air** 

### Extr **apolated Line 6 and 7 NOx NG Data** 

NOx line6 NOx line 7Fig 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

**Coa l D**  **ust**

**V el**  **oci ty**  **v s**  **Equi val**  **ence Rat**  **io S**  **to ic**  **hi om**  **et ric** 

**(F /A)ma**  **ss**

0 5 10 15 20 25 30 35

0 2 4 6 8 10 12 **Eq uiv**  **al en**  **ce R**  **at io** **c m** **/ s** **e c**  cm/se c



### Bur **nout fo**  **r S ubbi**  **tu min**  **ous Coal**  **C har**  **at 1**  **5% EA** 

0 10 20 30 40 50 60 70 80 90 100

0 0. 2 0. 4 0. 6 0. 8 1 **Se co**  **nds P** **e r** **c e** **n t** **b u** **r n** **o u** **t** 100 Micr on 150 Micr on 200 Micr on

### 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, Li ne 6, the burner coal nozzle is on the inside and stabilized by NG and a small amount of atomizing air near the gas inj ectors 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 alternatel y 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 may be 30#.hr or @12%. C ompletel y insignificant compared to NG. The variability +/
- @ 20 #/hr could be real or not. In fig 13, Li ne 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 #/h r or so. Result, over 50% EA range the NOx seems to decrease @25#/hr or @ 10%. Again insignificant compared to NG. The variabilit y 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%. L ine 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 Li ne 6 NO2 #/hr vs Stoc air in burner y = 2203.6x 3

- 5496.3x 2 + 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** **O 2** **/ h** **r**

**Stoc air** 

### Line 6 Raw Data NOx Emmissions (lbs/hr)@100MMB **Coal 95 MMB NG with**  **3KPPH A**

**tm air**

Line 6 Nox Emmissions (lbs/hr) Poly. (Line 6 Nox Emmissions (lbs/hr))

### F ig 13 Li ne 7 Raw NO2 #/hr vs Stoc in burner y = - 2578.9x 3 + 6096.1x 2

- 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**# /** **h r** **N O** **x**

**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 Li ne 6 Plotted in # NO2/MMB

Fig 15 Li ne 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 environmen ts vs stoc air. Figure 17 shows the #/MMB NOx contribution vs stoc air for the FBN. y = 6.7923x 3

- 17.127x 2 + 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 = 2.6773x 2

- 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** **O 2** **/ 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 = - 8.0153x 3 + 22.977x 2 + 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**% c** **o n** **v e** **r s** **t i** **o n** 

**Stoc air** 

### FBN **Converstion 0.6% FBN Coal No Air Preheat** 

FBN Converstion Poly. (FBN Converstion)

y = - 0.1467x 3 + 0.4204x 2 + 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** **O 2** **/ M** **M B** **f r** **o m** **F B** **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 plo t 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. L ine 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 1 80#/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 = - 11x 3 + 31.533x 2 + 1.9342x + 12.584

R2 = 1 y = - 14.666x 3 + 42.043x 2 + 2.5789x + 16.778

R2 = 1 y = - 19.066x 3 + 54.656x 2 + 3.3526x + 21.812

R2 = 1 y = - 24.933x 3 + 71.474x 2 + 4.3842x + 28.523

R2 = 1

0 20 40 60 80 100 120 140

0 0.5 1 1.5 2 2.5**# N** **O 2** **/ h** **r**

**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 Li ne 6 estimates of therm al and FBN contribution

Fig 20 Li ne 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** **O 2** **/ h** **r**

**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** **O 2** **/ h** **r**

**Stoc air** 

### Line7 130 MMB Coal 30 to 50 MMB NG

total NOx FBN contribution Thermal NOx

### Conclusions:

Both L ine 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 hourl y l imit 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 va riations 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 gr eater variabilit y 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. Any thing lower will result in non compliance due to variability , limit load and caus e 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 Com bustion Science 1 (1975): 33
- 45 3. Application of NOx Control technologys to Pow er Boilers, A. Ra wd on and S. Johnson 4. Coen Internal Documents 5. EPA document AP42 Section 11.23 Feb 1997 Table 11.23
- 4 6. Sm oot L.D., Horton M.D. and Williams, G.A. “Propagation of Laminar Pulverized Coal
- Air Flames” 16 th Symposium International on Combustion, The Combustion Institute, Pittsburgh, PA. 375 (1976)