Background: 

Summary Report for USS 

On 

NOx reduction for Kilns #6 and 7 

S. Londerville 10/26/11 

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 Fat 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 ofNOx 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 

entraimnent 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# N02/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 of220 to 270 # N02/hr 

with 240 # N02 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. 
3 

2.5 

2 al 

~ 

~ ......... 1.5 N 

0 

z 

# 1 

0.5 

0 

#N02/MMB vs firing rate 

!Ill lllJ II I II 

~~ ' ',

1 llillJ I lllL ll[ I ~~; ~: ,,~~ /IIIII 11111111 

I II II ~ ~ m±-1- c- -c-c1 1 rr-11 I , I I . -~ ... I""' 

I I I I I 

I'"" 

x3 + 9E-05x2 - 0.0285x + 3.8783 

R2 = 0.9991 

- 180#/hr target 

- 200#/hr 

- 240#/hr 

--Poly. (180#/hr target) 

100 150 200 250 

MMB Fired 

300 350 400 

Fig I #N02/MMB Vs Firing rate 

The proposed hourly limit of 180 #/hr will require reduced NOx in # N02/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 (refS). The NOx emission factor is 1.5 # N02/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#N02/hr. The emission factor is rated "D", meaning "below 

average" and may have variability. This EPA emission factor is equivalent to 3.29# N02/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# N02/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 f01mation. 
Background on NOx Formation, Thermal NOx 

Thermal NOx is formed at high temperatures when molecular N 2 and 0 2 dissociate and react to 

form NO. 

For formation, The Zeldovich equation is used as follows: 

and 

d(NO) = 2AJ- :T Jro) (N ) dt \ eq 2 

( ) Ko ( )0.5 

0 eq = (RT)0.5 02 eq 

One generally accepted practice is to assume o2 in equilibrium with 0 and 0 2 concentration using 

the Westenburg results for K0 see Westenberg, "Turbulence Modeling for CFD."(l) foro2 

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

Formation and Destruction in Combustion."(2). 

The1mal NOx increases exponentially with temperature as shown in Fig 2 and 3. 

NOx and CO vs time, Temperature at10°/oEA premixed 

5 

4 "'-. ~ I\ -+-CO .01sec 

3 ~ \ W""" H 2 1\ 

1 \ '\ Ar v v ----- CO .05 sec 

v ~ 

~v v 

a. 0 1\ -V 

a. CO .1 sec 

r- \ v _. 1 

+- ___! ~ Nox .01 -2 1\ sec 

-3 \ ::1( Nox .05sec 

-4 

-5 --.-Nox .1 sec 

1400 1900 2400 2900 3400 

Deg F 

Fig2 NOx and CO formation/destruction vs Temperature for premix Nat gas at 10%EA 
#N02/MMB vs Time and Temperature at 10% EA 

3.5 

3 

2.5 

al 

:!: 2 :!: 

........ 

N - 3000 DegF 

0 1.5 z 

'"' - 3200 DegF 

1 - 3400 Deg F 

0.5 

0 

0 0.2 0.4 0.6 0.8 

Seconds 

Fig 3 NOx Emissions as a function of Temperature and Time using refl 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. 

u.. 

bO 

QJ 

c 

Adiabatic Flame VS Excess air for coal and NG vs 

Air Preheat 

5000 

4500 

4000 

- NG 80 Deg air 

3500 - NG 2000 Deg air 

3000 - coal 80 Deg air 

2500 - coal 2000 Deg air 

2000 

0 20 40 60 80 100 

Excess air 

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

CH +N,-+ HCN BCN X - (not balanced) 

HCN B CN+02 -+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 fi·om 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 fornmtion 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 dit1er in that the intermediates are formed directly from pyrolysis 

of the parent molecule. Ambient nitrogen is unimportant. 

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

For medium temperature low 02 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 lbmN ) (1lbmoleN) (1lbmoleNOz) (46 lbmNOz) ( 1lbm Fuel) ( 106 BTU ) 

100 lbmFuel 14 lbmN 1lbmole N lbmoleN02 10300 BTU million BTU 

lbmNOz lbm 

= Ol.9 106 BTU (ZS%) = '

47 106 BTU 

Evaluation of Line 6 and Line 7 results NG firing: 

We will review the results ofNatural 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. 

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

600.00 

550.00 

500.00 

450.00 X 

~ 400.00 ... :E. 350.00 # 

300.00 

'i).---t--t--t--t--t--t--1---l ~ Line 6 Nox Em missions 

(lbs/hr) 

250.00 +-+-+-+-+-+-+-+--+---+--'111~ H--+-+-+---i Poly. (Line 6 Nox Em missions 

200.00 (lbs/hr)) 

150.00 

0.55 0.75 0.95 1.15 

Stoc air 

Fig 5 raw NOx data fi·om Line 6 
LINE 7 Raw data NOX (lbs/hr} 192 MMb avg NG with SKPPH atm air 

300.00 .-------.-------.-------,-------;------;r-------, 

260.00 ~lr-4~ +----:----+---v-::-~~1x.?. ... ~-<1760.5x + 1394 

R2 = d.8611 

160.00 

140.00 

120.00 

LINE 7 NOX (lbs/hr) 

-Poly. {LINE 7 NOX (lbs/hr)) 

100.00 +----t----1-----i-----+----ir------i 

0.90 0.95 1.00 

Fig 6 Raw NOx data fi·om line 7 

1.05 

Stoc air 

1.10 1.15 1.20 

We can clearly see a lot of scatter in the data. Tltis 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# N02/MMB. We 

observe line 7 has slightly better perfonnance 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. 
500 

450 

400 

X 350 

0 

~ 300 ..c 

........ 

# 250 

200 

150 

100 

#/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) 

0.5 0.7 0.9 1.1 

Stoc air 

Fig 7 Comparison of line 6 and 7 NO firing in #/ill: 

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

3 

2.5 

lXI 

- line 6 #/MMB :i: 2 :i: 

- Line 7#/MMB ........ 

N 

0 1.5 

z 

--Poly. (line 6 #/MMB) # 

1 

--Poly. (Line 7#/MMB) 

0.5 

0.7 0.8 0.9 1 1.1 1.2 

y = 2.6851x2 - 8.5287x + 6.9029 

R2 = 1 

Stoc air 

Fig 8 Comparison of line 6 and 7 firing NG in # N02/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# N02/hr would 

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

0.4 cmresponding to 800# N02/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# N02/hr or 4 # N02/MMB. 

Extrapolated Line 6 and 7 NOx NG Data 

1200 

1000 

.... 800 ..r. 

-

N 600 0 

z 

# 400 

200 

0 

I I I I I I I 

~ I I I I I ~ '"I I I I 

""''III 

~ ....... I 

I ....... i'k 

I I I ~ 

- NOxline6 

- NOxline7 

0.2 0.4 0.6 0.8 1 1.2 

Stoc air 

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

Background on Coal Combustion: 

The oxidation mechanism for coal involves heat up (fast) to devolitization and fmal 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 pa1i 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, Hmion 

and Willams, 1976 ref6 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. 
35 

30 

25 

() 

<II 

1/) 

-

20 

E 

() 15 

10 

5 

0 

0 

Coal Dust Velocity vs Equivalence Ratio Stoichiometric 

(F/A)mass 

2 4 6 8 10 12 

Equivalence Ratio 

[--.-. em/sec [ 

Fig 10 Coal flame velocity (related to flammability) VS E.R.= (F/A) O%EA= l, 100%EA=.5 

Burnout for Subbituminous Coal Char at 15% EA 

100 ~ H1 I u J e ' 

/ ~ ...- 90 ,.,. 

'I )~ I' 1., :;. 

80 i' I 70 .... v . 

:I 

0 

!: 60 ... - 100 Micron I :I 

.0 50 - 150 Micron .... I !: 

<II 40 1! - 200 Micron (.) - ... 

(I) 

ll. 30 -

20 ~ 10 I 

o, I 

0 0.2 0.4 0.6 0.8 1 

Seconds 

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

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# N02/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. 

Line 6 Raw Data NOx Emmissions (lbs/hr}@lOOMMB Coal 95 MMB NG 

with 3KPPH Atm air 

340.00 

320.00 

300.00 

l: 280.00 ......... 

N 

0 

z 

# 260.00 

240.00 

220.00 

0.50 

~~· t---1"'-'--'1~&- l-----:--- --::L...o.___ Line 6 Nox Emmissions 

(lbs/hr) 

111---+-:llk----+-- -------+~Orl:.llll----l-----=r----Po ly. (Line 6 Nox 

Emmissions (lbs/hr)) 

0.60 0.70 0.80 

Stoc air 

o_go 1.00 

Fig 12 Line 6 N02 #/hr vs Stoc air in burner 
270.00 

260.00 

250.00 

240.00 

)( 

0 

:: 230.00 ..c 

~ 

220.00 

210.00 

200.00 

190.00 

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

Jl = -25lji78.9J I + 6~ 6.1x 4769.7x + 1467.7 

+--i--11-.,...._.--:-- -:-----+---+---i--~"' = 0.236 

I i\ ~ I ~ II I ... I I -,........4 

~ ""~ """ ' I [ LINE 7 NOX (lbs/hr) 

1~ - Poly. (LINE 7 NOX (lbs/hr)) 

<J1 ~ i/ ~ 

I 

L ~ 

0.50 0.60 0.70 0.80 0.90 1.00 

Stoc air 

Fig 13 Line 7 Raw N02 #/hr vs Stoc in burner 
# N02/MMB Coai+NG line6 lOOMMB coai+95MMB NG 

1.60 .-------.-----.-- ----.----,-- ---, 

1·5° -r--t'l-Hl---l--f--+---+---Y-t- 6.7923x3 -17.127x2 + 13.508x1.40 2.0019 

~ R2 = 0.5304 ~ 1.30 

-

~ 1.20 

z 

# 1.10 

1.00 +---+--+-- --+---+--- ----1 

0.90 +---+----1----+--+------i 

0.50 0.60 0.70 0.80 0.90 1.00 

Stoc air 

Fig 14 Line 6 Plotted in# N02/MMB 

- #N02/MMB 

-Poly. (# N02/MMB) 

Line 7# N02/MMB 130 MMB coal+30 to SO MMB NG 

1.60 

I 

y = 2. 73x2 - .0041x + 2.7852 

1.50 +----A.'---+----- ----t----A-~)..1.3742 

al 

~ 1.40 

-

N 

~ 1.30 +---:-+-FtJ-M;;;I;;;!i # 

1.10 

0.50 0.60 0.70 0.80 0.90 1.00 

Stoc air 

Fig 15 Line 7 Plotted in #N02/MMB 

- #N02/MMB 

--Poly.(# N02/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. 
FBN Converstion 0.6% FBN Coal No Air Preheat 

45 ~~·~~~~~~~n 

40 

5 30 

·.;::; 

~ 25 (I) 

~ 20 0 

u '?R 15 -!--+-!--+-+::.~: 

10 

5 ~-++-+--+-+-+--

-t-'-L....L..!.-+-"--"--'--'-t-'--'-'-~---'-'-...L.f-L...L....l..--'--j 

0 0.5 1 1.5 2 2.5 

Stoc air 

- FBN Converstion 

- Poly. (FBN 

Converstion) 

Fig 16 FBN conversation vs stoc air medium the1mal environments 

#N02/MMB 0.6% FBN Coal No Air Preheat 

0.7 

z 

[f 0. 6 +-+ -+-+-+-: 

E .g 0.5 

~ 0.4 

~ 0.3 +-+--H -+ ~ N 

~ 0. 2 +-+--.l.,.-1-H-+-+-H-+-+-H-+-+-' 

- #N02/MMB 

- Poly. (#N02/MMB) 

:It 

0.1 

~~~ ~~~~ 

0 0.5 1 1.5 2 2.5 

Stoc air 

Fig 17 #N02/MMB vs stoc air from fig 16 

Fig 18 Converts the #N02/MMB to #N02/hr vs stoc air and coal firing rate. Our intent is to take 

the curve flt data and subtract out the FBN to determine what the thermal NOx is doing. 
14jl= 

120 

100 

... 80 ..1: 

......... 

N 

0 

z 

=II: 60 

40 

20 

0 

... "' 

~ 

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

.... ,) ... 

-j -~ 

Rz 

:-

... .. ·~· ~· ~ 

I , ,-r--

I 1,.... ,. ~ = 

~ ~/ -1t~~l 

1/ _.,I I 

19 56x2 + 3.3526x + 21.812 

R2 orrrp = 1 

- 75MMB 

- 100MMB 

- 130MMB ml { - 170MMB 

{lllJ-I __ r- ~ v .,,. I I lJl l.t,.J-rl l--~1 ........... ~ I I -- r-r-+ / 1./ ~w ~ :- L,-V v"l% /1 = Trl T lrt:n r 

-Poly. (75MMB) 

-Poly. (100MMB) 

0 

.....- ~ ••• ..~. .... ~ Rz = 

8 -Poly. (130MMB) 

-Poly. (170MMB) 

_ ...~ ITT,= 1,, +t' ,' gt, 

+ 12.584 

0.5 1 1.5 2 2.5 

Stoc air 

Fig 18 FBN contribution vs stoc air and firing rate 

Figure 19 and 20 are charts cmTected 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 conected 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#/ln. We fmiher 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 

fued NOx is directly related to the FBN contribution. 
... 

..c: 

....... 

300 

250 

200 

~ 150 

z 

:It 100 

so 

0 

Line 6 lOOMMB Coal95 MMB NG with 3KPPH atm air 

./"" -r-....... 

V" __ , -

I --- - total NOx 

- FBN contribution 

I I I - Thermal NOx 

I 

0.5 0.6 0.7 0.8 0.9 1 

Stoc air 

Fig 19 Line 6 estimates ofthermal and FBN contribution 

... 

..c: 

....... 

300 

250 

200 

~ 150 

z 

:It 100 

so 

0 

Line7 130 MMB Coal30 to SO MMB NG 

~ ~~ I Ill Ill I I I ,,~ I I ~~ 

II ... ~"-1"-

I I II I I 

- totaiNOx 

- FBN contribution 

Il l I I I I - Thermal NOx 

I I 

I I I I I 

0.5 0.6 0.7 0.8 0.9 1 

Stoc air 

Fig 20 Line 7 estimates of thermal and FBN contribution 
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 # N02/hr (@ .9#N02/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 san1e 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 fmiher than stoichiometric knowing the secondary air is 300% EA plus. 

This was also observed during testing, coal instability near I 00% combustion air in burner and as 

illustrated in fig I 0. 

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 I. 

This limit already requires I 00% combustion air to the burner. It is recommended to limit the 

NOxon 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. 

Fmiher 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#N02/MMB. Anything lower 

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

I. 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 ofNOx Control technologys to Power Boilers, A. Rawdon and S. Johnson 

4. Coen Internal Documents 

5. EPAdocumentAP42 Section 11.23 Feb 1997 Table 11.23-4 

6. Smoot L.D., Hmton M.D. and Williams, G.A. "Propagation of Laminar Pulverized Coal-Air Flames" 16'h 

Symposium International on Combustion, The Combustion Institute, Pittsburgh, PA. 375 (I976)