NOx Alternative Monitoring Method 11/12/2009 

Hibbing Taconite Company (HTC) requests Minnesota Pollution Control Agency (MPCA) approval to use the 

following equations to calculate hourly mass emissions of nitrogen oxides (NOx) for each of its three indurating 

furnaces (Lines 1 through 3). 

 3 ,2 ,1 ; [1] 

4

1

   jQE

i

j ji

Where Ej is NOx emissions from the entire furnace of Line j, lb NOx/hr; 

Cij is the predicted mass emission rate for NOx in the gas stream of Stack i at Line j, lb/hr, the algebraic 

detail of which can be found in [2] through [4]; Stacks 1 through 4 denote SV 021 through SV 024 for 

Line 1; SV 028 downward through SV 025 for Line 2; SV 029 through SV 032 for Line 3. 

The following multiple linear regression equations should be used to calculate Cij. 

C )FeTotal(171.66)22Hood(19721.0)Cool2(5925.5)Depth HL(399.18)SWF(064458.07.4607 nd

11  1    

C )FeWet(737.12)CaO(85.77Fe)Total(59.101)22Hood(17568.0)T Udd(06520.03.6732 21

     

31 )FeWet(122.12)FeTotal(24.5322)Hood(32661.0T) Udd(24631.0)Mspeed(874.133.3846C      

C41   4  4)SVen(540.38Gas) S(4258.16.5776 .5475(Wex P)  0.17165(Ho  )FeTotal(831.8022)od [2] 

   372.16Depth HL5.06713.0801C Mspeed 0.10259  37644.0T Udd   )FeSat(751.1318 Hood 12

     

  2 247114.0SVen840.24Feed17920.092.856C     )FeSat(493.4CAF770.218 Hood 22

     

32     Gas S0533.176.175C  0.10228 3        29527.0Thru B038051.0PWex 3126.3SWF 18 Hood 

42      4   4  926.41S WF08543.0SVen 004.32Gas N5379.325.618C     28753.0Mspeed 18 Hood [3] 

6105.896.29C     071846.0Cool24145.1Mspeed     )FeWet(495.6Silica258.6118 Hood nd

13

     

23 382.190C  0.139487      2    )18 Hood(077930.0PWex 5507.2S WF014698.0Gas S1165.1Feed

   762.41Depth HL9202.617.934C 208412.0Mspeed    GB(6.072T) MD10263.018Hood  33     

43 04.752C  0.28596       14823.0P Udd013.30Gas S3162.1Feed     CAF9963.418Hood [4] 

Where, 

WF S1 is the water flow rate of the wet scrubber at Stack 1 (gallons per minute); 

HL Depth is the furnace hearth layer depth (inches); 

2nd Cool is the second cooling hood pressure (inches of water column); 

Hood 22 is the hood temperature measured at Windbox 22 (degrees Fahrenheit); 

Total Fe is analyzed for fired pellets (percent); 

Udd T is the updraft drying (zone) discharge temperature (degrees Fahrenheit); 

CaO is analyzed for fired pellets (percent); 

Wet Fe++ is wet Fe++ of concentrate (percent); 

Mspeed is the linear moving speed of the traveling grate of the indurating machine (feet per minute); 

S Gas is the total nature gas usage of the south firing chamber (thousand cubic feet per hour); 

Ven S4 is the Venturi differential pressure for the wet scrubber at Stack 4 (inches of water column); 
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Wex P is the pressure measured at the windbox exhaust header (inches of water column); 

Hood 18 is the hood temperature measured at Windbox 18 (degrees Fahrenheit); 

Sat Fe++ is Satmagan Fe++, analyzed for concentrate (percent); 

Feed is the furnace green ball feed rate (wet long tons per hour); 

Ven S2 is the Venturi differential pressure for the wet scrubber at Stack 2 (inches of water column); 

CAF is the pressure measured at the cooling air fan header (inches of water column); 

WF S3 is the water flow rate of the wet scrubber at Stack 3 (gallons per minute); 

WF S4 is the water flow rate of the wet scrubber at Stack 4 (gallons per minute); 

B Thru is the temperature beneath the pellet bed in the firing zone (degrees Fahrenheit); 

N Gas is the total nature gas usage of the north firing chamber (thousand cubic feet per hour); 

Silica is analyzed for fired pellets (percent); 

WF S2 is the water flow rate of the wet scrubber at Stack 2 (gallons per minute); 

MD Temp is the machine discharge temperature (degrees Fahrenheit); 

GB is the moisture content of green balls (percent wet basis); 

Udd P is the pressure measured at the updraft drying zone (inches of water column); 

(Note that some variables in the right-hand side of [2] may appear to be identical to that of [3] and [4]. 

However, the variables found in [2], [3], and [4] are associated with Line 1, Line 2, and Line 3, respectively. 

With this understanding, the use of subscripts is minimized among the variables in the right-hand side of [2] 

through [4].) 

Figures 1, 2, and 3 compare predicted and measured NOx emissions at the furnace level. 

Line 1 NOx (lb/hr): Predicted vs. Measured

R2 = 0.906

300.00

400.00

500.00

300.00 400.00 500.00

Predicted

Figure 1. Furnace 1 NOx Predictions 
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Line 2 NOx (lb/hr): Predicted vs. Measured

R2 = 0.8666

300.00

400.00

500.00

600.00

300.00 400.00 500.00 600.00

Predicted

Measured

Figure 2. Furnace 2 NOx Predictions 

 

Line 3 NOx (lb/hr): Predicted vs. Measured

R2 = 0.9551

200.00

300.00

400.00

200.00 300.00 400.00

Predicted

Measured

Figure 3. Furnace 3 NOx Predictions 
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Alternative Method Proposals for NOx Emission Monitoring 

HTC originally submitted three alternative method proposals, one for each indurating furnace, with its letter to 

the Minnesota Pollution Control Agency (MPCA) dated June 30, 2008. A CD-ROM was also submitted to 

provide NOx performance testing results, process operating data, and regression modeling detail. The 

performance testing at Line 2 was conducted first (October 24 – November 3, 2007), followed with Lines 3 and 

1 (March 31 – April 8 and April 9 – 17, 2008, respectively). 

For purposes of the modified alternative monitoring method proposal, HTC performed best subsets regression 

analysis for each stack with NOx (lb/hr) as the dependent variable and 25 process operating variables as 

independent variables (predictors); and fitted NOx (lb/hr) with 5 selected process operating variables. In 

addition, HTC calculated leverage for each stack, with results summarized in Table 1. 

Furnace Line 1 Furnace Line 2 Furnace Line 3 

Variable Mean Standard 

Deviation Variable Mean Standard Deviation Variable Mean Standard Deviation 

Stack 1 

Leverage 0.04027 0.01928 Stack 1 Leverage 0.04225 0.01699 Stack 1 Leverage 0.04580 0.02708

Stack 2 

Leverage 0.04027 0.02265 Stack 2 Leverage 0.04225 0.02107 Stack 2 Leverage 0.04580 0.07425

Stack 3 

Leverage 0.04027 0.02234 Stack 3 Leverage 0.04225 0.01898 Stack 3 Leverage 0.04580 0.07332

Stack 4 

Leverage 0.04027 0.02601 Stack 4 Leverage 0.04225 0.01671 Stack 4 Leverage 0.04580 0.07260

Table 1. Leverage Calculations for Furnace Lines 1, 2, and 3
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Selection of Predictors for Regression Equations 
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