David Brooks

Process Engineer

Hibbing Taconite 

2012 PEMS work

Quick summary of NOx predictive equations for Lines 1, 2, & 3 based off 2007 & 2010 30 hour NOx stack 

tests. Predicted values are in “ppm dry”.

For each furnace, stack test hourly data was combined and entered into Minitab statistical software. 

2007 data was entered on its own sheet. 2010 data was entered on its own sheet. Both data sets then 

were combined into another sheet. Another sheet was built off all data (as needed), but was scrubbed 

to remove points where probes were plugged or calibrations where being.

Accompanying the stack data, hourly averaged process data was included from the OSI PI historian. 

Educated guesses were used to determine the process variables to include. Education and research says 

that temperatures and air flow are the main contributors to NOx generation. Scrubbing water and 

scrubber pressures are not part of the creation and would do little to remove or predict emissions.

It was chosen that only the highest emitting stack was to be predicted (stack 1 in data). The other three 

stacks were to be predicted off stack 1. The other three stacks seemed to follow a ratio, but did not 

always stay constant. All the stacks share a common header so there should be a relationship.

Once stack test data and process data were combined, best subset regressions were run to find the 

process variables with the highest correlation and/or effect on NOx emissions.

Overall summary:

Stack 1 was predicted reasonably well.

Stacks 2, 3, & 4 predicted off Stack 1 (actual) matched results reasonably well.

Time spent on analysis was stopped once the state decided they didn’t want to look at the data, and 

wanted to proceed with 4 CEMS. Weighting by factor, operating ranges, affect of ppm prediction failure 

against total lb/hr, etc. were not explored.

The plan is to add 2012 30 hour stack test data to the existing data to increase the predictive equations’ 

accuracy.

Results for predicting the highest emitting stack (Stack 1) compared to stack test data are listed in the 

following table. 

Note: it was found later that DDD hood pressure was not needed in the equations. The penalty to 

remove it is a couple tenths in the R squared value.

Intercept

DDD 

Hood 

Pressure

Firing 

Zone 

Pressure

Hood 18 

Temp

South 

Inlet 

Temp

South 

Trans 

Temp

Line 1 -453.635 10.92298 -47.4743 0.105587 -0.01072 0.126841 86.8% R

2

Line 2 -889.545 14.08401 50.08297 0.049291 -0.00216 0.430814 94.1% R

2

Line 3 -351.206 7.089283 -8.40658 0.070475 0.045536 0.086557 81.1% R

2
The following graphs compare the PPM (dry) prediction to stack test results (Stack 1).

1 56 112 168 224 280 336 392 448 504 560

140

120

100

80

60

40

20

Index

Data

S1 Nox_Bias

PEM

Variable

Line 1 Stack vs PEM

1 55 110 165 220 275 330 385 440 495 550

250

200

150

100

Index

Data

S1 Nox_Bias

PEM

Variable

Line 2 Stack vs PEM

1 53 106 159 212 265 318 371 424 477 530

120

100

80

60

40

20

Index

Data

S1 Nox_Bias

PEM

Variable

Line 3 Stack vs PEM
Using the highest emitting stack (stack 1), the other three stacks on each furnace were predicted.

Line 1 stack predictions

Stack 2 = 0.277417 + 0.87507 * Stack 1

Stack 3 = 7.37861 + 0.560859 *Stack 1

Stack 4 = 4.20415 + 0.46492 *Stack 1

1 19 38 57 76 95 114 133 152 171 190

130

120

110

100

90

80

70

60

50

40

Index

Data

S2 Nox_Bias

S2:S1 Fit

Variable

Line 1 Stack 2 vs PEM

1 19 38 57 76 95 114 133 152 171 190

90

80

70

60

50

40

30

Index

Data

S3 Nox_Bias

S3:S1 Fit

Variable

Line 1 Stack 2 vs PEM

1 19 38 57 76 95 114 133 152 171 190

70

60

50

40

30

20

Index

Data

S4 Nox_Bias

S4:S1 Fit

Variable

Line 1 Stack 4 vs PEM
Line 2 stack predictions

Stack 2 = 3.20504 + 0.804379 * Stack 1

Stack 3 = -11.8109 + 0.622549 * Stack 1

Stack 4 = -8.47991 + 0.484465 * Stack 1

1 55 110 165 220 275 330 385 440 495 550

200

150

100

50

0

Index

Data

S2 Nox_Bias

S2:S1 Fit

Variable

Line 2 Stack 2 vs PEM

1 55 110 165 220 275 330 385 440 495 550

140

120

100

80

60

40

20

0

Index

Data

S3 Nox_Bias

S3:S1 Fit

Variable

Line 2 Stack 3 vs PEM

1 55 110 165 220 275 330 385 440 495 550

110

100

90

80

70

60

50

40

30

20

Index

Data

S4 Nox_Bias

S4:S1 Fit

Variable

Line 2 Stack 4 vs PEM
Line 3 Stack Predictions

Stack 2 = 6.55725 + 0.745857 * Stack 1

Stack 3 = 15.3618 + 0.368988 * Stack 1

Stack 4 = 11.7028 + 0.238778 * Stack 1

1 53 106 159 212 265 318 371 424 477 530

60

50

40

30

20

Index

Data

S4 Nox_Bias

S4:S1 Fit

Variable

Line 3 Stack 4 vs PEM

1 53 106 159 212 265 318 371 424 477 530

100

90

80

70

60

50

40

30

Index

Data

S2 Nox_Bias

S2:S1 Fit

Variable

Line 3 Stack 2 vs PEM

1 53 106 159 212 265 318 371 424 477 530

60

50

40

30

20

Index

Data

S3 Nox_Bias

S3:S1 Fit

Variable

Line 3 Stack 3 vs PEM