Predictive Emission Monitoring System for Nitrogen Oxides 

December 9, 2009 

1.0 Scope and Application 

1.1 Hibbing Taconite Company (HTC), an unincorporated joint venture, is the owner and operator of a 

taconite (magnetite) ore mining and beneficiation facility located in Hibbing, Minnesota. According to 

the terms and conditions of HTC’s March 3, 2008 Administrative Order by Consent with the Minnesota 

Pollution Control Agency (MPCA), a predictive emission monitoring system (PEMS) is used to 

calculate emissions of nitrogen oxides (NOx) for each of HTC’s three pellet indurating furnaces. The 

PEMS complies with the requirements of 40 Code of Federal Regulations (CFR) Part 60, Appendix B 

(Performance Specification 16 – Specifications and Test Procedures for Predictive Emission Monitoring 

Systems in Stationary Sources). 

1.2 Following installation of the PEMS, HTC will complete an initial certification test to classify the PEMS 

as acceptable for use, and ongoing quality assurance tests will also be conducted to ensure the PEMS is 

operating properly. In addition, HTC has established an ongoing sensor evaluation procedure prior to 

PEMS certification. The PEMS initial certification test will be performed before reporting any PEMS 

data as quality assured. 

2.0 Summary of Performance Specifications 

2.1 For initial PEMS certification, HTC will perform a minimum 27-run, 3-level (9 runs at each level) 

relative accuracy test using U.S. Environmental Protection Agency (EPA) Method 7E (Determination of 

Nitrogen Oxides Emissions from Stationary Sources), which is detailed in 40 CFR Part 60, Appendix A. 

Additionally, the PEMS data will be evaluated for bias and by F-test and correlation analysis. Relative 

to periodic quality assurance, HTC will also conduct quarterly relative accuracy audits (RAAs) and 

annual relative accuracy test audits (RATAs) to assess ongoing PEMS operation. The frequency of the 

RAAs and RATAs may be shortened by successful operation of the PEMS during a prior year. 

 

3.0 Definitions 

3.1 Data Recorder means the equipment that provides a permanent record of the PEMS output. The data 

 recorder may include automatic data reduction capabilities and may include electronic data records, 

 paper records, or a combination of electronic data and paper records. 

3.2 Defective sensor means a sensor that is responsible for PEMS malfunction or that operates outside the 

 approved operating envelope. A defective sensor may be functioning properly, but because it is 

 operating outside the approved operating envelope, the resulting predicted emission is not validated. 
Page 2 of 36 

3.3 Operating envelope means the defined range of a parameter input that is established during PEMS 

 development. Emission data generated from parameter inputs that are beyond the operating envelope are 

 not considered quality assured and are therefore unacceptable. 

3.4 PEMS means all of the equipment required to predict an emission concentration or emission rate. The 

 system may consist of any of the following major subsystems: sensors and sensor interfaces, emission 

 model, algorithm, or equation that uses process data to generate an output that is proportional to the 

 emission concentration or emission rate, data recorder, and sensor evaluation system. 

3.5 PEMS training means the process of developing or confirming the operation of the PEMS against a 

 reference method under specified conditions. 

3.6 Quarter means a quarter of a calendar year in which there are at least 168 emission unit operating hours. 

3.7 Reconciled Process Data means substitute data that are generated by a sensor evaluation system to 

 replace that of a failed sensor. Reconciled process data may not be used without approval from the 

 Administrator. 

 

3.8 Relative Accuracy means the accuracy of the PEMS when compared to a reference method (RM) at the 

 source. The RA is the average difference between the pollutant PEMS and RM data for a specified 

 number of comparison runs plus a 2.5 percent confidence coefficient, divided by the average of the RM 

 tests. 

3.9 Relative Accuracy Audit means a quarterly audit of the PEMS against a portable analyzer meeting the 

 requirements of ASTM D6522–00 or a RM for a specified number of runs. A RM may be used in place 

 of the portable analyzer for the RAA. 

3.10 Relative Accuracy Test Audit means a RA test that is performed at least once every four calendar 

 quarters after the initial certification test while the PEMS is operating at the normal operating level. 

3.11 Reference Value means a PEMS baseline value that may be established by RM testing under conditions 

 when all sensors are functioning properly. This reference value may then be used in the sensor 

 evaluation system or in adjusting new sensors. 

3.12 Sensor Evaluation System means the equipment or procedure used to periodically assess the quality of 

 sensor input data. This system may be a sub-model that periodically cross-checks sensor inputs among 

 themselves or any other procedure that checks sensor integrity at least daily (when operated for more 

 than one hour in any calendar day). 

3.13 Sensors and Sensor Interface means the equipment that measures the process input signals and 

 transports them to the emission prediction system. 

4.0 Equipment and Supplies 

4.1 Number of Input Parameters 

 NOx emission rates are calculated as follows: 

 3 ,2 ,1 ; [1] 

4

1

   jQE

i

j ji
Page 3 of 36 

 Where Ej is NOx emissions from the entire furnace of Line j, lb NOx/hr; Qij 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 are used to calculate Qij. 

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

11  1    

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

     

Q )FeWet(122.12)FeTotal(24.5322)Hood(32661.0T) Udd(24631.0)Mspeed(874.133.3846 31

     

Q41   4  4)SVen(540.38Gas) S(4258.16.5776 .5475(Wex P)  0.17165(Ho  )FeTotal(831.8022)od [2] 

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

     

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

     

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

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

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

13

     

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

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

43 04.752Q  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 natural 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); 

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 natural 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); 
Page 4 of 36 

(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].) 

4.2 Parameter Operating Envelopes 

PEMS input parameter operating envelope specifications for Furnace Lines 1, 2, and 3 are defined in 

Tables 1, 2, and 3, respectively. Minimum and maximum values for input parameters were developed 

from historical process and quality data, which are supported by Figures 1-12 (for Furnace Line 1), 13-

27 (for Furnace Line 2), and 28-41 (for Furnace Line 3). 

Furnace Line 1 PEMS Input Parameters 

Parameter Description 

Minimum 

Value 

Maximum 

Value 

Stack 1 Scrubber Water Flow Rate 28 1020 

Hearth Layer Depth 2.5 9.0 

2nd Cooling Hood Pressure -7.0 -1.5 

Windbox 22 Hood Temperature 2150 2500 

Fired Pellet Total Fe 65.70 66.40 

Updraft Drying Discharge Temperature 400 700 

Fired Pellet CaO 0.25 0.85 

Concentrate Wet Fe++ 21.5 24.1 

Machine Grate Speed 3.5 12.0 

South Firing Chamber Natural Gas Usage 25 70 

Stack 4 Scrubber Venturi Differential Pressure 0.1 4.5 

Windbox Exhaust Header Pressure 10.0 25.0 

Table 1. Furnace Line 1 Input Parameter Minimum / Maximum Values 

WFS1

-200

0

200

400

600

800

1000

1200

Daily Average (2008)

Gallons per Minute

 Figure 1. Furnace Line 1 Stack 1 Scrubber Water Flow Rate 
Page 5 of 36 

HL Depth

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Daily Average (2008)

Inches

Figure 2. Furnace Line 1 Hearth Layer Depth 

2nd Cool

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

Daily Average (2008)

Inches of Water 

Column

Figure 3. Furnace Line 1 2nd Cooling Hood Pressure 

Hood 22

0

500

1000

1500

2000

2500

3000

Daily Average (2008)

Degrees Fahrenheit

Figure 4. Furnace Line 1 Windbox 22 Hood Temperature 
Page 6 of 36 

Total Fe

-20.00

0.00

20.00

40.00

60.00

80.00

Daily Average (2008)

Percent

 

Figure 5. Furnace Line 1 Fired Pellet Total Fe 

Udd Temp

0

100

200

300

400

500

600

700

Daily Average (2008)

Degrees Fahrenheit

Figure 6. Furnace Line 1 Updraft Drying Discharge Temperature 

CaO

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

Daily Average (2008)

Percent

Figure 7. Furnace Line 1 Fired Pellet CaO 
Page 7 of 36 

Wet Fe++

0.0

5.0

10.0

15.0

20.0

25.0

30.0

Daily Average (2008)

Percent

Figure 8. Furnace Line 1 Concentrate Wet Fe++ 

Mspeed

0.0

2.0

4.0

6.0

8.0

10.0

Daily Average (2008)

Feet per Minute

Figure 9. Furnace Line 1 Machine Grate Speed 

S Gas

-20

0

20

40

60

80

100

Daily Average (2008)

Thousand Cubic Feet 

per Hour

Figure 10. Furnace Line 1 South Firing Chamber Natural Gas Usage 
Page 8 of 36 

Ven S4

0.0

1.0

2.0

3.0

4.0

5.0

Daily Average (2008)

Inches of Water 

Column

Figure 11. Furnace Line 1 Stack 4 Scrubber Venturi Differential Pressure 

Wex P

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

Daily Average (2008)

Inches of Water 

Column

Figure 12. Furnace Line 1 Windbox Exhaust Header Pressure 

Furnace Line 2 PEMS Input Parameters 

Parameter Description 

Minimum 

Value 

Maximum 

Value 

Hearth Layer Depth 2.5 9.0 

Machine Grate Speed 3.5 12.0 

Updraft Drying Discharge Temperature 400 700 

Windbox 18 Hood Temperature 2150 2500 

Concentrate Satmagan Fe++ 21.0 22.6 

Green Ball Feed Rate 200 550 

Stack 2 Scrubber Venturi Differential Pressure 0.5 4.9 

Cooling Air Fan Header Pressure 10.0 25.0 

South Firing Chamber Natural Gas Usage 25 70 

Stack 3 Scrubber Water Flow Rate 325 827 

Windbox Exhaust Header Pressure 10.0 25.0 

Burn Thru Temperature 400 900 

North Firing Chamber Natural Gas Usage 25 70 

Stack 4 Scrubber Venturi Differential Pressure 0.4 4.2 

Stack 4 Scrubber Water Flow Rate 284 635 

Table 2. Furnace Line 2 Input Parameter Minimum / Maximum Values 
Page 9 of 36 

HL Depth

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Daily Average (2008)

Inches

Figure 13. Furnace Line 2 Hearth Layer Depth 

Mspeed

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Daily Average (2008)

Feet per Minute

Figure 14. Furnace Line 2 Machine Grate Speed 

Udd T

0

100

200

300

400

500

600

700

Daily Average (2008)

Degrees Fahrenheit

Figure 15. Furnace Line 2 Updraft Drying Discharge Temperature 
Page 10 of 36 

Hood 18

0

500

1000

1500

2000

2500

3000

Daily Average (2008)

Degrees Fahrenheit

Figure 16. Furnace Line 2 Windbox 18 Hood Temperature 

Sat Fe++

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

Daily Average (2008)

Percent

Figure 17. Furnace Line 2 Concentrate Satmagan Fe++ 

Feed

-100

0

100

200

300

400

500

600

Daily Average (2008)

Wet Long Tons per 

Hour

Figure 18. Furnace Line 2 Green Ball Feed Rate 
Page 11 of 36 

Ven S2

0

1

2

3

4

5

6

Daily Average (2008)

Inches of Water 

Column

Figure 19. Furnace Line 2 Stack 2 Scrubber Venturi Differential Pressure 

CAF

0.0

5.0

10.0

15.0

20.0

25.0

Daily Average (2008)

Inches of Water 

Column

Figure 20. Furnace Line 2 Cooling Air Fan Header Pressure 

S Gas

0

10

20

30

40

50

60

70

Daily Average (2008)

Thousand Cubic Feet 

per Hour

Figure 21. Furnace Line 2 South Firing Chamber Natural Gas Usage 
Page 12 of 36 

WFS3

0

200

400

600

800

1000

Daily Average (2008)

Gallons per Minute

Figure 22. Furnace Line 2 Stack 3 Scrubber Water Flow Rate 

Wex P

0.0

5.0

10.0

15.0

20.0

25.0

Daily Average (2008)

Inches of Water 

Column

Figure 23. Furnace Line 2 Windbox Exhaust Header Pressure 

B Thru

0

200

400

600

800

1000

Daily Average (2008)

Degrees Fahrenheit

Figure 24. Furnace Line 2 Burn Thru Temperature 
Page 13 of 36 

N Gas

0

10

20

30

40

50

60

70

Daily Average (2008)

Thousand Cubic Feet 

per Hour

Figure 25. Furnace Line 2 North Firing Chamber Natural Gas Usage 

Ven S4

0

1

2

3

4

5

Daily Average (2008)

Inches of Water 

Column

Figure 26. Furnace Line 2 Stack 4 Scrubber Venturi Differential Pressure 

WFS4

0

100

200

300

400

500

600

700

Daily Average (2008)

Gallons per Minute

Figure 27. Furnace Line 2 Stack 4 Scrubber Water Flow Rate 
Page 14 of 36 

Furnace Line 3 PEMS Input Parameters 

Parameter Description 

Minimum 

Value 

Maximum 

Value 

Machine Grate Speed 3.5 12.0 

2nd Cooling Hood Pressure -7.0 -1.5 

Windbox 18 Hood Temperature 2150 2500 

Fired Pellet Silica 4.16 4.80 

Concentrate Wet Fe++ 21.5 24.1 

Green Ball Feed Rate 200 550 

South Firing Chamber Natural Gas Usage 25 70 

Stack 2 Scrubber Water Flow Rate 0 711 

Windbox Exhaust Header Pressure 10.0 25.0 

Hearth Layer Depth 2.5 9.0 

Machine Discharge Temperature 250 900 

Green Ball Moisture 8.8 9.9 

Updraft Drying Zone Pressure -6.0 -1.0 

Cooling Air Fan Header Pressure 10.0 25.0 

Table 3. Furnace Line 3 Input Parameter Minimum / Maximum Values 

Mspeed

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Daily Average (2008)

Feet per Minute

Figure 28. Furnace Line 3 Machine Grate Speed 

2nd Cool

-8.0

-6.0

-4.0

-2.0

0.0

2.0

Daily Average (2008)

Inches of Water 

Column

Figure 29. Furnace Line 3 2nd Cooling Hood Pressure 
Page 15 of 36 

Hood 18

0

500

1000

1500

2000

2500

3000

Daily Average (2008)

Degrees Fahrenheit

Figure 30. Furnace Line 3 Windbox 18 Hood Temperature 

Silica

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

Daily Average (2008)

Percent

Figure 31. Furnace Line 3 Fired Pellet Silica 

Wet Fe++

0.0

5.0

10.0

15.0

20.0

25.0

30.0

Daily Average (2008)

Percent

Figure 32. Furnace Line 3 Concentrate Wet Fe++ 
Page 16 of 36 

Feed

-100

0

100

200

300

400

500

600

Daily Average (2008)

Wet Long Tons per 

Hour

Figure 33. Furnace Line 3 Green Ball Feed Rate 

S Gas

-20

0

20

40

60

80

Daily Average (2008)

Thousand Cubic Feet 

per Hour

Figure 34. Furnace Line 3 South Firing Chamber Natural Gas Usage 

WFS2

-200

0

200

400

600

800

Daily Average (2008)

Gallons per Minute

Figure 35. Furnace Line 3 Stack 2 Scrubber Water Flow Rate 
Page 17 of 36 

Wex P

0.0

5.0

10.0

15.0

20.0

25.0

Daily Average (2008)

Inches of Water 

Column

Figure 36. Furnace Line 3 Windbox Exhaust Header Pressure 

HL Depth

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Daily Average (2008)

Inches

Figure 37. Furnace Line 3 Hearth Layer Depth 

MD T

0

200

400

600

800

1000

Daily Average (2008)

Degrees Fahrenheit

Figure 38. Furnace Line 3 Machine Discharge Temperature 
Page 18 of 36 

GB

-2.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Daily Average (2008)

Percent

Figure 39. Furnace Line 3 Green Ball Moisture 

Udd P

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Daily Average (2008)

Inches of Water 

Column

Figure 40. Furnace Line 3 Updraft Drying Zone Pressure 

CAF

0.0

5.0

10.0

15.0

20.0

25.0

Daily Average (2008)

Inches of Water 

Column

Figure 41. Furnace Line 3 Cooling Air Fan Header Pressure 

4.3 Source-Specific Operating Conditions 

HTC produces two types of pellets: standard and high compression. The typical split annually is 75 

percent standard pellets and 25 percent high compression pellets. Limestone is added to each type of 

pellet to improve the pellet physical strength, and in particular, the compression strength. For both 

pellet types, less than one percent by weight limestone is added to the green (unfired) pellets, with high 

compression addition rates double in comparison to that of standard pellets. When producing high 
Page 19 of 36 

compression pellets, the natural gas burner temperature set points are increased to calcinate the 

increased limestone quantities that are added during a high compression production run. 

HTC has developed a process control strategy to optimize process air flow at less than 100 percent 

production rates. This optimization matches process air flow with production rate and minimizes excess 

process air that is not needed. An average of two maintenance shutdowns per furnace line occur each 

calendar year. The length of a shutdown is typically between 10 and 21 days and is dependant on the 

amount of scheduled maintenance performed. It should be noted that no specific operating conditions 

have been identified that will affect output of the PEMS. 

4.4 Ambient Conditions 

 

 Ambient conditions and seasonal changes will not affect the PEMS, so extrapolation to account for these 

conditions is not necessary. 

4.5 PEMS Principle of Operation 

 Best subsets regression analysis was performed for each stack with NOx (lb/hr) as the dependent 

variable and 25 process operating variables as independent variables (PEMS input parameters). NOx

(lb/hr) for each stack was fitted with 5 selected PEMS input parameters. 

In order to further evaluate whether the identified PEMS input parameters can be used to predict NOx 

emissions (lb/hr) for each stack, the residual standard deviation of the random differences between the 

actual measured NOx emissions and the regression equations’ predicted values were divided by the 

mean NOx emissions, with results summarized in Table 4. 

Stack

Standard 

Deviation of 

Residuals

Mean

NOx

(lb/hr)

Quotient 

(SDR / 

Mean)

Stack

Standard 

Deviation of 

Residuals

Mean

NOx

(lb/hr)

Quotient 

(SDR / 

Mean)

Stack

Standard 

Deviation of 

Residuals

Mean

NOx

(lb/hr)

Quotient 

(SDR / 

Mean)

Stack 1 

(SV 021) 4.3110 79.2800 0.0544 Stack 1 (SV 028) 7.9400 143.8200 0.0552 Stack 1 (SV 029) 3.0170 43.2590 0.0697

Stack 2 

(SV 022) 3.9670 90.8810 0.0437 Stack 2 (SV 027) 5.2260 105.2000 0.0497 Stack 2 (SV 030) 1.9520 59.6990 0.0327

Stack 3 

(SV 023) 4.3980 111.3100 0.0395 Stack 3 (SV 026) 4.8300 82.2190 0.0587 Stack 3 (SV 031) 3.2680 96.1930 0.0340

Stack 4 

(SV 024) 4.8620 124.0200 0.0392 Stack 4 (SV 025) 6.3250 62.9550 0.1005 Stack 4 (SV 032) 2.9130 118.7300 0.0245

Furnace Line 1 Furnace Line 2 Furnace Line 3

Table 4. Calculations for Furnace Lines 1, 2, and 3
Page 20 of 36 

Furnace Line 1, Stack 1 
Page 21 of 36 

Furnace Line 1, Stack 2 
Page 22 of 36 

Furnace Line 1, Stack 3 
Page 23 of 36 

Furnace Line 1, Stack 4 
Page 24 of 36 

Furnace Line 2, Stack 1 
Page 25 of 36 

Furnace Line 2, Stack 2 
Page 26 of 36 

Furnace Line 2, Stack 3 
Page 27 of 36 

Furnace Line 2, Stack 4 
Page 28 of 36 

Furnace Line 3, Stack 1
Page 29 of 36 

Furnace Line 3, Stack 2 
Page 30 of 36 

Furnace Line 3, Stack 3 
Page 31 of 36 

Furnace Line 3, Stack 4 
Page 32 of 36 

Figures 42, 43, and 44 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

Measured

Figure 42. Furnace 1 NOx Predictions 

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 43. Furnace 2 NOx Predictions 
Page 33 of 36 

 

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 44. Furnace 3 NOx Predictions 

4.6 Data Recorder Scale 

Data collected from PEMS input parameters is transmitted to the Fisher Rosemount Distributed Control 

System (DCS). DCS readings are transmitted to the OSI Soft PI Historian database, which facilitates 

storage and retrieval of historical data. PI Historian is configured to collect and process all PEMS input 

parameters, with NOx mass emission rates displayed visually in PI ProcessBook. 

4.7 Sensor Location and Repair 

 All PEMS input sensors are installed in accessible locations to facilitate repairs and replacement when 

needed. Following replacement of one of more sensors, a relative accuracy test audit will be conducted. 

Sensors are calibrated at a frequency consistent with manufacturers’ recommendations or alternatively 

on an annual basis. 

4.8 Sensor Evaluation System 

 The identified PEMS input parameters are also critical parameters used to maintain adherence to HTC’s 

customer-mandated fired pellet quality specifications. Consequently, instances of sensor failure and/or 

drift are promptly detected and resolved in order to maintain acceptable pellet quality. In addition, 

analytical laboratory analyses allow confirmation of conformance to pellet quality specifications, 

thereby ensuring that sensor failure or drift has not occurred. 

4.9 Parameter Envelope Exceedances 

 In the event that PEMS input parameter exceedances occur (outside specified operating envelopes), the 

associated NOx emissions data will not be classified as quality assured. 
Page 34 of 36 

5.0 Sample Collection, Preservation, Storage, and Transport 

5.1 Initial Certification / Relative Accuracy Test 

As previously mentioned, initial certification of the PEMS will occur using EPA Method 7E. The 

relative accuracy test for Furnace Lines 1, 2 and 3 will be conducted at low, mid, and high operating 

levels for the key parameters affecting NOx emissions (please refer to Table 5). 

 Furnace Line 1 Furnace Line 2 Furnace Line 3 

Key PEMS Input 

Parameter 

Windbox 22 Hood 

Temperature 

Windbox 18 Hood 

Temperature 

Windbox 18 Hood 

Temperature 

Low Operating Level 

(Approximate) 

2,300 Degrees 

Fahrenheit 

2,300 Degrees 

Fahrenheit 

2,300 Degrees 

Fahrenheit 

Mid Operating Level 

(Approximate) 

2,350 Degrees 

Fahrenheit 

2,350 Degrees 

Fahrenheit 

2,350 Degrees 

Fahrenheit 

High Operating Level 

(Approximate) 

2,400 Degrees 

Fahrenheit 

2,400 Degrees 

Fahrenheit 

2,400 Degrees 

Fahrenheit 

Table 5. Relative Accuracy Test Operating Levels for Key PEMS Input Parameters

5.2 Number of Reference Method Tests 

Reference method tests will be conducted using a minimum of nine runs at the low operating level, nine 

runs at the mid operating level, and nine runs at the high operating level. If more than nine runs are 

completed at a given operating level, a maximum of three runs may be rejected as long as the total 

number of runs at the given operating level is greater than or equal to nine. 

5.3 Reference Method Measurement Location 

Reference method measurement points are located at least two equivalent stack diameters downstream 

and one-half an equivalent stack diameter upstream from the nearest flow disturbance. 

5.4 Traverse Points 

Traverse points are selected to ensure representative samples are collected, with all reference method 

tests conducted within 3 centimeters of each selected traverse point but no closer than 3 centimeters to 

the stack wall. Measurement lines are established across the stack passing through the center in the 

direction of any expected stratification. A minimum of three traverse points are located on the 

measurement line at 16.7, 50.0, and 83.3 percent of the stack inside diameter. As an alternative for 

stacks greater than 2.4 meters, the three traverse points may be located on the measurement line at 0.4, 

1.2, and 2.0 meters from the stack wall. 

5.5 Relative Accuracy Procedure 

Relative accuracy tests are performed with a sample traverse of 21 minutes, sampling for 7 minutes at 

each traverse point. 
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5.6 Correlation of Reference Method and PEMS Data 

The exact time of day will be recorded for each reference method test run, with correlation of reference 

method and PEMS data to occur for each test run. The paired reference method and PEMS data will be 

used to calculate the relative accuracy for each operating level (low, mid, and high). 

5.7 Statistical Tests 

Using the relative accuracy data taken at the mid operating level, a calculation will be performed to 

determine if bias exists between the reference method and the PEMS. PEMS data is considered biased 

and must be adjusted if the arithmetic mean is greater than the absolute value of the confidence 

coefficient. In such cases, a bias factor must be used to correct the PEMS data. 

An F-test will be performed for the relative accuracy paired data at each operating level (low, mid, and 

high), to determine if the reference method and PEMS data differ for more than might be expected by 

chance. Calculated F-values must not be greater than the critical F-value at the 95-percent confidence 

level for the PEMS to be acceptable. 

 

Finally, correlation analysis will be performed using the relative accuracy paired data at each operating 

level (low, mid, and high) combined, to determine how well the reference method and PEMS data 

correlate. Calculated r-values must be greater than or equal to 0.8 for the PEMS to be acceptable. 

5.8 Reporting 

Results of the relative accuracy and statistical tests will be summarized in tabular form, to be 

accompanied by all data sheets, calculations, records of PEMS responses. Reports will also include 

documentation used to establish PEMS input parameter operating envelopes. 

5.9 Reevaluation of PEMS after a Failed Test, Change in Operations, or Change in PEMS Input Parameter 

Following initial certification, recertification of the PEMS will occur if: 

1. The PEMS fails a quarterly relative accuracy test audit; 

2. The PEMS fails a annual relative accuracy test audit; 

3. Process changes are made that could result in a significant increase in NOx emissions; 

Recertification tests will demonstrate that the PEMS provides acceptable data when operating in new 

operating ranges or with new PEMS input parameters. Recertification tests will be completed within 60 

emission unit operating days or 180 calendar days of occurrence of any one of the three aforementioned 

triggering events. 

6.0 Quality Control 

6.1 Daily Sensor Evaluation Check 

As previously discussed, the identified PEMS input parameters are also critical parameters used to 

maintain HTC’s fired pellet quality specifications and are therefore subject to continuous monitoring. 

Consequently, instances of sensor failure and/or drift are promptly detected and resolved. In addition, 

daily analytical laboratory analyses allow for confirmation of conformance to fired pellet quality 

specifications, thereby ensuring that sensor failure or drift has not occurred. 
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6.2 Quarterly Relative Accuracy Audits 

For the first year of PEMS operation, relative accuracy audits will be performed on a quarterly basis. 

Relative accuracy audits will consist of at least three 30-minute NOx portable analyzer or reference 

method determinations, with the average of the three determinations not to differ from the simultaneous 

PEMS average value by greater than 10 percent. 

Should the PEMS pass all three relative accuracy audits in the first year of operation, a single mid-year 

relative accuracy audit may be performed in place of the quarterly events. This option may be repeated 

in subsequent years, but only until the PEMS fails either a subsequent relative accuracy audit or relative 

accuracy test audit. When such a failure occurs, quarterly relative accuracy audits will resume in the 

quarter following failure and continue until the PEMS successfully passes both a year of relative 

accuracy audits and a subsequent relative accuracy test audit. 

 

6.3 Annual Relative Accuracy Test Audit 

A minimum nine-run relative accuracy test audit at the mid operating level will be performed on an 

annual basis (in the quarter in which a relative accuracy audit is not performed). The PEMS relative 

accuracy must not exceed 10 percent.