U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 1

© Hatch 2010/04 

U.S. Steel Minntac 

Line 7 NOx Reduction Pilot Project 

CFD Modeling 

Results of Variant Cases 

Distribution List 

Jason Berzansky – Hatch 

Cebert Clarke – Hatch 

Anastasiya Vershenya – Hatch 

John Visnesky – Hatch 

Jennifer Woloshyn – Hatch 

Darren Gietzen – USS Minntac 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 2

© Hatch 2010/04 

Table of Contents 

1. Executive Summary ................................................................................................................................. 3

2. Variant Cases .......................................................................................................................................... 6

2.1 Low-NOx Burner Case 6 – Coal and Natural Gas........................................................................... 7

2.1.1 Pellet Bed Temperature Profiles............................................................................................ 7

2.1.2 Gas Temperature Profiles ..................................................................................................... 8

2.1.3 NOx Emissions................................................................................................................... 10

2.2 Low-NOx Burner Case 7 – Coal, Biomass and Natural Gas.......................................................... 11

2.2.1 Pellet Bed Temperature Profiles.......................................................................................... 11

2.2.2 Gas Temperature Profiles ................................................................................................... 12

2.2.3 NOx Emissions................................................................................................................... 13

2.3 Low-NOx Burner Cases 8 and 9 – Coal, Biomass and Natural Gas............................................... 13

2.3.1 Gas Temperature Profiles ................................................................................................... 14

2.3.2 NOx Emissions................................................................................................................... 15

2.4 Summary of Variant Cases............................................................................................................ 16

3. Conclusions and Recommendations ...................................................................................................... 17
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 3

© Hatch 2010/04 

1. Executive Summary 

USS Minntac has initiated a pilot project to investigate NOx emission control for their induration 

systems. The present study investigates the impact of replacing the existing main kiln burner on the 

Line 7 induration system with a new low-NOx burner that has been designed by Coen. 

Computational fluid dynamics (CFD) and in-house developed process mathematical models have 

been used to estimate the expected level of NOx reduction with the proposed burner and to quantify 

the impact on the gas and pellet bed temperatures. In addition, potential modifications to the burner

operation are investigated to maintain acceptable process conditions. 

The existing operating conditions on Line 7 have been analyzed using the data collected on site 

during June 9 – 10, 2009. Three most commonly used fuel scenarios were considered: (1) coal and 

natural gas, (2) biomass and natural gas, (3) coal, biomass and natural gas. The total measured mass 

flow rate of NOx was 212 lb/hr, 386 lb/hr and 202 lb/hr for these three fuel scenarios, respectively. 

The presented results are valid for the given set of process conditions modeled, which are assumed 

to be representative of typical operation. In reality, the actual process conditions vary for many 

reasons, which will impact the actual process temperatures and related NOx production. 

Table 1-1 and Table 1-2 list boundary conditions for the first five cases that have been modeled. In 

addition to the fuel scenarios most commonly used on site, two alternate fuel mixes are considered: 

natural gas only, and fuel oil with natural gas. The additional fuel mixes are used primarily as 

emergency backup on site. 

Results for cases 1 and 3 show that a 62 – 96% decrease in NOx is expected with the low-NOx 

burner. Results for case 2 show that an 79 – 97% decrease in NOx is expected with the low-NOx 

burner. In all three cases the NOx decrease occurs because the peak flame temperature and gas 

volume of the kiln above the threshold for NOx generation decrease significantly. This predicted 

reduction in NOx is very specific to the particular process at Line 7 and operating condition 

simulated, and variation in these conditions will affect the level of NOx produced and the percent 

reduction from the existing operation of the equipment. 

Results for the low-NOx burner case 4 indicate a NOx generation rate of 230 lb/hr. This value is 

comparable to results predicted for the existing burner operation with both coal and natural gas and 

the tri-fuel mix. Since burning a greater amount of natural gas leads to higher NOx formation 

inherently, fuel scenario with 100% natural gas can be expected to produce similar predictions to the 

current operation with coal and natural gas fuel scenario. 

Results for the low-NOx burner case 5 predict NOx formation of 10 lb/hr. This result is comparable 

to the predictions for the most common fuel mixes. 

Results for cases 1 and 3 show a decrease in the gas temperature exiting the kiln into the preheat. 

This temperature decrease is a process concern because it represents a loss of process heat to the 

preheat section of the grate. To address this concern, variant cases have been defined by Hatch and 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 4

© Hatch 2010/04 

Coen. For these cases the burner settings were changed with the intention of increasing the gas 

temperature from kiln into the preheat without significantly increasing the peak flame temperatures. 

The changes to the burner settings include an increase in ambient make-up air, delivery of the 

natural gas through the core gun only, and removal of the make-up air swirl. Table 1-3 provides a 

summary of variable input conditions and key output parameters for four variant cases. Case 6 

represents coal and natural gas fuel scenario with increased make-up air flow rate, while case 7 

corresponds to the tri-fuel mix with increased make-up air flow rate. Cases 8 and 9 consider the 

impact of an adjusted biomass size distribution. 

Low-NOx burner case 6 showed a sufficient increase of the kiln exiting gas temperature with the 

increased make-up air to the main burner for coal and natural gas fuel scenario. However, for the trifuel mix (case 7), predictions show a decrease in the kiln exit temperature. 

Low-NOx cases 8 and 9 investigated the sensitivity of gas temperatures in the system to the reduction 

of the biomass size distribution with the original and variant burner settings. Both cases predicted 

slight elevation of the kiln to preheat gas temperature, although the results are still approximately 

45°F to 55°F below current operating conditions. Predictions for NOx formation in variant cases are 

in the range of 5 – 35 lb/hr, which is similar to the results obtained for the initial set of low-NOx 

burner cases 1 and 3. 

Based on these results, which show significant NOx reduction in all cases, pilot testing of the 

proposed burner is warranted. The kiln gas exit temperature has been identified as the main process 

condition affected by the burner. When coal is the main fuel, the results indicate that extra make-up

air should be considered to maintain the kiln exit temperature at the required level. However, when 

biomass is used, an increased firing rate or decreased make-up air flow rate may be required to 

achieve the desired kiln gas exit temperature. In this case NOx production is expected to slightly 

increase; however, it is still expected to be significantly less that currently observed conditions due to 

large mass flow rate of air at ambient temperature.
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 

Case 1: Coal & NG Case 2: Biomass & 

NG 

Case 3: Coal, 

Biomass & NG 

Parameter 

Validation LowNOx Validation LowNOx Validation LowNOx 

Total MMBTU/hr (Main & PH burners) 246.0 217.2 248.1 

Natural Gas to Main Burner 

(MMBTU/hr) 

22.8 53.2 24.8 

% Natural Gas to Main Burner of 

Total 

9.3% 24.5% 10% 

Natural Gas to Main Burner (SCFH) 22,170 51,610 27,090 

Natural Gas to PH Burners 

(MMBTU/hr) 

78.7 78.7 78.7 

% Natural Gas to PH Burners of Total 32.0% 36.2% 31.7% 

Coal to Main Burner (MMBTU/hr) 144.5 0 91.6 

% Coal to Main Burner of Total 58.7% 0% 36.9% 

Coal to Main Burner (lb/hr)1 14,200 0 9,000 

Coal Conveying Air Flow (lb/hr) 21,930 0 4,500 21,500 

Biomass to Main Burner (MMBTU/hr) 0 85.3 53.0 

%Biomass to Main Burner of Total 0% 39.3% 21.4% 

Biomass to Main Burner (lb/hr)2 0 12,000 7,460 

Biomass Conveying Air Flow Rate 

(SCFM) 

0 1,450 960 1,450 960 

Biomass Conveying Air Flow Rate 

(lb/hr) 

0 6,490 4,300 6,490 4,300 

Atomizing Air Flow Rate (SCFM) 1,750 0 1,750 0 1,750 0 

Make-up Air Flow Rate (SCFM) 140 11,880 1,630 14,445 1,850 11,540 

Secondary Air Mass Flow Rate (lb/hr) 595,104 595,104 595,104 

CFD Gas Temp from Kiln into PH (°F) 2,240 2,175 2,270 2,270 2,280 2,220 

Change in Gas Temp from Kiln to PH 

(°F) 

-65 0 -60 

Pellet Grate Discharge Temp (°F) 1,730 1,690 1,710 1,710 1,740 1,710 

Pellet Grate Discharge Oxidation % 70% 69% 70% 70% 71% 69% 

NOx Exiting the Preheat [lb/hr] 212 9-80 386 10-80 202 8-77 

Expected Decrease in Thermal NOx 

% 

62-97% 79-97% 62-96% 

Table 1-1: Summary of Model Parameters for Validation and Low-NOx Burner Cases 1 – 3 

 

1

 Coal Higher Heating Value = 10,180 BTU/lb 

2

 Biomass Higher Heating Value = 7,100 BTU/lb 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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Parameter Case 4: NG Case 5: Fuel Oil & NG 

Low-NOx Low-NOx 

Total MMBTU/hr (Main & PH burners) 246.0 246.0 

Natural Gas to Main Burner 

(MMBTU/hr) 

167.3 22.8 

% Natural Gas to Main Burner of Total 68.0% 9.3% 

Natural Gas to Main Burner (SCFH) 162,430 22,170 

Natural Gas to PH Burners (MMBTU/hr) 78.7 78.7 

% Natural Gas to PH Burners of Total 32.0% 32.0% 

Coal Conveying Air Flow (lb/hr) 4,500 4,500 

Fuel Oil to Main Burner (MMBTU/hr) 0 144.5 

%Fuel Oil to Main Burner of Total 0% 58.7% 

Fuel Oil to Main Burner (lb/hr) 0 7,540 

Atomizing Air Flow Rate (SCFM) 0 1,750 

Make-up Air Flow Rate (SCFM) 24,300 24,300 

Secondary Air Mass Flow Rate (lb/hr) 595,104 595,104 

CFD Gas Temp from Kiln into PH (°F) 2,310 2,210 

Pellet Grate Discharge Temp (°F) 1,710 1,700 

Pellet Grate Discharge Oxidation % 70% 69% 

NOx Exiting the Preheat [lb/hr] 230 10 

Table 1-2: Summary of Model Parameters for Low-NOx Burner Cases 4 – 5 

Parameter Case 6: Coal & 

NG 

Case 7: Coal, 

Biomass & NG 

Case 8: Coal, 

Biomass & NG3

Case 9: Coal, 

Biomass & NG3

Make-up Air Flow Rate (SCFM) 12,770 12,770 11,540 12,770 

CFD Gas Temp from Kiln into 

PH (°F) 

2,230 2,160 2,235 2,225 

Pellet Grate Discharge Temp (°F) 1,690 1,670 1,670 1,670 

Pellet Grate Discharge Oxidation 

% 

69% 67% 67% 67% 

NOx Exiting the Preheat [lb/hr] 9 5 35 8 

Table 1-3: Summary of Model Parameters for Variant Cases 6 – 9 

2. Variant Cases 

Results of modeling work for low-NOx burner cases 1 and 3 indicate a decrease in the kiln to 

preheat gas temperature as compared with current operating conditions. Based on model predictions, 

U.S. Steel provided feedback on the importance of maintaining the magnitude of gas temperature 

exiting the kiln into the grate because it provides the main source of heat for the pellet induration 

 

3

 Cases are modeled using reduced biomass distribution as discussed in Section 2. 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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process. Consequently, four variant cases have been defined to investigate the effect of modified 

burner conditions on the temperature of gas exiting the kiln. 

The boundary conditions for the proposed variant cases have been adjusted to decrease the observed 

drop in the kiln to preheat gas temperature for low-NOx burner cases 1 and 3. Coen has provided 

recommendations for new burner settings which are listed in Table 1-3 along with key output 

parameters. For all four variant cases, the changes are as follows: 

• make-up air flow rate has been increased to 12,770 scfm 

• the model of the new low-NOx burner has been altered to provide natural gas only through 

the core gun slot and not through the annulus gun clusters 

• the swirl vanes have been removed from the burner make-up air annulus. 

2.1 Low-NOx Burner Case 6 – Coal and Natural Gas 

Low-NOx burner case 1: coal and natural gas, predicted a gas temperature of 2175°F exiting the 

kiln, which is a 65°F drop from the currently observed temperature on site for the same fuel 

scenario. Coen recommended increasing the amount of make-up air from 11,880 scfm to 12,770 

scfm in order to increase the temperature of interest back to the observed operating conditions. Fuel 

feed rates and all other model settings, as determined through the validation procedure, remained 

fixed. 

2.1.1 Pellet Bed Temperature Profiles 

Figure 2-1 shows comparison between average pellet bed temperatures as predicted by 1D models 

for low-NOx burner case 1 and newly defined variant low-NOx burner case 6. 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 

0

500

1000

1500

2000

2500

-50 -40 -30 -20 -10 0 10 20 30 40 50

Axial Position [m]

Pellet Bed Temperature [°F]

CC6 1D Grate Model Average

CC6 1D Kiln Model

CC1 1D Grate Model Average

CC1 1D Kiln Model

Figure 2-1: Comparison of Traveling Grate and Kiln Pellet Bed Temperatures for Cases 1 and 6 

The grate exit temperature of the pellets for low-NOx burner case 6, as predicted by 1D Traveling 

Grate model, is 1690°F which is the same as the model prediction for case 1. The impact of 

additional make-up air on pellet temperature is more significant for 1D kiln model results where kiln 

discharge temperature drops by 10°F for case 6. Since pellets are adequately heated in order to 

undergo necessary 60 – 70% oxidation in the traveling grate, the small difference in the final pellet 

temperature does not impact the product quality. 

2.1.2 Gas Temperature Profiles 

Figure 2-2 shows gas temperature values, as predicted by the 3D CFD model, along the center line of 

the kiln and preheat section of the grate for the coal and natural gas fuel scenario. Two dimensional 

vertical cross sections displaying gas temperatures for both low-NOx burner cases 1 and 6 are 

presented in Figure 2-3. 

Grate Exit Temperature 

∆T (CC6 – CC1) = 0°F 

Kiln Discharge Temperature 

∆T (CC6 – CC1) = -10°F 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 9

© Hatch 2010/04 

0

500

1000

1500

2000

2500

3000

-25 -15 -5 5 15 25 35 45

Axial Position [m]

CFD Model Gas Temperature [°F]

CC1 3D CFD Model

CC6 3D CFD Model

Figure 2-2: Comparison of CFD Gas Temperatures for Cases 1 and 6 

The parameter of interest for the variant case is the gas temperature exiting the kiln. Figure 2-2 shows 

that the temperature has increased by 55°F as compared with low-NOx burner case 1. The resulting 

temperature of 2230°F is only 10°F below the temperature observed on site when firing coal and 

natural gas fuel mix. Consequently, the addition of make-up air to the originally modeled air flow 

rate increases the kiln exit gas temperature, thus satisfying process requirements as defined by 

U.S. Steel. Corresponding peak flame temperature decreases by 60°F to 2970°F. Changes to the 

pellet bed temperatures are small and are not expected to affect the process. 

Kiln to PH Gas Temperature 

∆T (CC6 – CC1) = 55°F 

Peak Centerline Temperature 

∆T (CC6 – CC1) = 40°F 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 

Figure 2-3: Low-NOx Burner Cases 1 and 6 Gas Temperature Profiles 

2.1.3 NOx Emissions 

Figure 2-4 shows volumes of the kiln where the temperature exceeds 2780°F for low-NOx burner 

cases 1 and 6. The figure additionally shows contours within the volumes to indicate areas where 

temperature is above 3000°F. In these regions, NOx formation increases exponentially with 

temperature. 

Figure 2-4: Kiln Volumes with Gas Temperatures Exceeding 2780°F for Cases 1 and 6 

Low-NOx burner case 6 predicts that 0.5% of the volume exceeds temperature of 2780°F, while 

0.7% of the volume for case 1 is greater than 2780°F. Additionally, the peak flame temperature in 

the kiln decreases from 3030°F to 2970°F. Model settings for cases 1 and 6 are the same except for 

the increase of the amount of make-up air being added to the system, thus the decrease in flame 

temperature and area of NOx formation within the flame is due to the addition of air at ambient 

T[°F]

T[°F]

Peak Flame Temperature= 3030°F

Peak Flame Temperature= 2970°F
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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temperature. Although small changes in temperatures are observed, they are not significant enough 

to affect NOx formation and both models predict 9 lb/hr NOx formation. 

2.2 Low-NOx Burner Case 7 – Coal, Biomass and Natural Gas 

Low-NOx burner case 3: coal, biomass and natural gas,, the prediction for gas temperature exiting 

the kiln is 2220°F, which is a 60°F drop from currently observed temperature on site for the same 

fuel scenario. Coen recommended increasing the amount of make-up air from 11,540 scfm to 12,770 

scfm in order to increase the temperature of interest back to currently observed operating conditions.

Fuel feed rates and all other model settings, as determined through the validation procedure, 

remained fixed. 

2.2.1 Pellet Bed Temperature Profiles 

Figure 2-5 shows comparison between average pellet bed temperatures as predicted by 1D models 

for low-NOx burner case 3 and newly defined variant low-NOx burner case 7. 

0

500

1000

1500

2000

2500

-50 -40 -30 -20 -10 0 10 20 30 40 50

Axial Position [m]

Pellet Bed Temperature [°F]

CC7 1D Grate Model Average

CC7 1D Kiln Model

CC3 1D Grate Model Average

CC3 1D Kiln Model

Figure 2-5: Comparison of Traveling Grate and Kiln Pellet Temperatures for Cases 3 and 7 

Overall, the addition of make-up air decreases pellet bed temperatures in the grate for the tri-fuel 

scenario. The grate exit temperature drops by 40°F due to the changes in the kiln exiting gas 

temperature as outlined in the section below. Final kiln discharge temperature for case 7 is only 5°F 

greater than predictions for case 3, which is within model prediction error. 

Grate Exit Temperature 

∆T (CC7 – CC3) = -40°F 

Kiln Discharge Temperature 

∆T (CC7 – CC3) = 5°F 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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2.2.2 Gas Temperature Profiles 

Figure 2-6 shows gas temperature values, as predicted by the 3D CFD model, along the center line of 

the kiln and preheat section of the grate for the coal and natural gas fuel scenario. Two dimensional 

vertical cross sections displaying gas temperatures for both low-NOx burner cases 3 and 7 are 

presented in Figure 2-7. 

0

500

1000

1500

2000

2500

3000

-25 -15 -5 5 15 25 35 45

Axial Position [m]

CFD Model Gas Temperature [°F]

CC3 3D CFD Model

CC7 3D CFD Model

Figure 2-6: Comparison of CFD Gas Temperatures for Cases 3 and 7 

The parameter of interest for this variant case is once again the gas temperature exiting the kiln. 

Figure 2-6 shows that the temperature has decreased by 60°F as compared with low-NOx burner 

case 3. The resulting temperature of 2160°F is 120°F below the temperature currently observed on 

site when firing coal, biomass and natural gas. Unlike coal and natural gas fuel variant case 6, 

increasing the amount of make-up air in the tri-fuel scenario results in a lower kiln exit temperature, 

which is further away from than the process requirements than the original burner. 

Kiln to PH Gas Temperature 

∆T (CC7 – CC3) = -60°F 

Peak Centerline Temperature 

∆T (CC7 – CC3) = -35°F 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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Figure 2-7: Low-NOx Burner Cases 3 and 7 Gas Temperature Profiles 

2.2.3 NOx Emissions 

Results for low-NOx burner case 7 show that less than 0.01% of the volume exceeds temperature of 

2780°F, while 0.02% of the volume for case 3 is greater than 2780°F. Additionally, the peak flame 

temperature in the kiln decreases from 3030°F to 2970°F. Model settings for cases 3 and 7 are the 

same except for the increase in the amount of make-up air being added to the system, thus the 

decrease in flame temperature and area of NOx formation within the flame is due to the increased 

addition of air at ambient temperature. Since the volume of gas above the threshold temperature is so 

small for both cases, the cross-sectional view through the kiln produced by the CFD model does not 

provide a useful visual comparison. 

The small changes in kiln gas temperatures result in minor reduction of the predicted NOx formation 

from 8 lb/hr for low-NOx burner case 3 to 5 lb/hr for case 7. 

2.3 Low-NOx Burner Cases 8 and 9 – Coal, Biomass and Natural Gas 

Results of low-NOx burner cases 6 and 7 indicated different trends in the kiln exit temperature due to

addition of extra make-up air to the system. For case 6, the temperature exiting the kiln has risen to

match the operating conditions observed on site; however, for case 7 the effect of additional makeup air was the opposite – the difference between the predicted temperature of gas exiting the kiln 

and the operating conditions increased. 

The main reason for this difference is believed to be the size distribution of the solids. The biomass is 

much coarser than the coal as documented in Appendix A, which changes the speed at which 

combustion takes place and alters the reaction to extra cold primary air. 

Coen suggested that biomass particles fed through the main burner break down into smaller particles 

immediately upon entering the kiln and before undergoing combustion. As a result, reduction of the 

biomass size distribution by a factor of 4 has been applied to the tri-fuel case scenario to check he 

sensitivity of model prediction to the biomass size distribution.. The fuel feed rate of biomass has not 

been altered. Based on the expected effects of the reduced size distribution, two additional variant 

T[°F]

Peak Flame Temperature= 2940°F

Peak Flame Temperature= 2790°F
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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cases for tri-fuel scenario have been defined. Low-NOx burner case 8 uses the same model settings 

and boundary conditions as case 3, while applying the newly defined biomass size distribution. LowNOx burner case 9 uses the same model settings and boundary conditions as case 7, which 

corresponds to tri-fuel scenario with increased make-up air flow, while reducing the original biomass 

size distribution by a factor of 4. This approach allows for direct comparison of cases 8 and 9 to 

understand the effect of extra make-up air given smaller biomass particles. 

2.3.1 Gas Temperature Profiles 

Figure 2-8 shows gas temperatures, as predicted by the 3D CFD model, along the center line of the 

kiln and preheat section of the grate for the coal, biomass and natural gas fuel scenarios, with 

reduced biomass size distribution. Two dimensional vertical cross sections displaying gas 

temperatures for both low-NOx burner cases 8 and 9 are presented in Figure 2-9. 

0

500

1000

1500

2000

2500

3000

-25 -15 -5 5 15 25 35 45

Axial Position [m]

CFD Model Gas Temperature [°F]

CC8 3D CFD Model

CC9 3D CFD Model

Figure 2-8: Comparison of CFD Gas Temperatures for Cases 8 and 9 with Reduced Biomass Distribution 

Both cases have predicted changes in the kiln to preheat gas temperature due to the smaller size of 

biomass particles. For case 8, that temperature has increased by 15°F, compared to case 3 due only 

to the adjustment in the biomass size distribution. For case 9, the effect of changing the biomass 

particle size has resulted in 65°F increase of the gas temperature compared to case 7. Therefore, as 

shown in Figure 2-8, the additional make-up air has the effect of decreasing the kiln exit temperature

by 10°F. 

Kiln to PH Gas Temperature 

∆T (CC9 – CC8) = -10°F 

Peak Centerline Temperature 

∆T (CC9 – CC8) = -45°F 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

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Model results of all low-NOx burner cases for tri-fuel scenario are summarized in Table 2-1. All 

modeled cases predict a decrease in the kiln to preheat gas temperature as compared to current 

operating conditions with specified fuel mix. The smallest difference of 45°F is predicted for case 8 

which corresponds to tri-fuel scenario with added make-up air and reduced biomass size distribution. 

Coal, Biomass and Natural Gas Fuel Scenario 

Case 

Temperature 

of Gas Exiting 

the Kiln [°F] 

Difference from 

Target 

Temperature [°F] 

Validation Case 3 – target temperature 2,280 – 

Coen Case 3 2,220 - 60 

Coen Case 7 – Variant Conditions 2,160 - 120 

Coen Case 8 – Reduced Biomass Distribution 2,235 - 45 

Coen Case 9 – Variant Conditions with 

Reduced Biomass Distribution 

2,225 - 55 

Table 2-1: Comparison of Gas Temperature Exiting the Kiln for Variant Cases 7 - 9 

Based on model predictions, the reduction of the biomass size distribution does not change the trend 

of decreased kiln exit temperature with increased make-up air for the tri-fuel scenario. 

Figure 2-9: Low-NOx Burner Cases 8 and 9 Gas Temperature Profiles 

2.3.2 NOx Emissions 

Table 2-2 displays predicted amounts of NOx mass flow rate for each of the variant cases with tri-fuel

scenario. 

T[°F]

Peak Flame Temperature= 3085°F

Peak Flame Temperature= 2860°F
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

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Coal, Biomass and Natural Gas Fuel Scenario 

Case NOx Concentration 

[lb/hr] 

Barr Measurements 202 

Coen Case 3 8 

Coen Case 8 – Reduced Biomass Distribution 35 

Coen Case 7 – Variant Conditions 5 

Coen Case 9 – Variant Conditions with 

Reduced Biomass Distribution 

8 

Table 2-2: Comparison of NOx Predictions for Tri-fuel Scenario 

The peak flame temperature for cases 8 and 9 is 3080°F and 2860°F respectively, indicating that 

NOx formation is expected to be greater for the case without the additional make-up air. 

Low-NOx burner case 8 predicts that 0.4% of the volume exceeds temperature of 2780°F, while 

only 0.1% of the volume for case 3 is greater than 2780°F. Additionally, the peak flame temperature 

in the kiln rises from 2940°F to 3085°F. All fuel and air mass flow rates for cases 3 and 8 are 

identical, thus the reduced biomass size distribution results in an increase of the main burner flame 

temperature and consequently an increase in NOx formation from 8lb/hr to 35 lb/hr. Similarly, lowNOx burner case 9 predicts that 0.2% of the volume exceeds temperature of 2780°F, while less than 

0.1% of the volume for case 7 is above that temperature. The peak flame temperature increases from 

2790°F to 2860°F. Both cases are modeled with increased make-up air flows, while case 9 takes into 

account reduced biomass size distribution. Models predict an increase in the amount of NOx 

generated in the system from 5 lb/hr to 8lb/hr, which is consistent with results obtained for cases 3 

and 8. 

2.4 Summary of Variant Cases 

Four variant cases have been modeled in order to investigate the effect of variant burner settings on 

the gas temperature traveling from the kiln to the preheat section of the grate. Results for low-NOx 

burner cases 1 and 3 indicated ~ 60 – 65°F drop in temperature as compared to current operating 

conditions. Due to process concerns, this temperature decrease is not considered acceptable by U.S. 

Steel. 

Low-NOx cases 6 and 7 have been defined to look into the effect of additional make-up air on the 

coal and natural gas as well as coal, biomass and natural gas fuel scenarios. For these cases, variant

burner setting haven been defined by Coen. Case 6 has produced favorable data as a result of 

addition of approximately 900 scfm of make-up air. The predicted kiln exiting gas temperature 

increased by 55°F, which is only 10°F below current conditions for this fuel mix. However, case 7 

indicated the opposite trend in the temperature of interest with the addition of make-up air. The kiln

to preheat temperature decreased by 60°F, putting it more than 120°F below the target temperature 

observed on site. 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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The most likely reason for the decrease in the kiln exit gas temperature for the tri-fuel case is the 

slower combustion of large biomass particles with the addition of make-up air as compared to coal. 

Two more cases have been defined to investigate the sensitivity of gas temperatures to the biomass 

combustion with reduced particle size. Although case 8 and case 9 both show slight increase in the 

kiln exit temperature due to the reduction in biomass size, the result of variant burner settings is still 

to decrease the temperature below current operating conditions. Based on temperature predictions 

for variant cases, Coen believes that the added make-up air will not have the desired effect when 

biomass is used because biomass has a lower flame temperature than coal. 

As a result, it is recommended to run the tri-fuel scenario with an increased firing rate or decreased

make-up air flow rate in order to raise the gas temperature exiting the kiln. NOx production is 

expected to increase in either case; however, the amount of NOx generated is still expected to be 

lower than for existing operation due to the addition of large amount of ambient air to the system. 

Further adjustments do not require CFD modeling and should be done during pilot testing. 

3. Conclusions and Recommendations 

NOx measurements have been collected in 1A and 1B ducts on Line 7 during June 9 – 10, 2009 site 

visit to Minntac facility. Three most commonly used fuel scenarios were considered: (1) coal and 

natural gas, (2) biomass and natural gas, (3) coal, biomass and natural gas. The total measured mass 

flow rate of NOx was 212 lb/hr, 386 lb/hr and 202 lb/hr for these three fuel scenarios, respectively. A 

CFD modeling study has been conducted to evaluate the effects on both NOx production and 

process conditions due to installation of a new low-NOx burner designed by Coen. 

The CFD model and complementary 1D models have been validated against the NOx concentrations 

measured in the 1A and 1B ducts as well as temperature measurements collected by Barr Engineering 

and USS Minntac trend data for the same time period. Upon completion of the validation procedure, 

the established model settings have been fixed, and applied to predict the burner performance with 

the new low-NOx burner and adjusted primary air flow rates. 

Based on the results of the analysis, significant reduction of NOx is expected for the three most 

common fuel scenarios. For these three fuel scenarios, model predictions show that the maximum 

gas temperature decreases with the low-NOx burner, as compared to the existing burner. 

Furthermore, the gas volume within the kiln, where the temperature exceeds 2780°F, decreases with 

the low-NOx burner as compared to the existing burner, thus further supporting the prediction that 

NOx production will significantly decrease. 

Key results for all modeled cases are provided in Table 1-1 , Table 1-2 and Table 1-3. For cases 1 and

3, CFD model predicts a 62 – 96% decrease in the amount of NOx leaving the preheat, while case 2 

predictions indicate 79 – 97% decrease. All three cases show a significant decline in thermal NOx 

formation that is due to a corresponding decrease in the flame temperature. 
U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project

Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 

Low-NOx burner cases 4 and 5 have not been validated, thus predicted NOx formation cannot be 

compared to the existing burner conditions. Results for the low-NOx burner case 4 indicate a NOx 

formation rate of 230 lb/hr, which is comparable to results predicted for existing burner operation 

with both coal and natural gas and the tri-fuel mix. Results for the low-NOx burner case 5 predict 

NOx formation of 10 lb/hr. This value is comparable to the predictions for the most common fuel 

mixes. 

Additionally, the results show a decrease in the kiln to preheat gas temperature for low-NOx cases 1 

and 3. It is important to maintain the magnitude of gas temperature exiting the kiln into the grate as it 

provides the main source of heat for the pellet induration process. Modeling of four additional 

variant cases has been completed. The boundary conditions for the proposed variant cases have been 

adjusted to decrease the observed drop in the gas temperature exiting the kiln, as the temperature 

variation for low-NOx burner cases 1 and 3 is a process related concern. 

Results of low-NOx burner case 6 showed a sufficient increase of the kiln exiting gas temperature 

with an addition of make-up air to the main burner for coal and natural gas fuel scenario. For the trifuel mix, low-NOx burner case 7 predicted the opposite effect on the temperature making it 120°F 

below current operating conditions. Two more cases have been defined, cases 8 and 9, to investigate 

the sensitivity of gas temperatures to the biomass combustion with reduced particle size. Although 

both cases show slight increase in the kiln exit temperature due to the reduction in biomass size, the

result of variant burner settings is still to decrease the temperature below current operating 

conditions. It is therefore recommended to run tri-fuel scenario with an increased firing rate or 

decreased make-up air flow rate. NOx production is expected to increase slightly; however, it is still

expected to be less than currently observed conditions due to the large amount of added ambient air. 

All variant cases predicted NOx formation in the range of 5 to 35 lb/hr. 

Based on predicted NOx reduction in all cases considered, further pilot testing of this burner is 

warranted. Additional adjustments to low-NOx burner operation, in order to obtain optimal kiln to 

preheat gas temperature, should be done during on site testing.