U.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
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 MinntacU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
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 G as................................................. ......... 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 Na tural 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 ....................

................................................... ............................... 17U.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
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 invest igate 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 dev eloped process mathematical models have been used to estimate the expected level of NOx red uction 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 p rocess conditions.

The existing operating conditions on Line 7 have be en 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 l b/hr for these three fuel scenarios, respectively. The presented results are valid for the given set o f process conditions modeled, which are assumed to be representative of typical operation. In reali ty, the actual process conditions vary for many reasons, which will impact the actual process tempe ratures and related NOx production.

Table 1-1 and Table 1-2 list boundary conditions fo r the first five cases that have been modeled. In addition to the fuel scenarios most commonly used o n site, two alternate fuel mixes are considered: natural gas only, and fuel oil with natural gas. Th e additional fuel mixes are used primarily as emergency backup on site.

Results for cases 1 and 3 show that a 62 – 96% decr ease in NOx is expected with the low-NOx burner. Results for case 2 show that an 79 – 97% d ecrease 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 gene ration 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 a ffect the level of NOx produced and the percent reduction from the existing operation of the equipm ent.

Results for the low-NOx burner case 4 indicate a NO x generation rate of 230 lb/hr. This value is comparable to results predicted for the existing bu rner 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 sc enario.

Results for the low-NOx burner case 5 predict NOx f ormation 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 ga s temperature exiting the kiln into the preheat. This temperature decrease is a process concern beca use it represents a loss of process heat to the preheat section of the grate. To address this conce rn, variant cases have been defined by Hatch andU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
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 chan ged with the intention of increasing the gas temperature from kiln into the preheat without sign ificantly increasing the peak flame temperatures. The changes to the burner settings include an incre ase 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 a nd natural gas fuel scenario. However, for the trifuel mix (case 7), predictions show a decrease in t he 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 tempera ture, although the results are still approximately 45°F to 55°F below current operating conditions. Pr edictions for NOx formation in variant cases are in the range of 5 – 35 lb/hr, which is similar to t he 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 te mperature 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 decrea sed make-up air flow rate may be required to achieve the desired kiln gas exit temperature. In t his case NOx production is expected to slightly increase; however, it is still expected to be signi ficantly less that currently observed conditions du e to large mass flow rate of air at ambient temperature.U.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 5

© 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 Validati on and Low-NOx Burner Cases 1 – 3

1 Coal Higher Heating Value = 10,180 BTU/lb

2 Biomass Higher Heating Value = 7,100 BTU/lbU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 6

© Hatch 2010/04 Case 4: NG Case 5: Fuel Oil & NG Parameter
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 & NG 3 Case 9: Coal,

Biomass & NG 3 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 op erating conditions. Based on model predictions, U.S. Steel provided feedback on the importance of m aintaining 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 distributi on as discussed in Section 2.U.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 7

© Hatch 2010/04 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 ca ses 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 l isted 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 altere d to provide natural gas only through the core gun slot and not through the annulus gun c lusters

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



### 2.1 Low-NOx Burner Case 6 – Coal and Natural Gas
Low-NOx burner case 1: coal and natural gas, predic ted a gas temperature of 2175°F exiting the kiln, which is a 65°F drop from the currently obser ved 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 intere st back to the observed operating conditions. Fuel feed rates and all other model settings, as determi ned 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 Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 8

© 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-N Ox burner case 6, as predicted by 1D Traveling Grate model, is 1690°F which is the same as the mod el prediction for case 1. The impact of additional make-up air on pellet temperature is mor e significant for 1D kiln model results where kiln discharge temperature drops by 10°F for case 6. Sin ce pellets are adequately heated in order to undergo necessary 60 – 70% oxidation in the traveli ng 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 predict ed by the 3D CFD model, along the center line of the kiln and preheat section of the grate for the c oal 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°FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
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 t he gas temperature exiting the kiln. Figure 2-2 sho ws that the temperature has increased by 55°F as compa red with low-NOx burner case 1. The resulting temperature of 2230°F is only 10°F below the temper ature 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 de creases by 60°F to 2970°F. Changes to the pellet bed temperatures are small and are not expec ted to affect the process.

Kiln to PH Gas Temperature **∆** T (CC6 – CC1) = 55°F Peak Centerline Temperature **∆** T (CC6 – CC1) = 40°FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 10

© Hatch 2010/04 Figure 2-3: Low-NOx Burner Cases 1 and 6 Gas Temper ature Profiles 

### 2.1.3 NOx Emissions
Figure 2-4 shows volumes of the kiln where the temp erature exceeds 2780°F for low-NOx burner cases 1 and 6. The figure additionally shows contou rs 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 Exce eding 2780°F for Cases 1 and 6
Low-NOx burner case 6 predicts that 0.5% of the vol ume 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 set tings for cases 1 and 6 are the same except for the increase of the amount of make-up air being add ed to the system, thus the decrease in flame temperature and area of NOx formation within the fl ame is due to the addition of air at ambient T[°F]

T[°F] Peak Flame Temperature = 3030 °F

Peak Flame Temperature = 2970 °FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 11

© Hatch 2010/04 temperature. Although small changes in temperatures are observed, they are not significant enough to affect NOx formation and both models predict 9 l b/hr NOx formation.



### 2.2 Low-NOx Burner Case 7 – Coal, Biomass and Natural G as Low-NOx burner case 3: coal, biomass and natural ga s,, the prediction for gas temperature exiting the kiln is 2220°F, which is a 60°F drop from curre ntly observed temperature on site for the same fuel scenario. Coen recommended increasing the amou nt of make-up air from 11,540 scfm to 12,770 scfm in order to increase the temperature of intere st back to currently observed operating conditions. Fuel feed rates and all other model settings, as de termined 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 pell et 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 withi n model prediction error.

Grate Exit Temperature **∆** T (CC7 – CC3) = -40°F
Kiln Discharge Temperature **∆** T (CC7 – CC3) = 5°FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 

### 2.2.2 Gas Temperature Profiles
Figure 2-6 shows gas temperature values, as predict ed by the 3D CFD model, along the center line of the kiln and preheat section of the grate for the c oal 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. Unl ike coal and natural gas fuel variant case 6, increasing the amount of make-up air in the tri-fue l scenario results in a lower kiln exit temperature
, which is further away from than the process require ments than the original burner.

Kiln to PH Gas Temperature **∆** T (CC7 – CC3) = -60°F

Peak Centerline Temperature **∆** T (CC7 – CC3) = -35°FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 13

© Hatch 2010/04 Figure 2-7: Low-NOx Burner Cases 3 and 7 Gas Temper ature Profiles



### 2.2.3 NOx Emissions

Results for low-NOx burner case 7 show that less th an 0.01% of the volume exceeds temperature of 2780°F, while 0.02% of the volume for case 3 is gre ater than 2780°F. Additionally, the peak flame temperature in the kiln decreases from 3030°F to 29 70°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 forma tion within the flame is due to the increased addition of air at ambient temperature. Since the v olume of gas above the threshold temperature is so small for both cases, the cross-sectional view thro ugh the kiln produced by the CFD model does not provide a useful visual comparison.

The small changes in kiln gas temperatures result i n minor reduction of the predicted NOx formation from 8 lb/hr for low-NOx burner case 3 to 5 lb/hr f or case 7.



### 2.3 Low-NOx Burner Cases 8 and 9 – Coal, Biomass and Na tural Gas

Results of low-NOx burner cases 6 and 7 indicated d ifferent trends in the kiln exit temperature due to addition of extra make-up air to the system. For ca se 6, the temperature exiting the kiln has risen to match the operating conditions observed on site; ho wever, for case 7 the effect of additional makeup air was the opposite – the difference between th e 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 Appendi x A, which changes the speed at which combustion takes place and alters the reaction to e xtra cold primary air.

Coen suggested that biomass particles fed through t he main burner break down into smaller particles immediately upon entering the kiln and before under going 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 n ot 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 °FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 14

© Hatch 2010/04 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 t he 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 mak e-up air flow, while reducing the original biomass size distribution by a factor of 4. This approach a llows for direct comparison of cases 8 and 9 to understand the effect of extra make-up air given sm aller 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 pr eheat 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°FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 Model results of all low-NOx burner cases for tri-f uel scenario are summarized in Table 2-1. All modeled cases predict a decrease in the kiln to pre heat gas temperature as compared to current operating conditions with specified fuel mix. The s mallest difference of 45°F is predicted for case 8 which corresponds to tri-fuel scenario with added m ake-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 th e Kiln for Variant Cases 7 - 9
Based on model predictions, the reduction of the bi omass size distribution does not change the trend of decreased kiln exit temperature with increased m ake-up air for the tri-fuel scenario.

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



### 2.3.2 NOx Emissions
Table 2-2 displays predicted amounts of NOx mass fl ow rate for each of the variant cases with tri-fuel scenario. T[°F] Peak Flame Temperature = 3085 °F

Peak Flame Temperature = 2860 °FU.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

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© Hatch 2010/04 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-fu el Scenario
The peak flame temperature for cases 8 and 9 is 308 0°F and 2860°F respectively, indicating that NOx formation is expected to be greater for the cas e without the additional make-up air.

Low-NOx burner case 8 predicts that 0.4% of the vol ume 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 a nd air mass flow rates for cases 3 and 8 are identical, thus the reduced biomass size distributi on results in an increase of the main burner flame temperature and consequently an increase in NOx for mation 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 tempera ture. The peak flame temperature increases from 2790°F to 2860°F. Both cases are modeled with incre ased make-up air flows, while case 9 takes into account reduced biomass size distribution. Models p redict an increase in the amount of NOx generated in the system from 5 lb/hr to 8lb/hr, whi ch 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 in vestigate 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 temperatu re decrease is not considered acceptable by U.S. Steel.

Low-NOx cases 6 and 7 have been defined to look int o the effect of additional make-up air on the coal and natural gas as well as coal, biomass and n atural gas fuel scenarios. For these cases, variant burner setting haven been defined by Coen. Case 6 h as 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 i t more than 120°F below the target temperature observed on site.U.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 17

© Hatch 2010/04 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 t he 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 cas e 8 and case 9 both show slight increase in the kiln exit temperature due to the reduction in bioma ss size, the result of variant burner settings is s till to decrease the temperature below current operating conditions. Based on temperature predictions for variant cases, Coen believes that the added mak e-up air will not have the desired effect when biomass is used because biomass has a lower flame t emperature 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 tem perature exiting the kiln. NOx production is expected to increase in either case; however, the a mount of NOx generated is still expected to be lower than for existing operation due to the additi on 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 d ucts 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 l b/hr for these three fuel scenarios, respectively. A CFD modeling study has been conducted to evaluate t he 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 temperat ure 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 flo w rates.

Based on the results of the analysis, significant r eduction of NOx is expected for the three most common fuel scenarios. For these three fuel scenari os, 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 burn er, thus further supporting the prediction that NOx production will significantly decrease.

Key results for all modeled cases are provided in T able 1-1 , Table 1-2 and Table 1-3. For cases 1 and 3, CFD model predicts a 62 – 96% decrease in the am ount of NOx leaving the preheat, while case 2 predictions indicate 79 – 97% decrease. All three c ases show a significant decline in thermal NOx formation that is due to a corresponding decrease i n the flame temperature.U.S. Steel Minntac - Line 7 NOx Reduction Pilot Pro ject
Results of Variant Cases - April 13, 2010

ISO 9001 H332571-07-55-124-0001, Rev. 1, Page 18

© Hatch 2010/04 Low-NOx burner cases 4 and 5 have not been validate d, 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 ki ln 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 ind uration process. Modeling of four additional variant cases has been completed. The boundary cond itions for the proposed variant cases have been adjusted to decrease the observed drop in the gas t emperature exiting the kiln, as the temperature variation for low-NOx burner cases 1 and 3 is a pro cess related concern.

Results of low-NOx burner case 6 showed a sufficien t 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 tri

fuel mix, low-NOx burner case 7 predicted the oppos ite 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 te mperature due to the reduction in biomass size, the result of variant burner settings is still to decre ase 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 conditi ons due to the large amount of added ambient air. All variant cases predicted NOx formation in the ra nge of 5 to 35 lb/hr.

Based on predicted NOx reduction in all cases consi dered, 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 s ite testing.