U. S. Steel Minntac

Line 6 Low NOx Main Burner

Final Report

& Facility NOx Management

December 1, 2011
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Introduction

Title V permit 13700005-005 was issued to U. S. Steel Minntac on December 22, 

2008. This permit action included not only NOx limits on a 24-hour block 

average and 365-rolling sum basis, but also put forth requirements to 

incrementally take steps towards proving a NOx control technology for indurating 

lines, as nothing was currently installed for this purpose anywhere in the taconite 

industry. The requirements included Computational Fluid Dynamics (CFD) 

modeling, a technology review, and ultimately selection of technologies to pilot 

test. The goal was to find a technically and economically feasible NOx control or 

reduction technology which would be successful on taconite facilities. 

In 2009 Minntac conducted a trial of the Nalco Mobotec technology, which is 

Selective Non-Catalytic Reduction, or SNCR. Through the testing it was 

determined that the technology was not technically feasible. After further 

evaluating other technologies, U. S. Steel elected to pilot test a low NOx main 

burner. This is a NOx reduction technology which has significant cross media 

benefits as compared to post-combustion controls in that the pollutant is reduced 

versus removed and transferred to another media.

The first installation occurred on Minntac Agglomerator Line 7 in May of 2010. 

Some design issues were encountered and incorporated into a second 

generation burner which was installed on Agglomerator Line 6 in April of 2011. 

The purpose of this report is not only to provide a report on the status of the Line 

6 burner, but also to discuss NOx management at the facility as a whole. Based 

on concepts learned both from the theory, design and operation of the new 

burners, Minntac was able to implement additional projects at the facility which 

resulted in NOx reductions.
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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NOx Reduction Goals

The permit contains a goal of a 70% NOx reduction from a baseline of 13,300 

tons/year using a 365-day rolling sum from all five waste gas stacks combined. 

Since not all five lines operate exactly the same, or with the same fuel blend, it 

was initially a struggle to decide how to evaluate a 70% reduction. A simplistic 

way to evaluate this is to assume the lines operate 100% of the year and would 

have equal emissions. This would be a worst case scenario since the lines do 

not operate at that high of an operational uptime. Therefore, to be ultimately 

conservative the internal NOx target for short term testing, was broken down as 

follows: 3,990 tons/year (13,300 * 0.3) goal to reflect one induration line would be 

182.2 lb/hr 

182.2 lbs/hr for each indurationline

5 indurationlines

1

*

24 hours

day *

ton

2000 lbs *

365 days

1 year *

year

3,990 tons

On February 1, 2011 a permit limit of 7,300 tons per year became effective that 

replaced the 13,300 tons/year limit. Although the pound per hour target is used 

for short term evaluation, the form of the limit being all five waste gas lines 

combined is critical. This allows for operational flexibility while ensuring 

compliance. The combined limit is especially important as additional controls are 

installed. This allows for a start up and shake down period with the new 

equipment while minimizing emissions from the facility and ensuring compliance.

Continuous Emissions Monitoring System (CEMS)

In order to reduce NOx emissions it is necessary to know what the emissions are 

on a short term basis. This enables real time data to be used when testing and 

tuning the equipment to better understand and evaluate how the changes are 

affecting NOx performance. As a result of having reliable CEMS on all of the 

lines, Minnesota Ore Operations has been effectively measuring NOx 

continuously. With this information, baseline NOx values are known which allows 

for effectively testing NOx reduction technologies. 

NOx emissions are measured by CEMS that are located in the waste gas stacks. 

CEMS have been installed on all the operating lines at Minnesota Ore Operation. 

The following is an outline of the CEMs installation:

 CEMS units were initially installed on Minntac lines 6 and 7 around 2000. 

The measurement was done with a Sick AG Model GM 31-6 which is an 

in-situ instrument with a 5 foot measurement probe. 

 CEMS were installed on Minntac lines 3, 4, and 5 in 2006. The 

measurement is done with an Ametek model 920 extractive analyzer. The 
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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decision was made to go to a different analyzer type because it was a 

newer technology which appeared to be more reliable.

 CEMS unit was installed at Keetac in 2008. Keetac uses the same 

measurement instrument as Minntac Lines 3, 4, and 5, the Ametek 

extractive analyzer.

 In spring 2010, procurement was initiated for replacing the CEMS on 

Minntac lines 6 and 7 with the same equipment as the other lines. The 

purpose of the installation was to improve CEMS reliability. In addition, 

having identical CEMS on all lines allows for standardization across the 

two facilities (six lines total) which improved spare parts management. 

The new CEMS have been installed and passing Certification Tests were 

completed on February 23 and 24, 2011 for Line 6 and 7, respectively.

o Prior to the installation of the new CEMS, the Line 7 CEMS failed in 

October of 2010. A rental CEMS was procured and installed to 

ensure continuous NOx readings. This quick action enabled testing 

to continue with the Line 7 burner without risk of inaccurate data.

o Since the installation of the new CEMS on Line 6 and 7, the 

downtime has been reduced. The downtime since installations has 

been excellent. The 2011 third quarter NOx CEMS downtime 

report contained zero percent downtime for both Lines 6 and 7.
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Line 7 Low NOx Main Burner

The first low NOx main burner was installed on Line 7 in May 2010. As 

previously stated, this was the first installation of a NOx technology on an 

indurating line in the United States. This was also the first design and installation 

of a quad fuel burner of this size by the manufacturer. Figure 1 shows the burner 

tip configuration of the Line 7 burner. 

Figure 1: Line 7 Low NOx Burner Tip

During the evaluation of the burner on Line 7 some issues were encountered. 

This was expected given that this was an initial design for an operation not 

currently utilizing the technology. The following outlines the issues, concerns, 

and actions taken to improve the line 7 burner NOx performance. 

 Issue #1: Unable to achieve desired combustion airflow

 Concern: Sufficient air volumes are an essential element to reach the 

proper stoichiometric ratio to maximize NOx emission reduction.

 Actions taken:

o Oversped the combustion air fan VFD according to manufacturer 

recommendations

o Increased the combustion air inlet duct to the burner to reduce 

friction loss

o Design modification made on the Line 6 low NOx burner
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 Issue #2: Unable to meet the NOx reduction goal on coal

 Concern: Fuel flexibility is a critical component to the successful operation 

of the facility.

 Action taken:

o Installed coal insert to restrict the coal displacement at the burner 

tip

o Installed a coal cone that was anticipated to improve NOx 

performance by 20%, however NOx performance did not improve

o Design modification made on the Line 6 low NOx main burner

 Issue #3: Unable to achieve steady rates of biomass at higher flow rates

due to the biomass settling out in the burner pipe

 Concern: Fuel flexibility is a critical component to the successful operation 

of the facility.

 Actions taken:

o Design modification made on the Line 6 low NOx main burner

Figure 2 shows the timeline of the modifications that were made. 

May-10 Dec-10

Jun-10 Jul-10 Aug-10 Sep-10 Oct-10 Nov-10

May-10

Line 7 Low NOx Burner

 Installation

Sep-10

Oversped 

Combustion Airfan

Jul-10

Installed Coal Insert

Nov-10

Installed Coal Cone

and Modified Combustion 

Air Duct to 

Increase Airflow

Figure 2: Line 7 Low NOx Burner Timeline of Modifications
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The short term testing results on Line 7 showed that the low NOx main burner is 

a technically feasible method to achieve the goal of a 70% NOx reduction. 

Graph 1 shows the burner achieving the NOx reduction target when combusting 

100% natural gas. Based on the successful NOx reductions achieved on Line 7, 

U. S. Steel made the decision to move forward with a low NOx main burner on 

Agglomerator Line 6. In addition, the information from the Line 7 low NOx main 

burner was used to improve the design for the Line 6 low NOx main burner.

Line 7 - Low NOx Main Burner 100% Natural Gas

100

120

140

160

180

200

220

240

260

12/16/1012/23/1012/30/10

1/6/11

1/13/111/20/111/27/11

2/3/11

2/10/112/17/112/24/11

3/3/11

NOx (lbs/hr)

Graph 1: Line 7 Low NOx Main Burner NOx Performance Running 100% 

Natural Gas

Average = 181 lbs/hr
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Line 6 Low NOx Main Burner

The Line 6 low NOx main burner was installed in April 2011. There were 3 

significant design changes that were made when compared to the Line 7 low 

NOx main burner. The first modification was increasing the size of the 

combustion air fans. The fan capacity was increased by 25% to ensure there 

was sufficient combustion air to ensure the appropriate stoichiometric ratio to 

achieve the NOx reductions. This required an addition to the north side of the 

Step 3 Agglomerator building in order to house the combustion fans.

The second modification was switching the locations of the combustion air and 

coal annulus. Figure 3 shows the Line 6 burner tip. The goal of relocating the 

combustion air on the outside was to improve the flame shape and NOx 

performance when combusting coal.

Figure 3: Line 6 Low NOx Burner Tip

The third modification was modifying the biomass annulus to eliminate the issue 

of plugging when running increased flow rates of biomass. 

Combustion Air

Coal

Biomass

Atomizing 

Air

Annulus Gas

Core Gas

Fuel Oil
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Variables to Control NOx

The primary means of NOx control with the low NOx main burners is by 

controlling the stoichiometric ratio of air to fuel. This is accomplished by 

adjusting the combustion air. The combustion air lengthens the flame and 

reduces the flame temperature which reduces NOx.

The other means of NOx control is by adjusting the atomizing air and the gas 

split between the core and annulus gas. The atomizing air adds swirl to the 

flame and affects the flame length and shape. The split between the core and 

annulus gas is also monitored because the core gas lengthens the flame and 

annulus gas stabilizes the flame.

All three variables were adjusted during burner testing to determine the optimum 

settings to meet the NOx goal. The optimum settings are different for each fuel 

type, therefore significant testing is necessary in order to fully understand the 

technical aspects of the system and ensure NOx minimization while meeting 

other obligations such as pellet quality and a safe working environment.

NOx Testing with Natural Gas

Similar to the success observed on the Line 7 burner, the Line 6 main burner is 

able to achieve the internal NOx target of <182.2 lbs/hr. However, in order to 

achieve the same level of NOx reduction,the Line 6 burner required more 

combustion air when compared to the Line 7 burner. It was determined that Line 

6 combustion air had 40% less thrust than Line 7 combustion air. The decrease 

in air thrust resulted in a poorer flame shape and higher NOx emissions at the 

same stoichiometric ratios when compared to Line 7. The goal is to have a long 

tight flame shape in order to minimize NOx while ensuring proper process 

operation. The burner manufacturer was consulted about this issue to 

recommend changes.

As a result, the manufacturer recommended installing rings on inner and outer 

edges of the combustion air annulus to increase the air thrust. The modification, 

which took several days, was made to the burner in mid September 2011. Figure 

4 shows the burner after the modification. The modification did provide the 

benefit of improved NOx performance.
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Figure 4: Line 6 Low NOx Burner after the Burner 

Modification

A summary of the testing on natural gas on 9/26/11 is shown in Table 1. The 

testing parameters consisted of burning 100% natural gas while varying the total

combustion air to reduce NOx. 

Line 6 Low NOx Burner - 100% Natural Gas 

Feed Set Point Coal Feed Kiln Gas Total Air Total Kiln Fuel NOx Stoich

(LTPH) (lbs/hr) (MSCFH) (lbs/hr) (lbs/hr) (lbs/hr) Ratio

560 0 167 66,912 8,359 585 0.50

560 0 166 83,720 8,328 451 0.63

560 0 166 99,571 8,309 324 0.75

560 0 168 117,327 8,379 242 0.88

560 0 170 135,775 8,496 178 1.01

Table 1: Line 6 Low NOx Burner Testing with 100% Natural Gas

Inner Ring

Outer Ring
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Graph 2 illustrates NOx emission levels versus the stoichiometric ratio. The 

graph demonstrates that NOx is reduced when the stoichiometric ratio increases. 

Therefore, the stoichiometric ratio is crucial to lowering NOx emissions. 

Line 6 Low NOx Burner with 100% Natural Gas: NOx vs. Stoich Ratio

0

100

200

300

400

500

600

700

0.00 0.20 0.40 0.60 0.80 1.00 1.20

Stoichiometric Ratio

NOx lbs/hr

Graph 2: Line 6 Low NOx Burner with 100% Natural Gas: NOx versus 

Stoichiometric Ratio (9/26/11)

Graph 3 confirms that the Line 6 low NOx burner can meet the 70% NOx 

reduction, averaging 182 lbs/hr from October 28, 2011 – November 7, 2011.

Line 6 - Low NOx Main Burner 100% Natural Gas

100

120

140

160

180

200

220

240

260

10/28/1110/29/1110/30/1110/31/11

11/1/1111/2/1111/3/1111/4/1111/5/1111/6/11

NOx (lbs/hr)

 Graph 3: Line 6 – Low NOx Main Burner 100% Natural Gas

Average = 182 lbs/hr
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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NOx Testing With Coal

Coal testing was delayed on Line 6 due to safety issues related to the coal mill. 

Once the safety issues were rectified, coal was able to be utilized as a fuel again. 

However, after restarting the coal, a leak was found in the pipe. The line was 

shutdown and the flame was shut off in order to repair the coal pipe. Coal testing 

was then able to be started in early June. 

Line 6 has been unable to achieve the NOx target goal of 182.2 lbs/hr of NOx 

when running a mix of coal and natural gas. The burner manufacturer was 

consulted multiple times about this issue. It was determined that Line 6 

combustion air had 20% less thrust than Line 7 combustion air when running 

coal. Similar to the issue when running natural gas, the decrease in air thrust 

resulted in a poorer flame shape and higher NOx emissions.

Therefore, the rings the manufacturer recommended installing on the inner and 

outer edges of the combustion air annulus were to improve NOx performance 

when running coal and natural gas. Figure 4 shows the burner after the rings 

were installed.

Table 2 shows the results of the testing on Line 6 low NOx burner on 10/7/11

when running 10,000 lbs/hr of coal and on 11/18/11 when running 8,000 lbs/hr of 

coal. The testing parameters consisted of holding the coal feed rates steady and 

burning reduced amount of kiln gas while varying the stoichiometric ratio to 

reduce NOx. After the modification there was an improvement in the coal flame 

shape; however, Line 6 is still unable to achieve the NOx goal when running a 

mix of coal and natural gas.

Line 6 Low NOx Burner - 10,000 lbs/hr of Coal

Feed Set Point Coal Feed Kiln Gas Total Air Total Kiln Fuel NOx Stoich

(LTPH) (lbs/hr) (MSCFH) (lbs/hr) (lbs/hr) (lbs/hr) Ratio

560 9,986 85 78,604 14,245 284 0.56

560 9,991 85 92,561 14,221 280 0.66

560 9,995 85 106,723 14,240 268 0.76

560 9,998 85 121,051 14,242 255 0.86

560 9,999 84 134,846 14,217 243 0.96

560 10,018 85 149,906 14,268 240 1.06

560 9,993 85 163,726 14,245 243 1.16

Line 6 Low NOx Burner - 8,000 lbs/hr of Coal

Feed Set Point Coal Feed Kiln Gas Total Air Total Kiln Fuel NOx Stoich

(LTPH) (lbs/hr) (MSCFH) (lbs/hr) (lbs/hr) (lbs/hr) Ratio

560 8,000 96 96,456 12,804 264 0.71

560 7,999 100 110,889 12,986 263 0.80

560 8,000 97 121,820 12,830 244 0.90

560 8,007 98 135,120 12,893 233 0.99

560 7,996 102 150,100 13,075 234 1.07

Table 2: Line 6 Low NOx burner – Running Coal
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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It was determined that NOx is the lowest at approximately a stoichiometric ratio 

of 1. This is depicted in Graph 4. As previously shown and discussed, this is 

different than how the burner operates on 100% gas to maximize NOx 

reductions.

Line 6 Low NOx Burner with Coal: NOx vs. Stoich Ratio

200

210

220

230

240

250

260

270

280

290

0.20 0.40 0.60 0.80 1.00 1.20 1.40

Stoichiometric Ratio

NOx lbs/hr

10,000 lbs/hr Coal

8,000 lbs/hr Coal

Graph 4: Line 6 Low NOx Burner with Coal: NOx versus Stoichiometric 

Ratio

NOx Testing With Biomass

The Line 6 low NOx burner was designed to burn 300 lbs/min of biomass. 

However, consistent biomass feed rates at levels greater than 125 lbs/min were 

unable to be achieved. The manufacturer was consulted on this issue and they 

recommended that changes be made to the biomass system to improve delivery. 

The biomass supplier completed the recommendations in June, but no 

improvements were seen in the volume of biomass through the burner. In 

addition, the biomass supplier assisted in additional testing by supplying a 

constant material of oat hulls, which has a more consistent size distribution. This 

is different than typical biomass feed which is not a constant material. Again, this 

did not improve performance. 

Due to these issues, the burner tip was inspected in July to determine if there 

was plugging and it was found that the biomass line was deformed. Figure 5 

shows the burner tip at the July inspection. The burner manufacturer 

investigated the cause of the deformation and found that is was a design 

malfunction. The biomass pipe was designed to be a division plate, but the 

location of the tabs resulted in the thin biomass pipe wall to see excessive 

stresses which caused it to deform. 
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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No further biomass testing is planned until the biomass line is repaired. The 

repair will be extensive as the burner will need to be removed from the kiln,

disassembled, reassembled and reinstalled. The burner manufacturer is 

providing the recommendations for the repair. Due to the time necessary to 

design the modification and lead time to construct, the modification is planned to 

coincide with the Line 6 major repair in early 2012.

Figure 5: Line 6 Low NOx Burner Tip after Biomass Line 

Deformation

Biomass pipe

Tabs
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NOx Testing With Fuel Oil

Fuel oil is used as a back-up fuel for when natural gas is curtailed. As a result, 

only a short test was conducted on the Line 6 low NOx main burner with fuel oil. 

Fuel oil was tested on 8/4/11 and was run for approximately two hours. Graph 5 

shows that fuel oil can meet the can meet the 70% NOx reduction, averaging 165 

lbs/hr.

Line 6 - Low NOx Main Burner 100% Fuel Oil

100

120

140

160

180

200

8:308:428:549:069:189:309:429:54

10:06

NOx (lbs/hr)

Graph 5: Line 6 – Low NOx Main Burner with Running 100% Fuel Oil.

Process Issues

The addition of combustion air from the low NOx main burners has had two 

negative affects on the process. The first is an increase in process fuel 

consumption. After the installation of the Line 7 low NOx burner, the process fuel 

increased by 10.5%. As a result of the fuel increase, a change was made to 

increase the waste gas fans on lines 6 and 7 to balance the air flow through the 

system to maintain fuel rates at values prior to the installation of the low NOx 

burners. The waste gas fans on both lines were run at maximum before the 

burner installations and with the addition of combustion air, the process efficiency 

decreased. Now with the increase in waste gas fan flow, the air in and out of the 

system is balanced.

The second issue that has been encountered is pellets dumping hot from the 

cooler. This created steam and poor visibility on the burner floor which is a 

safety concern. The reason for dumping hot from the cooler is the 3A and 3B 

cooling fans decrease with the addition of combustion air. Figure 6 shows the 

Average = 165 lbs/hr
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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kiln and cooler section of the Agglomerator process. The 3A and 3B fans are 

controlled by kiln hood pressure. 

Figure 6: Kiln and Cooler Section of the Agglomerator Process

With the addition of the combustion air through the burner, the pressure 

increases and forces the 3A and 3B cooling fans to decrease their airflow. The 

3A and 3B fans cannot increase because the pressure would increase which is 

also a safety concern. The cooler dumping hot also results in excessive 

mechanical wear, high fuel rates, and below target pellet quality. Projects have 

been completed in order to eliminate this issue.

With the projects implemented and through process optimization, Line 6 has not 

seen an increase in process fuel consumption and dumping hot from the cooler 

has been corrected.

Impact to Pellet Quality

Four quality parameters were examined to determine if there was an impact to 

final pellet quality. The four quality parameters are after tumble, compressions, 

reducibility, and low temperature disintegration (LTD). Table 3 shows the values 

before and after the installation of the Line 6 low NOx burner.

After Tumble Compression Reducibility LTD

Before 96.0 416 1.15 87.30

After 96.0 417 1.22 85.58

Table 3: Line 6 Quality Before and After the low NOx Burner Installation

An improvement was seen in the reducibility and is believed to be due to 

improved heat distribution in the kiln from the low NOx burner. 

A decreased in LTD was seen and is due to the issues of dumping hot from the 

cooler. When material is dumping hot, water is added and the hot pellets are 

cooled too quickly. When pellets are not allowed to cool slowly enough, the LTD 

decreases. Graph 6 shows that the LTD has been increasing since the Line 6 

KILN HOOD 

PRESSURE
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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low NOx burner installation in April 2011. This is due to resolving the issues of 

dumping hot from the cooler.

Low Temperature Disintegration (LTD)

79.0

80.0

81.0

82.0

83.0

84.0

85.0

86.0

87.0

88.0

89.0

May-11 Jun-11 Jul-11 Aug-11 Sep-11 Oct-11

%

Graph 6: Line 6 Low Temperature Disintegration Monthly Quality Averages

Additional NOx Education

In addition to the installation of the low NOx main burners, the manufacturer was 

procured to provide training to the key facility personnel to develop a better 

understanding of the technology. This included an extensive three day course 

that covered fundamentals of heat transfer, fluid dynamics, and chemistry to 

provide the rudiments of the technology. Furthermore, a short course was 

provided to the operators and maintenance personnel on the floor to provide 

them of a basic understanding of the burner operation. This time was also used 

to provide the burner design engineer an opportunity to view both lines in 

operation and evaluate their flame properties.
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Other NOx Projects

In addition to the installation of the low NOx main burners, other projects have 

been completed at Minntac to reduce overall NOx emissions. The following is 

the list of projects:

 CEMS Management (Lines 3, 4, 5, 6, and 7)

o Alarms have been added to the control room to alert the operators 

that the CEMS are not working properly. Technicians are then 

called out to correct the issue. This reduces the usage of 

substitution NOx values.

 Process Optimization (Lines 3, 4, 5, 6, and 7)

o By optimization the process, the line runs more efficiently and less 

NOx is emitted.

 Coal Management (Lines 6 and 7)

o A modification was made to the coal mills to alleviate the issue of 

plugging the coal pipes and stopping coal. When the coal pipes 

plugged, it created a process upset and increased NOx emissions.

o A new coal was procured and managed to improve performance.

 Biomass Management (Lines 3, 4, and 5)

o The biomass supplier has improved their biomass delivery by 

ensuring there is sufficient biomass to burn steady rates on the 

lines. Before, especially during holidays, biomass would run out 

and 100% natural gas would be used which increases NOx on lines 

without the new burners.

 Blowers Installed on the Burners (Lines 3, 4, and 5)

o Similar to the concept of the Line 6 and 7 low NOx main burners, 

blowers were installed on the other lines to improve the air to fuel 

ratio, thus reducing NOx.

 Burner Modifications (Lines 4 and 5)

o The burner draft tubes were modified so that additional air could be 

put through the existing burner.

 OCS© Installation (Line 4)

o OCS©, and expert control system, was installed and programmed 

to reduce NOx emissions.

 OCS© Upgrade (Line 5)

o Line 5 OCS © was upgraded with the same programming as Line 4 

to reduce NOx emissions.
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 Ovation Upgrade (Lines 3, 4, and 5)

o The Lines 3, 4, and 5 computer control system was reaching 

capacity where no new control logic could be installed. To ensure 

that there is sufficient capacity for future NOx projects, the 

computer control system was upgraded.

Graph 7 below shows the Minntac NOx model. This model is discussed 

frequently with the management team to ensure all are aware of the NOx 

emission levels we are currently at, and where we are projected to be. The 

assumptions for the model are as follows:

1. Continue to Optimize Line 6&7 Low NOx Burner

a. Operate Safe - Don't Dump Hot

b. Maintain NOx at 180 lb/hr

c. Reduce Process Fuel

d. Avoid Excess Wear to Equipment

e. Produce pellet tons within quality parameters to meet business 

plans

2. Increase Utilization of OCS 

3. Minimize CEM Downtime

Minntac’s NOx compliance has been and continues to be on target. This was 

accomplished by the installation and optimization of the low NOx main burners 

on Lines 6 and 7; in addition to all of the other NOx projects that have been 

completed on all five induration lines as discussed above. Graphs 8-12 show the 

NOx reductions on the individual induration line. As you will see when viewing 

the graphs, the Agglomerator lines have each shown a steady decrease in NOx 

emissions, and the NOx intensity level from the facility has reached record lows.
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Minntac NOx Model

4,600

5,500

6,400

7,300

8,200

9,100

10,000

10,900

11,800

12,700

13,600 2005 2006 2007 2008 2009 2010 2011 NOx Tons (365 Day Rolling Sum)

6,000

7,000

8,000

9,000

10,000

11,000

12,000

13,000

14,000

15,000

16,000

 Pellet Tons (1000's)

Yearly NOx Data

Prediction

Pellet Tons

NOx Limit = 13,300 Tons/Yr

Revised on 10/31/2011 

New NOx Limit = 

7,300 Tons/Yr

Started 2/1/11

Compliance on Target

Graph 7: Minntac NOx Model
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Line 3 NOx

0

2

4

6

8

10

12

Aug-09

Sep-09

Oct-09

Nov-09

Dec-09

Jan-10

Feb-10

Mar-10

Apr-10

May-10

Jun-10

Jul-10

Aug-10

Sep-10

Oct-10

Nov-10

Dec-10

Jan-11

Feb-11

Mar-11

Apr-11

May-11

Jun-11

Jul-11

Aug-11

Sep-11

Oct-11

NOx (tons/day)

Graph 8: Line 3 NOx Performance August 2009 – October 2011

Line 4 NOx

0

2

4

6

8

10

12

Aug-09

Sep-09

Oct-09

Nov-09

Dec-09

Jan-10

Feb-10

Mar-10

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1

Graph 9: Line 4 NOx Performance August 2009 – October 2011
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

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Line 5 NOx 

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Graph 10: Line 5 NOx Performance August 2009 – October 2011

Line 6 NOx

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Graph 11: Line 6 NOx Performance August 2009 – October 2011
U. S. Steel Minntac - Line 6 Low NOx Main Burner Final Report and Facility NOx Management

Page 23 of 23

Line 7 NOx 

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Graph 12: Line 7 NOx Performance January 2009 – October 2011 

Conclusions

Minntac has committed significant time, money and resources to not only meet 

permit requirements by complying with emission limits and moving forward with 

technology evaluations. Based on testing conducted to date, Minntac is 

confident that low NOx main burners are a technically and economically feasible 

mechanism to achieve NOx reductions. The installation of two of these burners, 

along with a significant amount of other projects has resulted in dramatic 

reductions in NOx emissions from the facility as a whole.