NOx Reduction Technology Evaluation and Mitigation 

Proposal for Keetac Expansion 

February 10, 2010 

Project 

United States Steel Corporation, Minnesota Ore Operations, is proposing to expand the production 

capacity of its facility in Keewatin, MN. The facility (Keetac) is located in Northern Minnesota, and is 

approximately 50 miles from the Boundary Waters Canoe Area Wilderness (BWCAW) and 100 miles 

from Voyageurs National Park in northeastern Minnesota. 

SO2 emissions from the proposed project will be controlled under Prevention of Significant Deterioration 

(PSD) Best Available Control Technology (BACT) requirements for emission controls. The NOx

emission increase for the facility with proposed permit limits is less than the major modification threshold 

and therefore the proposed project NOx emissions are not subject to PSD permitting requirements. 

The Project has a potential to emit (PTE) of 3,110 tons per year of nitrogen oxides (NOx). NOx emissions 

from the facility will exceed the 100 ton per year threshold established at 40 CFR 52.21(b)(1)(i)(a) 

therefore the Project would be classified as a major source for NOx emissions. Keetac has calculated the 

baseline emission for the existing facility to be 5,805 tons of NOx per year. Keetac has proposed permit 

limits that will limit emissions from both furnaces for a combined total of 5,840 tons per year. This results 

in an emission increase from the project of 35 tons per year, less than the major modification threshold of 

40 tons per year. Therefore the NOx emissions from the project are not subject to BACT or other PSD 

permitting requirements. 
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The PSD emission threshold is not triggered because the existing furnace historically was only fired with 

natural gas. In 2005 a permit was issued that allows the furnace to burn coal, which in a grate-kiln 

furnace leads to much lower NOx compared to natural gas combustion. Through a fueling strategy for 

both the new furnace that will utilize biomass and natural gas with coal and fuel oil as back-ups and the 

use of coal, natural gas, or fuel oil for the existing furnace, NOx emissions can be maintained less than the 

baseline. To meet the proposed NOx limits, permit conditions will require compliance demonstration 

through the use of a CEMS on both lines. This will allow for real time fuel choice decision making and 

ability to take advantage of operating information. The associated limit on the expansion kiln to meet 

netting requirements will be a 180 rolling day sum of 1157 tons which is equivalent to approximately 

2314 tons per year. 

Annual emissions will not increase above the threshold compared to historical baseline emissions. 

However, changes in stack parameters and the potential to burn 100 % natural gas for short periods of 

time need to be evaluated and compared to the impacts on visibility of the proposed project. Visibility 

standards in both parks can be affected by NOx and SO2 emissions from sources throughout the mid-west. 

 This NOx emission reduction technology evaluation and associated air quality modeling analysis are 

being used to demonstrate that NOx emissions from the proposed project do not adversely affect visibility 

in these parks. 

Normal operation of the facility is expected to be a 60/40 coal/natural gas mix on the existing line and a 

50/50 biomass/natural gas mix on the new line. Alternative fuel use is proven to reduce the generation of 

NOx. Typically 100% natural gas is utilized for short periods during startup or if there is an issue with 

alternative fuels or the process. Although issues can arise with the coal and/or biomass systems, it is 

unlikely that both systems would be down for an extended period of time. The new line will be capable 

of utilizing both biomass and coal which will further reduce the likelihood to burn 100% natural gas for 

extended periods of time. 

The proposed project includes renovation of a taconite induration furnace that has been shutdown for an 

extended period and is being permitted as a “new” furnace. The furnace is a grate/kiln design. NOx

emissions originate from fuel combustion in the kiln section of the furnace. This NOx reduction 

technology evaluation will consider potential NOx reduction measures which include: 

1. Reduced NOx generation from fuel combustion through improvements in process design that 

reduce energy consumption as compared to the existing Keetac furnace. 
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2. Reduced NOx generation by using combustion technology for minimizing NOx generation 

3. Control of NOx emissions through use of add-on emission control equipment. 

4. Fueling strategy to minimize NOx emissions 

Following the evaluation of potential control technologies is a summary of modeled impacts and a 

proposed approach for mitigating visibility impacts in the Class I areas. 

Overview of Emission Reduction Technologies 

Several control technologies are included in this NOx control technology review for Keetac’s Expansion 

Project. The emission reduction technologies evaluated include process designs, controls which reduce 

the generation of NOx, add-on controls and fueling strategies. 

Reduce NOx generation through process improvements that reduce 

energy use 

The taconite pellet production process is very energy intensive. The unfired pellets must be heated to 

2,300°F to 2,400°F in order to achieve the desired physical and metallurgical properties in the finished 

taconite pellets. Once the pellets have been fired, air is used to cool the pellets down to safe handling 

temperatures. Taconite induration furnaces are designed to recover heat from the process by using hot air 

exhausted from the pellet cooling to pre-heat unfired pellets coming into the furnace and to pre-heat 

combustion air used in the kiln burner. 

Because NOx emissions are directly related to fuel combustion, improvements in heat recovery design can 

reduce the generation of NOx emissions by reducing fuel consumption. The new taconite induration 

furnace is designed to use approximately 36% to 44% less fuel than the existing Keetac furnace, on an 

mmbtu per short ton of pellet produced basis. NOx emission rates used in the emission inventory were 

used as model inputs and were reviewed by the MPCA. The NOx emission rates are based on testing 

results from furnaces with lower heat recovery and a higher fuel use rate than expected for the new 

furnace. The reduced fuel usage numbers were developed using process design inputs from Hatch. These 

improvements are accomplished by: 

 Expanding the preheat and cooling sections of the furnace to increase the available pellet surface 

for heat transfer 
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 Improved air flow patterns for more efficient transfer of heat 

 Optimizing use of recovered energy between heating unfired pellets and combustion air 

 Improved air seals to minimize infiltration of cold air into the furnace 

 Improved insulation to minimize heat loss 

Process improvements are already incorporated into the design of the new furnace. Therefore, a direct 

cost feasibility of process improvement is not necessary. Typically, cost evaluations are not required of 

technologies that are selected for implementation. 

The exact relationship between reduced fuel usage and the resulting reduction in NOx emissions is not 

known. Due to the uncertainty associated with the exact reduction in NOx emissions that will be 

achieved, no change was made to the emission rates that were used for model inputs. 

Reduce NOx generation through use of combustion technology 

Two key factors that lead to the generation of NOx are temperature and amount of oxygen (O2) present. 

NOx emissions may be reduced by minimizing flame temperatures. This may be accomplished with flue 

gas recirculation and optimization of the burner design. NOx emissions can also be minimized by 

minimizing the amount of O2 present. However, in a taconite induration furnace, oxygen (O2) levels must 

be kept at 15% O2 so that sufficient oxygen is present for the oxidation of iron ore from magnetite to 

hematite. Oxidation of the ore is needed so that finished pellets have the correct metallurgical properties 

for use in a blast furnace. The oxidation process also provides most of the heat needed for proper pellet 

formation. In boilers and process heaters flue gas oxygen levels of 2% - 3% can be achieved thereby 

minimizing NOx formation. The same level of oxygen control is needed for NOx control by flue gas 

recirculation. To reach maximum energy efficiency, part of the heat recovery system must be used for 

preheating the combustion air. So, the ability to limit combustion air temperatures is limited. 

Due to the complex nature of controlling the combustion process in a rotary kiln, burner design is 

typically done by modeling the burner and kiln using computational fluid dynamic (CFD) models. At the 

U. S. Steel Minntac facility, an investigation of burner design options by CFD modeling is being 

conducted by Hatch, the project engineering group, and Coen, a burner manufacturer. Preliminary results 

for the grate/kiln systems at Minntac suggest that a NOx reduction upwards of 40% may be feasible with 

advanced burner designs for the kiln burner. Similar results may be possible on the new line at Keetac. 

However, differences in the grate/kiln design at Minntac vs. Keetac and fueling strategies may affect the 

level of NOx reduction achievable. Because of uncertainty associated with the exact reduction in NOx
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emissions that will be achieved, no change was made to the emission rates that were used for model 

inputs.

Reduce NOx emissions by using add-on emission control equipment 

Selective Catalytic Reduction (SCR) 

Selective catalytic reduction is a post combustion NOx control technology in which ammonia (NH3) is 

injected into the flue gas stream in the presence of a catalyst. SCR control efficiency on utility boilers is 

typically 70% - 90%. SCR requires an optimum temperature range of 570-850F. 

For the Keetac Expansion Project, there is the option of placing the SCR system in two different 

locations. These locations and their related feasibility and costs are described below. 

Selective Catalytic Reduction (SCR) on Pre-Heat (PH) Zone exhaust stream 

SCR is a potential candidate for control of NOx emissions from the PH exhaust stream because the design 

exhaust gas temperature is within the temperature range where SCR works properly. However, this 

technology has been identified as technically infeasible due to high solids loading as compared to power 

plant applications. There is also concern with hydrogen chloride and potassium compounds, as well as 

the SO2 and SO3 content of the gas stream. The high moisture content and ammonia injection would 

strongly promote the production of ammonium sulfate and ammonium bisulfate. Ammonium sulfate salts 

can cause catalyst deactivation and are also corrosive. Ammonium sulfate deposition could affect both 

the SCR system as well as the traveling grate system in DDD1 which would be downstream of an SCR 

system in this option. In addition, SCR catalyst will promote oxidation of SO2 to sulfuric acid (H2SO4). 

Sulfuric acid formed in an SCR reactor could cause additional corrosion problems in DDD1. Based on the 

factors listed above, installation of a SCR system at the PH exhaust is technically infeasible, and is not 

evaluated further. 

SCR following Electrostatic Precipitator (ESP) on PH exhaust stream 

LKAB, a taconite production facility located in Sweden, contracted with Alstom Power to construct a gas 

cleaning system on the PH exhaust stream at its KK4 plant. The gas cleaning system uses an ESP to 

remove particulates from the PH exhaust prior to NOx treatment by an SCR system. Reportedly the SCR 

system at LKAB was designed for 90% NOx removal. The SCR system is followed by heat recovery and 

SO2 controls. An ESP upstream of an SCR reactor would remove a significant amount of particulate 

matter and the contaminants associated with particulate matter that are catalyst poisons. This control 
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scheme does not address potential adverse impacts on SCR catalyst from SO2, SO3 and HCl described in 

the SCR analysis above. 

Representatives of U. S. Steel and LKAB met at Keetac during the summer of 2009 for a tour and 

meeting. Metso, LKAB’s design engineer, also attended. At the time of the meeting, LKAB’s KK4 line 

had been operating for approximately 6 months. The representatives from Metso and LKAB discussed the 

SCR technology installed on KK4. They discussed that while in theory SCR should work, the tight 

temperature profile didn’t allow for successful operation. In addition, LKAB representatives said they did 

not have any catalyst poisons in their gas stream. They also informed U. S. Steel that they would pursue 

more efficient low NOx burners in the future. 

Alstom Power, the company that designed the SCR system at LKAB, was requested by U. S. Steel to 

provide a bid for an SCR system at Minntac and subsequently declined to bid. Additionally, Mitsubishi 

declined to bid saying that SCR was not technically feasible. 

U. S. Steel previously has asked representatives of the MPCA for any information that the MPCA has 

received from the Swedish government on performance of the SCR system at LKAB. The MPCA has 

stated that they haven’t received any response from the Swedish government on the performance of SCR. 

Based on these factors, SCR with ESP pre-treatment is considered technically infeasible. 

Regenerative Selective Catalytic Reduction (RSCR) 

Regenerative Selective Catalytic Reduction (RSCR) is a form of clean-side SCR technology. The RSCR 

system is located after the particulate control device, which minimizes impact to the SCR catalyst. While 

the SO2 and SO3 may be removed from the flue gas stream with a GSA/ESP emission control system, 

other contaminants, which have the potential to poison the SCR catalysts, may not be removed. Ammonia 

is added to the flue gas stream in order to reduce NOx to N2 and H2O. For the DDD1 exhaust stream, the 

temperatures at this location are approximately 165F. Because this temperature is below the required 

temperature for NOx SCR operation, the flue gas temperature must be raised to between 570F and 650F. 

 Temperature elevation takes place in the RSCR, where a regenerative heater (RH) is used. The RH uses 

beds of ceramic media to achieve thermal efficiencies of >95%. The goal of the RSCR system is to 

combine heat recovery, temperature control, and catalyst elements of SCR into a single unit. RSCR is 

designed for heat efficiency and lower operating costs. 
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While this technology could theoretically work at a taconite facility, this technology has yet to be proven 

effective for this industry. There is the potential that constituents in the induration furnace exhaust, not 

present in coal fired utility boiler exhaust streams, would poison the RSCR catalyst. Poisons include 

calcium oxide, arsenic, potassium, alkali and alkali-earth metals and phosphorous. Calcium oxide, which 

is present in pellet fluxing reactions, could also poison the catalyst. The concentrations of these 

potential catalyst poisons at the RSCR inlet is unknown and could significantly impact the effectiveness 

of this control. The technical feasibility of RSCR is currently under investigation by U. S. Steel at 

Minntac, but RSCR has not been installed on a taconite furnace and therefore is an innovative and 

unproven technology. 

Selective Non-Catalytic Reduction (SNCR) 

Selective Non-Catalytic Reduction (SNCR) utilizes aqueous ammonia or urea to reduce NOx emissions. 

The optimal range for the flue gas temperature is 1800-2000 deg. F. This technology was pilot tested at 

Minntac and considered technically infeasible. 

Low Temperature Oxidation (LoTOx) 

BOC Gases’ LoTOx technology is a form of low temperature oxidation. LoTOx is an innovative NOx

control technology which, to date, has not been applied to a taconite induration furnace or a source with 

similar size and exhaust gas characteristics. This technology utilizes ozone, which is generated from pure 

oxygen on site with an electrically powered ozone generator. The ozone generation rate is controlled to 

match the needs for NOx control. The ozone used to oxidize NO to NO2 and NO2 to N2O5 prior to 

removal in a wet scrubber (absorber). The N2O5 is converted to HNO3 in a wet scrubber, and may be 

neutralized with lime or caustic. Additional wastewater treatment is necessary to remove the nitrates 

from the wastewater stream and would require further evaluation. Another concern is whether or not iron 

dust in furnace exhaust gas would react with ozone and reduce the control efficiency of LoTOx. The 

normal NOx control efficiency range for LoTOx is 80% to 90% with outlet NOx concentrations in the 

15ppm – 25 ppm range. This Project will utilize dry controls, whereas LoTOx requires the use of a wet 

scrubber. LoTOx has not been installed on a taconite furnace and therefore is an innovative and unproven 

technology. Therefore, this technology is considered technically infeasible. 
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Powerspan ECO 

The ECO is a multi-pollutant control system, controlling SO2, NOx, Hg, and PM. The ECO has only been 

applied to power plants, so the removal rates are not necessarily representative of what Keetac could 

expect. The product website cites 99% SO2, 90% NOx, 80-90% Hg, and 95% control of PM2.5 emissions. 

 The system relies on an activated carbon filter to remove aqueous mercury from the wastewater stream, 

which is primarily ammonium sulfate. Powerspan states that the stream will be pure enough to sell as 

fertilizer. There will be liquid and solid waste generated from the activated carbon filtration. 

The treatment train consists of the following: 

ESP → ECO Reactor (plasma oxidizer) → wet absorber and ammonia addition (removes SO2, NOx, and 

oxidized mercury) → wet ESP (final PM removal). 

Keetac’s proposed system will control 90% SO2, 0% NOx, 50% Hg, and 95% of PM2.5 emissions. 

Keetac’s current proposed system looks like the following: 

Spray Dryer Absorber → Activated carbon injection → ESP. 

Since this technology is innovative, this technology is technically infeasible. 

Fueling Scenarios 

NOx generation is maximized when the existing line and the new line are running 100% natural gas. 

Control technology evaluations routinely require evaluation of the effectiveness of fueling scenarios on 

pollutant generation. The range of fueling scenarios was evaluated to understand and define ranges 

reflective of operating conditions for the kilns. 

NOx emissions were calculated by multiplying fuel specific emission factors by the fuel mix used and the 

process throughput to get a total lb/hr rate. 

Fuel specific emission factor (lbs/ton pellet) x max pellet through put (tons/hr) = lbs/hr NOx

Max hourly rates: Expansion kiln - 450tph, Existing kiln - 830 tph 

Total annual production rates do not equate directly to instantaneous hourly max rates because it is not 

feasible to run the line continuously thought out the year. 
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The expansion kiln emission factor was modified by assuming a conservative 20% reduction in NOx

emissions due to the heat recovery design of the new furnace. These calculations show that an emission 

rate of 1,655 lb/hr is an approximate bound on where both kilns can run at the same time while one is 

firing 100% natural gas. The 1,655 lb/hr rate was determined primarily by evaluating kiln fueling 

scenarios by using this estimate of improved efficiency for the expansion kiln (applied to the existing 

emission factor with a 20% decrease). 

The attached graph shows these operating scenarios. The operating envelope that is contained within the 

boundaries of this limit is shown as the green area on the graph. The operating scenarios shown range 

from 100% natural gas for both kilns to 0% natural gas/100% solid fuel for both kilns. The red portion of 

the graph reflects operating scenarios where the 1655 lb/hr limit would be exceeded. Note that the target 

operating scenario (60% solid fuel existing/50% solid fuel expansion) is well below the 1655 limit. The 

emission rate for the 100% natural gas expansion kiln/60% solid fuel-40% natural gas existing kiln, 

which represents the solids target rate for the existing kiln, is also well below the 1655 lb/hr limit. When 

firing the expansion kiln at 100% natural gas, the natural gas component of the fuel mix of the existing 

line can be as high as 55% and still be under the 1655 lb/hr limit. 

The 100% natural gas existing/100% solid fuel expansion scenario is approximately 1655 lb/hr. See the 

following graph. 
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In summary the above graph shows the fueling scenarios corresponding to the green area of the graph will 

result in an emission rate less than 1655 lbs/hr. As explained below, U. S. Steel is proposing to have a 

NOx emission limit of 1655 lb/hr for the combination of emissions from the existing line and new line. As 

identified from the red area of the graph which defines emissions above the 1655 lb/hr limit, measures 

will be taken to ensure that the limit is not exceeded. 

Summary of Class I Modeling 

Class I modeling was completed in accordance with the protocol submitted in November 2008. Visibility 

modeling requires that the worst case scenario is used to determine the impact to Class I Areas. The 

worst case scenario for this project is when 100% natural gas is used on the existing line and new line. 

Modeling 100% natural gas on both lines resulted in a modeled impact. A NOx emission rate of 2,727 

lb/hr was studied in the draft Class I report from January 2009. As indicated in the Draft Environmental 

Impact Statement (DEIS), U. S. Steel will be required to propose a mitigation plan to address the modeled 

impact. The results reported in the Draft EIS were: 

o Days >10% = 6 
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o Days >5% = 31 

The modeling assessed what has been referred to as “Scenario 4-1”. This scenario consists of Scenario 1 

(the existing facility, as operated prior to solid fuel and the scrubber addition) subtracted using CALSUM 

from Scenario 4 (the full facility, existing and expansion sources, as proposed after the expansion). 

Emission rates reflect the 24-hour maximum rates. POSTUTIL was used on Scenarios 1 & 4 prior to their 

combination with CALSUM, using MNITRATE=1, with Ammonia background concentrations of 1 ppb. 

Several visibility analyses were performed using CALPOST. 

For FLAG 2000, Method 2 was used, with background concentrations based on the Annual Average 

Natural Background. Initial submittals used the default Eastern US background found in the FLAG 2000 

Appendices. FLAG 2000 noted that these background concentrations “...will serve until better estimates 

of natural condition are available.” Recent conversations have indicated that the site-specific background 

concentrations found in the Appendices to the proposed FLAG guidance (FLAG 2008) are acceptable, 

and these are used in the results provided here. 

For FLAG 2008: Method 8 was used, with background concentrations based on the 20% Best Days 

Natural Background, as provided in the Appendices to FLAG 2008. 

Subsequent modeling has been performed using the November 2008 protocol as the basis with revised 

emissions that reflect a 24-hour block average limit of 1655 lb/hr as identified in the fueling scenario 

evaluation. An additional analysis was added to this round of modeling reflecting a BART-like Analysis. 

Method 6 was used, with background concentrations based on the 20% Best Days Natural Background, as 

provided in the Appendices to FLAG 2008. Visibility modeling using an emission rate of 1,655 lb/hr 

from the fueling strategy analysis resulted in the following: 

o Days >10% = 0 

o Days >5% = 4 

o BART Method Days DdV 0.5 = 3 

for the total of the three years modeled. 

Mitigation Proposal 

U. S. Steel has committed in the permit application to have the following new mobile equipment meet 

Tier IV emission standards. The visibility modeling used the lower emission rates from the Tier IV 

mobile equipment. The applicable new equipment includes: 
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 Diesel-driven Haul Trucks 

 Diesel-driven Mining Shovels 

 Diesel-driven Front-end Loaders 

 Diesel-driven D10 Dozers 

 Diesel-driven Grader 

These commitments apply to equipment acquired for the expansion facility. U. S. Steel will include more 

information on the feasibility to retro fit existing trucks in the future, but can not provide this information 

currently. 

U. S. Steel will agree to a 24-hour block average limit of 1,655 lb/hr for sum total of emissions from both 

of the induration processes. Compliance with this emission limit will be demonstrated through the use of 

CEMs on both lines. 

In addition, U. S. Steel has agreed to a 180-day Rolling Limit to meet the PSD netting requirements. This 

limit will restrict the amount of time the facility is able to burn 100% natural gas while still ensuring both 

limits are met. 

U. S. Steel also considered the cumulative emission impact as compared to the goals of the NE Regional 

Haze Plan. See the attached Regional Haze SIP Northeast Minnesota Plan emission tracking spreadsheet. 

In summary, under a majority of the fueling scenarios as shown in the graph above, U. S. Steel would be 

below the proposed limit of 1655 lb/hr on a 24-hr block. Under the 100% natural gas on both lines 

scenario additional NOx mitigation would need to be employed in order to be in compliance with the 

proposed limit. Additional NOx mitigation includes but is not limited to identification and 

implementation of additional operating and maintenance practices after the unit is operating to further 

reduce NOx emissions, reduction of production, idling of a line until the issue requiring the use of 100% 

natural gas on both lines is resolved. 

Other Mitigation Opportunities Investigated by U. S. Steel 

The following other mitigation opportunities were investigated by U. S. Steel. These were evaluated by 

U. S. Steel and determined not to be feasible: 

1. Emission allowances and Green Energy 
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 Securing and retiring tradable emission allowances from National Emissions and on-site green 

energy generation are not attractive. Both options would be difficult for the MPCA to enforce 

and would not result in visibility improvement in the national parks of concern for this project. 

2. Enforceable reductions in emissions from nearby sources 

 A permit condition which restricts emissions from non-U. S. Steel facilities would be difficult for 

the Company to ensure permit conditions are being met by those facilities. 

Conclusion 

Based on the review of NOx reduction technology, all add-on controls are either innovative technologies, 

technically infeasible based on induration furnace operating conditions, or there are constituents in the 

DDD1 exhaust stream which, if present in sufficient quantities, would adversely affect the feasibility of 

add on controls. Fuel combustion improvements, process design and fueling strategies are the most 

feasible methods of NOx reduction. An emission limit of 1,655 lb/hr on a 24-hr block average will result 

in minimal visibility impacts.