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 Not1hern Minnesota, and is 

approximately 50 miles fi·om the Boundary Waters Canoe Area Wilderness (BWCA W) and 100 miles 

from Voyageurs National Park in not1heastern Minnesota. 

S02 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) of3,110 tons per year of nitrogen oxides (NOx). NOx emissions 

fi·om the facility will exceed the 100 ton per year threshold established at 40 CFR 52.21 (b)(l)(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 ofNOx 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 of35 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 fumace historically was only fired with 

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

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 shoti 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 S02 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. Altemative 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 

altemative 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 bum 100% natural gas for 

extended periods oftime. 

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

extended period and is being pe~mitted as a "new" furnace. The fumace is a grate/kiln design. NOx 

emissions originate from fuel combustion in the kiln section of the fumace. 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 fumace. 
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2. Reduced NOx generation by using combustion technology for minimizing NOx generation 

3. Control ofNOx 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 ofNOx, 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 ofNOx 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: 
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• Expanding the preheat and cooling sections of the furnace to increase the available pellet surface 

for heat transfer 

• 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 uncetiainty 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 ofNOx are temperature and amount of oxygen (02) 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 0 2 present. However, in a taconite induration furnace, oxygen (02) levels must 

be kept at 15% 0 2 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 of2%- 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, pati of the heat recovety 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. Preliminmy results for the 

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

reduction achievable. Because of 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 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 ofNOx 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 

S02 and S03 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 DDDl which would be downstream of an SCR system in 

this option. In addition, SCR catalyst will promote oxidation of S02 to sulfuric acid (H2S04). Sulfuric acid 

formed in an SCR reactor could cause additional corrosion problems in DDD 1. 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 pmticulates 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 

S02 controls. An ESP upstream of an SCR reactor would remove a significant amount of particulate matter 
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and the contaminants associated with patiiculate matter that are catalyst poisons. This control scheme does 

not address potential adverse impacts on SCR catalyst from S02, S03 and HCl described in the SCR 

analysis above. 

Representatives ofU. S. Steel and LKAB met at Keetac during the summer of2009 for a tour and meeting. 

Metso, LKAB 's design engineer, also attended. At the time of the meeting, LKAB 's KK 4 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 patiiculate control device, which minimizes impact to the SCR catalyst. While 

the SOz and S03 may be removed fi·om 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 H20. For the DDDl 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 RHuses 

beds of ceramic media to achieve thermal efficiencies of>95%. The goal of the RSCR system is to 
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combine heat recovery, temperature control, and catalyst elements of SCR into a single unit. RSCR is 

designed for heat efficiency and lower operating costs. 

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-eatih 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 ofRSCR 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 de g. 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 N02 and N02 to N20s prior to removal 

in a wet scrubber (absorber). The N20 5 is convmied to HN03 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 futiher evaluation. Another concern is whether or not iron dust in 

furnace exhaust gas would react with ozone and reduce the control efficiency ofLoTOx. The nonnal 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 S02, 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% S02, 90% NOx, 80-90% Hg, and 95% control ofPM2.s 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 

fettilizer. 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 S02, NOx, and 

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

Keetac's proposed system will control90% S02, 0% NOx, 50% Hg, and 95% ofPM25 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 totallb/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 detennined 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/1 00% 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 1 00% 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 1 00% 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/! 00% solid fuel expansion scenario is approximately 1655 lb/hr. See the 

following graph. 
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100% Solid Fuel 

100% NG Existing 

Total Kiln Emissions Related to Fueling Scenarios 

(w/Expansion Efficiency) 

0 

Existing Line Target Rate = 40% NG, 60% Solid Fuel 

Expansion Line Target Rate= 50% NG, 50% Solid Fuel 

. 1655-2055 

. 760·1655 

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 of2,727 Jb/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 repm1ed in the Draft EIS were: 

o Days > 1 0% = 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 
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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: 

• Diesel-driven Haul Trucks 

• Diesel-driven Mining Shovels 

• Diesel-driven Front-end Loaders 

• Diesel-driven D 1 0 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 1 00% natural gas on 

both lines is resolved. 

Other Mitigation Opportunities Investigated by U. S. Steel 
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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 

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 pennit conditions are being met by those facilities. 

Conclusion 

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

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

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

in minimal visibility impacts.