# U.S. Steel Minntac Minnesota 

# Line 7 NOx Reduction Pilot Project CFD Modeling Final Report
H 33257 1 Rev . 0 F

eb ru ary 1 8, 201 0

This document contains confid e ntial information in tended onlyfor the person(s) to whom it is addressed. The informa tion in th is document may not be disclo sed to, or used by, any other per son without Hatch's prior written consent .

# ~HATCH'" U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 Table of Contents 1. Executive Summary .................................................................................................................................3 2. Introduction ............................................................................................................................................5 3. Minntac Line 7 Induration Process ..........................................................................................................6 3.1 Process Summary ...........................................................................................................................6 3.2 Sources of NOx emissions .............................................................................................................. 7 3.3 Pellet Bed Reaction Kinetics ........................................................................................................... 9 3

### A Mass and Energy Transfer ............................................................................................................. 11 3.4. 1 Traveling Grate and Kiln Internal Heat Transfer .................................................................. 11 4. Current Operating Conditions ............................................................................................................... 12 4.1 Minntac Line 7 Operating Conditions ........................................................................................... 12 4.2 Existing Main Ki In Burner ............................................................................................................. 1 3 5. 3D CFD Model ...................................................................................................................................... 14 5.1 Kiln and Grate Model Geometry .................................................................................................. 14 5.2 Burner Tip Sub-Model .................................................................................................................. 16 6. Modeling Results ................................................................................................................................... 17 6.1 Low-NOx Burner Case 1 -Coal and Natural Gas .......................................................................... 18 6. 1 . 1 Pellet Tem perature Profiles ................................................................................................. 1 8 6.1 .2 Gas Temperature Profi Ies .................................................................................................... 1 9 6.1 .3 NOx Emissions ...................................................................................................................20 6.2 Low-NOx Burner Case 2- Biomass and Natural Gas ....................................................................21 6.2.1 PelletTem perature Profiles .................. ,.,.,.,.,.,. ...................................................................21 6.2.2 Gas Temperature Profiles ....................................................................................................22 6.2 .3 NOx Emissions ...................................................................................................................23 6.3 Low-NOx Burner Case 3 -Coal, Biomass and Natural Gas ........................................................... 24 6. 3. 1 Pellet Temperature Profiles .................................................................................................24 6. 3. 2 Gas Temperature Profi Ies ....................................................................................................2 5 6.3.3 NOx Emissions ...................................................................................................................26 7. Alternate Operating Scenarios ...............................................................................................................27 7.1 Low-NOx Burner Case 4- Natural Gas Only ...............................................................................27 7. 1 . 1 Pe I let Bed Temperature Profiles ..........................................................................................2 7 7.1 .2 Gas Temperature Profiles ....................................................................................................28 7.1 .3 NOx Emissions ...................................................................................................................29 7.2 Law-N Ox Burner Case 5 - Fuel Oi I .............................................................................................. 30 7.2. 1 Pel Iet Bed Temperature Profi Ies .......................................................................................... 30 7 .2.2 Gas Temperature Profiles .................................................................................................... 31 7 .2.3 NOx Emissions ...................................................................................................................32 8. Conclusions and Recommendations ......................................................................................................33
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## 1. Executive Summary U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project 

### Final Report- February 18, 2010

USS Minntac has initiated a pilot project to investigate NOx emission control for their induration systems. The present study investigates the impact of replacing the existing main kiln burner on the Line 7 

### induration system with a new low-NOx burner that has been designed by Coen. Computational fluid dynamics (CFD) and in-house developed process mathematical models have been used to estimate the expected level of NOx reduction with the proposed burner and to quantify the impact on the gas and pellet bed temperatures. In addition, potential modifications to the burner conditions are investigated to maintain acceptable process conditions. The existing operation conditions on Line 7 

### have been analyzed using the data collected on site during June 9-10, 2009. 

### Three most commonly used fuel scenarios were considered: (1) 

### coal and natural gas, (2) 

### biomass and natural I 

### biomass and natural gas. The total measured mass flow rate of NOx was for these three fuel scenarios, respectively. Table 1-1 

### and Table 1-2 

### define cases that have been modeled. In addition to the fuel scenarios most commonly used on site, two alternate fuel mixes are considered: natural gas only, and fuel oil with natural gas. The additional fuel mixes are used primarily as emergency backup on site. Table 1-1 

### and Table 1-2 

### also summarize key model predictions for each case. The presented results are valid for the given set of process conditions modeled, which are assumed to be representative of typical operation. In reality, the actual process conditions vary for many reasons, which will impact the actual process temperatures and related NOx production. Results for cases 1 and 3 show that a 62- 96% decrease in NOx is expected with the low-NOx burner. Results for case 2 show that an 79- 97% decrease in NOx is expected with the low-NOx burner. In all three cases the NOx decrease occurs because the peak flame temperature and gas volume of the kiln above the threshold for NOx generation decrease significantly. This predicted reduction in NOx is very specific to the particular process at Line 7 

### and operating condition simulated, and variation in these conditions will affect the level of NOx produced and the percent reduction from the existing operation of the equipment.

ISO 9001 Results for the low-NOx burner case 4 indicate a NOx generation rate of-· This value is comparable to results predicted for existing burner operation with both coal and natural gas and the tri-fuel mix. Since burning a greater amount of natural gas leads to higher NOx formation inherently, fuel scenario with 100% 

### natural gas can be expected to produce similar predictions to the current operation with coal and natural gas fuel mix. Results for the low-NOx burner case 5 

### predict NOx formation of-. This result is comparable to the predictions for the most common fuel mixes. Significant NOx reduction has been observed in all cases considered, thus warranting further pilot testing of the proposed low-NOx burner by Coen.

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# ~HATCH'" U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 A decrease was observed in the gas temperature exiting kiln into the preheat. This temperature decrease is a process concern because it represents a loss of process heat to the preheat section of the grate. To address this concern, variant cases have been defined by Hatch and Coen. These cases are intended to increase the gas temperature from kiln into the preheat without significantly increasing the peak flame temperatures. It is expected that the radiant heat transfer to the bed will decrease in these cases. Modeling of variant cases is in progress. Parameter Case 1: NG 

### & Coal Case 

### 2: NG 

### & Case 

### 3 : NG, Coal 

### & Biomass 

### Biomass Validation Low-NOx Validation Low- Validation LowNOx NOx Total MMBTU/hr (Main & PH burners) Natural Gas to Main Burner (MMBTU/hr) % Natural Gas to Main Burner of Total

Natural Gas to Main Burner (SCFH) Natural Gas to PH Burners (MMBTU/hr) %Natural Gas to PH Burners of Total

Coal to Main Burner (MMBTU/hr)

%Coal to Main Burner of Total

Coal to Main Burner (lb/hr) 1 Coal Conveying Air Flow (lb/hr) Biomass to Main Burner (MMBTU/hr)

%Biomass to Main Burner of Total

Biomass to Main Burner (lb/hr) 2 Biomass Conveying Air Flow Rate (SCFM) Biomass Conveying Air Flow Rate (lb/hr) Atomizing Air Flow Rate (SCFM)

Make-up Air Flow Rate (SCFM) Secondary Air Mass Flow Rate (lb/hr) CFD Gas Temp from Kiln into PH (°F) Change in Gas Temp from Kiln to PH (Of) Pellet Grate Discharge Temp (°F) Pellet Grate Discharge Oxidation % NOx Exiting the Preheat [lb/hr] Expected Decrease in Thermal NOx % 62-97% 79-97% 62-96% Table 

### 1-1: Summary of Model Parameters for Vahdallon and Low-NOx Burner Cases 

### 1-3
1 Coal Higher Heating Value**~** 10,180 BTU/Ib 2 Biomass Higher Heating Value**~** 7,100 BTU/Ib ISO 9001**~** WorkingTogether Q SAfnY H332571-07-55-124-0001, Rev. 0, Page 4
l1.l Hatch 2010/03Parameter Total MMBTU/hr (Main & PH burners) Natural Gas to Main Burner (MMBTU/hr) 

### % Natural Gas to Main Burner of Total

Natural Gas to Main Burner (SCFH) Natural Gas to PH Burners (MMBTU/hr)



### % Natural Gas to PH Burners of Total Coal Conveying Air Flow (lb/hr) Fuel Oil to Main Burner (MMBTU/hr)

%Fuel Oil to Main Burner of Total

Fuel Oil to Main Burner (lb/hr) Atomizing Air Flow Rate (SCFM) Make-up Air Flow Rate (SCFM) Secondary Air Mass Flow Rate (lb/hr) CFD Gas Temp from Kiln into PH (°F) Pellet Grate Discharge Temp (°F) Pellet Grate Discharge Oxidation% NOx Exiting the Preheat [lb/hr] U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 Case 

### 4: NG Case 

### 5: NG 

### & Fuel Oil

Low-NOx Low-NOx

Table 

### 1-2: Summary of Model Parameters for Low-NOx Burner Cases 

### 4-5 

## 2. Introduction A pilot project has been initiated to evaluate NOx emission control technologies for the USS Minntac Line 

### 7 induration system. The main focus of the project at this stage is to evaluate the NOx reduction achieved by utilizing a low-NOx burner designed by Coen in the kiln and the impact of this burner on the induration process.

ISO 9001This report discusses Phase 

### 1 of this project, which involves three-dimensional CFD modeling of the preheat grate and rotary kiln in combination with one-dimensional process models of the traveling grate furnace and rotary kiln. The purpose of the modeling work is: 

### 1. To determine the optimal burner settings or conditions to maximize NOx reduction while maintaining acceptable process conditions. In particular, the pellet temperature exiting the preheat grate into the kiln, the pellet temperature exiting the kiln, and the gas temperature exiting the kiln into the preheat will be examined. Specific target temperatures were not available. Therefore, direct feedback from US Steel regarding these temperature predictions is used to assess whether process conditions are acceptable. 

### 2. To estimate the expected NOx reduction that can be expected by switching to a low-NOx burner.

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### Final Report- February 18, 2010 3. To quantify the impact of the above changes on the gas, pellet bed and refractory temperatures. This document outlines the results of this modeling study and provides conclusions and recommendations based on the results. 

## 3. Minntac Line 7 Induration Process 3.1 Process Summary The purpose of the induration system is to dry and indurate iron ore green balls to form hardened pellets. The green balls first pass through a travelling grate furnace consisting of three sections: DD1, DD2, and preheat. They then travel down a chute into a rotary kiln and finally into a cooler. Throughout the process, the gas flow is counter-current to the solids flow. In the traveling grate, hot process gas (primarily air) travels down through the bed first drying the green balls in DD1 and DD2 and then initiating the calcination and oxidation reactions in the preheat. Hot gases exiting the kiln travel into the preheat and down through the bed. The gases exiting the preheat are fed into the DD1 section via the 1 A and 1 B ducts. Hot gases from the annular cooler are fed into the DD2 section via the recuperation duct. Gases exiting through the bottom of the DD1 and DD2 sections are treated and released via a stack. In addition to the hot gases, the DD2 section receives additional heat input from one natural gas burner located near the top of the grate adjacent to the preheat section. The preheat section includes fourteen natural gas burners for additional heat input. At the exit of the Preheat section, the pellets should have the physical integrity to survive both traveling down a chute into the kiln and the tumbling action of the kiln. It is estimated that 60% to 75% of the oxidation occurs in the preheat section. Due to all three sections being heated by down-draft air, the material near the top of the bed is generally at a higher temperature then material near the bottom of the bed. Similarly, drying, oxidation, and calcination reactions progress further near the top of the bed at a given location.

ISO 9001 In the kiln, the pellets are heated to their final induration temperature of 2265°F and held at temperature for an estimated four to eight minutes. Heat radiates from the main central burner to the bed and to the refractory lining of the kiln. The tumbling action of the pellets due to the rotation of the kiln ensures an even temperature distribution at a given downstream location and allows the pellets to pick up heat from the hot refractory. The Line 7 kiln, which is the focus of the present study, is a ported kiln. Ambient plant air enters the kiln through the bottom of the bed in the central third of the kiln. The purpose of the ported air is to control temperature and increase oxidation in the kiln and therefore decrease oxidation in the cooler. Hot gases from the cooler enter the kiln near the burner and represent secondary air to the burner. These gases enter the kiln via the parallel flow duct and the burner hood. In the cooler, the fired pellets encounter an updraft of ambient air from the 3A and 3B fans. The cooler will not be included in the models and is not discussed in detail. A schematic diagram of the overall process is shown in Figure 3-1.

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# ~HATCH'" U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010

Figure 3-1: Minntac Grate-Kiln Process Schematic 

## 3.2 Sources of NOx emissions The NOx formation is modeled as part of the 

### 3D CFD model of the preheat grate and kiln. NOx formation in the 

### DD1 and 

### DD2 sections of the grate is not included in the prediction. The NOx formation in the downdraft sections is expected to be negligible compared to the kiln and preheat sections due to the presence of only one small burner and consequently lower gas temperature. Measured NOx concentrations at the 

### 1A and 

### 1 B fans exiting the preheat will be compared to trend data at the stack to quantify any difference between the model output and the NOx levels at the stack.

ISO 9001 NOx can be formed by four distinct processes: (1) thermal NOx due to the oxidation of nitrogen in the air, (2) prompt NOx due to high-speed reactions at the flame front, (3) fuel NOx due to oxidation of nitrogen in the fuel, and (4) NOx from N2 via the intermediate N20. Both the prompt NOx and the NOx due to intermediate N20 may be safely neglected in this case. Prompt NOx formation becomes significant in low-temperature, fuel-rich conditions where residence times are short such as surface burners and gas turbines. The NOx due to intermediate N20 becomes significant at high pressures and low temperatures and for systems operated in flameless mode. The fuel NOx production is also expected to be much less than the thermal NOx production. The modeling approach is to consider only the production of thermal NOx. This assumption is reasonable because the fuel NOx production is expected to be less than 10% of the total NOx production. However, it is understood that the increased stoichiometric ratio in the proposed lowNOx burner may lead to some increases in fuel NOx, up to an estimated 20% of the total NOx production. Nevertheless, in all cases the relatively small increase in the production of fuel NOx at

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# ~HATCHTM U . S. Steel Minntac- Lin e 7 NOx Reduction Pilot Project Final R ep o rt - F ebruary 18, 2010 

### the high er s toichiometri c ratio s w ill be m o re than offset by the significant d ecline in th e rm al NOx formation. Th e pe rcentages of NOx decrease, as presented in thi s report, refl ect only th e thermal NOx predictions . These trends are consistent w i t h the overall decrease in tota l NOx due to the proposed burner . Actual decrease in total NOx m ay be slight l y less than reported here du e to th e e ffe c ts of fue l NO x that are not considered by the model . 5.0

4. 0 

### NO <IJ 3.0 1'1:1**~** 

### c:**~** 0 

### 0 

# > 2.0

1.0

0.0 0 500 1000 1 500 2000 25 00 3000 3500 4000 4500 

### Gas Temperature [°F] Figure 

### 3-2: NO x Eq uilibrium Composition in Air 

### As see n in Figure 3-2 , NO x fo rm ation in c re ases expo nenti a lly with t empe ra ture with si gnificant th e rm al NOx form ation occurring at temperatures above 2780 ° F. At temperatures hotte r th an 3500 ° F, the amount of produced th erm al NOx double s for eve ry 160 ° F in c rease in temperature. As a res u l t , the max imum gas t empe rature s and gas vo lum es of the kiln above 2780°F a re the primary indi ca tors of th e expected red uc tion in t h e am o unt of NOx ge nerated for eac h fue l sce nari o.

I SO 900 1 

### As exp l ain ed above, the amount of gene rated NO x i s extreme l y se nsiti ve to temperature va riation s. For exa mp l e, an in c rease in th e average burn zo ne te mpe rature of 3000 ° F by 5% ca n r esu l t in a co rre spondin g in crease of NOx concen tra ti o n by as muc h as 30 0 %. Con sequently, si gnifi ca nt c ha ng es in thermal NOx fo rm a tion ca n b e observed because of small e rrors in temperature pred i ction s. Therefore it i s impo rtant to po int out th e kno w n so urces of e rror in th e 3 D C FD m o d e l p redi cti o ns of gas tempe rature and cond u ct a sensitivity analysis to determine ho w these e rrors impact the NOx production.

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# ~HATCH~ U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 In order to complete a sensitivity analysis, modeling errors, input errors, and numerical errors for a given process have to be identified thus providing an estimate of variation in the output of the model. The CFD model of the kiln and grate preheat utilizes simplifications of the physical and chemical processes that allows a reduction in the time required to complete a simulation. These assumptions make the solution tractable, but also affect the accuracy of the temperature predictions. The key modeling assumptions that impact the temperature prediction are as follows:
• Combustion reaction mechanisms are not modeled in detail. Rather, a simplified two-step reaction mechanism is employed.
• The model assumes that the reaction chemistry is fast, and therefore the combustion is assumed to be controlled by the rate of mixing of air and fuel to the flame front.
• Dust elutriation is not physically modeled but instead represented through a temperaturedependant radiation absorption coefficient. Sources of input errors to the model include:
• The model assumes steady-state behaviour, which represents time average conditions. In reality, variations in process and operating conditions may result in temporary spikes in flame temperature and subsequent NOx formation.
• The inaccuracy in the physical property data of the materials. For example, the accuracy of the values of thermal conductivities, specific heat of materials, etc. inputted to the model. Numerical errors include:
• Precision of the model output is limited due to spatial resolution of the computational grid, which affects the detailed location prediction of flame temperature.
• Other discretization parameters as part of the numerical solution (differencing schemes, etc.)
• Level of convergence due to iterative solution of model equations. Based on an understanding of these errors and Hatch's extensive experience with combustion modeling, the predicted temperature from the CFD modeling is expected to be accurate to within 

### ±100°C (180°F). 

## 3.3 Pellet Bed Reaction Kinetics

ISO 9001 Both the traveling grate and the kiln models represent the induration process through three chemical reactions in the green ball/ pellet bed: evaporation, calcination and oxidation. All reactions are temperature controlled and are represented via Arrhenius equations.

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# :-e rll 0 

### c.. 

### E 0 

## u 

# .:s 0 f- 

# - 0 ?f. U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- Februar y 18 ,20 10 Based on composition of green balls entering t he kiln, absence of bound moisture is assumed; therefore, evaporation reaction is represented only through the removal of free moisture. The reaction occurs at relatively low temperatures, thus iron ore pellets lose all of the free moisture in downdraft sections of the grate as seen i n Figure 

### 3-3. The effect of evaporation reaction is not observed i n the preheat section of the traveling grate or the kiln which is consistent with normal operation of Lin e 7. The composition of green balls entering the traveling grate includes limesto ne and dolomite (fluxstone) mixed in 50% ratio by volume. Since dolomite undergoes a two step calcination reaction, the initial composition of green balls has been simplified such that calcium carbonate (CaC03) represents the entire frac tion of limestone and dolomite by weight percent. Increas ed weight f raction of calcium carbonate accounts for additional heat and C02 that is released in the first step of dolomite calcination reaction which is not modeled . Both steps of dolomite calcination reaction are exothermic and produce C02, thus dolomite can be modeled as an increased we ight fraction of calcium carbonate while accurately predicting heat and final products of the two s tep calcination. As shown in Figure 

### 3-3 below, 

### 1D traveling grate model predicts that calcination reaction converts all CaCO J contained in the green balls to CaO before they leave the preheat section and enter the kiln. Site data shows that pellet bed composition at the end of the preheat section no longer contains lim esto n e or dolomite, thus 

### 1 D trave ling grate model accurately est im ates full comp letion of calcination react i o n at that s tage of th e induration process.
12



### - % 

### H20 Content 10 

### -% 

### CaC03 conte nt

- % 

### CaO conte nt 8

6

4 

### DD1

-45 -40
-35
-30 -25 -20 - 15 - 10
-5 0 

### Grate pos iti on [m] Figure 3-3: 

### lD Grate Model Prediction of Average Green Ball Composition for Validation Cases 1-3
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### "#- U.S . Steel Minntac- Line 7 NOx Reduction Pilot Proj ect Fina l Report- February 18, 2010 Unlike evaporation or calcination, ox i dation reaction ta ke s place in the trave lin g grate as we ll as the rotary kiln. Ox id atio n of magnetite i nto hematite i s once again a thermally co ntrolled reacti on; howeve r, it occurs at high te mperatures t h a t are not achieved until pellets ente r the beginning of the seco n d downdraft section as ca n be seen in Fi gure 3-4. In order for t he pe llets to survive the fall into the kiln in tact, 60-70 % pe rce nt oxidation has to be ac hieved before pellets ex it the t rave lin g grate. 100 90



### - %Fe304 content

- % Fe203 content

- % Ox id at i on 

### DDl DD2

-45 -40 -35
-3 0
-25 - 20 - 1 5 -10 - 5 0 

### Grate position [m] Figure 3-4: 1 D Grate Model Prediction of Average Green Ball Composition for Validation 

### Cases 1-3 

### 3.4 Mass and Energy Transfer Energy en t ers the system as potent i a l c h em i ca l energy in the fuel which mixes with the air s tream thro ugh the main b urn e r in the k iln model and pre heat/downdraft burne rs in the trave li ng g rate model. Th e thermal en ergy produced from gas phase comb u st i on is then transferred to the so lid s in the packed bed and th e refractory wa ll s via convection and radiation. Th e grate m ode l a l so in cl ud es hot air s treams e nter in g the system from t h e kiln or recycled air from the coo l er. Both model s ref l ect energy loss due to th e h eat t rans fer through the refractory wa ll s an d subsequent shel l heat l osses to th e ambien t environment. 

### 3.4.1 Traveling Grate and Kiln Internal Heat Transfer

I S

O 9001 All t hree chemica l reaction s that take place i n the pellet bed are th e rm all y contro ll ed; th erefo re, both 

### 1 D and 3D models includ e multiple h eat transfer processes in order to correct l y pred i ct freeboard

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# ~HATCH~ U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 201 0 gas, refractory walls and pellet bed temperatures. Temperature calculations include the following heat transfer processes: 1. Heat transfer from gas to pellet bed and walls of the refractory: radiation and convection 2. Heat transfer from walls of the refractory to the pellet bed: radiation 3. Heat transfer from walls of the refractory which are in direct contact with pellets: conduction

- relevant to grate model only 4. Heat transfer through the refractory walls: conduction 5. Heat transfer from the outside shell to the environment: radiation and convection 6. Heat transfer between discretized pellet bed layers: radiation and conduction- relevant to grate model only 7. Heat transfer from elutriated dust particles to walls of the refractory and pellet bed: radiation -accounted for in the kiln model only 8. Heat transfer from ported air to pellet bed: convection- applicable only to the ported section of the kiln model 

## 4. Current Operating Conditions 4.1 Minntac line 7 Operating Conditions

ISO 9001A survey of the operating conditions on Line 7 was taken during the period from june 9 to 10, 200 for three most commonly used fuel scenarios: (1) coal and natural gas. (2) biomass and natural gas, (3) coal, biomass and natural gas. The data includes temperature and NOx measurements in 1 A and 1 B ducts taken by Barr Engineering, corresponding shell, plant and ambient air temperatures as well as trend data provided by US Steel for the same time period. For the first main burner fuel scenario- -the average NOx concentrations measured in the 1 A and 1 B ducts were respectively. When firing biomass and natural gas, the average NOx concentrations measured in the 1 A and 1 B ducts were- -· respectively. When firing the tri-fuel mix of I gas the average NOx concentrations measured in the 1 A and 1 B ducts were respectively. Simultaneously, Barr Engineering took gas temperature readings in the 1 A and 1 B ducts for each of the three tests. The temperature in the 1 A duct is consistently lower than the temperature in the 1 B duct. The asymmetry is attributable to an asymmetry in the flow rates. The flow rate through the 1A fan is 105,200 acfm while the flow rate through the 1 B fan is 163,100 acfm. When firing coal and natural gas in the main burner, the average temperatures measured in the 1 A and 1 B ducts were**~** WorkingTogether V SAFELY H332571-07-55-124-0001, Rev. 0, Page 12

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# ~HATCHTM U.S. Steel Minntac- Line 7 N Ox Reduction Pilot Project Final Report- February 18, 2010 645°F and 669°F, respectively. When firing biomass and natural gas the average temperatures measured in the 1A and 1 B ducts were 629°F and 666 ° F, respectively . When firing the tri-fuel mix the average temperatures measured in the 1A and 1 B ducts were 619 ° F and 656° F, respectively. The measured NOx concentration in ppm and measured flow rate were used to determine the mass flow rate of NOx exiting the system via the 1A and 1 B ducts . These values are summarized in Table Measured NOx Concentration (lb/hr) Fuel Scenario 1A 1B Total Coal (14,200 lb/hr ) and Natural Gas (22, 170 SCFH) 

# • I 

# • Biomass (12,000 lb/h r) and Natural Gas (51,61 0 SCFH) 

# • I 

# • Coal (9,000 lb/hr), Biomass (7,460 lb/hr) , and 

# • I 

# • Natural Gas (24,090 SCFH) Table 4-1: NOx Concentration Measurements in the 1 A and 1 B Ducts by Barr Engineering June 9-10, 2009 

## 4.2 Existing Main Kiln Burner The model of the existing kiln burner i s us ed for va lid ation cases, to match conditions during the testing carried out june 9-10, 2009. The kiln burner i s a multi-fuel burner by Aecometric Corp. Figure 4-1 shows the overall burner geo metry. Th e kiln burner is mounted such that refractorycove red s ection (48 in from burner tip) i s fully in se rted into the kiln. It i s located at th e centre of the kiln at th e material discharge end.

I S

O 900 1 izing Air Inlet 

# J Y iz ing Air Inl e t Figure 4-1: Isometric Views of the Existing Burner Model Geometry

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# ~HATCH~ 5. 3D CFD Model U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project 

### Final Report- February 18, 2010

The model of existing kiln burner has been used for the validation cases, which reflects the operating conditions on Line 7 during Hatch site visit to USS Minntac on june 9-10, 2009. Full description of the main burner and kiln-grate model geometry is included in Modeling Basis document (PRH332571 .001_ Revl ). The geometry of the proposed low-NOx kiln burner has been established and modeled by Coen. The output from the model has been forwarded to Hatch and implemented as boundary conditions to the existing model of the rotating kiln and traveling grate system. As a result of Coen's design of the natural gas annulus tip in the new low-NOx burner, natural gas flow can reach sonic**(Ma~** 

### 1) velocities when exiting the burner tip. Modeling sonic flow of natural gas, while capturing its interaction with primary and secondary air flow, presents a challenge. Hatch has created an additional 3D CFD sub-model in order to accurately predict the momentum of natural gas as it exits the burner and enters the main 3D CFD Kiln model. Details of this sub-model are presented in the following Section. 

### 5.1 Kiln and Grate Model Geometry Figure 5-1 shows the geometry of the kiln and preheat grate model. The model extends from baffle between the DD2 and preheat sections of the grate furnace to the tail end of the kiln. The parallel duct is also included in the model. For simplicity, the firing hood is not shown in the model; however, the secondary air entering through the firing hood is applied as a boundary condition into the appropriate opening in the end of the kiln. The preheat grate portion of the model is terminated at the green ball/pellet bed. The freeboard gases exit through the bed surface. Other boundary conditions on the bed surface are set to reflect the results of the 1 D model to account for the reactions in the bed. The kiln bed is assumed to be a flat surface at an angle of 35°. It is further assumed that 7% of the kiln volume is filled with pellets. The burner tips are included as inlet surfaces to the model. Each burner will be separately modeled and the output velocity, temperature, and gas species profiles will be applied to the appropriate surface.



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# ~HATCHTM

Locat i on of Preheat burn ers U . S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010

Secondary air: parallel duct Blast ports for infiltration air

Figure 5-1: CFD Model of the Preheat Grate and Kiln

I S

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5 .2 Burner Tip Sub-Model U . S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010

The o ve rall geometry of the burner tip sub-model is
The domain of the model can be seen in the profile view of the burner tip geometry in Figure 5-2 . The flow at the outlet of the burner tip submodel has been calculated as subsonic (M a 

### < 0 .3), thus all the compressibility effects are constricted to within the sub-model.

The annulus gun clusters

The core g un slot

Model Domain
Figure 

### 5-2: Profile and Front View of the Burner Tip Sub-Model Geometry The res ults of the CFD low-NOx burner model provid ed by Coen have bee n applied as boundary conditions to th e burner tip sub-mod el. Figure 5-3 outlines the f o ur annuli that require input from Coen low-NOx burner mod e l. Results from the sub-mod el are u sed as boundary cond ition s to the final 3D CFD model of the kiln and preheat s ection of th e grate. The sub- mode l geometry additionally include s fu e l o il gun which i s used as an emergency backup fuel. Since fuel oil is modeled as droplets of liquid , th e input from Coen i s applied directly to the main kiln-grate model as described in Section 7.2.

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# ~HATCHTM U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010
Coal Annulus Make-up Air Annulus Biomass Annulus Atomizing Air Annulus Natural Gas Annulus Gun Clusters Fuel Oil Gun Natural Gas Core Gun Slot

Figure 

### 5-3: Geometry of the 3D CFD Burner Tip Sub-model 

## 6. Modeling Results The models have been validated against measured data taken during the operation w ith t he existing burner on June 9-10, 2009. The validation procedure and result are discussed in Appendix A. After three fuel scena rio s have been successfu lly validated, the fixed mod e l settings have been applied to predict the effect of implementing the new low- NOx burner, designed by Coen. In all cases the proposed low-NOx burner is modeled with the same fuel feed rates and other process conditions as the corresponding validation cases to allow for direct comparison. The only modification to the process cond itions is primary air flow rate which is in c reas ed with the new burner as per Coen's design. All fuel flows and primary and seco nd a ry air flow rates used as input to th e n ew burner are li s

ted in Table 1-1. The presented re sults are valid for the g i ven set of process co nditions modelled , wh i c h are assumed to be rep res en tative of typical operation. In reality, the actual process conditions vary for many reasons, wh ich will impact the actual process temperatures and related NOx generation.

I SO 900 1 The low-NO x burner i s capable of operating with various combinations of coal, biomass, fu el oil and natural gas, which i s simil ar to the existing burner on Line 7 at U SS Minntac. Three most common operat ing fuel mixes are modeled and compared to three al re ady validated cases.**~** WorkingTogether V SAfELY H332571 -07-55 - 124-0001 , Rev. 0, Page 17

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# ~HATCHTM U.S . Steel Minntac- Li ne 7 NOx Reduct ion Pilot Project Final Report- February 18, 20 10 For each case, model o utputs h ave been compa red against pellet and gas temperatu re predictions obtained d uring the validation procedure . Resul ts of 1 D Traveling Grate, 1 D Kiln and 3D C FD models are compared fo r each of the three fuel mix scenarios. 

### 6.1 Low-NOx Burner Case 

### 1 -Coal and Natural Gas 6.1.1 Pellet Temperature Profiles Figure 

### 6-1 shows com pari son between average pellet bed te mperatu res as p redicted by 

### 1 D models for validation and low - NOx burner case 

### 1. Present ed trave ling g rate temperatures are the average va lues for four d i sc retized layers, w hil e kiln temp e rature s are ta ken as a direct o utput of th e 

### 1 D Kiln model.**~**

0

..........

"C (l,j CQ +"**~ ~**  Q.. 2500 Grate Exit Temperature 2000 llT (Low-NOx- Validation) = - 55° F

1000

500 

# I 

# r --**--** 

# ----, -

KH 

### n o ; sch arge T e mperatore 

# I \ llT ( L ow- NOx - Va li dati on) = -220 ° F



### I


### __,_ VC1 1 D Grate Mode l Average -+- VC1 1 

### D Kiln Model

-- CC1 1 

### D G rate Mode l Average



# -- cC1 1 

### D Kiln Model I 0**-- --** **-r --** **--~ ----** **---- --** **---- 4-** **---- -,--** **----------~------------~**

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

### Axial Position [m] Figure 6-1: Comparison of Traveling Grate and Kiln Pellet Bed Temperatures for Case 1 Overa ll , t h e observed t re n d i s a dec rease in t h e pellet b ed temperature. Th e grate exit temperature of th e pe ll e ts for Iow-NOx burner case 1, as predicted by 1 D Traveling Grat e model i s 

### 1680 ° F, which i s 55 °F lower than model p redicti on for val i dati on case 1. The more si gnifi ca nt impact o n pe ll e t tem perature ca n be see n w ithin the k iln. The ove rall drop in kiln di sc h arge temperature of pel l ets is 220°F, as predicted by 1 D Kiln model.

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6. 1.2 Gas Temperature Profiles U.S. Steel M inntac - Line 7 NOx R eduction Pilot Project Final Report- February 18 , 2 01 0
Figure 6- 2 shows gas temperature va lues, as predicted by the 3D CFD model , along the center lin e of the kiln and preheat section of the g rate for th e coal and natural gas fuel scenario . Two d im ens i onal vertical cross section s displaying gas temperatures for both val idatio n and low-NOx burner case 1 are presented in Figure 6-3.



### u:' 0
.......... 3500

3000

(l,l 2500 

# ... ::I 

# -**~** (l,l**~** 2000 (l,l 1- <ll r:l (,j 1 500**~** "C 0 

# § ,, I

500 Peak Ce nterline Tem p e rature 

### LH (Low-N Ox -Valid ation ) = -41 0° F 

# /
Kiln to PH Gas Tempe ratu re li.T (Low-NOx- Va lid at ion ) = -65 ° F

-+- 

### VC1 3D CFD Model 

# I -o- 

### CC1 3D CFD Model

### 0~**--~---- ~--** **-- -+----~--** **-- --** **~**

-25
- 1 5
-5 15 25 35 

### Axial Position [m] Figure 6-2: Comparison of CFD Gas Temperatures for Case 1 Th e key parameters i n Fi gure 6-2 are th e kiln t o preheat gas temperat u re and peak f l ame temperature. US Steel has exp ressed the importance of mainta i n in g the gas temperature exitin g t h e ki ln into t h e grate, as i t p rovides the main so urce of heat for th e pe ll et in durati o n p rocess. Pellets h ave to be h eated to a spec ifi c tempe rature in order to undergo 60-70% oxidation and surv ive the fall into the kiln without shattering. A decrease i n that gas temperature by 65 ° F from validat ion to l ow-N Ox b urn er ca s e i s pred icted , wh i c h i s cons id e r ed un acceptab l e for th e process .

IS

O 9001Another parameter of i nteres t i s the peak f l ame temperatu re . Si nce NOx for mati on is so se n sitive to temperat ure va riati ons, a drop in the peak flame temperature w ill result in a si gn ifi ca nt decrease in t h e amount of ge n erated NOx. Elon gati on of the flame from va lidat i on case to l ow-NOx b urner case i s shown i n Fi g ure 6-3. It red i s tributes th e h ea t of f u e l combust i on and re sults in 41 0 ° F peak flame temperature drop for coa l and natural gas fuel mix.**~** Work i ngTogether 

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1200 T[°F] 

### 1450 U.S . Steel Minntac- line 7 NOx Reduction Pilot Project Fina l Report - February 18, 2010



## "\



# ---------------- \

1700 1950 2200 2450 2700 2950 3200 Figure 6-3: Validation and Low-NOx Burner Case 1 Gas Temperature Profiles 

### 6.1.3 NOx Emissions As outlined in Sect ion 3 . 2, NOx formation increases expo nentiall y w ith temperature wit h signifi cant NOx formation at temperatures above 2780°F. Figure 6-4 shows vo lumes of the kiln w here the temperature exceeds 2780°F for val id ation case 1 and l ow-NOx burner case 1. Th e figure additiona ll y shows contours w ithin the vol u mes to indicate areas where temperature is above 3000°F. In th ese regions NOx in creases drastically with temperature.



### 2780 T[°F] 

### 2852 2924 2996 3068 3140 3212 3284 3356 Figure 6-4: Kiln Volumes with Gas Temperatures Exceeding 2780°F for Case 1 

### 3428 3500
Validation case 1 predicts that 2.4% of the vo lum e exceeds temperature of 2780°F whi l e on l y 0.6% of the volume for l ow-NOx burner case 1 is greater than 2780 ° F. Additi onal l y the peak flame temperature in the kiln drops from 3420°F for va li dat i on case 1 to 3030°F for l ow-NOx burner case 

# J

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# ~HATCHTM U.S . Steel Minntac- Line 7 NO x Reductio n Pilot Project Final Report- February 18, 20 10 1 . The drop in volume percentages and peak fl ame temperature implies that a significant decrease in the amount of generated NOx can be expected . Sensitivity analysis indicates that the thermal NOx generated in the kiln and preheat grate i s in the range from - · Based on the measurements taken by Barr, -of NOx is generated with the existing burner for the same fue l scenario. Th erefore, the expected decrease in NOx formation is 62- 97%. 

### 6.2 Low-NOx Burner Case 2 - Biomass and Natural Gas 6.2.1 Pellet Temperature Profiles Figure 6-5 shows a comparison between the a ve rage pellet bed temperatures as predicted by 1 D model s for va lid ation and low-NOx burner case 2. Presented traveling grate temperatures are the average va lu es for four discretized l ayers, w hile kiln temperatures are taken as a direct output of the 1 D Kiln model. 2500**~---- ~**  0
........ (1,1 

# ... ::I 2000

- 1500**~** (1,1 

### c.. 

## E (1,1 1- "C (1,1 1000 CQ 

# - .!! 

### "ai Q.. 

### soo 

## I Grate Exit Temperature**~T** (Low-NOx - Va li dation ) = - 70° F

Kiln Discharge Temperature**~T** ( Low -NOx- Val id ation ) = -180°F 

## I

-+- VC2 1 D Grate Mod e l Average -+-VC2 

### 1D Ki l n Mod e l

-- CC2 

### 1D Grate Mod e l Average

-- CC2 1 D Kiln Mode l 

# I I 0**,----- -.** **-- ----~------------** **,- ----** **---- ------** **,- ----** **-, ------** **4- ----** **-,**

I SO 900 1 - 50 -40 · 30
-20 ·1 0 0 10 20 30 40 

### Axial Position [m] Figure 6-5: Comparison of Trave ling Grate and Kiln Pellet Bed Tem peratu res fo r 

### Case 2 The overall trend of fo r biomass and natural gas fuel sce n ario i s a decrease in pe ll et bed te mpe rature with the l ow- NOx burn e r which simil ar to case 1. The grate ex it temp e rat ure of the pe llets is 1785 ° F, wh i c h i s 70° F lower than for va lid ation case 2. The more si gn ifi ca nt im pact on pe ll et temperature ca n be see n w ithin the kiln. The overa ll drop in kiln di sc harge tem p e rature of pellets i s 180 ° F. Th e

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### NOx Reduction Pilot Project Final Report- February 18, 2010 

### final kiln discharge temperature of the pellets for low-NOx burner case 2 i s 2090 ° F, which is 8 % below the average final pellet temperature for the three va lidation cases. 

### 6.2.2 Gas Temperature Profiles Figure 

### 6-6 shows predicted gas temperature values along the center line of the ki ln and preheat section of the grate for the biomass and natural gas fuel scenario . Two dimensional ve rt i ca l cross sections displaying gas temperatures for validation and low-NOx burner case 2 are prese nted in Figure 

### 6-7. 3500
3000**~** 0 

## ...... cu 2500 

# ... :::l

## ~

# ~ 2000 1

# ~ I**~ ~**  1500**~** "'C 0

# ~ 1000 loL. 

# u 500 

# l Peak Centerli ne Temperature t.T (low-NOx- Validation ) = 

### -55°F 

# /
Kiln to PH Gas Temperature 

### LH (Low-NOx- Validation) - 0 ° 

### F

-+- 

### VC2 30 CFO Mode l

-- CC2 3 0 CF 0 Model 0**~-- --** **---- ~----** **---,,---~---, ----** **---- -,---------,---------.----** **-- ---.** 



### I SO 900 1 -25 -1 5 -5 5 1 5 25 35 

### Axial Position [m] Figure 6-6: Comparison of CFD 

### Gas Temperatures for 

### Case 2 Key parameters for gas temperature predictio ns are the kiln to preheat gas temperature and the peak flame temperature. Low-NOx burner case 2 results show that there is no measureab l e decrease in the kiln to preheat gas temperature, as compared with validation case 2 results. The maintenance of this temperature parameter is an important process related concern for operation of Line 

### 7. Another val ue of interest is the peak flame temperature. Since NOx formation i s so sensiti ve to temperature va riation s, drop in the peak flame temperature results in significant decrease in th e amou nt of ge nerated NOx. El ongation of the flame from va lidation case 2 to l ow- N O x burner case 2 is shown in Figure 

### 6-7 . Thi s is the same trend seen for case 

### 1 . Th e implementation of new low-NOx burner and change in primary air f low rate resu l ts in 55 ° F drop of t he peak flame temperature fo r l ow- NOx burner case 2.**~** WorkingTogether Q SAFELY H332571-07-55-124-0001 , Rev. 0 , Page 22
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1200 T[°F] 

### 1450 U.S. Steel Minntac- Lin e 7 

### NOx Reduction Pilot Project Final Report- F ebruary 18, 20 10

# '--------------~----- 

# '

# --_,___----------

1700 1950 2200 2 450 2700 2950 3200 

### Figure 6 -7: Validation and low-NOx Burner Case 2 Gas Temperatures Profiles 

### 6.2. 3 NOx Emissions As outline in Section 3.2 , NOx fo rm at ion in c reas es expo ne ntial ly w i th te m perature . Figure 6-8 shows vo lume s of the kiln w here tempera ture exceeds 2780°F for both va lidati on case 2 and l ow- N O x burner case 2. Th e figure also shows temperature conto urs w ith i n thes e vo lum es to ind i cate areas w h ere temperature i s above 3000 ° F and NOx production increases sig ni f i ca ntl y w i th temperature.

I SO 9001 

### 2852 2924 2996 3068 3140 3212 3284 3356 3428 3500 

### Figure 6-8: Kiln Areas with Gas Temperatures Exceeding 2780 ° F for Case 2 Va lidation case 2 predicts that 1 .2% of th e vo lu me exceeds t emperatu re of 2780°F whi l e on l y 0.2% of the volum e i s greater th an 2780 ° F fo r l ow-NO x burner case 2. Th e relati ve drop in vol ume percentages indicates that a significant decrease in th e amoun t of generated NOx can be ex pected. Additiona ll y the maximum peak fl ame temperat ure in the kiln drop s from 3370°F fo r va lid a tion case 2 to 3040 ° F for low-NO x burner cas e 2. The greater maximum te m pe ratu re for bio m ass and natu ra l gas fuel scena rio as compared with case 1 predictions will result in greater NOx formation with in the**'::!":~** WorkingTogether Q SAFELY H 332571-07-55- 1 24-0001, R ev. 0 , P age 23 "Hatch 20 10/03

# ~HATCHTM U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 smaller vo lume of the kiln. However the relative drop in the maximum gas temperature sti ll indicates that there is a significant decrease of NOx production with the use of new low-NOx burner for case 2 . Sensitivity analysis indicates that the thermal NOx generated in the kiln and preheat grate is in the range from - · Based on the measurements taken by Barr, -of NOx are generated with the existing burner for the same fuel scenario. Therefore, the expected decrease in NOx formation is 79- 97%. 

### 6 .3 Low-NOx Burner Case 3 - Coal, Biomass and Natural Gas 6

. 3.1 Pellet Temperature Profiles Figure 6 - 9 shows a comparison between the average pellet bed temperatures as predicted by 1 D models for validation and low-NOx burner case 

### 1. Presented traveling grate temperatures are the average values for four discretized layers, wh il e kiln temperatures are taken as a direct output of the 1 D Kiln model .

aJ 1- :::l 2500

2000**~** 1500 1- a./ 

### c. 

## E aJ 1- "C aJ 1000 CQ 

### 'al Qj Q, 5 00 Grate Exit Temperature t.T (Low-NOx - Validation) = - 35 ° F 

# \ Kiln Discharge Temperature t.T (Low-NOx- Validation) = -245°F

--r- 

### VC3 1 D Grate Mode l Average --r- 

### VC3 1 D Kiln Model -+- 

### CC3 1 D Grate Model Average -e-- ((3 1 D Kiln Model 0

### --**-- -,------,-** **-----,----~------ 4---** **--~ ------** **,- -----.------** **.------.**

I S

O 9 001 -5 0 -40 ·30 -20 ·1 0 0 10 20 3 0 4 0 

### Axial Position [m] Figure 6-9 : Comparison of Travel i ng Grate and Kiln Pellet Bed Temperatures for Case 3 The trend for the tri-fuel scenario once again shows a decrease in the pellet bed temperatures. The grate exit temperature of the pellets with the l ow-NOx burner is 1 70 5° F, which is 3 5° F lower than predicted for va lid a tion case 3 with the exi s ting burner. 

### A substantial impact on pellet bed temperatures can be seen in the predictions of 1D kiln model. The pellet discharge temperature
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# ~HATCHTM U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project Final Report- February 18, 201 0 decreases by 245° F which is the greatest decrease observed for a ll three fuel mix cases considered . The final kiln discharge temperature of pellets for low-NOx burner case 3 is 2030 ° F, which is 10% be l ow the average fin al pellet temperature for the three va lid ation cases. 

### 6. 3.2 Gas Temperatu re Profiles Figure 6-10 shows gas temperature va lues as predicted by the 30 CFD model along the center l i ne of the kiln and preheat section of the grate for coal, biomass and natural gas fuel scenario. Two dimensional vertica l cross sections displaying gas temperatures for validat ion and low - NOx burner case 3 are presented in Figure 6-11.

,........, 1.1.. 0 

# - 3500 

# I _
3000 

## 1**~** 2500 

# ... :l**~** 

### ,_**~** e- 2000

### ~ <ll**~** 

### t.:l 1 500 

### "ii ""C 0**~** 

### 0 1000 1.1.. 

## u 500**~---·-·----** Peak Centerline Temperature LlT (Low-NOx - Validation)= -510°F

Kiln to PH Ga s Temperature LlT (Low-N Ox - Validation) = -60 ° F

-+- 

### VC 3 30 CFD Model

-- CC3 30 CFD Mode l 0**r- --** **---- -.-** **-------,----~-- -.** **-------- -,** **---- --** **---,-- -------,--** **-- --** **---.**

-25
- 15
-5 5 1 5 25 3 5 

### Axial Position [m] Figure 6-10: Comparison of CFD 

### Gas Temperatures fo r 

### Case 3 Key parameters for gas temperature predictions are the kiln to preheat gas temperature and the peak f l ame temperature. The results show a 60°F decrease in the kiln to pre heat gas temperature as compared with va lid ation case 3 results. As pointed out in the discussion of low-NOx burner case 1 results, this temperature drop has been deemed unacceptable by USS Minntac. They have in dicated the im portance of maintaining the kiln to preheat gas temperat ure with implementation of the new burner .

I S

O 9 001Another parameter of interes t is the peak flame temp e rature. Since NOx formation i s so sensitive to temperature variation s, a drop in the peak flame temperature resu lts in significant decrease in the amount of generated NOx. The elongation of the flame from va lid at ion case 3 to l ow-NOx burner**~** W orking Together Q SAFEL Y H 33257 1-0 7
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# ~HATCHTM U .S. Steel Minntac- Lin e 7 NOx Reduction Pilot Project Fin al Report- F eb ru ary 1 8, 2010
case 3 is similar to the two previously discussed cases as sh ow n in Fig u re 6-1 1 . The im p l emen tati o n of new l ow-N Ox burner and change in primary air flow rate results in a 620 ° F drop of the peak flam e temperature for lo w-NOx burner case 

### 3.

1200 T[°F] 1450 1700 

### 1950 2200 2450 2700 

### Figure 6-11: Validation and low-NOx Burner Case 3 Temperature Profile 2950 3200


### 6. 3 . 3 NOx Emissions Figure 6- 1 2 shows vo lum es within th e kiln where temperature exceeds 2780°F for both va lid a ti on a n d l ow- NOx burner case 

### 3. The f i g ure al so sh ows co ntours w i th in the vo lu mes to illu s trate t he areas w h ere significant NOx i s produced.

ISO 900 1**~-=-**



### 2780 T[°F] 

### 2852

. 

# ,... •.t.:c 

# --- -- -
2924 2996 3068 3140 

### 3212 3284 3356 3428 

### 3500 

### Figure 6-12: Kiln Areas with Gas Temperatures Exceeding 2780°F for Case 3 For va li dati on case 3, the tempe r ature exceeds 2780 ° F in 2.1% of the ki ln vo lum e . For low-NOx burne r case 3, t h e temperature exceeds 2780°F in only 0.1% of t h e ki ln vo lum e . The relative drop in

'V"::/ WorkingTogether V SAFE LY H 33257 1 -0 7-55- 1 24 -0001, R ev. 0 , Page 26 <>H a tch 201 0 /03

# ~HATCH~ U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 201 0 the volume where temperatures exceed the threshold for significant NOx formation indicates that a significant decrease in the amount of generated NOx can be expected. However the maximum gas temperature in the kiln increases from 3400'F for validation case 3 to 4940°F for low-NOx burner case 3. Therefore NOx reduction for tri-fuel mix scenario will not be as significant as the change in the expected amount of NOx produced for cases 

### 1 and 

### 2. Sensitivity analysis indicates that the thermal NOx generated in the kiln and preheat grate is in the range from -- Based on the measurements taken by Barr,- of NOx are generated with the existing burner for the same fuel scenario. Therefore, the expected decrease in NOx formation is 62 - 96% _ 

## 7. Alternate Operating Scenarios In addition to modeling three most common fuel scenarios, US Steel has expressed interest in predicting the impact of new low-NOx burner on the induration process with two more fuel combinations: natural gas only and fuel oil. Both fuel settings are in place for emergency backup situations and are therefore rarely used. As a result, there is no validation data that can be used for comparison for either natural gas only or fuel oil cases. 

### 7.1 Low-NOx Burner Case 

### 4- Natural Gas Only 

### 7.1.1 Pellet Bed Temperature Profiles Figure 7-1 shows pellet bed temperatures as predicted by 1 D models for low-NOx burner case 4. Similar to previous low-NOx burner cases, the bed thickness in 1 D traveling grate model is discretized into four segments while the 1 D Kiln model predicts a uniform temperature due to the kiln rotation.

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# ~HATCHTM

3 000 ,.- - 

# I 2 500 

# I**~** 

# I 0
.......... QJ 2000 G ree n ball fee d 

### ,_ 

## E temp erature = 116 ° F**~** Q,j 

### c. 

### E 15oo

## ~ "C Q,j Cl:l 

## 'a:i 1000 Q,j 

### a.. 500 

## I U.S. S teel Minntac - Lin e 7 N Ox Redu ction Pilot Project Final Report- Februa ry 18, 2010 

# I
Ki l n Disc harg e Te m pe rature 1 D Kiln Mod e l Pre dicti o n = 23 3 0 °F 

### 1 D Gra t e M o d e l - L ayer 1**---~r-** 

### 1 D Grate Model - Layer 2 --o-- 1 D Grate Mod e l - Laye r 3

-- 1 D Grat e Mod e l- La ye r 4 -+- 1 D K iln Model

# 1**--~ --** **--~-- ----** **-- --** **-- --** **-- ----** **-- ~** 

-50 -40 -30 -20 -1 0 0 1 0 20 30 40 

### Axial Position [m] Figure 7-1: Travel i ng Grate and Kiln Pellet Temperatures for Low- N Ox Burne r Case 4 T h e g ree n ba ll feed temperature u sed fo r nat ura l gas onl y low-NOx b urn e r case is th e sa m e as th e va lu e u sed fo r case 1 a t 11 6° F. O ut p ut o f 1 D Kil n m ode l fo r k iln di sc h arge te mperatu re of pe ll et i s 233 0 ° F w hi c h is 2% hi g h er than t h e ave rage fin a l pe ll e t tem pe rature fo r l ow- NOx cases 1 thro u g h 3 . 

### 7.1.2 Gas Temperature Profiles Fi g ure 7-2 sh ows t em pe ra ture con to u rs o n a two d imen sion a l verti cal cross secti o n thro u g h th e k iln a n d p reh ea t , as pre di cted b y th e 3 D CFD mode l. T h e gas te m pe rature alon g th e ce nte r lin e of the k il n and pre heat s ection of t h e grate are al so p rovid ed in th e g raph b e l ow.

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1450 1700 4ooo 

### I
3500 3000 

# 1 Average PH Temp: 3D CFD Model =2330°F 1000 500 1950 U.S. Steel Minntac - Line 7 NOx Reduction Pilot Project Final Report- February 18, 201 0

2200 2450 

### 2700 2950 3200

Peak Centerline Temp: 3D CFD Modei=3455°F 

# r

Burn Zone Temp: 3D CFD Modei = 3100 ° F



### -- CC4 3D CFD Model

# --**--~---- --~** **-- --~----** **~---- --** **1 ------** **---- ~** 

-25 -15 -5 5 1 5 25 35 

### Axial Position [m] Figure 

### 7-2: 30 CFD 

### Cas Temperature Profile for Low-NOx Burner 

### Case 4 The gas temperatures predicted by the CFD model for low-NO x burner case 4 are comparable to those seen for case s 1 through 3 with the existing burner. The peak flame tempera ture of 3490 ° F is significantly higher than the peak flame temperature observed for the first three l ow-N Ox burner cases, whose average va lu e is approximate l y 3000 ° F. The higher flame temperature i s an indication of relatively high amount of NOx formation within the flame. Due to absence of validation data, the gas temperatures for natural gas only fuel scenario cannot be compared to values measured on site. 

### 7. 1.3 NOx Emissions

I SO 9 001 A s shown in Figure 7-3, the kiln volume where gas temperature ex ceeds 2780 ° F is quite significant at 1.3% with peak flame temperature reaching 3490 ° F. The overa ll thermal NOx mass f l ow rate a s predicted by CFD model is- · This value is comparab l e with validation case 1 and 2 resu lts but significantly higher than NOx mass flow rates predicted for low- NOx burner cases 1 through 3. Since burning greater amount of natural gas l eads to higher NOx formation inherently, fuel scenario**~** WorkingTogether V SAF ELY H 33 2 5 71 -0 7-5 5- 1 24 -0001 , Rev. 0 , Pag e 29

<> H atc h 2010/ 0 3 45

# ~HATCHTM U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 with 100% natural gas ca n be expected to produce similar predictions to the current operation with coal and natural gas fuel mix. Due to the absence of validation data, error and sensitivity analysis cannot be completed for this case nor can this case be compared directly to natural gas operation with the existing burner.

2780 

### T[°F] 2852 2924 2996 3068 3140 3212 3284 3356 3428 3500 

### Figure 7-3: Kiln 

### Areas with Gas Temperatures Exceeding 2780°F for Low-NOx Burner Case 4 

### 7.2 low-NOx Burner Case 5 - Fuel Oil 

### 7.2.1 Pellet Bed Temperature Profiles Figure 7-4 shows the pellet bed temperatures as predicted by 1 D models for low-NO x burner case 5. As for previous low-NOx burner cases, th e bed thickness in the 1 D traveling grate model is discretized into four segme nts whi le the 1 D Kiln model predicts a uniform temperature due to kiln rotation.

I S

O 900 1**~** Workinglogether V SAFE LY H 3 3257 1-0 7-55 -1 24-0001, Rev. 0 , Page 30
e H atch 20 1 0/0 3 

# 1

# ~HATCH

# .. U . S. S t eel M inntac - Lin e 7 NOx R edu cti o n Pil o t Project Fin al Rep ort - Febru ary 18, 2 010

2500**~·---·---** ------ ---- --·**~** 0
........**~** 2000 I

.; 1soo G re en ba ll feed**~** te mpe ra t u re = 116° F 

### c. E 111 1- "C**~** 1000 I 

# - .!! Qj Q.. 5 00 Kiln Di sc harg e Temp e ratu re 1 D Ki l n Mode l Predi ction= 2230 ° F



### 1 

### D Grat e M od e l- Laye r 1 -o- 

### 1 

### D Grat e M ode l - Laye r 2 -(r- 

### 1 D G rat e Mode l - Laye r 3

-- 1 D G rat e Mode l - Layer 4 -+- 1 D K i l n Model
0**.- ----** **---- -.** **------ ----** **-- ------** **---- --** **------ .-** **---- -.** **-- ----** **.----- -.**  -50 -40
-30 -20 -1 0 0 1 0 20 30 40 

### Axial Position [m] Figure 

### 7-4: Traveling Grate and Kiln Pellet Temperatures for Low- N Ox Burner 

### Case 5 T he green ba ll feed tempe ratu re used for f u e l o il l ow- NOx b urn e r case i s t he sa m e as th e va l ue used fo r case 1 and case 4 a t 1 16° F. O utp ut of t h e 1 D Ki l n mode l fo r k iln d i sc h arge temperatu re of p e ll et i s 2230 ° F w hi c h i s co nsi s te nt w ith t h e f in a l pe llet temperat u re fo r cases 

### 1 t h ro ugh 3. 

### 7.2.2 Gas Temperature Profiles Figure 7 -5 sh ows t em p e ratu re co ntours o n a two d im ens i o n al ve rti ca l cross section t hro u g h t h e kiln a n d preheat , as p redi cted by th e 3D CFD mode L Gas te mperat ure al ong t h e center li ne of th e k iln and preheat se cti on of th e g rate is prov i ded i n the g rap h be l ow .

I SO 900 1**~** WorkingTogethe r V SAF£LY H 33257 1-0 7-55 - 1 24-0001, Rev. 0, Page 3 1 " Hatch 20 10/03 50

# ~HATCH

# ..

1200 T[°F] 1450 1700

3500
---
3000
2500**~ ~**  I

# ~ 2000 

# I**~** 

### .... <IJ Q.

## ~ 1 500 1- 

# "' I'!:S (,;, 1000
500 I 

# - 1 Average PH Temp: 3D CFD Model =2235 ° F 1950 U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 201 0

2200 2450

Peak Center! i ne Temp: 3D CFD Model= 3030°F 2700

Burn Zone Temp: 2950

3D CFD Modei=2560°F 3200



### -+- CC5 3D CFD Model 0**,---------.-------~----~-- -,---------,--------** **-,------ --** **-. ------** **-- -.** 

-25 -15 -5 5 15 25 35 

### Axial Position [m] Figure 7-5: 3D CFD Gas Temperature Profile for Low-NOx Burner Case 5 The predicted gas temperatures are comparable to those see n for low-NOx burner cases 1 through 3 . The peak centerline temperature i s 3030°F w hich within 50 ° F of predictions for low-NOx burner cases 1 through 3. Therefore, the amount of NOx formation in the kiln i s expected to be similar to results presented for these cases. Due to absence of validation data, the gas temp e ratures for fuel oil scenario cannot be co mpared to va lu es measured on site. 

### 7.2.3 NOx Emissions

I SO 9001As shown in Figure 7-6, the kiln vo lume where gas tempe rature exceeds 2780°F is 0.4% of the total volume with a peak flame t emperature of 3100 ° F. The overall thermal NOx mass flow rate as predicted by CFD model is ...This value i s much sma ll er than CFD model predictions for lowNOx burner case 4 and is comparable with results for low- NOx burner cases 1 through 3. Du e to absence of validation data, error and sensitivity analysis cannot be completed for thi s case.**~** WorkingTogether Q SAFELY H 3 32 5 71-07-5 5-1 24-0001, Rev . 0, Page 32 " H a tch 20 1 0 / 03 45

# ~HATCHTM
H 

# 1

2780 T[°F) 2852 

# i

2924 2996 3068 U.S. Stee l Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18,2010

F""C: 

# 7'1 .,:;.- .,_.,!!--?" 

## P="'



## - I .-c-' 

# .(r ..

# ::-· ·" II 3140 3212 3284 3356 3428 3500 Figure 7-6: Kiln Areas with Gas Temperatures Exceeding 2780 °F for Low-NOx Burner Case 5 

# 8. Conclusions and Recommendations NOx measurements have been col lected in 1A and 1 B ducts on Line 7 during June 9-10, 2009 site v i sit to Minntac fac ili ty . Three most common ly used fuel scenarios were considered: (1) coa l and natural gas, (2) biomass and natural (3) coa l biomass and natural gas. The total measured mass f l ow rate of NOx was for these th ree fuel scenarios, re specti ve ly. A CFD modeling study has been conducted to eva lu ate the effects on both NOx production and p rocess cond itions due to installation of a n ew low - NOx burner designed by Coen. Th e CFD model and complemen tary 1D mode l s have been va li dated against the NOx concentrations measured in the 1A and 1 B ducts as we ll as temperature measurements co ll ected by Barr Engi n eering and USS Minntac trend data for the same t im e pe ri od. Upon comp l etion of the va li dation procedure, the estab li sh ed model settin gs have been fixed, and app li ed to pred i ct the burner performance wit h the new l ow-NOx burner and adju sted prim ary air f l ow rates.

I S

O 9001 Based on the results of the ana l ys i s, signifi cant reduction of NOx is expected for the th ree most common fuel scenari os. Fo r these three f u el scenarios, model predictio n s show that the maximum gas temperature decreases w i th the l ow-NOx burner, as compared to t h e existing b urn er. Furth e rm ore, the gas vo lum e wit hin the k iln where the temperature exceeds 2780°F decreases with the l ow-NOx burner as co m pared to the ex i sting burner. Signifi cant thermal NOx form ation occurs at temperatures above 2780°F, and doub l es for eve ry150°F increase at temperatures greate r than 3500°F; therefore, these findings s u pport th e pred i ction that NOx producti on w ill si gnifi cant l y decrease. Key results for a ll modeled cases are provided in Table 1-1 and Table 1-2. For cases 1 and 3, CFD model predicts a 62-96% decrease in the amount of NOx leaving the preheat, whi l e case 2 predictions in d i cate 79- 97% decrease . All three cases show sign ificant decline in thermal NOx format i on that is due to a correspond in g decrease in flame temperature. L

ow-NOx burner cases 

### 4 and 5 have not been va l idated, thus p redicted NOx formation cannot be compa red to the ex i stin g burner cond iti ons . Res ults for the l ow- NOx burner case 4 indicate a NOx format i on rate of-' w hi ch i s comparab l e to results pred i cted for ex i sting burner operation with both coa l a nd natural gas and the tri-fuel mi x . Resu lts for th e l ow-N Ox burner case 5 predict**~** WorkingTogether V SAFELY H33 25 71 -07-5 5- 124- 0001 , Rev. 0, Page 33
e H atc h 2010/03U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 

### NOx formation of-- This value is comparable to the predictions for the most common fuel mixes. 

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

### Additionally, CFD model predicted a decrease in the kiln to preheat gas temperature for low-NOx cases 1 and 3. It is important to maintain the magnitude of gas temperature exiting the kiln into the grate as it provides the main source of heat for the pellet induration process. Hatch recommends that modeling of two additional variant cases be completed. The boundary conditions for the proposed variant cases should be adjusted to decrease the observed drop in the gas temperature exiting the kiln as the temperature variation for low-NOx burner cases 1 and 3 is a process related concern. Coen has provided recommendations for new burner settings and modeling of the proposed variant cases is currently in progress.

ISO 9001 """-"**WorkingTo~ther** 'V' SAFELY H332571-07-55-124-0001, Rev. 0, Page 34
© Hatch 2010/03

# ~HATCH~ U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010



# Appendix A Benchmark Comparison and Model Validation

ISO 9001**~** WorkingTo!{!ther " SAfElY H332571-07-55-124-0001, Rev. 0, Page 35 ""Hatch 2010/03

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

### Final Report- February 18, 2010 The models have been validated using the data obtained during the site visit to USS Minntac Line 7 on june 9-10, 2009 as well as additional trend data provided by the client for the same period. The primary kiln burner is capable of operating with various combinations of coal, biomass, fuel oil and natural gas. Site measurements have been collected for the three most common operating fuel mixes: 1. Coal and Natural Gas- Case 1 2. Biomass and Natural Gas
- Case 2 3. Coal, Biomass and Natural Gas (Tri-fuel)- Case 3 For each of the above scenarios, the temperature and NOx concentration in the 1 A and 1 B ducts exiting the preheat were measured concurrently with shell temperatures on the kiln and grate sidewalls as well as plant air conditions. Trend data available for the measurement period provides the boundary conditions during the testing periods including the green ball feed rate to the grate, the fuel firing rates for the main and preheat burners, and key air flow rates which are listed in the Modeling Basis document (PRH332571.001_Rev1). Based on direct comparison between trend data at the stack and data measured by Barr at 1N1 B fans during the same time period, a factor has been applied to relate predicted NOx output from the preheat grate to the expected value at the stack. The calculated multiplication factors for all three validation cases are shown in Table A-1. For direct comparison with trend data at the stack, the average value of 1.6 is applied as a multiplication factor to NOx model predictions.
Fuel Scenario Coal & 

### Natural Gas Biomass & 

### Natural Gas Biomass & 

### Natural Gas Stack Trend 1AI1B Fan by Multiplication**~U~S~S~JB~a~r~r~~~~~f---~Facto~r** 

### __ _,

A-1: NOx Mass Flow Rates for june 9-10,2009 

## A.1 Grate Basket Test Data Comparison

ISO 9001 1 D Traveling Grate and Kiln models are designed to provide predictions for green ball/pellet temperature and their composition throughout the induration process. During the june 2009 site visit, US Steel provided Hatch with several temperature measurements to be used for validation of pellet temperatures. For each fuel scenario, the green ball feed temperature as well as pellet discharge temperature from the kiln have been recorded; however, no direct measurements of pellet temperatures were available throughout the rest of the firing process. Pellets undergo initial heating in the traveling grate with temperatures progressively increasing as the grate moves from first downdraft section to the second continuing on to the preheat. The heating of the pellets is accomplished by blowing hot air from the top of the grate through the pellet bed. Since**~** Workinglogether Q SAFELY 

### H332571-07-55-124-0001, Rev. 0, Page 36 ©Hatch 2010/03

# ~HATCH~ U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 201 0 pellets stay relatively undisturbed until they reach the rotating kiln, the heat transfer to the top of the pellet bed is much greater than to the bottom. As part of the validation process, it was important to capture temperature variation with depth through the pellet bed. Figure A-1 shows results of the grate basket test conducted on Line 5 traveling grate at USS Minntac facility on October 

### 8, 2007. Four grate baskets collected temperature readings at various depths within the pellet bed throughout the length of the grate. Upon exiting the grate, pellet temperatures become uniform as a result of the constant kiln rotation. Unlike validation data collected on site, grate basket test results cannot be directly compared to 1 D traveling grate due the differences in grate geometries between operating lines as well as other material input and operating parameters. However, the grate basket test results display important trends in temperature profiles at different bed depths.

ISO 9001 

### u::'**~ ~**  

# =

2500 ,-----------------

2000 PH Temperature Drop Layer 4-Layer 1**~** 730°F

DD2 Temperature Drop Layer 4-Layer 1**~** 

### 5 70 oF

# ~**~** 1500**~** 

# "' Q.. 

### E

### ~ 

# 1:: 1000 

# "" DD1 Temperature Drop Layer4-Layer 1**~360°F** 

## OJ Qj


# .... 500 Test Data - Layer 

### 1 " Test Data - Layer 

### 2**-a~~** Test Data- Layer 

### 3 ---Test Data- Layer 4 0 ' ----·-·--·-·-···-·-----·· "'""""""'"""'"""""'""""''""''"'"'""'"'""'"'""'"'''""'"'"'--·-·--·---·--·-----------------·------·-·-------·-·----·---------------·------··--·-·---· 8:38:30 8:39:00 8:39:30 8:40:00 8:40:30 8:41:00 8:41:30 8:42:00 8:42:30 8:43:00 8:43:30 8:44:00 

### Time of Measurement [hrs] FigureA-1: Grate Basket Test Data taken October 8, 2007 

### on USS 

### Minntac line 5 Since direct comparison of temperature magnitudes of the grate basket test and 1 D model results cannot be done, the validation of pellet bed temperature profiles is accomplished by looking at temperature difference between different layers of the bed at downdraft 1, downdraft 2 and preheat sections of the grate. Figure A-2 shows average value of pellet bed temperatures produced by 1 D traveling model for validation cases 1-3. The model discretizes the thickness of the bed into four equal segments, thus producing temperatures for four different layers throughout the length of the grate.**~** WorkingTogether 0 SAFEtY H332571-07-55-124-0001, Rev. 0, Page 37 "'Hatch 2010/03

# ~HATCH~

Ci:'**~** 

# e :I 2500

2000**~** 1500 

# .. " Q. 

### E

### ~ 

# 1 1000 

# = ]; a:;


# .... 500

0 -50 U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project 

### Final Report- February 18, 2010

PH Temperature Drop Layer 4-Layer 1**~940°F** 

# \ DD2 Temperature Drop Layer 4-Layer 1**~** 

### 530°F

DD1 Temperature Drop Layer 4-Layer 1**~390** 

### °F 

# \

-45 -40 -35 

# \

-30 -25 -20 

### Axial Position [m] Grate Model
- Layer 1**~crate** 

### Model- Layer 2 -o- 

### Grate Model
- Layer 3 _._Grate Model
- Layer 4 -15 -10 -5 

### Figure A-2: Average Traveling Grate Temperatures of Pellets for Cases 1-3 As shown in Figure A-2, there is good correlation between grate basket test results and 1 D average modeling results for overall temperature drop from layer 4 to layer 1 in both downdraft sections. This approach provides validation for heat transfer methodology used in the 1 D model of the traveling grate. Model results show a higher temperature drop in the preheat section of 940°F as compared to 730°F for the grate basket test. Since the same heat transfer methodology is used for all three sections of the grate and the grate exiting oxidation percentage matches expected values as discussed in Section 3.3, the discrepancy in temperature magnitudes is thought to be due to variations between Line 5 and 7, as well as variations in material input and operating conditions. 

## A.2 Validation Case 1 

## -Coal and Natural Gas

A.2.1
ISO 

### 9001 Validation case 1 compares US Steel trend data to the results of three models for the coal and natural gas fuel mix. Main burner fuel, primary and secondary air flow rates are I isted in Table 1-1 . The models are validated against pellet bed, gas and shell temperature profiles as well as the amount of NOx generated in the grate/kiln induration system. 10 Model Pellet Bed Temperature Profiles

'QlCI WorkingTogether 'V SAFEtY 

### H332571-07-55-124-0001, Rev. 0, Page 38 "' Hatch 2010/03 0

# ~HATCHTM U.S. Stee l Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18 ,2 010 Figure A-3 shows pellet temperature profiles ge nerated by 1 D Travel in g G rate and 1 D Ki ln m ode l s. Due to discretization of bed thi ckness w ithin traveling grate model, the graph below displays tempe rat ures fo r all fo ur layers thro ughout th e l en gth of the g rate. O n ce pellets enter t he kiln , temperature p rofile become uniform due to contact rotation and o nl y sin gle temperature is displayed. 2500

2000 

### u:' 0 

### ........**~** :l 

## 1ti 1500

### ~ Green ball feed 

### c.. E temperature = 11 6° F aJ 1- "C aJ 1000 Kiln Discharge Te mperature USS Trend Data =2260 ° F 1 D Kiln Mode l Pred iction= 2270° F**~ ~**  

### - 1 D Grate Model - L aye r 1 

# - aJ aJ Q.. 5 00

0

- 50 -40
-30
-20 

### -b- 1 

### D Grate Mode l - Layer 2 -o- 

### 1 

### D G rate Mode l - Laye r 3 

### --1 D G rate Model- Laye r 

### 4 -+- 1 

### D Kiln Model



### • USS T re nd Data

- 10 0 1 0 20 30 40 

### Axial Position [m] Figure A-3: Travel ing Grate and Kiln Pellet Tempe ratures for Validation Case 1 On l y two va lid ation points are ava il able from USS T rend data co ll ected on site. The initi al green ball feed temperat ure of 11 6°F i s u sed as 1 D Trave lin g Grate mod e l in put . The final kiln di sc harge for fu e l case sce nario 1 h as been m easured at 2260° F w hil e 1D model pred i cted final kiln d i scharge temperature of pe ll ets to be 2270 ° F. Th erefore 1 D model p red i ct s pe ll et temp e rature w i t hi n 1 % erro r of th e data prov i ded by U S Stee l and i s expected to prov i de accu rate pe ll et temper a tures for the co rrespondin g l ow-NOx burner case, w ith a coa l and n atura l gas fuel mi x. A.2 .2 Cas T emperature Profil es

I S

O 900 1 Fi g ure A-4 shows 1 D Trave lin g Grate and Kiln model predict i ons of gas temperature through the entire l ength of th e system. US S tee l provided seve ral gas tem perature m eas ure m e nts in each secti o n of the g rate as we ll as w ithin the rotatin g kiln that h ave been u sed to va lid ate th e 1 D models .

'V5:I Workin g Together v SAFELY H 33257 1 -07- 55 - 1 24-0001, Rev. 0, Page 39 " Hatch 2 010/03 50

# ~HATCHTM

......... L.L. 0**~** 3500

3000

2500

:; 2000

## ~**~** 

### c..

## ~ 1500 1- <ll !';I (,J 1 000
SOD

0 U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report - February 18, 2010

1 Average PH Temp: USS Trend =2110°F 1 D Grate Model= 1950° F Peak Centerline Temp: USS Trend Data=2900 °F 1 D Grate Model =2860°F 

# / Average DD2 Temp:

# ~ •

-50USS Trend Data=1220°F 1 D Grate Model= 1200° F 

# \ • 

# • • 

# "" Average DD1 Temp: USS Trend Data = 570 ° F 1 D Grate Model = 600 ° F

-40
-30 -20
- 10 0 10 

### Axial Position [m] 

# \

Burn Zone Temp: USS Trend Data=2430°F 1 D Grate Model =2635°F

--1 D Grate Model

-- 1 D Kiln Model



# • USS Trend Data 20 30 40 Figure A-4: T raveling Grate and Kiln 

### Gas Temperatures for Validation 

### Case 1 A ll temperatures predicted by 1 D models are within 8% error from the validation points in both traveling grate and kiln. The temperature difference between va lidation points and model predictions are wi thin typical measurement errors. Validation case 

### 1 w hich u ses coal and natural gas fuel mix sce nario di splays good correlation between gas temperature results of 1 D Traveling Grate and 1 D Kiln models and validation data points co ll ected during Jun e 9- 10, 2009 site visit to Line 7 at US Steel Minntac facility.

I SO 900 1 In addition to validation of gas temperatures generated by one dimen sional models, similar procedure has been app lied to results produced by 3D CFD model of the preheat section of the grate and kiln. Figure A-5 shows two dimensional vertica l cross section through the center of the grate/kiln model as wel l as a graph of temperature values measured along the center lin e of th e preheat and the kiln.**~** WorkingTogether V SAFELY H33 257 1
-07
-55
- 1 24
-0001, Rev. 0, Page 40
0 Hatch 2010/0 3 

### so

# ~HATCHTM

1200 T[°F]



### c:' 0
.......... QJ 3500

3000

2500 

## 5 2000**~** 

### ,_ QJ Q.

## ~ 1500 1- rll 1':1 (,;, 1000

5 00 

# t

1450 1700



# • • • 

# v Average PH Temp: USS Trend =2115 ° F 3D CFD Modei = 2325 ° F U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18,2010



# ---------

1950 2200 2450 2700 2950
Peak Centerline Temp: USS Trend Data=2900 ° F ........__ 3D CFD Modei=3200°F --............. 

# I
Burn Zone Temp: USS Tren d Data = 2430 ° F 3D CFD Modei = 2410°F 3200



### -- 3D CFD Model

• USS Trend Data 0 ,- --------,- ------ --,- ---L---,-------- -, -------- -, ---------,---------,

-2 5
- 15 -5 5 15 25 35 

### Axial Position [m] Figure 

### A-5: 3D CFD 

### Gas Temperature Profile for Validation 

### Case 1 Simi l ar to 

### 1 D model results, all gas temperatures predicted by 3D CFD model fall within 10% error from the va li dation data points. The temperature difference between CFD model predictions and temperatures recorded on site for coal and natural gas fuel scenario fa ll with in an acceptab l e error band. Consequent l y gas temperatures are considered to be in good agreement with the trend data. 

## A.3 NOx emissions

ISO 9001 NOx concentrations were measured in 1 A and 1 B ducts by Barr Engineering concurrently with gas and shell temperature for coa l and natural gas fuel m i x. Table A- 2 shows mass flow rates of NOx in 1

A and 1 B ducts as measured on site as well as the total therma l NOx predicted by CFD Model for validation case 1.**~** Worki ngTogether Q SAFELY H332571 -07- 55- 124-0001, Rev . 0 , Page 41 " H a tch 20 10 /03 45

# ~HATCH~ U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010 

### Barr Measurements 

### 1A 18 Average NOx [ppm] 

# •• Total NOx Mass Flow

Only thermal NOx formation has been taken into account when performing NOx post processing analysis as described in the Modeling Basis document (PRH332571.001_Rev1). Due to high sensitivity of thermal NOx formation to the gas freeboard temperature, the NOx predictions can vary significantly with very small fluctuations in flame temperature predictions. Overall prediction error of 4% is an excellent result, since much smaller temperature errors can results in significant changes in the amount of NOx generated. 

## A.4 Validation Case 2 - Biomass and Natural Gas Validation case 2 compares US Steel trend data to the results of three models for biomass and natural gas fuel mix. Main burner fuel, primary and secondary air flow rates are listed in Table 1-1. The same validation procedure is used for this fuel scenario as the one applied to coal and natural gas fuel mix. A.4.1 Pellet Bed Temperature Profiles Figure A-6 shows pellet temperature profiles generated by 1 D Traveling Grate and 1 D Kiln models. Similar to validation case 1, bed thickness in 1 D traveling grate model is discretized into four segments, thus the graph below displays temperatures for all the layers throughout the length of the grate. Once pellets enter the kiln, temperature profile become uniform due to contact rotation and only single temperature is displayed.

ISO 9001 """WorkingTogether V' SAFflY H332571-07-55-124-0001, Rev. 0, Page 42 ©Hatch 2010/03

# ~ 

# HATCHTM

A.4.2 2500
---**~** 

# ... :1 2000

-; 1500 

# ...**~** Green ball feed 

### E**~** 1- '"C**~** 1000 

# =

500

0

-50 -40 -30 -20
- 10 U.S. Steel Minntac -line 7 NOx Reduction Pilot Project Final Report - F ebruary 18, 2010

0 K;ln o;scha<ge Temperature 

# I
USS Trend Data -2265°F 1 D Kiln Model Prediction- 2270 ° F

10 

### 1 D Grate Model - Layer 1 --tr- 1 D Grate Model- Layer 2 -o-1 D Grate Model- Layer 3 --1 D Grate Model- Laye r 4 -+- 

### 1 D Kil n Model

• USS T rend Data

20 30 40 

### Axial Position [m] Figure A-6: Traveling Grate and Kiln Pellet Temperatures for Validation Case 2 US Steel trend data prov id ed two pellet temperatures app li cable to validation case 2. In iti al green ball feed temperature of 125 ° F is used as 1 D Trave li ng Grate mode l input. 1 D Ki l n model predicted fina l ki l n di scharge temperature of 2270 ° F w hile US Steel measurements ind icated fina l pe ll et tempe rature of 2265 ° F. Results of 1 D models fa ll with i n 1% error of th e va li dat i on poin t fo r b i omass a nd natural gas fuel scenario. Based on above co mparison , 1 D tr ave lin g grate and kiln model s are acc urate l y matched wit h the pe ll et temperature validation data. Gas Temperature Profiles 

### Figure A-7 shows 1 D Traveling Grate and Ki ln mode l pred i cti ons of gas temperature th rough the entire l ength of the system. Mode li ng re sults have been compared to the measurements of gas temperatures in the grate and kiln during Hatch's site visit on June 9- 10, 2009.

I SO 9001 H332571-07-55 - 1 24-000 1, Rev. 0, P age 43 \::Jif:.l WorkingTogether V SAFELY ., Hatch 2010/03 so

# ~HATCHTM

3500

3000

2500 

### u::' 0

.......... Q,j 

## :5 2000

## ~ Q,j c.

## ~ 1500 1- 

### "' ra**~** 1000

500 U.S . Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- F eb ru ary 18, 201 0
Average PH Te mp : USS Tren d= 1 925 ° F 1D G rate Model= 1 935°F Average DD2 Temp: USS Trend Data= 1220 ° F 1D Grate Modei=1215°F 

# \

Ave rag e DD1 Temp:

### ~ USS Trend Data=565°F 1 D Grate Mode l = 590 ° F Peak Cen terline Temp: USS Trend D a ta=299 5°F 1 D Grate Model =271 0°F 

# ---. .

Burn Zone Temp : USS Trend Data = 241 0° F 1 D Grate Model =25 05°F



### ..._ 1 D Grate Model

___ , D Kiln Model

• USS Trend Data 0

### .-**-- --** **-- ----** **------ .-** **---- ----** **------.----- ----** **-- --** **-- 1**  ____ ___ ·5 0 · 40 ·30 · 20 · 1 0 0 10 20 30 40 

### Axial Position [m] Figure A-7: Trave ling Grate and Kiln Gas Temperatures for Validation Case 2 A ll temperatures pred i cted by t h e 1 D traveling grate mod el are w ithin 5% of measured va lu es and 1 D Kiln model are w ithin 1 0% o f m easured va lu es . Th e t emperatu re differences betwee n va lid ati on po ints and mode l pred i ct i ons are w ithin typical meas urement erro rs. Va li datio n case 2, w hi c h co r respo nd s to t h e biomass a nd n a tura l gas fue l mix sce nar i o, di splays good corre lati o n between the predicted gas fre eboard tempera ture s in th e 1 D Traveling Grate an d 1D Kiln and the va lid at ion data points.

I SO 90013D CFD model i s va lid ated using the same US S tee l trend temperatures that we re compa red with the 1 D model p redicti ons. Fi g ure A-8 displays two d im ens i o n a l verti ca l cross sect i o n through the center of the grate/kiln CFD model and a g rap h of gas temperatures a l ong th e cen ter lin e of the preheat an d the kiln .**~** WorkingTog ether Q SAFELY H33257 1
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1200 T[° F)

3500

3000 :**~** 0

..........25oo I
111 S2ooo**~** 

# ... 111 Q.

## ~ 1 500 I 1- fll 

### r:s 

### tJ 1450 1700

1000 Average 

### PH Temp : USS Tre nd = 1925 ° F 30 CFD Model = 2280 ° F 5oo 

## I 1950 U.S. Steel Minntac- Line 7 NOx Reduction Pilot Proj ect Final Report- February 18 ,20 10

2200 

### 2450 2700 2950
Peak Centerline Temp: USS Trend Data=2995°F -----.. 30 CFD Model =2985°F

Burn Zone Temp: USS Trend Data=2410 ° F 30 CFD Modei=2625 ° F



### • USS Trend 

### Data

-- 3D CFD Model 3200

0**,- --------** **,- --------** **,- --** **---- .-** **-------- ,-** **-------- ,---------.---** **-- --** **~**



### A.4.3 -2 5 - 1 5 -5 5 1 5 2 5 35 

### Axial Position [m] Figure A-8 : 30 CFD 

### Cas Temperature Profile for Validation Case 2 Gas tempe r at u res predicted by 30 CFD model display exce ll ent co rrelati o n wit h temperature va li dation points l ocated in the kiln . Measurements of gas temperature in th e preheat secti on of th e grate are 18% lower than res ults of CFD model. A lth o u gh thi s erro r i s greater than values obtained for va lid at i on case 1, the key parameter of kiln exit gas temperature is st ill w i th in 10% o f the meas ured va lu e. Sign ifi cant therma l NOx generation occurs at temperatures abov e 2780 ° F, th us el evated p reheat gas temperature predi cted by CFD model will not have a measurable affect on the amount of NOx format ion . 

### NOx Emissions NOx concentrations for biomass and natural gas fuel mix case were measured in 1

### A and 1 B d u ct s by Barr Eng in ee ring concurrent l y with gas and she ll temperature. Table A-3 shows mass f l ow rates of NOx in 1A and 1 B duc ts as measu red on site as well as the total therma l NOx p red i cted by CFD Model for va li dati o n ca s e 2. I SO 9 001 H 3 32 57 1 -0 7- 55-1 24- 0001, Rev. 0 , Pa ge 4 5**~** WorkingTogethe r V SAFELY " Hatch 20 10/ 03 45

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Table A-3: U.S. Steel Minntac**~Line** 7 NOx Reduction Pilot Project Final Report- February 18, 2010 Barr Measurements 

### Average NOx [ppm] NOx Mass Flow Rate
30 CFD Model Prediction Total Thermal NOx [lb/hr] Prediction Error Prediction of NOx Emissions 1A 

### 18 

# • •

Only thermal NOx formation has been taken into account during post processing calculations for this validation case. Total thermal NOx mass flow rate as predicted by CFD model is- in comparison with- as measured by Barr. The prediction error of 9% indicates the CFD model has accurately captured the temperature and NOx formation trends for validation case 2. 

## A.S Validation Case 3 -Coal, Biomass and Natural Gas (Tri-fuel) Validation case 3 

### compares US Steel trend data to the results of three models for coal, biomass and natural gas fuel mix (tri-fuel). In addition to primary and secondary flow rates, both solid fuel flow rates to the main burner are provided in Table 1-1. 

### The validation procedure established for previously discussed fuel mixes is once again applied to the modeling results to the tri-fuel scenario. A.S.i Pellet Bed Temperature Profiles 

### Figure A-9 shows pellet temperature profiles generated by 1 

### D Traveling Grate and 1 

### D Kiln models. As described for previous validation cases, bed thickness in 1 

### D traveling grate model is discretized into four segments while 1

### D Kiln model predicts a uniform temperature length due to kiln rotation.

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A.5.2 

### L:i:' 0

..........**~** 

# =

2500

2000**~** 1500**~** G reen ball feed

### ~ temperature = 127° F

## ~ "C Q.l 1000 

# = 4J**~** 

### c.. 500

0

-50 -40 -30
- 20 -1 0 U.S. Steel Minntac- Line 7 NOx Red ucti on P i lot Project Fina l Report- Februa ry 18, 2010

0 Kiln D i sc ha rge Temperature USS Trend Data - 2265 ° F 1 D Kiln Model Prediction- 2275 ° F

1 0 

### 1 D G rate Mode l - Layer 1 -a- 

### 1 D Grate Mode l - Layer 2 -o-1 D G r ate Mode l - L ayer 3

_._ 1 D Grate Model - L ayer 4

-- 1 D Ki ln Mode l

• USS T rend Data

20 30 40 

### Axial Position [m] Figure A-9: Traveling Grate and Kiln Pellet Temperatures for Validation Case 3 US Steel t rend data prov id ed two pellet temperatures applicable to va lid ation case 

### 3. The green ball feed tempe ratu re for this fuel mi x i s hi ghest o ut of a ll three validation cases at 127 ° F. The 1 D Kiln mode l predicts a pellet di scharge temperature of 2275 ° F compared to a measured va l ue of 2265 ° FTh e refore, mode l s of a ll t hree va lid ation cases have predicted fina l pellet temperatu re with i n 1 % of prov i ded US Steel trend data_ Th e 1 D m ode l s are cons id e red to provide an accurate representatio n of the pellet h eating t ren ds at Lin e 7 of US Stee l M in n tac, based on such a small pe llet tempe ratu re diffe ren ce between va lid ati on po ints and model predictions 

### Gas T emperature Profiles Fi gure A-1 0 displays 1 D Traveling Grate and 1 D Kiln model predict i ons of ga s freeboard tempe ratures. These predictions are compared to 

### 8 temperature measurements taken in both the traveling grate and kiln portion s of Li ne 7.

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3000
2500

1000
500 U.S. Steel Minntac - Li ne 7 NOx Reduction Pilot Project Final R eport - February 18, 20 10 

### Average 

### PH Temp: USS Trend - 2080 ° F 1 D Grate Modei - 1970° F Average 002 Temp: USS Trend Data - 1250 ° F 1 D Grate Model= 121 0 ° F • • •

# \~

Average 001 Temp: ' USS Trend Data = 560°F 1 D Grate Mode l = 580°F Peak Centerline Temp: USS Trend Data-2915 ° F 1 D Grate Mode l - 2865 ° F

Burn Zone Temp: USS Trend Data-2435 ° F 1 D Grate Modei-2630°F



### -- 1 D G rate 

### Mode l

-- 1 D Kiln Mode l • USS Trend Data 0**,-----~-- ----.-----~-----.----** **--~ --** **-- ----** **----------~------.----- -.** 

-50 -40 -30 -20 -10 0 1 0 20 30 40 

### Axial Position [m] Figure A-10: Traveling Grate and Kiln Gas Temperatures for Validation Case 3 All temperatures predicted by 1 D models for tri-f ue l sce na ri o are within 8 % error from the validation points w hi ch is consistent w ith re sult s for va lid ation cases 1 and 2 . Th e d i fferences between validat ion gas temperatures and corresponding mode l predictions are w i th in typ i ca l mea surement e rrors. Validation case 3 which models a mixture of all three fue l s: coal , biomass and natu ra l gas d i sp l ays a good correlation between 1 D Traveling Grate and 1 D Kiln mode l p redictions of gas freeboard temperatures and validation data points.

ISO 9001 

### Following established validation procedure, the 30 CFD model is compared to gas temperature trend data. Figure A-11 presents a two dimensional vertical cross section through the center of the traveling grate and kiln. The corresponding one d imensi onal graph displays temperature va lues taken a l ong the center l ine of the preheat and kiln.

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1200 

### T[°F] 3500

3000
2500

2000
1500 

### 1450 1700 ' •j,



# • 

# •
Average PH Temp: USS Trend=2080 ° F 1000 

### 3D CFD Mode i =2260°F
500 

# :
1950 2200 2450


### P

eak Centerl in e Temp: USS Trend Data=2915 °F 3D CFD Model= 3250 ° F

Burn Zone Temp: 

### 2700 2950 3200



### USS Trend Data =2435°F 3D CFD Mode l =2345°F



### --3D CFD Mode l

• USS Trend Data 0**r----- ----** **.----- --** **--.----L----,- ------~----** **---- -.~** **------~--------~**

-25 -15 -5 5 15 25 35 

### Axial Position [m] Figure A-11: 3D CFD Gas Temperature Profile for Validation Case 3 All gas temperature predictions produced by the CFD mode l fa ll within 10% of t he m easu red gas temperatures. These results are within an expected error measu rement band; therefo re , va li dation case 3 modeling output is in good agreement with va lid ation points provided by US Steel. Establishing accurate gas temperature predictions within reasonable error bands for a ll va lidation cases is a significant step in producing meaningfu l NOx results. 

### A.5.3 NOx Emissions 45



### NOx concentrat i ons .for the coa l , biomass and natural gas fue l mix case were measured in the 1 A and 1 B ducts by Barr Engineering co nc urre ntl y w ith gas and shell temperature measurements. Table A-4 shows mass flow rate s of NOx i n th e 1A and 1B ducts as measu red on site as well as the total thermal NOx predicted by CFD Mode l for va lidation case 3.

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A.5.4 U.S. Steel Minntac- Line 7 NOx Reduction Pilot Project Final Report- February 18, 2010



### Only thermal NOx formation has been taken into account during post processing calculations for validation case 3. Total thermal NOx mass flow rate as predicted by CFD model is- in comparison with- as measured by Barr. CFD model has under predicted the mass flow rate of NOx by 23% as shown in Table A-4 for the tri-fuel mix scenario. Although this is a significant percent error in comparison with results obtained for validation cases 1 and 2, the sensitivity of NOx formation to the temperature results in high percent NOx error due to very small changes in gas temperature. As described in Section A.5.2, all of the temperature predictions for both 1 D models and 3D CFD model were within 10% of the measured gas temperatures. Therefore the combination of both solid fuels and natural gas produces effects on NOx formation that could not be fully captured by the CFD model. Using the same post processing model for low-NOx burner case 3 allows to find a trend in NOx formation and provides an estimate on overall thermal NOx decrease due to the implementation of the new burner. Summary of Validation Findings 

### Appropriate methodology and input settings have been established based on the three validation scenarios. Modeling results that include the main burner design by Coen are described in Sections 6 & 

### 7 and have been generated using the fixed settings determined during the validation procedure. Consistent application of the established methodology allows for more precise determination of the impact of the new burner design on the temperature and consequent formation of thermal NOx. For direct comparison of trend data at the stack to the predicted thermal NOx leaving the preheat, multiplication factor of 1.6 has to be applied to the model predictions as discussed at the beginning of Appendix A.

Fuel Scenario Coal & 

### Natural Gas Biomass & 

### Natural Gas Stack Trend Thermal NOx 

# uss NOx Prediction

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