U.S. Steel Minntac 

Minnesota 

Line 7 NOx Reduction Pilot Project 

CFD Modeling Final Report 

H332571 

Rev. 0 

February 18, 201 0 

This document contains confidential information intended only for the person(s) to whom it is addressed. The information in this 

document may not be disclosed to, or used by, any other person 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 T em perature Profiles ................................................................................................. 1 8 

6.1 .2 Gas Temperature Profi I es .................................................................................................... 1 9 

6.1 .3 NOx Emissions ................................................................................................................... 20 

6.2 Low-NOx Burner Case 2- Biomass and Natural Gas .................................................................... 21 

6.2.1 PelletT em 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 I es .................................................................................................... 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 I et Bed Temperature Profi I es .......................................................................................... 30 

7 .2.2 Gas Temperature Profiles .................................................................................................... 31 

7 .2.3 NOx Emissions ................................................................................................................... 32 

8. Conclusions and Recommendations ...................................................................................................... 33 

ISO 9001 

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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 

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Parameter 

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 9001 

This 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 1 0% 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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Final Report - February 18, 2010 

the higher stoichiometric ratios will be more than offset by the significant decline in thermal NOx 

formation. 

The percentages of NOx decrease, as presented in this report, reflect only the thermal NOx 

predictions. These trends are consistent with the overall decrease in total NOx due to the proposed 

burner. Actual decrease in total NOx may be slightly less than reported here due to th e effects of fuel 

NOx 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 1500 2000 2500 3000 3500 4000 4500 

Gas Temperature [°F] 

Figure 3-2: NOx Equilibrium Composition in Air 

As seen in Figure 3-2, NOx formation increases exponentially with temperature with significant 

thermal NOx formation occurring at temperatures above 2780° F. At temperatures hotter than 

3500°F, the amount of produced thermal NOx doubles for every 160° F increase in temperature. As 

a result, the maximum gas temperatures and gas volumes of the kiln above 2780°F are the primary 

indicators of the expected reduction in the amount of NOx generated for each fuel scenario. 

ISO 9001 

As explained above, the amount of generated NOx is extremely sensitive to temperature variations. 

For example, an increase in the average burn zone temperature of 3000°F by 5% can result in a 

corresponding increase of NOx concentration by as much as 300%. Consequently, significant 

changes in thermal NOx formation can be observed because of small errors in temperature 

predictions. Therefore it is important to point out the known sources of error in the 3D CFD model 

predictions of gas temperature and conduct a sensitivity analysis to determine how these errors 

impact the NOx production. 

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

Final Report- February 18,2010 

Based on composition of green balls entering the 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 in Figure 3-3. The effect of evaporation reaction is not 

observed in the preheat section of the traveling grate or the kiln which is consistent with normal 

operation of Line 7. 

The composition of green balls entering the traveling grate includes limestone 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 fraction of limestone and dolomite by weight percent. Increased weight fraction 

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 weight fraction of 

calcium carbonate while accurately predicting heat and final products of the two step calcination. As 

shown in Figure 3-3 below, 1 D traveling grate model predicts that calcination reaction converts all 

CaCOJ 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 

limestone or dolomite, thus 1 D traveling grate model accurately estimates full completion of 

calcination reaction at that stage of the induration process. 

12 

- % H20 Content 10 

-% CaC03 content 

- % CaO content 8 

6 

4 

DD1 

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

Grate position [m] 

Figure 3-3: lD Grate Model Prediction of Average Green Ball Composition for Validation Cases 1-3 

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Final Report- February 18, 2010 

Unlike evaporation or calcination, oxidation reaction takes place in the traveling grate as well as the 

rotary kiln. Oxidation of magnetite into hematite is once again a thermally controlled reaction; 

however, it occurs at high temperatures that are not achieved until pellets enter the beginning of the 

second downdraft section as can be seen in Figure 3-4. 

In order for the pellets to survive the fall into the kiln in tact, 60-70% percent oxidation has to be 

achieved before pellets exit the traveling grate. 

100 

90 

- %Fe304 content 

- % Fe203 content 

- % Oxidation 

DDl DD2 

-45 -40 -35 -30 -25 -20 -15 -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 enters the system as potential chemical energy in the fuel which mixes with the air stream 

through the main burner in the kiln model and preheat/downdraft burners in the traveling grate 

model. The thermal energy produced from gas phase combustion is then transferred to the solids in 

the packed bed and the refractory walls via convection and radiation. The grate model also incl udes 

hot air streams entering the system from the kiln or recycled air from the cooler. Both models reflect 

energy loss due to the heat transfer through the refractory walls and subsequent shel l heat losses to 

the ambient environment. 

3.4.1 Traveling Grate and Kiln Internal Heat Transfer 

ISO 9001 

All three chemical reactions that take place in the pellet bed are thermally controlled; therefore, both 

1 D and 3D models include multiple heat transfer processes in order to correctly predict freeboard 

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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 9001 

A 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 

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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 1 A 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 1 A 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, 1 70 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 is used for validation cases, to match conditions during the 

testing carried out june 9-10, 2009. The kiln burner is a multi-fuel burner by Aecometric Corp. 

Figure 4-1 shows the overall burner geometry. Th e kiln burner is mounted such that refractorycovered section (48 in from burner tip) is fully inserted into the kiln. It is located at the centre of the 

kiln at the material discharge end. 

ISO 9001 

izing 

Air Inlet J Y izing Air Inlet 

Figure 4-1: Isometric Views of the Existing Burner Model Geometry 

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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. 

ISO 9001 

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Location of Preheat 

burners 

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 

ISO 9001 

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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 overall 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 (Ma < 0.3), thus all the compressibility effects are constricted 

to within the sub-model. 

The annulus gun clusters 

The core gun slot 

Model Domain 

Figure 5-2: Profile and Front View of the Burner Tip Sub-Model Geometry 

The results of the CFD low-NOx burner model provided by Coen have been applied as boundary 

conditions to th e burner tip sub-model. Figure 5-3 outlines the four annuli that require input from 

Coen low-NOx burner model. Results from the sub-model are used as boundary conditions to the 

final 3D CFD model of the kiln and preheat section of the grate. The sub-model geometry 

additionally includes fuel oil gun which is used as an emergency backup fuel. Since fuel oil is 

modeled as droplets of liquid, th e input from Coen is applied directly to the main kiln-grate model as 

described in Section 7.2. 

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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 the existing 

burner on June 9-10, 2009. The validation procedure and result are discussed in Appendix A. After 

three fuel scenarios have been successfully validated, the fixed model 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 conditions is primary air flow rate which is increased with the new burner as per Coen's 

design. All fuel flows and primary and secondary air flow rates used as input to the new burner are 

li sted in Table 1-1. The presented results are valid for the given set of process conditions modelled, 

which are assumed to be representative 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. 

ISO 9001 

The low-NOx burner is capable of operating with various combinations of coal, biomass, fuel oil and 

natural gas, which is similar to the existing burner on Line 7 at USS Minntac. Three most common 

operating fuel mixes are modeled and compared to three already validated cases. 

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For each case, model o utputs have been compared against pellet and gas temperature predictions 

obtained during the validation procedure. Results of 1 D Traveling Grate, 1 D Kiln and 3D CFD 

models are compared for 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 comparison between 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, while kiln temperatures are taken as a direct output of the 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 em peratore I \ 

llT (Low-NOx - Validation) = -220°F 

I 

__,_ VC1 1 D Grate Model Average 

-+- VC1 1 D Kiln Model 

--CC1 1 D Grate Model 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 

Overall, the observed trend is a decrease in the pellet bed temperature. The grate exit temperature of 

the pellets for I ow-N Ox burner case 1, as predicted by 1 D Traveling Grate model is 1680° F, which 

is 55 °F lower than model prediction for val idation case 1. The more significant impact on pellet 

temperature can be seen within the kiln. The overall drop in kiln discharge temperature of pel lets 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 Reduction Pilot Project 

Final Report- February 18, 201 0 

Figure 6-2 shows gas temperature values, as predicted by the 3D CFD model, along the center line of 

the kiln and preheat section of the grate for the coal and natural gas fuel scenario. Two d imensional 

vertical cross sections displaying gas temperatures for both val idation 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 1500 

~ 

"C 

0 § ,, I 

500 

Peak Centerline Temperature 

LH (Low-NOx -Validation) = -41 0° F 

/ 

Kiln to PH Gas Temperature 

li.T (Low-NOx- Validation) = -65° F 

-+-VC1 3D CFD Model 

I -o-CC1 3D CFD Model 

0~

-25 

--~---- -15 -5 ~-- ---+----~-- 15 25 -- --~ 35 

Axial Position [m] 

Figure 6-2: Comparison of CFD Gas Temperatures for Case 1 

The key parameters in Figure 6-2 are the kiln to preheat gas temperature and peak flame temperature. 

US Steel has expressed the importance of maintaining the gas temperature exiting the ki ln into the 

grate, as it provides the main source of heat for the pellet induration process. Pellets have to be 

heated to a specific temperature in order to undergo 60-70% oxidation and survive the fall into the 

kiln without shattering. A decrease in that gas temperature by 65 ° F from validation to low-NOx 

burner case is predicted, which is considered unacceptable for the process. 

ISO 9001 

Another parameter of interest is the peak flame temperature. Since NOx formation is so sensitive to 

temperature variations, a drop in the peak flame temperature will result in a significant decrease in 

the amount of generated NOx. Elongation of the flame from validation case to low-NOx burner case 

is shown in Figure 6-3. It red istributes the heat of fuel combustion and results in 41 0 ° F peak flame 

temperature drop for coal and natural gas fuel mix. 

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1200 

T[°F] 

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U.S. Steel Minntac- line 7 NOx Reduction Pilot Project 

Final 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 Section 3.2, NOx formation increases exponentially w ith temperature with significant 

NOx formation at temperatures above 2780°F. Figure 6-4 shows volumes of the kiln where the 

temperature exceeds 2780°F for val idation case 1 and low-NOx burner case 1. The figure 

additionally shows contours within the volumes to indicate areas where temperature is above 

3000°F. In these regions NOx increases 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 volume exceeds temperature of 2780°F whi le only 0.6% 

of the volume for low-NOx burner case 1 is greater than 2780° F. Additional ly the peak flame 

temperature in the kiln drops from 3420°F for validation case 1 to 3030°F for low-NOx burner case 

J 

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1. The drop in volume percentages and peak flame 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 is in the 

range from - · Based on the measurements taken by Barr, -of NOx is generated 

with the existing burner for the same fuel scenario. Therefore, 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 average pellet bed temperatures as predicted by 1 D 

models for validation and low-NOx burner case 2. Presented traveling grate temperatures are the 

average values for four discretized layers, 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 - Validation) = -70°F 

Kiln Discharge Temperature 

~T (Low-NOx- Val idation) = -180°F 

I 

-+- VC2 1 D Grate Model Average 

-+-VC2 1 D Ki ln Model 

--CC2 1 D Grate Model Average 

--CC2 1 D Kiln Model 

I I 

0 ,------.------~------------,----- ---- ------,------,------4------, 

ISO 9001 

-50 -40 ·30 -20 ·1 0 0 10 20 30 40 

Axial Position [m] 

Figure 6-5: Comparison of Traveling Grate and Kiln Pellet Bed Temperatures for Case 2 

The overall trend of for biomass and natural gas fuel scenario is a decrease in pellet bed temperature 

with the low-NOx burner which similar to case 1. The grate exit temperature of the pellets is 1785°F, 

which is 70° F lower than for validation case 2. The more significant impact on pellet temperature 

can be seen within the kiln. The overall drop in kiln discharge temperature of pellets is 180° F. The 

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final kiln discharge temperature of the pellets for low-NOx burner case 2 is 2090° F, which is 8% 

below the average final pellet temperature for the three validation 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 vertical cross 

sections displaying gas temperatures for validation and low-NOx burner case 2 are presented in 

Figure 6-7. 

3500 

3000 

~ 

0 

...... 

cu 2500 ... 

:::l 

~ 

~ 2000 1 

~ I ~ 

~ 1500 

~ 

"'C 

0 

~ 1000 

loL. 

u 

500 

l 

Peak Centerline Temperature 

t.T (low-NOx- Validation) = -55°F 

/ 

Kiln to PH Gas Temperature 

LH (Low-NOx- Validation) - 0° F 

-+- VC2 30 CFO Model 

--CC2 3 0 CF 0 Model 

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

ISO 9001 

-25 -1 5 -5 5 15 25 35 

Axial Position [m] 

Figure 6-6: Comparison of CFD Gas Temperatures for Case 2 

Key parameters for gas temperature predictions are the kiln to preheat gas temperature and the peak 

flame temperature. Low-NOx burner case 2 results show that there is no measureable 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 is so sensitive to 

temperature variations, drop in the peak flame temperature results in significant decrease in the 

amount of generated NOx. Elongation of the flame from validation case 2 to low-NO x burner case 2 

is shown in Figure 6-7. This is the same trend seen for case 1. The implementation of new low-NOx 

burner and change in primary air flow rate results in 55 ° F drop of the peak flame temperature for 

low-NOx burner case 2. 

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1200 

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Final Report- February 18, 201 0 

'--------------~-----

' --_,___----------

1700 1950 2200 2450 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 formation increases exponential ly with temperature. Figure 6-8 shows 

volumes of the kiln where temperature exceeds 2780°F for both validation case 2 and low-NOx 

burner case 2. The figure also shows temperature contours within these volumes to indicate areas 

where temperature is above 3000°F and NOx production increases significantly with temperature. 

ISO 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 

Validation case 2 predicts that 1.2% of the volume exceeds temperature of 2780°F while only 0.2% 

of the volume is greater than 2780° F for low-NOx burner case 2. The relative drop in volume 

percentages indicates that a significant decrease in the amount of generated NOx can be expected. 

Additionally the maximum peak fl ame temperature in the kiln drops from 3370°F for validation case 

2 to 3040°F for low-NOx burner case 2. The greater maximum temperature for biomass and natural 

gas fuel scenario as compared with case 1 predictions will result in greater NOx formation within the 

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smaller volume 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, while 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, 

500 

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 Model Average 

--r- VC3 1 D Kiln Model 

-+- CC3 1 D Grate Model Average 

-e-- ((3 1 D Kiln Model 

0 -----,------,------,----~------4-----~------,------.------.------. 

ISO 9001 

-50 -40 ·30 -20 ·1 0 0 10 20 30 40 

Axial Position [m] 

Figure 6-9: Comparison of Traveling 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 low-NOx burner is 1705°F, which is 35°F lower than 

predicted for validation case 3 with the existing burner. A substantial impact on pellet bed 

temperatures can be seen in the predictions of 1 D kiln model. The pellet discharge temperature 

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decreases by 245° F which is the greatest decrease observed for all three fuel mix cases considered. 

The final kiln discharge temperature of pellets for low-NOx burner case 3 is 2030° F, which is 10% 

below the average final pellet temperature for the three validation cases. 

6.3.2 Gas Temperature Profiles 

Figure 6-10 shows gas temperature values as predicted by the 30 CFD model along the center line of 

the kiln and preheat section of the grate for coal, biomass and natural gas fuel scenario. Two 

dimensional vertical cross sections displaying gas temperatures for validation 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 1500 

"ii 

""C 

0 

~ 

0 1000 

1.1.. 

u 

500 

~---·-·----

Peak Centerline Temperature 

LlT (Low-NOx - Validation)= -510°F 

Kiln to PH Gas Temperature 

LlT (Low-NOx - Validation) = -60° F 

-+- VC3 30 CFD Model 

--CC3 30 CFD Mode l 

0 r--------.--------,----~---.---------,---------,---------,---------. 

-25 -15 -5 5 15 25 35 

Axial Position [m] 

Figure 6-10: Comparison of CFD Gas Temperatures for Case 3 

Key parameters for gas temperature predictions are the kiln to preheat gas temperature and the peak 

flame temperature. The results show a 60°F decrease in the kiln to preheat gas temperature as 

compared with validation 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 indicated 

the importance of maintaining the kiln to preheat gas temperature with implementation of the new 

burner. 

ISO 9001 

Another parameter of interest is the peak flame temperature. Since NOx formation is so sensitive to 

temperature variations, a drop in the peak flame temperature results in significant decrease in the 

amount of generated NOx. The elongation of the flame from validation case 3 to low-NOx burner 

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case 3 is similar to the two previously discussed cases as shown in Figure 6-1 1. The implementati on 

of new low-NOx burner and change in primary air flow rate results in a 620° F drop of the peak 

flame temperature for low-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 6-12 shows volumes within th e kiln where temperature exceeds 2780°F for both validation 

and low-NOx burner case 3. The figure also shows contours w ithin the volumes to illustrate the areas 

where significant NOx is produced. 

ISO 9001 

~-=-

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 validation case 3, the temperature exceeds 2780°F in 2.1% of the ki ln volume. For low-NOx 

burner case 3, the temperature exceeds 2780°F in only 0.1% of the ki ln volume. The relative drop in 

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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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3000 ,.- -

I 

2500 I 

~ I 0 

.......... 

QJ 2000 Green ball feed ,_ 

E temperature = 116°F 

~ Q,j 

c. 

E 15oo 

~ 

"C 

Q,j 

Cl:l 

'a:i 1000 

Q,j 

a.. 

500 

I 

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

Final Report- February 18, 2010 

I 

Ki ln Discharge Temperature 

1 D Kiln Model Prediction = 2330°F 

1 D Grate Model - Layer 1 

---~r- 1 D Grate Model - Layer 2 

--o-- 1 D Grate Model - Layer 3 

-- 1 D Grate Model- Layer 4 

-+- 1 D Kiln Model 

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

Axial Position [m] 

Figure 7-1: Traveling Grate and Kiln Pellet Temperatures for Low-NOx Burner Case 4 

The green ball feed temperature used for natural gas only low-NOx burner case is th e same as the 

value used for case 1 at 11 6°F. O utput of 1 D Kiln model for kiln discharge temperature of pellet is 

2330 °F w hich is 2% higher than the average final pellet temperature for low-NOx cases 1 thro ugh 3. 

7.1.2 Gas Temperature Profiles 

Figure 7-2 shows temperature conto urs on a two d imensional vertical cross section through the kiln 

and preheat, as predicted by the 3D CFD model. The gas temperature along the center line of the 

kiln and preheat section of the grate are also provided in the graph below. 

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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 15 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-NOx burner case 4 are comparable to 

those seen for cases 1 through 3 with the existing burner. The peak flame temperature of 3490° F is 

significantly higher than the peak flame temperature observed for the first three low-NOx burner 

cases, whose average value is approximately 3000° F. The higher flame temperature is 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 

ISO 9001 

As shown in Figure 7-3, the kiln volume where gas temperature exceeds 2780°F is quite significant 

at 1.3% with peak flame temperature reaching 3490° F. The overall thermal NOx mass flow rate as 

predicted by CFD model is-· This value is comparable with validation case 1 and 2 results 

but significantly higher than NOx mass flow rates predicted for low-NOx burner cases 1 through 3. 

Since burning greater amount of natural gas leads to higher NOx formation inherently, fuel scenario 

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with 100% natural gas can 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-NOx burner case 5. 

As for previous low-NOx burner cases, the bed thickness in the 1 D traveling grate model is 

discretized into four segments whi le the 1 D Kiln model predicts a uniform temperature due to kiln 

rotation. 

ISO 9001 

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2500 ~·---·--- ------ ------· 

~ 

0 

........ 

~ 

2000 I 

.; 1soo Green ball feed 

~ temperature = 116°F 

c. 

E 

111 

1-

"C 

~ 1000 I 

.!! -

Qj 

Q.. 

500 

Kiln Di scharge Temperature 

1 D Ki ln Model Prediction= 2230° F 

1 D Grate M odel- Layer 1 

-o- 1 D Grate Model - Layer 2 

-(r- 1 D Grate Model - Layer 3 

-- 1 D Grate Model - Layer 4 

-+- 1 D Kiln Model 

0 .----- -----.------ ------------ ------------.------.------.------. 

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

Axial Position [m] 

Figure 7-4: Traveling Grate and Kiln Pellet Temperatures for Low-NOx Burner Case 5 

The green ball feed temperature used for fuel oil low-NOx burner case is the same as the value used 

for case 1 and case 4 at 116°F. O utput of the 1 D Kiln model for kiln discharge temperature of pellet 

is 2230° F which is consistent w ith the final pellet temperature for cases 1 through 3. 

7.2.2 Gas Temperature Profiles 

Figure 7-5 shows temperature contours on a two dimensional vertical cross section thro ugh the kiln 

and preheat, as predicted by the 3D CFD modeL Gas temperature along the center line of the kiln 

and preheat section of the grate is provided in the graph below. 

ISO 9001 

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" Hatch 201 0/03 

50 
~HATCH .. 

1200 

T[°F] 

1450 1700 

3500 ---

3000 

2500 

~ 

~ I 

~ 2000 I ~ .... 

<IJ 

Q. 

~ 1500 

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 seen for low-NOx burner cases 1 through 3. 

The peak centerline temperature is 3030°F which within 50°F of predictions for low-NOx burner 

cases 1 through 3. Therefore, the amount of NOx formation in the kiln is expected to be similar to 

results presented for these cases. Due to absence of validation data, the gas temperatures for fuel oil 

scenario cannot be compared to values measured on site. 

7.2.3 NOx Emissions 

ISO 9001 

As shown in Figure 7-6, the kiln volume where gas temperature exceeds 2780°F is 0.4% of the total 

volume with a peak flame temperature of 3100° F. The overall thermal NOx mass flow rate as 

predicted by CFD model is ... This value is much smaller than CFD model predictions for lowNOx burner case 4 and is comparable with results for low-NOx burner cases 1 through 3. Due to 

absence of validation data, error and sensitivity analysis cannot be completed for this case. 

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" Hatch 2010/03 

45 
~HATCHTM 

H 

1 

2780 

T[°F) 

2852 

i 

2924 2996 3068 

U.S. Steel 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 1 A and 1 B ducts on Line 7 during June 9-10, 2009 site 

visit to Minntac facility. Three most commonly used fuel scenarios were considered: (1) coal and 

natural gas, (2) biomass and natural (3) coal biomass and natural gas. The total measured mass 

flow rate of NOx was for these three fuel scenarios, respectively. A 

CFD modeling study has been conducted to evaluate the effects on both NOx production and 

process conditions due to installation of a new low-NOx burner designed by Coen. 

The CFD model and complementary 1 D models have been validated against the NOx concentrations 

measured in the 1A and 1 B ducts as well as temperature measurements collected by Barr Engineering 

and USS Minntac trend data for the same time peri od. Upon completion of the validation procedure, 

the established model settings have been fixed, and applied to predict the burner performance with 

the new low-NOx burner and adjusted primary air flow rates. 

ISO 9001 

Based on the results of the analysis, significant reduction of NOx is expected for the three most 

common fuel scenarios. For these three fuel scenarios, model predictions show that the maximum 

gas temperature decreases with the low-NOx burner, as compared to the existing burner. 

Furthermore, the gas volume within the kiln where the temperature exceeds 2780°F decreases with 

the low-NOx burner as compared to the existing burner. Significant thermal NOx formation occurs at 

temperatures above 2780°F, and doubles for every150°F increase at temperatures greater than 

3500°F; therefore, these findings support the prediction that NOx producti on will significantly 

decrease. 

Key results for all 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 le case 2 

predictions ind icate 79- 97% decrease. All three cases show significant decline in thermal NOx 

formation that is due to a corresponding decrease in flame temperature. 

Low-NOx burner cases 4 and 5 have not been validated, thus predicted NOx formation cannot be 

compared to the existing burner conditions. Results for the low-NOx burner case 4 indicate a NOx 

formation rate of-' which is comparable to results predicted for existing burner operation 

with both coal and natural gas and the tri-fuel mix. Results for the low-NOx burner case 5 predict 

~ WorkingTogether V SAFELY 

H3325 71 -07-55-124-0001 , Rev. 0, Page 33 

e Hatch 2010/03 
U.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 

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""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 1 N1 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 

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©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. 

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"'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 

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"' Hatch 2010/03 

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

Final Report- February 18,2010 

Figure A-3 shows pellet temperature profiles generated by 1 D Travel ing Grate and 1 D Ki ln models. 

Due to discretization of bed thickness w ithin traveling grate model, the graph below displays 

temperatures for all four layers throughout the length of the grate. O nce pellets enter the kiln, 

temperature profile become uniform due to contact rotation and only single 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 Temperature 

USS Trend Data =2260°F 

1 D Kiln Model Prediction= 2270° F 

~ ~ - 1 D Grate Model - Layer 1 

-

aJ 

aJ 

Q.. 

500 

0 

-50 -40 -30 -20 

-b- 1 D Grate Model - Layer 2 

-o- 1 D Grate Model - Layer 3 

--1 D Grate Model- Layer 4 

-+- 1 D Kiln Model 

• USS Trend Data 

-10 0 10 20 30 40 

Axial Position [m] 

Figure A-3: Traveling Grate and Kiln Pellet Tempe ratures for Validation Case 1 

Only two validation points are available from USS Trend data collected on site. The initial green ball 

feed temperature of 11 6°F is used as 1 D Traveling Grate model input. The final kiln discharge for 

fuel case scenario 1 has been measured at 2260° F w hile 1 D model predicted final kiln discharge 

temperature of pellets to be 2270° F. Therefore 1 D model predicts pellet temperature w ithin 1% 

error of the data provided by US Steel and is expected to provide accurate pellet temperatures for the 

corresponding low-NOx burner case, w ith a coal and natural gas fuel mix. 

A.2.2 Cas Temperature Profiles 

ISO 9001 

Figure A-4 shows 1 D Traveling Grate and Kiln model predictions of gas temperature through the 

entire length of the system. US Steel provided several gas temperature measurements in each sectio n 

of the grate as well as within the rotating kiln that have been used to validate the 1 D models. 

'V5:I WorkingTogether v SAFELY 

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" Hatch 2010/03 

50 
~HATCHTM 

......... 

L.L. 

0 

~ 

3500 

3000 

2500 

:; 2000 

~ ~ 

c.. 

~ 1500 

1-

<ll 

!';I 

(,J 

1000 

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: 

~ • 

-50 

USS 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: Traveling Grate and Kiln Gas Temperatures for Validation Case 1 

All temperatures predicted by 1 D models are within 8% error from the validation points in both 

traveling grate and kiln. The temperature difference between validation points and model predictions 

are within typical measurement errors. 

Validation case 1 which uses coal and natural gas fuel mix scenario di splays good correlation 

between gas temperature results of 1 D Traveling Grate and 1 D Kiln models and validation data 

points collected during June 9-10, 2009 site visit to Line 7 at US Steel Minntac facility. 

ISO 9001 

In addition to validation of gas temperatures generated by one dimensional models, similar 

procedure has been applied to results produced by 3D CFD model of the preheat section of the grate 

and kiln. Figure A-5 shows two dimensional vertical cross section through the center of the grate/kiln 

model as wel l as a graph of temperature values measured along the center line of th e preheat and the 

kiln. 

~ WorkingTogether V SAFELY 

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0 Hatch 2010/03 

so 
~HATCHTM 

1200 

T[°F] 

c:' 

0 

.......... 

QJ 

3500 

3000 

2500 

5 2000 

~ ,_ 

QJ 

Q. 

~ 1500 

1-

rll 

1':1 

(,;, 

1000 

500 

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 Trend Data = 2430°F 

3D CFD Modei = 2410°F 

3200 

-- 3D CFD Model 

• USS Trend Data 

0 ,---------,---------,----L---,---------,---------,---------,---------, 

-25 -15 -5 5 15 25 35 

Axial Position [m] 

Figure A-5: 3D CFD Gas Temperature Profile for Validation Case 1 

Simi lar to 1 D model results, all gas temperatures predicted by 3D CFD model fall within 10% error 

from the validation data points. The temperature difference between CFD model predictions and 

temperatures recorded on site for coal and natural gas fuel scenario fall within an acceptable error 

band. Consequently 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 coal and natural gas fuel mix. 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 thermal NOx predicted by CFD Model for 

validation case 1. 

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" Hatch 2010/03 

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 - February 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- Layer 4 

-+- 1 D Kiln Model 

• USS Trend Data 

20 30 40 

Axial Position [m] 

Figure A-6: Traveling Grate and Kiln Pellet Temperatures for Validation Case 2 

US Steel trend data provided two pellet temperatures applicable to validation case 2. Initial green 

ball feed temperature of 125° F is used as 1 D Traveling Grate model input. 1 D Ki ln model predicted 

final ki ln discharge temperature of 2270°F while US Steel measurements indicated final pellet 

temperature of 2265 °F. Results of 1 D models fall within 1% error of th e validation point for biomass 

and natural gas fuel scenario. Based on above comparison, 1 D traveling grate and kiln models are 

accurately matched with the pellet temperature validation data. 

Gas Temperature Profiles 

Figure A-7 shows 1 D Traveling Grate and Ki ln model predictions of gas temperature through the 

entire length of the system. Modeling results have been compared to the measurements of gas 

temperatures in the grate and kiln during Hatch's site visit on June 9-10, 2009. 

ISO 9001 H332571-07-55-124-0001, Rev. 0, Page 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- February 18, 201 0 

Average PH Temp: 

USS Trend= 1925 ° F 

1 D Grate Model= 1935°F 

Average DD2 Temp: 

USS Trend Data= 1220° F 

1D Grate Modei=1215°F 

\ 

Average DD1 Temp: 

~ USS Trend Data=565°F 

1 D Grate Model = 590° F 

Peak Centerline Temp: 

USS Trend Data=2995°F 

1 D Grate Model =271 0°F 

---. . 

Burn Zone Temp: 

USS Trend Data = 241 0° F 

1 D Grate Model =2505°F 

..._ 1 D Grate Model 

___ , D Kiln Model 

• USS Trend Data 

0 .----------- ------.----- ---- ------.----- ---- ------ 1 ____ ___ 

·50 ·40 ·30 · 20 ·10 0 10 20 30 40 

Axial Position [m] 

Figure A-7: Traveling Grate and Kiln Gas Temperatures for Validation Case 2 

All temperatures predicted by the 1 D traveling grate model are w ithin 5% of measured values and 

1 D Kiln model are w ithin 10% of measured values. The temperature differences between validation 

points and model predictions are within typical measurement errors. 

Validation case 2, which corresponds to the biomass and natural gas fuel mix scenario, displays good 

correlation between the predicted gas freeboard temperatures in the 1 D Traveling Grate and 1 D Kiln 

and the validation data points. 

ISO 9001 

3D CFD model is validated using the same US Steel trend temperatures that were compared with the 

1 D model predictions. Figure A-8 displays two dimensional vertical cross section through the center 

of the grate/kiln CFD model and a graph of gas temperatures along th e center line of the preheat and 

the kiln. 

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<> Hatc h 2010/03 

50 
~HATCHTM 

1200 

T[° F) 

3500 

3000 : 

~ 

0 

.......... 

25oo I 

111 

S2ooo 

~ ... 

111 

Q. 

~ 1500 I 

1-

fll 

r:s 

tJ 

1450 1700 

1000 Average PH Temp: 

USS Trend = 1925° F 

30 CFD Model = 2280° F 

5oo I 

1950 

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

Final Report- February 18,2010 

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 

-25 -15 -5 5 15 25 35 

Axial Position [m] 

Figure A-8: 30 CFD Cas Temperature Profile for Validation Case 2 

Gas temperatures predicted by 30 CFD model display excellent correlatio n with temperature 

validation points located in the kiln. Measurements of gas temperature in the preheat section of the 

grate are 18% lower than results of CFD model. Although this error is greater than values obtained 

for validation case 1, the key parameter of kiln exit gas temperature is still within 10% of the 

measured value. Signifi cant thermal NOx generation occurs at temperatures above 2780°F, thus 

elevated preheat gas temperature predi cted by CFD model will not have a measurable affect on the 

amount of NOx formation. 

NOx Emissions 

NOx concentrations for biomass and natural gas fuel mix case were measured in 1 A and 1 B ducts by 

Barr Engineering concurrently with gas and shell temperature. Table A-3 shows mass flow rates of 

NOx in 1 A and 1 B ducts as measured on site as well as the total thermal NOx predicted by CFD 

Model for validation case 2. 

ISO 9001 H332 571-07-55-1 24-0001, Rev. 0, Page 45 

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45 
~HATCH~ 

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. 

ISO 9001 

~ WorkingTogether V' SAFELY 

H332571-07-55-124-0001, Rev. 0, Page 46 

©Hatch 2010/03 
~ HATCHTM 

A.5.2 

L:i:' 

0 

.......... 

~ 

= 

2500 

2000 

~ 1500 

~ Green 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 Reduction Pilot Project 

Final Report- February 18, 2010 

0 

Kiln Discharge Temperature 

USS Trend Data - 2265°F 

1 D Kiln Model Prediction- 2275° F 

10 

1 D Grate Model - Layer 1 

-a- 1 D Grate Model - Layer 2 

-o-1 D Grate Model - Layer 3 

_._ 1 D Grate Model - Layer 4 

-- 1 D Ki ln Model 

• USS Trend Data 

20 30 40 

Axial Position [m] 

Figure A-9: Traveling Grate and Kiln Pellet Temperatures for Validation Case 3 

US Steel trend data provided two pellet temperatures applicable to validation case 3. The green ball 

feed temperature for this fuel mix is highest out of all three validation cases at 127° F. The 1 D Kiln 

model predicts a pellet discharge temperature of 2275°F compared to a measured value of 2265°FTherefore, models of all three validation cases have predicted final pellet temperature within 1% of 

provided US Steel trend data_ The 1 D models are considered to provide an accurate representation of 

the pellet heating trends at Line 7 of US Steel M inntac, based on such a small pellet temperature 

difference between validation points and model predictions 

Gas Temperature Profiles 

Figure A-1 0 displays 1 D Traveling Grate and 1 D Kiln model predictions of gas freeboard 

temperatures. These predictions are compared to 8 temperature measurements taken in both the 

traveling grate and kiln portions of Line 7. 

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0 

-

3500 

3000 

2500 

1000 

500 

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

Final Report - February 18, 2010 

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 Model = 580°F 

Peak Centerline Temp: 

USS Trend Data-2915°F 

1 D Grate Model - 2865°F 

Burn Zone Temp: 

USS Trend Data-2435°F 

1 D Grate Modei-2630°F 

-- 1 D G rate Model 

-- 1 D Kiln Model 

• USS Trend Data 

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

-50 -40 -30 -20 -10 0 10 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-fuel scenario are within 8% error from the validation 

points which is consistent with results for validation cases 1 and 2. The differences between 

validation gas temperatures and corresponding model predictions are within typical measurement 

errors. 

Validation case 3 which models a mixture of all three fuels: coal, biomass and natural gas displays a 

good correlation between 1 D Traveling Grate and 1 D Kiln model predictions 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 dimensional graph displays temperature values taken 

along the center line 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 Modei =2260°F 

500 

: 

1950 2200 2450 

Peak Centerl ine Temp: 

USS Trend Data=2915°F 

3D CFD Model= 3250°F 

Burn Zone Temp: 

2700 2950 3200 

USS Trend Data =2435°F 

3D CFD Model =2345°F 

--3D CFD Model 

• 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 model fall within 10% of the measured gas 

temperatures. These results are within an expected error measurement band; therefore, validation 

case 3 modeling output is in good agreement with validation points provided by US Steel. 

Establishing accurate gas temperature predictions within reasonable error bands for all validation 

cases is a significant step in producing meaningful NOx results. 

A.5.3 NOx Emissions 

45 

NOx concentrations .for the coal, biomass and natural gas fuel mix case were measured in the 1 A 

and 1 B ducts by Barr Engineering concurrently with gas and shell temperature measurements. Table 

A-4 shows mass flow rates of NOx in the 1 A and 1 B ducts as measured on site as well as the total 

thermal NOx predicted by CFD Model for validation 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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