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CLIFFS UTAC

LINE 2

PHASE 3 MODELING REPORT

0 7/20/14 Phase 3 TH/YF/TL/ BW Approved

3 3/10/14 Second Issue

Current Conditions

SD/TH/

YY/TL

BI Prelim

REV Date Description of Issue Originator CHKD Approved

JOB Report

S01CLUT01 RM1154
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Table Of Contents

FCT Combustion Gyro-Therm MKII Modeling Executive Summary ...................................................... 7

1. Introduction........................................................................................................................................... 8

2. Background......................................................................................................................................... 10

2.1 Iron Ore Processing at UTAC ..................................................................................................... 10

3. Modeling Process............................................................................................................................... 15

3.1 Kiln Model Methodology ............................................................................................................. 15

3.2 Kiln Model Inputs ........................................................................................................................ 17

3.3 Cooler Model............................................................................................................................... 20

4. Modeling Results................................................................................................................................ 22

4.1 Overview ..................................................................................................................................... 22

4.2 Kiln Aerodynamics ...................................................................................................................... 23

4.2.1 Current 100% Gas Firing ...................................................................................23

4.2.2 Current Gas/Coal Firing .....................................................................................27

4.3 Kiln Gas Temperature Maps....................................................................................................... 28

4.3.1 Current 100% Gas Firing ...................................................................................28

4.3.2 Current Coal/Gas Firing .....................................................................................29

4.3.3 Volume Averaged Image of Temperature Profile ...............................................29

4.4 Pellet Bed and Refractory Temperatures (Effect of Flame Heat Transfer) ................................ 30

4.4.1 Current 100% Gas Firing ...................................................................................32

4.4.2 Current Coal/Gas Firing .....................................................................................33

4.5 Cooler Model............................................................................................................................... 34

4.5.1 Current 100%Gas Firing ....................................................................................34

4.5.2 Current Coal/Gas Firing .....................................................................................34

4.6 NOx Model .................................................................................................................................. 35

4.7 Solution Runs with FCT Gyro-Therm Burner: 100% Gas........................................................... 38

4.7.1 FCT Combustion Burner at center of kiln ...........................................................41

4.7.2 FCT Gyro-Therm Burner at center of kiln, but extended 3ft into kiln...................43

4.7.3 FCT Gyro-Therm Burner located on East side, same location as current gas 

burner, and extended 3ft into kiln( 3 cases: 3% PA, 5% PA,12%PA) .................44

Additional runs and checks............................................................................................46

4.7.4 Fine Tuning Runs on 100% Gas ........................................................................46

4.7.5 Bed Heating Analysis with Gyro-Therm Burner..................................................46

4.7.6 Summary of NOx with FCT Gyro-Therm Burner.................................................49

4.8 Solution Runs – Phase Two: 100% Gas, and Co-fire 30% Gas 70% Coal................................ 49

4.8.1 Overview............................................................................................................49

4.8.2 Kiln Temperature Maps......................................................................................50

100% Gas Firing with new Gyro-Therm Burner .............................................................50

30% Gas + 70% Coal Firing with new Gyro-Therm Burner............................................52

4.8.3 NOx Prediction Results......................................................................................53

100% Gas Case ............................................................................................................53

30% Gas+70% Coal Case.............................................................................................54

4.8.4 Bed Heating and Cooler Analysis with Gyro-Therm Gas Burner ........................55

4.8.5 Bed Heating and Cooler Analysis with Gyro-Therm Co-fired Coal/Gas Burner...57

5. Discussion of Results ........................................................................................................................ 60
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5.1.1 Simulation of Current Conditions........................................................................60

5.1.2 Simulation of Solution Runs – Phase One..........................................................62

5.1.3 Simulation of Solution Runs – Phase Two..........................................................63

5.1.4 General Comments about Results at UTAC.......................................................63

5.1.5 Path Forward to Lower NOx Emissions at Cliffs UTAC Plant .............................64

6.0 Appendix: Background on Combustion, Heat Transfer, and Modeling........................................ 66

6.1 Combustion................................................................................................................................. 66

6.2 Fuels ........................................................................................................................................... 67

6.2.1 Gas 67

6.2.2 Coal 68

6.3 Combustion Modeling ................................................................................................................. 70

6.3.1 Gas Combustion Modeling.................................................................................70

6.3.2 Coal Combustion Modeling ................................................................................72

6.3.3 Turbulence Model ..............................................................................................72

6.3.4 Turbulence/Chemistry Interaction Model............................................................73

6.3.5 Radiation Model.................................................................................................73

6.3.6 CO Production Model.........................................................................................73

6.3.7 NOx Formation and Modeling ............................................................................73

6.4 Process Modeling ....................................................................................................................... 76

6.5 Material Motion and Heat Transfer in a Rotary Kiln.................................................................... 78
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Figures

Figure 1-1: FCT Combustion Approach to Burner Design and Process Guarantees ....................... 1

Figure 2-1: Iso-metric view of the UTAC pelletizing grate kiln........................................................ 10

Figure 2-2: Mass balance diagram of the UTAC grate kiln system ................................................ 11

Figure 2-3: Cross-sectional sketch of the UTAC coal burner......................................................... 13

Figure 2-4: Cross-sectional sketch of the UTAC gas burner and front view of both burners installed 

in the kiln....................................................................................................................................... 14

Figure 3-1: Mass Balance of UTAC Kiln: 100% Gas Firing............................................................ 19

Figure 3-2: Mass Balance of UTAC Kiln: Coal/gas firing ............................................................... 19

Figure 3-3: Overall view of plant showing cooler ........................................................................... 20

Figure 3-4: Process View of Cooler............................................................................................... 20

Figure 4-1: Secondary air streamlines – Lower inlet streamlines colored by velocity - Current Gas 

Firing Case ................................................................................................................................... 24

Figure 4-2: Secondary air streamlines – Upper inlet streamlines colored by velocity- Current Gas 

Firing Case ................................................................................................................................... 24

Figure 4-3: Secondary air velocity vectors colored by velocity – kiln vertical centerline- Current Gas 

Firing Case ................................................................................................................................... 25

Figure 4-4: Secondary air velocity vectors colored by velocity – gas burner vertical centerlineCurrent Gas Firing Case ............................................................................................................... 26

Figure 4-5: Secondary air velocity vectors colored by velocity – kiln horizontal centerline- Current 

Gas Firing Case............................................................................................................................ 26

Figure 4-6: Secondary air streamlines - Current Coal/Gas Firing Case ......................................... 27

Figure 4-7: Secondary air velocity vectors colored by velocity – kiln horizontal centerline - Current 

Coal/Gas Firing Case.................................................................................................................... 27

Figure 4-8: Kiln gas temperature map – kiln horizontal centerline - Current 100% gas case ......... 28

Figure 4-9: Kiln gas temperature map – kiln vertical centerline - Current 100% gas case ............. 28

Figure 4-10: Kiln gas temperature map – kiln horizontal centerline - Current Coal/Gas Firing Case

..................................................................................................................................................... 29

Figure 4-11: Kiln gas temperature map – kiln vertical centerline - Current Coal/Gas Firing case... 29

Figure 4-12: Kiln gas temperature map, based on mean volume temperatures – Current Coal/Gas 

Firing case .................................................................................................................................... 30

Figure 4-13: Computational mesh for the kiln model ..................................................................... 31

Figure 4-14: Mean temperature of pellets inside kiln - Current 100% Gas Firing Case.................. 32

Figure 4-15: Cross-sectional temperature distribution of pellets within kiln - Current Gas Firing 

Case ............................................................................................................................................. 32

Figure 4-16: Mean temperature of pellets inside kiln - Current Coal/Gas Firing Case.................... 33

Figure 4-17: Cross-sectional temperature distribution of pellets within kiln - Current 100% Coal/Gas 

Firing Case ................................................................................................................................... 33

Figure 4-18: Temperature Profile of pellets in cooler, 100% current gas firing case. (Discharge of 

kiln is 0ft)....................................................................................................................................... 34

Figure 4-19: Temperature Profile of pellets in cooler, current coal/gas case. (Discharge of kiln is 

0ft) ................................................................................................................................................ 35

Figure 4-20: NOx emissions showing hourly variations, and changes in Fuel ............................... 36

Figure 4-21: FCT Gyro-Therm Burner: Located at kiln center, 3% primary air. Note, flame 

impingement onto rear wall ........................................................................................................... 41

Figure 4-22: Flow streamlines showing intense recirculation at rear wall....................................... 41
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Figure 4-23: FCT Gyro-Therm Burner: Located at kiln center, 10% primary air............................. 42

Figure 4-24: FCT Gyro-Therm Burner: Located at kiln center, 3% primary air, 3ft forward into kiln.

..................................................................................................................................................... 43

Figure 4-25: FCT Gyro-Therm Burner: Located at kiln center, 3% primary air, 3ft forward into kiln.

..................................................................................................................................................... 43

Figure 4-26: FCT Gyro-Therm Burner: Located at same location as current gas burner, but 3ft 

forward into kiln. 3% primary air.................................................................................................... 44

Figure 4-27: FCT Gyro-Therm Burner: Located at same location as current gas burner, but 3ft 

forward into kiln. 3% primary air.................................................................................................... 44

Figure 4-28: FCT Gyro-Therm Burner: Located at same location as current gas burner, but 3ft 

forward into kiln. 12% primary air .................................................................................................. 45

Figure 4-29: FCT Gyro-Therm Burner: Located at same location as current gas burner, but 3ft 

forward into kiln. 12% primary air .................................................................................................. 45

Figure 4-30: Grid Test. FCT Gyro-Therm Burner, located at same location as current gas burner, 

but 3ft forward into kiln.................................................................................................................. 46

Figure 4-31: Heat flux to bed from Gyro-Therm burner.................................................................. 47

Figure 4-32: Bed temperature with Gyro-Therm burner................................................................ 47

Figure 4-33: Kiln temperature map-kiln horizontal centerline, 100% gas case............................... 50

Figure 4-34: Kiln temperature map-kiln vertical centerline, 100% gas case................................... 51

Figure 4-35: Kiln gas temperature map, volume rendering, 100% gas case.................................. 51

Figure 4-36: Kiln temperature map-kiln horizontal centerline, co-firing .......................................... 52

Figure 4-37: Kiln gas temperature map, volume rendering, co-fire case........................................ 53

Figure 4-38: NOx concentration, vertical slice, gas case ............................................................... 54

Figure 4-39: NOx concentration, horizontal slice, co-fire case....................................................... 55

Figure 4-40: Bed temperature with Gyro-Therm gas burner .......................................................... 56

Figure 4-41: Bed temperature within cooler for the gas Gyro-Therm burner.................................. 56

Figure 4-42: Heat flux to bed from Gyro-Therm gas burner........................................................... 57

Figure 4-43: Bed temperature within kiln for the coal/gas cofired burner ....................................... 58

Figure 4-44: Bed temperature within cooler for the coal/gas co-fired burner.................................. 58

Figure 5-1: Aerodynamics around rear burner wall, showing strong re-circulation zone ................ 62

Figure 5-2: Gyro-Therm Burner Sketch ......................................................................................... 65

Figure 6-1: Physical modeling of high (left) and low (right) momentum flames in a rotary kiln ....... 67

Figure 6-2: CFD simulation of a kiln with swirl in the secondary air flow and consequent flame 

deflection ...................................................................................................................................... 67

Figure 6-3: Schematic view of a simple Precessing Jet nozzle...................................................... 68

Figure 6-4: Solid fuel combustion process..................................................................................... 69

Figure 6-5: Principle NOx formation pathways .............................................................................. 76

Figure 6-6: Rolling bed motion in the transverse plane of a rotary kiln .......................................... 79
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Tables

Table 3-1: Summary of Plant Data Related to Kiln ........................................................................ 17

Table 3-2: Summary of Plant Data Related to Cooler.................................................................... 21

Table 4-1: Summary of Plant Data Related to Kiln ........................................................................ 22

Table 4-2: Summary of NOx Baseline data ................................................................................... 37

Table 4-3: Comparison of bed exit temperature based on burner configurations analyzed............ 48

Table 4-4: Comparison of NOx Emissions - Existing Vs New Predicted ........................................ 55

Table 4-5: Comparison of bed exit temperature (kiln & cooler) based on burner configurations 

analyzed ....................................................................................................................................... 59
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FCT Combustion Gyro-Therm MKII Modeling Executive Summary

FCT Combustion (FCT) has been asked to evaluate its low-NOx burner technology for application 

to Cliffs’ United Taconite (UTAC) Line 2 Grate Kiln Pelletizing Plant. FCT’s Gyro-Therm MKII 

burner technology has been demonstrated effective in other non-taconite applications at reducing 

NOx emissions by 30-70% while also improving energy efficiency. Cliffs contracted FCT to model 

the expected NOx reduction when the Gyro-Therm MKII is applied to UTAC Line 2. Although 

modeling was unable to demonstrate viable operation with the Gyro-Therm MKII installed in the 

current coal burner location, modeling with the Gyro-Therm MKII installed in the current gas burner 

location inserted inside the kiln hood indicates that the expected NOx reduction at UTAC Line 2 

would be 35% on natural gas while holding parameters steady to protect pellet quality and energy 

efficiency. 

As described in Section 2 of the report, the pelletizing process requires large amounts of oxygen to 

sustain pellet quality, which also elevates the baseline NOx rate from the furnace. This presents 

unique challenges for a low-NOx burner retrofit. To understand the dynamic physical and chemical 

processes in the UTAC Line 2 Furnace, FCT developed and validated a kiln model, as described in 

Section 3, to predict the variables contributing to NOx formation within the furnace. Section 4 of 

this report presents the modeling results for the current configuration and for proposed low-NOx 

solutions. Section 5 discusses the modeling results that predict the Gyro-Therm MKII burner for 

UTAC Line 2 will reduce NOx significantly (35%) and, would achieve NOx emission rates of 2.9

lb/mmBtu on natural gas. 
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1. Introduction

Cliffs Natural Resources Inc. owns and operates an iron ore mine and pellet plant in Northern

Minnesota known as United Taconite (UTAC). The mine is located in Eveleth, and the pellet plant 

is about ten miles away in Forbes. These sites are located in the Mesabi Iron Range of Minnesota. 

The ore is crushed at the mine site and transported by rail to the pellet plant, where it is again 

crushed, and then concentrated. The concentrated ore, mainly magnetite, is processed in a grate 

kiln which converts the magnetite to hematite. These hematite pellets are shipped out of Duluth, 

and sold to the steel industry to make tubular and flat rolled products.

The process consists of adding water and other binders to the concentrate, and producing 

spherical green pellets in balling drums. The pellets are dried and pre-heated on a grate and 

passed into a rotary kiln. Burners are located at the discharge end of the kiln that fire a coal/natural 

gas blend or 100% natural gas. The energy from the flames is used to further heat the pellets and 

indurate them. The pellets leave the kiln and drop into a grate cooler, before leaving the process. 

Pelletizing occurs at high temperature and with extremely high oxygen, so a considerable amount 

of energy is used to heat air and drive the process. As a consequence, the flame produces high 

NOx levels, perhaps some of the highest in rotary kilns. At these temperatures NOx is generated 

from the thermal mechanism. NOx is particularly high when natural gas is fired, about 2-3 times as 

much as when coal is fired. This is likely related to the burner design, and the nature of natural gas 

flames, which exhibit lower flame emissivity, and so burn hotter to achieve the same radiant heat 

transfer. The resulting hotter flame creates more NOx.

The current NOx levels are 5-6 lbs NOx/mmBTU when firing natural gas, and about 1.6-2.8 lbs 

NOx/mmBTU when firing coal; (there is a measurement of 5.4lb NOx/mmBTU that has been 

recorded when firing coal as well, but that was an upset condition that resulted in an outlier, as 

advised by Cliffs). The plant is required by the EPA to install and operate a low NOx burner on 

their Line 2. The NOx level targets are to be <1.2 lbs NOx/mmBTU when firing natural gas only, 

and <1.5 lbs NOx/mmBTU when firing coal or mixed fuels (including coal).

FCT COMBUSTION supplies a low NOx burner, called the FCT COMBUSTION Gyro-Therm MkII 

burner. This burner has been used in other iron ore pellet rotary kilns, and has produced NOx in 

the 2-2.5 lb/mmBTU range, when firing natural gas. It has been proposed as an option for the 

UTAC line 2 kiln. The FCT COMBUSTION Gyro-Therm MkII burner has been accepted as Best 

Available Technology for low NOx rotary kiln burners (in applications where the O2 concentration in 

the kiln is 2-3%) in some other industries in the US and around the world.

The FCT COMBUSTION Gyro-Therm MKII burner uses a special gas nozzle that injects the gas in 

a stirring type of motion. While the nozzle is mechanically simple, the fluid mechanics are complex 

and create a dramatically different flame. Through natural staging of the fuel-air mixing, NOx is 

greatly reduced.

In order to verify the operation of the FCT COMBUSTION Gyro-Therm MkII burner at UTAC Line 2, 

Cliffs has contracted FCT COMBUSTION to model it, to determine what the NOx will be, what the 

fuel efficiency is, and whether production will be maintained. The modeling process involves 

gathering data from the plant by FCT COMBUSTION engineers. The data are used to generate 
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complex computer models. The model is then validated, and tested for robustness, and predictive 

capability. Having completed this, the models are used to analyze operation with the new burner.

Figure 1-1: FCT Combustion Approach to Burner Design and Process Guarantees

FCT 

COMBUSTION 

APPROACH TO 

BURNER DESIGN
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2. Background

2.1 Iron Ore Processing at UTAC

The heart of the pellet plant process is a grate-kiln. The grate-kiln process is divided into three 

major sections – the grate, the rotary kiln, and the annular cooler. Feed material flows counter 

current to the process gases, and the system acts like a very large counter flow heat exchanger. 

The process gas flow is managed and balanced by a series of fans, while the material flow is 

managed by motorized conveyors and gravity. 

A detailed description of each part of the process follows.

Figure 2-1: Iso-metric view of the UTAC pelletizing grate kiln

Just prior to entering the grate, green balls of Magnetite exit the balling drums and are fed by a 

reciprocating conveyor to a roll feeder. The roll feeder serves to feed in a steady amount of 

greenballs into the travelling grate, and also to allow any fines or undersized material to fall through 

the gaps between the rolls. This material is collected and recycled back into the process to make 

more green balls. The greenballs entering the grate are thus of a consistent size range.
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Inside the grate, the travel speed is adjusted to achieve a uniform bed depth of about inches on 

top of the grate. This speed is typically around inches per minute.

The first sections of the grate are dedicated to drying. The greenballs come into contact with hot 

process air as the moisture is driven off. The ball moisture level entering the grate drying zones is 

about % by weight of the feed. The pellet moisture is at the exit of the drying zones. 

Following the drying zones, but still within the travelling grate, the balls are preheated further by 

contact with kiln exit gas and the material starts an oxidation reaction from magnetite to hematite 

and some induration takes place. By the time the material leaves the moving grate, the oxidation 

reaction is approximately % complete. The oxidation reaction is exothermic, so the reaction 

adds heat to the process.

Drying and oxidation are accomplished by forcing hot process air and flue gases down through the

bed and grate. The down draft is accomplished by maintaining a pressure gradient across the bed 

via the use of intermediate fans. 

Figure 2-2: Mass balance diagram of the UTAC grate kiln system

The material then enters a rotary kiln, which is direct fired with a coal/gas blend or natural gas. In 

the kiln, the material is exposed to very high temperatures from direct radiation from the flame, 

convective contact with hot kiln gases and conduction from the kiln refractory lined walls. The 

oxidation reaction, while continuing, is negligible in the kiln. The main processing step in the kiln is 

sintering, or hardening, of the pellets. It is here that the pellets get their strength. UTAC uses two 

burners in their kiln 
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The final section of the process is the annular cooler. Pellets drop into a rotating annular cooler 

and the oxidation reaction proceeds to almost 100% hematite. The cooler consists of an annular 

shaped rotating grate divided into two zones, with a fixed height bed of pellets on top of the grate. 

The bed depth is approximately inches, and the rotational speed is approximately inches per 

minute. Below the grate is a plenum of ambient air fed from two cooler fans. This air is forced up 

through the cooler grate and through the material, recovering as much heat as possible. This 

recovered heat is used for secondary air in the kiln, and is also sent through a recuperation duct to 

the drying zone in the grate section. The air which is forced through the primary cooling zone (Fan 

) is the hottest because it is located immediately following where the hot pellets exit the kiln. 

This air is delivered to the firing hood (upper and lower). The air through the secondary cooling 

zone is delivered through the recuperation duct and cooler vent stack.

The proper balance and management of gas flows in the grate kiln process is critical. Pressure 

differentials and thus flows must be maintained and controlled to make the product. Three sets of 

process fans are instrumental in making this happen. Each set of fans must be balanced with the 

others to achieve the proper flow. 

Fans and pull from below the preheat section of the grate. This draws the hot combustion 

gases and air down through the preheat material bed, and also through the kiln and secondary air 

through the cooler. Fans and discharge to the above grate portion of the drying zone. 

Fans and , known as the waste gas fans, remove gases from below the grate in the drying 

section and exhaust to atmosphere through a stack.

Fans and supply ambient air to the cooler. Fans and pull the air through the kiln. 

The Fans must be balanced with the Fans, as the Fans must keep the drying sections at 

negative pressure. The Fans must be balanced with the Fans, to keep the firing hood and 

above the grate in the annular cooler all at slightly negative pressure to reduce ingress leakage into 

the kiln hood.

While the Fans discharge to above the grate in the drying zone, this section is maintained at a 

negative pressure by the Fans which pull from below the drying zone and through a multi 

cyclone and wet scrubber. Because of the quenching of the air with water introduced into the wet 

scrubber, the Fans are relatively cold.

It is critical that the and Fans are controlled to keep all three major sections of the process 

which are hot at a negative pressure relative to outside ambient, otherwise there is a risk of hot gas 

egress which is dangerous. The only positive sections of this process are cold, and those are 1) 

between the discharge of the Fans and the bottom of the cooler grate, and 2) between the 

Fans and the stack. 

The kiln combustion process occurs with copious excess air, and the back end oxygen is typically 

16-18%. This large amount of preheated excess air flowing through the kiln is critical to promoting 

the oxidation reaction in the grate section. However, this copious hot excess air combined with 

high flame temperatures from the burner in the kiln lead to the formation of nitrogen oxides such as 

NO and NO2. It is these NOx compounds, among others, such as SOx, that are targeted for 

control and reduction by environmental agencies due to the links between them and photochemical 
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smog, regional haze and acid rain. In particular, US EPA has required Cliffs UTAC to reduce NOx 

emissions.

Cliffs burns both natural gas and coal in their kiln as both fuels are in abundant supply and are 

economical. Each fuel has a different effect on the process and heat recovery and various process 

flows can vary accordingly. 

The coal burner is a simple straight pipe with an internal diameter of inches. Coal and primary 

air are blown through the pipe and the coal is combusted in the kiln. A sketch of this burner is 

found in 

Figure 2-3 below.

Figure 2-3: Cross-sectional sketch of the UTAC coal burner

The gas burner is more complex. It consists of multiple channels, and uses for 

flame stability and shaping. 
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Figure 2-4: Cross-sectional sketch of the UTAC gas burner and front view of both burners installed in 

the kiln

While burning natural gas only, there is no need for coal conveying air. When burning coal, there is 

approximately Lb/min of air in the coal pipe along with the fuel. When on gas, the only 

air through this coal pipe is by natural convection. As a result of this an additional

pounds of air can flow through the cooler, and is thus pre-heated. Conversely, with coal there is 

less flow of air through the cooler and consequently lower heat recovery. However, tonnage 

changes in the kiln do also affect the fans, but also at constant tonnage fans do vary, due to heat 

recovery, oxidation etc. 

The preheat fans pull more recovered gas/heat while burning gas, and while burning 

coal the cooling fans push more air through the recoup duct and vent stack vis-à-vis 

the kiln.

When burning coal, there is a 10-20% fuel penalty, partly because of the extra cold air requirement 

of the coal burner. Accordingly, the plant typically burns mmBTU/hr of gas or mmBTU/hr 

of coal.

However, there are other differences with the two fuels. On coal, the cooler vent stack has higher 

flow and higher temperature than gas. Also, there is about 15% more secondary air into the kiln 

with coal through the parallel flow duct and the lower firing hood.

Burning coal results in lower NOx emissions. Typically the NOx generated while burning coal is 

1.5-2.5 lb/mmBTU, while it is 5-6 lb/mmBTU for gas. This is quite significant of course, as the NOx 

emissions are the target for the regulators. The plant has been given new limits of 1.2 lb/mmBTU 

for gas and 1.5 lb/mmBTU for coal or a fuel mixture including coal.

FCT Combustion has been contracted to study the process, and propose new low NOx technology 

burner(s) to allow Cliffs to continue to make top quality product, while achieving low NOx emission 

goals and avoiding significant fuel or thermal penalties..
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3. Modeling Process

3.1 Kiln Model Methodology

In the appendix (Section 6) a number of theoretical modeling techniques have been presented for 

combustion, heat transfer and chemical reaction. Selecting the models that capture the critical 

chemical and physical processes that represent the process is vital to ensuring they are reliable. In 

the current exercise, the importance is to capture the flame profiles, gas temperatures, material 

temperatures and NOx emissions.

For the 100% gas case, the flame profiles and gas phase temperatures were modeled with the 

following:

 A SST k−ω turbulence model is considered for RANS equations closure

 Mixture mass fraction with eddy-breakup (EBU) model for combustion in the gas phase

 30 species reaction model with a fast (instantaneous) rate for the gas phase reactions

 Discrete ordinate methods (DOM) with S4 approximation for radiation

Since coal burns in a different manner, the following was used for the coal/gas case

 A k−ε turbulence model is considered for RANS equations closure

 The P-1 radiation model

 Eulerian-Lagrangian approach for the coal particle tracking, with stochastic tracking for

turbulence

 Coal combustion modeled with drying, de-volatilization, volatile combustion and char 

burnout, with a two-stage (primary and secondary) pyrolysis

 Chemical Percolation De-volatilization (CPD) model for de-volatilization

 Non-premixed combustion model for homogeneous phase (natural gas and coal fuel), with 

chemical species concentrations derived from the predicted mixture fraction distribution 

(PDF), where 53 chemical species and 325 elementary reactions are considered.

The CFD models were prepared and ran using ANSYS Fluent ver. 15.0 ( and previously in ANSYS 

Fluent 14.7 version), which is a commercial code used for combustion modeling and NOx 

predictions. 

A separate model was developed to predict the temperatures within the kiln, including the pellet 

bed, refractory, and steel shell. The main goal of the model is to predict the temperature of the bed 

at the exit of the kiln as it heats up by convective and radiative heat transfer from the flame, and 

therefore capture critical information regarding processing of the iron ore pellets. The independent 

nature of the kiln model from the flame model reduces the demand on computer time.

The thermal kiln model considers advective heat transfer within the pellet bed, with enhancement in 

the active layer, based on experimental data, and accounts for the rotation of the kiln as well as 

heat loss from the steel shell to the environment.

In addition to predicting the bed temperature, the kiln model also provides predictions of the 

temperature at the internal surface of the refractory, allowing the kiln and flame model to be linked, 

in that the heat transfer from the flame heats the refractory, and this in turn influences the flame 

temperature. The modeling process involved running the flame model with an initial pellet bed 
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temperature profile, and using the heat transfer to calculate the pellet bed profile. The pellet bed 

profile then was used to update the profile in the flame model, until the pellet bed profiles in both 

models were similar.

The thermal kiln model was written using a combination of Matlab and Elmer, both commercial 

codes.

The cooler and grate models were also separate, decoupled models, created to ensure the 

pertinent processes in the cooler and grate were captured. These models analyze heat transfer 

between the air and pellets and the corresponding oxidation reaction.

The grate and cooler model were developed using Matlab, a commercial code.

The output of the flame model was compared with plant data, and to a previous model that was 

developed by Clean Combustion and Energy, for validation purposes. In addition, the outputs for 

all models were compared with overall plant data.

It should be noted that NOx formation is extremely sensitive to the way the flame mixes and the 

formation of a number of minor chemical species such as O and OH. Part of the reason for this 

behavior is that the NOx formed, even at UTAC, is just 0.01% of the flue gas flow. For example, an 

accepted NOx formation model is the GRI-Mech 3.0, which is optimized to model natural gas 

combustion, including NO formation and reburn chemistry. The chemistry of natural gas 

combustion is captured using 53 species and 325 reactions. In our initial results this model over 

predicted the NOx. As a consequence, an alternative model for NOx prediction method was used. 

The alternative is also an accepted method, and uses the extended Zeldovich mechanism, with 

turbulence interaction, using PDF (probability density function) for temperatures and an 

instantaneous rate for oxygen [O] formation. 

The best model is one that predicts NOx in a robust and precise manner, such that it captures the 

change of NOx with burner and kiln operating parameters. Such a model was developed in this 

study, and validated using plant data and by making burner adjustments, to ensure NOx changes 

were correctly predicted.
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3.2 Kiln Model Inputs

The model inputs were determined by visiting the plant and gathering data from the plant staff. The 

data is summarized in Table 3-1.

Table 3-1: Summary of Plant Data Related to Kiln

Parameter Measured US 

Gas

Measured US

Coal

Use for Model

Plant elevation above sea level 1374 feet 1374 1374 ft

Barometric pressure

Kiln firing rate BTU/Hr MM Btu/hr mmbtu/hr mmbtu/hr GROSS (Gas only)

mmbtu/hr GROSS (coal/gas)

Kiln Gas firing rate mmbtu/hr

cfm

 mmbtu/hr

cfm

 mmbtu/hr, cfm (Gas Only)

 mmbtu/hr, 

Kiln coal firing rate 0 stph mmbtu/hr

 tons/hr

0 (Gas only)

 mmbtu/hr, tons/hr 

(coal/gas)

Coal Primary air flow N/A lb/min 0 (Gas Only)

 lb/min (coal/gas)

Coal Primary air temperature N/A F F

Coal blend calorific value

35% Bowie

5% Pet Coke

60 Lo Sulfur

- Btu/lbs Btu/lb

Coal Blend stoich air/fuel ratio

Coal Blend PSD Mesh % retained

 

 

 

 

Coal Blend ultimate and 

proximate. As Fired

% Bowie

% Pet Coke

 Lo Sulfur

C %

H %

O %

N %

S %

Ash %

Volatile %

Moisture %

Btu/hr 

Gas primary air (called atomizing 

air) in core of burner

 Lb/min Always small 

amount of gas 

so stay the 

same

 Lb/min

Gas PA (AA) temp Ambient 30°F Varies for the 

time of the 

year

30°F

Gas flame shaping air also called 

‘cooling air’ outside channel

 Lb/min Always small 

amount of gas 

so stay the 

same

Lb/min

FSA temp Ambient 30°F Varies for the 

time of the 

year

30°F

Gas Calorific value 1000 Btu/ft3 1,000 Btu/ft3 1020 Btu/ft3

Gas Composition CH4 93.80

N2 1.38

CO2 0.83

CH4 93.80

N2 1.38

CO2 0.83
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C2H6 3.66

C3H8 0.32

Sum 100.00

C2H6 3.66

C3H8 0.32

Sum 100.00

Secondary Air Temp °F

Hood static pressure inches

Secondary air flow from cooler 

upper chamber (also known as 

parallel flow duct)

 #/min #/MIN lb/min (gas only)

 lb/min (coal/gas)

Secondary air flow to lower 

chamber from flow above screed 

wall.

#/min #/MIN lb/min (gas only)

 lb/min (coal/gas)

Inleakage into Hood 

Product pellet rate out of kiln LTPH LTPH LTPH 

Product pellet temp F

Pellet composition out of kiln

% oxidation in PH

% oxidation in kiln

% oxidation in cooler

% oxidation of 

magnetite

%oxidation 

of Magnetite

IN

OUT

Kiln RPH

Reaction Energy, Magnetite to 

Hematite

Btu/lb Fe3O4

Kiln Diam inside bricks feet ” ID of steel

Kiln length feet ft

Kiln burning zone temp F

Kiln Back End Oxygen % %

Kiln back end temp F F

Kiln back end CO

Back end CO2 %

Feed rate – green pellets LTPH LTPH LTPH 

Green Pellet Comp

Kiln pellet feed temp °F

Green pellet composition % oxidation % oxidation

Flue gas flow rate #/min #/min

Flue gas temp F F

Dust loading in flue gas

Using the data above, a mass balance was calculated, for each of the firing cases. The mass 

balance for 100% gas firing case is summarized in Figure 3-1. When coal and gas are co-fired the 

secondary air is slightly higher and there is additional air through the coal pipe. Also, the total kiln 

firing rate is increased to mmbtu/hr. These changes are captured in the mass balance shown 

in Figure 3-2.
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Figure 3-1: Mass Balance of UTAC Kiln: 100% Gas Firing

Figure 3-2: Mass Balance of UTAC Kiln: Coal/gas firing
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3.3 Cooler Model

The cooler supplies air to the pellet bed via two fans ( ) as shown Figure 3-3, below. 

Since the cooler is circular, Figure 3-4, illustrates the various stages of the cooling process by 

“unrolling” the cooler geometry. Air is fed into the first cooling section by fan , passes through 

the bed and exits through the two secondary air ducts. Fan supplies air to the second section, 

and that air passes out through the recoup duct and stack.

Figure 3-3: Overall view of plant showing cooler

Figure 3-4: Process View of Cooler

The data used for the cooler model is given in Table 3-2.
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Table 3-2: Summary of Plant Data Related to Cooler

Parameter Measured US

Gas 4/30/13

Measured US

Coal/Gas 3/20/12

Use for Model

Plant elevation FASL Elevation 

changes from 

grate

Elevation changes 

from grate

1374ft

Hot Pellet Feed Rate In LTPH LTPH LTPH 

Hot pellet temp F F °F 

°F 

Hot pellet feed composition % Oxidation % Oxidation %

Cool pellet product rate out LTPH LTPH LTPH (

Cool pellet temp F F

Cool pellet composition

3A Fan mass flow #/min #/min

3A fan temperature F F

3A fan pressure static “H2O ” H2O inches water

3B Fan mass flow #/min #/min

3B fan temperature

3B fan pressure static ” ”

Cooler stack flow #/min #/min

Cooler stack temperature F F

Cooler stack pressure

Cooler stack dust lost

Parallel flow duct (secondary 

air duct) Flow

#/min #/min

PFD/SAD temp F F °F

Recoup duct flow #/min #/min

Recoup duct temp

Recoup duct pressure inches water

Cooler discharge dust 

collector product rate

Cooler discharge DC air flow 

rate

Lb/min

Cooler bed depth ” ” inches

Cooler residence time min min minutes

’ Diameter 

cooler

’ diameter 

cooler

inch/min inch/min
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4. Modeling Results

4.1 Overview

The aerodynamics, combustion and NOx production of the kiln were modeled using a combination 

of Computational Fluid Dynamics (CFD) techniques. Two different CFD codes were used, with 2D 

and 3D models, to simulate the operation of the existing gas burner operating at typical conditions.

Having completed the 100% gas firing case, the model was adapted to simulate the coal/gas firing 

case. 

In addition a customized computational model was used to predict the temperature of the pellet bed 

material inside the kiln.

 

The cooler model is a separate model, and used to determine how the pellets complete the 

oxidation reaction and are cooled.

Prior to the analysis a basic burner comparison was made to get an understanding of where the 

burners sit relative to burner standards. This is summarized in Table 4-1.

Table 4-1: Summary of Plant Data Related to Kiln

Burner Burner Impulse 

(N/MW)

Primary Air as % 

of Stoichiometric 

Air Requirement of 

the Fuel

Comments

Current 100% Gas 

Firing

12% For natural gas firing the PA% normal, 

“standard” range is 3-10%.

Current Coal and 

Gas Firing

44% Typical range for PA is 25-40%, so PA% is 

high

Typically the range of Primary Air for natural gas is 3-10%, so it is quite high for the current 100% 

gas firing case. For the type of coal grinding and firing system used at UTAC, the PA range is 

typically 25-40%, and at UTAC it is 44%. So it is again high. These data are linked to higher NOx.

The burner impulse is a measure of the burner momentum, a parameter that determines flame 

shape and fuel/air mixing. But high momentum also leads to high NOx. The target range is 6-14 

N/MW [

1

], and so is on the mid to upper end of the target range for gas and high for coal.

Another factor relevant to combustion is excess oxygen. The process runs with about 15% excess 

oxygen (the actual oxygen concentration in the flue gas) leaving the kiln, which is typical for iron 

pelletizing, notoriously high in oxygen. At UTAC, the excess oxygen is 15%, so is within the range 

 

1 C. Helmreich: Characteristics of a High Momentum Burner. AUCBM, November 2013.
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typical for such kilns [

2

]. Most rotary kiln applications are designed to operate at 2-5%. As high 

oxygen leads to high NOx, iron ore kilns have a tendency to produce high NOx.

Dust loss is also a significant process that needs to be accounted for in the modeling approach in 

order to obtain realistic results which compare well with plant data. As the bed falls from the kiln to 

the cooler, the strong updraft coming from the secondary air likely leads to a significant amount of 

dust generated. Dust particles are also generated as air flows through the bed within the “first 

stage cooling” section of the cooler. The dust in entrained by the high velocity ( ft/s) secondary 

air flow in the lower SA duct and flows through the kiln. Some of the fine iron ore dust particles 

most likely get sticky as they come in contact with the flame, coalesce, and fall on the top of the 

bed in the exit region of the kiln, where they solidify as they come into contact with the cooler 

pellets. This process could be responsible for the observation that the pellets stick to each other in 

the kiln. The remaining part of the dust is entrained with the flue gases into the preheating section 

of the grate, where some of them are re-integrated to the iron-ore pellets. The influence of the dust 

on the process is accounted for in the various models that were developed using lump parameters.

Finally, it is expected and confirmed in airflow surveys that a significant amount of air will leak in 

from the outside of the grate, kiln, or cooler. This false air can also have a significant effect on the 

process and is accounted for in the models. 

4.2 Kiln Aerodynamics

The 3D CFD model extends from the secondary air ducts at the cooler outlet through to the kiln gas 

outlet at the travelling grate. The flow behavior of the secondary air is captured by extending the 

model geometry to include the full length of the secondary air ducts.

It is useful to examine the basic flow behaviors of the gas streams in a kiln combustion modeling 

exercise before looking at the details of the combustion behaviors because the kiln aerodynamics 

can have a direct effect on the quality of the combustion.

4.2.1 Current 100% Gas Firing

Figure 4-1 below shows a view of the secondary air ducts and the kiln hood with secondary air 

streamlines extending into the kiln. As an illustration of the way that the two secondary air streams 

flow into the kiln and the degree to which they mix, the streamlines from the lower inlet have been 

colored (according to velocity) and the upper inlet streamlines have been left grey. In this way it is 

possible to observe that even after a significant distance up the kiln, there is quite substantial 

segregation of these two flows. Figure 4-2, also below, shows the same information with the upper 

inlet streamlines colored.

 

2 C. Fredriksson: IFRF Finnish-Swedish Flame Days. Jyväskylä 17-18 April 2013.
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Figure 4-1: Secondary air streamlines – Lower inlet streamlines colored by velocity - Current Gas 

Firing Case 

Figure 4-2: Secondary air streamlines – Upper inlet streamlines colored by velocity- Current Gas 

Firing Case 

Notably, the turn imposed on the upper secondary air inlet flow does not cause 

significant disruption to the kiln aerodynamics. There are clear indications that the turn does bias 

the flow. 

The consequence is that the flow 

entering the kiln is overall biased towards the and this 

manifests as the creation of a swirl in the secondary air flow. The paths of the streamlines 

as they begin to intermingle demonstrate this effect. Other results showing the gas flame (later in 

the report) will demonstrate that the bias in the secondary air flow does deflect the flame towards 

.

It is also clear that the nature of the entry paths is quite different between the upper and lower 

inlets. 
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 The presence of the upper duct 

flow does assist in forcing the lower inlet flow to make this turn and initially constrains the lower 

inlet flow to the lower part of kiln.

A clear demonstration of the difference between the upper and lower flows is seen in Figure 4-3

below. 

 This kiln centerline 

slice is to one side of the gas burner, so the high velocity region that looks like the flame is actually 

along the far side of the flame (relative to the viewpoint). Figure 4-4, also below, shows the velocity 

vectors in a slice through the centerline of the gas burner. The coloring of the velocity vectors has 

been clipped at ft/s despite the velocity in the burner jet being up to ft/s in order to provide 

better detail in the lower velocity regions of the secondary air flow. 

If this 

kiln did not have the upper secondary air flow it is very likely that the burner jet would be pushed 

upward, as is commonly seen in other types of rotary kiln.

Figure 4-3: Secondary air velocity vectors colored by velocity – kiln vertical centerline- Current Gas 

Firing Case 
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Figure 4-4: Secondary air velocity vectors colored by velocity – gas burner vertical centerlineCurrent Gas Firing Case 

The effect of the upper secondary air inlet flow on the position of the flame is fully exposed in 

Figure 4-5 below. 

Kiln rotation is clockwise, 

This may have a small negative effect on heat transfer performance for the gas flame. 

The burner jet does not appear to be deflected as far as the kiln wall, which indicates that there is 

no danger to the refractory from direct flame impingement.

Figure 4-5: Secondary air velocity vectors colored by velocity – kiln horizontal centerline- Current 

Gas Firing Case 
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4.2.2 Current Gas/Coal Firing

When firing a combination of coal and gas, the airflow patterns were somewhat similar, with the 

upper secondary air limiting the upward motion of the lower secondary air, and a flow bias towards 

the gas burner side. For the coal/gas case, the streamlines are shown in Figure 4-6 and the 

velocity vectors at the burner level are shown in Figure 4-7. The obvious difference between the 

two cases is the double burner system 

Figure 4-6: Secondary air streamlines - Current Coal/Gas Firing Case 

Figure 4-7: Secondary air velocity vectors colored by velocity – kiln horizontal centerline - Current 

Coal/Gas Firing Case 
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4.3 Kiln Gas Temperature Maps

4.3.1 Current 100% Gas Firing

The effect of the upper secondary air flow on the position of the flame is more clearly demonstrated 

by looking at the kiln gas temperature map. Figure 4-8 below shows the kiln gas temperature on 

the horizontal centerline, with 100% gas under the current conditions. A slight flame deflection 

toward .

The kiln gas temperatures on the kiln vertical centerline are shown in Figure 4-9, also below. The 

shape of the flame appears different when viewed from the side. 

Of some note is that there is small region in the flame, visible in both views 

where the peak temperature is somewhat hotter than the remainder of the flame reaction sheath. 

This additional temperature may be responsible for the high NOx productivity of the gas flame in 

this kiln.

Figure 4-8: Kiln gas temperature map – kiln horizontal centerline - Current 100% gas case 

Figure 4-9: Kiln gas temperature map – kiln vertical centerline - Current 100% gas case 
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4.3.2 Current Coal/Gas Firing

The kiln gas temperatures when firing a combination of coal and gas are shown in Figure 4-10. 

 Figure 4-11 

shows the kiln vertical centerline, and in this case a slight upward deflection is noted.

Figure 4-10: Kiln gas temperature map – kiln horizontal centerline - Current Coal/Gas Firing Case

Figure 4-11: Kiln gas temperature map – kiln vertical centerline - Current Coal/Gas Firing case 

4.3.3 Volume Averaged Image of Temperature Profile

It should be noted that the above figures, for both the 100% gas case and the coal/gas combination 

firing case, are cross sections, which are not viewed by the normal eye. When the flame is viewed 

by a normal eye, the photons emitted by flame radicals are seen. This view is better approximated 
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by a volume average of temperature, as shown in Figure 4-12. In this image, the flame appears 

straighter, as is observed on site.

Figure 4-12: Kiln gas temperature map, based on mean volume temperatures – Current Coal/Gas 

Firing case 

4.4 Pellet Bed and Refractory Temperatures (Effect of Flame Heat Transfer)

The main function of a flame in a rotary kiln is to transfer heat to the product bed. Therefore, a

model was developed to compute the evolution of the iron ore pellets as they travel through the 

kiln. The model is coupled with the flame analysis as it uses the heat flux computed by the CFD 

model as an input to determine the energy input from the flame and flue gases by radiation and 

convection. The model also includes the refractory and has the ability to predict its temperature. 

The information can in turn be used by the flame model to obtain better predictions of gas 

temperatures.

A secondary and important feature of the bed model is that it captures the heat losses from the 

shell of the kiln. The model accounts for natural convection and radiation heat losses to the 

environment from the shell. The amount of this heat loss should not be underestimated. In fact 

literature [

3

] and [

4

] indicate that heat losses in rotary kilns can be up to 26% of the energy input. 

 

3 V. Karamarkovic, M. Marašević, R. Karamarković, M. Karamarković: Recuperator for waste heat 

recovery from rotary kilns, Applied Thermal Engineering 2013

4 T. N. Adams, Lime Kiln Principles and Operations. 

http://www.tappi.org/content/events/08kros/handouts/2-2.pdf
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Data from UTAC (United Taconite Pellet Plant Line 2 Process Airflow Survey, With Standard 

Pellets, Table 9) also suggests that heat losses by radiation could be as high as 

MMBtu/hr. Due to the excessive heat loss in this type of furnace, additional energy is required for 

the process, and additional temperature is required for to drive the reactions. The last two factors 

lead to higher NOx emissions.

The model accounts for the kiln rotation as well as the mixing pattern within the bed due to its 

rolling motion. In the model, the bed is divided into a passive (bottom) and active (top) layer. The 

passive layer is assumed to rotate as a bulk solid as the kiln rotates, and therefore exhibits low 

levels of intermixing between pellets, such that heat transfer only occurs by thermal conduction. 

The active layer, whose height is small compared to that of the passive layer (5-15%) quickly rolls 

atop the bottom layer, thus producing significant intermixing between pellets and enhancing heat 

transfer. This heat transfer by advection is modeled by enhancing the thermal conductivity of the 

material by a large (5-fold) factor.

The approach described above results in advection-diffusion governing equations which are solved 

on an unstructured grid. Figure 4-13 below illustrates the mesh used, the general major 

dimensions of the geometry, as well as the active and passive layers. 

Figure 4-13: Computational mesh for the kiln model

The main purpose of the kiln model is to establsh a baseline for temperatures and hence degree of 

processing. By taking a cross section, one also can observe temperature gradients and determine 

whether sufficent heat penetration occurs. The kiln model serves to highlight when flame changes 

improve or adversely affect processing and hence provides vital information in decision making. 
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4.4.1 Current 100% Gas Firing

Figure 4-14 below shows the computed evolution of the center bed temperature as it travels 

through the kiln as a function of the distance from the burner, for the current gas firing case. The 

initial bed temperature was set to about °F, as was measured as the kiln feed temperature. 

The temperature profile shows a peak in temperature in the vicinity (axially) of the flame. The 

temperature then decreases from the peak as the bed approaches the discharge point of the kiln 

due to the negative heat flux computed by the CFD. The exit bed temperature is about °F.

Figure 4-14: Mean temperature of pellets inside kiln - Current 100% Gas Firing Case

The pellet bed at UTAC is quite shallow and hence cross sectional temperaure gradients are 

minimal. Figure 4-15 shows the temperature of the bed as it enters the kiln (right image), the 

temperature at the maximum temperature (middle image which is about ft from the burner) and 

the temperaure when it leaves the bed (left image). While there are indeed minimal cross sectional 

temperature gradients, it can be seen that the bed begins to cool as it leaves the kiln (see the left 

image).

Figure 4-15: Cross-sectional temperature distribution of pellets within kiln - Current Gas Firing Case
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4.4.2 Current Coal/Gas Firing

For the current coal and gas, combination firing case Figure 4-16, shows the computed evolution of 

the center bed temperature as it travels through the kiln as a function of the distance from the 

burner. In this case the peak temperature is closer (about ft) from the burner before reducing in 

temperature slightly, with an exit bed temperature is about °F.

Figure 4-16: Mean temperature of pellets inside kiln - Current Coal/Gas Firing Case

Figure 4-17 shows the temperature of the bed for the coal/gas case as it enters the kiln (right 

image), the temperature at the maximum temperature (middle image which is about ft from the 

burner) and the temperaure when it leaves the bed (left image). Compared with gas, it can be 

seen that the hottest part of the pellet bed is closer to the burner, but still the bed cools off before 

exiting the kiln.

Figure 4-17: Cross-sectional temperature distribution of pellets within kiln - Current 100% Coal/Gas 

Firing Case
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4.5 Cooler Model

The cooler model was used to simulate how the pellets complete oxidation and are cooled before 

exiting the process. The model was first run with the 100% gas firing conditions and then with the 

current coal/gas firing conditions.

For validation, a 1D model was prepared first. This was used to ensure global parameters matched 

the process data.

4.5.1 Current 100%Gas Firing

Figure 4-18 shows the pellet temperature profile for the current 100% gas case. The pellets can be 

seen to heat up over a short distance before cooling. This initial heat up is due to oxidation, which 

is an exothermic reaction. The model also calculates the temperature of the secondary air, 

recuperative duct air, and cooler stack.

Figure 4-18: Temperature Profile of pellets in cooler, 100% current gas firing case. (Discharge of kiln 

is 0ft)

4.5.2 Current Coal/Gas Firing

Figure 4-19 shows the results for the current coal and gas firing case. A similar temperature profile, 

with initial heat up over a short distance is seen, as expected since there is little data to suggest 

otherwise.
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Figure 4-19: Temperature Profile of pellets in cooler, current coal/gas case. (Discharge of kiln is 0ft)

4.6 NOx Model

A number of NOx models were run and compared with the plant data, to ensure that a robust 

model was developed that adequately predicts the NOx. It is of particular importance to ensure the 

model can capture changes in burner design, fuel changes and the effects of turbulence. Then, 

having carried this out, the NOx model is fine tuned.

It is also to be noted that the NOx model is coupled with temperature and turbulence. Therefore, 

part of the fine tuning involves adjustments to the flame and kiln models. For example, if the model 

is over predicting NOx formation, in addition to the NOx model, the flame model and/or the kiln 

model are further tuned to ensure all flows and heat losses are captured.

While reviewing the NOx models, another challenge is to gain a good understanding of the actual 

NOx profile. UTAC plant NOx data is presented in Figure 4-20. It can be seen that the NOx 

reported varies by about 20% for gas and a factor of over 3 for coal. This exemplifies the challenge 

to create a predictive NOx model for burner changes. 

One comment would have to be added here - there is a data point outside the normal range 

corresponding to the normal operating conditions. Although it is reported and graphed here 

(5.4MMBTU/hr for coal/gas co-firing), the upset condition that generated the data point outside the 

range is obviously just an an outlier that may be due to variability in the measurement or it may 

indicate process variance or other errors; the outlier is included in the data set per se but not 

considered relevant for the modeling exercise and therefore ignored when concluding on the 

results.
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Figure 4-20: NOx emissions showing hourly variations, and changes in Fuel

The initial NOx model was two-dimensional (2D) in nature in order to decrease the computational 

time needed for the calculations while exploring the various modeling options available. This 

approach was devised in order to establish the best practice to obtain accurate NOx formation 

predictions for the problem at hand. Several approaches were considered, such as the Eddy 

Break Up (EBU) model for turbulent non-premixed flames along with the simplified Zeldovich 

mechanism for NOx formation. A more accurate model for turbulence/combustion interaction was 

added by implementing the flamelet model using the GRI-Mech 3.0 with 53 chemical species, while 

NOx formation was predicted using a post-processing approach with the extended Zeldovich and 

turbulence/chemistry interaction. The data produced by the model was validated with process 

data, and once an acceptable modeling approach was achieved, a more complex and 

representative 3-dimensional approach was considered. The extended Zeldovich equations were 

applied to the 3D model, and it was operated at various parameters. The results were thoroughly 

analyzed along 24 planes to confirm that the model accurately captured the physical and chemical 

phenomena observed at the plant. This task is necessary due to the complex reactions and flow 

patterns, especially from the secondary air from the upper and lower ducts.

When firing coal, the simulation was modified to include the coal particles, including particle size, 

particle injection velocity and frequency. The model includes devolatilization and particle/gas 

emission and radiation. The particle flow is also tracked, which is a function of air flow pattern, recirculation (reverse flow in burning zone), and entrainment of the gas flame which co-exists with the 

coal flame while coal is firing. As such the coal particle trajectories are highly turbulent. 

As coal is fired in a range of particle sizes, larger particles follow different flow patterns than smaller 

particles due to inertial differences. In fine-tuning, a representative size range had to be selected 

because it was found that insufficient sizes resulted in particles being excessively overcome by 

turbulence. By coupling the flow and turbulence, the final model used a particle size distribution 

with 20 mesh sizes before the intricacies of the UTAC coal mill were captured.

100% gas 

firing

Gas and 

Coal co-firing
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The dominant combustion process under the coal/gas scenario is coal, even though there is just 

over 20% gas co-firing. Therefore, the PDF model for turbulent diffusion flame was used, in 

conjunction and post processing to handle NOx formation. As for the gas-only case, the data was 

thoroughly examined and was validated based on the plant data. This process included coupling 

with all other models. For example, the initial results were over-predicting the bed temperature, 

and it was determined that the flame was too hot, because the kiln shell external wall losses were 

not sufficient. This resulted in fine tuning the coal flame model, and having done so, a more 

representative flame and pellet bed (in kiln and cooler) temperature profiles were developed.

This process of continuous development, validation with actual plant data, fine tuning, and 

upgrading, involved several iterations. It should be noted that this is not a progressive procedure, 

but rather an iterative process of continuous improvement. The net result was a robust model that 

provides adequate predictive ability for NOx emissions, fuel efficiency and pellet processing, and 

as such that will avoid unnecessary tweaking of data (a non-scientific approach that is against the 

first pass engineering principles that are considered extensively and at all times by FCT 

Combustion).

Other examples of robustness are the ability of the model to predict a lower NOx level with a 

reduction in primary air, and vice versa. Similarly, a reduction in fuel firing rate reduced NOx and 

vice versa.

Having completed the rigorous testing and validation, the baseline data shown in Table 4-2 was 

established.

Table 4-2: Summary of NOx Baseline data

Reported 

(lb NOx/MMBtu)

Model

(lb NOx/MMBtu)

Comments

100% Gas

Current Burner

5.3 to 6.4 4.5 The plant reports NOx 

to be in the 5-6 range 

at most times 

Coal/Gas

Current Burners

1.6-5.4 2.9 Plant reports that the 

typical range is closer 

to 2.5

 

Clearly it can be seen that the NOx levels measured at the plant vary considerably. This is due to 

natural turbulence of the process, and process fluctuations. The model results, together with the 

robust testing described above, are considered acceptable to predict the effect of burner changes.
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4.7 Solution Runs with FCT Gyro-Therm Burner: 100% Gas

RESOURCES 

All modeling work has been done using, almost continuously (7 d/week, 24 h/day), a 12 core XEON 

computer machine. A second similar machine was used to generate the structured and 

unstructured meshes, to run the pellet bed models (coupled with their respective flame) as well as 

to develop the grids and run the models for the cooler and the grate. Two senior process & CFD 

engineers were deployed exclusively to work on this project. One senior combustion process 

engineer – from our corporate office - with more than 15 years direct experience in mineral 

processing industries worldwide (including pelletizing plants) has been directly involved in 

validating every single case and confirming the boundary conditions and case files. Our corporate 

senior CFD engineer has been used in finding practical solutions for models that were either not 

converging or behaving erratically. This has been done using another 2 x 8 core XEON computer 

machine.

Other dedicated resources were timely allocated as well, such as one Project Manager and our 

most senior combustion and process engineer - with almost 30 years’ experience - our Technical 

Director that was tasked to oversee the successful delivery of the project.

SOFTWARE USED. ISSUES

All early cases in the modeling work were run using the commercial codes available using ANSYS 

FLUENT 14.7 software version. From early December 2013 onwards, we have moved to a beta 

version of ANSYS FLUENT 15.0. Consequently we have re-validated the previous runs using the 

new ANSYS FLUENT 15.0 version.

The main rationale for moving to a beta version of ANSYS FLUENT was that using version 14.7 

conflicting results were obtained at times and the model was either over-predicting or underpredicting (particularly for NOx). Sometimes (the model) was not even converging, or crashing 

whilst running.

One of the reasons why predictions were at times not accurate (or why the model was not 

converging, or crashing when running) is that the previous version (14.7) could better handle 

mainly steady state flames, however there is a lot of unsteady state flow and combustion behavior 

in the UTAC kiln. This is one of the reasons why version 15.0 has been released (apart from better 

predicting pollutants formation, such as NOx). It can better handle simulations that are (at least 

convergent to) unsteady states, but it adds additional complexity to every CFD run as every 

iteration has to be monitored actively by changing in real time relaxation factors, turning models on 

and off, etc. as well as gradually introducing flow, turbulence, velocity (on 3 axes), energy, 

continuity, etc. and allowing them to converge before coupling altogether. (in other words it hasn’t 

been possible to simply input the data, start the model and then return 8 hours later hoping to get 

the results; it requires continuous iterations and constant balancing of the model).

It has to be noted here that in all the aforementioned statements the subject is 3D modeling.

It also has to be understood here that it is not possible to know whether a model will converge or 

not until you have actually run the model in the machine. Almost always you have to run a model 
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(at least) twice to allow it to converge fully. Partial convergence (on just some of the parameterssuch as flow/ turbulence, etc.) doesn’t allow you to extract the necessary data or make 

recommendations. Partial convergence means also that the pollutants results in particular are not 

reliable; therefore a minimum 10˄-4 convergence factor is required to have confidence in the model.

One last statement (clarification) must be added here. Both unstructured meshes and structured 

meshes should (in theory) provide the same results. However, this has not been always the case. 

What we had found is that while we have started with unstructured meshes, which are much easier 

to mesh than structured meshes, in attempts to limit the time spent on this process when running 

the solutions (all the solutions that have had the Gyro-Therm MKII extended ft - compared to 

current burners location - inside the kiln) the results were “strange” compared to other cases that 

were run. Nearly all the solutions displayed NOx results in a similar range, showing less variation 

than known operation and also less variation than expected between different burner designs. 

Following discussions with ANSYS we have decided to use a hybrid style of mesh that has 

managed to get the NOx results closer to expected values, the models in this case exhibiting the 

theoretical robustness expected. One of the benefits of this specific type of mesh is that it allows for 

a more stable run, thus avoiding some of the unnecessary simulation crashes that may occur 

because of “gaps” in a purely unstructured or structured mesh. We have confirmed the validity of all 

our results runs using this hybrid style of mesh, starting with (re-running) the baseline cases - and 

as expected - the results were the same as originally generated. Once this was confirmed, we have 

used this style of mesh in all our solution runs. The outcome of each run became noticeably better. 

The NOx results showed appropriate differences, as they should, confirming the difference in 

boundary conditions (as per their respective/individual case-files set-up).

The thermal kiln bed model was written using a combination of Matlab and Elmer, which are both 

commercial codes, then validated in the ANSYS FLUENT combustion model (temperature of the 

kiln and bed was coupled with the flame).

The grate and cooler model were developed using Matlab, a commercial code, and again 

calibrated against the output of the ANSYS FLUENT model (2 way iteration).

METHOD. TIMING

For every modeled run, a set of parameters and boundary conditions were created.

Then the burner specifications were defined (including primary air%, axial air, axial and precessing

gas, momentum for both, etc.). Once these were finalized, the FCT Gyro-Therm burner nozzle(s) 

was (were) sized and designed to accommodate the parameters defined.

For every solution run FCT Combustion has first created a 3D negative volumetric drawing that 

was then used to create a computational mesh (either structured or unstructured and later hybrid).

That included the burner(s) (face-plate and 3D shape, including refractory), and the kiln.

Usually these meshes (either structured/unstructured) have 3.5-5.5 million cells, sometimes more. 

A bigger the mesh cell count will usually provide a more accurate prediction. This also comes with 

a time penalty, as it takes much longer for a case to converge. (The time required for a solution to 

converge is directly proportional with the number of cells in the mesh).
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Once the mesh is created, the set of conditions defined are introduced in the machine and the 

computer analyzes the data, plotting the convergence ratio for a number of factors. As soon as a 

model converges completely, we apply post-processing treatment to the flame to obtain the best 

possible NOx prediction.

If we are convinced that the data has been validated satisfactorily then we run, in parallel, the 

thermal kiln model, coupling the pellet reaction and the heat transferred to/from the kiln walls 

refractory and to/from the pellet bed with the burner flame and fine-tune the model. The model is 

then run again; allowed to converge again; post-processing applied again to fully substantiate the 

model run.

When this cycle is finished we analyze the results using TECPLOT - generating the necessary 

graphs showcasing the findings. Descriptive verbiage is added to describe the outcome of each 

model.

Timing: For 3D negative drawings and meshing: 3 days; for generating one set of conditions: 1-2 

days, for model set-up and run: 2-3 days. For post-processing and data validation: 1 day. For 

plotting and data analysis: 1 day. Total: 8-10 days for every run.

SOLUTION RUNS

The solution phase of the work involved investigating a number of burner set ups and locations.

The reason for investigating different burner locations was that the normal location for a kiln burner 

is central in the kiln and indeed Cliffs desired a central burner location, but early modeling showed 

that a central location was not optimal for this plant.
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4.7.1 FCT Combustion Burner at center of kiln 

(2 cases were run in this case: Gyro-Therm with 3% PA and Gyro-Therm with 10% PA) 

Figure 4-21 shows the flame with the FCT Combustion burner located at the center of the kiln. A 

high temperature zone is seen at the rear wall (where the burner ports are located), which is due to 

flame impingement. The flame is caught in the bluff body recirculation that exists downstream of 

the burner wall which causes it to deflect back towards the secondary air ducts, and to the West 

side (opposite the current gas burner).

Figure 4-21: FCT Gyro-Therm Burner: Located at kiln center, 3% primary air. Note, flame 

impingement onto rear wall

Figure 4-22: Flow streamlines showing intense recirculation at rear wall
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The above result was not expected, and so further analysis was carried out. Figure 4-22 shows a 

detail of the flow streamlines, around the rear wall, where the burners are located. The figure 

shows that intense recirculation zones occur as the secondary air leaves the duct and enters the 

kiln. There are strong flows in the directions, creating a low pressure region 

 With the FCT burner located in the center of the kiln, the 

flame is sucked in by this low pressure, causing it to deflect backwards and impinge on the wall. A 

gas burner located here and with these settings will not produce a satisfactory product, and will 

likely damage the refractory. This gas flame may be unstable because of this turbulence, 

particularly during light-up and warm-up.

One way to overcome the poor secondary air flows is to increase the burner momentum, by 

increasing the primary air quantity. Figure 4-23 shows the FCT Gyro-Therm burner operating at 

10% primary air. While the flame is no longer as severely adversely influenced by the secondary 

air aerodynamics, this flame is intense and creates high NOx, as well as not benefitting from the 

thermal advantages of low primary air operation.

Figure 4-23: FCT Gyro-Therm Burner: Located at kiln center, 10% primary air.
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4.7.2 FCT Gyro-Therm Burner at center of kiln, but extended ft into kiln

The next cases studied were the FCT Gyro-Therm burner pushed forward ft past the recirculation 

zone into the kiln, but at the same location as the current coal burner. Figure 4-24 shows a plan 

view of this case, and Figure 4-25 shows an elevation view. Figures 4-24 and 4-25 are both with 

3% primary air. In this case, the flame is deflected to the . 

This tendency to deflect along the is expected based on the velocity 

streamlines predicted by the modeling. Because of the position , there is also a 

noticeable improvement in overcoming the instability in the recirculation zone, but not enough to 

move it away from the . Additionally it can be seen in the plan view of the kiln that the 

flame is distorted into parts. It appears that a portion of the downstream part of the flame is 

captured by the and peeled off the main flame, as evidenced by the 

high temperature zone on the side of the kiln, . This is a 

somewhat surprising result and has the potential to result in high emissions of CO or other

indications of poor flame stability.

Figure 4-24: FCT Gyro-Therm Burner: Located at kiln center, 3% primary air, ft forward into kiln.

Figure 4-25: FCT Gyro-Therm Burner: Located at kiln center, 3% primary air, ft forward into kiln.
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4.7.3 FCT Gyro-Therm Burner located on East side, same location as current gas 

burner, and extended ft into kiln( 3 cases: 3% PA, 5% PA,12%PA) 

Due to the strong domination of the recirculation air flow over the flame shape, the FCT burner was 

moved to the same location as the current gas burner, but extended ft into the kiln. The gas 

burner position centerline is located of the coal burner centerline position. Figure 

4-26 and Figure 4-27 show the temperature profile in the plan and side views. The flame is wider, 

which indicates that it is less affected by the poor aerodynamics. The temperature profile shows 

that maximum temperatures are reached within the first 1/3 of the kiln length. This is consistent 

with the previous cases, and is typical for a well-developed, fully burned-out fuel. There still is a 

deflection towards the , however. We note here that this deflection shouldn’t pose any 

significant risk to the refractory as the peak temperature drops near the kiln wall 

 and is consistent with the flame position of the existing gas burner.

Moreover this doesn’t represent a risk for the pellet bed. Clearly the deflection of the flame toward 

the , as seen in the elevation view, will prevent flame washing directly on the bed.

Figure 4-26: FCT Gyro-Therm Burner: Located at same location as current gas burner, but ft forward 

into kiln. 3% primary air

Figure 4-27: FCT Gyro-Therm Burner: Located at same location as current gas burner, but ft forward 

into kiln. 3% primary air
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This above case was further investigated to see the effect of primary air adjustments. Three case 

studies were created and 3 different models run. The burner (face-plate) has been redesigned for 

each case. The primary air was increased to 5%, and then 12%. Figure 4-28 and Figure 4-29

show the 12% case for reference, but as primary air was increased, the flame became somewhat 

narrower, and longer. However, the general tendency for the flame to deflect was not changed, 

even with the higher primary air quantity. The higher primary air will come with a fuel penalty, and 

shows no marked improvement over the 3% and 5% cases in terms of flame position.

Figure 4-28: FCT Gyro-Therm Burner: Located at same location as current gas burner, but ft forward 

into kiln. 12% primary air

Figure 4-29: FCT Gyro-Therm Burner: Located at same location as current gas burner, but ft forward 

into kiln. 12% primary air
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Additional runs and checks

When complex aerodynamics like those found at UTAC exist, further evaluation is required. In 

particular, grid structure can impact results. As such, the FCT Gyro-Therm burner, under the 

conditions outlined in Section 4.7.3, was re-modeled but with a structured grid. The structured grid

has significantly more cells, and so is slow to converge. However, the results shown in Figure 4-30

indicate that the predicted flame shapes are similar.

Figure 4-30: Grid Test. FCT Gyro-Therm Burner, located at same location as current gas burner, but 

ft forward into kiln.

4.7.4 Fine Tuning Runs on 100% Gas

Since the case with the FCT Gyro-Therm, in the same location as the current gas burner but 

extended ft out, gave the most promising results, this case was fine tuned. The first fine tuning

involved adjustment of gas pressures, while keeping at 3% primary air. We also performed bed 

heating analysis using the FCT Gyro-Therm burner to understand if there is enough energy 

produced in order to make good, quality product. This was mandatory for our solution validation 

process. These results are presented below:

4.7.5 Bed Heating Analysis with Gyro-Therm Burner

An initial bed model run was performed based on the result of the CFD analysis for the 3% primary 

air Gyro-Therm burner moved ft forward and to the right

Figure 4-31 shows the distribution of the heat flux from the flame and flue gases to the kiln that was 

computed based on the CFD analysis.
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Figure 4-31: Heat flux to bed from Gyro-Therm burner

Figure 4-32 shows the evolution of the bed temperature as a function of the distance from kiln exit 

(i.e. where the pellets bed falls into the cooler). The results show that the final pellet temperature is 

calculated to be significantly higher than the values found for the existing gas and coal/gas burners. 

Indeed, the bed reaches °F as it exits the kiln in the model. 

Figure 4-32: Bed temperature with Gyro-Therm burner

This was to be expected since the amount of primary air used by the burner (3% of stoichiometric) 

is significantly lower than the amount of primary air used by the existing gas and coal/gas burners 
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(12% and 45%, respectively). This reduction in primary air, in turns, leads to an increase in the 

heat flux to the bed/refractory, and therefore to an enhancement in bed temperature. Table 4.3 

shows a comparison of the predicted pellets temperatures as they exit the kiln with the different 

burners studied at this point.

Table 4-3: Comparison of bed exit temperature based on burner configurations analyzed.

Note the Gyro-Therm burner will be fine-tuned to provide the optimum pellet bed temperature

Burner Configuration Pellet Kiln Exit Temperature (°F)

Existing Gas °F

Existing Coal/Gas °F

Gyro-Therm (un-optimized) To be determined but lower than °F, also 

potentially higher than °F

The above result shows that there is more than sufficient energy available to be transferred 

between the new flame and the pellet bed, because the calculated bed temperature is much higher 

than the current operating temperature. It also means that in fine tuning the flame, the firing rate 

will reduce slightly, to bring the calculated pellet bed temperature to a normal range for quality. 

The amount of energy saving is determined by fine tuning the flame and pellet bed model.

Note, the above is an initial calculated temperature on the surface of the pellets. It is not the actual 

bed temperature coupled with the flame temperature, as was carried out in the previous cases 

analyzed in the current report.

Even though the pellets will never reach °F, the cooler model was run, nevertheless, using the 

hypothetical high initial temperature predicted ( °F), to analyze the effectiveness of the cooler 

to deal with a surge in pellet bed temperature. With the higher inlet pellet bed temperature, the 

pellet temperature as the cooler exit was determined to be ( °F vs. °F, currently). Since the 

pellet discharge temperature from the cooler is only 92°F higher, when the pellets entering the 

cooler are about 750°F hotter, this indicates that the cooler is efficient and has additional cooling 

capacity, and can cope with hotter pellets, if needed.

While the above results indicate that the plant would be able to produce a similar quality product 

with the Gyro-Therm burner as designed, we plan to further improve the design of the burner by 

reducing the amount of fuel and keeping the same amount of primary air as a percentage of 

stoichiometric (i.e. 3%). We believe that the change will benefit the design by reducing the heat 

flux to the bed, thereby lowering the bed temperature at the exit of the kiln, but will also improve 

NOx emissions, since NOx formation is directly correlated to the amount of fuel consumed by the 

burner.
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4.7.6 Summary of NOx with FCT Gyro-Therm Burner

The lowest NOx runs were observed when the FCT Gyro-Therm burner was located in the same 

location as the current gas burner, but inserted ft further inside the kiln. Since the Gyro-Therm 

burner operates with significantly reduced primary air, a fuel saving is to be expected, and this 

should result in even lower overall NOx emissions.

With the FCT Gyro-Therm burner located in the same location as the current gas burner, and 

operating at 3% primary air, and fine-tuned, the NOx reduction from FCT baseline model (4.5 

lb/MMBTU) is expected to be 30-40% lower, which would bring the emissions to 2.7-3.1 

lb/MMBTU. 

4.8 Solution Runs – Phase Two: 100% Gas, and Co-fire 30% Gas 70% Coal

4.8.1 Overview

During April - June of 2014, FCT continued on the path to optimize Gyro-Therm burner design for a 

primary air flow of 5% of stoichiometric requirements, and further modelled the case. The results of 

this fine tuning design and modelling are favorable. The predicted NOx for 100% gas case GyroTherm is 2.91 lb/mmBTU, and for the 70% coal, 30% gas case is 2.04 lb/mmBTU. These NOx 

predictions and other modelling results are further discussed in more detail in the next report 

sections. The initial solution runs indicated acceptable performance at 3% PA, but subsequent 

design considerations for burner robustness and overall flame stability and shaping suggested that 

5% PA was the practical minimum design point, and this is hence the basis for the phase two 

solution runs going forward.

The aerodynamics, combustion, radiation and NOx production of the induration were modelled 

using a combination of Computational Fluid Dynamics (CFD) techniques, including physical 

modelling, CFD-bed model coupling, NOx post processing, coal particle tracking, etc. By combining 

and coupling all these models, two well validated models were developed and validated using 

current gas case and coal/gas firing cases. These same validated models were then used for the 

new designed Gyro-Therm burner modelling. 

For the new Gyro-Therm burner, two conditions are simulated, one with 100% percent gas, or 

mmBTU/hour gas firing; and a second one with 30% gas and 70% coal co-firing case, which is 

mmBTU/hour of gas and mmBTU/hour of coal. The modelling of the Gyro-Therm burner 

uses mmBTU/hour gas compared with mmBTU/hour of the current gas case. This is 

because the designed Gyro-Therm burner only requires primary air of 5% stoichiometric air 

required by the gas fuel compared with 12% for current case. From a process perspective, there is 

no need to have as much fuel to heat up cold primary air, thus a fuel reduction is possible, as 

suggested in section 4.7.5.

The cooler was also modelled using a separately developed model. It is used to determine how the 

pellets complete the oxidation reaction and are cooled.
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4.8.2 Kiln Temperature Maps

100% Gas Firing with new Gyro-Therm Burner

Gyro-Therm burners have the feature of generating short and bushy flames, and the CFD 

simulation verifies this. The top view temperature slice of the 100% gas case is shown in Figure 

4-33, and the side view of the temperature profile is shown in Figure 4-34. 

A slight deflection toward the is observed, but the hot flame region 

does not touch the refractory so the deflection will not cause any damage on the refractory. Also, 

the deflection has the tendency to be away from the coal burner, which ensures that the coal 

burner stays cool during 100% gas firing.

A more illustrative way to view the temperature profile is by volume rendering – a technique that 

can view flame in 3D rather than a 2D slice (a plane). The volume rendering graph of the 100% gas 

case is shown in Figure 4-35.

Figure 4-33: Kiln temperature map-kiln horizontal centerline, 100% gas case
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Figure 4-34: Kiln temperature map-kiln vertical centerline, 100% gas case

Figure 4-35: Kiln gas temperature map, volume rendering, 100% gas case
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30% Gas + 70% Coal Firing with new Gyro-Therm Burner

The co-firing case with designed Gyro-Therm burner is also simulated. The temperature profile 

viewed from the top is shown in Figure 4-36. The co-firing flame has the same trend as gas flame, 

being pushed towards one side of the kiln. The deflection is determined by secondary air flow. 

Similar to the gas case, the volume rendering graph is also included in Figure 4-37. 

Figure 4-36: Kiln temperature map-kiln horizontal centerline, co-firing
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Figure 4-37: Kiln gas temperature map, volume rendering, co-fire case

4.8.3 NOx Prediction Results

NOx prediction is crucial as the main target for this project is to reduce NOx. With the model 

validated using the current plant data, we are confident that the NOx prediction here is within a 

reasonable range of the actual case.

100% Gas Case

The NOx concentration is shown in Figure 4-38. The total NOx generated for 100% gas case is 

2.91 lb/mmBTU (comparing with existing plant reported 5.3-6.4 lb/mmBTU). With the low NOx 

nature of the Gyro-Therm burner, it is not surprising to see a large NOx reduction for this 100% gas 

firing case. The reason for the large NOX reduction is that Gyro-Therm has a relatively low flame 

temperature and high heat flux which results in low NOx generation and better process efficiency. 
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Figure 4-38: NOx concentration, vertical slice, gas case

30% Gas+70% Coal Case

The co-firing case generates even lower NOx, as expected, with NOx generation rate of 

2.04lb/mmBTU.

Figure 4-39 shows NOx concentration for the co-fire case. The gas flame still has a fairly high 

concentration of NOx compared with coal, but as with the trend shown in the previous 100% gas 

case, it will be much higher without the Gyro-Therm burner. To reinforce this point, the plant 

reports around 2.5lb/mmBTU for 20% gas co-firing which is still higher than the new burner with 

30% gas co-firing.

Table 4-4 shows a comparison of the predicted new burner NOx emissions versus existing 

measured emissions.
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Figure 4-39: NOx concentration, horizontal slice, co-fire case

Table 4-4: Comparison of NOx Emissions - Existing Vs New Predicted

Gas Only (lb/MM BTU) Co-Fire (lb/MM BTU)

Existing Burner 5.3-6.4 (modelled at 4.5) 1.6-5.4 (modelled at 2.9)

New Burner 2.91 (predicted) 2.02 (predicted)

4.8.4 Bed Heating and Cooler Analysis with Gyro-Therm Gas Burner

An initial bed model run was performed based on the result of the CFD analysis for the 5% primary 

air Gyro-Therm burner moved ft forward and located to the right.

Figure 4-40 shows the evolution of the bed temperature as a function of the distance from kiln exit 

(pellet discharge). The results show that the final pellet temperature is in line with the temperature 

reported for the existing gas burner ( °F). As was described above, since the burner is 

inserted ft inside the kiln, the bed temperature tends to drop significantly as the bed passes under 

the burner tip.
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Figure 4-40: Bed temperature with Gyro-Therm gas burner

The cooler model that was previously developed to study the existing plant was run using the newly 

determined exit bed temperature. Figure 4-41 shows the resulting bed temperature. The evolution 

of bed temperature as the bed travels through the cooler is very close to that in the existing plant 

because the bed temperature at the kiln exit is nearly the same as that produced by the existing 

burner.

Figure 4-41: Bed temperature within cooler for the gas Gyro-Therm burner
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4.8.5 Bed Heating and Cooler Analysis with Gyro-Therm Co-fired Coal/Gas Burner

An additional analysis was performed using the dual gas/coal co-fired burner. The heat flux from 

the flame to the bed computed for that case is shown in the Figure 4-42 below. The high radiant 

heat transfer from the coal flame to the bed is evident.

Figure 4-42: Heat flux to bed from Gyro-Therm gas burner

The bed temperature as it exits the kiln ( °F) is slightly higher than for the gas-only case or 

the existing burners. The high temperature of the pellets should not be a significant concern to the 

plant operation, as the pellets are already fully indurated in the grate and therefore strong enough 

to withstand the drop into the cooler. 
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Figure 4-43: Bed temperature within kiln for the coal/gas cofired burner

In addition, the cooler model that was previously developed to study the existing plant was run 

using the high initial temperature predicted ( °F) as the pellets are heated by the coal/gas cofired Gyro-Therm burner. The pellets temperature as they exit the cooler was determined to be 

similar to that in the current plant ( °F vs. °F). This indicates that the cooler provides a more 

than appropriate level of cooling of the pellets, and that there is not a significant correlation 

between the initial and final temperature of the bed within the cooler.

Figure 4-44: Bed temperature within cooler for the coal/gas co-fired burner
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Summary

The table below summarizes the results obtained for the newly designed burners and compares 

them to the existing burners’ performance.

Table 4-5: Comparison of bed exit temperature (kiln & cooler) based on burner configurations 

analyzed

Burner configuration Pellet exit temperature - kiln (F) Pellet exit temperature - cooler (F)

Existing gas

Existing coal

Gyro-therm gas

Gyro-therm cofired coal/gas
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5. Discussion of Results

Computational modeling has been carried out at the Cliffs North Central pellet plant in Minnesota,

known as United Taconite (UTAC). The purpose of the modeling work was to design a new burner 

that will reduce NOx, but maintain product quality, production and optimize fuel efficiency. Three 

sub models were developed, a flame/pellet bed inside the kiln, a pellet bed model in the cooler and 

a pellet model in the grate. The models were used in conjunction with each other to gain a picture 

of the process and the key parameters that impact pelletizing, combustion, fuel efficiency and NOx 

emissions.

The flame model was developed using ANSYS Fluent software ver. 15.0. This is a commercial 

software package that includes all the accepted sub models for turbulence, combustion reactions, 

emissions, heat transfer and radiation. Included also, are a range of post processing techniques to 

predict NOx changes. ANSYS Fluent is perhaps the most widely used and accepted software for 

computational modeling, and widely used in combustion modeling. While NOx modeling may not 

yield absolute agreement in all cases, ANSYS Fluent is often used to predict NOx changes.

The thermal kiln model and grate and cooler model were developed using Elmer and Matlab, which 

are both commercially available programs.

5.1.1 Simulation of Current Conditions

The process began with a site survey for data collection. A large amount of data was gathered and 

key data for the analysis was selected (Section 3). This data was used to establish a mass 

balance, to validate the data and ensure the correct inputs for the additional computational models.

Since the computational models are essentially detailed heat balances, only a basic heat balance 

is carried out, again to ensure data validation. At the same time, an estimate of the heat losses is 

made. The heat loss from a rotary kiln is a difficult measurement, but it is well known that it is 

significant. Literature ([3] and [4]) indicate that heat losses in rotary kilns can constitute up to 21-

26% of the energy input, and data from UTAC also shows that total heat losses are 

MMBtu/hr. These data lead to a high energy consumption, as well as a higher temperature 

demand to drive the chemical reactions. As such, the corresponding NOx is high. 

The first stage in the burner analysis is to review the basic kiln burner parameters. Factors that 

affect a rotary kiln flame are, (1) the type of fuel, (2) the amount of primary air flow as a function of 

total combustion air, (3) the amount of excess oxygen, (4) the relative momentum of the burner, 

and (5) the secondary air and recoup duct aerodynamics.

Natural gas can be fired without preparation. As such, a small amount of primary air is required for 

flame shaping – typically 3-10% of stoichiometric. However the UTAC plant uses about 12% of 

stoichiometric (see Table 4-1: Summary of Plant Data Related to Kiln), which is quite high. Coal 

must be dried and pulverized with hot air is used in the mill to dry the coal and sweep the mill. At 

UTAC, this air is also used to transport the coal to be burner. This way of grinding and firing coal is 

common, and referred to as direct firing, but it suffers from the drawback that a lot of primary air is 
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required. In addition, the amount of air cannot be changed without altering the coal mill 

performance. While a typical range is 25-40%, at UTAC it is 44%. Since NOx emissions are linked 

to primary air flow, and because in both firing cases, primary air is high, high levels of NOx are 

observed.

Iron ore pellet kilns run at extremely high excess oxygen, with about 14-18% oxygen where the 

pellets enter. Most other rotary kiln applications run at 2-5%, and as such the iron ore pellet kilns 

are notoriously high in oxygen. At UTAC, the kiln exit oxygen is 15%, so is within the range typical 

for iron pellet kilns [

5

]. Again, high oxygen leads to high NOx.

Another loss in rotary kilns is in-leakage of cold air. Cold air leaks in because rotary kilns are 

operated at negative pressure. This is necessary for safety issues, preventing hot gases exiting 

the process and burning personnel or equipment, and also to prevent dust emissions. Cold air 

entering the kiln is heated up to ore processing temperature and hence this takes heat from the 

flame that would otherwise be used for the reaction. In-leakage is a difficult measurement, but the 

amount can be linked to the heat balance and account for temperature over-predictions in the 

model.

Similarly, dust cycles impact heat flows. For example dust particles are generated in the cooler, 

then return back into the kiln. This transfers sensible heat from the kiln to the cooler. It also means 

that dust particles entrain into the kiln flame, which leads to stickiness and possible lump formation. 

The models need to accounts for this heat flow, otherwise an imbalance occurs.

The burner relative momentum is the difference between the momentum of the burner flow and the 

secondary air. Sufficient momentum is required to ensure good flame shape and fuel/air mixing, 

but high momentum also leads to high NOx. With the very high level of excess air in the kiln, fuel 

and air mixing is generally adequate, but one parameter that defines burner momentum is Impulse

[

6

], the units of which are N/MW. The current impulse with 100% gas is N/MW and is N/MW 

on coal/gas. The target range is 6-14 N/MW, and so the UTAC plant is on the mid to upper end of 

the target range.

When observing the kiln aerodynamics (Section 4.2) it was seen that the upper and lower

secondary air created a flow as they mixed and traveled along the kiln. This created a slight 

bias toward the , but it was not strongly evident in the flame. The primary reason 

was that the current burners, both gas and coal, have high primary air and momentum, as 

described above. These flames have sufficient force to punch through any adverse aerodynamics. 

While this is good for flame shape and heat transfer, the corresponding NOx emissions are high.

The pellet model results (Section 4.4) shows how the pellets heat up. Figure 4-14 and Figure 4-16

show that the hottest zone when firing coal is at about ft, compared to about ft when firing 

gas. This is very typical, as coal has higher flame luminosity and hence higher heat transfer. 

Discussions with plant staff also verify these findings. These results provide further confidence that 

the models are predictive with the required precision.

 

5 C. Fredriksson: IFRF Finnish-Swedish Flame Days. Jyväskylä 17-18 April 2013.

6 C. Helmreich: Characteristics of a High Momentum Burner. AUBCM, November 2013.
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The cooler model results (Section 4.5) show that as the pellets drop from the kiln, they heat up over 

about a ft length, before cooling off. This is due to the exothermic oxidation reaction occurring as 

the pellets are exposed to the air. Again, discussions with plant staff also verify these findings, 

providing further confidence that the models are adequately predictive.

The results of the 100% gas and coal/gas firing cases, using the current burners, are considered 

representative of the process. This leads to confidence that the models are predictive and suitable 

to observe the effect of new burners and other changes to the UTAC process.

5.1.2 Simulation of Solution Runs – Phase One

The solution phase of the modeling work posed some interesting challenges. With the FCT GyroTherm located in the center of the kiln, the flame was deflected strongly back upstream to the air 

flow, and towards the . Such a flame is deemed unacceptable as it would overheat and 

damage the refractory.

 

A closer investigation of the above phenomenon was therefore carried out, as shown in Figure 5-1. 

This indicated that a strong recirculation zone is generated as the air passes over the rear burner 

wall, creating flow , and negative pressure regions. With the burner 

located at the center of the kiln (Section 4.7.1), the flame is overwhelmed by the aerodynamics and 

deflected . Only by increasing the primary air was it possible for the flame to punch 

through the chaos and produce a suitable flame shape. However, the NOx levels were too high for 

this to be an acceptable solution. 

Figure 5-1: Aerodynamics around rear burner wall, showing strong re-circulation zone

In Section 4.7.2, the burner was pushed forward ft to try to keep the flame away from the 

recirculation zones, but the flame was overly deflected , and again deemed 

unacceptable.
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As such, the burner location was moved to the same location as the current burner , as 

shown in Section 4.7.3. The reason was that there was less intense recirculation on the . 

With the burner at this location, the flames appeared acceptable. The case was therefore further 

investigated by adjusting primary air. It was found that increasing primary air, from 3% to 5% and 

then 12%, raised NOx emissions. This trend is to be expected, as increasing primary air increases 

the oxygen concentration in the flame and the mixing intensity of the burner. As such, it was 

concluded that the NOx model was trending correctly.

Again to verify the flame shape in this last test (Section 4.7.3), the test was repeated with a 

different mesh format. Most of the runs were carried out using unstructured meshes, where 

polyhedral cells are used to generate the mesh. Another technique is to use a structured mesh, 

where 6 sided cells are used in the mesh. This latter technical is more time consuming, but the run 

was carried out to verify the flame shape results with the unstructured mesh. 

Further fine tuning of the case was carried out, by adjusting swirl (precessing) gas and axial gas on 

the burner. Then the thermal kiln model was run to verify the product was being processed 

correctly, and to determine what the energy efficiency would be. It should be noted that this case 

used 9% less primary air, so this in itself would reduce energy consumption. The NOx reduction 

from base case conditions on 100% gas is expected to be 30-40%.

5.1.3 Simulation of Solution Runs – Phase Two

Phase two simulations were conducted in May and June of 2014 in order to prove the optimized 

design of the new Gyro-Therm gas burner at 5% primary air, at the same location 

shown in earlier efforts to be best.

The scope included flow aerodynamics simulation in the kiln, combustion and heat transfer 

(including radiation modeling) for the newly validated Gyro-Therm burner. This included 100% 

natural gas firing and 30% natural gas firing rates. Both cases were post processed with species 

modeling to determine emission concentrations, as well as pellet bed heating analysis (on coal and 

gas and separately on gas) to ensure optimum heat transfer. The Gyro-Therm burner design 

parameters were optimized for consistent production of high quality product, regardless of 

proposed fuel mix. 

Post processed NOx predictions for these phase two design conditions are now at the confidence

level of +/-15% accuracy, and performance guarantee targets can now be set (to be included in the 

formal proposal for hardware) , and engineering of complete low NOx combustion solution can now 

begin.

5.1.4 General Comments about Results at UTAC

The FCT Gyro-Therm burner has already been used in the iron ore pelletizing industry. In a grate 

kiln application, firing 100% gas, NOx emissions of 2-2.5 lb NOx/MMBtu have been obtained. In 

other high temperature rotary kiln applications the FCT Gyro-Therm burner produces as low as 1-

1.5 lb NOx/MMbtu (in applications where the O2 concentration in the kiln is 2-3%).

In the case of UTAC, the modeling indicated a 35% NOx reduction from FCT modeled base case 

(2.9 lb NOx/MMBTU vs. 4.5 lb NOx/MMBtu). This number is higher than has been achieved in 

other installations and it is worth highlighting the reasons. 
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Iron ore pellet kilns, when compared to other high temperature applications like cement and 

ceramics, have a number of differences. Perhaps most significant is the amount of excess oxygen 

in the kiln. In the iron ore pellet kiln the kiln exit oxygen is about 15-16%. In most other rotary kiln 

applications, the kiln exit oxygen is 2-5%. Even though the temperatures in these other 

applications are similar, the vast amount of oxygen means that conditions increasingly favor 

thermal NOx generation. As such, NOx reduction remains a challenge.

A second factor is that not all iron ore pellet kilns are the same. For example, in the iron ore 

application where the FCT Gyro-Therm yielded 2-2.5 lb NOx/MMBtu, the secondary air 

temperature was about 200 °F lower than at UTAC. As such, it is to be expected that the lowest 

NOx at UTAC will be higher than the previous application, without some considerable development. 

Also, the hood was of a different design and did not have a rear wall to create the intense 

recirculation zone. As such the burner operated satisfactorily in the middle of the kiln.

Part of the reason for differences between different iron ore kilns (aside from process design) is 

due to the type of ore. This difference is best exemplified by comparing Cliff’s UTAC and Cliffs 

Tilden. At UTAC the ore is largely magnetite, which is exothermic when it oxidizes to hematite and 

before it pelletizes. Tilden has a mix of magnetite and hematite. As such, Tilden requires 

considerably more thermal energy to process each ton of product.

5.1.5 Path Forward to Lower NOx Emissions at Cliffs UTAC Plant

Now that the final modeling phase is complete and the simulation has indicated with a +/-15% 

accuracy the expected NOx emissions (2.9 lb NOx/MMBTU on gas only, and 2.02 lb NOx/MMBTU 

on coal/gas co-fire) and confirmed the burner design parameters, we can proceed with a 

performance guarantee and formal proposal for the burner hardware, and engineering of complete 

low NOx combustion solution can commence.

Engineering will cover calculations and all the design work and provide all the necessary 

guarantees related to preserving product quality, NOx reduction and energy input.

A preliminary sketch, Figure 5-2, of the proposed gas burner is included here for information 

purposes. It should be noted that sizing and final nozzle layout may vary slightly from this sketch, 

but it is expected that 

Included in the burner are 
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Figure 5-2: Gyro-Therm Burner Sketch
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6.0 Appendix: Background on Combustion, Heat Transfer, and 

Modeling

6.1 Combustion

Combustion is a specific group of chemical reactions where a fuel and oxygen burn together at 

sufficiently high temperature to evolve heat and combustion products. The fuel can be a gas (eg H2

or natural gas), a liquid (e.g. alcohol or oil), or a solid (e.g. coke, or coal). Combustion can vary in 

rate from a very slow decay to an instantaneous explosion. The objective of the combustion 

engineer and plant operator is to obtain a steady heat release at the required rate.

While chemistry of combustion is a very complex chain reaction, it can be reasonably considered 

as four basic reactions.

C + 2O2 -------> CO2+373 BTU/mol (complete oxidation of carbon) ( 6-1 )

2H2 + O2 -------> 2H2O + 542 BTU/mol (complete oxidation of hydrogen to water) ( 6-2 )

2C + O2 -----> 2CO + 209 BTU/mol (incomplete oxidation of carbon) ( 6-3 )

2CO + O2 <-----> 2CO2 + 163 BTU/mol (oxidation of CO) ( 6-4 )

CO (carbon monoxide) is result of incomplete combustion, and often the first indication of poor 

combustion. Not only does it result in loss of efficiency, it also is a poisonous gas.

Sulfur is also present in certain fuels and oxidizes to sulfur dioxide. The contribution to the heat of 

combustion of the fuel is small due to its low concentration in the fuel and is negligible in most 

industrial processes. Sulfur does have process and pollution implications, which are not discussed 

in this report.

To achieve complete combustion in practice it is often necessary to provide excess air. This is the 

amount of air over and above the minimum required for complete combustion.

None of the chemical reactions can take place until the oxygen in the air is brought into contact 

with the gas. All combustion processes therefore take place in the following stages:

MIXING - IGNITION - CHEMICAL REACTION - DISPERSAL OF PRODUCTS

The rate of combustion is dependent on the slowest of the above stages, which in this case is 

mixing. The rate and completeness of the combustion process is therefore controlled by the rate 

and completeness of fuel/air mixing. In fact burner flame shape and characteristics are dependent 

on how fuel and air mix.

For most rotary kiln burners, fuel/air mixing occurs as a result of jet entrainment. As fuel and 

primary air exit the nozzle, shear occurs at the boundary with the secondary air, causing the 

secondary air to be pulled into and mix with the fuel. This process is momentum controlled. The 
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greater the momentum of the jet, the faster the mixing and more intense the flame. Figure 6-1

shows a physical model of a high momentum (left) and low momentum (right) flame. 

 

Figure 6-1: Physical modeling of high (left) and low (right) momentum flames in a rotary kiln

Since the secondary air has to be entrained into the fuel/primary air jet the secondary air 

aerodynamics can have a huge effect on the fuel/air mixing. The secondary air flow patterns are 

considerably affected by the inlet ducting.

Figure 6-2 shows how swirling flow in a kiln can cause flame deflection, even with a well designed 

burner.

 

Figure 6-2: CFD simulation of a kiln with swirl in the secondary air flow and consequent flame 

deflection

From the above figures, it can be appreciated that modeling can play a vital part in understanding 

flame behavior and as a tool for designing burners for optimum performance.

6.2 Fuels

6.2.1 Gas 

The type of fuel also influences burner design. Natural gas is a convenient fuel. It is clean, easy to 

handle and there is no working capital in stock piles or tanks. The major drawback with gas is its 

low flame luminosity, which is much lower than oil or solid fuel. Gas flames have low emissivity 

and this in turn reduces the amount of radiation heat transfer, Qrad α ε(Ta

4

-Tb

4

).

FCT has a proven technology that overcomes this thermodynamic drawback - the Gyro Therm MkII 

burner. The Gyro-Therm MkII burner uses a “Precessing Jet” nozzle. This is a mechanically 
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simple nozzle that creates a complex fluid mechanic phenomenon. 

 

generated from it.

Figure 6-3: Schematic view of a simple Precessing Jet nozzle

The precession of a jet causes a dramatic change in the structure of the turbulent flow field that has 

a profound influence on the properties of a flame.

 

 

When burning gas this flow results in the stabilization of the flame close to the nozzle. The 

subsequent reduction of the strain in the jet is deduced to promote soot formation, so increasing 

flame emissivity and radiant heat transfer. At the same time, the characteristic flame temperature 

is reduced, because it releases its heat as radiation, which is consistent with a reduction in NOx 

emissions.

The Gyro Therm MkII incorporates a simple but extremely effective flame shaping technique. 

6.2.2 Coal

When solid fuel, like coal, burns the volatiles first vaporize, mix with oxygen and then burn. After 

the volatiles have burnt the remaining char particle will burn out. The reactions involved in the 

burning of particles and droplets take place in a layer at the surface, see Figure 6-4.
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Figure 6-4: Solid fuel combustion process

The following steps must occur:-

 

1. Oxygen has to diffuse in through a very thin, 10 micron, stagnant boundary layer.

2. Ignition must occur.

3. Chemical reaction then takes place at the hydrocarbon surface.

4. Next the products of combustion have to diffuse out again through the same boundary layer.

5. The final dispersion of flue gases take place.

As this is a surface reaction, surface area is maximized when the fuel particle is smallest. Typically 

the residence time in a flame is about 1 second, and a 200 μm takes about 1 second to burn. As 

such, the maximum particle size for a solid fuel is about 200 μm. For efficient combustion solid 

fuels are normally ground to a fineness of 10-15% residue on a 75 μm (200 mesh) sieve. The 

target residue depends on the coal volatile fraction.

In addition to particle size and injection velocity, other factors to consider when firing coal are: 

Volatile Content: The higher the volatile content the more rapidly the coal ignites and burns. High 

volatile coals (above 35%) tend to present a significantly higher explosion risk than those below 

25%. Coals with volatile contents above 45% require special precautions.

Swelling Properties: Once the volatiles have been driven off a char particle is left behind. If this is 

larger than the original particle then it is more open and the particle will burn more rapidly than if it 

shrinks.

Moisture Content: Coals have two types of moisture, surface moisture and inherent water. 

Generally the higher the inherent water the greater the reactivity of the coal and the higher the 

consequential fire and explosion risk. For pulverized coal firing the surface moisture has to be 

removed when grinding. Removal of the inherent water should be minimized otherwise moisture 

from the atmosphere recombines with the coal and causes spontaneous heating which can result 

in a fire or explosion.
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Ash Properties: The chemical composition of the ash has a significant effect on some processes 

and appropriate coals have to be selected accordingly, particularly for mineral processes. For 

boilers it is mainly the physical properties of the ash such as its softening and melting points which 

affect its deposition on tube banks etc.

Hardness and Abrasion Index: The hardness of the coal affects the capacity of coal mills, the 

harder the coal the less can be ground and/or coarse the resulting pulverized coal. The abrasion 

index is mainly dependent on the ash characteristics. Very abrasive coals with high silica ashes 

cause high wear rates in coal mill grinding elements.

Auto-ignition Temperature: An important characteristic of any fuel is the auto-ignition 

temperature. This is the temperature at which a fuel/air mixture will ignite without the need for any 

additional energy such as a spark. The lower the auto-ignition temperature the easier the fuel is to 

ignite and the higher the explosion risk it poses from extraneous ignition sources in the event of a 

fuel leak. However when used in a furnace, once the flame is lit, it is less likely to be accidentally 

extinguished than the flame of a fuel with a high auto-ignition temperature.

In general coal flames have very high emissivity, almost 1, due to the abundance of soot particles. 

As such, coal flames exhibit very high radiant heat transfer. Referring back to the radiant heat 

exchange formula, Qrad α ε(Ta

4

-Tb

4

), it can be noted that highly emissive flames, such as coal 

transfer heat (radiantly) at lower temperatures than low emissivity flames, like natural gas. A 

consequence is that NOx emissions (described later) are lower, even when the coal has nitrogen 

content.

A coal nozzle is essentially an open pipe or annulus. The key design parameters are the free area 

and the injection velocity. While there are basic guidelines for sizing a solid fuel annulus, FCT uses 

mathematical modeling to predict solid fuel particle burnout. 

6.3 Combustion Modeling

6.3.1 Gas Combustion Modeling

Rotary kiln burners are non-premixed combustion processes, which means the fuel and air mix 

outside the burner, entering the flame in distinct streams. The modeling process can be regarded 

as a mixing problem. One approach is the mixture fraction approach. Under these assumptions, 

the thermo-chemistry is reduced to a single parameter, the mixture fraction. The mixture fraction is 

the mass fraction that originated from the fuel stream. In other words, it is the local mass fraction of 

burnt and un-burnt fuel stream elements (C, H, etc.) in all the species (CO2, H2O, O2 etc.). The 

approach is elegant because atomic elements are conserved in chemical reactions. In turn, the 

mixture fraction is a conserved scalar quantity, and therefore its governing transport equation does 

not have a source term. Combustion is simplified to a mixing problem, and the difficulties 

associated with closing non-linear mean reaction rates are avoided. Once the fuel and air mix, the 

chemistry can be modeled as being in chemical equilibrium (Equilibrium model), or being near 

chemical equilibrium (Steady Diffusion Flamelet model), or significantly departing from chemical 

equilibrium (Unsteady Diffusion Flamelet model).

When modeling, it is normal to get a good understanding of which models most practically 

represent the real case. Consequently, often all models are used to analyze the data. The 

following sections describe the models in detail
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6.3.1.1 Mixture Fraction Approach

The basis of the non-premixed modeling approach is that under a certain set of simplifying 

assumptions, the instantaneous thermo-chemical state of the fluid is related to a conserved scalar 

quantity known as the mixture fraction. More detail can be found in the literatures [7].

Under the assumption of chemical equilibrium, we can describe the instantaneous relationships 

between mixture fraction and species fractions, density, and temperature. The prediction of the 

turbulent reacting flow, however, is concerned with prediction of the averaged values of these 

fluctuating scalars. How these averaged values are related to the instantaneous values depends on 

the turbulence-chemistry interaction model. One of the methods is to apply the assumed-shape 

probability density function (PDF) approach as its closure model when the non-premixed model is 

used.

6.3.1.2 The Diffusion Flamelet Models

The flamelet concept views the turbulent flame as an ensemble of thin, laminar, locally onedimensional flamelet structures embedded within the turbulent flow field. A common laminar flame 

type used to represent a flamelet in a turbulent flow is the counter-flow diffusion flame. This 

geometry consists of opposed, axi-symmetric, fuel and oxidizer jets. As the distance between the 

jets is decreased and/or the velocity of the jets increased, the flame is strained and increasingly 

departs from chemical equilibrium until it is eventually extinguished. The species mass fraction and 

temperature fields can be measured in laminar counter-flow diffusion flame experiments, or, most 

commonly, calculated. For the latter, a self-similar solution exists, and the governing equations can 

be simplified to one dimension along the axis of the fuel and oxidizer jets, where complex chemistry 

calculations can be affordably performed. The laminar counter-flow diffusion flame equations can 

be transformed from physical space to mixture fraction space.

The steady flamelet approach models a turbulent flame brush as an ensemble of discrete, steady 

laminar flames, called diffusion flamelets. The individual diffusion flamelets are assumed to have 

the same structure as laminar flames in simple configurations, and are obtained by experiments or 

calculations. Using detailed chemical mechanisms, we can calculate laminar opposed-flow 

diffusion flamelets for non-premixed combustion. The diffusion flamelets are then embedded in a 

turbulent flame using statistical PDF methods.

The advantage of the diffusion flamelet approach is that realistic chemical kinetic effects can be 

incorporated into turbulent flames. The chemistry can then be pre-processed and tabulated, 

offering tremendous computational savings. However, the steady diffusion flamelet model is limited 

to modeling combustion with relatively fast chemistry. The flame is assumed to respond 

instantaneously to the aerodynamic strain, and therefore the model cannot capture deep nonequilibrium effects such as ignition, extinction, and slow chemistry (like NOx).

6.3.1.3 The Unsteady Diffusion Flamelet Model

The steady diffusion flamelet model simulates local chemical non-equilibrium due to the straining 

effect of turbulence. In many combustors the strain is small at the outlet and the steady diffusion 

 

7 Y. R. Sivathanu and G. M. Faeth. "Generalized State Relationships for Scalar Properties in NonPremixed Hydrocarbon/AirFlames". Combustion and Flame. 82. 211–230. 1990.
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flamelet model predicts all species, including slow-forming species like NOx, to be near equilibrium, 

which is often inaccurate. The cause of this inaccuracy is the disparity between the flamelet timescale, which is the inverse of the scalar dissipation, and the slow-forming species time-scale, which 

is the residence time since the species started accumulating after mixing in the combustor.

The unsteady diffusion flamelet model can predict slow-forming species more accurately than the 

steady diffusion flamelet model. Computationally expensive chemical kinetics are reduced to one 

dimension and the model is significantly faster than the laminar-finite-rate, EDC or PDF transport 

models where kinetics are calculated in two or three dimensions. Typical examples of slow-forming 

species are gas-phase pollutants like NOx. By reducing the chemistry computation to one 

dimension, detailed kinetics with multiple species and stiff reactions can be economically simulated 

in complex 3D geometries.

6.3.2 Coal Combustion Modeling

As coal particles are discrete (non-fluid), the coal phase is modeled using an Eulerian-Lagrangian 

approach. The gas phase is solved separately considering the transport equations for continuous 

phase in the Eulerian frame of reference, while the secondary discrete solid coal phase is solved 

considering a Lagrangian frame. The trajectories of the particles are evaluated as a function of the 

force balance on them. The continuous phase flow pattern is impacted by the discrete phase (and 

vice versa) and the calculation of the main phase is alternated with the discrete phase until a 

converged coupled solution is achieved. As the trajectory of a particle is computed, the heat, mass 

and momentum gained or lost by the particle are evaluated, and these interactions are accounted 

for in the Eulerian equations of the primary phase by means of source terms. The dispersion of 

particles due to turbulence is accounted for by considering the stochastic tracking model, including 

the effect of instantaneous turbulent velocity fluctuations on particle trajectories.

Coal combustion itself is modeled according to the following phenomena sequence: drying, devolatilization, volatile combustion and char burnout. Coal already dried is introduced in the 

combustion chamber. The pyrolysis phenomena are regarded as a two-stage process: 

1. During primary pyrolysis, coal particles decompose and release volatile matter (devolatilization), composed by TAR, light hydrocarbons and gas; 

2. During secondary pyrolysis, TAR decomposes and produces soot, light hydrocarbons and 

gas (mainly CO and H2);

De-volatilization of coal is accounted for by considering the Chemical Percolation De-volatilization 

(CPD) model.

6.3.3 Turbulence Model

In the current system, turbulence has a strong influence on momentum, heat, and mass transfer. 

For these simulations, we used k-ω SST Model.

The standard k-ω model is an empirical model based on model transport equations for the 

turbulence kinetic energy (k) and the specific dissipation rate (ω). The shear-stress transport (SST) 

k-ω turbulence model is used to effectively blend the robust and accurate formulation of the k-ω
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model in the near-wall region with the free-stream independence of the k-ω model in the far field. 

The SST k-ω model combines advantages of both the standard k-ε model and the k-ω model. 

6.3.4 Turbulence/Chemistry Interaction Model

We use the Eddy Break-Up (EBU) Model to simulate the turbulence/chemistry interaction.

By setting up Mixture Mass Fraction approach, turbulence/chemistry interaction can be modeled 

through the eddy-breakup model. Because of strong interactions between the turbulent flow and 

reactions, the EBU model was based on the assumption that the reaction rate is controlled by 

turbulent mixing and the reaction rate was depended on the mixing of the turbulent eddies. 

6.3.5 Radiation Model

We use the Sn Discrete Ordinate Method (SnDOM) for the radiation simulations. The Sn 

quadrature schemes available in: S4, S6, S8, and S12. For example, the S4 approximation, which 

considers 12 ordinate directions in two dimensions (24 in three dimensions).

6.3.6 CO Production Model

For CO production, we use the Warnatz mechanism. CO concentrations in 2-D or 3-D reacting flow 

fields are calculated by assuming that the deviation of the calculated CO field from the equilibrium 

value is small or that the calculated CO concentration is small so that the post-processed CO 

concentration has negligible effect on the heat release and the overall flow field. It is also assumed 

that equilibrium values of CO2, CO and OH have been calculated for the Warnatz CO oxidation 

option.

6.3.7 NOx Formation and Modeling

Combustion NOx emission consists of mostly nitric oxide (NO), and to a lesser degree nitrogen 

dioxide (NO2) and nitrous oxide (N2O). NO is converted to NO2 in the atmosphere and so NOx is 

typically reported as NO2 by mathematically converting the mass of NO to the mass of NO2. NOx 

is a precursor for photochemical smog, contributes to acid rain, and causes ozone depletion. The 

NOx formation model provides a tool to understand the sources of NOx production and to aid in the 

design of NOx control measures.

In laminar flames and at the molecular level within turbulent flames, the formation of NOx can be 

attributed to four distinct chemical kinetic processes: thermal NOx formation, prompt NOx 

formation, fuel NOx formation, and intermediate NOx. Thermal NOx is formed by the oxidation of 

atmospheric nitrogen present in the combustion air. Prompt NOx is produced by high-speed 

reactions at the flame front. Fuel NOx is produced by oxidation of chemically combined nitrogen 

contained in the fuel. At elevated pressures and oxygen-rich conditions, NOx may also be formed 

from molecular nitrogen (N2) via N2O. 

6.3.7.1 Thermal NOx Formation

The formation of thermal NOx is determined by a set of highly temperature-dependent chemical 

reactions known as the extended Zeldovich mechanism. The principal reactions governing the 

formation of thermal NOx from molecular nitrogen are as follows:
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O + N2 <=> N + NO ( 6-5 )

N + O2 <=> O + NO ( 6-6 )

A third reaction has been shown to contribute to the formation of thermal NOx, particularly at nearstoichiometric conditions and in fuel-rich mixtures:

N + OH <=> O + NO ( 6-7 )

6.3.7.2 Prompt NOx Formation

It is known that during combustion of hydrocarbon fuels, the NOx formation rate can exceed that 

produced from direct oxidation of nitrogen molecules (or thermal NOx)

Prompt NOx is most prevalent in fuel rich flames (or fuel rich regions in flames). The actual 

formation involves a complex series of reactions and many possible intermediate species. The 

route now accepted is as follows:

CH + N2 <=> HCN + N ( 6-8 )

N + O2 <=> NO + O ( 6-9 )

HCN + OH <=> CN + H2O ( 6-10 )

CN + O2 <=> NO + CO ( 6-11 )

A number of species resulting from fuel fragmentation have been suggested as the source of 

prompt NOx in hydrocarbon flames (for example CH , CH2, C, C2H), but the major contribution is 

from CH (Equation 1–4) and CH2, via

CH2 + N2 <=> HCN + NH ( 6-12 )

The products of these reactions could lead to formation of amines and cyano compounds that 

subsequently react to form NO by reactions similar to those occurring in oxidation of fuel nitrogen, 

for example:

HCN + N <=> N2 + CH ( 6-13 )

6.3.7.3 Fuel-Bound Nitrogen

It is well known that nitrogen-containing organic compounds present in liquid or solid fossil fuels 

can contribute to the total NOx formed during the combustion process. This fuel nitrogen is a 

particularly important source of nitrogen oxide emissions for residual fuel oil and coal, which 

typically contain 0.3–2% nitrogen by weight. Studies have shown that most of the nitrogen in heavy 

fuel oils is in the form of heterocycles, and it is thought that the nitrogen components of coal are 

similar. It is believed that pyridine, quinoline, and amine type heterocyclic ring structures are of 

importance.
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6.3.7.4 NOx Formation from Intermediate N2O

Melte and Pratt [8] proposed the first intermediate mechanism for NOx formation from molecular 

nitrogen (N2) via nitrous oxide (N2O). Nitrogen enters combustion systems mainly as a component 

of the combustion and dilution air. Under favorable conditions, which are elevated pressures and 

oxygen-rich conditions, this intermediate mechanism can contribute as much as 90% of the NOx 

formed during combustion. This makes it particularly important in equipment such as gas turbines 

and compression-ignition engines. Because these devices are operated at increasingly low 

temperatures to prevent NOx formation via the thermal N2O- mechanism, the relative importance of 

the N2O-intermediate mechanism is increasing. It has been observed that about 30% of the NOx 

formed in these systems can be attributed to the N2O-intermediate mechanism.

The N2O-intermediate is not likely a strong contributor of NOx in the system currently being studied.

6.3.7.5 NOx Formation in Turbulent Flows

The kinetic mechanisms of NOx formation and destruction described in the preceding sections 

have all been obtained from laboratory experiments using either a laminar premixed flame or 

shock-tube studies where molecular diffusion conditions are well defined. In any practical 

combustion system, however, the flow is highly turbulent. The turbulent mixing process results in 

temporal fluctuations in temperature and species concentration that will influence the 

characteristics of the flame.

The relationships among NOx formation rate, temperature, and species concentration are highly 

nonlinear. Hence, if time-averaged composition and temperature are employed in any model to 

predict the mean NOx formation rate, significant errors will result. Temperature and composition 

fluctuations must be taken into account by considering the probability density functions which 

describe the time variation.

In turbulent combustion calculations, we solve the density-weighted time-averaged Navier-Stokes 

equations for temperature, velocity, and species concentrations or mean mixture fraction and 

variance. To calculate NO concentration, a time-averaged NO formation rate must be computed at 

each point in the domain using the averaged flow-field information. Methods of modeling the mean 

turbulent reaction rate can be based on either moment methods or probability density function 

(PDF) techniques.

6.3.7.6 Formation of Nitrogen Dioxide (NO2)

Nitrogen dioxide (NO2) is present in varying but generally low concentrations in the flue gases from 

all combustion systems, the major oxide of nitrogen is in the form of NO. However, as the flue 

gases cool most of the NO emitted is converted to NO2 in the atmosphere by slow reaction with O2. 

The NO2 molecule is also highly photochemically active and takes part in a complex photolytic 

cycle shown schematically in Figure 6-5. This reaction sequence results in ozone (O3) depletion 

and contributes to the formation of photochemical smog in the lower atmosphere. NO2 also reacts 

with water vapour to form nitric acid that is washed out as a contribution to acid rain.

 

8 P. C. Melte and D. T. Pratt. "Measurement of Atomic Oxygen and Nitrogen Oxides in Jet Stirred 

Combustion". In 15th Symposium (Int’l) on Combustion. The Combustion Institute. 1061–1070. 

1974.
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Figure 6-5: Principle NOx formation pathways

To predict NOx emissions, we solve a transport equation for nitric oxide (NO) concentration. When 

fuel NOx sources are present, we solve additional transport equations for intermediate species 

(HCN and/or NH3). When the N2O intermediate model is activated, an additional transport equation 

for N2O will be solved. The NOx transport equations are solved based on a given flow field and 

combustion solution. In other words, NOx is post processed from a combustion simulation. It is 

therefore evident that an accurate combustion solution becomes a prerequisite of NOx prediction. 

For example, thermal NOx production doubles for every 160°F (90°C) temperature increase when 

the flame temperature is about 3,500°F (1,930°C). Great care must be exercised to provide 

accurate thermo-physical data and boundary condition inputs for the combustion model. 

Appropriate turbulence, chemistry, radiation, and other sub-models must be employed.

Realistically, we can only expect results to be as accurate as the input data and the selected 

physical models. Under most circumstances, NOx variation trends can be accurately predicted, but 

the NOx quantity itself is subject to an offset. Accurate prediction of NOx parametric trends can cut 

down on the number of laboratory tests, allow more design variations to be studied, shorten the 

design cycle, and reduce product development cost. That is truly the power of the NOx simulation 

model and, in fact, the power of CFD in general.

6.4 Process Modeling

The heated phase change by oxidation is a process commonly used in iron-ore pelletizing. The 

pellets are hardened or changed owing to re-crystallization of iron oxides, formation of slag phases 

and secondary components. These processes take place at high temperatures.

The reaction that occurs in pelletizing is mainly oxidation of magnetite to hematite; the oxidation 

can be expressed as

2Fe3O4 +1/2 O2 <=> 3Fe2O3 + Q ( 6-14 )

where Q is equal to 264.55 kJ/mol
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As there are two modifications of Fe2O3, i.e. α-Fe2O3 (hematite) and γ-Fe2O3 (maghemite), the 

magnetite is oxidized gradually while several solid solutions of magnetite with maghemite, differing 

in composition, are being produced. This behavior is made possible because both have identical 

lattice structures. The oxidation of magnetite to hematite then proceeds according to equation 

(Meyer, 1980 9).

Fe3O4  Fe3O4. γ -Fe2O3(s)  γ - Fe2O3 . Fe3O4(s)  γ - Fe2O3  α-Fe2O3 ( 6-15 )

s – means surface phase

The completeness of oxidation depends on the conditions under which the oxidizing firing takes 

place, and on the chemical composition of raw material being processed.

The firing time and temperature are decisive for structural changes of the pellets; the nature of the 

changes affects, in turn, the mechanical properties of the pellets. In addition to time and 

temperature, structural changes are affected by the composition of the iron-bearing, especially by 

the content of gangue constituents, or by addition of basic substances [10].

Magnetite (Fe3O4) is not thermodynamically stable under oxidizing conditions and pure magnetite 

will be oxidized to α-Fe2O3 when above 400°C [

11].

According to these studies, we have established a three step reaction mechanism:

(1) first Fe3O4 transfer to surface active sites:

Fe3O4 => Fe3O4

X

( 6-16 )

(x represent to surface sites)

(2) surface sites absorb oxygen to transfer to Fe2O3 sites:

Fe3O4

X + 1/4 O2 => 3/2 Fe2O3X

( 6-17 )

(3) Fe2O3 site transfer to α-Fe2O3:

Fe2O3

X => α-Fe2O3 ( 6-18 )

Under atmospheric pressure, temperature ranging from 490 to 2590°C and volume fraction of 

oxygen between 0 and 30%, the following reaction rate expression has been evaluated:

( 6-19 )

 

9 Meyer, K., 1980, Pelletizing of Iron Ores, Frankfurt, Germany

10 Smrcek, K., Cejchar, O., J.Srb: Rudy, (1963), No.12, pp400-408

11 U. Colombo, G. Fagherazzi, F. Gazzarrini, G. Lanzavecchia, G. Sironi, Nature 219

(1968) 1036
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Where 

( 6-20 )

( 6-21 )

And

( 6-22 )

k0 in Equation (2-20) is 2.31·10−5(mol/(m3·s)).The unit for pressure in these equations are bar.

These reaction rate expressions are deduced follow the roles as those equations from carbon 

deposition reported in the literature [12]

These reaction rate coefficient data are taken from literatures such as [13,

14] etc.

6.5 Material Motion and Heat Transfer in a Rotary Kiln

As the purpose of the kiln is to process the pellets, heat transfer to the pellets is also modeled. 

Heat is transferred to/from the pellet bed from the flame gases (via radiation and convection) and 

to/from the refractory walls. Within the pellet bed heat is transferred primarily through advection, or 

material flow. Material flow studies [15] have identified a number of different modes of bed flow 

behavior. At low fills and speeds the bed behaves as a bulk. The bed travels up with the kiln until 

friction between its covered surface and wall is overcome and the bed slips. This mode is referred 

to as slipping. If friction is increased, the bulk continues to rotate with the kiln until the bed angle 

reaches the maximum angle of stability, an angle greater than the bed’s angle of repose. At this 

point the upper region breaks away and avalanches over the bed surface. As the kiln rotates 

further the maximum stability is once again reached and the process is repeated. This is called 

slumping. As speed is further increased the maximum angle of stability is reached before the 

avalanching material comes to rest, creating a continuous slump, and then the bed is said to be 

rolling. Further increases in kiln speed lead to cascading, cataracting and finally centrifuging. In the 

 

12 S.G. Zavarukhin and G.G. Kuvshinov, “The kinetic model of formation of nanofibrous carbon 

from CH4–H2 mixture over a high-loaded nickel catalyst with consideration for the catalyst 

deactivation,” J. Appl. Catal. A, vol. 272, pp. 219–227, 2004.

13 K. HANEDA and A. H. MORRISH, “MAGNETITE TO MAGHEMITE TRANSFORMATION IN 

ULTRAFINE PARTICLES”, JOURNAL DE PHYSIQUE, Colloque Cl, supplhettt au no 4, Tome 

38, Aoril 1977, page Cl-321

14 Guanghui Li, Tao Jiang, Yuanbo Zhang and Zhaokun Tang (2012). Recrystallization of Fe2O3 

During the Induration of Iron Ore Oxidation Pellets, Recrystallization, Prof. Krzysztof Sztwiertnia 

(Ed.), ISBN: 978-953-51- 0122-2, InTech, Available from: 

http://www.intechopen.com/books/recrystallization/recrystallization-of-fe2o3-during-theinduration-of-iron-ore-oxidized-pellets

15 H. Henein, J.K. Brimacombe, and A.P. Watkinson: Metall. Trans. B, 1983, col. 14B, pp. 191-

205.
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centrifuging mode, the particles’ centrifugal force overcomes gravity, and hence a rotational Froude 

number (r

2

/g) of 1.0 indicates the onset of centrifuging.

In the mineral processing industry rolling is the most desirable as it provides good mixing with a 

steady angle of repose. The latter ensures continuous surface renewal, which in turn facilitates 

efficient particle agitation. Heat transfer is maximized, allowing reactions to proceed, without 

causing attrition and excessive dust generation. 

In the rolling mode there are two distinct regions, see Figure 6-6. Nearer to the surface, the 

material avalanches downwards in a continuously shearing region (active region or layer), while 

close to the walls a plug flow region is established, where material moves in a rigid body motion, 

u/ = 0. Intimate mixing of particles occur in the active layer, which enhance heat transfer and 

experiments have shown that advective heat transfer is enhanced by a factor of up to 5 [16]

Figure 6-6: Rolling bed motion in the transverse plane of a rotary kiln

Axial motion regulates overall residence time in the kiln. The transverse motion is linked to the axial 

motion in that the material only advances axially when it is turned over in the cross section.

Heat transfer in the bed of a rotary kiln is similar to heat transfer in a packed bed but with an added 

component of particle advection. The heat transfer paths are:

 Within particle conduction.

 Particle-particle conduction.

 Particle-particle radiation.

 Convection by the interstitial gas.

 Advection by particle motion.

 

16 S.K. DHANJAL, P.V. Barr and A.P. Watkinson, “THE ROTARY KILN: AN INESTIGATION OF BED 

HEAT TRANSFER IN THE TRANSVERSE PLANE, Metallurgical and Material Transactions B, 

Volume 35B, December 2004, pp 1059-1070