Electronic documents, once printed, are uncontrolled and may become outdated. Page 1 of 79 

# CLIFFS UTAC

LINE 2

PHASE 3 MODELING REPORT

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

3 3/10/ 1 4 Second Issue

Current Conditions SD/ TH /

YY/T L BI Prelim

REV Date Description of Issue Originator CHKD Approved

JOB Report

S01CLUT01 RM11546

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 2 of 79 

### Table Of Contents

FCT Combustion Gyro

- Therm MKII Modeling Executive Summary

................................ ...................... 7

1. Introduction

................................ ................................ ................................ ................................ ........... 8

2. Backgroun d

................................ ................................ ................................ ................................ ......... 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 Over view

................................ ................................ ................................ ................................ ..... 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

................................ ................................ ................................ ....... 2 8
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 locate d 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

................................ ................................ ................................ ........................ 606

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 3 of 79 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

................................ ................................ .... 786

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

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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 Fi ring

................................ ............................ 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

- Cur rent 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 centerline

Current Gas Firin g 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

................................ ....... 416

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 5 of 79 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, 3f t 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

................................ .......... 796

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

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### Table s

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

................................ ................................ ................................ ................................ ....... 596

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

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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 con tracted 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 reduct ion 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 pr esents 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 emiss ion rates of 2. 9 lb/mmBtu on natural gas.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 8 of 79 

## 1. Introduction

Cliffs Natural Resources Inc. owns and operates an iron ore mine and pellet plant in North ern Minnesota known as United Taconite (UTAC). The mine is located in Eveleth, and the pellet plant

is abo ut 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 concen trated. 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.

T he process consists of adding wa ter and other b inders 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 1 00% 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 oxyge n, 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 f ired

. 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 ra diant h eat
transfer

. The resulting hotter flame creates more NOx.

The current NOx levels are 5

- 6 lbs NOx/ mm BTU when firing natural gas , and about 1.6

- 2.8 lbs

NOx/ mm BTU 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 lo w NOx b urner on their Line 2. The NOx level target s are to be <1.2 lbs NOx/ mm BTU when firing natural gas only

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

FCT COMBUSTION supplies a low NOx burner, called the FCT COMBUSTION Gy ro

- Therm MkII

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

the 2

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

UTAC line 2 kiln. The FCT COMBUSTION Gyro

- Therm M kII burner has been accepted as Best Available Technology for low NOx rotary kiln burners ( in applications where the O 2 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 u ses 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 dr amatically 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 mainta ined. The modeling process involves gathering data from the plant by FCT COMBUSTION engineers. The data are used to generate6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 9 of 79 complex computer models. T he model is then validated, and tested for robustness, and predictive capability. Having completed th is, 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 DESIGN6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 10 of 79 

## 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 th ree 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 f ollows.

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.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 11 of 79 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.

T he first sections of the grate are dedi cated to drying. The green balls come into contact with hot process air as the moisture is driven off. The ball moisture level e ntering 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 exi t 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 inte rmediate 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 k iln is sintering, or hardening, of the pellets. It is here that the pellets get their strength. UTAC uses two

burners in their kiln6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 12 of 79 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 se nt 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 delivere d 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

different ials 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 t he 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 F ans 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 sc rubber. Because of the quenching of the air with water introduced in to 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 pr essure 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) betwe en 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 NO 2 . It is these NOx compounds, among others, such as SOx, that are targete d for control and reduction by environmental agencies due to the links between them and photochemical6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 13 of 79 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 the ir 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 shap ing

.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 14 of 79 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, t here 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. A s a result of this an additional pounds of air can flow through the cooler, and is thus p re

- 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 throug h 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 mm BTU/hr

of coa l.

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/ mm BTU, while it is 5

- 6 lb/ mm BTU 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/ mm BTU for gas and 1.5 lb/ mm BTU 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 m ake top quality product, while achieving low NOx emission goals and avoiding significant fuel or thermal penalties..6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 15 of 79 

## 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 f lame 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 di fferent 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 c ombustion 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 homogeneou s 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 ANSY S 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, refractor y, 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 experimen tal 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 ref ractory, 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 bed6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 16 of 79 tem perature 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 adju stments, to ensure NOx changes were correctly predicted.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 17 of 79 

### 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

CO 2 0.83 CH4 93.80

N2 1.38

CO 2 0.836

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 18 of 79 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 CO 2 %

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

.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 19 of 79 Figure 3

- 1 : Mass Balance of UTAC Kiln: 100% Gas Firing

Figure 3

- 2 : Mass Balance of UTAC Kiln: Coal/gas firing6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 20 of 79 

### 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 t wo 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

.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 21 of 79 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/min6

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P

Electronic documents, once printed, are uncontrolled and may become outdated. Page 22 of 79 

## 4. Mode ling 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.

Havi ng 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 summariz ed in Table 4

- 1

.

Table 4

- 1 : Summary of Plant Data Related to Kiln

Burner Burner Impulse

(N/MW) Primary Air as %

of Stoi chiometric 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 li nked 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, n otoriously 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

.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 23 of 79 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 h igh 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) second ary 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 solid ify 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 t hat 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 second ary 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 t he 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 seconda ry 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

col ored (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

.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 24 of 79 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 seco ndary 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.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 25 of 79 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 secondar y 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 Case6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 26 of 79 Figure 4

- 4 : Secondary air velocity vectors colored by velocity

– gas burner vertical centerline

Current 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 refractor y from direct flame impingement.

Figure 4

- 5 : Secondary air velocity vectors colored by velocity

– kiln horizontal centerline

- Current Gas Firing Case6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 27 of 79 

### 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 Case6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 28 of 79 

### 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

tow ard

.

T he 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 somew hat 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 case6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 29 of 79 

### 4.3.2 Current Coal/Gas Firing

T he kiln g as 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

I t 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 approximated6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 30 of 79 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 tempe ratures

– 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 evolutio n 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 i t captures the heat losses from the

shell of the kiln. The model accounts for natural convection and radiation heat loss es 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 w aste 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.pdf6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 31 of 79 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 accoun ts 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 pr oducing 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 an d 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 32 of 79 

### 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 temp erature 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 po int 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 temper a ture ( 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 tha t 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 Case6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 33 of 79 

### 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 ex it 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 temper a ture ( 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 Case6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 34 of 79 

### 4.5 Cooler Model

The c ooler 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 wa s 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 tem perature profile, with initial heat up over a short distance is seen, as expected since there is little data to suggest

otherwise.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 35 of 79 Figure 4

- 19 : Temperature Profile of pellets in cooler, current coal/gas c ase. (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 in volves 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 rev iewing 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 . I t 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 a n outlier that may b e 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.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 36 of 79 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 computation al 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 c onsidered, 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 us ing 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

ana lyzed 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 lowe r 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 tracke d, which is a function of air flow pattern, re

circulation (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 i n 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

- firing6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 37 of 79 The dominant combustion process und er 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

thorou ghly 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 s hell 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 developm ent, 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 tha t 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 e xtensively 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 38 of 79 

### 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 pelletizi ng 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 behavi ng 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 Decem ber 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 under

predicting (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 bef ore 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 model6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 39 of 79 (at least) twice to allow it to converge fully. Partial convergence (on just some of the parameters

such 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 pr ocess 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 display ed 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 th e 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 cra shes 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 th e 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 (temperatur e 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 se t 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 (we re) 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).6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 40 of 79 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 predi ction.

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 burne r 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 41 of 79 

### 4.7.1 FCT Combustion Burner at center of ki ln

( 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 recirculatio n 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: Loca ted at kiln center, 3% primary air. Note, flame impingement onto rear wall

Figure 4

- 22 : Flow streamlines showing intense recirculation at rear wall6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 42 of 79 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 l ocated 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 r efractory. 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 43 of 79 

### 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 p lan view of this case, and Figure 4

- 25 shows a n 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 t o 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, b ut 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 G yro

- Therm Burner: Located at kiln center, 3% primary air, ft forward into kiln.6

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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 co al burner centerline position. Figure

4

- 26 and Figure 4

- 27 s how 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 t he 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 t he 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 o f 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 air6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 45 of 79 This above case was further investigat ed 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,

eve n 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

- The rm 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

in to kiln. 12% primary air6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 46 of 79 

### 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 H eating A nalysis with Gyro

- Therm B urner

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.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

P

Electronic documents, once printed, are uncontrolled and may become outdated. Page 47 of 79 Figure 4

- 31 : Heat flux t o 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 temp erature 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 burners6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 48 of 79 (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 t hey 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 n ever 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, t he pellet temp erature 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, t his indicates that the cooler is effici ent 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 t emperature 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.6

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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 i n 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% l ower, 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 Gyro

Therm is 2.91 lb/mmBTU, and for the 70% coal, 30% gas case is 2.04 lb/mmBTU. T hese 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 f lame 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 mode l led

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

mode l ling, CFD

- bed model coupling, NOx post processing, coal particle tracking, etc. By combining
and coupling all these models, two well validated model s were develope d and validated using
current gas case and coal/gas firing case s . These same validated models w ere 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

m mBTU/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 mmB TU/hour of the current gas case. T his 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 model l ed using a separat e ly developed model. It is used to determine how the
pellets complete the oxidation reaction and are cooled.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 50 of 79 

### 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 case6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 51 of 79 Figure 4

- 34 : Kiln temperature map

- kiln vertical centerline, 100% gas case

Figure 4

- 35 : Kiln gas temperature map, volume rendering, 100% gas case6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 52 of 79 

### 30% G as + 70% C oal F iring with new Gyro

- Therm B urner

The co

- firing case with designed Gyro

- Therm burner is also simulated. The temperature profile

view ed 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

- firing6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 53 of 79 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% G as C ase

The NOx con centration 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 G yro

- T herm 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 r esults in low NOx generation and better process efficiency.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 54 of 79 Figure 4

- 38 : NO x concentration, vertical slice, gas case



### 30% G as+70% C oal C ase

The co

- firing case generates even lower NOx

, as expected, with NO x 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

concent ration 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 th an 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 55 of 79 Figure 4

- 39 : NO x 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 H eating and C ooler A nalysis with Gyro

- Therm G as B urner

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 w as 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.6

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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 th e existing

burner.

Figure 4

- 41 : Bed temperature within cooler for the gas Gyro

- T herm burner6

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### 4.8.5 Bed H eating and C ooler A nalysis with Gyro

- Therm C o

- fired C oal/ G as B urner

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 tra nsfer 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 indu rated in the grate and therefore strong enough to withstand the drop into the cooler.6

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Figure 4

- 43 : Bed temperature within kiln for the coal/gas cofired burner

In addition

, the cooler model that was pre viously developed to study the existing plant was run using the high initial temperature predicted ( °F) as the pellets are heated by the coal/gas co

fired 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 th e cooler.

Figure 4

- 44 : Bed temperature within cooler for the coal/gas co

- fired burner6

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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 analyzed6

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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 use d 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 commerc ially 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 signi ficant. 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 amoun t 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 grindi ng and firing coal is common, and referred to as direct firing, but it suffers from the drawback that a lot of primary air is6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 61 of 79 required. In addition, the amount of air cannot be changed without altering the coal mill performance. While a typical range is 2 5

- 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 wher e 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 oxyge n 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

am ount 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 h eat 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 momen tum 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 k iln, 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 preci sion.

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

.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 62 of 79 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 Gyro

Therm 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 63 of 79 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 prim ary 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 technic al 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 op timum 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

- T herm 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 pro duces as low as 1

- 1.5 lb NOx/MMbtu (in applications where the O 2 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.6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 64 of 79 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

- 1 6%. 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 reduc tion 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 UT AC 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 t on of product.



### 5.1.5 Path F orward to L ower NOx E missions at Cliffs UTAC Plant

Now that the final modeling phase is complete and the simulation has indicate d 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 calcul ations 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 are6

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- 2 : Gyro

- Therm Burner Sketch6

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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 H 2 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 ve ry complex chain reaction, it can be reasonably considered

as four basic reactions.

C + 2O 2 ------- > CO 2 +373 BTU/mol (complete oxidation of carbon) ( 6

- 1 )

2H 2 + O 2 ------- > 2H 2 O + 542 BTU/mol (complete oxidation of hydrogen to water) ( 6

- 2 )

2C + O 2 ----- > 2CO + 209 BTU/mol (incomplete oxidation of carbon) ( 6

- 3 )

2CO + O 2 < ----- > 2CO 2 + 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 combust ion 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. The6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 67 of 79 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 model ing 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

aerod ynamics 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 simul ation 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 perform ance.



### 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, Q rad α ε(T
a 4

- T b 4 ).

FCT has a proven technology that overcomes this thermodynamic drawback

- the Gyro Therm MkII burner. The Gyro

- Therm MkII bu rner uses a “Precessing Jet” nozzle. This is a mechanically6

Cliffs UTAC Line 2 Phase 3 Preliminary Modeling Report

Electronic documents, once printed, are uncontrolled and may become outdated. Page 68 of 79 simple nozzle that creates a complex fluid mechanic phenomenon.

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 t his flow results in the stabiliz ation of the flame close to the nozzle. The subsequent reduction of the strain in the jet is deduced to promote so ot 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 M kII 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

.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 69 of 79 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 re sidence 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 μ m (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 co al 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 drive n 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.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 70 of 79 Ash Properties: The chemical composition of the ash has a significant effect on some processes and approp riate 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 Ind ex: 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 sili ca 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

a dditional 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 f lame 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, Q rad α ε(T
a 4

- T b 4 ) , it can be noted tha t 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 i s 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 Co mbustion 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. On e 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 loc al mass fraction of

burnt and un

- burnt fuel stream elements (C, H, etc.) in all the species (CO 2 , H 2 O, O 2 etc.). The approach is elegant because atomic elements are conserved in chemical reactions. In turn, the mixture fraction is a conserved scalar quanti ty, 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 norma l 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 detail6

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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 therm o

- chemical state of the fluid is related to a conserved scalar quantity known as the mixture fraction. More detail can be found in the literature s [ 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 predic tion 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 one

dimensional flamelet structures embedded within the turbule nt 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 exper iments, 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 perfo rmed. 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 flam elets. 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

diffusio n 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 tur bulent 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 non

equilibrium 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 Non

Premixed Hydrocarbon/AirFlames". Combustion and Flame. 82. 211

– 230. 1990.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 72 of 79 flamelet model predicts all species, including slow

- forming species like NOx, to be near equilibrium,

w hich is often inaccurate. The cause of this inaccuracy is the disparity between the flamelet time

scale, which is the inverse of the scalar dissipation, and the slow

- forming species time

- scale, which is the residence time since the species started accumula ting 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 on e 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 tra jectories 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

con verged 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 tu rbulent velocity fluctuations on particle trajectories

.

Coal combustion itself is modeled acc ording to the following phenomena sequence: drying, de

volatilization, 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 (de

volatilization), 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 sta ndard 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

- ω6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 73 of 79 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 m odel 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 t he 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 turbu lent 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 CO 2 , CO and OH have been calcula ted 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 (NO 2 ) and nitrous oxide (N 2 O). NO is converted to NO 2 in the atmosphere and so NOx is typically reported as NO 2 by mathematically converting the mass of NO to the mass of NO 2 . 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

f ormation, 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 chemic ally combine d nitrogen contained in the fuel. At elevated pressures and oxygen

- rich conditions, NOx may also be formed from molecular nitrogen (N 2 ) via N 2 O.



### 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:6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 74 of 79 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 near

stoichiometric 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 flam es) . The actual formation involves a complex series of reactions and many possible intermediate species. The route now accepted is as follows:

CH + N 2 <=> HCN + N ( 6

- 8 )

N + O 2 <=> NO + O ( 6

- 9 )

HCN + OH <=> CN + H 2 O ( 6

- 10 )

CN + O 2 <=> 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 , CH 2 , C, C 2 H), but the major contribution is from CH (Equation 1

– 4) and CH 2 , via

CH 2 + N 2 <=> HCN + NH ( 6

- 12 )

The products of these reactions could lead to formation of amines and cyano compounds that subsequently react to form N O by reactions similar to those occurring in oxidation of fuel nitrogen, for example:

HCN + N <=> N 2 + 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 importa nce.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 75 of 79 

### 6.3.7.4 NOx Formation from Intermediate N 2 

### O

Melte and Pratt [ 8 ] proposed the first intermediate mechanism for NOx formation from molecular nitrogen (N 2 ) via nitrous oxide (N 2 O). Nitrogen enters combustion systems mainly as a component

of the combustion and d ilution 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 t urbines and compression

- ignition engines. Because these devices are operated at increasingly low temperatures to prevent NOx formation via the thermal N 2 O

- mechanism, the relative importance of

the N 2 O

- intermediate mechanism is increasing. It has been obse rved that about 30% of the NOx formed in these systems can be attributed to the N 2 O

- intermediate mechanism.

The N 2 O

- 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 pr actical 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

f luctuations 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

turbule nt reaction rate can be based on either moment methods or probability density function (PDF) techniques.



### 6.3.7.6 Formation of Nitrogen Dioxide (NO 2 

### )

Nitrogen dioxide (NO 2 ) is present in varying but generally low concentrations in the flue gases from

all combustio n 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 NO 2 in the atmosphere by slow reaction with O 2

. The NO 2 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 (O 3 ) depletion and contributes to the formation of photochemical smog in the lower atmosphere. NO 2 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.

1974.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 76 of 79 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 NH 3 ). When the N 2 O intermediate model is activated, an additional transport equation

for N 2 O 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 th ermo

- 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 pelletiz ing is mainly oxidation of magnetite to hematite; the oxidation can be expressed as

2Fe 3 O 4 +1/2 O 2 <=> 3Fe 2 O 3 + Q ( 6

- 14 )

where Q is equal to 264.55 kJ/mol6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 77 of 79 As there are two modifications of Fe 2 O 3 , i.e. α

- Fe 2 O 3 (hematite) and γ

- Fe 2 O 3 (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 ).

Fe 3 O 4  Fe 3 O 4 . γ
- Fe 2 O 3 (s)  γ
- Fe 2 O 3 . Fe 3 O 4 (s)  γ
- Fe 2 O 3  α

- Fe 2 O 3 ( 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 (Fe 3 O 4 ) is not thermodynamically stable under oxidizing conditions and pure magnetite

will be oxidized to α

- Fe 2 O 3 when above 400 °C [ 11 ].

According to these studies, we have established a three step reaction mechanism:

(1) first Fe 3 O 4 transfer to sur face active sites:

Fe 3 O 4 => Fe 3 O 4 X ( 6

- 16 )

(x represent to surface sites)

(2) surface sites absorb oxygen to transfer to Fe 2 O 3 sites:

Fe 3 O 4 X + 1/4 O 2 => 3/2 Fe 2 O 3 X ( 6

- 17 )

(3) Fe 2 O 3 site transfer to α

- Fe 2 O 3 :

Fe 2 O 3 X => α

- Fe 2 O 3 ( 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) 10366

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 78 of 79 Where

( 6

- 20 )

( 6

- 21 )

And

( 6

- 22 )

k 0 in Equation (2

- 20) is 2.31·10

− 5 (mol/(m 3 ·s)). The unit for pressure in these e quations 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 r adiation 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 onc e 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

rolli ng. 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 CH 4



### – H 2 

### 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

- f e2o3

- during

- the

induration

- of

- iron

- ore

- oxidized

- pellets

15 

### H. Henein, J.K. Brimacombe, and A.P. Watkinson: Metall. Trans. B, 1983, col. 14B, pp. 191

- 205.6

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Electronic documents, once printed, are uncontrolled and may become outdated. Page 79 of 79 centrifuging mode, the particles’ centrifugal force overcomes gravity, and hence a rotational Froude number ( 

r 2 /g ) of 1.0 indicates the onset of centrifug ing.

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, a llowing 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