Oxygen Sensor Validation and Analysis of Process Air...

NRRI/

TR-2

007/17 OXYGEN SENSOR VALIDATION AND

ANALYSIS OF PROCESS AIR OXYGEN CONTENT AT UNITED TACONITE

By

David J. Englund and

Richard A. Davis August 2007 Technical Report NRRI/TR-2007/17 CMRL/TR-07-07 Natural Resources Research Institute University of Minnesota Duluth 5013 Miller Trunk Highway Duluth, MN 55811-1442 Coleraine Minerals Research Laboratory One Gayley Avenue PO Box 188 Coleraine, MN 55722

Recommended Citation Englund, D.J., and Davis, R.A., 2007, Oxygen sensor validation and analysis of process air oxygen content at United Taconite: University of Minnesota Duluth, Natural Resources Research Institute, Coleraine Minerals Research Laboratory, Technical Report NRRI/TR-2007/17, 41 p. Natural Resources Research Institute University of Minnesota, Duluth 5013 Miller Trunk Highway Duluth, MN 55811-1442 Telephone: 218-720-4272 Fax: 218-720-4329 e-mail: [email protected] Web site: http://www.nrri.umn.edu/egg ©2012 by the Regents of the University of Minnesota All rights reserved. The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation.

This publication is accessible from the home page of the Coleraine Minerals Research Laboratory or Economic Geology Group of the Center for Applied Research and Technology Development at the Natural Resources Research Institute, University of Minnesota, Duluth (www.nrri.umn.edu/cartd/cmrl or www.nrri.umn.edu/egg) as a PDF file readable with Adobe Acrobat 6.0.

Date of release: March 2012

OXYGEN SENSOR VALIDATION AND ANALYSIS OF PROCESS AIR OXYGEN CONTENT

AT UNITED TACONITE

COLERAINE MINERALS RESEARCH LABORATORY

August 14, 2007

By

David J. Englund Program Director

Process Fluid Flow and Heat Transfer

and

Richard A. Davis University of Minnesota Duluth

Chemical Engineering

Approved by David W. Hendrickson – Director

Coleraine Minerals Research Laboratory

CMRL/TR-07-07 NRRI/TR-2007/17

University of Minnesota Duluth Coleraine Minerals Research Laboratory Natural Resources Research Institute P O Box 188 5013 Miller Trunk Highway One Gayley Avenue Duluth, Minnesota 55811 Coleraine, Minnesota 55722

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Oxygen Sensor Validation and Analysis of Process Air Oxygen Content

at United Taconite Summary: CFX-TASCflow CFD Fortran formerly used to simulate magnetite oxidation and heat transfer between a pellet bed and cross flow gas stream has been revised and converted to ANSYS CFX 11 Command Expression Language for use in CFX11 CFD Cooler models. The conversion has been validated and found to yield consistent results when compared to the previous FORTRAN version. However, revision of the heat of reaction expression for magnetite oxidation now generates more heat release in the bed than the previous version, yielding higher predicted gas and solids temperatures under similar operating conditions. Future studies should focus on methods of estimating pellet entry temperature, incoming magnetite content, and cooling fan flow from plant measurements, to determine if current methods of estimation over-predict magnetite mass flow and solids entry temperatures, and under-predict cooling fan flow rates, as a means to resolve the trend toward higher temperatures in the model simulations. Most of the existing TASCflow cooler grids were found incompatible for CFX 11 simulations, necessitating development of new grids using Solid Works modeling software. Objective: Magnetite oxidation plays a key role in process fuel efficiency, pellet throughput and product quality, but there are no on-line methods to measure oxidation and correlate it with process operation. This project sought to use a high temperature oxygen probe located in the pellet cooler in conjunction with existing CFD models to correlate process oxygen concentration with production rate, gas flow and process temperature, for prediction of magnetite oxidation in the process. However, operational problems with the probe initially limited data generation, and major repair work on Line 2 during a plant outage caused by a major electrical failure in September 2006 resulted in mechanical failure of the probe. Upon restart, United Taconite decided not to replace the probe. In a parallel route, development of revised CFD code proceeded through a sub-contract with ANSYS Canada Ltd. ANSYS distributes CFX 11, the CFD code which replaces CFX-TASCflow. The existing CFD cooler models were developed using CFX-TASCflow. ANSYS provided necessary technical support in converting the CFX-TASCflow FORTRAN into Command Expression Language (CEL) for CFX. The project focused on code revision and validation of subroutines that simulate magnetite oxidation, heat transfer, and, now, oxygen uptake in the pellet bed. Background: This project initially consisted of two steps: 1. Convert CFD model code for prediction of oxygen concentration from CFX-TASCflow to CFX11.

2. Validate simulation results from ANSYS CFX 11 with an Oxygen probe installed at United Taconite Line 2.

- 2 -

Mechanical failure of the probe in 2006, and the subsequent decision not to replace it, shifted emphasis to conversion and validation of the CFD code. Conversion of the code proved to be considerably more difficult than anticipated, which delayed performing a large number of simulations. The code did not become available for validation until early June 2007, leaving only sufficient time for verification and validation with five UTAC CFX-TASCflow simulations from a previous UTAC funded study. It also became apparent that many of the existing CFX-TASCflow models would not easily run under the ANSYS CFX 11 conversion. This was attributable to grid complexity and incompatibility with the newer grid generation software, which limited the selection of simulations for validation. It will be necessary to rebuild cooler model grids using Solid Works, which is a three dimensional modeling package, before models can be run for Minntac and Keetac coolers. A project to accomplish this is planned under the Iron Ore Co-op for the 2007-2009 biennium. FORTRAN Code Revisions: The FORTRAN subroutine developed for CFX-TASCflow was limited to prediction of magnetite oxidation and subsequent heat generation and heat transfer between the pellet bed and cooling gases. Computational limitations in 1997 precluded calculation of oxygen concentration in the cooler gases when the original code was developed. Hence, it was first necessary to rewrite the FORTRAN to account for oxygen transfer from the gas stream. This was accomplished by the author of the original code, Dr. Richard Davis, University Minnesota Duluth, Chemical Engineering. A general description of the code revision is presented in the next section.

CFD Oxidation Model Revision:

The CFX-TASCflow CFD cooler model for pellet oxidation and heat transfer was converted to CFX-11. Additionally, the model was upgraded to account for the uptake of oxygen from the gas phase required by the magnetite oxidation.

The rate of magnetite oxidation is calculated from the shrinking core model. This model assumes that there is a reaction front between the hematite shell and magnetite core in a pellet. The pellets exiting the kiln are near their peak temperature. The rate of reaction is a function of the convective mass transfer of oxygen from the bulk gas phase to the pellet surface, the rate of diffusion through the hematite shell, and the rate of surface reaction between oxygen and magnetite at the interface between the shell and magnetite core. Under conditions of high temperature as found at the kiln exit, the solid-gas reaction rate is large. The magnetite oxidation reaction may be considered mass transfer limited.

The shrinking core model is integrated to calculate the time (or length) in the cooler for complete oxidation. The core concentration of magnetite is calculated as follows:

[ ] ( ) 3 43 4

3 4

1 p p Fe O

cFe O

XFe O

M

ε ρ−= (1)

The local rate of oxidation is interpreted in terms of the angular location along the bed.

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The flux of O2 is calculated as follows:

[ ] ( )3

3 4"2 3

14 144 3

3

bb c ciO

p

F e Ou rN

rx a

εππ−

=∆

(2)

where

b

xu∆

= time of reaction (3)

[ ] 33 4 44 3

c ciFe O rπ

= moles of O2 reacted based on moles of magnetite available for reaction in

a single pellet (4) 34

3prπ

= volume of a single pellet (5)

( )1 bε− = volume of pellets per bulk bed volume (6)

( )3 1 b

p

ar

ε−= = specific area for bed of uniform spheres (7)

This gives the flux of O2 and energy:

3

"2 212

b c ciO

p

u rN

x rρ

=∆

(8)

" "2rxn Oq H N= ∆ (9)

The CFX-TASCflow code is written in FORTAN. ANSYS programmers were consulted to convert the code to command line language in CFX.

Rate of Change of Local Core Radius = ARCLGRC

∂∂

= GDELXGOXF /− (10)

Local Core Volume = 3

34

GRCπ (11)

Rate of Change of Local Core Volume = GDELXGOXF

GRCARCLGRC

GRC 22 44 ππ −=∂∂

(12)

Rate of Change of Magnetite = Rate of Change of Volume * Magnetite Concentration * Bed Velocity

BVELGRHOCGDELXGOXF

GRC *4 2π−= (13)

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Heat Released = -Rate Change of Magnetite * Heat of Reaction * Core Volume Density. Heat of Reaction = GHRXN / 4(GHRXN is heat release for O2. Divide by 4 to get heat released per mole of magnetite)

Core Volume Density = Number of cores per unit volume = 3)2/(

34

1

PELDIA

BEDPOR

π

− (14)

Heat Released =

3

2

)2/(34

1*)4/(**4

PELDIA

BEDPORGHRXNBVELGRHOC

GDELXGOXF

GRCπ

π− (15)

32

)()1(6

***PELDIA

BEDPORGHRXNBVELGRHOC

GDELXGOXF

GRC−

= (16)

Heat Released is equivalent to GSOX * AUV in the CFX-TASCflow Fortran.

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Nomenclature

Variable Definition FORTRAN name

a Specific bed area, m2/m3 AUV

eD effective diffusivity of O2 in pellet, m2/s GDE

k O2 mass transfer coefficient, m/s GBK

3 4Fe OM Molecular weight of magnetite, 231.4 kg/kmol -

"q Heat flux from the reaction, W/m2 GSOX

cr unreacted pellet core radius, m GRC

cir initial unreacted pellet core radius, m GOXF

pr pellet radius, m PELDIA/2

"2ON Flux of O2 to bed, kmol/m2-s GSO2

bu bed velocity, m/s BVEL

3 4Fe OX Mass fraction of magnetite in green ball (dry basis)

x position along the cooler, m ARCL

x∆ length of cooler where oxidation occurs, m GDELX

rxnH∆ Heat of reaction, J/kmol O2 GHRXN

bε Bed porosity BEDPOR

pε Green pellet porosity (dry basis) -

pρ Density of green ball (dry basis), kg/m3 -

[ ]2 gO O2 concentration in the bulk gas, kmol/m3 GCO

[ ]3 4 cFe O Magnetite concentration in the core, kmol/m3 GRHOC

Additional Revisions: Once the basic CEL expressions were written and the TASCflow grids converted to a CFX11 compatible format, a lengthy process of verification began, as it was found that the new code initially predicted an excessive oxygen consumption rate when compared to a globally calculated value based on input boundary conditions. Ultimately, two additional revisions were incorporated into the code as follows: 1. The shrinking core magnetite mass in the pellet was adjusted for internal pellet void volume, which, in effect, decreased the mass of magnetite oxidizing in the model, bringing it in line with the expected mass found in the plant under similar conditions.

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2. Review of the original heat of reaction for magnetite as a function of temperature resulted in a revision, based on plots generated by HSC and FactSage Thermodynamic software. Figure 1 compares the original expression to those of HSC and FactSage. The validation runs used in the simulations in this report were based on an expression for the averaged value of HSC and FactSage. Future simulations will follow an expression based solely on the FactSage plot. The change in this expression effectively increased energy release in the pellet bed, which, in turn, yielded increased pellet and gas temperatures in the cooler outlet streams. In the original TASCflow code, temperature constraints were placed on the system to prevent excess temperatures from being generated. There are no constraints on temperatures generated in the CFX 11 version, which should result in more realistic predictions. However, the higher temperature predictions vs. actual measured values lead to more research to refine the prediction. One explanation may be that the amount of magnetite oxidizing in the cooler in the plant is over-estimated when setting up model boundary conditions. Another factor is that cooler and duct walls are treated as adiabatic (no heat loss), and, for future simulations, effect of heat loss through cooler walls should be incorporated into the simulations. This will tend to reduce temperatures in the cooler off gas streams. Discussion of Results: Code Evaluation: Two versions of the code incorporating the revisions previously discussed were tested; they are described as: 1. Simplified. The rate of oxygen uptake was treated as a linear function of the residence time in the cooler. The reaction was independent of the unreacted magnetite core radius. The simplified model assumes that the oxidation reaction goes to completion. The reaction rate is proportional to the velocity and initial core radius. 2. TASCflow Corrected is the preferred version and will be used in all future models. Corrected is used to signify corrections to the original TASCflow code: a. oxygen up-take b. new magnetite heat of reaction expression c. shrinking core mass adjustment for pellet void volume Table 1 presents a summary of comparisons between TASCflow simulation results (from 2004 study for United Taconite) and the two code versions described above. The simulations were chosen to test a range in cooling fan flow, production rate, and estimated magnetite entering the cooler. The original summary tables containing the TASCflow simulation results are given in Appendix I. Table 2 presents a comparison of oxygen concentration in the kiln gas stream for the TASCflow and CFX simulations. The values for TASCflow were calculated globally from the mass of magnetite oxidizing and primary cooler flow, as TASCflow was not set up to track oxygen concentration. The CFX values are extracted from the simulation results in two ways as follows: 1. A point value in the gas stream corresponding to the approximate oxygen sensor location in the plant cooler.

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2. The mass flow averaged (MF Ave.) concentration for the combined firing hood and parallel flow streams, which comprise the total kiln flow. Oxygen Consumption in Pellet Bed: The first step in verification of the CFX-11 model was to compare calculated oxygen consumed by the pellet bed with the value produced in the CFX-11 simulations. The calculated value was found from the known mass of magnetite oxidized in the cooler, which is calculated from the specified percentage of magnetite entering the cooler and the solids feed rate. The units for this comparison are kg/sec oxygen consumed by the bed, or depleted from the gas stream. Figure 2 plots the results as CFX value vs. calculated value; a line is plotted through the data points resulting from simulations with the Corrected TASCflow Code (CTC). A perfect correlation yields a slope of 1; in this figure, the slope of a trend line through the data points is 0.99. Also shown are the results from the Simplified Code (SC) which was not considered to be as accurate. The values are also tabulated in Table 1; a simple % error calculation based on (global calc - cfd pred)/global calc * 100 indicates most errors were within +/- 7%, with the exception of simulation UTAC 76 – CTC, which resulted in a 9% error. These levels were considered acceptable. Figure 3 shows a plot of predicted kiln flow oxygen concentration calculated from TASCflow simulations as described above, with values generated by CFX at the oxygen sensor location, and in the total stream flow to the kiln. The purpose of Figure 3 is to demonstrate the relationship between the oxygen sensor reading and the overall oxygen content entering the kiln. This relationship implies that a sensor located similarly in the plant should be capable of indicating oxygen concentrations in the cooler gases entering the kiln. A model could be developed that should relate the effects of operating variables on oxygen concentration, and then from this model one could derive the percentage of magnetite oxidizing in the cooler, assuming solids mass flows, temperatures, and cooling gas flows are measured reliably. Oxidation Heat Release in the Bed: Figure 4 plots heat of reaction (J/s) released in the pellet bed based on the thermodynamic heat of reaction at 298°K (= -485,110 J/mole O2) multiplied by the number of moles of oxygen reacting vs. the CFX simulations using both CTC and SC versions. The CTC version provides a more consistent correlation with the thermodynamic heat of reaction value. Cooling Zone Energy Balances: Figure 5 plots energy balances around both primary and secondary cooling zones for CFX vs. TASCflow simulations. The trend lines show reasonably good correlation. As expected, the slope for the primary cooling zone is greater than 1 (1.12) because there is more energy available from magnetite oxidation in the revised code. In the secondary cooling zone the slope is slightly less than one (0.92). Cooler Outlet Flows and Mass Flow Averaged Temperatures: Figures 6 and 7 graphically compare CFX and TASCflow results for firing hood flow and mass flow averaged temperature. Mass flow averaged temperature is based on mass flow weighted averaging of gas temperature at the outlet face of the opening. Error bars at the 6% level indicate that the CFX- generated flows fall within about 6% of the TASCflow values, whereas for temperature, error bars

- 8 -

are shown at the 4% level. The impact of greater energy release from the revised oxidation expression is clearly evident. Figures 8 and 9 show corresponding plots for the parallel flow duct flow. CFX flows are lower in the parallel flow duct relative to TASCflow, but still within 6%, while temperatures exceed those from TASCflow, but fall within the 4% range. Figures 10 and 11 make graphical comparisons between total mass flow and total energy flow, respectively, between CFX and TASCflow. The total flow to the kiln appears to balance within +/- 2%, while energy falls within 4%, being higher in the CFX simulations. Figures 12 - 14 plot similar comparisons for the recoup duct flow. Mass flows generated in CFX tend to fall within about 1% of the mass flows from TASCflow, while temperatures run hotter by as much as 10%. On a total energy flow basis, CFX tends to predict higher energy flow falling within 6% of the TASCflow value. The hotter recoup temperatures are most likely related to the higher temperatures generated in the bed, due to increased energy release from oxidation. Previous studies have shown that heat transfer within the bed can reach an upper limit, in which case no further cooling occurs until the bed location moves down stream, where colder gases can contact it. The CFX runs may be demonstrating this phenomenon, where the top of the bed and the gas stream are no longer exchanging energy, thus maintaining a hotter temperature in the bed as it enters the recoup portion of the cooler. Figures 15 - 17 complete the comparison around the cooler vent stack. Mass flows from CFX tend to be about 3% higher, with temperatures approximately 4% higher. Energy flow appears somewhat variable, which would be expected because of the upstream influences affecting cooling. The error bars show variability on the order of +/- 3% for the energy flow in the stack. Figures 18 - 20 are intended to show the full model assembly, consisting of cooler, parallel flow duct, firing hood, recoup duct, cooler vent stack, primary cooling fan duct and the secondary cooling fan duct, from various view points. Figures 21 - 23 show the location of wind box leakages in the primary and secondary cooling zones. Wind box leakages have been used to adjust leakage around the cooler. Primarily leakages have been located in the wind box, since this is where they are most likely to occur given the high pressures developed (15-20 inches H2O). Occasionally, leakages have been placed above the bed as well, but these are cases where plant personnel have specifically identified openings in the actual unit, usually the result of damaged seals. The purpose of leakages in CFD has been to "tune" the model outputs, to bring both flow and temperature into balance across the unit in all the streams. This is accomplished through a series of validation simulations with varied plant conditions. CFD simulations are made for each plant condition, and opening sizes are varied until a good fit is achieved for all of the plant tests. Once a good fit is achieved, the opening sizes are fixed, and a parametric study is performed to define cooler performance over a range of operating conditions. TASCflow simulations tended to require about 15 - 25% leakage of process fan flow out of the wind boxes to balance expected flows and temperatures. In part, the lower energy release from

- 9 -

oxidation in TASCflow would have influenced this problem. CFX, which generates a larger heat release per unit mass of magnetite, now results in higher temperatures under the same flow conditions. Thus, there now appears to be insufficient airflow to yield temperatures as measured in the plant. This opens the door for additional research related to tuning these models. There are several possible causes that warrant further investigation, as follows: 1. CFD cooler simulations are mass flow based, and often there is a lack of credible airflow data with regard to plant conditions being simulated. While every attempt is made to correctly estimate the fan flow, the data on which these estimates are based, such as air surveys, often have significant error due to turbulence and sampling point access. Thus, more reliable plant air flow measurements would improve the fit between prediction and operation. 2. Leakages will still play a role, and, with an accurate visual survey of the unit, most leakages can be readily identified and equivalent openings placed in the model grid. 3. The simulations to date have ignored heat transfer across exterior walls of ducts and cooler. It is possible to fix heat loss fluxes on these walls in the model. This would require a survey of surface temperatures and estimated heat fluxes in the plant. 4. The CFD simulations rely on an incoming pellet temperature and associated magnetite content at the kiln discharge. Temperature measurements are made, but accuracy of the measurement is subject to calibration, field of view, and location in the kiln. Magnetite content is usually unknown and, as a result, is estimated. The discharge stream can be sampled, but it is difficult to get representative samples. The uncertainty with these parameters means that CFD boundary conditions could be overestimating one or both of these variables. This could be easily tested over a number of simulations to determine the overall effect on cooler temperatures. Figures 24 - 26 show oxygen volume fractions at the pellet bed surface, three feet above the surface, and at six feet above the surface. Figure 27 shows oxygen mass fraction along a radial center line. An oxygen sensor should be placed along the roof between the firing hood and parallel flow outlets shown in this figure. Figures 28 and 29 compare pellet temperatures at the top of the bed for CFX and TASCflow results for the Utac 72 simulation, while Figures 30 and 31 compare pellet temperatures in the center of the bed for the same simulation. Two additional simulations were carried out using the baseline conditions of simulation Utac72; more would have been done, if time had permitted. One of these simulations eliminated all leakage from primary cooling; in other words, all fan flow goes through the bed, as opposed to a percentage leaking out of the wind box. In the other simulation, the pellet internal void volume was increased from 28% to 32%, which has the effect of decreasing incoming magnetite mass for a given feed rate. In Figures 32 - 36 the results are compared with the original TASCflow simulation and the CFX simulation with the same boundary conditions as the TASCflow run. The descriptions are given below.

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Utac 72-TASCflow --- Original simulation Utac 72-CFX --- Intended to duplicate the TASCflow run, with converted and corrected code. Utac 72-CFX-No Leak---Primary cooling zone leakage is eliminated to increase flow through bed. Utac 72-32% Void---Changed void volume parameter from 28% to 32%, primary cooling leakage is

active. The temperatures plotted are taken from Table 1 and represent mass flow averaged temperatures in in each stream. In Figure 32, the original TASCflow simulation yielded a firing hood temperature of 2342°F, which increased to 2462°F in the CFX version, decreasing to 2414°F if wind box leakage was eliminated, or to 2457°F if void volume were increased by 4% with leakage still active. In Figure 33, temperature in the parallel flow duct was originally predicted as 2387°F; conversion to CFX increased the temperature to 2574°F. Eliminating leakage limits the increase to 2482°F, and increased void volume limits the increase to 2560°F. The reader is left to review these trends for the recoup duct and the cooler vent stack in Figures 34 and Figure 35. In Figure 36, the maximum solids temperature occurring on the top of the bed along a radial centerline is presented; these numbers are also found in Table 3. In this case, the original TASCflow prediction was 2552°F, which compares with CFX values of 2618°F, 2586°F (no leakage) and 2604°F (increased void volume). It is recognized that absolute values for temperature should not necessarily be compared with plant measurements, because of the large level of uncertainty surrounding measurement accuracy, instrument calibration issues, errors in tonnage and chemistry for any given test condition, as well as errors in gas flow. Ultimately, what is desired is that trends are consistently predicted correctly between model and plant. More simulations are required to complete this analysis of the new code, and to develop new relationships between solids feed rate, magnetite content, system heat losses, and airflow. These relationships will be developed in a future Iron Ore Coop program to be completed by 2009.

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Table 1: 1 of 2CFX 11 Conversion and Comparison with Selected Tascflow Simulations

Primary SecondaryPellet heat of Reaction Cooling Zone Cooling Zone

Calc O2 CFD Energy J Net Energy Net Energy Firing Hoodconsumed O2 Reaction Calc J CFX in Gas streams in Gas streams Mass Flow MFAve Temp

Simulation kg/s kg/s % Error at 298K Pellet Source % Error J lbs/min Deg FTascflow Utac72 File description NA NA NA NA -9.64E+07 -7.30E+07 2,593 2342

Simplified cfx11_newcode_013 1.1239 1.1187 0.46 17,047,979 16,877,000 1.00 -9.56E+07 -7.86E+07 2,716 2401Corrected Tascflow cfx11corrected hxrxn 005 1.1239 1.2013 -6.89 17,047,979 18,096,000 -6.15 -9.69E+07 -7.99E+07 2,677 2462Corrected Tascflow cfx11corrected hxrxn noleakage 002 1.1239 1.1918 -6.04 17,047,979 17,954,000 -5.31 -1.15E+08 -6.13E+07 3,320 2414Corrected Tascflow cfx11_32_001 1.1239 1.1396 -1.40 17,047,979 17,164,000 -0.68 -9.68E+07 -7.91E+07 2,681 2457

Tascflow Utac 74 NA NA NA NA -1.19E+08 -6.91E+07 3,441 2,306Simplified cfx11_newcode_74_001 1.3045 1.2177 6.66 19,788,097 18,390,000 7.07 -1.23E+08 -7.20E+07 3,706 2,364

Corrected Tascflow cfx11corrected hxrxn 006 1.3045 1.3646 -4.61 19,788,097 20,564,000 -3.92 -1.26E+08 -7.26E+07 3,685 2,424

Tascflow Utac 76 NA NA NA NA -8.22E+07 -7.39E+07 2,301 2,332Simplified cfx11_newcode_76_001 0.7781 0.8058 -3.57 11,822,884 12,152,000 -2.78 -8.47E+07 -7.89E+07 2,488 2,382

Corrected Tascflow cfx11_corrhxrxn_76_001 0.7781 0.8485 -9.05 11,822,884 12,776,000 -8.06 -8.56E+07 -7.98E+07 2,466 2,429

Tascflow Utac 67 NA NA NA NA -1.26E+08 -7.22E+07 3,564 2,356Corrected Tascflow cfx11_corrhxrxn 67_006 0.9031 0.9609 -6.39 13,723,174 14,473,000 -5.46 -1.33E+08 -7.25E+07 3,794 2,464

Tascflow Utac 71 NA NA NA NA -1.05E+08 -7.56E+07 2,923 2,364Corrected Tascflow cfx11_corrhxrxn71_004 1.0114 1.0800 -6.78 15,369,003 16,268,000 -5.85 -1.10E+08 -7.67E+07 3,119 2,478

Table 1: 2 of 2CFX 11 Conversion and Comparison with Selected Tascflow Simulations

MaximumKiln Sec Air Total Kiln Flow Total Kiln Flow Recoup Outlet Total Recoup Stack Outlet Stack Pellet Bed

Mass Flow MFAve Temp Mass Flow energy Mass Flow MFAve Temp Energy Mass Flow MFAve Temp Energy TempSimulation lbs/min Deg F lbs/min J/s lbs/min Deg F J/s lbs/min Deg F J/s Deg F

Tascflow Utac72 6,921 2387 9514 115,160,000 7,100 1586 61,720,000 13,495 478 53780000 2552Simplified 6,518 2496 9234 115,062,000 7,006 1842 68,503,000 13,991 434 53152000 2546

Corrected Tascflow 6,428 2574 9105 116,313,000 7,028 1866 69,417,000 14,054 438 53587000 2618Corrected Tascflow 8,073 2482 11393 141,689,000 8,254 1327 62,657,000 14,008 244 41914000 2586Corrected Tascflow 6,449 2560 9130 116,201,000 7,012 1849 68,792,000 14,058 434 53410000 2604

Tascflow Utac 74 9,248 2,309 12689 149,240,000 6,174 1,508 51,615,000 7,942 740 40,490,000 2,529Simplified 8,979 2,411 12684 154,212,000 6,042 1,688 55,121,000 8,340 693 40,846,000 2,477

Corrected Tascflow 8,888 2,498 12573 157,145,000 6,076 1,699 55,705,000 8,359 693 40,928,000 2,593

Tascflow Utac 76 6,147 2,379 8447 101,440,000 4,951 1,857 48,738,000 7,925 960 47,830,000 2,487Simplified 5,993 2,459 8481 104,531,000 4,770 2,115 52,186,000 8,363 963 50,574,000 2,506

Corrected Tascflow 5,947 2,503 8413 105,296,000 4,797 2,131 52,783,000 8,375 967 50,797,000 2532

Tascflow Utac 67 9,634 2,315 13198 156,250,000 8,756 1,288 65,010,000 13,693 395 49,720,000 2566Corrected Tascflow 9,312 2,484 13106 163,762,000 8,765 1,393 69,000,000 13,955 327 46,675,000 2600

Tascflow Utac 71 7,847 2,387 10769 129,990,000 6,717 1,591 58,527,000 10,970 608 49,930,000 2559Corrected Tascflow 7,551 2,552 10670 135,726,000 6,635 1,769 62,791,000 11,316 529 47,528,000 2619

- 12 -

Table 2Tascflow Predicted Oxygen Concentration vs CFX Solution

Tascflow CFX CFX Calculated Sensor Location MF Ave Vol % O2

Simulation Vol % O2 Vol% O2 to KilnUtac 67 20.30 19.89 20.22Utac 71 20.02 19.51 19.97Utac 72 19.79 19.06 19.66Utac 74 19.95 19.38 19.9Utac 76 20.06 19.59 19.97

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Figure 1Heat of Reaction for Magnetite Oxidation

-520000

-500000

-480000

-460000

-440000

-420000

-400000

-380000

275 375 475 575 675 775 875 975 1075 1175 1275 1375 1475 1575 1675

Temperature, K

En

erg

y, J

/mo

le O

2

Factsage CFX Reaction Equation HSC Averaged (HSC & FSage low)

Averaged (HSC & Fsage high) JMP Low Temp Fit JMP4 High Temp Fit

Figure 2Oxygen Consumption in Cooler Pellet Bed

y = 0.9961x + 0.0709R2 = 0.9984

0.7000

0.8000

0.9000

1.0000

1.1000

1.2000

1.3000

1.4000

1.5000

0.7000 0.8000 0.9000 1.0000 1.1000 1.2000 1.3000 1.4000

Global Calculated O2 Reacting, kg/s

CF

X G

ener

ated

O2

Rea

ctin

g, k

g/s

CFX Simplified CFX Corr. Tascflow Linear (CFX Corr. Tascflow)

- 14 -

Figure 4Magnetite Oxidation Heat of Reaction J/s

y = 0.9939x + 979649R2 = 0.9984

10.000

12.000

14.000

16.000

18.000

20.000

22.000

10.000 12.000 14.000 16.000 18.000 20.000 22.000

Mill

ion

s

Millions

Global Calculated Energy, J/s

CF

X G

ener

ated

En

erg

y, J

/s

CFX Simplified CFX Corr. Tascflow Linear (CFX Corr. Tascflow)

Figure 3Tascflow Calculated Oxygen to Kiln vs CFX Model Predictions

y = 1.5989x - 12.531R2 = 0.9801

y = 1.0541x - 1.1628R2 = 0.9771

19

19.2

19.4

19.6

19.8

20

20.2

20.4

19.70 19.80 19.90 20.00 20.10 20.20 20.30 20.40

Tascflow Vol % Oxygen in Total Kiln Flow

CF

X O

xyg

en C

on

cen

trat

ion

Pre

dic

tion

, Vo

l % O

2

CFX Sensor Location CFX MF Ave to Kiln Linear (CFX Sensor Location) Linear (CFX MF Ave to Kiln)

- 15 -

Figure 5Energy Balance around Cooling Zones

y = 1.1186x + 8E+06

R2 = 0.992

y = 0.9294x - 9E+06

R2 = 0.3813

-140

-130

-120

-110

-100

-90

-80

-70

-60-130 -120 -110 -100 -90 -80 -70 -60

Mill

ion

s

Millions

Tascflow Energy, J/s

CFX

Ene

rgy

Flow

, J/s

CFX Simplified Pri. Cooling CFX Corr. Tascflow Pri Cooling

CFX Simplified Sec Cooling CFX Corr. Tascflow Sec Cooling

Linear (CFX Corr. Tascflow Pri Cooling) Linear (CFX Corr. Tascflow Sec Cooling)

Figure 6Firing Hood Mass Flow

2,200

2,400

2,600

2,800

3,000

3,200

3,400

3,600

3,800

Utac 67 Utac 71 Utac 72 Utac 74 Utac 76

Test #

Mas

s F

low

lbs/

min

Tascflow CFX Simplified CFX Corr Tascflow

Error Bars = +6%

- 16 -

Figure 8Kiln Secondary Air Flow

5,500

6,000

6,500

7,000

7,500

8,000

8,500

9,000

9,500

10,000


Test #

Mas

s F

low

lbs/

min


Error Bars = -6%

Figure 7Firing Hood MF Ave Temperature

2,300

2,325

2,350

2,375

2,400

2,425

2,450

2,475

2,500


Test #

MF

Ave

rag

ed T

emp

. F


Error Bars = +4%

- 17 -

Figure 10Total Kiln Flow

8,000

8,500

9,000

9,500

10,000

10,500

11,000

11,500

12,000

12,500

13,000

13,500


Test #

Mas

s F

low

lbs/

min


Error Bars = +/- 2%

Figure 9Kiln Sec Air MF Ave Temp

2,300

2,350

2,400

2,450

2,500

2,550

2,600


Test #

MF

Ave

rag

ed T

emp

. F


Error Bars = +4%

- 18 -

Figure 12Recoup Total Mass Flow

4,600

5,100

5,600

6,100

6,600

7,100

7,600

8,100

8,600


Test #

Mas

s F

low

lbs/

min


Error Bars = +/- 1%

Figure 11Total Energy Flow to Kiln

90

100

110

120

130

140

150

160

170


Mill

ion

s

Test #

J/s


Error Bars = +4%

- 19 -

Figure 14Total Energy Flow to Recoup

45

50

55

60

65

70


Mill

ion

s

Test #

J/s


Error Bars = +6%

Figure 13Recoup MF Ave Temp

1,200

1,300

1,400

1,500

1,600

1,700

1,800

1,900

2,000

2,100

2,200


Test #

MF

Ave

rag

ed T

emp

. F


Error Bars = +10%

- 20 -

Figure 15Stack Total Mass Flow

7,600

8,100

8,600

9,100

9,600

10,100

10,600

11,100

11,600

12,100

12,600

13,100

13,600

14,100

14,600


Test #

Mas

s F

low

lbs/

min


Error Bars = +3%

Figure 16Stack MF Ave Temp

300

350

400

450

500

550

600

650

700

750

800

850

900

950

1,000

1,050


Test #

MF

Ave

rag

ed T

emp

. F


Error Bars = +4%

- 21 -

Figure 17Total Energy Flow to Stack

40.0

42.5

45.0

47.5

50.0

52.5

55.0


Mill

ion

s

Test #

J/s


Error Bars = +3%

- 22 -

Figure 18. United Taconite Line 2 Cooler Model Assembly

- 23 -


- 24 -


- 25 -

Figure 21. United Taconite Line 2 Cooler - Primary Cooling Wind Box Leakage Openings

- 26 -

Figure 22. United Taconite Line 2 Cooler - Secondary Cooling Wind Box Leakage Openings

- 27 -

Figure 23. United Taconite Line 2 Cooler - Primary Cooling Wind Box Leakage Openings

- 28 -

Figure 24. Oxygen Mass Fraction in Air at Pellet Bed Surface

- 29 -

Figure 25. Oxygen Mass Fraction in Air Three Feet above Pellet Surface

- 30 -

Figure 26. Oxygen Mass Fraction in Air Six Feet above Pellet Surface

- 31 -

Figure 27. Oxygen Mass Fraction in Along Radial Center Line - Pellet Bed in Blue

- 32 -

Figure 28. CFX - Solids Temperature at Top of Pellet Bed

- 33 -

Figure 29. TASCflow - Solids Temperature at Top of Pellet Bed

- 34 -

Figure 30. CFX - Solids Temperature at Center of Pellet Bed

- 35 -

Figure 31. TASCflow - Solids Temperature at Center of Pellet Bed

- 36 -

Figure 32UTAC Run 72 - Firing Hood Temperatue Comparison

2300

2320

2340

2360

2380

2400

2420

2440

2460

2480

Utac72-TASCflow Utac72-CFX Utac72-CFX-NoLeak

Utac72-CFX-32%Void

Test Description

Gas

Tem

per

atu

re, F

Figure 33UTAC Run 72 - Parallel Flow Duct Temperatue

Comparison

2350

2400

2450

2500

2550

2600


Utac72-CFX-32%Void

Test Description

Gas

Tem

pera

ture

, F

- 37 -

Figure 34UTAC Run 72 - Recoup Duct Temperatue Comparison

1300

1350

1400

1450

1500

1550

1600

1650

1700

1750

1800

1850

1900


Utac72-CFX-32%Void

Test Description

Gas

Tem

pera

ture

, F

Figure 35 UTAC Run 72 - Vent StackTemperatue Comparison

200

250

300

350

400

450

500


Utac72-CFX-32%Void

Test Description

Gas

Tem

pera

ture

, F

- 38 -

Figure 36

UTAC Run 72 - Maximum Solids Temperatue Comparison

2500

2520

2540

2560

2580

2600

2620

2640


Utac72-CFX-32%Void

Test Description

Gas

Tem

pera

ture

, F

- 39 -

APPENDIX 1

Oxygen Sensor Validation and Analysis of Process Air...

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