Modeling of Rotary Kiln for Sponge Iron Processing Using CFD package (ANSYS 13.0) A Thesis Submitted for Partial Fulfillment of the Degree Award Of MASTER OF TECHNOLOGY In CHEMICAL ENGINEERING By Tapash Ranjan Majhi (210CH1204) Under the supervision of Dr. Shabina Khanam DEPARTMENT OF CHEMICAL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA-769008, INDIA 2012
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Modeling of Rotary Kiln for Sponge Iron Processing
Using CFD package (ANSYS 13.0)
A Thesis Submitted for Partial Fulfillment of the Degree Award
Of
MASTER OF TECHNOLOGY
In
CHEMICAL ENGINEERING
By
Tapash Ranjan Majhi
(210CH1204)
Under the supervision of
Dr. Shabina Khanam
DEPARTMENT OF CHEMICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY,
ROURKELA-769008, INDIA
2012
CERTIFICATE
This is to certify that the thesis entitled, “Modeling of Rotary Kiln for Sponge Iron
Processing Using CFD package (ANSYS 13.0)” submitted by Tapash Ranjan Majhi in
partial fulfilment of the requirements for the award of Master in Technology Degree in
Chemical Engineering with specialization in “Chemical Engineering” at the National
Institute of Technology, Rourkela (Deemed University) is an authentic work carried out by
him under my supervision and guidance. To the best of my knowledge, the matter embodied
in the thesis has not been submitted to any other University / Institute for the award of any
Degree or Diploma.
_____________________
Date: Signature of the Supervisor
Dr. Shabina Khanam
Assistant Professor
Department of Chemical Engineering
National Institute of Technology Rourkela
Rourkela-769 008, Orissa
ACKNOWLEDGEMENTS
I feel immense pleasure and privilege to express my deep sense of gratitude and feel
indebted towards all those people who have helped, inspired and encouraged me during the
preparation of this thesis.
First and the foremost, I would like to offer my sincere thanks and gratitude to my
thesis supervisor, Dr. Shabina Khanam for hER immense interest, enthusiasm, guidance
and assistance in this thesis work.
I owe a depth of gratitude to Prof. R.K. Singh, H.O.D, Department of Chemical
engineering for all the facilities provided during the course of my tenure. I would also like to
thank all the professors of Chemical engineering department for their constant support
throughout my project work. I want to acknowledge the support from non-teaching staff.
I want to acknowledge the support from all the friends of Chemical Engineering
department and other departments of NIT, Rourkela.
Last but not the least I want to owe a deep sense of thankfulness to my parents and
family members, for their support, encouragement and good wishes without those I would not
have been able to complete my thesis.
NAME: TapashRanjanMajhi
M. Tech (Chemical Engg. )
ROLL No.: 210CH1204
CONTENTS
CHAPTER
NO. CHAPTER NAME PAGE NO.
1 INTRODUCTION 1
2 LITERATURE REVIEW 7
3 SPONGE IRON PROCESSING 10
4 ROTARY KILN DESIGN & OPERATION 23
5 COMPUTATIONAL FLUID DYNAMICS (CFD) & ITS
APPLICATION
31
6 MODEL DEVELOPMENT 37
7 RESULT & DISCUSSION 43
8 CONCLUSION 54
9 REFERENCE 56
LIST OF FIGURES
FIGURE NO. FIGURE CAPTION PAGE NO.
1.1 Image of a Sponge Iron Granule 2
1.2 Microscopic view of sponge iron 4
3.1 Material Balance in a Rotary Kiln sponge iron plant 14
3.2 Energy Balance in a Rotary Kiln sponge based iron making 14
3.3 Schematic of optimised Rotary kiln sponge iron making process 21
4.1 Industrial view of Sponge Iron making Rotary Kiln 24
4.2 Schematic view of internal heat exchange in Rotary kiln 26
4.3 Schematic view of Lobe Blower. 27
4.4 Schematic view of Air & material flow in Rotary kiln 27
4.5 Filling of Bed Geometry 28
6.1 Schematic Drawing of discharge end of Kiln 38
6.2 2D & 3D model of Sponge iron Rotary Kiln 39
6.3 Faces of lobe blower. 39
6.4 Coal throw and solid out face at discharge end of kiln. 40
6.5 Cuts made to represent air pipes. 40
6.6 Wall of air pipe. 41
6.7 Face of air pipe. 41
6.8 Overall view and internal view of generated Mesh. 42
7.1 Three Dimensional PDF Table 44
7.2 Temperature profile at various after start up of flow ratio 9:1 & 8:2 46-50
7.3 Volume fraction of Phase-1(Air) 51
7.4 Volume fraction of Phase-2(Solid Bed material) 51
7.5 Velocity magnitude of Phase-2 with respect to Y-coordinate 52
7.6 Velocity magnitude of Phase-2 53
NOMENCLATURE
ρ → Density
ε → Surface emissivity
→ Gradient
σ → Stefan-Boltzmann constant
Φ → Flux at a boundary face
A → Surface area
Df → Diffusivity of flux
es → Surface emissivity.
E → Activation energy
f → Elemental mass fraction
h → Heat transfer co-efficient
H→ Enthalpy
→ Effective thermal conductivity.
T →Temperature
P → Pressure
Sh → Source term
qConduction → Heat transferred through conduction.
q → Amount of heat transferred.
→ Velocity vector
1
CHAPTER 01
INTRODUCTION
2
Sponge iron, also called Direct-reduced iron (DRI), is formed, when naturally available
Iron Ore which is an oxidised form of Iron (magnetite (Fe3O4) or hematite (Fe2O3)) is reduced to
its metallic form. This reduction
process occurs below the melting
temperature of both metallic iron
and its oxidised form. Though this
process is carried out at lower
temperature than melting point so
there is less volume reduction but a
large amount material get eliminated
during reduction reaction. Oxygen
removal form iron ore creates lots of
microscopic pores. This microscopic
pores gives the iron an sponge
texture as shown in fig. 1.1.
Therefore it is in another sense
known as sponge iron..
Fig 1.1. Image of a Sponge Iron Granule
STATUS OF STEEL PRODUCTION (in MT)
India China
Year 1952 1.5 1.5
Year 2005-06 43 340
Now in India nearly 283 DRI units have been operating over the states of Orissa, Jharkhand,
Chhattisgarh, West Bengal, Karnataka, Tamil Nadu, Andhra Pradesh, Gujarat and Goa.
3
In the year 2006 - India produced - 13.9 million ton
Venezuela - 6.2 million ton
Iran - 4.3 million ton
Mexico - 4.5 million ton
As per the National Steel Policy issued by the Ministry of Steel – India will produce 110
million tons of steel by 2020. The requirement of Sponge Iron as metallic will be 30 million tons.
Projection for metallic requirement in Year 2010-11 we require Melting Scrap 14 million
and DRI 18 million But availability of scrap is not likely to reach 11 million. So there is a huge
requirement of sponge iron production.
Today India produce 13.9 million tons of sponge iron, out of which 4.2 million ton is gas
based and remaining 9.7 million ton is coal based. India has a proven reserve of 410 million ton
of high grade iron ore, another 440 million ton of high grade iron ore which will be established.
India has total 9992 million ton of iron ore reserves [1]. India has sufficient non-coking coal
through of high ash low fixed carbon grade. Coal is used as a reducent for sponge iron making in
the furnace. The availability of scrap of required quantum is unlikely and therefore scraps needs
to be replaced more and more by DRI. Local supply of scrap is diminishing as generation of
scrap in India due to improvement of technology is getting continuously minimized. As per
World Steel Dynamics (WSD) – the Global shortage of scrap will reach 68 million tons in the
year 2010. That means the scrap price will go up and availability will be a problem. Due to
soaring price of iron ore and coke, blast furnace is being set up in the countries where iron ore or
coking coal is available. We must produce steel at a cheaper cost to remain competitive and
control over domestic market. DRI based steel making is therefore the only answer.
Sponge iron, also called Direct-reduced iron (DRI),[2] is produced from direct reduction
of iron ore (in the form of lumps, pellets or fines) by a reducing gas produced from natural gas or
coal. The reducing gas is a mixture majority of hydrogen (H2) and carbon monoxide (CO) which
acts as reducing agent. This process of directly reducing the iron ore in solid form by reducing
gases is called direct reduction. The porous structure of sponge iron clearly visible under optical
microscope as shown in fig. 1.2.
4
Fig1.2. Microscopic view of sponge iron[3]
Direct-reduced iron is richer in iron than pig iron, typically 90–94% total iron (depending
on the quality of the raw ore) [4] as opposed to about 93% for molten pig iron. Due to its high
purity it is an excellent feedstock for the electric furnaces used by mini mills. It allows them to
use lower grades of scrap for the rest of the charge or to produce higher grades of steel. Due to
following advantages sponge iron production is rising.
Pelletized iron ore or natural "lump" ore are used in direct reduction process.
Exceptionally in the fluidized bed process sized iron ore particles are used. Few selected
ores are suitable for direct reduction process.
Sponge iron is produced in a powdered form so, it acts as a good raw material that can
very well mixed with other metals in the production of different types of iron-based or
Ferro alleys.
HDRI (Hot Direct Reduced Iron) is iron not cooled before discharge from the reduction
furnace and immediately transported to a waiting electric arc furnace to be charged and
thereby saving energy.
Natural gas combined with little inert gases can be used in direct reduction process to
avoid the need of removal of these gases for other use. Presence of any inert gas along
the reducing gas lowers the effect (quality) of that gas stream and also the thermal
efficiency of the process.
5
Another most common uses for sponge iron is manufacturing of wrought iron. Iron of
this type is useful in the creation of ornamental objects for use around the house, like
decorative grills for screen doors, burglar bars for windows.
Sponge Iron which is produced in power form can be made into pellets, which is an
economic and useful substitute for the scrap metal sometimes used by steel
manufacturers. The amount of time and resources required to produce sponge iron is
minimal, so it is possible to manufacture large amounts quickly, a fact that only adds to
the advantages of this type of iron product.
Direct reduction, is most commonly practiced alternative route of iron making, has been
developed to overcome some of these difficulties of conventional blast furnaces. DRI is
successfully manufactured in various parts of the world through either natural gas or coal-based
technology. Iron ore is reduced in solid state at 800 to 1,050 °C (1,472 to 1,922 °F) either by
reducing gas (H2+CO) or coal. The specific investment and operating costs of direct reduction
plants are low compared to integrated steel plants and are more suitable for many developing
countries where supplies of coking coal are limited.
The direct reduction process is intrinsically more energy efficient than the blast furnace
because it operates at a lower temperature, and there are several other factors which make it
economical. Iron metal and its oxides have melting points close to each other and these are more
than 15000C. There are tendency for formation of formation of clusters , agglomerates and
accretion or ring formation during actual manufacture of sponge iron in the temperature range of
9000C to 1100
0C. Ring formation is a phenomenon occurring exclusively in a rotary kiln, while
clusters and agglomerates are common in both rotary kiln and shaft based processes. In the
presence of context we cannot conceive of a reactor to manufacture sponge iron, which operate
above 11000C, even though there have been prolonged attempts to produce sponge iron and
semi-fused iron at higher temperature. It is very easy to reduce the higher oxides of iron to FeO
stage. It is only needed to meet the heat demand. The reductant and the temperature level. The
reductant requirement and the temperature level. The key step in all DR processes is the
reduction of FeO to metallic iron form. If an iron oxide pellet or lump is exposed to reducing
gases like CO and H2 at suitable temperature, it gives rise to sponge iron. In other way if we
cover iron oxide lump with carbon or charcoal and heat in muffle furnace, it will give same
6
result. Physical contact with carbon should cause reduction, but the interior of iron oxide lump or
pellet also get reduced, which is not in contact with carbon. Further if we try to carry out the
same exercise under progressively reduced pressures, the rate reduction progressively reduces. It
is obvious, therefore, that reduction is effected by reducing gases, even if we keep iron oxide and
coal in contact. For facilitating quicker regeneration of these reducing gases from coal an
inclined rotary furnace is handy and convenient.
When an iron oxide pellet or lump or iron ore are charged into kiln along with coal,
reduction occurs in layers. A pellet of satisfactory metallization would not have any core Fe2O3
left in it, while the parallel reduced layer would be restricted to very limited area in core.
Iron ore of high grade is available in Orissa / Jharkhand and availability is not a problem.
Only the high cost of iron ore is causing a dent in the economics of scale of sponge iron industry.
the market requirement and the huge production practice there is great scope of development in
sponge iron making process. Computational fluid Dynamics is one of the emerging technology
used for estimation of various process. In this project some sections of sponge iron making
process were simulated. Some of the objectives of this project are
Developing models for the equipment engaged with sponge iron making process with
Computational fluid Dynamics package ANSYS 13.0.
Developing a virtual process environment by fixing parameters similar to the industrial
process.
Comparing the simulated outcome with industrial values.
Studding the behavior of process and its consequences towards changes in process
parameter.
Studding environmental effect of the process.
7
CHAPTER 02
LITERATURE REVIEW
8
In Sponge Iron industry Rotary kiln is the key equipment used to reduce iron ore to
sponge iron form. For design and optimization of rotary kiln, it is necessary to understand the
detailed processes that take place in the kiln. It is possible to get more insight, such as the
distributions of gas-solid flow, temperature, and composition of gases and particles within a
rotary kiln through mathematical modelling. However, only few expressions have existed so far
for the processes in a cement rotary kiln to model the fuel combustion, heat transfer, and
reduction chemistry. This is owing to the complexity of heat transfer that takes place
simultaneously along with chemical and mineralogical reactions. Moreover, the onsite
measurements for the detailed physical parameters are complicated and are not possible in many
cases.
CFD modelling of such a system proves to be beneficial to understand the fluid flow,
coal combustion and heat transfer phenomena in rotary kilns, and to improve the efficiency of
these units. A steady-state heat transfer model for drying and preheating of wet solids with
application to one reacting zone of a cement rotary kiln.[5] In the process conversion ratio of
material is linearly dependent on the final temperature of reactor. For a good conversion ratio the
temperature must be maintained between the minimum temperature and the limiting
temperature.[6,7] CFD predictions for cement rotary kilns including flame modelling, heat
transfer, and clinker chemistry were made by Mastorakos et al.,[8,9] in which a comprehensive
model for most of the processes occurring in a sponge iron rotary kiln was presented. The results
showed potential improvements in the models but only the temperature distribution was given,
the gas composition distribution has not been predicted. A heat flux function to take into account
the thermal effect of clinker formation. Combining the models of gas-solid flow, heat and mass
transfer, and pulverized coal combustion, a set of mathematical models for a full-scale cement
rotary kiln were established. In terms of CFD model, gas velocity, gas temperature, and gas
components in a cement rotary kiln were obtained by numerical simulation.[10]
Rotary kilns are complex systems that involve occurrence of several simultaneous
processes in both the bed and freeboard regions. It is thus essential to first identify key issues and
use appropriate methodology to develop tractable computational models for rotary kilns. Key
9
issues which need to be considered while developing a comprehensive model has been
developed for cement kilns.[11]
Due to the complexity of the physics involved, and the occurrence of multiple phases
with a large number of reactions in the bed/freeboard regions, very few CFD models have been
published for rotary kilns. Most of these computational models do not account for the main key
issues simultaneously in a single framework.[12] The bed and freeboard models were thus
treated as separate domains, and coupling between them is handled explicitly. The geometry of
the kiln was assumed to be axisymmetric in this work, and therefore, the boundary conditions
were applied only in an approximate manner. Moreover, this work assumed a formation of
coating throughout the kiln length. Karki et al . developed a 3D CFD-based model for simulating
simultaneous combustion and heat transfer in cement kilns. [13] They have used an effective
thermal conductivity to define degree of mixing in the bed region, developing a single
computational model for simulating cement kilns. Different values of effective thermal
conductivities at different locations in the kiln were used. However, there are no proper
guidelines to choose proper effective thermal conductivity, and the values used are based on
experience. It is also important to note that along with physical issues that need to be captured,
there are numerical issues involved in cement kiln modelling. The freeboard region of the kiln in
which combustion of coal takes place and the bed region of the kiln where reduction reactions
take place are strongly coupled with each other. However, the characteristic time-and-space
scales of the freeboard and bed regions are significantly different.[14]
10
CHAPTER 03
SPONGE IRON
PROCESSING
11
Sponge iron production is being practiced from ancient time. Due to complexity in
practice and lack of modern technology it was not so developed. Latter this was replaced by high
productive modern Direct Reduction process in the mid of 19th
century. Sponge iron is mainly
produced from Iron ore in two methods
a. Using reducing gases like CO and H2 in a shaft furnace.
b. Direct reduction by treating with coal as reductant.
In India companies adopt different technologies to reduce iron ore (Fe2O3) into sponge
iron(Fe) form. Some most commonly practiced technologies are Midrex, HyL III, SL/RN,