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Showing posts with label Refractory lining installation. Show all posts
Showing posts with label Refractory lining installation. Show all posts

Criteria for Furnace or Kiln Design and Selection of Refractories

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23-March-2010

The primary function of any refractory material is to withstand high temperature in a hostile environment. However, in actual application a refractory lining is required to perform many other functions depending upon the place of use and prevailing service conditions which in turn help us to decide what should be the outline or design of furnace, its parts followed by the selection of proper refractories. 

First let us see the various general requirements which a refractory material is to fulfill :
1. Ability to withstand high temperature.
2. Ability to withstand temperature fluctuation.
3. Ability to withstand the actions of processing materials and product of combustion.
4. Ability to withstand load under high temperature.
5. Ability to withstand impact and abrasion of solid, liquid and dust laden gases moving with high speed.
6. The refractory material should be volume stable.
7. It should not contaminate the finished product.
8. The refractory material should have low co-efficient of thermal expansion.
9. It should not conduct much heat.
For a proper design of any refractory lining system it is essential that the complete information of furnace or kiln type and prevailing service conditions are available.
The most important operational data required for the selection of refractories are as follows:
Furnace / Kiln Type :  For which industry the furnace or the kiln is to be used.
Process : Details of process to be adopted. Will the refractory material come in direct contact with slag, metal, dust, fluxing agent, gas or flame? Which part of the furnace or kiln will be subjected to the destructive actions of the above elements, etc.
Fuel  :  Type of fuel to be used for generation of heat energy. How the furnace will be heated.
Operation :  How the furnace (kiln) will be operated: continuous or intermittent. What is the extent of temperature fluctuation and over what period of time. To what extent the refractories will be exposed to thermal shock.
Operation - Temperature :  What will be the highest temperature to which refractories will be exposed. What will be the peaks.
Limiting - Temperature:  What are the maximum and minimum temperatures of the furnace or kiln design components e.g. steel shell temperature etc.
Heat Loss  :  What heat loss will take place? Is the heat to be conducted through refractories or retained within the furnace?
Surrounding Conditions   :  What are the surrounding conditions such as heat flux calculations, influence of any adjacent plant or component, maximum and minimum ambient temperatures, wind speed, radiation co-efficient etc.
Furnace Atmosphere :  Is it neutral, oxidizing, reducing or changing?
Furnace Pressure :  What operation pressure is expected? Is the furnace part under suction or under positive pressure.
In actual situation the refractories may have to work under some or all of the above conditions. They may act simultaneously and demand suitable refractories to withstand the destructive forces. No single refractory material can satisfy the entire requirement. Hence, a compromise is made and the most demanding requirements are first met at the cost of other lesser requirements. For example, in a hot air or gas carrying system the thermal conductivity would be the vital criteria. Therefore from every saving point of view insulating properties of the refractory material becomes more important than other properties for design considerations.
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Insulating Refractories (Part - I)

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18-March-2010

Insulating refractories are thermal barriers that keep in the heat and save energy. 
Furnaces used for melting, heat treatment, heat regeneration or for any other purpose demand maximum heat conservation so as to minimize heat losses for maximum heat efficiencies and minimum fuel consumption as well as high production as a result of maintaining high working temperatures. As the cost of energy has increased, the role of insulating refractories has become more important. Not too long ago, energy costs were low and stable, while the costs of insulating materials and, particularly, installation labour were moving northwards. Those circumstances dictated the use of minimal insulation. The situation is quite different now. The use of considerable quantity of refractories is socially and economically justified. With today’s energy costs at such higher levels has come the development of a wide range of new insulating refractory materials and technology of high-temperature insulation which are capable to restrict the escape of heat even at a much elevated temperature. Instead of going direct into the discussion of insulating refractories, their types, raw materials, manufacturing, properties and applications etc., here we will first review some of the fundamental technology of high-temperature insulation.     
The function of insulating refractory is to reduce the rate of heat flow (heat loss). Although it is not possible to totally prevent the flow of heat energy when there exists a temperature differential between two points, but it can be retarded. There are three mechanisms of heat transfer that we must understand. These are conduction, convection, and radiation. We must consider all these three mechanisms when we study the overall conductivity of a given material.
Heat transfer by Conduction occurs via the transfer of energy from atom to atom (or molecule to molecule) in a material. Atoms vibrate faster in higher temperature as they possess more energy. This energy will be passed to the adjacent atoms having lower energy. Since atoms and solids are bonded to one another and are in close contact, conduction in solids is higher than in liquids. Metals, especially, have high rates of conduction because both the atoms and their electrons conduct the electrons much more rapidly. Liquids generally have lower conduction rates than solids because of their lack of regular structure and strong bonding. Gases have much lower rates of conduction since their molecules exist at much lower concentrations and are in relatively infrequent contact. So, within metals, dense ceramics, and dense refractories Conductivity is the main mechanism of heat transfer.   
Energy transfer by Convection relies on the mass movement of a fluid. The moving fluid may be either a liquid or a gas. Convection does occur horizontally; but it depends on the gravitational force of the earth. Again, in case of dense refractory bricks heat transfer through this process can not happen since there is no fluid for convection.
Radiation process of heat transfer does not require the presence of any material. Radiation occurs most readily through empty space. The sun radiates energy through space to earth. Similarly all hot bodies radiate heat, and if they are hot enough they also radiate visible light which we call as glow.
When one studies heat transfer mechanisms in industrial processes, all three modes of heat (or energy) transfer must be considered. In a high temperature furnace or kiln, for example, energy is transferred from the heat source i.e. a burner to the material being heated and to the surrounding furnace refractory walls by all the three processes. The amount of energy transferred by radiation increases dramatically as the temperature increases. It is the dominant heat transfer mechanism at high temperatures. The load and the refractories of the furnace wall absorb energy, get hot, and re-radiate energy. The moving gases within the carry heat with them and transfer it when they come in contact with cooler solid. A small amount of gas conduction occurs, and conduction is the main process of transferring energy or heat from the surface of the solid or liquid load to its own interior.
One of the prime roles of a refractory is to withstand the effects of heat usually in a hostile environment. That is why for the selection of refractory and its designing Thermal Conductivity is one property which one has to consider. Usually one would like to have a refractory with low thermal conductivity so that heat may be more effectively contained within a furnace or kiln. Sometimes, however refractories and materials having high thermal conductivity are desired. For example, a protective muffle in certain ceramic kilns is designed to prevent combustion gases from reaching the ceramic ware. It must transfer as much heat to the ware as possible, so conductive ceramic materials like silicon carbide are often used for muffles.       
Since insulation refractories find application in processes involving thermal energy, an understanding of thermal properties especially, thermal conductivity of these refractories is quite important. Thermal Conductivity of a refractory material, k, is a measure of the amount of heat that it will allow to pass under certain conditions. Thermal conductivity can be defined as the quantity of heat transmitted through a material in unit time, per unit temperature gradient along the direction of flow and unit cross sectional area. First, let us understand the material conditions affecting this thermal property of a refractory brick whether it is insulating or normal brick, and then the most common method used to measure (or calculate) the same. While there are many factors affecting the thermal conductivity of refractories, some of the most important are [Reference: J.E. Burke, Progress in Ceramic Science, Vol. 2, Ed., Pergamon Press, Chapter 4, 1962]: 
1. Temperature
2. Complexity of structure (crystal and microstructure)
3. Defects (impurities, solid-solution, and stoichiometry)
Temperature dependence of thermal conductivity for several materials graph
                  Fig: Temperature dependence of thermal conductivity for several materials
The temperature dependence of thermal conductivity of several materials is shown in the adjacent figure. In general, the thermal conductivity is expected to decrease with increasing temperature when the temperature exceeds the Debye temperature. The Debye temperature is a characteristic temperature for a given material and may be below or above room temperature. The structural features such as, anisotropic arrangement of ions, relative mass difference between anion and cation, pores, and grain boundaries etc. do affect thermal conductivity of a material. Spinel (MgAl2O4) for instance, has a thermal conductivity lower than that for either MgO or Al2O3. Another example is reducing the thermal conductivity of a solid by introducing porosity and this is the most common technique of manufacturing insulating refractories.
Fortunately for us, the thermal conductivity of a refractory material is ordinarily measured in such a way as to account for all of the heat transfer processes that happen to be operating in that material. We do not have to unscramble them or deal with tem separately, for most ordinary purposes. Once that property is known for each material in the vessel, some very sophisticated calculations can be performed to find out where the heat goes in a given operation.
In the next following lines we will discuss only the simplest of these calculations. This will be enough to enable you or someone to select among various insulating refractories and also to measure what will be the refractory lining thickness.
Imagine a large flat slab or wall of refractory, whose hot face (hot side), is at some fixed temperature, Th. Its cold face (cold side) perhaps in contact with a steel shell, is at some lower temperature, Tc. We will call the thickness of the refractory X. Let us assume that the heat is supplied to the hot face at some fixed rate by process fluids, and that heat is removed from the cold face (may be by the steel shell and the air outside it) at exactly the same rate. Two things then follow: (a) heat flows through the refractory at exactly the same rate as well and (b) temperatures Th and Tc do not change with time. This is called Steady State situation. If we call some amount of heat H flows in time interval t then the rate of heat-flow Q would be H / t. If you think about it, you will understand that this rate of heat-flow or heat transport has to be proportional to the area of refractory wall, A, through which heat is flowing. One mathematical equation connects all of these things at once is:
Refractory Lining Technology

 where, k is the value of thermal conductivity.

To use this equation, we will adopt a set of English units that engineers in the fields of processing and refractories are familiar with. The unit of heat energy, the BTU (British thermal unit), is defined as the amount of heat that will raise the temperature of 1 pound of water by exactly 1OF. The unit of time will be hour (hr). We shall take units of area A in square feet (ft2), the thickness X in inches (in.) and temperature in OF. Clearly if the situation described by A, X, Th, and Tc is held fixed but different materials are studied, the rate of heat transport (Q or H/t) will be proportional to the k (thermal conductivity) of each material. Since k is a property of each material, we can get different values for the rate of heat transport by choosing different materials or mixtures of them. Thermal conductivities i.e. values of k for different materials are measured in the laboratory and published. We can use them in calculations with the above equation. Only we need to make sure that the units of k are (BTU.in)/(ft2.OF.hr).
In fact, k is numerically equal to the rate of heat transport when the slab area (here, area of the refractory or furnace wall) is exactly 1 ft2 and the temperature gradient is exactly 1OF/in. The table below lists some of the typical values of thermal conductivity (k) for different solid materials: some metals, some ordinary “working” refractories, some insulating and some highly conducting refractories. Given below are some examples of how to calculate Heat Loss or Heat Transport and Thickness of Refractory Lining:      
Suppose we have a furnace lined with Superduty refractory brick, and the total wall area of this furnace is 1350 ft2 and also suppose the refractory lining thickness is 12 inch. Say, the process we are conducting in this furnace keeps its hot-face temperature at 3000OF. With thermocouples we find that the cold-face is at a steady temperature 600OF. Then, what will be the rate of heat loss through all the walls of this furnace ?    
We find from the table given below that k for Superduty brick is 9.5. Then by putting all the given numbers into our heat transfer equation mentioned above we get the rate of heat flow (heat loss) Q as per -
Refractory Lining Technology

Refractory Lining Technology





It will be instructive to check here as how much less refractory it would take to match this heat loss keeping all the conditions same if we used, say, an insulating refractory firebrick whose thermal conductivity (k) value is 3.0, also taken from the table below. Suppose that this insulating brick can survive at 3000OF, to make the question reasonable. Here we will find out the required thickness of the insulating brick lining for which we first rearrange the heat transfer equation to be explicit in X so that we can solve it for the refractory thickness. Then by putting all the given numbers into the equation except 3.0 for k, we get -
Refractory Lining Technology
That is 3.8 inch of insulating firebrick has the same heat transfer resistance as 12 inch of conventional Superduty refractory firebrick ! We would be na├»ve to replace the one refractory by the other until we learn more; but the effectiveness of insulating refractories in containing heat is impressive. If we were to keep the refractory lining thickness at 12 in. for example, and solve our heat transfer equation with k = 3.0, we would find that the total rate of heat loss is only 810,000 BTU/hr., instead of 2,565,000 BTU/hr. Now imagine how much thousands of dollars we could save per month in fuel costs !     
However, on practical ground or real - life, calculations are never this simple for numerous reasons. For one thing, the value of thermal conductivity itself changes with temperature as the relative contributions of conduction, convection and radiation change. The second complication we will mention here is that in most cases the refractory lining of a furnace or kiln is done with several refractory layers of varying qualities:
1. A working face of refractory layer or, interior layer of refractory lining that is exposed to the process;
2. The refractory lining between the furnace or kiln shell and working lining, often referred to as the Safety Lining or Insulating Lining. Insulating linings are used to limit heat loss and to maintain the vessel (furnace) shell temperatures at reasonable levels.
Such refractory lining arrangements definitely complicate the heat transfer calculations. But even with the simple introduction about insulating refractories what we have given above, you can appreciate that a process operator can intelligently design a refractory lining that will endure its use temperature and chemistry, and at the same time meet the restrictions on refractory lining thickness or on heat loss that are specified for the situation.
In our next post Insulating Refractories (Part - II) we will look at the different types of insulating refractories and their manufacturing etc.                      
      Table :  Typical Thermal Conductivity Values
Refractories / Materials
k (BTU.in/ft2.OF.hr)
Metals (dense solid)
Copper
Aluminium
Gold
Silver
304 Stainless Steel
310 Stainless Steel
1020 Carbon Steel

2500
900 - 1500
2060
2900
113
96
360
Dense Refractories
Silica Brick
Superduty Brick
Periclase
High Alumina
Chrome - magnesite
Zirconia

13
9.5
20 - 50
10 - 40
14
5
Insulating Refractories
Insulating firebrick 2800
Insulating firebrick 2600
Insulating firebrick 2300
Ceramic Fiber Blanket 4 pcf (lb/ft3)
Ceramic Fiber Blanket 8 pcf (lb/ft3)
Vacuum formed board
Backup insulation

2.5 - 3.0
2.0 - 2.5
0.9 - 1.3
0.6 - 3.0
0.35 - 2.0
0.4 - 1.5
0.3 - 1.0
Conducting Refractories
Silicon Carbide
Baked Carbon
Graphite

100 - 200
300 - 800
500 - 1200

Blast Furnace - Refractory Lining Pattern

- 4 comments
27-July-2009
Refractory Technology: Different temperature zones of a Blast Furnace image Fig: Blast Furnace Temperature Zones

Refractory Technology: Blast Furnace image
Fig: Typical areas of a Blast Furnace
Blast Furnace - An Introduction
Blast Furnace is the focus of any integrated steel plant. Blast furnace is used to reduce the iron ore to iron. The charge, which consists of iron ore, coke and limestone etc. in the form of lumps and different ratios, is fed from the top. Air heated in the blast furnace stoves, is applied from the bottom of the furnace. The hot blast comes in contact with the descending charge in furnace and the iron ore gets reduced to iron due to reducing conditions on account of CO2 and CO in the furnace. CO provides further heat and a very high temperature is developed because of which the iron gets melted which, along with the slag is collected in the hearth from where these are tapped separately from different tap holes.
Ironmaking technology in general made great strides particularly, during the past few decades and as a result of which many alternative ironmaking processes such as Finmet, Fastmet, Hismelt, Romelt, COREX®, and FINEX® etc. have emerged. Nevertheless, the classical Blast Furnace, which has been around the longest, continues to be the dominant method of ironmaking till now. Improvements in burden quality, burden distribution, casting technology, and computer assisted supervision were realized throughout the world. To a great extent these operational improvements made it possible to install very sophisticated refractory lining systems in blast furnaces. The application condition of different sections of a blast furnace is different due to the very nature of its geometry and also pyrometallurgical process occurring at different stages (see adjacent Blast Furnace figures). Therefore, the Blast Furnace Bottom, Hearth, Taphole, Tuyeres, Bosh, Belly, Stack, Cast house, Blast Furnace Stoves all require different quality of refractories depending on the respective application conditions.
Selection of appropriate refractory combination depends on in-depth knowledge of ironmaking system and the physical, mechanical and chemical properties of the proposed refractories. An improper understanding of the above factors often leads to a refractory failure which, subsequently, becomes a complex problem to solve. Refractory linings whether it is of a Blast Furnace or any other furnace, usually fail due to any number or combination of such factors. For the convenience of understanding, here we will discuss the types of refractory lining required in a blast furnace area wise as well as the trend in the refractory lining pattern that has been observed during the last few years.

Blast Furnace RefractoriesRefractory Technology: Blast Furnace refractory lining pattern graphics
Fig: Conventional and New Refractory Lining along with Wear Mechanism
Now-a-days the campaign life of Blast Furnace is measured in terms of 10 - 15 yrs rather than 4 - 5 yrs while on the other hand, the trend is to replace smaller Blast Furnaces with large capacity Blast Furnaces, which are being subjected to even more stringent operating conditions. To achieve these goals, it is necessary to have a good combination of high grade refractories combined with highly efficient cooling systems and tight control on furnace operation to ensure high productivity without excessive wall working and with minimization of massive “slips” in the blast furnace which can cause excessive premature damage to the refractory linings. It is known that the bottom and a part of the hearth are corroded mainly by pig iron, slag and alkalies. Refractory bricks in these areas are subjected to high load and temperature. So it requires a refractory lining which should have high strength, lower creep in compression value and higher RUL and PCE values. Many furnaces still use low iron, dense 42-62% Alumina, Mullite refractory bricks, conventional Carbon blocks etc. in the bottom and lower hearth while the present trend is to replace it with super micro-pore Graphite bricks.
Research and data shows that Blast Furnace hearth life mainly depends on the following factors:
1. Operational Factors such as,
(a) High productivity leading to High heat loads
    (b) High fluid velocity causing more erosion
    (c) High coal injection means lower permeability
None of the above factors is under the control of furnace operator and hence, the only solution for this can be a robust refractory lining.
2. Refractory Lining System Design The entire refractory lining is also subjected to thermal stress which also plays a dominant role especially when the design is inadequate. The refractory lining system or design must take care of the following things -
(a) Optimize thermal resistance
(b) Provide expansion relief
(c) Prevent cracking
(d) Eliminate built-in barriers.
3. Refractory Properties
(a) High thermal conductivity
(b) Alkali resistance
(c) Low permeability
(d) Low thermal expansion
(e) Low elasticity.
The recent development of micro-porous carbon bricks and improvement in the quality of semi-graphite and graphite bricks has led to higher infiltration resistance to iron and slags, and thermal conductivity. The problem of brittle layer formation around 800OC isotherm by alkali condensation and thermal stresses have been addressed to by using smaller blocks, optimum expansion allowances etc. The carbon refractories are covered by fireclay or mullite bricks to protect it against oxidation. The design of this ‘Ceramic Cup’ is important, as the isotherms are altered depending on the quality and thickness of the cup material.
The stack bricks are particularly; exposed to high abrasion and erosion by charge material from top as well as high velocity fume and dust particles going out due to high blast pressure in a CO environment. Therefore, the application condition demands refractory materials which should have high strength, low permeability, high abrasion resistance and resistance to CO disintegration. Superduty fireclay refractory brick or dense alumina brick having Al2O3 around 39 - 42% can impart these characteristics required for stack application. The tuyere and bosh are attacked by temperature change, abrasion and alkalies; and the belly and lower shaft by thermal shock, abrasion and carbon monoxide attack etc. In the critical areas of the furnace, i.e. tuyere, bosh, belly and lower stack, silicon carbide, SiC-Si3N4 and corundum refractories have replaced carbon and 62% Al2O3 or Mullite bricks – taking advantage of the high thermal conductivity of SiC in combination with the stave coolers. However due to the problem of water leakage around taphole and tuyere area many blast furnaces are lined with high alumina or Alumina-Chrome corundum refractories.
Hot Blast Stove Refractories
The hot blast system, incorporating either three or four hot blast stoves per blast furnace, is the other major refractory installation in the blast furnace complex. With today’s large blast furnaces, the main trend in hot-blast stoves is toward high temperature and pressure ventilation with dome temperature around 1550OC, blast temperatures of 1250 - 1400OC, and furnace pressures of 3 - 5 kg/cm2. Therefore, selection of refractories for hot blast stoves depends primarily on their creep resistance properties, bulk density, specific heat, thermal shock resistance, cold crushing strength, thermal expansion and dimensional accuracy. Blast furnace stoves are generally designed by high alumina bricks and checkers. Silica bricks have been introduced in high temperature stoves operating over 1300OC and where the temperature is never allowed to drop below 600OC as silica bricks display poor thermal shock resistance at such low temperatures. Alternatively silica checker bricks can be used can be used in high temperature zone, high alumina bricks in the middle temperature range and hard fired fireclay bricks and other high strength bricks at the bottom checker level.
Table: Blast Furnace Refractories
Area
Present
Trend
Stack
39-42% Al2O3
Super-duty fireclay
Belly
39-42% Al2O3
Corundum, SiC-Si3N4
Bosh
62% Al2O3, Mullite
SiC-Si3N4
Tuyere
62% Al2O3, Mullite
SiC self-bonded, Al-Chrome (Corundum)
Lower Hearth
42-62% Al2O3, Mullite, Conventional Carbon block
Carbon/Graphite block with super micro-pores
Taphole
Fireclay tar bonded, High Alumina / SiC tar bonded
Fireclay tar bonded, High Alumina / SiC tar bonded
Main Trough
Pitch / water bonded, Clay / Grog / Tar bonded ramming masses, Castables
Ultra low cement castables, SiC / Alumina mixes, Gunning repairing technique
Tilting Spout
High alumina / SiC ramming masses / Low Cement Castables
High alumina / SiC / Carbon / ULCC
Hot Blast Stove
42-82% Al2O
70-82% Al2O3, 91% SiO2 checker bricks

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