Insulating Firebrick â Maximizing Energy Savings in Iron ...
Insulating Firebrick — Maximizing Energy Savings in Iron and Steel Applications Through Product Selection I
nsulating firebricks (IFBs) are well-established products for solving many problems of hightemperature heat containment in a wide variety of iron and steel applications, including blast furnace stoves, ductwork in direct reduction processes and reheat furnaces, and backup insulation in coke ovens, tundishes and ladles. IFBs are also used extensively to form the sidewalls, roofs and hearths of a wide variety of heat treatment, annealing and galvanizing lines (Figure 1). The volatile energy prices of recent years have increased the importance of maximizing energy savings in these applications. In some of these applications, energy has now become the most significant running cost. Since engineering design and lining material selection largely control the efficiency of energy usage in an application, it is vital that industrial designers understand the advantages and disadvantages of the various refractory materials available if they are to
design systems which minimize energy usage during operation of the installation. In particular, in order to optimize energy savings, the designer needs to know which IFB products provide the least energy losses. Since different suppliers manufacture insulating firebricks by different techniques (casting, slinger, extrusion, foaming, pressing, etc.), the brick chemistries and microstructures produced can be very different, leading to a wide variety of thermal conductivities available within products of the same temperature rating. This, in turn, leads to a wide variation in the ability of the different types of insulating firebricks to control energy loss from an application. This paper reports on the findings of a study to quantify the differences in energy usage that can be achieved with the three main types of insulating firebrick available on the market (cast, pressed and extruded) within class 23 and class 26 insulating
Figure 1
Abstract Differences in performance can be achieved by a wide range of insulating firebricks (IFBs) currently available. Since suppliers manufacture IFBs by different techniques, the brick microstructures can be very different, leading to a wide variety of thermal conductivities within the same class of product.
Authors Andy Wynn project director, Morgan Ceramics Dalian, Dalian, China [email protected]
Ermanno Magni manager — technical product application Europe, Thermal Ceramics Italiana s.r.l., Lodi, Italy ermanno.magni@morganplc. com
Massimiliano Marchetti Thermal Ceramics Italiana s.r.l., Lodi, Italy
Steve Chernack applications engineering manager, Morgan Thermal Ceramics North America, Augusta, Ga., USA [email protected]
Chris Johnson IFBs in coke oven stack (left) and tunnel kiln (right). This article is available online at AIST.org for 30 days following publication.
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product manager, Morgan Thermal Ceramics North America, Augusta, Ga., USA [email protected]
May 2012 ✦ 261
Table 1 Physical Properties of Class 23 and 26 IFBs Studied Class 23
Class 26
Manufacturing process
Cast
Slinger
Extrusion
Cast
Slinger
Extrusion
Density (kg/m3)
483
611
569
656
810
858
MOR (MPa)*
1
0.7
0.9
0.9
1.5
1.6
CCS (MPa)*
1.2
0.9
1.1
1.3
1.6
1.7
@ 1,230°C
–0.2
0.0
–0.2
—
—
—
@ 1,400°C
—
—
—
–0.3
–0.2
–0.7
Reversible linear expansion (%)
0.5
0.6
0.6
0.6
0.7
0.7
PLC (%)** after 24 hours
*ASTM C-93, **ASTM C-210
firebricks. Energy losses were measured using a standardized laboratory kiln arrangement, constructed using a variety of test bricks. This work demonstrates that as much as 37% difference in energy use can be measured between class 23 insulating firebricks manufactured by different methods and up to 38.5% for class 26 insulating firebricks.
Background To understand the effect of the different IFB manufacturing methods on thermal conductivity and energy loss behavior, three commercially available IFBs, each from classes 23 and 26, were selected, representing the three main manufacturing processes used by suppliers. Table 1 lists their physical properties. Further details concerning these manufacturing processes are available in the literature.1,2 1. The “cast” process uses gypsum plaster as a rapid-setting medium for a high-water-content clay mix, containing additional burnout additives. 2. The “slinger” process is a form of low-pressure extrusion of a wet clay mix containing high levels of burnout additives, with the additional
processing step that the semi-extruded material gets “slung” onto a continuous belt to generate additional porosity, before drying and firing. 3. The “extrusion” process forces a damp clay mixture containing burnout additives through an extrusion nozzle, where the extrudate is subsequently cut into bricks, dried and fired. As IFBs are primarily used for their insulating abilities, thermal conductivity is normally the most critical of the product properties. Thermal conductivity of all the products studied is displayed in Figure 2, measured to ASTM C-182. These graphs illustrate the large influence that manufacturing method has on thermal conductivity. A trend emerges from these data; in each class of IFB, the lowest thermal conductivity is always the cast brick, followed by the slinger-produced brick, with the extruded brick displaying the highest conductivity. Refractory engineers sometimes use density as a “rule of thumb” indicator of the insulating ability of an IFB, associating low density with better insulating properties. Comparing the density values of the IFBs in Table 1 with the thermal conductivity
Figure 2
(a)
(b)
Thermal conductivity for class 23 IFBs (a) and class 26 IFBs (b). 262 ✦ Iron & Steel Technology
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Figure 3 graphs in Figure 2 shows that this is not always the case. In particular, in the class 23 IFBs, the highest density product (slinger) has the intermediate set of thermal conductivity values. In the class 26 IFBs, the close densities of the slinger and extruded products do not give any indication of the large differences in thermal conductivity between them. Therefore, to ensure the best insulation possible when designing installations with IFBs, product selection should not be made on density values.
Experimental Two electrically heated laboratory Laboratory kilns built for the energy use study. muffle kilns of identical design and power rating were manufacFigure 4 tured to particular specifications by a laboratory kiln builder (Figure 3). For the class 23 IFB study, the first kiln was lined with the cast IFBs characterized in Table 1. This formed the benchmark, as this IFB has the lowest thermal conductivity in the class. The second kiln was lined with the class 23 slinger bricks and energy usage studies completed as detailed in Table 2. The lining of the second kiln was then replaced with the class 23 extrusion bricks and the same energy usage tests repeated. For the class 26 IFB study, the same procedure was followed, except that the first kiln was lined with the class 26 cast IFBs characterized in Table 1 as the benchmark, and the class 26 slinger and extrusion bricks were tested independently as the linings of the second kiln. For each kiln, to measure the energy usage during the test firings, power meters were set Infrared thermograph of muffle kilns during 1,000°C firing up between the power source and the kiln. For each test (“cast” IFB-lined kiln on the left). product comparison in the class 23 and 26 studies, two test firings were conducted: Test 2. Ramp at 3°C/minute from ambient to Test 1. Ramp at 3°C/minute from ambient to 800°C, 1,000°C, hold for 15 hours, natural cool back to hold for 15 hours, natural cool back to ambient. ambient.
Table 2
Results
Results of Energy Usage Tests With Class 23 and 26 IFBs kW used @ 800°C
%' to cast
% energy saved with cast
kW used @ 1,000°C
%' to cast
% energy saved with cast
Cast
11.2
—
—
16.0
—
—
Slinger
15.1
34.8
25.8
20.9
30.6
23.4
Extrusion
17.3
54.5
35.2
25.4
58.8
37.4
Cast
17.1
—
—
20.6
—
—
Slinger
20.3
18.7
15.8
27.9
35.4
26.2
Extrusion
22.7
32.7
24.7
33.5
62.6
38.5
IFB type Class 23
Class 26
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The results of the energy usage tests are shown in Table 2. By monitoring the kilns during the tests using an infrared camera (VarioCAM, FPA detector 320 x 240 pixel, 25 mm FOV 32° x 25°) the kiln surface temperatures could be measured. Figure 4 illustrates how much heat is wasted through the body of the kiln lined with the higher thermal conductivity IFB and how the surface temperature of the kiln becomes overheated. This behavior illustrates the combined effect in industrial applications of wasting energy costs and presenting May 2012 ✦ 263
Figure 5
Macrostructure of class 23 IFBs.
health and safety issues in terms of hazardous working temperatures.
thermal conductivity is controlled by the structure of the material. The different manufacturing methods of the IFBs studied produce materials with inherently different macro- and microstructures, and it is these which control the thermal behavior of the products. Figure 5 illustrates the differing macrostructures of the class 23 IFBs studied. Figure 5 shows that the texture of the IFBs is finest for the cast product and is coarsest for the extruded product. This is also observed in the microstructure under an electron microscope (Figure 6). The extruded IFB has large holes in the structure, ranging from 300 to 1,000 micron. Such large pore sizes are formed when combustible materials are added to the mix prior to extrusion and burnt out during the firing process. Typically, expanded polymer spheres of 0.5– 1.5 mm diameter are used by manufacturers to create
Discussion Table 2 shows that using different types of IFB in kiln linings can have a considerable effect on the energy needed to heat up the kiln. For the class 23 IFBs, more than 37% less energy was needed to run the test kiln through a 1,000°C firing cycle with the cast IFB compared to the extrusion IFB. For the class 26 IFBs, more than 38% less energy was needed to run the test kiln through a 1,000°C firing cycle with the cast IFB compared to the extrusion IFB. This illustrates the benefits of using the lowest conductivity IFB possible for maximizing energy savings. These differences in energy usage are due to the differing thermal conductivities of the IFBs. In materials of similar chemistry,
Figure 6
(a)
(b) Microstructure of class 23 cast IFB (a) and class 23 extrusion IFB (b). 264 ✦ Iron & Steel Technology
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Figure 7 such high levels of porosity in the fired product. This has the effect of reducing density, making the brick light in weight, but does not contribute so much toward the insulating properties of the IFB. The cast IFB has a much finer microtexture, which is created by decomposition of the binder during firing. Figure 7 illustrates the pore size distribution of the class 23 cast, slinger and extrusion IFBs as measured by mercury porosimetry. Microscopy and porosimetry studies show that the cast IFB has a much higher proportion of pore sizes in the < 10 micron range compared to slinger and extrusion. It is this combination of ultrafine pore structure, coupled with an absence Pore size distribution of class 23 IFBs. of very large pore sizes, that gives Figure 8 the cast product the lowest thermal conductivity. From macroscopic study, it is clearly seen that the extruded IFB has the largest volume of large holes in the structure (Figure 5), contrary to the porosimetry data (by 600–800 micron, mercury porosimetry reaches its capability limit in terms of measuring pore size accurately). Similar structural features are found in the class 26 products, with the cast IFB having a lot of ultrafine micronsized pores and the extrusion IFB having large mm-sized holes from burnout additives. These very different structures in cast vs. slinger vs. extrusion IFBs interfere with heat transfer mechanisms in different ways, which is why such big differences in energy Thermal conductivity vs. temperature for various refractories. usage are observed in the test kilns. IFBs are normally used in appliEnergy Savings Calculations cations > 1,000°C because, at these temperatures The laboratory test results demonstrate the potential they provide the most cost-effective insulation availto minimize energy usage by appropriate selection able, compared to alternative insulating refractories of IFB for an installation lining. To understand how (Figure 8). At temperatures above 1,000°C, the most this affects real, full-size industrial installations, heat important heat transfer mechanism becomes radiaflow calculations were performed (using the same cast tion, rather than conduction and convection, which and extrusion IFB types in the laboratory studies) to are the more significant heat transfer mechanisms at assess energy running costs in strategic locations of lower temperatures. The large pore sizes in the extrutwo annealing applications that utilize IFBs as the linsion IFB are inefficient at retarding energy transfer at ing material: a catenary strip annealing furnace and a the infrared wavelengths involved, and so this type of cast-iron part annealing furnace (Table 3). IFB displays a higher thermal conductivity compared Note that the modeled wall section of the catenary with cast IFBs. Conversely, the microporous structure strip annealing furnace is a layered composite of a of the cast IFB is much more efficient at interfering class 26 IFB hot face, backed up by class 23 IFB and with energy transfer at infrared wavelengths because vermiculite board, whereas the modeled roof section the pore sizes are a similar wavelength to infrared of the cast-iron part annealing furnace is one thick radiation, and so this type of IFB displays the lowest layer of class 23 IFB. These represent the two most thermal conductivity of all the IFB types.
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May 2012 ✦ 265
Table 3 Operating Conditions for Heat Flow Calculations Operating conditions Casing condition
Catenary strip annealing furnace
Cast-iron part annealing furnace
Wall
Roof
(m2)
127.3
46.5
Operating temperature (°C)
1,200
930
H2, CO, products of natural gas combustion
Products of natural gas combustion
20
20
Furnace area
Atmosphere Ambient temperature (°C) Ambient wind speed
(ms–1)
0
0
8,400
6,000
Layer 1, class 26 IFB thickness (mm)
114.3
—
Layer 2, class 23 IFB thickness (mm)
114.3
228.6
Layer 3, backup vermiculite board (mm)
50.8
—
Working hours per year Lining arrangement
common real-life IFB lining arrangements, where walls are normally built up using standard brick sizes, while roofs are constructed from special pre-assembled roof blocks. The thermal conductivity data displayed in Figure 2 was used to calculate the heat flow across the two furnace arrangements in two ways; one set of calculations where all IFB was produced by the cast process and one set of calculations where all IFB was produced by the extrusion process. The results are displayed graphically in Figures 9 and 10. Table 4 shows the results of the heat flow calculations and illustrates the significant differences that
can be achieved for casing temperatures when using different IFB types. Using cast IFB produces much lower casing temperatures than with extruded IFB. The lower surface temperature obtained using the cast IFB produces a more comfortable working environment for operators and minimizes the risk of burns due to operators coming into contact with the surface of the installation. Catenary Strip Annealing Furnace Wall — The heat flow calculations show that, for the catenary strip annealing furnace wall, the lining arrangement with
Figure 9
(a)
(b)
Heat flow calculations for catenary strip annealing furnace wall using cast IFB (a) and extrusion IFB (b). 266 ✦ Iron & Steel Technology
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Figure 10
(a)
(b)
Heat flow calculations for cast-iron part annealing furnace roof using cast IFB (a) and extrusion IFB (b).
the extrusion IFB requires 271 W/m2 more energy to maintain the 1,200°C operating temperature than the lining arrangement with the cast IFB, due to the lower thermal conductivity of the cast vs. extruded IFB. So, for the 127 m2 working area, the difference in energy consumption between the two simulated furnace walls equates to a savings of 42,450 m3 of natural gas per year using the cast IFB compared to the extrusion IFB. Assuming a gas price of $0.192/m3, this equates to an annual savings of $8,150/year for this wall section only. Assuming a furnace wall lining life of 10 years, the total savings over the life of the kiln lining would be $81,500. As this model studies only the wall arrangement, the savings that can be achieved on the complete structure can be significantly larger than this with appropriate IFB and refractory selection. A
127 m2 working area in the wall of the catenary strip annealing furnace would need ~7,200 standard sized IFBs. Given the current market price differential between the cast and extrusion IFBs, although the cast IFB price is a little higher, in this example this higher price would be paid back in just over three months. After this initial payback period, the rest of the 10-year service life delivers continuous cost savings due to the lower energy requirements. Cast-Iron Part Annealing Furnace Roof — For the cast-iron part annealing furnace roof, the lining arrangement with the extrusion IFB requires 434 W/ m2 more energy to maintain the 930°C operating temperature than the lining arrangement with the cast IFB. For the 46.5 m2 working area, the difference
Table 4 Results of Heat Flow and Energy Savings Calculations Catenary strip annealing furnace wall Heat flow calculations
Cast IFB
Extrusion IFB
Cast-iron part annealing furnace roof Cast IFB
Extrusion IFB
Casing temperature (°C)
70.3
87.2
71.9
94.7
Heat loss (W/m2)
598.8
869.9
713.4
1,147
Heat storage (MJ/m2)
110.6
135.6
72.92
79.8
Annual cost savings/m2
$64
—
$73
—
Total annual cost savings
$8,150
—
$3,382
—
Energy savings calculations*
*Assuming energy content of natural gas is 38.4 MJ/m3 and natural gas price is $0.192/m3
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May 2012 ✦ 267
in energy consumption between the two simulated furnace roofs equates to a savings of 17,615 m3 of natural gas per year using the cast IFB compared to the extrusion IFB, which equates to an annual savings of $3,382/year for this small roof section only. Assuming a furnace roof lining life of 10 years, the total savings over the life of the kiln lining would be $33,820. As this model studies only the roof arrangement, the savings that can be achieved on the complete structure can be significantly larger than this with appropriate IFB and refractory selection. The 46.5 m2 roof area would need ~2,600 standard sized IFBs; in this case, payback for using cast IFB is under three months. Additional Impact of IFB Selection — Another important consequence of the energy savings achieved using lower thermal conductivity IFB is the reduction in CO2 emissions. Using the cast IFB instead of the extrusion IFB reduces the environmental impact of running a furnace. In the examples above, since the catenary strip annealing furnace wall requires 42,450 m3/year less natural gas to run using cast vs. extruded IFB, as 1 m3 of natural gas produces ~1 m3 of CO2, there is a potential reduction in CO2 emissions of ~42,000 m3/ year. 1 m3 of CO2 equates to 1.96 kg, which equates to ~83 t/year reduction in CO2 produced, or 830 t over the life of the kiln lining. For the cast-iron part annealing furnace roof, 17,615 m3/year less natural gas is used in running the furnace when lined with cast compared to extruded IFB. This equates to a potential reduction in CO2 emissions of ~17,600 m3/ year or ~34.5 t/year reduction in CO2 produced. The choice of IFB in the furnace lining will also impact other practical aspects of using the furnace in a production environment. Selecting the cast IFB rather than the extrusion IFB will allow faster heating and cooling rates, because the lower-density cast IFB has a lower thermal mass, as evidenced in the lower heat storage values in Table 4. This effect was observed in the energy studies reported earlier in this paper. During both the 800°C and 1,000°C test firings, the cast IFB reached the programmed dwell temperature measurably faster than the extrusion IFB.
Conclusions By monitoring energy usage in laboratory kilns lined with IFBs manufactured by different process routes, and by modeling the effects on heat flow of using IFBs manufactured by different processes in two important iron and steel applications, it was demonstrated here that: For class 23 IFBs:
• Bricks manufactured by the cast process offer the lowest thermal conductivity in this class. • Bricks manufactured by the cast process can reduce energy usage by up to 37%. • Bricks manufactured by the cast process reduce casing temperatures and CO2 emissions and allow faster heating and cooling rates compared to IFBs manufactured by other processes. For class 26 IFBs: • Bricks manufactured by the cast process offer the lowest thermal conductivity in this class. • Bricks manufactured by the cast process can reduce energy usage by up to 38%. • Bricks manufactured by the cast process reduce casing temperatures and CO2 emissions and allow faster heating and cooling rates compared to IFBs manufactured by other processes. This study offers the following guidelines to industrial designers who wish to minimize heat losses from their installations: • IFBs should not be selected on the basis of bulk density, as there is no direct correlation between density and insulating capability. • Close attention should be paid to the reported thermal conductivity of IFB products. Commercially published thermal conductivity data vary in quality and accuracy, with some datasheets omitting the test method, which makes the data misleading when comparing and selecting products. • Published thermal conductivity should be measured to a recognized international standard (e.g., ASTM C-182) and be as low as possible. • Selecting an IFB due to price alone can turn out to be a false economy and a costly mistake in the long run. • IFBs manufactured by the cast process offer the lowest thermal conductivity IFBs available today at application temperatures and therefore provide the greatest energy savings. References 1. K.J. Moody, J.P. Street and E. Magni, “Insulating Firebrick: Manufacturing Processes and Product Quality,” Alafar Conference, Guatemala, 2004. 2. A.M. Wynn, M. Marchetti, J.P. Street and T. Yin, “Insulating Firebrick — Effect of Manufacturing Method on Product Performance,” UNITECR 09 Conference, Brazil, 2009. ✦
Nominate this paper Did you find this article to be of significant relevance to the advancement of steel technology? If so, please consider nominating it for the AIST Hunt-Kelly Outstanding Paper Award at AIST.org/huntkelly. This paper was presented at AISTech 2011 — The Iron & Steel Technology Conference and Exposition, Indianapolis, Ind., and published in the Conference Proceedings.
268 ✦ Iron & Steel Technology
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nsulating firebricks (IFBs) are well-established products for solving many problems of hightemperature heat containment in a wide variety of iron and steel applications, including blast furnace stoves, ductwork in direct reduction processes and reheat furnaces, and backup insulation in coke ovens, tundishes and ladles. IFBs are also used extensively to form the sidewalls, roofs and hearths of a wide variety of heat treatment, annealing and galvanizing lines (Figure 1). The volatile energy prices of recent years have increased the importance of maximizing energy savings in these applications. In some of these applications, energy has now become the most significant running cost. Since engineering design and lining material selection largely control the efficiency of energy usage in an application, it is vital that industrial designers understand the advantages and disadvantages of the various refractory materials available if they are to
design systems which minimize energy usage during operation of the installation. In particular, in order to optimize energy savings, the designer needs to know which IFB products provide the least energy losses. Since different suppliers manufacture insulating firebricks by different techniques (casting, slinger, extrusion, foaming, pressing, etc.), the brick chemistries and microstructures produced can be very different, leading to a wide variety of thermal conductivities available within products of the same temperature rating. This, in turn, leads to a wide variation in the ability of the different types of insulating firebricks to control energy loss from an application. This paper reports on the findings of a study to quantify the differences in energy usage that can be achieved with the three main types of insulating firebrick available on the market (cast, pressed and extruded) within class 23 and class 26 insulating
Figure 1
Abstract Differences in performance can be achieved by a wide range of insulating firebricks (IFBs) currently available. Since suppliers manufacture IFBs by different techniques, the brick microstructures can be very different, leading to a wide variety of thermal conductivities within the same class of product.
Authors Andy Wynn project director, Morgan Ceramics Dalian, Dalian, China [email protected]
Ermanno Magni manager — technical product application Europe, Thermal Ceramics Italiana s.r.l., Lodi, Italy ermanno.magni@morganplc. com
Massimiliano Marchetti Thermal Ceramics Italiana s.r.l., Lodi, Italy
Steve Chernack applications engineering manager, Morgan Thermal Ceramics North America, Augusta, Ga., USA [email protected]
Chris Johnson IFBs in coke oven stack (left) and tunnel kiln (right). This article is available online at AIST.org for 30 days following publication.
AIST.org
product manager, Morgan Thermal Ceramics North America, Augusta, Ga., USA [email protected]
May 2012 ✦ 261
Table 1 Physical Properties of Class 23 and 26 IFBs Studied Class 23
Class 26
Manufacturing process
Cast
Slinger
Extrusion
Cast
Slinger
Extrusion
Density (kg/m3)
483
611
569
656
810
858
MOR (MPa)*
1
0.7
0.9
0.9
1.5
1.6
CCS (MPa)*
1.2
0.9
1.1
1.3
1.6
1.7
@ 1,230°C
–0.2
0.0
–0.2
—
—
—
@ 1,400°C
—
—
—
–0.3
–0.2
–0.7
Reversible linear expansion (%)
0.5
0.6
0.6
0.6
0.7
0.7
PLC (%)** after 24 hours
*ASTM C-93, **ASTM C-210
firebricks. Energy losses were measured using a standardized laboratory kiln arrangement, constructed using a variety of test bricks. This work demonstrates that as much as 37% difference in energy use can be measured between class 23 insulating firebricks manufactured by different methods and up to 38.5% for class 26 insulating firebricks.
Background To understand the effect of the different IFB manufacturing methods on thermal conductivity and energy loss behavior, three commercially available IFBs, each from classes 23 and 26, were selected, representing the three main manufacturing processes used by suppliers. Table 1 lists their physical properties. Further details concerning these manufacturing processes are available in the literature.1,2 1. The “cast” process uses gypsum plaster as a rapid-setting medium for a high-water-content clay mix, containing additional burnout additives. 2. The “slinger” process is a form of low-pressure extrusion of a wet clay mix containing high levels of burnout additives, with the additional
processing step that the semi-extruded material gets “slung” onto a continuous belt to generate additional porosity, before drying and firing. 3. The “extrusion” process forces a damp clay mixture containing burnout additives through an extrusion nozzle, where the extrudate is subsequently cut into bricks, dried and fired. As IFBs are primarily used for their insulating abilities, thermal conductivity is normally the most critical of the product properties. Thermal conductivity of all the products studied is displayed in Figure 2, measured to ASTM C-182. These graphs illustrate the large influence that manufacturing method has on thermal conductivity. A trend emerges from these data; in each class of IFB, the lowest thermal conductivity is always the cast brick, followed by the slinger-produced brick, with the extruded brick displaying the highest conductivity. Refractory engineers sometimes use density as a “rule of thumb” indicator of the insulating ability of an IFB, associating low density with better insulating properties. Comparing the density values of the IFBs in Table 1 with the thermal conductivity
Figure 2
(a)
(b)
Thermal conductivity for class 23 IFBs (a) and class 26 IFBs (b). 262 ✦ Iron & Steel Technology
A Publication of the Association for Iron & Steel Technology
Figure 3 graphs in Figure 2 shows that this is not always the case. In particular, in the class 23 IFBs, the highest density product (slinger) has the intermediate set of thermal conductivity values. In the class 26 IFBs, the close densities of the slinger and extruded products do not give any indication of the large differences in thermal conductivity between them. Therefore, to ensure the best insulation possible when designing installations with IFBs, product selection should not be made on density values.
Experimental Two electrically heated laboratory Laboratory kilns built for the energy use study. muffle kilns of identical design and power rating were manufacFigure 4 tured to particular specifications by a laboratory kiln builder (Figure 3). For the class 23 IFB study, the first kiln was lined with the cast IFBs characterized in Table 1. This formed the benchmark, as this IFB has the lowest thermal conductivity in the class. The second kiln was lined with the class 23 slinger bricks and energy usage studies completed as detailed in Table 2. The lining of the second kiln was then replaced with the class 23 extrusion bricks and the same energy usage tests repeated. For the class 26 IFB study, the same procedure was followed, except that the first kiln was lined with the class 26 cast IFBs characterized in Table 1 as the benchmark, and the class 26 slinger and extrusion bricks were tested independently as the linings of the second kiln. For each kiln, to measure the energy usage during the test firings, power meters were set Infrared thermograph of muffle kilns during 1,000°C firing up between the power source and the kiln. For each test (“cast” IFB-lined kiln on the left). product comparison in the class 23 and 26 studies, two test firings were conducted: Test 2. Ramp at 3°C/minute from ambient to Test 1. Ramp at 3°C/minute from ambient to 800°C, 1,000°C, hold for 15 hours, natural cool back to hold for 15 hours, natural cool back to ambient. ambient.
Table 2
Results
Results of Energy Usage Tests With Class 23 and 26 IFBs kW used @ 800°C
%' to cast
% energy saved with cast
kW used @ 1,000°C
%' to cast
% energy saved with cast
Cast
11.2
—
—
16.0
—
—
Slinger
15.1
34.8
25.8
20.9
30.6
23.4
Extrusion
17.3
54.5
35.2
25.4
58.8
37.4
Cast
17.1
—
—
20.6
—
—
Slinger
20.3
18.7
15.8
27.9
35.4
26.2
Extrusion
22.7
32.7
24.7
33.5
62.6
38.5
IFB type Class 23
Class 26
AIST.org
The results of the energy usage tests are shown in Table 2. By monitoring the kilns during the tests using an infrared camera (VarioCAM, FPA detector 320 x 240 pixel, 25 mm FOV 32° x 25°) the kiln surface temperatures could be measured. Figure 4 illustrates how much heat is wasted through the body of the kiln lined with the higher thermal conductivity IFB and how the surface temperature of the kiln becomes overheated. This behavior illustrates the combined effect in industrial applications of wasting energy costs and presenting May 2012 ✦ 263
Figure 5
Macrostructure of class 23 IFBs.
health and safety issues in terms of hazardous working temperatures.
thermal conductivity is controlled by the structure of the material. The different manufacturing methods of the IFBs studied produce materials with inherently different macro- and microstructures, and it is these which control the thermal behavior of the products. Figure 5 illustrates the differing macrostructures of the class 23 IFBs studied. Figure 5 shows that the texture of the IFBs is finest for the cast product and is coarsest for the extruded product. This is also observed in the microstructure under an electron microscope (Figure 6). The extruded IFB has large holes in the structure, ranging from 300 to 1,000 micron. Such large pore sizes are formed when combustible materials are added to the mix prior to extrusion and burnt out during the firing process. Typically, expanded polymer spheres of 0.5– 1.5 mm diameter are used by manufacturers to create
Discussion Table 2 shows that using different types of IFB in kiln linings can have a considerable effect on the energy needed to heat up the kiln. For the class 23 IFBs, more than 37% less energy was needed to run the test kiln through a 1,000°C firing cycle with the cast IFB compared to the extrusion IFB. For the class 26 IFBs, more than 38% less energy was needed to run the test kiln through a 1,000°C firing cycle with the cast IFB compared to the extrusion IFB. This illustrates the benefits of using the lowest conductivity IFB possible for maximizing energy savings. These differences in energy usage are due to the differing thermal conductivities of the IFBs. In materials of similar chemistry,
Figure 6
(a)
(b) Microstructure of class 23 cast IFB (a) and class 23 extrusion IFB (b). 264 ✦ Iron & Steel Technology
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Figure 7 such high levels of porosity in the fired product. This has the effect of reducing density, making the brick light in weight, but does not contribute so much toward the insulating properties of the IFB. The cast IFB has a much finer microtexture, which is created by decomposition of the binder during firing. Figure 7 illustrates the pore size distribution of the class 23 cast, slinger and extrusion IFBs as measured by mercury porosimetry. Microscopy and porosimetry studies show that the cast IFB has a much higher proportion of pore sizes in the < 10 micron range compared to slinger and extrusion. It is this combination of ultrafine pore structure, coupled with an absence Pore size distribution of class 23 IFBs. of very large pore sizes, that gives Figure 8 the cast product the lowest thermal conductivity. From macroscopic study, it is clearly seen that the extruded IFB has the largest volume of large holes in the structure (Figure 5), contrary to the porosimetry data (by 600–800 micron, mercury porosimetry reaches its capability limit in terms of measuring pore size accurately). Similar structural features are found in the class 26 products, with the cast IFB having a lot of ultrafine micronsized pores and the extrusion IFB having large mm-sized holes from burnout additives. These very different structures in cast vs. slinger vs. extrusion IFBs interfere with heat transfer mechanisms in different ways, which is why such big differences in energy Thermal conductivity vs. temperature for various refractories. usage are observed in the test kilns. IFBs are normally used in appliEnergy Savings Calculations cations > 1,000°C because, at these temperatures The laboratory test results demonstrate the potential they provide the most cost-effective insulation availto minimize energy usage by appropriate selection able, compared to alternative insulating refractories of IFB for an installation lining. To understand how (Figure 8). At temperatures above 1,000°C, the most this affects real, full-size industrial installations, heat important heat transfer mechanism becomes radiaflow calculations were performed (using the same cast tion, rather than conduction and convection, which and extrusion IFB types in the laboratory studies) to are the more significant heat transfer mechanisms at assess energy running costs in strategic locations of lower temperatures. The large pore sizes in the extrutwo annealing applications that utilize IFBs as the linsion IFB are inefficient at retarding energy transfer at ing material: a catenary strip annealing furnace and a the infrared wavelengths involved, and so this type of cast-iron part annealing furnace (Table 3). IFB displays a higher thermal conductivity compared Note that the modeled wall section of the catenary with cast IFBs. Conversely, the microporous structure strip annealing furnace is a layered composite of a of the cast IFB is much more efficient at interfering class 26 IFB hot face, backed up by class 23 IFB and with energy transfer at infrared wavelengths because vermiculite board, whereas the modeled roof section the pore sizes are a similar wavelength to infrared of the cast-iron part annealing furnace is one thick radiation, and so this type of IFB displays the lowest layer of class 23 IFB. These represent the two most thermal conductivity of all the IFB types.
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Table 3 Operating Conditions for Heat Flow Calculations Operating conditions Casing condition
Catenary strip annealing furnace
Cast-iron part annealing furnace
Wall
Roof
(m2)
127.3
46.5
Operating temperature (°C)
1,200
930
H2, CO, products of natural gas combustion
Products of natural gas combustion
20
20
Furnace area
Atmosphere Ambient temperature (°C) Ambient wind speed
(ms–1)
0
0
8,400
6,000
Layer 1, class 26 IFB thickness (mm)
114.3
—
Layer 2, class 23 IFB thickness (mm)
114.3
228.6
Layer 3, backup vermiculite board (mm)
50.8
—
Working hours per year Lining arrangement
common real-life IFB lining arrangements, where walls are normally built up using standard brick sizes, while roofs are constructed from special pre-assembled roof blocks. The thermal conductivity data displayed in Figure 2 was used to calculate the heat flow across the two furnace arrangements in two ways; one set of calculations where all IFB was produced by the cast process and one set of calculations where all IFB was produced by the extrusion process. The results are displayed graphically in Figures 9 and 10. Table 4 shows the results of the heat flow calculations and illustrates the significant differences that
can be achieved for casing temperatures when using different IFB types. Using cast IFB produces much lower casing temperatures than with extruded IFB. The lower surface temperature obtained using the cast IFB produces a more comfortable working environment for operators and minimizes the risk of burns due to operators coming into contact with the surface of the installation. Catenary Strip Annealing Furnace Wall — The heat flow calculations show that, for the catenary strip annealing furnace wall, the lining arrangement with
Figure 9
(a)
(b)
Heat flow calculations for catenary strip annealing furnace wall using cast IFB (a) and extrusion IFB (b). 266 ✦ Iron & Steel Technology
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Figure 10
(a)
(b)
Heat flow calculations for cast-iron part annealing furnace roof using cast IFB (a) and extrusion IFB (b).
the extrusion IFB requires 271 W/m2 more energy to maintain the 1,200°C operating temperature than the lining arrangement with the cast IFB, due to the lower thermal conductivity of the cast vs. extruded IFB. So, for the 127 m2 working area, the difference in energy consumption between the two simulated furnace walls equates to a savings of 42,450 m3 of natural gas per year using the cast IFB compared to the extrusion IFB. Assuming a gas price of $0.192/m3, this equates to an annual savings of $8,150/year for this wall section only. Assuming a furnace wall lining life of 10 years, the total savings over the life of the kiln lining would be $81,500. As this model studies only the wall arrangement, the savings that can be achieved on the complete structure can be significantly larger than this with appropriate IFB and refractory selection. A
127 m2 working area in the wall of the catenary strip annealing furnace would need ~7,200 standard sized IFBs. Given the current market price differential between the cast and extrusion IFBs, although the cast IFB price is a little higher, in this example this higher price would be paid back in just over three months. After this initial payback period, the rest of the 10-year service life delivers continuous cost savings due to the lower energy requirements. Cast-Iron Part Annealing Furnace Roof — For the cast-iron part annealing furnace roof, the lining arrangement with the extrusion IFB requires 434 W/ m2 more energy to maintain the 930°C operating temperature than the lining arrangement with the cast IFB. For the 46.5 m2 working area, the difference
Table 4 Results of Heat Flow and Energy Savings Calculations Catenary strip annealing furnace wall Heat flow calculations
Cast IFB
Extrusion IFB
Cast-iron part annealing furnace roof Cast IFB
Extrusion IFB
Casing temperature (°C)
70.3
87.2
71.9
94.7
Heat loss (W/m2)
598.8
869.9
713.4
1,147
Heat storage (MJ/m2)
110.6
135.6
72.92
79.8
Annual cost savings/m2
$64
—
$73
—
Total annual cost savings
$8,150
—
$3,382
—
Energy savings calculations*
*Assuming energy content of natural gas is 38.4 MJ/m3 and natural gas price is $0.192/m3
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in energy consumption between the two simulated furnace roofs equates to a savings of 17,615 m3 of natural gas per year using the cast IFB compared to the extrusion IFB, which equates to an annual savings of $3,382/year for this small roof section only. Assuming a furnace roof lining life of 10 years, the total savings over the life of the kiln lining would be $33,820. As this model studies only the roof arrangement, the savings that can be achieved on the complete structure can be significantly larger than this with appropriate IFB and refractory selection. The 46.5 m2 roof area would need ~2,600 standard sized IFBs; in this case, payback for using cast IFB is under three months. Additional Impact of IFB Selection — Another important consequence of the energy savings achieved using lower thermal conductivity IFB is the reduction in CO2 emissions. Using the cast IFB instead of the extrusion IFB reduces the environmental impact of running a furnace. In the examples above, since the catenary strip annealing furnace wall requires 42,450 m3/year less natural gas to run using cast vs. extruded IFB, as 1 m3 of natural gas produces ~1 m3 of CO2, there is a potential reduction in CO2 emissions of ~42,000 m3/ year. 1 m3 of CO2 equates to 1.96 kg, which equates to ~83 t/year reduction in CO2 produced, or 830 t over the life of the kiln lining. For the cast-iron part annealing furnace roof, 17,615 m3/year less natural gas is used in running the furnace when lined with cast compared to extruded IFB. This equates to a potential reduction in CO2 emissions of ~17,600 m3/ year or ~34.5 t/year reduction in CO2 produced. The choice of IFB in the furnace lining will also impact other practical aspects of using the furnace in a production environment. Selecting the cast IFB rather than the extrusion IFB will allow faster heating and cooling rates, because the lower-density cast IFB has a lower thermal mass, as evidenced in the lower heat storage values in Table 4. This effect was observed in the energy studies reported earlier in this paper. During both the 800°C and 1,000°C test firings, the cast IFB reached the programmed dwell temperature measurably faster than the extrusion IFB.
Conclusions By monitoring energy usage in laboratory kilns lined with IFBs manufactured by different process routes, and by modeling the effects on heat flow of using IFBs manufactured by different processes in two important iron and steel applications, it was demonstrated here that: For class 23 IFBs:
• Bricks manufactured by the cast process offer the lowest thermal conductivity in this class. • Bricks manufactured by the cast process can reduce energy usage by up to 37%. • Bricks manufactured by the cast process reduce casing temperatures and CO2 emissions and allow faster heating and cooling rates compared to IFBs manufactured by other processes. For class 26 IFBs: • Bricks manufactured by the cast process offer the lowest thermal conductivity in this class. • Bricks manufactured by the cast process can reduce energy usage by up to 38%. • Bricks manufactured by the cast process reduce casing temperatures and CO2 emissions and allow faster heating and cooling rates compared to IFBs manufactured by other processes. This study offers the following guidelines to industrial designers who wish to minimize heat losses from their installations: • IFBs should not be selected on the basis of bulk density, as there is no direct correlation between density and insulating capability. • Close attention should be paid to the reported thermal conductivity of IFB products. Commercially published thermal conductivity data vary in quality and accuracy, with some datasheets omitting the test method, which makes the data misleading when comparing and selecting products. • Published thermal conductivity should be measured to a recognized international standard (e.g., ASTM C-182) and be as low as possible. • Selecting an IFB due to price alone can turn out to be a false economy and a costly mistake in the long run. • IFBs manufactured by the cast process offer the lowest thermal conductivity IFBs available today at application temperatures and therefore provide the greatest energy savings. References 1. K.J. Moody, J.P. Street and E. Magni, “Insulating Firebrick: Manufacturing Processes and Product Quality,” Alafar Conference, Guatemala, 2004. 2. A.M. Wynn, M. Marchetti, J.P. Street and T. Yin, “Insulating Firebrick — Effect of Manufacturing Method on Product Performance,” UNITECR 09 Conference, Brazil, 2009. ✦
Nominate this paper Did you find this article to be of significant relevance to the advancement of steel technology? If so, please consider nominating it for the AIST Hunt-Kelly Outstanding Paper Award at AIST.org/huntkelly. This paper was presented at AISTech 2011 — The Iron & Steel Technology Conference and Exposition, Indianapolis, Ind., and published in the Conference Proceedings.
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