Insulating firebricks (IFBs) are well-established products for solving many problems of high-temperature heat containment in industries ranging from ceramic-production kilns to anodes for primary aluminum.

The purpose of this work is to quantify the differences in performance that can be achieved by studying a wide range of IFBs currently available on the market. This is accomplished through laboratory-based measurements of energy losses from standard kiln arrangements constructed with a variety of test bricks. Since different suppliers manufacture IFBs by different techniques (casting, slinger, extrusion, foaming, pressing), the brick microstructures produced can be very different. This leads to a wide variety of thermal conductivities in the market within the same class of product, which in turn leads to a wide variation in the ability of the different types of IFBs to control energy loss from the kiln.

This work demonstrates that IFBs can display up to 37% difference in the energy savings achievable, depending on their method of manufacture. The article also presents further consequences of the manufacturing method on performance in terms of heating and cooling rates and reduction in CO2 emissions.

Background

IFB Manufacturing Methods
Table 1 lists the physical properties of four commercially available class-23 IFBs, representing the main manufacturing processes used by manufacturers. The “Cast” process uses gypsum plaster as a rapid-setting medium for a high-water-content clay mix containing some additional burnout additives. 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. 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. The “Cement” process is a form of casting using cement instead of plaster, which leads to a much slower set. Further details concerning these manufacturing processes are available in the literature.[1]

The density data reported in Table 1 is the average of measurements recorded on six bricks selected at random from a larger batch of product. The remainder of the physical-property data is generally an average of three measurements, while the thermal-conductivity data shown in Fig. 1 is measured on one sample selected at random from the batch.

IFB Thermal Conductivity
The different manufacturing methods for IFBs produce products with differing structure and chemistry, which in turn deliver different performance properties.[2] The primary performance parameter for IFBs is their ability to insulate, which, in terms of measureable properties, is assessed by the thermal conductivity of the product. Density is sometimes used as a rule-of-thumb indicator of the insulating ability of an IFB, but this can be misleading.

The difference in thermal conductivity between the different types of IFBs is shown in Fig. 1. It can be seen from this data that the thermal conductivity of the IFBs studied is not directly related to the density. For example, the highest-density product (slinger) has an intermediate set of thermal-conductivity values, but the IFB with the highest thermal conductivity (cement) actually has one of the lowest densities of the products studied. So, to maximize the insulating abilities of IFBs, product selection should not be made on density values.

Commercially published thermal-conductivity data varies in quality and accuracy with some datasheets omitting the test method, which makes the data misleading when comparing and selecting products. The thermal-conductivity data quoted in this work was measured independently to ASTM C-182, but what is not normally published is how the thermal-conductivity data translates to real conditions in service. If one IFB has lower thermal conductivity than another, how does that translate to heat loss in real applications in terms of energy costs? This work serves to answer this question by measuring actual energy use under controlled conditions using different IFBs.

Experimental

We commissioned a kiln builder to manufacture two electrically heated laboratory muffle kilns of identical design and power rating (Fig. 2). One was lined with the cast IFBs as characterized in Table 1, and the other was lined with the cement IFBs. We selected these two IFBs for the study as these represented the IFBs with the lowest and highest measured thermal conductivity.

For each kiln, power meters were set up between the power source and the kiln in order to measure the energy usage during the controlled test firings. Two test firings were conducted. Test 1 was a ramp at 3°C/minute from ambient to 800°C, hold for 15 hours, natural cool back to ambient. Test 2 was a ramp at 3°C/minute from ambient to 1000°C, hold for 15 hours, natural cool back to ambient.

Results

The results of the energy-usage tests are shown in Tables 2 and 3.

By monitoring the kilns during the tests using an infrared camera (VarioCAM, FPA detector 320x240 pixel, 25 mm FOV 32°x25°), the kiln surface temperatures could be measured. Figure 3 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 has the combined effect of wasting energy costs and presenting health and safety issues in terms of hazardous working temperatures.

Discussion

The results of the monitored test firings have demonstrated that there can be considerable differences in energy requirements to heat up kilns constructed using different types of IFB. With the IFB types studied under our test firing conditions, ~37% less energy was needed to run the test kiln through a 1000°C (1832°F) firing cycle with the cast IFB compared to the cement IFB. This difference in energy usage is a consequence of the different thermal conductivities, which are due to the differences in microstructure and pore size created by the manufacturing processes.[2] Figures 4a to 4c show the microstructure of the cast and cement IFBs used in the study as observed under an electron microscope.

Figures 4a-4c show that the cast IFB has a much finer microstructure. The cement IFB has large quantities of relatively large holes in the structure, ranging from 700-1300 micron. Such large pore sizes are formed when combustible materials are added to the mix for the cement-based casting process and are burned out during the firing process. Typically, expanded polymer spheres of ~1 mm diameter are used by manufacturers to create such high levels of porosity in the fired product. This has the effect of reducing density and making the brick light in weight, but it does not contribute so much toward the insulating properties of the IFB.

Both the cast and cement IFBs display similar pore sizes in the mid-size range, around 50 micron diameter. This is again due to use of burnout additives. But the cast IFB has a much higher proportion of pore sizes in the <10 micron range. Mercury porosimetry studies[2] indicate a significant presence of even finer porosity than this in the cast IFB. It is this combination of ultrafine pore structure, coupled with an absence of very large pore sizes that affords the cast IFB with lower thermal conductivity compared to the cement IFB.

IFBs are normally used in applications >1000°C because, at these temperatures, they provide the most cost-effective insulation available compared to alternative insulating refractories (Fig. 5). The structural nature of the products also means that they offer resistance to abrasion in high-temperature environments, coupled with chemical resistance (when the chemistry is tailored to cope with specific gases).

At application temperatures above 1000°C, the most important heat-transfer mechanism becomes radiation rather than conduction and convection, which are the more significant heat-transfer mechanisms at lower temperatures. The large pore sizes in the cement IFB are inefficient at retarding energy transfer at the infrared wavelengths involved, so this type of IFB displays a higher thermal conductivity compared with the cast. Conversely, the microporous structure of the cast IFB, with its small pore sizes, is much more efficient at interfering with energy transfer at infrared wavelengths, so this type of IFB displays low thermal conductivity. This is why the microstructure of the cast IFB provides superior insulation compared to the cement IFB.

Energy Savings The laboratory test results demonstrate the potential to minimize energy usage by appropriate selection of IFB for a kiln lining. To understand how this affects real, full-size kiln installations, we ran heat-transfer calculations (using the same cast and cement IFB types in the laboratory studies) to assess energy running costs for a typical roller kiln used by ceramic ware manufacturers (Table 4).

The hot-face model of the standard lining arrangement (layer 1) was set up based on data from commercially available class-26 IFBs (JM26, Thermal Ceramics). The backup insulation (layer 3) was set up using data from commercially available bio-soluble fiber board (Superwool 607, Thermal Ceramics). To assess the effect on energy consumption of using different IFB types in the lining arrangement, layer 2 was designated the test layer into which the data from different IFB types were input. The results of the heat-transfer calculations are shown in Fig. 6.

The heat-transfer calculations show that the lining arrangement with the cement IFB requires 152 W/m² more energy to maintain the 1300°C kiln temperature than the lining arrangement with the cast IFB in layer 2. So for the 150 m² heating area, the difference in energy consumption between the two simulated roller kilns is 22.8 kW. This equates to a savings of ~230,000 kW/year energy using the cast IFB compared to the cement IFB. Assuming a gas price of 0.035€/kWh, this equates to an annual savings of ~€8,000/year. Since the average life of a kiln lining is about 10 years, the total savings over the life of the kiln lining would be ~€80,000.

A 150 m² heating area in the kiln would need ~8,500 standard-sized IFBs. Although the cast-IFB price is higher than cement in this example, this higher price would be paid back in only four months. After the initial four-month payback period, the rest of the 10-year service life delivers continuous cost savings due to the lower energy requirements.

Additional Impact of IFB Selection Another important consequence of the energy savings achieved using the lower thermal-conductivity IFB is the reduction in CO2 emissions. Using the cast IFB instead of the cement IFB reduces the environmental impact of running the kiln. In the current kiln scenario, as the savings in this example using cast IFB is ~230,000 kW/year, a natural gas-fired roller kiln will require 22,000 m³/year less gas to fire it. As natural gas produces 37.8 MJ/m3, then 830,000 MJ/year will be saved. Since 1 m³ of natural gas produces ~1 m³ of CO2, there is a potential reduction in CO2 emissions of ~22,000 m³/year. The equivalent of 1 m³ of CO2 is 1.96 kg, which equates to ~43 tons/year reduction in CO2 produced or 430 tons over the life of the kiln lining.

A further benefit of using the lower thermal-conductivity cast IFB against the cement IFB is that the outer temperature of the kiln is lower. In the example calculated in this work, the skin temperature of the kiln utilizing cast IFB in layer 2 is 79°C, whereas the skin temperature of the kiln utilizing cement IFB in layer 2 is 88°C. 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 kiln compared to the higher thermal-conductivity cement IFB.

The choice of IFB in the kiln lining will also impact other practical aspects of using the kiln in a production environment. Selecting the cast IFB rather than the cement IFB will allow faster heating and cooling rates in the kiln, because the lower-density cast IFB has a lower thermal mass. This effect was observed in the energy studies reported in this paper. During both the 800°C and 1000°C test firings, the cast IFB reached the programmed dwell temperature faster than the cement IFB.

Conclusions

The work reported in this paper has demonstrated the following points:

• Differences in energy use as large as 37% were measured, under controlled laboratory conditions, between IFBs manufactured by different methods.
• When selecting insulating refractory products for furnace linings, close attention should be paid to the reported thermal conductivity of IFB products.
• The density of the products should not be used as a criterion to assess insulation ability, as this may lead to incorrect product selection.
• To minimize energy consumption in the kiln, the published thermal conductivity needs to 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 available today at application temperatures and, therefore, provide the greatest energy savings.

This paper has quantified the energy savings that are possible when using cast IFBs. The benefits of using the lowest thermal-conductivity IFBs available are:

1. Large cost-saving potential due to reduced energy usage
2. Lower CO2 emissions due to reduced energy usage
3. Reduced kiln surface temperatures offer operators safer working conditions IH

For more information: Contact Dr. Andy Wynn, Morgan Thermal Ceramics, Tebay Road, Bromborough, Merseyside, CH62 3PH, U.K.; tel: (+44) 151 482 7483; fax: (+44) 151 482 7426; e-mail: andy.wynn@morganplc.com; web: www.morganplc.com or Lance Caspersen, Morgan Thermal Ceramics, 2102 Old Savannah Rd, Augusta, GA 30906; tel: 706-796-4200; fax: 706-796-4328; e-mail: lance.caspersen@morganplc.com