In the brick and ceramics industries, fuel cost is a major component of operations cost. Operators of tunnel kilns can cut down operating costs by using EMISSHIELD high emissivity ceramic coatings. Using the coatings on tunnel kiln refractories will also provide increased productivity, even heating of load and longer refractory life.
The EMISSHIELD technology was developed by NASA for use in space vehicles (Figure 1). EMISSHIELD high emissivity coatings are manufactured for industrial applications by Wessex, Inc., which combined the NASA technology with its own binder systems.
Figure 1. X-33 Orbiter
In tunnel kilns, insulating refractories are placed at the cold face of dense refractories in order to reduce heat loss. In this arrangement, however, refractory materials need to withstand higher temperatures as a significant amount of heat gets stored in refractories. Moreover, valuable energy is lost as the working lining functions as a heat sink during cyclical operation (Figure 2).
Figure 2. Tunnel kiln cross-section with insulating refractory backing up dense refractory working lining
EMISSHIELD coatings are typically applied to the hot face of the tunnel kiln as shown (Figure 3). The heat energy radiated from the burners is absorbed by the high emissive coating and re-radiated back to the cooler load in the kiln.
Figure 3. Tunnel kiln cross-section with EMISSHIELD® high emissivity coating applied to the refractory hot face. The thermal energy absorbed by the coating, TC, is re-radiated and absorbed by the colder load, TL. The refractory lining is subsequently cooler and retains less heat energy.
EMISSHIELD Application in Tunnel Kilns
Emissivities of refractories typically range from 0.3 to 0.5 and can be increased to nearly 0.9 by coating them with EMISSHIELD. An increase in the emissivity of the refractory results in an increase in the heat energy absorbed by the kiln load. As this may result in over-firing that can affect the properties of the ware that is being fired, the burners need to be turned down. This results in significant fuel savings. When lower burner settings are being determined, an optical pyrometer may be used to ensure that the firing temperature in the kiln remains at the required value.
Alternatively, the push rate of the kiln car can be increased to ensure that the increased radiant heat in the kiln is absorbed without resulting in overheating of the load after EMISSHIELD is applied. Some operators may prefer this increase in productivity to fuel savings that can be achieved by turning down the burners.
EMISSHIELD high emissivity coatings absorb and re-radiate maximum heat energy when the temperature difference between the kiln load and the coating is maximum. Therefore, EMISSHIELD is applied only in preheat and soak zones in tunnel kilns. In the cooling zone of tunnel kilns, the temperature of fired ware is more than that of the refractory and so coating is employed in this zone.
EMISSHIELD coatings adhere well to insulating fire brick, dense refractories or refractory ceramic fibre products. The coating can be applied even if the substrate is worn or corroded. Figure 4 shows EMISSHIELD coated refractories whose surfaces were cracked, spalled and corroded. However, the surface has to be free of dirt and oil. It must not be covered with refractory wash coats or mineral deposits.
Figure 4. Tunnel kiln refractories that have been coated with EMISSHIELD
EMISSHIELD is sprayed by Certified EMISSHIELD Installers (Figure 5). Installation can be completed within a day unless excessive preparation of the surface by sandblasting is required. The coating will dry in about 24 hours and the kiln can be used the next day.
Figure 5. EMISSHIELD® being sprayed in a tunnel kiln
Expected Results When Using EMISSHIELD
High process efficiencies can be achieved by effective use of insulation in tunnel kilns. The chief benefit of using EMISSHIELD coatings in tunnel kiln refractories is fuel savings achieved through turning down the burners. Users of EMISSHIELD report fuel savings ranging from 6% to 20%, depending on how the kilns are designed and operated. Alternatively, the kiln push rate can be increased to utilize the excess heat energy available and productivity improved.
When EMISSHIELD is applied to linings, radiant heating is uniform and results in increased yield. Another benefit is that refractories stay cooler and are subjected to less thermal shock and stress. As a result, the service lives of refractories can be expected to increase.
This information has been sourced, reviewed and adapted from materials provided by ANH Refractories Europe.
For more information on this source please visit ANH Refractories Europe.