High-temperature, energy-efficient ceramic coatings for refractories—no longer “theoretical” technology—are being used successfully in kiln and furnace applications to reduce energy consumption, improve temperature uniformity, reduce maintenance, and increase production while improving product quality. By changing the re-radiative properties of a refractory lining in a furnace, these specialized ceramic coatings can provide energy savings of up to 15-20%, depending on the fuel being used, the furnace operation and configuration, and the production schedule. In addition, furnace heat-up time is decreased, and the service life of the high-temperature, ceramic-coated refractory is extended.

Ceramic Coating Technology

Water-based, spray-applied ceramic coatings can be used to coat both refractory and metal substrates. When applied, the coatings reradiate 90-95% of the radiant energy output by either burners or electrical elements used to heat the kiln. The key to the coating’s thermal performance is that it provides increased hemispherical emissivity (the ability to absorb energy and subsequently emit that energy back out from the surface). Using the equation below for radiation heat transfer, the coating’s performance increases with increasing temperatures:

Q = E_h x δ x (T_h⁴-T_c⁴)

where Q = re-radiated energy (BTU/hr-ft²), E_h = emissivity of hotter surface, δ = Stefan-Goltzmann constant, T_h = temperature of the hotter surface, and T_c = temperature of the colder surface. As the hot face of the refractory or metal surface receives the radiant energy, it transmits the energy back the colder body in the kiln. The efficiency with which it achieves this is related to the emissivity of the surface and the quantity of heat absorbed and re-emitted.

Figure 1 shows a comparison of an uncoated surface vs. a coated surface. As radiant heat waves bombard the surface of the coating, the waves bounce around the ceramic molecules and are then absorbed. The energy imparted on these molecules causes them to vibrate. The higher the temperature, the faster the ceramic molecules vibrate, making the thermal barrier more effective due to less heat being transferred to the underlying material. Due to the loss of the energy that is imparted on the molecules, the re-released heat is emitted at an elongated wavelength, resulting in a lower temperature coming back out from the surface.

The coating also decreases the catalytic efficiency of the surface, resulting in a decreased surface temperature and protecting the underlying substrate from heat-related damage. In addition, because less energy is transferred to the substrate, the coating increases the efficiency of the system by preventing heat transfer and dissipation.

Ceramic coatings are compatible with all types of refractories, as well as all ferrous and non-ferrous metals. The coatings account for expansion and contraction of any underlying material on which they are applied. Instead of designing the coatings to have a thermal expansion matching the intended substrate, the binder was formulated to optimize the bond strength between the coating and the substrate. Under thermal cycling conditions, the bonding of the coating allows it to move with the dimensional change of the substrate, thus preventing shearing between the coating and the substrate.

By increasing the thermal reflectivity of a refractory lining in a kiln or furnace, specialized ceramic coatings can provide energy savings of up to 20%, depending on the fuel type, furnace operation, furnace configuration, and production schedule. Ceramic-coated refractories also decrease furnace heat-up and turnaround times and extend service life.

The latest coatings are formulated based on water-soluble nanotechnologies, so no solvents are required for dilution or cleanup. They contain no volatile organic substances and are applicable to all types of refractories. Once cured, ceramic coatings are environmentally inert and do not require special handling or disposal. They are also resistant to most solvents and are unaffected by a wide range in pH.

The protection of metal substrates is achieved in a similar manner. A primer applied prior to other coatings helps prevent stainless steel and steel parts from erosion, oxidation and fatigue, while also protecting burner nozzles and electric heating elements from prolonged use at elevated temperatures. The resulting impervious, non-reactive barrier is resistant to combustion by-products, such as chlorides, and prevents these corrosive elements from coming into contact with the metal substrate.

Case in Point

In August 2017, International Technical Ceramics (ITC) was contracted by NC State University’s Craft Center to coat its cone 10, gas reduction car kiln and barrel raku kiln. The approximately 50 ft³ gas reduction kiln is a sprung arch design lined with 2,600°F insulating firebrick (IFB). The barrel raku kiln is lined with 2.5 in. of 2,600°F ceramic fiber blanket.

Surface preparation of both kilns consisted of removing (vacuuming) all dust and debris from the surface of the refractories. For the gas reduction kiln, the interior surface, sidewalls and roof, as well as the surface of the car, were coated with approximately  1/16 in. of 100HT ceramic coating. Completely coating the entire hot face surface area of the refractory lining increases the kiln’s efficiency by providing a more even temperature uniformity.

With additional heat available to reradiate energy, the ceramic coating shortened NC State’s firing time by 1 hr, reducing fuel consumption by approximately 3-4%. Shell temperatures on the kiln have also been reduced by 30-50°F, which has improved safety issues related to potential burns from accidentally touching the kiln while in use.

For the barrel raku kiln, the entire ceramic fiber surface area was coated with approximately 3/16 in. of 296A High Purity Top Coat to help protect the fiber lining against the damaging effects of the glazes used. The shell temperatures of the barrel raku kiln have been decreased by 50-80°F, which has again alleviated safety concerns. In addition, the firing time has been decreased by 50%, providing a reduction in fuel consumption of approximately 30%.

Lasting Benefits

The results realized by the Clay Studio of the Crafts Center at NC State University have provided a safer environment, along with more economical and cost-efficient operation. Both the gas reduction kiln and the barrel raku kiln will be maintenance free for years to come. This technology will enable the kilns to be more efficient while producing a higher quality product due to their ability to attain higher, more uniform firing temperatures.

Safety and power efficiency are the main goal for these protective coatings, which are designed to increase work environment safety, prolong equipment life, and reduce energy costs/environmental impact. The ceramic coatings are formulated based on technology that is environmentally friendly and water soluble, containing no volatile organic substances, and are applicable to both refractory and metal substrates.

For more information, contact the author at greg@itccoatings.com or visit www.itccoatings.com.

This article was originally posted on www.ceramicindustry.com.