Thermal cycling and its significance in an incinerator.

Thermal cycling is a common phenomenon that occurs in many high heat applications, such as in incinerators, industrial furnaces, kilns, and reactors. Refractory materials are commonly used in these high-temperature environments due to their ability to withstand high temperatures, thermal shock, and chemical corrosion. However, thermal cycling can have a significant impact on the life of refractory materials. As refractory materials are repeatedly exposed to high temperatures and rapid cooling cycles, they can develop cracks, spalling, and other forms of damage, which can reduce their lifespan and affect their overall performance. Understanding the effects of thermal cycling on refractory materials is critical in selecting and maintaining the right materials for high-heat applications.

The figures below represents an incinerator that operates in 12 hour shifts (12 hours on, 12 hours off) vs an incinerator in full operation for a week. It is evident from these images that operation at 12 hours a day, results in approximately a full thermal cycle per day. This thermal cycling as explained above, dramatically decreases the life of the refractory, resulting in more maintenace and repairs.

Steady State Conduction_A4 24 Hour

Figure 1: 24 Hour Operation for 5 Days

Transient Heat Conduction_ A4 12 Hour

Figure 2: 12 Hour Operation for 5 Days

Thermal cycling can occur due to various reasons, such as changes in operating conditions, thermal gradients, and mechanical stress. During the heating phase, refractory materials expand due to thermal expansion. As the temperature decreases, the materials contract. The repeated expansion and contraction cycles cause stress on the material, which can lead to micro-cracking, spalling, and erosion of the refractory material. Furthermore, the thermal cycling can cause thermal fatigue, which can increase the likelihood of cracking and spalling. This can compromise the structural integrity of the material, leading to reduced performance and eventually failure.

To mitigate the effects of thermal cycling on refractory materials, several strategies can be adopted. One of the most effective ways is to select materials that are specifically designed to withstand the thermal cycling. For example, some high-heat refractory materials are made from high-alumina, silicon carbide, or zirconium oxide, which are known to have high resistance to thermal shock and thermal cycling. Another strategy is to apply coatings to the surface of the material, which can provide an additional layer of protection against thermal shock and cyclic thermal stresses.

In addition, regular maintenance and inspection of refractory materials can help detect any signs of damage or wear, allowing for timely repairs or replacements. It is also important to ensure that the refractory materials are installed correctly, as improper installation can cause additional stress and damage during thermal cycling.

In conclusion, thermal cycling is a significant factor affecting the life of high-heat refractory materials. By understanding the effects of thermal cycling and adopting appropriate mitigation strategies, it is possible to improve the performance and extend the lifespan of refractory materials in high-heat applications.

Author: Devon Shepherd
Mechanical Engineer @ Macrotec