3. Heat Recovery Model for the Cooling Zone
In summary, there are two components of heat in the cooling zone of a tunnel kiln that can be recovered. The first is the high-temperature heat generated by the rapid cooling barrier, and the second is the low-temperature heat produced during accelerated cooling. Both of these heat components are clean and dry hot air, suitable for use in the drying chamber.
After the rapid cooling barrier is introduced into the kiln, a significant temperature difference occurs inside and outside the kiln products. To ensure uniformity in kiln temperature and product temperature, it is essential to extract the high-temperature gas generated by the rapid cooling barrier in a counter-current manner. This means that the outlet for extracting high-temperature gas should be closer to the exit end of the tunnel kiln than the inlet for introducing rapid cooling gas.
Due to the high temperature of the gas extracted from the kiln, the exhaust duct must be made of heat-resistant steel. Additionally, the exhaust duct must be equipped with special valves for distributing cold air. The main hot air duct may also have an inlet valve for cold air, and the hot air main duct must have an inlet valve for cold air. The temperature of the hot air entering the heat recovery fan should not exceed 300°C.
A large volume of air for accelerated cooling is introduced into the kiln from the tail and gradually extracted from the top after exchanging heat with the products. The temperature of the hot air entering the low-temperature heat recovery fan is approximately 150°C.
The drying chamber requires a completely different temperature and air volume for the drying medium. Therefore, it is necessary to install a separate hot air fan for drying and a preheating fan for the tunnel kiln, with an interposed container between them. This facilitates the mixing of high-temperature hot air, low-temperature hot air, and injected cold air, ensuring that the hot air fan can provide the drying chamber with a suitable temperature and air volume of the drying medium. This process can be automated. Additionally, the tunnel kiln working system and the drying chamber working system are completely separated, facilitating independent control and adjustment.
4. Other Recoverable Heat in the Tunnel Kiln
4.1 Smoke Heat
In the firing zone of the tunnel kiln, a large amount of smoke is generated by burning the fuel for firing products, and it flows to the preheating zone under the action of the exhaust fan, preheating the blanks. If high-temperature smoke heat is extracted at 350°C in the residual heat zone, it will undoubtedly affect the initial heating effect of the smoke in the preheating zone. The major drawback of utilizing high-temperature smoke is that the acidic gases in the smoke will severely corrode the drying equipment, and the outgoing gas from the drying chamber will pose a threat to the environment. Therefore, using smoke heat as a drying medium is not advisable. Instead, the smoke should be fully exchanged with the blanks inside the tunnel kiln until its temperature drops to 100-150°C. Then it should be expelled from the kiln and centrally treated through dust elimination equipment to meet the standard requirements before being discharged.
4.2 Kiln Cavity Heat Exchange
In the past, kiln cavity heat exchange was considered a method for kiln insulation. With the improvement of insulation materials, this heat exchange method has gradually been phased out, especially kiln wall cavity heat exchange. Some tunnel kilns still have kiln top cavity heat exchange to reduce the temperature of the kiln roof structure. Heat-exchanged hot air at approximately 50°C can be used as supplementary air volume for the drying medium.
4.3 Hot Air After Kiln Car Cooling
Modern tunnel kilns are equipped with an undercarriage pressure balance system. A cooling fan is installed at the position of maximum heat accumulation under the kiln car to blow air onto the structural parts of the kiln car for cooling. The gas after heat exchange flows in two directions, towards the kiln head and kiln tail. The gas flowing towards the kiln head is extracted at the corresponding exhaust position to achieve pressure balance between the upper and lower parts of the preheating zone. Due to imperfect sealing between kiln cars, some smoke components may infiltrate into the lower part of the kiln car. Therefore, the gas is generally treated together with the kiln exhaust gas through the kiln exhaust system. The gas flowing towards the kiln tail enters the low-temperature heat recovery pipeline through pipes.
5. Conclusion
1. Using the rapid cooling barrier can divide the tunnel kiln into two parts. Combining heat recovery with cooling process requirements can effectively recover the crystalline dry hot air as a drying medium in the tunnel kiln cooling zone.
2. Based on calculations and practical experience, the heat recovered from the tunnel kiln cooling zone can account for 35% to 45% of the total heat consumption of the tunnel kiln, meeting the heat requirements of the drying chamber. For example, in a tunnel kiln with a heat consumption index of 380 kcal/kg, the recovered heat can be as high as 152 kcal/kg. The heat consumption index of the drying chamber is approximately 1100 kcal/kg of water. When the moisture content of the wet blanks is reduced from 20% to a residual moisture of 2% during drying, the required heat is 160 kcal/kg, achieving a basic balance between supply and demand.
3. For internal combustion tunnel kilns, as long as they are not superheated, this heat recovery method is also applicable. It is advisable not to exceed an internal combustion ratio of 80%.
4. For secondary code-firing tunnel kilns, smoke should never be extracted as a drying medium. This approach is detrimental to the firing process of the tunnel kiln, environmentally unfriendly, and harmful to drying equipment and steel structure workshops.