By James Yang Dec 11, 2023
1. Current Situation
Almost all sintered brick factories use the heat recovered from tunnel kilns as the heat source for the drying chamber, which is a desirable means of energy recycling. However, due to various drawbacks in the current methods of heat recovery from tunnel kilns, it poses numerous challenges to product quality control and the external environment. Therefore, it is essential to conduct in-depth research on a rational approach to heat recovery from tunnel kilns and effectively utilize the residual heat.
With the increasing demands for energy efficiency and environmental protection in the brick-making industry, the application of secondary code-firing technology to produce hollow and insulation blocks is gaining traction. This underscores the urgency of addressing the aforementioned issues.
A popular method for tunnel kiln heat recovery involves extracting mid-to-low temperature residual heat from the tail section as the drying heat source and supplementing it with high-temperature heat. However, this method has several drawbacks:
- The double impact of the withdrawal of residual heat after the sintering zone affects the temperature distribution in the sintering zone, making it challenging to control the operation of the tunnel kiln effectively.
- The presence of water vapor in the flue gas results in the drying medium not being dry hot air. This leads to higher residual moisture in the dried product, which does not meet the ideal target of around 2%. Additionally, moisture from the green body enters the flue gas in the preheating zone, hindering thorough drying during subsequent stages, creating a harmful cycle.
In theory, the heat difference from the high firing temperature to the exit temperature after the sintering zone, excluding heat dissipation from the kiln body, kiln car, and product, can be recovered. However, the current methods of tunnel kiln heat recovery fail to maximize heat recovery, especially at high temperatures. The product temperature is excessively high, with an average temperature difference of about 150°C between the inside and outside of the kiln car, while the exit temperature is close to room temperature, which is not ideal. Consequently, it requires about 30% additional high-temperature flue gas to provide sufficient heat to the drying chamber, and significant heat loss occurs when the kiln car and product exit the kiln.
This heat extraction method disrupts the temperature curve of the cooling zone, posing a risk of cracking in the mid-temperature range of the product. Moreover, a large amount of flue gas entering the hot air pipeline system and during drying causes severe corrosion to the drying car and trays, and the gas escaping from the drying chamber, containing smoke and acid mist, poses a serious threat to the steel structure plant and the surrounding environment.
2. Rational Temperature Curve for the Cooling Zone
Based on the working principle of the tunnel kiln, its operating system is divided into the preheating-sintering zone (including insulation zone) and the cooling zone. These two zones should work separately without interference, with the preheating-sintering zone and the cooling zone being effectively isolated. A practical method to achieve this is by employing a rapid cooling barrier. The cooling zone is divided into three stages:
2.1 Rapid Cooling Stage
To separate the preheating zone from the cooling zone, a rapid cooling barrier must be set at the beginning of the cooling zone. Firstly, the rapid cooling barrier effectively prevents the reverse flow of airflow from the insulation area to the cooling area, facilitating smoother adjustment of the exhaust system. Secondly, rapid cooling improves the mechanical properties of the product and the apparent texture of the product significantly. Thirdly, the high-temperature gas generated by rapid cooling is conducive to the recovery of high-temperature residual heat.
It is crucial to note that the position of the rapid cooling stage must be at around 800°C to the high yield temperature (some raw materials can be reduced to 700°C). At this point, the product has good elasticity, and the shrinkage deformation of the product is elastic rather than plastic deformation, eliminating the risk of product cracking.
Therefore, automatic control of the rapid cooling barrier based on the defined kiln temperature is essential. For large-section tunnel kilns, rapid cooling barriers are arranged in several groups at the top of the kiln.
2.2 Slow Cooling Zone
Silica (SiO₂) in the product transforms from α-quartz to β-quartz at 573°C, with a volume change rate of 0.82%. Although the volume change of this low-temperature transformation is not significant, the transformation rate is fast, and it occurs under conditions of no liquid phase buffering, making it highly destructive. Therefore, no cooling measures are allowed between 500°C and 800°C, and the cooling of the product must be very slow within this temperature range. In other words, there must be a sufficient number of kiln cars in this segment of the tunnel kiln.
2.3 Accelerated Cooling Zone
From 500°C to the product exit temperature (around 50°C), the product can be rapidly cooled, typically by introducing cold air into the kiln through the use of a backpressure fan at the kiln tail. Since this cooling process takes a long time, the tunnel kiln must have sufficient length in this segment. Increasing the cooling air volume is also an effective method, providing more hot air to the drying chamber.