What is the working principle of the mixed lime vertical kiln
Blended lime vertical kiln (also known as lime vertical kiln) is one of the important equipment used for limestone calcination in industry, widely used in cement production, metallurgical industry, and chemical industry. It decomposes limestone (mainly composed of calcium carbonate) into calcium oxide (quicklime) and carbon dioxide gas by high-temperature heating. The working principle of the mixed fired lime vertical kiln is based on the principle of chemical reactions at high temperatures, and the specific operation method has certain specificity and complexity.
1. Structure and function of shaft kiln
The mixed fired lime vertical kiln is usually in a vertical shape, with an upper kiln mouth, a lower kiln mouth, a fire passage, and a uniformly distributed air intake system. The kiln body is equipped with multiple layers of brick insulation and refractory layers to prevent damage from high-temperature gases. The upper part of the shaft kiln is the inlet for limestone raw materials, and the lower part is the outlet for calcined products - quicklime and emitted carbon dioxide gas.
2. Calcination process of limestone
The main component of limestone is calcium carbonate (CaCO3), and the chemical reaction equation for its calcination process is as follows:
CaCO₃ + high temperature → CaO+CO ₂
In a lime kiln, limestone decomposes into calcium oxide (CaO, quicklime) and carbon dioxide gas (CO ₂) at high temperatures. This process typically occurs within the temperature range of 900 ° C to 1000 ° C.
3. Temperature control and layered firing inside the kiln
A significant feature of the mixed fired lime vertical kiln is that the temperature distribution shows a gradient change. Usually, the upper part of the kiln has a lower temperature for storing and entering limestone, while the lower part of the kiln has a higher temperature and is suitable for the calcination reaction of limestone.
Feeding and cooling area (upper part): Limestone first enters the kiln through the feeding port above. In the upper part of the kiln, the limestone gradually heats up and undergoes preliminary reactions such as dehydration. Due to the lower temperature, the chemical reaction rate of limestone is slower.
Calcination zone (middle): When limestone passes through the middle zone, the temperature reaches the range required for its decomposition (approximately 800 ° C to 1000 ° C). At this point, the calcium carbonate in limestone decomposes and is converted into calcium oxide (quicklime) and carbon dioxide.
Combustion zone (bottom): The temperature at the bottom of the kiln is usually above 1000 ° C, which is suitable for promoting thorough calcination of limestone. In this area, the interaction between calcium oxide and high-temperature airflow intensifies the reaction. The fuel is usually coal, natural gas, or heavy oil, and the heat generated by combustion is used to heat the lower area.
4. Airflow and fuel combustion inside the kiln
In order to maintain high temperature inside the kiln, direct or indirect heating with fuel is usually used. In a mixed fired lime kiln, the fuel is usually burned at the bottom of the kiln. The high-temperature gas rises upwards through the kiln body, driving the airflow inside the kiln and producing a good heat exchange effect. In order to improve the thermal efficiency of the kiln, flue gas recirculation technology is often used, which guides some of the kiln gas back to the top of the kiln body and contacts with newly input limestone through the feed inlet, thereby improving the thermal utilization efficiency.
5. Emission and treatment of calcined products
The calcined lime is transported to the bottom of the kiln and discharged through the discharge outlet. At this point, calcium oxide (quicklime) is ready for industrial applications. In addition, carbon dioxide gas will be discharged through the kiln roof and needs to be treated or emitted appropriately, especially in areas with strict environmental protection requirements, where further reduction of carbon dioxide emissions may be necessary.
6. Control and Optimization
The operation of lime kiln involves complex temperature control and airflow regulation. In order to maintain a good calcination effect, it is necessary to precisely control the temperature distribution, airflow direction, fuel supply, and limestone addition rate inside the kiln. Modern vertical kilns are usually equipped with automated control systems that can monitor various indicators inside the kiln in real time, such as temperature, pressure, gas composition, etc., and adjust various parameters according to actual needs, thereby achieving efficient and stable production.
7. Advantages and Challenges
The advantages of mixed fired lime vertical kiln are its simple structure, flexible operation, and low investment, making it suitable for small and medium-sized lime production enterprises. Through reasonable kiln design and operational control, high production efficiency and good product quality can be effectively achieved. However, the disadvantages of vertical kilns are also quite obvious, especially in terms of challenges such as thermal efficiency and environmental pollution. Vertical kilns are prone to high fuel consumption and significant carbon dioxide emissions, therefore, in modern production, more and more enterprises tend to adopt advanced rotary kilns and other equipment.
As a traditional lime production equipment, the mixed fired lime shaft kiln still occupies an important position in the lime industry due to its simple structure and high production efficiency. By rational design and optimized control, the utilization efficiency of kiln temperature can be improved, energy consumption can be reduced, and environmental pollution can be minimized. With the continuous improvement of environmental standards, the technological improvement and energy conservation and emission reduction of lime shaft kilns are still important directions for future development.
