The production technology of tunnel kiln

The type of tunnel kiln is like a long and narrow tunnel. The side walls and top of the kiln are made of firebricks. In the kiln, there are several mobile kiln cars running along the track buried on the foundation. From the end of the cart, it is divided into pre-tropical zone, firing zone and cooling zone along the length direction of the kiln. According to the technical requirements, the products are stacked on the kiln cart, and then pushed into the kiln by the cart machine at the kiln head. The products are preheated, burned and cooled in the kiln, and finally the finished products are obtained from the kiln tail.

In order to ensure the normal operation of the tunnel kiln and make its thermal system meet the requirements of product firing, the following fan systems are also set up outside the kiln: (1) Exhaust fan system: exhaust gas in the kiln and adjust the pressure in the kiln. (2) Pre-tropical circulation system: Adjust the temperature and pressure at the entrance of the kiln to improve the yield. (3) Vehicle underpressure balance system: Adjust the pressure in and under the kiln to reduce the heat escaping into the kiln and damage the kiln vehicle. (4) Combustion air system: The hot air from the cooling zone is pumped to the burning zone for combustion, saving energy. (5) Blast cold air system: cold air is sent to the cooling belt in the kiln, and heat exchange is carried out with the passing products. The products are cooled, the cold air is heated, and then the hot air is used to assist combustion.

The above are some basic fan Settings of tunnel kilns. According to different kiln types and different process requirements, there will be other auxiliary systems, which will not be described here.

The main characteristics of tunnel kiln are continuous production, large production capacity, low fuel consumption, high degree of automation and low labor intensity. Because the temperature of kiln can be controlled accurately, the quality of fired products is stable, so it is suitable for the single variety of refractory products with large production capacity. The deficiency has the following two points: one is the large investment; Secondly, when the temperature system of tunnel kiln needs to be changed, a certain number of transition vehicles should be promoted, which may cause some waste products. Meanwhile, the working lining in the kiln is easy to be damaged under the frequent temperature rise and fall, which will affect the service life of the kiln. Therefore, tunnel kilns are generally not suitable for the production of small-batch, multi-variety and special-shaped refractory products.

According to the maximum operating temperature of tunnel kilns, tunnel kilns are generally divided into the following three types: (1) low-temperature tunnel kilns: the firing temperature is about 1000℃, which are mainly used for baking sliding plate bricks and other products with special technological requirements. (2) Medium-temperature tunnel kiln: the firing temperature is 1300~1650℃, mainly used for firing ordinary alkaline brick, clay brick, high aluminum brick, sliding plate brick, water mouth brick, silicon brick and other products. (3) High temperature tunnel kiln: the firing temperature is more than 1700℃, generally between 1800 and 1900℃, mainly used for firing middle-grade magnesia bricks, high-purity magnesia bricks, directly combined magnesia chrome bricks, magnesia aluminum and rigid jade products.

Also some people according to the name of fired products classification, this is because different products, firing technology is not the same, the length of the kiln, height is not the same. For example, the tunnel kiln of silicon brick is generally 150~180m long, and the height from the table to the top of the kiln is 1.6~1.9m. The length of the alkaline brick tunnel kiln is 80~100m, and the height from the table to the top of the kiln is about 1m.

With the popularization and application of new technology in metallurgy and building materials industry, the use conditions of refractories are becoming more and more severe. A large number of high purity and high quality refractory materials need to be fired by high-temperature tunnel kiln. At present, dozens of tunnel kilns have been built at high temperature between 1800 and 1900℃ in China. Their main technical performance is as follows:

Kiln length: 50~156m; Width of kiln: 1.1~3.2m;

Production capacity: 5000~32000t per year;

Heat consumption per unit product: 6600~6900kJ/kg.

The following measures are generally adopted in the design of high temperature tunnel kilns at 1800~1900℃ :

(1) Double-layer arch roof structure is adopted in the sintering zone of the kiln. Hot air is extracted from the cooling zone of the tunnel kiln with a high temperature fan and sent to the double-layer arch of the sintering zone for secondary heating. The hot air is heated to about 1000℃, and then sent to each burner through the kiln wall air passage. In this way, it not only cools the working lining of the burning belt arch, improves the service life, but also improves the temperature of combustion air to ensure the high temperature in the kiln.

(2) Reduce the lower space of kiln car, so as to achieve the pressure balance between kiln and kiln, and reduce the hot gas in the kiln and the gas flowing under the kiln. The kiln car is constructed with high strength lightweight materials to reduce heat loss of kiln car.

(3) The fuel and the combustion-supporting high temperature air coming down from the double-deck vault are directly sprayed into the burning space between the brick stacks, and the high-speed mixed combustion is referred to as the direct combustion mode. Can ensure the high temperature firing, and improve the thermal efficiency.

(4) A circulating fan system shall be installed in the pre-tropical zone and the cooling zone of the kiln. The circulation fan of pre-tropical zone is used to ensure the temperature at the entrance of kiln, which is not too high, so as to avoid the brick crack caused by rapid overheating. The circulating fan with cooling belt is used to ensure the product is cooled quickly and evenly and improve the quality of the product. And can reduce the length of the cooling belt, reduce investment.