Clinker kiln, also known as cement kiln, is the equipment used by cement plant for calcinating raw materials into clinker at extremely high temperature. In the production process, raw materials are fed from the cold end of the clinker kiln. Because the kiln has a certain inclination and constantly rotating, raw materials will move toward the hot end continuously. At the same time, the fuel is injected from the hot end, which releases heat and generates high-temperature gas under air-assisted combustion. Driven by the fan, the hot gas flows from the hot end to the opposite end, and the material and flue gas exchange heat in the process of reverse movement so that raw materials are converted into clinker. It can be seen from the above content that the calcining process of cement kiln mainly focuses on the movement of materials in the kiln, the flow of gas in the kiln, the combustion of fuel and the heat transfer between materials and gas.
In the last article The Calcining Technology Of Clinker Kiln, we talked about controlling the negative pressure and airflow velocity in cement kiln and the influence of coal quality on clinker calcining. In this article, we will put our focus on other aspects.
Calcining Technology of Clinker
Increase the Primary Air Flow and Blowout Speed
Primary air flow can be divided into high-speed axial air, cyclone air and a small amount of low-speed central air, which will transport pulverized coal into the kiln, forming a powerful “willow leaf” flame. If the primary air flow decreases, we can increase the secondary high temperature air flow instead. At present, the feeding coal air and the total amount of primary air flow account for about 10%-15% of the total combustion air in kilns, which is the actual level that can be reached. In addition, the unique structure of placing coal air inside the cyclone air is adopted to form a flame with moderate intensity and reasonable thermal distribution in clinker kiln, which is convenient for timely adjustment and control in production. When we use the kiln system with high-efficiency grate cooler, the primary air flow ratio is controlled at about 8%. While when we use the kiln system with the single cooler, we should increase the primary air flow ratio to 14%-18% so that the kiln system can enter a good state.
The Role of Stable Kiln Tail Feeding
A stable feeding device at the kiln tail is very important for cement rotary kiln. In this stable feeding device (as shown below), the screw conveyor is used for metering and adjusting. It directly reflects the feeding amount to the instrument panel at the kiln head through the sensor, giving the operator an intuitive quantitative, so as to facilitate the control of the whole kiln parameters. However, the conveying capacity of raw material screw conveyor is always greater than that of metering screw conveyor. The redundant raw materials will enter the recycle pipe and be sent into the elevator. In addition, the raw material silo is equipped with a film switch alarm to control the material level, so as to prevent an excessive fluctuation. When the material level is higher than the control upper limit, the feeding of the elevator and screw conveyor will stop. And when the material level in the silo is lower than the lower limit of the control point, the feeding of the elevator and screw conveyor will start. An arc pressing sleeve is installed at the inlet of the raw material screw conveyor at the bottom of the silo to prevent raw materials from directly rushing into the metering screw conveyor and the raw material screw conveyor, thereby causing a blockage.
The Coal Circle and Kilneye Brick
In the process of calcination in clinker kiln, the coal circle in the sintering zone is very difficult to form. Under normal conditions, if the calcination environment in the kiln is good, the output will be high. Therefore, we should pay more attention to the formation and shedding of kiln skin in the sintering zone, so as to prevent the bad effect caused by carelessness. Moreover, the coal circle should not be too thick. If we find it is difficult for clinker to pass through, we should clean up in time otherwise it will affect the production.
According to the actual calcination experience, we found that the formation of coal circle is beneficial to some extent. Especially when the thickness of the circle is moderate, it will be helpful for the clinker calcination. For example, when the pulverized coal enters the sintering zone from the air pipe, the coal circle will make the flame fully burned and get a higher flame temperature. In addition, we should also increase the height of kilneye brick, thicken the material layer of the cooling zone and increase the residence time of clinker in high-temperature zone. In this way, the kilneye temperature of the rotary kiln can be raised, the combustion of pulverized coal in the kiln can be well promoted, and the combustion speed can be accelerated.