- Time:Feb 16, 2023
Due to the high temperature in the rotary kiln, continuous rotation and concentrated chemical reactions, internal stress and mechanical stress will be generated on the surface of the kiln lining, and the kiln lining will be eroded by the melt, causing the refractory bricks to fall off and causing the surface temperature of the kiln body to be too high. The method can prolong the service life of the fire brick, prevent the shell of the rotary kiln from being damaged, reduce the heat loss to the outside, and improve the thermal efficiency. By discussing how to hang the kiln skin of the new dry-process rotary kiln, analyzing the influencing factors of the kiln skin hanging and seeking the protection measures of the kiln skin, we have a deeper understanding of the rotary kiln hanging kiln skin, and lay a theoretical foundation for future study, production and practice.
1. What is hanging kiln skin
Rotary kiln is the main equipment for cement production. The cement rotary kiln is an inclined rotating cylinder placed on several pairs of supporting wheels through several belts, and its inner wall is inlaid with refractory bricks. Raw meal powder is fed into the kiln shell body from the high-end feeding pipe at the kiln tail shell. Due to the inclination and gentle rotation of the kiln shell body, the material will roll along the circumferential direction and flow from high temperature to low temperature along the axial direction. The composite movement of end movement, the raw material goes through preheating, decomposition, firing and other processes in the kiln. After the cement clinker is fired, it is discharged from the bottom of the kiln shell and enters the cooler. Fuel is sprayed from the kiln head and burned in the kiln. The heat generated heats the raw meal, making the raw meal calcined into clinker. The hot air formed during the exchange with the material enters the kiln system from the feed end of the kiln, and finally passes through the kiln. The chimney vents into the atmosphere.
The rotary kiln is mainly composed of kiln shell, transmission device, support device, retaining wheel device, kiln head sealing device, kiln tail sealing device, kiln head cover and so on. The cylinder of the cement rotary kiln is a cylinder made of steel plates. In order to protect the cylinder of the rotary kiln, a layer of refractory bricks is embedded inside it as a kiln lining.
The rotary kiln is a device that integrates fuel combustion, heat exchange, high-temperature chemical reaction and conveying material airflow. Since the temperature of the gas in the kiln is much higher than the temperature of the material, the high temperature above 1450 °C is often maintained in the container. The kiln Every revolution, the surface of the kiln lining is subject to periodic thermal shocks, and the temperature range is 150-250°C, generating thermal stress within the surface range of 10-20mm of the kiln lining; the kiln lining also bears the alternation of brick masonry due to rotation. The changing axial and radial mechanical stress, as well as the erosion and wear of the calcined material; because the silicate melt produced at the same time has a certain erosive effect on the refractory material, it will cause the refractory material to fall off, and the surface temperature of the kiln body will be too high Or "red kiln".
The refractoriness and thickness of refractory bricks are limited, and they cannot withstand the attack of long-term high temperature and the corrosion of chemical reactions of materials. In order to prolong the service life of refractory bricks, a solid protective layer is added to the surface, that is, hanging The upper layer of clinker is called "kiln skin" by Kanhuogong.
The purpose of hanging the kiln skin is to prolong the service life of the fire bricks, so that the rotary kiln shell is not damaged, and it can reduce the heat loss and improve the thermal efficiency.
2. The principle of hanging kiln skin
The kiln skin is hung according to the characteristics of the combination of the adhesive properties of the material during calcination and the surface-fused refractory bricks. After the material enters the burning zone, a liquid phase appears, and the liquid phase increases with the temperature rise. When the surface of the refractory bricks melts slightly, with the rotation of the kiln, the clinker with certain adhesiveness will press the refractory bricks underneath, and absorb a certain amount of heat from the refractory bricks to make them stick together and react chemically. The first layer of kiln skin is formed with the temperature drop, and the kiln skin becomes thicker and thicker with the prolongation of the kiln operation time, and the surface temperature of the kiln skin is also higher and higher. The skin is firm and dense.
3. The role of kiln skin
Kiln skin is changed from clinker or dust from liquid phase or semi-liquid phase to solid. Its main functions are:
（1）Protect the refractory bricks so that the refractory bricks are not directly subjected to high temperature and chemical erosion.
（2）Store heat energy, reduce the heat loss from the kiln shell to the surroundings, and improve the thermal efficiency of the rotary kiln.
（3） As a heat transfer medium, when the kiln skin is exposed to the air and in contact with high-temperature air, it absorbs heat through radiation or convection. When the kiln skin is in contact with the material at the bottom, it transfers heat to the raw material by conduction. material.
（4）The surface of the kiln skin is rough, which can reduce the flow rate of the powder and prolong the reaction time of the material in the kiln.
4. How to hang the kiln skin
The general steps of hanging the kiln skin in the new dry process kiln are as follows:
When the temperature of the kiln tail reaches about 950 degrees, the temperature of the vortex decomposition chamber reaches 650-700 degrees, and the fire bricks at the kiln head start to glow and whitish, the raw material fed first is about to enter the firing zone. At this time, the kiln head is kept on fire to keep the firing zone at a high enough temperature, and the two suspended C4 ash valves are restored. Open the valve of the tertiary air pipe to about 10%, open the valves of the vortex combustion chamber and decomposition chamber, and start spraying light diesel oil and a small amount of coal powder into the vortex decomposition chamber. When the outlet temperature of C1 reaches 400-450 degrees, open the cold air valve placed on the exhaust gas pipeline from the outlet of C1 to the high-temperature fan, and mix cold air to adjust the temperature of the exhaust gas to protect the high-temperature fan. When the gas temperature at the outlet of C5 reaches 900 degrees, the material can be unloaded after the valve of the air pipe is properly opened three times. The feeding amount is 30-40% of the design capacity, and gradually close the cold air valve to appropriately increase the coal feeding amount and system exhaust air volume, turn the kiln at a slow speed, and start to hang the kiln skin; the system is normal and the temperature of the calciner is stable, and it can be dismantled Ignition fuel injector; if burning anthracite, the use time of the ignition fuel injector should be extended appropriately, but the amount of oil can be reduced to support the combustion of the smokeless. The key to hanging the kiln skin well is to adjust the coal feeding amount and kiln speed in time when the raw material reaches the firing zone, so as to ensure that the temperature of the firing zone is stable, and the kiln speed is compatible with the feeding amount, so that the thickness of the kiln skin that is stuck is consistent, smooth. If the temperature is too high, the hanging kiln skin is easy to be burnt down, the raw material is easy to burn, and the refractory brick is severely eroded; if the temperature suddenly drops, the raw material will run away, and the formed kiln skin will be loose and easy to collapse. Generally, it takes 3-4 shifts to hang the kiln skin.
5. Influencing factors of hanging kiln skin
（1） Chemical composition of raw meal
The so-called hanging kiln skin is the process of liquid phase solidification to the surface of refractory bricks. Therefore, the amount of liquid phase in clinker firing and the viscosity of the liquid phase directly affect the formation of the kiln skin, and the chemical composition of the raw meal directly affects the liquid phase and its viscosity. In the past wet kilns, people advocated that the silicic acid rate of the raw material during the kiln skin hanging period should be relatively low, and the saturation ratio should be relatively high. But for the precalcining kiln, the kiln head currently uses three-air or four-air burners. When the rotary kiln is in normal operation, the primary air volume is small and the secondary air temperature is high. Therefore, the combustion speed and flame temperature of pulverized coal are much higher than those of wet kiln. If the silicic acid rate is reduced, the amount of liquid phase will increase correspondingly, the material will burn easily, and the hanging kiln skin will fall off easily if it is not exposed to fire. Therefore, it is generally advocated that the raw material for hanging the kiln skin should have the same composition as the normal raw material.
（2）Firing temperature and flame control
The main factors for hanging the fired belt kiln skin are not only a certain amount of liquid phase and liquid phase viscosity, but also an appropriate temperature, and there must be a certain temperature difference between the air flow, materials and refractory bricks. Generally, the firing temperature during normal production should be controlled. Grasp the small and uniform clinker grains, do not burn large pieces, let alone burn, and strictly prohibit running raw materials. The lifting weight is controlled within the normal production index. It is necessary to keep the firing temperature stable, the kiln speed stable, and the flame shape complete and smooth. The thickness of the kiln skin hanging out in this way is consistent, flat, uniform and firm.
（3）Feeding amount and kiln speed
In order to make the kiln skin hang firm, even and smooth, a stable thermal system in the kiln is a prerequisite. During hanging the kiln skin, stable feed rate and stable kiln speed are crucial. If the amount of feed is too much or the kiln speed is too fast, the temperature in the kiln will not be easy to control, and the kiln skin that sticks to it will be uneven and not firm. Therefore, it is best to control the initial feeding amount and kiln speed of the new kiln for the first time to hang the kiln skin at about 35% of the designed output. After hanging to a certain extent, it will gradually increase slowly depending on the sticking situation of the kiln skin. In this way, the material layer can be kept from thin to thick, the temperature from low to high, and the residence time of the material in the kiln from long to short, so as to keep the firepower moderate and stable, stick the kiln skin and burn the clinker well. . As the kiln skin continues to thicken, gradually reduce the residence time of materials in the kiln, thicken the material layer, and increase the firepower of the firing zone. This is also a measure to prevent the kiln skin from loosening due to fast hanging.
（4）The position of the nozzle during hanging the kiln skin
Under normal circumstances, the position of the nozzle should be as far forward as possible (pull out), and at the same time, the material is biased, and the flame should be short rather than long. In this way, the high-temperature area is relatively concentrated, and the high-temperature point is closer to the front, so that the kiln skin is gradually pushed from the front of the kiln to the inside of the kiln. With the gradual increase of the amount of raw material, the nozzle should move into the kiln accordingly. When the output of the kiln increases to the normal situation, the nozzle will also move to the normal production position. Do not hang the flame too long during the hanging of the kiln skin, otherwise the high temperature area will not be concentrated, the kiln skin will be hung far away or the front will be thinner and the rear will be thicker, or even unfavorable situations such as the front kiln skin has not been hung yet and the rear has formed a ring.
（5）There should be three days of operation time for the new kiln to hang the kiln skin
In the process of hanging the kiln skin, it is required that the kiln skin gradually thicken, not too fast, so as to avoid the hanging kiln skin being loose and not strong, which will affect the long-term safe operation of the rotary kiln in the future. To judge whether the kiln skin is smooth, first scan according to the temperature of the cylinder to see if there is a peak in the curve fluctuation; secondly, according to the kiln regular curve, to see if there is a periodic change in width; The color of the kiln skin is slightly white, the front and back are flat, the color is consistent, and there is no convexity and light and shade, which indicates that the kiln skin is relatively smooth.
6. How to protect the kiln skin
In order to protect the kiln skin, the following points should be paid attention to:
（1）First of all, hang the kiln skin well, there is a guarantee in the hanging, and there is a hanging in the guarantee.
（2）Establish and build the responsibility system for the storage of kiln lining materials and bricklaying, improve the quality of lining materials and bricklaying, and each belt kiln lining should be matched accordingly.
（3）Strengthen the control of calcination, avoid burning high fire, burning top fire, strictly prohibit burning flow or raw materials, and ensure that the clinker particles are fine and uniform.
（4） Improve the structure of the coal injection nozzle, maintain the complete flame shape, frequently move the position of the coal injection nozzle, and adjust the high temperature area.
（5）Strictly prevent loop formation, and deal with loop formation in time if found.
（6） Take measures in time when the kiln skin is found to be bad.
（7） Strengthen equipment maintenance, reduce the number of kiln shutdowns, and prevent the kiln skin from falling off due to sudden cold and hot.
Hang the kiln skin according to the characteristics of the adhesive properties of the material during calcination and the combination of the refractory bricks on the surface. Pay attention to the key points such as the amount of feed, kiln speed, flame length and hanging time of the kiln skin when hanging the kiln skin. , considering various factors comprehensively, hanging the kiln skin can reduce the heat loss to the outside, improve thermal efficiency, and prolong the service life of refractory materials, which has a very positive impact on the rotary kiln. In addition, equipment maintenance should be strengthened, and measures should be taken in time to protect the kiln skin when it is found that the kiln skin is not good.
ZHENJIN REFRACTORIES is committed to the research and development of refractory materials in cement, lime, non-ferrous metals, steel, glass and chemical industries, technical solution design, construction and supply of supporting products and other one-stop general contracting services, integrating production, sales, research and export Kiln overall solution service provider. The service chain runs through the entire life cycle of the project, including preliminary consultation, scheme design, R&D and production, furnace construction and safety production. Through the optimized combination of various brand products, it can meet different process requirements and create value for customers.
