Crusting, accumulation, clogging and control of kiln tail system of rotary kiln decomposition kiln

　　Crusting, accumulation, clogging and control of kiln tail system of rotary kiln decomposition kiln

　　The popularization and application of kiln decomposition kiln has promoted the rapid development of China's cement industry, significantly increased the scale and production capacity of cement production, played an extremely important role in reducing consumption and improving product quality, and is the current development direction of China's cement industry. However, there is a common problem in the production process line of pre-calciner kiln which has been put into production in China, that is, the kiln tail system - preheating system and decomposer crusting, material accumulation and blockage. System crust, accumulation of material, affecting the ventilation and material flow, resulting in blockage; once the system is blocked, the thermal system is disrupted, seriously affecting the quality of kiln production, and dealing with blockage, it is more difficult to resume production, and worse, the blockage of collapsed material caused by personal injury or death. How to correctly understand and seriously treat this objective phenomenon, recognize the various hazards it will bring to the production; effectively through some necessary control means and certain process processing measures, scientific prediction and prevention, is the key to ensure the smooth production, to ensure the safety of the process facilities, the advantages of the system.

　　First, the blockage of the more frequent parts and high frequency time

　　Blockage may occur in many parts of the precalciner system, but mainly occurs in the three-stage and four-stage cyclone; all levels of the downcomer tube and flap valve, if not dealt with in a timely manner, sometimes from the downcomer tube to the preheater cone, and even the entire cyclone; then the decomposer and its slope, linking tube, variant or reducer tube, etc..

　　From the time point of view, most of the blockage occurs shortly after ignition, kiln operation is not normal, the system thermal regime is not stable and so on. In addition, there are many accidents in the system, frequent opening and stopping of the kiln, improper matching of wind and material, incomplete combustion of pulverized coal and other external factors, which also easily cause blockage.

　　Second, the cause of blockage

　　There are many and complex factors that cause blockage, so it must be carefully analyzed and studied from the process, raw combustion materials, equipment, thermal system, operation and management. According to the production experience of some plants, the main reasons for crust blockage are roughly the following.

　　1. Blockage caused by crusting

　　Crust is high temperature materials in the smoke chamber, rising pipe, all levels (mainly three, four) cyclone cylinder cone wall bonded layer of hard skin, serious place is a circle shrinkage. Impede the normal operation of the material, bonding and firing alternately, so that the number and thickness of the skin layer gradually increased, affecting the kiln ventilation, changing the speed and direction of operation of the material and airflow in the preheater, and finally lead to blockage. The main reasons for this phenomenon are three.

　　(1) the impact of ash return

　　Electric dust collection (including humidification tower) to collect down the material, has gone through a high-temperature physicochemical reaction, this material re-enter the preheater, it is easy to cause early decomposition of materials, early liquid phase, too late to reach the kiln, the formation of molten state in the preheater, adhering to the inner wall of the cyclone, the formation of crust, leading to blockage in serious cases. This situation mainly occurs when the kiln tail system temperature is high and the back ash is unevenly blended or blended in too large amount. Therefore, those cyclone dust collector dust collection efficiency is not high, electric dust collection under the back of the ash and not into the raw material storage homogenization system, but directly from the lifting pump, etc. into the kiln, or back to the ash blending when there is no stable material metering facilities or the failure of such facilities in the production line, more should strengthen the operation to prevent high temperature. It is also necessary to adjust and modify the back ash blending system. Improve the system cyclone, especially the top cyclone dust collector dust collection efficiency, reduce the dust content into the electric dust collection, in order to reduce back to ash.

　　(2) The influence of harmful elements

　　When the content of harmful elements such as K, Na, Cl and S in the raw fuel is high, a large number of appearing alkali will then volatilize from the high temperature zone of the firing zone and enter the gas phase to react with other components, firstly with chlorine and sulfur dioxide, which is carried to the kiln tail system with the airflow and condensed on the raw material in the form of sulfate and chloride after the temperature is reduced. This precipitate appears as a molten phase at a lower temperature, forming a microfine melt, and then solidification of solid particles occurs. They evaporate through multiple high-temperature, low-temperature coalescence cycle and adhesion, adhering to the preheater, decomposition furnace and coupling pipeline to form a crust, if not handled in a timely manner, continue the cycle of adhesion, eventually leading to blockage.

　　(3) Local high temperature causes crust blockage

　　The pre-decomposition system temperature is high, and there are many factors that lead to crusting. Such as material flow fluctuations; coal powder due to incomplete combustion into the preheater to produce secondary combustion; system operation is not stable, etc. will lead to high local temperature, so that the liquid phase in advance, the formation of cohesive material crust. Material flow is large and small, it is easy to disrupt the normal work of the preheater, decomposition furnace and kiln. The operation often lags behind, can not keep up with the changes in the material flow, add or reduce coal in a timely manner, or even a short period of time to cut off the material, can not reduce coal in a timely manner, resulting in a high temperature of the system due to less material, resulting in crusting, blockage. Ignition due to incomplete combustion of coal powder in the kiln or decomposition furnace, part of the run to the preheater attached to the cone and the lower material pipe, the temperature rises when the fire, the formation of local high temperature. Operation, one-sided emphasis on the rate of decomposition into the kiln, the decomposition furnace with too much coal, the ratio of the two fires is out of proportion, resulting in high temperature in the furnace, the premature appearance of the liquid phase, coupled with the high tangential speed of the material in the furnace, centrifugal force is large, it is easy to cause the melt to adhere to the furnace wall, the formation of crust in the furnace; because the residence time of the material in the decomposition furnace is very short, excessive coal powder in the furnace in time to burn, was brought to the four-level cyclone to form a secondary Combustion, resulting in high temperature crusting in the cyclone.

　　2. Blockage caused by air leakage

　　It not only reduces the separation efficiency of the cyclone and increases heat consumption, but also is a major factor causing blockage of the system.

　　(1) Clogging caused by internal air leakage

　　Preheater at all levels of the ash discharge valve is not closed tightly, burned or malfunctioning, can not play a good role in locking the wind, not only reduce the cyclone dust collection efficiency, and will cause a short circuit, collapse of material and blockage. Because the lower material tube ash valve lock wind is not strict, the next level of gas will pass through the lower material tube, so that the material collected in the preheater and re-rise, can not be smoothly discharged, resulting in internal circulation. Due to the high wind speed at the mouth of the lower material, does not reach a certain amount, the material will not settle, but once too much material has the conditions for settling, it is a large group of fall, resulting in uneven under the material, dispersion is not good, resulting in blockage.

　　(2) Blockage caused by external wind leakage

　　External wind leakage refers to the cold air leaking into the system from outside the system. It is mainly from all levels of the cyclone cylinder inspection door, the lower material pipe ash valve shaft, the flange of the connecting pipeline, the preheater top cover, the measurement points, etc. leakage into. Cyclone preheater within the airflow movement is complex, coupled with a wider distribution of powder particle size, so that its internal material movement is more complex, randomness is greater. If the system is not well sealed, leakage into the cold air, changing the trajectory of the material in the preheater, reducing the speed of its rotational movement, centrifugal dumping to the wall of the centrifugal force is reduced, part of the material with the airflow back to the upper level, resulting in material circulation, and eventually pile up blockage. On the other hand, cold air leakage into contact with hot materials, it is very easy to cause cold and hot material cohesion, adhesion in the preheater barrel wall, resulting in crust or produce large blocks, stuck down the material tube or ash discharge valve caused by blockage. In addition, when the fuel is not completely burned, combustible materials and leakage of O2 re-combustion, resulting in local high temperature, overheating so that the material on the inner wall melt adhesion, crust blockage.

　　3. Blockage caused by improper operation

　　(1)The feeding is not timely. When the decomposing furnace ignites and reaches the feeding temperature, it must be fed in time, otherwise it will cause the system temperature to be high, and the material quantity is smaller at that time, which is more likely to cause crusting.

　　(2) Improper exhaust air volume when opening and stopping the kiln. For this reason, when the kiln needs to stop material, the amount of exhaust air can not be reduced by large spokes, otherwise it is easy to make the material deposited in the tube (mainly in the horizontal tube) due to the wind speed is too small, resulting in the accumulation. When the kiln is reopened and fed, the discharge air volume is too small at the beginning, and the accumulated materials cannot be taken away smoothly, and with the increasing amount of feeding, the accumulation of materials in the tube increases, which will also lead to blockage of the accumulation when serious.

　　(3) The amount of downstream material is not synchronized with the kiln speed. Kiln operation is not normal, the thermal system is not stable, the need to pre-fight small slow or slow kiln, reduce the material is not timely is easy to feed the amount of material and kiln speed is not synchronized, resulting in the accumulation of materials in the kiln tail smoke chamber. At this time, even if the kiln is still running, but the materials piled up in the kiln tail can not be quickly transported out, the piled up materials by high temperature melt adhesion in the interior wall of the kiln tail flue chamber, the formation of a shed material in the flue chamber and kiln connection phenomenon, resulting in the flue chamber and the upper level of pre-heater blockage.

　　(4) improper control of the exhaust air volume. When the exhaust volume is too large, the pre-decomposition system airflow speed is high, the material in the preheater was thrown to the wall of the centrifugal force is larger, the material along the wall rotating down speed, the material and high-temperature air contact time is relatively long, easy to sticky paste in the preheater wall, the formation of layers from loose to solid cover, resulting in blockage; when the exhaust volume is too small, the airflow speed is reduced, it is difficult to make the material group washed away, the formation of collapse blockage, and the material It is easy to stay in the horizontal connection pipe, resulting in horizontal pipe blockage.

　　(5) Uneven distribution of kiln and furnace air volume and uncoordinated operation. The operation adjustment is unreasonable, and the opening degree of the kiln tailgate gate and the opening degree of the three wind gates into the decomposer are not appropriate, which will easily lead to uneven distribution of kiln and furnace air volume. If the wind speed of the kiln tailgate is too low, or the wind speed of the decomposer inlet is too low or too high, it will cause the material to crust, shed material, collapse material, pile up and blockage in the precalciner system. Kiln and furnace operation can not take into account before and after; poor coordination; one-sided emphasis on the kiln ventilation, system negative pressure; inappropriate pursuit of decomposition rate into the kiln, two fires with bad, also easy to cause high-temperature crusting, accumulation of material, collapse of material, blockage.

　　4.Blockage caused by foreign matter

　　The system of inspection door brick mosaic does not labor collapse; cyclone cylinder, decomposition furnace top cover and lining material peeling; cyclone cylinder inner tube or scattering plate burned down; ash discharge valve burned or rotation does not work; maintenance refractory bricks or iron and other objects left in the preheater is not cleared out and easy to cause mechanical blockage of the preheater.

　　5. Improper design, the congenital deficiency caused by blockage

　　System design to create good conditions for the production, not because of some parts of the design is not reasonable, resulting in congenital deficiencies, affecting production. The system blockage caused by congenital deficiency is difficult to deal with in production and must be avoided. For example: the horizontal connection pipe is too long, the connection pipe angle is too small; all levels of pre-heater inlet height to width ratio is small; cone angle is small; the inner tube is too long; back to the ash can not be evenly mixed into the production will affect the production.

　　Three, prevention and treatment

　　1. Improve the inspection before starting and opening the kiln

　　After system overhaul, make sure to check the system in detail, clean up all the debris inside the system, and confirm whether the refractory bricks and other lining materials are firm. Before starting the kiln, all ash discharge valves should be checked to confirm whether they are flexible or damaged; check whether the weighting of ash discharge valves at all levels is reasonable to prevent the mechanical rotation from being too light or too heavy, resulting in poor rotation or sealing, forming air leakage and causing blockage. When the ash discharge valve flutters slightly during normal production, the counterweight is reasonable. When opening the kiln, you should check whether all the inspection doors, flanges, measuring holes and ash discharge valve shafts are sealed in time to prevent the blockage caused by external wind leakage. Problems found in time to deal with, can not wait for the "next time". When the temperature rises and the material can be fed, it should be fed in time. Before feeding, the ash discharge valve should be active and the blowing device should be opened to prevent the cone volume material.

　　2. Strengthen the operation

　　During normal production, the operation should be strictly operated to maintain a reasonable distribution of temperature and pressure, taking into account the front and back, and closely coordinated; operators should have a good sense of responsibility and foresight. Add and reduce material in time, wind and coal material with reasonable, feeding kiln speed synchronization; diligent inspection, diligent contact, diligent observation, diligent activity.

　　3. Keep the original, combustion materials, reasonable ingredients, improve the quality of coal powder

　　Strictly control the harmful components of raw and combustion materials, generally require alkali content (Na2O+K2O) <1.5%, chlorine content Cl<0.02%, sulfur to alkali ratio control at 0.85 or so. Adjust the clinker rate value, optimize the ingredients, the amount of liquid phase control at 24~27% is more suitable; adopt two high and one medium ingredients program, make the firing product soft but not caked, hard but not scattered. Control the fineness and moisture of coal powder, avoid high sulfur coal and poor quality coal.

　　4. Improve process facilities, comprehensive management, eliminate hidden problems

　　Production line with frequent blockage should diagnose the whole process, find out various factors that may lead to crusting, and effectively manage it. In production, once blockage is found, the cause should be found out as soon as possible and dealt with in time to prevent crusting and increase the difficulty of treatment.

　　IV. Conclusion

　　The factors that cause crusting, accumulation and blockage in kiln tail system of kiln decomposition kiln are many and very complicated, there are single and multiple, to operate this kiln well, we must strengthen the management and operation, stabilize the system thermal system; improve the system sealing; improve the quality of raw fuel, strictly control the harmful components of raw fuel; reasonable ingredients. Mastering the law, preventing and dealing with crust blockage is an important guarantee for the normal operation of kiln, furnace and apparatus, improving the operation rate of kiln, ensuring the quality of production, ensuring the safety of equipment and people; it is the key to give full play to the advantages of precalciner kiln.