- Time:Jan 20, 2023
Preheater collapse is a common problem in the actual production of kiln systems. It not only affects the thermal efficiency of firing system and kiln operating rate, but also causes blockage of pre-decomposition system or kiln head backfire in serious cases and personal injury accidents.
A. Analysis of the causes of collapsed material
The so-called collapse is the uneven flow or gathering of raw material in the preheater, resulting in a large amount of raw material concentration from the cyclone cone through the ash discharge valve, down the material pipe into the next cyclone outlet pipe, and its pipeline wind speed and the lack of large strands of raw material will be blown apart, to lift, raw material directly through the cyclone or decomposition furnace collapse into the kiln. The main reasons for the collapse of the precalciner system are as follows.
1、Uneven feeding volume
The long time of raw material compression, large moisture, poor looseness; feed bin arch, feeder raw material from time to time; screw feeder speed is unstable, resulting in uneven feeding, etc. are easy to cause collapse of material. This is because it is difficult to stabilize the thermal regime of the precalciner system when the feeding quantity of raw material is more and less. When the feeding quantity is small, the amount of raw material suspended in the hot air flow is reduced, and the heat released from the fuel combustion cannot be absorbed by the raw material, resulting in local high temperature easily adhering to the wall of the cyclone, cone and lower material tube, forming crust.
When the feeding volume is high and the system pipeline wind speed is lacking to blow the raw material apart, the raw material cannot be evenly dispersed and suspended in the airflow, which not only reduces the heat transfer efficiency, but also easily causes the raw material to pile up and collapse. In addition, the feeding quantity is high and low, so that the raw material temperature, decomposition rate and kiln material load rate fluctuate greatly, and the clinker out of the kiln is either raw or over-burned, and the relationship between the operator's wind, coal, material and kiln speed is difficult to grasp, and the heat balance of the system is destroyed, which is very easy to produce collapsed material or crust and blockage.
2、The cyclone construction form is not reasonable
The slope of the cyclone inlet and the bottom of the vortex shell is too small, and the horizontal section is too long. In the early stage of feeding, due to the small air volume of the system and the low cross-sectional air speed, a large amount of raw material is easy to settle and accumulate in the small inclination and horizontal section. When the system airflow disturbance or pressure changes, a large amount of material suddenly slips down and causes collapse of material. Cyclone cone local cone angle is too small, especially when the part of the refractory lining material flatness is poor, will also cause the accumulation of material. After a long time, it will either cause blockage or collapse of large strands of material.
3、Cyclone cone outlet, ash discharge valve and lower material pipe leakage
There are two forms of air leakage from the predecomposition system: external air leakage and air leakage. The so-called external air leakage refers to the cold air leaking in from the inspection hole, measuring hole, ash discharge valve, even receiving channel flange and cyclone manhole door under the negative pressure of the system. When the ash discharge valve is deformed due to burned or the counterweight is too light, the hot airflow from the next cyclone outlet directly enters the cyclone by the cone outlet through the ash discharge valve, so the air leakage is called air leakage.
In general, the negative pressure in the cone of the cyclone is gradually increasing from top to bottom. In other words, the negative pressure at the bottom of the cone and the outlet is larger, and the cold air around the system is easy to leak in. The bottom of the cone, especially at the outlet, the airflow has a small radius of rotation and a small centrifugal force, which is easily disturbed by wind leakage and produces a turbulent gas field that is not conducive to gas-solid separation, thus causing the material to be collected to return to the mix and reducing the dust collection efficiency of the cyclone.
In addition, the leakage of cold air into the cyclone cone center local upward axial wind speed increases, so that the raw material has been separated from the airflow to produce a large reverse flutter will also reduce the efficiency of dust collection, resulting in a sharp increase in the concentration of cyclone gas dust. As time goes by, the concentration of raw material in the cyclone cone becomes larger and larger, so that it exceeds the upward support capacity of the airflow, and the large strand of raw material suddenly rushes to the discharge port and collapses into the kiln through the ash discharge valve, the next cyclone cone or the decomposition furnace.
As for the air leakage of the precalciner system, it is not generally valued and not easily detected, and usually can only be judged by the operator's experience. In fact, the seriousness of air leakage leading to material collapse is greater than that of external air leakage. This is because the next level of cyclone outlet gas through the lower material pipe, ash valve and cone discharge port leakage into the cyclone, will make the cyclone collect down the raw material re-rise in the preheater caused by the cycle. As the ash discharge valve loses the role of locking wind, so that the rising wind speed at the cone outlet of the cyclone is higher and the airflow buoyancy is larger, when the raw material of the cyclone reaches sufficient quantity suddenly sinks downward, causing serious collapse of material. Generally speaking, a large strand of material suddenly sinks downward and generates a large negative pressure, then the material gathered in the horizontal section of the cyclone cylinder inlet and the bottom of the vortex shell is disturbed by the powerful vortex airflow and collapses together with the large strand of material, when the collapse of material is serious, several ash discharge valve stems will be seen to buckle high, and soon there will be a large tempering at the kiln head, and the spouting dragon can continue for several seconds.
System air leakage not only affects the operation of the NSP kiln system, so that the separation efficiency of the cyclone drops sharply, the pre-heated material to the upper level of the lower temperature cyclone to increase the return mix, reducing thermal efficiency, and the admixture of cold air into the system to increase the amount of exhaust gas, fan power consumption and heat loss taken away by the exhaust gas to increase, but also to reduce the system gas temperature, thereby reducing the integrated heat transfer coefficient between the exhaust gas and the material, the result of which makes into the kiln The result is that the temperature and decomposition rate of the incoming material are obviously low. Generally speaking, the thermal index of NSP kiln in China is mostly lower than the index of similar foreign kilns, and the serious air leakage of the system is one of the many important factors, which should be paid sufficient attention to.
4、The operation of NSP kiln at low output is more likely to cause collapse of material.
The entire precalciner system, including the kiln tail rise flue, the decomposer, the cyclone and its connecting channel, have certain requirements for the cross-sectional air speed to ensure that the raw material is suspended in the hot air stream and has high thermal efficiency and separation efficiency. However, the equipment specification is based on the design capacity, that is to say, the most correct feeding volume to reach the design capacity, the section air speed of each place is also in the most correct state.
Of course, the equipment selection generally leaves a certain amount of capacity, for example, the kiln tail high temperature fan air volume and pressure are above 15%. As long as the fan operates within its set parameters, it is not uncommon for the actual production capacity of the kiln to exceed the designed output. This is because the increase in production, although the system resistance caused by higher power consumption correspondingly increased, but the total output increased, unit clinker electricity consumption, heat consumption is not necessarily high, there is a comprehensive balance, economic accounting issues. However, the yield is too low, first of all, the yield is too low, the wind speed in various parts of the system is low, the material in the pre-heater system of the horizontal pipe, cyclone vortex shell bottom and cone and other parts of the accumulation. Material accumulation to a certain extent, when the system temperature, pressure, air volume changes slightly will collapse downward.
In addition, the output is low, the heat consumption is high, the waste gas volume per unit of clinker is large, and the excess air coefficient of the pre-decomposition system is also large in this case, but the air velocity of the import and export of cyclones at all levels is still far below the design value. The air to material ratio in the system is large, i.e., the concentration is small, the cyclone separation efficiency is sharply reduced, and the partial raw material is circulated in the preheater system. With the continuous feeding of raw material, the amount of raw material circulating in the system also gradually increases, and the concentration of raw material reaches a certain level, the airflow wind speed lacks to blow and lift the material, but collapses directly into the kiln. This is an important reason why the lower the output of the NSP kiln, the more frequently the raw material collapses and the kiln speed does not increase, making it easy to run raw material.
B. To reduce the collapse of the material of several measures
1、Strictly control the moisture of raw material out of the mill and stabilize the feeding amount of raw material into the kiln
The uniformity of raw material composition is an important factor in determining the quality of cement clinker. However, if the moisture of raw material is too large, it will not only make the top slope of raw material homogenizing bank easy to block, the wall of bank easy to bond, reduce the effective volume of homogenizing bank, the center mixing room at the bottom of bank easy to appear ditch flow phenomenon, raw material can not be fully fluidized, which directly affects the homogenizing effect of raw material, and the raw material with high moisture is also easy to arch in the small bin of kiln end feeding, the raw material falls unevenly or collapses, resulting in the amount of material entering the pre-decomposition system is sometimes more and less Therefore, it is easy to cause collapse of material. Therefore, the moisture of raw material should be strictly controlled at ≤0.5%.
With a uniform composition and smooth flow of raw material, it is difficult to stabilize the raw material feeding into the kiln, ensure the stability of the kiln system thermal regime and burn high quality cement clinker, assuming that the feeding volume control is not timely and the measurement is not accurate. At present, most large and medium-sized new dry process cement plants in China generally use weighing bin with Shenke electronic belt scale raw material feeding metering system or use Japanese powder research rotor scale, they are not only accurate measurement, the amount of incoming material can also be timely control, operation rarely collapse phenomenon, clinker quality are relatively high.
However, most small and medium-sized new dry process cement plants, due to the constraints of the conditions, into the kiln raw material feeding amount generally fluctuate more, coupled with the operation is not law, the precalciner system collapse material frequently, kiln speed can not be raised, firing system thermal system is not stable, clinker production quality is generally low. For these cement plants, under the premise that the moisture of raw material is not large and the material surface of the small bin is constant, if the electromagnetic speed motor of the double tube screw feeder is changed to an asynchronous motor controlled by a frequency converter and the influence of the airflow of the pneumatic lifting pump on the metering equipment is eliminated, the amount of raw material fed into the pre-decomposition system can be more uniform. Then strengthen the job training, change the operation method, the collapse phenomenon of the system and the quality of clinker production can be improved.
2、Change the horizontal section of the cyclone for the angle of inclination, to prevent the accumulation of materials
NSP technology carried out to date, the country's various design units have also developed their own unique pre-decomposition system. The small size of the cyclone, low resistance, good preheating effect is then the common goal pursued by all. Some have changed the cyclone inlet and the bottom of the vortex shell to a certain degree of inclination, thus eliminating the possibility of a large amount of accumulated material. But there are still no changes, especially in the early construction of the precalciner kiln production line, most of them save the horizontal section of the cyclone inlet and the bottom of the volute. It is suggested that these cement plants should improve the cyclone inlet wind speed, change the bottom of the vortex shell inclination ≥ 45 °, so that the collapse of the system will certainly reduce the phenomenon of material.
3、Conical expansion bin is set under the cone outlet of the cyclone
Conical expansion bin can be set below the cone of cyclone to expand the diameter of the cone outlet, so as to effectively reduce the possibility of material accumulation and arching at the bottom of the cone. In addition, the section of the expansion bin is larger than the cyclone cone outlet section, the material into the expansion bin due to its sudden expansion of the section, the material drop speed slowed down, playing a buffer role. It can also reduce the negative pressure of the ash discharge valve and the lower material pipe, which is conducive to alleviating the phenomenon of collapsing materials caused by air leakage and external air leakage.
4、Rational adjustment of the angle of the ash discharge valve stem and its counterweight
The role of the ash discharge valve is to simultaneously lock the air and discharge the material. However, if the angle of the valve stem and its counterweight are not appropriate, either the material above the valve plate will easily pile up and cause blockage, or the valve plate will not be in place to produce a large air leakage.
The position of the balancing rod of the ash discharge valve should be below the horizontal line and the angle between the horizontal line is less than 30°, and it is best to adjust to about 15°. Because at this time, the line displacement of the center of gravity of the balancing rod changes very little, and with the increase in the valve plate opening, the center of gravity of the balancing rod and the axis spacing increases at the same time, the torque increases, the valve plate reset time required to shorten. As for the counterweight, it should be adjusted at the beginning of the cold state, adjusted to a gentle hand lift the balance rod up, a release of the balance rod on the reset, the hot state only need to individual ash valve for micro-adjustment can be. In this way, the balance lever of ash discharge valve has a small swing and high frequency, which can not only discharge the material in time but also reduce the air leakage, and the phenomenon of collapsing material can be improved.
5、As soon as possible to jump over the low production of large collapse of material danger zone
The most important feature of the precalciner production process is that about 60% of the fuel is burned in the decomposer, and the general raw material temperature in the kiln can reach 830-850℃, with a decomposition rate of more than 90%. This creates favorable conditions for fast turning kiln, thin material and long flame calcination. Therefore, the starting value of the NSP kiln should be high, generally not less than 60% of the design output. Later gradually increase the amount of feeding, but should try to prevent the delay of low feeding operation time.
At the stage of gradually increasing feeding quantity, it is crucial to master the relationship between wind, coal, material and kiln speed, and the operation steps should be to raise the wind first and then add coal, and raise the kiln speed first and then add material. In the initial stage, the range of feeding can be bigger, and after 80% of feeding quantity, it will be slowed down appropriately. As long as the thermal parameters of the system are at the upper limit of reasonable circumference, operate boldly. In this way, even a large scale NSP kiln can be added to the design feeding amount in 1h.
In general, the feeding volume is more stable when it is increased to 80% of the design value. With a normal kiln skin, the feeding from opening to the highest output can usually be done in 1h or less. If it is said that below 70% feeding is the danger zone of collapsing material, then it takes only a few minutes to jump over this danger zone by increasing the feeding from 60% to 70%, and the kiln condition will be stable afterwards. At this time, the amount of material in the precalciner system has reached a certain level, the material flow is smooth, the cyclone cone outlet, ash discharge valve and the lower material pipe have a large amount of material passing through at any time, and the air leakage and leakage from the above parts can be suppressed, so there is little collapse of material, and even if there is, it is very small and has no effect on the operation. This is why people say that the higher the output of NSP kiln, the easier it is to operate.
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