Cause analysis of preheater collapse

Date: 2019-07-26 Views:

Preheater collapse is a common problem in kiln production.It not only affects the thermal efficiency of the firing system and the operation rate of the kiln, but also causes clogging of the pre-decomposition system or kiln head tempering and personal injury accidents.

Collapse material refers to the uneven flow or accumulation of raw material in the preheater, which causes a large amount of raw material to be concentrated from the cone of the cyclone tube through the ash discharge valve and the discharge pipe to join the outlet pipe of the next cyclone tube. However, the wind speed of the pipe is not enough to blow away and hold up the large amount of raw material. Raw material directly collapses into the kiln through the cyclone tube or decomposition furnace.The main causes of material collapse in the pre-decomposition system are as follows:

 

1 uneven feeding quantity


Raw material storage time is long, moisture is large, loose degree is poor;Feeding bin arch, feeding machine raw material sometimes not;Spiral feeding machine speed instability, resulting in uneven feeding and other prone to collapse.This is because it is difficult to stabilize the thermal system of the predecomposition system due to the amount of raw material feeding.When the amount of feed is small, the amount of raw material suspended in the hot gas stream decreases, and most of the heat from fuel combustion cannot be absorbed by the raw material, resulting in local high temperature which is easy to adhere to the wall surface, cone and feeding pipe of the cyclone tube, forming skin.When the amount of raw material is large and the wind speed of the system pipeline is not enough to blow the raw material away, 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 accumulation and collapse.In addition, the feeding quantity is high or low, which makes the raw material temperature, decomposition rate, and material load rate in the kiln fluctuate greatly, and the raw or overburned clinker comes out of the kiln, which makes it difficult for the operator to grasp the relationship between wind, coal, material and kiln speed. The thermal balance of the system is destroyed, and it is easy to produce collapse, skilling and blockage.

2 unreasonable structure of cyclone tube


The inclination Angle at the inlet of the cyclone cylinder and the bottom of the vortex shell is too small, and the horizontal section is too long. In the initial feeding stage, due to the small system air volume and low section wind speed, a large number of raw materials are easy to settle and accumulate in the small inclination and horizontal section.When the turbulence or pressure of the system flow changes, a large number of materials suddenly slide down and cause collapse.The cone Angle of cyclone cone cone is too small, especially when the flatness of refractory lining is poor, it will also cause accumulation.Over time, it was either a blockage or a collapse of large stocks of material.
 

3 cyclone cone outlet, dust discharge valve and discharge pipe leak air


There are two types of air leakage in pre-decomposition system: external air leakage and internal air leakage.External air leakage refers to the cold air leakage from inspection hole, measuring hole, ash discharge valve, connecting pipe flange and manhole door of cyclone tube under the system negative pressure.Internal air leakage means that when the ash discharge valve is out of shape due to burning or the balance weight is too light, the hot air flow from the outlet of the lower stage cyclone tube directly passes through the discharge pipe and enters into the cyclone tube from the cone outlet through the ash discharge valve.

In general, the negative pressure in the cone of the cyclone cylinder increases gradually from top to bottom.That is to say, the bottom of the cone and outlet negative pressure is larger, the system around the cold air is easy to leak in.At the bottom of the cone, especially at the outlet, the rotating radius of airflow is small and the centrifugal force is small. It is easy to be disturbed by air leakage and produce disordered gas flow field that is not conducive to gas and solid separation, so that the material to be collected will be mixed back and reduce the dust collecting efficiency of the cyclone tube.In addition, the leakage of cold air increases the axial wind speed in the central part of the cone of the cyclone tube, and causes the raw material that has been separated from the flow to produce a large reverse flying, which will also reduce the dust collection efficiency, leading to a sharp increase in the concentration of dust in the cyclone tube.With the passage of time, the raw material concentration in the cyclone tube becomes larger and larger, so that it exceeds the supporting capacity of the airflow. The large amount of raw material suddenly rushes to the discharge port, and collapses into the kiln through the ash discharge valve, the lower cyclone tube or the decomposition furnace.

 

4 NSP kiln is more prone to collapse under low production


The whole pre-decomposition system, including the rising flue at the end of the kiln, the decomposition furnace, all levels of cyclone tubes and their connecting pipes, has certain requirements for the wind speed in the section to ensure that the raw material is suspended in the hot gas flow, and has high thermal efficiency and separation efficiency.However, the determination of equipment specifications is based on the design capacity, that is to say, to achieve the optimal feeding capacity, the wind speed of the section is also in the best state.Of course, the equipment selection generally has a certain amount of spare capacity, such as kiln tail high temperature fan air volume and pressure are more than 15%.As long as the fan operates within its set parameters, it is often the case that the actual production capacity of the kiln exceeds the designed output.This is because of the increase in output. Although the system resistance increases and the electricity consumption increases correspondingly, the total output increases. The electricity consumption and heat consumption per unit of clinker are not necessarily high.However, the situation is different when the output is too low. Firstly, the output is too low, the wind speed is low everywhere in the system, and the material accumulates in the horizontal pipe of the preheater system, the bottom of the cyclone cylinder vortex shell and the cone.Material accumulation to a certain extent, when the system temperature, pressure, air volume will change slightly will collapse.In addition, the output is low, the heat consumption is high, the waste gas volume per unit clinker is large, in this case, the excess air coefficient of the pre-decomposition system is also large, but the wind speed of the inlet and outlet of the cyclone tube at all levels is still far below the design value.The material ratio of the system is large, that is, the concentration is small, the separation efficiency of the cyclone tube is sharply reduced, and some raw material circulates in the preheater system.With the continuous feeding of raw material, the amount of raw material circulation in the system gradually increases, and the raw material concentration reaches a certain level. The air speed is not enough to blow and lift the material, but directly collapse into the kiln.This is an important reason why the lower the output of NSP kiln, the more frequent the collapse of materials, and the higher the speed of the kiln, the easier it is to run raw materials.

System air leakage not only affects the operation of the kiln system, the separation efficiency of cyclone tube fell sharply, has preheating material at the next higher level of lower temperature cyclone tube backmixing increase, lower thermal efficiency, with the increase of the make the exhaust system and cooling, fan power consumption and exhaust away heat loss increase, also make the system gas temperature drop, thus reducing the integrated heat transfer coefficient between the gas and material, as a result, make the material into the kiln temperature and decomposition rate is significantly lower.Generally speaking, the thermal indexes of NSP kilns in China are mostly lower than those of similar kilns abroad, and the serious air leakage in the system is one of the important factors, which should be paid enough attention to by people.

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