“Dust accumulation and blockage” is a phenomenon and a problem in the use of dust bags, and it is one of the reasons why the dust bag loses its efficacy. There is no uniform definition within the industry. It is generally believed that “dust accumulation and clogging” means that during the long-term operation or outage of the dust bag, in the working condition of high humidity or oily substances in contact with the filter material, the dust condenses on the filter surface of the dust bag or inside the filter material. , adhesion or encrustation, and cannot be effectively removed by the online dust cleaning system, resulting in a significant increase in running resistance.
After the bag dust is accumulated and blocked, the dust covers the surface of the filter bag densely, which greatly reduces the effective filtration area of the filter material, the air permeability of the filter material drops sharply, and the running resistance becomes high, which increases the load of the induced draft fan and causes energy consumption. In severe cases, the induced draft fan will be overwhelmed and unable to operate. At the same time, the high-pressure drop of the dust bag makes the online dust cleaning system frequently clean, which not only consumes a lot of compressed air but also causes a serious loss of the mechanical strength of the filter material and greatly shortens the service life. Therefore, if the dust bag is often operated under the condition of dust accumulation and clogging, the user will have to pay a higher operating cost for the operation of the system. If the problem of dust accumulation and clogging cannot be solved in a timely and effective manner, the cloth bags will have to be replaced in batches.
Find solutions for the causes of dust accumulation and blockage:
In actual working conditions, the factors that may cause dust accumulation and blockage are more complicated. For specific cases, we must analyze the flue gas, dust collector, filter material, operation control and other related factors in order to understand it. The real inducing factor of dust accumulation and clogging, so as to avoid and solve the problem.
A – Condensation dust accumulation blockage:
In general, the operating temperature of the bag filter must be 25K higher than the acid dew point temperature, which is relatively safe for the bag filter. This is mainly to avoid the occurrence of condensation. Liquid water forms when the operating temperature is below the dew point. The liquid water mixes with the dust and accumulates on the surface of the filter bag to form a dust accumulation blockage.
The higher the content of moisture and sulfur trioxide in the flue gas, the higher the dew point temperature. This means that the continuous operating temperature of the dust collector must be higher. However, due to the limitation of the physical properties of the dust bag filter material and the operating economy, the continuous operation temperature of the dust collector must be controlled within a certain suitable range. If the two cannot be coordinated, condensation will be inevitable. Therefore, water content, SO3 content and operating temperature are the three main factors that determine whether condensation occurs.
The operating temperature of a certain dust collector is stable at about 150°C, but since the main incinerated substances are wastes from chemical enterprises such as solids, liquids and gases with complex components, the content of water and sulfide in the flue gas varies greatly (water content even exceeds 25%), and the dew point temperature fluctuates greatly. Condensation is inevitable, and dust accumulation is extremely serious. At the same time, due to the penetration of acidic substances and long-term operation, the dust gradually crusts on the surface of the filter bag, resulting in the loss of air permeability of the filter bag.
Secondly, the compressed air of pulse cleaning is also one of the possible causes of condensation. The temperature of compressed air is generally much lower than the operating temperature of the dust collector. At the moment of pulse, the low-temperature compressed air will cause the temperature of the upper part of the filter bag to drop rapidly. When it is lower than the dew point temperature, it will cause condensation on the outer surface of the filter bag at this part . And as the compressed air descends, its influence on the air temperature in the bag becomes smaller and smaller. Therefore, the phenomenon of dust accumulation and blockage within one meter below the mouth of the dust bag is more serious.
Again, the air leakage rate of the dust collector is also a possible factor causing condensation. Generally, the air leakage rate of the dust collector is about 1-2%. But with the prolongation of running time, the air leakage rate will increase. If it is in the north where the winter is severe, the problem becomes even bigger. The higher the air leakage rate, the greater the impact on the local temperature in the dust collector, and the higher the risk of dust accumulation and clogging of the filter bag at the leakage site due to condensation.
In addition, although the dust collector walls are equipped with insulation layers, since the four walls are the forefront of contact with the outside world, the temperature must be lower than other parts. In actual service, we found that the dust accumulation of the outer ring dust bag near the four walls is usually clogged. much higher than other parts.
B – cohesive dust accumulation blockage:
It mainly means that due to the relatively high adhesion of dust, although no dew condensation occurs, the dust still adheres to the surface of the fiber, and the online dust cleaning system cannot remove it. The cause of cohesive dust accumulation and clogging may come from the following situations.
- The dust itself has a relatively high viscosity, and when it contacts with the filter fiber, the intermolecular force is relatively strong. For example, oily particles, ammonium sulfate produced by denitrification, and salt lime used in desulfurization.
- Although some dust is not viscous, it is easy to deliquesce. When it is trapped on the surface of the fiber, it will absorb moisture in the air and form a solution on the surface of the fiber. Like powdered sugar.
- Some dust may not have strong adhesion, but it can absorb moisture in the flue gas and undergo a chemical process of recrystallization to generate new hydraulic substances or crystals, and the formed “crust” covers the surface of the filter material . For example: cement clinker, desulfurization products – calcium sulfate.
- Even if the dust itself is not viscous, if the dust particles are fine and the water content of the dust is relatively large, the dust is easy to be evenly adsorbed on the surface of the filter material, forming a layer of “floating ash”. The adhesion is not very strong, but Difficult to remove. The longer the running time, the thicker the “floating dust”. This situation may occur in general coal-fired power plants, especially for filter bags with a length of more than 6 meters. A certain northern power plant once caused the operating resistance of the dust collector to rise from about 1500 to 2200Pa in a short period of time due to this reason, resulting in a serious shutdown of the entire line.
C – Structural dust accumulation and blockage: We classify all dust accumulation blockages caused by dust collector design and related component structure and dust accumulation blockages caused by the improper operation as structural dust accumulation blockages.
- Filter material structure and treatment: Some filter materials have loose fibers on the filter surface due to insufficient needle punching density. Even if the surface has been singeed, fine dust can still easily enter the interior of the filter material and reside in it. Dust accumulates to a certain extent inside, coupled with the influence of water vapor in the flue gas, gradually forms a blockage from the inside out. A power plant in the north used the cloth bags for less than a year, and had to consider replacing the cloth bags in batches because of this problem. In addition, some filter materials do not have or cannot be singed or calendered, and the fiber ends are retained on the surface of the filter material, which actually provides “condensation nuclei” for condensation, so that the condensation phenomenon starts from the fiber ends first. Dust will also form dust clusters at this location first, and when the dust clusters gradually become larger, they bridge each other. A large area of dust accumulation and blockage is gradually formed. In waste incineration plants using pure PTFE filter media, if there is no other treatment on the surface, stubborn dust encrustation at the end of the fiber can often be found. In addition, for working conditions with high humidity or the risk of acid condensation, the filter material has not been treated with hydrophobicity. Clogging by dust buildup is relatively common.
- Filter bag fit: We know that pulse cleaning actually uses the impact of pulse airflow and secondary airflow on the filter bag to cause the filter bag to deform from top to bottom in the weft direction. The inertia of deformation and the filter bag and cage The impact of the bone creates an “avalanche effect” on the surface dust layer, thereby removing the surface dust. The cleaning effect largely depends on the size of the deformation inertia and the strength of the collision between the filter bag and the cage. In practical applications, due to the fact that the weft dimension of the filter bag is too small or the heat shrinkage of the filter material is too large during use, after a period of use, the filter bag is “bundled” on the cage and cannot be separated. At this time, the deformation of the filter bag caused by pulse cleaning is very small, the collision intensity between the filter bag and the cage is limited, and the dust on the surface cannot be effectively removed. Long-term accumulation and compaction of dust on the surface of the filter bag may cause dust accumulation and blockage.