Analysis and treatment method of filter bag blockage

1. Overview of the blockage bag phenomenon
"blockage bag" refers to the working conditions where the oily or high-humidity material comes into contact with the filter material during the long-term operation or shutdown of the dust-removing cloth bag, and the dust is condensed, adhered, or crusted on the filter surface of the dust-removing cloth bag or inside the filter material. It is effectively removed by the online dust removal system, causing the phenomenon of a substantial increase in operating resistance. It is one of the reasons that cause the dust bag to lose its effectiveness.

After the filter bag blockage, the dust is densely covered on the surface of the filter bag, so that the effective filtering area of the filter material is greatly reduced, the air permeability of the filter material drops sharply, the running resistance becomes high, and the load of the induced draft fan increases and the energy consumption increases. In severe cases, the induced draft fan may be overwhelmed and unable to operate. At the same time, the pressure drop of the dust-removing bag remains high, making the online dust-removing system frequent dust removal, which not only requires a large amount of compressed air but also reduces the mechanical strength of the filter material and greatly shortens the service life. Therefore, if the dust-removing cloth bag is operated under the bag-gluing state for a long time, the production enterprise will have to pay higher operating costs for the operation of the system. If the bag glue problem cannot be solved in a timely and effective manner, the company will have to replace the filter bags in batches.

2. Common causes and treatment methods of blockage bags
In actual working conditions, the reasons for the blockage bag are more complicated. For specific cases, analysis can be made from the dust collector, filter material, operation control, and other related factors in order to understand the real cause of the blockage bag, so as to better reduce the occurrence of the blockage bag phenomenon.

2.1. Condensation blockage bag

Condensation blockage bag refers to the phenomenon of blockage bag caused by condensation. When the operating temperature of the bag filter is lower than the dew point, liquid water will form. Liquid water mixes with dust and gathers on the surface of the filter bag to form a blockage bag. Condensation and bag sticking always occur at the same time. For such problems, we can study and prevent them together. Generally, the operating temperature of the bag filter must be higher than the dew point temperature, so that the bag operation will be safer.

Another possible cause of condensation is compressed air for pulse cleaning. The compressed air temperature is generally much lower than the operating temperature of the dust collector. At the moment of pulse, the low-temperature compressed air will rapidly reduce the temperature of the upper part of the filter bag. When the temperature is lower than the dew point temperature, condensation will appear on the outer surface of the filter bag.  As the compressed air descends, its influence on the air temperature in the bag becomes less and less. Generally, the phenomenon of bag sticking within 1m below the mouth of the dust bag is more serious.

To prevent the bag filter bag from condensing, it is necessary to ensure that the temperature of the compressed air is higher than the dew point temperature (25~35 ℃). When the dust collector is used to process high-temperature and high-humidity gas, a temperature detection alarm device should be installed at the entrance for monitoring, and thermal insulation materials such as rock wool should be installed outside the shell for insulation. The suitable temperature in the mill of this cement company is controlled at 50~110℃.

2.2. Increased air leakage rate
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 1%~2%. But with the extension of operating time, the air leakage rate will increase. If the bag filter under negative pressure leaks, it will inhale a large amount of outside air and mist, which will cause the filter bag to become damp, which will cause paste bag adhesion and increase the running resistance. If it is the rainy season, this situation will be more serious. 

Therefore, when the dust collector is installed, the air leakage rate should be less than 3%. The welding quality of the dust collector must be implemented in strict accordance with the corresponding specifications and standards. After the welding is completed, use kerosene, fluorescent powder, etc. for leakage inspection, and the leak must be re-welded. At the same time, air leakage points such as trachoma must be eliminated. The inspection door of the dust collector must be sealed with rubber strips and checked and replaced frequently. Check each unloader and flange carefully to make a seal. Reducing the air leakage of the dust collector can effectively protect the filter bag, avoid the occurrence of bagging, and ensure the smooth operation of the equipment under low resistance. Before the entire dust removal system shuts down, the fan should continue to run to exhaust all the moisture in the system.

2.3. Adhesive blockage bag

Adhesive blockage bag mainly refers to the relatively high adhesion of dust. Although no condensation occurs, the dust still adheres to the surface of the fiber, causing the online dust removal system to be unable to remove it. The reasons for sticky blockage bags may come from the following situations.

One is that the dust itself (such as oily particles, ammonium sulfate generated by denitration, nitrate lime used for desulfurization, etc.) is relatively viscous. When it comes in contact with the filter fiber, the intermolecular force is relatively strong and cause adhesion.

The second is that some dust is not very adherent, but they can absorb moisture in the flue gas and undergo a chemical reaction of recrystallization to form new hydraulic substances or crystals (such as cement clinker, calcium sulfate, the product of desulfurization), The formed "crust" covers the surface of the filter material. Even if the dust itself is not viscous, if the dust particles are finer and have a relatively large water content, the dust can easily be uniformly adsorbed on the surface of the filter material, forming a layer of "floating dust". The adhesion is not very strong, but it is difficult to remove. As the running time increases, the "floating dust" will accumulate thicker. At present, the blockage bags in this situation are mainly handled by manual cleaning or washing. However, it has problems such as complicated disassembly and assembly, easy damage to filter bags and bag cages, long construction period, and secondary pollution (water pollution). Some domestic manufacturers also provide this kind of service, but it takes a long time, and it is impossible to avoid the damage to the filter material structure caused by the damage of the bag in the treatment and the water washing.

2.4. The cleaning process is not in place
Pulse bag filter generally uses compressed air to blow dust. Compressed air contains a lot of oil, water, and other impurities. If it is sprayed directly into the filter bag without purification, the filter bag will be contaminated and damp, resulting in condensation. If the dust collector processes high-temperature, high-humidity gas, once compressed air with a lower temperature is injected, condensation will easily occur on the surface of the filter bag, and a large amount of dust will stick to the bag and cause a blockage. In order to prevent the oil and water in the compressed air from causing the filter bag to stick together, the corresponding work system must be formulated and implemented:

(1) Open the gas storage tank, pulse valve airbag, and other exhaust and sewage valves every day to remove oil and water pollution;

(2) Install an air heater in front of the high-pressure gas tank, so that the compressed air is dehydrated and heated before being sprayed into the filter bag for dust removal.

At the same time, the short pulse time and the long cycle will cause the dust on the bag to enter the circulating filter state without removing all the dust, resulting in a rapid increase in resistance, reduced ventilation, and delayed dust cleaning, resulting in sticky bags. For the company's bag filter, the pulse valve injection time (cleaning process) can be adjusted in the interval of 0.05~0.50s, and the cleaning cycle at the same position is about 5~10min.

2.5. Structured blockage bag

blockage bags caused by the design of the dust collector and related components and those caused by improper operation can be classified as structural blockage bags.

One is an unreasonable filter structure. Due to the insufficient needle punching density of some filter materials, the fiber on the filter surface is relatively loose, and fine dust is easy to enter and stay in the filter material. Dust accumulates in the interior to a certain extent, coupled with the influence of water vapor in the flue gas, will gradually form a blockage from the inside out. Therefore, when selecting a filter bag, it is necessary to select a reasonable classification according to the characteristics of the exhaust gas, and the filter material is treated with hydrophobicity, and the surface is singed and calendered.

Second, the degree of cooperation of the filter bag is insufficient. Pulse cleaning is actually using the impact of pulsed airflow and secondary airflow on the filter bag to make the filter bag deform in the latitude direction from top to bottom. The inertia of the deformation and the collision between the filter bag and the cage bone cause an "avalanche" on the surface dust layer. The effect, thereby removing surface dust. The cleaning effect depends largely on the size of the deformation inertia and the strength of the collision between the filter bag and the cage bone. In practical applications, due to the small size of the filter bag in the weft direction or the excessive heat shrinkage of the filter material in use, after a period of use, the filter bag is "bound" on the cage bone and cannot be separated. At this time, the deformation of the filter bag caused by the pulse cleaning becomes smaller, the collision between the filter bag and the cage bone is limited, and the surface dust cannot be effectively removed. Long-term accumulation and compaction of dust on the surface of the filter bag may cause the bag to stick. For this situation, it is necessary to make appropriate adjustments based on experience in actual applications and select appropriate filter bags.

2.6. Improper operation
In addition, improper operation is also one of the causes of blockage bags. For example, when the machine is switched on and off (especially before the long-term shutdown), the dust removal operation is not performed according to the correct procedure. Therefore, during the startup and shutdown process (especially long-term shutdown), the program operation must be carried out in strict accordance with the guidelines for switching on and off. Before the dust removal system stops running, completely exhaust the wet gas in the dust collector and replace it with dry air to prevent condensation.
In addition, when unexpected situations such as boiler tube burst occur during operation, failure to properly handle and remediate the filter bag can also easily cause bag sticking. When an accident such as a pipe burst occurs, a large amount of water vapor will enter the bag filter. At this time, the flue gas should be switched to the bypass in time (if there is a bypass channel), and the burst pipe should be repaired in time. While the filter bag is forced to clean the dust online, the fan should be turned on to discharge the humid gas in the dust collector.

3. Conclusion
To sum up, there are many reasons for the blockage of the bag filter. In the daily operation and maintenance process, attention should be paid to avoid these situations and avoid the filter bag blockage from the source. In the dust removal process, if the bagging phenomenon has occurred, relevant emergency treatment should be carried out in time to prevent the bagging phenomenon from further deterioration. In the cement production process, if the bag filter is scientifically managed and maintained, and the operation specifications are strictly implemented, the adhesion phenomenon can be minimized and controlled, the dust removal efficiency can be improved, and the dust removal effect of the bag filter can be maximized.

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