Causes of Baghouse Filter Bag Clogging

What is "bag clogging"?

"Bag clogging" is a phenomenon and a problem that occurs during the use of dust collector filter bags, and is one of the reasons why dust collector filter bags lose their effectiveness. 

There is no unified definition in the industry. Generally, "bag clogging" refers to the phenomenon where, during long-term operation or shutdown of the dust collector filter bag, 

in conditions with high humidity or contact with oily substances, dust condenses, adheres, or forms a crust on the filter surface or inside the filter material, and cannot be effectively removed by the online cleaning system, 

resulting in a significant increase in operating resistance.

After bag clogging, dust densely covers the surface of the filter bag, significantly reducing the effective filtration area of the filter material, drastically decreasing the air permeability of the filter material, and resulting in very high operating resistance. 

This increases the load on the induced draft fan, leading to increased energy consumption, and in severe cases, may even cause the induced draft fan to be overloaded and unable to operate. At the same time, the persistently high pressure drop of the dust collector filter bag leads to frequent cleaning by the online cleaning system, 

which not only consumes a large amount of compressed air but also causes serious loss of mechanical strength of the filter material, greatly shortening its service life. Therefore, if the dust collector filter bag operates in a clogged state for a long time, 

the user will have to pay higher operating costs for the system. If the bag clogging problem is not solved in a timely and effective manner, the entire batch of filter bags will have to be replaced.

Finding solutions from the causes of bag clogging:

In actual working conditions, the factors that may cause bag clogging are complex. For specific cases, we must analyze the flue gas, dust collector, filter material, operating control, and other related factors to understand the true inducing factors of bag clogging, so as to avoid and solve the problem.

I. Condensation-induced bag clogging:

Generally, the operating temperature of a baghouse dust collector must be at least 25K above the acid dew point temperature for safe operation of the filter bags. This is mainly to avoid condensation. 

When the operating temperature is below the dew point, liquid water will condense. The liquid water mixes with dust and accumulates on the surface of the filter bag, forming bag clogging.

The higher the water content and sulfur trioxide content 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 limitations of the physical properties of the filter bag material and operating economics, the continuous operating temperature of the dust collector must be controlled within a suitable range. 

If these two factors cannot be reconciled, condensation is inevitable. Therefore, water content, SO3 content, and operating temperature are the three main factors determining whether condensation occurs.

A certain dust collector operates at a stable temperature of around 150°C, but because the main substances it incinerates are complex solid, liquid, and gaseous waste from chemical companies, the water and sulfide content in the flue gas varies greatly (water content even exceeding 25%), 

and the dew point temperature fluctuates significantly. Condensation is unavoidable, and bag clogging is extremely severe. At the same time, due to the penetration of acidic substances and prolonged operation, dust gradually forms a crust on the surface of the filter bags, causing a loss of air permeability.

Secondly, the compressed air used for 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. 

During the pulse, the low-temperature compressed air rapidly lowers the temperature of the upper part of the filter bag. When it falls below the dew point temperature, it will cause condensation on the outer surface of the filter bag in that area. 

As the compressed air moves downwards, its impact on the air temperature inside the bag becomes smaller. Therefore, bag clogging is generally more severe within one meter below the dust collector bag opening.

Furthermore, the air leakage rate of the dust collector is also a possible factor causing condensation. The air leakage rate of a typical dust collector is around 1-2%. However, with prolonged operation, the leakage rate will increase. If it is in the harsh winter of the north, the problem becomes more significant. 

The higher the leakage rate, the greater the impact on the local temperature inside the dust collector, and the higher the risk of bag clogging due to condensation at the leakage point.

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 is certainly lower than other parts. 

In actual service, we found that the degree of bag clogging in the outer circle of dust collector bags near the four walls is usually much higher than in other parts.

II. Adhesive Bag Blinding:

This mainly refers to situations where, due to the high adhesiveness of the dust, even without condensation, the dust still adheres to the fiber surface and cannot be removed by the online cleaning system. 

The reasons for adhesive bag blinding may stem from the following situations:

1. The dust itself has high viscosity, resulting in strong intermolecular forces when it comes into contact with the filter media fibers. Examples include oily particles, ammonium sulfate produced during denitrification, and quicklime used in desulfurization.

2. Some dusts, although not inherently sticky, are easily deliquescent. When trapped on the fiber surface, they absorb moisture from the air and form a solution on the fiber surface. For example, powdered sugar.

3. Some dusts may not be highly adhesive, but they can absorb moisture from the flue gas and undergo a recrystallization chemical process, generating new hydraulic substances or crystals, forming a "crust" that covers the filter media surface. For example, the desulfurization product—calcium sulfate.

4. Even if the dust itself is not sticky, if the dust particles are fine and the dust has a high moisture content, the dust can easily and evenly adhere to the filter media surface, forming a layer of "floating dust." The adhesion is not very strong, but it is difficult to remove. The longer the operating time, the thicker the "floating dust" layer becomes. 

This situation is common in coal-fired power plants, especially with filter bags longer than 6 meters. The differential pressure of the bag filter should be controlled below 1500 Pa during operation.

III. Structural Bag Blinding:

We categorize all bag blinding caused by the design of the dust collector and related component structures, as well as bag blinding caused by improper operation, as structural bag blinding.

1. Filter Media Structure and Treatment: Some filter media have insufficient needle-punching density, resulting in loose fibers on the filtering surface. Even with surface singeing treatment, fine dust can easily enter the filter media and remain there. 

As the dust accumulates internally to a certain extent, coupled with the influence of water vapor in the flue gas, a blockage gradually forms from the inside out. At a power plant in northern China, the filter bags had to be replaced after less than a year of use due to this problem.

Furthermore, some filter materials are not or cannot be singed or calendered, leaving fiber ends on the surface of the filter material. This actually provides "condensation nuclei" for condensation, causing condensation to begin at the fiber ends. 

Dust also forms dust clumps at these locations, and as the dust clumps grow larger, they bridge together. Large-scale bag blinding gradually forms. In waste incineration power plants using pure PTFE filter materials, if the surface is not treated in any other way, stubborn dust crusts are often found at the fiber ends.

In addition, in working conditions with high humidity or a risk of acid condensation, if the filter material is not treated for hydrophobicity, bag blinding is quite common.

2. Filter bag fit: We know that pulse cleaning actually utilizes the impact of the pulse airflow and secondary airflow on the filter bag, causing the filter bag to deform in the weft direction from top to bottom. 

The inertia of the deformation and the collision between the filter bag and the cage create an "avalanche effect" on the surface dust layer, thereby removing surface dust. The cleaning effect largely depends on the magnitude of the deformation inertia and the intensity of the collision between the filter bag and the cage. 

In practical applications, due to reasons such as the filter bag's weft dimension being too small or excessive thermal shrinkage of the filter material during use, after a period of use, the filter bag becomes "bound" to the cage and cannot be separated. 

At this time, the deformation of the filter bag caused by pulse cleaning is very small, and the intensity of the collision between the filter bag and the cage is limited, so the surface dust cannot be effectively removed. Long-term accumulation and compaction of dust on the surface of the filter bag can lead to bag blinding.

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