1. What is “paste bag”:
“Paste bag” is a phenomenon and a problem in the use of dust bag. It is one of the reasons that cause dust bag to lose its effectiveness. There is no unified definition in the industry. The author thinks: “paste bag” is a long-term operation of dust bag. During or out of operation, in the conditions in which high humidity or oily substances come into contact with the filter media, the dust condenses, adheres or crusts on the filter surface of the dust bag or inside the filter media and cannot be effectively removed by the on-line cleaning system. Cause a substantial increase in operating resistance.
After the bag pastes the bag, the dust densely covers the surface of the filter bag, so that the effective filtration area of the filter material is greatly reduced, the air permeability of the filter material is drastically reduced, the running resistance becomes very high, and the load of the induced draft fan is increased to cause an increase in energy consumption. Seriously, even the draught fan is overburdened and unable to operate. At the same time, the high pressure drop of the dust bag makes frequent cleaning of the on-line ash cleaning system not only consumes a lot of compressed air, but also causes mechanical strength of the filter media. Severe loss, the service life is greatly shortened. Therefore, if the dust bag is run under the paste bag for a long period of time, the owner will have to pay a higher operating cost for the operation of the system. If the problem of paste bags cannot be resolved in time, the owner Will have to replace the whole bag.
2.. Reasons for the formation of paste bags:
In actual working conditions, the factors that may cause paste bags are more complicated. For specific cases, we must analyze the conditions of smoke, dust collectors, filter materials, operation control and other related factors to understand the paste. The real inducing factor of the bag so that the problem can be avoided and solved.
(1). Condensation paste bags:
In general, the operating temperature of the bag filter must be more than 25K above the acid dew point temperature. It is safer to operate the bag. This is mainly to avoid the occurrence of condensation. When the operating temperature is lower than the dew point, it will form a liquid state. Water. Liquid water mixed with dust and collected on the surface of the filter bag will form a paste bag.
Dew point calculation formula: ts = 186 +20 lg ψ H2O +26 lg ψ SO3 (1) where ts is the dew point temperature, ψH2O and ψSO3 are the volume fraction of H2O and SO3, respectively.
It can be seen that 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 precipitator must be higher, and due to the physical properties of the filter bag material and the economical operation, the continuous operation temperature of the precipitator must be controlled within a suitable range. Coordination, condensation will be unavoidable. Therefore, water content, SO3 content and operating temperature are the three main factors that determine whether condensation occurs.
An industrial baggage incinerator removes the bag’s acid condensation to form a paste bag. The operating temperature of the filter is stable at about 150°C, but the main incineration material is the waste of chemical companies with complex components such as solids, liquids, and gases. Therefore, the content of water and sulphide in the flue gas varies greatly (the water content even exceeds 25%), and the dew point temperature fluctuates greatly. Condensation is unavoidable and the bag is extremely serious. At the same time, due to the infiltration of acid 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.
Second, the pulsed cleaning of the compressed air is also one of the possible causes of condensation. Compressed air temperature is generally much lower than the operating temperature of the dust collector. At the moment of pulse, the temperature of the upper part of the filter bag will be rapidly reduced by the low temperature compressed air, and when the temperature is lower than the dew point temperature, condensation on the outer surface of the filter bag will occur at that part. . As the compressed air goes down, its influence on the air temperature inside the bag becomes smaller and smaller. Therefore, the phenomenon of paste bags in the range of one meter below the outlet of the dust bag is more serious.
Again, the air leakage rate of the dust collector is also a possible contributor to condensation. General dust collector air leakage rate is about 1-2%. However, as the running time increases, the air leakage rate will increase. If it is in the cold winter, the problem becomes bigger. The higher the air leakage rate, the greater the influence on the local temperature in the dust collector, and the risk of the filter bags of the leaked part becoming higher due to condensation.
In addition, although the dust collector wall is provided with insulation layer, but because the four walls are the most frontier with the outside world, the temperature is certainly lower than other parts, the actual service we found is usually close to the four-walled outer dust bag bag paste level It is much higher than other parts.
(2), adhesive paste bags
Mainly because the dust adhesion is relatively large, although no condensation occurs, but the dust still adheres to the fiber surface, and the on-line cleaning system cannot remove it. The reasons for adhesive paste bags may come from the following situations.
A The dust itself has a relatively large viscosity, and when it is in contact with the filter fiber, the intermolecular force is relatively strong. Such as oily particles, denitrification generated ammonium sulfate, desulfurization use of lime and so on.
B Although some dusts are not sticky, they are easily deliquescent. When they are trapped on the surface of the fiber, they absorb the moisture in the air and form a solution on the surface of the fiber. For example, sugar powder.
C. Some dust adhesion may not be strong, but it can absorb the moisture in the smoke and recrystallize the chemical process to generate new hydraulic materials or crystals, forming a “crust” covering the surface of the filter. For example: cement clinker, desulphurization products – calcium sulfate.
D Even though the dust itself is not sticky, if the dust particles are fine and the moisture content of the dust is relatively large, the dust can easily adsorb on the surface of the filter material to form a layer of “floating dust” and the adhesion is not very strong, but it is very 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 longer than 6 meters. A certain northern power plant once caused the dust collector’s operating resistance to rise to 2200 Pa in a short period of time from about 1500 due to this reason, resulting in the serious situation of the entire line being out of service.
(3), structural paste bags:
We classify all paste bags caused by paste bags and improper handling caused by the design of dust collectors and related parts into structural paste bags.
a, filter structure and processing:
In some filter materials, due to insufficient needle punch density, the fibers of the filter surface are relatively loose, and even if the surface is singed, fine dust can easily enter and stay inside the filter material. Dust builds up to a certain degree in the interior, and together with the influence of water vapor in the flue gas, it gradually becomes blocked from the inside out. A power plant in northern China used bags for less than a year, because of this problem had to consider the entire batch of replacement bags.
In addition, some filter materials do not have or cannot be singed or calendered, and the ends of the fibers are retained on the surface of the filter material. This actually provides a “condensation nucleus” for condensation, so that condensation begins first at the fiber ends. Dust also forms dust clusters at this location first, as the dust mass gradually becomes larger and bridges each other. A large area of paste bags gradually formed. In a waste incineration power plant using pure PTFE filter material, if the surface is not treated any more, stubborn dust encrustations at the fiber ends are often found.
In addition, the filter material was not treated with hydrophobicity for conditions with high humidity or risk of acid condensation. Bags are more common.
b, filter bag fit:
We know that pulse cleaning is actually the use of pulsed air and secondary air flow on the impact of the filter bag, so that the filter bag deformation from top to bottom in the weft, deformation inertia and filter bag and cage bone collision in the surface dust layer Causes an “avalanche effect” to remove surface dust. The cleaning effect largely depends on the magnitude of the inertia of the deformation and the strength of the collision between the filter bag and the cage. In practical applications, due to the reason that the latitudinal dimension of the filter bag is too small or the heat-shrinkage degree of the filter material in use is too large, the filter bag “bundles” cannot be separated on the cage bone after a period of use. At this time, the deformation of the filter bag caused by the pulse cleaning is very small, the collision intensity between the filter bag and the cage 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 result in a paste bag.