According to the mechanism of filter dust removal, the ash-attached layer plays a very important role in the actual dust removal process. The ash-attached layer on the surface of the filter cloth is composed of dust particles with different sizes and has various structural and void properties. Under normal circumstances, it affects the dust collection efficiency and resistance of the bag filter, and determines the performance of the operation. The filter cloth is a skeleton that forms a dust layer and supports a dust layer.
1, in the filter dust removal process, the role of the ash layer
(1) When the ash layer is thick enough, high dust removal efficiency can be achieved.
(2) When the ash layer is thin or has many voids, the air permeability is good, and the dust removal resistance and the dust removal efficiency are low.
(3) When the ratio of ash to ash is large, the inertia of ash removal is large, and it is easy to separate from the surface of the filter bag after the shock and strengthen the cleaning effect.
(4) When the ash-adhesive layer has a large viscosity, it is not easy to remove dust, and the resistance is also high.
The extent to which the remaining ash-attached layer enters the interior of the filter cloth or adheres to the surface of the filter cloth directly affects the resistance of the dust remover and the dust removal effect.
2. The formation process of the attached gray layer can be roughly divided into three sections.
(1) Within a few minutes or a few hours after the new filter cloth is used, dust fills the gap of the filter cloth.
(2) The filter cloth is cleaned several times after being used for one week or longer until a stable remaining ash layer is established.
(3) Each time the amount of ash removed from the filter bag is approximately equal to the amount of ash deposited on the bag before the next cleaning, the resistance is the same under the same cleaning conditions.
The adhesion in the unstable ash layer is related to the treatment of sulfur trioxide, water vapor, calcium or other alkali metal components, particle size, and charge in the flue gas, during the dust removal process. High-sulfur fuels have greater resistance to filter bags than low-sulfur fuels. As the thickness of the ash layer increases, the resistance of the filter bag also increases. From the beginning to the maximum resistance, the relationship between the two is not completely linear, but there is no abrupt change and most of the time is stable.
3. The effect of different cleaning methods on the attached ash layer.
The separation of the unstable ash-coating layer from the filter cloth during the cleaning process occurs on the outer surface of the filter cloth. In the process of use, the holes and grooves of the filter cloth’s diameter and weft interweaving yarns are first filled with ash and appear prismatic, and then gradually develop to the outer surface of the filter cloth to form a ash-adherent layer. Under normal circumstances, the adhesion between the filter cloth and the gray layer, the gray layer and the dust particles, and the latter is much larger, so the unstable ash layer is separated from the filter cloth just outside the filter cloth. Occurred on the surface, this is the weakest binding place, where the sticky gray strength is not as strong as the gray shear force.
The thickness of the ash layer on the surface of the filter cloth varies from 5 to 20 times. When the back-flushing method is adopted, the remaining ash-coating layer has a high degree of unevenness and tends to appear as a speckled detachment. Most of the remaining ash-attached layer remains on the cloth bag, so the remaining ash content is large, and the ash attached to the filter cloth section is large. Layer profiles such as rolling hills. When used again, the pressure loss rises quickly. This shows that although the resistance loss of the filter bag after cleaning has decreased, it does not mean that the cleaning has reached a satisfactory level. Because of the large remaining ash, it still works. The remaining ash-attached layer of the filter cloth for vibration cleaning is relatively thin and uniform. The amount of ash attached per unit area is small, and the profile of the attached ash layer on the cross-section of the filter cloth is also relatively flat.
The flue gas of the combustion boiler is dusted with cloth bag, and it is back blown or shaken and bagged to remove dust. The cross-section shape of the ash layer attached to the glass fiber filter bag is: the ash layer near the surface of the filter bag is very thin and uniform. The cracks on the ash layer are very small, or not at all, and have little effect on the size of the filter bag blowback or vibration force. Farther away, due to vibration or blowback, there are large cracks in the ash layer, there is no ash in the cracks, it is clean and there is not much resistance. Therefore, it can be said that the resistance of the thick and thin ash-coating layer is not proportional to the structure of the ash-adhering layer, and the resistance depends on the compactness of the remaining ash-coating layer of the filter cloth, that is, the surface structure of the filter bag. The remaining ash-attached layer of blowback ash is denser and has higher resistance, and it is more difficult to fall off. Because the ash layer is continuously filled through the forward and reverse airflows of the filter cloth, and multiple backflushes are not detached from the ash. The layer is filled with dust more firmly and forms a gray tumour. This also explains the reason for the gradual accumulation of the residual ash layer of the blowback ash. The residual resistance rises over time and there is no fixed upper limit. For the back-fed bag filter used in power plant pulverized coal boilers, the commonly used glass fiber woven filter cloths have a filtration rate of 0.4-0.6 m/min, and the accumulated dust on the filter surface is about 2.3-3.7 kg/m2.
The rapping dust removal is carried out under the condition that the air flow is static, and the filter cloth is smooth, and the resistance is also small, so that there is less remaining ash layer after rapping and cleaning, and the emission concentration is lower than that of the back blowing in a very short time. Ash removal is bigger. It is usually about 3 times in a few minutes but its cleaning cycle is very long.
Counter-blow cleaning often causes perforation of the ash layer to permeate the air, sharply reducing the counter-blow pressure, and the ability to continue to blow off the ash after pressure relief, resulting in less dust per blow-back, and therefore maintaining the same resistance conditions. Counter-blow cleaning is more frequent than rapping and vibration cleaning. The speed of backflushing is less than 0.61m/min. Increasing the number of backflushing does not reduce the resistance of the ash layer. Instead, it increases the emission concentration and reduces the dust removal efficiency. Using a high backflushing speed may cause damage to the filter bag.
The ash-attached layer that thrashed and cleaned the ash began to fall. Afterwards, it gradually decreased, but the drop was always relatively uniform, and the remaining ash layer was evenly distributed.
The ash attached to the bag filter of the bag filter is a gradual accumulation process. The accumulated dust reaches a certain thickness, and the dust removal efficiency can reach normal. At this time, the resistance of the filter bag is relatively stable for a period of time, and the resistance of the filter bag and the thickness of the attached ash layer are reached. Clear the dust after the predetermined limit.
From the mechanism of rapping and cleaning, the rapping generates high-frequency vibration force, which is uniformly transmitted on the rigid body of the draw bag frame. The tensioned filter bag also vibrates. In the vibration process, the attached gray layer has inertial force, such as Sufficient to overcome the adhesion between the bag and the filter bag, one impact shock will be able to fall off the filter bag. In the process of impact rapping, the dust falling from one impact occupies the majority of the ash layer and continues to be beaten to attach the ash attached to the filter bag, especially a denser layer of ash on the surface of the filter bag. The vibration energy is absorbed, and the tight adhesion becomes loosely attached. The more energy is absorbed, the more adsorbed gas is, the looser, thus reducing the friction between the dust particles, making it gradually fluidized, and the adhesive force of the filter bag is smaller than that of the falling gravity. Finally separated from the filter bag surface.
