Reasons for bag clogging in bag filter dust removal and solutions

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What is a \"failed filter bag\"? A \"failed filter bag\" is a phenomenon or problem that occurs during the use of dust removal filter bags, and it is one of the reasons why these filters lose their effectiveness. There is no unified definition within the industry. It is generally believed that \"bag fouling\" occurs when, during the long-term operation or shutdown of dust collection bags, under conditions of high humidity or when oily substances come into contact with the filter media, dust accumulates, adheres, or forms a crust on the filtering surface of the bags or inside the filter media, and cannot be effectively removed by the online cleaning systems, resulting in a significant increase in operating resistance. 　After the filter bags are lined, dust covers the surface of these bags densely, which significantly reduces the effective filtering area of the filter media. The air permeability of the filter media drops sharply, and the operating resistance increases greatly. This leads to an increased load on the exhaust fan, resulting in higher energy consumption; in severe cases, the exhaust fan may become overloaded and stop functioning. At the same time, the high pressure drop across the dust collection bags forces the online cleaning system to clean them frequently; this not only consumes a large amount of compressed air but also leads to a significant reduction in the mechanical strength of the filter media, thereby **reducing its service life**. Therefore, if the dust collection bags operate in a clogged state for an extended period, users will have to bear higher operating costs for the system. If the bag clogging problem is not resolved promptly and effectively, the bags will have to be replaced in their entirety. 　Find solutions by examining the causes of bag fouling: In actual operating conditions, the factors that can lead to bag fouling are quite complex. For specific cases, we must analyze the flue gas conditions, the dust collector, the filter media, operational controls, and other related factors in order to identify the true causes of bag fouling, thus enabling the prevention and resolution of such problems. 　　I. Dew formation-prone filter bags: Under normal circumstances, the operating temperature of bag filters must be at least 25 K above the acid dew point temperature, which ensures safer operation for the filter bags. This is mainly to prevent dew formation. Liquid water forms when the operating temperature is below the dew point. When liquid water mixes with dust and accumulates on the surface of the filter bag, it causes the bag to become clogged. 　　The higher the levels of moisture and sulfur trioxide in the flue gas, the higher the dew point temperature. This means that the operating temperature of the dust collector must be higher. Due to the limitations imposed by the physical properties of the filter media used in dust collectors and their operational economics, the operating temperature of such collectors must be kept within a suitable range. If these two cannot be reconciled, dew formation will be inevitable. Therefore, water content, SO3 content, and operating temperature are the three main factors that determine whether dew formation will occur. 　　The operating temperature of a certain dust collector remains stable at around 150°C. However, since the materials it primarily incinerates are waste products from chemical industries in the form of solids, liquids, and gases with complex compositions, the levels of water and sulfides in the flue gas vary greatly (with the water content sometimes exceeding 25%), and the dew point temperature fluctuates significantly. Dew formation is inevitable, 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, resulting in a loss of air permeability in those bags. 　　Secondly, the compressed air used for pulse dust cleaning is also one of the possible causes of dew formation. The temperature of compressed air is generally much lower than the operating temperature of the dust collector. At the moment of pulse operation, the cold compressed air causes the temperature at the upper part of the filter bags to drop rapidly; when it falls below the dew point temperature, condensation appears on the outer surface of the filter bags in that area. As the compressed air moves downward, its impact on the temperature of the air inside the bag becomes increasingly minor. Therefore, the problem of bag clogging is more severe within a range of one meter below the mouth of the dust removal bag. 　　Again, the air leakage rate of the dust collector is also a possible factor contributing to dew formation. The air leakage rate of ordinary dust collectors is around 1–2%. However, as the operating time increases, the air leakage rate rises. If it is in the cold northern regions during winter, the problem becomes more serious. The higher the air leakage rate, the greater the impact on the local temperature inside the dust collector, and the greater the risk that the filter bags at those leaking areas will become clogged due to dew formation. 　　Furthermore, although the walls of the dust collector are equipped with insulation layers, since these walls are in direct contact with the outside environment, their temperature is definitely lower than that of other areas. In practical operation, we have found that the dust collection bags located near the walls tend to suffer from more severe clogging compared to those in other areas. 　　II. Adhesive dust bags: These refer to situations where, due to the high adhesiveness of the dust, although no dew formation occurs, the dust still adheres to the surface of the fibers, and the online dust removal system is unable to remove it. The reasons for sticky paste bags may stem from the following situations. 　　1. Dust itself has relatively high viscosity; when it comes into contact with the fibers of the filter medium, the intermolecular forces are strong. Such as oily particles, sulfuric acid an produced by denitration, and nitrocalceous lime used in desulfurization. 　　2. Some dusts, although not sticky, are highly deliquescent; when trapped on the surface of fibers, they absorb moisture from the air and form a solution on that surface. For example, powdered sugar. 　　3. Some dusts may not have strong adhesiveness, but they can undergo a chemical process in which they absorb moisture from the flue gas and recrystallize, forming new hydraulic substances or crystals; these form a \"crust\" that covers the surface of the filter media. For example: cement clinker, and the by-product of desulfurization—calcium sulfate. 　　4. Even if the dust itself is not sticky, if its particles are fine and it contains a high amount of moisture, it can easily adhere evenly to the surface of the filter material, forming a layer of \"floating dust.\" This layer does not have very strong adhesion, but it is difficult to remove. The longer it runs, the thicker the “dust layer” becomes. This situation can occur in conventional coal-fired power plants, especially with filter bags that are longer than 6 meters. A power plant in the north once experienced this issue, which caused the operating resistance of the dust collector to rise from around 1500 Pa to 2200 Pa in a short period of time, leading to a serious situation where the entire production line had to be shut down. 　　III. Structural bag clogging: We classify all cases of bag clogging caused by the design of the dust collector and the structure of its related components, as well as those resulting from improper operation, under structural bag clogging. 　　1. Filter media structure and treatment: Some filter media have a low needle-punching density, resulting in relatively loose fibers on their filtering surface; even after singeing treatment, fine dust can still easily penetrate into the filter media and remain there. As dust accumulates to a certain extent inside, combined with the effect of water vapor in the flue gas, blockages gradually form from the inside out. At a power plant in the north, the bag filters were not used for even a year before it became necessary to consider replacing them all due to this issue. 　　Furthermore, some filter media do not undergo or cannot be subjected to singeing or calendering treatments, leaving the fiber ends on their surface; these actually serve as \"nuclei\" for dew formation, causing dew to appear first at the fiber ends. Dust also first forms dust clumps at this location; as these clumps grow larger, bridges form between them. Large areas of bagged paste were gradually formed. In waste incineration plants using pure PTFE filters, if no other treatments are applied to the surface, persistent dust crusts at the ends of the fibers can often be observed. 　　Furthermore, in conditions with high humidity or a risk of acidic dew formation, the filter media was not treated to make it hydrophobic. Pouched bags are quite common. 　　2. Filter bag fit: As we know, pulse cleaning actually utilizes pulse air currents and secondary air currents to impact the filter bags, causing them to deform from top to bottom in the transverse direction. The inertia of this deformation, along with the collisions between the filter bags and the cage frame, creates an \"avalanche effect\" in the surface dust layer, thereby removing the surface dust. The dust removal efficiency depends to a large extent on the magnitude of the deformation inertia and the intensity of the collision between the filter bags and the cage frame. In practical applications, due to reasons such as the overly small transverse dimension of the filter bags or excessive thermal contraction of the filter media during use, after a period of time, the filter bags become \"bonded\" to the cage frame and cannot be separated. At this time, the deformation of the filter bags caused by pulse cleaning is minimal; the impact between the filter bags and the cage frame is limited, and the dust on their surfaces cannot be effectively removed. Prolonged accumulation and caking of dust on the surface of the filter bags can lead to bag fouling.