The chemical corrosion of filter bags mainly includes acid dew point corrosion and sulfuric acid aerosol corrosion. Due to changes in the coal market, the types of coal used in coal combustion have undergone changes, resulting in a significant increase in sulfur content. The concentration of SO ₂ in the flue gas entering the filter bag has increased from 800~3000mg/Nm ³ to 800~4500mg/Nm ³, and for a large portion of the time, the SO ₂ in the flue gas entering the filter bag is around 4000mg/m ³. Under working conditions, the higher the SO ₂ in the flue gas, the higher the dew point of the flue gas, and there is condensation in the dust collector during use. In addition, acidic gases corrode the filter bag, causing it to lose strength easily. To cope with the impact of chemical corrosion, PTFE is a commonly used filter material in power plants, which is superior to PPS and P84.
2. Oxidative factors
power plantBoiler dust collector bagThe filter material selected for the filter bag is PPS fiber+PTFE base cloth, and the surface layer is PPS fiber. PPS filter material has good alkali and acid resistance, but its weak point is poor antioxidant performance. To ensure that the service life of PPS filter material is not less than 3000 hours, it should be used under the conditions of flue gas temperature not exceeding 150 ℃, oxygen content not exceeding 8%, and NO ₂ content not exceeding 15mg/m ³.
PTFE fiber and P84 fiber have better oxidation resistance than PPS fiber.
3. Hydrolysis factors
Hydrolysis is the decomposition process that occurs when a compound reacts with water. PPS and PTFE do not hydrolyze, while P84 is easily hydrolyzed. In the application of filter bags, some power plants use a small amount of P84 fiber mixed needle punched into the dust facing surface of PPS filter material to improve the filtration accuracy of the filter bag. However, the issue of easy hydrolysis should be carefully considered for coal combustion with high moisture content and condensation. PPS and PTFE will not hydrolyze when it comes to hydrolysis issues.
4. Temperature factor
The ability of filter media made from different raw materials to withstand long-term continuous temperature is different. Some old power plants have high exhaust temperatures, with summer temperatures around 170 ℃. After long-term high temperature operation, their filter bags were only used for 14000 hours, and in less than two years, a large number of broken bags were found. The reason for this was that the exhaust temperature was too high, which damaged the filter bags.
PPS, P84, and PTFE are three types of filter media. PPS can withstand temperatures of less than 160 ℃ for a long time, while P84 and PTFE can withstand temperatures of around 200 ℃ for a long time. Therefore, when selecting filter bags, the influence of boiler exhaust temperature on filter materials should be considered.
5. Factors affecting the thermal shrinkage rate of filter media
Due to the fact that most of the bag filters for power plant boiler dust collectors are made of synthetic fibers, the filter material will undergo thermal shrinkage under certain temperature conditions. Therefore, the heat setting process is crucial in the production of filter material. Some power plant boiler bag filter bags, from the inspection report, it can be seen that the mutual wear between the filter bag and the bag cage has caused multiple holes in the filter bag from the inside out and multiple cracks in the vertical direction of the ribs, which is judged to be related to its thermal shrinkage rate. Therefore, during the production of filter media, it is required that the thermal shrinkage rate of the filter media at 200 ℃ be less than 1%. The thermal shrinkage rate of the filter media is an important indicator that cannot be ignored.