In recent years, air quality has been deteriorating, hazy weather is serious, and air environment pollution is becoming increasingly serious. At present, China's industrial base is relatively weak, coal-based energy structure occupies a major position and has long existed, the proportion of coal in various energy sources is greater than 70%, coal-fired power plants soot emissions account for 40%-50% of the total national emissions, PM2.5 emissions account for 20%-30% of the country, soot-type pollution is still one of the main forms of air pollution in China. Since July 1, 2014, coal-fired units in the main urban areas of 47 cities in key control areas have implemented special emission limits with soot emission requirements of 20 mg/m3 and 10 mg/m3 in some areas. In a sense, the current increasing severity of air pollutants is related to the backward filtration means of terminal plants.
1、Superfine surface layer gradient structure filter needle felt
It is mainly divided into four parts, from top to bottom: ultra-fine fiber layer, fine fiber layer, base cloth and coarse fiber layer. The traditional filter needle felt adopts the symmetric structure of sandwich layer, i.e. the fiber material of the upper and lower layers of the base cloth is exactly the same, and the filtration method is deep filtration; the filter needle felt adopts the gradient structure of graded combination of fiber/density, and the specific structure is shown in Figure 1-3: the surface filter layer is formed by the ultra-fine PPS fiber to form a dense layer, the dust layer is filled with density gradient structure, the middle layer is high-strength and low-stretch PTFE base cloth, and the inner layer and the dust layer form a gradient structure of fiber. The coarse pore layer is easy for air to pass through.
The smaller the linear density of microfiber, the finer the fiber diameter, the more dense the air permeability channel. The pore size of the gradient structure filter needle felt made of microfiber is finer, which greatly improves the filtration accuracy of the filter needle felt and obtains higher emission value. The design of the gradient laminar structure is conducive to the formation of filter cake, making the filter needle felt filtering precision high and maintain a large air permeability, improve the efficiency of dust capture and reduce the operating resistance of the equipment.
2、Superfine surface ladder
2.1 Introduction to the experiment
2.1.1 Main instruments
YG026H type multi-functional electronic strength machine (Wuhan); YG461E fabric permeability meter permeability meter (Wuhan); Topas AFC-132 filter material test bench (Germany); Topas PSM165 pore size tester (Germany); high precision electronic scale 0.001g.
2.1.2 Experimental method
Market purchase of aluminum trioxide dust and dust samples provided by the power plant, the third party to provide particle size analysis report; conventional testing using the strength machine and permeability instrument using GB/T 3923. 1 textile fabric tensile properties, GB/T 5453 textile fabric permeability of the standard test method for testing; filtration performance test using AFC-133 filter material tester using VDI3926 evaluation can The standard test method of cleanable filter needle felt is used for testing; the pore size of filter needle felt is tested by PSM-165 pore size tester using ISO4003 international standard.
2.1.3 Testing device
The AFC-132 filter needle felt test bench is of great importance for the optimization and development of filter needle felt and for quality assurance in the production process. Among them, dust capacity and filtration efficiency are among the most concerned properties of filter materials, which are determined by the pressure difference between upstream and downstream of the filter media under test and the particle size or mass of the tested particles.
Figure 2-1 shows the filter needle felt performance test device, which can simulate the actual working conditions, and select the alumina dust for the filter needle felt filtration performance characteristics test.
2.2 Experimental results and analysis
2.2.1 General Performance Test
Figure 2-2 Weft tensile fracture curve
Table 2-1 shows the conventional test results of the ultrafine surface gradient structure PTFE composite filter needle felt, through the test results, we can see that the average warp strength of ultrafine surface gradient structure PTFE composite filter needle felt is 1267.8N, and the average weft strength is 1812.6N, both meet the national standard requirements. The weft strength of the filter needle felt is mainly related to the holding force and entanglement degree between the fibers, and the warp strength is related to the strength of the PTFE base fabric. From Figure 2-1 and 2-2, it can be seen that the fracture curves of the warp and weft filter needle felt are different, the warp filter needle felt fracture is mainly by the base cloth to bear the tension, the weft filter needle felt fracture the first peak for the base cloth force, the second peak for the fiber fracture force. The good air permeability is good for the isolation of dust. This indicates that the conventional performance is related to the fiber type, ratio, and production process, and not much related to the structure of the filter needle felt.
2.2.2 Filtering performance test
Table 2-2 VDI test results of gradient structure filter needle felt
As can be seen from Table 2-2, due to the superfine surface layer gradient structure test results, the first 30 cycles of filter media filtration time is longer, while the emission concentration of 0.12mg/m3, much higher than the normal filter needle felt filtration emission concentration; after the stabilization phase, equivalent to the normal use of dust cake layer formation, 1000Pa fixed pressure blowing phase emission concentration reached 0.027mg/m3, 1800Pa fixed pressure blowing 2 hours, the average cleaning cycle is 655s, it can be seen that the filter needle felt resistance rise is relatively stable, the emission concentration is 0, to achieve the ultra-clean emissions. This is directly related to the structure of the ultra-fine surface layer of the filter needle felt, the surface layer of ultra-fine fiber layer, fiber specific surface area, the filter needle felt surface filtration area increases, significantly improving the filtration performance of the filter needle felt.
As can be seen from Figure 2-3, the resistance of the filter needle felt rises relatively smoothly, with an initial resistance of 30 Pa and a residual resistance of 358 Pa at the end of the aging stage. It can effectively trap the dust in the surface layer of the filter needle felt, reduce the penetration of dust, and at the same time facilitate the cleaning of ash, which greatly reduces the rise time of the resistance.
Figure 2-4 after aging 1000Pa constant pressure spraying 2 hours process
As can be seen from Figure 2-4, the filter media resistance rise time is uniform, reflecting the filter media has reached a stable filtration stage, the filter needle felt surface powder cake layer has been formed, through 2h constant pressure blowing test, the filtration efficiency reached 99.999%, has achieved ultra-clean emissions.
Figure 2-5 after aging 1800Pa constant pressure spraying 2 hours process
As can be seen from Figure 2-5, the filter needle felt resistance rise time is uniform, reflecting the filter needle felt has reached a stable filtration stage, the filter needle felt surface powder cake layer has been formed, through 2h 1800Pa constant pressure blowing test, the filtration efficiency reached 100%, to meet the standard requirements of ultra-clean emissions.
2.2.3 Scanning electron microscope test
From Figure 2-6, it can be seen that the superfine surface layer of gradient structure filter needle felt is obviously layered, and it can be clearly seen that the gradient structure filter needle felt has four layers: superfine surface layer, fine fiber layer, base cloth and bottom layer, and the superfine fibers are mainly concentrated on the surface of the filter needle felt, which also verifies the structural characteristics of the gradient structure filter needle felt and is fully in line with the "surface-like filtration" mechanism.
3 Conclusion
1) Temperature consideration: the long-term use temperature of gradient structure filter needle felt is 160℃, which can meet the long-term use conditions in terms of temperature.
2) Mechanical properties: The conventional performance test of gradient structure filter needle felt shows that it fully meets the requirements of national standards.
3)Consideration of oxidation resistance: Under the environment of power plant, the working condition of nitrogen and sulfur content is low, and the gradient structure filter needle felt can fully meet the working condition; for the gradient structure filter needle felt, high efficiency PTFE permeation film treatment is used to improve its acid resistance and oxidation resistance corrosion ability.
4) Consideration of operating resistance and filtration efficiency: the use of gradient structure filter needle felt, filter needle felt structure hierarchy, to achieve "class surface filtration", can effectively trap dust in the surface layer of the filter needle felt, reduce the infiltration of fine particles of dust, greatly improve the filtration precision, to achieve ultra-clean ultra-low emission requirements, but also to ensure that the later use of equipment Stable operating resistance.
