Key words: electric bag composite dust collector bag filter dust emission concentration 1 Introduction With the rapid development of China's national economy, the problem of atmospheric environmental pollution is becoming more and more serious. The smog and acid rains in large and medium-sized cities are frequent, which seriously affects people's physical and mental health and normal life. Coal-fired power plants as one of the major sources of air pollutant emissions has been the focus of attention. Now, the “ultra-low emission†of atmospheric pollutants in coal-fired power plants has become a hot topic. The core component of the bag-type and electric bag composite dust collector is the filter material. Today, the emission efficiency of the filter material has received great attention. However, the final discharge concentration of the precipitator depends not only on the filtration efficiency of the filter itself, but also on the filtration efficiency of the filter bag after sewing and whether there is leakage in the body of the precipitator, etc., which will greatly reduce the final dust emission of the precipitator. influences. In this paper, through the laboratory test results of a large number of filter materials, as well as the CFD numerical simulation calculation, the initial quantitative analysis of the factors affecting the dust emission of the dust collector is carried out, in order to achieve the ultra-low emission of the bag/electric bag composite dust collector. "plays a certain guiding role. 2 Factors affecting dust emissions from dust collectors 2.1 Filter material filtration performance 2.1.1 Filtration performance and laboratory test results of filter media The filtration mechanism of the filter material includes screening, inertial collision, interception, diffusion and electrostatic attraction. Screening is one of the main dust filter mechanisms for filter media. According to the structure of the filter material, it can be mainly divided into three types: woven fabric, non-woven fabric and coated filter material. For woven fabrics, after the start of filtration, due to the adhesion force, dust bridging occurs between the meshes of the warp and weft, and a dust layer is formed on the surface of the filter material. Since the particle size of the dust is generally smaller than the fiber diameter, the sieving effect is also stronger, and the dust removal efficiency can be significantly improved. Compared with woven fabrics, the needle felt filter material has a finer, evenly distributed and somewhat deep pore structure. The dust is less likely to penetrate the filter material, and it is easier to form a dust layer on the surface of the filter material, so that it can be better. Dust removal efficiency. The membrane filter material is a filter material which is covered with a film having a large number of minute pores on the surface of the woven fabric or the needle felt. The pore size of the membrane is so small that almost all of the dust is retained on the surface of the filter material, that is, the dust is directly collected by the action of the filter material. It does not rely on the action of the dust layer, nor does it allow the particles to enter the deep layer of the filter material. It also makes it easier to remove the dust while achieving higher dust removal efficiency, thus maintaining a lower pressure loss. The filter material dynamic filtration performance tester is the test equipment recommended by China's national standard GB/T 6719-2009 and international standard ISO 11057-2011, German standard VDI/DIN 3926, and Japanese and American related standards to test the filter material filtration performance. The tester can simulate the dust removal process under field conditions through repeated filtration-cleaning process to obtain the filtration performance of the filter material. The tester structure is shown in Figure 1. Tester structure Figure 1 filter material dynamic filtration performance tester The tester consists of four parts: dusting, cleaning, self-control and recording. The dust quantitatively supplied by the dust collector is dispersed and mixed by compressed air to form a uniform and stable dust-containing gas flow; when the dust-containing gas flows through the filter material, most of the dust in the gas stream is trapped on the surface of the filter material, and the gas is filtered. The resistance of the filter material is continuously increased due to the accumulation of dust; when the resistance of the filter material increases to a certain set value, the control system automatically starts the pulse blowing process, and the compressed air is blown through the pulse blowing valve to remove the filter material. The accumulated dust makes the resistance of the filter material immediately reduced, and then the next cycle of filtration-cleaning process is automatically performed. The filter material performance test needs to go through four stages of the initial stage, the aging stage, the stabilization stage and the post-aging stage. During the whole test process, the resistance on both sides of the filter material is automatically recorded by the system. The trace dust of the filter material is captured by the filter in the sampler. The weight of the filter membrane and the filtration time recorded by the system are obtained. Emission concentration and dust removal efficiency. Zhejiang Feida uses the tester according to the VDI 3926 standard. In recent years, it has carried out filtration performance tests on more than 100 filter materials of major domestic filter manufacturers, and has mastered a large amount of data. Table 1 gives the basic conditions for testing the filter performance test. Table 2 shows the dust discharge statistics for the 100 test media in the initial and post-aging phases. It can be seen from Table 2 that the dust discharge concentration of the filter material itself is extremely low under the test conditions. The average emission concentration of the initial stage (clean filter material, 30 cleaning cycles) is less than 1.5mg/m3. After the aging test, the average concentration of all filter materials is less than 0.5mg/m3, which is much lower than The discharge concentration of the bag filter or electric bag composite dust collector on site. The conditions of the working conditions vary widely. The main factors affecting dust emission, such as dust characteristics, operating conditions (filtration speed, cleaning frequency), etc., are also quite different from those in the laboratory. It is difficult to quantitatively evaluate the impact of these factors. According to some test results, it is qualitatively judged, in order to play a guiding role in controlling dust emissions. 2.1.2 Effect of dust particle size The particle size of the dust directly affects the filtration efficiency of the filter material. Figure 2 shows the grading filtration efficiency of a certain filter material under different conditions. In addition to the membrane filter material, the general filter material mainly relies on the filtering effect of the surface dust layer. Therefore, as the dust layer is deposited on the surface of the filter material, the filtration efficiency of the filter material is also different, and the cleaning filter has the lowest dust removal efficiency. The dust layer is the highest after it is piled up, and it drops after the ash is removed. In addition, for dust with a particle size of about 0.2 to 0.4 μm, the filtration efficiency is the lowest under different conditions, because the dust in this particle size range is at the lower limit of the range of inertial collision and interception, and the upper limit of the diffusion range. . However, whether it is for clean filter materials or filter materials with powder layer, the general trend is the same, that is, the filter material has better filtration efficiency for large-diameter dust. When the particle size is larger than 0.5μm, the trend is very obvious. . 1—filter material after dust accumulation; 2—filter material after rapping; 3—cleaning filter material Figure 2 Classification efficiency of filter media under different conditions [1] The standard dust used in this paper is 4.5μm, and the median diameter of fly ash in power plant coal-fired boilers is much larger than this value, generally reaching tens of μm. That is to say, under the same conditions, the laboratory test conditions are more likely to cause dust leakage. 2.1.3 Impact of filtration speed The filtration speed is an extremely important parameter in the design of the baghouse/electric bag composite dust collector. The filtration speed has a great influence on the resistance of the equipment, and also has a certain influence on the discharge concentration of the equipment. Under woven fabric conditions, a smaller filtration rate helps to establish a dust layer, which significantly improves dust removal efficiency (Figure 3). Figure 3 Relationship between filtration speed and penetration [2] When using needle felt or film filter material, the effect of filtration speed on dust removal efficiency is not as obvious as that of woven fabric, but as the filtration speed increases, the dust will penetrate the filter material more easily, and the cleaning frequency will also increase. The latter is also a factor in the increase in dust emissions. That is to say, regardless of the filter material used, an increase in the filtration speed will result in an increase in dust emissions. In the laboratory test, the standard specified filtration speed is 2m/min, which is much larger than the typical 1m/min~1.2m/min on site. From this point of view, under the same conditions, the laboratory test conditions are more likely to cause dust leakage. 2.1.4 Impact of the cleaning cycle Generally, the filter material mainly depends on the powder layer on the surface to filter, but each time the cleaning will destroy the powder layer on the surface, the dust leakage is the largest for the general filter material, and then With the increase of filtration time, the powder layer is repaired and the amount of dust leakage is rapidly reduced. Figure 4 shows the correlation between the amount of particulate matter penetrating through the filter material and the filtration process measured by the particle counter during the dynamic filtration performance test of the filter. It can be seen from the figure that during the entire filtration process, the dust leakage mainly occurs after a short period of time after the cleaning. During a cleaning cycle, the particulate matter of the filter material is almost zero during most of the time. In other words, under the same conditions, more cleaning times, or a shorter cleaning cycle, means higher dust emission concentration. As shown in Table 3, the filter material of plant A, if the other test conditions are the same, if the cleaning set resistance is increased from 1000Pa to 1800Pa, the cleaning cycle will be extended from 206s to 514s, and the dust discharge will be from 0.081 mg/m3 was reduced to 0.027 mg/m3. A large number of laboratory tests have shown that under standard test conditions, most of the filter material has a cleaning cycle of only 2 to 3 minutes, while on-site bag filters have a low filtration rate, and the cleaning cycle generally has tens of minutes. The bag composite dust collector is even longer, and the cleaning cycle of the laboratory test is much shorter than the on-site cleaning cycle. That is to say, in terms of the cleaning cycle, laboratory test conditions are more likely to cause dust leakage. In summary, the above test results can be concluded that although the operating conditions of the on-site precipitator differ from the laboratory test conditions, the laboratory test conditions are more conducive to dust leakage, so dust leakage due to filter material can be considered. The actual emissions on site will not be higher than the laboratory test results. 2.1.5 Influence of dust concentration In various industrial dust collectors that are widely used, in general, as the dust concentration at the inlet of the dust collector increases, the outlet concentration of the dust collector also increases. Such as electrostatic precipitators, cyclone dust collectors are such "constant efficiency" dust collectors. For example, the bag filter or the electric bag composite dust collector is to use the filter material to ensure the final discharge concentration. It is generally considered to be a “constant discharge concentration†dust collector, that is, the discharge concentration of the dust collector is independent of or related to the inlet dust concentration of the dust collector. Big. In order to determine the influence of dust concentration on the concentration of filter material, this paper tested the dust leakage of filter materials under different dust concentration conditions with particle counters under the same conditions. Figure 5 shows the relationship between the amount of dust leakage through the filter and the time. It can be seen from the figure that when the dust concentration is increased from 2.5 mg/m3 to 7.5 mg/m3, the cleaning cycle is significantly shortened and the peak of dust leakage is increased. Figure 6 shows the amount of dust emissions accumulated in each cleaning cycle corresponding to the six cleaning cycles of Figure 5. It can be seen from Fig. 6 that although the dust concentration is increased to three times as much as the original, the peak value of the dust leakage is increased, but the cumulative leakage amount of the dust does not increase in one cleaning cycle. This can be understood as the damage of the primary dust layer due to the increase of the dust concentration can be repaired more quickly, and once the repair is completed, the dust leakage is almost reduced to zero, thus, a cleaning cycle In other words, the amount of dust leakage does not increase. As can be seen from the above test results, if the other conditions are the same, if the dust concentration is increased, the cleaning cycle of the filter material is shortened to cause an increase in the discharge concentration. That is, the effect of the dust concentration on the discharge concentration of the filter material is caused by an increase in the number of cleaning operations. Although the concentration of soot in the flue gas will be much greater than the dust concentration under the test conditions under the operating conditions, as mentioned above, the cleaning frequency of the laboratory test is much larger than that of the bag filter or electric bag composite dust collector in the field. Therefore, from this point of view, laboratory test conditions are more likely to cause dust leakage. 2.2 Influence of filter bag suture on dust emission The difference between the filter performance of the filter bag used in the field and the filter material tested in the laboratory is that the former has suture and the latter does not. In order to grasp the influence of the suture on the filtration performance of the filter bag, this paper also adopts the filter material dynamic filtration performance tester. It was tested. The test selected three filter bags of the same material produced by different manufacturers for the filtration performance test, and compared with the test results of the filter material (seamless line). Among them, the seam of the filter bag of Plant B is glued and sealed to reduce dust leakage. The stitching (machine) means that the machine is glued, and the stitching (hand) means that the hand is applied by glue. There is no glue applied to the seams of the A and C filter bags. According to the test method specified by the standard, the filter material performance test passes through the initial stage, the aging stage, the stabilization stage, and the post-aging test stage. In these four stages, the dust emission test results after aging are shown in Table 3. The filter material (with suture) sample of factory C, due to the sewing problem (Fig. 7), the pinhole is very obvious. In the initial stage of the test, the filter of the sampler is blocked due to serious dust penetration. It cannot be continued, and the dust emission concentration is the average value at the end of the test. The test results of the three sample seamless lines are not much different, but the test results are quite different after the stitching. From the test results, the filter bags of the three manufacturers A, B and C represent three different levels of excellent, good and bad. A manufacturer has a seamless line with similar discharge concentrations, sewing. The process is good; B manufacturer, the effect of coating sealing with machine is better, can see that the suture has no additional impact on dust discharge, but the effect of hand-sealing sealing is not satisfactory, the stitching process Need to be improved; C manufacturer's filter bag stitching pinhole is very obvious, the discharge concentration of the suture is more than 200 times of the seamless line, even if the test filtration speed is reduced from 2m/min to 1m/mim, the dust emission concentration remains At 47.1 mg/m3, such a filter bag is clearly unacceptable. For the filter material of Factory C, under the condition of filtration speed of 2m/min, the discharge concentration of the seamless line is less than 0.5% of the suture. It can be considered that for the suture sample, the dust basically leaks from the suture. of. If a filter bag with a diameter of Φ130mm is assumed, the suture has the same leak rate as the current sample, and the dust leakage of the filter material portion is neglected, and the discharge concentration of the filter bag outlet can be calculated to be 16.9 mg/m3. The test results in this paper show that the impact of the quality of the filter bag on the dust emission is much greater than the impact of the filter material itself. Filter manufacturers or bag manufacturers should pay great attention. 2.3 Influence of leakage welding of dust collector housing Affecting the dust discharge of the dust collector, in addition to the dust that passes through the filter material itself, the dust that passes through the filter bag seam, and also includes some leaks inside the dust collector that should be sealed but not sealed. Assume that there is a leak weld at the duster plate, the length is 1m, the width is 3mm, and the pressure difference between the plate and the plate is 1000Pa. According to the CFD numerical simulation, the airflow velocity at the leakage weld can reach 35m/s. For a bag filter with an inlet concentration of 20 g/m3 and a flue gas volume of 2,000,000 m3/h, the amount of dust contributed by such a leak weld can increase the dust discharge concentration of the dust collector by about 3.8 mg/m3. If there is a hole with a diameter of 100mm on a filter bag of the precipitator, the dust emission will increase by about 10mg/m3, and if it is converted to the standard condition, it can be increased by about 15mg/Nm3. It can be seen from the above rough calculation that the leakage of the main part of the dust collector can have a great impact on the dust discharge, and the dust collector manufacturer must strictly control the installation quality, and it is necessary to prevent the occurrence of the bag breaking phenomenon. 3 Conclusion Based on the above findings, the following conclusions can be drawn: 1) Needle felt filter material has high dust removal efficiency. Under laboratory conditions, the dust emission concentration of most filter materials after aging is less than 0.5mg/m3, and the dust emission concentration of most filter materials is more than 1mg/m3. 2) The sewing quality of the filter bag has a significant influence on the dust discharge. The dust leakage from the poorly-filtered filter bag through the seam is much larger than the leakage caused by the filter material itself. The use of glue sealing method has a good effect on preventing leakage, but the quality of the glue also has a great influence on the anti-leakage effect, and the durability of the glue is also a problem worthy of attention. 3) Leakage inside the precipitator can have a significant impact on dust emissions. A small gap can increase the dust emission of the dust collector by several mg/m3. 4) Dust leakage from the filter bag seam and the inside of the dust collector may be much larger than the amount of dust leakage generated by the filter itself. It is necessary to ensure that the dust emission of the bag filter or the electric bag composite dust collector meets the "ultra-low emission" requirement of the coal-fired power plant, and it is extremely important to ensure the sewing quality of the filter bag and the installation quality of the dust collector body. Simply put, a single luozhu is a luozhu and two star wheels. 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The dust discharge of the bag filter and the electric bag composite dust collector comes from three aspects: the dust passing through the filter material, the dust passing through the seam of the filter bag and the dust passing through the leak point on the body of the dust collector. Generally, people pay more attention to the collection efficiency of filter materials, but there are few studies on other two types of dust leakage sources, especially the dust leakage effect on filter bag sutures. In this paper, through a large number of laboratory test data and numerical simulation calculations, the influence degree of these three leakage sources on dust emission of dust collector is analyzed. On this basis, the main technology that should be adopted when dust meets “ultra-low emission†is proposed. Measures.