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Technical author of low-pressure pulse bag dust collector from Lurgi Company in Germany: Abstract by He Xinping, School of Power Machinery, Wuhan University: This article introduces the application and selection suggestions of the low-pressure rotary pulse bag dust collector of the German Lurgi Company in the coal-fired boiler of the 200MW power generation unit in my country. keywords: German Lurgi Company ； Low pressure rotary pulse bag dust collector ； Oblate filter bags and filter bag bundles The low-pressure rotary pulse bag dust collector technology of German Lurgi Company was used for the first time in my country, and it has a reference role for the development and application of dust removal technology in my country. Since the dust collection efficiency of the electrostatic precipitator is greatly affected by the nature of the dust, and the composition of coal in my country is ever-changing and the properties of the dust are also different, it is difficult for some factories to achieve long-term, efficient and stable dust removal requirements using electrostatic precipitators in coal-fired boilers in power plants. In order to solve this problem, domestic attempts have been made to use bag dust collectors, but they have failed due to various reasons. This limits the application and development of bag dust collectors in coal-fired power plants. along with * * With the improvement of dust emission indicators and the development of bag dust removal technology, the advantages of bag dust collectors, such as strong adaptability, high dust removal efficiency, and reliable operation, have gradually emerged. This has established a technical foundation for the application of bag dust collectors in coal-fired boilers in power plants. 1. Introduction to the bag dust collector system for coal-fired power plants. The equipment is used for exhaust dust removal from coal-fired boilers of 200MW generating units. The process parameters are as follows: Processing air volume 1738000m3/h Normal flue gas operating temperature 140~170 ℃ Population concentration 25~30 g/m3 Required emission concentration ≤50 mg/N m3 Air leakage rate ≤1.5 % Operating resistance ≤2100 Pa Shell design pressure +5000, -6000Pa The flue gas discharged from the boiler air preheater first enters the diffuser through the air duct. An air flow distribution device is installed in the diffuser to reduce the flow rate of the flue gas and evenly enter the bag chamber. The flue gas is filtered through the filter bag from the outside to the inside, then enters the upper box (clean air chamber) of the bag chamber from the bag mouth, and is then discharged through the exhaust fan. As filtration proceeds, the pressure difference inside and outside the filter bag gradually increases. When the pressure difference reaches the set value, the dust cleaning system works until the pressure difference is lower than the set value. The cleared dust is discharged through the ash hopper to achieve the purpose of dust removal (see Figure 1). Figure 1 Structural diagram of bag dust collector 2. Technical characteristics Compared with similar bag dust collectors, the low-pressure rotary pulse bag dust collector of German Lurgi Company has many technical features.: 1. Use oblate filter bags and arrange them into filter bag bundles in concentric circles (see Figure 2). Figure 2 Oblate round filter bags and filter bag bundles. Each filter bag bundle can be arranged with up to 1156 filter bags. The length of the filter bag is 8 meters. The maximum filtration area of each filter bag bundle is 3700 square meters. It is fixed with the flower board using elastic rolls and sealing gaskets. There is an oblate cage inside the filter bag. To facilitate installation, the bag cage is divided into three sections. Good design and processing quality make docking and disassembly very convenient. Each equipment can be designed into single chamber, double chamber or multiple chambers according to the site conditions. Each chamber can be composed of one or more filter bag bundles, which is a typical modular design. The advantage of using the oblate type is that the filter bag arrangement is more compact and takes up less space under the same filtration area. The oblate filter bag used in this equipment is mainly for rotary dust cleaning, and the blowing port can have a greater chance of being aligned above the bag mouth. The disadvantage is that in a square shell, the space at the four corners cannot be fully utilized. 2. Adopt diffuser plus lateral air intake method. Currently, bag dust collectors are mainly upper air intake type or lower air intake type, and lateral air intake mode is relatively rare. This equipment adopts a diffuser plus lateral air intake method, and its characteristics are:: (1) Make the air flow entering the bag chamber more uniform, and the mechanical running resistance of the main body is low ； (2) It avoids the problem of dust falling in the opposite direction to the air flow during dust cleaning using the lower air intake method, and increases the dust cleaning effect. ； (3) The side air intake method can also pass through multiple filter bag bundles at one time. For equipment that handles large amounts of flue gas, its compact structure can effectively reduce the floor space and one-time investment. ； (4) The lateral air intake mode makes the flow rate entering the bag chamber lower (lower than 1.4m/s for this equipment), which not only speeds up the pre-separation of coarse particles, but also ensures that the overall flow before the flue gas enters the filter bag bundle is evenly distributed in and around the center of the filter bag bundle, making the filtration load of each filter bag uniform. ； (5) The lower flow rate reduces the erosion and wear of the filter bag by dust and extends the service life of the filter bag. 3. Use online rotary low-pressure pulse dust cleaning (see Figure 3) Figure 3 Schematic diagram of low-voltage pulse dust cleaning. Dust cleaning of pulse bag dust collectors is divided into two types: online type and offline type. Each has advantages and disadvantages in practical applications. Online dust cleaning has a simple structure, easy control, small mechanical resistance, and small changes in system resistance. However, since dust cleaning and filtering are performed at the same time, it affects the dust cleaning effect and is not conducive to non-stop maintenance. The advantages and disadvantages of offline dust cleaning are exactly the opposite. The reason why this equipment uses online dust cleaning is mainly due to the low smoke concentration and easy dust cleaning of the boiler, which achieves the purpose of maximizing strengths and avoiding weaknesses. At the same time, this equipment adopts a structure of four chambers running in parallel in the design. The air inlet and outlet of each chamber is equipped with a louvered baffle door, which can still achieve non-stop maintenance. The dust cleaning of pulse bag dust collector can be divided into two types: injection dust cleaning and box dust cleaning. Injection cleaning requires a blow pipe and a pulse valve on each row of filter bags. It has high dust cleaning intensity and good effect, but requires a large number of pulse valves and it is inconvenient to replace the filter bags. Box-type dust cleaning is just the opposite. The rotary pulse cleaning used in this equipment is a jet cleaning in principle, but it combines the advantages of both. A filter bag bundle only requires one pulse valve, and the rotary blow pipe is used for blowing and dust cleaning. Therefore, the number of pulse valves is small and the dust cleaning intensity is high. During shutdown maintenance, the rotary blow pipe can be rotated freely by hand, making it easy to replace the filter bag. At the same time, there is no Venturi tube at the bag mouth of this equipment, which does not make the wind speed at the bag mouth too high and ensures the dust cleaning effect. This is a problem that must be solved when using long filter bags. The compressed air required for pulse bag dust collector cleaning is divided into two types: high-pressure cleaning and low-pressure cleaning. The compressed air pressure required for high-pressure cleaning should be above 0.5Mpa, while low-pressure cleaning is generally 0.2 to 0.4Mpa (0.3Mpa is also used as the dividing line between high and low-pressure cleaning in China), but this equipment only requires 0.08Mpa, which can be described as ultra-low pressure cleaning. Since it is difficult for many users to ensure a compressed air source above 0.5Mpa, low-pressure pulse is more popular. This equipment takes advantage of the low pressure and large flow characteristics of the Roots blower, so it can achieve the same dust cleaning effect as high-voltage pulse. 4. Use a wide bag chamber upper box. The bag chamber upper box refers to the structure of the upper part of the filter bag mouth. Generally, this structure is designed to be larger only when the upper air intake method is used. The upper box of the bag chamber of this equipment is as large as the entire bag chamber, and the height is about 3m. Although this structure increases weight, it has the following benefits:: (1) Inspection and replacement of filter bags can be completed inside the upper box of the bag chamber. The entire upper box of the bag chamber only requires one access door, which is of great benefit in reducing the total air leakage rate of the dust collector. ； (2) Due to the large volume of the upper box of the bag chamber, the wind speed on the clean air side is also low, which is beneficial to the air flow distribution of the filter bag bundle and reducing the mechanical operating resistance. In addition, a sealed observation window with lighting is designed on the upper box of the bag chamber. When the equipment is running, you can observe the conditions inside the upper box of the entire bag chamber without entering the dust collector, including whether the filter bag is damaged and whether the dust cleaning device is working normally, etc., making daily maintenance convenient and timely. 5. Use a unique cleaning control mode. Using PLC control is not necessarily advanced, because this only refers to the hardware equipment. What is important is whether the program design meets the on-site conditions, whether it meets the dust cleaning requirements, and whether it saves the most energy. The dust cleaning control of this equipment has the following characteristics:: (1) Pressure difference cleaning is used, that is, whether to clean and stop dust cleaning is determined based on the pressure difference inside and outside the filter bag. This cleaning method is especially suitable for occasions with low dust concentration and low dust viscosity. (2) Adopt three cleaning modes: slow, medium and fast, which are automatically adjusted by PLC. When the dust load is large, the fast mode is automatically started, and when the dust load is low, the slow mode is automatically started. ； When the resistance of the filter bag is low at the beginning of operation, the slow mode is automatically started. ； When the resistance of the filter bag increases in the later stages of operation, the fast mode will be activated. The above dust cleaning control mode can extend the effective life of the filter bag and ensure that the lowest dust collector resistance, lowest energy consumption and minimal fluctuation of system resistance are maintained throughout the entire life cycle of the filter bag. And there are pre-programmed programs that are debugged according to the actual situation and run automatically. 6. A complete protection system. First, a water spray device and a temperature testing device are designed in the air inlet duct. When the flue gas temperature exceeds the specified value, an alarm will be issued. If the temperature continues to rise, different numbers of water spray devices will be automatically activated to cool down. The water spray device can reduce the flue gas temperature by about 40 degrees to protect the filter bag from damage. When the water spray device is not in use, the nozzle relies on sucking in fresh air for automation to keep the nozzle clean. There is a bypass flue with 50% of the flue gas volume before and after the dust collector. When the flue gas temperature rises rapidly or a large amount of water vapor penetrates, the bypass flue can be opened to divert the flue gas to protect the filter bag. The bypass flue also serves as a temporary passage for flue gas when the dust collector fails. Since this equipment uses Ryton filter material, which has poor oxidation resistance, an aerobic analyzer is installed in the entire system to ensure that the oxygen content in the flue gas entering the dust collector is within 10%. Fuel is needed when starting the generator set and stabilizing combustion at low load, and oil fumes can easily cause blockage of the filter bag. Therefore, this equipment is equipped with a suction port for pre-spraying dust at the air inlet. Before using the bag dust collector, pre-spray new filter bags or filter bags that have been out of service for a long time to effectively prevent filter bags from being blocked. 3. Actual operation situation This dust collector has four chambers and eight units. Each unit is equipped with a filter bag bundle. Each filter bag bundle contains 1,000 filter bags and a set of rotary dust cleaning devices. The total filtration area is 25,600 square meters and the filtration speed is 1.13m/min. The first equipment was put into operation at the end of 2001 and has been running for half a year. Although formal testing has not yet been carried out, the readings from the online dust concentration tester set up by the environmental protection department show that it will not exceed the standard, and no dust can be seen from the chimney mouth. The operating resistance has always been around 80Pa, which is far lower than the design requirement, indicating that the dust cleaning effect is very good. The reason why the operating resistance is low is that the operating resistance of new equipment is usually low, Lurgi Company's explanation is, but their guarantee is that it will not be greater than the design value during the life of the filter bag. During the debugging, Lurgi Company also tested the wind speed at the filter bag mouth. From the test results, although the air intake was from the side, the wind speed at the bag mouth deviated by at most double, and it had nothing to do with the air flow direction, indicating that the layout and structural design of the filter bag were reasonable. After half a year of operation, two shutdown inspections were conducted due to boiler problems, and no problems were found, indicating that the quality of design, manufacturing and installation is good, and the technical equipment is mature and reliable. It has been proved that the bag dust collector can be used to remove dust from the exhaust of coal-fired boilers in power plants. 4. Comparison with electrostatic precipitators 1. Investment analysis According to international convention, investment in dust collectors will be compared based on one-time investment, operating costs and maintenance costs (see Table 1) Table 1 Comparison of conventional investment between bag dust collectors and electrostatic precipitators. However, judging from the current domestic market, this is not the case, especially for dust removal equipment with a flue gas processing volume greater than 200,000m3/h. The one-time investment of bag dust collectors is higher than that of electrostatic precipitators. The main reason is that: (1) Due to competition in the industry, the price of electrostatic precipitators has dropped to the point where steel is sold based on weight, and the high technical content cannot be reflected in the price. ； (2) Since my country’s environmental protection requirements are relatively low, the one-time investment in electrostatic precipitators is related to dust removal efficiency. In this project, because the emission concentration required by the user cannot be higher than 50 mg/Nm3, and the local coal quality is not suitable for electrostatic precipitators, although this equipment uses Lurgi technology, and most of the key components (such as filter bags, bag cages, pulse valves, etc.) are imported from abroad, if an electrostatic precipitator is used, even if based on the current domestic electrostatic precipitator market price, the situation will still change greatly after multiple inquiries from users (see Table 2). Table 2 The cost comparison of the two dust collectors in this project shows that this project objectively analyzed the process conditions and the characteristics of the two dust collectors, and then chose the solution with low one-time investment, low operating costs and high dust removal efficiency. 2. Structural comparison with similar products (see Table 3) Table 3 Structural comparison between Lurgi and other products Notes: 1) The filling speed and embedding speed are calculated based on the filtration speed in the table 2) The embedding speed of Lurgi products is calculated based on the area of concentric circles. As can be seen from the table, Lurgi products have the following characteristics compared with Fuller and similar domestic products.: (1) The maximum filtering area of each pulse valve is 24 times larger than that of Fuller's air box pulse ； (2) The filling speed is the largest, indicating that the length-to-diameter ratio of the filter bag is very large, reaching 63. The embedding speed is within the normal range, indicating that the arrangement of the filter bags is reasonable. ； (3) The filter bag has a long guaranteed life, up to 30,000 hours (3.4 years), while the internationally generally guaranteed life is 2 years, saving users maintenance costs. ； (4) The air leakage rate is low, only 1.5%, which is an internationally advanced indicator. The reduction of air leakage rate can effectively reduce the energy loss of the main fan. However, due to structural design, production and installation, it is difficult for domestic products to achieve this. (5) Low steel consumption. Although the steel consumption ratio is related to the specifications of the dust collector, according to Lurgi's consistent pragmatic and steady style in product design, such steel consumption is undoubtedly very advanced. 5. Recommendations for selecting bag dust collectors for coal-fired boilers in power plants. Bag dust collectors are divided into many varieties according to structure and dust removal method. Each variety is suitable for certain process conditions due to its own characteristics. Therefore, the following suggestions are put forward: (1) Priority should be given to the pulse type. Due to its high filtration speed, it is very suitable for process conditions with large flue gas volumes. At the same time, the use of three-dimensional filter materials can not only improve dust removal efficiency and reduce the operating resistance of the filter material, but also has a longer service life than two-dimensional filter materials. (2) The dust concentration of coal-fired boiler smoke in power plants is low and the dust viscosity is low. The selected pulse bag dust collector can be designed with a larger length-to-diameter ratio of filter bags, and the filter bags can also be arranged relatively densely to reduce the weight of the equipment and the floor space. (3) The filtration speed should not be too high during model selection and should generally not be greater than 1.2m/min. Excessively high filtration speed may reduce one-time investment, but it will increase operating resistance, increase equipment failures, and shorten the life of the filter material. (4) For key components, such as filter bags, pulse valves, etc., high-quality ones should be selected, especially filter bags. Not only do the filter materials need to be suitable for the working conditions and of good quality, but also the processing quality of the filter bags must be paid attention to. These are important factors that affect the performance and operation rate of the dust collector and must be paid enough attention to. (5) The bag dust collector is greatly affected by the properties of the flue gas. Careful protection measures should be considered in the process design. The air inlet flue should be as long as possible in the process layout, so that the protection device can work better in case of problems with the boiler. (6) Attention must be paid to the design, production and installation quality of the product. (7) The equipment must be operated in strict accordance with the regulations. The bag dust collector is greatly affected by the properties of the flue gas and is more delicate than the electrostatic precipitator. Routine maintenance should be strengthened, especially in the initial stage of operation, and problems should be discovered and dealt with promptly.