Failure and management of bag filters in coal-fired power plants

I. Introduction

In terms of 2002 as a bag filter Speaking at the start of large-scale use of coal-fired power plants, coal-fired power plant bag filter applications in our country has reached more than ten years, but its use was not widespread. In the metallurgical industry, cement industry bag filter have been widely used why they can not be widely used in the power industry to promote it? Although the reasons are many, but one damaged filter bag is not open around the crux. Over the years, in order to prevent the early damage of the filter material in the power industry and improve the service life of the filter bag, the researchers have done a lot of work, analyzed various factors affecting the life of the filter bag, and summarized the damage of the filter bag of the coal-fired power plant. For important reasons, various methods for preventing breakage of the filter bag have been proposed. Most power plant filter bags have been in use for more than three years, or even five to six years.

There are many reasons for the damage of the filter bag, but it can be divided into two categories, namely physical cause and chemical cause.

Second, the filter bag physical damage caused by damage

Physical causes are mainly mechanical (mechanical) causes and thermal causes.

Mechanical (mechanical) causes include friction, dusty airflow, mechanical cutting, and the like. The cause of mechanics is mostly local damage, which can occur in the mouth of the bag, the bottom of the bag and other parts.

The damage of the bag mouth is mostly within 400cm of the bag mouth, and the damage is mostly shown from the inside to the outside. This type of damage begins with the back-blown compressed air flow deviating from the center of the filter bag and directly scouring the inside of the filter bag. The compressed air on one side of the filter is continuously flushed. First, the surface layer on the inside of the filter bag is blown off by compressed air. The filter surface layer is then blown to form a hole. Experiments have shown that under the action of 0.5MPa air pressure injection, about 3000 injections, the needle felt can be blown out of the hole, see photo.

3000 blows are generally about one month for a normal bag filter. Some dust collectors use smoke from a half month to a month or so to find smoke in the chimney. If you look closely, you will find that the flower board is gray on the clean side of the dust collector. If the filter bag is not found and replaced at this time, after a loophole is generated in the filter bag, since the airflow is short-circuited here, the resistance is zero, so a large amount of dusty air on the dust side passes directly through here, in view of the airflow here. The flow rate is very high, and the dusty airflow will directly flush the opposite side of the hole. In the form of more holes and dusty flue gas inlets, the number of voids will increase, which will eventually cause the bag to be ring-shaped.

At the same time, after the dusty airflow enters the clean air chamber, the flow rate is greatly reduced due to the expansion of the space, and the dust settles on the flower board. When the dust accumulation exceeds the natural accumulation angle of the dust, the dust will fall to the filter bag. In this case, two effects will occur at the same time. One is the enema. At the same time, the dust at the bottom of the bag is blown by the compressed air, which impacts the bottom of the filter bag, causing damage to the bottom of the filter bag. Second, when the compressed air is blown back, it will drive the flower plate. The dust on it hits the filter bag together. Further, under the scouring of the high-pressure dust-containing airflow, the damage of the bag mouth of the filter bag is further accelerated, that is, the shortening of the time includes the increase of the area and the portion, and sometimes the bag mouth portion is formed into a sieve shape.

The main reason for the damage of the bag mouth is that the short pipe is twisted, the deformation of the flower plate, the straightening of the filter bag is not straightened, the pressure of the compressed air is too high, etc., and the installation of the blow pipe is rough, the skew of the blow pipe is not moderate, and there are quite Due to the poor process level of some of the blow-off pipes of the dust collector production plant, the position of the air outlet of the blow pipe is not accurate and the bag mouth is damaged.

The wear of the bag body of the filter bag, the vertical strip wear marks in the middle of the filter bag, in several cases:

If the damage marks are from the inside to the outside, the damage is in the contact between the vertical ribs of the bag cage, and the vertical ribs of the bag cage are desoldered or rusted. The filter bag is ground or the pressure is too high, and the spray is too frequent, so that the filter bag and the bag cage The contact position is broken and broken to cause breakage. This type of wear can be resolved by changing the bag cage or adjusting the blow.

If the damage marks are from the outside to the inside, break the contact at the vertical ribs of the bag cage. It is often seen that the outside of the filter bag (adjacent filter bag or box member) has traces of rubbing against the filter bag. This kind of wear is the deformation of the bag cage or the diameter of the filter bag is too large, causing the filter bag and the filter bag, the filter bag and the dust box body member to rub against each other during the back blowing process. It is generally necessary to replace the bag cage and ensure the quality of the installation. During the design of the bag filter, the distance between the filter bag and the distance between the filter bag and the box structure should be kept too small.

The inside of the filter bag is sometimes damaged because the outer diameter of the bag cage and the inner diameter of the filter bag are too large, causing the swinging range of the filter bag to be cleaned and filtered when the filter bag is too large, and the inside of the filter bag and the bag cage are rubbed, causing the inner side of the filter bag to wear. It is important to match the bag cage to the filter bag.

Wear of the lower part of the filter bag: the outer wear of the lower part of the filter bag is more than 300mm at the bottom of the filter bag. The wear is mostly on one side, the lower part is the most serious, and the upward direction is gradually reduced. The partial sewing thread will be worn off, and the sewing thread will be strong in the position where it is not worn. Such wear is caused by deformation of the flower plate, too small hole spacing, deformation of the bag cage, and excessively long filter bags. Individual filter bags and dust collector walls are damaged by friction. Such damage is strictly checked for the level of the panels and a well-made bag cage should be used. The inside of the lower part of the filter bag is worn: it is more common in the contact position with the bottom of the bag cage. Most of the difference between the diameter of the bag cage and the bottom of the bag cage is too large, resulting in excessive change of the filter bag during cleaning and filtration, and the bag cage is worn and damaged, and the bag is replaced. The cage can be solved.

Wear at the bottom of the filter bag: Wear on the outside of the bottom of the filter bag: There are obvious signs of wear on the damaged surface, and the bottom sewing thread and the filter base cloth are worn off. In severe cases, the bottom of the bag is damaged or the whole is peeled off. This kind of wear is caused by the high level of material in the hopper, and the airflow raises the dust particles directly to wash the bottom of the bag, causing wear. By increasing the ash discharge capacity of the ash bucket and adjusting the ash discharge operation system of the ash hopper, the dust accumulation is prevented from being too high, and the eddy current wear filter bag can be avoided in the ash hopper.

The filter bag has a certain applicable temperature for inorganic materials including glass fiber and basalt fiber, synthetic fiber, including polyphenylene sulfide (PPS), polyimide, and polytetrafluoroethylene, and is suitable for long-term use. The temperature used also has an extreme temperature. At long-term use temperatures, the filter bag can maintain a relatively long service life (life), once it exceeds this temperature, the fiber will quickly age or even destroy under the action of heat. There are two types of cases of thermal damage caused by bag filter bags in large coal-fired power plants. One type is that the temperature of the flue gas is too high. Generally, in order to improve the thermal efficiency of coal-fired boilers, the outlet temperature of the coal preheater of coal-fired boilers is generally designed to be around 140-150 °C. At present, the filter materials used in coal-fired power plants can meet the long-term use at this temperature, that is, 30,000 hours according to common requirements. However, the temperature of the flue gas exceeds this temperature range, and it often occurs. In most cases, it is caused by the accumulation of ash and even fouling of the superheater and the air preheater wall tube, which seriously hinders the heat exchange between the superheater and the air preheater. Causes the temperature of the smoke to rise. Most of this ash or scale is caused by improper cleaning or incomplete cleaning. For the rotary air preheater, if the jam occurs, the temperature of the flue gas will rise very high due to the loss of the heat exchange function. If the proper measures are not taken in time, the filter bag will usually burn out. Therefore, for a boiler using a rotary air preheater, it is necessary to have a device for detecting whether the air preheater is stuck, and there are emergency measures and equipment for preventing the smoke temperature from rising due to the empty preloader jam, burning the filter bag, such as equipment. Automatic water spray cooling equipment, set air inlet valve with automatic cold air.

Another type of situation in which the filter material is subjected to excessive temperature causes damage and loss occurs in the dust hopper of the dust collector. When the ash is too high, even the bottom of the filter bag is buried, causing the bottom of the filter bag to be heated for a long time, causing damage to the filter bag. The excessively high ash accumulation in the ash bucket usually occurs when the ash conveying system fails, including the failure of the ash valve under the ash hopper, the ash in the ash hopper cannot fall, or the ash ash canopy cannot fall. However, if the ash bucket is equipped with a high level gauge and the level gauge is working properly, this condition can be found and processed in time without causing damage to the filter bag. However, some enterprises have neglected the setting of high-level level gauges in order to save costs. Some enterprises have neglected the periodic verification of the level gauges, causing the level gauges to fail and not discovering them in time. These are often high gray buckets. The ash is buried in hidden dangers.

Due to the heat loss caused by the filter material, it is basically avoidable by strengthening the operation management, but the enterprise also needs to strengthen the management of the filter bag, especially to prevent the filter material from mixing into the normal temperature fiber.

Third, the filter bag chemical damage caused by damage

The second type of cause of damage to filter materials in coal-fired power plants is the chemical cause. The chemical cause is that the chemical components in the flue gas cause corrosion, hydrolysis and oxidation of the filter fiber, which causes chemical changes in the filter fiber, which causes the filter material (bag) to be damaged. The chemical cause is an important aspect of the loss of efficiency of filter materials in coal-fired power plants. The flue gas of coal-fired power plants contains SOx, NOx, O2, etc., and also contains water vapor of relatively high temperature. Under the action of these substances, one or the other kind of chemical reaction will inevitably occur in the filter fiber. of course. The intensity of the reaction varies depending on the type of material contained in the flue gas, temperature and humidity. These reactions cause strong damage to the filter fiber, prompting or accelerating the aging of the filter material, and even causing early damage of the filter material.

Practice has shown that in the case of proper monitoring of flue gas temperature conditions, the aging rate of the filter material is relatively slow, not only can meet the requirements of an overhaul cycle of 30,000 hours, and can even exceed the limit value. This problem can be explained by the operation time of multiple filter bags in China for more than 5 years. That is to say, at the time of various conditions of control, the aging speed of the filter material itself can fully meet the economic requirements of dust removal in coal-fired power plants.

In the early years, when the bag filter entered the large power plant, the filter bag of the dust collector of the coal-fired power plant was caused by chemical causes, and the early damage occurred frequently. This is mainly as a new thing, whether it is the design department and the power plant side, the dust collector manufacturer, or the filter material (bag) production plant, are in the exploration stage. Through years of exploration, both the design department and the power plant side, as well as the dust collector manufacturer and the filter bag producer have accumulated considerable experience, basically in different regions of the country, different coal types, different boiler forms and different emissions. Requirements and other basic environmental and technical requirements, how to design, select, manufacture suitable filter materials, and how to use and manage, have a rich and full understanding and understanding. From the design, manufacture, installation and operation management of the dust removal system, to the design, manufacture, selection and testing of the filter material, some experience has been accumulated, so that the early damage of the filter bag is greatly reduced. However, the further understanding of the aging and damage of the filter bag due to the chemical cause of the dust collector of the coal-fired power plant and the corresponding countermeasures are still an important issue for a long time to come.

A coal dust collector of a coal-fired thermal power plant started operation in March 2013, and in January 2014, it found a large area leaking bag. The Filter Center of Northeastern University passed the on-site observation and testing, and carried out laboratory tests on the filter bags sampled on site. After a series of work, it was found that the damage of the filter bag was mainly caused by the corrosion of the flue gas. The filter bag strength test is as follows:

According to the data in the table, the breaking strength is substantially lost in both the warp direction and the weft direction, and the latitudinal elongation at break is greatly reduced, which indicates that the molecular structure of the filter fiber has changed.

The filter bag test showed a PH value of 5.64, which was strongly acidic. The dust of the dust collector was tested, and the sulfur oxide content in the flue gas reached 1247 mg/m3. It is further understood that the inlet air temperature fluctuates greatly during the initial operation of the precipitator, and sometimes the temperature is as low as 80 to 90 °C. That is to say, sometimes working at a temperature far below the acid dew point, the filter bag is subjected to strong acid corrosion, so that the strength is rapidly reduced, or the filter material repeatedly shuttles above and below the acid dew point, and the filter material is further accelerated. The process of damage, see the photo.

According to the technical diagnosis of multiple bag filters in China, it is found that the early failure of the filter bag, especially the damage problem, is often not a single cause, and often multiple problems coexist.

Fourth, the filter material selection monitoring and failure treatment.

The core of the bag filter is the filter bag. The efficiency of the filter bag is related to the level of the discharge. The resistance of the filter bag affects the energy consumption of the system. The life of the filter bag affects the reliability of the system, that is, the trouble-free running time.

The efficiency, resistance and life of the filter material are directly related to the performance and quality of the filter material itself, and are also inseparable from the operation. Only by effectively and strictly controlling the performance, quality and operation of the filter itself can the technical performance and economic performance of the bag dust removal be fully improved. This article describes the practices of some power plants.

1. When purchasing, it is clearly stated that the technical requirements for filter bags and filter materials should meet the requirements of the filter materials in the technical requirements of the national standard GB/T 6719-2008 bag filter.

The manufacture of filter media is a complex process, and every detail of the process affects the technical content and product quality of the product. The level of equipment, process level and management level of filter material production in various filter materials enterprises in China are not the same, the level of products is also very different, and the stability of products also changes. According to this condition, low-level products that do not meet international requirements can be rejected.

Second, the tendering process is preferred to the filter material production plant. In the bidding process, the sample of the bidding filter bag is sent to a professional testing institution for testing, and the test result is used as the basis for evaluating whether the technical standard is short-listed, ensuring that the good product is highly scored and selected; the filter bag that will be produced by the winning bidder After being delivered to the factory, according to the sampling method, an appropriate amount of filter bags are taken and sampled again to check the quality stability and reliability of the filter production products. Such procedures should be the basic procedures for corporate procurement, but they are often ignored by some companies, and some companies have suffered huge losses. Northeastern University Filter Materials Testing Center handles at least dozens of tests involving quality disputes in filter materials every year. However, if you grasp the quality of products first in bidding and procurement, how much economic benefit will be added in the future, which will be managed in the future. How much trouble is reduced in work.

Third, regular monitoring and evaluation of the operation of the filter bag. The practice is that after the filter bag is operated, 1~2 filter bags are regularly taken every year and sent to a professional testing institution, which performs a series of tests on the filter bag, and through series testing, evaluates the current state of the filter bag and proposes related construction. Sexual advice, improve the operation of the bag filter, and make predictions on the life of the filter bag. Under the guidance of this opinion, the company can arrange the production plan in an orderly manner, make a series of arrangements for replacing the filter bags in advance, and reduce or avoid a series of problems such as violations and super-discharge caused by temporary failure of the filter bag. Unprepared temporary machine change bags are often easy to cause filter bag manufacturers. Manufacturers are prone to quality problems due to temporary arrangement, and the operation of changing filter bags on site is often rough. The shortcomings of this series often lead to a significant reduction in the life of the filter bag afterwards. Even some companies have to take down the machine every two or two months to check and replace the filter bag.

Enterprises using baghouses can regularly monitor and grasp the characteristics of the filter bag in the bag dust removal process and the process and law of failure grading through regular monitoring and evaluation of the filter bag, so that the bag filter system can be scientifically operated. The purpose of energy saving and emission reduction.

The following table shows the monitoring data of the filter bag after two, three or four years of use of the filter bag of a power plant.

The above indicates that the mechanization change of the filter material of the power plant is very small, and the strength performance is basically not reduced after 4 years of use.

The filter material is not treated properly.

The so-called abnormal failure of the filter material means that the filter material fails before it reaches the life expectancy index. When this happens, most users require the filter bag supplier to compensate for some or all of them. In most cases, both parties tend to pay more attention to economic aspects and ignore technical work. However, it is often difficult to obtain deeper excavations due to the neglect of technical work, and it is difficult to obtain a scientific and fair solution. Northeastern University Filter Materials Testing Center summarizes the following points for enterprise reference through years of dust detector failure detection and evaluation.

1. When purchasing filter bags, enterprises should randomly select one or two filter bags from the purchased filter bags, and seal them by the supplier and the buyer until the contract is cancelled. The sealed filter bag is used as a two-way detection mechanism to provide a sample of the initial filter material for detection in the event of an abnormal failure in the future. Of course, if the abnormal failure time of the filter material is longer than the initial working time (for example, 2 years), the detection data at this time may be slightly lower than the initial state, which is due to the natural aging of the filter material. If it is not properly preserved, it will accelerate this natural aging. Therefore, as mentioned above, it is most appropriate to randomly select a sending inspection mechanism from the purchased filter bag for comprehensive testing, and sign the two sides of the test results.

2. Once the bag filter has increased its discharge concentration, it basically indicates that the filter bag is damaged. In this case, the damaged filter bag must be found and replaced as soon as possible, and the cause of the damage must be found to prevent large damage to the filter bag afterwards.

Most users do not have to reduce the load for various reasons, find and replace the broken filter bag, and then the broken filter bag is broken. The so-called one needle does not make up, and the ten needle is difficult to seam. This is mainly caused by the damage of the filter bag, as described in the previous article, that is, a large amount of dust is deposited on the flower board, which in turn causes the filter bag to be broken into pieces. Therefore, the filter bag is damaged and replaced as soon as possible. Enterprises must do it. Enterprises should make every shift carefully observe and find problems in a timely manner.

If the filter bag is damaged, the real-time status of the dust collector and filter bag should be fully recorded. When it is found that one or even several filter bags are damaged, they should be disposed as follows. Otherwise, it is almost the same as no disposal. The disposal method is as follows.

Check the room on the net gas side. If there is damage to the filter bag, dust must be deposited on the bag flower board. At this time, firstly, the dust in the chamber flower board is completely removed by an industrial vacuum cleaner, and then all the flower plates other than the damaged filter bag are tightly covered. The purpose of the above work is to prevent people from entering the clean air filter chamber to extract the damaged filter bag, and to drop the dust on the outside of the clean air chamber flower plate and the damaged filter bag into other filter bags, or to drop the dust inside the bag to the peripheral filter. In the bag. When all the damaged filter bags and the nearly damaged filter bags are taken out, all the dust dropped in the clean air chamber is removed. It is necessary to remove all the dust in the intact filter bag of the chamber. The simpler method is to use an industrial vacuum cleaner to extend into the bag through a long straw and pump it from the top to the bottom of the bag to remove the dust attached to the bag, especially the bottom of the bag. After the completion, the damaged filter bag is replaced with a new filter bag. Although this work is time-consuming and laborious, it can ensure stable operation for a long time after the damaged filter bag is replaced.

At present, most enterprises in China have basically failed to follow the above procedures after breaking the bag. Most of them open the clean air chamber, find the broken bag, directly extract it, and then change the new bag. The result is that basically no more filter bag breakage will occur in a month, so cycle back and forth.

Another point is the analysis of the cause. If the company has the ability, it can handle it on its own. If there are many aspects involved, it is best to have an experienced professional organization to analyze and analyze. In order to enable professional analysts to fully understand the state of the filter bag when it is damaged, the damage related information should be fully and completely recorded. After taking the bag, the filter bag is sealed to the professional institution with a plastic bag as far as possible under the original state. Many units will be placed outside for many days, and it will be difficult to obtain scientific and accurate results if they are exposed to sunlight or rainy spray bags and sent to professional testing institutions for analysis.

V. Conclusion

Filter material (filter bag) is the most important thing for the long-term stable work of coal-fired power plants. This article only introduces some problems in filter bag selection and monitoring management. Due to space limitations, other related issues will be discussed later. It should be said that only the problems mentioned in the article will cause the relevant personnel of the enterprise to pay attention to it, so that the bag filter can be applied more and more widely in the power industry.