In order to achieve this goal, it is necessary to further deepen the theoretical basis of dust removal, invent new high-efficiency electrostatic precipitators, study effective measures to reduce high specific resistance dust back corona and develop anti-corrosion conductive coatings on the surface modification technology of dust collecting plates, and Actively explore practical and feasible dust collection technology, which is of great significance for energy saving and emission reduction in the field of flue gas dust purification.
Development status of electrostatic precipitator technology
Electrostatic precipitators have attracted much attention due to their high efficiency of dust removal, low operating resistance, large amount of flue gas treatment and low energy consumption, and quickly occupied the dust removal market. With the continuous rapid development of the economy and the improvement of environmental protection standards, countries have conducted in-depth research on the application technology and equipment of electrostatic precipitators, and have achieved certain results.
Wide pole electrostatic precipitator. As early as 1977, Cooperman of the United States used the gradient relation to study the influence of wide pole distance on dust airflow, and made a sufficient theoretical explanation for the wide pole pitch to improve the dust removal efficiency. In 1980, H. Hoegh-Petersn doubled the channel on the original equipment, using a 400mm pitch and keeping the field strength constant. The experimental research shows that the wide-pitch technology can reduce the power consumption and improve the dust removal efficiency. The improvement factor of the wide-pitch to the dust drive speed is 1.05~1.40. At the same time, it has obvious superiority to the processing of fly ash with high specific resistance. Sex. Chinese scholars have also conducted related research on wide-pitch electrostatic precipitators since the 1980s.
The advantages of high efficiency, low resistance and low consumables of the wide-pole electrostatic precipitator have been widely recognized. At the same time, however, the current density of the wide-pole electrostatic precipitator is low, the dust charge is slow and insufficient, the cost of the rectification equipment is high, the investment of the power supply equipment is too large, and the dust collection area is correspondingly reduced. Therefore, it is also subject to certain constraints in actual production. It is necessary to determine the optimal spacing through comprehensive comparison of economic and technical technologies for different working conditions.
Move the electrode ESP (MEEP-ESP). The mobile electrode technology successfully solves the problem that the high specific resistance fine dust deposited on the dust collecting plate is difficult to remove by the rapping method. The basic principle is that the dust collecting plate is made into a form that can move up and down, and then the rotating dust is brushed off the inside of the lower hopper to keep the collected dust relatively clean, and the dust is cleaned. The non-airflow zone is carried out, thereby effectively preventing the formation of back corona and the occurrence of secondary flying of the dust vibrating, and ensuring efficient dust removal.
At present, several companies in China have independently developed similar technologies and have tried them on several projects. The situation is good and further improvement is being made. Show actual application abroad, MEEP-ESP is not only difficult to apply the conventional ESP collection resistivity dust, fine dust and dust high viscosity ratio, but also particularly suitable for the special difficulties of burning coal and coal species changing the kiln The generated dust, as well as the limited equipment site, can achieve higher dust collection efficiency with a relatively small dust collection area, and achieve greater economic benefits with relatively less equipment investment.
Cohesion technology. The coagulation technology that has appeared in recent years is an effective measure to remove fine particles in the flue gas and improve the dust removal performance. The main idea of ​​the technology is to install a coalescing device at the inlet flue of 5 m in front of the precipitator, which is a pretreatment device before the high-speed flue gas enters the precipitator. The agglomerator comprises a set of parallel channels between the positive and negative phases. When the smoke and dust pass, respectively, positive or negative charges are obtained, and the flue gases of different channels are mixed together when entering the dust collector, and the positive particles in the gas are positively charged. Mixing with the negatively charged coarse particles flowing out from the adjacent negative polarity channels, while the negatively charged fine particles are mixed with the positively charged coarse particles, thereby reducing the number of fine particles and forming a relatively easy to remove particle size larger than 10 μm. Dust particles improve dust removal efficiency.
High-flow flue gas keeps the grounding plate clean without the need for rapping like an electric precipitator, saving maintenance costs. For a 100 MW generator set, the agglomerator requires only about 5 kW of electricity. For the induced draft fan, the added resistance is only 200Pa. Due to the low investment, operating costs and maintenance fees, cohesion technology has broad application prospects.
Electric bag composite dust collector. Combined with other dust removal mechanisms to improve dust removal performance, the combined dust removal method has great advantages. It can overcome the unfavorable factors of single dust collector operation, and achieve advantages and avoid weaknesses and complementary advantages. The most common one is the composite ESP combined with the electrostatic precipitator and other dust removal methods. Among them, the electric bag composite dust collector is the most effective one.
In the late 1980s, the Palo Alto Electric Power Research Institute in California developed COHPAC electrical bags, mainly as an improvement to ESP. The solution is to add a bag filter downstream of the original ESP to keep the dust concentration less than 10mg/Nm3. In 2002, an environmental protection company in China transformed a 70m2 ESP in a rotary kiln with a daily output of 1000t into a series electric bag at the Pudong Cement Plant in Shanghai. The treated flue gas volume was 240,000m3/h, and the first electric field of the original dust collector was retained. (Anode side rapping technology), the second and third electric fields were changed to bag dust removal (long bag low-pressure pulse technology). The battery was put into operation on April 2, 2003, and the emission concentration was stable below 30mg/Nm3 for a long time.
With the increasingly strict environmental protection standards in China, electric bags have shown strong advantages as new dust removal equipment for improving ESP dust removal efficiency and effectively controlling fine dust. It is not only suitable for new projects, but also suitable for the transformation of old ESP.
Problems and directions for improvement
Deepen the study of dust removal theory. The dust collection mechanism of the existing electrostatic precipitator is based on the basic assumption that the dust collecting plate is always kept clean during the dust collecting process, and the electrostatic precipitator in the actual dust collecting process, along with the charged dust in the dust collecting plate The sedimentation on the surface forms a gradually thickened dust layer on the surface of the plate. The thickness of the dust layer and the difference between the specific resistance and the corona current will inevitably affect the conduction and release effects of the current in the dust layer, resulting in different amounts of charge remaining in the dust layer. The accumulated charge in the dust layer will dust the space. The electric field forms a counteracting effect. At the same time, the charge distribution characteristics in the dust layer directly determine the possibility of anti-corona generation. Therefore, the dust collection of the electrostatic precipitator is an unsteady dust collection process. The existing electrostatic dust removal theory lacks an accurate description of this process, which weakens its scientificity to some extent, resulting in the lack of pertinence in the design and operation parameters of electrostatic precipitators. In-depth theoretical and practical research on the unsteady collection process of electrostatic precipitators can not only enrich the theory of electrostatic dust collection, but also guide the exploration of key issues that seek to overcome the dust collection effect of electrostatic precipitators.
Improve the dust removal effect of high specific resistance dust. Another main reason for the dust removal efficiency of many electrostatic precipitators is that the specific resistance value of flue gas dust is higher under operating conditions. For example, the specific resistance of the flue gas dust of the sintering head is 1011 Ω ~ cm ~ 1012 Ω ̇ cm. When the conventional electrostatic precipitator is used to purify the high specific resistance dust fumes, the dust layer formed on the surface of the dust collecting pole exhibits a high resistance value, and forms a large hysteresis effect on the conduction of the electric field current, and accumulates in the dust layer. The amount of charge will increase, causing a repulsive effect on the subsequent charge of the charged particles to the plate. In severe cases, back corona will occur, that is, the surface of the dust layer will generate discharge, resulting in secondary backflow of dust and dust collection effect. deterioration. The structural characteristics of the conventional electrostatic precipitator are difficult to overcome the effects of anti-corona, and there is no trapping ability for positively charged dust, so the running effect will be poor. Exploring the key technologies that can effectively improve the dust removal effect of high specific resistance dust is still an urgent problem to be solved by the electrostatic precipitator technology.
Plate surface modification technology. The operation practice of many electrostatic precipitators presents a very obvious feature: the electrostatic precipitator has a good dust removal effect in the first two years after it is put into use, and the dust removal efficiency can reach more than 99%, but with the extension of the running time, the dust removal effect decreasing gradually. Under many operating conditions, the dust and dust often cause corrosion of the dust collecting plate, and the surface of the plate forms a strong “dust and rust composite layerâ€. The rough surface morphology of the composite layer further aggravates the coring. The difficulty of gray. Experiments have shown that the "dust rust composite layer" on the surface of the old plate has a negative impact on the dust collection effect. Seeking the surface modification technology of the electrode plate, preventing the corrosion of the electrode plate, maintaining the good conductivity and cleaning effect of the electrode plate, and preventing the dust collection effect of the electrostatic precipitator from being operated for a long time is a new subject faced by the electric dust removal technology.
Dust forced collection technology. Cooperman pointed out in 1970 that there was a dust mass concentration gradient across the cross section of the electrostatic precipitator. At present, relevant scholars in China have obtained the theoretical and actual electric field dust concentration distribution formula by establishing the mathematical model of electric field dust transmission and the regression of the dust concentration distribution curve of the measured section. The results show that the dust concentration distribution in the electric field is related to the position of the section. The mass concentration from the corona wire to the dust collecting plate on each section of the electric field is gradually increased, and the dust concentration on the surface close to the plate is the highest. How to effectively collect the dust in the high-concentration airflow close to the surface of the plate and the secondary flying dust caused by the coring and cleaning of the plate, which invents a practical dust-collecting technology to effectively reduce the dust penetration rate. It can have an immediate effect, which may be the most realistic problem in achieving breakthroughs in ESP technology.
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