At present, there are three kinds of ultra-low emission combination routes, namely wet electrostatic dust removal technology, integrated desulfurization dust removal technology and electric bag composite dust removal technology. In addition, through the analysis, it is concluded that the main problems faced by ultra-low smoke emission are optimized operation and accurate measurement, and the defects need to be optimized in the short term, and the dust removal technology with low energy consumption and high efficiency should be developed in the long term.
By the end of 20 15, the installed capacity of thermal power in China reached 100554 million kilowatts, accounting for 65.9% of the total installed capacity, and the total power generation was 42,307×108 kilowatts, accounting for 73.7% of the total power generation. For a long time in the future, the power supply pattern dominated by coal-fired power generation in China will not change fundamentally, and coal is still the main energy source in China.
Coal-fired power generation will emit a lot of pollutants. Figure 1 describes the pollutant emission ratio of various industries in recent three years [2]. As can be seen from the figure 1, in 20 14 years, SO2 emissions from thermal power industry accounted for 34.6 1% of the total annual emissions, NOx emissions accounted for 37.69% of the total annual emissions, and soot emissions accounted for 13.53% of the total annual emissions. Through comparison, it can be seen that although the proportion of pollutants discharged from coal-fired power plants has decreased, it still accounts for.
Therefore, according to the high share, coal-fired power plants are still an important source of various pollutants in China's atmosphere.
At the same time, the current domestic air pollution situation is grim: according to statistics, among the 338 cities above the prefecture level in 20 15, 73 cities met the ambient air quality standards, accounting for 21.6%; The ambient air quality in 265 cities exceeded the standard, accounting for 78.4%. In order to prevent and control air pollution, the state has strengthened the control of pollutant emission from coal-fired power plants, and has successively promulgated a series of policies and regulations, such as Emission Standard of Air Pollutants from Thermal Power Plants (GB13223-2011) and Emission Standard of Air Pollutants from Coal-fired Power Plants.
Action plan for upgrading and transformation of energy conservation and emission reduction (20 14-2020), and the work plan for ultra-low emission and energy-saving transformation of coal-fired power plants will be fully implemented. According to the latest policy, the eastern, central and western provinces should complete the ultra-low emission transformation of all coal-fired units by 20 17, 20 18 and 2020 respectively, namely
50 mg/m3.
Since the release of ultra-low emission policy, the ultra-low emission transformation of coal-fired units has made rapid progress. According to statistics, in 20 15, the national ultra-low emission transformation volume is about 1.4× 108 kW, and it is planned to implement ultra-low emission transformation in 20 16. In many ultra-low reconstruction projects, it is the most difficult to meet the emission standards of smoke and dust. At the same time, there are many problems in the operation of coal-fired power plants that have completed the transformation of ultra-low smoke and dust. This paper analyzes the existing problem of smoke overload.
The present situation and application of low emission technology are summarized, and the problems existing in the process of technology application are summarized, which provides reference for the optimization of ultra-low emission technology of soot in coal-fired power plants.
1 ultra-low emission technology
There is no concept of ultra-low emission abroad. As shown in table 1, the smoke emission concentrations of coal-fired power plants in some countries are compared. In China, the smoke emission concentration of coal-fired power plants is required to be lower than 10mg/m3, while the smoke emission concentration of power plants is generally controlled below 5mg/m3, which is far lower than the allowable smoke emission concentration of the United States, Japan and other related countries. Therefore, there is a lack of application experience of foreign related ultra-low emission technologies.
The ultra-low emission technology of soot in China has not made a major breakthrough, which is an improvement and combination of the existing dust removal technology. As shown in Figure 2, the existing ultra-low smoke emission technology is a combination of primary dust removal technology and deep dust removal technology. Primary dust removal technology can remove most of the dust, but it can't meet the discharge standard or has high cost, including electrostatic dust removal technology, bag dust removal technology and electric bag composite dust removal technology, in which the traditional electrostatic dust removal technology captures coarse particles.
The efficiency can be as high as 99.9%, but the collection efficiency of submicron fine particles is low, so the collaborative technology of electrostatic precipitation technology has been developed, including low temperature electrostatic precipitation technology, high frequency power supply technology and rotating electrode electrostatic precipitation technology; Deep dust removal technology is to further remove dust on the basis of primary dust removal to make the flue gas meet the standard, including integrated desulfurization and dust removal technology (including SPC-3D) and wet electrostatic dust removal technology.
1. 1 primary dust removal technology
1. 1. 1 electrostatic precipitation technology
(1) low temperature electrostatic precipitator technology
Low-temperature electrostatic precipitation technology refers to arranging a low-temperature economizer in front of the electrostatic precipitator, so that the flue gas temperature at the inlet of the precipitator is reduced from the conventional 120~ 160℃ to the low-temperature state below the acid dew point (within 100℃, generally at 85~95℃). According to the literature report [10], low-temperature electrostatic precipitation technology can
The essence of low-temperature electrostatic dust removal technology is flue gas conditioning, which plays a synergistic role in dust removal by reducing flue gas temperature, dust specific resistance, flue gas volume flow and breakdown voltage. When the flue gas temperature drops below the acid dew point, SO3 condenses on the surface of fine particles, which enhances the surface conductivity of fine particles and promotes the agglomeration and growth of fine particles. But the application of this technology will lead to the increase of secondary dust in electrostatic precipitator, low temperature heat exchanger and electrostatic precipitator.
Acid corrosion of dust collector, reduced ash fluidity (and change of water balance in desulfurization system) and other adverse effects.
(2) Rotating electrode electrostatic dust removal technology.
Rotating electrode electrostatic precipitator technology divides the electric field of the precipitator into two parts: the first-stage fixed electrode field and the second-stage rotating electrode field. The anode part is provided with a rotating anode plate, which is cleaned by a rotating dust removal brush. When the dust and the rotating anode plate move to the non-dust collecting area, they are brushed by a pair of rotating dust cleaning brushes.
The essence of rotating electrode electrostatic dust removal technology is the transformation of electrode plate, the main purpose of which is to reduce secondary dust, remove high specific resistance and sticky dust and avoid back corona, but its structure is complex, prone to failure and the system reliability and stability are poor.
(3) High frequency power supply technology
High-frequency power supply technology is to use the rectifier bridge to rectify the power frequency power supply into a DC power supply of about 530V, and then turn it into a high-frequency AC power supply of more than 20kHz through an inverter circuit, and then boost it through a high-frequency transformer, and then rectify and filter it through a high-frequency rectifier to form a high-frequency current of more than 40kHz.
The essence of high frequency power supply technology is the transformation of electrostatic precipitator power supply. Compared with the power frequency power supply, the high frequency power supply improves the power supply voltage and current, increases the input of electric power, and improves the dust charge and field strength, thus improving the dust removal efficiency. After the transformation of the high frequency power supply of electrostatic precipitator in a power plant, the smoke emission concentration decreased from 42mg/m3 before the transformation to 17mg/m3, and the emission reduction effect was obvious.
At present, the transformation of electrostatic precipitator in coal-fired power plants is generally to increase the number of electric fields and carry out the transformation of high-frequency power supply. At the same time, according to the actual situation of the power plant, flue gas conditioning (low temperature electrostatic precipitator technology) or electrode plate transformation (rotating electrode electrostatic precipitator technology) is carried out to reduce the dust concentration to a certain level before the flue gas enters the deep precipitator.
1. 1.2 bag dust removal technology
Bag dust removal technology is a technology to filter dusty gas by using the synergistic effect of interception, inertia, diffusion, gravity and static electricity of fiber fabric. Bag filter is an unstable process in which filtration and dust removal alternate: when dusty gas enters the bag filter, the dust with large particles and high relative density falls into the ash hopper due to gravity settlement, and the gas containing fine dust is intercepted when passing through the filter material, and the gas is purified; along with
With the progress of filtration, the resistance is rising, so it is necessary to clean and regenerate the ash.
At present, the bag dust removal technology has not made a breakthrough. In order to meet the requirements of ultra-low dust emission, it is necessary to increase the number of bags, which leads to the pressure loss and energy consumption of the dust collector. With the increase of the number of discarded cloth bags, its harmless treatment will be a difficult problem in the future.
1. 1.3 electric bag composite dust removal technology
Electric bag composite dust removal technology is a kind of dust removal technology which organically combines the filtering mechanism of electric dust removal and bag dust removal. The pre-dust collection in the front electric field removes most of the dust, at the same time, the fine particles are charged and condensed, and the extremely fine particles are condensed to form large-size particles.
Electric bag composite dust removal technology includes integrated electric bag dust removal technology and split electric bag dust removal technology. * * * The same advantages are: it is not affected by the composition of coal and fly ash, the concentration of flue gas at the outlet is low and stable, and the impact of bag breakage on emission is less than that of bag filter.
* * * The same disadvantages are: large system pressure loss, sensitivity to flue gas temperature and composition, low utilization rate of old filter bags, high equipment cost, high annual operating cost and poor economy. Compared with the split electric bag dust removal technology, the integrated electric bag dust removal technology occupies a smaller area, but it cannot be overhauled online under 100% load.
1.2 advanced dust removal technology
1.2. 1 wet electrostatic precipitator technology
Compared with dry electrostatic dust removal technology, the working principle of wet electrostatic dust removal technology is basically the same, but wet electrostatic dust removal technology uses water film to clean ash instead of traditional vibration dust removal.
The dust removal efficiency of wet electrostatic dust removal technology is not affected by dust specific resistance, which can effectively avoid secondary dust and corona. At the same time, the dust in wesp is not only affected by electrostatic force and fluid resistance, but also by thermophoresis force and liquid bridge force, which improves the removal of fine dust. In addition, wesp plates are formed.
The formed water film will greatly improve the discharge current in electrostatic precipitator, enhance the charging ability of fine particles, and further improve the removal efficiency.
Although wet electrostatic dust removal technology can realize low concentration emission of smoke and dust, the reuse of washing water in desulfurization system will change the water balance of desulfurization system. At the same time, the washing water containing smoke and dust will have a certain impact on the slurry performance and increase the discharge of desulfurization wastewater. In addition, the construction and operation cost of wet electrostatic precipitator is high, and the polar plate and polar wire are easy to corrode, which greatly limits the popularization of wet electrostatic precipitator.
1.2.2 comprehensive desulfurization and dust removal technology
The dust at the outlet of wet desulfurization system consists of three parts: residual dust after washing and absorption by desulfurization tower, slurry droplets containing solid particles such as gypsum and limestone carried by flue gas through demister, and soluble salts. According to the research of Wang Hui, the limestone and gypsum particles newly added in the flue gas of wet desulfurization account for 47.5% and 7.9% of the total particle mass respectively.
The dust removal efficiency of desulfurization system is related to the operation of desulfurization tower, dust concentration and particle size. It is generally believed that the dust removal efficiency of desulfurization system can reach 50%, and the removal effect of wet desulfurization system on ultrafine particles, SO3 aerosol, toxic heavy metals and gypsum droplets is generally poor. According to literature reports, the removal efficiency of total particles in flue gas by wet desulfurization system is 46%~6 1.7%, and that of PM 1 2.61%~-1.58%.
The removal rate of PM2.5 is between-2.02% and 8.50%, and the removal rate of PM 10 is between 42.63% and 58.68%.
In order to enhance the dust removal effect of desulfurization system, it can be reformed from the following two aspects.
(1) Improve the demisting performance of demister and reduce the slurry carried by flue gas.
(2) When designing the desulfurization system, we should not only consider the desulfurization efficiency, but also consider the effect of synergistic removal of smoke and dust. The general measures are: increasing the slurry coverage of spray layer and nozzle of absorption tower, improving the uniformity of flue gas distribution in the tower, adopting efficient atomizing nozzle, reducing the flue gas flow of absorption tower and ensuring the uniform inlet pressure of nozzle.
Based on the above principles, there are two main ways of transformation in China at present.
(1) ultra-clean desulfurization and dust removal integrated technology (SPC-3D) This is an efficient combination of cyclone coupling device, tube bundle dust removal device and high-efficiency energy-saving spraying device. According to the test results, when the dust emission concentration at the outlet of electrostatic precipitator is less than 30 mg/m3, the dust emission concentration at the outlet of absorption tower after desulfurization can be reduced to less than 5mg/m3.
(2) Efficient demister
When optimizing the absorption in the tower, the flat demister is replaced by the 2~3 stage roof demister, or the tube demister and the roof demister are connected in series. According to the relevant test results, the dust removal efficiency can be improved by 30%. After the above transformation, the mass concentration of gypsum droplets in a power plant in Zhejiang decreased from 32mg/m3 to 13mg/m3, and the removal effect was remarkable.
The construction and operation cost of integrated desulfurization and dust removal device is low, but the change of working conditions has a great influence on the outlet flue gas concentration, and its stability is lower than that of wet electrostatic precipitator. In addition, the increase of demister flushing volume will also affect the water balance of desulfurization system.
1.3 application status of ultra-low emission technology
In June 2065438 +0654381October+February 2065438 +06, the Ministry of Environmental Protection investigated 80 power plants and 287 coal-fired units, and the application of dust removal technology is shown in Figure 3. Electrostatic dust removal technology is the main dust removal technology adopted by various research units, with a total of 186 sets, accounting for 60% of the total number of research.
As shown in Figure 4, as of 20 15 and 12, some dust removal technologies have been put into operation and the installed capacity of units under construction. As can be seen from Figure 4, among the primary dust removal technologies, the low-temperature electrostatic dust removal technology is the most widely used, with a total installed capacity of 95000MW. Among the advanced dust removal technologies, the wet electrostatic dust removal technology is the most widely used, with a total installed capacity of190,000 MW.
At present, the typical ultra-low emission routes of soot mainly include: the ultra-low emission route with wet electrostatic precipitator as secondary dust removal, the ultra-low emission route with wet desulfurization and cooperative dust removal as secondary dust removal, and the ultra-low emission route based on ultra-clean electric bag compound dust removal without secondary dust removal. According to the actual operation, the ultra-low emission of smoke and dust has been realized.
According to the best feasibility guide for pollution prevention and control in thermal power plants, the technical route selection of ultra-low emission of smoke and dust is shown in Table 2.
Note: ① The choice of primary dust removal method should first judge whether it is suitable for electrostatic precipitation according to the properties of coal quality and ash. If it is not suitable, electric bag composite dust removal or bag dust removal should be preferred;
(2) when the dust concentration is less than 10mg/m3 or 5mg/m3 at a time, the electric bag composite dust collector should be preferred;
(3) When the flue gas concentration at the outlet of the primary dust collector is 30~50mg/m3, WESP should be selected for secondary dust removal; When the concentration of flue gas at the outlet of primary dust collector is 10~30mg/m3, wet desulfurization (WFGD) should be adopted for secondary dust removal.
④ Numbers in the table indicate technical suitability: 0 is not suitable; 1 appropriate; ② More suitable; 3 is the most suitable.
2 status and development of ultra-low emission technology of smoke and dust
At present, a large number of coal-fired units in China have completed ultra-low emission transformation. For ultra-low emission soot, its technical status and development are shown in Figure 5. The main problems faced by ultra-low emission of soot are operation optimization and soot measurement.
(1) Operation optimization smoke control comes from the cooperation of various technologies. Different dust removal technologies have different dust removal efficiency and energy consumption. It is necessary to optimize the distribution of dust removal efficiency of various technologies to achieve ultra-low smoke emission and reduce energy consumption.
(2) Measurement of smoke and dust At present, according to HJ/T76—2007 "Technical Requirements and Detection Methods of National Pollution Source Smoke and Dust Emission Continuous Monitoring System", on-line monitoring (CEMS) and laboratory weighing check are adopted to realize the supervision and management of pollutant emission data, but the following problems are faced.
① The existing on-line continuous monitoring smoke concentration meters are generally not calibrated, and the relationship determined by separate calibration cannot be used.
HJ/T76—2007 《 Technical Requirements and Test Methods of National Continuous Monitoring System for Flue Gas Emission from Pollution Sources 》 formulated the CEMS comparison method, which stipulated that when the particle emission concentration was ≤50mg/m3 and the absolute error was less than 15mg/m3, the measurement accuracy of the smoke meter was within the rated range, and it was unnecessary to calibrate the CEMS of ultra-low emission units. In addition, the uneven arrangement of CEMS measuring points will also produce smoke measurement.
Have a great influence.
② Weighing method is generally used to measure smoke and dust in field experiments, and the sampling basis is GB/T 16 157 "Determination of Particulate Matter in Exhaust Gas from Stationary Pollution Sources and Sampling Method of Gaseous Pollutants", but it is not applicable to the case where the concentration of particulate matter is less than 50mg/m3.
At present, referring to foreign low-concentration sampling standards, China has begun to formulate its own low-concentration sampling standards, and issued the Gravimetric Method for Determining Low-concentration Particles in Exhaust from Stationary Pollution Sources (draft for comments). This method is determined by membrane method, and the detection limit is 65438 0 mg/m3.
The measurement results of this method are greatly influenced by the operation level of the experimenter. Under the background that the emission concentration of some power plants is lower than 1mg/m3, it is very difficult to accurately measure the smoke and dust, and the accuracy of this method needs to be further verified. In addition, Hebei Province and Shandong Province have also issued local standards for the determination of low-concentration particulate matter. The sampling method is similar to that in the literature, and it needs to be verified and improved in practice. In the future, the ultra-low emission of soot from coal-fired power plants needs to complete the following work.
Work.
(1) Summarize the existing problems in operation, and put forward optimized operation schemes under different application backgrounds of different ultra-low emission technologies, so as to reduce operation energy consumption and operation cost.
(2) Summarize the problems existing in online detection and sampling detection of low-concentration smoke and dust, modify or formulate relevant standards, and actively promote the accuracy of measurement.
(3) Carry out life cycle assessment of ultra-low emission technology of soot. Life cycle assessment (LCA) is a good method to evaluate technology energy consumption and its impact on the environment. At present, LCA has been used in desulfurization, dust removal, denitrification and other fields. With the help of LCA, the list of energy consumption and environmental impact can be listed, the environmental impact can be defined, and the key to reducing energy consumption can be found out.
The problem of high energy consumption of the existing combined technology cannot be solved in the short term, so in the long run, it is necessary to develop a new dust removal technology with low energy consumption and high efficiency to replace the existing combined technology.
3 Conclusion
This paper introduces the background and development of ultra-low emission of coal-fired power plants, and summarizes the ultra-low emission technology of smoke and dust.
(1) Ultra-low dust emission technology mainly includes primary dust removal technology and advanced dust removal technology. The primary dust removal technology includes electrostatic dust removal technology, bag dust removal technology and electric bag composite dust removal technology, and the deep dust removal technology includes desulfurization and dust removal integration technology (including SPC-3D) and wet electrostatic dust removal technology. The advantages and disadvantages of each technology are summarized.
(2) The existing ultra-low dust emission technology is an efficient combination of primary dust removal technology and deep dust removal technology, and the transformation scheme should be diversified according to the actual situation of the power plant.
(3) The main problems of ultra-low smoke emission are operation optimization and smoke measurement. In the short term, the defects should be optimized, and in the long term, low energy consumption and high efficiency dust removal technology should be developed.
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