Smoke and dust is one of the main factors that cause air pollution, and the fundamental measure to reduce air pollution is to reduce the emission of harmful substances into the atmosphere. At present, the main equipment for treating particulate pollutants such as smoke and dust in China is bag filter and electrostatic precipitator. When selecting dust removal equipment, its economy, reliability, applicability and sociality should be fully considered. When choosing dust removal technology, we should consider the local conditions, field conditions, coal combustion characteristics, emission standards and dust removal efficiency to be achieved. In view of the current national requirements for environmental protection, the performance differences between bag filter and electrostatic precipitator, and the practical application in all walks of life, this paper makes a simple and objective comparison of the basic characteristics of the two kinds of dust collectors in practical application.

2 principle comparison

(1) bag filter

Bag-shaped filter elements (i.e. filter bags) are made of different porous filter materials. When the dusty gas passes through the filter bag, the dust particles are intercepted due to inertia, and the fine dust particles (particle size 1μm or less) constantly change their movement direction due to diffusion (Brownian motion), which increases the chance of dust particles contacting the filter bag. When dust particles collide with the filter bag, the adhesion and electrostatic action make dust particles accumulate on the surface of the filter bag to form a filter cake (or filter bed), and finer dust particles can be captured by screening. If the filter mode of the dust collector is internal filtration, dust particles will stay on the inner surface of the filter bag, and clean gas will escape to the outside of the bag through the gaps between the fibers of the filter bag; If the filter mode of the vacuum cleaner is external filtering; otherwise. When the dust particles accumulate to a certain extent, the dust particles are removed from the filter bag by pneumatic or mechanical methods with the help of gravity, and the dust is collected and transported away.

(2) Electrostatic precipitator

High-voltage direct current is applied to the positive and negative electrodes of the electrostatic precipitator, so that an electric field sufficient to ionize the gas is maintained between the two electrodes. When the dusty gas passes through the high-voltage electric field, the dust particles are charged (generally negatively charged). Under the action of electric field force, the charged dust particles move to the dust collecting electrode (positive electrode) with opposite polarity and are deposited on the dust collecting electrode, so that the dust particles are separated from the dusty gas, and then the dust falls into the collecting hopper at the lower part of the electrostatic precipitator for collection and transportation.

3 Comparison of dust removal efficiency

The dust removal efficiency of bag filter is higher than that of electrostatic precipitator, and the collection of heavy metal particles and submicron dust particles which have serious influence on human body is more effective. Generally, the dust removal efficiency can reach above 99.99%, and the concentration of discharged smoke and dust can be stably lower than 50mg/Nm3, even lower than 10mg/Nm3, thus achieving almost zero emission.

Judging from the current application of coal-fired boilers in domestic power industry, the smoke emission concentration after bag filter treatment can be guaranteed to be below 30mg/Nm3. For example, two 200MW units in Hohhot Power Plant use bag filter for flue gas purification. From the long-term automatic monitoring results of CEMS system and the manual sampling results of authoritative testing institutions, the emission concentration is lower than 27mg/Nm3.

The efficient filtration mechanism of bag filter determines that it is not affected by the change of physical and chemical properties of coal, and has stable dust removal efficiency. In view of the current national emission standards and the collection method of emission expenses, the economic benefits brought by bag filter are obvious.

Although the dust removal efficiency of electrostatic precipitator can reach above 99.9%, its dust removal efficiency is unstable due to the high technical requirements of control and maintenance and the sensitivity of electrostatic precipitator to dust, but it can also meet the emission requirements as a whole. With the further improvement of national environmental protection standards, more and more power plants burn low-sulfur coal (or after high-efficiency desulfurization), and it will be more and more difficult for electrostatic precipitators to meet the discharge standards.

4 Comparison of the influence of boiler system change on dust collector

The coal type of coal-fired power plant is relatively stable, but it will also encounter changes in coal type or coal quality; The boiler system is a system that will change and adjust frequently, so the physical and chemical properties, flue gas concentration, temperature and other parameters of the flue gas produced by the boiler will also change. These changes will also cause different changes in the dust collector. The effects of several main changes of the system on different dust collectors are as follows:

(1) The air volume of the blower and induced draft fan remains unchanged, but the flue gas concentration at the boiler outlet changes.

For bag filter: the change of flue gas concentration will only cause the change of filter bag load of bag filter, which will lead to the change of dust cleaning frequency (automatic adjustment). The filter bag with high dust concentration has fast dust accumulation speed, high dust cleaning frequency and low dust cleaning frequency, which will not cause the change of discharge concentration.

For electrostatic precipitator: the change of smoke concentration will directly affect the charging of dust, so it will directly affect the dust removal efficiency of electrostatic precipitator, which is finally reflected in the change of emission concentration. Usually, with the increase of smoke concentration, the dust removal efficiency is improved, and the emission concentration will also increase accordingly; When the dust concentration decreases, the dust removal efficiency decreases, and the emission concentration will also decrease accordingly.

(2) The amount of boiler smoke and dust is constant, and the air volume of air supply and induced draft fan changes.

For bag filter: the change of air volume will directly cause the change of filtration wind speed, thus causing the change of equipment resistance, but it has little effect on dust removal efficiency. With the increase of air volume, the resistance of equipment increases and the output of induced draft fan increases; On the contrary, the output of induced draft fan decreases.

For electrostatic precipitator: the change of air volume has little effect on the equipment, but the dust removal efficiency of electrostatic precipitator will change obviously with the change of air volume. With the increase of air volume, the electric field wind speed of electrostatic precipitator increases, and the residence time of dust in the electric field shortens. Although the wind disturbance in the electric field will enhance the effective driving speed of charged dust, it is not enough to make up for the negative impact caused by the shortening of dust residence time in the electric field and the intensification of secondary dust, so the dust removal efficiency will be very obvious. On the contrary, the dust removal efficiency will be improved, but the improvement is not great.

(3) the change of temperature

For bag filter: condensation will occur when the flue gas temperature is too low, which may cause "bag sticking" and shell corrosion; If the flue gas temperature is too high and exceeds the allowable temperature of the filter material, it will cause "bag burning" and damage the filter bag. However, if the temperature changes within the temperature range of the filter material, it will not affect the dust removal efficiency. The temperature causing adverse consequences is extreme temperature (accident/abnormal state), so the bag filter must be equipped with effective protection measures to control the extreme temperature.

For electrostatic precipitator: if the flue gas temperature is too low, condensation will cause shell corrosion or high voltage creepage, but it is beneficial to improve dust removal efficiency; The increase of flue gas temperature will cause the increase of dust specific resistance, which is not conducive to dust removal. Therefore, the flue gas temperature will directly affect the dust removal efficiency, and the influence is obvious.

(4) Changes of physical and chemical composition of flue gas (or coal burning)

For bag filter: the physical and chemical composition of flue gas has no effect on the dust removal efficiency of bag filter. However, if the flue gas contains components that are corrosive to all filter materials, it will directly affect the service life of filter materials.

For electrostatic precipitator: the physical and chemical composition of flue gas will directly cause the change of dust specific resistance, thus affecting the dust removal efficiency, which has a great impact. The most direct influence is the content of sulfur oxides in flue gas. Generally, the higher the content of sulfur oxides, the lower the specific resistance of dust, the easier it is to collect dust and the higher the dust removal efficiency; Conversely, the lower the dust removal efficiency. In addition, changes in chemical composition (such as silicon, aluminum, potassium, sodium, etc. ) will also cause obvious changes in dust removal efficiency.

(5) Air distribution

For bag filter, the dust removal efficiency is not directly related to the air distribution, that is, the air distribution does not affect the dust removal efficiency. However, the local airflow distribution in the dust collector should be as uniform as possible, and the deviation should not be too large, otherwise the local bag will be broken due to uneven local load or jet wear, which will affect the normal service life of the filter bag of the dust collector.

For electrostatic precipitator: electrostatic precipitator is very sensitive to the airflow distribution in electric field, which directly affects the dust removal efficiency. In the performance evaluation of electrostatic precipitator, the root mean square index of airflow distribution is usually one of the important indexes to evaluate the quality of electrostatic precipitator.

5 Comparison of operation and management

(1) Operation management

Bag filter: stable operation, simple control, no high-pressure equipment, good safety, few interference factors affecting dust removal efficiency and stable discharge. Because the filter bag is the core component of the bag filter, and it is relatively fragile, the equipment management requirements are strict.

For electrostatic precipitator: there are many interference factors on dust removal efficiency in operation, and the discharge is unstable; The control is relatively complex, which requires high safety protection for high-voltage equipment. Because electrostatic precipitators are all steel structures, they are not easy to be damaged. Compared with the bag filter, the equipment management requirements are not very strict.

(2) Stop and start

Bag filter: it is convenient, but it needs to protect the filter bag when it is stopped for a long time.

For electrostatic precipitator: it is convenient and can be stopped at any time.

(3) Repair and maintenance

For bag filter: it can realize non-stop maintenance, that is, online maintenance.

For electrostatic precipitator: be sure to stop when overhauling.

6 equipment investment comparison

(1) For conventional flue gas and dust conditions (mainly flue gas suitable for electrostatic precipitator), the emission concentration of the two kinds of dust collectors should meet the current low environmental protection requirements (such as 120mg/m3), and the initial investment of bag filter is 20% ~ 35% higher than that of electrostatic precipitator.

(2) For dust with low sulfur and high specific resistance and dust with high SiO2 _ 2 and Al _ 2O _ 3, the two kinds of dust collectors should meet the current low environmental protection requirements (such as 1.20mg/m3), and the initial investment of electrostatic precipitator and bag filter is equivalent or the investment of electrostatic precipitator is high.

For boilers of typical coal types, such as Zhungeer coalfield and Bohai Bay coalfield, the investment of electrostatic precipitator is higher than that of bag filter.

(3) Under normal circumstances, to achieve the same dust removal efficiency or the same emission concentration, the investment of electrostatic precipitator is usually higher than that of bag filter. Taking the 200MW unit of Hohhot Power Plant as an example, under the same conditions, under the premise of ensuring the emission concentration reaches the standard:

Bag filter: the investment of each unit filter is less than 20 million yuan; Ensure that the discharge concentration is lower than 50mg/m3.

Electrostatic precipitator: according to electric field 4, the specific dust collection area is1.30m2/m3 s, and the standard discharge concentration is 1.20mg/m3. Each electrostatic precipitator needs to invest about 25 million yuan.

7 Comparison of operation and maintenance costs

(1) Operating energy consumption

Bag filter: the energy consumption of the fan is large, and the energy consumption of dust removal is small.

For electrostatic precipitator: the fan consumes less energy and the electric field consumes more energy.

But generally speaking, the power consumption of the two kinds of dust collectors is equivalent. For dust that is difficult to be collected by electrostatic precipitator, or when the electric field of electrostatic precipitator exceeds four electric fields, the energy consumption of electrostatic precipitator is higher than that of bag precipitator, that is to say, the operating cost of electrostatic precipitator is higher than that of bag precipitator at this time. If calculated according to the environmental protection standard of 1.20mg/m3, the electric field of electrostatic precipitator must be above 4 to ensure the discharge standard, so its power consumption must be higher than that of bag filter.

(2) Maintenance cost

The maintenance cost of bag filter is mainly the replacement cost of filter bag. Because the pollution discharge cost of bag filter is much lower than that of electrostatic precipitator, if the bag filter is used for 1.5 ~ 2 years, it can offset the cost of replacing the filter bag once.

The maintenance cost of electrostatic precipitator is mainly the replacement of collecting electrode (anode plate), cathode wire and rapping hammer. This cost is high, but the replacement interval is relatively long, about 6 years.

(3) Economic benefit analysis

In actual operation, the emission concentration of bag filter is about 10% of that of electrostatic precipitator, so the actual sewage charge paid by bag filter in power plant is about110 of that of electrostatic precipitator. Judging from the current situation of sewage charges levied by the state, the sewage charges paid per furnace/year after adopting bag filter are considerable, reaching one million yuan. In addition, the desulfurization efficiency of bag filter is about 5%, which can also reduce the sewage charge of sulfur dioxide in power plant.

In a word, with the increasingly strict requirements of national environmental protection standards, electrostatic precipitators must adopt electrostatic precipitators with electric fields above level 4 if they want to meet the emission standards. At this time, the initial investment of electrostatic precipitator has been higher than that of bag filter, and the operating power consumption of electrostatic precipitator above 4 electric fields (or 4 electric fields with high specific dust area) is much higher than that of bag filter. Therefore, even if the electrostatic precipitator meets the discharge standard, its initial investment and operating cost are higher than that of bag filter. In addition, the emission concentration of electrostatic precipitator is always about 10 times that of bag filter. According to the current new sewage charging system, even if the emission concentration reaches the standard, it will be charged according to the dust emission. Therefore, under the same emission standards, electrostatic precipitator needs to pay more than bag precipitator. Therefore, from the perspective of cost performance, bag filter will become the first choice for industrial dust control.

8 abstract

To sum up, although both bag filter and electrostatic precipitator can deal with particulate pollutants such as smoke and dust, they have their own advantages and disadvantages due to different principles. Therefore, when choosing a vacuum cleaner, it is necessary to choose a suitable vacuum cleaner according to the specific situation and conditions and the actual needs of users in order to achieve the ideal effect.

References:/jswt/n1446.html.