Issue 12/2015 - Research Progress on PM2.5 Control Technology in Flue Gas

PM2.
5 pollution status
PM2.
5 mainly comes from human emissions, from the form of pollution sources can be divided into fixed pollution sources and mobile pollution sources, fixed pollution sources mainly include coal-fired power plants and industrial boilers, mobile pollution sources include automobile exhaust, dust, etc.

From the perspective of pollutants, PM2.
5 mainly comes from the combustion of coal, which is the most important energy substance in China, so it can be seen that the coal-dominated PM2.
5 pollution situation will not change
in recent years.

According to the "January 2015 Beijing-Tianjin-Hebei, Yangtze River Delta and Pearl River Delta Regional and Municipal Directly Central Government, Provincial Capitals and Cities with Independent Planning Status" released by the China Environmental Monitoring Station, since January 1, 2013, the proportion of 74 cities (referred to as 74 cities) in the Beijing-Tianjin-Hebei, Yangtze River Delta and Pearl River Delta regions and municipalities directly under the central government, provincial capitals and cities with independent planning status is between 3.
2%~100%, and the average proportion of days to meet the standard is 48.
2%.
The proportion of days with light pollution was 25.
6%, moderate pollution was 13.
3%, heavy pollution was 11.
0%, and severe pollution was 1.
9%.

Among the days exceeding the standard, PM2.
5 was the primary pollutant, followed by PM10
.
The pollution situation is very serious
.

For the prevention and control of PM2.
5, it is not only necessary to control a single pollutant, but also to carry out coordinated control of various pollutants in order to effectively control the PM2.
5 pollution situation
.
This is a protracted battle
for China that needs to be constantly explored.

Flue gas PM2.
5 control technology
1.
Wet control technology
electrostatic precipitator is the use of strong electric field corona discharge, gas ionization, dust charge, under the action of electric field force to separate dust from the gas device, with high dust removal efficiency, gas pressure loss and other characteristics
.
The capture mechanism of wet electrostatic precipitator technology is the same as that of dry electrostatic precipitator, but it is different from the dry electrostatic precipitator in the removal method of capturing dust, the former uses flushing liquid to flush the electrode to remove the dust in the form of a slurry, and the latter uses mechanical vibration and other methods to remove the dust accumulation on the electrode, so they have obvious differences
in structure and dust removal characteristics 。 Due to the improvement of ash cleaning technology, wet electrostatic precipitator does not cause secondary dust flying due to vibration, electric wind and air flow, which not only ensures low emissions, especially for PM2.
5 fine particles, SOX, dioxins, heavy metals and alkaline chloride particles can be effectively
captured.
With the improvement of environmental protection requirements in various countries, developed countries such as the United States, Japan and Germany have increased the treatment of pollutants such as fine dust and SO3 acid mist, and the development and application of wet electrostatic precipitators have also become the development direction
of the dust removal industry.

Wet flue gas desulfurization (WFGD) is the main desulfurization process currently used, and relevant studies at home and abroad have shown that WFGD has a good removal effect on large particles, but the removal efficiency of fine particles (particle size < 2.
5μm) is very low
.

If the inhalable particulate matter can be removed at the same time during the wet desulfurization process, it will be of great significance
for flue gas pollution control.
In the process of wet desulfurization, the high-temperature flue gas is in contact with the absorption liquid of the low-temperature desulfurizer, and a strong mass transfer and heat transfer effect occurs, which vaporizes part of the absorbent liquid, increases the moisture content of the flue gas at the outlet of the scrubber, and also reduces
the temperature of the flue gas.
Usually at the outlet of the WFGD system, the moisture content of the flue gas can reach more than 85%~90%, if the conditions of the outlet flue gas of the WFGD system can be changed, the supersaturated environment required for the condensation of fine particles can be obtained, so as to control the emission
of PM2.
5 of the WFGD system.
By adding steam in the desulfurization tower and using the steam phase change to promote the condensation and growth of PM2.
5, the removal efficiency
of PM10 and PM2.
5 mass and quantity concentration can be effectively improved.

2.
Electrocoagulation and merging technology
Coagulation technology is to make the dispersed fine particles contact each other through physical or chemical action to combine into larger particles, according to the different coagulation mechanism, mainly divided into thermal coagulation, acoustic coagulation and electrocoagulation
.
Because it is difficult to implement thermocoagulation and acoustic coagulation, electrocoagulation and merging technology
are commonly used at present.

Electrocoagulation and merging technology is a technology
proposed in recent years to use different polarity discharges to cause dust particles to be charged with different charges, and then condense and become larger under the combined action of turbulent transport and electrostatic force.
The working principle of electrocoagulation technology is to place the electrode at the same time with positive and negative two, that is, the particles in the electric field (after the electric field) will be partially charged positive, partially negatively charged, and the dust particles charged with heterogeneous charges will be effectively condensed under the appropriate flow field speed and flow field turbulence, so that the particle size of fine particles becomes larger, the driving speed is increased, and it is conducive to being removed
.
This technology overcomes the problem of insufficient PM2.
5 charge of the electrostatic precipitator, improves the dust removal efficiency of the electrostatic precipitator, and significantly reduces the total mass emissions, especially the emission
of PM2.
5.
At the same time, high-voltage pulse discharge can also remove some harmful components
in the flue gas.

Many domestic companies are studying dust agglomeration technology, and have carried out relevant beneficial experiments, obtained national patents, and mastered core technologies
.
Foreign companies began to study particulate polymerization technology in 1999, began industrial application in 2002, so far the application performance is less, its test results show that PM2.
5 emissions can be reduced by 80%, turbidity can be reduced by 50%~80%, and total mass emissions can be reduced by 1/3~2/3
.

3.
Electrostatic composite dust removal technology
At present, conventional high-efficiency dust collectors cannot effectively capture
particles with particle size between 0.
1~2.
0μm.
The traditional electrostatic precipitator can reach more than 99% for particles above 10μm, but when the particle diameter is less than 2μm, the dust removal efficiency will be significantly reduced, less than 90%, or even less than 50%.

Most of the particulate matter removed by the bag filter is less than 2.
5μm
.
Therefore, in order to effectively remove PM2.
5, it is necessary to improve the dust removal efficiency of existing dust collectors and develop a new and efficient dust removal process
.

Electric bag composite dust collector can combine the advantages of two high-efficiency dust removal equipment, make up for each other's shortcomings, and obtain a more efficient and practical dust removal equipment
.
This is of great significance
for improving China's dust removal technology and equipment, reducing industrial dust emission concentration, and reducing air pollution.

In order to meet the current dust removal requirements and clean production, the focus at home and abroad is on the research of hybrid dust removal system, which is generally composed of
a combination of static electricity and other dust removal mechanisms.
Such as static electricity + cyclone dust collector, static electricity + particle layer dust collector, static electricity + cloth bag, etc
.

With funding from the U.
S.
Department of Energy, Miller et al.
of the U.
S.
Center for Energy and Environmental Research have developed a compact system called the Advanced Hybrid Particulate Collector
.
The basic idea is to collect electrostatic precipitator and bag dust removal in a cavity, and place the filter bag between the electrostatic plate and the polar wire, realizing a real mix
.
Since there are very few particles in the flue gas after the electrostatic precipitator, plus because these particles are repellent with the same charge, more pores and condensed particle layers can be formed on the surface of the filter bag, so that the filtration resistance is small, the surface cleaning is easy, the pulse cleaning time is increased, and the energy consumption is reduced, so the filtration speed can be increased to 4~8 times the traditional speed, and the spacing of the filter bag can be compressed, whether it is space or investment will be greatly reduced, and the particle charging enhances the effect of the dust layer and the fiber layer on the fine particles, at the same time, Its small size and low
investment in filter bags and plates.
In addition, the two are fully mixed in a chamber, the secondary dust generated by electrostatic ash cleaning can be captured by the filter bag, and the dust of the filter bag pulse ash cleaning will enter the electric field area, these dust due to the surface of the filter bag condensation, particle size increases, so it is easy to be captured by the electric field after entering the electric field area, which overcomes the efficiency reduction caused by the cleaning of the two, thereby greatly improving the dust removal efficiency.
In particular, the removal efficiency of PM2.
5 can be used with a better performance membrane filter material, which will further increase the removal efficiency
of PM2.
5.
However, one of the difficulties of this technology is the protection
of filter bags.
Since the filter bag is in a high-voltage electric field, coupled with the continuous accumulation of charged particles on the surface of the filter bag, it is easy to break down the keel behind the filter bag and the dust layer on the surface of the filter bag to generate electric sparks and burn the filter material
.
Miller et al.
adopted the following solutions: (1) Add a plate with an opening rate of about 40% between the filter bag and the electrode, so that the equipotential line around the bag is reduced by nearly 100 times, which not only protects the filter bag, but also can be used as a plate for dust collection; (2) Use conductive filter material and lamination, and add a row of grounding wires or grounding mesh
between the high-voltage electrode and the cloth bag.
This will prevent static electricity from accumulating
on the filter bag.
Through the above technology, the incidence of electric sparks around the filter bag is almost zero, which basically solves the problem of the filter bag being static technicallyThe problem of
electrical destruction.

In addition, in recent years, there are many topics and research and development directions for PM2.
5 treatment technology in China
.
Such as screen dust removal technology, airflow steering technology, bipolar charging technology, etc
.

Conclusion
Based on the current technology, the condenser has the following advantages: (1) the operating cost is relatively low, the pressure loss is small, the operation is reliable, the maintenance is less, and the adaptability is good; (2) Reduce the total mass emission of smoke and dust and improve the dust removal efficiency of dust removal equipment; Significantly reduce the emission of PM2.
5 and toxic heavy metals, reduce the harm caused by particulate emissions, and improve atmospheric visibility; (3) improve the adaptability of electrostatic precipitators to different coal types within a certain range; (4) create conditions
for the reduction of electrostatic precipitators.
The combination of pulse charging and DC dust collection is an effective way to control nanoscale fly ash particle pollutants, but when positive pulse discharge is combined with negative DC dust collection, there is a transition voltage, and when the pulse peak voltage is higher than this voltage, the removal efficiency of particles less than 0.
1μm will be greatly improved
.
The transition voltage of positive pulse discharge is lower than that of negative pulse, which has better safety and economy, and is a better choice
for modifying traditional electrostatic precipitators.
In general, condensers to remove PM2.
5 in the air have considerable development prospects
.

At the same time, the removal of a variety of pollutants, the realization of desulfurization, destocking, particulate matter removal integration is also a focus of current research, with the existing dust removal, desulfurization, denitrification and other pollution control technologies as the core, combined with coal pretreatment technology, combustion process pollutant control technology, the formation of a complete pollutant control chain from the source to the terminal control, minimize investment costs, save energy, and reduce pollutant emissions
from the source.