1. Basic concept of PM2.5
In recent years, China has become one of the most smoggy regions in the world, with a high concentration of PM2.5. According to a research report by Ma Guojun, professor senior engineer of Guodian Environmental Protection Research Institute and a consultant to the World Bank’s FGD project, there is still much room for improvement in the reduction of PM2.5 in coal-fired power plants.
PM2.5: particulate matter with an aerodynamic equivalent diameter of 2.5 microns or less;
PM2.5 comes from natural and man-made sources of pollution;
PM2.5 is not a single component of air pollutants, but a complex and variable air pollutant composed of a large number of different chemical components
The main components of PM2.5 are sulfuric acid, ammonium bisulfate, ammonium sulfate, ammonium nitrate, elemental carbon C and organic carbon. PM2.5 emitted directly by pollution sources is called “primary particle”; Gaseous pollutants discharged from pollution sources condense or undergo complex chemical reactions in the atmosphere to produce PM2.5, which is called “secondary particles”.
According to the “Beijing Air Quality Standard Strategic Study” carried out to ensure the air quality of the 2008 Beijing Olympic Games, the composition of PM2.5 in Beijing is as follows:
29% Industrial coal
15% biomass combustion
13% secondary sulfate
10% motor vehicle
9% secondary nitrate
8% Dust
16% Others
2. Influence of PM2.5
2.1 Health hazards of PM2.5
PM 2.5 can be directly into the human alveoli, also known as pulmonary particulate matter;
PM2.5 causes three respiratory diseases:
As the carrier of viruses and germs, PM2.5 causes colds, tuberculosis and pneumonia;
PM2.5 can cause respiratory allergies, such as asthma and alveolitis.
PM2.5 can lead to weakened immunity and increase the incidence of lung cancer.
2.2 Influence of PM2.5 on atmospheric visibility
Atmospheric visibility is determined by the scattering and absorption of light by particles in the atmosphere:
If there are no particles at all, the scattering of light by atmospheric molecules is very small, and the visibility can reach 100-300km. Visibility can reach 30km in the ultra-clean city air; When the atmospheric visibility decreases to l0km and the relative humidity of the atmosphere is below 80%, it is defined as haze.
3. Coal burning flue gas and PM2.5
The contribution of coal flue gas to PM 2.5 varies between 10% and 25% depending on the region.
Characteristics of flue gas emission from coal burning:
Fixed point source; Large flue gas discharge; Organized discharge; Controllable emissions;
Reasonable inference: Coal enterprises will become the “leader” of strict control.
PM 2.5 emissions from coal flue gas can be divided into:
Filter particles (primary PM) : mechanical particles; The particles are large, 1~ 2.5μm;
Condensable particles (secondary PM’) : gas, vapor state; Generate a second PM in the environment.
The ratio of filtrable particles to condensable particles is about 1:3~1:1.
There is currently no control technology specifically for PM 2.5. Existing and mature APCs in the industrial sector are capable of removing PM2.5. But there are limits to efficiency or capacity.
4. SCR flue gas denitrification system
Impact of SCR flue gas denitrification system:
While reducing NOx in flue gas, SCR catalyst will catalyze the oxidation of about 1~ 3% SO2 into SO3, which will increase the condensable sulfuric acid aerosol in the coal flue gas with high sulfur content. The escape of excess ammonia will increase the concentration of ammonia salt in the flue gas.
There are the following methods to reduce the impact of SO3 generated by SCR on PM2.5:
Catalysts with low oxidation capacity for SO2 were selected. Disadvantages: expensive, unfavorable to flue gas mercury removal;
The disadvantages of setting DSI device in front of air preheater to remove SO3 in flue gas are: increased investment is needed, high price of absorbent and high operating cost.
5. cloth bag filter
In 2003, Lillieblad tested PM2.5 and mercury emissions from a high gas ratio bag filter in a Finnish coal-fired power plant. The bag was made of PTFE-coated PPS (polyphenylene sulfide) and had been in good condition for 31,000 hours. The test results are shown in the table:
The test results show that:
In the flue gas of bag filter outlet, PM1.0 is 3 ~ 6%, PM2.5 is 15 ~ 20% and PM10 is 79 ~ 88%. The increased emission of sub-micron particles in the tested bag dust collector is due to the small hole with a diameter of about 0.4μm in the bag coating.
The conclusion is that the collection efficiency of cloth bag filter is about 99.5% when the particle diameter is 0.2-3 μm.
6. Condensable particulate matter
6.1 Aerosols and colored plumes
The main sources of condensable particles from coal flue gas are SOx and NOx, and SOx is the main one. SO3 in flue gas combines with water in flue gas to form sulfuric acid aerosol with particle size < 0.3μm;After the sulfuric acid aerosol is discharged from the chimney and first enters the atmosphere, it reacts with the positive ionization in the environment to produce secondary PM2.5 (near-ground pollution). SO2 in flue gas is oxidized to SO3 by photochemical oxidation, liquid phase oxidation and particle oxidation in the environment, and sulfate aerosol is formed. While SO2 has a lifetime of several days in the atmosphere, sulfate aerosols can migrate more than 1000km (long-range pollution).
Dry ESP and bag precipitator cannot remove condensable PM2.5!
6.2 Wet scrubber
The conventional wet FGD can not effectively remove fine particles: the conventional wet FGD will increase the concentration of discharged particles due to the removal of solid matter in the slurry. Conventional wet FGD could not effectively remove SO3.
Therefore, the wet scrubber of flue gas desulfurization has low removal efficiency for PM2.5. Wet desulphurization may also increase the emission of particulate matter, and ammonia desulphurization increases the risk of ammonia escape. Effective measures to reduce PM2.5 in the export flue gas of wet FGD:
1) WESP installed downstream of FGD has limited capacity and poor effect on ammonia desulfurization;
2) Install the dry FGD upstream of the FGD.
6.3 Dry FGD
Dry FGD can effectively remove acid gases such as SO3, HCl and HF while removing SO2. Most of the terminal equipment of dry FGD is dust collector. If cloth bag dust collector is used, the problem of PM2.5 emission can be solved.
The dry FGD products of Solway, a Belgian soda manufacturer, introduced by Aier Environmental Protection, combined with its rich engineering technology and experience in atmospheric management, have built dozens of sets of dry FGD devices in China and run them reliably, making a contribution to the cause of atmospheric environment management in China!