The bag filter is a filter method to separate and collect solid particles in the flue gas. The dust collection process mainly includes screening, inertial collision, interception, diffusion and electrostatic adsorption. The dust collection process in a short time after the new filter bag or dust is cleaned mainly includes inertial collision, interception, diffusion and electrostatic adsorption. After the dust adhesion layer is formed on the filter cloth, the collision, interception, diffusion and electrostatic adsorption will gradually become smaller (Figure 1 ). The factors affecting the filter efficiency of the bag filter include dust characteristics, flue gas characteristics, filter bag characteristics, unit operating conditions, the thickness of the dust layer on the surface of the filter bag, the pressure loss of the filter bag, and the filtration speed. The commonly used filter bag treatment processes for bag filter in thermal power plants include PPS + PTFE coating, PPS + 15% P84, and PPS + surface treatment. The resistance of the dust removal system is less than 1500Pa, and the exhaust concentration of the filtered flue gas is less than 50mg/m30, and the life of the filter bag is less than 50mg/m30. 1. Filtration speed The filter wind speed v of the bag filter is the average speed of the flue gas passing through the surface of the filter bag, vu003dQt/60F. Among them, Qt is the flue gas flow through the filter bag, m3/h; F is the total area of u200bu200bthe filter bag, m2. The selection of filtration speed should consider factors such as economy, filter bag service life and filtration efficiency. When the same dust quality is accumulated on the surface of the filter bag, the cumulative dust collection rate decreases significantly with the increase of the filtering speed. In addition, the selection of filtration speed is closely related to factors such as dust removal method, dust removal system, dust characteristics, and inlet dust concentration. Generally, for the low-pressure pulse jet dust removal method, the filtration speed is ≤1.0m/min, and the filter speed is ≤1.1m/min after the addition of the dust pretreatment device; the filtration speed of the back blow cleaning method is ≤0.8m/min; dust When the particles are small, or the inlet dust concentration is high, and the dust viscosity is high, the filtration speed should be reduced. Lowering the filtration speed can improve the filtration efficiency and prolong the service life of the filter bag. Second, the resistance of the bag filter The resistance of the bag filter determines the energy consumption, filtration efficiency and cleaning cycle of the dust removal system. It is related to the structure of the dust collector, the characteristics of the filter bag, the filtration rate, the dust concentration, the cleaning method, the flue gas temperature and the smoke. Factors such as gas viscosity are related. The resistance Δp of the bag filter is composed of the structural resistance Δpc, the resistance of the cleaning filter bag Δpf and the resistance of the dust layer on the filter bag Δpd: Δp u003d Δpc + Δpf + Δpd (1) Δpc refers to the inlet and outlet of the dust collector, the air flow distribution device, and the internal channel block The flow resistance caused by the plate, etc., usually Δpc<300Pa. The cleaning filter bag resistance Δpf is: Δpf u003d ξfμυ (2) where: ξf is the resistance coefficient of the filter bag, m-1; μ is the viscosity of the gas, Pa·s; υ is the filtering wind speed, m/s. Generally Δpf<300Pa, the resistance of the dust layer Δpd is related to factors such as filter bag, dust particle size distribution, chemical composition, flue gas temperature, moisture content, etc., and needs to be determined through experiments. The resistance of the dust layer under certain conditions is shown in Figure 2. At the same filtration speed, as the thickness of the dust deposition increases, the resistance of the dust layer increases; when the filtration speed is less than 0.5m/min, the thickness of the dust layer has nothing to do with the resistance; When the filtration speed is less than 1.2m/min and greater than 0.5m/min, after the thickness of the dust layer increases to a certain extent, the resistance changes linearly when the thickness of the dust layer continues to increase. The increase in the resistance of the bag filter can increase the filtration efficiency, but the increase in the operating resistance will increase the energy consumption. Therefore, there is the best balance point between the resistance of the bag filter and the filtration efficiency. It is recommended to control the resistance of the bag filter between 1200 Pa and 1500 Pa. 3. The influence of dust particle size on filtration efficiency The size of flue gas dust particle size directly affects the filtration efficiency of the bag filter. The classification efficiency of dust particle diameter under different conditions is shown in Figure 3 . It can be seen from Figure 3 that the filtration efficiency increases with the increase of the particle size, but the dust with a particle size of 0.2 to 0.4 μm has the lowest filtration efficiency under different filter bag conditions. Compared with the filter bag after dust collection and cleaning, the filter efficiency of the clean filter bag is the lowest. Its filter efficiency mainly depends on the filter bag. The filter bag surface area dust filter efficiency is improved, especially for dust with a particle size of less than 0.5μm. The improvement is obvious. 4. Dust pretreatment Dust pretreatment is mainly to reduce the dust load of the filter bag by reducing the dust concentration and changing the dust particle size distribution, thereby reducing the number of dust cleaning times and extending the service life of the filter bag. The dust concentration can be appropriately reduced by installing electric precipitators, cyclone dust collectors, shutters, humidification towers, etc. in front of the bag filter. Changing the particle size distribution of dust is an important way to solve the problem of ultrafine dust collection, that is, through electrocoagulation, liquid coagulation, and chemical coagulation, small dust particles become larger dust particles for utilization and collection. More about: filter material test
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