The function of the dust removal equipment is to reduce dust emission concentration and protect the atmosphere. The dust collectors for asphalt concrete mixing equipment include a primary dust collector and a secondary dust collector. The general small-scale mixing equipment is only equipped with one-stage dust remover; the large-scale mixing equipment adopts two-stage dust removal to meet the requirements of environmental protection dust removal. The first level generally adopts the gravity type or centrifugal type dry dust collector, while the second level often adopts the wet type dust collection or bag type dust removal. . After the wet dedusting, the amount of dust discharged can reach 400mg/m3, and the amount of dust discharged after the bag type dedusting can reach 50mg/m3.
Early asphalt mixing equipment was equipped with a dry cyclone dust filter to remove dust.
Due to the dry cyclone dust removal, only large particles (>5 μm) of dust can be removed, and the content of large dust particles in the dust is very small. A large amount of fine dust less than 5 μm is discharged. With the improvement of environmental protection requirements and the development of dust removal technology, asphalt mixture mixing equipment can only meet the requirements of exhaust emissions by adopting secondary dust removal. The methods of secondary dust removal include wet dust removal and bag dust removal. Different dust removal methods are adopted according to different requirements.
Primary dust collector
There are two kinds of dust collectors, cyclone dust collector and gravity dust collector. The cyclone dust collector has a simple structure, and its dust collection device is a cylinder type, and the airflow enters tangentially, and the dust is separated under the action of centrifugal force. The cyclone dust collector can collect dust with a particle size of 5 μm or more. The dust removal efficiency is generally 5% to 85%.
Gravity dust collector is to use the expansion tank to reduce the speed of the air flow, so that large particles of dust can be reduced by gravity. Generally, particles larger than 40 μm are trapped.
Baghouse
Bag type dust collector is a kind of high efficiency dust removal device. It uses organic fiber or inorganic fiber fabric to make filter bag and filter the dust in the flue gas to trap dust with particle size of 0.3μm or more. The dust removal efficiency can reach 95%~ 99%. The dust concentration can reach 50mg/Nm3.
At present, bag type dust collectors have three types of atmospheric backflush, blower blowback and pulse injection. Atmospheric backflush uses the negative pressure of the system and the pressure difference of the atmosphere to allow the atmosphere to enter the bag for cleaning. Since the pressure difference is small, the disturbance of dust removal is small, so the cleaning time must be extended, and the filter area must be increased in order to meet the needs of dust removal. Therefore, the volume of the atmospheric reverse dust remover is generally large. Pulse jet cleaning is the use of high-pressure air injection bags within a short time, making it shake and remove the dust attached to the bag. Because the injection time is short and the injection force is large, the dust adhered to the bag is easy to drop. The short-time cleaning and cleaning of the dust almost does not affect the operation of the dust collector. The volume of pulse jet dust collector is generally small, but it needs to be equipped with high pressure gas source and pulse valve.
When the bag filter is working, under the suction of the fan, the dusty flue gas enters into the box 1, the flue gas is dispersed under the interception of the baffle, and the large particles are discharged from the exit 2 and used as aggregates. Dust-laden gas enters into the filter bag 4 from the outside 3 of each filter bag. Under the effect of screening, grilling, impact, interception, diffusion, and electrostatic attraction of the filter bag, fine dust adheres to the gap between the filter cloths. Dust is separated from the smoke. As the dust accumulates on the filter bag, a dust layer with a certain thickness is formed, so that the air permeability of the filter cloth is reduced, and the dust collector is prevented from working normally. Therefore, bag filter must always clear the dust on the filter bag in the process of work. There are mechanical rapping and blowing methods to remove accumulated dust, and there are different methods such as pulsed high-pressure injection and atmospheric back-blowing.
Atmospheric backflushing method: The controller controls the pulse valve to open the vent above the filter bag at regular intervals, causing pressure difference between the inside and outside of the filter bag, causing the filter bag to shake and vibrate. The dust on the bag falls to the bottom of the box, and then it is set at the bottom of the box. The screw conveyor is sent out of the box. In order to ensure the filtering effect of the filter bag, a certain pressure difference must be maintained between the dust-laden flue gas and the filtered purge gas, and a differential pressure gauge is installed on the bag-type dust collector. If the pressure difference is too large, it indicates that the filter bag has too much dust and the filter resistance is too large; the pressure difference is too small, indicating that the filter bag has been damaged. The fan is arranged at the chimney mouth by means of suction to avoid the accelerated wear of the fan by the dust.
Because the main component bag of the bag filter is operated under high temperature conditions for a long time, the sulfur content in the fuel also causes the acid material to corrode the bag during the combustion process. Therefore, the material of the bag must be able to withstand temperature and acid resistance. In addition, the rust prevention of the dust box is also very important. There are elements such as sulfur and nitrogen in the fuel, which become acidic in the process of combustion and oxidation. After entering the dust box, it is easy to corrode the inner wall of the box. Therefore, it is required to coat the inner wall of the box with anti-corrosion coating or use anti-corrosion materials. The improvement of the new bag filter technology focuses on how to improve the cleaning effect of the bag, such as changing the position of the bag disturbance and increasing the air volume of the pulse nozzle.