Cyclones
Cyclones or cyclone separators are devices used in the removal of particles from air or gas using the principle of inertia. Cyclone separation is a more efficient method as compared to other collection techniques. More than one cyclone separators can be used in a system known as a multi-cyclone, where they operate together in a parallel manner to remove more particles at a faster rate. During the collection of particles in the air, a continuous feed of air is passed into the cyclone. Air is passed into a chamber with a spiral vortex. The system is set in a way that, the lighter component of air will go up following the high-speed helical movement of the air in the vortex. The lighter components of air, with less inertia, travel up the chamber while the vortex does not quite easily influence the larger particles with more inertia. The large particles, having difficulty in moving up the vortex, hit the inside walls of the container and drop into the collection hopper. These particles are then collected and observed as air pollution particles (Chu et al. 2011).
Most cyclone separators have the capacity to remove particulate matter in the size of ten micrometers and above in diameter. More powerful cyclones also exist that are designed to remove air pollution particles with as small as three micrometers in diameter. One limitation of using the cyclones is that they are not effective with particles larger than two hundred micrometers. For particles larger than 200 micrometers, other collection techniques and devices can be used. The standard cyclone separators cannot pick up particles less than 10 micrometers in size. One advantage of using cyclones is that they are easier and less expensive to install and maintain. The operating cost of using the device is also lower as compared to other devices. The removed particles are also collected while dry making it easier to dispose of.
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Baghouse
A baghouse is another air pollution control device or technique which used in the collection of particulate matter out of air. The method of collection using the baghouse is applied in a wide range of areas such as commercial processes and removing particles from gas released from combustion for electricity generation. Baghouse technique of collection is quite efficient, and standard quality baghouses have an efficiency of 99%. In operation, air is filtered by bags which are made of various materials. The bags are periodically cleaned up to remove accumulated dust or particles of air pollution. Air enters that baghouse through a hopper and is directed into a compartment. Particles or dust is trapped by a filter media on the inside surface of the compartment. Cleaning takes place during filtering or when filtering has stopped. The dust collects on the filter media on the surface of the bags through mechanisms such as the inertial collection, Brownian motion and also through interception mechanism (Vehlow, 2015).
The efficiency of the baghouse comes from the combination of the three mechanisms which increases the dust formation on the filter media and the resistance to gas flow. Baghouse devices come in different types such as the pulse jet and the reverse air baghouses. One disadvantage of using the baghouse is that the filter bags have to be replaced periodically. The other drawback is the fact that the whole process is quite time-consuming, and replacing the filter bags is always messy and involving. Baghouses are highly efficient, and the pulse jet type can continuously clean itself while in use. The reverse air type can handle very high temperatures. Generally, the efficiency level of collecting dust particles in air using the baghouse is very high as compared to other collection techniques. The filter media can catch both small and very large particles in the bags.
Electric Precipitator
The electric precipitator uses an electric charge to clean and remove particulate matter, dust of impurities in air. Electrostatic precipitators were initially designed to recover valuable industrial process materials. The precipitators are also used in air pollution control, in the removal of particulate matter or dust particles from waste gases in industries and power-generating stations. In operation, the precipitator functions by channeling energy only to the particulate matter in the air which is to be collected, without interfering with the flow of the gases or air. The principle is to attract charged particles after energy is applied to them. Charged particles are attracted and collected on plates and then shaken off into the hopper at the bottom of the unit. The collected particles can then be transported for disposal or recycling (Ji et al. 2013).
The efficiency of the electric precipitator is quite high and removes particles as small as 0.01 micron. High-quality precipitators can be adjusted to give the required purifying power by the operator. The device can process very large gas ranges and can also allow higher operating temperatures. The electric precipitator, however, comes with a lot of limitations and disadvantages such as costly installation and management. The equipment requires proper handling and protection. The operation of the electric precipitator can be affected by changes in the air such as humidity and temperature. Acquiring and the installation of the electric precipitator also require a large capital investment. Comparably to the other collection devises such as the baghouse and the cyclone, the electric precipitator is more expensive to acquire, install and maintain. The other two are also easier to use as compared to the electric precipitator.
References
Chu, K. W., Wang, B., Xu, D. L., Chen, Y. X., & Yu, A. B. (2011). CFD–DEM simulation of the gas-solid flow in a cyclone separator. Chemical Engineering Science , 66 (5), 834-847.
Ji, J. H., Yasuhiko, K., Noh, H. S., Yun, S. Y., & Cho, H. W. (2013). U.S. Patent No. 8,470,084 . Washington, DC: U.S. Patent and Trademark Office.
Vehlow, J. (2015). Air pollution control systems in WtE units: an overview. Waste Management , 37 , 58-74.
