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These fine particles emitted from the power plants disperse over a wide area and are harmful to human beings as they enter the respiratory tract and cause many chronic health problems. Based on their size, they are known as PM10 and PM2.5.
Pollution due to particulate matter in the air is one of the six criteria pollutants in the National Air Quality Standard by the US EPA and also by other regulatory authorities worldwide. Documented health hazards world-wide prompted regulators to specify special regulations to contain the emission of theses small particles into the atmosphere. Due to the very small size, this finds its way into the respiratory tract of humans and is identified as a potential health hazard for the population exposed to it. These particles embedded in the respiratory tract can cause respiratory tract infections, asthmatic complaints, and chronic bronchitis.
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Dust from power plants was one of the main emission problems of old coal fired power plants. Visibility and ophthalmic problems were the result of exposure to this dust content. Effects on respiratory diseases were not documented. Power plants then had only mechanical cyclones that separated around sixty to seventy percent of the ash.
As the power plant numbers and capacities increased regulations were in place limiting the dust emission from these plants. The advent of reliable Electrostatic Precipitators (ESP’s) and reliable fabric filters helped in considerably reducing dust or particulate emission. Reduction of the earlier limits of 350 μg/m3 to the current values of 150 μg/m3 or below is to eliminate these health hazards. Together with regulations that require higher chimney stacks, dust fallout on ground level is predicted to be at acceptable levels.
Even in countries like India which fire high ash content coals, the new breed of ESPs and Fabric Filters, coupled with high power electronics and software are able to contain the emission limits within the current regulatory limits. Proper sizing, design operation and maintenance of ESP’s are the main criteria. Technology is available. ESPs today can capture 99.5 % of the ash that enters the ESP.
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Earlier standards specified only the total dust emission without considering the size of the particles. Considering the documented evidence of health hazards associated with very fine particles, particulate emission regulations are now made in two parts namely PM10, particulate matter less than 10 micrometers and PM2.5 particulate matter less than 2.5 micrometers. Particles greater than 10 microns settle to the ground with less chance of entering the respiratory tract.
The current National Ambient Air Quality Standards of US EPA identifies PM10 and PM2.5 as part of the six criteria pollutants. The limits of PM10 are 150 μg/m3 average on a 24 hour basis and PM2.5 is 15 μg/m3 on an annual average basis. The Indian Ambient Air Quality Standards for PM10 is 100 μg/m3 and PM2.5 at 60 μg/m3.
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The particulate matter themselves can be divided into two as to their formation.
First is the primary particle that forms directly at the source like the ash formed during the combustion of the Coal or dust formed during combustion.
The other category is the secondary particles formed due to chemical reactions from gaseous emissions from the primary source. Majority is from vehicular emissions.
Most of the particulate emissions from Coal fired plants fall in the PM10 category. The contribution to PM2.5 from power plants is the secondary particles formation due to the SOXand NOX emissions from the plant. The current emission criteria do not include the formation of secondary particle by the sources, but in the future, these may also be part of the pollution criteria.