Different devices used for Controlling particulate/flue gas emissions
The most effective method of controlling air pollution is definitely to prevent the formation of air pollutants or to reduce their emission at the source itself.
This can be achieved by using pure grade materials, modifying the process of formation, as well as the technologies.
Even if all measures are taken, emissions of pollutants cannot be controlled fully and pollution controlled devices should, therefore be used. Some of the different devices for particulate emissions are:
(ii) Cyclone separator
(iv) Catalytic converter
(v) Electrostatic precipitator
For controlling gaseous pollutants, the flue gases are flame combusted to have C02 and water vapour (almost full oxidation), for example, in a catalytic converter or if the flue gas contains enough S02, it is removed by its absorption or adsorption as in the scrubber.
Such devices are used for collecting very small particles (~ 1 pm) through fabric filtration. A large chamber with several compartments containing fabric filter bags is called baghouse. The bags are distributed in the compartments so that when one compartment is under operation the other can be cleaned.
Dust containing gases are passed through the bags and the dust particles are trapped (particle size > 1 pm), after a certain time the particles adhered to the bags disallow particles of more smaller size (~0.01 pm). The efficiency of removal is as high as 100 per cent.
(ii) Cyclone separator:
For relatively large particles (5 – 20 pm), the control device used is cyclone separator. Gaseous emissions containing suspended particles enter tangentially near the top of the cyclone.
As the gas spins, the centrifugal force causes dust particles to collide with the outer walls and then gravity causes them to fall down into the collector.
The cleaned gas now escapes from the top. Whereas for particles larger than 5 pm the efficiency of cyclone separator is about 90 per cent, for smaller particles it is less efficient. However, it is not expensive and is maintenance free.
Scrubber is used for removal of S02 from flue gas. There are two methods:
(a) Wet method
(b) Dry method
(a) Wet method:
In the wet method, pulverized limestone (CaC03) is mixed with water; slurry is made and sprayed into flue gases.
Chemical reactions produce calcium sulphite (CaS03. 2H20) and precipitate it out. The precipitate is removed as a sludge, which is later used in the construction industry.
(b) Dry method:
In this process, lime (CaO) is used for removal of S02.
(iv) Catalytic converter:
Catalytic converter is used in the automobile engine for controlling emissions very effectively. Three-way catalytic converter is now available in the market.
Three way means removal of three pollutants such as CO, hydrocarbons and NOx. In this type of catalytic converter, CO and hydrocarbon are oxidized to C02 and NO, is reduced to N2 in the same catalytic bed.
The catalytic converters not only control emissions but also allow engines to operate at near stoichiometric conditions. The efficiency o catalytic converter however, gets reduced drastically when leaded petrol is used.
(v) Electrostatic precipitator:
This device is generally used in power plants for removal of very small particles (~1 pm). In this process, high voltage (100,000V) is applied to the wires connected to the grounded plates.
The corona discharge in the wire ionizes the incoming gas, the positively charged gas molecules and electrons on their way to grounded plate adhere to the particulate matter.
The particulate matter becomes charged and is attracted to the grounded surface, where they are removed either by flushing with liquid or by gravitational force.