Particulate matter in gaseous effluents consists of particles of size ranging from 0.1 um to 100 um. The device selection is based upon physical and chemical characteristic of particulate, size, temperature and humidity. Following equipments can be used.
(a) Gravitational settling chamber:
This technique is generally employed to remove solid particulates of size > 50 um. It consists of a chamber in which the velocity of carrier gas is reduced so that heavy particles settle down. The collector efficiency is improved by dividing the settling chambers by number of trays or horizontal shelves. It is advantageous owing to its low cost, simple construction and low maintenance requirement.
(b) Cyclone separators:
In this method gas containing particulates is allowed to into tight circular spiral-fitted chamber. Now the particulates present in the gaseous stream possess greater inertia than the gas molecules and experience greater centrifugal force, which drive these particulates towards the wall of the power plants and industries dealing with rock products.
The cyclone collector is most efficient for the removal of large particle of size ranging from (5-20 um) because centrifugal force is stronger on particles in a spinning gas.
(c) Fabric filters:
Small solid particles of size (1 um) are removed effectively by use of fabric filters. The exhaust gases are allowed to pass through bags of very fine cloth-like mesh which are capable of retaining small particles.
Fabric filters generally consist of cotton, wool, nylon, Dacron or cloth. It is used in industries dealing with rock products pigments.
(d) Electrostatic Precipitators:
It is very efficient and widely used device for controlling particulate emissions in power plants, cement and paper industry and oil refinery.
In this technique particles present in gaseous exhaust are charged electrically and a separated from the gas stream under the influence of electric field.
The Electrostatic precipitator consists of a series of grounded thin plates which are charged to high voltages, alternately +and- ve.
Particles approaching these plates become charged and are attracted towards opposite plates. From these plates they fall into a collector (hopper) below.
(e) Wet Scrubbers:
This is a natural cleansing mechanism through which some pollutants are washed out of the atmosphere.
The same principle is employed here in wet scrubbers. The exhaust gases are passed through a fine spray of water to increase effective size of particles due to formation of water particulate agglomerates.
These are then separated from gas stream by any of collection mechanism.