The control of particulate matter is an important aspect of industrial air pollution engineering. Particles are collected by a combination of several mechanisms. The six available mechanisms are gravitational settling, centrifugal impaction, inertial impaction, direct interception, diffusion and the electrostatic attraction.
The large particles in the gas stream have too much inertia to follow the gas streamlines around the impactor and are impacted on the impactor surface, while the small particles and the gas tend to diverge and pass around the interceptor.
Gravity settling chambers
This is a simple particulate collection device using the principle of gravity to settle the particulate matter in a gas stream passing through its long chamber. The primary requirement of such a device would be a chamber in which the carrier gas velocity is reduced so as to allow the particulate matter to settle out of the moving gas stream under the action of gravity. This particulate matter is then collected at the bottom of the chamber. The chamber is cleaned manually to dispose the waste.
Settling chambers discussed above are not effective in removing small particles. Therefore, one needs a device that can exert more force than gravity force on the particles so that they can be removed from the gas stream. Cyclones use centrifugal forces for removing the fine particles. They are also known as centrifugal or inertial separators.
The cyclone consists of a vertically placed cylinder which has an inverted cone attached to its base. The particulate laden gas stream enters tangentially at the inlet point to the cylinder. The velocity of this inlet gas stream is then transformed into a confined vortex, from which centrifugal forces tend to drive the suspended particles to the walls of the cyclone. The vortex turns upward after reaching at the bottom of the cylinder in a narrower inner spiral. The clean gas is removed from a central cylindrical opening at the top, while the dust particles are collected at the bottom in a storage hopper
The efficiency of a cyclone mainly depends upon the cyclone diameter. For a given pressure drop, smaller the diameter, greater is the efficiency, because centrifugal action increases with decreasing radius of rotation. Centrifugal forces employed in modern designs vary from 5 to 2500 times gravity depending on the diameter of the cyclone. Cyclone efficiencies are greater than 90 per cent for the particles with the diameter of the order of 10 µ. For particles with diameter higher than 20 µ, efficiency is about 95 per cent. The efficiency of a cyclone can be increased by the use of cyclones either in parallel or in series.
The advantages of cyclones are low initial cost; simple in construction and operation; low pressure drop; low maintenance requirements; continuous disposal of solid particulate matter, and use of any material in their construction that can withstand the temperature and pressure requirements.
The disadvantages of cyclones include low collection efficiency for particles below 5-10 µ in diameter; severe abrasion problems can occur during the striking of particles on the walls of the cyclone, and a decrease in efficiency at low particulate concentration.
Typical applications of cyclones are:
For the control of gas borne particulate matter in industrial operations such as cement manufacture, food and beverage, mineral processing and textile industries
To separate dust in the disintegration operations, such as rock crushing, ore handling and sand conditioning in industries
To recover catalyst dusts in the petroleum industry
To reduce the fly ash emissions
Scrubbers are devices that remove particulate matter by contacting the dirty gas stream with liquid drops. Generally, water is used as the scrubbing fluid. In a wet collector, the dust is agglomerated with water and then separated from the gas together with the water.
The simpler types of scrubbers with low energy inputs are effective in collecting particles above 5-10 µ in diameter, while the more efficient, high energy input scrubbers will perform efficiently for collection of particles as small as 1-2 µ in diameter.
The advantages of scrubbers are low initial cost; moderately high collection efficiency for small particles; applicable for high temperature installations; they can simultaneously remove particles and gases, and there is no particle re-entrainment.
The disadvantages of scrubbers are high power consumption for higher efficiency; moderate to high maintenance costs owing to corrosion and abrasion; and wet disposal of the collected material.
The scrubbers are used in a variety of applications, some of them are:
They’re particularly useful in the case of a hot gas that must be cooled for some reason.
If the particulate matter is combustible or if any flammable gas is present, even in trace amounts, in the bulk gas phase, a scrubber is preferred to an electrostatic precipitator.
Scrubbers can be used when there are wastewater treatment systems available on the site, with adequate reserve capacity to handle the liquid effluent.
Scrubbers are also used when gas reaction and absorption are required simultaneously with particulate control.
Fabric filtration is one of the most common techniques to collect particulate matter from industrial waste gases. The use of fabric filters is based on the principle of filtration, which is a reliable, efficient and economic methods to remove particulate matter from the gases. The air pollution control equipment using fabric filters are known as
A bag house or a bag filter consists of numerous vertically hanging, tubular bags, 4 to 18 inches in diameter and 10 to 40 feet long. They are suspended with their open ends attached to a manifold. The number of bags can vary from a few hundreds to a thousand or more depending upon the size of the bag house. Bag houses are constructed as single or compartmental units. In both cases, the bags are housed in a shell made of rigid metal material.
The advantages of a fabric filter are high collection efficiencies for all particle sizes, especially for particles smaller than 10 micron in diameter; simple construction and operation; nominal power consumption; and dry disposal of collected material.
The disadvantages of a fabric filter are operating limits are imposed by high carrier gas temperatures, high humidity and other parameters; high maintenance and fabric replacement costs; large size of equipment; and problems in handling dusts which may abrade, corrode, or blind the cloth.
Fabric filters find extensive application in metallurgical industry, foundries, cement industry, chalk and lime plants, brick works, ceramic industry, flour mills, etc.
The latest of them, electrostatic precipitators (ESP) are particulate collection devices that use electrostatic force to remove the particles less than 5 micron in diameter. It is difficult to use gravity settlers and cyclones effectively for the said range of particles. Particles as small as one-tenth of a micrometer can be removed with almost 100 per cent efficiency using electrostatic precipitators.
Collection principle of ESP
Dust particles suspended in the gas are electrically charged through discharge electrodes (DE) and migrate under the influence of a strong electrical field (by Transformer Rectifier ie TR Set) towards the collecting electrode (CE) where they are deposited. The collecting electrodes are connected to the Earth via the precipitator casing. The discharge electrodes are suspended from insulators and have negative polarity. They carry 72-110 kV DC as per precipitator design. In the immediate vicinity to discharge electrodes Corona discharges are produced due to high field strength and electrons are set free. The negative gas ion produced charge the dust particles in the flue gas passing through the ESP which migrate under the influence of the electrical field towards CE, where they release part of their charge and are captured. Through various rapping systems (mostly Tumbling Hammer or MIGI) the dust particles come out of the hoppers through dust discharge valve and are discharged through suitable conveyors to ash pit/bin.
For sizing of ESP, following parameters are given:
Flue gas flow rate in m3/sec at discharge from boiler, ie at the inlet of ESP
ESP inlet dust concentration (IDC) in mg/Nm3
Desired emission limit or maximum outlet dust concentration (ODC) in mg/Nm3
ESP dust collection can be calculated by the Deutsch formula:
E = 1 - e -?f
Where E = Efficiency in % [(IDC – ODC)/ * 100%]
e = Base of the natural Logarithm
f = Specific collection area in m2/m3/sec.
= Migration Velocity in
m/sec. ('?' is the main part of the equation which is determined from experience and reflects the dust collection efficiency).
The degree of separation therefore depends exponentially on the migration velocity and specific collection area i.e. total collection area in m2 (projected) / gas flow rate in m3/sec. The greater the '?f' value the higher the dust collection efficiency. A great 'f' value simply means more collection area which in the long run will be the safest way with regard to ageing and low emission of the ESP. The alignment is very important ie the spacing between Emitting and Collecting Electrode must be same over the whole precipitator in order to maintain the high voltage. Optimum gas distribution is also equally important.
The advantages of using the ESP are high collection efficiency; particles as small as 0.1 micron can be removed; low maintenance and operating cost; low pressure drop (0.25-1.25 cm of water); satisfactory handling of a large volume of high temperature gas; treatment time is negligible (0.1-10s); cleaning is easy by removing the units of precipitator from operation; and there is no
limit to solid, liquid or corrosive chemical usage.
The disadvantages of using the ESP are:
High initial cost
Space requirement is more because of the large size of the equipment.
Possible explosion hazards during collection of combustible gases or particulate.
Precautions are necessary to maintain safety during operation. Proper gas flow distribution, particulate conductivity and corona spark over rate must be carefully maintained.
The negatively charged electrodes during gas ionization produce ozone.
The important applications of ESPs are cement industry, pulp and paper mills, metallurgical industry, chemical industry, petroleum industry, carbon black industry, electric power industry, etc.
About the Author:
RK Sikdar is the Managing Director of Air Control Engineering Pvt Ltd. He has worked for 35 years in project, vendor development & procurement, marketing, contracts, works and general management. Sikdar was also Chairman of Eastern India of Indian Electrical & Electronics Manufacturers' Association (IEEMA). Air Control Engineering Pvt Ltd undertakes turn-key projects of air pollution control and industrial air handling systems, such as electrostatic precipitator, bag filter, cage vent, industrial fan and other auxilliaries. It also has expertise in offering optimum solution related to gas cleaning and handling system. For more details, contact on Tel: 033 2262 8767 or Mob: 098367 88787 or email@example.com