Equipments for the control of particulate pollutants are broadly divided into 5:
1. Gravity settling chamber.
2. Mechanical collectors.
3. Fabric filters.
4. Particulate wet scrubbers.
5. Electrostatic precipitators
Each of these five categories include several types of devices. A brief description of the five main classes and the various devices included in them is given below.
2. BRIEF DESCRIPTION
2.1 Gravity Settling Chamber
As the name implies , this category of control devices relies upon gravity settling to remove particles from the gas stream. These are usually used as precleaners for more efficient collectors.
2.2 Mechanical Collectors
Mechanical collectors use the inertia of the particles for collection. The particulate-laden gas stream is forced to spin in a cyclonic manner. The centrifugal force on particles in a spinning stream is much greater than gravity and hence cyclonic mechanical collectors are very effective.
The prominent types of mechanical collectors are:-
1. Large diameter cyclones
2. Small diameter multi-cyclones
2.3 Particulate Wet Scrubbers
Particulate wet scrubbers utilize liquids to assist in the removal of particulates from the carrier gas stream. The object of the scrubber is to transfer suspended particulate matter in the gas to the scrubbing liquid which can be readily removed by the gas clearing device. Following are the commonly used types of scrubbers:-
1. Spray towers
2. Venturi scrubber
3. Impingement plate scrubber
2.4 Fabric Filters
Fabric filters consist of a tubular bag or an envelope, suspended or mounted with open ends in such a way that when the dirty gas passes through it, the particulate matter is arrested on the inside fabric surface of the bag. The whole system is generally known as baghouse.
Some of the popular types are:-
1. Reverse air baghouse
2. Reverse jet baghouse
2.5 Electrostatic Precipitation
Electrostatic precipitation is a physical process by which particles suspended in a gas stream are charged electrically and, under influence of electrical field, separated from the gas stream. Electrostatic precipitators are used to remove aerosol from large volumes of gas.
3. APPLICATIONS OF THE VARIOUS TYPES OF CONTROL EQUIPMENTS
3.1Gravity Settling Chamber
These devices are used in pre-treatment of gases before any other type of collector.
3.2 Mechanical Collector
Mechanical collectors are used if the particulate matter is primarily dry.
3.3Particulate Wet Scrubber
These are used if there is a high concentration of wet and/or sticky particulate matter.
Filtration baghouses are used if the particulates are dry and if these particulates, i.e. the gases or vapors in the gas stream are combustible or explosive.
Electrostatic precipitators are used for dry particulate matter and for vapors which are combustible. ESPs’ are usually widely applicable.
4.MERITS AND DEMERITS OF THE DIFFERENT EQUIPMENT TYPES
4.1 Gravity Settling Chamber
· Low initial cost
· Simple construction
· Low maintenance cost
· Low pressure drop
· Large space requirement
· Only larger sized particles can be collected
4.2 Mechanical Collectors
· Low cost
· Cyclonic collectors can handle a very wide range of physical and chemical conditions of operation, as compared to other devices.
· During cyclonic separation , the carrier gas rotational velocity may exceed several times the average inlet gas velocity.
4.3 Electrostatic Precipitator
· Pressure drop is less
· Power requirement is less
· Large gas volumes can be handled
· Ability to operate over a wide range of conditions
· High efficiency to remove very small particles even as small as 0.1 micron.
· Negligible treatment time
· High initial cost
· Special safety precautions required
· Large space required
· Possible explosion hazards when combustible particulates or gases enter.
4.4 Fabric Filters
· High collection efficiency, for particles smaller than 10 micron
· Low pressure drop
· Nominal power consumption
· Simple operational procedure
· Large size
· High construction cost
· High maintenance cost
4.5 Particulate Scrubbers
· Can be used on sources that have potentially explosive gases or particulate matter
· Small space requirements
· Very compact and can be retrofitted into existing plants.
· They require makeup water to replace the water vaporized into the gas stream and lost to plurge liquid and sludge removed from the scrubber system
· These devices generate a waste stream that needs to be treated properly
5.STUDY ON A FEW PROMINENT DEVICES
5.1Gravity Settling Chamber
A settling chamber consists of a huge box installed in the ductwork. The sudden expansion of size of the chamber reduces the speed of the dust filled airstream and heavier particles settle out. Simple chambers are simple in design and can be manufactured from almost any material.
5.2Large Diameter Cyclone
This is a mechanical collector. A typical large diameter cyclone system is shown in the adjoining figure. The gas stream enters the cyclone tangentially and creates a weak vortex of spinning gas in the cyclone body. Large diameter particles move toward the cyclone body wall and then serttle into the hopper of the cyclone.
The cleaned gas turns and exits the cyclone. Large diameter cyclones are used to collect particles ranging in diameter from one sixteenth inch to more than 6 inches.
5.3Spray Tower Scrubbers
This is the simplest and most common particulate wet scrubber in commercial service. Sets of spray nozzles located at near the top of the scrub vessel generate water droplets that impact with particles in the gas stream as the gas stream moves upwards.
A typical spray tower is shown in the figure.
5.4Reverse Air Type Fabric Filter
This is one of the major categories of fabric filters. It is used mainly for large industrial sources. A typical reverse air type fabric filter is shown in the adjoining figure.
In this type of unit, the particle laden gas stream enters from the bottom and passes into the inside of the bags. Filtered gas passes through the bags and is exhausted from the unit. When cleaning is necessary, dampers are used to isolate a compartment of bags from the inlet gas flow. Then some of the filtered gas passes in the reverse direction (from the outside of the bag to the inside) in order to remove some of the dust cake. The gas use for reverse cleaning is then re-filtered and released.
The figure attached shows the scale of a typical electrostatic precipitator used at a coal-fired boiler.
Essentially, all of these units are divided into a number of separately energized areas that are termed fields. Most precipitators have between 3 and 10 fields in series along the gas floor path. On large units, the precipitators are divided into a number of separate, parallel chambers, each of which has an equal number of fields in series.
The gas enters these fields and a high voltage electrical charge is applied to the small diameter electrode in the center of the gas passage. The large vertical surfaces on both sides of the electrode are electrically grounded collection plates. The particles in the gas stream, which is moving horizontally through the unit, become charged and then move to either side.
Since pollution these days, is at an alarmingly high level, the technologies and equipments we use to check and control pollution should also be state-of-the-art.
Equipments available for the control of particulate pollutants are widely effective and user-friendly. These equipments are with no doubt, a vital part of today’s world, n order to providea pollution free atmosphere.
1. Dust Collector, Wikipedia, < www.wikipedia.org >
2. Air pollution control devices, Gary. M. Hutter
3. Environmental Protection Agency – Reference sheet
4. Basic Concepts in Environmental Sciences, Gerard Sinclair
5. Air Pollution Control Equipment Selection Guide, Kenneth Schiffer