The following information provides brief details on the most common types of control devices for removing particulate matter.
1. Gravity settlers
A gravity settler removes particulate matter by slowing the velocity of discharge so that the solid particulates will drop out of the gas stream the gravity settler is a simple and inexpensive device, often used as a precleaner to remove coarse particles, in conjunction with other air pollution control devices. Typically gravity settlers are more effective on particle
Cyclone collectors employ centrifugal force instead of gravity to separate particles from the gas stream. Because centrifugal force, can be generated, that is several times greater than gravitational force, particles that are much smaller can be removed.
A cyclone consists of a cylindrical shell with tangential gas entry and an inverted cone attached to the base the tangential entry imparts a whirling motion which causes the particles to be thrown towards the wall on which they collect and slide towards the conical collector. The collection efficiency of a cyclone depends on the magnitude of centrifugal force exerted on the particles.
3. Wet Collectors or Scrubbers
Scrubber is a collection device in which the particles are washed out of the gas flow by a water spray. In general, particles are removed from the gas stream by one or a combination of the following mechanisms.
1. Inertial impaction. A particle because of its inertia may be unable to adjust to the rapidly changing curvature of streamlines in the vicinity of a droplet and may cross the streamlines to hit the droplet.
2. Direct interception. The gas flow path in which the particle is flowing may pass sufficiently close to the water droplet making contact with the surface and be collected.
3. Diffusion. Sub-micron particles do not flow in streamlines but wobble randomly due to collisions with molecules of gas. Random Brownian motion causes incidental contact between the particle and the droplet. At low flow velocity, particles spend more time near droplet surfaces, thus enhancing diffusion collection.
4. Electrostatic attraction. Particles having opposite charges are attracted to each other and get captured.
5. Gravitational force. Some of the larger particles are removed by gravitational pull. Gravity is important for larger particles at low flow velocities.
Types of scrubbers
(a) Spray towers
(b) Packed bed towers
(c) Venturi-scrubbers. Spray towers
Spray towers are low-cost scrubbers that can be used to remove both gaseous and particulate contaminants. The spray tower is a vertical column in which the gas passes upwards and the liquid solvent passes downwards. Liquid droplets are formed by the process of atomization. As the gas flows upwards, entrained particles collide with liquid droplets sprayed across the flow passage. By the time the reaches the top of the column, most of the gaseous contaminants dissolve in the solvent and the particulates get collected on the liquid droplets. The loaded solvent settles by gravity at the bottom of the chamber
In general, the smaller the droplet size and the greater the turbulence, the more the chance for absorption of the gas production of fine droplets requires the use of high-pressure spray nozzles. If water is the solvent, the application is limited to a few inorganic gases such as NH3, Cl2 and S02.
4. Electrostatic precipitators (ESP)
Electrostatic precipitation uses the force of an electric field on electrically charged particles to segregate particulate matter from a waste gas stream. The particles are deliberately charged and passed through an electric field causing the particles to migrate towards an oppositely charged electrode, which acts as a collection surface. The accumulated deposits are removed from the collection surface from time to time.
The ESPs are of two types, i.e., pipe type and plate type. The plate type consists of parallel vertical grounded steel plates called collecting electrodes together with an array of parallel discharge wires mounted on a plane midway between the plates the dirty gas with particulates passes between the plates. As the gas flows in between parallel plates, electrostatic forces cause the particulates to migrate to the collector electrode where they stick. The clean gas then emerges on the other side. The collected particulates are removed from the collector electrode by either water washing or rapping it periodically.
5. Fabric filters
Filter is a membrane (cloth, wire mesh, etc.) with holes smaller than the dimensions of the particles to be retained. As the fine particles are caught on the sides of the holes of the filter, they tend to close the holes and make them smaller. As the amount of collected particles increases, the cake of collected materials becomes the filter and the filter medium (fabric) now supports the cake.
When a dirty gas stream flows through the filter medium, the solid particles deposit on the face of the medium. The cleaned gas flows through both the filter cake and the medium, and has a small change in velocity, because the pressure drops, causing the gas to expand.
As the most commonly used surface filters have an enclosing sheet metal structure in the shape of a house, it is also called a bag house. It consists of a number of cylindrical cloth bags that are closed at the top and they hang from a support.
The bag is made of either natural or synthetic fibres. Synthetic fibres are widely used for filtration because of their low cost, better temperature and chemical resistance characteristics, and small fibre diameter. Bag life varies between 1 and 5 years. The hopper at the bottom serves as a collector for dust.
The gas enters through the inlet chamber and larger particulates fall into the hopper due to gravity. The gas then flows upwards into the filter bags and then outwards through the fabric, leaving the particulate matter as a cake on the inside of the bags. A dust cake builds on the inside bag surface during filtration. Efficiency during cake formation (pre-coat) is low, but increases as the cake is formed, until a final efficiency of 99% is obtained.
The accumulation of particulates increases the air resistance of the filter and therefore, filter bags should be cleaned periodically. They can be cleaned by rapping, shaking or vibration, or by reverse air flow, causing the filter cake to be loosened and to fall into the hopper below.
While selecting a filter medium for bag houses, the following properties are taken into consideration:
(i) gas temperature
(ii) gas composition
(iii) gas flow rate
(iv) size of the particulate matter
(v) concentration of particulate matter.
The commercially available fabric materials are cotton, wool, nylon, Dacron, polypropylene, fibre glass, etc.
Control of Gaseous Contaminants
Major air pollutants are gases such as carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx) and volatile organic compounds (VOCs). In general, the concentration of pollutants in waste air stream is relatively low, but the emissions can still exceed the regulatory limits. Removal of gaseous pollutants can be achieved by the following methods:
Absorption, also called scrubbing, involves transferring pollutants from a gas phase to a contacting solvent. This is a mass transfer phenomenon in which the gas dissolves in the liquid. Mass transfer is a diffusion process wherein the pollutant gas moves from points of higher concentration to points of lower concentration. The removal of the pollutant gas takes place in three steps:
(i) diffusion of the pollutant gas to the surface of the liquid
(ii) transfer across the gas-liquid interface (dissolution)
(iii) diffusion of the dissolved gas away from the interface into the liquid.
Equipments such as spray towers, packed columns, cyclone scrubbers and venture scrubbers are employed to absorb pollutant gases. Absorption is used extensively in the separation of corrosive and hazardous pollutants from waste gases.
Spray towers can handle a fairly large volume of gas with relatively little pressure drop and reasonably high efficiency of removal. Spray towers are also effective for dual removal of particulate and gaseous contaminants.
Generally absorbed materials are dissolved into the absorbent, like sugar in water, whereas adsorbed materials are attached onto the surface of a material, like dust on a wall.
This section deals with adsorption which is also a mass transfer process in which the gas is bonded to a solid. Adsorption is a surface phenomenon. The mechanism of adsorption can be classified as either physical adsorption or chemisorption. The bond may be physical or chemical. Electrostatic force (van der Waals) holds the pollutant gas when physical bonding is significant. Chemical bonding is by reaction with the surface. Activated carbon, molecular sieves, silver gel and activated alumina are the most common adsorbents. The common property of the adsorbent is a large active surface area per unit volume. They are very effective for hydrocarbon pollutants. This process is particularly suitable when the pollutants are:
(ii) Unstable in liquid
(iii) Low in concentration
A compound will condense at a given temperature if the partial pressure is increased until it is equal to or greater than its vapour pressure at that temperature. If the temperature of the gas mixture is reduced to its saturation temperature, its vapour pressure equals its partial pressure and condensation will occur.
We can remove VOCs from the gas stream by cooling it to a lower temperature so that most of the VOCs are condensed as liquid and then separated from the gas by gravity.
Some air pollutants contain materials such as compounds of carbon, hydrogen, oxygen and sulphur which, when burned, produce less harmful materials.
Carbon monoxide which is harmful to health and benzene which is a reactive hydrocarbon are converted to harmless materials.
The combustion equipment used to control pollution emission is designed to promote oxidation reactions as close as possible to completion, leaving a minimum of unburned compounds. For proper combustion to occur, it is necessary to have a proper combination of four basic parameters such as oxygen, temperature, turbulence and time. Depending upon the contaminant being oxidized, the combustion method can be classified as:
1. Direct flame combustion
2. Thermal combustion
3. Catalytic combustion.
1. Direct flame combustion
In direct flame combustion, the waste gases are burned directly in an incinerator, with or without addition of supplementary fuel. In some cases, heat value and oxygen content are sufficient to allow them to burn on their own. In other cases, introducing air and adding a small amount of supplementary fuel will bring the gaseous mixture to its combustion point.
2. Thermal combustion
Thermal incinerators are used when the concentration of combustible gaseous pollutants is too low to make direct flame combustion feasible. Generally the waste gas is preheated, and the preheated gas is directed into a combustion zone, equipped with a burner supplied with supplementary fuel. The temperature of the operation depends upon the nature of the pollutant in the waste gas.
3. Catalytic combustion
A catalytic combustion unit generally consists of a preheating section and a catalytic section Catalytic combustion process has been used to control SOx, NOx, HC and CO. In catalytic removal of S02, the gas is thoroughly cleaned in a dust precipitator, and then passed through an S02 oxidation catalyst vanadium pentoxide at high temperature (454°C). The process yields sulphuric acid mist. The process has an estimated capacity for removing 90% of original S02 present in the flue gas.
The biological treatment of VOCs and other pollutants has received increasing attention in recent years. Biofiltration involves the removal and oxidation of organic compounds from contaminated air by beds of compost, peat or soil. This treatment offers an inexpensive alternative to conventional air treatment technologies such as adsorption and incineration.
The schematic representation of biofiltration is A biofiltration process uses microorganisms immobilised in the form of a bio film layer on adsorptive materials such as compost, peat or soil. As the contaminated vapour stream passes through the filter bed, pollutants are transferred from the gaseous phase to the liquid bio layer and oxidised. Bio filters can be used for treating vapour containing about 1500 mg/m3 of biodegradable VOCs.