Air Pollution & Its Control Abstract

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Chapter 1

AIR POLLUTION AND ITS CONTROL

B. C. Meikap, Ph.D. Chemical Engineering Department, Indian Institute of Technology, Kharagpur, West Bengal, India School of Chemical Engineering, University of Kwazulu-Natal, Howard College Campus, Keng George V Avenue, Dunban, Pin-4041, South Africa [email protected] Akhila Kumar Swar, Ph.D. State Pollution Control Board, Orissa Unit — VIII, Bhubaneswar 751012, India [email protected] Chittaranjan Mohanty, Ph.D. Civil Engineering Department, Veer Surendra Sai University of Technology, Burla, Sambalpur, Orissa, India [email protected] J. N. Sahu, Ph.D. Chemical Engineering Department, University Malaya, Kuala Lumpur-50603, Malaysia jay [email protected] Yung-Tse Hung, Ph.D., P.E., DEE Department of Civil and Environmental Engineering, Cleveland State University, 16945 Deerfield Dr. Strongsville, Ohio 44136-6214, USA [email protected], [email protected]

Abstract Chemical and allied process industries emit huge air pollutants and causes severe degradation to the environment. The control option of air pollutants greatly depends on the nature of pollutants and type of sources. There is a tremendous demand for air 1

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pollution control in fertilizer, coal-fired thermal power plants, petroleum refinery, cement, steel industries, etc. to meet the stringent demands. The topics covered are air pollution sources, nature, transport, pollution problems encountered, and control technologies usually used with design consideration. Keywords: Air pollution, particulate matters, bag filters, electro static precipitator, environmental pollution, clean technology.

1.

Introduction

The “air pollutant” means any solid, liquid, or gaseous substance present in the atmosphere in such concentration as may be or tend to be injurious to human beings or other living creatures or plants, property, or environment. Air pollution means the presence of air pollutants in the atmosphere of any air. The atmosphere contains a number of air pollutants generated from either natural or anthropogenic sources. Pollutants are released into our atmosphere by many methods. Smokestacks from factories have been increasing due to the rise of industry in the last century. These are known as stationary sources. Mobile sources, on the other hand, include exhaust from motor vehicles, trains, airplanes, etc. The pollution from these sources is measured by the amount emitted as well as by the amount of pollution in the ambient air. Both of these sources contribute significant amounts of contaminants into the air that we breathe. All such pollutants are called primary pollutants.

1.1.

Source of Air Pollutants1−5

There are many natural sources of air pollution such as eruption of volcanoes, biological decay, and lightning-caused forest fire. Naturally, the Earth already has its own air pollution loading. However, industrialization or just everyday routines has become added burden to the existing air pollution loading. Most air pollutants originate from human-made sources, including mobile sources (e.g., cars, trucks, buses, ships, train, airplanes, aircrafts, etc.) and stationary sources (e.g., factories, power plants, fertilizer, petroleum refinery, cement, steel industries, etc.), as well as indoor sources (e.g., building materials and activities such as cleaning). Some general pollutants are carbon monoxide, sulfur dioxide, nitrogen oxide, carbon dioxide, ozone, lead, particulate matter (PM), and synthetic compounds (i.e., chlorofluorocarbons (CFCs)]. They are discussed in detail below.

1.1.1. Carbon Monoxide (CO) It is a colorless, odorless gas formed when carbon in fuel is not burned completely. Motor vehicle exhaust contributes about 60% of all CO emissions nationwide (Latest Finding on National Air Quality 2002). Other nonroad engines and vehicles (such

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as construction equipment and boats) contribute about 22% of all CO emissions nationwide. Higher levels of CO generally occur in areas with heavy traffic congestion. In cities, 95% of all CO emissions may come from motor vehicle exhaust. Other sources of CO emissions include industrial processes (such as metal processing and chemical manufacturing), residential wood burning, and natural sources such as forest fires. Woodstoves, gas stoves, cigarette smoke, and unvented gas and kerosene space heaters are sources of CO indoors. The highest levels of CO in the outside air typically occur during the colder months of the year when CO automotive emissions are greater and nighttime inversion conditions are more frequent. In inversion conditions, the air pollution becomes trapped near the ground beneath a layer of warm air.

1.1.2. Sulfur Oxides (SOx ) These are colorless gases formed by burning sulfur. SOx gases are formed when fuel containing sulfur, such as coal and oil, is burned, and when gasoline is extracted from oil or metals are extracted from ore. Sulfur dioxide (SO2 ) is the criteria pollutant that is the indicator of sulfur oxide concentrations in the ambient air. SO2 dissolves in water vapor to form acid and interacts with other gases and particles in the air to form sulfates and other products that can be harmful to people and their environment.

1.1.3. Nitrogen Oxides (NOx ) These compounds pose problems, for the most part, in the forms of nitrogen oxide (NO) and nitrogen dioxide (NO2 ). They combine with hydrocarbons and other volatile organic compounds (VOCs) in the presence of ultraviolet (UV) sunlight to produce photochemical smog, mainly ozone (O3 ), which has adverse health effects.

1.1.4. Carbon Dioxide (CO2 ) This chemical, a byproduct of most fuel combustion, is a greenhouse gas and a major contributor to global warming.

1.1.5. Ozone (O3 ) It is a gas composed of three oxygen atoms. It is a colorless compound that has an electric discharge-type odor. It is a unique criteria pollutant in that it is exclusively a secondary pollutant. It is not usually emitted directly into the air, but at ground level it is created by a chemical reaction between oxides of nitrogen (NOx ) and VOCs in the presence of heat and sunlight. The concentration of ozone in a given locality is influenced by many factors, including the concentration of NO2 and VOCs in the area, the intensity of the sunlight, and the local weather conditions. Ozone and the chemicals that react to form it can be carried hundreds of miles from their origins, causing air pollution over wide regions.

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1.1.6. Synthetic Compounds (i.e., CFCs) Until banned by new provisions of the Montreal Protocol, these compounds were produced and released by industries for air conditioning, insulation, cleaning fluids, etc. Some have been known to contribute to the destruction of stratospheric ozone. CFCs are lowering the average concentration of ozone in the stratosphere. Since 1978 the use of CFCs in aerosol cans has been banned in the United States, Canada, and most Scandinavian countries. Depending on the type, CFCs stay in the atmosphere from 22 to 111 years. CFCs move up to the stratosphere gradually over several decades. Under high-energy UV radiation, they break down and release chlorine atoms, which speed up the breakdown of ozone (O3 ) into oxygen gas (O2 ).

1.1.7. Lead (Pb) It is a metal found naturally in the environment as well as in manufactured products. Because of unique physical properties that allow it to be easily formed and molded, lead has been used in many applications. The major sources of lead emissions have historically been motor vehicles (such as cars and trucks) and industrial sources. Due to the phase out of leaded gasoline, metal processing is the major source of lead emissions to the air today. The highest levels of lead in the air are generally found near lead smelters. Other stationary sources are waste incinerators, utilities, and lead-acid battery manufacturers.

1.1.8. Particulate Matter It is the general term used for a heterogeneous mixture of solid particles and liquid droplets found in the air, including dust, dirt, soot, smoke, and liquid droplets. Particles can be suspended in the air for long periods of time. Some particles are large or dark enough to be seen as soot or smoke. Others are so small that individually they can only be detected with an electron microscope. PM can be a primary or secondary pollutant. “Primary” particles, such as dust or black carbon (soot), are directly emitted into the air. They come from a variety of sources such as cars, trucks, buses, factories, construction sites, tilled fields, unpaved roads, stone crushing, and burning of wood. “Secondary” particles are formed in the air from the chemical change of primary gaseous emissions. They are indirectly formed when gases from burning fuels react with sunlight and water vapor. These can result from fuel combustion in motor vehicles, at power plants, and in other industrial processes. PM2.5 describes the “fine” particles that are less than or equal to 2.5 µm in diameter. PM10 refers to all particles less than or equal to 10 µm in diameter (about one-seventh the diameter of a human hair).

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1.2.

5

Important Primary Pollutants

The primary pollutants are SOx, CO, NOx , Pb; hydrocarbons including the photochemically reactive aliphatic hydrocarbons, like alkenes and nonreactive alkenes, and carcinogenic substances mainly consisting of aromatic hydrocarbons; allergic agents like pollens and spores; and radioactive substances. These primary pollutants often react with one another or with water vapor in the presence of sunlight to form an entirely new set of pollutants, called secondary pollutants. These secondary pollutants are the chemical substances, which are produced from the chemical reactions of natural or anthropogenic pollutants or due to their oxidation, etc., caused by the energy of the sun. These new pollutants are often more harmful than the original basic chemicals that produce them.

1.3.

Important Secondary Pollutants

Once the smoke, containing air pollutants, is released into the atmosphere from a source like an automobile or a factory chimney, it gets dispersed into the atmosphere into various directions depending upon the prevailing winds and temperature and the pressure conditions in the environment. Generally secondary pollutants are sulfuric acid (H2 SO4 ), ozone (O3 ), formaldehydes, and peroxy–acyl-nitrate (PAN), etc.

1.4. Air Pollutant Transport and Dispersion4 Air pollution does not always stay where it was made. It can make its way around the Earth. This is called transport and dispersion and is very complex. There are many things that affect the way pollution is spread, including wind and atmospheric stability. Wind is caused by differences in pressure in the atmosphere. Wind can carry pollutants away from sources, and sometimes can bring pollutants to clean regions. Atmospheric stability is the up and down motion of the atmosphere. The air near the surface of the Earth is usually warmer in the day because it absorbs the sun’s rays. The warmer air at the surface rises and mixes with the cooler air in the upper atmosphere. This is known as convection. This movement also spreads the polluted air. Air is stable when warm air is above cool air. This is also called a temperature inversion. During a temperature inversion, air pollution released near the ground is trapped there and can only be moved by strong winds. When a temperature inversion happens over an industrial area, it usually causes smog. Other weather factors can affect air pollution, like solar radiation, precipitation, and humidity. Solar radiation helps make ozone a secondary air pollutant. Humidity and precipitation can also help create other dangerous secondary pollutants, like acid rain. Precipitation can also help by washing pollution from the air, removing PM.

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1.5. Air Quality Management — National Ambient Air Quality Standards (NAAQS)2 In 1970, President Nixon created the Environmental Protection Agency (EPA) by Executive Order. An executive order is an order issued by a government’s chief executive, intended to give attention to a certain law or body of laws and to direct federal agencies how to implement them. The formation of EPA marked a dramatic change in national policy regarding the control of air pollution. Whereas previous federal involvement had been mostly in advisory and educational roles, the new EPA emphasized stringent enforcement of air pollution laws. The EPA was assigned the daunting task of repairing the damage already done to the natural environment and establishing new criteria to guide Americans in making a cleaner environment a reality. A few weeks later, the United States Congress passed the Clean Air Act Amendments (CAAA) of 1970. The passage of the CAAA of 1970 marked the beginning of modern efforts to control air pollution. The CAAAmendments of 1970 serve as the principal source of statutory authority for controlling air pollution and establish the basic United States program for controlling air pollution. The CAA Amendments require EPA to set NAAQS for certain pollutants, to develop programs to address specific air quality problems, to establish EPA enforcement authority, and to provide for air quality research. These amendments placed the major responsibility for achieving NAAQS by 1975 on the states via their implementation plans. The EPA established the NAAQS for six common air pollutants, called criteria pollutants. The criteria pollutants are carbon monoxide (CO), nitrogen dioxide (NO2 ), sulfur dioxide (SO2 ), lead (Pb), PM, and ozone (O3 ). Ambient air is the air to which the general public has access, as opposed to air within a facility or at a smokestack. The NAAQS is based on comprehensive studies of available ambient air monitoring data, health effects data, and material effects studies. NAAQS regulates criteria pollutants by setting ambient air concentration and time standards and taking actions to attain these standards. Most pollutants regulated by the NAAQS have two limits. One limit, the “primary” standard, protects everyone including children, people with asthma, and the elderly from health risk. The other limit, the “secondary” standard, prevents unacceptable effects on the public welfare, e.g., unacceptable damage to crops and vegetation, buildings and property, and ecosystems. The primary and secondary standards for each of the criteria pollutants are shown in Table 1. Each NAAQS corresponds to a specific averaging time, and some pollutants have standards for more than one averaging time. The averaging time is the time period over which air pollutant concentrations are averaged for the purpose of determining attainment with the NAAQS.

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Table 1.

Primary and Secondary Standards for Each of the Criteria Pollutants.

Primary Standard (Health-Based) Pollutant Type of Average

Standard Level Concentration

PM10

Annual arithmetic mean 24-hour average not to be exceeded more than once per year on average over three years Spatial and annual arithmetic mean in area 98th percentile of the 24-hour average Maximum daily one-hour average to be exceeded no more than once per year averaged over three consecutive years Three-year average of the annual fourth highest daily eight-hour average Annual arithmetic mean

PM25

Oa3

NO2 SO2 CO

Lead

7

Annual arithmetic mean 24-hour average Eight-hours (not to be exceeded more than once per year) one hour (not to be exceeded more than once per year) Maximum quarterly average

Secondary Standard (Welfare-Based) Type of Average

Standard Level Concentration

50 µg/m3

—

150 µg/m3

—

Same as primary standard Same as primary standard

15 µg/m3

—

65 µg/m3

—

0.12 mg/L

—

0.08 mg/L

—

Same as primary standard

0.053 mg/L

—

0.03 mg/L 0.14 mg/L 9 mg/L

Three hours — —

Same as primary standard 0.50 mg/L — No secondary standard

Same as primary standard Same as primary standard Same as primary standard

35 mg/L

—

No secondary standard

1.5 µg/m3

—

Same as primary standard

Source: USEPA, 2007.2 a EPA is phasing out the one-hour, 0.12-mg/L standards (primary and secondary) and putting in place the eight-hour, 0.08-mg/L standards. However, the 0.12-mg/L standards will not be revoked in a given area until that area has achieved three consecutive years of air quality data meeting the one-hour standard.

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1.6.

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Effects on Health and the Environment6−8

Like photochemical pollutants, sulfur oxides contribute to the incidence of respiratory diseases. Acid rain, a form of precipitation that contains high levels of sulfuric or nitric acids, can contaminate drinking water and vegetation, damage aquatic life, and erode buildings. When a weather condition known as a temperature inversion prevents dispersal of smog, inhabitants of the area, especially children and the elderly and chronically ill, are warned to stay indoors and avoid physical stress. The dramatic and debilitating effects of severe air pollution episodes in cities throughout the world—such as the London smog of 1952 that resulted in 4000 deaths—have alerted governments to the necessity for crisis procedures. Even everyday levels of air pollution may insidiously affect health and behavior. Indoor air pollution is a problem in developed countries, where efficient insulation keeps pollutants inside the structure. In less developed nations, the lack of running water and indoor sanitation can encourage respiratory infections. Carbon monoxide, for example, by driving oxygen out of the bloodstream, causes apathy, fatigue, headache, disorientation, and decreased muscular coordination and visual acuity. Air pollution may possibly harm populations in ways so subtle or slow that they have not yet been detected. For that reason research is now underway to assess the long-term effects of chronic exposure to low levels of air pollution — what most people experience — as well as to determine how air pollutants interact with one another in the body and with physical factors such as nutrition, stress, alcohol, cigarette smoking, and common medicines. Another subject of investigation is the relation of air pollution to cancer, birth defects, and genetic mutations. A recently discovered result of air pollution is seasonal “holes” in the ozone layer in the atmosphere above Antarctica and the Arctic, coupled with growing evidence of global ozone depletion. This can increase the amount of UV radiation reaching the Earth, where it damages crops and plants and can lead to skin cancer and cataracts. This depletion has been caused largely by the emission of CFCs from refrigerators, air conditioners, and aerosols. The Montreal Protocol of 1987 required that developed nations signing the accord not exceed 1986 CFC levels. Several more meetings were held from 1990 to 1997 to adopt agreements to accelerate the phasing out of ozone-depleting substances.

2. Air Pollution Control The atmosphere, just like a river, do possesses self-cleansing properties, which continuously dilute/clean/remove the pollutants from the atmosphere under natural processes. So long as the pollutants discharged by human into the environment is lower than the natural cleansing capacity of the environment, we live comfortably without

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any air pollution problem. But as and when the discharged pollutants exceed the natural cleansing capacity, our environment becomes polluted, giving us diseases, spoiling our clothes, plants, eatables, buildings, etc.Attempts are then made to reduce the emission of pollutants from the automobiles or factories by adopting mechanical means, or by using high-rise chimneys for better dispersion and dilution of pollutants over a longer range of environment. The natural self-cleansing process of the environment, and the engineering measures adopted to artificially clean the industrial and vehicular gases, before they are emitted into the environment.

2.1.

The Natural Self-Cleansing Properties of the Environment

The various natural properties, which continuously clean the environment, automatically are: • • • • •

dispersion; gravitational settling with or without flocculation of particles; absorption includes washout and scavenging; rain washout; and adsorption.

2.2.

Dilution Method for Controlling Air Pollution from Stationary Sources (Factories)

The emitted smokes can be spread over a larger area, through the use of highrise chimneys, thereby transporting the pollutants over larger distance, and thus, reducing the pollution near the emission source. This method is largely adopted in developing countries, because the pollution is generally confined to a smaller environment near cities and industrial towns only. The neighboring environment, which is free from emissions, is thus made to share some of the pollutant burden, thereby causing somewhat equitable distribution of the pollutants in the surrounding area. Nevertheless, this method only reduces the concentration of pollutants at particular place(s), rather than reducing or removing the pollution load from the total environment, as a whole.

2.3.

Controlling Air Pollution from Stationary Sources by Installing Engineering Devices

In order to reduce the pollution load entering the environment from stationary sources, several measures may be taken; out of them, replacement of burning fuel by electricity or solar energy is by far the best method, as it will eliminate the very production of pollutants in the combustion process. Besides this, we can use better

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quality of fuels and efficient engines, for reducing pollution loads from emissions. Say for example, LPG (liquid petroleum gas) and LNG (liquefied natural gas) may be used in industries in place of coal, as they will produce much less pollutants in the emission. Replacement of old obsolete processes in industries with the new efficient processes may also lead to reduced pollution emissions. Besides such innovations and precautions, certain mechanical devices may be installed in the industrial processes, which may help in reducing the emission of pollutants. Such mechanical devices are generally divided into two categories: (i) those devices which help in reducing PM; and (ii) those devices which help in reducing gaseous pollutants. These devices are discussed below: Basically, four ways are available for control of emission of air pollutants discharged by the industries into the atmosphere to control their detrimental effects on the surrounding environment. These are: 1. Reduction of pollutant discharge at the source by the application of control equipment. 2. Reduction at the source through raw material changes, operational changes, or modification, or replacement of process equipment. 3. Dilution of the source emission using tall stacks. 4. Proper planning and zoning of industrial areas. But the most effective methods are reduction at the source by the application of control equipment and process control. Air pollution problems should be properly considered when an industry is designed and built to get the real benefit. But in most cases, air pollution control is an afterthought, and ways and means have to be devised to treat the polluted emissions leading to retrofitting problems. To remove the PM from flue gas, various types of control equipment are available. But to select the required equipment, certain basic data must be available: • • • • •

Quantity of gas to be treated and its variation with time. Nature and concentration of the PM to be removed. Temperature and pressure of the gas stream. Nature of the gas phase (for solubility and corrosive effects). Desired quality of the treated emission, i.e., removal efficiency of particulates required.

2.3.1. Objectives of Using Control Equipment • • • •

Prevention of nuisance. Prevention of physical damage to property. Elimination of health hazards to plant personnel and to the general population. Recovery of valuable waste products.

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• Minimization of economic losses through the reduction of plant maintenance. • Improvement of product quality.

2.4.

Particulate and Gaseous Pollutant Control Devices3−8

The important devices, which are used to control PM and gaseous pollutants, are gravitational settling chambers, cyclone, wet scrubbers (collectors) and Venturi scrubbers, fabric filters, electrostatic precipitators (ESPs), gas absorption, gas adsorption, condensation, incineration, biofiltration, etc.

2.4.1. Gravitational Settling Chambers The settling chamber is the simplest type of equipment used for the collection of solid particulates. It consists of a chamber in which the carrier gas velocity is reduced so as to allow the particulates to settle out of the moving stream under the action of gravity. The most common form is a long box-like structure, with an inlet at one end and an outlet at the other, set horizontally, often on the ground. It can be constructed from brick and concrete. The carrier gas is made to pass at low velocities. The solid particulates having higher density than the surrounding gas settle under the influence of gravity on the base of the chamber, from where they are removed through hoppers. The gas velocity must be sufficiently low (less than about 3 m/s to prevent re-entrainment of the settled particles; less than 0.5 m/s for good results). To minimize turbulence and ensure uniform velocity, curtains, rods, and wire mesh screens may be suspended in the chamber. The pressure drop through the settling chamber is usually small and consists mostly of entrance and exit losses. Installation costs are low because of the simple structure. A simplified analysis of the gravity settling chamber assumes that the solids move along the chamber with the velocity of the gas and also settle with Stoke’s velocity. A particle entering the chamber at the top will be collected by the chamber, if its settling time is the same (or less than) the time the gas takes to pass through the chamber. To achieve this, horizontal trays or shelves are sometimes incorporated in the chambers. The trays (plates) are fitted at about 1–3 cm height intervals as shown in Fig. 1. The increase in efficiency obtained by the insertion of horizontal trays is directly proportional to the number of trays. Even with such equipment, however, the minimum particle size which can be removed in practice is about 10 µ. Also, the use of this modified settling chamber is limited by difficulties in cleaning the closely spaced trays and by their tendency to warp during high-temperature operation.

2.4.1.1. Advantages • Low initial cost. • Simple construction.

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GRAVITATIONAL SETTLING CHAMBERS

GAS OUTLET GAS INLET

DUST COLLECTION HOOPERS

Figure 1.

Components Air inlet Mid casing Collection hopper Dust discharge devise Baffles or Louvers Air outlet

Schematics of Gravitational Settling Chambers.

Source: Kumar and Chenchaiah, 2006.4

• • • • •

Low maintenance cost. Low pressure drop. Dry and continuous disposal of solid particulates. It can be constructed out of almost any material. Temperature and pressure limitations imposed only by materials of construction used.

2.4.1.2.

Disadvantages

• Large space requirements. • Only comparatively large particles (definitely not less than 10 µ, if very dense and 40 µ, if of low density) can be collected.

2.4.1.3. Applications Industrial application of this equipment is limited. Settling chambers are used generally to remove particulates above 40 µ in diameter. However, fine materials such as carbon black and various metallurgical fumes form agglomerates which have enough mass to permit collection in settling chambers. Settling chambers are used widely for the removal of large solid particulates from natural draft furnaces, kilns, etc. They are also sometimes used in the process industries, particularly the food and metallurgical industries, as a first step in dust control. Because of simplicity of construction and low maintenance costs, gravity settling chambers have found quite widespread application as pre-cleaners for high-efficiency collectors. This reduces the inlet dust loadings to the second-stage cleaner and can remove large highly abrasive materials,

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thus reducing maintenance costs of high-efficiency equipment which is more subject to abrasive deterioration.

2.4.2. Cyclones The cyclonic separator is an important and popular type of dust-removal equipment. This class of separators is the most common of a general group of separators that are classified as centrifugal or inertial separators. It depends upon centrifugal force for its action. They produce a continuous centrifugal force as a means of exerting the greater inertial effects of the dispersoid. A cyclone collector can be defined as a structure without moving parts in which the velocity of an inlet gas stream is transformed into a confined vortex from which centrifugal forces tend to drive the suspended particles to the wall of the cyclone body. It consists of vertically placed cylinders which has an inverted cone attached to its base. The particulate-laden gas stream enters tangentially at the inlet point into the cylinder. The outlet pipe for the purified gas is a central cylindrical opening at the top. The dust particulates are collected at the bottom in a storage hopper (Fig. 2). The gas path generally follows a double vortex. First, the gas spirals downward at the outer periphery of the cylindrical portion, continues through the conical portion,

CYCLONE Outlet Inlet Vortex Finder Cylinder

Cone

Dust Discharger

Figure 2.

Schematic of a Cyclone Separator.

Source: Kumar and Chenchaiah, 2006.4

Cyclone body

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and reaches the bottom. The gas stream then moves upward in a narrower inner spiral, concentric with the first, and leaves through the outlet pipe. Due to the rapid spiraling movement of the gas, the dispersoids are projected toward the wall by the centrifugal force and then they drop by gravity to the bottom of the body, where they are collected in the storage hopper. During cyclonic separation, carrier gas rotational velocity may exceed several times the average inlet gas velocity. Cyclones are not sized from theory but are normally designed by set procedures. One set of sizes of various parts is as follows: The design factor having the greatest effect on the collection efficiency is the cyclone diameter. For a given pressure drop, smaller the diameter, higher is the efficiency, because centrifugal action increases with decreasing radius of rotation. Centrifugal forces employed in modern designs vary from 5 to 2500 times the gravity depending on the diameter of the cyclone. Cyclone efficiencies are greater than 90% for particles with diameter of the order of 10 µ. For particles with diameter higher than 20 µ, efficiency is about 95%. In practice, cyclonic separators may be designed for the satisfactory collection of particles over wide ranges of size and concentration, and over wide ranges of pressure and temperature. They can be operated at temperatures as high as 1000◦ C and pressures 500 atmospheres. They can handle gas volumes ranging from about 0.85 to 700 cubic meters per minute. Particles of diameter 50–10 µ can be easily separated. If particles are large (5–200 µ), a properly designed cyclone will perform adequately with moderate power requirement. For particles larger than 200 µ, a settling chamber is desirable, as it is more resistant to abrasion. An important precaution to be taken in operating a cyclone is to prevent gas leakage. A 15% gas leakage can bring down the efficiency to virtually zero.

2.4.2.1. Efficiency Cyclones are generally divided into two classes, “conventional” and “high efficiency.” High-efficiency cyclones merely have a smaller body diameter to create greater separating forces, and there is no sharp dividing line between the two groups. High-efficiency cyclones are generally considered to be those with body diameters up to about 0.25 m. One particular cyclone efficiency problem is the formation of eddies at the top of the unit where the dirty gas is introduced. The turbulence in the eddies causes some of the incoming dirty gas to be mixed with the outgoing clean gas stream. The effect of this problem can be minimized by removing the exit gas stream at a point below the zone of maximum turbulence. This is done by adding a central tube called a vortex finder which projects into the cyclone body below the turbulent entry region to confine the rising inner gas spiral. In general, increase in collection efficiency will result if there is an increase in any of the following: dust particle size, dust particle density, gas inlet velocity, inlet

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dust loading, cyclone body length (number of gas revolutions), and ratio of body diameter to gas outlet tube diameter. On contrary, collector efficiency will decrease if there is an increase in gas viscosity or density, cyclone diameter, gas outlet diameter, inlet width, and inlet area. To get increased efficiency, especially for the collection of smaller sized particles, a small-diameter, long taper cyclone should be used.

2.4.2.2. Tangential inlet and involute inlet — a comparison Inlets are of two types — tangential and involute. A straight tangential entry creates quite a bit of turbulence which will lead to back mixing and loss of efficiency even when a vortex finder is included in the cyclone design. On the other hand, the involute design (Fig. 2) brings in the gas parallel to the outer edge of the cyclone (tangent at that point) and leads it around a spiral for 180◦ to enter the top section with minimum turbulence. The other advantages of the involute design are better particle projection to the wall and a decrease in the loss of finer particles. The higher velocity in the central core can cause a slight increase in the Bernoulli effect, drawing more fine particles from the wall toward the central core. However, fine loses at the top and the pressure loss are much less and the efficiency is much higher for the involute design than for the tangential entry design.

2.4.2.3.

Operating problems

There are three important operating problems associated with cyclones. They are erosion, corrosion, and material build-up. • Erosion — Heavy, hard, sharp-edged particles, in a high concentration, moving at high velocity in the cyclone, continuously scrape against the wall and can erode the metallic surface unless suitable materials are used. • Corrosion — It is a problem if the cyclone is operating below the condensation point when reactive gases are present in the effluent stream. The best solution to any corrosion problem in a cyclone is to keep the product above the dew point. If the gas and dust are corrosive at low temperatures then perhaps the only alternative is to use a stainless alloy. • Build-up — Build-up of dust cake on the cyclone walls, especially around the vortex finder, at the ends of any internal vanes, and opposite the entry can become a severe problem. It occurs most frequently with hygroscopic dusts. In case the dust builds up on the wall of the cyclone cone, a simple solution is to pound on the cone with a sledge hammer. Another solution is to hang chains inside; this works, but reduces efficiency. A better solution is to flange a section between the dust-collecting hopper and the cyclone body. It can be removed periodically and scraped.

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2.4.2.4. Advantages of cyclones • • • • • • •

Low initial cost. Simple construction and operation. Low pressure drop. Low maintenance requirements. It has no moving parts. Continuous disposal of solid particulates. They can be constructed of any material which will meet the temperature and pressure requirements and the corrosion potential of the carrier gas stream.

2.4.2.5.

Disadvantages of cyclones

• Low collection efficiency for particles below 5–10 in diameter. • Equipment is subject to severe abrasive deterioration. • Decreasing collection efficiencies for decreasing dispersoid concentrations in the gas stream.

2.4.2.6. Applications Cyclones are widely used for the control of gas-borne particulates in such industrial operations as cement manufacture, feed and grain processing, food and beverage processing, mineral processing, paper and textile industries, and wood working industries. Cyclones are also used to separate dust in disintegration operations, such as rock crushing, ore handling, and sand conditioning in industries. They are also used in the recovery of catalyst dust in the petroleum industry, and in the reduction of fly ash emissions.

2.4.3. Multiple Cyclones For high efficiency at reasonable capacity, a battery of smaller cyclones operating in parallel is used in preference to a large single unit. This battery of smaller cyclones is known as “multiple cyclones.” It is also referred to as multi-clones, tubular cyclones, tubular collectors, and multiple collectors. Multiple cyclones consist of rows of parallel tubes, about 25 cm in diameter, with a common inlet chamber, a common outlet plenum, and a common dust-collection system as shown in Fig. 3. These chambers must be designed for a constant pressure drop in each, to avoid any channeling of the dirty gas to any particular single cyclone or group of cyclones. Such a situation leads immediately to overloading which increases the pressure drop in that section and causes further overloading in other parts of the system. The hopper (dust bin) must be designed with more care than that designed for a single

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Clean Gas Outlet Clean Gas Outlet

Raw Gas Inlet

Raw Gas Axial Inlet Baffle Plates Cyclone Body

High Efficiency Dust Collection Dust Discharge

Small Cyclones Common Hopper Dust Discharge

Figure 3.

Schematic of a Multicyclone Separator.

Source: Kumar and Chenchaiah, 2006.4

cyclone because the rate of dust collection will be generally much larger (because of greater centrifugal action due to smaller diameter of the cyclones). The velocities are much higher in the smaller cyclones which can cause re-entrainment problems to be more severe than in larger units. Continuous dust bin emptying and very often a deeper hopper are required to reduce the potential for dust re-entrainment. Multiple cyclones have good abrasion resistance, are compact, and have convenient inlet and outlet arrangements. They have low pressure drops and are highly efficient in collecting heavy particles. But their main disadvantage is they kplug (materials build-up). Multiple cyclones are used as collectors for cement clinkers, steel mill sinter, and stone dust in quarry and asphalt operations.

2.4.3.1.

Cyclone in series

The second cyclone serves to remove particles which were not collected in the first cyclone owing to a statistical distribution across the inlet, accidental re-entrainment due to eddy currents, and re-entrainment in the vortex core. The efficiency of the second cyclone will be less than that of the first cyclone. Cyclones operated in series are advantageous under the following circumstances: 1. Cyclones in series may be used to maintain a large degree of dust collection even if the dust outlet of the primary cyclone plugs. In such a condition, the secondary cyclone acts as a primary cyclone as far as collection efficiency is concerned.

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2. A primary large-diameter cyclone may be used to collect coarse material which would otherwise clog the smaller passages of more efficient small-diameter secondary cyclones. Cyclone collectors are often installed in series with other type of dust-collecting equipment. For example, to install high-efficiency cyclones ahead of ESPs. In an installation of this type, the cyclone exhibits an increased efficiency with an increase of gas load or dust load. On the other hand, the precipitator shows an increase in efficiency with a reduced gas load or dust load. Thus the characteristics of the two types of equipment compensate for each other, and this results in maintaining efficiency over a wide range of gas flow and dust loading. In general, if the particle-size distribution is such that most of the particulates can be removed in cyclones, then in such a case, very high overall efficiencies can be obtained by operating cyclones in series.

2.4.4. Wet Scrubbers In these devices, the flue gas is made to push up against a down failing water (liquid) current. The particulate matter mixes up with water droplets and, thus, falls down and gets removed. Water solutions, when replaced with other aqueous chemical solutions, like lime, potassium carbonate, slurry of MnO, and MgO, etc. do help in removing gaseous pollutants also from the flue gases. Venturi scrubbers offer high-performance collection of fine particles, usually smaller than 2–3 µm in diameter. They are particularly suitable when the particular matter is sticky, flammable, or highly corrosive. The high performance of the Venturi scrubbers is achieved by accelerating the gas stream to very high velocities, of the order of 60–120 m/s. The high-speed action atomizes the feed liquid, generally introduced in a uniform fashion across the throat through several low-pressure spray nozzles directed radically inward as shown in Fig. 4. The droplets accelerate in the throat section, and due to the velocity difference between the particles and the droplets the particles are impacted against the slowmoving droplets. This acceleration of the droplets is not likely to be complete at the end of the throat, so that particle collection continues to some extent into the diverging section of theVenturi. The gas liquid mixture is then directed to a separation device such as a cyclone separator where the droplets carrying the particulate matter are separated from the gas stream. The mechanisms affecting the collection of particulates in the Venturi scrubber are inertial impaction, diffusion, electrostatic — phenomenon and condensation and agglomeration. • Uses energy of moving gas stream to atomize liquid into droplets. • Gas flow through a narrow throat where high velocity is produced. • Water introduced into throat is sheared to droplets because of gas.

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Figure 4. Typical Industrial Venturi Scrubber. Source: Panda and Sharma, 2006.5

• Droplets become targets for particle collection. • Large-bottom inlet cyclonic separators are used for collecting particulate matter leaving Venturi. • Mist eliminators are used to remove liquid droplets before gas exit into atmosphere.

2.4.4.1. Advantages • They can be made to remove gaseous pollutants also, along with the remaining particulate matter. • Hot gases can be cooled down. • Corrosive gases can be removed and neutralized. • The separated gases through contact with aqueous chemicals may produce useful byproducts, as chemicals and fertilizers.

2.4.4.2.

Disadvantages

• A lot of wastewater, needing disposal, may be produced. • Maintenance cost is high, when corrosive materials are collected. • Wet outlet gases cannot rise high from the stack.

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• Poses freezing problems in cold countries. • Plume may sometimes be visible in the sky due to the presence of water vapor.

2.4.5. Fabric Filters Filtration is one of the most reliable, efficient, and economic methods by which particulate matter can be removed from gases. Filters can be broadly divided into the following two types. 1. Fabric or cloth filters. 2. Fibrous or deep-bed filters. In cloth filters, the filter is in the form of a fabric bag arrangement — tubular bags or as cloth envelopes — and is suitable for a dust loading of the order of 1 g/cu m. In the case of deep-bed filters, a fibrous medium like mats of wool, cellulose, etc., acts as a separator and the collection takes place in the interstices of the bed and is suitable for light dust loads of the order of 1 mg/cu m.

2.4.5.1.

Fabric or cloth filters

The most common type of fabric collector is the tubular type, consisting of tubular bags. A bag house or bag filter consists of numerous vertical bags 120–400 mm diameter and 2–10 m long. They are suspended with open ends attached to a manifold. The hopper at the bottom serves as a collector for the dust (Fig. 5). The gas entering through the inlet pipe strikes a baffle plate, which causes the larger particles to fall into a hopper due to gravity. The carrier gas then follows upward into the tubes and then outward through the fabric leaving the particulate matter as a “cake” on the inside of the bags. Efficiency during pre-coat forms part of the filtering medium, which helps in further removal of the particulates. Thus dust becomes the actual filtering medium! The bags, in effect, act primarily as a matrix to support the dust cake. The cake is usually formed in a matter of minutes, or sometimes even seconds. The accumulation of dust increases the air resistance of the filter, and therefore filter bags have to be periodically cleaned. 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. The normal velocities at which the gas is passed through the bags is 0.4–1 m/min. There are many types of “filter bags” depending on the bag shape, the type of housing, and the method of cleaning the fabric.

2.4.5.2.

Factors affecting efficiency

Efficiency of bag filters may decrease on account of the following factors: • Excessive filter ratios — “Filter ratio” is defined as the ratio of the carrier gas volume to gross filter area, per minute flow of the gas. Excessive filter ratios

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Figure 5. Typical Industrial Bag House of the Pulse-Jet Design. Source: Panda and Sharma, 2006.5

lower particulate removal efficiency and result in increased bag wear. Therefore, low filter ratios are recommended for high concentration of particulates. • Improper selection of filter media — While selecting filter media, properties like temperature resistance, resistance to chemical attack, and abrasion resistance should be taken into consideration.

2.4.5.3.

Reverse jet filter

Recently, bag filters cleaned by an air jet from a traversing ring have been developed. This unit differs basically from the usual tubular type unit with regard to the method of cleaning and the type of filter medium employed. The tubular filters in this unit are cleaned by a high-velocity air jet discharged from the inner side of a traversing ring which moves on the outside of the filter tube. The air jet passes through the fabric in a direction reverse to the normal flow and removes cake continuously from the filter surface. Current designs employ felt as the filtration medium. Following are the advantages and disadvantages of a reverse jet filter. • Requires no shutdowns or programming for cleaning. Blow rings are a continuous cleaning mechanism in that some part of each bag is being cleaned at all times. • Filter works at a higher velocity of 3–6 m/min as compared to 0.5–2 m/min for the usual cloth filters.

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• The cloth resistance can be maintained at a nearly constant value in contrast to other cloth filters in which the resistance builds up to the design value. But the main disadvantages of blow rings are the mechanical linkages and the individual air hose attachments required for each bag. The degree of maintenance required is also high. The bag house specifications for a reverse jet filter must include linear ring travel rate, number of rings per bag, and the flow rate and pressure of the cleaning air.

2.4.5.4.

Envelope type fabric filter

This type of filter is in the form of a cloth envelop supported on a wire screen frame. The individual units range from 0.6 to 1 m wide and 3 to 5 cm thick. In contrast to the bag filter, gas is introduced on the outside of each envelop, passes through the fabric in the frame of the unite, and then out of the collector. The filters are prevented from collapsing due to the inner framework. As in bag filters, the envelop panels mounted in multiples.

2.4.5.5.

Multi-compartment-type bag house

If the requirements of the process being controlled are such that continuous operation is necessary, the bag filter must be of a multicompartment type to allow individual units of the bag filter to be successively off-stream during shaking. This is achieved either manually in small units or by programming control in large, fully automatic units. In this case, sufficient cloth area must be provided to ensure that filtering capacity will not be reduced during shaking periods when any one unit of the filter is off-stream.

2.4.5.6.

Fibrous or deep-bed filters

In the deep-bed filters, a fibrous medium acts as the separator, and the collection takes place in the interstices of the bed. Fibrous filters may be composed of mats of wool, asbestos, cellulose, glass, or iron fibers. They find their most extensive use in air conditioning and heating and ventilating systems, but high-efficiency mat filters are also employed as after-cleaners and less efficient ones roughing units to protect still more efficient equipment for pollution-control purposes. They are most suitable for light dust loads, of the order of mg/cu m. The filters can be classified as either viscous or dry. In the viscous type the filter is coated with a sticky material or “adhesive” to help in catching the particles and prevent re-entrainment. The adhesive is usually an oil or grease of high flash point and low volatility, and it should be a good wetting agent. The filter medium, generally glass wool, is placed on metal or cardboard with a wire mesh, approximately

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50 cm square and several centimeters deep. The fibers will be generally packed with increasing density from front to back so most of the large particles are removed before the more efficient part is reached. This helps in prolonging the life of the filter. When resistance to air flow becomes excessive, glass wool media are usually discarded while the wire mesh is washed with hot water and then recoiled. In fact, as a general rule, used filters are discarded rather then cleaned and reused. Automatic viscous filters are also available. They can be used for handling large quantities of atmospheric air containing high concentration of dust (about two grains per 1000 cu ft). The fiber is formed into an endless belt, which continuously moves through an oil bath at the bottom of the housing. They can be operated at high air velocities of the order of 90–150 m/min. Dry filters for air conditioning are supplied in units similar in size to the viscous type except that the depth of the dry cell is usually greater. The filter materials may be paper, glass fibers, or cotton batting. Dry filters which are automatically vibrated at intervals to dislodge the dust are also available. They handle higher concentrations than the usual dry-cell filters but not the heavy loadings of the cloth filters. High-efficiency dry fibrous filters have been developed for special applications, such as the removal of radioactive or toxic particles or the cleaning of air in industrial plants manufacturing photographic film or fine instruments. Loosely packed pads of glass fibers have been found particularly suitable for such cases. Another important advantage of deep-bed filters is that they are generally very good for service in a corrosive atmosphere because of their “throwaway” nature.

2.4.6. Electrostatic Precipitators Electrostatic precipitators (ESPs) are particulate collection devices that utilize electrical energy directly to assist in the removal of the particulate mater. They have been successfully used for the removal of fine dusts from all kinds of waste gases with very high efficiency. Particles as small as a tenth of a micron can be removed. The principle on which this equipment operates is that, when a gas containing aerosols is passed between two electrodes that are electrically insulated from each other and between which there is a considerable difference in electric potential, aerosol particles precipitate on the low potential electrode. There are various types of ESPs. They are mainly used for industrial purposes. They can also be used for air cleaning in public buildings, theatres, railway cars, etc.

2.4.6.1.

Description of general system

Basically, an ESP consists of six major components as shown in Fig 6. 1. A source of high voltage. 2. Discharge electrodes and collecting electrodes.

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Rectifiers

Raw Gas Distribution Plate

Gas Outlet

Collecting Electrode Plates

Discharge Electrodes

Gas Inlet

Rapping Hammers

Figure 6.

Hoppers

Cutaway View of a Large Modern ESP Showing the Various Parts.

Source: Panda and Sharma, 2006.5

3. 4. 5. 6.

Inlet and outlet for the gas. A means (generally a hopper) for disposal of the collected material. An electronic cleaning system. An outer casing to form an enclosure around the electrodes (precipitator shell).

The gases entering an ESP may or may not have pretreatment before entering the unit. Pretreatment could consist of removing part of the dust loading by use of certain mechanical collectors, or by adding chemicals to the gas to change the physical properties of the gas to enhance precipitator action. The entire ESP is enclosed, in a casing. The actual geometric configuration may be either rectangular or cylindrical. In all cases, there is an inlet and outlet connection for gases, hoppers to collect the precipitated matter, and the necessary discharge electrode and collector surfaces. Usually, there will be a penthouse on the precipitator. This is a weatherproof gastight enclosure over the precipitator that houses the high-voltage insulators. Precipitators are usually built with a number of auxiliary components, which include access doors, dampers, safety devices, and gas distribution systems. The access provisions can be either a door or a plate which are closed and bolted under normal operating conditions but which can be opened when necessary for inspection and maintenance. Dampers are the means by which the quantity of gas is controlled. They may be either a guillotine, a louver, or some similar device which opens and closes to adjust gas flow. In addition, the safety grounding system is extremely

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important and must always be in place during operation and especially during periods of inspection. The most common grounding system consists of a conductor, one end of which is grounded to the casing, and the other end is attached to the highvoltage system by an insulated operating lever. The collecting system consists of the collecting surfaces where the particulate matter is deposited. Also, a device for rapping or vibrating the collecting surfaces is included so that the deposited particles can be dislodged. The high-voltage system includes the discharge electrode and the insulators. Electrical insulation must be done for safety purposes. The precipitator hopper is integral with the precipitator shell and is made from the same materials. The function of the hopper is to collect the precipitated material for final disposal. ESPs require a high-voltage direct current source of energy for operation. Therefore, transformers are required to step up the normal service voltages to high voltages. Rectifiers convert the alternating current to unidirectional current.

2.4.6.2.

Principle of electrostatic precipitation

In ESPs, the gas stream is passed between two electrodes, across which a high potential difference is maintained. Out of the two electrodes, one is a discharging electrode and the other a collecting electrode. Because of a high potential difference and the discharge system, a powerful ionizing field is formed. Potentials as high as 100 kilovolts (but usually 40–60 kV) are used. Consequently, ionization creates an active glow zone (blue electric discharge) called the “corona” or “corona glow.” Gas ionization is the dissociation of gas molecules into free ions. As the particulates in the carrier gas pass through this field, they get charged and migrate to the oppositely charged collecting electrode. The particles, once deposited on the collecting electrode, lose their charge and are removed mechanically by rapping, vibrating, or washing to a hopper below. Thus, the four steps in the process are: 1. 2. 3. 4.

Place the charge on the particle to be collected. Migrate the particle to the collector. Neutralize the charge at the collector. Remove the collected particle.

2.4.6.3.

Single-state and two-stage precipitators

ESPs can be either single stage or two stage in design. In a single-stage precipitator, gas ionization and particulate collection are combined in a single step. Whereas in the case of a two-stage precipitator, particles are ionized in the first chamber and collected in the second chamber. Almost all industrial precipitators are of the singlestage design. Usually the two-stage precipitator is used for lightly loaded gases and the single stage, for more heavily loaded industrial gas stream.

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Pipe-type and plate-type precipitators

There are two basic designs of the single 3-stage precipitator. They are the pipe-type precipitator and the plate-type precipitator.

2.4.6.4.1.

Pipe-type precipitator

In the pipe-type precipitator, the collecting electrodes are formed by a nest of parallel pipes which maybe round, square octagonal. Generally, the pipe is about 30 cm in diameter or less. The most common form of a discharge electrode is a wire with small radius of curvature, suspended along the axis of each pipe. They must be weighted or supported to retain proper physical tension and location, electrically insulated from the support grid and strong enough to withstand rapping or vibrating for cleaning purpose. The gas flow is axial from bottom to top. Pipe electrodes may be 2–5 m high. Spacing between the discharge electrode and collecting electrode ranges from 8 to 15 cm or even 20 cm. Precipitation of the aerosol particles occurs on the inner pipe walls, from which the material can be removed by periodic rapping of the pipes or flushing with water. Generally, the pipe-type precipitator is used for the removal of liquid particulates.

2.4.6.4.2.

Plate-type precipitator

In the plate-type precipitator, the collecting electrodes consist of parallel plates. The discharge electrodes are similar to those used in the pipe-type precipitators, i.e., they are wires with a small curvature. Sometimes square roads (approximately 4.8 mm) and twisted square road (from 3.2 to 6.4 mm) are used. The wires are suspended midway between the parallel collecting electrodes and usually hang free with a weight suspended at the bottom to keep them straight. Discharge electrodes are made from noncorrosive materials like tungsten, and alloys of steel and copper. The gas flow is parallel to the plates. Plates may be 1–2 m wide and 3–6 m high. The parallel plates should be spaced at equal intervals (between 15 and 35 cm). The collection of the aerosols takes place on the inner sides of the parallel plates. The dust material colleted can be removed by rapping either periodically or continuously. The dust particles thus removed fall into the hoppers at the base of the precipitator. Collection electrodes should have a maximum amount of collection surface, bulking resistance, resistance to corrosion, and have a consistent economic design. Plate-type precipitators may be classified further on the basis of the direction of gas flow as horizontal-flow and verticalflow precipitators. Gas velocities are normally maintained at 0.5–0.6 m/s in these precipitators.

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Efficiency

Generally the collection efficiency of the ESP is high, approaching 100%. Many installations operate at 98 and 99% efficiency. Some materials ionize more readily than others and are thus more adapted to removal by electrostatic precipitation. Acid mists and catalyst recovery units often have efficiencies in excess of 99%; carbon black, because of its agglomerating tendency, has a normal collection efficiency of less than 35%. However by proper combination of an ESP with a cyclonic collector, high efficiencies may be obtained in collecting carbon black.

2.4.6.6.

Particle re-entrainment

Particle re-entrainment is a problem associated with particle charging. It occurs primarily in two situations — due to either inadequate precipitator area, or inadequate dust removal from the hopper. Re-entrainment reduces ESP performance because of the necessity of re-collecting dust which had been previously removed from the carrier gas. The problem can be overcome by a proper design of the ESP.

2.4.6.7.

Design parameters

The design parameters relating to the carrier gas are volumetric flow rate, composition, temperature, and dew point. The design parameters in relation to the dust particles are concentration, size distribution, resistivity, bulk density, composition, hygroscopicity, and tendency to agglomerate. Other important factors affecting the efficiency of the ESP are velocity distribution at the entrance of the precipitator, design of duct work, collection electrode area, ionization potential, etc.

2.4.6.8. Advantages of ESPs • • • • • • • •

High collection efficiency. Particles as small as 0.1 m can be removed. Low maintenance and operating costs. Low pressure drop (0.25–1.25 cm of water). Satisfactory handling of a large volume of high-temperature gas. Treatment time is negligible (0.1–10 s). Cleaning is easy by removing units of the precipitator from operation. There is no limit to solid, liquid, or corrosive chemical usage.

2.4.6.9.

Disadvantages of ESPs

• High initial cost. • Space requirement is more because of the large size of the equipment. • Possible explosion hazards during collection of combustible gases or particulates.

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• Precautions are necessary to maintain safety during operation. Proper gas flow distribution, gas resistivity, particulate conductivity, and corona spark over rate must be maintained carefully. • The poisonous gas, ozone, is produced by the negatively charged discharge electrodes during gas ionization.

2.4.6.10.

ESP for air cleaning

Air cleaning precipitators are used for cleaning air in public buildings, theatres, ships, railway cars, and in some private homes and club house. The object of using air cleaning precipitators is to make the air more healthy or pleasant to breathe by removing tobacco smoke, pollen, etc., or to prevent dust from interfering with delicate industrial operation such as manufacture of watches or electronic tubes, or to prevent soiling of walls, paintings, etc.

2.4.7. Gas Absorption Equipment The removal of one or more selected components from a gas mixture by absorption is probably the most important operation in the control of gaseous pollutant emissions. Absorption is a unit operation as shown in Fig. 7, in which a substance is transferred from gaseous phase to a liquid phase. The substance may simply be dissolved in the liquid phase, or may react with the liquid or a specific substance in the liquid. Water is the most commonly used absorbent liquid. For example, sulfur dioxide in a flue gas may be absorbed in water containing calcium hydroxide to form calcium sulfate, which can then be scrubbed from the gas stream by more water. As the gas stream passes through the liquid, the liquid absorbs the gas, in much the same way that sugar is absorbed in a glass of water when stirred. Absorption is commonly used

Figure 7. Typical Packed Column Diagram. Source: USEPA, 2007.2

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to recover products or to purify gas streams that have high concentrations of organic compounds. Absorption equipment is designed to get as much mixing between the gas and liquid as possible. Absorbers are often referred to as scrubbers, and there are various types of absorption equipment. The principal types of gas absorption equipment include spray towers, packed columns, plate towers, spray chambers, and venture scrubbers. The packed column is by far the most commonly used for the absorption of gaseous pollutants. The packed column absorber has a column filled with an inert (nonreactive) substance, such as plastic or ceramic, which increases the liquid surface area for the liquid/gas interface. The inert material helps to maximize the absorption capability of the column. In addition, the introduction of the gas and liquid at opposite ends of the column causes mixing to be more efficient because of the counter-current flow through the column. In general, absorbers can achieve removal efficiencies grater than 95%. One potential problem with absorption is the generation of waste-water, which converts an air pollution problem to a water pollution problem.

2.4.8. Gas Adsorption Equipment Here, the effluent gas is passed through solid adsorbers contained in collection equipment. When a gas or vapor is brought into contact with a solid part of it is taken up by the solid. The molecules that disappear from the gas either enter the inside of the solid or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption. Adsorption is the binding of molecules or particles to a surface. In this phenomenon, molecules from a gas or liquid will be attached in a physical way to a surface. The binding to the surface is usually weak and reversible. Adsorption may be physical or chemical (chemisorption). In physical adsorption, the sorbed layer is loosely bound to the adsorber solid and may be removed easily, for example, by reducing pressure, etc. In chemisorption, the adsorbed layer is found more firmly to the solid surface and greater energy is required to remove the adsorbed molecules. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they have enormous surface areas per unit weight. The common types of adsorption equipment are thin-bed adsorbers and deep-bed adsorbers. Thin-bed adsorber equipment consists of a thin layer or bed of adsorbed solid, like activated alumina. The gaseous stream entering the device meets the solid surface and gets rapidly adsorbed. Since the bed is thin, the resistance to flow is less and hence, adsorption is quick. This type of adsorber is used for air steams containing trace quantities or low volumes of pollutants. Another type deep-bed adsorber has a bed of a depth of 2.5–4 cm and uses activated charcoal to adsorb air that is comparatively heavily polluted.

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Activated carbon is the universal standard for purification and removal of trace organic contaminants from liquid and vapor streams. Carbon adsorption uses activated carbon to control and/or recover gaseous pollutant emissions. In carbon adsorption, the gas is attracted and adheres to the porous surface of the activated carbon. Removal efficiencies of 95–99% can be achieved by using this process. Carbon adsorption is used in cases where the recovered organics are valuable. For example, carbon adsorption is often used to recover perchloroethylene, a compound used in the dry cleaning process. Carbon adsorption systems are either regenerative or nonregenerative as shown in Figs. 8 and 9. A regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use. Steam is used to purge captured pollutants from the bed to a pollutant recovery

Figure 8.

Regenerative Carbon Adsorption System.

Source: USEPA, 2007.2

Figure 9. Source: USEPA, 2007.2

Non-Regenerative Carbon Adsorption System.

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device. By “regenerating” the carbon bed, the same activated carbon particles can be used again and again. Regenerative systems are used when the concentration of the pollutant in the gas stream is relatively high. Nonregenerative systems have thinner beds of activated carbon. In a nonregenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant. Because of the solid waste problem generated by this type of system, nonregenerative carbon adsorbers are usually used when the pollutant concentration is extremely low.

2.4.9. Condensation Equipment Condensation is the process of converting a gas or vapor to liquid. Any gas can be reduced to a liquid by lowering its temperature and/or increasing its pressure. The most common approach is to reduce the temperature of the gas stream, since increasing the pressure of a gas can be expensive. A simple example of the condensation process is droplets of water forming on the outside of a glass of cold water. The cold temperature of the glass causes water vapor from the surrounding air to pass into the liquid state on the surface of the glass. Condensers are widely used to recover valuable products in a waste stream. Condensers are simple, relatively inexpensive devices that normally use water or air to cool and condense a vapor stream. Condensers are typically used as pretreatment devices. They can be used ahead of adsorbers, absorbers, and incinerators to reduce the total gas volume to be treated by more expensive control equipment. Condensers used for pollution control are contact condensers and surface condensers. In a contact condenser as shown in Fig. 10, the gas comes into contact with cold liquid. In a surface condenser as shown in Fig. 11, the gas contacts a cooled surface in which cooled liquid or gas is circulated, such as the outside of the tube. Removal efficiencies of

Figure 10. Source: USEPA, 2007.2

Contact Condenser.

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Figure 11.

Surface Condenser.

Source: USEPA, 2007.2

condensers typically range from 50% to more than 95%, depending on design and applications.

2.4.10. Incineration Equipment Incineration, also known as combustion, is mostly used to control the emissions of organic compounds from process industries. This control technique refers to the rapid oxidation of a substance through the combination of oxygen with a combustible material in the presence of heat. When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor. Incomplete combustion will result in some pollutants being released into the atmosphere. Smoke is one indication of incomplete combustion. Equipment used to control waste gases by combustion can be divided into three categories: direct combustion or flaring, thermal incineration, and catalytic incineration. Choosing the proper device depends on many factors, including the type of hazardous contaminants in the waste stream, concentration of combustibles in the stream, process flow rate, control requirements, and an economic evaluation. A direct combustor or flare as shown in Fig. 12, is a device in which air and all the combustible waste gases react at the burner. Complete combustion must occur instantaneously since there is no residence chamber. Flares are commonly used for disposal of waste gases during process upsets, such as those that take place when a process is started or shutdown. A flare can be used to control almost any emission stream containing (VOCs). Studies conducted by EPA have shown that the destruction efficiency of a flare is about 98%. In thermal incinerators, the combustible waste gases pass over or around a burner flame into a residence chamber where oxidation of the waste gases is completed. For thermal incineration, it is important that the vapor stream directed to the thermal

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Figure 12. Thermal Incinerator General Case. Source: USEPA, 2007.2

incinerator has a constant combustible gas concentration and flow rate. These devices are not well-suited to vapor streams that fluctuate, because the efficiency of the combustion process depends on the proper mixing of vapors and a specific residence time in the combustion chamber. Residence time is the amount of time the fuel mixture remains in the combustion chamber. Often, supplementary fuel is added to a thermal incinerator to supplement the quantity of pollutant gases being burned by the incinerator. Energy and heat produced by the incineration process can be recovered and put to beneficial uses at a facility. Thermal incinerators can destroy gaseous pollutants at efficiencies greater than 99% when operated correctly. Catalytic incinerators are very similar to thermal incinerators as shown in Fig. 13. The main difference is that after passing through the flame area, the gases pass over a catalyst bed. A catalyst is a substance that enhances a chemical reaction without being changed or consumed by the reaction. A catalyst promotes oxidation at lower temperatures, thereby reducing fuel costs. Destruction efficiencies greater than 95% are possible using a catalytic incinerator. Higher efficiencies are possible if larger catalyst volumes or higher temperatures are used. Catalytic incinerators are best suited for emission streams with low VOC content.

2.4.11. Biofiltration Equipment9−12 Air biofiltration has been practiced since the early decades of the last century and has gained much interest in recent years for controlling odorous emissions into

Figure 13. Source: USEPA, 2007.2

Catalytic Incinerator.

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the air, toxic compounds, VOCs. The principle of biofiltration is relatively simple: a contaminated air stream is passed through a porous packed bed on which pollutantdegrading cultures of microorganisms are immobilized, and air biotreatment relies on microbial reactions for the degradation of waste compounds. As the odorous and contaminated air passes through the media, the contaminants in the air stream are absorbed by the biofilm. These contaminants are then oxidized to produce biomass, CO2 , H2 O, NO3 , and SO4 . Biofiltration is an emerging technology, and in comparison with traditional methods of air pollution control, it offers a number of advantages for the treatment of low concentrations of polluted air streams. Besides its high removal efficiency, low capital and operating costs, safe operating conditions, and low energy consumption, it does not generate undesirable byproducts and converts many organic and inorganic compounds into harmless oxidation products. A biofilter normally consists of a simple bed of material that is conductive to the support of microbe growth through which the pollutant gaseous passes at a low velocity as shown in Fig. 14. The bed material filter normally compost, sphagnum peat, wood chips, polystyrene, fiberglass wool, clay, soil, or granular activated carbon. The bed material filters are surrounded by a film of aqueous liquor, which

Figure 14. Source: Pandey et al., 2006.13

Schematics of Biofilter Equipment on a Bench Scale.

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is teeming with microorganisms. To promote microbe growth and activity levels, the bed is usually kept moist and the gas is humidified before entry into the filter. Biofilters may have one or more beds of biologically active materials. As the stream of contaminated air flows through the filter, contaminates dissolve into the aqueous liquor (driven by entropy). These dissolved contaminates are then consumed as food by the microorganisms. Carbon dioxide, water, oxidized organic compounds, and more microorganisms are the end products. Biofilter can be used for removing wide range of organic (hydrocarbons, chlorinated hydrocarbons, ketones, esters, aldehydes, alcohols, and odors) and inorganic compounds (hydrogen sulfide, carbon disulfide, ammonia, and nitrogen oxides). Bioscrubing, trickling biofiltration, and biofiltration are typical waste gas cleaning technologies. Biofiltration appears to be the cheapest and also the most studied and most extensively used technology. Bioscrubing and trickling biofiltration are used rather for special applications. Trickling biofilter and biofilter are packed columns with organic or anorganic carrier material witch is covered with biocatalyst. Degration rates of 50–100 (g/h m3 ) are typical. The chlorine content in chloinates hydrocarbon significantly reduces the rate of degration. Removal efficiences of over 90% have been achieved on these contaminates.

2.5.

Controlling Air Pollution from Moving Sources (Automobiles)

Automobiles chiefly emit carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx ). The contents of these pollutants in the smoke emitted by the automobiles are combustion gaseous — i.e., CO, HC, H2 , — hundreds of oxygenated hydrocarbons, as well as small fractions of nitrogen oxides. These pollutants are highly dangerous to the overall environment, and to the life in general; and hence the automobiles are nowadays seen as a symbol of technological menace. The emissions from petrol engines of two, three, and four wheelers (including cars, having four-stroke engines) contain heavier concentrations of HC and CO, whereas the four-stroke diesel engines of diesel vehicles (buses and trucks) contain heavier concentrations of NO along with thick smoke and particles. The auto emissions also contain gaseous pollutants, like SO2 and lead compounds, especially when lead-containing fuel is used, as in India.∗ Besides the above improvements required in the gasoline, the upkeep and maintenance of the auto engines is also of utmost importance. The proper tuning of the engine and carburetor is very essential, in order to control pollutant emissions. Central Motor Vehicles Rules, 1989, framed under Motor Vehicles Act, 1988, has, therefore, stipulated permissible auto emission levels. The maximum permissible CO emission, while idling, is limited to 3.0% for cars, and 4.5% for two and three wheelers. Such stipulation for improved tuning and adjustment of the engines is

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only an initial step to limit the vehicular pollution; as in fact, there is no other choice left now, but to change the basic design of our automobiles, incorporating catalytic convertors in their exhaust pipes, and to adopt multi-point fuel injection (MPFi) system with dual intake valves to limit the fuel intake exactly to the needs of the engine, with no excess fuel usage at any point. The catalytic convertors are usually made of noble metals, like platinum, palladium, etc., and help in oxidizing CO and HC into their final product CO2 , and also in reducing NO into nitrogen. These noble metal catalysts are highly active, and resist sulfur poisoning. A catalytic convertor is generally placed inside the tail exhaust pipe of the automobile, so as to pass through it the “partially oxidized emissions” before they are let out into the atmosphere.

3.

Universal Air Pollution Control Measures for Industries I. To avoid generation of black smoke/dense particulate matter emission from the stack of the kilns, boilers, furnaces, etc., the industry must install adequately designed ESP/Gas Cleaning Plant and continuously operate the same to meet the prescribed standard of the pollution control agencies/authorities. II. The existing bag filters should be periodically overhauled and upgraded/ replaced with new bag filter to meet the norms in respect of particulate matter emission. Portholes and platforms along with the ladder at each stack attached to the individual bag filters and ESPs shall be provided to facilitate the Pollution Control Board/ environmental authorities for stack monitoring. III. D.G. sets should be equipped with AMF. (Auto Mains Failure Panel) for auto changeover of power supply from grid power to D.G. power in the event of power failure. The AMF Panel should preferably be PLC (programmable logic control) based. Dedicated D.G. sets of adequate capacity shall be installed to ensure sufficient standby power supply to run all pollution control devices of the plant in the event of power failure to ensure continuous operation of pollution control equipments. IV. All raw materials, product, and waste materials should be transferred through covered vehicles without any spillage or leakages on the way. In the case of any accidental spillage on the road, the materials/wastes shall be lifted by the industry and suitably disposed off in designated solid waste dumping area. V. All sources of fugitive dust emission generated from material stack yards, material transfer points of conveyors, bottom of the bag filters, ESP hopper, and intermediate bin should be fully enclosed. VI. Inventory of excess spare parts air pollution control systems/spare bags shall be in the store to meet emergency need of ESPs and bag filters.

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VII. Approach roads and internal roads should be black topped or concreted and good housekeeping practices should be followed to prevent the generation of fugitive dust. VIII. Appropriate devices like pneumatic dust handling system/mechanical dust handling system/Pug mill shall be provided at the hoppers of ESPs and pulse jet bag filters for continuous evacuation of dust from the hoppers without creating fugitive emission near the ESP and bag filter area. The collected dust from air pollution control equipments should be utilized or disposed off in a designed land fill area. Until capping of land fill, the dust shall be kept in wet condition with water sprinklers to avoid re-entrainment into the surrounding area due to wind. IX. Raw materials like iron ore and coal fines should be stored under covered shed. X. Accumulation of dust in the work zone and nondumping area inside the factory premises shall be avoided. The work zone area shall be properly cleaned either manually or mechanically every day and the dust so collected shall be disposed off in the designated dump site. XI. Accretion material/dust, waste material generated due to cleaning of the kilns/ furnaces/ boilers/ equipments should not be dumped haphazardly but it should be temporarily kept in an earmarked area and subsequently removed and transferred to the designated dump site within two days of their generation. XII. Dust from approach roads and internal roads should be removed every day and taken to the dump site. The approach road and internal roads shall be cleaned by water hose periodically to avoid the accumulation of dust and to control fugitive dust emission during plying of vehicles. XIII. Permanent type of high-pressure water spraying system with nozzles should be installed for regular spraying of water on all roads, work zone, and solid waste dumping area. XIV. Proper housekeeping should be maintained by a dedicated team. XV. The industry must constitute a team of responsible and technically qualified personnel who will ensure continuous operation of all pollution control devices round the clock (including night hours). XVI. In no case, leakage of flue gas should be allowed through the emergency caps of process equipments bypassing the pollution control devices. XVII. Air pollution control measures installed at crushers, screens, material transfer points, product handling area, cooler discharge, magnetic separator, and other potential dust generating points shall be operated continuously and effectively to control fugitive dust emission.

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XVIII. Additional bag filters should be provided at coal injection point, raw material feeding point, storage bins, intermediate bins, and any other material transfer points (if not provided yet) to control the emission of fugitive dust. Slip rings of kilns shall be maintained properly to prevent dust leakages. XIX. The industry should install separate energy meters for ESP/GCP and other pollution control devices. A logbook shall be maintained to record the energy meter readings, and a monthly statement of such energy meter readings of power consumption along with monthly electricity bill shall be furnished to the Pollution Control Board. XX. Maintenance of all the pollution control devices shall be taken up during normal shutdown of kilns. In case of failure of any air pollution control devices at any point of time, the fact shall be intimated to the Board immediately by fax, and immediate action shall be taken to resume the proper functioning of the devices. XXI. Boundary shall be provided around the existing/proposed solid waste dumping area, and to prevent the generation of wind-borne dust, the height of the dump shall not be more than the height of the boundary wall under any circumstances. XXII. The ambient air quality shall confirm to the prescribed standard. Areas within the premises of the plant shall be considered as industrial area and areas outside the premises shall be treated as residential and rural areas. XXIII. The D.G. set shall be installed in an acoustically designed enclosure to control noise level and over anti-vibration pads to avoid vibration. XXIV. The height of the stack attached to the D.G. sets shall confirm to the fol√ lowing. H = h + 0.2 KVA where h = height of the building where it is installed in meter, KVA = capacity of the D.G. set in KVA, H = height of the stack in meter above ground level. XXV. Green belt shall be developed along the boundary of factory premises during the forthcoming monsoon. A plan showing the green belt area shall be submitted to the Board within one month. Number of species and existing number of surviving trees shall be reported to the Board. More information on air pollution control can be found from the literature.14−16

References 1. Garg, S.K. and Garg, R. (1999). Sewage Disposal and Air Pollution Engineering, New Delhi, India: Khanna Publishers. 2. USEPA, U.S. Environmental Protection Agency. (2007). Air Pollution Control Orientation Course. SI: 422 Air Pollution Control Orientation Course Self-Instruction Manual. epa.gov/oar/oaqps/eog/course422/index.html.

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3. Biswas, D.K. and Mishra, P.C. (2006). Clean air is a national requirement. In: Proceedings of International Conference on Impact of Industrialization on environmental pollution — Its Control and Abatement, Orissa, India, pp. 42–48. 4. Kumar, K.S. and Chenchaiah, S. (2006). Controlling industrial air pollution emissions. In: Proceedings of International Conference on Impact of Industrialization on Environmental Pollution — Its Control and Abatement, Orissa, India, pp. 86–92. 5. Panda, B. and Sharma, U. (2006). Air pollution. In: Proceedings of International Conference on Impact of Industrialization on Environmental Pollution — Its Control and Abatement, Orissa, India, pp. 98–104. 6. Rao, C.S. (1992). Environmental Pollution Control Engineering, New Delhi, India: Wiley Eastern Limited. 7. Master, G.M. (2007). Introduction to Environmental Engineering and Science, New Delhi, India: Pearson Education. 8. Buonicore, A.J. and Davis, W.T. (1992). Air Pollution Control Manual, Air and Waste Management Association, Van Nostrand Reinhold, New York. 9. Davis, W.T. (2000). Air Pollution Engineering Manual, Second Edition, A Wiley Interscience Publication. 10. Deshusses, M.A. and Cox, H.H.J. (2000). Biotrickling Filters for Air Pollution Control, Department of Chemical and Environmental Engineering, University of California. 11. Sheridan, B., Curran, T., Dodd, U., and Couigan, J. (2002). Biofiltration of odor and ammonia from a pig unit: a pilot-scale study. Biosystems Engineering 82(4): 441–453. 12. Deving, J.S., Deshusses, M.A., and Webster, T.S. (1999). Biofiltration for Air Pollution Control, Lewis Publishers. 13. Pandey, R.A., Gangane, R., Mudliar, S.N., and Rajvaidya, A.S. (2006). Treatment of waste gas containing monomethylamine in a biofilter enriched with Pseudomonas mendocina, Waste Management 26(3), 233–244. 14. Wang, L.K., Pereira, N.C. and Hung, Y.T. (Eds.) (2004). Air Pollution Control Engineering. New Jersey: Humana Press, 504 pp. 15. Wang, L.K., Pereira, N.C. and Hung, Y.T. (Eds.) (2005). Advanced Air and Noise Pollution Control. New Jersey: Humana Press, 526 pp. 16. USEPA (2011). The Phasecut of Ozone-Depleting Substances. www.epa.gov/ozone/ title6/phaseout/.