. However, in the presence of sunlight or other pollutants, such innocuous emissions can be transformed into hazardous pollutants that present a threat to mankind and the ecology. In addition, pollutants can be transported over long distances from their sources, causing impacts hundreds or even thousands of kilometres downwind. For these reasons, research focuses on basic kinetic studies to determine reaction rate constants; smog chamber studies to establish the reactivity, reaction products, and persistence of chemicals in various atmospheric situations; ground level and airborne field experiments to define the rates and products of atmospheric reactions; and modelling studies to predict the impact of atmospheric reactions. This requires extensive experience in the application of aircraft, chemical tracers, and dispersion modelling to assess the extent and importance of pollutant transport.
Local and regional pollution takes place in the lowest layer of the atmosphere (Fig), the troposphere, which extends from the earth's surface to about 16 km.
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The troposphere is the region in which most weather occurs. If the load of pollutants added to the troposphere were equally distributed, the pollutants would be spread over vast areas and the air pollution might almost escape our notice. Also, pollution sources tend to be concentrated, especially in cities (Figs).
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In the weather phenomenon known as a thermal inversion, a layer of cooler air is trapped near the ground by a layer of warmer air above. When this occurs, normal air mixing almost ceases and pollutants are trapped in the lower layer. Local topography, or the shape of the land, can worsen this effect-an area ringed by mountains, for example, can become a pollution trap.
Burning gasoline in motor vehicles is the main source of smog in most regions of the world today (Fig).
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Powered by sunlight, oxides of nitrogen and volatile organic compounds react in the atmosphere to produce photochemical smog. Under adverse weather conditions, accidental releases of other toxic substances can be disastrous. The worst such accident occurred in 1984 in Bhopal, India, when methyl isocyanate released from an American-owned factory during a thermal inversion caused at least 3300 deaths.
Most particles emitted by anthropogenic sources are less than 2.5 mm in diameter and include a larger variety of toxic elements than particles emitted by natural sources. Fossil fuel combustion generates metal and sulphur particulate emissions, depending on the chemical composition of the fuel used. The EPA estimates that more than 90% of fine particulates emitted from stationary combustion sources are combined with sulphur dioxide. Sulphates, however, do not necessarily form the largest fraction of fine particulates. In locations such as Bangkok, Chongqing (China), and Sao Paulo (Brazil), organic carbon compounds account for a larger fraction of fine particulates, reflecting the role of emissions from diesel and two stroke vehicles or of smoke from burning coal and charcoal. Although sulphates represent a significant share (30 to 40%) of fine particulates in these cases, caution is required before making general assertions about the relationship between sulphates and fine particulates, since the sources and species characteristics of fine particulates may vary significantly across locations. Combustion devices may emit particulates comprised of products of incomplete combustion and toxic metals, which are present in the fuel and in some cases may also be carcinogenic. Particulates emitted by thermal power generation may contain lead, mercury, and other heavy metals.
The main objective of air quality guidelines and standards is the protection of human health (Fig).
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Since fine particulates (PM,,) are more likely to cause adverse health effects than coarse particulates, guidelines and standards referring to fine particulate concentrations are preferred to those referring to TSP, which includes coarse particulate concentrations. Scientific studies provide ample evidence of the relationship between exposure to short-term and long-term ambient particulate concentrations and human mortality and morbidity effects. However, the dose-response mechanism is not yet fully understood. Furthermore, according to the WHO, there is no safe threshold level below which health damage does not occur.
Airborne particulate matter emissions can, to a great extent, be minimized by pollution prevention and emission control measures. Prevention is frequently more cost-effective than control and, therefore, should be emphasized. Measures such as improved process design, operation, maintenance, housekeeping, and other management practices can reduce emissions. By improving combustion efficiency, the amount of products of incomplete combustion (PICs), a component of particulate matter, can be significantly reduced.
Atmospheric particulate emissions can be reduced by choosing cleaner fuels. Natural gas used as fuel emits negligible amounts of particulate matter. Oil-based processes also emit significantly fewer particulates than coal-fired combustion processes. Low-ash fossil fuels contain less non-combustible, ash-forming mineral matter and thus generate lower levels of particulate emissions. Lighter distillate oil-based combustion results in lower levels of particulate emissions than heavier residual oils. However, the choice of fuel is usually influenced by economic as well as environmental considerations.
Inertial or impingement separators rely on the inertial properties of the particles to separate them from the carrier gas stream. Inertial separators are primarily used for the collection of medium-size and coarse particles. They include settling chambers and centrifugal cyclones (straight-through, or the more frequently used reverse-flow cyclones). Cyclones are low-cost, low-maintenance centrifugal collectors that are typically used to remove particulates in the size range of 10-100 p. The fine-dust removal efficiency of cyclones is typically below 70 %, whereas electrostatic precipitators (ESPs) and bag-houses can have removal efficiencies of 99.9% or more.
Electrostatic precipitators (ESPs) remove particles by using an electrostatic field to attract the particles onto the electrodes. Collection efficiencies for well-designed, well-operated, and well-maintained systems are typically in the order of 99.9% or more of the inlet dust loading. ESPs are especially efficient in collecting fine particulates and can also capture trace emissions of some toxic metals with an efficiency of 99%.
Filters and dust collectors collect dust by passing flue gases through a fabric that acts as a filter. The most commonly used is the bag filter, or bag-house. The various types of filter media include woven fabric, needled felt, plastic, ceramic, and metal. The operating temperature of the bag-house gas influences the choice of fabric. Accumulated particles are removed by mechanical shaking, reversal of the gas flow, or a stream of high-pressure air. Fabric filters are efficient (99.9% removal) for both high and low concentrations of particles but are suitable only for dry and free-flowing particles. Their efficiency in removing toxic metals such as arsenic, chromium, lead, and nickel is greater than 99%.
Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are Venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 p or less) is lower than that of a Venturi scrubber.
When designing control technology, environmental factors include (a) the impact of control technology on ambient air quality; (b) the contribution of the pollution control system to the volume and characteristics of wastewater and solid waste generation; and (c) maximum allowable emissions requirements.
Economic factors include (a) the capital cost of the control technology; (b) the operating and maintenance costs of the technology; and (c) the expected lifetime and salvage value of the equipment.
Engineering factors include (a) contaminant characteristics such as physical and chemical properties - concentration, particulate shape, size distribution, chemical reactivity, corrosivity, abrasiveness, and toxicity; (b) gas stream characteristics such as volume flow rate, dust loading, temperature, pressure, humidity, composition, viscosity, density, reactivity, combustibility, corrosivity, and toxicity; and (c) design and performance characteristics of the control system such as pressure drop, reliability, dependability, compliance with utility and maintenance requirements, and temperature limitations, as well as size, weight, and fractional efficiency curves for particulates and mass transfer or contaminant destruction capability for gases or vapours.
Nitrogen oxides are produced in the combustion process by two different mechanisms: (a) the burning the nitrogen in the fuel, primarily coal or heavy oil fuel NO, and (b) high-temperature oxidation of the molecular nitrogen in the air used for combustion (thermal NO,). Formation of fuel NO, depends on combustion conditions, such as oxygen concentration and mixing patterns, and on the nitrogen content of the fuel. Formation of thermal NO, depends on combustion temperature. Above 1,538' C, NO, formation rises exponentially with increasing temperature. The relative contributions of fuel NO, and thermal NO, to emissions from a particular plant depend on the combustion conditions, the type of burner, and the type of fuel.
Combustion control may involve any of three strategies: (a) reducing peak temperatures in the combustion zone; (b) reducing the gas residence time in the high-temperature zone; and (c) reducing oxygen concentrations in the combustion zone.
Thermal power plants burning high-sulphur coal or heating oil are generally the main sources of anthropogenic sulphur dioxide emissions worldwide, followed by industrial boilers and nonferrous metal smelters. Emissions from domestic coal burning and from vehicles can also contribute to high local ambient concentrations of sulphur dioxide. The principal approaches to controlling SO, emissions include use of low-sulphur fuel; reduction or removal of sulphur in the feed; use of appropriate combustion technologies; and emissions control technologies such as sorbent injection and flue gas desulphurization (FGD).
Since sulphur emissions are proportional to the sulphur content of the fuel, an effective means of reducing SOx, emissions is to burn low-sulphur fuel such as natural gas, low-sulphur oil, or low-sulphur coal. Natural gas has the added advantage of emitting no particulate matter when burned. Today's major emissions control methods are sorbent injection and flue gas desulphurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulphur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulphur oxide emissions.
Local and regional pollution takes place in the lowest layer of the atmosphere (Fig), the troposphere, which extends from the earth's surface to about 16 km.
The troposphere is the region in which most weather occurs. If the load of pollutants added to the troposphere were equally distributed, the pollutants would be spread over vast areas and the air pollution might almost escape our notice. Also, pollution sources tend to be concentrated, especially in cities (Figs).
In the weather phenomenon known as a thermal inversion, a layer of cooler air is trapped near the ground by a layer of warmer air above. When this occurs, normal air mixing almost ceases and pollutants are trapped in the lower layer. Local topography, or the shape of the land, can worsen this effect-an area ringed by mountains, for example, can become a pollution trap.
Burning gasoline in motor vehicles is the main source of smog in most regions of the world today (Fig).
Powered by sunlight, oxides of nitrogen and volatile organic compounds react in the atmosphere to produce photochemical smog. Under adverse weather conditions, accidental releases of other toxic substances can be disastrous. The worst such accident occurred in 1984 in Bhopal, India, when methyl isocyanate released from an American-owned factory during a thermal inversion caused at least 3300 deaths.
Most particles emitted by anthropogenic sources are less than 2.5 mm in diameter and include a larger variety of toxic elements than particles emitted by natural sources. Fossil fuel combustion generates metal and sulphur particulate emissions, depending on the chemical composition of the fuel used. The EPA estimates that more than 90% of fine particulates emitted from stationary combustion sources are combined with sulphur dioxide. Sulphates, however, do not necessarily form the largest fraction of fine particulates. In locations such as Bangkok, Chongqing (China), and Sao Paulo (Brazil), organic carbon compounds account for a larger fraction of fine particulates, reflecting the role of emissions from diesel and two stroke vehicles or of smoke from burning coal and charcoal. Although sulphates represent a significant share (30 to 40%) of fine particulates in these cases, caution is required before making general assertions about the relationship between sulphates and fine particulates, since the sources and species characteristics of fine particulates may vary significantly across locations. Combustion devices may emit particulates comprised of products of incomplete combustion and toxic metals, which are present in the fuel and in some cases may also be carcinogenic. Particulates emitted by thermal power generation may contain lead, mercury, and other heavy metals.
The main objective of air quality guidelines and standards is the protection of human health (Fig).
Since fine particulates (PM,,) are more likely to cause adverse health effects than coarse particulates, guidelines and standards referring to fine particulate concentrations are preferred to those referring to TSP, which includes coarse particulate concentrations. Scientific studies provide ample evidence of the relationship between exposure to short-term and long-term ambient particulate concentrations and human mortality and morbidity effects. However, the dose-response mechanism is not yet fully understood. Furthermore, according to the WHO, there is no safe threshold level below which health damage does not occur.
Airborne particulate matter emissions can, to a great extent, be minimized by pollution prevention and emission control measures. Prevention is frequently more cost-effective than control and, therefore, should be emphasized. Measures such as improved process design, operation, maintenance, housekeeping, and other management practices can reduce emissions. By improving combustion efficiency, the amount of products of incomplete combustion (PICs), a component of particulate matter, can be significantly reduced.
Atmospheric particulate emissions can be reduced by choosing cleaner fuels. Natural gas used as fuel emits negligible amounts of particulate matter. Oil-based processes also emit significantly fewer particulates than coal-fired combustion processes. Low-ash fossil fuels contain less non-combustible, ash-forming mineral matter and thus generate lower levels of particulate emissions. Lighter distillate oil-based combustion results in lower levels of particulate emissions than heavier residual oils. However, the choice of fuel is usually influenced by economic as well as environmental considerations.
Inertial or impingement separators rely on the inertial properties of the particles to separate them from the carrier gas stream. Inertial separators are primarily used for the collection of medium-size and coarse particles. They include settling chambers and centrifugal cyclones (straight-through, or the more frequently used reverse-flow cyclones). Cyclones are low-cost, low-maintenance centrifugal collectors that are typically used to remove particulates in the size range of 10-100 p. The fine-dust removal efficiency of cyclones is typically below 70 %, whereas electrostatic precipitators (ESPs) and bag-houses can have removal efficiencies of 99.9% or more.
Electrostatic precipitators (ESPs) remove particles by using an electrostatic field to attract the particles onto the electrodes. Collection efficiencies for well-designed, well-operated, and well-maintained systems are typically in the order of 99.9% or more of the inlet dust loading. ESPs are especially efficient in collecting fine particulates and can also capture trace emissions of some toxic metals with an efficiency of 99%.
Filters and dust collectors collect dust by passing flue gases through a fabric that acts as a filter. The most commonly used is the bag filter, or bag-house. The various types of filter media include woven fabric, needled felt, plastic, ceramic, and metal. The operating temperature of the bag-house gas influences the choice of fabric. Accumulated particles are removed by mechanical shaking, reversal of the gas flow, or a stream of high-pressure air. Fabric filters are efficient (99.9% removal) for both high and low concentrations of particles but are suitable only for dry and free-flowing particles. Their efficiency in removing toxic metals such as arsenic, chromium, lead, and nickel is greater than 99%.
Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are Venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 p or less) is lower than that of a Venturi scrubber.
When designing control technology, environmental factors include (a) the impact of control technology on ambient air quality; (b) the contribution of the pollution control system to the volume and characteristics of wastewater and solid waste generation; and (c) maximum allowable emissions requirements.
Economic factors include (a) the capital cost of the control technology; (b) the operating and maintenance costs of the technology; and (c) the expected lifetime and salvage value of the equipment.
Engineering factors include (a) contaminant characteristics such as physical and chemical properties - concentration, particulate shape, size distribution, chemical reactivity, corrosivity, abrasiveness, and toxicity; (b) gas stream characteristics such as volume flow rate, dust loading, temperature, pressure, humidity, composition, viscosity, density, reactivity, combustibility, corrosivity, and toxicity; and (c) design and performance characteristics of the control system such as pressure drop, reliability, dependability, compliance with utility and maintenance requirements, and temperature limitations, as well as size, weight, and fractional efficiency curves for particulates and mass transfer or contaminant destruction capability for gases or vapours.
Nitrogen oxides are produced in the combustion process by two different mechanisms: (a) the burning the nitrogen in the fuel, primarily coal or heavy oil fuel NO, and (b) high-temperature oxidation of the molecular nitrogen in the air used for combustion (thermal NO,). Formation of fuel NO, depends on combustion conditions, such as oxygen concentration and mixing patterns, and on the nitrogen content of the fuel. Formation of thermal NO, depends on combustion temperature. Above 1,538' C, NO, formation rises exponentially with increasing temperature. The relative contributions of fuel NO, and thermal NO, to emissions from a particular plant depend on the combustion conditions, the type of burner, and the type of fuel.
Combustion control may involve any of three strategies: (a) reducing peak temperatures in the combustion zone; (b) reducing the gas residence time in the high-temperature zone; and (c) reducing oxygen concentrations in the combustion zone.
Thermal power plants burning high-sulphur coal or heating oil are generally the main sources of anthropogenic sulphur dioxide emissions worldwide, followed by industrial boilers and nonferrous metal smelters. Emissions from domestic coal burning and from vehicles can also contribute to high local ambient concentrations of sulphur dioxide. The principal approaches to controlling SO, emissions include use of low-sulphur fuel; reduction or removal of sulphur in the feed; use of appropriate combustion technologies; and emissions control technologies such as sorbent injection and flue gas desulphurization (FGD).
Since sulphur emissions are proportional to the sulphur content of the fuel, an effective means of reducing SOx, emissions is to burn low-sulphur fuel such as natural gas, low-sulphur oil, or low-sulphur coal. Natural gas has the added advantage of emitting no particulate matter when burned. Today's major emissions control methods are sorbent injection and flue gas desulphurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulphur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulphur oxide emissions.