DEFINING AIR POLLUTIONThe presence of contaminants or pollutant substances in the air that interfere with human health or welfare, or produce other harmful environmental effects referred as Air Pollution United States Environmental Protection Agency (2007) Terms of Environment: Glossary, Abbreviations and AcronymsTYPES OF AIR POLLUTANTSThere are generally two types of air pollutants:I. Primary pollutantsII. Secondary pollutants Primary pollutantsSubstances emitted directly from sources, are called primary pollutants. They are certainly not innocuous.A substantial portion of the gas and vapors emitted to the atmosphere in appreciable quantity from anthropogenic sources tends to be relatively simple in chemical structure: carbon dioxide, carbon monoxide, sulfur dioxide, and nitric oxide from combustion processes; hydrogen sulfide, ammonia, hydrogen chloride, and hydrogen fluoride from industrial processes. The solvents and gasoline fractions that evaporate are alkanes, alkenes, and aromatics with relatively simple structures. In addition, more complex molecules such as polycyclic aromatic hydrocarbons (PAHs) and dioxins are released from industrial processes and combustion sources and are referred to as toxic pollutants. Substances such as these, emitted directly from sources, are called primary pollutants. However, the primary pollutants do not, of themselves, produce all of the adverse effects of air pollution. Secondary pollutantsThe pollutants manufactured in the air by Chemical and photochemical reactions among the primary pollutants and the constituents of the unpolluted atmosphere are called secondary pollutants. SO3 sulphur trioxide NO2 nitrogen dioxide PAN (peroxylacetyl nitrate) Ozone Aldehydes & Ketones Various Sulphates & nitrate saltsChemical reactions may occur among the primary pollutants and the constituents of the unpolluted atmosphere. The atmosphere may be viewed as a reaction vessel into which are poured reactable constituents and in which are produced a tremendous array of new chemical compounds, generated by gases and vapors reacting with each other and with the particles in the air. The pollutants manufactured in the air are called secondary pollutants; they are responsible for most of the smog, haze, and eye irritation and for many of the forms of plant and material damage attributed to air pollution. With the knowledge that each secondary pollutant arises from specific chemical reactions involving specific primary reactants, we must control secondary pollutants by controlling how much of each primary pollutant is allowed to be emitted.Factors influencing formation of secondary pollutants:The reaction mechanisms and various steps involved in the formation of secondary pollutants are influenced by many factors such as: Concentration of reactants Amount of moisture present in the atmosphere Degree of photo activation Metrological forces Local topography

Sources of Air PollutionI.General sourcesThe sources of air pollution are nearly as numerous as the grains of sand. In fact, the grains of sand themselves are air pollutants when the wind entrains them and they become airborne.A. Natural SourcesAn erupting volcano emits particulate matter. Pollutant gases such as SO2, H2 S, and methane are also emitted. The emission from an eruption may be of such magnitude as to harm the environment for a considerable distance from the volcanic source. Clouds of volcanic particulate matter and gases have remained airborne for very long periods of time.Accidental fires in forests and on the prairies are usually classified as natural sources even though they may have been originally ignited by human activities. In many cases foresters intentionally set fires in forestlands to burn off the residue, but lightning setting off a fire in a large section of forestland could only be classed as natural. Such a fire emits large quantities of pollutants in the form of smoke, unburned hydrocarbons, carbon monoxide, carbon dioxide, oxides of nitrogen, and ash.Other natural sources, such as alkaline and saltwater lakes, are usually quite local in their effect on the environment. Sulfurous gases from hot springs also fall into this category in that the odor is extremely strong when close to the source but disappears a few kilometers away.B. Anthropogenic Sources

1. Industrial Sources The reliance of modern people on industry to produce their needs has resulted in transfer of the pollution sources from the individual to industryA great deal of industrial pollution comes from manufacturing products from raw materials:(1) iron and steel from ore, (2) lumber from trees, (3) gasoline and other fuels from crude oil, and (4) stone from quarries. Each of these manufacturing processes produces a product, along with several waste products which we term pollutantsIndustrial pollution is also emitted by industries that convert products to other products(1) automobile bodies from steel, (2) furniture from lumber, (3) paint from solids and solvents, and (4) asphaltic paving from rock and oil. Industrial sources are stationary, and each emits relatively consistent qualities and quantities of pollutants.2. Utilities The utilities in our modern society are so much a part of our lives that it is hard to imagine how we survived without them. An electric power plant generates electricity to heat and light our homes in addition to providing power for the personal computer, television, refrigerator, and the recharging of the batteries for the laptop, cell phone etc.Utilities are in the business of converting energy from one form to another and transporting that energy. If a large steam generating plant, producing 2000MW, burns a million kilograms per hour of 4% ash coal, it must somehow dispose of 40 000 kg of ash per hour. For a typical generating plant, the gaseous emissions would include 341 000 kg of oxides of sulfur per day and 185 000 kg of oxides of nitrogen per day. Another type of utility that is a serious air pollution source is the one that handles the wastes of modern society. An overloaded, poorly designed, or poorly operated sewage treatment plant can cause an air pollution problem. In many countries around the world, open dumps still exist. These may catch fire and release harmful plumes of smoke and fumes. Even in more economically developed nations, landfills remain sources of dust and smoke, as are fires in abandoned mines shafts and in industrial waste disposal and reclamation sites.3. Personal Sources Even though society has moved toward centralized industries and utilities, we still have many personal sources of air pollution for which we alone can answer(1) automobiles, (2) home furnaces, (3) home fireplaces and stoves, (4) backyard barbecue grills, and (5) open burning of refuse and leaves319How Air Pollution Is ControlledPollution control equipment can reduce emissions by cleaning exhaust and dirty air before it leaves the business. A wide variety of equipment can be used to clean dirty air. DNR engineers carefully study and review how these controls may work and the methods and requirements are put into a permit - a major duty performed by the DNR.How common control equipment works is explained below: Bag Houses or Fabric Filters Scrubbers or Wet Collectors Adsorbers Cyclones Vapor Condensers Electrostatic Precipitators Flares Afterburners Catalytic OxidizersBag Houses Or Fabric FiltersThe fabric filters, commonly called baghouses, are widely used industrial strength "vacuum cleaners." They remove particulate matter found in smoke, vapors, dust or mists.The filters remove particles from exhaust gases, leaving the particles on the filter while the cleaner air passes through. Collected particulates form a "dust cake" on the filter that is routinely cleaned off by a blast of air in the opposite direction or by mechanical shaking. The dust cake falls into a hopper for disposal or reuse in the industrial process.Filter bags hang in a sturdy house. Sometimes the house is insulated when cleaning hot gases to prevent corrosive moisture or acid mists from condensing and harming the equipment. Sometimes dozens, even hundreds of cylindrical filters, each eight to 40 feet long may hang in a series of houses at one location.Filters are made of woven cotton, wool or synthetic materials. Some synthetic materials can withstand high temperatures or are resistant to chemical reaction. Each baghouse must meet the needs of the particular industry process. Gas temperature, moisture content and chemical reactivity decide what filter material is used.When an industry applies for an air quality permit to install a baghouse, DNR air quality engineers must determine if the baghouse has enough filters and surface area to remove an adequate amount of pollutants before issuing the permit. The DNR also reviews the design to ensure the proposed fan size is adequate to pull or push air through the series of filters. This is determined by the "pressure drop" or the measurement of air resistance across the filters. This can be learned from similar baghouses or calculated using mathematical equations and laws of physics.A baghouse with a high-pressure drop needs a larger fan or more energy to move dirty air through the filters. Proper design, reflected in a good permit, helps ensure the right equipment is installed to remove enough pollutants. This can reduce the number of operating problems when using the baghouse and prevent possible air pollution violations. DNR staff also reviews the bag cleaning method, hopper design, and other factors.Baghouse benefits:Effectively removes large percentages of particulate matter.

Scrubbers Or Wet CollectorsScrubbers or wet collectors remove particles or gases from the exhaust stream by using water sprays. Gases can be absorbed if they are water-soluble or by adding various chemicals to the spray. Particles of dust or soot can also be captured in microscopic liquid mists. Before the exhaust leaves the scrubber, the liquid mists must be collected before the exhaust enters the public air.Generally, high-powered scrubbers remove more particles but are more costly to operate due to added energy costs. Scrubbers that remove gases like sulfur oxides, nitrogen oxides or hydrochloric acid depend more heavily on the mechanical and chemical engineering design, not as much on power. Scrubbers are generally better at removing particles than cyclones, but not as good as electrostatic precipitators or baghouses unless operated at high power.Scrubbers collect microscopic particles by injecting fine mists of water into the exhaust. The moving particles cannot avoid impacting into the droplets, making them easier to collect. Scrubbers must do two things: bring pollutants into contact with the water mist, then remove the airborne water mist. Removing mist sounds simple, but the water droplets are extremely small and the air flows through the scrubber very quickly.The DNR air quality engineers use physics, chemistry and engineering skills to determine how the system will operate before an air quality permit is issued. The DNR also works with professionals from the business too. Temperature, flue gas and pollutant composition, air pressure, solubility and the chemical reactivity must be calculated to figure out how much pollution will be removed.Scrubbers come in all shapes and sizes. Sometimes chambers are filled with water spray nozzles. Others use complex systems of baffles, motors, sprays and nozzles. Others use chambers packed with small, odd shaped fill material to increase surface area for particles and water to hit for collection. Some force pollutants through a small nozzle passage to increase the gas flow as it squeezes through the small opening. Here water spray is injected and sheared into fine droplets for the particles to hit.Scrubbers offer many design options to meet a variety of air pollution control needs. For example, "flue gas desulfurization" scrubbers inject lime or limestone to react with sulfur dioxide gas to form sulfates, which are then removed.Scrubber benefits:It can collect both particles and gases and can handle high temperature gases. Fire and explosion hazards found in some dry-collection systems are eliminated with wet collection. Once the pollutants are collected, they cannot escape easily, unlike dry collection systems where dust can be released from hoppers. The water slurry can sometimes be easier to handle than dry dust.

AdsorbersAdsorption is similar to using a sponge to soak up water. A porous solid material is used to soak up gaseous air contaminants from the exhaust air. Inside an adsorber, air passes through a layer of materials for pollutants to adhere or "stick." Eventually the adsorbent material becomes saturated or filled with the pollutant and can hold no more. Similar to squeezing a sponge for reuse, the adsorbent material must be refreshed for reuse or disposed.Activated carbon, silica gel and alumina oxide are common adsorbing agents. The chemical nature of the adsorbent, the total surface area (how porous it is), and pore diameter are given careful consideration before adsorbers are used. If the gas contains particulate matter, the adsorbent bed can become clogged. Some gas is precleaned by a baghouse filter, electrostatic precipitator or cyclone to remove particulate matter before entering the adsorber.How are Adsorbers Cleaned?Like squeezing a sponge for reuse, adsorbers can be refreshed too. There are several chemical principles that apply to how adsorbers work. For example, as gas temperature increases, adsorption decreases. As gas pressure increases, so does adsorption. The slower the gas moves through adsorption materials, the more gas removed. These same principles are used to regenerate or refresh saturated adsorbers.Temperature can be increased to release pollutants from the activated charcoal or the pressure can be decreased. "Steam stripping" uses injected steam to remove pollutants from activated charcoal. Many organic compounds can be condensed, distilled or decanted from captured steam and reused instead of emitted into the air. Charcoal canisters are used to capture emissions from some dry-cleaning machines to remove perchloroethylene, a toxic chemical.Degreasing, rubber processing, and printing operations sometimes use adsorbers. Toxic or odorous vapors from food processing, rendering plants, sewage treatment plants and many chemical manufacturing process use adsorbers too.Activated carbon can be made from wood, coal, coconut husks or other nutshells and petroleum byproducts. To activate or make it porous, the material is heated in a chamber with little air. This produces a material with a surface area so great that one gram may have two to five football fields of surface! Activated carbon is often used for controlling organic pollutants such as solvents, odors, toxic gases and gasoline vapors.Cyclone

o most Iowans, the word "cyclone" refers to Iowa State University-the home of the Cyclones. But in air pollution control, a cyclone is a device used to remove larger size particles, about 20 microns in diameter, from the air stream. Often, more than 80 percent of particles are removed. More efficient equipment like baghouses or electrostatic precipitators can then remove the smaller particles.Cyclones are often found at feed mills, crushers, dryers, grain elevators, and even high school industrial arts classrooms. Photos. In industrial uses, cyclones are often used as precleaners for more expensive and sophisticated control equipment such as electrostatic precipitators or baghouses.Dirty air is forced into the cyclone where it moves in a circular motion in increasingly tighter circles. Centrifugal force causes the larger particles to move toward the outside wall. Like a large, fast moving car attempting a tight curve, the large particles cannot make the turn. They impact the wall and fall to the bottom for collection. Particles are also knocked out of the air stream when they collide with each other. Groups of cyclones hooked together are called multicyclones. Multicyclones are more efficient at removing fine particulate matter.Cyclone benefits:Few moving parts, very low capital and operating cost, materials can withstand acids, high heat and pressure.

Electrostatic PrecipitatorsElectrostatic Precipitators or ESPs have been used in industry for over 60 years. They can collect particles sized 0.1 to 10 microns very efficiently. They are generally more efficient at collecting fine particles than scrubbers or cyclones.Electrostatic precipitators take advantage of the electrical principle that opposites attract. A high voltage electrode negatively charges airborne particles in the exhaust stream. As the exhaust gas passes through this electrified field, the particles are charged. Typically 20,000 to 70,000 volts are used. A large, grounded flat metal surface acts as a collection electrode. Microscopic particles are attracted to this surface where they build-up to form a dust cake. Periodically, a rapper strikes the plate to knock the dust cake into a collection hopper.Because no filters are used, ESP's can handle hot gases from 350 to 1,300 degrees Fahrenheit.A shell or house contains the electrodes, exhaust gases and rapper. The shell must be well built with a rigid frame to hold the components in their proper place. Hot temperatures inside the shell can vary greatly from subzero Iowa winter temperatures outside the shell. Such tremendous temperature differences can cause expansion and contraction to stress joints and welds. Often shells are insulated to minimize temperature differences and prevent gases from condensing into corrosive liquids.To ensure ESPs work well, engineers must figure out a number of things. "Resistivity" means the particles have some resistance to electricity. This can reduce the effectiveness of an ESP in cleaning exhaust gas. Sometimes changing the gas temperature or changing the water vapor in the gas can reduce resistivity. To make these changes, the correct amounts of water or steam must be added. Sometimes "conditioning agents" such as sulfuric acid, ammonia, and soda ash can be added. Each option can require professional study and review.The DNR's air quality engineers review air pollution permit applications to ensure the pollution control equipment is the correct size and many other factors. For example, the collection plates must be large enough to clean the volume of gas placed through the ESP. Many technical items must be calculated and agreed upon by DNR engineers and industry professionals before the DNR issues an air quality permit.BenefitESPs can handle large volumes of hot exhaust gases - beneficial for high-temperature exhausts found at Portland Cement plants, steel industry furnaces and industrial and utility boilers.Catalytic OxidizersA catalyst is a chemical that causes or speeds up chemical reactions without the catalyst itself changing. A catalyst can speed up the burning of organic gases or require lower temperatures to save fuel usage and reduce costs. Platinum or palladium are two elements often used as catalysts. Automobile catalytic converters operate on the same principles to reduce tailpipe emissions.http://www.iowadnr.gov/Environment/AirQuality/HowAirPollutionIsControlledgarden.lovetoknow.com

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