How 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 Oxidizers

Process Controls

There are other ways to reduce emissions besides using pollution control equipment--prevent emissions to begin with. Air quality permits help minimize, reduce or prevent emissions as much as possible by placing requirements on how things are done.

Permits can specify the quantity, type, or quality of fuel or other substance used in a process. For example, a permit might specify the maximum percent of sulfur that can exist in the coal to reduce sulfur dioxide emissions. A permit may specify the quantity of volatile chemicals in paint, solvent, adhesive or other product used in large quantity during manufacturing. Permits can also help reduce the impact of emitted pollutants on local air by specifying smokestack height and other factors.

Engineers can also set combustion specifications to minimize emissions. For example, to help reduce nitrogen oxide formation, the combustion conditions in the furnace can be altered. The flame temperature can be lowered or raised, the amount of time air remains in the combustion chamber can be altered, or the mixing rate of fuel and air can be changed. These options are often reviewed, studied and best choices made depending upon cost, plant design and many other variables.

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Bag Houses Or Fabric FiltersIn Iowa, 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.

Potential concerns:

Bag wear or failure, holes in the house can reduce the 'vacuum' and overall efficiency. Collected dry materials must be carefully handled to prevent release into the air.

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:

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.

Scrubbers also have disadvantages:

Water and absorbed gases can become very corrosive, so the scrubber must be properly designed to meet each specific industrial process. Because scrubbers use water, high-humidity air leaving the scrubber can cause large water vapor plumes when emitted into cold Iowa winter air. Fog and precipitation can cause local meteorological problems or driving hazards near the industry. And because water is used to clean the air, the dirty water also needs to be cleaned. Settling ponds and sludge handlers are often needed to clarify the water slurry. High-powered scrubbers are costly to operate when using high fan speeds.

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.

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.

Vapor CondensersCondensers turn a gaseous vapor to a liquid. Any gas can be liquefied if the temperature is lowered enough or if it is pressurized. Condensers cool vapors enough to turn them into liquid.

Dry cleaning machines may use vapor condensers to cool evaporated cleaning solvents such as perchloroethylene for reuse. Without the condenser, the chemical vapor would be lost into the air and more chemicals purchased. Large storage tanks may use condensers to capture evaporated gases and return them to storage. This prevents Iowans from breathing the vapors and allows businesses to use the liquid for its intended purpose, reducing emissions and sometimes saving cost.

Condensers often act as pre-cleaners to remove gas vapors before the air is sent to more expensive control equipment such as incinerators or adsorbers. This reduces the volume of gas needing treatment to reduce costs.

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.

ESPs 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 found across Iowa.

Flares

Combustion can also be used to control emissions of hydrocarbons and other organic vapors such as chlorine, fluorine and particulate matter (soot). If a process emits volatile organic compounds such as ink fumes from a large commercial printing operation, the fumes can be destroyed or burned.

Flares dispose of intermittent or emergency releases of combustible gases from industrial sources. They are often found in refineries or chemical plants. In a flare, combustible gases are burned in a flame. They are designed to do so with minimal smoke (smoke is made up of dense amounts of soot particulates.)

AfterburnersAfterburners use a flame enclosed within a chamber. Combustion by-products include water vapor and carbon dioxide gas. They are commonly used to destroy volatile organic compounds. Heat tolerant refractory bricks line the chamber. Pollutant-laden gases are passed through the chamber and burned at temperatures between 1300 to 1500 degrees Fahrenheit. Because of rising fuel costs, heat recovery systems can use waste heat for useful purposes.

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.

One concern with catalytic oxidizers is contamination or deactivation of the catalyst material. Particulate matter like soot and dust can coat the catalyst surface. Certain chemicals can combine or react with the catalyst to deactivate it. Sulfur in gasoline for example, reduces the longevity of auto catalytic converters. The U.S. EPA is pushing for reduced amounts of sulfur in gasoline and diesel fuel. This will reduce the 'poisoning' of catalytic converters for better emission controls and reduced sulfur oxides in the air.

Measures against Air Pollution

by Vehicle Exhaust

Automobiles play a major role in urban traffi c and have

contributed to making our lives more convenient. However, in

exchange for this convenience, they are causing air pollution.

Due to loose national emission limits on old diesel vehicles,

which have resulted in preventing the popularization of new

diesel vehicles meeting stricter limits and generating chronic

traffi c congestion, air pollution from automobiles has not

improved at all and countermeasures have been lagging for a

long time.

Given these factors, TMG has been working on the

improvement of air pollution and steadily making achievements

by developing measures against environmental pollution from

automobiles including: implementing regulations for

operations of old diesel vehicles based on the Tokyo

Metropolitan Environmental Security Ordinance; promoting

the popularization of low-emission vehicles; and promoting

transportation demand management (TDM). 20

Measures against Diesel

Vehicle Exhaust

●Emissions of Nitrogen Oxide (NOx) and Particulate Matter (PM) in Tokyo

and approximately 30% of PM are emissions from automobiles.

Of the emissions from automobiles, approximately 80% of

nitrogen oxide and almost all PM are from diesel vehicles.

The PM inherent in exhaust gas from diesel vehicles is

known to be carcinogenic and is related to respiratory

diseases and pollen allergies. To protect the health of Tokyo

residents, measures against exhaust gas from diesel vehicles

play an important role.

✔Regulation on Diesel Vehicle Emission

In accordance with the Tokyo Metropolitan Environmental

Security Ordinance, TMG enforced regulations on diesel

vehicle emissions in October 2003.

Regulated diesel vehicles include diesel vehicles that do

not satisfy the nation’s new short-term regulation (by March

2006, the nation’s long-term regulation applies) of buses,

trucks, and special automobiles including concrete mixer

vehicles, sanitation vehicles, and refrigerator and freezer

vehicles (excluding passenger vehicles).

Since the enforcement of the regulations, diesel vehicles

✔Importance of Measures against Diesel Vehicle Exhaust

Looking at emissions of nitrogen oxide (NOx) and

particulate matter (PM) in Tokyo, approximately 50% of NOx that do not satisfy PM emissions standards designated by the

ordinance cannot be driven in Tokyo. However, for vehicles

fi rst-registered in the last 7 years, application of the regulations

will be waived. Vehicles failing to meet the standards are

required to be replaced by vehicles satisfying the latest

regulations or low-emission vehicles or to install PM reduction

equipment designated by the Governor of Tokyo.

Additionally, similar regulations are being enforced in

neighboring Saitama, Chiba, and Kanagawa Prefectures.

After the enforcement of regulations in 2003, PM emission

standards designated by the ordinance employed the same

values used for former long-term emission regulations applied

for new cars by the country. However, since April 1st, 2006,

Saitama Prefecture and TMG have imposed stricter limits, the

same values as the new short-term country regulations.

✔Regulating Illegal Diesel Vehicles

To secure the effectiveness of various regulations related to

automobiles, in accordance with the Tokyo Metropolitan

Environmental Security Ordinance, TMG assigned Tokyo vehicle

pollution regulators (also known as illegal diesel vehicle regulators),

regulating vehicles failing to meet the standards. Contents of the

regulatory actions by the regulators include vehicle inspections on

the street and at distribution centers and regulating illegal vehicles

by recording vehicles with video cameras. Additionally, from June

2004, by utilizing fi xed cameras installed on the Metropolitan

Expressway, measures

against vehicles entering

Tokyo from other prefectures

have been strengthened.

Furthermore, TMG opened

a stop the diesel black

smoke (dry soot) hotline

so that Tokyo residents

can report illegal diesel vehicles.

From the enforcement of the regulations to the end of

March 2011, regulatory actions were taken at a total of 930

locations on the street and at distribution centers (including

truck terminals, and piers). Additionally, inspections recorded

with video cameras were conducted at a total of 791 locations.

(URL) http://www.kankyo.metro.tokyo.jp/vehicle/air_pollution/diesel/regulation/g_

men.html (in Japanese)

Where it has been confi rmed that a noncompliant vehicle

has been driven within Tokyo, prompt compliance with the

applicable regulations will be urged, and the person

responsible for operation of the vehicle will be banned from

driving in Tokyo as his or her administration penalty for

noncompliance.

If the person banned from driving fails to comply with this

order, he or she will be subject to punishment involving the

public disclosure of his or her identity as an offender and a

fi ne of up to 500,000 yen.