Air Pollutants

Criteria Air Pollutants

The Clean Air Act regulates six common air pollutants: particle pollution (particulate matter), ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. These are called “criteria” air pollutants because the U.S Environmental Protection Agency (EPA) sets human health-based and environmentally-based criteria to limit the amount of these pollutants that are permissible in the ambient air. These limits are called primary and secondary national ambient air quality standards (NAAQS).

In 1997 particulate matter (PM) pollution was divided into two divisions, particles equal to or smaller than 10 micrometers in diameter (PM10), and particles equal to or smaller than 2.5 micrometers in diameter (PM2.5). At one time the belief was that if you pulverized particles, the danger went away. Health care professionals learned that pulverizing particles actually increased the danger because microscopic particles get deep into the lungs.

Ozone and particle pollution are the most widespread health threats.

Hazardous Air Pollutants

U.S. EPA and Iowa DNR regulate 188 air pollutants known or suspected to cause cancer or other serious health effects such as reproductive effects or birth defects, or adverse environmental consequences. These pollutants are called hazardous air pollutants (HAP) or air toxics.

Air toxics are generally more localized than the criteria pollutants and the highest levels are close to their sources. Most air toxics originate from human-made sources, including cars and trucks, factories, power plants and refineries, as well as some building materials and cleaning solvents.

U.S. EPA is responsible for setting national standards for regulating sources of HAP, called National Emission Standards for Hazardous Air Pollutants (NESHAP).

Because of the difficulty in assessing air toxic health risks with the original Clean Air Act enacted in 1970, Congress amended the Clean Air Act in 1990 to emphasize controlling emissions of air toxics through available control technology, and then periodically evaluating any remaining risk from air toxics.

Pollution 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.

There are other ways to reduce emissions besides using pollution control equipment, such as first preventing emissions. 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 certain paints, solvents, adhesives or other products 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.

DNR Engineers can also set combustion specifications to minimize emissions. For example, the combustion conditions in the furnace can be altered to help reduce nitrogen oxide formation. 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 evaluated and best choices made depending upon cost, plant design, and many other variables.

Common control equipment and how it works is explained below:

Adsorption 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 processes use adsorbers too.

Activated carbon is the most common adsorbent material and 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.

Afterburners use a flame enclosed within a chamber. They are commonly used to destroy volatile organic compounds. Afterburners are commonly used as a control device for incinerators. 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. Combustion by-products include water vapor and carbon dioxide gas. Because of rising fuel costs, heat recovery systems can use waste heat for useful purposes.

Fabric filters, commonly called baghouses, are industrial strength "vacuum cleaners." They remove particulate matter found in smoke, vapors, dust, or mists. They are used by many industries for many different processes in Iowa.

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 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 are factors used to determine the filter material to be 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. An appropriate pressure drop range can be calculated using engineering principles or obtained from prior uses of similar baghouses.

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

A 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 and diesel fuels reduces the effectiveness and longevity of automobile catalytic converters. To help reduce vehicle pollution, the U.S. EPA now requires the use of ultra-low sulfur diesel (ULSD) fuel in diesel engines. The sulfur content of ULSD is limited to 15 ppm. EPA limits the sulfur content of gasoline to 10 ppm.

In the air pollution control realm, a cyclone isn't a weather phenomenon but a device used to remove larger size particles from an air stream, down to about 20 microns in diameter. Often, more than 80 percent of these larger 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.

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.

Potential Concerns:

Sticky materials can clog cyclones, hard or sharp edged particles can wear them out. fails to control ultra-fine particles.

Electrostatic 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 satisfies 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.

Combustion can be used to control emissions of hydrocarbons and other organic vapors such as chlorine, fluorine, and carbon-based particulate matter (soot). If a process emits volatile organic compounds such as ink fumes from a large commercial printing operation, the fumes can be 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 generally made up of dense amounts of soot particulates.)

Scrubbers 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.

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. 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.

Condensers 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.