Air Pollution Control Technology in the United States

Air Pollution Control Technology in Environmental Law

Methods used to reduce the amount of air contaminants discharged from a source. “Sources” originate air pollutants. Examples of sources include industrial plants, automobiles, and numerous small businesses, such as dry cleaners. Technology is the application of knowledge to a particular problem.

Air pollutants appear in two types: particles (called particulates) and gases. Selection of proper mechanisms hinges on their form and, in the case of gases, their chemical composition.

When air pollution was first regulated, most regulators focused on particulates. Particulates impact visibility by contributing to smog, and they affect breathing. Because of government emphasis, technology to reduce particulates developed sooner than it did for gaseous air pollution.

Controlling air pollution at the source may be accomplished at three different times in a process. The first is the beginning: substitution of materials or fuel may impact the creation of air contamination. Second, changes made to the process itself can eliminate the formation of pollutants. Finally, at the end of the process, the air pollutants may be captured and destroyed or reduced to an acceptable level.

To illustrate the first option, a printing company may be using petroleum based inks and generating air pollution during the printing process and when cleaning equipment. By switching to a vegetable based ink (most commonly soy based), pollution will be drastically reduced.

Process design controls are called “in process” technology. An example would be a system that captures solvents in a closed system, cleans them, and returns them to use within the process.

The treatment of air pollution at the end of the process, known as end of pipe treatment, occurs when pollutants are captured and treated before release to the atmosphere. For example, a scrubber maybe added at the end.

Add-on or endofpipe treatment has been the method of choice for many years. The major reason is cost: it is cheaper to change an industrial system at the end than to redesign an entire process. However, with the air emission limitations becoming more stringent, industries that want to build new plants (or modify old ones) are forced to consider air pollution control while designing their structures.

Several types of technology used for air pollution control are examined below. Particulates and gaseous emissions are treated separately since they must be the focus of a chosen method. However, many air pollution streams contain both particulates and gases, so treatment may involve more than one technology.

Technology Applied to Particulates

Particulates are differentiated by their size, concentration, and some physical properties (like density). Use of specific control equipment follows an analysis of the particulate’s characteristics. Four categories of equipment address most air particulate pollution: cyclones, fabric filters, wet scrubbers, and electrostatic precipitators.

Cyclones operate as one might expect. The incoming pollutant stream is forced into a vortex spin. The coarser particulates fall out and are collected.

Fabric filters operate in the same manner as straining a liquid through a cheesecloth. The filter, a bag often made of cotton, fiberglass, or teflon, acts as a surface upon which the particulates collect as the air stream is pushed through. The dust cake is removed periodically.

Wet scrubbers use a spray liquid to remove particulates from an air stream. The liquid is introduced countercurrent to the air flow.

Electrostatic precipitators use electricity to separate the particulates from the air stream. The particulates are given an electrical charge and then they are attracted to an oppositely charged plate, where they collect until removed as a dust cake.

Electrostatic precipitators use electricity to separate the particulates from the air stream. The particulates are given an electrical charge and then they are attracted to an oppositely charged plate, where they collect until removed as a dust cake.

Technology Applied to Gaseous Pollutants

Gaseous pollutants vary according to their chemical composition. For that reason, analysis of the proper control technology is more complex and not as easily broken out into categories as particulate control technology. However, certain processes recur within treatments to remove gaseous pollutants.

Absorption involves assimilation of the gas into a liquid. It is not a surface phenomenon, as adsorption is. The gas must be capable of being dispersed in the liquid. In some cases, a chemical reaction may also be engineered by the selection of a liquid that will absorb the gas. Examples of commonly used absorbing liquids are water, caustic, sodium carbonate, and certain nonvolatile oils.

Catalytic oxidation is primarily a thermal (heat) process, but it uses a catalyst to enhance the efficiency of the thermal unit. The technology operates at lower temperatures than a thermal oxidizer. Catalytic oxidizers operate within a range of 400° to 1100° Fahrenheit. They are quite efficient in treating volatile organic compounds. A catalyst is a substance that enables a chemical reaction to occur faster or under different conditions than those normally required. The catalyst itself does not change during the reaction. A mixture of metals is the common catalyst for a catalytic oxidizer. It is placed at the bottom of the unit and is designed to allow maximum surface contact. Honeycomb patterns are not uncommon.

Thermal oxidation requires more fuel to destroy air pollutants because it operates solely on the heat provided. The temperatures of a thermal oxidation unit (commonly called an incinerator) is at least 1500° Fahrenheit. Thermal oxidation is cheaper to set up, but more expensive to operate, than a catalytic oxidizer. Their efficiency of destruction is roughly the same.
Based on “Environment and the Law. A Dictionary”.