Air Pollution Control Methods
Authors: Crocker Burton B. 
Published: July 2004

Air pollution may be rendered less harmful by reducing the concentration of contaminants, the exposure time, or both. Selection of pollution control methods is generally based on the need to control ambient air quality in order to achieve compliance with standards for criteria pollutants, the need to reduce emission to the atmosphere of a hazardous air pollutant, or in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem: a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. The classes of pollutants include gases and particulates; odors are often discussed separately. To achieve air pollution control, reliable measurements are needed to quantify both the pollutant concentration and the contribution of individual sources. These data are necessary for designing control equipment, for monitoring emissions, and for maintaining acceptable ambient air quality. Two categories of measurement techniques are ambient and source measurement. Ambient sampling may help to establish and operate a pollution alert network, monitor the effect of an emission source, predict the effect of a proposed installation, locate the source of an undesirable pollutant, or obtain permanent sampling records for legal action or for modifying regulations. Source sampling problems are distinct from those of ambient sampling. Source gas may have a high temperature or contain high concentrations of water vapor or entrained mist, dust, or other interfering substances. Typical objectives of source sampling are demonstrating compliance with regulations, obtaining emission data, and determining the need for maintenance of process or control equipment. The Clean Air Act in 1990 has three areas of emphasis: acid rain reduction in the northeastern United States; severe limitation on atmospheric emissions of 189 chemicals on the hazardous or Toxic Substance list; and tightened regulations on vehicular exhaust (ozone compliance and smog reduction). Five methods are available for controlling gaseous emissions: absorption, adsorption, condensation, chemical reaction, and incineration. Atmospheric dispersion from a tall stack is now less viable. Adsorption is desirable for contaminant removal down to extremely low levels. Condensation is best for substances having rather high vapor pressures. Incineration is used to remove organic pollutants and small quantities of H2S, CO, and NH3. Specific problem gases such as sulfur and nitrogen oxides require combinations of methods. Major sources of sulfur dioxide are the combustion of sulfur‐containing fossil fuels, sulfur recovery from petroleum processing, and pulp and paper manufacture. Combustion emissions are controlled by substituting a low sulfur fuel source, by fuel desulfurization and refining, and by sulfur removal either in the combustion process or from the flue gas. Major sources of nitrogen oxide emission are nitrogen fixation during high temperature combustion, nitric acid manufacture and concentration, and vehicular emissions. NO formation in combustion may be reduced by maintaining low excess air, employing two‐stage combustion, flue gas recirculation, and burner placement. The removal of particles (liquids, solids, or mixtures) from a gas stream requires deposition and attachment to a surface. Devices for particle collection by filtration can be divided into three categories: cloth filtration, paper and mat filters, and in‐depth aggregate bed filtration. Scrubbers can be highly effective for both particulate collection and gas absorption. Scrubbers make use of a combination of particulate collection mechanisms. Wet‐scrubber collection efficiency may be unexpectedly enhanced by particle growth. Vapor condensation produced by cooling, can lead to particle growth or agglomeration.