Industrial production generates a host of air pollutants that must be mitigated using various air-pollution control (APC) technologies. This one-page reference provides information on the most significant and prevalent categories of air pollutants from industrial sites.
Acid gases
Acid gases are so named due to their tendency to form strong acids in the atmosphere. This class of pollutants generally results from the combustion of fossil fuels and once emitted, can react with water and oxygen in the atmosphere to form sulfuric, nitric or hydrochloric acid. The presence of these acids can lower the pH of rainwater, which damages plants, soil and freshwater bodies. The gases themselves can also have a direct impact on human health with exposure. Wet scrubbers using basic solutions of sodium hydroxide, for example, are commonly used to treat this type of pollution.
Sulfur oxides (SOx). Sulfur oxide pollutants, primarily SO2 and SO3, are generated when sulfur-containing fossil fuels are burned at power plants, or combusted at industrial facilities to heat boilers. Smelting of metal ores and petroleum refining are examples of processes that can produce SOx.
Nitrogen oxides (NOx). Nitrogen oxides are reactive gases primarily produced from the high-temperature combustion of fossil fuels. These pollutants, mainly nitric oxide (NO) and nitrogen dioxide (NO2), cause respiratory issues, form acid rain, and create ground-level ozone. Key industrial sources include power-generation facilities, industrial boilers and furnaces, and waste-incineration plants.
Hydrochloric acid (HCl). Most HCl results from the combustion of fossil fuels, especially coal, but it is also generated by waste-incineration and waste-to-energy processes, as well as the production of chlorinated chemicals, such as polyvinyl chloride. HCl is highly corrosive to metal.
Hydrogen sulfide (H2S). A number of industrial processes generate H2S, including pulp and paper manufacturing, petroleum refining and wastewater treatment.
Hydrofluoric acid (HF). HF is formed in much lower volumes than SOx or NOx pollutants, but HF is far more harmful at lower concentrations than the previous two. HF is used in alkylation units at petroleum refineries and can be released during manufacturing of bricks and ceramics.
Particulate materials
Particulate matter (PM) generally refers to airborne solid material and liquid droplets. They are often classified by particle size, such as PM2.5 or PM10. PM2.5 particles have diameters smaller than 2.5 microns, while PM10 are those with diameters less than or equal to 10 microns (Figure 1). Both can present health issues, such as asthma and difficulty breathing in the short term, and chronic lung issues with long-term exposure. PM2.5 particles are considered more dangerous because they are able to travel deeper into the lungs and can enter the bloodstream, while PM10 materials are largely trapped in the nose, throat and upper airways.
FIGURE 1. Particulate matter pollution is generally categorized by particle size, with PM2.5 particles considered more dangerous due to their ability to move deeper into respiratory airways
PM may be either directly emitted from sources (primary particles) or formed in the atmosphere through chemical reactions of gases (secondary particles), such as SO2 and NOx. Power generation and cement, steel and chemical manufacturing are industrial producers of PM.
HAPs and VOCs
Volatile organic chemicals (VOCs) are defined as carbon-based compounds that readily vaporize at ambient temperatures and near atmospheric pressures. Not all VOCs carry specific health risks, but those that have been found to cause (or are suspected of causing) health problems, such as respiratory distress, cancer, birth defects and others, as well as environmental damage, are considered hazardous air pollutants (HAPs).
Common industrially emitted HAPs include benzene, formaldehyde, acetaldehyde, toluene and xylenes, methanol, ethylene oxide and hexanes. Other HAPs are metal-containing compounds, such as those incorporating mercury, lead, cobalt, chromium, arsenic and other heavy metals. The U.S. Environmental Protection Agency (EPA; Washington, D.C.; www.epa.gov) lists 188 substances as HAPs [1].
The petroleum-refining, chemical manufacturing, oil-and-gas, pulp and paper and steel-making sectors generate the most HAPs.
Reference
1. U.S. Environmental Protection Agency. https://www.epa.gov/haps/initial-list-hazardous-air-pollutants-modifications/.