Sources of Pollutants in the Ambient Air - Stationary Sources

Figure: Stationary sources

Stationary sources are non-moving sources, fixed-site producers of pollution such as power plants, chemical plants, oil refineries, manufacturing facilities, and other industrial facilities. There are hundreds of thousands of stationary sources of air pollution in the United States. Stationary sources emit both criteria pollutants and hazardous air pollutants (HAPs). Air pollution from stationary sources is produced by two primary activities. These activities are stationary combustion of fuel such as coal and oil at power generating facilities, and the pollutant losses from industrial processes. Industrial processes include refineries, chemical manufacturing facilities, and smelters.

Stationary sources have many possible emission points. An emission point is the specific place or piece of equipment from which a pollutant is emitted. Air pollutants can be emitted from smokestacks, storage tanks, equipment leaks, process wastewater handling/treatment area, loading and unloading facilities, and process vents. A process vent is basically an opening where substances (mostly in gaseous form) are "vented" into the atmosphere. Common process vents in a chemical plant are distillation columns and oxidation vents. Emissions from storage tanks are due to pollutants that can leak through the roofs, and can leak through tank openings when liquids expand or cool because of outdoor temperature changes. Also, air pollutants can escape during the filling and emptying of a storage tank. Air pollution produced from wastewater occurs when wastewater containing volatile chemicals comes in contact with the air. Volatile means that it can be evaporated, or pass from a liquid state to a gaseous state.

Large, stationary sources of emissions that have specific locations and release pollutants in quantities above an emission threshold are known as point sources. Those facilities or activities whose individual emissions do not qualify them as point sources are called area sources. Area sources represent numerous facilities or activities that individually release small amounts of a given pollutant, but collectively can release significant amounts of a pollutant. For example; dry cleaners, vehicle refinishing and gasoline dispensing facilities, and residential heating will not typically qualify as point sources, but collectively the various emissions from these sources are classified as area sources.

Stationary sources are also classified as major and minor sources. A major source is one that emits, or has the potential to emit, pollutants over a major source threshold. A minor source is any source which emits less pollutants than the major source threshold.