Major air pollutants are gases such as carbon monoxide, nitrogen oxides, sulfur oxides, and VOCs. Generally, the pollutant concentrations in waste air streams are relatively low, but emissions can still exceed the regulatory limits. Removing air pollutants is achieved by the following methods:
Absorption by a liquid solution
Condensation of pollutants by cooling the gas stream Adsorption on a porous adsorbent
Chemical conversion of pollutants into harmless compounds
Sometimes methods are combined to treat a feed stream. For example, the absorption of SO2 can be performed in an absorber using an aqueous lime solution. The key step in the separation is absorption although reactions occur between the SO2 and lime in the absorber. Therefore, this SO2 separation is considered absorption.
Choosing the air pollutant removal method depends mostly on the physical and chemical properties of the pollutant and the conditions (i.e., temperature, pressure, volume, and concentration) under which the pollutant is treated. The methods chosen for reducing air pollution must not increase pollution in other sectors of the environment. For example, transferring the air pollutant into liquid or solid absorption agents that subsequently contaminate the environment is not a solution to the problem.
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