Acid gas

Acid gas is natural gas or any other gas mixture which contains significant amounts of hydrogen sulfide (H2S), carbon dioxide (CO2), or similar contaminants. Although the terms acid gas and sour gas are used interchangeably, strictly speaking, a sour gas is any gas that contains hydrogen sulfide in significant amounts. Whereas an acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide (CO2) or hydrogen sulfide. Thus, carbon dioxide by itself is an acid gas but it is not a sour gas.

Before a raw natural gas containing hydrogen sulfide and/or carbon dioxide can be used, the raw gas must be treated to remove those impurities to acceptable levels and that is commonly done by using an amine gas treating process. The removed H2S is most often subsequently converted to by-product elemental sulfur in a Claus process.

Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide.

Hydrogen sulfide is a toxic gas. It also places restrictions on the materials that can be used for piping and other equipment handling sour gas, as many metals are sensitive to sulfide stress cracking.