Sour gas is natural gas, petroleum refinery byproduct gas or any other gas containing significant amounts of hydrogen sulfide (H2S). Natural gas is usually considered sour if there are more than 5.7 milligrams of H2S per cubic metre of natural gas at 20 °C, which is equivalent to approximately 4 ppm by volume.
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, commonly by an amine gas treater or similar process. The removed H2S is most often subsequently converted to by-product elemental sulfur in a Claus process. Any mercaptans (RSH) present are commonly removed in a sweetening process.
Processes within petroleum 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 than can be used for piping and other equipment handling sour gas, as many metals are sensitive to sulfide stress cracking. The presence of hydrogen sulfide in gas causes lower quality burning and the production of sulfur dioxide, and so is regulated in commercially sold gas.
Carbon dioxide at concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour. Concentrations above 17% are lethal with exposure of 1 minute or more.
- NaturalGas.org website page Processing Natural Gas
- Energy Information Agency website page Natural Gas Processing: The Crucial Link Between Natural Gas Production and Its Transportation to Market
- U.S. Environmental Protection Agency: "Carbon Dioxide as a Fire Suppressant: Examining the Risks"