Sour gas is a naturally occurring form of raw natural gas found deep within the earth. It poses a significant hazard because it contains a substantial quantity of contaminants, primarily the highly toxic compound hydrogen sulfide (H2S). Sour gas must undergo extensive processing before it can be safely used.
Defining Sour Gas and its Composition
Sour gas is natural gas, predominantly methane, that contains significant amounts of hydrogen sulfide (H2S). The presence of this compound classifies the gas as “sour,” distinguishing it from “sweet gas,” which has negligible amounts of the contaminant. The term “sour” derived from the rotten-egg odor of hydrogen sulfide and the sour taste of metals exposed to it.
The exact threshold for classification varies by regulatory body and application. Natural gas is commonly considered sour if it contains above 4 parts per million (ppm) of H2S by volume, though some regulations use a much higher threshold, such as 100 ppm, to trigger additional safety requirements. Hydrogen sulfide is often accompanied by carbon dioxide (CO2). Together, these are sometimes referred to as “acid gases,” but only H2S makes the gas “sour.”
Health Risks of Hydrogen Sulfide Exposure
The danger of sour gas is the toxicity of hydrogen sulfide, a broad-spectrum poison affecting the nervous and respiratory systems. It is a colorless, flammable gas slightly heavier than air, allowing it to accumulate in low-lying, poorly ventilated areas. Hydrogen sulfide is toxic because it inhibits cellular respiration, preventing the body’s cells from utilizing oxygen.
At low concentrations, exposure to H2S can cause irritation to the eyes and respiratory tract, along with symptoms like nausea, headaches, and dizziness. Moderate exposure, around 50 to 100 ppm, can cause marked eye inflammation, coughing, and digestive upset. Prolonged low-level exposure may result in chronic effects such as fatigue, poor memory, and loss of appetite.
A particularly dangerous characteristic of hydrogen sulfide is a phenomenon called “olfactory fatigue” or “olfactory paralysis.” Although the gas has a distinctive rotten-egg smell at low concentrations, continuous or high-level exposure rapidly deadens the sense of smell. This loss of smell can occur at concentrations around 100 to 150 ppm, creating a false and deadly sense of safety for the exposed individual.
At high concentrations, typically above a few hundred ppm, hydrogen sulfide is immediately life-threatening. Exposure to extremely high levels, around 1,000 ppm, can cause immediate unconsciousness, respiratory arrest, and death within minutes, sometimes after only a single breath. This rapid onset of severe symptoms makes hydrogen sulfide the second most common cause of fatal gas inhalation in the workplace, after carbon monoxide.
Industrial Handling and Treatment
Because of its corrosiveness and extreme toxicity, sour gas cannot be transported or used until the hydrogen sulfide is removed. The process of removing H2S and other contaminants is known as “sweetening,” a purification step performed at specialized gas processing plants to meet safety and pipeline specifications.
The most common method for sweetening is called amine treating or amine scrubbing, which uses various aqueous solutions of chemicals called alkanolamines. The sour gas flows upward through an absorption tower, where the down-flowing amine solution chemically reacts with and absorbs the H2S and CO2. This process produces a “sweetened” gas stream that is safe for transport and a “rich” amine solution containing the absorbed contaminants.
The rich amine is then sent to a regeneration unit, where heat is applied to reverse the chemical reaction, stripping the H2S out of the solvent. The recovered hydrogen sulfide, now in a concentrated stream, is often processed further using the Claus process. The Claus process converts the H2S into elemental sulfur, which is a valuable, non-hazardous industrial product that can be sold commercially.