Natural gas, biogas, and water that contain sulfur compounds or ammonia are considered “sour.” In low concentrations, these contaminants can be the source of maleficent odors, while in higher concentrations they can present hazards to infrastructure from corrosion and human health from respiratory illnesses and in some cases death. Treating sourness in fluids is called gas sweetening, because it makes it “less sour” by removing the sulfur and ammonia contaminants.
When it comes to methane gas, whether it is produced from a well drilled deep into the earth or generated on the surface in an anaerobic digester, being sour usually refers to the presence of hydrogen sulfide (H2S). Whether in the context of natural gas or biogas, gas sweetening is synonymous with hydrogen sulfide removal, also known as desulfurization.
There are several strategies for gas sweetening based on several factors, including:
- The volume of gas to be treated
- The concentration of H2S in the gas stream
- How the gas is being used
- What applicable environmental regulations will allow.
One traditional method for using or disposing of sour gas is to burn it either as a fuel source for a generator or steam boiler or flared it off as a waste product. Burning sour gas eliminates the H2S, however, the reaction also creates sulfur dioxide, which is considered a harmful pollutant and a major contributor to acid rain.
Gas sweetening decontaminates gas streams so they can be commercialized or disposed of in a way that does not generate harmful byproducts, like SO2.
There are three main groupings of gas sweetening methods:
- Scavengers and Adsorbents
- Catalytic reactions
- Mechanical destruction or Injection
Scavengers and Adsorbents. Scavengers and adsorbents are typically one-and-done solutions. When using a scavenger, a chemical reaction between the scavenger element and H2S “traps” the hydrogen sulfide, but it does remove it and the spent scavenger media must be disposed of as a hazardous waste. In the case of adsorbents, the H2S is chemically bonded to the surface of the media, leaving solid wastes for disposal.
Amine scrubbers. For high volumes of gas, sweetening means using an amine scrubber or other selective gas stripping process to concentrate the H2S, after which the resulting “acid gas” stream can then be combusted (thermal oxidation), injected into a specially permitted Acid Gas Injection well (AGI), or destroyed mechanically, chemically, or biologically.
Claus process. For very large volumes of gas and high H2S concentrations, such as at a gas plant or a refinery, the Claus process is employed to mechanically destroy H2S. The Claus process is a large-scale method that is not suitable for projects that are small, short term, or have high variability in volume.
Biologics. Biological sweetening consists of stripping the H2S out of the gas with an aqueous soda solution and then exposing the H2S -rich solution to thiobacillus bacteria which consume the H2S, leaving elemental sulfur. This process can be slow to adapt to changes in gas volumes or H2S concentrations.
Acid Gas Injection. AGI or Acid Gas Injection involves injected concentrated H2S gas mixed with CO2 (Acid Gas) into a formation deep in the earth. While this can be a safe and permanent solution, it is expensive to get started and requires ongoing monitoring, as well as the transportation of the acid gas to the injection site.
