What Is an Acid Gas?

The most straightforward definition of an acid gas is any gaseous compound which, when dissolved in water, will form an acidic solution. The most common types of acid gas are carbon dioxide (CO2) and hydrogen sulphide (H2S), though there are many other varieties as well, including hydrogen chloride (HCI), hydrogen fluoride (HF), sulphur oxides (SO2 and SO3) and nitrogen oxides (NOx).

Although these substances are often found in natural gas in varying concentrations, they can be damaging to both the pipes which contain them and the humans who work in their vicinity. As a result, there are strict regulations about the concentrations of acid gas in power plants, cement kilns and waste incineration plants which require the use of sophisticated environmental technology to achieve compliance.

Acid gas vs sour gas

Although the two terms acid gas and sour gas are often used interchangeably, doing so is not strictly correct. That’s because sour gas refers specifically to gas compounds which contain high concentrations of H2S alone, so named for the sour odour which H2S gives off.

Acid gas, on the other hand, can refer to a gas containing high concentrations of any of the other acidic gases mentioned above. As such, H2S is both an acid gas and a sour gas, but HCI and HF are acid gases but not sour gases. For this reason, it is not correct to regard the terms as synonyms for one another, since sour gas is a mere subset of acid gas.

The dangers of acid gas

Acid gas has been known to cause significant damage to the pipeline through which it runs and the equipment which it serves. Not only can it bring about extensive corrosion and rusting, it can significantly shorten the lifespan of both pipelines and equipment, costing the companies in question millions of pounds in repairs and replacements.

In addition to their corrosive nature, acid gases can also cause serious health complications among humans. Prolonged exposure to acid gases can bring about severe illness or exacerbate existing conditions, even causing death in the most extreme situations. Finally, the release of acid gases into the atmosphere can do untold damage to the environment, manifesting itself in such phenomena as acid rain and global warming.

Quantifying and treating acid gases

Due to their dangerous nature, acid gases must be highly controlled wherever they are found. The European Standards (EN) have been set up specifically to regulate such issues, imposing strict requirements on the instrumentation and methods employed to measure acid gases. The educational e-learning session Acid Gases: HF and HCI - what impact will new EN standards for these gases have? is the ideal way to gain an overview of current test methods and techniques.

Treating acid gases involves reducing the levels of such impurities to bring them down to acceptable concentrations. This is most commonly achieved via an amine gas treating process, sometimes known as “sweetening”, which is so named for the removal of the sour odours mentioned above.