Acid Gas (H₂S) Removal

The earth is an intriguing home for humanity. It brings forth abundant food and fiber, on the one hand, while destructively shaking at its fault lines and spewing forth molten lava from active volcanoes. At once nurturing and threatening, this planet has nevertheless failed at obliterating human beings. One reason for that is ingenuity, that unique capacity of homo sapiens to solve problems through innovation. Having learned of the power of combustion to generate electricity, we nevertheless had to protect ourselves from poisons that comingle with the raw materials we burn. Enter new technology for acid gas removal and management.

What is Acid Gas?

Very strictly, acid gas is any gaseous compound that makes an acidic solution when dissolved in water. Although there are numerous kinds of acid gas, the most ubiquitous are carbon dioxide (CO2) and hydrogen sulfide (H2S). These chemicals are often found in natural gas deposits yet render natural gas unusable. Not are they toxic upon exposure to people, they are also caustic agents in pipes and vessels, causing corrosion and sometimes irreparable damage. Needless to say, the management and disposal of these substances is rigorously regulated for both public health reasons and environmental protection. H2S and CO2 removal from natural gas is one such regulation.

How Bad Are H2S and CO2?

Carbon dioxide, though a necessary element for life, is nevertheless implicated in atmospheric pollution when released in large amounts. Yet there is another problem with excess CO2 — it lowers the heating value of the combustible gas. Sometimes measured in joules per kilogram — and other times expressed in British Thermal Units per cubic foot — heating value gauges the amount of heat the combustion of a fuel actually produces. In a nutshell, when CO2 concentrations are higher, heating value goes lower. It is therefore a chemical compound in need of isolation.

Hydrogen sulfide is a dangerous toxin for human beings. Depending on the density of accumulation, it can cause dizziness and respiratory distress — on one end of the illness spectrum — or convulsions and death in extreme cases. The U.S. Centers for Disease Control warn that prolonged exposure to H2S brings about the worst results. It is no surprise that H2S is so harmful to the body, given everything it does to other materials. Well documented is the cracking, blistering and corrosion of sturdy metals by this acerbic, and very flammable, acid gas. This is why natural gas can not be transported long distances until H2S removal.

How Is Removal Accomplished?

Hydrogen sulfide removal, and that of other acid gas components, requires the compound to come into contact with chemical solvents, physical solvents or a combination of both. These solvents occupy an absorption column of an acid gas removal unit. When the acid gas — sometimes called sour gas — moves through the column, its molecules are attracted to those in the liquid solvents, and are thus osmosed from the natural gas stream. While there are other ways, this H2S removal system is fairly common and usually quite effective.

Does It Work the Same for Biogas?

Unlike natural gas, biogas is not a fossil fuel. Instead, biogas is formed as organic matter — dead foliage or animals, food waste, manure, sewage etc. — decays and ferments. The process of anaerobic digestion makes capturing the methane from biogas possible and productive. In turn, the methane (CH4) is combustible and used to generate electricity. As with natural gas, H2S is present in biogas in concentrations of up to 1,000 parts per million (PPM). For effective operation of internal combustion engines, this density needs to drop to 100PPM at a maximum. Hence, H2S removal from biogas is as necessary as H2S removal from natural gas.

Anaerobic digesters use the absence of oxygen to allow micro-organisms to break down the organic matter within. Once sufficiently disintegrated, the matter yields methane and carbon dioxide — along with H2S biogas. By injecting small doses of oxygen into the digester contents, technicians allow the formation of micro-organisms that successfully oxidize the sulfide. Meanwhile, iron chloride is applied to the substrate matter to create iron sulfide salt. In so doing, technicians reduce the presence of H2S to about 10 PPM.

The removal and neutralization of hydrogen sulfide is often called H2S scrubbing. A hydrogen sulfide scrubber for natural gas may use different reacting agent than an H2S scrubber for biogas. Yet the principle is the same: the H2S scrubber uses reagents to alter the destructive composition of the hydrogen sulfide. Another such absorbent is activated carbon, a charcoal-like substance that have a high capacity for micro-porosity, i.e. very tiny pores, invite H2S adhesion to its surface areas. Hydrogen sulfide removal activated carbon enhanced is yet another method of ridding useful gas of this contaminant.


In Summary

There are myriad procedures available for the efficient dispatch of acid gas. Those making the decisions as to how must doubtless look at factors like concentration, temperature, pressure and substrate. The good news is that there is always an optimal way to extract this detrimental presence.


AirFacts Magazine