Biogas treatment with activated carbon; how hydrogen sulfide (H₂S) is removed (Gas sweetening).
Biogas is a very interesting form of renewable energy. It is produced from organic wastes, such as manure and others. It can also be generated from wastewater whose organic compounds are susceptible to anaerobic degradation. Biogas can be converted into electricity or heat.
But there is one detail that often determines whether a biogas production project is successful and safe, or becomes a risk: hydrogen sulfide (H₂S).
This compound is responsible for the “rotten egg” smell. The biggest problem is not the odor, but its toxicity and corrosiveness. Therefore, one of the most common steps in biogas production plants is the treatment of this gas, not with a standard activated carbon, but with a particular type of activated carbon.
What is H₂S and why should it be separated from biogas?
Biogas is usually a mixture of:
- Methane (CH₄), which is the fuel,
- Carbon dioxide (CO₂),
- Sulfur-containing compounds, such as H₂S and mercaptans, which are found in small concentrations in biogas. Mercaptans are formed when hydrogen sulfide reacts with organic compounds. Sulfur comes from the same organic matter of which it is a part.
As mentioned, the problem is that the H₂S:
- It is toxic,
- Corrodes pipes and equipment,
- May affect engines, boilers or turbines.
What type of activated carbon traps hydrogen sulfide and other sulfur-containing compounds?
Activated carbon is a solid that adsorbs (traps) molecules with covalent and non-polar character. This is the case of most organic molecules. The high adsorption capacity of an activated carbon is due to the enormous surface area provided by its pores.
Hydrogen sulfide and low molar mass mercaptans are neither covalent nor apolar. They are polar compounds, which, as a particular characteristic, are acidic in character. If the surface area of the activated carbon is adequately impregnated with an alkali (without clogging the pores), an activated carbon is obtained that will chemically absorb these acidic compounds.
Carbotecnia produces this type of coal under the trade name of Vapacid.
What other characteristics does Vapacid activated carbon have?
One is made from carbon obtained from coconut shell and thermally activated. This type of activated carbon is characterized by the fact that most of its pores are smaller than 2 nanometers (these are known as micropores). Micropores are best suited to retain small compounds, which are those found in the gas phase.
On the other hand, Vapacid is not only offered in granular form, but also in the form of pellets, which are 4 mm diameter cylinders. This form generates a low resistance to gas flow, thus causing a low pressure drop in the gas flowing through the fixed bed formed by the carbon.
The key reaction: converting H₂S to elemental sulfur.
The reaction product of hydrogen sulfide and other sulfur-containing compounds and the Vapacid impregnant is elemental sulfur. The reaction can be expressed by the following chemical equation:
8H2S+ 4 O2 🡲S8 + 8H2O
Unlike other coals that retain acid gases, Vapacid inhibits side reactions that form sulfuric acid. This is important, as sulfuric acid is particularly corrosive.
How do you know if your project needs desulfurization with activated carbon?
Typical signs:
- Strong “rotten egg” odor near vents or equipment,
- Accelerated corrosion.
- Recurrent failures in metal parts.
- Need to meet a maximum H₂S limit for a motor/turbine.
