Hydrogen sulfide removal from biogas – Part 2
DESOTEC innovation: cost-effective hydrogen sulfide removal
It is well known that every activated carbon will remove hydrogen sulfide from biogas to some extent. Hydrogen sulfide will adsorb on the internal carbon surface where it can be oxidised in presence of oxygen to sulphur. Ash constituents like iron oxide, can accelerate and improve the H2S removal. The H2S removal capacity of plain activated carbon is usually not attractive due to the low removal capacities. For cost-effective hydrogen sulfide removal from biogas, the removal capacities and rates are boosted by impregnating the activated carbon with suitable chemicals.
DESOTEC’s R&D chemists and engineers studied different base materials, different salts, metal oxides or alkalines and impregnation routes to improve the hydrogen sulfide removal properties of the products used in the biogas purification process.
Hydrogen sulfide removal process
In the hydrogen sulfide removal process, the H2S is initially physically absorbed into the adsorbed water undergoing the dissociation according to following reactions:
H2S (g) + H2O ↔ H2S (aq)
H2S (aq) ↔ H+ + HS-
HS- ↔ H+ + S2-
The formation of sulphur and water occurs by means of sulfide oxidation in presence of free oxygen:
2 H2S + O2 ↔ ¼ S8 + 2 H2O (1)
2 H2S + 3 O2 ↔ 2 SO2 + 2 H2O (2)
2 H2S + SO2 ↔ 3 S + 2 H2O (3)
¼ S8 + 2 O2 → 2 SO2
2 SO2 + O2 + 2 H2O → 2 H2SO4
At lower temperatures, reaction (1) is most likely to happen. The hydrogen sulfide removal capacity of an activated carbon depends on the operating conditions in which the product is used such as:
- Relative humidity;
- Molar ratio of H2S to O2;
- The temperature;
- Sulphur loading on the carbon (and thus pore volume);
- Type of impregnant;
- And other operating parameters.
DESOTEC’s R&D chemists and engineers did a lot of research to design, develop, and produce an activated carbon with unique properties for H2S removal from biogas. Hydrogen sulfide removal in biogas can be done on the wet biogas leaving the digester, or on the dried gas. Since the first step in the H2S removal process is the absorption of the H2S into the adsorbed water, it is important to understand the impact on the relative humidity on the biogas purification process.
For more info on hydrogen sulfide removal and biogas treatment solutions in general, don’t hesitate contact us!
At DESOTEC’s facilities, all used carbon is analysed so the right measures can be taken for handling and removing the saturated carbon out of the mobile filters. All molecules that were adsorbed on the activated carbon at the customers’ site, are desorbed inside DESOTEC’s reactivation furnaces. These contaminants are then fully destroyed, in accordance with National and European legislation, by an incineration and neutralisation setup. The entire installation and it's emissions are under continuous on-line monitoring, which guarantees that only harmless water vapour is seen exiting the chimney.