Energy efficient H2S treatment and enhanced methane upgrade
The presence of hydrogen sulfide (H2S) is an issue in many cases, especially in biogas plants. In this paragraph, we focus in particular on two-stage biogas plants, where the anaerobic digester is separated from the methane upgrading unit. In this case, high concentrations of H2S are already formed in the digester, where ozone can be injected to reduce this issue.
Since the digester is operating in anaerobic conditions i.e. absence of oxygen, ozone should be injected with care. The injection is done in the air pocket above the microbial reacting bed. In alternative, an additional tank for the ozonation can be added between the digester and the methane upgrade unit. This solution is safer for the anaerobic conditions but requires extra installation costs.
The effectiveness of ozonation in digester pockets
The air pocket above the reacting media offers quite a large volume for the ozone to react with the H2S. Therefore, the oxidation reaction with ozone is particularly effective. The H2S conversion for a reference case of ozonated digester is summarized in the figure.
As shown, ozone successfully treat H2S with conversions above 80 %. This means that ozone is capable of reducing the H2S concentration from 500 – 1000 ppm to 75 – 150 ppm already before the methane upgrading stage. Based on the incoming conditions and requirements, we offer a range of tailor-made solutions, optimizing the H2S removal and minimizing the energy consumption. H2S is converted into SO2, greatly reducing the odor and corrosion problems, according to the reaction shown.
Compared to other technologies, ozone offers several benefits for reducing H2S in digesters. For example, the use of iron chloride in the bio-bed has been the common practice for several years. However, this practice requires high operational costs, since the iron chloride is continuously consumed during the anaerobic digestion. In addition, there may be corrosion problems due to the high corrosion potential of iron chloride.
Compared to air or oxygen injection, ozonation is much more effective due to its high oxidation potential. When air is used, the presence of nitrogen significantly decrease the biogas grade, since it has a high concentration and it cannot be removed in the following steps of the process. The use of oxygen solves the problem of lowering the biogas grade; however, the performance is inferior compared to ozone. Since oxygen has a lower oxidation potential, the oxidation reaction happens slower compared to ozone. This means that an oxygen injection requires much larger volumes and reaction times. Therefore, the injection of oxygen cannot be applied in plants with limited space. On the contrary, ozone has a much faster reaction rate, resulting in an effective treatment even for small volumes.