Biological biogas upgrading to biomethane

The innovative biomethane biogas upgrading technology is one of the two technological solutions dedicated to the biogas purification that are being tested as part of the PerFORM WATER 2030 project.

What does biogas biological upgrading mean?

What is going to be tested in the PerFORM WATER 2030 project?

A pilot plant was built for experimentation: how is it made?


What does biogas biological upgrading mean?

Generic placeholder image Biogas upgrading by biological means is based on the metabolism of microorganisms commonly present in digesters, the Archaea metanigeni idrogenotrofi. They can perform the following reaction with a negative value of the free energy of Gibbs ΔG0 (-130,7 kJ/mole): Generic placeholder image Thus, adding hydrogen, produced for instance by water electrolysis using other renewable energies, to the biogas and maintaining the temperature and pH conditions that allow hydrogenotrophic methanogens to grow (T = 37°C and 55 pH = 6.8-7.8) results in the reduction of carbon dioxide to methane, with the production of water.

From the application point of view, it can operate in two main ways: in-situ and ex-situ. In the former, pure hydrogen (H2) is introduced directly into an existing anaerobic digester, so that the CO2 present in the biogas is converted to methane. In the ex-situ mode, the upgrading process is carried out in a stand-alone anaerobic reactor, containing a biomass enriched with methanogenic hydrogenotrophs, which is fed by biogas and H2 appropriately mixed. In the first case, there are lower investment costs, but more controls are needed to avoid negative effects on sludge digestion. In the second case, on the other hand, there is less need for control and greater plant flexibility in the face of a greater investment.

It should be noted that unlike traditional upgrading technologies that separate CO2 from the methane flow, the biological conversion of CO2 allows for a significant increase in methane production and can also be used for the conversion of CO2 to methane alone. The presence of hydrogenotrophic methanogens, a key element for an efficient upgrading process in both modes, can be increased through different strategies (bioaugmentation or endogenous enrichment of hydrogenotrophic populations) with mixed or pure cultures. The use of mixed adapted cultures is more advantageous than pure ones, as microbial communities are more robust and do not need sterile conditions.

What does biogas biological upgrading mean?

Generic placeholder image Biogas upgrading by biological means is based on the metabolism of microorganisms commonly present in digesters, the Archaea metanigeni idrogenotrofi. They can perform the following reaction with a negative value of the free energy of Gibbs ΔG0 (-130,7 kJ/mole): Generic placeholder image Thus, adding hydrogen, produced for instance by water electrolysis using other renewable energies, to the biogas and maintaining the temperature and pH conditions that allow hydrogenotrophic methanogens to grow (T = 37°C and 55 pH = 6.8-7.8) results in the reduction of carbon dioxide to methane, with the production of water.

From the application point of view, it can operate in two main ways: in-situ and ex-situ. In the former, pure hydrogen (H2) is introduced directly into an existing anaerobic digester, so that the CO2 present in the biogas is converted to methane. In the ex-situ mode, the upgrading process is carried out in a stand-alone anaerobic reactor, containing a biomass enriched with methanogenic hydrogenotrophs, which is fed by biogas and H2 appropriately mixed. In the first case, there are lower investment costs, but more controls are needed to avoid negative effects on sludge digestion. In the second case, on the other hand, there is less need for control and greater plant flexibility in the face of a greater investment.

It should be noted that unlike traditional upgrading technologies that separate CO2 from the methane flow, the biological conversion of CO2 allows for a significant increase in methane production and can also be used for the conversion of CO2 to methane alone. The presence of hydrogenotrophic methanogens, a key element for an efficient upgrading process in both modes, can be increased through different strategies (bioaugmentation or endogenous enrichment of hydrogenotrophic populations) with mixed or pure cultures. The use of mixed adapted cultures is more advantageous than pure ones, as microbial communities are more robust and do not need sterile conditions.


What is going to be tested in the PerFORM WATER 2030 project?

Within the PerFORM WATER 2030 project, one of the two technological solutions being experimented regarding biogas upgrading is the biological one, via the ex-situ hydrogenotrophic methanogenesis. SEAM engineering Srl & DICA Department of Politecnico di Milano designed, built and installed the pilot plant inside a container within the San Giuliano Milanese West wastewater treatment plant (WWTP).

The pilot is feed by biogas, coming from the full-scale digesters in the WWTP, and by hydrogen, produced on-site by a latest-generation electrolyser with anionic exchange membrane furnished by HyDEP srl. The pilot is inoculated with an anaerobic sludge already enriched with hydrogenotrophic methanogens, coming from other researches previously carried out by Politecnico di Milano.

The overall conceptual scheme of the biological upgrading reactor and of the units connected to it is shown in the figure below:

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During the experimentations, hydrogen is mixed in different proportions to the biogas in order to acquire a better understanding of the process. The main objectives of this experiment are to identify the ideal operating parameters to achieve biogas conversion yields in the range 90-99% and to define the key parameters to design the process at full-scale and control its operation. To achieve these objectives, in addition to continuously monitoring the process parameters, a simulation model specially developed for process simulation, that will be calibrated and validated with experimental data gradually obtained under dynamic conditions.

Biological biogas upgrading to biomethane

Project details


A pilot plant was built for experimentation: how is it made?

The ex-situ biogas upgrading pilot plant was placed together with the electrolyser in a container (ATEX zone type 2) close to the two anaerobic digesters of San Giuliano West WWTP operated by CAP Group.

Here below two pictures taken at the ex-situ biological biogas upgrading pilot installed at San Giuliano Ovest wastewater treatment plant; on the left the biogas pre-treatment unit and the biogas and H2 pre-mixing chambers while on the right the upgrading pilot plant (500 litres).

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The reactor installed has a working volume of 0.5 m3 and operates continuously. The flow rates of biogas and hydrogen will vary between 1.5-3 m3/d e 2.5-5 m3/d respectively. The real biogas, coming from the two full-scale digesters of the plant, is pre-treated for the H2S and steam removal before entering the reactor. The gas mixture is injected through both bottom diffusers and ejectors. The reactor is also equipped with a sludge recirculation system, which ensures very high mixing together with gas blowing.


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