Innovative Solutions for Biogas Purification

Development of the NitroSX Process for Using Nitrate from Agricultural Digestate as an Oxygen Donor for a Biological Oxidative Biogas Desulfurization

Interior view of the NitroSX pilot plant.  © FiW e. V.
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Process diagram of the NitroSX pilot plant.  © FiW e. V.
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Pilot plant at the Simmerath site on a farm with a biogas production plant.  © FiW e. V.
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Biogas, a high-energy combustible gas, is the end product of the anaerobic degradation of organic matter. Its main component is the energy-rich methane (CH₄). However, biogas also contains significant amounts of carbon dioxide (CO₂) and other trace gases. Particularly problematic is hydrogen sulfide (H₂S), which is formed in larger quantities when protein-rich substrates are digested. H₂S concentrations in biogas can easily range from 200 to 5,000 ppm (0.02 to 0.5 vol.-%).

During combustion, H₂S is converted to sulfur dioxide (SO₂), which acts as a greenhouse gas in the atmosphere. It also causes corrosion and acidification of lubricants, damaging gas engines. Catalytic exhaust cleaning systems are impaired by sulfur oxides (SOx). Even at very low concentrations, H₂S is highly toxic to many organisms.

Current developments, including increasingly stringent legal emission requirements and the need for energy optimization, call for innovative biogas purification methods, particularly for H₂S removal. The use of a biological oxidative cleaning process provides a sustainable and holistic approach to biogas desulfurization, ensuring the required efficiency for gas engines and enabling low-maintenance operation without reducing operational lifespan.

Description of the NitroSX process

The NitroSX process is based on a biotrickling filter operated with digestate serving both as inoculum and as scrubbing liquid. Its goal is the microbiological removal of H₂S from biogas. Naturally occurring microorganisms and the nutrients present in the digestate are utilized for this purpose. These chemolithotrophic bacteria oxidize reduced sulfur compounds to gain energy.

In the biological desulfurization stage, these microbial strains degrade H₂S in the biogas in the presence of nitrate (NO₃⁻). The conversion proceeds stepwise from elemental sulfur to sulfate. The energy released during this process is used by the microorganisms for metabolism and growth.

The nitrate required for the process is supplied in a preceding nitrification stage. In this stage, the ammonium (NH₄⁺) naturally present in the digestate is microbiologically converted under aerobic conditions—similar to a wastewater treatment plant—into nitrite (NO₂⁻) and nitrate (NO₃⁻).

Development of the NitroSX process

The development of the NitroSX process was carried out within the framework of four consecutive research projects. All investigations were conducted at pilot scale on a commercial agricultural facility. The system was operated with real biogas and digestate to demonstrate the scalability of the process.

In the first project, the pilot plant was planned and dimensioned, and initial process data were collected under low H₂S concentrations.

In the two subsequent projects, process stability during steady-state operation was demonstrated, even at H₂S concentrations exceeding 3,000 ppm. Up to this stage, KNO₃ and Ca(NO₃)₂ were dosed as external oxidizing agents.

In the fourth research project, nitrification of the digestate was tested to enable its use as both inoculum and scrubbing liquid in the biotrickling filter. The results demonstrated high stability and robustness of the process, even under strongly fluctuating H₂S concentrations. 

Outlook

The research activities are ongoing. In the currently running follow-up project Enbian, supported by the Industrial Bioeconomy funding program of the BMWK and running until September 2027, the focus is on scaling the process. The goal is to demonstrate the NitroSX process at industrial scale.

The large-scale plant is being realized in collaboration with SH Sulphtec GmbH, which is responsible for implementation. The cooperation between the institute and SH Sulphtec aims to develop the NitroSX process to market readiness, providing an economically viable and environmentally friendly technology for the construction and operation of biogas desulfurization facilities.

Within the project, the following aspects are being addressed:

  • Development of a full-scale plant concept, including chemical calculations, reactor design, and component sizing
  • Detailed design of control and automation systems for the plant
  • Economic evaluation of operational and process-engineering aspects