Biogas desulphurisation is key to safe and profitable renewable energy production. Biogas contains hydrogen sulphide (H₂S), a toxic and corrosive compound that compromises equipment, safety and regulatory compliance. Among the most commonly used methods for H₂S removal are iron hydroxides and oxygen injection. Both work, but differ in efficiency, safety, cost and sustainability.
Table of contents
- Why is it necessary to remove H₂S from biogas?
- Desulphurisation with iron hydroxide: operation and advantages
- Oxygen injection in biogas: benefits and risks
- Comparison: iron hydroxide vs oxygen in biogas
- Which option should you choose for your biogas plant?
- Frequently asked questions about iron hydroxide in biogas
- Conclusion: iron hydroxide, the safest and most effective option
Why is it necessary to remove H₂S from biogas?
H₂S in biogas is a natural by-product of anaerobic digestion. Its removal brings direct benefits:
- Equipment protection (engines, compressors, pipes): reduces corrosion and downtime.
- Regulatory and contractual compliance (CHP, boilers, biomethane upgrading).
- Biogas quality: more stable and safer operation.
- Environmental impact: fewer emissions of harmful compounds.
The decision is not whether to desulphurise, but how to desulphurise with the best balance between OPEX, safety and reliability.
Desulphurisation with iron hydroxide: operation and advantages
What is iron hydroxide and how does it act against H₂S?
Iron hydroxides react actively with the H₂S present in biogas, converting it into iron sulphide (FeS), a stable solid that is removed from the gas. It is a simple and effective reaction that takes place under normal operating conditions and without adding oxygen to the system.
Depending on the product formulation, it is possible to partially regenerate the medium through controlled aeration. In addition, a buffering effect is observed that helps to smooth out H₂S peaks when dosing is temporarily interrupted.
Our N-Bio solution is designed to be dosed in situ in the digester or in the reactor feed. This strategy captures H₂S early on, stabilises the biogas stream and prevents oxygen from entering the gas line. When required by the project, we can also integrate external iron media beds as a complementary stage.
Main advantages: efficiency, safety, easy maintenance
Iron hydroxides offer clear advantages over other desulphurisation methods:
- Proven efficiency across a wide range of concentrations and flow rates, with good tolerance to fluctuations.
- Operational safety: no O₂ in biogas → lower ATEX risk.
- Simple integration: on-site dosing (N-Bio) without complex equipment; fixed bed option where applicable.
- Predictable maintenance: monitoring of H₂S at the outlet and pressure drop (in beds) to schedule replacements.
- Compatibility with upgrading to biomethane and with pre-treatment schemes.
Limitations and how to manage them.
Although this is a highly effective solution, it is important to bear certain aspects in mind to ensure maximum performance:
- Media saturation: requires monitoring H₂S at the outlet and planning replacements.
- Waste management (FeS): must be handled in accordance with local regulations.
- Regeneration: its viability depends on the H₂S load and the product.
Oxygen injection in biogas: benefits and risks
How oxidation with oxygen/air works
The injection of oxygen (O₂) or air is an alternative method for biogas desulphurisation. It works by oxidising hydrogen sulphide (H₂S): when oxygen is added in controlled doses, the H₂S is transformed into elemental sulphur or sulphates, reducing its concentration in the gas.
Advantages: low reagent cost
- Low reagent cost: oxygen or air is accessible and inexpensive.
- Initial simplicity: does not require the installation of complex reactors or significant modifications to the biogas system.
- Specific applications: it is suitable for constant biogas flows, where operation remains stable and predictable.
Risks: ATEX safety, complex control, impact on digester
Although it may seem like a simple option, oxygen injection involves risks that must be carefully assessed:
- ATEX safety: the mixture of methane and oxygen can pose a risk of explosion or combustion if not strictly controlled.
- Constant monitoring: requires precise sensors and control loops to ensure that oxygen never exceeds safe limits.
- Impact on the biological process: in some cases, injection into the digester can alter the activity of methanogenic bacteria, reducing digestion efficiency.
- Sulphur deposits: oxidation generates elemental sulphur, which can accumulate in equipment and pipes, affecting operation.
Comparison: iron hydroxide vs oxygen in biogas
| Criterion | Iron hydroxide(s) | Oxygen injection (O₂/air) |
| H₂S removal efficiency | High and predictable across wide ranges | Good with steady flows and fine control |
| Biogas safety | Very high (no O₂ in gas line) | Requires ATEX, O₂ limits, and strict protocols |
| Operational complexity | Low (load/media replacement) | Medium-high (sensors, control loops, maintenance) |
| CAPEX | Low–medium (filters/beds) | Medium-high (dosage, safety, instrumentation) |
| OPEX | Predictable replacement/regeneration | Inexpensive reagent, but constant monitoring and safety required |
| By-product management | Solid, manageable FeS | Elemental sulphur/sulphates, potential for deposits |
| Suitability | Small to medium-sized plants and variable loads | Plants with high control and stable conditions |
Operational conclusion: if you prioritise safety, simplicity and compliance, iron hydroxide —especially in situ dosing with N-Bio— is usually the preferred option. Oxygen injection may be viable with advanced engineering and rigorous control.
Which option should you choose for your biogas plant?
Choose iron hydroxide(s) such as N-Bio when you need reliable desulphurisation, rapid start-up, tolerance to variations and minimal risk in the gas line.
Consider oxygen injection if you have very stable flows, specialised technical equipment and can handle the complexity of ATEX.
Frequently asked questions about iron hydroxide in biogas
Is iron hydroxide safe?
Yes. Desulphurisation with iron hydroxide is carried out without injecting oxygen into the biogas, which significantly reduces ATEX risks and simplifies operation.
What maintenance does it require?
Low. Monitor H₂S at the outlet and bed pressure drop. Plan media replacement before rupture; basic filter cleaning and seal verification.
Can the environment be regenerated?
Some iron hydroxide media allow regeneration with air under controlled conditions. Viability (number of cycles, performance) depends on the product and the H₂S load. We advise you on a case-by-case basis.
What is its cost compared to oxygen?
The CAPEX for iron hydroxide is usually lower and the OPEX more predictable (planned replacements). Oxygen injection may have inexpensive reagents, but it requires sensors, continuous monitoring and safety measures that increase complexity.
Conclusion: iron hydroxide, the safest and most effective option
For most plants seeking reliable and safe biogas desulphurisation, iron hydroxides offer stable efficiency, simple operation and reduced risk.
Oxygen injection can work in very stable scenarios with a high level of control, but it adds complexity and ATEX requirements.
If you want a predictable solution geared towards plant availability, N-Bio is your best starting point. Let’s talk and size up your system.
