Biogas is positioned as a renewable energy source with great potential. However, raw biogas contains impurities that pose technical and environmental problems. The main one is hydrogen sulphide (H₂S), a corrosive sulphur compound that must be removed. Biogas desulphurisation is the process that removes these sulphur compounds, ensuring that biogas can truly serve as a clean and sustainable energy source.
Biogas and the sulphur challenge
Biogas is produced from the anaerobic digestion of organic waste, and its main composition is methane (CH₄) and carbon dioxide (CO₂). However, it also usually contains hydrogen sulphide (H₂S), whose concentration varies depending on the substrate, ranging from 0.1% to 3% (approx. 1,000–30,000 ppm).
The presence of H₂S poses a serious challenge: during combustion, it transforms into sulphuric acid (H₂SO₄), which accelerates corrosion in engines, pipes and equipment. Even at low levels, it causes cumulative damage and increases maintenance costs. Furthermore, when burned, it generates sulphur dioxide (SO₂) emissions, which are responsible for acid rain and unpleasant odours, impacting the health of workers and nearby communities as well as the environment.
Therefore, the removal of hydrogen sulphide is an essential step in ensuring that biogas can be used as a safe and truly sustainable fuel.
What is biogas desulphurisation?
Biogas desulphurisation consists of removing H₂S and other sulphur compounds from biogas. It is a key purification stage that produces safer, more stable biogas with better energy efficiency.
By reducing the H₂S content, infrastructure is protected against corrosion, polluting emissions are avoided, and the use of biogas in applications such as electricity generation, heating, or vehicle fuel is facilitated.
In addition, desulphurisation ensures that the gas retains its energy value, transforming it into higher quality biogas or even biomethane, a renewable fuel that can be injected into natural gas networks or used directly in sustainable mobility. In short, solving the “sulphur problem” is what allows biogas to go from being a raw resource to becoming a clean, profitable energy source that is aligned with sustainability goals.
Methods for Biogas Desulphurisation
There are various techniques for biogas desulphurisation, and the choice depends on the H₂S level, flow rate and conditions at each plant. Among the most common approaches are:
- In-situ desulphurisation: The most widespread strategy in the industry is the dosing of iron compounds directly into the anaerobic digester, before the H₂S is released with the biogas.
- Iron hydroxides (N-Bio): Compared to ferric salts, iron hydroxides from the N-Bio Solutions range offer a safe and efficient alternative. Their application in solid form (powder or pellets) allows for a gradual reaction with H₂S, reducing its concentration in a stable manner without affecting the pH of the digester. In addition, they add iron and sulphur to the digestate, improving its value as a fertiliser.
- Ferric salts (such as FeCl₃): Their main advantage is their immediate reaction with H₂S, which allows for rapid desulphurisation. However, they have significant limitations: they are corrosive, acidify the medium, require specific liquid dosing systems and do not generate a buffering effect, so if dosing is interrupted, H₂S levels can rise rapidly.
- Oxygen injection: Some plants choose to introduce micro-amounts of oxygen into the digester to oxidise H₂S into elemental sulphur using bacteria. This method can partially reduce the sulphur content without chemical additives, but it involves risks: overdosing can negatively affect methane production and generate explosive mixtures.
- Other methods (biological and physical): There are alternatives such as biofiltration or biotrickling, which use microorganisms to oxidise H₂S, and physical processes such as activated carbon adsorption or liquid scrubbing. Although effective in certain contexts, they tend to require higher operating and maintenance costs and are not always practical for large-scale biogas plants.
Why biogas desulphurisation is key to clean energy
Ultimately, removing sulphur from biogas is what makes this renewable fuel truly clean and practical. Biogas desulphurisation is vital for several reasons:
- Equipment protection: Desulphurisation prevents severe corrosion of engines, turbines and pipes. Otherwise, H₂S would form acids during combustion that would corrode metal surfaces. By cleaning the gas, operators extend the service life of biogas generators and avoid costly damage. Gas engine manufacturers typically require H₂S levels below 50-250 ppm to ensure reliable operation, underscoring the importance of sulphur removal for machinery longevity.
- Reduction of harmful emissions: Clean biogas produces far fewer air pollutants. If H₂S is not removed, combustion of the gas releases sulphur dioxide (SO₂), which contributes to acid rain and air pollution. Desulphurisation of biogas eliminates these sulphur emissions, meaning that biogas can be burned with minimal environmental impact: a much greener alternative to fossil fuels.
- Improved safety and odour control: Hydrogen sulphide has a noxious odour and is highly toxic, even at low concentrations. Removing H₂S makes biogas odourless and non-toxic, protecting workers and communities. This improves overall safety and eliminates the rotten egg smell associated with raw biogas, making biogas projects more neighbourhood-friendly.
- Improved energy quality: By removing H₂S and other impurities, the resulting biogas has a higher percentage of methane. This increases the calorific value (energy content) of the fuel. In other words, each cubic metre of clean biogas contains more usable energy. The fuel burns more efficiently and cleanly, which is especially important for applications such as vehicle fuel or power generation, where fuel quality is important.
- Ensuring compatibility and compliance: Many advanced uses of biogas require it to be as clean as natural gas from pipelines. For example, to inject biomethane into the national gas grid or use it in vehicles, sulphur levels must be extremely low (often only a few ppm). In some regions, regulations limit H₂S in biogas to less than 10 ppm for injection into the grid. Biogas desulphurisation enables these strict standards to be met and allows renewable biogas to seamlessly replace fossil natural gas in pipelines and engines. It also means that the CO₂ by-product of biogas upgrading can be released or used without causing odour or corrosion problems.
In summary, biogas desulphurisation is a key step in harnessing the full environmental benefits of biogas. By actively removing sulphur compounds, biogas is transformed from a raw waste by-product into a clean and reliable source of energy. This process ensures that biogas can be used in the same way as traditional natural gas, but without the drawbacks of corrosion or pollution. It also reaffirms the role of biogas in the transition to clean energy, converting organic waste into useful energy with minimal emissions. Through the effective removal of H₂S, biogas becomes not only renewable, but truly clean, helping to power our world while protecting our equipment, our air and our communities.
Frequently asked questions about biogas desulphurisation
¿Qué es el sulfuro de hidrógeno (H₂S) y por qué es un problema en el biogás?
Hydrogen sulphide (H₂S) is a colourless, toxic and highly corrosive gas that forms naturally during the anaerobic digestion of organic matter. Its presence in biogas is common, as sulphate-reducing bacteria generate it from the sulphur compounds present in waste. Although it may seem like a minor impurity, H₂S poses a major challenge for the use of biogas: it has a characteristic “rotten egg” odour, is harmful to health even in low concentrations, and when combusted, it transforms into sulphuric acid (H₂SO₄), which accelerates the corrosion of engines, pipes and boilers. In addition, the combustion of biogas with H₂S produces sulphur dioxide (SO₂), a pollutant associated with acid rain. For these reasons, the removal of H₂S is essential to ensure safety, extend the life of equipment and promote biogas as a renewable and sustainable fuel.
¿Por qué la eliminación de H₂S es clave para la energía limpia?
Removing H₂S from biogas is a fundamental step in ensuring that this renewable resource can be considered a true source of clean energy. Firstly, its removal protects facilities from corrosion: engines, boilers, turbines and pipes are seriously affected if biogas contains sulphur, which increases maintenance costs and reduces plant efficiency. Secondly, desulphurisation prevents polluting emissions. If not removed, H₂S is converted into sulphur dioxide (SO₂) during combustion, contributing to acid rain and environmental degradation. It also improves safety, as H₂S is a poisonous and strong-smelling gas, harmful to both workers and nearby communities. Finally, purifying biogas increases its energy value, making it possible to obtain biomethane of a quality comparable to natural gas. In this way, the removal of H₂S makes biogas a sustainable, safe and competitive fuel in the transition to clean energy.
¿Qué métodos existen para la desulfuración del biogás?
Biogas desulphurisation can be carried out using different techniques, and the choice depends on factors such as H₂S concentration, gas flow rate and the conditions at each plant. Among the most commonly used methods is the dosing of iron compounds, both ferric salts (such as ferric chloride, which acts immediately) and iron hydroxides, which react progressively and offer a more stable buffering effect, as well as improving the fertilising value of the digestate. Another strategy is oxygen injection or microaeration, which promotes the biological oxidation of H₂S, although it requires strict control to avoid the risk of explosion or methane losses. There are also adsorption systems using activated carbon or iron oxides, which are effective in reducing H₂S to very low levels, although they require regeneration or replacement of the material. Finally, biological methods, such as biofilters or biotrickling, use microorganisms to oxidise H₂S in a sustainable manner. In many cases, plants combine several techniques to ensure clean, safe biogas suitable for advanced energy applications.
