The role of oxygen in biogas production

Oxygen (O₂) plays the most important role in biogas production.One of the undesirable byproducts of biogas production is hydrogen sulphide (H₂S), which must be removed from the biogas flow across the plant as it is corrosive and can damage downstream equipment. There are different methods to eliminate H₂S, such as iron oxide dosing, adsorption with activated carbon or molecular sieves, water scrubbing, and membrane separation. The disadvantage of these methods is that they require a huge installation to filter out a trace amount of H₂S.The most sustainable and cost-effective method of removing H₂S is oxygen dosing, also known as micro dosing or micro oxygenation. By injecting a small amount of oxygen into the digester, H₂S can undergo a biochemical reaction with oxygen, producing harmless byproducts such as sulphur and water.

Dosing is often managed to maintain a steady ratio of oxygen in the process, which is typically in the range of 0.3-3%. H₂S is reduced to less than 5 mg/m³ with oxygen dosing. This can increase the calorific content of the biogas, making it more useful for energy production.Ideally, oxygen is supplied by an on-site oxygen generator. This is the most cost-effective way to remove hydrogen sulphide (H₂S) and the solution with the lowest total cost of ownership (TCO) and the lowest impact on the environment. On-site gas generation systems require installing a compressor and a gas generator to ensure that the optimal amount of oxygen is introduced into the system.

How oxygen (O₂) improves biogas quality

One of the main issues in the production of biogas is the resulting hydrogen sulphide (H₂S).

Biogas with high levels of hydrogen sulphide (H₂S) is corrosive to downstream equipment. In addition, H₂S can reduce the calorific value of biogas, making it less efficient as a fuel. It can also pose a safety risk to plant operators. For all of these reasons, biogas plants need to remove as much hydrogen sulphide (H₂S) as possible from the gas.

Using oxygen in biogas plants can help to remove H₂S from the gas, resulting in a more stable and safe biogas production process. The oxygen reacts with hydrogen sulphide (H₂S) to form sulphur dioxide (SO₂), a process called desulphurisation.

On-site oxygen generation is a cost-effective and reliable solution for biogas plants to achieve desulphurisation. With an on-site oxygen generator, biogas plants can produce their own oxygen on demand, eliminating the need for expensive and cumbersome gas deliveries.

Level of purity needed for biogas plants

Biogas production requires a certain level of purity for the compressed air and oxygen used in the process. The purity level needed will depend on the specific requirements of the biogas plant and the intended use of the biogas.

For example, if the biogas is intended for injection into the natural gas grid, it must meet the purity requirements set by the local regulations. These purity requirements usually require that the biogas has a methane content of at least 96%, and that it contains no more than 4% impurities, such as carbon dioxide, oxygen, and nitrogen.

To achieve this level of purity, a PSA oxygen generator, such as the Atlas Copco OGP⁺ 3-30, can be used to produce oxygen with a purity of up to 99.999%. This high level of purity ensures that the biogas meets the required specifications for injection into the natural gas grid.

However, if the biogas is only intended for use on-site, such as for heating or electricity generation, a lower level of purity may be acceptable. In this case, a membrane oxygen generator may be a more cost-effective option, as it can produce oxygen with a purity of up to 95%. This level of purity is usually sufficient for on-site use of the biogas.

It is important to determine the specific purity requirements for your biogas plant and choose the appropriate compressor, blower, and oxygen generator to ensure the quality and safety of the biogas production process.