Gas-to-Grid Biomethane Upgrading using Oxygen Injection & Control
Optimising biogas upgrading performance through controlled oxygen injection for efficient H₂S reduction and gas quality compliance
Over 45 Years in Operation
1000's of UK Installations
Supporting 100's of UK Sites
The Challenge
Biogas produced from anaerobic digestion contains hydrogen sulphide (H₂S), which must be removed to protect downstream equipment, meet gas-to-grid specifications and ensure safe operation.
Biological desulphurisation typically used air injection and relied on aerobic bacteria to break down H2S into elemental sulphur & water. But in the case of Gas to Grid, the upgrader will not allow the excess Nitrogen to enter the system and so we use very low levels of pure oxygen.
For operators upgrading biogas to biomethane, maintaining consistent gas quality is critical. Excess H₂S can:
- Damage compressors and upgrading systems
- Increase maintenance costs and downtime
- Prevent compliance with gas-to-grid injection standards
- Lead to inefficient operation of upgrading systems
At the same time, over-injection of oxygen can dilute the gas, reduce calorific value and introduce safety risks.
Achieving the correct balance requires accurate monitoring and controlled oxygen dosing.
The Solution
Oxygen injection (or controlled air dosing) is a widely used and cost-effective method of reducing H₂S in biogas systems.
By introducing small, controlled amounts of oxygen into the digester or gas stream, biological processes convert H₂S into elemental sulphur, significantly reducing sulphur content before downstream treatment.
When properly controlled, oxygen injection systems provide:
- Stable and effective H₂S reduction
- Reduced reliance on chemical dosing
- Improved gas quality for upgrading and grid injection
- Lower operating costs compared to alternative methods
The key to success is accurate measurement and control, ensuring oxygen is injected in proportion to gas flow and H₂S concentration.
How It Works
A typical gas-to-grid upgrading system using oxygen injection includes:
- Biogas flow measurement to determine total gas production
- Gas analysis (H₂S, CH₄, CO₂, O₂) to monitor composition
- Oxygen or air injection system controlled via process signals
- Control logic to adjust dosing based on real-time conditions
Oxygen is injected either:
- Directly into the digester (biological desulphurisation), or
- Into the gas line upstream of upgrading systems
Control strategies often use:
- H₂S concentration feedback
- Biogas flow rate
- Combined control logic for stable operation
This ensures efficient sulphur removal without over-injection or gas dilution.
Allison Engineering Experience & Insight
We have supported a large number of anaerobic digestion plants across the UK, providing solutions for biogas monitoring, desulphurisation and upgrading systems.
In practice, the success of oxygen injection systems depends on:
- Correct positioning of injection points
- Reliable gas flow measurement
- Accurate and stable gas analysis
- Robust control strategies tailored to the plant
Poorly controlled systems can lead to unstable H₂S levels, excessive oxygen use or reduced gas quality. By taking an application-led approach, we help ensure systems are correctly specified and operate reliably over the long term.
Recommended Products
Biogas Analysers (AwiFlex / AwiEco)
Continuous monitoring of CH₄, CO₂, H₂S and O₂ to provide real-time feedback for control systems and ensure gas quality compliance.
Thermal Mass Flow Meters (ST Series)
Accurate biogas flow measurement to support control of oxygen dosing and overall process monitoring.
Oxygen Generators (PSA / VSA Systems)
On-site oxygen generation providing a safe, reliable and cost-effective supply for injection systems without reliance on delivered gas.
Control & Integration Systems
Integrated systems linking flow, gas analysis and oxygen injection to provide automated, responsive process control.
Key Benefits
- Effective reduction of H₂S to protect upgrading systems and downstream equipment
- Improved gas quality for reliable gas-to-grid compliance
- Reduced reliance on chemical dosing and associated operating costs
- Optimised oxygen injection through accurate measurement and control
- Lower operating costs with efficient on-site oxygen generation options
- Enhanced process stability with real-time monitoring and automated control
Need information or advice?
Speak with our friendly, expert team to discuss your requirements and find out how we can help you.
Gas-to-Grid Upgrading FAQs
Common questions about oxygen injection, biogas upgrading and process control for gas-to-grid applications.
Why is oxygen injection used in biogas upgrading?
Oxygen injection is used to reduce hydrogen sulphide (H₂S) levels in biogas through biological desulphurisation. By introducing controlled amounts of oxygen, naturally occurring bacteria convert H₂S into elemental sulphur.
This process helps protect downstream equipment, improve gas quality and reduce the need for chemical treatment, making it a cost-effective solution for many AD plants.
How is oxygen injection controlled in gas-to-grid systems?
Oxygen injection is typically controlled using a combination of biogas flow measurement and gas analysis. The system adjusts oxygen dosing based on parameters such as H₂S concentration and total gas flow.
Advanced control strategies use real-time feedback to maintain stable H₂S levels while avoiding over-injection, ensuring efficient and safe operation.
What happens if too much oxygen is injected into biogas?
Excess oxygen can dilute the biogas, reducing its calorific value and potentially affecting upgrading system performance. It can also introduce safety concerns if oxygen levels exceed acceptable limits.
This is why accurate measurement and controlled dosing are essential to maintain the correct balance.
Can oxygen be generated on-site for biogas upgrading?
Yes, many systems use on-site oxygen generation via PSA or VSA technology. This provides a reliable and cost-effective alternative to delivered oxygen, reducing logistics and ensuring consistent supply.
On-site generation also allows better integration with control systems for responsive dosing.
How does oxygen injection improve gas-to-grid compliance?
By reducing H₂S levels before upgrading, oxygen injection helps ensure that the final biomethane meets grid injection specifications. This protects upgrading equipment and supports consistent gas quality.
Reliable monitoring and control are essential to maintain compliance over time.
How do I choose the right system for oxygen injection?
The correct system depends on factors such as plant size, gas production rate, H₂S levels and process configuration. It is important to consider both measurement and control, not just the injection method.
An application-led approach ensures the system is correctly specified for long-term performance, efficiency and reliability.
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Need Help Optimising Your Gas-to-Grid Upgrading System?
Speak to our team to design and implement a reliable oxygen injection and control solution tailored to your application, improving gas quality, reducing costs and ensuring long-term performance.