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Reducing Biogas Losses to Improve Safety, Efficiency and Sustainability
Industry News
Sustainability
While AD plants are designed to provide renewable energy, ongoing optimisation and expert management are essential to maximise their environmental and operational performance. Methane leakage remains a significant challenge, increasing greenhouse gas (GHG) emissions while also creating safety risks and impacting process efficiency.
Measuring escaping emissions can be difficult, as they often originate from parts of the process that are not routinely monitored or controlled. However, emerging technologies are helping operators identify emission sources more effectively, creating new opportunities to optimise performance and reduce environmental impact.
Alpheus is gaining first-hand experience of the potential these technologies can offer, from extending the life of critical biogas assets through long-term methane monitoring to supporting innovative approaches for recovering residual methane from digestate.
Methane leakage isn't just an environmental issue; it reduces energy output and creates potential safety risks.
As a highly flammable gas, methane can create explosive atmospheres during the handling, storage and transfer of sludge, while escaping biogas represents lost revenue and a missed opportunity to generate heat and electricity. Capturing and retaining this gas improves safety, increases energy recovery and enhances overall site performance.
Leakage tends to occur at specific points such as digester roofs, seals, and vents, or due to random equipment failures. Open-topped vessels and storage lagoons for digestate are designed to allow for dissipation of residual methane and the associated explosion risk, but are a major source of methane losses.
Monitoring the escaping methane is necessary to maintain plant stability and ensure that equipment, such as gas upgrade units and CHP engines, operate efficiently.
There are two approaches to measuring gas emissions – either looking at an entire geographic area, or looking at a specific point in the process.
Using these techniques, a large body of academic research has revealed significant methane losses across the entire biogas sector, with some process activities, such as tank filling, emerging as a particularly high source of emissions.
Alpheus partnered with a client to build an AD plant at a pharmaceutical site in Scotland in 2013, which we have continued to maintain, operate and improve ever since.
A common issue with AD plants in the chemical sector is the presence of high levels of chemical species (such as solvents, sulphide or ammonia), a characteristic that AD equipment suppliers are less familiar with, resulting in shorter service-life guarantees. In the case of a membrane gas holder, with high levels of chemical species, the membrane may stiffen with age or experience chemical attack from components in the biogas, causing microscopic tears in the membrane and increasing permeability. As these tears increase in number, overall membrane integrity reduces. It is impossible [FW1.1]to determine the membrane condition through direct inspection, so we sought an indirect method.
Working with Mettler Toledo, Alpheus identified a new application for their TDL analyser: to measure the very low levels of methane permeating through the flexible gas membrane into the air used to inflate the gas holder. The device uses a laser beam which crosses the path of air passing through the gas holder vent. After this is reflected by a mirror, the device can measure the methane content down to parts-per-billion levels.
This TDL Analyser device was permanently installed on the gas holder in 2020 to continuously monitor the condition of the membrane over time. Factors such as temperature, operating pressure and gas volume can all affect membrane performance, so it is important to understand how these changes influence the readings obtained. A single inspection may not provide an accurate picture, but continuous monitoring allows trends to be tracked, giving operators greater confidence in the health of the asset and helping them make informed maintenance and end-of-life replacement decisions.
The graph below shows methane emissions over 12 months.
Emissions increase during warmer weather but fall back afterwards, demonstrating why continuous monitoring provides a more accurate picture than spot checks.After 13 years of operation, the site chose to replace the membrane; the data analysis of asset condition over its life allowed the membrane to be used for longer than the manufacturer was prepared to guarantee, with significant savings in capital expenditure over the lifetime of the biogas plant.
Since privatisation of the municipal wastewater sector in England, anaerobic digestion has become a key process, reducing the amount of sludge that requires disposal and allowing the recovery of energy.
One of the issues that has resulted from this technology change is the need for digestate storage. The process safety risks that come from the formation of explosive atmospheres associated with digestate storage were tragically illustrated at the Wessex Water Avonmouth explosion in December 2020.
While details are still somewhat unclear, what is known is that methane had collected in the headspace of a vessel that contained digested sludge, and that a fugitive emission was ignited, resulting in four deaths.
If methane had been removed from the sludge and the sludge had been deactivated before storage, the incident would not have occurred in the way it did. Alpheus have been supporting Anglian Water and Bluemethane to operate a trial of technology that aims to capture and reduce methane emissions from post-digested sludge. If successful, this solution would also deliver soft benefits of increased biogas output and lower GHG emissions.
As Principal Designer (PD) and Principal Contractor (PC), our team has been responsible for the site installation, integration of the pilot plant, and will also manage operations throughout the trial.
As methane emissions come under increasing scrutiny, operators are facing growing expectations around monitoring, measurement and reporting. While current UK sustainability reporting requirements do not capture all emission sources, future regulation is likely to place greater emphasis on understanding and reducing methane losses.
For AD operators, having robust monitoring and management processes in place will be increasingly important to demonstrate good environmental performance, support compliance, and meet evolving industry standards.
As the industry continues to focus on improving sustainability, increased monitoring and reducing methane losses offer a clear opportunity to improve safety, increase energy recovery and strengthen environmental performance.
1 "Methodology to Assess Methane Leakage from AD” Part 1 produced for the UK Government Department of BEIS (2016) gives a large range (0-10% of methane produced) for methane emissions from AD. The report suggests that a default value of 5% can be used in the absence of data.
