How to protect an AD plant from biogas leakage

The UK’s anaerobic digestion (AD) industry has come a long way in a short space of time, growing by 350% in a decade to 648 operational facilities . Yet while many efficiency and health & safety advancements have been made across the industry in recent years, there remains room for improvement.

In particular, the issue of biogas leakage is one which many AD operators are still failing to address; often because the problem is invisible. However, the dangers associated with it – from diminished profits to environmental pollution and health & safety risks – should not be underestimated.

Here, Tim Elsome, General Manager for AD specialists FM BioEnergy, outlines the real cost of unidentified biogas leaks – and the inexpensive steps you can take to reduce the risks on your plant…

The scale of the problem

While most responsible plant operators will be monitoring key parameters such as temperature, digester biology and biogas production on a regular basis, the vast majority are not checking for gas leaks, believing it’s an issue which doesn’t affect their plant.

The evidence proves otherwise. Over the last eight years, 85% of the 964 plants we have surveyed in the UK and Germany were suffering from biogas leakage. A quarter of these were deemed ‘significant’ (>1,000l CH4/h), causing serious financial losses and safety concerns; half had only minor leakages (< 100l CH4/h); while the rest were deemed ‘medium’ (< 1,000l CH4/h). In most cases, more than one leakage type was present.

Translating this to the UK as a whole could mean that 550 plants are currently at risk; with 137 in danger of a serious financial or safety breach. Furthermore, if each of these 550 plants was to leak an average of just 0.5% of their capacity, it could equate to a potential loss of 37 GWhe-e a year, resulting in 6,000 tonnes of methane escaping into the atmosphere annually.

The risks of doing nothing

The implications of this volume of methane being released are significant. According to the latest IPCC Assessment Report, methane is 34 times more potent than CO2 as a greenhouse gas over a 100-year period. For any industry to be emitting this volume of methane would be a concern; but for a renewable sector, whose entire premise is based on being green, this is catastrophic.

Aside from the considerable environmental impact, biogas leaks bring other risks. In the worst-case scenario, biogas in combination with air can form an explosive gas mixture which, in a confined space near an ignition source, can result in explosion. While explosions are thankfully extremely rare, they bring a high risk of serious injuries and fatalities and, as a result, are something no plant owner ever wants to experience on their site.

Biogas also contains hydrogen sulphide (H2S), a toxic gas which has been the cause of a number of deaths in the UK agricultural industry in relation to slurry tank management. As H2S is heavier than air, it will fall to the ground. In confined, poorly-ventilated spaces it can accumulate and remain unnoticed until someone enters, resulting in sometimes fatal effects.

Gas leaks on AD plants also have a financial impact. Any volume of biogas leaking into the atmosphere will subsequently reduce a plant’s gas yield; and therefore, the owner’s profit margin. In fact, losing just 1m3 of methane per hour will result in a financial loss in the region of £5,000 per year.

There is also the issue of sustainability criteria to consider. In order to receive payments through either the Feed-in Tariff (FIT) or Renewable Heat Incentive (RHI) schemes, AD operators must demonstrate that their plant is operating sustainably. Regulators have considered clamping down on this area, as some industry reports mention very high levels of fugitive emissions. Site operators can therefore use gas leakage surveys as a way to protect against potential loss of incentives and demonstrate to the authorities that their plant is well-managed, with leaks kept to a minimum.

Leakage hotspots

While an AD operator may believe that their plant is operating at a high standard, all anaerobic digesters have inherent weak points which make them susceptible to biogas leakage. Potential hotspots include:

• Gas membrane connections;
• Cable grommets (where a submersible stirrer cable passes through the digester wall);
• Flange connections;
• Viewing windows;
• Carbon filters;
• Any areas where maintenance is carried out.

Reducing your risk

The risks of gas leakage are clearly significant and often expensive. However, identifying a leak is a simple and affordable process which can help prevent a serious incident from occurring. A gas leakage detection service should therefore form part of any responsible plant operator’s ongoing maintenance programme.

For example, the FM BioEnergy service covers a full AD plant survey with a methane-sensitive monitor and laser, as well as infra-red devices, including:

• Survey of all tanks, CHP, biogas upgrading equipment, roof membranes, pipes and flanges;
• Analysis of emissions from CHP and double-membrane covers;
• Report with images, videos and repair priority table.

While the majority of our audits to date have uncovered minor leaks, 25% were found to have serious failings; fixing these not only prevents a more serious and costly incident from occurring, it often results in a 12-month payback on the price of the survey.

The best times to conduct a detection survey are at the start of full operation; after significant maintenance work; if your feed-to-gas conversion is lower than expected (and the biology remains stable); and of course, if you can smell biogas. After all, the cost of detecting a potential leak is minimal but the implications of leaving it to chance could be massive.

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