Emissions and climateAnalysis
Methane emissions from biogas plants: where they come from and how large the losses are
Methane escapes from biogas plants through safety valves, membranes, open digestate tanks, CHP engines and off-gas. We discuss the sources, ranges of losses from measurement campaigns and ways of reducing them.
Other language versions: polski.
A biogas plant is one of the tools for reducing methane emissions, but it can also emit methane itself if it is leaky or badly operated. The main sources include: digestate tanks (especially open ones), safety and overpressure valves of the digesters and gas tanks, leaking joints and fittings, unburned methane in the exhaust of CHP engines, off-gas from upgrading, and service and emergency operations. Measurement campaigns carried out in Europe point to a wide spread of losses – from fractions of a per cent to a dozen or so per cent of the methane produced – with a significant part of the emissions coming from a few points that can be quickly repaired once detected.
Sources of emissions
| Source | Mechanism | Possibility of reduction |
|---|---|---|
| Digestate in open tanks | residual methane production after leaving the digester | gas-tight cover with gas recovery, longer retention time |
| Overpressure and vacuum valves | opening at too high a pressure (overpressure valve) or too low (vacuum valve) | emergency flare, a gas store of adequate capacity, automation |
| Membranes of gas tank and digester roofs | micro-leaks, damage | inspections, replacement |
| Leaks in fittings, pipes, compressors | seal wear, corrosion | LDAR, seal replacement |
| CHP exhaust | unburned methane (so-called methane slip) | engine selection, catalytic oxidiser |
| Off-gas from upgrading | methane slip | afterburning, technology selection |
| Loading/unloading, service | venting, opening of equipment | procedures, gas recovery |
What measurements show
Reports and studies (including work carried out under IEA Bioenergy Task 37 and campaigns in Denmark, Germany, Sweden and other countries) confirm that:
- methane losses are distributed unevenly: many installations have small losses, and a small group – very high ones;
- the largest single emissions result from venting, damaged membranes and open digestate stores;
- for agricultural installations the average values in publications usually lie in the range of about 1% to a few per cent of methane production, and the spread between installations is very large;
- remote measurements (e.g. by the tracer gas method or plume analysis) capture the total leak well, and contact measurements – point sources.
The exact figures depend on the country, technology and measurement period, so in your own analyses you have to use data from specific campaigns, not generalisations.
Worked example
Assumptions: a biogas plant produces 2.2 million m³ of methane a year. Total losses of 3% mean 66 thousand m³ of CH₄, i.e. about 47 t. At GWP₁₀₀ = 28 this is about 1325 t CO₂e a year (at GWP₂₀ = 84 – about 3970 t CO₂e). In energy terms: 660 MWh a year (10 kWh/m³), and at an assumed price of PLN 200/MWh – PLN 132 thousand of lost revenue. If 70% of these losses come from one leaking valve and an open tank, repairing them lowers the losses to about 0.9% and saves about PLN 92 thousand a year. Whether this pays off has to be assessed individually, comparing the cost of covering the tank and replacing the valve with the value of the avoided losses.
How to reduce them
- Gas-tight covering of digestate tanks and connecting them to the gas system.
- An emergency flare and a large gas store, so that venting does not occur.
- Inspections and LDAR: regular detection of leaks with a detector or optical camera and their repair.
- Catalytic oxidation of methane in CHP exhaust where the values are high.
- Afterburning of the off-gas from upgrading.
- Operating procedures: start-ups, shutdowns, filter replacements, so as to limit venting.
- Staff training and quality control of repairs.
Impact on the GHG balance
Methane leakage enters the emissions balance of biomethane or of energy from biogas and can decide whether the reduction threshold in RED is met. Small losses (fractions of a per cent) usually do not change the assessment, but losses of a few per cent can noticeably lower the reduction. Producers are therefore increasingly documenting losses with measurements.
The role of design and acceptance of the installation
Most leak problems can be limited at the design stage: an appropriately sized gas store, a flare of sufficient capacity, valves that can discharge to the flare instead of to the atmosphere, gas-tight covers and access to inspection points. When accepting the installation it is worth demanding leak-tightness tests and an inspection with an optical camera, and writing regular checks into the service agreement. A baseline leak measurement after start-up provides a reference point for later comparisons.
Why measurements pay off
Installations that measure losses usually reduce them – knowing where methane escapes makes it possible to set repair priorities. In addition, reliable documentation improves standing in certification and in talks with the bank or the gas customer.
Summary
Methane emissions from biogas plants are a technical problem that can be significantly reduced with small investments and good practice. The best strategy is to combine a gas-tight design, systematic measurements and quick repairs.
Sources
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