BiomethaneAnalysis
Methane slip in biogas upgrading: how much we lose and what it costs
Methane slip is the part of the CH₄ from biogas that goes into the off-gas instead of the biomethane. We calculate how many tonnes of CO₂e and how many zloty this means, using a 500 m³/h installation as an example.
Other language versions: polski.
Methane slip is the part of the methane contained in the raw biogas that, in the upgrading process, does not end up in the biomethane but escapes with the off-gas, i.e. the waste stream rich in CO₂. It is expressed as a percentage of the feed. It is both a loss of revenue (you sell less gas) and a greenhouse gas emission: methane is a greenhouse gas several dozen times stronger than CO₂ over a 100-year horizon. For a biomethane plant this indicator is among the most important technical and environmental parameters.
Where slip comes from
In every separation technology some of the methane passes into the CO₂ stream:
- in membranes methane permeates slowly, but not zero; the higher the product purity and the higher the yield, the larger the required area and recirculation;
- in PSA methane is adsorbed to a small extent and remains in the column or in the purge gas;
- in water scrubbing methane dissolves in water (weakly, but it is a fraction of a percent), and in stripping it goes to the off-gas;
- in amine scrubbing methane dissolves physically in trace amounts;
- in cryogenics losses depend on the efficiency of separation.
Besides slip there are also losses not resulting from the separation itself, which must be added to the total methane losses at the installation: leaks from fittings, venting during start-ups and shutdowns, and emergency burning of biogas in the flare.
Typical values
The table below shows the orders of magnitude found in the literature and in suppliers' offers (modern systems, steady operation). Actual values must be confirmed in the guarantee and in measurements.
| Technology | CH₄ slip in the off-gas |
|---|---|
| Multi-stage membranes | usually below approx. 1% |
| PSA | approx. 1% or less; older installations more |
| Water scrubbing | approx. 0.5–1.5% |
| Amine scrubbing | usually below 0.1–0.2% |
| Cryogenics | depending on the configuration, low |
A calculation example
Assumptions: an installation of 500 m³/h of biogas with 55% CH₄ (275 m³/h of methane), 8000 hours of operation per year, slip of 1%. Annual methane production in the biogas: 2.2 million m³. Loss: 22 thousand m³ CH₄. At a methane density of approx. 0.717 kg/m³ this is approx. 15.8 t CH₄. With GWP₁₀₀ = 28 (the value from the fifth IPCC report) this gives approx. 442 t CO₂e per year. In energy terms the loss corresponds to approx. 220 MWh (at approx. 10 kWh/m³), which at an assumed gas price of PLN 200/MWh gives approx. PLN 44 thousand per year.
For comparison: slip of 0.1% would mean 44 t CO₂e and PLN 4.4 thousand. The difference of approx. 400 t CO₂e per year corresponds to the emissions of a few hundred passenger cars driven for a year – which is why losses matter not only financially but also in the emissions calculation of the biomethane.
How to limit slip
- Choice of technology to suit emission requirements (e.g. amines or multi-stage systems).
- Afterburning of the off-gas in a thermal or catalytic oxidiser; the methane is converted into CO₂, which reduces the greenhouse effect. At a low CH₄ concentration, supplementary fuel or heat recovery is required.
- Recirculation of gas from the flash, the permeate or the purge gas.
- Operation: avoiding operation outside the design range, tightness of fittings, leak control.
- Monitoring: measurement of the CH₄ concentration in the off-gas and a mass balance of the installation.
Slip in the biomethane emissions balance
In the RED methodology, methane emissions from upgrading are included in the GHG balance of the biomethane (as process emissions). In default calculations assumptions about the level of losses are adopted, and an actual low slip can improve the result in actual calculations. This is the link between technology and the value of the certified gas: a low methane loss improves the carbon footprint and the commercial value of the biomethane.
Contractual provisions
In the contract with the technology supplier, write down: the guaranteed slip (%), the conditions of measurement (load, composition, method), a penalty for exceedance and an obligation of correction. It is also worth including periodic measurements after start-up and during operation, because slip usually grows as the membranes or sorbent age.
The difference between slip and actual emission
Slip describes what leaves the separator in the off-gas, but not always the same thing reaches the atmosphere. If the off-gas is directed to a thermal oxidiser, the methane emission falls by the afterburning efficiency (usually over 99% in correct operation), and additional biogenic CO₂ appears. Conversely, leaks unrelated to separation must be added to the emission. That is why GHG balances distinguish two indicators: technological slip (a property of the device) and the total methane losses of the installation (the sum of slip, leaks, start-ups and failures after taking afterburning into account). The second indicator is more important for the climate and should be the subject of verification measurements.
Summary
Methane slip is an indicator that combines economics and climate. Small percentage differences translate into hundreds of tonnes of CO₂e and tens of thousands of zloty per year, so it is worth measuring, guaranteeing and limiting them at every stage of the installation's life.
Sources
Questions and answers
What methane slip is acceptable?
There is no single legal threshold for all countries. In practice, modern plants aim for values below 1%, and where off-gas afterburning is used, the effective emission is even lower.
