How to Calculate Biogas Methane Recovery Rate
Biogas plants monetize methane, not total biogas volume. Accurately translating raw flow, composition, and downtime into delivered methane energy underpins revenue forecasts, interconnection studies, and emissions reporting. An overly optimistic capture assumption can exaggerate return-on-investment claims and undermine crediting programs, while conservative estimates may leave financing on the table. A transparent calculation keeps both technical and financial teams aligned.
The steps below define variables in consistent units, apply deterministic formulas, and outline validation against chromatograph and SCADA data. They pair naturally with pipeline capacity checks in the hydrogen linepack flexibility guide and with traceability expectations seen in the battery passport traceability walkthrough when you need auditable environmental attributes.
Definition and scope
Methane recovery rate expresses how much methane leaves the upgrading system relative to what entered the biogas feed. It incorporates feed composition, separation efficiency, and real-world uptime. The output is commonly reported in Nm³/h of methane and converted to energy using the lower heating value (LHV), aligning with pipeline and combined heat-and-power contracts. The calculation does not address CO₂ purity or water dew point compliance, which must be handled separately.
This walkthrough focuses on steady-state operation. If your plant experiences seasonal feed swings or frequent feedstock shifts, repeat the calculation with scenario inputs to bracket expected performance. For projects that inject renewable natural gas into multiple pipelines, compute recovery separately per delivery point to reflect differing uptime and pressure constraints.
Variables and units
- Q – Raw biogas volumetric flow at normal conditions (Nm³/h).
- yCH4 – Methane fraction in raw biogas (dimensionless, 0–1).
- ηcap – Capture efficiency of the upgrading process (dimensionless, 0–1).
- U – Operating uptime over the period analyzed (dimensionless, 0–1).
- QCH4 – Methane volume entering upgrading (Nm³/h).
- Qdel – Methane volume delivered after capture and uptime (Nm³/h).
- Edel – Energy delivered on an LHV basis (kWh/h).
Keep composition measurements in volume percent and convert to fractions for calculations. Uptime should reflect real availability, not just planned availability, and should include downtime from maintenance, feed interruptions, or grid outages that halt compression. When methane slip is penalized, adjust ηcap downward to reflect the portion lost to flare or venting.
Core formulas
QCH4 = Q × yCH4
Qdel = QCH4 × ηcap × U
Edel = Qdel × LHVCH4
LHVCH4 is typically 9.97 kWh/Nm³. If your contracts reference higher heating value (HHV), multiply by 11.06 kWh/Nm³ instead and document the change. Capture efficiency should combine both stage separation performance (membrane or PSA) and downstream polishing steps that may reject off-spec gas. When uptime is low, the model properly scales down delivered energy without altering instantaneous purity.
Step-by-step workflow
1. Characterize the feed
Gather recent gas chromatograph data to determine yCH4. If only historical averages are available, use the lowest observed methane fraction to stay conservative. Confirm that volumetric flow Q is temperature- and pressure-normalized; if not, convert to Nm³/h to match energy constants.
2. Measure or specify capture efficiency
Use vendor guarantees or commissioning data to set ηcap. Adjust for membrane stage cut, PSA tail gas losses, and any slip permitted to meet oxygen or CO₂ specifications. If CO₂ removal is coupled with sulfur polishing, ensure the measurement boundary matches what the meter will record for gas sales.
3. Quantify uptime
Derive U from SCADA availability logs over the same period as the flow data. Exclude planned shutdowns only if contracts allow their exclusion when reporting delivered volumes. For plants with redundancy, compute the weighted uptime of the path that limits throughput.
4. Compute methane delivery and energy
Multiply Q by yCH4 to get QCH4. Apply ηcap and U to obtain Qdel. Multiply by 9.97 kWh/Nm³ to derive Edel. Present both volumetric and energy results; counterparties may prefer one over the other depending on pipeline nominations or CHP sizing.
5. Document uncertainties and reconcile
Compare calculated Qdel against custody-transfer meter readings. Reconcile differences by checking calibration, pressure base conditions, and flare logs. If the plant participates in crediting schemes, align the reconciliation process with the documentation rigor highlighted in the carbon removal delivery reserve article to maintain auditability.
Validation and monitoring
Validate yCH4 monthly with spot sampling, and cross-check with online analyzers where available. Track ηcap against vendor curves; a sudden drop often signals membrane fouling or PSA valve issues. Monitor U alongside causes of downtime to separate grid outages from process faults. When delivering into multiple offtake points, reconcile each with its own meter to avoid masking localized constraints.
Trending Edel against feedstock mix changes helps catch digestion instability early. If co-digestion is expanding, simulate expected methane fractions so procurement teams can negotiate feed contracts with realistic recovery expectations.
Limits and interpretation
The calculation assumes steady-state operation and does not capture transient spikes in methane concentration during start-up or upset. It also omits electrical parasitics and compression power, which affect net plant efficiency but not methane recovery directly. For projects claiming greenhouse gas reductions, remember that methane slip to atmosphere carries high warming potential; low ηcap may undermine environmental claims even if Edel remains high.
To account for uncertainty, apply confidence intervals to yCH4 and ηcap. Present P50 and P90 cases when negotiating financing. When uptime volatility is high, consider probability-weighting U by root causes so maintenance plans can target the dominant contributors to lost energy.
Embed: Biogas methane recovery calculator
Input raw flow, composition, capture efficiency, and uptime to calculate delivered methane volume and LHV energy in one step.