Biogas volume, methane recovery and saleable CBG

Keep three quantities separate

Raw biogas volume, methane content and saleable CBG mass describe different things. Raw biogas contains methane, carbon dioxide and other constituents. Upgrading removes components, and some methane can leave in the residual gas. The product must then meet the agreed sale specification.

When comparing proposals, ask which quantity each figure represents and whether the gas measurement is wet or dry, at normal or actual conditions. A daily volume with no stated basis cannot be reliably compared with a compressor's hourly rating.

Reconcile the operating window

A plant producing 10,000 Nm³ of raw gas per day and processing it over 18 hours has an average processing duty of approximately 556 Nm³/hour before any design margin. This arithmetic does not establish the right equipment size by itself. Peaks, turndown, maintenance and the gas-storage profile must also be checked.

Check recovery as well as purity

A high methane percentage in the product does not show how much of the inlet methane was retained. Ask vendors for an explicit recovery guarantee and a measurement method covering the relevant inlet, product and residual streams.

• Use one agreed feed-gas analysis and flow basis.
• Separate gross production from gas that can actually be sold.
• Define quality rejection and downtime assumptions.
• Agree metering accuracy, calibration and test duration.

Put the balance into the contract

The performance schedule should connect feed quality and quantity to gas output, product quality, recovery and utility use. Unspecified assumptions become difficult to resolve at handover. A design review before ordering provides a practical opportunity to close these gaps.

Keep an auditable gas balance

Record the location, reference conditions and time period for each gas measurement. Inlet raw gas, product gas and residual gas should be compared on compatible dry or wet and pressure/temperature bases. Where direct measurement is unavailable, identify the estimate and its uncertainty.

A useful acceptance protocol defines instrument calibration, inlet conditions, the product-quality limits and how recycled or off-spec gas is counted. It also identifies equipment stops and excludes ambiguous double counting. Product purity and methane recovery must remain separate results.

Read the production promise as a chain of assumptions

A daily CBG commitment depends on feed availability, organic input, biological conversion, gas capture, upgrading recovery and operating availability. Ask which of these are supported by testing, which are covered by vendor guarantees and which remain owner responsibilities.

The engineering review should connect each assumption to a measurement or an enforceable condition. That is more useful than a single conversion factor copied between proposals, especially when the proposed raw-gas composition or processing schedule changes.

• Identify whether flow is raw biogas, methane or product gas.
• State gas reference conditions and the product composition.
• Define the complete package's electricity and recovery boundaries.
• Reconcile production with the dispatch and storage schedule.