Use a consistent gas basis, separate component flow from total gas flow, and avoid counting capture, recovery or operating losses twice.
Write the calculation boundary first
A useful mass balance follows CO₂ from a defined source point to a defined product point. Available source CO₂, accepted feed CO₂, recovered liquid and marketable product are different quantities. Before entering numbers, decide which quantity the flow meter reports and which losses the recovery fraction includes. This prevents a seemingly precise result from mixing different boundaries.
For a simple first-pass estimate, annual recovered tonnes equal available CO₂ in kilograms per hour multiplied by the overall recovery fraction and operating hours, then divided by 1,000. If the input already describes delivered liquid-product capacity, do not apply the recovery fraction again. Similarly, if operating hours already exclude planned downtime, do not subtract that same downtime twice.
Convert volume on its declared conditions
For the ideal-gas screening model used here, CO₂ mass flow equals total gas volume flow multiplied by absolute pressure, CO₂ mole fraction and molecular mass, divided by the gas constant and absolute temperature. A compressibility factor can be included when an appropriate real-gas value is available. The website’s normalized example uses 0°C, 101.325 kPa absolute and Z = 1.
Using a CO₂ molecular mass of 44.0095 kg/kmol and R = 8.314462618 kPa·m³/(kmol·K) gives approximately 1.9635 kg of ideal CO₂ per cubic metre at that reference condition. This is a declared approximation, not a universal density for every temperature or pressure. A detailed plant calculation should use an appropriate property model.
Worked example: a CO₂-rich gas stream
Assume an illustrative dry mixed-gas flow of 1,000 Nm³/h, with 98% CO₂ on the same dry basis. Available CO₂ is approximately 1,000 × 0.98 × 1.9635 = 1,924.2 kg/h. At an assumed overall recovery of 90%, product is approximately 1,731.8 kg/h. Over 8,000 operating hours, the arithmetic gives approximately 13,854 tonnes per year.
These assumptions do not describe a measured result from our plants. They show how the units connect. The result still needs comparison with collection availability, peak flow, product acceptance, storage and customer demand. The equipment design capacity may need to differ from the mean product rate because the source varies over time.
Separate capture from downstream recovery
A dilute source can involve two distinct fractions: the CO₂ captured from the mixed gas and the captured CO₂ retained as final product. If capture is assumed to be 85% and downstream recovery 95%, their combined fraction is 0.85 × 0.95 = 0.8075, or 80.75%. Do not subsequently multiply by another overall recovery fraction intended to represent those same losses.
A collection fraction may also be relevant if only part of the generated source gas is admitted. It belongs in the balance only when it is not already included in the measured available flow. Write each stream and loss once. This simple discipline is more valuable than adding more decimal places to uncertain input data.
Know what the result cannot establish
The arithmetic estimates quantity. It does not demonstrate food-grade suitability, contaminant removal, refrigeration duty, compressor power or a safe operating envelope. A source with incomplete analysis should produce a list of missing data rather than a quality approval. A correct mass balance can still describe an uneconomic or unsuitable project.
For commercial assessment, compare annual acceptable product with usable internal demand or credible external offtake. If the plant could recover 8,000 tonnes but the market accepts only 5,000, assigning full selling value to all 8,000 overstates the opportunity. Preserve the distinction between technical recovery potential, planned production and marketable tonnes when discussing the result with us.
Common technical questions
Is Nm³/h the same as actual m³/h?
No. A normalized volume refers to declared reference conditions. Actual volume refers to conditions at the measurement point.
Should CO₂ concentration be applied to a kg CO₂/h input?
No. That input already describes the CO₂ component. Concentration is needed when converting an appropriate total mixed-gas measurement.
Check the calculation before converting it into equipment capacity
| Check | Correct interpretation |
|---|---|
| 1,000 Nm³/h is dry gas; CO₂ is 98% on a dry basis | Use the stated dry flow with the dry CO₂ fraction. |
| 1,000 m³/h is actual wet gas | Use actual absolute pressure, temperature, compressibility and wet CO₂ fraction; do not multiply by the normal-state density. |
| Recovery is 90% of collected CO₂ | Do not apply the same 10% loss again in annual operating hours. |
| 8,000 operating hours already exclude downtime | Apply a separate availability factor only if it represents additional, clearly identified time losses. |
At 15°C instead of 0°C, the same ideal-gas volume contains about 5.2% less mass at the same pressure. A label such as “standard” without a reference temperature can therefore cause a material sizing error. At elevated pressure, use a suitable equation of state or validated property calculation rather than assuming Z = 1.
Technical references
- NIST Chemistry WebBook: Carbon dioxide — Molecular weight 44.0095 and CO₂ phase-change data.
- NIST CODATA: fundamental physical constants — Molar gas constant R = 8.314462618 J mol−1 K−1 on the stated molar basis.
Apply this to your project
Open the source assessment tool to record the mass-balance and economic assumptions. For a project-specific design, share the gas analysis and product specification with us.
We publish this guide as YTC. Our editorial policy explains the source checks, AI assistance and how to report a correction. Examples do not establish a plant guarantee.