Calculator inputs
Formula used
- mg/Nm³ (from ppmv) ≈ ppmv × MW / Vm, where MW is g/mol and Vm is L/mol.
- O₂ correction: Ccorr = C × (20.9 − O₂ref) / (20.9 − O₂meas).
- Emission rate: kg/h = (Ccorr × Q) / 1,000,000, with C in mg/Nm³ and Q in Nm³/h.
- g/s = kg/h × 1000 / 3600; t/year = kg/h × hours/year ÷ 1000.
How to use
- Select your concentration unit: mg/Nm³ or ppmv.
- Enter the measured concentration and normalized flow rate.
- If using ppmv, enter molecular weight and keep molar volume default unless required.
- Enable oxygen correction only when your limit uses a reference oxygen.
- Enter operating hours per year to estimate annual tonnage.
- Click calculate to view results above the form, then export.
Example data table
| Pollutant | Method | Concentration | Flow (Nm³/h) | O₂ meas / ref | kg/h | t/year (8,000 h) |
|---|---|---|---|---|---|---|
| SO₂ | mg/Nm³ | 180 mg/Nm³ | 55,000 | 6% / 0% | 0.0108 | 0.0864 |
| NOx | ppmv | 75 ppmv (MW 46) | 40,000 | 3% / 3% | 0.0062 | 0.0496 |
| HCl | mg/Nm³ | 25 mg/Nm³ | 20,000 | — | 0.0005 | 0.0040 |
Example values are illustrative and may not match your facility conditions.
Why stack emission rates matter
Stack concentration alone cannot show total release. Converting to a mass rate links monitoring data with permit limits, dispersion modeling inputs, and emissions inventories. A stable mass rate also helps prioritize control projects, compare units, and track performance across seasons and loads. Many agencies accept g/s and kg/h for engineering assessments, while annual totals support fees, sustainability metrics, and corporate reporting dashboards.
Concentration and flow integration
The calculator multiplies corrected concentration (mg/Nm³) by normalized flow (Nm³/h) to obtain mg/h, then converts to kg/h and g/s. Normalized flow reduces bias from temperature and pressure changes, so values remain comparable between tests. Example: 100 mg/Nm³ at 50,000 Nm³/h equals 5,000,000 mg/h, or 5.0 kg/h, which is 1.389 g/s. If flow is 10% higher, emissions rise 10%.
Using ppmv with molecular properties
When instruments report ppmv, the tool estimates mg/Nm³ using ppmv × MW ÷ Vm. With MW in g/mol and Vm near 22.414 L/mol at normal conditions, the conversion matches common stack testing practice. Example: 50 ppmv SO₂ (MW 64.07) converts to about 143 mg/Nm³. For 75 ppmv NO₂-equivalent (MW 46), the result is about 154 mg/Nm³. Adjust Vm only when your standard specifies different reference conditions.
Oxygen correction for comparable limits
Many permits normalize to a reference oxygen to compare combustion sources fairly. The correction factor uses (20.9 − O₂ref)/(20.9 − O₂meas). If measured O₂ is 6% and reference is 3%, the factor is about 1.16, increasing concentration and emission rate. If measured O₂ is lower than the reference, the factor can reduce results. Apply correction only for oxygen-based limits and ensure O₂ is reported on the same dry basis as the concentration.
Quality checks and annual reporting
Good results depend on aligned averaging periods and reliable input units. Use synchronized time windows for concentration and flow, confirm that “Nm³” is truly normalized, and document any moisture or dry-gas conversions required by your method. For annual totals, the calculator multiplies kg/h by operating hours and converts to tonnes. Example: 0.50 kg/h over 8,000 hours equals 4.0 t/year. Keep operating hours realistic for inventories, audits, and change-control reviews. If you are reconciling with monitors, export CSV results by hour, then aggregate in your spreadsheet. For reporting, retain raw values, correction settings, and calibration dates for traceability during inspections or third-party review.
FAQs
1) What does Nm³/h represent?
It is gas flow normalized to reference temperature and pressure. Using normalized flow keeps emission rates consistent when stack temperature, moisture, or barometric pressure vary.
2) Should I always apply oxygen correction?
No. Use it only when your permit or standard specifies a reference oxygen. For mass inventory calculations, use uncorrected concentration unless required.
3) Why is 20.9 used in the oxygen formula?
20.9% is the typical oxygen content of dry ambient air. The correction adjusts measured concentrations to the chosen reference oxygen basis.
4) How do I pick molecular weight for NOx?
Use the value defined by your method. Some programs use NO₂-equivalent (46 g/mol). Others treat NO and NO₂ separately. Match your reporting requirement.
5) My analyzer reports mg/m³, not mg/Nm³. What now?
Convert to a normalized basis using your standard’s temperature and pressure correction, or use a normalized flow that matches your measurement basis before calculating.
6) Why do my annual tonnes look too high?
Check operating hours, flow units, and whether concentration averages match the same time period as flow. Small unit mismatches can scale totals dramatically.