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When do you need to measure NOx?
- Permit conditions. The environmental permit sets an emission limit value for NOx and how often it is to be checked.
- Medium combustion plants (1–50 MW): the Swedish Ordinance on Medium Combustion Plants (SFS 2018:471) requires periodic measurement of NOx, at least every three years.
- Large combustion plants (the Swedish Ordinance on Large Combustion Plants, SFS 2013:252): continuous measurement from 100 MW, otherwise periodic. Automated measuring systems are calibrated and checked with QAL2 and AST.
- Waste incineration (the Swedish Ordinance on Waste Incineration, SFS 2013:253): NOx is measured continuously, and the measuring system is checked every year by parallel measurements with the reference method EN 14792.
- The Swedish NOx charge. The plant's own NOx measuring equipment is checked at least once per calendar year with a comparison measurement according to the Swedish EPA's regulations NFS 2016:13.
- Performance tests and tuning: guarantee values after a plant has been built or rebuilt, and adjusting the combustion, SNCR or SCR.
How the sampling train is built
A sample stream of the flue gas is drawn out through a probe with a coarse filter and carried in a heated sample line to a gas cooler, where the water vapour condenses and is drained off. The dry sample gas passes a pump, a fine filter and a flow meter (rotameter) on its way to the analyser. There NO is determined by chemiluminescence: NO reacts with ozone and emits light in proportion to the concentration. For NOx the gas is first led through a converter that reduces NO2 to NO. The values are logged continuously, and zero and span gas can be connected at the probe.
- 1. Probe with coarse filter
- 2. Heated sample line
- 3. Gas cooler that dries the sample gas
- 4. Condensate drain
- 5. Pump
- 6. Fine filter
- 7. Rotameter
- 8. Analysers for nitrogen oxides and oxygen; the NOx analyser has a converter that turns NO2 into NO
- 9. Data acquisition
- 10. Zero and span gas
NO2 is the sensitive part. It dissolves in the condensate in the gas cooler, which is why the pump sits after the cooler: overpressure in the cooler increases the loss. We do not use gas coolers that lose more than 10 % NO2 when the NO2 concentration may exceed 20 mg/Nm3 or a quarter of the NOx. At flue gas temperatures above 250 °C stainless steel can change the ratio between NO and NO2, so ceramics, glass or titanium are used instead.
How is a NOx measurement done?
Measurement plane and measurement point
The measurement is planned according to EN 15259. If the flue gas is homogeneous over the cross-section, one representative measurement point is enough; otherwise we measure at several points. The plant must run stably in the operating condition we have agreed on.
Connection and calibration
The system is connected, heated up and run on flue gas for about an hour. Zero gas (nitrogen) and span gas with a known NO concentration are then fed directly to the analyser, which is adjusted. The concentration of the span gas is determined with an uncertainty of at most 2 %.
Check of the sampling system
The same gases are then fed in at the probe, through the whole system. If the reading differs by more than 2 % of the span gas value from the direct calibration, we look for leaks or contamination before the measurement starts.
Measurement
NOx is measured continuously together with oxygen, and the values are logged at least once a minute. The sample gas flow and the temperatures are monitored throughout. A measurement period is at least 30 minutes.
Check after the measurement
After the measurement period, and at least once a day, the zero and span points are checked again. If they have drifted by more than 2 % of the span gas value, the measured values are corrected; above 5 % the measurement is rejected.
Calculation and report
The measured values are corrected for the quenching of the light signal by carbon dioxide and converted to mg/Nm3 dry gas at the reference oxygen content. You receive the result with its measurement uncertainty in an accredited, digitally signed report.
The requirements in EN 14792
These requirements in the standard decide whether a NOx measurement is valid. Leaks and drift are checked at every measurement, the converter and the linearity at least once a year.
| Requirement | Value |
|---|---|
| Leak test | At most 2 % deviation when the span gas is fed through the whole sampling system, or a leak rate of at most 2 % of the sample gas flow |
| Zero and span drift | Above 2 % of the span gas value the measured values are corrected, above 5 % the measurement is rejected |
| Converter efficiency | At least 95 % of the NO2 is converted to NO |
| Linearity | Deviation of at most 2 % of the measuring range |
| Dew point after the gas cooler | At most 4 °C |
| Response time | At most 200 seconds for the whole measuring system |
| Measurement uncertainty | Expanded uncertainty at most 10 % of the daily emission limit value, before conversion to the reference oxygen content |
NO, NO2 and NOx: what is reported?
The analyser measures only NO. When the sample gas is first led through the converter, it shows the sum, NOx, and NO2 is the difference. In flue gas from ordinary combustion more than 95 % of the nitrogen oxides are NO, but in the atmosphere NO is oxidised further to NO2. NOx is therefore reported as if all of it were NO2: 1 ppm NOx is 2.05 mg/Nm3, although 1 ppm NO weighs only 1.34 mg/Nm3. Converting NO and NO2 separately and adding them up gives a value that is too low, in the example 141 instead of 205 mg/Nm3.
| Step | How we calculate | Example |
|---|---|---|
| 1. Add up in ppm | NOx = NO + NO2 | 90 + 10 = 100 ppm |
| 2. Convert to mg/Nm3 as NO2 | ppm × 2.05 | 100 × 2.05 = 205 mg/Nm3 |
| 3. Convert to the reference oxygen content | Concentration × (21 − reference O2) / (21 − measured O2), with oxygen measured at the same time | From 8 to 6 % O2: 205 × 15/13 = 237 mg/Nm3 |
What affects the NOx concentration?
Nitrogen oxides form in two main ways during combustion: from the nitrogen in the fuel, and from the nitrogen in the air when the flame temperature is high (thermal NOx). The concentration therefore depends on the fuel, the temperature and the oxygen supply in the combustion zone. Air staging, low-NOx burners and flue gas recirculation lower the temperature or the oxygen content there.
If that is not enough, NOx is reduced to nitrogen with ammonia or urea: without a catalyst at 800–1,000 °C (SNCR) or over a catalyst at a lower temperature (SCR). Ammonia that does not react leaves with the flue gas, and with urea in SNCR nitrous oxide can also form. Where SNCR or SCR is used, ammonia must therefore also be monitored according to the BAT conclusions.
Which emission limit values apply to NOx?
The emission limit value for NOx is in the permit, in the ordinance for the type of plant or in a BAT conclusion. The BAT conclusions give ranges, where the upper value applies as the limit value under the Swedish Industrial Emissions Ordinance (SFS 2013:250). The ordinances apply in addition to the permit conditions. The values are NOx expressed as NO2 in mg/Nm3 dry gas. What counts as a new plant is defined in each rule. Engines and gas turbines have their own values, and existing medium combustion plants of 5 MW or less are covered only from 2030.
| Plant | NOx, mg/Nm3 | Rule |
|---|---|---|
| Medium combustion plant 1–50 MW, solid fuels | 300 at 6 % O2 | SFS 2018:471, annexes 1–3 and 12–14 |
| Medium combustion plant, natural gas | 100 for new plants, 200–250 for existing ones. At 3 % O2 | SFS 2018:471, annexes 1–3 and 12–14 |
| Large combustion plant 50–100 MW, biomass or peat | 300, or 250 for new plants. At 6 % O2 | SFS 2013:252, sections 55 and 63 |
| The same plant under the BAT conclusions | Yearly average 70–225, or 70–150 for new plants. At 6 % O2 | BAT 24, large combustion plants |
| Waste incineration | 200 as a daily average and 400 as a half-hourly average. At 11 % O2 | SFS 2013:253, sections 57 and 63 |
| Waste incineration under the BAT conclusions | Daily average 50–150 (180 where SCR cannot be used), or 50–120 for new plants. At 11 % O2 | BAT 29, waste incineration |
What does a NOx measurement cost?
The price depends on the number of stacks or ducts and operating conditions, how long the measurement is to run, the conditions at the measurement site and the travel distance. NOx is measured with the same measuring system as oxygen, carbon monoxide and carbon dioxide, so the cost per parameter is low when the gases are measured together. There is no laboratory analysis. The time is spent connecting, calibrating and checking. Request a quote – we reply within two working days. Approximate prices for the most common measurements are in the price guide.
How often must NOx be measured?
As often as the permit says. The ordinances set the minimum. Medium combustion plants measure NOx at least every three years up to 20 MW and every year above 20 MW, the first time within four months of registration or commissioning. Large combustion plants without continuous measurement measure at least every six months. Where NOx is measured continuously, as at waste incineration plants, the measuring system is checked against the reference method at least once a year.
Frequently asked questions about NOx measurement
Which standard applies to NOx measurement?
EN 14792, the European standard reference method with chemiluminescence. It is used both for periodic measurements and when automated measuring systems are calibrated and checked. The older Swedish standard SS 028425, with an IR or UV analyser, is also within our accreditation.
Is the measurement accredited?
Yes. Both sampling and analysis of NO, NO2 and NOx according to EN 14792 are within METLAB's Swedac accreditation. The accredited measuring range for NO and NOx is 0.1–5,000 ppm.
How long does the measurement take?
It depends on the purpose. A measurement period is at least 30 minutes, and the BAT conclusions use the average of three such periods. A comparison measurement for the Swedish NOx charge requires at least ten data pairs (a reference value and the value from the plant's measuring system for the same period), normally of 60 minutes each. In addition, the system runs on flue gas for about an hour before the calibration.
Can NOx be measured at the same time as other parameters?
Yes. The same sample gas can be led to several analysers, for example for carbon monoxide and carbon dioxide, and oxygen is always measured at the same time for the conversion to the reference oxygen content. Dust, gas flow and wet chemical samples are taken with their own equipment during the same visit.
What is required of the measurement site?
A sampling port where the probe reaches a representative point in the duct, a safe working platform and electricity for the heated sample line, the gas cooler and the analysers. The requirements are in EN 15259. The line to the analyser should be short, otherwise the response time becomes long.
Does carbon dioxide or ammonia interfere with the measurement?
Carbon dioxide quenches the light signal in the analyser, normally by about 5 % of the reading, and we correct for that. Ammonia from SNCR or SCR can both reduce and form NO on hot steel surfaces in the measuring system, but below 5 ppm the effect is negligible.