On this page
When do you need to measure nitrous oxide?
- Permit conditions. The environmental permit for a fluidised bed boiler, a waste incinerator with SNCR or a process plant may set a condition for N2O and how it is to be checked.
- Large combustion plants under the BAT conclusions: N2O is measured once a year in circulating fluidised bed boilers firing coal, biomass or peat.
- Waste incineration under the BAT conclusions: N2O is measured once a year in fluidised bed boilers and where SNCR with urea is used.
- Emissions trading. In the production of nitric acid, adipic acid, glyoxal and glyoxylic acid, N2O is part of the EU Emissions Trading System (EU ETS) and is normally determined by measurement according to the EU Monitoring and Reporting Regulation.
- Process control: tuning of SNCR with urea and of the bed temperature in a fluidised bed boiler, where NOx is to be reduced without N2O and CO increasing.
How the sampling train is built
A sample stream of the flue gas is drawn out through a probe with a coarse filter and led in a heated sample line to a gas cooler, where the water vapour condenses at 2–5 °C and is drained off. The dry sample gas passes through a pump, a fine filter and a flow meter to the analyser, which determines N2O by the absorption of infrared light: the more light the gas absorbs at the wavelength of nitrous oxide, the higher the concentration. Oxygen is measured in the same sample gas. 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 nitrous oxide and oxygen
- 9. Data acquisition
- 10. Zero and span gas
The cooler is the sensitive part. If NO2 and SO2 dissolve in the condensate, nitrous oxide can form there, which is why the gas is cooled thoroughly and the condensate is drained off. Carbon monoxide absorbs light at almost the same wavelength as N2O, so the sample gas is led through a converter that oxidises CO to CO2 at no more than 300 °C, where the nitrous oxide does not yet decompose.
How is a nitrous oxide measurement done?
Measurement plan and operating conditions
The measurement is planned according to EN 15259 and a measurement programme that we agree with you: measurement site, operating conditions and measurement periods. For N2O in large combustion plants the BAT conclusions want two measurement series, one above and one below 70 % load.
Choice of measuring range
The instrument's range is chosen for the concentration you expect: the readings should fall at 20–80 % of the range, and peaks must fit. So we ask in advance about the expected concentration and the temperature, moisture and dust concentration of the flue gas.
Connection and calibration
The system is heated up and runs on flue gas for about an hour. Zero gas (nitrogen) and span gas with a known N2O concentration are then led straight to the analyser, which is adjusted. The span gas concentration is determined with an uncertainty of at most 2 %.
Check of the sampling system
The same gases are then led in at the probe, through the whole system. If the reading differs by more than 2 % of the span gas value, we look for leaks or contamination. If a leak is suspected, the probe is plugged and the leak rate is measured.
Measurement
N2O is measured continuously together with oxygen and carbon dioxide, and the values are logged at least every ten seconds. The sample gas flow is kept constant within 10 %. 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 readings are corrected; at more than 5 % the measurement is rejected.
Calculation and report
The N2O values are corrected for the effect of carbon dioxide and converted to mg/Nm3 (dry gas at 0 °C and 101.3 kPa), for combustion at the reference oxygen content: 1 ppm N2O is 1.96 mg/Nm3. You receive the result with its measurement uncertainty in an accredited, digitally signed report.
The requirements in EN ISO 21258
These requirements decide whether a nitrous oxide measurement is valid. Leak tightness and drift are checked at every measurement; linearity, repeatability, interferences and the CO converter at least once a year.
| Requirement | Value |
|---|---|
| Leak test | At most 2 % deviation when the span gas is led through the whole sampling system, or a leak rate of at most 2 % |
| Zero and span drift | Above 2 % of the span gas value the readings are corrected; above 5 % the measurement is rejected |
| Response time | At most 200 seconds for the whole measuring system |
| Detection limit | At most 2 % of the measuring range |
| Linearity and repeatability | Deviation at most 2 % of the measuring range |
| Interfering gases | Combined effect at most 6 % of the measuring range, tested with CO2, CO, SO2 and NO. CO is removed in the converter, CO2 is corrected for |
| CO converter | At least 95 % of the carbon monoxide is oxidised, checked at 1,000 ppm CO (our own requirement) |
| Span gas | N2O in nitrogen, concentration determined with an uncertainty of at most 2 % |
The standard compares the drift with the measured value, 2 and 10 % respectively. Since concentrations in flue gas are often close to zero, we compare with the span gas value instead, as for NOx, CO and O2. That is the practice in Sweden and gives a stricter requirement at low concentrations.
Where does nitrous oxide form?
Nitrous oxide forms from the fuel's nitrogen when combustion takes place at low temperature, below about 900 °C. That is why fluidised bed boilers, where the bed is kept cool, are the boiler type the BAT conclusions single out. The other source is SNCR: when NOx is reduced with urea, nitrous oxide forms as a by-product, with ammonia practically none.
In the process industry, nitrous oxide is measured above all in nitric acid production. It is a by-product that cannot be calculated from the raw materials, and for emissions trading EN ISO 21258 is specified as the measurement standard. Nitrous oxide is not part of NOx: a NOx analyser measures NO and NO2, not N2O.
What does nitrous oxide mean for the climate impact?
Over a hundred years one kilogram of nitrous oxide corresponds to 265 kilograms of carbon dioxide, the value the EU's climate reporting takes from the IPCC's Fifth Assessment Report. Nitrous oxide and carbon dioxide have almost the same molecular mass, so 40 ppm N2O in the flue gas corresponds to about one per cent by volume of CO2. In a boiler with 10 % carbon dioxide in the flue gas, that adds ten per cent to the climate impact. Methane is counted the same way, with the factor 28, and is measured with a method of its own: methane measurement.
Which emission limit values apply to nitrous oxide?
Nitrous oxide has no limit value in the ordinances on medium and large combustion plants or on waste incineration, and the BAT conclusions set no BAT-associated emission level (BAT-AEL) for N2O: they require measurement and that the emission is limited through the combustion technique. A limit value therefore exists only if the permit sets one.
| Plant | Level | Rule |
|---|---|---|
| Medium and large combustion plants, waste incineration | No limit value for N2O in the ordinances | SFS 2018:471, 2013:252 and 2013:253 |
| Circulating fluidised bed boilers firing coal, biomass or peat, large combustion plant | No BAT-AEL. Measurement once a year, and the combustion is set so that N2O and CO are limited | BAT 4, 20 and 24, large combustion plants |
| Waste incineration in a fluidised bed or with SNCR with urea | No BAT-AEL. Measurement once a year, and N2O is limited through the combustion technique | BAT 4 and 29, waste incineration |
| Production of nitric acid, adipic acid, glyoxal and glyoxylic acid | No limit value. The emission is measured and is part of emissions trading as carbon dioxide equivalents | The EU Monitoring and Reporting Regulation for emissions trading, Annex IV |
What does a nitrous oxide 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. Nitrous oxide is measured in the same dried sample gas as oxygen, carbon dioxide, NOx and CO, so the cost per parameter is low when the gases are measured together. There is no laboratory analysis; the time goes on connection, calibration and checks. Request a quote – we reply within two working days. Approximate prices for the most common measurements are in the price guide.
How often must nitrous oxide be measured?
As often as the permit says. The BAT conclusions specify once a year: for N2O in circulating fluidised bed boilers in large combustion plants, with two measurement series above and below 70 % load, and at waste incineration in a fluidised bed or with SNCR with urea. The ordinances have no interval of their own for N2O. Nitrous oxide from nitric acid production is measured continuously for emissions trading.
Frequently asked questions about nitrous oxide measurement
Is the measurement accredited?
Yes. Both sampling and analysis of N2O according to EN ISO 21258 are within METLAB's Swedac accreditation. The accredited measuring range is 0.5–100 ppm according to our method list, up to around 200 mg/Nm3.
Does a NOx analyser measure nitrous oxide?
No. NOx is the sum of NO and NO2, and the chemiluminescence analyser measures only NO, with a converter also NO2. Nitrous oxide needs an infrared analyser of its own, which measures in the same dried sample gas and is connected at the same time. A low NOx value says nothing about the nitrous oxide.
Why does nitrous oxide form in a fluidised bed boiler?
The bed temperature is low, below about 900 °C, and then part of the fuel's nitrogen is converted to N2O. A higher temperature gives less nitrous oxide but more NOx, which is why the BAT conclusions want the boiler set so that NOx falls without N2O and CO rising.
Can nitrous oxide be measured at the same time as other parameters?
Yes. Nitrous oxide is measured in the same dried sample gas as oxygen, carbon dioxide, NOx and CO, and the carbon dioxide is needed for the correction anyway. Methane and TOC are measured with their own heated equipment at the same time, as are dust, gas flow and wet chemical samples.
Do carbon dioxide or carbon monoxide interfere with the nitrous oxide measurement?
Yes, both absorb infrared light close to the wavelength of nitrous oxide. Carbon monoxide is therefore oxidised to carbon dioxide in a converter before the analyser, and we determine the effect of carbon dioxide for each instrument and subtract it using the carbon dioxide concentration measured at the same time.