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When do you need to measure CO?
- Permit conditions. The environmental permit sets an emission limit value for CO 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 CO to be measured at every periodic measurement, at least every three years, although the ordinance has no emission limit value for CO.
- Waste incineration (the Swedish Ordinance on Waste Incineration, SFS 2013:253): CO is measured continuously, and the measuring system is checked every year against the reference method EN 15058 (QAL2 and AST).
- Large combustion plants (the Swedish Ordinance on Large Combustion Plants, SFS 2013:252): gas-fired plants measure CO continuously from 100 MW, otherwise every six months. The BAT conclusions require continuous monitoring of CO, except in small plants with short operating hours.
- Tuning and performance tests: CO shows when there is not enough air, and is measured when the combustion is set up and when guarantee values are checked after a plant has been built or rebuilt.
How the sampling train is built
The measuring system is extractive. A probe with a coarse filter draws out a sample stream of the flue gas, and a heated sample line carries it to a gas cooler that condenses out the water vapour. The dry sample gas passes a pump, a fine filter and a rotameter on its way to the analyser. There infrared light passes through a measuring cell containing sample gas. CO absorbs light at its own wavelengths, and the higher the concentration, the less light reaches the detector (non-dispersive infrared spectrometry, NDIR). Zero and span gas are connected at the probe, and the values are logged continuously.
- 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 carbon monoxide and oxygen
- 9. Data acquisition
- 10. Zero and span gas
Water vapour is the main interferent for an IR analyser, so the sample gas is dried to a dew point of at most 4 °C before the analysis. Nor should the CO analyser sit downstream of the NOx analyser's converter, which can itself form CO. At very high concentrations we dilute the sample gas 1:10 with dry air immediately before the analyser, and the measuring range then extends to 5 % CO by volume.
How is a CO measurement done?
Measurement plan and operating condition
The measurement is planned according to EN 15259: the measurement site, the operating conditions and the measurement periods are decided together with you. If the flue gas is homogeneous over the cross-section, one representative measurement point is enough; otherwise we measure at several points.
Measuring range and span gas
The measuring range is chosen so that it covers the highest peaks and at least 150 % of the half-hourly emission limit value. The span gas lies at about 90 % of the measuring range, and its concentration is determined with an uncertainty of at most 2 %.
Calibration and leak test
The system runs on flue gas for about an hour. Zero gas (nitrogen) and span gas are then fed first directly to the analyser, which is adjusted, and then in at the probe. If the readings differ by more than 2 % of the span gas value, we look for leaks before the measurement starts.
Measurement
CO is measured continuously together with oxygen. The values are logged at least once a minute, and more often when short CO peaks need to be captured. The sample gas flow and the temperatures are monitored. 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 concentration in ppm is 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 15058
These requirements decide whether a CO measurement is valid. Leaks and drift are checked at every measurement, linearity and interference at least once a year.
| Requirement | Value |
|---|---|
| Validated range | 0–400 mg/m3 (large combustion plants) and 0–740 mg/m3 (waste incineration), measurement periods of 30 minutes |
| 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 |
| Linearity | Deviation of at most 2 % of the measuring range |
| Interference | Other gases may together affect the reading by at most 4 % of the measuring range |
| Dew point after the gas cooler | At most 4 °C |
| Measurement uncertainty | Expanded uncertainty at most 6 % of the daily emission limit value, before conversion to the reference oxygen content |
What does CO say about the combustion?
Carbon monoxide forms when the carbon in the fuel does not burn all the way to carbon dioxide. That happens when there is not enough air, when air and combustible gases mix poorly so that pockets of low oxygen content pass through the furnace, when the temperature is too low or the residence time too short. During load increases and when the moisture content of the fuel varies, the furnace temperature can fall, and CO then often comes in short peaks.
CO is therefore an indicator of the combustion as a whole. According to a report from Värmeforsk (the Swedish thermal engineering research institute), hydrocarbons (TOC) and heavier compounds such as PAH follow when CO rises. It is probably the peaks that matter, not the average, and the relationship is different for every boiler. At the same time NOx falls when the excess air is reduced. Tuning is therefore about finding the operating point where both CO and NOx are low, and then CO, NOx and oxygen are measured at the same time.
Which emission limit values apply to CO?
The emission limit value for CO is in the permit, in the ordinance for the type of plant or in a BAT conclusion. For solid fuels the ordinances on medium and large combustion plants have no emission limit value for CO. There the permit conditions apply. The values are mg/Nm3 dry gas.
| Plant | CO, mg/Nm3 | Rule |
|---|---|---|
| Waste incineration | 50 as a daily average, to be met on 97 % of the days of the year. 100 as a half-hourly average, or 150 for 95 % of the ten-minute values of the day. At 11 % O2 | SFS 2013:253, section 66 |
| Waste incineration under the BAT conclusions | Daily average 10–50. At 11 % O2 | BAT 29, waste incineration |
| Large combustion plant firing natural gas: boiler, gas engine or gas turbine | 100. At 3 % O2 in boilers and 15 % O2 in gas turbines and gas engines | SFS 2013:252, sections 60, 62, 66 and 67 |
| Large combustion plant, solid biomass or peat | No emission limit value. Indicative yearly averages in the BAT conclusions, from below 30 up to 250 (50–100 MW), 160 (100–300 MW) and 80 (from 300 MW), for plants operated at least 1,500 hours per year. At 6 % O2 | BAT 24, large combustion plants |
| Medium combustion plant 1–50 MW | No emission limit value, but CO must always be measured | SFS 2018:471, section 43 |
The value 100 mg/Nm3 applies in new plants to all gaseous fuels, and in gas turbines also to light and medium distillates. It does not apply to gas turbines and gas engines for emergency use that run at most 500 operating hours per year, nor to gas-fired gas turbines at loads below 70 %. In the ranges of the BAT conclusions the upper value is binding, under the Swedish Industrial Emissions Ordinance (SFS 2013:250).
What does a CO 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. CO is measured with the same measuring system as oxygen, NOx and carbon dioxide, so the cost per parameter is low when the gases are measured together. There is no laboratory analysis. Request a quote – we reply within two working days. Approximate prices for the most common measurements are in the price guide.
How often must CO be measured?
As often as the permit says. The ordinances set the minimum. Medium combustion plants measure CO 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. Gas-fired large combustion plants without continuous measurement measure at least every six months. At waste incineration plants CO is measured continuously, and the measuring system is checked against the reference method at least once a year.
Frequently asked questions about CO measurement
Is the measurement accredited?
Yes. Both sampling and analysis of CO according to EN 15058 are within METLAB's Swedac accreditation. The accredited measuring range is 1–5,000 ppm, with dilution up to 50,000 ppm.
How is ppm converted to mg/Nm3?
1 ppm CO corresponds to 1.25 mg/Nm3 (the molar mass of 28 g/mol divided by the molar volume of 22.4 l/mol). 100 ppm is therefore 125 mg/Nm3. The concentration is then converted to the reference oxygen content: measured at 9 % O2 and with a reference of 6 %, it becomes 125 × 15 / 12 = 156 mg/Nm3.
Why must CO be measured when the ordinance has no emission limit value?
The Swedish Ordinance on Medium Combustion Plants (SFS 2018:471) requires it for all medium combustion plants, whatever the fuel. Plants that run at most 500 operating hours per year and are exempt from the emission limit values also measure CO: at the latest after 1,500 operating hours, 500 if the rated thermal input is above 20 MW, and at least every five years.
How long does the measurement take?
A measurement period is at least 30 minutes, and the BAT conclusions use the average of three consecutive periods. In addition, the system runs on flue gas for about an hour before the calibration. For tuning we measure for as long as needed.
Can CO be measured at the same time as other parameters?
Yes. The same sample gas is led to several analysers: oxygen is always measured at the same time, and NOx and carbon dioxide in the same system. Dust, TOC and wet chemical samples are taken with their own equipment during the same visit.
What interferes with a CO measurement?
Mainly water vapour, which is why the sample gas is dried. Carbon dioxide, hydrocarbons and nitrous oxide also absorb infrared light. Nitrous oxide occurs in flue gas from fluidised bed boilers and with SNCR using urea, and the analyser's sensitivity to nitrous oxide must then be known.
How accurate must a permanently installed CO monitor be?
At waste incineration plants and large combustion plants the measurement uncertainty of an automated measuring system may be at most 10 % of the daily emission limit value for CO, the strictest requirement among the continuously measured parameters. The averages are multiplied by 0.90 before they are compared with the emission limit value.
What is required of the measurement site?
A sampling port where the probe reaches a representative point, a safe working platform according to EN 15259 and electricity for the heated sample line, the gas cooler and the analysers. The analysers should stand close to the sampling port: a short line gives a short response time, and then the CO peaks are visible.