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When do you need to measure dust?
- Permit conditions. The environmental permit sets an emission limit value for dust and how often it is to be checked.
- Medium combustion plants (1–50 MW) under the Swedish Ordinance on Medium Combustion Plants (SFS 2018:471): periodic measurement at least every three years up to 20 MW and every year above 20 MW.
- Large combustion plants (the Swedish ordinance SFS 2013:252) and waste incineration (SFS 2013:253): calibration and surveillance measurements of automated measuring systems (QAL2 and AST).
- Performance tests and guarantee checks after a boiler or dust collector has been built or rebuilt.
- Process control: troubleshooting bag filters, cyclones and electrostatic precipitators, or the basis for an investment.
How the sampling train is built
The dust in a known flow of sample gas is collected on a plane filter. The sample gas is drawn isokinetically through a sharp-edged nozzle facing the gas stream, passes the probe tube and the filter, is dried and led through a control valve, pump and flow meter to a dry gas meter that measures the volume sampled. At the same time the gas velocity is measured with a pitot tube, as are the temperature and pressure of the flue gas.
- 1. Nozzle, sharp-edged and facing the gas stream
- 2. Probe tube, heated
- 3. Filter holder with plane filter, heated
- 4. Drying equipment: condensate bottle in a cooling bath and a drying tower with silica gel
- 5. Control valve
- 6. Gas-tight pump
- 7. Flow meter (rotameter)
- 8. Dry gas meter
- 9. Temperature sensor at the gas meter
- 10. Temperature sensor, flue gas temperature (type K thermocouple)
- 11. Pressure gauge, static pressure in the duct
- 12. Pressure gauge, dynamic pressure (micromanometer)
- 13. Pitot tube (L- or S-type) for gas velocity
- 14. Barometer
The filter holder sits either in the duct directly after the nozzle (in-stack) or outside the duct after the probe tube (out-stack). We choose according to the dust and the gas: sticky dust is sampled with the filter in the duct, without bends where dust can be deposited before the filter. After a scrubber or flue gas condenser, where the gas contains droplets, the filter sits outside the duct and is heated so that the droplets evaporate; otherwise the filter can break or water-soluble dust can pass through it. At high concentrations or with fine dust, a filter holder with a larger filter area is used.
How is a dust measurement done?
Measurement plane and measurement points
According to EN 15259 the measurement plane must lie on a straight run of duct, normally at least five hydraulic diameters downstream of the nearest disturbance, two upstream of the next and five from the top of the stack. The number of measurement points is set by the cross-sectional area of the duct, and the points are spread over two measurement lines at right angles to each other.
Preliminary measurement and choice of nozzle
Gas velocity and temperature are measured at the measurement points with a pitot tube, and the composition of the gas (oxygen, carbon dioxide and moisture) is determined. The nozzle is then chosen so that sampling stays isokinetic even at the point with the highest velocity.
Leak test
With the nozzle sealed, the whole sampling train is tested for leaks. The leak rate may be at most 2 % of the expected sample gas flow.
Isokinetic sampling
The probe is moved between the measurement points, at least 3 minutes at each point and at least 30 minutes in total. The sample gas flow is adjusted so that the deviation from isokinetic sampling stays within −5 to +15 %, and the filter is kept at the flue gas temperature or at least 160 °C.
Rinsing and field blank
Everything upstream of the filter is rinsed with deionised water, and with acetone where needed, and the rinse residue is weighed together with the filter. A field blank is taken every measurement day: the same handling, but without drawing gas through the equipment.
Conditioning and weighing
Filters and rinse residues are dried at 160 °C, cooled in a desiccator and weighed on the same balance before and after sampling. Control filters that travel with the samples all the way correct for apparent changes in weight.
Calculation and report
The dust concentration is the mass of dust divided by the volume of gas sampled, converted to mg/Nm3 (dry gas at 0 °C and 101.3 kPa) at the plant's reference oxygen content. The result is delivered with its measurement uncertainty in an accredited, digitally signed report.
The requirements in EN 13284-1
These requirements decide whether a dust measurement has been carried out correctly. They are set out in the standard and in our accredited method description, and we document them at every measurement.
| Requirement | Value |
|---|---|
| Measuring range | Below 50 mg/m3 – at higher concentrations ISO 9096 is used |
| Isokinetic deviation | −5 to +15 % |
| Sampling time | At least 3 min per measurement point and at least 30 min in total |
| Filtration temperature | The flue gas temperature or at least 160 °C |
| Filter | Retains more than 99.5 % of particles of 0.3 µm |
| Leak rate | At most 2 % of the expected sample gas flow |
| Field blank | At most 10 % of the emission limit value or 0.5 mg/m3, whichever is higher |
| Measurement uncertainty | Expanded uncertainty at most 20 % of the emission limit value – under good measurement conditions we are below 11 % |
How long do we have to sample?
The weighing has an uncertainty of about 1 mg. For that to be a small part of the result, the filter needs to collect at least 20 mg of dust, and how long that takes depends on how low the concentration is.
At an emission limit value of 10 mg/Nm3, 2 Nm3 of sample gas is enough. At 3 mg/Nm3 about 7 Nm3 is needed, two hours at 3.5 Nm3/h, and at 1 mg/Nm3 at least 20 Nm3, four hours at 5 Nm3/h. The lower the limit value, the longer the sampling time, and that shows in the price.
Which emission limit values apply?
The emission limit value is set in the plant's environmental permit or in the ordinance that applies to the type of plant. These are the levels for medium combustion plants (1–50 MW) firing solid fuel:
| Boiler firing solid fuel | Dust, mg/Nm3 at 6 % O2 |
|---|---|
| New, 5 MW or less | 50 |
| New, more than 5 to 20 MW | 30 |
| New, more than 20 MW | 20 |
| Existing, 20 MW or less | 50 |
| Existing, more than 20 MW | 30 |
The values are in annexes 1–3 and 12–14 to the Swedish Ordinance on Medium Combustion Plants (SFS 2018:471). Existing means that the boiler was put into operation no later than 19 December 2018; for such boilers of 5 MW or less the values apply from 1 January 2030. All values, exemptions and dates are on the page about medium combustion plants. For waste incineration the daily emission limit value for dust is 10 mg/Nm3 at 11 % O2.
What does a dust measurement cost?
The price depends on the number of stacks or ducts and operating conditions, how many parameters are measured at the same time, what the measurement site is like and the travel distance. A dust measurement at one sampling location costs about SEK 15,000–30,000 excluding VAT and travel. When dust is measured together with gases and flow as one package, the price per parameter is lower. Read the price guide or request a quote – we reply within two working days.
How often must dust be measured?
As often as the permit says. For medium combustion plants the rule is at least every three years up to 20 MW and every year above 20 MW. Many permits for larger plants require a measurement every year, and plants with a continuous dust monitor (an automated measuring system, AMS or CEMS) also need calibration according to EN 14181 – QAL2 and AST.
We measure dust across Sweden
METLAB measures dust at plants across Sweden, from Skåne to Norrbotten. The technicians work from the offices in Enköping, Falun and Skellefteå and measure the same way wherever the plant is: the same method, the same report.
Stockholm, Gothenburg, Malmö or a mill town far from the nearest city: we come to you. What sets the schedule is when the plant can run at the load the dust measurement is to represent. Request a quote or contact the nearest office.
Measure dust yourself
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ADS-1ADS-1 dust sampler
Automatic isokinetic control, 7-inch display, built in our workshop in Enköping.
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Course · METLAB AcademyDust sampling and gas flow measurement
Two days of theory and field exercises with our measurement engineers. The next course is planned once enough people have registered their interest.
Frequently asked questions about dust measurement
What does isokinetic sampling mean?
That the gas is drawn into the nozzle at the same velocity as it flows in the duct. If the velocity in the nozzle is too low, too many heavy particles are caught; if it is too high, too few. That is why the gas velocity is measured first and the sample gas flow is adjusted throughout the sampling period.
How many sub-samples are needed?
The standard sets no fixed number. A result is based on at least two equivalent sub-samples under the same operating condition, and we normally take three. The sampling time is adapted to the dust concentration so that the filter collects enough mass for a reliable weighing.
Why is the filter kept at 160 °C?
Some dusts contain compounds that are solid at low temperature but evaporate at higher temperature: sulphuric acid when firing heavy fuel oil, hydrates after desulphurisation and boron compounds from glass furnaces, for example. The result can then differ by a factor of up to ten depending on the temperature. That is why 160 °C is used as a common temperature, so that measurements are comparable.
What is a field blank?
A control sample that is handled exactly like a real sample (the filter is fitted, the probe is inserted and rinsed), but without drawing gas through the equipment. It shows how much dust the handling itself contributes and may be at most 10 % of the emission limit value. Concentrations below the field blank are reported as less than the field blank.
Can you measure after a scrubber or flue gas condenser?
Yes. When the flue gas contains droplets, the filter sits outside the duct and is heated, so that the droplets evaporate before the gas reaches the filter.
Can you measure at very high dust concentrations, for example upstream of a filter?
Yes. At high concentrations ISO 9096 and a filter holder with a large filter area are used.
What is required of the measurement site?
Sampling ports on a straight run of duct, a safe working platform and electricity. The requirements are in EN 15259; we are happy to help before the ports are cut.
Do we get an accredited report?
Yes. Dust according to EN 13284-1 is within METLAB's Swedac accreditation, and the report is delivered digitally signed.