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Where do the emissions come from?
- The recovery boiler burns the black liquor and emits SO2, TRS, NOx and dust. The gases are measured continuously, dust periodically or continuously (BAT 9).
- The lime kiln burns lime mud to lime, often with strong gases as fuel. SO2, TRS, NOx and dust are measured periodically or continuously as the permit says.
- The dedicated TRS burner (the strong gas boiler or NCG incinerator) burns the strong malodorous gases (concentrated non-condensable gases) from the digester and the evaporation plant; SO2, TRS and NOx are measured periodically or continuously.
- Diffuse sources such as the fibre line, tanks and chip bins give off weak gases with TRS. They are collected and burnt (BAT 20); the emissions are estimated with periodic measurements (BAT 11).
Which rules apply?
| Plant | Regulation | What it means for the measurement |
|---|---|---|
| Recovery boiler | The BAT conclusions for pulp and paper, BAT 9 and 21–23 | NOx, SO2 and TRS continuously, dust periodically or continuously. O2, CO, temperature, pressure and water vapour continuously (BAT 8). NH3 periodically with SNCR. |
| Lime kiln, dedicated TRS burner (strong gas boiler or NCG incinerator) | BAT 9 and 24–29 | NOx, SO2, TRS and dust periodically or continuously; the permit says which and how often. |
| Sulphite recovery boiler | BAT 9 and 36–37 | NOx and SO2 continuously, dust periodically, NH3 periodically with SNCR. |
| Bark boiler and other boilers, at least 50 MW in total | The Swedish Ordinance on Large Combustion Plants (SFS 2013:252) and the BAT conclusions for large combustion plants | SO2, NOx and dust continuously from 100 MW, below that at least every six months. BAT 4 adds CO, HCl, HF and mercury. AST every year, QAL2 at least every five years. |
| Steam boilers of 1–50 MW | The Swedish Ordinance on Medium Combustion Plants (SFS 2018:471) | Periodic measurement of NOx, dust and SO2 where there is a limit value, and always CO: every year above 20 MW, every three years below. |
| Boilers with at least 25 GWh of useful energy per year | The Swedish NOx charge, NFS 2016:13 | A comparison measurement of the NOx equipment by an accredited laboratory every calendar year. Recovery boilers, kraft and sulphite, are exempt; a lime kiln is not a boiler in the sense of the Act. |
BAT-associated emission levels (BAT-AELs) have applied as limit values under normal operation since 30 September 2018 (chapter 1, section 8 of the Swedish Industrial Emissions Ordinance). Mechanical pulp and paper machines have no BAT-AELs for air; the requirements are in the permit. The permit can set stricter requirements and more frequent measurement; the text of the permit and the statutes is what applies.
What is measured – and what is typical?
| Parameter | Why in this industry | Typical level or requirement | Method |
|---|---|---|---|
| SO2 | Sulphur in the liquor and the strong gases | BAT-AEL recovery boiler 5–50 yearly average, 10–70 daily average, lower above 75 % dry solids; lime kiln 5–70, with strong gases 55–120; dedicated TRS burner 20–120 at 9 % O2 | EN 14791 |
| TRS (including H2S) | Malodorous sulphur compounds from cooking and evaporation | BAT-AEL recovery boiler 1–5 yearly average, 1–10 daily average; lime kiln < 1–10 mg S/Nm3, with strong gases up to 40; dedicated TRS burner 1–5 at 9 % O2 | The mill's TRS instrument · H2S according to VDI 3486-2 |
| NOx | Nitrogen in the liquor and the strong gases; high dry solids give more NOx | BAT-AEL yearly average: recovery boiler 120–200; lime kiln 100–200 with liquid, 100–350 with gaseous fuel; dedicated TRS burner 50–400 at 9 % O2 | EN 14792 |
| Dust | Sodium salts from the recovery boiler, lime dust from the lime kiln | BAT-AEL yearly average: recovery boiler 10–25, existing 10–40; lime kiln 10–25, existing 10–30 | EN 13284-1 |
| NOx, SO2 and dust from the bark boiler | Biomass-fired boiler under the BAT conclusions for large combustion plants | BAT-AEL yearly average, existing boiler of 50–100 MW: NOx 70–225, SO2 15–100, dust 2–15 | EN 14792 · EN 14791 · EN 13284-1 |
| NH3 | Ammonia slip from SNCR | Periodically under BAT 9; BAT-AEL sulphite recovery boiler < 5 at 5 % O2, bark boiler < 3–10 | EN ISO 21877 |
BAT-AELs are the levels the EU's reference documents say well-run mills achieve; the upper value of the range applies as the limit value. mg/Nm3 (dry gas at 0 °C and 101.3 kPa) at 6 % O2 unless stated otherwise. Concentrations depend on the liquor's dry solids content, the fuel and the flue gas cleaning.
What the measurement year looks like
Go through the permit and the monitoring programme
The permit states which sources are measured continuously, which periodically and how often. Check that BAT 9 is covered and that the bark boiler is classed correctly by total rated thermal input.
AST every year, QAL2 at least every five years
You run the ongoing checks of the automated measuring systems according to EN 14181 (QAL3). On bark boilers under SFS 2013:252 we do the AST in one day and the QAL2 with at least 15 parallel measurements over at least three days.
Periodic measurements at the pulp mill
We measure dust, SO2, NOx, H2S and NH3 on the lime kiln, the dedicated TRS burner and the recovery boiler as often as the permit says, and steam boilers under SFS 2018:471 every year or every three years.
Comparison measurement for the NOx charge
On boilers with at least 25 GWh we check the NOx equipment every calendar year; the flue gas flow is always included. The declaration is submitted by 25 January.
Annual environmental report by 31 March
There you report the measurement results and how the BAT conclusions are complied with (section 5 b of NFS 2016:8). A new measuring system needs a new QAL2, a new or rebuilt boiler a performance test against the supplier's guarantees.
What is changing, and what often goes wrong?
The BAT conclusions from 2014 apply unchanged; the Swedish Environmental Protection Agency updated its guidance on them in December 2025. The amended Industrial Emissions Directive (EU) 2024/1785 was to be transposed by 1 July 2026, but the Government's proposals were out for consultation until 1 October 2026 and have not been decided. Three things are easily confused: the bark boiler follows the combustion plant rules, not the BAT conclusions for pulp and paper; the lime kiln is not a boiler in the sense of the NOx charge; and a TRS condition cannot be checked with an H2S sampling alone.
What does a measurement cost at a pulp and paper mill?
The price depends on the number of sources and parameters, whether it is a QAL2 with three measurement days or an AST, whether H2S and NH3 are to be sampled wet chemically, the accessibility of the measurement sites and the travel distance. If you coordinate the bark boiler's comparison measurement with the pulp mill's periodic measurements you save measurement days. Request a quote – we reply within two working days. Approximate prices for the most common measurements are in the price guide.
Frequently asked questions
Must TRS be measured continuously on the recovery boiler?
Yes. Under BAT 9, TRS (including H2S) is measured continuously on the recovery boiler, as are NOx and SO2. On the lime kiln and the dedicated TRS burner, periodic measurement is enough if the permit allows it.
Is the lime kiln covered by the NOx charge?
No. The charge applies to boilers, gas turbines and combustion engines with at least 25 GWh of useful energy per year. A lime kiln produces neither steam nor hot water. Recovery boilers, kraft and sulphite, are exempt; the bark boiler is covered.
How are the measured values compared with the BAT-AELs?
The upper value of the range applies under normal operation. With continuous measurement, daily and yearly averages are compared; with periodic measurement, the average of three measurements of at least 30 minutes. Periods of abnormally low dry solids content during an evaporation plant stop are not counted.
What counts as TRS, and does METLAB measure it?
TRS is the sum of hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide, expressed as sulphur. The mills measure TRS with their own instruments. We sample H2S according to VDI 3486-2 with accredited sampling; the analysis is done by an accredited laboratory.