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When do you need to measure HCN?
- Permit conditions. The environmental permit sets the emission limit value for HCN and how often it is to be checked. The combustion ordinances have no limit value of their own for HCN.
- Chemical and pharmaceutical industry: the BAT conclusions (EU) 2022/2427 require measurement of HCN at least once a year at every stack where HCN is relevant. The BAT-associated emission level (BAT-AEL) of at most 1 mg/Nm3 becomes a limit value on 12 December 2026.
- Acrylonitrile, carbon fibre and coke: HCN is a by-product of the ammoxidation of propene, is present in the exhaust air from carbon fibre furnaces and is carried along with the coke oven gas.
- Waste and fuels rich in nitrogen, such as polyurethane, nylon and acrylonitrile-based plastics: HCN forms in the flame, and the permit may set a condition.
- Performance tests and troubleshooting: checks of a scrubber or thermal oxidiser, or comparison with an FTIR instrument that measures HCN continuously.
How the sampling train is built
The measuring principle is wet chemical absorption, with the same equipment as for HCl. The sample gas is drawn through a heated probe of borosilicate or quartz glass with a quartz fibre filter, which stops the particles, and then bubbles through two absorption bottles with 0.5 M sodium hydroxide in a cooling bath, where the hydrogen cyanide is bound as cyanide ions. An empty knock-out bottle catches splashes and a drying tower with silica gel dries the gas. After the valve, pump and flow meter, a dry gas meter with a thermometer measures the sample gas volume.
- 1. Probe with filter holder, heated
- 2. Absorption bottles with absorption solution, in a cooling bath
- 3. Knock-out bottle
- 4. Drying tower
- 5. Valve
- 6. Sampling pump
- 7. Flow meter
- 8. Gas meter
- 9. Thermometer at the gas meter
The probe and filter are kept at 175 °C or more, and at least 20 °C above the dew point of the flue gas, so that no condensate forms before the bottles, and the gas after the bottles must be below 20 °C. Sodium hydroxide is corrosive and is handled with gloves and safety glasses.
How is an HCN measurement done?
Measurement plan
In homogeneous flue gas one measurement point is enough; otherwise the sample is taken at several. Sampling time, flow and the amount of solution are chosen according to the concentration expected, so that the cyanide content of the solution is at least ten times the detection limit of the analysis.
Field blank and leak test
Before each measurement series, and at least once per day and per measurement point, a field blank is taken: the bottles are filled, the equipment is connected and rinsed, but the pump is not started. Before each sub-sample the train is tested for leaks; the leak rate may be at most 2 % of the sample gas flow.
Sampling
About 40 ml of sodium hydroxide goes into each bottle. The sample gas is drawn through the bottles at normally 4–10 l/min for at least 30 minutes. The gas meter reading and temperature are noted every ten minutes, and the filter temperature and the flow are checked during the sample.
Rinsing
Both solutions are poured into one sample bottle. The system from the filter holder onwards is rinsed with about 20 ml of sodium hydroxide, which is passed through the bottles and added to the sample.
Analysis
An accredited laboratory determines total cyanide in the sample and the field blank: spectrophotometrically according to SS 028176 (Swedish standard), or by titration when NOx, SO2 or hydrogen sulphide in the sample gas interferes with the photometric method.
Calculation and report
The amount of cyanide is converted to HCN (factor 27/26) and divided by the volume of dry sample gas at standard conditions. The result is reported in mg/Nm3 (dry gas at 0 °C and 101.3 kPa), for combustion processes converted to the reference oxygen content, with its measurement uncertainty.
The requirements in chapter 5.16 of the Emissionsmäthandbok
Chapter 5.16 of the handbook sets the requirements, with the general requirements for wet chemical sampling in chapter 5.10. These decide whether an HCN measurement is valid.
| Requirement | Value |
|---|---|
| Absorption solution | 0.5 M sodium hydroxide, about 40 ml in each of two bottles in series |
| Materials | Probe of borosilicate or quartz glass, filter of quartz fibre |
| Filtration temperature | At least 175 °C and at least 20 °C above the dew point of the flue gas |
| Absorption | At most 5 % of the cyanide in the second bottle, or at low concentrations at most five times the detection limit of the analysis |
| Leak rate | At most 2 % of the sample gas flow |
| Sampling time | At least 30 min per sub-sample |
| Field blank | At most 10 % of the emission limit value; lower sample results are reported as at most equal to the field blank |
| Sample gas volume | Determined with an uncertainty below 1 % |
| Concentration in the solution | At least ten times the detection limit of the analysis: above 0.1 mg/l for photometric analysis |
What does the method measure: HCN or cyanides?
The method determines all gaseous cyanides that pass the filter, expressed as HCN. In flue gas from combustion, hydrogen cyanide dominates completely according to the handbook, so the result can be read as HCN. Aerosols with cyanide salts can pass the filter and are then included. An instrument that measures optically, with FTIR or a diode laser, determines HCN specifically and can show something other than the manual method.
The photometric method in SS 028176 has a detection limit of 0.01 mg/l, so the solution should contain at least 0.1 mg/l: at low concentrations the sampling time is extended. According to the handbook the photometric method cannot be used when the sample gas contains NOx, SO2 or hydrogen sulphide, or when the solution is coloured or turbid; titration is then used, with a potentiometric end point if the solution is coloured. 1 ppm HCN corresponds to 1.21 mg/Nm3.
Where does HCN come from?
HCN forms when nitrogen-containing materials burn or are heated. Polyurethane foam, nylon and polyacrylonitrile (acrylic fibres) give off hydrogen cyanide in pyrolysis and combustion, and the yield grows with the nitrogen content of the material and with the temperature; polyacrylonitrile gives off HCN even at 300 °C. In the flame, HCN is also an intermediate when the fuel's nitrogen becomes nitrogen oxides, and with complete combustion it is oxidised further to NO.
In the process industry HCN is a by-product in the manufacture of acrylonitrile by ammoxidation of propene, about one kilogram of HCN per nine kilograms of acrylonitrile. The exhaust air from the furnaces in the manufacture of carbon fibre from polyacrylonitrile contains several hundred ppm of HCN. At coke oven plants HCN is carried along with the coke oven gas; cyanide is one of the parameters in the BAT conclusions for the waste water from gas cleaning at coke oven plants.
The emission is limited by absorption in a scrubber or by thermal oxidation, the two techniques the BAT conclusions for the chemical industry name for HCN.
Which emission limit values apply to HCN?
The only general value is the BAT conclusions' BAT-AEL for the chemical industry, a limit value under the Swedish Industrial Emissions Ordinance (SFS 2013:250) from 12 December 2026. The Swedish ordinances on waste incineration (SFS 2013:253) and on large and medium combustion plants (SFS 2013:252 and SFS 2018:471) do not mention hydrogen cyanide, and the BAT conclusions for waste incineration and large combustion plants have no BAT-AELs for HCN. There the limit value is set in the permit.
| Plant and averaging period | HCN, mg/Nm3 | Source |
|---|---|---|
| Chemical industry, channelled emissions, daily or sampling period (no oxygen correction; process furnaces 3 % O2) | < 0.1–1 | BAT conclusions (EU) 2022/2427, BAT 18 |
| Waste incineration, large and medium combustion plants | No general limit value; the permit applies | SFS 2013:253, 2013:252, 2018:471; (EU) 2019/2010, 2021/2326 |
The BAT-AEL for the chemical industry does not apply to minor emissions, with a mass flow of HCN below about 5 g/h.
What does an HCN measurement cost?
The price depends on the number of stacks or ducts and operating conditions, the number of sub-samples and the sampling time, and on the laboratory analysis of each sample and field blank. Low concentrations need a longer sampling time. When HCN is measured together with HCl, ammonia or dust at the same measurement site, the probe, travel and set-up are shared, and the price per parameter is lower. Isokinetic sampling, difficult access and a long journey add to the cost. Request a quote and we will reply within two working days. Approximate prices for the most common measurements are in the price guide.
How often must HCN be measured?
As often as the permit says. In the chemical industry the BAT conclusions require measurement at least once a year at every stack where HCN is relevant, if possible when the emissions are expected to be at their highest during normal operation; the frequency can be reduced to every three years if the levels have proved stable. The combustion ordinances set no frequency for HCN.
Frequently asked questions about HCN measurement
Is the HCN measurement accredited?
Yes. Sampling according to chapter 5.16 of the Emissionsmäthandbok is within METLAB's Swedac accreditation, with hydrogen cyanide as a line of its own in Swedac's register. The analysis is done by an accredited laboratory, and the report gives the result with its measurement uncertainty.
Is there a standard for HCN?
No EN standard, as the BAT conclusions for the chemical industry also note. The method in Energiforsk's Emissionsmäthandbok is the one we are accredited for, and the equipment is the same as in EN 1911 for HCl.
Is HCN dangerous to sample?
HCN is toxic, and the sampling is done by trained staff according to the handbook's safety rules: the extracted sample gas is led away from the workplace, the need for a gas detector is assessed in advance, and the sodium hydroxide is handled with gloves and safety glasses. The sample gas flow is a few litres per minute.
Can HCN be measured at the same time as HCl, HF and ammonia?
Yes. From the same heated probe and filter, the sample gas is split between parallel trains of absorption bottles, each with its own absorption solution, pump and gas meter. The filter is then kept at the highest temperature any of the methods requires, 175 °C for HCN and ammonia.
How long does an HCN measurement take?
Each sub-sample takes at least 30 minutes, and the BAT conclusions use the average of three consecutive sub-samples. At low concentrations the sampling time is extended so that the solution reaches at least ten times the detection limit of the analysis.
What is required of the measurement site?
Sampling ports, a safe working platform and electricity for the probe heating and the pump, according to EN 15259. If the gas contains droplets, as after a wet scrubber, the sample is taken isokinetically at several points, and the measurement site must be suitable for dust measurement.
Do you measure hydrogen bromide too?
Yes. Hydrogen bromide (HBr) is sampled with the same equipment as HCl, according to EN 1911 in modified form: see HBr measurement.