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When do you need to measure NMHC?
- Permit conditions. The environmental permit for a gas engine, gas turbine or biogas plant may set a condition for NMVOC, hydrocarbons other than methane, and how it is to be checked.
- Large combustion plants (from 50 MW): the BAT conclusions give lean-burn gas engines firing natural gas BAT-associated emission levels (BAT-AELs) for formaldehyde and methane, measured once a year.
- The annual environmental report. NMVOC is reported above 5,000 kg per year, methane above 100,000 kg per year. A measurement of TOC and methane gives the basis for both.
- Checking the abatement plant: an oxidation catalyst oxidises carbon monoxide and unburnt hydrocarbons, but not methane. Measurement before and after shows what it removes.
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 through a heated sample line and a heated fine filter to the instrument, without cooling or drying. The whole line is kept at a minimum of 180 °C, and the gas reaches the detector in under 60 seconds. In the flame ionisation detector (FID) the sample gas is burnt in a hydrogen flame: organically bound carbon forms ions, and the current between two electrodes is proportional to the carbon concentration. For methane, the sample gas first passes a catalytic converter at about 300 °C, which burns off all other organic compounds.
- 1. Probe with coarse filter
- 2. Heated sample line
- 3. Fine filter, heated
- 4. Converter in the analyser that oxidises all hydrocarbons except methane
- 5. Flame ionisation detector (FID), heated
- 6. Data acquisition
- 7. Zero and span gas (methane in synthetic air)
The instrument can have two detectors, one that receives the sample gas directly and one that receives it through the converter, so that TOC and methane are measured at the same time. The converter is the sensitive part: sulphur, nitrogen and chlorine compounds can poison the catalyst, and very high concentrations of other hydrocarbons can overheat it.
How is an NMHC measurement done?
Measurement plan and operating conditions
The measurement is planned according to EN 15259 together with you. For a gas engine under the BAT conclusions, methane is measured at more than 70 % load, and the BAT-AEL applies at full load.
Converter check
Before every job, ethane in synthetic air is led through the instrument with and without the converter. At least 98 % of the ethane must be burnt off.
Calibration
Zero gas and span gas are led straight to the instrument: propane for the TOC channel, methane in synthetic air for the methane channel. The gases are then led in at the probe, and the difference may be at most 2 % of the span gas value.
Measurement
TOC and methane are measured continuously and logged. A measurement period is at least 30 minutes, and the BAT conclusions use the average of three in a row. Moisture and oxygen are measured at the same time.
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 invalid.
Calculation and report
TOC and methane are converted to mg C/Nm3 (Nm3: dry gas at 0 °C and 101.3 kPa), and NMHC is the difference. The values are converted to dry gas and, for combustion, to the reference oxygen content, 15 % O2 for engines and gas turbines. You receive the result with its measurement uncertainty in an accredited, digitally signed report.
The requirements in EN ISO 25140 and EN 12619
The converter, the sampling system and drift are checked at every measurement; linearity, the effect of oxygen and interferences at least once a year.
| Requirement | Methane, EN ISO 25140 | TOC, EN 12619 |
|---|---|---|
| Converter | Burns off at least 98 % of the ethane; methane loss at most 15 %, compensated at the calibration | No converter |
| Response time | At most 60 seconds | At most 200 seconds |
| The sampling line | Heated, at least 20 °C above the flue gas temperature, at most 200 °C | At least 180 °C all the way to the detector |
| Span gas | Methane in synthetic air | Propane, uncertainty at most 2 % |
| Check of the sampling system | At most 2 % difference; leak rate at most 2 % | At most 2 % difference, relative to the span gas value |
| Zero and span drift | At most 2 % of the measuring range, checked every three hours | Above 2 % the readings are corrected; above 5 % the measurement is invalid |
| Linearity and interferences | At most 2 % of the measuring range; other interferences at most 4 % | At most 2 % |
For methane we compare the drift with the span gas value, 2 and 5 % respectively, as for the other gases. That is the practice in Sweden: the standard's 2 % of the lowest measuring range is seldom achievable in the field.
What is the difference between TOC, methane and NMHC?
The flame ionisation detector counts carbon atoms, and for saturated hydrocarbons such as methane the signal is broadly proportional to their number.
| Term | What it means |
|---|---|
| TOC (TVOC) | Total organic carbon: the sum of all gaseous organic compounds, methane included, measured by FID according to EN 12619 and expressed as carbon in mg C/Nm3. |
| Methane, CH4 | The simplest hydrocarbon and a greenhouse gas. Measured separately according to EN ISO 25140, with a converter that removes everything else. At gas engines, unburnt methane is the largest part of TOC. |
| NMHC (NMVOC) | Non-methane hydrocarbons, volatile organic compounds other than methane: TOC minus methane, both counted as carbon. The compounds form ground-level ozone together with nitrogen oxides and sunlight, and NMVOC has its own threshold in the annual environmental report. |
How NMHC is calculated
The TOC channel shows ppm propane, the methane channel ppm methane. Both are converted to carbon before one is subtracted from the other: a propane molecule has three carbon atoms, a methane molecule one.
| Step | How we calculate | Worked example, gas engine |
|---|---|---|
| 1. TOC to carbon | ppm propane × 3 × 12 / 22.4 = ppm propane × 1.61 | 260 ppm propane = 418 mg C/Nm3 |
| 2. Methane to carbon | ppm methane × 12 / 22.4 = ppm methane × 0.54 (as CH4: × 0.71) | 700 ppm methane = 375 mg C/Nm3, or 500 mg CH4/Nm3 |
| 3. NMHC | TOC as carbon minus methane as carbon | 418 − 375 = 43 mg C/Nm3 |
The conversion to dry gas and the reference oxygen content is then done as for TOC. The figures in the example are chosen for the arithmetic, not measured.
Where is NMHC requested?
At gas engines and gas turbines firing natural gas, biogas or landfill gas. A lean-burn gas engine lets part of the methane through unburnt, and an oxidation catalyst removes carbon monoxide and other hydrocarbons but not methane. If only TOC is measured, what you see is therefore mostly methane slip, and both the BAT conclusions and the annual environmental report keep methane and NMVOC apart.
Where TOC is regulated, no NMHC measurement is needed: the ordinances on waste incineration and on organic solvents set limit values for total organic carbon, methane included, and there the TOC measurement is enough. The ordinances on medium and large combustion plants have no limit value for hydrocarbons at all.
Which emission limit values apply to NMHC?
NMHC has no limit value in the ordinances on combustion plants, and the BAT conclusions give no BAT-AEL for NMVOC as a whole, so a limit value exists only if the permit sets one. The BAT-AELs for formaldehyde and methane apply with their upper value as the limit value under the Swedish Industrial Emissions Ordinance (SFS 2013:250).
| Plant | Level | Rule |
|---|---|---|
| Lean-burn gas engines on natural gas, from 50 MW | NMVOC: no BAT-AEL. Formaldehyde 5–15 mg/Nm3; methane 215–500 for new and 215–560 for existing plants, at full load and 15 % O2 | BAT 45, table 26, large combustion plants |
| Medium combustion plants, 1–50 MW | No limit value for NMHC, methane or TOC; engines and gas turbines have values for SO2, NOx and dust | SFS 2018:471, sections 25–32 |
| Waste incineration | TOC 10 mg C/Nm3 as a daily average, methane included, at 11 % O2 | SFS 2013:253, section 57 |
| The annual environmental report | No limit value; NMVOC is reported above 5,000 kg per year, methane above 100,000 kg per year | The Swedish EPA's regulations NFS 2016:8 |
What does an NMHC 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. There is no laboratory analysis: TOC and methane are measured with instruments on site. If NOx, CO and oxygen are measured at the same time, the cost per parameter is lower. Request a quote – we reply within two working days. Approximate prices for the most common measurements are in the price guide.
How often must NMHC be measured?
As often as the permit says. The BAT conclusions for large combustion plants specify once a year for formaldehyde and methane from natural gas engines, with the methane measured at more than 70 % load. The annual environmental report is submitted every year, and NMVOC above 5,000 kg or methane above 100,000 kg per year is reported there. The Swedish Ordinance on Medium Combustion Plants requires no measurement of hydrocarbons.
Frequently asked questions about NMHC measurement
Is the measurement accredited?
Yes. TOC according to EN 12619, methane according to EN ISO 25140 and NMHC as a calculation from the two are within METLAB's Swedac accreditation, for both sampling and analysis. The accredited measuring range for the TOC channel is 0.05–10,000 ppm.
Why can NMHC not be measured directly?
The flame ionisation detector counts every carbon atom that passes the flame, methane's included. What can be removed is everything except methane, with a catalytic converter. So TOC and methane are measured separately, and Swedac lists NMHC as a calculation.
Is NMHC the same as NMVOC?
In flue gas measurement, yes: both are the organic carbon the FID instrument measures, minus methane. NMHC stands for non-methane hydrocarbons, NMVOC for volatile organic compounds other than methane. The BAT conclusions and the annual environmental report write NMVOC, Swedac and our method description NMHC.
Can you measure on a biogas engine or a landfill gas engine?
Yes. At biogas plants, some gas streams can have very high concentrations of carbon dioxide, and then the instrument's sensitivity to carbon dioxide needs to be known. Sulphur, nitrogen and chlorine compounds can damage the converter, which is why it is checked with ethane before every job.
Can NMHC be measured at the same time as other parameters?
Yes. NOx, CO, oxygen and nitrous oxide are measured in a separate, cooled measuring system, and the moisture content for the conversion to dry gas. With the gas flow, the emission can be given in kg per hour, and with the operating hours in kg per year for the annual environmental report.
What is required of the measurement site?
A sampling port where the probe reaches a representative point, a safe place to work and electricity for the heated line and the instrument, according to EN 15259. The instrument must stand close to the sampling port, since the sample gas must reach the detector in under a minute.