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When do you need to measure methane?
- Permit conditions. The environmental permit for a gas engine, a biogas plant or a boiler may set a condition for methane, for biogas often as a share of the production.
- Large combustion plants under the BAT conclusions: methane is measured once a year in engines fired with natural gas, at more than 70 % load.
- Biogas plants. In the industry's voluntary scheme Egenkontroll Metanemissioner (self-monitoring of methane emissions), the emissions are quantified every three years by an independent measurement consultant.
- Climate reporting. One kilogram of methane counts as 28 kilograms of carbon dioxide in the EU's climate reporting, and a measurement gives the plant's own value.
- Gas engines: an oxidation catalyst reduces other hydrocarbons but not methane, so the methane slip needs to be measured separately.
How the sampling train is built
A sample stream of the 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 analyser. Nothing is cooled or dried: the whole sampling line is kept hot, and the sample gas must reach the detector in less than a minute. Before the detector the gas passes a catalytic converter at about 300 °C, where all other hydrocarbons are burnt while the methane passes through unaffected. In the flame ionisation detector the methane is burnt in a hydrogen flame, and the current between two electrodes is proportional to the concentration. Zero and span gas can be connected at the probe.
- 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 converter is the sensitive part. Sulphur, nitrogen and chlorine compounds, silicones and dust can poison the catalyst, and very high concentrations of other hydrocarbons can overheat it. That is why we check its efficiency with ethane before every job: at least 98 % must be burnt off. Part of the methane, up to 15 %, can also be burnt in the converter; that is included in the calibration, which goes the same way.
How is a methane measurement done?
Measurement plan and operating conditions
The measurement is planned according to EN 15259 and a measurement programme that we agree with you. For gas engines the BAT conclusions want the measurement made at more than 70 % load.
Heating and calibration
The sampling line is heated up, and the converter to about 300 °C. Zero gas and span gas, methane in synthetic air of known concentration, are then led straight to the analyser through the converter, and the instrument is adjusted. The span gas concentration is determined with an uncertainty of at most 2 %.
Check of the sampling system
The same gases are then led in at the probe, through the whole sampling line. If the reading differs by more than 2 % of the span gas value, we look for leaks or contamination.
Measurement
Methane is measured continuously and logged. The moisture content is measured at the same time, and for combustion also oxygen, for the conversion to dry gas and the reference oxygen content. A measurement period is at least 30 minutes. If TOC is measured in parallel, you also get NMHC.
Check after the measurement
After the measurement period, and at least once a day, the zero and span points are checked again. For methane the requirement is stricter than for TOC: the drift may be at most 2 % of the span gas value, otherwise the measurement is not valid.
Calculation and report
The readings are converted from ppm to mg/Nm3 (dry gas at 0 °C and 101.3 kPa) and where needed to the reference oxygen content: 1 ppm CH4 is 0.71 mg/Nm3. You receive the result with its measurement uncertainty in an accredited, digitally signed report.
The requirements in EN ISO 25140
These requirements decide whether a methane measurement is valid. The converter, leak tightness and drift are checked at every measurement series, the others at least once a year.
| Requirement | Value |
|---|---|
| Residence time and response time | The sample gas reaches the detector in less than 60 seconds, and the response time is at most 60 seconds |
| Temperature in the sampling line | Heated all the way, at least 20 °C above the gas temperature and at most 200 °C. The converter at around 300 °C |
| The converter | At least 98 % of ethane is burnt, checked before every measurement series. Methane loss at most 15 %, compensated at the calibration |
| Span gas | Methane in synthetic air, concentration determined with an uncertainty of at most 2 % |
| Leak test | At most 2 % difference when the span gas is led through the whole sampling line |
| Zero and span drift | At most 2 % of the span gas value, otherwise the measurement is not valid |
| Linearity, repeatability and the effect of oxygen | At most 2 % of the measuring range each, repeatability at zero 1 % |
| Interfering gases | Combined effect at most 4 % of the measuring range, tested with SO2, NO, CO, CO2 and water vapour |
The standard states the drift requirement as 2 % of the measuring range and wants the check every three hours. We compare with the span gas value and check at the start and end of every measurement day, more often if there is a risk that the requirement is not met.
Methane, TOC and NMHC: what is the difference?
The same kind of detector measures all three, with different pre-treatment of the gas and in different units: methane in mg CH4/Nm3, TOC as carbon in mg C/Nm3. When the methane is subtracted, it is converted to carbon, 0.54 mg C/Nm3 per ppm.
| Parameter | What is measured |
|---|---|
| Methane (CH4) | Measured after the converter, which burns off everything else. Unit mg/Nm3, or mg C/Nm3 when the condition specifies carbon |
| TOC (TVOC) | All gaseous organic carbon, measured without a converter and calibrated with propane. Unit mg C/Nm3 |
| NMHC | TOC minus methane, both counted as carbon, with two detectors at the same time. Unit mg C/Nm3 |
Where does the methane come from?
Methane is unburnt fuel. In a lean-burn gas engine part of the natural gas slips through, and an oxidation catalyst does not help: it does not remove saturated hydrocarbons with fewer than four carbon atoms. That is why the BAT conclusions have a BAT-associated emission level (BAT-AEL) of its own for methane from gas engines. In boilers firing biomass, methane comes along when combustion is incomplete, together with CO.
At biogas plants the methane is the product, 55–75 per cent by volume of the raw gas, and it can leak from digesters, digestate tanks, digestate stores, ventilation and the off-gas from upgrading. In Egenkontroll Metanemissioner the plant looks for leaks itself, and an independent measurement consultant quantifies the emissions every three years.
In climate reporting one kilogram of methane counts as 28 kilograms of carbon dioxide, over a hundred years according to the IPCC's Fifth Assessment Report. Other values occur: 21 and 25 in older rules, 34 in the industry's own scheme. Nitrous oxide, the other greenhouse gas from combustion, is counted with the factor 265 and measured with a method of its own: nitrous oxide measurement.
Which emission limit values apply to methane?
Methane has no limit value in the ordinances on combustion plants or on waste incineration. Limit values come from the permit, for biogas plants often as a share of the production, and from the BAT conclusions, where the upper value applies as the limit value under the Swedish Industrial Emissions Ordinance (SFS 2013:250).
| Plant | Level | Rule |
|---|---|---|
| New lean-burn gas engines on natural gas, from 50 MW | 215–500 mg/Nm3 as the average over the sampling period, at full load and 15 % O2 | BAT 45, table 26, large combustion plants |
| The same engines in existing plants | 215–560 mg/Nm3, indicative for engines operated less than 500 hours per year | BAT 45, table 26, large combustion plants |
| Biogas plants | Condition in the permit, often a maximum share of the methane produced | The permit. Egenkontroll Metanemissioner |
| Medium and large combustion plants, waste incineration | No limit value for methane in the ordinances | SFS 2018:471, 2013:252 and 2013:253 |
What does a methane 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. Methane is measured with a separate heated measuring system with FID and converter, and if NMHC is to be reported a second detector for TOC is needed. If moisture, oxygen and gas flow are measured at the same time, the cost per parameter is lower. 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 methane be measured?
As often as the permit says. The BAT conclusions for large combustion plants specify once a year for methane from natural gas engines, measured at more than 70 % load. Biogas plants in Egenkontroll Metanemissioner quantify their emissions every three years, with their own leak detection in between. The ordinances have no interval for methane.
Frequently asked questions about methane measurement
Is the measurement accredited?
Yes. Both sampling and analysis of methane according to EN ISO 25140 are within METLAB's Swedac accreditation, as is NMHC calculated from TOC and methane. The method list gives no measuring range of its own for methane; the instrument's range is chosen for the concentration expected.
In which unit is methane reported?
Normally in mg CH4/Nm3 dry gas: 1 ppm methane is 0.71 mg/Nm3. If the condition specifies carbon, the methane is converted to mg C/Nm3, 0.54 mg C/Nm3 per ppm. For combustion the concentration is also converted to the reference oxygen content, 15 % O2 for gas engines.
Why is it not enough to measure TOC?
TOC shows the sum of all organic carbon, not how much of it is methane. For gas engines and biogas the condition applies to the methane, and NMHC is the difference. If both are to be reported, they are measured with a detector each.
How long does the measurement take?
That depends on the number of operating conditions. The sampling line must be heated up and calibrated, and every measurement period is at least 30 minutes. The converter is checked with ethane before we set off.
What is required of the measurement site?
A sampling port where the probe reaches a representative point in the duct, a safe place to work and electricity for the heated line and the instrument. The requirements are in EN 15259. The instrument must stand close to the sampling port, since the sample gas must reach the detector in less than a minute.