Methane and CO2 in biogas are measured by NDIR, separately or in one dual-channel analyser, and oxygen is measured to show air ingress. Raw biogas is wet and carries H2S, so a conditioning step is needed before the analyser. ACCURRA-S 4030 reads methane from 0 to 100 % with a minimum range of 0 to 5 %.
- List the components of raw biogas and landfill gas
- Choose the measurements for each stage: raw, cleaned, upgraded
- Explain why H2S and moisture shape the sample system
What is in biogas
Raw biogas from digesters is mostly methane (commonly around 50 to 70 %) and CO2 (30 to 50 %), saturated with water vapour and carrying traces of hydrogen sulphide, and sometimes oxygen and nitrogen from air ingress. Landfill gas is similar, with air drawn into the wells by the extraction system. Upgrading removes CO2 and the contaminants to make biomethane with a high methane content, often above 90 %.
What to measure and why
| Point | Measure | Why |
|---|---|---|
| Digester gas | CH4 and CO2 | Process health, gas yield |
| Landfill well | CH4, CO2 and O2 | Well condition; oxygen shows air ingress and a fire risk |
| Before the engine | CH4 and CO2 | Fuel quality for gas engines |
| Upgrader outlet | CH4 (and CO2, O2) | Product quality for compressed biogas |
NDIR is the right principle for both gases; ACCURRA-S 4030 reads methane and 6030 reads CO2, and a dual-channel version reads both from one sample: What is an NDIR gas analyser.
The sample is the hard part
- Moisture. Raw biogas is saturated. Cool the sample to a stable dew point and drain the condensate: Condensate, moisture and sample cooling for gas analysis.
- Hydrogen sulphide. H2S is corrosive and toxic. It lies outside the standard sample limits of the ACCURRA-S, i-Cons and SGI-320. We review the stream and add conditioning or specify a suitable arrangement before proposing an analyser.
- Siloxanes and particles in landfill gas coat sensors and optics; filters and conditioning are needed.
- Safe area. Biogas is flammable; the analysis location and the classification of the area must be decided at the start.
Putting an analyser on raw wet biogas without conditioning. It will work briefly, then drift or fail as water and H2S attack the cell and lines.
Why oxygen matters
Oxygen should be near zero in a digester gas. Rising oxygen means air is getting in through a leak or from over-extraction on a landfill, and it signals both a quality problem and a safety risk near flammable gas. Many plants therefore measure oxygen alongside methane and CO2: Biogas, landfill gas and digesters explained.
- Biogas is mainly methane and CO2; NDIR measures both.
- Oxygen in the gas shows air ingress and a safety risk.
- Moisture and H2S shape the sample system and are outside standard sample limits.
- Dual-channel analysers read two gases from one sample.
Check your understanding
Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading.
Which two gases give the basic quality of biogas?
Methane and CO2.
What does a rising oxygen reading in a landfill gas well indicate?
Air is entering the well, often from over-extraction, which lowers quality and raises the fire risk.
Why can a standard ACCURRA-S not take raw biogas directly?
Raw biogas is wet and carries H2S, outside the non-corrosive, non-condensing sample specification.
Key terms
Frequently asked questions
How do you measure methane in biogas?
By NDIR on a conditioned sample, for example ACCURRA-S 4030.
Can one analyser measure methane and CO2 together?
Yes, a dual-channel enclosure reads both from one sample.
What does biogas contain besides methane?
CO2, water vapour, and traces of H2S, hydrogen, nitrogen and oxygen.
Is a biogas analyser safe-area equipment?
The standard ACCURRA-S is for safe areas. For classified locations, tell us the zone and we will advise.
Related products
ACCURRA-S
Online analyser for trace, percent and purity O2, CO, CO2, CH4 and dew point; can measure two gases from one sample.
View ACCURRA-S
i-Cons
Smart Peltier sample conditioner with diagnostics, auto blowback and Modbus, in a compact 19 inch cabinet.
View i-ConsWhere this applies
- Biogas and compressed biogas (CBG)
- Landfill gas wells and flares
- ACCURRA-S methane analyser
- Dual gas CO2 and CH4 (percent)