Oxygen in biogas and renewable natural gas (RNG) comes mostly from air: air pulled into landfill wellfields and collection piping under vacuum, and air dosed into digesters to control H₂S. RNG injected into a utility or pipeline must meet that system's oxygen limit, for example 0.2 % by volume under SoCalGas Rule 30 and 0.1 % under PG&E Gas Rule 21, and some interstate pipelines publish limits as low as 10 ppm. Measuring oxygen continuously from the raw gas to the injection point shows where air enters and whether the upgrader is removing it.
Oxygen in biogas comes from air, because anaerobic digestion and landfill decomposition do not produce it. The two main routes are air drawn into collection systems that run under vacuum and air added on purpose to digesters.
Landfill gas. A landfill gas collection system applies vacuum to the wellfield, and pulling too hard draws air through the cover and into the waste mass. EPA's Landfill Methane Outreach Program notes that tuning a typical landfill for 48 to 52 % methane results in some air infiltration, with 0 to 2 % oxygen in the collected gas, and its wellhead procedures treat 2 % or more oxygen as high for operational practice. EPA also suggests a quick leak check: a balance-gas-to-oxygen ratio near 4 to 1, the ratio found in air, points to an air leak.
Digester gas. Some digesters control H₂S by microaeration, which adds a small, controlled amount of air to the headspace so that bacteria convert H₂S to elemental sulfur. A University of Maryland Extension fact sheet describes oxygen dosing of about 0.3 to 3 % of produced biogas, supplied as air at about 1.5 to 15 % of biogas flow. Any oxygen the bacteria do not consume stays in the gas and carries on to the upgrader.
Leaks at low pressure. Blower suction, condensate knockouts, sample ports and fittings on low-pressure gas piping can all let air in, in the same way that air enters low-pressure equipment in oil and gas.
The oxygen limit for RNG is set by the utility or pipeline that receives the gas, and published values differ by system. The examples below come from published tariffs and fact sheets.
| Receiving system | Published oxygen limit | In ppm by volume |
|---|---|---|
| SoCalGas, Rule 30 | 0.2 % | 2,000 |
| PG&E, Gas Rule 21 | 0.1 % | 1,000 |
| Interstate pipelines with the tightest published limits | 10 ppm (0.001 %) | 10 |
Oxygen often also counts toward a total inerts limit. The SoCalGas fact sheet lists a 4 % total inerts limit alongside its 0.2 % oxygen limit, and nitrogen from air ingress counts against the same total. For more published values, see natural gas pipeline oxygen limits.
The SoCalGas fact sheet also states that attributes such as carbon dioxide, total inerts and heating value are monitored continuously at the point of utility interconnection. An RNG producer's own continuous oxygen reading shows a rising trend before the gas reaches that interconnection.
Oxygen matters in RNG upgrading because the upgrader has to deliver gas under the receiving system's limit, and because some upgrading steps cannot accept oxygen at all. SWANA notes that medium-pressure cryogenic systems cannot accept oxygen, because an explosive mixture can result, so those plants need an oxygen removal step upstream. That removal step makes water, which then adds a water removal step.
Oxygen also counts against the heating value and the inerts total, and in pipelines oxygen accelerates internal corrosion where water is present. In the raw gas, a high oxygen reading at the wellfield or digester can signal a process problem before it reaches the upgrader, such as over-pulled wells or air dosing set too high.
Upgrading technologies remove oxygen to different degrees, so the oxygen in the product gas depends on the process train. Published descriptions include the following points:
Because removal varies by process, measure oxygen after the upgrader as well as before it.
| Measurement point | What the reading shows | Range to consider |
|---|---|---|
| Wellfield header or digester outlet | The reading shows air pulled in by vacuum or added by microaeration. | Percent, 0–10 %vol* or 0–21 %vol* |
| Upgrader inlet | The reading shows the oxygen load the upgrader has to handle. | Percent, 0–1 %vol* or 0–10 %vol* |
| After oxygen removal | The reading shows whether the removal step is working. | Trace, 0–1000 ppm*, or Ultra-Trace, 0–50 ppm* |
| Product gas before injection | The reading monitors oxygen against the receiving system's limit. | Percent 0–1 %vol* for a 0.1 % or 0.2 % limit, or Ultra-Trace 0–50 ppm* for a 10 ppm limit |
On the 0–1 %vol range, accuracy of ±0.5 % of full scale equals ±0.005 %vol*, which is small relative to a 0.1 % or 0.2 % product-gas limit.
*Based on controlled lab conditions.
Reading at more than one point is what makes the data useful. The first point where oxygen rises is where air is getting in, and the difference across the upgrader shows how much oxygen the process removes.
An oxygen analyzer for biogas reads oxygen in a background that is mostly methane and CO₂, often with H₂S. The University of Maryland fact sheet describes biogas as 50 to 80 % methane and 20 to 50 % CO₂.
That background is hard on electrochemical cells. One galvanic sensor maker states that CO₂ above 1 % requires an acid-electrolyte sensor for continuous use, and that H₂S must be scrubbed to below 10 ppm in the sample gas. See how long an electrochemical oxygen sensor lasts for the effect on replacement intervals.
OxyScout has no H₂S or CO₂ cross-sensitivity, and as an optical analyzer it has no electrochemical cell to replace. OxyScout is engineered for Class 1 Division 2 (Zone 2) hazardous areas, is rated up to 250 psi without the flowmeter installed, and operates from 0 to 50 °C*. Higher pressure ratings are available if a regulator is installed upstream. The analyzer is specified for non-condensing conditions only, so remove liquid water from the biogas sample before the analyzer. Accuracy is ±2 ppm or ±5 % of reading, whichever is greater*, on the Ultra-Trace range (0–50 ppm). Accuracy is ±0.5 % of full scale* on the 0–1 %vol Percent range, and ±5 % of full scale* on the Trace range (0–1000 ppm) and the 0–10 % and 0–21 %vol ranges. Typical response is under 10 seconds*.
*Based on controlled lab conditions.
See the optical oxygen analyzer page for full specifications and the optional sample system. OxyScout is priced comparably to electrochemical analyzers. To review your gas, measurement points and injection limit with an engineer, book a quote meeting.
The limit depends on the receiving system. SoCalGas Rule 30 allows 0.2 % by volume, PG&E Gas Rule 21 allows 0.1 %, and some interstate pipelines publish limits as low as 10 ppm.
Collection systems run under vacuum, and pulling too hard draws air through the landfill cover into the waste mass and the collection piping.
Yes. Microaeration doses a small amount of air into the digester to control H₂S, and any oxygen the bacteria do not consume remains in the biogas.
CO₂ can affect electrochemical cells, and one sensor maker requires an acid-electrolyte cell above 1 % CO₂. OxyScout has no CO₂ or H₂S cross-sensitivity.
Measure at the raw gas, at the upgrader inlet, after any oxygen removal step and at the product gas before injection, so the first rise in oxygen shows where air enters.
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