Oxygen Generators for Biogas Plants
Hydrogen sulphide is the one component of your biogas that attacks everything downstream of the digester. Biological desulphurisation removes most of it inside the plant, and it runs on a small, continuous dose of oxygen. OXYBERG® produces that oxygen from compressed air on your own site, so the sulphur leaves the gas without a load of inert nitrogen arriving in its place.
What oxygen does in a biogas plant
Raw biogas leaves the digester with hydrogen sulphide in it, and how much depends on the substrate rather than on the plant. Sulphur-rich and protein-rich feedstocks such as slurry, stillage or food waste produce far more than energy crops, and published figures for agricultural plants span a few hundred to several thousand ppm.
That gas then has to run an engine. Hydrogen sulphide burns to sulphur dioxide and condenses as acid on every cool surface downstream, working through gas lines, heat exchangers, the engine oil and the engine itself. Engine manufacturers publish an intake limit, and above it the answer is shorter oil change intervals, faster wear or a warranty discussion. On plants that upgrade to biomethane the same sulphur reappears as adsorbent consumption: whatever the digester does not remove, the cleaning stage has to.
Biological desulphurisation deals with the bulk of it before any of that. A small, controlled amount of oxygen goes into the gas space of the digester or, more usually, the secondary digester. Sulphur-oxidising bacteria already live on the wet surfaces there, and with oxygen available they oxidise dissolved sulphide to elemental sulphur, which settles out and leaves with the digestate. No reagent is bought and no medium is disposed of, which is why this is the first sulphur removal stage on most plants that have one.
The oxygen itself comes from a PSA generator: compressed air passes through a molecular sieve that holds back nitrogen while oxygen passes, with one vessel regenerating while the other produces. The PSA oxygen generator page covers the process itself.
Air or oxygen, and what the nitrogen costs
The bacteria need oxygen. Air is only about a fifth oxygen, and the remaining four fifths are nitrogen that nothing in the plant consumes. Dose air, and that nitrogen stays in the biogas all the way to the engine or the upgrading unit.
The arithmetic is worth doing on your own numbers. A digester producing 500 Nm³/h of biogas, dosed with air at five per cent of the gas flow, takes in 25 Nm³/h of air, of which roughly 20 Nm³/h is nitrogen. Close to four per cent of what then leaves the plant is an inert gas that was never in the digester, and the methane fraction falls by about the same proportion.
For a plant burning its gas on site that is a measurable loss of calorific value. For a plant upgrading to biomethane it is more than that, because nitrogen is the component the common upgrading technologies do not remove: membrane and amine systems separate carbon dioxide, not nitrogen. Grid specifications leave little room either, with European limits for oxygen in injected biomethane in the region of one per cent and far tighter values where a network connects to underground storage.
Oxygen at 90 to 95 per cent delivers the same oxidation with roughly a fifth of the gas volume, and its balance is a few per cent of argon and nitrogen rather than the bulk of the stream. The inert gas you add to your own product drops from percentage points to a fraction of one.
How much oxygen a digester needs
The dose follows the sulphur load, not the size of the plant. Two digesters with the same electrical output can sit far apart because their substrates carry different sulphur, and the same digester moves through the year as the feed mix changes. As an order of magnitude, the technical literature puts the requirement at roughly 0.5 to 1.5 per cent of the biogas flow as pure oxygen, equivalent to something like 3 to 6 per cent if air is used. Treat that as a starting bracket and confirm it against your own H₂S readings.
Both directions have a cost. Too little oxygen and the sulphide stays in the gas. Too much and the oxidation runs on past elemental sulphur to sulphuric acid, which attacks concrete in the headspace, while unused oxygen shows up in the gas analysis, where an upgrading plant is the first to notice it. That is why the dose is controlled against a measured H₂S value and a residual oxygen reading rather than set once at commissioning.
How to size an oxygen supply for a biogas plant
Start from the sulphur load
Biogas flow multiplied by H₂S concentration gives the load the bacteria have to work through. Take both from your own gas analyser across a full feed cycle, including the worst substrate mix you run, not from the design values in the commissioning file.
Define what the gas has to meet
An engine intake limit and an upgrading feed specification are different targets, and they lead to different residual H₂S values and different doses. Decide which one governs before anything gets sized.
Convert the load into an oxygen flow
Dose as a percentage of the biogas flow, then size for the peak rather than the annual average. A generator sized on the mean is short exactly when a sulphur-rich batch goes in.
Check the compressed air
Oxygen flow multiplied by the air factor of 11 gives the demand, at any purity in the range. Quality counts as well as volume: oil carry-over and residual moisture shorten sieve life, and OXYBERG® expects compressed air to ISO 8573-1:2010 Class 1.4.1 at 7 bar(g).
Need sizing for your application?
You do not need your full readings to start. Tell us your plant size and how you handle sulphur today, and we will say whether an oxygen generator is worth a closer look. If you have your H2S figures and the gas specification to hand, send them and you get a sizing proposal back.
Where biological desulphurisation ends and other methods start
Biological desulphurisation is rarely the only sulphur stage on a plant, and it is not meant to be. Iron salts dosed into the substrate bind sulphide in the liquid phase before it ever reaches the gas. They work, they are simple to retrofit, and they are a consumable with a running cost per tonne of substrate and an iron load that ends up in the digestate. Activated carbon sits at the other end of the chain as a polishing stage: very effective down to low ppm, and a medium that is replaced on a schedule and disposed of.
External biological units, biotrickling filters and two-stage scrubbers, run the same biology outside the digester with tighter control, and a two-stage design keeps the process air out of the gas altogether at a higher capital cost.
On most plants the practical answer is a combination. Biological desulphurisation inside the digester takes the bulk of the sulphur at the lowest running cost, and a polishing stage takes the last few ppm where a tight specification applies. Oxygen supply affects the first of those, which is also the one carrying most of the load, and a well-run first stage is what keeps the consumables in the second one from becoming the dominant operating cost.
Why generate the oxygen on site
No nitrogen ballast in your gas
The whole point of using oxygen instead of air. The same oxidation happens without four fifths of the dosed volume being an inert gas, so methane content, calorific value and the inert load of your biomethane stay where they were.
Continuous duty suits PSA
Desulphurisation runs whenever the digester runs, which is most hours of the year. A small, constant, uninterrupted flow is the load profile a PSA generator is built for, and the one a delivery schedule fits worst. Most biogas plants do not buy oxygen today. They blow in air, or they dose iron salts, so the comparison is not a gas bill: it is the inert nitrogen you carry in your gas and the consumables you keep buying. What a generator is worth on your plant depends on your sulphur load and your electricity price, and that calculation is worth running together before anything else.
A dose you can move
Substrate changes and H₂S climbs with it. With your own generator you change a setpoint and check the analysis, rather than renegotiating a supply volume and waiting for the next delivery window.
Standard wear parts, open service
Filters and wear parts are off-the-shelf components you can buy on the open market, so service stays open to competition for the life of the machine. Warranty runs three years as standard and up to five years with a service contract.
OXYBERG® oxygen generators for biogas duty
OXYBERG covers 18 sizes in total, listed in full on the oxygen generator overview. The selection below spans the band most digesters fall into, shown at 90 % purity because that is where biological desulphurisation sits.
| Model | O₂ at 90 % | Compressed air at 90 % |
|---|---|---|
| OXYBERG® 500 | 2.1 Nm³/h | 23.1 Nm³/h |
| OXYBERG® 900 | 9.2 Nm³/h | 101.2 Nm³/h |
| OXYBERG® 1200 | 18.9 Nm³/h | 207.9 Nm³/h |
| OXYBERG® 1500 | 44.6 Nm³/h | 490.6 Nm³/h |
| OXYBERG® 1700 | 65.8 Nm³/h | 723.8 Nm³/h |
Which size fits follows from the sulphur load rather than from the installed electrical capacity, and a buffer vessel belongs in the plan even where the average demand looks comfortable, because a fresh charge of sulphur-rich substrate does not wait for the generator.
Where a site has no suitable compressed air, the generator is planned as a complete plant rather than as a single machine: compressor station, air treatment, buffer, generator and oxygen storage as one chain from one source within the BERG Group. The PSA oxygen plant page covers that configuration. Applicable NITROBERG® and OXYBERG® standard generator assemblies are conformity assessed under PED 2014/68/EU using Modules B + D. Engineering and manufacturing are Made in Germany.
FAQ: oxygen generators for biogas plants
For biological desulphurisation. Sulphur-oxidising bacteria in the digester headspace convert dissolved sulphide into elemental sulphur, and they need a small, continuous supply of oxygen to do it. The sulphur settles out and leaves with the digestate, which keeps hydrogen sulphide out of the gas before it reaches the engine or the upgrading unit.
Biologically rather than chemically. Given oxygen, the bacteria already present on the wet surfaces oxidise sulphide to elemental sulphur, and with more oxygen further to sulphate. Nothing is dosed in that has to be bought or disposed of, which is what makes this the cheapest sulphur removal stage a biogas plant has. Control of the dose is what decides where the reaction stops.
Because of the nitrogen that arrives with the air and that nothing in the plant consumes. It stays in the biogas, lowering the methane fraction and the calorific value, and it is the component the usual upgrading technologies do not separate out. Oxygen at 90 to 95 per cent achieves the same oxidation with far less gas going in, so what stays behind afterwards is a fraction of what air leaves.
It follows the sulphur load, so biogas flow multiplied by H₂S concentration, not the electrical output. The technical literature puts the requirement in the region of 0.5 to 1.5 per cent of the biogas flow as pure oxygen, and that is a bracket to start from rather than a design figure. The dose is then trimmed in operation against the measured H₂S and the residual oxygen in the gas.
The lower end of the range is enough. Between 90 % and 93 % the gas is well past the point where oxygen availability limits the bacteria, and a higher grade does not make them work faster. On OXYBERG the air factor is 11 at every purity, so a higher grade does not cost you compressed air. What it costs is output: the same machine delivers between 10 and 40 % more oxygen at 90 % than at 95 %.
At these doses the oxygen is consumed close to where it enters, and the bulk of the digester stays anaerobic. Published trials on micro-aeration report methane yields unchanged or slightly improved. Overdosing is the case to avoid, which is why the dose is interlocked with the gas analysis and why the explosion protection concept for the plant belongs in its design rather than in the gas supply.
Usually it reduces what they have to do rather than removing them. Biological desulphurisation takes the bulk of the sulphur at the lowest running cost, and a polishing stage still handles the last few ppm where a tight specification applies. Where iron salts are dosed today, a well-controlled biological stage is what makes it possible to reduce that consumable rather than the other way around.