The Swedish Gas Technology Centre's review of commercial biogas upgrading, SGC Rapport 2013:270, puts the mean and median methane slip from pressure swing adsorption at 1.8 to 2 percent. Amine scrubbers are guaranteed at 0.1 percent. Both figures describe methane lost from the product. Read from the other side of the machine, they describe what a CO₂ plant receives.
The review compares technologies on biogas at 60 percent methane and 40 percent CO₂. On that feed, a 2 percent slip leaves roughly 3 percent methane in the off-gas. A 0.1 percent slip leaves about 0.15 percent. The raw gas is the same and so is the pipeline product, yet the stream handed to a capture skid differs twentyfold in methane.
Methane in the Off-Gas Costs CO₂
Methane will not condense at CO₂ liquefaction conditions. It leaves through the liquefier vent along with nitrogen and oxygen, and it takes CO₂ with it. Our September analysis of liquefier vent losses put the ratio at about 2.7 moles of CO₂ for every mole of methane at minus 20°F and 300 psig.
Run the upgrader figures through that ratio. Methane alone costs a PSA stream about 8 percent of its CO₂ at the vent. A three stage membrane design, which the review credits with about 0.5 percent slip, gives up about 2 percent. An amine stream loses under half a percent. All of that comes before anyone counts nitrogen or oxygen.
Four Technologies, Four Off-Gases
Each upgrading method pulls CO₂ away from methane through a different mechanism, and each mechanism decides what else travels with the CO₂. Four of the commercial methods in the review show how far apart the results can land.
- Membranes. CO₂ permeates faster than methane, and water vapor, hydrogen and part of the oxygen permeate with it. Nitrogen moves at nearly the same rate as methane, so it stays in the product gas.
- Pressure swing adsorption. An adsorbent holds CO₂ at pressure and releases it when the pressure drops. Standard configurations remove neither oxygen nor nitrogen, so both stay with the methane, and the off-gas carries the highest slip of the group.
- Amine scrubbing. A solvent binds CO₂ chemically and a heated stripper releases it, with the lowest slip of the group. Oxygen destroys the amine irreversibly and hydrogen sulfide is normally taken out with activated carbon first, so the feed is cleaned before separation. The released CO₂ can carry traces of the solvent and its degradation products.
- Water scrubbing. CO₂ dissolves into water at 6 to 10 bar and comes back out in a desorption column, where air is blown through the water at atmospheric pressure. Hydrogen sulfide follows the same path. The off-gas is CO₂ mixed into a stream of air.
The water scrubber is the outlier. Air is mostly nitrogen, and every unit of nitrogen in a liquefier feed carries nearly three units of CO₂ out the vent. A stream that leaves the upgrader diluted in stripping air sits closer to a flue gas problem than a purification problem.
Landfill Gas Brings Its Own Air
Feedstock moves the numbers again. A digester normally carries little air. A landfill collection system runs its wells under vacuum, and the vacuum pulls air into the waste and into the gas. The review treats oxygen and nitrogen as a landfill gas issue, and as a digester issue only where air is dosed in for biological sulfur removal.
On a landfill site with membranes, part of that oxygen leaves through the membranes with the CO₂. The deoxo catalyst a landfill project installs to meet pipeline oxygen limits treats the methane product. Oxygen that went the other way stays in the off-gas, and at the liquefier it behaves almost exactly like methane, costing about 2.7 moles of CO₂ per mole at the vent.
Two Separate Cleanup Jobs
Beverage grade is 99.9 percent CO₂ minimum, which leaves 0.1 percent for everything else combined. Methane, nitrogen and oxygen are light gases. Cryogenic distillation strips them from the liquid and sends them out the vent. Sulfur compounds and aromatics are heavier. Benzene has to be held under 20 parts per billion, and those compounds come out on adsorbent beds before the gas reaches the cold section.
The upgrader shifts the load between those two jobs. An amine stream arrives with almost no methane and its sulfur already removed upstream, saturated with water and carrying solvent traces. A PSA or membrane stream brings more methane and needs a vent sized for it. A water scrubber stream can carry the hydrogen sulfide the upgrader never removed, spread through a volume of air.
Our cryogenic purification, now patent pending, was designed around biogas streams where these inerts are always present. CleanCycleCarbon entered continuous commercial beverage grade production in December 2024 on that kind of feed.
Start With the Upgrader Datasheet
A gas analysis taken on one day shows one day. The upgrader's design shows what the stream will look like across a year of operation, and it answers most of the questions a purification designer will ask first.
- Which technology, and for membranes, how many stages and where the permeate is recycled.
- The guaranteed methane slip, and the slip actually measured in operation.
- The feedstock, and on a digester, whether air is dosed for sulfur control.
- Where hydrogen sulfide is removed, whether upstream of the upgrader or not at all.
- Oxygen and nitrogen in the off-gas, each broken out separately and sampled ahead of the oxidizer.
Two RNG plants running the same biogas to the same pipeline spec can produce CO₂ streams that differ twentyfold in methane. The upgrader was chosen to make methane, usually years before anyone priced the CO₂, and the day it was chosen it set how much of that CO₂ a liquefier can keep.
