Biomethane
Biomethane is the same molecule as the gas already in the pipe. Once it is injected there is no physical way to tell the two apart, and no meter anywhere in the network can do it.
So the entire carbon claim rests on paperwork. Not on the chemistry, not on the meter — on whether the contract behind the gas describes something that was actually made and actually delivered.
Biomethane is biogas upgraded to near-pure methane and injected into the gas grid. Its Scope 1 figure is near-zero because the carbon is biogenic, not because nothing was emitted — and the claim holds only if the gas you bought was physically delivered.
Every other fuel on this site can be identified in the tank. Biomethane cannot be identified in the pipe, which is why it is the one fuel whose emission factor is decided by a contract.
What biomethane is
Biomethane is biogas that has been upgraded — the carbon dioxide, water, hydrogen sulphide and siloxanes stripped out — until what remains is almost entirely methane, at a specification that allows it to be injected into the natural gas grid or compressed for vehicle use. It is also sold as renewable natural gas.
Two things it is not. It is not biogas: raw biogas is roughly half carbon dioxide and cannot go into the grid. And it is not chemically different from the fossil gas it displaces — for the physics of combustion, see natural gas, which covers the per-gas split, the calorific value traps and the unit bases. This page is about the part that page does not settle: what you are allowed to report.
| Property | Detail |
|---|---|
| Source | Anaerobic digestion of food waste, farm residues, manure or sewage sludge; occasionally biomass gasification. |
| Upgrading | CO2 and contaminants removed until methane content reaches roughly 96–98%. |
| Delivery | Grid injection, or compressed (bio-CNG) / liquefied (bio-LNG) for transport. |
| Why it is low carbon | The combustion CO2 is biogenic, so it is reported outside the scopes rather than inside Scope 1. |
| What decides the claim | Chain of custody — whether the gas was physically delivered or only certificated. |
The numbers DEFRA actually publishes
There is a widespread assumption that DEFRA publishes a biomethane emission factor in the same shape as the natural gas one. It does not, and the shape of what it does publish tells you how the gas is meant to be accounted.
| Row | Value | Where it sits |
|---|---|---|
| Natural gas, per kWh (GCV) | 0.18231 kg CO2e/kWh | Scope 1, the full fossil comparison |
| Biogas — combustion non-CO2, per kWh | 0.00022 kg CO2e/kWh | Scope 1 — the CH4 and N2O only |
| Biogas — well-to-tank, per kWh | 0.01851 kg CO2e/kWh | Scope 3 Category 3 |
| Biomethane (compressed), per kg | 2.71 kg CO2/kg | Biogenic memo — outside the scopes |
| Biomethane (liquefied), per kg | 2.78 kg CO2/kg | Biogenic memo — outside the scopes |
The filing tells you the treatment. The two biomethane rows are not on a fuels sheet at all. In the DEFRA workbook they sit on the sheet named Outside of scopes — cells D39 and D63. The source document has already made the accounting decision, in its own structure, before anyone reads a number off it.
Note what is missing: there is no per-kWh Scope 1 biomethane combustion factor. The near-zero combustion treatment comes through the bioenergy route, where the CO2 is set aside as biogenic and only the methane and nitrous oxide remain in Scope 1. That residual is small but it is not nothing, and rounding it to zero is the single most common error on this fuel.
Why “zero” is the wrong word
Burning biomethane emits carbon dioxide. Roughly the same amount, per unit of energy, as burning fossil gas. The molecule does not know where it came from.
What changes is where that carbon dioxide is reported. Because the carbon was drawn out of the atmosphere within the last growing season rather than out of a reservoir, the GHG Protocol treats it as biogenic and requires it to be reported outside the scopes, as a memo item, rather than inside Scope 1.
Three consequences follow, and all three are routinely missed:
- The memo line is mandatory, not optional. Biogenic CO2 reported nowhere is an incomplete inventory, not a clean one.
- Scope 1 does not reach zero. The non-CO2 residual stays, at 0.00022 kg CO2e/kWh on the biogas route.
- Scope 3 does not fall the same way. The well-to-tank burden of producing, upgrading and distributing the gas is real and belongs in Category 3.
The chain-of-custody question
This is the part that decides whether the number is defensible, and it has almost nothing to do with the gas.
Once biomethane enters the grid it is indistinguishable from fossil gas. So the market runs on certificates, in the same way the electricity market runs on energy attribute certificates and guarantees of origin. A producer injects gas, a registry issues a certificate for it, and a buyer retires that certificate against their own consumption.
The question every assurance provider asks is what the certificate is attached to.
| Arrangement | What it means | How it tends to be treated |
|---|---|---|
| Physical supply with certificates | Gas injected into the same grid you draw from, certificates retired against your volume. | The strongest position. Accepted by most assurance providers. |
| Book and claim, same market | Certificates bought separately from the gas supply, within one connected network. | Contested. Increasingly requires evidence of injection into the same network. |
| Certificates with no injection link | An attribute purchased with no demonstrable physical delivery. | Weakest. Excluded under tightening science-based target guidance. |
The discipline is identical to market-based Scope 2 reporting, and the failure mode is identical too: double counting. If a certificate is retired by you and the underlying gas is also counted as renewable by the producer or the grid operator, one injection has been claimed twice. The registry exists to prevent exactly that, which is why an unregistered claim is worth very little.
The practical test. Ask the supplier which registry issued the certificates, what volume was retired against your account, and on what date the gas was injected. If those three facts cannot be produced, the near-zero factor cannot be defended, and a verifier will say so. See chain of custody for the general form of the argument.
The upgrading step has its own footprint
Turning biogas into biomethane is not free. The upgrading plant separates carbon dioxide from methane using water scrubbing, membranes, pressure-swing adsorption or amine washing, and every one of those consumes energy — typically electricity, which carries the grid’s own carbon intensity.
The step also leaks. Some methane leaves with the separated carbon dioxide stream, and that loss is called methane slip. It matters out of all proportion to its volume, because global warming potential multiplies every escaped kilogram of methane by a large factor. A plant losing a few per cent of its output uncombusted can erase a noticeable share of the benefit it was built to deliver.
This is why the well-to-tank figure is the one to watch rather than the combustion figure. The combustion side is a reporting convention; the upstream side is where the real physical emissions of the fuel sit.
Feedstock decides the lifecycle answer
Two identical molecules of biomethane can have very different lifecycle footprints, because the argument for each starts in a different place.
- Waste feedstocks — food waste, manure, sewage sludge. The strongest case. The material would have decomposed and emitted anyway, frequently as uncontrolled methane; see landfill gas for what the alternative looks like.
- Residues — straw, crop trimmings. Generally good, provided removing the residue does not degrade the soil it came from.
- Purpose-grown energy crops — maize in particular. The weakest case, because land, fertiliser and displaced food production all enter the calculation. Nitrogen fertiliser brings nitrous oxide with it.
A supplier who will not say which feedstock the gas came from is not in a position to support a lifecycle claim about it. The same logic runs through every biofuel argument.
Worked example — switching 500,000 kWh
A site burns 500,000 kWh of gas a year and switches to biomethane on a physically-supplied contract.
| Line | Before (fossil gas) | After (biomethane) |
|---|---|---|
| Scope 1 combustion | 500,000 × 0.18231 = ~91.2 t CO2e | 500,000 × 0.00022 = ~0.11 t CO2e |
| Scope 3 Cat 3 (well-to-tank) | Fossil upstream burden | 500,000 × 0.01851 = ~9.3 t CO2e |
| Outside scopes | None | A biogenic CO2 memo line, roughly the tonnage Scope 1 just lost |
The headline is a Scope 1 reduction of about 99.9%. The honest reading is narrower: the combustion carbon did not disappear, it moved to a memo line below the inventory; a real upstream burden of roughly nine tonnes remains in Scope 3; and every part of it depends on the contract surviving scrutiny.
On the arithmetic. DEFRA publishes no per-kWh Scope 1 factor for biomethane, so the combustion line above uses the biogas non-CO2 row — the closest published equivalent, and the route by which the biogenic treatment is applied. Both biogas rows are cited to their own cells in the DEFRA workbook. The tonnages are rounded.
Common confusions
| The belief | What is actually true |
|---|---|
| Biomethane is zero carbon | Its Scope 1 combustion CO2 is reported outside the scopes. The gas still emits, a non-CO2 residual stays in Scope 1, and the upstream burden stays in Scope 3. |
| Biogas and biomethane are the same | Biogas is roughly half CO2 and cannot enter the grid. Biomethane is biogas upgraded, at the cost of energy and some methane slip. |
| A certificate is enough | Certificates without a demonstrable injection link are the weakest form of the claim and are being excluded from credible target-setting guidance. |
| The biogenic CO2 need not be reported | It is a required memo item. Omitting it makes the inventory incomplete. |
| All biomethane is equally good | Waste-fed and maize-fed biomethane have materially different lifecycle footprints. |
Frequently Asked Questions
No, and the word causes real reporting errors. Burning biomethane releases roughly as much carbon dioxide per kWh as burning fossil gas. Because that carbon is biogenic it is reported outside the scopes as a memo item rather than inside Scope 1, which is a change of location on the report, not a change in the physics. A non-CO2 residual remains in Scope 1, and the upstream burden of producing and upgrading the gas remains in Scope 3 Category 3.
It depends entirely on what the certificates are attached to. Certificates retired against gas physically injected into the network you draw from are the strongest position and are generally accepted. Certificates bought with no demonstrable link to an injection are the weakest, and tightening target-setting guidance is moving against them. The practical test is whether your supplier can name the registry, the volume retired against your account, and the injection date.
On a separate memo line outside Scopes 1, 2 and 3. The GHG Protocol requires biogenic combustion CO2 to be disclosed but kept out of the scope totals, so that a fuel switch does not appear to make carbon vanish. Leaving the line off altogether is a completeness failure, and it is one an assurance provider will look for precisely because the fuel is being claimed as low carbon.
Because that is the accounting treatment, expressed in the structure of the workbook. The biomethane rows are biogenic CO2 memo figures, per kilogram, and they sit on the sheet named for the place they belong on the report. There is deliberately no per-kWh Scope 1 biomethane combustion factor, because under the convention the combustion CO2 does not belong in Scope 1 at all.
It can, as a Scope 1 reduction, but the evidentiary bar is the same one applied to renewable electricity claims rather than a lower one. A physically supplied contract with retired certificates from a recognised registry is the defensible form. A certificate-only arrangement with no injection link is increasingly excluded. Treat the chain of custody as the deliverable, not the purchase.
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