Biogas
Biogas has three emission factors, not one. They are published in three different places, they go to three different lines on the report, and the largest of the three is the one that does not count towards your total.
Nearly every biogas reporting error is a version of putting one of those three numbers where another one belongs.
Biogas is the raw gas from anaerobic digestion — roughly half to two-thirds methane, most of the rest carbon dioxide. Burning it carries three separate factors: a small Scope 1 residual, a large biogenic CO2 memo reported outside the scopes, and an upstream figure in Scope 3.
The question “what is the emission factor for biogas” has no single answer, and a page that gives you one number has almost certainly given you the wrong one.
What biogas is
Biogas is the gas produced when organic matter breaks down without oxygen. It comes out of a sealed digester, a covered lagoon or a capped landfill cell, and it is a mixture rather than a single substance: roughly 50–65% methane, most of the balance carbon dioxide, with water vapour, hydrogen sulphide and traces of other gases.
For the process that makes it deliberately, see anaerobic digestion. For the same gas arising uncontrolled under a landfill cap, see landfill gas. For what it becomes once the carbon dioxide is stripped out, see biomethane. This page is about the raw gas itself and, specifically, about its numbers.
The three numbers, and where each one goes
DEFRA publishes biogas three times over, on three different sheets. This is not duplication — each figure answers a different reporting question, and they are not interchangeable.
| Figure | Per kWh | Per tonne | Where it goes |
|---|---|---|---|
| Combustion, non-CO2 | 0.00022 | 1.22851 | Scope 1 — the CH4 and N2O only |
| Biogenic CO2 | 0.19902 | 1105.6695 | Outside the scopes — a required memo item |
| Well-to-tank | 0.01851 | 102.85714 | Scope 3 Category 3 |
All three are DEFRA 2026, and each is cited to its own cell. The combustion rows sit on the Bioenergy sheet, the biogenic rows on the sheet named Outside of scopes, and the upstream rows on WTT–bioenergy. The filing is the accounting instruction: the workbook has already decided where each number belongs, before anyone reads a value off it.
The biggest number is the one that does not count
Set the first two figures beside each other and the shape of biogas accounting becomes obvious. The biogenic carbon dioxide is roughly 900 times the Scope 1 residual — and it is the one that never enters your total.
That is not a loophole. The carbon in biogas was pulled out of the atmosphere by a plant within the last growing season, so releasing it returns carbon that was recently taken. The GHG Protocol therefore requires it to be disclosed outside the scopes rather than inside them. See biogenic CO2 for the underlying convention.
Two consequences follow:
- The memo line is mandatory. Reporting biogas with no biogenic CO2 disclosure is an incomplete inventory, not a clean one. The number is large and leaving it off is conspicuous.
- Scope 1 is not zero. The residual methane and nitrous oxide stay in, and a factor of almost zero is not the same as zero. Rounding it away is the single most common biogas error.
Per kWh or per tonne
Both bases are published, and picking the wrong one is a silent error because both produce a plausible answer.
Use per kWh when your meter reads energy — a CHP unit, a boiler, anything with a gas meter downstream of the digester. Use per tonne when your records are mass, which is usual when gas is sold or transferred rather than burnt on site. The two are linked by the gas’s calorific value, and biogas calorific value varies with methane content, so the conversion is site-specific rather than a constant you can borrow.
If you have both, prefer the basis your evidence is strongest in. An auditor will ask to see the meter reading or the weighbridge ticket behind whichever number you used; see kilowatt-hour for the unit traps that come with energy bases.
Flaring, utilisation, and doing neither
What you do with the gas changes the answer more than any factor choice.
| Route | What happens to the methane | Reporting effect |
|---|---|---|
| Burnt for energy | Converted to CO2, minus a small residual | Scope 1 residual + biogenic memo; displaces other fuel |
| Flared | Converted to CO2, no energy recovered | Same emission treatment, no displacement benefit |
| Vented or leaked | Released as methane | The worst case by a wide margin |
The reason venting is so much worse is global warming potential: methane released intact is counted at many times the warming of the carbon dioxide it would have become. Burning biogas is not primarily about the energy. It is about destroying the methane.
Flaring therefore has a real climate benefit even though it produces nothing useful, which is why a flare running during a CHP outage is good practice rather than waste.
Biogas, landfill gas and biomethane
| Term | What it is | Key difference |
|---|---|---|
| Biogas | Raw gas from a sealed digester | Contained and collected by design |
| Landfill gas | The same reaction under a landfill cap | Partly uncontrolled; capture is never complete |
| Biomethane | Biogas upgraded to near-pure methane | Grid-injectable; the claim depends on chain of custody |
DEFRA publishes the biogenic CO2 memo for landfill gas at the same per-kWh value as biogas, which makes sense — it is the same gas — but the two are not interchangeable in an inventory, because the question of whose emission it is differs entirely between a digester you operate and a landfill you sent waste to.
Worked example — 400,000 kWh of digester gas
An on-site digester produces 400,000 kWh of biogas in a year, all of it burnt in a CHP unit.
| Line | Calculation | Goes to |
|---|---|---|
| Scope 1 | 400,000 × 0.00022 = ~88 kg CO2e | Inside the total |
| Biogenic CO2 | 400,000 × 0.19902 = ~79.6 t CO2 | Memo, outside the scopes |
| Well-to-tank | 400,000 × 0.01851 = ~7.4 t CO2e | Scope 3 Category 3 |
The number inside the total is 88 kilograms. The number outside it is 79.6 tonnes. Both are correct, both are required, and reporting either one in the other’s place misstates the inventory by roughly three orders of magnitude. Tonnages are rounded.
Common confusions
| The belief | What is actually true |
|---|---|
| Biogas has an emission factor | It has three, on three sheets, for three lines of the report. |
| Biogas is zero carbon | Its Scope 1 figure is very small, not zero, and its biogenic CO2 is roughly 900 times larger. |
| The biogenic CO2 can be ignored | It is a required memo disclosure. Omitting it makes the inventory incomplete. |
| Flaring is wasteful and therefore bad | Flaring destroys methane. It is far better than venting, even though it recovers nothing. |
| Biogas and biomethane are the same | Biogas is roughly half CO2. Upgrading it to biomethane costs energy and leaks some methane. |
Frequently Asked Questions
There are three, and which one you want depends on the line you are filling in. The Scope 1 combustion figure covers the residual methane and nitrous oxide only. The biogenic carbon dioxide figure is far larger and is reported outside the scopes as a memo item. The well-to-tank figure covers producing and delivering the gas and belongs in Scope 3 Category 3. A source offering a single biogas factor has not told you which of the three it is.
Because they measure different things. Burning biogas converts its methane to carbon dioxide, and that carbon dioxide is the bulk of the emission by mass. It is excluded from the scope totals because the carbon was recently taken from the atmosphere by a plant. What remains inside Scope 1 is only the small quantity of methane that escapes combustion unburnt plus the nitrous oxide formed in the process — roughly one nine-hundredth the size.
Match the basis to your evidence. Use per kWh where a gas meter reads energy, which covers most on-site combustion in a boiler or CHP unit. Use per tonne where your records are mass, which is usual when gas is transferred or sold. The two are linked by calorific value, and biogas calorific value depends on its methane content, so do not borrow a generic conversion — an auditor will ask to see the meter reading or weighbridge ticket behind the figure you used.
Substantially, and it is worth running a flare even when no energy is recovered. Flaring converts methane to carbon dioxide; venting releases the methane intact, and methane is counted at many times the warming effect of the carbon dioxide it would have become. A flare operating during a generator outage is good practice rather than waste, and an inventory should record the difference rather than treat all uncombusted gas alike.
Chemically they are the same gas from the same reaction, and DEFRA publishes the biogenic carbon dioxide memo at the same value per kWh for both. In an inventory they are not interchangeable, because the ownership question differs. Biogas from a digester you operate is your direct emission. Landfill gas arises from waste you sent elsewhere, which places it in Scope 3 and makes capture rate, not combustion, the decisive variable.
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