Fuel Oil — Definition and GHG Accounting Context
Every barrel of crude oil is split, by boiling point, into fractions — the light stuff at the top becomes petrol and jet fuel, and something heavy and tar-like is left at the bottom. That residue does not go to waste: thickened, sometimes pre-heated to flow at all, it fires the boilers of factories and power stations and drives the great diesel engines of ocean-going ships.
That residue is fuel oil. Fuel oil is a heavy petroleum-derived liquid fuel burned for heat, steam, power or propulsion — in its strict sense the thick residual fraction left after the lighter products have been distilled off.
Fuel oil is a heavy, viscous liquid fuel from the bottom of the crude-oil barrel, burned in industrial boilers and marine engines. UK fuel oil emits about 3.17492 kg CO₂e per litre (or ~3228.89019 kg per tonne) at combustion (DEFRA, live), plus an upstream well-to-tank factor. On an energy basis it is one of the highest-carbon liquid fuels — and, unlike pump petrol and diesel, it carries no biofuel blend, so all of its CO₂ is fossil.
Definition — The Heavy End of the Barrel
Fuel oil is a general term for the heavier liquid fuels produced from crude oil and burned to release heat — as opposed to the lighter transport fuels (petrol, jet fuel) refined from the same barrel. In its strict sense, “fuel oil” means residual fuel oil: the thick, high-viscosity fraction that remains at the bottom of the distillation column after the lighter, more valuable products have been boiled off. It is dense, high in carbon and sulphur, and so viscous that the heaviest grades must be heated before they will flow through a pump or burner.
Because fuel oil is a distillation residue rather than a precisely engineered product, it has no single chemical formula — it is a broad mixture of long-chain hydrocarbons, typically from around C₂₀ upward. That heaviness is exactly why it packs so much carbon per litre: it is denser and more carbon-rich than the middle distillates like diesel. It is used where cost matters more than cleanliness and a big steady burner can tolerate a dirty fuel: industrial boilers and furnaces, older or standby power generation, and — as marine fuel oil — the propulsion of large ships.
Definition at a glance
| What it is | A heavy petroleum-derived liquid fuel; strictly, the residual fraction left after distillation |
|---|---|
| Also called | Residual fuel oil, heavy fuel oil (HFO); marine grades are “bunker fuel” / bunker C; ASTM grades No. 5–6 |
| Burned in | Industrial boilers, furnaces, power generation, marine engines |
| Blend | None — fuel oil is 100% fossil (no bioethanol/biodiesel blend), so no biogenic memo |
| Combustion factor | ~3.17492 kg CO₂e/litre · ~3228.89019 kg/tonne (DEFRA, live) |
| In accounting | Scope 1 (own boilers = stationary; own ships = mobile), plus WTT in Scope 3 |
Distillate vs Residual — the Fuel-Oil Family
“Fuel oil” is used both narrowly and broadly, and the confusion is worth clearing up. Petroleum liquids split into two families by how they leave the refinery:
- Distillate fuel oils — the lighter, cleaner cuts that boil off and condense: kerosene / heating oil, gas oil, and diesel. Thin enough to burn without pre-heating.
- Residual fuel oils — the heavy tail left behind: high viscosity, high sulphur, cheap. This is “fuel oil” in the strict sense used by emission-factor sources.
National emission-factor sets keep these as separate line items, because their carbon and energy content differ. The table below shows the UK DEFRA family with live per-litre combustion factors — note how the heavy residual grade sits above the middle distillates:
| Line item | What it is | CO₂ per litre kg CO₂e · live, DEFRA |
|---|---|---|
| Fuel oil | Heavy residual fuel oil — boilers, power, industry | 3.17492 |
| Marine fuel oil | Residual “bunker” fuel for ships (HFO) | 3.10202 |
| Gas oil | Distillate — red diesel, off-road, some heating | 2.75541 |
| Diesel | Distillate — road transport (average blend) | 2.58354 |
| Burning oil | Distillate — kerosene for domestic heating | 2.54016 |
Because these grades all sit around 2.5–3.2 kg CO₂e/litre but are not interchangeable, a common accounting error is applying a generic “oil” factor to the wrong one. If a boiler or ship burns heavy residual oil, use the fuel oil (or marine fuel oil) factor — not the gas-oil or diesel figure. When in doubt, match the delivery note’s fuel description to the source’s exact line item.
Fuel Oil’s Emission Factors (Live)
Fuel oil’s emissions arrive at two stages — burning it, and producing and delivering it — and can be expressed per litre, per tonne, or per kWh of energy. The live DEFRA factors, split by stage:
| Stage | Per litre | Per tonne | Scope |
|---|---|---|---|
| Combustion | 3.17492 | 3228.89019 | Scope 1 |
| Well-to-tank (upstream) | 0.69539 | 714.86545 | Scope 3 |
kg CO₂e · DEFRA 2026, shown live. AR5 GWP-100 basis (the DEFRA convention).
Notice that on a mass basis (per tonne) fuel oil, gas oil and diesel are all close to ~3.2 tonnes of CO₂e per tonne of fuel — carbon content per kilogram is similar across petroleum liquids. The differences you see per litre and per kWh come from density and calorific value: fuel oil is denser (near 1 kg/litre) and slightly lower in energy per kilogram, which is why it looks heaviest per litre. This is exactly why the basis of an emission factor — per litre, per tonne, or per kWh — must always be stated.
Carbon intensity by fuel — energy basis
Combustion factors, net calorific value. DEFRA 2026 snapshot (AR5 GWP-100). Fuel oil is the highest-carbon common liquid fuel per unit of energy — only solid coal is higher, and switching a boiler from fuel oil to natural gas cuts CO₂ per kWh by roughly a third.
Fuel Oil in GHG Accounting
It depends on where it is burned. Fuel oil combusted in a company’s own or controlled boilers, furnaces or generators is stationary combustion and sits in Scope 1. Fuel oil burned to propel the company’s own or controlled ships is mobile combustion — still Scope 1, just the mobile sub-category. In every case, the well-to-tank emissions from extracting, refining and delivering the fuel are Scope 3. And because fuel oil carries no biofuel blend, there is no biogenic memo to separate out — all of its combustion CO₂ is fossil and stays in the scope total. The Scope 1 Stationary Combustion Calculator applies the boiler factors; the Marine & Aviation Fuel calculator handles fuel burned in owned ships.
Worked micro-example
A factory burns 20 tonnes of fuel oil in its steam boiler over a year. Using the DEFRA 2026 factors (snapshot):
- Combustion (Scope 1): 20 t × 3,228.89 kg/t = 64,578 kg ≈ 64.6 t CO₂e
- Well-to-tank (Scope 3): 20 t × 714.87 kg/t = 14,297 kg ≈ 14.3 t CO₂e
- Total (well-to-tank + combustion): ≈ 78.9 t CO₂e
Snapshot using DEFRA 2026 (AR5 GWP-100). The live per-tonne factors above will keep the on-page figures current; this worked chain is fixed to its stated inputs.
Common Confusions
- Treating “fuel oil” as one interchangeable product. Heavy residual fuel oil, gas oil, diesel and burning oil are separate line items with different factors — match the fuel description to the exact source row.
- Confusing fuel oil with heating oil. In the UK, domestic “heating oil” is usually kerosene (burning oil) or gas oil — the lighter distillates — not heavy residual fuel oil. In the US, “No. 2 heating oil” is essentially diesel.
- Using a per-litre factor for a mass measurement. Fuel oil is dense and usually delivered by the tonne; applying a per-litre factor to a tonnage (or vice versa) is a large error. Use the matching per-tonne factor.
- Expecting a biogenic slice. Unlike pump petrol (E10) and road diesel (B7), fuel oil has no biofuel blend — there is no biogenic portion to report separately; all its CO₂ is fossil.
- Thinking the IMO 2020 sulphur cap cut CO₂. The 0.5% sulphur limit on marine fuel oil targets sulphur-oxide air pollution, not carbon — low-sulphur fuel oil emits essentially the same CO₂ per tonne burned.
Frequently Asked Questions
Fuel oil is a heavy, viscous liquid fuel refined from crude oil and burned to produce heat, steam, power or propulsion. In its strict sense it means residual fuel oil — the thick fraction left at the bottom of the distillation column after the lighter products (petrol, jet fuel, diesel) have been boiled off. It is dense, high in carbon and sulphur, and the heaviest grades must be pre-heated to flow. Fuel oil is used mainly in industrial boilers and furnaces, in some power generation, and — as marine fuel oil or “bunker fuel” — to power large ships. Broadly, the term “fuel oil” can also cover the lighter distillate grades used for heating.
Burning UK fuel oil emits about 3.17492 kg CO₂e per litre, or roughly 3228.89019 kg CO₂e per tonne, at combustion (the DEFRA factor, shown live). Because fuel oil is dense and usually traded by mass, the per-tonne figure is the one most industrial users apply. Producing and delivering the fuel adds an upstream well-to-tank factor of about 714.86545 kg CO₂e per tonne. On an energy basis fuel oil is about 0.28523 kg CO₂e per kWh (net calorific value) — one of the highest of any liquid fuel, and well above natural gas at about 0.20199 kg per kWh.
They come from the same barrel but different fractions. Diesel (and gas oil) are middle distillates — lighter cuts that boil off and condense, thin enough to burn without pre-heating, and used in road and off-road engines. Fuel oil, strictly, is the residual fraction left behind: heavier, more viscous, higher in sulphur, and cheaper, used in big industrial burners and ships’ engines. Per tonne of fuel their combustion CO₂ is similar (both near 3.2 t CO₂e/tonne), but fuel oil is denser, so per litre it emits more — about 3.17492 kg CO₂e versus 2.58354 kg for diesel (DEFRA, live). Diesel also carries a small biofuel blend; fuel oil does not.
Broadly, yes. “Heavy fuel oil” (HFO) is the residual fuel oil burned in large marine engines, and “bunker fuel” is the shipping industry’s name for the fuel loaded into a ship’s bunkers — most of which, historically, has been heavy fuel oil (the heaviest marine grade is sometimes called “bunker C”). Strictly, “bunker fuel” is a slightly broader term that can also include lighter marine gas oil, so they are not perfectly synonymous. In emission-factor terms, ship’s residual fuel is reported under a dedicated marine fuel oil line item — about 3.10202 kg CO₂e per litre at combustion (DEFRA, live) — separate from the stationary “fuel oil” figure.
It depends on where it is burned, but it is almost always Scope 1. Fuel oil combusted in an organisation’s own or controlled boilers, furnaces or generators is stationary combustion in Scope 1. Fuel oil burned to propel the organisation’s own ships is mobile combustion — still Scope 1. In both cases the well-to-tank emissions from producing and delivering the fuel are a separate Scope 3 item. Because fuel oil has no biofuel blend, there is no biogenic portion to report as a memo — all of its combustion CO₂ is fossil and counts fully in the scope total.