Combustion Emissions — Definition and GHG Accounting Context
Almost everything about modern life runs on fire. We burn gas to heat buildings, diesel to move goods, coal and gas to make electricity, jet fuel to fly. Each of those flames turns the carbon locked in a fuel into carbon dioxide and sends it into the atmosphere — and added together, that is the largest single reason the climate is changing.
The emissions from all that burning have a name. Combustion emissions are the greenhouse gases released when a fuel is burned — chiefly the carbon dioxide from oxidising the carbon in the fuel.
Combustion emissions are the greenhouse gases produced when a fuel is burned: mainly CO₂ from the carbon in the fuel, plus small amounts of methane and nitrous oxide. Burning fossil fuels this way produces around 90% of global CO₂ emissions. They scale with the fuel’s carbon content — coal emits far more per unit of energy than natural gas — and are distinct from process and fugitive emissions. In accounting they are almost always Scope 1: stationary combustion in boilers, and mobile combustion in vehicles.
Definition — Greenhouse Gases from Burning Fuel
Combustion emissions are the greenhouse gases released when a fuel is burned to produce energy. In combustion, the carbon and hydrogen in a fuel react with oxygen and release heat; the carbon ends up as carbon dioxide (C + O₂ → CO₂) and the hydrogen as water. Every fossil fuel — coal, oil, natural gas and their derivatives — carries carbon that was locked underground for millions of years, and burning it returns that carbon to the atmosphere as CO₂. This is the mechanism behind most of the world’s emissions.
Combustion emissions are one of three broad kinds of direct emission, and it is worth keeping them apart. Combustion comes from burning fuel for energy. Process emissions come from chemical reactions that are not combustion — such as the calcination of limestone in a cement kiln. Fugitive emissions come from leaks and venting, where gas escapes without being burned at all. A single site can have all three; a cement kiln, for instance, has both combustion emissions from its fuel and process emissions from its raw material.
Definition at a glance
| What they are | Greenhouse gases released by burning a fuel for energy |
|---|---|
| Main reaction | C + O₂ → CO₂ (plus H₂O from the hydrogen in the fuel) |
| Gases | Mostly CO₂; small amounts of CH₄ and N₂O, counted via GWP as CO₂e |
| Two settings | Stationary (boilers, furnaces, power) and mobile (vehicles) |
| Not the same as | Process (reactions) or fugitive (leaks) emissions |
| In accounting | Almost always Scope 1 for the burner |
The Three Gases of Combustion
Burning a fuel produces three greenhouse gases, in very different quantities. They are combined into a single CO₂-equivalent figure using each gas’s global warming potential:
| Gas | Where it comes from | Share of the CO₂e |
|---|---|---|
| CO₂ | The carbon in the fuel oxidising completely | The overwhelming majority — typically well over 99% |
| CH₄ (methane) | Incomplete combustion — a little fuel escapes unburned | Small, but its GWP is far higher than CO₂ |
| N₂O (nitrous oxide) | High-temperature reaction of nitrogen in the air/fuel | Very small, but a very high GWP |
For almost every fuel, CO₂ dominates the combustion footprint; the methane and nitrous oxide are minor contributors that a full emission factor still includes. The completeness of combustion matters: complete combustion converts the fuel cleanly to CO₂ and water, while incomplete combustion — from a poorly-tuned burner or a cold engine — produces carbon monoxide, soot and unburned methane, wasting energy and adding pollutants. Efficient, complete combustion is both cleaner and lower-carbon per unit of useful energy.
Combustion Emission Factors by Fuel
How much CO₂ a fuel emits when burned depends on its carbon content: the more carbon per unit of energy, the higher the emission factor. Comparing fuels on the same energy basis (per kWh of net calorific value) shows the ladder clearly, with live DEFRA factors:
| Fuel (combustion) | Emission factor kg CO₂e / kWh (NCV) · live, DEFRA |
|---|---|
| Industrial coal | 0.34223 |
| Fuel oil | 0.28523 |
| Diesel | 0.26071 |
| Natural gas | 0.20199 |
Natural gas emits the least CO₂ per unit of energy because much of its energy comes from hydrogen (which produces only water), while coal is almost pure carbon and emits the most — roughly two-thirds more per kWh than gas. This is why switching from coal to gas, and from gas to electrification, cuts combustion emissions. The factor is always tied to a basis — per litre, per kWh, per tonne — so a combustion factor must be applied to fuel measured on the matching basis, and never compared across different bases.
Combustion Emissions in GHG Accounting
Fuel a company burns in its own or controlled equipment is Scope 1 — split into stationary combustion (boilers, furnaces, generators) and mobile combustion (its vehicles). The stationary combustion calculator and mobile combustion calculator apply the factors by fuel. When someone else burns fuel on your behalf, the combustion lands in a different scope: the fuel burned at the power station behind your purchased electricity is your Scope 2, and fuel burned by your suppliers or by users of products you sell is Scope 3. One important split: burning biomass or biofuels releases biogenic CO₂, which is reported as a separate memo item rather than in the Scope 1 total, whereas fossil combustion CO₂ counts in full.
Worked micro-example
A building burns 10,000 kWh of natural gas in its boiler over a winter (net calorific value basis):
- 10,000 kWh × 0.202 kg CO₂e/kWh = ≈ 2,020 kg CO₂e of combustion emissions (Scope 1)
Snapshot using the live natural-gas factor above (0.20199 kg CO₂e/kWh). Almost all of it is CO₂; a fraction of a percent is CH₄ and N₂O. Producing and delivering the gas adds a separate upstream (well-to-tank) figure, and running the same heat from a heat pump would move the emissions to Scope 2 instead.
Common Confusions
- Confusing combustion with process emissions. Combustion is burning fuel; process emissions come from chemical reactions like calcination. A cement kiln has both, counted separately.
- Counting only CO₂. Combustion also releases small amounts of methane and nitrous oxide; because of their high GWP, a complete factor includes them as CO₂e.
- Comparing fuels on different bases. A per-litre factor and a per-kWh factor are not comparable. Put fuels on the same energy basis before ranking them.
- Treating biomass combustion as zero. Burning biomass still emits CO₂; it is reported as biogenic in a separate memo, not simply ignored.
- Assuming combustion is always Scope 1. It is Scope 1 only for whoever owns the equipment; the same combustion is Scope 2 (purchased electricity) or Scope 3 (suppliers, sold products) for others.
Frequently Asked Questions
Combustion emissions are the greenhouse gases released when a fuel is burned to produce energy. The carbon in the fuel reacts with oxygen to form carbon dioxide (C + O₂ → CO₂), and small amounts of methane and nitrous oxide are also produced. Carbon dioxide makes up the overwhelming majority of the total. Combustion emissions come from burning coal, oil, gas and their derivatives for heat, electricity, transport and industry, and are the largest single source of human greenhouse-gas emissions — burning fossil fuels produces around 90% of global CO₂. They are distinct from process emissions (from chemical reactions) and fugitive emissions (from leaks).
Combustion emissions come from burning a fuel for energy — oxidising its carbon to CO₂. Process emissions come from chemical or physical reactions that are not combustion, such as the calcination of limestone in cement-making, which releases CO₂ from the mineral rather than from a fuel. The distinction matters because they respond to different solutions: combustion emissions can be cut by switching to a cleaner fuel or electrifying, whereas process emissions are fixed by chemistry and need carbon capture or material substitution. Many industrial sites, such as cement and lime plants, have both — the kiln fuel produces combustion emissions and the raw material produces process emissions, and they are reported separately.
Among common fossil fuels, natural gas has the lowest combustion emissions per unit of energy, because a large part of its energy comes from burning hydrogen (which produces only water) rather than carbon. On a net-calorific-value basis, natural gas emits about 0.20199 kg CO₂e per kWh, diesel about 0.26071 kg, fuel oil about 0.28523 kg, and industrial coal about 0.34223 kg (DEFRA, live) — so coal emits roughly two-thirds more than gas for the same energy. The lowest-carbon option of all is to avoid combustion entirely, using electricity from low-carbon sources.
It depends on who burns the fuel. Fuel a company burns in equipment it owns or controls is Scope 1 — either stationary combustion (boilers, furnaces, generators) or mobile combustion (its own vehicles). When someone else burns the fuel on the company’s behalf, the same combustion falls elsewhere: the fuel burned at the power plant supplying its purchased electricity is Scope 2, and fuel burned by suppliers or by customers using its sold products is Scope 3. So the physical act of combustion is the same, but the scope depends entirely on where the boundary of ownership and control sits.
Yes. Although carbon dioxide dominates, burning fuel also releases small quantities of methane (CH₄) — from fuel that escapes unburned in incomplete combustion — and nitrous oxide (N₂O), formed at high temperatures. Both have a much higher global warming potential than CO₂, so even in tiny amounts they make a measurable contribution. A complete combustion emission factor therefore combines all three gases into one CO₂-equivalent figure using their GWP values. For most fuels the CH₄ and N₂O together add well under one percent to the CO₂ total, but they are included for accuracy and can matter more for specific fuels and technologies.