Well-to-wake vs tank-to-wake

The boundary determines what the number means.

Tank-to-wake describes emissions associated with using a fuel on board. Well-to-tank describes its upstream supply chain. Well-to-wake brings those stages together. A comparison is only meaningful when the boundaries and calculation methods match.

IMO's lifecycle guidance considers CO₂, methane and nitrous oxide across marine-fuel pathways. It separates production and delivery from onboard use so that a reduction in one stage does not conceal emissions in another. [1]

Well-to-tank · WtT

Upstream stages such as feedstock extraction or cultivation, production, processing, transport and distribution to the ship.

Tank-to-wake · TtW

The onboard use stage, including the greenhouse gases relevant to the fuel and conversion technology.

Well-to-wake · WtW

The combined lifecycle boundary, bringing upstream and onboard components onto a consistent basis.

A lifecycle value usually expresses emissions per unit of energy. The unit gCO₂e/MJ means grams of carbon dioxide equivalent per megajoule. It describes intensity. A figure in tonnes CO₂e describes an emissions quantity. Switching between the two requires an explicit energy quantity.

One consistent calculation

Assume a hypothetical fuel batch with 1,000 GJ of energy. Its upstream intensity is 20 gCO₂e/MJ and its onboard intensity is 40 gCO₂e/MJ. Both components use the same energy basis, greenhouse-gas equivalence method and system boundary assumptions.

The total already contains both components. Adding all three rows would double-count the emissions. The numbers are teaching inputs, not default factors, a certified fuel pathway or a statutory benchmark.

Five checks before comparing fuels

1. Match the boundary. Compare WtW with WtW. An upstream-inclusive value and an onboard-only value answer different questions.
2. Match the energy basis. Record the lower calorific value and units used. A tonne of one fuel does not necessarily contain the same energy as a tonne of another.
3. Match the gases and equivalence method. State which gases are included and the method used to express them as CO₂ equivalent.
4. Identify the fuel pathway and technology. Feedstock, production energy, distribution and onboard conversion can affect the result. A fuel name alone is an incomplete description.
5. Trace the factor. Distinguish default from measured or certified values. Record the source, version and conditions under which the value may be used.

Unburned methane passing through an engine is commonly described as methane slip. Omitting a relevant non-CO₂ gas can distort a climate comparison even if the CO₂ calculation is correct. An engine-specific measurement and a generic factor should not be substituted without checking the governing method. [1] [2]

Physical emissions and regulatory treatment

EU ETS maritime uses direct emissions under its accounting rules. FuelEU Maritime assesses the annual lifecycle GHG intensity of onboard energy. Their different boundaries are one reason a single fuel value cannot simply be copied into both calculations. [3]

A fuel may qualify for a particular regulatory accounting treatment if the prescribed evidence and conditions are satisfied. That treatment does not establish that the fuel has physically zero exhaust emissions or zero lifecycle emissions. Keep the measured activity, physical interpretation and regulatory adjustment identifiable in the record.

The EU ETS and FuelEU comparison explains the separate compliance mechanisms. The calculation above does not determine allowances, FuelEU surplus or a tradeable carbon credit.

What should accompany an intensity claim?

A usable claim should identify its boundary, value and unit; the fuel batch and pathway; the energy basis; the method and factor source; and the scope of any supporting verification. If those elements are missing, record the gap before relying on the number.

For example, “40 gCO₂e/MJ” is insufficient by itself. Is it the upstream component or the lifecycle total? Which assessment version applies? Does the evidence cover the delivered batch? A small ambiguity at this stage can become a material disagreement in reporting or settlement.

The Carbon Basel Accord's proposed Global Measurement pillar addresses this comparability problem. Its digital fuel passport concept would keep the interpretation attached to the evidence. The existing worked example follows a correction from an outdated fuel assessment through all three pillars.