A marine fuel cannot be judged at the funnel alone. Ammonia burns with no carbon dioxide coming out of the stack, yet ammonia made from unabated natural gas can carry a heavier climate burden than the heavy fuel oil it was meant to replace, because almost all of its emissions happen before the fuel ever reaches the ship. This is the problem well-to-wake (WtW) accounting exists to solve, & it is why both the IMO Net-Zero Framework and FuelEU Maritime score fuels across their whole life cycle rather than at the point of combustion. This article is the hub for the well-to-wake fuel-pathways cluster: it sets out how the life-cycle accounting works, what the IMO LCA Guidelines and the FuelEU Annex do with it, and why the same fuel name can span a range of intensities, then routes down to the fourteen leaf articles that carry the numbers for each fuel. The cluster sits under decarbonization and alternative fuels and feeds the regulatory metrics in the IMO Net-Zero Framework and GFI and EU maritime carbon pricing.
The accounting has one logic worth stating once. A fuel’s climate impact is the sum of everything emitted to make it and everything emitted to burn it, divided by the useful energy it delivers. Make that into a single number, in grams of CO2-equivalent per megajoule, and fuels of wildly different chemistry & energy density line up on one scale. The questions that recur at every step are the same four: where do the upstream emissions come from, what comes out of the engine, which greenhouse gases count and at what weight, and how was this particular batch actually produced. Hold those four and the whole framework reads as one method applied fuel by fuel.
Well-to-tank, tank-to-wake, well-to-wake
The life cycle of a marine fuel splits into two stretches, and the boundary between them is the ship’s bunker tank. Well-to-tank (WtT) covers the upstream: extracting the crude or growing the feedstock, refining or synthesizing the fuel, liquefying it where needed, and shipping it to the bunker port and into the tank. Tank-to-wake (TtW) covers the onboard part: the greenhouse gases that come out of the engine when the fuel burns, plus any fuel that escapes unburned. Well-to-wake is simply WtT plus TtW, the whole chain from the well, the field, or the wind farm through to the wake behind the ship.
The reason the split matters is that the two stretches behave differently for different fuels. For a fossil oil the TtW combustion carbon dominates & the WtT refining adds a modest tail; for a green e-fuel the TtW carbon can be near zero while the WtT figure carries whatever the production took. A fuel that looks clean at the funnel can be dirty at the well, and the only honest comparison counts both. That is the conceptual core of the well-to-wake intensity deep-dive, which works the arithmetic of combining the two stretches into one number.
Why energy is the denominator
The figure is expressed per megajoule of energy delivered, not per tonne of fuel, because fuels carry very different amounts of energy in a kilogram. Heavy fuel oil holds roughly 41 megajoules per kilogram of lower heating value; methanol holds about 19.9; ammonia about 18.6; liquid hydrogen about 120. A ship burning ammonia needs more than twice the mass that it would burn of oil to do the same work, so comparing the two per tonne of fuel tells you nothing useful. Dividing the life-cycle emissions by the energy actually delivered normalizes the energy-density difference out, & gCO2eq/MJ becomes the common currency. Both the IMO LCA Guidelines adopted under MEPC.376(80) and the FuelEU Maritime Annex use this unit for that reason.
The CO2-equivalent and the GWP100 basis
The “eq” in gCO2eq does real work. Combustion and the upstream chain release three greenhouse gases that matter here: carbon dioxide, methane, and nitrous oxide. They warm the planet at very different rates per kilogram, so each non-CO2 gas is converted into the mass of CO2 that would warm equally over a chosen horizon, using its global warming potential. The IMO LCA Guidelines & FuelEU Maritime both use the 100-year horizon (GWP100) on the values from the IPCC Fifth Assessment Report (AR5): carbon dioxide counts as 1, methane as 28, and nitrous oxide as 265. So a gram of methane in the life cycle is scored as 28 grams of CO2-equivalent, and a gram of nitrous oxide as 265.
The choice of a 100-year horizon is itself a policy decision with consequences, because methane is a short-lived gas that warms far harder over 20 years than over 100. A 20-year horizon (GWP20) puts methane at roughly 80 to 84 rather than 28, which would change the relative standing of LNG sharply. The regulators settled on GWP100 for the binding metrics, and the methane-slip question is treated separately in the dedicated methane slip and N2O hub and the narrower methane slip deep dive. The contrast between the two horizons is exactly what the LNG well-to-wake calculator surfaces, scoring Otto and diesel-cycle slip side by side, & it is why an LNG figure quoted without a horizon is incomplete.
What goes into the well-to-tank number
The WtT figure is not a single measurement; it is a sum of emissions across each stage of the supply chain, built up in a life-cycle inventory under the ISO 14040 framework. For a fossil oil the stages are extraction of the crude, transport to the refinery, refining into the fuel grade, and distribution to the bunker port, with the refinery step carrying the largest share because hydrotreating and cracking burn energy. For LNG the chain adds two heavy steps that oil does not have: liquefaction, which chills the gas to minus 162 degrees Celsius and consumes a meaningful fraction of the energy in the cargo, and the venting and flaring of methane along the extraction and transport chain, which is counted at the GWP100 weight of 28. This is why the FuelEU Annex II WtT default for LNG (18.5 gCO2eq/MJ) sits above the HFO default (13.5) and the MGO default (14.4) even before any onboard methane slip is counted.
For the synthesized fuels the WtT figure is dominated by the carbon intensity of the input energy. Green hydrogen made by electrolysis carries almost no WtT carbon when the electricity is renewable, but the same electrolyzer running on a grid that is half coal produces hydrogen dirtier than steam reforming, because the WtT emissions track the grid’s gCO2/kWh directly. The synthesis steps that turn hydrogen into ammonia or methanol add their own energy demand on top. So the WtT number for a synthetic fuel is really a statement about the electricity behind it, which is the reason the IMO LCA Guidelines and FuelEU both insist on certified, documented supply-chain data before a low WtT figure can be claimed for these fuels.
Where each input value comes from
A practitioner assembling a ship’s attained intensity pulls the inputs from specific documents, not from a general table. The WtT default factors for fossil fuels come straight from FuelEU Maritime Annex II or the IMO LCA Guidelines default tables, keyed to the fuel grade. The TtW combustion factors (the CO2, CH4, and N2O emitted per gram of fuel burned) also come from those Annexes, keyed to the fuel and, for gas, the engine type. The lower heating value that converts a tonne of bunkers into megajoules of energy comes from the same Annex or from the bunker delivery note for the actual stem. The methane-slip percentage comes from the engine’s Annex classification or, where a maker has measured it, from a certified engine test. The energy consumed on the voyage comes from the ship’s own fuel-consumption records reported under the EU MRV or the IMO Data Collection System. Each value has a documented source, and a compliance figure built on guessed inputs does not survive verification.
Methane slip and the nitrous-oxide tail
The tank-to-wake side is not just combustion CO2. Two other gases ride along, and for some fuels they decide the verdict. Methane slip is the unburned methane that passes through an engine without combusting, & it is the central weakness of gas-fueled propulsion. A low-pressure dual-fuel engine running the Otto cycle slips more methane than a high-pressure diesel-cycle engine, because the lean premixed charge leaves methane in the cylinder crevices and in the valve-overlap scavenge. FuelEU Maritime assigns engine-specific slip percentages in its Annex II, ranging from about 1.7% for a slow-speed diesel-cycle engine up to about 3.1% for a four-stroke medium-speed Otto-cycle engine, and each slipped gram of methane is then multiplied by 28 in the WtW sum.
That multiplier is why LNG’s funnel advantage shrinks once the life cycle is counted. LNG burns with roughly 25% less CO2 per unit energy than heavy fuel oil at the point of combustion, but the upstream liquefaction and the engine methane slip claw a large part of that back, and on a high-slip engine the well-to-wake gap against oil can close to single digits. The four engine families that FuelEU Annex II distinguishes for gas (slow-speed diesel-cycle, medium-speed four-stroke Otto, lean-burn spark-ignited, and low-pressure dual-fuel) each carry their own slip default, and the spread between the cleanest and the dirtiest is the difference between LNG comfortably beating oil and barely matching it. The two main families are pulled apart in LNG well-to-wake intensity, Otto versus diesel cycle, and the bio-derived version that cuts the upstream carbon is in bio-LNG well-to-wake intensity.
Methane slip is also where the GWP horizon bites hardest. The same slipped methane scored at 28 (GWP100) reads at about 80 on GWP20, so a regime that adopted the short horizon would push every gas-fueled ship’s attained intensity up sharply and could flip LNG from a compliant choice to a failing one on the high-slip engines. This is the reason the choice of horizon is not a technicality but a lever on the whole gas fleet’s compliance position, and why the per-fuel LNG articles state the horizon with every figure. Nitrous oxide is the smaller but real tail: a combustion product of nitrogen-bearing fuels & lean burn, scored at 265, and a particular watch-item for ammonia, where incomplete combustion and after-treatment can generate N2O that undoes part of the carbon-free combustion benefit. An ammonia engine that controls NOx with a selective catalytic reducer must do so without generating N2O across the catalyst, because a small N2O slip at a 265 weight can offset a large slice of the zero-carbon combustion gain.
The IMO LCA Guidelines: default versus certified pathways
The IMO adopted the Guidelines on the Life Cycle GHG Intensity of Marine Fuels under resolution MEPC.376(80) on 7 July 2023, and they are the method behind the GHG Fuel Intensity (GFI) metric in the IMO Net-Zero Framework. The Guidelines classify fuels into pathways by feedstock, origin, production route, and the energy used to make them, and they set out how to compute a well-to-wake intensity for each. The framework gives a fuel two ways to get a number, and the difference between them is where most of the practical decisions sit.
A default value is the figure the Guidelines assign to a representative pathway when no audited supply-chain data is available. The defaults are set on conservative assumptions, meaning they are deliberately on the high-emission side so that a fuel cannot claim a benefit it has not proved. A certified pathway value is the actual intensity of a specific fuel batch, calculated on real supply-chain data and verified through a recognized sustainability certification scheme against the Guidelines’ methodology. The gap between the two can be large for renewable fuels: a generic default for a bio or e-fuel pathway is far higher than a genuinely clean, audited batch, so producers of low-carbon fuel pursue certification to claim the real number. The same default-versus-certified structure governs FuelEU Maritime, where a ship using fossil fuel takes the Annex II default & a ship using a renewable fuel must hold certification to claim a lower WtT figure. The methodology that ties this into the GFI metric is worked in the marine GFS methodology.
The certification runs through a voluntary scheme recognized for the purpose, audited against the Guidelines’ calculation rules, that issues a proof of sustainability for each batch tying the fuel to its documented feedstock and production route. The proof states the certified WtT intensity, and the ship presents it to claim the lower figure; without it, the ship falls back to the conservative default. For a renewable fuel the difference decides compliance, because the default for a generic bio or e-fuel pathway can sit several times above a genuinely clean certified batch. The certification also guards against double-counting, so the same renewable molecule cannot be claimed for compliance credit in two places at once, which is a real risk where a fuel could be claimed under both a road-transport renewable scheme and the marine one.
The Guidelines rest on the standard life-cycle-assessment framework codified in ISO 14040 and ISO 14044: define the goal and scope, set the system boundary, build the life-cycle inventory of flows, then translate those into impact. The maritime application fixes the functional unit (a megajoule of energy delivered to the ship), the system boundary (well to wake), and the impact category (GWP100 CO2-equivalent), so that every fuel is scored on the same terms. The Guidelines also fix how a biogenic carbon flow is treated: the CO2 released when a biofuel burns is counted as zero in the TtW step because it was taken up from the atmosphere when the feedstock grew, so a biofuel’s climate burden lands almost entirely in its WtT cultivation, land-use, and processing emissions. That convention is the reason a waste-feedstock biofuel and a virgin-crop biofuel of the same chemistry can sit at opposite ends of the intensity scale.
Feedstock dependence: the color codes
The single most important idea in well-to-wake accounting is that the fuel’s name does not fix its emissions; the production route does. The industry’s color shorthand captures this for the hydrogen-derived fuels. Grey hydrogen is made by steam-methane reforming of natural gas with the CO2 vented, and it carries heavy upstream emissions. Blue hydrogen is the same reforming with carbon capture and storage on the process CO2, which cuts the upstream figure by the capture rate, typically claimed at 60% to 90%. Green hydrogen is made by electrolysis of water using renewable electricity, and its WtT carbon is near zero when the power is genuinely renewable. The same logic carries straight into the derivatives.
Ammonia, synthesized from hydrogen and nitrogen by the Haber-Bosch process, inherits the color of its hydrogen: grey ammonia from grey hydrogen is high-carbon upstream, green ammonia from green hydrogen is near-zero upstream, & blue ammonia sits between. Methanol splits three ways: fossil methanol from natural gas, bio-methanol from biomass or biogas, and e-methanol synthesized from green hydrogen and captured CO2. The per-fuel treatments work each split: ammonia grades, methanol grades, and hydrogen for the carrier itself, each backed by a pathway calculator that runs the grey-to-green span on its own inputs: the ammonia WtW calculator, the methanol WtW calculator, and the hydrogen WtW calculator. The lesson is the same across all three: a fuel that is carbon-free at the funnel is only as clean as the energy that made it, which is why the WtW figure, not the TtW figure, is the one that governs compliance.
Blue is the color that hides the most variation, because the capture rate is a claim and not a fixed number. A blue-hydrogen plant capturing 60% of its process CO2 leaves a large residual, while one capturing 90% leaves little, so the WtT figure swings with the rate and with whether the upstream methane from the gas supply was also controlled. A blue pathway that captures the reformer CO2 but ignores fugitive methane along the gas chain can still carry a high WtW figure once that methane is counted at 28. This is why the IMO LCA Guidelines and FuelEU require the blue figure to be certified on real plant data rather than assumed from a headline capture rate; the capture percentage on a brochure is not the certified WtW intensity. The same caution applies to the storage side, since capture without permanent storage moves the CO2 rather than removing it.
The green pathways are clean only to the degree the electricity is. An electrolyzer drawing from a renewable power-purchase agreement with additionality and time-matching produces hydrogen near the floor of the range, but one drawing from a mixed grid inherits that grid’s carbon intensity stage by stage. Regulators address this with strict rules on what counts as renewable electricity for fuel production, because a loose rule would let a fuel claim green status on grid power that was largely fossil. The detail of those electricity-accounting rules sits in the per-fuel hydrogen and e-fuel articles, and it is the single largest source of uncertainty in any green-fuel WtW claim.
The drop-in biofuels
Not every alternative needs a new engine. Two drop-in biofuels run in conventional diesel machinery with little or no modification, & their WtW figures turn entirely on the feedstock. FAME (fatty acid methyl ester) biodiesel, made by transesterifying vegetable oils or animal fats, has near-zero biogenic combustion carbon counted under the LCA convention but carries WtT emissions from cultivation, land-use, and processing that vary widely by feedstock; the detail is in FAME well-to-wake intensity. HVO (hydrotreated vegetable oil), a renewable diesel made by hydrotreating the same fats and oils, is a cleaner-burning paraffinic fuel with a similar feedstock dependence, worked in HVO well-to-wake intensity. Waste-and-residue feedstocks generally certify far lower than virgin crops, and a used-cooking-oil HVO can reach a deep WtW cut while a virgin-palm FAME with land-use change can be worse than fossil diesel.
Synthetic e-fuels and the energy penalty
E-fuels (also called synthetic fuels or Power-to-Liquid) are made by combining green hydrogen with captured carbon to synthesize a drop-in or near-drop-in fuel. Their WtW carbon can be near zero when the hydrogen is green and the CO2 is from a sustainable source, but they carry a heavy energy penalty: each conversion step (electrolysis, synthesis, liquefaction) loses energy, so the renewable electricity needed per megajoule of fuel delivered is several times the energy in the fuel itself. That penalty does not show up in the gCO2eq/MJ figure but it dominates the cost and the renewable-power demand, and it is the subject of e-fuel well-to-wake intensity. The conventional oils that e-fuels aim to replace, the incumbent baselines, sit in VLSFO and MGO well-to-wake intensity and HFO well-to-wake intensity, while the petroleum-gas option is in LPG well-to-wake intensity.
Representative well-to-wake intensities by pathway
The table gives representative well-to-wake intensities in gCO2eq/MJ for the main marine-fuel pathways, on the GWP100 (AR5) basis the IMO LCA Guidelines and FuelEU Maritime both use. The fossil-fuel figures align with the FuelEU Maritime Annex II default factors, where the WtT default is fixed and the TtW combustion adds to it: HFO carries a 13.5 gCO2eq/MJ WtT default, MGO 14.4, and fossil LNG 18.5, before the combustion CO2, CH4, and N2O are added. The FuelEU Maritime 2020 reference baseline against which reduction targets are set is 91.16 gCO2eq/MJ, which anchors the conventional-oil column. The renewable and zero-carbon pathways are shown as ranges because they depend on the feedstock & the production route, and a certified clean batch lands far below a conservative default. Treat the ranges as illustrative of the method; the binding figure for any specific batch is the certified pathway value or the regulatory default, not a representative number.
| Fuel and pathway | WtW intensity (gCO2eq/MJ) | Basis and attribution |
|---|---|---|
| HFO (heavy fuel oil) | about 91.6 | FuelEU Annex II: 13.5 WtT + combustion TtW |
| VLSFO / MGO (marine gas oil) | about 91 to 94 | FuelEU Annex II: 14.4 WtT (MGO) + combustion; baseline 91.16 |
| LPG | about 75 to 80 | FuelEU Annex II default factors |
| Fossil LNG, diesel-cycle (low slip) | about 76 to 84 | FuelEU Annex II: 18.5 WtT + TtW with engine-specific slip |
| Fossil LNG, Otto-cycle (high slip) | about 84 to 90 | FuelEU Annex II: 18.5 WtT + higher methane-slip TtW |
| Fossil methanol | about 95 to 100 | FuelEU Annex II default factors |
| Grey ammonia (fossil-gas hydrogen) | about 100 to 120 | IMO LCA / FuelEU default, unabated pathway |
| Grey hydrogen (steam reforming) | about 100 to 130 | IMO LCA default, unabated reforming |
| FAME / HVO (waste feedstock) | about 15 to 35 | Certified pathway, feedstock dependent |
| Bio-LNG (biomethane) | about 10 to 35 | Certified pathway, feedstock dependent |
| Bio-methanol | about 5 to 30 | Certified pathway, feedstock dependent |
| Blue ammonia / blue hydrogen | about 20 to 60 | Certified pathway, capture-rate dependent |
| Green ammonia / green hydrogen | near 0 to 20 | Certified renewable-electrolysis pathway |
| E-methanol / e-fuel (green H2 + captured CO2) | near 0 to 25 | Certified Power-to-Liquid pathway |
| Nuclear marine propulsion | near 0 (operational) | No combustion GHG; see leaf article |
The pattern in the table is the substance of the whole cluster. The fossil column clusters near the 91 gCO2eq/MJ baseline because the combustion CO2 dominates and the upstream is a modest tail. The carbon-free-at-the-funnel fuels (ammonia, hydrogen) span from worse-than-oil to near-zero depending entirely on whether they are grey or green, which is the single clearest demonstration that the funnel reading is not the climate reading. The bio & e-fuel rows reach deep cuts only on certified clean feedstocks and pathways. Nuclear marine propulsion carries no combustion greenhouse gas and is treated separately in nuclear well-to-wake intensity, where the life-cycle boundary covers the fuel cycle and the construction rather than combustion.
When a ship burns more than one fuel, as a dual-fuel vessel running gas with a liquid pilot does, the vessel’s overall intensity is the energy-weighted average of the fuels’ WtW intensities. That blending arithmetic, which is how a compliance figure is actually assembled for a mixed-fuel voyage, is set out in the blend methodology and runs directly in the WtW blend calculator, which takes the per-fuel intensities and energy shares and returns the energy-weighted whole-ship figure.
Reading the table against the funnel reading
The contrast that runs through the table is the gap between what a fuel does at the funnel and what it does across the life cycle. Take fossil LNG against heavy fuel oil. At the point of combustion LNG cuts CO2 by about a quarter per unit energy, which would put it near 68 gCO2eq/MJ if combustion were all that mattered. Add the 18.5 gCO2eq/MJ WtT default for liquefied gas and the methane slip at GWP100, and a high-slip four-stroke installation climbs back toward 90, almost level with oil. The same molecule on a low-slip slow-speed diesel-cycle engine lands lower, in the high-70s to mid-80s, because the slip term is smaller. One fuel, two engine choices, a swing of a dozen points or more, and none of it visible at the funnel.
The ammonia and hydrogen rows make the point harder still. Both burn with zero combustion CO2, so a funnel-only reading scores them at zero. The WtW reading scores grey ammonia at 100 to 120, worse than the oil it replaces, because the hydrogen behind it came from unabated natural gas reforming and the synthesis added more fossil energy. The green versions of the same fuels reach near zero. The entire span, from worse-than-oil to near-zero, comes from the production route, which is the case for measuring well to wake rather than at the stack. A regulator that scored only the funnel would reward grey ammonia and grey hydrogen for emissions they merely moved upstream.
How the metric drives the regulation
Well-to-wake intensity is not an academic figure; it is the quantity two binding regimes measure a ship against. The IMO Net-Zero Framework sets a GHG Fuel Intensity (GFI) limit that tightens over time, and a ship’s attained GFI is its energy-weighted WtW intensity computed on the LCA Guidelines; the attained GFI calculator builds that number from a ship’s fuel mix and checks it against the tier limit. A ship above the limit must surrender units; a ship below it can earn them. FuelEU Maritime runs a parallel mechanism for voyages touching the European Union, measuring the same WtW intensity against a baseline of 91.16 gCO2eq/MJ that reduces in steps: 2% from 2025, rising to 6% in 2030 and steeper cuts to 2050, with a penalty on the gap, which the FuelEU GHG-intensity calculator scores against the reducing target. Both regimes are worked in the IMO Net-Zero Framework and GFI and EU maritime carbon pricing.
The consequence for fuel selection is direct. A shipowner choosing between LNG and a bio or e-fuel is not comparing combustion carbon; the regulator scores the WtW number, so a high-slip LNG installation that looked clean at the funnel can fall short of a target a certified bio-methanol meets easily. This is why the per-fuel pathway numbers, not the marketing labels, decide a newbuild’s fuel strategy, and why the same fuel can be a compliant choice on one production route & a stranded asset on another. The fuels themselves, their handling, and their engine implications are the subject of the alternative marine fuels hub, which this cluster supplies with the life-cycle numbers.
The one place the WtW basis does not yet reach is the EU Emissions Trading System (ETS), which prices a ship’s CO2 on a tank-to-wake basis rather than well to wake. So a ship pays for ETS allowances on the carbon out of its funnel while FuelEU Maritime and the IMO GFI score it on the full life cycle, which means a grey-ammonia ship that emits no funnel CO2 escapes the ETS bill on that fuel even though its WtW intensity is high. That mismatch between a TtW-priced carbon market and a WtW-scored fuel standard is a known seam in the regime, and it is treated in EU maritime carbon pricing. The three instruments also differ in geographic scope: the IMO GFI applies globally to ships above the size threshold on international voyages, FuelEU Maritime applies to energy used on voyages into, out of, and within the EU on a 50% or 100% basis depending on the leg, and the EU ETS follows the same EU-linked scope. A single ship trading worldwide can therefore face all three at once, each scoring a different slice of the same fuel on a different boundary.
Why the same WtW number can win in one regime and lose in another
The regimes share the WtW intensity metric but set their limits and penalties differently, so a fuel that clears one can miss another. FuelEU Maritime measures the gap below its reducing baseline and applies a penalty in euros per tonne of CO2-equivalent on the shortfall, with a pooling and banking mechanism that lets an over-compliant ship lend credit to an under-compliant one. The IMO GFI sets a tiered limit with a remedial-unit cost on the gap above the limit and a reward for ships below it. A certified e-methanol batch near 10 gCO2eq/MJ clears both with room to spare; a grey-ammonia stem at 110 fails both badly. The interesting cases are the fuels in the 70-to-90 band, where a low-slip LNG installation can sit just inside one regime’s current limit and just outside another’s, and where the year matters because the baselines step down on a schedule. The blend methodology that combines fuels across a mixed voyage, worked in the blend methodology, is how an operator manages a ship into the compliant band by mixing a small share of certified renewable fuel with the conventional bulk.
How the fourteen leaf articles fit together
This hub is the map; the numbers live one level down. The conceptual anchor is well-to-wake intensity, which works the WtT-plus-TtW arithmetic and the GWP100 conversion in full. The incumbent baselines sit in VLSFO and MGO well-to-wake intensity and HFO well-to-wake intensity, the figures every alternative is measured against. The gas pathways run through LNG, Otto versus diesel cycle and its renewable counterpart bio-LNG, with LPG as the petroleum-gas option.
The carbon-carrier fuels each get their own treatment because each spans a grey-to-green range: methanol grades, ammonia grades, and hydrogen. The drop-in biofuels are in FAME and HVO, the synthetic route in e-fuels, and the no-combustion option in nuclear marine propulsion. The arithmetic that combines several fuels on one voyage is in the blend methodology. Read together, the fourteen articles let an owner or charterer assemble a ship’s attained WtW intensity fuel by fuel & route by route, then check it against the IMO GFI and FuelEU targets the regulation sets.
The order to read them in follows the decision a real fuel choice makes. Start with well-to-wake intensity for the method, then the two oil baselines to fix the reference point every alternative is measured against. Read the candidate fuel’s own article next for its grey-to-green span and its WtT drivers, and the methane slip deep dive if the candidate is gas. Finish with the blend methodology to see how a small renewable share moves the whole-ship figure, because in practice most ships will not switch fuel outright but blend a certified low-carbon fraction into a conventional bulk to step down the attained intensity year by year as the baselines tighten. The cluster is built to support that incremental decision, not a one-shot fuel swap.
Limitations
This article maps the well-to-wake accounting method and gives representative pathway intensities; it is not a substitute for the IMO LCA Guidelines text adopted under MEPC.376(80) and its later amendments, the FuelEU Maritime Annex factors, or the certified pathway documentation for a specific fuel batch. The representative figures here are illustrative of where each pathway sits on the gCO2eq/MJ scale; the binding number for compliance is the regulatory default for the fuel or the certified pathway value verified for the actual batch, and these differ. A fuel scored on a conservative default can be far worse than the same fuel certified, and a producer’s certified claim depends on the integrity of the certification scheme behind it.
The GWP100 (AR5) basis used by both regimes is a policy choice that understates short-term methane warming relative to a 20-year horizon; a WtW figure that looks favorable for LNG on GWP100 reads differently on GWP20, and the methane-slip percentage assigned to an engine is itself an Annex default that a real engine can beat or miss. The renewable-pathway ranges in the table depend on feedstock, land-use, & the carbon intensity of the production electricity, all of which vary by source and region and change as supply chains develop. The IMO LCA Guidelines and the FuelEU factors are themselves under periodic revision through the IMO Marine Environment Protection Committee and the European Commission, so the current instrument and its in-force amendments govern, not any single figure quoted here. None of the per-fuel articles replaces a verified pathway calculation for a specific bunker stem on a specific voyage.
See also
- Well-to-wake intensity: the WtT-plus-TtW arithmetic and the GWP100 conversion in full.
- VLSFO and MGO well-to-wake intensity: the conventional-oil baselines every alternative is measured against.
- HFO well-to-wake intensity: heavy fuel oil on the well-to-wake basis.
- LNG well-to-wake intensity, Otto versus diesel cycle: how engine type and methane slip set the LNG figure.
- Bio-LNG well-to-wake intensity: biomethane as the renewable gas pathway.
- LPG well-to-wake intensity: liquefied petroleum gas on the well-to-wake basis.
- Methanol grades well-to-wake intensity: fossil, bio, and e-methanol split out.
- Ammonia grades well-to-wake intensity: grey, blue, and green ammonia.
- Hydrogen well-to-wake intensity: grey, blue, and green hydrogen.
- FAME well-to-wake intensity: biodiesel and its feedstock dependence.
- HVO well-to-wake intensity: hydrotreated vegetable oil as renewable diesel.
- E-fuel well-to-wake intensity: synthetic Power-to-Liquid fuels and the energy penalty.
- Nuclear marine propulsion well-to-wake intensity: the no-combustion option and its life-cycle boundary.
- Blend methodology: the energy-weighted average for a multi-fuel voyage.
- Decarbonization and alternative fuels: the parent subject hub.
- IMO Net-Zero Framework and GFI: the GHG Fuel Intensity metric the WtW number feeds.
- EU maritime carbon pricing: FuelEU Maritime and the EU ETS that score the same WtW intensity.
- Alternative marine fuels: the fuels themselves, their handling, and engine implications.
- Methane slip deep dive: the unburned-methane problem behind the LNG figures.
- Marine GFS methodology: how the WtW intensity ties into the GHG Fuel Standard.