Per-fuel well-to-wake intensity: bio-LNG

Background: bio-LNG as a near-drop-in renewable

Bio-LNG sits at the intersection of two mature industries. Anaerobic digestion of organic waste streams to produce biogas is a 1970s-vintage technology with several thousand operating plants across Europe, North America, and East Asia. Liquefaction of natural gas is a 1960s-vintage technology with several dozen baseload export terminals and a much larger fleet of small-scale liquefiers serving regional markets. The marine bio-LNG value chain bolts the two together: organic feedstock arrives at a digester, the resulting raw biogas is upgraded by removing carbon dioxide and trace contaminants until it meets the same Wobbe-index and methane-content envelope as pipeline natural gas, and the upgraded biomethane is liquefied either at the upgrading plant itself, at a co-located small-scale liquefier, or after injection into the gas grid and withdrawal at any liquefier connected to that grid.

The drop-in characteristic is what makes bio-LNG operationally important. A vessel built for fossil LNG and certified under the IGF Code can take a bio-LNG bunker without modification to the fuel-gas supply system, the engine control software, the boil-off management chain, or the safety case, because the molecule is methane in both cases and the upgrading specification ensures contaminant levels are within the same envelope. Methane number, lower heating value, density at minus 162 degrees Celsius, and combustion stoichiometry are all unchanged. The shipowner therefore captures the GHG benefit without a capital expenditure on the asset. That is a structurally different proposition from methanol as a marine fuel, ammonia as a marine fuel, or hydrogen, each of which requires a purpose-built fuel system.

The constraint is supply. European biomethane production reached approximately 6.4 billion cubic metres in 2024 according to the European Biogas Association, equivalent to roughly 4.6 million tonnes of LNG-equivalent if the entire output were liquefied and bunkered. The marine sector competes for that volume against road transport, gas-grid blending obligations, and industrial heat. Liquefaction capacity is the second constraint: only a fraction of biomethane production sites are equipped or contracted with a small-scale liquefier, and the volumes that physically reach a marine bunker barge in 2026 are an order of magnitude smaller than the production figure suggests. The market price reflects scarcity, with bio-LNG typically trading at a 200 to 600 percent premium to fossil LNG depending on the certification chain and the contract horizon.

Feedstock pathways

Bio-LNG can be produced from a wide spectrum of feedstocks, and the WtT intensity assigned by MEPC.391(82) Annex 1 and FuelEU Annex II depends entirely on which feedstock the certificate names. The categories matter because they determine both the default intensity value and the eligibility under the RED III feedstock caps.

Manure is the highest-impact feedstock from a GHG perspective. Liquid manure stored in open lagoons or solid manure stockpiled at the farm vents methane to atmosphere as native anaerobic decomposition proceeds. The IPCC inventory methodology for agriculture quantifies this baseline emission. Diverting the manure into a sealed digester captures that methane and combusts it as fuel, which both eliminates the baseline emission and substitutes for fossil LNG. The MEPC.391(82) Annex 1 framework recognises this through an avoided-methane credit that can drive the reported WtT intensity below zero. A typical default for manure-based bio-LNG with the credit applied is approximately minus 60 to minus 100 gCO2eq/MJ before WtT energy inputs are added back, netting to a WtT figure in the minus 20 to minus 50 gCO2eq/MJ range when liquefaction and transport energy is included.

Food waste from supermarket back-of-store collections, hospitality streams, household separate collection, and food-processing residues is a second high-value feedstock. The baseline counterfactual is typically landfill or incineration, both of which carry their own emission profile. Food-waste bio-LNG default intensities under MEPC.391(82) sit in the 15 to 25 gCO2eq/MJ WtT range, depending on the inclusion of avoided-emission credits and the energy intensity of the digestion plant. Most food-waste pathways qualify as RED III Annex IX-A advanced feedstocks.

Agricultural residue includes cereal straw, corn stover, sugar-beet pulp, fruit-processing residue, and livestock bedding. The counterfactual is field decomposition or low-value combustion. Default WtT intensities are typically 20 to 30 gCO2eq/MJ. Most agricultural residue qualifies as Annex IX-A.

Sewage sludge from municipal wastewater treatment plants is a long-established digester feedstock. Many large European treatment plants already operate on-site digesters and recover the resulting biogas for plant heat or grid injection. Liquefaction adds a step. Default WtT intensities are similar to food waste at 15 to 25 gCO2eq/MJ.

Energy crops such as maize silage, rye, sorghum, and grasses grown on agricultural land carry a higher WtT intensity because of fertiliser, diesel, and land-use emissions. Energy-crop bio-LNG defaults sit in the 35 to 55 gCO2eq/MJ WtT range. Critically, energy crops fall under the RED III food and feed crop cap of 7 percent of the renewable transport target by member state, which limits their contribution to the marine bio-LNG pool.

Woody biomass via thermal gasification is a separate technology pathway. Lignocellulosic biomass (wood chips, forest residue, short-rotation coppice) is gasified at high temperature to produce a synthesis gas, which is then methanated to produce bio-methane. Thermal gasification is at a lower technology readiness level than anaerobic digestion and is currently represented by a small number of demonstration and early-commercial plants. Default WtT intensities under MEPC.391(82) are scenario-dependent but typically 25 to 40 gCO2eq/MJ.

The feedstock label is therefore not a marketing detail. It determines the entire WtW outcome, the RED III eligibility, and the FuelEU Annex II allocation.

Anaerobic digestion vs thermal gasification

The two production routes deliver the same end molecule by very different physics, and the differences map onto cost, scalability, and feedstock flexibility.

Anaerobic digestion is a mesophilic or thermophilic biological process. Organic feedstock is fed into a sealed tank, kept at roughly 35 to 55 degrees Celsius, and decomposed by methanogenic archaea over a hydraulic retention time of 20 to 60 days depending on the feedstock and the reactor design. The output is raw biogas, typically 50 to 65 percent methane with the balance carbon dioxide and trace hydrogen sulphide, ammonia, and water. Upgrading to biomethane requires removing the carbon dioxide and contaminants, generally by water scrubbing, pressure-swing adsorption, membrane separation, or amine scrubbing. Upgraded biomethane is greater than 96 percent methane and meets pipeline gas specification.

The technology is mature, the capital cost per cubic metre of installed capacity is well-characterised, and operating risk is low. The fundamental limit is feedstock supply: a digester is a slow-throughput device and the feedstock catchment radius rarely exceeds 50 to 100 kilometres for economic transport. Plant sizes therefore range from a few hundred to a few thousand cubic metres of biomethane per hour, far below the scale of fossil LNG liquefaction trains.

Thermal gasification is a high-temperature thermochemical process. Lignocellulosic biomass is heated to 700 to 1,000 degrees Celsius in a controlled-oxygen environment, decomposing into a synthesis gas of carbon monoxide, hydrogen, methane, carbon dioxide, and water vapour. The synthesis gas is cleaned, conditioned, and routed through a methanation reactor where carbon monoxide and hydrogen are catalytically combined to form additional methane. The product gas is upgraded and liquefied like anaerobic-digestion biomethane.

Gasification can in principle access a much larger feedstock pool than anaerobic digestion because it accepts woody biomass, which anaerobic digestion cannot effectively process. Plant scales can also be larger because the reactor is energy-density-limited rather than residence-time-limited. The trade-off is technological maturity: only a handful of commercial-scale gasification-to-biomethane plants have entered service, capital costs per unit of capacity remain higher than anaerobic digestion, and operational availability data are still being established.

Both pathways feed into the same MEPC.391(82) Annex 1 default tables, but with distinct entries that recognise the different upstream emission profiles.

WtT credit mechanism: avoided unmanaged methane from manure

The negative WtT intensity that manure-based bio-LNG can attract under MEPC.391(82) is a function of an avoided-emission accounting rule, and the rule is worth understanding because it is both technically sound and politically contested.

The starting point is the IPCC inventory methodology for agriculture. Liquid manure stored in open lagoons or solid manure heaped at the farm releases methane through native anaerobic activity. The methane release rate depends on storage type, temperature, retention time, and animal species. Typical IPCC default values for liquid dairy manure are equivalent to 30 to 60 kilograms of CH4 per cow per year, which on a GWP100 basis is 840 to 1,680 kilograms of CO2-equivalent.

Diverting that manure into a sealed digester eliminates the open-lagoon release because the methane is now captured and combusted. Combustion converts methane to carbon dioxide and water, and the carbon in that CO2 is biogenic (it was photosynthesised by the cattle’s grazing pasture or feed crop), so it does not appear on the greenhouse-gas balance. The net effect is that the GHG balance of the dairy farm has improved by the full GWP100 value of the diverted methane.

The MEPC.391(82) accounting rule allocates that improvement to the bio-LNG pathway as a credit. The numerical magnitude is large because methane is 28 times more potent than CO2 on a GWP100 basis, and the avoided emission was the methane molecule itself rather than CO2. The credit is applied at the WtT stage and can drive the WtT figure below zero, sometimes substantially so.

The contested element is the counterfactual. The credit is only valid if the manure would, in fact, have been left to vent in an open lagoon absent the bio-LNG project. If the same manure would otherwise have been spread on fields immediately, dried for solid storage, or already routed to a digester for grid injection, the avoided emission is smaller or zero. MEPC.391(82) Annex 1 default values build in conservative assumptions about the counterfactual, but project-specific values certified through ISCC EU or RedCert can differ. Auditing the counterfactual is the central task of the certification body, and the resulting paperwork is what travels with the bio-LNG molecule through the mass-balance chain.

The policy implication is that manure-based bio-LNG is the only marine fuel pathway that can attract a negative WtW intensity under the current IMO and EU frameworks. That makes it disproportionately attractive on a compliance basis and explains the price premium it commands.

MEPC.391(82) Annex 1 default values

The IMO Lifecycle GHG Assessment Guidelines, adopted as MEPC.391(82) at MEPC 82 in October 2024, provide default WtW intensity values that apply to a fuel pathway in the absence of a verified actual value. The bio-LNG entries are pathway-specific and feedstock-specific.

Typical Annex 1 default values for bio-LNG, in gCO2eq/MJ on the AR5 GWP100 basis used by the IMO LCA Guidelines and treated in /wiki/per-fuel-wtw-lng-otto-diesel:

• Bio-LNG from manure (with avoided-methane credit): WtT in the range minus 70 to minus 100 gCO2eq/MJ before liquefaction and transport, netting to roughly minus 20 to minus 50 gCO2eq/MJ WtT after liquefaction; WtW after combustion CO2 (biogenic, zero) plus methane slip equivalent to the host engine technology
• Bio-LNG from food waste: WtT roughly 15 to 25 gCO2eq/MJ; WtW for an Otto-cycle engine roughly 22 to 35 gCO2eq/MJ once methane slip is added
• Bio-LNG from agricultural residue: WtT roughly 20 to 30 gCO2eq/MJ; WtW band similar to food waste
• Bio-LNG from sewage sludge: WtT roughly 15 to 25 gCO2eq/MJ
• Bio-LNG from energy crops: WtT roughly 35 to 55 gCO2eq/MJ; WtW values approach fossil LNG once methane slip is added
• Bio-LNG from woody biomass via thermal gasification: WtT roughly 25 to 40 gCO2eq/MJ

The combustion CO2 is treated as biogenic under the IMO Guidelines, which means it carries a TtW CO2 emission factor of zero on a CO2eq/MJ basis. Combustion does still produce CO2 in the funnel stack, but the carbon was photosynthesised within the prior biological cycle and is therefore counted as recycled rather than fossil. This is the same accounting logic that underpins the RED III treatment of biofuels generally.

The methane slip term is identical to the host engine technology. A bio-LNG bunker burned in an Otto-cycle four-stroke engine attracts the same slip factor as a fossil LNG bunker in the same engine, treated in /wiki/per-fuel-wtw-lng-otto-diesel. The WtW comparison between bio-LNG and fossil LNG therefore reduces almost entirely to the WtT difference, which is the entire reason bio-LNG is interesting.

A vessel using bio-LNG must declare the feedstock pathway in the SEEMP Part III and in the Statement of Compliance under the IMO Net-Zero Framework, and the declared pathway must be supported by a sustainability certificate from a recognised scheme. The default value is used unless an actual value is verified, and an actual value can only be lower than the default if the producer has invested in a robust LCA documentation chain.

FuelEU Annex II categories: Annex IX-A vs IX-B; food and feed cap

FuelEU Maritime Annex II imports the RED III feedstock taxonomy into the marine compliance regime. The two key categories for bio-LNG are Annex IX-A and Annex IX-B, and a third bucket of food-and-feed crops carries a hard cap.

Annex IX-A advanced feedstocks include manure, sewage sludge, food waste, and most agricultural residues. Bio-LNG produced from Annex IX-A feedstocks counts toward the FuelEU GHG intensity reduction target without restriction and is eligible for the lifecycle GHG savings calculations. The RED III Annex IX list is reviewed periodically and can be expanded.

Annex IX-B feedstocks include used cooking oil and animal fats Category 1 and 2. These are more relevant to bio-diesel and HVO pathways than to bio-LNG, but they appear in the same regulatory architecture and warrant noting.

Food and feed crops (maize silage, rye, sugar beet, sugar cane, cereals) are subject to the RED III 7 percent cap on the contribution toward the renewable transport target by member state. The cap was set at 7 percent in RED II and retained in RED III, with the option for member states to set lower caps. For marine bio-LNG, the practical implication is that energy-crop pathways are the marginal source: they can fill out a national bio-LNG supply once advanced feedstock streams are exhausted, but they do not enjoy the same regulatory headroom as Annex IX-A material. Maritime certificates that name energy-crop feedstock face member-state-level scrutiny on whether the cap has been respected.

The FuelEU compliance balance for a vessel using bio-LNG is calculated on the actual or default WtW intensity for the certified pathway, applied to the energy quantity bunkered, and netted against the GHG intensity limit for the year. Bio-LNG does not receive the FuelEU RFNBO multiplier, which applies only to renewable fuels of non-biological origin (typically electrolytic hydrogen and its derivatives). The bio-LNG benefit comes entirely from the low or negative WtW number, not from a multiplier.

RED III sustainability criteria: ≥80 percent GHG savings

Directive (EU) 2023/2413, the Renewable Energy Directive III recast adopted in October 2023, raised the GHG saving threshold for new biofuel and biomethane installations from the RED II value of 65 percent to 80 percent for installations starting operation after 1 January 2026. The threshold is calculated relative to a fossil fuel comparator of 94 gCO2eq/MJ, which is the same number that anchors the FuelEU intensity reduction trajectory.

The 80 percent threshold means a new bio-LNG pathway must demonstrate a WtW intensity of at most 0.20 times 94 = 18.8 gCO2eq/MJ to qualify under RED III for installations brought online from 2026 onward. Older installations that started operation before 2026 retain the prior thresholds (50 percent or 65 percent depending on commissioning date).

The threshold has practical consequences. Energy-crop bio-LNG pathways with WtT intensities of 35 to 55 gCO2eq/MJ struggle to clear the 80 percent threshold once methane slip is added, and many fall outside RED III eligibility for new plants. Manure and food-waste pathways comfortably clear the threshold, sometimes by a wide margin. The threshold therefore acts as a structural filter that pushes the new-build biomethane fleet toward Annex IX-A feedstocks.

RED III also imposes:

• Land-use sustainability criteria: feedstock cannot come from primary forest, highly biodiverse grassland, or wetland converted after 2008
• Mass balance accounting: the only allowed allocation method between physical molecules and sustainability claims at the production and trading level
• Verification by recognised certification schemes: ISCC EU, RedCert, 2BSvs, and a small number of others
• GHG saving documentation: actual values calculated under the RED III methodology or default values from the directive’s Annex VI

Bio-LNG entering the marine bunker pool must carry a proof of sustainability (PoS) issued under one of the recognised schemes. Without a PoS, the bunker reverts to the FuelEU default value for fossil LNG and the GHG benefit is forfeited.

Mass balance certification: ISCC EU, RedCert, 2BSvs

Mass balance is the accounting backbone of the bio-LNG market. The principle is simple: at any point in the supply chain, the volume of sustainable product claimed downstream must equal the volume of certified sustainable input received upstream, allowing for documented losses and conversions. The physical molecules can mix freely with non-sustainable molecules, but the sustainability attribute travels through the chain on paper, with each handover audited.

ISCC EU is the largest of the EU-recognised schemes and is the dominant certification used for marine bio-LNG bunkers in the Amsterdam-Rotterdam-Antwerp (ARA) range. ISCC EU certifies feedstock origin, production plant operations, trader chain-of-custody, and end-customer delivery. A vessel taking a bio-LNG bunker under ISCC EU receives a Sustainability Declaration that names the feedstock category, the WtW intensity, the volume in megajoules, and the chain identifier. That declaration is the document used for FuelEU and IMO Net-Zero Framework reporting.

RedCert is a German scheme widely used for biomethane injected into the German gas grid. It is recognised by the European Commission for RED II and RED III compliance and offers similar mass-balance accounting to ISCC EU, with feature emphasis on grid-injection chains.

2BSvs (Biomass Biofuels Sustainability voluntary scheme) is a French scheme historically focused on bioethanol and biodiesel but extended to biomethane. It is recognised by the European Commission and used by some traders operating across the Belgian and French grid networks.

All three schemes operate under the EU mass-balance rule as implemented through Commission Implementing Regulation (EU) 2022/996 on rules to verify sustainability and greenhouse gas emissions saving criteria. The rules require:

• Physical mixing only within a defined balancing area (often a single production plant, a single trader’s tank inventory, or a defined grid section)
• Conservation of energy quantity and sustainability attributes within the balancing area
• Audit trail from feedstock to end customer
• Annual surveillance audits by accredited certification bodies

The mass-balance allocation is the legal basis on which a marine bunker barge can deliver fossil LNG molecules to a vessel and accompany the delivery with a bio-LNG sustainability claim. The molecules in the barge tank are not physically green; they were produced as fossil LNG. The corresponding green attribute was injected into the grid (or accounted within the trader’s book) at a certified upgrading plant elsewhere, and the bunker delivery documentation transfers that attribute to the receiving vessel. This is mass balance, not book-and-claim, because the gas remains within a defined physical balancing area (the European gas grid).

Book-and-claim controversies for marine bio-LNG

The terminology distinction between mass balance and book-and-claim is central to the legitimacy of marine bio-LNG and is currently unresolved at the IMO level.

Mass balance requires the physical product and the sustainability attribute to travel through a defined balancing area together, with the attribute allocated to one or more physical deliveries totalling no more than the certified input. The European gas grid functions as such a balancing area, which is why bio-LNG bunkers in Northwest Europe can be supplied with mass-balance documentation even when the physical molecules at the bunker barge are fossil LNG.

Book-and-claim decouples the physical product entirely from the sustainability attribute. A producer in country A injects biomethane into the local gas grid, generates a certificate, and sells the certificate independently to a marine bunker buyer in country B. The buyer takes a fossil LNG bunker, retires the certificate, and reports the corresponding GHG benefit. The certificate has travelled through a market, not through a physical chain.

Book-and-claim is not currently accepted under FuelEU Maritime or under the IMO Net-Zero Framework default rules. Both regimes require mass balance through a connected physical infrastructure or a directly delivered physical bunker. The European gas grid is large enough that mass balance can stretch to most EU bunker ports, but it does not extend to ports outside the European gas grid (Singapore, Fujairah, Houston, Yokohama). A vessel calling those ports cannot currently take a mass-balanced bio-LNG bunker unless a physical bio-LNG cargo has been delivered to a local liquefier and bunker barge, which is rare outside Europe and parts of North America.

The result is a regulatory geography: bio-LNG bunkering is concentrated in ARA, Marseille, Genoa, Algeciras, and a handful of North American ports, while major Asian and Middle Eastern bunkering hubs cannot offer mass-balance bio-LNG at scale. Some industry voices argue for a managed expansion of book-and-claim to globalise access, while regulators and several member states resist on the grounds that book-and-claim invites attribute inflation, double counting, and weaker counterfactuals. The debate is active at IMO MEPC and at the European Commission.

The practical guidance for shipowners is to assume mass-balance bio-LNG is available only at recognised European and limited North American bunker ports, to verify each certificate against the recognised scheme, and to plan voyage routing accordingly when bio-LNG is part of the FuelEU compliance strategy.

Commercial supply, green premium, bunkering infrastructure

Marine bio-LNG entered the commercial supply chain in 2021 with the first ARA bunker by Titan, and has expanded to a handful of suppliers and a growing volume since.

Titan, headquartered in Amsterdam, is the largest dedicated marine bio-LNG supplier in 2026. Titan offers bio-LNG and bio-LNG blends across the ARA range and has expanded service to Marseille, Hamburg, and Gibraltar. The company’s mass-balance accounting is ISCC EU certified. Volumes supplied to the marine sector have grown from a few thousand tonnes in 2021 to several hundred thousand tonnes annually by 2026.

Shell offers bio-LNG and bio-LNG blends through its Shell Marine business, sourced from the European gas grid and supplied via its bunker barge fleet in the ARA range and Singapore (Singapore deliveries are physical fossil LNG with mass-balance allocation back to European injection points, a structure that the FuelEU framework treats with care).

Chevron Shipping has integrated bio-LNG into its fleet decarbonisation pathway and offers bio-LNG bunker delivery to its own fleet and to selected third-party customers in the European trade.

Other suppliers include Gasum (Nordic markets), TotalEnergies (Marseille and Belgium), Engie (Belgium and France), and a growing list of regional players in Spain, Italy, and Germany.

The green premium is the differential between bio-LNG and fossil LNG bunker prices. Typical 2024-2026 ranges:

• Fossil LNG at the bunker barge: USD 500 to 800 per tonne LNG-equivalent
• Bio-LNG with ISCC EU mass-balance certification: USD 1,200 to 2,400 per tonne LNG-equivalent
• Premium: 200 to 600 percent over fossil LNG, narrowing toward the lower end as supply expands

The premium is justified by the regulatory benefit. A vessel paying USD 1,500 per tonne extra for bio-LNG over fossil LNG receives a WtW intensity reduction of roughly 50 to 80 gCO2eq/MJ, which translates into a FuelEU compliance balance worth USD 700 to 1,300 per tonne at the 2026 penalty rate, plus the IMO Net-Zero Framework GFI position. The economic case sharpens as the FuelEU intensity reduction trajectory tightens past 2030 and as the IMO GFI benchmarks bite.

Bunkering infrastructure is the same physical equipment as fossil LNG: bunker barges, ship-to-ship transfer protocols, IGF-Code-compliant fuel-gas supply, and the entire safety case treated for fossil LNG. No vessel modification, no new bunker barge fleet, no parallel safety case. That is the structural advantage of bio-LNG over methanol as a marine fuel or ammonia as a marine fuel, where new bunker chains must be built from scratch.

Methane slip: identical to fossil LNG

The methane slip from a dual-fuel engine running on bio-LNG is physically identical to the slip from the same engine running on fossil LNG, because the molecule is methane in both cases and the combustion chemistry is the same. The full slip discussion in /wiki/per-fuel-wtw-lng-otto-diesel applies without modification.

The numerical impact is significant. An Otto-cycle four-stroke engine slipping 3 to 4 percent of fuel energy as unburned methane attracts a methane-slip contribution of approximately 25 to 35 gCO2eq/MJ on the AR5 GWP100 basis used by MEPC.391(82) and FuelEU Annex II. That contribution adds to the bio-LNG WtW number, eating into the WtT advantage. A bio-LNG bunker with a manure-based WtT of minus 30 gCO2eq/MJ burned in an Otto engine with 30 gCO2eq/MJ of methane slip ends up at roughly 0 gCO2eq/MJ WtW, which is still excellent but not dramatically negative.

Diesel-cycle high-pressure dual-fuel engines (MAN ME-GI and the Diesel-cycle mode of ME-GA) carry roughly 0.2 to 0.5 percent slip and therefore preserve more of the bio-LNG benefit. A vessel selecting engine technology with bio-LNG in mind has a structural reason to favour the Diesel cycle.

The interaction between feedstock pathway and engine cycle is therefore the central sizing question for a bio-LNG strategy. A premium manure-based bio-LNG bunker burned in an Otto-cycle engine yields a smaller compliance benefit than the same bunker burned in a Diesel-cycle engine, and the operator decision must reflect both fuel cost and engine economics.

The IMO is reviewing methane-slip default values at MEPC, with a potential update to lower the slip factors as engine technology matures. Lower defaults would amplify the bio-LNG benefit on a WtW basis. Operators sizing future strategy should track this technical work alongside the FuelEU revision cycle.

Blending with fossil LNG: proportionate WtW reduction

Bio-LNG is fully miscible with fossil LNG at the molecular level and can be blended in any ratio. Most marine bio-LNG bunkers in 2026 are delivered as blends rather than as 100 percent bio-LNG, because supply constraints and price differentials make partial substitution the more common commercial structure.

The WtW intensity of a blend follows a linear interpolation between the fossil-LNG and bio-LNG endpoints, weighted by energy share rather than mass share (since both fuels have effectively identical lower heating values once liquefied). For a blend with bio-LNG energy fraction $x_{\text{bio}}$:

A 20 percent bio-LNG blend with fossil LNG at 85 gCO2eq/MJ Otto-cycle WtW and bio-LNG at 5 gCO2eq/MJ WtW yields:

A reduction of 16 gCO2eq/MJ from the fossil-LNG baseline, achieved at 20 percent of the green premium of pure bio-LNG. The trade-off between volume and intensity reduction is the central commercial parameter, and operators size the optimum using /calculators/fuel-wtw-blend against the FuelEU intensity reduction trajectory and the IMO GFI benchmarks.

The mass-balance documentation must accompany the bio-LNG fraction of the blend. A bunker barge delivering 100 tonnes of fossil LNG and 25 tonnes of mass-balance-allocated bio-LNG (a 20 percent energy-share blend) issues a single bunker delivery note for 125 tonnes total with the bio-LNG fraction explicitly identified and the corresponding sustainability declaration attached. The vessel reports the blend composition in its FuelEU and IMO Net-Zero submissions, and the WtW intensity used in the compliance balance is the energy-weighted blend value calculated above.

Blends scale the bio-LNG benefit linearly. They do not unlock additional benefit beyond the linear interpolation. They are therefore a pure economic optimisation: for a given bio-LNG green premium and a given FuelEU penalty rate, the optimum blend ratio is the one where marginal compliance value equals marginal premium cost.

Formula, assumptions, and limits

Formula

The well-to-wake CO2-equivalent intensity of a bio-LNG bunker is the sum of three components:

where $\text{EF}_{\text{WtT,bio}}$ is the well-to-tank intensity for the certified bio-LNG pathway in gCO2eq/MJ, $\text{EF}_{\text{TtW,CO}_2,\text{bio}}$ is the tank-to-wake CO2 emission factor (zero for biogenic combustion under MEPC.391(82) and FuelEU Annex II), $\text{GWP}_{100,\text{CH}_4}$ is the AR5 100-year global warming potential for methane (28), and $\text{slip}$ is the unburned methane slip rate of the host engine in gCH4/MJ fuel input.

The blend formula for fossil LNG mixed with bio-LNG at energy fraction $x_{\text{bio}}$:

Derivation

The WtW formulation follows the IMO LCA Guidelines, treated in /wiki/marine-gfs-methodology, which decompose the lifecycle into a WtT stage covering all energy and material flows up to the bunker manifold, and a TtW stage covering combustion and slip from the moment the fuel enters the engine.

The WtT term for bio-LNG aggregates feedstock collection (manure transport, food-waste haulage, residue collection), digestion plant energy consumption (heating, mixing, parasitic load), upgrading energy (compressor for water scrubbing or membrane, regeneration heat for amine), liquefaction energy (compression chain to minus 162 degrees Celsius), and any pipeline or truck transport between upgrading and liquefaction. For manure pathways, an avoided-methane credit is applied at this stage, calculated as the GWP100-weighted methane that would have vented from the open-lagoon counterfactual.

The TtW combustion CO2 term is set to zero because the carbon in bio-methane is biogenic. The carbon was photosynthesised within the recent biological cycle (months for food waste, years for energy crops, decades at most for woody biomass), and the IMO and EU frameworks treat that carbon as recycled. The combustion CO2 is still physically present in the funnel exhaust, but it does not appear on the GHG balance.

The TtW slip term applies the AR5 GWP100 factor of 28 to unburned methane. A slip rate of 1 gCH4 per MJ of fuel input contributes 28 gCO2eq/MJ to the WtW total. Slip is a function of engine cycle (Otto vs Diesel), engine load, charge-air conditioning, and combustion control, and is treated in detail in /wiki/per-fuel-wtw-lng-otto-diesel. The IMO defaults under MEPC.391(82) Annex 1 are 1.7 gCH4/MJ for low-pressure four-stroke Otto engines, 1.0 gCH4/MJ for low-pressure two-stroke Otto, and 0.2 gCH4/MJ for high-pressure Diesel-cycle engines, with verified actual values permitted.

Assumptions

1. Biogenic CO2 is zero on the WtW balance. This is the foundational assumption of the bio-LNG benefit and is shared by IMO MEPC.391(82), FuelEU Annex II, and RED III. It rests on the premise that biogenic carbon is recycled within the relevant biological timescale.
2. AR5 GWP100 of 28 for methane. Both IMO and FuelEU use this value. Switching to AR5 GWP20 (84) or to AR6 GWP100 (29.8) materially changes the slip contribution and would therefore change the bio-LNG WtW number.
3. Mass-balance certification is honest. The framework relies on accredited certification bodies to verify feedstock origin, conversion factors, and chain of custody. Fraudulent or weakly audited certificates undermine the entire benefit.
4. Avoided-methane counterfactual is conservative. For manure pathways, the credit is valid only if the manure would otherwise have vented in an open lagoon. Default values build conservatism in, but actual values must be verified.
5. Engine slip is at MEPC.391(82) Annex 1 default or below. Slip is the largest TtW term and is sensitive to engine load and operational practice.

Worked example

A 35,000 dwt container feeder fitted with a 6-cylinder MAN ME-GA two-stroke low-pressure Otto-cycle dual-fuel main engine takes a 200-tonne bio-LNG bunker in Rotterdam. The bunker is certified ISCC EU mass-balance, sourced from a 50/50 mix of dairy manure and food-waste digester biomethane, with an Annex 1 default WtT of minus 15 gCO2eq/MJ for the blended feedstock pathway. The host engine is rated at 1.0 gCH4/MJ slip.

WtT term: $-15$ gCO2eq/MJ TtW combustion CO2 term: $0$ gCO2eq/MJ (biogenic) TtW slip term: $28 \cdot 1.0 = 28$ gCO2eq/MJ WtW total: $-15 + 0 + 28 = 13$ gCO2eq/MJ

Compared with the same engine on fossil LNG at WtT of 18 gCO2eq/MJ, TtW combustion CO2 of 56 gCO2eq/MJ, and slip of 28 gCO2eq/MJ for a WtW of 102 gCO2eq/MJ, the bio-LNG bunker delivers a reduction of approximately 89 gCO2eq/MJ, an 87 percent improvement.

For 200 tonnes at an LHV of 50 MJ/kg, the energy is $200 \cdot 1000 \cdot 50 = 10$ TJ, and the GHG saved on a WtW basis is $89 \cdot 10^{12} \cdot 10^{-9} \cdot 10^{-3} = 890$ tonnes CO2eq versus fossil LNG. At a fossil-LNG bunker price of USD 700/t and a bio-LNG bunker price of USD 1,800/t, the green premium for the 200-tonne lift is USD 220,000, which equates to approximately USD 247 per tonne of CO2eq avoided. That figure is comparable to the 2026 FuelEU non-compliance penalty rate.

Edge cases and limits

Negative WtW outcomes. Pure manure-based bio-LNG burned in a Diesel-cycle engine can produce a WtW figure below zero. Both IMO and FuelEU frameworks handle negative values by allowing the surplus to be used elsewhere in the compliance balance, but operators should verify the specific pooling and banking rules in /wiki/fueleu-intensity-formula-breakdown before relying on the credit.

Energy-crop bio-LNG. Pathways with WtT above 35 gCO2eq/MJ may not clear the RED III 80 percent threshold for new installations, and may attract scrutiny on the food-and-feed cap. A certificate naming energy-crop feedstock should be tested against the RED III thresholds before reliance.

Non-EU bunker ports. Mass-balance bio-LNG is currently available only where the physical balancing area extends. A vessel taking bunkers in Singapore, Fujairah, or Yokohama cannot rely on European mass-balance documentation under FuelEU rules. The book-and-claim debate at IMO is unresolved as of MEPC 84.

Methane slip at low load. Otto-cycle engines slip more at low load than at design point. A vessel operating at 30 percent MCR for a long period attracts a higher slip factor, eroding the bio-LNG benefit. Operational planning should account for the slip-load curve, treated in /wiki/per-fuel-wtw-lng-otto-diesel.

GWP horizon sensitivity. AR5 GWP20 of 84 for methane (rather than GWP100 of 28) materially worsens the slip contribution. Some advocacy frames argue for GWP20 reporting alongside GWP100. Neither IMO nor FuelEU currently uses GWP20 in the binding calculation, but a vessel exposed to GWP20 pressure (regional regulators, charterers, lenders) should size both metrics.

Certification fraud risk. Documented cases of irregular mass-balance allocation in adjacent biofuel chains have led to ISCC EU enforcement actions and, in some cases, certificate withdrawal. Operators relying on bio-LNG documentation should verify certification status via the issuing scheme and demand current proof-of-sustainability.

Regulatory basis

• IMO MEPC.391(82): Annex 1 default emission factors for liquefied biomethane pathways, biogenic CO2 treatment, methane-slip defaults
• FuelEU Maritime (Regulation (EU) 2023/1805): Annex II default values for biomethane and bio-LNG, mass-balance verification requirements, RED III feedstock taxonomy import
• RED III (Directive (EU) 2023/2413): Sustainability and GHG-saving criteria, Annex IX-A and IX-B feedstock lists, food-and-feed cap, 80 percent GHG saving threshold for new installations from 2026
• Commission Implementing Regulation (EU) 2022/996: Mass-balance verification rules, certification scheme recognition criteria
• MARPOL Annex VI Regulation 14 and Regulation 13: Operational compliance for SOx and NOx, separate from the lifecycle GHG path; treated in /wiki/marpol-annex-vi
• IMO IGF Code: Safety case for gas-fuelled ships, applied identically to fossil LNG and bio-LNG
• AR5 GWP values: GWP100 of 28 for methane, used by both IMO and FuelEU

Common errors

1. Counting TtW combustion CO2. A common error is treating the bio-LNG combustion CO2 as non-zero. Under MEPC.391(82) and FuelEU Annex II it is zero (biogenic).
2. Ignoring methane slip. A second common error is treating bio-LNG as a zero-emission fuel. Methane slip is identical to fossil LNG and contributes meaningfully on a CO2eq basis.
3. Confusing book-and-claim with mass balance. Marine bunker certificates must come through a mass-balance chain to qualify under FuelEU. Book-and-claim certificates from disconnected jurisdictions do not currently qualify.
4. Applying the FuelEU RFNBO multiplier. Bio-LNG is a biofuel, not a renewable fuel of non-biological origin. The RFNBO multiplier in /wiki/fueleu-rfnbo-multiplier does not apply.
5. Using GWP20 in the IMO calculation. IMO and FuelEU both use AR5 GWP100. GWP20 reporting is informational and does not change the binding number.
6. Treating energy-crop bio-LNG as Annex IX-A. Energy crops are in the food-and-feed bucket and subject to the 7 percent cap. Only manure, sewage sludge, food waste, agricultural residue, and similar wastes are Annex IX-A.

See also

• /wiki/per-fuel-wtw-lng-otto-diesel
• /wiki/per-fuel-wtw-vlsfo-mgo
• /wiki/marine-gfs-methodology
• /wiki/fueleu-intensity-formula-breakdown
• /wiki/fueleu-rfnbo-multiplier
• /wiki/imo-net-zero-framework
• /wiki/marpol-annex-vi
• /wiki/methanol-as-marine-fuel
• /wiki/ammonia-as-marine-fuel
• /calculators/fuel-wtw-bio-lng
• /calculators/fuel-wtw-lng
• /calculators/fuel-wtw-blend
• /calculators/fueleu-ghg-intensity
• /calculators/fueleu-rfnbo-multiplier
• /calculators/gfi-attained

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