The Global Centre for Maritime Decarbonisation (GCMD) has released a report setting out a practical approach to help shipowners verify that they receive both the total quantity of marine biofuel and the renewable content they have contracted for. The report, Quantity assurance in marine biofuel supply chains, published on 27 August, draws on evidence from GCMD’s end-to-end marine biofuel supply chain trials conducted under commercial operating conditions.

Biofuels are purchased not only for their energy content, but also for the renewable fraction and associated greenhouse gas (GHG) emissions reductions they represent. Blend ratios — such as B24 or B30 — are typically supplier-declared rather than independently verified. According to GCMD, closing this verification gap would strengthen confidence in marine biofuel transactions, given that the renewable fraction determines the green premium and underpins GHG accounting and regulatory compliance.

Mass flow meter considerations

On the question of total quantity transferred, the report points to mass flow meters (MFMs) as the preferred method for custody transfer, noting that they provide a transparent and auditable basis for determining the quantity of fuel transferred.

However, the report highlights a technical complication: adding fatty acid methyl ester (FAME) to conventional marine fuels — such as very low-sulphur fuel oil (VLSFO) or high-sulphur fuel oil (HSFO) — can reduce the final blend’s viscosity, potentially pushing it outside the validated operating range of the MFM installed on the bunker barge. GCMD recommends that the viscosity of the final blend be assessed under actual transfer conditions against the applicable MFM operating range, and that this can be managed through appropriate transfer temperature control or by using MFMs validated for broader ranges.

Verifying the renewable fraction

Verifying the renewable content requires different analytical approaches depending on the biofuel type. According to the report, FAME-based blends can be verified through compositional analysis, while hydrotreated vegetable oil (HVO)-based blends require radiocarbon analysis, as renewable and fossil hydrocarbons cannot be distinguished through compositional testing alone.

The report also cautions that interpreting analytical measurements is not straightforward. Neat FAME may contain up to 3.5% non-ester material, meaning the measurable FAME content of a finished blend may not exactly match its declared blend ratio. Laboratory methods also have finite measurement precision, so a difference between a measured result and a declared blend ratio does not necessarily indicate incorrect blending or under-delivery.

Completing a three-pillar framework

The report completes what GCMD describes as a three-pillar assurance framework covering quality, quantity, and GHG emissions abatement for marine biofuels. Together, according to GCMD, the three pillars address whether fuel is fit for use, whether it delivers the claimed emissions reductions, and whether it contains the contracted fuel quantity and renewable content.

Professor Lynn Loo, CEO of GCMD, said: “Buyers need confidence that they have received both the fuel and the renewable content they paid for. This report provides a practical, evidence-backed basis for verifying renewable fuel claims, protecting commercial value and supporting credible GHG accounting as marine biofuel use scales.”