Biofuel Blending Challenges MFM Accuracy, GCMD Study Finds

Blending FAME-based biofuels with residual marine fuel grades such as VLSFO and HSFO can lead to the blend's viscosity falling below the validated operating range of mass flow meters (MFMs), according to a study by the Global Centre for Maritime Decarbonisation (GCMD).

The study, cited by Hellenic Shipping News Worldwide, examines difficulties shipowners encounter in ensuring accurate quantities of bio-blended bunker supplies and verifying the renewable fraction for which they pay. MFMs are designed to provide reliable measurements within specific validated operating ranges.

Barges are typically configured with MFM validation ranges based on the type of marine fuel they handle. For instance, barges primarily supplying residual grades like VLSFO and HSFO are generally validated for a higher viscosity range, while those handling distillates such as LSMGO are validated for a lower range. Although pure biofuel grades (B100) can be bunkered using distillate-fuel barges, biofuel blends with residual grades are typically bunkered using residual fuel barges.

Viscosity and Measurement Accuracy

The GCMD study indicates that the lower viscosity of biofuel components, when blended with residual fuels, can reduce the overall viscosity of the blend. This reduction can cause the blend's viscosity to fall outside the validated range of the MFMs on the bunkering barges. Within their validated range, MFMs typically achieve a measurement uncertainty of +/- 0.5%, but this accuracy is not guaranteed when the viscosity is outside this range.

To address this, GCMD recommends measuring the blend's viscosity at transfer conditions prior to delivery. Other suggested controls include managing transfer temperature, using higher-viscosity blend components, or employing MFMs that are validated over a broader viscosity range.

Verifying Renewable Content

Beyond viscosity, the GCMD report also emphasises the importance of the renewable fraction in bio-blended fuels. This fraction dictates the green premium paid by buyers and is crucial for greenhouse gas accounting and regulatory compliance. The study notes that declared blend ratios, such as B24 or B30, are typically provided by suppliers and are not independently verified. Land-based investigations have previously revealed that some biodiesel blends contained significantly less renewable content than declared, occasionally consisting mostly of fossil fuel.

While FAME blends can be analysed using standard laboratory tests, HVO blends require radiocarbon dating. GCMD points out that both methods carry a margin of error that must be considered. For example, pure FAME may contain up to 3.5% non-ester material, meaning the measurable FAME content in a finished blend might not precisely match its declared blend ratio. Laboratory test results below a declared ratio do not, by themselves, constitute proof of incorrect blending.

As a measure to improve transparency, GCMD recommends that renewable fractions should be explicitly declared in supply documents. These declarations should specify whether the fraction is measured by mass or by volume and include the full fuel specification.