VPS has published a white paper where it asserts that the existing ISO 8217 formula for calculating energy content — a key test parameter — is not accurate for fuels containing FAME, such as biodiesel.^[1]

Energy content represents the amount of heat transferred within the combustion chamber during the burn process and indicates the available energy from the fuel. Higher energy content results in higher power generation and better combustion efficiency. The energy content has a direct impact on fuel economy and greenhouse gas emissions and is therefore an important parameter for ship operators to consider in preparation for the EU ETS.

In the VPS research paper, a number of conventional fossil fuels and biofuels with varying FAME content were measured using the ASTM D240 method and the energy content compared with that obtained using the ISO 8217 calculation method (which uses a formula specified in Appendix H of the International Marine Fuel Standard ISO 8217:2017).

The results show good correlation for conventional fuels and distillates, indicating that the ISO 8217 calculation method provides an acceptable degree of accuracy for measuring energy content in conventional fuel types. However, according to VPS, this correlation falls away at FAME content above 10%, thus demonstrating that the ISO 8217 calculation method cannot be relied upon to provide accurate energy content for fuel blends containing more than 10% FAME and that, for these blends, the accuracy of the calculation formula is not acceptable, and accurate energy content can only be determined by measurement using calorimetry.

"This inaccuracy," VPS says, "is due to the greater oxygen content of FAME ... as compared to conventional fuels (which typically contain very little oxygen)."

The ISO 8217 formula is only based on the linear relationship between density and energy content, VPS explains, but does not account for oxygen present in FAME.

As the FAME and the oxygen in the FAME are both variables, a linear relationship between the FAME and the energy content could therefore not be established in studies carried out on biofuels with varying FAME content.

For blends containing more than 10% FAME, VPS concludes that the accurate assessment of energy content requires direct measurement (rather than an equation calculation) using the ASTM D240 method, which uses an oxygen bomb calorimeter to determine the energy content.