Lloyd’s Register’s Fuel Oil Bunkering Analysis and Advisory Service (FOBAS) on Wednesday (2 April) released a bulletin to elaborate on the importance of accurate energy content determination and strongly recommended specifying the Net Specific Energy (NSE) according to ASTM D240 method when a bunker fuel contains FAME: 

Last year, ISO TC28/SC4/WG6 introduced the latest edition of the ISO 8217 standard, marking a significant shift in marine fuel quality specifications. One of the most notable changes is the integration of drop-in biofuels from renewable sources into the marine fuel mix. These bio-derived fuels are gaining traction due to their ability to reduce greenhouse gas emissions on a life cycle basis, all without requiring modifications to marine engines or the existing fuel supply infrastructure.

Among the predominant bio-derived fuels are fatty acid methyl esters (FAME), already standardized in EN 14214 and ASTM D6751. The latest edition of ISO 8217 (2024) introduces new fuel grades, specifications, and testing requirements under Table 3 for residual blends containing FAME, while Table 1 permits up to 100% FAME in DF grades. However, conventional DM and RM grades must remain FAME-free, with only trace levels allowed. Additionally, hydrotreated vegetable oil (HVO), another bio-derived drop-in fuel, can be used as a blend component in any ratio (up to 100%).

Despite these provisions, the most common biofuel blend in the market is 30% FAME mixed with 70% conventional fuel.

For conventional petroleum-based fuels, ISO 8217:2024 – Annex J provides a calculation method for Net Specific Energy (NSE) and Gross Specific Energy (GSE) based on tested density, sulfur, water, and ash contents. This method has been sufficiently precise for ship operators to estimate fuel consumption and adjust engine settings accordingly.

However, as biofuels have inherently lower calorific values than fossil fuels, their energy content varies depending on the blend ratio. The current ISO 8217 formula does not account for biofuels, as the energy content when FAME is present is overestimated. This discrepancy can lead to inefficiencies in fuel management. Some of the electronically controlled engines require fuels NSE values to be used as an input to the engine management system. Hence, if the value is inaccurate, it may result in inefficient engine performance.

To address this, the ASTM D240 Bomb Calorimeter method is recommended for accurate determination of heat of combustion, particularly for biofuel blends. For example, FAME has a typical energy value of 37 MJ/kg, whereas residual fuels average around 41 MJ/kg. Chart 1 highlights the discrepancies between the calculated NSE (using ISO 8217’s calculation method) and the measured values (ASTM D240) for VLSFO biofuel blends containing 10-100 %v/v FAME. The data reveals that the calculated values consistently overestimate the actual energy content, with discrepancies ranging from 0.83 MJ/kg up to 5.22 MJ/kg, increasing as the FAME content rises. 

It is worth mentioning that for B30 blend, i.e. containing FAME at 30 %v/v, which is considered as the most popular marine bioblend, the discrepancy is approx. 1.50 MJ/kg. This overestimation can lead to inaccurate fuel consumption predictions, potentially causing operational inefficiencies, miscalculations in voyage planning, and higher-than-expected fuel costs. The findings reinforce the limitations of ISO 8217’s formula and emphasize the importance of direct Net Specific Energy value using ASTM D240 to ensure accurate energy assessments for biofuel blends. This point has also been emphasized in clause 6.18 of ISO 8217:2024.

The ASTM D240 method involves burning a weighed sample of the test fuel in an oxygen bomb calorimeter under controlled conditions. ASTM D240 is utilised for fuels consisting only of carbon, hydrogen, nitrogen, oxygen and sulphur, so it is suitable for both distillate and residual fuel containing FAME as well as B100. The gross heat of combustion (at constant volume) is calculated using the measured temperature rise (corrected), the measured energy (MJ/°C), and relevant thermochemical corrections, one of which allows for the correction for the heat of formation of sulphuric acid.

To apply this correction, the mass percent of Sulphur within the sample must be known. The net heat of combustion is subsequently calculated using the gross heat of combustion at constant volume (MJ/kg) and the mass percent of hydrogen in the sample which is determined using a suitable method, typically ASTM D5291 or ASTM D1018. 

As the maritime industry transitions toward sustainable fuels, accurate energy content determination is crucial for efficient fuel management. Since ISO 8217’s calculation method for Net Specific Energy (NSE) is no longer applicable when FAME is present, the ISO 8217:2024 standard specifies the ASTM D240 method for determining the NSE of biofuel blends, whether RF or DF, containing FAME. 

Given the increasing adoption of biofuels to meet decarbonization goals, reliable energy measurement is essential for achieving operational efficiency. In light of this, FOBAS has adopted this change and strongly recommends specifying the NSE according to ASTM D240 when a bunker fuel contains FAME.

Photo credit: Louis Reed from Unsplash
Published: 4 April, 2025