Biofuels are appealing to shipowners as they are ‘drop-in’ fuel, meaning that they can be a direct replacement for the traditional fossil fuels without the need for any major modifications. This reduces capex costs as well as avoiding lengthy downtime periods while changes are made.
The two most popular biofuel options are Fatty Acid Methyl Esters (FAME), which is also known as biodiesel, and hydrotreated vegetable oils (HVO). These can be used as a 100% product (B100) or as a blend component with traditional fossil fuels.
The environmental case for biofuels
Biofuels are hydrocarbons and are therefore not zero-carbon fuels. However, if the biofuel component is sustainably sourced, the lifecycle emissions can be much lower than traditional marine fuels.
Greenhouse gas (GHG) emissions from shipping, and in the case of biofuels we are primarily concerned with carbon dioxide (CO2), can be measured two ways: Tank-to-Wake and Well-to-Wake. Tank-to-Wake (TtW) concerns only the combustion of the fuel, while Well-to-Wake (WtW) also considers its sourcing, production and transportation.
When considering biofuels on a Tank-to-Wake basis, any reduction in GHG emissions is modest. However, more significant reductions are realised when looking at it on a Well-to-Wake basis. Actual reductions depend on the nature and production of the biofuel, but it is reported that some biofuels derived from woody biomass can reduce GHG emissions by more than 90%.
Biofuel characteristics and quality
The composition and characteristics of a biofuel depends on the source of the feedstock and its processing. Similarly, the quality and characteristics of biofuel blends (such as B30 where the biofuel component is 30%) is determined by both the fossil fuel and bio components.
The international standard for marine fuels is ISO 8217, which was last updated in 2024 where it now allows the use of FAME up to a concentration of 100% in a specific grade. But ISO 8217 applies to the quality and characteristics of the product that is delivered to the vessel, which may be a blend. It does verify the quality or FAME component. This falls under the standard EN 14214 “Fatty Acid Methyl Esters (FAME) for use in diesel engines and heating applications — Requirements and test methods” or ASTM D6751 “Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels”. Further, ISO 8217 is concerned with quality not sustainability criteria.
HVO quality is not included in the ISO 8217 marine fuel standard, but the British Standard for automotive paraffinic diesel fuels EN 15940:2016 can be used as a reference for quality.
Properties and characteristics of biofuels
Biofuels are considered a ‘drop-in’ fuel, which might suggest a straightforward transition to using these fuels from traditional marine fossil fuels. But they are not without their challenges, and the ship operator should carry out a risk assessment before first using biofuels.
Cold flow properties
The cold flow properties of FAME (essentially its ability to resist solidifying or waxing up in cooler temperatures) depend on its source and its saturated content. The usual parameter for this is the Pour Point; the temperature that it loses its flow state, although the new ISO 8217:2024 includes Cloud Point and Cold Filter Plugging Point for bio-distillate marine blends. A highly saturated FAME has the worse cold flow performance. According to CIMAC, there is a significant difference in Pour Points of tallow methyl ester (TME) with rapeseed methyl ester (RME); 12°C vs -15°C, respectively.
The cold flow characteristics of FAME and FAME blends can be improved with cold flow improver additives. However, the most important mitigating measure is to keep the temperature of the fuel comfortably above its Pour Point. This presents challenges when storing biofuels in a vessel’s MGO tank as they are typically not fitted with heating coils. Proper planning and anticipating the temperatures likely to be experienced on forthcoming voyages will prove important in making sure the fuel stays fit for use on board the vessel.
Stability
Bio-residual fuels containing FAME can lose their stability in the same way traditional residual marine fuels can, where asphaltenic sludge is created, blocking pipes and components and potentially damaging the engine. There are several types of fuel stability, but for biofuels we are most interested in ‘thermal stability’ and ‘oxidisation stability’.
Thermal stability relates to its susceptibility to degradation when the fuel is exposed to temperatures much higher than ambient, such as when heated. For this, the usual Total Sediment tests (TSP, TSE, TSA) remain relevant. However, CIMAC warns that these test methods might be precise for FAME content.
Oxidation stability refers to the tendency of the fuel to react with oxygen at ambient temperatures, which is relevant to storage conditions. FAME is less resistant to oxidation than fossil fuels and is therefore more prone to degrade over time.
Storage and treatment
The storage life of biofuels is generally shorter than that of traditional marine fossil fuels. As mentioned earlier, oxidation can affect the fuel’s stability and therefore reduce its practical lifespan.
Also, as previously noted, is the risk of solidification in the storage tank if the required minimum temperatures are not maintained.
There are other aspects to consider when it comes to storage and transfer on board the vessel. FAME is a good solvent, which can have two negative effects. The first is that tank coatings and system materials could become damaged, and some metals become corroded if they are not compatible with the biofuel. Secondly, it can dislodge and disperse any sludge or dirt in the tanks and system, which can cause issues further along the system, such filter clogging or adding load on to the centrifugal separators (purifiers/clarifiers).
Staying with the centrifugal separators, some biofuels may be of a density that is lower than the design criteria of the separator. This is more pronounced with HVO biofuels where its density can be even lower. In such cases, the owner should contact the manufacturer for guidance.
There is also the risk of microbial growth with FAME, which is increased if there is any free water present in the tank or system. This microbial degradation forms slime and will usually require full system flushing to prevent any reoccurrence. HVO is less prone to microbial growth and therefore at less of a risk of degradation.
As with traditional marine residual fuels, always try to avoid the comingling of different fuel stems.
Engine operation
According to CIMAC, early experiences of using biofuels and blends in both slow speed (two-stroke) and medium-speed (four-stroke) marine diesel engines have been successful. They do warn however, that checks should be made before using biofuels that the seals in the engine are compatible.
Owners are recommended to consult the engine manufacturer, the fuel provider and lubricating oil supplier when considering using biofuels.
A nutty issue
Cashew Nut Shell Oil (CNSO), sometimes referred to as Cashew Nut Shell Liquid, is a naturally occurring byproduct of the cashew nut industry. Because of its low cost, wide availability and characteristics, CNSO has recently been mooted as a viable future source of biofuel.
It is, however, a non-FAME biofuel and therefore would not naturally fall under the scope of ISO 8217. Furthermore, its characteristics can introduce risks. The fuel analysis company Maritec reported operational problems when a batch of CNSO found its way into some VLSFO deliveries at Rotterdam and Singapore, which led to fuel sludging, injector failure, filter clogging, system deposits and corrosion of turbocharger nozzle rings.
However, these reported issues do not necessarily mean there isn’t a future for CNSO as a marine fuel. Vehicle carrier fleet UECC are trialling a CNSO-based biofuel known as FSI.100, collaborating with FOBAS (Lloyd’s Register’s fuel testing division), Wartsila, and biofuel supplier ACT Group.
Owners considering using CNSO as a biofuel should do so only following a thorough assessment of the risks and in full conjunction with the engine manufacturer, a trusted fuel supplier and Class.
Coming up
In the next instalment, we will look deeper into biofuels, focusing on sustainability, calculating emissions, charterparty considerations and the pollution risks.
Recommended Reading
https://www.cimac.com/cms/upload/workinggroups/WG7/CIMAC_Guideline_Marine-fuels_containing_FAME_04-2024.pdf
https://www.lr.org/en/knowledge/research-reports/2024/fuel-for-thought-biofuel-report/
https://www.zerocarbonshipping.com/energy-carriers/bio-diesel/?section=fuel-production