Singapore: Maritec releases whitepaper reviewing VLSFO bunker fuel 

Bunker fuel testing and marine surveying business Maritec Pte Ltd (Maritec) on Friday (29 September) published a whitepaper on reviewing fuel stability, fuel stability reserve, and the corresponding analysis techniques of Very Low Sulphur Fuel Oils (VLSFOs).

The following are excerpts from the ‘Analysis Methods for Measuring Stability, Stability Reserve & Compatibility of Residual Marine Fuels’:

Introduction

Since 1 January 2020, the International Maritime Organization (IMO) has enforced a 0.50% global sulphur cap for the shipping industry to reduce sulphur oxide emissions. A comparison of pre-IMO 2020 fuels and post-IMO 2020 fuels reveals that the latter exhibit greater instability, waxiness, lower density & viscosity, lower micro carbon residue (MCR), lower calculated carbon aromaticity index (CCAI), lower vanadium content, higher net specific energy, higher pour point, and higher acid number. The decreased stability reserve (higher paraffinic and lower aromatic content) of post-IMO 2020 fuels also raises concerns about compatibility issues when different fuels are mixed.

To address these challenges, Maritec lab is equipped with the necessary equipment and testing methods to assess the cleanliness, stability, stability reserve, compatibility, and cold flow properties of post-IMO fuels. Given that fuel stability is the primary concern with Very Low Sulphur Fuel Oils (VLSFOs), this article focuses on reviewing fuel stability, fuel stability reserve, and the corresponding analysis techniques.

Blending of Very Low Sulphur Fuel Oil (VLSFO, IMO 2020 Compliant Fuel)

IMO global sulphur cap of 0.5%S has changed the primary blending target from viscosity and density to sulphur. Marine fuels post-IMO 2020 has seen a wide variability of fuel formulations and characteristics.

Typically, very low sulphur fuel oils can be blended using three categories of blend stocks (1):

Blending of Very Low Sulphur Fuel Oil (VLSFO, IMO 2020 Compliant Fuel)

IMO global sulphur cap of 0.5%S has changed the primary blending target from viscosity and density to sulphur. Marine fuels post-IMO 2020 has seen a wide variability of fuel formulations and characteristics.

Typically, very low sulphur fuel oils can be blended using three categories of blend stocks (1):

Based on the available blend stocks, very low sulphur fuel oils can generally be blended into four major types/groups:

Fuel characteristics evolution and potential quality issues due to 0.5%S Limit

Post-IMO 2020 fuels (VLSFOs) exhibit lower density, lower viscosity, lower MCR (Micro Carbon Residue), lower CCAI (Calculated Carbon Aromaticity Index), lower vanadium content, higher net specific energy, higher pour point, and higher acid number compared to pre-IMO 2020 fuels

It is also noted that VLSFOs generally have better ignition and combustion properties, which contribute to efficient fuel utilization and combustion processes.

However, there is a rise in cold flow property issues within paraffinic-grade very low sulphur fuel oils (VLSFOs), characterized by higher pour points, can potentially introduce concerns regarding cold flow problems, particularly wax formation.

An increase in fuel stability issues is also observed which arises from two factors. Firstly, the use of hydrotreated blend stocks reduces the stability reserve of the fuel due to lower aromaticity and asphaltene solubility. Secondly, when low sulphur aromatic and paraffinic blend stocks are mixed, fuel instability may occur if the blend lacks adequate aromatic content to dissolve the asphaltenes present. Additionally, an increase in fuel compatibility issues can occur when aromatic and paraffinic-grade VLSFOs are co-mingled, leading to incompatibility and potential flocculation of asphaltenes if the blend lacks sufficient aromatics.

Conclusion

The factors that influence stability of residual fuels are fuel formulation (an internal factor, fuel blend itself – on whether the fuel has sufficient aromaticity or contains any appreciable amount on non-hydrocarbons especially non-hydrocarbons containing hydroxyl group such as alkylresorcianols/alkyl-1,3-benzenediols and others) and the external factors such as thermal & mechanical stress and storage time/duration.

The data collated in this article indicates that most of the unstable fuels or marginally stable fuels that cause sludging and filter clogging issues were mostly due to fuel formulation (an internal factor) – either high or borderline high total sediment due to the presence of chemical contaminants, fuels containing appreciable amount of non-hydrocarbons that contain hydroxyl group such as alkylresorcianols/alkyl-1,3-benzenediols, phenolic compounds and others.

Our past historical data indicated that there are also instances that fuels are detected with certain concentration of alkylresorcinols, phenolic compounds and slightly reactive hydrocarbons however the vessels are able to consume the fuels without any operational issues. It is recommended that when a fuel is detected with the presence of alkylresorcinols, phenolic compounds and slightly reactive hydrocarbons, it is important that fuel stability reserve for the fuel shall also be evaluated, if the fuel is identified to contain alkylresorcinols, phenolic compounds & slightly reactive hydrocarbons and the fuel is also unstable or marginally stable, the vessel is recommended practice the aforementioned preventive measures to prevent, reduce and solve the potential sludging and filter clogging issues.

Moving Forward

In the near future when biofuels is widely adopted as drop-in marine fuel to achieve the regulatory requirements on reduction of carbon/GHG emission, due to the diversity of the liquid cutter stocks derived from biomass, marine fuel stability and marine fuel stability reserve will become critical test parameters.

Note: The full version of the ‘Analysis Methods for Measuring Stability, Stability Reserve & Compatibility of Residual Marine Fuels’ whitepaper can be viewed here.

Photo credit: Maritec Pte Ltd
Published: 2 October, 2023