The Importance of Understanding the Chemical Differences Between Distillate and Residual Fuels In recent years, the maritime industry has seen a surge of advice from so-called fuel experts—people whose backgrounds are...
The Importance of Understanding the Chemical Differences Between Distillate and Residual Fuels
In recent years, the maritime industry has seen a surge of advice from so-called fuel experts—people whose backgrounds are mainly in finance or sales, rather than in marine operations or engineering. These individuals often tell marine professionals that they lack understanding of fuels and their impact on engine performance.
This claim can be amusing to anyone who's actually managed a fuel system at sea, and in some cases, it feels offensive. Therefore, it's time to clarify the technology behind marine fuels and explain why heavy fuel oil (HFO) and marine gas oil (MGO) require different additive strategies, and why no single product can effectively serve both.
Most maritime professionals recognize that HFO and MGO are fundamentally different products. HFO is a thick, viscous substance that needs to be heated to flow, while MGO resembles cleaner, pourable road diesel. While the handling and storage differences are clear, what’s less discussed is how these differences extend to the molecular level, which greatly impacts the selection of appropriate additives.
This is important because the additive market is filled with products that claim to work for various fuel types: universal treatments that claim to fit HFO, VLSFO, MDO, MGO, and even biofuels. The appeal of a one-size-fits-all solution is tempting, but the chemistry that works for distillate fuels is not only different in amount, but also fundamentally incompatible with that of residual fuels.
MGO and MDO are distillate fuels that come from the distillation process and have undergone hydro-treatment to meet sulfur regulations. These fuels are chemically stable, featuring well-ordered hydrocarbons mainly in the C10 to C25 range. The issues that arise with them usually come from what refining removes. Desulfurization eliminates polar sulfur compounds that previously helped to lubricate injection equipment. Additionally, storage and circulation can lead to oxidative degradation, resulting in deposits on injector nozzles, while microbial growth can occur in water at the tank’s bottom in warm climates.
These issues are manageable with targeted solutions: lubricity additives, detergents, cetane improvers, cold flow agents, and biocides. Each of these uses clear chemistry with understood mechanisms of action.
HFO is Not Just a Dirtier Version of MGO; It's Chemically Different.
Heavy fuel oil, on the other hand, is a residual product—what’s left after valuable components are extracted in a refinery. It contains high molecular weight hydrocarbons, complex polycyclic aromatic structures, asphaltenes, resins, sulfur compounds, and organo-metallic contaminants like vanadium and sodium. Fuels from catalytic processes also bring in abrasive catalytic fines, which can damage injection system components if not filtered out.
The defining feature of HFO is that it doesn’t exist as a true solution. Instead, it appears as a colloidal dispersion in which asphaltene molecules are suspended by resin molecules and aromatic solvents. This balance is delicate; blending with paraffinic cutter stock to meet sulfur specifications can reduce aromatic solvency, causing asphaltenes to cluster and form sludge. The IMO 2020 sulfur cap has exacerbated this issue across many vessels.
Treating HFO focuses on maintaining colloidal stability first. Asphaltene dispersants—high molecular weight compounds—play a crucial role here. They attach to asphaltene particles through various methods, creating a barrier to prevent agglomeration. If this chemistry is mismanaged, it can destabilize the fuel.
Vanadium, a common element in residual fuels, poses another challenge. During combustion, it oxidizes to vanadium pentoxide, which can become corrosive when combined with sodium from seawater. The solution involves using magnesium-based chemicals that form solid compounds, effectively mitigating corrosion risks. However, such solutions are not present in distillate fuel additives, which lack the need for them.
The Demulsifier Used for Distillate Fuels May Not Work for HFO!
Even the chemicals that deal with water-in-fuel emulsions differ between fuel types. Distillate emulsions have weak stabilizing films that can be easily displaced, while HFO emulsions are stabilized by stronger films that require high molecular weight compounds to displace. Thus, a one-size-fits-all product simply isn't viable.
A valuable tool that should be given more consideration is the fuel homogenizer. This device uses high shear forces to break down agglomerated asphaltenes, creating a more uniform fuel suspension. This enhancement aids additive performance, as more surface area is available for dispersants, leading to improved fuel stability and more efficient purification processes.
Regarding water accumulation in fuel systems, most vessels face minor water intrusion through various means. This incidental water acts as a contaminant. If the water is dispersed in residual fuel, it complicates purification and can lead to engine issues. The right response involves removing this water, and a homogenizer helps facilitate that by improving separation.
On the other hand, a deliberate water-in-HFO emulsion can have engineering advantages. Injecting such an emulsion allows micro-explosions that enhance fuel combustion, improving efficiency. Creating a stable emulsion requires specific emulsifier chemistry and a homogenizer to ensure stable droplet distribution.
The operational challenges surrounding emulsion fuels are notable. Implementing them requires reworking existing purification systems to avoid conflict with the emulsion stability, which can raise cold corrosion risks. The strategy is not a straightforward modification; it calls for dedicated equipment and careful management to function correctly.
In summary, understanding the distinct roles of these fuels and the associated technologies is crucial for effective HFO management. Discerning between incidental water contamination, emulsion strategies, and the role of homogenizers is vital for optimizing performance in marine fuel systems.