Not All Bunkers Are Created Equal, And Neither Are the Additives

Why the chemical gap between distillate and residual fuels matters more than most operators realize, why a single fuel additive product can’t bridge it, and why homogenisers are the underrated tool in the HFO armory.

By Paul Morgan (gCaptain) – In recent years the maritime industry has been bombarded with explanations and sales pitches from pseudo fuel experts, individuals whose careers sit in banking or sales rather than on the water or in an engine room, telling marine professionals they don’t really understand fuels or how engines relate to fuel consumption and power output. 

Much of that rhetoric is laughable to anyone who has actually managed a fuel system at sea. Some of it is frankly offensive. So it’s time to put some meat on the bone with proven technology, and explain clearly why the two main categories of marine fuel demand fundamentally different additive strategies, why no single product can do both jobs well, and, of course, where a fuel homogeniser fits into the picture.

Every maritime professional knows that heavy fuel oil and marine gas oil are different products. That much is obvious from the moment you handle them. One is a thick, viscous material that has to be heated before it’ll flow through a pipe. The other is a clean, pourable liquid not unlike road diesel. The practical differences in handling, storage, purification, and heating are well understood. What’s less often discussed is just how deep those differences run at the molecular level, and why that depth matters enormously when it comes to choosing the right additive treatment.

It matters because the additive market is full of products claiming to work across the board: universal treatments suitable for HFO, VLSFO, MDO MGO, and even Biofuels in a single formulation. The temptation of simplicity is understandable. But the chemistry effective in distillate service and the chemistry effective in residual fuel service aren’t just different in degree. In several key areas they’re fundamentally incompatible.

MGO and MDO are distillate products, drawn from the refinery’s distillation columns and hydrotreated to meet sulphur specifications. Their hydrocarbon composition is well-ordered, mostly paraffinic and naphthenic molecules in the C10 to C25 range, chemically stable and reasonably uniform. The problems that do arise come from what refining removes rather than what it leaves. Desulphurisation strips out the polar sulphur compounds that historically lubricated injection equipment. Oxidative degradation during storage and recirculation produces gums and lacquer deposits on injector nozzle tips. Microbial colonisation takes hold wherever free water sits at the bottom of a tank in warm climates. 

These are targeted problems with targeted solutions: fatty acid or synthetic ester lubricity additives, polyisobutylene succinimide or polyetheramine detergents, cetane improvers, cold flow additives, biocides. Lower molecular weight compounds working through well-understood, single-mechanism interactions. Good, proven chemistry.

HFO isn’t just a dirtier version of MGO, it’s a chemically different category of material, and it demands a different category of solution.

Heavy fuel oil is something else entirely. It’s a residual product, what remains after the refinery has extracted everything commercially valuable. That residue carries very high molecular weight hydrocarbons, complex polycyclic aromatic structures, asphaltenes, resins, sulphur compounds, and organo-metallic contaminants, principally vanadium and sodium. Fuels from catalytic cracking streams also carry catalytic fines, hard aluminium silicate particles that abrade injection system components and cylinder liners if the purifier doesn’t catch them. 

The defining characteristic of HFO is that it isn’t a true solution. It’s a colloidal dispersion, with asphaltene molecules held in suspension by resin molecules and the aromatic character of the surrounding medium. That balance is fragile. Blend with paraffinic cutter stock to hit a sulphur specification, as VLSFO blends routinely do, and the aromatic solvency drops to the point where asphaltenes flocculate, precipitate, and form sludge. The IMO 2020 sulphur cap drove exactly this problem across a wide swathe of the fleet.

Treating HFO means addressing colloidal stability first. Asphaltene dispersants, the primary tool for this, are high molecular weight amphiphilic molecules, oligomeric and polymeric compounds with molecular weights from several hundred to tens of thousands of grams per mole. They adsorb onto asphaltene particle surfaces through multiple simultaneous mechanisms: pi-pi stacking with aromatic regions, hydrogen bonding with heteroatomic groups, and coordination with vanadium and nickel porphyrin centres. The dispersant coats each particle and presents a steric barrier that prevents agglomeration. Getting this chemistry wrong, or applying it at the wrong treat rate, can actually destabilise the fuel. Which brings us to vanadium.

Vanadium is present in virtually all residual fuels as organo-metallic porphyrin complexes, typically 20 to several hundred milligrams per kilogram. During combustion it oxidises to vanadium pentoxide, melting point 690 degrees Celsius, and in the presence of sodium from seawater contamination, forms eutectic compounds melting as low as 530 degrees, well within the operating temperature of exhaust valve seats and turbocharger nozzle rings. Molten vanadium oxide is aggressively corrosive to the nickel alloys used in those components. 

The solution is magnesium-based combustion modification chemistry: oil-soluble magnesium sulphonate or oxide dosed to a molar ratio of magnesium to vanadium of at least three to one, forming high-melting-point magnesium vanadate that remains solid and non-adherent. A distillate fuel additive contains none of this, because it has no reason to. And conversely, the cetane improver that addresses ignition delay in MGO has no useful mechanism in HFO, where the ignition challenge is governed by CCAI rather than conventional cetane chemistry. The combustion challenge in HFO is incomplete oxidation of heavy asphaltenic fractions, addressed by organometallic catalysts, iron octoate or cerium naphthenate at 5 to 50 milligrams per litre as elemental metal, which have nothing to act on in a clean distillate fuel

The demulsifier that resolves a distillate water-in-fuel emulsion cleanly and quickly may have no appreciable effect on an HFO emulsion stabilised by viscoelastic asphaltene-resin film!

Even the demulsification chemistry that both fuel types require can’t simply be shared. Distillate fuel emulsions are stabilised by relatively weak interfacial films of oxidised polar compounds and metallic soaps, which low molecular weight ethylene oxide and propylene oxide block copolymers displace readily at modest treat rates. HFO emulsions are stabilised by viscoelastic asphaltene-resin films of far greater mechanical strength. Displacing them requires high molecular weight alkylphenol-formaldehyde resin ethoxylates, with molecular weights after ethoxylation reaching 10,000 to 20,000 grams per mole. 

A product that does one job well won’t do the other at all. This is why the universal fuel additive doesn’t hold up to scrutiny: the molecular weight requirements, the polarity requirements, and the target problems are too different across too many additive categories. A universal product addresses some requirements adequately and others inadequately, or includes chemistry for both at concentrations too dilute to be effective in either. The cost difference between two correctly specified fuel-specific products and one compromised universal is small. The performance difference, measured in injector life, separator efficiency, and fuel system reliability, is not.

One tool that deserves more attention alongside the additive programme is the fuel homogeniser, a high-shear rotor-stator assembly or ultrasonic cavitation system installed in-line between the settling and service tanks. It applies intense localised mechanical energy to the fuel stream, breaking down agglomerated asphaltene clusters, reducing sludge precursor sizes, and producing a finer, more uniform suspension. 

The benefit to additive performance is direct: asphaltene dispersants adsorb onto particle surfaces, and the efficiency of that adsorption is proportional to available surface area. Homogenisation gives the dispersant more surface to work on. The dispersant goes further, the colloidal stability of the treated fuel is better, and water droplets in the fuel coalesce more readily, making centrifugal purification faster and more complete. Combustion also benefits: finer, more uniform heavy fraction particles at the injector produce more complete oxidation, with documented reductions in exhaust smoke opacity, piston crown deposit accumulation, and in some cases specific fuel oil consumption. All of this is achieved by conditioning the hydrocarbon fuel itself, with no water involved and no compromise to the purifier or injection system.

Which leads to the final and most easily confused area: the relationship between water, HFO, and the homogeniser. Every vessel accumulates small quantities of water in its fuel system. Tank surfaces sweat. Condensation works back through vent lines. Bunkering introduces moisture. This incidental water is a contaminant and nothing else. Dispersed through residual fuel as fine droplets, stabilised by the natural surfactancy of the asphaltene-resin fraction, it resists separation in the purifier, travels toward the injection system, and causes cold corrosion in the cylinder, hydraulic shock if it accumulates, and injector erosion from cavitation as droplets flash to steam. The correct response is demulsification and removal. The homogeniser supports that removal by promoting coalescence and improving purifier efficiency. This is the opposite of emulsification.

Deliberate emulsion fuel is a different matter with a genuine engineering basis. When a stable water-in-HFO emulsion containing 10 to 20 per cent water is injected into a cylinder, each fuel droplet carries dispersed water micro-droplets that flash to steam on contact with the combustion zone, expanding roughly 1,700 times in volume. The resulting micro-explosion shatters the surrounding fuel droplet, dramatically increasing surface area and improving hydrocarbon-oxygen contact. More complete combustion of the heavy asphaltenic fractions, lower peak temperatures, reduced NOx, reduced particulate. 

These effects have been measured and published. They’re not fabricated. Producing a stable emulsion fuel requires specific emulsifier chemistry, high HLB non-ionic surfactants such as ethylene oxide and propylene oxide block copolymers or polysorbates, that form a mechanically stable interfacial film holding water droplets at a target size of 1 to 10 microns. And it requires a homogeniser, because simple mixing produces a coarse, non-uniform, unstable droplet distribution that no emulsifier can reliably hold. The homogeniser sets the droplet size. The emulsifier holds it. Without both working together at matched specifications, you have an unstable mixture, not an emulsion fuel.

The operational constraints are significant and should be understood honestly. The centrifugal purifier must be bypassed or substantially reconfigured for emulsion fuel operation, because a functioning purifier breaks the emulsion and removes the water before it reaches the engine, negating the strategy entirely. Cold corrosion risk rises with elevated combustion gas water content, particularly at reduced loads where liner temperatures fall. These risks are manageable in a purpose-designed, purpose-commissioned installation with dedicated mixing and dosing systems and continuous monitoring. They’re considerably less manageable in a standard HFO system adapted on the fly. Emulsion fuel is a specialist strategy requiring specialist equipment and chemistry. It is not a bolt-on modification, and it is entirely distinct from both the incidental water contamination that demands removal and from standard homogeniser operation that improves fuel quality without any water addition at all. Keeping those three situations clearly separated is not a technical nicety. It’s a basic requirement for managing an HFO fuel system properly.

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