Biodiesel: The Fuel Reshaping Marine Decarbonisation
Walk the quayside of any forward-thinking port today and you will hear the same question being asked in boardrooms and engine rooms alike: what actually goes in the tank once heavy fuel oil falls out of favour? For a growing number of shipowners, part of the answer is biodiesel. Derived from organic feedstocks rather than crude oil, biodiesel is already being blended into marine fuel supplies from Rotterdam to Singapore, offering a near-term route to lower lifecycle carbon emissions without ripping out existing engines.
Biodiesel is a renewable diesel substitute produced through a chemical process called transesterification, in which fats or vegetable oils react with an alcohol, typically methanol, in the presence of a catalyst. The result is a mix of fatty acid methyl esters, commonly shortened to FAME, along with glycerine as a byproduct. Feedstocks range from rapeseed and soybean oil to used cooking oil, animal fats, and increasingly algae-based oils that avoid competing with food crops. Because the finished product is an ester rather than a hydrocarbon, it behaves slightly differently from conventional diesel, but closely enough that it can be used in existing compression-ignition engines with minimal modification.
How Biodiesel Works as a Marine Fuel
For vessel operators, the appeal of biodiesel lies in its compatibility with the machinery already installed on board. Most marine diesel engines can run on blends of biodiesel and conventional marine gas oil without hardware changes, provided the blend ratio stays within manufacturer tolerances. Common blends include B7, B20, B30, and even B100, pure biodiesel, which some ferry operators and inland fleets have trialled successfully. The fuel has a higher cetane number than fossil diesel in many cases, which can improve combustion efficiency, and it contains virtually no sulphur, a meaningful advantage for operators navigating emission control areas.
There are tradeoffs engineers watch closely. Biodiesel has a lower energy density than marine gas oil, meaning slightly higher fuel consumption for the same power output. It also has a tendency to absorb moisture, oxidise during long storage, and gel at low temperatures, all of which demand attention to tank maintenance, fuel filtration, and bunkering logistics. Shipboard fuel systems originally designed around petroleum diesel sometimes need upgraded seals and gaskets, since the solvent properties of FAME can degrade certain older elastomers. None of these issues are deal-breakers, but they explain why classification societies and engine manufacturers, including Wärtsilä, have published detailed guidance on blend limits and handling procedures.
Where Biodiesel Is Making an Impact in Shipping
The real-world momentum behind biodiesel in maritime circles has been building steadily over the past five years. Major bunkering hubs now offer biodiesel blends as a drop-in option, and several container lines and cruise operators have run high-profile voyages on B30 or higher blends to demonstrate feasibility ahead of tightening International Maritime Organization carbon intensity rules. The appeal is straightforward: unlike ammonia, methanol, or hydrogen, biodiesel does not require new engines, new storage tanks, or new safety systems. It slots into the existing bunkering infrastructure, which makes it one of the fastest levers available for reducing a fleet’s carbon footprint right now rather than in a decade.
Short sea shipping, ferries, and offshore support vessels operating in coastal and inland waters have been particularly active adopters, partly because feedstock supply chains are more localised and partly because these vessels often operate under stricter regional emissions mandates. Port authorities in Northern Europe have also pushed biodiesel blending programmes as a way to cut harbour craft emissions without waiting for new propulsion technology to mature.
Challenges and the Road Ahead
Scaling biodiesel for deep-sea shipping remains constrained by feedstock availability. Global supplies of used cooking oil and waste fats are finite, and diverting large volumes of virgin vegetable oil raises land-use and food-security concerns that regulators are watching carefully. Lifecycle carbon accounting also varies significantly depending on feedstock origin, so not all biodiesel delivers the same emissions benefit. Industry groups are now pushing for stricter certification standards to ensure claimed carbon savings hold up under scrutiny, while research into algae and lignocellulosic feedstocks aims to widen the supply base beyond food-linked crops.
Biodiesel will not single-handedly decarbonise global shipping, and few in the industry claim otherwise. But as a transitional fuel that works with the fleet already on the water, it buys shipowners time and credibility while longer-term solutions mature. Expect its role to expand steadily as certification tightens and feedstock innovation catches up with demand.
