New Tide for Marine Propulsion

(Biofuels International) Biofuels are emerging as one of the most promising green alternatives to conventional marine fuels. Their production from renewable feedstocks significantly reduces lifecycle greenhouse-gas emissions compared with fossil fuel alternatives, and many biofuels are “drop-in” compatible.

Stringent regulations like the European Union’s FuelEU Maritime directive are driving demand for lower-carbon marine energy and biofuels offer a proven pathway to compliance.

The directive targets mandates that progressively increase the share of renewables in ships’ energy mix, rising from 2% in 2030 to 40% by 2050.

By incorporating FAME, HVO or renewable synthetic diesel, operators can immediately reduce their fuel carbon intensity (FCI) and avoid penalties linked to non-compliance.

Together, these factors make biofuels a scalable, practical bridge to fully decarbonised shipping.

Yet beneath the optimism lies a stubborn challenge: the lower energy density of most biofuels.

This is not just a frustrating technicality, it directly impacts a vessel’s range, power and operating costs.

Nevertheless, thanks to increasing green regulation at regional and global level, the tide is turning in favour of sustainable marine fuels.

However, the question remains. How long will it take for next generation technologies to close the biofuel performance and cost gap and become a fully credible alternative?

Energy densities

A ship’s range and efficiency hinges on how much energy each litre of fuel contains. Conventional marine diesel delivers about 38–40 megajoules (MJ) per litre, while today’s leading biofuels fall short. Even the most advanced “drop-in” biofuels typically trail conventional diesel by 3–5%. Ethanol-based fuels, with their high oxygen content and low energy density, represent the greatest shortfall.

If the tank capacity stays the same, the result is a proportional drop in voyage range unless additional storage is retrofitted.

Lower-density fuels also produce slightly less peak power in unmodified engines.

Some biodiesel blends, especially those rich in fatty acid esters, are prone to oxidation and deposit formation. Inline filters can clog more quickly and injector nozzles require extra cleaning, driving up maintenance intervals. HVO and synthetic fuels, by contrast, burn cleaner, but they come at a premium.

Next generation biofuels

The silver lining lies in emerging technologies designed to push energy densities closer to - or even matching - that of fossil diesel.

Power-to-liquid processes combine captured CO2 with green hydrogen to synthesise hydrocarbons almost identical to diesel.

These fuels routinely achieve MJ/l like diesel, effectively reaching energy-density parity. The obstacle? High capital investment and the need for abundant low-carbon electricity.

Algae has long been touted as the ideal biofuel feedstock, with lipid profiles that could rival fossil fuel hydrocarbons.

Cutting-edge bioreactor designs and genetic engineering are boosting algal oil yields. However, scaling up remains a hurdle.

In the interim, many shipowners are adopting dual-fuel systems or blending ratios that combine diesel with varying proportions of biofuel.

This hybrid approach maintains much of diesel’s performance while reducing carbon intensity by up to 50%, depending on the blend.

FuelEU Maritime’s primary aim is to foster consistent use of renewable and low-carbon fuels across European shipping lanes.

Trade-off or transition?

In the near term, some density penalties are unavoidable. Ships might require retrofits, larger tanks, adjusted fuel-injection systems or upgraded filters, to integrate higher volumes of biofuel. These incremental investments serve as a bridge until truly drop-in biofuels mature.

Yet the long-term outlook is optimistic. Renewable synthetic diesel already offers energy-density parity and algal innovations are progressing. READ MORE