The International Maritime Organization (IMO) recently revised its greenhouse gas (GHG) reduction strategy and adopted more ambitious goals towards reducing the carbon footprint of the global fleet. Specifically, the revised strategy maintains the CII reduction by 40% by 2030 compared to 2008 and calls for reduction in absolute GHG emissions from marine vessels: 20% by 2030 and 70% by 2040 all the way to net zero by 2050.
The shift from carbon intensity goals to absolute GHG reduction goals makes the challenge more substantial for shipowners and operators. The CII is a metric that can be reduced based on the operational utilization of a vessel and thus may not fully support the efforts of IMO for decarbonizing the global fleet. On the other hand, absolute GHG reduction goals require solid changes to the technology onboard the vessel and/or the fuel used for propulsion and power generation.
Alternative fuels can offer direct reductions in the absolute GHG emissions of a vessel. So far, the regulations focus on the tank-to-wake component of the emissions, so these are calculated based on the published carbon factors from IMO. Future biofuels and e-fuels will necessitate lifecycle accounting of carbon emissions, but this has not been regulated yet by IMO. Regardless of regulations though, the global production and infrastructure of low- and zero-carbon fuels is still very small to support the decarbonization of the global fleet. The recent uptake of methanol by new buildings in the containership sector reveals the benefits of liner shipping in adopting low-carbon fuels based on the ports they call and their fixed routes on the globe. However, wide adoption of low- and zero-carbon fuels by the global tanker and bulk carrier fleet will require a considerable increase in the global production and infrastructure, unlikely to happen in time to support the 2030 target, potentially even not the 2040 one.
Therefore, reducing the absolute GHG emissions of a marine vessel can be achieved through certain means that are either currently available or will be made available in the near future. First, through operational means that reduce fuel consumption, such as slow steaming and weather routing, which are already done to a large extent, and just-in-time arrival, which is more difficult to implement. Second, through technology use onboard that reduces fuel consumption or CO2 emissions, such as energy efficiency technologies (e.g. hydrodynamics, wind assisted propulsion) or carbon capture that is gaining significant momentum. Third, through alternative fuels that can be made available and cost effective in the midterm.

Many of the energy efficiency technologies are currently either deployed on marine vessels, such as hull and propeller technologies, or are under development and evaluation in pilot projects, such as wind assisted propulsion systems. Onboard carbon capture is still under development with several pilot projects taking place around the globe. Most of the carbon capture systems proposed are based on the absorption-desorption principle and utilize scrubber-like columns for capturing the CO2 from the vessel’s exhaust gas. However, this concept is likely too bulky and expensive for effective use onboard. Marine vessels will require more compact and modular carbon capture systems that will enable them to capture 20-30% of their CO2 emissions by 2030, and gradually increase this amount to 60-70% by 2040. This solution combined with energy efficiency technologies can help shipowners and operators to comply with the 2030 and 2040 GHG regulations without changing their fuels and by making investment choices that increase the value of their vessels.
The above technologies can also be combined with biofuels that are drop-in solutions for vessels in operation. Biodiesel (FAME) is an alternative fuel that blends well with fuel oil and has been demonstrated through multiple trials at sea. Depending on the feedstock, biodiesel can be produced with a lifecycle carbon footprint of < 33 gCO2e/MJ, which will make it eligible for use based on the expected IMO regulations and the FuelEU Maritime regulations. Additional promising biofuels, such as bio-oil and bio-crude are currently under development but will need a significant increase in their production volumes in order to cover part of the global bunker fuel consumption. Current bunker prices of biodiesel require a 15- 30% premium compared to VLSFO, but this is expected to drop as production and supply increases. Nevertheless, utilizing drop-in fuels such as biodiesel will be much more cost effective than using zero-carbon e-fuels in the future.
Overall, the current technology landscape presents great opportunities for decarbonization of the global fleet at reasonable investments and useful timelines. Marine vessels can reduce their GHG emission by (i) adopting some necessary energy efficiency technologies, mostly for their hull and propellers, (ii) installing modular carbon capture systems starting with low capture rates (20-30%), and/or (iii) utilizing drop-in biofuels at low blends (also 20-30%). This combination is likely sufficient to make the vessels compliant with the 2030 regulations and approach the 2040 ones without breakthroughs needed in e-fuel technology and production. These solutions are also cost-effective, increase the value of the assets, and can be applied to a wide range of vessels serving a wide range of needs
Sotirios Mamalis, PhD, Senior Development Engineer, CLEOS This email address is being protected from spambots. You need JavaScript enabled to view it.