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Posted: April 29th, 2023
Ship emissions reduction
Shipping contributes significantly to global emissions of air pollutants and greenhouse gases. It is estimated that international shipping accounts for approximately 2-3% of global carbon dioxide (CO2) emissions from fossil fuel combustion (IMO, 2020). However, shipping emissions are projected to increase substantially in the coming decades due to anticipated growth in global seaborne trade volumes. This poses serious challenges for efforts to mitigate climate change and reduce other environmental impacts from the maritime sector. Fortunately, a variety of technical and operational measures exist that can help reduce ship emissions. This article provides an overview of key strategies for reducing emissions from ships, including examples of their application and estimated emission reduction potentials.
Engine and Fuel Improvements
Many strategies for reducing ship emissions involve improvements to main and auxiliary engines or changes to fuel types. Slow steaming, where ships reduce speed to save fuel and cut emissions, has been widely adopted in the industry. Estimates indicate slow steaming of approximately 20-30% can reduce CO2 emissions per transport work by 10-30% (Buhaug et al., 2009). Other engine-related measures include engine tuning and maintenance to optimize performance and efficiency. Waste heat recovery systems capture unused heat from engines and generators to power auxiliary equipment, cutting both fuel use and emissions.
Alternative low-carbon and zero-carbon fuels are also being developed to replace conventional heavy fuel oil and marine gas oil. Liquefied natural gas (LNG) emits less CO2, particulate matter, and sulfur oxides compared to heavy fuel oil when combusted. Several major shipping lines have ordered LNG-fueled ships in recent years. Biofuels produced from biomass or waste oils can provide a low-carbon drop-in fuel alternative, though high production costs remain a challenge (IMO, 2020). Ammonia is also gaining attention as a potential zero-carbon fuel that can be produced from renewable hydrogen. Fuel cells powered by hydrogen may enable fully electric propulsion on some ship types in the future.
Operational Measures
In addition to engine and fuel changes, optimizing ship operations can yield significant emissions benefits. Just-in-time operations and optimized speed profiles based on weather and sea conditions help ships arrive on schedule while burning less fuel. Route planning tools utilize real-time data on ocean currents, waves, and weather to determine the most fuel-efficient transit routes. Speed limits in coastal and Emission Control Areas further reduce emissions from ships sailing in these sensitive regions.
Hull and Propeller Design
Advances in hull and propeller design have the potential to boost fuel efficiency by several percentage points. Hull coatings that inhibit marine growth can maintain hydrodynamic performance over time. Finer hull lines and optimized propeller designs tailored for specific ship types and operational profiles also enhance efficiency. Newer ships increasingly feature such efficiency-enabling designs. Retrofitting older vessels with upgrades like propeller boss cap fins or hull air lubrication systems also provides benefits, though retrofits are more challenging than incorporating technologies during newbuild construction.
Exhaust Gas Cleaning Systems
Exhaust gas cleaning systems, also known as scrubbers, are installed onboard ships to remove sulfur and particulate matter from engine exhaust before it is emitted into the air. Open-loop scrubbers clean exhaust by mixing it with seawater, while closed-loop systems treat exhaust with alkaline chemicals. When fitted to ships using high-sulfur fuel, scrubbers enable compliance with stringent sulfur emission limits without requiring fuel changes. They have been rapidly adopted in recent years, with over 4,000 scrubbers now installed globally (DNV GL, 2021). While scrubbers reduce local air pollution, they do not cut CO2 or other greenhouse gases.
Market-Based Measures
A variety of market-based measures aim to incentivize emissions reductions from shipping. The International Maritime Organization’s (IMO) Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) regulations will require ships to meet progressively stricter energy efficiency standards over time to discourage inefficient designs. The European Union’s Emissions Trading System now covers emissions from 50% of cargo ships arriving and departing EU ports. Revenues generated can support low-carbon R&D and deployment in the sector. Port state control of ship emissions also plays a role by enforcing international regulations. Looking ahead, a global market-based measure, such as an international fuel levy or emissions trading system, could accelerate decarbonization of international shipping if implemented through the IMO.
Conclusion
The maritime industry faces considerable challenges to achieve the deep emissions cuts needed to align with climate targets. However, a diverse portfolio of technical and operational solutions exists today that can significantly reduce emissions from ships. Widespread deployment of these measures supported by enabling policies and investments presents the best strategy for putting the sector on a sustainable path. Areas ripe for further progress include alternative fuels, energy efficiency innovations, optimized newbuild designs, and market-based policies. With coordinated global action, shipping can curb its environmental impacts while continuing to enable global trade and economic development in a low-carbon future.
References
Buhaug, Ø., Corbett, J. J., Endresen, Ø., Eyring, V., Faber, J., Hanayama, S., … & Winebrake, J. J. (2009). Second IMO GHG study. London, UK: International Maritime Organization (IMO).
DNV GL. (2021). Scrubber insights annual report 2021. Høvik, Norway: DNV GL. https://www.dnv.com/publications/scrubber-insights-annual-report-2021-205051
International Maritime Organization. (2020). Fourth IMO greenhouse gas study 2020. London, UK: International Maritime Organization (IMO). https://www.imo.org/en/OurWork/Environment/Pages/Fourth-IMO-Greenhouse-Gas-Study-2020.aspx
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