Managing Emissions from Ammonia-Fueled Vessels

Published — March 3, 2023

Executive Summary

The Mærsk Mc-Kinney Møller Center for Zero Carbon Shipping (MMMCZCS) has identified blue and electro ammonia as potential low-emission alternative fuel pathways. The emissions profile for ammonia fuels is currently unknown, as ammonia engines are still under development. However, emissions from ammonia internal combustion engines (ICEs) may present safety, climate, and regulatory risks, necessitating onboard vessel emission management technologies and solutions.

While ammonia combustion presents emission risks that are not fully known today, a combination of emission management technologies are already available or under development. A dedicated MMMCZCS working group was established to study potential emission scenarios for ammonia ICEs and technologies that can reduce emissions to acceptable levels.

The working group made the following conclusions:

Ammonia combustion presents specific emissions risks related to safety, health, and climate:

  • Ammonia slip is highly toxic, presenting a safety risk for crew and passengers on board the vessel.
  • NOX formed by incomplete ammonia combustion presents a health risk to local communities where vessels operate and must be managed to maintain regulatory compliance.
  • N2O is a potent greenhouse gas (GHG) impacting the global climate (1 gram of N2O is equivalent to 265 grams of CO2).
  • Due to poor ammonia combustion characteristics, secondary or pilot fuel is required. If the pilot fuel is fossil based, it will result in CO2 emissions

Onboard ammonia emission sources require a combination of emission management technologies:

We defined three emissions scenarios based on potential emission profiles, and all scenarios required 3-4 different treatment technologies to achieve acceptable emissions levels. Emission management technologies are needed to treat ammonia boil-off gas (BOG) from fuel tanks, ammonia mixtures from purging and venting operations, and combustion emissions from the engine(s). While such combinations would enable significant emissions reduction, they would also increase the cost and complexity of vessel design compared with vessels operating on conventional fuels.

Managing ammonia emissions is possible and management technology development timelines are expected to align with ammonia ICE development:

Some emission management technologies are already commercially available for maritime use, including reliquefication and selective catalytic reduction (SCR). Others are based on existing maritime or shore-based concepts that need to be adapted for ammonia as a fuel, including engines, gas combustion units (GCU)/ boilers, catalysts, and water catchers/chemical absorbers.

Industry-wide collaboration during engine and emission management technology development is needed to optimize ammonia-fueled vessel designs:

All stakeholders, including engine manufacturers and emission management technology suppliers, must work together to develop ammonia-fueled vessel designs and optimize the use of materials, costs, and overall system efficacy. Without collaboration, specific parts of the vessel design will be developed in isolation, and interconnected systems and technologies could end up unnecessarily oversized, inefficient, or costly. Regulators should follow upcoming tests and technology development closely to ensure that practical, effective, and realistic targets and goals are set from the beginning.

Acceptable ammonia emission levels are not yet clearly defined:

Given the broad range of exposure limits in literature and the lack of knowledge on ammonia as a fuel for the maritime sector, there is a need to be conservative when defining guidelines as an additional safeguard. Thus, low limits are generally included in Classification Society (Class) guidelines, ahead of mandatory International Maritime Organization (IMO) instruments in response to the industry’s interest in ammonia as a fuel. The operational ammonia limits defined in existing Class guidelines vary. Coordinated alignment on thresholds for adequate risk management is required to secure standardization and industry guidance.

Our analysis showed that, with industry-wide collaboration across ammonia engine development, emission management, and vessel design, emission risks will not be a showstopper for ammonia-based fuel pathways. However, well-to-tank (WTT) emissions from ammonia fuels still need to be better understood to assess the overall viability of ammonia-based alternative fuel pathways.

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