Focus Shift: NOx Emissions and Why the World Needs to Pay Immediate Attention

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NOx Emissions

Marine vessels release different kinds of harmful emissions (SOx, NOx, and particulate matter) into the air which have severe side effects on the environment and on human health. The demands of the thriving logistics and marine industries drive the need for more marine vessels, so it is essential to ensure safeguards to limit its impact on the environment. In an effort to curb the harmful impacts of these enormous global industries, the International Maritime Organization (IMO) has released strict guidelines on emissions released from marine vessels. The stringent regulations have forced the industry to re-invent ways to tackle emissions.

Understanding the environmental impacts of NOx

In the third IMO greenhouse gas study, the Marine Environment Protection Committee (MEPC) argued that shipping contributes to 15% of the global nitrogen oxides (NOx) emissions[1]. Unlike Sulphur oxides (SOx), NOx gases are not generated as a direct derivate of the fuel burned, but by the process of burning the fuel. NOx forms when nitrogen reacts with oxygen at high combustion temperatures. The major impact of it entering the environmental ecosystem is that it catalyzes the breakdown of ozone. NOx reacts with other substances and therefore plays a role in forming smog, acid rain, ground-level ozone, and increased levels of fine particles (PM), which are associated with deforestation, surface water acidification, reduced crop yield, and adverse health effects. Health effects are mostly related to the human respiratory system causing inflammation of the airway, respiratory diseases, and increased sensitization to allergens.

Current Scenario in the Marine Industry

On 1st January 2020, a new global cap by the IMO on sulphur content in marine fuels came into effect. The new regulations, known as IMO 2020, mandates a maximum sulphur content of 0.5% in marine fuels globally. Vessel owners could choose to either switch to Very Low Sulphur Fuels (VLSFO) or install exhaust gas scrubbers. SOx scrubbers are widely used for removing harmful sulphur particulates from the exhaust gases before releasing it to the external environment. There is a provision of fines if the vessels do not adhere to the regulations.

Back in 2008, the IMO also adopted the NOx Technical Code to regulate the amount of NOx in released emissions. There have been many subsequent amendments, notably the IMO NOX Tier II and the Tier III. Several technologies, or a combination of technologies, can reduce emissions to Tier III levels like Dual fuel engines (DF)/pure gas engines, Selective catalytic reduction (SCR) and Exhaust gas recirculation (EGR). These solutions have a large physical footprint and require extensive modification works within the engine room. EGRs also involves a corrosion risk for the engine.

Looking towards the decisions made by the various bodies, one can only expect greater reforms towards the environment-friendly operation of large vessel fleets in the near future.

IMO Tier II and Tier III Standards

Tier II and Tier III control set respective NOx limits for ship operation using low-sulphur fuels (0.5%), both outside and inside NOx Emission Control Areas (ECAs). For operation inside ECAs, a ship engine must be able to switch operations to meet Tier III standards. Tier II, which is a global regulation came into effect in 2011, while Tier III was enforced since 2016 in the North American and US Caribbean ECAs. Tier III is an 80% reduction in emissions relative to Tier I. The North Sea and Baltic Sea are to be added to the ECA from January 1, 2021.


IMO Marine Engine Regulations for NOx Limits for Ships at Sea [2]

Date effective



NOx limit, g/kWh, n < 130


Tier II


14.4 g/kWh


Tier III

NOx Emission Control Areas

3.4 g/kWh


Tier III

The North Sea & Baltic Sea

3.4 g/kWh



Global NOx Emissions - Why the World Needs to Pay Immediate Attention

EURO 6 standards and why the marine industry should be striving towards it

EURO 6 is a set of limits for harmful exhaust emissions produced by virtually any vehicle powered by petrol or diesel combustion units. Since September 2015, European vehicle manufacturers have been obliged to ensure that new car emissions do not exceed those limits – as measured in an official test. The Euro 6 standard gained increased importance in 2019 because it encapsulates the criteria with which new ultra-low emission zones (ULEZ) and clean air zones (CAZ) will be enforced [4]. EURO 6 NOx emission limit is around 0.4 g/kWh.

EURO 6 is the most stringent emission qualification for land-based traffic. It is stricter than any requirements for landside power utilities with a power-producing capacity equivalent to large cruise ships. Attaining these ambitious standards would be justification for allowing cruise vessels and any large/medium vessels access to central city ports.

Current Solutions to Tackle NOx Emissions

There is no commercially viable solution (without project-specific external funding) available for large ships to reach EURO-6 emission levels, neither for newbuilds nor retrofits. Current solutions for meeting EURO 6 standards onboard include:

  • Pilot scheme externally subsidised small fully electric ferries built for short-distance transit
  • Pilot scheme externally subsidised fuel cell installations built for producing small auxiliary power

These are not commercially available for cruise vessel loads where the requirement is can be several megawatts even at the port.

Alternatively, there is also a possibility of connecting ships to local landside electricity network while in port to meet EURO 6 limits. But this raises massive challenges like insufficient availability of electricity and lack of expensive related infrastructure in ports as no standards exist for connecting marine vessels with landside electricity networks. The equipment installed onboard only function in certain ports and not elsewhere. The investment cost for each port, typically over the $50 million range, is only viable if all ports apply the same solution.

All existing solutions are either too small scale (and still require external subsidies) or require heavy infrastructure changes with massive investments.

Future Ahead

As the world moves towards environment safe practices and more stringent regulations, it is quite crucial to study the harmful impacts and then act to alleviate them by investing in suitable solutions for the same. IMO 2020 compliant fuels may ensure lesser sulphur emissions, but the growing fuel costs and upgrading of engine systems can be a major problem for vessel operators. Wisely investing in emission abatement technologies may give them an adequate option to ensure compliance as well as getting the best return on investment. NOx emissions from ships will be an increasingly important topic for the coming years for retrofits and new-builts. Novel cost and space-efficient solutions would be needed.




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