Nitrogen oxide (NOx) emissions from ships are a significant contributor to air pollution and climate change. These concerns have led to stricter regulations on emissions.
But let us start with the basics: What is NOx?
Nitrogen oxides (NOx) are gases that play a major role in air pollution and the creation of smog. When NOx is exposed to sunlight, it breaks down and produces ozone (O₃). Unlike its protective role in the stratosphere’s ozone layer, when at ground level O₃ acts as a harmful pollutant. Additionally, when NOx interacts with rain, it forms nitric acid, which contributes to acid rain. These substances can easily enter the lungs and cause significant damage to delicate lung tissue. Even brief exposure may irritate the lungs in otherwise healthy individuals.
What are the requirements?
The environmental and health concerns have led to stricter global regulations on NOx emissions in shipping. To address this, the International Maritime Organization (IMO) has introduced a three-tier system, where Tier I and Tier II are global and Tier III apply only in certain NOx Emission Control Areas (ECAs).
- Tier I applies for ships constructed on or after 1 January 2000, and limits the engine NOx emission;
- Tier II applies for ships constructed on or after 1 January 2011 and requires a reduction in NOx emissions by 20% compared to the previous Tier I values;
- Tier III applies for marine diesel engines with an output of 130 kW or higher installed on ships constructed on or after 1 January 2016 as well as for new engines installed in all ships on or after 1 January 2016. Tier III requires ships to reduce their NOx emissions by 80% compared to Tier I requirements, when operating in the North American ECA and the US Caribbean Sea ECA – and for ships constructed on or after 1 January 2021, also when operating in the Baltic Sea or North Sea ECAs.
How can the shipping industry reduce these risks?
The most common technology options to achieve NOx Tier III compliance on two-stroke main engines are Selective Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR).
SCR works by injecting marine-grade urea into the exhaust stream, where it reacts over a catalyst to convert NOx into harmless nitrogen and water. EGR, on the other hand, reduces NOx formation by recirculating a portion of cooled exhaust gas back into the engine, lowering combustion temperatures and thereby limiting NOx formation at the source.
The challenges for shipowners particularly concern the choice of NOx reduction technology, as SCR systems require availability and cost management of marine-grade urea. This also comes with the need for proper training and support for the crew to ensure suitable handling of the new technology onboard.
To sum up:
Ships built after 2016 must comply with NOx Tier III rules when sailing in the North American and US Caribbean Sea ECAs, and those built after 2021 must also comply in the Baltic and North Sea ECAs. Compliance means cutting NOx emissions in shipping by 80% compared to Tier I, using technologies such as SCR or EGR. And though this comes with operational and cost challenges for shipowners, it is a major step towards cleaner air.
Did you know that alongside NOx the other major pollutant from the shipping industry is Sulphur Oxides (SOx)?
Fuel oil has a relative high SOx content and is emitted with the ship’s exhaust gas and can lead to acid rain when released into the atmosphere. Regulations on SOx are therefore primarily focused on limiting the sulphur content in marine fuel, by using Low-Sulphur Fuels or Exhaust Gas Cleaning Systems (Scrubbers) to remove SOx from the exhaust gases to prevent releasing them into the atmosphere.