NOx and SOx emissions from ships contribute significantly to air pollution, human health issues, and marine environmental degradation. MARPOL Annex VI regulates these emissions, with stricter limits in Emission Control Areas (ECAs) compared to open seas.

• SOx ECAs: Maximum fuel sulfur content = 0.10% m/m.
• Outside ECAs (open waters): Maximum fuel sulfur content = 0.50% m/m.
• NOx: Tier III standards, the most stringent, apply in NOx ECAs. Specific limits depend on engine construction date.

Environmental Impacts

Sulfur Oxides (SOx):

• Acid Rain: SOx leads to acid rain formation, which harms sensitive ecosystems and damages forests.
• Air Quality: Contributes to urban smog and regional haze, reducing visibility.
• Health: High concentrations adversely affect human respiratory health.

Nitrogen Oxides (NOx):

• Smog and Ozone: Promote ground-level ozone (smog), aggravating asthma and respiratory conditions.
• Vegetation Damage: High NOx levels reduce crop yields and damage plant growth.
• Eutrophication: Nitrogen deposition accelerates eutrophication in coastal waters, causing algal blooms, oxygen depletion, and harm to aquatic life.

NOx and SOx Limitations as per MARPOL Annex VI

NOx Limitations

The latest NOx emission Tier III has been enforced from January 1, 2016, in Emission Control Areas (ECAs). Tier II emission limits apply to engines installed on or after January 1, 2011, while diesel engines installed on or after January 1, 2000, are required to comply with Tier I emission limits.

The NOx emission limits (in g/kWh) are as follows:

Note: NOx Tier III is applicable only in ECAs. Outside ECAs, Tier II limits apply.

SOx Limitations

The fuel oil sulphur limit for SOx control is as follows:

Methods to Control NOx Emissions

NOx control methods can be divided into two categories:

1. Primary Methods:

Primary NOx reduction measures are implemented within the combustion chamber itself to reduce nitrogen oxide (NOx) emissions at their source. These measures include the use of:

• Low-NOx burners
• Improved fuel oil quality
• Combustion air preheating
• Fuel-water emulsions

These techniques aim to lower the peak combustion temperature, thereby reducing NOx formation. While highly effective, primary methods typically require significant investment and can affect engine performance and operational characteristics.

2. Secondary Methods:

• Selective Catalytic Reduction (SCR): In this system, urea or ammonia is injected into the exhaust gas before it passes through a unit consisting of a special catalyst layer. A chemical reaction between urea/ammonia and NOx reduces NOx to nitrogen (N2​) at temperatures between 300-400°C. SCR units are typically installed between the manifold and the turbocharger.
• Exhaust Gas Recirculation (EGR): In this system, a portion of the exhaust gas is recirculated to the scavenge air receiver after passing it through a scrubber unit. EGR is claimed to achieve around 50-60% NOx reduction from Tier I levels. However, the discharge of the cleaning water from the scrubber requires treatment.