What is Nitrogen Oxides or NOx air pollution from Ships?

With more and more ships traveling each day to different parts of the world, the air pollution caused by them is on the rise and is one of the major global concerns. The two main pollutants from the ship’s emission are Nitrogen oxides (NOx) and Sulphur oxides (SOx). These gases have adverse effects on the ozone layer in the troposphere area of the earth’s atmosphere which results in the green house effect and global warming.

Both NOx and SOx are combustion products that are emitted in to the environment in the form of smoke. MARPOL Annex VI is strictly followed to have the air pollution under control limits.

ship pollution

Nitrogen Oxides or NOx Pollution

Marine fuel in an I.C engine is burnt inside the combustion chamber by the correct mixture of fuel and air in the presence of heat or ignition source.

The ignition source in the marine engine is the compression stroke of the piston, after which, the combustion begins.

As the air mixture is 21 % Oxygen and 78% Nitrogen, nitrogen reacts with oxygen under certain engine operating conditions to form Nitrogen oxides or NOx.

What Causes NOx in Marine Engines?

  • High Cylinder Temperature and Pressure during combustion proce
  • Heavy Load on the engine or engine unit.
  • Improper air and fuel ratio for combustion.
  • High Temperature of intake or scavenge air inside the cylinder.
  • Over heated cylinder jacket due to poor heat transfer by jacket cooler.
  • Jacket water temperature is on the higher side.
  • Bad quality of fuel used for engine.

Limits for NOx Emission For merchant vessel

-17.0 g/kwh when engine rpm (n) is less then 130 rpm.

-45.0 x n (-0.2) g/kwh when ‘n’ is between 130 rpm or above 130 rpm but less then 2000rpm.

-9.8 g/kwh when ‘n’ is 2000 rpm and above.


n- Rated engine speed

g/kwh-  gram per kilo watt hour

rpm   –  revolution per minute

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References: IMO

About Author

An ardent sailor and a techie, Anish Wankhede has voyaged on a number of ships as a marine engineer officer. He loves multitasking, networking, and troubleshooting. He is the one behind the unique creativity and aesthetics at Marine Insight.

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