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Formation of NOx

Nitrogen oxides are the major air pollutants and pose a challenge to engine manufacturers to meet the limits set forth by the MARPOL Annex VI. 

Combustion of fuels is the primary anthropogenic source of formation of nitrogen oxides, While Nitric oxide (NO) and nitrogendi-oxide are usually grouped together as NOx emissions, nitric oxide is the predominant oxide of nitrogen produced by combustion process. Therefore, it is an important minor species in the combustion because of its contribution to air pollution. 

The principal source of NO is the oxidation of atmospheric (molecular) nitrogen. However, if the fuel contains significant nitrogen, the oxidation of the fuel nitrogen-containing compounds is an additional source. In the combustion of fuel that contains no nitrogen , nitric oxide is formed by three chemical mechanisms that involve nitrogen from the air. 

The three principal mechanisms are:
i) The thermal or Zeldovich mechanism
ii) The Fennimore or prompt mechanism
iii) N2O – the intermediate mechanism

The formation of NOx is a function of combustion temperature (i.e. high peak pressure, high compression ratio, high rate of fuel delivery, etc) and residence time. Therefore, the fundamental principle is regardless of what measures you take to bring down the NOx emission levels, they must be effective in keeping the peak temperature down, low and residence
time to minimum.

What Causes NOx in Marine Engines?
  1. High Cylinder Temperature and Pressure during combustion process
  2. Heavy Load on the engine or engine unit.
  3. Improper air and fuel ratio for combustion.
  4. High Temperature of intake or scavenge air inside the cylinder.
  5. Over heated cylinder jacket due to poor heat transfer by jacket cooler.
  6. Jacket water temperature is on the higher side.
  7. Bad quality of fuel used for engine.


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