How may NOx be reduced in exhaust gases onboard?

06 Sep '18, 11:34

Sept. 6, 2018, 11:34 a.m.
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NOx content reduced by use of Catalytic Conversion Of Exhaust Gases. The principle behind catalytic reduction is to combine nitrogen atoms of NOx and NH3 to provide a stable nitrogen molecule. To do this at normal exhaust temperatures a reducing the NOx catalyst is used. The type of catalyst used will vary according to NOx levels, type of fuel and temperatures. The catalyst core usually takes the form of a heat-resistant ceramic with the catalyst carried on the surface. The catalyst must present a large area to maximize the chance of every NOx and NH3 molecule coming into contact with it. The open area in the catalyst chamber must also be large to prevent excessive back pressure on exhaust gas flow. Catalysts act either by themselves of introduced as a coating applied to a carrier material. For marine purposes, the selective catalytic reducer (SCR) has been found to be most suitable for use with diesel, engines whose exhaust is rich in oxygen.
The SCRF controls emissions by using a de-NOx and oxidation catalyst to convert NOx, HC and CO to molecular nitrogen, water and carbon dioxide. It also oxidizes some diesel soot to form normal constituents of air that are considered harmless.
For the marine system urea was chosen as the reducing agent as opposed to ammonia which requires greater quantities is more expensive and toxic. Urea comes in a white crystalline form, is harmless to store, melts at 113½C and dissolves easily in water.
In practice, the urea acts as a carrier of N atoms so that when it comes into contact with NOx and N atoms combine to form N2. The system works best with four-stroke medium and high-speed engines the slow speed two-stroke engines requiring a different approach. alt text

In the first part of the system, NOx is oxidized to form N2 and H2 further along the system excess ammonia is removed and CO2 and HC are oxidized to CO2 and H20. Some soot is also burnt.

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06 Sep '18, 11:41

Sept. 6, 2018, 11:41 a.m.
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