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Module 6: Air Pollutants and Control Techniques - Nitrogen Oxides - Formation Mechanisms
Lesson Material
Practice
Problems
Objective
- Describe the basic formation mechanisms for nitrogen oxides.
Nitrogen oxides are formed during the combustion of fuel in the presence of air. At elevated temperatures, nitrogen from the air and the fuel reacts with oxygen to form nitrogen oxides. Approximately 90 to 95% of the nitrogen oxides generated in combustion processes are in the form of nitric oxide (NO). Once in the atmosphere, the NO reacts in a variety of photochemical and thermal reactions to form NO2. Accordingly, the total mass emissions of nitrogen oxides from the unit are usually expressed in the form of NO2.
Nitrogen oxides are formed from two different sources:
- Atmospheric nitrogen (N2) entering the combustion zone as part of the combustion air
- Nitrogen present in the fuel or waste
The nitrogen oxides generated from atmospheric nitrogen are often termed thermal NOx because they are formed in the high temperature areas around burner flames in combustion chambers. Nitrogen oxides generated from the fuel or waste are termed fuel NOx. Not all of the fuel nitrogen compounds are released during combustion. Unlike sulfur, a significant fraction of the fuel nitrogen remains in the bottom ash or in the fly ash.
The complex sets of reactions responsible for nitrogen oxides generation are very sensitive to high oxygen concentrations and high gas temperatures in the combustion zone. Nitrogen oxides emissions are highest during high boiler or incinerator loads because the highest gas temperatures occur under these conditions.
Practice Problems
Nitrogen Oxides - Formation Mechanisms
- Instructions:
- Complete the Practice Problems before proceeding to the next section. Click on the button below.