I have read in analyzer vendor literature that NOx formation is the indication for best combustion in boilers (than Oxygen in flue gas). But we have to limit it. How far is it correct? can anybody give technical reference?
keith bowers, B and B Consulting, email@example.com
NOx is a function of peak flame temperature and oxygen content at that point. IF your objective is maximum flame temperature, perfect stoichometry at the flame front is necessary. However, nature rules, and it is very difficult to achieve this ideal ratio AND achieve complete consumption. This is the reason 'excess air' is always required in process furnaces.
The imperfect mixing of air (oxygen and nitrogen) with fuel and turbulence in the flame causes some fuel and oxygen to move out of the flame front before combustion is complete.
Design of the burners and burner tile are critical to achieving high combustion efficiency, low NOx, short flame length, and stable combustion at different firing duties. Staged air inlet, induced combustion gas recycle in the flame front, and proper burner tile design will achieve very high combustion efficiency (low CO and HC in the flue gas) AND low NOx. John Zink and Callidus are two good firms very experienced burner/burner tile design and fabrication.
Low NOX levels in the flue gas, by itself, does not mean good combustion. It simply means there was either low temperature in the combustion zone, there was no excess oxygen in the combustion zone, or both. High excess oxygen in the flue gas also is of little value by itself since it can result from air in-leakage anywhere and does not imply there was enough, much less, excess air in the combustion zone.
I have often seen adequate or more 'excess air' in the furnace flue gas, coincident with significant CO and unburned hydrocarbons!! Very low CO, no unburned hydrocarbons, low NOX, and low oxygen in the flue gas DOES imply excellent combustion conditions.