OCR Text |
Show F O R M A T I O N O F N Ox Although nitrogen forms many oxides, the only ones present in measurable quantities in the NOx in combustion gases are nitric oxide (NO, colorless, odorless, toxic, eventually forms NO2 at ambient conditions) and nitrogen dioxide (NO2/ reddish brown, pungent, toxic, forms photochemical smog in sunlight), butatnormal combustion temperatures and residence times, the formation of NO is strongly favored over NO2 both thermodynamically and kinetically. Therefore, under typical industrial combustion conditions, NO is predominant in the NOx content of stack gases from fired furnaces. The exact processes by which NOx is formed are very complex and dependent on the outcome of competing reactions in which atomic oxygen , atomic nitrogen and nitrogen compounds are formed, destroyed and reformed. Nitrogen, oxygen and other molecules undergo thermal dissociation at normal combustion temperatures (2800°F+). These and the production of certain intermediate hydrocarbon radicals provide the constituents necessary to initiate the sequence of interractions which end up producing NO. It is important to appreciate the differences as well as the similarities between NOx formation from nitrogen in the air, often called nitrogen fixation, and that resulting from fuel bound nitrogen. Past work has shown that nitrogen fixation during combustion is increased by high flame temperatures and the presence of excess air (oxygen). However, low excess air is required in many practical applications. Temperature rises as excess air is reduced which increases NOx even though the reduced availability of oxygen may tend to dampen this. Operating substoichiometrically, as is possible in staged burners, provides both 13/3 |