Prompt nitrogen oxides, formation

Prompt Nitrogen Oxides emerge because of the lack of oxidizer in the reaction medium. Their formation (Fenimore mechanism) is based on the following reactions ... 

The amount of prompt NO produced in combustion systems is relatively small compared with the total NO formation. However, prompt NO is still formed at low temperatures and is one of the features in producing ultra-low NO burners. The nitric oxide reacts with other species in the atmosphere to give various other nitrogen oxides, namely NO2 and nitrogen pollutants. 

The relative importance of these three mechanisms in NO formation and the total amount of prompt NO formed depend on conditions in the combustor. Acceleration of NO formation by nonequilibrium radical concentrations appears to be more important in non-premixed flames, in stirred reactors for lean conditions, and in low-pressure premixed flames, accounting for up to 80% of the total NO formation. Prompt NO formation by the hydrocarbon radical-molecular nitrogen mechanism is dominant in fuel-rich premixed hydrocarbon combustion and in hydrocarbon diffusion flames, accounting for greater than 50% of the total NO formation. Nitric oxide formation by the N20 mechanism increases in importance as the fuel-air ratio decreases, as the burned gas temperature decreases, or as pressure increases. The N20 mechanism is most important under conditions where the total NO formation rate is relatively low [1],... 

In the flue gases of power installations with gas flow rates of 25,000—32,000 m /h, nitrogen oxides concentration is 400—800 mg/m, whereas the figures for installations with gas flow rates of 35,000—80,000 m /h are higher, 700—1500 mg/m [296]. Of the three major kinetic mechanisms of the formation of nitrogen oxides during the combustion of hydrocarbon fuels, thermal (Zeldovich mechanism), prompt (Fenimore mechanism) and fuel NOx, the major role in the combustion of natural gas belongs to the thermal mechanism [297], This mechanism, described by the two main reactions... 

During combustion processes the molecular nitrogen in the combustion air and the fuel nitrogen that may be present in the fuel is converted into nitric oxide and some nitrogen dioxide4 when NO and residual O2 are cooled together. The NO formation is also controlled by (1) thermal NO, (2) prompt NO and (3) N2O to NO routes5-7. 

Nitric oxide (NOx) is one of the main pollutants in combustion systems and rotary kilns are no exception particularly for pulverized fuel combustion. NOx formation depends on three factors, namely (i) the amount of nitrogen present in the fuel, (ii) the combustion temperature, and (iii) the stoichiometric conditions for the combustion reaction. Hence NOx production is classified into fuel NOx, thermal NOx, and prompt NOx. Some of the mechanisms for the formation of these species during pulverized coal combustion in rotary cement kilns have been described in commercial CFD packages (e.g., FLUENT, CINAR). 

Prompt NO is the third, but usually small, source and is attributed to the reaction of atmospheric N2 with radicals such as CH2 fragments derived from the fuel. This results in the formation of nitrogen species such as HCN that can be further oxidized to NO. 

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