Modeling of Nitric Oxide NOx

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). 

Most models assume that the fuel-bound nitrogen that is released by the devolatilization of coal is in the form of HCN, or some instantaneous transforms of HCN, which in turn form the base species of NO formation. It is believed that the HCN not only contributes to fuel NOx formation but also to some destruction of NOx and that the net formation might depend on the chemical as well as the thermal state of the mixture. The global chemical reactions involved for coal flames might therefore be expressed as 

Kinetic studies have shown that the principal pathways of NO formation and reduction by hydrocarbon radicals are 

The concentration of H radicals in the post flame regions of hydrocarbon diffusion flames have been observed to be on the same order as the H2 concentration, and the OH radical can be estimated by the observed partial equilibrium of the reaction 

These fast chemistry multi-mixture fraction combustion models and similar models have been incorporated into several CFD packages including CINAR, FLUENT, and others. 

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