Basis for NOV Prevention

In the combustion of fuels that do not contain nitrogen compounds, NO.v compounds (primarily NO) are formed by two main mechanisms, the thermal mechanism proposed by Zeldovitch and the prompt mechanism [5]. In the thermal mechanism NO is formed by the oxidation of molecular nitrogen through the following reactions  

The prompt mechanism is initiated by hydrocarbon radicals, predominantly 

The HCN and N are converted rapidly to NO by reaction with oxygen and hydrogen atoms in the flame. 

The prompt mechanism predominates at low temperatures under fuel-rich conditions, whereas the thermal mechanism becomes important at temperatures above 1500 °C. Due to the onset of the thermal mechanism the formation of NO. in the combustion of fuel/air mixtures increases rapidly with temperature above 1500°C and also increases with residence time in the combustor (Fig. 1). The thermal and prompt mechanisms are discussed in detail in the review by Miller and Bowman [5]. 

In catalytic combustion of a fuel/air mixture the fuel reacts on the surface of the catalyst by a heterogeneous mechanism. The catalyst can stabilize the combustion of ultra-lean fuel/air mixtures with adiabatic combustion temperatures below 1500°C. Thus, the gas temperature will remain below 1500°C and very little thermal NO. will be formed, as can be seen from Fig. 1. However, the observed reduction in NO. in catalytic combustors is much greater than that expected from the lower combustion temperature. The reaction on the catalytic surface apparently produces no NO. directly, although some NO.v may be produced by homogeneous reactions in the gas phase initiated by the catalyst. 

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