The formation rates of both thermal NOx and fuel NOx are kinetically or aerodynamically limited, with the amount of NOx formed being less than the equilibrium value (Wark and Warner 1981). Combustion conditions dominate the formation rate of NOx, and modifying the combustion process can suppress it. Rapidly mixing oxygen with the fuel promotes both thermal and fuel NOx formation.
MacKinnon (1974) developed a kinetic model from experimental studies of a heated mixture of N2, O2, and air as well as air. His kinetic equations provide insight to the strategies for controlling the formation of thermal NOx as follows:
The peak temperature should be reduced. The gas residence time at the peak temperature should be reduced.
The oxygen concentration in the highest temperature zone should be reduced.
Regardless of the mechanisms, several general statements can be made about fuel NOx. It depends highly on the air/fuel ratio. The percent conversion to fuel NOx declines rapidly with an increasing fuel equivalent ratio. (The fuel equivalent ratio is a multiple of the theoretical fuel/air ratio and is the inverse of the stoichiometric ratio. The sto-ichiometric ratio is unity when the actual air/fuel ratio equals the theoretical air to fuel needed for complete combustion.) The fuel equivalent ratio primarily affects the oxidation of the volatile R-N fraction (where R represents an organic fragment) rather than the nitrogen remaining in the char. The degree of fuel-air mixing also strongly affects the percent conversion of fuel nitrogen to NOx, with greater mixing resulting in a greater percent conversion.
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