Mechanisms Governing the Formation and Destruction of NO_x and Other Nitrogenous Species in Low NO_x Coal Combustion Systems¹

Abstract

The salient fundamental factors that influence the design of low NO _x burners and of NO _x abatement schemes are reviewed. They have been examined through laboratory combustor studies, which simulate some of the salient features of practical systems, while still preserving flexibility and the ability to extract mechanisms. These studies have shown that coal ignition properties and flame attachment to the burner have great influence on coal nitrogen convention to NO _x in axial turbulent diffusion flames. They have also shown that distributed air addition is more effective at reducing NO _x formation than simple air staging, and these results conform to field data from full scale applications

Short time scale phenomena in the ignition zone involve heterogeneous and micro-scale diffusion phenomena, and the total volatile nitrogen released is only partially converted to NO _x even in the presence of excess quantities of oxygen. Micro-mixing also complicates predictions of short time scale phenomena during reburning. However, for both pulverized coal staged combustion, and gas reburning situations, long time scale concentration profiles of NO, HCN and NH₃ can be well modelled by the same simplified kinetic mechanism, which is based on known detailed gas phase reactions and partial equilibrium assumptions.

Key Words:

• NO _x coal combustion
• staged combustion
• mechanisms

Notes

¹ Invited lecture, presented at the Second International Conference on Combustion Technologies for a Clean Environment, Calouste Gulbenkian Foundation, Lisbon, Portugal, 19-22 July 1993.