MECHANISMS GOVERNING THE FORMATION AND DESTRUCTION OF NOX AND OTHER NITROGENOUS SPECIES IN LOW NOX COAL COMBUSTION SYSTEMS

The salient fundamental factors that influence the design of low NOx b urners and of NOx abatement schemes are reviewed. They have been exami ned through laboratory combustor studies, which simulate some of the s alient features of practical systems, while still preserving flexibili ty and the ability to extract mechanisms. These studies have shown tha t coal ignition properties and flame attachment to the burner have gre at influence on coal nitrogen conversion to NOx in axial turbulent dif fusion flames. They have also shown that distributed air addition is m ore effective at reducing NOx formation than simple air staging, and t hese results conform to field data from full scale applications. Short time scale phenomena in the ignition zone involve heterogeneous and m icro-scale diffusion phenomena, and the total volatile nitrogen releas ed is only partially converted to NOx, even in the presence of excess quantities of oxygen. Micro-mixing also complicates predictions of sho rt 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 g as phase reactions and partial equilibrium assumptions.