Nitrogen chemistry during burnout in fuel-staged combustion

A parametric study of the chemistry of the burnout zone in reburning has been performed in laboratory plug flow reactors in the temperature range 800-1350 K. Inlet mole fractions of NO, NH3, HCN, CO, and O-2 were varied, together with different temperatures and residence times to simulate reaction conditions in practical systems. Under lean conditions, a minimum in NO emission exists as a function of temperature. Both HCN and NH3 can act as either NO reductants or as sources for NO by oxidation. Reactions and selectivities for HCN and NH3 are controlled by the radical pool produced by fuel (CO) oxidation. As increasing amounts of CO were added, temperatures for both ignition and the minimum in NO became lower. At 2% CO, 4% O-2, and 100 ms residence time, the minimum in NO was found at approximately 1000 K. At low temperatures, significant amounts of N2O were measured in the reactor outlet. This is attributed to N2O formation by HCN/NO reactions and to the slow decomposition of N2O at these temperatures. Large reductions in NO were seen under fuel-rich conditions and at high temperatures. The observed NO reduction was very much dependent on the inlet mole fraction of O-2. Detailed chemical kinetic modeling of the experiments showed reasonable predictions for overall fuel-lean conditions, but the model failed to predict experimental results under fuel-rich conditions. The present results provide guidelines for optimizing the conditions for the burnout process of reburning, as well as other processes for NOx reduction by staged combustion. The results also provide a test basis for verifying kinetic models for nitrogen chemistry at low temperatures (800-1350 K). Copyright (C) 1996 by The Combustion Institute