Prediction of Nitric Oxide Destruction by Advanced Reburning

Abstract

Advanced reburning refers to a process wherein injection of a hydrocarbon fuel such as natural gas aft of the combustion zone is followed by injection of a nitrogen-containing species such as ammonia. In recent work, the authors used a systematic reduction method to develop a four-step, eight-species reduced mechanism from a 312-step, 50-species full mechanism for advanced reburning processes. The four-step model has been integrated into a comprehensive computational fluid dynamics combustion code, PCGC-3. In this work, the integrated model for advanced reburning has been evaluated through comparisons with experimental data for species and temperature profiles as well as effluent NO concentrations. Comparisons are shown herein for a base case, with reburning only (natural gas addition) and with advanced natural gas reburning (natural gas addition followed by NH3 addition). Profile comparisons show that the predicted flow-average axial NO concentration with advanced reburning followed the trends of experimental data, though the predicted initial NO level was lower than experimental data by 20−30%, partly due to both inaccurate prediction of flame structure and experimental error. Comparisons of effluent molar flux of NO with variation in swirl number, (NH3/NO)in and location of NH3 injection show that predicted trends and magnitudes of change were consistent with measured trends, though the predicted NO reduction was typically much higher than measured. Measurements and predictions both showed that NO reduction increased with increasing swirl number, (NH3/NO)in and reburning zone residence time. Most of the NO reduction is caused by chemical reaction, and only a small part of the NO reduction is due to dilution. Study results provide directions for further research.