Updated effect of carbon monoxide on the interaction between NO and char bound nitrogen: A combined thermodynamic and kinetic study

Carbon monoxide (CO), as one of the major gaseous product in the oxy-fuel combustion boiler, is found to exert great influence on nitric oxide (NO)-char reactions. Unfortunately, the detailed mechanism of CO enhancement still remains obscure. In this work, starting from the armchair model with embedded nitrogen, we propose some updated mechanisms with details at the molecular level that can describe the role of CO through energetic and kinetic evaluations based on the combination of density functional theory (DFT), transition state theory (TST) and Chemkin calculations. The preferred pathways are identified with 54 intermediates and 68 transition states. The addition of CO to the surface carbyne site with C-down mode is the most favorable and triggers the low-temperature desorption of CO2 via C-C bond dissociation due to its low barrier of approximately 40 kJ/mol. One important role of CO is to react with the surface oxygen atom in the species C(NO) to clean the surface. More specifically, the energy barrier for nitrogen exposure reduced from 311.8 kJ/mol to 243.4 kJ/mol if the aromatic armchair edge is clean, and further reduced to 211.8 kJ/mol with the existence of nearby carbine type site. In addition, N-2 desorption, is sensitive to, and largely facilitated by, the presence of CO. The results are of great value to interpret various reported experimental phenomena, including but not limited to, the small effect of CO at high temperatures, the initial NO reduction temperature at char surface, and the important impact of CO on the reaction activation energy. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.