Understanding the effect of CaO on HCN conversion and NOx formation during the circulating fluidized combustion process using DFT calculations

Hydrogen cyanide (HCN) is an important intermediate during the conversion of fuel nitrogen to NO_x. The mechanism of HCN oxidation to NO, N₂, and N₂O on the CaO (100) surface model was investigated using density functional theory calculations to elucidate the effect of in-furnace SO_x removal on HCN oxidation in circulating fluidized bed boilers. HCN adsorption on the CaO (100) surface releases as high as 1.396 eV and the HC bond is strongly activated. The CaO (100) surface could catalyze the oxidation of CN radical to NCO with the energy barrier decreasing from 1.560 eV for the homogeneous case to 0.766 eV on the CaO (100) surface. The succeeding oxidation of NCO by O₂ forming NO is catalyzed by the CaO (100) surface with the energy barrier decreasing from 0.349 eV (homogeneous process) to 0.026 eV on the CaO (100) surface, while the reaction between NCO and NO forming either NO or N₂ is prohibited in comparison with corresponding homogeneous routes. The rate constants of these reactions under fluidized bed combustion temperature range are provided, and the calculation results lead to the conclusion that CaO (100) surface catalyzes the HCN conversion and improves the NO selectivity during HCN oxidation in the HCN/O₂/NO atmosphere, which could well explain previous experimental observations. Kinetic parameters of HCN oxidation on the CaO (100) surface are provided in the Arrhenius form for future kinetic model development.