Catalytic oxidation of NH3 over circulating ash in the selective non-catalytic reduction process during circulating fluidized bed combustion

Abstract

Selective non-catalytic reduction (SNCR) with NH3 as the reducing agent is widely used for the denitrification of flue gas in circulating fluidized bed combustion (CFB) process, where the circulating ash can cause the oxidation of NH3. In this study, the behaviors and mechanisms of NH3 catalytic oxidation over circulating ash were systematically investigated. The results revealed that NH3 could be catalytically oxidized to form NO efficiently by circulating ash. Along with the temperature increasing from 550 °C to 950 °C, the formation of NO increased at first and then decreased. Oxygen was crucial to NH3 catalytic oxidation, while its effects on NO formation were slight at high concentrations. Meanwhile, as the initial NH3 concentration increased, the concentration of NO in the flue gas increased while the selectivity of NO decreased because of the enhanced homogeneous reduction between the residual NH3 and NO. Fe2O3 was confirmed to be one of the principal mineral components in circulating ash to catalyze NH3 oxidation. In addition, the mechanisms of NH3 oxidation over circulating ash were also investigated. It was validated that NH2 on Lewis acid sites were the main NH3 adsorbed species (ad-species), and the main oxidation intermediates of NH3 ad-species were bidentate and bridged nitrates in the presence of O2.