Adsorption and catalytic oxidation of residual NH₃ on coal ash after selective non-catalytic reduction in coal-fired boilers

Selective non-catalytic reduction (SNCR) with NH₃ as the reducing agent is widely used for the denitrification of flue gas in coal-fired boilers, where fly ash significantly influences the conversion of the residual NH₃ that does not participate in denitrification. However, there have been few studies on the exact nature of this influence, particularly the adsorption and reaction mechanisms of NH₃ on fly ash. In this study, temperature-programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study the mechanisms of NH₃ adsorption and reactions over coal ash. In the absence of oxygen, in the temperature range of 50-450 °C, NH₃ was adsorbed on the surface of the coal ash. The adsorption capacity of lignite ash was higher than that of anthracite ash. This difference was attributed to the large specific surface area and surface acidity of the lignite ash. However, between 450-850 °C, coal ash had a catalytic effect on NH₃ decomposition and oxidation. Due to the high surface lattice oxygen content of lignite ash, its catalytic oxidative ability was superior to anthracite ash. Moreover, NH₃ was first adsorbed over Lewis and Brønsted acid sites on the surface of coal ash and later underwent hydrogen abstraction to produce either the NH₂ or the NH intermediate. The intermediates further reacted with the surface lattice oxygen of coal ash to produce NO and N₂O. These results might be helpful for the management of NH₃ residues from SNCR processes and the utilization of amino reducing agents in coal-fired boilers.