The selective catalytic reduction (SCR) of nitric oxides with ammonia as reductant is one of the best-proven and world-wide used method for the removal of NOX from stationary sources due to its efficiency, selectivity and economics. The De-NOX process has to be installed to prevent the evolution of NOX from the coal, oil and orimulsion power plants since the late 1990s in the Republic of Korea. V2O5-WO3 as an active catalyst and TiO2 as a high surface area support are main components of a commercial catalyst for the SCR unit. When the SCR catalyst is employed in the de-NOx unit, the catalyst would undergo the deactivation during the commercial operation.
Extensive research work has been carried out for better understanding about deactivation of the SCR catalyst during a commercial operation. Alkali metals such as potassium and sodium have been proposed as the major poisoning elements to accelerate the deactivation of SCR catalysts. Among the alkali metals, potassium is the most important element for the deactivation. It was well known that the deposition of the alkali metals can decrease both the number and the strength of Brønsted acid sites, which are largely responsible to the NOX removal efficiency. Also SCR catalyst plugging can come from the deposition of ammonium sulfates and fly ash. Other reasons of the catalyst deactivation may be a loss of surface area for sintering and rutilization of titania after long-term operation at high temperature and poisoning by arsenic in the case of wet bottom boilers.
The deactivated SCR catalyst has to be regenerated for reuse in terms of process economics and environmental impact of heavy metals contained in the used catalyst. In the present work, the deactivation of a commercial SCR catalyst used in a coal-fired power plant has been investigated and its regeneration has been carried out. The SCR efficiency over the fresh, the deactivated and the regenerated catalysts has been determined at various operating conditions. The SCR catalysts have been characterized by a scanning electron microscope (SEM), a temperature-programmed desoprtion (TPD) and an X-ray diffractometer. An inductively coupled plasma-atomic emission spectrometer (ICP-AES) was used for elemental analyses of the catalyst and a washing solution.
The deactivated catalyst was regenerated by H2SO4 solutions with various concentrations (0.005 to 5 wt%). A large amount of deposits was observed on the deactivated catalyst, which resulted in a drastic decrease in the surface area of the deactivated catalyst. The poisoning elements (Ca, K, Na, and S) were detected in the deactivated catalyst. The catalytic activity was recovered by regenerating the catalyst with acidic solutions such as sulfuric acid, hydrochloric acid and so on. A high catalytic activity over the regenerated catalyst came from an increase in the surface sulfate, which could provide a formation of the polymeric vanadate and acid sites on the catalyst surface. Higher concentration of H2SO4 brought about dissolution of active material in the catalyst, which caused a decrease of the SCR efficiency and a loss of surface area. Based on the present experimental result, the pilot plant facility for the regeneration will be constructed and demonstrated for reuse of the commercial SCR catalyst.