Abstract
SCR still represents the most widely applied technique to remove nitrogen oxides from flue gas from both stationary and mobile sources. The catalyst lifetime is greatly affected by the presence of poisoning compounds in the exhaust gas that deactivate the catalysts over time on stream. The progressive and widespread transition towards bio-derived fuels is pushing research efforts to deeply understand and contrast the deactivating effects of some specific poisons among those commonly found in the emissions from combustion processes. In particular, exhaust gases from the combustion of bio-fuels, as well as from municipal waste incineration plants and marine engines, contain large amounts of alkali and alkaline earth metals that can severely affect the acid, redox, and physical properties of the SCR catalysts. This review analyzes recent studies on the effects of alkali and alkaline earth metals on different types of SCR catalysts divided into three main categories (conventional V2O5-WO3/TiO2, supported non-vanadium catalysts and zeolite-based catalysts) specifically focusing on the impact of poisons on the reaction mechanism while highlighting the different type of deactivation affecting each group of catalysts. An overview of the different regeneration techniques aimed at recovering as much as possible the original performance of the catalysts, highlighting the pros and cons, is given. Finally, current research trends aiming to improve the tolerance towards alkali-poisoning of SCR catalysts are reported.
Highlights
Nitrogen oxides are among the most widespread polluting species produced by both industry and transportation
The most common catalysts for the NOx -SCR process are V2 O5 -WO3 /TiO2 [1], often named VWT, and many literature reports have focused on the poisoning effects due to alkali and alkaline earth metals on these materials
This paper reviews the poisoning effect of both alkali and alkaline earth metals, analyzing the poisoning mechanism, the possible regeneration strategies, and the approaches to limit deactivation of SCR catalysts divided into three main categories: (i) conventional V2 O5 -WO3 /TiO2 catalysts; (ii) systems based on metals different from vanadium; (iii) zeolite-type catalysts
Summary
Nitrogen oxides are among the most widespread polluting species produced by both industry and transportation. SCR is the most widely used deNOx process to clean exhaust gases involving the reduction of NOx to N2 by a suitable reducing agent, generally ammonia This is a well-established catalytic technique for the treatment of industrial and power plant emissions. The most common catalysts for the NOx -SCR process are V2 O5 -WO3 /TiO2 [1], often named VWT, and many literature reports have focused on the poisoning effects due to alkali and alkaline earth metals on these materials. This paper reviews the poisoning effect of both alkali and alkaline earth metals, analyzing the poisoning mechanism, the possible regeneration strategies, and the approaches to limit deactivation of SCR catalysts divided into three main categories: (i) conventional V2 O5 -WO3 /TiO2 catalysts; (ii) systems based on metals different from vanadium; (iii) zeolite-type catalysts
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