Standard SCR Process

The standard SCR process is based on the reduction of NO with ammonia to water and nitrogen according to the following main reaction  

In fact, NO accounts for 90-95% of NO in exhausts. Aqueous ammonia or urea, as ammonia precursor, are typically employed. The term selective refers to the unique ability of ammonia to react selectively with N instead of being oxidized by oxygen. 

In the case of sulfur-containing fuels (e.g. coal or oil), SO2 is produced during combustion in the boiler along with minor percentages of SO3 SO2 can further be oxidized to SO3 over the catalyst  

Reaction (13.2) is highly undesired because SO, reacts with water present in the flue gas in large excess and with ammonia to form sulfuric acid and ammonium sulfate salts. The ammonium sulfate salts deposit and accumulate on the catalyst if the temperature is not high enough, leading to catalyst deactivation, and on the cold equipment downstream of the catalytic reactor, causing corrosion and pressure drop problems. The catalyst deactivation by deposition of ammonium sulfate salts can be reversed upon heating. 

Commercial SCR catalysts are made of homogeneous mixtures of titania, tungsta and vanadia (or molybdena). Titania in the anatase form is used as a high surface and sulfur-resistant carrier to disperse the active components. Tungsta or molybdena is employed in large amounts (10 and 6% w/w, respectively) to increase the surface acidity and the thermal stability of the catalyst and to limit the oxidation of SO2. Vanadia is responsible for the activity in the reduction of NO, but it is also active in the oxidation of SO2. Accordingly, its content is kept low, usually below 1-2% w/w. 

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