Selective Catalytic Reduction The SCR Process

however, can only be removed by adding a reductant, ammonia, and using a catalyst. The process is called selective catalytic reduction, or SCR. The catalyst consists of vanadia and titania and works in the temperature interval 600-700 K according to the overall reaction  

The ammonia is either injected as pure ammonia under pressure or in an aqueous solution at atmospheric pressure. Instead of ammonia, urea can also be used. The challenge of the process is to efficiently remove as much NOx as possible at full conversion of the reductant, as emission of NH3 from the SCR reactor would of course be highly undesirable. 

The combined approach of removing both the sulfur and the NOx from the flue gas is called SNOX (Haldor Topsoe A/S) or DESONOX (Degussa). An example of the setup for this process is shown in Fig. 10.11, where 99% of the NOx is converted in the SCR reactor and the SO2 is converted into sulfuric acid. 

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