In hot processes, oxides of nitrogen are often formed and these are harmful pollutants which are of increasing concern. Selective catalytic reduction works by introducing another compound, termed a ‘reductant’, which encourages the oxygen atoms to ‘jump ship’ from the NOx molecules to react with the reductant instead. In most cases, a catalytic converter is used to promote this reaction, in which case it occurs in the temperature range of 150-600oC. Without a catalytic converter, a higher temperature is required.
The most common reductant used in SCR processes is ammonia. This can be bought in the form of ammonia solution, or alternatively a solution of urea such as Adblue can be used instead. Urea solutions are easier to store and handle than ammonia solution and also more widely available, however if they are injected directly into the flue gas they require a minimum temperature of 300oC.
It is very important that the quantity of reductant injected matches the quantity of NOx present in the flue gas. If the quantity is too low, NOx reduction will be compromised, whilst if it is too great, ammonia will be emitted. Therefore a sophisticated injection system for the reductant is required.
A well-designed SCR system is capable of reducing NOx emissions by over 90%