Selectivity refers to the preferential reduction and removal reaction between NH3 and NOx under the action of a catalyst and in the presence of oxygen, generating nitrogen and water instead of undergoing oxidation reaction with oxygen in the flue gas. Its main reaction equation is:

4NO+4NH3+O2 →4 N2+6H2O(1)

2NO2+4NH3 +O2 → 3 N2+6H2O(2)

In the absence of a catalyst, the above chemical reaction only occurs within a very narrow temperature range (around 980 ℃). When using a catalyst, the reaction temperature can be controlled between 300-400 ℃, which is equivalent to the flue gas temperature between the boiler economizer and the air preheater. The above reaction is an exothermic reaction, and due to the low concentration of NOx in the flue gas,

The increase in catalyst temperature caused by the reaction can be ignored.