SCR (Selective Catalytic Reduction) is a selective catalytic reduction technology. At present, ammonia catalytic reduction method is the most widely used technology. It has no by-products, no secondary pollution, simple device structure, high removal efficiency (up to 90%), reliable operation, and easy maintenance. Selectivity means that in the presence of oxygen and in the presence of oxygen, NH3 preferentially undergoes a reduction and removal reaction with NOx to form nitrogen and water, without oxidizing with the oxygen in the flue gas.

Its main reaction formula is:

(1) 4NO + 4NH3 + O2 → 4N2 + 6H2O

(2) 2NO2 + 4NH3 + O2 → 3N2 + 6H2O

In the absence of a catalyst, the above-mentioned chemical reaction is only carried out in a very narrow temperature range (about 980 ° C). When the catalyst is used, the reaction temperature can be controlled at 300-400 ° C, which is equivalent to the boiler economizer and air pre-conditioning. The temperature of the flue gas between the heaters is exothermic. Due to the low concentration of NOx in the flue gas, the increase in catalyst temperature caused by the reaction can be ignored.
SNCR (Selective Non-Catalytic Reduction) is a selective non-catalytic reduction method. It is an economical and practical NOx removal technology. Its principle is based on NH3, urea [CO (NH2) 2], etc. Before the boiler is atomized or injected into the boiler, it is atomized by the heat in the furnace. Sprayed into the furnace temperature range of 800 ºC ~ 1100 ºC, the reducing agent quickly thermally decomposed into NH3 and other by-products, and then NHCR and NOx in the flue gas undergo SNCR reaction to generate N2. It can therefore be considered a selective chemical process. The reducing agent has different reaction temperature ranges. This temperature range is called a temperature window, which has a great influence on the denitration efficiency of the method.