SNCR and SCR denitration technology is to use SNCR technology such as reducing agent such as ammonia, urea solution into the area of furnace temperature is 850-1100 ℃, first carries on the denitration reaction, excess ammonia in the catalyst can be 280-420 ℃ under the action of the flue gas temperature range will become a N2 and H2O NOX in flue gas decomposition. The combined denitrification system combines the advantages of the two technologies to reduce denitrification costs and reduce ammonia leakage while increasing the removal rate of NOX. In the combined denitrification system, ammonia leakage in the SNCR process provides the required reductant for SCR. More NOX can be removed through the SCR denitrification process, and the opportunity of ammonia leakage can be further reduced. The catalyst used in the combined denitrification system is much less than that in the SCR denitrification system alone, and a higher NOX removal rate can be achieved.

SCR/SNCR combined denitrification technology is a newly developed dry flue gas denitrification technology, which combines the characteristics of high efficiency of selective catalytic reduction denitrification technology (SCR) and low investment of selective non-catalytic reduction denitrification technology (SNCR).

Selective catalytic reduction denitrification technology refers to the selective reaction of reducing agents (NH3 and urea) with NOx under the action of metal catalyst to generate N2 and H2O instead of being oxidized by O2, so it is called "selective". NH3, CO, H2, methane, ethylene, propane and propylene can be used as SCR reaction reductants.

Selective non-catalytic reduction denitrification requires no catalyst, and the selected reductant is the same as SCR, also NH3, ammonia water or urea. SNCR is sprayed with a reducing agent over the boiler and in the horizontal flue, and selectively reacts with NOx in the flue gas to generate harmless N2 and H2O.

The advantages of the former are that the denitrification rate can be greater than 80%, and the NH3 consumption can be optimized, but the disadvantages are that the design requirements are higher. The design is more complex; Higher manufacturing and operating costs. The latter has the advantage of low investment cost and is more suitable for SNCR modification of existing boilers, but has the disadvantage of high ammonia consumption. The ammonia escaping quantity is large; Mixing well is difficult. So its denitrification rate is low, only 50 ~ 60%.

SNCR/SCR mixed flue gas denitrification technology is to further remove NOx by combining SNCR process reducing agent injection into the furnace with SCR process using escaped ammonia for catalytic reaction. The "combined process" was first tested on a fuel oil plant in Japan in the 1970s, and tests showed that the technology was feasible. In recent years, with the development of the chimney denitration technology, many new types of chimney denitration technology is applied to the production, and achieved very good results, at the same time, combined with the chimney flue gas denitration technology has been constantly in the process of reform of optimization, performance and effect is also constantly improve in this reform, so many new chimney flue gas denitration technique has been used as a formal in production technology projects. It is an effective combination of SNCR process and SCR. While maintaining and promoting the advantages of the two processes, it USES the advantages of each other to restrain and overcome the defects of high cost of single SCR and low efficiency of single SNCR. SNCR and SCR processes complement each other and complement each other.