1. Selection of reducing agent for SNCR denitration system:

There are three reducing agents for SNCR denitration system: liquid ammonia, ammonia and urea.

(1) Liquid ammonia:

Advantages: after being sprayed into the furnace, it will quickly volatilize into gas, and will not cause wet wall and corrosion of the heating surface in the furnace;

Disadvantages: ammonia is toxic, combustible and explosive, and the safety protection requirements for storage are high. It can be stored and used in large quantities only after being approved by relevant fire safety departments; The SNCR system using liquid ammonia is relatively complex, with high initial investment cost, high operation and maintenance cost, large pipeline loss and frequent liquid ammonia leakage accidents. In terms of safety, it is recommended not to use liquid ammonia as reducing agent;

(2) Ammonia:

Advantages: the injection rigidity and penetration are higher than that of ammonia injection;

Disadvantages: ammonia has strong odor, volatility and corrosivity, and has certain operation safety requirements. Due to a large amount of dilution water, the storage and transportation system is complex;

(3) Urea:

General industrial and agricultural urea is used as reducing agent, and its nitrogen content is more than 46%. No special safety protection measures are required for its transportation, storage and transportation.

Urea is selected as NOx reducing agent from the aspects of safety, procurement, investment, operation cost and system complexity.
 

2. Selection of reductant injection point:

The NOx removal efficiency of SNCR method mainly depends on the reaction temperature, stoichiometric ratio of NH3 to NOx, mixing degree, reaction time, etc. the flue gas temperature of CFB boiler is generally below 920 ℃, and the flue gas temperature in separator area is between 870-900 ℃. The temperature control of SNCR process is very important. If the temperature is too low, NH3 reaction is incomplete, which is easy to cause NH3 escape; When the temperature is too high, NH3 is easy to be oxidized to no, which offsets the denitration effect of NH3. Too high or too low temperature will lead to the loss of reductant and the decrease of NOx removal rate.

For SNCR denitration reducing agent (urea or liquid ammonia), CFB boiler can provide a suitable temperature range for the reaction between reducing agent and NOx to achieve the purpose of removing NOx.

According to the research of current technicians on circulating fluidized bed boiler, the best reductant injection point of SNCR is the inlet of cyclone separator. According to the flow field calculation and actual measurement, the average residence time of flue gas in the cyclone separator will be approximately greater than 1s, while the temperature in the cyclone separator basically does not change. The residence time of reducing agent in the appropriate temperature range will exceed 1s, exceeding the optimal reaction residence time by 0.5 seconds, which is enough for its full reaction.

In addition to the reaction time, it also needs the full mixing of denitration reducing agent and flue gas. In the cyclone separator of CFB boiler, the flow field of air flow is relatively complex, including the steering and acceleration of separator inlet, the rotation and steering of main air flow along the inner wall of separator, etc. With the separation of the solid phase, the gas also rotates against the wall, and a reflux zone is formed during the rotation, which creates very good conditions for the diffusion and mixing of the gas phase.

The strong mixing of gas phase in cyclone separator is very beneficial to ammonia injection denitration reaction. In the cyclone separator of CFB boiler, reducing agent and flue gas will be well mixed, which is conducive to improving denitration efficiency.

According to the influencing factors of NOx removal efficiency of SNCR method, the reducing agent injection point is selected at the inlet of cyclone separator in terms of improving denitration efficiency.
 

3. Summary

SNCR denitration technology has little impact on boiler efficiency, exhaust gas temperature, boiler heating surface and downstream equipment of boiler, does not affect the safety of unit operation, and does not need targeted equipment transformation; Compared with SCR denitration technology, SNCR denitration technology has the advantages of simple and easy project implementation, low investment and operation cost, small floor area and short construction period. Therefore, the equipment type can be reasonably selected according to different working conditions to obtain the highest input-output benefit.