Numerical Simulation Of SNCR Denitrification Characteristics Of Circulating Fluidized Bed Boiler Flue Gas

NO_x is one of the three major pollutants emitted by flue gas from coal-fired power plants.With the increasingly stringent national and local environmental protection policies,the flue gas of the furnace must be denitrification in order to meet the NO_xemission standard.SNCR denitration efficiency is affected by reducing agent atomization quality,mixing situation with flue gas,ammonia nitrogen molar ratio,smoke temperature and other operating parameters,as well as the structural parameters such as connecting flue and separator.It is an effective way to improve the denitration efficiency and reduce the ammonia escape amount by using computer simulation to carry out the influence of these parameters on the denitration characteristics and optimize them.Due to the lack of in-depth research on the influence of nozzle structure parameters and operating parameters on atomization quality and denitrification efficiency,the design and efficient operation of denitrification system cannot be well supported.In this thesis,based on the conventional design of internal mixing air atomizing nozzle structure parameters,the simulation calculation model of atomizing nozzle is established.The influence of structure parameters and operation parameters on the atomization characteristics is studied.Then a simulation model is established for the connected flue-cyclone separator system to study the influence of atomization quality and structure parameters of cyclone separator on the denitrification characteristics.The concrete research content is as follows:（1）The internal mixing air atomizing nozzle is selected as the atomizing device.According to the exhaust volume of 300 MW circulating fluidized bed boiler,the structural parameters of the internal mixing air atomizing nozzle are designed according to the conventional design method.The atomization model of atomizing nozzle and the denitrification model of connecting flue-cyclone separator are established.（2）The influences of four main structural parameters on the atomization characteristics of the nozzle are studied,including impactor length L_c,outlet diameter D,mixing chamber length L,and gas-liquid inlet angleθ.The simulation results show that liquid fluid is mixed with air quickly after colliding with the impactor,which improves the atomization effect of nozzle.The diameter of the outlet directly affects the atomization effect of the nozzle.Reducing the diameter of the outlet can greatly improve the atomization quality.The change of the length of the mixing chamber has limited atomization change of the nozzle.The interaction between the two phase flows at the entrance of the mixing chamber can be improved by adjusting the gas phase inlet Angle.（3）The influences of four operating parameters on the atomization characteristics of the nozzle are studied,including gas consumption rateτ,gas phase pressure P_g,liquid phase pressure P_l and working medium temperature T.The simulation results show that the increasing atmospheric consumption rate can improve the atomizing agent velocity and atomizing quality at the entrance of the mixing chamber.In the case that the liquid phase pressure remains unchanged,increasing the gas phase pressure can increase the gas-liquid velocity and improve the atomization effect.The influence of temperature on atomization is mainly reflected in the change of working medium’s viscosity with the change of temperature.With the increase of liquid pressure,the atomization effect of nozzle becomes worse.The larger the viscosity of the liquid phase working medium,the larger the SMD.（4）The influence of three operating parameters of the cyclone separator,temperature T_r,NO initial concentrationω_NO and NSR on the denitrification efficiency are studied.It is found that the maximum denitrification efficiency is achieved when the mass fraction of NO is 0.08%at 950℃with the NSR_max=1.5.Through orthogonal analysis,the primary and secondary relationship between nozzle structure and denitration efficiency is obtained,and the optimum operating condition is obtained.The effect of three structural parameters of the cyclone separator on the denitrification efficiency is studied,which contains the connecting pipe length L_p,the central tube insertion depth S and the central cylinder diameter D_e.The results show that the denitrification efficiency increases with the increase of connecting pipe length,decreases with the increase of central tube insertion depth,and increases with the increase of central cylinder diameter.