Performance Study On SCR Denitrification System Of 630 MW Unit Before And After Low NO_x Reform

The SCR denitrification system of #2 unit in a power plant is studied from two aspects: SCR catalyst for denitrification,distribution of flue gas parameters.The flue gas parameters of SCR denitrification system before and after NOx modification were studied.The influences of flue gas parameters on the performance of denitrification system were analyzed,and the operation characteristics of SCR denitrification system were revealed.In order to ensure the denitrification efficiency,the flue gas temperature of the boiler should be controlled in the active window temperature of the catalyst;the flue gas velocity and the mass concentration of the flue dust should be reduced,and the pressure difference between the front and back of the denitrification system should be paid close attention to,so as to prevent the deposition or blockage of the flue dust in the catalyst;the concentration of NOx in the flue gas inlet should be kept below the design value to prevent the continuous operation or instantaneous overrun of the design value.According to the denitrification efficiency requirements of SCR system,adjust ammonia injection amount of equipment,make ammonia nitrogen molar ratio slightly larger than the requirement of denitrification efficiency,and pay close attention to the amount of ammonia escaping from the system to prevent a large number of ammonia escaping.Catalyst is the core of SCR denitrification system.Based on the single-layer catalyst denitrification efficiency model and catalyst deactivation model of SCR system,the denitrification efficiency model of SCR system was established,and the influence of SCR denitrification catalyst on denitrification efficiency was analyzed.When the molar ratio of ammonia to nitrogen in SCR system is less than 1,at a certain point,the denitrification efficiency of each layer of catalyst is proportional to the molar ratio of ammonia to nitrogen.The calculated results of the model are basically in agreement with the actual denitrification effect.Due to the different deactivation rate of each layer of catalyst,the denitrification efficiency of each layer of catalyst varies in a curve with the change of operation time.With the addition of the third layer catalyst,the time for the unit to meet the emission standards becomes longer,which is conducive to the long-term stable operation of the unit.Based on the SCR system denitrification efficiency model,assuming that the reactants in SCR flue gas are randomly averaged in a certain range,the simulation results show that the non-uniformity of ammonia-nitrogen molar ratio distribution in flue gas has a greater impact on the denitrification efficiency and ammonia escape of SCR system.When the molar ratio of ammonia to nitrogen is 0.8,0.815,0.9 and 0.915,the relative standard deviations of the molar ratio distribution of ammonia to nitrogen are greater than 14.70%,13.37%,6.65% and 5.36%,respectively,the simulation results show that the ammonia escape in denitrification system will begin to increase rapidly,and the larger the molar ratio of ammonia to nitrogen in SCR system,the larger the ammonia escape.Comparing the model with the industrial test data,the model conforms to the actual production situation.Finally,based on the SCR system denitrification efficiency model and catalyst deactivation model,combined with the emission requirements of SCR denitrification system,the life and renewal cycle of SCR catalyst were predicted.The three layer of the catalyst,the catalyst layer is in the front,the flue gas erosion is bigger,the most serious wear,the faster the rate of inactivation.When the catalyst layers can't be replaced each other,the service life of the catalyst is predicted by the model.It is found that replacing the first layer catalyst is not the longest running time plan,and it is necessary to determine which layer of catalyst to replace by calculation and prediction.If the catalyst layers can be replaced with each other,the most economical way is to replace the catalyst with the worst activity,and according to the activity of the replaced catalyst,from front to back,according to the adjustment of catalyst activity from bad to good.