Study On Low Nitrogen Combustion Characteristics Of 85t/h Gas Boiler Based On Flue Gas Recirculation

Natural gas combustion process is widely used in chemical,metallurgical,thermal,brewing and other industries.The main pollutant is NOx in the combustion process.In China NOx emission for gas boulers should be less than30mg/Nm³?converted to 3.5%O₂?.At present,gas boilers are difficult to meet the above emission standards,so it is necessary to study gas boilers with low NOx combustion characteristics.There are many problems in gas boilers,for example high initial NOx emission,the occurrence of shocks and even flameout with the external flue gas recirculation in gas boilers.A new low-nitrogen burner is designed based on internal flue gas recirculation.The experiment and numerical simulation are combined to study the combustion characteristics of this technology.On this basis,a gas boiler in capacity of 85t/h was modified by the technology of internal flue gas recirculation low nitrogen burner and external flue gas recirculation.The analysis shows that the internal flue gas recirculation mainly draws the flue gas into the burner through the high-speed jet and the flue gas pressure drop inside the furnace.The high-temperature flue gas reduces the combustion intensity and temperature so that NOx formation decreases,and the air preheated improves combustion stability.The external flue gas recirculation sends the low-temperature flue gas into the furnace through the flue gas recirculation fan,which lowers the combustion temperature and effectively suppresses the generation of NOx.Based on the 0.8MW experimental system the experimental results show that the initial NOx emission of the internal flue gas recirculation burner is60⁷0mg/Nm³,which is lower than that of the traditional low nitrogen burner of80¹20mg/Nm³.When the external flue gas recirculation rate is greater than20%,NOx emission concentration is lower than 30 mg/Nm³.Fluent numerical simulation results show that the generation of thermal NOx decreases due to the lower furnace center flame temperature which is caused by the mixture of the air and hot flue gas when the load is 70%of the rated load and the excess air coefficient is 1.2 without no external flue gas recirculation.The volume fraction of hot flue gas is 20%of air volume.The simulation of external flue gas recirculation is conducted.The result shows that under the same load,as the external flue gas recirculation rate increases from 0 to 24%,the peak temperature of combustion decreased from2120K to 1860K,and the peak oxygen concentration decreased from 21%to17.7%.Meanwhile the peak rate of thermal NOx formation is decreased from7.2 mol/?m³?s?to 1.08 mol/?m³·s?,a decrease of about 85%.And the peak rate of rapid NOx formation decreased from 4.33 mol/?m³?s?to 1.59 mol/?m³?s?,a decrease of about 62%.The results calculated by Chemkin-Pro show that when the excess air coefficient is less than 1,fast NO is dominant,accounting for 43.2%of the total NO.And when the excess air coefficient is greater than 1,the thermal NO produces accounting for 64.2%of the total NO.Low-nitrogen conversion for a gas boiler in capacity of 85t/h was carried out using internal flue gas recirculation low nitrogen burner with external flue gas recirculation.The results show that NOx emission concentration is99.8mg/Nm³ by the internal flue gas recirculation low-nitrogen burner when the external flue gas is not recirculated under the rated load.NOx emission concentration is lower than 30 mg/Nm³ and the combustion is stable when the external flue gas recirculation rate is greater than 15%.