Numerical Study On The Modification Of 350MW Coal-fired Boiler Using Hot Primary Air As High Speed OFA To Reduce NO_x Emission

As the main primary energy source in my country,most of coal is used for direct combustion,such as thermal power generation and industrial production.The pollutants produced by combustion such as sulfur oxide and nitrogen oxide will not only threaten human health,but also cause serious environmental problems.In order to meet the increasingly stringent emission standards and achieve the requirements of"ultra-low emission",thermal power plants have invested a lot of cost in the research and development of low-NO_X technology and the transformation of boilers to reduce NO_X emissions in the exhaust gas.Most of the 300 MW units in service are not designed with advanced low-nitrogen combustion technology,and the NO_Xemission concentration is relatively high,which will cause problems such as large ammonia injection in the SCR denitrification system,increased denitrification costs,and air preheater blockage.Although the NO_X emission has been reduced after the low-nitrogen burner modification,compared with the newly-built units that considered advanced low-NO_Xcombustion technology in the initial design,the NOx emission of the original flue gas is still about50-100 mg/Nm³ higher.A 350 MW coal-fired boiler in service has such a problem.After low-nitrogen burner modification and coating treatment,the average NO_X emission concentration at the outlet of the economizer is still higher than that of the same type units when the boiler is operating at full load with four coal mills.Therefore,according to the design parameters and operating status of this 350 MW boiler in service,this article proposes and studies a new technology,high-speed over fire air technology（HOFA）,which uses the surplus primary air as the high-speed jet over-fire air to deeply reduce the NO_X concentration at the furnace outlet.This article mainly uses FLUENT software to carry out numerical simulation on the boiler,and combines field test data to study HOFA technology.This paper first conducted a cold numerical simulation study,established a model of the furnace area above 26 m,simulated the influence of single-sided and double-sided arrangement of HOFA on the cold flow field,and determined a preliminary modification plan.It is found that although the double-sided HOFA has advantages in the diffusion capacity of the upper furnace and the coverage capacity of the front wall area,the single-sided HOFA performs better in the penetration capacity and coverage area of the key area,and the single-sided HOFA shows better rigidity.The comprehensive comparison draws the conclusion that the high-speed over fired air of the single-sided HOFA scheme has a stronger influence on the flow field,and the technical route of the actual boiler transformation adopting the single-sided HOFA scheme is determined.In the thermal numerical simulation study,a single-sided HOFA scheme is adopted,and hot primary air from the ABC layer and AB layer is extracted respectively as high-speed over fired air,and the influence of different air extraction methods on the combustion in the furnace is compared,so as to determine the specific transformation plan and operation suggestions.It is found that the oxygen concentration at the outlet of the ABC layer case（case one）increased a little,and the carbon content of fly ash increased significantly,but the NO_X concentration and CO concentration decreased.Although the carbon content of fly ash has a large drop in the AB layer case（case two）and the CO concentration is the lowest,the NO_X emission is the highest among the three operating conditions.Comparing the original operating case with the outlet NO_X of 335mg/Nm³,the outlet NO_X of case one decreased by 26.43 mg/Nm³,and the outlet NO_X of case two increased by 5.07 mg/Nm³.It is concluded that the case one is more appropriate,that is,the separate ventilation from each layer of ABC is more effective in reducing NO_X emissions.Based on the results of numerical simulations,the implementation and performance test of the on-site HOFA technical transformation were carried out,and the parameters before and after the transformation were compared.It is found that after HOFA was put into use,the boiler combustion efficiency is basically the same as that before the transformation.The temperature distribution and oxygen distribution at the furnace outlet are more uniform,and the NO_X emission is significantly reduced.After the transformation,the NO_X（converted to 6%oxygen）at full load and 75%load are 364.65 mg/Nm³ and 242.60 mg/Nm³,respectively,which is about 33-53mg/Nm³ lower than that before the transformation,and the denitrification efficiency is 8.3% and 17.9%,respectively.It shows that the HOFA technology can effectively reduce NO_X deeply in practical applications.After the on-site test data verified the accuracy of the numerical simulation method and results,the numerical simulation method was used to conduct an in-depth study on the HOFA in order to provide references for other units to apply HOFA technology.Mainly study the performance of two loads of load 75%and 100%and two arrangements of single sided HOFA and double-sided HOFA in combustion and emission reduction.Research shows that under single-sided HOFA cases,the outlet NO_X at 75%load and 100%load is 228 mg/Nm³ and 334 mg/Nm³,respectively,while the corresponding measured data before the transformation are 296 mg/Nm³and 398 mg/Nm³,respectively,which are reduced 68 mg/Nm³ and 64 mg/Nm³,respectively.And the corresponding emission reduction effect is 23%and 16%.It is believed that the NO_Xgeneration and suppression effect of the boiler is better under lower load operation.Under 100%load,the single-sided HOFA has a larger high-concentration CO area,less O₂ concentration area,and smaller high-temperature area in the main combustion zone,which makes the NO_X generation lower.Although the carbon content of fly ash in double-sided case has dropped,its NO_Xconcentration is also the highest of all the cases at 454 mg/Nm³.Comparing with the outlet NO_Xconcentration of 398 mg/Nm³ before the transformation,the emissions did not decrease as expected,but increased,which again confirming that the emission reduction effect of single-sided HOFA arrangement is better than that of double-sided HOFA arrangement.