Numerical Simulation Of Low NO_x Reformation Of An800MW Swirl-opposed Firing Boiler

With the increasing stern of environmental problems, the emission of air pollutants like NOx attracts more and more concern. This dissertation presents a transformation using air-staged combustion method will conducted by adding OFA nozzles so as to solve the problem of high NOx emission existing in an800MW ultra-supercritical utility boiler. While air-staged combustion can make a remarkable reduction of NOx emission, it can inevitably cause certain unfavorable influence at the same time. Combustion situation and NOx emission of this boiler in different conditions were numerically simulated using FLUENT software. By weighing the advantages and disadvantages, key parameters were determined and transformation scheme was improved.Firstly, the original condition was numerically simulated with reasonable calculation method based on proper physical and mathematical model. The predictive velocity field, temperature field and the distribution of species were discussed in detail to show the rationality of calculation results. Calculated values and measured values of some parameters were compared to verify the accuracy. As a result, the calculation model used above was reasonable and reliable for the prediction of transformation condition.Secondly, deep analysis of NOx formation characteristics was conducted to guide the improvement of air-staged combustion transformation. Not only was the comparison between the origional condition and the transformative condition with Over-Fire Air conducted,but also was the disscussion of the concentation of NOx and its chemical reaction speed distrbution porceeded. Thus the influence of air-staged combustion on the NOx generating character was analysed in all aspects. Also, it is thoroughly discussed that variation of NOx generating speed with air-staged combustion considering Fuel NOx and Thermal NOx repectively. Meanwhile, the controlling factor of NOx formation rate at various spot in the furnace is discovered.At last, different conditions were designed and simulated to study the effect of the over-fire air ratio and the height of OFA nozzles on the combustion and emission. It was showed that the emission decreased with the increase of the over-fire air ratio and the height of OFA nozzles. However, these increase had an negitive impact on the buring-out and the temperature in the upper furnace. It was anlysed when the over-fire air ratio was20%and the height of OFA nozzles was44m, the decrease of NOx emission reached the maximum value and the the adverse effects was controled within the acceptable range. What’s more, an improved scheme was proposed in the dissertation considering the actural situation which burner at main combusting area should be moved downward with the transformation of OFA. After it had been verified, this modification scheme could cause greater NOx reduction and least adverse effects, it is considered to be the best one.