| As the demand for energy saving and emission reduction becomes more and more urgent, the development of high-capacity ultra-supercritical boilers has also been significantly promoted. Taking one 660MW ultra-supercritical power plant as an example, this work utilizes the thermal calculation and analysis method to investigate the root causes resulting in the high exhaust gas temperature (EGT) and the large thermal deviation encountered in high temperature steam generation components. The obtained results can provide valuable reference for improving the safety and economy of ultra-supercritical power plants.This work proposes an improved calculation method for the tri-sectional air preheater installed in ultra-supercritical once-through boilers based on previous research efforts. A micro-element model is established firstly in this method, and then analytical expressions for each section are derived mathematically. Through performing iterative computations, the mean temperature at the outlet of each section as well as the mean metal temperature is obtained. One highlight that makes this work different from previous works is the accurate evaluation of the influence of gas leakage on the performance of the tri-sectional air preheater. Besides, through dividing the preheater into more computational regions along the gas flow direction, more detailed description about the temperature distribution of the metal and flow can be achieved.Taking the former 73 standard calculation procedure as a basis, a new systematic calculation procedure is established, employing the newly presented thermal calculation methods for furnace and platen superheater by other researchers as well as the currently established new calculation method for tri-sectional air preheater. The accuracy of this calculation procedure has been confirmed through comparing the its calculation results with the original design parameters. In addition, with this calculation procedure, the operation characteristics of the studied components have also been investigated, and they can benefit the operation analysis and retrofit of boilers.Focusing on the undesired high EGT, this work firstly analyzes the boiler’s operation characteristics under various conditions, and then analyzes the effects of coal quality, environmental temperature, area deviation of back-end heating surface, and air-leakage in both the furnace and coal culverizine svstem on the EGT. By using field data, the practical operating condition is simulated, and lacking of sufficient heating surface in the economizer is finally determined to be the main reason. The analysis results can serve as theory reference for performing retrofit aiming at reducing the EGT.Through employing the thermal calculation results, a model for evaluating the thermal deviation is also established. Calculation is then performed, from the aspects of nonuniform thermal load coefficient, nonuniform flow coefficient, and thermal deviation coefficient, on the final superheater to get the distribution of steam temperature and tube temperature. In addition to evaluating the mechanical strength of the tubes, the calculation results can also be used for predicting the growth of steam-side oxide scale. |
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