Modeling the occurrence and methods of reducing thermal deviations of upper furnace heating surfaces in a 1000 MW dual circle tangential firing single furnace ultra-supercritical boiler

The FLUENT computational fluid dynamics software package was used to model outlet velocity and temperature inhomogeneity in a 1000 MW dual circle tangential firing single furnace ultra-supercritical boiler. These computations allowed a theoretical analysis of thermal deviations at the furnace outlet and suggested means of reducing such deviations. This work involved study of radiative and convective heat transfer of the upper furnace platen superheaters, the radiative-convective heating surfaces above the furnace nose and the convective heating surfaces in the horizontal flue. The results demonstrated that the radiant heat load of the heating surfaces of the platen superheaters is related to the sectional dimensions of the furnace and exhibits a bimodal distribution in the boiler modeled during this work. It was also determined that a large recirculation zone is formed in the central section of the horizontal flue owing to velocity superposition. After establishing the thermal load distribution characteristics and the causes of thermal deviations at the various heating surfaces, further modeling was performed to assess the extent to which thermal deviations could be reduced by decreasing residual rotation at the furnace outlet via horizontally swinging the over fire air (OFA). The effects of OFA swing angles on velocity and temperature inhomogeneity at the furnace outlet were subsequently analyzed, and an OFA swing of-10° was found to be optimal based on considerations of thermal deviations at the furnace outlet, the airflow field in the furnace, and safe operation of the boiler.