Application of Computational Fluid Dynamics and Process Modeling to Investigate Low-Load Operation of a Subcritical Utility-Scale Boiler

To enable the inclusion of intermittent renewable energy sources on existing power grids requires base-load coal-fired power plants to operate flexibly, including fast load changes and continuous low-load operation. Low-load operation of a 620 MWe subcritical boiler is analyzed with the aid of a cosimulation methodology that incorporates a detailed 3D computational fluid dynamics (CFD) model together with a one-dimensional process model. A discretized one-dimensional model of the water/steam circuit was developed for the furnace, the radiant superheaters, and the convective/back pass heat exchangers. This is coupled with a detailed CFD model incorporating the furnace and radiant superheaters using an Eulerian–Eulerian reference frame. The coupled model is used to investigate the combustion stability, water/steam side effects, and the radiant heat-exchanger operational limits for six different burner firing arrangements at a low boiler load of 32% of maximum continuous rating. The results show that certain firing arrangements can lead to a high risk of fire-side corrosion and overheating of heat-exchanger components. Based on the analyses of combustion stability, boiler efficiency, and the safe operation of heat-exchanger components, a mixed firing arrangement with a higher secondary air mass flowrate for nonfiring burners was selected as the best operational strategy at this low load for the boiler under investigation.