Computer simulation has been proven to provide a good understanding of engineering phenomenon. This work presents numerical simulation results on secondary air jet penetration into a dense phase of a three-dimensional fluidized bed at a commercial scale. Initial model as a reference and four modified models which are called as case A, B, C, and D were created by modifying the angle of secondary air. Evaluation of combustion process is based on mass fraction distribution of H2O and CO2 at center line of the furnace. Generally, modified geometry improves the performance of furnace compared to reference. We also present data of total energy and temperature to get a comprehensive insight of the furnace performance. The simulation results can be used as a consideration to improve the efficiency of steam power plants by adjusting the direction of secondary air flow.