A computational fluid dynamics (CFD) model of a 300 MWe down-fired furnace equipped with overfire air (OFA) has been developed using Fluent 6.3. A level of confidence in the current CM model has been established by performing mesh-independence tests and verified by comparing furnace temperature simulations with actual furnace data. The validated CFD model is then applied in the investigation of the effects of several operating conditions at full load with different OFA nozzle angles. In the down-fired furnace, the primary air/fuel was found unable to penetrate the horizontal secondary air flow zone and reach the furnace hopper; thus, no combustion occurs in the furnace hopper zone. The peak-temperature zone appears in the upper furnace, contrary to the original design concept. Simulation results also indicate that an OFA nozzle angle set below 30 degrees moves the mixing point of the OFA flow and the up-flowing gas downward, further aiding combustion in the upper furnace. However, when the angle increases above 30 degrees, OFA is unable to reach the furnace center, weakening combustion in the furnace, because no reaction with coal can take place. This study provides a basis to assess in-depth future operations of down-fired boilers and help in designing OFA equipment.