Calcination, where CaCO3 is thermally decomposed to CaO and CO2, is one of the most important chemical reactions. In this work, a CFD model is developed to investigate the flash calcination behavior of lime mud under various temperatures and gas atmospheres. Simulation results are compared with experimental data obtained from a pilot-scale flash calcination experimental campaign conducted in a drop tube furnace at the RISE site in Piteå. The simulated flash calcination temperature ranges from 700 to 1350 °C, with 50 °C increments, under different atmospheric conditions: 100% N2, CO2, and H2O vapor, as well as a 50/50 mixture of CO2 and H2O vapor. The comparison shows overall good agreement between simulations and experiments, with most flash calcination thresholds accurately captured, particularly in cases involving CO2-containing atmospheres. The particle residence time, temperature, and the presence of CO2 and H2O vapor were identified as the most influential parameters affecting flash calcination conversion and the evolution of the specific surface area.