A large eddy simulation (LES)-discrete element method (DEM) investigation was carried out to study particle clusters in a canonical downer reactor during the fluid catalytic cracking of gasoil. The analysis considered two cases: with and without chemical reactions. The results of the system under cold-flow conditions for the frequency and mean lifetime of clusters were in good agreement with experimental data. The changes in the hydrodynamic conditions generated by the cracking reactions led to clusters with shorter mean lifetime but higher frequency compared to the system without chemical reactions. Additionally, the frequency and mean lifetime of clusters were successfully correlated to an empirical expression as a function of the particle Reynolds number and the Stokes number, which summarize the hydrodynamic conditions of the downer.