Computational examination of pulverized coal combustion in an entrained flow downer reactor exhibited the existence of two distinct combustion regions referred to as near burner combustion (NBC) and far region combustion (FRC) with lack of mediate state. Operating conditions and material properties were altered within a broad range in which the reactor's behavior was studied. A transition mechanism between the two combustion structures was recognized and isolated to illustrate ranges of stable combustion. NBC was mainly affected by the,hydrodynamics while FRC was mainly affected by the devolatilization kinetics. Non dimensional analysis of the gas solid behavior yielded Stokes values close to unity for the transition cutoff. A new methodology for the selection of a distribution representative particle diameter is presented and assumed to be applicable for various types of two phase exothermic reacting flows. The pulverized coal combustion inside the reactor was simulated using the Euler-Lagrange approach and validated against experimental results. Special attention was given to the backmix of hot gases combined with the inherent recirculation zone near the reactor's entrance for their effects on the transition mechanism between combustion regions. Understanding these mechanisms will lead to better control over processes regarding phase continuous mixing vs short contact within entrained flow downer reactors.