Gasification technology is an important part of clean coal technology. Further development of this technology requires understanding the processes and interactions of gas and solid fuel particles injected into the gasifier. In this study, a numerical simulation of an entrained flow coal gasifier has been investigated using experimental operating conditions. The reactions and kinetic parameters of the gasification process have been extracted using coal gasification data obtained from similar published papers. Comparison of the simulation results with experimental data and two other similar studies confirm the accuracy of the developed model. The focus of this study is on the accuracy of the models presented for the devolatilization process and the effect of the oxidizer change from oxygen to air on the gasifier performance. Four devolatilization models including chemical percolation devolatilization, single rate, Kobayashi and constant rate models have been investigated. The predicted trends of species changes are similar in different devolatilization models but the amount of produced syngas is somewhat different depending on the accuracy of each model. The Kobayashi and constant rate models predict the devolatilization rate lower than the other two models. The results obtained from the chemical percolation devolatilization model are more consistent with the experimental data compared to the other models but require higher computational times. The use of air oxidizing agents reduces the concentration of produced syngas rather than oxygen and hence reduces the gasifier efficiency.