A one-dimensional mathematical model was developed for devolatilization and heat transfer of lump coal falling in the freeboard zone of the COREX melter gasifier. The distributed activation energy model (DAEM) was used to depict the coal devolatilization. The heat transfer and temperature variation of lump coal were modeled by a transient energy conservation equation. The coupled comprehensive model was validated by previous experimental data of Pittsburgh No. 8 coal with different heating rates and bituminous coal of large particles. The model was used to investigate lump coal falling in the freeboard zone of the COREX melter gasifier. The fractional volatile yield and temperature distribution of lump coal were obtained, as well as the transient velocity and acceleration. Furthermore, the effect of the lump coal diameter was studied on devolatilization and temperature distribution of lump coal during the course of falling.