Transformation of the graphite−MnCl2 complex/ammonia system was investigated in a thermochemical reactor through numerical simulation. Thermophysical properties such as effective thermal conductivity and gas permeability were measured for the reactive medium with a bulk density of 100−250 kg m-3, and the values obtained were in the range of 14.0−25.6 W m-1 K-1 and (8.0 × 10-15)−10-12 m2, respectively. A model for the thermochemical reactor was developed on a macroscopic scale, and then the measured thermophysical properties were used in the model equations. The results of the simulation showed that, when the operating pressure was 200 kg m-3, the rate of global conversion was significantly impeded by the mass-transfer limitation. The absence of a distinctive heat front in the reactor shows that heat transfer is not a limiting factor with the graphite−MnCl2 complex.