Heat storage systems using reversible chemical solid-fluid reactions to store and release thermal energy operates in charging and discharging phases. During last three decades, discussions on thermal decomposition of several salt-hydrates were done (experimentally and numerically) [1,2]. A mathematical model of heat and mass transfer in fixed bed reactor for heat storage is proposed based on a set of partial differential equations (PDEs). Beside the physical phenomena, the chemical reaction is considered via the balances or conservations of mass, extent conversion and energy in the reactor. These PDEs are numerically solved by means of the finite element method using Comsol Multiphysics 4.3a. The objective of this paper is to describe an adaptive modeling approach and establish a correct set of PDEs describing the physical system and appropriate parameters for simulating the thermal decomposition process. In this paper, kinetic behavior as stated by the ICTAC committee [3] to understand transport phenomena and reactions mechanism in gas and solid phases is taking into account using the generalized Prout-Tompkins equation with modifications based on thermal analysis experiments. The model is then applied to two thermochemical materials CaCl2 and MgCl2 with experimental activation energies and a comparison is made with TGA-DSC measurement results.