A thermodynamic model was developed to associate the agglomeration tendency with the inherent properties of bed materials. Solid surface energy of metallic iron was calculated by the elastic modulus and lattice constant. Based on the relationship between solid surface energy and temperature, the thermodynamic spontaneity for particle agglomeration was explained, and the limiting temperature to defluidize was determined in a good agreement with the experimental data. Consequently, a criterion for the defluidization was proposed as a reference to select the bed materials. (C) 2016 Elsevier B.V. All rights reserved.