THE EFFECT OF POROSITY ON HEAT TRANSFER AND MASS TRANSFER OF Mg-3Ni-2MnO₂ HYDROGEN STORAGE MATERIALS REACTION BED

Hydrided Mg-3Ni-2MnO₂ composite powders were prepared by mechanical milling under hydrogen atmosphere. Heat and mass transfer, the effective thermal conductivity (ETC) of the hydrided Mg-3Ni-2MnO₂ powder reaction bed with various porosities were measured using a self-made apparatus. The effect of porosity on the bed is also analysized. The results show that the ETC of reaction bed is poor and it increases with decreasing porosity. Three porosities, 0.37, 0.53, 0.63 were selected in the present work. The bed with 0.53 porosity exhibits relatively fast reaction rates in both hydrogenation and dehydrogenation process. The hydrogenation process is a fast exothermic reaction resulting in a quick increase if the temperature of the bed during this process, and there is a temperature gradient: the temperature close to the bed wall is lower but higher at the center of bed. In dehydrogenation of the bed, the temperature of hydrided bed decreases due to the endothermic reaction, and the temperature at the center falls the lowest and keep at that temperature for a long time. The analyses reveal that increase of ETC don't always helps to improve the bed's hydriding and dehydriding rates. There should be an optimal porosity which helps to transfer both the heat and the mass.