Thermochemical performance of magnesium hydroxide–expanded graphite pellets for chemical heat pump

Expanded graphite (EG) was used to enhance the thermal conductivity in the packed bed reactors of magnesium oxide–water (MgO–H2O) chemical heat pumps (CHP). An expanded graphite–magnesium hydroxide composite (EM) was obtained by mixing a precursor CHP material of magnesium hydroxide (Mg(OH)2) powder and EG. The composite was pelletized to achieve a diameter (ϕ) of 7.1 mm and thickness (l) ranging of 3.5–4.5 mm. Mg(OH)2 dehydration and MgO hydration in the EM pellets were investigated on the packed bed reactor of a 100-W-scale CHP experimental apparatus. The temperatures measured in the packed beds of the EM pellets, as well as the dehydration and hydration kinetics, were compared with the results obtained using a packed bed of pure Mg(OH)2 pellets (ϕ = 1.9 mm, l = 5–10 mm). The thermochemical performances of the EM pellets were analyzed as a function of the mass mixing ratio of Mg(OH)2 to EG (4:1, 8:1, and 16:1) used for preparing the EM. In both dehydration and hydration, the EM pellets showed a higher reaction rate and more homogeneous temperature distribution in the packed bed than did the pure Mg(OH)2 pellets. Consequently, a greater final material conversion was achieved. A higher reactivity enhancement was measured with the addition of a greater quantity of EG in the EM preparation. From the experimental results, it was calculated that the bed comprising EM pellets (having a mass mixing ratio of 8:1) had a heat-storage capacity of 881 kJ / kg Mg ( OH ) 2 , gross heat output of 714 kJ kg Mg ( OH ) 2 , and mean power output rate of 132 W / kg Mg ( OH ) 2 . These values were higher correspondingly than the 755, 505, and 94 W / kg Mg ( OH ) 2 values calculated for the bed comprising pure Mg(OH)2 pellets. It was demonstrated that the EM pellets had higher reactivity than pure Mg(OH)2 pellets because of their higher thermal conductivity.