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 881kJ/kgMg(OH)2, gross heat output of 714kJkgMg(OH)2, and mean power output rate of 132W/kgMg(OH)2. These values were higher correspondingly than the 755, 505, and 94W/kgMg(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.