Abstract: Magnesium and calcium hydroxides are promising for thermochemical heat storage (THS) and chemical heat pumping since they undergo a reversible reaction of de-/rehydration. The pure hydroxides lose water with acceptable for THS rate at T > 300oC (Mg(OH)2) and T > 400oC (Ca(OH)2) which restricts a range of possible sources of surplus heat to be stored. The dehydration reactivity enhancement is desirable because it can increase the efficiency of hydroxide-based thermochemical heat storage systems. In the present work effect of inorganic salts of alkali metals on de-/rehydration of Mg(OH)2 and Ca(OH)2 is studied. Mixtures of Mg(OH)2 and Ca(OH)2 with various inorganic salts (XCl, XNO3 and XOAc, X2SO4, where X = Li, Na, K) were prepared. The salt screening showed that modification with nitrates, acetates and LiCl results in considerable (30-80oC) decrease in dehydration temperature (Td) for Mg(OH)2. Lithium nitrate (LiNO3) was found to have the most profound effect (~80oC). Potassium nitrate (KNO3) had the maximal effect on Ca(OH)2 (~35oC). The study of LiNO3/Mg(OH)2 and KNO3/Ca(OH)2 with the salt content Y = 0.5-20 wt. % reveals that the Td-decrease depends on the salt content. Dehydration kinetics study was performed at P(H2O) = 24 mbar that is typical for closed THS cycle. The study showed that the dehydration of XNO3/Mg(OH)2 (X = Li, Na) at T = 230-280oC and KNO3/Ca(OH)2 at T = 380-400oC is considerably faster as compared to the pure hydroxides. Rehydration kinetics study showed lower half-conversion times for salt-modified materials. De- and rehydration were studied by XRD, vibrational spectroscopy, HRTEM and NMR methods. The pathways of the salts effect on the dehydration of the hydroxides were discussed. One of the possible explanations of dehydration temperature decrease is a drastic decrease of the surface areas of the resulting oxides which brings the system closer to the calculated equilibrium of bulk phases. Thus, screening of salts showed maximal effect of LiNO3 and KNO3 on Mg(OH)2 (~80oC) and Ca(OH)2 (~35oC) dehydration, respectively. The dehydration and rehydration kinetics study showed that these materials may be promising for THS. The mechanism of the salt effect is discussed.

Cite: Shkatulov A.I. , Aristov Y.I.
Thermochemical Heat Storage by Magnesium and Calcium Hydroxides Doped by Inorganic Salts
Materials Science and Engineering Congress (MSE 2016) 27-29 Sep 2016