In-situ visualization of thermochemical storage material Ca(OH)2/CaO: Understanding agglomeration and channeling in bulk powder

Kurzfassung

Among several gas-solid pairs in thermochemical reactions, the reversible conversion of Ca(OH)2 to CaO and water vapor presents benefits of high energy density, ample availability, inexpensiveness and environmental safety. However, the formation of agglomerates during cyclic de-/hydration of Ca(OH)2/CaO causes channeling and disturbs bulk homogeneity, which influences reaction rate and conversion. The methods suggested to pre­vent agglomeration, like encapsulation/additives, decrease the economic efficiency and also reduce energy density. Thus, this work emphasizes studying agglomeration in Ca(OH)2/CaO powder bulks to improve under­standing of the behavior of unmodified material. An in-situ visualization technique is developed to examine the structural modifications during Ca(OH)2/CaO cyclic de-/hydration. Unlike existing methods of visualization where real-time investigation is not possible, cyclic reactions are captured in-situ at real time for the first time. The dehydration reaction was conducted at 500 and 550 °C which are very much relevant to the storage of solar thermal energy in concentrating power plant. The hydration temperatures of 350, 400 and 450 °C are significant in regard to generation of steam and heat pump applications. Dehydration and rehydration result in contraction and expansion respectively, leading to agglomeration/cracks in the bulk during successive cycles. Development of these agglomerates/cracks is observed as a common occurrence irrespective of varying operating conditions, like reaction temperature and H2O vapor flow rate. The crucial information observed during cyclic reaction is that strong agglomerates and powder bulk clogging can be avoided by performing the dehydration in a diluted water vapor atmosphere, e.g. by using an inert carrier gas.