Endothermic Performances of Fe- and Mn-modified Carbide Slags in CaCO3/CaO Thermochemical Heat Storage Cycles

The modification with dark metallic oxide is identified as the crucial strategy to enhance optical absorptions of calcium-based materials for the direct solar-driven thermochemical energy storage. The effect of modification on the heat release behavior in carbonation of calcium-based material has been widely investigated, but its effect on the heat storage behavior in calcination is lacking of sufficient research, typically for low-cost calcium resource such as carbide slag. The Fe-modified and Mn-modified carbide slags for CaCO 3 /CaO heat storage were synthesized and their optimum decomposition temperatures, effective heat storage conversions, heat flows and heat storage rates in endothermic stage were investigated. Although the Fe modification exacerbates the CaO sintering due to the formation of Ca 2 Fe 2 O 5 , that is still effective in reducing the regeneration temperature of CaO in CaCO 3 /CaO cycles. The Mn modification enhances significantly sintering resistance by forming the CaMnO 3 and its transformation into Ca 2 MnO 4 . The effective heat storage conversion of Mn-modified carbide slag after 30 cycles is 3.2 times as high as that of untreated carbide slag. Mn-modified carbide slag exhibits the lowest regeneration temperature and the highest heat storage rate after cycles. The loose and stable porous structure of Mn-modified carbide slag contributes to its superior endothermic performance. Therefore, Mn-modified carbide slag seems to be the potential candidate for calcium looping thermochemical heat storage.