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The grain growth mechanism of nano-CaO-based CO 2 adsorbents in calcium looping (CaL) process was studied to figure out the main factors affecting sorption durability. First, the thermal growth characteristics of nano-CaCO 3 grains and nano-CaO grains at 750-850 °C was measured to fit the grain growth kinetic models. The activation energy data of grain growth of nano-CaCO 3 and nano-CaO were obtained as 104.8 kJ mol -1 and 212.8 kJ mol -1 respectively, which indicated that the grain growth of nano-CaCO 3 was easier than that of nano-CaO. Then, the grain sizes of regenerated nano-CaO undergoing 10 CaL cycles were compared with those derived from nano-CaCO 3 suffering high temperature heat-treatment under the same looping temperature and time. It was found that CaCO 3 -CaO chemical conversion could accelerate the grain growth of regenerated nano-CaO. Based on these results, the grain growth mechanism of regenerated nano-CaO grain in CaL process was proposed. The thermal growth of nano-CaCO 3 grain was the key issue to influence the grain growth of regenerated nano-CaO. Therefore, shortening the high temperature residence time as well as preventing the interface contact of nano-CaCO 3 grains were good for limiting the grain growth of regenerated nano-CaO., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)