Decoupling Thermal-Driven and Reaction-Induced Effects on the Strength Evolution of CaO Pellets during Calcium Looping CO2 Capture

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Calcium looping is a promising CO2 capture technology, and the mechanical stability of calcium-based adsorbent pellets is critical for their industrial application. Although pelletization processes and formulation optimization can improve the initial strength, the strength evolution of pellets during carbonation/calcination cycles and the underlying dominant mechanism remains little understood. This study identified two major effects governing pellet strength evolution during cyclic reactions: the thermal-driven effect (thermal shock damage and high-temperature sintering) and the reaction-induced effect (volume change damage and reactive sintering). Their respective roles and relative contributions at different cycling stages were systematically investigated by combining carbonation/calcination cycling tests, short-term thermal shock tests, and nonreactive long-term thermal cycling control experiments. The results indicate that the CaO pellets show an early decline in strength, followed by a partial recovery in later cycles. Mechanistic analysis indicates that thermal shock promotes pellet cracking, while cyclic volume expansion/contraction caused by the CaO/CaCO3 phase transformation can induce local stress concentration, both of which cause strength degradation. In contrast, sintering-induced densification enhances structural strength, thereby contributing to late-stage strength recovery. Relative contribution analysis reveals a clear stage-dependent transition in the factors governing strength evolution. The thermal-driven effect dominates the early stage, contributing over 70% in the 0–5 cycle, whereas the reaction-induced effect gradually increased to over 60% in the 20–25 cycle and became dominant later. This study provides mechanistic insights into the strength evolution of calcium-based pellets during calcium looping and offers guidance for improving the mechanical stability of calcium-based pellets.