Dual Effects of In Situ Coal Combustion on CaO Pellets for CO2 Capture: High-Temperature Sintering and Ash Stabilization

High-temperature CaO-based CO2 capture technology, energized by in situ coal combustion, exhibits substantial promise owing to its high energy efficiency, strong compatibility, and maturity. However, sorbent deactivation mechanisms under complex coal combustion conditions, particularly for industrially required pelletized sorbents, are unclear. Pelletized sorbents were co-fired with four representative coals (differing in Na-K, S, and Al-Si content) in this study. Key factors were decoupled, and two competing mechanisms were revealed: (1) High-temperature sintering deactivation: Single co-firing triggers localized overheating (>900 °C), causing severe sintering and pore collapse. This reduces the specific surface area by 29% and pore volume by 50%, occludes meso-/macropores, and leads to a significant drop in initial CO2 capture capacity to 0.266–0.297 g/g. Coal types and minor residual surface impurities (<1.7%) are secondary factors. (2) Si-Al ash stabilization: During repeated co-firing (1–9 cycles), Si-Al ash components enrich on sorbents (0.1–7.6%), forming a thermally protective layer. After 20 adsorption–desorption cycles, the CO2 capture capacity loss drops from 17.6% to 3.9%, improving cycle stability. These findings clarify these dual mechanisms, providing a theoretical basis for system optimization and highlighting precise control of the combustion temperature field as critical for industrial deployment.