Effect of CaZrO₃ dopant on structural evolution and CO₂ diffusion transfer of calcium-based sorbents

Calcium-based sorbents are widely employed in sorption enhanced CH₄/H₂O reforming for hydrogen production, but they suffer from gradual degradation in both sorption capacity and rate in cyclic processes. Previous studies have demonstrated that CaZrO₃ dopant could significantly enhance the stability of sorbents, but the underlying mechanism remains unclear. A multi-physics coupled model of the carbonation process was established, integrating experimentally characterized structural parameters to investigate the effect of CaZrO₃ dopant on CO₂ diffusion and the mechanism for improving sorption stability. The model combined the changing grain size model with transient heat and mass conservation equations. Results indicated that CaZrO₃ dopant （dopant volume fraction is 36%） maintained the relative stability of pore volume during the carbonation process, and increased the CO₂ diffusion coefficient by 146%, thereby significantly enhancing the average CO₂ concentration within the particle and promoting the rapid and uniform CaO conversion. Compared with pure CaO sorbents, the CaO/CaZrO₃ composite retained a loose and porous structure in cyclic processes. Its pore volume and CaO grain size remained stable after multiple regeneration cycles, with CaO conversion decreasing by only 3.6% after 10 cycles. By using dimensionless parameters to decouple the effects of distinct structural parameters, it was found that mitigating pore loss reduced CO₂ diffusion resistance, while suppressing CaO grain growth decreased chemical reaction resistance, thereby enhancing CaO conversion and carbonation rate. The CaZrO₃ dopant demonstrated dual regulatory effects on sorption performance. Increasing the CaZrO₃ dopant amount enhanced structural stability and CaO conversion, but concomitantly increased reaction resistance, thereby limiting the carbonation reaction rate.