Sintering and agglomeration mechanisms of Fe2O3-CaO-K2CO3 ternary system in biomass-fueled chemical looping combustion process: Experimental and molecular dynamics simulation study

YF Zheng and YL Shao and T Song and XD Wang and L Pang and RK Agarwal, CHEMICAL ENGINEERING JOURNAL, 524, 169315 (2025).

DOI: 10.1016/j.cej.2025.169315

Biomass-fueled chemical looping combustion (CLC) is an innovative combustion technology characterized by inherent CO2 separation. Iron (Fe)-based oxygen carriers are widely employed due to their abundance and high reactivity. However, interactions between alkaline/alkaline- earth metals in biomass ash and oxygen carriers often induce sintering and agglomeration, potentially compromising particle circulation stability and carbon capture efficiency. To address the challenges, this study focuses on the Fe2O3-CaO-K2CO3 ternary system in the fuel reactor of a CLC system. Through a combined approach of experimental characterization, thermodynamic analysis, and molecular dynamics simulations, this work comprehensively examines product components and morphology, agglomeration mechanisms, ion migration behavior, and structural evolution patterns. The results reveal that K2CO3 decomposition products preferentially react with Fe2O3 to form KFeO2, which further coats the oxygen carrier surfaces. Additionally, CaO reacts with Fe2O3 to produce Ca2Fe2O5, forming solid-phase "bridges" between particles. When the temperature increases from 1073 K to 1273 K, particle agglomeration intensifies, resulting in shrinkage increased to 36.8 % and normalized surface area decreased to 78 %. Notably, although K+ exhibits a higher diffusion activation energy (Ed = 29.001 kJ/mol) compared to Ca2+ and Fe3+, its migration capability is significantly greater, with a diffusion coefficient of 0.626 & Aring;2/ps at 1273 K, which is 13.7 times higher than that of Fe3+. The enhanced diffusivity is attributed to the molten phase formed by K2CO3 at high temperatures, which facilitates K+ transport via interstitial diffusion mechanisms. In contrast, while Fe atoms undergo local coordination changes, their long- range migration remains constrained. Consequently, the growth of sintering necks (Ca2Fe2O5) slows down in later stages. These findings provide a theoretical foundation for developing effective strategies to mitigate agglomeration of oxygen carriers in CLC process.

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