Insights into the behavior and mechanism of K retention governed by Si-Al network structure during biomass thermal...

Understanding the migration characteristics of potassium (K) during biomass thermal conversion is crucial for effectively mitigating the ash-related issues. To investigate the release and retention behavior of K, a reactive force field molecular dynamics (ReaxFF MD) model was established and validated against relevant experimental data. Concurrently, the underlying mechanism of K retention by the silicon-aluminum (Si-Al) network was elucidated through ReaxFF MD simulations and thermodynamic calculations. The modeling results indicated that increasing the Al/(Si + Al) molar ratio from 0.40 to 0.77 resulted in a significant decrease in the ratio of non-bridging oxygen to tetrahedrally coordinated Al (NBO/T) from 3.68 to 1.16. This change signified that higher Al/(Si + Al) ratios facilitated the polymerization of Si-Al network, which also enhanced the retention of K. Notably, potassium meta-aluminate (KAlO₂), which captured K⁺ into the Al-O-Al(K) and Al-O-K structures via charge compensation to tetrahedrally coordinated Al ([AlO₄]), exhibited the highest retention capacity, followed by kalsilite (KAlSiO₄). The analysis of the radial distribution function (RDF) revealed that the coordination number (CN) of Al increased from 2.9 to 3.7 as the Al/(Si + Al) molar ratio increased. Concurrently, the proportion of [AlO₄] rose from 30% to 59%, thereby enhancing the K retention capacity by 30%. Additionally, the K-containing intermediates formed at elevated concentration of O₂ were captured by the Si-Al network, resulting in a 17% improvement in K retention. These findings regarding the elementary reactions and retention pathways associated with potassium provide a theoretical foundation for the development of advanced ash control technologies in biomass utilization.