Silicate Rings in Woody Biomass Ash Melts Based on Molecular Dynamics Simulations

The medium-range ordering of the molten CaO-K₂O-SiO₂ system was investigated by studying the primitive silicate rings extracted from molecular dynamics simulations. Variations in the ring size distribution and the interconnectivity between the rings due to the different contents of the basic cations (Ca²⁺ and K⁺) were identified. With increasing basic cation content, i.e., depolymerization, the overall number of rings decreased, but increasingly larger rings (>10-membered) occurred while small-sized rings (4- to 7-membered) remained the most common. The share of 5-membered rings increased with depolymerization, particularly with K content. In terms of ring structure, compositions with more K cations also tended to produce rings with longer Si-O bonds and wider Si-O-Si angles; i.e., rings that were weaker and more stretched. A method was introduced to characterize the interconnectivity between rings, which indicated that the most common small-sized rings have a tendency to be interconnected to rings of specific larger sizes, depending on the composition. Visualization and counting of the basic cations inside of these rings showed that they were cation clusters. It suggests that these basic cation clusters tend to be located adjacent to Si-rich regions where small-sized rings interconnect with each other.