Experimental and thermodynamic investigation of ash chemistry and combustion conditions governing biogenic silica production from agricultural biomass residues

<title>Abstract</title> <p> Agricultural biomass residues represent an abundant and underutilized resource for the production of biogenic silica. However, the combined influence of feedstock chemistry, combustion conditions, and alkali and alkaline earth metal (AAEM) content on silica quality remains insufficiently understood. This study investigated the effects of ash chemistry, citric-acid pretreatment, combustion temperature (400–700°C), and residence time (1–4 h) on biogenic silica production from cassava peel, yam peel, coconut husk, corncob, and cornhusk. Silica quality was evaluated using elemental composition, specific surface area (S _BET ), pore volume (PV), slagging indices, principal component analysis (PCA), Pearson correlation analysis, response surface methodology (RSM), and FactSage thermodynamic modeling. PCA identified SiO₂ content, S _BET , PV, combustion temperature, and AAEM oxides as the principal variables contributing to variability in silica quality. PC1 and PC2 explained 28.4% and 14.2% of the total variance, respectively, distinguishing silica-rich systems from AAEM-rich ash compositions associated with increased slagging propensity. Citric acid pretreatment substantially increased silica contents from 20.01 to 38.62 wt.% in cassava peel, 26.70 to 45.22 wt.% in yam peel, 11.53 to 44.57 wt.% in coconut husk, 20.82 to 49.53 wt.% in corncob, and 39.94 to 70.74 wt.% in cornhusk. Corresponding reductions in alkali concentrations lowered slagging indices and delayed liquid-phase formation. Pearson correlation and RSM analyzes showed that combustion temperature was the dominant factor affecting silica textural properties. Response surface optimization identified an optimal combustion window of approximately 550–650°C and around 2 h residence time, providing the most favorable balance between silica purity, specific surface area, and pore volume preservation. These findings provide predictive guidelines for producing high-value biogenic silica from underutilized biomass residues while mitigating slagging. </p>