Dual role of silicon in potassium-induced ash deposition: Low-Temperature mitigation and high-temperature aggravation mechanisms

Potassium-induced ash deposition constrains biomass utilization. Although silicon addition can mitigate ash deposition by immobilizing potassium, it may also intensify melt-induced deposition, and the relative severity of potassium-release- versus silicate-melting-induced deposition remains unresolved. To address this gap, this study introduces a new quantitative ash-adhesion-strength parameter and, coupled with molecular dynamics simulations, demonstrates that strongly adherent, melt-driven K–silicate deposits are more detrimental than potassium condensation deposits. Analyses of silicon-poor sunflower hulls (SH) and silicon-rich rice straw (RS) are contrasted to clarify the dual effects of silicon. For SH, deposition is dominated by potassium condensation. Ash shrinkage begins near 800 °C but adhesion remains weak because KCl and K2SO4 show low binding energies on Fe2O3(110). The low Si/Al ratio favors high-melting KAlSiO4, giving a stepwise shrinkage curve and delaying melt-induced deposition to higher temperatures. For RS, potassium release still governs deposition below 900 °C but is suppressed by its higher silicon content. However, the deposition mechanism transitions to a melt-dominated process above 900 °C: the formation of low-melting K2Si4O9 and K–Ca–Si phases trigger rapid ash shrinkage and melting around 900 °C. Strong Fe–O–Si covalent bonds at the K2Si4O9/Fe2O3(110) interface yield intrinsic adsorption energies far exceeding those of KCl and K2SO4; hence, capillary forces and interfacial bonding jointly amplify adhesion strength. Overall, silicon additives suppress chloride/sulfate activity and condensation-induced deposition within low-temperature window (<900 °C). At elevated temperatures, they exacerbate melt-induced deposition, which can be more detrimental than condensation-induced deposition and therefore require stringent control of silicon speciation and operating conditions.