Effect of SiO2/CaO on viscosity, structure and phase transition of biomass ash slag by experimental study, thermodynamic analysis and MD simulations

Abstract

The melting properties of biomass ash slag in entrained-flow gasifiers are an important control parameter for determining the service life and operational stability of gasifier refractories. This study used melting experiments to evaluate the impact of SiO₂ and CaO mass ratios, as well as K₂O content, on slag melting. The study found that ash slag with low SiO₂/CaO (0.50–0.75) is challenging to meet the need for liquid slag discharge due to its difficulty melting. Increased K₂O level improves meltability in high SiO₂/CaO (=12.5) ash slag but not in low SiO₂/CaO (=0.5) ash slag. Thermodynamic calculations revealed that high melting points of Ca₂SiO₄ and CaO in the low SiO₂/CaO ash slag hinder the melting process. High SiO₂/CaO ratios can effectively fix potassium while improving fluidity. Furthermore, MD simulations confirmed that Ca²⁺ functions as a network modifier, promoting the formation of FO and NBO, depolymerizing the [SiO₄]^4- silicate network, and significantly reducing slag viscosity. Elevated SiO₂/CaO ratios increase Si-O-Si bridging bonds within the network structure, enhancing the degree of polymerization and thereby raising viscosity. When the K₂O level is 30 %, ash slag with SiO₂/CaO = 4.00–6.00 can meet the requirement of continuous slag discharge. The melting properties of biomass ash slag are significantly enhanced by modifying SiO₂/CaO. Appropriate SiO₂/CaO ratios can enable smooth slag discharge from the gasifier and provide theoretical support for the long-cycle operation of biomass gasification systems.