Study on High-Temperature Ash Behavior Characteristics of Biomass

In the biomass utilization process, the high-temperature behavior of ash significantly influences the operational efficiency of gasifiers. This study examined the variations in the high-temperature behavior of straw ash and the underlying mechanisms using an intelligent ash fusion analyzer, a Theta high-temperature rotating viscometer, X-ray diffraction (XRD), SEM-EDS, and FactSage thermodynamic simulations. The results revealed that the high-temperature behavior of corn straw ash is mainly controlled by thermodynamic equilibrium, whereas that of wheat straw ash is primarily governed by kinetic factors. Both types of biomass ash contain a high proportion of K₂O (>30%), which leads to relatively low ash flow temperatures (<1300 °C). Interestingly, corn straw ash exhibited a higher flow temperature than wheat straw ash, despite its a higher K2O and lower SiO2 content. This anomalous behavior was attributed to the elevated levels of CaO (10.39%) and MgO (7.33%) in corn straw ash, which promote the formation of high-melting-point silicates such as K2MgSiO4, K2Ca2Si2O7, and CaSiO3 at elevated temperatures. In contrast, wheat straw ash, with comparatively lower CaO (4.92%) and MgO (2.82%) contents, tended to generate low-melting-point potassium silicates under similar thermal conditions. At high temperatures, both corn and wheat straw ashes exhibited typical crystalline slag melting characteristics, with viscosity increasing linearly as the temperature decreased below a certain threshold. The abrupt viscosity increase in corn straw ash slag was primarily caused by the rapid nucleation and growth of silicate crystals during cooling. Due to the “chemical dilution effect” caused by the high P2O5 content, KAlSiO4 persisted throughout the cooling process in wheat straw ash slag, which resulted in a generally higher viscosity and a sharp viscosity increase at the final stage of cooling. This study elucidated the mechanism by which the ash chemical composition, through determining high-temperature phase equilibrium and non-equilibrium kinetics, macroscopically influences the ash melting and flowing behaviors. This research provided a theoretical foundation for further exploration of the high-temperature properties of biomass ash.