Investigation into the fusibility of biomass ashes and their mineral phase transformations at elevated temperatures by using the HT-XRD technique

In this paper, the fusibility of biomass ashes with varying chemical and mineral compositions were investigated and their mineral phase transformations were characterised in-situ by using the HT-XRD technique. Results showed that ash fusion temperatures (AFTs) of the analysed biomass increased in the order of walnut shell (WS) < wheat straw (WHS) < peanut shell (PES) < pine sawdust (PS). This is attributed partially to their chemical compositions, since WS and WHS ashes had high K₂O contents (30.77%), but the PES and PS ashes were abundant in SiO₂, Al₂O₃ and/or CaO. HT-XRD analysis showed that K-bearing minerals were firstly identified but then diminished in WS ash as temperature, confirming their role for slag formation. For WHS ash, the release of KCl and the formation of K silicates or aluminosilicate at different temperatures were confirmed, leading consequently to a decreased AFTs. In the PES ash, SiO₂, MgO, CaAl₂Si₂O₈, CaAl₂SiO₆ were identified, which also did not melt at the temperatures examined. Likewise, minerals in PS ash including CaO, Ca₃SiO₅ and Ca₂SiO₄ remain identified at elevated temperatures. Macro- and micro-morphological analysis shows that the WS and WHS ashes were prone to be sintered at temperatures as low as 650 °C, yet PES and PS ashes were less sintered even at 850 °C. These were confirmed by thermodynamic calculations when the phase and mineral transitions were considered. These mineral phases were also compared with those from conventional XRD, confirming the advantages of HT-XRD in determining mineral phase transformation and comprehending fusibility of the biomass ashes.