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.