Chemical, mineralogical and petrological characterization of ash formers in high-temperature sintering processes in wood pellet ash

• Ash sintering in wood pellets analyzed using advanced multi-scale techniques. • Feldspar dissolves at ∼700 °C, forming amorphous melts and triggering sintering. • K-feldspar crystallizes during cooling, confirming potassium’s key role. • Sintering temperature identified between 850 °C and 900 °C by CCS and XRD. • Reflected-light microscopy validated for amorphous phase characterization. Operational disruptions in thermochemical conversion processes frequently result from ash sintering and slagging. A deeper understanding of these sintering mechanisms is crucial for maintaining reliable plant operations and optimizing efficiency. This study provides guidance for mineralogical characterization and transformation in thermochemical processes, specifically focusing on sintering mechanisms in wood pellets through integrated analytical methods, including cold compression strength tests (CCS), reflected-light microscopy, scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), X-ray fluorescence analysis (XRF), X-ray powder diffraction analysis (XRD), and high-temperature X-ray diffraction analysis (HT-XRD). Reflected-light microscopy revealed critical mineral transformations beginning with feldspar dissolution around 700 °C, followed by quartz dissolution and the formation of amorphous melts. Subsequent cooling processes resulted in the secondary crystallization of K-feldspar from these melts. Detailed chemical analyses via SEM-EDX confirmed the spatial distribution of chemical constituents, identifying K-feldspar as the ultimate sintering product. CCS analysis established the sintering temperature between 850 °C and 900 °C, corroborated by quantitative XRD and HT-XRD data, which demonstrated increased amorphous phase content with rising temperature. The identification of K-feldspar secondary crystallization underscores potassium’s significant influence on ash sintering behaviors, directly affecting combustion efficiency and boiler performance. This research also confirms the efficiency of reflected-light microscopy, particularly in analyzing amorphous processes, complemented by SEM-EDX and XRD analyses. These combined analytical insights are essential for optimizing biomass ash processing and addressing operational challenges.