Comparative observation of the flow behavior of low- and high-temperature ashes of biomass

Biomass ashes are frequently known for their limiting effects in thermochemical conversion processes. Thus, a comprehensive understanding and prediction of the temperature-depending behavior of the ashes is indispensable for practical applications. In the present study, the flow behavior of low-temperature ash (LTA) and high-temperature ash (HTA) of biomass is investigated in detail and monitored by in situ methods. For this purpose, the mineral matter formation and transformation of both ashes are studied by X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. A combination of heating-stage microscope and scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX) has enabled a systematic identification of the microstructures and observations of LTA and HTA. The results have illustrated an associated release of K and Cl and the formation of new mineral phases in the remaining material during ashing at increased temperature. Both ashes have exhibited flow temperatures below 1100 °C and a rarely observed behavior of higher characteristic temperatures for LTA than for HTA is detected. From the heating-stage microscopy, the ashes have given evidence to a significant formation of slag at the characteristic temperatures and a viscosity-driven behavior at the characteristic points of ash fusion. Based on these results, a conclusion can be drawn regarding the usability of this biomass for high-temperature conversion processes on the industrial scale.