Preparation of oxynitride glasses from woody biofuel ashes

Oxynitride glasses have been prepared by melting woody biofuel ash from a power plant in south Sweden with addition of calcium metal as an extra modifier in a nitrogen atmosphere at 1350–1500 °C. The glasses were characterized by X-ray powder diffraction, differential thermal analysis and scanning electron microscopy. Cation and anion glass compositions were determined by energy dispersive X-ray analysis and combustion analysis, respectively. The glasses were found to be homogenous, translucent gray to black, and to contain up to 23 e/o of calcium and 5 e/o of nitrogen. The glass formation depends on the ratio of calcium metal introduction to the ash precursors. A strong exothermic reaction was observed at 650–850 °C, leading to the formation of amorphous and crystalline oxynitride phases that melt at high temperatures upon further heating. The glass densities vary between 2.76 g/cm3 and 2.92 g/cm3. The glass transition temperature was found to vary from 670 °C to 749 °C and increase linearly with the nitrogen content.