Structural characteristics and thermal conversion performance of ash and slag from circulating fluidized bed gasifier

Based on the circulating fluidized bed (CFB), Xinjiang Zhundong coal (ZDC) gasification ash (FA: fly ash; BA: bottom slag) was analyzed by industrial analysis, ultimate analysis and Fourier infrared spectroscopy to determine the basic properties and functional group species. The results show that BA contains up to 99.30% in ash, while FA shows high fixed carbon and C content of 69.30% and 73.78% respectively. Furthermore, the carbonaceous forms and surface morphology of ZDC and FA were characterized by Raman, XRPES and SEM, and the pyrolysis, combustion and gasification characteristics of ZDC and FA were studied with TG-DTG methods. XRPES show that C content of the FA surface is 89.42%, and exists primarily as >C–C< and >C–H, while O was in the form of >C=O. Alkaline earth metals Ca bound to the above-mentioned carbon-involved functional groups cause high disorder in FA. SEM observed that the rough and porous FA surface occurs due to spherical particles of molten mineral attached and embedded surface and pore channels. The thermal conversion characteristics show that the maximum weight loss rate peak temperature of pyrolysis and combustion of FA is significantly higher than that of ZDC, indicating that the pyrolysis and combustion performance of FA is reduced. However, 100% carbon conversion of FA uses about half the time compared with ZDC and the gasification performance has improved significantly since it has well-developed pore structures, more amorphous carbon and abundant active sites, enhancing diffusion of CO₂ from gasifiers. Briefly, FA has the potential and ability to be recycled for direct utilization in CFB as gasification feed.