Raman spectroscopy and X-ray diffraction analysis of the structural characteristics of fluidized bed gasification fly ash

Previous experiments using thermo-gravimetric analysis (TGA) concluded a high carbon fly ash obtained from a circulating fluidized bed (CFB) gasifier by firing an anthracite coal to be less reactive compared to other high ash content chars that were prepared under laboratory conditions in a drop tube furnace (DTF) from a parent bituminous coal. This fly ash had internal surface (BET) area and pore volume of an order of magnitude larger than the laboratory prepared chars and so a further investigation was carried out by determining the influence of the fly ash residence time (10-20s) in the CFB on its structural ordering. The structural analysis was carried out by using Raman spectroscopy and X-ray diffraction (XRD). Raman spectroscopy with a 457.9nm laser, was used to compare the structural ordering of the fly ash carbon with those of its parent (anthracite) coal; a char prepared from the same parent coal at the same heat treatment temperature as the fly ash but under laboratory conditions; and the other chars prepared from a bituminous parent coal (under laboratory conditions) whose reactivities were found to be higher than the fly ash. It was concluded that the long residence times enhanced the decomposition of the anthracite coal, consumption of the amorphous carbon and the ordering of aromatic units and the crystalline structure as gasification progressed. The structural behavior of fly ash was also investigated after different conversions in DTF through reactions with oxygen at 5% and 20% concentration at a temperature of 1300°C. At low residence times, the fly ash became less ordered due to further decomposition and release of loose organic matrix and after longer residence time the fly ash then became more ordered due to aromatic ring growth. XRD experiments were used to validate the structural analysis result from Raman spectroscopy. The results concluded fly ash and a laboratory char prepared at high temperature to be the most ordered. There was also the linear relationship between the volatile content of the chars including the fly ash when compared with their structural parameters.