The Occurrence of Quartz in Coal Fly Ash Particles

Abstract

In spite of the presence of quartz, coal fly ash can be considered as a nuisance or inert dust. Respirable crystalline silica (e.g., quartz) is notorious for the induction of, for example, progressive massive fibrosis (PMF); besides, in 1997 the IARC stated that crystalline silica in the form of quartz or cristobalite from occupational sources is carcinogenic to humans. Quartz is present in both coal and residual ash. Ash originates from combustion of pulverised coal and, once removed from the flue gases by electrostatic precipitators (ESPs), it is called pulverized fuel ash (PFA). Thus, occupational exposure to PFA could also include exposure to silica. However, epidemiological studies did not show evidence of progressive massive fibrosis (PMF). In vitro tests demonstrated that PFA is less toxic than silica, and in vivo data of PFA did not support the importance of silica content for toxicity. Commissioned by the Dutch coal-fired power plants, KEMA has started a research project to determine the quartz content in coal and the corresponding PFA. It appears that on average 50% of the a-quartz in coal is found again in the total fraction of PFA (D_50(ae) 31 µm, where Dsotae) is the aerodynamically mass median diameter), whereas 16% is found in an even finer fraction (D_50(ae) 10 µm). The remaining part of the quartz is embedded in a glass phase. Scanning electron microscopy (SEM) with x-ray microanalyses (XMA) of cross-sections of 11,130 ash particles showed that quartz in PFA is present as unmelted sand particles. These quartz particles are angularly shaped. However, two types are to be distinguished: free coarse angular quartz particles (not respirable) and small angular quartz particles within the PFA particles. From the SEM/XMA results, it has to be concluded that the quartz in the respirable fraction is predominantly present within the original molten PFA particle. Since the effects of quartz are surface related, this elucidates the negative results of quartz-related effects of PFA in epidemiological, in vitro and in vivo studies. Besides, the amount of the total α-quartz in the respirable fraction of the ashes studied is less than 0.2%, so probably the Dutch occupational quartz standard of 0.075 mg m⁻³ will not be exceeded.