Characterization of optical and physiochemical properties of reclaimed fly ash for use as sensible thermal energy storage material in concentrating solar power systems

Abstract

It is critical to develop strategies to reduce embodied emissions associated with the production and processing of thermal energy storage (TES) materials used in concentrating solar power (CSP), particularly as CSP becomes more prevalent. One such strategy involves reclamation of existing waste materials for use as solid TES particles. In this study, solar weighted absorptance, elemental composition, crystalline abundance, and microscopic appearance of samples were characterized for eight unconventional and three conventional coal fly ash waste samples. Four of the samples were subjected to a series of thermal cycling protocols of 1–4 cycles from 800 °C to 1400 °C to mimic CSP operating conditions, and then recharacterized to elucidate changes to the measured properties. The results show that weighted solar absorptance of the samples ranges from roughly 55% to 87% (the latter of which is comparable to Carbobead CP), and that absorptance is positively correlated with crystal phase iron enrichment in the as-received samples. However, following thermal cycling, physical characteristics such as surface roughness and degree of crystallinity have a greater impact on the weighted absorptance values. A bulk iron content of >5% is needed to achieve absorptance values appropriate for TES irrespective of whether an ash is conventional or unconventional. While a small number of past studies have reported that fly ash may be suitable for TES, this is the first known study to investigate unconventional ashes and to separate effects of crystalline versus bulk iron. The results of this study can be used as a basis for optimizing fly ash properties for this purpose.