Valorization of phosphogypsum by-product as thermochemical energy storage material

Abstract

Phosphogypsum (PG), a phosphate industry by-product, emerges as a promising candidate among numerous materials capable of reversibly storing thermal energy via endo-exothermic reactions. By utilizing this resource, not only its environmental impact will be reduced, but the overall cost-effectiveness of the thermochemical energy storage (TcES) system will be improved. PG as characterized in this study is composed mainly of calcium sulfate (CS) dihydrate, CaSO4·2H2O. Its dehydration proceeds in two steps: first, transforming it into CS hemihydrate, CaSO4∙0.5H2O, with an enthalpy of 441 J/g, followed by dehydration to CS anhydrate, CaSO4, with an enthalpy of 147 J/g. Rehydrating the material allows complete energy recovery. The reversibility of PG's hydration and dehydration reactions for the working pair CaSO4/H2O is demonstrated in this study. The cycling stability of PG and pure CS as thermochemical energy storage materials is investigated on a mg-scale as well as in a TcES lab-scale reactor of 20 g storage material bulk. PG shows better cyclability characteristics compared to CS. Over 60 cycles of hydration/dehydration at 140 °C, PG exhibited good cycling stability, whereas a decrease in conversion was observed in CS. However, increasing the dehydration temperature resulted in reduced cyclability for both materials. This reduced cyclability is attributed primarily to the formation of an inactive anhydrous CS phase, anhydrite II. This phase formation occurs at lower temperatures in the case of CS when compared to PG, which explains the distinct behaviors of these materials during cycling. Additionally, CS tends to agglomerate more easily, while PG presents reduced agglomeration. This difference is attributed to the presence of SiO2 in PG, which was confirmed through a three-cycle hydration/dehydration experiment comparing CS and CS with the addition of 10 wt% SiO2. PG also allows for a significant thermal heat upgrade, which was demonstrated in the lab-scale reactor during 26 cycles of hydration and dehydration, holding significant potential for various industrial applications, including waste heat recovery.