Modeling Li isotope fractionation during the coal-combustion process and evaluating its capacity as an environmental tracer

Abstract

Coal-fired power plants (CFPPs) utilizing lithium-rich coal are significant sources of anthropogenic Li emissions, highlighting the importance of effective tracing of potentially toxic Li for environmental monitoring and regulation. Lithium isotopes are a promising tracer, yet the dynamics of isotopic behavior during coal combustion are not well understood. This study investigates the elemental and isotopic behavior of Li in feed coals and combustion products from two pulverized coal-fired boilers (PC-H and PC-X) and one circulating fluidized bed boiler (CFB-H). The results showed that Li was primarily captured in fly ash (66.4 % to 84.8 %), with a fraction (1.3 % to 25.2 %) emitted into the atmosphere (stack emission). The isotopic composition of Li in feed coal ranged from −0.6 ‰ to +0.3 ‰, contrasting with δ⁷Li values in bottom ash and fly ash, which range from −2.7 ‰ to +0.1 ‰ and − 1.9 ‰ to +0.5 ‰, respectively. Notably, bottom ash samples consistently exhibit lower δ⁷Li values than their corresponding feed coal, suggesting the volatilization of heavier Li isotopes during combustion. Mass balance calculations reveal that stack emissions are enriched in heavier isotopes, with δ⁷Li values reaching up to +17.8 ‰. A Rayleigh fractionation model, with a fractionation factor of α = 1.0044, indicates that light lithium isotopes preferentially condense onto fly ash, while heavier isotopes are predominantly found in stack emissions. This model provides a quantitative framework for understanding isotopic segregation during cooling, significantly advancing methodologies for tracing and quantifying Li emissions for environmental impact assessments and source attribution of anthropogenic Li from CFPPs.