Behavior of thallium in pulverized coal utility boiler installations in Southwest China

Abstract

ABSTRACT Thallium (Tl) is a toxic element that exists in coal at trace level. Coal-fired power plants (CFPPs) consume large amounts of coal and can potentially release this element into surrounding environment. However, knowledge of fates of Tl inside CFPPs and associated atmospheric emissions is still very limited. In this study, five CFPPs with pulverized coal boilers (PC) in Guizhou province, southwest China, were selected for investigation. All input and output solid materials and the stack flue gas samples were collected simultaneously. Tl concentrations in feed coal (0.10–0.34 mg·kg−1) of the five CFPPS were only a third to one half of the national average value. Tl concentrations were obviously higher in fly ash (0.39–1.13 mg·kg−1) than in bottom ash (0.09–0.25 mg·kg−1), indicating notable redistribution of Tl during coal combustion. Tl concentrations were low in limestone (0.01–0.02 mg·kg−1), flue gas desulfurization gypsum (0.01–0.03 mg·kg−1), and the stack flue gas (0.006–0.011 μg·Nm−3). Most Tl inside these CFPPs was captured by electrostatic precipitator or electrostatic precipitator-fabric filter (ESP/ESP-FF) fly ash (88.66%–97.44%), followed by bottom ash (2.13%–10.73%), gypsum (<3.89%), and stack emissions (0.01%–0.05%). Atmospheric emission factors of Tl from different CFPPs are in the range of 0.04–0.09 mg Tl·t−1 coal, 0.02–0.04 μg·(kW·h)−1, or 0.002–0.004 g Tl·TJ−1. Using these emission factors, a total of 3.96 ± 1.32 kg (range: 2.64–5.94 kg) Tl is estimated to be released into the atmosphere annually from CFPPs in Guizhou in 2017. To avoid the cross-media contamination from the combustion products, careful treatment of the captured fly ash, bottom ash, and gypsum is needed, considering that a large amount of Tl (average: 728 kg·yr−1; range: 664–792 kg·yr−1) is retained in these solid combustion products and the possibility of formation of more toxic Tl3+ during the combustion process. Implications: Thallium (Tl) is a rare but toxic element. Identifying and quantifying its source are high priorities for controling its contamination. Coal-fired power plants (CFPPs) were thought an important source of Tl, but few field studies had been conducted for this area. In this article, we investigated the fate of Tl in five pulverized coal utility power plants in Guizhou province, Southwest China, and found the Tl concentration in stack gas is in low levels of 0.006–0.011 μg·Nm−3, and less than 0.05% of total input of Tl is escaped into the ambient atmosphere. The majority of Tl (88.7%–97.4% of the total output) is detained by the ESP/ESP-FF fly ashes. Compared to the little amount (~4 kg·yr−1) of Tl that discharged into the atmosphere from Guizhuo’s CFPPs in 2017, more Tl (over 700 kg) ends up in the solid coal combustion products each year, which makes the need of careful disposal of this solid combustion waste to prevent the mobilization of Tl into the environment.