Speciation and Thermal Stability of Mercury in Solid Products from Ultralow Emission Air Pollution Control Devices

The thermal stability of mercury (Hg) is a key factor that influences its solidification. However, the thermal stability of Hg in ultralow emission coal-fired power plants remains unclear. This study aimed to employ the temperature-programmed desorption technique to identify the forms and mass distribution of Hg species in solid products across air pollution control devices (APCDs) in typical ultralow emission coal-fired power plants. The migration mechanism of Hg across APCDs was determined by comparing the Hg species present in different solid samples. Furthermore, the impacts of different ultralow emission pollution control devices on Hg migration were obtained by comparing Hg compounds in solid products from different units. Results showed that Hg in the fly ash was mainly in the form of HgCl2 (41-47%) and HgS (43-47%). HgCl2 tended to be abundant in fine particles, and HgCl2 accounted for 90.68% of Hg in wet electrostatic precipitator (WESP) slag. HgCl2 (28-29%) and HgS (21-85%) were the dominant Hg species in gypsum. Hg in the waste sludge was mainly HgS. The corresponding temperatures of the release rate of 90% Hg in fly ash, gypsum, waste sludge, and WESP slag were in the range of 333-362, 350-637, 290-654, and 188-455 degrees C, respectively. The total Hg thermal stability of solid products increased in the order of gypsum > waste sludge > fly ash > WESP slag.