Effect of HBr formation on mercury oxidation via CaBr2 addition to coal during combustion

Abstract

Adding CaBr₂ to coal to enhance elemental mercury (Hg⁰) oxidation during combustion has been an effective mercury control technology, but the added CaBr₂ may increase levels of noxious Br₂ or HBr gases in flue gas. Temperature-programmed decomposition (TPD) experiments were conducted to verify the effect of CaBr₂ addition on Hg⁰ oxidation. The results indicated that the amount of Hg⁰ released initially decreased with increasing amounts of CaBr₂ additive and then held steady. The optimal amount of additive was 200 μg g⁻¹. CaBr₂ addition effectively oxidized Hg⁰ released at relatively low temperatures only. The generation of HBr was confirmed by mass spectrometry. The formation of HBr occurred over a temperature range of 250 °C to 400 °C, and the HBr concentration first increased and then remained stable as levels of CaBr₂ additive were increased in coal. The maximum concentration of HBr was 18 ppm and corresponded to 200 μg g⁻¹ CaBr₂. Further analysis indicated a strong, negative linear correlation between the amount of Hg⁰ released and the HBr concentration in flue gas. Based on these findings and previous studies, the possible mechanism of oxidation of Hg⁰ by CaBr₂ was analyzed.