Ab initio-based Mercury Oxidation Kinetics via Bromine at Postcombustion Flue Gas Conditions

Abstract

Because direct mercury measurements are difficult throughout the postcombustion flue gas environment, it is practical to use mercury reaction kinetics to theoretically determine mercury speciation based upon coal composition, plant equipment, and operating conditions. Elemental mercury cannot be captured in wet scrubbers; however, its oxidized forms can. Mercury−bromine oxidation kinetics have been studied with electronic structure calculations using a broad range of ab initio methods. Reaction enthalpies, equilibrium bond distances, vibrational frequencies, and rate constants have all been predicted using density functional theory as well as coupled cluster methods. Upon the basis of comparisons to available experimental and high-level theoretical data in the literature, quantum-based methods for the following Hg−Br reactions have been validated: Hg + Br ↔ HgBr, Hg + Br2 ↔ HgBr + Br, and HgBr + Br ↔ HgBr2. The use of these methods for predicting forward and reverse rate constants for the following Hg−Br reactions has been carried out for the first time: Hg + HBr ↔ HgBr + H, HgBr + Br2 ↔ HgBr2 + Br, and HgBr + HBr ↔ HgBr2 + H. Understanding the speciation of mercury in the flue gases of coal combustion is paramount to the development of efficient control technologies.