(244a) Submechanism for Mercury Oxidation Via Bromine and Chlorine In Coal Combustion Flue Gas

In order to reduce mercury from coal-fired plants, it would be beneficial to know the forms of mercury in the flue gas. Elemental mercury cannot be captured in wet scrubbers; however, its oxidized forms can. Chlorine is a good oxidizing agent and is naturally found in bituminous coal, but bromine is expected to be an even better oxidizing agent because of its larger size; this causes it to have greater London dispersion force interactions with mercury, resulting in easier outer valence electron removal. Bromine additive technologies have recently been implemented in several companies to enhance mercury oxidation. Because capture technologies are highly dependent upon the form of mercury that is present, investigations into their speciation are extremely important.

Though there have been numerous efforts to study mercury compounds as relevant to atmospheric studies, there is little data currently available for mercury compounds found in combustion flue gases. It would be particularly beneficial to obtain kinetic rate constants at various high temperature and pressure conditions typical for a combustion system. Prevalent species of mercury containing bromine and chlorine in coal combustion flue gases were studied using density functional theory and a broad range of ab initio methods. Reaction enthalpies, equilibrium bond distances, and vibrational frequencies were all predicted using coupled cluster (CC) methods. All electronic calculations were carried out using the Gaussian03 or MOLPRO software.

Kinetic predictions of the reactions shown below involving the formation of oxidized mercury via bromine and chlorine containing compounds are presented.

          1. Hg + Br → HgBr₂                   9. Hg + Cl → HgCl

          2. HgBr + Br → HgBr₂               10. HgCl + Cl → HgCl₂

          3. HgBr + Br₂ → HgBr₂ + Br      11. HgCl + Cl₂ → HgCl₂ + Cl

          4. HgBr + Br → Hg + Br₂           12. HgCl + Cl → Hg + Cl₂

          5. HgBr + H → HBr + Hg           13. HgCl + H → HCl + Hg

          6. Hg + HOBr → HgBr + OH      14. Hg + HOCl → HgCl + OH

          7. HgBr + HBr → HgBr₂ + H      15. HgCl + HCl → HgCl₂ + H

          8. HgBr + Cl → HgBrCl             16. HgCl + Br → HgBrCl

These kinetics are used in reaction route graph theory to create a submechanism for the oxidation of mercury via bromine and chlorine compounds in an existing global combustion model. Understanding the speciation of mercury in the flue gases of coal combustion is paramount in developing efficient technologies to ensure its capture.