On the path to elucidating the speciation of mercury in the flue gases of coal combustion

Abstract

The goal of this research is to understand the speciation of mercury in the flue gases of coal combustion. As the flue gas cools, thermochemical equilibrium calculations indicate that elemental mercury, Hg⁰, is converted to oxidized mercury, Hg²⁺, in the form of HgO or HgCl₂. Hg⁰ is insoluble in water, HgO has low solubility in water and HgCl₂ is highly soluble in water. Since HgCl₂ is water-soluble, it can be captured in wet chemical scrubbers to prevent its release to the atmosphere. Therefore, the understanding of the mechanisms of mercury's oxidation in flue gases is paramount when considering mercury capture. This research attempts to elucidate the mechanisms of oxidation through a detailed kinetic and thermodynamic analysis. The current research focuses specifically on the oxidation of mercury via chlorine-containing compounds. Future research will involve the oxidation via oxygen-containing compounds and the effect of SO₂ and NOₓ compounds on mercury's oxidation. Quantum chemistry is used to determine accurate transition structures, which are required for the calculation of activation energies and rate constants from theory. Simultaneous to the theoretical work, an experimental apparatus has been designed and fabricated with the inclusion of a quadrupole mass spectrometer. The mass spectrometer is used in conjunction with a laminar flow reactor to simulate the oxidation of mercury via chlorine-containing compounds in flue gases. The ultimate goal of this research is to obtain a potential mercury oxidation mechanism based upon theoretically predicted kinetic parameters, which are then validated through concentration profiles obtained from experimental measurements. In addition, results from the experimental work indicate that at ambient conditions, the oxidation of mercury via chlorine may result as a consequence of heterogeneous reactions involving the Pyrex reactor surface. This work not only allows for a more thorough understanding of mercury's speciation in the flue gas environment, but also questions current sampling devices and their potential interference with reactivity measurements involving mercury-chlorine species.