Effects of hydrogen injection and temperature on enhancing homogeneous mercury oxidation by chlorine

Experiments and models are used to investigate the potential of H₂ injection to encourage mercury oxidation to enhance removal from coal-fired power plant exhaust streams. A laboratory-scale quartz reactor creates a surrogate combustor exhaust stream within which mercury oxidation is measured. The experiments focus on the effects of multiple reactor temperatures and varying concentrations of O₂, H₂O, HCl, and H₂. The modeling study first evaluated several chemical kinetic mechanisms for mercury oxidation and identified a set of reaction parameters that replicated experimental trends. The models then explore the chemical kinetic basis for increased mercury oxidation due to H₂ injection. The models indicate that injected H₂ increases the radical pool of H and OH, provided sufficient time is allowed at an elevated temperature. The radicals then react with HCl to produce a pool of Cl (and subsequently Cl₂). The increased Cl₂ concentration reacts with Hg and HgCl, improving mercury oxidation to HgCl₂. Water-soluble HgCl₂ is much more easily removed from combustion exhaust gases by standard air pollution control equipment than elemental Hg.Implications: In this study, the authors present experimental data and chemical kinetic models to investigate the opportunity of reducing mercury emissions from coal-fired power plants by injecting the exhaust with H₂. The study concludes that injecting H₂ at an elevated temperature in a temporary isothermal environment is a way to enhance mercury oxidation and, ultimately, mercury removal. The implications of this work could be a reduction in mercury emissions by leveraging a new technological solution.