Understanding Mercury Transformations in Coal-Fired Power Plants: Evaluation of Homogeneous Hg Oxidation Mechanisms

摘要

Homogeneous mercury oxidation mechanisms described by Niksa and Qiu, and three theoretical mercury oxidation reaction rate constants developed by Wilcox were evaluated for their predictions of the extent of mercury oxidation under coal combustion conditions. Predictions were compared to results from bench-scale experiments to determine whether such models were suitable for predicting measured levels of homogeneous mercury oxidation. Experiments considered different flue gas compositions (O_2, Cl, NO, and SO_2) and quench rates to provide a broad range of conditions for analysis. Regardless of the mechanism chosen, most mercury oxidation was predicted to occur at temperatures below 900 K. The Niksa mechanism predicted Hg oxidation to occur only in systems that were close to isothermal at temperatures above 900 K followed by a rapid gas quench. This mechanism provided the best agreement with the experimental data of Sliger. The Qiu mechanism predicted Hg oxidation in several experimental systems and conditions fairly accurately although it did not provide the best agreement in all cases. Qiu mechanism predictions for the experimental system at the University of Connecticut operating at an equivalence ratio of 0.9 in the presence of HCl/Cl_2 and also SO_2 were within the bounds of experimental uncertainty. Additionally, for an experimental dataset obtained from the University of Utah for quench rates of 210 and 440 K/s in the presence of HCl, the Qiu model predicted the experimental observations with a high degree of accuracy. The effects of flue gas composition and quench on Hg oxidation are qualitatively represented by the Qiu mechanism suggesting a relative robustness of the model, although there is still need to refine rate constants to achieve greater accuracy. The Wilcox rate constants when substituted in the Qiu mechanism predicted near-complete oxidation of Hg irrespective of HCl concentrations in systems that involve flue gas quench below temperatures of 450 K.