Understanding mercury transformations in coal-fired power plants: evaluation of homogeneous Hg oxidation mechanisms.

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 (O2, Cl, NO, and SO2) 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/Cl2 and also SO2 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 HCI, 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.

A Model for the Viscous Coalescence of Amorphous Particles.

A model is presented for coalescence by viscous flow in two-particle systems, a phenomenon of interest in such diverse processes as the coagulation of air pollutant aerosols, glass and polymer sintering, and the coagulation of liquid suspensions. The model achieves computational efficiency by incorporating an empirical analytical function to describe the fluid interface, making it particularly suited to incorporation into aerosol dynamics models under conditions where coalescence-limited growth can occur. The dynamics of the interfacial evolution in the coalescing pair are approximated by employing a modification to the boundary integral approach in which a single surface point is used to solve the equations for the surface velocities. Model predictions of dimensionless shrinkage and neck growth obtained using this modeling approach compare favorably to those of existing finite-element models reported in the literature and provide a basis for more complex models to approximate multiple particle coalescence. Copyright 2001 Academic Press.