Reaction mechanisms and chemical kinetics of mercury transformation during coal combustion

Abstract

Mercury (Hg) emission from coal combustion has attracted considerable public concern because of its harmful effects on human health and the ecosystem. The fundamental understanding of the reaction mechanisms and chemical kinetics that govern the transformation of Hg0 to Hg2+ and Hgp in coal-fired flue gas is crucial for mercury emission control. Kinetic calculations with quantitative predictability are critical to scaling up laboratory experiments to pilot- or full-scale tests. Despite extensive research over the last two decades, there remain many unresolved issues that can limit our ability to make useful engineering predictions and hence the potential for mercury emission control. This review discusses recent progress in the study of reaction mechanisms and kinetics of mercury oxidation over a wide temperature range, with a specific focus on the heterogeneous reaction mechanism of mercury adsorption, conversion and desorption on solid surfaces. Thermochemical properties of relevant mercury species are the basis of thermodynamic predictions. They are reviewed and provided first. Various methods and theories for evaluating and estimating kinetic rate parameters of elementary reactions are surveyed from existing experiments and theoretical studies. Further, the chemical reaction kinetics of mercury oxidation is discussed with an emphasis on two primary aspects of the problem: (i) the development of homogeneous reaction mechanisms using quantum chemistry calculations and (ii) the advancement of heterogeneous reaction mechanisms in which kinetic parameters of surface reactions are fitted to experimental data. Various kinetic models are tested against selected experimental data, the corresponding performance of each kinetic model is compared and evaluated. Finally, we provide an outlook on the reaction mechanisms and kinetics of mercury transformation during coal combustion.