Atomistic-Level Models

Understanding the speciation of mercury throughout the coal-combustion process is crucial to the design of efficient and effective mercury removal technologies. Mercury oxidation takes place through combined homogeneous (i.e., strictly in the gas phase) and heterogeneous (i.e., gas–surface interactions) pathways. Both bench-scale combustion experiments [1] and quantumchemistry-based theoretical model efforts [2, 3] indicate that homogeneous mercury oxidation is responsible for, at most, 10% of the overall oxidation in a typical coal-fired flue gas with chlorine levels at 500 ppmv (e.g., HCl equivalent). These studies have been additionally confirmed by recent work that compares model predictions to bench-scale experiments, with the simulation predictions dependent on previous kinetic submodels developed specifically for Hg oxidation [4, 5]. Mercury speciation in coal-fired flue gases are extremely complex and depend on many factors, some of which include the mineralogy and chemistry of the coal, combustion conditions, power plant configuration, flue-gas composition, and temperature-time history from the boiler to the stack. The extent to which particulate-bound mercury (Hgp), gaseous oxidized mercury (Hg2+), and gaseous elemental mercury (Hg0) are emitted from the stack is also dependent on the existing pollution controls in a given power plant. The effective removal of Hg through existing flue-gas control technologies acts as a co-benefit (as discussed elsewhere in this book). For instance, electrostatic precipitators (ESPs), in particular cold-side ESPs and hot-side ESPs, capture, on average, 27% and 4% of Hg, while fabric filters (FFs) are more effective with approximately 58%Hg removal [6]. In general, Hgp or Hg2+ is easier to capture using one of these control technologies. In addition, oxidized Hg may also be captured in existing flue-gas desulfurization (FGD) units as the oxidized form is water-soluble. Activated carbon injection (ACI) is a direct method used for Hg capture, in which powdered activated carbon (PAC) is injected into the plant’s flue-gas streamwhere it adsorbs gaseous Hg and is collected in downstream particulate control devices, such as FFs or ESPs. Heterogeneous investigations include both adsorption and