Predicting Extents of Mercury Oxidation in Coal-Derived Flue Gases

Abstract

The proposed mercury (Hg) oxidation mechanism consists of a 168-step gas phase mechanism that accounts for interaction among all important flue gas species and a heterogeneous oxidation mechanism on unburned carbon (UBC) particles, similar to established chemistry for dioxin production under comparable conditions. The mechanism was incorporated into a gas cleaning system simulator to predict the proportions of elemental and oxidized Hg species in the flue gases, given relevant coal properties (C/H/O/N/S/Cl/Hg), flue gas composition (O2, H2O, HCl), emissions (NO(X), SO(X), CO), the recovery of fly ash, fly ash loss-on-ignition (LOI), and a thermal history. Predictions are validated without parameter adjustments against datasets from lab-scale and from pilot-scale coal furnaces at 1 and 29 MWt. Collectively, the evaluations cover 16 coals representing ranks from sub-bituminous through high-volatile bituminous, including cases with Cl2 and CaCl2 injection. The predictions are, therefore, validated over virtually the entire domain of Cl-species concentrations and UBC levels of commercial interest. Additional predictions identify the most important operating conditions in the furnace and gas cleaning system, including stoichiometric ratio, NO(X), LOI, and residence time, as well as the most important coal properties, including coal-Cl.

Oxidation of Mercury in Products of Coal Combustion

“…Dr. Krishnakumar is now with Niksa Energy Associates, Belmont, CA. The calculations were performed using the semi-empirical 8-step mechanism for homogeneous reactions of mercury and chlorine species that has evolved from work by Widmer et al (2000), Niksa et al (2001), Qiu et al (2003), Niksa and Fujiwara (2005a), and Krishnakumar and Helble (2007). The calculations were performed for a simulated flue gas from an Ohio bituminous coal (130 ppmv HCl) using both a constant quench rate and a full-scale industrial boiler post-flame temperature profile.…”

Mercury Control with Calcium-Based Sorbents and Oxidizing Agents

“…This thermodynamic driving force does not begin to change significantly until below ~1100 °F. At this point, Hg-speciation begins to be influenced by kinetic-rate limitations [1][2][3][4][5][6][7][8], within the temperature/time histories of the CRF and full-scale boilers. The driving force however, quickly (by at least 600 °F for the vast majority of coal types, see Fig.…”

“…This can be accomplished by altering the concentration of gas-phase reactants or by increasing the heterogeneous reaction rate (i.e., catalytic enhancement -UBC, vanadium pentoxide, gold/palladium, etc.). The overall heterogeneous reaction rate for mercury oxidation and capture depends on interrelated mechanisms for both chlorine speciation and in turn Hg-speciation, some of which have been discussed in the literature [1][2][3][4][5][6][7][8].…”

“…Loss on ignition (LOI) is much more commonly used when characterizing power-plant operation, and LOI is obtained as part of the ash mineral analysis. Hence, modelers endeavoring to fit their model predictions with experimental data have desired to use LOI as an indictor of UBC or just to use LOI directly in their model instead of UBC [8].…”

A Kinetic Investigation of High-Temperature Mercury Oxidation by Chlorine

“…5 Attempts have been made to model homogeneous mercury oxidation in the flue gas environment through reaction channels involving chlorine explicitly. [6][7][8] Experimentally and theoretically derived rate data available in the literature for modeling mercury-chlorine reactions at combustion conditions are limited.…”