A Mechanism for Mercury Oxidation in Coal-Derived Exhausts

Abstract

This paper evaluates an elementary reaction mechanism for Hg0 oxidation in coal-derived exhausts consisting of a previously formulated homogeneous mechanism with 102 steps and a new three-step heterogeneous mechanism for unburned carbon (UBC) particles. Model predictions were evaluated with the extents of Hg oxidation monitored in the exhausts from a pilot-scale coal flame fired with five different coals. Exhaust conditions in the tests were very similar to those in full-scale systems. The predictions were quantitatively consistent with the reported coal-quality impacts over the full range of residence times. The role of Cl atoms in the homogeneous mechanism is hereby supplanted with carbon sites that have been chlorinated by HCl. The large storage capacity of carbon for Cl provided a source of Cl for Hg oxidation over a broad temperature range, so initiation was not problematic. Super-equilibrium levels of Cl atoms were not required, so Hg was predicted to oxidize in systems with realistic quench rates. Whereas many fundamental aspects of the heterogeneous chemistry remain uncertain, the information needed to characterize Hg oxidation in coal-derived exhausts is now evident: complete gas compositions (CO, hydrocarbons, H2O, O2 NOx, SOx), UBC properties (size, total surface area), and the ash partitioning throughout the exhaust system are required.

Mass Transfer within Electrostatic Precipitators: Trace Gas Adsorption by Sorbent-Covered Plate Electrodes

“…However, like coal ore itself, the composition of coal combustion products is complex and variable. As a result, chemical kinetic models for Hg [5][6][7][8] are not yet complete, particularly with respect to heterogeneous chemistry on particulate matter.…”

Mercury Control with Calcium-Based Sorbents and Oxidizing Agents

“…This may be especially true if the HgS is settled in a baghouse filter cake. Hence, chlorine may re-suspend mercury in the flue gas by the reverse of the reaction described by equation (4) or by other reactions, especially reactions involving chlorinated-carbon sites. Reentrainment reactions may be quite significant in the filter cake, especially if they are enhanced by catalysis, analogous to that represented in equation (1).…”

“…A system of reactions, which include significant chlorine-speciation reactions, has been proposed to describe homogeneous Hg-oxidation [4]. This set of governing reactions allows direct oxidation of Hg o to HgCl and HgCl to HgCl 2 by the following four chlorine species with different reaction rates: Cl, Cl 2 , HCl, and HOCl [4].…”

“…H 2 S + HgCl 2 HgS + 2HCl (4) Thermal decomposition of Na 2 S 4 can produce a large independent concentration of elemental sulfur vapor, thus promoting reaction (2) above. However, higher temperatures also enhance reactions competing to deplete the injected and generated concentrations of Na 2 S 4 , H 2 S, and elemental sulfur, to form Na 2 SO 3 and SO x .…”

“…This set of governing reactions allows direct oxidation of Hg o to HgCl and HgCl to HgCl 2 by the following four chlorine species with different reaction rates: Cl, Cl 2 , HCl, and HOCl [4]. This system of equations has been shown to effectively predict mercury speciation for specific homogeneous systems [2].…”