Date of Award

1-1-2009

Document Type

Campus Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Joseph R. Flora

Abstract

Heterogeneous reactions of elemental Hg adsorption and oxidation in a simulated flue gas system were studied. Density functional theory using B3LYP paramerization were used to calculate binding energies, thermodynamic parameters, and kinetic constants. Binding energy calculations show that HCl, SO2, or NO2 adsorption is favorable on a carbon surface. H or Cl atom attachment on the carbon surface does not significantly enhance elemental Hg adsorption. SO2 attachment on the carbon surface does not enhance elemental Hg adsorption. NO2 attachment improves elemental Hg adsorption on the zigzag carbon edge, but doesn't have a significant effect on the armchair edge. Because Hg is likely to be oxidized by Cl species, subsequent calculations focused primarily on reactions of Hg with HCl.

Thermochemistry calculations were performed to obtain Gibb's free energies and equilibrium constants for Hg oxidation and reaction with HCl. HCl initially reacts with the carbon surface to produce Cl species, which acts as the primary oxidizing agent in the system. A reaction mechanism was proposed based on the thermodynamic study. Kinetic constants for heterogeneous reactions of Hg in the presence of HCl and carbon were estimated. The transition state was estimated from potential energy surface scans, and the canonical transition state theory was used to obtain to obtain estimates of the kinetic constants. Adsorption reactions on carbon black and some of the oxidation reactions in the system are barrierless. HCl reactions on carbon surfaces have barriers. Intermediate surface compounds were found on the reaction path of HCl reactions on the carbon surfaces. The intermediate Cl-H-Carm is proposed to be the major oxidant, which initiates the Hg oxidation reactions in the system.

CSTR and column reactor process models were developed to simulate the Hg oxidation and reaction with HCl in the presence of carbon black. The process models were to predict general trends for the effluent elemental and oxidized Hg levels. The kinetic model predictions were closer to the experimental results compared to the equilibrium model, indicating that the reaction system is kinetically controlled. Armchair carbon was found to be more active than zigzag carbon, and Cl-H-Carm was the major oxidant. The first step of Hg oxidation by Cl-H-Carm initiated the overall reaction. Sensitivity analyses showed that sufficient Cl-H-Carm oxidant was generated and Hg oxidation reaction on armchair carbon surface was the rate limiting reaction for elemental Hg removal. The estimated rates of HgCl2 adsorption on both armchair and zigzag carbon surface and the elemental Hg oxidation need to be increased to fit the experimental results better.

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