Reaction Mechanism Study Of Mercury Removal By Carbon Sorbents Based On Quantum Chemistry Calculation

Mercury emitted from coal-fired boiler will do great harm to human health and environmental, which has attracted much attention as a very important global pollutant. More and more solid adsorbents were used to remove mercury, of which the most widely used is carbon-based sorbents. Unburned carbon as the main mercury adsorbent in fly ash. Mercury is released by the heat treatment in the process of fly ash re-use that will cause environmental pollution. Mercury having different forms in the fly ash, so it has different thermal stability. Due to the factors of complexity of the system and low mercury content, so more complex experimental study and results unstable. Therefore we can study the reaction characteristics from the microscopic point of material. Due to the carbon-based adsorbents are expensive, more and more scholars focus on modification of activated carbon, of which the reaction path and the reaction mechanism has not been systematically studied.Investigation on the desorption mechanism of halogenated mercury from unburned carbon surface is significant. Quantum chemistry calculation software was used in the paper to study microscopic mechanism which mercury compounds desorption reaction on the unburned carbon surface. It was made at B3PW91 level by density functional calculations of quantum chemistry that theoretical exploration about reaction of mercury compounds desorption on the unburned carbon surface was conducted on the level of atoms and molecules. The geometry optimization and the frequency analysis was made to ensure reliability. The activation energies were calculated by thermal energy calibration(including zero point energy calibration). The reaction rate constants of the reaction in the temperature scale of 298-1500 K were calculated from transition state theory, the variation trend of which were analyzed with the changes in temperature.Thermodynamic parameters were calculated of reaction of the mercury compound desorption on unburned carbon surface. The Gibbs free energy and enthalpy values of the reaction in the temperature scale of 298-1500 K were calculated to analysis the spontaneous and absorption or exothermic of each reactions. The results showed that the reaction of the mercury compound desorption on unburned carbon surface is difficult to carry out, need to raise the temperature and increase environmental conditions.Strong oxidizing substances can change the adsorption of solid sorbent, this paper select Mn O2 to modified the activated carbon. Theoretical exploration of mercury heterogeneous adsorption reaction by activated carbon and MnO2 modified activated carbon was conducted at B3PW91 level by density functional theory calculations of quantum chemistry. The activation energies were calculated by thermal energy calibration(including zero point energy calibration). The reaction rate constants of the reaction in the temperature scale of 298-1500 K were calculated from transition state theory, the variation trend of which were analyzed with the changes in temperature. The results showed that modified active carbon can greatly improve efficiency of mercury removal, saving cost, which provide a theoretical basis for find a more affordable, effective mercury sorbent.