Study On Trace Elements Release And Reaction Mechanism During Coal Combustion

Trace elements emitted from coal-fired boiler would do great harm to human health, global agricultural and social sustainability, but the mechanism of release, emission and control is still not clear. Trace elements emitted from coal combustion have become an increasingly important environmental concern. In this work, we focused on describing the evaporation, reaction mechanism, on-line analysis and kinetic modeling of trace elements during coal combustion with computational and experimental methods.The quantum chemistry theories were introduced into the field of combustion. The geometry optimizations of molecular, heat of reactions, change of entropy and vibration frequencies were calculated by different levels of ab initio, DFT theory of quantum chemistry and typical ECP basis sets for combustion flue gas containing mercury system. The calculating results were compared with the NIST experimental results in order to validate the quantum mechanical method and basis set combination. The results show that the QCISD/Stevens combination is the most accurate, and than is the B3PW91/Stevens combination, B3LYP/Stuttgart combination. It improves the calculation results by appointing different basis set for metal atom and nonmetal atoms. The results provide a base for investigating kinetic mechanism of mercury interaction with combustion-generated flue gas by quantum chemistry.Theoretical exploration on mercury oxidation reaction mechanism in flue gas was conducted on the level of atoms and molecules. The geometry optimizations of reactant, transition state, intermediate and product were made at QCISD level by ab initio calculations of quantum chemistry. The basis set of Stevens was used for Hg, and the basis set of 6-311++G(3df,3pd) was used for nonmetal atoms (Cl, H, O and N). The property of stable minimums were validated by vibration frequencies analysis. The activation energies were calculated by thermal energy calibration (including zero point energy calibration). The reaction rate constants in the temperature scale of 293-2000K were calculated from transition state theory. The calculated parameters can provide new foundation for emission model of trace elements during coal combustion.The adsorptions of different species of mercury (Hg0,HgCl2 and HgCl) on the unburned carbon (UBC) surface were investigated by the density functional theory (DFT). The cluster models of zigzag, armchair and tip were set up to well representing the UBC surface. The unsaturated carbon atoms in the edge of the cluster model were used to simulate the active sites of the UBC surface, and other carbon atoms were closed by H atom. The present calculations show that UBC could substantially reducing gaseous mercury chloride (HgCl2 and HgCl), but have no apparent effect on Hg0, which is compatible with the available experimental results. Cl atom and surface functionality (lactone, carbonyl and semiquinone)increase the adsorption of mercury on UBC surface. The research method will provide the valuable information for the optimizing and selecting sorbent of the trace element in flue gas.An inductively coupled plasma atomic emission spectrometer (ICP-AES) was developed to continuously measure the heavy metal concentrations in order to track the metal release process. The system is devoted to the thermal treatment of metal-spiked mineral matrix, coal and municipal solid wastes in fluidized bed (850°C). This method was used to study the kinetics of heavy metal vaporization. The optimum values of the gas flows for the 1200 W power generator are 0.1 L/min for the sample gas and 0.2 L/min for Ar. During the thermal treatment of coal and municipal solid waste, the release process of Cd and Pb is short; Zn vaporizes lower than Cd and Pb. The formation of stable compounds such as ZnO·Al2O3 and CdO·Al2O3 could decrease the metals vaporization. In all cases, the experimental setup was successfully used to monitor the metal evaporation process during coal and solid waste thermal treatment. A study was carried out to investigate the kinetic law of toxic metal release during thermal treatment in a fluidized bed. Both direct and inverse models were developed in transient conditions. The direct model predicts the time course of the metal concentration in the gas from the vaporization rate profile, based on the Kunii and Levenspiel's 2-phase flow model for Geldart Group B particles. The inverse model was developed and validated to predict the metal's vaporization rate from its concentration in the outlet gas. A method to derive the kinetic law of heavy metal vaporization during fluidized bed thermal treatment of coal from the global model and the experimental measurements is derived and illustrated. A first order law was fitted for the mineral matrix and a second order law (simplified) was fitted for coal. This method can be applied to any matrix, whatever mineral matrix or organic matrix.