Study On The Reaction Mechanism Of Trace Element Pb In The Coal Combustion Process

Emission of poisonous trace elements from coal combustion, which are serious pollutants considered after SO2, NOX and CO2, has attracted worldwide attention. Control of trace elements and their compounds has been an emerging and frontier area in the research fields of pollutants emission and control from coal combustion. As yet, study on trace elements is concentrated on such areas as enrichment, distribution and transformation principle, as well as sampling devices and control techniques. The study about reaction mechanisms of trace elements with coal combustion products and other matters, which is helpful to conceive better control technique with lower cost and higher efficiency, is short.This paper particularly studied the trace element Pb in the coal combustion from different aspects and with three different methods. The main contents are as follows:Calculation models of chemical thermal dynamical equilibrium are used to predict the chemical speciation of Pb in the coal combustion flue gas. The software used is F*A*C*T, the temperature range is from 400K to 2000K, the press is 0.1MPa. Calculation models are based on the different flue setting, and different Cl concentration. Possible reactions in the different models are predicted.The ab initio calculations of quantum chemisty are used to studied the seven reaction mechanisms which are related to Pb chlorides and oxide. The geometry optimization of reactants, transition states, intermediates and products are made at MP2/SDD level and the isoelectronic density graphs of some molecules are plotted. Then the reaction potential barriers are calculated and the zero point energy is corrected at the same level. Finally, the reaction heat, entropy, active energy and absolute rate constants are calculated and compared with reference data. Based on the perious study, the detail kinetic model of Pb/O/H/Cl system is constructed, and calculation is made by the CHEMKIN software package. Results of kinetic calculation are compared to that of chemical thermal dynamical calculation, proving the reliability of kinetic model. The effect of Cl concentration to Pb chloride is analyzed. Sensitivities of PbCl4 and PbO to reactions are calculated and analyzed.