| Coal as a primary energy occupies a leading position in China’s energy structure, pollution caused by coal combustion is the largest source of pollution damage to China’s ecological environment In addition to sulfur dioxide, nitrogen oxide and carbon dioxide, the emissions of trace elements and their compounds have great damage to human beings and the environment, which has caused more and more attention of domestic and foreign scholars.The emissions and control of trace elements in coal combustion flue gas are closely related to its morphology. Based on chemical kinetics, a kinetic model of Hg was proposed, which includes 7 kinds of elements, C, H, O, N, Hg, S, Cl, and 172 elementary reactions,and verified with existing experimental results. The influence of residence time, reaction temperature and the composition of flue gas on the oxidation of mercury were studied by using the model, and the sensitivity of the model was analyzed. This provides a theoretical basis for the effective control of mercury emissions.By dynamic calculation, HCl and Cl2 are the main substances that are beneficial to the oxidation of Hg, and Cl2 is more advantageous to the oxidation of mercury than HCl. In addition to Cl2 and HCl, NO, NO2, SO2 three gas components in the coal gas can inhibit the oxidation of mercury. SO2 gas has a slight inhibition on the oxidation of mercury, and the inhibition of NO2 and NO is stronger. However, the synergistic effect between NO2 and SO2 can further inhibit the oxidation of Hg. The sensitivity analysis of the kinetic model showed that the influence of Cl2, HCl, NO, NO2 and SO2 on the oxidation of mercury in the flue gas decreased. HOCl is the most important intermediate substance that affects the formation of Hg Cl2, and the oxidation ability of Cl2 to Hg is only after HOCl. Hg was first oxidized by HOCl and Cl to generate Hg Cl, and then the Hg Cl was oxidized to Hg Cl2 by the action of Cl2. The main oxidation pathway of Hg was Hgâ†'Hg Cl â†'Hg Cl2.Because of the synergistic effect between NO2 and SO2 in coal fired flue gas, it could further inhibit the oxidation of mercury. In this paper, we use the method of sulfur fixation of calcium based on the coal combustion, and study the effect of the addition of calcium carbonate on the oxidation of Hg by chemical thermodynamic equilibrium. The speciation of mercury and other components of flue gas are analyzed under different calcium carbonate dosing ratio.The thermodynamic calculations by Chemistry HSC and the gas phase equilibrium analysis by Chemkin showed that the Ca elements affect the contents of Cl2, HCl, SO3 and SO2 in the flue gas to influence the oxidation of mercury, and S and Ca are not directly related to the reaction of mercury. At the high temperature stage, Ca O reacts with SO2/SO3 first to form stable Ca SO4, and the excess Ca O in the low temperature stage will react with HCl to produce Ca Cl2, the consumption of Cl element in the low temperature stage is the main factor of Hg oxidation Inhibition. Excessive calcium carbonate addition not only consumes the S element but also the Cl element, it will inhibit the oxidation of mercury. Need to pay attention to the dosing ratio of calcium carbonate in actual combustion process, which is conductive to the oxidation of mercury. |
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