Gaseous Mercury Absorption From Simulated Flue Gas

Mercury and its compounds are of the important contaminations in the environment. Coal-combustion is the most main source of mercury in atmosphere, and study on mercury treatment becomes one of the focuses in environmental field.In this paper, contamination, harm, physical and chemical characters and properties in coal-combustion flue gases of mercury are discussed. The emphasis is on the control methods of gaseous mercury in coal-combustion flue gases, which include absorption, wet scrubbing and impulse plasma advanced technology. Mercury treatment using adsorption materials such as activated carbon from coal-fired flue gases is promising in combustion field. Effect of flue gas constituents on mercury adsorption was also studied. But trace mercury from coal-combusted flue gases removal with mass transfer and reaction kinetics in chemical engineering was hardly reported.Two types of absorbents, namely potassium permanganate and potassium persulfate, with known overall superior performance was selected in a bubble reactor. Effect of initial potassium permanganate concentration, pH, and mercury concentration in the inlet of reactor, reaction temperature and gas flow on mercury oxidative absorption in potassium permanganate solution was investigated. Based on mass transfer parameters of reactor measured by chemical method, mass transfer and reactor kinetics was also studied. The primary conclusions were as follows.1. The efficiency is greater in acidic and alkaline than neutral solutions. Furthermore, H₂SO₄ contributes to Hg⁰ removal, absorbing on MnO₂. Nevertheless, free radical, OH·, is produced to oxidize Hg⁰ in alkaline solutions. Initial mercury concentrations are relatively not affective, and higher temperature inhabits Hg° removal.2. Mass transfer reaction kinetics was also discussed with surface renewable theory. Reaction rate constants are 8.94×10⁴ and 2.44×10⁵m³/(mol·s), corresponding to the reaction temperatures on 298 and 328 K when pH = 0, respectivley. The second order rate constant decreases to 1.95×10⁴ m³/(mol·s) with 298 K when pH = 7. Activation energy, which indicates rate constant is sensitive to temperature if activation energy is very large, of two-order reaction is 27.22 kJ/mol, and the frequency factor is 5.25×10¹² m /(mol·s). Moreover, Ha>3, which prove reaction is afast reaction.3. The strong oxidative ability of potassium permanganate is the mechanism of Hg° removal. The reduction product of potassium permanganate (Mn2+) probably can self-catalyze oxidation of Hg° in acidic solutions. But the product of potassium permanganate is MnO2 that can absorb Hg2+ in solutions, and free radical, OH-, takes part in oxidation of Hg° in concentrated alkaline solutions.Influence of some factors, such as initial concentration of potassium persulfate, catalyst, mercury, reaction temperature, pH, tertiary butanol (TBA) and sodium thiosulfate, on mercury oxidized in potassium persulfate was examined. Furthermore, kinetics and reaction mechanism were also discussed. The following conclusions were listed.1. Hg° removal efficiency is enhanced when potassium persulfate and catalysts concentrations, and decreases with TBA. Sodium thiosulfate is positive for removal efficiency. Lower reaction temperature and higher mercury concentration are relatively favorable. Furthermore, the removal efficiency of Hg° is much favorable in neutral than both strong acidic or alkaline solution. But, 0.3 mmol/L Ag+ dramatically eliminates the effect of pH.2. The reaction between Hg° and potassium persulfate obeys a peso-first order with the non-catalyzed rate constant (k) 3.53175><10"5 s"1 at 328.15 K. The ratio of the catalyzed reaction rate constant to the overall rate constant is increased with increasing catalysts concentrations, but decreased with high temperature. The ratio for silver ion catalysis is larger than that for copper ions at the same temperature and catalysts concentrations. The results show that the enthalpy of activation is just less than activation energy and the former lower evidently in the presence of catalysts. Furthermore, the enthalpy of activation decreases sharply with silver ion as catalyst, whereas reduces a little. The frequency factor, activation energy, the enthalpy of activation and the entropy of activation of uncatalyzed reaction is 5.93><1017 s"1, 139.62 kJ/mol, 137.01 kJ/mol and 86.62 J/(mol-K), respectively. The frequency factor, activation energy, the enthalpy of activation and the entropy of activation of silver ion catalyzed reaction is 5.37xl010 L/(mol-s), 72.92 kJ/mol, 70.32 kJ/mol and 9.22 J/(mol-K), respectively. The frequency factor, activation energy, the enthalpy of activation and the entropy of activation of copper ion catalyzed reaction is 1.18*1014 L/(mols), 94.89 kJ/mol, 92.29 kJ/mol and 73.20 J/(mol-K), respectively.3. The free radicals reaction mechanism catalyzed by metal ions was alsodiscussed. Cu2+ is probably an effective chelate catalyst in K^Os-N^SaOa redox system. Hg° oxidation by K2S2O8 can be achieved by two processes involve in direct oxidation by K2S2O8 and indirect reaction by free radicals. K2S2O8 decomposed into SO4 ? which oxidize metal ions to reactive valent species and H2O to OH-. Both of them oxidize quickly Hg° to Hg^+ which can be oxidized to the final product, Hg2+, in solutions.