| Coal combustion is the main source of anthropogenic mercury emissions.Due to its toxicity and ecological effects,the control of mercury pollution in coal combustion flue gas has been paid more and more attention in the world.Mercury exists in three forms in the flue gas of coal combustion which respectively are granular mercury(HgP),oxidized mercury(Hg2+)and elemental mercury(Hg0).The former two can be removed through the existing pollutant control system of flue gas,but Hg0 is difficult to remove from the flue gas,due to its volatile and insolubility in water.In order to improve the removal efficiency of Hg0 from flue gas,the oxidation of Hg0 into oxidized mercury(Hg2+)which can be absorpted is considered to be a feasible method for the mercury emission control from coal combustion flue gas.Therefore,the research and development of new technologies which are efficient,economical and lower secondary pollution for mercury removal from flue gas by oxidation has become a research focus in the field of air pollution control.Research in this area has important theoretical significance and application value.Super valent transition metal compounds refer to the compounds which are the super oxidation states of some transition metals,the cation of super valent transition metals often are not stabilized in a independently shape,but some super valence metals can exist stably by means of appropriate multitooth ligand,such as diperiodatoargentate(?)(DPA),diperiodatocuprate(?)(DPC)and dyhydroxy diperiodatonickelate(?)(DPN).Super valent transition metal compounds generally have high redox potential,theoretical larger electrochemical capacity,and the compounds themselves and the discharged product are environmentally friendly.The super valent transition metal compounds are good oxidants under some conditions.Generally speaking,the oxidation reaction rate of complexes is slow,while some trace metal ions doping as the catalysts,such as Ru(?),Ir(?),Os(?),Cr(?),Ni(?),Cu(?),etc.,can reduce the activation energy of the reaction,accelerate the reaction and improve the reaction efficiency.In this study,super valent transition metals and transition metal ion catalyst were combined to form a catalytic oxidation system,which was applied to the oxidation and absorption of Hg0.The specific work is as follows:In this paper,a series of catalytic oxidation systems with high valence state transition metal compounds as oxidants were constructed,and the efficient conversion from Hg0 to Hg2+ in flue gas was achieved by gas-liquid phase catalytic oxidation.By means of characterization of oxidant,catalyst and removal product,combined with the macroscopic kinetics of catalytic oxidation on mercury removal,the reaction mechanism of removing Hg0 in flue gas by catalytic oxidation was put forward.(1)The experimental results of mercury removal from flue gas using DPA(?)-Ru(?)catalytic oxidation system showed that under the condition of adding trace Ru(?),the oxidation efficiency of Hg0 was significantly improved,and the optimal reaction conditions were obtained:DPA(?)concentration was 1.03 mmol/L,Ru(?)concentration was 2.0?mol/L,DPA solution pH value was 8.5,reaction temperature of 40?.The experimental results also showed that due to competitive oxidation,SO2 in flue gas,especially at a high concentration,has a greater impact on the mercury removal efficiency,NO in the flue gas has a smaller impact on the mercury removal efficiency,and the O2 in the flue gas has almost no impact on the mercury removal efficiency.Ultraviolet-visible spectrophotometry(Uv-Vis)was used to characterize the oxidant,catalyst solution,and a mercury breakthrough experiment was carried out.The experimental results confirmed that the catalyst action of Ru(?)was realized by forming the intermediate complex with Hg0 primarily,and then being oxidized by the active species of oxidant to obtain the product,while Ru(?)was recovered.Cold Vapour Atomic Fluorescence(CVAFS)was used to analyze the oxidation products,and the oxidation products were confirmed to be Hg2+.The material balance was calculated according to this measured results.According to the experimental results and related literature,the mechanism of catalytic oxidation for mercury removal as one-step double electron transfer was speculated.(2)The DPC(?)-Ni(?)catalytic oxidation combination system was selected by contrast test,and the optimal reaction conditions were systematically obtained in the Hgo removal experiment:DPC(?)concentration was 0.18mmol/L,Ni(?)concentration was 4×10-5mmol/L,the reaction temperature was 45?,and the solution pH was 8.5.Under optimal conditions,the removal efficiency of Hg0 reached 93.3%.At the same time,the related reaction mechanism of mercury removal from flue gas in DPC(?)-Ni(?)catalytic oxidation system was proposed for the first time.MPC(monoperiodatocuprate(?))which is the dissociated product of DPA,is considered to be the main oxidizing reactivity species.According to the literature and the present study,the existence form of catalyst Ni(?)was[Ni(H2O)6OH]+,which formed an intermediate complex with Hg0 in the catalytic oxidation reaction,and then was oxidized by the active oxidizing species MPC,and the oxidation reaction was consider to be a one-step single electron reaction.(3)The DPN(?)-Ni(?)catalytic oxidation system was selected to consummate the catalytic oxidation system.The mercury removal experiment of flue gas was conduced to investigate the influence of the factors on the mercury removal efficiency,and the optimal experimental conditions were obtained:DPN(?)concentration was 0.5mmol/L.Ni(?)concentration was 2×10-5 mmol/L,reaction temperature was 40?,solution pH was 9.According to relevant literature and experimental data in this study,the oxidant DPN(?)existed in the form of low protons in alkaline medium,that is,[Ni(OH)2(H2IO6)2]4-,the activation center of the reaction is[Ni(OH)2(HIO6)]2-,and the oxidation reaction between Ni(?)and Hg0 is a one-step double electron transfer.(4)The macroscopic reaction kinetics on mercury removal in flue gas with super valence metal compounds catalytic oxidation system was carried by experimental study and the relevant calculation.The experimental results showed that mercury removal with super valence metal compounds met first order kinetics reaction.The catalytic oxidation reaction rate constant by Ag(?)-Ru(?)was 0.0237min-1,the apparent activation energy was 25.32kJ/mol.While without catalyst,the reaction rate constant was 0.0154min-1,and apparent activation energy was 42.96kJ/mol,Ru(?)increased the reaction rate and decreased the activation energy.The catalytic oxidation reaction rate constants of Cu(?)-Ni(?)and Ni(IV)-Ni(?)systems were 0.0291min-1 and 0.0371mm-1,respectively.The apparent activation energy of those two systems was 28.26kJ/mol and 33.33kJ/mol,respectively.The oxidant concentration has little influence on the reaction rate,but the increase of the catalyst concentration can increase the reaction rate.However,the increasing effect gradually weakened with the increase of the concentration,hence the optimal concentration should be determined jointly according to the counter factor experiment in the early study.The pH value has a great influence on the reaction rate,and the reaction rate is very low under acidic conditions,that is,the catalytic oxidation system depended on the alkaline reaction environment.The high concentration of impurity gas,especially SO2,has a great inhibitory effect on the reaction rate. |
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