Study On Elmental Mercury Oxiation And Modified Sorbent For Mercury Removal By Non-thermal Plasma

Mercury is one kind of trace heavy metal pollutant.Because of its stable chemical properties and strong volatility,which can be long-distance transported in the atmosphere,and gradually settled down away from the pollution source region,it damages the ecological environment and the human health,and becomes another major coal burning pollutant following the dust,SO₂ and NO_x.There are three forms of mercury in the coal combustion flue gas:element mercury,oxidized mercury and particle mercury.Hg⁰,which accounted for a large proportion in the flue gas,is difficult to be removed by air pollution control devices due to its strong volatility and insolubility,and leads to the total mercury removal efficiency lowered.The activated carbon injection?ACI?technology has been recognized as the most promising control technology for removing mercury from the coal-fired flue gas,but the removal cost is too high.The non-thermal plasma?NTP?is recognized as one of the most promising technologies for Hg⁰ removal,and it can realize the simultaneous removal for multi-pollutants.The present work is devoted to solving some of these scientific problems related to the mercury oxidation mechanism,the key reactions,and the main influencial factors on the Hg⁰ oxidation efficiency.In addition,the effects of non-thermal plasma modification on physic-chemical properties of the adsorbent surface were investigated,and the enhanced mechanism of mercury removal and desulfurization was also revealed.A new experimental system of the Hg⁰ oxidation by the non-thermal plasma was set up.The relationship between the discharge parameters of non-thermal plasma reactor with dielectric barrier discharge,as well as the mercury gas phase distribution and the influence of gas composition on the Hg⁰ oxidation was investigated.The results show that the energy density of the reaction system is positively related to the discharge voltage.The 03 concentration produced by the discharge process increases linearly with increase of the energy density,but can be completely decomposed when the gas temperature is higher than 300?.There is an optimal correlation value between the Hg⁰ oxidation efficiency and input energy?SED?,and the maximum Hg⁰ oxidation efficiency is about 81%when the input energy density was 240J/L.The total mass of mercury in the simulated flue gas after the non-thermal plasma reactor is balanced,and it mainly exists in the form of oxidized mercury.The increase of O₂ concentration in the simulated flue gas is beneficial for Hg⁰ oxidation.H2O?g?and CO₂ can promote Hg⁰ oxidation in the absence of O₂,while in the presence of O₂,they decrease the oxidation efficiency of Hg⁰.SO₂ and NO inhibit Hg⁰ oxidation.In addition,HCl can prompt Hg⁰ oxidation under the non-thermal treatment,and Hg⁰ in the simulated flue gas can be completely oxidized when the concentration of HCl was higher than 50 ppm.The kinetic model of Hg⁰ oxidation is established by the elemental reaction and the kinetic parameters.The variation of the free radical concentration,the main influence factors,the key reactions were studied,which was beneficial for revealing the Hg⁰ oxidation mechanism by the non-thermal plasma.The results show that the electrons migrate to the high energy region with the increase of the average electron energy,which facilitates the dissociation of O₂ molecules and the formation of free radicals in the flue gas.The concentration of O and Hg⁰ oxidation efficiency increases with the increase of the energetic electron number,and the activated free radical O is the key factor for Hg⁰ oxidation.The increase of O₂ concentration is beneficial for the Hg⁰ oxidation.The concentration of O quickly decreases in the presence of H2O,but the Hg⁰ oxidation efficiency increases with the increase of the H20 concentration.NO and SO₂ lead to the decrease of the O concentration and Hg⁰ oxidation efficiency.The Hg⁰ oxidation was intensified by CaCl₂ coupling non-thermal plasma.The main Hg⁰ oxidation products and their distribution were analyzed by the Temperature Programmed Desorption?TPD?technology,to reveal the Hg⁰ oxidation mechanism.The results show that the CaCl₂ can obviously promote Hg⁰ oxidation under the non-thermal plasma treatment.The Hg⁰ oxidation efficiency increases with the increase of the energy density,and Hg⁰ can be completely oxidized when the energy density was higher than 240 J/L.O₂ in the simulated flue gas has an important effect on the Hg⁰ oxidation,and the oxidation efficiency of Hg⁰ increases with the increase of the O₂ concentration.Both NO and SO₂ inhibit the oxidation of Hg⁰ and the effect of SO₂ is significantly higher than NO.However,compared with the conventional non-thermal plasma,the effects of NO and SO₂ on Hg⁰ oxidation are obviously declined with the CaCl₂ coupling non-thermal plasma treatment.The mercury oxidation products deposited on the CaCl₂ surface are HgCl₂,Hg2O and HgO,while there is only HgO in the gas phase.In addition,the concentration of the Cl element on the CaCl₂ surface is not obviously lost.Another new experimental apparatus of AC modification by the non-thermal plasma was established.The effects of non-thermal plasma ionization on the physic-chemical properties of the AC surface were investigated,and the mercury removal performance of modified AC was evaluated in a fixed bed.The results show that the specific surface area and the pore volume of AC are slightly decreased after the non-thermal plasma modification,while the content of acidic oxygen function groups on the AC surface is obviously increased.The mercury adsorption capacity of AC increases after modification,and it reaches the maximum when the modification power and time were 15 W and 20 min respectively.The adsorption of Hg⁰ on the modified AC surface is mainly controlled by the external mass transfer and the surface chemisorption.The equilibrium adsorption capacity of Hg⁰ on AC surface increases after the non-thermal plasma treatment,and the external diffusion and the surface chemisorption are enhanced.A nonel calcium-based sorbent was prepared and modified by the non-thermal plasma for mercury removal,featuring synergistic desulfurization and denitrification,which optimizes the optimum conditions for preparation and modification.The results show that the specific surface area of the calcium-based sorbent is higher than that of CaO.With the increase of CaO,the specific area of the calcium-based sorbent decreases while the Ca content on the sorbent surface increases.The content and types of acidic oxygen function groups on the calcium-based sorbent are obviously higher than those on AC,when the mass ratio of AC to CaO,the hydrothermal time and the.temperature were 1,1 hour and 70? respectively,the content of acidic oxygen function groups on the calcium-based sorbent surface reaches the maximum,and the removal performance of Hg⁰ and SO₂ is the optimal.After the non-thermal plasma modification,the particle size and the content of C on the calcium-based sorbent surface decreases while the content of O and the acidic oxygen function groups increase.When the modification power and time are 20 watt and 10 minutes,the acidic oxygen function groups reach the maximum and the calcium-based sorbent adsorption performance of Hg⁰ and SO₂ are the optimal.