Study On Characteristics Of Regenerated Activated Carbon Modified By SO₂ For Mercury Adsorption

The implementation of the Minamata Convention indicates that the international community has made comprehensive restrictions on the emission of mercury,and the mercury pollution control technology has made great research achievements since1995.Among research and develpment of mercury removal technologies,the flue gas activatied carbon injection is known as the hot spot both at home and abroad.The pore structure of activated carbon is well developed with strong adsorption capacity and has the characters of fast adsorption kinetics and relatively easy to be regenerated.Modified by chemical method with penetration of sulfur,chlorine,iodine,bromine or other elements or even loading precious metals,the activated carbon can be improved with surface pore structure and surface chemical properties changed to enhance the directional chemisorption ability.Based on the carbothermal reduction of SO₂reaction,the activated carbons were activated at high temperature by low concentration SO₂ to prepare to sulfur-loaded activated carbon mercuric adsorbent,which regeneration cheracteristics was also investigated.With ordinary commercial activated carbon as raw material and low concentration SO₂ as active modifier,high efficiency activated carbon adsorbent was prepared on a tube furnace experimental device.N₂ adsorption/desorption,elemental analysis and scanning electron microscopy were combined to investigate the influence of modified conditions to adsorbents’pore structure,element content and surface morphology,and the mechanism of C-SO₂ reaction was explored for further.The results show that the micropores of activated carbon are filled in non-oxidizing sulfur after reaction,although the pore volume of adsorbent decreases,they still show the characteristics of microporous adsorbent.The change of the modified temperature has a complex effect on the adsorbents’pore structure,prolonging the activation time can effectively promote the increase of the specific surface area and the pore volume of the activated carbon adsorbent.The activation has the effects of both physical activation and chemical modification,leading the content of S and O in the adsorbent increased obviously.Scanning electron microscopy also showed that the surface of the activated carbon adsorbent was coarse adhesion after sulfur impregnation,which indicated that the modified activation had a certain damage to the micropors of the activated carbon adsorbent.The mercury removal performance of activated carbon adsorbents modified by SO₂ with different preparation conditions was investigated on a fixed bed mercury adsorption experimental device,and the effects of factors such as adsorption temperature,initial mercury concentration,O₂ and SO₂ on the mercury removal properties were investigated,and the desorption experiments of partially adsorbed samples were carried out on the TPD experimental platform.The characteristics and mechanism of mercury removal from activated carbon adsorbent were analyzed by combining with the previous characterization results.The results showed that the original activated carbon has large specific surface area,abundant functional groups and mercury removal ability.After the high temperature modification by SO₂,the physical and chemical properties of the activated carbon adsorbent were further improved,showing excellent chemical adsorption on mercury.The influence of modification temperature,modification time and SO₂ concentration on the mercury removal performance are complex.Prolonging the modification time will affect the main reaction and produce some C-SO₂ reaction byproducts.Under the high SO₂ concentration,SO₂ will occupy the limited active site of activated carbon and make the physical adsorption of activated carbon saturated.120-150℃is a suitable adsorption temperature range and fitted to the reaction temperature of the flue gas injection demercuration in the coal-fired power plant.The increase of Hg⁰ concentration in the flue gas will reduce the efficiency of the adsorbent,while promote the removal rate and improve the utilization efficiency of the adsorbent as a result.The addition of 6%O₂ and low concentration of SO₂ in simulated flue gas can effectively promote the removal of Hg⁰.Thermal desorption regeneration and sulfur reloading regeneration were carried out on the TPD test facility and the tube furnace respectively,in ordert to explore the mercury removal characteristics of the regenerated adsorbents.The effects of sulfur reloading regeneration on physicochemical properties of the adsorbents and the cycling number in Hg⁰ removal were systematically investigated by means of characterization techniques.The results show that the adsorption efficiency is greatly reduced after the thermal regeneration,because both the pore structure and most of the sulfurous functional group on the surface of the adsorbent was destroyed at the high temperature.The adsorption efficiency after 2 hours was 63.65%after thermal regeneration at constant lower temperature.It indicated that the constant temperature regeneration could retain the partial chemical adsorption capacity and avoid the erosion and collapse of the pore structure of adsorbent at high temperature.However the thermal desorption was always accompanied with the loss of sulfur atoms in the surface of adsorbents.The loading of sulfur can make up for the sulfur content lost during the pyrolysis,and the sulfur reloading regeneration is good to realize sulfur atom supplement.The average Hg⁰ removal efficiency of the adsorbent is 82.62%after five times of regeneration,showing good regeneration property.