Studies Of S/SO₂ Modified Activated Carbon On Flue Gas Mercury Removal And Regeneration Characteristics

Mercury features their characteristics of volatility,accumulation in organism and persistence in harm,which seriously threatens the human being health and ecological environment and attracts widespread concern from the world.Coal combustion is the most significant source of anthropogenic mercury emissions.As the world’s largest coal producer and consumer,China faces more serious mercury emission and control problems.At present,the commonly used technology of mercury removal is activated carbon injection（ACI）into flue gas.It has high efficiency but large consumption of adsorbent,high operating cost and not recycling of the adsorbent that may result in potential secondary pollution.Therefore,it is crucial to develop the cost effective and renewable mercury removal adsorbents for achieving mercury emission reduction and realizing the regeneration of adsorbents of the ACI technology in coal-fired power plant.Accordingly,in this paper,the activated carbon is selected as the adsorbent carrier,and S/SO₂sulfur-loading modification and thermal regeneration are used to realize the preparation and recycling of renewable mercury removal adsorbents.Combining experiments and theories,macroscopic and microscopic research aspect,this paper carried out deep research and discussion on scientific issues such as sulfur-loading modification,mercury removal characteristics,structure-effect relationship between mercury adsorption and sulfur morphology,characteristics and mechanism of adsorption and desorption,and deactivation and regeneration.SO₂ was used as the modifier to prepare the sulfur-loaded activated carbon renewable mercury based on the carbothermal reaction.The experiments on mercury removal and regeneration characteristics were carried out on a fixed bed.Characterized by pore structure and XPS the mechanisms of mercury adsorption and oxidation and morphology of adsorbent before and after the thermal regeneration cycle were explored.The results showed that with the increase of the temperature of adsorbent’preparation,the specific surface area decreased,the content of active sulfur,active oxygen,oxygen-containing functional groups and sulfur-containing functional groups generated by the carbothermal reduction increased,which promoted the mercury adsorption efficiency.Especially the newly formed sulfide group can greatly improve the chemical adsorption of Hg⁰on the surface of the adsorbent.The mercury saturated adsorbent was regenerated by SO₂loading and the results showed that the average mercury removal efficiency of the adsorbents after three cycles of regeneration was 89.71%,85.48%and 79.92%,respectively.It indicated that after the recycle tests the pore structure collapsed,the specific surface area lessened and the thermal regeneration performance declined.Elemental sulfur was used as the modifier to prepare the sulfur-loaded activated carbon renewable mercury adsorbent.The mercury removal and regeneration performance were experimented on a fixed bed.The structure-effect relationship between sulfur loading and mercury removal behavior and the characteristics and mechanism of deactivation and regeneration of adsorbent were studied by BET,EDS,FTIR and thermogravimetric analysis（TGA）.The results showed that Hg⁰penetration rate was lower than 8.58%at 150℃.Elemental sulfur was fully loaded on surface of the activated carbon.It was found that there were interactive active sites between S and Hg,and a stable C-Hg S structure was formed through electron rearrangement and transfer,which can improve the chemical adsorption.With progress of the adsorption,the inactivation phenomenon occurred for the sulfur-loaded activated carbon.It deduced that the decrease of non-oxidizing sulfur and the decrease of oxidized sulfur was an important reason for the inactivation.Through fixed-bed experiments,the effects of oxidized sulfur and non-oxidized sulfur on mercury removal performance were further explored.It verified that the mercury removal ability of elemental sulfur and sodium sulfide,and thiosulfate is bigger than that of sodium sulfite and sodium sulfate.The recirculated mercury removal efficiency of the S regenerated adsorbents was 80.22%,81.75%,79.43%and 72.96%,respectively.It was found that the main reasons of decay in regeneration performance originated form variation of the sulfur-loaded activated carbon are due to changes in the pore structure,surface sulfur content and sulfur speciation.It was the migration and conversion of the non-oxidized sulfur to oxidized sulfur on surface of the adsorbent that led to collapse of the inner-channels and decline of the mercury removal capacity.Based on the density functional theory,the mechanism of mercury adsorption of the S/SO₂modified activated carbon and impacts of various sulfur forms and active sites on mercury adsorption sites were deeply studied.A cluster model of activated carbon surface was established.The bonding characteristics,adsorption energy,electrostatic potential and molecular orbital of Hg⁰under different substrate structures of activated carbon were obtained.The results showed that one to two C-S bonds may be formed when a single sulfur atom is supported on the surface of the carbon cluster,suggesting that there is both chemisorption and physisorption adsorption in this process.The adsorption energy of SAC-Arm-1 configuration is the largest and is-65.47 k J/mol.The adsorption bonds of SO₂modified activated carbon are mainly C-O-S and C-S-C.After the carbon cluster model adsorbs SO₂molecules,S in SO₂has a strong positive potential,and Hg tends to lose electrons under the action of potential difference to form Hg O,which makes it easier to be adsorbed on the carbon cluster surface.The molecular orbitals calculations were conducted on the mercury adsorption of the two cluster models based on SO₂and elemental sulfur modified activated carbon.The results demonstrated that the exchanging orbitals obviously appeared around the adsorbed Hg atoms in the LUMO of SOAC-Arm-1,and the HOMO-LUMO energy gap is small,only 0.01713 e V,while the free electrons on the molecule are prone to orbital transitions that impel the adsorption reaction to be occured.Under the SOAC-Arm-3 conformation,C-Hg bond length is the shortest,and the adsorption energy is high as-70.42 k J/mol,indicating the chemical bonding ability of the two is stronger.The mechanism of mercury adsorption by chain isomers and cyclic isomers of different sulfur elements supported on surface of activated carbon was investigated.The results revealed that chain S₇sulfur molecules were loaded on surface of activated carbon and formed new mercury removal active sites and chemisorption of mercury occurred.