Mechanisms Of Mercury Removal By Sulfur-Rich Adsorbent Derived From High-Sulfur Petroleum Coke

Mercury pollution is increasingly recognized as a severe and worldwide public health concern due to its high volatility,long persistence and bioaccumulation in the environment.Mercury pollution from coal-fired power plants is one of the major anthropogenic sources globally.To cope with the increasingly stringent mercury emissions,many efforts have been focused on developing low-cost adsorbent for high-efficiency removal of mercury,which has become an important research area for promoting the activated carbon injection technology and controlling mercury emission from coal-fired power plants.In this study,high-sulfur petroleum coke was chosen as the precursor to prepare sulfur-rich mercury removal adsorbents.The main academic questions addressed in this dissertation are:the transformation of sulfur forms during pyrolysis,KOH/SO₂activation,S⁰ impregnation and influence of sulfur incorporation on mercury removal;the impact of different sulfur species on mercury removal as well as kinetics,equilibrium and thermodynamics of mercury adsorption on different sulfur forms;the interaction mechanism between mercury and different sulfur-containing functional groups.Firstly,transformation behaviors of sulfur-containing groups during pyrolysis,KOH/SO₂activation,S⁰ impregnation and influence of sulfur form distributions on mercury removal performance were studied.It was revealed that the inherent sulfur in high-sulfur petroleum coke was mostly in the form of thiophenes,which is very thermally stable and unreactive.Therefore,the content of thiophenes changed little over temperature during the pyrolysis process.The study of KOH-activated coke suggested that KOH was effective in converting thiophenes to water-soluble K₂S and K₂SO₄ which were readily removed by water washing.As thiophene sulfur had little effect on mercury adsorption,additional functional groups should be loaded to improve the mercury removal performance.The mercury adsorptive capacity of elemental sulfur impreganated adsorbent was highly temperature-dependent.It was found that the thermal escape of S⁰ would become significant at high temperatures,and sulfur impregnation promoted the formation of a small amount of sulfide and sulfoxide apart from S⁰.Moreover,sulfur was added into the carbon matrix mainly in the form of organic sulfide via the reaction of SO₂ with petroleum coke,as well as a small amount of sulfane,sulfoxide,sulfone and sulfate.The mercury removal performance of sulfur-rich adsorbent derived from high-sulfur petroleum coke was co-affected by pore structure,sulfur content and sulfur form distributions.Secondly,the modification mechanism of different SO₂ activation approaches and the impact of different sulfur species on mercury removal were investigated.Three distinct SO₂ activated petroleum cokes PC-S,PC-NS and PC-SC were prepared via different sulfurization protocols.The three typical sulfur forms generated during different sulfurization processes were sulfide,sulfane and elemental sulfur respectively.Mercury adsorption experiments suggested that chemisorption of Hg⁰ onto different sulfur species was the key adsorption mechanism of Hg⁰removal by SO₂activated cokes.It was found that the increase of adsorption temperature extremely shorten the equilibrium time and reduced the amount of Hg⁰ adsorbed in PC-S,PC-NS and PC-SC.But sorbents with different certain sulfur groups appeared to be affected by temperature changes with varying degrees.The order of sensitivity to adsorption temperature in Hg⁰ removal performance was S⁰>sulfide>sulfane.Furthermore,the ranking of the favorability of Hg⁰ adsorption at 120?was as follows:sulfane>sulfide>S⁰.According to TPD experimental results,it can be deduced that the primary adsorbed mercury species on sulfide,sulfane and S⁰ were Hg-SR,Hg S(red)and Hg S(black)respectively.Then,the kinetics,equilibrium and thermodynamics of gas-phase mercury adsorption on different SO₂activated petroleum cokes were studied.Although mercury adsorption is limited by external mass transfer and intraparticle diffusion,chemisorption at active sites is the rate-controlling step of mercury adsorption on SO₂ activated petroleum cokes.Results indicated that the pseudo-first-order model and Langmuir model could accurately describe the adsorption process of mercury adsorption on SO₂ activated petroleum cokes.It was indicated that the ranking of active Hg⁰ chemisorption sites according to concentration was PC-NS<PC-S<PC-SC.In general,mercury was favorably adsorbed on the surface of SO₂ activated cokes,and PC-NS showed the highest adsorption ability with Hg⁰.The thermodynamics results suggested that adsorption of mercury on SO₂ activated cokes was a process of spontaneous,endothermic and entropy increment.The adsorption activation energy of PC-S,PC-NS and PC-SC were 68.61,54.18 and 96.49 k J/mol respectively.Finally,a series of sulfur-containing models such as thiophene,sulfide,sulfane and elemental sulfur has been established using density functional theory,the basic structure considered here is a zigzag benzene rings which consists of three cells.Then,the adsorption structures of mercury on different sulfur-containing species were optimized and the adsorption energies for each structure were calculated.Based on the calculated results,various methods were used to interpret the interaction between the Hg atom with different sulfur-containing species,such as ADCH atomic charge,bond order,surface electrostatic potential and molecular orbital composition analysis.The reaction mechanism and Hg S formation process of mercury adsorption on different sulfur-containing species were clarified according to the minimum energy principles.The results showed that the adsorption of the Hg atom on the thiophene sulfur molecule was dominated by electrostatic interaction.As the result of the weak adsorption force,thiophene sulfur performed a poor mercury adsorption capacity.The mercury adsorption activity of sulfur atoms in cyclic polysulfides was lower than that of chain sulfides.Electrons from Hg atom were transferred into the terminal sulfur atom via chemisorption of mercury.The nature of Hg atom adsorption on the terminal sulfur atom in polysulfides is Coulombic force between the positively charged Hg atom and the negatively charged terminal sulfur atom.It was revealed that the order of their adsorption activity is sulfane sulfur>elemental sulfur>unsaturated carbon atoms>disulfides among different sulfur-containing functional groups.For elemental sulfur molecules,the mercury adsorption activity gradually decreased with the increase of molecular size.