Mechanism Study On Mercury Removal Of Modified Biochar Sorbent

As a kind of toxic and harmful substances that are difficult to eliminate,mercury and its compunds can not only transport and circulate long distances in the ecosystem but also cause persistent pollution to the environment.They also have irreversible biological neurotoxicity and are potentially harmful to human health.Thus they are attracting increasing attention globally.Coal combustion has been one of the largest anthropogenic atmospheric sources of mercury.China is the largest producer and consumer of coal in the world,as well as one of the first-round signatories of the Minamata Convention on Mercury.Therefore,China is facing a great challenge in the field of mercury pollution control.In terms of removal efficiency and equipment upgrading,adsorbent in-duct injection technology remains the most competitive overall the mercury removal technologies from coal-fired flue gas.The most commonly used adsorbent is activated carbon,but its operation cost is high.The development of novel economic and efficient mercury removal adsorbent is still one of the cutting-edge issues in this field.In this study,renewable biomass resources were prepared to adsorbent with high mercury removal performance through pyrolysis,activation,and halogen modification.Following the methodology of“experiment to theory,macrocosm to microcosm”,the evolutionary of char structure during pyrolysis and the transformation mechanism of mercury adsorption on the surface of biochar were thoroughly studied.Quantitative evaluation of the performance of biochar adsorbent in the actual coal-fired flue gas was carried out.With advanced material characterization techniques and computational quantum chemistry,the mechanism of the halogen-modification improvement was revealed and the model of mercury adsorption on the surface of carbon adsorbent was proposed,providing important basic data and industrial application reference for the development of biochar adsorbent and mercury control technology.Rice husk,coconut shell and almond shell were selected as the research objects.TGA was carried out to learn their pyrolysis and charring characteristics,and distributed activation energy model of biomass pyrolysis was established by genetic algorithm.The joined use of elemental analysis,FTIR,Raman spectra,TEM,nitrogen adsorption as well as SEM were applied for more detailed physicochemical properties of biochar,such as surface functional groups,crystallite growth and pore distribution.To reveal the close correlation between physicochemical properties and mercury adsorption,the influence of pyrolysis temperature on mercury removal properties of biochar was investigated on a fixed-bed column reactor.For better understanding,hemicellulose,cellulose and lignin were chosen as three different referential single component model compounds,then correspondence of physicochemical properties and pyrolysis behavior between these single component model compounds and natural received biomass materials was established.The decreased content of surface oxygen-containing functional groups with the increasing of pyrolysis temperature indicates that its promotion on mercury adsorption might have been overestimated.At high pyrolysis temperatures,more unsaturated edge carbon atom could be obtained during the formation of partial-ordered microcrystalline graphite,which would act as the active adsorption site during mercury removal.For another,single/multiple-hole defects introduced by deoxidation reaction under high temperature may also enhanced mercury adsorption on solid surface.Microporous structures with high adsorption potential played a leading role in mercury adsorption on biochar.Optimal pyrolysis temperature was identified as 600 ? while the prepared biochar reached its maximum specific surface area and micropore volume respectively.As channel pores,the mesoporous and macropores enlarged with temperature increasing,thereby reducing mass transfer resistance of impregnation and mercury adsorption process.Lignin had a higher contribution to the yield of biochar,while hemicellulose was beneficial to the formation of microporous structure with smaller pore diameter on the surface during biomass pyrolysis.The multi-peak distributed activation energy model could more accurately describe and distinguish the multiple reaction mechanisms of biomass pyrolysis,such as the generation of volatile products from the pyrolysis of macromolecule and charring through aromatization and condensation.Due to the limitation of mercury removal performance of raw biochar,modification with ammonium halides and activation with H₃PO₄/CO₂ were put forward.Systematic experimental studies on the mercury removal characteristics of modified biochar were carried out in a fixed-bed device,which then compared with various commercial activated carbons.The influence mechanism of physical pore structure and surface element distribution on the mercury removal performance of biochar and activated carbon without and with modification were investigated,combined with characterization approaches like N₂ adsorption/desorption,SEM and EDX.Various forms of mercury in adsorption process,i.e.adsorbed mercury Hg_ads,evolved gaseous elementary mercury Hg⁰?g?and oxidized mercury Hg²⁺?g?,were quantitative researched by on-line mercury analysis equipment and OHM.Temperature programmed decomposition desorption was also conducted to verify the adsorption configuration and stability of mercury on biochar adsorbent.With the analysis of XPS,mercury species transformation model was established finally.Mercury removal performance of biochar was significantly improved after impregnated by NH₄Cl/NH₄Br solution with low concentration.Its mercury adsorption efficiency maintained up to 80⁹0%,on par with commercial activated carbon.Activation by H₃PO₄ and CO₂ mainly improved the channel porous structure to facilitate the loading of modified components in deep position of the particles,thereby further improving the adsorption efficiency of mercury.Physisorption played a leading role for untreated sorbent,and was significantly inhibited by shortage of super fine micropores.The removal of mercury by modified samples was mainly chemical adsorption.Up to 80%of inlet mercury was adsorbed by sorbent,while escaped Hg⁰?g?and Hg²⁺?g?accounted for only 10%respectively under certain temperature and atmosphere conditions.Stable HgBr adsorption configuration could be formed from HgO and OH-Hg-Br intermediates in the presence of O₂.The escaped Hg²⁺?g?might be produced from disproportionation reaction of HgBr.Lower adsorption temperature and the absence of O₂ made more mercury adsorption and less Hg²⁺?g?escape accordingly.Excess O₂ not only brought competitive adsorption but also consumed Br₂ molecular,then resulting in adsorption capacity deterioration.In order to further verify the mercury removal performance of modified biochar in the condition of actual coal-burning flue gas,a small circulating fluidized bed coal combustion device was designed and built,and the experiments on mercury removal by biochar injection in flue gas was carried out.The mechanism of sampling,analyzing and evaluating different forms of mercury in flue gas was established and improved based on OHM.The operation parameters of adsorbent injection for mercury removal were optimized,and the synergistic removal performance of modified adsorbent for SO₂,NO and hazardous trace elements were investigated.NH₄Br modified rice husk char achieved80%mercury removal efficiency,similar to that of activated carbon with the same modification method.Mercury removal efficiency could be promoted as the retention time extended.Both modified rice husk char and activated carbon had almost the same SO₂ removal efficiency?about35%?,while NO removal efficiency of the former was less than 40%of the latter,reflecting different mechanisms of SO₂ and NO captured by adsorbent.The distribution of hazardous trace elements in ash phase and their enrichment characteristics on the surface of fly ash and adsorbent were mainly affected by its occurrence form,thermal stability and other factors.The content of hazardous trace elements enriched on the surface of particle phase was increased by 20%³60%after adsorbent injection.The enrichment factor of semi-volatile trace elements was generally higher than that of non-volatile trace elements.Based on density functional theory,the geometric structure optimization and energy calculation of carbon cluster molecular models and adsorption system were carried out by using computational quantum chemistry method.The adsorption mechanism of mercury on carbon-based adsorbent was systematically investigated from the perspective of electronic structure.The difference of electron spin structure with various spin multiplicities was compared by molecular plane spin population analysis.The characteristics of shell structure,chemical bond and lone pair electron are presented visually by electron localization function.The weak interaction was quantitatively analyzed by electrostatic field analysis and van der Waals surface penetration distance.The covalent interaction between mercury and zigzag carbon edge,as well as the weak interaction between mercury and the armchair carbon edge were clarified combined with molecular orbital composition analysis,bond order analysis and atomic charge analysis.The multi-atoms adsorption path of mercury on the carbon cluster was constructed for the first time.Generation of?hole had a positive effect on subsequent mercury adsorption.As a kind of electron-withdrawing group,halogen realized mercury adsorption promotion by changing the electron distribution of its ortho-position carbon atom and improving its mercury adsorption activity.The remarkable improvement of calculated adsorption energy and the experimental phenomenon of modified carbon-based adsorbent to improve the performance of mercury removal were mutually confirmed.Mercury oxidation and adsorption mechanism of carbon-based adsorbent was established and improved by introducing carbocation and chlorine radical as intermediate products.Based on the characterization of microcrystalline structure,defective carbon cluster model and hetero-atom modification model were constructed reasonably,and its mechanism on mercury adsorption was also explored.