Catalytic Adsorption And Oxidation Study On Hg⁰ Removal From Syngas By Fe(NO₃)₃ Laden Biochar

Coal gasification is one of the most efficient and inevitable trend of clean coal utilization technologies for modern power generation purposes.Nevertheless,in the process of coal gasification,different toxic pollutants(H₂S,HCl,HCN,Hg etc)discharged into the atmosphere.Among these toxic pollutants,Mercury(Hg)caused severe harms to environment and the human health.Mercury is highly volatile and persistent pollutant which is capable of spreading and sustaining in the atmosphere over a year.Because of its extreme toxicity and bioaccumulation it can damage the nervous system or may lead to death in acute condition.Approximately 65%of anthropogenic mercury emission to the atmosphere come from coal combustion/heating power plants.Thus,the Hg emission control from coal combustion/heating power plants is inevitable.In the coal utilization process Mercury emits in its three gaseous states(i)Particulate mercury(Hg^p),(ii)Oxidized mercury(Hg²⁺),and elemental mercury(Hg⁰).The oxidized mercury and particulate mercury can be captured in conventional dust and gas purification devices.However,the elemental mercury cannot be captured by the existing flue gas purification equipment due to its high volatility,insolubility(25⁰C 60?g/m³),and chemical inertness.Thus,generates the threatful human health and ecological mercury hazards.Therefore,the removal of elemental mercury is the key to removing mercury from the coal syngas.This paper does the following work:(1)Various highly competitive and regenerable magnetic tea biochars(MTBCs)were developed by one-step pyrolysis of Fe(NO₃)₃ laden waste tea leaves at 500?and characterized by analytical techniques such as BET,XRD,VSM,SEM,TGA,and XPS.The Hg⁰ removal performance via adsorption/oxidation from syngas was investigated under a wide reaction temperature range(50??400?).Results showed that MTBC with an optimal loading of 0.46 mol/L Fe(NO₃)₃,attained an approximately 86%of Hg⁰removal efficiency(?^T)at 200?under syngas(20%CO,20%H₂,10ppm HCl,and 400ppm H₂S).After ten repeated adsorption/regeneration cycles,the?^T value was still above 71%.(2)The Hg⁰ removal from hot gas is challenging since the reducing atmosphere is unfavorable for oxidizing Hg⁰.Besides,during the thermal process the Fe₃O₄/?-Fe₂O₃particles instability over the sorbent surface may affect the catalytic activity of the sorbent.Therefore,a highly resistive and competent magnetic tea biochar was prepared to capture the Hg⁰ from syngas.In this work,a facile yet innovative method was developed to synthesize an efficient,thermally stable and highly regenerable magnetic tea-biochar to capture the Hg⁰ from syngas.The magnetic sorbent was synthesized by a simple sonochemically assisted one-step pyrolysis method to capture the Hg⁰from syngas.The sonochemically synthesized sorbent(TBC-U_{Fe 0.46}X)with optimal loading of 0.46 mol L^-1Fe(NO₃)₃and 0.3M citric acid solution,achieved more prodigious S_BET with developed pore structures,strong thermal stability,ultra-paramagnetic property and high dispersion of iron oxide particles.Adsorption/oxidation bench-test revealed that the Hg⁰removal efficiency of TBC-U_{Fe 0.46}X was much higher than earlier developed sorbents at 200?.Moreover,after the ten repeated adsorption/regeneration cycles?^T value was above 90%.(3)Coal gasification process yields strong reducing gases CO,H₂,H₂S,HCl,H₂O;as previously reported,many exhausted gases can be effectively adsorbed by sorbents.Thus,the effects of syngas elements were tested over TBC-U_{Fe 0.46}X,to investigate whether there is an adsorption competition between Hg⁰ and other gases,e.g.,H₂S,H₂,CO,HCl,and H₂O.Besides,the Hg⁰ removal mechanism and breakthrough curve analysis were also studied.Results revealed that TBC-U_{Fe 0.46}X demonstrated a high Hg⁰ capturing performance of?98%with an adsorption capacity of 106.81?g/m³ within 60 minutes at 200?C under complex syngas which is much larger than previously synthesized sorbents.Besides at 23.1%Hg⁰breakthrough,the TBC-U_{Fe 0.46}X achieved an average Hg⁰ adsorption capacity of 16.58mg/g within 24 h under syngas.Remarkably,the adsorption capacity of TBC-U_{Fe 0.46}X under complex syngas environment was still 97.68?g/m³ after ten repeated adsorption/regeneration cycles.