Mechanism Study On The Mercury Removal From Coal Gas By Titanium Dioxide Supported Cerium Manganese Oxides And Its Regeneration

Coal gasification,one of the utilization methods for coal,has the characteristics of cleanness,high efficiency,and strong adaptability to different coal types,and therefore,huge application potential.However,compared with the flue gas from direct coal combustion,the coal gas contains a higher concentration of elemental mercury,and it is difficult to oxidize and remove due to the high temperature and the reducing atmosphere.Mercury,with highly biologically toxic,poses a serious threat to the human body and the environment.Therefore,it is necessary to carry out economic and efficient researches on mercury removal from the coal gas.Compared to the mercury removal from coal-fired flue gas,the current research on mercury removal from coal gas needs to be further studied,and the catalytic oxidation mechanism of mercury in the reducing atmosphere still needs to be improved.Therefore,the development of economical and efficient renewable sorbents and the clarification of their reaction mechanisms are of great significance for the application of coal gasification technology and environmental protection.Based on analysis,in order to improve the catalytic oxidation ability of the adsorbent in a reducing atmosphere,the composite metal oxides CeMn/Ti adsorbent?using MnO₂ with strong oxidation capacity and CeO₂ with strong oxygen storage capacity as components,TiO₂ as carrier?was proposed in this study.A series of CeMn/Ti adsorbents were prepared by co-precipitation method,and their mercury removal performance and regeneration performance were investigated by a fixed bed reaction system.Combined with other characterization methods such as BET,XRD,SEM,XPS and TPR,the mechanism of mercury removal from the coal gas and the regeneration was explored.Firstly,the composite metal oxide CeMn/Ti adsorbents were prepared by loading CeO₂ and MnO₂ with different proportions on the nano-TiO₂ supportter by a co-precipitation method.The mercury removal experiments carried by the fixed-bed showed that the CeMn/Ti adsorbent could effectively remove elemental mercury from the coal gas.The best calcination temperature of the adsorbent was 500?,and the BET and SEM characterizations showed that when the best mass ratio of CeO₂/TiO₂ was 0.2and MnO₂/TiO₂ was 0.1,the physicochemical properties and surface morphology of the adsorbent were optimal.In the presence of H₂S,the mercury removal experiment at160?also verified that the Ce_0.2Mn_0.1Ti adsorbent had the best mercury removal performance,and the average Hg⁰ removal efficiency was 91.55%.The CeMn/Ti adsorbent had good temperature adaptability,and the average Hg⁰ efficiency can be maintained above 80%in the range of 80?160?.With the BET,XRD,and XPS characterization,the synergistic modification mechanism of cerium and manganese in the adsorbent was revealed:Ce doping increased the proportion of Mn⁴⁺in manganese oxides,and Mn doping was beneficial to improve the dispersion of cerium oxides on the adsorbent surface and increase the specific surface area.Then,the effect of gas components on mercury removal performance were analyzed.H₂S can greatly improve the mercury removal efficiency by generating active sulfur,while the temperature will inhibit the reaction.At the same time,part of H₂S generated stable metal sulfates and sulfites due to the strong oxidizing ability of Ce_0.2Mn_0.1Ti adsorbent.H₂ and CO consumed surface active oxygen and active sulfur and thus had obvious inhibitory effect on mercury removal.NH₃ could consumed active oxygen on the adsorbent surface and bind with H₂S,so it also inhibited the removal of mercury.HCl and H₂S were similar to the adsorption of mercury by generating active chlorine.When it coexists with H₂S,however,the promotion effect of H₂S on mercury removal was weakened due to the competitive adsorption relationship.O₂ was conducive to the adsorption and oxidation of elemental mercury.Finally,by the thermal desorption regeneration,the mercury desorption rules on Ce_0.2Mn_0.1Ti adsorbent and the regeneration performance were investigated.The thermal desorption process indicated that the mercury compounds on the adsorbent surface were mainly HgO and HgS,and they were completely desorbed and decomposed at 500?.The regeneration at 500?could maximize the recovery of the adsorption activity.The CeMn/Ti adsorbent does not require much oxygen content during the regeneration process,and 5%oxygen can restore most of oxidation ability.The decrease of mercury adsorption efficiency after CeMn/Ti adsorbent regeneration was mainly due to the occupied active sites and the formation of metal sulfates and sulfites with some active components.After multiple regeneration cycles,the mercury removal efficiency of the CeMn/Ti adsorbent gradually kept stable with the increase in regeneration cycles,and it can still reach 48.86% after 4 cycles,which exhibited good thermal regeneration performance.