| Arsenic,a heavy metal pollutant emitted from coal combustion,has attracted wide attention due to its carcinogenicity,teratogenicity and bioaccumulation.Arsenic in flue gas mainly exists in the form of As2O3(g).The use of metal oxide oxidation-adsorption arsenic removal is a promising arsenic emission reduction technology.However,the adsorbents represented by CaO,Fe2O3 and y-Al2O3 can effectively oxidize and adsorb As2O3(g)only at high temperatures of 500～700℃,and the activity at medium and low temperatures is not ideal.In recent years,the rapid development of renewable energy has made it normal for coal-fired power units to continue to operate at low load or deep peak shaving.When the unit runs at low load,the flue gas temperature decreases obviously.Therefore,the development of arsenic pollution control technology suitable for medium and low temperature(200～400℃)has become an important problem to be solved urgently.On the other hand,coal-fired flue gas contains a large amount of SO2,which is toxic to the catalyst and seriously affects the arsenic adsorption characteristics on the catalyst surface.Therefore,it is of great scientific significance and application value to develop adsorbents with arsenic removal and sulfur poisoning resistance at medium and low temperature.In this paper,seven kinds of composite adsorbents such as FeCeLaO were prepared.The effects of temperature and flue gas components on the removal of arsenic by adsorbents were studied.The role and synergistic mechanism of Fe,Ce and La in the process of arsenic removal from flue gas at medium and low temperature(200～400℃)were revealed.The effects of temperature,SO2 concentration and flue gas composition on the anti-sulfur poisoning ability of the adsorbent were further investigated,and the anti-sulfur poisoning mechanism of the adsorbent was revealed.The main work is as follows:(1)Seven adsorbents including pure Fe2O3,pure CeO2,pure La2O2CO3,FeCeO,FeLaO,CeLaO and FeCeLaO were prepared by co-precipitation method.The samples were characterized by XRD,H2-TPR and Rama techniques.The results suggest that Fe and La can be completely or partially incorporated into CeO2 lattice to form solid solution,the synergistic effect of Fe,Ce,and La is conducive to the formation of more low-temperature active sites and oxygen vacancy defects.(2)The oxidation and enhanced adsorption characteristics of the adsorbents on As2O3(g)in flue gas at different temperatures were investigated.The results show that both Fe2O3 and La2O2CO3 are effective active components for As2O3 adsorption,CeO2 is a good structural assistant.FeCeLaO can completely oxidize As(III)to As(V)at 200～400℃,and the arsenic removal efficiency is up to 89%.This is attributed to the synergistic effect of Fe,Ce and La,which makes the composite oxide have stronger low temperature oxidation activity and adsorption capacity.(3)The effects of NO and CO2 in flue gas on arsenic removal by FeCeLaO were explored.The results showed that 0-1000ppm NO and 0%-9%CO2 promoted the arsenic removal of FeCeLaO adsorbent.1000ppm-1500ppm NO and 9%-12%CO2 inhibit the arsenic removal by FeCeLaO adsorbent.(4)The effect of SO2 in flue gas on the active sites of adsorbents was analyzed.The fresh adsorbent was pre-sulfated and characterized by FTIR,SO2-TPD,TG and XPS techniques.The results of infrared analysis showed that SO42-or HSO4-were formed on the surface of all adsorbents.TPD results show that Fe and La elements in FeLaO have undergone a large degree of sulfation.In CeLaO and FeCeLaO,almost only cerium sulfate desorption peaks were observed,it shows that SO2 preferentially reacts with Ce and protects the active sites of Fe and La.TG analysis showed that the higher the temperature,the greater the degree of sulfation of the adsorbent.XPS results showed that the content of chemisorbed oxygen on the surface of Fe2O3,CeLaO and FeCeLaO increased after presulfurization,while La2O2CO3 and FeLaO decreased.The proportion of Ce3+ increased significantly after presulfurization of CeLaO and FeCeLaO.(5)The effects of SO2 concentration,temperature and flue gas composition on the sulfur poisoning resistance of arsenic removal adsorbents were studied.The results show that SO2 in the concentration range of 0-1500ppm can promote the removal of arsenic by Fe2O3,CeLaO and FeCeLaO.This is attributed to the catalytic oxidation of gaseous arsenic by active sites contailing SO42-or HSO4-.SO2 in the concentration range of 1500-2000ppm is an inhibitory effect.This is attributed to the competitive adsorption of SO2 and AS2O3.The presence of SO2 inhibited the removal of arsenic by La2O2CO3 and FeLaO,which was attributed to the consumption of chemical active oxygen species on the surface of the adsorbent during the SO2 sulfation process.The lanthanum sulfate formed by the reaction of SO2 and the active component La does not have arsenic adsorption activity,resulting in deactivation of the adsorbent.In the range of 200-500℃,the higher the temperature,the more obvious the promoting effect of SO2 on the arsenic removal of Fe2O3,CeLaO and FeCeLaO,and the stronger the sulfur poisoning resistance of the adsorbent.However,its inhibition on the dearsenication of La2O2CO3 and FeLaO is also more significant.The presence of NO in flue gas has little effect on the sulfur resistance of FeCeLaO.When the concentration of O2 is lower than 15%,O2 can promote the arsenic removal performance of FeCeLaO after presulfurization,and the higher the concentration,the stronger the promotion effect.This is attributed to the fact that O2 can supplement the active oxygen sites on the surface of the adsorbent.The excessive O2 concentration(15%-21%)has an inhibitory effect. |
