Study On Desulfurization And Regeneration Performance Of Samarium Doped Cerium Oxide Composite Adsorbent At High Temperature

During the process of preparing synthesis gas by hot gas reforming in the integrated gasification combined cycle power generation system?IGCC?,the sulfur element in the coal is converted into hydrogen sulfide?H₂S?during gasification/reformation process,unlike sulfur oxides from the traditional thermal power plants.The H₂S existed in the exhaust gas is highly toxic and corrosive.It will cause great harm to the human body if the H₂S-emission discharge into the air without purification treatment,and it is also an influencing factor for kinds of environmental problems such as smoke plume and acid rain.Removal of H₂S in tail gas,therebefore,becomes hot research topics.The operating temperature of the conventional desulfurization process is much lower than the coal gasification temperature,which leads to additional energy loss due to the hot gas cooling and reheating process.The industrial desulfurization adsorbent has some drawbacks such as low sulfur capacity,poor thermal stability and difficult regeneration.Therefore,it is particularly important to develop an adsorbent with good thermal stability and efficient regeneration performance,which is suitable for high-temperature gas desulfurization.In this paper,the composite rare earth oxide?samarium-doped cerium?was the research object.Firstly,a model,as a theoretical basis for testing experiment optimization,was constructed and calculated by density functional theory.And then,a series of SDC adsorbents were prepared to remove the H₂S in hot gas.The study investigated the effect of the different factors?such as doping-amount,reducing gas,H₂S concentration and so on?on the desulfurization property of the adsorbents.What's more,the study further discussed the effect of different temperature,gas atmosphere,desulfurization/regeneration on the regenerability of the SDC sorbents.Based on DFT+U theory,the study investigated the electronic structure,charge transfer,energy band distribution of Ce O₂ and SDC using the optimized unit cell model,and the adsorption process of the H₂S molecules at different positions of the surface of Ce O₂?111?and SDC?111?were also studied.The research found that the energy band structure of SDC and Ce O₂ is very close,but the density of states of SDC is obviously enhanced near the Fermi energy,and its Fermi energy enters the valence band,forming a large number of holes,which helps to improve the electronic transfer in the adsorption process.The doping of Sm significantly reduces the adsorption energy?-0.21317 e V?of S in H₂S on the Ce top of the SDC?111?surface than the adsorption energy?-0.17478 e V?of Ce on the Ce O₂?111?surface,which is more conducive to H₂S adsorption.And after doping with Sm,the adsorption energy of S in H₂S on the top of Sm is-0.14377 e V,which can also form adsorption,indicating that SDC has better desulfurization performance than Ce O₂.Especially,binary rare earth oxide has a mutual promotion between each other,which plays a positive role in the regeneration ability of the adsorbent.The results of the study revealed that SDC will interact strongly with H₂S,showing a significant electron transfer phenomenon.This suggests that a strong chemical adsorption occurs during the entire desulfurization process.Theoretical calculations proved that SDC adsorbent has a good adsorption performance,which lays the foundation for precise testing and optimization of conditions in the laboratory.SDC adsorbent was prepared by oxalic acid co-precipitation method and its high temperature desulfurization performance was studied.Doping amounts of active components in adsorbent plays a key role in desulfurization performance.The desulfurization capacity of the SDC adsorbent increased first and then decreased with the increase of the Sm-doping amount.When the doping amount of Sm was 20%,the composite rare earth oxide adsorbent showed the highest H₂S breakthrough sulfur capacity,reaching12.1 g S/100 g adsorbent.The study found that the SDC adsorbent has the smallest grain size and the largest surface area when the doping amount of Sm reaches up to 20%.The SDC adsorbent has excellent desulfurization performance due to the synergy between Ce O₂ and Sm₂O₃.The breakthrough sulfur capacity of the adsorbent is the largest at 800?,indicating that 800?is the optimal desulfurization temperature of the SDC adsorbent.It is suitable for high-temperature gas desulfurization.This suggests that the adsorbent has good thermal stability at high temperature.H₂ has a negative effect on the desulfurization performance of the SDC adsorbent.The H₂ significantly shorten the breakthrough time and reduce the breakthrough sulfur capacity.In contrast,CO has no obvious effect on the desulfurization performance of the SDC adsorbent.In addition,adding steam,increasing the reaction space velocity and increasing the concentration of H₂S will reduce the desulfurization performance of the SDC adsorbent.During the desulfurization process,tetravalent cerium is reduced to trivalent cerium by hydrogen sulfide and hydrogen,and the lattice oxygen is significantly reduced after desulfurization.Sulfur of different valence exists on the surface of SDC adsorbent after desulfurization,and Ce₂O₂S is the main S-containing species of SDC adsorbent after desulfurization.Doping an appropriate amount of Sm in SDC can enhance the adsorption activity of the adsorbent for reducing gas?H₂,H₂S?and hence improving desulfurization performance.The regeneration path and mechanism of the adsorbent were determined by investigation of the desulfurization/regeneration cycle of the SDC adsorbent.The study found that SDC adsorbent has excellent regeneration performance and shows good cycle stability.The used SDC adsorbent can be efficiently regenerated in O₂ atmosphere.The breakthrough sulfur capacity of the regenerated adsorbent is the largest after using 800?for regeneration,indicating that the optimal regeneration temperature of the SDC adsorbent is 800?.The adsorbents have obvious regeneration effect at the different O₂ concentrations.The desulfurization breakthrough time of the second-adsorption is the longest when the O₂concentration is 5%.The breakthrough times of the adsorbent are similar after six consecutive desulfurization/regeneration cycles,which is close to that of the fresh adsorbent.This suggests that the SDC sorbent has stable desulfurization and good industrial application potential.In addition,the key to the regeneration mechanism of the adsorbent is the regeneration path of Ce₂O₂S because Ce₂O₂S is the main sulfur-containing product after desulfurization of the adsorbent.There are two paths for Ce₂O₂S regeneration in O₂/N₂mixed gas at high temperature:In an air atmosphere,Ce₂O₂S is first oxidized to Ce₂O₂SO₄,and then decomposed into Ce O₂ and SO₂.However,Ce₂O₂S undergoes an oxidation reaction to directly generate Ce O₂ and elemental sulfur in a 2%O₂/N₂ atmosphere.Therefore,the precise control of the oxygen content in the regeneration process can obtain the elemental sulfur,which provides a new idea for the regeneration of the composite rare earth oxide adsorbent.