Study On The Preparation & Modification And Pelletization Of Sodium-based CO₂ Solid Sorbents

The global warming,deduced by massive greenhouse gases?mainly CO₂?emissions,is getting worse and worse.And it has become an important issue to reduce CO₂ emissions worldwide in recent years.It is imperative to develop CO₂ capture technologies to reduce the CO₂ emissions from coal-fired power plants which is the main stationary CO₂ emission sources.Among various CO₂ capture technologies,using dry regenerable alkali-metal-based solid sorbents,due to the metrics of high sorbent utilization,low energy consumption and no secondary pollution,has been considered to be a particularly promising option for removing CO₂ from flue gas.And using sodium-based sorbent for CO₂ capture will be more commercially viable because of its relatively cheap price,high theoretical CO₂ capture capacity and lower regeneration temperature.However,the CO₂ sorption capacity of sodium-based sorbent in previous investigations is not competitive enough,and research on preparation&modification is insufficient.Meanwhile,it is rare to explore the molding process which is a key step into industrial application.To address the issues above,the preparation and modification mechanism of powder sodium-based sorbents were studied with a fixed-bed reactor.Furthermore,the molding process of sodium-based sorbents was explored by extrusion-spheronization method.The specific research contents are as follows:The effects of calcination temperature and impregnation methods on powder Na₂CO₃/?-Al₂O₃ sorbents were investigated,and proper preparation conditions were selected.Results show that the ligand environment of surface partial hydroxyls on support?-Al₂O₃ changes due to the existence of Na₂CO₃,making their removal temperature advance to 300-400°C.When calcination temperature is between 200 and 400°C,owing to the removal of crystal water and surface hydroxyls,the sorbent microstructures are improved with the increase of calcination temperature,and so is the CO₂ sorption performance.When the temperature further increases to 500 and 600°C,the sorbents sinters gradually with the decrease of pore channels;the CO₂sorption performance degrades.Therefore,400°C is the appropriate calcination temperature.The Na₂CO₃/?-Al₂O₃ sorbents,prepared by ultrasonic and conventional impregnation,have little difference in microstructures and CO₂ sorption performances;so is the sorbents prepared by incipient wetness and wet impregnation.After comprehensive consideration,conventional impregnation is selected;and the sorbent with Na₂CO₃ loading below 33 wt%is prepared by incipient wetness impregnation,while the sorbent with higher loading is prepared by wet impregnation.According to the selected preparation process,the effects of carbonation temperature and Na₂CO₃ loading amount on the CO₂ sorption performance of Na₂CO₃/?-Al₂O₃ sorbent were studied,and the sodium-based sorbent with high CO₂ sorption capacity was obtained.The CO₂sorption capacity decreases with increasing carbonation temperature from 50 to 80°C.When the target loading amount of Na₂CO₃ is 20-50 wt%,the sorption capacity increases with increasing loading and then deceases.The CO₂ sorption capacity is maintained at 2.2-2.4mmol·g^-1 with the loading between 33 and 40 wt%.The CO₂ adsorption capacity is the highest with 35 wt%loading,reaching approximately 2.4 mmol·g^-1.With the increase of loading amount,the microstructures deteriorate and the carbonation conversion decreases.When the loading amount is above 40 wt%,the carbonation conversion of Na₂CO₃ is reduced evidently.The effect mechanism of MgO on CO₂ sorption of sodium-based sorbent was verified and the influence of MgO doping amount on the CO₂ sorption performance of sorbent with high Na₂CO₃ loading?30 wt%?was obtained.It is found that after MgO doping,some Mg²⁺will enter the lattice defect sites of?-Al₂O₃ to form Mg-Al composite oxide.Combined with TG-FTIR,it is further confirmed that,besides NaHCO₃,Mg₆Al₂CO₃?OH?₁₆·4H₂O is generated after the carbonation reaction,which plays a crucial role in facilitating the conversion of Na₂CO₃ to NaHCO₃.When doped with 1 wt%MgO,the CO₂ sorption performance is improved;if the doping amount continues to increase,the sorption uptake is lower than that of undoped sorbent.Also,the sorption behaviors are well described with Avrami's fractional-order kinetic model.The pelletization of sodium-based sorbents was explored via extrusion-spheronization method,and the pelletization technic route was preliminarily established.Through experimental groping,the operating parameters of extrusion and spheronization were obtained,and sorbent pellets with good sphericity were prepared.Due to the densification of molding,compared with the directly crushed pellets,the mechanical strength of the pellets prepared by extrusion-spheronization?without calcination?is enhanced,but the CO₂ sorption performance is decreased.Due to the effect of pore enlargement resulting from sodium carboxymethyl cellulose?CMC-Na?pyrolysis during calcination,the macropores of molding pellets added with CMC-Na gradually increase with increasing calcination temperature,and the gas diffusion resistance decreases,leading to the increase of Na₂CO₃ amount for CO₂ sorption.With the calcination temperature increasing from 200 to 500°C,the CO₂ sorption performance is gradually enhanced and compressive strength declines slowly.Due to fine particles sintering,the CO₂ sorption performance of pellets calcinated at 600°C remains almost unchanged compared with pellets calcinated at 500°C,but the compressive strength decreases greatly.There was no significant difference in the mechanical strength and sorption performance of the molding pellets with different calcination time.The appropriate calcination temperature and time are 500°C and 2 h,respectively.When adding CMC-Na,with the increase of addition amount?0-2 wt%?,the CO₂ sorption capacity of the molding pellets increases and the mechanical strength degrades slightly;the sorption capacity reaches 1.98 mmol·g^-1 with CMC-Na addition amount of 2 wt%.The promotion extent of microcrystalline cellulose on the CO₂adsorption performance of the molding pellets is worse than that of CMC-Na,and the pellets are poor in mechanical strength after calcination.Finally,CMC-Na is selected as additive with the amount of 2 wt%,and the corresponding pellets exhibits good CO₂ sorption-desorption cycle characteristics.