Synthesis Of Ordered Mesoporous Silica And Alumina From Coal Fly Ash And Utilization Of Residues For Carbon Capture

Since the large production amount of coal fly ash（CFA）and the low rate of multipurpose utilization in China,it is urgently imperative to study the high-value recycling technology of CFA.Meanwhile,the abatement of CO2 emission is also the most emergency problem faced by the coal-fired power plants.Considering the high-value and total utilization,this thesis innovatively proposed an original idea for the recycling of CFA,including（i）the extraction of silicon and aluminum,（ii）the synthesis of mesoporous nano materials,and（iii）the on-site CO2 capture using the Si-Ca slag.For the first time,the simulated flue gas is introduced to synthesize the mesoporous SiO2 and mesoporousγ-Al₂O₃ from the extraction liquid of CFA,using a novel CO₂-assistant precipitation technology.Thus,this strategy provides a facile and promising approach to the scalable production of mesoporous nano materials from CFA,as well as the on-site CO2 capture and utilization.The main conclusions of this thesis are as follows:Considering the high contents of both silicon and aluminum for Chinese CFA,we creatively developed an alkali-dissolution method and a lime-sinter method,for the simultaneous recycling of both silicon and aluminum.The key parameters of operational conditions were systematically investigated and optimized,and the migration and the crystalline transformation rules of silicon and aluminum were clarified through various characterization;thus,the extraction efficiency of silicon and aluminum could reach up to 46.62%and 87.42%,respectively.Besides,the Si-Ca slag could be further used for on-site CO2 capture in the coal-fired power plants,due to the high CaO content.The simulated purified flue gas,for the first time,was introduced into the extraction liquid（Na2SiO3）to obtain the mesoporous SiO2.The effect of silicon concentration and template agent were investigated to improve the hydroxy content and the mesoporous structure of the mesoporous SiO₂.The as-synthesized mesoporous SiO₂（“SiO₂-0.16”）has high purity（99.35%）,low hydroxy content(1.28 OH·nm^-2),high specific surface area(1,157 m²·g^-1),and a highly ordered hexagonal mesostructure（2.9 nm）.This CO2-assistant precipitation technology could achieve the shorter reaction time of 3 h and the production of by-product（Na2CO3）,as well as obtain the product with high purity and excellent mesoporous structure properties.The simulated purified flue gas was also used for the synthesis of mesoporousγ-Al2O3 from the extraction liquid（NaAlO2）.The effect of precipitation temperature and calcination temperature were discussed to clarify the formation mechanism of the mesoporous structure and the crystallineγ-Al₂O₃ phase.The as-synthesized mesoporousγ-Al₂O₃（“Al₂O₃-65/550”）has high surface area(230.3 m²·g^-1),crystallineγ-Al₂O₃ phase and ordered hexagonal mesostructure（3.8 nm）.This CO2-assistant precipitation technology could synthesize the ordered hexagonal mesostructure without any template agent and obtain the crystallineγ-Al₂O₃ phase under comparatively low temperature（550°C）,as well as achieve the shorter reaction time of 1 h and the production of by-product（Na2CO3）.To investigate cyclic CO2 sorption property of Si-Ca slag,the calcite content was firstly determined using the reference intensity ratio method,resulting in the theoretical CO2 uptake of 0.136 g-CO2·g-sorbent^-1.The reaction kinetic model,for the CO2 sorption of Si-Ca slag,was proposed to clarify the effect of CO2 concentration,sorption temperature,and trace acid gas;the first-order reaction kinetic equation and the Ginst-ling equation were used to fit the kinetically controlled stage and the diffusion-controlled stage of CO2 sorption process,respectively.The cyclic CO2 uptakes of Si-Ca slag were also investigated using a thermal gravimetric analyzer,the Si-Ca slag possessed a final CO2 uptake of 0.0858 g-CO2·g-sorbent^-1 after 30 cycles,and the average deactivation rate over the 30 cycles was merely 0.72%per cycle.The Si-Ca slag possessed both superior cyclic stability and high cyclic CO2 uptake,and thus could be well used for on-site CO2capure from the coal-fired power plants.