The Study On CO₂ Gasification Of Dem-coal Char Catalyzed By K⁺-modified Perovskite

Coal is the main fossil energy in China.Coal gasification as a clean utilization technology can effectively improve the utilization rate of coal carbon and reduce pollution,which makes it become an inevitable choice for achieving green development.In addition,a large number of carbon dioxide emissions have a significant impact on global climate change,and the existing coal gasification technology is not conducive to the enrichment and utilization of carbon dioxide.Therefore,some scholars put forward a theory of CO₂capture from the source of fuel conversion to achieve the effective utilization of coal carbon and CO₂,and how to make the key step of coal char gasification reaction more efficient has been widely concerned by researchers.In this thesis,a new idea of using perovskite-type oxides ABO₃ with controllable structure and high activity as catalysts for CO₂ gasification of coal char is proposed.In order to further improve the catalytic activity of the catalysts,K⁺doping at A-site of perovskite-type oxides is carried out to varying degrees.A series of K⁺modified perovskite oxides La_1-xK_xBO₃?B=Fe,Co,Mn,Ni,x=0-0.9?were prepared by sol-gel method.Catalytic and cyclic gasification experiments were carried out in a thermogravimetric reactor and a tube furnace reactor.XRD,XPS,SEM,EDS,SEM and laser particle size analyzer were used to analyze the effect of K⁺doping and gasification on the crystal structure,surface morphology and surface element composition of perovskite.The results show that the doping amount of K⁺has a significant effect on the structure of different B-site elemental catalysts.When the molar ratio of K⁺at A-site is less than 0.7,the catalyst with Fe at B site shows a stable perovskite crystal structure,which means that K⁺enters the perovskite lattice and occupies the A site of perovskite oxide together with the transition metal oxide La.However,the catalysts with Co,Mn or Ni at B site with obvious non-perovskite impurities.The catalytic activity of perovskite-type oxides increases with the increase of K⁺amounts.When x increases from 0 to 0.7,the breakthrough curve temperature of the initial reaction of catalytic gasification of Fe-based perovskite-type oxides decreases from 825?to 650?.The kinetic calculation results show that the activation energy of gasification reaction decreases significantly with the increase of K⁺amounts.Cyclic gasification of modified Fe-based perovskite oxides was carried out.The results show that the cyclic properties of modified perovskite oxides are gradually improved with the increase of K⁺amounts at A-site.Among them,the catalytic rate of La_0.3K_0.7FeO₃ decreased slightly after several cyclic reactions,but the time required to achieve 50%carbon conversion in the tenth gasification reaction was still only 31%of that of undoped LaFeO₃ perovskite,which means it still showed good catalytic performance.Moreover,when x reaches 0.7,the further increase of K⁺ions at A-site has little effect on the catalytic performance.The characterization test results of modified perovskite La_0.3K_0.7FeO₃ show that K⁺doping at A-site can effectively increase the number of oxygen vacancies and the content of Fe⁴⁺,which makes the catalyst have more active sites and stronger oxidation performance,and free K⁺ions on the surface of the reaction can participate in the gasification reaction,thus further improving the gasification reaction rate.But the formation of oxide impurities such as Fe₃O₄ in circulating gasification makes the surface of catalyst agglomerate and particle size increase gradually,the effective contact area between catalyst and coal char decreases,and the catalytic performance was gradually weakened during circulating experiments.However,the crystal structure of perovskite La_0.3K_0.7FeO₃ is stable and still exhibited perovskite-type structure after ten cycles,which helps it maintain good catalytic performance during cyclic experiments.Thus,K⁺-modified perovskite La_0.3K_0.7FeO₃ is a kind of equipment-friendly,efficient and recyclable catalyst and can be used as a promising catalyst for coal char CO₂ gasification.