The Oxygen Release Law Of The Typical Oxygen Carrier Lattice And The Optimization Of Its Chemical Chain Combustion Performance

In recent years,it is paid great attention that the increasing emission of carbon dioxide?CO₂?from the combustion of fossil fuel is the major contributor to the greenhouse effect and global warming.Conventional carbon capture technologies,such as Pre-combustion,Post-combustion and Oxy-fuel combustion technology,will bring high carbon capture costs and high energy consumption.Chemical looping combustion?CLC?can realize the integration of CO₂ capture in flue gas with nearly zero energy consumption during the energy production process,and has been regarded as the most potential"second generation"carbon capture technology.In the CLC system,oxygen carriers?OCs?play the role of oxygen and heat transfer,while the reactor is the place where various reactions take place.Thus,the physical and chemical characteristics of the OCs and the structure design of reactor are the key factors to affect the reaction performance and CO₂ capture efficiency of the CLC system.From the“proof of concept”of CLC technology to the demonstration plant with 3MW thermal power,most of the large-scale CLC pilot plants were based on the circulating fluidized bed reactor,and the screening of OCs has been done around fluidized bed reactor.It is found that fluidized bed reactor has the advantages of high gas-solid heat and mass transfer efficiency and has uniform temperature field distribution.However,there are still many problems such as high temperature sintering,agglomeration,carbon deposition,OC attrition,extra requirement for the dust removal equipment with high efficiency and unable to achieve high pressure operation.Fixed-bed reactor also has broad application prospects due to its unique advantages of pressure operation,no attrition of the OCs filled in the packed bed,no requirement for additional gas-solid separation device,simple structure and flexible operation.However,there are few studies on the large-scale CLC system based on fixed-bed reactor at present.In this thesis,a laboratory scale-up fixed-bed reactor CLC reactor was designed and constructed,and the usage of OCs reached kilogram level.In addition,the stationary OCs in the packed bed determine that the use of OCs is no longer limited by their mechanical strength.Thus,many OCs with good reactivity but poor mechanical strength can be used in the fixed bed reactor.Many oxygen carriers with good reactivity but poor strength can be used in the fixed bed reactor.Based on the previous researches,CuO/SiO₂,Fe₂O₃/Al₂O₃,Mn₃O₄/ZrO₂ and NiO/ZrO₂ OCs were selected for the chemical looping combustion and a series of studies were carried out on the activity distribution of these OCs in the fixed reactor bed.A series of CuO/SiO₂,Fe₂O₃/Al₂O₃,Mn₂O₃/ZrO₂ and NiO/ZrO₂ OCs were prepared by the mechanical ball milling method.The CLC reactivity was studied in a fixed-bed reactor to investigate the effects of preparation methods and particle size on the CLC performance.The results show that OCs prepared by mechanical ball milling has the inferior reaction activity and lower effective utilization of lattice oxygen.The powder OCs is prone to agglomeration,and the particle OCs with diameter of 2 mm and length of 4 mm has the best reaction performance.In addition,four OCs prepared by the three preparation methods showed similar reaction performance:NiO/ZrO₂ OC sticks had the highest CH₄ conversion and can achieve almost complete conversions of CH₄,but CO₂selectivity was low,especially at the end of the reaction stage when large amount of NiO has been consumed,partial oxidation would take place to produce CO and H₂,and metallic Ni would catalyze CH₄ cracking resulting in serious carbon deposition;CuO/SiO₂ OCs exhibited high CH₄ conversion?80%⁹0%?,excellent CO₂ selectivity?>96%?and high resistance to carbon deposition,and had the optimum comprehensive performance;Fe₂O₃/Al₂O₃ OCs had poor reactivity and reacted with CH₄ followed the process step by step,and complete combustion of CH₄ occurred when Fe₂O₃ is reduced to Fe₃O₄,and can obtain high CH₄ conversion and CO₂ selectivity.While the further reduction to FeO/Fe will be accompanied by partial oxidation and pyrolysis of CH₄ to produce a large amount of CO and H₂,accompanied by carbon deposition,thus reducing the CH₄ conversion and CO₂ selectivity;Mn₂O₃/ZrO₂ OCs showed excellent CO₂selectivity?>95%?and high resistance to carbon deposition,but has the lowest CH₄conversion.It is difficult to meet all the requirements for high reaction rate,high CH₄ conversion and CO₂ selectivity,and high resistance to carbon deposition by using these four OCs independently.The four OCs were considered to fill in the packed bed in special sequence to realize the complementary advantages among the OCs and improve the CLC reaction performance.The results showed that the average CH₄ conversion and average CO₂ selectivity can be effectively enhanced by composite filling operation,and also improve the resistance to carbon deposition of the whole reaction system and have better cyclability.Among these composite filling,the OCs arranged by CuNiFeMn and FeCuNiMn sequences showed the best performance.NiO/ZrO₂ OCs can be used as additives to improve the CH₄ conversion and CuO/SiO₂ OCs can be used as modifiers to improve the resistance to carbon deposition of the whole CLC system.Their position in the sequence is the key factor to affect the CLC reaction performance.Furthermore,the promotion mechanism on the CLC performance by the composite filling in the packed bed with different OCs in is analyzed from the micro-level.The results show that the migrating rate of lattice oxygen determines the final products.CH₄molecules are adsorbed on the OCs surface active sites by internal and external diffusion and activated into H and CHx.The lattice oxygen located on the surface and sub-surface layer oxidizes H and CHx to CO₂ and H₂O with high selective.The consumption of lattice oxygen will produce more oxygen vacancies in the lattice,while the bulk lattice oxygen migrates outside to supplement the oxygen vacancies.High migration rate can replenish oxygen vacancies in time,so it is easier to promote the formation of C-O bonds and eventually produce CO and CO₂.While low migration rate cannot replenish the oxygen vacancies generated timely.Oxygen deficiency makes it easier to form C-C bonds,resulting in carbon deposition.Through the composite filling operations,the OCs with high activity at the front of the sequence?such as CuO/SiO₂?first contacted with CH₄ and oxidized it to CO₂ and H₂O.CO₂ and H₂O acted as oxygen source and continued to diffuse forward to contact with the OCs?such as NiO/ZrO₂?which were easy to produce carbon deposition to replenish oxygen vacancies in the lattice timely,thus inhibiting or eliminating carbon deposition to a certain extent.Finally,considering that the endothermic and exothermic reactions of different OCs during the CLC oxidation and reduction process may cause fluctuations in the bed temperature field and bring the temperature run-away phenomenon.The temperature shock may lead to powdering,sintering,agglomeration of the OCs and make them rapidly deactivated and reduced the CLC reactivity and shorten the OCs service life.Based on these consideration,this thesis designed and constructed a set of fixed reactor experimental device which can monitor the bed temperature change in real time,and tests the bed temperature field under different OCs filling conditions.By monitoring the fixed-point temperature,the effects of the endothermic and exothermic behavior of OCs in different stages of CLC on the bed temperature filed was investigated,which provided a basis for optimizing the OCs activity distribution in the reactor bed and stabilizing the bed temperature field.The results show that the endothermic and exothermic behavior of the OCs will bring fluctuations to the bed temperature field,and the synergistic effect between different OCs can be utilized to stabilize the bed temperature field.