Mechanism And Performance Enhancement Of Steel Slag-based Ca/Fe Oxygen Carrier For Chemical Looping Combustion

CO₂ abatement technology has attracted worldwide attention due to the greenhouse effect.As one of emerging technologies for carbon capture,Chemical Looping Combustion?CLC?has unique features such as high combustion efficiency and internal CO₂ separation,which demonstrates excellent advantages in both the low energy penalty of carbon capture and the total cost of system.Oxygen carrier is one of key component for developing and optimizing of the CLC technology.Accordingly,a comprehensive exploration on Ca/Fe-based oxygen carriers has been conducted in this dissertation,which mainly includes oxygen transfer mechanism for synthetic materials,the regulation and modification methods for steel slag-based materials into Ca/Fe-based oxygen carrier.Furthermore,both the reaction characteristics and performance enhancement of steel slag-based oxygen carrier have been carried out for the CLC process.The main contents and conclusions of this thesis are given below.Firstly,the path of reduction reaction for the superior pure reagents of CaSO₄ and CaSO₄/Fe₂O₃ was examined by combing the quantum-chemical calculation together with a series of characterization methods,including thermogravimetric analyzer,programmed temperature reduction,in-situ X-ray diffractometer and electron microscope-spectroscopy.Based on the density functional theory,the oxygen transfer mechanism of CaSO₄/Fe₂O₃compound oxygen carrier was revealed at the level of electrons or atoms,and the improved reactivity and stability of CaSO₄ was clarified when Fe₂O₃was introduced.It was found that the reaction path of CaSO₄ is: CaSO₄ ? CaSO₃ ?CaSO₂ ? CaSO ? Ca S compared to that of CaSO₄/Fe₂O₃oxygen carrier that is: CaSO₄/Fe₂O₃? CaSO₄/Fe ?CaFeSO ? Ca S/Fe₂O₃.The essential difference lies in the intermediate products of CaSO₃ and CaFeSO,respectively.Because the electronic property of CaFeSO is more active and likely to undergo charge transfer with fuel,CaSO₄/Fe₂O₃exhibits better reactivity than CaSO₄.In addition,the characteristics of the S atom is a dominant position in the side decomposition reactions of CaSO₄,which identifies that CaSO₄/Fe₂O₃shows better stability than CaSO₄ largely because the S atom in CaFeSO is more stable than that in CaSO₃.Secondly,the actual steel slags with high Ca/Fe component were investigated byusing X-ray diffractometer and X-ray fluorescence spectrometer.A routine has been suggested for replacing the pure chemical reagents of CaSO₄ and Fe₂O₃as oxygen carrier in the CLC process.The thermodynamic analysis was carried out to clarify the oxygen-carrying characteristics of the steel slag system,and then the CLC process reaction model was established by employed Aspen Plus software for optimizing the reaction condition of steel slag-based Ca/Fe oxygen carrier in the CLC process.It was found that the chemical compositions within steel slag are mainly Ca and Fe and the reaction of SO₂ with steel slag at high temperature could effectively adjust the combined formation of Ca and Fe.A reasonable ratio of Ca/Fe-based compound oxygen carrier was obtained with the contents of CaSO₄,Fe₂O₃and Mg Fe2O5 as 40.49%,3.32% and 2.41%,respectively.Clearly,the modified steel slag could be used as an Ca/Fe oxygen carrier in the CLC process,which almost completely converted the fuel and its corresponding theoretical oxygen transport capability was about 20%.The CaSO₄ component in the steel slag underwent a side decomposition reaction during the reaction process,resulting in the inability to regenerate the oxygen carrier.In order to ensure the reaction behavior of Ca/Fe oxygen carrier and to suppress the decomposition of CaSO₄,the optimized ratios of fuel/steel slag and air/steel slag were about 3 mol/mol and 9 mol/mol,respectively.The suitable operation temperatures for the steel slag-based Ca/Fe oxygen carrier should be set around 800? for the reduction stage and 1100? or for the regeneration stage.Thirdly,the reactivity behavior,oxygen transport capacity and circulation stability of steel slag-based Ca/Fe oxygen carrier were investigated by using fixed-bed reactor together with the evaluation of an actual CLC process in a fluidized bed reactor.The detailed evolution of Ca and Fe components and mechanism of reduction/oxidation reaction were explored by combined with scanning electron microscopy and X-ray diffractometer.It was found that sulphated steel slag had excellent reactivity in both fixed-bed and fluidized bed reactors.Moreover,the reaction rate of sulphated steel slag was about 16 times of that of pure CaSO₄ oxygen carrier in the fixed-bed reactor,while fuel was almost completely converted during the initial cycles in the fluidized bed reactor.Relatively satisfactory answers in the fixed-and fluidized bed reactors indicate that the reactivity of the sulphated steel slag was mainly determined by the property of thematerial in nature,however the effect of mass transfer could be ignored.Therefore,the excellent kinetics of the sulphated steel slag is mainly attributed to the catalytic function of Fe₂O₃and Mg Fe2O4 for reducing of CaSO₄.The oxygen transport capability of sulphated steel slag was as high as 19.60% at the initial stage,meeting with the requirement for the ordinary utilization of oxygen carrier.Main troubles such as crushing,melting and agglomeration were not found after several cycles of CLC,which indicated that the oxygen carrier had excellent flow property.In a words,steel slag exhibits brilliant kinetic performance,which can be used as substitute for synthetic oxygen carrier from the pure reagents of CaSO₄ and Fe₂O₃.Similar with the result from thermodynamic analysis,however,the steel slag-based oxygen carrier has a relatively poor circulation reactivity.Finally,the focus was on the circulation reactivity of steel slag-based Ca/Fe oxygen carriers.The effects of CaO modification on the release and cycle performance of SO₂ were investigated during the reduction and regeneration process of sulphated steel slag.The CaO modification method was constructed to improve the cycle performance of steel slag-based oxygen carriers in a thermogravimetric analyzer and a fluidized bed reactor.Compared to several other low-cost oxygen carriers,the modified steel slag shows batter reaction performance and can be used as one of potential oxygen carriers in the CLC process.It was found that the CaO modified steel slag could significantly inhibit the release of SO₂ during the reduction and regeneration processes,especially almost no SO₂ was released below 850? during the CLC process of the modified steel slag.The modified steel slag with an optimized ratio of about 40% CaO had excellent cycle performance and the corresponding oxygen transport capacity was about 10%.When the temperature was above 850?,however,the problem of poor cyclic stability was difficult to completely overcome by CaO modification,the recommended operating temperature should not exceed 850?.By comparing the commonly used cheap oxygen carriers,including iron ore,manganese ore and iron oxide from steel plants,the modified steel slag showed the highest fuel conversion and oxygen transport capacity,and could achieve more than 97% of fuel conversion even at the temperature of 750?.In addition,the modified steel slag exhibited excellent cycle performance at temperatures below 850?,which was almost comparable to some pure metal-based oxygen carriers.In summary,the modifiedsteel slag-based Ca/Fe oxygen carrier shows balanced and excellent reaction performance for further application of the CLC process.