Investigation On Chemical-looping Gasification Using Calcium-ferrum Mixed Oxide As Oxygen Carriers

The problems of greenhouse effect and air pollution caused by the massive use of fossil fuels have been restricting the healthy development of human and even the global ecological environment.Nowadays,many scholars in various countries are trying to find the alternative energy,while the clean and efficient use of fossil fuels is also the focus of environmental crisis.Coal gasification technology is of great significance for promoting the comprehensive utilization of fossil fuels,improving the current shortage of resources,and reducing environmental pollution.Chemical looping combustion?CLC?,is achieved through the reaction of oxygen carrier?OC?with fuel,which avoids the high CO₂ capture cost and inhibits the formation of thermal NOx due to the lower reaction temperature.It also achieves the gradient utilization of energy.Chemical-looping gasification?CLG?is a new gasification technology orginated from CLC,and its target product is synthetic gas rather than heat.The development of oxygen carrier has always been the key issue in the chemical looping technology.In this work,calcium-iron mixed oxygen carriers have been systematically investigated in CLG process.Firstly,four kinds of CaO/Fe₂O₃ mixed OCs were prepared by the impregnation method.The chemical characterization showed that the combination of CaO and Fe₂O₃mainly formed two new crystalline structures of CaFe₂O₄ and Ca₂Fe₂O₅.The addition of CaO improved the specific surface area and pore structure of Fe based OC,but reduced its mechanical strength.HSC Chemistry 6.0 was used to analyze the reactions of two new phases of CaFe₂O₄ and Ca₂Fe₂O₅ with conventional fuels.It showed that the reduction of CaFe₂O₄ was accompanied by the generation of Ca₂Fe₂O₅,and the latter was more capable of transforming solid fuel.The actual reactions of these OCs with H₂ and C were studied in TGA and chemisorption apparatus.It was found that the participation of CaO had great influences on the reduction process of Fe based OC.The FeCa50?CaFe₂O₄?showed similar chemical properties with Fe₂O₃ at the early stage of reaction while the latter reaction was similar to that of FeCa70?Ca₂Fe₂O₅?.The performances of these four kinds of CaO/Fe₂O₃ OCs in CLG processes as well as their hydrogen production capacity and CO conversion ability were tested in a batch fluidized bed and a fixed bed reactors.The participation of CaO can significantly increase the hydrogen production rate of Fe based OC.FeCa50?CaFe₂O₄?OC exhibited higher carbon conversion rate and product quality in the gasification of coal and char,and its reduced product Ca₂Fe₂O₅ showed obvious catalytic effect on the transformation of CO with steam,so it was more suitable for application in CLG process.The 20 redox cycle tests on the thermogravimetry showed that the CaFe₂O₄ oxygen carrier had a strong thermal stability.The condition optimization experiments showed that when the temperature was 900?C and O/C was 1,relatively high carbon conversion rate and gas production quality can be obtained.And in the experiments with CO₂ as a gasification agent,CaFe₂O₄ still exhibited high coal char catalytic conversion and CO production capacity.The analysis of the system simulation showed that CaFe₂O₄ can achieve the auto-thermal balance of the CLG as well as high syngas output.