| Chemical looping gasification?CLG?is an innovative and low-cost gasification technology.In coal-fuelled CLG reaction system,oxygen carrier?OC?is the key to resolve the contradiction between rapid conversion and partial oxidation of coal.Therefore,the exploitation for high-performance OC is one of the significant issues of the development of CLG.As a high-performance OC,cooper ferrite?CuFe2O4?had been prepared in the paper.Based on the study of the reaction characteristics of the OC,the thermodynamic simulation and experimental research on CLG of coal were carried out focusing on the evolution behavior of material in the fuel reactor.The CLG characteristics of coal with CuFe2O4 as OC were investigated from two aspects of thermodynamics and kinetics.Firstly,the phase composition,surface topography and temperature-programmed reduction characteristics of CuFe2O4 OC were analyzed by XRD,SEM and H2-TPR respectively.The thermal stability of CuFe2O4 OC was investigated by means of TGA and fluidized bed reactor,and the redox performance of CuFe2O4 OC was investigated in a fluidized bed reactor.The results show that the CuFe2O4 OC prepared by sol-gel-combustion synthesis method is a typical tetragonal crystal material.The surface microstructure of CuFe2O4 OC is lamellar and barely microporous.The reduction of CuFe2O4 OC by H2 is characterized by stepwise reduction as well as tend to deep reduction.The multistage oxygen release of CuFe2O4OC in oxygen-poor environment is related to temperature.Oxygen deficiency due to the migration and transformation of copper ions is the main reason for oxygen release of CuFe2O4OC between 750°C and 950°C.There is a cubic phase and a cubic phase in the oxygen-deficient body produced from oxygen release process,which have a correspondence with the Cu2+concentration on octahedral sites and the Cu+concentration on interstitial sites,and the trigonal phase increase while cubic phase decrease with the increase of temperature.The oxygen loss ratio is a macroscopic manifestation of oxygen deficiency.The higher the temperature,the greater the oxygen loss ratio.There is relative poor reduction reactivity of CO to CuFe2O4 OC with the formation of metallic Cu and Fe3O4 and 16.9%of the lattice oxygen has not been yet converted until the final of reduction stage.While the regeneration conversion efficiency of the reduced oxygen carrier is up to 97.3%,showing a good regeneration performance.Secondly,the thermodynamic calculation of the CLG of coal with CuFe2O4 OC was carried out by HSC Chemistry.The results show that CLG of coal is a process with complex multi-step chain reaction system.Compared with the Fe2O3 OC,the thermodynamic limit of the CuFe2O4 OC redox reaction shifts to the low temperature.The thermodynamic fesibility CLG system of coal with CuFe2O4 OC has been confirmed by the Multi-Step Reaction model established based on Aspen Plus.According to the contour distribution of the unit net heat release,the whole working conditions are divided into three zones,Restricted Autothermal Equilibrium Zone,Complete Autothermal Equilibrium Zone and Transition Zone.Inherent separation of reducing gas and CO2 can be achieved in the Restricted Autothermal Equilibrium Zone and complete conversion of fuel can be achieved in the Complete Autothermal Equilibrium Zone.While Transition Zone should be avoided when designing experimental conditions in which CLG system cannot be operated spontaneously.Co-analysis of temperature and OC-fuel mass ratio shows that the Transition Zone expands while the Restricted Autothermal Equilibrium Zone narrow in the direction of increasing temperature.CLG system is less restricted by the Transition Zone with an easier tendency to autothermal balance at low gasification temperature.However,a higher oxygen carrier circulation is needed and cold gas efficiency is inherently limited simultaneously.A single positive correlation between OC-fuel mass ratio and carbon conversion while a parabolic relationship between CLG system unit net heat release as well as cold gas efficiency and OC-fuel mass ratio are stated respectively.The selection of the best working condition should be in the Restricted Autothermal Equilibrium Zone where near the Transition Zone.The recommended gasification temperature range is800900°C.Finally,the effects of steam flow rate,OC-fuel mass ratio,temperature,particle size and oxygen loss ratio of OC on CLG of coal with CuFe2O4 as OC was investigated,and reaction kinetic analysis for the rate-determining step of CLG was performed.The results show that the true oxygen transport capacity of CuFe2O4 OC is 0.111 with the formation of Fe3O4 and FeCu4alloy in the CLG process of coal,which consists of pyrolysis conversion stage and fixed carbon conversion stage.Among the influencing factors,the increase of temperature as well as steam flow rate and the decrease of the particle size of OC are beneficial to the complete conversion of coal and the improvement of cold gas efficiency.The increase in temperature can cause exponential increase in the CLG reaction rate.The increase of steam flow rate is conducive to fixed carbon conversion but adverse to pyrolysis conversion.As a result,the average fixed carbon conversion rate is increased by about 1.3 times while the pyrolysis conversion rate is reduced by about 0.8 times with the increased in steam flow rate from 0.25 g/min to 1.25 g/min.The CLG reaction rate as well as carbon conversion rate increase while quality of syngas decline when OC-fuel mass ratio increases.When oxygen loss ratio increase from 0 to 0.4,H2concentration increased by 8.15%while tiny difference occurs in CLG reaction rate and carbon conversion rate and cold gas efficiency.The reaction kinetics analysis shows that the fixed carbon conversion is the rate-determining step of the CLG process of coal with CuFe2O4 as oxygen carrier.The spherical symmetric shrinking core model reveals the dynamic characteristics of the rate-determining step,the mechanism function of which is G?7??C?8??28?1-?7?1-?C?8?13,and the corresponding activiation energy is 188.48kJ/mol. |
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