Study Of Biomass Chemical Looping Gasification With CoFe₂O₄ As Oxygen Carrier

Biomass is a kind of clean and renewable energy and it plays an important role in the transformation of energy system.Biomass chemical looping gasification is a kind of novel gasification technology with the advantages of low cost,high calorific value and low tar content.Oxygen carrier is the key of chemical looping technology and its reactivity is closely related to the operation of the whole reaction system.CoFe₂O₄ is a kind of bimetallic Fe-Co composite oxide which has both the advantages of cobalt and iron based oxygen carrier and it is a good candidate in chemical looping technology.However,there is no research on biomass chemical looping gasification with CoFe₂O₄ as oxygen carrier.The reactivity of biomass with CoFe₂O₄ as oxygen carrier was investigated through the thermodynamic analysis,thermogravimetric analyzer and fixed bed reactor.This can provide theoretical guidance for the application of CoFe₂O₄ in biomass chemical looping gasification.The influence of different factors on reactivity of biomass with CoFe₂O₄ can be investigated through the thermodynamic calculation and it can provide theoretical guidance for subsequent experiments.The effects including the amount of oxygen carrier,temperature,pressure,steam content,carbon dioxide,nitrogen and sulfur in biomass on gasification characteristics were examined through the thermodynamic analysis with pine sawdust as fuel and CoFe₂O₄ as oxygen carrier.The oxidation process of reduced oxygen carrier was also studied.Thermodynamic analysis results show that CoFe₂O₄ can provide lattice oxygen to effectively improve the gasification of biomass.With the amount of oxygen carrier increases,the reduction degree of CoFe₂O₄ decreases.As the temperature goes up,the amount of H₂and CO increases,while the amount of CO₂ decreases.The increase of pressure is not conductive to the gasification.With the amount of steam increases,H₂ and CO₂ yield increases while CO yield decreases.The ratio of H₂ to CO increases and high quality syngas can be obtained with the addition of steam.In the atmosphere of CO₂,the production of CO and H₂O increases while the amount of H₂ decreases.The nitrogen and sulfur in biomass has little effect on the performance of CoFe₂O₄.The reduced CoFe₂O₄ oxygen carrier can be reoxidized by O₂ to CoFe₂O₄ in air reactor.The study of reaction kinetics and mechanism of biomass chemical looping gasification is of great significance to both the design of reactor and optimization of operation conditions.The effects of oxygen carrier,heating rate,biomass particle size,mass ratio of oxygen carrier to biomass（O/B）and CO₂ were investigated through thermogravimetric experiments.The reaction mechanism of oxygen carrier was studied together with XRD.In addition,the reaction kinetic model was established and kinetic parameters were calculated based on the thermogravimetric results.Thermogravimetric results show that the gasification process can be divided into three stages of drying,pyrolysis and char gasification.Biomass particle size and heating rate have great influence on the heat and mass transfer between CoFe₂O₄ and biomass.Smaller biomass particle is more conductive to the gasification reaction while gasification is restrained under CO₂ atmosphere.During the reaction,cobalt can be first reduced which can promote the further reduction of iron due to the synergistic effect.With the amount of oxygen carrier increases,the reduction degree of oxygen carrier decreases.The optimal mass ratio of oxygen carrier to biomass is 0.8.Compared with the pyrolysis process,the char gasification process is more difficult to happen,which is the key in the whole chemical looping gasification.In order to study the component of the syngas and gasification characteristics in depth,the evaluation of biomass chemical looping gasification was conducted via a fixed bed reactor.The effects of mass ratio（oxygen carrier to biomass）,steam content and reaction temperature on reactivity were explored and the recyclability of oxygen carrier was also studied.The oxygen carriers of fresh and reacted were characterized by X-ray diffraction（XRD）and scanning electron microscopy/energy dispersive spectrometer（SEM-EDS）.The results show that CoFe₂O₄ can effectively improve the gasification of biomass and carbon conversion efficiency.The optimal performance can be achieved when the mass ratio,steam content and temperature are 0.8,50%,and 900℃,respectively.High quality syngas can still be obtained after five redox cycles.Meanwhile,oxygen carrier can be well regenerated without obvious sintering agglomeration and it has good application prospect in chemical looping gasification.