Characteristic Of Biomass Chemical Looping Gasification And The Optimization For Ca₂Fe₂O₅ Oxygen Carrier

Biomass is a kind of clean and renewable energy resource,and biomass gasfication of syngas production is an important approach for its energy use.Chemical looping gasification（CLG）is an application of chemical looping technology for biomass energy use.In CLG,oxygen carrier（OC）provides the oxygen for the biomass gasification process,and syngas mainly composed of CO and H₂ is produced.Compared with the conventional gasification approaches like air gasification and pure oxygen gasification,CLG can avoid the low heat value of syngas resulted from massive nitrogen introduction in air gasification,and save the high energy consumption and high cost in pure oxygen preparation.Nowadays,biomass CLG technology is still in a initial stage,and there are plenty of problems to be solved before industrial use,especially for oxygen carrier material and reactor.Therefore,this study researched about the basic characteristics of biomass CLG,as well as the influence of CaO additive in this process.Then calcium ferrites were prepared as OC material for biomass CLG to improve the syngas selectivity.Later on,Ca₂Fe₂O₅ OCs with Sr substitution at A site,Co substitution at B site and inert supports were developed.The study contains as follow:（1）The basic characteristics of biomass CLG was investigated using Fe₂O₃ as OC in fixed bed reactor,and the effects of OC content,gasification temperature and steam flow on syngas production and OC property were obtained.The results showed that OC as an oxidant provided oxygen for the biomass pyrolysis products,and also had an effect on catalytic crack in pyrolysis,thus syngas production could be greatly imporved.As oxygen source,OC in appropriate content could effectively improve the syngas production performance,but excess OC would cause the further oxidation of syngas into CO₂ and H₂O.High temperature is conducive to the biomass deep crack,as well as the reactions among OC and pyrolysis products,thus resulted in higher syngas yield.On one hand,steam as gasification agent promoted H₂ production,but excess steam restricted CO production and caused the high energy consumption;on the other hand,steam could greatly improve the production of H₂ through iron-steam reaction.（2）The influence of CaO additive on biomass CLG performance using Fe₂O₃ OC was analyzed.The results showed that CaO as additive could significantly improve the syngas production,and decreased CO₂ production.CaO could elevate the gasification efficiency from55.2%into 66.2%in 850°C.CaO played as CO₂ sorbent at a lower temperature,and promoted the H₂ production through absorbing CO₂ and pushing the shift of WGS.At a higher temperature,CaO played as catalyst and elevated the selectivity of syngas.The reactivity variation in multiple redox mainly caused by CaO coverage on Fe₂O₃ surface,the formation of Ca₂Fe₂O₅ and CaO deactivation.Overall,CaO as additive could improve the syngas production performance in biomass CLG.（3）Calcium ferrites（including CaFe₂O₄ and Ca₂Fe₂O₅）were prepared as OCs for biomass CLG process,and their characterization and property in biomass CLG were analyzed through SEM,XRD,XPS,thermodynamics,TG-FTIR and fixed bed.The results showed that CaFe₂O₄and Ca₂Fe₂O₅ had lower oxidation ablitity than Fe₂O₃,and brought greater syngas selectivity in biomass CLG.CaFe₂O₄ and Ca₂Fe₂O₅ had more adsorbed oxygen with high activity on the surface,thus kept a great activity in biomass CLG.Compared with Fe₂O₃,Ca₂Fe₂O₅ as OC could increase the gasification efficiency by about 10%in steam CLG.The reduction product Fe0 from Ca₂Fe₂O₅ could revive into Fe³⁺,which was conducive to the enhancement of H₂production in the steam oxidation stage.The oxidation ability of calcium ferrites could be described as:Fe₂O₃>CaFe₂O₄>Fe₃O₄>FeO>Ca₂Fe₂O₅,and their performance in CLG followed the order:Ca₂Fe₂O₅>CaFe₂O₄>Fe₂O₃.（4）Ca₂Fe₂O₅ with Co doping at A site and Sr doping at B site were developed to further improve its performance in CLG.The OCs with doping were characterized by XRD,XPS,SEM and BET,and their properties in biomass CLG was evaluated in TGA and fixed bed test,then the mechanism of reactivity enhancement was discussed.The results showed that the doping content x of Sr at A site and Co at B site in Ca₂Fe₂O₅ unit cell is finite,and their maxium were both at the range of 0.6-1.0.Both the substitution of Sr for Ca and Co for Fe could improve the activity of Fe cation in Ca₂Fe₂O₅,and produced more metal Fe in the reduction,thus the greater H₂ production could be obtained in the iron-steam reactions.For the Co-doped Ca₂Fe₂O₅ OCs,due to the higher activity of Co cation,excess Co doping content would lead to the further oxidation of syngas,lowering the syngas production.Among them,Ca₂Fe_1.8Co_0.2O₅ had a greatest syngas production performance in biomass CLG.After Sr substitution,the syngas production improvement is mainly attributed to the lattice distortion,which weakened Fe-O bond,and Ca_1.4Sr_0.6Fe₂O₅ as OC had greatest gasification efficiency.The deactivation of these OCs in multiple redox is mainly caused by the ash deposit on their surface.The ash melting and the migration of Ca from Ca₂Fe₂O₅ into phosphates,caused the weakness of reactivity and syngas selectivity in multiple redox.（5）Mg/Al/Zn oxides were added into Ca₂Fe₂O₅ OC as supports,and the potential effects of different supports on OC performance in CLG were evaluated.The results showed that the addition of Al oxide broke the brownmillerite-type structure of OC,and greatly enhanced the oxygen release rate,but weakened the syngas selectivity of biomass CLG.ZnO addition presented higher reactivity in biomass CLG,but ZnO would be reduced first before the reduction of Ca₂Fe₂O₅ in CFZ5 OC,and converted into metal Zn,thus was unacceptable for biomass CLG.MgO addition enhanced the oxygen release ability of Ca₂Fe₂O₅,and improved gasification performance in CLG.With MgO addition,the gasification efficiency could increase from 85.1%into 98.4%using CFM3 as OC.MgO addition also effectively elevated the melting temperature of the reduced OC by the dissolution of FeO into MgO structure,which would enhance the stability of OC in multiple redox.In OC material design,dissolving its reduction products into another materials with high melting temperature,is also a strategy to improvie its stability.Overall,this study provides the high-reactivity and high-selectivity oxygen carriers,as well as their optimization path for biomass CLG application.