The Reaction Character And Modification Of Iron Based Oxygen Carriers During Biomass Chemical Looping Gasification

Biomass is one of the most important renewable energy resources,and the highly efficient gasification technology of biomass is wildly concerned.Compared with traditional fossil energy,biomass will generate more steam and tar during its pyrolysis/gasification process,and lead to the damage of the downstream equipment of the gasification system.Based on the research status of advanced gasification technology,combined with chemical looping gasification(CLG)technology and tar/syngas reforming technology,a two-step biomass gasification system was proposed which can suppress tar yield and improve gasification efficiency.This paper mainly focused on the oxygen-carrying characteristics,regenerative capacity of Fe-based oxygen carriers(OCs)during the CLG process,reaction between steam and OCs,and the effect of OCs on the tar formation during gasification are mainly investigated.The feasibility of Fe-based OCs in the process of biomass chemical looping gasification(BCLG)was comprehensively analyzed.Firstly,the four pure metal oxides(Fe2O3,CuO,NiO and CaSO4)were chosen as the OC for CLG with CO in order to select the most potential OC for further study in this work.The results showed that Fe2O3 possessed the maximum regenerative capacity(74.39%),and the actual oxygen carrying capacity during the single redox reaction showed the law of CaSO4> Fe2O3> CuO>NiO.Though the pure Fe2O3 was found seriously sintered after reaction,considering the reaction character,cost and environmental factors,Fe2O3 was chosen as the main OC for further investigation.For different Fe-based OCs prepared by different modification methods(preparing method,loading ratio,adding addition active material,etc.),thermal gravimetric analysises(TGA)were conducted to analyze their reaction character and redox reaction character,and the structure variation during reaction were studied through X-ray diffractometer(XRD),scanning electron microscope(SEM)coupled with energy dispersive X-Ray spectroscopy(EDX)and N2 adsorption/desorption analysis.The results indicated that among all Fe2O3/Al2O3 OCs,moral ratio of Fe:Al2O3 between 1.3:1 and 2:1 prepared via sol-gel method supposed to be the best.The addition of NiO could not only further increase the reactivity,but also promote the further reduction of of Fe-based OCs.NiO-Fe2O3/Al2O3 prepared by sol-gel method was proposed to be the best OC amoung the candidates.CLG using different Fe-based OCs with H2O/CO was investigated further to discover the interactions between Fe-based OCs and steam.Based on the Mossbaure spectroscopy and gas release character,it is found that the proper steam concentration could improve the reduction of Fe2O3 to Fe3O4,but it hindered the further reduction of Fe-oxides,therefore decreased the oxygen carrying capacity of pure Fe2O3.The further investigation indicated that the modified OCs could maintain good oxygen carrying capacity during the reaction though the appearance of steam would decrease their reduction rate.On the other hand,Fe3O4 and FeO,which is generated after reduction of Fe2O3,were found to possess promoting effect on WGS reaction between CO and H2 O,hence,to improve the H2 yield.According to the reaction condition determined by the experiments above,the gasification of different biomass fuels using Fe-based OCs as gasifying medium on fixed bed were conducted.It proved that Fe-based OCs benefited the further oxidation of biomass fuels as well as promoted the thermal cracking of macromolecular tar.But the ash in biomass fuels could also have effect on the component and yield of tars.Ruling out the effect of ash,the reaction of cellulose with different Ni/Fe doping ratios(molar ratio)showed an obvious increasing in H2 yield(57.41%、151.30% and 59.50% for NFA、NFA0.5 and NFA2,respectively).The further anlaysis on tar component also indicated that iron based OCs possessed good promoting effect on macromolecular tars(M3-M4),and meanwhile,increased the ratio of M2 tars(about 31%、36% and 59% for NFA、NFA0.5 and NFA2,respectively).For saw dust and rice straw,as the alkali/alkaline earth metal content in the ash could promote the biomass pyrolysis/gasification and tar cracking,the addition of iron base OCs did not apparently affect on WGS reaction and tar cracking.Finally,the CLG reaction of cellulose using NiO-Fe2O3/Al2O3 as OC for multiple cycles on a fluidized bed reactor was carried out to simulate the real CLG process of biomass.The oxidation protective coating of the Fe-base OC prepared via sol-gel method had some selectivity for tar catalytic cracking.And the sample feeding rate also affected the tar formation as well as cracking.In addition,according the XRD analysis on the OC after reaction with the fuels,it revealed that the strong interaction between Fe and Ni led to the formation of Fe-Ni crystal structure,and meanwhile limited the generation of Fe-Al crystal.However,the components generated during reaction of Fe-based OC also promoted the Boudaurd reaction and Fischer-Tropsch synthesis,which led to the yield increase of carbon deposition and CH4,respectively.