The Study On Biomass Chemical Looping Gasification With Fe Based Oxygen Carriers

Low calorific heat biomass can be converted into high calorific heat fuel gas through biomass chemical looping gasification technology,which has high energy conversion rate.The syngas product can be used for power generation and heating or other chemicals manufacturing.The preparation of high-quality syngas needs oxygen for biomass gasification,while removing or reducing CO₂ concentration.It has been reported that Fe₂O₃ can provide oxygen for biomass gasification.And CaO is a good CO₂ absorbent and catalyst.But there are few studies on coupling Fe₂O₃ and CaO to prepare high-quality syngas by biomass chemical looping gasification.In this paper,CaO was introduced to Fe₂O₃ to prepare high-quality syngas.Fe-based oxygen carriers were prepared by loading Fe on CaO,Al₂O₃,SiO₂ and Mg O carriers.The effects of the types of Fe-based oxygen carriers and their preparation methods,the ratio of oxygen carriers to biomass and reaction temperatures on the biomass chemical looping gasification were studied.Furthermore,multi techniques,such as BET,XRD,CO₂-TPD,H₂-TPR and SEM were performed to analyse the characterization of Fe based oxygen carriers to guide the selection.TGA-FTIR experiments confirmed that Fe₂O₃ and CaO play different roles in biomass chemical looping gasification.When Fe₂O₃ is used as oxygen carrier,the weight loss peak caused by gasification appears obviously at 770 ^oC.Infrared spectroscopy shows that CO and CO₂ are produced from biomass gasification at this time.When CaO acts on biomass gasification,the results showed that CaO can absorb CO₂ at temperatures lower than 700 ^oC,and mainly plays the role of catalyze when the temperature higher than 700 ^oC.The effects of mixture of Fe₂O₃ and CaO,different mixing ratios of Fe₂O₃ and CaO and different reaction temperatures on the syngas were analyzed in a fixed-bed reaction system.The combined oxygen carriers of iron oxide and calcium oxide demonstrated the best activity for chemical looping gasification of biomass.Fe₂O₃ could provide oxygen for biomass gasification,while CaO played a dual role of absorbing CO₂ and catalysis at mild temperatures and reforming tars to syngas at high temperature.As the temperature rises from650 ^oC to 850 ^oC,the gas yield increases significantly.It could be seen from the H₂-TPR and XRD characterizations of the oxygen carriers that Fe₂O₃ supplied oxygen,and the reduction of Fe₂O₃ occured step by step.In addition,10 cycles of biomass chemical looping gasification with the mixture of Fe₂O₃ and CaO as oxygen carriers were carried out.The results of gas production and SEM showed that the mixed oxygen carriers of Fe₂O₃ and CaO exhibited good cyclic reaction characteristics,and the oxygen carriers could maintain good reactivity and carbon deposition resistance.Fe/CaO oxygen carriers with different Fe₂O₃ loadings were prepared by impregnation method and mechanical mixing method.From the H₂-TPR and CO₂-TPD characterizations,it was found that the reduction peak increased and the absorption of CO₂ decreased with the increase of Fe loading.The oxygen carriers prepared by mechanical mixing method had better performance than those prepared by impregnation method.The results of biomass chemical looping gasification on fixed bed showed that mix 40%Fe/CaO oxygen carrier had the highest gas yield with 663.5m L/g dry biomass and the best syngas quality with the concentration was68.5%.Iron-based oxygen carriers loaded on Al₂O₃,SiO₂ and Mg O inert carriers were prepared by impregnation method and mechanical mixing method,respectively.H₂-TPR characterization showed that all oxygen carriers exhibited better reducibility,and mechanical mixing method of the same carrier showed higher reducibility than impregnation method;CO₂-TPD characterization showed that SiO₂ as carrier had weaker CO₂ absorption,while CO₂absorption of oxygen carriers obtained by alkaline carrier was higher.The fixed bed experiments showed that alkaline carrier（Mg O）is beneficial for biomass gasification to syngas,and the gas yeilld and syngas concentration are larger than those of acidic and neutral carriers.The chemical looping gasification simulation was established on Aspen Plus software.By coparing the experimental results with simulation results,the simulation is accurate and reliable.The model was used to simulate the effect of reaction temperature and the mass ratio of oxygen carrier to biomass,the optimal parameters were that the reaction temperature was850^oC and the ratio of oxygen carrier to biomass was 1.