Study On The Characteristics Of Biomass Gasification Using Iron-based Modified Calcium Oxide Sorbent

Compared with fossil fuels,biomass energy is an ideal clean energy,which is renewable and can achieve zero carbon dioxide emissions.Hydrogen rich syngas from biomass gasification is an effective way of thermochemical conversion and utilization of biomass.Using steam as the gasification medium and CaO as the catalyst is an effective way to increase H₂content of syngas in biomass gasification.CaO,as an effective absorbent in biomass gasification,can absorb the generated CO₂,and play a catalytic role in gasification.It can promote the gasification reaction and achieve CO₂ capture.However,in the case of large tar content,CaO is easy to lose its activity.It is considered that CaO can be modified by adding active components.In order to increase the production of H₂,to reduce the tar content,and to inhibit the deactivation of CaO absorbent,CaO was modified in this theisis,by adding the active component of Fe,and the gasification characteristics of Fe/CaO absorbent were studied.Firstly,the pyrolysis and gasification thermogravities of sawdust,sawdust with CaO,sawdust with Fe/CaO were compared,and the pyrolysis gas products were analyzed by TG-FTIR.All the stages of pyrolysis and gasification under three conditions were revealed.The addition of CaO and Fe/CaO,compared with pure sawdust,existed CO₂ absorption and release processes.It was found that both CaO and Fe/CaO can increase the yield of H_2.The results show that CaO is beneficial to CO₂absorption and Fe/CaO can increase the yield of H₂ more significantly.Secondly,the optimal preparation method of Fe/CaO absorbent was discussed,including different preparation methods,different calcium precursors and iron ion salts,and different iron loadings.All the prepared Fe/CaO absorbents were tested by XRD,SEM-EDS,BET,CO₂-TPD and H₂-TPR to determine the composition,microscopic morphology,element distribution of calcium and iron,particle surface area,CO₂ absorption capacity and H₂ reduction characteristics with temperature of the absorbent.The different prepared absorbents were used in steam biomass gasification experiments to investigate their gasification characteristics.The characteristics of Fe/CaO absorbent prepared by wet mixing,sol-gel and impregnation were studied in three ways.The components of Fe/Ca absorbents prepared by three methods are CaO and Ca₂Fe₂O₅.It was found that the impregnation method has better CO₂ absorption performance.The absorbents prepared by the three methods can improve the hydrogen and total gas yields of biomass gasification,and the best one is the absorbent prepared by impregnation method.Based on the impregnation method,the properties of absorbents prepared by different precursors were studied.The main components of these absorbents are CaO and Ca₂Fe₂O₅.Among them,absorbents prepared with Ca(C₂H₃O₂)₂·H₂O or Fe(C₂H₃O₂)₂ contain Fe₂O₃as well.From the gasification results,the absorbent prepared with CaO and Fe(NO₃)₃·9H₂O obtained the highest H₂ yield.The effects of Fe content on the properties of absorbents with different Fe loading were studied.The results show that with the increase of Fe loading,Ca₂Fe₂O₅ in the component increases,and the CO₂ absorption performance of the absorbent decreases.When the mass ratio of Fe/CaO is 5%,the highest yield of syngas and H₂ can be obtained.The highest syngas yield of 38.21 mol/kg biomass,H₂ yield of 26.40 mol/kg biomass,LHV values of 8.69 MJ/kg and gasification efficiency of 49.15%were obtained at an optimized mass ratio of Fe/CaO=5%.The gasification results show that the promotion of Fe/CaO absorbent to biomass gasification is the result of the synergistic action of Cao and Fe.CaO mainly promotes the production of H₂through the absorption of CO₂,and Fe mainly promotes the steam reforming of tar through the catalysis.Finally,the prepared Fe/CaO absorbent was applied to the steam reforming of toluene,a tar model compound.The effects of temperature,S/T ratio of water vapor to toluene,and catalyst on tar reforming were studied.The results show that temperature is the most important factor in the steam reforming of tar,and the higher the temperature is,the greater the tar conversion is.Fe/CaO catalyst can effectively promote the steam reforming of tar.At the temperature of 850?,the catalyst with Fe/CaO mass ratio of 10%has the best catalytic performance for tar.A novel calcium looping compact reactor configuration was proposed.The dual fluidized bed is suitable for calcium looping biomass gasification to produce hydrogen-rich syngas.The riser above the gasifier can promote the cracking of the tar in the gasification product,thereby increasing the production of syngas and reducing the amount of tar.This system was simulated using Aspen Plus software,which verified the beneficial effects of the riser.For the biomass feeding of 1kg/s,when the steam flow rate is 40mol/s,the CaO circulation flow rate is 17.5mol/s,and the carbon conversion rate is 0.5,the hydrogen production in the outlet syngas can reach197kg/h,and the hydrogen concentration can reach 79%.