Investigation On Sorption-Enhanced Steam Gasification Of Biomass Using Ca/Ce-Based Materials

The sorption-enhanced steam gasification of biomass(SESGB)using Ca-looping takes advantages of in-situ CO2 capture by CaO-based materials,which is considered as a promising technology.This technology can not only obtain high H2 concentration and yield,but the enrichment of CO2 also can be achieved.Thus,the high reactivity and stable CO2 capture performances of CaO-based materials is the key to high H2 production.However,CO2 capture performances of natural CaO-based materials decrease rapidly with the number of cycles due to the sintering,resulting in low H2 production,which inhibit the industrial application of SESGB using CaO-based materials.In order to obtain higher CO2 capture and H2 production efficiencies,and a series of Ca/Ce-based materials were designed with CaO as the CO2 sorbent,CeO2 as the catalyst for steam gasification of biomass in this work.Ca12Al14O33 and red mud were used to enhance the sintering resistance and catalytic effect on tar reduction,respectively,and the cyclic CO2 capture and H2 production performances in SESGB using the materials were further improved.In order to elucidate the mechanism for high CO2 capture and H2 production performances during the SESGB using the Ca/Ce-based materials,the microstructure characterizations and density functional theory calculations of the were performed,aiming at providing theoretical guidance and basis for industrial application.The CaCe materials were prepared by wet-mixing method,and the H2 production performance from SESGB using the materials were studied.The most appropriate reaction conditions for SESGB using the materials were determined by the L12(4×23)orthogonal experiments.The effects of CeO2 on sizes of CaO grains and particles in the material were analyzed.The effects of biomass ash on reacticity of the materials were elucidated.The mechanism of high CO2 capture and enhancing H2 production of CaCe materials in SESGB were determined.When the mass ration of CaO/CeO2 is 90:10,the H2 concentration and yield using the material achieve 77.8vol.%and 141.8mL/g,respectively after 10 cycles,which are 1.49 and 1.81 times as high as those of CaO material,and lower operating cost was obtained.In order to further enhance the sintering resistance of CaCe material during SESGB cycles,the Ca12Al14O33 was used as the inert supporter to enhance the sintering resistance.The CaAlCe materials were prepared by wet-mixing method.The effect of Ca12Al14O33 on cyclic CO2 capture and H2 production performances in SESGB using the CaAlCe material were studied.The active sites,chemical states,and electron properties of the CaAlCe material were discussed.The evolutions of carbonation kinetics,microstructure morphologies,and pore structure of the CaAlCe material were analyzed.The synergistic effects of Ca12Al14O33 and CeO2 on improving CO2 capture and H2 production during multiple SESGB cycles using the CaAlCe material were elucidated.The CaAlCe material possesses higher cyclic stability and sintering resistance than CaCe material.Ca12Al14O33 enhances the cyclic stability of CaAlCe material,and CeO2 improves the carbonation reaction rates during the cycles.The CO2 capture capacity of CaAlCe material is 2.4 times as high as that in the absence of Ca12Al14O33 and CeO2.The improved H2 production and reduced CO2 concentration were achieved by the CaAlCe material.The H2 concentration and yield are 78.7vol.%and 120mL/g after 10 cycles,which are 2.5 and 4.8 times higher than those of limestone,respectively.To further enhance the catalytic tar reduction during SESGB cycles using Ca/Ce-based materials,the industrial waste-red mud(RM)was used as the dopant for synthesis of RM-CaAlCe materials by wet-mechanical mixing method.The effects of pre-treatment methods of RM on carbonation activity of the RM-CaAlCe materials were analyzed.The CO2 capture,syngas compositions,and tar yields during multiple SESGB cycles using the RM-CaAlCe materials were studied.The evolutions for phase compositions,electron properties,surface and microstructure morphologies of RM-CaAlCe materials during the cycles were discussed.The mechanisms of improved H2 production and tar reduction during SESGB cycles from RM addition were elucidated.The Fe2O3 from RM reacts with CaO into Ca2Fe2O5.The Ca2Fe2O5 improves the concetration of oxygen vacancy and enhances the interaction between CaO and CeO,thus the CO2 capture,water gas shift(WGS),and tar reduction reactions,etc are promoted.The H2 concentration and yield are 69.1vol.%and 596.2mL/g,respectively after 10 cycles,which are 43%and 65%higher than those in the absence of RM.Using the density functional theory calculations,the CO2 capture characteristics on CaCe material in steam gasification of biomass condition was investigated,the mechanism of Ce doping and H2O promoting CO2 adsorption was elucidated.The reaction pathways of WGS reaction,which is the typical reaction during steam gasification of biomass,were studied.The effect of CaO surface on WGS reaction for H2 production was clarified.The presence of Ce improves the activity of adjacent atomic O,which improves the CO2 adsorption performance,and the H2O can enhance the interaction between the CaO surface and CO2.The dissociation of H2O is the rate-limiting step of WGS reaction,which occurs spontaneously on the CaO surface.Ce doping decreases the activation energy of O-H bond broken.The activation energies of CO2 generation and adsorption are reduced,and H2 desorbs from the surface spontaneously.