Supercritical water gasification is a promising technology for wet biomass utilization.In this paper,Ni and other metal catalysts were synthesized by wet impregnation.The stability and catalytic activities of Ni catal...Supercritical water gasification is a promising technology for wet biomass utilization.In this paper,Ni and other metal catalysts were synthesized by wet impregnation.The stability and catalytic activities of Ni catalysts were evaluated.Firstly,catalytic activities of Ni,Fe,Cu catalysts supported on MgO were tested using wheat straw as raw material in a batch reactor at 723 K and water density of 0.07 cm^(3)/g.Experimental results showed that the order of metal catalyst activity for hydrogen generation was Ni/MgO>Fe/MgO>Cu/MgO.Secondly,the influence of different supports on Ni catalysts performance was investigated.The results showed that the order of the Ni catalysts’activity with different supports was Ni/MgO>Ni/ZnO>Ni/Al_(2)O_(3)>Ni/ZrO_(2).Finally,the effects of Ni loading and the amount of Ni catalyst addition on hydrogen production,and the stability of Ni/MgO catalyst were studied.It was found that serious deactivation of Ni catalyst in the process of supercritical water gasification took place.Even if carbon deposited on the catalyst surface was removed by high temperature calcination and the catalyst was reduced with hydrogen,the activity of used catalyst was only partially restored.展开更多
Supercritical water fluidized bed(SCWFB)is a promising reactor to gasify biomass or coal.Its optimization design is closely related to wall-to-bed heat transfer,where particle convective heat transfer plays an importa...Supercritical water fluidized bed(SCWFB)is a promising reactor to gasify biomass or coal.Its optimization design is closely related to wall-to-bed heat transfer,where particle convective heat transfer plays an important role.This paper evaluates the particle convective heat transfer coefficient(h_(pc))at the wall in SCWFB using the single particle model.The critical parameters in the single particle model which is difficult to get experimentally are obtained by the computational fluid dynamics-discrete element method(CFD-DEM).The contact statistics related to particle-to-wall heat transfer,such as contact number and contact distance,are also presented.The results show that particle residence time(τ),as the key parameter to evaluate h_(pc),is found to decrease with rising velocity,while increase with larger thermal boundary layer thickness.τfollows a gamma function initially adopted in the gas-solid fluidized bed,making it possible to evaluate h_(pc) in SCWFB by a simplified single particle model.The theoretical predicted h_(pc) tends to increase with rising thermal gradient thickness at a lower velocity(1.5 U_(mf)),while first decreases and then increases at higher velocity(1.75 and 2 U_(mf)).h_(pc) occupies 30%-57%of the overall wall-to-bed heat transfer coefficient for a particle diameter of 0.25 mm.The results are helpful to predict the overall wall-to-bed heat transfer coefficient in SCWFB combined with a reasonable fluid convective heat transfer model from a theoretical perspective.展开更多
Supercritical water fluidized bed(SCWFB)reactors are designed to gasify biomass or coal with high efficiency.In this paper,the wall-to-bed heat transfer characteristics in SCWFB are studied using the computational flu...Supercritical water fluidized bed(SCWFB)reactors are designed to gasify biomass or coal with high efficiency.In this paper,the wall-to-bed heat transfer characteristics in SCWFB are studied using the computational fluid dynamics and discrete element method(CFD-DEM)coupled with a constant heat flux boundary.Two different methods are considered to deal with the multiphase heat flux boundary in CFD-DEM because there is currently no single widely accepted approach.Zhang’s method predicts a more accurate wall-to-bed heat transfer coefficient in SCWFB than Lattanzi’s method according to comparisons of the simulation results with an empirical correlation.The influences of temperature,pressure,velocity,and the solid phase properties,such as the particle diameter,particle heat capacity,particle thermal conductivity,and particle density,on the SCWFB wall-to-bed heat transfer characteristics are studied based on Zhang’s method.The simulation results help reveal the SCWFB heat transfer characteristics.展开更多
基金This work is currently supported by the Key Project of Chinese National Programs for Research and Development(No.2016YFB0600102)National Nature Science Foundation of China(No.51676158)Shaanxi Science&Technology Co-ordination&Innovation Project(No.2016KTZDGY08-02).
文摘Supercritical water gasification is a promising technology for wet biomass utilization.In this paper,Ni and other metal catalysts were synthesized by wet impregnation.The stability and catalytic activities of Ni catalysts were evaluated.Firstly,catalytic activities of Ni,Fe,Cu catalysts supported on MgO were tested using wheat straw as raw material in a batch reactor at 723 K and water density of 0.07 cm^(3)/g.Experimental results showed that the order of metal catalyst activity for hydrogen generation was Ni/MgO>Fe/MgO>Cu/MgO.Secondly,the influence of different supports on Ni catalysts performance was investigated.The results showed that the order of the Ni catalysts’activity with different supports was Ni/MgO>Ni/ZnO>Ni/Al_(2)O_(3)>Ni/ZrO_(2).Finally,the effects of Ni loading and the amount of Ni catalyst addition on hydrogen production,and the stability of Ni/MgO catalyst were studied.It was found that serious deactivation of Ni catalyst in the process of supercritical water gasification took place.Even if carbon deposited on the catalyst surface was removed by high temperature calcination and the catalyst was reduced with hydrogen,the activity of used catalyst was only partially restored.
基金supported by the National Key Research and Development Program of China (grant No.2020YFA0714400)the National Natural Science Foundation of China (grant No.51925602).
文摘Supercritical water fluidized bed(SCWFB)is a promising reactor to gasify biomass or coal.Its optimization design is closely related to wall-to-bed heat transfer,where particle convective heat transfer plays an important role.This paper evaluates the particle convective heat transfer coefficient(h_(pc))at the wall in SCWFB using the single particle model.The critical parameters in the single particle model which is difficult to get experimentally are obtained by the computational fluid dynamics-discrete element method(CFD-DEM).The contact statistics related to particle-to-wall heat transfer,such as contact number and contact distance,are also presented.The results show that particle residence time(τ),as the key parameter to evaluate h_(pc),is found to decrease with rising velocity,while increase with larger thermal boundary layer thickness.τfollows a gamma function initially adopted in the gas-solid fluidized bed,making it possible to evaluate h_(pc) in SCWFB by a simplified single particle model.The theoretical predicted h_(pc) tends to increase with rising thermal gradient thickness at a lower velocity(1.5 U_(mf)),while first decreases and then increases at higher velocity(1.75 and 2 U_(mf)).h_(pc) occupies 30%-57%of the overall wall-to-bed heat transfer coefficient for a particle diameter of 0.25 mm.The results are helpful to predict the overall wall-to-bed heat transfer coefficient in SCWFB combined with a reasonable fluid convective heat transfer model from a theoretical perspective.
基金This work was supported by the National Natural Science Foundation of China through grant Nos.51925602 and 51888103.
文摘Supercritical water fluidized bed(SCWFB)reactors are designed to gasify biomass or coal with high efficiency.In this paper,the wall-to-bed heat transfer characteristics in SCWFB are studied using the computational fluid dynamics and discrete element method(CFD-DEM)coupled with a constant heat flux boundary.Two different methods are considered to deal with the multiphase heat flux boundary in CFD-DEM because there is currently no single widely accepted approach.Zhang’s method predicts a more accurate wall-to-bed heat transfer coefficient in SCWFB than Lattanzi’s method according to comparisons of the simulation results with an empirical correlation.The influences of temperature,pressure,velocity,and the solid phase properties,such as the particle diameter,particle heat capacity,particle thermal conductivity,and particle density,on the SCWFB wall-to-bed heat transfer characteristics are studied based on Zhang’s method.The simulation results help reveal the SCWFB heat transfer characteristics.