| The by-product tar produced during the biomass pyrolysis process not only severely limits the resource efficiency of biomass,but also poses the risk of environmental pollution and harm to human health.In this study,activated biochar was used as a carrier to prepare multi-element metal nano-catalysts to remove tar in situ,which had the great significance of the further development of tar steam catalytic reforming technology and biomass pyrolysis gasification technology.In this study,a series of multi-element metal nano-catalysts were prepared by the citric acid sol-gel method,and the catalytic steam reforming of the tar model compound（toluene or toluene/naphthalene/phenol）was investigated in a self-made two-stage biomass catalytic pyrolysis fixed-bed reactor,and the anti-deactivation efficiency and mechanism of the catalyst were studied in depth.The effects of activation temperature and activation time on the physicochemical properties of biochar were investigated,and a series of multi-element metal nano-catalysts were prepared with activated biochar as a carrier.The characterization results of biochars showed that activated biochar had a rich porous morphology and contains a small amount of alkali metal K.The optimal activated biochar was obtained under the conditions of activation temperature of 800℃and activation time of 60min,and its specific surface area,total pore volume and average pore diameters were945.04 m2/g,0.535 cm3/g and 2.263 nm,respectively.The characterization results of catalysts showed that the multi-metal nano-catalysts prepared by the sol-gel method were more successful,and the addition of promoter metals improved thegrain sintering problem of monometallic nickel-based catalyst by increasing the specific surface area of the catalyst,improving the dispersion of active components,and enhancing the interaction force between metals.The effects of reaction temperature,metal promoters and its loading amount on the catalytic steam reforming performance of toluene as tar model compound were studied.Comparing the catalytic performance of multi-element metal nano-catalysts for steam reforming of toluene under the optimal reaction conditions,it can be seen that the catalytic activity of the binary metal nano-catalyst of 6%Ni-4%Co/char modified by Co metal promoter was optimal at the reaction temperature of 700℃.At this time,the conversion rate of toluene,gas production,low calorific value of gas,H2 production,H2/CO,H2 selectivity and H2 volume content were 99.59%,1.849Nm3/kg,11.60 MJ/Nm3,111.46 g H2/kg toluene,2.63,80.77%and 67.50%,respectively,followed by 6%Ni-6%Fe/char（at reaction temperature 800℃）and 6%Ni-6%Mg/char（at reaction temperature 700℃）.When 6%Ni-4%Co/char was further used to treat the toluene/naphthalene/phenol mixture under optimal conditions,nearly 100%of the phenol was removed,and the catalytic cracking conversion rates of toluene and naphthalene were 99.47%and 90.91%,respectively.Besides,gas production,H2/CO,and H2 selectivity of 6%Ni-4%Co/char were 4.237 Nm3/kg,3.88and 89.63%,respectively,which were significantly improved compared to 6%Ni/char.The anti-deactivation efficiency and mechanism of the modified binary metal nano-catalyst of 6%Ni-4%Co/char were studied in depth from the aspects of the service life,the amount of carbon deposits,the reduction performance,and the microstructure and types of carbon deposits.The research results of catalyst service life showed that when catalyst of 6%Ni-4%Co/char was recycled third time,the conversion rates of toluene,naphthalene and phenol were 94.30%,85.18%and96.02%,respectively.Besides,there were no significant change in gas components and gas production,and catalyst still maintained high catalytic activity.However,the catalytic activity of the monometallic nanocatalyst of 6%Ni/char was apparently reduced when it was recycled two times.Research on mechanism described that the amount of carbon deposit on the catalyst,the proportion of graphitic carbon in the carbon deposit,and the size of active particles all increased with the increase the times of the catalyst was used.Besides,graphite carbon and grain agglomeration were the main reasons for the deactivation of catalyst carbon deposit and grain sintering respectively.However,the addition of the promoter metal Co promoted the gasification reaction of carbon deposition,increased the specific surface area of the catalyst,improved the dispersion of active particles and enhanced the interaction force between metals to improve the anti-carbon deposition and anti-sintering performance of the monometallic nano-catalyst,but it did not effectively solve the problem of oxidative deactivation of nickel-based catalysts. |
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