Catalytic Decomposition Of Tar Derived From Biomass Pyrolysis Using LaNi_xFe_1-xO₃ Catalysts

Tar is a by-product of biomass gasification,tar removal can not only save energy,but also has important environmental benefits.Perovskite oxides have many advantages,such as high thermal stability,diverse structure and good regeneration performance.In this paper,perovskite oxides,LaNi_xFe_1-xO_3,were used to study the catalytic decomposition of tar derived from biomass pyrolysis.In order to further explore the mechanism of tar cracking,the kinetic model of tar pyrolysis was studied by using the distributed activation energy model（DAEM）.The main research contents and results are as follows:（1）Perovskite catalysts LaNi_x Fe_1-xO₃ were prepared by the sol-gel method,and the catalytic performance of LaNi_xFe_1-xO₃ was investigated.The results showed that LaNi_xFe_1-x-x O₃ had good catalytic performance for tar cracking.At the same time,the experimental temperature and Ni/Fe ratio have some effects on the total gas production,carbon deposition,tar conversion and catalyst stability.The temperature exhibited significant effect on the total gas production at the range of500°C-800°C,the influences of temperature decreased gradually with the further increase in temperature.At the same temperature,the tar conversion increased with the increasing of Ni/Fe ratio at first,and then decreased.At 800°C,the tar conversion increased from 79.00%for Ni1Fe0 to 94.82%for Ni0.2Fe0.8,and finally decreased to 85.23%for Ni1Fe0.The characterization of the fresh and used catalysts showed that no obvious carbon deposition and sintering were observed for the Ni0.2Fe0.8 catalyst.（2）The experiments of tar steam reforming using LaNi_xFe_1-xO₃ as catalysts were performed at the temperature of 800°C.Compared with the catalytic cracking of tar,introducing steam into the reactor increased remarkable tar conversion and decreased sharply carbon deposition.The total gas production increased rapidly with the steam/carbon ratio（S/C）at the range of 0.5-2.0,and then maintained stable with the S/C increased to 4.0.The values of maximum tar conversion were observed for LaNi_x Fe_1-xO₃ catalysts at S/C=2.0.The Ni0.8Fe0.2 exhibited the best catalytic activity,and the maximum value of increase in total gas production,tar conversion and concentration of H₂ were 3850.6mL,98.52%and 72.02%.respectively.A slight sintering and little change of perovskite structure of Ni0.8Fe0.2 were observed.The Ni0.8Fe0.2 remained higher activity after 8h,and exhibited a lower carbon deposition than that of Ni1Fe0.（3）The mechanism study on thermal cracking kinetics of tar:For n-th order DAEM,different probability density functions were used,different models such as methods of iso-conversional procedure,two-steps model were also compared.The results show that single-step reaction could not explain the kinetic of tar thermal cracking weight loss curve.Two-steps model was investigated,which 1^stt step attributed to diffusions mechanism of water and light organics volatilization,and 2^ndstep followed a reaction order mechanism of f（α）=（1-α）^2.26.DAEM with Rayleigh distribution gave the most suitable agreement with experimental data of tar weight loss.