| Biomass efficient utilization is always a scientific problem which serious attracts researchers a lot. The main purpose of biomass pyrolysis/gasification is to produce high-quality fuels, but during this conversion process tar and biochar are evitable by-products. Tar will cause kinds of problems to both the operation system and environment; meanwhile, tar also contains parts of the energy which will reduce the overall conversion efficiency of pyrolysis/gasification if not further utilization. Biochar is a kind of porous material which can be used not only good adsorption material, but also excellent catalyst support, and, gaining economical and environmental-friendly benefits.In this paper pine sawdust was used as pyrolysis raw material, and the product of biochar was used as the support of biochar based Fe-Ni catalyst. The effect of biomass catalytic pyrolysis conditions such as reaction temperature, metal loading amounts, the gas residence time and so on, was studied to the effect of products characteristics and gas compositions. Meanwhile, the stability of catalyst was evaluated. Characterization techniques (BET, XRD, SEM, XPS) were adopted to analyze the catalyst before and after pyrolysis reaction, which could further explain the mechanism of biochar based catalyst for biomass catalytic pyrolysis; the effect of different catalysts on tar chemical components was also examined. According above content, the research work of this paper as follows:(1) Pine sawdust from wood processing factory was selected as raw material of pyrolysis in the pilot project of our experiment group and by-product, biochar, was used as the catalyst support. Through the element analysis, industrial analysis and heating value calculation of these two kinds of materials, the results showed that:pine sawdust has high content of volatiles while biochar has high content of fixed carbon; the two main elements in the raw materials composition are C、H and O; biochar from pyrolysis has advanced porous structure with surface area of252.13m2/g; in addition, the low heating value of these two kinds of raw materials are16.52MJ/kg and25.92MJ/kg, respectively.(2) The pyrolysis characteristics and kinetics of pyrolysis of raw materials was studied by thermal analysis method, and the results displyed that:sawdust pyrolysis can be divided into three stages:the first stage is mainly the moisture loss and light volatile emission; the second stage is the main pyrolytic stage, in which large part of volatiles were discomposed with high loss rate; the third stage is the weak decomposition the carbon residue and inorganic compounds. In the kinetic study, single step reaction model was established first, and the kinetics parameters obtained were further used for the distribution multistep reaction activation energy model (Distributed Activation Energy Model, DAEM) to calculate the iterative initial value for the least square method, the results illustrated that:in the conversion rate of between of0.02-0.98, average activation energy of pine sawdust was179.23kJ/mol, which was well fitted to the D3diffusion model. The isoconversion method MKAS can well match with the experimental data, but inevitably ignored the complexity of biomass pyrolysis due to the model was a single step reaction. Distributed activation energy model (DAEM) can avoid this problem by descretting infinite200parallel first-order reactions, and the simulation results in TG and DTG curves are in good agreement with the experimental data.(3) With biochar from pine sawdust as the catalyst support, the preparation of12kinds of biochar based catalyst was prepared by immersion method. At the same time, BET, XRD, SEM-EDS, XPS and other characterization methods were adopted to analysis in the optimal conditions. The results showed that:the surface area of biochar before loading was252.13m2/g, and after loading Fe and Ni, the specific surface area, pore volume and average adsorption diameter decreased, which was evidenced by SEM pictures. The existence of iron and nickel in prepared catalyst was proved by XRD and XPS.(4) By using the two-stage biomass catalytic pyrolysis equipment in laboratory, the influence of catalytic temperature, different metal loading amount and gas residence time to characterization and compostions of gaseous products were researched. And the catalyst before and after reaction under optimal conditions were characterized by BET, XRD, SEM, XPS, etc. The results were:from the analysis of BET and SEM, the amount of pores in catalyst surface after reaction were reduced, which would decreased the adsorption ability; from the analysis of XRD and XPS, the catalyst after reaction was proved the appearance of metal elemental on the surface; gas yield was increased with the rise of temperature, but the tar and biochar production were decreased, with H2, CO content increased and CO2, CH4content decreased in the final gaseous production; when the metal loading amount increased from0to8wt%, gas yield showed first increased and then gradually decreased, while the content of tar and biochar showed a downward trend, and H2content increased with the increase of load amount, but the content of CO reached to the peak value in6wt%; the optimal load amount is6wt%, and Ni/char exhibit relatively good catalytic activity. Gas yield was increased with the increasing of gas residence time, but the peak value of tar yield appeared at the time0.5s. The content of H2also reached the highest at0.5s, and CO showed a slow increase. Catalyst lifetime evaluation was evaluated for8hours under the best condition, H2and CO maintained at a relatively high level over different catalyst, and the sequence of tar removal rate were:Fe-6/char:78.5%, Fe-6-Ni-6/char:85.67%, Ni-6/char:87.15%.(5) The effect of different catalysts (char, Fe-6/char, Fe-6-Ni-6/char, Ni-6/char) to the tar composition was tested at the temperature of800℃and gas residence time0.5s. Elemental analysis, FTIR and GC/MS were carried out to characterize tar, and the result was that the main elements were C and O, the content of S was scarce. After the catalytic reaction of tar over different catalysts (char, Fe-6/char, Fe-6-Ni-6/char, Ni-6/char), C content was gradually increased, while O decreased; FTIR showed that oxygen containing compounds with functional group of O-H were significantly decreased. Wave-number range of690-900cm-1represented aromatic compound had been certain strengthened, and1500-1700cm-1represented PAHs weakened. After the tar catalytic reaction of Ni-6/char catalyst for catalytic degradation, the peak number in FTIR spectrum was significantly reduced; through the GC/MS analysis, after tar catalytic decomposition single ring aromatic hydrocarbons were obvious increase, while oxygen containing compounds, polycyclic aromatic hydrocarbons and heterocyclic decreased. |
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