| Biomass steam gasification process mainly aims to obtain the high quality syngas. However, the tar is inevitably generated during such process. The generation of the tar is unfavorable to the normal running and even reduce energy conversion efficiency of the gasification system because the tar contained a part of energy in biomass. Steam reforming, which takes the advantages of low operational temperature and high conversion for tar removal, is considered as an effective way to convert the tar into syngas.In this study, a loaded type of catalyst NiO-MoO3/?-Al2O3 was prepared. The NiO-MoO3/?-Al2O3 was further used for the steam reforming of toluene which is a model tar. The effects of Ni and Mo loading, water/carbon ratio, reforming temperature condition on the carbon conversion, gas volume and product distribution were investigated during the steam reforming of toluene. Base on the investigation, the preparation conditions for NiO-MoO3/?-Al2O3 catalyst was obtained. Sawdust was selected as a biomass feedstock for steam gasification, which is performed on a self-design three-stage fixed bed reactor. Base on the gasification results, the effects of temperature, steam/biomass ratio as well as NiO-MoO3/?-Al2O3 catalyst on the volume of syngas, higher heating values of syngas, carbon conversion of feedstock, gas.product distribution were discussed. Moreover, the energy balance of the three-stage fixed bed reactor was analyzed. In addition, the thermodynamic equilibrium of the reforming of gasification derived-tar was investigated by using the minimization of Gibbs free energy method. Corresponding to the the above research content, the work of this study are summerized as follows:(1) In this study, the physico-chemical characteristics, pyrolysis behaviors and apparent kinetic of sawdust feedstock were investigated firstly. According to the ultimate analysis, proximate analysis and calorific value results, the sawdust has a LHV of 16.35kJ/mol and can be characterized high volatiles, low ash content, low sulfur content feedstock, which is suitable for steam gasification. The behaviors and apparent kinetic of sawdust pyrolysis were investigated by using thermogravimetric analysis (TGA). The DTG curves of sawdust pyrolysis can be considered as the overlaps of the DTG curves of hemicelluloses, cellulose and lignin. Based on the overlaps, the pyrolysis behaviors of sawdust can be divided into three stages, each of which respectively covers a conversion range of 0-0.30,0.30-0.80,0.80-1, and mainly is contributed by hemicelluloses, cellulose and lignin decomposition. The apparent activation energy of sawdust pyrolysis is 172.56kJ/mol, with a preferable combined mechanism model of f{a)=(1-a)1.1252a-13369. Subsequently, pyrolytic kinetic of sawdust was investigated by using Fraser-Suzuki deconvolution and combined mechanism method. Result showed that the pyrolysis mechanisms of hemicelluloses and lignin composition presented a n-th order reaction and diffusion, however, the mechanism of cellulose met the nucleation-nuclear growth mechanism very well.(2) NiO-MoO3/?-Al2O3 catalyst was prepared by impregnation method and then was used for the toluene steam reforming experiments. Results showed that the best loading for Ni and Mo were 10% and 15%, respectively. Based on such loading, carbon conversion of toluene steam reforming reached about 99% under the water/toluene ratio of S/C7=9 at 700?. The components of the product mainly are benzene, naphthalene and biphenyl type of hydrocarbons. In the catalyst, two forms of NiMoO4 are the main crystalline phases for the Ni and Mo active components. According to XPS, XRD and ESEM-EDS characterization results, the chemical valences of Ni and Mo were reduced alter steam reforming, which lead to the formations of new crystalline phases of Ni2MO3O8, Mo2C, Ni. The generation of new crytalline phase led to a more dense feature of apparent morphology of catalysts surface.(3) A three-stage fixed bed reactor was designed for the hydrogen-rich gas preparation using steam gasification of sawdust and NiO-MoO3/?-Al2O3 catalyst. Compared with common NiO/?-Al2O3 catalyst, NiO-MoO3/?-Al2O3 catalyst showed better performance on tar yield reduce, syngas production yield improvement, H2 content increase in syngas. According to the calculation, the syngas energy efficiency, energy recovery efficiency and energy consumption ratio of the three-stage fixed bed reactor was 84.26%,86.39% and 2.33, respectively.(4) Base on the minimization of Gibbs free energy method, the thermodynamic equilibrium simulation was perform for the tar (CH1.63O0.47), which is derived from the steam gasification of sawdust Further more, Response Surface Method was used to investigate the influences of water/carbon ratio (S/C) and temperature parameters on the product distribution, dry gas yield during tar steam reforming. Results showed that the main components of syngas of tar steam reforming is H2, which has a linear relationship to the dry gas yield. It denotes that the steam reforming of tar is suitable for the preparation of hydrogen-rich gas. S/C ratio is a important parameter for tar steam reforming. When the S/C ratio is in low level, the high yield of solid carbon is obtained and denotes that low S/C is not conducive to the ascension of carbon conversion. However, high S/C ratio level denotes a steam surplus will happen. The products of tar steam reforming are sensitive to the parameters of S/C ratio, temperature and even the interaction between S/C ratio and temperature. For the purpose of design which aims to obtain a largest dry gas yield, steam reforming conditions for tar may be in the temperature of 694.6 ?, S/C= 1.7. Under such conditions, the optimal dry gas yield can be obtained by 4.758L/g. |
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