【作者】 曹凌燕；

【导师】 季生福；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2011， 硕士

【摘要】 在日趋严重的能源危机与环境污染的背景下,清洁能源氢的开发和研究受到了广泛关注。生物质气化制富氢合成气是一项富有前景的制氢技术。我国城市中的落叶等生物质废弃物数量巨大。最大程度的利用这部分生物质资源,在气化介质的氛围下,利用催化剂将这部分废弃生物质转化制氢气,不仅可以缓解环境压力,还能创造能源效益。在一般的生物质气化中,产物中氢气含量较低,并且含有较多的焦油导致生物质气化率较低,限制了气化产物的进一步利用。采用水蒸气作为气化介质和使用催化剂是提高氢气含量并降低产焦量的有效方法。Ni基催化剂是对生物质水蒸气重整具有优良催化性能的催化体系之一。本文以介孔分子筛SBA-15为载体,采用浸渍法制备了Ni质量含量为5%-20(wt)%的Ni/SBA-15系列催化剂以及碱土金属(Mg、Ca、Sr、Ba)和稀土金属(La、Ce)、过渡金属(Co)等不同助剂改性后的Ni/SBA-15催化剂。以城市比较普遍的杨树落叶作为目标生物质,将其破碎成粉末,在常压固定床反应器上考察了温度、水蒸气的添加量、Ni含量、生物质与催化剂添加比,以及助剂等因素对杨树叶生物质水蒸气催化重整制富氢合成气反应性能的影响,采用X射线衍射、N2吸脱附和高分辨透射电镜对催化剂的结构进行了表征。结果表明:Ni/SBA-15系列催化剂中,Ni在载体SBA-15上分散良好,比表面积、孔径分布保持较好。Ni含量为12.5wt%时,催化剂性能最好,产氢最多,在反应温度800℃、水蒸气流量为60 ml.h-1、生物质与催化剂质量比为5：1的条件下,12.5wt%Ni/SB-15催化剂的氢气收率为158.50 mlH2/g生物质,氢气体积含量达到31.08%。对于M/Ni/SBA-15系列催化剂体系,添加Ca助剂的CaO-Ni/SBA-15催化剂显示了优异的催化性能。CaO含量为7.0 wt%时,相同反应条件下1g生物质更是产生了273.30 ml氢气,产物中氢气体积含量达到54.5%。MgO-Ni/SBA-15催化剂在杨树叶制氢上同样表现出了不俗的性能,MgO含量为5.0 wt%时,相同反应条件下1g生物质更是产生了272.85 ml氢气,氢气体积含量达到57.05%。而CaO、MgO的同时加入,使得催化剂表现出了更好的产氢性能。MgO含量为5.0 wt%, CaO含量为4.2 wt%时,1g生物质更是产生了321.16 ml氢气,氢气体积含量超过了60%。CaO、MgO的添加改变了杨树叶产物分配,CO含量明显降低。深入研究表明,CaO在镍基催化剂上呈高度分散状态,保持了介孔分子筛孔道大小。CaO在杨树叶重整制氢反应中起到吸收CO2的作用,在水蒸气过量的条件下,使得水汽变化反应吉布斯自由能降低,反应朝着CO降低、H2增加的方向进行。CaO对CO2的吸收是有限的,本实验CaO添加量为7%的催化剂吸收了30%的C02气体。CaO除了起到C02吸收剂的作用,还充当着提高产氢量的助催化剂的作用。更多还原

【Abstract】 Hydrogen is regarded as the energy for future with its cleanness and high calorific value and has become a focus of renewed interest in many parts of the world. As the fossil energy resource reduces sharply and the pollution of the environment becomes more serious, biomass has been considered as one of the most probable source for hydrogen production. The number of disused biomass proved to be enormous in our national city. Coversion from this part of biomass to hydrogen-rich syngas using an effective catalyst and gasifying agent is thought to be a feasible option. This production pathway is an environmentally clean and economically viable solution which results in scarcely any CO2 emissions with greenhouse effect. Gasfication of biomass often results in heavy tar and low hydrogen yield in gas product. These two notable problems confine the varity use of gas production to application. Stream has been recognized as a effective gasifying agent for tar removal and higher hydrogen yield. It is investigated that Ni-based catalyst has been recognized as an effective catalyst with high activity for catalytic steam reforming from biomass to hydrogen.In this paper, the poplar leaves biomass materials as the feedstock for the steam reforming were collected and ground to powder. The proximate analysis and ultimate analysis of biomass sample were made as well as TGA. A series of Ni-based SBA-15 catalysts with Ni content from 5wt% to 20wt% and the modifications with alkaline earth metal (Mg, Ca, Sr, Ba), rare earth metal (La, Ce) and transition metal(Co) were prepared by impregnation method. A great many factors affected the performance of catalytic steam reforming of the poplar leaves to the hydrogen-rich syngas was investgated in fixed-bed reactor under atmospheric pressure, such as temperature, steam flow, Ni content, mass ratio of biomass and catalyst. The structure of catalysts was characterized by X-ray diffraction (XRD), N2-adsorption/desorption, transmission electron microscopy (TEM).The results indicated that the 12.5wt%Ni/SBA-15 catalyst exhibited the best performance for the catalytic steam reforming of poplar leaves to hydrogen-rich syngas. A maximum hydrogen yield and hydrogen volume fraction among gas products was observed to 158.50 ml H2/g biomass and 31.08% under the optimal reaction conditions that reaction temperature was 800℃, steam flow was 60ml.h-1, biomass/catalyst mass ratio was 5. It could be acquainted with the fact that the prepared molecular sieves SBA-15 maintained a good meso-porous structure and the structure of SBA-15 modified by Ni kept in good condition. The interaction force between active component and carrier was moderate, which promoted the catalystic reforming. Noteworthily, CaO-12.5%Ni/SBA-15 catalysts exhibited the highest performance among the series of M/Ni/SBA-15 catalysts. It was indicated that the CaO content of 7wt% favored the maximum hydrogen production which increased to 273.30 ml H2/g biomass and 54.5% in volume under the same condition. The addition of MgO also largely improved the product distribution and gas quality of poplar leaves. Hydrogen yield reached 272.85ml H2/g biomass and hydrogen volume fraction to 57.05%. Both addition of CaO and MgO promoted the reaction to a finer performance.1 g biomass generated 321.16ml H2. In the meantime, hydrogen volume fraction reached over 60%.The addition of CaO and MgO improved the product distribution and gas quality of poplar leaves, resulting in the decrease of CO. Future investigation suggested that the addition of CaO increased the dispersity of NiO and CaO was in a high degree of disparity. Meanwhile, molecular sieves SBA-15 maintained a good meso-porous structure with the modification of CaO. CO2 was absorbed by CaO and largely lowered in the gas system. In the presence of excessive steam, the free energy of Gibbs of system declined substantially, which played a promotive role in pushing water-gas shift reaction. It changed the original equilibrium of gas phase and pushed water-gas shift reaction to take place on the direction of hydrogen production to produce the additional H2.30% CO2 was absorbed while CaO content increased to 7wt%, which attributed to the limited capability of CaO for CO2 absorption under high temperature. CaO played the role not only of a CO2 sorbent but also that of a catalyst for biomass gasification.更多还原