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生物质催化制氢特性研究
其他题名Study on the characteristics of hydrogen production from biomass catalytic pyrolysis and gasification
吕鹏梅
学位类型学士
导师陈勇
2003
学位授予单位中国科学院广州能源研究所
学位授予地点中国科学院广州能源研究所
学位名称博士
学位专业热能工程
关键词生物质 催化裂解 催化气化 制氢
摘要在人类面临严重的能源危机与环境污染的背景下,世界各国都在致力于对洁净能源氢的开发和研究,并取得了一定的研究成果。生物质气化制氢是一项富有前景的制氢技术,已引起了世界各国研究者的普遍关注。但在一般的生物质空气气化中,燃气中氢气含量较低,且含有较多的焦油,限制了生物质气化气的进一步利用。加入水蒸汽和使用催化剂是2种提高燃气氢气含量并降低焦油产量的有效手段。本研究中,作者首次对生物质快速升温催化裂解特性和动力学模型进行了探讨,取得了重要结论。并设计一小型流化床作为生物质催化气化反应器,创新性地将流态化催化剂和固定床用催化剂联合使用,简化了实验流程,取得了较好的裂解焦油和制取富氢燃气结果。催化裂解是生物质催化气化过程中发生的主要反应,通过对其特性进行研究,为后续的生物质流化床催化气化工艺研究提供指导。首先采用程序升温热重法对生物质的低温催化裂解特性进行了分析。结果表明:以热解方式使生物质析出气体为目的时,温度选取600-700℃较合适;在实际气化过程中,提高热解温度有利于气化效率的提高,适当延长停留时间有利于气相产物的增加;纤维素的热解速率在所研究的物料中是最高的,因而比较适合于进行气化以获取燃料气为目的产物的利用过程;锻烧白云石在600℃以上表现出焦油裂解作用,因而在物料的热解过程中会出现2次较明显的失重峰;镍基催化剂在较低的温度下即可表现出焦油裂解作用,在物料热解失重曲线上观察不到镍基催化剂的单独作用;2种锯末、木质素和纤维素的热裂解或催化裂解过程均可用一级反应动力学来描述;纤维素的热解活化能最高,为142-156kJ/mol,木质素的热解活化能最低,为25kJ/mol左右。根据生物质在流化床中气化受热速率较高的特点,设计了生物质快速升温催化裂解实验系统,提出生物质催化裂解动力学模型,并对快速升温催化裂解特性进行了分析。结果表明:白云石和镍基催化剂均可显著地提高热解气中HZ的含量,镍基催化剂的作用更大,将H2含量提高约一倍左右;镍基催化剂具有强烈地降低热解气中CH4含量的作用,白云石也起到降低热解气中CH4含量的作用,但降低的幅度较小;研究假定的模型综合了三竞争反应模型和二次反应模型的特点,即生物质首先进行三个平行的裂解反应,生成气体、焦炭和焦油,焦油再经二次裂解生成气体和焦炭;模型对于锯末、纤维素和木质素的催化裂解适用比较准确;动力学反应级数n的数值在0.66-1.57之间,用镍基催化剂时,其n值要高于使用锻烧白云石时求出的n值;当白云石应用温度低于800℃时,焦油裂解活化能要高于生物质热解活化能,即白云石的使用温度宜高于800℃。再以流化床为反应器,对生物质空气一水蒸汽气化制取富氢燃气的特性进行了研究,探讨了一些主要工艺条件对气体成分和氢产率的影响,并首次提出了生物质潜在氢产率的概念,对生物质气化制氢的特性进行了较深入的分析。结果表明:较高的反应器温度,适当的空气当量比ER和水蒸汽/生物质比S/B(在本实验研究中分别为0.23,2.02),以及较小的生物质颗粒比较有利于氢的产出;最高的氢产率:719/kg biomass是在反应器温度为900℃,ER为0.22和S忍为2.7的条件下取得的。最后,分别在流化床中使用流态化催化剂白云石和在流化床下游固定床中使用镍基催化剂,对生物质流化床催化气化制氢特性进行了研究。结果表明:在固定床催化反应器出口,H2,CO2含量增加,CH4,CO,C2组分含量减少;气体产物中,H2平均含量超过50vol%,C2组分含量降低到Ivol%以下,CH4转化率接近50vol%;在实验条件范围内,最高气体产率可以达到3.31 Nm3/kg biomass,最高氢产率可达到130.28 gH2/Kg biomass;镍基催化剂对焦油的裂解率在实验条件范围内均达到50%以上,对镍基催化剂350 min的寿命测试表明:较高的温度有利于催化剂寿命的保持;在催化反应器内的氢气转换和焦油裂解均可用一级反应动力学来描述。以上研究成果的取得,实现了对生物质催化裂解和催化气化制取富氢燃气特性较系统的认识,为生物质催化气化制氢工艺提供了一定的理论和实验基础。
其他摘要Under the background of being confronted with serious energy crisis and environment pollution, the world is dedicated in the research and development of hydrogen energy and has got some achievements. The prospective future of hydrogen rich gas from biomass gasification makes it a major concern of researchers all over the world. The technology of biomass air gasification produces a gas with low hydrogen content and quite a lot of tar, which impedes its further application. Steam introduction and catalysts application are two effective methods to increase hydrogen content and decrease tar yield. In this study, an original investigation on the properties of biomass fast catalytic pyrolysis and its kinetics model has been performed and important conclusions are achieved. Then one small-scale fluidized bed is developed to perform biomass catalytic gasification and fluid-cracking catalysts and catalysts used in fixed bed reactor are employed together in the process. This innovative method simplifies the experimental facility and gets good results of cracking tar and producing hydrogen-rich gas. Biomass catalytic pyrolysis is the main reaction that occurs in the process of biomass catalytic gasification. To have a study on its characteristics is helpful to optimize the technique of biomass catalytic gasification in the fluidized bed. By means of thermogravimetry, the properties of biomass catalytic pyrolysis are investigated. The results reveal that a temperature range of 600-700 ℃ is suitable for fuel gas production from biomass pyrolysis. A higher temperature is favorable for higher gasification efficiency and a longer gas residence time benefits more gaseous products. The pyrolysis rate of cellulose is the fastest among the samples tested in this study, from which it can be inferred that it is suitable for the process of gasification aimed at getting gaseous products. Calcined dolomite takes effect on tar cracking when temperature is over 600 ℃ and then 2 weight loss peaks are observed on the thermogravimetry curve of biomass pyrolysis. Nickel-based catalyst has effect on tar cracking even at a rather low temperature and its function can't be observed on the curve of weight loss curve. The catalytic pyrolysis of 2 kinds of wood sawdust, one kind of lignose and cellulose can be modeled by one first-order reaction. The activation energy of cellulose pyrolysis is the highest, being 142-156 kJ/mol, and the one of lignose is the lowest, being about 25 kJ/mol. Then an apparatus for fast pyrolysis of biomass is designed and set up to simulate the fast heating rate in the fluidized bed. The features of biomass fast catalytic pyrolysis in the apparatus are studied, while the kinetics model of biomass catalytic pyrolysis is put forward. The results show that both of calcined dolomite and nickel-based catalyst can elevate the hydrogen content greatly. The nickel-based catalyst has stronger effect and the hydrogen content is nearly doubled. The content of CH4 can be reduced a lot through the use of nickel-based catalysts. Calcined dolomite can also decrease the content of CH4 to a smaller extent. The model is a combination of three-stage model and second-reaction model and it assumes that biomass first decomposes to gaseous products, tars and chars via three competitive reactions and then tars go through a second cracking reaction to produce gases and chars. The proposed model fits well with the calculated data got from pyrolysis tests of wood sawdust, lignose and cellulose. The calculated reaction order of n ranges between 0.66 and 1.57; the value of n calculated from biomass pyrolysis with nickel-based catalyst is larger than the one calculated from the use of calcined dolomite. When dolomite is used with temperature being lower than 800 ℃, the activation energy of tar cracking is higher than biomass pyrolysis. Therefore it is concluded that calcined dolomite needs to be used at a temperature higher than 800 ℃. And then the characteristics of hydrogen rich gas production from biomass air-steam gasification in a fluidized bed are studied. The concept of Hydrogen Yield Potential is put forward to have a deeper analysis on hydrogen production from biomass. From the experimental results, it can be seen that higher reactor temperature, proper equivalence ratio (ER) and steam to biomass ratio (S/B), being 0.23 and 2.02 respectively in the current study, is more favorable for hydrogen production. The smaller biomass particle size will also contribute to more hydrogen yield. The highest hydrogen yield, 71 g Ha/kg biomass (wet basis), is achieved at a reactor temperature of 900 ℃, ER of 0.22, S/B of 2.70. At last, with use of dolomite in the fluidized-bed gasifier and a use of nickel-based catalysts in the fixed bed reactor downstream the gasifier, the properties of hydrogen yield from biomass are investigated. At the exit of the catalytic reactor, the content of H2 and CO2 is increased and the content of CH4, CO and C2 is decreased. In the gaseous products, the average content of H2 exceeds over 50 vol %; C2 content is lowered to below 1 vol % and nearly half of CH4 is converted after the catalytic reactor. Over the ranges of experimental conditions examined, the highest gas yield reaches 3.31 Nm3/kg biomass, wet basis; the maximum hydrogen yield gets to 130.28 g H2/kg biomass, wet basis. The tar cracked by nickel-based catalysts is over 50 wt %. The lifetime test of 350 minutes for nickel-based catalyst shows that a higher temperature will extend its lifetime. The hydrogen conversion and tar cracking in the catalytic reactor can be described by one first-order reaction model. The above study provides a quite systematic understanding on the properties of hydrogen-rich gas production from biomass catalytic pyrolysis and gasification. It supplies some fundamental idea on the theory and experiments of hydrogen production from biomass.
页数142
语种中文
文献类型学位论文
条目标识符http://ir.giec.ac.cn/handle/344007/3875
专题中国科学院广州能源研究所
生物质能源生化转化实验室
推荐引用方式
GB/T 7714
吕鹏梅. 生物质催化制氢特性研究[D]. 中国科学院广州能源研究所. 中国科学院广州能源研究所,2003.
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