【作者】 黄浩；

【导师】 胡国新；

【作者基本信息】 上海交通大学 ， 工程热物理， 2010， 博士

【摘要】 为了改善能源结构和缓解目前严重的环境污染问题,人们越来越重视清洁能源—氢能的开发和利用。生物质蒸汽气化制氢是富有发展前景的可再生资源制氢技术。本文针对生物质蒸汽气化产氢浓度低、能耗高的缺点,提出了湿生物质定向气化制取高浓度氢气工艺。该工艺将湿生物质干燥、热解、蒸汽气化、高温原位CO2分离集中在一个反应器内进行,利用高温原位CO2分离技术突破反应器内热力学平衡的限制,定向推动蒸汽重整反应向着产氢方向进行,应用湿份干燥产生的自发蒸汽进行蒸汽气化,在获取产氢浓度较高的气化气的同时简化操作、降低能耗,具有较好的经济性。围绕该工艺的构建,本论文开展以下研究工作:(1)自发蒸汽气氛内生物质热解/气化研究。在自制的两套反应器内进行湿生物质热解/气化研究,研究湿度、温度、加热速率、停留时间对产物分布、产气组分分布、碳转化率等参数的影响规律,重点讨论应用自发蒸汽进行蒸汽重整的可行性。结果表明加热速率对应用自发蒸汽进行蒸汽重整具有决定作用。采用高加热速率时,干燥和热解发生在一个相对较短的时间段内,强化了自发蒸汽与热解中间产物(焦油、半焦)的接触频率,从而提高产氢量以及产氢浓度。增加载气流量降低了自发蒸汽、热解挥发份在反应器内的停留时间不利于蒸汽重整。此外,还研究了热解/气化中半焦微晶结构的变化规律。研究表明饱和结构(γ带)的分解集中在573~673K之间。芳香结构的分解贯穿整个反应过程,但不能完全分解,是构成焦炭的主要结构。低温(<573K)时,γ带衍射线增加以及芳香度降低从微晶结构证明了生物质热解初期活性中间态物质的存在。(2)湿生物质定向气化热力学分析及实验研究。运用热力学分析和实验研究两种方法对湿生物质定向气化进行研究。热力学分析表明CaO碳酸化反应能突破热平衡对气相反应的限制,定向推动气相反应向着产氢方向进行。随着[Ca]/[C]、S/B的增加,产氢量、产氢浓度增加。CaO碳酸化反应进行的程度直接关系到定向气化的效果。CO2分压低是制约CaO碳酸化反应的主要因素。实验研究表明湿度、温度、[Ca]/[C]等参数对定向气化过程的影响与热分析结论基本相同。高温强化了生物质、焦油热分解等反应产生较多氢气,但是不利于CaO碳酸化反应。高温时生成的CaO活性较差,这一点在XRD谱图和SEM图片上也有所体现。定向气化操作的最佳温度在923~973K。此外,还发现Ca(OH)2对水煤气反应的促进作用明显大于对甲烷蒸汽重整的影响。(3) CaO吸收剂循环性能改进实验研究。围绕钙基吸收剂不完全转化以及循环吸收能力下降,结合湿生物质定向气化实际操作条件,开展以下研究:定向气化过程中钙基吸收剂孔隙结构变化;钙基吸收剂蒸汽活化和水合活化;煅烧条件对h-CaO吸收能力的影响规律;实际生物质定向气化中的钙基吸收剂循环特性。研究表明Ca(OH)2分解能引起孔隙结构的增加,特别是中孔范围内的孔;CaCO3所造成的孔堵塞主要是针对孔径小于25nm的孔。活化温度对蒸汽活化效果有较大的影响。随着活化温度的升高活化效果逐渐下降,最高活化温度为598K。煅烧条件对Ca(OH)2分解产物的CO2吸收能力有较大的影响。首次发现煅烧初期延长煅烧时间会导致其分解产物的CO2吸收能力大幅度下降。随着温度的增加,吸收能力下降幅度减少。相对于氮气气氛,在蒸汽气氛中煅烧,吸收能力下降幅度较大。在生物质定向气化中采用可蒸汽活化可以对CaO进行有效活化。(4)移动填充床内湿生物质定向气化理论分析。从动力学角度对移动填充床内湿生物质定向气化进行理论研究。建立了系统的湿生物质定向气化动力学模型,对影响生物质定向气化制氢的几个关键参数进行研究,分析气化过程中一次蒸汽、挥发份、二次蒸汽析出规律以及CaO碳酸化反应对气化反应的作用机理。此外,通过理论计算获取反应器设计相关参数也是本章模拟计算的一个目的,以期为生物质定向气化制氢工业化应用提供理论依据。更多还原

【Abstract】 Both the growing awareness of the decreasing availability of fossil fuels and the increasing pressure on the environment from production and combustion of fossil have nowadays led to a deeper interest in sustainable energy generation using biomass. In the past two decades, hydrogen production from steam gasification of biomass has become the subject of extensive research in the field of biomass utilization. However, there are still many obstacles required to resolve until the commercial breakthrough could be obtained, such as low hydrogen content in gas product, high levels of energy consumption. In this thesis, a novel process of directional gasification of wet biomass for hydrogen production was proposed. In the process, the drying of wet biomass, pyrolysis, gasification, steam production and in-situ CO2 capture are occurred in the same reactor. The removing of CO2 by in-situ capture will change the original equilibriums of gas phase and push reactions, such as, steam reforming of carbonaceous fuel, water gas shift reaction, to take place on the direction of hydrogen production to produce additional H2. Furthermore, the steam auto-generated from the drying of wet biomass is utilized as reactant to react with the intermediate product of pyrolysis to produce more hydrogen. As a result, the pre-drying process and the specific steam generation, which involves lots of energy consumption are removed consequently in this process. For developing the novel process, investigations were conducted in this thesis as following:(1) Mechanism study of biomass pyrolysis in an auto-generated steam atmosphere.Hydrogen-rich gas production from pyrolysis of biomass in an auto-generated steam atmosphere was proposed. The scheme aims to utilize steam auto-generated from biomass moisture as a reactant to react with the intermediate products of pyrolysis to produce additional hydrogen. The effects of moisture content, temperature, heating rate, and residence time on product distributions, gas composition, carbon conversion, and other parameters were investigated experimentally. The results show that heating rate is a key role in the process. Under fast-heating conditions, drying and pyrolysis occurred in a relatively shorter time, which enhances the interactions between the auto-generated steam and the intermediate products of pyrolysis and hence produces more hydrogen. The use of sweeping gas is unfavorable to hydrogen production due to the reduced residence time of both the auto-generated steam and the volatile. Moisture content has a great effect on hydrogen production. The H2 yield and content increases with the moisture content. Under the conditions of fast-heating rate and without the use of sweeping gas, the pyrolysis of wet biomass with a moisture content of 47.4% exhibits higher H2 yield of 495 mL/g, H2 content of 38.1 vol%, and carbon conversion efficiency of 87.3% than those (267 mL/g, 26.9 vol%, and 68.2%) from the pyrolysis of the pre-dried biomass with a moisture content of 7.9%, which represents the conventional biomass pyrolysis.Measurement of the atomic structures of chars produced from pyrolysis of wet biomass at different temperature was carried out with XRD. The presented results indicate that most aliphatic chains (γband) are decomposed before 673K during pyrolysis, while only parts of the aromatic system ((002) band) are decomposed, which is the main structure of char. The increase of X-ray intensity ofγband below 573K and the decrease of aromaticity in the same temperature range confirmed the existence of activated intermediate product during pyrolysis in terms of atomic structures.(2) Directional gasification of wet biomass for hydrogen production with in-situ CO2 capture.The directional gasification of wet biomass for hydrogen production with in-situ CO2 capture proposed in the present thesis was investigated with a thermodynamic analysis and experimental studies, respectively. The thermodynamic analysis was done using Aspen Plus software (version 11.1) and the Gibbs energy minimization approach was followed. The analysis data indicate that the directional gasification of wet biomass for hydrogen production proposed in this theis is feasible. The CO2 absorption by CaO plays an important role in the process, which directionally pushes the reactions to take place towards the direction of hydrogen production. High temperature is unfavorable to the CO2 absorption by CaO. It should be noticed that the decrease of CO2 partial pressure resulted from the occurrence of CO2 absorption and/or the dilutedness by excessive steam significantly restrict the CO2 absorption by CaO.The experimental results show that the effects of moisture content, temperature, and [Ca]/[C] on the process is similar with the results of the analysis. The presence of the sorbent greatly promotes both hydrogen production from biomass gasification and CO2 capture. In this process, CaO plays the dual role of catalyst and sorbent. Furthermore, it is noteworthy that the sorbent reveals a stronger effect on the water gas shift reaction than on the steam reforming of methane. The reactor temperature reveals different effects on hydrogen production and CO2 absorption. High temperature favors enhancing the H2 yield while goes against CO2 capture. The negative effect of high temperature on CO2 capture is also proved by XRD spectrum and SEM image. For the novel method, the optimal operating temperature is in the 923–973K range.(3) Study on the improvement of Ca-based sobent in hydrogen production from directional gasification of wet biomass.Aimed to the incomplete conversion and absorption capacity decay of CaO, a series of study were performed. The results show that h-CaO generated from the dehydration of Ca(OH)2 has larger ABET and Vpore, than its former Ca(OH)2. Although the burning of SR sample can realize CaO regeneration, the regenerated CaO with a totally deteriorated pore networks suggests poor reactivity. The pore less than 25 nm in CaO powder are easier be blocked by CaCO3, which will cause incomplete utilization. The effect of steam reactivation gradually decreased as steam reactivation temperature increases. The optimal steam reactivation temperature is 598K. For water reactivation, the increase of water amount will result in a better reactivation effect. A fast decay of absorption capacity during Ca(OH)2 calcination was observed first time. Results show that the CO2 capture capacity decreases significantly as calcination time increases. At 923K, the decay leads to a 27.3% of the CO2 capture capacity loss, while calcination time increases from 2.5 minutes to 5 minutes. High calcination temperature is helpful to weaken the extent of the decay, whereas the presence of steam during calcination makes the decay deteriorated. During the decomposition of Ca(OH)2, an intermediate product was generated, which is of high reactivity but very unstable.(4) Theoretical study on the directional gasification of wet biomass in a moving bed.A mathematical model on the directional gasification of wet biomass in a moving bed was proposed. The drying of wet biomass, the decomposition of biomass and Ca(OH)2, and the CO2 capture by CaO were considered in the model. The model can predict the time-product (first steam, volatile matter, and second steam) released and time-temperature history of feedstock. The result had a good agreement with the experimental results, as could be beneficial for the optimization of reaction parameter and reactor design.更多还原