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生物质热解挥发特性的比较研究
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摘要
生物质能开发利用是当前国内外广泛关注的重大课题,既涉及农业和农村经济,又关系到能源安全。大力开发利用生物质能,既是我国开拓新能源途径,缓解能源供需矛盾的战略措施,也是解决“三农”问题,保证社会经济持续发展的重要任务。随着国际社会对温室气体减排联合行动付之实施,大力开发生物质能源,对于改善我国以化石燃料为主的能源结构,特别是为农村地区因地制宜地提供清洁方便能源,具有十分重要的意义。
     本文首先阐述了发展生物质能源的重要意义,介绍了国内外生物质能源转换和利用技术的研究现状,并重点介绍了生物质能和生物质能资源的概念和特点,从宏观和微观两个方面论述了开发生物质能资源和生物质能利用技术的必要性和现实意义,得出了本文的研究方向、主要内容以及指导意义,说明了具体的研究思路和研究方法。本文在查阅了大量文献资料的基础上,主要采用两种方法在不同设备和工作条件下进行实验研究,分别从慢速热解和快速热解两个方面研究生物质的热解挥发特性和反应机理,并对两者进行比较。
     本研究利用WCT-1C型差热分析仪对玉米秸、花生壳和棉花秆等三种生物质颗粒进行了不同升温速率下的慢速热解实验,得到了它们在不同升温速率下热解挥发百分比和失重速率与时间的关系曲线。由曲线可知,在5℃/min、10℃/min、15℃/min和20℃/min四种升温速率下,玉米秸、花生壳和棉花秆三种生物质的最大挥发百分比分别接近40%、50%和60%;最大失重速率的变化范围分别在8×10-3mg.min-1-20×10-3mg.min-1、8×10-3mg.min-1-30×10-3mg.min-1和8×10-3mg.min-1-35×10-3mg.min-1之间;三种生物质在不同升温速率下达到最大挥发百分比的时间相同,大约分别为60min、35min、20min、15min。由此可知,在程序升温慢速热解条件下,生物质的热解反应主要受升温速率的影响。随着升温速率的提高,失重速率逐渐增大,达到最大挥发百分比的时间逐渐提前。
     本研究对原层流炉热解装置进行了有针对性的改造,延长了反应管的长度,对延长部分采用缠绕电阻丝的方法进行加热,以使反应管内温度尽可能的保持一致,并且增加了热电偶的数量,实现对整个反应管内温度的实时监控。同时,重新设计了喂料器,改善了喂料量和喂料均匀程度。在设备改进和参数优化的基础上,以玉米秸为原料进行了不同温度和不同收集距离条件下的快速热解实验,研究了玉米秸的快速热解挥发特性,测定在快速热解条件下玉米秸的最终挥发百分比是85%。
     本研究在以上两个实验的基础上,对生物质慢速热解与快速热解挥发特性和反应机理进行了比较,发现了两者的不同和联系。实验结果表明,在慢速热解条件下,生物质的热解反应主要受升温速率的影响,升温速率越高,失重速率就越大,达到最大挥发百分比的时间就越短;当加热速率提高到某一定数值之后,生物质的热解挥发百分比就会趋向于达到一个固定不变的值。
Recently, the development and utilization of biomass energy is concerned by the domestic and abroad governments and research institutes, not only involved in the agriculture and rural economy, but also related to the energy security. On the one hand, developing and using biomass energy is the strategic measures to develop new energy sources and alleviate the contradiction between the energy supply and demand in China. On the other hand, it is the important task to solve the "three rural" issue for the sustainable development of Chinese-socio-economic. With the implementation of joint action to pay for the greenhouse gas emissions of the world by the international community, the vigorous development of biomass energy resources is of great significance to improve our energy structure based on fossil fuel, especially to provide clean and convenient energy for the rural areas in local conditions.
     In this paper, the importance of biomass energy development is described firstly. Then, the research status of biomass energy conversion and utilization technologies and the concepts and features focused on biomass resources are introduced in this paper. From the macro and micro aspects, the necessity and significance of the biomass energy resources development and the biomass energy utilization technologies are discussed. At last, the main direction and research method of this study is stated. This study is of the important practical significance to the further research in biomass research field.
     In this paper, based on a large amount of literature and experimental studies, two methods are used to study the volatilization characteristics and reaction mechanism of slow and fast pyrolysis of biomass respectively. The differences between slow and fast biomass pyrolysis are compared in the end.
     In this study, using the WCT-1C type differential thermal analyzer, the slow pyrolysis experiments of corn stalks, peanut shells and cotton stalks particles under different heating rates (5℃/min,10℃/min,15℃/min and 20℃/min) were conducted. The curves of the percentage of pyrolysis volatilization and the rate of weight loss with the time at different heating rates in slow pyrolysis experiments were determined. The curves indicated that, in the temperature programmed conditions, pyrolysis of biomass is mainly affected by heating rate. As the heating rate increased, the time of percentage of maximum pyrolysis volatilization gradually advanced, and the rate of weight loss also increased.
     In this study, the fast pyrolysis device, laminar entrained flow reactor, was improved by extending the length of the reaction tube in order to extend the reaction time of the biomass. The extended tube was heated by an electric heater and controlled by a PID controller, so that the reaction tube temperature could keep consistent as much as possible. More thermocouples were used to realize the real-time monitoring of temperature in the reaction tube. At the same time, the feeder was re-designed and the feeding rate and its uniformity were improved. We obtained a suitable parameter of feeding volume. Corn stalk was used to conduct the fast pyrolysis experiments at different temperatures and different collection distance based on the above improvement and optimization of the device in this study. By the fast pyrolysis experiments, the devolatilization characteristics of biomass fast pyrolysis were studied, and the final percentage of the devolatilization was determined. It will validate and optimize the previous proposed model of the devolatilization characteristics of biomass fast pyrolysis. We measured the final volatilization percentage of corn stalk and came to the conclusion of 85%.
     In this study, based on two experiments in the above, the volatile characteristics and reaction mechanism of the slow pyrolysis and fast pyrolysis of biomass in different conditions was compared. The experimental results showed that, under the conditions of slow pyrolysis, the pyrolysis of biomass is mainly affected by the heating rate. The higher heating rate is accompanied by the greater rate of weight loss and the shorter time of maximum volatilization percentage. When the heating rate increased to a certain value, the pyrolysis of biomass volatilization rate will tend to reach a constant value, too. This value is named as the final volatilization percentage of biomass. The fast pyrolysis of biomass is under a very high heating rates, and the experimental results showed that, in the fast pyrolysis of biomass under the conditions of the high heating rate, the volatile characteristics is no longer affected by the heating rate any more, but only be related to the type of biomass. Different types of biomass will have the different final evaporation percentage.
引文
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