【作者】 徐朝芬；

【导师】 向军；

【作者基本信息】 华中科技大学 ， 热能工程， 2014， 博士

【摘要】 生物质作为一种可再生清洁能源具有硫含量低、二氧化碳“零排放”等特点,已得到越来越多研究者的关注。然而生物质单独作为燃料直接利用(如燃烧)或间接利用(如气化、液化等热转化)还存在诸多问题,如单位体积发热量低、气化液化产品品质较差等,煤与生物质共气化是解决上述问题的有效途径之一。由于煤与生物质共气化的交互作用导致反应过程非常复杂,因此开展煤与生物质共气化反应特性及机理研究,对煤与生物质的洁净高效利用十分必要。本文以我国三种典型煤和三种常见生物质为研究对象,对煤与生物质共热解、共气化特性及其动力学开展研究,探讨转化过程中产物释放,同时对焦炭颗粒的表面形态结构和焦炭碳骨架的演变机制进行了详细分析,揭示了煤与生物质共气化协同反应特性及反应机制。本文主要展开了如下几个方面的工作。研究了三种煤(褐煤、烟煤和贫煤)与三种生物质(木屑、稻秆和玉米秆)及其混合样的共热解特性,采用TGA-FTIR联用技术对共热解产物释放特性进行在线分析,对生物质和煤在转化过程中的协同效应和动力学进行了研究。结果发现褐煤与木屑共热解的挥发分开始析出的温度和最大失重峰温度都明显低于纯褐煤；随着反应温度的升高,生物质对共热解过程的促进作用更加显著；质量掺混比在某一范围内,两者的协同作用最为显著；煤与生物质共热解协同作用对气态产物中CO、CO2和CH4等的影响较为明显,而对含芳环结构化合物的影响程度较弱；利用DAEM法计算了煤与生物质共热解活化能随温度和转化率的变化规律,发现活化能不是简单地随反应的加深而单调递增,而是随转化率变化的一个函数；在相同条件下,共热解的活化能在木屑和褐煤单独热解所需活化能之间,并根据褐煤与木屑掺混反应活化能的变化特征证明协同效应的存在。研究了三种煤和三种生物质及其混合样的水蒸气共气化反应特性。研究发现三种生物质中木屑与褐煤混合物共气化过程中也存在协同效应,并在两者掺混比在一定范围时协同效应最显著；通过比较动力学参数发现褐煤中掺混木屑降低了共气化反应的活化能；煤与生物质共气化协同作用对气态产物中CO释放的影响较为明显。采用氮气等温吸附/脱附分析法、傅里叶红外分析和拉曼光谱分析等对煤与生物质共气化焦的结构进行了详细表征,深入分析了煤与生物质共气化过程中焦颗粒物理化学结构的演化规律。研究发现热解阶段混合焦中孔隙特性表现出随机生长模式；气化阶段气化剂首先在中孔和微孔区域与焦炭上的活性点发生气固反应并形成中孔,共气化焦活性点消耗完毕后气化剂则对焦炭碳骨架进行侵蚀,中孔再逐步汇聚直至碳骨架坍塌。促使共气化反应活性点增加的主要原因是生物质羟基的引入。基于共气化焦理化特性分析,发现温度是控制气化焦炭内部结构转变路径的主要因素,得出整个共气化过程可分为三个阶段：a.600℃以前发生的缩聚反应快于小分子气化反应阶段；b.600℃～900℃之间发生的小分子气化反应为主体的气化阶段；c.900℃以上发生的大分子碳骨架直接被气化消耗阶段。最后在自行设计搭建的小型台架上开展了煤与生物质的水蒸气共气化实验。结果发现随着木屑掺混比例的升高,共气化产气中H2和CO2的含量增大,CO和CH4的含量降低,碳转化率和气化效率随着木屑掺混比例增加而先升后降,并在质量掺混比一定范围时达最大值,协同效应表征参数也在该范围时达最大,表明此时木屑与褐煤的共气化协同效应最显著；不同煤种和生物质种类对共气化气体产率影响明显,其中最低阶的褐煤与木屑共气化协同作用最为明显,两者共气化气体产量最高,碳转化率最大。利用ASPEN PLUS软件建立了煤与生物质小型台架共气化的反应模型,模拟计算结果与实验结果有较好的一致性。本论文通过对煤与生物质共气化的热解段和气化段的协同效应,混合焦结构随气化反应的变化规律,以及在小型台架上进行的煤与生物质共气化等方面较深入的研究,获得了大量的基础数据,对煤与生物质共气化机理有了较深入的认识。为煤与生物质共气化工艺过程的设计与优化提供基础理论指导。更多还原

【Abstract】 Biomass as a renewable clean energy resource has several advantages, such as low sulfur content, CO2neutral and so on. Thus, it has attracted considerable attentions from many researchers. However, direct utilization (eg.combustion) or indirect utilization (eg. gasification, liquefaction etc.) of biomass as a fuel still exist many problems, such as low volume heating value and poor quality of its products. Co-gasification of coal and biomass is one of the technologies to solve these problems. Co-gasification, as one of the co-conversion technologies, is believed to be the most promising technologies for the efficient utilization of coal and biomass. During the process of co-gasification, interactions between coal and biomass may occur, resulting in a very complex reaction process. Thus, this research focuses on the mechanism and the synergistic effect of the co-gasification of coal and biomass. This thesis can be categorized into several parts:The pyrolysis of three kinds of coals (lignite, bituminous coal and lean coal) and three kinds of biomass (sawdust, rice straw and corn straw) and their blends were studied by TGA. The formative products were monitored online by using FTIR contacted to the TGA. The kinetic mechanism and synergistic effect between coal and biomass during the pyrolysis were investigated. The results showed that the temperature at both starting weight loss and maximum weight loss peak of the blends pyrolysis were lower than those of the single coal pyrolysis. Biomass might promote the pyrolysis of coal. The promotion was more significant at higher temperature. The synergistic effect was most significant for the blend pyrolysis with the mass ratio in a certain range. The formations of CO, CO2and CH4gaseous products were affected obviously by blending coal and biomass. The kinetic parameters were calculated by DAEM method. The calculation results showed that the activation energy was not only simply to deepen monotonically increasing with reaction, but also was a function with conversion changing. And the synergistic effect was proved by variety characteristics of reaction activation energy when the coal to biomass blends.The steam gasification of three kinds of coals (lignite, bituminous coal and lean coal) and three kinds of biomass (corn straw, rice straw and sawdust) and their blends were studied by TGA. The kinetic of the gasification and synergistic effect between coal and biomass were investigated. The study found that the synergistic effect was most significant for blend of lignite and sawdust in a certain range. Kinetic calculation showed that the activation energy of lignite was decreased by blending sawdust. The formations of CO were affected obviously by blending coal and biomass.Using nitrogen isothermal adsorption/stripping, and Fourier infrared and Raman spectroscopy, the structures of the chars from single coal and biomass and their blends were analyzed to investigate the evolution of the physical and chemical structure of char during the co-gasification. The study found a random growth pattern during the pyrolysis stage of char pore; the gasifying agent firstly reacts with the active site around the area of mesopore and micropore to form mesopore. After the active site has been completely consumed, the gasifying agent begins to erode carbon skeleton, and the mesopore converges again until the carbon skeleton collapses. The main reason for the increase of active site during co-gasification was the introduction of hydroxyl groups from biomass. Based on the analysis of the physicochemical properties of the chars during co-gasification, the temperature is found to be one of the most significant parameters on controlling the evolution of the char structures. This results that the co-gasification process can be mainly divided into three stages:a) the stage when the poly condensation reaction is faster than micromolecule gasification reaction at temperatures under600℃; b) the stage when the micromolecule gasification dominates at temperatures between600℃-900℃; c) the stage when macromolecule carbon skeleton is gasified directly at temperatures above900℃.Finally, steam co-gasifications of coal, biomass and their blends were carried out on our self-design bench-scale gasification rig. The results showed that with the increase of sawdust mixed ratio, CO2and H2increases, CO, CH4reduces, and the carbon conversion, the gasification efficiency and char gasification activity firstly increases with sawdust mixing ratio and then decreases, reaching a maximum value with the mass ratio in a certain range, at the same time the highest co-gasification synergistic effect between sawdust and lignite occurs. The types of coal and biomass also influence the co-gasification rate obviously. The most obvious co-gasification synergistic effect was found between the lowest-rank coal lignite and sawdust, showing the highest gas production and carbon conversion rate. By using the ASPEN PLUS software, the coal and biomass gasification reaction model on this bench-scale reactor was built, showing good consistency between modeling results and experimental results.In this study, the synergistic effect between coal and biomass during co-pyrolysis and co-gasification, the chemical and physical properties of the char prepared at different pyrolysis condition and the changing of the char property during the gasification, and the gasification of single coal, biomass and their blends conducted on a bench-scale reactor were investigated. A large amount of fundamental and valuable data of the coal, biomass and their blends gasification were obtained and the mechanism of the co-gasification was studied. It is believed that the results obtained by this research can provide theoretical guidance for practical applications of coal and biomass co-gasification.更多还原