【作者】 于敦喜；

【导师】 徐明厚；

【作者基本信息】 华中科技大学 ， 热能工程， 2007， 博士

【摘要】 颗粒物(Particulate Matter,PM)是我国城市空气的首要污染物,对人体健康和生态环境造成了严重危害。燃煤过程是大气颗粒物的重要来源,其贡献已超过33%,对燃煤颗粒物的深入研究是进行颗粒物危害评价、法规制定和有效控制的重要基础。国内外学者虽然在此方面已经开展了大量研究,取得了较大进展,但是相关理论还远未完善。一方面,传统的基于质量或体积粒径分布的颗粒物模态识别方法往往不能有效识别颗粒物中间模态,并且不能准确定义各模态的粒径范围,因此难以深入揭示颗粒物的形成机理,而且常常存在研究结果不一致的情况;另一方面,由于常用体分析(bulk analysis)技术的局限性,对燃煤颗粒物形成机理的大多数研究都还只能停留在定性的层面,因此阻碍了认知水平的提高。本论文研究的主要目标在于开发更为有效的颗粒物模态识别方法,验证燃煤颗粒物三模态分布的合理性,并基于此颗粒物模态识别方法,对各模态(尤其是中间模态)颗粒物的形成机理及其影响因素进行定性揭示;同时,利用逐粒统计分析技术对颗粒物的形成机理进行更深入的定量揭示,从而为燃煤颗粒物控制技术的研究与开发奠定基础。本论文取得的主要研究成果包括:(1)提出了全新的基于Al元素粒径分布的颗粒物模态识别方法,利用该方法对实验室沉降炉和电厂锅炉燃煤颗粒物的形成模态进行了研究,证明燃煤颗粒物实际上均呈三模态(超细模态、中间模态和粗模态)分布,而非传统的双模态(超细模态和粗模态)分布。研究表明,基于Al元素粒径分布的颗粒物模态识别方法不仅能有效识别颗粒物模态的数目,而且能更为准确地定义各模态的粒径范围,比传统的基于质量或体积粒径分布的颗粒物模态识别方法更为合理有效,因此具有十分重要的理论价值。(2)利用基于Al元素粒径分布的颗粒物模态识别方法对各模态颗粒物的形成机理进行了研究,结果表明,燃煤颗粒物超细模态和粗模态的形成可用已有的理论进行解释,即超细模态主要由无机物的气化-凝结形成,粗模态主要是熔融矿物聚合的结果,而对中间模态的形成还没有现成的理论。本论文根据中间模态颗粒物中Al元素的分布规律,首次提出气态组分在细小残灰颗粒上的异相冷凝/反应是其主要形成机理;并通过系统的实验研究深入揭示了煤粒破碎、煤中细小颗粒以及外在矿物颗粒的燃烧转化在中间模态颗粒物形成中的重要作用,不仅提升了对PM2.5形成机理的认识,而且有助于PM2.5控制技术的研究与开发。(3)鉴于传统的基于质量或体积粒径分布的颗粒物模态识别方法往往不能有效识别颗粒物中间模态、不能准确定义各模态粒径范围,利用基于Al元素粒径分布的颗粒物模态识别方法对颗粒物三模态进行了详细表征,准确定义了各模态的粒径范围;并在此基础上,系统、深入地揭示了燃烧温度、氧含量和煤粉粒径对超细模态、中间模态、PM2.5和PM10生成的影响规律,对于燃烧过程中颗粒物生成的预测具有重要的参考价值。(4)针对传统体分析技术难以开展定量研究的局限性,利用逐粒统计分析技术(即CCSEM技术)对矿物分布、转化行为以及颗粒物的形成机理进行了详细、定量揭示。研究表明,煤中矿物的含量、内/外分布和粒径分布等都具有高度非均一性特点,不同种类的矿物具有不同的转化行为。熔融矿物的聚合是导致颗粒物粒径增大的主要原因,黄铁矿和部分碳酸盐的破碎是细微颗粒物的重要来源。杂质元素的存在可促进硅酸盐矿物的熔融和聚合,有利于粗颗粒的生成;硅酸盐化是大多数矿物和无机元素向颗粒物转化的主要途径。更多还原

【Abstract】 Particulate matter (PM) is the dominant pollutant in most Chinese cities, and has brought about serious impacts on human health and the ecological environment. Coal combustion is the major source of PM in Chinese ambient air and accounts for more than 33% in PM mass. The knowledge of PM formation and emission contributes a lot to the risk assessment of PM pollution, legislation and emission control. Many investigators have carried out a number of studies and much progress has been achieved. However, it is still far from complete at present. On one hand, the conventional method for particle mode identification, based on mass or volume size distributions, is often unable to effectively identify the newly-found central particle mode and accurately define the particle mode size range. Therefore, particle formation mechanisms have not yet been demonstrated clearly and some conflicting results might be obtained. On the other hand, particle formation mechanisms can only be qualitatively studied in most literature due to the limitations of bulk analysis techniques often used, which cannot provide an insight into the problems of particle formation.The first objective of this dissertation is to develop an effective method for particle mode identification and verify the tri-modal fly ash particle size distribution reported recently. Second, the formation mechanisms of different particle modes (especially the central mode) and the influence factors are to be studied qualitatively based on the developed method for particle mode identification. Third, an advanced CCSEM technique, rather than conventional bulk analysis techniques, is used to quantitatively investigate particle formation mechanisms during coal combustion. The key findings of this dissertation are as follows:(1) A new method based on the size distribution of the aluminum (Al) is developed for particle mode identification, and used to characterize particulate matter generated from a laboratory drop tube furnace and two real coal-fired boilers. All combustion particle samples are proven to have three modes (the ultrafine, central and coarse mode), rather than just mere two (the ultrafine and coarse mode). By the developed particle mode identification method, not only can the three particle modes be identified, but also can their size ranges be accurately defined. It is shown that, this method is more effective and reasonable than the conventional particle mode identification method based on mass or volume size distributions.(2) By the developed particle mode identification method based on the size distribution of the Al, it is found that the formation of the ultrafine and coarse modes can be well explained by established theories, i.e., the ultrafine mode is generated by vaporization-condensation processes, while the coarse mode is the result of coalescence of molten minerals. However, the formation of the newly found central mode can not be interpreted by existing theories. This dissertation first suggests that the central mode is formed by the heterogeneous condensation or reaction of vaporized species on the surfaces of fine residual ash particles. A systematic study on the origins of the inner cores of the central mode shows that, coal particle fragmentation, the transformation of small particles and excluded minerals present in the original coal are important sources of the central mode.(3) Rather than the conventional particle mode identification method based on mass or volume size distributions, the developed particle mode identification method based on the size distribution of the Al is first used to characterize the three particle modes and their size ranges. Based on these results, the influences of combustion temperature, oxygen concentration and coal particle size on particle formation are further investigated. An important result is that, the formation of the ultrafine mode, the central mode, PM2.5 and PM10 can be enhanced by increasing combustion temperature, oxygen concentration and decreasing coal particle size under most conditions. (4) Due to the limitations of the conventional bulk analysis techniques, an advanced CCSEM technique is used to quantitatively investigate the heterogeneous distribution of minerals in coal, their transformation and PM formation. The highly heterogeneous nature of coal minerals and their transformation behavior are characterized in detailed. The results show that, coalescence of molten mineral particles leads to the increase in PM size, while fragmentation of pyrite and some carbonates during combustion is an important source of fine PM. Clay minerals in coal have a complex composition and contain some impurities, which result in melting and coalescence of alumino-silicate minerals at lower temperatures and formation of larger PM. Most minerals and elements are transformed into PM through reactions with alumino-silicates.更多还原