【作者】 魏凤；

【导师】 郑楚光； 张军营；

【作者基本信息】 华中科技大学 ， 热能工程， 2005， 博士

【摘要】 燃煤锅炉排放烟气中含有大量的亚微米颗粒,富集了较多的有毒痕量元素,如Hg、As、Se、Cd 和Cr 等,由此所带来的严重环境污染问题已引起了国内外的极大关注。但是由于煤燃烧过程的复杂性、亚微米颗粒生成与排放的介观性、现有除尘装置的局限性,对于燃烧过程中亚微米颗粒的形成机理和有效控制方法的研究一直进展缓慢。因此,对燃烧中亚微米颗粒形成机理及其控制方面开展研究,定量、准确地进行相关的描述,使之能够应用于生产实践,将有助于进一步丰富和完善化石燃料燃烧机理与污染控制理论,对我国能源与环境的可持续发展产生深远的影响。本文首先论述燃煤亚微米颗粒的排放对人类及其居住环境所造成的严重危害,说明了研究亚微米颗粒形成机理和控制方法的重要性和必要性。从对燃煤过程中亚微米颗粒形成机理的研究方面入手,系统综述了国内外在数值模拟和实验研究方面所开展的各项工作,分析和比较相关的实验和模拟研究的优势及其不足; 在分析燃煤电站采用的常规控制方法的基础上,对最新的亚微米颗粒团聚方法(如光、声、电、磁、热等)进行分析,提出采用化学团聚促使亚微米颗粒团聚成较大颗粒后加以清除的方法,并以此作为本文的研究思路,进行系统而深入的研究工作。本文主要从以下几个方面开展研究。本文基于气溶胶动力学理论,模拟了燃烧过程中亚微米颗粒的形成演化过程。通过描述两类亚微米颗粒的形成:自由分子颗粒和链状分子结构的颗粒,系统分析了亚微米颗粒形成过程中成核机理、冷凝机理、凝聚机理的作用特点和规律。通过模型与不同颗粒形成实验结果的对比,充分证实了该模拟能够定量、准确地反映颗粒物的形成演化。这些工作在国内还属于较少开展的领域,该模型的运用对于认识与描述燃烧过程中亚微米颗粒的形成具有十分积极的意义。本文还深入研究了亚微米颗粒富集痕量元素的规律和机理。通过不同煤样的滴管炉燃烧实验,研究和分析了飞灰的主要组成元素、不同挥发性的痕量元素(以砷、铅和铬为例)在不同粒径飞灰颗粒上的富集规律,证实不同粒径颗粒的质量呈双峰分布,砷的含量分布与煤种无关,而与煤中含量、易挥发性密切相关,富集机理主要以化学吸附为主; 铬的分布主要与其赋存形态有关。针对亚微米颗粒富集痕量元素的过程中,亚微米颗粒粒径是否变化的问题,结合亚微米颗粒的形成演化模型,比较系统地分析更多还原

【Abstract】 Plenty of sub-micron particles emitted from coal-fired boiler are enriched with many trace elements, such as Hg, As, Se, Pb, Cd, and Cr, which would do great harm to environment and mankind health。The serious pollutant problem brings the maximum attention of many science researchers. However little information about the formation mechanism of sub-micron particles and its controlling during the combustion methods was reported because of the complexity of combustion course, microcosmic of sub-micron particles formation and limitation of removing devices. Then the paper carried out systemic research about the formation and controlling of the sub-micron particles enriched with trace elements during the coal combustion, which is helpful for perfecting fossil fuel combustion mechanism and pollution controlling theory and has a far-reaching impact on continuing development of energy sources and environment. Firstly, this thesis discusses the serious harm of sub-micron particle emission on human and inhabitation environment, which means it important and necessary to study sub-micron particle formation and controlling method. According to the research work on the formation fundamental theories and control ways of sub-micron particles enriched with trace elements up to date in detail, their advantage and its disadvantages are systematically analyzed about the latest developments in numerical simulation and experiment study on formation theories and control ways (including electricity, lightening, sound, magnetism, heating and so on ) of sub-micron particles. A new way about adopting chemical agglomeration controlling sub-micron from combustion processes is proposed. Based on this idea, the relevant research word follows. Based on aerosol theory, a mathematical model about sub-micron particle formation during combustion is developed successfully and the research is lack in our country. The model describes the formation of two sub-micron particles, including free molecule and chain molecule, analyzes the effect rule of nucleation, condensation and coagulation. Through the contrast of model calculation and particle formation experimental result, the model feasibility has been validated and can rationally reflect the formation rule of sub-micron particles. So the model can direct the formation of sub-micron particles during combustion. In this thesis, synthetic study about the enrichment of trace elements (including As, Cr and Pb) and main elements of fly ash , K and so on) on different diameter of sub-micron particles are carried out. The results through drop coal combustion experiment show that particles mass of different diameter is distributing by two-apex, that As content distribution is independent of coal kinds and is relating with As volatility and content in coal. Cr distribution is main relating with existing form. So the model about the connection of particle diameter and trace element is developed. Through the calculation and validate, calculation is consistent with the test result under the condition of 0 < Kn< ∞, though the connection is not consistent with the weakening volatility of trace element. The study is of meaningful direction for describing the transferring and translation of sub-micron particle enriching with trace element. In order to study the adsorption rule of fly ash composition on As, Al2O3、CaO、MgO and SiO2 are selected and tested about As volatilization and their adsorption to As. At the same time, base on adsorption mechanism and quality diffuse theorya, the model about the adsorption of CaO to As has been developed according to absorb theory and mass pervasion theory. Test result show As vapor can easily react with Cao, MgO and make arsenic-compound. The model can discovery that As gasification rate, translation rate, temperatur, CaO transform rate and CaO particle surface area are important for absorbing As. Both CaO transform rate and CaO particle surface area are first great and after lessening. The model can well direct the study of As adsorption mechanism. A new way about adopting chemical agglomeration controlling sub-micron from combustion processes is proposed to decrease sub-micron particle emission during coal combustion. Three models are developed to study the agglomeration mechanism between sub-micron particles and atomized particles of agglomerator, including particle self-agglomerating physical model without adding agglomerators, agglomerating between ash particles and agglomerator particles, also agglomerating effect forces between ash particles and agglomerator particles, which is proved to be a very useful tool in the process. The first self-agglomerating model results show that particle number, diameter, flow rate and mass are changing with the length of agglomerating tube and the agglomeration is not obvious. The second model results are consistent with the test results and better reflect the effect rules of agglomerator. The effect force model results show that resistance and viscosity force of agglomerator particles have main effect on agglomeration processes, gravitation and elasticity force are in the next place. Finally, In allusion to the issue of efficiently controlling sub-micron particle emissions through agglomeration way in coal combustion, a minitype agglomeration of fly ash stovethat simulated gas flow is made up in order to do experiment researches. The test results show that the agglomerator adding can greatly decrease gas concentration at the exit and that gas flux、gas concentration、agglomerator flux, PH and concentration are the important factors impacting on the agglomeration of sub-micron particle, which proves the way validity. The agglomeration way can not change running operation of boiler and removing device and is of low investment, simple technique and easy operation.更多还原