【作者】 赵青玲；

【导师】 张百良；

【作者基本信息】 河南农业大学 ， 农业生物环境与能源工程， 2007， 博士

【摘要】 随着化石能源的短缺及其带来的环境问题日益严重、秸秆直接燃烧发电技术的推广和普及成为国内外关注的热点。但是秸秆的形成条件及过程与化石能源不同,秸秆具有与其它能源不同的成分和结构,因此,秸秆直接燃烧将给燃烧系统带来了很多问题,其中,以秸秆直接燃烧在锅炉受热面上形成沉积的问题最为严重,它不但影响受热面内的热量传递,而且对受热面造成严重的腐蚀、影响锅炉的操作运行,目前,秸秆燃烧在受热面上形成沉积腐蚀的问题已成为制约秸秆直接燃烧发电技术推广应用的主要障碍。尽管国内外对秸秆燃烧过程中在锅炉受热面上形成沉积的研究取得了一定的进展,但是由于沉积的形成是一个极其复杂的物理化学过程,涉及锅炉原理、燃料及灰渣化学和反应动力学、多相流体力学、传热传质学、燃烧原理与技术、材料科学等众多学科,同时还受灰熔点、灰成分、灰粘度、炉膛热力参数燃烧器的结构与布置炉内空气动力工况锅炉运行参数等因素的影响,加之受到研究手段和研究方法的限制,迄今为止沉积的形成机理及其对受热面腐蚀的原理还未彻底弄清,因此,由沉积引起的锅炉问题依然存在。为了寻求降低沉积腐蚀的方法和措施,消除沉积给锅炉带来的危害,推动秸秆直接燃烧技术的发展,本文在查阅了大量国内外相关文献的基础上,设计了模拟秸秆成型燃料燃烧在锅炉受热面上形成沉积的试验装置,并研究和分析了原料成分、炉膛温度、进风量等因素的变化对沉积形成的影响。结果表明:秸秆中较高的碱金属及氯含量是燃烧秸秆比木材更易在受热面上形成沉积腐蚀的主要原因;秸秆燃烧过程中,碱金属的逸出量随炉膛温度的升高而增加,炉膛温度的升高也有利于碱金属与SiO₂结合生成低熔点的共晶体;进风量的大小对炉膛内的空气动力场,烟气的速度及方向,烟气中飞灰颗粒的运动速度、方向及沉积位置有重要的影响,在秸秆燃烧过程中,受热面上的沉积随风速的增大而提高,但是,当风速超过12m/s时,烟气中含有较多气体组分的飞灰来不及与受热面接触,就随烟气排出,而且初始沉积在受热面上的灰粒也会重新回到烟气中,受热面上的沉积量出现下降趋势;沉积率随着受热面的温度的升高而下降,但是一旦粘性最大的沉积层全面形成时,受热面温度对沉积率的影响就大大降低最终随着沉积物的增长而完全被消除,另外,受热面温度的升高也促进了结垢层的形成;秸秆的组织和结构的变化对沉积的形成有重要影响,经过高压形成的秸秆成型燃料燃烧以后,形成了密实的焦炭骨架,改变了沉积的形成规律,降低了沉积率。为了弄清秸秆燃烧过程中在锅炉受热面上形成沉积的过程和机理,首先,分别对拦火圈内表面和水箱底面上形成的沉积进行了研究和分析,结果表明,炉膛内不同位置,其表面上的沉积是不一样的:拦火圈内表面上的沉积颗粒细腻,反射白光,其成分中没有Si、Al、Fe、Ti等难熔化合物,水箱表面上的沉积颜色为银灰色、颗粒粗大,且Ca、Si、Ti、Mg等矿物质含量较高。然后根据上述分析,对二者的形成过程和机理分别进行了推理,认为:沉积的形成主要是秸秆中的灰分在燃烧过程中的形态变化和输送作用的结果,其形成过程也是灰粒沉积的过程。拦火圈内表面上沉积的形成机制是以凝结和化学反应为主,水箱表面上的沉积主要是通过颗粒撞击和热迁移两种方式形成的。针对沉积引起腐蚀的机理,笔者对受热面上脱落的沉积块（脱落处含有壁面）进行了分析,结果表明:腐蚀过程就是将Fe通过化学反应被逐步转移到沉积中去,使受热面原来的保护膜遭到破坏。在腐蚀过程中氯化物和硫酸盐扮演着重要角色,其机理就是沉积中氯化物穿过氧化层与管壁中的Fe反应生成FeCl₃,硫酸盐与FeCl₃反应生成FeS,降低了FeCl₃的浓度,使反应朝着生成FeCl₃的方向进行。为了对秸秆直接燃烧过程中受热面上沉积腐蚀的危害有一个清醒的认识,同时也为我国研发秸秆直接燃烧技术提供借鉴。本文首先通过试验研究了沉积对受热面换热效率的影响及对受热面的腐蚀作用,然后结合燃烧秸秆的实际锅炉探讨了沉积对锅炉操作的影响,研究表明:秸秆燃烧在锅炉受热面上形成的沉积比燃煤形成的沉积,更能抑制受热面上的热量传递。一般地,燃煤锅炉受热面上积有3mm疏松灰或10mm熔融渣时,才可造成炉膛传热下降40%,秸秆燃烧形成的灰沉积厚度从0mm增加到0.56mm,受热面的传热系数下降了51%;大量的氯富集在沉积中,对锅炉受热面造成了严重的腐蚀,降低了受热面的使用寿命;随着受热面上沉积的增厚,易于引起塌灰等问题,严重影响了锅炉的安全运行,同时沉积的增厚,使得排烟温度的升高,降低了锅炉出力。针对目前各种降低沉积的方法和措施,本文根据它们的特点进行了归类,并一一进行了分析和评价,分析认为目前的这些方法在使用中各有利弊,必须根据实际情况、原则选择合适的方式降低沉积及其引起的腐蚀问题。更多还原

【Abstract】 With fossil fuels exhausting and environment pollution increasingly serious, Generating technology by straw direct combustion is becoming the focus of attention at home and abroad. But the direct combustion of straw in boiler causes a lot of problems to combustion system owing to its special form conditions and process making the composition and structure of Straw different from fossil energy, among them, the deposit and corrosive on the heat-transfer surface is most severe, which not only influences the transfer of heat in the heat-transfer surface, but also seriously corrodes the heat-transfer surface and impacts on the running of boiler. Currently, the deposits and corrosive on the heat-transfer surface caused by the direct combustion of straw has become a main obstacle of the development of generating technology by straw direct combustion.Although at home and abroad the study on deposits on the heat-transfer surface in boiler caused by straw combustion has made some progress, owing to the forming of deposits being a complicated physical chemistry process which comes down to furnace principle, fuel and chemistry and reaction dynamics of ash and slagging, multiphase hydrodynamics、heat and mass transferring、burning principle and technology、material and so on, in addition, which is affected by many factors such as the melting point、elements、viscidity of ash、thermal parameter of hearth、structure of combustion and air dynamical condition in furnace、running parameter of combustion and so on. At the time, current study means is also limited. As a result, the forming process and corrosion principle of deposits is still unclear and the problems caused by deposit still exist.In order to seek the Methods and measures of reducing Corrosion and deposit quantity, eliminate the harm caused by deposit and impel the development of generating technology by straw direct combustion, on the basis of consulting a great deal of correlative Literatures of home and abroad, the author designed a test device to simulation the formation of deposit on the heat-transfer surface caused by the direct combustion of straw, and studied and analyzed the influence of the component change of materials, the temperature change of hearth and the speed change of wind on the formation of deposits and corrosion. The result shows: the main cause that the quantity of deposit during straw combustion higher than that during lumber combustion is the quantity of alkali metals and Cl in straw is higher than that in lumber; The quantity of alkali metals separating from straw fuel increases along with the temperature of hearth rising, which is beneficial to the formation of the low melting point share-crystal by alkali metals and SiO₂; Wind speed influences on the aerodynamic field of hearth, smoke speed and direction, changes the velocity, direction and depositing location of fly ash particles, the upper wend speed is good to the separation of alkali metals and Cl from straw, and is easy to the formation of fouling. But when wend speed exceeds 12m/s, the ash in smoke including a lot of gas compositions has no chance to touch the heat-transfer surface and is directly given out, under the condition, the granule that have adhered to heat-transfer surface renewably return to smoke, as a result, the amount of deposits on the heat-transfer surface declines; The rate of deposits declines with the heat-transfer surface temperature enhancing, when the whole surface of deposits with viscosity is formed, the influence of heat-transfer surface temperature falls, and at last, it is eliminated with the amount of deposits increasing. Besides, with the heat-transfer surface temperature increasing, fouling is going to be appearing in deposits. The organization and structure change of straw has an important influence on the deposits. Compared to raw straw, after SDBF burns, the close-grained coke framework is formed which not only changes the rule of the formation of deposits but also declines the rate of deposits.In order to get to the bottom of process and mechanism of deposit formation on heat-transfer surface during straw combustion, the author first studied and analyzed separately the formation of deposit on the inner surface of barring fire loop and the surface of water tank, the results show: the deposits on the different surface in the hearth is different, the deposit on the inner surface of barring fire loop is made of fine particles and reflects white light, there are not Si、Al、Fe、Ti or other Refractory compounds found in its components, however the deposit on the surface of water tank is silver gray and made of coarse grains, in it, the content of Ca、Si、Ti、Mg and other Minerals is higher. According to above analysis, the author reasoned separately their formation process and mechanism and thought that the formation of deposits is caused by the configuration change and transportation of ash, which is also the depositing process of ash. The main mechanism of deposit on the inner surface of barring fire loop is condensation and chemical reaction and the deposit on the surface of water tank is formed by thermophoresis、particle impaction.To seek the mechanism of corrosion caused by deposit, the author analyzed the deposit shelled from the heat-transfer surface, the result shows that the process of corrosive is also the process of gradually transferring Fe to deposit by chemical reaction, which destroys the original protective film of the heat-transfer surface. During corrosion, chloride and sulphate play an important role, first, chloride goes . through oxide and reacts with Fe to make FeCl₃, then, sulphate reacts with FeCl₃ to make FeS, so that, the concentration of FeCl₃ reduces and the reactor is towards the direction of FeCl₃In order to have a clear understanding of harm caused by deposit and corrosive and to provide some reference to straw direct combustion of China, the paper first studied the influence of deposit on heat transfer efficiency of heat-transfer surface and corrosion on heat-transfer surface by test, then, discussed the influence of deposit on the running of boiler with the actual straw burning boiler. Research shows that the deposits formed by straw combustion more seriously inhibits heat transfer than that formed by limber combustion. Generally, when deposit is 3mm loose ash or 10mm slagging, heat transferred through the heat-transfer surface decreases 40%, however, when the thickness of deposit formed by straw combustion adds from 0mm to 0.56mm, heat transferred by the heat-transfer surface decreases 51%; a great deal of Cl who enriches in deposit erodes seriously heat-transfer surface, reduces the life time of heat-transfer surface; at the time, the deposit impacts on the running of the boiler safely.In view of the current various methods and measures that reduce deposit, this paper classified them according to their characteristic and analyzed and evaluated them separately, by this way, the author thought that these methods in use have their pros and cons, thus a suitable method must be chose from them to reduce deposit according to the actual situation and Principle更多还原