【作者】 杨光；

【导师】 廖艳芬；

【作者基本信息】 华南理工大学 ， 工程热物理， 2012， 硕士

【摘要】 生物质能源是一种储量大、可再生的洁净能源，有着多种转化利用方式，为缓解当今世界日益严峻的能源问题与环境压力提供了一条有利途径。在众多生物质热化学利用方式中生物质燃烧是一种重要的途径，同时生物质高效燃烧利用影响因素也得到了越来越多的关注，而碱金属/碱土金属作为生物质内富含的元素，在燃烧利用过程中可能存在床料聚团、受热面沾污腐蚀、结渣以及沉积等多种问题，成为阻碍生物质燃料广泛利用的主要因素之一。因此了解碱金属在燃烧过程中的行为及影响，掌握其析出和转化的规律，对解决碱金属问题具有极其重要的意义。本文研究重点在生物质燃烧过程中碱金属的迁移。在大量文献综述与对比分析的基础上，归纳了国内外在生物质能能源化利用过程中碱金属相关问题的研究成果。通过管式炉燃烧试验，结合FactSage化学热力平衡软件模拟生物质燃烧过程中碱金属的迁移过程，找到生物质燃烧过程中碱金属迁移规律，同时研究了碱金属对生物质燃烧的催化影响，为减轻生物质利用过程中碱金属问题提供理论依据。通过生物质燃烧热重实验，探讨研究不同气氛、升温速率及催化剂对生物质物质催化燃烧的影响。分析表明高升温率将加重热滞后现象，并且富氧对后段固定碳的燃烧促进作用有限。催化剂能使燃烧速率增加、着火点提前，有利于燃烧的进行和充分燃烧。添加催化剂后样品燃烧活化能降低主要发生在转率α小于0.6区间，CuO的效果最明显，最大下降值为66.41Kj/mol，添加剂降低活化能主要是由于对挥发份的析出和燃烧催化作用。同时催化剂对于固定碳燃烧的催化作用不大，催化剂能够促使固定碳开始燃烧，能将固定碳燃烧提前至与挥发份燃烧大时间段重合。通过秸秆管式炉燃烧实验，研究秸秆基本燃烧特性，检测燃烧残留物中碱金属分布。分析表明，秸秆中固定碳和挥发份的两种不同的燃烧特性，挥发份具有非常快燃烧速度，固定碳的燃烧相对较慢，而且温度越低，固定碳燃烧越向后迟延，可以通过控制反应器温度来实现生物质秸秆低温气相燃烧和高温固相燃烧的分离。低温段或者燃烧初始阶段，钾元素析出规律与挥发份析出接近，研究认为热稳定性差的有机钾是随生物质挥发份快速析出；而高温阶段，钾释放速度明显减慢，主要是因为蒸汽压的升高促使钾元素以无机盐蒸汽的形式逃逸。燃烧残留物分析表明，燃烧残留物中碱氯摩尔比大于1，并且是随着温度的提高出现上升趋势，表明氯具有比碱金属更高的挥发性。同时XRD分析表明，KCl和K2SO4是生物质秸秆燃烧残渣中碱金属主要存在形式，并且生物质秸秆燃烧过程中的碱金属的蒸发可能主要是以KCl和K2SO4蒸汽的形式向气相中迁移，并以KCl迁移为主。FactSage模拟可从热平衡角度计算秸秆燃烧过程中可能的燃烧产物及其分布。结果表明，低温有利于钾元素以水溶盐的形式固留在生物质残留物中，低温下钾K主要以KCl（s）、K₂SO₄（s）、K₂Si₄O₉（s）固态形式存在，随温度的升高，由于KCl蒸汽压的增加，KCl（s）向KCl（g）、（KCl）₂（g）和KCl（salat）转化。同时，生物质内部高硅含量或者认为掺混硅添加剂将有利于抑制气态K的释放，将碱金属固留在残渣中，并且在650～800℃温度范围内K2SO4与KCl之间的低温共熔现象对于低温下的粘结也有部分贡献。变工况模拟结果表明：随着氧浓度的提高，将有利于减弱中温段（低于750℃）碱金属向气相迁移，并且能够降低腐蚀性气体HCl生成，有利于减轻受热热面的积灰、结渣和腐蚀问题。同时由于高温度段气相氯含量变化不大，故高温段提高秸秆燃烧气氛氧浓度对于解决腐蚀问题没有特别重要的意义。更多还原

【Abstract】 The utilization of biomass is very helpful for alleviating pressure from energy shortageand pollution in the world. Combustion is one important way of the biomass utilization. Theinfluence factors of the combustion are attracting more and more attention. Among thesefactors, as the important contents of the alkali metals affects the combustion of biomass verymuch. Potassium, always brings agglomeration, corrosion and deposition on the heatingsurfaces in combustion system, and becomes the main reason which limits the utilization ofstraw in large scale. Consequently, to better understand the alkali behaviors during thecombustion of agricultural residue, and seize the law of alkali emission and transformation isof great importance to solve the alkali problems.This paper pays some attention on the transformation behavior of alkali in biomasscombustion process. And base on the comparative analysis of extensive literatures, issuesrelated to alkali metal during the energy utilization process of biomass in the world, also beenmade a comprehensive discussion. The equilibrium analysis software FactSage, is used todetermine the transformation behavior of alkali, and combined with the combustionexperiments, the alkali behavior characteristics of alkali are be revealed. In order to provide atheoretical basis for reducing the alkali metal problem, the characteristic of straw catalyticcombustion also been studied.The catalytic combustion characteristics of biomass straw under conditions of differentatmosphere, heating rate and catalyst are studied on the thermal analyzer. Combustionparameters such as ignition temperature, peak temperature of maximum weight loss rate,burnout temperature, maximum rate of combustion, and combustibility index were analyzed.The result shows that: high-heating rate contributes to the thermal hysteresis. And theoxygen-enriched atmosphere also can enhance the fixed-carbon combustion, while thecontribution of it is limited.Combustion rate increased by adding catalyst, and ignition temperature reduced, soadding catalyst is conducive to reduce ignition. The kinetic parameters are obtained bynon-linear dynamic model. The activation energy reduced mainly below α <0.6when catalystis added, adding CuO is better than other ones, the maximum reduction is66.41Kj/mol, socatalysts reduce the activation energy is mainly due to the volatile combustion catalysis. Thecontribution of catalyst to fixed-carbon combustion is limited, while the fixed-carboncombustion begins earlier during straw combustion because of the catalyst, it means there isoverlapping on time among volatilization and fixed-carbon combustion process. By combustion experiments in fixed-bed reactor, the influence of combustiontemperature and time on devolatilization, char combustion, potassium transformation andevolution behaviors were investigated. The result shows that: volatile components of strawhave rapid devolatilization and good combustion features, while the residual char burns outrelatively slowly. At lower temperatures there is a greater backward delay for char combustion.Therefore, the gas phase combustion and char combustion of the straw biomass can beseparated by controlling the reactor temperature.Under low combustion conditions or during the initial burning stage, the evolution ofpotassium is associated with decomposition of the organic structure, while the emissionsunder high temperature conditions correspond to the evaporation of inorganic potassium saltas a result of the increased vapor pressure. The ash residue analysis shows the molar ratio of（K+Na）/Cl increased with temperature, indicating that chlorine in the straw has highervolatility than alkali metals under the combustion conditions. XRD analysis revealed that KCland K2SO4were the main compounds associated with potassium in combustion residuals, andKCl was shown to play a dominant role in potassium evolution, while the contribution ofK2SO4is limited.The equilibrium analysis software FactSage, is used to simulate thermodynamics state ofthe transformation behavior of alkali during the straw combustion. The result shows that: thewater soluble K+may react with anions that have low thermal stability, forming inorganicsalts such as KCl（s）、K₂SO₄（s）、K₂Si₄O₉（s） at low temperature. The amounts of KCl（s）decreased with increasing temperature. The rapid evolution of KCl（s） began at about600oC,and it was transformed into KCl（g）,（KCl）₂（g）, KCl（salat）, and K₂O（slagd）, which indicatesthat combustion should be conducted at less than600oC to retain potassium within theresidual straw. At the same time, high silica content in biomass would restrain the release ofgaseous K. However, more agglomeration of straw ash may occur in such cases, and theeutectic phenomenon appears between KCl and K₂SO₄leading to formation of a eutecticmixture, which also contributes to the slagging problem that was observed at themid-temperature range （670～800oC）.Variable conditions simulation result shows that: the evolution of alkali element wouldbe reduced mid-temperature range （below750oC） with the increasing of the oxygenconcentration. And, it also able to reduce the mount of the corrosive gas HCl, that would beconducive to reduce the problem of deposition, fouling, slagging and agglomeration.Improving the oxygen concentration for straw combustion had no particular importance for solving corrosion problems, at high-temperature range, but would help alleviate thedeposition problem on heating surface.更多还原