【作者】 齐永锋；

【导师】 章明川；

【作者基本信息】 上海交通大学 ， 热能工程， 2009， 博士

【摘要】 本文在开发煤粉着火理论预报方法的基础上,应用实验研究与数值模拟相结合的方法,对煤粉再燃过程中极其复杂、具有高度非线性的两种临界现象-挥发分及煤焦还原脱硝临界现象与煤粉着火临界现象的相互作用进行了研究。首次发现了煤粉再燃脱硝效率随过量空气系数变化而显现的突变性质与煤粉燃烧典型的临界现象-“着火”有关,并从理论上阐明了煤粉再燃脱硝与煤粉着火过程相互作用的物理本质,对再燃脱硝效率随过量空气系数发生突跃变化的临界现象作出了合理解释。本文研究成果对煤粉着火及燃烧特性的预报、以及工程应用中确定再燃运行参数有重要现实意义,并为煤粉再燃技术的工业应用提供了坚实的科学指导。本文采用居里点裂解发生器与气相色谱仪研究了神木烟煤、二号烟煤及晋城无烟煤煤粉的快速热解特性。实验发现:煤粉的快速热解失重主要发生在升温阶段,烟煤与无烟煤挥发分中焦油的质量分数均最大,其中烟煤焦油释放量占挥发分的质量分数达到50%以上,高于无烟煤。根据热解产物的释放数据,采用单方程反应模型计算出了热解反应动力学参数。依据这些参数,本文开发了考虑包括焦油在内的具体热解成分的单颗粒煤粉着火过程、传热、传质、化学反应全耦合瞬态精确模型,并对上述三种煤的着火及燃烧过程进行了严格的数值模拟。发现三种煤粉的着火模式均为联合着火,计算结果验证了炭粒着火初期颗粒表面CO火焰所引起的高温,而且给出了挥发分火焰引燃颗粒表面一次反应产物CO的证据。此外,本文还采用该模型计算了煤粉发生联合着火时的能量分配系数。针对全耦合燃烧瞬态模型计算过程复杂、计算时间太长等缺点,本文开发了基于可燃气体着火极限理论的单颗粒煤粉非稳态均相着火简化模型,并采用全耦合瞬态模型对简化模型的计算精度进行了验证。针对三种煤的计算结果表明,简化模型理论假设合理、计算精度较高、计算过程简单、计算时间很短,可以满足工业应用。基于上述简化的均相着火理论,本文开发了适合工程应用的煤粉气流均相着火模型。上述关于模型的工作解决了煤粉燃烧领域中以前一直未能很好解决的一个基础理论问题,研究成果具有学术理论和实际应用的双重意义。最终,本文结合关于炭粒的热力着火模型,开发了再燃条件下煤粉气流发生着火时的临界条件的预报方法。本文设计建立了煤粉再燃实验系统,对煤粉脱硝效率、烟气中主要气体含量和部分残焦随再燃区过量空气系数及停留时间的变化规律进行了实验研究,对再燃区炉膛图像进行了在线采集,并采用上面开发的方法预报了再燃煤粉的着火状态。从理论上说明了煤粉再燃脱硝与煤粉着火过程相互作用的内在本质,阐述了再燃区各运行参数影响脱硝时的内在关联。再燃温度较低时,煤粉脱硝效率及烟气中H2、CO含量曲线随再燃区过量空气系数增大均呈现不规则“M”型的变化规律,煤粉着火状态对脱硝效率有关键影响。当煤粉尚未着火时,少量氧气的存在有利于煤粉的同相脱硝反应,此时煤粉在再燃区的最佳停留时间与烟气中碳氢化合物的消耗速率有关。但当过量空气系数较高以致引起煤粉发生均相着火时,碳氢化合物及其中间产物被氧化反应大量消耗掉,脱硝效率明显降低,煤粉在再燃区的最佳停留时间与煤粉着火时间基本吻合。当再燃区过量空气系数进一步增大时,挥发分燃烧产生的热量加热并引燃煤焦,此时煤粉着火方式为联合着火,颗粒大幅升温,残留挥发分释放及表面氧化反应速率快速上升,有利于煤焦表面CO和自由活性点的生成。NO的异相还原反应速率随颗粒温度升高也会快速上升,此时异相还原反应开始占优,总体脱硝效率上升,在实验研究的停留时间范围内脱硝效率持续上升,此时最佳停留时间的确定应与煤粉燃尽一起来考虑。太高的过量空气系数会导致煤焦燃烧开始受到扩散控制,火焰离开颗粒表面外移,氧气难以到达颗粒表面,不利于煤焦升温和表面自由活性点的生成,异相脱硝作用开始减弱,总体脱硝效率再次下降。较高再燃温度下煤焦颗粒升温对异相脱硝的促进作用不如低温工况明显。再燃温度固定时较粗煤粉发生均相着火及联合着火时的临界过量空气系数下降,导致出现上述现象时的过量空气系数亦有所降低。针对上述研究结论,本文认为,在实际锅炉的大多数再燃工况下,由于煤粉的均相着火,挥发分对NOx的均相还原效用并没有得到充分的发挥。在煤粉再燃初期,应尽量避免发生着火,以使煤粉热解释放出的挥发分能够主要用于对NOx的同相还原反应,而不是被燃烧反应消耗掉;在再燃过程后期,应该使煤焦刚好发生着火,从而促进颗粒升温和异相还原反应的进行。本文最终提出了理想的再燃方案:极少量空气或部分来源于尾部烟道的烟气携带再燃煤粉进入再燃区起始段,该区段过量空气系数很低,从而避免煤粉发生着火,确保同相还原反应效果占优并最大化。在再燃区第二段,注入少量空气,确保起始段形成的煤焦刚好发生非均相着火,从而使得异相脱硝反应效果占优并最大化。采用该方案应该能够同时有效解决“均相着火”对同相脱硝的抑制作用、以及“非均相不着火”对异相脱硝的不利影响。本文研究结论对实际工程应用中确定再燃区各运行参数、实施再燃方案有重要的指导意义。更多还原

【Abstract】 In this paper, based on the theoretical prediction method investigation of homogeneous ignition temperature of pulverized coal, the interaction between two extremely complicated and highly non-linear critical phenomena during pulverized coal reburning, i.e., that on homogeneous and heterogeneous denitration, and that on pulverized coal ignition, was investigated by combining experiment and numerical simulation. For the first time, the abrupt variations of denitration efficiency with excess air coefficient during coal reburning, were found to be relevant with the typical critical phenomenon during coal combustion, i.e., "ignition", the physical essence of the interaction between pulverized coal denitration and ignition was theoretically elucidated, and a reasonable explanation was made on the critical phenomenon about the variation jump of denitration efficiency with excess air coefficient. The investigation results of this paper have important practical significance for the prediction of pulverized coal ignition and combustion characteristics, and the determination of the operating parameters during industrial reburning. Those results also provide a solid scientific guidance for the industrial application of pulverized coal reburning.The fast pyrolysis characteristics of Shenmu bitiminous coal, Erhao bitiminous coal and Jincheng anthracite coal were investigated with curie point pyrolyzer and gas chromatography, and the following experimental results were found. The mass loss of coal pyrolysis mainly occurs in the temperature rising stage, the mass fraction of tar in volatiles is ranked the first for both bituminous coal and anthracite coal, which exceeds 50% for bituminous coal and is higher than that for anthracite coal. According to the release data of pyrolysis products, the single-equation model was used for calculating the kinetic parameters of pyrolysis reactions. Based on these parameters, the exact whole-coupling transient model on single pulverized coal combustion was developed, with various pyrolysis products including tar, heat transfer, mass transfer, and chemical reactions considered, and the igntion and combustion processes of the three coals mentioned above were simulated exactly. The ignition modes of those were found all to be joint heterohomogeneous ignition, the high temperature caused by CO flame on char surface at the beginning of char ignition was validated by simulation results, which also proved that the primary surface reaction product, i.e., CO was ignited by volatile flame. In addition, the energy distribution coefficient during joint heterohomogeneous ignition was also calculated with the above mentioned model.Considering some disadvantages of the whole-coupling transient model, such as too complicated simulation process and too long simulation time, a simplified unsteady homogeneous ignition model of single pulverized coal was developed, based on the ignition limit theory of combustible gas, whose calculation precision was verified with that whole-coupling model. The simulation results of above three coals showed that the theoretical assumptions of the simplified model were reasonable, and it owned high precision, simple process, and short calculation time, which can meet the industrial demand. Based on the above simplified homogeneous ignition theory, the homogeneous ignition model of pulverized coal stream was developed, which can be used in industrial applications. The above investigation about models solves a basic theoretical problem in coal combustion field, which has never been solved well in the past, the research results are much significant for both academic theory and practical application. Ultimately, combined with heterogeneous ignition model (TET) of coal char, the prediction method on critical ignition condition of pulverized coal stream during reburning was developed.In this paper, the experimental system of pulverized coal reburning was designed and established, the variations of denitration efficiency, main gas concentrations in flue gas, and some residual char with excess air coefficient and residence time were investigated, and the on-line images in furnace were also acquired. The model mentioned above was also used to predict the ignition status of reburning coal. The physical essence of the interaction between denitration and ignition during pulverized coal reburning was elucidated, and the intrinsic relationship of the influences of operating parameters on denitration was discussed.At lower reburning temperature, the curves of denitration efficiency, H2, and CO volume fractions in flue gas all present irregular M-shaped variation with increasing excess air coefficient, the ignition status of pulverized coal has key influences on denitration efficiency. When the coal is not ignited yet, the appropriate amount of oxygen is in favor of homogeneous NO reduction, and the optimal residence time of coal in reburning zone is related to the consumption rates of hydrocarbons in fuel gas. When the excess air coefficient is relatively high to cause homogeneous ignition, the hydrocarbons released from pyrolysis and their relevant intermediates important for homogeneous NO reduction are consumed greatly by the volatile flame; then, the denitration efficiency decreases abruptly, and the optimal residence time coincides with the ignition time of volatiles on the whole. At higher excess air coefficient, the coal char is ignited by the volatile flame, and the ignition mode is joint heterohomogeneous ignition mode. The particle temperature rises greatly, the rates of residual volatile release and surface oxidation reaction also increase greatly, which is favorable for the generation of CO and free active sites on char surface. As a result the heterogeneous reducing reactions begin to strengthen and dominate the whole denitration process, and the denitration efficiency increases again. Here, that efficiency increases continuously with prolonging residence time in this experiment, and the determination of optimal residence time should be considered together with the burnout of pulverized coal. At even higher excess air coefficient, the char combustion begins to be controlled by diffusion, the flame begins to move away from char surface, and it is difficult for the oxygen to reach that surface, which goes against char temperature rising and free active site generation. As a result, the heterogeneous reducing reactions are weakened, and the whole denitration efficiency decreases again. At higher reburning temperature the promotion effects of char ignition on denitration are less obvious than those at lower temperature. As to bigger coal particles, the excess air coefficients, at which the above-mentioned phenomena appear, are slightly lower due to their lower ignition temperatures.Aiming at the above results, it is considered that the homogeneous reducing reactions of volatiles do not give their full play due to homogeneous ignition at most industrial reburning conditions. In the initial stage of pulverized coal reburning, the ignition should be avoided as much as possible so that the volatiles are mainly used for NO reduction, instead of consumed by combustion reactions. In the latter stage of reburning, the char ignition should be just ensured, which is in favor of char temperature rising and heterogeneous NO reduction.According to the above analyses, the ideal scheme of pulverized coal reburning is presented and described below. At first little air or part flue gas from tail duct of boiler enters the upstream region of reburning zone with pulverized coal, so that the excess air coefficient in this region is very low, the ignition is avoided, and the homogeneous NO-reduction reactions will maximize and dominate the whole denitration process. Then some air enters the downstream region of reburning zone, the coal char formed in the upstream region will be just ignited, and the heterogeneous NO-reduction reactions will maximize and dominate the whole denitration process. Based on the above scheme, the homogeneous NO-reduction reactions and the heterogeneous NO-reduction reactions will play their roles in two time-stages respectively, which can avoid not only the unfavorable influences of homogeneous ignition on homogeneous NO reduction, but also the unfavorable influences of no ignition on heterogeneous NO reduction. In a word, the above mentioned investigation results have much guiding significance for the determination of operating parameters and the implement of reburning scheme in industry.更多还原