【作者】 艾元方；

【导师】 梅炽； 蒋绍坚；

【作者基本信息】 中南大学 ， 热能工程， 2006， 博士

【摘要】 (低氧)高温空气燃烧具有火焰峰值温度降低、温度场均匀、氧化烧损减弱和NO_x排放大幅度降低等技术优势,为低品位或低密度能源燃料(如低热值煤气、低热值煤、生物质和城市生活垃圾)利用提供了一种新途径。深入研究这种燃烧过程物理化学特征和薄壁蓄热特性,可提高控制NO_x和CO_x排放和节能水平,促进社会和经济的可持续发展。论文主要内容和结论包括:(1)搭建了“低氧弥散燃烧”新体系。低氧弥散燃烧是一种以燃气自燃为稳定燃烧条件,反应点弥散且燃烧过程延时可控的燃烧。这种燃烧的热力学特征是均匀燃烧放热和单位燃烧空间燃烧放热减小,动力学特征是反应区弥散。可用弥散度和炉温不均匀度衡量弥散性能。空气高速喷入炉内,炉内气流快速切换,燃气和空气从空间或时间上错开喷入,炉外预先部分或全部稀释等措施可强化弥散性能。(2)提出了一种低氧弥散燃烧数值模拟新方法。引用等效比定压热容可减小因装置散热和多原子气体高温离解引起的仿真误差。选用涡耗散燃烧模型进行燃烧模拟时引用等效比定压热容,能获得和实验基本一致的温度分布,能获得“火焰体积大且边界不清晰,温度分布均匀,燃烧放热均匀,单位反应区体积燃烧放热少”等弥散特征。恰当的等效比热容可通过实际燃烧热平衡测试试算获得。(3)数字仿真和热态实验相结合验证了低氧弥散燃烧动力学条件。低氧燃烧火焰能由传统火焰转变到弥散火焰。两种火焰的分界线受到涡耗散燃烧模型速度阈值影响。涡耗散燃烧模型不能精确预测此分界线。空气含氧浓度越低,燃气流量越大或空气过剩系数越高,弥散燃烧最高炉温就越低。(4)提出了一种求解薄壁蓄热过程的摄动解析—数值计算新方法。基于薄壁假设和单参数摄动法,求出了沿气流流动方向弱固体导热条件下蓄热体温度分布一阶摄动解,解析—数值解和实验及纯数值计算吻合良好。研制了一种新型薄壁蓄热过程数字仿真系统。该仿真系统以空气和烟气进口温度和若干操作条件为信息源,迅速计算出气固温度分布变化以适应蓄热体热过程设计和操控需要。可优化设计出满足“低温端不结露、低氧稳燃、运行经济、无大温度应力和以炉内滞留烟气稀释为主建立燃烧低氧条件”要求的切换周期。(5)用摄动法解析研究了薄壁蓄热体温度效率和切换周期变化规律。存在最大温度效率和相应的最佳切换周期。沿气流流动方向固体导热和单位质量固体蓄热能力不影响温度效率峰值和最佳切换周期。空气含氧浓度或通道内周长降低导致温度效率峰值降低。最大温度效率与通道长度成正比,最佳切换周期与间壁厚度成正比。温度效率解析和弥散燃烧工业实验一致,最佳切换周期和高温气化中间实验基本吻合。对于双预热型薄壁蓄热系统,短的切换周期会明显降低运行经济性,并导致热效率峰值推迟出现。烟气流过过渡管道时间越长,切换周期越短,忽略燃气排放损失的热效率误差越大。更多还原

【Abstract】 High temperature air combustion (with low oxygen-content) has many advantages such as lower flame peak temperature, uniform temperature distribution, weaker oxidation loss and lower NO_x emission, and provides a new way to process the low-grade or low-density fuel such as low-heat-value gas, low-heat-value coal, biomass and municipal live waste. Deeper researches on its physical and chemical characteristics and thin-walled regeneration characteristics can improve the control level on NO_x and CO_x emission and energy saving in the combustion process and promote the sustained development of society and economy.Main contents and specialties of this work are summarized as following:(1) A new combustion system named "dispersion combustion with low oxygen-content" had been constructed. Dispersion combustion with low oxygen-content is a kind of combustion that the reaction spot is dispersed and the combustion process is delayed and controlled with the self-ignition of fuel mixture as the stable combustion condition. Its thermodynamic characteristics are the uniform heat release and the lower heat release per volume of reaction region. Its kinetic characteristic is the flame dispersion. Its dispersion performance can be evaluated by the dispersivity and the furnace-temperature unevenness and intensified by the methods that the air injects into the furnace at a high speed, the gas organization in the furnace is alternatively changed at a high frequency, the fuel gas and the air inject into the furnace from different positions or at different time and the dilution of flue gas partially or fully on the outside of furnace.(2) A new numerical simulation method for dispersion combustion with low oxygen-content had been proposed. The introduction of an equivalent specific heat capacity at constant pressure can decrease the simulation error because of the heat loss from combustion chamber and the decomposition heat of polyatomic gas. The combustion simulation based on eddy dissipation model and equivalent specific heat capacity can achieve temperature profiles that basically agree with the experimental ones and dispersion behaviors such as flat flame, unclear flame boundary, uniform temperature distribution, uniform heat release and lower heat release per volume of reaction region. The suitable equivalent specific heat capacity can obtain through the trial calculation on the actual combustion heat-balance test.(3) Kinetic conditions of dispersion combustion with low oxygen-content had been proved by the digital simulations as well as the hot tests. The combustion flame with low oxygen concentration can be converted from tradition to dispersion. The boundary between two flames is affected by the velocity threshold of eddy dissipation model. Eddy dissipation model can not exactly predict the boundary. The higher the fuel flow rate is, the larger the excess air coefficient is or the lower the oxygen concentration is, the lower the maximal furnace-temperature for dispersion combustion is.(4) A new perturbation analytic and numerical method for the thin-walled regenerative process had been presented. Based on a thin-walled assumption and a single parameter perturbation method, the first order asymptotic solution to the regenerator temperature distribution under weak solid heat conduction along the gas flow direction have been obtained. Analytic and numerical solutions agree well with tests and finite-difference numerical solutions. A new kind of digital simulation software of thin-walled regenerative process has been developed. The inlet temperature of air and flue gas and some operation conditions are taken as the information sources, the software can calculate regenerator temperature rapidly and meet the design and operation-control optimization on regenerative process. The optimization of switch time has been designed according to the demands of "steady low-oxygen-content combustion, no moisture condensation on the low-temperature end, economical operation, any large thermal stress and low oxygen concentration mainly from the dilution of remained furnace gas".(5) The temperature efficiency and the switch time of thin-walled regenerator had been optimized by the perturbation. There exists the maximal temperature efficiency and the corresponding optimal switch time. The solid heat conduction along the flow direction and the regenerator heat storage capacity for the unit volume has no impact on temperature efficiency peak and optimal switch time. The decrease of oxygen concentration in the air or circumferential length in the passage leads to the decrease of temperature efficiency peak. Maximal temperature efficiency is directly proportional to the passage length and optimal switch time is directly proportional to the matrix thickness. Temperature efficiency tendency from semi-analysis is the same as that in dispersion combustion industrial tests, and optimal switch time is basically in agreement with that in high-temperature gasification intermediate tests. As to thin-walled regenerators preheating the air and the fuel, the shorter switch time can result in the lower heat efficiency obviously and its later peak of heat efficiency. The longer time the flue gas passing through transition pipes has or the shorter switch time is, the higher heat efficiency error because of the fuel gas loss has.更多还原