【作者】 梁立刚；

【导师】 吴少华；

【作者基本信息】 哈尔滨工业大学 ， 热能工程， 2008， 博士

【摘要】 城市生活垃圾焚烧在能源发展战略中是一个主要的研究领域。为使垃圾焚烧处理技术真正达到无害化的目的,了解焚烧过程中污染物的产生机理和控制技术是十分必要的。本文研究了城市生活垃圾的焚烧过程,并针对固定床内城市生活垃圾焚烧过程进行了系统的试验研究及数值模拟,为深入认识床层内垃圾的焚烧过程和减少垃圾焚烧过程中污染物的排放奠定了基础,通过本文的研究可以提供更可靠的数据用来设计和优化城市生活垃圾焚烧炉。利用热天平研究城市生活垃圾的焚烧过程,采用FTIR红外光谱仪测得CO2、CO、CH4三种气体在热解过程中的定量析出曲线,并利用积分法计算出热解过程中各气体总的析出质量同时采用Coats-Redfern法,得到了模拟城市生活垃圾焚烧过程中的活化能和指前因子,确定了城市生活垃圾的焚烧表观动力学模型。为了深入了解和研究城市生活垃圾焚烧过程的特性,建立了一维固定床热态实验台。城市生活垃圾试样由蔬菜、细沙和纸板组成,它们具有和真实的城市生活垃圾相似的灰分、水分、固定碳含量。固定床燃烧发生在一个垂直的圆柱体燃烧室,它由称重传感器悬挂着。通过改变一次风量、一次风温和物料本身特性对垃圾床层燃烧的影响规律进行了详细的实验研究。实验研究主要针对焚烧过程中固定床内温度、气氛浓度、床层重量及NO生成特性的变化规律来进行的。同时,利用计算流体力学的方法对层燃炉排上城市垃圾焚烧过程及NO生成特性进行了数值研究,采用一维非稳态模型对床层内气固两相介质分别建立控制方程,确定了水分蒸发、挥发分析出及燃烧和焦炭燃烧反应速率,建立了污染物NO生成及还原和二噁英生成反应简化模型,得出了床层中温度分布、床层重量和高度变化、气相中反应物组分和污染物NO浓度分布,得出了水分蒸发速率、挥发分析出速率、垃圾着火锋面位置及其推进速度等特性参数。实验结果表明:随着一次风量的增加,床层表面CO平均排放浓度是一直降低的,造成C生成CO转化率降低,而N生成NO的转化率是先升高后降低的;随着一次风温度的升高,CO和NO的平均排放浓度逐渐升高,NO最高浓度峰值没有发生明显的变化;随着水分比例的增加,CO2、NO和CO平均排放浓度逐渐减小;CO2、CO的平均排放浓度随着物料中灰分所占比例的增加而减小,NO的平均排放浓度随着灰分所占比例的增加先增大后降低。随着一次风流量的增加,火焰锋面的传播速度和模拟垃圾的焚烧速率先增加后降低,但是火焰锋面的传播速度降到一定的数值后保持不变;随着一次风温的增加火焰平均传播速度略有增加,而在底部物料燃烧火焰传播速度增加较多,同时模拟垃圾的平均燃烧速率和水分蒸发速率有小幅度提高;随着模拟垃圾物料中水分和灰分含量的增加,火焰传播速度和模拟垃圾的焚烧速率是降低的。模拟结果和实验对比表明:模拟床层物料质量和床层高度随时间变化曲线与实验值吻合较好,在燃烧中段,床料质量近似按直线变化,燃烧峰面匀速向下传播,水分蒸发和挥发分析出速率近似为常数;而后期由于剩余焦炭与氧反应速度较慢,燃烧速度减低,质量变化接近水平线。从实验和模拟两方面对床层内温度分布对比分析,整体的分布规律一致,火焰峰面和燃烧最高温度的预报与实验值相吻合,模拟程序准确预报了床层中热量传递和火焰峰面传播过程。在垃圾焚烧的过程中,水分蒸发阶段占据的时间最长,约占整个焚烧实验时间2/3,是影响垃圾焚烧进程的重要因素,当料层温度上升至100℃以上,热解和燃烧区内物料温度梯度明显增大,温升速率迅速提高。床表面气体成分浓度模拟和实验结果对比表明:床层内物料燃烧产生气体总体变化趋势的模拟能够反映床层内物料燃烧过程,且O2和CO2模拟结果与实验值吻合较好。采用De’Soete提出的HCN衰减的总体反应机理可以较为准确的模拟固定床上垃圾焚烧过程中NO的生成,在不同一次风量下的模拟结果与实验结果吻合较好。NO主要来源于燃料型NO,垃圾中氮含量是影响NO生成的主要因素。本文的研究表明,通过实验研究能够更好地掌握和了解垃圾焚烧过程,并为改进垃圾焚烧数值模型和为层燃式垃圾焚烧工程应用提供依据;采用数学模型进行模拟计算,可以深入认识垃圾焚烧的过程,充分了解热量的传递过程,气体生成过程,也可以预测到垃圾焚烧的最高温度和垃圾焚烧过程的趋势。更多还原

【Abstract】 Incineration of municipal solid waste (MSW) is one of the key areas in the global cleaner energy strategy. In order to realize non-hazardous treatment technology of MSW incineration, understanding the generation mechanism and controlling technology are very important during MSW non-hazardous treatment. In this paper, systemic simulation and experimental study on combustion process in a fixed bed are investigated, and the base is established on the study of combustion process in a fixed bed and reducing pollutants emission in MSW incineration. The experiment and simulation results provide direction for design and optimization of the moving grate of MSW.The thermal gravimetric experiments are done to analysis thermal decomposition and combustion characteristics of MSW. The components of pyrolysis gas production are tested with a Fourier Transform Infrared spectrometry (FTIR) to get the fixed quantify Curve of pyrolysis gas(CO2、CO、CH4) emission. The quality of pyrolysis gas is calculated by integral method. According to TG data, kinetics parameters E、A for combustion of MSW samples are calculated by Coats-Redfern method to get the apparent kinetics model for combustion of MSW.In order to reveal the features of the combustion process in the porous bed of waste incinerator, a fixed-bed experimental reactor is employed to reveal the combustion characteristics in simulated municipal solid waste (MSW) beds. Simulated municipal solid waste consists of vegetable, sand, and paper card, with the similar ash, moisture, fixed carbon contents as actual municipal solid waste in China. The reactor shown is a vertical cylindrical combustion chamber suspended from a weighing scale. The effects of primary air, primary air temperature and material characteristics on the burning process of simulated MSW are investigated. Temperature distributions, ignition front velocity, bed weight, characteristics of NO release are measured during the combustion process. At the same time, the combustion process of MSW and characteristics of NO release on the moving grate are studied and simulated by CFD codes. In this paper, one dimensional unsteady batch mathmetics model is also established to simulate combustion process in bed. The mass, momentum, energy conservation equations of two phases in the waste bed are proposed, and the models of moisture evaporation, volatile matter devolatilizaton, char combustion, NOx production and reduction and dioxin formation are simplified and established. Velocity, temperature and gas species distribution in the bed are obtained together with rates of moisture evaporation, volatiles devolatilization and carbon burnout and ignition front position.It is found that the average concentration of CO and the conversion ratio of C to CO are inversely proportional to primary air flow rate; the conversion ratio of N to NO rises as the primary air flow increases and reaches a peak point at the critical air flow rate, after that the conversion ratio of N to NO declines as the air flow further increases; with the increase of primary air temperature, the average concentration of CO and NO, but the highest concentration of NO changes hardly; the average emission concentration of CO and CO2 is inversely proportional to the moisture and the ash level in the fuel; the conversion ratio of N in MSW to NO and average emission concentration NO decrease with the increase of moisture content; with the increase of ash content, the average emission concentration of NO increases firstly, then decreases. At a certain moisture level, the average flame propagation speed and the burning rate increase as the airflow rate increases until a peak point is reached, beyond which further increase in the air flow results in a fall in the average flame propagation speed, but when the flame propagation speed decreases to a certain extent, it keeps constant; when primary air temperature is lower than 100°C, the average flame front increases only a little, but the flame front increases faster near the grate; the total mass loss rate and moisture evaporation rate increase only a little with the increase of primary air temperature; with the increase of moisture and ash content, the flame propagation speed and the burning rate decrease.The simulation results are compared with experimental data, which shows that the incineration process of waste in the fixed bed is reasonably simulated. During the middle of combustion period, bed-mass decreases linear, the front of flame propagates down at a steady velocity. Rate of moisture evaporation and devolatilation are steady at a constant. The simulation results of weight loss, flame front, solid maximum temperature in bed are accordant with experimental data well, which shows that waste burning rate is approximately constant in the middle of incineration process, and moisture evaporation occupies the dominant time fraction of overall incineration experiment. The residual char reacts with O2 slowly so that the crest of bed mass is level approximately. The simulation results are compared with experimental data, which shows that bed temperature versus time at different height from the grate in the fixed bed is reasonably simulated. The simulation can predict the trend about combustion, heat transfer in bed and the propagation of flame front. With heat transferring, fuel goes through the drying, pyrolysis, char combustion and burnout. The combustion process of municipal solid waste is simulated accurately by the model. Due to the high water content of fuel, moisture evaporation consumes a great deal of heat and evaporation takes up the mostly combustion time (about 2/3 of the whole combustion process) so that it has an important effect on the time of the combustion, and evaporation of moisture mainly occurs in the bed temperature below 100℃. The gradient of temperature is steep in the pyrolysis zone and a small thickness of pyrolysis front is found. The results of simulations and experiments are compared and analyzed, which shows that simulation and experiment can reflect the process of waste incineration in bed. At the same time, the simulating results of O2 and CO2 are accordant with experiment results. In the process of NO formation, only fuel NO is considered and modeled by overall reaction mechanism in which HCN is used as intermediate product, which is suggested by De’Soete. With different primary air, the simulating results of NO are accordant with experiment results. Fuel NO is main source of NO in MSW combustion,and the N content is the key to NO formation.Through this experimental investigation, the combustion process of simulated MSW in bed can be better understood and the experiment results can be used to amend the mathematics model and be consulted by the application in project. The results from modeling can show the combustion process, and make us deeply know how the heat transfers in the fuel, gas yields from fuel. At the same time, the simulation can predict the maximum temperature of waste incineration and the trend about combustion.更多还原