低二次风量条件下强化浓缩型旋流低NOx燃烧机理研究

Swirl combustion technology based on the staged combustion will have widely future application in the coal-fired boiler. When the slump in secondary air flow of the swirl burner (The secondary air flow decreases of 42.8% when the over fire air rate is 30%), However, operational results revealed that furnaces suffer similarly from various problems such as late coal ignition, poor combustion stability, low burnout and high NOx emissions. To find out a solution for improving the status, an enhanced-concentrated type low NOx combustion concept is proposed in this research program. Accordingly, cold air experiments using a pulverized-coal swirl burner model was performed within a laboratory-scale facility to study the effects of the structural parameters of the burner and the air dynamical parameters on the distribution of jet velocity vector, and turbulent intensity and the particle concentration distribution in the combustion zone under the strong swirling turbulent flow. Based on the analysis of the characteristics of the central recirculation zone and the trajectory of particle flow, the mechanism of controlling the mixing and diffusion of the gas flow is determined, and the method to realize the orderly flow of particles is explored. The distribution of gas temperature and concentrations of gaseous species and char physical structure and element is measured in a 0.5MW pulverized coal-fired furnace with air-staging with different secondary air flow, swirl number and concentration ratio. The influence mechanism of the particle dispersion process under the strong swirling turbulent flow on the NO formation was revealed based on the analyzing of the effect of CO on the formation of NO. These investigations are accepted to establish a reasonable parameter setup for enhanced-concentrated type low NOx burner. The results of this study will provide important basic data and methods for the swirl combustion technology.

基于分级燃烧的旋流燃烧技术在燃煤锅炉具有广阔应用前景。但当燃烧器二次风风量大幅降低（燃尽风率为30%时，二次风量降幅达42.8%）时，面临着火晚、燃烧稳定性差、燃尽差和NOx排放高等问题。为较好解决这些问题，本项目提出强化浓缩型旋流低NOx燃烧思想，拟借助冷态实验研究低二次风量下强化浓缩型旋流燃烧器射流速度和湍流强度的变化规律和强旋湍流对燃烧区间颗粒浓度分布的影响规律；通过分析回流区特征及颗粒流动轨迹，确定控制气流混合和颗粒扩散机制，探索实现颗粒有序流动的方法。利用0.5MW热态实验台研究低二次风风量下强化浓缩型旋流燃烧器温度场、气氛场和煤焦物理结构的分布规律，分析其对NO催化还原和CO催化氧化的影响，揭示强旋湍流在真实燃烧条件下颗粒弥散过程与NO生成的关系,实现低二次风量下高效低NOx强化浓缩型旋流燃烧器参数设计。研究结果将为低二次风量下旋流燃烧技术设计提供基础数据和方法。

当炉内空气分级技术与旋流燃烧器结合后，旋流燃烧器着火及燃烧稳定性性能可能变差。本项目以某电厂600 MW机组墙式对冲锅炉所配套旋流煤粉燃烧器和燃尽风为原型，分别建立冷态燃烧器及燃尽风模型，探索影响低二次风量条件下燃烧器出口回流区形成关键因素及一、二次风混合特性；同时，对燃尽风出口射流流动特性进行测量；尔后，通过数值模拟计算研究不同一次风结构对流动、燃烧及NOx生成特性的影响；最后，在实际锅炉上进行研究不同煤粉浓度对燃烧及NOx生成特性的影响。.1）通过对燃烧器扩口结构研究发现：一、二次风扩口结构对回流区影响较大；一次风扩口结构对回流区的形成影响较小，但是影响一次风扩散及颗粒流动轨迹的关键因素；单纯改变一次风扩口位置，在实际锅炉上经测试，锅炉NOx排放量由344mg/m3降低至313mg/m3。.2）低二次风量条件关乎燃尽风率，通过气固两相流实验研究不同燃尽风率对出口射流影响发现：燃尽风率对射流扩展角影响不明显，但对穿透深度影响较大； .3）前面研究发现一次风结构是影响燃烧器一次风扩散及颗粒流动轨迹的关键因素，因此通过数值模拟手段对一次风结构进行研究，研究发现：所有工况中，设置直径74mm和59mm的2级浓缩环结构的工况，相比无浓缩环工况可由461mg/m3降低至439mg/m3。就炉膛整体燃烧状况和NOx排放量而言，最优结构为设置直径为74mm和59mm，倾斜角度为20°的2级浓缩环结构。.4）在真实燃烧条件下，燃烧器出口NO浓度峰值与着火位置之间距离约为0.2m；煤粉浓度越高，着火性能和NO还原性能越好；随着燃烧器一次风煤粉浓度增加，锅炉飞灰含碳量减少，炉渣含碳量增加，NOx排放量从481.1 mg/m3降至351.4 mg/m3。