【作者】 汪华剑；

【导师】 周怀春；

【作者基本信息】 华中科技大学 ， 热能工程， 2010， 博士

【摘要】 发展W型火焰锅炉高效低NOx的燃烧技术,优化炉内燃烧,是我国低挥发分煤的燃烧利用中一个急需解决的重要课题。详细的试验检测和准确的燃烧过程数值模拟是正确分析炉内的煤粉燃烧过程,揭示炉内NOx的生成机理,准确把握不同因素对炉内燃烧特性的影响的关键,能够为炉内的燃烧优化和发展新型W型火焰锅炉高效低NOx燃烧技术提供指导和依据。本文通过基于图像处理的燃烧检测技术对W型火焰锅炉内的燃烧进行检测,并对炉内燃烧过程进行详细的数值模拟分析,研究了优化炉内燃烧的技术,并在电站锅炉上通过改造进行了实践。本文的主要研究内容如下：W型火焰锅炉燃烧过程的检测研究。通过使用基于图像处理的炉内燃烧过程检测技术,对Foster Wheeler (FW)技术W型300MW和600MW级的锅炉进行检测。对一台300MW级W型火焰锅炉进行了沿炉膛高度方向的温度和火焰发射率的检测,得到了炉内火焰温度和发射率沿炉膛高度方向上分布的主要规律。对一台600MW级W型火焰锅炉进行三维温度场重建的研究,并将三维重建温度分布的信息与采用便携式图像温度检测系统以及红外高温计的检测结果进行了比较,结果显示检测结果的差别在10%以内,表明检测结果的可靠性。检测结果表明W型火焰锅炉目前存在的飞灰含碳量高,NOx排放超标等问题与实际燃烧过程中火焰中心偏上,煤粉停留时间不足,炉内空气分级燃烧不足有直接关系。对FW技术的W型火焰锅炉燃烧过程进行数值模拟研究并提出一种提高NOx预测准确性的方法。通过建立适合W型火焰锅炉特点的贴体网格,采用Computational Fluid Dynamics (CFD)方法对一台310MW W型火焰锅炉内的燃烧过程进行了模拟,得到了典型的炉内温度分布,速度场和颗粒轨迹等,并通过将模拟得到流场和沿炉膛高度方向的温度分布结果与试验结果进行对比,表明了模拟方法和结果的可靠性。以此为基础分析了目前炉内燃烧中存在的一些主要问题和原因。通过将CFD燃烧过程模拟得到燃烧器出口的升温曲线,结合具有氮释放预测能力的化学渗透脱挥发分(Chemical Percolation Devolatilization) CPD模型,计算得到煤粉在燃烧过程中挥发分氮和焦炭氮的比例,以及挥发分中NOx的前驱物HCN和NH3的份额。用这些参数取代传统的经验参数,取得了对NOx生成的预测的更高精度。在以上研究的基础上,对四种不同运行条件下的W型火焰锅炉燃烧优化开展了数值模拟研究。通过CFD数值模拟结合现场燃烧调整试验,研究了乏气比例的调整、C风(油枪风)比例调整和混煤掺烧方式调整对炉内燃烧过程和NOx排放的影响。研究不同燃烧调整方式对动量比、煤粉着火、火焰下冲深度、煤粉燃尽和NOx排放的影响,给出了优化运行的指导意见。由于常规的燃烧优化方式不能从根本上解决炉内空气分级不足造成NOx排放高的问题,通过数值模拟研究拱上燃尽风的实施方案,指出在拱上实施高速燃尽风,能够在保证煤粉燃尽的同时有效降低NOx排放。由于常规的燃烧优化方式对W型火焰锅炉的高效低NOx运行只能起到有限的作用,而且往往难以兼顾稳燃、高效和低NOx燃烧的要求,所以研究了在F层二次风风箱中加入下倾的导流叶片,使得占二次风60-70%的F风以一个下倾的角度进入下炉膛,从而改进炉内的燃烧。该技术可以在不改变拱上动量的情况下,增加向下／水平动量比,还能通过下倾的二次风对下行的一次风起到引射作用,这样可以保证主要的二次风不会直接水平冲击下冲的煤粉气流,使得二次风逐渐沿一次风火焰行程补充氧量,有利于延长火焰行程,扩大下炉膛还原区,有利于煤粉燃尽和降低NOx排放。通过燃烧过程数值模拟优化F层二次风下倾角度,得到最佳下倾角度为25。。该改造措施在电厂的实际应用效果表明,该项新技术的应用,可以有效降低飞灰含碳量,高负荷下提高燃烧效率超过1%,降低NOx排放超过20%。更多还原

【Abstract】 The development of coal combustion technology to optimize the coal combustion to improve the efficiency and reduce NOx emission of arch fired boilers, is an important and urgent issue in the combustion utilization of low-volatile coal. Detailed tests and accurate numerical simulation of combustion process is the key point to accurate grasp coal combustion process in furnaces, reveale the mechanism of NOx formation, and comprehensively understand the effects of various factors on the combustion characteristics. They are the the guidance and basis to optimize the coal combustion and develop new high efficiency and low NOx emission combustion technology. This work, based on the analysis of combustion process in the arch fired furnaces by detailed flame detection technology employing image processing technology and numerical simulation, studied a pratical technology to improve the combustion in the arch fired furnace. The detailed description is as below:Combustion detection technology by image processing technique has been applied to study the combustion process in two typical Foster Wheeler (FW) technology arch fired furnaces, each of which is representive for 300MW and 600MW grade boilers. The measurements of the 300MW furnace, show general rules of temperature distribution and flame emissivity along the height of the furnace. Three-dimensional temperature field reconstruction study on a 600MW grade boiler has been performed. The three-dimensional temperature distribution measurements have been verified by a portable image temperature detection system and an infrared pyrometer test results. The comparison shows the temperature measurement differences between the three instruments are within 10%, which indicates the reliability of the mesurements. Numerical simulation method reliability and results are also confirmed by the temperature measurements. The analysis based on above results, shows that the high unburnt carbon and NOx emissions are mainly caused by the actual high flame center in the furnace, which reduce the residence time of pulverized coal and decrease the staged air combustion.A numerical simulation is applied to a FW technology arch fired boiler combustion process, and a method to improve the prediction of NOx emission is presented as well. By the establishment of body-fitted grid of a 310MW FW arch fired furnace, the CFD method is performed to simulate the combustion characteristics in the furnace, and the typical furnace temperature distribution, velocity field and particle trajectory have been presented. The comparison of simulated flow field and temperature distribution along the furnace height with the experimental results, shows the reliability of the simulation methods and results, and the prediction reveal the reasons account for the bad combustion in the furnace. By the CFD combustion results, the heating progress of coal particles in the furnace can be account to predict the nitrogen release by Chemical Percolation model for Devolatilization (CPD) model. Then, the volatile nitrogen and char nitrogen ratio and the proportion of volatile NOx precursors in form of HCN and NH3 are able to be calculated for the actual combustion process in the furnace. Using these computed parameters instead of the traditional experiential parameters, the NOx emission prediction accuracy is improved.On the basis of the above results, four combustion optimization methods for FW arch fired furnace are researched by numerical modeling and field tests. The effects of the proportion adjustments of vent air, C air (oil gun air) and the method to burn blended coal in the furnace on coal combustion process and NOx emission are studied, by comparasions of the momentum ratio, coal igniton, flame depth, coal burnout and NOx emission variations, and optimal operation are suggested. As the conventional combustion optimization methods are not able to fundamentally solve the shortage of the high NOx emission as a reason of lack air-staged combustion. So deep air combustion method by the independent over fire air (OFA) set above the arch of furnace is presented. The numerical simulation is employed to study the effects of the independent OFA strategy on the combustion characteristic, and introduce high-speed OFA above the arch to guarantee the coal burnout with effective reduction of NOx emission.As the conventional optimization methods not only play a limited role to improve combustion efficiency and reduce NOx emission for the arch fired furnace, but also hard to maintain stable combustion while achieve high efficiency and low NOx emission. So a technology to improve the combustion is studied by set guide vanes in the F layer secondary air box to decline the 60-70% secondary air with a certain angle into the furnace. The method is able to increase the downward/horizontal momentum ratio without any change of the flow on the arch. The declination of the secondary air also plays an injection effect on the down-flow primary air, this ensures that the main secondary air gradually offer oxygen for the coal flame along the flame path, not directly impacts the down-flowing primary air. As a result, the flame path is extended and the reduction zone in the down furnace is enlarged, which improve the coal burnout and reduce the NOx emission. Numerical simulation of combustion optimization of different F layer secondary declined angles shows the best angle is 25°. The industrial application of this technology on a FW furnace shows that this method effectively reduces unburnt carbon in fly ash, and increases combustion efficiency more than 1% with more than 20% NOx emission reduction on high load.更多还原