摘要
燃烧器是电站锅炉中的重要热工设备,对于燃料的稳定燃烧和燃烧效率有很大影响。旋流燃烧器作为一种高效燃烧器已经广泛应用于锅炉燃烧中,但其通常导致大量的NOx生成。为了在提高能源利用效率的同时适应越来越严格的环境保护要求,对于高效清洁的能源利用方式的需求日益受到重视。因此,开发一种低NOx旋流燃烧器具有重要意义。
首先,本文借鉴德国SM-4型煤粉旋流燃烧器的建立了可应用于甲烷-空气,煤粉-空气燃烧的旋流燃烧器和小型燃烧室。实验台的设计考虑了在该实验台上能够开展不同燃烧工况的研究,以及如何便于采用目前先进的光学测试手段对燃烧流场进行实验研究。
其次,在不同的外二次风率和外二次风旋流数的工况下,利用烟气分析仪和热电偶对燃烧室内温度场和烟气组分浓度进行了测量,在以天然气为燃料时,保持过剩空气系数和旋流数等条件不变,增大二次旋流风率可降低燃烧过程中NO的生成,但却会增加CO的生成。增大外二次风旋流数可以降低燃烧过程NO的生成。在以煤粉为燃料时,增大二次旋流风率会增加NO的生成。
然后,利用二维粒子图像速度仪(PIV)系统对变工况甲烷/空气、煤粉/空气的燃烧速度场进行了一系列测量,得到了较好的速度场信息。从PIV实验可以看出,在以天然气为燃料燃烧的流场中,保持在总风量不变的情况下,增大外二次风率使速度梯度更大,对火焰结构影响更大,从而使得火焰的收缩更明显。在保持外二次风率不变的情况下,增大外二次风旋流数将会削弱前期混合,加强后期混合。对于煤粉燃烧而言,增大外二次风率能增强空气与煤粉混合。在保持外二次风率不变的情况下,增加外二次风的旋流数将加强煤粉与空气混合。
最后,利用商用CFD软件Fluent,采用大涡模拟方法结合简化PDF燃烧模型和Smagorinsky-Lilly亚网格湍流模型对选定实验工况下的扩散燃烧进行了数值研究。通过大涡模拟计算旋流燃烧室内天然气的燃烧过程,并能较好的符合实验规律。大涡模拟可用于追踪复杂湍流的瞬态结构和时均场。
Burner is important thermal equipment in utility boiler, and it has a significant impact on stable combustion and combustion efficiency. Swirl burner has been widely used in boiler as a highly efficient burner. However, it usually generates a large number of NOx. In order to improve the efficiency of energy utilization and adapt to the increasingly strict restriction on the requirements of environment protection, it is important to develop clear and efficient methods for energy utilization. Therefore, it is meaningful to develop a low-NOx swirl burner.
First of all, an experimental facility which can used methane-air and coal-air as fuel has been designed and set up. Author placed high premiums on the following two aspects. Firstly, experiment on the facility in different combustion conditions. Secondly, it will be convenient for the application of advanced laser diagnostics during the experimental research of combustion flow fields.
Secondly, apply flue-gas analyzer and thermocouple to do experiment in chamber which can obtain gas species concentration and temperature field in different external secondary swirl air ratios and swirl numbers. In nature gas combustion, maintained the excess air ratio and swirl number as constant, increasing external secondary swirl air ratios or increasing external secondary swirl number can reduce amount of NOx generation. in pulverized coal combustion, increasing external secondary swirl air ratios can increase amount of NOx generation.
Then, a 2-D Dantec's PIV(particle image velocimetry) system was used to measure the combustion flow fields of methane/air and pulverized coal/air in different conditions, and reasonable results has been obtained. In nature gas combustion, maintain total air volume and increase the external secondary swirl air ratio result in increasing velocity gradient and shorten flame length. When maintain external secondary swirl air ratio as constant, increase external secondary swirl number can weaken former mixed and strengthening the latter mixed. For pulverized coal combustion, increase external secondary swirl air ratio can enhance mixing of air and coal. When maintain external secondary swirl air ration as constant, increase external secondary air swirl number will strengthen the mixing of pulverized coal and air.
Methane/air combustion process has been studied with commercial CFD soft-ware Fluent. Using large-eddy simulation, simplify PDF combustion model and the Smagorinsky-Lilly sub-grid turbulence model to study the combustion process. Through large-eddy simulation on methane/air combustion process in the chamber, its result meets the experiment data appropriately. Large-eddy simulation can be used to track the complex turbulence structure and transient field.
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