摘要
本文通过冷态实验和数值模拟对W型火焰锅炉拱上燃尽风结构及布置进行研究,实验分别通过热线风速仪测定流场速度和乙二醇示踪方法测得气流流动踪迹,数值模拟以FLUET商用软件为计算平台。
为降低NOx而在W型火焰炉的拱上通入OFA气流,则拱下供入的二次风量就会减小,势必会影响到拱下炉膛流场结构,进而影响到整个炉膛的燃烧状况,因而有必要在加装OFA装置之前先对拱下流场发生的变化进行研究。主要从以下开展研究工作:加装燃尽风(OFA)装置对拱下流场的影响,包括二次风不下倾和下倾25°的情况。对此问题的研究采用的外旋流内直流OFA管,在二次风不下倾和下倾25°的情况下,对OFA所占份额进行调整得出四种工况0%,10%,20%,30%。在二次风不下倾的情况下,随着OFA风率的增加,下炉膛一次风依然能形成稳定的W形。且随着OFA风率的增加,一次风射流衰减变慢,穿透深度增加,增大煤粉行程,有利于煤粉的燃尽;在二次风下倾的情况下,随着OFA风率的增加,下炉膛的流场总体改变较小,一次风均能打破底部的死滞区。
在W型火焰炉上炉膛引入OFA射流时,必须使OFA射流能与炉内各处烟气充分混合,对于此问题的研究采用的也是在冷态模化实验台上,通过用乙二醇示踪方法得到OFA射流的流动轨迹,从而判断OFA气流在炉膛深度方向和宽度方向是否与烟气气流充分混合。对此分别研究OFA风率、倾角及喷口结构变化时,对OFA射流与烟气混合的影响。对三种结构的OFA喷口:直流结构、内直流外旋流、福斯特惠勒结构安排工况。由示踪图片可以看出达到最佳效果时,OFA风率需至少在20%以上,OFA喷口倾角为30°,采用内直流外旋流OFA喷口,外层旋流部分喷口所安装的叶片应选用弯曲叶片。
本文采用拉格朗日随机颗粒轨道模型对炉膛进行了气固两相的数值研究,主要研究了OFA倾角和风率变化时,对炉膛温度分布、煤粉燃尽程度及NOx排放浓度的影响。加装OFA喷口后,炉膛出口烟温和飞灰含碳量明显提高,NOx排放量有一定程度降低。随着OFA喷口下倾角度由0°变化到30°,炉膛出口烟温和飞灰含碳量逐渐降低,当角度增大到40°时,出口烟温和飞灰含碳量又有较大幅度反弹,这说明OFA射流下倾在30°内时,有利于增大煤粉燃烧行程,提高煤粉燃尽率,但当下倾增加至40°时,由于OFA无法冲入炉膛中部,因而和煤粉的混合变差,混合点升高。
The configuration and collocation of OFA in down-fired boiler were studied by experiments and numerical simulation in the paper. An IFA300 constant temperature anemometers system was used to measure the air velocity at the measurement points in cold aerodynamic experiments. Trace glycol was used to acquire the contrail of OFA in the trace experiments. Numerical simulation was completed by FLUET.
An addition of OFA above the arch in down-fired boiler for the low-NOx emission will decrease the secondary air. This will influent the air velocity of fuel-burning zone and the combustion of the boiler. Thus it’s essential for studying the air velocity of fuel-burning zone before installing OFA. The contents mainly contain two situations : the angel of the secondary air jet was 0 and the angel of the secondary air jet was 25. In the experiment we use the OFA jet formed by two pipes in which the air was vortical in the outer pipe and the other was straight. Four experiments were completed by arranging the proportion of OFA, 10% , 20% , 30% , 40% .When the angel of the secondary air jet was 0, the primary air still formed W-shape air flow in the furnace along with the proportion of OFA increased. And it also increased the burn-out rate. When the angel of the secondary air jet was 25, the aerodynamic field in the down-fired furnace had few change. And the primary air could reach the bottom in the furnace.
It’s necessary that the OFA air could blend into the other air in the furnace when installing the OFA pipes. The standard is the contrail of the OFA by way of trace glycol. Thus the emphases is the effect of the rate, the angel and the structure of the OFA. There are three kinds of structure in the experiments: one pipe from which the jet is straight, two pipes in which the air was vertical in the outer pipe and the other was straight, FW pipe. From the trace photographs we can conclude: the rate is 20% at lowest, the best angel is 30, the best structure is the second kind in which the vane should be curving.
The numerical simulation of gas-solid flow was carried out by Lagrange stochastic particle tracking model. The influences of the rate and the angel of the OFA on the temperature distribution in the furnace, the burn-out rate of the coal powder and NOx emission were studied. The conclusion is that the temperature of the outlet in the furnace and carbon content in the fly ash increased evidently after adding the OFA air jet. And NO_x emission decreased in a certain extent. When the angel of the OFA air jet is 0 to 30, the out-let temperature in the furnace and carbon content in the fly ash decreased gradually. When the angel increased to 40, they had a big extent of increase. This is because that when the angel is under30, the OFA is propitious to increase the burning journey and the burning time of the coal powder. When the angel is 40, OFA couldn’t crush the central section of the furnace. This made the OFA couldn’t had a full mix with the coal powder which made the mix point increase.
引文
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