新型烟气再循环在流化床中的行为特性及NO_(x)排放分析

Behavior characteristics and NO_(x)emission analysis of new flue gas recirculation in fluidized bed

作为低氮燃烧的重要技术手段,烟气再循环(FGR)技术在煤粉炉和链条炉中应用广泛,但对于循环流化床(CFB)锅炉,单一的底部通入方式严重限制了机组低负荷稳燃能力。为保证床温的同时降低NO_(x)排放水平,提出一种采用炉膛稀相区FGR与补燃风协同作用下的新型烟气再循环方法,在1台0.2 t/h CFB燃烧器上开展热态试验,分析了运行参数和喷射位置对炉内燃烧特性和NO_(x)排放的影响。结果表明:通入再循环烟气会导致该区域烟温下降40~60℃,炉温分布先下降后升高,引起炉膛出口10℃左右的温降,顶部通入补燃风能有效提升出口烟温。随烟气再循环率增加,中部通入再循环烟气使得NO_(x)排放逐渐下降,当烟气再循环率达16%时,可在不影响床温的前提下使NO_(x)排放降低32%,而顶部通入再循环烟气会使NO_(x)排放先下降后上升。炉膛顶部送入不同再循环烟气/补燃风时,随补燃风量增加和烟气再循环率降低,NO_(x)质量浓度从249增至304 mg/m^(3),NO_(x)转化率从17%逐渐增至33%。“中部FGR+顶部补燃风”方式能降低NO_(x)排放38%,同时燃烧效率维持在98.7%以上。最后基于再循环烟气通入位置、过量空气系数和烟气再循环率,提出了烟气再循环背景下的NO_(x)排放预测模型,为CFB锅炉低负荷运行过程中强化低氮燃烧提供依据。

As an important technical means of low-NO_(x)combustion,flue gas recirculation(FGR)technology has been widely adopted in pulverized coal boilers and chain boilers.However,in circulating fluidized bed(CFB)boilers,the stable combustion capacity under low load is seriously limited due to the conventional injection mode.In order to reduce NO_(x)emission under the premise of ensuring bed temperature,a new FGR method based on the synergistic effect of FGR in dilute phase zone and supplementary combustion air was proposed.The thermal experiments were carried out on a 0.2 t/h CFB combustor,and the effects of operating parameters and injection positions on combustion characteristics and NO_(x)emission in the furnace were analyzed in detail.The key results show that the flue gas temperature in this area will decrease by 40-60℃,and the furnace temperature profiles will first decrease and then increase,resulting in a temperature drop of about 10℃at the furnace outlet.The supplementary combustion air from the upper part of the furnace can effectively improve the gas temperature at furnace outlet.With the increase of FGR ratio,the middle FGR injection can gradually reduce the NO_(x)emission.When the FGR ratio reaches 16%,the NO_(x)emission can be reduced by 32%and the bed temperature is kept unchanged.With the increase of top FGR ratio,the NO_(x)emission will decrease first and then increase.When different FGR/supplementary combustion air is sent into the top of the furnace,the mass concentration of NO_(x)increases from 249 to 304 mg/m^(3)and the conversion rate of NO_(x)increases from 17%to 33%gradually with the increase of supplementary combustion air volume and the decrease of FGR ratio.The mode of middle FGR+top supplementary combustion air could reduce the NO_(x)emission by 38%and maintain the combustion efficiency over 98.7%.Eventually,a NO_(x)emission prediction model under FGR mode was proposed based on FGR injection position,excess air coefficient and FGR ratio,which could provide technical basis for better low-NO_(x)combustion during low-load operation of CFB boilers.