内/外烟气再循环对天然气燃烧NOx生成的影响

doi:10.16085/j.issn.1000-6613.2018-2321

摘要/Abstract

摘要：

烟气再循环作为一种有效抑制NO_x生成的技术，广泛应用于化工、制药和酿酒等行业的天然气低氮燃烧过程，其作用机理是学术界和工业界关注的热点。本文根据内/外烟气再循环的作用不同，设计了弱/强内烟气再循环两种扩散式燃烧器，结合外烟气再循环技术，在0.8MW中试实验台上研究了内/外烟气再循环对天然气燃烧过程NO_x生成的影响规律，同时开展了Fluent数值模拟。结果表明：内烟气再循环能够有效抑制NO_x的生成，同时保证燃烧稳定。相同工况下，带内烟气再循环燃烧器初始排放远低于传统低氮燃烧器。同时结合外烟气再循环技术，可使NO_x排放量维持在30mg/m³以下。Fluent模拟结果显示了外烟气再循环能够显著地降低天然气燃烧火焰区的温度和O₂浓度。其中温度的降低对于NO_x生成的抑制起到了决定性的作用。当NO_x降低到约30mg/m³时，快速型NO_x对整体NO_x生成的贡献超过热力型NO_x，此情况下必须考虑对快速型NO_x进行抑制。

关键词: 污染, 天然气, 低氮燃烧, 数值模拟, 烟气再循环

Abstract:

Flue gas recirculation, an effective way to inhabit NO_x formation, is widely used in the natural gas low-NO_x combustion process in chemical engineering, pharmaceutical and brewing industries. Its working mechanism attracts attentions from both academia and industry. Considering the effects of the internal/external flue gas recirculation (i-/e-FGR), two types of diffusion-combustion burners with weak/strong i-FGR were designed in this study. Combined with the e-FGR technology, pilot-scale (0.8MW) experiments were conducted to reveal the i-/e-FGR effect on the NO_x emission of the natural gas combustion. Additionally, Fluent simulation was also conducted. Results showed that i-FGR significantly restrained the NO_x formation while guaranteed a stable combustion. The initial emission of the burner with i-/e-FGR was far below that of the traditional burner. Combined with e-FGR, NO_x emission was maintained below 30mg/m³. Fluent simulation indicated that e-FGR dramatically decreased the flame temperature and O₂ concentration. The decrease of temperature played a decisive role in the suppression of the NO_x formation. When the NO_x emission was around 30mg/m³, the prompt NO_x mechanism dominated the overall NO_x formation, which drove a need to depress the prompt NO_x formation to further decrease the overall NO_x formation.

Key words: pollution, natural gas, low NO_x combustion, numerical simulation, flue gas recirculation

中图分类号:

TK16

王志宁,杨协和,张扬,金燕,张海,吕俊复. 内/外烟气再循环对天然气燃烧NO_x生成的影响[J]. 化工进展, 2019, 38(9): 4327-4334.

Zhining WANG,Xiehe YANG,Yang ZHANG,Yan JIN,Hai ZHANG,Junfu LÜ. I-/e-FGR effect on NO_x emission of natural gas combustion[J]. Chemical Industry and Engineering Progress, 2019, 38(9): 4327-4334.

图/表 15

图1 中试实验台系统

图2 中试实验两种燃烧器示意图及模型

图3 不同工况下Type A/B燃烧器的NOx排放特性

图4 不同工况下Type A/B燃烧器的CO排放特性

图5 外烟气再循环率（Re-FGR）对NOx生成的影响

图6 Fluent计算几何模型及网格

图7 Type B燃烧器炉膛中心截面及局部燃烧器流场

图8 Type B燃烧器的炉膛中心截面及燃烧器局部CO分布（Pc=0.56MW，Re-FGR=0）

图9 Type B燃烧器局部温度分布

图10 Type B燃烧器外烟气再循环率对内烟气再循环率的影响

图11 Type B燃烧器外烟气再循环率对NOx生成的影响（NOx排放数值换算到3.5% O2条件下）

图12 Type B燃烧器外烟气再循环率对炉膛温度的影响

图13 Type B外烟气再循环率对炉膛氧浓度的影响

图14 外烟气再循环率对炉膛中心横截面NOx生成速率的影响

图15 Type B燃烧器外烟气再循环率对炉膛NOx生成速率峰值的影响

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