内/外烟气再循环对天然气燃烧NO_x生成的影响

I-/e-FGR effect on NO_x emission of natural gas combustion

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/m^3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/m^3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。

Flue gas recirculation, an effective way to inhabit NOx formation, is widely used in the natural gas low-NOx 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 NOx emission of the natural gas combustion. Additionally, Fluent simulation was also conducted. Results showed that i-FGR significantly restrained the NOx 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, NOx emission was maintained below 30mg/m^3. 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 NOx formation. When the NOx emission was around 30mg/m^3, the prompt NOx mechanism dominated the overall NOx formation, which drove a need to depress the prompt NOx formation to further decrease the overall NOx formation.