燃气锅炉中NO2的生成规律研究

doi:10.11949/0438-1157.20190111

摘要/Abstract

摘要：

天然气燃烧过程主要污染物是NO和NO₂。针对部分燃气锅炉NO₂排放较高的情况，开展了中试实验和数值模拟，研究天然气燃烧过程NO₂生成规律。结果表明当锅炉采用“燃烧器分级燃烧+烟气再循环”的低氮燃烧策略时，如果燃烧器的伴流风与燃料的混合被推迟，则NO₂的生成量较大，最高超过总NO _x 生成量的50%，空气和燃料充分混合的条件下NO₂生成量较少。NO₂在热燃烧产物与冷空气碰撞的交界面上大量生成，NO₂生成的温度窗口是800~900 K。NO₂主要通过NO与HO₂的反应生成，NO₂分解后又生成NO，NO₂的生成不会显著影响燃烧过程总NO _x 的排放。

关键词: 污染, 数值模拟, 天然气, 二氧化氮, 对冲火焰

Abstract:

The main pollutants of natural gas-fired boilers are NO and NO₂. The emission data of two natural gas-fired boilers were reported in the present paper and NO₂ emission was found to be high. To understand the high NO₂ emission in these natural gas-fired boilers, pilot-scale experiments were conducted and detailed chemical kinetic studies were carried out numerically using Chemkin-Pro software. The results showed that the NO₂ emission would be high if the low NO _x strategy “burner staged-combustion + flue gas recirculation” was applied and the mixing of the co-flow air with the staged fuel was delayed. In this situation, the amount of NO₂ emission even exceeded 50% of the amount of the total NO _x emission. When the air and fuel were well mixed, NO₂ emission was suppressed. NO₂ mainly formed near the contact surface of the hot burned product flow and cold air flow. A narrow temperature window, 800—900 K, was found for NO₂ formation. The dominating pathway for NO₂ formation was the reaction between NO and HO₂. When NO₂ decomposed, NO formed. Therefore, the formation of NO₂ would not significantly affect the total NO _x emission of the natural gas-fired boiler.

Key words: pollution, numerical simulation, natural gas, nitrogen dioxide, opposed-flow flame

中图分类号:

TK 16

王志宁, 杨协和, 张扬, 金燕, 张海, 吕俊复. 燃气锅炉中NO₂的生成规律研究[J]. 化工学报, 2019, 70(8): 3121-3131.

Zhining WANG, Xiehe YANG, Yang ZHANG, Yan JIN, Hai ZHANG, Junfu LYU. NO₂ formation in natural gas-fired boilers[J]. CIESC Journal, 2019, 70(8): 3121-3131.

图/表 17

图1 某两台燃气锅炉现场测试NO和NO2排放情况

Fig.1 NO and NO2 emissions of two natural gas-fired boilers in field tests

表1 现场测试的两台燃气锅炉概况

Table 1 Specifications of two natural gas-fired boilers in field tests

参数	锅炉Ⅰ（boiler Ⅰ）	锅炉Ⅱ（boiler Ⅱ）
运行地点	北京某生活小区	河北某汽车修理厂
用途	供热用热水锅炉	工业用蒸汽锅炉
容量	2 t/h	1 t/h
当地NO _x 排放要求	< 30 mg/m³	< 80 mg/m³
低氮技术	分级燃烧烟气再循环	分级燃烧烟气再循环
再循环烟气取气位置	烟囱底部	烟囱底部
过量空气系数（α）	1.20~1.25	1.3~1.4
排烟温度	70~100℃	150~200℃

图2 中试实验燃烧器样机原理示意

Fig.2 Schematic diagram of conceptual design of burners used in pilot-scale experiments

图3 中试实验台系统

Fig.3 System diagram of pilot-scale experimental rig

图4 不同燃烧负荷下两种燃烧器NO和NO2排放浓度对比（无FGR；α = 1.4）

Fig.4 NO and NO2 emissions of two burners at various combustion loads

图5 不同烟气再循环率下两种燃烧器NO和NO2排放浓度对比（P c = 0.5 MW；α = 1.2）

Fig.5 NO and NO2 emissions of two burners at various flue gas recirculation ratios

图6 不同过量空气系数下两种燃烧器NO和NO2排放浓度对比（α = 1.3; P c=0.32 MWth）

Fig.6 NO and NO2 emissions of two burners at various excess air ratios

图7 不同温度和过量空气系数下全混流反应器中甲烷燃烧NO与NO2生成规律（常压，3.5% O2标准）

Fig. 7 NO and NO2 emissions of CH4/air combustion in perfectly stirred reactor under various temperatures and excess air ratios (ambient pressure, 3.5% O2 standard)

图8 Chemkin-Pro数值模拟对象示意图

Fig 8 Schematic diagram of object in Chemkin-Pro simulation

表2 对冲火焰计算中热烟气来流组分摩尔分数

Table 2 Species mole fraction of hot product stream in opposed-flame computation

组分	α = 0.8	α = 1.0	α = 1.2
CO	5.20×10^-2	1.85×10^-2	5.80×10^-3
CO₂	7.04×10^-2	7.90×10^-2	7.82×10^-2
H	1.70×10^-3	1.91×10^-3	1.04×10^-3
H₂O	2.17×10^-1	1.82×10^-1	1.52×10^-1
HO₂	8.74×10^-8	6.48×10^-7	3.92×10^-7
N	6.68×10^-9	5.35×10^-9	8.99×10^-10
O₂	1.54×10^-4	7.55×10^-3	3.80×10^-2
N₂	6.59×10^-1	7.13×10^-1	7.50×10^-1
NO	3.51×10^-5	3.51×10^-5	3.51×10^-5
NO₂	4.46×10^-9	1.28×10^-8	5.08×10^-9

图9 不同温度下NO和NO2在热烟气-冷空气对冲流中的分布规律

Fig.9 NO and NO2 mole fraction distribution in hot combustion products-air opposed flow at different temperatures

图10 不同来流速度下NO和NO2在热烟气-冷空气对冲流中的分布规律

Fig.10 NO and NO2 mole fraction distribution in hot combustion products-air opposed flow at different temperatures

图11 不同过量空气系数的热燃烧产物与冷空气对撞后NO和NO2的分布规律

Fig.11 NO and NO2 mole fraction distribution in hot combustion products-air opposed flow at different excess air ratios

图12 热燃烧产物与不同组分的冷气体对撞后NO和NO2的分布规律

Fig.12 NO and NO2 mole fraction distribution in hot combustion products -cold gases of different components

图13 不同过量空气系数和热烟气温度下EINO和EINO2对比

Fig.13 EINO and EINO2 at different excess air ratio and hot combustion product temperature

图14 NO2生成路径分析（图中数据单位：mol/(cm3·s)）

Fig.14 NO2 formation path analysis

图15 不同过量空气系数的热燃烧产物与冷空气对撞后HO2生成速率的分布规律

Fig.15 HO2 production rate distribution in hot combustion products-air opposed flow at different excess air ratios

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燃气锅炉中NO₂的生成规律研究

NO₂ formation in natural gas-fired boilers

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