NO2 formation in natural gas-fired boilers

doi:10.11949/0438-1157.20190111

Abstract

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

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

CLC Number:

TK 16

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.

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.

Figures/Tables 17

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

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℃

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

Fig.3 System diagram of pilot-scale experimental rig

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

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

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

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)

Fig 8 Schematic diagram of object in Chemkin-Pro simulation

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

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

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

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

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

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

Fig.14 NO2 formation path analysis

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

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NO₂ formation in natural gas-fired boilers

燃气锅炉中NO₂的生成规律研究

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