CFD simulation of low-temperature NO oxidation using ozone in sintering flue gas

doi:10.11949/0438-1157.20190418

Abstract

Abstract:

The flue gas from a 256 m² sintering machine was selected to explore the characteristics of NO oxidized by ozone at low temperature. The flow characteristics of O₃ injection gas and sintering flue gas and NO oxidation characteristics were investigated by CFD numerical simulation method. By compared with 76-step complex reaction mechanism, 11-step simplified mechanism was verified to be reasonable for the above process. The effects of process conditions, including reaction temperature, O₃/NO molar ratio and O₃ distribution, on the conversion rates of NO _x with different chemical valences were examined. The results from the simulation for the reactor, which is simple in structure, show that NO₂ and N₂O₅ are the main oxidation products in the flue gas, whilst NO₃ is consumed due to its poor chemical stability. The oxidation efficiency of NO changes little with the increase of reaction temperature, the conversion rate of NO₂ increases and performs more significant change at higher temperature. However, the conversion rate of N₂O₅ presents an opposite trend compared with that of NO₂. With the raise of O₃/NO molar ratio, the oxidation rate of NO increases slowly and that of NO₂ increases firstly and then decreases when the molar ratio higher than 1.25, while N₂O₅ emerges in the condition even the O₃/NO molar ratio less than 1.0 and the N₂O₅ production increases gradually, resulting from a higher O₃/NO ratio in the injection core regions. Furthermore, the structural optimization for the ozone distributor could improve the homogeneity of O₃ in the sintering flue gas. Afterwards the oxidation efficiency of NO increases by 12.8% approximately when the O₃/NO molar ratio is 1.0 and the average residence time is 0.87 s, while the increase for N₂O₅ conversion rate of about 15.6% was obtained when the O₃/NO molar ratio is 2.0 and the average residence time is 1.73 s.

Key words: sintering flue gas, NO _x , ozone, oxidation, hydrodynamics, numerical simulation

摘要：

以256 m²烧结机O₃氧化烧结烟气中NO过程为研究对象，采用CFD数值模拟方法考察了含O₃喷射气体与烧结烟气流动及NO低温氧化特性。通过与76步复杂反应机理的对比验证了11步简化机理的适用性，分析了反应温度、O₃/NO摩尔比以及O₃分布特性对NO氧化效率和不同价态NO _x 转化率的影响规律。通过对简单结构反应器的模拟结果表明：NO₃稳定性较差，烟道内主要氧化产物为NO₂与N₂O₅；随反应温度升高，NO氧化效率基本保持不变，NO₂转化率提高且提升速率逐渐增大而N₂O₅呈相反规律；随O₃/NO摩尔比增大，NO氧化效率提高但提升速率逐渐减小，NO₂转化率先增大后在摩尔比高于1.25时开始减小，而各工况均产生N₂O₅且生成量逐渐增大，其原因为射流核心区可提供高O₃/NO摩尔比条件；通过优化O₃分布器结构改善O₃与烟气接触与混合条件，O₃与NO摩尔比为1.0、停留时间为0.87 s时NO氧化率可提高约12.8%，摩尔比为2.0、停留时间为1.73 s时N₂O₅转化率可提高约15.6%。

关键词: 烧结烟气, 氮氧化物, 臭氧, 氧化, 流体动力学, 数值模拟

CLC Number:

X 511

Jiangyuan QU, Xiaolong LIU, Yanjun GUAN, Nana QI, Yang TENG, Wenqing XU, Tingyu ZHU, Kai ZHANG. CFD simulation of low-temperature NO oxidation using ozone in sintering flue gas[J]. CIESC Journal, 2019, 70(11): 4387-4396.

曲江源, 刘霄龙, 关彦军, 齐娜娜, 滕阳, 徐文青, 朱廷钰, 张锴. 基于CFD模拟的臭氧低温氧化烧结烟气中NO过程分析[J]. 化工学报, 2019, 70(11): 4387-4396.

Figures/Tables 14

Fig.1 Schematic of O3-NO x reactions and computational domain

Table 1 O3-NO x simplified reaction mechanism and kinetic parameters

Reaction		A	β	E _a	Order
R1	O₃+NO $?$ NO₂+O₂	8.43×10¹¹	0	10.98	2
R2	O₃+NO₂ $?$ O₂+NO₃	8.43×10¹⁰	0	20.54	2
R3	NO₂+NO₃ $?$ N₂O₅	7.89×10¹⁷	-3.90	0	3
R4	H₂O+NO+NO₂ $?$ HNO₂+HNO₂	1.60×10⁸	0	0	3
R5	H₂O+N₂O₅ $?$ HNO₃+HNO₃	1.51×10²	0	0	2
R6	HNO₂+NO₃ $?$ HNO₃+NO₂	1.21×10⁹	0	0	2
R7	HNO₂+O₃ $?$ HNO₃+O₂	3.01×10⁵	0	0	2
R8	NO₃ $?$ NO+O₂	2.50×10⁶	0	50.72	1
R9	NO₃+NO₃ $?$ NO₂+NO₂+O₂	5.12×10¹¹	0	20.37	2
R10	NO₂+NO₃ $?$ NO+NO₂+O₂	2.71×10¹⁰	0	10.48	2
R11	NO+NO₃ $?$ NO₂+NO₂	1.08×10¹³	0	-0.91	2

Table 1 O3-NO x simplified reaction mechanism and kinetic parameters

Reaction		A	β	E _a	Order
R1	O₃+NO $?$ NO₂+O₂	8.43×10¹¹	0	10.98	2
R2	O₃+NO₂ $?$ O₂+NO₃	8.43×10¹⁰	0	20.54	2
R3	NO₂+NO₃ $?$ N₂O₅	7.89×10¹⁷	-3.90	0	3
R4	H₂O+NO+NO₂ $?$ HNO₂+HNO₂	1.60×10⁸	0	0	3
R5	H₂O+N₂O₅ $?$ HNO₃+HNO₃	1.51×10²	0	0	2
R6	HNO₂+NO₃ $?$ HNO₃+NO₂	1.21×10⁹	0	0	2
R7	HNO₂+O₃ $?$ HNO₃+O₂	3.01×10⁵	0	0	2
R8	NO₃ $?$ NO+O₂	2.50×10⁶	0	50.72	1
R9	NO₃+NO₃ $?$ NO₂+NO₂+O₂	5.12×10¹¹	0	20.37	2
R10	NO₂+NO₃ $?$ NO+NO₂+O₂	2.71×10¹⁰	0	10.48	2
R11	NO+NO₃ $?$ NO₂+NO₂	1.08×10¹³	0	-0.91	2

Fig.2 Validation and comparison of different reaction mechanisms

Fig.3 Residence time for gaseous mixture of flue gas and oxidant

Fig.4 Mole fraction distribution of NO and O3

Fig.5 Mole fraction distribution of NO x in different chemical valence

Fig.6 Relative mole fraction distribution of species in flue gas along axis direction

Fig.7 Effect of temperature on conversion rate of NO x

Fig.8 Effect of O3/NO molar ratio on conversion rate of NO x

Fig.9 Schematic of optimization for oxidant distributor

Table 2 Mixing and reaction conditions for different cases (T=378 K, [NO]in=2.0×10-4)

Case index	Mixing condition	Reactor length L	Res. time t _m/s	[O₃]/[NO]
A	distributor Ⅰ	3D	0.87	1.0
B	distributor Ⅱ	3D	0.87	1.0
C	distributor Ⅰ	6D	1.73	2.0
D	distributor Ⅱ	6D	1.73	2.0
E	well-mixed	6D	1.73	2.0

Fig.10 Effect of mixing condition for gaseous mixture on oxidation of NO

Fig.11 Effect of mixing condition for gaseous mixture on conversion of N2O5

Fig.12 Comparison of NO x conversion rate in different cases

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