助燃空气对乙烯裂解炉NOx排放的影响

doi:10.11949/0438-1157.20190771

摘要/Abstract

摘要：

乙烯裂解炉内复杂物理化学过程耦合模拟与优化能够满足乙烯装置对高效率、低污染和低成本的设计和操作要求，对提高乙烯工业的竞争力具有重要意义。针对简单燃烧机理难以准确预测炉膛燃烧生成NO_x浓度分布的弊端，提出了在裂解炉使用更准确的简化GRI-Mesh 3.0机理结合涡耗散概念(EDC)模型的方法，并对Sandia Flame D的燃烧过程进行计算流体力学(CFD)模拟，验证了此耦合模型的可靠性。在已建立的燃烧模型的基础上，研究了助燃空气对降低裂解炉NO排放的影响，结果表明：在满足裂解炉热效率的情况下，空气预热温度为300~600 K、过量空气系数为1.1时降低NO的效果最佳。

关键词: 乙烯裂解炉, 模拟, 优化, 简化GRI- Mesh 3.0机理, 计算流体力学, 助燃空气, NO_x排放

Abstract:

Coupled simulation and optimization of complex physical and chemical processes in ethylene cracking furnace can meet the demand for design and operation of high efficiency, low pollution and low cost in ethylene plant. Therefore, it has great significance for improving the competitiveness of the ethylene industry. Aiming at the disadvantage of simple combustion mechanism that it is difficult to accurately predict the distribution of NO_x concentration produced by furnace combustion, are reduced GRI-Mesh 3.0 mechanism combined with eddy dissipation concept (EDC) model for cracking furnace was proposed. The combustion process of Sandia Flame D was simulated and the reliability of the coupled model was verified. Based on the established combustion model, the effect of combustion-supporting air on reducing NO emission of cracking furnace was studied. The results show that the best effect of reducing NO is when the air preheating temperature is 300—600 K and the excess air coefficient is 1.1 when the thermal efficiency of the cracking furnace is satisfied.

Key words: ethylene cracking furnace, simulation, optimization, reduced GRI-Mesh 3.0 mechanism, computational fluid dynamics, combustion-supporting air, NO_x emission

中图分类号:

TQ 021.1

胡贵华, 叶贞成, 杜文莉. 助燃空气对乙烯裂解炉NO_x排放的影响[J]. 化工学报, 2020, 71(2): 698-707.

Guihua HU, Zhencheng YE, Wenli DU. Effect of combustion-supporting air on NO_x emission of ethylene cracking furnace[J]. CIESC Journal, 2020, 71(2): 698-707.

图/表 16

图1 Sandia Flame D燃烧器结构示意图

Fig.1 Schematic diagram of Sandia Flame D burner

表1 Flame D入口速度和组分条件

Table 1 Inlet velocity and composition conditions of Sandia Flame D

结构	直径/长×宽/mm	温度/K	速度/(m/s)	组分/(质量分数)
结构	直径/长×宽/mm	温度/K	速度/(m/s)	CH₄	O₂	N₂	CO₂	H₂O
主火焰	7.2	294	49.6	0.156	0.197	0.647	0	0
值班火焰	18.2	1880	11.4	0	0.054	0.742	0.11	0.094
空气伴流	300×300	291	0.9	0	0.23	0.77	0	0

图2 Flame D模型网格

Fig.2 Grid of Flame D model

图3 Flame D燃烧速度与温度云图

Fig.3 Velocity and temperature contours of Flame D combustion

图4 Flame D温度分布模拟值与实验数据比较

Fig.4 Comparison of temperature distributions with simulation results and experimental data for Flame D

图5 Flame D组分分布云图

Fig.5 Species distributions contours of Flame D

图6 NO质量分数分布

Fig.6 NO mass fraction distribution

表2 裂解炉炉膛结构尺寸和操作条件

Table 2 Structure dimension and operating conditions of cracking furnace

Firebox structure

Firing condition

Fuel composition/%(mass)

Length (x-direction)/m

Width

(y-direction)/m

Height

(z-direction) /m

Number of floor burners

Number of wall burners

Fuel gas ?ow rate in bottom/(kg/s)

Fuel gas ?ow rate in side/(kg/s)

CH₄

H₂

C₂H₄

图7 不同空气预热温度下燃烧器上方烟气平均温度和NO分布规律

Fig.7 Flue-gas average temperature and NO distribution above burners at different preheating temperatures

图8 不同空气预热温度下的NO摩尔生成率和质量流率

Fig.8 Mole production rate and mass flow rate of NO at different air preheating temperatures

表3 不同过量空气系数下的入口条件

Table 3 Inlet conditions under different excess air coefficients

α	底部烧嘴燃料流量/(kg/s)	底部风门入口流量/(kg/s)	侧壁烧嘴总流量/(kg/s)
1.05	0.09375	1.70036	1.44857
1.07	0.09375	1.73274	1.47472
1.09	0.09375	1.76513	1.50087
1.10	0.09375	1.78132	1.51395
1.13	0.09375	1.82991	1.55317
1.16	0.09375	1.87849	1.59240
1.20	0.09375	1.94326	1.64470

表4 不同过量空气系数下的模拟结果

Table 4 Simulation results under different excess air coefficients

α	反应净生成热/(J/(cm³?s))	CH₄摩尔分数	温度/K	NO摩尔生成率/(mol/(cm³?s))
1.05	36.09	1.59×10^-5	2225.53	3.03×10^-7
1.07	36.05	1.60×10^-5	2218.58	2.89×10^-7
1.09	35.93	1.62×10^-5	2211.42	2.75×10^-7
1.1	35.88	1.63×10^-5	2207.03	2.68×10^-7
1.13	35.56	1.63×10^-5	2203.82	2.62×10^-7
1.16	35.34	1.65×10^-5	2197.88	2.51×10^-7
1.2	35.02	1.66×10^-5	2192.88	2.42×10^-7

图9 不同过量空气系数下的NO摩尔生成率和反应净生成热

Fig.9 NO molar production rate and net heat production of reaction under different excess air coefficients

图10 两种工况下沿炉膛高度方向NO摩尔分数分布

Fig.10 NO molar fraction distribution along furnace height under two conditions

图11 两种工况下沿炉膛高度截面的平均NO摩尔分数分布

Fig.11 Average NO molar fraction distribution along furnace height section under two conditions

图12 两种工况下炉膛出口处的NO质量分数分布

Fig.12 NO mass fraction distribution at furnace outlet under two conditions

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