CFD simulation of transfer and chemical reaction process in wet flue gas desulfurization tower

doi:10.11949/j.issn.0438-1157.20181368

Abstract

Abstract:

The Eulerian-Lagrangian method is employed to describe the gas-liquid flow and the heat and mass transfer process in a full-scale wet flue gas desulfurization(WFGD) tower, which is in operation at a 330 MW coal-fired power unit. Instantaneous chemical reactions involving 13 dissolved species in aqueous phase are assumed to be at equilibrium obeying mass balance for the specific chemical compositions and electro neutrality. The gas-liquid flow model is coupled with mass transfer and chemical reactions procedures through user defined functions (UDFs). The gas-liquid hydrodynamics, droplets evaporation and SO₂ chemical absorption is predicted in the spray scrubber. Radial distribution characteristics of SO₂ concentration and pH of the droplets are obtained and the effect of gas-liquid flow behavior on chemical absorption process is analyzed. The results show that the local liquid-gas ratio distribution is one of the key factors affecting the radial distribution of SO₂. Furthermore, the absorption performance of the desulfurization tower can be improved by controlling the two-phase flow pattern in the area around the wall and that beneath the main pipe. The drop rates of pH increase with the raise of SO₂ concentration in flue gas or the decrease of droplet diameter. Besides, the higher concentration of SO₂ in gas phase or the smaller characteristic size of droplet can accelerate the process that the concentration of each chemical component reaches a stable state.

Key words: wet flue gas desulfurization, gas-liquid flow, heat and mass transfer, absorption, numerical simulation

摘要：

以某330 MW燃煤发电机组湿法烟气脱硫塔为研究对象，采用欧拉-拉格朗日方法建立了塔内气-液两相流动、热质交换模型，以溶解平衡、质量守恒及电荷守恒描述浆液内13种溶质组分瞬时化学反应特性，通过用户自定义函数实现流动模型与传质模型、化学反应模型的耦合。基于上述模型预测了脱硫塔内气-液两相流动、液滴蒸发与SO₂化学吸收过程，获得了SO₂浓度与浆液pH的径向分布特性，详细分析了气-液流动对化学吸收特性的影响，结果表明局部液气比分布特性是影响SO₂径向分布的关键因素之一，可通过调控近壁区及主管道区的两相流动状态提高脱硫塔的吸收性能；随着气相侧SO₂浓度提高或液滴粒径减小，浆液pH下降速率增大且各化学组分浓度达到稳定状态用时缩短。

关键词: 湿法烟气脱硫, 气液两相流, 热质交换, 吸收, 数值模拟

CLC Number:

X 701.3

Jiangyuan QU, Nana QI, Yanjun GUAN, Yang TENG, Wenqing XU, Tingyu ZHU, Kai ZHANG. CFD simulation of transfer and chemical reaction process in wet flue gas desulfurization tower[J]. CIESC Journal, 2019, 70(6): 2117-2128.

曲江源, 齐娜娜, 关彦军, 滕阳, 徐文青, 朱廷钰, 张锴. 湿法烟气脱硫塔内传递与化学反应过程CFD模拟[J]. 化工学报, 2019, 70(6): 2117-2128.

Figures/Tables 12

Fig.1 Schematic diagram of desulfurization tower and computational domain

Table 1 Reaction equilibrium constants in droplets

Parameter	Formula	Eq.
$K H 2 O$ /(mol/L)²	[H⁺][OH^-]	(19)
$K S O 2, a q$ /(mol/L)	[H⁺][HSO₃ ^-]/[SO_2,aq]	(20)
$K H S O 3 -$ /(mol/L)	[H⁺][SO₃ ^2-]/[HSO₃ ^-]	(21)
$K H S O 4 -$ /(mol/L)	[H⁺][SO₄ ^2-]/[HSO₄ ^-]	(22)
$K C O 2, a q$ /(mol/L)	[H⁺][HCO₃ ^-]/[CO_2,aq]	(23)
$K H C O 3 -$ /(mol/L)	[H⁺][CO₃ ^2-]/[HCO₃ ^-]	(24)

Table 1 Reaction equilibrium constants in droplets

Parameter	Formula	Eq.
$K H 2 O$ /(mol/L)²	[H⁺][OH^-]	(19)
$K S O 2, a q$ /(mol/L)	[H⁺][HSO₃ ^-]/[SO_2,aq]	(20)
$K H S O 3 -$ /(mol/L)	[H⁺][SO₃ ^2-]/[HSO₃ ^-]	(21)
$K H S O 4 -$ /(mol/L)	[H⁺][SO₄ ^2-]/[HSO₄ ^-]	(22)
$K C O 2, a q$ /(mol/L)	[H⁺][HCO₃ ^-]/[CO_2,aq]	(23)
$K H C O 3 -$ /(mol/L)	[H⁺][CO₃ ^2-]/[HCO₃ ^-]	(24)

Fig.2 Schematic diagram of geometry discretization and relative errors profiles of average SO2 concentration at different XY planes

Fig.3 Contour of gas velocity distribution

Fig.4 Dimensionless axial velocity radial profiles

Fig.5 Contour of discrete phase concentration

Fig.6 Diameter distribution character of droplets in different cross sections

Table 2 Measured values and simulation results of gas temperature and vapor volume fraction

项目	T _g,in /℃	T _g,out /℃	$V H 2 O, i n$ /%	$V H 2 O, o u t / %$
实测 Ⅰ	125.0	48.0	7.5	13.2
实测 Ⅱ	135.0	50.0	6.2	13.7
实测 Ⅲ	131.0	51.0	7.1	13.4
平均值	130.3	49.7	7.0	13.4
模拟值	130.0	50.0	7.0	13.8
相对误差	—	0.60%	—	2.99%

Table 2 Measured values and simulation results of gas temperature and vapor volume fraction

项目	T _g,in /℃	T _g,out /℃	$V H 2 O, i n$ /%	$V H 2 O, o u t / %$
实测 Ⅰ	125.0	48.0	7.5	13.2
实测 Ⅱ	135.0	50.0	6.2	13.7
实测 Ⅲ	131.0	51.0	7.1	13.4
平均值	130.3	49.7	7.0	13.4
模拟值	130.0	50.0	7.0	13.8
相对误差	—	0.60%	—	2.99%

Fig.7 Influence of initial gas temperature on temperature and relative humidity of flue gas in tower

Fig.8 Radial profiles of SO2 concentration in gas phase

Fig.9 Slurry distribution in spray scrubber colored by pH

Fig.10 Influences of injection location and droplet diameter on pH

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