Simulation investigation on marine exhaust gas SO₂ absorption by seawater scrubbing

ABSTRACT

Ships have become an important source of SO₂ emission in coastal areas with the rapid development of maritime transport. It is of great significance to develop a marine scrubber for reducing SO₂ emission of ships. In this study, numerical simulation of a full-scale marine spray scrubber is conducted to investigate two-phase flow pattern and SO₂ absorption process in the scrubber. A desulfurization model based on seawater absorbent is coupled into the simulation, which considers the mass transfer between phases and seawater aqueous phase chemistry simultaneously. A distribution ring is introduced in the scrubber to enhance the desulfurization performance of the scrubber. The result of simulation shows that the distribution ring can optimize effectively the distribution of gas–liquid phases and enhance the SO₂ absorption. Under vertical condition, the desulfurization efficiency could be promoted approximate 6% after installing a distribution ring. The inclined condition resulting from the ship swinging could lead to the uneven distribution of droplets and an obvious decrease (8.7%) of desulfurization efficiency, whereas the desulfurization performance of the scrubber could be ensured with a distribution ring installed even under an inclined condition. Finally, a spray scrubber design scheme has been developed and successfully applied in the exhaust gas cleaning system (EGCS) of a container ship. Test result shows the outlet average value of SO₂/CO₂ can be reduced to 3.55. Meanwhile, the consistency of test data and calculation result indicates the applicability of the numerical model established for the simulation and optimization of the scrubber in industrial applications also.

Implications: EGCS is an effective method to reduce SO₂ emission of marine industry. However, different from a land desulfurization tower, the application of a spray scrubber in EGCS faces more problems due to the different application scenarios and complex sea conditions (inclined condition resulting from ships swinging and so on) during sailing. In this work, a numerical model capable of investigating physical and chemical phenomena in the scrubber simultaneously is established, which can produce a great amount of data for the operation instruction of EGCS and the design and optimization of the marine spray scrubber. The distribution ring is introduced in the marine spray scrubber to intensify the SO₂ absorption and enhance the desulfurization performance of the scrubber under different working conditions.

Nomenclature

$A_p$	=	surface area of droplet, m2
Cd	=	drag coefficient
Cp	=	specific heat of droplet, kJ/(kg·K)
Ci	=	concentration of i, mol/m3
${\mathrm{C}}_{\mathrm{S}{\mathrm{O}}_2,\mathrm{l}}$	=	concentration of SO2 in liquid phase, mol/m3
$[\mathrm{C}]$	=	carbon species in liquid phase
${\mathrm{d}}_{\mathrm{p}}$	=	diameter of droplet, m
${\mathrm{D}}_{\mathrm{S}{\mathrm{O}}_2,\mathrm{g}}$	=	diffusion coefficient of SO2 in air, m2/s
${\mathrm{D}}_{\mathrm{S}{\mathrm{O}}_2,\mathrm{l}}$	=	diffusion coefficient of SO2 in liquid phase, m2/s
e	=	internal energy per unit mass, kJ/kg
${\mathrm{E}}_{\mathrm{S}{\mathrm{O}}_2}$	=	mass-transfer enhancement factor
$F_p$	=	drag force between phases, N
g	=	gravitational acceleration, m/s2
$h$	=	convective heat transfer coefficient, W/(m2·K)
${\mathrm{H}}_{\mathrm{S}{\mathrm{O}}_2}$	=	Henry coefficient of SO2, Pa··m3/mol
${\mathrm{k}}_{\mathrm{g}}$	=	gas-side mass transfer coefficient, mol/(m2·s·Pa)
$k_l$	=	liquid-side mass transfer coefficient, m/s
${\mathrm{K}}_{\mathrm{i}}$	=	equilibrium constant of chemical reactions
${\mathrm{K}}_{\mathrm{G}}$	=	global SO2 mass transfer coefficient, mol/(m2·s·Pa)
${\mathrm{m}}_{\mathrm{p}}$	=	mass of droplet, kg
$M_{S{O}_2}$	=	molecular weight of SO2, g/mol
$N_{S{O}_2}$	=	molar mass transfer of SO2 between phases, mol/(m2·s)
$Nu$	=	Nusselt number
$P_{S{O}_2,\mathrm{g}}$	=	partial pressure of $\mathrm{S}{\mathrm{O}}_{2(\mathrm{g})}$ in the gas bulk, Pa
$\mathrm{P}\mathrm{r}$	=	Prandtl number
R	=	universal gas constant, J/(mol $\cdot$ K)
$\mathrm{R}{\mathrm{e}}_{\mathrm{p}}$	=	Reynolds number of droplet
$S_{ss,p}$	=	mass source term for droplet
$S_{mom,p}$	=	momentum source term for droplet
$S_{en,p}$	=	energy source term for droplet
$\mathrm{S}\mathrm{h}$	=	Sherwood number
$Sc$	=	Schmidt number
$\left[\mathrm{S}\right]$	=	sulfur species in liquid phase
$T$	=	temperature of gas phase, K
$T_p$	=	temperature of droplet, K
${\vec{u}}_{\mathrm{g}}$	=	velocity of gas phase, m/s
$\overrightarrow{u_p}$	=	velocity of droplet, m/s
$w_k$	=	mass fraction of $k$ species
$\rho$g	=	density of gas, kg/m3
${\rho }_p$	=	density of droplet, kg/m3
$\mu$	=	dynamic viscosity, Pa·s
${\sigma }_p$	=	the surface tension of liquid phase, N/m
${\lambda }_{\mathrm{g}}$	=	thermal conductivity, W/(m·K)
$\stackrel{=}{\tau }$	=	shear stress, kg/(m·s2)

Abbreviation

EGCS	=	exhaust gas cleaning system
DR	=	distribution ring
SL	=	sprayer layer

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work is supported by the National Natural Science Foundation of China (No. U1609212).

Notes on contributors

Wenjun Li

Wenjun Li is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Yongxin Zhang

Yongxin Zhang is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Zhongyang Zhao

Zhongyang Zhao is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Chang Liu

Chang Liu is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Yifan Wang

Yifan Wang is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Mingqiang Shen

Mingqiang Shen is affiliated with Zhejiang Energy Marine Environmental Technology Co., Ltd, Hangzhou, People’s Republic of China.Republic of China.

Haobo Dai

Haobo Dai is affiliated with Zhejiang Tiandi Environmental Protection Engineering Co., Ltd, Hangzhou, People’s Republic of China

Yang Yang

Yang Yang is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Chenghang Zheng

Chenghang Zheng is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.

Xiang Gao

Xiang Gao is affiliated with State Key Laboratory of Clean Energy Utilization, State Environmental Protection Engineering Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou, People’s Republic of China.