Fouling of Impurities in Desulfurized Flue Gas on Hollow Fiber

Dec 31, 2015 - ABSTRACT: Membrane-based absorption shows numerous potential applications for CO2 capture from flue gas. However, during the process, m...
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Fouling of Impurities in Desulfurized Flue Gas on Hollow Fiber Membrane Absorption for CO2 Capture Lin Zhang, Juan Li, Lei Zhou, Rui Liu, Xia Wang, and Linjun Yang* Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, P R China ABSTRACT: Membrane-based absorption shows numerous potential applications for CO2 capture from flue gas. However, during the process, membrane fouling is not only caused by the wetting but also induced by gaseous components (SOx, NOx, and water vapor) and fine particles in the flue gas. In this work, the synergetic effects of water vapor coupling with SOx and NOx as well as the fine particles were investigated to better understand the membrane fouling and the performance of polypropylene hollow fiber membrane modules for CO2 capture in actual industrial conditions. The CO2 removal efficiency and the masstransfer rate over time were studied under different feed gas flows of various gaseous composition schemes. The membrane performance suffered from a notable decrease after exposure to SO3 and particles, and in a wet gas stream, the effects of SO3 and particles were irreversible, whereas the influence of SO2 and NO2 can be recovered by N2 sweeping. The characterization results of membrane modules confirm that the membranes are polluted by gaseous and particle impurities to different extents, and after long-term operation in flue gas containing impurities, the morphology of materials changed substantially. The reduction of contact angles illustrated that the hydrophobic property decreases obviously after exposure to SOx. Moreover, the results of Energy dispersive spectrometry and Fourier transform infrared spectroscopy indicated that the interaction of stranded SO3 and the membrane formed a new group. During the process of membrane CO2 adsorption from flue gas containing fine particles, the pores of the membrane were blocked by fine particles and the surfaces were covered by a particle layer, which occupies the efficient contact area and leads to an increase of mass-transfer resistance and reduced membrane performance.

1. INTRODUCTION The conclusion that the increasing concentration of CO2 in the atmosphere leads to global warming is the consensus of the world. In 2010, the International Energy Agency (IEA) presented the carbon emissions reduction target scheme to reduce emissions to 14 Gt of CO2 by 2050.1 To achieve the goal, carbon capture and storage (CCS) from major emission sources is of extreme importance. With a 40% share of total emissions, coal-fired power plants are the main greenhouse gas producer.2 This underlines the essential nature of CO2 capture from power plants. Postcombustion CO2 capture technology is one of the direct and efficient approaches for reducing the CO2 emission from flue gas. At present, a wide range of technologies is available or under development to capture CO2 from the flue gas, such as chemical solvent (e.g., MEA, DEA),3,4 solid sorbents,5,6 cryogenic separation,7,8 membrane technology,9−11 and so on. Amine stripping as a widely studied technology has been in commercial operation. However, it shows several disadvantages in terms of huge stripper size, corrosion problems, and high energy consumption as well as undesirable gas−liquid contact issues, including flooding, liquid channeling, and foaming.12−15 Membrane-based absorption combining the advantages of chemical absorption and membrane technology overcomes the problems of conventional technologies and has become a sought-after method for CO2 capture. In the application of membrane-based gas absorption, although the porous membrane itself offers no selectivity, it provides a desirable gas−liquid contact area for mass transfer between gas and absorbent. With CO2 absorbent flowing in counter-current through the shell or tube side of the membrane module, CO2 in the gas stream on the opposite side is absorbed © XXXX American Chemical Society

rapidly without interpenetration of gas and liquid phase. Hence, the gas and liquid flow rate can be optimized independently, avoiding operational problems.14 During this process, the transportation of gas spans gas boundary and liquid boundary layer consisting of gas phase, membrane phase, and liquid phase resistances. The resistance-in-series model was presented to describe how the mass transfer of gas molecules overcomes a series of resistances from gas bulk to liquid phase.15 These resistances play a directly role in the diffusion of gas molecules and the mass transfer. In the application of membrane absorption for CO2 capture from coal-fired power plant, the gaseous impurities (such as SOx, NOx, and saturated water vapor) and fine particles in flue gas can adhere to the membrane, interact with the membrane material, and enhance the membrane resistance, which may result in membrane fouling resulting in degradation of membrane performance. Membrane fouling is widely studied in the wastewater treatment using membranes; however, it seldom receives attention with respect to gas separation processes. Numerous reports focus on membrane wetting, which we think is one kind of membrane-fouling problem. Wang et al.16 found that the CO2 flux decreased about 20% in the initial 4 days of operation when using diethanolamine (DEA) as the absorbent because of Special Issue: International Conference on Carbon Dioxide Utilization 2015 Received: October 26, 2015 Revised: December 14, 2015 Accepted: December 31, 2015

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zhou, PRC). The fly ash fine particles provided by Sinopec Yangzi Petrochemical Co., Ltd. (Nanjing,PRC) were introduced into the feed gas to simulated actual flue gas. The specifications of the membrane module and parameters of simulated flue gas are listed in Table 1.

the membrane wetting. The study of Lv et al.17 defined that absorbent molecules diffused into polypropylene (PP) polymers leading to swelling of the membranes and the reduction of contact angle. Comparing the performance of PP and polytetrafluoroethylene (PTFE) membranes, Chabanont et al.18 considered that the mass-transfer resistance of PP and PTFE porous fibers is more sensitivity to wetting effect. All the above results highlight the effect of liquid phase on membrane absorption processes and membrane materials; however, the impurities in the gas phase of actual flue gas also lead to membrane fouling, which exerts influence on the membrane performance and damages the membrane materials and modules. Condensation of water vapor can induce the plasticization of the membrane resulting in lifespan reduction.19 Fang et al.20 considered that supersaturated water forming droplets on the gas side of the membrane compromised the gas transfer with the increasing resistance of the membrane. Furthermore, Bram et al.21 found that the membrane properties decreased dramatically because of a particle layer formed after long-term exposure in flue gas. In the study of Brands et al.,22 corrosion defects were observed at the membrane module and separation behavior changed because of the interaction of the membrane and water vapor. The long-term test of the membrane in the power plant indicated that membrane fouling is a notable problem for industrial application of membrane modules in terms of CO2 capture from coal-fired flue gas. Moreover, SOx in the desulfurized flue gas can combine with H2O to generate acid fog and aerosol, which may damage the stable operation of the membrane module. In our previous work, the independent effects of SO2, relative humidity, and fine particles on the performance of membranes for CO2 capture were studied preliminarily.23 However, these effects and the detailed variations of membrane materials and microstructure were not investigated, being left for investigation in the present work. Additional experimental and mechanism studies are necessary to achieve a better understanding of membrane fouling in power plants. The main objectives of this work are investigating experimentally the synergetic effects of water vapor coupling with SOx and NOx as well as fine particles and studying the changing of membrane material properties to characterize the interaction of membranes and impurities. CO2 capture processes using PP hollow fiber membrane modules were operated under SO2 and H2O, SO3 and H2O, and fine particles and H2O conditions. The trends of CO2 removal efficiency and mass-transfer rate versus time were studied. The variation of membrane properties, in terms of microstructure and hydrophobic properties, was characterized by field emission scanning electron microscopy (FESEM) and contact angle measurement, respectively. In addition, the interaction mechanism behind the membrane-fouling process was studied using Fourier transform infrared spectroscopy (FT-IR) and Energy dispersive spectrometry (EDS).

Table 1. Specifications of PP Hollow Fiber Membrane Contactor parameter

value

outer diameter inner diameter effective length fiber inner diameter fiber outer diameter CO2/N2 SO2/H2O SO3/H2O NO2/H2O

94 mm 80 mm 380 mm 300 μm 400 μm 12/88 vol% 857 g·m−3, RH = 85% 112 g·m−3, RH = 85% 616 mg·m−3, RH = 85%

2.2. Apparatus and Procedure. Schematic diagrams for the membrane contactor and the experimental apparatus for CO2 absorption by a hollow fiber membrane module are shown in Figure 1. A membrane contactor generally consists of a bundle of hollow fibers and cylindrical cavity shell, and both ends of the contactor are sealed by glue. The inner wall of the hollow fibers is called “tube side”, and the contactor shell and the outer wall of the fibers make up the “shell side”. During the membrane absorption process, the gas and the absorbent solution flow on opposite sides of the microporous membrane fibers to ensure the noncontact of gas and liquid phase. CO2 is removed from the mixture through concentration gradient driven diffusion. Three gas streams (CO2, N2, and SO2) controlled by the mass flow controllers are mixed to prepare the simulated flu gas (12% vol CO2/88% vol N2). To simulate the environment after desulfurization, SO3 was introduced by a SO3 generator, and an extra water supply device was installed to ensure a high relative humidity (RH). The RH values were monitored online by an HMT337 humidity transmitter. There was a SAG-410 aerosol generator (TOPAS, Germany) for testing the membrane performance under particle conditions. Wet feed gas at a flow rate of 500 L·h−1 was fully mixed in the buffer tank and various component concentrations were deteremined before entering the tube side of the membrane at room temperature (25 °C) and ambient pressure. The concentrations of each component at the inlet and outlet of the membrane module were measured by an online gas analyzer (Tsubaki Emerson) and the flue gas analyzer (MRU, Delta 2000CD-IV). The MEA absorbent with the flow rate of 24 L· h−1 was pumped through the shell side of the membrane without desorption cycling. The operation of CO2 membrane absorption from desulfurized flue gas was conducted at the outlet of the wet flue gas desulfurization (WFGD) system. The particle size distribution and concentration parameters were measured by an electrical low-pressure impactor (ELPI, Dekati Ltd., Finland). 2.3. Membrane Characterization Methods. Fourier transform infrared spectroscopy (Thermo Nicolet 6700) and Energy dispersive X-ray detector were used to analyze membrane composition and functional group before and after the membrane fouling. The hydrophobic properties of fresh and fouled membranes were tested by a DropMeter A-100 contact angle measurement instrument. SEM images of PP

2. MATERIALS AND METHODS 2.1. Materials. CO2, N2, SO2, and NO2 with purities of 99.99%, 99.9%, 99.99%, and 99.99%, respectively (Nanjing Shangyuan industrial gas plant, PRC) were used to generate the simulated flue gas. Monoethanolamine (MEA) with purity of 99.5% was dissolved in deionized water with the concentration of 0.5 mol·L−1 for CO2 membrane absorption. Polypropylene hollow fiber membrane modules were purchased from Hangzhou Kaihong Membrane Technology Co., Ltd. (HangB

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Figure 1. Schematic diagrams for the (a) membrane contactor and (b) membrane CO2 absorption test system.

Figure 2. Synergetic effects of SO2 and water vapor on the membrane CO2 absorption performance: (a) CO2 removal efficiency and (b) masstransfer rate (CCO2 = 12%, CSO2 = 857 mg·m−3, CNO2 = 616 mg·m−3, RH = 85%, Qg,in = 500 L·h−1, QL,in = 12 L·h−1).

hollow fiber membranes. These quantities can be calculated by eqs 1 and 2 respectively.

hollow fibers before and after particles exposure were observed using a field emission scanning electron microscopy instrument (Ultra Plus) to study changes in the microstructure. 2.4. Evaluation Parameters of Membrane Absorption. In this study, the CO2 removal efficiency (xd) and the masstransfer rate (JCO2) were used to study the performance of PP

xd =

(Cg,in − Cg,out) Cg ,in

⎛ Cg ,out ⎞ ⎟ × 100% × 100% = ⎜⎜1 − Cg ,in ⎟⎠ ⎝ (1)

C

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Figure 3. Spectrogram for PP membrane fibers after SO2 exposure for 60 h: (a) EDS analysis and (b) FT-IR analysis.

JCO = 2

reduction of mass-transfer rate shown in Figure 2b indicates that the properties of the PP membrane have a notable influence under the coexistence of SO2 and high RH water vapor. In addition, although membrane wetting under the CO2/N2 feed gas (depicted as a red line in Figure 2) conditions was limited during 60 h of operation,17 the synergetic SO2 and water vapor may contribute positively to the wetting effects of MEA due to changes in the hydrophilic property. When the retrieval test was conducted by blowing N2 cylinder gas for 5 h, a rebound in the CO2 removal efficiency from 65.6% to 78% indicated that the nonpermanent damage of the membrane was caused by the synergetic SO2 and water vapor. To better understand the synergetic effects of SO2 and water vapor, the fresh membrane and the membrane after exposure to SO2 and water vapor were analyzed by FT-IR and EDS. The EDS results of used membrane in Figure 3a indicate that there are sulfur and oxygen elements on the membrane surface, which indicates that the SO2 was not totally absorbed by MEA and still adhered on the membrane. The diffusion channel for CO2 transfer into the membrane pores was partly occupied by the residual SO2, which results in the decrease of the masstransfer rate. Figure 3b shows the FT-IR spectra for PP fibers after exposure to SO2/H2O. The peaks at 1460 and 1378 cm−1 and tetrad peaks in the range of 2960−2800 cm−1 are the carbon skeleton peaks of PP. Furthermore, the C−H bending vibration absorption peak at 1166 cm−1 and the peaks at around 998 and 973 cm−1 are the characteristic peaks of PP. No obvious groups change was found in the FT-IR results, which illustrated that the synergetic effects of SO2 and H2O would not cause any chemical change of the membrane and explains the performance recovery of the membrane after N2 blowing. As presented in Figure 4, the contact angles were measured to study the hydrophobic property of the PP membrane before and after experiment. PP is a hydrophobic material, and a small decrease of the contact angle of the fresh membrane from 114.1° to 104.2° was obtained as a result of SO2 (857 mg·

(Q g ,inCg ,in − Q g ,outCg ,out) × 273.15 0.0224 × Tg × S

(2)

where xd is the CO2 removal efficiency, %; JCO2 is the masstransfer rate, mol·m−2·h−1); and Cg,in and Cg,out are the volumetric fraction in the inlet and outlet gas respectively, %. Qg,in and Qg,out are the gas flow rate in the inlet and outlet gas, respectively; Tg is the gas temperature, K; and S represents the effective membrane area, m2.

3. RESULTS AND DISCUSSION 3.1. Synergetic Effects of SO2/H2O and NO2/H2O. The concentrations of gaseous components in the desulfurized flue gas are 10−20% vol CO2, SO2 below 100 mg·m−3, NOx 50− 100 mg·m−3 (NOx includes NO2 and NO; NO2 accounts for about 90−95%, NO accounts for about 5−10%) and saturated water vapor. To investigate the influence of these gas impurities in a short time, the simulated flue gas with NO2 616 mg·m−3, RH = 85%, 857 mg·m−3 SO2 as well as RH = 85% and 616 mg· m−3 NO2 was prepared for experiments. As shown in Figure 2, NO2 showed negligible effect on the membrane absorption for CO2 capture and CO2 removal efficiency, and the mass-transfer rate remained stable during 60 h of operation. However, the NO2 and water vapor mixture slightly decreased the CO2 removal efficiency and the mass-transfer rate, which may be mainly caused by the blockage of the transfer channel for CO2 due to water condensation in the membrane pores. The performance of a membrane shows significant decrease after 60 h of operation in the presence of SO2 and H2O (red line in Figure 2). The CO2 removal efficiency and the masstransfer rate reduced from 80.2% and 0.353 mol·m−2·h−1 to 65.6% and 0.273, respectively. In our previous experimental work using a PP hollow fibers membrane module,23 the independent presence of SO2 slightly decreased the CO2 removal efficiency and the influence of water vapor with RH = 85% became stable after 6 h of operation. Compared with the results presented in ref 23, the coexistence of SO2 and water vapor promoted the performance deterioration, which was presumably due to the following reasons. First, the effect of capillary condensation makes water transfer into pores of the membrane, and with the presence of SO2, SO2 molecules combine much easier with water drops24 in the pore leading to the greater increase of membrane resistance. Moreover, SO2 absorbing on the membrane decreases the mass-transfer coefficient of the membrane, reducing the CO2 flux.25 The

Figure 4. Contact angle of the PP hollow fiber membranes: (a) fresh membrane, (b) fouled membrane after SO2 exposure, and (c) fouled membrane after retrieval test by blowing N2 gas. D

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Figure 5. Synergetic effects of SO2 and water vapor on the membrane CO2 absorption performance: (a) the CO2 removal efficiency and (b) masstransfer rate (CCO2 = 12%, CSO3 = 112 mg·m−3, RH = 85%, Qg,in = 500 L·h−1, QL,in = 12 L·h−1).

Figure 6. Spectrogram for PP membrane fibers after SO3 exposure for 60 h: (a) EDS analysis and (b) FT-IR analysis.

m−3)/H2O (RH = 85%) exposure. This indicates that the synergetic effects of SO2 and water vapor altered the surface energy of membranes and thereby reduced the hydrophobic property of the PP material. However, the contact angle was recovered (Figure 4c) by sweeping with N2 cylinder gas, which could as well offer an interpretation for the retrieval of the CO2 removal efficiency after the retrieval test. 3.2. Synergetic Effects of SO3 and H2O. The SCR catalyst can convert typically 0.5− 2% of SO2 to SO3 in addition to 0.5−1.5% of the SO2 in the flue gas being oxidized to SO3 during boiler combustion. That SO3 is hardly removed by the electrostatic precipitator and the WFGD system and may result in the corrosion of the membrane module and materials. Generally, the concentration of SO3 is below 20−30 mg·m−3 in desulfurized flue gas; to investigate the influence over a short time, CO2 separation from the flue gas containing 112 mg·m−3 SO3 was operated for 60 h. Figure 5 shows the trend of membrane performance versus time under the condition of SO3 (112 mg·m−3)/ H2O (RH = 85%). The CO2 removal efficiency and the mass-transfer rate reduced by 26.7% and 34.9%, respectively, after the 60 h experiment. During 20−30 h of operation, the CO2 removal efficiency jumped from 75.5% to 63.3%. For the overall trend, the parameters declined significantly within the first 30 h, then the further reduction was moderate. By correlating Figure 2 with Figure 5, the decrease of the membrane performance for CO2 absorption could be attributed to SO3 sulfuric acid mist in

spite of the capillary condensation of water vapor on the surface and pore of microstructure membrane. It is extremely easy for SO3 to absorb water to form sulphurous acid,26 which exerts a strong corrosion ability on the membrane material; furthermore, SO3 and H2O tend to transport into H2SO4 aerosol by homogeneous or heterogeneous nucleation27,28 especially in moist circumstances, leading to severe membrane fouling. In the gas phase, the gas film was thickened by condensation of H2SO4 aerosol with high adhesiveness and resulted in the increase of gas resistance. In addition, the hydrophobic property of the membrane may be changed because acid molecules transport into membrane pores and adsorb on pore walls after long-term exposure to SO3 sulfuric acid mist, which could make the membrane more likely to be wetted by MEA, and membrane resistance increased. Consequently, the CO2 removal efficiency and the mass-transfer rate obviously reduced. Although the membrane could not be defined to work under nonwetted conditions, compared with the wetting effect on membrane properties in refs 16−18, the SO3 and H2O contribute to a major decrease in membrane properties. The retrieval test was also conducted, and the CO2 removal efficiency remained at 65%, which indicates that the synergetic effect of SO3 with H2O is irreversible. The FITR and EDS spectra for the used membrane were analyzed and the contact angles were measured to investigate the irreversible changes in the membrane performance. A small amount of sulfur was stranded on the membrane, shown in E

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the opposite sides of the membrane to measure the gas-phase parameters. It can be seen from Figure 8b that the initial CO2 removal efficiency for gas flow in the shell side is lower than that in the tube side, which consistently matches the study of Lai et al.29 In the presence of fly ash particles, the membrane absorption properties for tube side and shell side reduced continuously as the time increased, with a reduction of 16.4% and 27.9%, respectively, after 25 h of operation. The downward trend in the CO2 removal efficiency for both flow schemes showed that fly ash particles serious deteriorate the membrane performance, especially when gas flow is in the shell side. Particle depositioninduced decrease on the membrane properties was verified and discussed in our previous work. To determine how the CO2 removal efficiency exhibited a different decreasing rate, microscale analyses were performed using ELPI and FESEM to study the parameters of fly ash particles in the gas stream before and after membrane modules under the two flow schemes and the changing of membrane morphology. As depicted in Figure 9, the number concentration of particles is 4.8 × 105 cm−3 and the peak particle size is 1.362 μm at the inlet of the hollow fiber membrane module. At the outlet of the tube side and shell side modules, the number concentration decreased by 91.5% and 99.9%, respectively. Severe membrane fouling was observed from the SEM image of the outer wall of the membrane fibers after exposure to fly ash (see Figure 10a). The results demonstrate that a greater amount of particles flowing through the shell side of the membrane module were intercepted and the deposited particles adhere on the membrane surface to form a particle cake layer that prevents CO2 molecules from diffusing and absorbing. Nevertheless, particle size shows no obvious changes. By contrast, the membrane fouling due to fly ash particle deposition for the tube side was less serious and less evenly distributed; moreover, these particles were likely to aggregate in clusters to block the flow channel. Therefore, the different degree of membrane fouling contributes to different rates of decrease of membrane performance. The possible reasons for the different degree of membrane fouling could be as follows. The flow of particles due to the gas entrainment under the tube side flow scheme is similar to that of microscale forced tube flow, which is affected by one-way resistance. The flow of particles under the shell side flow scheme is similar to that of forced external flow passing tube bundles, which is affected by

Figure 6a. As depicted in Figure 6b, the peaks at 1177 and 1068 cm−1 may be attributed to the sulfonic group; moreover, superposition peaks due to the stretching vibration of CH2 and CH3 at 2960−2800 cm−1 were weakened, which may be due to the CH3 being partly oxidized by SO3. The C−N bending vibration peak appearing at 580 cm−1 may caused by MEA wetting. It could be conjectured from the strong sulfonic group that coexistence of SO3 and gaseous H2O may acidize the PP membrane fiber surface and seriously weaken the hydrophobicity of PP. In addition, the contact angle of PP fibers (Figure 7) decreased from 114.1° to 88.8° and became 92.8°

Figure 7. Contact angle of the PP hollow fiber membranes: (a) fresh membrane, (b) fouled membrane after SO2 exposure, and (c) fouled membrane after retrieval test by blowing N2 gas.

after N2 cylinder gas sweeping, which verifies that the surface wettability of the PP membrane changed from hydrophobic to hydrophilic. The change of contact angle explains the irreversible synergetic effect of SO3 with H2O on PP membrane properties for CO2 capture. The aforementioned results illustrate that the gaseous impurities in desulfurized flue gas exert negative influence on the performance of membrane absorption for CO2 capture, bring about membrane fouling problems, and even shorten the membrane life span. 3.3. Effect of Particles on Membrane CO2 Absorption under Two Flow Schemes. Gas stream flowing in the tube side and in the shell side of the membrane module are two typical flow schemes for membrane gas absorption (shown in Figure 8a). To investigate the effect of particles on membrane CO2 absorption under different flow schemes, a stream of fly ash particles with mass concentration below 200 mg·m−3 was introduced by aerosol generator into the CO2/N2 mixture to prepare the simulated flue gas. The flow rate of the feed gas was maintained at 0.5 m3·h−1, and the concentration of CO2 was kept at 12 vol %. A “non-absorbent” gas−membrane contact method was contrived to remove the distractions of MEA wetting as much as possible. The simulated flue gas was first supplied into the tube or shell side of the membrane module for 2 h, then MEA aqueous solution was introduced to fully fill

Figure 8. Membrane CO2 absorption process in the presence of fly ash particles: (a) typical flow schemes and (b) CO2 removal efficiency (CCO2 = 12%, Qg,in = 500 L·h−1, Ql,in = 12 L·h−1). F

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Figure 9. Fly ash particle parameters at the inlet and outlet of membrane module for two typical flow schemes: (a) number concentration and (b) particle size distribution.

Figure 10. FESEM images of fouled membrane fibers by coal-fired fly ash particle deposition: (a) outside and (b) inner side.

Figure 11. FESEM images of membrane fibers surface and pores fouled by coal-fired fly ash particle deposition after tube side exposure.

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analyzed. It could be concluded that gaseous and particulate impurities can lead to membrane fouling and changes in the membrane properties to different extents. The effect of dry NO2 is negligible as long as the concentration of NO2 is much lower than that of CO2, while the membrane performance slightly decreased after introduction of water vapor. In the coexistence of SO2 and H2O, the CO2 removal efficiency and the mass-transfer rate reduced by 18.2% and 29.3%, respectively, whose reductions were promoted presumably because of capillary condensation effect of H2O molecules, the competition absorption of SO2, and the interaction of water vapor and acid gas. The low concentration of SO3 could cause serious decrease in the membrane performance after 60 h of operation, probably caused by the SO3 sulfuric acid mist and H2SO4 aerosol. Moreover, the EDS and FT-IR analyses for membrane fibers after gaseous impurities exposure indicated that SO3 could not be absorbed by MEA totally, which may generate new sulfonic group and may acidize the PP membrane fiber surface, seriously weakening the hydrophobic property. The contact angle of PP membrane has dramatic reduction after 60 h of operation in the presence of SO3, and the surface wettability of the membrane fundamentally changed, while the decrease after SO2 exposure was relatively small and could be recovered by sweeping N2 cylinder gas. In addition, severe membrane fouling was observed after exposure to fly ash whether gas was in the tube side or shell side; furthermore, higher CO2 removal efficiency and less reduction was obtained under the tube side flow scheme because of different interception rate for particles and resistance of membrane module. Finally, fine fly ash particles were observed not only adhering to the surface of the membrane but also entering into and blocking the membrane pores, which was the main reason for the decrease of membrane performance.

horizontal and vertical stresses. The resistance for the latter is much higher, particularly for thousands of compactly staggered membrane fibers in the module. Thus, the membrane is easier to foul because of the interception and deposition of particles, which occupy the effective mass-transfer area and destroy the surface structure of the membrane, resulting in the severe deterioration of the performance of the membrane module. It is concluded from the present results that in view of the influence of fine particles, the tube side flow scheme better suits the industrial application for reducing the membrane fouling. Cross-sectional SEM images of PP hollow fiber membranes for the tube side flow scheme were analyzed to study the mechanism for fine particle fouling and reducing the membrane performance. As shown in Figure 11, particles not only adhere to the inner face of the membrane but also enter into the membrane pores, which thickens the membrane and blocks the transfer channel. The membrane mass-transfer resistance, 1/ Km, is given by eq 3:30 τR i(ln R o/R i) 1 = Km εDp

(3)

where τ is the membrane tortuosity and ε is the membrane porosity; Ri and Ro are the inner and outer radius of the membrane fiber, respectively. Dp, expressed by eq 4, is the overall diffusion coefficient consisting of the molecular diffusion and Knudsen diffusion. DKn is given by eq 5.31 Dp =

DKn

1 (1/Dm) + (1/DKn)

0.5 4 d p ⎛ 2R gTg ⎞ = ⎟ ⎜ 3 2 ⎝ πMA ⎠

(4)

(5)

■

where dp is the pore diameter and MA is the molecular weight of the diffusing component. When the membrane was fouled by fine particles, membrane thickness was enhanced leading to the increase of membrane mass-transfer resistance; however, the porosity was reduced because of particle adhesion onto the pores. Furthermore, the reduction of pore diameter weakens the Knudsen diffusivity coefficient, which increases the membrane mass-transfer resistance. The composition of the particles is the oxide of silicon, iron, aluminum, calcium, magnesium, sodium, and sulfur. The major component is SiO2, which is hydrophilic. The van der Waals force, electrostatic force, and liquid bridge force are the main reasons leading to the adhesion of particles. The hydrophilic particles are more likely to adhere on the surface of the membrane because of the liquid bridge in nonvacuum environment. A more specific study of the adhesive force will be undertaken in further work. Therefore, the influence of particles appears in the terms of cake layer effects and increase of masstransfer resistance, causing the decrease of CO2 removal efficiency and reducting the membrane performance. Consequently, the membrane fouling due to fine particles is a notable problem for postcombustion CO2 capture from desulfurized flue gas.

AUTHOR INFORMATION

Corresponding Author

*Tel: +86-025-83795824. Fax: +86-025-83795824. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (51176034), Doctoral Scientific Fund Project of the Ministry of Education of China (20130092110005), Ordinary University Graduate Student Scientific Research Innovation Projects of Jiangsu province (KYLX15-0072), and Scientific Research Foundation of Graduate School of Southeast University (YBJJ1508). The authors gratefully acknowledge this financial support.

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REFERENCES

(1) IEA. Energy Technology Perspectives: Scenarios & Strategies to 2050; International Energy Agency OECD/IEA: Paris, 2010; www.iea. org. (2) Schivley, G.; Ingwersen, W. W.; Marriott, J.; Hawkins, T. R.; Skone, T. J. Identifying/quantifying Environmental Trade-offs Inherent in GHG Reduction Strategies for Coal-fired Power. Environ. Sci. Technol. 2015, 49 (13), 7562−7570. (3) Ahmad, A. L.; Sunarti, A. R.; Lee, K. T.; Fernando, W. J. N. CO2 Removal Using Membrane Gas Absorption. Int. J. Greenhouse Gas Control 2010, 4 (8), 495−498.

4. CONCLUSION Based on the WFGD conditions, the synergetic effects of water vapor coupling with SOx and NOx as well as fine particles on PP hollow fiber membrane module performance and the changing of membrane material properties were studied and H

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DOI: 10.1021/acs.iecr.5b04021 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX