Published at : 09 May 2023
Volume : IJtech Vol 14, No 3 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i3.5544
|Sutrasno Kartohardjono||Process Intensification Laboratory, Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok 16424, Indonesia|
|Eva Fathul Karamah||Process Intensification Laboratory, Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok 16424, Indonesia|
|Grace Nathalie Talenta||Process Intensification Laboratory, Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok 16424, Indonesia|
|Thariq Ahmad Ghazali||Process Intensification Laboratory, Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok 16424, Indonesia|
|Woei Jye Lau||Advance Membrane Technology Research Center, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia|
Hazardous pollutants such as NOx (NO and NO2) and SO2 generally come from fossil fuel combustion, harm the human respiratory system, and damage environmental ecosystems. The conventional technology that has been used so far consists of two methods: FGD (Flue Gas Desulfurization) and SCR (Selective Catalytic Reduction) or SNCR (Selective Non-Catalytic Reduction) to remove SO2 and NOx. The study aims to examine the performance of polysulfone membranes in removing NOx and SO2 simultaneously using hydrogen peroxide (H2O2) and sodium hydroxide (NaOH) solutions as absorbents. The presence of H2O2 and NaOH in absorbent solutions plays a role in oxidizing NOx into soluble species in water and in absorbing SO2 gas, respectively. During the experiment, the feed gas flowed through the lumen fiber and then passed through the fiber to the shell side of the membrane module, where the reaction happened between NOx and SO2 and the absorbent. The experimental results showed that the presence of SO2 affected the NOx reduction efficiency. The NOx and SO2 removal efficiencies decreased with the feed gas flow. This study's maximum NOx and SO2 reduction efficiencies were 93.9 and 99.8%, respectively.
Air pollution; H2O2; NOx; Removal efficiency; SO2
Air pollution in Indonesia increases yearly due to growing public energy consumption. Hazardous pollutants such as NOx (NO and NO2) and SO2 generally come from fossil combustion, harm the human respiratory system, and damage environmental ecosystems (Manisalidis et al., 2020; Wang, Wang, and Shammas, 2020; Sharma et al., 2013). Based on a study on the emissions prediction from the coal-fired power plants in Indonesia in 2016-2020, there was an exponential increase of 120.0 and 798.5 ktons of NOx and SO2, respectively, in that period (Sunarno, Purwanto, and Suryono, 2021). The Indonesian Government's efforts to prevent air pollution nationally set Ambient Air Quality Standards (BMUA) in Government Regulation No. 41 of 1999 (RI, 1999). However, the NOx and SO2 emissions produced by the coal-fired power plants in Indonesia are above the value of the BMUA, so efforts are needed to reduce emissions in PLTUs made from burning coal. Reducing NOx and SO2 emissions requires two different technologies, namely SO2 reduction using FGD (Flue Gas Desulfurization) and NOx reduction using SCR (Selective Catalytic Reduction) or SNCR (Selective Non-Catalytic Reduction) (Chang et al., 2004; Brandenberger et al., 2008; Wu et al., 2019). Simultaneous removal of NOx and SO2 with two separate technologies requires complex processes, high operational costs, and investment (Zhao et al., 2021; Cheng and Zhang, 2018; Krzyzynska and Hutson, 2012). Both NOx and SO2 are acid gases (Li et al., 2020); however, it needs to use different techniques to remove both gases due to differences in solubility, where the solubility of NOx in water is lower than that of SO2 (Fang et al., 2011).
A previous study (Kartohardjono et al., 2019; Kartohardjono et al., 2017) has shown that the HFMM (hollow fiber membrane module) can be used as a bubble reactor to remove NOx using H2O2 and HNO3 solutions. The fibers’ role is to distribute the feed gas into the solutions on the shell side of the HFMM so that reactions happen between NOx and the absorbent. NO (nitrogen monoxide) in NOx is an insoluble gas in water, so it needs to be oxidized to increase its solubility. One of the solutions that can be used to oxidize NO is hydrogen peroxide (H2O2). The H2O2 is superior as it is very stable under normal conditions, environmentally friendly, does not leave harmful residues, and the operating costs are pretty affordable. No conventional technology in the power generation industry can reduce NOx and SO2 simultaneously (Park et al., 2019; Si et al., 2019). In order to remove NOx and SO2 simultaneously, an absorbent that can oxidize NOx into water-soluble species and an alkaline solution that can absorb SO2 are required. This study utilized the polysulfone hollow fiber membrane module to remove NOx and SO2 simultaneously using absorbents consisting of H2O2 as an oxidant and sodium hydroxide (NaOH) as a base solution. The polysulfone membrane module was chosen because of its excellent stability over a wide pH range (2-13) and oxidant resistance (Febriasari et al., 2021; Serbanescu, Voicu, and Thakur, 2021). Therefore, it can be expected to see the effect of SO2 in the feed gas on NOx removal compared to NOx removal alone. The NaOH solution absorbs the reaction products between NOx gas and SO2 and H2O2. Reactions (1-7) are reactions that may occur in the process of simultaneously removing NOx and SO2 using a mixture of H2O2 and NaOH as absorbents: (Kartohardjono et al., 2020; Sun, Zwoli?ska, and Chmielewski, 2016):
The hollow fiber membrane module used
contains 50 polysulfone fibers with a diameter of 3 cm and an effective length
of 25 cm, supplied from GDP Filter Bandung, Indonesia. The fibers are 1.8 and 2
mm in the inside and outside diameters, respectively. The feed gas, which
contained 600 ppm of NOx and 500 ppm of SO2 in nitrogen, was supplied from
PT EIN Jakarta, Indonesia. The chemicals used, H2O2 and NaOH, are analytical grades
Merck Indonesia supplies. The feed gas flowed inside the fiber in the membrane
module throughout the experiment. The flow rate was adjusted using a CX Series
mass flow controller. The feed gas diffused across the membrane pores to the
shell side of the HFMM and contacted absorbent solutions so that reactions
occurred between NOx, SO2, H2O2, and NaOH, as shown in Reaction
(1-8). The concentrations of NOx and SO2 gases entering and leaving the
HFMM were recorded by the Gas Analyzer ECOM-D. The schematic of the experiment
is presented in Figure 1.
The amount of absorbed NOx and SO2 gases, GasAbs, removal efficiency, R, fluxes, J, and NOx and SO2 loading, Gas-loading, can be calculated by Equations 8-11 (Kartohardjono et al., 2020; Ding et al., 2014):
Where Xin and Xout, QG,in, T, P, and R are the concentration of gas inlet and outlet of the membrane module, feed gas flow rate, temperature, pressure, and gas constant, respectively.
Figure 1 The experimental diagram schematic
Figure 2 shows the effect of the feed gas flow rate, containing 500 ppm SO2 and 600 ppm NOx, on the simultaneous removal of SO2 and NOx in the HFMM, which contains 0.1M of H2O2 and 0.5 M NaOH each of 200 ml. As demonstrated in Figure 2, the NOx removal efficiency declines with increasing the feed gas flow rate due to the reduced gas residence time in the HFMM (Kartohardjono et al., 2019). Meanwhile, the SO2 removal efficiency is relatively constant to the feed gas flow rate changes because it is already close to 100%. The removal efficiency of SO2 depends not only on the oxidant (H2O2), as expressed in Eq. 6, but also mainly on the alkaline solution present in the adsorbent (NaOH) so that it can be removed entirely (removal efficiency » 100%) (Chen, Chen, and Chiang, 2020; Liu et al., 2019; Huang, Ding, and Zhong, 2015). The NOx absorption efficiency decreases from 93.9 to 81.3% by increasing the feed gas flow from 100 to 200 mL/min. The NOx removal was more complex than the SO2 removal, as the SO2 solubility in water was about 700 times higher than that of NO (Fang et al., 2011). A previous study showed a slight decrease in single NOx removal efficiency from 94.6 to 94.0% by increasing the same feed gas flow rate in a polysulfone HFMM containing 48 fibers using absorbents of H2O2 and HNO3 solutions. It reveals that the presence of SO2 in the feed gas decreases the efficiency of NOx removal due to the competition factor in consuming H2O2 as an oxidation agent, as expressed in Equations 5 and 6 (Chen, Chen, and Chiang, 2020; Kartohardjono et al., 2020).
Figure 2 The dependency of NOx and SO2 reduction efficiencies, R, on the feed gas flow, QG
The amount of absorbed NOx and SO2 and mass transfer flux, J, rise with the feed gas flow, as presented in Figure 3. The increase in the feed gas flow increases the number of gas molecules and resulting a higher concentration of the bulk gas. This condition creates a higher concentration driving force, bringing the higher absorbed NOx and SO2 and mass transfer flux (Liu et al., 2019). The absorbed NOx and SO2 rose from 3.8 to 6.6 x 10-5 mmol/s and 4.1 to 8.1 x 10-5 mmol/s, respectively, by increasing the feed gas flow from 100 to 200 mL/min. Meanwhile, the NOx and SO2 flux increased from 4.9 to 8.4 x 10-8 mmol/cm2.s and 5.2 x 10-8 to 1.0 x 10-7 mmol/cm2.s, respectively, when the feed gas flow rate was increased from 100 to 200 mL/min. A previous study exhibited a similar result: single NOx flux increased from 5.6 x 10-8 to 1.1 x 10-7 by doubling the feed gas flow from 100 to 200 mL/min in a polysulfone HFMM with 48 fibers containing absorbents of H2O2 and HNO3 solutions. It is also revealed that the existence of SO2 in the feed gas affects the transfer flux of NOx (Kartohardjono et al., 2019).
Figure 3 The dependency of absorbed NOx and SO2, and mass transfer fluxes, J, on the feed gas flow, QG
As with flux, NOx and SO2 loading increases with the feed gas flow rate due to the increased amount of absorbed NOx and SO2. Figure 4 shows the dependency of NOx and SO2 loading on the feed gas flow. The NOx and SO2 loading increased from 0.0019 to 0.0033 mmol/mol.s and 0.0020 to 0.0041 mmol/mol.s, respectively, by doubling the feed gas flow from 100 to 200 mL/min. Similar results were also reported that the NOx removal increased from 0.002 to 0.004 mmol/mol.s by doubling the feed gas flow rate, containing NOx 600 ppm, in the PVDF HFMM containing 0.5 wt.% H2O2 and 0.5M HNO3 each of 25 ml (Purnawan et al., 2021).
Figure 4 The absorbed NOx and SO2 loading dependency on the feed gas flow, QG
Figure 5 shows the absorption efficiency (%R) for NOx and SO2 as a function of H2O2 concentration. The efficiency of NOX removal increases with the increase in H2O2 concentration, while in SO2 gas, the efficiency is relatively constant with the addition of H2O2 concentration. The increase in the concentration of H2O2 causes an increase in the number of moles of O2 produced in the solution to oxidize NOx. The highest absorption efficiency achieved was 97.53% for NOx and 99.79% for SO2 at a 0.1 M H2O2 concentration. This study of simultaneous removal of mixed gases (NOx and SO2) resulted in a lower %R compared to the utilization of hollow fiber membranes on single-gas NOx by H2O2/HNO3 solvents in previous studies (Kartohardjono et al., 2019). The absorption efficiency of NOx gas is about 95% at 0.25% H2O2 by mass. In the same polysulfone membrane module and H2O2 solvent, it is seen that the absorption efficiency decreases between mixed gas (NOx and SO2) and single gas (NOx only) due to SO2 compounds competing with NOx in consuming the absorbent (i.e., H2O2 and NaOH).
Figure 5 The dependency of NOx and SO2 reduction efficiencies, R, on the H2O2 concentration in the absorbent solutions at the feed gas flow rate of 0.1 L/min
Figure 6 shows the effect of the concentration of H2O2 on the amount of gas absorbed and the mass transfer flux at a feed gas flow rate of 0.1 L/min. The amount of SO2 gas absorbed and the mass transfer flux of SO2 was constant, at about 4.05 x 10-5 mmol/s and 8.98 x 10-8 mmol/cm2.s. Meanwhile, the amount of NOx absorbed and the mass transfer flux of NOx increased with the increase in the concentration of H2O2 in the absorbent solution. The amount of NOx absorbed and the mass transfer flux of NOx increased from 3.72 to 3.96 x 10-5 mmol/s and from 8.24 to 8.77 x 10-8 mmol/cm2.s. The increase in the amount of absorbed gas and flux is relatively small, so it can be categorized as the concentration of H2O2 does not have much effect on the amount of gas absorbed and the flux of NOx and SO2. The increasing concentration of H2O2 only affects the reaction rate between NOx with H2O2. Compared with other studies (Kartohardjono, 2019), the results also show an insignificant mass transfer flux from 1.153 x 10-9 mmol/cm2.s at 0.1% w/w H2O2to 1.486 x 10-9 mmol/cm2.s at 0.4% w/w H2O2.
Figure 6 The dependency of NOx and SO2 absorbed and flux, J, on the H2O2 concentration in the absorbent solutions at the feed gas flow rate of 0.1 L/min.
NOX and SO2 loading decreased drastically as the feed gas flow rate increased, as shown in Figure 7. The NOx loading at a feed gas flow rate of 100 mL/min was 1.86 x 10-3 mmol NOx per mole H2O2 per second. It decreased drastically to 1.98 x 10-4 mmol NOx per mole H2O2 per second if the concentration of H2O2 in the absorbent solution increased from 0.001 to 0.1 M. Meanwhile, SO2 loading decreased drastically from 2.03 x 10-2 mmol SO2 per mole H2O2 per second to 2.03 x 10-4 mmol SO2 per mole H2O2 per second if the concentration of H2O2 in the absorbent solution increases from 0.001 to 0.1 M. This decrease occurs because the increase in the amount of NOx and SO2 gas absorbed is not proportional to the increase in the concentration of H2O2 in the absorbent. Similar results were also reported: the NOx loading decreased with increasing the concentration of absorbents (Karamah et al., 2021; Purnawan et al., 2021).
Figure 7 The dependency of NOx and SO2loading on the H2O2 concentration in the absorbent solutions at the feed gas flow rate of 0.1 L/min
The elimination of NOx and SO2 simultaneously can be conducted in an HFMM using absorbents such as H2O2 and NaOH. The presence of SO2 in the feed gas could reduce the removal efficiency of NOx because of the competition factor in consuming H2O2 in the process. The NOx and SO2 removal efficiencies decrease with the feed gas flow rate, while the NOx and SO2 absorbed, fluxes, and loadings increase with the feed gas flow. This study's maximum NOx reduction efficiency was 93.9%, while SO2 can be almost entirely removed. In future work, the methods would be applied to remove NOx and SO2 simultaneously from the flue gas resulting from fossil fuel combustion.
The authors wish to acknowledge the PDUPT Project via the Directorate of Research and Services Universitas Indonesia through Contract No. NKB-213/UN2.RST/ HKP.05.00/2021.
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