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Effects of Concentration and Adsorption Product on the Adsorption of SO2 and NO on Activated Carbon Yangyang Guo,†,‡ Yuran Li,† Tingyu Zhu,*,† and Meng Ye† †

Research Center for Process Pollution Control, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100049, China ABSTRACT: The effects of various SO2 and NO concentrations on the adsorption capacity of activated carbon were measured experimentally for sequential and simultaneous adsorption of SO2 and NO in a fixed bed using an area integral calculation method. The results show that higher NO concentrations increase SO2 adsorption, especially for CNO > 200 ppm; however, higher SO2 concentrations restrict NO adsorption with little NO adsorbed for CSO2 > 700 ppm. The NO adsorption capacity decreases as the SO2/NO ratio increases, while the total adsorption capacity for SO2 and NO has a peak at SO2/NO = 1.7. The product analysis by XPS supports that NO promotes SO2 chemisorption, SO2 occupies both C−O and CO groups, but NO occupies the C−O group. The TPD-MS measurement shows that most of the SO2 is chemisorbed, and the ion chromatography measurement indicates that 66.0% of the total adsorbed SO2 is oxidized to SO3. SO2 replaces the physisorbed NO, which is 94.3% of the total adsorbed NO, but cannot desorb the chemisorbed NO. TPD experiments show two NO desorption peaks at 200 and 280 °C, and almost all the NO desorbed as the temperature increases to 500 °C, with the chemisorbed NO being 5.6% of the total adsorbed NO.

1. INTRODUCTION

while chemically adsorbed NO promotes the adsorption of SO2 through the following reaction:9

SO2 and NOx emissions, mainly from stationary sources, have received more and more attention in China in recent years, with SO2 emissions of 21.85 Mt and NOx emissions of 18.52 Mt in 2010.1 Limestone-gypsum wet flue gas desulfurization (WFGD) and selective catalytic reduction (SCR) are the most effective methods for separate removal of SO2 and NOx. Compared to these separate adsorption steps, simultaneous adsorption of SO2 and NOx on activated carbon is more economic, and the activated carbon can also capture the volatile organic compounds (VOCs), heavy metals, and other substances.2,3 Previous studies have shown that the adsorption capacity of activated carbons for SO2 can reach 355 mg/g4 and that for NOx can reach 200 mg/g5 using various treatments of the activated carbon. Therefore, researchers have shown more interest in investigating the simultaneous removal of SO2 and NOx on activated carbon. Previous studies revealed that SO2 and NOx can affect each other’s adsorption. The presence of SO 2 inhibits NO 2 adsorption due to blocking of the reaction sites involved in the conversion of NO to NO2, while the coadsorption of NO2 promotes SO2 adsorption.5 SO2 shows a greater adsorption affinity than NO-NO2, with more SO2 adsorbed as less NONO2 is adsorbed.6 The Henry’s law constant for SO2 is almost 20 times larger than that for NO, and SO2 has strong electrostatic and dispersion interactions with the adsorbent due to a higher permanent dipole moment and polarizability.7 However, other studies showed that NO has no effect on SO2 adsorption, which can be caused by different activated carbon and experimental conditions.8 The adsorption mechanism has also been clearly described. Physically adsorbed NO can be replaced and desorbed by SO2, © 2012 American Chemical Society

SO2 + * → SO2 *

(* means an active site)

NO + 1/2O2 + * → NO2 *

NO2 * + 1/2O2 + SO2 * → [(NO2 )(SO3)*] + *

When the SO2 concentration is below 300 ppm, the amount of chemically adsorbed NO will increase as the fixed-bed length increases. The adsorption of NO-NO2 and SO2 on activated carbon impregnated with KOH has also been investigated, with the results indicating that the adsorbed SO2 is predominantly found on the external surface, producing mainly K2SO4 with some H2SO4 and K2SO3.6 NO can be oxidized on the carbon surface, and the adsorbed NO, O2, and NO2 can generate the intermediate product [NO-O-NO2]ads, which transforms to HNO3 in the presence of water.10 However, the concentration effects on the SO2 and NO adsorption have not been well-investigated with these effects being important due to the fluctuating concentrations in practical applications. This work described two kinds of experiments to investigate the effects of the one component's concentrations on the other’s adsorption, especially the mole ratio of SO2 to NO (SO2/NO). Although the NO adsorption process has been clearly described, the NO chemical adsorption products have not been discussed in detail and their proportions in the total NO adsorption capacity have not yet been reported. The adsorption products are also measured in Received: June 2, 2012 Revised: November 27, 2012 Published: December 5, 2012 360

dx.doi.org/10.1021/ef3016975 | Energy Fuels 2013, 27, 360−366

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Ion chromatography (Dx500, Dionex) was used to measure the absolute quantity of SO32− and SO42− washed from the activated carbon after SO2 adsorption. A 0.50 g portion of AC-C after adsorption was inmersed into 0.1 mol/L NaOH and stirred for 10 min, and the eluate was analyzed by ion chromatography. Na2SO4 and Na 2 SO 3 standard solutions were measured to quantify the concentration of SO32− and SO42− in the sample solutions. The unadsorbed AC-C was measured as the blank sample, and each sample was repeated three times. 2.3. Experimental Procedure. The effects of the SO2 and NO concentrations on the adsorption selectivity were measured using sequential and simultaneous adsorption. Sequential adsorption refers to adsorption of a single component for several hours, and then adsorption of another component at various concentrations to investigate the effects of the later component concentration on the initially adsorbed component. Simultaneous adsorption refers to simultaneous input of SO2 and NO into the reactor. For the sequential adsorption, simultaneous adsorption, and single component adsorption, all the simulated flue gas included 5 vol % O2 and a balance of N2. The experimental procedure is shown graphically in Figure 1.

these tests to describe the effects of the products on the subsequent adsorption processes.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. The commercial coconut shell activated carbon (AC-C) used in the experiment was from the Gongyi activated carbon plant in Henan province. The AC-C was dried at 120 °C for 10 h before the experiment, with the properties shown in Table 1. N2 adsorption was performed at 77 K through an

Table 1. Texture Characterization of AC-C BET surface area HK micropore width micropore volume bulk density granule diameter

892.6 m2/g 0.498 nm 0.36 mL/g 0.68 g/cm3 1.0−2.0 mm

automatic surface area and porosity analyzer (Autosorb iQ, Quantachrome). The surface area was calculated from the N2 adsorption isotherms using the BET equation, with the micropore volume and characteristic size calculated using the Horvath−Kawazoe (HK) method. The sample is mainly microporous, with the micropore volume accounting for about 89% of the total volume. The micropore size of 0.498 nm is a little larger than the equivalent molecular diameters of SO2 and NO (0.35 and 0.317 nm).11 The surface chemical state of the AC-C and the products were measured by X-ray photoelectron spectroscopy (XPS, ESCALab 250, Thermo Electron) with an Al Kα X-ray source (1486.6 eV) at a constant recording ratio of 40. The X-ray source was run at a reduced power of 150 W. The pressure in the analysis chamber was maintained at 10−8 Torr or lower during each measurement. To compensate for surface charging effects, all binding energies were referenced to the C 1s hydrocarbon peak at 284.6 eV. 2.2. Experimental System. The adsorption of SO2 and NO on the AC-C was investigated in a fixed-bed reactor.The quartz tube reactor was 20 mm in diameter and 500 mm in height with a sieve plate in the middle. A 4.0 g portion of AC-C was loaded on the plate for each experiment. The reaction temperature was 120 °C. The gas flow rate at standard state was 300 mL/min, and the gaseous hourly space velocity (GHSV) was about 1800 h−1. The simulated flue gas consisted of SO2, NO, O2, and a balance of N2. After being mixed in the mixing vessel, the gas was fed into the reactor and the effluent gas was continuously detected by a quadruple mass spectrometer (GAM200, IPI). Before each test, the mass spectrometer was purged with high-purity nitrogen for 6 h. SO2 is identified using the major mass ion of 64. The major mass ion for both NO and NO2 is 30. In the presence of O2, activated carbon acts as an adsorbent and catalyst and can oxidize NO to NO2.12 The NO oxidation rate decreases with increasing temperature in the atmosphere containing O2 without water, and the concentration of oxidized NO2 is only a few parts per million at 125 °C.13 A NOx analyzer (CLD 60 Series, ECO Physics) and the mass spectrometer were used to measure the NO adsorption for the same adsorption conditions and reactors. Both methods gave the NO2 concentration as only 2−3 ppm at 120 °C, due to the decreasing amount of physically adsorbed NO at higher temperatures. For all the experiments in sections 3.1 and 3.2 performed at 120 °C, only a mass ion of 30 was monitored for the NO concentration. For the TPD experiments in section 3.3 performed at 120−500 °C, a mass ion of 46 was also monitored for NO2. Temperature-programmed desorption coupled with mass spectroscopy (TPD-MS) was adopted to investigate the SO2 adsorption products. The heating rate was set at 10 °C/min, and the desorption measurements were performed in the range of 30−900 °C. The effluent gases of CO2, SO2, and CO were monitored continuously by the mass spectroscopy. CO was identified by the major mass ion of 12, and the CO signal values were obtained by the deduction of CO2 from the total C.

Figure 1. Graphical representation of the experimental procedure.

3. RESULTS AND DISCUSSION 3.1. Effect of NO Concentration on the SO 2 Adsorption. The sequential adsorption results for SO2 and

Figure 2. NO concentration effects on the SO2 adsorption for sequential adsorption. (a) Mass spectral profile of SO2 and NO breakthrough curves. (b) Partially enlarged curves for various NO concentrations (all initial SO2 concentrations were 1000 ppm, with the NO concentration increasing from 100 to 667 ppm).

NO on the AC-C are shown in Figure 2. Initially, the outlet SO2 concentration increases up to a constant. As the NO is input, the outlet SO2 concentration begins to decrease to a minimum and then increases again. The minimum outlet SO2 concentration decreases as the inlet NO concentration increases, as shown in the enlarged panels in Figure 2b. The 361


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Figure 3. SO2 breakthrough curves at various NO concentrations in simultaneous adsorption. Figure 6. SO2 concentration effect on NO adsorption for sequential adsorption. (a) Mass spectrometer profiles of SO2 and NO breakthrough curves. (b) Enlarged curves for different SO 2 concentrations (all initial NO concentrations were 500 ppm, and the SO2 concentrations were increased from 50 to 1000 ppm).

Figure 4. Area integral calculation for the effect of NO on the SO2 adsorption. Figure 7. NO breakthrough curves for various SO2 concentrations in simultaneous adsorption.

Figure 5. Enhancement of NO on SO2 adsorption with increasing NO concentrations.

outlet NO concentration increases up to a constant as time increases. The results indicate that the activated carbon still adsorbs SO2 after being saturated with SO2 as NO is input, and that NO improves the adsorption of SO2. As Tang et al.9 indicated, the presence of NO enhances the chemical adsorption of SO2 based on the formation of [(NO2)(SO3)]. The results for the simultaneous adsorption of SO2 and NO are shown in Figure 3. The outlet SO2 concentration increases to a constant as time increases. The outlet SO2 concentration decreases as the inlet NO concentration increases, which shows that a higher NO concentration improves the SO2 adsorption.

Figure 8. Areas illustrating the effect of SO2 on the NO adsorption.

An area integral method was used to analyze the experimental data to quantitatively compare the enhancement due to NO on the SO2 adsorption between the sequential and the simultaneous adsorption tests. The cross-hatched area in Figure 4 is determined from the experimental data for the different processes and denotes the increased SO2 adsorption 362


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the NO concentration increases, while the enhancement is more obvious for the simultaneous adsorption. For the sequential adsorption, the enhancement is relatively weak, with the NO still enhancing the SO2 adsorption even though the activated carbon has been saturated with SO2. The reason for the enhancement effects of NO on the SO2 adsorption is that the reaction NO + O2 + SO2** → [(NO2)(SO3)*] + * occurs as a sulfate/bisulfate species SO2** is formed by the chemically adsorbed SO2, which then creates an active site with surplus positive ions.9 For simultaneous adsorption, not only the chemically adsorbed SO2 but also the intermediate product (NO-O-NO2)* from the partial oxidation of NO promote chemical adsorption of both species,14 so the enhancement is more obvious. As the dotted lines in Figure 5 show, NO has a large effect on the SO2 adsorption for NO concentrations above 200 ppm. Thus, larger NO concentrations should be used to enhance the SO2 adsorption. 3.2. Effect of SO2 Concentration on NO Adsorption. The sequential adsorption results for SO2 and NO are shown in Figure 6. Initially, the outlet NO concentration increases and up to a constant. When SO2 is added, the outlet NO concentration rapidly increases up to a peak and then gradually decreases. The peak height of the outlet NO concentration increases as the inlet SO2 concentration increases, as shown in the enlarged panels in Figure 6b. The outlet SO2 concentration also increases up to a constant as time increases, which shows that the activated carbon still adsorbs SO2 after being saturated with NO. These results indicate that SO2 inhibits the adsorption of NO and replaces the already adsorbed NO on the activated carbon, while the extent of the inhibition becomes stronger with higher SO2 concentrations. The physical adsorption process is similar to vapor−liquid phase transitions.15 Vapor−liquid phase transitions are easier for substances with higher boiling points than for those with lower boiling points due to greater intermolecular forces. The boiling points of SO2 and NO are 263 and 121 K; therefore, SO2 will adsorb on the carbon surface before the NO. The van der Waals force for the initially adsorbed NO is weaker than that for the SO2. Therefore, NO is replaced by SO2, and the replacement reaction rate is related to the SO2 adsorption rate. The dynamics rate of the SO2 adsorption is proportional to the SO2 concentration,16 so the adsorption of NO becomes even more different at high SO2 concentrations. The tests of the simultaneous adsorption of SO2 and NO were conducted to investigate the effects of SO2 on the NO adsorption, with the results shown in Figure 7. As shown in Figure 3, the outlet NO concentration increases up to a constant as time increases, with the NO concentration more easily reaching equilibrium than the SO2. The equilibrium NO concentration indicates that the sorbent is quickly saturated with NO, and the adsorption then rapidly decreases. The outlet NO concentration increases as the inlet SO2 concentration increases, which shows that a higher SO2 concentration has a significant negative effect on the NO adsorption. The area calculation method was used in the same way as in section 3.1 to calculate the negative effects of SO2 on the NO adsorption between the sequential and simultaneous adsorption, whereas the area here stands for the negative effect of SO2 on the NO adsorption and were caculated to the decreased NO adsorption amount. As shown in Figure 8, the adsorption curve with only NO is taken as the baseline with an integration time of 240 min. The calculated results are shown in Figure 9.

Figure 9. Inhibitive effects of SO2 on the NO adsorption with increasing SO2 concentrations.

Figure 10. Effect of SO2/NO ratio on the adsorption capacities of SO2 and NO.

Figure 11. Surface elements on the AC-C using XPS after various adsorption procedures (1, blank sample; 2, only 1000 ppm of SO2; 3, only 500 ppm of NO; 4, 1000 ppm of SO2 and 500 ppm of NO simultaneously). .

due to the NO over a period of 120 min. Taking the adsorption curve of only SO2 as the baseline, the area between the red line and the black line is the difference for the sequential adsorption, whereas the area between the red line and the blue line is the difference for the simultaneous adsorption. These areas are calculated to evaluate the increased SO2 adsorption amount due to the enhancement effect of NO on the SO2 adsorption. The calculated results are shown in Figure 5. As shown in Figure 5, for both sequential and simultaneous adsorption, the NO gradually enhances the SO2 adsorption as 363


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Figure 12. Oxygen functional groups analysis. (a) O 1s spectrum of the O2 adsorbed on activated carbon. (b) The area percentage change before and after the adsorption.

occupied by NO almost entirely reoccupied by SO2 for SO2 concentrations higher than 300 ppm. With the simultaneous adsorption, the NO adsorption is greatly restricted with little NO adsorbed at SO2 concentrations higher than 700 ppm. As the dotted lines show, the SO2 has a weak negative effect on the NO adsorption at SO2 concentrations lower than 300 ppm. Thus, a lower SO2 concentration should be used to reduce the restrictive effects on the NO adsorption. For the simultaneous adsorption, the SO2/NO volume ratio is more useful in practical application. The effect of the SO2/ NO ratio on the adsorption capacity for SO2 and NO is shown in Figure 10. The NO adsorption capacity decreases with increasing SO2/NO ratio, with little NO adsorbed at high SO2 concentrations, while SO2/NO ≥ 5. The SO2 adsorption capacity has a peak at about SO2/NO = 1.7, which indicates the optimum SO2/NO ratio for SO2 removal at around 1.7. The total adsorption capacity for SO2 and NO is also shown, and the curve trend is almost the same as SO 2 . The SO 2 concentration in the most coal-fired flue gas is 340−1120 ppm, calculated based on the typical sulfur content in coal ranging from 0.6% to 2.0% in China, and the NO x concentration is 290−580 ppm (NOx refers to NO2),17 with the actual SO2/NO volume ratios of 0.57−3.82 as shown in the shaded part in Figure 10. SO2 adsorption has not been affected much in this region, while it is unfavorable for NO adsorption. The actual SO2/NO ratio is not far from the optimum 1.7, indicating that the simultaneous removal of SO2 and NO in practical applications is not much affected by the components' concentrations; however, higher SO2 concentrations should be avoided to reduce the restrictive effect on the NO adsorption. 3.3. Adsorption Product Analysis. The surface elements on the activated carbon after the various adsorption procedures (as shown in Figure 1) measured using XPS are shown in Figure 11. The simultaneous adsorption product for No. 4 increases the S adsorption amounts compared with the single component adsorption (No. 2). The N adsorption amount compared with the single component adsorption (No. 3) only increases a little. The results indicate that NO promotes the chemisorption of SO2, and the effect of NO on the SO2 adsorption is very obvious (1.28% → 1.63%). The O 1s spectrum was analyzed for the oxygen functional groups, which are very crucial to the chemisorption, with the results shown in Figure 12. The main peak was divided into three peaks according to the binding energies of 531.1, 533.2,

Figure 13. TPD profiles of the SO2 desorption.

Table 2. Concentrations of SO32− and SO42− by Ion Chromatography SO32− (mg/L)

SO42− (mg/L)

sample

av

RSD %

av

RSD %

AC-C AC-C-S

6.23 6.31

2.04 2.61

20.43 173.24

1.62 1.77

Figure 14. NO desorption curves using 500 ppm of SO2.

As shown in Figure 9, for these two kinds of adsorption, the negative effects of SO2 on the NO adsorption increase as the SO2 concentration increases. For the sequential adsorption, the inhibitive effects of SO2 on the NO do not change for SO2 concentrations above 300 ppm. The results show that the initiallly adsorbed NO amounts are equal at the same adsorption condition, with the adsorption sites originally 364


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Figure 15. NO adsorption and temperature-programmed desorption curves in the fixed bed.

desorbed NO were then calculated from the area between the line of the control experiment and the test without activated carbon given by the gray line. The area for the adsorbed NO is 1.0804 × 10−10 [A·min], and the area for the desorbed NO is 1.0183 × 10−10 [A·min], which is 94.3% of the adsorbed value. These results verify that SO2 replaces most of the NO, but still a small amount of NO cannot be desorbed by the SO2. Temperature-programmed desorption (TPD) was also used to desorb the adsorbed NO, with the results shown in Figure 15. In this experiment, 500 ppm of NO flowed into the system for 6 h with the flow then shut off to keep the front 8 h the same as in Figure 14. At 8 h, the furnace was heated from 120 to 500 °C at a heating speed of 10 °C/min. NO is rapidly desorbed with increasing temperature, and two desorption peaks appear at 200 and 280 °C, as shown in the right magnified figure. The higher peak corresponds to the physisorption, and the lower peak corresponds to the chemisorptions.4 The entire desorption peak area relative to the control experiment given as the gray line in Figure 14 is 1.0798 × 10−10 [A·min], which is 99.9% of the total adsorbed NO (1.0804 × 10−10 [A·min], as shown in Figure 15). Therefore, the proportion of the chemisorbed NO is only 5.6%, and the TPD experiment desorbs almost all the NO. At 120 °C, no NO2 was detected, as shown in Figure 14, whereas NO2 was detected when the temperature increased to 160−270 °C (Figure 15). The desorbed NO2 may include the adsorbed NO2 from the gas and the intermediate product ([NO-O-NO2]ads). This difference provides additional evidence that SO2 can only replace the physically adsorbed NO.

and 535.2 eV corresponding to the chemisorbed O, C−O, and CO groups.18 The percentages of peak areas stand for the relative contents of these groups, with the results shown in Figure 12b. The chemisorbed O increases after the single adsorption of SO2 or NO, the C−O and CO groups both decrease for SO2 adsorption, while only the C−O group obviously decreases for NO adsorption. The results show that SO2 occupies both C−O and CO groups, but NO occupies the C−O group. The TPD-MS method was used to decompose the adsorbed SO2 on carbon, with the results shown in Figure 13. The desorption process of SO2 was accompanied with CO and CO2 detected, which were decomposed from the functional groups of carbon. SO2 began to desorb as T > 480 K, and almost all the adsorbed SO2 was decomposed at higher temperature, which could be related to the strongly adsorbed SO2 or being oxidized to SO3, corresponding to different decomposition temperatures at 600 and 670 K.19 The total SO2 desorption peak was divided into two peaks according to decomposition temperatures of 600 and 670 K by Gauss-fitting analysis, and the calculated area percentage of the peak at 670 K (SO3) is 64.3%. The TPD-MS method provided that most of the SO2 was chemisorbed on the activated carbon and the absolute quantity of the sulfate was washed from the SO3 by the catalytic reaction (SO2 + 1/2O2 → SO3). The sulfites were measured by ion chromatography; the results showed that there were almost no changes of SO32− detected, whereas the concentration of SO42− increased significantly, as shown in Table 2. The total adsorbed SO2 capacity is 30.85 mg/g based on the breakthrough curves in Figure 3, and the chemisorbed SO2 converted to sulfate is 20.37 mg/g based on the concentration data in Table 2, Therefore, the oxidized SO2 is almost 66.0% of the total adsorbed SO2 and is close to 64.3% calculated by the TPD-MS. Various activated carbon candidates and adsorption conditions have resulted in various SO2 adsorption capacities in previous studies.4,20 The results of two desorption experiments designed to obtain more information about the NO chemisorption products are shown in Figures 14 and 15. In Figure 14, 500 ppm of NO flowed into the system for 6 h and was stopped for 2 h, and then 500 ppm of SO2 was input for 3 h to desorb the previously adsorbed NO. When the NO flow was stopped at 360 min, the NO ion current value returned to the initial value, which shows that no NO was desorbed. When the SO2 flow began at 480 min, the NO ion current value rapidly increased up to a peak and then gradually decreased, which shows that NO was desorbed and replaced by SO2. The quantities of adsorbed and

4. CONCLUSIONS Activated carbon still adsorbs SO2 after its saturation in the presence of NO. NO enhances the chemical adsorption of SO2, with higher NO concentrations further increasing the SO2 adsorption, especially for CNO > 200 ppm. SO2 has a weak restrictive effect on NO adsorption for CSO2 < 300 ppm. The restrictive effect increases with the SO2 concentration increasing, and very little NO is adsorbed for CSO2 > 700 ppm. The adsorption capacity for NO decreases as the SO2/ NO ratio increases, while the adsorption capacity for SO2 and NO has a peak at the SO2/NO = 1.7. The actual SO2/NO ratio has not drifted much from 1.7, so the one stage simultaneous removal of SO2 and NO by activated carbon is practicable. The amount of S in the adsorption products obviously increased in the simultaneous adsorption, verifying that NO promotes the chemisorption of SO2. The changes of the surface 365


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oxygen functional groups show that SO2 occupies both C−O and CO groups, but NO occupies the C−O group. The adsorption product with only SO2 flow has a total adsorbed SO2 capacity of 30.85 mg/g with almost all the adsorbed SO2 desorbed at above 480 K, being that 66.0% of the total adsorbed SO2 has been oxidized as SO3. Desorption tests show that SO2 replaces the physisorbed NO, which is 94.3% of the total adsorbed NO, with little of the chemisorbed NO desorbed by the SO2. TPD tests show that almost all the NO is desorbed as the temperature is increased up to 500 °C, with two desorption peaks at 200 and 280 °C. The chemisorbed NO is only 5.6% of the total adsorbed NO.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-82544821. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the State 863 projects (No. 2012AA062501) and the Project of the Natural Science Foundation of China (No. 21177129).

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REFERENCES

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