Sodium Lactate Aqueous Solution, A Green and Stable Absorbent for

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Sodium Lactate Aqueous Solution, a Green and Stable Absorbent for Desulfurization of Flue Gas Kai Zhang, Shuhang Ren, Yucui Hou, Weize Wu, and Yuyun Bao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02023 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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Industrial & Engineering Chemistry Research

Sodium Lactate Aqueous Solution, a Green and Stable Absorbent for Desulfurization of Flue Gas Kai Zhanga, Shuhang Rena, Yucui Houb, Weize Wua,*, Yuyun Baoa a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China; b

Department of Chemistry, Taiyuan Normal University, Taiyuan 030031, China

ABSTRACT: A novel absorbent sodium lactate (NaLa) aqueous solution (aq) for SO2 absorption is reported. SO2 in simulated flue gas is reversibly absorbed in NaLa aq. The absorption capacity of SO2 in the absorbent increases with increasing NaLa content and SO2 concentration, and decreasing temperature. Compared with the absorbents reported in the literature, the absorption capacity of SO2 in NaLa aq. is much high, for example, 50 wt % NaLa aqueous solution can absorb 0.130 g SO2 / g absorbent at a SO2 concentration of 2.5 vol % and 40 oC. Importantly, NaLa (aq) exhibits high reversibility and long-term stability, indicating a promise for the desulfurization of flue gas. The absorption mechanism is proposed to be the replacement of lactic acid (HLa) by sulfurous acid (H2SO3), which is generated by dissolving SO2 in water.

*

Corresponding author. Tel./Fax: +86 10 64427603. E-mail address: [email protected] (W.

Wu) 1

ACS Paragon Plus Environment


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INTRODUCTION The removal and recycling of hazardous gas SO2 from flue gas have drawn much attention in both academic and industrial research. Up to now, wet flue gas desulfurization (FGD) using calcium-based absorbents is the most ubiquitous technique.1 However, a large amount of wastewater and by-product CaSO4 produced in the process do not realize the recovery of SO2. Recently, the FGD processes with regenerated absorbents (abs), such as sodium sulfite

2

and magnesium oxide3, 4, can

significantly decrease the production of by-product. Nevertheless, the disadvantages in the above FGD processes with regenerated absorbents are the easy oxidation of absorbents and high energy consumption in absorbent regeneration. Several strategies have been proposed to solve these insufficiencies. Ionic liquids (ILs), especially functional ILs,5-9 are proposed as solvents for SO2 capture and recovery because of their moderate energy need in transport and regeneration process and benign physicochemical properties such as negligible vapor pressure, non-flammability and tunable structure. In spite of these possible benefits, many reports have shown clearly the poor biodegradability,10, physical properties such as high viscosity

13

11

toxicity,12 unfavorable

and high manufacturing cost

14

of ILs.

Moreover, ILs cannot withstand long-term high temperature conditions.15, 16 These drawbacks of ILs may limit their application for FGD. A desirable and possible way to solve these drawbacks is to design a pleasant candidate that combines the advantages of physicochemical properties of industrial and academic absorbents, especially chemical SO2 absorption, due to the relatively low concentrations of SO2 (for instance 0.2 vol %) in flue gas. The SO2 absorption mechanism in several protonated ILs (e. g., 1,1,3,3-tetramethylguanidinium lactate-[TMG]La) and aprotic ILs (e. g., tetraethylammonium lactate-[N2222]La) 2


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presented in the literatures 17, 18 can guide designing the candidate. The anions in these kinds of ILs are always organic acids (like monocarboxylic acid, RCOOH), and their pKa is larger than that of sulfurous acid, which means that strong acid H2SO3 can replace these weak organic acids.17 Hence, based on the above absorption mechanism, the organic acids chosen for candidate synthesis should have a weaker acidity than H2SO3, and be non-volatile. If not, the candidate cannot capture SO2 efficiently in flue gas by chemical interaction, nor can it be regenerated.19 In this work, lactic acid (HLa) was chosen because of its low molecular weight and extremely low volatility. On the other hand, in order to enhance the thermostability of this candidate, the preferred method is to select a kind of inorganic hydroxide with high water-solubility to neutralize lactic acid. Due to the high water content in flue gas, the absorbent used to capture SO2 in flue gas must be an aqueous solution. Based on the information above, we synthesized a novel SO2 absorbent, sodium lactate (NaLa) aqueous solution (aq) formed by a neutralization of NaOH and lactic acid. NaLa has not only the advantages of these absorbents above, such as low-cost production, non-flammability and negligible vapor pressure, but also biodegradability and non-toxicity, and it is more stable than ILs because of the introduction of sodium cation instead of organic cations. Moreover, the addition of water in NaLa will greatly reduce the viscosity of the absorbents, which is more suitable for practical desulfurization in flue gas and high-temperature treatment, for instance, the steam stripping method.

EXPERIMENTAL SECTION Materials. SO2 (99.95 %), NO (10 %, with 90 % N2), O2 (99.99 %), CO2 (99.99 %) and N2 (99.999 %) were obtained from Beijing Haipu Gases Co., Ltd., Beijing, China. Simulated flue gases with different SO2 concentrations were obtained 3



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by mixing SO2 and N2. NaOH (97 %) and lactic acid (80−85 % in aqueous solution) were purchased from Aladdin Co., Ltd., Shanghai, China. Sodium lactate (NaLa) was obtained by neutralization between NaOH and lactic acid with equal molar amounts. The water contents in samples were determined by Karl Fischer titration (ZDJ-400S, Xianquweifeng Co., Ltd., Beijing, China). The viscosity of NaLa (aq) with different NaLa contents was measured by a viscometer (Lovis 2000M, Anton Paar Gmbh, Austria).

SO2 absorption and desorption experiments. The schematic diagram of experimental system for absorption and desorption of SO2 is shown in Figure 1. The SO2 absorption and desorption experiments were conducted using an absorption tube with a length of 100 mm and an inner diameter of 15 mm under the ambient pressure. The simulated flue gas of 100 cm3/min was bubbled through water in a tube before the absorption in order to offset the water evaporated from the absorption tube. The gas flow was monitored by a rotameter and calibrated by a soap-film flow meter. The absorbents with different NaLa concentrations were prepared by adjusting the proportions of NaLa and water, and wNaLa represents the mass fraction of NaLa. The tube containing absorbents was immersed in a constant temperature water bath. A constant-temperature oil bath was used in desorption experiments treated with 100 cm3/min pure N2. The temperatures of the water bath and the oil bath were maintained within ± 0.5 oC. The concentrations of SO2 in absorbents were determined by an iodine titration method (HJ/T 56–2000, a standard method of Ministry of Environmental Protection of P. R. China). The absorption/desorption efficiency (DE) of the absorbents was determined using equation (1).

DE =

Ra − Rd × 100% Ra 4


(1)

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where Ra is the mole ratio of SO2 to NaLa when the absorbent was saturated, Rd is the mole ratio of SO2 to NaLa after desorption. The removal efficiency (RE) is determined using the following equation (2).

RE =

C0 − C × 100% C0

(2)

where C0 is the inlet SO2 concentration (in the example it is 960 ppm) and C is the outlet SO2 concentration at determination time. The outlet SO2 concentration was detected by a flue gas analyzer (KM9106, Kane International Co., Ltd., UK). 1

H NMR and FTIR analyses. 1H NMR of virgin and reused absorbents

for different days were conducted on a 400 MHz spectrometer (Bruke Avance III, Switzerland). FTIR spectra of samples was recorded on a Fourier transform spectrometer (Nicolet 6700, USA) with wavenumbers from 400 to 4000 cm−1.

Long term stability of NaLa. A NaLa aqueous solution was first saturated with SO2 (2.5 vol %) and kept for 12 h in an oil bath at 40 oC, and then the temperature of oil bath was increased to 100 oC keeping for another 12 h. Lastly, the absorbent was treated with 100 cm3/min N2 at 100 oC to desorb SO2. After SO2 desorption, the oil bath was cooled to 40 oC, and the absorbent was reused to absorb SO2 (2.5 vol %). The process was repeated for 60 days.

RESULTS AND DISCUSSION NaLa (aq) for SO2 capture was investigated at different NaLa mass fractions,

absorption temperatures and SO2 concentrations. As expected, NaLa (aq) has a high SO2 absorption capacity of 0.46 mol SO2 / mol NaLa, corresponding to mass absorption capacity of 0.13 g SO2 / g abs with wNaLa = 50 %, C(SO2) = 2.5 vol % at 40 o

C. NaLa content affects the mass absorption capacity of SO2 in absorbent, but not

5



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molar absorption capacity (Figure 2). The addition of water in NaLa greatly reduces the viscosity of the absorbents. For example, the viscosity of NaLa (aq) is 28.97 mPa·s with wNaLa = 60 % and 12.89 mPa·s with wNaLa = 50 % at 40 oC (Figure 3). The result suggests that NaLa and water play different roles on SO2 capacity and viscosity. The absorption capacities of SO2 in some anion functional ionic liquids are listed in Table 1 for comparison. It can be found that the absorption capacity of SO2 in NaLa (aq, wNaLa = 50 %) is comparable with those of functional ILs reported in the literature. As expected, SO2 absorption capacity decreases with an increase in temperature (Figure 4), suggesting that absorbed SO2 can be recovered and the absorbent can be regenerated by heat treatment. In addition, the SO2 absorption capacity approximately keeps a constant at temperatures from 30 oC to 40 oC, because the absorption capacity of SO2 in NaLa (aq, wNaLa = 50 %) almost achieve theoretical absorption capacity (0.5 mol SO2 / mol NaLa). While at temperatures greater than 40 oC, temperature has a significant influence on the SO2 absorption capacity. SO2 absorption capacity decreases with decreasing SO2 concentration (Figure 5), and still exhibits high absorption capacities of SO2 with low concentrations, for instance, 0.070 g SO2 / g abs and 0.031 g SO2 / g abs with SO2 concentrations as low as 0.60 vol % (6000 ppm) and 0.096 vol % (960 ppm), respectively. Figure 6 shows the outlet SO2 concentration and RE during absorption of SO2 with C0 = 0.096 vol % (960 ppm) as a function of time in NaLa (aq, wNaLa = 50 %) at 40 oC. It can be found that SO2 charged into NaLa (aq) was completely captured at initial time, and RE can reach 100%. The reason that RE decreases at the later absorption period is that the absorbent reaches closely to its saturated absorption capacity. However, if the SO2 absorption process is performed in an absorption 6


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column, an increase in stage number and a decrease in flow ratio of flue gas to absorbent are favorable to increase RE. On the base of above results, we speculate that during the absorption process, 2 mole NaLa absorbs stoichiometrically 1 mole SO2 at with a SO2 concentration of 2.5 vol % and 40 oC, and the absorbed SO2 can be regenerated through increasing temperature and reducing SO2 partial pressure. To test the reversibility of the absorbent, N2 was bubbled into NaLa + SO2 (aq) at 100 oC to regenerate the absorbent. There is a negligible decrease of SO2 absorption capacity in NaLa (aq) after five absorption and desorption cycles (Figure 7), and the DE are 92.6%, 90.0%, 92.1%, 91.7% and 92.9%, which demonstrates that the NaLa (aq) has high reversibility. To further understand the absorption process, the absorption mechanism was analyzed by FTIR. As we know, −COO− with a negative charge is obtained by COOH dissociating one H atom, and the negative charge is not concentrated upon one oxygen atom, but equally spread on the two oxygen atoms, which leads the two carbon– oxygen bonds to be equivalent. As expected, there is no band at about 1700 cm−1 assigned to C=O (Figure 8). The spectra (a) show that the two carbon–oxygen bonds in NaLa aqueous solutions were equivalent. However, two new absorption bands at 1660 cm−1 and 960 cm−1, which represents bond C=O for former and −SO32− for latter, appear after NaLa (aq) absorbs SO2. Namely, the negative charge is deflected from one oxygen atom to another after SO2 absorption because of the formation of –COOH and Na2SO3. Based on the above results and absorption capacities of SO2, the proposed absorption mechanism is shown in Scheme 1. Absorption Na2SO3 + 2HLa

2NaLa + SO2 + H2O Desorption 7



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Scheme 1. Proposed absorption mechanism between NaLa (aq) and SO2.

In addition, the effect of gas velocity on SO2 absorption and desorption was investigated, and the results are shown in Figure 9. It can be seen that the gas velocity has almost no effect on the SO2 absorption and desorption. Figure 10 shows SO2 absorption capacity in NaLa (aq, wNaLa = 50 %) when NO, O2, or CO2 exist in the simulate flue gas. It can be found that NO, O2 or CO2 has almost no effect on the SO2 absorption. So, the effects of NO, O2 or CO2 on SO2 absorption can be ignored. Another important property of absorbent for industry application is its long-term stability. In this work, 1H NMR was used to analyze the stability of NaLa. There is no obvious change in characteristic bands in 1H NMR between virgin sample and samples with 60 days of reuse (Figure 11), which directly reflects the high stability of NaLa (aq). Furthermore, SO2 absorption capacity of the resued absorbent is 0.42 mol SO2/mol NaLa at 40 oC with CSO2 = 2.5 %, which is closed to the SO2 absorption capacity in fresh absorbent (0.46 mol SO2/mol NaLa). Obviously, the high SO2 absorption capacity, high reversibility and high long-term stability of the absorbent can greatly reduce the long term cost for application. A process for the technology using NaLa (aq) as desulfurization absorbent is proposed and shown in Figure 12. It mainly contains the process of pre-purification of flue gas, SO2 absorption, SO2 desorption, and SO2-H2O separation. Compared with wet limestone desulfurization process used in the industry,1 the advantages of the technology of using NaLa (aq) as desulfurization absorbent are the reuse of absorbent, recovery of SO2, and environmentally benignancy of absorbent. Compared with the desulfurization process using ILs, the advantages of the technology of using NaLa (aq) as desulfurization absorbent are low cost and environmentally benignancy of 8


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absorbent, and high long-term stability of absorbent. The properties of NaLa (aq) desulfurization process show that it has a bright application in the future.

CONCLUSIONS In conclusion, NaLa (aq) has been found to be a suitable absorbent for capturing

SO2 in flue gas. Inherently, lactate plays a key role on SO2 absorption capacity and absorbent reversibility in the absorption process according to absorption mechanism of replacement of lactic acid with sulphurous acid. The existence of cation Na+ makes the absorbent exhibit high thermal stability. Water in the absorbent can greatly reduce its viscosity and then increase the transportation property of absorbent. Externally, the desulfurization process can be operated at temperatures about 40 oC, which is close to the present one using calcium-based absorbents, with a high absorption capacity 0.07 g SO2 / g abs and 0.031 g SO2 / g abs at SO2 concentrations of 0.60 vol % (6000 ppm) and 0.096 vol % (960 ppm), respectively. We believe NaLa (aq) has a great potential for desulfurization in flue gas.

AUTHOR INFORMATION

Corresponding Author * Tel./Fax: +86 10 64427603. E-mail: [email protected].

Funding Sources The project is financially supported by the National Natural Science Foundation of China (No. 21176020), and the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT).

ACKNOWLEDGMENTS The authors thank Prof. Zhenyu Liu and Qingya Liu for their helpful discussion

and suggestions. 9



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Table 1. Absorption capacity of SO2 in ILs reported in the literature and comparison with the result of this work Absorbent

T /oC

CSO2 /% in volume fraction

Absorption capacity

Ref.

[TMG]P

40

3

0.51a, 0.17b

[20]

[N2222]succinate

40

3

0.68a, 0.18b

[20]

[N2222]malate

40

3

0.70a, 0.18b

[20]

[P66614]HBO

30

10

1.03a, 0.09b

[21]

[P66614]Tetz

20

10

1.54a; 0.178b

[6]

[P66614]Im

20

10

2.07a; 0.241b

[6]

[P4442][Tetz]

20

0.2

0.90a, 0.19b

[22]

[P66614][BenIm]

20

0.2

1.62a, 0.173b

[23]

[N2222][diglutarate]

40

0.4

0.55a, 0.13b

[24]

NaLa

40

2.5

0.46c, 0.26d

This work

NaLa

40

0.096

0.11c, 0.062d

This work

a

, mol SO2 / mol IL; b, g SO2 / g IL; c, mol SO2/ mol NaLa; d, g SO2 / g NaLa

Abbreviations: [TMG]P, 1,1,3,3-tetramethylguanidinium propanoate; [N2222]succinate, tetraethylammonium [P66614]HBO,

succinate;

[N2222]malate,

trihexyl(tetradecyl)phosphonium;

tetraethylammonium [P66614]Tetz,

malate;

trihexyl(tetradecyl)

phosphonium tetrazole; [P66614]Im, trihexyl(tetradecyl)phosphonium imidazole; [N2222]diglutarate,

tetraethylammonium

diglutarate;

trihexyl(tetradecyl)phosphonium benzimidazolate.

13


[P66614][BenIm],


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Figure 1. Schematic diagram of experimental system for absorption and desorption. 1, SO2-contained N2 or N2 gas cylinder; 2, pressure reducing valve; 3, rotometer; 4, glass tube with water; 5, glass tube with absorbent; 6, water or boil bath; 7, temperature controller; 8, tail gas absorption device.

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0.16

0.6 0.5

0.14

0.4 0.12 0.3 0.10 0.2 0.08 0.06

0.1

30

35

40

45

50

Mole ratio of SO2 to NaL

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SO2 absorption capacity(g/g abs)

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0.0

NaLa content (wt%)

Figure 2. Effect of NaLa mass fraction on SO2 absorption at a temperature of 40 oC with a SO2 concentration of 2.5 vol %.

15



60 o

30 C o 40 C o 50 C o 60 C

50

η (mPas)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 30 20 10 0

60

50

40

30

NaLa content (wt%)

Figure 3. Viscosity of NaLa (aq) as a function of NaLa content.

16


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0.16

0.12

0.08

0.04

0.00

30

35

40

45

50

55

o

Temperature( C)

Figure 4. Effect of temperature on the absorption of SO2 with a SO2 concentration of 2.5 vol % at wNaLa = 50 %.

17


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0.16

0.12

0.08

0.04

0.00 0

1

2

3

4

5

SO2 concentration(%)

Figure 5. Effect of SO2 concentration on SO2 absorption at a temperature of 40 oC, and wNaLa = 50 %.

18


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250 100 200 95 150 90 100

RE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Outlet SO2 concentration(ppm)

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85 50 80 0 0

50

100

150

200

75

Time(min)

Figure 6. Outlet SO2 concentration and RE during absorption of SO2 with C0 = 0.096 vol % (960 ppm) as a function of time at 40 oC by NaLa (aq, wNaLa = 50 %).

19


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0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

1

2

3 Cycle

4

5

Figure 7. Absorption and desorption cycles of SO2 in NaLa aqueous solution (wNaLa = 50 %). In each cycle, SO2 (2.5 vol %) was absorbed at 40 oC and desorbed at 100 oC. , Absorption; □, Desorption.

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1660

Absorbance (a.u.)

Page 21 of 26

960

(b)

(a)

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm )

Figure 8. FTIR spectra of (a) NaLa aqueous solution (wNaLa = 50 %) and (b) NaLa aqueous solution (wNaLa = 50 %) + SO2.

21


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SO2 absorption capacity (g/g abs)


0.15 0.12 0.09 0.06 0.03 0.00

60

80

100

120

140 3

Simulated flue gas flow (cm /min)

Figure 9. Effect of simulated flue gas flow on SO2 absorption and desorption in NaLa (aq, wNaLa = 50 %) at an absorption temperature of 40 oC and a desorption temperature of 100 oC, and CSO2 = 2.5 vol %. ■, Absorption; □, Desorption.

22


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SO2 absorption capacity (g/g abs)

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0.12 0.10 0.08 0.06 0.04 0.02 0.00 SO2+N2 SO2+NO+N2 SO2+O2+N2 SO2+CO2+N2

Simulate flue gas

Figure 10. Effect of NO, O2 and CO2 on SO2 absorption and desorption in NaLa (aq, wNaLa = 50 %).

, Absorption; □, Desorption. Conditions: absorption temperature, 40

o

C; desorption temperature 100 oC; CSO2 = 2.1 vol %; CNO = 0.06 vol %; CO2 = 14.1

vol %; and CCO2 = 14.5 vol %.

23



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(b) 60 day

4.10

1.26

47 day

4.10

1.26

35 day

4.12

1.28

28 day

4.12

1.28

21 day

4.12

1.27

5

4

3

2

1

δ / ppm

(a) 14 day

4.12

1.28

7 day

4.11

1.27

4 day

4.07

1.27 -CH3

-CHvirgin

5

4.05

4

1.26

3

2

1

δ / ppm

Figure 11. 1H NMR spectra for NaLa (aq) reused for 60 days: (a) from 0 to 14 days; (b) from 21 to 60 days.

24


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Figure 12. A proposed process for the technology using NaLa (aq) as absorbent for flue gas desulfurization. 1, chimney; 2 and 8, heat exchanger; 3, scrubber; 4, water cleaning device; 5, 10, 14, cooler; 6, absorption tower; 7 and 9, pump; 11, purification and separation device; 12, heater; 13, desorption tower; 15, separator.

25



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Environmentally benign and stable sodium lactate aqueous solution was used to efficiently capture SO2 in flue gas.


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Sodium Lactate Aqueous Solution, A Green and Stable Absorbent for

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