Properties of carbon dioxide absorption and reduction by sodium borohydride under atmospheric pressure

Fuel 142 (2015) 1–8

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Properties of carbon dioxide absorption and reduction by sodium borohydride under atmospheric pressure Yi Zhao ⇑, Zili Zhang, Xinfeng Qian, Yinguang Han School of Environmental Science & Engineering, North China Electric Power University, Baoding 071003, PR China

h i g h l i g h t s  An innovative method for CO2 conversion by sodium borohydride was developed.  41.5% of CO2 was reduced into formate under atmospheric pressure.  The mechanism of CO2 reaction with sodium borohydride was proposed.  Thermodynamics and kinetics of the reaction CO2 between were investigated.

a r t i c l e

i n f o

Article history: Received 8 May 2014 Received in revised form 11 September 2014 Accepted 23 October 2014 Available online 8 November 2014 Keywords: Carbon dioxide Sodium borohydride Formate Thermodynamics and kinetics

a b s t r a c t An innovative method has been developed to reduce carbon dioxide into formate under atmospheric pressure and ambient temperature, in which, sodium borohydride solution was used as a reducing agent. The effects of various factors on carbon dioxide reduction were investigated experimentally, from which, the optimal experimental conditions were obtained with 80% of ethanol content, 0.44 mol L1 of sodium borohydride concentration, 318 K of reaction temperature and 9.0 of solution pH. And the average absorption efficiency of carbon dioxide was achieved at about 54.1%. According to the analysis results of products, the comparison of electrode potentials and the relevant references, formate was determined as the main reduction product; [BHi(OH)4i]1 (i = 4, 3, 2, 1) and [BHj(HCO2)(OH)3j]1 (j = 3, 2, 1, 0) were considered as the vital active intermediates for carbon dioxide reduction. The thermodynamic parameters such as enthalpy, Gibbs free energy and equilibrium constant were calculated at about 7.84  102 kJ mol1, 5.78  102 kJ mol1 and 95.0, respectively, indicating that this reduction was feasible in thermodynamics. Meanwhile, the kinetics of carbon dioxide reduction was investigated, from which, the reaction order of 1.03 and the apparent activation energy of 11.1 kJ mol1 were obtained, respectively. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Among the major greenhouse gases (GHGs), carbon dioxide (CO2) has the most important impact on climatic changes because of its greatest emissions, and power plants may account for at least one quarter of its total emissions every year [1]. By the middle of this century, the atmospheric concentration of CO2 will reach up to 550 ppm [2], resulting in more and more serious greenhouse effects [3]. With the increasing concern on energy saving and emission reduction in thermal power industry, developing more effective technologies of CO2 capture, storage and utilization at low cost has become an urgent issue. Under atmospheric pressure, the rapid ⇑ Corresponding author. Tel./fax: +86 0312 7522343. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.fuel.2014.10.070 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

absorption of CO2 by ammonia or alkanolamine solutions has been widely studied and partially applied to industry engineering [4–6]. However, this process is energy-intensive for the regeneration of solvent and is also plagued by corrosion problems. Currently, more attention has been focused on CO2 absorption by ionic liquids (ILs) [7]. Nevertheless, the complicated design and synthesis for highly effective ILs and poor economics may be major obstacles to industrial applications and future developments [8]. As one of methods for CO2 utilizations, the hydrogenation of CO2 into formate (HCOO) by CO2-expanded solvent was carried out under 4–20 MPa [9]. Through homogeneously catalyzed hydrogenation, Himeda developed a process for CO2 reduction into HCOO, and pressures of CO2 and hydrogen gas (H2) were kept in the range of 1–6 MPa [10]. Under 1.01 MPa, Pérez-Alonso tested the performance of CO2 reaction with H2 [11]. Apart from H2, other hydrogen donors such as isopropyl alcohol, dimethylamine-borane,

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Y. Zhao et al. / Fuel 142 (2015) 1–8

and ammonia-borane were also applied to CO2 hydrogenation under dozens of atmospheric pressure [12–17]. Apparently, high pressure may bring two vital issues in consideration of industrializations. One is that the investment cost will be raised due to the need of large amounts of pressure-resistant metal materials. Secondly, the energy consumption of operating processes will be significantly increased. Thus the development of technologies with high efficiency at mild conditions is greatly meaningful to CO2 utilization. For years, as a hydrogen donor, sodium borohydride (NaBH4) has been utilized in chemical synthesis [18,19]. The boron-doped porous carbon was produced from CO2 reaction with NaBH4 under ambient pressure and 500 °C [20,21]. Nevertheless, the absorption reaction between CO2 and NaBH4 at low temperature and atmospheric pressure has not been reported yet. In this study, the effects of ethanol content, NaBH4 concentration, reaction temperature, solution pH and coexisting gases such as sulfur dioxide (SO2), nitric oxide (NO) and oxygen (O2) in flue gas on CO2 absorption were investigated to obtain the optimal experimental conditions. The results of product analyses and the comparison of electrode potentials were used to deduce the mechanism of HCOO production from CO2 absorption. The reaction characteristics were examined on the basis of thermodynamic and kinetic theories. Although cost is an issue for using NaBH4 to CO2 absorption, this process has the following advantages: (1) both the energy consumption in operation and the investment on devices are decreased, because of the operating conditions of atmospheric pressure and moderate temperature; (2) the building of hydrogen storage facilities is avoided and the security in operation is improved, compared with the usage of H2 as a reducing agent for CO2 hydrogenation. 2. Experimental

the range of 288–338 K by an electrical heater device in accordance with the temperature of actual flue gas after desulphurization processes. The absorption reaction occurred when simulated flue gas entered into the bubbling reactor containing NaBH4 solution. The reaction temperature was adjusted from 288 K to 338 K by an electric-heated thermostatic water bath. The solution pH was regulated by mixed acid and alkali (phosphoric acid, acetic acid, boric acid and sodium hydroxide) buffer, and measured by a pH meter (PHSJ-5, Shanghai Leici Instrument Company, China). The stirrer speed was controlled in the range of 20–30 rpm. After passing through the tail gas treatment part, the spent simulated flue gas was discharged into atmosphere. In order to improve the validity of experimental results, the effects of influencing factors such as ethanol content, NaBH4 concentration, reaction temperature and solution pH on CO2 absorption were investigated consecutively, in which, the experiments were implemented by increasing the numerical values of each influencing factor, and then by decreasing these values. The experimental data were obtained by averaging the two measurements. 2.3. Analysis methods An infrared gas analyzer (Xibi GXH510, China) was used to determine the inlet and outlet concentrations of CO2, from which, the absorption efficiency can be calculated according to Eq. (1).

gce ¼ ð1  u0 =ui Þ  100%

ð1Þ

where gce represents the absorption efficiency of CO2, %; ui and u0 are the inlet and outlet CO2 concentrations, respectively, %. An ion chromatograph (IC, Metrohm 792, Switzerland) and a Fourier Transform Infrared Spectroscopy (FT-IR, NICOLET 380, USA) were applied to characterize reaction products. A multifunctional flue gas analyzer (MRU95/3 CD, Germany) was used to detect the concentrations of SO2, NO and O2.

2.1. Materials All gases in experiments were supplied by compressed gas steel cylinders (North Special Gas Co., Ltd., China), with the purity of >99.8%. All chemicals with analytical reagent (AR) were used as received without further purification (Tianjin Chemical Reagents Company). The high purity water that was applied to prepare the absorption solutions was produced by the lab water purification system (Changfeng Co., Ltd., Beijing), with the specific resistance of >18.25 MX/cm. The absorption solution of NaBH4 was prepared as follows: (I) putting ethanol and water with an appropriate ratio into a beaker to form a mixture solution; (II) adjusting the solution pH from 8 to 13 by sodium hydroxide (1 mol L1) to inhibit the self-hydrolysis of NaBH4; (III) adding certain amounts of NaBH4 and making it dissolve completely by magnetic stirring at ambient temperature before applying. 2.2. Experimental procedure The key part of experimental system is a bubble reactor (selfmade) with 250 mL of effective volume and 15.5 cm of height, and a gas blanket of micron porous core fabric is located at 1.5 cm far from the bottom of reactor to evenly distribute gas flow, as shown in Fig. 1. During the experiments, N2, CO2, O2, SO2 and NO were metered through mass flow controllers (LZB, Tianjin Flow Meter Co., Ltd., China) and mixed in a buffer bottle, in which CO2, O2, SO2 and NO were diluted by N2 to desired concentrations, and then the simulated flue gas with 1 atm was formed. The total gas flow was kept around 400–750 mL min1. The gas temperature was controlled in

3. Results and discussion 3.1. Effect of influencing factors on CO2 absorption In protic solvents, for example water, NaBH4 can undergo the self-hydrolysis process [22]. Nonetheless, the process will be weakened in organic solvents because of their smaller autoprotolysis constants [23]. Generally, the solubilities of NaBH4 in organic solvents such as ethanol, acetonitrile, pyridine and tetrahydrofuran are 0.04 g g1 at 298 K, 0.01 g g1 at 301 K, 0.03 g g1 at 298 K and 0.001 g g1 at 293 K, respectively. By comparison, ethanol was selected as the suitable solvent in the views of the solubility and economics for the preparation of NaBH4 solution [22,23]. The effect of ethanol content (r0, %) on CO2 absorption was studied, as shown in Fig. 2a. The absorption efficiency is about 6% when water is used alone as the solvent, possibly owing to the self-hydrolysis of NaBH4 and the slight dissolution of CO2 in weak alkalinity [23]. As ethanol content ranges from 0% to 80%, the absorption efficiency is enhanced rapidly and the highest absorption efficiency appears at 80%, which can be interpreted as that the inhibition of self-hydrolysis of NaBH4 is increased with an increase of ethanol content, which will make more NaBH4 participate in CO2 absorption. However, the absorption efficiency decreases slightly when ethanol content exceeds 80%, mainly due to the decreasing solubilities of NaBH4 and CO2 in the high concentrations of ethanol [24]. Hence, the optimal ethanol content was set at 80%. In order to verify the absorption characteristics of NaBH4 in the mixture of ethanol and water (r0 = 80%), the experiments were carried out by using NaBH4 solution as the absorbent and then, also

Y. Zhao et al. / Fuel 142 (2015) 1–8

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Fig. 1. Experimental apparatus. 1 – N2 gas cylinder; 2 – CO2 gas cylinder; 3 – O2 gas cylinder; 4 – SO2 gas cylinder; 5 – NO gas cylinder; 6 – pressure relief valve; 7 – glassrotor flow meter; 8 – buffer bottle; 9 – flow control valve; 10 – gas electric heater; 11 – bubbling reactor; 12 – stirrer; 13 – water-bath heater; 14 – drier; 15 – infrared gas analyzer of CO2; 16 – gas analysis instrument; 17 – exhausted gas processing unit.

Fig. 2. (a) Effect of ethanol content on CO2 absorption. T: 298 K; NaBH4: 0.09 mol L1; pH: 8.0; total gas flow: 0.5 L min1; SO2: 50 mg m3; NO: 100 mg m3; CO2: 12%; O2: 5.7%. (b) Effect of NaBH4 concentration on CO2 absorption. Ethanol content: 80%; T: 298 K; pH: 8.0; total flow: 0.5 L min1; SO2: 50 mg m3; NO: 100 mg m3; CO2: 12%; O2: 5.7%. (c) Effect of reaction temperature on CO2 absorption. Ethanol content: 80%; NaBH4: 0.44 mol L1; pH: 8.0; total flow: 0.5 L min1; SO2: 50 mg m3; NO: 100 mg m3; CO2: 12%; O2: 5.7%. (d) Effect of pH on CO2 absorption. Ethanol content: 80%; T: 318 K; NaBH4: 0.44 mol L1; total flow: 0.5 L min1; CO2: 12%; O2: 5.7%; SO2: 50 mg m3; NO: 100 mg m3.

by using ethanol and water as the absorbent (without NaBH4) under the same conditions. Fig. 3 shows that the highest absorption efficiency of CO2 is 26.2% and lasts for about 280 s, by using NaBH4 solution, whereas the efficiency is about 6.7% when using ethanol and water as the absorbent, which indicates the excellent ability of NaBH4 solution for CO2 absorption. For determining the optimal NaBH4 concentration, the effect of NaBH4 concentration

on CO2 absorption was investigated. Fig. 2b shows that the absorption efficiency is improved with increasing NaBH4 concentration, especially within the range of 0.09–0.44 mol L1. However, the absorption efficiency remains constant in the range of 0.44–0.61 mol L1, because of the near saturation of NaBH4 at about 0.44 mol L1 [24,25]. Accordingly, the optimal concentration of NaBH4 was selected at 0.44 mol L1.

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Fig. 3. Effect of NaBH4 solution on CO2 absorption. T: 298 K; NaBH4: 0.09 mol L1; pH: 8.0; total flow: 0.5 L min1; SO2: 50 mg m3; NO: 100 mg m3; CO2: 16.4%; O2: 5.7%.

At high temperature, the molecular diffusion across a gas–liquid interface is fast and NaBH4 reactivity is increased [26], which will be favorable to CO2 absorption. As a key influencing factor in gas–liquid reactions, reaction temperature affects both the selfhydrolysis of NaBH4 and the liquid concentration of CO2. According to the previous works, the relationship between reaction temperature and liquid concentration or gas partial pressure of CO2 could be expressed as Eq. (2) [27,28]. By using Eq. (3) and the optimum ethanol content of 80%, the molar fraction of ethanol (rE) in Eq. (2) was calculated at 0.58, from which, Eq. (2) was simplified as Eq. (4). It can be seen from Eq. (4) that increasing temperature will bring lower CO2 solubility in liquid phase and higher CO2 concentration in gas phase, which may directly influence CO2 absorption. Conversely, decreasing temperature will make more CO2 to be absorbed by NaBH4 solution.

ln PCO2 =xCO2 ¼ 22:22  2:30r E þ 3:75e4:31rE  ð1139:41  135:69r E þ 963:48e4:46rE Þ=T 1

1

ð2Þ

 1ÞM1 q0 =ðM0 q1 Þ

ð3Þ

ln PCO2 =xCO2 ¼ 21:21  1135:07=T

ð4Þ

r E ¼ ½1 þ ðr 0

where PCO2 , CO2 partial pressure in gas phase, Pa; xCO2 , CO2 solubility in liquid phase, g g1; M0 and M1, molecular weights of water and ethanol, respectively, g mol1; q0 and q1, densities of water and ethanol, respectively, g mL1. Fig. 2c shows that the absorption efficiency rises as temperature changes from 288 K to 318 K, owing to the acceleration of molecule diffusion and the improvement of NaBH4 reactivity. But it decreases gradually when temperature is higher than 318 K, being attributed to the aggravation of NaBH4 self-hydrolysis and the decrease of CO2 solubility at higher temperature. Hence, the optimal reaction temperature was determined at 318 K. According to the previous report, the reaction activity of NaBH4 could be influenced by solution pH [24]. Fig. 2d illustrates that the absorption efficiency is only 20% as solution pH varies from 2.0 to 4.0, because of the stronger self-hydrolysis of NaBH4 under acidic conditions. After that, the absorption efficiency increases rapidly with pH from 4.0 to 9.0 and the efficiency of about 50% is achieved at 9.0, which can be attributed to the inhibitory effect of high pH on NaBH4 self-hydrolysis. However, the absorption efficiency keeps basically unchanged within the range of 9.0–13.0, implying that the self-hydrolysis of NaBH4 is inhibited at high pH, which is favorable to CO2 absorption, meanwhile its reactivity is also decreased [25,29,30]. Based on the works of Iwakura et al. [31] and Fenimore and Jones [32], the excessive amounts of hydroxide (OH) at high

pH were disadvantageous to CO2 reduction. Therefore, the optimal solution pH was determined at 9.0. Generally, flue gas after desulphurization devices usually contains SO2, NO and O2 with the concentrations of 50–500 mg m3, 100–500 mg m3 and 5–6%, respectively. Therefore, the surveys about the impacts of SO2, NO and O2 on CO2 absorption were implemented. The results showed that the coexisting gases had no obvious competition or inhibition effects on CO2 absorption (Figs. 4 and 5). On the basis of the previous investigation, sulfite ion (HSO3) was deoxidized to sodium dithionite (Na2S2O4) by NaBH4 in the presence of large amounts of sodium metabisulfite (Na2S2O5) [33]. In our work, nonetheless, the reaction between NaBH4 and SO2 was inhibited on account of the absence of Na2S2O5. The report on the reaction between S-nitro glutathione and NaBH4 indicated that this reaction was a NO release process [34], implying indirectly that there was no obvious reaction between NaBH4 and NO. For the impact of O2, Hill [35] reported that O2 molecule was chemically inert to NaBH4 if there were no external constrained conditions, being similar to the interpretation of Jeong et al. [36]. Under the optimal conditions, in which ethanol content was 80%, NaBH4 concentration was 0.44 mol L1, reaction temperature was 318 K and solution pH was 9.0, five parallel experiments were carried out when CO2, O2, SO2 and NO were 12%, 5.7%, 50 mg m3 and 100 mg m3, respectively. The average absorption efficiency of CO2 was obtained at about 54.1%, and the absorption was stable for 30 min without replenishing the solution. In addition, the variance of 1.21 in parallel tests was reached, which showed the reliability of this process. Based on the concentration of HCOO determined by IC, the selectivity of CO2 absorption for HCOO was calculated at 41.5% by using Eq. (5).

a ¼ cVðv tgce c0 Þ1

ð5Þ

where a, CO2 selectivity, %; c, HCOO concentration, mol L1; V, solution volume, L; v, gas flow, L min1; t, reaction time, min; cCO2 ;0 , inlet molar concentration of CO2, mol L1. For purifying the product of formic acid (HCOOH), the separation process was proposed as follows. A certain amount of water was added into the product mixture for dissolving HCO3 and HCOO completely, and then the solution was distilled at 351 K (boiling point of ethanol) to reuse ethanol. The residual solution was acidized by hydrochloric acid, and HCO3 was decomposed into CO2 that would be recycled to the reactor. After the azeotropy distillation at 373 K, the dilute HCOOH was separated from the crystallization of sodium chloride, and the purified HCOOH was obtained accordingly. The experimental results showed that the

Fig. 4. Effect of SO2 and NO on CO2 absorption. Ethanol content: 80%; T: 318 K; NaBH4: 0.44 mol L1; pH: 9.0; total flow: 0.5 L min1; O2: 5.7%; CO2: 12%.

Y. Zhao et al. / Fuel 142 (2015) 1–8

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Fig. 5. Effect of O2 on CO2 absorption. Ethanol content: 80%; T: 318 K; NaBH4: 0.44 mol L1; pH: 9.0; total flow: 0.5 L min1; CO2: 12%; SO2: 50 mg m3; NO: 100 mg m3.

proposed purifying method was feasible, which provided significant reference for the industrial application of this process.

3.2. Reaction mechanism As a result of the alkalescence conditions, CO2 may be initially absorbed into bicarbonate (HCO3), and then be reduced into HCOO by NaBH4, on the basis of CO2 reaction with lithium aluminum hydride or lithium borohydride [37]. In order to understand the reaction paths, the product samples taken from the reactor were analyzed by FT-IR and IC, as shown in Figs. 6 and 7. Fig. 6 demonstrates that the characteristic peaks of HCO3 and HCOO in the standard sample and the product sample are basically accordant with each other. Peaks around 3357 cm1 are owing to the stretching vibration of OAH of HCO3, and the absorptions at 1473 cm1 and 1348 cm1 are attributed to the CAH single bond of HCOO, all of which are important symbols

Fig. 6. FT-IR spectra. Window: potassium bromide (AR); wave: 4000–400 cm1; precision: 0.1 cm1; temperature: ambient; relative humidity: 650%.

Fig. 7. Ion chromatograms. 1: HCO3; 2: HCOO. Chromatographic column: Metrosep Organic Acids-250/7.8; eluent: 0.5 mmol L1 H2SO4; flow rate: 0.5 mL min1; detection: conductivity; pressure: 3–4 MPa; temperature: ambient; sample volume: 20 lL.

for distinguishing HCO3 from HCOO [38]. The sharp and strong absorptions at about 1595 cm1 are ascribed to the vibration superposition of double bonds (C@O) of HCO3 and HCOO. Peaks at 781 cm1 and 2500–2700 cm1 are caused by the combined actions of single band of CAO and hydrogen bond. Hence, there was HCOO being produced from CO2 absorption. It can also be found from Fig. 6 that no sulfur or nitrogen compounds are contained in absorption solutions, which may further verify that SO2, NO and O2 in flue gas do not affect CO2 absorption. Fig. 7 indicates that the retention times for HCO3 and HCOO are separately 7.48 min and 13.99 min, which are the same as those in the standard samples. Thus there are HCO3 and HCOO in the products. The former comes from the alkali absorption of CO2, and the latter is mainly from CO2 reduction by NaBH4. Additionally, from the perspective of electrochemistry, H2BO3/ BH4 and CO2/HCOOH at pH 9.0 were calculated at respectively 0.94V and 0.73V by using Nernst formula [39]. Obviously, the latter absolute value is smaller than the former, which demonstrates the feasibility of this reduction, namely, CO2 can be reduced into HCOOH by NaBH4. Based on the analysis results of products, the comparison of electrode potentials and the relevant references, the reaction mechanism of CO2 with NaBH4 in alkalinity was proposed, as shown in Eqs. (6)–(15) [19–21,34,37,40,41], in which the production of HCOO was deduced as four recycling reactions of BHi(OH)4i1 (i = 4, 3, 2, 1) and HCO2BHj(OH)3j1 (j = 3, 2, 1, 0). In liquid phase, soluble CO2 primarily combined with OH to form HCO3 that would participate in the following cycle reactions. For the first cycle, HCO3 was firstly reduced to BH3CO2H1 (1) by BH4; from the dissociation of (1), HCOO and BH3OH1 (2) were produced in alkaline conditions. In the second cycle, intermediate (2) reacted with HCO3 to produce HCO2BH2OH1 (3); from the dissolution of (3), HCOO and BH2(OH)21 (4) were generated. In the third cycle, HCO3 was continued to be reduced by intermediate (4) into HCO2BH(OH)21 (5), and then, HCOO and BH(OH)31 (6) were produced from the dissolution of (5). In the fourth cycle, HCO3 was reduced by intermediate (6) into HCO2B(OH)31 that was dissociated into HCOO and B(OH)41. The total reaction can be expressed as Eq. (16).

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Y. Zhao et al. / Fuel 142 (2015) 1–8

Dissolving of CO2:

Table 2 Fitting coefficients in regression equations.

CO2 ðgÞ ! CO2 ðaqÞ

ð6Þ

CO2 ðaqÞ þ OH ðaqÞ ! HCO3 ðaqÞ

ð7Þ

cCO2 ;0 /(103 mol L1)

Coefficient

Value

Std. errora

4.73

a b c adj. R2b

5.38 5.92  103 1.32  105 0.983

1.20  102 2.70  104 1.14  106

5.18

a b c adj. R2

5.28 6.83  103 1.74  105 0.977

1.38  102 3.14  104 1.32  106

5.58

a b c adj. R2

5.19 5.85  103 1.32  105 0.992

7.83  103 1.76  104 7.42  107

6.47

a b c adj. R2

5.03 7.05  103 1.72  105 0.989

1.03  102 2.39  104 1.01  106

6.92

a b c adj. R2

4.99 6.04  103 1.36  105 0.992

8.31  103 1.88  104 7.96  107

First cycle:

BH4 ðaqÞ þ HCO3 ðaqÞ ! BH3 CO2 H ðaqÞ þ OH ðaqÞ

ð8Þ

BH3 CO2 H ðaqÞ þ OH ðaqÞ ! HCOO ðaqÞ þ HOBH3 ðaqÞ

ð9Þ

Second cycle:

HOBH3 ðaqÞ þ HCO3 ðaqÞ ! HCO2 BH2 OH ðaqÞ þ OH ðaqÞ 





HCO2 BH2 OH ðaqÞ þ OH ðaqÞ ! HCOO ðaqÞ þ

BH2 ðOHÞ2 ðaqÞ

ð10Þ ð11Þ

Third cycle:

BH2 ðOHÞ2 ðaqÞ þ HCO3 ðaqÞ ! HCO2 BHðOHÞ2 ðaqÞ þ OH ðaqÞ ð12Þ HCO2 BHðOHÞ2 ðaqÞ þ OH ðaqÞ ! HCOO ðaqÞ þ BHðOHÞ3 ðaqÞ ð13Þ Fourth cycle:

BHðOHÞ3 ðaqÞ þ HCO3 ðaqÞ ! HCO2 BðOHÞ3 ðaqÞ þ OH ðaqÞ

ð14Þ

HCO2 BðOHÞ3 ðaqÞ þ OH ðaqÞ ! HCOO ðaqÞ þ BðOHÞ4 ðaqÞ

ð15Þ

Total reaction:

a b

4CO2 ðgÞ þ 4NaOHðaqÞ þ NaBH4 ðaqÞ ! 4HCOONaðaqÞ þ NaBðOHÞ4 ðaqÞ ð16Þ

3.3. Thermodynamics In accordance with the principles of chemical thermodynamics [42,43], the parameters such as enthalpy change (DrHh), Gibbs free energy (DrGh) and equilibrium constant (log Khp) in Eq. (16) were all calculated. As shown in Table 1, the numeric values of enthalpy change and Gibbs free energy decrease greatly with different temperatures, and the equilibrium constants are all positive, which shows that the reactions of CO2 with NaBH4 solutions are exothermic and spontaneous processes. In general, a reaction with an equilibrium constant greater than 3.04 is usually assumed to proceed in completion [44]. Therefore, the complete reduction between CO2 and NaBH4 may occur in thermodynamics. However, it should be noticed that the equilibrium constant has a markedly decreasing trend as temperature increases, forecasting that increasing temperature may be disadvantageous to CO2 reduction. 3.4. Kinetics At the initial absorption, the concentration of NaBH4 which was excessive to that of CO2 could be deemed to maintain basically invariable. Based on the chemical theory in kinetics [45,46], the logarithm of variation rate of CO2 concentration at initial period is linear with its concentration, hence, the reaction order of CO2 absorption can be obtained from the slope of Eq. (17).

lgðdcCO2 ;0 =dtÞ ¼ lgk þ nlgcCO2 ;0

Standard error. Adjusted R-square.

where cCO2 ;0 , initial CO2 concentration, mol L1; k, rate constant, (mol L1)1n s1; n, reaction order with dimensionless quantity; t, reaction time, s. In order to acquire the reaction order, the experiments were proceeded under optimal conditions with different initial concentrations of CO2 (cCO2 ;0 /103 mol L1 = 4.73, 5.18, 5.58, 6.47 and 6.92). The regression of CO2 concentration (y) with time (x) can be expressed by Eq. (18).

y ¼ eaþbxþcx

2

ð18Þ

where a, b and c are fitting coefficients, as shown in Table 2. From the derivation of Eq. (18), the variation rate of CO2 concentration at any time can be denoted as Eq. (19).

dcCO2 =dt ¼ y0 ¼ ðb þ 2cxÞeaþbxþcx

2

The variation rate of CO2 concentration at initial time is expressed by Eq. (20).

a dcCO2 ;0 =dt ¼ dcCO2 =dt t¼0 ¼ y0 x¼0 ¼ be

Table 1 Thermodynamic data at different temperatures.

DrHh/(102kJ mol1)

DrGh/(102kJ mol1)

log Khp/(102)

278 288 298 308 318 328 338

7.79 7.80 7.81 7.83 7.84 7.86 7.87

6.04 5.98 5.91 5.85 5.78 5.72 5.65

1.13 1.08 1.04 0.991 0.950 0.910 0.873

ð20Þ

By plugging the data in Table 2 into Eq. (20), the reaction order of 1.03 (Fig. 8) was calculated out, which illustrated the response of reaction rate on CO2 concentration, and the fraction order of CO2 could be considered as pseudo-zero-order.

ð17Þ

T/K

ð19Þ

Fig. 8. Linear regression of variation rate of initial CO2 concentration.

Y. Zhao et al. / Fuel 142 (2015) 1–8

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(IRT1127) and Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Hangzhou, P.R. China (311202). References

Fig. 9. An Arrhenius plot of ln k vs. 1/T.

As for apparent activation energy, its relationship with rate constant and temperature can be demonstrated by Arrhenius formula of Eq. (21) [45,46].

ln k ¼ 

Ea þC RT

ð21Þ

where Ea, apparent activation energy, kJ mol1; C, the intercept of Arrhenius plot of ln k vs. 1/T; R, molar gas constant, 8.31 J mol1. In order to obtain the apparent activation energy of this absorption, the experiments were carried out at different temperatures (T/K = 290, 298, 308 and 318) as initial CO2 concentration was settled at 6.92  103 mol L1. According to the intercept of Eq. (17), the rate constants at different temperatures were calculated by using Eqs. (17)–(20). And then, from the slope of Arrhenius plot of ln k vs. 1/T in Fig. 9, the apparent activation energy of 11.1 kJ mol1 (<40 kJ mol1) was obtained. In general, the reactions with the activation energy of less than 40 kJ mol1 will have greater reaction rates [47]. Hence, the obtained datum of the apparent activation energy indicated that CO2 could be rapidly absorbed and reduced by NaBH4 solutions in this work. 4. Conclusions At atmospheric pressure and moderate temperature, CO2 from simulated flue gas was effectively reduced into the raw industrial material of HCOOH by NaBH4 solution. The average absorption efficiency of CO2 was about 54.1% under optimal experimental conditions in which ethanol content was 80%, NaBH4 concentration was 0.44 mol L1, reaction temperature was 318 K and solution pH was 9.0. The developed process may effectively reduce the cost of CO2 emission reduction and provides a new idea for carrying simultaneously out the capture and utilization of CO2 from coalfired flue gas. The reaction mechanism between CO2 and NaBH4 was proposed, in which HCOO with the selectivity of 41.5% was confirmed as the primary reduction product. The intermediates of [BHi(OH)4i]1 (i = 4, 3, 2, 1) and [BHj(HCO2)(OH)3j]1 (j = 3, 2, 1, 0) were considered as the vital active mediums for HCOO production from CO2 reduction, which could enrich the theories of CO2 absorption. The reaction characteristics were investigated in thermodynamics and kinetics. The results showed that the reduction of CO2 by NaBH4 solution could proceed in completion with a fast reaction rate. Acknowledgments We appreciate the financial supports of the Program for Changjiang Scholars and Innovative Research Team in University

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