CO2 Fixation Process with Waste Cement Powder via Regeneration of

Jun 5, 2015 - Research Center for Sustainable Science and Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku Universi...
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CO2 Fixation Process with Waste Cement Powder via Regeneration of Alkali and Acid by Electrodialysis: Effect of Operation Conditions Daiki Shuto,† Kan Igarashi,† Hiroki Nagasawa,‡ Atsushi Iizuka,§ Motoki Inoue,† Miyuki Noguchi,† and Akihiro Yamasaki*,† †

Department of Materials and Life Sciences, Graduate School of Science and Engineering, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8633, Japan ‡ Department of Chemical Engineering, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8527, Japan § Research Center for Sustainable Science and Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai, Miyagi 980-8577, Japan ABSTRACT: The effect of the operation conditions on the recovery of acid−alkali pairs by electrodialysis was studied to improve the efficiency of the newly developed mineral carbonation process for carbon dioxide fixation. Sodium nitrate (NaOH + HNO3), sodium chloride (NaOH + HCl), and potassium chloride (KOH + HCl) showed almost equal performance in the recovery process for the two fixed membrane configurations. For the same salt, the configuration of anion-exchange membrane bipolar membrane (AEM−BPM) showed higher recovery rate and lower power consumption than the cation-exchange membrane bipolar membrane (CEM−BPM) configuration. Significantly higher recovery rates were observed when potassium acetate was used for the CEM−BPM configuration. A higher recovery rate was observed for a lower initial concentration, but the power consumption was lower for higher initial concentrations. Calcium in waste cement powder can be rapidly leached with acetic acid. The results showed that the process performance would be significantly improved by using acetic acid for calcium leaching, and sodium hydroxide for CO2 capture.

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INTRODUCTION It is recognized that carbon capture and storage (CCS) is a vital countermeasure for global warming to reduce anthropogenic CO2 emission under the premise of continual use of fossil fuels as energy resources.1,2 CCS by mineral carbonation is one of the promising options for CCS.3−5 CCS by mineral carbonation is based on the formation of calcium or magnesium carbonate by the reaction of carbon dioxide with calcium or magnesium ions in alkali earth metal sources, including ultramafic rocks6−9 such as serpentine and wollastonite and industrial wastes10−16 such as waste cement and concrete or steel slag. Mineral carbonation reactions produce carbonates, which can realize safe and permanent carbon storage. In addition, the high-quality carbonates produced could be sold, which would reduce the process cost. The rate of the mineral carbonation reaction needs to be increased because of the slow rates of extraction of alkali earth metal ions from alkali earth metal sources. A number of ways have been proposed to increase the reaction rate, including the use of high pressure, high temperature, or chemicals such as acids, alkalis, or chelating agents.17−23 The solution containing extracted alkali earth metal ions would react with gaseous CO2 or solutions containing carbonate ions to form carbonates. After the carbonation reaction, the precipitated carbonates would be separated from the solution. When an acid or alkali is used for the process, they should be recovered and reused for the extraction reactions. Otherwise, the one-path use of these chemicals would increase the process cost, and more importantly, the power consumption to produce new chemicals would cause CO2 emission.24 Thus, recycling and reusing the chemicals used for increasing the reaction rate is essential, and © 2015 American Chemical Society

development of effective recovery methods for these chemicals is required for practical CCS by mineral carbonation. The electrochemical method is a potential way to recover the chemicals used in accelerating the reaction. Electrodialysis is a membrane separation process using ion-exchange membranes under electric potential differences. Ion-exchange membranes are set in the electrodialysis cell in an appropriate configuration, and target ions can be selectively transported through the ionexchange membrane under an electric potential difference.25,26 Among the variety of applications of electrodialysis, recovery of acid−alkali pairs from a salt solution could be applied to the present system.27 The bipolar membrane (BPM), a laminated membrane of an anion-exchange membrane (AEM) and a cation-exchange membrane (CEM), should be suitable for this purpose because it generates protons and hydroxide ions under an electric potential higher than the water dissociation potential.25,26 In a previous paper, we proposed a new type of CCS process by mineral carbonation with recovery of acid and alkali by the electrodialysis process. The process is composed of four steps (Figure 1).27 (i) Extraction of calcium. An acid solution is used to extract calcium ions from calcium or magnesium sources. Ca (or Mg) in sources → Ca 2 + (or Mg 2 +)

Received: Revised: Accepted: Published: 6569

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February 20, 2015 May 29, 2015 June 5, 2015 June 5, 2015 DOI: 10.1021/acs.iecr.5b00717 Ind. Eng. Chem. Res. 2015, 54, 6569−6577

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Industrial & Engineering Chemistry Research

operation conditions, such as acid−alkali combinations and concentrations.

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EXPERIMENTAL METHOD AND CONDITIONS Acid−Alkali Recovery by Bipolar Membrane Electrodialysis. The experimental setup was the same as we used for BMED27 with some modifications. A schematic diagram of the electrodialysis apparatus is shown in Figure 2. The electro-

Figure 1. Schematic flow diagram of the CO2 fixation process.

(ii) CO2 absorption. CO2 is captured from flue gas into an alkaline hydroxide solution to form a sodium or potassium carbonate solution. 2Na + (or 2K+) + 2OH− + CO2 (g) → 2Na + (or 2K+) + CO32 − + H 2O

(2)

(iii) Precipitation of carbonate. Calcium or magnesium carbonate is crystallized and precipitated by mixing the solutions from steps i and ii. Ca 2 + (or Mg 2 +) + CO32 − → CaCO3 (or MgCO3) (3)

(iv) Regeneration of acid and alkali by bipolar membrane electrodialysis (BMED). The neutralized solution is treated by BMED to regenerate the acid and the alkali, which can then be reused for steps i and ii.

Figure 2. Schematic diagram of the electrodialysis apparatus for recovery of nitric acid and sodium hydroxide from sodium nitrate solution. BPM, bipolar membrane; CEM, cation-exchange membrane; AEM, anion-exchange membrane.

Na + (or K+) + NO3− + H 2O → {Na + (or K+) + OH−} + {H+ + NO3−}

dialysis apparatus was composed of 10 cell units sandwiched between two electrode cells. Each cell unit was composed of two BPMs (Neosepta BP-1B, ASTOM Co., Tokyo, Japan) and either a CEM (Selemion CMV, Asahi Glass Co., Tokyo, Japan) or an AEM (Selemion AMV, Asahi Glass Co.) depending on the membrane configuration of the cell. The effective membrane area was 0.021 m2, and the gap between membranes was set at 0.75 mm. The membranes alternately form acid cells and alkali recovery cells. The feed salt solutions were pumped into the cells with magnetic pumps (Master Flex L/S economy variable speed drive, Cole Parmer, Vernon Hills, IL, U.S.A.) from feed tanks (0.5 L) at a flow rate of 0.5 L/min, and then circulated. Thus, the system was a batch-type system. The power was supplied by a direct-current power supply unit (PK36-11, Matsusada Co., Tokyo, Japan). Sodium sulfate solution (0.1 M) was circulated in the electrode solution with a magnetic pump at a flow rate of 0.4 L/ min. After confirming that steady-state flow had been achieved, voltage was applied to the system under constant voltage conditions. The pH of the solutions was monitored and recorded with a pH meter (D-51, Horiba Co., Kyoto, Japan). The change of the voltage between the electrodes was measured with a voltmeter (Data Mini, Hioki Co., Tokyo, Japan). The solution was sampled, and the concentrations of the acid and alkali were determined either by the titration or back-titration method. The electrodialysis experiments was

(4)

The overall reaction is mineral carbonation producing alkali metal carbonates such as calcium carbonate and magnesium carbonate.3−6 (Ca, Mg) + CO2 → CaCO3 (MgCO3)

(5)

The power consumption of the process can be mainly attributed to the BMED step, because the other steps proceed spontaneously with negative Gibbs energy changes.27 Reduction of the power consumption of the electrodialysis step to recover the acid and alkali is the key to make the process practical. In a previous paper,27 laboratory-scale experimental results showed that the power consumption for electrodialysis was lower than the specific power generation associated with the equivalent CO2 emission for low recovery yield. The efficiency of the performance of the BMDE step can be increased in various ways: changing the configuration of the electrodialysis equipment such as the number of cells, changing the membrane configuration, using more selective ion-exchange membranes, and using a combination of acid and alkali with higher recovery rates. In this study, we conducted laboratory experiments to investigate the possibility of improving the efficiency of the electrodialysis step by changing various 6570

DOI: 10.1021/acs.iecr.5b00717 Ind. Eng. Chem. Res. 2015, 54, 6569−6577

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Industrial & Engineering Chemistry Research performed for 120 min under a given condition and repeated at least three times to confirm reproducibility. The power consumption of electrodialysis was estimated by the net cell potential, V(t), which is the electric potential difference between the outermost bipolar membranes. The net cell potential was monitored with a voltmeter (Data Mini, Hioki Co., Tokyo, Japan) during electrodialysis. The power consumption for the recovery process of acid and alkali by BMED, P(t), was determined as follows: P(t ) =

I (t )V (t ) n(t )

(6)

where P(t) is the power consumption required for the recovery of 1 mol of acid, I(t) is the current, V(t) is the net cell potential difference excluding the electrode potential, and n(t) is the recovery rate of the acid or the alkali in moles per second at a given time t. Because both I(t) and n(t) generally varied with time, the power consumption at time t was calculated with a step-by-step method by dividing the average power consumption IVav for time interval t to t + Δt by the acid recovered for the given time interval: P(t ) =

IVav {I(t + Δt )V (t + Δt ) + I(t )V (t )}/2 = ΔnHNO3 nHNO3(t + Δt ) − nHNO3(t ) (7)

Note that the membrane potential V(t) also changed with time because of the change of the electrode potential even though the total voltage was kept constant at 13.5 V. To estimate the power consumption, we ignore the electrode potential because the contribution of the electrode potential will be negligible when a number of membrane stacks are used for the practical process. Two types of the membrane configuration were tested: AEM−BPM type and CEM−BPM type, as shown in Figure 3. The unit of the AEM−BPM type was composed of one AEM sandwiched between two BPMs, while that of the CEM−BPM type was composed of one CEM sandwiched between two BPMs. For both types, the feed salt solution was introduced into the cells between the membranes. For the AEM−BPMtype unit, the anions (A−) in the alkali recovery cell will be transported through the AEM driven by the electric potential applied along the cell and couple with protons generated in BPM2 in the acid recovery cell to recover the acid solution (HA(aq)). In the alkali recovery cell, the remaining metal ions (M+) will couple with hydroxide ions generated in BPM1 to recover the alkali solution (MOH). For the CEM−BPM unit, the metal cations (M+) in the acid recovery cell will be transported through the CEM to the alkali recovery cell, where alkali solution (MOH) will form by coupling with hydroxide ions generated in BPM1. The anions in the acid recovery cell will couple with protons generated in BPM1 to form acid. The electrodialysis experiments were performed with 10 cell units under various electric potentials. Several types of feed solution were used to recover the alkali and acid, including sodium nitrate, sodium chloride, potassium chloride, and potassium acetate. The concentration of the salt solution ranged from 0.5 to 2.5 M depending on the case. Calcium extraction experiments were conducted in a plastic beaker with an inner volume of 500 mL. Waste cement powder, which was generated in a recycling plant and supplied by Tateish Construction Co. (Chiba, Japan), and acetic acid (99.9%, special grade, Wako Chemical Co., Osaka, Japan), with

Figure 3. Schematic diagrams for the two configurations of the electrodialysis units: (a) AEM−BPM type; (b) CEM−BPM type.

concentration adjusted by deionized water, were mixed in the beaker under stirring with a magnetic stirrer. The weight ratio of waste cement powder to acetic acid solution in the feed (solid/liquid or S/L ratio) was 35.1−126.4 g (S/L = 0.28), which corresponds to the stoichiometric condition to form a saturated solution of calcium acetate with 4.4 M acetic acid solution based on the elemental analysis of waste cement powder shown in Table 1.

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RESULTS AND DISCUSSION Effect of Type of Anion in the AEM−BPM Configuration. Figure 4 shows the change of the recovery yield of acid−alkali with time for various types of salt. The electrodialysis system was composed of five pairs of AEM−BPM, which form 10 unit cells. The voltage was kept constant at 13.5 V, and the initial concentrations of the salt in the feed solution were fixed at 2.5 M. The recovery yield R is defined as follows: R=

moles of acid recovered moles of acid ions in the feed (conjugate base in the feed salt) (8)

The recovery yield increased with time for both salts tested, but leveled off at the later stage of the operation. The recovery yield for a given operation time was slightly lower for sodium nitrate than for sodium chloride. This result can be explained by the difference in the ion conductivities. The ion conductivity of the chloride ion (7.631 S m2 mol−1) is slightly higher than that of the nitrate ion (7.142 S m2 mol−1).28 The acid would be recovered by the transport of these anions through the AEM in competition with hydroxide ions, which have a conductivity of 19.8 S m2 mol−1. The higher conductivity of the chloride ion should result in a higher recovery rate for the chloride ion than 6571

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Industrial & Engineering Chemistry Research Table 1. Elemental Analysis of Waste Cement Powder with XRF element weight percent (wt %)

Ca 32.3

Si 11.5

Al 2.88

Figure 4. Change of the recovery yield of acid−alkali with time for various salts. The electrodialysis system was composed of five AEM− BPM pairs (10 cell units). The voltage was 13.5 V, and the initial concentration of all of the salt solutions was 2.5 M.

Fe 5.74

P 5.74

S 0.26

others 45.6

Figure 6. Change of the recovery yield of acid−alkali with time for electrodialysis with AEM−BPM or CEM−BPM configurations and either KCl or NaCl. The voltage was 13.5 V, and the initial concentration of all of the salt solutions was 2.5 M.

the nitrate ion. However, at the later stage of the recovery process, the transport of hydroxide ions would impede the recovery. Figure 5 shows the power consumption with time for the recovery of 1 mol of acid−alkali from the salt in the

configuration for a given time and salt. For a given configuration and time, the effect of the alkali ion was almost negligible. Although the recovery yields for the systems with the CEM−BPM configuration leveled off at about 40%, the recovery yield for the system with AEM−BPM reached about 70%. The difference of the recovery yield between the two configurations can be explained by the competitive transport of protons (CEM−BPM configuration) and hydroxide ions (AEM−BPM configuration). For the CEM−BPM configuration, transport of protons from the acid recovery cell to the alkali recovery cell through the CEM would increase with time because the proton concentration in the acid recovery cell increases with time, and consequently the recovery rate of acid would level off (or even decrease) with time. For the AEM− BPM configuration, transport of hydroxide ions from the alkali recovery cell to the acid recovery cell through the AEM would become significant with the progress of the recovery. The conductivity of protons through the membrane is 34.96 S m2 mol−1, which is much higher than other ions, such as hydroxide (19.8 S m2 mol−1), potassium (7.348 S m2 mol−1), and sodium ions (5.008 S m2 mol−1).28 Thus, the effect of proton transport on the recovery yield for the CEM−BPM configuration is more significant than that of the hydroxide ion for the AEM−BPM configuration. The leveling off of the recovery yield can be explained by the proton transport through the CEM being balanced with the transport rate of protons from the BPM at the alkali recovery cell. Figure 7 shows the power consumption for the electrodialysis systems in Figure 6, where the power consumed for the recovery of 1 mol acid and 1 mol alkali from 1 mol salt. The power consumption increased with time, reflecting the decrease in ions in the feed cells, and the power consumption for a given time was lower for the AEM−BPM configuration than for CEM−BPM. For a given recovery yield, the difference is more apparent. These results can be explained by the leakage of protons through the CEM membrane, because protons have higher ion conductivity than alkali metal ions. Consequently, the transport numbers of alkali ions would decrease with increasing operation time.

Figure 5. Change of the power consumption (membrane potential based) of acid−alkali recovery with time with various salts. The electrodialysis conditions were the same as those in Figure 4.

electrodialysis process shown in Figure 3. The power consumption gradually increased with time, but the trend was almost the same for both salts. The increase in the power consumption can be attributed to the decrease in the transport number of the anions through the AEM membrane because of the leakage of hydroxide ions. Effect of Cell Configuration (AEM−BPM or CEM− BPM). Figure 6 shows the change of the recovery yield of acid− alkali with time for the two different membrane configurations (AEM−BPM and CEM−BPM) with either potassium chloride or sodium chloride. The initial concentration of the feed salt was fixed at 2.5 M, and the voltage was kept constant at 13.5 V. The behavior of the acid−alkali recovery depended on the membrane configuration, and the recovery yields were higher for the AEM−BPM configuration than for the CEM−BPM 6572

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Figure 7. Change of the power consumption of acid−alkali recovery with time for electrodialysis with AEM−BPM or CEM−BPM configurations and either KCl or NaCl. The operation conditions were the same as those in Figure 6.

Figure 9. Change in the transfer number of potassium ion with electrodialysis time. The conditions were the same as those in Figure 8.

Effect of Acid Strength and Conjugate Base. Figure 8 shows the change of the recovery yield of acid−alkali with time

Figure 10. Change of the power consumption of acid−alkali recovery with time for the same operation conditions as those in Figure 8. Figure 8. Change of the recovery yield with time for salts of a strong acid and strong alkali (KCl) and a weak acid and strong alkali (CH3COOK) by electrodialysis. The membrane configuration was CEM−BPM with 10 cell units. The voltage was 13.5 V, and the initial concentration of all of the salt solutions was 2.5 M.

consumption increased with time, and increased with increasing recovery yield for potassium chloride. The power consumption for potassium acetate was almost constant up to 90 min, where the recovery yield was about 90% as shown in Figure 11. For potassium acetate, the higher efficiency and lower power consumption of acid−alkali recovery by electrodialysis can be mainly explained by the buffering effect of acetic acid. The salt

for potassium chloride (a salt of hydrochloric acid (a strong acid) and potassium hydroxide (a strong alkali)) and potassium acetate [a salt of acetic acid (a weak acid) and potassium hydroxide (a strong alkali)]. The membrane configuration for the electrodialysis was CEM−BPM. The voltage was kept at 13.5 V, and the initial concentration of all salt solutions was 2.5 M. The recovery yield was dramatically higher for potassium acetate than for potassium chloride, and the recovery yield of potassium acetate reached about 90% in 90 min. Figure 9 shows the change of the transfer number of the potassium ion with time during electrodialysis. For potassium chloride, the transport number decreased with time: the transfer number was as low as about 10% after 75 min operation, where the recovery yield was about 35%. For potassium acetate, the transfer number of the potassium ion slightly increased up to 30 min, and then gradually decreased with time. The transfer number was greater than 30% after 90 min, where the recovery yield was about 90%. Figure 10 shows the change in power consumption for the recovery of 1 mol acid and 1 mol alkali with time for the electrodialysis processes shown in Figure 8. The power

Figure 11. Power consumption for the electrodialysis process plotted against the acid−alkali pair recovery yield for the same operation conditions as those in Figure 8. 6573

DOI: 10.1021/acs.iecr.5b00717 Ind. Eng. Chem. Res. 2015, 54, 6569−6577

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Industrial & Engineering Chemistry Research solution of potassium acetate introduced into the acid recovery cell completely dissociated into potassium and acetate ions. Because of the electric potential applied to the cell, the potassium ions would be transported to the alkali recovery cell through the CEM. The BPM would generate protons in the acid recovery cell. The protons would combine with acetate ions to form acetic acid, where the degree of the dissociation is extremely low because of the small acid dissociation constant. The leakage of protons from the acid recovery cell to the alkali recovery through the CEM could be impeded by capturing of protons by acetate ions. For the CEM−BPM configuration, because the leakage of protons through the CEM would be the main reason for the decrease of the process efficiency, the recovery rate as well as the power consumption for acid−alkali recovery by electrodialysis would be significantly improved by the immobilization of protons by a weak acid. Effect of the Concentration of Acetic Acid. In the previous section, we showed that using a salt of a weak acid and strong alkali is effective for recovery of the acid−alkali pair with BPM−CEM electrodialysis in terms of the recovery rate and the power consumption. In this section, the effect of the initial concentration of the potassium acetate solution on the recovery rate and power consumption for the recovery of potassium hydroxide and acetic acid with the BPM−CEM process will be discussed based on the experimental results. Figures 12 and 13 show the effect of the initial concentration of potassium acetate on the recovery yield and power

Figure 13. Variation of the power consumption with time for recovery of the acid−alkali pair by BPM−CEM electrodialysis from potassium acetate solutions with various initial concentrations. The conditions were the same as those in Figure 12.

Figure 14. Relationship between power consumption and recovery yield of the acid−alkali pair for the electrodialysis process. The operation conditions were the same as those in Figure 12.

4.09 S m2 mol−1,28 which is about half of the mobilities of the other anions, and leakage of hydroxide ions would significantly reduce the efficiency (i.e., the transfer number of acetate ion). The use of a weak alkali or acid is another option to improve the recovery efficiency. Extraction of Calcium from Waste Cement Powder with Acetic Acid. In the previous section, the efficiency of the recovery of the acid and alkali pair by electrodialysis would be greatly improved by using acetic acid (a weak acid) rather than a strong acid. However, the use of a weak acid may affect the efficiency of the extraction step of calcium or magnesium ions from the solid sources. In this section, we investigated the extraction of calcium ions from waste cement powder, a potential candidate as a calcium source for CCS, by mineral carbonation using acetic acid.12 Figure 15 shows the change of calcium concentration extracted into the acetic acid phase with time, where initial acetic acid concentrations in the range of 0.5−4.4 M. The calcium concentration immediately increased upon contact of the waste cement powder with the acetic acid solution, and the concentration reached a plateau in 5 min. The final concentration (after 120 min) increased with increasing initial concentration of acetic acid. The maximum extraction ratio obtained for 4.4 M acetic acid was about 40%, as shown in Figure 16, indicating that calcium in waste cement can be partially extracted into the acetic acid phase under the present

Figure 12. Change of the recovery yield of the acid−alkali pair with time for various initial concentrations of potassium acetate by BPM− CEM electrodialysis. The BPM−CEM system contained 10 cell units, and the voltage was 13.5 V.

consumption, respectively. The recovery yield increased with time for all the concentration conditions, and the recovery yield for a given time was higher for lower initial concentration. Note that, for the lowest initial concentration at 0.5 M, the recovery yield showed a peak at about 20 min and then gradually decreased. This can be explained by the low concentration not being sufficient to capture protons produced at the bipolar membrane. The power consumption decreased with increasing concentration. The increase in the power consumption is because the higher acetate ion concentration would result in a higher capture rate of protons, which would impede the leakage of protons. When the power consumption is plotted against the recovery yield, the effect of the initial concentration is almost negligible, as shown in Figure 14. The BPM−AEM configuration with potassium acetate was not investigated in this study. The mobility of the acetate ion is 6574

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time up to 60 min and was almost unchanged after 60 min for all of the concentration conditions. Considering the precipitation step for calcium carbonate formation, mixing the extracted solution containing calcium ions with alkali metal carbonate solution, the pH of the extracted solution should be higher than 6.1, which is pKa1 for carbonic acid.29 Otherwise, mixing would result in the generation of gaseous carbon dioxide from the carbonate or bicarbonate ions. This criterion is satisfied for the extracted solution with an initial concentration of acetic acid of less than 1.0 M for the fixed S/L ratio of 0.28. The higher the S/L ratio, the higher the final pH of the extracted solution. For the initial concentration of 4.4 M acetic acid, with increasing S/L ratio, the pH of the extracted solution increased, and the final pH was 7.24 for the S/L ratio at 0.86. Under this condition, the calcium concentration reached 7000 ppm, which is about 90% of the saturation solution of acetic acid. Another advantage of a higher final pH of the extracted solution is that the dissolution of impurities in the solid source would be dramatically reduced.27 Calcium carbonate precipitation was observed when the extracted solution was mixed with an equimolar concentration of potassium carbonate solution to leave a potassium acetate solution. Using acetic acid, the main impurities in the waste cement powder (i.e., iron, aluminum, and silicon) were extracted with calcium. The final concentration of these impurities can be reduced by using a more dilute solution of acetic acid or a higher S/L ratio. In particular, an S/L ratio higher than 0.86 resulted in an extremely low concentration of impurities extracted in the aqueous phase: the total concentration of the three impurities was about 200 ppm. Details of the precipitation method can be found in a previous paper.23 The concentration of the salt solution of potassium acetate can be controlled by the S/L ratio and the initial concentration of acetic acid in the extraction step. Considering the performance of the acid−alkali recovery step by BPM electrodialysis, a low concentration of the salt solution would decrease the recovery time, but a high concentration would be more appropriate for the absorption of gaseous CO2 from flue gas. To determine the optimum conditions, a more detailed design is required. Note that the initial salt concentration can be adjusted up to the saturated concentration of calcium acetate (4.4 M) while maintaining neutral pH conditions for the precipitation step. Power Consumption for the CCS Process. The power consumption of CO2 fixation can be mostly attributed to the power consumption of the electrodialysis step, because steps i− iii are spontaneous processes and do not consume power. The power consumption for pumping solutions, solid−liquid separation by filtration or thickener, and transportation of waste cement powder and products should be included. However, these contributions are excluded to give a rough estimate of the power consumption. In the present process, the amount of CO2 fixed depends on the valence numbers of the alkali and acid used. Assuming that the electric power with specific CO2 emission at 400 g/kWh (9.09 mol/kWh),30 which is equivalent to the power generation at 400 kJ/mol-CO2 emission, is used for the electrodialysis, the power consumption for electrodialysis should be less than twice this value for the recovery of monovalent acid. Otherwise, it would be impossible to achieve net reduction of CO2 emission. When potassium acetate was used, the power consumption of electrodialysis was slightly lower than 200 kJ/mol for the recovery of 1 mol of alkali−acid pair (Figures 11 and 14). Note

Figure 15. Change of the calcium concentration in the acetic acid phase with time (stirring rate = 300 rpm, room temperature, atmospheric pressure).

Figure 16. Change of the calcium extraction ratio from waste cement powder to acetic acid solution with initial concentrations of acetic acid ranging from 0.5 to 4.4 M (stirring rate = 300 rpm, room temperature, atmospheric pressure).

conditions. The extraction ratio decreased with decreasing initial concentration of acetic acid. Note that the S/L ratio was fixed for all of the runs shown in Figures 15 and 16. Figure 17 shows the pH change during the extraction experiment. The pH in the extracted solution increased with

Figure 17. Variation of the pH of the acetic acid solution during the extraction process of waste cement powder. The initial acetic acid concentration was varied from 1.0 to 4.4 M (stirring rate = 300 rpm, room temperature, atmospheric pressure). 6575

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increasing recovery yield until ∼80% recovery yield, at which net reduction of CO2 emission can be realized for the present system. On the other hand, the power consumption gradually decreases with time when using strong acids. The cost for CCS is about 175 USD (about €200) per metric ton of CO2. 4. For acid/alkali recovery by electrodialysis with the CEM−BPM configuration and potassium acetate, a higher recovery rate is obtained for a lower initial concentration of potassium acetate, but the power consumption is lower for a higher initial concentration. 5. Extraction experiments of the leaching of calcium from waste cement powder (a calcium source) with acetic acid show that calcium in the waste cement powder can be rapidly leached and the amount of impurities is negligible. Thus, the present mineral carbonation process is more feasible in terms of power consumption when acetic acid is used for calcium leaching and potassium hydroxide is used for CO2 capture from flue gas than when a strong acid is used for leaching.

because 2 mol of each acid and alkali should be required for the treatment of 1 mol of CO2, the power consumption for the recovery process is 200 kJ × 2 = 400 kJ/mol CO2. This power consumption is almost equivalent to the power consumption associated with the CO2 emission, 400 kJ/mol-CO2 emission. On the other hand, when a strong acid such as hydrochloric acid or nitric acid was used, the power consumption increased with increasing recovery yield and outnumbered the value of 400 kJ/mol-CO2 emission, although the power consumption at the earlier stage was almost equivalent to the case using acetic acid. The power consumption for the present case may not necessarily be suitable for CCS, but there is still some room for improvement in the power consumption for electrodialysis, such as by increasing the number of cells and increasing the current efficiency by using more selective ion-exchange membranes. Note that the theoretical power consumption for the BMED is 80.1 kJ/mol for the alkali−acid pair based on the dissociation energy of water in the BPM, which may be the minimum value of the present system. The cost of the regeneration of acid and alkaline can be estimated by the price of electric power (0.07 USD per kWh), ignoring the makeup cost for lost chemicals. On the basis of the potassium acetate process (Figures 11 and 14), where the power consumption for the recovery of 1 mol of acid and alkali pair is 200 kJ, the recovery process of 1 mol of CO2 would consume 400 kJ/mol-CO2, which is equivalent to about 2500 kWh per metric ton of CO2. The cost of this power is about 175 USD (about €200). According to published studies on the cost estimation of CCS with accelerated carbonation processes using alkali industrial waste, the total cost of CCS is €77,31 €46−270,32 or 105 USD per metric ton of CO2.4 Therefore, there is a difference between the cost of the present process and other reported costs. However, the cost estimation studies for CCS by mineral carbonation reactions were based on different frameworks, and direct comparison is difficult. Nevertheless, the cost for the recovery of the acid−alkali pair would be significantly reduced by using acetic acid and potassium hydroxide compared with the process with one-path use of an acid and an alkali. In addition, the calcium carbonate product could be sold (100 USD per metric ton). Because one metric ton of CO2 can be converted to about 100/44 [Mr(CaCO3)/ Mr(CO2)] metric tons of calcium carbonate, the process would be profitable by about 50 USD excluding other process costs.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-422-37-3887. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS The following conclusions can be drawn from the present study. 1. For AEM−BPM electrodialysis, the recovery rate and power consumption for a given recovery yield are almost unaffected by the type of salt. 2. For acid/alkali recovery with sodium chloride and potassium chloride, the AEM−BPM electrodialysis configuration gives a higher recovery rate than the CEM−BPM configuration, and the power consumption for a given recovery rate is higher for the CEM−BPM configuration. 3. The acid/alkali recovery rate by electrodialysis with the CEM−BPM configuration is significantly higher with potassium acetate (a salt of a strong alkali and weak acid) than for other salts of a strong acid and a strong alkali. For potassium acetate, the power consumption is almost constant at slightly less than 400 kJ/mol-CO2 with 6576

DOI: 10.1021/acs.iecr.5b00717 Ind. Eng. Chem. Res. 2015, 54, 6569−6577

Article

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DOI: 10.1021/acs.iecr.5b00717 Ind. Eng. Chem. Res. 2015, 54, 6569−6577