Study on the Mechanism and Energy Consumption of CO2

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Study on the Mechanism and Energy Consumption of CO2 Regeneration Process by Membrane Electrolysis Xinglei Zhao,† Na Liu,† Yundong Wang,† Weiyang Fei,*,† and Geoffrey W. Stevens‡ † ‡

The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia ABSTRACT: A significant barrier for carbon capture and sequestration (CCS) deployment is the high capital cost and the high energy consumption for capturing CO2 from a coal fired power station. Novel membrane electrolytic regeneration technology has been reported to significantly reduce the energy consumption compared with the conventional thermal regeneration process [Martin, F. J.; Kubic Jr., W. L. Green Freedom: A Concept for Producing Carbon-Neutral Synthetic Fuels and Chemicals; LA-UR-077897; Los Alamos National Laboratory, Los Alamos, NM, USA, 2007]. The mechanism and energy consumption for this novel process have been studied in this paper. CO2 regeneration was achieved through the membrane electrolysis process, and CO2 regeneration efficiency reached as high as 100%. At room temperature, the membrane electrolysis process is considered in three stages: no CO2 release stage, enhanced CO2 release stage, and steady CO2 release stage. The mechanisms for each stage and the whole process have been presented. Compared with the traditional CCS process, the energy requirement is significantly reduced to 16.6% for the regeneration process and 25.8% for the total CO2 capture and compression process using the electrolysis regeneration process based on ASPEN PLUS simulations. Further work on the membrane electrolytic regeneration is needed to examine the impact of promoters, the optimization of electrolysis conditions, and the equipment cost.

1. INTRODUCTION Global warming has become a challenge as a result of massive emissions of greenhouse gases, such as CO2, CH4, etc. The overabundance of atmospheric CO2 is believed to be a major contributor. Carbon capture and sequestration (CCS) provides a means to substantially reduce the amount of CO2 emission into the atmosphere from the flue gas generated by fossil fuel power plants. Due to the low CO2 concentrations in the flue gas, only chemical solvents show an absorption capacity large enough to be applicable for CO2 capture.1 Chemical absorption of CO2 with aqueous solvents including alkanolamines and hot potassium carbonate (K2CO3) solutions are well-known and effective processes for removing CO2 from power plant flue gases.2 Compared with alkanolamines, K2CO3 solution is less toxic, is less prone to oxidative degradation, and has a low heat of regeneration.3,4 Reaction of K2CO3 with CO2 occurs via the following overall equation: K 2 CO3 þ CO2 þ H2 O f 2KHCO3

ð1Þ

Equation 1 can be more realistically represented in ionic terms as eqs 24. H2 O f Hþ þ OH

ð2Þ

CO2 þ OH f HCO3

ð3Þ

CO3 2 þ Hþ f HCO3

ð4Þ

However, when K2CO3 solution is used to absorb CO2, the process has a low CO2 absorption rate and relatively high capital r 2011 American Chemical Society

cost and energy consumption. Therefore, various methods to intensify the process are needed to make CO2 absorption process more feasible and affordable. Martin et al. at Los Alamos National Laboratory in the United States presented a low-risk transformational concept called Green Freedom in 2007.5 The heart of this technology is a new process for stripping CO2 through ion-exchange-membrane electrolysis after CO2 was absorbed using K2CO3 solution. The new electrolysis process was reported to drastically reduce the energy requirement for stripping CO2 by 96% compared with the conventional process of heating. However, it is unknown if the new regeneration method was economical because of the lack of detailed information. In this paper, the mechanism for this novel process is investigated and the energy consumption is estimated from a design example for a carbon capture project using the software ASPEN PLUS.

2. MECHANISM OF THE ELECTROLYSIS PROCESS 2.1. Experiments. The electrolysis experiments were performed using a membrane electrolysis unit with an active volume of 150 cm3 as shown in Figure 1. The anode and cathode of the cells were separated by the cation-selective membrane Nafion 324 (N324, DuPont). The electrode material was Ti and Ni. Before the gas and liquid entered the electrolytic cell, they were preheated to the reaction temperature. The nitrogen gas Received: December 8, 2010 Accepted: April 28, 2011 Revised: April 27, 2011 Published: April 28, 2011 8620

dx.doi.org/10.1021/ie1024529 | Ind. Eng. Chem. Res. 2011, 50, 8620–8631

Industrial & Engineering Chemistry Research

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Figure 1. Experimental apparatus for the membrane electrolytic regeneration. (1) Anode chamber. (2) Cathode chamber. (3) Cation-selective membrane. (4) Electrode. (5) Gas mass controller. (6) Thermometer. (7) Drain sleeve. (8) Recycled water.

stream flowed inward from the inlet of the electrolytic cell by a pressure-driving force, and the solution was directly fed to the anode and cathode chambers, where the electrolysis process in the solution took place at constant current density. Current density (j) was defined as the ratio of the current passing through the solution to the membrane area between the anode and cathode of the cells.6 The stirrer was used to reduce concentration gradients in the solution. Electrolytic regeneration was first tested for the rich solution (CO2 loading: R ≈ 0.65) after CO2 was absorbed into K2CO3 solution. A current was passed through this solution, and the current density (j) was controlled at 500 and 1000 A/m2, respectively. The definition of R is shown in eq 5. R¼

½HCO3 ½HCO3 þ 2½CO3 2

Figure 2. Change of Raman spectra for the solution containing 1 mol/L KHCO3 and 0.2 mol/L K2CO3 when KOH is added.

ð5Þ

To study the mechanism of electrolytic regeneration at room temperature, 80 mL of solution containing KHCO3 and K2CO3 at different CO2 loadings was added to the anode chamber and 80 mL of solution containing KOH was added to the cathode chamber initially. The concentration of Kþ ion for all the solutions added in the chamber was kept between 1.8 and 2.6 mol/L. Latter, the impact of different concentrations of the actual solution in the cathode chamber was investigated, which was completed by adding different concentrations of KOH and K2CO3 or KHCO3 and K2CO3. The effect of the temperature has also been studied. 2.2. Gas and Liquid Analysis. The CO2 concentration in the outlet gas streams was measured by an infrared gas analyzer (GXH-3011, Huayun Analytical Instrument Institution, CO2 mole fraction = 05%). When the current was passed through the solution, the electrolysis of H2O occurred and H2 and O2 were formed. The concentrations of H2 and O2 were respectively determined by an H2 analyzer (Htr-100S, H2 mole fraction = 03%) and an O2 analyzer (Oxy-30S, O2 mole fraction = 02%). Both analyzers were purchased from Beijing Hangtian Ketuo Co. In the solution, CO32 and HCO3 ions were present when CO2 was absorbed into K2CO3 solution as shown in eq 1. Also, Kþ existed in the solution because of the dissociation reaction of the electrolyte. Thus, CO32, HCO3, OH, and Kþ could be present in the solution, and it is necessary to know if they can exist simultaneously. The coexistence of HCO3 and OH was

Figure 3. Change of concentration of the solution at j = 500 A/m2 (anode and cathode: 80 mL of solution containing 1.56 mol/L KHCO3 and 0.38 mol/L K2CO3).

studied by adding different amounts of KOH into the solution consisting of K2CO3 and KHCO3 with the help of a Raman spectrometer (R-3000, Agiltron, Inc.). The typical wavenumber for the HCO3 ion is 1012 cm1 and that for the CO32 ion is 1063 cm1.7 The peak of the SO42 ion is at a wavenumber of 978 cm1 as a reference to determine the concentration of other ions. The changes in the Raman spectrum are shown in Figure 2. With the increase of amount of KOH added in the solution, the intensity of the HCO3 ion decreases and that of the CO32 ion increases. This indicates that HCO3 does not coexist with OH under these conditions; thus HCO3 þ OH f CO3 2 þ H2 O

ð6Þ

In this paper, the acidbase titration method reported by Luo8 was used to determine the concentrations of the components, such as K2CO3, KHCO3, and KOH. 8621

dx.doi.org/10.1021/ie1024529 |Ind. Eng. Chem. Res. 2011, 50, 8620–8631


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2

Figure 4. Change of concentration of the solution at j = 1000 A/m (anode and cathode: 80 mL of solution containing 1.47 mol/L KHCO3 and 0.38 mol/L K2CO3).

2.3. Process Confirmation for Electrolytic Regeneration. The change of the concentration of the solution is shown in Figures 3 and 4 when current is passed through a solution consisting of KHCO3 and K2CO3. The concentrations of KHCO3 and K2CO3 in the anode chamber reduce as a result of electrolysis, while in the cathode chamber, K2CO3 concentration increases and KHCO3 concentration decreases. KOH is detected when KHCO3 is depleted (see Figure 4). The reason for this phenomenon is that both KHCO3 and K2CO3 participate in electrolysis processes in the anode chamber. In the cathode chamber, KOH is produced9 and it reacts with KHCO3 as shown in eq 6. Correspondingly, K2CO3 is formed. KOH appears as soon as KHCO3 is depleted. As the electrolysis reaction occurs, the total amount of Kþ ion is constant during the electrolysis process. Moreover, when the solutions in the anode and cathode chambers are mixed following the reaction, the OH ion is not present when the amount of KHCO3 is more than that of KOH. Thus, a total ion balance, eq 7, is relevant for this situation. According to eq 7, the reduction of [HCO3] is accompanied by the increase of [CO32]. The conversion ratio of KHCO3 to K2CO3 is expressed as ηKHCO3, which is calculated as eq 8. The value of ηKHCO3 is 51.5 or 33.2%, respectively, in the process as shown in Figures 3 and 4. Therefore, after CO2 is absorbed using K2CO3 solution, the regeneration of CO2 from the rich solution occurs through the membrane electrolysis process.

½K þ ¼ ½HCO3 þ 2½CO3 2 ηKHCO3 ¼

½HCO3 final ½HCO3 initial

ð7Þ

Figure 5. Change of CO2 release rate under different current densities (anode: 80 mL of solution containing 1.56 mol/L KHCO3 and 0.21 mol/L K2CO3; cathode: 80 mL of solution containing 1.76 mol/L KOH; R = 0.8).

Figure 6. Change of ηKHCO3 under different current densities (anode: 80 mL of solution containing 1.56 mol/L KHCO3 and 0.21 mol/L K2CO3; cathode: 80 mL of solution containing 1.76 mol/L KOH; R = 0.8).

2.4.1. Impact of Chemical Regeneration Reaction on Electrolysis Process. Equation 9 (the reverse reaction of eq 1) occurs when the conventional thermal regeneration process is used. 2HCO3 f CO3 2 þ CO2 v þ H2 O The equilibrium constant (K) for eq 9 is as follows:10,11

ð8Þ

K ¼

2.4. Mechanism for the Electrolytic Regeneration. HCO3 and CO32 ions participated in the electrolysis process as described previously. Stuchi9 reported that CO2 was released when the Kþ ion present in solution of K2CO3 and KHCO3 was replaced by Hþ ion. The purpose is the conversion of K2CO3 to KOH in their system, while the conversion of KHCO3 to K2CO3 is considered for the current study. In this section, the mechanism for the electrolytic regeneration process is presented in detail.

ð9Þ

RCO3 2 RCO2 RH2 O ¼ e15:4 340=T þ 1:3 ln T RHCO3 2

ð10Þ

The value of K increased with the temperature as shown in eq 10. Thus, the impact of the chemical reaction on electrolytic regeneration is lower at room temperature than at higher temperatures. The changes of CO2 release rate and the conversion ratio of KHCO3 to K2CO3 under different current densities at room temperature are shown in Figures 5 and 6. 8622



Figure 7. Change of gas release rate and concentration of the solution at j = 1250 A/m2 (anode: 80 mL of solution containing KHCO3 and K2CO3 solution for [Kþ] = 2.5 mol/L; cathode: 80 mL of solution containing 2 mol/L KOH; R = 0.21).

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Figure 9. Change of CO2 release rate with R at j = 1250 A/m2 (anode: 80 mL of solution containing KHCO3 and K2CO3 for [Kþ] = 2.5 mol/L; cathode: 80 mL of solution containing 2 mol/L KOH).

atmospheric temperature is presented. Figure 7 shows the change of gas release rate and the concentration of the solution at j = 1250 A/m2. The release rate of H2 and O2 reaches the steady release very quickly during the electrolysis, and the release ratio of H2 to O2 is about 2, showing the electrolysis of H2O is occurring. Moreover, the current efficiency for the electrolysis of H2O is about 99%, which means only this reaction takes place. The definition of the current efficiency (ηt) is given in eq 11: ηt ¼

Figure 8. Three stages of CO2 release at j = 1250 A/m2 (anode: 80 mL of solution containing KHCO3 and K2CO3 for [Kþ] = 2.5 mol/L; cathode: 80 mL of solution containing 2 mol/L KOH; R = 0.21).

When the current is passed through solution with high CO2 loading (R = 0.8), the CO2 release rate increases linearly with time and then remains almost constant. The larger the current density, the higher the CO2 release rate at the steady state. The conversion ratio of KHCO3 to K2CO3 is about 61% at j = 1250 A/m2 after 150 min. When no current is passed through the solution and only the chemical regeneration reaction (eq 9) occurs, the CO2 release rate reduces with the time. The conversion ratio of KHCO3 to K2CO3 is about 4.8% within 150 min. Also, the CO2 release rate resulting from the chemical regeneration reaction (eq 9) could be further reduced with the decrease of KHCO3 concentration during the electrolysis process. Thus, the impact of chemical regeneration reaction on the electrolysis process is neglected at j = 1250 A/m2 at room temperature. 2.4.2. Mechanism of Electrolytic Regeneration Process at Room Temperature. A study of the electrolysis mechanism at the

zNH2 F 100% It

ð11Þ

where z, NH2, F, I, and t represent the electron transfer number, H2 release rate, Faraday constant, current, and electrolysis time, respectively. When a solution with a CO2 loading (R) of 0.21 is used, CO2 is not detected initially. After some time, CO2 begins to be released. CO2 is released at a steady rate, and the release ratio of CO2 to H2 is approximately 2 after 90 min. The electrolysis process has been classified into three stages (see Figure 8). They are the no CO2 release stage, the enhanced CO2 release stage, and the steady CO2 release stage. As shown in Figure 9, with the increase of CO2 loading in the solution, the time for the no CO2 release stage reduces and even disappears; the time for the enhanced CO2 release stage is between 30 and 90 min. Take et al.12 studied the electrolysis of H2O. They reported þ H ion was produced in the anode chamber and then transferred to the cathode chamber through the membrane electrolyte between two chambers, while electrons produced in the anode chamber arrived at the cathode chamber by the external circuit. H2 was released when Hþ ion obtained an electron in the cathode chamber. During the electrolysis process in the current study, if all the Hþ ions in the anode chamber enter the cathode chamber, Kþ ion present in solution of KHCO3 is not replaced and CO2 is not released in the anode chamber as shown in Figures 79. Therefore, there could be a competitive effect between the replacement of Kþ ion by Hþ ion and the penetration of Hþ ion through the membrane into the cathode chamber. 8623



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Figure 10. Ratio of amount of increased KOH to H2 release rate at j = 1250 A/m2 (anode: 80 mL of solution containing 1.19 mol/L K2CO3; cathode: 80 mL of solution containing 1.73 mol/L KOH).

This question is explained when the electrolysis mechanism is discussed at the different stages of CO2 release as follows. 2.4.2.1. No CO2 Release Stage. When only a solution of K2CO3 or the mixture of K2CO3 and KHCO3 exists in the anode chamber and no CO2 gas is released, the stage is regarded as the no CO2 release stage. When CO2 is not released in the electrolysis process, the concentration of K2CO3 decreases at the same rate as the increase in the concentration of KHCO3 as shown by the relationship d[KHCO3]Anode/dt ≈ d[K2CO3]Anode/dt in Figure 7, which indicates that K2CO3 is converted to KHCO3 from eq 6. It is assumed that Hþ ion in the anode chamber forces eq 6 to the left and Kþ ion enters the cathode chamber through the cation-selective membrane as the concentration of Hþ ion is kept lower at pH 10.5. According to the electroneutrality, Kþ ion should move from the anode chamber to the cathode chamber at an equal rate with the electron through the external circuit. In the cathode chamber, H2O obtains one electron and becomes OH ion and 1/2H2; thus the amount of KOH increases at twice the rate with the release of H2. Figure 10 confirms the assumption presented here. Otherwise, the molar ratio of KOH to H2 should be less than 2 if Hþ ion migrates through the membrane. Also, as shown in Figure 11, the rate of change of Kþ ion is the same for the anode and cathode chambers, which further indicates the movement of Kþ ion through the membrane from the anode chamber to the cathode chamber. Therefore, the electrolysis mechanism in the first stage is proposed as follows: Anode : H2 O 2e f 1=2O2 v þ 2H þ

ð12Þ

2H þ þ 2OH f 2H2 O

ð13Þ

2K2 CO3 þ 2H2 O f 2KHCO3 þ 2OH þ 2K þ

Figure 11. Change of Kþ ion during the electrolysis process at j = 1250 A/m2 (anode: 80 mL of solution containing 1 mol/L K2CO3; cathode: 80 mL of solution containing 1.79 mol/L KOH).

Because the cation-selective membrane (N324) is used, 2Kþ ions enter the cathode chamber through the membrane. ð16Þ

2K þ þ 2OH f 2KOH

ð17Þ

2K þ þ 2H2 O þ 2e f 2KOH þ H2 v

ð18Þ

Total:

The electrolysis reaction is 2K 2 CO3 þ 3H2 O f 2KHCO3 þ 2KOH þ H2 v þ 1=2O2 v ð19Þ The electrolysis schematic diagram for the no CO2 release stage is shown in Figure 12. 2.4.2.2. Steady CO2 Release Stage. When the compound KHCO3 (or with low concentration of K2CO3) is present in the anode chamber, the Kþ ion present in solution of KHCO 3 is replaced by the Hþ ion (producing H2CO3) and correspondingly CO2 is released, which is shown by the release ratio of CO2 to H2 equal to 2 as shown in Figure 7 (at the electrolysis time of 110 min and later). As introduced previously in the no CO2 release stage, the movement of Kþ ion from the anode chamber to the cathode chamber is verified by the amount of KOH increasing at twice the rate with the release of H2. The electrolysis mechanism in this stage is presented as follows:

ð14Þ

Total:

Cathode : 2H2 O þ 2e f H2 v þ 2OH

Anode : H2 O 2e f 1=2O2 v þ 2H þ

ð20Þ

2H þ þ 2KHCO3 f 2H2 CO3 þ 2K þ

ð21Þ

2H2 CO3 f 2CO2 V þ 2H2 O

ð22Þ

Total:

2K 2 CO3 þ H2 O 2e f 2KHCO3 þ 2K þ þ 1=2O2 v ð15Þ

2KHCO3 2e f 2CO2 v þ 1=2O2 v þ 2K þ þ H2 O ð23Þ 8624



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Figure 12. Electrolysis schematic diagram for no CO2 release stage.

Figure 13. Electrolysis schematic diagram for steady CO2 release stage.

As introduced previously, 2Kþ ions enter the cathode chamber through the membrane. Cathode : 2H2 O þ 2e f H2 v þ 2OH

ð24Þ

2K þ þ 2OH f 2KOH

ð25Þ

2K þ þ 2H2 O þ 2e f H2 v þ 2KOH

ð26Þ

Total: The electrolysis reaction is 2KHCO3 þ H2 O f 2CO2 v þ 1=2O2 v þ H2 v þ 2KOH ð27Þ The electrolysis schematic diagram for steady CO2 release stage is shown in Figure 13. 2.4.2.3. Enhanced CO2 Release Stage. When the mixture of K2CO3 and KHCO3 exists in the anode chamber and CO2 is released at an increasing rate before the rate reaches the steady state, this stage is regarded as the enhanced CO2 release stage. When solutions of K2CO3 and KHCO3 are present, the concentration of K2CO3 gradually decreases with the electrolysis.

The concentration of KHCO3 reduces after it increases to a maximum value (see Figure 7). The change of the concentration of KHCO3 is due to the competitive reactions between the conversions of CO32 to HCO3 (eq 15) and HCO3 to CO2 (eq 23) by electrolysis. The release ratio of CO2 to H2 increases with the electrolysis time, which shows that the rate of the replacement of Kþ ion present in solution of KHCO3 by Hþ ion (eq 23) increases. The reaction rate is respectively assumed to be x for eq 15 and y for eq 23. The electrolysis mechanism in this stage is concluded as follows: Anode : xK 2 CO3 þ x=2H2 O xe f xKHCO3 þ xK þ þ x=4O2 v ð28Þ yKHCO3 ye f yCO2 v þ y=4O2 v þ yK þ þ y=2H2 O ð29Þ Total: xK 2 CO3 þ ðx yÞ=2H2 O ðx þ yÞe f ðx yÞKHCO3 þ yCO2 v

þ ðx þ yÞ=4O2 v þ ðx þ yÞK þ 8625

ð30Þ



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Figure 14. Electrolysis schematic diagram for enhanced CO2 release stage.

The rate of electron transfer should be same as in the no CO2 release stage and the enhanced CO2 release stage at the constant current density. Therefore, the number of electrons transferred in eq 30 should be 2 as in eqs 15 and 23. That is, x þ y = 2. Equation 30 is then rearranged as follows: ð2 yÞK 2 CO3 þ ð1 yÞH2 O 2e f 2ð1 yÞKHCO3 þ yCO2 v þ 1=2O2 v þ 2K þ

ð31Þ

As introduced previously, 2Kþ ions enter the cathode chamber through the membrane. Cathode : 2H2 O þ 2e f H2 v þ 2OH

ð32Þ

2K þ þ 2OH f 2KOH

ð33Þ

2K þ þ 2H2 O þ 2e f H2 v þ 2KOH

ð34Þ

Total:

The electrolysis reaction is ð2 yÞK 2 CO3 þ ð3 yÞH2 O f yCO2 v þ H2 v þ 1=2O2 v þ 2ð1 yÞKHCO3 þ 2KOH ð35Þ The electrolysis schematic diagram for the enhanced CO2 release stage is shown in Figure 14. The electrolysis reaction at the no CO2 release stage and the steady CO2 release stage is also described using eq 35 under the following conditions: When y = 0, the electrolysis process stays at the no CO2 release stage. Equation 35 is transformed to eq 19. When y = 2, the electrolysis process is in the steady CO2 release stage. Equation 35 is converted to eq 27. It is emphasized that ηKHCO3 reaches 100% when the electrolysis reaction is continued past the steady CO2 release stage, which is the advantage for electrolytic regeneration over other regeneration processes. When the solutions in the anode and cathode chambers are mixed, the neutralization reaction takes place between KHCO3 and KOH as shown in eq 6. Therefore, the following reaction is

Figure 15. Impact of composition in the cathode chamber on the release ratio of H2 to O2 at j = 1250 A/m2 (anode: 80 mL of solution containing 0.20.3 mol/L KHCO3 and 1.11.2 mol/LK2CO3; cathode: 80 mL of solution containing K2CO3 and KOH or KHCO3 for [Kþ] = 2.1 mol/L).

obtained by combining eqs 6 and 35: 2yKHCO3 þ ð1 yÞH2 O f yK 2 CO3 þ yCO2 v þ H2 v þ 1=2O2 v

ð36Þ This further indicates that the conversion of KHCO3 to K2CO3 is achieved through the electrolysis process when y > 0. In order to examine the electrolysis mechanism, the solution only consisting of KOH was used in the cathode chamber. In this part, the impact of the concentration of the solution in the cathode chamber on the electrolysis process is studied. When the concentration of the solution in the cathode chamber is varied, the release ratios of H2 to O2 and CO2 to H2 have no apparent change and the changes of the concentrations of K2CO3 and KHCO3 in the anode chamber remain almost constant (see Figures 1517). Therefore, the change of KOH concentration in the cathode chamber has no apparent effect on the electrolysis reaction in the anode chamber. Although the initial concentration 8626



Figure 16. Impact of composition of the solution in the cathode chamber on the release ratio of CO2 to H2 at j = 1250 A/m2 (anode: 80 mL of solution containing 0.20.3 mol/L KHCO3 and 1.11.2 mol/ L K2CO3; cathode: 80 mL of solution containing K2CO3 and KOH or KHCO3 for [Kþ] = 2.1 mol/L).

Figure 17. Impact of composition of the solution in the cathode chamber on concentration of the solution in the anode chamber at j = 1250 A/m2 (anode: 80 mL of solution containing 0.20.3 mol/L KHCO3 and 1.11.2 mol/L K2CO3; cathode: 80 mL of solution containing K2CO3 and KOH or KHCO3 for [Kþ] = 2.1 mol/L).

of KOH has been changed, the formation rate of KOH remains unchanged at the constant current density and the reaction between KOH and KHCO3 (eq 6) occurs, which is inferred by the constant ratio of KOH to KHCO3 (see Figure 18). 2.4.3. Mechanism of Electrolytic Regeneration Process at Higher Temperatures. The mechanism of the electrolysis reaction at room temperature has been studied in order to avoid the impact of chemical regeneration (eq 9) on the electrolysis process. However, the preferred temperature for the electrolysis of H2O is around 80 °C. Increasing the temperature reduces the voltage loss. Furthermore, when K2CO3 solution is used to absorb CO2, the absorption temperature is at 6070 °C.

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Figure 18. Impact of initial composition on change of concentration of the solution in the cathode chamber at j = 1250 A/m2 (anode: 80 mL of solution containing 0.20.3 mol/L KHCO3 and 1.11.2 mol/L K2CO3; cathode: 80 mL of solution containing K2CO3 and KOH or KHCO3 for [Kþ] = 2.1 mol/L).

Figure 19. Impact of temperature on release ratio of H2 to O2 at j = 1250 A/m2 (anode: 80 mL of solution containing 1.3 mol/L K2CO3; cathode: 80 mL of solution containing 2.2 mol/L KOH).

The small difference in the temperature for the absorption and regeneration will reduce the energy required for the heat exchanger. Thus, the impact of temperature on the electrolysis process is shown in Figures 1921. As shown in Figure 19, the release ratio of H2 to O2 is around 2 at all temperatures investigated, showing that the electrolysis of H2O takes place. The amount of KOH in the cathode chamber increases at twice the rate with the release of H2 (see Figure 20), which indicates that the movement rate of Kþ ion remains unchanged from the anode chamber to the cathode chamber. The change of CO2 release rate with the increase of temperature is shown in Figure 21. When the electrolysis temperature increases, the time consumed for the no CO2 release stage decreases. There is a slower increase in the ratio of CO2 to H2 released at the enhanced CO2 release stage. A lower release 8627



Figure 20. Impact of temperature on ratio of amount of increased KOH to H2 release rate at j = 1250 A/m2 (anode: 80 mL of solution containing 1.3 mol/L K2CO3; cathode: 80 mL of solution containing 2.2 mol/L KOH).

Figure 21. Impact of temperature on release ratio of CO2 to H2 at j = 1250 A/m2 (anode: 80 mL of solution containing 1.3 mol/L K2CO3; cathode: 80 mL of solution containing 2.2 mol/L KOH).

ratio of CO2 to H2 is achieved at the steady CO2 release stage. For example, the ratio of CO2 to H2 at the steady stage reduces to 1.52 at 82 °C from 2 at 25 °C. The reason for the above phenomena is that the conversion of KHCO3 to K2CO3 also occurs through the chemical regeneration (eq 9) at higher temperatures and higher concentration of KHCO3. Equation 9 is another path for the release of CO2. Because of eq 9, CO2 is easier to release in the electrolysis process at higher temperatures, which enables a slower increase of CO2 release through the reaction shown in eq 27 in the enhanced CO2 release stage. Although the total electrolysis rate is not affected at the constant current density at higher temperature, the proportion of eq 19 is increased because of eq 9. If eq 19 is not significant in the steady CO2 release stage, the release ratio of CO2 to H2 should be greater than 2 at higher temperatures as a result of eqs 9 and 27.

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3. ENERGY CONSUMPTION FOR THE ELECTROLYSIS PROCESS The energy consumption of the process is estimated for a design example from a carbon capture project using the software ASPEN PLUS. 3.1. Development of ASPEN Model. An ASPEN model for the traditional CO2 capture and compression process is developed as shown in Figure 22. The model consists of one absorber (which is represented as ABSORBER), one stripper (STRIPPER), one CO2 compressor (CO2COMP), two separators (SEP1 and SEP2), one blower (BLOWER), one liquidliquid mixer (MIX), three heat exchangers (HX1, HX2, and HX3), and several pumps. Flue gas (FLUEGAS) enters the absorber after it passes through the blower for increasing its pressure to some degree, and then the water inside the flue gas is separated at the block SEP1. Gas and liquid (LEANSOL) phases flow through the column countercurrently. The solvent from the absorber is brought to the stripper for CO2 regeneration after passing through the heat exchanger (HX2) for using the heat of the rich solvent from the stripper. Stripped CO2 is compressed at block CO2COMP after the water inside CO2 is separated at the block SEP2. In order to realize the water balance, the water of stream H2OA, H2OB, and H2OC should be returned to the system and more water should be added if necessary. The flue gas composition for a CO2 capture and sequestration project in a 60 MW power plant is shown in Table 1. The design conditions for the absorber, stripper, and CO2 compressor are respectively shown in Tables 2, 3, and 4. The absorbent used here is 30 wt % K2CO3 solution. The energy consumption for the total process is obtained by accumulating the value of every block. In order to compare the traditional CCS process and those with electrolytic regeneration, the new ASPEN model with the electrolytic regeneration technology has been developed as shown in Figure 23. The electrolysis model is composed of three parts: electrolysis reaction (ELEREG), liquidgas separation (SEP3), and combustor (COMBUST). In the electrolysis reaction, eqs 9, 19, and 27 are selected and the ratio of reaction rates for these reactions is determined as described above. The neutralization reaction (eq 6) after the electrolysis process has also been used in this model. The reaction heats for eqs 6, 19, and 27 have not been included because of lack of data. In the electrolysis process, a mixture of CO2 (∼70%) and O2 is produced in the anode chamber. Thus it is important to separate them prior to CO2 compression. However, no discussion was presented in the report by Martin et al.5 If the method to separate CO2 from the mixture of CO2 and N2 is similar to that for separating CO2 from the mixture of CO2 and O2, the energy consumption is increased significantly. Hence, a combustor (COMBUST) is designed to burn H2 in the presence of the mixture of CO2 and O2, by which pure CO2 is obtained. The combustion heat is used for the production of the stream in the power plant. 3.2. Comparison of the Energy Consumption between the Electrolysis and Traditional Process. The energy consumption

for CO2 capture and compression using the traditional regeneration method is considered. According to the simulation result, the rate of CO2 release is shown in eq 37. The energy required for the traditional CO2 capture and compression is obtained as shown in eq 38.

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MCO2 ¼ 1068:2 106:8 ¼ 961:4 ðmol=sÞ

ð37Þ

E1 ¼ 652117:6=961:4 ¼ 678:3 ðkJ=mol CO2 Þ

ð38Þ



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Figure 22. ASPEN model for traditional CO2 capture and compression process.

Table 1. Flue Gas Composition for a CO2 Capture Project flue gas

Table 4. Design Conditions for CO2 Compressor

value

design conditions

gas flow rate, kg/s

218.87

operating temperature, °C

40

gas temperature, °C

84.23

final pressure, kPa

100.00

gas pressure, kPa

100

CO2 mol %

14.42

N2 mol % O2 mol %

73.81 4.01

H2O mol %

7.76

Table 2. Design Conditions for Absorber design conditions

value

column height, m

20

theoretical stages

20

operating pressure, kPa CO2 removal efficiency

101.3 90%

packing type

50 mm diameter Pall ring

value

product of the voltage, the current, and the residence time of the reactant in the electrolysis chamber. The calculation process has been shown as follows: As discussed previously, the regeneration of KHCO3 was achieved with the help of water electrolysis. The theoretical voltage for water electrolysis is 1.23 V.13 The real voltage could reach 2 V and the corresponding current is 1.68 A.14 According to the simulation result, the release rate of H 2 is shown in eq 39. MH2 ¼ 625:14 ðmol=sÞ

ð39Þ

The current efficiency for the electrolysis of H2O is assumed to be 97%. Therefore, the residence time of the reactant is calculated as follows: τ ¼ 2ð625:14Þð96500=1:68=97%Þ ¼ 7:404 107 ðsÞ ð40Þ

Table 3. Design Conditions for Stripper design conditions

a

The electric energy is then obtained as shown in eq 41. value

column height, m

20

theoretical stages

21a

operating pressure, kPa packing type

110 50 mm diameter Pall ring

EE ¼ 2ð1:68Þð7:404 107 Þ=103 ¼ 248775 ðkJ=sÞ

The combustion heat of H2 of 68.3 kcal/mol is used in the model. If the heat is recovered, the following energy is obtained according to the simulation result: EH2 ¼ 178645 ðkJ=sÞ

Bottom reboiler is the 21th stage.

Figure 24 shows the amount of the energy consumption. The energy required for the traditional regeneration accounts for 89% of the total energy. The energy consumed for the electrolysis process mainly comes from the electric energy, which is calculated by the

ð41Þ

15

ð42Þ

Thus, the total energy required for the electrolytic regeneration process per mole of CO2 is as follows: E2 ¼ ð248775 178645Þ=961:4 ¼ 72:9 ðkJ=mol CO2 Þ ð43Þ 8629



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Figure 23. ASPEN model for novel CO2 capture and compression process with electrolytic regeneration.

because the impact of the promoter on the electrolytic regeneration has not been investigated as yet. In the CO2 capture process with the electrolysis regeneration, the energy requirement could be decreased if the heat of reaction from the electrolysis and acidbase reaction is recovered. This paper offers the preliminary research on the membrane electrolytic regeneration. Further work is needed to examine the impact of the promoter, the electrolysis reaction heat, the optimization of electrolysis condition, the equipment cost, and so on.

Figure 24. Comparison of energy consumption for CO2 capture and compression process with traditional or electrolytic regeneration.

The total energy required for the electrolysis process including the heat exchanger (HX3) per mole of CO2 is as follows: E3 ¼ 72:9 þ 27:2 ¼ 100:1 ðkJ=mol CO2 Þ

ð44Þ

Energy consumption of CO2 regeneration of the traditional method and that of the electrolysis method have been compared in Figure 24. The energy required for the electrolysis process reduces to 16.6% compared with the traditional thermal regeneration. When the total process of CO2 capture and compression is considered, the energy consumption decreases to 25.8% using electrolytic regeneration compared to the traditional method. It is emphasized that the solvent of K2CO3 with at least one promoter was used in the real CO2 capture process. However, the solvent of K2CO3 without the promoter was studied in this paper

4. CONCLUSIONS The mechanism and the energy consumption of the membrane electrolysis process for CO2 regeneration have been studied in this paper. The main conclusions were obtained as follows: 1. CO2 regeneration is achieved through the membrane electrolysis process with a high efficiency when K2CO3 solution is used as CO2 absorbent. 2. At room temperature, the membrane electrolysis process is divided into three stages: the no CO 2 release stage, the enhanced CO2 release stage, and the steady CO2 release stage. The mechanisms for each stage and the whole electrolysis process have been presented. At the steady CO2 release stage, the CO 2 release rate at higher temperatures is lower than those at room temperature because the electrolysis reaction of K 2 CO3 has been prompted by the occurrence of chemical regeneration. 3. Compared with the traditional CCS process, the energy requirement is respectively reduced to 16.6% for electrolytic regeneration and 25.8% for the total CO2 capture and compression process using electrolytic regeneration. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86 10 6278 9637. Fax: þ86 10 6277 0304. E-mail: fwy-dce@ tsinghua.edu.cn. 8630



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’ ACKNOWLEDGMENT This research was carried out under the Specialised Research Fund for Doctoral Programme of Higher Education of MOE of China (Nos. 200700033154 and 200800030095) and the National Natural Science Foundation of China (No. 20836008) in the State Key Laboratory of Chemical Engineering of Tsinghua University, Beijing, People's Republic of China. ’ REFERENCES (1) Abu-Zahra, M. R.M.; Schneiders, L. H. J.; Niederer, J. P. M.; Feron, P. H. M.; Versteegb, G. F. CO2 capture from power plants. Part I. A parametric study of the technical performance based on monoethanolamine. Int. J. Greenhouse Gas Control 2007, 1, 37–46. (2) Jassim, M. S.; Rochelle, G. T. Innovative absorber/stripper configurations for CO2 capture by aqueous monoethanolamine. Ind. Eng. Chem. Res. 2006, 45, 2465–2472. (3) Uyanga, I. J.; Idem, R. O. Studies of SO2 and O2 induced degradation of aqueous MEA during CO2 capture from power plant flue gas streams. Ind. Eng. Chem. Res. 2007, 46, 2558–2566. (4) Cullinane, J. T.; Rochelle, G. T. Carbon dioxide absorption with aqueous potassium carbonate promoted by piperazine. Chem. Eng. Sci. 2004, 59, 3619–3930. (5) Martin, F. J.; Kubic, W. L., Jr. Green Freedom: A Concept for Producing Carbon-Neutral Synthetic Fuels and Chemicals; LA-UR-077897; Los Alamos National Laboratory, Los Alamos, NM, USA, 2007. (6) Tanaka, Y. Mass transport and energy consumption in ionexchange membrane electrodialysis of seawater. J. Membr. Sci. 2003, 215 (12), 265–279. (7) Frantz, J. D. Raman spectra of potassium carbonate and bicarbonate aqueous fluids at elevated temperatures and pressures: comparison with theoretical simulations. Chem. Geol. 1998, 152, 211–225. (8) Luo, S. M. Dtermination of NaOH, Na2CO3 or NaHCO3 in the solution containing vanadium. Chin. J. Process Eng. 1983, 3 (4), 152–155. (9) Stuchi, S.; Schuler, A.; Constantinescu, M. Coupled CO2 recovery from the atmosphere and water electrolysis: feasibility of a new process for hydrogen storage. Int. J. Hydrogen Energy 1995, 20 (8), 653–663. (10) Posey, M. L. Thermodynamics Model for Acid Gas Loaded Aqueous Alkanolamine Solutions. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, USA, 1996. (11) Edwards, T. J.; Maurer, G.; Newman, J.; Praunitz, J. M. Vaporliquid equilibria in multicomponent aqueous solutions of volatile weak electrolytes. AIChE J. 1978, 24 (6), 966–976. (12) Take, T.; Tsurutani, K.; Umeda, M. Hydrogen production by methanolwater solution electrolysis. J. Power Sources 2007, 164 (1), 9–16. (13) Jomarda, F.; Ferauda, J. P.; Caire, J. P. Numerical modeling for preliminary design of the hydrogen production electrolyzer in the Westinghouse hybrid cycle. Int. J. Hydrogen Energy 2008, 33 (4), 1142–1152. (14) Qin, J. G.; Zhang, Y. H. The energy saving analysis of the water electrolysis techniques into the hydrogen. Jiangxin Energy 2001, 18 (3), 23–25. (15) Wuhan University; Jilin University. Inorganic Chemistry, 1st ed.; Higher Education Press: Beijing, 2000.

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Study on the Mechanism and Energy Consumption of CO2

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