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Regeneration energy analysis of NH3-based CO2 capture process integrated with a flow-by capacitive ion separation device
Energy 125 (2017) 178e185
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Regeneration energy analysis of NH3-based CO2 capture process integrated with a flow-by capacitive ion separation device Minkai Zhang, Yincheng Guo* Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2016 Received in revised form 22 February 2017 Accepted 24 February 2017 Available online 27 February 2017
The flow-by capacitive ion separation (CIS) device was introduced into the NH3-based CO2 capture process for reducing the regeneration energy. Regeneration energy analysis of the NH3-based CO2 capture process integrated with the CIS device under different flow rates, NH3 concentrations and CO2 loadings of the rich solvent (Richout) flowing into the CIS device was performed. The flow rates, NH3 concentrations and CO2 loadings of Richout considered in this paper are 120e150 L/min, 2.0e3.0 mol/L, and 0.3e0.5, respectively. When choosing suitable operating parameters of the CIS device, the flow rate of the concentrated ion stream flowing out of the CIS device decreases. Therefore, the introduction of the CIS device into the NH3-based CO2 capture process can lead to a significant reduction of the regeneration energy, and the regeneration energy can be reduced by above 20%. Particularly, for the case that the NH3 concentration of Richout is 2.0 mol/L, the regeneration energy can be reduced by up to 35%. © 2017 Elsevier Ltd. All rights reserved.
Keywords: CO2 capture Aqueous ammonia Rich solvent Capacitive ion separation Regeneration energy
1. Introduction For reducing the CO2 emissions from coal-fired power plants, the NH3-based post-combustion CO2 capture process is considered as a feasible technical route [1,2], which is due to its advantages of high CO2 loading capacity, no absorbent degradation, no equipment corrosion, and simultaneous removal of CO2, SO2 and NOx [3e6]. But, the industrial application of the NH3-based CO2 capture process is limited to its high regeneration energy [7,8]. In the NH3based CO2 capture process of the Munmorah pilot plant [8], the experimental results showed that more than 50% of the reboiler heat duty was used for heating the rich solvent (Richin) flowing into the stripper, and thus reducing the flow rate of Richin can provide a solution for reducing the regeneration energy of the NH3based CO2 capture process. Since the NH3- and MEA-based CO2 capture processes are two typical chemical absorption methods for CO2 capture [9], the ideas for reducing the flow rate of Richin used in the MEA-based process can be referred for the NH3-based process. Yan et al. [10] suggested that for the MEA-based process, the rich solvent (Richout) flowing out of the absorber is separated into diluted and concentrated ion streams, and then just let the concentrated ion stream flow into the
* Corresponding author. E-mail address: [email protected] (Y. Guo). http://dx.doi.org/10.1016/j.energy.2017.02.141 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
stripper. From Richout to the concentrated ion stream, the mass fraction of MEA was increased from 30% to 60% while the CO2 loading was kept unchanged, the regeneration energy can be reduced by 34.78%. However, Yan et al. [10] did not test these methods for separating Richout while directly assumed the mass fraction of MEA in the concentrated ion stream. Jande et al. [11,12] introduced a kind of flow-by capacitive ion separation (CIS) technology into the MEA-based process for separating Richout into the diluted and concentrated ion streams. Here, CIS varies the ion concentration of the effluent solvent by changing the direction of the electric field between the flow-by capacitor alternatively. Further, Jande et al. [11] simulated the MEA-based CO2 capture process integrated with CIS, and the simulation results showed that when the CO2 loading of the lean solvent flowing into the absorber stayed in the range of 0e0.0323, the energy saving could reach 10%e45%. We proposed a novel process for the NH3-based CO2 capture by integrating CIS [13], and developed rigorous electrolyte ion adsorption and desorption models, which were verified with the experimental results from the desalination process [13,14]. However, the regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by CIS device under different operating parameters of the rich solvent flowing into the CIS device was lacked. In this paper, the regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by CIS device was carried out for testing whether the reduction of the regeneration
M. Zhang, Y. Guo / Energy 125 (2017) 178e185
179
Nomenclature
Prc
C
Capacitance of the capacitor, F
C efa
Integral average of the effluent ion concentration in the adsorption process, mol/L
Pc RE0
C efd Cf
Integral average of the effluent ion concentration in the desorption process, mol/L Ion concentration of Richout, mol/L
Cp
Ion concentration of the diluted ion stream, mol/L
ta Ut V
Cr
Ion concentration of the concentrated ion stream, mol/ L Faraday’s constant, C/mol CO2 output of the NH3-based CO2 capture with CIS, kg/s Applied current of the capacitor, A Number of ion separation units Reboiler heat duty of the NH3-based CO2 capture without CIS, kW
Greek symbols Power loss coefficient of CIS Energy saving Charge efficiency of the capacitor 4t Flow rate of Richout, L/min 4p Flow rate of the diluted ion stream, L/min 4r Flow rate of the concentrated ion stream, L/min
F g I n Pr0
energy can be achieved. Effects of the operating parameters of the CIS device on the energy saving when integrating CIS under different operating parameters of Richout were obtained. The operating parameters of CIS include the applied current, capacitance and target voltage of the capacitor and the ion separation unit number. The operating parameters of Richout include its flow rate, NH3 concentration and CO2 loading. These results can provide technical guidance for choosing suitable CIS devices integrating into different NH3-based CO2 capture processes.
2. NH3-based CO2 capture process integrated with a flow-by CIS device Fig. 1 presents the schematic diagram of the NH3-based CO2 capture process integrated with a flow-by CIS device. The flue gas and the lean solvent (Leanin) flow into the absorber from the bottom and top, respectively. Leanin absorbs CO2 in the flue gas and then turns into the rich solvent (Richout) flowing out of the absorber. In this paper, the properties of flue gas after the desulfurization from a typical 500 MW coal-fired power plants [15] were used, the flow rate of the flue gas is chosen as 915 kg/h as shown in Table 1. Further, Richout flows into the flow-by CIS device, where
Fig. 1. Schematic diagram of the NH3-based CO2 capture process integrated with a flow-by CIS device.
REc
Reboiler heat duty of the NH3-based CO2 capture with CIS, kW Energy consumption of CIS, kW Regeneration energy of the NH3-based CO2 capture without CIS, MJ/kg CO2 Regeneration energy of the NH3-based CO2 capture with CIS, MJ/kg CO2 Longest time for the ion adsorption process, s Target voltage of the capacitor, V Total volume of the solvent in the channel, mL
b h l
Table 1 Properties of the flue gas from a typical coal-fired power plant. Flow rate, kg/h
915
Pressure, kPa
101.3
Temperature, C
50.0
Component volume fraction, vol% N2
CO2
H2O
O2
71.29
12.43
11.84
4.44
Richout is separated into a diluted ion stream and a concentrated ion stream. After the heat-exchanger and heater, the concentrated ion stream is sent to the stripper for CO2 regeneration. The gas product after the condenser is almost pure CO2. The lean solvent (Leanout) after the heat-exchanger and the diluted ion stream are mixed, cooled and further sent to the absorber for recycle. For the flow-by CIS device, it includes n same ion separation units. Richout flowing out of the absorber is separated into n streams equally and each stream flows into one ion separation unit accordingly. Each ion separation unit consists two flow-by capacitors #1 and #2, and it undergoes conditions (a) and (b) alternatively. For the condition (a), capacitor #1 stays at the electrolyte ion adsorption mode and produces a branched diluted ion stream, while capacitor #2 stays at the electrolyte ion desorption mode and produces a branched concentrated ion stream. Meanwhile, the gate of the capacitor #1 to the diluted ion stream is open but the gate of the capacitor #1 to the concentrated ion stream is closed; meanwhile, the gate of the capacitor #2 to the diluted ion stream is closed but the gate of the capacitor #2 to the concentrated ion stream is opened. On the contrary, for the condition (b), capacitor #1 stays at the electrolyte ion desorption mode and produces a branched diluted ion stream, while capacitor #2 stays at the electrolyte ion adsorption mode and produces a branched concentrated ion stream. The gate of the capacitor #1 to the diluted ion stream is closed but the gate of the capacitor #1 to the concentrated ion stream is opened; meanwhile, the gate of the capacitor #2 to the diluted ion stream is opened but the gate of the capacitor #2 to the concentrated ion stream is closed. By alternating the conditions (a) and (b), a steady branched diluted ion stream and a steady branched concentrated ion stream are produced from each ion separation unit. Further, the n same branched diluted ion streams and concentrated ion streams converge together and turn into the diluted stream and concentrated ion stream flowing out of the
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flow-by CIS device, respectively.
Table 2 Kinetic reaction model for the NH3eCO2eH2O system.
3. Mind map for regeneration energy analysis Based on the design of the NH3-based CO2 capture process integrated with a flow-by CIS device, the mind map for the regeneration energy analysis of this process was determined, as shown in Fig. 2. The properties of Richout flowing out of the absorber were predicted with the comprehensive model developed for the NH3based CO2 capture process in our previous work [16], which considered the hydrodynamics, non-ideal thermodynamic behaviors, rate-based heat/mass transfer, and finite reaction rates simultaneously. The kinetic reaction model for the NH3eCO2eH2O system shown in Table 2 is chosen for describing the chemical reactions in the NH3-based CO2 capture process, which considers that the reactions of CO2 with NH3 and the reactions of CO2 with OH are kinetically controlled. The calculation methods and related parameters for the equilibrium constants of the chemical reactions and reaction rates of the kinetic reactions can be found in Ref. [16]. The properties of the diluted ion stream and the concentrated ion stream were predicted with the electrolyte ion adsorption and desorption models for a type of flow-by capacitor [13], given as followings:
1 4p z4r z 4t ; 2
(1)
C p ¼ C efa zCf
C r ¼ C efd zCf þ
lI
zF4a
lI zF4d
12
V ; 4a t a
(2)
12
V ; 4d ta
(3)
where 4t and Cf are the flow rate and ion concentration of Richout flowing into the CIS device, respectively. 4p and C p are the flow rate and ion concentration of the diluted ion stream flowing out of the CIS device, respectively. 4r and C r are the flow rate and ion concentration of the concentrated ion stream flowing out of the CIS device, respectively. C efa and C efd are integral averages of the effluent ion concentration in the adsorption and desorption processes, respectively. l is the charge efficiency of the capacitor, I is the applied current of the capacitor, z is the average of the partial molar ionic valences of the feed solvent, and F is the Faraday’s
Reaction ID
Reaction type
1
Equilibrium
2 3
Equilibrium Equilibrium
4 5 6 7 8
Kinetic Kinetic Kinetic Kinetic Salt
Chemical equation NH3 þH2 O4NHþ 4 þOH
2H2 O4H3 Oþ þOH
2 þ HCO 3 þH2 O4CO3 þH3 O CO2 þOH /HCO 3 HCO 3 /CO2 þOH
NH3 þCO2 þH2 O/NH2 COO þH3 Oþ NH2 COO þH3 Oþ /NH3 þCO2 þH2 O NH4 HCO3 ðSÞ4NHþ 4 þHCO3
constant (96485 C/mol). V is the total volume of the solvent in the channel between the two electrodes of a single flow-by capacitor. The longest time for the ion adsorption process, ta ¼ CUt =I, where C and Ut are the capacitance and target voltage of the capacitor, respectively. Besides, since the ions in Richout flowing into the CIS device are mostly the monovalent ions, according to the CIS studies of Jande et al. [11], the mole fractions of ions before and after the CIS device are assumed as the same. The temperatures of the concentrated ion stream and the diluted ion stream are assumed to be same to that of Richout. Thus, the properties of the diluted ion stream and the concentrated ion stream can be given. Based on the comprehensive model for the NH3-based CO2 capture process developed in our previous work [16] and the predicted properties of the concentrated ion stream, the regeneration process of NH3based CO2 capture integrated with the flow-by CIS device can be simulated, and therefore its regeneration energy can be obtained. When performing regeneration energy analysis of the NH3based CO2 capture process integrated with a flow-by CIS device, for the convenience of the research, the properties of the rich solvent (Richout) flowing out of the absorber can be directly set. The base operating parameters of Richout and the flow-by CIS device are shown in Tables 3 and 4, respectively. The high applied current of the capacitor in Table 4 stays in the range of the industrial applications, taking the chrome plating power source in the metallurgical industry as example, the current for processing 1 m3 plating
Table 3 Base operating parameters of the rich solvent (Richout) flowing out of the absorber. Flow rate, L/min
NH3 concentration, mol/L
CO2 loading
Temperature, C
134
2.5
0.4
24.2
Diluted ion stream Gas product
Richout CIS
Concentrated ion stream Stripper
Predicted by the comprehensive model for NH3-based CO2 capture [16]
Predicted by the electrolyte ion adsorption and desorption models for a type of flow-by capacitor [13]
Leanout
Predicted by the comprehensive model for NH3-based CO2 capture [16]
Fig. 2. Mind map for the regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by CIS device.
M. Zhang, Y. Guo / Energy 125 (2017) 178e185 Table 4 Base operating parameters of the flow-by CIS device. Number of ion separation units Charge efficiency of the capacitor Applied current of the capacitor, A Capacitance of the capacitor, F Target voltage of the capacitor, V Total volume of the solvent between the capacitor, mL
2000 1.0 96 8000 1.2 4.0
solution is up to 3000 A [17]. The high capacitance of the capacitor can be ensured by adopting MnO2/nanoporous carbon composite electrodes [18]. When changing one operating parameter of Richout, other operating parameters of Richout were assumed unchanged. Meanwhile, when changing one operating parameter of the flow-by CIS device, other operating parameter of the CIS device were also assumed unchanged. In general, each tested operating parameter of the CIS device should include cases considering both higher and lower values than the corresponding value of the base case. The values of the tested applied current and capacitance of the capacitor obey this. The tested values of the number of the ion separation units are always lower than that of the base case, which is due to the consideration of reducing the complexity of the CIS device. The few ion separation units mean the simpler CIS device. The reason for that the tested values of the target voltage of the capacitor are always lower than that of the base case is to avoid the electrolysis of water. At the ambient temperature and pressure, the theoretical minimum voltage for the electrolysis is 1.23 V, and thus the target voltage can be set as 1.2 V or a little lower than 1.2 V. According to the typical NH3-based CO2 regeneration process in our previous work [16], the temperature of the rich solvent (Richin) flowing into the stripper was chosen as 80 C, the condenser temperature was chosen as 25 C, the reboiler heat duty was chosen as 160 kW. For the convenience of regeneration energy analysis, on the one hand, for the NH3-based CO2 capture process without integrated with CIS, when changing the operating parameters (flow rate, NH3 concentration, CO2 loading) of Richout, the reboiler heat duty was kept constant, which was always 160 kW. As the reboiler heat duty is kept constant for the NH3-based CO2 capture process without integrated with CIS, the CO2 output of these cases will change accordingly when changing the operating parameters of Richout. Thus, the different values of the regeneration energy can be obtained by changing the operating parameters of Richout. On the other hand, for each case of the NH3-based CO2 capture process integrated with CIS device, when changing the operating parameters of the CIS device, the CO2 output of these cases is kept as the same as that of the corresponding cases of the NH3-based CO2 capture process without integrated with CIS. For ensuring this, the reboiler heat duty needs to be adjusted accordingly when changing the operating parameters of CIS device. Then, the different values of the regeneration energy can be obtained by changing the operating parameters of CIS device. The energy saving for the NH3-based CO2 capture process when integrating a flow-by CIS device was defined, given as following:
h¼
RE0 REc 100%: RE0
(4)
In Eq. (4), h is the energy saving, RE0 is the regeneration energy of the NH3-based CO2 capture process without integrated a CIS device, and REc is the regeneration energy of the NH3-based CO2 capture process integrated with a CIS device, here, the regeneration energy is defined as the ratio of the reboiler heat duty to the CO2 output. When the energy saving h is larger than zero, it means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process would result in the decrease of the regeneration
181
energy. When the energy saving h is smaller than zero, it means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process would result in the increase of the regeneration energy. When the energy saving h is zero, it means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process has no effect on the regeneration energy. The reboiler heat duty of the NH3-based CO2 capture process without integrated a CIS device is termed as Pr0 , and the reboiler heat duty of the NH3-based CO2 capture process integrated with a CIS device is termed as Prc . The energy consumption of the flow-by CIS device is termed as Pc . It should be pointed out that the reboiler heat duty Pr0 was kept as 160 kW for the NH3-based CO2 capture process without integrated with CIS when changing the operating parameters of Richout. Whereas, when analyzing the NH3-based CO2 capture process integrated with CIS device, the CO2 output of the NH3-based CO2 capture process integrated with a CIS device is kept as the same as that of the corresponding cases of the NH3-based CO2 capture process without integrated a CIS device, which is termed as g. Therefore,
RE0 ¼
Pr0 ; g
(5)
REc ¼
Prc þ Pc : g
(6)
The energy consumption of the flow-by CIS device Pc is calculated with the following equation:
1 Pc ¼ b nIUt ; 2
(7)
where b is the power loss coefficient of the CIS device, and n is the number of ion separation units. The introduction of b is due to that the energy released in the electrolyte ion desorption process can be partly reused for charging in the electrolyte ion adsorption process. Dlugolecki and van der Wal [19] found that up to 83% of the energy released in the electrolyte ion desorption process can be recovered in the ion desorption process. Therefore, the value of b was chosen as 20% in this paper. 4. Results and discussion When investigating the effect of integration of the flow-by CIS device on the regeneration energy, it should be pointed out that the values of the CO2 output are different under different parameters of Richout (flow rate, NH3 concentration, CO2 loading). But, the CO2 output has the same value for the NH3-based CO2 capture process integrated with/without CIS device under the same corresponding parameter of Richout. 4.1. Effects of operating parameters of CIS device under different flow rates of Richout Fig. 3 presents the effects of the operating parameters of the CIS device, applied current of the capacitor, ion separation unit number, capacitance of the capacitor, and target voltage of the capacitor on the energy saving h under different flow rates of Richout. In Fig. 3a, for the cases that the flow rates of Richout are 120 L/min and 134 L/min, when the applied current of the capacitor is lower than 40 A, with the applied current of the capacitor increasing, the ion concentration of the concentrated ion stream flowing out of the CIS device increases, the required reboiler heat duty for keeping the CO2 output unchanged decreases significantly, the energy consumption of the CIS device increases little, and therefore the energy
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M. Zhang, Y. Guo / Energy 125 (2017) 178e185
(a)
(b)
30
20
Energy saving, %
Energy saving, %
20 10 0 Flow rate: 120 L/min Flow rate: 134 L/min Flow rate: 150 L/min
-10 -20
30
0
20
40
60
80
100
120
10 0
-20
140
Flow rate: 120 L/min Flow rate: 134 L/min Flow rate: 150 L/min
-10
0
Applied current of the capacitor, A
26
28
22 20
Flow rate: 120 L/min Flow rate: 134 L/min Flow rate: 150 L/min
18 2000 4000 6000 8000 10000 12000 14000 16000
Capacitance of the capacitor, F
Energy saving, %
(d) 30
Energy saving, %
(c) 28
24
500
1000
1500
2000
2500
Number of the ion separation units
26 24 22 20 0.4
Flow rate: 120 L/min Flow rate: 134 L/min Flow rate: 150 L/min 0.6
0.8
1.0
1.2
1.4
Target voltage of the capacitor, V
Fig. 3. Effects of the operating parameters of the CIS device: (a) applied current of the capacitor, (b) number of the ion separation units, (c) capacitance of the capacitor, and (d) target voltage of the capacitor on the energy saving under different flow rates of Richout.
saving when integrating the CIS device increases significantly. Whereas, when the applied current of the capacitor is higher than 40 A, with the applied current of the capacitor increasing, the decrease of the reboiler heat duty is almost the same to the increase of the energy consumption of the CIS device. Thus, the energy saving changes little and stays in the range of 20%e30%. Here, the energy savings stay in the range of that obtained by Jande et al. [11] when performing regeneration energy analysis of the MEA-based CO2 capture process integrated with CIS. Particularly, for the case that the flow rate of Richout is 120 L/ min, when the applied currents of the capacitor are 30 A and 32 A, the energy savings are smaller than zero for these cases, this indicates that the introduction of the flow-by CIS device into the NH3based CO2 capture process does not result in the decrease of the regeneration energy, but results in the increase of the regeneration energy. This is due to that the increase of the ion concentration of the concentrated ion stream flowing out of the CIS device is weak when the applied current of the capacitor is small under the condition of 120 L/min for the flow rate of Richout, but the decrease of the CO2 amount of the concentrated ion stream is large. This gives rise to that the required reboiler heat duty increases significantly for keeping the CO2 output unchanged when integrated with CIS device under the same flow rate of 120 L/min for Richout, further considering the increase of the energy consumption of the CIS device, thus, the energy saving when integrating the CIS device is smaller than zero for this case. In our previous work [13], this phenomenon, that the introduction of the CIS device does not result in the reduction of the regeneration energy, while gives rise to the increase of the regeneration energy, was not observed. But,
this phenomenon was observed in the work of Jande et al. [11] when studying the MEA-based CO2 capture process integrated with CIS. For the case that the flow rate of Richout is 150 L/min, with the applied current of the capacitor increasing, the difference between the decrease of the reboiler heat duty and the increase of the energy consumption of the CIS device is small, and therefore the energy saving when integrating the CIS device changes little and stays in the range of 20%e30%. By comparing Fig. 3a and b, it can be found that the effects of the applied current of the capacitor on the energy saving are similar to those of the ion separation unit number for the cases that the flow rates of Richout are 120 L/min, 134 L/min and 150 L/min, respectively. The reason for this is that the effects of the applied current of the capacitor and number of ion separation units on the ion concentration of the concentrated ion stream, and further on the reboiler heat duty are almost same [13], meanwhile those effects on the energy consumption of the CIS device are same, as shown in Eq. (7). In Fig. 3c, for the cases that the flow rates of Richout are 120 L/min, 134 L/min and 150 L/min, with the capacitance of the capacitor increasing, the ion concentration of the concentrated ion stream increases, thus the required reboiler heat duty decreases in order to keep the CO2 output unchanged when integrated with CIS device under the corresponding flow rates of Richout. From Eq. (7), it can be seen that the capacitance of the capacitor does not affect the energy consumption of the CIS device. Therefore, as the capacitance of the capacitor increases, the energy saving increases gradually. In Fig. 3d, for the cases that the flow rates of Richout are 120 L/min, 134 L/min and 150 L/min, with the target voltage of the capacitor increasing, the ion concentration of
M. Zhang, Y. Guo / Energy 125 (2017) 178e185
the concentrated ion stream increases, thus, in order to keep the CO2 output unchanged when integrated with CIS device under the corresponding flow rates of Richout, the required reboiler heat duty decreases. With the target voltage of the capacitor increasing, since the decrease of the reboiler heat duty is weaker than the increase of the energy consumption of the CIS device, the energy saving decreases gradually. In general, for the cases that the flow rates of Richout are 120 L/ min, 134 L/min and 150 L/min, when choosing the suitable operating parameters of the flow-by CIS device, the introduction of the CIS device into the NH3-based CO2 capture process can result in a significant reduction of the regeneration energy, the energy saving when integrating the CIS device can reach above 20%. But, for the case that the flow rate of Richout is 120 L/min, when the applied current of the capacitor or the ion separation unit number is small, the introduction of the CIS device into the NH3-based CO2 capture process does not result in the reduction of the regeneration energy, while results in the increase of the regeneration energy. This is due to that the CO2 amount of the concentrated ion stream flowing out of the CIS device is quite low, in order to keep the CO2 output unchanged when integrated with CIS device under this flow rate of Richout, the required reboiler heat duty increases.
4.2. Effects of operating parameters of CIS device under different NH3 concentrations of Richout Fig. 4 presents the effects of the operating parameters of the CIS device, applied current of the capacitor, ion separation unit number, capacitance of the capacitor, and target voltage of the capacitor
(a)
on the energy saving h under different NH3 concentrations of Richout. In Fig. 4a, for the cases that the NH3 concentrations of Richout are 2.5 mol/L and 3.0 mol/L, when the applied current of the capacitor is lower than 40 A, with the applied current of the capacitor increasing, the energy saving increases significantly when integrating the CIS device. This is due to that the increase of the ion concentration of the concentrated ion stream is strong, and the decrease of the reboiler heat duty is quite evident for these cases. However, when the applied current of the capacitor is higher than 40 A, the applied current of the capacitor has little effects on the energy saving when integrating the CIS device. Particularly, for the case that the NH3 concentration of Richout is 3.0 mol/L, when the applied current of the capacitor is 17 A, the energy saving is smaller than zero, which means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process leads to the increase of the regeneration energy. This is due to that the CO2 output of the NH3-based CO2 capture process without integrated the CIS device is quite high for this case. When integrating the CIS device, as the applied current of the capacitor is small, the ion concentration of the concentrated ion stream flowing out of the CIS device is low, in order to keep the CO2 output unchanged, the required reboiler heat duty should be increased, thus, the energy saving is smaller than zero. For the case that the NH3 concentration of Richout is 2.0 mol/ L, with the applied current of the capacitor increasing, the energy saving increases gradually when integrating the CIS device. By comparing Fig. 4a and b, it can be found that the effects of the applied current of the capacitor on the energy saving are similar to those of the ion separation unit number for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L,
(b)
40
183
30
20
Energy saving, %
Energy saving, %
30 20 10 0
NH3 conc.: 2.0 mol/L NH3 conc.: 2.5 mol/L
-10 -20
10 NH3 conc.: 2.0 mol/L
0
NH3 conc.: 2.5 mol/L
NH3 conc.: 3.0 mol/L 0
20
40
60
80
100
120
NH3 conc.: 3.0 mol/L -10
140
0
Applied current of the capacitor, A
(c) 30
500
1000
1500
2000
2500
Number of the ion separation units
(d) 40 35
20
15
NH3 conc.: 2.0 mol/L NH3 conc.: 2.5 mol/L NH3 conc.: 3.0 mol/L
10 2000 4000 6000 8000 10000 12000 14000 16000
Capacitance of the capacitor, F
Energy saving, %
Energy saving, %
25
30 25 20 15 10 5 0.4
NH3 conc.: 2.0 mol/L NH3 conc.: 2.5 mol/L NH3 conc.: 3.0 mol/L 0.6
0.8
1.0
1.2
1.4
Target voltage of the capacitor, V
Fig. 4. Effects of the operating parameters of the CIS device: (a) applied current of the capacitor, (b) number of the ion separation units, (c) capacitance of the capacitor, and (d) target voltage of the capacitor on the energy saving under different NH3 concentrations of Richout.
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M. Zhang, Y. Guo / Energy 125 (2017) 178e185
respectively. Similarly, for the case that the NH3 concentration of Richout is 3.0 mol/L, when the ion separation unit is 400, the increase of the ion concentration of the concentrated ion stream is weak, the energy saving is smaller than zero, thus, the introduction of the CIS device results in the increase of the regeneration energy. In Fig. 4c, for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L, with the capacitance of the capacitor increasing, the energy saving increases accordingly. This is due to that the capacitance of the capacitor does not affect the energy consumption of the CIS device, but the reboiler heat duty reduces with the capacitance of the capacitor increasing. In Fig. 4d, for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L, as the target voltage of the capacitor increases, the energy saving decreases gradually. This is due to that the increase of the energy consumption of the CIS device is stronger than the decrease of the reboiler heat duty for these cases. Besides, under low NH3 concentration of Richout, for the NH3-based CO2 capture process integrated with the flow-by CIS device, choosing the suitable applied current or target voltage of the CIS device can result in more significant reduction of the regeneration energy. In general, for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L, the introduction of the CIS device into the NH3-based CO2 capture process can result in a significant reduction of the regeneration energy, and the energy saving can reach above 20% when choosing the suitable operating parameters of the flow-by CIS device. Comparing to the cases that the NH3 concentrations of Richout are 2.5 mol/L and 3.0 mol/L, for the case that the NH3 concentration of Richout is 2.0 mol/L, choosing the appropriate operating parameters of the CIS device can result in more significant reduction of the regeneration energy, the energy saving can reach above 35%. Besides, for the case that the NH3 concentration of Richout is 3.0 mol/L, when the applied current of the capacitor or the ion separation unit number is quite small, the introduction of the CIS device will lead to the increase of the regeneration energy of the NH3-based CO2 capture process. 4.3. Effects of operating parameters of CIS device under different CO2 loadings of Richout Fig. 5 presents the effects of the operating parameters of the CIS device, applied current of the capacitor, ion separation unit number, capacitance of the capacitor, and target voltage of the capacitor on the energy saving h under different CO2 loadings of Richout. Here, the values of CO2 loadings of Richout were chosen according to the experimental values [3,4]. But in the work of Jande et al. [11], the values of CO2 loadings considered are obviously lower than the experimental values [2,9]. In Fig. 5a, for the cases that the CO2 loading of Richout are 0.3 and 0.4, with the applied current of the capacitor increasing, the energy saving increases significantly firstly when integrating the CIS device, and then changes a little. For the case that the CO2 loading of Richout is 0.5, with the applied current of the capacitor increasing, the energy saving increase significantly firstly, and then increase slowly when the applied current of the capacitor is higher than 60 A. Particularly, for the case that the CO2 loading of Richout is 0.3, when the applied currents of the capacitor are 35 A and 37 A, the energy savings are smaller than zero when integrating the CIS device. This is due to that the CO2 amount of the concentrated ion stream flowing out of the CIS device is small when the CO2 loading of Richout and the applied current of the capacitor are both small. Thus, this gives rise to that the required reboiler heat duty increases significantly for keeping the CO2 output unchanged when integrated with CIS device under the same CO2 loading of 0.3 for Richout, and therefore the energy saving is smaller than zero. For the case that the CO2 loading of Richout is 0.5, when the applied current of the capacitor is 20 A, the
energy saving is also smaller than zero when integrating the CIS device. This is due to that when the CO2 loading of Richout is high, the CO2 output of the NH3-based CO2 capture process without integrated the CIS device is large. When integrating the CIS device, as applied current of the capacitor is small, the required reboiler heat duty for keeping the CO2 output constant will increase, which further results in that the energy saving is smaller than zero. By comparing Fig. 5a and b, it can be found that the effects of the applied current of the capacitor on the energy saving are similar to those of the ion separation unit number for the cases that the CO2 loadings of Richout are 0.3, 0.4 and 0.5, respectively. Similarly, for the case that the CO2 loading of Richout is 0.3, when the ion separation unit numbers are 750 and 770, the energy saving is smaller than zero. For the case that the CO2 loading of Richout is 0.5, when the ion separation unit number is 400, the energy saving is also smaller than zero. In Fig. 5c, for the cases that the CO2 loadings of Richout are 0.3, 0.4 and 0.5, with the capacitance of the capacitor increasing, the energy saving increases gradually. In Fig. 5d, for the cases that the CO2 loadings of Richout are 0.3 and 0.4, as the target voltage of the capacitor increases, the energy saving decreases gradually, this is due to the weak increase of the ion concentration of the concentrated ion stream. For the case that the CO2 loading of Richout is 0.5, with the target voltage of the capacitor increasing, the energy saving increases a little firstly, and decreases gradually. In general, for the cases that the CO2 loadings of Richout are 0.3, 0.4 and 0.5, when choosing the suitable operating parameters of the flow-by CIS device, the introduction of the CIS device into the NH3-based CO2 capture process can lead to the significant reduction of the regeneration energy, and the energy saving can reach above 20% when integrating the CIS device. Whereas, for cases that the CO2 loadings of Richout are 0.3 and 0.5, when the applied current of the capacitor or the ion separation unit number is small, the introduction of the CIS device results in the increase of the regeneration energy of the NH3-based CO2 capture process.
5. Conclusions In this paper, regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by capacitive ion separation (CIS) device under different flow rates, NH3 concentrations and CO2 loadings of the rich solvent (Richout) flowing into the CIS device was carried out. Effects of the operating parameters of the CIS device on the energy saving were obtained. The following conclusions can be drawn: (1) For the NH3-based CO2 capture process integrated with the flow-by CIS device, when choosing the suitable operating parameters of the CIS device, the reduction of the flow rate of the concentrated ion stream flowing out of the CIS device can result in a significant reduction of the regeneration energy, and the energy saving can reach above 20%. (2) Particularly, for the case that the NH3 concentration of Richout is 2.0 mol/L, when choosing the suitable operating parameters of the CIS device, the energy saving can be up to 35%. (3) However, for the cases that the flow rate of Richout is 120 L/ min, the NH3 concentration of Richout is 3.0 mol/L, or the CO2 loadings of Richout are 0.3 and 0.5, when the applied current of the capacitor or the ion separation unit number is small, the introduction of the CIS device into the NH3-based CO2 capture process results in the increase of the regeneration energy, which is due to the low CO2 amount of the concentrated ion stream flowing out of the CIS device.
M. Zhang, Y. Guo / Energy 125 (2017) 178e185
(a)
(b)
40
20 10 0
CO2 loading: 0.3 CO2 loading: 0.4
-10 -20
40 30
Energy saving, %
Energy saving, %
30
185
CO2 loading: 0.5 0
20
40
60
80
100
120
20 10 0
CO2 loading: 0.4
-10 -20
140
CO2 loading: 0.3 CO2 loading: 0.5 0
Applied current of the capacitor, A
500
1000
1500
2000
2500
Number of the ion separation units
(c) 35
(d) 40 CO2 loading: 0.3
25
20
CO2 loading: 0.4
35
Energy saving, %
Energy saving, %
30
CO2 loading: 0.3 CO2 loading: 0.4
CO2 loading: 0.5
30
25
CO2 loading: 0.5 15 2000 4000 6000 8000 10000 12000 14000 16000
20 0.4
Capacitance of the capacitor, F
0.6
0.8
1.0
1.2
1.4
Target voltage of the capacitor, V
Fig. 5. Effects of the operating parameters of the CIS device: (a) applied current of the capacitor, (b) number of the ion separation units, (c) capacitance of the capacitor, and (d) target voltage of the capacitor on the energy saving under different CO2 loadings of Richout.
Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 51390493, the Wang Yongzhi Academician Research Innovation Foundation of Tsinghua University, and the Beijing Municipal Commission for Science & Technology under Grant No. Z08040902950803. References [1] Olajire AA. CO2 capture and separation technologies for end-of-pipe applications-a review. Energy 2010;35:2610e28. [2] Pellegrini G, Strube R, Manfrida G. Comparative study of chemical absorbents in postcombustion CO2 capture. Energy 2010;35:851e7. [3] Bai H, Yeh AC. Removal of CO2 greenhouse gas by ammonia scrubbing. Ind Eng Chem Res 1997;36:2490e3. [4] Yu H, Morgan S, Allport A, Cottrell A, Do T, McGregor J, et al. Results from trialling aqueous NH3 based post-combustion capture in a pilot plant at Munmorah power station: absorption. Chem Eng Res Des 2011;89:1204e15. [5] Zhao BT, Su YX, Tao WW, Li LL, Peng YC. Post-combustion CO2 capture by aqueous ammonia: a state-of-the-art review. Int J Greenh Gas Control 2012;9: 355e71. [6] Dong RF, Lu HF, Yu YS, Zhang ZX. A feasible process for simultaneous removal of CO2, SO2 and NOx in the cement industry by NH3 scrubbing. Appl Energy 2012;97:185e91. [7] Yu JW, Wang SJ, Yu H, Wardhaugh L, Feron P. Rate-based modelling of CO2 regeneration in ammonia based CO2 capture process. Int J Greenh Gas Control 2014;28:203e15. [8] Yu H, Li LC, Morgan S, Allport A, Cottrell A, McGregor J, et al. Results from trialling aqueous NH3 based post combustion capture in a pilot plant at
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Munmorah power station: solvent regeneration energy. Chemeca 2012, Wellington, Australia. 2012. p. 1097e107. Strube R, Pellegrini G, Manfrida G. The environmental impact of postcombustion CO2 capture with MEA, with aqueous ammonia, and with an aqueous ammonia-ethanol mixture for a coal-fired power plant. Energy 2011;36:3763e70. Yan SP, He QY, Ai P, Wang YY, Zhang YL. Regeneration performance of concentrated CO2-rich alkanolamine solvents: the first step study of a novel concept for reducing regeneration heat consumption by using concentration swing absorption technology. Chem Eng Process 2013;70:86e94. Jande YAC, Asif M, Shim SM, Kim WS. Energy minimization in monoethanolamine-based CO2 capture using capacitive deionization. Int J Energy Res 2014;38:1531e40. Jande YAC, Kim WS. Desalination using capacitive deionization at constant current. Desalin 2013;329:29e34. Zhang MK, Guo YC. A novel process for NH3-based CO2 capture by integrating flow-by capacitive ion separation. Int J Greenh Gas Control 2016;54:50e8. Zhao R, Biesheuvel PM, van der Wal A. Energy consumption and constant current operation in membrane capacitive deionization. Energy Environ Sci 2012;5:9520e7. Zeng XZ, Chen CH, Gao BC. Technical progress in recovering carbon dioxide from flue gas. Environ Prot Chem Ind 2000;20:12e7. Zhang MK, Guo YC. A comprehensive model for regeneration process of CO2 capture using aqueous ammonia solution. Int J Greenh Gas Control 2014;25: 22e34. Zhao JB. On low voltage high current parallel high frequency DC switching power module. Master’s thesis. China: Xi’an Shiyou University; 2011. Yang J, Zou LD, Song HH, Hao ZP. Development of novel MnO2/nanoporous carbon composite electrodes in capacitive deionization technology. Desalin 2011;276:199e206. Dlugolecki P, van der Wal A. Energy recovery in membrane capacitive deionization. Environ Sci Technol 2013;47:4904e10.
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Regeneration energy analysis of NH3-based CO2 capture process integrated with a flow-by capacitive ion separation device Minkai Zhang, Yincheng Guo* Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2016 Received in revised form 22 February 2017 Accepted 24 February 2017 Available online 27 February 2017
The flow-by capacitive ion separation (CIS) device was introduced into the NH3-based CO2 capture process for reducing the regeneration energy. Regeneration energy analysis of the NH3-based CO2 capture process integrated with the CIS device under different flow rates, NH3 concentrations and CO2 loadings of the rich solvent (Richout) flowing into the CIS device was performed. The flow rates, NH3 concentrations and CO2 loadings of Richout considered in this paper are 120e150 L/min, 2.0e3.0 mol/L, and 0.3e0.5, respectively. When choosing suitable operating parameters of the CIS device, the flow rate of the concentrated ion stream flowing out of the CIS device decreases. Therefore, the introduction of the CIS device into the NH3-based CO2 capture process can lead to a significant reduction of the regeneration energy, and the regeneration energy can be reduced by above 20%. Particularly, for the case that the NH3 concentration of Richout is 2.0 mol/L, the regeneration energy can be reduced by up to 35%. © 2017 Elsevier Ltd. All rights reserved.
Keywords: CO2 capture Aqueous ammonia Rich solvent Capacitive ion separation Regeneration energy
1. Introduction For reducing the CO2 emissions from coal-fired power plants, the NH3-based post-combustion CO2 capture process is considered as a feasible technical route [1,2], which is due to its advantages of high CO2 loading capacity, no absorbent degradation, no equipment corrosion, and simultaneous removal of CO2, SO2 and NOx [3e6]. But, the industrial application of the NH3-based CO2 capture process is limited to its high regeneration energy [7,8]. In the NH3based CO2 capture process of the Munmorah pilot plant [8], the experimental results showed that more than 50% of the reboiler heat duty was used for heating the rich solvent (Richin) flowing into the stripper, and thus reducing the flow rate of Richin can provide a solution for reducing the regeneration energy of the NH3based CO2 capture process. Since the NH3- and MEA-based CO2 capture processes are two typical chemical absorption methods for CO2 capture [9], the ideas for reducing the flow rate of Richin used in the MEA-based process can be referred for the NH3-based process. Yan et al. [10] suggested that for the MEA-based process, the rich solvent (Richout) flowing out of the absorber is separated into diluted and concentrated ion streams, and then just let the concentrated ion stream flow into the
* Corresponding author. E-mail address: [email protected] (Y. Guo). http://dx.doi.org/10.1016/j.energy.2017.02.141 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
stripper. From Richout to the concentrated ion stream, the mass fraction of MEA was increased from 30% to 60% while the CO2 loading was kept unchanged, the regeneration energy can be reduced by 34.78%. However, Yan et al. [10] did not test these methods for separating Richout while directly assumed the mass fraction of MEA in the concentrated ion stream. Jande et al. [11,12] introduced a kind of flow-by capacitive ion separation (CIS) technology into the MEA-based process for separating Richout into the diluted and concentrated ion streams. Here, CIS varies the ion concentration of the effluent solvent by changing the direction of the electric field between the flow-by capacitor alternatively. Further, Jande et al. [11] simulated the MEA-based CO2 capture process integrated with CIS, and the simulation results showed that when the CO2 loading of the lean solvent flowing into the absorber stayed in the range of 0e0.0323, the energy saving could reach 10%e45%. We proposed a novel process for the NH3-based CO2 capture by integrating CIS [13], and developed rigorous electrolyte ion adsorption and desorption models, which were verified with the experimental results from the desalination process [13,14]. However, the regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by CIS device under different operating parameters of the rich solvent flowing into the CIS device was lacked. In this paper, the regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by CIS device was carried out for testing whether the reduction of the regeneration
M. Zhang, Y. Guo / Energy 125 (2017) 178e185
179
Nomenclature
Prc
C
Capacitance of the capacitor, F
C efa
Integral average of the effluent ion concentration in the adsorption process, mol/L
Pc RE0
C efd Cf
Integral average of the effluent ion concentration in the desorption process, mol/L Ion concentration of Richout, mol/L
Cp
Ion concentration of the diluted ion stream, mol/L
ta Ut V
Cr
Ion concentration of the concentrated ion stream, mol/ L Faraday’s constant, C/mol CO2 output of the NH3-based CO2 capture with CIS, kg/s Applied current of the capacitor, A Number of ion separation units Reboiler heat duty of the NH3-based CO2 capture without CIS, kW
Greek symbols Power loss coefficient of CIS Energy saving Charge efficiency of the capacitor 4t Flow rate of Richout, L/min 4p Flow rate of the diluted ion stream, L/min 4r Flow rate of the concentrated ion stream, L/min
F g I n Pr0
energy can be achieved. Effects of the operating parameters of the CIS device on the energy saving when integrating CIS under different operating parameters of Richout were obtained. The operating parameters of CIS include the applied current, capacitance and target voltage of the capacitor and the ion separation unit number. The operating parameters of Richout include its flow rate, NH3 concentration and CO2 loading. These results can provide technical guidance for choosing suitable CIS devices integrating into different NH3-based CO2 capture processes.
2. NH3-based CO2 capture process integrated with a flow-by CIS device Fig. 1 presents the schematic diagram of the NH3-based CO2 capture process integrated with a flow-by CIS device. The flue gas and the lean solvent (Leanin) flow into the absorber from the bottom and top, respectively. Leanin absorbs CO2 in the flue gas and then turns into the rich solvent (Richout) flowing out of the absorber. In this paper, the properties of flue gas after the desulfurization from a typical 500 MW coal-fired power plants [15] were used, the flow rate of the flue gas is chosen as 915 kg/h as shown in Table 1. Further, Richout flows into the flow-by CIS device, where
Fig. 1. Schematic diagram of the NH3-based CO2 capture process integrated with a flow-by CIS device.
REc
Reboiler heat duty of the NH3-based CO2 capture with CIS, kW Energy consumption of CIS, kW Regeneration energy of the NH3-based CO2 capture without CIS, MJ/kg CO2 Regeneration energy of the NH3-based CO2 capture with CIS, MJ/kg CO2 Longest time for the ion adsorption process, s Target voltage of the capacitor, V Total volume of the solvent in the channel, mL
b h l
Table 1 Properties of the flue gas from a typical coal-fired power plant. Flow rate, kg/h
915
Pressure, kPa
101.3
Temperature, C
50.0
Component volume fraction, vol% N2
CO2
H2O
O2
71.29
12.43
11.84
4.44
Richout is separated into a diluted ion stream and a concentrated ion stream. After the heat-exchanger and heater, the concentrated ion stream is sent to the stripper for CO2 regeneration. The gas product after the condenser is almost pure CO2. The lean solvent (Leanout) after the heat-exchanger and the diluted ion stream are mixed, cooled and further sent to the absorber for recycle. For the flow-by CIS device, it includes n same ion separation units. Richout flowing out of the absorber is separated into n streams equally and each stream flows into one ion separation unit accordingly. Each ion separation unit consists two flow-by capacitors #1 and #2, and it undergoes conditions (a) and (b) alternatively. For the condition (a), capacitor #1 stays at the electrolyte ion adsorption mode and produces a branched diluted ion stream, while capacitor #2 stays at the electrolyte ion desorption mode and produces a branched concentrated ion stream. Meanwhile, the gate of the capacitor #1 to the diluted ion stream is open but the gate of the capacitor #1 to the concentrated ion stream is closed; meanwhile, the gate of the capacitor #2 to the diluted ion stream is closed but the gate of the capacitor #2 to the concentrated ion stream is opened. On the contrary, for the condition (b), capacitor #1 stays at the electrolyte ion desorption mode and produces a branched diluted ion stream, while capacitor #2 stays at the electrolyte ion adsorption mode and produces a branched concentrated ion stream. The gate of the capacitor #1 to the diluted ion stream is closed but the gate of the capacitor #1 to the concentrated ion stream is opened; meanwhile, the gate of the capacitor #2 to the diluted ion stream is opened but the gate of the capacitor #2 to the concentrated ion stream is closed. By alternating the conditions (a) and (b), a steady branched diluted ion stream and a steady branched concentrated ion stream are produced from each ion separation unit. Further, the n same branched diluted ion streams and concentrated ion streams converge together and turn into the diluted stream and concentrated ion stream flowing out of the
180
M. Zhang, Y. Guo / Energy 125 (2017) 178e185
flow-by CIS device, respectively.
Table 2 Kinetic reaction model for the NH3eCO2eH2O system.
3. Mind map for regeneration energy analysis Based on the design of the NH3-based CO2 capture process integrated with a flow-by CIS device, the mind map for the regeneration energy analysis of this process was determined, as shown in Fig. 2. The properties of Richout flowing out of the absorber were predicted with the comprehensive model developed for the NH3based CO2 capture process in our previous work [16], which considered the hydrodynamics, non-ideal thermodynamic behaviors, rate-based heat/mass transfer, and finite reaction rates simultaneously. The kinetic reaction model for the NH3eCO2eH2O system shown in Table 2 is chosen for describing the chemical reactions in the NH3-based CO2 capture process, which considers that the reactions of CO2 with NH3 and the reactions of CO2 with OH are kinetically controlled. The calculation methods and related parameters for the equilibrium constants of the chemical reactions and reaction rates of the kinetic reactions can be found in Ref. [16]. The properties of the diluted ion stream and the concentrated ion stream were predicted with the electrolyte ion adsorption and desorption models for a type of flow-by capacitor [13], given as followings:
1 4p z4r z 4t ; 2
(1)
C p ¼ C efa zCf
C r ¼ C efd zCf þ
lI
zF4a
lI zF4d
12
V ; 4a t a
(2)
12
V ; 4d ta
(3)
where 4t and Cf are the flow rate and ion concentration of Richout flowing into the CIS device, respectively. 4p and C p are the flow rate and ion concentration of the diluted ion stream flowing out of the CIS device, respectively. 4r and C r are the flow rate and ion concentration of the concentrated ion stream flowing out of the CIS device, respectively. C efa and C efd are integral averages of the effluent ion concentration in the adsorption and desorption processes, respectively. l is the charge efficiency of the capacitor, I is the applied current of the capacitor, z is the average of the partial molar ionic valences of the feed solvent, and F is the Faraday’s
Reaction ID
Reaction type
1
Equilibrium
2 3
Equilibrium Equilibrium
4 5 6 7 8
Kinetic Kinetic Kinetic Kinetic Salt
Chemical equation NH3 þH2 O4NHþ 4 þOH
2H2 O4H3 Oþ þOH
2 þ HCO 3 þH2 O4CO3 þH3 O CO2 þOH /HCO 3 HCO 3 /CO2 þOH
NH3 þCO2 þH2 O/NH2 COO þH3 Oþ NH2 COO þH3 Oþ /NH3 þCO2 þH2 O NH4 HCO3 ðSÞ4NHþ 4 þHCO3
constant (96485 C/mol). V is the total volume of the solvent in the channel between the two electrodes of a single flow-by capacitor. The longest time for the ion adsorption process, ta ¼ CUt =I, where C and Ut are the capacitance and target voltage of the capacitor, respectively. Besides, since the ions in Richout flowing into the CIS device are mostly the monovalent ions, according to the CIS studies of Jande et al. [11], the mole fractions of ions before and after the CIS device are assumed as the same. The temperatures of the concentrated ion stream and the diluted ion stream are assumed to be same to that of Richout. Thus, the properties of the diluted ion stream and the concentrated ion stream can be given. Based on the comprehensive model for the NH3-based CO2 capture process developed in our previous work [16] and the predicted properties of the concentrated ion stream, the regeneration process of NH3based CO2 capture integrated with the flow-by CIS device can be simulated, and therefore its regeneration energy can be obtained. When performing regeneration energy analysis of the NH3based CO2 capture process integrated with a flow-by CIS device, for the convenience of the research, the properties of the rich solvent (Richout) flowing out of the absorber can be directly set. The base operating parameters of Richout and the flow-by CIS device are shown in Tables 3 and 4, respectively. The high applied current of the capacitor in Table 4 stays in the range of the industrial applications, taking the chrome plating power source in the metallurgical industry as example, the current for processing 1 m3 plating
Table 3 Base operating parameters of the rich solvent (Richout) flowing out of the absorber. Flow rate, L/min
NH3 concentration, mol/L
CO2 loading
Temperature, C
134
2.5
0.4
24.2
Diluted ion stream Gas product
Richout CIS
Concentrated ion stream Stripper
Predicted by the comprehensive model for NH3-based CO2 capture [16]
Predicted by the electrolyte ion adsorption and desorption models for a type of flow-by capacitor [13]
Leanout
Predicted by the comprehensive model for NH3-based CO2 capture [16]
Fig. 2. Mind map for the regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by CIS device.
M. Zhang, Y. Guo / Energy 125 (2017) 178e185 Table 4 Base operating parameters of the flow-by CIS device. Number of ion separation units Charge efficiency of the capacitor Applied current of the capacitor, A Capacitance of the capacitor, F Target voltage of the capacitor, V Total volume of the solvent between the capacitor, mL
2000 1.0 96 8000 1.2 4.0
solution is up to 3000 A [17]. The high capacitance of the capacitor can be ensured by adopting MnO2/nanoporous carbon composite electrodes [18]. When changing one operating parameter of Richout, other operating parameters of Richout were assumed unchanged. Meanwhile, when changing one operating parameter of the flow-by CIS device, other operating parameter of the CIS device were also assumed unchanged. In general, each tested operating parameter of the CIS device should include cases considering both higher and lower values than the corresponding value of the base case. The values of the tested applied current and capacitance of the capacitor obey this. The tested values of the number of the ion separation units are always lower than that of the base case, which is due to the consideration of reducing the complexity of the CIS device. The few ion separation units mean the simpler CIS device. The reason for that the tested values of the target voltage of the capacitor are always lower than that of the base case is to avoid the electrolysis of water. At the ambient temperature and pressure, the theoretical minimum voltage for the electrolysis is 1.23 V, and thus the target voltage can be set as 1.2 V or a little lower than 1.2 V. According to the typical NH3-based CO2 regeneration process in our previous work [16], the temperature of the rich solvent (Richin) flowing into the stripper was chosen as 80 C, the condenser temperature was chosen as 25 C, the reboiler heat duty was chosen as 160 kW. For the convenience of regeneration energy analysis, on the one hand, for the NH3-based CO2 capture process without integrated with CIS, when changing the operating parameters (flow rate, NH3 concentration, CO2 loading) of Richout, the reboiler heat duty was kept constant, which was always 160 kW. As the reboiler heat duty is kept constant for the NH3-based CO2 capture process without integrated with CIS, the CO2 output of these cases will change accordingly when changing the operating parameters of Richout. Thus, the different values of the regeneration energy can be obtained by changing the operating parameters of Richout. On the other hand, for each case of the NH3-based CO2 capture process integrated with CIS device, when changing the operating parameters of the CIS device, the CO2 output of these cases is kept as the same as that of the corresponding cases of the NH3-based CO2 capture process without integrated with CIS. For ensuring this, the reboiler heat duty needs to be adjusted accordingly when changing the operating parameters of CIS device. Then, the different values of the regeneration energy can be obtained by changing the operating parameters of CIS device. The energy saving for the NH3-based CO2 capture process when integrating a flow-by CIS device was defined, given as following:
h¼
RE0 REc 100%: RE0
(4)
In Eq. (4), h is the energy saving, RE0 is the regeneration energy of the NH3-based CO2 capture process without integrated a CIS device, and REc is the regeneration energy of the NH3-based CO2 capture process integrated with a CIS device, here, the regeneration energy is defined as the ratio of the reboiler heat duty to the CO2 output. When the energy saving h is larger than zero, it means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process would result in the decrease of the regeneration
181
energy. When the energy saving h is smaller than zero, it means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process would result in the increase of the regeneration energy. When the energy saving h is zero, it means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process has no effect on the regeneration energy. The reboiler heat duty of the NH3-based CO2 capture process without integrated a CIS device is termed as Pr0 , and the reboiler heat duty of the NH3-based CO2 capture process integrated with a CIS device is termed as Prc . The energy consumption of the flow-by CIS device is termed as Pc . It should be pointed out that the reboiler heat duty Pr0 was kept as 160 kW for the NH3-based CO2 capture process without integrated with CIS when changing the operating parameters of Richout. Whereas, when analyzing the NH3-based CO2 capture process integrated with CIS device, the CO2 output of the NH3-based CO2 capture process integrated with a CIS device is kept as the same as that of the corresponding cases of the NH3-based CO2 capture process without integrated a CIS device, which is termed as g. Therefore,
RE0 ¼
Pr0 ; g
(5)
REc ¼
Prc þ Pc : g
(6)
The energy consumption of the flow-by CIS device Pc is calculated with the following equation:
1 Pc ¼ b nIUt ; 2
(7)
where b is the power loss coefficient of the CIS device, and n is the number of ion separation units. The introduction of b is due to that the energy released in the electrolyte ion desorption process can be partly reused for charging in the electrolyte ion adsorption process. Dlugolecki and van der Wal [19] found that up to 83% of the energy released in the electrolyte ion desorption process can be recovered in the ion desorption process. Therefore, the value of b was chosen as 20% in this paper. 4. Results and discussion When investigating the effect of integration of the flow-by CIS device on the regeneration energy, it should be pointed out that the values of the CO2 output are different under different parameters of Richout (flow rate, NH3 concentration, CO2 loading). But, the CO2 output has the same value for the NH3-based CO2 capture process integrated with/without CIS device under the same corresponding parameter of Richout. 4.1. Effects of operating parameters of CIS device under different flow rates of Richout Fig. 3 presents the effects of the operating parameters of the CIS device, applied current of the capacitor, ion separation unit number, capacitance of the capacitor, and target voltage of the capacitor on the energy saving h under different flow rates of Richout. In Fig. 3a, for the cases that the flow rates of Richout are 120 L/min and 134 L/min, when the applied current of the capacitor is lower than 40 A, with the applied current of the capacitor increasing, the ion concentration of the concentrated ion stream flowing out of the CIS device increases, the required reboiler heat duty for keeping the CO2 output unchanged decreases significantly, the energy consumption of the CIS device increases little, and therefore the energy
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Fig. 3. Effects of the operating parameters of the CIS device: (a) applied current of the capacitor, (b) number of the ion separation units, (c) capacitance of the capacitor, and (d) target voltage of the capacitor on the energy saving under different flow rates of Richout.
saving when integrating the CIS device increases significantly. Whereas, when the applied current of the capacitor is higher than 40 A, with the applied current of the capacitor increasing, the decrease of the reboiler heat duty is almost the same to the increase of the energy consumption of the CIS device. Thus, the energy saving changes little and stays in the range of 20%e30%. Here, the energy savings stay in the range of that obtained by Jande et al. [11] when performing regeneration energy analysis of the MEA-based CO2 capture process integrated with CIS. Particularly, for the case that the flow rate of Richout is 120 L/ min, when the applied currents of the capacitor are 30 A and 32 A, the energy savings are smaller than zero for these cases, this indicates that the introduction of the flow-by CIS device into the NH3based CO2 capture process does not result in the decrease of the regeneration energy, but results in the increase of the regeneration energy. This is due to that the increase of the ion concentration of the concentrated ion stream flowing out of the CIS device is weak when the applied current of the capacitor is small under the condition of 120 L/min for the flow rate of Richout, but the decrease of the CO2 amount of the concentrated ion stream is large. This gives rise to that the required reboiler heat duty increases significantly for keeping the CO2 output unchanged when integrated with CIS device under the same flow rate of 120 L/min for Richout, further considering the increase of the energy consumption of the CIS device, thus, the energy saving when integrating the CIS device is smaller than zero for this case. In our previous work [13], this phenomenon, that the introduction of the CIS device does not result in the reduction of the regeneration energy, while gives rise to the increase of the regeneration energy, was not observed. But,
this phenomenon was observed in the work of Jande et al. [11] when studying the MEA-based CO2 capture process integrated with CIS. For the case that the flow rate of Richout is 150 L/min, with the applied current of the capacitor increasing, the difference between the decrease of the reboiler heat duty and the increase of the energy consumption of the CIS device is small, and therefore the energy saving when integrating the CIS device changes little and stays in the range of 20%e30%. By comparing Fig. 3a and b, it can be found that the effects of the applied current of the capacitor on the energy saving are similar to those of the ion separation unit number for the cases that the flow rates of Richout are 120 L/min, 134 L/min and 150 L/min, respectively. The reason for this is that the effects of the applied current of the capacitor and number of ion separation units on the ion concentration of the concentrated ion stream, and further on the reboiler heat duty are almost same [13], meanwhile those effects on the energy consumption of the CIS device are same, as shown in Eq. (7). In Fig. 3c, for the cases that the flow rates of Richout are 120 L/min, 134 L/min and 150 L/min, with the capacitance of the capacitor increasing, the ion concentration of the concentrated ion stream increases, thus the required reboiler heat duty decreases in order to keep the CO2 output unchanged when integrated with CIS device under the corresponding flow rates of Richout. From Eq. (7), it can be seen that the capacitance of the capacitor does not affect the energy consumption of the CIS device. Therefore, as the capacitance of the capacitor increases, the energy saving increases gradually. In Fig. 3d, for the cases that the flow rates of Richout are 120 L/min, 134 L/min and 150 L/min, with the target voltage of the capacitor increasing, the ion concentration of
M. Zhang, Y. Guo / Energy 125 (2017) 178e185
the concentrated ion stream increases, thus, in order to keep the CO2 output unchanged when integrated with CIS device under the corresponding flow rates of Richout, the required reboiler heat duty decreases. With the target voltage of the capacitor increasing, since the decrease of the reboiler heat duty is weaker than the increase of the energy consumption of the CIS device, the energy saving decreases gradually. In general, for the cases that the flow rates of Richout are 120 L/ min, 134 L/min and 150 L/min, when choosing the suitable operating parameters of the flow-by CIS device, the introduction of the CIS device into the NH3-based CO2 capture process can result in a significant reduction of the regeneration energy, the energy saving when integrating the CIS device can reach above 20%. But, for the case that the flow rate of Richout is 120 L/min, when the applied current of the capacitor or the ion separation unit number is small, the introduction of the CIS device into the NH3-based CO2 capture process does not result in the reduction of the regeneration energy, while results in the increase of the regeneration energy. This is due to that the CO2 amount of the concentrated ion stream flowing out of the CIS device is quite low, in order to keep the CO2 output unchanged when integrated with CIS device under this flow rate of Richout, the required reboiler heat duty increases.
4.2. Effects of operating parameters of CIS device under different NH3 concentrations of Richout Fig. 4 presents the effects of the operating parameters of the CIS device, applied current of the capacitor, ion separation unit number, capacitance of the capacitor, and target voltage of the capacitor
(a)
on the energy saving h under different NH3 concentrations of Richout. In Fig. 4a, for the cases that the NH3 concentrations of Richout are 2.5 mol/L and 3.0 mol/L, when the applied current of the capacitor is lower than 40 A, with the applied current of the capacitor increasing, the energy saving increases significantly when integrating the CIS device. This is due to that the increase of the ion concentration of the concentrated ion stream is strong, and the decrease of the reboiler heat duty is quite evident for these cases. However, when the applied current of the capacitor is higher than 40 A, the applied current of the capacitor has little effects on the energy saving when integrating the CIS device. Particularly, for the case that the NH3 concentration of Richout is 3.0 mol/L, when the applied current of the capacitor is 17 A, the energy saving is smaller than zero, which means that the introduction of the flow-by CIS device into the NH3-based CO2 capture process leads to the increase of the regeneration energy. This is due to that the CO2 output of the NH3-based CO2 capture process without integrated the CIS device is quite high for this case. When integrating the CIS device, as the applied current of the capacitor is small, the ion concentration of the concentrated ion stream flowing out of the CIS device is low, in order to keep the CO2 output unchanged, the required reboiler heat duty should be increased, thus, the energy saving is smaller than zero. For the case that the NH3 concentration of Richout is 2.0 mol/ L, with the applied current of the capacitor increasing, the energy saving increases gradually when integrating the CIS device. By comparing Fig. 4a and b, it can be found that the effects of the applied current of the capacitor on the energy saving are similar to those of the ion separation unit number for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L,
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Fig. 4. Effects of the operating parameters of the CIS device: (a) applied current of the capacitor, (b) number of the ion separation units, (c) capacitance of the capacitor, and (d) target voltage of the capacitor on the energy saving under different NH3 concentrations of Richout.
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respectively. Similarly, for the case that the NH3 concentration of Richout is 3.0 mol/L, when the ion separation unit is 400, the increase of the ion concentration of the concentrated ion stream is weak, the energy saving is smaller than zero, thus, the introduction of the CIS device results in the increase of the regeneration energy. In Fig. 4c, for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L, with the capacitance of the capacitor increasing, the energy saving increases accordingly. This is due to that the capacitance of the capacitor does not affect the energy consumption of the CIS device, but the reboiler heat duty reduces with the capacitance of the capacitor increasing. In Fig. 4d, for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L, as the target voltage of the capacitor increases, the energy saving decreases gradually. This is due to that the increase of the energy consumption of the CIS device is stronger than the decrease of the reboiler heat duty for these cases. Besides, under low NH3 concentration of Richout, for the NH3-based CO2 capture process integrated with the flow-by CIS device, choosing the suitable applied current or target voltage of the CIS device can result in more significant reduction of the regeneration energy. In general, for the cases that the NH3 concentrations of Richout are 2.0 mol/L, 2.5 mol/L and 3.0 mol/L, the introduction of the CIS device into the NH3-based CO2 capture process can result in a significant reduction of the regeneration energy, and the energy saving can reach above 20% when choosing the suitable operating parameters of the flow-by CIS device. Comparing to the cases that the NH3 concentrations of Richout are 2.5 mol/L and 3.0 mol/L, for the case that the NH3 concentration of Richout is 2.0 mol/L, choosing the appropriate operating parameters of the CIS device can result in more significant reduction of the regeneration energy, the energy saving can reach above 35%. Besides, for the case that the NH3 concentration of Richout is 3.0 mol/L, when the applied current of the capacitor or the ion separation unit number is quite small, the introduction of the CIS device will lead to the increase of the regeneration energy of the NH3-based CO2 capture process. 4.3. Effects of operating parameters of CIS device under different CO2 loadings of Richout Fig. 5 presents the effects of the operating parameters of the CIS device, applied current of the capacitor, ion separation unit number, capacitance of the capacitor, and target voltage of the capacitor on the energy saving h under different CO2 loadings of Richout. Here, the values of CO2 loadings of Richout were chosen according to the experimental values [3,4]. But in the work of Jande et al. [11], the values of CO2 loadings considered are obviously lower than the experimental values [2,9]. In Fig. 5a, for the cases that the CO2 loading of Richout are 0.3 and 0.4, with the applied current of the capacitor increasing, the energy saving increases significantly firstly when integrating the CIS device, and then changes a little. For the case that the CO2 loading of Richout is 0.5, with the applied current of the capacitor increasing, the energy saving increase significantly firstly, and then increase slowly when the applied current of the capacitor is higher than 60 A. Particularly, for the case that the CO2 loading of Richout is 0.3, when the applied currents of the capacitor are 35 A and 37 A, the energy savings are smaller than zero when integrating the CIS device. This is due to that the CO2 amount of the concentrated ion stream flowing out of the CIS device is small when the CO2 loading of Richout and the applied current of the capacitor are both small. Thus, this gives rise to that the required reboiler heat duty increases significantly for keeping the CO2 output unchanged when integrated with CIS device under the same CO2 loading of 0.3 for Richout, and therefore the energy saving is smaller than zero. For the case that the CO2 loading of Richout is 0.5, when the applied current of the capacitor is 20 A, the
energy saving is also smaller than zero when integrating the CIS device. This is due to that when the CO2 loading of Richout is high, the CO2 output of the NH3-based CO2 capture process without integrated the CIS device is large. When integrating the CIS device, as applied current of the capacitor is small, the required reboiler heat duty for keeping the CO2 output constant will increase, which further results in that the energy saving is smaller than zero. By comparing Fig. 5a and b, it can be found that the effects of the applied current of the capacitor on the energy saving are similar to those of the ion separation unit number for the cases that the CO2 loadings of Richout are 0.3, 0.4 and 0.5, respectively. Similarly, for the case that the CO2 loading of Richout is 0.3, when the ion separation unit numbers are 750 and 770, the energy saving is smaller than zero. For the case that the CO2 loading of Richout is 0.5, when the ion separation unit number is 400, the energy saving is also smaller than zero. In Fig. 5c, for the cases that the CO2 loadings of Richout are 0.3, 0.4 and 0.5, with the capacitance of the capacitor increasing, the energy saving increases gradually. In Fig. 5d, for the cases that the CO2 loadings of Richout are 0.3 and 0.4, as the target voltage of the capacitor increases, the energy saving decreases gradually, this is due to the weak increase of the ion concentration of the concentrated ion stream. For the case that the CO2 loading of Richout is 0.5, with the target voltage of the capacitor increasing, the energy saving increases a little firstly, and decreases gradually. In general, for the cases that the CO2 loadings of Richout are 0.3, 0.4 and 0.5, when choosing the suitable operating parameters of the flow-by CIS device, the introduction of the CIS device into the NH3-based CO2 capture process can lead to the significant reduction of the regeneration energy, and the energy saving can reach above 20% when integrating the CIS device. Whereas, for cases that the CO2 loadings of Richout are 0.3 and 0.5, when the applied current of the capacitor or the ion separation unit number is small, the introduction of the CIS device results in the increase of the regeneration energy of the NH3-based CO2 capture process.
5. Conclusions In this paper, regeneration energy analysis of the NH3-based CO2 capture process integrated with a flow-by capacitive ion separation (CIS) device under different flow rates, NH3 concentrations and CO2 loadings of the rich solvent (Richout) flowing into the CIS device was carried out. Effects of the operating parameters of the CIS device on the energy saving were obtained. The following conclusions can be drawn: (1) For the NH3-based CO2 capture process integrated with the flow-by CIS device, when choosing the suitable operating parameters of the CIS device, the reduction of the flow rate of the concentrated ion stream flowing out of the CIS device can result in a significant reduction of the regeneration energy, and the energy saving can reach above 20%. (2) Particularly, for the case that the NH3 concentration of Richout is 2.0 mol/L, when choosing the suitable operating parameters of the CIS device, the energy saving can be up to 35%. (3) However, for the cases that the flow rate of Richout is 120 L/ min, the NH3 concentration of Richout is 3.0 mol/L, or the CO2 loadings of Richout are 0.3 and 0.5, when the applied current of the capacitor or the ion separation unit number is small, the introduction of the CIS device into the NH3-based CO2 capture process results in the increase of the regeneration energy, which is due to the low CO2 amount of the concentrated ion stream flowing out of the CIS device.
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Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 51390493, the Wang Yongzhi Academician Research Innovation Foundation of Tsinghua University, and the Beijing Municipal Commission for Science & Technology under Grant No. Z08040902950803. References [1] Olajire AA. CO2 capture and separation technologies for end-of-pipe applications-a review. Energy 2010;35:2610e28. [2] Pellegrini G, Strube R, Manfrida G. Comparative study of chemical absorbents in postcombustion CO2 capture. Energy 2010;35:851e7. [3] Bai H, Yeh AC. Removal of CO2 greenhouse gas by ammonia scrubbing. Ind Eng Chem Res 1997;36:2490e3. [4] Yu H, Morgan S, Allport A, Cottrell A, Do T, McGregor J, et al. Results from trialling aqueous NH3 based post-combustion capture in a pilot plant at Munmorah power station: absorption. Chem Eng Res Des 2011;89:1204e15. [5] Zhao BT, Su YX, Tao WW, Li LL, Peng YC. Post-combustion CO2 capture by aqueous ammonia: a state-of-the-art review. Int J Greenh Gas Control 2012;9: 355e71. [6] Dong RF, Lu HF, Yu YS, Zhang ZX. A feasible process for simultaneous removal of CO2, SO2 and NOx in the cement industry by NH3 scrubbing. Appl Energy 2012;97:185e91. [7] Yu JW, Wang SJ, Yu H, Wardhaugh L, Feron P. Rate-based modelling of CO2 regeneration in ammonia based CO2 capture process. Int J Greenh Gas Control 2014;28:203e15. [8] Yu H, Li LC, Morgan S, Allport A, Cottrell A, McGregor J, et al. Results from trialling aqueous NH3 based post combustion capture in a pilot plant at
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