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Effect of K2CO3·1.5H2O on the regeneration energy consumption of potassium-based sorbents for CO2 capture
Applied Energy 112 (2013) 381–387
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Effect of K2CO31.5H2O on the regeneration energy consumption of potassium-based sorbents for CO2 capture Wenying Zhao a, Gerald Sprachmann b, Zhenshan Li a,⇑, Ningsheng Cai a, Xiaohui Zhang b a Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization & Reduction, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China b Researcher Gas Separation Technologies, Shell Global Solutions International BV, 1030 BN Amsterdam, The Netherlands
h i g h l i g h t s The formation condition of K2CO31.5H2O was obtained through thermodynamic equilibrium calculation. High-pressure fixed bed experiment was carried out to verify the thermodynamic calculation. The system energy consumption for K2CO3 as CO2 sorbent was analyzed. One novel process was put forward to reduce the energy consumption.
a r t i c l e
i n f o
Article history: Received 7 April 2013 Received in revised form 28 May 2013 Accepted 12 June 2013 Available online 13 July 2013 Keywords: CO2 capture K2CO31.5H2O Energy consumption High-pressure fixed bed
a b s t r a c t A high-pressure fixed bed reactor was used to study the formation condition of K2CO31.5H2O and the significance of K2CO31.5H2O in reducing the regeneration energy required for potassium-based sorbents. The reaction heat of K2CO3 converted into KHCO3 in the following reaction: K2CO3(s) + CO2(g) + H2O(g) M 2KHCO3(s), is approximately 143 kJ mol1-CO2. This value is much larger than that of amine with CO2 (60 kJ mol1-CO2). K2CO31.5H2O can absorb CO2 with the reaction heat of 42 kJ mol1-CO2 in the following reaction: K2CO31.5H2O(s) + CO2(g) M 2KHCO3(s) + 0.5H2O(g). This result indicates that a large amount of heat (99 kJ mol1-CO2) is released during the formation of K2CO31.5H2O in the following reaction: K2CO3(s) + 1.5H2O (g) M K2CO31.5H2O(s). The energy required for potassium-based sorbents can be potentially reduced when KHCO3 is converted into K2CO31.5H2O in the regeneration process or when the heat released during the formation of K2CO31.5H2O can be reused. Consequently, this work is focused on the investigation of the formation condition of K2CO31.5H2O and the potential effect of K2CO31.5H2O on the reduction of the energy required for potassium-based sorbents. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction As is widely known, the CO2 released from the burning of fossil fuels has greatly exacerbated the greenhouse effect, which in turn has led to disastrous consequences on global climate change [1,2]. A potential approach to reduce CO2 emission from large point sources is CO2 capture and storage (CCS). The estimated costs of CO2 transportation (US$1 – 3 per ton per 100 km) [1] and storage (US$4 – 8 per ton of CO2) [1] are much lower compared with the cost of CO2 capture, which is minimally estimated at US$35 – 55 per ton of CO2 captured [3]. Therefore, reducing the cost of CO2 capture will significantly make CCS more economically attractive. The high cost of CO2 capture is due to the large amount of energy required in the separation process [3]. The state-of-the-art technology for CO2 capture from gas streams uses aqueous amine
solutions (such as ca. 40% monoethanolamine and ca. 60% water) to absorb CO2. Although mature, the liquid amine scrubbing technology suffers from inherently high regeneration energy consumption, equipment corrosion and amine oxidative degradation [4]. Besides, amine-promoted hot potassium carbonate solution, Benfield solution, is also employed as the absorbent to remove CO2 and H2S from the synthesis gas [5,6]. However, the hot potassium carbonate process introduces major process concerns of corrosion, erosion, and column instability which affect the capital and maintenance costs in the form of design and operation [7]. The chemical absorption of CO2 by solid sorbents is one of the improved techniques to overcome these shortcomings. Researchers [8–12] have claimed that potassium-based sorbents are a promising technology for the removal of CO2 through the following reaction:
K2 CO3 ðsÞ þ CO2 ðgÞ þ H2 OðgÞ $ 2KHCO3 ðsÞ ⇑ Corresponding author. Tel./fax: +86 10 62789955. E-mail address: [email protected] (Z. Li). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.06.018
1
DH60 C ¼ 143 kJ mol
ð1Þ
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W. Zhao et al. / Applied Energy 112 (2013) 381–387
After the absorption reaction, the sorbent needs to be regenerated for it to be used repeatedly:
2KHCO3 ðsÞ $ K2 CO3 ðsÞ þ CO2 ðgÞ þ H2 OðgÞ 1
DH150 C ¼ 143 kJ mol
ð2Þ
The carbonation/regeneration cycles of potassium-based sorbents consist of one absorber and one regenerator. The flue gas that contains CO2 is introduced into the absorber, whose temperature (TA) is kept between 60 °C and 90 °C. In the absorber, CO2 is absorbed through the exothermic reaction (Eq. (1)). The utilization of released heat is difficult given the low reaction temperature. The produced KHCO3 is transported to the regenerator, where the temperature is maintained at approximately 150 °C (TR). During the regeneration process, the energy consumption is mainly allotted to overcome the sensible heat of sorbents with the temperature rising from TA to TR and to provide the heat for KHCO3 decomposition in the endothermic reaction (Eq. (2)). Most published results have focused on the effect of various supports [13–16] and experimental conditions [16–20] on the absorption and regeneration performance of potassium-based sorbents by using thermogravimetric analysis (TGA) [13,21], fixedbed reactor [22], single fluidized-bed reactor [18,19,23], dual fluidized bed reactors [24], and pilot plant in a practical power plant [17]. However, studies on the system energy consumption of potassium-based sorbents are few. The reaction heat of K2CO3-converted KHCO3 (as shown in Eq. (2)) is approximately 143 kJ mol1CO2, which is much larger than that of amine with CO2 (60 kJ mol1-CO2). This large reaction heat (143 kJ mol1-CO2) results in high regeneration energy consumption of approximately 3250 kJ tkg1-CO2. Considering the sensible heat of sorbents for the increase in temperature from the absorber to the regenerator (DT 100 K), the total regeneration energy consumption is more than 3700 kJ kg1-CO2 . In contrast, the heat required for CO2 recovery with hindered amine (Kansai Electric Power Co.,) and alkanolamine (Gas/Spec FS-1L) is 2930 [25,26] and 3693 [25] kJ kg1CO2, respectively. Therefore, given such a large reaction heat to be overcome, potassium-based sorbents are not competitive compared with the current liquid amine method in terms of energy consumption. Sesquihydrated potassium carbonate crystals (K2CO31.5H2O) can be grown by slow evaporation of its aqueous solution maintained at 40 ± 1 °C [27]. And it has also been reported that K2CO31.5H2O can be formed with the addition of water between 50 °C and 70 °C [12,19,28,29].
K2 CO3 ðsÞ þ 1:5H2 OðgÞ $ K2 CO3 1:5H2 OðsÞ
DH60 C ¼ 99 kJ mol
1
ð3Þ
K2CO31.5H2O has an important function in the absorption process of potassium-based sorbents, which can react with CO2 with a fast kinetic rate [13,28] through the following reaction [22]:
K2 CO3 1:5H2 OðsÞ þ CO2 ðgÞ $ 2KHCO3 ðsÞ þ 0:5H2 OðgÞ
DH60 C ¼ 44 kJ mol
1
ð4Þ
The reaction heat for K2CO31.5H2O formation is approximately 99 kJ mol1. Forming K2CO31.5H2O or reusing the released heat during the regeneration step can significantly reduce the energy consumption of regeneration. Shigemoto et al. [25] reported that the energy consumption could be reduced to approximately 1978 kJ kg1-CO2 if the regeneration process is carried out based on reaction (Eq. (4)). Therefore, the formation of K2CO31.5H2O is a crucial step in reducing the energy requirement in the regeneration process of potassium-based sorbents. Deshpande et al. [27] reported that K2CO31.5H2O loses 5.59% of water at 100 °C forming K2CO3H2O and loses all the crystallization water in the range of
130–135 °C. And it was also found that K2CO31.5H2O starts to lose its crystallization water at the temperature higher than 100 °C forming K2CO3 [30]. However, there is still not sufficient information about the formation condition of K2CO31.5H2O especially under various pressures. In particular, there have been very few reports investigating or validating the possibility of K2CO31.5H2O formation during the regeneration process of potassium based sorbents as well as the effect of K2CO31.5H2O on reducing regeneration energy required for potassium-based sorbents. One of the main objectives of this study is to investigate the formation conditions of K2CO31.5H2O. For this purpose, the existing condition of K2CO31.5H2O was initially investigated via thermodynamic equilibrium analysis by using HSC Chemistry 7.0. The experiment was conducted in a high-pressure fixed bed reactor to validate the thermodynamic equilibrium calculation result. Thereafter, the system energy consumption of potassium-based sorbents was analyzed based on the two proposed processes. 2. Materials and methods 2.1. Thermodynamic equilibrium calculation Thermodynamic equilibrium calculation was conducted using HSC-Chemistry software 7.0, a chemistry software program. The calculation of multicomponent thermodynamic system can be carried out via this software based on the Gibbs free energy minimization method. The representative composition used in this calculation includes K2CO3, KHCO3, K2CO31.5H2O, CO2(g), and H2O(g). 2.2. Experimental setup Fig. 1 shows the scheme of the experimental apparatus for the high-pressure fixed bed test with a range of 16 MPa. The set-up includes a gas/water injection system, a heater to produce steam, a high-pressure reactor, and a post-process system. Deionized water was fed to the system by using a micro-piston pump, which was then heated into saturated steam at high pressure in the heater with a height of 200 mm and an internal diameter of 16 mm. The high-pressure fixed bed reactor is made of stainless steel with a height of 500 mm and internal and external diameters of 8 and 16 mm, respectively. The temperature of the reactor was controlled by a three-stage furnace and was measured using four thermocouples, three of which were uniformly distributed in the furnace wall and one was placed at the center of the bed. The gas post-treatment system includes a condenser and a mass spectrograph. 2.3. Procedure for studying the formation condition of K2CO31.5H2O Experiments were conducted to investigate if K2CO31.5H2O could be formed at high temperatures (190 °C, 200 °C, 215 °C, 235 °C) at different pressures (1, 1.5, 2, and 3 MPa) through the reaction (Eq. (3)). Approximately 3 g of pure KHCO3 was packed in the fixed bed and was initially calcinated to convert it completely into K2CO3. Mass spectrograph was used to check the released gas until CO2 could no longer be detected, which indicated that KHCO3 had completely decomposed. The system pressure was then increased to the set value (1, 1.5, 2, and 3 MPa) by introducing an inert gas (N2). The temperature of the heater and reactor was increased to the set point (190 °C, 200 °C, 215 °C, 235 °C), which was above the saturation temperature of the steam at a particular vapor pressure. The water was injected at a rate of approximately 0.1 g min1. The experiment lasted for 1 h. The solid product was then analyzed
W. Zhao et al. / Applied Energy 112 (2013) 381–387
383
Fig. 1. Scheme of the high-pressure fixed bed test system. MS: mass spectrograph.
via X-ray diffraction (XRD) and TGA to evaluate the amount of K2CO31.5H2O based on the weight loss caused by dehydration. For the TGA test, approximately 10 mg of the solid product (K2CO3/K2CO31.5H2O) was placed in pure N2 atmosphere with a flow rate of 100 ml min1. The temperature was increased from the ambient temperature to 105 °C, which was maintained for approximately 30 min to remove moisture. Thereafter, the temperature was increased to 200 °C at a rate of 30 °C tmin1, which was maintained for approximately 20 min until the weight was stable. 2.4. Procedure for studying the decomposition of KHCO3 The experiment was carried out to determine if K2CO31.5H2O could be directly formed from KHCO3 in the regenerator. Approximately 3 g of pure KHCO3 was packed in the reactor. The system pressure was increased to 3 MPa by adjusting the back pressure valve of the fixed bed and by filling the system with N2. Thereafter, the temperature of the heater and reactor was increased to 235 °C, which was above the saturation temperature of the steam at 3 MPa. The water was fed through a micropiston pump at a rate of approximately 0.1 g min1. The experiment lasted for 1 h. The solid product was analyzed via XRD and TGA based on the procedures mentioned above. 3. Results and discussion 3.1. Thermodynamic equilibrium calculation Fig. 2 shows the calculated equilibrium pressure of H2O and CO2 over K2CO31.5H2O and KHCO3 at various temperatures. In a practical process, the temperature should be higher than that of the saturated steam to prevent the condensation of the steam. The saturation vapor pressure of the steam vs. temperature is plotted in Fig. 2 (curve I). Curve II in Fig. 2 represents the equilibrium partial pressure of H2O vs. temperature for the reaction given by Eq. (3). Curve III represents the equilibrium pressure of gaseous CO2 and H2O for the reaction given by Eq. (2). The calculation result
Fig. 2. Equilibrium pressures of gaseous substances as a function of temperature.
indicates that the saturation vapor pressure of the steam is higher than the equilibrium pressure of H2O for the formation of K2CO31.5H2O, as shown in Fig. 2. Therefore, the yield of K2CO31.5H2O is the dominant reaction in the zone between curves I and II. The decomposition of K2CO31.5H2O takes place in the zone under curve II. In the zone above curve III, K2CO3 reacts with CO2 and H2O to form KHCO3. In the zone below curve III, the decomposition of KHCO3 is dominant. For the global reaction given in Eq. (4), KHCO3 directly reacts with the steam to produce K2CO31.5H2O, which includes the decomposition of KHCO3 through Eq. (2) and the formation of K2CO31.5H2O through Eq. (3). Therefore, Eq. (4) only takes place below curve III and above curve II, as shown in Fig. 2. Based on the thermodynamic equilibrium calculation result, curve III is below curve II when the equilibrium pressure of the steam is lower than 1 bar, which indicates that Eq. (4) cannot occur. Therefore, KHCO3 cannot directly react with the steam to produce K2CO31.5H2O at atmospheric pressure. In addition, the zone below curve III and above curve II is narrow at high pressure,
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which indicates that the direct formation of K2CO31.5H2O from KHCO3 is difficult. The experiment was carried out in four representative conditions (four square points shown in Fig. 2), namely, 190 °C, 200 °C, 215 °C, and 235 °C, with corresponding pressures of 1, 1.5, 2, and 3 MPa, respectively, to investigate the formation condition and evaluate the formation amount of K2CO3 hydrate. The formation amount of K2CO31.5H2O from KHCO3 (Eq. (4)) was estimated at 235 °C and 3 MPa. 3.2. Experimental validation of thermodynamic equilibrium calculation results 3.2.1. Formation of K2CO31.5H2O from K2CO3 Based on the thermodynamic equilibrium analysis results, K2CO3 can react with the steam to produce K2CO31.5H2O in the zone between curves I and II, as shown in Fig. 2. Three representative conditions in that zone, namely, 190 °C/1 MPa, 200 °C/1.5 MPa, and 215 °C/2 MPa, were selected to investigate the formation of K2CO31.5H2O. Fig. 3 shows the XRD patterns of the experiment product at 190 °C/1 MPa and 215 °C/2 MPa. The amount of K2CO31.5H2O phase was substantial, which indicates that K2CO31.5H2O was indeed produced. Fig. 4 shows that in the three conditions, the mole fractions of K2CO31.5H2O in the product are 75%, 63%, and 67%, which indicates that most of K2CO3 could be converted to K2CO31.5H2O in the selected conditions. Based on thermodynamic equilibrium calculation results, theoretically, K2CO3 can be completely converted to K2CO31.5H2O in the selected conditions. However, only 75%, 63%, and 67% of K2CO3 were converted to K2CO31.5H2O in the experiment at 190 °C/1 MPa, 200 °C/ 1.5 MPa, and 215 °C/2 MPa. The incomplete conversion was mainly due to two factors. First, the conversion of K2CO3 was affected by the reaction kinetics between K2CO3 and the steam. Second, a layer of the K2CO31.5H2O product was formed on the surface of K2CO3, which increased the diffusion resistance of the steam through the product layer to the interface of K2CO3 to form K2CO31.5H2O. 3.2.2. Formation of K2CO31.5H2O from KHCO3 Fig. 5 shows the XRD patterns of the product of Eq. (4) between pure KHCO3 and the steam. Four kinds of substance were observed. The KHCO3 phase was observed in the product, which indicates that it did not completely react with the steam. The K2CO31.5H2O
Fig. 4. Mole fraction of K2CO31.5H2O formed in the product.
Fig. 5. XRD patterns of the product after the decomposition of KHCO3 in steam at 235 °C/3 MPa; () K4H2(CO3)31.5H2O, (d) K2CO31.5H2O, (j) KHCO3, (.) K2CO3.
and K2CO3 phases were also observed because K2CO31.5H2O and K2CO3 were in equilibrium with each other. A new phase K4H2(CO3)31.5H2O, which can be converted into K2CO31.5H2O as reported in previous studies [19,28], was also observed. Based on TGA analysis results, only 0.13 mol K2CO31.5H2O was formed per molar KHCO3 and the corresponding conversion rate of KHCO3 to K2CO31.5H2O was approximately 26%. In addition, Fig. 2 shows a narrow zone between curves II and III, which indicates that Eq. (4) would be difficult to execute in practical processes. 4. System energy consumption analysis for K-based sorbents 4.1. Process I: Application of potassium carbonate for CO2 capture
Fig. 3. XRD patterns of the reaction product of K2CO3 and steam (a) 190 °C/1 MPa; and (b) 215 °C/2 MPa; (j) K2CO31.5H2O, (d) K2CO3.
Fig. 6 shows process I, which is proposed to capture CO2 from the flue gas by using potassium carbonate. Two reactors are needed: one for absorption and the other for regeneration. In the absorber with a temperature of approximately 60 °C and pressure P, potassium carbonate absorbs CO2 from the flue gas and is converted into KHCO3 through Eq. (1). The reaction heat is then released and removed. The formed KHCO3 is transported to the regenerator and converted into K2CO3 in a CO2 atmosphere through Eq. (2) with the release of CO2 and steam. Highly pure
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W. Zhao et al. / Applied Energy 112 (2013) 381–387 Table 2 Energy consumption calculation for process I. P
1 bar
1.5 MPa
2.0 MPa
2.5 MPa
Units
Treg Dc Sensible heat
170 99% 22.4
250 99% 38.7
250 99% 38.7
300 99% 49
°C
Reaction heat
141.3
142.5
141.2
142.4
Total energy required
163.7
179.8
180
190
Total energy required
3720
4086
4092
4318
Compression energy avoided Net energy required
0
478
555
617
3720
3608
3536
3701
kJ t mol1CO2 kJ mol1CO2 kJ mol1CO2 kJ kg1CO2 kJ kg1CO2 kJ kg1CO2
Fig. 6. Process I: application of potassium carbonate in CO2 capture.
CO2 can be produced after the condensation of the steam. The regeneration process is conducted at temperature T and at the same pressure P in the absorber. In the regenerator, the required energy is mainly used to overcome the reaction heat, and the sensible heat of the solid sorbents for the increase in temperature from the absorption temperature to the regeneration temperature. Moreover, the sensible heat of the gas could be avoided because its specific heat capacity is very small. The heat released in the absorber is not taken into account because the absorption temperature is low (60 °C). High-pressure CO2 can be produced when the regenerator is operated at high pressure, which can reduce the compression energy consumption for the CCS process. Therefore, the energy required for process I is evaluated with the assumption that 1 mol CO2 is captured from the flue gas by the K2CO3 sorbent at different pressures. The required energy can be calculated using the following equation:
Q reg ¼ mKHCO3 C p ðT reg T abs ÞNK2 CO3 DH Dc
ð5Þ
where Qreg is the regeneration energy required; Cp is the specific heat capacity of the K2CO3 sorbent; Treg and Tabs are the temperatures of the absorber and regenerator, respectively; N K2 CO3 is the mole number of the K2CO3 sorbent; DH is the reaction heat of (Eq. (2)) (143 kJ mol1-CO2); and Dc is the regeneration extent of KHCO3 in the regenerator. The values of these parameters are shown in Table 1. In Table 2, the temperature of the regenerator (Treg) is the complete decomposition temperature of KHCO3 in a CO2 atmosphere and Dc is assumed to be 99%. The sensible heat of the solid sorbents and the reaction heat can be calculated by the first and second term in the right part of Eq. (5), respectively. Net energy required represents the equivalent energy required for CO2 production at 1 bar. Compression energy required is the estimated energy based on the polytrophic process of the following equations [31]:
W th ¼ n=ðn 1Þ RT ½1 ðP2 =P 1 Þðn1Þ=n
pressures of the gas before and after compression, respectively; and n is the polytropic constant (n is 1.25 for CO2).
Q c ¼ W th =g
where Qc is the compression energy required and g is the compression efficiency (85%, assumed). Overall, the energy consumption for the whole process is approximately 3720 kJ kg1-CO2 under atmosphere pressure. And the energy consumption can be reduced to approximately 3600 kJ kg1-CO2 if the reactors are operated under 1.5 MPa or 2.0 MPa with high-pressure CO2 stream produced, which reduces the compression power for CCS. However, the energy consumption for CO2 capture by aqueous MEA is approximately 4200 kJ kg1CO2 [32], which indicates that the proposed process does not have any obvious advantage over the matured MEA process in terms of system energy consumption. 4.2. Process II: Application of potassium carbonate for CO2 capture
4.2.1. Process description Another process in reducing the energy consumption further is proposed to use the heat released during the formation of K2CO31.5H2O. As shown in Fig. 7, three reactors are needed for process II: one for absorption, one for regeneration and another temporary reactor for the formation of K2CO31.5H2O. In the absorber, K2CO31.5H2O/K2CO3 absorbs CO2 and is converted into KHCO3 at 60 °C/
ð6Þ
where Wth is the theoretical compression work; R is the ideal gas constant; T is the temperature of the gas; P1 and P2 represent the Table 1 Values of parameters in Eq. (5) where 1 mol CO2 was assumed to be captured.
Mass of KHCO3 Specific heat capacity Mole number of K2CO3 Reaction heat to overcome
Symbol
Value
Units
mKHCO3 Cp N K2 CO3 DH
200 1.02 103 1 143
g kJ tg1 K1 mol kJ mol1
ð7Þ
Fig. 7. Process II: application of potassium carbonate in CO2 capture.
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1 bar. The formed KHCO3 is then transported to the regenerator and converted into K2CO3 through Eq. (2) at 170 °C/1 bar with the release of CO2. The K2CO3 produced in the regenerator is transported to the temporary reactor and reacts with the steam to produce K2CO31.5H2O through (Eq. (3)) at 200 °C/1.5 MPa with a significant amount of reaction heat released. 4.2.2. Material balance The assumption is that 1 mol CO2 from the flue gas is captured in the process, and the reactant (K2CO31.5H2O/K2CO3) needed is 1 mol. The product (KHCO3) transported to regenerator is 2 mol. In the regenerator, the sorbent is assumed to be completely regenerated. Therefore, 1 mol K2CO3 is produced in the regenerator and transported to the temporary reactor. Based on the high-pressure experiment result, 70% of K2CO3 is assumed to be converted to K2CO31.5H2O at 200 °C/1.5 MPa in the temporary reactor. Therefore, 0.7 mol K2CO31.5H2O was formed in the temporary reactor. Based on Eq. (3), 1.05 mol steam (200 °C/1.5 MPa) should be supplied to react with K2CO3. For the whole process, solid sorbents are transported and circulated among the three reactors, with the assumption that no loss of sorbents occurs. 4.2.3. Heat balance In the absorber, the reaction between K2CO31.5H2O/K2CO3 and CO2 is exothermic. However, reusing the heat released is difficult given the low temperature (60 °C). In the regenerator, the decomposition of KHCO3 is endothermic. The energy required is mainly used for the sensible heat of KHCO3 from 60 °C to 170 °C and to overcome the reaction heat (DH1). Therefore, the energy consumption in the regenerator (Qreg) can be expressed as the following equation:
Q reg ¼ mKHCO3 Cp1 ðT reg T abs ÞNKHCO3 =2 DH1
ð8Þ
In the temporary reactor, the energy consumption includes the sensible heat of K2CO3 from 170 °C to 200 °C and the energy required to produce the steam (1.5 MPa, 200 °C). The energy consumption in the temporary reactor (Qtemp) can be expressed as the following equation:
Q temp ¼ mK2 CO3 Cp2 ðT temp T reg Þ þ Q steam
ð9Þ
At the same time, K2CO31.5H2O is produced in the temporary reactor with a significant amount of heat released, which can be utilized because the temperature is high. The released heat can be transported by the steam to the regenerator to reduce the energy required for the decomposition of KHCO3. The heat transfer efficiency (f) is assumed to be 0.8. Therefore, the available heat released during the formation of K2CO31.5H2O in the temporary reactor (Qhydrate) can be expressed as the following equation:
Q hydrate ¼ NK2 CO3 DH2 Dc n
ð10Þ
For the whole process, the energy required (Qtotal) includes the energy consumption in the regenerator and the temporary reactor, which can be calculated using the following equation:
Q total ¼ Q reg þ Q temp Q hydrate
ð11Þ
Based on the material and heat balance, the meaning and value of the symbols in Eqs. (8)–(10) are represented in Table 3. In the temporary reactor, 1.05 mol steam (1.5 MPa, 200 °C) should be supplied, which consumes energy to produce steam. The required energy (Qsteam) can be calculated based on the enthalpy of steam (1.5 MPa, 200 °C) compared with the enthalpy of water (1 bar, 30 °C). Assuming that water (1 bar, 30 °C) is the original condition, the pressure of water was initially increased to 1.5 MPa by using the pump. The high-pressure water is then heated and vaporized to produce steam (1.5 MPa, 200 °C). Based
Table 3 Energy consumption calculation for process II.
Mass of KHCO3 Specific heat of KHCO3 Temperature of regenerator Temperature of absorber Temperature of temp reactor Mole number of KHCO3 Reaction heat to overcome Energy required in regenerator Mass of K2CO3 Mole number of K2CO3 Specific heat of K2CO3 Reaction heat to overcome Heat transfer efficiency Conversion of K2CO3 Heat released in temp reactor Energy required for steam Energy required in temp reactor
Symbol
Value
Units
mKHCO3 Cp1 Treg Tabs Ttemp N KHCO3 D H1 Qreg mK2 CO3 N K2 CO3 Cp2 D H2 n Dc Qhydrate Qsteam Qtemp
200 1.02 103 170 60 200 2 143 157.9 138 1 1.02 103 99 0.8 0.7 55.4 49 53.2
g kJ g1 K1 °C °C °C mol kJ mol1 kJ mol1-CO2 g mol kJ g1 K1 kJ mol1
kJ mol1-CO2 kJ mol1-CO2 kJ mol1-CO2
on the enthalpy of H2O at different conditions, the heat required to produce steam (1.05 mol) is estimated to be approximately 49 kJ mol1-CO2. Therefore, based on Eq. (11), the energy consumption for the whole process is estimated to be 155.7 kJ mol1-CO2 or 3539 kJ kg1-CO2. For the system energy consumption, the energy required to overcome the regeneration reaction heat and the sensible heat of the sorbent is 71% and 7%, respectively, which cannot be avoided. On the other hand, the energy required to produce the steam (1.5 MPa, 200 °C) is 22%. The steam (1.5 MPa, 200 °C) can be produced from low grade heat source or utilization of waste heat emissions to reduce the cost in practical processes. Excluding the energy required to produce steam, the least energy required for the potassium-based sorbents is approximately 2425 kJ kg1-CO2 while making use of the heat released during the formation of K2CO31.5H2O. 5. Conclusions The formation condition of K2CO31.5H2O was determined through thermodynamic equilibrium calculation. High-pressure fixed bed experiment was carried out at four representative conditions, namely, 190 °C/1 MPa, 200 °C/1.5 MPa, 215 °C/2 MPa, and 235 °C/3 MPa. The thermodynamic equilibrium calculation result indicates that K2CO31.5H2O was indeed produced through the reaction between K2CO3 and the steam. Approximately 75%, 63%, and 67% of K2CO3 were converted to K2CO31.5H2O in the experiment at 190 °C/1 MPa, 200 °C /1.5 MPa, and 215 °C/2 MPa. In addition, the thermodynamic equilibrium analysis results show that KHCO3 cannot directly react with the steam to produce K2CO31.5H2O with the equilibrium pressure of H2O and CO2 lower than 1 bar. Even at high pressure, the direct formation of K2CO31.5H2O from KHCO3 is difficult because the formation zone of K2CO31.5H2O is narrow. The system energy consumption for potassium carbonate as a CO2 sorbent was analyzed based on the two proposed processes. The energy required for the regeneration of potassium carbonate is approximately 3600 kJ kg1-CO2. One novel process was proposed to reduce the energy consumption and use the heat released during the formation of K2CO31.5H2O. The energy consumption for the whole process is estimated to be 3539 kJ kg1-CO2. The produce of steam covers 22% of the total energy consumption. The steam can be produced from low grade heat source or utilization of waste heat emissions to reduce the cost. Excluding the energy required to produce steam, the least energy required for potassium-based sorbents is approximately 2425 kJ kg1-CO2. However,
W. Zhao et al. / Applied Energy 112 (2013) 381–387
three reactors are needed and operated at different pressures, which might be challenging from a technical point of view. Acknowledgments Financial supports from Shell Global Solutions International B.V. (SSR161) and the National High Technology Research and Development Program of China (2012AA06A115) are sincerely acknowledged. References [1] Freund P. Making deep reductions in CO2 emissions from coal-fired power plant using capture and storage of CO2. Proc Inst Mech Eng 2003;217:1–7. [2] Herzog HJ. What future for carbon capture and sequestration? Environ Sci Technol 2001;35:148A–53A. [3] Singh D, Croiset E, Douglas PL, Douglas MA. Techno-economic study of CO2 capture from an existing coal-fired power plant: MEA scrubbing vs. O2/CO2 recycle combustion. Energy Convers Manage 2003;44:3073–91. [4] Veawab A, Tontiwachwuthikul P, Chakma A. Corrosion behavior of carbon steel in the CO2 absorption process using aqueous amine solutions. Ind Eng Chem Res 1999;38:3917–24. [5] Benson HE, Field JH, Jimeson RM. CO2 absorption employing hot potassium carbonate solutions. Chem Eng Prog 1954;50:356–64. [6] Yi F, Zou H, Chu G, Shao L, Chen J. Modeling and experimental studies on absorption of CO2 by Benfield solution in rotating packed bed. Chem Eng J 2009;145:377–84. [7] Maddox RN. Gas and liquid sweetening. 2nd ed. Norman: M. Campbell; 1974. [8] Lee SC, Choi BY, Lee TJ, Ryu CK, Ahn YS, Kim JC. CO2 absorption and regeneration of alkali metal-based solid sorbents. Catal Today 2006;111:385–90. [9] Lee S, Kim J. Dry potassium-based sorbents for CO2 capture. Catal Surv Asia 2007;11:171–85. [10] Hoffman JS, Pennline HW. Study of regenerable sorbents for CO2 capture.. [11] Zhao CW, Chen XP, Zhao CS. Study on CO2 capture using dry potassium-based sorbents through orthogonal test method. Int J Greenhouse Gas Control 2010;4:655–8. [12] Lee S, Choi B, Ryu C, Ahn Y, Lee T, Kim J. The effect of water on the activation and the CO2 capture capacities of alkali metal-based sorbents. Korean J Chem Eng 2006;23:374–9. [13] Zhao CW, Chen XP, Zhao CS. Carbonation and active-component-distribution behaviors of several potassium-based sorbents. Ind Eng Chem Res 2011;50:4464–70. [14] Zhao CW, Chen XP, Zhao CS. Carbonation behavior of K2CO3 with different microstructure used as an active component of dry sorbents for CO2 capture. Ind Eng Chem Res 2010;49:12212–6. [15] Li L, Li Y, Wen X, Wang F, Zhao N, Xiao FK, et al. CO2 capture over K2CO3/MgO/ Al2O3 dry sorbent in a fluidized bed. Energy Fuels 2011;25:3835–42.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Effect of K2CO31.5H2O on the regeneration energy consumption of potassium-based sorbents for CO2 capture Wenying Zhao a, Gerald Sprachmann b, Zhenshan Li a,⇑, Ningsheng Cai a, Xiaohui Zhang b a Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Municipal Key Laboratory for CO2 Utilization & Reduction, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China b Researcher Gas Separation Technologies, Shell Global Solutions International BV, 1030 BN Amsterdam, The Netherlands
h i g h l i g h t s The formation condition of K2CO31.5H2O was obtained through thermodynamic equilibrium calculation. High-pressure fixed bed experiment was carried out to verify the thermodynamic calculation. The system energy consumption for K2CO3 as CO2 sorbent was analyzed. One novel process was put forward to reduce the energy consumption.
a r t i c l e
i n f o
Article history: Received 7 April 2013 Received in revised form 28 May 2013 Accepted 12 June 2013 Available online 13 July 2013 Keywords: CO2 capture K2CO31.5H2O Energy consumption High-pressure fixed bed
a b s t r a c t A high-pressure fixed bed reactor was used to study the formation condition of K2CO31.5H2O and the significance of K2CO31.5H2O in reducing the regeneration energy required for potassium-based sorbents. The reaction heat of K2CO3 converted into KHCO3 in the following reaction: K2CO3(s) + CO2(g) + H2O(g) M 2KHCO3(s), is approximately 143 kJ mol1-CO2. This value is much larger than that of amine with CO2 (60 kJ mol1-CO2). K2CO31.5H2O can absorb CO2 with the reaction heat of 42 kJ mol1-CO2 in the following reaction: K2CO31.5H2O(s) + CO2(g) M 2KHCO3(s) + 0.5H2O(g). This result indicates that a large amount of heat (99 kJ mol1-CO2) is released during the formation of K2CO31.5H2O in the following reaction: K2CO3(s) + 1.5H2O (g) M K2CO31.5H2O(s). The energy required for potassium-based sorbents can be potentially reduced when KHCO3 is converted into K2CO31.5H2O in the regeneration process or when the heat released during the formation of K2CO31.5H2O can be reused. Consequently, this work is focused on the investigation of the formation condition of K2CO31.5H2O and the potential effect of K2CO31.5H2O on the reduction of the energy required for potassium-based sorbents. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction As is widely known, the CO2 released from the burning of fossil fuels has greatly exacerbated the greenhouse effect, which in turn has led to disastrous consequences on global climate change [1,2]. A potential approach to reduce CO2 emission from large point sources is CO2 capture and storage (CCS). The estimated costs of CO2 transportation (US$1 – 3 per ton per 100 km) [1] and storage (US$4 – 8 per ton of CO2) [1] are much lower compared with the cost of CO2 capture, which is minimally estimated at US$35 – 55 per ton of CO2 captured [3]. Therefore, reducing the cost of CO2 capture will significantly make CCS more economically attractive. The high cost of CO2 capture is due to the large amount of energy required in the separation process [3]. The state-of-the-art technology for CO2 capture from gas streams uses aqueous amine
solutions (such as ca. 40% monoethanolamine and ca. 60% water) to absorb CO2. Although mature, the liquid amine scrubbing technology suffers from inherently high regeneration energy consumption, equipment corrosion and amine oxidative degradation [4]. Besides, amine-promoted hot potassium carbonate solution, Benfield solution, is also employed as the absorbent to remove CO2 and H2S from the synthesis gas [5,6]. However, the hot potassium carbonate process introduces major process concerns of corrosion, erosion, and column instability which affect the capital and maintenance costs in the form of design and operation [7]. The chemical absorption of CO2 by solid sorbents is one of the improved techniques to overcome these shortcomings. Researchers [8–12] have claimed that potassium-based sorbents are a promising technology for the removal of CO2 through the following reaction:
K2 CO3 ðsÞ þ CO2 ðgÞ þ H2 OðgÞ $ 2KHCO3 ðsÞ ⇑ Corresponding author. Tel./fax: +86 10 62789955. E-mail address: [email protected] (Z. Li). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.06.018
1
DH60 C ¼ 143 kJ mol
ð1Þ
382
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After the absorption reaction, the sorbent needs to be regenerated for it to be used repeatedly:
2KHCO3 ðsÞ $ K2 CO3 ðsÞ þ CO2 ðgÞ þ H2 OðgÞ 1
DH150 C ¼ 143 kJ mol
ð2Þ
The carbonation/regeneration cycles of potassium-based sorbents consist of one absorber and one regenerator. The flue gas that contains CO2 is introduced into the absorber, whose temperature (TA) is kept between 60 °C and 90 °C. In the absorber, CO2 is absorbed through the exothermic reaction (Eq. (1)). The utilization of released heat is difficult given the low reaction temperature. The produced KHCO3 is transported to the regenerator, where the temperature is maintained at approximately 150 °C (TR). During the regeneration process, the energy consumption is mainly allotted to overcome the sensible heat of sorbents with the temperature rising from TA to TR and to provide the heat for KHCO3 decomposition in the endothermic reaction (Eq. (2)). Most published results have focused on the effect of various supports [13–16] and experimental conditions [16–20] on the absorption and regeneration performance of potassium-based sorbents by using thermogravimetric analysis (TGA) [13,21], fixedbed reactor [22], single fluidized-bed reactor [18,19,23], dual fluidized bed reactors [24], and pilot plant in a practical power plant [17]. However, studies on the system energy consumption of potassium-based sorbents are few. The reaction heat of K2CO3-converted KHCO3 (as shown in Eq. (2)) is approximately 143 kJ mol1CO2, which is much larger than that of amine with CO2 (60 kJ mol1-CO2). This large reaction heat (143 kJ mol1-CO2) results in high regeneration energy consumption of approximately 3250 kJ tkg1-CO2. Considering the sensible heat of sorbents for the increase in temperature from the absorber to the regenerator (DT 100 K), the total regeneration energy consumption is more than 3700 kJ kg1-CO2 . In contrast, the heat required for CO2 recovery with hindered amine (Kansai Electric Power Co.,) and alkanolamine (Gas/Spec FS-1L) is 2930 [25,26] and 3693 [25] kJ kg1CO2, respectively. Therefore, given such a large reaction heat to be overcome, potassium-based sorbents are not competitive compared with the current liquid amine method in terms of energy consumption. Sesquihydrated potassium carbonate crystals (K2CO31.5H2O) can be grown by slow evaporation of its aqueous solution maintained at 40 ± 1 °C [27]. And it has also been reported that K2CO31.5H2O can be formed with the addition of water between 50 °C and 70 °C [12,19,28,29].
K2 CO3 ðsÞ þ 1:5H2 OðgÞ $ K2 CO3 1:5H2 OðsÞ
DH60 C ¼ 99 kJ mol
1
ð3Þ
K2CO31.5H2O has an important function in the absorption process of potassium-based sorbents, which can react with CO2 with a fast kinetic rate [13,28] through the following reaction [22]:
K2 CO3 1:5H2 OðsÞ þ CO2 ðgÞ $ 2KHCO3 ðsÞ þ 0:5H2 OðgÞ
DH60 C ¼ 44 kJ mol
1
ð4Þ
The reaction heat for K2CO31.5H2O formation is approximately 99 kJ mol1. Forming K2CO31.5H2O or reusing the released heat during the regeneration step can significantly reduce the energy consumption of regeneration. Shigemoto et al. [25] reported that the energy consumption could be reduced to approximately 1978 kJ kg1-CO2 if the regeneration process is carried out based on reaction (Eq. (4)). Therefore, the formation of K2CO31.5H2O is a crucial step in reducing the energy requirement in the regeneration process of potassium-based sorbents. Deshpande et al. [27] reported that K2CO31.5H2O loses 5.59% of water at 100 °C forming K2CO3H2O and loses all the crystallization water in the range of
130–135 °C. And it was also found that K2CO31.5H2O starts to lose its crystallization water at the temperature higher than 100 °C forming K2CO3 [30]. However, there is still not sufficient information about the formation condition of K2CO31.5H2O especially under various pressures. In particular, there have been very few reports investigating or validating the possibility of K2CO31.5H2O formation during the regeneration process of potassium based sorbents as well as the effect of K2CO31.5H2O on reducing regeneration energy required for potassium-based sorbents. One of the main objectives of this study is to investigate the formation conditions of K2CO31.5H2O. For this purpose, the existing condition of K2CO31.5H2O was initially investigated via thermodynamic equilibrium analysis by using HSC Chemistry 7.0. The experiment was conducted in a high-pressure fixed bed reactor to validate the thermodynamic equilibrium calculation result. Thereafter, the system energy consumption of potassium-based sorbents was analyzed based on the two proposed processes. 2. Materials and methods 2.1. Thermodynamic equilibrium calculation Thermodynamic equilibrium calculation was conducted using HSC-Chemistry software 7.0, a chemistry software program. The calculation of multicomponent thermodynamic system can be carried out via this software based on the Gibbs free energy minimization method. The representative composition used in this calculation includes K2CO3, KHCO3, K2CO31.5H2O, CO2(g), and H2O(g). 2.2. Experimental setup Fig. 1 shows the scheme of the experimental apparatus for the high-pressure fixed bed test with a range of 16 MPa. The set-up includes a gas/water injection system, a heater to produce steam, a high-pressure reactor, and a post-process system. Deionized water was fed to the system by using a micro-piston pump, which was then heated into saturated steam at high pressure in the heater with a height of 200 mm and an internal diameter of 16 mm. The high-pressure fixed bed reactor is made of stainless steel with a height of 500 mm and internal and external diameters of 8 and 16 mm, respectively. The temperature of the reactor was controlled by a three-stage furnace and was measured using four thermocouples, three of which were uniformly distributed in the furnace wall and one was placed at the center of the bed. The gas post-treatment system includes a condenser and a mass spectrograph. 2.3. Procedure for studying the formation condition of K2CO31.5H2O Experiments were conducted to investigate if K2CO31.5H2O could be formed at high temperatures (190 °C, 200 °C, 215 °C, 235 °C) at different pressures (1, 1.5, 2, and 3 MPa) through the reaction (Eq. (3)). Approximately 3 g of pure KHCO3 was packed in the fixed bed and was initially calcinated to convert it completely into K2CO3. Mass spectrograph was used to check the released gas until CO2 could no longer be detected, which indicated that KHCO3 had completely decomposed. The system pressure was then increased to the set value (1, 1.5, 2, and 3 MPa) by introducing an inert gas (N2). The temperature of the heater and reactor was increased to the set point (190 °C, 200 °C, 215 °C, 235 °C), which was above the saturation temperature of the steam at a particular vapor pressure. The water was injected at a rate of approximately 0.1 g min1. The experiment lasted for 1 h. The solid product was then analyzed
W. Zhao et al. / Applied Energy 112 (2013) 381–387
383
Fig. 1. Scheme of the high-pressure fixed bed test system. MS: mass spectrograph.
via X-ray diffraction (XRD) and TGA to evaluate the amount of K2CO31.5H2O based on the weight loss caused by dehydration. For the TGA test, approximately 10 mg of the solid product (K2CO3/K2CO31.5H2O) was placed in pure N2 atmosphere with a flow rate of 100 ml min1. The temperature was increased from the ambient temperature to 105 °C, which was maintained for approximately 30 min to remove moisture. Thereafter, the temperature was increased to 200 °C at a rate of 30 °C tmin1, which was maintained for approximately 20 min until the weight was stable. 2.4. Procedure for studying the decomposition of KHCO3 The experiment was carried out to determine if K2CO31.5H2O could be directly formed from KHCO3 in the regenerator. Approximately 3 g of pure KHCO3 was packed in the reactor. The system pressure was increased to 3 MPa by adjusting the back pressure valve of the fixed bed and by filling the system with N2. Thereafter, the temperature of the heater and reactor was increased to 235 °C, which was above the saturation temperature of the steam at 3 MPa. The water was fed through a micropiston pump at a rate of approximately 0.1 g min1. The experiment lasted for 1 h. The solid product was analyzed via XRD and TGA based on the procedures mentioned above. 3. Results and discussion 3.1. Thermodynamic equilibrium calculation Fig. 2 shows the calculated equilibrium pressure of H2O and CO2 over K2CO31.5H2O and KHCO3 at various temperatures. In a practical process, the temperature should be higher than that of the saturated steam to prevent the condensation of the steam. The saturation vapor pressure of the steam vs. temperature is plotted in Fig. 2 (curve I). Curve II in Fig. 2 represents the equilibrium partial pressure of H2O vs. temperature for the reaction given by Eq. (3). Curve III represents the equilibrium pressure of gaseous CO2 and H2O for the reaction given by Eq. (2). The calculation result
Fig. 2. Equilibrium pressures of gaseous substances as a function of temperature.
indicates that the saturation vapor pressure of the steam is higher than the equilibrium pressure of H2O for the formation of K2CO31.5H2O, as shown in Fig. 2. Therefore, the yield of K2CO31.5H2O is the dominant reaction in the zone between curves I and II. The decomposition of K2CO31.5H2O takes place in the zone under curve II. In the zone above curve III, K2CO3 reacts with CO2 and H2O to form KHCO3. In the zone below curve III, the decomposition of KHCO3 is dominant. For the global reaction given in Eq. (4), KHCO3 directly reacts with the steam to produce K2CO31.5H2O, which includes the decomposition of KHCO3 through Eq. (2) and the formation of K2CO31.5H2O through Eq. (3). Therefore, Eq. (4) only takes place below curve III and above curve II, as shown in Fig. 2. Based on the thermodynamic equilibrium calculation result, curve III is below curve II when the equilibrium pressure of the steam is lower than 1 bar, which indicates that Eq. (4) cannot occur. Therefore, KHCO3 cannot directly react with the steam to produce K2CO31.5H2O at atmospheric pressure. In addition, the zone below curve III and above curve II is narrow at high pressure,
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which indicates that the direct formation of K2CO31.5H2O from KHCO3 is difficult. The experiment was carried out in four representative conditions (four square points shown in Fig. 2), namely, 190 °C, 200 °C, 215 °C, and 235 °C, with corresponding pressures of 1, 1.5, 2, and 3 MPa, respectively, to investigate the formation condition and evaluate the formation amount of K2CO3 hydrate. The formation amount of K2CO31.5H2O from KHCO3 (Eq. (4)) was estimated at 235 °C and 3 MPa. 3.2. Experimental validation of thermodynamic equilibrium calculation results 3.2.1. Formation of K2CO31.5H2O from K2CO3 Based on the thermodynamic equilibrium analysis results, K2CO3 can react with the steam to produce K2CO31.5H2O in the zone between curves I and II, as shown in Fig. 2. Three representative conditions in that zone, namely, 190 °C/1 MPa, 200 °C/1.5 MPa, and 215 °C/2 MPa, were selected to investigate the formation of K2CO31.5H2O. Fig. 3 shows the XRD patterns of the experiment product at 190 °C/1 MPa and 215 °C/2 MPa. The amount of K2CO31.5H2O phase was substantial, which indicates that K2CO31.5H2O was indeed produced. Fig. 4 shows that in the three conditions, the mole fractions of K2CO31.5H2O in the product are 75%, 63%, and 67%, which indicates that most of K2CO3 could be converted to K2CO31.5H2O in the selected conditions. Based on thermodynamic equilibrium calculation results, theoretically, K2CO3 can be completely converted to K2CO31.5H2O in the selected conditions. However, only 75%, 63%, and 67% of K2CO3 were converted to K2CO31.5H2O in the experiment at 190 °C/1 MPa, 200 °C/ 1.5 MPa, and 215 °C/2 MPa. The incomplete conversion was mainly due to two factors. First, the conversion of K2CO3 was affected by the reaction kinetics between K2CO3 and the steam. Second, a layer of the K2CO31.5H2O product was formed on the surface of K2CO3, which increased the diffusion resistance of the steam through the product layer to the interface of K2CO3 to form K2CO31.5H2O. 3.2.2. Formation of K2CO31.5H2O from KHCO3 Fig. 5 shows the XRD patterns of the product of Eq. (4) between pure KHCO3 and the steam. Four kinds of substance were observed. The KHCO3 phase was observed in the product, which indicates that it did not completely react with the steam. The K2CO31.5H2O
Fig. 4. Mole fraction of K2CO31.5H2O formed in the product.
Fig. 5. XRD patterns of the product after the decomposition of KHCO3 in steam at 235 °C/3 MPa; () K4H2(CO3)31.5H2O, (d) K2CO31.5H2O, (j) KHCO3, (.) K2CO3.
and K2CO3 phases were also observed because K2CO31.5H2O and K2CO3 were in equilibrium with each other. A new phase K4H2(CO3)31.5H2O, which can be converted into K2CO31.5H2O as reported in previous studies [19,28], was also observed. Based on TGA analysis results, only 0.13 mol K2CO31.5H2O was formed per molar KHCO3 and the corresponding conversion rate of KHCO3 to K2CO31.5H2O was approximately 26%. In addition, Fig. 2 shows a narrow zone between curves II and III, which indicates that Eq. (4) would be difficult to execute in practical processes. 4. System energy consumption analysis for K-based sorbents 4.1. Process I: Application of potassium carbonate for CO2 capture
Fig. 3. XRD patterns of the reaction product of K2CO3 and steam (a) 190 °C/1 MPa; and (b) 215 °C/2 MPa; (j) K2CO31.5H2O, (d) K2CO3.
Fig. 6 shows process I, which is proposed to capture CO2 from the flue gas by using potassium carbonate. Two reactors are needed: one for absorption and the other for regeneration. In the absorber with a temperature of approximately 60 °C and pressure P, potassium carbonate absorbs CO2 from the flue gas and is converted into KHCO3 through Eq. (1). The reaction heat is then released and removed. The formed KHCO3 is transported to the regenerator and converted into K2CO3 in a CO2 atmosphere through Eq. (2) with the release of CO2 and steam. Highly pure
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W. Zhao et al. / Applied Energy 112 (2013) 381–387 Table 2 Energy consumption calculation for process I. P
1 bar
1.5 MPa
2.0 MPa
2.5 MPa
Units
Treg Dc Sensible heat
170 99% 22.4
250 99% 38.7
250 99% 38.7
300 99% 49
°C
Reaction heat
141.3
142.5
141.2
142.4
Total energy required
163.7
179.8
180
190
Total energy required
3720
4086
4092
4318
Compression energy avoided Net energy required
0
478
555
617
3720
3608
3536
3701
kJ t mol1CO2 kJ mol1CO2 kJ mol1CO2 kJ kg1CO2 kJ kg1CO2 kJ kg1CO2
Fig. 6. Process I: application of potassium carbonate in CO2 capture.
CO2 can be produced after the condensation of the steam. The regeneration process is conducted at temperature T and at the same pressure P in the absorber. In the regenerator, the required energy is mainly used to overcome the reaction heat, and the sensible heat of the solid sorbents for the increase in temperature from the absorption temperature to the regeneration temperature. Moreover, the sensible heat of the gas could be avoided because its specific heat capacity is very small. The heat released in the absorber is not taken into account because the absorption temperature is low (60 °C). High-pressure CO2 can be produced when the regenerator is operated at high pressure, which can reduce the compression energy consumption for the CCS process. Therefore, the energy required for process I is evaluated with the assumption that 1 mol CO2 is captured from the flue gas by the K2CO3 sorbent at different pressures. The required energy can be calculated using the following equation:
Q reg ¼ mKHCO3 C p ðT reg T abs ÞNK2 CO3 DH Dc
ð5Þ
where Qreg is the regeneration energy required; Cp is the specific heat capacity of the K2CO3 sorbent; Treg and Tabs are the temperatures of the absorber and regenerator, respectively; N K2 CO3 is the mole number of the K2CO3 sorbent; DH is the reaction heat of (Eq. (2)) (143 kJ mol1-CO2); and Dc is the regeneration extent of KHCO3 in the regenerator. The values of these parameters are shown in Table 1. In Table 2, the temperature of the regenerator (Treg) is the complete decomposition temperature of KHCO3 in a CO2 atmosphere and Dc is assumed to be 99%. The sensible heat of the solid sorbents and the reaction heat can be calculated by the first and second term in the right part of Eq. (5), respectively. Net energy required represents the equivalent energy required for CO2 production at 1 bar. Compression energy required is the estimated energy based on the polytrophic process of the following equations [31]:
W th ¼ n=ðn 1Þ RT ½1 ðP2 =P 1 Þðn1Þ=n
pressures of the gas before and after compression, respectively; and n is the polytropic constant (n is 1.25 for CO2).
Q c ¼ W th =g
where Qc is the compression energy required and g is the compression efficiency (85%, assumed). Overall, the energy consumption for the whole process is approximately 3720 kJ kg1-CO2 under atmosphere pressure. And the energy consumption can be reduced to approximately 3600 kJ kg1-CO2 if the reactors are operated under 1.5 MPa or 2.0 MPa with high-pressure CO2 stream produced, which reduces the compression power for CCS. However, the energy consumption for CO2 capture by aqueous MEA is approximately 4200 kJ kg1CO2 [32], which indicates that the proposed process does not have any obvious advantage over the matured MEA process in terms of system energy consumption. 4.2. Process II: Application of potassium carbonate for CO2 capture
4.2.1. Process description Another process in reducing the energy consumption further is proposed to use the heat released during the formation of K2CO31.5H2O. As shown in Fig. 7, three reactors are needed for process II: one for absorption, one for regeneration and another temporary reactor for the formation of K2CO31.5H2O. In the absorber, K2CO31.5H2O/K2CO3 absorbs CO2 and is converted into KHCO3 at 60 °C/
ð6Þ
where Wth is the theoretical compression work; R is the ideal gas constant; T is the temperature of the gas; P1 and P2 represent the Table 1 Values of parameters in Eq. (5) where 1 mol CO2 was assumed to be captured.
Mass of KHCO3 Specific heat capacity Mole number of K2CO3 Reaction heat to overcome
Symbol
Value
Units
mKHCO3 Cp N K2 CO3 DH
200 1.02 103 1 143
g kJ tg1 K1 mol kJ mol1
ð7Þ
Fig. 7. Process II: application of potassium carbonate in CO2 capture.
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1 bar. The formed KHCO3 is then transported to the regenerator and converted into K2CO3 through Eq. (2) at 170 °C/1 bar with the release of CO2. The K2CO3 produced in the regenerator is transported to the temporary reactor and reacts with the steam to produce K2CO31.5H2O through (Eq. (3)) at 200 °C/1.5 MPa with a significant amount of reaction heat released. 4.2.2. Material balance The assumption is that 1 mol CO2 from the flue gas is captured in the process, and the reactant (K2CO31.5H2O/K2CO3) needed is 1 mol. The product (KHCO3) transported to regenerator is 2 mol. In the regenerator, the sorbent is assumed to be completely regenerated. Therefore, 1 mol K2CO3 is produced in the regenerator and transported to the temporary reactor. Based on the high-pressure experiment result, 70% of K2CO3 is assumed to be converted to K2CO31.5H2O at 200 °C/1.5 MPa in the temporary reactor. Therefore, 0.7 mol K2CO31.5H2O was formed in the temporary reactor. Based on Eq. (3), 1.05 mol steam (200 °C/1.5 MPa) should be supplied to react with K2CO3. For the whole process, solid sorbents are transported and circulated among the three reactors, with the assumption that no loss of sorbents occurs. 4.2.3. Heat balance In the absorber, the reaction between K2CO31.5H2O/K2CO3 and CO2 is exothermic. However, reusing the heat released is difficult given the low temperature (60 °C). In the regenerator, the decomposition of KHCO3 is endothermic. The energy required is mainly used for the sensible heat of KHCO3 from 60 °C to 170 °C and to overcome the reaction heat (DH1). Therefore, the energy consumption in the regenerator (Qreg) can be expressed as the following equation:
Q reg ¼ mKHCO3 Cp1 ðT reg T abs ÞNKHCO3 =2 DH1
ð8Þ
In the temporary reactor, the energy consumption includes the sensible heat of K2CO3 from 170 °C to 200 °C and the energy required to produce the steam (1.5 MPa, 200 °C). The energy consumption in the temporary reactor (Qtemp) can be expressed as the following equation:
Q temp ¼ mK2 CO3 Cp2 ðT temp T reg Þ þ Q steam
ð9Þ
At the same time, K2CO31.5H2O is produced in the temporary reactor with a significant amount of heat released, which can be utilized because the temperature is high. The released heat can be transported by the steam to the regenerator to reduce the energy required for the decomposition of KHCO3. The heat transfer efficiency (f) is assumed to be 0.8. Therefore, the available heat released during the formation of K2CO31.5H2O in the temporary reactor (Qhydrate) can be expressed as the following equation:
Q hydrate ¼ NK2 CO3 DH2 Dc n
ð10Þ
For the whole process, the energy required (Qtotal) includes the energy consumption in the regenerator and the temporary reactor, which can be calculated using the following equation:
Q total ¼ Q reg þ Q temp Q hydrate
ð11Þ
Based on the material and heat balance, the meaning and value of the symbols in Eqs. (8)–(10) are represented in Table 3. In the temporary reactor, 1.05 mol steam (1.5 MPa, 200 °C) should be supplied, which consumes energy to produce steam. The required energy (Qsteam) can be calculated based on the enthalpy of steam (1.5 MPa, 200 °C) compared with the enthalpy of water (1 bar, 30 °C). Assuming that water (1 bar, 30 °C) is the original condition, the pressure of water was initially increased to 1.5 MPa by using the pump. The high-pressure water is then heated and vaporized to produce steam (1.5 MPa, 200 °C). Based
Table 3 Energy consumption calculation for process II.
Mass of KHCO3 Specific heat of KHCO3 Temperature of regenerator Temperature of absorber Temperature of temp reactor Mole number of KHCO3 Reaction heat to overcome Energy required in regenerator Mass of K2CO3 Mole number of K2CO3 Specific heat of K2CO3 Reaction heat to overcome Heat transfer efficiency Conversion of K2CO3 Heat released in temp reactor Energy required for steam Energy required in temp reactor
Symbol
Value
Units
mKHCO3 Cp1 Treg Tabs Ttemp N KHCO3 D H1 Qreg mK2 CO3 N K2 CO3 Cp2 D H2 n Dc Qhydrate Qsteam Qtemp
200 1.02 103 170 60 200 2 143 157.9 138 1 1.02 103 99 0.8 0.7 55.4 49 53.2
g kJ g1 K1 °C °C °C mol kJ mol1 kJ mol1-CO2 g mol kJ g1 K1 kJ mol1
kJ mol1-CO2 kJ mol1-CO2 kJ mol1-CO2
on the enthalpy of H2O at different conditions, the heat required to produce steam (1.05 mol) is estimated to be approximately 49 kJ mol1-CO2. Therefore, based on Eq. (11), the energy consumption for the whole process is estimated to be 155.7 kJ mol1-CO2 or 3539 kJ kg1-CO2. For the system energy consumption, the energy required to overcome the regeneration reaction heat and the sensible heat of the sorbent is 71% and 7%, respectively, which cannot be avoided. On the other hand, the energy required to produce the steam (1.5 MPa, 200 °C) is 22%. The steam (1.5 MPa, 200 °C) can be produced from low grade heat source or utilization of waste heat emissions to reduce the cost in practical processes. Excluding the energy required to produce steam, the least energy required for the potassium-based sorbents is approximately 2425 kJ kg1-CO2 while making use of the heat released during the formation of K2CO31.5H2O. 5. Conclusions The formation condition of K2CO31.5H2O was determined through thermodynamic equilibrium calculation. High-pressure fixed bed experiment was carried out at four representative conditions, namely, 190 °C/1 MPa, 200 °C/1.5 MPa, 215 °C/2 MPa, and 235 °C/3 MPa. The thermodynamic equilibrium calculation result indicates that K2CO31.5H2O was indeed produced through the reaction between K2CO3 and the steam. Approximately 75%, 63%, and 67% of K2CO3 were converted to K2CO31.5H2O in the experiment at 190 °C/1 MPa, 200 °C /1.5 MPa, and 215 °C/2 MPa. In addition, the thermodynamic equilibrium analysis results show that KHCO3 cannot directly react with the steam to produce K2CO31.5H2O with the equilibrium pressure of H2O and CO2 lower than 1 bar. Even at high pressure, the direct formation of K2CO31.5H2O from KHCO3 is difficult because the formation zone of K2CO31.5H2O is narrow. The system energy consumption for potassium carbonate as a CO2 sorbent was analyzed based on the two proposed processes. The energy required for the regeneration of potassium carbonate is approximately 3600 kJ kg1-CO2. One novel process was proposed to reduce the energy consumption and use the heat released during the formation of K2CO31.5H2O. The energy consumption for the whole process is estimated to be 3539 kJ kg1-CO2. The produce of steam covers 22% of the total energy consumption. The steam can be produced from low grade heat source or utilization of waste heat emissions to reduce the cost. Excluding the energy required to produce steam, the least energy required for potassium-based sorbents is approximately 2425 kJ kg1-CO2. However,
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