Potassium Carbonate Slurry-Based CO2 Capture Technology

Sep 15, 2015 - Absorption heat is directly related to energy required for CO2 regeneration. However, other factors such as viscosity, reaction rate, a...
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Potassium Carbonate Slurry-Based CO2 Capture Technology Shiwang Gao,*,† Dongfang Guo,‡ Hongguang Jin,† Sheng Li,† Jinyi Wang,‡ and Shiqing Wang‡ †

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Haidian District, Beijing 100190, China Huaneng Clean Energy Research Institute, Changping District, Beijing 102209, China

‡

ABSTRACT: Carbonate slurry process is considered to be one of the potential technologies for large-scale CO2 capture from coal-based power stations, as it can reduce the participation of water in stripper. In this work, constant volume method was adopted to measure the CO2 loading in aqueous potassium carbonate solutions at the temperatures of 40, 70, and 120 °C for CO2 partial pressure from 0.4 to 240 kPa. Absorption heats of CO2 in aqueous solutions of K2CO3, MEA, and MDEA were measured and compared using true heat flow method. The cyclic CO2 equilibrium loading increases with K2CO3 concentration increase, which is a benefit for CO2 capture. However, the increased viscosity leads to a decrease of the apparent absorption rate. Absorption heat depends on temperature, K2CO3 concentration, and CO2 loading. Compared to MEA, potassium carbonate solution has a lower absorption heat. Generated potassium bicarbonate crystallization can enhance CO2 absorption but also increase absorption heat. Slurry desorption can reduce 34% regeneration energy and 37% cooling duty of CO2-rich gas compared to the traditional carbonate process without precipitation at the same regeneration condition of temperature elevation from 70 to 130 °C. crystallization, and the concentrated KHCO3 slurry is regenerated for the purpose of lowering water involvement during regeneration, which is expected to significantly reduce the energy cost for desorption. Anderson et al.12 studied the K2CO3 slurry-based CO2 capture process by commercial simulation tool ASPEN. Simulation results indicated that the desorption energy of the K2CO3 slurry-based CO2 capture process is within 2.0−2.5 GJ/(t of CO2) when the CO2 concentration in coal-fired flue gas is 11% and CO2 capture efficiency is 90%. Normally, the desorption energy is about 4.0GJ/(t of CO2) for conventional MEA absorption process. Smith et al.13 conducted a bench-scale test on the K2CO3 slurry-based CO2 capture process and studied the performance parameters such as pressure drop, liquid holdup, and CO2 removal efficiency. Studies on thermodynamics and kinetics of CO2 absorption in concentrated K2CO3 solution are quite limited. Experimental data for CO2 loading and absorption heat are not available. This work measured equilibrium CO2 loading and heat of absorption in K2CO3 solutions in different concentrations, utilizing a gas− liquid equilibrium reactor and a real heat flux method, which provides critical basic parameters for design of the K2CO3 slurry-based CO2 capture process and assessment of process energy consumption.

Chemical absorption process is widely used for removal of acidic gases such as CO2 and H2S. Meanwhile, it is the most mature technology for CO2 emission reduction at present, and it is expected to be a prospective technology for large-scale commercialization.1−4 In conventional chemical absorption process, the capture and separation of CO2 from gas mixture is achieved due to the variation of CO2 solubility at different temperatures. Specifically, absorption process occurs at low temperature and desorption occurs at high temperature. Common chemical absorbents include ethanolamine, carbonate, amino acid salt, piperazine, and their derivatives.4,5 Previous studies show that the operation cost of a power generation unit will increase 0.212 RMB/(kW/h) if monoethanolamine (MEA) is used as the solvent for CO2 removal in the power plant, which is about 70−80% higher than that of a conventional power generation unit.6 Chemical absorption process, as illustrated in Figure 1, requires more energy for CO2 regeneration and results in higher operation cost,2,7 which is mainly due to the high water content in the solvent (usually above 70%). A large part of the energy is consumed by heating and evaporating the water in the solvent. Therefore, developing a new absorption process and reducing the water involvement in the regeneration process are effective approaches to lowering the energy cost. K2CO3 solution is widely used for CO2 capture in ammonia synthesis, hydrogen production, and natural gas industries.8−11 K2CO3 mass fraction in conventional hot potassium carbonate process is normally 30%, and the full process only involves gas phase and liquid phase flows. The absorption reaction is shown as follows: CO2 (g) + H 2O + K 2CO3 → 2KHCO3

1. EXPERIMENTAL SETUP 1.1. Gas−Liquid Equilibrium Reactor. Constant volume method is used to determine how much gas is absorbed by measuring the volume of gas before and after absorption, which can avoid disturbing the established gas−liquid equilibrium due to gas phase and liquid phase sampling. This method has many other advantages such as high precision, simple operation, and analysis.14−16

(1)

K2CO3 slurry-based CO2 absorption technology is a CO2 capture process that involves gas, solid, and liquid phases (Figure 2). The solubility difference between K2CO3 and KHCO 3 allows for deposit of KHCO3 by means of © 2015 American Chemical Society

Received: June 29, 2015 Revised: September 5, 2015 Published: September 15, 2015 6656

DOI: 10.1021/acs.energyfuels.5b01421 Energy Fuels 2015, 29, 6656−6663

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Figure 1. Traditional CO2 capture process by chemical absorption.

Figure 2. Novel CO2 capture process by carbonate slurry. Constant volume method is used to measure CO2 equilibrium loading in this work. As shown in Figure 3, the device is mainly composed of a gas−liquid reactor, a gas holder, a data acquisition system, and other supported equipment. The gas−liquid reactor is thread hermetically sealed, and the temperature is controlled by electric heating. A gas−liquid agitator is installed inside the reactor.

The gas holder is used to store CO2 from cylinders, and its temperature is controlled by electric heating. Reactor and gas holder are both fabricated by 316L stainless steel. The temperatures are measured by resistance thermometer Pt100, and the pressures are measured by a diffused silicon pressure sensor. Parameters such as temperature, pressure, and agitating speed are acquired by online and real-time measurements that are controlled by computers. During the test, nitrogen is first used to purge the reactor, and then a certain volume of K2CO3 solution with known concentration is injected into the reactor and kept at a constant temperature for approximately 10 h after which the solution can be considered to be in gas−liquid equilibrium status. The corresponding pressure is assumed as the initial pressure (Pv) of the system. Then, CO2 is injected into reactor from the gas holder and reacted with K2CO3 in solution. The pressure in the reactor continuously drops until a new gas−liquid equilibrium is reached, and the pressure is indicated as PT. The partial pressure of CO2 in the gas phase at equilibrium, PE, is calculated by the following equation: pE = pT − pV

(2)

nCO2 and ngCO2 are the molar quantities of CO2 in the gas holder before and after the CO2 injection, which can be calculated by the PR equation of state. CO2 loading in solution is calculated by the following equation:

Figure 3. Diagram of vapor−liquid equilibrium apparatus. 6657

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Figure 4. Diagram of reaction calorimeter apparatus.

αCO2 =

g nCO2 − nCO 2

nK2CO3

The integral heat (−ΔHint) is defined as the ratio of the total heat released to the total CO2 absorbed (kJ/(mol of CO2)) during the absorption process, starting from the first CO2 injection. The differential heat (−ΔHdiff) is defined as the ratio of the heat released to CO2 absorbed during single CO2 injection (kJ/(mol of CO2)).19 In the desorption test on rich solution, a condenser is provided for the calorimeter. The heat fluxes of both hot flow and cold flow are recorded at the same time during the desorption process.

(3)

where nK2CO3 refers to the initial molar quantity of K2CO3. CO2 equilibrium loading includes both dissolved CO2 and reacted CO2. 1.2. True Heat Flux Calorimeter. A true heat flux calorimeter is used to measure the heat of absorption and heat of desorption of CO2 in K2CO3 solution and ethanolamine solution.17−19 A true heat flux calorimeter can conduct real-time monitoring and recording on the heat flux, temperature, and pressure during the process of reaction. In this work, a ChemsiSens CPA201 calorimeter is used to precisely measure the heat variation in the reactor. The reactor in the CPA201 calorimeter has an effective volume of 250 mL. The baseplate and head are fabricated by 316L stainless steel, and a blade agitator is installed inside. The reactor has a two-layer glass wall with a vacuum inside. During testing, the reactor is submerged into a liquid at constant temperature. The liquid temperature is properly controlled to be always consistent with the temperature in the reactor in the process of reaction. As shown in Figure 4, the calorimeter setup includes a CPA201 calorimeter, a VCR202 dose controller, a gas mass flow controller, and automatic process control software. When measuring the absorption heat of CO2 in K2CO3 solution, the heat recorded by the true heat flux calorimeter, Q0, includes the following parts: (1) heat required to increase the CO2 temperature from room temperature to reaction temperature, Q1, Q 1 = cm(T2 − T1)

2. RESULTS AND DISCUSSION 2.1. Equilibrium CO2 Loading in K2CO3 Solution. Tables 1 and 2 give the CO2 equilibrium loading in 30% and Table 1. CO2 Equilibrium Loading in 30% K2CO3 Solutions 40 °C

(4) −1

−1

where c refers to the specific heat of CO2 (J·g ·K ), m refers to the mass of CO2 injected (g), T2 refers to reaction temperature, and T1 refers to room temperature; (2) heat generated by adiabatic compression, Q2,

Q 2 = ΔpV

Q − Q1 − Q2 Q = 0 nabs nabs

120 °C

PCO2/kPa

αCO2

PCO2/kPa

αCO2

PCO2/kPa

αCO2

0.57 0.90 1.26 1.72 2.32 3.36 5.52 7.48 10.75

0.28 0.332 0.372 0.409 0.452 0.498 0.574 0.647 0.710

0.37 1.99 4.53 10.33 23.35 40.85 83.43

0.133 0.278 0.383 0.503 0.629 0.719 0.812

6.8 15.9 28.3 39.3 56.1 72.8 105.1 134.3 154.2 185.3 216.3 237.9

0.140 0.235 0.292 0.351 0.417 0.481 0.546 0.592 0.622 0.655 0.681 0.698

50% K2CO3 solutions, respectively, which are measured by constant volume method (solution concentration refers to mass concentration unless otherwise specified) and agree with literature’s data by Tosh et al.20 As illustrated in Figure 5, if the K2CO3 concentration remains constant, the equilibrium curve moves toward the left and CO2 loading decreases when the temperature increases. The CO2 is captured and separated by absorbing CO2 at low temperature and desorbing CO2 at high temperature in the K2CO3 slurry-based CO2 capture process. At 40 and 70 °C, the equilibrium CO2 loading curve for 50% K2CO3 solution moves toward the right compared to that of 30% K2CO3 solution, which indicates that the CO2 loading increases with K2CO3

(5)

where Δp refers to the pressure change in reactor (Pa) and V refers to the gas phase volume in the reactor (m3); (3) heat generated by physical dissolution of CO2 in solution; (4) heat generated by chemical reactions between CO2 and K2CO3; (5) heat generated by KHCO3 crystallization. Absorption heat (−ΔHabs) is defined as −ΔHabs =

70 °C

(6)

where Q (kJ) refers to the heat released due to absorption reaction in the reactor and nabs (mol) refers to absorbed CO2. 6658

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Energy & Fuels Table 2. CO2 Equilibrium Loading in 50% K2CO3 Solutions 40 °C

70 °C

120 °C

PCO2/kPa

αCO2

PCO2/kPa

αCO2

PCO2/kPa

αCO2

0.36 0.51 0.75 1.64 4.08

0.448 0.531 0.613 0.693 0.772

0.37 0.94 3.50 7.14 15.07 28.02 59.74

0.347 0.421 0.496 0.580 0.659 0.730 0.790

11.5 21.6 29.7 42.6 57.3 74.2 88.2 110.8 135.8 166.3

0.163 0.219 0.266 0.323 0.374 0.415 0.456 0.497 0.533 0.567

concentration under constant CO2 partial pressure. At 120 °C, CO2 equilibrium loading curve for 50% K2CO3 solution moves toward the left, which indicates that CO2 loading is small at high temperature. Results show that a solution with high K2CO3 concentration can provide high CO2 equilibrium loading. However, the viscosity of the solution will increase and other physical properties such as surface extension, heat capacity, and crystallization effect will change when increasing the K2CO3 concentration. All of these factors will impose an impact on process parameters such as apparent absorption rate and heat load. 2.2. Effect of K2CO3 Concentration on CO2 Absorption Rate. Pressure changes during the CO2 absorption in 30% and 50% K2CO3 solution at 70 °C are given in Figure 6, where the vertical coordinate, p/p0, expresses the ratio of the reactor pressure to the initial pressure after injection of CO2. As illustrated in Figure 6, the apparent CO2 absorption rate of K2CO3 solution with a concentration higher than 30% is lower than that of conventional K2CO3 solution which has a concentration lower than 30%. This finding is consistent with the studies by Knuutila et al. 21,22 Increasing reagent concentrations should lead to higher reaction rate. However, we found that CO2 absorption rate decreases with K2CO3 concentration increase. It should be caused by the increased

Figure 6. Pressure change during CO2 absorption.

viscosity under higher K2CO3 concentration condition, which would lead to the diffusion coefficient of CO2 into the liquid phase increasing significantly and result in a lower CO2 absorption rate. Therefore, it is critical to select the proper activator to increase the absorption rate for the K2CO3 slurrybased CO2 capture process. Development of absorber packing with high performance can also increase CO2 absorption efficiency. 2.3. Absorption Heat of CO2 in K2CO3 Solution. The pressure and heat flux recorded by the calorimeter in the process of CO2 absorption in 30% K2CO3 solution at 40 and 70 °C are given in Figure 7 and 8, respectively. A 1 g amount of CO2 is injected each time. The valley point on the pressure and heat flux curves can be approximately considered as the reaction equilibrium point after which CO2 is injected again. At 40 °C (Figure 7), the peak shape of the heat flux diverges after the seventh CO2 injection, and the peak value of the heat flux increases significantly. Crystallization is observed in the reactor of the calorimeter, and the corresponding valley point of the pressure drops significantly, which indicates that crystallization enhances CO2 absorption. At 70 °C (Figure 8), the valley point

Figure 5. CO2 equilibrium loading in aqueous K2CO3 solutions. Tosh et al. data taken from ref 20. 6659

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Table 3. Heat of CO2 Absorption into Different Solutions at 40 °Ca 30% K2CO3

Figure 7. Pressure and heat flow during CO2 absorption into 30% potassium carbonate solutions at 40 °C.

30% MDEA

30% MEA

α

−ΔHint

α

−ΔHint

α

−ΔHint

0.076 0.157 0.238 0.318 0.400 0.481 0.567 0.648 0.727 0.804

55.0 49.6 47.4 46.4 45.5 44.9 49.2 51.3 52.8 53.8

0.090 0.173 0.256 0.340 0.423 0.506 0.589 0.668 0.743 0.812

67.4 67.5 67.3 66.8 66.6 66.3 65.9 65.7 65.5 65.2

0.045 0.090 0.133 0.174 0.219 0.265 0.311 0.354 0.398 0.441 0.484 0.525

107.9 101.4 100.9 101.2 99.7 98.6 97.3 97.3 96.9 96.5 94.9 92.3

a Note: α, CO2 loading (mol of CO2/(mol of K2CO3)) ; −ΔHint, integral heat (kJ/(mol of CO2)).

Figure 8. Pressure and heat flow during CO2 absorption into 30% potassium carbonate solutions at 70 °C. Figure 9. Integral heat of CO2 absorption into different solutions (30% concentration).

on the pressure curve rises after each CO2 injection, and the peak value of the heat flux drops gradually. No crystallization is observed during the entire absorption process. A true heat flux calorimeter is used to measure the absorption heat of CO2 in MEA, MDEA, and K2CO3 solutions. A comparison of the measured absorption heat of CO2 in these three solutions is given in Table 3 and Figure 9. It is obvious that the absorption heat is the smallest in the K2CO3 solution. At 40 °C, the absorption heat of CO2 in 30% K2CO3 solution drops first and then rises, which is due to the heat released from crystallization. In the case without crystallization, the absorption heat of CO2 in K2CO3 and ethanolamine solutions decreases with increasing CO2 loading. The integral heat and differential heat of CO2 absorption in K2CO3 solutions with different concentrations at 70 °C are given in Table 4 and Figure 10. Under constant temperature, both integral heat and differential heat increase with increasing K2CO3 concentration. For 40% and 50% K2CO3 solutions, the integral heat first drops and then increases with increasing the CO2 loading; the differential heat increases suddenly and then drops gradually with increasing CO2 loading, but it still maintains at a higher level. This is caused by the heat released from KHCO3 crystallization in the process of CO2 absorption in highly concentrated K2CO3 solution.

Absorption heat is directly related to energy required for CO2 regeneration. However, other factors such as viscosity, reaction rate, and CO2 loading should also be considered when selecting absorbents. In the concentrated K2CO3 slurry-based CO2 capture process, CO2 loading in the solution cycle is increased, less solution is cycled, and less water is involved in CO2 regeneration, but the viscosity of the solution is increased, more cooling water is required due to crystallization heat, and additional heat is required for dissolving the crystals in the desorption process. All of these factors should be considered for assessment of operating cost. 2.4. Desorption Heat of Rich Solvent. During the CO2 desorption process, external heat is needed to increase the rich solvent, maintain its temperature at a relatively high level, and compensate the heat consumed by endothermic desorption reaction. The condenser at the top of the stripper is used to cool the steam and desorbed CO2. The true heat flux method is used to simulate the CO2 regeneration process. The effect of various desorption methods on regeneration energy is analyzed and compared. The process of preparing rich solvent is similar to that of the absorption heat measurement. Rich solvent for desorption 6660

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Energy & Fuels Table 4. Heat of CO2 Absorption into K2CO3 Solutions at 70 °Ca 30% K2CO3

a

40% K2CO3

50% K2CO3

α

−ΔHdiff

−ΔHint

α

−ΔHdiff

−ΔHint

α

−ΔHdiff

−ΔHint

0.076 0.158 0.239 0.320 0.400 0.480 0.558 0.634

58.7 44.4 44.3 41.1 41.4 40.8 41.0 40.6

58.7 51.3 48.9 46.9 45.8 45.0 44.4 44.0

0.077 0.156 0.235 0.313 0.391 0.470 0.548 0.625 0.700 0.773

53.1 50.6 51.1 50.8 60.2 75.8 74.4 72.6 70.4 67.6

53.1 51.9 51.6 51.4 53.2 57.0 59.5 61.1 62.1 62.6

0.078 0.157 0.235 0.314 0.392 0.470 0.548 0.627

75.0 68.4 90.8 98.5 96.9 90.7 84.4 77.6

75.0 71.7 78.0 83.2 85.9 86.7 86.4 85.3

Note: α, CO2 loading (mol of CO2/(mol of K2CO3)) ; −ΔHdiff, differential heat (kJ/(mol of CO2)); −ΔHint, integral heat (kJ/(mol of CO2)).

Figure 11. Heat curves of CO2-rich-solution desorption. Figure 10. Heat of CO2 absorption into K2CO3 solutions with different concentrations.

experiments is prepared based on CO2 loading under simulated flue gas conditions, and then it is heated for the desorption test. The rich solvent with a K2CO3 concentration of 40% and a CO2 loading of 0.418 is used for the regeneration test. Such a CO2 loading is consistent with the equilibrium loading under simulated flue gas conditions. In the K2CO3 slurry-based CO2 capture process, the CO2-rich crystal slurry which is produced by crystallization of such a rich solvent at 35 °C. Figure 11 and Figure 12 show the heat flux recorded during the process of CO2 desorption from the rich solvent and from the crystal slurry, respectively. For CO2 desorption from the crystal slurry, the water involvement is reduced since the water content is lowered. The effect of reactor pressure on cooling water heat flux is also reduced. Therefore, the effect of pressure on required regeneration heat flux is very small in the K2CO3 slurry-based CO2 capture process. Table 5 gives the data of heat recoded during the process of CO2 desorption from rich solvent and from crystal slurry, respectively. No. 1 is the data obtained from the first desorption test and CO2 being released; no. 2 is the data obtained from the desorption test after the first test and no CO2 being released. When temperature is increased from 70 to 130 °C, the heat required for heating the crystal slurry is 36080 J, which is lower than the heat required for heating the rich solvent (50884 J). During the heating process, the cooling duty for stripped gases for both cases is very small. During a period of 4060 s, the integral heat of CO2 desorption from the crystal slurry is 39666

Figure 12. Heat curves of slurry desorption.

J which is 34% lower than the integral heat of CO2 desorption from the rich solvent (60086 J). Meanwhile, the cooling duty for the stripped gases is reduced by 37%. These data indicate that water involvement is reduced and desorption of the crystal slurry significantly reduces the energy required for CO2 regeneration and cooling duty for stripped gases. For both desorption methods, the second desorption (no. 2) process has a faster heating rate, lower energy cost, and lower cooling duty compared to the first desorption process (no. 1), which also indicates that the reduction of water involvement in the regeneration process can reduce the energy cost effectively. 6661

DOI: 10.1021/acs.energyfuels.5b01421 Energy Fuels 2015, 29, 6656−6663

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Energy & Fuels Table 5. Heat of CO2 Desorption Measured by Calorimeter Apparatus no.

start time/s

end time/s

Δtime/s

heat input/J

condensation/J

sum/Ja

1

1680 1680 1980 1980 2440 2440 2660 2660

3360 5740 3610 6040 4140 6500 4310 6720

1680 4060 1630 4060 1700 4060 1650 4060

−50884 −60086 −50173 −50532 −36080 −39666 −35301 −35995

782 12073 448 1757 926 7616 933 3000

−50102 −48013 −49724 −48775 −35153 −32050 −34368 −32995

rich solution desorption

2 crystal slurry desorption

1 2

a

note 70−130 70−130 70−130 70−130 70−130 70−130 70−130 70−130

°C °C, °C °C, °C °C, °C °C,

then maintain some time then maintain some time then maintain some time then maintain some time

Note: Sum, heat input plus condensation. (2) Fei, W.; Ai, N.; Chen, J. Capture and Separation of Greenhouse Gases CO2The Challenge and Opportunity for Separation Technology. Chem. Ind. Eng. Prog. 2005, 24 (01), 1−4 (in Chinese).. (3) Figueroa, J. D.; Fout, T.; Plasynski, S.; et al. Advances in CO2 capture technology-The U.S. Department of energy’s carbon sequestration program[J]. Int. J. Greenhouse Gas Control 2008, 2 (1), 9−20. (4) International energy outlook 2004, DOE/EIA-0484; U. S. Department of Energy: Washington, DC, USA, 2004. (5) Dongfang, G. U. O.; Jinyi, W.; da Silva, G.; Shiwang, G. A. O. Reactivity and Mechanism Study of CO2 With Amino Acids as Carbon Capture Solvents. Proce. CSEE (in Chinese) 2013, 33 (32), 29−33. (6) Liu, Y.; Zhu, L.; Yan, W. Economic Assessment for the CO2 Capture Technologies Applied in the Coal-firing Power Plant[J]. Proc. CSEE 2010, S1, 59−64. (7) Feng, Q. I. N.; Shujuan, W.; Svendsen, H. V. F.; Changhe, C. Research on heat requirement of aqua ammonia regeneration for CO2 capture. CIESC J. 2010, 05, 1233−1240. (8) Rochelle, G. T.; Seibe,R. T. F.; Closmann, F., et al. CO2 Capture by Absorption with Potassium CarbonateFinal Roport; The University of Taxas at Austin and the University of Regina: Austin, TX, USA, and Regina, Saskatchewan, Canada, 2007. (9) Guo, D.; Thee, H.; da Silva, G.; et al. Borate-Catalyzed Carbon Dioxide Hydration via the Carbonic Anhydrase Mechanism. Environ. Sci. Technol. 2011, 45 (11), 4802−4807. (10) Dongfang, G. U. O.; Shiwang, G. A. O.; Ming, C. A. I.; Jian, C.; Weiyang, F. E. I. Effect of Borate Catalysis on CO2 Absorption by Potassium Carbonate Solutions. Proc. CSEE 2014, 34 (11), 1741− 1747. (11) MUMFORD, K. A.; SMITH, K. H.; ANDERSON, C. J.; et al. Post-combustion capture of CO2: Results from the solvent absorption capture plant at hazewood power station using potassium carbonate solvent [J]. Energy Fuels 2012, 26, 138−146. (12) Anderson, C.; Harkin, T.; Ho, M.; et al. Developments in the CO2CRC UNO MK3 process: A multi-component solvent process for large scale CO2 capture. Energy Procedia 2013, 37, 225−232. (13) Smith, K.; Lee, A.; Mumford, K.; Li, S.; Indrawan; Thanumurthy, N.; Temple, N.; Anderson, C.; Hooper, B.; Kentish, S.; Stevens, G. Pilot plant results for a precipitating potassium carbonate solvent absorption process promoted with glycine for enhanced CO2 capture. Fuel Process. Technol. 2015, 135, 60−65. (14) Dong, L.; Chen, J.; Gao, G. Solubility of carbon dioxide in aqueous solutions of 3-amino-1-propanol. J. Chem. Eng. Data 2010, 55, 1030−1034. (15) Ai, N.; Chen, J.; Fei, W. Solubility of carbon dioxide in four mixed solvents. J. Chem. Eng. Data 2005, 50, 492−496. (16) Ma'mun, S.; Nilsen, R.; Svendsen, H. F.; Juliussen, O. Solubility of carbon dioxide in 30 mass % monoethanolamine and 50 mass % methyldiethanolamine solutions. J. Chem. Eng. Data 2005, 50, 630− 634. (17) Kierzkowska-Pawlak, H.; Zarzycki, R. Calorimetric measurements of CO2 absorption into aqueous N-methyldiethanolamine solutions. Chem. Pap. 2002, 56 (4), 219−227.

In the conventional process, the temperature of rich solvent from absorber is approximately 70 °C. In the K2CO3 slurrybased CO2 capture process, rich solvent is cooled to about 30− 40 °C for the purpose of crystallization. A lower temperature will generate more crystal, but it requires larger cooling duty during the crystallization process and higher heating duty during the heating process before entering the stripper. Therefore, the optimization of the heating duty and cooling duty in the K2CO3 slurry-based CO2 capture process is of critical importance. Related studies, such as heat recovery from the lean solvent at the bottom of the stripper and from the condensed liquid of the reboiler, are still needed to be done.

3. CONCLUSIONS Gas−liquid equilibrium reactor and true heat flux method are used to measure equilibrium CO2 loading and absorption heat in K2CO3 solutions with different concentrations. The major conclusions of this work are as follows: (1) When increasing the K2CO3 concentration, the CO2 equilibrium loading will increase during absorption and decrease during desorption. With higher K2CO3 concentration, the CO2 loading in the solution cycle is larger and less solution is cycled, which is better for CO2 capture and separation. (2) Under high K2CO3 concentration condition, the viscosity of solution will increase and the dispersion coefficients of components will decrease with increasing K2CO3 concentration, resulting in a lower CO2 absorption rate. (3) Absorption heat of CO2 in K2CO3 solution is affected by temperature, concentration, CO 2 loading, and KHCO 3 crystallization. The heat generated by KHCO3 crystallization will cause the increase of absorption heat. Absorption heat of CO2 in K2CO3 solution is lower than that in ethanolamine solvent such as MEA. (4) When the solvent temperature is increased from 70 °C to desorption temperature 130 °C, the desorption energy can be reduced by 34% and the cooling water for stripped gases can be reduced by 37% when the CO2 is desorbed from crystal slurry instead of conventional rich solvent.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-89181102. Fax: +86-10-89181000. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.5b01421 Energy Fuels 2015, 29, 6656−6663

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DOI: 10.1021/acs.energyfuels.5b01421 Energy Fuels 2015, 29, 6656−6663