Environ. Sci. Technol. 2003, 37, 1670-1675
Effect of Steric Hindrance on Carbon Dioxide Absorption into New Amine Solutions: Thermodynamic and Spectroscopic Verification through Solubility and NMR Analysis JUNG-YEON PARK, SANG JUN YOON, AND HUEN LEE* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea
Acid gas absorption technology is of great importance in these days for the prevention of global warming and the resulting worldwide climate change. More efficient process design and development for the removal of acid gases has become important, together with the development of new absorbents as one of urgent areas of research in addressing global-warming problems. In the present work, aqueous solutions of 2-amino-2-hydroxymethyl-1,3propanediol (AHPD), a sterically hindered amine, has been examined as a potential CO2 absorbent and compared with the most commonly used absorbent, monoethanolamine (MEA) solution, through equilibrium solubility measurements and 13C NMR spectroscopic analyses. The solubilities of CO2 in aqueous 10 mass % AHPD solutions were higher than those in aqueous 10 mass % MEA solutions above 4 kPa at 298.15 K, but below 4 kPa, the solubility behavior appeared to be the opposite. The solubility difference between these two solutions increased with the CO2 partial pressures above the crossover pressure. Equilibrated CO2-MEAH2O and CO2-AHPD-H2O solutions at various CO2 partial pressures ranging from 0.01 to 3000 kPa were analyzed by 13C NMR spectroscopy to provide a more microscopic understanding of the reaction mechanisms in the two solutions. In the CO2-amine-H2O solutions, amine reacted with CO2 to form mainly the protonated amine (AMH+), bicarbonate ion (HCO3-), and carbamate anion (AMCO2-), where the quantitative ratio of bicarbonate ion to carbamate anion strongly influenced the CO2 loading in the amine solutions. A profusion of bicarbonate ions, but a very small amount of carbamate anions, was identified in the CO2-AHPD-H2O solution, whereas a considerable amount of carbamate anions was formed in the CO2-MEAH2O solution. AHPD contains more hydroxyl groups than nonhindered MEA, and hence, the chemical shifts in its 13C NMR spectra were strongly influenced by the solution pH values. In contrast, MEA appeared to be insensitive to pH. The strong interrelations among CO2 solubility, CO2 partial pressure, bulkiness of the amine structure, and pH identified through the present experimental investigations can provide basic guidelines for finding new potential organic * Corresponding author phone: +82-42-8693917; fax: +82-428693910; e-mail:
[email protected]. 1670
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absorbents, including specifically designed amine chemicals.
Introduction Various alkanolamines have been used for the bulk removal of CO2 from gas streams through absorption processes. In particular, monoethanolamine (MEA) has been the most widely adopted as a liquid absorption agent. CO2-alkanolamine-H2O solutions at equilibrium generally go through several reactions and form the carbamate anion (AMCO2-), protonated cation (AMH+), bicarbonate ion (HCO3-), and carbonate ion (CO32-) (1).
Moreover, it has been well recognized that the relative formation of carbamate anion and bicarbonate ion has a crucial effect on the solubility of CO2 in aqueous alkanolamine solutions. The more bicarbonate ions form in equilibrium CO2-alkanolamine-H2O solutions, the more free amines exist, and these free amines are able to react with CO2 molecules again, which finally leads to a remarkable enhancement in the CO2 loading capacity. The overall reaction stoichiometry indicates that 2 mol of amine are required per mole of CO2 reacted for the carbamate anion, whereas a one-to-one ratio is required for the bicarbonate ion (2). The degree of hydrolysis of the carbamate anion is determined by reaction parameters such as the amine concentration, solution pH, and chemical stability of the carbamate anion (3-5). Recently, the sterically hindered amine of 2-amino-2methyl-1-propanol (AMP) has been chosen as a potential candidate absorbent recommended for its high CO2 loading capacity and easy regeneration. The steric hindrance of the bulky substituent adjacent to the amino group of the sterically hindered amine lowers the stability of the carbamate anion formed by the CO2-amine reaction (6). Eventually, an aqueous solution of hindered amine reacts with CO2 to form mostly bicarbonate ions and shows a high solubility of CO2. It must be noted, however, that the sterically hindered amines do not show a higher CO2 loading capacity than conventional amines under all conditions. Some published literature reports have shown that the solubilities of CO2 in aqueous solutions of CO2 and hindered amines (such as AMP and AEPD) are even lower than those in CO2-MEA-H2O solutions below a certain CO2 partial pressure (7, 8). There are certain discrepancies between this experimental phenomenon and the theory that hindered amines have excellent CO2 loading capacities because of their structures. The present study has started to examine this issue closely. Actual processes for removing CO2 are run at atmospheric pressure or at CO2 partial pressures even lower than atmospheric pressure, so a high solubility of carbon dioxide at lower pressure is an essential factor for a good absorbent. Therefore, this work will provide some technological insight into the identification or design of economical acid gas absorbents to prevent global warming effects. 10.1021/es0260519 CCC: $25.00
2003 American Chemical Society Published on Web 03/11/2003
FIGURE 1. (a) High-pressure NMR sample tube. (b) Connection setup for 1/8-in.-o.d. vacuum tubing. In this work, 2-amino-2-hydroxymethyl-1,3-propanediol (AHPD), a sterically hindered amine, was chosen as a new CO2 absorption agent, and the solubilities of CO2 in AHPDH2O and MEA-H2O solutions were measured to compare the CO2 loading capacities. The carbamate anion and bicarbonate ion of the equilibrated CO2-AHPD-H2O and CO2-MEA-H2O solutions were identified by 13C NMR spectroscopy. All experiments were performed at 298.15 K and at CO2 partial pressures ranging from 0.01 to 3000 kPa. Some studies (9-11) involving NMR analysis of CO2-amineH2O solutions have previously been published, but no available data are for CO2 partial pressures higher than atmospheric pressure. In the present study, we first examined the spectroscopic behavior of CO2 absorption in the range of high CO2 partial pressures, as well as at low pressures, including atmospheric, and interrelated this behavior with the corresponding macroscopic properties.
Experimental Section Equilibrium Solubility Measurements. Materials. Aqueous alkanolamine solutions were prepared with distilled water, >99.5% monoethanolamine (MEA), and >99% 2-amino-2hydroxymethyl-1,3-propanediol (AHPD) from Aldrich Chemical Co. All chemicals were used without further purification. The carbon dioxide and nitrogen gases were of commercial grade with a purity of 99.9%. Apparatus and Procedure. A schematic diagram of the apparatus used for making samples of the equilibrated CO2amine-H2O solutions for both solubility and NMR analyses is shown in our previous work (12). The apparatus consisted of an equilibrium cell; a gas chromatograph; a pressure gauge; a thermometer; a circulation pump; valves; and cylinders of carbon dioxide, nitrogen, and helium. The equilibrium cell was made of 316 stainless steel with an internal volume of about 450 mL. This cell was connected to a gas chromatograph (Hewlett-Packard, 5890 Series II Plus) by a vapor sampling valve (Rheodyne, 7410) with a loop of about 500 µL to determine the composition of the vapor phase. The column installed in the GC was 1.8 m long with an internal diameter of 3 mm and was packed with Porapak Q. For GC analysis, helium was used as the carrier gas at a flow rate of 30 mL/min, and a thermal conductivity detector (TCD) was used. The equilibrium cell was immersed in a water bath maintained within (0.1 °C of the set-point temperature by a refrigerator/heater (Jeio Tech, RBC-20). The actual solution temperature in the equilibrium cell was measured by a K-type thermocouple with an accuracy of (0.1 K. To measure the system pressure, a Heise gauge (CM 118324, 0-3500 kPa
range) was used with an accuracy of (0.1% of the gauge range. The cell, filled with 300 mL of aqueous amine solution, was initially completely purged with nitrogen gas to remove the remaining air. A sufficient amount of CO2 was then directly fed into the cell until the CO2 partial pressure approached the desired system pressure. In particular, at CO2 partial pressures lower than atmospheric pressure, a certain amount of nitrogen was added to enhance the total solution pressure. The relative compositions of the mixed gas were accurately determined using a gas chromatograph. When the system pressure and vapor composition stopped changing, the liquid and vapor phases in the cell were considered to have reached an equilibrium state. The liquid samples withdrawn from the equilibrium cell were analyzed by the titration method as described in the literature (13). High-Pressure 13C NMR Spectroscopy. 13C NMR analyses on equilibrated samples of the CO2-amine-H2O solutions were performed at 298.15 K and various CO2 partial pressures using a Bruker AMX FT 500 MHz NMR spectrometer. The normal 5-mm-o.d. NMR sample tubes obtained from Newera Co. were used at CO2 partial pressures lower than atmospheric pressure, whereas high-pressure NMR sample tubes from Wilmad Glass Co. were used at high CO2 partial pressures. The high-pressure NMR sample tube is shown in Figure 1. It was designed to be connected to a 1/8-in. stainless steel vacuum line using SwageLok fittings and had a 5-mm outer diameter, 1.4-mm wall, 0.18-m length, and 3500 kPa maximum pressure. Each tube was supplied with a PV-ANV valve made out of Teflon, and all other parts were Pyrex or equivalent glass, thus ensuring chemical resistivity. The valve was opened simply by being turned counterclockwise, so the tubes could be kept under vacuum or pressure for a considerable length of time. Quantitative spectra were obtained by using Bruker inverse-gated decoupling (decoupled spectrum without NOE) with a delay of 30 s and a pulse width of 9 µs. The number of scans (NS) was 20003000 so that accurate peaks could be obtained in the NMR spectra.
Results and Discussion The main objective of the present study is to estimate thermodynamic characteristics of an aqueous AHPD solution as a potential CO2 absorbent and to investigate its reaction mechanism with CO2 over various CO2 partial pressures. To estimate the CO2 loading capacity, the equilibrium solubilities of CO2 in aqueous AHPD solutions were measured and compared with those in aqueous MEA solutions. In particular, two important species in the CO2-AHPD-H2O solutions, VOL. 37, NO. 8, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Solubilities of Carbon Dioxide in Aqueous 10 Mass % MEA and AHPD Solutions at 298.15 K 10 mass % MEA
10 mass % AHPD
P (kPa)
r (mol of CO2/ mol of MEA)
P (kPa)
r (mol of CO2/ mol of AHPD)
0.07 0.30 3.30 7.10 19.8 49.7 96.8 303.7 855.3
0.408 0.464 0.577 0.622 0.665 0.731 0.769 0.826 0.913
0.90 3.30 9.20 19.0 96.8 401.6 967.6 2427.3
0.454 0.615 0.750 0.865 1.181 1.397 1.548 1.649
FIGURE 2. Solubility of carbon dioxide in aqueous solutions of 10 mass % alkanolamines at 298.15 K. namely, bicarbonate ion and carbamate anion, were identified by 13C NMR spectroscopy, and the resulting NMR spectra were compared with those of the CO2-MEA-H2O solutions to understand the reaction mechanisms of the solutions at various CO2 partial pressures. Equilibrium Solubility Measurements. The equilibrium solubilities of CO2 in the aqueous 10 mass % AHPD/MEA solutions were measured at 298.15 K and at CO2 partial pressures ranging from 0.01 to 3000 kPa, and the overall results are presented in Table 1 and plotted in Figure 2. At this specific temperature, the AHPD-H2O solution showed a better CO2 absorption capacity than the MEA-H2O solution above 4 kPa CO2 partial pressure. The solubility difference between the MEA-H2O and AHPD-H2O solutions was augmented with increasing CO2 partial pressure. This high solubility of CO2 in aqueous AHPD solutions is considered to be due to the molecular structure of AHPD. The steric hindrance of a bulky group in AHPD lowers the stability of the carbamate anions produced from the CO2-amine reaction. Accordingly, CO2 loadings in aqueous AHPD solutions far exceed those in aqueous MEA solutions in which stable carbamate anions formed. It must be noted, however, that the solubility of CO2 in aqueous AHPD solution decreases rapidly as the CO2 partial pressure is decreased. At CO2 partial pressures below 4 kPa, the solubilities of CO2 in the AHPDH2O solutions were even lower than those in the MEA-H2O solutions. In a previous work, the solubility curves of CO2 in aqueous 10 mass % AHPD and MEA solutions at 313.15 K 1672
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were also found to exhibit a crossover point at about 50 kPa CO2 partial pressure (12). It might be expected from both experimental results that the solubility crossover occurs at higher CO2 partial pressures when temperature becomes higher. The appearance of crossover point in the solubility curve has not yet been clarified either theoretically or experimentally, even though a great deal of solubility data have confirmed this abnormal phenomenon. High-Pressure 13C NMR Spectra. For a better understanding of the absorption reaction mechanism of equilibrated CO2-amine-H2O solutions at various CO2 partial pressures, the most influential chemical species, namely, bicarbonate ion and carbamate anion, were identified by both quantitative and qualitative analyses using 13C NMR spectroscopy. The 13C NMR spectra of both the CO2-10 mass % AHPD-H2O and CO2-10 mass % MEA-H2O solutions were detemined at 298.15 K and over CO2 partial pressure ranging from 0.01 to 3000 kPa. First, to assign protonated cations in the equilibrated CO2-amine-H2O solutions, the NMR spectroscopic results of the aqueous 10 mass % amine solutions without any dissolved CO2 were compared with those of the CO2-amine-H2O solutions at atmospheric pressure. However, no identical chemical shifts were found in the two 13C NMR spectra, which might be due to the difference in pH values between the aqueous amine solutions (AHPD, pH ) 11.76; MEA, pH ) 12.44) and the CO2-amineH2O solutions (AHPD, pH ) 7.43; MEA, pH ) 7.75). It must be noted that the chemical shifts are considered to depend strongly on pH and that, in general, amines exist almost completely in the protonated form at pH 7-8 (14). Therefore, the definite identification of protonated cations was made possible by comparing the 13C NMR spectra of the CO2amine-H2O solutions with those of aqueous amine solutions (AHPD, pH ) 7.43; MEA, pH ) 7.75) whose pH values had been adjusted by the addition of aliquots of standard HCl solutions. For chemical shifts of 40-80 ppm, carbon peaks of protonated and carbamate amines (AMH+, AMCO2-) were observed, and hence, the remaining peaks could be identified with the carbamate form of the amines. Moreover, the carbon peaks of carbamate anion (AM13CO2-) and bicarbonate ion (H13CO3-) appeared at 140-180 ppm. The quantitative analysis of carbamate anion (AM13CO2-) was made by considering the ratio of the peak areas of the carbons in the carbamate anion. Bicarbonate ions of the CO2-AHPD-H2O and CO2-MEA-H2O solutions showed similar chemical shifts at the same carbon dioxide absorbing conditions. The chemical shifts and peak areas of species in various samples at 298.15 K and atmospheric pressure are summarized in Table 2. The formation of carbonate ions is not likely to occur because the basicities of both the CO2-AHPD-H2O and CO2MEA-H2O solutions are low enough (pH ) 7-10) to guarantee that the carbonate-bicarbonate equilibrium is shifted more toward the bicarbonate side at various CO2 partial pressures. In particular, the pH values of the CO2amine-H2O solutions were additionally measured over CO2 partial pressure ranging from 0.01 to 100 kPa, as shown in Figure 3, which, of course, indicates that, at higher partial pressures, the pH decreases because of the strong absorption of CO2. The peak areas of bicarbonate ion and carbamate anion obtained from the quantitative 13C NMR analyses of the CO2amine-H2O solutions were normalized to the initial amount of amine. The normalized NMR peak areas and chemical shifts of bicarbonate ion and carbamate anion in the CO2AHPD-H2O and CO2-MEA-H2O solutions at various CO2 partial pressures are also summarized in Table 3. The amounts of bicarbonate ion and carbamate anion in the CO2amine-H2O solutions decisively influence the corresponding CO2 loadings, as plotted in Figure 4. For both cases of MEA and AHPD, the amount of bicarbonate ion monotonically
TABLE 2. Chemical Shifts (δ) and Peak Areas of Species in Aqueous Amine Solutions at 298.15 K and 1 atm δ (ppm) (peak areas) aC
system
bC
cC
dC
eC
fC
NH3+aCH2bCH2OH NH3+aCH2bCH2OH eCOO-NHcCH dCH OH 2 2 HfCO3NH3+aCH2bCH2OH
44.13 42.81 (8.09) 43.14
64.85 59.21 (9.08) 59.46
44.56 (1.76) -
62.87 (2.00) -
166.28 (1.91) -
162.06 (6.52) -
AHPD-H2O NH3+aC(bCH2OH)3 CO2-AHPD-H2O NH3+aC(bCH2OH)3 eCOO-NHcC(dCH OH) 2 3 HfCO3+a b AHPD-H2O-HCl NH3 C( CH2OH)3
64.81 62.35 (9.64) 62.24
58.10 61.45 (27.44) 61.97
65.58 (0.35) -
61.88 (0.97) -
160.21 (0.36) -
162.13 (8.16) -
MEA-H2O CO2-MEA-H2O MEA-H2O-HCl
TABLE 3. Chemical Shifts (δ) and Peak Areas of Bicarbonate Ion and Carbamate Anion in the CO2-Amine-H2O Solutions at Various Partial Pressures of Carbon Dioxide and 298.15 K CO2-MEA-H2O
CO2-AHPD-H2O
δ (ppm) (peak areas)
P (kPa)
MEA13CO2-
H13CO3-
0.07 0.30 3.30 7.10 19.8 49.7 96.8 303.7 855.3
166.37 (3.03) 166.36 (3.38) 166.35 (3.78) 166.33 (3.35) 166.32 (3.10) 166.30 (2.30) 166.28 (1.91) 166.24 (1.39) 166.18 (0.75)
163.84 (0.78) 163.34 (1.34) 162.70 (2.10) 162.43 (3.40) 162.24 (3.94) 162.11 (5.68) 162.06 (6.52) 162.01 (7.80) 161.96 (9.46)
FIGURE 3. pH vs CO2 partial pressure of the equilibrated CO2amine-H2O solutions. increases with CO2 loading, whereas the amount of carbamate anion increases at low CO2 loading, reaches a maximum, and then decreases at high CO2 loading. The general features drawn from Figure 4a for the case of CO2-MEA-H2O solutions were compared with the modeling results of refs 15 and 16, and the two were found to be almost identical qualitatively. As mentioned earlier, the decrease of the amount of carbamate anions directly after the maximum was reached is related to the change in pH of the solution. As the solution pH decreased, the hydronium ions formed increased and reacted with carbamate ions to make them unstable. The decarboxylation of these unstable intermedi-
δ (ppm) (peak areas)
P (kPa)
AHPD13CO2-
H13CO3-
0.90 3.30 9.20 19.0 96.8 401.6 967.6 2427.3
161.15 (0.07) 160.94 (0.19) 160.70 (0.23) 160.51 (0.29) 160.21 (0.36) 160.07 (0.44) 160.02 (0.39) 159.98 (0.34)
163.06 (1.19) 162.71 (2.85) 162.43 (4.79) 162.32 (6.44) 162.13 (8.16) 162.04 (8.73) 162.02 (9.30) 162.00 (9.86)
ates changed the carbamates into bicarbonate ions through a hydrolysis reaction. As the CO2 loading increased, the pH value decreased from about 10 to 7, and the carbamate anions of MEA and AHPD began to decrease at pH 8.7 and pH 7.5, respectively. The most remarkable contrast of these two amine solutions can be found in the large difference in the amounts of carbamate anions formed at equilibrium. Here, it must again be emphasized that a significant amount of carbamate anion was formed in the MEA solution, whereas in the AHPD solution, only a small amount of carbamate anion was detected for the whole range of CO2 partial pressures. This phenomenon might be due to the difference in the molecular structures of MEA and AHPD, as previously stated. However, particular attention must be paid to the fact that the bulky structure of AHPD does not always act favorably on the solubility of CO2 in aqueous AHPD solutions. At low CO2 partial pressures, only small amounts of both bicarbonate ions and carbamate anions were observed in the NMR spectra of the equilibrated CO2-AHPD-H2O solutions, which severely lowered the resulting CO2 absorption capacity, but considerable amounts of bicarbonate ions were formed and led to more favorable CO2 absorption at high partial pressures. However, we must note from technological aspect of CO2 absorption processes that the bulky molecular structure of AHPD hindered the formation of bicarbonate ions as well as carbamate anions at relatively low CO2 partial pressures. The much larger quantity of MEA carbamate anions than AHPD carbamate anions at low CO2 partial pressures played a key role in the better CO2 loading capacity of aqueous MEA solutions. Accordingly, the generally accepted conclusion that aqueous solutions of sterically hindered amines always provide positive absorption advantages over aqueous MEA solutions on account of molecular structure differences of the two amines must be limited to high CO2 partial pressures. Chemical shifts (ppm) of bicarbonate ion and carbamate anion in the 13C NMR spectra for the CO2-amine-H2O VOL. 37, NO. 8, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. CO2 loading vs normalized NMR peak areas of bicarbonate ion and carbamate anion in (a) CO2-MEA-H2O and (b) CO2-AHPDH2O solutions at 298.15 K. solutions decreased with increasing CO2 partial pressure, as shown in Figure 5. Exceptionally, carbamate anions in the MEA solutions showed very little change in chemical shift in comparison with other ions. One of the most interesting experimental results of the present study can be stated as follows: The chemical shifts of ions except MEA carbamate were greatly changed at low CO2 partial pressures and very slightly changed at high pressures. This chemical shift behavior was expected to be closely related to both the pH in solution and the functional groups, such as the hydroxyl groups of the amines. At lower CO2 partial pressures, the amine solutions had higher pH’s (OH- . H+) and therefore, the hydroxyl groups of the amines were likely to be attacked more easily by hydroxyl ions in solution, which made the amine charges more negative and the chemical shifts more sensitive. Of course, the sterically hindered amine of AHPD contains more hydroxyl groups than the nonhindered MEA, and hence, its chemical shifts were strongly influenced by solution pH, as clearly indicated in the 13C NMR spectra. In contrast, MEA appeared to be insensitive to pH. The present study of solubility and 13C NMR spectra for the CO2-MEA-H2O and CO2-AHPD-H2O solutions was 1674
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FIGURE 5. Chemical shifts and normalized NMR peak areas of bicarbonate ion and carbamate anion in (a) CO2-MEA-H2O and (b) CO2-AHPD-H2O solutions at various partial pressures of carbon dioxide and 298.15 K. performed to examine CO2 absorption characteristics of sterically hindered AHPD and nonhindered MEA at various CO2 partial pressures. Thermodynamic CO2 loadings of aqueous AHPD solution in the high CO2 partial pressure range far exceeded those in aqueous MEA solution in which stable carbamate anions were formed. The steric hindrance of the bulky molecular structure of AHPD lowered the stability of the carbamate anions produced from the CO2-amine reaction. It must be noted, however, that the solubility of CO2 in aqueous AHPD solution decreased rapidly as the CO2 partial pressure decreased. Very small quantities of both bicarbonate ions and carbamate anions were observed in the NMR spectra of the CO2-AHPD-H2O solutions, which might explain the lower solubility of CO2 in aqueous AHPD solution than in aqueous MEA solutions at low CO2 partial pressures. The chemical shifts of bicarbonate ion and carbamate anion in 13C NMR spectra for the CO2-amineH2O solutions decreased with increasing CO2 partial pressure.
One of the most interesting experimental results of the present study can be stated as follows: The chemical shifts of ions except MEA carbamate were greatly changed at low CO2 partial pressures and very slightly changed at high pressures. The sterically hindered amine of AHPD contains more hydroxyl groups than the nonhindered MEA, and hence, its chemical shifts were strongly influenced by solution pH values, as clearly indicated in the 13C NMR spectra, but MEA appeared to be pretty much insensitive to pH. The overall NMR results also provide technological insight for CO2 separation and recovery that the chemical shifts of various types of amines, either hindered or nonhindered, determined from the NMR spectra can be used as a key process parameter for reliable and better prediction of CO2 loading capacities without the use of any corresponding solubility measurements and additionally required physical and chemical properties.
Acknowledgments This research was performed for the Carbon Dioxide Reduction & Sequestration Center, one of the 21st Century Frontier R&D Programs and the Greenhouse Gas Research Center, one of the Critical Technology-21 Programs funded by the Ministry of Science and Technology of Korea, and also partially supported by the Brain Korea 21 Project.
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Received for review August 12, 2002. Revised manuscript received November 24, 2002. Accepted January 21, 2003. ES0260519
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