Experimental Verification of the Reaction Mechanism of Solid K2CO3

Sep 29, 2016 - Experimental Verification of the Reaction Mechanism of Solid K2CO3 during Postcombustion CO2 Capture. Arturo Gomez†, Abhimanyu Jayaku...
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Experimental Verification of the Reaction Mechanism of Solid K2CO3 during Postcombustion CO2 Capture Arturo Gomez,† Abhimanyu Jayakumar,† and Nader Mahinpey* Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada ABSTRACT: Previous research has not conclusively demonstrated whether KHCO3 formation occurs through the direct reaction of K2CO3 with gas phase CO2 and H2O or via the intermediate formation of K2CO3·(1.5H2O). In this study, four sets of experiments, using regenerated and prehydrated K2CO3, were performed in different flue gas component mixtures to fully elucidate the reaction mechanism involved. The extents of carbonation and hydration were quantified by separately tracking CO2 and/or H2O uptakes and releases. Test results revealed regenerated K2CO3 to be most active for KHCO3 formation, while the carbonation of prehydrated samples was limited by their drying rates, proving that K2CO3· (1.5 H2O) cannot be directly converted to KHCO3. Therefore, K2CO3 carbonation and hydration are competing reversible reactions which occur in parallel through direct reaction with the flue gas components. The knowledge gained about the reaction mechanism provides the fundamental direction for future process design to make this technology practical. Upon calcination at temperatures greater than 120 °C,4 KHCO3 decomposes to regenerate active K2CO3 and releases a gas stream, mainly composed of CO2 and steam, from which steam can be condensed out. The concentrated CO2 stream is then ready to be collected and sent to storage. Although, at this stage, gaining an understanding of the reaction mechanism involved is essential for the design of an efficient postcombustion CO2 capture process using K2CO3based solid sorbents. The predominant view in the literature is that CO2 capture by solid K2CO3 occurs via a sequential reaction mechanism involving two main steps.5−9 According to this reaction mechanism, K2CO3 undergoes hydration in the first step to form K2CO3·(1.5H2O), as shown by reaction 2. In the second step, the intermediate species, K2CO3·(1.5 H2O), then reacts with CO2 to form KHCO3 as per reaction 3.

1. INTRODUCTION Anthropogenic CO2 emissions are significantly contributing to global climate change.1 Hence, in order to control the CO2 emissions from fossil fuel combustion processes, it has now become critical to develop an efficient, cost-effective, and retrofittable postcombustion CO2 capture technology. Thus far, the only mature and commercialized postcombustion CO2 capture technology involves the use of aqueous solutions of amines. This technology, however, has a high energy demand for solvent regeneration due to the presence of large amounts of water.2,3 The use of solid sorbents for CO2 capture offers an attractive alternative to aqueous systems, since solid sorbents have the potential to have lower regeneration energy requirements, and they could also result in a more compact process, thus decreasing operating and capital costs. A promising type of solid chemisorbents for CO2 are alkali metal carbonates, such as K2CO3, which have several advantageous characteristics, such as high theoretical CO2 capture capacities (7.24 mmol·g−1 of K2CO3), low cost, abundance, and no toxicity. When exposed to combustion flue gases at low temperatures (40−80 °C), potassium carbonate (K2CO3) reacts with H2O and CO2 to form potassium bicarbonate (KHCO3), as shown by the overall carbonation reaction (Reaction 1). The H2O required for the occurrence of reaction 1 is provided by the release of the trapped moisture contents and/or the oxidation of the hydrogen contents of the fuels, during combustion. K 2CO3(s) + H 2O(g) + CO2 (g) ⇋ 2KHCO3(s) © 2016 American Chemical Society

K 2CO3(s) + 1.5H 2O(g) ⇋ K 2CO3 · (1.5H 2O)(s)

(2)

K 2CO3 · (1.5H 2O)(s) + CO2 (g) ⇋ 2KHCO3(s) + 0.5H 2O(g)

(3)

X-ray diffraction (XRD) analysis of carbonated K2CO3 samples in the literature show the formation of several products, which include KHCO3, K2CO3·(1.5 H2O), and K 4H 2 (CO 3 ) 3·(1.5 H 2 O), present along with unreacted Received: Revised: Accepted: Published:

(1) 11022

July 29, 2016 September 27, 2016 September 29, 2016 September 29, 2016 DOI: 10.1021/acs.iecr.6b02916 Ind. Eng. Chem. Res. 2016, 55, 11022−11028

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Industrial & Engineering Chemistry Research K2CO3.10,11 K4H2(CO3)3·(1.5 H2O) is likely an additive combination of the bicarbonate and hydrate. Other possible reactions involving this adduct can be expressed as simple combinations of reactions 1, 2, and 3 and will not change the nature of the overall reaction mechanism involved. From the support material and process design standpoints, the important characteristic of the reaction mechanism that needs to be confirmed is how KHCO3 formation mainly occurs. Possible scenarios include the direct reaction of K2CO3 with gas phase CO2 and H2O (reaction 1), the formation of K2CO3·(1.5 H2O) as the intermediate followed by its subsequent reaction with CO2 (reactions 2 and 3), or a combined reaction mechanism involving all three reactions. In the literature, notably, Zhao et al. have conducted a series of fundamental studies which provide some useful preliminary insights into the carbonation behavior of unsupported K2CO3.11−13 They showed that the carbonation reactivity and yield of K2CO3 are dependent upon the nature of the precursor from which it is regenerated. K2CO3 regenerated from KHCO3 showed faster kinetics and a much higher CO2 uptake than K2CO3 regenerated from K2CO3·(1.5 H2O).11−13 This result was also confirmed as a part of our previous work, where the difference in carbonation behavior was seen to be due to the formation of different K2CO3 phases during the regeneration of KHCO3 (phase I) and K2CO3·(1.5 H2O) (phase II), which have slight differences in their monoclinic structures.14 The carbonation performances of K2CO3 from different precursors were seen to stabilize at a similar intermediate level over multiple regeneration−carbonation cycles, independent of the precursor, and based on the thermodynamics associated with the reaction conditions and flue gas composition used. This eventual stabilization in the carbonation performance, after sufficient cycling, was a consequence of the stabilization in the constituent phase compositions of the K2CO3 samples, produced by regeneration.14 Conversely, other important results from the studies of Zhao et al. cast doubts on the validity of the sequential reaction mechanism.12 First, the K2CO3·(1.5 H2O) (analytical reagent) showed no weight change during carbonation in thermogravimetric analyzer (TGA) tests, likely implying no CO2 uptake. Moreover, various regenerated precursors after prehydration showed minor weight changes when exposed to a mixture of dry CO2 and N2 to test for reaction 3. The minor weight changes observed do not provide evidence of significant CO2 uptake via reaction 3, yet the authors did not question the validity of the sequential reaction mechanism. This apparent lack of reactivity of K2CO3·(1.5 H2O) with CO2 is in direct disagreement with the possibility of the series mechanism. Also, the reliance on only TGA weight data provided the authors with no information on the amounts of bicarbonate formed in their samples. In this work, a coupled thermogravimetric analyzer−mass spectrometer (TGA-MS) analysis system is used to overcome the experimental limitations in the literature.14 This experimental system enables the separate tracking of CO2 and H2O uptake/release quantities during K2CO3 sample carbonation, drying, and regeneration. Using this setup, the carbonation behaviors of regenerated and prehydrated K2CO3 samples, derived from two different precursors, are compared in humidified N2 + CO2 and dry N2 + CO2 atmospheres, at 30 and 50 °C, to investigate the occurrence of reaction 3. The drying rates of the prehydrated samples, in dry N2, are also determined at both temperatures. A comparison of the

proportionality of the drying rates and CO2 uptakes of the prehydrated samples helps show whether the KHCO3 formed was through reaction 3 or due to the carbonation of the drying sample using the released or ambient H2O via reaction 1. The determination of the reaction mechanism involved provides vital fundamental information for the future design of a practical process for postcombustion CO2 capture using K2CO3-based solid sorbents.

2. EXPERIMENTAL SECTION The starting samples for the carbonation tests carried out in this study were the regenerated and prehydrated K2CO3 samples obtained from the KHCO3 (Sigma-Aldrich, ACS reagent, 99.7%, granular) and K2CO3·(1.5 H2O) (Alfa Aesar, ACS reagent, 98.5−101.0%, crystalline) analytical reagent precursors. All experiments were started by loading around 120−125 mg of K2CO3 weight equivalent of the analytical reagents in the stainless steel crucible of the TGA with dry N2 flowing in the reactor. The wide stainless steel crucible of the TGA allowed for the given, larger-than-usual sample quantities to be kept in almost a mono- or bilayer particle distribution. A detailed description of the TGA-MS experimental setup is provided in our recent work.14 This setup allows for accurate CO2 and H2O uptake quantification while minimizing the mass transfer effects in the larger sample quantities used. 2.1. Experimental Procedure. Four sets of experiments were carried out using each of the two precursors at the carbonation temperatures of 30 and 50 °C. Two different carbonation temperatures were chosen in order to explore how the CO2 reactivity of a prehydrated sample, derived from a given precursor, was impacted by the difference in its drying rates at these two temperatures. In particular, 30 and 50 °C were chosen, since the drying rates of the prehydrated samples are much lower at 30 °C than at 50 °C. Sample regeneration was carried out at 170 °C. The gas compositions used for the various steps of sample hydration, carbonation, regeneration, and drying are as described below for the different experimental sets. 2.1.1. Sample Regeneration and Wet Carbonation. This set of experiments is the base case which is more representative of a typical CO2 capture cycle of the sorbent. In this case, both the KHCO3 and K2CO3·(1.5 H2O) precursors were first completely regenerated at 170 °C in dry N2 to produce K2CO3. The K2CO3 produced was then carbonated for 3 h in a model flue gas atmosphere composed of CO2 (2.34%), H2O (2.13%), and balance N2, at both 30 and 50 °C. The gas compositions mentioned are in the units of standard volume % for the reference conditions of 25 °C and 1 atm. For this set of experiments, the CO2 and H2O uptakes of the samples were calculated during carbonation as described previously.14 2.1.2. Sample Prehydration, Dry Carbonation, and Regeneration. For this experimental set, only the KHCO3 precursor needed to be regenerated first at 170 °C in dry N2, in order to be subsequently hydrated. The regenerated KHCO3 precursor and the K2CO3·(1.5 H2O) reagent were then maintained in a humidified N2 atmosphere (2.18% H2O and balance N2) at 50 °C, until their weights stabilized at the level corresponding to 100% K2CO3·(1.5 H2O). The hydrated samples were then either maintained at 50 °C or cooled to 30 °C. After reaching the desired reaction temperature, the reactor system was purged with dry N2 for 30 min to remove any ambient moisture. The hydrated samples were then exposed to a dry mixture of CO2 and N2 (2.39% CO2 and balance N2) for 11023

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Industrial & Engineering Chemistry Research 3 h in order to investigate the occurrence of reaction 3. The CO2 uptakes during dry carbonation were calculated as shown previously;14 however, the H2O contents of the samples could not be tracked due to the nature of the experiment. After the dry carbonation step, the samples were maintained in the same gas atmosphere and regenerated at 170 °C. This approach enabled the quantification of the net amount of CO2 released during regeneration. The quantification of the amount of CO2 released during regeneration follows a very similar procedure to the CO2 uptake quantification during carbonation as described in section 2.2. Since the very gradual uptake of CO2 during dry carbonation could not be accurately quantified over the baseline signal noise of the MS, the CO2 uptake was validated by quantification during regeneration in order to get a more accurate measure of the CO2 captured by the sample. During regeneration, CO2 is released more quickly, producing a more pronounced signal change in the MS readings, which makes it easier to detect and quantify. 2.1.3. Sample Prehydration, Wet Carbonation, and Regeneration. These experiments follow a similar procedure to that described in section 2.1.2. The only differences were that the hydrated samples were maintained in dry N2 for 15 min, until the concentration of CO2 stabilized in the gas line previously used for hydration, and then they were carbonated and regenerated in the model flue gas atmosphere of CO2 (2.34%), H2O (2.13%) and balance N2. 2.1.4. Drying of Hydrated Samples. Hydration of the regenerated KHCO3 precursor and the K2CO3·(1.5 H2O) reagent were performed in the humidified N2 atmosphere (2.18% H2O and balance N2) at 50 °C as mentioned in section 2.1.2. The hydrated samples were then dried in a 100% N2 atmosphere at both the reaction temperatures of 30 and 50 °C, in order to compare their drying rates. 2.2. Quantification of the CO2 Released during Regeneration. During regeneration, the net amount of CO2 released was calculated in the same way as during carbonation, as presented elsewhere.14 It was calculated by integrating the difference between the observed CO2 flow rates at the reactor outlet for the blank reactor and the reactor containing the sample. The MS CO2 signal readings observed were converted into their corresponding CO2 flow rates by calibration of the MS CO2 signal response for a set of known CO2 feed flow rates in the gas mixture over the appropriate signal range. CO2 calibrations were carried out at the end of every experiment in the corresponding carbonation gas mixture used for the experiment. Figure 1 shows the observed CO2 outlet flow rates during the regeneration of the prehydrated K2CO3 sample derived from the KHCO3 precursor, after dry carbonation at 30 °C, compared to its corresponding blank run. As seen in Figure 1, at the start of the regeneration step as the reactor starts heating up, initial flow rate fluctuations are observed for the blank and sample run cases. These fluctuations are most likely due to the seals of the glass reactor system being affected during the temperature ramp, and eventually stabilize. As the reactor temperature increased initially, a drop in CO2 outlet flow rate was observed for the sample case. The drop is indicative of CO2 uptake by the sample. This CO2 uptake most likely occurs due to the release of hydration water by the sample at higher temperatures. A portion of the hydration water released, along with CO2 from the dry carbonation gas, can still react with the sample to form KHCO3 via reaction 1 at intermediate temperatures.

Figure 1. CO2 flow rates at the reactor outlet during the regeneration step for the prehydrated K2CO3 sample derived from KHCO3, after dry carbonation at 30 °C, and its corresponding blank run, as an example for quantifying the net CO2 released. Shaded areas indicate CO2 uptake/release relative to the blank run.

Once the temperature was sufficiently high, all of the CO2 adsorbed by the sample was subsequently released, resulting in an upward spike in the CO2 outlet flow rate. After all of the CO2 was released, the CO2 outlet flow rate returned back to the blank baseline level corresponding to the CO2 feed flow rate. Integration of the downward and upward spikes provide the net amount/volume of CO2 released during regeneration, which corresponds to the amount of CO2 that was adsorbed during the dry carbonation of the prehydrated sample over 3 h. The volumetric CO2 flow rates and volume of CO2 adsorbed by the sample can be easily converted to molar or weight units by simple calculations.

3. RESULTS AND DISCUSSION The following results presented from the four sets of experiments, using the KHCO3 and K2CO3·(1.5 H2O) analytical reagents as K2CO3 precursors, are used to understand the role of reaction 3 in the postcombustion capture of CO2 by solid K2CO3 at low temperatures. 3.1. Wet Carbonation of Regenerated Precursors. After complete regeneration at 170 °C in dry N2, both K2CO3 precursors were carbonated in the model flue gas mixture of CO2 (2.34%), H2O (2.13%), and balance N2, at 30 and 50 °C. The weights of the samples from the TGA during carbonation are shown in the form of ratios, with respect to the initial weight of completely regenerated K2CO3, in Figure 2(a). The observed total weight gains in the TGA results are the summation of the weight gains due to KHCO3 and K2CO3·(1.5 H2O) formation during wet carbonation. The CO2 uptake rates of the K2CO3 samples during carbonation, as calculated from the MS data, are shown in Figure 2(b). The hydration rates of the samples were computed by the difference between the TGA and MS data.14 The bicarbonate and hydrate conversions of the samples are reported after 6000 s of carbonation, since the CO2 uptakes were negligible at greater times. After 6000 s of carbonation at 30 °C, the observed bicarbonate and hydrate conversions for the regenerated K2CO3·(1.5 H2O) precursor were 11.14% and 58.5%, respectively. At 50 °C, the corresponding conversions were 16.61% and 10.69%. In the case of the regenerated KHCO3 precursor, the bicarbonate and hydrate conversions were observed to be 51.97% and 38.26% at 30 °C, and 70.17% and 26.25% at 50 °C, respectively. Carbonation results obtained in this set of experiments evidence that the K2CO3 sample derived from KHCO3 has a much higher carbonation yield compared to the K2CO3 sample 11024

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Figure 2. (a) Sample weight ratios with respect to regenerated K2CO3 from TGA and, (b) sample CO2 uptake rates from MS, during wet carbonation of the regenerated precursors.

Figure 3. (a) Sample weight ratios with respect to regenerated K2CO3 from TGA for the prehydrated samples during dry carbonation and, (b) sample CO2 uptake/release rates from MS during regeneration after dry carbonation.

derived from K2CO3·(1.5 H2O). This observation is consistent with results presented in the literature and is due to slight differences in the monoclinic structures of the two derived K2CO3 phases.11−14 It is also clear from Figures 2(a) and 2(b) that most of the weight gain and CO2 uptake by the regenerated samples occur within the first 2000 s of carbonation. This result shows that regenerated K2CO3 is indeed very active for postcombustion CO2 capture. Another interesting finding is that the ultimate bicarbonate conversions, calculated from the area under the curve in Figure 2(b), of both the derived K2CO3 samples are higher at 50 °C than at 30 °C, but their hydration conversions are seen to decrease with an increase in temperature. Since both bicarbonate formation and hydration reactions are exothermic reactions, it would be expected that the extents of both bicarbonate and hydrate conversions would decrease with an increase in temperature. Based on thermodynamics, lower conversions to both hydrate and bicarbonate at higher temperatures would be especially expected if the sequential reaction mechanism is valid. However, our observations could possibly indicate that the carbonation and hydration reactions may be occurring in parallel to one another, where a reduction in temperature may be promoting hydration by a higher extent than carbonation, thus resulting in a lower bicarbonate conversion at lower temperatures due to a lower number of available active sites. 3.2. Dry Carbonation of Prehydrated Samples. As described in section 2.1.2, prehydrated K2CO3 samples derived from the KHCO3 and K2CO3·(1.5 H2O) precursors were carbonated for 3 h in a dry CO2 (2.39%) and balance N2 gas mixture at 30 and 50 °C. This set of experiments was carried

out in order to isolate the occurrence of reaction 3 and evaluate its contribution to bicarbonate formation. After sample prehydration, the reactor system was first purged with pure N2 for 30 min, in order to remove any ambient H2O, during which the samples dried to certain minor extents. During the dry carbonation of the prehydrated samples, however, no CO2 uptake was detected by the MS for any of the samples, over and above the baseline noise fluctuations of the MS CO2 signal. Moreover, the TGA weight changes during dry carbonation, presented in Figure 3(a), were very marginal for all the samples. If the sequential reaction mechanism is valid, in which K2CO3· (1.5 H2O) is supposed to be the active intermediate, a significant increase in sample weight would be expected due to the uptake of the heavier molecule of CO2 through reaction 3. Additionally, the final sample weight ratios would be closer to 1.449, corresponding to 100% KHCO3. Nonetheless, the TGA results in Figure 3(a) alone do not conclusively disprove the sequential mechanism, since it is possible that carbonation could be occurring simultaneously with the drying of the hydrate. On the other hand, the CO2 uptake over the course of 3 h was undetectable by the MS during dry carbonation, which indicates that the carbonation rate and the overall extent of carbonation were likely very low. In order to more accurately determine the amount of CO2 adsorbed by the samples during dry carbonation, the carbonated samples were maintained in the same dry carbonation gas atmosphere and regenerated by heating to 170 °C. Quantification of the net amount of CO2 released during regeneration is explained in section 2.2. Figure 3(b) shows the initial CO2 uptake and eventual CO2 release for all 11025

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Figure 4. (a) Sample weight ratios with respect to regenerated K2CO3 from TGA for the prehydrated samples during wet carbonation, (b) sample CO2 uptake rates from MS during wet carbonation of the prehydrated samples, and (c) sample CO2 uptake/release rates from MS during regeneration after wet carbonation.

This possibility is evaluated in the following subsections. The observations so far appear to be in contradiction with the sequential reaction mechanism proposed in the literature, where the prehydrated sample containing K2CO3·(1.5 H2O) is claimed to be the active intermediate species for bicarbonate formation via reaction 3.5−9 3.3. Wet Carbonation of Prehydrated Samples. In order to minimize their drying, the prehydrated samples derived from both K2CO3 precursors were carbonated for 3 h in the model flue gas mixture of CO2 (2.34%), H2O (2.13%), and balance N2, at 30 and 50 °C. Before the wet carbonation step, the prehydrated samples were maintained in pure N2 for 15 min while the CO2 concentration stabilized in the humidified N2 gas line, used for hydration prior, to maintain the accuracy of the CO2 uptake quantification upon switching to the wet carbonation step. Figure 4(a) shows the weight ratios of the samples in the TGA with respect to regenerated K2CO3 during wet carbonation. For the K2CO3·(1.5 H2O) precursor, no significant weight change was observed during wet carbonation at both 30 and 50 °C. In the case of prehydrated K2CO3 derived from the KHCO3 precursor, a marginal weight increase was observed at 30 °C. At 50 °C, however, a very significant weight increase was observed for the same sample, with its final weight ratio being almost similar to the final weight ratio of its corresponding case in section 3.1. The CO2 uptake data from the MS during wet carbonation, in Figure 4(b), shows that the K2CO3·(1.5 H2O) precursor exhibited no significant CO2 uptake at both temperatures, likely due to its much lower carbonation reactivity and possible yield. Prehydrated K2CO3 derived from the KHCO3 precursor had a marginal CO2 uptake rate initially which then decreased soon after at 30 °C. The same sample at 50 °C showed much higher

the dry carbonated samples during regeneration. The net amounts of CO2 released by the samples during regeneration were calculated from the plots shown in Figure 3(b), and they correspond to the total CO2 uptakes by the samples during dry carbonation. In the case of the K2CO3·(1.5 H2O) precursor dry carbonated at 30 and 50 °C, the net amounts of CO2 released during regeneration were negligibly small and could not be accurately quantified over the baseline MS CO2 signal noise. The K2CO3·(1.5 H2O) precursor has a very low carbonation reactivity and yield, and very likely captured negligibly small amounts of CO2 during dry carbonation. The prehydrated sample derived from the KHCO3 precursor and dry carbonated at 30 °C showed a net CO2 release during regeneration corresponding to only 3.08% conversion to bicarbonate during dry carbonation, while the sample dry carbonated at 50 °C showed a relatively higher net CO2 release corresponding to just 8.75% conversion to bicarbonate during dry carbonation. The bicarbonate conversions observed for the prehydrated precursors after dry carbonation were much lower than those for the completely regenerated K2CO3 samples after wet carbonation in section 3.1. This finding shows that reaction 1 may be the dominant pathway for bicarbonate formation. The contribution of reaction 3 to bicarbonate formation may be very limited or negligible. Reaction 3 could even be nonexistent in this gas−solid reaction system, if bicarbonate formation during the dry carbonation experiments is related to the drying rate of the prehydrated sample. In this scenario, bicarbonate formation may only be occurring through reaction 1, where the drying sample reacts with gas phase CO2 and a portion of the limited amount of hydration H2O released. At the same time, it can still be argued that bicarbonate formation during dry carbonation is being limited due to the drying of the sample. 11026

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expectedly lower than those at 50 °C. However, at a given temperature it was observed that the rate of drying for the hydrated sample derived from KHCO3 was higher compared to that of the K2 CO3 ·(1.5 H2O) precursor sample. The monoclinic K2CO3 phases derived from the KHCO3 and K2CO3·(1.5 H2O) precursors have been found to be slightly different in the XRD results from our previous work.14 These structural differences could lead to H2O being included in a different way in each of the distinct K2CO3 solid phases, and result in the two phases having different affinities for H2O. Nonetheless, further investigation is required to determine the real nature of the observed difference in dehydration behaviors of the different K2CO3 phases. After 3 h of drying, the K2CO3· (1.5 H2O) precursor remained 90.12% hydrated at 30 °C and 56.23% hydrated at 50 °C. Hydrated K2CO3 derived from the KHCO3 precursor remained 79.08% hydrated at 30 °C after 3 h; however, at 50 °C the sample was completely dry after 8000 s. The observations from the drying experiments explain why the prehydrated sample derived from the KHCO3 precursor showed higher CO2 uptakes during dry carbonation (section 3.2) and wet carbonation (section 3.3) at 50 °C. Since it has the highest drying rate, it releases the most hydration H2O into the reaction atmosphere for reaction 1 to occur during dry carbonation as a parallel reaction with the gas phase. During wet carbonation at 50 °C, its stronger tendency to dry allows reaction 1 to proceed to a much greater extent. In the other cases involving the prehydrated samples, low sample drying rates were the reason for the limited bicarbonate conversions observed. From the fundamental point of view, the times required for the carbonation of the regenerated and corresponding prehydrated samples are compared, in order to verify the reaction mechanism. The time taken for the carbonation of a sample to reach equilibrium or its bicarbonate conversion at the end of the 3-h carbonation step represents the overall carbonation reactivity of that sample. If the sequential carbonation mechanism were true, it would be expected that the prehydrated sample during wet carbonation (section 3.3) would exhibit a similar or higher carbonation reactivity (shorter time to reach equilibrium) compared to its corresponding regenerated sample from the same precursor during wet carbonation (section 3.1) at the same temperature. However, in this study, the opposite is found to be true, where the regenerated samples show higher carbonation reactivities than their corresponding prehydrated cases. Clear evidence of this reactivity difference is observed between the regenerated and prehydrated samples from the KHCO3 precursor at 50 °C, and this difference is much larger at 30 °C when the drying rates of the prehydrated sample are much lower. A comparison of the results from all the sections reveals that the regenerated K2CO3 precursors are the most active form of K2CO3 for CO2 capture, while the prehydrated samples containing K2CO3·(1.5 H2O) cannot be directly converted to KHCO3 through reaction 3 and need to undergo drying first in order to be active for carbonation via reaction 1. Therefore, the direct carbonation reaction 1 is the main reaction contributing to bicarbonate formation. Reaction 3 does not occur, and hence, the sequential reaction mechanism is invalid. The experimental results obtained in this paper verify the results from our previous work14 and prove that the carbonation reaction 1 and hydration reaction 2 are competing reversible reactions which occur in parallel in this gas−solid reaction

CO2 uptake and the uptake rate gradually decreased below the MS CO2 signal noise level after about 5000 s of wet carbonation. Figure 4(c) shows the net CO2 released by the carbonated samples during regeneration in the same wet carbonation gas, and confirms the same uptake trend observed in Figure 4(b). However, the CO2 uptakes estimated from Figures 4(b) and 4(c) for the samples derived from the KHCO3 precursor differ significantly. For the sample at 30 °C, quantification during carbonation estimated 3.01% bicarbonate conversion while the quantification of its regeneration plot estimated 11.34% bicarbonate conversion. For the same sample at 50 °C, these conversion estimates were 54.14% and 73.56%, respectively. The regeneration estimates were much higher and more accurate, since the very low CO2 uptake rates for long durations during the latter part of the wet carbonation step were either comparable to or not detectable over the MS CO2 signal noise. Also, during the initial period of regeneration, there were no significant upward spikes, indicating CO2 uptake at intermediate temperatures, observed in Figure 4(c), as were observed in Figure 3(b) in section 3.2. This observation is most likely due to the suppression of sample drying in the wet carbonation gas at the intermediate temperatures. The results from this experimental set show that only the prehydrated sample derived from the KHCO3 precursor during wet carbonation at 50 °C achieved a similar bicarbonate conversion compared to its analogous case in section 3.1. However, it took much longer to reach the same conversion level. In the other three cases, the prehydrated samples had very limited conversions to bicarbonate compared to the performances of their corresponding regenerated samples in section 3.1. Also, the drastic difference in bicarbonate conversions observed for the prehydrated sample derived from the KHCO3 precursor at 30 and 50 °C makes it very clear that the carbonation of the prehydrated sample was being limited by the rate of its backward hydration or drying reaction. 3.4. Drying of Hydrated Samples. Hydrated K2CO3 samples derived from the KHCO3 (after regeneration) and K2CO3·(1.5 H2O) precursors were dried in a 100% N2 atmosphere at both the operating temperatures of 30 and 50 °C. The amount of hydrate remaining in the K2CO3 samples was monitored by tracking the sample weight in the TGA over 3 h. The extents of drying for the two hydrated precursors at the different carbonation temperatures are shown in Figure 5. The drying rates for the hydrated samples at 30 °C were

Figure 5. Percent hydrate remaining from TGA during the drying of the hydrated samples derived from the KHCO3 and K2CO3·(1.5 H2O) precursors, at 30 and 50 °C, in dry N2 over 3 h. 11027

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Article

Industrial & Engineering Chemistry Research

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system under the postcombustion conditions studied. The parallel reaction mechanism for K2CO3 carbonation and hydration provides very important fundamental information for the design of a practical postcombustion CO2 capture process using K2CO3-based solid sorbents. Mainly, it shows that hydrate formation via reaction 2 will limit the CO2 capture efficiency of solid K2CO3 and at the same time increase the regeneration energy requirement per mole of CO2 captured due to the additional energy required for sorbent dehydration. Hence, in terms of process development, it will be necessary to optimize the carbonation temperature and H2O content in the flue gas to minimize hydrate formation and design a suitable support configuration for K2CO3 dispersion which can regulate its interaction with H2O in the process.

4. CONCLUSION The carbonation performances of regenerated and prehydrated K2CO3 samples were analyzed and compared in four sets of experiments which revealed that the regenerated samples are much more active for CO2 capture through bicarbonate formation. The prehydrated samples showed very low CO2 uptakes during dry carbonation and low CO2 uptake rates during wet carbonation. These observations helped verify that K2CO3·(1.5 H2O) in the prehydrated samples cannot be directly converted to KHCO3 via the sequential carbonation reaction 3, and that CO2 uptake was limited by the drying rate of the prehydrated sample. Since reaction 3 does not occur, the sequential reaction mechanism for K2CO3 carbonation is invalid. K2CO3 carbonation only occurs through the direct carbonation reaction 1, and K2CO3 carbonation and hydration occur simultaneously in a parallel reaction mechanism, where reversible reactions 1 and (2) are competing for the active sites. In order to design a practical postcombustion CO2 capture process using K2CO3-based solid sorbents, knowledge of the parallel reaction mechanism involved provides vital fundamental information for process optimization. Process and energy efficiencies can be increased by optimizing the process conditions and designing a suitable support material for K2CO3 dispersion to minimize hydrate formation, while still maintaining practical carbonation rates.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (403) 210-6503. Fax: (403) 284-4852. E-mail: nader. [email protected]. Author Contributions †

A.G. and A.J. contributed almost equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding this study.

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02916 Ind. Eng. Chem. Res. 2016, 55, 11022−11028