Towards the insights into fast CO2 absorption over novel

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Towards the insights into fast CO2 absorption over novel nanostructured MgO-based sorbent Lixia Yang, Dan Liu, Pingping Wang, Hwimin Seo, Jianzhou Gui, and Yong-Ki Park Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01294 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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Industrial & Engineering Chemistry Research

Towards the insights into fast CO2 absorption over novel nanostructured MgO-based sorbent Lixia Yanga, Dan Liub*, Pingping Wangb, Hwimin Seoc, Jianzhou Guia,b*, Yong-Ki Parkc,* a

State Key Laboratory of Separation Membranes & Membrane Processes, School of

Material Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, P.R. China b

School of Environmental and Chemical Engineering, Tianjin Polytechnic University,

Tianjin, 300387, P.R. China c

Green Chemistry Division, Korea Research Institute of Chemical Technology,

Daejeon, 305-343, Republic of

Korea

*Corresponding Authors: E-mail addresses: [email protected] (D. Liu), [email protected] (J. Gui), [email protected] (Y. K. Park)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT A uniform nanoplate-like MgO-based sorbents were prepared with eitelite and hydromagnesite intergrowing each other, and their CO2 capture capacity was evaluated for fast CO2 absorption in thermal swing absorption process, with model wet industrial flue gas at a CO2 concentration of 14.5 vol%, at ambient pressure. Fast absorption and desorption rate, acceptable dynamic absorption capacity and good cyclic stability (9wt% after recycling 100 times) have been obtained. Some theoretical work has been carried out and found eitelite was the only origin of the active MgO species for fast CO2 uptake in TSA process. Nitrate was also demonstrated to have some promotive effect in CO2 fast absorption on the active MgO species. The effectiveness of eitelite was close related to its special crystal structure, i.e., after calcination, the special layer assembling with the proximity of Na+ and O2- at a molecular level probably contributed to the fast absorption. Meanwhile, the void space caused by decarbonization of hydromagnesite may also facilitate the absorption and diffusion of CO2. Therefore, this sorbent has been demonstrated to be a potential absorbent for post-combustion CO2 capture from fossil-fuel-fired power plant. Keywords: CO2 capture, thermal swing absorption (TSA), sorbents, double salt, MgO, nanostructured

1. INTRODUCTION There is growing concern that anthropogenic carbon dioxide (CO2) emissions are contributing to global climate change1-2. Wet absorption processes based on 2


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amine-based solvents are proven post-combustion CO2 capture technologies commercially available now, however, high energy requirement and efficiency penalty, corrosion, solvent instability and preliminary cooling of flue gas are the key problems existing in a conventional wet method. A promising alternative technology is the thermal-swing fluidized bed process with solid sorbents. In thermal-swing absorption (TSA) process, the sorbent selectively absorbs CO2 from a gas, and then with additional heat, the reaction is reversed to generate a concentrated stream of CO2 for storage. Compared with pressure swing absorption (PSA), TSA is often less expensive to operate, and causes low energy-penalty; solid sorbents show the advantages on lower-cost, abundant, environmentally benign3-4; Meanwhile, fluidized bed process is a mature efficient technology industrially. Finally, the colleagues in KRICT proposed a new concept, multi-stage energy exchangeable fluidized bed process5. To be an effective sorbent for this TSA fluidized bed process, one must have (1) adequate absorption/desorption kinetics at ambient pressure; (2) acceptable absorption capacity; (3) good stability after the absorption-desorption cyclic test. CO2 capturing from the medium-temperature gas stream (200-500oC) in some particular process streams, e.g. Integrated Gasification Combined Cycle (IGCC) process, is receiving increasing attention6-7. Usually, the pretreatment of flue gas to suitable absorption temperature by cooling or heating is needed before CO2 capture using low- or high-temperature sorbent, which will greatly increase the energy penalty of CCS process. Therefore, to develop an efficient and effective medium temperature 3



sorbent for this process is highly pursued. Dry sorbent candidates for CO2 capture have been reviewed8-12. In medium temperature range, no many effective candidates except MgO-based materials have been reported, and most recently a review on MgO-based materials for CO2 capture has been published13. Nanosized/ mesoporous MgO14-16, Mg(OH)217-19 and hydrotalcite20-23 all have been reported as medium temperature sorbents for PSA process, however, they all have the poor dynamic capacity for CO2, i.e., absorption capacity substantially decreases when slightly increasing absorption temperature, so none of them are good candidates for TSA process. Alkali-promoted MgO-based sorbents have been demonstrated to be one kind of intermediate temperature sorbents24. However, most of the previous studies show static absorption performance of sorbents in PSA process, i.e., pressure swing absorption of CO2 at a fixed temperature for a period of time. Mayorga et al. reported that the CO2 absorption capacity of prepared Na/K double salts (Na2Mg(CO3)2/ K2Mg(CO3)2) in a PSA cycle could vary from 4.8 to 57wt% at 375oC, which was determined by the preparation conditions and final solid compositions. The absorption capacity increased much under high-pressure CO2 stream, and about 75% of theoretic capacity could be maintained at 10 atmospheres of stream25. Singh et al. reported Mg double salts, especially Mg-Na double salt show promising capacity (20.68wt%) at 375oC in PSA process26. Looking for a good sorbent candidate for TSA fluidized bed process is challenging due to the slow absorption kinetic of MgO-based materials. We noticed 4


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that most sorbents for TSA process were evaluated under concentrated CO2 gas stream with long equilibrium absorption time of 30min or even longer. LiNO3-(Na-K)NO2-MgO shows an extremely high capacity of 69wt% under 100% CO2 gas stream, however, long CO2 absorption time(240min) is needed27-28. Sodium nitrate promoted MgO based sorbent give an absorption capacity could reach to 15.43wt% in 100% CO2 stream at 380oC for 1h, however，there is a long induction period of 20min needed, which must be shortened before the large-scale commercialization in fluidized bed process29-31. As we know that flue gas stream from fossil-fuel-fired power plant usually contains about 10-20% of CO2, 5-10% of steamed H2O and the others8, 15, 32. The dilution in CO2 and ambient pressure of the industrial flue gas is the big barrier for the post-combustion method33-35. Static absorption test in pure CO2 flow with time duration (>20min) under pressure is far different from dynamic CO2 absorption at ambient pressure in fluidized bed TSA process for industrial flue gas (i.e., wet diluted CO2). Therefore, it is of great significance to find a nice absorbent with fast absorption rate under wet diluted CO2 stream at the ambient pressure in TSA fluidized bed process. To increase the absorption rate of MgO sorbents, a homogenous nanoplate-like MgO-based sorbent was prepared and evaluated for dynamic cyclic CO2 capturing in 300-450oC at the ambient pressure for TSA process under a wet model flue gas stream. Meanwhile, a series of sorbents including pure eitelite had been prepared for CO2 capturing evaluation to determine the origin of active MgO species and the role of 5



nitrate in as-synthesized sorbents in TSA process, which was very important for the design of efficient MgO-based sorbent. Finally, the reason of eitelite, as the only origin of active MgO phase, had been studied from its crystal structure at a molecular level.

2. EXPERIMENTAL SECTION 2.1. Preparation of the sorbents Highly stable MgO-based sorbents were prepared by precipitation methods. At room temperature, the Na2CO3 aqueous saturated solution was gradually added to Mg(NO3)2 aqueous solution under stirring, with the Na/Mg mole ratio 6, and white slurry formed immediately. The stirring continued for additional 1h after feeding Na2CO3, and then the mixture was allowed to settle for 24 h. The precipitate was separated by filtration without supplementary water washing. The wet cake was dried at 120oC overnight. 2.2. Characterization of the sorbents (I) XRD: The analysis was carried out on Rigaku X-ray diffractometer operated at 30kV and 15mA, with Cu Kámonochromatized radiation (λ=0.15468nm). (II) SEM: SEM images were obtained with a high-resolution scanning electron microscope (Philips, XL-30S FEG Scanning Electron Microscope) equipped with energy dispersive X-ray spectroscopy (EDS). (III) TG-DSC: These techniques were used to monitor the weight and heat flow changes with SETSYS -1750 CS Evolution from Setaram Incorp. In this study, the TG-DSC curve was recorded by heating the sample from 50 to 700oC in the 6


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N2 atmosphere at a rate of 5oC/min. 2.3. Dynamic absorption-desorption cyclic test of the sorbents Dynamic absorption-desorption cyclic test of CO2 was performed using Autochem-2920 device (Micromeritics Inc.), which is equipped with a thermal conductivity detector for quantifying the amount of CO2 during the absorption and the desorption. First, a calibration has been applied to TCD signal, converting signal form arbitrary unit (a.u.) to the calibrated unit (cm3/min). Through the peak edit tool, the peak area was then calculated and converted to the calibrated unit, which is divided by the mass of the sample to obtain the gas uptake (cm3/g). The temperature range from 300oC to 500oC ramping at the rate of 10oC/min under a mixed gas flow of He (50mL/min) and CO2 (10mL/min) humidified by passing through a water bubbler at 53oC. At this temperature, the calculated moisture flow is about 10ml/min, so the vol. ratio CO2: H2O: He of the gas flow is 14.5:14.5:71, which can be well representing the compositions of flue gas. Before TCD detection, mixed gas flow passed through a cold trap for water removal. A two-cycle dynamic absorption-desorption cyclic test was carried out for the determination of absorption conditions. The temperature increased initially from room temperature to 500oC at 10oC/min then decreased to 250oC at the same rate, then swing the temperature to 500oC and finally decreased to 250oC, and there was no equilibrium time for absorption and desorption. After that cyclic stability test was carried out at the temperature of working ranges between 310 and 450oC, temperature ramping rate is 10oC/min for both heating and cooling step.

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The absorption time duration was10 min, and there was no holding time for desorption. In order to check the optimum absorption temperature after regeneration at high temperature, temperature-swing CO2 absorption test at different absorption temperature has also been carried out by temperature gravimetric (TG) analysis. The initial heating from room temperature to 500oC for 30min was conducted in 100% N2 to avoid the absorption before reaching the desired temperature, the remaining steps during the temperature swing were exposed to model flue gas flow of N2 (50ml/min) and CO2 (10ml/min). Different absorption temperature of 310, 305, 300, 295, 285 and 280oC and a desorption temperature of 450oC were selected for TSA tests, the absorption time durations was 10 min and there was no holding time for desorption, and the heating and cooling rate was 10oC/min. Static CO2 isotherms were also tested at different temperature (305, 300 and 290oC) for 2h under model mixed flue gas stream of N2 (50ml/min) and CO2 (10ml/min).

3. RESULTS AND DISCUSSION 3.1.CO2 absorption performance The effect of absorption temperature has been carried out by TG method, and the results are shown in Fig. 1. The sorbents were pretreated in the N2 flow at 500oC for 30min, and then swung the temperature from 500oC to 310oC at the rate of 10oC/min, and a slight weight increase (6.45%) occurred after holding at 310oC for 30min under dry flue gas flow. The sorbents were regenerated again with swinging temperature to 8


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450oC, and then cooling down to the desired temperature of 305, 300, 295, 285 and 280oC for absorption, Both of heating and cooling steps are carried out in the dry flue gas flow(He: 50ml/min, CO2: 10ml/min). The absorbent prepared in this study revealed high absorption capacity which easily reached 10wt% when absorption temperature was below 305oC, The suitable absorption temperature range is within 280-305oC. That is, no holding time for absorption is required if the absorption of CO2 is carried out below 300oC. The absorbent revealed high CO2 desorption rate at low temperature. Even at 420oC, the absorbent can be fully regenerated in the given flue gas composition. Usually, the absorption and desorption temperatures are strongly influenced by the test conditions and the nature of materials. Especially, the desorption temperature and time is influenced by the partial pressure of CO2 during desorption. As the concentration of CO2 increases, it becomes more difficult to desorb CO2 from the absorbent. According to the Zhang's result, to induce full desorption of CO2 in the N2 environment, about 470oC for 10min is required. So, to induce desorption in the high concentration 14.5vol% of CO2, the higher temperature (> 470oC ) or the longer desorption time (> 10min) is required29. However, the sorbent prepared in this study revealed full CO2 desorption even in 14.5vol% CO2 gas stream by increasing temperature to 450oC at a ramping rate of 10oC/min without holding time. Lower desorption temperature of our sample may be caused by the nanostructure morphology of the sample, facilitating CO2 desorption due to low diffusion limitation. Absorption temperature is also influenced by the absorption environment, 9



especially CO2 concentration. As the concentration of CO2 increases, it becomes easier for the absorbent to absorb CO2. According to the Zhang's result, the absorbent absorbs CO2 at 380oC for 60min in 100% CO2 environment29. If the concentration of CO2 decreases to 14.5wt%, the absorption temperature has to be much lowered. However, the nanostructured absorbent in this study revealed fast CO2 absorption at 305oC, short absorption time even in 14.5wt% CO2. Therefore, we can conclude that the morphology of the samples really makes the difference in the absorption and desorption speed and temperature. 25

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Fig. 1 CO2 absorption behavior measured through thermal gravimetric analysis at different temperatures from 310℃ to 280℃ In order to evaluate long-term stability, dynamic absorption-desorption cycles were carried out in Autochem-2920 equipped with TCD detector under the model flue gas composition exhausted in the electric power plant. The temperature range was within 310-450oC, the temperature swing rate is 10oC/min. The sorbents showed a very high absorption capacity of 9wt% even after 100 cycles, and narrow absorption 10


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and desorption temperature windows in the TSA of the low partial pressure of wet CO2 (14.5vol%). Meanwhile，dynamic absorption test has been done for a lower partial pressure of CO2 (10vol%), and the absorption capacity decreased 22% due to the further dilution of CO2. This means that the absorbent has good cyclic stability and fast absorption and desorption rate of CO2, which can be applied as a sorbent for the post-combustion CO2 capture process, especially for the coal-fired power plant. (See Fig. 2). TCD Signal

Temperature (°C)

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Fig. 2 Dynamic absorption-desorption cyclic test under mixed gas stream (He=50 ml/min, CO2=10ml/min, H2O=10ml/min) in 310-450℃ range ramping at 10℃/mi n (89th to 100th cycles)

Temperature gravimetric (TG) analysis absorption was carried out under the flowing condition of 60ml/min dry CO2 (vol. ratio He:CO2 = 5:1) at ambient pressure for 2h. The isotherm at different temperatures of 290oC, 300oC, and 305oC has been shown in Fig. 3. The absorption occurs in two steps. The first absorption within 10 min occurs very fast, which can be attributed to the formation of Eitelite phase. The next absorption proceeds very slowly and its rate is affected inversely by the

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temperature. That is, a slight increase in temperature led to an obvious decrease of absorption rate. At the beginning of absorption, only Eitelite phase was observed but both of Eitelite and MgCO3 phase were detected after 2h. From this result, it could be suggested that the formation rate of MgCO3 phase is a slow process and the stability of this phase is strongly influenced by the temperature. Regardless of the absorption temperatures, almost same amount of CO2 absorption was obtained after 10 min; the absorption capacities of 9.89, 9.73, and 9.31wt% were obtained after 10min absorption at 290, 300, and 305oC, respectively. This means that only certain amount of Eitelite is formed at the beginning of absorption and its formation rate is one order of magnitude faster than that of MgCO3 phase in the dry CO2 environment. 25

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Fig. 3 Static absorption test of the sorbent at different temperatures for 2 hours (A) enlarged figure in the first 10 minutes (B) In summary, the nano-structured MgO can be suggested as an efficient CO2 absorbent working in medium-temperature range (300-450oC) and regenerated easily by thermal swing. That is, the high absorption/desorption rate, good cycle stability, high absorption capacity (9wt% after 100 cycles) at low CO2 partial pressure make it be a promising sorbent for industrial application. Especially, this sorbent is useful for the fluidized bed TSA process which requires very fast sorption and desorption rates 12


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and has great potential to reduce energy penalty and operation cost. 3.2. Characterization of as-synthesized samples From the SEM images in Fig. 4A and B, the fresh sorbent is homogeneous assembled with nanostructured round plate, about 4μm in diameter and 50nm in thickness. The XRD pattern of the sorbent in Fig.4C shows that three different phases exist in the as-synthesized sample, including eitelite (Na2Mg (CO3)2, pdf:83-1591), hydromagnesite (Mg5(CO3)4(OH)2∙4H2O, pdf:25-0513) and sodium nitrate (NaNO3, pdf: 36-1474), among which the first phase was a major phase. From the homogeneity of the sample, the sorbents were formed by the intergrowth of eitelite and hydromagnesite phase. A

B

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*: Na2Mg(CO3)2 ^: Mg5(CO3)4(OH)2 4H2O

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Fig. 4 SEM images (A and its enlarged image B), XRD pattern (C) and EDS analysis results of fresh sorbent (D) The atomic ratio of Na to Mg has been tested by both EDS and ICP methods. EDS results were listed in Fig. 4D, and the atomic ratio of Na to Mg was 1:1. The ICP 13



test of the sample was in good agreement with the EDS result, and the atomic ratio of Na to Mg was 0.9:1. The atom ratio of Na to Mg in eitelite unit should be 2:1, much higher than Na to Mg atom ratio of the sample (about 0.9:1), so it further confirmed that eitelite and hydromagnesite phases were intergrowing together to form homogeneous nanoplate-like phase. BET surface area of this sorbent has been tested to be 22.82m2/g by N2 adsorption-desorption isotherm shown in Fig. 5. The N2 adsorption and desorption isotherms are the typical IVa with H3a type hysteresis loop, which reveals the non-rigid aggregates of plate-like particles36. TG-DSC analysis of sorbents and eitelite materials shown in Figure 6 is to study the amount of weight loss and thermal flow change in different temperature regimes. The weight loss over 650oC and an endothermic peak at about 655oC have been attributed to the decomposition of NaNO3 phase37, and the thermal decomposition process (

Towards the insights into fast CO2 absorption over novel

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