Low concentration CO2 capture in fluidized beds of Ca(OH)2 as affected by storage humidity

Low concentration CO2 capture in fluidized beds of Ca(OH)2 as affected by storage humidity Helena Moreno a, Francisco Pontiga a,*, José M. Valverde b a Departamento de Física Aplicada II, ETS Ingeniería de Edificación, Universidad de Sevilla, Av. Reina Mercedes 4, 41012 Sevilla, Spain b Departamento de Electrónica y Electromagnetismo, Facultad de Física, Universidad de Sevilla, Av. Reina Mercedes s/n, 41012 Sevilla, Spain. * Corresponding author. E-mail addresses: [email protected] (H. Moreno), [email protected] (F. Pontiga), jmill[email protected] (J. M. Valverde). © 2020. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/ You can view the Version of Record for this article at: https://doi.org/10.1016/j.cej.2020.127179 Abstract Carbon negative emission technologies (NETs) such as CO2 direct air capture (DAC) are being already considered as necessary for climate change mitigation. This paper investigates CO2 capture at room temperature and atmospheric pressure by a fluidized bed of Ca(OH)2 powders as influenced by the relative humidity (RH) to which this sorbent was exposed during storage. Humidity is precisely controlled by means of several supersaturared salt solutions, and FTIR spectrometry is used to measure accurately the time evolution of CO2 and H2O concentrations in the effluent gas. Results show that CO2 capture is promoted by an increase of the storage RH. The observed effect is particularly important for very high storage RH (~ 100%, near the dew point). After the capture tests, quantitative analyses of samples composition were carried out using X-ray powder diffractometry. These analyses revealed that the CaCO3 content after CO2 capture occurs via carbonation in samples that were stored under ~100% RH. On the other hand, in samples stored under low to moderate RH, the presence of CaCO3 was significantly reduced, indicating that most of CO2 capture takes place through physical adsorption. Keywords CO2 capture; calcium hydroxide; carbonation; direct air capture; pre-hydration; storage conditions; fluidized bed 1. Introduction The recent UN Climate Change Conference COP 25 held in Madrid in 2019 has urged governments around the world to comply with the recommendation of Paris agreement to keep the global mean temperature rise below 2 ºC relative to pre-industrial levels. Reaching such objective will require strong mitigation measures in all sectors that contribute to CO2 emissions [1]. In this regard, carbon capture and sequestration (CCS) technologies are being developed to reduce CO2 emissions from high carbon intensity sources such as fossil fuel based power plants and cement production, where flue gases are released with concentrations of the order of 12-15 vol% [2, 3]. However, because of the limited efficiency of capture technologies, a residual CO2 concentration is still liberated in cannot be used to mitigate distributed CO2 emissions from other sectors such as transport, residential, agriculture, etc., which account for a significant fraction of total CO2 emissions (~ 40%) [4]. An increasing number of studies are pointing out that the 2 ºC goal will hardly be achieved without large-scale implementation of carbon negative emission technologies (NETs) during the second half of this century [5, 6, 7]. Sustainable bioenergy with carbon capture and storage has been usually considered the most feasible option of negative-emission technologies [8, 9], albeit direct air capture (DAC) technologies are thought to play a significant role in the mid-term [10, 11]. In fact, some prototypes and commercial DAC plants are already in operation worldwide showing the techno-economic feasibility of this technology 2 by 2040 [12]. Importantly, the increasing levels of CO2 in the atmosphere is also a concern for the construction sector, since CaCO3 formed in the reaction of CO2 with Ca(OH)2 in the cement lowers the pH in reinforced concrete structures, which accelerates corrosion [13]. CO2 capture at low concentration is generally based in the use of a variety of sorbents, such as inorganic chemisorbents (solids or in solution), zeolites, organoamine adsorbents, etc. [14, 15]. Alkaline metal oxides and alkaline earth metal oxides have been reported as good sorbents of CO2. Calcium-based oxides have the additional advantage of being the most abundant among alkaline earth metal oxides in nature. For that reason, the Calcium Looping (CaL) process, based on the cyclic calcination/carbonation of CaCO3/CaO at high temperature, is gaining attention in the last years to capture CO2 from high carbon intensity sources, where very large flow rates with a relative high CO2 vol% (around 15%) must be processed [2, 16, 17]. The need for high carbonation temperatures in this application imposes a fundamental limitation on the capture efficiency, since CO2 concentration levels in the effluent gas from the carbonator reactor cannot be decreased below the equilibrium CO2 vol% (around 1% for T ~ 650ºC) [18]. The performance of CaO-based sorbents is also being tested for ambient temperature CO2 capture from low-CO2 streams, as potential candidates for DAC [19]. Although the carbonation reaction is very slow at low temperatures, the CO2 vol% at equilibrium is virtually zero at ambient temperature, which would allow full CO2 removal from the inlet gas by using small gas flow rates, considering also the relatively low concentration of CO2 present in the atmosphere. Thus, recent studies have concluded that, by choosing suitable operating conditions, the use of CaO-based sorbents can be a viable solution for DAC [20]. Among other factors, the relative humidity (RH) in the CO2 loaded stream is known to play a key role on the capture performance of Ca-based sorbents at low temperatures. Shih et al. [21] investigated the reaction of Ca(OH)2 with CO2 in humid N2 at 60-90 ºC. They used a fixed bed reactor, and the bed was humidified before the capture experiments using a certain RH, at which capture was to be performed afterwards. They observed that the reaction of Ca(OH)2 with CO2 to form CaCO3 only occurs when the RH is above a critical value of 8%. However, since their experiments were carried out with relatively large concentrations of CO2 (up to 12 %) and high gas flow rates (4 L/min), observations could not be made about the impact of the RH on the CO2 breakthrough time. Ridha et al. [19] have conducted a similar research, at room temperature, using lime and pelletized lime-cement particles as sorbents of CO2. They used a fixedbed reactor and, before experiments, the sample was hydrated up to 8 hours by passing through it a flow of humid N2 at a flow rate of 650 mL/min. Then, CO2 was incorporated to the humid gas flow in a low concentration that varied from 0.5% to 2%. Although the RH to which the sample was exposed was not measured, their results showed that the pre-hydration phase had a significant effect on the CO2 capture rate. For example, they observed that the CO2 breakthrough time was increased from 21 min to 660 min for lime when an inlet concentration of 0.5% of CO2 was used. Samari et al. [22] have pursued further this work using an average CO2 concentration of ~0.04% in the inlet air. They pre-hydrated for 3 h the sample by passing nitrogen through a water bubbler and then through the fixed bed. The air containing CO2 was then introduced in the reactor using gas flow rates of 0.5 and 1.0 L/min. They observed that the absence of humidity in the inlet air led to a significantly shorter breakthrough time. However, pre-hydration was always perfomed with a single relative humidity (RH ~ 55%). Dheilly et al. [23] have exposed slaked lime to a controlled atmosphere of CO2 with different RHs (30%, 60% y 100%), and measured the time evolution of carbonation of Ca(OH)2 for a period of 10 days. The experiments were made inside an environmental chamber, in which the atmosphere was continuously renewed at a rate of 3 L/h. In samples exposed to very low CO2 concentrations (~0.03%), RH of 30%, and temperature above 20-30 ºC, carbonation of Ca(OH)2 was not observed. However, CO2 capture occurred progressively as the ambient RH was raised, and the effect was more intense at the lower temperatures. The authors also conducted other experiments in which Ca(OH)2 was previously humidified in an oxygen atmosphere for 25 days with a RH of 100%. Then, the sample was exposed to the CO2 loaded atmosphere. They observed a substantial CO2 capture in the first moments. After 25 days, no more hydroxide was present. In a very recent paper, Erans et al. [20] have used lime and hydrated lime samples exposed for a prolonged time to ambient air in different environments with the aim of studying its carbonation performance. Small amounts of the samples were periodically analyzed in order to determine their hydration and carbonation degree. They observed that hydrated lime, which was prepared by calcining limestone and then hydrating the sample using a mechanical granulator and deionised sprayed water, captures CO2 much faster than lime: 50% carbonation conversion was reached in 168 h using hydrated lime, compared to 453 h using lime. The present work deals with the use of hydrated lime, Ca(OH)2, as CO2 sorbent from gas streams with low CO2 concentration at room temperature. In particular, this study aims to investigate the effect of passive pre-hydration on the CO2 capture performance of Ca(OH)2. As discussed above, the effect of forced prehydration of lime and hydrated lime on CO2 capture has been already considered in previous studies [19, 21, 22]. Forced pre-hydration is usually achieved by passing a humid gas flow across the sorbent. In contrast, passive pre-hydration consists of a prolonged exposure of the sorbent to a controlled humidity environment, as might occur during sorbent storage in practice. Here, a CO2-free storage atmosphere will be used during the passive hydration phase, which may lead to hydration but prevents carbonation unlike in other studies [22, 23]. A variety of humidity storage environments will be considered, ranging from a very dry atmosphere (RH 24%) to a very humid one (~ 100%). Additionally, most of previous investigations on CO2 capture that include pre-hydration also consider the presence of water in the gas stream across the sample during the CO2 capture test [19, 21, 22]. While this may be advantageous for enhancing CO2 capture, it can be a confounding factor to determine the role that prehydration has played on its own. In this work, CO2 capture tests will be carried out using a dry mixture of CO2 (1 vol%) and N2 at room temperature to prevent the sample from being exposed to additional moisture during CO2 capture experiments. The concentrations of CO2 and H2O at the exit of the reactor will be continuously and accurately monitored by means of FTIR spectroscopy. This technique allowed us measuring the CO2 breakthrough time as a function of sorbent storage RH with high precision. Since CO2 capture can be influenced by the gas-solid contact area, analyses of BET surface area and particle size distribution have been carried out on samples prepared for the capture tests. Figure 2: Frequency curve of the volume of particles as a function of the particle size in selected samples prepared for CO2 capture tests using Ca(OH)2 stored at different relative humidities as indicated. Figure 3: SEM images of nanosilica agglomerates (grey) covered by Ca(OH)2 particles (white) of two selected samples prepared for the CO2 capture test. (a) Ca(OH)2 stored at RH 24%. (b) Ca(OH)2 stored at RH 100%. significant reduction in the strength of attractive van der Waals forces between the particles, as shown in previous works [36]. Moreover, the hydrophobicity of nanosilica is expected to minimize the role of adsorbed moisture on the fluidization of the samples used in this study compared to raw Ca(OH)2. In order to test the possible effect of moisture on the powder fluidization behavior, three samples were prepared using Ca(OH)2 stored at different RH (24%, 75.5% and 100%), and their bed expansion curves were measured. These measurements were carried out using dry N2, and following the same procedure as in CO2 capture tests. That is, an initial gas flow rate of 1 slpm was imposed through the sample for a period of 3 minutes to initialize the sample in a reproducible state. Then, once the gas flow had stopped and the sample had settled, the gas flow rate was increased in steps 0.01 slpm every 15 s. The results of these tests are shown in figure 4. As can be seen, the relative bed expansions of the samples stored at RH below 100% follow a close trend. At flow rates above 0.2 slpm, the bed expansions reach a plateau, which is higher for the sample stored at 100% RH. Presumably, a high level of humidity can enhance fluidization because it hinders surface tribocharging, which might lead to attractive forces between the particles and between the powder and the internal vessel walls, as seen from visual inspection in figure 5. In CO2 capture experiments, a flow rate of 0.1 slpm was used in all tests, for which a bubble-free fluidization is observed. At this flow rate, the samples prepared using the Ca(OH)2 stored at RH 24% and 75.5% Figure 4: Relative bed expansion as a function of the gas flow rate of Ca(OH)2/nanosilica powder mixtures employed in the CO2 capture tests, which were prepared using Ca(OH)2 stored at different relative humidities as indicated. Figure 5: Photographs of the fluidized bed in Ca(OH)2/nanosilica samples prepared with Ca(OH)2 stored at (a) RH 24% and (b) RH 100%, for a gas flow rate of 0.1 slpm. exhibit very similar values of the relative bed expansion: (16 ± 3)% and (18 ± 3)%, respectively. In contrast, the sample prepared using Ca(OH)2 stored at RH 100% shows a higher relative bed expansion, (29 ± 2)%, which would favor the gas-solid interaction and therefore promote further CO2 capture. 4. Experimental set-up Figure 6 shows a schematic representation of experimental set-up used in the experiments. The powder bed was confined in a 20 cm long glass tube, with inner diameter of 23.4 mm. The bottom side of the glass tube was closed with a glass microfiber filter (Whatman Grade GF/A), with a nominal particle retention of 1.6 µm. The sample to be tested (3.45 grams) was deposited upon the glass microfiber filter, which was replaced after every test. The exit of the glass tube was closed with a foam filter, to prevent elutriated particles from exiting the fluidized bed reactor. An additional external pneumatic filter (filtration size 0.01 µm) was inserted between the reactor and the Fourier-transform infrared (FTIR) spectrophotometer (Bruker Vertex 70) that was used for the gas analysis. In order to help fluidization of the sample, the reactor was placed on a vibration exciter (TIRA TV51110), which was driven with a frequency of 50 Hz. Dry N2 (gas purity > 99.999%) and CO2 (gas purity > 99.995%) were employed for the CO2/N2 mixture. A system of three mass flow controllers (Alicat MC-5SLPM-D, MC-200SCCM-D, MC-2SCCM-D) and solenoid valves were used to prepare the mixture of these gases and to divert the flow either directly to the spectrophotometer or to the bed reactor and then to the spectrophotometer. The CO2 capture experiment was carried out at room temperature and atmospheric pressure as follows. Firstly, a gas flow of 1 slpm of N2 was imposed through the sample for a period of 3 minutes, in order to initialize the sample in a reproducible state. Then, a gas mixture of 1% CO2 and 99% N2, with a gas flow rate of 0.1 slpm, was sent directly to the spectrophotometer, and the absorption spectrum was registered to obtain a calibration reference for this combination of gases. Finally, the same gas mixture and flow rate was diverted Figure 6: Experimental setup used in the CO2 capture 2 and H2O are detected in the effluent gas by means of a FTIR spectrometer. towards the bed reactor, and the effluent gas was analyzed in the spectrophotometer. During this period, IR spectra were recorded every 40 s, in order to track the temporal evolution of the CO2 and H2O contents in the gas leaving the reactor. The timestamp of each FTIR spectrum was later corrected for the finite transit time of the gas from the reactor to the spectrometer cell. This transit time was measured at the end of the CO2 capture experiment. To this purpose, the CO2/N2 gas flow was suddenly replaced by pure N2, and the elapsed time span to observe this change on the FTIR spectra was measured. A transit time of 85 s was obtained. The infrared spectrophotometer used in the experiments has a spectral resolution better than 0.16 cm-1, which allows the individual peaks in CO2 and H2O spectra to be accurately resolved. The uncertainty in CO2 vol% measurements was below 10 ppm. The optics and the sample compartments of the FTIR spectrophotometer were continuously purged with CO2-free (less than 1 ppm) ultra-dry air ( 73 ºC dew point) to minimize the interference of ambient CO2 and water vapor. Prior to the tests, the base line of the spectrophotometer was recorded after filling the gas cell with N2 in order to suppress the contribution of any impurity that might be present in the buffer gas. For the spectral determination of H2O vol% in the effluent gas, a calibration curve was previously built using a dew point and pressure transmitter (Vaisala DPT146). Finally, X-ray powder diffractometry (Bruker D8 Advance A25) was used as a complementary technique to analyze the composition of some samples after the CO2 capture tests. Rietveld refinement technique was applied to quantify the percentage of calcite (CaCO3) and portlandite (Ca(OH)2) in these samples. 5. Experimental results and discussion In order to assess the effect of passive pre-hydration on the CO2 capture capacity of hydrated lime for its use in DAC, a gas mixture with low CO2 concentration (1% in volume) was used in all the experiments. Figure 7 shows the time evolution of the CO2 volume fraction at the exit of the reactor for Ca(OH)2 samples, previously stored in inert environments of different RH levels following the procedure described in section 2. As can be seen, CO2 is completely removed from the inlet gas stream for a certain period of time (the so-called breakthrough time, BT), which depends critically on the RH storage conditions. Once complete CO2 capture is ended, the CO2 vol% in the effluent gas raises very fast in a short period of just a few seconds, after which it approaches asymptotically the CO2 vol% in the inlet gas mixture. As shown in previous works [24, 31, 32], the sharp rise of CO2 after the BT is helped by the nanosilica additive added to the sorbent, which promotes the gas-solid contacting efficiency. The phase of complete CO2 capture until BT is then prolonged in time, and CO2 appears abruptly in the effluent gas once the reaction sites become saturated. Arguably, this special feature would facilitate in practice switching on/off the capture/sorbent regeneration regimes. As it will be shown later, most of CO2 capture takes place before the BT, when the effluent gas is completely CO2 clean. The influence of the RH during the storage of Ca(OH)2 on the breakthrough time can be clearly appreciated in figure 8. For values of the storage RH up to 75.5%, the BT and, therefore, the capture capacity of samples, increases moderately as the storage RH is raised. In contrast, the sample stored at a RH close to the dew point (RH ~ 100%) exhibits a BT substantially higher than the Figure 7: Time evolution of CO2 volume fraction in the effluent gas during CO2 capture by a bed of Ca(OH)2 fluidized with a dry CO2/N2 gas mixture (1 vol.% CO2). Calcium hydroxide samples were stored at different RH levels, as indicated. The vertical arrow points to the breakthrough time corresponding to the sample stored at RH of 100%. rest of samples: more than twice of that found at a storage RH of 75.5% and more than three times the BT observed at storage RH of 24%. The surface of sorbent actually exposed to CO2 is a main limiting factor for the capture reaction. As shown in section 3, both the BET surface area and the mean diameter of sample particles tend to increase slightly with the RH to which Ca(OH)2 was exposed during the pre-hydration phase. However, the increase of surface area of samples with RH was very small and approximately linear, which does not explain the highly nonlinear augmentation of the BT measured during CO2 capture tests for samples stored at very high RH. In contrast, bed Figure 8: Breakthrough time (BT) measured in the CO2 capture tests as a function of the relative humidity at which the sorbent was previously stored. expansion in samples exposed to very high RH (~ 100%) was found to be higher than in the rest of samples (figure 4). Expectedly, the enhancement of fluidization facilitates further CO2 capture by promoting the gas-solid interaction. However, the present results seem to be more consistent with the findings by Beruto and Botter [26]. These authors measured the equilibrium adsorption isotherm of H2O on Ca(OH)2 at 20 ºC, and observed a smooth linearly increase of water adsorption in the range of relative pressures from 20% to 70%. Then, for RH above 80%, the rate of water adsorption increased very sharply. That result is indicative of the formation of a thick multilayer adsorbed film of H2O, which behaves as a bidimensional liquid-like interface that facilitates the reaction of CO2 with Ca(OH)2. It has been argued that the physicochemical processes leading to the carbonation of Ca(OH)2 at low temperatures consist of several steps mediated by the presence of physisorbed water [26, 37]. Firstly, Ca(OH)2 is partially dissolved and dissociated inside adsorbed water layers, releasing Ca2+ and OH ions, Ca(OH)2(s) Ca2+(aq) + 2OH (aq) (1) and rendering the solution alkaline. Since CO2 is slightly soluble in H2O at low temperatures, aqueous CO2(aq) is formed, and its reaction with the liquid H2O layer gives rise to carbonic acid, concentration follows a different decay pattern. A short initial transient, in which drying is the main source of H2O, is followed by a much slower decay that extends up to the BT (vertical arrow), approximately. The increase of H2O vol% with respect to the drying test evidences that carbonation is occurring in this sample, and the same conclusion can also be applied to tests carried out in samples stored at RH of 58% and 75.5%. Nevertheless, in all these samples, carbonation is much less significant than that observed in Ca(OH)2 stored at a RH of 100%, as suggested from the huge presence of water vapor measured in the effluent gas (red squares in figure 10). In order to quantify precisely the amount of CO2 that was captured by the carbonation reaction, Ca(OH)2 samples were analyzed using X-ray powder diffractometry after the conclusion of the CO2 capture tests. During the time between the end of the capture test and the X-ray analysis, the samples were stored in airtight bags, in order to prevent any further carbonation. A Ca(OH)2 sample, as received, was also analyzed for a reference. The diffractograms obtained from the XRD analysis are shown in figure 12. Intensity peaks corresponding to portlandite, Ca(OH)2, and calcite, CaCO3, have been identified with markers inside the figure. Calcite has a very intense main diffraction peak at 2 29.5º. The inset in figure 12 shows a magnified view of this peak to appreciate the role played by the storage RH conditions. Rietveld refinement was used to calculate quantitatively the portlandite and calcite contents in the analyzed samples. The variation in CaCO3 mass content with respect to the as received sample can be seen in figure 13 as green bars. Also, the estimated increase in CaCO3 mass content in the samples, assuming that the capture of CO2 occurred entirely according to the carbonation reaction (6), is presented in figure 13. These data have been calculated from the FTIR measurement of CO2 Figure 12: X-Ray diffraction patterns of Ca(OH)2 samples as received, and after being stored at different relative humidities (as indicated) and exposed to a flow of dry CO2/N2 (1 vol.% CO2 main figure. The inset shows a subset of the scattering angle, corresponding to the most intense peak of calcite (CaCO3). Markers identify the main intensity peaks of portlandite Ca(OH)2 ( ), calcite ( ), and overlapping peaks of portlandite and calcite ( ). concentration in the effluent gas for the entire duration of the capture test (blue bars) and for the duration of the BT (red bars). X-ray analysis confirms that carbonation of Ca(OH)2 increases with the RH at which Ca(OH)2 was maintained during the pre-hydration phase. However, while the observed carbonation is weak (below 1%) in samples stored at RH of 24% and 75.5%, it rises up sharply up to 7.5% for 100% storage RH. This last value is similar to the one derived from the FTIR measurement in the BT (8.4%). In contrast, in the other samples (RH of 24% and 75.5%), the expected increase of Figure 13: Expected increase in CaCO3 mass according to FTIR analysis (red and blue bars) and actual increase according to X-Ray diffraction analysis (green bars) after CO2 capture in Ca(OH)2 samples previously stored at different RH conditions (as indicated). CaCO3 according to the FTIR analysis should be much higher than that found from the XRD Rietveld analysis. This apparent disagreement seems to indicate that the capture of CO2 in these samples occurs mainly through physical adsorption, or that chemisorption of CO2, if occurs, does not end up as precipitated CaCO3. The contrary is true when the Ca(OH)2 sample was prehydrated using a RH of about 100%. In that case, nearly 80% of the CO2 captured was in the form of CaCO3, which is consistent with the high concentration of H2O observed in the effluent, as can be expected from the carbonation global reaction expressed by reaction (6). As previously discussed, water physisorbed during pre-hydration of the samples is the main factor that determines carbonation in experiments. As the carbonation reaction proceeds, a layer of precipitated CaCO3 is formed on the surface of the Ca(OH)2 particles, which hinders the dissolution of further Ca(OH)2 and precludes the diffusion of CO2 across this barrier. However, in CO2 capture applications, this limitation can be partially overcome by incorporating water vapor into the CO2 gas stream. In such a case, physisorption of this extra humidity in non-passivated sites of Ca(OH)2 particles would provide new reactions sites for further carbonation to proceed. 6. Conclusions This work has investigated how CO2 capture by a fluidized bed of Ca(OH)2 is affected by previous storage of the sorbent in a controlled humidity environment. The CO2 capture tests were carried out at ambient temperature and atmospheric pressure using a low CO2 vol% in the inlet gas flow, in order to assess the performance of these samples for DAC. Pre-hydration of the sorbent during storage was achieved by passive exposure to a variety of accurately controlled relative humidities, belonging to a wide range of environments from very dry to close to dew point. Results from CO2 capture tests have shown that the storage RH has a critical effect on the CO2 breakthrough time and hence on the overall CO2 capture capacity of the sample: the higher the storage RH, the longer the breakthrough time and the greater the mass of CO2 captured. However, the correlation between the CO2 capture capacity and the storage RH is not linear, which is related to the mechanism that governs physisorption of water in Ca(OH)2 at room temperature. Samples stored at RH close to 100% exhibited much higher capture capacities and extraordinarily prolonged CO2 breakthrough times. X-ray powder diffractometry has revealed that, in these samples, most of CO2 capture occurs by carbonation. In contrast, for lower storage RH contributes only by a small fraction to CO2 capture. In this case most of the CO2 captured is probably dissolved or in the form of bicarbonate ions in the physisorbed water present in Ca(OH)2. 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