Unlocking synergistic benefits of the calcium looping-calcium hydroxide integration for energy storage: A perspective on sorbent performance

Unlocking synergistic benefits of the calcium looping-calcium hydroxide integration for energy storage: A perspective on sorbent performance Juan Arcenegui-Troya a,* , Andr´ es Carro b , Carlos Ortiz d , Ricardo Chacartegui b,f , Antonio Perej´ on c,e , Luis A. P´ erez-Maqueda c,* , Pedro E. S´ anchez-Jim´ enez c,* a Departamento de Electr´ onica y Electromagnetismo, Facultad de Física, Universidad de Sevilla, Avda. Reina Mercedes s/n, 41012 Sevilla, Spain b Departamento de Ingeniería Energ´ etica. Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain c Instituto de Ciencia de Materiales de Sevilla, C. S. I. C.-Universidad de Sevilla, C. Am´ erico Vespucio n o 49, 41092 Sevilla, Spain d Department of Engineering, Universidad Loyola Andalucía, Avda. de las Universidades s/n, Dos Hermanas, 41704 Seville, Spain e Departamento de Química Inorg´ anica, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain f Universidad de Sevilla, Laboratory of Engineering for Energy and Environmental Sustainability, 41092 Seville, Spain ARTICLE INFO Keywords: Calcium Looping Thermochemical energy storage Calcium hydroxide Calcium hydroxylation Calcium carbonate Carbonation ABSTRACT Calcium Looping has recently attracted attention as a high temperature thermochemical energy storage system. However, significant sintering due to the high temperatures hampers the recyclability of CaO. Hydration and hydroxylation has been explored as a method to regenerate the spent CaO. This study investigates a novel synergistic integration of carbonation (CaCO 3 /CaO) and hydroxylation (CaO/Ca(OH) 2 ) reactions. Calcination was conducted in N 2 and N 2 /H 2 O mixtures with 29 % steam content. Carbonation was conducted in CO 2 /H 2 O mixture with similar steam concentrations. Results show that steam plays a dual role: during calcination, it promotes the formation of large pores on the CaO surface, and during carbonation, it enhances mineralization, resulting in larger CaCO 3 grains. Also, steam promotes CO 2 diffusion through the CaCO 3 layer and, at the same time, significantly mitigates the deactivation of CaO along the cycles. Specifically, sequential calcination/ carbonation cycles without steam yield a residual conversion value of 0.14. Steam injection improved residual conversion to 0.27. Alternatively, the interleaving of hydroxylation/dehydroxylation cycles in the sequence further increased this value to 0.64 without steam and up to 0.76 with steam injection. Hydroxylation/dehydroxylation cycles alone demonstrated high stability, with a residual conversion of 0.98 when interleaved with calcination/carbonation cycles under 29 % steam conditions. Additionally, frequent hydroxylation/dehydroxylation cycles improve overall conversion stability, highlighting their synergistic benefits within the integrated process. This work underscores the potential of integrating Calcium Looping with Calcium Hydroxide for improved multicycle performance and opens pathways for scaling experiments to pilot systems, alongside assessing the efficiency and economic viability of this integrated approach. 1. Introduction Renewable energy sources have inherent drawbacks related to their dispatchability. Consequently, their deployment on a world scale depends on the development of energy storage technologies that can ensure the effective dispatchability of power whenever needed, regardless of prevailing weather conditions [1,2]. One of the proposals that has garnered significant interest in recent years is thermochemical energy storage (TCES) [3,4]. Compared to other storage systems based on phase change materials or sensible heat storage, TCES stands out for much higher energy densities and the potential for long-term storage with negligible losses [3]. In the TCES scheme, thermal energy from a renewable source is utilised to drive an endothermic chemical reaction. The resulting products are stored separately and can be recombined opportunistically to produce the reverse exothermic reaction whenever power is demanded [4,5]. Since the reaction is reversible, the material can be reused once recombination is completed to store and provide energy in a new cycle. Among the various systems under consideration for TCES, particular attention has been drawn to one based on the reversible calcination/ carbonation process of CaCO 3 /CaO, commonly referred to as calcium looping (CaL). Under standard operating conditions, the process is * Corresponding author. E-mail address: [email protected] (P.E. S´ anchez-Jim´ enez). Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej https://doi.org/10.1016/j.cej.2024.158775 Received 15 October 2024; Received in revised form 11 December 2024; Accepted 18 December 2024 Chemical Engineering Journal 504 (2025) 158775 Available online 22 December 2024 1385-8947/© 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/bync-nd/4.0/ ). described by Eq. (1). The many advantages exhibited by the CaL process comprise a high energy storage density and the potential for hightemperature heat release. Furthermore, the raw materials required for the process are earth-abundant and non-toxic, such as CaCO 3 -containing minerals (i.e., limestone or dolomite) [6,7], industrial wastes and byproducts [8–10]. CaCO3(s)⇌CaO(s) + CO2(g),ΔHr=178kJ/mol (1) Typically, the CaL technology as TCES has been proposed for its integration in large-scale concentrating solar power (CSP) plants [11,12]. However, integrating wind or PV-based electricity could also be considered [13]. Ongoing projects aim to advance the technology to a Technology Readiness Level (TRL) 5 level, thereby assessing its feasibility as a practical solution for efficient solar power storage [14]. In literature, calcination is typically conducted either in CO 2 at around 930 ◦C or in an inert gas (i.e., N 2 or He) at around 750 ◦C, while carbonation is carried out within 750–850 ◦C in pure CO 2 [15–17]. While calcination in inert atmosphere is often used to mitigate sintering in lab scale experimental research, feasible CaL-CSP integration scheme has to contemplate in closed CO 2 loop to avoid costly CO 2 separation procedures [17–20]. Due to the severe conditions endured during calcination-carbonation, CaO undergoes sintering that lead to a reduction of the reactive surface area, leading to the material’s deactivation in subsequent cycles [16,21]. Additionally, when carbonation takes place in CO 2 -rich environment, the nascent CaCO 3 rapidly forms a thick blocking layer on the CaO surface that impedes CO 2 from reaching the unreacted core and prevents further carbonation [7,22]. Since CaO deactivation eventually requires the removal of the spent sorbent to be replaced by fresh CaO precursor, it decreases the energy efficiency of the CaL-CSP technology and increases its cost [11]. Therefore, considerable effort has been devoted to finding strategies to prevent the loss in reactivity and circumvent pore plugging [23]. The inclusion of refractory additives is used to hinder grain growth and sintering, and also to form stable mixed oxides such as silicates or zirconates which act as structural supports for the active CaO/CaCO 3 particles [24–28]. However, they raise the cost of the raw material and lower the energy density. Also, it has been reported that additives eventually segregate after a number of cycles [7,29]. Thermal, mechanical and chemical treatments can be used to modify the reactivity of CaO: introducing structural defects, modifying crystallinity or reducing the particle size [26,30,31]. The use of smaller CaO particles improve conversion due to favourable surface to volume ratio [7,32]. However, often the higher activity is offset by enhanced sinterability and cohesiveness [33,34]. The addition of water has been used to reactivate spent CaO through transformation into Ca(OH) 2 ; the associated change in volume produces cracks in the particle that improves the reactivity in subsequent cycles [35,36]. However, steam generation entails an important energy penalty [23]. Finally, alternate carbonate systems such as BaCO 3 /BaO and SrCO 3 /SrO are currently under study to develop systems capable to operate at even higher temperatures [23,28,29,37]. Besides, the presence of steam at higher temperatures strongly influence the calcination and carbonation processes [38–40]. Initial studies investigating its effect on the CO 2 uptake capacity of CaO were conducted under CO 2 capture conditions [38,41–43]. It was generally observed that steam promoted CO 2 diffusion during the carbonation stage [38,40,44]. However, steam also displays a strong mineralising and sintering effect that offset any possible benefits when carbonation is carried out at high temperatures, as required in most CaL-CSP schemes [40]. Nevertheless, divergent results have also been reported; the CaO carbonation reaction being highly sensitive to thermodynamic equilibrium and the specific experimental conditions and experimental setups employed [45–47]. Considering conditions compatible with a CaL-CSP scheme, recent studies suggest that nascent CaO arising out of the CaCO 3 calcination in the presence of steam exhibits a more open and porous microstructure that is less prone to pore plugging, and also more reactive for subsequent carbonation stages [39,41,48]. Furthermore, steam promotes the decomposition of CaCO 3 , allowing a significant reduction in the calcination temperature [39,49]. This feature entails substantial benefits, as the calcination temperature conditions many technological aspects, including the reactors’ size, the receiver’s material, the heliostat field’s extension, the number of collectors and the radiative losses [19,50]. The reversible reaction of CaO and H 2 O can also be advantaged as a TCES at intermediate temperatures as the associated reaction enthalpy is also notable [51–53]: Ca(OH)2(s)⇌CaO(s) + H2O(g),ΔHr=109kJ/mol (2) Even though research on TCES concepts based on the CaO/Ca(OH) 2 system has advanced significantly in the last years, the attained TRL yet remains at laboratory stage [54–57]. The working temperatures lie within the 350 to 600 ◦C range [56,58], depending on the water pressure employed. As is the case with CaL, the energy storage capability of the material declines with the number of cycles. The deactivation is related to structural changes and particle agglomeration that lead to poor cyclability, poor mechanical properties and slow hydroxylation kinetics [55,59,60] Both calcination/carbonation and hydroxylation/dehydroxylation processes have been extensively explored as TCES systems for CSP plants. However, this study presents, for the first time, an experimental investigation of the potential synergistic benefits of integrating both technologies focusing specifically on optimizing the sorbent’s multicyclic performance. Specifically, the steam released during the dehydroxylation of Ca(OH) 2 can be utilized to enhance CO 2 capture efficiency and reduce sorbent attrition, as suggested in previous works [61,62]. The technical feasibility of an operational scheme that combines calcination/carbonation cycles with dehydroxylation/hydroxylation cycles has been recently studied by means of numerical simulations [63]. However, the concept still needs to be demonstrated experimentally, and the optimal conditions for harnessing steam effectively in terms of reaction kinetics and sorbent reactivation needs to be studied. This work further explores, at a laboratory scale, various operational conditions and their influence on the multicyclic performance of the storage system, aiming to identify the most favorable conditions for maximizing synergistic benefits. The use of hydroxylation/dehydroxylation cycles between carbonation/calcination cycles results in an improvement in multicyclic CaO conversion in both processes. This strategy can avoid the loss of reactivity and deactivation of CaO, the replacement by a new precursor, and the decrease in energy efficiency of the CaL-CSP technology. 2. Experimental The multicycle tests reported in this study were conducted with Fig. 1. Scheme of the experimental setup utilised in this work. J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 2 Limestone ESKAL60 supplied by KSL Staubtechnik GmbH. The samples used consisted of high-purity CaCO 3 (99.9 %), with a average particle size of 60 μ m. Particle size distribution can be found in Fig. S1, but also in reference [40]. The multicycle tests were carried out using a customised thermogravimetric analyser (TGA) LINSEIS STA PT 1600 with the capability for steam injection into the furnace. The experimental setup employed is schematised in Fig. 1. Liquid water was directed from a nitrogen-pressurised tank to a vaporiser through a liquid flow controller (LFC) Bronkhorst Liqui-Flow L13V12. The amount of water injected into the vaporizer is continuously monitored. The high accuracy of the flow controller (99 %) ensures the reproducibility of the steam injection conditions. Following vaporisation, the resulting steam was combined with nitrogen or carbon dioxide and introduced into the furnace at a 200 ml/ min flow rate. To prevent condensation, the transfer line was maintained at 165 ◦C. Mass flow controllers (MFC) regulate the gas flow into the vaporiser. Nitrogen was used as the purge gas, with a 10 ml/min flow rate. The impact of steam on the calcination and the carbonation kinetics was studied under linear heating conditions at a rate of 5 ◦C/min. For comparative analysis, these reactions were examined in the presence and absence of steam. While calcination was conducted in a nitrogen and water vapour mixture, with varying steam content (either 0 % or 29 %), carbonation experiments were performed using a mixture of carbon dioxide and water vapour, with various levels of H 2 O content (either 0 % or 29 % H 2 O). 29 % is the maximum steam volume that can be used safely to prevent damage to the thermobalance. Multicycle tests were undertaken, employing various combinations of conditions, to elucidate the impact of steam presence at different stages on the performance of CaO as a sorbent. Thus, the schemes tested were: •Cal 0: 20 calcination/carbonation cycles with 10-min calcination stages in 100 % N 2 at 750 ◦C, and 10-min carbonation stages in 100 % CO 2 at 850 ◦C. •Cal 29: 20 calcination/carbonation cycles with 10-min calcination stages in 71 % N 2 /29 % H 2 O at 750 ◦C and 10-min carbonation stages in 71 % CO 2 /29 % steam at 850 ◦C. •Cal 29-Hyd 29: 20 calcination/carbonation cycles with the intercalation of 20 hydroxylation/dihydroxylation cycles between two consecutive calcination/carbonation cycles: 10-min calcination stages in 71 % N 2 /29 % H 2 O at 750 ◦C, 10-min carbonation stages in 71 % CO 2 /29 % H 2 O at 850 ◦C for 10 min, hydroxylation at 400 ◦C for 10 min and dehydroxylation at 500 ◦C for 10 min, both in 71 % N 2 /29 % H 2 O. •Cal 0-Hyd 29: 20 calcination/carbonation cycles with the intercalation of 20 hydroxylation/dehydroxylation cycles between two consecutive calcination/carbonation cycles. 10-min calcination stages in 100 % N 2 at 750 ◦C, 10-min carbonation stages in 100 % CO 2 at 850 ◦C, hydroxylation at 400 ◦C and dehydroxylation at 500 ◦C, both in 71 % N 2 /29 % H 2 O. •Hyd 29: 20 hydroxylation/dehydroxylation cycles. Hydroxylation at 400 ◦C and dehydroxylation at 500 ◦C, both in 71 % N 2 /29 % steam. In all cases, the transition between stages at different temperatures consisted of linear heating or cooling at 30 ◦C/min with the consequent change of gas atmosphere. The influence of cycling on the microstructure of the samples under the examined diverse conditions was analysed utilising a HITACHI S4800 scanning electron microscope (SEM). Before analysis, the samples were coated with a layer of gold using an Emitech K550 Telstar sputtercoating machine (30 s, 30 mA). The specific surface area (S BET ) and pore size distribution of CaO after five cycles, conducted under varying atmospheric compositions, were determined through N 2 physisorption analysis. Before performing the physisorption analysis, the sample was degassed at 350 ◦C for 2 h. 3. Results and discussion 3.1. Calcination and carbonation in linear heating conditions Fig. 2a shows the time evolution of the sample weight during calcination in experiments conducted under linear heating conditions at a rate of 5 ◦C/min in N 2 . A temperature shift of approximately 50 ◦C is evident when comparing the results recorded in the presence of steam with those obtained without water vapour. This reduction in the required calcination temperature agrees with earlier findings on the effect of steam injection during calcination in the CaL-CSP scheme [39]. There is no consensus on the mechanism responsible of the increase in the decarbonation rate. Some authors ascribe it to the much higher heat transfer coefficient of steam [64,65], while others propose that H 2 O has a catalytic effect;. the stronger affinity of H 2 O for CaO promotes the displacement of CO 2 in favour of forming intermediate Ca(OH) 2 [66–68]. However, this is quite debatable at the high calcination temperatures employed, which are significantly above the equilibrium temperature for Ca(OH) 2 formation. In any case, CaL-CSP scheme might leverage this effect for reducing the CaO calcination temperature or the time required to attain full calcination and facilitate the implementation of flash vertical calciners [69]. Fig. 2b depicts the mass increase during carbonation under a linear cooling rate of 5 ◦C/min. Observe that the temperature (indicated on the upper scale of the graph) diminishes as time progresses. In this case, the Fig. 2. a) Weight evolution during calcination under linear heating at 5 ◦C/ min. The atmosphere composition consisted of 100 % N 2 in the case of the continuous blue line and 71 % N 2 /29 % H 2 O in the case of the dashed blue line. b) Weight evolution during carbonation under linear cooling at 5 ◦C/min. The atmosphere composition consisted of 100 % CO 2 in the case of the continuous green line and 71 % CO 2 /29 % H 2 O in the case of the dashed green line. J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 3 addition of steam lowers the carbonation temperature by approximately 25 ◦C. This change can be satisfactorily explained in terms of the reaction equilibrium temperature, which decreases ought to the dilution of CO 2 with steam. The equilibrium temperature for the reaction represented by equation (1) is given by [70,71]: Teq = − α ln(pCO2/A)(3) with A =4.083⋅107 atm, α =20474 K and pCO2 expressed in atm. The equilibrium temperature as a function of the CO 2 partial pressure has been plotted in Fig. 3 as a continuous blue line. The red and green dots mark the experimental conditions under which the results depicted in Fig. 2b were obtained. As might be seen, when CO 2 gets diluted from 100 % to 71 % by volume, equivalent to a partial pressure of 0.71 atm, the equilibrium temperature reduces from 895.2 to 872.9 ◦C. This change in 22.3 ◦C agrees with the shift observed in Fig. 2b. As highlighted in a recent study, the influence of dilution with steam on carbonation kinetics equals the effect of dilution with inert gas, such as nitrogen [40]. Thus, steam does not seem to play any role besides changing the pCO2. It is also noticeable in Fig. 2b the decrease in CaO conversion when the carbonation temperature diminishes. This agrees with the results reported in previous work that studied the conversion during the first cycle as a function of temperature. It is attributed to reduced CaCO 3 product layer thickness following an Arrhenius dependence [72]. Fig. 4a and 4b show SEM micrographs of the samples after calcination with and without steam, respectively. There is good agreement with previous results showing the presence of steam promotes the formation of larger surface pores [39,48]. On the other hand, Fig. 4c and 4d include micrographs of samples after carbonation. The mineralising effect of steam is evidenced by the larger size of CaCO 3 grains [40]. 3.2. Multicycle tests Fig. 5a illustrates the progression of conversion XC during the first and the twentieth cycles of calcination/carbonation. These cycles were conducted under two different set of conditions, as specified in section 2, namely Cal 0 and Cal 29-Hyd 29. The conversion values were derived from the mass measurements obtained through TGA, as per the following equations: Xc=(mass(%) − 56 56 )MCaO MCO2 (4) where MCaO and MCO2 are the molar masses of CaO and CO 2 , respectively. Two distinct stages are observed during the carbonation process. A first stage with a rapid escalation in the amount of CaO converted, typically referred to as a fast kinetic-controlled phase, followed by a slower carbonation stage characterised by a sharp decrease in the slope of the curve [22,73]. This latter is commonly termed the diffusioncontrolled phase, as carbonation proceeds through the diffusion of CO 2 through the CaCO 3 layer formed during the kinetic-controlled phase Fig. 3. Equilibrium temperature as a function of the CO 2 partial pressure. The coloured points correspond to the conditions imposed in our experiments. Fig. 4. SEM micrographs of (a,b) CaO samples arising after calcination in N 2 and N 2 /H 2 O atmosphere and (c, d) CaCO 3 formed after the subsequent carbonation in CO 2 and CO 2 /H 2 O. Experiments performed under linear heating conditions as depicted in Fig. 2. All the images use the same scale bar. J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 4 [74]. Two facts are remarkable in sight of the results presented in Fig. 2a. On the one hand, while steam does not affect the conversion rate during the kinetic-controlled phase, the slope of the curve during the diffusioncontrolled phase significantly increases in the presence of water vapour. This agrees with previous studies indicating that steam promotes the diffusion of CO 3 2– across the CaCO 3 layer, accelerating the carbonation process [40,44]. It has been also suggested that the creation of OH – ions after the dissociation of H 2 O could play a significant role [75]. As observed in the figure, the decay in conversion with the cycle number is much more limited when the tests are conducted in the presence of steam. Likewise, Fig. 5b shows the time evolution of conversion Xc during the first and the twentieth cycles of hydroxylation/dehydroxylation conducted under the conditions Hyd 29 and Cal 29-Hyd 29. In these cases, conversion was calculated according to: XH=(mass(%) − 56 56 )MCaO MH2O (5) being MH2O the molar mass of H 2 O. In both experimental schemes, the conversion attained during hydroxylation remains quite stable throughout the entire multicycle test. Still, the intercalation of one calcination/carbonation cycle, accompanied by steam injection, between two consecutive dehydroxylation/hydroxylation reduces the deactivation of the sorbent and leads to nearly full conversion values. Fig. 6a shows the conversion values attained during carbonation as a function of the cycle number N for the different conditions tested. Each point was calculated as the average of three different measurements. The error bars were estimated from the difference between the maximum and minimum conversion values reached at each cycle. It has been shown that conversion values can be fitted by the next equation [76,77]: XC=Xr+X1 k(N−1) + (1−Xr/X1)−1(6) being Xr the residual conversion, k the deactivation constant and X1 the conversion at the first cycle. As in the real application the material is expected to be cycled hundreds of times, it is crucial to know its multicycle performance after many cycles. In this sense, Xr represents the conversion achieved after an infinite number of cycles. Thus, it is a valuable indicator to compare performances. The fitting curves are plotted as continuous lines in Fig. 6, while the residual conversion values resulting from the fitting are collected in Table 1. Data depicted as open blue triangles correspond to the test conducted in the typical CaL-CSP conditions, excluding the presence of steam. The decay profile is typical of CaO-based materials precursors under these CSP compatible experimental conditions [17,48]. The deactivation over successive cycles is clear, with only 25% of the sorbent converted by the twentieth cycle and a residual conversion value of Xr=0.14±0.01. The injection of water vapour during the calcination/carbonation cycles enhances the residual conversion by 93 %, reaching Xr=0.27±0.01, what concurs with previous results [48]. The positive impact is mostly related to the increased surface porosity of the CaO particles formed after calcination, which exhibits a more open microstructure less prone to pore plugging and the loss of reactive surface by sintering [39,48]. The plot formed by green squares in Fig. 6a shows the effect of intercalating one hydroxylation/dehydroxylation cycle between two consecutive calcination/carbonation cycles (conditions Cal 0-Hyd 29). The sintering-induced decay is minimized by the intercalated hydroxylation, which serves to reactivate the CaO. The conversion stabilizes after 4–6 cycles, with a very high residual conversion Xr=0.64±0.01. Furthermore, the improvement is even more notable when the intercalation of hydroxylation/dehydroxylation cycles is combined with the addition of water vapour during the calcination/carbonation cycles (red squares). Using such experimental conditions, the estimated residual conversion is Xr=0.76±0.01. In summary, while steam addition during calcination has a positive impact on the multicycle performance, the results plotted in Fig. 6a demonstrate that the intercalation of the hydroxylation/dehydroxylation cycle accounts for most of the improvement observed. The positive synergy is also observed in the multicycle performance exhibited during the hydroxylation/dehydroxylation cycles, as shown in Fig. 6b. The conversion attained during hydroxylation has been depicted as a function of the cycle number for the various conditions tested. The conversion values achieved in the twentieth hydroxylation are collected in Table 1. The XH values remain high throughout the multicycle tests, demonstrating that, consistently with the conclusion of previous studies [54,57], hydroxylation is less prone to deactivation compared to carbonation. Such stability can be even enhanced by intercalating one calcination/carbonation cycle (with 29 % steam by volume) between two consecutive hydroxylation/dehydroxylation cycles, maintaining Fig. 5. a) Time evolution of experimental conditions (temperature and atmosphere) and Xc during the first and the twentieth cycles of calcination/ carbonation conducted in the conditions Cal 0 and Cal 29-Hyd 29. b) Progress of experimental conditions (temperature and atmosphere) and conversion XH during the first and the twentieth cycles of dehydroxylation/hydroxylation carried out in the conditions Hyd 29 and Cal 29-Hyd 29. J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 5 nearly full conversion (XH=0.98 ±0.02) after twenty cycles. Fig. 7a and b depict micrographs illustrating the surface of a CaO particle after five cycles conducted under two distinct conditions: Cal 29-Hyd 29 and Cal 0. These conditions correspond to the highest and lowest values of residual conversion, respectively. It is evident that the sample tested under Cal 29-Hyd 29 conditions displays a less sintered surface, with larger pores, making it less prone to pore plugging. This observation aligns well with findings reported in the literature and may account for the high conversion values achieved. Moreover, this is confirmed by the results of the N 2 physisorption analysis of these samples. While the sample tested in (29 % H2O)-Hyd/Deh(29 % H2O) possessed a specific surface area value of S BET =(7.3 ±0.2) m 2 /g and a specific volume of pores value of V p =(3.3 ±0.7)⋅10 -2 cm 3 /g, the one cycled in Cal 0 exhibited reduced values for both parameters: S BET = (1.15 ±0.12) m 2 /g and V p =(1.6 ±0.4)⋅10 -3 cm 3 /g. 3.3. Impact of intercalation frequency In a preceding investigation focused on the integration of calcium looping and calcium hydroxide thermochemical systems for energy storage, it was demonstrated that the intercalation of a hydroxylation/ dehydroxylation cycle every fith calcination/carbonation cycle led an improvement in carbonation conversion [63]. Nonetheless, this enhancement was relatively modest compared to the markedly positive Fig. 6. a) and b) conversion values achieved during the calcination/carbonation and the hydroxylation/dehydroxylation cycles, respectively, in the multicycle tests conducted in various experimental conditions. Table 1 Values of residual carbonation conversion determined from the fitting of Eq. (6) to data plotted in Fig. 6a and conversion values achieved during the twentieth hydroxylation. Conditions XrR2XH(N=20) Cal 29-Hyd 29 0.76 ±0.01 0.997 0.98 ±0.02 Cal 0-Hyd 29 0.64 ±0.01 0.984 0.88 ±0.02 Cal 29 0.27 ±0.01 0.996 — Cal 0 0.14 ±0.01 0.995 — Hyd 29 — — 0.86 ±0.02 J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 6 impact reported here. This prompts the inquiry into determining the optimal frequency of such intercalation to yield maximal benefits. Fig. 8a and 8b show the multicycle performance during carbonation and hydroxylation in the multicycle tests conducted intercalating one hydroxylation/dehydroxylation cycle with variable frequency, namely after one, two and three calcination/carbonation cycles. The residual conversion values resulting from the fitting of equation (6) to the data plotted are collected in Table 2. As might be observed, the optimal results are achieved when intercalation is carried out between two consecutive calcination/carbonation cycles. This observation reinforces the conclusions drawn from the analysis conducted in sections 3.2 and 3.3; most synergy benefits can be attributed to the intercalation of the hydroxylation/dehydroxylation cycles. Hence, increasing the frequency of these cycles enhances reactivity stability. 3.4. Process configuration and challenges in scaling-up the process Evaluating the potential scaling-up of the process, a key challenge of the combined system here proposed is to ensure a proper heat integration along the hydroxylation and dehydroxylation steps. If the condensation heat is not recovered, losses of 36 % of the solar thermal energy available in the dehydroxylation reactor could occur [78]. The condensation heat recovered during the charging phase can be used as evaporation heat during the discharging phase, allowing that the charging and discharging processes can be decoupled by storing the reaction products at ambient temperature and pressure [63]. Another option could be storing saturated steam in pressure vessels during the charging phase and recovering it during the discharging phase [52]. Fig. 9 shows a schematic of the streams involved in the integrated calcium looping and calcium hydroxide formation (CaL-Hydrox) process for TCES in CSP plants. Similar to the CSP-CaL process in large-scale energy storage systems [11], starting from a limestone stream (CaCO 3 ), solar thermal energy can be used to trigger the calcination reaction. However, in this case the CaO produced can go to the hydroxylation reactor instead of to the carbonator, depending on the ratio between hydroxylation/dehydroxylation cycles per carbonation/calcination cycle. The CaO produced in the dehydroxylation (using solar thermal energy) can go to the carbonator, completing the carbonation/calcination and hydroxylation/ dehydroxylation cycles tested in the laboratory. This process configuration poses additional challenges in scaling-up, such as the possibility of performing simultaneous or consecutive phases, depending on the scheduling of the power storage/production system. Numerical simulations demonstrated the technical feasibility of the integrated CaCO 3 / Ca(OH) 2 process, achieving efficiencies ranging from 38 % to 46 % [63]. If proper thermal integration is ensured, this efficiency range can be improved by increasing the multicyclic conversion of calcium oxide in the carbonator due to the positive effect of the hydroxylation step. Fig. 7. a) and b) sem micrographs of the resulting CaO after five cycles conducted under Cal 29-Hyd 29 and Cal 0 conditions, respectively. The results of the N 2 physisorption analysis are provided in the figure. Fig. 8. a) and b) conversion values achieved during the calcination/carbonation and the hydroxylation/dehydroxylation cycles, respectively, in the multicycles tests conducted intercalating the hydroxylation/dehydroxylation cycle after different numbers of calcinations/carbonations. Table 2 Values of residual carbonation conversion determined from the fitting of Eq. (6) to data plotted in Fig. 6a and conversion values achieved during the twentieth hydroxylation. Conditions XrR2 1 Cal/Carb – 1 Hyd/Deh 0.76 ±0.01 0.997 2 Cal/Carb – 1 Hyd/Deh 0.40 ±0.03 0.989 3 Cal/Carb – 1 Hyd/Deh 0.34 ±0.01 0.992 J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 7 4. Conclusions This work examines the potential integration of carbonation and hydroxylation cycles in a combined calcium looping system for TCES. Multicycle tests were performed using limestone in a custom thermogravimetric analyzer with steam injection capability. Calcination was conducted in a nitrogen and water vapor mixture with 29 % steam and without steam. Carbonation used a CO 2 and steam mixture with similar H 2 O content variations. In standalone calcination and carbonation tests, steam affected the CaO surface by increasing the presence of large pores. During carbonation, steam had a mineralizing effect, forming larger CaCO 3 grains. Multicycle results show that steam does not impact conversion during the kinetically controlled phase. However, during the diffusion-controlled phase, steam significantly increased the carbonation rate, indicating enhanced CO 2 diffusion through the CaCO 3 layer. While conversion decreased over cycles in all conditions, the loss of reactivity was more pronounced without steam. In pure calcium looping multicycle tests (calcination/carbonation), conversion dropped to 25 % after 20 cycles, with a residual conversion of 0.14. Steam injection improved the residual conversion to 0.27, highlighting its positive impact. Intercalating hydroxylation/dehydroxylation cycles between calcination/carbonation steps further increased residual conversion to 0.64, even without steam. The combined use of hydroxylation/dehydroxylation cycles and steam injection during calcination/carbonation cycles led to a residual conversion of 0.76. SEM micrographs revealed that samples tested under Cal 29-Hyd 29 conditions exhibited alleviated sintering and larger pores, making the material less prone to pore blockage. Hydroxylation/dehydroxylation cycles also showed strong multicycle performance. Conversion remained high throughout the tests, as hydroxylation is less prone to deactivation than carbonation. Stability improved further when a calcination/carbonation cycle with 29 % steam was added between hydroxylation/dehydroxylation cycles, achieving nearly complete conversion (0.98 residual conversion) after 20 cycles. This confirms that the intermediate hydroxylation/ dehydroxylation cycles drive most of the synergistic benefits. Increasing the frequency of these cycles further enhanced reaction conversion and maintained higher residual conversion stability. This work opens up new research avenues. From a materials perspective, further studies should explore the influence of particle size, temperature, gas composition and the impact of various steam fractions. From a process engineering perspective, based on the experimental results obtained, it would be required to optimize the system to determine the optimal amount of CaO to be sent for hydroxylation cycles versus the amount of CaO from the calciner that could directly proceed to the carbonation stage. Regarding the industrial application of the technology, it is crucial to refine the heat recovery system in both the CaO/ CaCO 3 and the hydroxylation/dehydroxylation cycles. In the former, the high temperature of the reactions presents a challenge, while in the latter, significant energy is required for steam generation, making efficient heat management critical. Although the proposed integrated cycle significantly improves the multi-cycle conversion of the material (addressing one of the main drawbacks of the technology), the industrial application and scaling of this technology will face additional challenges besides the optimization and adaptation of the operational conditions to a larger scale. These include efficient integration of the power cycle during carbonation and, most importantly, the development of efficient calcination systems using renewable energy sources. CRediT authorship contribution statement Juan Arcenegui-Troya: Writing – original draft, Investigation, Formal analysis, Data curation. Andr´ es Carro: Formal analysis. Carlos Ortiz: Validation, Methodology. Ricardo Chacartegui: Supervision, Conceptualization. Antonio Perej´ on: Software, Methodology, Formal analysis. Luis A. P´ erez-Maqueda: Project administration, Funding acquisition, Conceptualization. Pedro E. S´ anchez-Jim´ enez: Writing – review & editing, Supervision, Project administration. Fig. 9. Conceptual scheme of the streams in a CSP-TCES plant using the integrated CaL-Hydrox process. J. Arcenegui-Troya et al. Chemical Engineering Journal 504 (2025) 158775 8 Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Financial support is acknowledged from grants TED2021-131839BC22 and PDC2021-121552-C21 funded by MCIN/AEI/10.13039/ 501100011033 and by European Union NextGenerationEU/PRTR, and the grant PID2022-140815OB-C22 funded by MCIN/AEI/10.13039/ 501100011033 and ERDF A way of making Europe. Appendix A. 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