New approaches on carbonate based thermochemical energy storage systems integrated with concentrated solar energy

NEW APPROACHES ON CARBONATE-BASED THERMOCHEMICAL ENERGY STORAGE SYSTEMS INTEGRATED WITH CONCENTRATED SOLAR ENERGY Nabil Mohamed Amghar Department of Reactivity of Solids Institute of Materials Science of Seville University of Seville Sevilla, 2023 NEW APPROACHES ON CARBONATE-BASED THERMOCHEMICAL ENERGY STORAGE SYSTEMS WITH CONCENTRATED SOLAR ENERGY Author: Nabil Mohamed Amghar Director: Luis Allan Pérez Maqueda Profesor de Investigación Director: Pedro Enrique Sánchez Jiménez Científico Titular Tutor: María Dolores Alcalá González Profesora Titular Instituto de Ciencias de los Materiales de Sevilla (ICMS-CSIC) Universidad de Sevilla Sevilla, 2023 Abstract PhD Thesis i ABSTRACT The extensive rely on fossil fuel sources, although it is the primarily responsible for the technological advancements since the industrial revolution, it has also contributed to the acceleration of climate change era. Among the main current societal challenges is the reduction of CO2 emissions into the atmosphere, which is derived from the burning of fossil fuels used mainly in thermal plants to produce energy. Renewable energies have emerged as a clean and viable alternative to decreasing the rate of greenhouse gases (GHG) and CO2 emissions. The deployment of renewable energy sources worldwide is extensive, although their intermittency is a significant hindrance to their universal application. Solar radiation is considered one of the most promising sources for reducing emissions and its integration with current available technologies is relevant. The Concentrated Solar Power (CSP) tower technology involves the concentration of solar radiation by heliostats onto a receiver on the top of a tower, allowing for high temperatures to be achieved. The greatest challenge for solar technology is the limited duration of solar time during the day (roughly, 8 h). This issue can be partially overcome through Thermal Energy Storage (TES) solutions, among which, the thermochemical energy storage (TCES) is the least developed but holds the highest energy density. This thesis examines different TCES solutions as a complement to CSP tower plants. The proposed approach involves utilizing solid-state chemical reactions to store heat. While there are a numerous TCES systems available, natural carbonate materials are considered the most promising option. Of these, the CaCO3/CaO system has been amply studied due to its high energy density, wide availability and low-cost. Moreover, the involved temperatures are suitable for integration into CSP. Specifically, this doctoral thesis evaluates the impact of closed-loop CO2 conditions on TCES applications. The calcination temperature exceeds 900 °C to accomplish rapid rates of decomposition, resulting in stronger sintering-induced deactivation. Here, we propose the use of reduced absolute CO2 pressure to match the optimal operational CSP tower temperature. Calcination under mild vacuum conditions (at 0.1 and 0.01 bar) is similar to the use of an inert atmosphere (such as N2, Ar and He) for the decomposition of limestone without altering the reaction atmosphere. This contributes to the reduction in plant expenses, as gas separation is not required. Also, the use of acicular calcium acetates as CaO precursors in storage materials with sintering-resistant morphology that improves the multicycle calcination/carbonation performance has been investigated. An alternative carbonate system, the SrCO3/SrO pair, has been less studied as the complete decomposition within a CO2 close circuit requires ultra-high temperatures (> 1200 °C), surpassing the temperatures required by CSP towers. Ultra-high temperature involves significant sintering and a large growth of grains, ultimately leading to lower efficiencies. For the improvement of the system, it is considered the addition of high Abstract ii PhD Thesis Tammann temperature materials as dopants. Moreover, the reduction of the calcination temperature is also contemplated lowering the absolute CO2 pressure of the system to enable a decrease in temperature to 900 °C under mild vacuum conditions which could make the system approachable for CSP technology. Furthermore, the synthesis of strontium acetates is also suggested as a means to improve the SrO multicyclic activity. Moreover, one of the greatest advantages of the TCES systems is the capability to store the by-products at room temperature, although any possible effect on subsequent cycles has yet to be studied. A thorough assessment is carried out on the behaviour of the CaO sorbent during storage, taking into account several temperatures, durations and atmospheres. Furthermore, the impact of limestone particle size is also taken into consideration. Eventually, the utilization of litter from cigarette butts, which has a severe impact on the environment, is regarded as the initial step in obtaining highly porous CaO. Cigarette butts are made of cellulose acetate which is impregnated with calcium nitrate and by means of solution combustion synthesis (SCS) gives rise to a more porous CaO particles. This approach has the potential to significantly reduce waste and produce energy through TCES application. Additionally, it can contribute to the decarbonization of society through the capture and sequestration of CO2. x FIGURES INDEX Figure 1.1.1: Global energy-related CO2 emissions and the increase from 1990 to 2021. Data source: Global Energy Review 2021 [4]. ....................................................... 2 Figure 1.3.1: Types of Concentrated Solar Power (CSP) technologies (obtained from [16]). ........................................................................................................................ 5 Figure 1.3.2: Operational under construction and development CSP projects (adapted from [19]). .......................................................................................................... 6 Figure 1.3.3: CSP global capacity per country (adapted from [20]). ................... 7 Figure 1.3.4.: Current thermal CSP plants in Spain (collected from [20]). ......... 7 Figure 1.3.5: Electricity consumption and solar power production (obtained for [22]). ................................................................................................................................. 8 Figure 1.4.1: Scheme of TES process (adapted from [24]). ................................ 9 Figure 1.4.2: TES methods: sensible, latent and thermochemical storage. ....... 10 Figure 1.4.1.1: Schematic diagram of the molten salts process with CSP tower technology. Figure acquired from [51]. .......................................................................... 12 Figure 1.4.2.1: Energy density for the most common PCMs (extracted from [52]). ........................................................................................................................................ 13 Figure 1.4.3.1: Scheme of a TCES system (adapted from [25]). ....................... 14 Figure 1.5.1: Equilibrium curve of the CaCO3/CaO process extracted from [81]. ........................................................................................................................................ 16 Figure 1.5.2: CaCO3/CaO reaction phases. ........................................................ 17 Figure 1.5.3.: Example of the CaO deactivation. ............................................... 18 Figure 1.6.1.1: Diagram of CaL-CCS scheme for CO2 sequestration from flue gas. .................................................................................................................................. 21 Figure 1.6.1.2: Element abundance on Earth's surface. Figure extracted from [137]. .............................................................................................................................. 22 Figure 1.6.2.1.: Schematic diagram of the CaL-TCES system (obtained from [142]). ............................................................................................................................. 24 Figure 1.7.1: Equilibrium curve of the SrCO3/SrO system adapted from [149].28 Figure 1.7.2: SrCO3 decomposition under CO2 atmosphere. ............................ 29 Figure 3.2.1.1: Scheme of the mixing process for the SrZr10 sample. ............. 51 Figure 3.2.2.1: Schematic diagram of the synthesis of the AcSrMg10 sample. 52 xi Figure 3.2.2.3: Schematic diagram of the Ca-based derived from cigarette butts. ........................................................................................................................................ 53 Figure 3.3.3.1.1: Scheme of the thermobalance assembled for vacuum operation. ........................................................................................................................................ 55 Figure 3.3.3.1.2: Time evolution of sample mass (%), temperature (°C) and absolute pressure (bar) for a calcination/carbonation cycle of limestone. In this test, carbonation was carried out at 1 bar CO2 and 850 °C, and calcination at 0.1 bar CO2 and 700 °C. ............................................................................................................................ 56 Figure 3.3.3.2.1: STA 449 F5 Jupiter from NETZSCH. ................................... 58 Figure 3.3.3.2.2: Time evolution of sample mass (%) and temperature (°C) for a calcination/carbonation cycle of SrCO3. In this test, carbonation was carried out 1200 °C, and calcination 1400 °C in a CO2 closed-loop. .............................................................. 58 Figure 3.3.3.3.1: Perkin Elmer 8000. ................................................................. 59 Figure 3.3.3.3.2: Time evolution of sample mass (%) and temperature (°C) for a calcination/carbonation cycle of CaL-TCES tests for TCBs 0.1Ca sample. In this experiment, carbonation was carried out 800 °C, and calcination 950 °C in a CO2 closedloop. ................................................................................................................................ 60 Figure 3.3.3.4.1: Time evolution of sample mass (%) and temperature (°C) for a calcination/carbonation cycle of CaL-CCS conditions for TCBs 0.1Ca sample. In this test, carbonation was carried out at 650 °C for 5 min in an atmosphere of 15 % CO2 / 85 % N2 (vol/vol), and calcination at 900 °C for 5 min in 70 % vol. CO2 and 30 % vol. of N2. .. 61 Figure 4.1.1: (a) Schematic diagram and (b) photograph of the homemade thermobalance ................................................................................................................. 75 Figure 4.1.2: Time evolution of sample mass (%), temperature (ºC) and absolute pressure (bar) for a calcination/carbonation cycle of limestone. In this test, carbonation was carried out at 1 bar and 850 ºC, and calcination at 0.1 bar CO2 and 700 ºC. .......... 77 Figure 4.1.3: Time evolution of effective conversion (Xeff) measured during calcination/carbonation cycles in test T2........................................................................ 78 Figure 4.1.4: Multicycle effective conversion data measured for natural limestone tested under the calcination/carbonation conditions listed in Table 4.1.1. (a) Calcinations under 0.1 bar CO2. (b) Calcinations under 0.01 bar CO2. Solid lines represent the best fits of Equation (8) to data (best fitting parameters are shown in Table 4.1.2). ................. 79 Figure 4.1.5: Time evolution of CaO conversion during the carbonation and calcination stages of the 1st and 19th cycles for the different tests. (a,b) Calcinations under 0.1 bar CO2. (c) and (d) calcinations under 0.01 bar CO2. ............................................. 82 Figure 4.1.6: SEM and TEM micrographs of CaO derived from natural limestone after one calcination stage at 765 ºC and under an absolute pressure of (a,b) 0.1 bar (test T1), and (c,d) 0.01 bar (test T3). .................................................................................... 83 xii Figure 4.1.7: SEM and TEM micrographs of CaO derived from natural limestone after 20 cycles at 765 ºC. (a,b) 0.1 bar (test T1), and (c,d) 0.01 bar (test T3). ............... 84 Figure 4.1.8: Comparison of the main diffraction peak (2 0 0) corresponding to CaO (2Ɵ =37.3 º) for limestone samples calcined in vacuum and air at 765 ºC. ........... 86 Figure 4.2.1: XRD patterns of (a) dolomite, and the different acetates prepared in this work: (b) CaAc, (c) Ca80Mg20Ac and (d) Ca95Mg05Ac. ................................... 104 Figure 4.2.2: SEM micrographs of (a) dolomite, (b) Ca95Mg05Ac, (c) Ca95Mg05Ac after decomposition to the carbonate, and (d) Ca95Mg05Ac after decomposition to the oxides. ........................................................................................ 105 Figure 4.2.3: Scheme of the in-house thermogravimetric instrument used in this work. ............................................................................................................................. 106 Figure 4.2.4: Time evolution of the mass and temperature for the sample Ca80Mg20Ac, measured during calcination/carbonation cycles under experimental conditions E2. ............................................................................................................... 108 Figure 4.2.5: Effective conversion as a function of the cycle number measured for the four samples studied in this work, tested under experimental conditions E1-E5: (a) dolomite; (b) CaAc; (c) Ca80Mg20Ac; (d) Ca95Mg05Ac. The Xeff values of limestone calcined at 765 ºC under 0.1 bar CO2 and carbonated at the same temperature at an absolute pressure of 1 bar CO2 is also included for comparison. ................................. 110 Figure 4.2.6: Effective conversion during the first 90 s of carbonation at the first, fifth and tenth cycle for the samples tested under experimental conditions E4. .......... 112 Figure 4.2.7: Accumulated Dv for dolomite, CaAc, Ca80Mg20Ac and Ca95Mg05Ac. The value for limestone is also included, calculated from the data reported in [46]. .......................................................................................................................... 114 Figure 4.2.8: In-situ XRD patterns of Ca95Mg05Ac, measured from 400 ºC to 750 ºC in vacuum. ........................................................................................................ 115 Figure 4.2.9: (a,b) SEM micrographs of Ca95Mg05Ac after first calcination at 0.1 bar CO2. (c,d,e) EDX analysis of the sample subjected to 10 calcination/carbonation cycles, ending in calcination. ........................................................................................ 116 Figure 4.2.10: HRTEM micrographs of Ca95Mg05Ac: (a-d) after calcination at 0.1 bar CO2; (e-h) subjected to 10 calcination/carbonation cycles ending in calcination. ...................................................................................................................................... 117 Figure 4.2.11: HRTEM micrographs and HAADF-STEM mappings of Ca95Mg05Ac: (a-d) after calcination at 0.1 bar CO2; (e-h) subjected to 10 calcination/carbonation cycles ending in calcination. .................................................. 117 Figure 4.3.1: Schematic diagram of the preparation of treated cigarette butts (TCBs)-derived CaO materials. .................................................................................... 133 xiii Figure 4.3.2: X-ray diffractograms of the samples obtained after the flaming combustion step. ........................................................................................................... 136 Figure 4.3.3: N2 adsorption-desorption isotherms of TCBs 0.1Ca, TCBs 0.5Ca and TCBs 0.1Ca10Mg. ................................................................................................. 138 Figure 4.3.4: SEM images of (a) CBs before flaming combustion, (b) TCBs 0.1Ca as produced after flaming combustion and (c) particle size distribution of TCBs 0.1Ca after flaming combustion. ............................................................................................. 139 Figure 4.3.5: Time evolution of temperature and effective conversion for multicycle CaL-TCES tests of (a) TCBs 0.1Ca, (b) TCBs 0.5Ca, (c) TCBs 1Ca and (d) TCBs 0.1Ca10Mg. The experiments consisted of 5-min calcination stages at 950 °C and 5-min carbonations at 800 °C. ...................................................................................... 140 Figure 4.3.6: Multicycle effective conversion of impregnated TCBs samples: (a) with calcium nitrate and (b) with 0.1 mol/L of calcium nitrate (TCBs 0.1Ca) and different magnesium nitrate concentrations (5 mol %, 10 mol % and 20 mol %) with respect to the molar amount of calcium. TCBs 0.1Ca is presented in both graphics for comparison (red circles). ......................................................................................................................... 142 Figure 4.3.7: SEM images of CaO derived from impregnated TCBs subjected to 20 calcination/carbonation cycles under CaL-TCES conditions, ending in calcination. ab) TCBs 0.1Ca, c-d) TCBs 0.5Ca and e-f) TCBs 0.1Ca10Mg. .................................... 144 Figure 4.3.8: Secondary electron micrographs (SEM at 2 and 20 kV) and compositional mapping of CaO derived from TCBs 0.1Ca10Mg showing Ca and Mg after 1 cycle (a-c) and 20 cycles (d-f). .................................................................................. 145 Figure 4.3.9: Accumulated storage energy (ASE) of each sample considering 1000 calcination/carbonation cycles. ............................................................................ 146 Figure 4.3.10: Time evolution of temperature and effective conversion obtained in multicycle tests performed according to CaL-CCS operating conditions: calcination stages at 900 °C in 70 % CO2/30 % N2 (vol./vol.) and carbonation stages at 650 ºC in 15 % CO2/85 % N2 (vol./vol.). (a) TCBs 0.1Ca, (b) TCBs 0.5Ca, (c) TCBs 1Ca and (d) TCBs 0.1Ca10Mg. .................................................................................................................. 147 Figure 4.3.11: Multicycle evolution of the CO2 uptake of the TCBs samples impregnated (a) with different calcium nitrate concentrations and (b) with 0.1mol/L of calcium nitrate (TCBs 0.1Ca) and different magnesium nitrate concentrations (5 mol %, 10 mol % and 20 mol %) with respect to the molar amount of calcium. TCBs 0.1Ca is presented in both graphics for comparison (red circles)............................................... 149 Figure 4.3.12: SEM pictures of CaO derived from impregnated TCBs subjected to 20 calcination/carbonation cycles under CaL-CCS conditions, ending in calcination. ab) TCBs 0.1Ca, c-d) TCBs 0.5Ca and e-f) TCBs 0.1Ca10Mg. .................................... 151 Figure 4.4.1: Particle size distribution measured for C80 and C150. .............. 166 xiv Figure 4.4.2: Schematic diagram of the experimental procedure for evaluating storage tests. Blue and yellow indicate which experiment segments were carried out in N2 or CO2 atmospheres. ..................................................................................................... 167 Figure 4.4.3: Time evolution of the storage conversion for calcined C80 samples, maintained in CO2 atmosphere at different temperatures. ............................................ 171 Figure 4.4.4: Mass percentage gains as a function of time for samples tested at 50 °C and 200 °C in CO2. a) C80; b) C150. Legend is shared for both graphics and represents the temperature of the storage step. The red ellipse highlights the value for 1-week storage. .......................................................................................................................... 172 Figure 4.4.5: a) Time evolution of temperature and sample mass (C80 sample) recorded in the TGA during multicycle calcination/carbonation tests using a 60-min storage step at 50 °C. b) Close-up view of the first cycle. Calcination and carbonation were carried out in a CO2 atmosphere for 5 min at 950 °C and 800 °C, respectively. Blue and yellow highlight the segments under N2 or CO2 atmospheres. .............................. 172 Figure 4.4.6: a) Multicycle evolution of the CaO conversion, calculated from Equation (22) and Equation (23), for C80 and C150. b) Close-up of the last ten cycles for C80 and C150. Legend is shared for both graphs. Unfilled symbols represent samples submitted to the 200 °C storage step and cross-filled symbols to the 50 °C storage step. Solid symbols represent CaO conversion when a storage step at a high temperature (800 °C) in N2 is considered (5-min calcination and carbonation at 950 °C and 800 °C, respectively). ................................................................................................................ 173 Figure 4.4.7: Volumetric energy density values as a function of the cycle number for C80 particles tested by including in the multicycle experiment a storage step a) 50 °C, b) 200 °C in CO2 and c) 800°C in N2. Values were calculated using Equation (25). . 175 Figure 4.4.8: Process Flow Diagrams (PFDs) evaluated: a) storage at low temperature (based on [13]); b) storage at high temperature (based on [15]). ............. 178 Figure 4.4.9: a) Plant efficiency and b) overall energy density as a function of the storage temperature and the particle size. ..................................................................... 180 Figure 4.5.1: Diffractograms after the mechanical treatment of a) SrCO3, b) SrZr10, c) SrSi10 and d) SrMg10. ................................................................................ 199 Figure 4.5.2: SEM micrographs of SrCO3 derived from 10 wt % loads after mechanical milling for a) SrZr10, b) SrSi10 and c) SrMg10. ...................................... 200 Figure 4.5.3: Time evolution of temperature and sample mass (SrZr10 composite) obtained from TGA measurements during multicycle calcination/carbonation tests. Experimental conditions were 5-min calcination stages at 1400 °C and 5-min carbonation at 1200 °C, both in a CO2 atmosphere. ......................................................................... 201 Figure 4.5.4: Multicycle effective conversion measured for a) composites with 5 wt % additive loads and b) 10 wt % additive loads. Data corresponding to milled SrCO3 are shown in both figures for comparison (black squares). .......................................... 202 xv Figure 4.5.5: X-Ray Diffractograms of a) SrZr10, b) SrSi10 and c) SrMg10 after the multicycle tests. ...................................................................................................... 204 Figure 4.5.6: Time evolution of the derivative of effective conversion during the a) first, b) tenth and c) nineteenth cycles for 10 % load composites. Legend is shared for all graphs. ..................................................................................................................... 206 Figure 4.5.7: Cycle evolution of the fast and diffusive reaction-controlled stage ratio for composites with a) 5 wt % loads and b) 10 wt % loads. ................................ 207 Figure 4.5.8: SEM micrographs of SrCO3 derived from 10 wt % loads after multicyclic activity for a) SrZr10, b) SrSi10 and c) SrMg10. ...................................... 208 Figure 4.5.9: a), d), g) Secondary electron micrograph (SE at 2 and 20 kV) and compositional mapping after 20 cycles for Sr (b, e, h) and c) Zr of the SrZr10, f) Si of the SrSi10 and i) Mg of the SrMg10. ................................................................................. 209 Figure 4.5.10: a) Volumetric energy density as a function of the number of cycles and b) accumulated volumetric energy density per sample during 20 cycles. Sr composites have been tested at 1400 °C/1200 °C for calcination/carbonation in CO2 closed circuit for 5 min each. For the sake of comparison, calcium carbonate has been tested at 950 °C/850 °C for calcination/carbonation in CO2 closed-loop for 5 min each (brown squares and line). .............................................................................................................................. 210 Figure 4.5.11: Process Flow Diagram. ‘M’ stands for either Sr or Ca. Adapted with permission from [30]. ........................................................................................... 213 Figure 4.6.1: Schematic diagram of the setup employed for performing the calcination/carbonation cycles. Calcinations and carbonations were conducted at absolute CO2 pressures of 0.01 bar and 1 bar, respectively. ....................................................... 233 Figure 4.6.2: Time evolution of temperature and sample mass for SrZr10. Experimental conditions involve an isotherm at 900 ºC and 10 min of calcination and carbonation at absolute CO2 pressures of 0.01 bar and 1 bar, respectively. ................. 234 Figure 4.6.3: Effective conversion as a function of the cycle number for the samples studied in this work: (a) SrCO3, SrZr10, SrSi10, SrMg10 and SrAl10; (b) SrCO3, AcSr and AcSrMg10. ................................................................................................... 236 Figure 4.6.4: Cycle evolution of the energy storage density for: (a) SrCO3, SrZr10 and AcSrMg10, subjected to 20 calcination/carbonation cycles, using absolute pressures of 0.01 bar and 1 bar of CO2 for calcination and carbonation, respectively; (b) SrCO3 and SrZr10 subjected to 20 calcination/carbonation cycles at an absolute pressure of 1 bar CO2. .............................................................................................................................. 238 Figure 4.6.5: XRD patterns of (a) SrCO3, (b) SrZr10 and (c) AcSrMg10 after being subjected to ten calcination/carbonation cycles ending in carbonation. ............. 240 Figure 4.6.6: XRD patterns of (a) SrMg10, (b) SrAl10 and (c) SrSi10 after being subjected to ten calcination/carbonation cycles ending in carbonation. ....................... 241 xvi Figure 4.6.7: SEM micrographs of SrZr10 (a), and AcSrMg10 (d), and the corresponding EDX analysis (b,c,e,f) of the samples subjected to 10 calcination/carbonation cycles, ending in calcination. ................................................. 242 Figure 4.6.8: CSP-TCES conceptual scheme. Me represents either Sr or Ca, while XO represents the additive (MgO or ZrO2). ................................................................. 243 xvii xviii TABLES INDEX Table 1.4.1.1: Most common materials for SHS. ............................................... 11 Table 1.4.3.1: List of the most common TCES systems. ................................... 15 Table 3.2.1.1: Materials composition of the different composite made by drymilling. ............................................................................................................................ 51 Table 3.2.2.1: Mass and molar composition of the different acetates synthesized. ........................................................................................................................................ 52 Table 3.2.2.3: Composition of cigarette butts after submitted to flaming combustion. .................................................................................................................... 53 Table 3.3.3.1.1: Operation conditions used for the multicycle test of limestone. ........................................................................................................................................ 56 Table 3.3.3.1.2: Operation conditions of the experimental exposure used for CaO sorbents. .......................................................................................................................... 57 Table 4.1.1: Operating conditions for the different calcination/carbonation tests carried out in this work, and their corresponding acronyms for their identification throughout the text. ......................................................................................................... 76 Table 4.1.2: Values of deactivation rate (k) and residual conversion (X) obtained by fitting Equation 8 to multicycle conversion experimental data (Figure 4.1.4). ....... 80 Table 4.1.3: Specific surface area (SBET) and porosity data (Vp) of the CaO samples, measured after the first, third and fifth cycles under the conditions corresponding to tests T1 and T3. .......................................................................................................... 85 Table 4.2.1: Molar and mass ratio of CaO and MgO for the four samples used in this work. The theoretical densities of the corresponding calcined materials are also included. ....................................................................................................................... 103 Table 4.2.2: Operating conditions used for calcination and carbonation. The acronyms have been used to identify the experimental conditions in the text. ............ 106 Table 4.2.3: Effective conversion in the first and tenth cycle, and the difference between them (Xeff) for the four samples studied in this work under experimental conditions E1-E5. ......................................................................................................... 111 Table 4.2.4: Surface area and pore volume of the samples after calcination at 0.1 bar CO2. ........................................................................................................................ 113 Table 4.3.1: Molar and mass percentage of CaO and MgO in the TCBs-derived CaO materials prepared in this work. ........................................................................... 133 Table 4.3.2: XRF analysis of untreated CBs and TCBs 0.1Ca, TCBs 0.5Ca, TCBs 1Ca and TCBs 0.1Ca10Mg samples after the flaming combustion synthesis. ............. 136 xix Table 4.3.3: BET surface area, pore volume and pore size of TCBs 0.1Ca, TCBs 0.5Ca and TCBs 0.1Ca10Mg samples. ......................................................................... 139 Table 4.3.4: CaO conversion at the first and twentieth cycles, and residual conversion (Xr), deactivation constant (k) and R-squared (R2) values obtained from fitting to Equation 13 the conversion values presented in Figure 4.3.6.. .............................. 143 Table 4.3.5: CO2 uptake, expressed as g CO2/g calcined material, at the first and twentieth cycles, residual CO2 uptake, deactivation constant and R-squared obtained for the samples tested under CaL-CCS conditions. ........................................................... 150 Table 4.4.1: Particle size distribution (PSD) parameters of the two limestone samples. Dv (10), Dv(50), and Dv(90) indicates the percentiles meaning the 10 %, 50 % and 90 % of the sample is smaller than the given size. ................................................ 165 Table 4.4.2: Experimental conditions for the different calcination/carbonation tests carried out in this work. ........................................................................................ 168 Table 4.4.3: Values of CaO conversion for the first (XCaO,1) and twentieth (XCaO,20) cycles. ............................................................................................................ 173 Table 4.4.4: Accumulated volumetric energy density of limestone samples with a storage step at 200 °C and 50 °C in CO2 and 800 °C in an inert atmosphere (N2). ..... 174 Table 4.4.5: Main assumptions in the CSP-CaL model. .................................. 177 Table 4.5.1: Composition and particle size distribution (PSD) parameters of the different samples studied. ............................................................................................. 198 Table 4.5.2: Experimental estimation of the effective conversion measured in the first and twentieth cycles for all samples tested. .......................................................... 205 Table 4.5.3: Values of volumetric energy density for the twentieth cycle and cumulative energy density stored per sample. Solid density use for each composite is also displayed. ...................................................................................................................... 211 Table 4.5.4: Energy balance for the CSP-TCES integration using the SrCO3/SrO and CaCO3 /CaO systems. ............................................................................................ 215 Table 4.5.A1: Main stream data for the CSP-TCES integration based either on CaCO3 or SrCO3. .......................................................................................................... 218 Table 4.6.1: Precursors, mass ratio, and methodology employed to prepare each sample tested in this work. ........................................................................................... 232 Table 4.6.2: Xeff at the first and tenth cycle for the samples tested in this work. ...................................................................................................................................... 236 Table 4.6.3: Energy balance for the CSP-TCES integration (Figure 4.6.8). ... 245 Introduction 4 PhD Thesis accessible, largely due to the mounting environmental concerns and the drive to cut the carbon footprints and shift away from more conventional energy sources. A modest investment in researching and developing new technologies to mitigate the impact of climate change. 1.2. Renewable energies During the latest era, renewable energies have become the most widely used technology globally. This occurred due to the ever-increasing installation capacity and priority dispatch [13,14]. Moreover, it is predicted that renewable energies will reach new heights due to an ambitious upscaling of building capacity. This capacity accounts for 95 % of global power capacity increase through 2026 [5]. Solar photovoltaics (PV), followed by wind and hydropower, are the systems which benefit the most and have grown at a faster pace than in previous years. Although commodity prices have inflated, it is forecasted that installation capacity will grow by 17 % [5]. Despite efforts to expand renewable energies power capacity and displace fossil fuels for energy production, there are significant drawbacks that slow down full implementation of the technology. These include the large space required to build energy stations which has subsequent negative effects on the environment. In addition, the intermittency of the natural source means that it can only operate for limited hours per day and at low-effective levels. Finally, expensive surplus storage also entails a significant challenge to the investment in renewable energy. Therefore, renewable energies represent the most optimal choice available, albeit not without limitations for large-scale GHG emission reduction in energy production. Hence, prioritizing the discovery of novel clean energy sources is imperative. 1.3. Solar energy Among all renewable sources, solar energy is a promising option because of the high radiation flux that reaches the Earth's surface from the sun. Two kinds of solar radiation reach the surface: diffuse and direct radiation. The latter can be concentrated, resulting in stronger light beams. PV cells made of semiconductor materials are used in solar technology to convert solar radiation into electrical energy directly using the photoelectric effect. This process is accomplished through the use of certain materials that possess the ability to absorb photons and emit electrons, being silicon the most extensively employed material [15]. The capture of free electrons leads to the generation of an electric current that can served as a source of electricity. These cells are connected in a series or in parallel circuit to produce a solar panel, which is tasked with supplying voltage and current as dictated by energy requirements. Introduction PhD Thesis 5 On the other hand, Concentrated Solar Power (CSP) facilities comprise a set of mirrors called heliostats. These mirrors concentrate the solar radiation onto a receiver or collector which then converts it into heat. This heat is used to produce steam, which can either drive a turbine to generate electricity or be directly utilised as process heat in manufacturing industries. CSP technology is gaining increasing attention for its great potential for massive deployment and commercialization in the near future, particularly for low-cost thermal heat storage solutions. There are four types of CSP technologies (Figure 1.3.1) which differ in the type of collector used [16]. These are: • Power Tower: it is also known as central receiver systems, they use sun-tracking heliostats to focus sunlight onto a receiver situated at the top of a tower. • Parabolic Trough: where solar radiation is concentrated by parabolically curved trough-shaped reflectors onto a receiver pipe containing heat transfer fluid. The fluid flowing through the pipe is heated and the resulting steam is used in a power block. • Linear Fresnel: it is similar to the parabolic trough system, it comprises many collectors in parallel rows with mirrors situated flat on the ground reflecting sunlight onto the heat-transfer fluid pipe above. • Parabolic Dish: it consists of a dish-shaped, with a parabolic point focus concentrator that focuses solar radiation onto a receiver situated at the focal point. Figure 1.3.1: Types of Concentrated Solar Power (CSP) technologies (obtained from [16]). Introduction 6 PhD Thesis The initial CSP plant technology that emerged was the parabolic trough, but the tower system has been growing rapidly. The tower collectors’ capacity to achieve higher temperatures (∼ 1000 °C) is the primary driver for this system’s development, as it increases the plant efficiency and lowers the storage costs. The electricity generation from CSP plants (∼114 $/MWh [17]) is competitive in the current energy crisis where fossil fuel prices are soaring [18]. However, investment in the development and construction of such plants is restrained by feedstock expenses and other factors. The CSP plant industry is still in its early stages of market expansion. According to the International Energy Agency’s (IEA) portfolio, the installation capacity of CSP plants was twice that of 2017 in 2020. The forecasted contribution is predicted to be 11 % of the world's energy production by 2050 [4]. The adoption of CSP plants is increasing worldwide, with almost 70 % of plants currently operational as of 2021. Spain and the USA currently possess 65 % of the global installed capacity, whilst South Africa, Morocco and China have reached or exceeded 500 MW. Unfortunately, the construction and development of CSP stations have been negatively impact by health, feedstock and energy crises. China, the Middle East and North Africa (MENA) and to a lesser extent Europe are the only regions showing signs of continued development of CSP plants [19]. Figure 1.3.2 depicts the global status of CSP in 2021. Figure 1.3.2: Operational under construction and development CSP projects (adapted from [19]). Figure 1.3.3 illustrates the development of the CSP thermal global capacity from 1984. Notably, the majority of the growth in this technology occurred between 2008 and 2014. Afterwards, a deceleration is evident between 2014 and 2017 when other nations, Introduction PhD Thesis 7 including China, Morocco and MENA countries began to invest in and build CSP power plants. Currently, approximately half of the world's global capacity has been constructed or is under construction in Spain and the USA. Figure 1.3.3: CSP global capacity per country (adapted from [20]). Spain is at the forefront of CSP implementation with 2.3 GW of installed and fully operational stations, which can generate up to 9 % of the country’s electricity during certain periods. Moreover, Spain has been a pioneer in CSP tower technology since 2007, when the first station with a nominal capacity of 11 MWe was built in southern Spain. Figure 1.3.4 summarises the CSP plants currently in operation in Spain. Figure 1.3.4.: Current thermal CSP plants in Spain (collected from [20]). Introduction 8 PhD Thesis Since energy is continuously generated, it is not feasible to store the excess energy beyond the battery capacity, which is inadequate for enabling flexible energy consumption [21]. Figure 1.3.5 displays a daily profile of electricity usage and solar output. The current production of electricity and new alternative sources to mitigate the insufficient generation of solid fuels energy, such as heating electrification by heat pumps, present other issues such as grid collapse due to increased power peak demand. Although grid expansion is technically feasible, it incurs in significant expenses and other problematic issues such as cut-off periods. Therefore, TES integration is being evaluated as an alternative to reduce energy consumption [22,23]. Figure 1.3.5: Electricity consumption and solar power production (obtained for [22]). 1.4. Thermal Energy Storage (TES) As is well-established, solar energy is limited to sunlight hours (8 h/day) and does not allow for constant energy generation. Moreover, during daylight periods, other energy sources have precedence due to their lower cost and availability. Therefore, the key goal is to align energy acquisition during periods of sunlight with high-demand energy production. Hence, solar heat storage could be a viable option for enhancing energy flexibility and generating energy as required. As a result, energy provision becomes more consistent with minimal interference from external factors (such as solar radiation), leading to a decrease in demand curve fluctuations and the ability to regulate the energy generation rate, thereby overcoming the issue of intermittency. TES systems involved three main steps: Introduction PhD Thesis 9 • Heat charging: solar energy is used as a clean source for obtaining heat. • Storage step: this is the crucial stage of the system. Materials can be stored for the long term until energy production is needed. • Heat discharging: when demand requires it, the heat is available for energy production. TES systems are suitable for addressing the discrepancy between the intermittent provision of solar energy and continuous electricity production by extending plant operation periods beyond daylight hours and managing energy generation more efficiently. TES involves accumulating energy in the form of heat during the charge step, storing it and releasing it (during the discharge step) when the demand requested it without any thermal losses. Figure 1.4.1 illustrates this process. Currently, 90 % of commercial-scale CSP plants integrate TES with the use of molten salts for sensible heat storage. Figure 1.4.1: Scheme of TES process (adapted from [24]). TES is an emerging synergy with renewable energies for energy production, as it extends and adapts the energy generation to more favourable periods. TES involves obtaining heat from three different mechanisms (Figure 1.4.2) [25–27]: • Sensible Heat Storage: based on the capacity of the material to heat up without undergoing a phase change [26,27]. • Latent Heat Storage: based on the phase change of a material [28,29]. • Thermochemical Energy Storage: based on the energy generated by chemical reactions [23,30]. Introduction 10 PhD Thesis Figure 1.4.2: TES methods: sensible, latent and thermochemical storage. 1.4.1. Sensible Heat Storage (SHS) Among TES systems, SHS is the most commonly used. Thus, a material possessing high heat capacity is necessary. Solar energy is employed to elevate the energy content (charging step) of a fluid or a solid, which is then stored at the charging temperature. In the discharging step, the heat is transferred to another heat transfer fluid (HTF), mainly water, and utilized in a power cycle to obtain energy. The initial HTF’s temperature decreases which correlates to the sensible heat. The amount of heat stored is contingent on the substance’s heat capacity, temperature variance and quantity [31]. The formula for sensible heat is shown as follows [32]: 𝑄 =𝑚·𝐶𝑝· ∆𝑇 (1) where m represents the mass of the material (kg), Cp denotes the heat capacity over the temperature range of operation (kJ/kg·K) and ΔT is the temperature difference between the charging and discharging steps (K). The list of materials appropriate for this application is extensive and includes, but is not limited to, water, air, oil, brine, concrete, sand, soil, molten salts or rocks. Table 1.4.1.1 summarises the most popular materials for SHS [33,34]. Water is a widely used and inexpensive substance known for its non-toxic properties and high heat capacity of 4.18 kJ/kg·K making it suitable for both residential and industrial applications. Nonetheless, at temperatures exceeding 100 °C, other materials such as oil, liquid metals and molten salts are preferred [35]. Introduction PhD Thesis 11 Table 1.4.1.1: Most common materials for SHS. Materials Temperature Range (°C) Specific Heat (kJ/kg·K) Sand 20 0.80 Rock 20 0.88 Concrete 20 0.88 Granite 20 0.82 Water 0-100 4.18 Engine oil < 160 1.88 Ethanol < 78 2.40 Propane < 97 2.50 Butane < 118 2.40 Octane < 126 2.40 Molten salts 265-565 1.53 Molten salts are widely used for TES integrated with CSP plants due to their low cost, high heat capacity and great energy storage density. Currently, only a limited number of CSP plants operate using TES solutions to generate electricity in the absence of direct solar radiation [36]. The procedure consists of warming up a HTF with direct solar radiation. HTF typically consist of a blend of various molten salts, usually nitrates salts such as sodium nitrate (NaNO3) and potassium nitrate (KNO3), in 60/40 ratio, known as solar salt, due to its high heating capacity [37–41]. Molten salt systems have several limitations that impact the competitiveness of CSP with regards to conventional electric power plants. Among these systems, SHS employing molten salts is the most commonly used technology in CSP tower plants. Solar salt is the most widely used commercially, although it has significant disadvantages that hinder complete technology deployment such as: • Operational temperature: thermal degradation of the molten salts occurs at temperatures above 550 °C which can be easily reachable by CSP systems. Once the materials are degraded, their activity cannot be recovered. Consequently, process control is indispensable for the feasibility of this technology [42,43]. • Salts costs: these solar salts are scarce and well-localized in South America (mainly in Chile), which is the primary reason for their high price. • Storage complexity: it is difficult to recover the original powder morphology when the salts melt for the first time and then solidifies, which is a significant problem. Thus, solidification of the salts is not recommended as it tightens up and impedes fluidization of the material. For this reason, temperatures below 200 °C are not recommended [44]. • Salts' chemical nature: nitrate salts are highly corrosive and requires the use of chemical-resistant materials for plants, thereby increasing construction costs. Introduction 12 PhD Thesis In general, industrial plant efficiency is enhanced with high operational temperatures. This holds particularly true for CSP plants. Hence, the use of molten salts is restricted, given their limitations in this regard. There is a great scope for research in developing new heat transfer fluids with improved properties compared to molten salts, such as higher calorific capacity, lower solidification point and greater degradation temperature. Currently, molten salts are widely used system for SHS and energy production to meet overnight and high peak demands [37,38]. However, there is a continuous search for alternatives to compete with fossil fuel energy generation [45–47]. Progressively, plants were constructed to generate steam and produce electricity using heat. Gemasolar was the initial commercial CSP tower plant to adopt TES for energy production with molten salts. With nominal capacity of 17 MWe, the plant can produce 80 GWh annually. Due to the application of molten salts as HTF, the plant has an autonomy of 15 h operation without solar radiation. In this process, molten salts are heated in the tower collector and the resulting solar salt is directed towards a hot salt tank. The energy is stored in the form of sensible heat at a density of 0.8 GJ/m3 until it is required for power production [48–51]. Subsequently, the heat is transferred to a stream of water (steam generation) which can be used in a power cycle. The cooled down salts are stored in a cold silo for recirculation in the collector. The process is schematically represented in Figure 1.4.1.1. Figure 1.4.1.1: Schematic diagram of the molten salts process with CSP tower technology. Figure acquired from [51]. Introduction PhD Thesis 13 1.4.2. Latent heat storage (LHS) Materials for LHS are also referred to phase change materials (PCMs). Solar energy is utilized to heat the material and induce a phase change such as fusion, evaporation or crystallization. It should be noted that the temperature range for PCM is restricted [52,53]. Subsequently, the medium is stored in its new phase at the temperature of the charging step. When the energy is released (discharge step), the medium’s returns changes to its former state. The phase change is associated with latent heat. The stored thermal energy can be expressed as: 𝑄 =𝑚·𝐿 (2) where m is the mass of the material (kg) and L is the latent heat of the material (kJ/kg). The solid-liquid phase change is commonly used for latent heat storage due to the large difference in volume between liquid and gas. However, this drastic volume increase poses challenges for practical operation, despite the potential for greater heat obtained [54]. In comparison, the solid-solid phase change (crystallization) releases considerably less latent heat than the solid-liquid phase change [54]. In contrast to sensible heat storage, latent heat storage offers a significantly greater heat storage density, with minimal temperature shift between the storage and heat discharge phases [55]. The greater heat storage density allows for smaller storage tanks, which in turn yields a smaller external surface area and decreases heat loss. For effective heat storage, materials must fulfil specific criteria, including high latent heat, high thermal conductivity, chemical stability, cost-effectiveness, non-toxicity and non-corrosiveness [55]. These materials can be classified into two categories for this purpose (Figure 1.4.2.1): organic (such as paraffins, fatty acids, alcohols and glycols) and inorganic (such as salts, hydrated salts, metals, metal alloys and eutectics). Figure 1.4.2.1: Energy density for the most common PCMs (extracted from [52]). Introduction 20 PhD Thesis • Sorbent synthesis: the morphological structure is crucial as different microstructures increase the specific surface area and enhance the reactivity of the CaO sorbent. For instance, the acicular shape of calcium acetate significantly improves the conversion achieved for CaO multicyclic experiments over a larger number of cycles. The synthesis of acetates is inexpensive and industrially scalable since only acetic acid and water are needed. The optimal deployment of the CaL-CSP technology globally hinges on the synergy between the most suitable reaction conditions and the various techniques to boost CaO conversion. 1.6. CaL operation modes The operating conditions are highly dependent on the specific application. The CaL process serves two main purposes, namely CO2 capture and sequestration (CCS) and TCES. While carrying out the CCS process under real power plants conditions, the main parameter to consider is the concentration of flue gases. However, optimal conditions for TCES are still under investigated with the aim of achieving the highest yield while maintaining CaO activity. 1.6.1. Calcium Looping for CO2 capture and sequestration (CaL- CCS) The CaL process for CO2 capture and sequestration (CaL-CCS) is based on the carbonation of the CaO at temperatures close to 650 °C and the subsequent calcination recovering the sorbent at higher temperatures (900 °C) as shown in Figure 1.6.1.1. In the carbonator, flue gases, with a CO2 concentration of 15 % vol./vol., fluidize CaO during the capture process. The fluidized bed ensures effective gas-solid contact and heat-mass transfer in reactors operating at a fast fluidification regime with a gas velocity of 5 to 10 m/s [124,125]. The partially carbonated particles are directed to a second reactor (calciner) to give rise in short residence times to the decarbonation of the material under high partial pressure of CO2 (above 70 % vol./vol. [77]). Hence, the CO2 resulting from the combustion of fossil fuels is captured in a high concentrations, comprised, stored and utilized in other applications [126–128]. The regenerated CaO is then reintroduced back into the carbonator to begin a new cycle. Introduction PhD Thesis 21 Figure 1.6.1.1: Diagram of CaL-CCS scheme for CO2 sequestration from flue gas. The efficiency of the CO2 capture technology is determined by the process conditions. These conditions depend on several factors, including: • The CO2 concentration of the flue gases. • The short residence time in the reactors caused by high combustion gas flow. • The higher partial pressure of CO2 in the calciner. • The presence of impurities (SO2 or ashes) reduces the CaO reactivity as the cycles progress. The challenging conditions cause deactivation of the CaO sorbent, which is regenerated during calcination. This leads to a rapid decline in conversion [99,129–131], with only 10 % of the theoretical maximum value being achieved [101,132–136]. Therefore, periodic replacement of the inactivated sorbent with fresh CaCO3 is necessary. Nevertheless, using natural high purity limestone as a CaO precursor due to its availability and cost-effectiveness (see Figure 1.6.1.2), improves the competitiveness of the process. Introduction 22 PhD Thesis Figure 1.6.1.2: Element abundance on Earth's surface. Figure extracted from [137]. The feasibility of the CaL-CCS technology has been demonstrated through the integration of several bench-scale plants in recent years [77]. Therefore, it is necessary to assess potential drawbacks of the process or materials before scaling-up to industrial levels. La Pereda (Asturias, Spain) is highlighted as one of the pioneers in CaL-CCS among the current pilot-scale plants. It comprises a 1.7 MWth power plant design intended for processing approximately 1 % of the flue gases generated from a commercial coal plant of 50 MWth [138]. The Technological Industrial Research Institute (Taiwan) collaborated with Cement Cooperation to create a bench-scale plant integrated with a cement plant, having a nominal power of 1.9 MWth and reaching a CO2 capture value of 1 t/h [77,139]. Moreover, this organization designed a pilot-scale plant with a capacity of 3 MWth and a CO2 capture efficiency of up to 85 %. Other pilot plants are located at the Darmstadt Technical University (Germany) with a capacity of 1 MWth [140]. The multicyclic capture conditions for limestone result in a low residual effective conversion value when it is calcined at temperatures above 900 °C in high CO2 concentration and subsequently carbonated at 650 °C in a 15 % vol/vol CO2 ratio. Introduction PhD Thesis 23 1.6.2. Calcium Looping for Thermochemical Energy Storage (CaL-TCES) Recently, a novel alternative concept for the CaL system integrated with CSP has been proposed. A significant advantage of the TCES systems over molten salts (SHS) is the considerably higher energy density potentially attainable (3.2 GJ/m3 and 0.8 GJ/m3, respectively). Therefore, in combination with lower heat losses and the utilization of naturally occurring minerals that are non-toxic, inexpensive and widely available (such as carbonates), this system offers great benefits. While this system has been in existence since the late 1970s, it has only recently gained interest as a potential TCES system because the operating conditions have been optimized and its cyclability improved. Furthermore, TCES systems offer the possibility of long-term storage of the reaction byproducts at room temperature, making it an intriguing process for coupling energy production with demand. Previous research and simulations indicate that adding natural limestone to have the CSP plant results in an efficiency improvement of over 20 % compared to the molten salt system [141]. Exergy efficiency measure under reversible conditions is an effective way to characterise the plant’s thermal efficiency for power generation from heat [23]. This is defined as the ratio of the recovered and supplied heat. Reducing heat losses and utilizing temperatures for storing and releasing energy as closely as feasible maximizes exergy efficiency. Moreover, in line with the second law of thermodynamics and the Carnot cycle, the greater the operational temperature, the higher the maximum efficiency that can be achieved for producing electrical power from the released heat. Thus, a specific objective in power plants is to enhance heat to power conversion efficiency. The projected CaL-TCES system scheme (Figure 1.6.2.1) consists of a solar calciner, a CO2 storage system equipped with a compressor, two solid silos for storing CaCO3 and CaO, a carbonator and a power unit. The temperatures potentially attainable in the solar receptor exceed 1000 °C ensuring the complete endothermic decomposition of CaCO3. The high-temperature CaO and CO2 streams pass through a heat exchanger to transfer their sensible heat and are then cooled and stored at room temperature. The solid CaO is transported to a silo for storage, while the CO2 gas stream is compressed and stored in supercritical conditions. Thus, under practical conditions, the reaction products can be stored for several weeks or months. When necessary, they are reassembled to boost the reverse exothermic reaction regenerating the CaCO3 which is stored and transported to the calciner starting the cycle again. The cost of the heat is manageable through the use of a power cycle (Brayton or Rankine cycle), based on a turbine that utilizes a heat transfer fluid (water or CO2) to generate energy. Introduction 24 PhD Thesis Figure 1.6.2.1.: Schematic diagram of the CaL-TCES system (obtained from [142]). Recently, the process has progressed to a bench scale phase that surpasses the concept phase, where the materials’ evaluation has been investigated. Consequently, it is feasible to consider the advantages and drawbacks of the technology. The CO2 capture and sequestration process offers valuable insights to achieve optimal system integration. Nonetheless, despite the entirely different operational conditions, indication of the materials' performance is evident. On the other hand, integration of CaL-TCES can be optimized in CSP tower plants by meeting certain requirements: ➢ Minimize calciner temperature: • Minimising heat loss by radiation and convection within the receiver is crucial for maximising calciner yield with operating temperature being the primary determining factor. • The potential for utilising cost-effective materials: as temperatures increase, the materials become more costly and specialised. The present solar receiver is capable of attaining temperatures around 1000 °C. • The lower the calciner temperature, the simpler the operation. ➢ Maximize carbonator operation temperature: • The energy production efficiency is enhanced. As the cycle temperature rises, power cycles yield greater heat-to-energy output. • The carbonation reaction kinetics slow down as the temperature approaches the equilibrium. It is desirable to operate at the highest temperature possible to achieve a fast reaction and reduce the residence time. Introduction PhD Thesis 25 Following these requirements, the integration scheme comprises boosting the exothermic reaction, which operates at high temperatures (850 °C), within a high partial pressure atmosphere of CO2. In contrast, the calcination reaction occurs in an atmosphere devoid of CO2 to carry out the endothermic reaction, which takes place at a lower temperature (700-750 °C) conveniently reachable by the CSP tower’s solar receptor. Thus, it is envisaged that the sintering and the deactivation of the sorbent can be prevented. Therefore, the use of more cost-effective materials for the metallic alloy solar receptor and plant will significantly reduce the expenses. Additionally, it is important to consider the storage of the by-products of the reaction. The CaCO3 obtained from carbonation and CaO and CO2 (from the limestone decomposition) can be stored separately at room temperature for extended periods of time. This is one of the primary benefits of integrating CaL-TCES with CSP. Storage of CaO and CaCO3 can be easily accomplished in silos at atmospheric conditions. CO2 storage, on the other hand, presents challenges due to the need for high compression pressure to reduce the tank’s volume. It has recently been discovered that CO2 can be stored at room temperature (25 °C) under supercritical conditions (minimum pressure of 65 bar) in storage tanks allowing for the recovery of about 140 kJ/kg of heat through CO2 compression. In typical operating conditions, assuming a CaO conversion of 0.5, the thermal-to-energy global efficiency of CaL-TCES in is 45 % assuming. The higher the conversion, the higher the efficiency. Compared to the molten salt system for SHS, CaL has several advantages: • Theoretically, Ca precursors can achieve higher energy density. • Operating at a high temperature of 850 °C improves the system’s thermal- to-energy efficiency and reduces heat losses. • The material can be stored at room temperature with CaL, whereas the molten salt system requires continuous warming of the vessels since its solidification point is at 200 °C. Therefore, CaCO3, CaO and CO2 can be stored separately at ambient temperature and pressure. As outlined previously, the process seeks to determine the optimal conditions for enhancing the sorbent’s activity while also improving its multicyclic activity. Therefore, a thorough search for the best conditions is crucial for the industrial development of the CaL technique. Several alternatives have been assessed to prevent the deactivation of the CaO sorbent and enhance its multicyclic activity. The reversibility of the CaCO3/CaO system enables operation under diverse conditions, whilst the CaL system is mainly dependent on the CO2 partial pressure, determined by thermodynamic equilibrium of the reaction. As such, higher CO2 concentrations permit carbonation at higher temperatures (> 800 °C). Accordingly, the higher the CO2 concentration, the greater the operational temperature. The equilibrium temperature for a pure CO2 atmosphere is ∼ 895 °C. This temperature range allows for greater power efficiency cycles such as the supercritical H2O Rankine cycle or supercritical CO2 Brayton cycle. Optimizing of the CaL conditions Introduction 26 PhD Thesis with an appropriate thermoelectric energy production cycle can improve the efficiency of the CSP tower plants and reduce CaO inactivation. Moreover, increasing the partial pressure of CO2 can be applied to the calcination process enable operation at temperatures above 900 °C. CSP tower technology is currently in its development phase; however, it is expected that it will operate at these temperatures soon, allowing the CaL system to operate without gas mixtures, which eliminates the complexity of gas separation, simplifies plant design and enables operation at greater temperatures, thereby enhancing the thermal-to-electric conversion. It should be noted that the CaCO3 decomposition rate is accelerated when temperatures reach 950 °C, significantly affecting the CaO activity. The tough operating conditions result in greater sintering-induced deactivation of the CaO particles, yet the thermal-to-energy efficiency remains comparable to other conditions owing to the higher temperature attained. Moreover, the efficiency is enhanced by avoiding gas separations methods such as membranes [143,144]. Meanwhile, alternative calcination conditions are feasible for CSP systems. The use of a restricted partial pressure of CO2 accords with CSP tower technology, meaning that operating temperatures between 700 °C and 800 °C can be achieved, ideal for efficient tower technology function. To achieve these calcination conditions, decarbonation must be undertaken under an inert atmosphere. Helium is frequently employed due to its high thermal conductivity (0.15 W/K·m). The higher CO2 diffusivity in this gas facilitates the integration of the CSP systems, as decarbonation occurs at temperatures below 750 °C. The reduction of calcination conditions alleviates the CaO inactivation, hindering sintering and maintaining a high CaO-specific surface area available for reaction with CO2. This enables the sorbent to be used for a larger number of cycles. The lower calcination temperature also allows for the use of cost-effective solar receivers made of metal alloys. On the contrary, performing the reverse exothermic reaction in high partial pressure CO2 conditions requires gas separation through membranes so as to store He and CO2 separately. The separation process is energy-intensive and entails an increase in plant costs due to the selective membrane management. There are selective permeable membranes that are commercially available and have been tested for H2-CO2 separation. The similarities between H2 and He, due to their small size and lightweight molecules, suggest a potential application of these membranes for separation purposes. However, it is necessary to further evaluate their performance under relevant conditions (such as high flows, CO2 purity, etc). In regard to CaL performance, calcination within a pure helium atmosphere is rapidly achieved in short residence times, leading to enhanced multicycle activity. Another effective strategy for reducing CO2 partial pressure involves operating in mild vacuum conditions. This approach offers two dual benefits: Introduction PhD Thesis 27 • Allows for working in a pure CO2 atmosphere with a limited absolute pressure, without the intricacy and additional cost of gas membranes separation and preventing sintering of CaO particles. • The carbonation process carried out at a high temperature, achieving excellent thermal-to-electric efficiency. In this scenario, the separation of the gases is unnecessary, thereby reducing energy penalties and plant costs. Currently, these conditions are of great interest because operating in a softer vacuum (0.01 or 0.1 bar) is easily achievable and does not require additional plant construction costs, since common materials for reactors, pipelines and other equipment are suitable for such conditions. Moreover, decreasing the absolute CO2 pressure of the system permits calcination at temperatures lower than 750 °C, thereby increasing the potential utilization of the CaO sorbent for higher decarbonation/carbonation cycles. These conditions offer significant advantages and few drawbacks in contrast to other CaL conditions described previously. 1.7. Alternative TCES systems Alternative metal carbonates have been evaluated based on an analogue process to the CaCO3/CaO system. Along with limestone and dolomite, other natural carbonates like strontianite (SrCO3) also bear similarities in the reaction mechanism with the CaCO3/CaO duo. The process is based on the following reaction: 𝑆𝑟𝐶𝑂3⇄ 𝑆𝑟𝑂+ 𝐶𝑂2 ∆𝐻𝑟0= −234 𝑘𝐽/𝑚𝑜𝑙 (6) The higher thermal stability and higher reaction enthalpy allows for higher turning temperatures and, therefore, higher energy densities. The maximum theoretical energy storage density (10.61 GJ/m3 for SrO/SrCO3, 10.60 GJ/m3 for MgO/Mg(OH)2 and 10.4 GJ/m3 for CaO/CaCO3) [145–147]. Additionally, the overall efficiency of the plant would improve with higher carbonation temperature [148]. SrCO3 is a material with a high energy density and high dissociation temperature when exposed to pure CO2 atmosphere, which can provide a significant amount of heat release. As the required temperature for the reaction to take place increases, the higher the energy density. Temperatures greater than 1200 °C are categorized as being in the ultra-high temperature range. These systems rely on CO2 equilibrium for determining both calcination and carbonation conditions. As shown in Figure 1.7.1, the equilibrium temperature rises as the atomic number of the alkaline earth metal increase. The prevailing trend in the industry is to operate at higher temperatures to reduce heat losses. However, this system shares the same limitations as the CaL process. In particular, SrO experiences a severe deactivation after undergoing multicyclic activity. Introduction 28 PhD Thesis Figure 1.7.1: Equilibrium curve of the SrCO3/SrO system adapted from [149]. The research of SrCO3/SrO duo is limited due to the technical difficulties associated with ultra-high temperature testing (above 1200 °C), which incurs costly material expenses and the use of slightly corrosive gases such as CO2. The calcination conditions are of crucial importance, similar to the CaL system, as they impact the subsequent carbonation reaction and reducing sintering, thereby enhancing energy storage capacity. Hence, prior research implemented an inert atmosphere (N2 or Ar), or low to moderate CO2 concentrations to enhance the calcination kinetics and accomplish complete reaction at temperatures under 1200 °C in short residence times. As elaborated on in CaL, this requires an expensive gas separation process to acquire a high-purity CO2 stream, which is then stored under high pressure for further carbonation at higher temperatures. Currently, CSP tower plants have a temperature limitation, as they cannot operate at temperatures above 1000 °C. Despite this, it is believed that the technology will be improved in the near future to achieve this goal. Nevertheless, there are other alternatives available for energy storage at ultra-high operational temperatures, such as thermophotovoltaics (TPV), thermionic or hybrid thermionic-thermophotovoltaics although these are still in an early investigation step. For instance, Datas et al. examined the effectiveness of using silicon and silicon-boron as a PCM for LHS at temperatures exceeding 1400 °C [150,151]. This ultra-high temperature is relevant to various industrial applications such as blast furnaces, metallurgy, ceramics and others. Figure 1.7.2 illustrates the decomposition of the SrCO3 under a CO2 atmosphere. Harsher calcination conditions (pure CO2 atmosphere) imply operation at ultra-high Introduction PhD Thesis 29 temperatures (1400 °C) to ensure the rapid decomposition of SrCO3, which leads to aggressive sintering in the SrO particle. The efficiency of the plant’s thermal-to-electric conversion is boosted, avoiding the need for gas separation using membranes and enhancing system integration for energy storage [59,142]. Figure 1.7.2: SrCO3 decomposition under CO2 atmosphere. Moreover, the enhancement of the SrO activity can also be achieved by incorporating materials with high thermal resistance. Introducing materials with high Tammann temperature stabilizes the structure and enables SrO to be exposed to CO2. Additionally, the introduction of dopants reduces the induced-sintering effect, thereby mitigating the particle growth. A range of materials have demonstrated efficacy, such as polymorphic materials (CaSO4 and Sr3(PO4)2) [152], mixed oxides (SrZrO3, yttriastabilized SrZO3 and SrSiO3) [145,153], or oxides like MgO, SiO2, Al2O3 and ZrO2 which react with SrO at high temperatures giving rise to the mixed oxide (with the exception of MgO) [148,149,154–156]. Introduction 36 PhD Thesis storage based on the Ca(OH)2/CaO cycle, Appl. Therm. Eng. 23 (2003) 733–741. https://doi.org/10.1016/S1359-4311(03)00015-2. [75] C. Ortiz, J.M. Valverde, R. Chacartegui, L.A. Perez-Maqueda, P. Giménez, The Calcium-Looping (CaCO3/CaO) process for thermochemical energy storage in Concentrating Solar Power plants, Renew. Sustain. 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The goal is to achieve a significant improvement in terms of operational plant costs by avoiding gas separation complexity during calcination in an inert atmosphere. As a costeffective and easily synthesized option, natural carbonate-derived acetates are explored as an alternative. This study evaluates the multicycle performance of acetates and the impact of their acicular structure. 2. The influence of the addition of low ratios of high Tammann temperature materials (MgO, SiO2, ZrO2, Al2O3) as structure-stabilizing additives for a steady performance during reaction cycles is evaluated. The dopants have been included using an easily scalable process (mechanical treatment) to ensure material homogeneity. 3. The reduction of calcination temperature is also under consideration. Therefore, it is essential to lower the CO2 partial pressure for achieving this aim. Two options are available: either to the reduce of the absolute pressure of the system while keeping CO2 as a calcination atmosphere or to employ an inert atmosphere (N2, Ar, He). This thesis investigates the first option in order to retain the closed-loop system benefits of CO2. At plant scale, the use of low-medium vacuum conditions (0.1-0.01 bar) is generally unproblematic given that most plant construction materials are suitable for these conditions. 4. Furthermore, this thesis examines the feasibility of using waste as a material support for manufacturing highly porous sorbents based on calcium, which can be employed in the Calcium Looping (CaL) process. Specifically, cigarette butts were impregnated with various ratios of calcium and magnesium nitrates in order to investigate energy storage solutions. Materials and Methods 52 PhD Thesis Figure 3.2.2.1: Schematic diagram of the synthesis of the AcSrMg10 sample. Table 3.2.2.1: Mass and molar composition of the different acetates synthesized. Molar (%) Mass (%) Sample CaO MgO SrO CaO MgO SrO CaAc 100 0 - 100 0 - Ca80Mg20Ac 80 20 - 84.8 15.2 - Ca95Mg05Ac 95 5 - 96.4 3.6 - SrAc - 0 100 - 0 100 AcSrMg10 - 20 80 - 91.1 8.9 3.2.3. Impregnation methodology First, used cigarette butts were peeled and washed at 75 °C in distilled water under stirring for 30 min. This process was repeated 3 times to ensure maximum removal of waste from the filters. Then, the clean cigarette butts were dried at 100 °C for 12 h. The impregnation was carried out by soaking 24 g of cigarette butts in 192 mL of a Ca(NO3)2 solution with different molar ratios. Conversely, to improve the performance, a Mg(NO3)2 solution at different molar concentrations was added to the Ca(NO3)2 solution. The cigarette butts were then left at room temperature for two days to complete the absorption of the calcium and magnesium solutions by the cellulose acetate of the filters. Hereafter, the water was removed using a freeze-dryer for 24 h. After that, the dryimpregnated cigarette butts were submitted to Solution Combustion Synthesis (SCS) also known as flaming combustion. This process consists of heating the furnace to 300 °C and introducing the cigarette butts, where the burning of the organic cellulose acetate support is produced, obtaining CaO and MgO in the molar ratios previously considered. The process for Ca-based derived sorbent is shown in Figure 3.2.2.3. This mechanism helps obtaining high porous materials resumed in Table 3.2.2.3. Materials and Methods PhD Thesis 53 Figure 3.2.2.3: Schematic diagram of the Ca-based derived from cigarette butts. Table 3.2.2.3: Composition of cigarette butts after submitted to flaming combustion. Molar (%) Mass (%) Sample CaO MgO CaO MgO TCBs 0.05Ca 100 - 100 - TCBs 0.1Ca 100 - 100 - TCBs 0.25Ca 100 - 100 - TCBs 0.5Ca 100 - 100 - TCBs 1Ca 100 - 100 - TCBs 0.1Ca5Mg 95 5 96.4 3.6 TCBs 0.1Ca10Mg 90 10 92.6 7.4 TCBs 0.1Ca20Mg 80 20 84.8 15.2 3.3. Methods The main methods used to characterize and analyse the performance of the materials are described. 3.3.1. Thermogravimetric Analysis (TGA) The role of operational conditions in material performance is crucial. Moreover, thermodynamic equilibrium aids in determining optimal working conditions for each sample. In summary, the following conditions were employed: • Reduced pressure conditions for TCES: under these conditions, calcination takes place at reduced CO2 absolute pressure with the temperature set at 765 °C and 700 °C (for CO2 absolute pressure of 0.1 and 0.01 bar respectively) for Cabased sorbents. Regarding Sr-based precursors, lower calcination temperatures enable operation at 900 °C. Consequently, these mild calcination conditions assist in mitigating the sintering-induced mechanism. Different carbonation temperatures were evaluated for Ca-based sorbents under 1 bar of absolute pressure of CO2 (700 °C, 765 °C and 850 °C) and the optimal results were Materials and Methods 54 PhD Thesis achieved with higher carbonatation temperatures. Carbonation for Sr-derived samples, was conducted at the same temperature as calcination, which was isothermal at 900 °C. • CO2 closed cycle at ultra-high temperature for TCES: the thermodynamic equilibrium at an absolute pressure of 1 bar of CO2 indicates that calcination cannot occur at temperatures lower than 1200 °C, implying ultra-high temperature for fast calcinations (in this case, 1400 °C). Carbonation is conducted at 1200 °C. These harsh conditions notably impact the SrO activity, with sintering having a substantial effect. • CO2 closed cycle for TCES conditions (CaL-TCES): in this instance, the calcination and carbonation processes are both carried out at an absolute pressure of 1 bar of CO2. Consequently, the temperature for the rapid decomposition of the Ca-based sorbent is set at 950 °C. In addition, carbonation is conducted at 850 °C. • CO2 capture and sequestration conditions (CaL-CCS): the selected conditions were taken from real operating power plants, and the atmosphere used reflects the concentration of the average flue gas streams. Hence, in line with equilibrium thermodynamics, carbonation ocurred at 650 °C for 5 min in an atmosphere of 15 % CO2 / 85 % N2 (vol/vol), and calcination took place at 900 °C for 5 min in 70 % vol. CO2 and 30 % vol. of N2 atmosphere. In summary, utilizing a CO2 closed cycle at a plant scale can prove advantageous in avoiding gas separation complexities. Conversely, high temperatures and high partial pressure of CO2 can hasten the deactivation of the calcium and strontium oxide. Moreover, calcination under reduced absolute pressure of CO2 can alleviate sintering and sorbent deactivation while enhancing the energy yield of the process. 3.3.3.1. Reduced pressure conditions for TCES Multicycle calcination/carbonation tests were carried out by means of thermogravimetry analysis (TGA). A novel in-house thermobalance was designed and constructed to operate from low pressures up to 5 bar in order to test the material behaviour under specific CaL-TCES conditions. Figure 3.3.3.1.1 shows a schematic diagram and a photograph of the device. The system basically consists of a highsensitivity CI Electronics microbalance (2 × 10−7 g) and a reactor. The reactor is composed of a Watlow heater and a non-porous mullite tube, connected to the microbalance by KF-flanges sealed with a clamp and an O-ring. The temperature of the reactor is controlled by thermocouple placed on the wall of the Watlow heater, and the temperature of the sample is measured by a second thermocouple placed under the sample crucible. A flat ceramic crucible (0.154 cm3) was used to hold the sample. A vacuum Materials and Methods PhD Thesis 55 pump and a pressure gauge were incorporated into the system in order to perform the experiments under a controlled CO2 pressure (absolute pressures of 0.01, 0.1 or 1 bar in the tests performed). The CO2 pressure was controlled by low-pressure needle valves connected between the vacuum pump and the thermobalance. Rotameters were used to adjust the gas flow through the thermobalance during the experiments. Figure 3.3.3.1.1: Scheme of the thermobalance assembled for vacuum operation. The mass was accurately calibrated using a set of calibration weights ranging from 1 mg to 20 mg. Temperature calibration was performed using the well-known decomposition reaction of hydrous calcium oxalate heated at a heating rate of 10 °C/min. Alumina powder was used as a standard to correct for buoyancy effects. The operating conditions were considered as a function of the reduced CO2 atmosphere according to the equilibrium curve (see Figure 1.5.1 for CaCO3/CaO system and Figure 1.7.1 for SrCO3/SrO) and the samples used. For limestone, carbonation was always carried out at an absolute pressure of 1 bar of pure CO2 (Figure 3.3.3.1.2). According to the thermodynamic equilibrium of the Materials and Methods 56 PhD Thesis CaCO3/ CaO system, the maximum carbonation temperature under these conditions is 895 °C. Three different carbonation temperatures, i.e. 850 °C, 765 °C and 700 °C, were investigated in order to accelerate the carbonation kinetics. Ideally, the higher the carbonation temperature the higher the overall efficiency of the CaL-TCES process. However, the reaction kinetics slow down significantly as the equilibrium temperature is approached, which is detrimental to the process. Calcination was carried out under an absolute pressure of 1 bar of pure CO2, as well as under controlled absolute CO2 pressures of 0.1 bar and 0.01 bar. The calcination temperatures were selected by considering the equilibrium temperatures at these CO2 pressures (760 °C at 0.1 bar and 652 °C at 0.01 bar). Table 3.3.3.1.1 resumes the calcination and carbonation conditions for all the samples tested. Figure 3.3.3.1.2: Time evolution of sample mass (%), temperature (°C) and absolute pressure (bar) for a calcination/carbonation cycle of limestone. In this test, carbonation was carried out at 1 bar CO2 and 850 °C, and calcination at 0.1 bar CO2 and 700 °C. Table 3.3.3.1.1: Operation conditions used for the multicycle test of limestone. Calcination Carbonation Test Temperature (°C) Absolute CO2 Pressure (bar) Temperature (°C) Absolute CO2 Pressure (bar) T1 765 0.1 765 1 T2 765 0.1 850 1 T3 765 0.01 765 1 T4 700 0.01 700 1 T5 700 0.01 850 1 Materials and Methods PhD Thesis 57 Similarly, multicycle calcination/carbonation tests of CaO sorbents were performed with 40 mg of the samples in all cases, operating in a closed CO2 cycle using absolute pressures of 0.01 bar and 0.1 bar CO2 to carry out the calcination stage. The temperatures used for the calcination were 700 °C and 760 °C for 0.01 bar and 0.1 bar of absolute CO2 pressure, respectively, chosen according to the thermodynamic equilibrium of the CaCO3/CaO system and in order to optimize the multicycle performance of the materials at different temperatures achievable by conventional solar receivers. An absolute pressure of 1 bar CO2 was used for carbonation in all tests, at 700 °C, 760 °C and 850 °C. Table 3.3.3.1.2 summarizes the operating conditions tested for these samples. Table 3.3.3.1.2: Operation conditions of the experimental exposure used for CaO sorbents. Calcination Carbonation Test Temperature (°C) Absolute CO2 Pressure (bar) Temperature (°C) Absolute CO2 Pressure (bar) E1 700 0.01 700 1 E2 760 1 E3 850 1 E4 760 0.1 760 1 E5 850 1 For the preparation of SrO sorbents, tests were carried out under isothermal conditions (900 °C) for both calcination and carbonation, employing a closed-loop pressure swing approach. The first stage of the system was stabilised at an absolute CO2 pressure of 0.01 bar by means of the vacuum pump. Then, the temperature was raised to 900 °C at a heating rate of 10 °C/min. At this temperature, complete calcination of the samples was achieved in less than 10 min in all cases. At this point, the vacuum pump was disconnected and the CO2 pressure was increased to an absolute pressure of 1 bar in order to conduct the carbonation stage, which again lasted 10 min. Then, new calcinations and carbonations stages were alternated. The amount of sample used in the experiments was 40 mg in all cases. 3.3.3.2. CO2 closed cycle at ultra-high temperature for TCES. Considering the ultra-high temperature approach for the SrCO3/SrO system, the experiments were carried out using a simultaneous thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC) STA 449 F5 Jupiter from NETZSCH (see Figure 3.3.3.2.1). This analyzer is equipped with a highly sensitive balance (< 0.1 μm) and a silicon carbide furnace that allows operation at temperatures up to 1600 °C. The Materials and Methods 58 PhD Thesis sample mass used in each experiment was 25 mg. The experiments were carried out under pure CO2 at atmospheric pressure. Figure 3.3.3.2.1: STA 449 F5 Jupiter from NETZSCH. The scheme consisted of an initial calcination stage in which the sample was heated at 20 °C/min up to 1400 °C, and then kept for 5 min to achieve complete calcination. Then, the temperature was reduced at the same rate (20 °C/min) to 1200 °C, followed by a 5-min carbonation stage. These stages were repeated 20 times to analyze the multicycle conversion behaviour of the sample according to the scheme in Figure 3.3.3.2.2. 020 40 60 80 0 200 400 600 800 1000 1200 1400 Time (min) Temperature (ºC) 70 75 80 85 90 95 100 105 Mass (%) Calcination Carbonation Figure 3.3.3.2.2: Time evolution of sample mass (%) and temperature (°C) for a calcination/carbonation cycle of SrCO3. In this test, carbonation was carried out 1200 °C, and calcination 1400 °C in a CO2 closed-loop. Materials and Methods PhD Thesis 59 3.3.3.3. CO2 closed cycle for TCES conditions (CaL-TCES) The calcination/carbonation tests were carried out in a Perkin Elmer 8000 (see Figure 3.3.3.3.1). This instrument is equipped with a high sensibility balance (0.1 µm) and a furnace capable of achieving high heating rates and stable isotherms. Both, calcination and carbonation steps, lasted 5 min to mimic the short residence times expected in the calciner and the carbonator. To reduce the effects related to mass and heat transfer phenomena and to obtain reliable data on the real process kinetics, small amounts of sample (10 mg) were used in each experiment. Figure 3.3.3.3.1: Perkin Elmer 8000. Tests compatible with CaL-TCES were carried out according to the closed CO2 CaL concept. In this case, the tests started with calcination in the CO2 atmosphere using a ramp of 100 °C/min from room temperature up to 950 °C. The temperature was then lowered to 800 °C under the same atmosphere to perform the carbonation stage. Afterward, the temperature was increased again to 950 °C to start another cycle. These steps were repeated to complete 20 cycles (Figure 3.3.3.3.2). The entire experiment was performed under a CO2 atmosphere. Heating and cooling rates were done at 100 °C/min. Materials and Methods 60 PhD Thesis 0 5 10 15 20 0 200 400 600 800 1000 Time (min) Temperature (ºc) 40 50 60 70 80 90 100 Mass (%) Calcination Carbonation Figure 3.3.3.3.2: Time evolution of sample mass (%) and temperature (°C) for a calcination/carbonation cycle of CaL-TCES tests for TCBs 0.1Ca sample. In this experiment, carbonation was carried out 800 °C, and calcination 950 °C in a CO2 closed-loop. 3.3.3.4. CO2 capture and sequestration conditions (CaL-CCS) Here, tests were performed by using a TGA Perkin Elmer 8000 (see Section 3.3.3.3.). For the CO2 capture experiments (Figure 3.3.3.4.1), realistic conditions were chosen to mimic those proposed for real plants. Thus, carbonation was performed at 650 °C for 5 min in an atmosphere of 15 % CO2 / 85 % N2 (vol./vol.), and calcination at 900 °C for 5 min in 70 % vol. CO2 and 30 % vol. of N2. Heating and cooling rates were also set at 100 °C/min. Materials and Methods PhD Thesis 61 010 20 30 0 200 400 600 800 1000 Time (min) Temperature (ºc) 50 60 70 80 90 100 Mass (%) Calcination Calcination Carbonation Carbonation Figure 3.3.3.4.1: Time evolution of sample mass (%) and temperature (°C) for a calcination/carbonation cycle of CaL-CCS conditions for TCBs 0.1Ca sample. In this test, carbonation was carried out at 650 °C for 5 min in an atmosphere of 15 % CO2 / 85 % N2 (vol/vol), and calcination at 900 °C for 5 min in 70 % vol. CO2 and 30 % vol. of N2. 3.3.2. X-ray Diffraction (XRD) A Rigaku Miniflex600 diffractometer with Ni-filtered Cukα radiation (λ = 1.5406 Å) working at 40 kV and 15 mA was used. The 2θ range was settled depending on the sample used. For example, the 2θ range was set from 20 ° to 70 ° for dolomite and limestone and from 10 ° to 70 ° for strontianite. For the initial characterization of calcium and calcium magnesium acetates a 2θ range of 5 ° to 30 ° was used to enlarge the search area of interest. 3.3.3. Scanning Electron Microscopy (SEM) Scanning Electron Microscopy (SEM) was used to study the microstructure of the initial and cycled samples. An ultra high-resolution HITACHI S4800 instrument was used. To facilitate the imaging, the samples were first sputter coated with a thin layer of gold employing an Emitech K550 Telstar sputter-coating equipment (30 s and 30 mA). 3.3.4. Energy Dispersive X-ray Spectroscopy (EDS) The HITACHI S4800 SEM unit is equipped with an EDX spectroscopy unit (Bruker AXS Microanalysis GmbH) for the chemical characterization of the sample. Results and Discussion PhD Thesis 69 4.1. Calcination under low CO2 pressure enhances the Calcium Looping performance of limestone for thermochemical energy storage. The CaL process has been extensively studied under various conditions. Calcination reactions have been performed in diverse atmospheres, from CO2 to inert atmospheres such as N2, Ar or He. Operating under an inert atmosphere entails issues such as gas separation complexity. The uptake of CO2 within the loop can be achieved by recycling it and using it to drive the carbonation process. Moreover, while CO2 can power the CaO particle’s deactivation, the inert atmosphere can help mitigate the sinteringinduced mechanism. Thus, the application of reduced CO2 absolute pressures during the calcination reaction has been investigated in this work. Results and Discussion 70 PhD Thesis Results and Discussion PhD Thesis 71 Calcination under low CO2 pressure enhances the Calcium Looping performance of limestone for thermochemical energy storage Abstract The Calcium Looping performance of limestone for thermochemical energy storage has been investigated under novel favorable conditions, which involve calcination at moderate temperatures under CO2 at low pressure (0.01 and 0.1 bar) and carbonation at high temperature under CO2 at atmospheric pressure. Calcining at low CO2 pressures allows to substantially reduce the temperature to achieve full calcination in short residence times. Moreover, it notably enhances CaO multicycle conversion. The highest values of conversion are obtained for limestone samples calcined under 0.01 bar CO2 at 765 ºC. Under these conditions, the residual conversion is increased by a factor of 10 as compared to conditions involving calcination under CO2 at atmospheric pressure. The enhancement of CaO conversion is correlated to the microstructure of the CaO samples obtained after calcination. As seen from SEM, BET surface and XRD analysis, calcination under low CO2 pressure leads to a remarkable decrease of pore volume and CaO crystallite size. Consequently, CaO surface area available for carbonation in the fast reaction-controlled regime and therefore reactivity in short residence times is promoted. Keywords: Concentrated Solar Power; limestone; thermochemical energy storage; Calcium-Looping; low CO2 pressure. Results and Discussion 72 PhD Thesis 1. Introduction Thermochemical energy storage (TCES) systems, based upon the heat absorbed and released in reversible chemical reactions with high turning temperature, have a great potential in concentrated solar power (CSP) plants [1-3]. One of the main advantages of TCES is that it allows decoupling the generation of power from demand. In general terms, the heat generated in the CSP plant is used to carry out a reversible endothermic chemical reaction whose byproducts are stored separately. On demand, the exothermic reverse reaction is triggered, which releases the chemically stored heat to be used for generating electrical power [4-6]. The Calcium-Looping (CaL) process, based on the decarbonation and carbonation reactions of calcium carbonate (CaCO3) is being currently the subject of several lab-scale and process engineering studies for thermochemical energy storage in CSP plants with tower technology (CaL-TCES), with small pilot-scale plants under construction [7-18]. Large pilot-scale demonstrations (1-2 MWth) have already shown that the CaL process can be an efficient low-cost technology to capture CO2 from fossil fuel thermoelectric plants [19-21]. The CaL process for CO2 capture and storage (CaL-CCS) is based on the carbonation reaction of CaO to capture CO2 from flue gases at temperatures around 650 °C and the subsequent regeneration of the carbonated solids by calcination at temperatures above 900 °C in a high CO2 concentration environment [22-23]. In the CaO capture process carried out in the carbonator, the post-combustion gas (with a CO2 concentration of ~ 15 %vol. at atmospheric pressure) is used to fluidize a bed of CaO particles. The now partially carbonated particles are then transported to a second fluidized bed reactor (calciner) where calcination proceeds in short residence times. In this way, the CO2 resulting from the combustion of fossil fuels is recovered in the calciner at high concentration to be subsequently compressed and stored or transported for other uses. After calcination, the regenerated CaO is circulated back to the carbonator for its use in a new cycle [24-25]. The efficiency of the CaL technology for CO2 capture is limited though by process conditions, such as the low CO2 concentration in the post-combustion gases and the necessarily short residence times in the reactor due to the high mass flow rates involved [26]. Moreover, the high temperatures and high CO2 concentration in the calciner, added to inactivation by irreversible sulphation and ashes, greatly reduce the reactivity of the regenerated CaO in each cycle [27]. These adverse conditions lead to a rapid irreversible loss of CaO reactivity with the number of cycles [28-30]. The proposed integration of the CaL technology in CSP plants as a TCES system is also based on the reversibility and endothermic nature of the CaCO3 decarbonation reaction [7-8]. CaCO3 has a significantly higher energy density than that of molten salts, currently used as the state-of-the-art thermal energy storage system in CPS commercial plants [8,31-33]. Moreover, molten salts have serious limitations, such as a limited maximum operation temperature (550 ºC to avoid degradation), their high cost, and the need of storage temperature above a certain minimum (150-200 °C) to avoid solidification and the corrosion caused in the materials used to transport and storage these fluids [34- Results and Discussion PhD Thesis 73 36]. On the other hand, natural CaCO3 rich minerals such as limestone and dolomite are non-toxic, very abundant and inexpensive. In addition, both calcium carbonate and the calcium oxide obtained after calcination can be stored indefinitely without thermal losses [8,37,38]. It must be remarked that the most adequate calcination/carbonation conditions in the CaL-CSP integration are not necessarily the same as in the CaL-CCS process since the reversibility of the process permits to adjust the working temperatures depending on the CO2 partial pressure, and according to the thermodynamic equilibrium of the reaction [7,39]. Thus, it has been proposed to carry out carbonation at a high temperature (> 800 °C) in an environment of high CO2 concentration, while the calcination temperature could be lowered using inert gases, atmospheres with a low concentration of CO2 or low CO2 pressures to avoid CaO deactivation [8]. In the CaL-TCES process, concentrated solar energy is used to decompose CaCO3 at high temperatures on a stream of particles in a solar reactor. The products of the reaction, CaO and CO2, are then transported and stored separately. When energy production is demanded, these products are circulated to another reactor to carry out the exothermic carbonation reaction. The CO2 in excess over the stoichiometric need is released as a high pressure and high temperature stream from the carbonator to a gas turbine for power production using a CO2 closed cycle. Thus, carbonation should be ideally carried out at high temperature and high CO2 pressure to enhance the thermoelectric efficiency of the cycle [10,40-42]. The use of CaL technology for energy storage faces challenges related to systems development. In the calciner, different reactor technologies are candidates to be applied depending on the size of the plant and operation conditions [8,16,43-45]. One possibility is using helium in the calciner atmosphere, which significantly reduces the calcination temperature in short residence times below ~ 725 °C and could be separated from the CO2 released during calcination using state of the art membrane technologies [37,46]. Calcination at this low temperature would reduce the loss of multicycle CaO activity caused by sintering. However, the need for adding a CO2-He separation step increases the technical complexity, energy penalty and cost of the technology [15,16]. A further option is to calcine under pure CO2, which would simplify the process, as it would allow carrying out the whole cycle through a closed CO2 circuit. In this case, the greatest inconvenient would be the rapid decay of CaO activity with the number of cycles [10,15]. In order to achieve complete calcination of CaCO3 under pure CO2 at atmospheric pressure, the temperatures required are around 950 °C, which further promotes the sintering-induced deactivation of the CaO particles. Consequently, CaO reactivity is considerably decreased in each cycle, as seen in the CaL-CCS process [15,22]. As demonstrated in recent works having data on the multicycle CaL activity of the CaO precursor at realistic process conditions is of paramount relevance for plant Results and Discussion 74 PhD Thesis modelling analysis to yield reliable results. Thus, lab-scale experimental measurements should be performed at practical conditions imposed by the process. Regarding calcination in the CaL-TCES system, a further possibility suggested in the engineering literature is using a pure CO2 environment at reduced absolute pressure, which is technically feasible still allowing to carry out the whole process in a closed CO2 circuit [16,47]. Expectedly, the calcination temperature could be decreased at reduced absolute CO2 pressure thus mitigating the decay of CaO activity with the number of cycles [48]. An entrained flow reactor (downer reactor) is considered in this work. This technology allows continuous flow, and it is suitable for application at reduced pressures. In an entrained flow reactor calciner, with particles falling by gravity, no additional issues are expected regarding operating at a partial vacuum. Regarding particles transport, particles pneumatic transport is not expected to be significantly affected by the total pressure, but by the pressure differences. If mechanical devices were used for solids transport, no additional issues would be expected by the pressure. Vacuum operation would need additional effort regarding isolation to avoid excessive air leakages into the system. To the best of our knowledge, for energy storage applications using calcium looping, the effect of calcination at reduced pressure (and therefore, lower temperatures) on the multicycle behavior has not been presented before in the literature, ant it is of high interest for future plant designs. Thus, results presented in this study could contribute to support future CaL TCES designs. 2. Materials and methods 2.1. Materials Natural limestone of high purity (99.3 wt % CaCO3) received from Taljedi quarry (Seville, Spain) has been used in this work. The material was sieved to yield a particle size in the range of 160-200 μm [21,49,50]. 2.2. Multicycle calcination/carbonation tests Multicycle calcination/carbonation tests have been carried out through thermogravimetry analysis (TGA). A novel in-house thermobalance has been specially assembled to operate from low pressures up to 5 bar in order to test the material behavior at specific CaL-TCES conditions. Figure 4.1.1 shows a schematic diagram and a photograph of the device. The system consists basically of a high sensitivity CI Electronics microbalance (2x10-7 g) and a reactor. The reactor is composed of a Watlow heater and a non-porous mullite tube, connected with the microbalance by KF-flanges sealed using a clamp and an o-ring. The temperature of the reactor is controlled by placing a thermocouple on the wall of the Watlow heater, and the temperature of the sample is measured using a second thermocouple positioned underneath the sample crucible. A flat ceramic crucible (0.154 cm3) was used to hold the sample. A vacuum pump and a pressure gauge were incorporated to the system in order to perform the experiments under a Results and Discussion PhD Thesis 75 controlled CO2 pressure (absolute pressures of 0.01, 0.1 or 1 bar in the tests carried out in the present work). CO2 pressure was controlled utilizing low-pressure needle valves connected between the vacuum pump and the thermobalance. Rotameters were used to adjust the gas flow through the thermobalance during the experiments. Figure 4.1.1: (a) Schematic diagram and (b) photograph of the homemade thermobalance Mass was accurately calibrated by means of a set of calibration weights ranging from 1 mg to 20 mg. Temperature calibration was performed using the well-known decomposition reaction of hydrous calcium oxalate, which was heated at a heating rate of 10 °C/min. Alumina powder was used as a standard to correct buoyancy effects. Calcination/carbonation tests were carried out in a closed CO2 cycle. Table 4.1.1 lists the diverse operating conditions tested. In this study, carbonation was always performed at an absolute pressure of 1 bar of pure CO2. According to the thermodynamic equilibrium of the CaCO3/CaO system, the maximum carbonation temperature at these conditions is 895 °C [39,51,52]. Three different carbonation temperatures, i.e. 850 °C, 765 °C and 700 °C, were explored to hasten the carbonation kinetics. Ideally, the higher the carbonation temperature the higher the global efficiency of the CaL-TCES process [14,16]. However, the reaction kinetics slows down significantly as the equilibrium Results and Discussion 76 PhD Thesis temperature is approached, which is detrimental to the process [14]. Calcination was carried out under an absolute pressure of 1 bar of pure CO2, as well as under controlled absolute CO2 pressures of 0.1 bar and 0.01 bar. Calcination temperatures were selected by considering the equilibrium temperatures at these CO2 pressures (760 °C at 0.1 bar and 652 °C at 0.01 bar) [39]. These values were also selected accordingly to the solar receiver current state of the art. The calciner operation conditions were considered for the layouts presented in references [16] and [41]. Both layouts are based on two closed circuits, one for calcination and another for carbonation. This configuration allows operation under different pressure conditions, adjusting them to the optimum design. A high carbonation temperature will allow integration of high-efficiency thermal power cycle, operating at higher temperatures (for instance SCO2). Lower receiver temperatures will allow controlling radiation losses with less intricate receiver designs and geometries and less technical requirements in the selection of materials. For this application, the calciner is an entrained flow reactor (downer reactor), which allows a continuous high mass flow. The operation at different CO2 pressures in the calciner affects energy consumption and involved costs. Additional equipment to those included in the reference layouts [16,41] is required. Thus, a vacuum pump must be added to the circuit to start operation. Besides, additional insulation in piping and vessels will be required to avoid excessive air leakages (although a controlled level of air in the mixture is not expected to penalize operation strongly). It implies an additional investment cost and extra power consumption. Once a reduced pressure is achieved and limestone is supplied, the obtained CO2 is transported by a compressor that maintains a continuous operation at reduced pressure, but penalizing plant output with an increase in energy consumption due to the higher compression work. Table 4.1.1: Operating conditions for the different calcination/carbonation tests carried out in this work, and their corresponding acronyms for their identification throughout the text. Calcination Carbonation Test Temperature (°C) Absolute pressure (bar) Temperature (°C) Absolute pressure (bar) 1 765 0.1 765 1 2 765 0.1 850 1 3 765 0.01 765 1 4 700 0.01 700 1 5 700 0.01 850 1 Multicycle calcination/carbonation tests were started with a precalcination stage of the limestone sample at the selected CO2 pressure. As a first step, pressure was reduced using the vacuum pump. Once the system was stabilized, the temperature was increased at 10 °C/min to the selected calcination temperature. After the sample was fully calcined, the temperature was varied at 50 °C/min rate to the carbonation temperature, and then the CO2 pressure was increased to an absolute pressure of 1 bar to carry out the carbonation Results and Discussion PhD Thesis 77 stage (Figure 4.1.2). Then, the temperature and the CO2 pressure were again changed to the calcination conditions to start a new cycle (Figure 4.1.2). Residence times of 10 min were employed in both the calcination and carbonation stages. Figure 4.1.2: Time evolution of sample mass (%), temperature (ºC) and absolute pressure (bar) for a calcination/carbonation cycle of limestone. In this test, carbonation was carried out at 1 bar and 850 ºC, and calcination at 0.1 bar CO2 and 700 ºC. The multicycle activity of limestone obtained from tests carried out at the conditions shown in Table 4.1.1 was assessed by means of the effective conversion (Xeff), defined as the ratio of the mass of CaO converted to CaCO3 in the carbonation stage to the total sample mass before carbonation (Equation (7)): 𝑋𝑒𝑓𝑓(𝑁)= 𝑚𝑐𝑎𝑟𝑏𝑁 − 𝑚𝑁 𝑚𝑁 · 𝑊𝐶𝑎𝑂 𝑊𝐶𝑂2 (7) Here, 𝑚𝑐𝑎𝑟𝑏𝑁 and 𝑚𝑁 are the sample masses after and before carbonation in the Nth cycle, and 𝑊𝐶𝑂2 = 44 g/mol and 𝑊𝐶𝑎𝑂 = 56 g/mol are the molar masses of CO2 and CaO, respectively. As an example, Figure 4.1.3 shows a complete thermogram corresponding to the test T2, in which carbonation and calcination were performed at 765 °C and 0.1 bar CO2, and 850 °C and 1 bar CO2, respectively. Remarkably, this figure illustrates that a stable, accurate measurement of the sample mass can be achieved with the in-house developed thermobalance used in this work, which is of great relevance for the reliability of TGA tests. Results and Discussion 84 PhD Thesis The microstructure of the samples drastically changes after 20 cycles (Figure 4.1.7). Thus, the size of the CaO grains shows marked sintering after the cycles at 765 °C, which would lead to a significant decrease in the surface area available for carbonation in the fast reaction-controlled phase as revealed by the TGA tests. This effect is much more noticeable when calcinations were performed under a CO2 pressure of 0.1 bar (Figures. 4.1.7a and 4.1.7b). Figure 4.1.7: SEM and TEM micrographs of CaO derived from natural limestone after 20 cycles at 765 ºC. (a,b) 0.1 bar (test T1), and (c,d) 0.01 bar (test T3). SEM and TEM observations were complemented by surface area (BET) and pore volume measurements (Vp). These parameters were determined for the CaO samples after the first calcination and after the third and fifth cycles ending in calcination, carried out under the conditions corresponding to tests T1 and T3. Results of these measurements are collected in Table 4.1.3, together with the corresponding conversion values measured by TGA (Figure 4.1.4). For example, after the first calcination, conversion values obtained in the first carbonation for tests T1 and T3 are 0.81 and 1.00, respectively. Typical CaO surface area obtained from natural untreated limestone is in the range 3-20 m2/g, with low porosity values when calcined under an inert gas or under CO2 [38,61-66]. Much higher values of CaO surface area and porosity have been reported for limestone samples calcined in vacuum at low temperatures [67-69]. Surface area and pore volume after calcination are higher for the sample tested under the operating conditions of test T3, as compared to the sample tested under the Results and Discussion PhD Thesis 85 operating conditions of T1. Thus, it is clear that calcining under an absolute pressure of 0.01 bar CO2 leads to an increased surface area available for carbonation as would be expected from the TGA results. Then, the favorable carbonation conditions used (high temperature and high CO2 concentration) promotes carbonation in the fast reactioncontrolled phase. The characterization analysis indicates that the high conversion values obtained from the first carbonation cycle for test T3 (Figures. 4.1.4 and 4.1.5) are a consequence of the effect of the calcination conditions on the CaO microstructure (Figures. 4.1.6 and 4.1.7). CaO surface area and therefore conversion are higher for operating conditions corresponding to test T3. Table 4.1.3: Specific surface area (SBET) and porosity data (Vp) of the CaO samples, measured after the first, third and fifth cycles under the conditions corresponding to tests T1 and T3. Calcination/carbonation Test Calcination T1 (T = 765 °C; carbonation at 1 bar CO2, calcination at 0.1 bar CO2) T3 (T = 765 °C; carbonation at 1 bar CO2, calcination at 0.01 bar CO2) SBET (m2/g) Vp (cm3/g) Xeff SBET (m2/g) Vp (cm3/g) Xeff 1 55.50 0.161 0.81 74.94 0.225 1.00 3 16.22 0.075 0.57 17.74 0.091 0.82 5 9.64 0.047 0.47 14.63 0.077 0.72 In order to investigate the effect of the absolute gas pressure in the calciner environment on the microstructure of the nascent CaO, temperature-dependent XRD tests were carried in which limestone samples were calcined under vacuum (0.01 bar) and in air (absolute pressure of 1 bar and a flux of 100 cm3/min). The CaO crystallite size of the samples calcined in vacuum and airflow at 765 °C were estimated using the Scherrer method. This temperature was used as a reference because, as discussed above, the multicycle experiments performed at 765 °C provide the best results in terms of CaO conversion (tests T1 and T3). Figure 4.1.8 shows a comparison of the main diffraction peak (2 0 0) corresponding to CaO (2Ɵ = 37.3°) calcined in vacuum and air, respectively. It is clear from the figure that the peak width is greater for the sample calcined under vacuum. Moreover, the intensity of the diffraction peak is higher for the sample calcined in air. The Shcherrer equation gives a CaO crystallite size of 13 nm for the sample calcined in vacuum and 24 nm for the sample calcined in air. Thus, the absolute pressure in the calciner environment has an important effect on the size of the CaO crystallites and therefore on CaO reactivity. Similar results were reported by Beruto and Searcy [70], who furthermore correlated the size of CaO crystals resulting from limestone calcination to the reactivity Results and Discussion 86 PhD Thesis of the calcined limestone towards hydroxylation. More recently, the reactivity of CaO towards carbonation that results from the crystallographic CaCO3/CaO transformation has been observed to be inversely correlated to the size of CaO crystallites [71,72]. In situ XRD analysis reported elsewhere [72] indicates that the growth of CaO crystallites occurs through two stages. The first stage is driven by the aggregation of nascent CaO nanocrystals that attract each other by surface van der Waals forces, which is followed by sintering of the aggregated nanocrystals. Arguably, calcination under high CO2 partial pressure enhances the adsorption of CO2 molecules on the surface of the nascent CaO nanocrystals, which increases the strength of van der Waals attractive forces between them, thus enhancing aggregation. Accordingly, by reducing the CO2 absolute pressure in the calciner environment, CO2 adsorption is minimized, which diminishes the strength of surface attractive forces thus hindering aggregation of CaO nanocrystals. Thus, a favorable strategy to boost the reactivity of the CaO regenerated in each cycle after calcination under CO2 would be to reduce the absolute CO2 pressure to avoid aggregation of the nascent CaO nanocrystals and shorten the calcination time to minimize subsequent sintering. As confirmed by the TGA results reported in the present work reducing the CO2 pressure is a highly efficient technique to mitigate the loss of CaO reactivity after calcination. Figure 4.1.8: Comparison of the main diffraction peak (2 0 0) corresponding to CaO (2Ɵ =37.3 º) for limestone samples calcined in vacuum and air at 765 ºC. 4. Conclusions In this work, the multicyclic performance of natural limestone has been investigated when subjected to novel Calcium Looping conditions for thermochemical energy storage in CSP plants with tower technology. These conditions involve a CO2 closed circuit for carbonation and calcination, which would reduce the technical Results and Discussion PhD Thesis 87 complexity of the technology. The main novelty lies in calcining at absolute low pressures of CO2 (0.01 and 0.1 bar) whereas carbonation is performed at high temperature under CO2 at atmospheric pressure as proposed in previous schemes. A main benefit of carrying out the calcination stage at reduced CO2 pressure is that the temperature to achieve full calcination in short residence times can be substantially decreased, which would facilitate the design of the solar reactor. Furthermore, our study demonstrates that the absolute CO2 pressure used for calcination has an important effect on the multicyclic CaO conversion. Thus, calcining at low CO2 pressures enhances notably conversion and mitigates its progressive decay with the number of cycles. The highest values of effective conversion were obtained for limestone samples calcined at 765 °C (test T3) and 700 °C (test T5) at an absolute pressure of 0.01 bar of CO2, with carbonations carried out at 850 °C under CO2 at atmospheric pressure as corresponding to CaL conditions for thermochemical energy storage in CSP plants. The residual conversion that results from these conditions is enhanced by a factor of 10 compared to that obtained when calcination is carried out under CO2 at atmospheric pressure. SEM, TEM, specific surface area (SBET) and temperature-dependent XRD measurements indicate that the enhancement of CaO conversion is correlated to the microstructure of calcined CaO samples. Calcination under reduced CO2 pressure leads to a decreased size of the CaO grains thus increasing the surface area available for carbonation in the fast reaction-controlled phase. Enhanced carbonation takes place in very short residence times, which would allow shortening the residence time of the CaO solids in the carbonator. The effect of calcination pressure on the effective conversion analyzed in this paper affects the global performance of the plant in two ways. On the one hand, the higher conversion reduces the required amount of solids and thus the required energy for transport. On the other hand, a lower pressure reduces the required size of components, the receiver itself, but also the cyclones, storage tanks and heat exchangers. For a specific plant, a set of calciner operation conditions optimizes the balance between global plant design, operation, efficiency and costs. These optimum conditions must be derived from the balance between material conversion, solar receiver temperatures and energy required in solids and gases transport. Acknowledgements This work has been supported by the Spanish Government Agency Ministerio de Economıa y Competitividad (contracts CTQ2017-83602-C2-1-R and -2-R) and Junta de Andalucıa-Consejerıa de Economıa, Conocimiento, Empresas y Universidad-Fondo Europeo de Desarrollo Regional (FEDER) (Programa Operativo FEDER Andalucıa 2014-2020, project US-1262507). 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Benitez-Guerrero, J. M. Valverde, A. Perejon, P. E. Sanchez-Jimenez, L. A. Perez-Maqueda, Effect of milling mechanism on the CO2 capture performance of limestone in the Calcium Looping process, Chem. Eng. J. 346 (2018) 549-556. Results and Discussion 100 PhD Thesis 1. Introduction Improving the deployment and efficiency of renewable energies is currently the most viable solution towards the aim of global decarbonization. Among renewable energies, concentrated solar power (CSP), which exploits the energy from the sun to generate electricity, has attracted a lot of attention in terms of investment and research in the last decades. Thus, a total of 135 concentrated solar power projects have been under operation, construction or development from 2006 to 2021 around the world in 10 countries [1]. They contribute to mitigate the environmental impact related to electricity production, as the massive reduction in CO2 emissions is one of the main worldwide challenges nowadays [2,3]. However, this challenge can only be met if the problem of direct solar radiation variability is addressed through the development of low-cost solar energy storage technologies based on abundant, non-toxic materials that enable the commercial expansion of solar energy on a large scale [4-6]. Currently, molten salts (mixtures of NaNO3/KNO3) are used as sensible heat thermal energy storage system integrated in the first and second generation concentrated solar power (CSP) plants [7,8]. It is, therefore, a mature technology that allows decoupling production and demand [8]. However, molten salts present serious limitations related to their cost, corrosiveness, the maximum operation temperatures (~ 560 °C to avoid degradation) and the requirement of storing them at temperatures over 220 °C to avoid their solidification, which penalize the CSP plant performance [9-13]. Next generation of CSP plants contemplate operation at temperatures over 600 °C, which would significantly boost the power-to-heat efficiency [14-15]. Furthermore, storage concepts with higher energy densities are under consideration. Thermochemical energy storage (TCES), while still in the research and development phase, has great potential. It is based on the heat exchanged in reversible chemical reactions, which inherently high enthalpies that results in very high energy densities [16-19]. The integration of a TCES system in a CSP plant is devised as follows; the heat generated by concentrated solar power is used to carry out the endothermic chemical reaction whose products would be stored separately. When energy demand requires it, the reverse exothermic reaction would be carried out and the stored heat would be released to produce energy. The capability for long-term storage of the reaction products without thermal losses is another advantage of TCES systems over concepts that rely on latent or sensible heat storage [20- 23]. Several thermochemical energy storage systems have been proposed, such redox reactions of metallic oxides, ammonia decomposition, hydration/dehydration reactions, sulphur cycles and carbonation/decarbonation reactions [22,24,25]. Among the latter, extensive research has been devoted to the calcium looping (CaL) process for thermochemical energy storage in CSP plants, due to the high theoretical energy storage density of CaCO3/CaO, 4.82 GJ/(m3 CaCO3), which corresponds to a value of 10.62 GJ/(m3 CaO) if the calcined solid is considered in the calculations [26-32]. The CaL-CSP integration consist in using the concentrated solar energy on a bed or stream of CaCO3 to Results and Discussion PhD Thesis 101 induce its calcination at high temperature (about 800 °C in inert gas or 950 °C in CO2) [33-37]. The reaction products, CaO and CO2, are transported out of the calcinator and stored separately. When demand requires energy production, these products are brought back together in the carbonator to carry out the exothermic carbonation reaction to regenerate the CaCO3. As the process can be carried out at temperatures as high as 800 °C [33,38], gas turbines could be used for electricity production with higher efficiency than the steam turbines used in conventional CSP plants [39-41]. Another advantage of the CaL process is that CaO could be exploited in other industrial applications requiring heat [38,42]. However, the CaL technology presents some drawbacks for its integration in CSP plants, mainly the high temperature needed to carry out the calcination reaction, which promote the progressive sintering-induced deactivation of CaO along the carbonation and calcination cycles [34,43,44]. Previous results from theoretical models and numerical simulations estimate that stable sorbent conversion values above 20 % are required for CaL-CSP plants to best the efficiencies currently attained in CSP plant equipped with molten salt storage systems [29]. Two main general approaches have been used to enhance the reactivity of CaO during the multiple carbonation/calcination cycles: (i) modifying the operating conditions in terms of temperature and atmosphere in order to attenuate sintering [45-48], and the use of other natural or synthetic Ca-based materials with improved structural stability [49- 52]. Among these materials, dolomite, CaMg(CO3)2, exhibits very stable CaO multicycle performance [53-55] due to the role of inert MgO arising during the first calcination. However, MgO constitutes 41.8 % in mass, which significantly reduces the maximum attainable energy density. In addition, calcium acetate derived CaO shows promising results as its decomposition leads to highly reactive CaO particles of small size [56,57]. However, the decay in activity is also fast unless structural stability is improved, normally by the use of additives [56]. Regarding the operating conditions, it has been proposed to carry out the calcination under inert gases such as helium, nitrogen or argon, allowing the calcination temperatures to be reduced below 750 °C, according to the thermodynamic equilibrium of the CaCO3/CaO system [27,54,58]. However, the use of an inert gas during calcination entails implementing a costly separation stage that imposes a significant energy penalty on the process [59]. Steam has also proven effective in reducing the calcination temperature as it combines high thermal conductivity and certain catalytic effects promoting the desorption of CO2 during calcination [60-64]. Another alternative is the operation under a closed CO2 cycle, in which both carbonation and calcination would be carried out in pure CO2 atmosphere [27,45]. This permits avoiding gas separation processes or condensation steps but it requires increasing the calcination temperature up to 950 °C in order to achieve the complete calcination of CaCO3 in reasonable residence times. These extreme operation conditions severely deteriorate the multicycle activity of Results and Discussion 102 PhD Thesis the CaO due to extensive sintering and loss of surface area available for carbonation [27,28,45]. Very recently, it has been proposed a new operating scheme that involves calcination at moderate temperatures under low absolute CO2 pressures (0.1-0.01 bar) and carbonation at high temperatures under CO2 at atmospheric pressure [46,47]. Previous results have shown these conditions results in notably enhanced reactivity for limestone derived CaO, as compared to previously reported conditions. Thus, it is of the most interest to study the multicycle performance of other CaO-based materials, with inherently better performance under these milder operation conditions. In this work, the reactivity of CaO obtained from dolomite and acetate precursors with different CaO/MgO ratios has been tested. 2. Materials and methods 2.1. Materials Natural limestone, dolomite and magnesite, provided by Matagallar (Pedrera, Spain), Bueres (Asturias, Spain) and Magnesitas Navarras (Navarra, Spain), respectively, were used in this work. Acetic acid of high purity (99.9 %) from VWR chemicals (CAS: 64-19-7) was used for the synthesis of the acetates. 2.2. Characterization methods X-ray diffractograms were collected in the 2θ range from 20 ° to 70 ° for dolomite and from 5 ° to 30 ° for the acetates, using a Rigaku Miniflex diffractometer working at 40 kV and 15 mA. Scanning Electron Microscopy (SEM) was used to study the microstructure of the as prepared and cycled samples. An ultra-high resolution HITACHI S4800 instrument was used. HRTEM and HAADF-STEM micrographs were registered using a Talos F200S FEG microscope. For this purpose, the powder samples were deposited on copper grids. In situ X-ray diffractograms (XRD) during the calcination process were acquired in vacuum using a powder diffractometer (Bruker D8 Advance) fitted with a hightemperature chamber (Anton Paar XRK 900) and a fast response/high sensitivity detector (Bruker Vantec 1). The temperature during the in situ XRD experiment was increased at 10 °C/min from 400 °C up to 750 °C. XRD scans of 295 s were recorded in the range of 20 ° < 2θ < 60 ° (0.03 ° per step), each 25 °C. Scans were all registered at constant temperature. The BET surface areas and total pore volumes (determined from the N2 absorbed at p/p0 = 0.99) of the samples were measured in an ASAP2420 Micromeritics instrument. The samples were degassed at 400 °C in vacuum for 2 h and then measured at a temperature of -196 °C. 2.3. Synthesis of the CaO-based materials Results and Discussion PhD Thesis 103 Magnesite and limestone were calcined in air in a furnace at 800 °C for 2 h in order to obtain the corresponding calcium and magnesium oxides. Three acetates with different Ca/Mg molar ratios were prepared: calcium acetate (CaAc) using calcined limestone (CaO) as a precursor, Ca80Mg20Ac using a mixture of calcined limestone and calcined magnesite (MgO) in a molar ratio of 80 %CaO/20 %MgO, and Ca95Mg05Ac from a mixture in a molar ratio of 95 %CaO/5 %MgO. For the synthesis of each acetate, 1 g of the mixtures of CaO and MgO were treated with 50 mL of an aqueous solution of acetic acid (25 % in volume), and stirred for 2 h at room temperature. Then, the solution was dried at 120 °C for approximately 2 h to obtain the calcium and magnesium acetate crystals. Further details can be found in [56]. Table 4.2.1 includes the molar and mass ratio of CaO and MgO corresponding to the four samples used in this work. Table 4.2.1: Molar and mass ratio of CaO and MgO for the four samples used in this work. The theoretical densities of the corresponding calcined materials are also included. Sample CaO (%molar) MgO (%molar) CaO (%mass) MgO (%mass)  calcined (kg/m3) Dolomite 50 50 58.2 41.8 3440 CaAc 100 0 100 0 3340 Ca80Mg20Ac 80 20 84.8 15.2 3377 Ca95Mg05Ac 95 5 96.4 3.6 3349 Figure 4.2.1 shows the XRD patterns of the four samples studied in this work. From this data, the dolomite sample is a phase pure material, as well as the CaAc sample, composed only of hydrated calcium acetate. On the other hand, Ca80Mg20Ac and Ca95Mg05Ac are composed of a mixture of calcium acetate and calcium magnesium acetate in different proportions, with no other crystalline phases. From the intensity of the XRD peaks, a higher amount of calcium magnesium acetate was detected for Ca80Mg20Ac, which is consistent with the relative amounts of calcium oxide and magnesium oxide used as precursors. Results and Discussion 104 PhD Thesis Figure 4.2.1: XRD patterns of (a) dolomite, and the different acetates prepared in this work: (b) CaAc, (c) Ca80Mg20Ac and (d) Ca95Mg05Ac. Figure 4.2.2 shows the representative SEM images of dolomite (Figure 4.2.2a) and Ca95Mg05Ac (Figure 4.2.2b). Dolomite is composed of platelets, while Ca95Mg05Ac presents an acicular morphology, which is typical of the acetates, with a particle size in the range 200-150 μm. Moreover, the microstructure and particle size of the acetate is maintained after decomposition to the carbonate (Figure 4.2.2c) and to the oxides (Figure 4.2.2d). The same particle sizes were obtained for CaAc and Ca80Mg20Ac. Results and Discussion PhD Thesis 105 Figure 4.2.2: SEM micrographs of (a) dolomite, (b) Ca95Mg05Ac, (c) Ca95Mg05Ac after decomposition to the carbonate, and (d) Ca95Mg05Ac after decomposition to the oxides. 2.4. Multicycle experiments Multicycle experiments were performed in an in-house thermogravimetric instrument prepared to work from low pressures (0.01 bar) up to 5 bar. Figure 4.2.3 shows a scheme of the instrument. It consists of a microbalance and a reactor composed of a furnace and a non-porous mullite tube. A flat crucible was used to hold the sample in the microbalance in order to facilitate the contact of the sample with the gases. The CO2 pressure was controlled by means of a vacuum pump, a pressure gauge and a pressure measurement system. Further details of the instrument can be found in [46,65]. The multicycle calcination/carbonation tests were performed using 40 mg of the samples in all cases, and operating in a closed CO2 cycle in which absolute pressures of 0.01 bar and 0.1 bar CO2 were employed for calcination. The calcination temperatures were 700 °C and 760 °C respectively, selected according to the thermodynamic equilibrium of the CaCO3/CaO system [66,67] and in order to optimize the multicycle performance of the materials at different temperatures attainable by conventional solar receivers [68]. An absolute pressure of 1 bar CO2 was used for carbonation in all tests, at 700 °C, 760 °C and 850 °C. Table 4.2.2 summarizes the operating conditions tested in this work. Results and Discussion 106 PhD Thesis Figure 4.2.3: Scheme of the in-house thermogravimetric instrument used in this work. Table 4.2.2: Operating conditions used for calcination and carbonation. The acronyms have been used to identify the experimental conditions in the text. Calcination Carbonation Test Temperature (°C) Absolute CO2 Pressure (bar) Temperature (°C) Absolute CO2 Pressure (bar) E1 700 0.01 700 1 E2 760 E3 850 E4 760 0.1 760 E5 850 Prior to the start of the multicycle tests, the system was stabilized at the selected absolute CO2 pressure, using the vacuum pump. Then, the calcination stage was started by increasing linearly the temperature up to the corresponding calcination temperature, which was hold for 10 min to ensure a complete calcination. Then, the temperature was changed to the carbonation temperature. Once the temperature was stable, the vacuum pump was disconnected and the CO2 pressure was then increased to 1 bar to initiate the carbonation reaction, lasting 10 min. At that point, the temperature was again decreased to the calcination temperature for a new cycle. Heating and cooling rates of 10 °C/min were always employed. The multicycle performance of the samples was evaluated by means of the effective conversion, defined as the mass ratio of calcium oxide converted in the carbonation stage at each N-cycle to the total mass of the sample before carbonation (𝑚𝑖), Results and Discussion PhD Thesis 107 including solids inert to carbonation. The term 𝑚𝑐𝑎𝑟𝑏(𝑡)−𝑚𝑖 is therefore the CO2 uptake (𝑚𝐶𝑂2(𝑡)) in each cycle: 𝑋𝑒𝑓𝑓 =(𝑚𝑐𝑎𝑟𝑏(𝑡)−𝑚𝑖) 𝑚𝑖𝑊𝐶𝑎𝑂 𝑊𝐶𝑂2=𝑚𝐶𝑂2(𝑡) 𝑚𝑖 𝑊𝐶𝑎𝑂 𝑊𝐶𝑂2 (9) The terms 𝑊𝐶𝑎𝑂 and 𝑊𝐶𝑂2 are the molar masses of CaO and CO2, respectively. It is important to remark that the effective conversion takes into account the presence of solids that are inert to carbonation under the experimental conditions used in this work. This parameter is more interesting for practical purposes than the CaO conversion, since the inert solids will be also circulated through the system and will influence the efficiency of the process. From Equation (9), the energy storage capacity (𝐷𝑚) of the tested materials in kJ/kg for each cycle can be quantified using Equation (10): 𝐷𝑚=𝑚𝐶𝑂2(𝑡)· ∆𝐻𝑅 𝑚𝑖 (10) where 𝑚𝐶𝑂2is CO2 uptake during carbonation, as stated above, and ΔHR is the enthalpy of the reaction (4045.5 kJ/kg CO2). The energy storage density (𝐷𝑣) in GJ/m3 was obtained by multiplying the energy storage capacity and the density of the calcined materials in kg/m3 (Equation (11)): 𝐷𝑣=𝐷𝑚 × ρ (11) As a first approximation, the theoretical densities of the calcined materials were considered for the calculations of 𝐷𝑣, included in Table 4.2.1. 3. Results and discussion 3.1. Multicycle activity Figure 4.2.4 shows, as an example, the evolution of the effective conversion during 10 calcination/carbonation cycles as a function of time for a test E2 on the sample Ca80Mg20Ac, in which calcination was performed at 700 °C and 0.01 bar CO2, and calcination was carried out at 760 °C and 1 bar CO2 (Table 4.2.2). In the first calcination, three mass losses are observed, ascribed to dehydration, decomposition of the acetate to obtain the carbonate, and finally the subsequent decomposition of the carbonate to yield CaO. From this point, the successive CaO carbonation and CaCO3 calcination stages were alternated. As may be observed, under the calcination conditions corresponding to test E2, fast calcinations and carbonations were attained. The mass gain is similar in each carbonation stage, which is an indication of a stable multicycle performance. Results and Discussion 108 PhD Thesis Figure 4.2.4: Time evolution of the mass and temperature for the sample Ca80Mg20Ac, measured during calcination/carbonation cycles under experimental conditions E2. Figure 4.2.5 shows the values of effective conversion at the end of carbonation as a function of the cycle number for the four samples tested in this work under the five tests included in Table 4.2.2. The theoretical maximum effective conversion values are also included as horizontal lines in the figure for comparison purposes (Xeff max). For each sample, similar conversion values were obtained regardless the performed test, with the exception of CaAc that presents a more obvious decrease in effective conversion with the cycle number for the experiment E3. These results are in contrast with those presented for natural limestone, with a particle size in the range 160-200 μm, tested under similar conditions, in which the reactivity strongly depends on the absolute CO2 pressure used for calcination and the temperature of carbonation [46]. Table 4.2.3 includes the values of effective conversion in the first (Xeff(1)) and tenth cycle (Xeff(10)) for all the test carried out, and the differences between them, expressed as Xeff. It can be observed that Xeff values are higher for dolomite and CaAc than for the magnesium calcium acetates. The low values of effective conversion attained for dolomite (Figure 4.2.5a), as compared with the rest of samples, are due to the presence of MgO (one mol per mol of CaO) on the calcined sample that is inert to carbonation under the experimental conditions used in this work. The amount of MgO in calcined dolomite is approximately 42 % in mass. For this reason, the maximum value of effective conversion attainable for dolomite is 0.58. It has been amply demonstrated that the MgO grains hampers the sintering of CaO-based materials during the carbonation/calcination cycles, which explains the stable Results and Discussion PhD Thesis 109 effective conversion attained for dolomite [54,69,70]. Thus, Xeff is in the range from - 0.11 to -0.16, depending on the test. In CaAc (Figure 4.2.5b), the calcined sample is composed only of CaO and serves as a reference of a sample with no MgO addition. The values of effective conversion in the first cycle (Xeff(1)) are obviously much higher than that of dolomite, but, in contrast, the decay in reactivity is significant as can be observed from the values of Xeff (Table 4.2.3). It has been previously observed that the CaO obtained from the decomposition of CaAc is very reactive to carbonation, but at the expense of intense sintering-induced deactivation in the presence of CO2 [56,71,72]. As compared to CaAc, the Ca80Mg20Ac sample (Figure 4.2.5c) presents only slightly smaller initial values of Xeff(1), attributed to its 20 % molar content of MgO. This corresponds to a mass percentage of 15.2 % in MgO, which is inert to carbonation and thereby reduce the maximum Xeff attainable. However, the deactivation along the calcination/carbonation cycles is almost completely suppressed, as can be inferred from the values of Xeff, independently of the type of test carried out. Taking into account the promising results obtained for Ca80Mg20Ac, a sample with even lower content in MgO was prepared, Ca95Mg05Ac. The aim was to reduce the load of inert MgO while still preserving its stabilizing role. The sample was cycled under the all test conditions (E1-E5). As may be seen in Figure 4.2.5d, very high and stable values of effective conversion were obtained during 10 cycles regardless the test performed, with Xeff(1) in the range 0.84-0.90 and Xeff(10) above 0.78. This material presents an outstanding multicycle performance with Xeff of -0.04 for the tests E2 and E4. For the tests E3 and E5, in which carbonations were carried out at 850 °C, higher values of Xeff were obtained. The effective conversion of natural limestone, calcined at 765 °C under 0.1 bar CO2 and carbonated at the same temperature at an absolute pressure of 1 bar CO2 is also included for comparison [46]. A marked decrease on the effective conversion is observed with the cycle number for limestone, in contrast with Ca95Mg05Ac. Results and Discussion 116 PhD Thesis Figure 4.2.9: (a,b) SEM micrographs of Ca95Mg05Ac after first calcination at 0.1 bar CO2. (c,d,e) EDX analysis of the sample subjected to 10 calcination/carbonation cycles, ending in calcination. Further insights about the microstructure of the sample were obtained by the transmission electron microscopy analysis, as shown in Figures 4.2.10 and 4.2.11. Figures 4.2.10a-4.2.10d illustrate HRTEM micrographs of Ca95Mg05Ac after the first calcination at 0.1 bar CO2. It is confirmed that the sample is composed of nanometric CaO grains aggregated in particles of several microns. The grain size is in the order of 100 nm and the MgO grains are much smaller. Figures 4.2.10e-4.2.10h present HRTEM the micrographs of the sample subjected to 10 calcination/carbonation cycles ending in calcination. As expected, the microstructure changes in such a way that CaO grain size looks more heterogeneous and increases, due to sintering. The small spots seen in Figures 4.2.10e-4.2.10h may be attributed to MgO segregated from the CaO matrix. Results and Discussion PhD Thesis 117 Figure 4.2.10: HRTEM micrographs of Ca95Mg05Ac: (a-d) after calcination at 0.1 bar CO2; (e-h) subjected to 10 calcination/carbonation cycles ending in calcination. Figure 4.2.11 shows the HRTEM micrographs and the corresponding HAADFSTEM mappings of Ca95Mg05Ac subjected to a first calcination (Figures 4.2.11a- 4.2.11d) and subjected to 10 calcination/carbonation cycles (Figures 4.2.11e-4.2.11h). The HAADF-STEM mappings of the sample after the first calcination illustrate a homogeneous distribution of the MgO nanograins in the CaO matrix. On the other hand, segregation and aggregation of the MgO nanograins is evident in Figure 4.2.11f, which are located in the CaO grain boundaries. This justifies again the small decrease in reactivity on this sample along the cycles (Figures 4.2.5-4.2.6). Figure 4.2.11: HRTEM micrographs and HAADF-STEM mappings of Ca95Mg05Ac: (a-d) after calcination at 0.1 bar CO2; (e-h) subjected to 10 calcination/carbonation cycles ending in calcination. Results and Discussion 118 PhD Thesis 4. Conclusions The CaL multicycle performance of dolomite and calcium-magnesium acetates has been studied under reaction conditions involving calcination under an absolute CO2 pressure of 0.1 bar or 0.01 bar and carbonation under 1 bar CO2. These conditions imply lower calcination temperatures thereby minimizing the sintering-induced deactivation of CaO. At absolute CO2 pressures of 0.01 bar and 0.1 bar, the starting carbonates can be fully calcined in short residence times at 700 °C or 760 °C, respectively. Furthermore, the nascent CaO particles obtained under these calcination conditions are nanometric size and highly reactive towards subsequent carbonation. Different temperatures were explored for carbonation (700 °C, 760 °C and 850 °C) as depending on the calcination temperatures, to obtain five types of operating conditions explored. In all cases, CaO carbonation takes place in less than 1 min, which is a very relevant result for the practical application of the process. Samples with different MgO content were tested under these novel process conditions. Overall, high cycling stability for the active materials was achieved, save for the CaAc samples that shows a slight decay trend due to the absence of MgO which proves very effective in avoiding the CaO deactivation. The accumulated energy density of the sample derived from the calcium-magnesium acetate Ca95Mg05Ac is higher than that of dolomite, CaAc and Ca80Mg20Ac. In particular, Ca95Mg05Ac presents an exceptional multicycle performance even after subjected to 10 calcination/carbonation cycles, which gives an accumulated energy storage density of 90.9 GJ/m3. It is demonstrated by surface area measurements, SEM and TEM that the excellent results of Ca95Mg05Ac are related with the microstructure of the sample after calcination, and that just 5 % molar addition of MgO is enough to prevent a marked sintering of the CaO grains. The much-reduced calcination temperature and the avoidance of gas separation systems makes this CaL concept attractive for thermochemical energy storage applications. Acknowledgements This work has been funded by EU Next Generation funds and Spanish Ministry of Science and Innovation (projects TED2021-131839B-C22 and PDC2021-121552- C21), and by the grant CTQ2017-83602-C2-1-R (MCIN/AEI/10.13039/501100011033 and ERDF A way for making Europe by the European Union). Results and Discussion PhD Thesis 119 References [1] A. 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Pérez-Maqueda, P. E. Sánchez-Jiménez, CO2 capture performance of Ca-Mg acetates at realistic Calcium Looping conditions, Fuel 196 (2017) 497-507, doi: 10.1016/j.fuel.2017.01.119. [72] D. S. Sultan, C. R. Müller, J. S. Dennis, Capture of CO2 using sorbents of calcium magnesium acetate (CMA), Energy & Fuels 24 (6) (2010) 3687-3697, doi: 10.1021/ef100072q. [73] P. E. Sanchez-Jimenez, J. M. Valverde, L. A. Perez-Maqueda, Multicyclic conversion of limestone at Ca-looping conditions: The role of solid-sate diffusion controlled carbonation, Fuel 127 (2014) 131-140, doi: 10.1016/j.fuel.2013.09.064. Results and Discussion 228 PhD Thesis Results and Discussion PhD Thesis 229 Efficient SrO-based thermochemical energy storage using a closed-loop pressure swing Abstract The SrCO3/SrO system has recently attracted interest for thermochemical energy storage due to the high energy densities attainable. However, the high temperatures needed to promote calcination involve a sintering-induced deactivation of SrO to carbonation. In this work, SrO-based samples have been tested using a closed-loop pressure swing approach involving calcinations and carbonations at absolute pressures of 0.01 bar and 1 bar CO2, respectively. Using low CO2 absolute pressure for calcination decreases the reaction temperature to 900 °C, thus reducing the deactivation of SrO. Moreover, the use of additives further improves the reactivity of the samples. The addition of ZrO2 and MgO by mechanical mixing and acetic acid treatment, respectively, results in samples with very high multicycle performance, yielding material energy storage densities after twenty cycles above 5.0 GJ/m3. These results significantly improve those obtained for similar samples in which calcinations and carbonations were carried out at an absolute pressure of 1 bar CO2. Regarding the integration of the thermochemical energy storage into concentrating solar power plants, calcining SrO-based materials at low pressure increases the net thermal-to-electric efficiencies by up to 6 % points compared to CaO-based materials calcined at the same conditions. The importance of experimental conditions and precursors in the multicycle behaviour of SrO-based materials for thermochemical energy storage is emphasized. Keywords: Thermochemical Energy Storage; low absolute CO2 pressure; SrCO3; Strontium-looping; Concentrated Solar Power; Closed loop. Results and Discussion 230 PhD Thesis 1. Introduction The massive deployment of renewable energies depends on using efficient, costeffective and scalable energy storage systems [1]. Thermal Energy Storage (TES) integrated into power plants based on renewables has attracted interest due to the potential high efficiency and reduction of costs [2,3]. Thermochemical energy storage (TCES), based on the use of reversible chemical reactions, emerges as a promising thermal energy storage (TES) system due to its great advantages over other thermal energy systems, such as the high energy density and the possibility of storing the products of the reactions at room temperature without thermal losses [4]. Thus, different TCES systems have been proposed, such as hydroxides [4,5], redox reactions [6] and carbonation reactions [7,8]. Solid-gas reactions are good candidates for thermochemical storage as they are suitable for high temperatures [9,10]. Among these reactions, the reversible calcination/carbonation reaction of CaO has been extensively explored due to a combination of high enthalpy, reasonable cycle stability and wide availability and inexpensiveness of the raw minerals [11,12]. Another alkali earth carbonate suitable for TCES is SrCO3 [13,14] This system has a resemblance to the deeply studied CaCO3/CaO [15,16], but the enthalpy of the reaction is higher, and the maximum theoretical energy storage density is 10.6 GJ/(m3 SrO) [17]. Furthermore, higher carbonation temperatures may be employed, which would increase the overall efficiency of the plant [18]: 𝑆𝑟𝐶𝑂3⇄ 𝑆𝑟𝑂+ 𝐶𝑂2 ∆𝐻𝑟0= 234 𝑘𝐽/𝑚𝑜𝑙 (31) However, the temperature required to regenerate active SrO by calcining SrCO3 is much higher than for the CaCO3/CaO system [19,20], which results in extensive sintering of the material, with the corresponding decay in activity along the ensuing cycles [14]. Consequently, the influence of different atmospheres on the calcination temperature and reactivity of SrCO3 has been studied, including inert atmospheres and gas mixtures containing different CO2 partial pressures up to 0.5 bar [13]. Theoretically, using an inert atmosphere decreases the calcination temperature, reducing the deactivation of SrO towards carbonation and minimizing the formation of mixed oxides during the cycles. However, a gas separation stage would be necessary to remove the CO2 obtained in the calcination step, increasing the technical complexity and the cost of the application. The system scheme can be simplified using pure CO2 for both calcination and carbonation [14,21]. Calcination under pure CO2 increases the calcination temperature up to 1400 °C, which promotes extensive sintering-induced deactivation. The deactivation of SrO towards carbonation is evident from the first cycles. However, this behavior can be partially reversed by employing additives with a high Tammann temperature, such as ZrO2, SiO2 and MgO [14]. On the other hand, it has been proven that the calcination temperature of SrCO3–SrSiO3 composites can be reduced to 700 °C under an absolute pressure of 0.1 bar CO2 [21]. The samples were carbonated at the same temperature under 6 bar CO2 with very promising results in terms of reversibility and a significant improvement in the reaction kinetics with the addition of NaCl and MgCl2 [21]. Results and Discussion PhD Thesis 231 Thus, additives have been used to prevent particle sintering, facilitating the contact between SrO and CO2. For example, polymorphic materials such as CaSO4 and Sr3(PO4)2 have been added, and it was found that they both hinder sintering [13]. Moreover, SrO has been supported on yttria-stabilized strontium zirconate and strontium zirconate, which allows for obtaining relatively stable energy density values with the cycle number [17]. The use of MgO as support for SrO has also been explored, with full stability in long cyclic tests for samples prepared by wet-mixing [22]. The addition of different amounts of Al2O3 has been also considered, with the best results in energy density and fluidizability obtained for 34 wt % Al2O3 [23,24]. Moreover, coarse granules of SrO/Al2O3 have been prepared with the addition of hydroxyapatite. These granules showed weaker consolidation than the SrO granules [25]. In all these studies, the best results were obtained for additive/SrO mass ratios above 30 %, and due to the high temperatures needed for calcination, mixed oxides were formed that are inert to carbonation and reduce the maximum attainable reactivity. This work assesses the multicycle performance of SrO-based materials for TCES. The calcination/carbonation cycles were performed using a novel TCES scheme that contemplates carrying out the calcination stage under a low absolute pressure of CO2, with carbonations at 1 bar of absolute pressure of CO2 [20,26]. Thus, the cycles were carried out at 900 °C using a closed-loop pressure swing approach, which involved calcinations and carbonations at absolute pressures of 0.01 bar and 1 bar CO2, respectively, avoiding a gas separation stage [13,17]. Characterization of the samples is presented to correlate the structure and microstructure before and after the cycles with the reactivity. The impact of integrating SrO-based materials under a closed-loop pressure swing approach into concentrated solar power (CSP) plants is assessed from a process engineering perspective. 2. Materials and methods 2.1. Materials A dry powder of SrCO3 (98 wt % SrCO3) was provided by Sigma Aldrich (Ref. 289833). In addition, the following oxides were used as additives: ZrO2 (Tosoh, Ref.TZ- 0), SiO2 (99.5 % Strem Chemicals, Ref. 931436), MgO (99 % Panreac, Ref. 211276) and Al2O3 (99.8 % Aldrich, Ref. 9813924). Moreover, high-quality natural magnesite (MgCO3) provided by Magnesitas Navarras (Navarra, Spain) was used. Acetic acid of high purity (99.9 %) from VWR Chemicals (CAS: 64-19-7) was employed to prepare the acetates. 2.2. Preparation of SrCO3 based composites and acetates Results and Discussion 232 PhD Thesis The different additives (either ZrO2, SiO2, MgO or Al2O3) were mechanically mixed by ball milling in a mass ratio 1:9 with SrCO3 to prepare the composites following the procedure described in [14]. In that work, it was demonstrated that the reactivity of the samples was higher for a mass ratio of 1:9 than for 0.5:0.95. Moreover, the mass ratio 1:9 has been used in this work for comparison purposes with the results presented in [14]. For the preparation of strontium-based acetates, a similar procedure to that reported for calcium acetates was used [27,28]. Two samples were prepared: pure strontium acetate and a mixed strontium and magnesium acetate with a mass ratio of 1MgO/9SrCO3. Thus, the results of the multicycle performance were comparable with those of the mechanically mixed samples. MgCO3 and SrCO3 were calcined at 950 °C for 2 h in air and used as raw materials for the acetate preparation. In each case, 1 g SrO or the mixture of MgO and SrO was dispersed in 50 mL of an aqueous solution of acetic acid (25 %) and magnetically stirred for 2 h at room temperature. Then, the samples were heated up to 120 °C and left for 2 h until they were completely dry, yielding acetate crystals. Table 4.6.1 summarizes the precursors, the mass ratio, the methodology employed to prepare the samples and their corresponding names. Table 4.6.1: Precursors, mass ratio, and methodology employed to prepare each sample tested in this work. Sample name Precursors Mass ratio of the additives and SrCO3 Methodology SrCO3 -- -- -- SrZr10 ZrO2 and SrCO3 1:9 Mechanical mixing SrSi10 SiO2 and SrCO3 1:9 Mechanical mixing SrMg10 MgO and SrCO3 1:9 Mechanical mixing SrAl10 Al2O3 and SrCO3 1:9 Mechanical mixing AcSr SrO -- Acetic acid treatment AcSrMg10 MgO and SrO 1:9 Acetic acid treatment 2.3. Methods Scanning Electron Microscopy (SEM) was employed to analyse the microstructure of the samples before and after the calcination/carbonation cycles. A HITACHI S4800 instrument was used for this purpose. The crystal structure of the samples was studied by X-ray diffraction, in a Rigaku Miniflex diffractometer using Cu- K𝛼 radiation. The multicycle calcination/carbonation experiments were carried out in an in-house thermogravimetric instrument that allows working from vacuum to a pressure Results and Discussion PhD Thesis 233 of 5 bar of different gases. The instrument consists of a microbalance, a reactor and a vacuum pump, connected to a pressure gauge to monitor the pressure of the gases inside the system, in this case CO2. Figure 4.6.1 shows a schematic of the setup; further details can be found in [20]. The tests were performed under isothermal conditions (900 °C) for both calcination and carbonation, employing a closed-loop pressure swing approach. Thus, in a first stage, the system was stabilized at an absolute CO2 pressure of 0.01 bar by means of the vacuum pump (Figure 4.6.1). Then, the temperature was increased at a heating rate of 10 °C /min to 900 °C. At this temperature, full calcination of the samples was achieved in all cases in less than 10 min. At this point, the vacuum pump was disconnected and the CO2 pressure raised to an absolute pressure of 1 bar in order to conduct the carbonation stage, which was lasted again for 10 min. Then, new calcinations and carbonations stages were alternated. The amount of sample used in the experiments was 40 mg in all cases. Figure 4.6.1: Schematic diagram of the setup employed for performing the calcination/carbonation cycles. Calcinations and carbonations were conducted at absolute CO2 pressures of 0.01 bar and 1 bar, respectively. Figure 4.6.2 shows, as an example, the thermogravimetric experiment registered for the sample SrZr10. Interestingly, fast calcinations and carbonations were attained at 900 °C. Thus, calcination at an absolute pressure of 0.01 bar CO2 would allow working at temperatures that match those of the operation of receivers in concentrated solar power plants [29-31]. A slight decay in the carbonation extension is evident along the cycles, Results and Discussion 234 PhD Thesis which is observed in the progressively smaller mass gains. This can be attributed to inactivation of SrO due to sintering. Figure 4.6.2: Time evolution of temperature and sample mass for SrZr10. Experimental conditions involve an isotherm at 900 ºC and 10 min of calcination and carbonation at absolute CO2 pressures of 0.01 bar and 1 bar, respectively. 2.4. Effective conversion The effective conversion is the main parameter used in this work to evaluate the reactivity of the samples during the calcination/carbonation cycles. The effective conversion is defined as mass the ratio of strontium oxide converted in the carbonation stage of each N-cycle to the total sample mass before carbonation, including the additives that are inert to carbonation [32]: 𝑋𝑒𝑓𝑓,𝑁 = (𝑚𝐶𝑎𝑟𝑏,𝑖 −𝑚𝑖 𝑚𝑖)𝑁·𝑊𝑆𝑟𝑂 𝑊𝐶𝑂2= (𝑚𝐶𝑂2 𝑚𝑖)𝑁·𝑊𝑆𝑟𝑂 𝑊𝐶𝑂2 (32) where 𝑚𝑖 and 𝑚𝐶𝑎𝑟𝑏,𝑖 are the sample masses before and after carbonation at the Nth-cycle, and therefore the parameter 𝑚𝐶𝑂2 is the CO2 uptake in each carbonation stage. 𝑊𝑆𝑟𝑂, 𝑊𝐶𝑂2 are the molar masses of SrO and CO2, respectively. Results and Discussion PhD Thesis 235 2.5. Energy storage density The energy storage capacity of the calcined materials at each N-cycle, in Gigajoules per ton (GJ/t), was calculated from Equation (32) using the following equation [17,33]: 𝐷𝑚=(𝑚𝐶𝑂2 𝑚𝑖)𝑁·∆𝐻𝑅 (33) Where ∆𝐻𝑅 is the enthalpy of reaction (1) in kJ/kg CO2 (5318.2 kJ/kg) and the values of (𝑚𝐶𝑂2 𝑚𝑖)𝑁correspond to the CO2 mass uptake divided by the mass of the calcined sample (Equation (32)). The energy storage density of the calcined materials (GJ/m3) can be obtained from Equation (34) [33]: 𝐷𝑣=𝐷𝑚· 𝜌 (34) being 𝜌 the theoretical densities of the calcined materials. 3. Results and discussion 3.1. Multicycle performance The multicycle performance of all samples, measured during 10 calcination/carbonation cycles, was studied in the form of effective conversion (Equation (32)). Figures 4.6.3a and 4.6.3b show the values of effective conversion as a function of the cycle number. The effective conversion of SrCO3 is presented in both figures for comparative purposes. As may be seen, the samples present a different behavior depending on the additive used and the preparation method. Thus, for SrCO3 almost full conversion is obtained in the first two cycles that is followed by a sharp decay in the subsequent cycles with a Xeff value in the tenth cycle of 0.11. This deactivation for carbonation can be associated to sintering of the material along the cycles. The addition of ten percent by mass of either ZrO2 or MgO drastically prevents the deactivation of SrO for carbonation. Thus, for SrZr10, full conversion is obtained during the first four cycles (as may be also appreciated in Figure 4.6.2), considering that the maximum effective conversion attainable for a sample that contains ten percent by mass of additive is 0.86. From the fifth cycle, the decay in conversion starts, but it is much less pronounced than for pure SrCO3, with a Xeff value in the tenth cycle of 0.69. A very high multicycle performance was also achieved for the sample SrMg10, with conversions in the range 0.86-0.78 in the first three cycles and a decay from the fourth cycle that ends in a conversion of 0.58 in cycle number ten. On the other hand, different deactivation trends were obtained for SrAl10 and SrSi10. Thus, SrAl10 presents an intermediate reactivity between SrCO3 and SrZr10. In the first cycle, the effective conversion is high (0.80), but it decays with the cycle number to 0.46 for the tenth cycle. The sample SrSi10 shows much lower values of Xeff from the first cycle, but a quasi-steady multicycle performance, Results and Discussion 236 PhD Thesis since the decay in conversion between the first and the tenth cycle is almost negligible (from 0.44 to 0.41). The multicycle performances of strontium acetate (AcSr) and the mixture of strontium and magnesium acetates (AcSrMg10) are shown in Figure 4.6.3b. Strontium acetate clearly presents a better performance than SrCO3, even though Xeff in the first and second cycles is much lower (0.69). However, the drop in conversion is not as pronounced as for SrCO3, and Xeff in the tenth cycle is 0.43. On the other hand, AcSrMg10 also presents a high multicycle performance. Thus, the maximum attainable conversion is obtained in the first four cycles (Xeff = 0.86). From the fifth cycle, the reactivity decreases, and a value of Xeff of 0.67 is obtained in the tenth cycle. Therefore, the performances of raw SrCO3 and the sample SrMg10, prepared by ball milling, are improved when the corresponding acetates are used, as also observed for Ca-based samples [27,28]. Moreover, Xeff is above 0.4 in all samples, improving the results reported for other SrO-based systems studied under different experimental conditions [14,23,24]. Table 4.6.2 compares the Xeff values for the first and tenth cycle of all the samples tested in this work. Figure 4.6.3: Effective conversion as a function of the cycle number for the samples studied in this work: (a) SrCO3, SrZr10, SrSi10, SrMg10 and SrAl10; (b) SrCO3, AcSr and AcSrMg10. Table 4.6.2: Xeff at the first and tenth cycle for the samples tested in this work. Sample Xeff,1 Xeff,10 SrCO3 1.00 0.11 SrZr10 0.86 0.69 SrMg10 0.86 0.58 SrSi10 0.44 0.41 SrAl10 0.80 0.46 AcSr 0.69 0.43 AcSrMg10 0.86 0.67 Results and Discussion PhD Thesis 237 3.2. Energy density Figure 4.6.4a shows the energy density of SrCO3, SrZr10 and AcSrMg10 as a function of the cycle number, calculated from 20 calcination/carbonation cycles and according to Equation (34). The maximum achievable energy density for the SrCO3/SrO system (10.6 GJ/m3) is obtained in the first two cycles. From the third cycle, a dramatic drop in the energy density is observed due to the decay in conversion, with a minimum energy density as low as 0.8 GJ/m3. The energy densities of the SrZr10 and AcSrMg10 samples are lower than that of SrCO3 in the first and second cycles. In the third cycle, the three samples present comparable values of energy density, but from this cycle, the best performance of SrZr10 and AcSrMg10 in terms of effective conversion as compared to SrCO3 is also reflected in much higher values of energy density. Both samples (SrZr10 and AcSrMg10) present similar values of Dv as a function of the cycle number for the first eight cycles, and from cycle number nine a better performance is obtained for SrZr10. The energy density values are higher than those previously reported for other SrO-based systems for thermochemical energy storage tested under different experimental conditions [14,17]. Figure 4.6.4b shows, for comparison purposes, the energy storage density as a function of the cycle number for SrCO3 and SrZr10, calculated from the results presented in [14]. In that work, calcinations and carbonations were carried out under an absolute pressure of 1 bar CO2. It is clearly seen that much higher Dv values are obtained for the sample SrZr10 when calcined under an absolute pressure of 0.01 bar CO2 (from 9 GJ/m3 to 6.1 GJ/m3) as compared with the sample calcined at high temperatures under 1 bar CO2 (from 7.5 GJ/m3 to 2.4 GJ/m3). Interestingly, a different trend is obtained for SrCO3 that presents a less pronounced drop in Dv with the cycle number when calcined at 1 bar CO2 (from 5.3 GJ/m3 to 1.2 GJ/m3), even though the initial values are lower than when calcined at 0.01 bar CO2. This behaviour may be attributed to the fact that sintering of this sample may be favoured when calcined at low CO2 pressure. Results and Discussion 244 PhD Thesis performance, compromising the plant operation. High-temperature particle receivers are being intensively investigated [38]. Indirectly heated receivers seem to be the most interesting option to deal with the low absorptivity of carbonates [39]. For direct solar irradiation, strategies could be preparing dark composites using additives (i,e., iron, manganese, SiC) with notably enhanced absorptivity [40]. Other thermal properties, such as specific heat or thermal conductivity are in the same order of magnitude than solar salts or KCl-MgCl2 [37]. To quantify the impact of the different materials proposed in this research, simulations have been conducted using Aspen Plus software, considering the scheme proposed in Figure 4.6.8 (with more details on the process in [34]). Thus, a comparative analysis has been carried out between SrCO3, AcSrMg10, SrZr10 and CaCO3 under the same conditions. Experimental data for CaCO3 tested under low-pressure calcination have been taken from [20]. A net thermal power of 100 MWth at the receiver is considered for all cases. Solar field efficiency is regarded as 0.7 [41], and the solar receiver efficiency is calculated as a function of the receiver temperature according to [26]. It is assumed that the receiver temperature is 50 °C higher than the calcination temperature for all cases. CaCO3-based simulations consider calcination at 765 °C (0.01 bar) and carbonation at 850 °C (1 bar), while the rest of the cases use the conditions and the experimental results described in Sections 2 and 3.1. Two operating modes are considered under design conditions: 8 h of constant solar thermal input (‘sun mode’) and 16 h without solar input (‘night mode’). To assess efficiency, a weighted average of plant performance for both operating modes is considered [7]. Table 4.6.3 shows the energy balance of the integration. Results and Discussion PhD Thesis 245 Table 4.6.3: Energy balance for the CSP-TCES integration (Figure 4.6.8). Parameter CaCO3 SrCO3 AcSrMg10 SrZr10 ‘Sun mode ’ ‘Night mode’ ‘Sun mode’ ‘Night mode’ ‘Sun mode’ ‘Night mode’ ‘Sun mode’ ‘Night mode’ Power consumption (MWe) Net CSP power (MWth) 100 0 100 0 100 0 100 0 Steam cycle pump -0.01 - -0.02 - -0.02 - -0.02 - Power cycle compresso r - 16.59 -12.64 -14.99 -12.68 -15.35 -12.54 -15.37 -12.52 Storage compresso r - 11.79 - -6.88 - -8.38 - -8.50 - Auxiliaries calciner -0.20 - -0.13 - -0.15 - -0.15 - Auxiliaries carbonator -0.14 -0.14 -0.13 -0.13 -0.13 -0.15 -0.13 -0.15 Solids conveying calciner -0.89 - -2.87 - -0.71 - -0.70 - Solids conveying carbonator -0.30 -0.30 -0.96 -0.96 -0.24 -0.24 -0.23 -0.23 Power production (MWe) Storage turbine - 1.32 - 0.77 - 0.94 - 0.95 Power cycle turbine 26.46 26.46 27.76 27.76 27.44 27.44 27.40 27.40 Steam turbine 3.42 - 2.44 - 2.66 - 2.69 - Summary Net power (MWe) - 0.036 14.704 4.23 14.76 5.136 15.47 4.984 15.47 thermal-to electric efficiency 29.4 % 33.7 % 36.1 % 35.9 % Solar-to electric efficiency 15.9 % 17.1 % 18.2 % 18.2 % Simulation results show a higher thermal-to-electric performance for SrO-based composites than for natural carbonates, propelled by significantly improved multicycle Results and Discussion 246 PhD Thesis conversion (Section 3.1). Remarkably, considering low pressure in the calciner implies higher energy consumption in compressing CO2, as it increases the necessary pressure ratio to reach the storage pressure. This penalizes operation during sunlight hours compared to traditional systems that operate at atmospheric calcination pressure, resulting in lower overall efficiency throughout the day (38 % thermal-to-electric efficiency for CaCO3 by considering ambient pressure in both reactors) [34]. According to [14], the thermal-to-electric efficiency reaches 49.5 % by considering harsh calcination (1400 °C, 1 bar) and carbonation (1200 °C, 3 bar). However, the low pressure in the calcination is essential when considering SrCO3 materials to avoid excessive sintering and receiver heat temperature, which involves higher thermal losses. Using SrO-based composites promotes higher solar-to-electric efficiency above 2 % points higher than CaCO3 under the same conditions (closed-loop pressure swing approach), reducing the impact of calcination under low-pressure compared to typical CSP-CaL schemes. This, combined with the significantly higher energy density of these SrO compounds compared to CaO- based materials (more than double), justifies the potential use of these TCES materials in CSP plants. 4. Conclusions In this work, SrO-based samples have been tested using a closed-loop pressure swing approach involving calcinations and carbonations at absolute pressures of 0.01 bar and 1 bar CO2, respectively, with the objective of decreasing the calcination temperature to 900 °C, thus reducing the deactivation of SrO. It is emphasized the importance of experimental conditions and precursors in the cyclability and efficiency of SrO-based systems for thermochemical energy storage. Experimental results under realistic conditions are used to estimate the overall solar-to-electric efficiency when integrating the TCES system into concentrated solar power plants. It is demonstrated that, according to the thermodynamic equilibrium of the SrO/SrCO3 system, fast calcinations and carbonations are attainable at 900 °C. Despite the low temperature used for calcination, the effective conversion of SrCO3 decays from the second calcination/carbonation cycle. However, using additives drastically improves the efficiency of the SrO-based systems. The samples have been prepared by ball milling and by acetic acid treatment. The multicycle performance of the SrO-derived samples depends on the reactivity of each additive with SrO at the temperature of operation (900 °C). Thus, since MgO does not react with SrO at this temperature and the reactivity of ZrO2 with SrO is low, these samples present the best cyclability and energy storage density. Specifically, AcSrMg10 (prepared by the acetic acid method) and SrZr10 show high effective conversions and the energy storage densities at the twentieth cycle are 5.3 GJ/m3 and 6.1 GJ/m3, respectively. These results significantly improve those obtained for similar samples in which calcinations and carbonations were carried out at an absolute pressure of 1 bar CO2. Results and Discussion PhD Thesis 247 Process simulation results demonstrate that SrO-based composites outperform natural carbonates in thermal-to-electric performance due to the improved multicycle conversion and the potential interest of a closed-loop pressure swing approach for plant operation. Acknowledgements This work has been funded by the grant TED2021-131839B-C22 (Ministerio de Ciencia e Innovación). Financial support from projects PDC2021-121552-C21 and PDC2021-121552-C22 (MCIN/AEI/10.13039/501100011033 and European Union Next Generation EU/PRTR) is also acknowledged. N. 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Therefore, the following general conclusions are acknowledged for low CO2 pressure conditions: • A closed CO2 cycle for calcination and carbonation would reduce the technical complexity of the technology, avoiding the intricacy and costly gas separation. • The importance of reaction conditions is highlighted. Operating under closed CO2 loop entailed several challenges, such as the sintering-induced due to the harsh conditions required for calcination. • Working at low CO2 pressure allows the calcination temperature to be reduced to a temperature suitable for Concentrated Solar Power tower systems. • Under low CO2 pressure conditions, sintering is mitigated improving the performance of the materials and increasing the conversion. • In terms of plant construction, the materials used for atmospheric conditions are the same as those used for mild vacuum conditions, so there would be no additional cost. The main conclusions of each paper are summarised in the following points: 5.1. Calcination under low CO2 pressure enhances the calcium Looping performance of limestone for thermochemical energy storage. The reduction in the absolute pressure of CO2 (0.01 and 0.1 bar) impacts the calcination temperature (700 and 765 °C, respectively) of the limestone. This collaborates to a notable enhance in the effective conversion and a reduction in the deactivation of CaO. 5.2. Thermochemical energy storage using calcium magnesium acetates under low CO2 pressure conditions. Morphology is a key aspect of CaO sorbent performance of. Therefore, a low-cost synthesis is carried out to obtain calcium and calcium magnesium acetates. The acicular structure of the acetates improves the performance while reducing the inactivation of the