Low-cost Ca-based composites synthesized by biotemplate method for thermochemical energy storage of concentrated solar power

1 Low-cost Ca-based composites synthesized by biotemplate method for Thermochemical Energy Storage of Concentrated Solar Power Monica Benitez-Guerrero 1, 2, Jose Manuel Valverde 1*, Antonio Perejon 2, 3, Pedro E. Sanchez-Jimenez 2, Luis A. Perez-Maqueda 2. 1 Facultad de Física, Universidad de Sevilla, Avenida Reina Mercedes s/n, 41012 Sevilla, Spain. 2 Instituto de Ciencia de Materiales de Sevilla, C.S.I.C.-Universidad de Sevilla, C. Américo Vespucio nº49, 41092 Sevilla, Spain. 3 Facultad de Química, Universidad de Sevilla, Avenida Reina Mercedes s/n, 41012 Sevilla Spain. *Prof. Dr. J.M. Valverde Facultad de Física Universidad de Sevilla Avenida Reina Mercedes s/n, 41012 Sevilla (Spain) Tel +34 954550960 Fax +34 954239434 E-mail: [email protected] 2 Low-cost Ca-based composites synthesized by biotemplate method for thermochemical energy storage of concentrated solar power Highlights * CaO/SiO2 composites were synthesized by a low cost biomimetic process from rice husk * CaO/SiO2 composites are highly efficient for concentrated solar energy storage *The composites show significantly better performance than natural limestone *Pore-plugging is reduced by the composition and microstructure of the CaO/SiO2 composites Abstract An ever more environmentally conscious society demands the use of green, sustainable and high-efficiency renewable energy resources. However, large-scale energy storage remains a challenge for a deep penetration of power produced from renewables into the grid. The Calcium-Looping (CaL) process, based on the reversible carbonation/calcination of CaO, is a promising technology for thermochemical energy storage (TCES) in Concentrated Solar Power (CSP) plants. Natural limestone to be used as CaO precursor is cheap, non-toxic and abundant. Nevertheless, recent works have shown that carbonation of CaO derived limestone at optimum conditions for TCES is limited by pore-plugging, which leads to severe deactivation for large enough particles to be employed in practice. In our work, we have synthesized inexpensive CaO/SiO2 composites by means of a biotemplate method using rice husk as support. The morphological and compositional features of the biomorphic materials synthesized help improve the CaO multicycle activity under optimum CSP storage conditions and for particles sufficiently large to be managed in practical processes. Keywords: Renewable resources; Energy conversion; Thermochemical Energy storage; Calcium Looping; Biomorphic composites. 3 1. Introduction The Calcium-Looping (CaL) process based on the reversible carbonation/calcination reaction of CaO: CaO(s)+CO2(g) ⇄ CaCO3(s) +∆𝐻𝑟 0 ; ∆𝐻𝑟 0=178 𝑘𝐽𝑚𝑜𝑙−1 (1) has been extensively studied in the last years to capture CO2 from fossil fuel-fired power plants [1-3] as originally proposed by Shimizu et al. [4]. In the late 1970s the CaL process was early investigated for thermochemical energy storage (TCES) of concentrated solar power (CSP) [5, 6]. In the last years most of the works on energy storage in CSP has been focused on molten salts technology [7-10] and redox-based systems [11-13]. The use of the CaL process to this end has not attracted a wide interest until quite recently as it would allow long term storage of massive amounts of energy at low cost and using abundant and non-toxic materials such as limestone [14]. A flow diagram of the integration of the CaL process in CSP plants recently proposed elsewhere [14] built upon a closed CO2 cycle is shown in Fig. 1. It basically consists of a solar calciner, a carbonation reactor, a CO2 compression-storage system, two reservoirs for CaO and CaCO3 storage and a power unit. After calcination of CaCO3 in the calciner using concentrated solar energy, the sensible heat of CaO and CO2 is recovered and these products are stored independently. On demand, CaO and CO2 are circulated into the carbonator wherein heat is released by the exothermic carbonation reaction. This heat is transported by the CO2 in excess to a gas turbine where electricity is generated while the effluent CO2 is sent to storage. Optimum CaL conditions to obtain the maximum overall efficiency from the CaL-CSP integration involve high temperature carbonation (above 850 ºC) under high CO2 concentration. Remarkably, these conditions differ from those corresponding for CO2 capture in which calcination is carried out under high CO2 concentration at high temperature (above 900 ºC) and carbonation occurs at relatively lower temperature (around 650 ºC) under low CO2 concentration (around 15% vol.) [14]. Among calcium oxide based sorbents, natural limestone (nearly 100% CaCO3) the most preferable CaO precursors since it is environmentally friendly, inexpensive, 4 abundant and widely available [15]. However, a major challenge to use limestone is the severe loss of activity of the regenerated CaO after just a few cycles of carbonation/decarbonation. Deactivation at CaL-CO2 capture conditions is due to the marked sintering suffered by the CaO grains under the necessary harsh calcination conditions, which involve high temperatures and high CO2 partial pressure [16]. On the other hand, pore-plugging is a critical limiting mechanism at CaL-CSP storage conditions as carbonation at high temperature and under high temperature is very fast, which makes it likely that the thick CaCO3 layer blocks the inner pores of the CaO particles [17, 18]. In order to avoid deactivation, the use of different additives such as Al2O3, La2O3, Li2CO3, MgO, SiO2, TiO2, Y2O3 and ZrO2 has been investigated in previous works [19-25]. These studies have been mostly focused on the use of the CaL process for CO2 capture [2, 3, 15]. Taking into account the large scale of the application (typically around 500 tons of raw limestone would be needed in the CaL process to capture the CO2 released from a commercial coal fired power plant) [26], a compromise between sorbent cost production and its multicycle activity performance is required [27]. Thus, silica is a promising additive as it is not expensive and a wide spread reliable inert material. Nanostructured silica, both in the form of molecular sieves [28] and nanoparticles [29] serves to improve the dispersibility of CaO agglomerates [29] and mitigate sintering [30], which is attributed to the formation of calcium silicates at high temperatures that confer thermal stability to the CaO skeleton at the harsh calcination conditions used for CO2 capture [31]. Nonetheless, the synthesis of nanostructured silica from either sol-gel routes or flame synthesis, which is employed in commercially available products such as Aerosil®, has a high cost. In contrast, the use of rice husk biomass (a by-product of rice husk industry), as proposed in the present work, is an alternative cost-effective method to produce nanosilica at massive amounts. Hence, rice husk ash has been used as additive to synthesize CaO sorbents for CO2 capture [32, 33]. In this sense, several works have been reported on the use of raw rice husk to generate porous CaO pellets for CO2 capture with improved CO2 sorption performance [34, 35]. 5 However, the effect of silica in SiO2/CaO mixtures on the activity of CaO cycled at CaL conditions for CSP storage remains largely unexplored yet. Recently, the CaL-CSP multicycle performance of a nano-SiO2/CaO composite derived by physical mixing of a commercial SiO2 nanopowder and limestone was analyzed by our group [17]. Quite surprisingly, it was found that the presence of nanosilica caused a significant deterioration of the multicycle CaO activity, in contrast with previous results obtained at CaL conditions for CO2 capture [30]. The action of nanosilica, by shifting the CaO mesoporous size distribution to smaller pore sizes, was found to hinder CaO carbonation by promoting pore-plugging, which is the main limiting factor on CaO conversion at CaL conditions for CSP storage. In the present work, rice husk has been employed to synthesize CaO/SiO2 composites by a biotemplate route to generate a microstructural porosity in CaO, while at the same time the composites reproduce the micro and nanostructure of the husk. To the best of our knowledge, no further studies have described the use of such kind of composites for thermochemical energy storage of CSP. The multicycle performance of the CaO/SiO2 composites synthesized in this way is significantly improved as compared to limestone at CaL-CSP conditions. These results are significantly important as they are obtained for particles of size large enough to be used in the practical application in circulating fluidized bed reactors. 2. Experimental Section 2.1. Chemicals and Materials Ca(NO3)2·4H2O (Sigma-Aldrich) and raw rice husk (Herba Ricemills S.L.) were used in our work to synthesize CaO/SiO2 composites. Natural limestone (>99 %wt CaCO3) from Matagallar quarry in Pedrera (Sevilla, Spain) was also used for the sake of comparison. Two size fractions of this limestone (less than 45 m and 45-160 m) were employed (Fig.A1 in Appendix A, Supplementary Content). 6 2.2. Synthesis of CaO/SiO2 composites Raw rice husk was pretreated to be used as biotemplate as detailed in Appendix A in order to obtain high purity nanostructured SiO2 of large specific surface area. For this purpose 0.1L Ca(NO3)2·4H2O solutions of 0.5 and 2 M were prepared for the infiltration of 10 g of as-pretreated rice husk. The infiltrating solutions and the pretreated rice husk were maintained at 50 ºC during 24 h under stirring, until infiltration and water evaporation were achieved. Finally, the products were oven-dried at 120 ºC before thermal treatment. The two steps thermal treatment was conducted at the same conditions detailed in the Appendix for the ash synthesis. The nominal compositions of the biotemplated composites are 30wt%SiO2/70wt%CaO and 10wt%SiO2/90wt%CaO (labeled hereafter as 70%CaO and 90%CaO, respectively). 2.3. Material Characterization Compositional analysis was carried out by X-ray fluorescence (XRF) using an Axios PW4400 (PANalytical) instrument and energy-dispersion X-ray (EDX) using a Bruker-X Flash4010 detector. Powder X-ray diffraction (XRD) was performed using a MiniFlex600 (Rigaku) operated with Ni filtered CuKα radiation (λ = 1.5406 Å) at 40 kV and 15 mA for a scan range 2=5–90° at a step rate of 5° min-1. Particle size distributions (PSDs) were obtained by laser diffractometry using a Mastersizer 2000 (Malvern). To this end, the samples were previously dispersed in 2-propanol (as recommended for Ca-based materials according to ISO 14887 [36]) and sonicated for 30 s to loose particle agglomerates. Nitrogen adsorption-desorption isotherms at 77 K were acquired by means of a TriStar II 3020 (Micromeritics) instrument. The samples were degassed at 150 ºC for 2 h. Specific surface area (SBET) was calculated using the BET equation [37]. Total pore volume (Vsp) was determined from the amount of gas adsorbed at a P/P0 value of 0.97. Mesopore distributions were derived by the BJH method applied to the adsorption branch of the isotherms [38]. The average pore sizes (wp and wpBJH) were determined by approximating the pore geometry to a cylinder. 7 Scanning Electron Microscopy (SEM) micrographs and compositional EDX mapping were acquired using a Hitachi S4800 FEG microscope on gold-sputtered samples. Transmission Electron Microscopy (TEM) and High-angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) micrographs were registered using a Talos F200S FEG microscope (FEI company) in which the powder samples were deposited on copper grids. 2.4. Calcium Looping (CaL) Multicycle Conversion CaL multicycle tests were carried out using a Q5000IR thermogravimetric analyzer (TA Instruments), which is provided with a high sensitivity balance (<0.1 g) and a furnace heated by IR halogen lamps that allow for high heating and cooling rates (up to 300 ºC min-1) to be expected under practical conditions. The samples were subjected to CaL conditions leading to high efficiency for TCES of CSP as reported elsewhere [14]. Each run was initiated with a precalcination stage under helium atmosphere from room temperature to the calcination temperature of 725 ºC, with a heating ramp of 300 ºC min-1. Afterwards, the temperature was quickly (300 ºC min-1) increased to 850 ºC for carbonation under pure CO2 for 5 min. Carbonation was followed by a rapid decrease (300 ºC min-1) of temperature to 725 ºC for calcination under pure He during 5 min. In order to simulate the extraction of sensible heat from the solids in a real plant before they are stored, the temperature was decreased (at 100ºC min-1) to 300 ºC and kept there for 2 min under He between the carbonation and calcination stages. A total of 20 carbonation/calcination cycles were run in this way for the CaO/SiO2 composites and natural limestone. A schematic representation of the procedure is given in Fig. A2. Additional tests extended up to 50 cycles were carried out in order to analyze the long-term performance of the materials as well as data reproducibility. In the practical application, the material would not be cycled indefinitely. Due to the significant drop of conversion of limestone derived activity after just 20 cycles, a certain amount of material should be periodically purged while fresh limestone is fed into the system to balance out mass. The average number of cycles 8 that a particle undergoes in the CaL process will depend on the recirculation flow rate of solids between the carbonator and the calciner and the flow rate of fresh limestone introduced [39]. Thus, the analysis of the multicycle activity of the material along a finite number of cycles yields useful information on its performance for practical purposes. The multicycle CaO conversion, XCaO, for the CaO/SiO2 composites is calculated as 𝑋𝐶𝑎𝑂 𝑁 = 𝑚𝐶𝑎𝑟𝑏 𝑁−𝑚𝑁 𝑚 𝑁 ·𝑊𝐶𝑎𝑂 𝑊𝐶𝑂2·( 1 𝑓 ) (2) where mN and mCarb N are the sample masses before and after carbonation at the Nth-cycle and WCaO and WCO2 are the molar masses of CaO and CO2, respectively. In Eq 2, f is the fraction of CaO in the mixture. The very small amount of calcium silicates formed, as observed from XRD, is neglected. A more relevant parameter for practical purposes to assess the performance of the samples is the effective conversion, Xef N, defined as the ratio of the CaO mass converted in the carbonation stage of each N-cycle to the total sample mass before carbonation 𝑋𝑒𝑓 𝑁 = 𝑚𝐶𝑎𝑟𝑏 𝑁−𝑚𝑁 𝑚 𝑁 ·𝑊𝐶𝑎𝑂 𝑊𝐶𝑂2 (3) The use of Xef takes into account the presence of inert compounds in the composites. Thus, the specific energy released in the carbonation stage per mass unit would be given by Xef times (∆Hr0/WCaO) (kJ g-1), where ∆Hr0 is the reaction enthalpy (-178 kJ mol-1). 3. Results and Discussion The pretreated rice husk (Appendix A), employed as biotemplate to synthesize the CaO/SiO2 composites, is characterized by a globular external surface and the presence of inner channels (Fig. A3). After combustion of the carbonaceous material, the compositional analysis of the obtained ash reveals that Si is the main element of the rice husk ash, followed by Ca, Mg, Fe, K and Mn in minor proportions as well as other trace elements such as Na, Al, S, P (as shows Table 1). In agreement with previous studies [40-43], the main ash component generated after 9 thermal treatment of rice husk is nanostructured silica (SiO2), which is mainly amorphous in our case as shown in Fig. A5a. The specific surface area of the obtained ash is significantly high, SBET = 218 m2 g-1 as well as the corresponding pore volume, Vp = 0.35 cm3 g-1. The typical pore size is about 3 nm as observed in Fig. A5b. These values are similar to data reported elsewhere [40]. SEM micrographs in Fig. A6 show that the ash replicates the microstructure of rice husk with a shrinkage factor due to the loss of biomass. SEM analysis on the rice husk ash shows a porous structure in which the bulbous forms and channels observed in the raw rice husk are still clearly identified. The nanoparticles that compose the whole structure, mainly amorphous silica, have been analyzed by HRTEM. The size of these nanoparticles is around 10-40 nm, which is similar to the size of the intraparticle pores (Fig. A7). Table 1. Compositional analysis of rice husk ash as determined from XRF (shown as oxides in weight percent). % Wt % Wt Al2O3 0.07 Na2O 0.06 CaO 1.90 NiO 0.02 Cr2O3 0.09 P2O5 0.17 Fe2O3 0.31 SO3 0.28 K2O 0.34 SiO2 95.8 MgO 0.78 ZnO 0.02 MnO 0.15 For infiltration a Ca(NO3)2·4H2O solution is employed to fill the pores of the biotemplate matrix up to cover completely the inner and outer surface of the pretreated rice husk. During the first step of the thermal treatment, the infiltrated mixture is heated up to 600 °C in nitrogen. 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Pore size distribution (BJH) (a) and particle size frequency and cumulative distributions (b) measured for as-prepared composite materials. 110 100 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 Pore Volumen (cm3/g/nm) 3 nm Pore Diameter (nm) 70% CaO 90% CaO 6 nm a b a 70% CaO 90% CaO 0.1 1 10 100 1000 0 1 2 3 4 5 6 70% CaO 90% CaO Volume (%) Particle Size (m) 0 20 40 60 80 100 Cumulative Fraction (%) Particle Size (m) 22 Fig. 4. Time evolution of CaO conversion and temperature during 1st (a) and 20th (b) cycles for the CaO/SiO2 composites tested under CaL-CSP storage conditions. Colored regions I and II correspond to fast reaction and slow solid-state diffusion controlled phases respectively. 560 565 570 575 580 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Limestone 70% CaO 90% CaO XCaO Time (min) N=20 0 200 400 600 800 Temperature (°C) a b 40 45 50 55 60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Limestone 70% CaO 90% CaO XCaO Time (min) N=1 0 200 400 600 800 Temperature (°C) 40 45 50 55 60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Limestone 70% CaO 90% CaO XCaO Time (min) N=1 0 200 400 600 800 Temperature (°C) 40 45 50 55 60 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Limestone 70% CaO 90% CaO XCaO Time (min) N=1 0 200 400 600 800 Temperature (°C) N = 1 N = 20 I II III 23 Fig. 5. CaO conversion (a) and effective conversion (b) at the end of the carbonation stage as a function of the cycle number for the CaO/SiO2 composites tested under CaL-CSP storage conditions. Multicycle conversion data for sieved limestone samples reported elsewhere are plotted for comparison 24 Fig. 6. Effective conversion as a function of the cycle number for both synthetic composites and limestone samples of similar particle size tested under CaL-CSP storage conditions for 20 and 50 cycles. Fig. 7. SEM picture of a limestone sample after 20 CaL cycles ending in calcination and with a broken surface that illustrates a porous inner CaO skeleton onto which a CaO sintered layer is formed. 0 5 10 15 20 25 30 35 40 45 50 0.0 0.2 0.4 0.6 0.8 1.0 Limestone (20 cycles) Limestone (50 cycles) 70% CaO (20 cycles) 70% CaO (50 cycles) 90% CaO (20 cycles) 90% CaO (50 cycles) Xef N 25 Fig. 8. SEM micrographs of 70%CaO (a, b) and 90%CaO (c, d) composites after 20 CaL cycles. Fig. 9. Secondary electron micrographs (SE at 2 and 20 kV) and compositional mapping (Ca and Si) of the 70%CaO composite sample after 20 CaL cycles. 32 Fig. A7. TEM micrographs of rice husk ash Fig. A8. Powder X-ray diffractograms of as-prepared composites with different CaO loads. 33 Fig. A9. HRTEM micrographs (a-d) and HAADF-STEM mapping for Si and Ca elements (e) of asprepared 70%CaO composite. Fig.A10. HRTEM micrographs (a-d) and HAADF-STEM mapping for Si and Ca elements (e) of asprepared 90%CaO composite. 34 Fig.A11. Secondary electron micrographs (SE at 2 and 20kV) and compositional mapping (Ca and Si) of 70%CaO composite after being subjected to 20 cycles under CaL-CSP conditions. Fig.A12. Secondary electron micrographs (SE at 2 and 20kV) and compositional mapping (Ca and Si) of 90%CaO composite after being subjected to 20 cycles under CaL-CSP conditions. 35 Fig. A13. HRTEM micrographs and HAADF-STEM mapping for Si and Ca elements of the 90%CaO composite sample after 20 cycles.