Low-pressure calcination to enhance the calcium looping process for thermochemical energy storage

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in: JOURNAL OF CLEANER PRODUCTION on 2022, available at: https://doi.org/10.1016/j.jclepro.2022.132295” Low-Pressure Calcination to Enhance the Calcium Looping Process for 1 Thermochemical Energy Storage 2 3 C. Ortiz a,*, A. Carro b, R. Chacartegui b, J.M. Valverde c, A. Perejón d,e, P.E. Sánchez-Jiménezd, 4 e, L. Pérez-Maqueda e 5 a Materials and Sustainability Group, Department of Engineering, Universidad Loyola Andalucía, Avda. de las 6 Universidades s/n, 41704 Dos Hermanas, Seville, Spain 7 b Dpto. Ingeniería Energética. Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain 8 c Dpto. Electrónica y Elecromagnetismo, Universidad de Sevilla, Avenida Reina Mercedes s/n, 41012 Sevilla, Spain 9 d Dpto. de Química Inorgánica, Facultad de Química, Universidad de Sevilla, 41012 Sevilla, Spain 10 e Instituto de Ciencia de Materiales de Sevilla, C.S.I.C.-Universidad de Sevilla, Americo Vespucio 49, 41092 Sevilla 11 12 * Corresponding author: [email protected] (+34) 655783930 13 Abstract 14 The Calcium-Looping (CaL) process, based on the multicyclic calcination-carbonation of 15 CaCO3/CaO, is considered a promising Thermochemical Energy Storage (TCES) 16 technology to be integrated in Concenrtrating Solar Power (CSP) plants. As a novelty, 17 this work proposes a CaL integration that considers low-pressure calcination under pure 18 CO2 at a reduced temperature. Low-pressure calcination (0.01 bar) provides a suitable 19 solution to mitigate CaO sintering and its consequent loss of reactivity in the carbonation 20 stage. Since the temperature for quick calcination in a pure CO2 atmosphere is decreased 21 (from around 950ºC at 1 bar to 765ºC at 0.01 bar), the energy losses at the receiver are 22 minimized. In addition, a reduced calcination temperature allows for the use of metallic 23 receivers already tested at the MW-scale, which significantly increases the CSP-CaL 24 integration reliability. Moreover, multicycle CaO reactivity is promoted in short residence 25 times, allowing the use of a simpler reactor design. Furthermore, there is an increase of 26 85% in the energy storage density of the system. The proposed plant proposes a smooth 27 integration of the CaL process in CSP plants, with a moderate storage level and supported 28 by a natural gas backup system (solar share higher than 50%). The results show that the 29 solar thermal-to electric efficiency is above 30%. 30 Keywords 31 Thermochemical Energy Storage, Concentrating Solar Power, Calcination temperature, 32 Calcium-Looping, low-pressure, energy density 33 1. Introduction 34 Thermochemical energy storage is a promising technique for Concentrating Solar Power 35 (CSP) plants to improve their dispatchability (Carrillo et al., 2019). Among the diverse 36 options, the Calcium-Looping (CaL) process, based on the cyclic calcination-carbonation 37 of CaCO3/CaO has been intensively investigated due to its inherent advantages such as 38 high energy density -as typical in TCES systems (Li et al., 2020)-, very high turning 39 temperature, low price, and wide availability of the raw materials (i.e., limestone, 40 dolomite) (Chacartegui et al., 2016) and similarity to the widely developed cement 41 industry (Vatopoulos and Tzimas, 2012). Currently, the technology has been validated in 42 relevant environment (SOCRATCES H2020 project). However, some challenges and 43 drawbacks must be overcome to bring the technology to a commercial scale. 44 45 Calcination under pure CO2 and atmospheric pressure, as proposed in previous works to 1 simplify the CaL-CSP integration, occurs sufficiently fast only at temperatures around 2 or over 950ºC (Valverde and Medina, 2015), which causes severe sintering of the CaO 3 unreacted particles that leads to CaO deactivation after several cycles (Benitez-Guerrero 4 et al., 2017). Therefore, the CaO conversion decays to just about 15-18% after 20 cycles 5 (Sarrión et al., 2018). CaO deactivation reduces the efficiency of the process (Chacartegui 6 et al., 2016), increases the size of the equipment of the plant (due to the large amount of 7 poorly reactive material that must be transported) and reduces the energy density of the 8 system (Ortiz et al., 2018). Interestingly, residual CaO conversion is highly dependent on 9 the process conditions and the CaO precursors used, reaching values in a wide range from 10 ~0.07 to approximately 0.82 (Ortiz et al., 2019). In general, sintering is minimized by 11 reducing the calcination temperature, which leads to improved multicyclic performance. 12 The CSP-CaL integration is simplified by working in a CO2 closed circuit involving 13 calcination, carbonation, and power generation under pure CO2 (Ortiz et al., 2019). A 14 widely researched option to reduce the calcination temperature is to reduce the partial 15 pressure of CO2 in the calciner (Barin, 1989). Most of these works propose working at 16 atmospheric pressure under steam-enriched atmospheres (Berger, 1927; Valverde and 17 Medina, 2017) or helium (Valverde and Medina, 2016). Despite the promising potential 18 of this option, helium and steam should be separated from the CO2 released during 19 decarbonation to reduce the volume of gas storage, which poses a technological challenge 20 from an energy integration perspective. As an alternative, calcination under low absolute 21 CO2 pressure reduces the equilibrium temperature, and there is no need for gas separation 22 (Sarrión et al., 2020). 23 24 The benefits of reducing the calcination temperature are not limited to enhancing the CaO 25 multicyclic conversion. One of the main technological challenges for scaling up the CSP-26 CaL integration is the development of solar calciners (Carlos Ortiz et al., 2021). Different 27 solar particle receivers based on falling particles (Ho et al., 2016), fluidized beds (Flamant 28 et al., 1980) or rotary kilns (Abanades and André, 2018) have been investigated. 29 Moreover, indirectly heated reactors from pressurized air or CO2 as HTF have been 30 proposed as an alternative to solar particle receivers (Bailera et al., 2021). Regardless of 31 the type of solar receiver and reactor, the efficiency of CSP-CaL integration is hindered 32 by the high radiation losses at high temperatures. In this regard, low-pressure calcination 33 also provides a suitable solution to increase the thermal efficiency of the process by 34 decreasing the required calcination temperature. 35 36 This paper analyzes a novel CSP-CaL scheme for TCES based on an air-pressurized 37 cavity receiver integrated with an indirectly heated low-pressure calciner where solar 38 energy storage takes place. The work carried out combines testing at laboratory scale to 39 evaluate the performance of the materials under these novel conditions, which 40 simultaneously provides detailed data for the process enginering characterization. The 41 proposed plant aims at a smooth integration of the Calcium-Looping process in CSP 42 plants, with a moderate storage level and supported by a natural gas back-up system (solar 43 share higher than 50%). The results show a potential interest in low-pressure calcination 44 to increase the solar-to-electric efficiency of the plant above 30% by using elements of 45 already commercial or widely tested solar technology. 46 2. Low-Pressure Calcination 47 Reducing the partial pressure of CO2 in the calciner allows the equilibrium temperature 48 of calcination-carbonation to decrease, which brings several advantages to improve the 49 reliability of the CSP-CaL integration. Many works have investigated the calcination 1 process at atmospheric pressure (1 bar) in inert gases. Thus, in 1929, Berger proposed 2 using steam to improve the decomposition of limestone, which would occur at 700-750°C 3 (Berger, 1927). More recent works have corroborated that steam calcination significantly 4 enhances CaO reactivity (Champagne et al., 2013; Valverde and Medina, 2017). A partial 5 pressure of 3% of steam in the calciner allows a notable improvement in kinetics, while 6 29% of steam increases the residual CaO conversion value by 50% (Jesús Arcenegui 7 Troya et al., 2021). However, a higher partial steam pressure is required to reduce the 8 calcination temperature below 800°C in a CO2/H2O atmosphere, which causes a 9 significant energy penalty to bring the water (after condensation for CO2/H2O 10 separation) up to the calcination temperature (Ortiz et al., 2019). Furthermore, steam 11 calcination hinders the mechanical strength of CaO, leading to particle attrition (Valverde 12 and Medina, 2017). Alternatively, helium has been proposed to reduce the partial pressure 13 of CO2. Calcination under a helium atmosphere occurs fast at around 725ºC, promoting 14 the calcination kinetics presumably due to its high thermal conductivity and the high 15 diffusivity of CO2 in He (Valverde and Medina, 2016). However, as in the case of steam, 16 the separation of He/CO2 to reduce the storage volume is energetically expensive, which 17 penalizes the whole process,while at the same time the plant complexity increases. 18 19 As an alternative to calcination under gas mixtures, reducing the absolute CO2 pressure 20 in the calciner allows reducing the equilibrium temperature below 800°C in a pure CO2 21 atmosphere (Sarrión et al., 2020). According to recent work (Sarrión et al., 2020), a 22 residual CaO conversion (Xeff) as high as 0.42 is obtained for limestone samples calcined 23 under 0.01 bar CO2 at 765ºC. These results were replicated in this work. Thus, natural 24 limestone from Taljedi quarry (Seville, Spain) with a particle size in the range 160-200 25 nm was subjected to two multicycle calcination/carbonation tests in a in-house 26 themogravimetric instrument constructed to work from low pressures up to 5 bar. A 27 detailed description of the instrument was provided elsewhere (Sarrión et al., 2020). The 28 first test consisted on performing calcination and carbonation under an aboslute pressure 29 of 1 bar CO2 at temperatures of 950ºC and 850ºC, respectively (Figure 1a). Besides, for 30 the test conducted under reduced CO2 pressure, calcination was carried out under a 31 controlled absolute pressure of 0.01 bar CO2 and carbonation at 1 bar CO2. Considering 32 the thermodynamic equilibrium of CaCO3/CaO, the temperature employed for calcination 33 and carbonation was 765ºC (Figure 1a). 34 35 As can be seen in Figure 1a, operating under low-pressure calcination the multiclic 36 conversion is almost six times higher than under typical CaO conversion of limestone 37 calcined under pure CO2 at atmospheric pressure. Additionally, calcination under these 38 conditions far from equilibrium would occur fast (Valverde, 2015). Figure 2 compares 39 different limestone calcination conditions. 40 41 42 0 5 10 15 20 0.0 0.2 0.4 0.6 0.8 1.0 N Xeff Calc. 765ºC 0.01 bar CO2 Carb. 765ºC 1 bar CO2 Calc. 950ºC 1 bar CO2 Carb. 850ºC 1 bar CO2 (a) 1 0 10 20 30 0.0 0.2 0.4 0.6 0.8 1.0 Xeff 1st Cycle t (min) (b) 2 012345 0.0 0.5 1.0 1.5 2.0 2.5 3.0 dX eff /dt (min -1 ) t (min) (c) 3 Figure 1: a) Effective multyciclic CaO conversion (Xeff) of limestione calcined in pure 4 CO2 atmosphere; b) Evolution of conversion for the first cycle; c) Carbonation rate during 5 the first carbonation 6 7 10 times higher Carbonation of the CaO obtained from limestone calcined at low absolute pressure occurs 1 mainly in the fast reaction-controlled phase, which involves carbonation times below 1 2 minute to facilitate the reactor design and operation. Figure 1b shows the evolution of the 3 conversion for the first cycle. In addition to a higher conversion from the first cycle, it is 4 observed that there is no diffusive part for the sample calcined at 0.01 bar of CO2, unlike 5 the sample calcined at 1 bar of CO2. Interestingly, the carbonation rate is 10 times faster 6 for the sample calcined at 0.01 bar CO2 (Figure 1c). The high reaction rate for low-pressure 7 calcination means that both fluidized bed reactors (FB) and entrained flow reactors (EF) 8 are potentially suitable for the process (Carlos Ortiz et al., 2021). Although low absolute 9 pressure calcination requires an extra effort to prevent air leakages, continuous operation 10 of large-scale reactors and heat exchangers under partial vacuum is technically feasible 11 (for example, condensers in steam-based power plants operate at 0.05-0.08 bar) (NETL, 12 2015). 13 14 15 Figure 2: Comparison of limestone calcination conditions. Orange and blue dots represent 16 absolute or partial pressure, respectively. 17 18 3. CSP-CaL process integration 19 This section covers the CSP-CaL integration process developed from the laboratory-scale 20 results presented in Section 2. Interestingly, the performance of CaL is highly dependent 21 on the process conditions, namely reactor pressure, temperature, and gas composition, as 22 well as on the CaO precursor and its particle size (Ortiz et al., 2019). Therefore, novel 23 process configurations as proposed in this work require reliable data obtained from 24 experimental measurements. 25 3.1. Process Flow Diagram description 26 Figure 3 shows the conceptual process scheme of the plant. The layout is based on a solar 27 regenerative Brayton cycle (pathway 1-8), in which pressurized air (stream 3a) is heated 28 0,000 0,001 0,010 0,100 1,000 10,000 600 700 800 900 1000 CO2pressure (bar) T (K) equilibrium This work (absolute pressure) Typical (absolute pressure) (Ortiz et al. 2019) Sarrión et al. 2020 (absolute pressure) Benitez-Guerrero et al. 2019 (partial pressure, He atmosphere) Arcenegui et al. 2021 (partial pressure, air-steam atmosphere) carbonation calcination in a cavity receiver (which could be a tube-based system or a volumetric system) (Stein 1 and Buck, 2017). Similar solar gas turbine systems with TCES have recently been 2 proposed in the literature (C Ortiz et al., 2021b; Yang et al., 2021). Valves V1 and V2 3 allow the pressurized air stream to be diverted to avoid its passage through the receiver 4 when solar radiation is not enough, as well as from the sunset. The heated air stream 5 passes in series through the calciner (low-pressure endothermic reactor) to give up 6 sensible heat indirectly for calcination to be carried out. Thus, pressurized air is used as 7 Heat Transfer Fluid -HTF- for calcination. After the reactor, the air passes through a 8 natural gas combustor (back-up system) to provide extra heat (just if needed) to enter the 9 gas turbine at the design temperature. A similar scheme was proposed for a solar 10 combined cycle in (C Ortiz et al., 2021a). Using the proposed configuration, the Solar 11 Multiple (SM) is limited to SM≈1.3 (C Ortiz et al., 2021a), although in the present work 12 it is considered sufficient to increase the solar share of typical solar-based Brayton cycles 13 by around 20-25%. Figure 4 illustrates the T-s diagram of the solar regenerative Brayton 14 cycle. Note that points 5 and 6 in the diagram are the same in the base case, since natural 15 gas back-up system only provides extra heat if solar radiation is not enough to heat the 16 air up to 780ºC. 17 18 19 20 Figure 3: Conceptual process scheme 21 22 The heat provided by the HTF (pressurized air) is used for calcining a CaCO3 stream 23 entering the reactor. Note that since the carbonation reaction is incomplete (Valverde, 24 2013), a solid stream (stream s1) consisting of a specific mole flow of CaCO3 (X) and 25 the rest of CaO (1-X) will enter the calciner. An Entrained Flow (EF) calciner is used 26 where particles react as they fall freely. Previous works have shown that calcination 27 would be produced entirely if the residence time of the particles is sufficiently long 28 (Lisbona et al., 2020; Ortiz et al., 2020). Using an EF reactor, it is possible to use fine 29 particles (average particle size less than 45 µm) which allows calcination to take place 30 completely in just several seconds (Carlos Ortiz et al., 2021). Fine particles operation 31 leads to agglomeration, gas channeling and plugging phenomena, hindering the 32 performance in FB reactors (Raganati et al., 2018). Note that in this case calcination and 33 carbonation occurs alternatively in the same reactor, and therefore the carbonation would 34 also occurs quickly by using fine particles (Durán-Martín et al., 2020). 35 36 Once calcination occurs, CaO (stream c1) is sent to a high-temperature storage tank by a 1 pneumatic conveyor. CO2 released in calcination (stream g1) is cooled and compressed 2 to 75 bar before storage (Chacartegui et al., 2016). Since CO2 exits the calciner at a very 3 high temperature, two thermal energy storage vessels are proposed consecutively placed 4 and filled with steel slag. The hot CO2 stream (g1) would heat up the steel slag beds, 5 which provide afterwards the stored energy to heat the CO2 entering the reactor (stream 6 g7). This would be the high-temperature Thermal Energy Storage (TES) -TES1- system. 7 Low-temperature storage -TES 2- would be placed after the CO2 compressor to take 8 advantage of the heat released in the compression. Steel slag has previously been 9 proposed as an efficient TES at temperatures as high as 1000°C (Ortega-Fernández et al., 10 2015; C Ortiz et al., 2021a). 11 12 Figure 4: T-s diagram of the solar regenerative Brayton cycle (see pathway 1-8 in Figure 13 3) 14 15 When solar energy is not available (i.e., at night or on cloudy days), the plant can be 16 operated in a similar way by using the previously stored solar heat. The stored CO2 and 17 CaO are sent again to the reactor, where carbonation occurs under atmospheric 18 conditions. In this case, the pressurized HTF stream (3b) enters directly the entrained flow 19 to take the heat released by the exothermic carbonation reaction. After that, the HTF 20 closes the cycle by passing through the natural gas combustor, the gas turbine, and the 21 regenerator. Note that only one reactor is proposed that operates in a reversible way for 22 either calcination or carbonation processes. 23 3.2. Process Modelling 24 The novel process scheme proposed has been modelled in Thermoflex TM (Thermoflow 25 Inc., n.d.) and Aspen Plus TM (Aspen Technology Inc., n.d.). The authors have previously 26 developed integration models for different power cycles (Ortiz et al., 2017) and operating 27 conditions (Chacartegui et al., 2016), among others. However, this work is quite novel, 28 since it proposed to carry out the calcination in vacuum, performing hourly simulations 1 of the entire plant under these conditions. Vacuum calcination allows operating at 2 significantly lower calcination temperatures, which has an important impact on receiver 3 efficiency and therefore on overall plant performance. Remarkably, novel systems 4 integration and simulation conditions are proposed in this work adapted to the new 5 process flow diagram, including the integration of a solar gas turbine on the solar side for 6 calcination, a single reactor that works alternately as calciner and carbonatator, and 7 quasistationary hourly simulation. 8 9 In the modelling, the assumed solar field efficiency is 70%, as calculated for similar CSP 10 plants (Binotti et al., 2017; Rovense et al., 2018). The receiver efficiency depends on the 11 receiver temperature as a result of the high radiation losses associated with this type of 12 reactor. The receiver efficiency assumed in this work is based on Eq. 1 (Ho, 2016) as well 13 as on the results obtained in the operation of similar solar receivers (Figure 5) (Buck et 14 al., 2002; Korzynietz et al., 2016). Thus, a constant solar receiver efficiency of 72% is 15 used for the low-pressure calcination case (HTF outlet temperature of 850ºC) and of 60% 16 for the atmospheric pressure calcination case (HTF outlet temperature of 1050ºC). Further 17 development of CFD models, which is out of the scope of this work, would be required 18 to evaluate the receiver efficiency hourly as a function of the incident solar flux and the 19 ambient conditions (C Ortiz et al., 2021b). 20 21 (1) Here, T is the receiver temperature in K, α and ε are the absorptivity and emissivity of the 22 receiver material (0.86 and 0.6 respectively for Ni-based alloy tubes (Balat-Pichelin et 23 al., 2021)) and it is assumed that Q is 600 kW/m2 (this value could be as high as 900 24 kW/m2 for volumetric air receivers) (Ho, 2016). 25 26 27 Figure 5: Receiver efficiency as a function of the HTF receiver outlet temperature 28 29 Turbines are modelled providing an adiabatic expansion. The air and CO2 properties are 30 taken from the REFPROSP (NIST) database, while IFC-67 is used for the formulation of 31 50% 55% 60% 65% 70% 75% 80% 85% 850 1050 1250 1450 receiver efficiency Receiver HTF oulet temperature (K) Equation Solugas average Solugas expected SOLUGAS test REFOS project average REFOS project expected (improved design) this work (low Pressure calcination) this work (atmospheric pressure calcination) 1 2 3 Figure 8: Analysis of the hourly flow of materials to and from storage tanks for typical days of 4 spring (day 1) and summer (day 2). A negative sign refers to the difference in the amount of 5 material stored with respect to the beginning of the day. Note that Case 1 and Case 2 represent 6 low-pressure and atmospheric-pressure calcination, respectively. 7 As may be seen, maximum and minimum storage levels would occur in the highest solar 8 radiation days. Note that the days represented in Figure 8 have different Y-scales. Taking 9 into account day 2 (summer), in the first 6 hours of the day, the CO2 and CaO vessels are 10 emptied as they are sent to the carbonator for power production, while the solid storage 11 (CaO/CaCO3) is filled with solids from carbonation. After these hours, the solids tank has 12 increased the material stored by 800 tons. The proposed operation considers that all energy 13 stored during the day is consumed at night (the amount of material stored is the same at the 14 beginning as that at the end of the day), but this might not be the case. An analysis of 15 operation strategies for certain demand patterns would be necessary to optimise the system, 16 but this task is beyond the scope of this work. It is notable in Figure 8 the significant 17 difference between solids and CaO flow for the cases of low-pressure calcination (case 1) 18 and atmospheric-pressure calcination (Case 2). Although the amount of CO2 produced 19 during calcination is slightly lower in case 2 (as there is less solar energy available, as shown 20 in Table 2), the CaO and solid streams are significantly higher. This is due to the lower CaO 21 multicyclic conversion in Case 2, which in addition to leading to a lower energy density, 22 which entails a higher cost of storage systems, leads to a higher cost of solid conveying 23 systems. In addition, if storage of solids at room temperature is considered, the cost of the 1 heat exchangers would also be notably higher in the case of atmospheric calcination than in 2 the case of low-pressure calcination. 3 4.3. Sensitivity analysis 4 This section compares the performance of both systems under changes in relevant 5 parameters for the design assumed in Section 3. The parameters chosen for the analysis 6 are the efficiency of the solar receiver, the temperature difference to provide to provide 7 energy for calcination, the efficiency of the main compressor and turbine and the 8 temperature chosen for the auxiliary combustor. 9 10 PENDIENTE DE COMPLETAR LOS CÁLCULOS, LAS FIGURAS Y SU 11 DESCRIPCIÓN 12 13 14 15 16 17 5. Conclusions 18 The results of this work show the potential benefits of performing calcination under a CO2 19 atmosphere at reduced absolute pressure (0.01 bar) in the Calcium Looping process for 20 Thermochemical Energy Storage. The lower temperature required in this case for calcination at 21 reduced pressure compared to atmospheric pressure (~765ºC vs. ~950ºC) notably reduces the 22 20% 24% 28% 32% 36% -15% 0% 15% Solar-to-electric efficciency Parameter variation Eta receiver Low Pressure 20% 24% 28% 32% 36% -15% 0% 15% Solar-to-electric efficciency Parameter variation ΔT reactor Low Pressure 20% 24% 28% 32% 36% -5% 0% 5% Solar-to-electric efficciency Parameter variation Air turbomachinery Isentropic efficiency Low Pressure 20% 24% 28% 32% 36% 40% -15% 0% 15% Solar-to-electric efficciency Parameter variation Combustor temperature Low Pressure complexity of the plant, allowing the use of mostly mature solar receivers (metallic receivers 1 tested even at the MW scale), which is the main challenge for the deployment of the technology. 2 Although in the integration proposed in this work, a lower calcination temperature causes a lower 3 energy generation efficiency (up to 8%point less), this is compensated by the lower radiation 4 losses in the solar receiver when calcining at low pressure (lower temperature in the heater as in 5 the solar receiver), which could boost the solar receiver efficiency in more than 10%points. Thus, 6 similar solar-to-electric efficiencies are obtained. In addition, low-pressure calcination presents 7 better results in terms of solar share and, above all, energy storage density due to the higher 8 multicyclic conversion of CaO, which varies from around X=0.15 under typical calcination 9 conditions for TCES to X=0.42 under low-pressure calcination, accoridng TGA data obtained at 10 laboratory scale. 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