The Ca-looping process for CO2 capture and energy storage: role of nanoparticle technology
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The calcium looping (CaL) process, based on the cyclic carbonation/calcination of CaO, has come into scene in the last years with a high potential to be used in large-scale technologies aimed at mitigating global warming. In the CaL process for CO2 capture, the CO2-loaded flue gas is used to fluidize a bed of CaO particles at temperatures around ~ 650 °C. The carbonated particles are then circulated into a calciner reactor wherein the CaO solids are regenerated at temperatures near ~ 950 °C under high CO2 concentration. Calcination at such harsh conditions causes a marked sintering and loss of reactivity of the regenerated CaO. This main drawback could be however compensated from the very low cost of natural CaO precursors such as limestone or dolomite. Another emerging application of the CaL process is thermochemical energy storage (TCES) in concentrated solar power (CSP) plants. Importantly, carbonation/calcination conditions to maximize the global CaL-CSP plant efficiency could differ radically from those used for CO2 capture. Thus, carbonation could be carried out at high temperatures under high CO2 partial pressure for maximum efficiency, whereas the solids could be calcined at relatively low temperatures in the absence of CO2 to promote calcination. Our work highlights the critical role of carbonation/calcination conditions on the performance of CaO derived from natural precursors. While conditions in the CaL process for CO2 capture lead to a severe CaO deactivation with the number of cycles, the same material may exhibit a high and stable conversion at optimum CaL-CSP conditions. Moreover, the type of CaL conditions influences critically the reaction kinetics, which plays a main role on the optimization of relevant operation parameters such as the residence time in the reactors. This paper is devoted to a brief review on the latest research activity in our group concerning these issues as well as the possible role of nanoparticle technology to enhance the activity of Ca-based materials at CaL conditions for CO2 capture and energy storage.
KeywordsCO2 capture Energy storage Calcium looping Nanocomposites
Another emerging application of the CaL process is thermochemical energy storage (TCES) in concentrated solar power (CSP) plants, which remains still at the concept stage (Chacartegui et al. 2016; Alovisio et al. 2017). Currently, a few CSP demonstration plants are under operation worldwide incorporating thermal energy storage to generate electricity in the absence of direct solar irradiation (González-Roubaud et al. 2017). When available, direct solar irradiation is used in these plants to heat a so-called HTF (heat transfer fluid) usually consisting of a mixture of molten salts with high heat capacity. The solar salt is carried to a hot salt tank, where energy is stored in the form of sensible heat with an energy density near 0.8 GJ/m3. However, the use of molten salts poses serious inconveniences that hinder the competitiveness of CSP with this type of energy storage as sensible heat against coal power plants. One of them is the limitation of the solar salt upper temperature due to its decomposition near 600 °C, which does not allow achieving sufficiently high thermoelectric efficiencies as compared with fossil fuel-fired power plants. Another relevant issue is the high solidification point of the molten salts (between 120 and 220 °C), which makes it necessary to keep the salts at temperatures above these values leading to important heat losses especially during the night hours in desert zones at high altitude where CSP technology allocation is more appropriate. In addition, molten salts are very corrosive, which requires the installation of costly materials for transport and storage.
Essentially, TCES consists of using the potentially high temperatures attainable in the solar receiver to carry out an endothermic chemical reaction (Prieto et al. 2016). The reaction by-products are stored individually and, when energy is needed, they are brought together to drive the exothermic reverse reaction. In the CaL-CSP integration (Fig. 1b), calcination/carbonation conditions under which the solids would be cycled to maximize the global CaL-CSP plant efficiency may differ radically from those used in the CaL process for CO2 capture (Fig. 1a). Thus, according to a recently proposed integration scheme (Chacartegui et al. 2016; Alovisio et al. 2017), in the former case, carbonation would be carried out at high temperatures (> ~ 850 °C) under high CO2 partial pressure, whereas the solids could be calcined in the solar receiver at a relatively low temperature in the absence of CO2 to minimize sintering and lower solar receptor costs (Fig. 2).
In the CaL-CSP integration scheme, concentrated solar energy is used to carry out the calcination reaction. The CO2 and CaO streams stemming from calcination are passed through a heat exchanger network to extract their sensible heat after which they are stored at ambient temperature. CaO solids are circulated into a solids reservoir, whereas the CO2 gas stream is stored under high pressure at supercritical conditions by means of intercooling compression. Thus, besides of sensible and thermochemical energy storage, this integration includes energy storage also in the form of compressed gas. Thermochemical stored energy is released in the carbonator on demand through the exothermic carbonation reaction. The CO2 over the stoichiometric ratio exiting the carbonator at high temperature and high pressure is used to generate electric power by means of a gas turbine and is recycled in a closed circuit. Carbonation may be carried out at high temperature (> 850 °C) under pure CO2 to maximize the thermoelectric efficiency, which allows overcoming current temperature limits (T ~ 550–600 °C) in commercial CSP plants where heat is stored in molten salts. The use of the CaL process for TCES in CSP plants has many potential advantages such as the wide availability of limestone, its low cost, non-toxicity, and the high energy density attainable from the reaction enthalpy (~ 3.2 GJ/m3). Fast calcination at reduced temperatures (~ 700 °C) can be achieved under helium due to the high thermal conductivity and the very high CO2 diffusivity in this gas. He can be separated from the CO2 released during calcination by means of membranes, which allows recycling it in the calciner, whereas CO2 is sent to storage. Calcination at low temperature under low CO2 pressure would prevent the regenerated CaO from excessive sintering (Fig. 2), thus preserving a high CaO surface area available for carbonation in the next cycle. It would also allow the use of relatively cheap solar receivers based on metal alloys. Another advantage of the CaL-CSP integration is that the reactants can be stored at ambient temperature as opposed to molten salts, which must be kept always at temperatures over ~ 200 °C to avoid solidification.
This work is focused on a review of recently reported experimental results from our research group demonstrating the relevant role of calcination/carbonation conditions on the multicycle performance of the CaO derived from calcination of natural carbonate minerals, steel slag, and synthetic composites. The type of CaL conditions determines fundamentally the role of the two main mechanisms by which carbonation becomes limited such as grain sintering, which reduces drastically the CaO surface available for fast carbonation, and pore plugging, which prevents CO2 access into the pores of the CaO particles. The possible role of nanoparticle technology on mitigating these limiting mechanisms is highlighted.
Experimental materials and methods
Some of the experimental results presented in this work were derived from thermogravimetric tests made on natural CaO precursors such as limestone (99.8 wt% CaCO3) from Matagallar quarry in Pedrera (Seville, Spain) supplied by Segura S.L., calcitic marble (99.4 wt% CaCO3) from Purchena (Almería, Spain) supplied by Omya Clariana, and dolomite (94.4 wt% CaMg(CO3)2 and 5 wt% CaCO3) from Bueres (Asturias, Spain) provided by Dolomitas del Norte (Spain). All the samples were originally received in powder form and were sieved (mesh size 45 μm) in order to analyze the multicycle CaO carbonation behavior for particles of different size ranges (< 45 and > 45 μm) at CaL conditions for CO2 capture and CSP storage. This mesh size has been chosen as a typical low limit to ensure efficient gas-solid separation in commercial cyclones. Results will be also shown for powdered steel slag (Acerinox Europe S.A.U., Los Barrios, Spain) (Perejón et al. 2017), which is massively produced in the steel and iron industry from the use of limestone to remove non-ferrous oxides during the process of reducing iron ore. When treated with dilute acetic acid, calcium acetate is obtained that subsequently decomposes into CaCO3 upon calcination. Results obtained using nanosilica/CaO composites prepared from diverse methods as explained below will be also reviewed.
Multicycle carbonation/calcination tests have been carried out using a Q5000IR thermogravimetric analyzer (TA Instruments) provided with a high sensitivity balance (< 0.1 μg). Heat transfer phenomena are minimized by placing the sample inside a SiC enclosure heated with four symmetrically positioned IR halogen lamps, which ensures consistent and uniform heating. Active water cooling of the surrounding furnace body provides an efficient heat sink and favors accurate temperature and heating/cooling rate control up to 300 °C/min. These technical characteristics allow imposing experimental conditions that mimic realistic operating conditions especially concerning heating and cooling very high rates. The tests carried out under CaL-CO2 capture conditions were started with a precalcination stage rapidly raising the temperature at 300 °C/min from room to a temperature between 900 and 950 °C under a 70% CO2/30% air v/v atmosphere. Then, the temperature was quickly decreased (300 °C/min) to 650 °C to introduce a carbonation stage under a 15% CO2/85% air v/v atmosphere after which the sample was again calcined by quickly increasing the temperature (300 °C/min) to 900–950 °C under high CO2 concentration (70% CO2/30% air v/v) to restart a new cycle. On the other hand, tests at CaL-CSP storage conditions were initiated by a precalcination stage at 725 °C, heating the sample from room temperature at a heating rate of 300 °C/min under pure He. Then, the temperature was quickly increased (300 °C/min) to 850 °C and the gas switched to pure CO2 for carbonation. Once the carbonation stage was ended, the temperature was quickly decreased to 150 °C and maintained for 2 min. Then, the gas was changed to pure He and the sample was calcined again by increasing the temperature (300 °C/min) to 725 °C for CaO regeneration. The intermediate step at 150 °C in these tests was introduced in order to mimic the extraction of sensible heat from the solids exiting the carbonator and calciner before storage, which leads to a rapid cooling of the solids. Short residence times, as expected in practice for both calcination and carbonation stages, were used (5 min unless otherwise specified). In order to avoid undesired effects due to CO2 diffusion resistance across the sample, a small and fixed sample mass (10 mg) was used in all tests.
Calcium looping performance of natural minerals
Figure 4c–d shows that the effective conversion of dolomite, which is rather high, is not limited by particle size (note that the effective conversion takes into account the total mass of the sorbent including the inert MgO nanoparticles). In this case, it may be argued that the inert MgO domains, which become increasingly segregated from the CaO grains with the number of cycles, provide a path for the CO2 molecules to percolate inside the inner pores of the particles.
Calcium looping performance of steel slag
Calcium looping performance of CaO/nanosilica composites
Nanoparticle technology has been widely employed in the last years to synthesize CaO-based materials with enhanced CO2 capture capacity. In most cases, inert nanoparticle additives are employed as mechanical support and to provide thermal stabilization to the CaO skeleton. Some additives successfully employed to this end include alumina, titania, titanium ethoxide, calcium titanate, zirconia, magnesia, alkali metals, potassium permanganate, lanthanum oxide, mesoporous silicate, chromium oxide, cobalt oxide, ceria, and silica. The interested reader may find an extensive review in Valverde (2013). However, a main concern regarding the use of synthetic sorbents is cost. Typically, the limestone inventory needed for CO2 capture in a commercial coal-fired power plant, which annually releases to the atmosphere 1–3 Mt of CO2, can be as large as 500 tons (Ortiz et al. 2015). The competitiveness of using modified CaO-based sorbents with enhanced capture capacity compared to raw limestone has been assessed in Romeo et al. (2009). Generally, the use of synthetic sorbents would be feasible only if the multicycle activity performance is notably enhanced and the sorbent production cost is extraordinarily cheap. In this regard, the use of nanostructured silica, which may be massively produced at a relatively low cost by a simple method from rice husk (Monica Benitez-Guerrero et al. 2018), may be an interesting choice. Nanosilica is in fact already employed in other large-scale industrial applications as for the production of concrete mixtures with high compressive strength (Sobolev et al. 2009).
In the next sections, we review our recent work on the use of nanosilica/CaO composites, prepared by different methods, in the CaL process. As seen above for the treated steel slags, the presence of nanosilica may play a diverse role on the CaL process as depending on the type of calcination/carbonation conditions. Thermal stabilization provided by nanosilica, and the consequent preservation of pores in the mesoporous range, can be beneficial at CaL conditions for CO2 capture, where the main limiting mechanism is the loss of surface area by sintering during calcination at harsh conditions. On the other hand, this same effect may be harmful at CaL conditions for CSP storage, where it is the likely plugging of small pores during carbonation at high temperature under high CO2 concentration what chiefly limits CaO conversion. The relative role of nanosilica on the CaL process as depending on the type of CaL conditions will be illustrated in the next sections for nanosilica/CaO composites prepared by diverse methods.
Nanosilica/CaO physical mixtures
In another work (Valverde et al. 2017a), we tested the multicycle performance of a physical mixture of limestone and the same nanosilica (Aerosil R974) at CaL conditions for CSP storage involving carbonation at high temperature under high CO2 concentration and calcination at relatively lower temperature. In contrast with the effect observed at conditions for CO2 capture (Fig. 10 top), the presence of nanosilica yielded an adverse effect at CaL conditions for CSP storage (Fig. 10 bottom). As seen above for the steel slag samples, thermal stabilization provided by nanosilica, and the consequent preservation of porosity in the mesoporous range, promotes pore plugging, which severely limits carbonation at CaL conditions for energy storage.
Nanosilica supported CaO by wet impregnation
Nanosilica/CaO composites prepared by biotemplate method
Both CO2 capture and energy storage are large-scale applications that require massive amounts of solids inventory. Thus, the use of low-cost materials and simple synthesis methods is of paramount relevance. In this sense, the use of a waste such as rice husk can have a potential benefit. The interested reader may find a detailed study in Romeo et al. (2009) about justifiable costs of synthetic Ca-based materials depending on the improvement of CaO conversion achieved. In the case of coal-fired power plants, the use of inexpensive raw limestone, with a residual conversion around 0.075, adjusts well to the integrated plant operation (Romeo et al. 2009). High capture efficiency has been demonstrated at pilot-scale plants where the deactivated CaO is periodically purged while fresh limestone is introduced into the system for mass balance (Perejón et al. 2016). Another cause of CaO deactivation which must be taken into account is the irreversible sulfation of CaO by the reaction with SO2 naturally present in flue gases. Sulfation may be especially relevant for coal-fired power plants. On the other hand, it is judged that economic benefits can be expected in the case of natural gas-fired power plants from the use of synthetic materials where the enhanced sorbent would react with low content SO2 flue gases thus not being deactivated by sulfation (Romeo et al. 2009). Likewise, these synthetic sorbents could be beneficial in the CaL application for energy storage where gases are also free of SO2 that would lead to deactivation thus negating the benefits yielded by the enhanced reactivity. Nevertheless, it must be considered that irreversible CaO sulfation is promoted as the size of CaO pores is increased, since sulfation at CO2 capture conditions is mainly limited by pore plugging (Blamey et al. 2010). In fact, the CaO purged from the calciner in pilot tests, which is highly sintered, shows a high SO2 capture capacity (Cordero et al. 2014). Thus, achieving thermal stabilization by means of nanoparticle technology, which would aid preserving pores in the mesoporous range, could be an effective strategy also in reducing undesired sulfation. This would serve to minimize the need of periodically introducing fresh solids into the system and thus help reduce energy cost to heat the solids up to high temperatures.
The CaL process, based on the calcination/carbonation of cheap, abundant, and non-toxic natural carbonate minerals such as limestone and dolomite, has attracted much interest in the last years to be integrated in fossil fuel-fired power plants for CO2 capture. CaL conditions in this application involve calcination under high CO2 concentration and high temperature, which cause marked sintering of the regenerated CaO skeleton. Such loss of porosity together with the low CO2 concentration in the carbonation environment at relatively lower temperature leads to a severe drop of CaO conversion in the fast reaction-controlled phase with the number of cycles. Nonetheless, carbonation in the subsequent solid-state diffusion controlled stage becomes relatively enhanced particularly for dolomite and steel slag-derived sorbents. Thus, prolonging the solids residence time in the carbonator would be a technical solution to significantly reduce the energy penalty for CO2 capture while a high capture efficiency is maintained.
A further application wherein the CaL process shows a great potential consists of thermochemical energy storage in concentrated solar power plants. Remarkably, the calcination/carbonation conditions under which the CaL-CSP integration could achieve a high global efficiency differ radically from those used in the CaL process for CO2 capture. These involve carbonation under high temperature and high CO2 concentration, whereas calcination could be carried out at a relatively lower temperature in the absence of CO2. Under these conditions, the natural CaO precursors analyzed (limestone, dolomite, and steel slag) exhibit a high multicycle CaO conversion. However, pore plugging poses an important limitation in the case of limestone for particles of size to be used in practical applications (> ~ 100 μm). One such important limitation is mitigated in the case of dolomite arguably due to the presence of domains in the calcined product of inert MgO grains that allow diffusion of CO2 into the inner pores of the CaO grains. Similarly, an industrial waste such as steel slag, after being subjected to an acetic acid treatment and a separation process, shows a high and stable multicycle activity, which can be explained by the formation of a macroporous structure thermally stable at CaL-CSP conditions.
A main conclusion drawn from our work is that the type of CaL conditions is a main factor that determines the multicycle CaO conversion behavior. Thus, any study aimed at assessing the performance of either natural or synthesized CaO precursors from nanoparticle technology must be based on experimental tests made at realistic CaL conditions according to the particular application wherein this process is integrated. In this regard, nanoparticle technology may play a role in improving the performance of naturally occurring minerals provided that the synthetic Ca-based materials can be massively produced at low cost. This can be the case of nanosilica, which can be obtained from rice husk. The use of nanosilica as additive has been shown to promote the CaL performance of nanosilica/CaO composites as compared to natural limestone by mitigating CaO sintering at CaL conditions for CO2 capture. Moreover, the mechanical strength is significantly enhanced for the composites, which can be a relevant benefit in industrial applications based on circulating fluidized beds where particle attrition is a relevant issue. Thermal stabilization provided by nanosilica in samples prepared from physical mixtures leads however to an adverse effect at CaL conditions for CSP storage. In this case, small pores are preserved, which are susceptible to plugging when carbonation is carried out at high temperature under high CO2 concentration as in the CaL application for energy storage. Nonetheless, pore plugging can be avoided by preparing the composites via infiltration of nanosilica derived from rice husk using a biotemplate method. In this case, nanosilica domains mimicking rice husk would allow diffusion of CO2 into the inner CaO pores much like MgO domains do in the case of dolomite. A further advantage of thermal stabilization provided by nanosilica would be the mitigation of irreversible CaO sulfation, which is enhanced as the size of the pores is increased with the number of cycles due to sintering. This may be a relevant feature especially for CO2 capture, since the flue gas usually contains SO2, which is an important cause of CaO deactivation.
The microscopy service of the Innovation, Technology and Research Center of the University of Seville (CITIUS) and the characterization services of the Institute of Materials Science of Seville (ICMS) are sincerely acknowledged.
This work was supported by the Spanish Government Agency Ministerio de Economia y Competitividad (FEDER funds, contracts CTQ2014-52763-C2, CTQ2017-83602-C2).
Compliance with ethical standards
Conflict of interest
The author declares that he has no conflict of interest.
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