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The Ca-looping process for CO2 capture and energy storage: role of nanoparticle technology

Review
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Abstract

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.

Keywords

CO2 capture Energy storage Calcium looping Nanocomposites 

Introduction

The calcination/carbonation reaction of limestone,
$$ {\mathrm{CaO}}_{\left(\mathrm{s}\right)}+{\mathrm{CO}}_{2\left(\mathrm{g}\right)}\rightleftarrows {\mathrm{CaCO}}_{3\left(\mathrm{s}\right)\kern1em };\kern0.5em \Delta {{\mathrm{H}}_{\mathrm{r}}}^0=-178\ \mathrm{kJ}/\mathrm{mol} $$
(1)
lies at the heart of a vast number of industrial applications and natural processes. In the past few years, the calcium looping (CaL) process, based on the multicycle carbonation/calcination of CaO, has come into scene with a high potential to be used either for CO2 capture or thermochemical energy storage. Relevant properties of the CaO stemming from limestone calcination are its porosity, reactivity, crystal structure, and mechanical strength, which are critically determined by the environmental conditions under which the reaction evolves depending on the specific application.
The integration of the CaL process into coal-fired power plants for CO2 capture (Fig. 1a) has been already demonstrated at the pilot scale (1–2 Mwth) level (Blamey et al. 2010; Perejón et al. 2016). In this process, the CO2-loaded flue gas (~ 15% CO2 v/v concentration) is used to fluidize a bed of CaO particles at temperatures around 650 °C, which leads to fast carbonation of the solids. The carbonated particles are then circulated into a second reactor wherein the CaO solids are regenerated by calcination at temperatures typically near ~ 950 °C. CO2 concentration in the calciner must be necessarily high in order to extract it as pure as possible for compression and the subsequent storage. To attain fast calcination under a high CO2 partial environment, the calciner temperature has to be increased well over the equilibrium temperature, which yields a marked sintering and loss of reactivity of the regenerated CaO (Fig. 2) (Valverde et al. 2015).
Fig. 1

Flow diagrams of the calcium looping process for CO2 capture in fossil fuel-fired power plants (a) and for thermochemical energy storage in concentrated solar power plants (b)

Fig. 2

SEM images of CaO resulting from calcination of limestone at diverse conditions of temperature and CO2 concentration (Valverde et al. 2015). Calcination temperatures and ratios of CO2 partial pressure to equilibrium partial pressure are indicated

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 3 shows the time evolution of temperature and sample mass percentage recorded during the first cycle of TGA tests carried out under CaL-CSP storage and CaL-CO2 capture conditions, respectively, for marble powder (CaCO3). As can be seen, the type of CaL conditions determines fundamentally the kinetics of the carbonation stage. Thus, carbonation under CaL-CSP storage conditions takes place mainly in a very short initial stage, which would be controlled by the kinetics of the reaction on the surface of the CaO particles. Subsequent carbonation, which would be determined by solid-state diffusion of CO2 across the CaCO3 layer, is fully negligible (Fig. 3a). In contrast, carbonation in this slower phase takes a relevant role under CaL-CO2 capture conditions (Fig. 3b).
Fig. 3

Time evolution of temperature and sample mass % for the first carbonation cycle using marble samples of different particle sizes, under CaL-CSP storage (a) and CaL-CO2 capture conditions (b) (Benitez-Guerrero et al. 2017)

Data on the multicycle effective conversion under CaL-CSP storage and CaL-CO2 capture conditions for sieved natural minerals are shown in Fig. 4 (effective conversion is defined as the ratio of CaO converted to the total mass of the sorbent before carbonation including inert compounds if present). Remarkably, a clear difference is observed in the behavior of the different carbonates, as well as in the behavior of the different granulometric fractions tested, depending on the type of CaL conditions. On the other hand, limestone and marble manifest a quite similar performance. In the case of CSP storage conditions, multicycle conversion is severely hindered for both natural carbonates for particles larger than 45 um, reaching a conversion value at the 20th cycle close to 0.16. However, for particles smaller than 45 um, it is substantially higher indicating that such particle size poses a limitation to carbonation under conditions for CSP storage. In contrast, there is no effect of particle size on the multicycle conversion of CaO derived from limestone and marble when tested under CO2 capture conditions (Fig. 4b). The much lower conversion values measured under CaL-CO2 capture conditions (Fig. 4b) as compared to those under CaL-CSP storage conditions can be explained from the harsh calcination conditions (high temperature under high CO2 concentration) used in the former case, which lead to a severe sintering of the CaO grains as the number of cycles increases. Thus, calcination conditions for CO2 capture cause a drastic loss of the regenerated CaO activity.
Fig. 4

Multicycle effective conversion (ratio of CaO converted to total mass of sorbent) of sieved limestone and marble samples subjected to CaL cycles under CaL-CSP storage (a) and CaL-CO2 capture conditions (b), and sieved dolomite compared to limestone under CaL-CSP (c) and CaL-CO2 capture conditions (d) (Benitez-Guerrero et al. 2017)

The influence of the type of CaL conditions on the multicycle conversion performance as depending on particle size is explainable from the relative thickness of the CaCO3 layer built upon the CaO surface as compared to the size of the pores in the CaO skeleton and from the carbonation kinetics in the fast reaction-controlled (FR) stage. The FR stage ends up when the CaCO3 layer built upon the CaO exposed surface reaches a 40–50 nm thickness at carbonation conditions for CO2 capture (T ~ 650 °C and 15% CO2 vol. concentration). The thickness of this product layer can be increased over ~ 100 nm (Li et al. 2012) if the carbonation temperature and CO2 vol. concentration are increased as is the case at CaL conditions for CSP storage. On the other hand, under CaL-CO2 capture conditions, the CaO structure is severely sintered by the harsh calcination conditions and the typical size of the pores is typically over 100 nm as measured by mercury intrusion porosimetry (Fig. 5), which is smaller than the thickness of the carbonate layer at the end of the chemically controlled FR stage. Thus, the CO2 molecules can access into the pores of the CaO particles and pore plugging does not pose a limitation to carbonation in the fast reaction-controlled stage. Otherwise, the size of the pores generated in the CaO skeleton formed by calcination in the absence of CO2 in the calciner environment at relatively low temperatures (as is the case for CSP storage conditions) is typically on the order of tens of nanometers (Fig. 5), whereas the thickness of the CaCO3 layer generated at the high carbonation temperature under high CO2 concentration can be over 100 nm. Moreover, this carbonate layer is very rapidly formed due to the fast reaction kinetics at the high carbonation temperatures used as seen in Fig. 3. The external surface area of the CaO grains derived from calcination at CaL-CSP storage conditions would thus become quickly plugged, which impedes the access of CO2 to the internal surface area of the CaO skeleton.
Fig. 5

Pore size distribution measured by Hg intrusion porosimetry of CaO obtained from calcination of limestone under pure N2 at 750 °C and pure CO2 at 950 °C. The insets schematize the CaCO3 layer built up on porous CaO particles under CaL-CSP storage and CaL-CO2 capture conditions leading to pore plugging for relatively large CaO particles under CSP storage conditions involving a relatively low calcination temperature (Benitez-Guerrero et al. 2017)

The SEM picture in Fig. 6 shows the surface of a limestone particle cycled at CaL conditions for CSP energy storage (ending in calcination) which further supports the argument on the important limitation posed by pore plugging on the CaO multicycle activity at CaL-CSP storage conditions. As may be seen, a part of the particle’s surface has been broken during sample manipulation, which has left exposed a relatively porous CaO skeleton inside the particle whereas the rest of the surface is covered by a sintered CaO layer. This figure suggests that only a relatively small fraction of the material at the particle’s surface would be active along the carbonation/calcination cycles while the relatively porous inner CaO skeleton remains unreacted as it becomes inaccessible to the CO2. Pore plugging would be therefore a main limiting mechanism on carbonation in the fast reaction-controlled regime, leading to a decay of the sorbent activity in short residence times with the number of cycles, which is further enhanced the larger the particle is.
Fig. 6

Top: SEM image of CaO resulting from limestone after CaL cycles at conditions for CSP energy storage (ending in calcination). Bottom: SEM image of sintered micron-sized CaO grains and inert MgO nanosized grains resulting from dolomite calcination

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

Figure 7 shows data on multicycle CaO conversion measured for steel slag treated with acetic acid and tested under CaL-CSP storage and CaL-CO2 capture conditions (Perejón et al. 2017). As may be seen, results for carbonation/calcination cycles performed under CaL-CO2 capture conditions demonstrate a small, although quite stable, conversion. In contrast, results from the tests performed under CaL-CSP storage conditions show a rather high and stable conversion.
Fig. 7

Multicycle conversion of CaO derived from steel slag treated with acetic acid without and with an intermediate separation step (see Miranda-Pizarro et al. (2016) and Perejón et al. (2017) for further experimental details). Tests carried out under CaL-CSP storage conditions and CaL-CO2 capture conditions. Values of the residual conversion are shown in the inset

The multicycle conversion at CaL-CSP storage conditions is kept at a higher level if the steel slag, after being subjected to the acetic acid treatment, is further subjected to separation by means of filtration (Fig. 7), which removes from the treated slag metal oxide nanoparticles (see SEM micrographs in Fig. 8) and mainly nanosilica as revealed by XRF analysis (Valverde et al. 2017b). The presence of nanostructured silica promotes the preservation of pores in the mesoporous range (pore diameters between 2 and 50 nm). Thus, the presence of nanosilica would play an adverse role at CaL conditions for CSP storage, since small pores are prone to be plugged. Accordingly, porosity in the macroporous range (pores of size greater than 50 nm are less prone to plugging) is enhanced in the sample subjected to the filtration process, which serves to mitigate pore plugging and therefore favors conversion at CaL-CSP storage conditions.
Fig. 8

SEM micrographs of steel slag samples pretreated with acetic acid and subjected to 21 cycles at CaL conditions for CSP storage ending in carbonation. CaCO3-sintered grains and nanosized particles of metal oxide impurities (mainly Si, Al, Fe, and Mg oxides) are indicated

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 this study (Valverde et al. 2012), nanosilica/Ca(OH)2 composites were prepared by physically mixing a commercial nanosilica from Evonik (Aerosil R974) with a Ca(OH)2 powder. After heating at 850 °C in air, SEM, EDX, and XRD analyses indicated that the nanostructured silica and CaO reacted locally leading to the formation of calcium silicate at contact points (Fig. 9). As the melting point of calcium silicate locally formed is higher as compared to that of CaO, the resistance to sintering was improved. TGA results demonstrated that the addition of nanosilica promoted significantly the CaL performance at conditions for CO2 capture due to enhanced mass/heat transfer area and improved thermal stability. This may be seen in Fig. 10 (top), where the time evolution of CaO conversion is plotted for the 1st and 100th CaL cycle. Note that the rate of conversion in the fast reaction-controlled regime is remarkably increased already in the 1st cycle for the nanosilica/CaO composite, whereas after 100 cycles, the overall conversion in this fast regime is much higher for the composite as compared to that for pure CaO.
Fig. 9

SEM picture of calcined nanosilica/CaO composite prepared from a physical mixture of a commercial nanosilica (Aerosil R974) and Ca(OH)2 (the arrow indicates a reaction spot where calcium silicate is formed)

Fig. 10

Top: CaO conversion as a function of time measured at CaL conditions for CO2 capture during the 1st and 100th carbonation/calcination cycle for CaO and a nanosilica/CaO composite obtained by physically mixing Ca(OH)2 and a commercial nanosilica (Aerosil R974 from Evonik, 15 wt%) (Valverde et al. 2012). Bottom: CaO conversion as a function of the cycle number measured at CaL conditions for CSP storage for limestone and nanosilica/limestone physical mixture samples (Valverde et al. 2017a)

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

In this work (Sanchez-Jimenez et al. 2014), a synthetic sorbent was prepared by incipient wetness impregnation of molten calcium nitrate on a nanosilica support (Aerosil 300 from Evonik). After calcination, the CaO derived from calcium nitrate fully reacted with the SiO2 matrix to form a nanostructured calcium silicate. The amount of calcium nitrate to be impregnated on the SiO2 support was calculated as to balance the CaO/SiO2 stoichiometric ratio for the formation of calcium silicate. This yielded a calcium silicate nanostructured matrix, which was coated with a layer of CaO by additional impregnation as seen in Fig. 11. Thus, the porosity of the synthesized sorbent was controlled by the amount of CaO employed in following impregnations after the formation of the calcium silicate matrix (Fig. 12). The synthetic composites were shown to exhibit a stable conversion well above the residual conversion of natural limestones at CaL conditions for CO2 capture (Fig. 13). Furthermore, particle size distribution (PSD) measurements of samples subjected to prolonged high energy ultrasonication, capable of producing particle fragmentation, demonstrated that the mechanical strength of the CaO-impregnated sorbent was enhanced as compared to CaO derived from natural limestone. This is an added benefit as particle attrition due to high energy impacts and thermal stresses would lead to the formation of very fine fragments that cannot be recovered by cyclones in the commercial application. Even though multicycle tests at CaL-CSP storage conditions were not carried out using this synthetic composite, pore plugging would be expected to be promoted from the small size of the pores. The next section describes a preparation method which serves to minimize pore plugging thus enhancing the composite multicycle performance also at energy storage conditions.
Fig. 11

SEM picture of a Ca-based synthetic sorbent obtained by impregnation of molten calcium nitrate on nanosilica (Sanchez-Jimenez et al. 2014)

Fig. 12

BJH desorption (dV/dD) pore volume distributions of nanosilica and nanosilica impregnated with molten calcium nitrate and calcined. The 0 wt% CaO case corresponds to a sample prepared using the stoichiometric CaO/SiO2 ratio to yield only calcium silicate after calcination. The weight percentage of CaO indicated in the other cases corresponds to the weight percentage of CaO built up in excess on the calcium silicate support after subsequent impregnations (Sanchez-Jimenez et al. 2014)

Fig. 13

CaO conversion vs. cycle number for synthetic sorbents obtained by impregnation of nanosilica with calcium nitrate (10, 23, 30, and 40 wt% CaO) and for natural limestone at CaL conditions for CO2 capture

Nanosilica/CaO composites prepared by biotemplate method

In this study (Monica Benitez-Guerrero et al. 2018), nanosilica was initially prepared from raw rice husk, which was thermally pretreated to obtain high purity nanostructured SiO2 after pyrolysis and combustion (Fig. 14). Then, a calcium nitrate solution was employed to fill the pores of the biotemplate nanosilica matrix up to cover completely the inner and outer surfaces of the pretreated rice husk. Further thermal treatment produced a nanosilica/CaO composite (Fig. 15) with enhanced performance as compared to limestone at CaL conditions for CSP storage. The results indicated that pore plugging was avoided by the composition and microstructure of the CaO/SiO2 composites. SEM-TEM analyses (Fig. 15) indicated that the porous structure of the composite and the deep cavities formed between the SiO2 plates and CaO grains would facilitate the diffusion of CO2 into the inner pores of the particles during carbonation. Consequently, the multicycle performance of the composite was improved as compared to limestone also at CaL conditions for energy storage (Fig. 16). This illustrates how nanoparticle technology can be useful to tune the performance of the material at given conditions once the limiting mechanism at these conditions is well understood.
Fig. 14

TEM micrographs of nanostructured SiO2 synthesized from rice husk ash at different magnifications (a, b, c, d)

Fig. 15

Secondary electron micrographs (SE at 2 and 20 kV) and compositional mapping (Ca and Si) of 70% CaO composite after being subjected to 20 cycles under CaL-CSP conditions

Fig. 16

CaO conversion as a function of the cycle number for the CaO/SiO2 composites prepared by the biotemplate method and tested at CaL conditions for energy storage. Multicycle conversion data for sieved limestone samples are plotted for comparison

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.

Conclusions

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.

Notes

Acknowledgements

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.

Funding information

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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© Springer Science+Business Media B.V., part of Springer Nature 2018

Authors and Affiliations

  1. 1.Facultad de FisicaUniversidad de SevillaSevilleSpain

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