Ca-based materials derived from calcined cigarette butts for CO2 capture and thermochemical energy storage

Cigarette butts (CBs) are one of the most common types of litter in the world. Due to the toxic substances they contain, the waste generated poses a harmful risk to the environment, and therefore there is an urgent need for alternative solutions to landfill storage. Thus, this work presents a possible revalorization of this waste material, which implies interesting environmental benefits. CBs were used as sacrificial templates for the preparation of CaO-based materials by impregnation with calcium and magnesium nitrates followed by flaming combustion. These materials presented enhanced porosity for their use in the Calcium Looping process applied either to thermochemical energy storage or CO2 capture applications. The influence of the concentration of Ca and Mg in the impregnating solutions on the multicycle reactivity of the samples was studied. An improved multicycle performance was obtained in terms of conversion for both applications.

Influence of Long-Term CaO Storage Conditions on the Calcium Looping Thermochemical Reactivity

Long-term storage capability is often claimed as one of the distinct advantages of the calcium looping process as a potential thermochemical energy storage system for integration into solar power plants. However, the influence of storage conditions on the looping performance has seldom been evaluated experimentally. The storage conditions must be carefully considered as any potential carbonation at the CaO storage tank would reduce the energy released during the subsequent carbonation, thereby penalizing the round-trip efficiency. From lab-scale to conceptual process engineering, this work considers the effects of storing solids at low temperatures (50-200 °C) in a CO2 atmosphere or at high temperatures (800 °C) in N2. Experimental results show that carbonation at temperatures below 200 °C is limited; thus, the solids could be stored during long times even in CO2. It is also demonstrated at the lab scale that the multicycle performance is not substantially altered by storing the solids at low temperatures (under CO2) or high temperatures (N2 atmosphere). From an overall process perspective, keeping solids at high temperatures leads to easier heat integration, a better plant efficiency (+2-4%), and a significantly higher energy density (+40-62%) than considering low-temperature storage. The smooth difference in the overall plant efficiency with the temperature suggests a proper long-term energy storage performance if adequate energy integration is carried out.

Biotemplating of Al2O3-Doped, CaO-Based Material from Bamboo Fiber for Efficient Solar Energy Storage

The high-temperature sintering of CaO-based materials leads to the serious decay of energy storage performance during the calcination/carbonation cycle. To overcome the loss in porosity problem, an efficient CaO-based material for thermal energy storage was synthesized using bamboo fiber as the biotemplate. The synthesis parameters (bamboo fiber addition, pyrolysis, Al2O3 loading) and the energy storage reaction characteristics of CaO-based energy storage material were optimized on the basis of cyclic calcination/carbonation experiments. The results show that the sacrificed biotemplate enhances the porosity of the synthetic material, denoting improved energy storage density. The cumulative energy storage density of the templated material over 50 cycles is 24,131.44 kJ/kg higher than that of limestone. The carbonation conversion and energy storage density of the templated CaO-based material doped with 5 wt.% Al2O3 and 0.5 g bamboo fiber reach 0.75 mol/mol and 2368.82 kJ/kg after 10 cycles, respectively, which is 2.7 times as high as that of original limestone. The maximum apparent carbonation rate of the templated CaO-based materials in the 1st cycle corresponds to a 240% increment compared to limestone. The maximum calcination rate of the synthetic CaO-based material in the 12th cycle remains 93%, as compared with the initial cycle. The microstructure analysis reveals that the hierarchically-stable structure during the cycle is beneficial for a more effective exposure of surface reactive sites for CaO and inward/outward diffusion for CO2 molecules through CaO. The method using the sacrificed biological template provides an advanced approach to fabricate porous materials, and the composite CaO-based material provides high-return solar energy storage for a potential application in industrial scale.

Acquiring an effective CaO-based CO2 sorbent and achieving selective methanation of CO2

CO₂ capture, utilization, and storage are promising strategies to solving the problems of superfluous CO₂ or energy shortage. Here, mechanochemical reduction of CO₂ by a MgH₂/CaH₂ mixture was first performed, by which we achieve selective methanation of CO₂ and acquire an effective CaO-based CO₂ sorbent, simultaneously. The selectivity of methanation is near 100% and the yield of CH₄ reaches 30%. Four MgO and carbon-doped CaO-based CO₂ sorbents (MgO/CaO/C, MgO/2CaO/C, MgO/4CaO/C, and MgO/8CaO/C) were formed as solid products in these reactions. Among them, the MgO/4CaO/C sorbent shows high initial adsorption amount of 59.3 wt% and low average activity loss of 1.6% after 30 cycles. This work provides a novel, well-scalable, and sustainable approach to prepare an efficient inert additive-including CaO-based CO₂ sorbent and selectively convert CO₂ to CH₄ at the same time.

We achieve selective methanation of CO₂ and acquire an effective CaO-based CO₂ sorbent by reduction of CO₂ with a MgH₂/CaH₂ mixture.

Role of particle size on the multicycle calcium looping activity of limestone for thermochemical energy storage

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Thermal energy performance of narrow particle size distribution limestones is studied.

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Multicyclic activity is better for small particles under all the different studied conditions.

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This effect is particularly relevant for particles smaller than 15 μm median particle size.

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Particle size effect is not relevant for particles between 15 and 900 μm.

The calcium looping process, based on the reversible reaction between CaCO₃ and CaO, is recently attracting a great deal of interest as a promising thermochemical energy storage system to be integrated in Concentrated Solar Power plants (CaL-CSP). The main drawbacks of the system are the incomplete conversion of CaO and its sintering-induced deactivation. In this work, the influence of particle size in these deactivation mechanisms has been assessed by performing experimental multicycle tests using standard limestone particles of well-defined and narrow particle size distributions. The results indicate that CaO multicycle conversion benefits from the use of small particles mainly when the calcination is carried out in helium at low temperature. Yet, the enhancement is only significant for particles below 15 μm. On the other hand, the strong sintering induced by calcining in CO₂ at high temperatures makes particle size much less relevant for the multicycle performance. Finally, SEM imaging reveals that the mechanism responsible for the loss of activity is mainly pore-plugging when calcination is performed in helium, whereas extensive loss of surface area due to sintering is responsible for the deactivation when calcination is carried out in CO₂ at high temperature.

Thermally induced carbonation of Ca(OH)2 in a CO2 atmosphere: kinetic simulation of overlapping mass-loss and mass-gain processes in a solid-gas system

Thermally induced carbonation of Ca(OH)₂ in a CO₂ atmosphere is a reaction exhibiting particular features, including stoichiometric completeness to form CaCO₃ and a kinetic advantage over the carbonation of CaO particles. This study aims to gain further insight into the reaction mechanisms of CO₂ capture by Ca(OH)₂ and CaO. It focuses on the kinetic modeling of the carbonation of Ca(OH)₂ as a consecutive reaction in a solid-gas system. The kinetic behaviors of the thermal decomposition of Ca(OH)₂ in an inert gas atmosphere and of the overall process of thermally induced carbonation of Ca(OH)₂ in a CO₂ atmosphere were investigated using thermal analyses and other complementary techniques. Based on kinetic results, the overall reaction of the thermally induced carbonation of Ca(OH)₂ in a CO₂ atmosphere was separated by a kinetic deconvolution analysis into two consecutive reaction steps: the thermal decomposition of Ca(OH)₂ and the subsequent carbonation of the CaO intermediate. The relationship between the two component reaction processes was well illustrated by a consecutive shrinkage of the dual reaction interfaces of Ca(OH)₂-CaO and CaO-CaCO₃. The continuous supply of water vapor and CO₂ to the CaO-CaCO₃ interface from different directions was suggested to be the physico-geometrical advantageous feature of the carbonation of Ca(OH)₂.

A Model to Stabilize CO2 Uptake Capacity during Carbonation-Calcination Cycles and its Case of CaO-MgO

Nowadays, capturing anthropogenic CO₂ in a highly efficient and cost-effective way is one of the most challenging issues. Herein, the key parameters to stabilize CO₂ uptake capacity have been studied based on four kinds of pure calcium oxides (CaO) prepared by a simple calcination method with four different calcium precursors. A simple ideal particle model was proposed to illustrate the uniform distribution of pure CaO, in which the CO₂ uptake capacity is positively related with surface area of CaO particles and the stability is opposite to the distance between two CaO particles after carbonation. The adsorption capacity of the best sample with a distance of 398 nm between two CaO particles after carbonation only lost 0.344% per cycle, which is originated from the low possibility of the agglomeration between neighboring particles. On the basis of the proposed model, the composite with magnesium oxide (MgO) distributed uniformly in CaO was fabricated by a simple ball milling method, which possessed an excellent stability with a decay rate of only 3.9% over 100 carbonation-calcination cycles. In this case, MgO played as inert to increase the distance between CaO particles for agglomeration prevention.