Synthesis of CaO-based sorbents through incorporation of alumina/aluminate and their CO2 capture performance

Post-combustion CO2 capture via a variety of temperature ranges and material adsorption process: A review

Carbon dioxide (CO₂) emissions from fossil fuel combustion have been linked to increased average global temperatures, a global challenge for many decades. Mitigating CO₂ concentration in the atmosphere is a priority for the protection of the environment. This is a comparison of the three main technological categories available for CO₂ capture and storage. They include: oxy-fuel combustion, pre-combustion, and post-combustion. Each capture technology has inherent benefits and disadvantages in cost, implementation, and flexibility, but post-combustion CO₂ capture has demonstrated the most promising results in typical power plant configurations. This paper presents a review of different post-combustion CO₂ capture materials; solvents, membranes, and adsorbents, focusing on economical and environmentally safe low to high temperature solid adsorbents. Furthermore, the authors summarize the advantages and limitations of the materials investigated to provide insight into the challenges and opportunities currently facing the development of post-combustion CO₂ capture technologies. The solid sorbents currently available for CO₂ capture are also reviewed in detail, including physical and chemical properties, reactions, and current research efforts on improvement.

CO2 Sorption and Regeneration Properties of K2CO3/Al2O3-Based Sorbent at High Pressure and Moderate Temperature

In this study, the CO2 sorption mechanisms and regeneration properties of alumina-based sorbent using K2CO3 loading under high-pressure and moderate temperature conditions were examined. To investigate the mechanism of CO2 sorption, a zirconium-based sorbent was compared with an alumina-based sorbent. The CO2 capture capacities of the PAI10, 20, 30, and 40 were 32.3, 63.0, 95.4, and 124.5 mg CO2/g sorbent, respectively. To investigate the CO2 sorption mechanism of an alumina-based sorbent, we performed XRD, TG/DTG, and FTIR analyses after CO2 sorption in the presence of 10 vol% CO2 and H2O each at 200 °C and 20 atm. For PAI10–40 sorbents, KHCO3 and KAl(CO3)(OH)2 phases were observed by TG/DTG and FTIR analysis. For PAI-x sorbents, it was confirmed that the captured CO2 is desorbed completely at a temperature below 400 °C at 20 atm.

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

Carbon dioxide capture and mitigation form a key part of the technological response to combat climate change and reduce CO₂ emissions. Solid materials capable of reversibly absorbing CO₂ have been the focus of intense research for the past two decades, with promising stability and low energy costs to implement and operate compared to the more widely used liquid amines. In this review, we explore the fundamental aspects underpinning solid CO₂ sorbents based on alkali and alkaline earth metal oxides operating at medium to high temperature: how their structure, chemical composition, and morphology impact their performance and long-term use. Various optimization strategies are outlined to improve upon the most promising materials, and we combine recent advances across disparate scientific disciplines, including materials discovery, synthesis, and in situ characterization, to present a coherent understanding of the mechanisms of CO₂ absorption both at surfaces and within solid materials.

Advanced High-Temperature CO2 Sorbents with Improved Long-Term Cycling Stability

Developing novel sorbents with maximum carbonation efficiency and good cycling stability for CO₂ capture is a promising route to sequester anthropogenic CO₂. In this work, we have employed a green synthesis method to synthesize CaO-based sorbents suitably stabilized by MgO and supported by in situ generated carbon under inert atmosphere. The varied amounts (10-30 wt %) of MgO were used to stabilize the CaO. The supported mixed metal oxide (MMO) sorbents were screened for high-temperature CO₂ capture under CO₂ rich (86% CO₂) and lean (14% CO₂) gas streams at 650 °C and atmospheric pressure. The MMO sorbents captured 53-63 wt % of CO₂ per gram of sorbent under 86 and 14% CO₂, accounting for about 98% carbonation efficiency, which outperforms the CO₂ capture capacity of limestone derived CaO (L-CaO) sorbents (22.8 wt %). All of the synthetic MMO sorbents showed greater capture capacity and cyclic stability when compared to benchmark L-CaO. Because of the high carbonation efficiency and cycling stability of g-Ca_0.69Mg_0.3O sorbent, it was tested for 100 carbonation/regeneration cycles of 5 min each under CO₂ lean conditions. The g-Ca_0.69Mg_0.3O sorbent showed exceptional CO₂ capture capacity and cycling stability and retained about 65% of its initial capture capacity after 100 cycles.

The effect of the layer-interlayer chemistry of LDHs on developing high temperature carbon capture materials

(a) SEM image of the fresh MMOs, (b) carbonation/regeneration cycles, and (c) SEM image of the MMOs after 60 carbonation/regeneration cycles of the Ca–Al-ada LDHs.

A Carbide Slag-Based, Ca12Al14O33-Stabilized Sorbent Prepared by the Hydrothermal Template Method Enabling Efficient CO2 Capture

Calcium looping is a promising technology to capture CO2 from the process of coal-fired power generation and gasification of coal/biomass for hydrogen production. The decay of CO2 capture activities of calcium-based sorbents is one of the main problems holding back the development of the technology. Taking carbide slag as a main raw material and Ca12Al14O33 as a support, highly active CO2 sorbents were prepared using the hydrothermal template method in this work. The effects of support ratio, cycle number, and reaction conditions were evaluated. The results show that Ca12Al14O33 generated effectively improves the cyclic stability of CO2 capture by synthetic sorbents. When the Al2O3 addition is 5%, or the Ca12Al14O33 content is 10%, the synthetic sorbent possesses the highest cyclic CO2 capture performance. Under harsh calcination conditions, the CO2 capture capacity of the synthetic sorbent after 30 cycles is 0.29 g/g, which is 80% higher than that of carbide slag. The superiority of the synthetic sorbent on the CO2 capture kinetics mainly reflects at the diffusion-controlled stage. The cumulative pore volume of the synthetic sorbent within the range of 10–100 nm is 2.4 times as high as that of calcined carbide slag. The structure of the synthetic sorbent reduces the CO2 diffusion resistance, and thus leads to better CO2 capture performance and reaction rate.

Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted MgO-CaCO3 Sorbents

As a potential candidate for precombustion CO₂ capture at intermediate temperatures (200-400 °C), MgO-based sorbents usually suffer from low kinetics and poor cyclic stability. Herein, a general and facile approach is proposed for the fabrication of high-performance MgO-based sorbents via incorporation of CaCO₃ into MgO followed by deposition of a mixed alkali metal salt (AMS). The AMS-promoted MgO-CaCO₃ sorbents are capable of adsorbing CO₂ at an ultrafast rate, high capacity, and good stability. The CO₂ uptake of sorbent can reach as high as above 0.5 g_CO2 g_sorbent^-1 after only 5 min of sorption at 350 °C, accounting for vast majority of the total uptake. In addition, the sorbents are very stable even under severe but more realistic conditions (desorption in CO₂ at 500 °C), where the CO₂ uptake of the best sorbent is stabilized at 0.58 g_CO2 g_sorbent^-1 in 20 consecutive cycles. The excellent CO₂ capture performance of the sorbent is mainly due to the promoting effect of molten AMS, the rapid formation of CaMg(CO₃)₂, and the plate-like structure of sorbent. The exceptional ultrafast rate and the good stability of the AMS-promoted MgO-CaCO₃ sorbents promise high potential for practical applications, such as precombustion CO₂ capture from integrated gasification combined cycle plants and sorption-enhanced water gas shift process.

Multishelled CaO Microspheres Stabilized by Atomic Layer Deposition of Al2 O3 for Enhanced CO2 Capture Performance

CO₂ capture and storage is a promising concept to reduce anthropogenic CO₂ emissions. The most established technology for capturing CO₂ relies on amine scrubbing that is, however, associated with high costs. Technoeconomic studies show that using CaO as a high-temperature CO₂ sorbent can significantly reduce the costs of CO₂ capture. A serious disadvantage of CaO derived from earth-abundant precursors, e.g., limestone, is the rapid, sintering-induced decay of its cyclic CO₂ uptake. Here, a template-assisted hydrothermal approach to develop CaO-based sorbents exhibiting a very high and cyclically stable CO₂ uptake is exploited. The morphological characteristics of these sorbents, i.e., a porous shell comprised of CaO nanoparticles coated by a thin layer of Al₂ O₃ (<3 nm) containing a central void, ensure (i) minimal diffusion limitations, (ii) space to accompany the substantial volumetric changes during CO₂ capture and release, and (iii) a minimal quantity of Al₂ O₃ for structural stabilization, thus maximizing the fraction of CO₂ -capture-active CaO.

Super-dry reforming of methane intensifies CO2 utilization via Le Chatelier's principle

Efficient CO₂ transformation from a waste product to a carbon source for chemicals and fuels will require reaction conditions that effect its reduction. We developed a "super-dry" CH₄ reforming reaction for enhanced CO production from CH₄ and CO₂ We used Ni/MgAl₂O₄ as a CH₄-reforming catalyst, Fe₂O₃/MgAl₂O₄ as a solid oxygen carrier, and CaO/Al₂O₃ as a CO₂ sorbent. The isothermal coupling of these three different processes resulted in higher CO production as compared with that of conventional dry reforming, by avoiding back reactions with water. The reduction of iron oxide was intensified through CH₄ conversion to syngas over Ni and CO₂ extraction and storage as CaCO₃ CO₂ is then used for iron reoxidation and CO production, exploiting equilibrium shifts effected with inert gas sweeping (Le Chatelier's principle). Super-dry reforming uses up to three CO₂ molecules per CH₄ and offers a high CO space-time yield of 7.5 millimole CO per second per kilogram of iron at 1023 kelvin.

Thermal dechlorination of PCB-209 over Ca species-doped Fe₂O₃

Degradation reaction of decachlorobiphenyl (PCB-209) was investigated over the synthesized Ca species-doped Fe2O3 at 300 °C. The 1%Ca-Fe2O3 exhibited the highest activity among the four catalysts prepared with the pseudo-first order reaction at k(obs) = 0.103 min(-1). PCB-207, PCB-197, PCB-176, PCB-184, PCB-150, PCB-136, PCB-148, PCB-104, PCB-96, PCB-54, PCB-19, PCB-4 and PCB-1 were identified as the dominant isomers in their respective nonachlorobiphenyl (NonaCB) to monochlorobiphenyl (MonoCB) homologue groups. Analysis of the hydrodechlorination products indicated that dechlorination was much more favored on meta- and para-than on ortho-positions. The formation of significantly predominant NonaCB and octachlorobiphenyl (OctaCB) isomers was attributed to lower energy principles and to the 90° dihedral angles of two aromatic rings which prevented the hydrodechlorination at ortho-positions. When the number of chlorine atoms is not more than 7, the steric effect supports the formation of predominant PCB isomers having chlorines at four ortho-positions. During the dechlorination of tetrachlorobiphenyl (TetraCB) formed to generate monochlorobiphenyl (MonoCB) isomers, the chlorine atoms fully substituted at the ortho-positions have to be successively removed, with the first two dechlorinations preferentially occurring at the two different benzene rings. This is dissimilar to that of octachloronaphthalene (PCN-75) in which the hydrodechlorination reaction happened preferentially at ortho-position due to the existence of steric effects. The opposite roles of the steric effect in ortho-position between PCB-209 and PCN-75 might be due to the difference of the π-conjugated plane caused by the dihedral angle of 90° and 0° of the two aromatic rings.

Study of CO2 cyclic absorption stability of CaO-based sorbents derived from lime mud purified by sucrose method

Using lime mud (LM) purified by sucrose method, derived from paper-making industry, as calcium precursor, and using mineral rejects-bauxite-tailings (BTs) from aluminum production as dopant, the CaO-based sorbents for high-temperature CO2 capture were prepared. Effects of BTs content, precalcining time, and temperature on CO2 cyclic absorption stability were illustrated. The cyclic carbonation behavior was investigated in a thermogravimetric analyzer (TGA). Phase composition and morphologies were analyzed by XRD and SEM. The results reflected that the as-synthesized CaO-based sorbent doped with 10 wt% BTs showed a superior CO2 cyclic absorption-desorption conversion during multiple cycles, with conversion being >38 % after 50 cycles. Occurrence of Ca12Al14O33 phase during precalcination was probably responsible for the excellent CO2 cyclic stability.

Improved CO2 adsorption capacity and cyclic stability of CaO sorbents incorporated with MgO

The CO₂ adsorption capacity of CaO sorbents with different MgO wt% (calcination temperature 800 °C, carbonation at 650 °C with 100% CO₂, and de-carbonation at 800 °C in 100% N₂).

Solar Thermochemical Energy Storage Through Carbonation Cycles of SrCO3/SrO Supported on SrZrO3

Solar thermochemical energy storage has enormous potential for enabling cost-effective concentrated solar power (CSP). A thermochemical storage system based on a SrO/SrCO3 carbonation cycle offers the ability to store and release high temperature (≈1200 °C) heat. The energy density of SrCO3/SrO systems supported by zirconia-based sintering inhibitors was investigated for 15 cycles of exothermic carbonation at 1150 °C followed by decomposition at 1235 °C. A sample with 40 wt % of SrO supported by yttria-stabilized zirconia (YSZ) shows good energy storage stability at 1450 MJ m(-3) over fifteen cycles at the same cycling temperatures. After further testing over 45 cycles, a decrease in energy storage capacity to 1260 MJ m(-3) is observed during the final cycle. The decrease is due to slowing carbonation kinetics, and the original value of energy density may be obtained by lengthening the carbonation steps.

Monitoring solid oxide CO2 capture sorbents in action

The separation, capture, and storage of CO2 , the major greenhouse gas, from industrial gas streams has received considerable attention in recent years because of concerns about environmental effects of increasing CO2 concentration in the atmosphere. An emerging area of research utilizes reversible CO2 sorbents to increase conversion and rate of forward reactions for equilibrium-controlled reactions (sorption-enhanced reactions). Little fundamental information, however, is known about the nature of the sorbent surface sites, sorbent surface-CO2 complexes, and the CO2 adsorption/desorption mechanisms. The present study directly spectroscopically monitors Na2 O/Al2 O3 sorbent-CO2 surface complexes during adsorption/desorption with simultaneous analysis of desorbed CO2 gas, allowing establishment of molecular level structure-sorption relationships between individual surface carbonate complexes and the CO2 working capacity of sorbents at different temperatures.

Porous spherical CaO-based sorbents via PSS-assisted fast precipitation for CO2 capture

In this paper, we report the development of synthetic CaO-based sorbents via a fast precipitation method with the assistance of sodium poly(styrenesulfonate) (PSS). The effect of PSS on physical properties of the CaO sorbents and their CO2 capture performance were investigated. The presence of PSS dispersed the CaO particles effectively as well as increased their specific surface area and pore volume remarkably. The obtained porous spherical structure facilitated CO2 to diffuse and react with inner CaO effectively, resulting in a significant improvement in initial CO2 carbonation capacity. A proper amount of Mg(2+) precursor solution was doped during a fast precipitation process to gain CaO-based sorbents with a high anti-sintering property, which maintained the porous spherical structure with the high specific surface area. CaO-based sorbents derived from a MgxCa1-xCO3 precursor existed in the form of CaO and MgO. The homogeneous distribution of MgO in the CaO-based sorbents effectively prevented the CaO crystallite from growing and sintering, further resulting in the favorable long-term durability with carbonation capacity of about 52.0% after 30 carbonation/calcination cycles.

CaO-based CO2 sorbents: from fundamentals to the development of new, highly effective materials

The enormous anthropogenic emission of the greenhouse gas CO2 is most likely the main reason for climate change. Considering the continuing and indeed growing utilisation of fossil fuels for electricity generation and transportation purposes, development and implementation of processes that avoid the associated emissions of CO2 are urgently needed. CO2 capture and storage, commonly termed CCS, would be a possible mid-term solution to reduce the emissions of CO2 into the atmosphere. However, the costs associated with the currently available CO2 capture technology, that is, amine scrubbing, are prohibitively high, thus making the development of new CO2 sorbents a highly important research challenge. Indeed, CaO, readily obtained through the calcination of naturally occurring limestone, has been proposed as an alternative CO2 sorbent that could substantially reduce the costs of CO2 capture. However, one of the major drawbacks of using CaO derived from natural sources is its rapidly decreasing CO2 uptake capacity with repeated carbonation-calcination reactions. Here, we review the current understanding of fundamental aspects of the cyclic carbonation-calcination reactions of CaO such as its reversibility and kinetics. Subsequently, recent attempts to develop synthetic, CaO-based sorbents that possess high and cyclically stable CO2 uptakes are presented.