Applications of fly ash for CO2 capture, utilization, and storage

Harnessing Ash for Sustainable CO2 Absorption: Current Strategies and Future Prospects

This review explores the potential of using different types of ash, namely fly ash, biomass ash, and coal ash etc., as mediums for CO2 capture and sequestration. The diverse origins of these ash types - municipal waste, organic biomass, and coal combustion - impart unique physicochemical properties that influence their suitability and efficiency in CO2 absorption. This review first discusses the environmental and economic implications of using ash wastes, emphasizing the reduction in landfill usage and the transformation of waste into value-added products. Then the chemical/physical treatments of ash wastes and their inherent capabilities in binding or reacting with CO2 are introduced, along with current methodologies utilize these ashes for CO2 sequestration, including mineral carbonation and direct air capture techniques. The application of using ash wastes for CO2 capture are highlighted, followed by the discussion regarding challenges associated with ash-based CO2 absorption approach. Finally, the article projects into the future, proposing innovative approaches and technological advancements needed to enhance the efficacy of ash in combating the increasing CO2 levels. By providing a comprehensive analysis of current strategies and envisioning future prospects, this review aims to contribute to the field of sustainable CO2 absorption and environmental management.

A new strategy to stabilize the heavy metals in carbonized MSWI-fly ash using an acid-resistant oligomeric dithiocarbamate chelator

Fly ash (FA) derived from municipal solid waste incineration (MSWI) requires safe handling before landfilling due to its extremely high salt content and the risk of leaching heavy metals (HMs) under acidic conditions. Herein, aimed at improving the acid stability of dithiocarbamates, a cost-effective oligomeric dithiocarbamate (ODTC) was developed to stabilize HMs from carbonated MSWI-FA. Spiking of 3.6 wt% ODTC reduced the HM leaching below landfill standards in China, even across the pH range of 2.0-13.0 or 8-week exposure to the natural environment. Stabilization decreased the acid-soluble/exchangeable fractions of Cd, Pb, and Zn from 22.2%, 4.49%, and 21.9% to 0.14%, 0.11%, and 12.2%, respectively, resulting in safe levels for Pb and Cd with risk assessments. Compared to DDTC and SDD, ODTC exhibited higher stability under acidic conditions after chelation with the HMs, minimized the risk of HM leaching, and significantly reduced stabilization costs. In-depth studies proved that the stabilization mechanism involved the ability of ODTC to chelate HMs strongly and form acid-resistant ODTC-HM complexes, agglomeration of the MSWI-FA grains to encapsulate the ODTC-HM complexes, transformations of the HMs from acid-soluble species to stable oxidizable and residual species, and specifically ODTC reducing high-valent Pb to more stable Pb(II) species.

Synthesis and Surface Strengthening Modification of Silica Aerogel from Fly Ash

This study focuses on using activated fly ash to preparate silica aerogel by the acid solution-alkali leaching method and ambient pressure drying. Additionally, to improve the performance of silica aerogel, C6H16O3Si (KH-570) and CH3Si(CH3O)3 (MTMS) modifiers were used. Finally, this paper investigated the factors affecting the desilication rate of fly ash and analyzed the structure and performance of silica aerogel. The experimental results show that: (1) The factors affecting the desilication rate are ranked as follows: hydrochloric acid concentration > solid-liquid ratio > reaction temperature > reaction time. (2) KH-570 showed the best performance, and when the volume ratio of the silica solution to it was 10:1, the density of silica aerogel reached a minimum of 183 mg/cm3. (3) The optimal process conditions are a hydrochloric acid concentration of 20 wt%, a solid-liquid ratio of 1:4, a reaction time of two hours, and a reaction temperature of 100 °C. (4) The optimal performance parameters of silica aerogel were the thermal conductivity, specific surface area, pore volume, average pore size, and contact angle values, with 0.0421 W·(m·K)-1, 487.9 m2·g-1, 1.107 cm3·g-1, 9.075 nm, and 123°, respectively. This study not only achieves the high-value utilization of fly ash, but also facilitates the effective recovery and utilization of industrial waste.

Research on Utilizable Calcium from Calcium Carbide Slag with Different Extractors and Its Effect on CO2 Mineralization

With the increasing accumulation of alkaline industrial solid waste, the mineralization of CO2 using alkaline industrial solid waste has broad application prospects. Carbide slag is highly alkaline and contains a large amount of calcium elements, making it an excellent material for CO2 mineralization. Our idea was to acquire qualified products and fast kinetics by integrating carbide slag utilization and carbon reduction. The reaction route was divided into two steps: calcium extraction and carbonization. In order to achieve efficient extraction of utilizable calcium, we selected NH4Ac as the extraction agent, which has the advantage of buffer protection and environmental friendliness due to being an acetate radical. The extraction efficiency of utilizable calcium exceeded 90% under the conditions of L/S 20:1 and NH4+/Ca2+ 2:1. In the carbonization process, the crystal forms of CaCO3 synthesized by direct carbonation, acid extraction, and ammonium salt were characterized. The formation mechanism of vaterite in ammonium solution and the influence of impurities (Al3+, Mg2+) on the crystal transformation were revealed. This study provides technical support for using alkaline industrial waste to prepare high-purity vaterite. Therefore, alkaline industrial waste can be efficiently and sustainably utilized through CO2 mineralization.

Facile Route for Effective Separation and Full-Scale Recycling of Fly Ash and Unburned Carbon

The synchronous production of high-quality unburned carbon concentrate and cleaned ash from high LOI (loss on ignition) fly ash without yielding secondary solid waste is a dilemma issue. In this study, a viable flotation process with one rougher and two cleaners is developed for simultaneously obtaining carbon concentrate with a yield of 18.00% and an ash content of 17.49% and cleaned ash with a yield of 82.00% and a LOI of 4.63% from fly ash, reaching 84.72% of combustible substance recovery and 80.66% of flotation perfection index. The characteristic analyses of the stage by stage releasing products using laser particle size analysis, XRF, XRD, and SEM-EDS demonstrate that the inevitable factors that lead to a remaining higher ash content in the one-step flotation carbon concentrate are the random entrainment of mineral particles in the size range of 0-20 μm, especially the quasi-colloidal parts within 0-12.5 μm and the weak selective collection of fine-grained conjoined granules in the size range of 0-40 μm. Consequently, at least two cleaning steps are required for the effective separation of unburned carbon and ash. Furthermore, batch flotation test results show that diesel is superior to kerosene in the collection of unburned carbon, with an optimum dosage of 800 g/t; no. 2 oil acts more positively than MIBC for the separation of unburned carbon and ash, with an optimal dosing amount of 600 g/t; the optimum pulp concentration and flotation time are as follows: 100 g/L and 3.5 min for the rougher, 45 g/L and 2 min for the first cleaning, and 30 g/L and 3 min for the second cleaning. This study provides an economically feasible technological solution for the full-scale recovery of high-LOI fly ash in one step and avoids the problem of secondary solid waste that would have been generated in previous studies.

Performance Assessment of Carbon Dioxide Sequestration in Cement Composites with a Granulation Technique

The cement industry emits a significant amount of carbon dioxide (CO2). Therefore, the cement industry should recycle the emitted CO2. However, sequestration by carbonation in cement composites absorbs a very small amount of CO2. Therefore, a direct way of achieving this is to improve the absorption performance of CO2 in cement composites. In this study, to improve absorption, unlike in existing studies, a granulation technique was applied, and the material used was calcium hydroxide (CH). In addition, granulated CH was coated to prevent a reaction during the curing of cement paste. The coated CH granule (CCHG) was applied to 5% of the cement weight as an additive material, and the specimens were cured for 91 days to wait for the coating of CCHG to fully phase-change. The experiment of CO2 absorption showed an unexpected result, where the use of blast furnace slag (BFS) and fly ash (FA) had a negative effect on CO2 sequestration. This was because BFS and FA had a filler effect in the cement matrix, and the filler effect caused the blocking of the path of CO2. In addition, BFS and FA are well-known pozzolanic materials; the pozzolan reaction caused a reduction in the amount of CH because the pozzolan reaction consumed the CH to produce a calcium silicate hydrate. Therefore, the pozzolan reaction also had a negative effect on the CO2 sequestration performance combined with the filler effect. The CO2 sequestration efficiency was decreased between ordinary cement paste and BFS-applied specimens by 45.45%. In addition, compared to cases of ordinary cement paste and FA-applied specimens, the CO2 sequestration performance was decreased by 63.64%. Comprehensively, CO2 sequestration performance depends on the porosity and amount of CH.

Membrane Distillation-Crystallization for Sustainable Carbon Utilization and Storage

Anthropogenic greenhouse gas emissions from power plants can be limited using postcombustion carbon dioxide capture by amine-based solvents. However, sustainable strategies for the simultaneous utilization and storage of carbon dioxide are limited. In this study, membrane distillation-crystallization is used to facilitate the controllable production of carbonate minerals directly from carbon dioxide-loaded amine solutions and waste materials such as fly ash residues and waste brines from desalination. To identify the most suitable conditions for carbon mineralization, we vary the membrane type, operating conditions, and system configuration. Feed solutions with 30 wt % monoethanolamine are loaded with 5-15% CO2 and heated to 40-50 °C before being dosed with 0.18 M Ca2+ and Mg2+. Membranes with lower surface energy and greater roughness are found to more rapidly promote mineralization due to up to 20% greater vapor flux. Lower operating temperature improves membrane wetting tolerance by 96.2% but simultaneously reduces crystal growth rate by 48.3%. Sweeping gas membrane distillation demonstrates a 71.6% reduction in the mineralization rate and a marginal improvement (37.5%) on membrane wetting tolerance. Mineral identity and growth characteristics are presented, and the analysis is extended to explore the potential improvements for carbon mineralization as well as the feasibility of future implementation.

Decarbonatization of Energy Sector by CO2 Sequestration in Waste Incineration Fly Ash and Its Utilization as Raw Material for Alkali Activation

In this study, municipal solid waste incineration (MSWI) fly ash was subjected to mineral carbonation with the aim of investigating CO2 sequestration in waste material. The conducted study follows the trend of searching for alternatives to natural mineral materials with the ability to sequestrate CO2. The mineral carbonation of MSWI fly ash allowed for the storage of up to 0.25 mmol CO2 g-1. Next, both carbonated and uncarbonated MSWI fly ashes were activated using an alkaline activation method by means of two different activation agents, namely potassium hydroxide and potassium silicate or sodium hydroxide and sodium silicate. Mineral carbonation caused a drop in the compressive strength of alkali-activated materials, probably due to the formation of sodium and/or potassium carbonates. The maximum compressive strength obtained was 3.93 MPa after 28 days for uncarbonated fly ash activated using 8 mol dm-3 KOH and potassium hydroxide (ratio 3:1). The relative ratio of hydroxide:silicate also influenced the mechanical properties of the materials. Both carbonated and uncarbonated fly ashes, as well as their alkali-activated derivatives, were characterized in detail by means of XRD, XRF, and FTIR. Both uncarbonated and carbonated fly ashes were subjected to TG analysis. The obtained results have proved the importance of further research in terms of high-calcium fly ash (HCFA) utilization.

Effect of Water on CO2 Adsorption on CaNaY Zeolite: Formation of Ca2+(H2O)(CO2), Ca2+(H2O)(CO2)2 and Ca2+(H2O)2(CO2) Complexes

Efficient CO2 capture materials must possess a high adsorption capacity, suitable CO2 adsorption enthalpy and resistance to water vapor. We have recently reported that Ca2+ cations exchanged in FAU zeolite can attach up to three CO2 molecules. Here we report the effect of water on the adsorption of CO2. Formation of Ca2+(H2O)(CO2), Ca2+(H2O)(CO2)2 and Ca2+(H2O)2(CO2) mixed ligand complexes were established. The Ca2+(H2O)(CO2) species are readily formed even at ambient temperature and are characterized by ν(12CO2) and ν(13CO2) infrared bands at 2358 and 2293 cm-1, respectively. The Ca2+(H2O)(CO2)2 species are produced at low temperature and are identified by a ν(13CO2) band at 2291 cm-1. In the presence of large amounts of water, Ca2+(H2O)2(CO2) complexes were also evidenced by ν(12CO2) and ν(13CO2) bands at 2348 and 2283 cm-1, respectively. The results demonstrate that, although it has a negative effect on CO2 adsorption uptake, water in moderate amounts does not block CO2 adsorption sites.

Emerging waste-to-wealth applications of fly ash for environmental remediation: A review

The considerable increase in world energy consumption owing to rising global population, intercontinental transportation and industrialization has posed numerous environmental concerns. Particularly, in order to meet the required electricity supply, thermal power plants for electricity generation are widely used in many countries. However, an annually excessive quantity of waste fly ash up to 1 billion tones was globally discarded from the combustion of various carbon-containing feedstocks in thermoelectricity plants. About half of the industrially generated fly ash is dumped into landfills and hence causing soil and water contamination. Nonetheless, fly ash still contains many valuable components and possesses outstanding physicochemical properties. Utilizing waste fly ash for producing value-added products has gained significant interests. Therefore, in this work, we reviewed the current implementation of fly ash-derived materials, namely, zeolite and geopolymer as efficient adsorbents for the environmental treatment of flue gas and polluted water. Additionally, the usage of fly ash as a catalyst support for the photodegradation of organic pollutants and reforming processes for the corresponding wastewater remediation and H₂ energy generation is thoroughly covered. In comparison with conventional carbon-based adsorbents, fly ash-derived geopolymer and zeolite materials reportedly exhibited greater heavy metal ions removal and reached the maximum adsorption capacity of about 150 mg g^-1. As a support for biogas reforming process, fly ash could enhance the activity of Ni catalyst with 96% and 97% of CO₂ and CH₄ conversions, respectively.

Geochemical carbon dioxide removal potential of Spain

Many countries have made pledges to reduce CO₂ emissions over the upcoming decades to meet the Paris Agreement targets of limiting warming to no >1.5 °C, aiming for net zero by mid-century. To achieve national reduction targets, there is a further need for CO₂ removal (CDR) approaches on a scale of millions of tonnes, necessitating a better understanding of feasible methods. One approach that is gaining attention is geochemical CDR, encompassing (1) in-situ injection of CO₂-rich gases into Ca and Mg-rich rocks for geological storage by mineral carbonation, (2) ex-situ ocean alkalinity enhancement, enhanced weathering and mineral carbonation of alkaline-rich materials, and (3) electrochemical separation processes. In this context, Spain may host a notionally high geochemical CDR capacity thanks to its varied geological setting, including extensive mafic-ultramafic and carbonate rocks. However, pilot schemes and large-scale strategies for CDR implementation are presently absent in-country, partly due to gaps in current knowledge and lack of attention paid by regulatory bodies. Here, we identify possible materials, localities and avenues for future geochemical CDR research and implementation strategies within Spain. This study highlights the kilotonne to million tonne scale CDR options for Spain over the rest of the century, with attention paid to chemically and mineralogically appropriate materials, suitable implementation sites and potential strategies that could be followed. Mafic, ultramafic and carbonate rocks, mine tailings, fly ashes, slag by-products, desalination brines and ceramic wastes hosted and produced in Spain are of key interest, with industrial, agricultural and coastal areas providing opportunities to launch pilot schemes. Though there are obstacles to reaching the maximum CDR potential, this study helps to identify focused targets that will facilitate overcoming such barriers. The CDR potential of Spain warrants dedicated investigations to achieve the highest possible CDR to make valuable contributions to national reduction targets.

High pressure CO2 adsorption onto Malaysian Mukah-Balingian coals: Adsorption isotherms, thermodynamic and kinetic investigations

CO₂ sequestration into coalbed seams is one of the practical routes for mitigating CO₂ emissions. The adsorption mechanisms of CO₂ onto Malaysian coals, however, are not yet investigated. In this research CO₂ adsorption isotherms were first performed on dry and wet Mukah-Balingian coal samples at temperatures ranging from 300 to 348 K and pressures up to 6 MPa using volumetric technique. The dry S1 coal showed the highest CO₂ adsorption capacity of 1.3 mmol g^-1, at 300 K and 6 MPa among the other coal samples. The experimental results of CO₂ adsorption were investigated using adsorption isotherms, thermodynamics, and kinetic models. Nonlinear analysis has been employed to investigate the data of CO₂ adsorption onto coal samples via three parameter isotherm equilibrium models, namely Redlich Peterson, Koble Corrigan, Toth, Sips, and Hill, and four parameter equilibrium model, namely Jensen Seaton. The results of adsorption isotherm suggested that the Jensen Seaton model described the experimental data well. Gibb's free energy change values are negative, suggesting that CO₂ adsorption onto the coal occurred randomly. Enthalpy change values in the negative range established that CO₂ adsorption onto coal is an exothermic mechanism. Webber's pore-diffusion model, in particular, demonstrated that pore-diffusion was the main controlling stage in CO₂ adsorption onto coal matrix. The activation energy of the coals was calculated to be below -13 kJ mol^-1, indicating that adsorption of CO₂ onto coals occurred through physisorption. The results demonstrate that CO₂ adsorption onto coal matrix is favorable, spontaneous, and the adsorbed CO₂ molecules accumulate more onto coal matrix. The observations of this investigation have significant implications for a more accurate measurement of CO₂ injection into Malaysian coalbed seams.

Multi-Scale Computer-Aided Design of Covalent Organic Frameworks for CO2 Capture in Wet Flue Gas

Discovery of remarkable porous materials for CO₂ capture from wet flue gas is of great significance to reduce the CO₂ emissions, but elucidating the most critical structure features for boosting CO₂ capture capabilities remains a great challenge. Here, machine-learning-assisted Monte Carlo computational screening on 516 experimental covalent organic frameworks (COFs) identifies the superior secondary building units (SBUs) for wet flue gas separation using COFs, which are tetraphenylporphyrin units for boosting CO₂ adsorption uptake and functional groups for boosting CO₂/N₂ selectivity. Accordingly, 1233 COFs are assembled using the identified superior SBUs. Density functional theory calculation analysis on frontier orbitals, electrostatic potential, and binding energy reveals the influencing mechanism of the SBUs on the wet flue gas separation performance. The "electron-donating-induced vdW interaction" effect is discovered to construct the better-performing COFs, which can achieve high CO₂ uptake of 4.4 mmol·g^-1 with CO₂/N₂ selectivity of 104.8. Meanwhile, the "electron-withdrawing-induced vdW + electrostatic coupling interaction" effect is unearthed to construct the better-performing COFs with superior CO₂/N₂ selectivity, which can reach 277.6 with CO₂ uptake of 2.2 mmol·g^-1; in this case, H₂O plays a positive contribution in improving CO₂/N₂ selectivity. This work provides useful guidelines for designing optimized two-dimensional-COF adsorbents for wet flue gas separation.

Preparation and CO2 Adsorption Properties of Porous Particles Based on Alkaline Solid Wastes

Goaf filling is an effective method of preventing goaf disasters in mines. If the filling material can mineralize and absorb a large amount of CO₂, then the goaf will provide a huge amount of space for carbon storage, which will help to achieve carbon peaking and carbon neutrality. The purpose of this article is to prepare a kind of porous particle with high porosity and a large specific surface area that can be used to fill the mined-out area and adsorb a large amount of CO₂ by using the mineralized solid waste as the aggregate. Taking the compressive strength and CO₂ adsorption capacity as the objective function, a chemical foaming method and an orthogonal experiment were used to determine the optimal ratio of porous particles. The results showed that when the masses of carbide slag, oleic acid, cetyltrimethylammonium bromide, sodium hypochlorite, and water were 19.5, 0.3, 0.07, 1.35, and 15 g, respectively, the pore sizes of the prepared porous particles had a gradient distribution, i.e., micropores, mesopores, and macropores accounted for 18.75, 80.93, and 0.32%, respectively. In addition, the compressive strength reached 79.4 N, and the static CO₂ adsorption capacity was 117.43 cm³/g. The superimposed calculation of the adsorption capacity and mineralization capacity showed that 1 ton of solid waste can theoretically store approximately 0.66 ton of CO₂. The pseudo-first-order kinetic model can well fit the adsorption process of an adsorbent for CO₂, which proves that the adsorption process of adsorbent for CO₂ is a physical adsorption mechanism. The porous particles prepared based on solid waste are harmless and simple in the preparation process. They can fill the underground space of mines after adsorbing CO₂ and realize the integration of solid waste utilization, carbon storage, and underground space disaster management, with significant social, economic, and ecological benefits.

Carbon mineralization with concurrent critical metal recovery from olivine

Carbon dioxide utilization for enhanced metal recovery (EMR) during mineralization has been recently developed as part of CCUS (carbon capture, utilization, and storage). This paper describes fundamental studies on integrating CO₂ mineralization and concurrent selective metal extraction from natural olivine. Nearly 90% of nickel and cobalt extraction and mineral carbonation efficiency are achieved in a highly selective, single-step process. Direct aqueous mineral carbonation releases Ni²⁺ and Co²⁺ into aqueous solution for subsequent recovery, while Mg²⁺ and Fe²⁺ simultaneously convert to stable mineral carbonates for permanent CO₂ storage. This integrated process can be completed in neutral aqueous solution. Introduction of a metal-complexing ligand during mineral carbonation aids the highly selective extraction of Ni and Co over Fe and Mg. The ligand must have higher stability for Ni-/Co- complex ions compared with the Fe(II)-/Mg- complex ions and divalent metal carbonates. This single-step process with a suitable metal-complexing ligand is robust and utilizes carbonation processes under various kinetic regimes. This fundamental study provides a framework for further development and successful application of direct aqueous mineral carbonation with concurrent EMR. The enhanced metal extraction and CO₂ mineralization process may have implications for the clean energy transition, CO₂ storage and utilization, and development of new critical metal resources.

The Global Carbon Footprint and How New Carbon Mineralization Technologies Can Be Used to Reduce CO2 Emissions

Carbon dioxide is a byproduct of our industrial society. It is released into the atmosphere, which has an adverse effect on the environment. Carbon dioxide management is necessary to limit the global average temperature increase to 1.5 degrees Celsius and mitigate the effects of climate change, as outlined in the Paris Agreement. To accomplish this objective realistically, the emissions gap must be closed by 2030. Additionally, 10–20 Gt of CO2 per year must be removed from the atmosphere within the next century, necessitating large-scale carbon management strategies. The present procedures and technologies for CO2 carbonation, including direct and indirect carbonation and certain industrial instances, have been explored in length. This paper highlights novel technologies to capture CO2, convert it to other valuable products, and permanently remove it from the atmosphere. Additionally, the constraints and difficulties associated with carbon mineralization have been discussed. These techniques may permanently remove the CO2 emitted due to industrial society, which has an unfavorable influence on the environment, from the atmosphere. These technologies create solutions for both climate change and economic development.

Recent Progress of SAPO-34 Zeolite Membranes for CO2 Separation: A Review

In the zeolite family, the silicoaluminophosphate (SAPO)-34 zeolite has a unique chemical structure, distinctive pore size, adsorption characteristics, as well as chemical and thermal stability, and recently, has attracted much research attention. Increasing global carbon dioxide (CO₂) emissions pose a serious environmental threat to humans, animals, plants, and the entire environment. This mini-review summarizes the role of SAPO-34 zeolite membranes, including mixed matrix membranes (MMMs) and pure SAPO-34 membranes in CO₂ separation. Specifically, this paper summarizes significant developments in SAPO-34 membranes for CO₂ removal from air and natural gas. Consideration is given to a variety of successes in SAPO-34 membranes, and future ideas are described in detail to foresee how SAPO-34 could be employed to mitigate greenhouse gas emissions. We hope that this study will serve as a detailed guide to the use of SAPO-34 membranes in industrial CO₂ separation.

A Review on SAPO-34 Zeolite Materials for CO2 Capture and Conversion

Among several known zeolites, silicoaluminophosphate (SAPO)-34 zeolite exhibits a distinct chemical structure, unique pore size distribution, and chemical, thermal, and ion exchange capabilities, which have recently attracted considerable research attention. Global carbon dioxide (CO₂ ) emissions are a serious environmental issue. Current atmospheric CO₂ level exceeds 414 parts per million (ppm), which greatly influences humans, fauna, flora, and the ecosystem as a whole. Zeolites play a vital role in CO₂ removal, recycling, and utilization. This review summarizes the properties of the SAPO-34 zeolite and its role in CO₂ capture and separation from air and natural gas. In addition, due to their high thermal stability and catalytic nature, CO₂ conversions into valuable products over single metal, bi-metallic, and tri-metallic catalysts and their oxides supported on SAPO-34 were also summarized. Considering these accomplishments, substantial problems related to SAPO-34 are discussed, and future recommendations are offered in detail to predict how SAPO-34 could be employed for greenhouse gas mitigation.

Removal of Inorganic Salts in Municipal Solid Waste Incineration Fly Ash Using a Washing Ejector and Its Application for CO2 Capture

This study investigated the effects of washing equipment for inorganic salts, such as NaCl, KCl, and CaClOH, to decontaminate municipal solid waste incineration fly ash (MSW-IFA). Based on the feature of hydrodynamic cavitation, the device developed in this study (referred to as a ‘washing ejector’) utilizes the cavitation bubbles. A washing ejector was analyzed under a range of conditions, employing as little water as possible. In hydrodynamic cavitation, the increase in fluid pressure with increasing static pressure is mainly attributed to the increase in particle–bubble collisions via the cavitation flow. The results revealed that the fluid pressure influenced the removal of inorganic salts during cavitation in water. This is because during the washing process from the collapse of cavitation bubbles, the release is achieved through the dissolution of inorganic salts weakly bound to the surface. After treatment by a washing ejector, the removal of soluble salts elements such as Cl, Na, and K was reduced by approximately 90%. Removing the inorganic salts in the IFA altered the characteristics of the Ca-related phase, and amorphous CaCO₃ was formed as the cavitation flow reacted with CO₂ in the ambient air. Furthermore, the washing effluent produced by washing IFA was found to be beneficial for CO₂ capture. The washing effluent was enriched with dissolved Ca from the IFA, and the initial pH was the most favorable condition for the formation of CaCO₃; thus, the effluent was sufficient for use as a CO₂ sequestration medium and substitute for the reuse of water. Overall, the process presented herein could be effective for removing soluble salts from IFA, and this process is conducive to utilizing IFA as a resource.

Carbon dioxide emission control of a vermicompost process using fly ash

Composting and vermicomposting generate a valuable product rich in plant nutrients and at the same time, reduce environmental pollution. However, along with these processes and in relation to the metabolism of the microorganism and the worms present in the vermicomposting, CO₂ is emitted to the atmosphere, contributing to the greenhouse effect. Taking these issues into account, different masses of fly ash were used to study the control of the CO₂ of the gas coming from a vermicomposting process and to evaluate the possibility of using the adsorbent as fertilizer in the culture of lettuce Lactuca sativa. Along the vermicomposting process, an increase in the concentration of CO₂ emissions was observed, with a maximum level of emission at the day 20 of the process and an average of 770 mg/L in air. After the adsorption process, the CO₂ concentration was lower due to the effect of the fly ash that was able to trap the emitted gas. The percentage of CO₂ adsorption shows maximum values of 55.5, 58.1 and 63.8% with 0.5, 1 and 1.5 kg of fly ash, respectively. The CO₂ uptake capacities of the different loads of fly ash used were 3.39, 7.03 and 6.84 mmol CO₂/g sorbent with 0.5, 1 and 1.5 kg of fly ash, respectively. After five weeks of sowing L. sativa, it was observed that when no fly ash was used in the soil, the length of the stem was 10.2 cm. Then, the length of the stem was 22 cm, and 16 cm when 10% of fly ash was applied and not applied in the adsorption process, getting a significant correlation between the load of fly ash and the length of the stem. The r when fly ash was used in the adsorption process was 0.9817 and 0.9811 when no ash fly was used in the process.

CO2 adsorption on PEHA-functionalized geothermal silica waste: a kinetic study and quantum chemistry approach

The use of geothermal silica waste to prepare amine-modified CO2 adsorbent materials was succesfully tested.

Performance and mechanisms of fly ash for graphene oxide removal from aqueous solution

The potential wide use of graphene oxide in various fields results in the possibility of its dispersion throughout natural water systems, with a negative impact on organisms and ecosystems. This study evaluated the removal of graphene oxide (GO) from water by fly ash (FA). The effects of various conditions (including the initial concentration of graphene oxide, the pH of the initial solution, the amount of absorbent, and temperature) on the removal rate of GO were investigated in detail. The results show that the maximum removal rate of graphene oxide by fly ash is 93%; the isotherm adsorption process conforms to a Langmuir model; the adsorption reaction is a spontaneous exothermic process. Under optimal conditions, the pH of the solution was adjusted to 6, the amount of fly ash was 5 mg, the initial concentration of GO was 60 mg·L^-1, and the temperature was 303 K. Using X-ray diffraction (XRD), Raman spectroscopy (Raman), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscopy (TEM), Zeta potential and X-ray electron spectroscopy (XPS), the adsorption mechanism was characterized. The experimental results demonstrate that fly ash is a good material for GO removal from aqueous solution.

Numerical characterization of the plasma arc with various Ar-CO2 mixtures

Plasma technology finds applications in many industrial areas. To improve the process efficiency, different plasma forming gases and their mixtures are used. Since CO₂ is economical and has high thermal conductivity, it is identified as a potential candidate in plasma material processing. However, the characteristics and merits of CO₂ plasma arc in material processing are scanty. In this work, a 2D model is built to simulate plasma arc with CO₂ and its mixtures with Ar. Effects of arc current, gas composition and arc length on the arc heating efficiency and arc characteristics are discussed. The arc size is strongly influenced by the gas composition at 200 A than at 100 A. Temperature distribution in the arcs is wider when CO₂ content is decreased. The effect of arc current on the arc heating efficiency is stronger at lower current. Results predicted by the present model are compared with measured data for model validation.

Advanced strategies in Metal-Organic Frameworks for CO2 Capture and Separation

The continuous carbon dioxide (CO₂ ) gas emissions associated with fossil fuel production, valorization, and utilization are serious challenges to the global environment. Therefore, several developments of CO₂ capture, separation, transportation, storage, and valorization have been explored. Consequently, we documented a comprehensive review of the most advanced strategies adopted in metal-organic frameworks (MOFs) for CO₂ capture and separation. The enhancements in CO₂ capture and separation are generally achieved due to the chemistry of MOFs by controlling pore window, pore size, open-metal sites, acidity, chemical doping, post or pre-synthetic modifications. The chemistry of defects engineering, breathing in MOFs, functionalization in MOFs, hydrophobicity, and topology are the salient advanced strategies, recently reported in MOFs for CO₂ capture and separation. Therefore, this review summarizes MOF materials' advancement explaining different strategies and their role in the CO₂ mitigations. The study also provided useful insights into key areas for further investigations.

Current and future perspectives on catalytic-based integrated carbon capture and utilization

There exist several well-known methods with varying maturity for capturing carbon dioxide from emission sources of different concentrations, including absorption, adsorption, cryogenics and membrane separation, among others. The capture and separation steps can produce almost pure CO₂, but at substantial cost for being conditioned for transport and final utilization, with high economical risks to be considered. A possible way for the elimination of this conditioning and cost is direct CO₂ utilization, whether on-site in a further process but within the same plant, or in-situ, coupling both capture and conversion in the same unit. This approach is usually called integrated carbon capture and utilization (ICCU) or integrated carbon capture and conversion (ICCC), and has lately started receiving considerable attention in many circles. As CO₂ is already industrially employed in other sectors, such as food preservation, water treatment and conversion to high added-value chemicals and fuels such as methanol, methane, etc., among others, it is of great interest to explore the global ICCC approach. Catalytic-based processes play a key role in CO₂ conversion, and different technologies are gaining great attention from both academia and industry. However, the 'big picture of ICCU' and in which technology the efforts should focus on at large scale is still unclear. This review analyzes some promising concepts of ICCU specifically on CO₂ catalytic conversion, highlighting their current commercial relevance as well as challenges that have to be faced today and in the next future.

Decarbonization in ammonia production, new technological methods in industrial scale ammonia production and critical evaluations

With the synthesis of ammonia with chemical methods, global carbon emission is the biggest threat to global warming. However, the dependence of the agricultural industry on ammonia production brings with it various research studies in order to minimize the carbon emission that occurs with the ammonia synthesis process. In order to completely eliminate the carbon emissions from ammonia production, both the hydrogen and the energy needed for the operation of the process must be obtained from renewable sources. Thus, hydrogen can be produced commercially in a variety of ways. Many processes are discussed to accompany the Haber Bosch process in ammonia production as potential competitors. In addition to parameters such as temperature and pressure, various plasma catalysts are being studied to accelerate the ammonia production reaction. In this study, various alternative processes for the capture, storage and complete removal of carbon gas released during the current ammonia production are evaluated and the current conditions related to the applicability of these processes are discussed. In addition, it has been discussed under which conditions it is possible to produce larger capacities as needed in the processes studied in order to reduce carbon gas emissions during ammonia production in order to provide raw material source for fertilizer production and energy sector. However, if the hydrogen gas required for ammonia production is produced using a solid oxide electrolysis cell, the reduction in the energy requirement of the process and in this case the reduction of energy costs shows that it will play an important role in determining the method to be used for ammonia production. In addition, it is predicted that working at lower temperature (<400 °C) and pressure (<10 bar) values in existing ammonia production technologies, despite increasing possible energy costs, will significantly reduce process operating costs.

Utilization of low-calcium fly ash via direct aqueous carbonation with a low-energy input: Determination of carbonation reaction and evaluation of the potential for CO2 sequestration and utilization

Environmental impacts from coal-fired power generation that produces large amounts of CO₂ and fly ash are of great interest. To reduce negative environmental impacts, fly ash utilization was investigated via a direct aqueous carbonation with a low-energy input in which the alkali calcium content in the fly ash reacted with CO₂ to form carbonate. Raw fly ash was characterized to understand the potential for direct aqueous carbonation of fly ash. The performance of the fly ash as a calcium source for direct aqueous carbonation at atmospheric pressure was investigated for different solid-liquid ratios and introduced CO₂ concentrations. Variations in fly ash elemental composition, reaction solution pH, CO₂ concentration in the reactor outlet, CO₂ uptake efficiency, CaCO₃ content and degree of carbonation were used to illustrate this process reaction. The maximum CO₂ uptake efficiency was ~0.016 g-CO₂/g-fly ash. This value was compared with previous studies, and the CO₂ uptake efficiency was comparable despite the use of a low-energy input method, i.e., direct aqueous carbonation with atmospheric pressure and unconcentrated CO₂. The calculated maximum degree of carbonation was 31.0%, which corresponds to 0.0063 g-CO₂/g-fly ash. Carbonated product characterization confirmed the carbonation reaction mechanism and safety for further utilization. A comparison of CO₂ uptake efficiency in this work with previous work, and considering the energy input and reactive species content, is provided. An assessment of the CO₂ reduction potential is provided.

Visible-light-driven photocatalysis of carbon dioxide and organic pollutants by MFeO2 (M = Li, Na, or K)

In recent years, lithium-containing ceramic materials have attracted considerable research attention as high-temperature adsorbents of carbon dioxide. The recycling of electrode materials from spent lithium-ion batteries for use as photocatalysts in recovering CO₂ and degrading organic pollutants is worthy of exploration. Solid, magnetic ferrite-containing photocatalysts are easily separated from reaction solutions by using magnetic devices. Solid catalysts (e.g., LiFeO₂, LiFe₅O₈, NaFeO₂, and K₂Fe₂O₄) were prepared through the calcination of Fe₂O₃ and M₂CO₃. CO₂ was photoreduced and crystal violet (CV) and 2-hydroxybenzoic acid (2-HBA) were photodegraded under visible light irradiation. The optimized K₂Fe₂O₄ photocatalyst increased the rate of photocatalytic conversion from CO₂ to methane at 20.9 µmol g^-1 h^-1. The catalytic efficiency indicated that the optimized reaction rate constants of CV with LiFeO₂, NaFeO₂, and K₂Fe₂O₄ were 2.98 × 10^-1, 5.32 × 10^-1, and 4.36 × 10^-1 h^-1, respectively. The quenching effect achieved through the use of various scavengers and the electron paramagnetic resonance in CV degradation revealed the substantial contribution of the reactive superoxide anion radical O₂^- and the minor roles of h⁺ and the OH radical. Its usefulness in the synthesis of solid-base catalyst MFeO₂ is promising for environmental control and relevant applications, particularly in solar energy manufacturing.

The Potential Use of Fly Ash from the Pulp and Paper Industry as Thermochemical Energy and CO2 Storage Material

As a part of our research in the field of thermochemical energy storage, this study aims to investigate the potential of three fly ash samples derived from the fluidized bed reactors of three different pulp and paper plants in Austria for their use as thermochemical energy (TCES) and CO2 storage materials. The selected samples were analyzed by different physical and chemical analytical techniques such as X-ray fluorescence spectroscopy (XRF), X-ray diffraction (XRD), particle size distribution (PSD), scanning electron microscopy (SEM), inductively coupled plasma atomic emission spectroscopy (ICP-OES), and simultaneous thermal analysis (STA) under different atmospheres (N2, CO2, and H2O/CO2). To evaluate the environmental impact, leaching tests were also performed. The amount of CaO as a promising candidate for TCES was verified by XRF analysis, which was in the range of 25–63% (w/w). XRD results indicate that the CaO lies as free lime (3–32%), calcite (21–29%), and silicate in all fly ash samples. The results of STA show that all fly ash samples could fulfill the requirements for TCES (i.e., charging and discharging). A cycling stability test of three cycles was demonstrated for all samples which indicates a reduction of conversion in the first three reaction cycles. The energy content of the examined samples was up to 504 kJ/kg according to the STA results. More energy (~1090 kJ/kg) in the first discharging step in the CO2/H2O atmosphere could be released through two kinds of fly ash samples due to the already existing free lime (CaO) in those samples. The CO2 storage capacity of these fly ash samples ranged between 18 and 110 kg per ton of fly ash, based on the direct and dry method. The leaching tests showed that all heavy metals were below the limit values of the Austrian landfill ordinance. It is viable to say that the valorization of fly ash from the pulp and paper industries via TCES and CO2 storage is plausible. However, further investigations such as cycling stability improvement, system integration and a life cycle assessment (LCA) still need to be conducted.

Current Developments of Carbon Capture Storage and/or Utilization–Looking for Net-Zero Emissions Defined in the Paris Agreement

An essential line of worldwide research towards a sustainable energy future is the materials and processes for carbon dioxide capture and storage. Energy from fossil fuels combustion always generates carbon dioxide, leading to a considerable environmental concern with the values of CO2 produced in the world. The increase in emissions leads to a significant challenge in reducing the quantity of this gas in the atmosphere. Many research areas are involved solving this problem, such as process engineering, materials science, chemistry, waste management, and politics and public engagement. To decrease this problem, green and efficient solutions have been extensively studied, such as Carbon Capture Utilization and Storage (CCUS) processes. In 2015, the Paris Agreement was established, wherein the global temperature increase limit of 1.5 °C above pre-industrial levels was defined as maximum. To achieve this goal, a global balance between anthropogenic emissions and capture of greenhouse gases in the second half of the 21st century is imperative, i.e., net-zero emissions. Several projects and strategies have been implemented in the existing systems and facilities for greenhouse gas reduction, and new processes have been studied. This review starts with the current data of CO2 emissions to understand the need for drastic reduction. After that, the study reviews the recent progress of CCUS facilities and the implementation of climate-positive solutions, such as Bioenergy with Carbon Capture and Storage and Direct Air Capture. Future changes in industrial processes are also discussed.

Fixation stability of glass matrix co-existent with crystal phases for heavy metals formed by high-temperature vitrification

Vitrification is an effective solidification method for heavy metal-containing wastes. However, most investigations focused on the formation of glass matrix. Seldom report discussed the influence of co-existing crystals on heavy metal stabilizations. In this work, Ca-Al-Si phase was formed in the glass matrix by adjusting the composition of feeding ingredient and melting temperature. As a result, when molar ratio of CaO/(SiO₂+Al₂O₃) was lower than 0.97 and reaction temperature was bigger than 1300 °C, small-size Ca-Al-Si phase (Ca₂Al₂SiO₇ and CaAl₂Si₂O₈) was homogeneously distributed in vitreous matrix. At the same time, Cr, Zn, and Pb leaching concentrations were the lowest, far lower than the leaching standard values. According to theoretical calculations, Zn and Pb replaced Ca atom; Cr replaced Al atom in Ca-Al-Si phase under thermal conditions. These replacements resulted in the fixation and stabilization of heavy metals. When the CaO/(SiO₂+Al₂O₃) molar ratio was bigger than 1.00, neither glass nor Ca-Al-Si was formed. Similarly, when the melting temperature was decreased, Ca-Al-Si phase formed a bigger size. Both these went against the stabilization, resulting in high leaching concentrations of heavy metals. The main of this work will help the development of high-temperature melting for the treatment of hazardous wastes.

Uncovering the dynamics in global carbon dioxide utilization research: a bibliometric analysis (1995-2019)

The anthropogenic emission of carbon dioxide (CO₂) into the atmosphere is recognized as the main contributor to global climate change. To date, scientists have developed various strategies, including CO₂ utilization technologies, to reduce global carbon emissions. This paper presents the global scientific landscape of the CO₂ utilization research from 1995 to 2019 based on a bibliometric analysis of 1875 publications extracted from Web of Science. The findings indicate a major increase in the number of publications and citations received from 2015 to 2019, denoting a fast-emerging research trend. The dynamics of global CO₂ utilization research is partly driven by China's policies and research funding to promote low-carbon economic development. Applied Energy is recognized as a core journal in this research topic. The utilization of CO₂ is a multidisciplinary topic that has progressed by multidimensional collaborations at the country and organizations levels, while the formation of co-authorship networks at the individual level is mostly influenced by the authors' affiliations. Keyword co-occurrence analysis reveals a rapid evolution in the CO₂ utilization strategies from chemical fixation in carbonates and epoxides to pilot-scale testing of power-to-gas technologies in Europe and the USA. The development of efficient power-to-fuel technologies and biological utilization routes (using microalgae and bacteria) will probably be the next research priorities in CO₂ utilization research.

Green Synthesis of 2-Oxazolidinones by an Efficient and Recyclable CuBr/Ionic Liquid System via CO2, Propargylic Alcohols, and 2-Aminoethanols

With the aim of profitable conversion of carbon dioxide (CO2) in an efficient, economical, and sustainable manner, we developed a CuBr/ionic liquid (1-butyl-3-methylimidazolium acetate) catalytic system that could efficiently catalyze the three-component reactions of propargylic alcohols, 2-aminoethanols, and CO2 to produce 2-oxazolidinones and α-hydroxy ketones. Remarkably, this catalytic system employed lower metal loading (0.0125–0.5 mol%) but exhibited the highest turnover number (2960) ever reported, demonstrating its excellent activity and sustainability. Moreover, our catalytic system could efficiently work under 1 atm of CO2 pressure and recycle among the metal-catalyzed systems.

A technical review of bioenergy and resource recovery from municipal solid waste

Population growth, rapid urbanization, industrialization and economic development have led to the magnified municipal solid waste generation at an alarming rate on a global scale. Municipal solid waste seems to be an economically viable and attractive resource to produce green fuels through different waste-to-energy conversion routes. This paper reviews the different waste-to-energy technologies as well as thermochemical and biological conversion technologies for the valorization of municipal solid waste and diversion for recycling. The key waste-to-energy technologies discussed in this review include conventional thermal incineration and the modern hydrothermal incineration. The thermochemical treatments (e.g. pyrolysis, liquefaction and gasification) and biological treatments (e.g. anaerobic digestion and composting) are also elaborated for the transformation of solid wastes to biofuel products. The current status of municipal solid waste management for effective disposal and diversion along with the opportunities and challenges has been comprehensively reviewed. The merits and technical challenges of the waste-to-energy technologies are systematically discussed to promote the diversion of solid wastes from landfill disposal to biorefineries.

Sound-Assisted Fluidization for Temperature Swing Adsorption and Calcium Looping: A Review

Fine/ultra-fine cohesive powders find application in different industrial and chemical sectors. For example, they are considered in the framework of the Carbon Capture and Storage (CCS), for the reduction of the carbon dioxide emissions to the atmosphere, and in the framework of the thermochemical energy storage (TCES) in concentrated solar power (CSP) plants. Therefore, developing of technologies able to handle/process big amounts of these materials is of great importance. In this context, the sound-assisted fluidized bed reactor (SAFB) designed and set-up in Naples represents a useful device to study the behavior of cohesive powders also in the framework of low and high temperature chemical processes, such as CO₂ adsorption and Ca-looping. The present manuscript reviews the main results obtained so far using the SAFB. More specifically, the role played by the acoustic perturbation and its effect on the fluid dynamics of the system and on the performances/outcomes of the specific chemical processes are pointed out.

Coal Fly Ash Derived Silica Nanomaterial for MMMs-Application in CO2/CH4 Separation

In order to obtained high selective membrane for industrial applications (such as natural gas purification), mixed matrix membranes (MMMs) were developed based on polysulfone as matrix and MCM-41-type silica material (obtained from coal fly ash) as filler. As a consequence, various quantities of filler were used to determine the membranes efficiency on CO₂/CH₄ separation. The coal fly ash derived silica nanomaterial and the membranes were characterized in terms of thermal stability, homogeneity, and pore size distribution. There were observed similar properties of the obtained nanomaterial with a typical MCM-41 (obtained from commercial silicates), such as high surface area and pore size distribution. The permeability tests highlighted that the synthesized membranes can be applicable for CO₂ removal from CH₄, due to unnoticeable differences between real and ideal selectivity. Additionally, the membranes showed high resistance to CO₂ plasticization, due to permeability decrease even at high feed pressure, up to 16 bar.

Mechanistic insight into methane dry reforming over cobalt: a density functional theory study

Cobalt-based catalysts are a potential candidate among non-noble metal catalysts in dry reformation of methane (DRM), while the detailed mechanism of the DRM reaction is still largely unknown. In this contribution, the rather complicated reaction network for DRM is explored by density functional theory calculations. The most favorable adsorption structures of all species involved in the DRM reaction over Co(0001) have been identified. For CO2 activation, its direct dissociation to generate CO and O is the dominant reaction pathway. For CH4 direct dissociation, CH dehydrogenation into atomic C and H is the rate-determining step (RDS). It is predicted that the CH is the most abundant species among CHx (x = 0-3) over Co(0001). O acts as an oxidant and reacts with CH to produce CHO, and subsequently, CHO decomposes into CO and H. Atomic C may directly react with O to produce CO, or be oxidized by OH to COH, followed by the COH decomposition to CO and H. Thus, three possible pathways for DRM over the Co(0001) surface are proposed in our study, and the oxidation step is suggested as the RDS. The dominant route is identified as CH4 successive dissociation into CH, and CH oxidizing by O to form CHO, then CHO decomposition to CO and H.