Synthesis of highly efficient CaO-based, self-stabilizing CO2 sorbents via structure-reforming of steel slag

Review on the modifications of natural and industrial waste CaO based sorbent of calcium looping with enhanced CO2 capture capacity

The calcium looping cycle (CaL) possesses outstanding CO2 capture capacity for future carbon-capturing technologies that utilise CaO sorbents to capture the CO2 in a looping cycle. However, sorbent degradation and the presence of inert materials stabilise the sorbent, thereby reducing the CO2 capture capacity. Consequently, the CaO sorbent that has degraded must be replenished, increasing the operational cost for industrial use. CaO sorbents have been modified to enhance their CO2 capture capacity and stability. However, various CaO sorbents, including limestone, dolomite, biogenesis calcium waste and industrial waste, exhibit distinct behaviour in response to these modifications. Thus, this work comprehensively reviews the CO2 capture capacity of sorbent improvement based on various CaO sorbents. Furthermore, this study provides an understanding of the effects of CO2 capture capacity based on the properties of the CaO sorbent. The properties of various CaO sorbents, such as surface area, pore volume, particle size and morphology, are influential in exhibiting high CO2 capture capacity. This review provides insights into the future development of CaL technology, particularly for carbon-capturing technologies that focus on the modifications of CaO sorbents and the properties that affect the CO2 capture capacity.

Application of iron and steel slags in mitigating greenhouse gas emissions: A review

The comprehensive consideration of climate warming and by-product management in the iron and steel industry, has a significant impact on the realization of environmental protection and green production. Blast furnace slag (BFS) and steel slag (SS), collectively called iron and steel slags, are the main by-products of steelmaking. The economical and efficient use of iron and steel slags to reduce greenhouse gas (GHG) emissions is an urgent problem to be solved. This paper reviewed the carbonization and waste heat recovery of iron and steel slags, and the utilization of iron and steel slags as soil amendments, discussed their application status and limitations in GHG reduction. Iron and steel slags are rich in CaO, which can be used as CO₂ adsorbents to achieve a maximum concentration of 0.4-0.5 kg CO₂/kg SS. Blast furnace molten slag contains a considerable amount of waste heat, and thermal methods can recover more than 60 % of the heat energy. Chemical methods can use waste heat in the reaction to generate gas fuel, and iron in slags can be used as a catalytic component to promote chemical reaction. Waste heat recovery saves fuel and reduces the CO₂ emissions caused by combustion. When iron and steel slags are used as soil amendments, the iron oxides, alkaline substances, and SiO₂ in iron and steel slags can affect the emission of CH₄, N₂O, and CO₂ from soil, microorganisms, and crops, and achieve a maximum reduction of more than 60 % of the overall GHG of paddy fields. Finally, This paper provided valuable suggestions for future GHG reduction studies of iron and steel slags in energy, industry, and agriculture.

The asymmetric effect of technology shocks on CO2 emissions: a panel analysis of BRICS economies

Technological innovation positively contributes to economic development in BRICS countries; their environmental consequences cannot be ignored. Thus, it is imperious to explore the impact of technological shocks on environmental quality. We used ARDL and NARDL models to draw empirical consensus on the data set from 1990 to 2019 for BRICS economies. The results of ARDL model reveal that technological shocks positively affect carbon emissions in the long-run and short-run. The findings of NARDL model reveal that positive shocks in technology positively affect carbon emissions in the long-run and short-run, implying that an increase in technological development triggers an increase in carbon emissions. However, the negative shocks in technology have a negative impact on carbon emissions in the long-run, inferring that a reduction in technological development leads to a decrease in carbon emissions. The negative shock in technology has no significant impact on carbon emissions in the short-run. The findings emphasize the importance of environmental friendly technology to achieving sustainable development goals.

Decarbonising the iron and steel sector for a 2 °C target using inherent waste streams

The decarbonisation of the iron and steel industry, contributing approximately 8% of current global anthropogenic CO₂ emissions, is challenged by the persistently growing global steel demand and limitations of techno-economically feasible options for low-carbon steelmaking. Here we explore the inherent potential of recovering energy and re-using materials from waste streams, high-temperature slag, and re-investing the revenues for carbon capture and storage. In a pathway based on energy recovery and resource recycling of glassy blast furnace slag and crystalline steel slag, we show that a reduction of 28.5 ± 5.7% CO₂ emissions to the sectoral 2 °C target requirements in the iron and steel industry could be realized in 2050 under strong decarbonization policy consistent with low warming targets. The technological schemes applied to engineer this high-potential pathway could generate a revenue of US$35 ± 16 and US$40 ± 18 billion globally in 2035 and 2050, respectively. If this revenue is used for carbon capture and storage implementation, equivalent CO₂ emission to the 2 °C sectoral target requirements is expected to be reduced before 2050, without any external investments.

The iron and steel industry is emissions intensive. Here the authors explore its decarbonisation potential based on recovering energy and recycling materials from waste streams in 2020-2050. 28.5% of CO₂ emissions under sectoral 2 °C target requirements can be reduced in a high-potential pathway.

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.

Utilization of Malaysia EAF slags for effective application in direct aqueous sequestration of carbon dioxide under ambient temperature

Iron and steel industries are among the contributors of CO₂ emission in large volume into the atmosphere, causing detrimental effects to the environment and the ecosystem at large scale. These industries also generate solid wastes in the form of electric arc furnace (EAF) slag during operations which result in about 10–15% slag wastes per ton of steel produced. In this study, the EAF slags from an iron and steel-making factory in Klang, Malaysia was utilized for CO₂ sequestration through direct aqueous mineral carbonation. According to the surface area analysis, the fresh EAF slag has a mesoporous structure, its elemental composition shows the presence of 20.91 wt.% of CaO that was used for the sequestration of CO₂ through carbonation. The sequestration capacity was found to be 58.36 g CO₂/kg of slag at ambient temperature in 3 h, with the liquid/solid (L/S) ratio of 5:1 and using <63μm particle size. Moreover, the shrinking core model (SCM) was used to analyze the solid-fluid reaction in a heterogeneous phase and the CO₂ sequestration shows to be controlled by the product layer phase. The EAF slag is demonstrated to have the potential of CO₂ sequestration at ambient temperature.

Solid-Waste-Derived Carbon Dioxide-Capturing Materials

Solid sorbents are considered to be promising materials for carbon dioxide capture. In recent years, many studies have focused on the use of solid waste as carbon dioxide sorbents. The use of waste resources as carbon dioxide sorbents not only leads to the development of relatively low-cost materials, but also eliminates waste simultaneously. Different types of waste materials from biomass, industrial waste, household waste, and so forth were used as carbon dioxide sorbents with sufficient carbon dioxide capture capacities. Herein, progress on the development of carbon dioxide sorbents produced from waste materials is reviewed and covers key factors, such as the type of waste, preparation method, further modification method, carbon dioxide sorption performance, and kinetics studies. In addition, a new research direction for further study is proposed. It is hoped that this critical review will not merely sum up the major research directions in this field, but also provide significant suggestions for future work.

Accurate Control of Cage-Like CaO Hollow Microspheres for Enhanced CO2 Capture in Calcium Looping via a Template-Assisted Synthesis Approach

Herein we report the development of synthetic CaO-based sorbents for enhanced CO₂ capture in calcium looping via a template-assisted synthesis approach, where carbonaceous spheres (CSs) derived from hydrothermal reaction of starch are used as the templates. Cage-like CaO hollow microspheres are successfully synthesized only using urea as the precipitant, and the formation mechanism of this unique hollow microsphere structure is discussed deeply. Moreover, cage-like CaO hollow microspheres possess an initial carbonation conversion of 98.2% and 82.5% under a mild and harsh conditions, respectively. After the 15 cycles, cage-like CaO hollow microspheres still possess a carbonation value of 49.2% and 39.7% under the corresponding conditions, exceeding the reference limestone by 85.7% and 148.1%, respectively. Two kinetic models are used to explore the mechanism of carbonation reaction for cage-like CaO hollow microspheres, which are subsequently proved to be feasible for analysis of chemical-controlled stage and diffusion-controlled stage in the carbonation process. It is found the unique hollow microsphere structure can significantly reduce the activation energy of carbonation reaction according to the kinetic calculation. Furthermore, the energy and raw material consumptions related to the synthesis of cage-like CaO hollow microspheres are analyzed by the life cycle assessment (LCA) method.

Inorganic carbonate composites as potential high temperature CO2 sorbents with enhanced cycle stability

A calcium magnesium carbonate composite (CMC) material containing highly porous amorphous calcium carbonate (HPACC) and mesoporous magnesium carbonate (MMC) was synthesized. CMCs with varying HPACC : MMC mol ratios and high BET surface area (over 490 m² g⁻¹) were produced. The CMCs retained the morphology shared by HPACC and MMC. All these materials were built up of aggregated nanometer-sized particles. We tested the CO₂ uptake properties of the synthesized materials. The CMCs were calcined at 850 °C to obtain the corresponding calcium magnesium oxide composites (CMOs) that contained CaO : MgO at different mol ratios. CMO with CaO : MgO = 3 : 1 (CMO-3) showed comparable CO₂ uptake at 650 °C (0.586 g g⁻¹) to CaO sorbents obtained from pure HPACC (0.658 g g⁻¹) and the commercial CaCO₃ (0.562 g g⁻¹). Over 23 adsorption–desorption cycles CMOs also showed a lower CO₂ uptake capacity loss (35.7%) than CaO from HPACC (51.3%) and commercial CaCO₃ (79.7%). Al was introduced to CMO by the addition of Al(NO₃)₃ in the synthesis of CMC-3 to give ACMO after calcination. The presence of ∼19 mol% of Al(NO₃)₃ in ACMO-4 significantly enhanced its stability over 23 cycles (capacity loss of 5.2%) when compared with CMO-3 (calcined CMC-3) without adversely affecting the CO₂ uptake. After 100 cycles, ACMO-4 still had a CO₂ uptake of 0.219 g g⁻¹. Scanning electron microscope images clearly showed that the presence of Mg and Al in CMO hindered the sintering of CaCO₃ at high temperatures and therefore, enhanced the cycle stability of the CMO sorbents. We tested the CO₂ uptake properties of CMO and ACMO only under ideal laboratory testing environment, but our results indicated that these materials can be further optimized as good CO₂ sorbents for various applications.

A Ca/Mg/Al oxide composite was synthesised and showed a high CO₂ uptake of 0.537 g g⁻¹ at 650 °C with high uptake even after 100 cycles.

BOF steel slag as a low-cost sorbent for vanadium (V) removal from soil washing effluent

Soil washing is an effective remediation method to remove heavy metals from contaminated soil. However, it produces wastewater that contains large amounts of heavy metals, which lead to serious pollution. This study investigated the removal of vanadium (V) from synthetic soil washing effluent using BOF steel slag. The effects of particle size, slag dosage, initial pH, and initial vanadium concentration on removal behavior were studied. Adsorption kinetics and isotherms were also analyzed. The results showed that the vanadium removal efficiency increased as the steel slag particle size decreased and as the amount of slag increased. The initial pH and vanadium concentration did not play key roles. At the optimum particle size (<0.15 mm) and dosage (50 g/L), the removal rate reached 97.1% when treating 100 mg/L of vanadium. The influence of the washing reagent residue was studied to simulate real conditions. Citric acid, tartaric acid, and Na₂EDTA all decreased the removal rate. While oxalic acid did not have negative effects on vanadium removal at concentrations of 0.05–0.2 mol/L, which was proved by experiments using real washing effluents. Considering both soil washing effect and effluent treatment, oxalic acid of 0.2 mol/L is recommended as soil washing reagent.

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.

Direct mineral carbonation of steelmaking slag for CO2 sequestration at room temperature

Rapid increase of CO2 concentration in the atmosphere has forced the international community towards adopting actions to restrain from the impacts of climate change. Moreover, in India, the dependence on fossil fuels is projected to increase in the future, implying the necessity of capturing CO2 in a safe manner. Alkaline solid wastes can be utilized for CO2 sequestration by which its disposal issues in the country could also be met. The present work focuses to study direct mineral carbonation of steelmaking slag (SS) at room temperature and low-pressure conditions (<10 bar). Direct mineral carbonation of SS was carried out in a batch reactor with pure CO2 gas. The process parameters that may influence the carbonation of SS, namely, CO2 gas pressure, liquid to solid ratio (L/S) and reaction time were also studied. The results showed that maximum sequestration of SS was attained in the aqueous route with a capacity of 82 g of CO2/kg (6 bar, L/S ratio of 10 and 3 h). In the gas-solid route, maximum sequestration capacity of about 11.1 g of CO2/kg of SS (3 bar and 3 h) was achieved indicating that aqueous route is the better one under the conditions studied. These findings demonstrate that SS is a promising resource and this approach could be further developed and used for CO2 sequestration in the country. The carbonation process was evidenced using FT-IR, XRD, SEM and TG analysis.

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.

Use of steel slag for CO2 capture under realistic calcium-looping conditions

CaO derived from steel slag pretreated with acetic acid can be regenerated in short residence times at 900 °C and its capture capacity is greatly improved.

Characteristics and performance of CaO-based high temperature CO2 sorbents derived from a sol–gel process with different supports

10 CaO-based sorbents were synthesized by a sol–gel process supported with various materials, and their cyclic behavior was investigated under the same reaction conditions.