Calcium-Rich Industrial Wastes as Potential Sorbents for Cyclic CO₂ Capture in the Gas–Solid Carbonation–Calcination Looping

Abstract

Calcium-rich industrial wastes may serve as low-cost sorbents for near-source CO₂ capture, but their practical potential depends on the reactive calcium species and cyclic stability. This study compares steel slag (SS), air pollution control residue (APCr), and cement kiln dust (CKD) under controlled thermogravimetric analysis (TGA) conditions for direct gas–solid carbonation and calcination looping. X-ray diffraction identified Ca(OH)₂ in SS, CaClOH in APCr, and calcite-derived CaO in the calcined CKD as the principal reactive calcium species, corresponding to theoretical CO₂ sequestration capacities of 117.0, 58.2, and 365.2 g CO₂ kg⁻¹ waste, respectively, based on the reference intensity ratio method. After the isothermal carbonation for 1 h, experimental carbon sequestration capacities or CO₂ uptakes were 84, 39, and 201 g CO₂ kg⁻¹ waste, equivalent to conversion rates of 71.8, 67.0, and 55.0%. The CO₂ uptake curves showed that the carbonation kinetics of these wastes obeys a two-stage regime featuring an initial rapid carbonation stage followed by a slower one restricted by the product-layer diffusion of CO₂. Among the investigated industrial wastes, CKD exhibited the highest cyclic uptake of CO₂ and the lowest observed cyclic deactivation, whereas the lower cyclic performance of SS and APCr was largely limited by the calcium encapsulation and chloride-induced high-temperature sintering, respectively. This study provides an alternative solution for the valorization of industrial solid waste according to the “waste-for-waste” concept.

1. Introduction

Carbonation of calcium-rich industrial solid wastes has been investigated as a route that may couple CO₂ capture, mineral storage, and resource utilization, while still requiring a clearly defined process boundary and net-emission assessment. Considering that the thermodynamic basis for stable carbonate formation, the acceleration of natural carbonation, and the replacement of naturally occurring minerals by alkaline industrial residues have been well established [1,2,3,4,5,6,7,8,9,10,11], recent reviews of industrial-waste mineral carbonation and medium- to high-temperature solid-oxide sorbents show that the value of these routes depends not only on CO₂ uptake, but also on energy input, water demand, sorbent deactivation, product utilization, and the net CO₂ boundary [12,13,14,15,16]. Reported CO₂ capture capacities for steel slag, municipal solid-waste incineration residues, cement kiln dust, fly ash, and other alkaline residues vary from several kilograms to several hundred kilograms of CO₂ per tonne of waste. The controlling factors are therefore not limited to total calcium content, but include reactive Ca/Mg species, mass-transfer conditions, reaction pathways, cyclic CO₂ capture stability, and downstream product utilization and environmental risk [17,18,19,20]. Among these, the reactive calcium species and cyclic stability are crucial factors in determining whether an industrial solid waste can be developed into a CO₂ sorbent.

The ability of calcium-rich solid wastes to capture CO₂ is based on the CaO, Ca(OH)₂, calcium silicates, calcium aluminates, CaClOH, and other alkaline calcium-bearing phases that can directly react with CO₂ or release reactive calcium species. The core reaction is the formation of thermodynamically stable CaCO₃ via the carbonation reaction, but the source and accessibility of reactive calcium species differ markedly among wastes. In steel slag, the calcium-containing phases mainly include free CaO, Ca(OH)₂, and calcium silicates that form after the lime or limestone flux is added during steelmaking; part of the calcium is encapsulated in complex silicate or Fe–Mg phases, so the effective reactivity is lower than that implied by the total content of calcium [21]. In municipal solid-waste incineration air pollution control residue (APCr), calcium is mainly derived from slaked lime or lime slurry used for semi-dry or dry acid-gas removal. In addition to unreacted Ca(OH)₂, APCr commonly contains CaClOH, CaSO₄, NaCl, KCl, and other salts. These components may provide reactive calcium species for carbonation, but can also promote melting, sintering, and pore blockage during the high-temperature gas–solid carbonation process [22,23]. For example, after 20 carbonation–calcination cycles, the CO₂ uptake of APCr could be decreased to 6% to 94% relative to that in the first cycle [22]. Cement kiln dust (CKD) commonly contains CaCO₃, CaO, Ca(OH)₂, and clinker minerals; its fine particle size and high alkalinity have led to its evaluation as a promising material for CO₂ fixation [24,25]. Importantly, the carbonation stabilization of these alkaline industrial wastes helps prevent the secondary environmental exposure of heavy metals and other potentially hazardous trace elements [17,18,20,23,24,25].

According to the reaction medium and process route, CO₂ capture or fixation using calcium-rich wastes can be classified as direct gas–solid carbonation, direct aqueous carbonation, and indirect carbonation, with the main differences among these routes summarized in

Table 1. Comparison of the main CO₂ sequestration routes using calcium-rich industrial wastes.

Route	Main Advantages	Main Limitations	Most Suitable Use
Direct gas–solid carbonation	Simple route; little wastewater; suitable for near-source flue-gas coupling	Product-layer diffusion and limited late-stage conversion	Rapid screening and dry mineralization
Direct aqueous carbonation	Enhanced CO2 dissolution and Ca2+ migration	Water use, salt migration, liquid–solid separation and wastewater treatment	High-rate fixation and stabilization
Indirect carbonation	Higher Ca extraction and better product control	Leaching-agent consumption, regeneration and by-product handling	High-value CaCO3 or coupled resource recovery
Carbonation–calcination looping	Regenerable CaO and high cumulative capture capacity	Sintering, pore collapse and the energy-intensive calcination for CO2 release	High-temperature cyclic capture when CO2 stream is managed

. Direct gas–solid carbonation has a simple flowsheet and low water demand, which is suitable for near-source coupling with CO₂-containing gases from cement, steel, and incineration plants. However, it faces a main drawback of the increasingly worse carbonation kinetics as the CaCO₃ product layer forms. Aqueous carbonation can enhance CO₂ dissolution, Ca²⁺ leaching, and ionic transport, but requires liquid–solid separation, control of salt migration, and wastewater treatment. Studies on steel slag emphasize the effects of pressure, liquid-to-solid ratio, particle size, and mineralogy; high total Ca does not guarantee high carbonation efficiency when calcium is locked in silicate or Fe–Mg phases [21,24,26]. Indirect carbonation may improve calcium extraction and product purity, but its efficiency depends on the leaching-agent consumption, solvent regeneration, and by-product management. It is noteworthy that the yet unresolved issue of these sequestration-based routes is the insufficient CO₂ abatement ability due to the limited content of reactive calcium species for single use. By contrast, utilization of the calcium-rich industrial wastes as a CO₂ sorbent to be operated in a carbonation–calcination looping mode can effectively increase the cumulative gross capture over repeated cycles, producing a high-purity CO₂ stream during regeneration of the reactive calcium species through CaCO₃ decomposition. Because calcination releases CO₂, this route is more accurately described as cyclic CO₂ capture and sorbent regeneration rather than single-use permanent sequestration. Despite this, the high-temperature regeneration step will cause grain growth, pore-structure collapse, and loss of reactivity; for example, APCr with high chloride and alkali-salt contents is particularly susceptible to melting, sintering, and pore blockage [12,13,16,22].

Recently, CO₂ abatement through carbonation–calcination looping of calcium-rich industrial wastes has received increasing academic and industrial interest [22,27,28]. For instance, repeated carbonation–calcination of the Ca-rich APCr has been experimentally demonstrated to be feasible, but the impurities, such as chloride and alkali salts, can weaken the cyclic CO₂ capture stability due to melting and sintering [22]. Further investigation revealed that the wet dechlorination combined with high-gravity carbonation of such residues can increase the CO₂ uptake while promoting the stability of the whole process [29]. However, these previous studies mostly focused on an individual waste and investigated the cyclic CO₂ capture potential under different operating conditions; the carbonation–calcination looping performance of various waste candidates lacks a direct comparison based on a unified investigation protocol. Moreover, it is not simply whether the calcium-rich wastes can react with CO₂, but which reactive calcium species can be repeatedly regenerated and what the CO₂ capture capacity and cyclic stability of these residues are when used as a potential sorbent. Steel slag (SS), air pollution control residue (APCr), and cement kiln dust (CKD) are selected as three typical calcium-rich industrial wastes because they represent by-products of steelmaking, incineration for power generation, and cement production, respectively, and differ markedly in the calcium source, impurity composition, and thermal stability. This work investigated the reactive calcium phases, carbonation kinetics, and cyclic deactivation of three typical wastes under the gas–solid TGA basis, therefore laying a research foundation for developing such Ca-rich alkaline wastes into potential CO₂ sorbents for the near-source CO₂ abatement.

2. Materials and Methods

2.1. Sample Preparation

The SS sample used in this study was obtained after the treatment of water washing, crushing, and magnetic separation from the converter of a steel plant in Beijing, and the APCr sample was collected from the cloth-bag dedusting system of a municipal solid-waste incineration power plant in Shenzhen, whilst the CKD sample was obtained from the kiln of a cement plant in Beijing. All samples were ground at ambient temperature into smaller particles that could pass through a 200-mesh screen (particle size < 74 μm), which could improve the sample homogeneity and reduce particle-size-related mass-transfer differences during the TGA measurement [30].

2.2. Material Characterization

The elemental composition of the investigated industrial wastes was analyzed by X-ray fluorescence (XRF) using an XRF-1800 Analyzer (Shimadzu Corp., Kyoto, Japan). Morphology of the samples was observed under a JSM-6460LV scanning electron microscope (SEM; JEOL Ltd., Akishima, Tokyo, Japan). X-ray diffraction (XRD) analysis of the sample was performed to identify the calcium-bearing crystalline phases and evaluate their potential contribution to carbonation. The XRD patterns were collected using a Smartlab X-ray diffractometer (Rigaku Corp., Tokyo, Japan) with the following operating parameters: CuKα radiation (λ = 1.5418 Å), 40 kV, and 200 mA power generator. An angular range of 5–100° 2θ was measured with a step size of 0.02° and a 2 s counting time per step. The identification of crystalline phases was undertaken referring to the PDF-2 2004 database (International Centre for Diffraction Data, Newtown Square, PA, USA). The surface area and pore volume of the investigated wastes were measured by using an Autosorb IQ surface area and porosity analyzer(Quantachrome Instruments, Boynton Beach, FL, USA), and each sample was pretreated by degassing at 300 °C for 3 h before the measurement.

The well-established reference intensity ratio (RIR) method was used to semi-quantify the mineral phase containing the reactive Ca available for carbonation in the industrial waste samples, with corundum(α-Al₂O₃ powder; Alfa Aesar, Ward Hill, MA, USA) used as an internal standard [27,31,32,33]. The content of any crystal phase i in the sample, added with a known amount of corundum, is given by Equation (1):

$$x_i = {\displaystyle \frac{I_i}{I_{cor} k_{i,cor}}} \times {\displaystyle \frac{x_{cor}}{1 - x_{cor}}},$$

(1)

where x is the weight fraction, I is the intensity of the most intense line of a crystal phase, and the subscripts i and cor are phase i and the standard corundum phase, respectively. The value k_i,cor represents the reference intensity ratio of phase i to corundum, which is determined according to convention from the most intense line of phase i and corundum in a 50:50 mixture by weight.

2.3. Carbonation Experiments

Thermogravimetric analysis (TGA) was used to investigate temperature-programmed carbonation, individual carbonation, and carbonation–calcination looping of the samples with a TGA/DSC 1 STARe thermogravimetric analyzer (version 20; Mettler-Toledo, LLC, Columbus, OH, USA). During the temperature-programmed carbonation test, approximately 10 mg of the pretreated sample was placed on an alumina 150 μL pan in a uniform layer of about 1 mm, and then heated to 900 °C at a rate of 20 °C/min in a CO₂ flow of 60 mL/min. During the individual carbonation test, the sample was heated to the target temperature at a rate of 20 °C/min in a N₂ flow, and then the temperature was stabilized for 60 min after the atmosphere was switched to a gas flow of 60 mL min⁻¹ containing pure CO₂ or CO₂ at a concentration of 10 vol% (balance in N₂). During the carbonation–calcination looping test, the fresh sample was heated to 600 °C in a N₂ flow and then carbonated for 5 min after the atmosphere was switched to 100 vol% CO₂. Then, the temperature was increased to 800 °C and the reaction atmosphere was switched from the CO₂ flow to the N₂ flow to calcine the carbonated sample for another 5 min at 800 °C. After that, the temperature was reduced to 600 °C to repeat the above looping procedure 20 times.

3. Results and Discussion

3.1. Physicochemical Characterization of the Sampled Wastes

As shown in

Table 2. Elemental composition of the investigated waste samples.

SS (%)		APCr (%)		CKD (%)
Ca	28.34	Ca	32.67	Ca	31.17
Al	6.42	Cl	32.66	C	9.97
Mg	6.03	Na	7.96	Si	5.83
Fe	5.34	K	5.10	Al	2.62
Si	5.08	S	3.95	Fe	1.86
F	1.02	Si	2.39	Mg	1.71
Mn	0.61	Zn	1.24	K	1.24
Ti	0.38	Mg	1.25	Ti	0.17
K	0.20	Fe	1.07	Cl	0.16

, calcium was the dominant element in all three samples, accounting for 32.67% in APCr, 31.17% in CKD, and 28.34% in SS, but the composition of remaining elements differed substantially. The high content of Ca in SS is attributed to the use of lime or limestone as a flux to deal with iron ore. The slag also contained Mg, Al, Si, and Fe as main elements with a concentration above 5%; the application of magnetic separation pretreatment explains the lower residual Fe content relative to some reported steel slags [34,35]. APCr had a more complex composition of elements, with Cl (32.66%), Na (7.96%), and K (5.10%) appearing as major components in addition to Ca. Its high Ca content is a result of the use of Ca(OH)₂ for semi-dry flue-gas treatment. The high Cl content reflected the wide chlorine-bearing feed sources in the municipal solid waste which are fixed in the residues through treatment of the incineration flue gas with Ca(OH)₂. CKD mainly contained Ca, Si, and Al, which are associated with kiln feed and clinker phases in cement, while the reported carbon content at 9.97% indicated the existence of considerable amounts of calcite in the dust.

Figure 1 shows the morphology of fresh samples, which exhibited distinct surface microstructures. The SEM image of SS showed the formation of angular and layer-like particles, which is the characteristic shape of Ca(OH)₂. APCr was dominated by finer granular agglomerates, which could provide a better porosity for the APCr to be used as a potential sorbent. By contrast, CKD showed a broader particle-size distribution and included particles with hexagonal or rhombohedral outlines, indicating the formation of CaCO₃ phase in the CKD.

The XRD patterns shown in Figure 2 revealed that SS mainly contained portlandite, Mg–Al hydrotalcite, olivine, and periclase, with andradite, katoite, Ca₂SiO₄, and rutile as minor phases [31]. The principal phases identified in APCr included NaCl, CaClOH, and indialite, with CaSO₄, KCl, CaF₂, and Ca₂SiO₄ as minor phases [32]. Notably, portlandite and CaClOH, which are typically reactive calcium species available for carbonation to sequestrate CO₂, were confirmed in the SS and APCr, respectively, theoretically verifying the CO₂ sequestration potential of both wastes. The RIR method based on XRD patterns gave portlandite and CaClOH contents of 18.95% and 24.44%, respectively, in the corresponding waste samples. Although highly reactive calcium species were not detected in the raw CKD, calcite with a calculated content of 83.1% appeared as the crucial Ca-based species, which is readily converted into the reactive CaO species upon calcination at elevated temperatures higher than 700 °C.

3.2. Evaluation of the CO₂ Uptake Capacity

Direct gas–solid carbonation of the sampled wastes exhibited different reaction behaviors (Figure 3a). Although the carbonation rates, indicated by the increase in the net sample weight, were all very slow below 300 °C, APCr achieved its maximum carbonation rate between 300 and 400 °C, which was lower than the temperature ranges corresponding to the highest carbonation rates of SS and CKD at approximately 600–700 °C. When the temperature was continuously increased, the surface carbonation gradually reached saturation, and the derived carbonates began to decompose at elevated temperatures of around 850 °C, which was indicated by the decrease in the sample weight. Figure 3b shows the CO₂ uptake of different wastes at their respective optimal temperatures as determined in Figure 3a. Regardless of waste type, the uptake curves showed a similar two-stage CO₂ capture behavior, namely an initial rapid stage controlled by the carbonation kinetics and a following slow stage restricted to the diffusion of CO₂ molecules through the CaCO₃ product layer. It is noteworthy that the three waste samples showed similar early-stage uptake of CO₂, but their later-stage CO₂ capture behavior differed markedly. CKD maintained a significantly faster diffusion-controlled CO₂ uptake and continued to capture CO₂ throughout the tested duration, whereas SS and APCr gradually approached saturation after 10 min carbonation. The resulting CO₂ uptakes of SS, APCr, and CKD were 8.4, 3.9, and 20.1%, respectively, corresponding to the carbonation conversion rate in the range of 55.0–71.8%. Under the tested conditions, APCr and SS presented similar CO₂ uptake when the CO₂ concentration of the reaction gas was decreased from 100 to 10 vol%, suggesting potential tolerance to the flue-gas-level CO₂ concentrations. It is important to point out that real flue gas also contains water vapor, SO_x, NO_x, and particulates as impurities, which will impact the practical CO₂ uptake of these wastes when considering their practical implementation. For instance, water vapor can enhance the product-layer transport of CO₂ but also induces hydration or agglomeration of the waste particles, while the presence of SO₂, even at low concentrations, will compete with CO₂ via persistent sulfation to consume regenerable Ca species [12,16,31,32]. Hence, the scaling-up test of these wastes under real flue-gas conditions is a crucial future step for further demonstration of the commercial potential.

3.3. Performance of the Sampled Wastes in Carbonation–Calcination Looping

It is shown in Figure 4 that CKD presented better cyclic CO₂ capture performance compared to SS and APCr, with the uptake of CO₂ nearly 3.5 times higher than SS, and 8 times higher than APCr. Furthermore, CKD showed acceptable cyclic stability over the 20 carbonation–calcination cycles, with the average deactivation rate at 1.5% per cycle. The performance of APCr was the worst among the three wastes: only 14.1 mg CO₂ per gram of the sample was achieved in the first cycle, and the CO₂ uptake continuously decreased with the repeated cycles. This is likely associated with the existence of alkali-metal chlorides such as KCl and NaCl in the APCr, which have a melting point below 800 °C and could cause the sintering of the residue to limit the accessibility to capture CO₂. In addition to the inferior cyclic CO₂ capture performance, the occurrence of chloride volatilization or migration during the repeated calcination of APCr at elevated temperatures is another issue adverse to the application of APCr as a CO₂ sorbent. Studies have revealed that the carbonation process of APCr can induce a transformation of the CaClOH and KCl-bearing phases [36], and the thermal treatment alone can also alter the solid-phase distributions of chlorine [37,38,39]. Despite this, the accumulative CO₂ uptake of the wastes was improved at least fivefold compared to the conventional technical route of direct carbonation sequestration, even if only 10 carbonation–calcination looping cycles were considered.

In order to further investigate the deactivation mechanism for CO₂ capture over repeated carbonation–calcination cycles, XRD patterns, SEM images, and BET surface areas of the best-performing CKD samples before and after the looping process are compared in Figure 5, Figure 6 and Figure 7 and

Table 3. Surface area and porosity characteristics of the calcined CKD and that tested after 20 carbonation–calcination cycles.

Parameter	Unit	Calcined CKD	Cycled CKD
BET specific surface area	m2 g−1	9.395	5.869
Total pore volume	cm3 g−1	0.039	0.024
Average pore diameter	nm	16.644	16.181
DFT pore-size mode	nm	2.897	3.169
DFT surface area	m2 g−1	7.409	5.250

. As shown in Figure 5, the diffraction peaks of calcite disappeared after the calcination pretreatment of the CKD sample, and the appearance of the characteristic peaks corresponding to CaO indicates the effective activation of CKD for carbonation to capture CO₂. Encouragingly, all CaO diffraction peaks remained after repeated carbonation–calcination for 20 cycles, confirming the persistence of regenerated CaO phases under the tested conditions. The retained CaO phases provided reactive calcium species for carbonation in each CO₂ capture cycle, which is consistent with the sustained cyclic CO₂ uptake of CKD.

Relative to the calcined CKD (Figure 6), the cycled sample (Figure 7) exhibited localized particle coarsening, enhanced agglomeration, and partial densification. These changes indicated limited sintering of the CKD sample during the looping process; however, the main morphological structure was preserved to make CKD continuously available for CO₂ capture.

The surface area and porosity characteristics shown in Table 3 supported the SEM observations, as evidenced by the decrease in the BET specific surface area and total pore volume of the tested CKD sample by 37.5 and 39.3%, respectively, after 20 carbonation–calcination cycles. The loss of accessible surface area and pore volume is consistent with the enhanced agglomeration and localized densification rather than complete pore collapse.

Table 4. Comparison of the main results in this study with other recent studies on CO₂ abatement with calcium-rich wastes.

Study	Material/Route	Key Results	Comparison with the Present Results
Dal Pozzo et al. [22]	Ca-based APC residues; carbonation–calcination looping	Repeated uptake was feasible with composition-dependent effects of chlorinated phases on the diffusion resistance and uptake retention.	Total chloride alone does not determine the cyclic uptake retention.
Chen et al. [29]	MSWI fly ash; wet dechlorination and high-gravity carbonation	258.5 g CO2 kg−1 fly ash with coupled dechlorination and stabilization.	Higher than CKD in this study, but the intensified wet route is not directly comparable with dry TGA carbonation.
Biava et al. [21]	Steel-slag carbonation review and material valorization	Temperature, pressure, liquid-to-solid ratio, CO2 partial pressure, particle size, and mineralogy control performance.	Supports the interpretation that limited exposure of reactive Ca constrains SS under the present conditions.
Elyasi Gomari et al. [26]	Steel slag; response-surface optimization	Optimized uptake of 149.32 g CO2 kg−1.	Higher than 84 g CO2 kg−1 for SS here, demonstrating the importance of operating-condition optimization.
Flipkens et al. [25]	CKD/LKD; ocean alkalinity enhancement	Kiln dust showed CO2-removal potential, but turbidity and environmental risks required attention.	Both studies indicate CKD potential, but ocean-alkalinity metrics cannot be equated with gas–solid carbonation.

compares the present results with other studies according to the technical route, reaction medium, pretreatment, and process boundary. The study on repeated carbonation–calcination of Ca-based APCr reported by Dal Pozzo et al. supports the interpretation of the results that the reactive calcium species in APCr can enable cyclic CO₂ uptake, but the high chloride content promotes melting, sintering, and pore blockage of the material [22]. For steel slag, Elyasi Gomari et al. reported an optimized uptake of 149.32 g CO₂ kg⁻¹ [26], and the lower value of the SS investigated in this study (84 g CO₂ kg⁻¹) is consistent with a substantial fraction of calcium being locked in silicate, aluminate, or Fe–Mg-bearing phases. The present result does not mean that steel slag is unsuitable for CO₂ capture; rather, it indicates that grinding, hydrothermal activation, or aqueous intensification may be required to improve exposure of the reactive calcium species to CO₂. Overall, under the tested conditions, CKD is the strongest candidate for high-temperature cyclic CO₂ capture. SS is better suited to mineralization after pretreatment or reaction intensification, whereas APCr is more appropriately coupled with dechlorination, stabilization, and controlled carbonation.

When considering the practical application, regeneration energy due to CaCO₃ calcination is central to these investigated wastes. The endothermic decomposition of CaCO₃ requires approximately 178 kJ mol⁻¹, corresponding to about 4.0 MJ per kg of CO₂ released [12,16]. However, the carbonation step releases a comparable amount of heat, so the net energy penalty depends on the heat recovery efficiency, which is influenced by the regeneration atmosphere, solids circulation, and gas heating. Some advanced-process heat integration measures, such as autothermal Ca–Fe concepts, may also be available for reducing the energy penalty but require techno-economic analysis based on the experimental data from scaling-up demonstrations [40].

4. Conclusions

Under the tested TGA conditions, carbonation–calcination looping increased the cumulative gross CO₂ capture capacity of calcium-rich industrial wastes compared with direct gas–solid carbonation for single use. The comparison among SS, APCr, and CKD shows that total calcium content alone is insufficient for evaluating the CO₂ capture potential. CKD was the most suitable candidate for high-temperature cyclic CO₂ capture because the calcination of abundant CaCO₃ produced reactive CaO, resulting in the highest CO₂ uptake and most stable cyclic performance as evidenced by the higher retention of CO₂ uptake, with an average loss rate of approximately 1.5% per cycle, than SS and APCr over the looping process. SS contained considerable calcium but showed lower effective carbonation because part of its calcium was locked in silicate, aluminate, or Fe–Mg-bearing phases. APCr contained the carbonation-active CaClOH phase, but its high chloride content made it unfavorable for stable high-temperature carbonation–calcination looping. From a process-development viewpoint, SS should be activated to expose more reactive calcium species, and APCr should undergo dechlorination and stabilization before carbonation-oriented utilization. The main limitations of this study are the use of single-source samples and idealized TGA atmospheres; therefore, the present results should be regarded as a comparative TGA-based sorbent/process-potential study. Practical deployment will nevertheless depend on many techno-economic factors such as continuous solids handling, heat integration, impurity control, sorbent pretreatment and makeup, CO₂ purification, and the management or utilization of cycled solids. The use of existing wastes may reduce the purchase and disposal burdens of sorbents, but grinding, activation, regeneration heat, environmental controls, and capacity decay will affect the net cost. These factors require reliable techno-economic analysis based on the mass and energy balance data acquired from scaling-up or pilot-scale demonstrations. Future work should focus on evaluating feedstock variability, real flue-gas impurities, and environmental safety before scale-up or quantitative comparison with conventional carbon capture technologies.