Alkaline Element Leaching from Fly Ash for Direct CO₂ Fixation

Abstract

Fly ash (FA), a major by-product of coal combustion, has long been regarded as a challenging industrial solid waste. Its inherent abundance of alkaline-earth oxides positioned it as a promising candidate for CO₂ sequestration through mineral carbonation. This study systematically investigated the effects of key operational parameters, including time, stirring rate, ultrasonic treatment, and solid-to-liquid ratio, on the leaching efficiency of calcium ions and subsequent CO₂ fixation. Ultrasonic treatment, a solid-to-liquid ratio of 1:7, a stirring speed of 600 rpm, and 7% monoethanolamine (MEA) collectively enhanced the calcium leaching efficiency (χ_e) to 16.7%, thereby supplying a substantial reservoir of calcium ions for CO₂ fixation. Additionally, the CO₂ injection into fly ash slurry and the slurry spraying into CO₂ gas were investigated to optimize reactor configurations. The latter method demonstrated superior performance, attaining a CO₂ fixation efficiency of 7.23%. This corresponds to a carbonation conversion efficiency (ηc) of approximately 44.5%, indicating that nearly half of the leached calcium ions were successfully converted into stable carbonates. Advanced characterization techniques (SEM-EDS, XRD, FTIR) confirmed the formation of stable carbonates and highlighted the role of additives in enhancing reactivity. The environmental benefit of this approach is addressing fly ash wastes and transforming fly ash into a CO₂ fixation material. These findings provided critical insights for calcium leaching and CO₂ fixation of fly ash.

1. Introduction

The most recent statistics from the International Energy Agency (IEA) demonstrate that global energy-related CO₂ emissions increased by 1.1% in 2023, reaching an unprecedented peak of 37.4 Gt. It is noteworthy that coal emissions accounted for more than 65% of this increase [1,2]. The process of CO₂ fixation is defined as the carbonation reaction of CO₂ into carbonates with natural Ca/Mg silicate minerals or alkaline residues [3]. In comparison with alternative utilization methodologies, CO₂ fixation possesses distinct advantages that are attributable to its spontaneous nature, including its cost-effectiveness and high feasibility.

Fly ash (FA) is a significant by-product of coal-fired power plants and is regarded as one of the most complex human-made waste materials [4]. The production of fly ash is globally estimated to be approximately 1.1 billion tonnages per annum. In China, the emission of fly ash is estimated to be approximately 900 million tonnages with a continued increase [5]. In addition, the FA can also be used for Al recovery, Si recovery, iron oxides recovery, zeolite preparation, adsorbents, and cementing material [6,7]. Critically, beyond its role as a bulking agent, FA possesses inherent agronomic value due to its composition. It is a source of essential plant nutrients such as calcium, magnesium, and potassium, alongside silicon and trace elements. Analogous to nutrient-rich biomass ash from natural fires—which is documented to elevate soil pH, increase electrical conductivity, and enhance the bioavailability of exchangeable calcium (exch. Ca), magnesium (exch. Mg), and mineral nitrogen [8]—appropriately processed FA holds significant potential as a soil amendment to improve fertility and physical structure. The improper disposal of FA has had consequences for the environment and resulted in the wastage of potentially recoverable resources. Consequently, there is an urgent and continuous demand to explore innovative recycling approaches for FA. Because the FA is rich in earth-alkali oxides, the carbonation of FA provides a potential candidate for the sequestration of CO₂. The substantial by-product has the potential to facilitate the sequestration of approximately 38 million tonnages of CO₂ per annum [9]. The CO₂ fixation can synergistically solve two main environmental problems associated with coal combustion.

According to process characteristics, the CO₂ fixation can be regarded as an in situ and ex situ mineral carbonation process [10]. The former refers to in situ mineral carbonation, as defined in IPCC reports: a process where CO₂ reacts with Ca-Mg bearing minerals in their native geological formations. Specifically, CO₂ is injected into deep subsurface geological reservoirs, including saline aquifers, depleted oil and gas fields, and unmineable coal seams [11], where it reacts with naturally occurring Ca-Mg bearing minerals within the formation to form stable Ca and Mg carbonates. This reaction immobilizes CO₂ and prevents its emission into the atmosphere. That said, stringent operational management is required for in situ mineral carbonation to mitigate potential risks, such as groundwater contamination, low rock reactivity, and unintended CO₂ migration [12]. The latter refers to ex situ mineral carbonation, which occurs outside the original geological formation. In this process, Ca-Mg bearing minerals are first extracted from the geological formation and transported to a purpose-built reaction site, where they are subjected to reactions with CO₂ to generate Ca and Mg carbonates. Based on the source of free Ca/Mg ions, the ex situ process can be divided into direct fixation (with abundant Ca/Mg present in the reaction solvent) and indirect fixation (with abundant Ca/Mg present in solid residues) [13,14]. Compared with indirect fixation, direct fixation accomplishes CO₂ fixation in one single step, with fixation capacity as the exclusive performance indicator [15]. The straightforward process offers economic viability for the large-scale utilization of FA.

Despite the different fixation processes, CO₂ sequestration is achieved by two main mechanisms: the leakage of ions and the carbonation of calcium hydroxide [16]. Despite the fact that CO₂ has the capacity to react directly with oxides under dry conditions, the high pressure and slow reaction rate render dry fixation a challenging process to apply. Furthermore, the reduction in operating pressure would result in the release of some free CO₂ into the atmosphere [17,18]. Indeed, the fixation rate and capacity may be dictated by the release of alkaline metals from such residues. This finding has been corroborated by numerous researchers [19,20]. Pressure, temperature, stirring speed, ultrasonic treatment, and the solid-to-liquid ratio (the mass ratio of FA in slurry) affect the CO₂ sequestration in alkaline residues by regulating the ion transportation. Some researchers even proposed chemical modification and physical activation methods to improve the carbonation potential of FA by enlarging the specific surface, changing the chemical environments, and destabilizing the crystal structure [21]. Therefore, it is necessary to clarify and improve the transportation of ions in the CO₂ stability system for a specific FA specimen.

In the present study, the research group endeavored to explore the transportation of ions during the process of CO₂ fixation. A series of systematic bench-scale experiments was conducted for the purpose of parameter optimization, with the objective of advancing specific FA towards the realm of fixation practices. Following the characterization of the sample, the parameters were considered in the experimental design: pressure, temperature, stirring speed, ultrasonic treatment, solid-to-liquid ratio, and contact modes. Two stages of ion leakage and CO₂ fixation were investigated by means of monitoring the following parameters: ion concentration, surface texture, crystal structure, and element distribution. Ultimately, the study developed a spray system to improve CO₂ fixation capacity further.

2. Materials and Methods

2.1. FA Samples Reagents

The FA was retrieved from the baghouse filters following the desulfurization process and subsequently conveyed from the Shanghai Thermal Power Plant to Xinjiang University. To obtain samples that were representative of the population, approximately 50 kg of FA was collected per day over a 20-day period, and the samples were thoroughly amalgamated. In the laboratory, the FA was utilized for CO₂ fixation without undergoing any pretreatment, thus simulating the practical situation. The surface textures, element distribution, and minerals were analyzed using a scanning electron microscope (SEM-EDS), X-ray fluorescence (XRF), and X-ray diffraction (XRD). In accordance with the national standard GB/T 212-2008—“Methods for approximate analysis of coal” [22], the approximate analysis of FA was conducted to determine its moisture content, ash content, volatile matter, and fixed carbon. Ethanolamine (C₂H₇NO, MEA) was purchased from the Sinopharm Co., Ltd. (Shanghai, China). The reagent was not treated before the experiments.

2.2. Fixation System

As demonstrated in Figure 1, an artificial fixation system was devised for the purpose of simulating the CO₂ fixation process. The supply of pure CO₂ to the autoclave reactor was facilitated by a steel pipe, with the inner pressure of the autoclave reactor being regulated by a pressure-reducing valve. Its internal volume was 0.6 L, and the presence of a substantial reaction cavity was conducive to pilot-scale experimentation. The autoclave reactor was equipped with a stirrer, an ultrasonicator, an inlet valve, a nozzle, an observation window, and an exhaust valve, which facilitated the optimization of experimental parameters. The entire system (with the exception of a gas cylinder) was placed on a large electronic balance, with the weight increment thereby representing the carbon sequestration. In order to facilitate the sampling process, a sampling port was installed at the bottom of the reactor.

2.3. Leaching and Fixation Operation

The CO₂ fixation system has the capacity to facilitate two distinct modes of contact. The first mode involves the injection of CO₂ gas into the FA suspension located at the base of the autoclave reactor. The second mode entails the spraying of FA suspension into the CO₂ gas from the upper region of the reactor. In the former mode, the FA powder of 2.5 kg and a specific amount of water were added into the autoclave reactor, and fixation commenced with CO₂ pumping. The effect of solid-to-liquid ratio (1:3–1:7), time (10–60 min), stirring speed (200 r/min–1000 r/min), and ultrasonication on ions leaching and CO₂ fixation were investigated through single-factor experiments. The MEA (1–10%) was utilized as an additive to ascertain its impact on ion leaching and CO₂ fixation. The optimal solution conditions identified in the preliminary phase were subsequently applied in the subsequent fixation. In other words, the FA was pre-activated by stirring in a 7% MEA solution for 1 h, employing a combination of ultrasonication (2400 W) and mechanical agitation at 600 rpm. The 7% of MEA additive was driven by considerations of cost-effectiveness.

2.4. Sample Characterization

The elemental distribution, surface morphology, crystal phase, functional groups, and chemical bonds of residues were characterized using X-ray fluorescence (XRF, PANalytical Axios, Almelo, The Netherlands), scanning electron microscopy (SEM-EDS, JSM-7000F, JEOL, Tokyo, Japan), X-ray diffraction (XRD, Bruker D8 ADVANCE, Bruker Corporation, Billerica, MA, USA), and Fourier-transform infrared spectroscopy (FTIR, Bruker VERTEX 70v, Bruker Corporation, Billerica, MA, USA), respectively. The residues resulting from the leaching and carbonation processes were designated L-Residues and C-Residues, respectively. In addition, the calcium ions of the solution after leaching and carbonation were analyzed using an ion meter (PXSJ-216F, Shanghai Leici Instrument Co., Ltd., Shanghai, China).

The XRD data were supplied with Cu-K α radiation (λ = 1.5406 Å) over a 2θ range from 5° to 80° with a scanning speed of 5 °/min. The wavenumber range of FT-IR was 4000–400 cm⁻¹, and the detection applies a resolution of 0.09 cm⁻¹ and fast scanning at 65 times per second. The SEM-EDS analysis was performed with the working voltage of 5–20 kV and the amplification factors of 500–2000. The EDS analysis set the acceleration voltage to 20 kV, the extraction voltage to 4.9 kV, and the emission current to 10 μA. The XRF semi-quantitative analysis was performed with the analysis voltage of 40 kV, X-ray tube bundle current of 95 mA, and Rh target power of 4000 W.

2.5. Calculation Methods

The fixation was considered as two stages: ion leaching and carbonation conversion. As shown in Equations (1)–(5), leaching efficiency (χ_e) is the ratio between leaching calcium (W_Ca-leahing) and total calcium (W_Ca-total). The quantity of CO₂ fixation was indicated by the discrepancy in slurry quality prior to and following the reaction (m_post and m_ex). Additionally, to evaluate the efficiency of converting the leached calcium into carbonate, the carbonation conversion efficiency (η_C) is defined as the molar ratio of fixed CO₂ to leached calcium.

$$W_{Ca-leaching}={\displaystyle \frac{C_{C{a}^{2+}}}{1000}}\times {\displaystyle \frac{v}{1000}}(\mathrm{g})$$

(1)

$$W_{Ca-total}={\displaystyle \frac{C_{Ca}}{100}}\times m$$

(2)

$${\chi }_e={\displaystyle \frac{W_{Ca-leaching}}{W_{Ca-total}}}\times 100(\%)$$

(3)

$$Fixation={\displaystyle \frac{m_{post}-m_{ex}}{m_{ex}}}$$

(4)

$${\mathit{\eta }}_C={\displaystyle \frac{\left(m_{post}-m_{ex}\right)/44.01}{W_{Ca-leaching}/40.08}}\times 100\%$$

(5)

3. Results and Discussion

3.1. FA Samples

The suitability of FA for CO₂ fixation and sequestration was largely attributed to the presence of alkaline oxides such as CaO and MgO. Nevertheless, the CaO content in FA may be subject to considerable variation, owing to the disparities in combustion processes, coal types, and desulfurization methods. This element constitutes a pivotal factor in the diverse CO₂ sequestration capacities observed among the various types of FA [23]. Therefore, it is necessary to explore the chemical and physical properties of pristine FA samples. The photograph, SEM-EDS images, element distribution, and mineral crystals are shown in Figure 2. The samples obtained from the FA exhibited a uniform size distribution and appeared as brown powders. The images obtained using SEM appear to demonstrate that the FA particles exhibit a characteristic polymorphic morphology. According to the principles of large particle adsorption and small particle agglomeration, small spherical particles adhered to the surface of larger spheres [24], and the small particles could provide a large specific surface for CO₂ fixation.

The primary constituents of the FA include C, Si, Al, Ca, S, Fe, O, and Ti, whose oxides collectively account for over 95% of the total ash mass. Notably, the CaO content was about 15.59%, which was an active component for CO₂ fixation, while K and Mg were observed as minor elements. The element groups demonstrated concurrence with the mean value of the industrial FA recently published in the academic literature [25]. After full peak fit and Rietveld refinement, the main calcium minerals in FA were CaSO₄ and CaO. In view of the rigorous preparation protocols, it was reasonable to infer that S in the samples originated exclusively from the desulfurization section [26]. Based on XRF and XRD data, elemental analysis indicated 6.35% of SO₃ from CaSO₄. In the course of the processes of hydration and carbonation, S exhibited a marked preference for reacting with CaO and Ca(OH)₂, resulting in the formation of stable CaSO₄. This implied that S competed with CO₂ for binding sites on CaO and Ca(OH)₂, and the formation of stable CaSO₄ on the FA surface might hinder CO₂ diffusion [27]. This has the potential to reduce the availability of these compounds for CO₂ fixation, thereby impairing overall carbonation efficiency.

According to the approximate analysis, the water content, volatile matter, ash, and fixed carbon were 5.60%, 10.23%, 82.47%, and 1.70% (

Table 1. Elemental composition and approximate analysis of fly ash.

Elemental Composition
Items	SiO2	Al2O3	CaO	SO3	Fe2O3	MgO	K2O	TiO2	P2O5
wt.%	19.36	29.43	15.59	6.35	3.55	6.67	1.44	17.45	0.16
Approximate analysis
Items	Mad	Ad	Vdaf	FCdaf	\	\	\	\	\
wt.%	5.60	82.47	10.23	1.70	\	\	\	\	\

). The incomplete combustion of coal results in the presence of residual coal particles in FA samples. Even so, the FA contained a certain amount of moisture, but it was insufficient to meet the substantial carbonation demands of the FA.

3.2. Ion Leaching

3.2.1. Influence of Leaching Time

The migration of calcium ions into the solution from the FA samples was a pivotal stage for the process of CO₂ fixation [28]. Thus, an organic amine was normally used as a leaching agent to assist in ion leaching in order to chemically activate FA and ameliorate the CO₂ fixation [29]. Consequently, the MEA was selected as an auxiliary agent to accelerate and increase the rate of ion migration. The water medium is the control group. As shown in Figure 3a, the ions leaching are positively correlated with treatment time. The ions leaching from both solutions exhibit a rapid initial phase within the first 30 min, followed by a deceleration in the subsequent period. The leaching efficiency initially surged and then plateaued after approximately 1 h, with the final leaching efficiencies of 9.60% (Figure 3a). The ion leaching could be divided into two stages: an initial fast leaching and subsequent slow leaching. The former stage is governed by chemical reactions, while the latter is driven by the diffusion mechanism [30].

During the initial rapid phase, the calcium primarily originated from calcium-containing compounds located on the particle surfaces. The surface-bound species were directly contacting the solution and reacting with the leaching agent. As the leaching progresses, the calcium content on the particle surface swiftly diminishes, exposing an inert surface layer of insoluble SiO₂ and Al₂O₃ [31]. The inert layer had been shown to impede the contact between the reactive sites in the particle core, thereby significantly reducing the leaching rate. Finally, the ions extraction shifted from the chemical-reactions-controlled stage to the diffusion-controlled stage.

3.2.2. Influence of Solid-to-Liquid Ratio

The initial moisture content of the material appears to be sufficient to facilitate the carbonation reaction. For example, it is optimal for municipal solid waste incinerator bottom ash for CO₂ sequestration with an ash moisture content of 15% (w/w) [32]. However, the optimal moisture content for FA fixation was determined to exceed that of bottom ash derived from municipal solid waste. In conditions of elevated moisture levels, water molecules have the potential to engage in carbonation reactions through two distinct mechanisms: (1) the dissolution of CO₂ gas within the water film that accumulates on residual surfaces, and (2) the hydration of alkaline oxides present in FA [33]. As shown in Figure 3b, the ion extraction efficiency decreased with an increasing solid-to-liquid ratio. When the solid-to-liquid ratio decreased from 1:3 to 1:7, the leaching efficiency increased from 7.5% to 10.30% (Figure 3b).

Obviously, the fixation took a very long time, and the carbonation suffered from unsatisfied reaction kinetics when the water content was incommensurate with a threshold value of CO₂ fixation. It is evident that water has a significant role in facilitating the leaching of calcium ions, as well as the diffusion of CO₂ [34]. In other words, a high solid-to-liquid ratio meant insufficient water in the fixation system, inhibiting the ion extraction reaction [35]. In instances where water participated in the process of fixation, the gas-solid reaction system, which comprises two phases, transitioned into a gas-liquid-solid reaction system, encompassing three phases. A portion of the gaseous CO₂ dissolved in the aqueous phase to form carbonic acid, which subsequently ionizes to produce H⁺, HCO₃⁻, and CO₃²⁻ (Equations (6)–(8)). The dissolution of minerals is promoted by these ions, which also enhance their subsequent carbonation. He et al. [36] also demonstrated that the leaching efficiency in the solution decreased rapidly with a decreasing solid-to-liquid ratio, monotonically. Thus, the CO₂ fixation is still plagued by the role of water molecules.

$$H_{2}O+C{O}_2\to H_{2}C{O}_3\to 2H^{+}+C{O}_3^{2-}$$

(6)

$$CaO+H_{2}O\to C{a}^{2+}+2O{H}^{-}$$

(7)

$$C{a}^{2+}+C{O}_3^{2-}\to CaC{O}_3$$

(8)

3.2.3. Influence of Reactor Design

In the context of a CO₂ fixation system, the reactor design constitutes a pivotal site at which alkaline residues react with CO₂. Notable among the reactor design factors that are typically employed to enhance CO₂ fixation are stirring speed and ultrasonic pretreatment [37]. As demonstrated in Figure 3c,d, when the stirring speed increased from 200 rpm to 600 rpm, the leaching efficiency of FA increased from 7.4% to 9.60%. However, a marginal decline to 9.1% was discerned at 1000 rpm. In the experimental investigation of ultrasonic pretreatment, the leaching efficiency of FA was observed to be 10.8% in the absence of ultrasonic pretreatment, whereas it increased to 12.7% when ultrasonic pretreatment was applied in ion leaching. In the ultrasonic reaction system, the growth and subsequent explosion of microbubbles resulted in the dispersion of particle clusters in liquid media. This process also led to an increase in the specific surface area of powders and an enhancement of the chemical reactivity of FA [38]. Chen et al. [39] reported an increase of carbonation efficiency from 14.1% to 18.8% through ultrasound enhancement. The stirring could improve the diffusion and mass transfer efficiency of CO₂ and the leaching efficiency of alkaline metal elements.

3.2.4. Influence of Agent Concentration

The integration of CO₂ absorption and fixation was proposed by scholars as a means of combining the rapid CO₂ absorption rate characteristic of amine solutions with the low energy consumption of fixation [40]. The introduction of exogenous substances was required to increase the CO₂ uptake further. It has been established by scholars that conventional acid leaching not only extracts alkaline elements, but also heavy metal pollutants [41]. Furthermore, the addition of industrial alkali is necessary to ensure the desired pH level for CO₂ fixation [42]. In order to leave heavy metals in the leached residue and avoid the frequent pH swing, amine media were proposed to promote CO₂ fixation.

As shown in Figure 3a, a significant increase in leaching efficiency was observed by 66.67% as the agent concentration increased to 10%. The presence of ammonium ions has been demonstrated to facilitate the transformation of inactive calcium to active calcium within the reaction system. In this process, a plausible explanation is that a Ca(CH₃COO)⁺ complex was formed to reduce free ions in the solution. It was hypothesized that this would promote the dissolution of calcium ions from FA [43]. However, the MEA was in favor of carbonation in the mode of absorbing CO₂ to zwitterions [44]. Considering the lost effectiveness and leaching performance, ultrasonic treatment, solid-to-liquid ratio of 1:7, stirring speed of 600 rpm, and MEA of 7% enhanced the leaching efficiency by 73.95%, which provided a substantial amount of calcium ions for CO₂ fixation.

3.2.5. Characterization of Leaching FA Samples

As depicted in Figure 4a,b, several key absorption bands are revealed in the FT-IR spectra. The absorption band at approximately 1100 cm⁻¹, corresponding to the stretching vibration of Si-O bonds, is indicative of the presence of quartz and aluminosilicates. The unshifted wavenumbers of Si-O bonds demonstrated stable aluminosilicates throughout the experimental process. Additionally, leaching residues exhibited a high-intensity absorption band at about 3400 cm⁻¹, which is characteristic of the hydroxyl groups (O-H) in Ca(OH)₂ [45]. The XRD patterns of products derived from the leaching process are presented in Figure 4c. The predominant mineral phases in leaching products comprised TiO2, CaSO₄, CaO, Al₂O₃, and 3Al₂O₃·2SiO₂, which was consistent with the raw FA. The morphology of leaching products was depicted in Figure 4d. The reduction in the sphericity of fly ash indicates that ions at the active sites have been dissolved. Furthermore, the mass ratio of calcium in leaching FA decreased from 8.34% to 6.86% because calcium ions were dissolved in the FA slurry. The extraction of ions was employed as a pretreatment stage for the fixation of CO₂. Calcium sources present in the FA samples have been shown to promote CO₂ retention through the process of leaching precipitation [24].

3.3. CO₂ Fixation

3.3.1. Effects of Contact Modes and MEA on FA CO₂ Fixation Efficiency

Following the ions leaching, the CO₂ fixation should be processed under conditions that ensure sufficient exposure. To this end, a range of reactors have been designed to enhance the contact between the CO₂ gas and the leaching solution. This study gave two types of contact modes whose phase interfaces were optimized to reduce the mass transfer resistance during CO₂ fixation. The fixation efficiency was evaluated subsequent to the process of CO₂ fixation.

The experiment commenced with the introduction of CO₂ gas into the FA slurry. This method resulted in a fixation ratio of 6.80% with a rapid initial increase in fixation occurring within the first 10 to 30 min. Subsequently, the FA slurry was sprayed into CO₂ gas. Despite similar trends in the curves, this approach achieved a fixation ratio of 7.23% (Figure 5a). Notably, this fixation efficiency was achieved starting from the optimized leaching condition with χ_e = 16.7%. The corresponding carbonation conversion efficiency (η_C), calculated via Equation (5), was approximately 44.5%. This indicates that under the optimal spraying condition, roughly half of the calcium ions made available through leaching were successfully converted into stable carbonate. Additionally, the solution with 7% MEA increased the CO₂ fixation of FA samples from 2.10% to 11.18%, which could be attributed to the promoted dissolution of calcium ions from FA and the enhanced CO₂ absorption capacity of the amine (Figure 5b).

3.3.2. Characterization of Mineralizing FA Samples

As shown in Figure 6a,b, the dominant absorption bands in the range of 1410–1430 cm⁻¹ and the subsequent band at 860–875 cm⁻¹ were associated with the presence of carbonate (C-O) bonds, with significant increases in peak intensities suggesting the formation of calcite. As demonstrated in Figure 6c,d, subsequent to the fixation reaction, a multitude of fine-grained crystals manifested morphologically rhombohedral and spherical structures within carbonation products. The crystals exhibited a tendency to aggregate into clusters, displaying a variety of morphologies, including spindle-shaped (fibrous and spear-like) and cuboid. These observations are indicative of the formation of vaterite, aragonite, and calcite [46]. After CO₂ fixation, the intensity of the CaO crystal structure collapsed, and the peak intensity of the fixation product CaCO₃ was obviously enhanced at 2θ = 29.4°, 48.5°, and 36.0°. As observed in the relevant literature, analogous XRD patterns were identified [47,48]. In consideration of the XRD analysis, the precipitation of CaCO₃ is characterized by a sequential transformation from thermodynamically unstable temporary amorphous CaCO₃ to metastable vaterite or aragonite, ultimately stabilizing as calcite [49].

4. Process Analysis and Economic Considerations

4.1. Analysis of Rate-Limiting Steps and Reactor Performance

The comparative analysis of reactor configurations revealed a modest performance gain for the spray method (7.23%) over bubbling (6.80%). While the spray method fundamentally enhances the gas-liquid interfacial area for CO₂ absorption, the small magnitude of improvement indicates that the overall carbonation rate was not solely limited by the gas-phase mass transfer of CO₂. Instead, the process appears to be co-limited by the supply and diffusion of Ca²⁺ ions from the fly ash particles and the kinetics of carbonate precipitation. This is consistent with the calculated carbonation conversion efficiency (η_C) of ~44.5%, which quantifies the yield limitation inherent to the precipitation step. Thus, while reactor design optimizing gas-liquid contact (e.g., spraying) is beneficial, significant further efficiency gains will require strategies to address these subsequent solid-liquid reaction and crystallization bottlenecks.

4.2. Economic Assessment and the Role of MEA

The use of monoethanolamine (MEA) as a consumable additive presents a significant economic barrier for scale-up. As an expensive solvent designed for regenerative cycles, its consumption as a single-pass additive would render the process economically unviable at scale. The marginal gain in overall fixation efficiency (7.23%) does not justify the high cost of MEA. Herein, MEA was employed primarily as a model chemical agent to establish the system’s maximum theoretical performance and to elucidate mechanisms, not as a proposed commercial additive. Therefore, achieving a favorable cost-benefit ratio for fly ash carbonation necessitates future research focused on additive-free process intensification, such as advanced reactor design (e.g., spray systems) or physical activation methods to overcome the kinetic limitations identified.

5. Conclusions

This study systematically elucidated the mechanisms and optimization strategies for CO₂ fixation using coal-derived fly ash. Key operational parameters, including ultrasonic pretreatment, solid-to-liquid ratio, reactor configuration, and additive selection, were rigorously evaluated to enhance calcium leaching and subsequent CO₂ fixation. Ultrasonic treatment significantly improved ion leaching efficiency by disrupting particle agglomerations, achieving an enhancement of up to 12.70%. A solid-to-liquid ratio of 1:7 was identified as optimal, balancing ion diffusion and CO₂ dissolution to attain a leaching efficiency of 10.30%. The combined application of ultrasonic treatment, a solid-to-liquid ratio of 1:7, a stirring speed of 600 rpm, and 7% MEA enhanced the calcium leaching efficiency (χ_e) to 16.7%, which provided a substantial reservoir of calcium ions for CO₂ fixation. Comparative analysis of reactor designs revealed that spraying FA suspension into CO₂ gas outperformed gas injection into FA slurry, achieving a CO₂ fixation efficiency of 7.23%. The carbonation conversion efficiency (η_C) under this condition was calculated to be ~44.5%, effectively linking the leaching and fixation stages by quantifying the proportion of leached calcium ultimately sequestered as carbonate.

The economic feasibility of the process is constrained by the high cost of monoethanolamine (MEA) as a consumable additive. For large-scale applications, achieving a favorable cost-benefit ratio would require either a significant increase in the carbonation conversion efficiency or the development of additive-free process intensification routes. The reliable equipment and optimized process parameters established herein provide a foundational framework for such technological advancement, supporting the potential future integration of carbonation technology within coal-based power systems.