A Review on CO₂ Sequestration via Mineralization of Coal Fly Ash

Abstract

Coal fly ashes (COFA) are readily available and reactive materials suitable for CO₂ sequestration due to their substantial alkali components. Therefore, the onsite collaborative technology of COFA disposal and CO₂ sequestration in coal-fired power plants appears to have potential. This work provides an overview of the state-of-the-art research studies in the literature on CO₂ sequestration via the mineralization of COFA. The various CO₂ sequestration routes of COFA are summarized, mainly including direct and indirect wet carbonation, the synthesis of porous CO₂ adsorbents derived from COFA, and the development of COFA-derived inert supports for gas-solid adsorbents. The direct and indirect wet carbonation of COFA is the most concerned research technology route, which can obtain valued Ca-based by-products while achieving CO₂ sequestration. Moreover, the Al and Si components rich in fly ash can be adapted to produce zeolite, hierarchical porous nano-silica, and nano-silicon/aluminum aerogels for producing highly efficient CO₂ adsorbents. The prospects of CO₂ sequestration technologies using COFA are also discussed. The objective of this work is to help researchers from academia and industry keep abreast of the latest progress in the study of CO₂ sequestration by COFA.

1. Introduction

COFA is a type of solid waste from power plants derived from the combustion of coal [1]. Every year, a large amount of COFA is generated as the demand for cheap electricity increases [2]. As the world’s largest consumer of coal, nearly 80% of coal is consumed to produce electricity each year in China [3], producing 827 million tons of COFA in 2022. Although partial COFA is used as supplementary feedstocks to make roadbed and water-permeable bricks or as the raw materials to synthesize zeolite [4], a good deal of COFA is still accumulated or disposed of in landfills, causing harmful impacts on the local environment [5]. In addition, because heat power plants are also major sources of CO₂ emissions, it is urgent to achieve CO₂ reduction in coal-fired power plants via CO₂ capture and storage technologies [6,7]. COFAes are inexpensive, convenient, and reactive materials suitable for mineral carbonation due to their substantial alkali and alkali earth metals [1,8]. Therefore, the onsite collaborative treatment of COFA disposal and CO₂ sequestration in heat power plants can gain significant advantages [2,9]. It is similar to CO₂ storage by enhanced oil recovery (EOR) and enhanced gas recovery (EGR) technologies [10,11,12], which provide additional benefits while achieving CO₂ sequestration.

Generally, mineral CO₂ sequestration using COFA is mainly classified into two types, including direct and indirect carbonation. Typically, direct carbonation of COFA can be accomplished by aqueous mineral carbonation or gas-solid reactions. Indirect carbonation contains the extraction of Ca²⁺ and Mg²⁺ from COFA by leaching agents (i.e., acid, ammonium salt, etc.) followed by carbonation reactions [13]. While sequestering CO₂, the final carbonated products of COFA via direct carbonation can be used as construction fillers because their physical structure and leaching resistance are enhanced as well. The indirect carbonation of COFA allows for the production of pure carbonates due to the removal of impurities (i.e., silica and iron) prior to carbonate precipitation. Hence, these can enhance the economy of the carbonation treatment while decreasing the overall environmental influence of COFA. Moreover, COFA can also be used to prepare zeolites [4,14,15] and hierarchical porous nano silica [16] for CO₂ adsorption. The COFA-derived inert supports for K₂CO₃-based, CaO-based, or amine-based sorbents for CO₂ capture are also reported [17,18,19].

In this paper, the literature on CO₂ sequestration using COFA has been comprehensively reviewed. The reviewed CO₂ sequestration technologies majorly include direct and indirect wet carbonation, synthesis of porous CO₂ adsorbents derived from COFA, and the development of COFA-derived inert supports for gas-solid adsorbents. The detailed technical route and important findings regarding the above CO₂ sequestration techniques are described. Moreover, the carbonation efficiency of COFA using different sequestration technologies and the practicality of these potential technical routes are comparatively discussed. The major aim of this review is to timely grasp the development status and technical challenges of CO₂ sequestration technologies using COFA and provide suggestions on the future development of these technologies.

2. CO₂ Mineralization

The specific flow chart of direct and indirect CO₂ mineralization using COFA is depicted in Figure 1. The CO₂ sequestration is mainly accomplished by mineralization in a slurry (direct carbonation) or aqueous solution (indirect carbonation). The relevant research on direct and indirect CO₂ mineralization using COFA is described in detail below.

2.1. Direct Carbonation

Direct CO₂ mineralization is achieved by the reaction of COFA and CO₂ either in a gaseous or aqueous phase environment. The direct CO₂ mineralization route is simple and potential, and it possesses the advantages of eliminating the extraction step of reactive components and the minimal consumption of chemical reagents. As listed in Table 1, research on the direct CO₂ mineralization of COFA is mainly centered on the investigation of CO₂ sequestration parameters such as COFA component, carbonation temperature, carbonation pressure, carbonation time, liquid-to-solid ratio, etc. [1,9]. Moreover, there are also studies on adding calcium and magnesium ion extractants or using supercritical CO₂ to promote fly ash direct mineralization to fix CO₂.

As illustrated in Figure 2, the carbonation process of COFA mineralizing CO₂ mainly includes three stages [20]: (I) dissolution of CO₂ in fly ash slurry and ionization of H₂CO₃ produces CO₃²⁻; (II) Ca²⁺ and Mg²⁺ leaching into solution; (III) generation of CaCO₃/MgCO₃ grains that are wrapped on the surface of fly ash particles or dispersed in slurry. In fact, the carbonation reaction gradually penetrates from the outside surface to the inner core of COFA particles. Gaseous CO₂ is diffused and dissolved in the liquid film surrounding the fly ash particles and reacts by leaching out Ca²⁺ and Mg²⁺. The generated carbonate products deposit on the surface of fly ash particles, leading to an uncarbonated inner core surrounded by a growing edge of carbonate products [21]. Therefore, the parameters of Ca²⁺/Mg²⁺ leaching and transportation, CO₂ diffusion, carbonate product coating, and pore blockage will affect the overall rate and degree of the carbonation reaction. Among them, Ca²⁺/Mg²⁺ leaching is the rate-limiting step of COFA mineralizing CO₂.

The in-situ naturally weathered COFA in a wet-dumped ash dam can achieve CO₂ sequestration via carbonation to some extent; however, the carbonation reaction rate is too slow. Muriithi et al. [22] found that a period of 20 years was required to capture 6.8 wt % CO₂ by in-situ natural carbonation of wet disposed ash. In contrast, the fresh COFA fixed 6.5 wt% CO₂ via ex-situ accelerated carbonation (i.e., 4 Mpa, 90 °C, and liquid/solid ratio of 1 mL/g) in merely 2 h. Therefore, ex-situ accelerated carbonation of COFAes is a promising large-scale CO₂ sequestration technology.

There are great discrepancies in the components of COFA due to the different coal types and the ways of flue gas desulfurization. The content of alkaline earth metals within COFAes closely affects their CO₂ sequestration capacity (C_n), especially the Ca content. Ji et al. [23] dedicated themselves to studying the COFA properties’ influence on the carbonation reactions, and five types of Chinese and Australian coal combustion fly ashes were used as the samples for the study. It was found that the rich reactive Ca/Mg-bearing crystalline phases in COFA, such as CaO, Ca(OH)₂, MgO, and Ca₂Fe₂O₅ and Mg(OH)₂, contributed to the increased C_n. Particularly, the Ca-bearing phases were superior to the Mg-bearing phases in improving the kinetics of carbonation reactions due to their higher reactivity with CO₂. The COFA containing 32.4 wt% of CaO and 29.3 wt% of MgO exhibited the highest C_n of 132 g-CO₂/kg-fly ash via accelerated carbonation (carried out at 220 °C, initial CO₂ pressure of 2 MPa, 2 h and a liquid/solid ratio of 5 mL/g).

Moreover, Yuan et al. [24,25] selected four types of fly ashes derived from the subcritical, supercritical, and ultra-supercritical units, respectively, and the circulating fluidized bed, drum, and once-through boiler units. It was found that the raw fly ashes exhibited extremely low C_n of 1.8 g CO₂/kg fly ash via dry mineralization. Mechanical ball milling modification could effectively enhance the C_n of fly ashes due to producing more fresh surfaces and pores within fly ashes. The CO₂ sequestration capacities of wet and dry milling-modified ashes were, respectively, increased to 12.9 and 37.1 g CO₂/kg fly ash [24]. In addition, wet mineralization is markedly superior to dry mineralization in improving the C_n of fly ashes. Even the raw fly ashes displayed outstanding C_n under the wet mineralization route, which was significantly better than that of mechanically modified fly ashes under dry mineralization conditions. It was mainly ascribed to the presence of water effectively accelerating the leaching of Ca²⁺/Mg²⁺ from the solid COFA particles in wet mineralization conditions [26], consequently the superior C_n. Although the water availability promotes the leaching of Ca²⁺/Mg²⁺, excess water slowed the CO₂ diffusion rate due to the formed mass transfer barrier blocking the pores and cavities on the fly ash particles’ surfaces, instead reducing the carbonation efficiency. Moreover, the high reaction temperature contributed to the accelerated carbonation reaction due to the improved mass transfer rate, thermal movement of molecules, and average reaction kinetics [27,28]. However, the low CO₂ solubility in water and the exothermic carbonation reaction were not conducive to CO₂ sequestration at high temperatures.

To further break through the bottleneck of slow carbonation rate in the CO₂ mineralization process of COFA, supercritical CO₂ was adopted to enhance carbonation [24,25]. The COFA exhibited a markedly higher C_n under supercritical CO₂ (54.9 g CO₂/kg fly ash) compared to non-supercritical CO₂ (42.3 g CO₂/kg fly ash) due to the supercritical CO₂ possessing the advantages of strong diffusion character and desirable permeability. The CO₂ mineralization experiments of block CaO were designed by Yuan et al. [25] to measure the diffusion characteristics of supercritical and non-supercritical CO₂. As illustrated in Figure 3, the supercritical CO₂ (128.6 μm) exhibited a significantly deeper diffusion depth than the non-supercritical CO₂ (105.5 μm) within block CaO, intuitively demonstrating the strong diffusion characteristics.

Siriwardena et al. [29] conducted experiments to measure the carbonation coefficient of cylindrical specimens (19 × 38 mm) of fly ashes. The fly ash specimens were placed in the carbonation chamber under certain conditions (reactor temperature of 40 ± 1 °C, relative humidity of 65 ± 5%, and CO₂ concentration of 5 ± 0.5%). The carbonated specimens were split lengthwise, and the newly cut section was sprayed with 1% phenolphthalein indicator. The depth of carbonation could be evaluated by measuring the depth to the color boundary line between the carbonated (pH < 9.2, colorless) and uncarbonated (pH > 9.2, purple) regions, as shown in Figure 4. The following diffusion equation can describe the relationship between carbonation depth and exposure period.

$$\mathrm{X}=\mathrm{C}\sqrt{t}+a$$

(1)

where $\mathrm{X}$ refers to the depth of carbonation (cm); $\mathrm{C}$ is the coefficient of carbonation ($\mathrm{c}\mathrm{m}\sqrt{\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}}$); $t$ represents the duration of accelerated carbonation (days); and $a$ is the empirical constant (cm). Although the COFA exhibited relatively low C_n (~41.6 g CO₂/kg fly ash), the progression of carbonation in COFA was rapid, exhibiting a significantly high carbonation coefficient of 1.66$\mathrm{c}\mathrm{m}\sqrt{\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}}$. It was mainly attributed to the less dense matrix in fly ash specimens, which allowed rapid diffusion of CO₂ because of the generation of fewer reaction products.

The other researchers also conducted many experimental studies to reveal the effect of CO₂ mineralization parameters on the C_n of coal fly ashes. However, the reported values of C_n varied in different studies due to the different selected fly ashes and CO₂ mineralization parameters (Table 1) [30,31,32,33,34,35,36,37,38,39,40,41]. As shown in Figure 5, Pan et al. [32] proposed an integrated route for CO₂ sequestration, fly ash stabilization, and by-product application using a high-gravity carbonation treatment method. The influence of different operating parameters on the carbonation conversion of COFA was investigated by response surface methodology. The maximal carbonation conversion of fly ash was 77.2% (i.e., C_n of 249.4 g CO₂/kg fly ash) using a rotation rate of 743 rpm and a liquid/solid ratio of 18.9 mL/g at 57.3 °C. Moreover, the physicochemical properties (i.e., heavy metal leaching and volume expansion) of carbonated fly ash were upgraded, making them suitable for green materials in construction engineering.

The leaching of Ca²⁺ and Mg²⁺ into solution is an important parameter that affects carbonation efficiency. Therefore, the addition of Ca²⁺ and Mg²⁺ extractants is an effective approach to promote the leaching of Ca²⁺ and Mg²⁺ into the solution, resulting in improved C_n. Ji et al. [42] studied the effect of various 0.5 mol/L additives (i.e., Na₂CO₃, Na₂CO_3, and NaCl, NaHCO₃) on the CO₂ sequestration efficiency of coal fly ashes. The carbonation efficiency for different additives was ranked in the following order: 0.5 mol/L Na₂CO₃ > 0.5 mol/L Na₂CO₃ and 0.5 mol/L NaCl > 0.5 mol/L NaHCO₃. The C_n was as high as 102 g CO₂/kg fly ash under the conditions of 0.5 mol/L Na₂CO₃ and liquid/solid ratio of 10 mL/g at 275 °C and 2 MPa for 2 h. It was mainly attributed to Na₂CO₃, which can provide abundant CO₃²⁻ in the liquid phase, which results in superior C_n. The C_n of COFA with the addition of 1 mol/L NH₄Cl or sea salt was comparatively studied by Jo et al. [43]. It was suggested that the additives increase the solubility of Ca-bearing minerals, resulting in more CO₂ sequestration to some extent. Therefore, the C_n in the presence of 1 mol/L NH₄Cl with a liquid/solid ratio of 3 mL/g at 25 °C for 18 h was approximately 23 g CO₂/kg fly ash, compared to 19 g CO₂/kg fly ash in its absence.

The amine-looping process using coal fly ash could achieve efficient CO₂ mineralization as well, which was confirmed in the study by Ji et al. [44,45]. They mainly investigated the CO₂ fixation performance of COFA in various typical amine solutions (i.e., monoethanolamine, diethanolamine, triethanolamine, 2-amino-2-methy-1-propanol, and piperazine). It revealed that amines contributed to enhancing the mass transfer of CO₂, promoting Ca²⁺ leaching, and generating small CaCO₃ precipitation particles, especially the fly ash in 0.5 mol/L piperazine solution, which exhibited the desirable C_n of 102.9 g CO₂/kg fly ash. The mechanism of amine-looping for high-efficiency CO₂ carbonation in the water system using COFA was illustrated in Figure 6. The CO₂ sequestration was a three-step process: mass transfer of the gaseous CO₂ phase to the aqueous CO₂ phase, leaching of Ca²⁺, and precipitation of CaCO₃. Amines promoted the mass transfer of the gaseous CO₂ phase to the aqueous CO₂ phase by providing extra CO₂ reaction pathways. Meanwhile, the protonated amines formed from carbamate generation enhanced the leaching of Ca²⁺, therefore being beneficial for the precipitation of CaCO₃. Piperazine (a typical diamine) could offer two amino groups used to adsorb CO₂, which can maintain a markedly higher CO₂ concentration in the solution compared to the other four monoamines, resulting in superior CO₂ sequestration performance.

To overcome the issue of the large energy consumption of absorbent regeneration for the amine-looping process, an integrated process combining CO₂ absorption with mineralization was proposed [45]. A chemical method was used to regenerate the amine absorbent in the integrated process combining CO₂ absorption with mineralization rather than the traditional thermal method. As shown in Figure 7, the CO₂-rich solution was sent to the carbonation sink, where carbonation reactions occurred between the CaO-rich wastes and the CO₂-rich solution. Afterward, the CO₂ was sequestered as a solid CaCO₃ phase, and the amine solvent was regenerated and returned to the absorption reactor for continuous CO₂ absorption. The practical application of using COFA as a feedstock for the regeneration of absorbent in an integrated CO₂ absorption-mineralization process was feasible. Results indicated that piperazine showed a larger cyclic loading of 0.42 mol/mol in the integrated process, which was approximately more than 1.1 times that of the traditional regeneration process by a thermal method.

Table 1. Summary of direct CO₂ mineralization using COFA.

Reference	Carbonation Condition	CaO and MgO Content in Fly Ash	Cn (g-CO2/kg-Fly Ash)
Shao et al. [20]	liquid/solid ratio of 6 mL/g, 30 °C, initial CO2 pressure of 2 MPa, 15 min	8.8 wt% Ca, 0.36 wt% Mg	54.9
Muriithi et al. [22]	brine, liquid/solid ratio of 1 mL/g, 90 °C, initial CO2 pressure of 4 MPa, 30 vol % CO2, 2 h	9.2 wt% Ca, 2.44 wt% Mg	65
	in-situ natural carbonation, 20 years		68
Yuan et al. [24]	100 rpm, liquid/solid ratio of 100 mL/g, 80 °C, initial CO2 pressure of 1 MPa, 5 h	25.83 wt% CaO, 2.17 wt% MgO	42.3
	100 rpm, liquid/solid ratio of 30 mL/g, 40 °C, initial CO2 pressure of 8 MPa, 5 h		54.9
	100 rpm, dry mineralization, 40 °C, initial CO2 pressure of 8 MPa, 5 h		1.8 (raw ashes)
			12.9 (dry milled ashes)
			37.1 (wet milled ashes)
Yuan et al. [25]	100 rpm, dry mineralization, 40 °C, initial CO2 pressure of 8 MPa, 1 h	10.37 wt% CaO, 2.17 wt% MgO	1.8
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	10.37 wt% CaO, 2.17 wt% MgO	55 (3 MPa), 58.2 (5 MPa), 89.3 (8 MPa)
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	6.67 wt% CaO, 0.84 wt% MgO	19.7 (3 MPa), 23.8 (5 MPa), 38.3 (8 MPa)
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	6.55 wt% CaO, 1.14 wt% MgO	17.5 (3 MPa), 19.4 (5 MPa), 35.5 (8 MPa)
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	4.68 wt% CaO, 0.24 wt% MgO	10.7 (3 MPa), 11.7 (5 MPa), 13.9 (8 MPa)
Ji et al. [23]	liquid/solid ratio of 5 mL/g, 220 °C, initial CO2 pressure of 2 MPa, 2 h	16.4 wt% CaO, 1.2 wt% MgO	58
	liquid/solid ratio of 5 mL/g, 220 °C, initial CO2 pressure of 2 MPa, 2 h	9.4 wt% CaO, 27.9 wt% MgO	125
	liquid/solid ratio of 5 mL/g, 140 °C, initial CO2 pressure of 2 MPa, 2 h	3.6 wt% CaO, 7.1 wt% MgO	13
	liquid/solid ratio of 5 mL/g, 140 °C, initial CO2 pressure of 2 MPa, 2 h	13.4 wt% CaO, 0.5 wt% MgO	26.5
	liquid/solid ratio of 5 mL/g, 220 °C, initial CO2 pressure of 2 MPa, 2 h	32.4 wt% CaO, 29.3 wt% MgO	132
Siriwardena et al. [29]	liquid/solid ratio of 0.3 mL/g, 40 °C, 10 vol % CO2, humidity of 65%, 28 days	22.75 wt% CaO, 4.48 wt% MgO	41.6
Back et al. [30]	600 rpm, liquid/solid ratio of 20 mL/g, 75 °C, initial CO2 pressure of 0.01 MPa, 4.5 h	37.3 wt% CaO,15.4 wt% MgO	230
La Plante et al. [31]	liquid/solid ratio of 100 mL/g, atmospheric pressure, 60 °C, 100 vol % CO2, 72 h	28.5 wt% CaO, 6.6 wt% MgO	95.0
Pan et al. [32]	743 rpm, liquid/solid ratio of 18.9 mL/g, 57.3 °C, 15 vol % CO2	62.8 wt% CaO, 0.83 wt% MgO	249.4
Revathy [33]	liquid/solid ratio of 13.35 mL/g, 61.6 °C, 4.87 MPa, 100 vol % CO2, 50 min	6.74 wt% CaO	50.72
Ukwattage et al. [34]	liquid/solid ratio of 5 mL/g, 40 °C, initial CO2 pressure of 6 MPa, 10 h	39.8 wt% CaO, 7.3 wt% MgO	7.66
Miao et al. [35]	liquid/solid ratio of 10 mL/g, 5 kWh/m3 energy input in slurry, 60 °C, 15 vol % CO2, 2 h	33.1 wt% CaO, 0.95 wt% MgO	128
Montes-Hernandez et al. [36]	liquid/solid ratio of 10 mL/g, 30 °C, initial CO2 pressure of 1 MPa, 18 h	5.0 wt% CaO	26.2
Bauer et al. [37]	1500 rpm, liquid/solid ratio of 0.12 mL/g, 25–80 °C, initial CO2 pressure of 0.015 MPa, 2 h	28.4 wt% Ca, 0.92 wt% Mg	211
Ukwattage et al. [38]	60 rpm, liquid/solid ratio of 3 mL/g, 60 °C, initial CO2 pressure of 3 MPa, 10 h	24.8 wt% CaO, 13 wt% MgO	27.1
Dananjayan et al. [39]	900 rpm, liquid/solid ratio of 15 mL/g, 30 °C, initial CO2 pressure of 0.4 MPa, 2 h	6.74 wt% CaO, 2.22 wt% MgO	50.3
Patel et al. [40]	liquid/solid ratio of 0.24 mL/g, 50 °C, 30 vol % CO2	37.25 wt% CaO, 0.45 wt% MgO	40
Ho et al. [41]	liquid/solid ratio of 100 mL/g, atmospheric pressure room temperature, 30 vol % CO2, 30 min	3.44 wt% Ca, 0.82 wt% Mg	16
Nyambura et al. [46]	brine, 600 rpm, liquid/solid ratio of 2 mL/g, 30 °C, initial CO2 pressure of 4 MPa, 2 h	9.2 wt% Ca, 2.44 wt% Mg	71.8
Jo et al. [43]	deionized water, liquid/solid ratio of 3 mL/g, 25 °C, 15 vol % CO2, 18 h	7.2 wt% CaO, 1.5 wt% MgO	19
	1 M NH4 Cl, liquid/solid ratio of 3 mL/g, 25 °C, 15 vol % CO2, 18 h		23
	sea water, liquid/solid ratio of 3 mL/g, 25 °C, 15 vol % CO2, 18 h		20
Ji et al. [42]	0.5 M Na2CO3, liquid/solid ratio of 10 mL/g, 275 °C, initial CO2 pressure of 2 MPa, 2 h	16.4 wt% CaO, 1.2 wt% MgO	102
Ji et al. [44]	0.5 mol/L piperazine solution, liquid/solid ratio of 10 mL/g, atmospheric pressure, 55 °C, 40 vol % CO2, 1.5 h	24.3 wt% CaO, 0.9 wt% MgO	102.9

Table 1. Summary of direct CO₂ mineralization using COFA.

Reference	Carbonation Condition	CaO and MgO Content in Fly Ash	Cn (g-CO2/kg-Fly Ash)
Shao et al. [20]	liquid/solid ratio of 6 mL/g, 30 °C, initial CO2 pressure of 2 MPa, 15 min	8.8 wt% Ca, 0.36 wt% Mg	54.9
Muriithi et al. [22]	brine, liquid/solid ratio of 1 mL/g, 90 °C, initial CO2 pressure of 4 MPa, 30 vol % CO2, 2 h	9.2 wt% Ca, 2.44 wt% Mg	65
	in-situ natural carbonation, 20 years		68
Yuan et al. [24]	100 rpm, liquid/solid ratio of 100 mL/g, 80 °C, initial CO2 pressure of 1 MPa, 5 h	25.83 wt% CaO, 2.17 wt% MgO	42.3
	100 rpm, liquid/solid ratio of 30 mL/g, 40 °C, initial CO2 pressure of 8 MPa, 5 h		54.9
	100 rpm, dry mineralization, 40 °C, initial CO2 pressure of 8 MPa, 5 h		1.8 (raw ashes)
			12.9 (dry milled ashes)
			37.1 (wet milled ashes)
Yuan et al. [25]	100 rpm, dry mineralization, 40 °C, initial CO2 pressure of 8 MPa, 1 h	10.37 wt% CaO, 2.17 wt% MgO	1.8
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	10.37 wt% CaO, 2.17 wt% MgO	55 (3 MPa), 58.2 (5 MPa), 89.3 (8 MPa)
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	6.67 wt% CaO, 0.84 wt% MgO	19.7 (3 MPa), 23.8 (5 MPa), 38.3 (8 MPa)
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	6.55 wt% CaO, 1.14 wt% MgO	17.5 (3 MPa), 19.4 (5 MPa), 35.5 (8 MPa)
	100 rpm, liquid/solid ratio of 10 mL/g, 40 °C, initial CO2 pressure of 3, 5 and 8 MPa, 1 h	4.68 wt% CaO, 0.24 wt% MgO	10.7 (3 MPa), 11.7 (5 MPa), 13.9 (8 MPa)
Ji et al. [23]	liquid/solid ratio of 5 mL/g, 220 °C, initial CO2 pressure of 2 MPa, 2 h	16.4 wt% CaO, 1.2 wt% MgO	58
	liquid/solid ratio of 5 mL/g, 220 °C, initial CO2 pressure of 2 MPa, 2 h	9.4 wt% CaO, 27.9 wt% MgO	125
	liquid/solid ratio of 5 mL/g, 140 °C, initial CO2 pressure of 2 MPa, 2 h	3.6 wt% CaO, 7.1 wt% MgO	13
	liquid/solid ratio of 5 mL/g, 140 °C, initial CO2 pressure of 2 MPa, 2 h	13.4 wt% CaO, 0.5 wt% MgO	26.5
	liquid/solid ratio of 5 mL/g, 220 °C, initial CO2 pressure of 2 MPa, 2 h	32.4 wt% CaO, 29.3 wt% MgO	132
Siriwardena et al. [29]	liquid/solid ratio of 0.3 mL/g, 40 °C, 10 vol % CO2, humidity of 65%, 28 days	22.75 wt% CaO, 4.48 wt% MgO	41.6
Back et al. [30]	600 rpm, liquid/solid ratio of 20 mL/g, 75 °C, initial CO2 pressure of 0.01 MPa, 4.5 h	37.3 wt% CaO,15.4 wt% MgO	230
La Plante et al. [31]	liquid/solid ratio of 100 mL/g, atmospheric pressure, 60 °C, 100 vol % CO2, 72 h	28.5 wt% CaO, 6.6 wt% MgO	95.0
Pan et al. [32]	743 rpm, liquid/solid ratio of 18.9 mL/g, 57.3 °C, 15 vol % CO2	62.8 wt% CaO, 0.83 wt% MgO	249.4
Revathy [33]	liquid/solid ratio of 13.35 mL/g, 61.6 °C, 4.87 MPa, 100 vol % CO2, 50 min	6.74 wt% CaO	50.72
Ukwattage et al. [34]	liquid/solid ratio of 5 mL/g, 40 °C, initial CO2 pressure of 6 MPa, 10 h	39.8 wt% CaO, 7.3 wt% MgO	7.66
Miao et al. [35]	liquid/solid ratio of 10 mL/g, 5 kWh/m3 energy input in slurry, 60 °C, 15 vol % CO2, 2 h	33.1 wt% CaO, 0.95 wt% MgO	128
Montes-Hernandez et al. [36]	liquid/solid ratio of 10 mL/g, 30 °C, initial CO2 pressure of 1 MPa, 18 h	5.0 wt% CaO	26.2
Bauer et al. [37]	1500 rpm, liquid/solid ratio of 0.12 mL/g, 25–80 °C, initial CO2 pressure of 0.015 MPa, 2 h	28.4 wt% Ca, 0.92 wt% Mg	211
Ukwattage et al. [38]	60 rpm, liquid/solid ratio of 3 mL/g, 60 °C, initial CO2 pressure of 3 MPa, 10 h	24.8 wt% CaO, 13 wt% MgO	27.1
Dananjayan et al. [39]	900 rpm, liquid/solid ratio of 15 mL/g, 30 °C, initial CO2 pressure of 0.4 MPa, 2 h	6.74 wt% CaO, 2.22 wt% MgO	50.3
Patel et al. [40]	liquid/solid ratio of 0.24 mL/g, 50 °C, 30 vol % CO2	37.25 wt% CaO, 0.45 wt% MgO	40
Ho et al. [41]	liquid/solid ratio of 100 mL/g, atmospheric pressure room temperature, 30 vol % CO2, 30 min	3.44 wt% Ca, 0.82 wt% Mg	16
Nyambura et al. [46]	brine, 600 rpm, liquid/solid ratio of 2 mL/g, 30 °C, initial CO2 pressure of 4 MPa, 2 h	9.2 wt% Ca, 2.44 wt% Mg	71.8
Jo et al. [43]	deionized water, liquid/solid ratio of 3 mL/g, 25 °C, 15 vol % CO2, 18 h	7.2 wt% CaO, 1.5 wt% MgO	19
	1 M NH4 Cl, liquid/solid ratio of 3 mL/g, 25 °C, 15 vol % CO2, 18 h		23
	sea water, liquid/solid ratio of 3 mL/g, 25 °C, 15 vol % CO2, 18 h		20
Ji et al. [42]	0.5 M Na2CO3, liquid/solid ratio of 10 mL/g, 275 °C, initial CO2 pressure of 2 MPa, 2 h	16.4 wt% CaO, 1.2 wt% MgO	102
Ji et al. [44]	0.5 mol/L piperazine solution, liquid/solid ratio of 10 mL/g, atmospheric pressure, 55 °C, 40 vol % CO2, 1.5 h	24.3 wt% CaO, 0.9 wt% MgO	102.9

2.2. Indirect Carbonation

Indirect mineral carbonation of coal fly ashes mainly involves the extraction of Ca²⁺ by acids or other solvents into an aqueous solution, followed by the carbonation reaction between the extracted Ca²⁺ and injected CO₂. Indirect mineral carbonation allows for the production of pure carbonates due to the impurities (i.e., Si and Fe) that can be filtered out prior to carbonate precipitation [47].

He et al. [48,49] explored the effect of various extraction agents (i.e., NH₄Cl, NH₄NO₃, and CH₃COONH₄) on Ca²⁺ extraction efficiency from COFA particles. The above three ammonia salts were all effective Ca²⁺ extraction agents, achieving a calcium leaching efficiency of about 35–40% within 1 h. CH₄COONH₄ was superior to the other two ammonia salts in promoting Ca²⁺ extraction of COFA. A carbonation efficiency of 41–47% was achieved when carbonating the leachate with CO₂. Furthermore, a higher carbonation efficiency of ~90–93% can be obtained when NH₄HCO₃ is used instead of CO₂ as the source of CO₃²⁻. By this method, 1 ton of fly ash could sequester 0.075 t of CO₂, simultaneously producing 0.17 t of precipitated CaCO₃. Jo et al. [50] investigated the effect of solid dosage, CaO content, CO₂ flow rate, and solvent type on the extraction efficiency of Ca²⁺ from coal fly ashes. In fact, the extraction efficiency of Ca²⁺ was closely associated with the ultimate carbonation efficiency of fly ash. The results indicated that the C_n of fly ash was about 8 g-CO₂/kg-fly ash using deionized water as the solvent at ambient temperature and pressure conditions with a solid dosage of 100 g/L and a CO₂ flow rate of 2 mL/min.

Moreover, acetic acid was also used as the leachate to extract Ca²⁺ and Mg²⁺ from brown COFA to achieve indirect CO₂ mineralization sequestration [51]. Most of the Ca and Mg within COFA could be dissolved into the solution at the initial stage of leaching, and the carbonation of the leached solution was conducted using a continuously stirred high-pressure autoclave. The CO₂ was sequestered mainly in carbonate precipitates (i.e., CaCO₃ and MgCO₃) and water-soluble bicarbonate (i.e., Mg(HCO₃)₂). The carbonation of brown COFA-derived leachate exhibited a markedly lower global activation energy of 12.7 kJ/mol compared to that of natural minerals, contributing to maintaining a fast reaction rate. As shown in Figure 8, the maximum C_n of the fly ash was obtained at 60 ℃ for either CO₂ being sequestrated in the form of the solid or liquid phase. Therefore, the maximum C_n could reach 264 g CO₂/kg fly ash when considering both the contributions of carbonate precipitates and water-soluble bicarbonate. In addition, multiple-cycle leaching-carbonation and Mg²⁺ leaching kinetic modeling were studied during indirect carbonation of Victorian brown COFA for CO₂ fixation [47]. It revealed that the extraction efficiency of Ca²⁺ and Mg²⁺ markedly decreased with the increased cycling number of ammonium chloride. The carbonation efficiency gradually dropped with the increase in ammonium chloride cycles as well.

An integrated circular system combining indirect carbonation of COFA with solution regeneration by bipolar membrane electrodialysis (BPED) was put forward by Ho et al. [52]. As shown in Figure 9, the system consisted of three main blocks, i.e., Ca²⁺ leaching from fly ash, CaCO₃ precipitation, and acid and alkaline solution regeneration. The CO₂ sequestration mechanism of COFA is described in Equation (2).

$$\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left({\mathrm{C}\mathrm{a}}^{2+}+2\mathrm{N}{\mathrm{O}}_3^{-}\right)+\mathrm{C}{\mathrm{O}}_2 \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e} \mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(2{\mathrm{N}\mathrm{a}}^{+}+\mathrm{C}{\mathrm{O}}_3^{2-}\right)\to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{s}\right)}+2\mathrm{N}\mathrm{a}\mathrm{N}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}$$

(2)

BPED could regenerate the HNO₃ and NaOH solutions (Equation (3)). The bipolar membranes could decompose the water into H⁺ and OH^–, and simultaneously dissociate the salt into Na⁺ and NO₃^– through electrodialysis, consequently regenerating the HNO₃ and NaOH solutions.

$$\mathrm{N}\mathrm{a}\mathrm{N}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H}(\mathrm{f}\mathrm{o}\mathrm{r} \mathrm{C}{\mathrm{O}}_2 \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e})+\mathrm{H}\mathrm{N}{\mathrm{O}}_3(\mathrm{f}\mathrm{o}\mathrm{r} \mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{n}\mathrm{g})$$

(3)

The experiment results demonstrated that the C_n was ~11 g CO₂/g fly ash, corresponding to a CO₂ conversion of 92.9%. During the leaching stage, the adoption of a relatively lower nitric acid/calcium ratio was beneficial to the generation of a higher-purity CaCO₃ precipitate.

A novel electrolytic carbonation system for the collaborative treatment of COFA, brine wastewater, and CO₂ was proposed by Lu et al. [53]. As shown in Figure 10, the acidity produced by the electrolysis of brine electrolyte at the anode directly leached Ca²⁺ or Mg²⁺ from COFA. The Ca²⁺ or Mg²⁺ balanced the OH⁻ generated at the cathode to form Ca(OH)₂ or Mg(OH)₂, which then sequestered CO₂ and simultaneously produced high-purity carbonate precipitates. The electrolysis contributed to the enhanced dissolution of fly ash, and then it increased the C_n from 9.75 to 18.42 g CO₂/kg fly ash in the NaCl electrolyte.

In addition, protonated amine [54] and glycine [55,56] were used as reagents during the COFA-based leaching-mineralization course, which avoided the large consumption of exogenous acid-base reagents. The protonated amine could promote Ca²⁺ leaching from COFA, and simultaneously be converted into free amine [54]. Then, the free amine adsorbed the protons released by the precipitation reaction of CaCO₃ and could achieve in-situ regeneration of protonated amine. The authors systematically investigated the CO₂ mineralization performance of 13 typical amine-mediated COFA and the polymorph selection of CaCO₃. Triethanolamine (TEA) exhibited the largest CaCO₃ yield of 56.8%, and the corresponding Ca utilization efficiency was 16.8%. Moreover, the amines had a pronounced control effect on the CaCO₃ product’s size, polymorph, and morphology. As shown in Figure 11, the primary amino group was superior to the secondary and tertiary ones in promoting the formation of vaterite. The introduction of a side chain was prone to make the vaterite transform to calcite, and the grain size of the vaterite was inversely proportional to the length of the side chain.

To achieve CO₂ sequestration and CaCO₃ production, Zheng et al. [55,56] designed a glycine-mediated leaching-mineralization method for COFA. The process can simultaneously achieve the desirable CO₂ sequestration efficiency of COFA and produce high-purity CaCO₃ products in an in-situ recyclable glycine solution. After leaching for 1 h, a mineralization efficiency of 74.4% and a maximum CaCO₃ yield of 98.8 g/kg fly ash were achieved in a 2 M glycine solution with a fly ash dosage of 200 g/L. The reaction mechanism of the glycine-mediated leaching-mineralization approach through glycine and COFA (COFA) was illustrated in Figure 12. During the leaching stage, Gly0 could act as the proton donor and chelating agent that markedly promoted Ca²⁺ leaching, simultaneously producing Gly⁻. As an excellent CO₂ absorbent, Gly⁻ accelerates the mass transfer of gaseous CO₂ to the liquid phase by offering extra CO₂ reaction pathways. The carboxyl of Gly⁻ could also bind to the protons derived from carbonation reactions in the mineralization step, acting as a proton receptor. Moreover, the Gly species acted as a desirable crystal regulator in carbonate precipitation, which contributed to extending the continuous conversion and growth of CaCO₃ crystals.

An integrated system coupling mineral carbonation with direct air capture under atmospheric conditions was put forward by Ragipani et al. [57], as depicted in Figure 13. The evenly mixed slurries of COFA and concentrated alkali carbonate were used as recyclable solvents. Sodium hydroxide solvent was used to sequestrate CO₂ (Equation (4)) during the DAC process. The Equations (5) and (6) described the reaction generating NaOH solution via the carbonation between the COFA and Na₂CO₃. conditions. It was found that the coupled system achieved a high CaCO₃ conversion of ~80% in a 1.9 M Na₂CO₃ solution for 1 h. Moreover, the cost of sequestering 1 ton of CO₂ was about $116−133, while the process based on life cycle assessment emitted 0.03−0.25 t-CO₂e/t-CO₂-sequestered.

$$2\mathrm{N}\mathrm{a}{\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{C}\mathrm{O}}_{2\left(\mathrm{g}\right)}\to \mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{H}}_2\mathrm{O}$$

(4)

$$2\mathrm{C}\mathrm{a}\mathrm{O}.\mathrm{S}\mathrm{i}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+2\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+{2\mathrm{H}}_2\mathrm{O}\to 2\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{s}\right)}+\mathrm{S}\mathrm{i}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+4\mathrm{N}\mathrm{a}{\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}$$

(5)

$$\mathrm{C}\mathrm{a}\mathrm{O}.{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3.2\mathrm{S}\mathrm{i}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+{4\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{s}\right)}+2\mathrm{A}\mathrm{l}({\mathrm{O}\mathrm{H})}_3+2\mathrm{S}\mathrm{i}{\mathrm{O}}_{2\left(\mathrm{s}\right)}+2\mathrm{N}\mathrm{a}{\mathrm{O}\mathrm{H}}_{\left(\mathrm{a}\mathrm{q}\right)}$$

(6)

3. CO₂ Mineralization in Conjunction with Fly Ash-Derived by-Products Utilization

As mentioned above, the carbonation of COFAes can effectively achieve the fixation of CO₂. In addition, the further utilization of fly ash-derived by-products has also received extensive attention from researchers. Ren et al. [58] investigated the feasibility of combining the preparation of composite gravel with CO₂ absorption using COFA. The composite gravel pellets were prepared from mixtures of COFA, gypsum, Portland Cement and water, and the pellets were subjected to carbonation curing using flue gas. Figure 14a illustrates the prepared composite gravel pellets. The composite gravel pellets contained a 20 wt% cement content and 1:11(A11) ratio of gypsum to COFA, exhibiting the highest compressive strength of 11.11 MPa at the age of 28 days, as shown in Figure 14b. It was positively correlated with its CO₂ adsorption capacity, and the CO₂ adsorption for composite gravel pellets reached 4.789% with a 1:11 ratio of gypsum to COFA (Figure 14c).

Moreover, a new approach of employing the carbonated slurry of coal fly ashes during the CO₂ sequestration process to inhibit spontaneous combustion of coal (SCC) in the coal mining process was put forward [20]. The COFA reached a C_n of 54.9 g CO₂/kg fly ash with a liquid/solid ratio of 6 at 2 MPa. The carbonated fly ash slurry demonstrated a superior capability to inhibit SCC compared to the uncarbonated slurry. The temperature increase at the crossing point reached 13.8 °C in comparison to the origin coal when the carbonated fly ash slurry-to-coal ratio was set at 3:1. It greatly diminished the risk of SCC, indicating carbonated fly ash slurry possessing an apparent suppression function on SCC. The carbonated coal fly ashes were used as the mineral admixture for concrete or cement mortar [59,60]. Chen et al. [60] studied the property of cement mortar containing carbonated coal fly ashes and the combined impact of carbonation curing. It showed that carbonation treatment decreased the proportion of free CaO and the hydration heat of coal fly ashes. Therefore, the expansion of mortar specimens incorporating carbonated fly ash was greatly mitigated in comparison to that of specimens using fresh fly ash [59]. In addition, Freire et al. [61] produced geopolymers with COFA and rice husk ash, which were adopted as the CO₂ capture materials, and various geopolymer formulations were tried. The calcined rice husk ash activated via NaOH was the best precursor to prepare geopolymer that exhibited superior CO₂ capture performance.

The dual needs of CO₂ emission reduction and COFA treatment require the design of an innovative pathway to simultaneously achieve CO₂ sequestration and further conversion into useful carbonates. Yin et al. [62] conducted a range of experiments using fly ash and a mixture of fly ash and bottom ash to study CO₂ mineralization function with regenerable solvents (i.e., 2.5 M sodium glycinate and 30 wt% MEA). A mixture of fly ash and bottom ash was superior to sole fly ash, which obtained the highest CO₂ mineralization degree of non-calcium carbonate content. Moreover, nanoscale calcium carbonate could be successfully produced from dissolved calcium through CO₂-loaded sodium glycinate and CTAB surfactant. The detailed reaction mechanism for the integrated approach of CO₂ mineralization and nanoscale calcium carbonate production using COFA was demonstrated in Figure 15. Therefore, the route of directly sequestering CO₂ in COFA and simultaneously producing nanoscale CaCO₃ with regenerable CO₂ capture solvents was feasible.

In addition, Monasterio-Guillot et al. [63] were the first to explore the combined effects of CO₂ sequestration, zeolite production, and simultaneous heavy metal element trapping at the carbonation stage of COFA. The carbonation efficiency was as high as 79% (a net C_n of 45 g CO₂/kg fly ash) c under mild hydrothermal conditions for Class F fly ash (3.72 wt% CaO, 1.74 wt% MgO). Simultaneously, the amorphous precursors of zeolite and different crystalline zeolites with a yield as high as 60 wt% were produced. The potentially toxic elements within COFA were effectively solidified in the newly generated calcite and zeolite products, avoiding the leaching of toxic elements.

As illustrated in Figure 16, a green and facile pathway to synthesize mesoporous γ-Al₂O₃ from COFA while simultaneously achieving on-site CO₂ utilization was investigated by Yan et al. [64]. The lime-sinter route was used to extract aluminum from COFA. The COFA mixed with CaCO₃ was calcined at 1390 °C and then the calcined products were dissolved in Na₂CO₃ solution at 70 °C, achieving a high aluminum extraction efficiency of 87.42%. Then, the flue gas containing 15 vol% CO₂ and 85 vol% N₂ after being purified was injected into the extracted liquid to achieve CO₂-assisted precipitation of Al(OH)₃ precursors (Equation (7)). Ultimately, the mesoporous γ-Al₂O₃ (high surface area of 230.3 m².g⁻¹) was produced by calcining the Al(OH)₃ at 400 °C or 550 °C (Equation (8)). This proposed pathway appeared to be desirable for scaled-up production of mesoporous γ-Al₂O₃ by integrating the on-site recycling of COFA and CO₂ utilization.

$$2\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_{2\left(aq\right)}+{\mathrm{C}\mathrm{O}}_{2\left(\mathrm{g}\right)}+3{\mathrm{H}}_2\mathrm{O}\to \mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_{3\left(\mathrm{a}\mathrm{q}\right)}+2\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}{)}_{3\left(s\right)}$$

(7)

$$2\mathrm{A}\mathrm{l}(\mathrm{O}\mathrm{H}{)}_{3\left(s\right)}\to \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+3{\mathrm{H}}_2\mathrm{O}$$

(8)

4. CO₂ Sequestration of COFA by Dry Carbonation Process

In gas-solid carbonation, CO₂ reacts with free CaO within COFA to generate CaCO₃. Patel et al. [40] studied the CO₂ sequestration performance of COFA with reaction temperatures ranging from 500°C to 750 °C. Figure 17 demonstrated that the CO₂ sequestration level apparently increased with the temperature being leveled up to 700 °C in the initial stage. However, the CO₂ sequestration efficiency gradually dropped to below ~50% after the reaction time exceeding 10 min. Moreover, an apparent reduction in CO₂ sequestration efficiency was observed in the rapid reaction regime when the temperature reached 750 °C due to the simultaneous partial decomposition of CaCO₃.

Revathy et al. [33] designed the carbonation experiments of (Indian COFA (class F type-CaO < 10 wt%) via the gas-solid route using response surface methodology, and temperature and pressure were the main factors studied. The interaction between the factors (i.e., temperature and pressure) and their impacts on response (the reduction of CO₂ concentration) was analyzed. The temperature affected the carbonation efficiency of COFA because of the remarkable influence of temperature change on CO₂ concentration. At a certain temperature, the pressure played a profound role in the gas-solid carbonation of COFA. It was noted that the influence of temperature seemed to be more remarkable compared to that of pressure on the carbonation of fly ash when both temperature and pressure were changed. The global optimum solution was identified as a temperature of 44.86 °C and pressure of 24 bar, obtaining a reduction in CO₂ concentration of 18.2%. The COFA exhibited a maximum C_n of 20.0 g CO₂/kg fly ash under gas-solid carbonation.

Liu et al. [65] studied the influence of temperature, CO₂ concentration, steam concentration, and reaction duration on the CO₂ fixation performance of COFA via a direct gas-solid carbonation approach. It was found that the increased temperature and the concentration of CO₂ and H₂O(g) contributed to the improved CO₂ fixation efficiency. The effects of temperature and H₂O(g) were more apparent than that of CO₂ content. The COFA achieved a maximum C_n of 60 g CO₂/kg fly ash corresponding to a fixation efficiency of 28.74% at 600 ^oC with 20% H₂O(g) addition. Moreover, Ćwik et al. [66] carried out a mineral carbonation experiment of high-calcium COFA using a continuous flow reactor with different temperatures and CO₂ pressures. The dry and moist conditions were comparatively investigated. Results exhibited that COFA achieved a maximal C_n of 117.7 g CO₂/kg fly ash (corresponding to a carbonation efficiency of 48.14%) under gas-solid conditions. Moderate amounts of H₂O(g) contained in the CO₂ gas flow were beneficial to the improved carbonation efficiency. The promotion effect of steam resulted from the formation of Ca(OH)₂ or transient Ca(OH)₂ and the enhanced CO₂ molecular mobility at the carbonation stage [67,68].

Moreover, the CO₂ mineral carbonation of COFA doped with carbide slag was conducted in a pilot-scale circulating entrained-flow bed (CEB) rector (Figure 18). In a semi-dry atmosphere, the carbonation efficiency was enhanced 4 times higher than that of carbide-slag-doped COFA. It was mainly attributed to a thin liquid film formed on the surface of fly ash that facilitated CO₂ diffusion. At 550 °C, a maximal carbonation efficiency of 55% was obtained when adding 15% steam to the feed and injecting 6 kg/h of water into the rector system [69].

5. CO₂ Sequestration Materials Derived from COFA

Usually, COFA is rich in silica and alumina, which is beneficial to enhance the high-temperature CO₂ capture stability of CaO-based sorbents. Chen et al. [18,70] spared efforts to seeking a potentially effective approach to improve the capability of CaO-based sorbents mixed with COFA for CO₂ capture. The comparative studies on the effect of pretreatments (i.e., grinding, calcination, hydration, and leaching) of COFA on CO₂ capture performance of fly ash-modified CaO-based sorbents were conducted. The pretreatments of grinding and calcination for fly ash contributed to the improved CO₂ uptake capability for CaO-based sorbents. Acidic conditions were superior to basic conditions in enhancing CO₂ capture capacity and reactivity for CaO-based sorbents hydrated with fly ash [70]. The improved pore structure and the formation of inert stabilizers (i.e., Ca₁₂Al₁₄O₃₃ and CaSiO₃) within the CaO-based sorbents are the main reasons [71,72,73]. Particularly, the fly ash-modified sorbents with a mass ratio of slag/calcined calcium carbonate of 1:2, exhibited the CO₂ capture capacity of 0.19 g CO₂/g sorbent in the 30th cycle under severe calcination conditions, which was 3.8 times that of pure CaO [18].

Yan et al. [17] developed a green approach that involved silicon extraction from COFA by the method of alkali dissolution. Then, the synthetic nano silica was incorporated into CaO-based sorbents to improve both their cyclic CO₂ capacities and their sorption kinetics. Results exhibited that the synthetic nano silica-stabilized CaO-based sorbent still showed a higher CO₂ uptake of 0.2 g CO₂/g sorbent in short carbonation duration after 30 cycles (Figure 19), apparently increased by 155% over pure CaO. What was more important, the synthetic nano silica-stabilized CaO-based sorbent achieved approximately 90% of the total CO₂ uptake within ~20 s. The extremely fast CO₂ capture rate was very key for practical applications. The superior CO₂ capture performance of synthetic nano silica-stabilized sorbent was mainly ascribed to the evenly distributed Ca₂SiO₄, yielding a large amount of small pores directly exposed to CO₂ during cycles.

The fly ash-stabilized, CaO-based sorbents were prepared via a physically dry-mixing method with different CaO precursors (i.e., CaO, Ca(OH)₂, CaCO_3, and CaC₂O₄.H₂O) [74]. After 30 cycles, the sorbent synthesized from CaC₂O₄.H₂O (90 wt% of CaO in the sorbent) exhibited the highest CO₂ uptake of 0.38 g CO₂/g CaO. This sorbent still showed a desirable capacity of 0.27 g CO₂/g CaO even after being calcined at 920 °C in pure CO₂. The excellent performance of the fly ash-stabilized, CaO-based sorbents for cyclic CO₂ capture mainly resulted from the formation of highly dispersed inert Ca₂Al₂SiO₇ within the sorbent. Sreenivasulu et al. [75] further conducted the thermodynamic and kinetic studies of CO₂ capture using a COFA-doped sorbent (50 wt% CaO, 10 wt% MgO, and 40 wt% COFA). The thermodynamic studies confirmed both the feasibility of the reaction and the positive role of COFA in improving CO₂ capture and reducing regeneration temperatures. They also proposed a model for both the reaction and diffusion kinetic control stages, which had been verified through the experimental data obtained. It was found that the rate constants of 2.5 and 0.8 min⁻¹ and activation energies of 23 and 30 kJ/mol were evaluated for reaction- and diffusion-controlled stages at 650 °C, respectively. The results clearly indicated the improved rate achieved by COFA-doped CaO-based sorbents along with their thermal stability.

Moreover, the CO₂ uptake capacity of hydrated lime mixed with fly ash under low-temperature and humid conditions was also investigated [76]. The reaction condition was set at 60 °C with 70% relative humidity. The fly ash-doped CaO-based sorbents containing <50 wt% of fly ash exhibited higher CO₂ uptake than hydrated lime without doping fly ash (~0.15 g CO₂/g sorbent). The sorbent doped with 30 wt% of fly ash showed a particularly high maximum CO₂ uptake of 0.26 g of CO₂/g sorbent. The enhanced microstructure resulting from the generation of calcium silicate hydrates was responsible for the improved reactivities of the fly ash-doped sorbents.

Moreover, various kinds of zeolites synthesized from COFAes were used to capture CO₂ in the flue gas. As shown in Figure 20, the typical octahedral prism and cube-shaped crystals were observed for the synthetic zeolites derived from COFAes, indicating the good generation of the desired zeolitic materials. Aquino et al. [77] comparatively measured the adsorption capacity of two synthetic zeolites (i.e., NaX and NaA types) from COFA and commercial zeolites and further assessed their performance in the temperature swing adsorption (TSA) testing process. It was found that NaX and NaA-type zeolites exhibited CO₂ adsorption capacities of 1.97 and 1.37 mmol CO₂/g adsorbent at 303 K, which were close to those of commercial zeolites. More importantly, the synthetic zeolites showed highly stable adsorption capacity during five cycles, demonstrating the possibility of practical application in TSA processes.

Verrecchia et al. [14] focused on studying the effect of NaOH/COFA mass ratio, crystallization temperature, and duration on the synthesis of COFA-derived zeolites by the combined approach of fusion and hydrothermal. The major products contained NaX and amorphous compounds that were produced at 90 °C with a NaOH/COFA ratio of 1.2 for 7 h. They exhibited a capacity of 2.18 mol CO₂/kg adsorbent for CO₂ adsorption, accounting for 57% of the capacity of commercial 13X zeolites. Further, the central composite full factorial method was used to design experiments to optimize the synthesis parameters, aiming to improve the capacity of COFA-derived zeolites for CO₂ adsorption. It indicated that the synthetic zeolite produced with the experimental conditions of 1.4 NaOH/COFA, 80 °C, and 7 h exhibited a markedly increased adsorption capacity of 3.3 mol CO₂/kg adsorbent (~86% of commercial 13X zeolites). Muriithi et al. [15] also successfully synthesized two types of zeolites (NaX and NaA type) from COFAes used for CO₂ capture. The synthetic NaX-type zeolite possessed hierarchical morphology with platy structures arranged in spherical clusters and exhibited desirable CO₂ physisorption properties.

Moreover, the collaborative activation method of microwave and ultrasound was adopted to synthesize K-MER zeolites with desirable crystallinity and high purity [78]. Comprehensively adjusting the crystallization duration, pressures, and dynamic pressure programs could produce K-MER zeolites exhibiting various morphologies. At 25 °C, the as-synthetic K-MER zeolites exhibited a maximal adsorption volume of CO₂ of 47.58 cm³/g. The as-synthetic zeolites could also be used as catalysts to promote the cyanoethylatic reaction between methanol and acrylonitrile, which achieved a high acrylonitrile conversion rate of 99 wt% for 0.5 h under a pre-pressure of 1.5 MPa.

Furthermore, the COFA could be adopted as the raw material for producing synthetic hierarchical porous nano silica [16], nanoparticles MCM-41 [79], calcium silicate hydrate support [80], and silica-alumina aerogel [19] for CO₂ captured adsorbents. The hierarchical porous nano silica was mainly prepared through the collaborative method of microwave-assisted alkaline extraction and hydrothermal synthesis from fly ash derived from coal gasification [16]. The hierarchical porous nano silica had the isosteric adsorption heat of −30.06 kJ/mol, demonstrating that CO₂ adsorption involved both physisorption and chemisorption. The hierarchical porous nano silica exhibited a CO₂ adsorption capacity of 1.63 mmol CO₂/g adsorbent at 30 °C and 1 bar. The nanoparticle MCM-41 was functionalized with (3-aminopropyl) triethoxysilane and dispersed in diethylenetriamine (DETA) solution through ultrasonic dispersion method to prepare nanofluids for CO₂ capture [79]. The nanofluids coupled with high gravity technology markedly enhanced the CO₂ capture efficiency because of the improvement of grazing and hydrodynamic effect in a high gravity field (as shown in Figure 21), reaching ~95.7%. Qu et al. [80] synthesized highly porous COFA-derived calcium silicate hydrate (CSH) using the pore-expanded assistance of azeotropic distillation. The COFA-derived calcium silicate hydrate was then used as the support to prepare PEI@CSH adsorbents via impregnation. The 60% [email protected] adsorbent exhibited the excellent CO₂ uptake of 198 mg CO₂/g adsorbent under ideal regeneration conditions, and after 10 cycles, the adsorption capacity remained at 103 mg CO₂/g adsorbent even under severe regeneration conditions.

The silica-alumina aerogel derived from COFA possessed good microstructure, which could also be used as the inert support to prepare K₂CO₃-based CO₂ adsorbent with a wetness impregnation method [19]. The synthetic silica-alumina aerogel support exhibited a relatively high specific surface area of 400 m²/g and a specific pore volume of 1.9 cm³/g, and the proportion of mesopores within the support was over 99%. Therefore, the K₂CO₃-based adsorbent with K₂CO₃ load of 30 wt% achieved the maximal adsorption capacity of 2.02 mmol CO₂/g adsorbent under the conditions of 60 °C, 15 vol% water vapor, and 15 vol% CO₂.

6. Conclusions and Prospects

This paper summarizes the CO₂ sequestration technologies using COFA reported in the literature. The relevant technical routes and representative research results from previous studies in this field are discussed and analyzed. The mineral CO₂ sequestration using COFA is one of the most promising technologies, which can be classified into direct and indirect carbonation. The direct CO₂ mineralization route is simple and highly practicable, which eliminates the extraction step of reactive components and minimizes the consumption of chemical reagents. Indirect CO₂ mineralization involves the extraction of reactive Ca²⁺ from COFA with leaching agents (i.e., acid, ammonium salt, etc.) followed by carbonation reactions, which allow the production of high-value pure carbonates.

Moreover, the by-products derived from the CO₂ mineralization using COFA can be further utilized as feedstock to prepare composite gravel, cement mortar, geopolymers, mesoporous γ-Al₂O₃, and zeolite for toxic element trapping. The free CaO within COFA makes it possible to sequester CO₂ by dry process, and steam addition can effectively enhance carbonation efficiency. In addition, the silica and alumina contained in COFA can be used as materials for the synthesis of highly efficient CO₂ adsorbents. The COFA-modified CaO-based sorbents exhibit highly stable CO₂ capture performance due to the formation of extensive inert stabilizers, such as Ca₁₂Al₁₄O₃₃, Ca₂Al₂SiO₇, CaSiO₃, and Ca₂SiO₄. The CO₂ adsorption capability of the synthetic zeolites (i.e., NaX, NaA, and K-MER type) from coal fly ashes is comparable to that of commercial zeolites. Moreover, the nanoparticles MCM-41, calcium silicate hydrate, and silica-alumina aerogel derived from COFA can be used as the porous support of amine- or K₂CO₃-based adsorbents for low-temperature CO₂ capture.

The research on the development of CO₂ sequestration technologies using COFA in the past 10 years is summarized and analyzed. There are still some issues to be studied and solved.

(1)

Although extensive research has been carried out to enhance the CO₂ mineralization performance of COFA, the current research is limited to the optimization of experimental parameters at the laboratory scale. In the future, this technology needs to be promoted for industry, which needs to consider the cost of various raw materials and energy consumption. Moreover, the carbon assets obtained from CO₂ fixation and the additional economic benefits of valued carbonation by-products also need to be considered. Therefore, it is urgent to achieve an integrated economic feasibility analysis or life cycle assessment concerning the industrial-scale CO₂ mineralization of COFA. The researchers can obtain the technical cost of CO₂ sequestration for direct and indirect carbonation of COFA via the above technical process economic analysis. This will be able to better demonstrate the practicality of the direct and indirect carbonation of COFA technology and the potential for industrial-scale promotion.

(2)

COFA contains major associated elevated soluble trace elements (e.g., As, Cr, Mn, Cu, Sr, Ce, and V, etc.) that are potentially toxic to the biological system [63,81,82]. The heavy metal toxic elements are released into the surrounding environment (adversely affecting the plant and soil quality) by leaching of COFA once these are ponded or landfilled. Furthermore, these potentially toxic trace elements can reenter the food chain and human life cycle from these disposal sites via certain pathways [83]. Relevant scholars have investigated the accelerated carbonation of phosphogypsum [84] and municipal solid waste incineration (MSWI) fly ash [21,81,85], and found that the carbonation reaction can solidify the heavy metal elements within waste to a certain extent in the realization of CO₂ sequestration. Hence, it is necessary to investigate the leaching characteristics of potentially toxic elements in COFA during the wet CO₂ mineralization process.

(3)

For the synthesis of zeolites for CO₂ capture, the types of zeolite products are not rich enough. More technologies and processes should be developed to prepare low-cost and high-grade zeolites. Furthermore, the development of zeolites with highly selective adsorption and multi-effect functions should be the focus of research.