A Comprehensive Review of CO₂ Mineral Sequestration Methods Using Coal Fly Ash for Carbon Capture, Utilisation, and Storage (CCUS) Technology

Abstract

CO₂ emissions from fossil fuel combustion are the main source of anthropogenic greenhouse gases (GHGs). A method of reducing CO₂ emissions is CCUS (carbon capture, utilisation, and storage) technology. One part of CCUS technology involves mineral sequestration as its final stage, utilisation, which can be carried out using natural raw materials or waste. This is a particularly interesting option for power and CHP plants that use coal as their primary fuel. Combustion processes produce fly ash as a waste by-product, which has a high potential for CO₂ sequestration. Calcium fly ash from lignite combustion and fly ash from fluidised bed boilers have particularly high potential due to their high CaO content. Fly ash can be used in the mineral sequestration of CO₂ via direct and indirect carbonation. Both methods use CO₂ and flue gases. Studies conducted so far have analysed the influence of factors such as temperature, pressure, and the liquid-to-solid (L/S) ratio on the carbonation process, which have shown different effects depending on the ash used and the form of the process. Due to the large differences found in the properties of fly ash, related primarily to the type of fuel and boiler used, the process of mineral CO₂ sequestration requires much research into its feasibility on an industrial scale. However, the method is promising for industrial applications due to the possibility of reducing CO₂ emissions and, at the same time, recovering waste.

1. Introduction

The achievement of a low-emission economy or society will require the severe limitation of CO₂ emissions in the mid-21st century. This process needs to be accompanied by the implementation of low-emission technologies, as well as changes in some social models and lifestyles.

Human activities are estimated to have caused the Earth to have warmed by approximately 1.0 °C above the temperature levels observed before the industrial era [1]. It is important to take all possible actions to limit greenhouse gas emissions. The Paris Agreement determined a global action plan, which intends to limit global warming to well below 2 °C, preferably to 1.5 °C, relative to pre-industrial temperature levels [2]. Recent anthropogenic greenhouse gas emissions have been the highest in history and are probably the main cause of the climate change observed since the mid-20th century (i.e., global warming) [1].

Global CO₂ emissions have increased by approximately 850 Mt since 2019, and emissions from coal combustion have increased by 900 Mt. Coal combustion has been the largest contributor to the increase in global CO₂ emissions in the post-pandemic era. In 2023, coal combustion was responsible for approximately 70% of the increase in global emissions from energy combustion in 2023. Global CO₂ emissions from energy generation increased by 1.1% in 2023 to 37.4 billion tons (Gt), and coal combustion emissions accounted for more than 65% of this increase [3].

Coal combustion generates fly ash, which is captured in gas cleaning systems as a by-product of the combustion process [4].

Limiting climate warming to 2 °C or 1.5°C requires changes in energy systems over the next 30 years, such as reducing fossil fuel consumption, increasing production from low- and zero-carbon energy sources, and increasing the use of alternative energy carriers [5].

One of the key actions to mitigate climate change is the implementation of carbon capture, utilisation, and storage (CCUS) technologies [6]. CCUS is a type of technology for capturing CO₂ (from large point sources, such as fossil fuel or biomass power plants, cement plants, etc.); its transport (by pipeline, ship, rail, etc.); and storage in deep geological formations or utilisation [7,8].

Under CCUS, CO₂ utilisation involves the use of CO₂ in concentrations above atmospheric levels, either directly or as a raw material in industrial or chemical processes (for example, the conversion of CO₂ into liquid fuels and chemicals [9] and mineral sequestration [10]), to produce valuable carbon-containing products [10].

The use of carbon dioxide can be realised through a number of applications in which CO₂ is used directly (i.e., without chemical modification) or indirectly (i.e., converted into various products [7]).

The mineral sequestration method is an option for CO₂ utilisation through the following processes [10]:

• the mineral carbonation of natural resources and wastes;
• the naturally occurring carbonation processes of concrete;
• the production of sodium bicarbonate (NaHCO₃);
• the CO₂ treatment of concrete;
• the carbonation of bauxite residues.

For power plants using coal as their primary fuel, the mineral sequestration of CO₂ using fly ash is a particularly interesting option [11].

Mineral carbonation–mineral sequestration as a method of carbon dioxide sequestration was first proposed by Seifritz (1990) in Nature [12], but the first study on this matter was published in 1995 by Lackner, K.S.; Wendt, C.H.; Butt, D.P.; Joyce, E.L. Jr.; and Sharp, D.H. [13].

Mineral carbonation is a reaction of CO₂ with metallic oxides such as Ca, Mg, or Fe, as a result of which, the insoluble carbonates are created [14]:

MO + CO₂ → MCO₃ + heat

(1)

For the two basic oxides, CaO and MgO, the reactions can be described by the following equations [13,15]:

CaO + CO₂ → CaCO₃ + 179 kJ/mol

(2)

MgO + CO₂ → MgCO₃ + 118 kJ/mol

(3)

CO₂ mineral sequestration is an ecologically safe method. As a result of the occurring processes, carbon dioxide is permanently bonded in the form of carbonates that exist in the natural environment. The products of carbonation are mainly carbonates of calcium (calcite and aragonite); magnesium, calcium, and magnesium (dolomite); potassium; and rare-earth elements.

Natural resources (olivine, talc, serpentinite) as well as mineral wastes (fly ash, steel slag, concrete and cement wastes, CKD) can be used in the mineral carbonation process [16].

In this article, issues related to the mineral sequestration of CO₂ using fly ash from power plants (power plants and combined heat and power plants) are presented while taking into account the type of coal used as fuel.

An innovative approach to mineral sequestration presented in the article is the analysis of the process by the type of fuel burned (hard coal, lignite, or coal with biomass) and the type of boiler used. This is a very important approach to the topic, since fly ash from lignite combustion and ash from fluidised bed boilers especially are waste materials with limited recovery, which are characterised by a high CaO content.

The article also touches on the possibility of implementing the mineral sequestration of CO₂ on an industrial scale, which is a very important part of the analysis for a complete picture of the whole process.

The applicability of fly ash from coal combustion is analysed from the point of view of fuel being used as a factor to determine its potential for CO₂ fixation, which is a completely new and previously unpublished approach to this topic.

2. Mineral Carbonation—Fundamentals of the Process

The process of mineral carbonation under normal conditions is very slow, especially in the case of natural minerals; therefore, in order to improve the speed and yield of a reaction with CO₂, a so-called pretreatment can be used. The pretreatment methods for natural and waste raw materials that can be used to bind CO₂ include the following [17,18]:

• grinding—the rate of the carbonation reaction increases with an increase in surface area, and a reduction in grain size results in an increase in permeation by 10 to 90%;
• magnetic separation—as a result of this separation, iron compounds are removed, so the reaction rate increases;
• thermal treatment—the carbonation of gaseous CO₂ is very slow at room temperature, but thermal treatment can accelerate the reaction;
• surface activation techniques—the specific surface area of the mineral is increased by applying acids or steam.

Mineral sequestration can be carried out as a direct method or as an indirect method.

Sequestration using the direct method can be carried out via the following processes:

• CO₂—aqueous suspension of mineral resources (natural or waste);
• CO₂—raw mineral materials (natural or waste);
• flue gases—aqueous suspension of raw mineral materials (natural or waste);
• flue gases—raw mineral materials (natural or waste).

The processes using waste are carried out most often as a direct carbonation method by using aqueous waste suspensions with different water-to-waste ratios [19].

The indirect carbonation process involves the prior extraction of reactive carbon dioxide components from a mineral matrix. Extraction can be carried out via the use of the following [13,15,17,18,19,20,21,22,23,24]:

• HCl;
• calcium hydroxide—Ca(OH)₂;
• a double leaching process;
• acetic acid;
• MgCl₂-3.5H₂O;
• NaOH;
• NH₄Cl;
• seawater.

The efficiency of carbonation depends on the efficiency of Ca extraction, suggesting that the Ca extraction stage is the most important factor controlling the speed and extent of the process.

The primary product of CO₂ mineral sequestration when using fly ash is calcite CaCO₃, but vaterite or aragonite can also be produced [25,26,27,28,29,30].

Carbonation occurs primarily in the phases of portlandite (4), calcium silicates (5), (6), and ettringite (7), as follows [31]:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(4)

CaSiO₃ + CO₂ → CaCO₃ + H₂O

(5)

CaO∙nSiO₂∙mH₂O (C−S−H) + CO₂ → CaCO₃ + SiO₂ + mH₂O

(6)

1/3(CaO∙Al₂O₃3CaSO₄∙32H₂O) + CO₂ → CaCO₃ + sCaSO₄∙2H₂O + 2/3Al(OH)₃ + 23/3H₂O

(7)

Fly ash is most often used for the sequestration of CO₂ in the form of a water suspension, with mineral carbonation conducted following the direct method, i.e., treating the suspension with CO₂. The process can be separated into the following three phases [26]:

CaO (s) + H₂O (l) → Ca²⁺ (aq) + 2OH⁻ (aq)

(8)

CO₂ (aq) + H₂O ↔ HCO³⁻ (aq) + H⁺ → CO₃²⁻ (aq) + 2H⁺

(9)

Ca²⁺ + CO₃²⁻ ↔ CaCO₃ ↓

(10)

MgCO₃ can also be formed from the reaction of brucite and periclase [26].

The mineral sequestration of CO₂ is a complex process, in which the following stages can be separated for suspensions [32,33,34]:

• diffusion of CO₂ into a suspension;
• solvation of CO₂(g) to CO₂(aq);
• formation of H₂CO₃;
• dissociation of H₂CO₃ into H⁺, HCO³⁻, and CO₃²⁻;
• migration of Ca²⁺ ions from the sorbent phases into a solution;
• nucleation of carbonates;
• precipitation of carbonates.

The slowest stage is the dissolution of CO₂ in water and its diffusion. The limiting factor for CaCO₃ formation is the maximum amount of calcium ions available in the aqueous environment, which strongly affects the water parameters (temperature, pH, etc.) and the type of calcium speciation (hydroxide, sulphate, chloride, etc.). In waste subjected to mineral carbonation, the Ca(OH)₂ content gradually decreases until it disappears. The resulting calcite is absorbed and precipitates on the surface of the particles, forming the passivation layer [35].

Boundary layers directly affect the reaction rate and CO₂-binding capacity by limiting the diffusion of CO₂ through the resulting process products; these layers block or hinder the access of reactive components to the solid matrix on the particle surface [36,37,38,39].

3. Factors Influencing the Mineral Carbonation Process

The absorption and binding of carbon dioxide depend on the conditions under which the mineral carbonation process is carried out; these are primarily temperature and pressure but also the pH, time, and liquid-to-solid ratio (L/S) (Table 1 and Table 2).

3.1. Fly Ash Type

The controlling factor in determining the mineral sequestration process of CO₂ is the chemical composition of the fly ash, particularly the CaO and free CaO contents [24,25]. The CaO content is a factor affecting the process performance for both the direct [30,40] and indirect mineral carbonation methods [24,41].

The largest quantities of CaO are contained in fly ashes generated by lignite combustion (Table 1) and those collected from fluidised beds (Table 1 and Table 2) [42].

The CaO content of ashes from lignite combustion was as high as 64.7% (Table 1), while that for ashes from hard coal combustion was 32.4% (Table 2).

The problem with analysing the mineral carbonation potential is the lack of information in most publications on the type of boiler in which the coal was burnt. The composition of fly ash is highly dependent not only on the type of coal used but also on the type of boiler. For fluidised bed boilers, the CaO content increases dramatically because of the presence of desulphurisation products. Fly ash from boilers using a dry desulphurisation method will also have a high CaO content.

Table 1. CO₂ mineral sequestration using lignite and sub-bituminous fly ash.

Fly Ash Type	CaO/Ca Content	Process Characterisation					Reference
			Temperature	Duration	Other Process Parameters	Results of process
		Suspension—CO2				Maximum conversion: 5.2 moles CO2/kg FA	[40]
		Semi-dry reaction conditions L/S = 0.03–0.36 dm3/kg				Carbonation efficiency: 13.1–52.8%; cumulative CO2 uptake: 2.2–4.8 mmol/g	[43]
		Aqueous carbon sequestration process		1; 4 h		Results of process: amorphous calcium carbonate	[44]
	CaO—29.7%	Suspension—CO2	25; 60 °C	30–60 min.	p: 0.1 MPa	CO2 capture capacity: 26.4 kg CO2/Mg FA	[45]
	CaO—29.6; 19.6; 24.7; 64.7%	Suspension—flue gas (CO2; SO2; NOx)	200–600 °C waters	1–6 h	p: 0.1; 0.2 MPa	Carbonation of ash slurries and carbonation yield products: 19–37%	[46]
		Suspension—CO2 L/S = 0.1−0.7	40 °C	24 h	p: 3.0 MPa	FA can store up to 23 kg CO2/Mg FA	[47]
		Suspension (brine and water)—CO2	40 °C	48 h	p: 3.0 MPa	Sequestration: 10.03, 19.93, and 25.66 kg CO2/Mg FA for water saturation; 10.60, 20.50, and 26.23 kg CO2/Mg FA for brine saturation; sequestration capacity: 7.66 kg CO2/Mg FA	[48]
		Suspension—CO2 S/L = 0.8; 1.0; 1.25; 2.5				CO2 absorption: 1.4–8.8 g CO2/100 g	[49]
		Suspension—CO2 L/S = 0.7:1; 0.8:1				CO2 absorption: 4.71–9.33 g CO2/100 g	[50]
		Fly ash—CO2				Best carbonation efficiency: 10.5%	[51]
	Ca—24.85%	Suspension—CO2	30–80 °C; optimum—75 °C	10–56 min. Optimal —60 min.	-	%CO2 capture: max. 19.6 % efficiency: max. 78.62	[52]
	CaO—8.28%	Suspension—CO2 L/S = 3~7	30 °C	0.25 h	p: 0.05, 0.1, 0.15, 0.2, 0.25	Max. sequestration capacity: 67.92 g/kg	[35]
	CaO—9.4; 32.4%	One- or five-cycle leaching; carbonation of ammonium chloride	25; 40; 60; 80° C	10–60 min.		Carbonation percentage of Mg and Ca decreased with increasing cycle number	[53]
	CaO—15.0%	Slurries of Ca–FA-AEW		86 d		Sequestration efficiency: up to 32.3 g CO2/kg	[54]
	CaO—38.77%	Suspension—CO2 L/S = 10 l/kg	25; 50; 100; 150; 200 °C	24 h	p: 0.1–1.5	Sequestration efficiency: 212.57 kg CO2/Mg FA	[55]
Class F	CaO—8.80%	Suspension—CO2 L/S = 6	30 °C	900 s	p: 0.20 MPa	CO2 sequestration: 54.9 g/kg; initial rate of CO2 sequestration: 0.035 mol/kg/s	[56]
Class F	CaO—2.08%	Suspension (fly ash + cement)—CO2		20 min.		Max. CO2 uptake efficiency: 1.39 mg CO2/g fly ash + cement	[57]
	CaO—18.0%	Suspension—CO2			p: atmospheric	CO2 sequestration capacity: 81.70 g CO2/kg fly ash; CO2 sequestration capacity of treated fly ash: 81.44 g CO2/kg FA	[58]
high-alkali coal	CaO—36.30%	Fly ash: CaO—3:1 ratio carbonation furnace flue gas (10% CO2 and 90% N2)	750 °C	1 h	Analytical reagents: KOH, NaOH, Fe(OH)3, NaCl, KCl, Fe(Cl)3; doping ratios: 10%; 20%; 30%	Carbonation conversion after 1 h = range from about 60% to almost 100%	[59]
	CaO—13.4%	Suspension—CO2	40; 140 °C	120 min.	p: 0.2 MPa	103.0 and 102.0 g CO2/kg FA	[60]
	CaO—16.4%					43.2 g CO2/kg FA
FBC	CaO—15.5%	Suspension—CO2 S/L = 1.0		28 d		CO2 storage capacity: 15.7%; degree of CO2 binding: 11.4%	[11]

Table 1. CO₂ mineral sequestration using lignite and sub-bituminous fly ash.

Fly Ash Type	CaO/Ca Content	Process Characterisation					Reference
			Temperature	Duration	Other Process Parameters	Results of process
		Suspension—CO2				Maximum conversion: 5.2 moles CO2/kg FA	[40]
		Semi-dry reaction conditions L/S = 0.03–0.36 dm3/kg				Carbonation efficiency: 13.1–52.8%; cumulative CO2 uptake: 2.2–4.8 mmol/g	[43]
		Aqueous carbon sequestration process		1; 4 h		Results of process: amorphous calcium carbonate	[44]
	CaO—29.7%	Suspension—CO2	25; 60 °C	30–60 min.	p: 0.1 MPa	CO2 capture capacity: 26.4 kg CO2/Mg FA	[45]
	CaO—29.6; 19.6; 24.7; 64.7%	Suspension—flue gas (CO2; SO2; NOx)	200–600 °C waters	1–6 h	p: 0.1; 0.2 MPa	Carbonation of ash slurries and carbonation yield products: 19–37%	[46]
		Suspension—CO2 L/S = 0.1−0.7	40 °C	24 h	p: 3.0 MPa	FA can store up to 23 kg CO2/Mg FA	[47]
		Suspension (brine and water)—CO2	40 °C	48 h	p: 3.0 MPa	Sequestration: 10.03, 19.93, and 25.66 kg CO2/Mg FA for water saturation; 10.60, 20.50, and 26.23 kg CO2/Mg FA for brine saturation; sequestration capacity: 7.66 kg CO2/Mg FA	[48]
		Suspension—CO2 S/L = 0.8; 1.0; 1.25; 2.5				CO2 absorption: 1.4–8.8 g CO2/100 g	[49]
		Suspension—CO2 L/S = 0.7:1; 0.8:1				CO2 absorption: 4.71–9.33 g CO2/100 g	[50]
		Fly ash—CO2				Best carbonation efficiency: 10.5%	[51]
	Ca—24.85%	Suspension—CO2	30–80 °C; optimum—75 °C	10–56 min. Optimal —60 min.	-	%CO2 capture: max. 19.6 % efficiency: max. 78.62	[52]
	CaO—8.28%	Suspension—CO2 L/S = 3~7	30 °C	0.25 h	p: 0.05, 0.1, 0.15, 0.2, 0.25	Max. sequestration capacity: 67.92 g/kg	[35]
	CaO—9.4; 32.4%	One- or five-cycle leaching; carbonation of ammonium chloride	25; 40; 60; 80° C	10–60 min.		Carbonation percentage of Mg and Ca decreased with increasing cycle number	[53]
	CaO—15.0%	Slurries of Ca–FA-AEW		86 d		Sequestration efficiency: up to 32.3 g CO2/kg	[54]
	CaO—38.77%	Suspension—CO2 L/S = 10 l/kg	25; 50; 100; 150; 200 °C	24 h	p: 0.1–1.5	Sequestration efficiency: 212.57 kg CO2/Mg FA	[55]
Class F	CaO—8.80%	Suspension—CO2 L/S = 6	30 °C	900 s	p: 0.20 MPa	CO2 sequestration: 54.9 g/kg; initial rate of CO2 sequestration: 0.035 mol/kg/s	[56]
Class F	CaO—2.08%	Suspension (fly ash + cement)—CO2		20 min.		Max. CO2 uptake efficiency: 1.39 mg CO2/g fly ash + cement	[57]
	CaO—18.0%	Suspension—CO2			p: atmospheric	CO2 sequestration capacity: 81.70 g CO2/kg fly ash; CO2 sequestration capacity of treated fly ash: 81.44 g CO2/kg FA	[58]
high-alkali coal	CaO—36.30%	Fly ash: CaO—3:1 ratio carbonation furnace flue gas (10% CO2 and 90% N2)	750 °C	1 h	Analytical reagents: KOH, NaOH, Fe(OH)3, NaCl, KCl, Fe(Cl)3; doping ratios: 10%; 20%; 30%	Carbonation conversion after 1 h = range from about 60% to almost 100%	[59]
	CaO—13.4%	Suspension—CO2	40; 140 °C	120 min.	p: 0.2 MPa	103.0 and 102.0 g CO2/kg FA	[60]
	CaO—16.4%					43.2 g CO2/kg FA
FBC	CaO—15.5%	Suspension—CO2 S/L = 1.0		28 d		CO2 storage capacity: 15.7%; degree of CO2 binding: 11.4%	[11]

Table 2. Mineral sequestration of CO₂ using hard coal fly ash.

Fly Ash Type	CaO/Ca Content	Process Characterisation					Reference
			Temperature	Duration	Other Process Parameters	Results of the process
		Suspension—CO2				Carbonation efficiency of 83.5%—final CO2 3.2%, i.e., 32 g CO2/kg FA	[61]
	CaO—3.72 %	carbonation/zeolitization				1 Mg–45 kg of CO2 79% carbonation efficiency	[62]
		Suspension—CO2; 0.5 M NaHCO3		4 h		Results of the process: amorphous calcium carbonate	[44]
		Suspension—CO2				Aqueous carbonation capacity to sequester CO2: 26 kg CO2/Mg FA	[63]
						Accelerated mineral carbonation: 3.86 ± 1.28 CaCO3	[64]
		Solid—CO2				0.29–4.29 mmol CO2 capture/g FA	[65]
		Solid—CO2	25 °C 45 °C	42.9; 52.3; 68.7; 106.7; 163.1; 177.0; 209.1 min. 24.9; 54.0; 79.5; 96.0; 101.1; 111.3; 134.8 min.	p: 0.1; 0.25; 0.5; 0.75; 1;0; 1.25; 1.5 MPa	CO2 uptake: 18.2 wt. %; max. carbonation efficiency: 74%	[30]
		Suspension—CO2			24.9	10.71–27.05 kg CO2/Mg FA	[48]
		Suspension—CO2				CO2 absorption: 0.42–1.31 g CO2/100 g	[66]
							[11]
	CaO—9.198%	CO2—FA/brine slurry S/L = 0.1; 0.5; 1.0	30; 90 °C		p: 4.0 MPa	CO2 sequestration potential: 36.47 and 71.84 kg of CO2/Mg FA	[67]
		Fly ash brine dispersion—CO2 S/L = 0.1; 0.5; 1.0	30; 90 °C		p: 1.0; 4.0 MPa	CO2 content: 2.75–6.5 %wt.	[68]
	CaO—7.2%	Indirect aqueous carbonation			1 M NH4Cl; seawater	CO2 sequestration: 0.008 kg CO2/kg FA	[41]
	Ca—42.28 mg/g	Indirect mineral carbonation		2 h	Gas mixtures: 15% and 33% CO2	CO2 storage capacity: 31.1 mg CO2/g FA	[69]
	CaO—4.74%	Suspension—CO2 S/L = 50–200 g/L; optimum—100 g/L	25–60 °C optimum—40 °C	90 min.	(MEA)/N-methyldiethanolamine (MDEA); mixed amine solution (MAS)	Mineralisation efficiency: max. 64.8%	[24]
		Sonochemically enhanced carbonation				Max. conversion to carbonate: 50.5%	[33]
	CaO—22.75%	Suspension—CO2 L/S = 0.2; 0.3	40 °C			CO2 sequestration: 0.2; 0.21% by mass	[25]
	Ca—3.44%	Indirect carbonation process L/S = 6.25; 8.9; 12.5; 20; and 25 for 0.06 M HNO3			HNO3/Ca = 0.44; 0.62; 0.87; 1.4; 1.74	CO2 uptake efficiency: 0.011 g CO2/g FA	[70]
	CaO—6.74%	Solid—CO2	30 ± 3°C	0.5; 1; 2; 3; 4 h; optimum: 1 h	p: 0.2; 0.4; 0.6; 0.8; 1.0 MPa optimum pressure: 0.1 MPa	Max. carbonation capacity: 50.3 g CO2/kg FA	[71]
		Suspension—CO2 L/S = 2; 5; 10; 15; 20		0.5; 1; 2; 3; 4 h; optimum: 2 h	p: 0.2; 0.4; 0.6; 0.8; 1.0 MPa optimum pressure: 0.4 MPa	Max. carbonation capacity: 26.3 g CO2/kg FA
	CaO—6.74%	Solid—flue gas	31.72; 40; 60; 80; 88.28 °C	0.5; 1; 2; 3; 4; 5; 6 h	p: 0.257; 1.5; 4.5; 7.5 MPa; optimum pressure: 4.87 MPa	Max. sequestration capacity: 21.03 g of CO2 /kg FA	[72]
		Suspension—CO2 L/S = 13.35	33.92; 62.5; 45; 80; 91.08 °C optimum—61.6 °C	0–180 min; optimum—50 min.	p: 0.418; 2;0; 4.5; 7.0; 8.583 L/S = 6.0; 1.57; 13.0; 20.0; 24.43 MPa; optimum pressure—0.481 MPa	Max. sequestration capacity: 50.72 g of CO2 /kg
	CaO—6.14%	Solid—CO2 L/S ratio of 0.15 H2O and NaOH solutions (1 and 3 M)	25; 50; 100; 150 °C	1 h	p: 1.0 MPa	Carbonation efficiency: 43.57%	[73]
	CaO—9.4%	Suspension—CO2	40 °C 140 °C	120 min.	p: 0.2 MPa	33.3 g-CO2/kg FA; 93.1 g-CO2/kg FA	[60]
	CaO—3.20%	Suspension (fly ash + red mud)—CO2			Ratio of red mud and fly ash ranged from 10:0 to 5:5	Carbonation capacity: 135.51 g CO2/kg fly ash + red mud	[74]
	CaO—1.24%	Solid—supercritical CO2	40 °C	3 h	Hydrothermally activated at 220 °C—30 min.	CO2 mineralization sequestration: 25.03 kg/Mg CO2 mineralization sequestration alter hydrothermal activation: 154.10 kg/Mg	[75]
	CaO—1.24%		100; 140; 180; 220 °C	3 h	p: 2.0 MPa	Adsorption energy of CO2: −66.424 kcal/mol; adsorption energy of CO2 (hot steam): −65.037 kcal/mol	[76]
	CaO—4.16%	Fly ash + ground granulated blast furnace slag + alkaline activators + superplasticizer + C12H25SO4Na + H2O2 (alkali activated foam concrete)—CO2 (20%)	20 ± 2 °C		Relative humidity: 70 ± 5%	Max. CO2 sequestration capacity of alkali activated foam concrete: 26.41 kg/m3	[77]
	CaO—3.36%	Solid—CO2	20 °C	3 d	p: 3.0 MPa	Carbon sequestered: 1.68 g	[78]
		Suspension—CO2				Carbon sequestered: 1.83 g
	CaO—6.01%	Indirect		0.5; 1.0; 2.0; 4.0; 8.0;16.0 h; 1 d; 2 d; 4 d; 8 d; 16 d	1 g of dried CFA was reacted in a 250 mL solution with 0.1 mol/L oxalic acid	1 Mg of reacted fly ash could store over 34 kg of carbon	[79]
CFB fly ash	CaO—28.42%	Solid—CO2	300–800 °C	5; 10; 30; 60 min.	CO2 content: 5%; 10%; 15%; 20%; 100%	Max. CO2 sequestration capacity: 60 g CO2/kg FA; max. sequestration efficiency: 28.74%	[80]
CFBC fly ash	CaO—24.40%					CO2 sequestration: 1.27; 2.50%	[25]
CFB fly ash	CaO—33.06%	Suspension—CO2 S/L = 50; 100; 150; 200	20; 40; 60; 80°C optimum—60°C		p: 0.1 flue gas—CO2 conc.—14.85%	Carbonation efficiency: 78.17%; CO2 sequestration capacity: 0.128 g CO2/g CFA	[28]
CFB fly ash	CaO—28.83%	Suspension— exhaust gases L/S = 10:1	Ambient	30 min.	Exhaust gases: 72–78% N2; 12–15% CO2; 6–8% H2O; 3–4% O2; 300–500 ppm SO2; 150–220 ppm NOx; 10–50 ppm CO p: atmospheric	Carbon fixation effect: 58.14 kg CO2/ton for fly ash	[81]
CFB fly ash	CaO—38.35%	Suspension—CO2 L/S = 3:1			L/S = 3:1		[82]
CFB fly ash	CaO—38.42%	Suspension—CO2 L/S = 3:1; 5			L/S = 3:1; 5		[83]
CFB fly ash	CaO—7.03%	Suspension—CO2	40 °C	1 h	Liquid-to-solid ratio of 10 mL/g p: 3.0; 5.0; 7.0; 7.1; 7.2; 7.3; 7.4; 7.5; 7.6; 7.7; 7.8; 7.9; 8.0 MPa	Max. CO2 sequestration capacities: 89.3 g/kg	[84]
	CaO—5.02; 5.07; 4.10%					Max. CO2 sequestration capacities: 38.3; 35.5; 13.9 g/kg
CFB fly ash	Ca—18.46%	Suspension—CO2 L/S = 10 mL/g	40 °C	1 h	p: 1; 3; 5; 7; 8 MPa	Max. carbonation efficiency: ~90%	[85]
Fly ash from the co-combustion of coal and biomass	CaO—5.17%	Solid—CO2				Degree of carbonation: 1.51%	[86]

Table 2. Mineral sequestration of CO₂ using hard coal fly ash.

Fly Ash Type	CaO/Ca Content	Process Characterisation					Reference
			Temperature	Duration	Other Process Parameters	Results of the process
		Suspension—CO2				Carbonation efficiency of 83.5%—final CO2 3.2%, i.e., 32 g CO2/kg FA	[61]
	CaO—3.72 %	carbonation/zeolitization				1 Mg–45 kg of CO2 79% carbonation efficiency	[62]
		Suspension—CO2; 0.5 M NaHCO3		4 h		Results of the process: amorphous calcium carbonate	[44]
		Suspension—CO2				Aqueous carbonation capacity to sequester CO2: 26 kg CO2/Mg FA	[63]
						Accelerated mineral carbonation: 3.86 ± 1.28 CaCO3	[64]
		Solid—CO2				0.29–4.29 mmol CO2 capture/g FA	[65]
		Solid—CO2	25 °C 45 °C	42.9; 52.3; 68.7; 106.7; 163.1; 177.0; 209.1 min. 24.9; 54.0; 79.5; 96.0; 101.1; 111.3; 134.8 min.	p: 0.1; 0.25; 0.5; 0.75; 1;0; 1.25; 1.5 MPa	CO2 uptake: 18.2 wt. %; max. carbonation efficiency: 74%	[30]
		Suspension—CO2			24.9	10.71–27.05 kg CO2/Mg FA	[48]
		Suspension—CO2				CO2 absorption: 0.42–1.31 g CO2/100 g	[66]
							[11]
	CaO—9.198%	CO2—FA/brine slurry S/L = 0.1; 0.5; 1.0	30; 90 °C		p: 4.0 MPa	CO2 sequestration potential: 36.47 and 71.84 kg of CO2/Mg FA	[67]
		Fly ash brine dispersion—CO2 S/L = 0.1; 0.5; 1.0	30; 90 °C		p: 1.0; 4.0 MPa	CO2 content: 2.75–6.5 %wt.	[68]
	CaO—7.2%	Indirect aqueous carbonation			1 M NH4Cl; seawater	CO2 sequestration: 0.008 kg CO2/kg FA	[41]
	Ca—42.28 mg/g	Indirect mineral carbonation		2 h	Gas mixtures: 15% and 33% CO2	CO2 storage capacity: 31.1 mg CO2/g FA	[69]
	CaO—4.74%	Suspension—CO2 S/L = 50–200 g/L; optimum—100 g/L	25–60 °C optimum—40 °C	90 min.	(MEA)/N-methyldiethanolamine (MDEA); mixed amine solution (MAS)	Mineralisation efficiency: max. 64.8%	[24]
		Sonochemically enhanced carbonation				Max. conversion to carbonate: 50.5%	[33]
	CaO—22.75%	Suspension—CO2 L/S = 0.2; 0.3	40 °C			CO2 sequestration: 0.2; 0.21% by mass	[25]
	Ca—3.44%	Indirect carbonation process L/S = 6.25; 8.9; 12.5; 20; and 25 for 0.06 M HNO3			HNO3/Ca = 0.44; 0.62; 0.87; 1.4; 1.74	CO2 uptake efficiency: 0.011 g CO2/g FA	[70]
	CaO—6.74%	Solid—CO2	30 ± 3°C	0.5; 1; 2; 3; 4 h; optimum: 1 h	p: 0.2; 0.4; 0.6; 0.8; 1.0 MPa optimum pressure: 0.1 MPa	Max. carbonation capacity: 50.3 g CO2/kg FA	[71]
		Suspension—CO2 L/S = 2; 5; 10; 15; 20		0.5; 1; 2; 3; 4 h; optimum: 2 h	p: 0.2; 0.4; 0.6; 0.8; 1.0 MPa optimum pressure: 0.4 MPa	Max. carbonation capacity: 26.3 g CO2/kg FA
	CaO—6.74%	Solid—flue gas	31.72; 40; 60; 80; 88.28 °C	0.5; 1; 2; 3; 4; 5; 6 h	p: 0.257; 1.5; 4.5; 7.5 MPa; optimum pressure: 4.87 MPa	Max. sequestration capacity: 21.03 g of CO2 /kg FA	[72]
		Suspension—CO2 L/S = 13.35	33.92; 62.5; 45; 80; 91.08 °C optimum—61.6 °C	0–180 min; optimum—50 min.	p: 0.418; 2;0; 4.5; 7.0; 8.583 L/S = 6.0; 1.57; 13.0; 20.0; 24.43 MPa; optimum pressure—0.481 MPa	Max. sequestration capacity: 50.72 g of CO2 /kg
	CaO—6.14%	Solid—CO2 L/S ratio of 0.15 H2O and NaOH solutions (1 and 3 M)	25; 50; 100; 150 °C	1 h	p: 1.0 MPa	Carbonation efficiency: 43.57%	[73]
	CaO—9.4%	Suspension—CO2	40 °C 140 °C	120 min.	p: 0.2 MPa	33.3 g-CO2/kg FA; 93.1 g-CO2/kg FA	[60]
	CaO—3.20%	Suspension (fly ash + red mud)—CO2			Ratio of red mud and fly ash ranged from 10:0 to 5:5	Carbonation capacity: 135.51 g CO2/kg fly ash + red mud	[74]
	CaO—1.24%	Solid—supercritical CO2	40 °C	3 h	Hydrothermally activated at 220 °C—30 min.	CO2 mineralization sequestration: 25.03 kg/Mg CO2 mineralization sequestration alter hydrothermal activation: 154.10 kg/Mg	[75]
	CaO—1.24%		100; 140; 180; 220 °C	3 h	p: 2.0 MPa	Adsorption energy of CO2: −66.424 kcal/mol; adsorption energy of CO2 (hot steam): −65.037 kcal/mol	[76]
	CaO—4.16%	Fly ash + ground granulated blast furnace slag + alkaline activators + superplasticizer + C12H25SO4Na + H2O2 (alkali activated foam concrete)—CO2 (20%)	20 ± 2 °C		Relative humidity: 70 ± 5%	Max. CO2 sequestration capacity of alkali activated foam concrete: 26.41 kg/m3	[77]
	CaO—3.36%	Solid—CO2	20 °C	3 d	p: 3.0 MPa	Carbon sequestered: 1.68 g	[78]
		Suspension—CO2				Carbon sequestered: 1.83 g
	CaO—6.01%	Indirect		0.5; 1.0; 2.0; 4.0; 8.0;16.0 h; 1 d; 2 d; 4 d; 8 d; 16 d	1 g of dried CFA was reacted in a 250 mL solution with 0.1 mol/L oxalic acid	1 Mg of reacted fly ash could store over 34 kg of carbon	[79]
CFB fly ash	CaO—28.42%	Solid—CO2	300–800 °C	5; 10; 30; 60 min.	CO2 content: 5%; 10%; 15%; 20%; 100%	Max. CO2 sequestration capacity: 60 g CO2/kg FA; max. sequestration efficiency: 28.74%	[80]
CFBC fly ash	CaO—24.40%					CO2 sequestration: 1.27; 2.50%	[25]
CFB fly ash	CaO—33.06%	Suspension—CO2 S/L = 50; 100; 150; 200	20; 40; 60; 80°C optimum—60°C		p: 0.1 flue gas—CO2 conc.—14.85%	Carbonation efficiency: 78.17%; CO2 sequestration capacity: 0.128 g CO2/g CFA	[28]
CFB fly ash	CaO—28.83%	Suspension— exhaust gases L/S = 10:1	Ambient	30 min.	Exhaust gases: 72–78% N2; 12–15% CO2; 6–8% H2O; 3–4% O2; 300–500 ppm SO2; 150–220 ppm NOx; 10–50 ppm CO p: atmospheric	Carbon fixation effect: 58.14 kg CO2/ton for fly ash	[81]
CFB fly ash	CaO—38.35%	Suspension—CO2 L/S = 3:1			L/S = 3:1		[82]
CFB fly ash	CaO—38.42%	Suspension—CO2 L/S = 3:1; 5			L/S = 3:1; 5		[83]
CFB fly ash	CaO—7.03%	Suspension—CO2	40 °C	1 h	Liquid-to-solid ratio of 10 mL/g p: 3.0; 5.0; 7.0; 7.1; 7.2; 7.3; 7.4; 7.5; 7.6; 7.7; 7.8; 7.9; 8.0 MPa	Max. CO2 sequestration capacities: 89.3 g/kg	[84]
	CaO—5.02; 5.07; 4.10%					Max. CO2 sequestration capacities: 38.3; 35.5; 13.9 g/kg
CFB fly ash	Ca—18.46%	Suspension—CO2 L/S = 10 mL/g	40 °C	1 h	p: 1; 3; 5; 7; 8 MPa	Max. carbonation efficiency: ~90%	[85]
Fly ash from the co-combustion of coal and biomass	CaO—5.17%	Solid—CO2				Degree of carbonation: 1.51%	[86]

3.2. Liquid/Solid Ratio (L/S)

When analysing the degree of CO₂ binding by individual suspensions, it should also be taken into account that the degree of carbonation is also affected by the liquid/solid ratio (L/S) [28]. The liquid/solid ratio (L/S) determines the ability to leach Ca ions from fly ash into a solution.

When the solid-to-water (s/w) ratio is low, the gas permeability is high, and CO₂ diffuses efficiently. However, when the water content increases, the pores in the ash are blocked, and the diffusion of the gas into the pore system is stopped, resulting in an inhibition of the reaction [32].

3.3. Temperature and Pressure

Temperature has a significant effect on the course of carbonation, especially during the first few hours of the process. Each carbonation reaction process shows different sensitivities to the effect of the reaction temperature [36].

The mineral carbonation process has been studied over a wide temperature range, from ambient temperatures to 750 °C (Table 1 and Table 2).

Investigations into the effect of temperature in the range of 40 to 140 °C showed a large variation depending on the ash used [60]. Fly ash from lignite combustion showed the highest sequestration potential, but, in its case, increasing the temperature to 140 °C did not increase the efficiency of the process, which the authors explained was due to the fact that the unreacted Ca/Mg phases were stable in this temperature range (40–140 °C). In the case of another coal combustion origin fly ash studied by these authors, they found that it had a significant increase in carbonation efficiency when the temperature was increased from 40 to 140 °C, which was likely due to the low weight fraction of the Ca-bearing phases but high weight proportion of the Mg-containing phases. In the case of coal combustion fly ash, the increase in temperature had a significant effect on the process yield, the reactants being mainly lime and portlandite, which are reactive to CO₂ and were almost completely reacted to form calcite [60].

Increasing the temperature to 75 °C resulted in an initial increase (to 55%) [74,83] in the efficiency of the carbonation process, followed by a decrease, mainly due to the exothermic nature of the reaction, which does not favour the reaction at high temperatures. Excessively high temperatures can reduce the solubility of CO₂ in water, which can also reduce the carbonation process.

The hot steam activation of the carbonation process resulted in a 2.5-times higher process efficiency. The authors linked this effect to a reduction in the sintering effect and an increase in the activation effect of the process, leading to an increase in the surface activity of the fly ash pores and facilitating the transport of CO₂ molecules [76].

Higher process temperatures can delay the precipitation of CaCO₃ due to the reduced solubility of CO₂ [36]. However, the reaction rate can be significantly increased as the reaction temperature increases. The optimal temperatures are 60 °C for direct carbonation and 40 °C for indirect carbonation (Table 2).

The carbonation efficiency can also decrease with the increasing temperature, apparent gas velocity, and solid-to-liquid ratio, due to the rapid coating of active CaO by the passivation layer [28].

For direct gas–solid carbonation, temperature, pressure, and H₂O are key factors that affect the process. Increasing the temperature and pressure under the gas–solid process conditions has a significant increasing effect on the process yield, as confirmed in the study of Liu et al. [80]. Although high temperature and pressure can improve the kinetics of the process, their impact may be limited [87].

In the case of an indirect process and the leaching of CaO, increasing the temperature can increase the kinetics of calcium dissolution [36].

Pressure is also a factor that increases the efficiency of the carbonation process [35]. Increasing pressure to a supercritical level can significantly improve the sequestration capacity [84].

The increase in CO₂ sequestration capacity found by Shao et al. [35] when increasing the pressure from 0.5 to 2.5 MPa was explained using thermodynamics and Henry’s Law. In addition, increasing the pressure increased the amount of CO₂ dissolved in the solution and maintained a higher concentration of CO₃²⁻ ions, accelerating the precipitation of CaCO₃ and MgCO₃. The concentration of Ca²⁺ and Mg²⁺ ions at the end of the reaction also decreased, and more Ca and Mg were converted to carbonate precipitation. Another effect was the maintenance of a lower pH as a result of the large amount of dissolved CO₂, which contributed to the increase in Ca²⁺ and Mg²⁺ [35].

An improvement in the performance of the carbonation process with an increase in pressure from the non-supercritical (1 MPa) to the supercritical (8 MPa) state was found by the team of Yuan et al. [85]. This was due to the increased amount of CaCO₃ precipitated by adsorption precipitation [85].

Increasing the pressure in the mineral suspension–CO₂ sequestration process can improve the carbonation process as more CO₂ dissolves in the solution. However, the initial CO₂ pressure and temperature do not affect the efficiency of the process [63,71,72,88].

The effect of temperature on the efficiency of the carbonation process is very complex [87], and the results of studies vary quite significantly, from finding an initial upward trend followed by a decrease in process efficiency [48] to finding an initial decrease in process efficiency followed by an increase [60].

The conclusion put forward by Wang et al. 2024 [87] is that the low-temperature process is more efficient when the fly ash is characterised by a high CaO/MgO content.

4. The Effect of Mineral Sequestration on the Leachability of Contaminants

Carbonation may cause a reduction in the leachable volume of contaminants that are mainly leached at high pH values, while, for elements that are easily leached at medium and low pH values, carbonation results in the even easier release of these contaminants.

The reduction in the amount of leachability of a number of heavy metals is probably due to the formation of insoluble carbonates, the immobilisation of heavy metals by the C-S-H phase, or adsorption and co-precipitation following the formation of a solid solution with calcite.

Leaching is the result of interdependent processes: hydrolysis, hydration, dissolution/precipitation, oxidation/reduction, complex formation, sorption, and the formation of solid solutions and new minerals [89]. Among the factors induced by the carbonation process that have an effect on reducing leachability, the following are the most commonly cited [90]:

• lowering of the leachate pH;
• changes in solubility due to carbonate precipitation or the formation of oxygen anions;
• lowering the release of selected metals through their sorption by newly formed minerals;
• reduction in matrix porosity due to calcite formation.

The basic carbonation reaction of Ca(OH)₂ with carbon dioxide, resulting in the formation of carbonates, causes a reduction in pH. In the case of fly ash, this is a reduction in pH from 12–13 to about 8–9. The consequence of this reduction in pH is a change in solubility and leachability [48]. Therefore, carbonisation can cause a reduction in the leachability of elements that are leached at a high pH. For elements that are easily leached at medium and low pHs, carbonation causes them to move more easily into a solution [91].

Carbonation is an important factor in lowering the leachability of some heavy metal ions (Cd, Zn, Mn, Co, Ni, Pb, Sr, Ba, Cr, Cu, Se, As, and Hg) [30,48,87,92,93]. Regarding the leachability of Cd, Cu, Ni, Pb, and Zn, their minimum solubility is reached within the pH range of 7–10. Carbonation reduces the leachability of Pb, Ni, and Cu to the greatest extent [30]. Carbonation can increase the leachability of elements that form oxyanions, i.e., Cr, Mo, Sb, and Se, which show maximum leachability in the pH range of 7–10 [30,94].

A lower pH can cause the co-precipitation of some metals such as carbonates, which can lead to reduced leachability of these elements [87].

In the case of carbonation, an important factor in reducing the leachability of some heavy metal ions (Cd, Zn, Mn, Co, Ni, Pb, or Sr) and SO₄²⁻ ions is their sorption on calcite, leading to co-precipitation [11,95,96].

Sorption is potentially the most important mechanism controlling the leachability of Zn, Pb, Cd, and Cu, which have a strong affinity for calcite and are reduced by sorption onto CaCO₃, leading to co-precipitation. The affinity of these metals for CaCO₃ increases with an increasing pH [11,34].

The reduction in Zn and Cr can also be explained by the immobilisation of heavy metals in the C-S-H phase. The most important mechanism of immobilisation is probably adsorption on the silicate surface, incorporation into the crystal matrix, or chemical bonding [97,98].

Carbonation can increase the leachability of SO₄²⁻ ions resulting from the decomposition of ettringite and the formation of soluble CaSO₄ (Equation (7)) [98].

Regarding the structure of calcite, positions occupied by Ca²⁺ cations can be substituted for other divalent metals, especially during its growth. A solid solution is formed when ions of one type are substituted for ions of another type in the crystal lattice (structure). A solid solution of CdCO₃/CaCO₃ is formed when Cd²⁺ ions substitute Ca²⁺ ions. Among the ions that are adsorbed on the calcite are Cd, Zn, Mn, Co, Ni, Pb, and Sr [95].

The arsenic leachability may be reduced due to adsorption and co-precipitation following the formation of a solid solution with calcite [99] or increased sorption by iron oxides at a reduced pH [54,100].

The reduction in Cu leachability is explained by the formation of copper carbonate [101]. Heavy metal carbonates that form as a result of the carbonation process are undetectable using the X-ray method due to their trace amounts.

The authors [85] suggest that physical encapsulation is a stabilising phenomenon for all heavy metals. Increasing the pressure improves the carbonation efficiency and increases the stabilisation effect for all heavy metals. However, increasing the pressure to a supercritical level increases the solubility of CO₂, and the heavy metal form changes to an easily leachable form [85]. The formation of a passivation layer as a result of subsequent carbonation, on the one hand, limits the progress of the process and, on the other hand, may be a factor in limiting the leachability of heavy metals [87,102,103].

5. Fly Ash in CCUS Technology—Product Disposal and Utilisation

A very important feature of mineral carbonation is the possibility of using the products to benefit the economy.

Mineral carbonation can be used in various applications, as follows [31]:

• in situ: underground mineral sequestration of CO₂ combined with the geological storage of CO₂;
• ex situ: aboveground industrial processes;
• end-of-pipe technology (CO₂ is converted into solid carbonates that are stored for sequestration);
• process-integrated technology (this applies, for example, to the use of CO₂ in the production of construction materials).

In the case of mineral carbonation, the process can be carried out using both pure CO₂ and flue gases, as demonstrated by pilot- and industrial-scale examples of method implementation [61,104,105].

An example of research into the implementation of mineral sequestration in construction materials is the production of aggregates using gypsum and cement [106]. The authors [106] found a positive effect of mineral carbonation on the compressive strength of aggregates.

Another proposal for the use of fly ash subjected to mineral carbonation is to make mortars from it and then cure these mortars in a CO₂ atmosphere [107].

Research is being conducted around the world to optimise the process for implementation at the pilot and industrial scales. The technology developed by Blue Planet Systems uses fly ash in the mineral carbonation process to produce fine and coarse artificial aggregates and is based on an indirect carbonation process. The process can use CO₂ or flue gases with more than 5% contents. Currently, the technology is at the stage of a pilot plant operation [105]. Waste concrete, cement kiln plant dust, steelmaking slag, fly ash, bauxite residues, and silicate rocks are calcium-rich sources of so-called geomasses, which produce a CO₂-binding aggregate coating. The Blue Planet process reduces CO₂ emissions by 100 kg per ton of concrete, equivalent to about 220 kg CO₂/m³ of concrete. Taking into account the cement content in 320 kg/m³ of concrete, this results in a reduction of about 0.65 tons of CO₂ per ton of Portland cement [105].

6. Conclusions

Fly ash from coal combustion is a waste material that, due to its properties, is often considered a material for CO₂ bonding via mineral carbonation in CCUS technology. The rate of fly ash generation is 660 Mt/year, while the storage capacity is estimated at 144 Mt CO₂/year [108].

The average amount of CO₂ that can theoretically be absorbed via the mineral carbonation of fly ash from lignite combustion is 14.6% ± 2.8, while the experimental potential is 26.4–23.0%. For fly ash from coal combustion, these potentials are lower at 3.6% ± 0.6 and 2.0–3.0%, respectively, mainly due to the CaO content [109,110,111].

In recent years, the use of fly ash for the mineral sequestration of CO₂ has often been analysed and identified as a future technology due to its many advantages. At the same time, the mineral sequestration of CO₂ can be considered a waste recovery method [11].

Particular attention has been paid to the process of direct carbonation. However, carbonation is a very complex process and requires further research, especially in its optimisation, the improvement of mass transfer, its efficiency, and better understanding of the mechanics. Attention should also be paid to the production of value-added products or the efficient use of its products [103].

Fly ash from the combustion of coal and lignite and co-combustion with biomass, with widely varying CaO contents ranging from a few to several tens of per cent, is used for mineral sequestration studies.

So far, research has focused primarily on process analysis in the direct suspension–CO₂ pathway. More and more research is also being carried out with gases of similar composition to flue gases.

Various liquid-to-solid ratios are used with aqueous suspensions, and L/S is identified as one of the most important factors influencing the carbonation process due to its effect on the ability to leach Ca ions from fly ash into a solution.

Another process parameter that has been analysed is temperature, the influence of which is not unequivocal in the carbonation process due to the complexity of the process, as well as the variability of the nature of the fly ash material. Different optimum temperature values were found for the CO₂ suspension and CO₂ fly ash processes, resulting from the nature of the process and the rate of formation of the CaCO₃.

Pressure is a process parameter that also has a positive effect on mineral sequestration efficiency; this has been confirmed by numerous studies.

Future Work

The use of fly ash for CO₂ sequestration on an industrial scale remains a research challenge due to the nature of this waste and, in particular, the variability of its composition.

The key issue to be researched is increasing the efficiency of the direct CO₂ (flue gas)–fly ash carbonation process. It is necessary to develop optimal conditions for the mineral carbonation process that would enable the implementation of the technology on an industrial scale. Future research should also focus on technical solutions for process chambers to maximise CO₂ fixation and prevent gas leakage (Figure 1).

A direction of research that should also be continued is the economic application of the process products. The economic application of carbonation products as part of the CCUS technology should also be addressed in future research.