Energy Sector Derived Combustion Products Utilization—Current Advances in Carbon Dioxide Mineralization

Abstract

Carbon dioxide and combustion products are among the main waste streams deriving from the energy sector. Efficient and cost-effective methods of solid waste valorization and carbon capture, storage and utilization are needed in the transition period towards carbon neutrality in light of the recent scenarios forecasting energy demand and energy supply mix under dynamic social, economic and political circumstances. Within this paper, the current advances in carbon dioxide mineralization, combining carbon dioxide utilization and combustion products valorization, are presented in terms of the recognized methodological options of carbonation methods, process efficiency and effects on the process product properties. Special attention is given to the studies on the valorization of fluidized bed boilers fly ash, differing in a range of parameters from the conventional boilers fly ash, as well as the effects of the carbonation process on the stabilization and improvement of its properties and the resulting extended range of applicability. The relevant research fields needing further investigations, as well as the desired decision makers’ supporting actions, are also specified.

1. Introduction

1.1. Energy Demand and Carbon Dioxide Emission Forecasts

The reduction in carbon dioxide emissions and in the volume of combustion products are the major energy policy targets of European countries, along with the development of the technologies to increase energy efficiency and implementation of less carbon-intensive energy production technologies [1,2,3,4]. New waste valorization techniques and novel applications of various solid fuels’ derived conversion products are also sought [5,6,7]. This is because it seems that the role of thermochemical conversion of solid fossil fuels will be still unquestionably dominant in the transition period, before reaching carbon neutrality. This is especially the case in the light of the recent disturbances in the energy sector and the world economy in general, resulting from the COVID-19 pandemic and new political and economic challenges resulting from the military conflict close to the eastern frontier of the European Union.

In an International Energy Agency (IEA) document of 2019, a yearly increase in power demand of 2.1% by 2040 was forecasted, from approximately 26,600 TWh to 41,400 TWh, as well as the rise in power demand in the final energy consumption from 19% in 2018 to 24% in 2040 [8]. The main contributors to this were predicted to be industry (over 30% of the expected world increase in power demand by 2040), cooling systems (17%), as well as large and small appliances and electric vehicles (10% each). Furthermore, a rise in the share of renewables in power generation was expected, from 26% in 2018 to 44% in 2040, with a relevant increase in photovoltaics and wind energy systems from 7% to 24%. The world coal-based energy generation capacity was assumed to be maintained at a level of approximately 10,000 TWh, mainly because the developments in Asian countries are expected to exceed the relevant decline in developed countries. It was assumed that coal will still provide as much as 25% of power generation in 2040 (38% in 2018), while natural gas and nuclear power were expected to represent 20 and 10% shares, respectively.

World carbon dioxide emissions were also expected to decline to 80 g/kWh by 2040, i.e., 80% below the level of 2018, as a result of the wider implementation of renewables and nuclear energy systems as well as carbon capture, storage and utilization (CCUS) technologies. The power and heat generation sector has been providing safe, flexible and cost competitive supplies, but also contributing significantly to greenhouse gas emissions. Approximately 60% of the total 2080 GW of the existing coal-fired power plants installed capacity has been under operation for over 20 years now [8]. Therefore, in the IEA forecast, special attention was also given to the retrofitting of about 240 GW of the installed capacity of coal-fired power plants by employing carbon capture and storage systems or by the application of the co-combustion with biomass, methods which would require the investments of 225 billion USD. The expected world yearly coal demand for the period of 2018–2040 was projected to be stable and to amount to 5400 Mt [8].

The economic disturbances related to the COVID-19 pandemic imposed a revision of these forecasts, at least regarding the period of 2020–2021, which was taken into account in the next IEA outlook [9]. The world power demand decreased in 2020 by 1% on average when compared to the value of 2019, but an estimated increase of 4.5% for 2021 was assumed. The demand for non-renewable energy resources in power generation decreased by approximately 3% in total, with the highest value for coal, of 440 TWh, also being caused by the relatively low prices of natural gas. The world carbon dioxide emissions for 2020 also decreased by 5.8% (i.e., 2 GtCO₂) to 31.5 Gt. However, at the same time, the concentration of this gas in the atmosphere was at the highest level ever reported, at 412.5 ppm. The expected increase in carbon dioxide emission from energy sector in 2021 was 4.8%, i.e., 1500 MtCO₂. Of this amount, 640 MtCO₂ was forecasted to be related to coal utilization and 215 MtCO₂ to the natural gas use. The expected rise in power demand was to be covered in 50% with renewables, in 2% with nuclear energy sources and in 48% with fossil fuels (coal and natural gas), with the parallel rise in coal-based power generation of 480 TWh [9]. Despite the consequences and disturbances observed in the fuel and energy sectors related to the COVID-19 pandemic, the targets of the European Union countries’ in the field of carbon dioxide emission mitigation and atmospheric concentration reduction remained valid, along with the focus on the technological developments in low-emission and highly efficient energy technologies, as well as carbon capture and storage (CCS) and carbon capture, storage and utilization (CCUS) technologies.

1.2. Carbon Dioxide Capture, Storage and Utlization Technologies

The currently applied post-combustion capture technologies usually employ the process of the chemical absorption of carbon dioxide in amine solution, most commonly monoethanolamine. The capture efficiency of such systems is over 90%, and they may be used for flue gas streams of carbon dioxide concentrations of 5–15% vol. Among the pre-combustion capture technologies which are applicable to exhaust gases of higher contents of carbon dioxide, of 30% vol., achievable in gasification technologies, physical absorption and adsorption as well as membrane and cryogenic techniques should be mentioned. Nowadays, the industrial-scale carbon capture of reasonable profitability is performed by the so-called enhanced oil recovery (EOR) systems. Carbon capture and utilization technologies (CCU) are considered to be complementary to carbon capture and storage. They cover chemical synthesis application in the production of polymers, fuels and chemicals, like, e.g., methane, methanol, dimethyl ether, olefins, formaldehyde, urea or synthesis gas and mineralization technologies in the production of carbonates and construction materials. he latter applications have the highest long-term capture potential among the all current CCU technologies. Because of the limited demand for carbon dioxide in both EOR applications and chemical synthesis, combined with high costs of the hydrogen that needs to be provided for chemical synthesis and the higher costs of chemicals produced with the use of captured, anthropogenic carbon dioxide when compared to conventional technologies, the CCU technologies have recently been considered to be more economically feasible when constituting elements of the so-called Power-to-X technologies. The latter technologies make use of excess renewable electricity, under the conditions of the insufficient flexibility of power grids and energy production systems, in the production of hydrogen in the process of electrolysis. Hydrogen may here be either the final product or a substrate for the production of synthetic natural gas (SNG) in the reaction with carbon dioxide. Storage and distribution of SNG is less problematic than that of hydrogen and is feasible with the use of the existing natural gas storage and transportation systems. A widespread implementation of CCS and CCU technologies is hindered by still high costs of carbon dioxide capture and utilization, a lack of the social acceptance for carbon dioxide storage as well as an unwillingness to bear the higher costs of CCU-derived chemical synthesis products when compared to products generated with the use of conventional methods. There are also still the technological barriers and challenges that need to be addressed in terms of the integration process: from the emission source, through carbon dioxide capture, transport, to carbon dioxide storage or utilization.

1.3. Solid Combustion Products

Power and heat production in combustion systems fed with solid fuels also results in the generation of solid combustion products, like ash, slags and gypsum. Before they can be marketed, they need to be proven to comply with REACH standards [10]. Nowadays, gypsum and conventional boiler fly and bottom ash are mostly valorized, and their utilization routes cover the production of cement, binders, and concrete as well as application as aggregates and fillers in the construction industry.

Fluidized bed fly ash differs considerably from conventional boiler fly ash in physical, chemical and mineralogical properties. In particularly, the content of free calcium oxide in fluidized bed fly ash is increased and amounts to a few weight percent or higher when flue gas desulfurization process residues are also present in the fluidized bed combustion products. This creates new scientific and practical challenges. Processes are needed which aim at the reduction in potentially hazardous component content in fluidized bed fly ash and the stabilization of its properties before the subsequent implementation, both in traditional and innovative applications. The free calcium oxide content over 1% w/w limits the conventional utilization pathways of fly ash, and results in its categorization as a hazardous substance with the relevant codes H315 (causes skin irritation), H318 (causes serious eye damage) and H335 (may cause respiratory irritation) [11]. The process of carbon dioxide mineralization with the use of alkaline earth-rich fluidized bed fly ash is therefore considered as a technological solution enabling improvement in environmental safety of this type of combustion products and widening its application range, while also contributing to the development of carbon dioxide utilization techniques.

1.4. European Union on the Way to Carbon Neutrality

One of the European Union’s ambitions is to become carbon neutral by 2050 [12]. This target is also the basis of the European Green Deal [13], covering the set of actions leading to the effective use of resources and the mitigation of emissions in circular economies via investments in the development of new, pro-environmental technologies, including those reducing the emissions from the power sector. This vision of the European Commission presented in 2018 is in line with the objectives of the Paris Agreement concluded in 2015 during the UN Climate Change Conference (COP21) and implemented in 2018 (COP24). These consist of the reduction in the average world temperature rise to 1.5 °C and greenhouse gases emissions by at least 40% to 2030 when compared to the values of 1990 [14]. These targets are to be reached principally by increasing energy efficiency, widening implementation of renewables, the development and implementation of the CCUS technologies in energy-intensive systems, and the zero-emission production of hydrogen and biomass-based energy production. All EU countries committed to prepare and implement the national plans for energy sectors and climate by 2030 [15].

In 2020, the Council of Europe set even more restrictive targets, of at least 55% greenhouse gases net emission reduction by 2030, with a requirement of at least a 32% share of renewables in the gross final energy consumption, an increase in energy efficiency to 32.5% and the creation of an internal EU energy market. In the EC communication “Fit for 55” of 2021, the new agenda of adapting the existing climate and energy legislation was proposed on the basis of the COP26 conclusions, including the Renewable Energy Directive modification, consisting of setting a new target for share of renewables in the EU energy mix (40% instead of the recent 32%) [16]. The modification of the Energy Efficiency Directive, Effort Sharing Regulations and the EU Emission Trading System by the inclusion of new sectors was also proposed, as well as the revision of carbon dioxide emission standards for new cars, which are to become zero-emission by 2035 [17].

All the above, as well as new political and economic challenges resulting from the military conflict close to the eastern frontier of the European Union, make the efforts undertaken to develop and implement the technologies enabling more the environmentally friendly operation of fossil fuel-based energy systems still valid and important. These cover the technologies of carbon dioxide capture, storage and utilization, enabling greenhouse gases emission mitigation, with the major source of these emissions being the sector of heat and power generation and the most targeted gas, carbon dioxide, as well as the technologies of combustion products valorization.

2. Carbon Capture, Storage and Utilization

2.1. Carbon Dioxide Emission from Energy Sector

Approximately 40% of the world carbon dioxide emissions derive from the energy sector, of which 75% come from coal-fired power plants. In 2020, because of the effects of COVID-19 pandemic on the economy, world carbon dioxide emissions dropped by 5.8%, i.e., 2 Gt, to 31.5 Gt and this reduction was 5 times higher than the previous one caused by the economic crisis in 2009 [9]. Still, the atmospheric concentration of carbon dioxide was the highest ever reported, of 412.5 ppm. The expected increase in carbon dioxide emission with the post-pandemic economic recovery and increase in the demand for fossil fuels in 2021 was approximately 4.8%, including 650 MtCO₂ resulting from the renewed oil use and 640 MtCO₂ from coal utilization. The world power sector was responsible for less than half of the carbon dioxide emission reduction in 2020, and was expected to account for approximately 80% of the forecasted increase in 2021, related mainly to the operation of coal-fired power plants in Asia. The expected carbon dioxide emissions caused by natural gas combustion were also assumed to be higher in 2021 and to amount to 7.35 Gt [9].

The world electricity demand in 2020 dropped on average by approximately 1% when compared to 2019, with monthly decreases of 10% in China, 20% in India, 15% in Germany, France and Great Britain and 25% in Spain and Italy, when compared to the respective periods of 2019. It was expected that in 2021 the world electricity demand would increase by 4.5%, with a 2% rise in USA, 8% in China and India and 3% in the largest EU consumers: Germany, France, Italy and Spain [9].

The record rise in use of renewables, in particularly wind and photovoltaics in 2020, of 12 and 23%, respectively, combined with the decrease in electricity demand, caused a fall in the demand for fossil fuels in the power generation sector of 3%, with the highest value reported for coal, of 440 TWh. The decrease in electricity generation in coal-fired power plants in 2020 was the highest ever observed (i.e., 4.4%), with over half of this value reported for USA and 23% for EU countries. To some extent, this effect was caused also by relatively low prices of natural gas. Natural gas-fired power plants experienced lower decreases in production, of 1.6%, when compared to 2019. The demand for oil in electricity generation has been falling gradually since 2012, and the decrease in 2020 was 4.4% [9].

Approximately 48% of the increase in the world electricity demand in 2021 was expected to be covered by fossil fuels-fired power plants, predominantly coal-fired, in about 50% by renewables and in 2% by nuclear sources. China would account for over half of the expected rise in coal-based electricity in 2021, of 330 TWh. In India, about 70% of the rise in the electricity demand was expected to be covered by coal-fired power plants [9].

The forecasted world demand for coal in the energy sector in 2040, according to the scenario of the current energy policies, is 3400 Mt, which will provide about 10,400 TWh and secure 25% of the world electricity demand in 2040 [8].

2.2. Carbon Dioxide Capture

The technological approaches to carbon capture applicable for large point emission sources, like fossil fuel-based power plants, include the pre-combustion, the post-combustion and the oxy-combustion options. The most matured and most commonly applied is the post-combustion capture method with the use of amine solutions, predominantly monoethanolamine, and having a capture efficiency of over 90% [18]. This is a relatively energy-intensive process, which is caused by the need for amine regeneration and carbon dioxide compression. It lowers the energy production efficiency by about 10 percentage points and increases the production costs by approximately 70–80% [19,20]. Other compounds applied in chemical absorption of carbon dioxide from flue gases include water solutions of ammonia, sodium and potassium carbonates and sodium hydroxide. The physical absorption is performed with the use of commercially available absorbents, like, e.g., Selexol, Rectisol or Sulfinoll.

The research and development works in the field of carbon capture are focused on cost reduction by the implementation of less energy intensive solutions, including the development of innovative solvents substituting amine solutions [21,22], as well as process modifications and optimization [23,24,25]. Separation techniques, recognized and tested within other industrial processes, are also considered. They have potential, particularly in the case of systems of higher carbon dioxide concentrations, where the integration of the carbon capture and water gas shift reaction, converting carbon dioxide into hydrogen and carbon monoxide, is possible, including membrane separation and solid sorbents [26,27,28,29,30]. The current research areas in the field of membrane separation cover innovative materials for ceramic, polymer and hybrid membranes, of a desired permeability and selectivity, as well as membrane configurations, ensuring adequate module packing density [9,30].

Among the solid sorbents systems, the systems based on fixed-beds and temperature (TSA) or pressure swing adsorption (PSA), vacuum swing adsorption (VSA), electric swing adsorption (ESA) as well as fluidized beds [31,32] and moving beds [33,34] may be distinguished. The biotechnological routes are also tested with the use of bacteria or algae using carbon dioxide in life processes [35] as well as the cryogenic separation systems, where carbon dioxide is separated at low temperature of −100 to −135 °C and under elevated pressure in a solid state, with the capture efficiency of 90–95% and energy intensity of 600–660 kWh per Mg of carbon dioxide separated [18]. The technology of hydrate formation is another example of carbon capture options, considered with the use of various promoters of phase equilibrium and thermodynamic conditions [36]. The chemical looping combustion idea, first proposed by Shimizu et al. [37], is considered as potentially applicable to the gasification process and making use of oxide carriers, e.g., calcium carbonates or sulfates, or iron oxides [38,39,40,41]. Hybrid systems, combining chemical absorption and membrane separation [42] or cryogenic separation methods [43] are also investigated.

The concentrations of carbon dioxide in flue gases from conventional combustion systems are relatively low, of 12–15% vol., and the flow rates are approximately 10 times higher than in other industrial processes, i.e., in the chemical industry, which makes the direct adoption of the separation techniques employed there for the purposes of carbon capture technologies in the energy sector unfeasible [44,45]. The current post-combustion capture also requires the removal of nitrogen and sulfur compounds before carbon dioxide sequestration [46].

The oxy-combustion option, with the use of cryogenic air separation units, is perceived as the most competitive to amine absorption, with the loss in energy efficiency of 8%, and with bottlenecks consisting of high investment costs and limited feasibility of application in already existing power plants [20]. In such systems, fuel is combusted in the atmosphere of oxygen, carbon dioxide and steam, and the final product after combustion, decompression, and heat recovery is carbon dioxide. This product, after final treatment, may be directed to further storage or utilization [45]. The research efforts are devoted to the development of pressurized systems, including the supercritical carbon dioxide systems, ceramic membranes with the potential for the reduction in oxygen separation costs, as well as construction solutions of the appliances operated in a high-oxygen content environment [45,47]. The level of contaminations of the carbon dioxide stream separated in the oxy-combustion option is the highest among the capture options considered, and covers sulfate and nitrogen oxides, hydrogen sulfide, oxygen, argon, hydrogen, methane and mercury, all derived from the fuel combusted or the inflowing air, which is particularly important when the subsequent utilization of carbon dioxide captured in the EOR process is intended [48].

The costs of carbon dioxide capture are estimated to be approximately 80% of the costs of the entire CCS chain, i.e., carbon capture, transport and storage [49]. The highest potential in terms of cost reduction is currently seen in technologies integrating the carbon capture and energy production processes, i.e., chemical looping combustion, offering relatively low energy efficiency loss and enabling avoidance of investment costs related to the separate technological stage of carbon capture. Among the technologies of the highest potential for implementation in the existing power plants, the PSA techniques with the use of membrane systems and solid sorbents are proposed, provided that the carbon capture efficiency of at least 90% could be proven [19,50].

2.3. Carbon Dioxide Storage

The deep saline aquifers and depleted oil and natural gas fields are considered to be the potential locations of captured carbon dioxide storage of the largest storage capacity. The usability of a given geological formation in carbon dioxide capture is determined by its porosity, thickness, permeability, overburden tightness and stability of the geological conditions in the storage vicinity [51]. The detailed aspects of the assessment of storage feasibility in particular geological formations cover tectonics, the geothermal and hydrogeological regime, hydrocarbon resources, distance from the capture location and socio-economic settings [52]. Carbon dioxide capture at an industrial-scale is implemented in the enhanced oil recovery process, enabling an increase in the oil recovery of 30–60% when compared to the output expected without this process [53,54]. The storage potential is estimated to be 400–10,000 Gt for deep saline aquifers, 675–920 Gt for depleted oil and gas fields, and 3–200 Gt for coal deposits [55,56]. An example of carbon dioxide storage combined with the enhanced oil recovery is the Weyburn project initiated in 2000 in Saskatchewan, Canada, of the total storage potential of over 30 Mt of carbon dioxide captured in a gasification plant located in North Dakota, USA, and transported 320 km via pipeline to the storage location, of the yearly storage capacity of 3 MtCO₂/year [57,58]. The first implementation of this process took place in 1970s in Permian Basin, West Texas and New Mexico, and since then about 140 such plants have been operated [59]. The enhanced coalbed methane recovery (ECB) was tested at the demonstration scale in coal mines of New Mexico, USA, in Alberta, Canada, and also in China, Japan and Poland [18,60]. The main factor limiting the applicability of this process is the permeability of rocks [61].

The mechanisms taking place in carbon dioxide storage, in deep saline aquifers inland and offshore at the depths of 700–1000 m, are diverse and complex. They cover, among other things, the hydrodynamic mechanisms of carbon dioxide binding in the form of supercritical fluid, the dissolution of gaseous carbon dioxide in liquid or the replacement of water in porous structures as well as the mechanism of mineralization and permanent binding of carbon dioxide in the form of calcium, magnesium, or iron carbonates [62]. An example of this technology being implemented in deep saline aquifers Utsira Sand at the industrial-scale natural gas field location is the Sleipner project in the North Sea, Norway, of a storage capacity of 1 MtCO₂/year at the depth of approximately 1000 m [63]. The first large-scale venture in North America was the QUEST project, in Alberta, Canada. Carbon dioxide was captured from the process of hydrogen production and stored at the amount of 1 MtCO₂/year. It was fully integrated at industrial-scale, and this was started in 2015 [64]. Among the other carbon dioxide capture and storage projects, the Clean Gas Project (Net Zero Teeside Project), with the storage capacity of 10 MtCO₂/year, located close to Teeside, Great Britain, should be mentioned. The project covers the storage of carbon dioxide in deep saline aquifers in the North Sea at a location with a total storage capacity of over 1000 Mt of carbon dioxide, which is captured with the use of the amine absorption process at the amounts of 1.8 MtCO₂/year in the combined gas steam system with a 700 MW gas turbine. The ambition of the project is to demonstrate the first full decarbonization of industrial cluster in the UK by 2030 [65]. The TUNDRA project aims at the development of the world largest installation of carbon dioxide capture, with an efficiency of up to 90% (which is the equivalent of 4 MtCO₂/year) from the Milton R. Young Station generator, North Dakota, USA, and its storage in porous geological formations containing saline water at the depth of about 4700 feet [66]. Other projects include: Snøhvit project at the Barentsa Sea, Norway (0.7 MtCO₂/year); Illinois Industrial Carbon Capture Project, USA (1 MtCO₂/year); ZeroGen project in Queensland, Australia (0.7 MtCO₂/year); Gorgon project, Barrow Island, WA, Australia (4.5 MtCO₂/year) and Latrobe Valley project, Victoria, Australia (13 MtCO₂/year) [18,59].

The oceans, being sinks of 1.7 GtCO₂ per year, are also being considered as carbon dioxide storage options. At the depth below 3 km, carbon dioxide could be stored in the liquid form of a higher density than water, which would cause its deposition at the ocean floor [67]. The idea of carbon dioxide storage in oceans is, however, more controversial than with the use of geological formations since the potential negative impact of water acidification affecting the ecosystem is not fully recognized. Other options of carbon dioxide storage in geological formations include magma storage and mineralization with the formation of carbonates [68].

The wide implementation of carbon capture and storage technologies is hindered, principally by insufficient market mechanisms and incentives for the investments in such technological developments, the lack of legal regulations determining the conditions of carbon dioxide storage at the inland and offshore locations and ensuring the financial stability of CCS undertakings, as well as the lack of the industrial-scale examples of ocean carbon dioxide storage [18,69]. The major technological limitations cover the integration of CCS installation with energy production systems or other utilization sites, as well as the integration of the entire carbon dioxide management chain: from the emission sources, through transport to storage. The so-called CCS clusters, combining the decarbonization efforts of a larger number of industrial partners by creating common carbon dioxide transport and storage networks are sought as the ways of CCS investments costs mitigation and process optimization. There are several examples of such clusters in Alberta ACTL, Denver, the Gulf Coast, the Rocky Mountains or Rotterdam [70]. Within the Porthos project in Rotterdam, the yearly storage of 2.5 to 5–10 Mt of carbon dioxide, captured in various industrial processes, is to be demonstrated. The gas is to be transported via a pipeline to a sea platform located about 20 km from the coast, where it is to be injected into the depleted natural gas fields, consisting of tight porous sandstone at the depth of about 3000 m at the North Sea [71]. The Northern Lights project is part of the Norwegian industrial-scale CCS implementation, covering carbon dioxide capture from the cement and energy production plant in Oslo region, the transportation of liquid carbon dioxide to a land terminal located at the industrial area Naturgassparken in Øygarden on the west coast of Norway, and next via a pipeline to the injection site at the North Sea (Oseberg A platform). The project is to be started in 2024 with the initial, yearly carbon dioxide capture and transport capacity of 1.5 MtCO₂/year. Carbon dioxide is to be stored at the depth of about 2500 m below the seabed [72].

2.4. Carbon Dioxide Utilization

The carbon dioxide utilization technologies are considered to be complementary to the storage technologies for carbon dioxide captured from large point emission sources. They offer the advantage of higher social acceptance for carbon dioxide transformation into marketed products and environmental safety without the need for storage site monitoring and the prevention of potential carbon dioxide leakages [18,73]. The current utilization options cover already recognized processes of ammonia production, greenhouse growing, food industry, brewing industry, production of coolants and fire agents. Carbon dioxide may be also applied as a substrate for chemical synthesis, in the production of polymers, fuels and construction materials. The amount of carbon dioxide that could be utilized in CCU technologies is limited by the demand for chemical synthesis products, estimated to be 0.18 Gt/year (in the long-term perspective: 1–2 Gt/year), orders of magnitudes lower than the yearly total world carbon dioxide emission (about 41 Gt/year in 2017 or 8 Gt/year–forecasted for 2050) or even the emission from large point sources (14 Gt/year) [74]. The demand for anthropogenic carbon dioxide in the EOR process, although not considered to be a CCU technology since no new chemical compound is synthesized in this process, is also limited to 18 Gt in the USA for the economically reasonable recovery of 67 billion barrels of oil [75]. The limiting factor for a widespread implementation of CCU technologies is also the need for a supply of hydrogen for chemical synthesis and higher production costs when compared to conventional production routes. This is why the CCU technologies are nowadays often considered as an element of the so-called Power-to-X technologies, which enable the use of excess renewable electricity, that cannot be efficiently stored in the amounts required, for the production of hydrogen in the process of electrolysis. Hydrogen generated in this way may be next utilized as an energy carrier or substrate for synthetic natural gas (SNG) production (in Power-to-Methane option). The storage and transportation of the latter one are less problematic than these of hydrogen, and are possible with the use of the existing natural gas storage and transport system. The following innovative technological options of the anthropogenic carbon dioxide utilization are currently being considered:

−

electrochemical methods of carbon dioxide conversion into the synthesis gas, methane, as well as methanol and/or dimethyl ether with the use of renewable electricity, and with the particular focus on the development of economically attractive, highly efficient and selective catalysts;

−

photochemical methods requiring the development of photocatalytic materials, laboratory-scale reaction studies and improvements in carbon dioxide conversion efficiency;

−

methods of carbon dioxide and hydrogen synthesis requiring highly efficient and highly selective and stable catalysts [76,77,78];

−

economically efficient methods of hydrogen production [79];

−

biological methods for which bacteria cultures or enzymes increasing the efficiency, selectivity and conversion rate of carbon dioxide in natural biochemical processes are needed;

−

efficient and selective catalysts for copolymerization processes;

−

carbon dioxide mineralization [80], including the production of construction materials with the use of waste streams of relatively high calcium and magnesium content, e.g., combustion products, smelting slags, cement kilns dust and carbonation process of a considerable potential for carbon dioxide emission reduction resulting from stable binding of carbon dioxide in carbonates and application potential, with the main research focus being on the development of simple, effective and energy-efficient technological options of carbonation process [81,82,83].

The synthesis gas produced with the use of the electrochemical method may be further processed into synthetic fuels or methane [84,85,86]. Other applications of electrochemical methods cover the production of formic acid [87], carbon nanotubes and nanofibers [88,89]. In the photochemical process performed with the application of a counter-rotating ring receiver reactor recuperator CR5, solar energy is employed for heating ceramic oxide material to the temperature of 1500 °C and its chemical reduction, with the release of a part of oxygen into the hot section of the reactor. In the cold section of the reactor, the oxidation reaction of the material is performed with the use of carbon dioxide with a release of carbon monoxide. The photochemical system may also generate hydrogen when carbon dioxide is replaced with water in the cold section of the reactor, with the products of such a reaction (carbon monoxide and hydrogen) being the components of the synthesis gas [89]. The most recognized methods of the catalytic conversion of carbon dioxide cover the low-temperature and low-pressure alkaline electrolysis for the production of hydrogen and the catalytic synthesis of methanol as a gasoline additive [90]. Biotechnological methods of carbon dioxide utilization are mostly based on the anaerobic digestion performed by anaerobic microorganisms in an oxygen-free environment [91]. The most common copolymerization products include polycarbonates and monoethylene glycol [92]. Research studies have also been performed on improving the mechanical properties of construction materials, including the compressive strength, with the carbon dioxide utilization capacity of approximately 3.5 kgCO₂/m³ of concrete [93], as well as on valorization of waste materials, like cement dust, smelting slags or combustion ash, in the process of carbonation. These studies have resulted in the production of construction materials, e.g., concrete additives [94,95,96].

3. Carbon Dioxide Mineralization

3.1. Carbonation Mechanism

The mechanism of carbon dioxide mineralization is based on the reactions involved in the natural weathering of siliceous rocks, like olivine, serpentinite or wollastonite (reactions (1)–(3)) [97]:

Mg₂SiO₄ + 2CO₂ → 2MgCO₃ + SiO₂         ΔH = +89 kJ/mol

(1)

Mg₃Si₂O₅(OH)₄ + 3CO₂ → 3MgCO₃ + 2SiO₂ + 2H₂O  ΔH = +64 kJ/mol

(2)

CaSiO₃ + CO₂ → CaCO₃ + SiO₂           ΔH = +90 kJ/mol

(3)

The idea of the process intensification for carbon dioxide sequestration purposes was proposed in the 1990s [80]. The amount of a siliceous rock required for the binding of 1 Mg of carbon dioxide is estimated to be about 2–4 Mg, which means that the world resources of the respective minerals would be sufficient for the sequestration of the entire coal-derived emissions of carbon dioxide [98]. The limitations lie, however, in the process kinetics, the need for rocks pretreatment in order to reach the required levels of carbon dioxide conversion and/or additives that need to be recovered and recycled within the process, which make the process energy- and cost intensive. The technologies of carbonation of natural alkaline earth metals sources are at the testing stage and the cost estimates for the best recognized process of carbon dioxide mineralization with the use of olivine range from approximately 50–100 USD/tCO₂, combined with 30–50% energy penalty for mineralization itself and 10–40% energy penalty for carbon dioxide capture [98]. Furthermore, the long-term carbon capture target of 8 GtCO₂/year would mean that, apart from the need for natural rocks mining and pretreatment, there is also the need for the management of 32 Gt/year of the process product. Because of the scale, the expected utilization routes would cover land reclamation and remediation, the transformation of desert areas into semi-deserts, and the production of soil additives and construction materials. The assumed mineralization of 8 GtCO₂/year would give the product amount enabling its application to the area of approximately 250 km²/year with the potential for turning deserts into semi-deserts estimated to be millions of km² [75,97]. The estimated cost of the mineralization product is 10–30 USD/t, comparable with the cost of sand used in the construction industry [97]. The calcium or magnesium carbonate as the mineralization product is stable, even in acid environment; in the environment of pH 1, less than 1.5% of the bound carbon dioxide was reported to be released [99,100].

3.2. Carbonation Methods

The main technological approaches to carbon dioxide mineralization tested today cover the direct dry carbonation in a fixed-bed or a fluidized bed reactor, where the reaction takes place between the reagents in a solid and a gaseous phase; these were direct wet carbonation with carbon dioxide dissociated in water (carbonate anions) and the indirect wet method, consisting of the extraction of calcium ions and/or the dissociation of carbon dioxide preceding carbonation in a water solution [101].

The dry carbonation method is based on the exothermic reaction between oxides, hydroxides and metastable silicates with carbon dioxide [102]. The factor limiting the process efficiency to a value usually below 80% is the creation of a non-porous layer of carbonates on the solid particles’ surface, hindering the diffusion of carbon dioxide. The parameters facilitating the process kinetics are an increased pressure (1 MPa) and temperature (170–500 °C) [103], as well as a moisture content of several weight percent [104]. An example of the process tested in the aspect of the in situ carbon capture in the thermochemical processing of solid fuels for energy purposes is the chemical looping combustion with the use of limestone or dolomite at the temperature range of 600–950 °C, and with the sorption capacity of the sorbents decreasing with the number of carbonation–calcination cycles [105].

The wet direct carbonation methods cover the one-stage process, performed without the preliminary phase of alkaline earth metals extraction, including: precipitation by mixing two solutions containing calcium and carbonate ions; the reverse micro-emulsion method, in which the oil–water emulsion is mixed with a surfactant, and next with the emulsion containing calcium and carbonate ions; carbon dioxide bubbling in solutions containing calcium ions; and the slow carbonation method, consisting of ammonia carbonate hydrolysis, with carbonate anions reacting with calcium ions [106,107]. The wet indirect methods are performed at a few stages with the use of such materials like NaHCO₃/Na₂CO₃ and NH₄HCO₃/(NH₄)₂CO₃ reacting with the extracted cations of alkaline earth metals to carbonates. The leaching agents employed cover acids (acetic, formic, and lactic) or ammonia salts (in particularly in the case of smelting slags). The advantage of the indirect wet methods is high purity of the carbonates produced [103,108]. He et al. demonstrated the extraction of 35% of calcium contained in fly ash in the wet indirect method and the conversion of half of this amount, i.e., about 16%, into carbonates with a high purity of 97% w/w [109]. The major drawback of the wet indirect method is the need for the regeneration of the chemicals used (acids, ammonia salts). An additional issue is that the technical and economic aspects are mostly neglected in the scientific literature reporting on the field concerned [97].

The product of the carbonation process may represent a few polymorphic forms. The anhydrous calcite, aragonite, or vaterite are created based on a hydrated, amorphous form. They differ in terms of the specific surface area: from about 30–45 m²/g for calcite and aragonite to 100–200 m²/g for vaterite and the amorphous form [109]; thermodynamic stability: from the most stable calcite through aragonite to vaterite, as well as the degree and rate of dissolution, hardness, density and optical properties—all affecting their application potential. The less stable forms are made more stable via the mechanisms of sequential dissolution and recrystallization: the amorphous form is dissolved and recrystallized in the form of vaterite, which is next dissolved and recrystallized in the form of calcite with the maximum below 40 °C [110]. At the temperature above 60 °C, the stabilization of aragonite takes place [111]. The controlled creation of particular polymorphic forms of calcium carbonate depends also on the pH, pressure, time, and chemical additives [112,113]. An increase in pH was shown to facilitate the creation of more stable forms; a pH of over 10 was proven to be favorable for calcite formation, while a pH below 8 for vaterite and a pH of 9–10 for aragonite, with other process parameters, such as the temperature, carbon dioxide flow rate and reaction time also affecting the final process results [114,115]. The research efforts in terms of carbonates production in the process of carbon dioxide mineralization are focused, among others things, on the reduction in the product particle size, an increase in its specific surface area, and a widening of its application range by various chemical modifications [116].

3.3. Waste Materials as Alkaline Earth Metals Source in Carbonation Process

A special level of attention, when considering the enhanced carbon dioxide mineralization process of the CCU technologies, is given to the utilization of inexpensive waste materials as sources of alkaline earth metals with additional benefits of their valorization. The waste streams of interest cover smelting slags, flue gas treatment waste, energy sector ash, concrete and cement production waste, mineral processing waste, e.g., bauxite, and paper industry waste [97,117,118,119]. Such materials are often more reactive and of the right grain size when compared to the natural mineral resources of calcium or magnesium, e.g., serpentinite, which requires preliminary grinding as well as chemical and physical activation to improve its carbonation efficiency. Additionally, liquid industrial waste or brines are considered in the industrial solid waste wet indirect carbonation method, with the initial phase of carbon dioxide dissolution and calcium ions extraction, followed by calcium carbonate precipitation in order to reduce the water demand of the process and to facilitate the decomposition of siliceous rocks [120]. The chemical and mineralogical complexity of solid industrial waste containing alkaline earth metals, applicable in the process of carbon dioxide mineralization, means the further studies are still needed on the recognition of the mechanisms of the reactions taking place at the materials interface in order to potentially improve reaction kinetics and mass transport [121].

3.4. Fly Ash as Alkaline Earth Metals Source in Carbonation Process

The process of carbon dioxide mineralization with the use of combustion ash may follow the wet or dry direct carbonation method at ambient temperatures and low carbon dioxide pressure, with the use of alkaline earth metals sources provided at the required grain size class without the need for preliminary grinding [122]. Fly ash is the combustion product of the lowest grain size, for which new utilization options are sought as alternatives to environmentally burdensome storage. The applications of coal fly ash currently consider cover pozzolanic fillers for cement blends [123,124,125], raw materials for clinker production, binder substitutes in concrete and grout production [125,126,127], the production of bricks [128], road works [129], agriculture [130], water and wastewater treatment [131], production of zeolites [132] and injection mixtures for application in mining techniques [133]. The world combustion products generation in 2010 was approximately 750–780 Mt, of which about 67% was in China, 18% in India, 10% in USA and 5% in Europe (i.e., about 38 Mt, of which fly ash contributed to 68%) [134,135]. Approximately 53% of the world ash was utilized, of which 20% was used in concrete production [135]. In Europe, even 94% of fly ash, bottom ash, slags and fluidized bed fly ash was reused, of which about 49% was used in land remediation, and 25% in construction industry [135]. According to the data of 2020, the world production of combustion products in 2016 increased to 1.22 Gt with the highest share for China (46%), India (16%) and Europe (11%, of which EU-15 countries accounted for 3%) [136]. The world level of their utilization also increased, to 63%, and remained at the level of 94% for EU-15 countries, with the main utilization routes in the construction industry (for cement and concrete production—class F ash) being in gypsum production, in road works and land remediation.

The properties of coal ash, including the chemical and mineralogical composition as well as the structure, depend on the coal properties, the technical parameters of the combustion process, and the carbon capture methods employed, all of which affect the applicable ash utilization pathways, and are often specific for a given country [125]. The main components of fly ash constitute oxides of silicon, aluminum, iron, calcium and magnesium (95–99% w/w) with the rest being titanium, sodium, potassium and sulfur oxides. The ASTM C618-8 standard divides fly ash into class C, of high calcium content and class F, of pozzolanic properties, depending on the total amount of silicon, aluminum and iron oxides (≥50% or ≥70%, respectively) as well as the values of sulfur oxide, moisture and loss on ignition [137]. The pozzolanic properties make fly ash useful as a cementitious material in the construction industry. Class F fly ash, applied as a cement substitute, reduces the hydration heat and therefore mitigates the risk of concrete cracking [138]. Properly designed concretes produced with the use of fly ash were also proven to be of an increased compressive strength and reduced permeability [139]. Fly ash may be applied in soil stabilization, the improvement of roadbeds, the production of filling aggregates, additives to bituminous surfaces and mineral fillings for asphalt concrete, reducing the costs of roadworks by about 10–20% and 30–40% in the case of roadsides [140]. Additionally, its use helps to avoid social and economic costs related to the reduced land degradation, mineral resources extraction and waste landfilling. The substitution of cement, as the most expensive and energy-intensive concrete component, with fly ash reduces production costs and improves the properties of concrete by reducing water demand, heat of hydration and bleeding, while retaining or improving concrete workability. The fly ash application range covers the production of concretes with Portland cements, pozzolanic cements and setting time regulators [140]. Bituminous coal combustion-derived fly ash was tested in the amounts of 15–25% w/w as a cement substitute in a binder, and of up to 75% w/w of cement substitution in roadworks [140]. It was proven that replacing 35–50% of cement with fly ash reduces the water demand tested with the cone fall method by 5–7% in comparison with the reference sample [141]. An increase in a compressive strength after 90 days and the reduced shrinkage were also reported for concretes used in dam production, with the increase in the level of cement replacements with fly ash ranging from 0 or 30 to 50% w/w [142]. An application of chemically modified fly ash with the use of 0.2% w/w of CaCO₃ in water solution of HNO₃ was reported to improve properties of cement–fly ash binders in terms of setting time [143], as well as a compressive strength, with the assumed mechanism related to the change in hydration time of calcium trisilicates and trialuminates as a results of creation of a more compact ash structure [144]. The mechanically processed fly ash used as a cement clinker substitute (50–50%) gave the effect of maintaining or improving the compressive strength and compliance with the remaining cement assessment criteria for construction applications, including the setting time, volume stability and steel corrosion resistance in cement construction [140]. The use of fly ash in a roller-compacted concrete (ROCC) for road, dam and large floor surfaces affects its properties both in fresh and hardened states, impacting its workability, compressive strength and shrinkage. It reduces the impact of concrete bleeding, heat of hydration, permeability and porosity, and increases concrete abrasion resistance. Additionally, it reduces the alkali silica reaction (ASR) between potassium and sodium ions from Portland binders and aggregates, resulting in concrete cracking [140,145]. An application of fly ash as a cement substitute in a reactive powder concrete (RPC) in which cement was used in amounts of 800–1000 kg/m³, as well additives in the form of powdered quartz and microsillica, reduced the negative impacts of high cement content on heat of hydration and shrinkage, and improved concrete durability [146,147]. Numerous laboratory-scale works also reported the feasibility of the application of fly ash which does not comply with the standards of physical and chemical properties according to ASTM C-618, including the fly ash of high calcium compounds content [148,149]. Fly ash applied in the amounts of 20–50% w/w of clay in bricks production was reported to increase their porosity from 20% to 50%, reduce their weight and improve their strength, water and low-temperature resistance [150]. In order to mitigate the problem of the limited early compressive strength of concrete produced with the use of conventional boiler fly ash, limiting their content in a binder to approximately 30% w/w, the use of cement of high early strength achieved with grinding is advised. This is expected to result in a reduced binding rate, and therefore the improved hydrates’ structure and in the increased early compressive strength, making the use of a higher proportion of fly ash in a binder possible.

3.5. Carbonated Fly Ash Potential as a Cementitious Material

The applicability of fly ash carbonated with the use of the indirect wet method as a cement substitute and the effect of the carbon dioxide flow rate between 2 and 4 mL/min and weight ratio of water to fly ash of 15 and 7.5 mL/g was assessed by Ebrahimi et al. [151]. The slurry produced with fly ash in deionized water was mixed for 1 h at 30 °C, and next the carbon dioxide was dosed until pH of 8.3 was reached, below which the decomposition of calcium carbonate would take place. The increase in carbon dioxide flow rate was reported to increase the carbonation reaction rate. The ratio of water to fly ash was observed to improve the final process efficiency, irrespectively of the carbon dioxide flow rate. The maximum final carbonation process efficiency was approximately 0.03 kgCO₂/kg of fly ash tested. The thermograms showed water release at 25–100 °C, decomposition of organic compounds and magnesium carbonate at 105–500 °C and calcium carbonate at 500–1000 °C [151,152]. Cement mortars cubes of the dimensions 2 × 2 × 2 cm³ were formed next, with the substitution of Portland cement in the ratio of 10 or 30% w/w with the filtered and dried carbonated fly ash and with the use of a water-to-binder ratio of 0.4 as and seasoned for 14 days under ambient conditions. A slight decrease was reported in compressive strength values from 30 MPa to 29 MPa with 10% w/w substitution of cement in a binder, and to approximately 20 MPa when 30% w/w substitution ratio was applied. The calculated pozzolanic activity index values were 97 and 67%, respectively. It was also concluded that the economic and environmental benefits related to the valorization and utilization of fly ash and carbon dioxide counterbalanced the slight decrease in compressive strength of the materials tested [151].

The technical and economic efficiency of the simulated carbon dioxide mineralization with the use of fly ash from lignite combustion, indirect carbonation method with the cation extraction step in an NH₄Cl solution, or NH₄Cl and HCl mixture, followed by carbonation with the use of flue gas stream of the temperature of 150 °C was assessed by Hosseini et al. [153]. The estimated demand for fly ash was 25 Mg/h and the product capacity assumed was 100,000 Mg/year. The case of NH₄Cl solution application required the employment of low-pressure steam as a heat source for the endothermic reaction of calcium and magnesium chlorides formation at 80 °C. The leachate from the extraction stage was assumed to be directed to the carbonation reactor and cooled continuously to the temperature of 60 °C, which was expected to be the optimal temperature value under the process conditions adopted [154]. The reaction product was to be filtered and the NH4Cl solution regenerated and recycled in the process. With the use of the mixture of NH4Cl and HCl, no supplies of the external heat were required, but a correction of the pH value before the carbonation process was needed. In the first case, therefore, the calculated amount of magnesium carbonate was 5 Mg/h and that of calcium carbonate was 4.9 Mg/h, with a utilization rate of 0.17 Mg of carbon dioxide per 1 Mg of fly ash. The respective values for the second option were estimated to be 9 and 7 Mg/h of magnesium and calcium carbonates and 0.3 MgCO₂/Mg of fly ash, with the additional use of approximately 20 Mg/h of HCl. The efficiency of cations extraction and carbonation was simulated to be higher in the second option, but it required higher amounts of chemicals to be employed, and therefore also higher energy inputs, even with the avoidance of the external heat supply to the cation extraction step.

The costs estimations available in the literature are based on laboratory-scale experimental studies, and as such do not cover important factors, like profit margin, tax rates, government incentives and carbon dioxide emission charges [153,155]. Several demonstration installations of carbon dioxide mineralization are under operation worldwide with the use of natural minerals or industrial waste [107,152]. The estimated costs of mineral sequestration in the optimistic scenario were reported to be from 48–97 Euro/MgCO₂ [156] to 50–210 USD/MgCO₂ for a direct carbonation method [157]. In the case of slag utilization, when used as a source of alkaline earth metals and as an indirect carbonation method, the cost was approximately 600–4500 USD/MgCO₂ [107]. The simulated value [153] for lignite fly ash application (103 USD/MgCO₂) was comparable to the values reported for natural minerals and higher than for bituminous coal fly ash (11–21 USD/MgCO₂) [158]. The sales price of the process product as a synthetic dolomite was estimated to be 130 USD/Mg, while that for the cement substitute was 110 USD/Mg. The sensitivity analysis showed the highest effect of the sale price, followed by the effect of the production costs, on the net present value (NPV) and the internal rate of return (IRR) [153].

3.5.1. Changes in Physical and Chemical Properties of Carbonated Fly Ash

The particle size of fly ash and its porous structure determine the contact surface between the solid and gaseous reactants in a carbonation reaction. Most of the ash described in the literature was said to be of a particle size of 0.5–300 µm, an average pore diameter below 20 µm, a density of 0.54–0.86 kg/m³, a specific surface area of 0.3–0.5 m²/g and a pH value of 1.2–12.5 [135]. In terms of the morphological properties, the conventional boilers’ fly ash particles are characterized by a regular, spherical shape and a smooth surface of amorphous structures. The presence of cenospheres filled with amorphous particles and crystals–plerospheres, as well as of porous unburnt carbon is also possible [103,135].

Fly ash from fluidized bed boilers on the other hand contain the solid residue after the combustion performed at relatively low temperature (800–900 °C) when compared to conventional boilers (1400–1600 °C) and calcium compound-based flue gas desulphurization methods [135,159]. The presence of irregular and porous particles is characteristic for fluidized bed fly ash [135] as well as good compressive strength resulting from its pozzolanic properties. In the process of flue gas cooling, calcium and magnesium oxides present in fly ash react with steam, carbon dioxide and sulfur oxides to form portlandite (Ca(OH)₂), calcite and gypsum (CaSO₄·2H₂O). Pyrite is transformed into hematite at the temperature range of 400–700 °C, and into magnetite at a temperature over 1390 °C. Silicon, calcium and magnesium may be present in fly ash in the form of separate crystalline phases (like quartz, periclase), diopside (CaMgSi₂O₆) or amorphous phases [103]. Calcium and magnesium oxides and hydroxides are the most reactive in carbonation process, while silicates and alumina silicates are less reactive, depending on their structure and chemical properties. Among the over 300 minerals and 188 mineral groups forming coal combustion fly ash, the most reactive are calcium oxide, portlandite, brucite and periclase, even under mild process conditions, at the temperature below 100 °C and under the atmospheric pressure [160,161]. The conversion rate in carbonation process of other calcium and magnesium compounds, including basanite (CaSO₄·0.5H₂O), gypsum and brownmillerite (Ca₂(Al,Fe)₂O₅) is lower.

Changes in physical and chemical properties of fly ash, resulting from the carbonation process, were reported to have beneficial effects on its applicability as a cement substitute in concrete production. The number of particles of the diameter over 20 µm and below 5 µm increases in carbonated fly ash [162]. It becomes less prone to heavy metal leaching and swelling as a result of water binding by calcium compounds. The amounts of calcium oxides, hydroxides and sulfates are also reduced since they are transformed into calcium carbonate [103,161]. The specific surface area of carbonated fly ash increases, and the surface of the carbonated fly ash particles is covered with needle and block crystals of aragonite and calcite [161], which in turn results in higher mechanical strength of a binder being produced with the use of carbonated fly ash. The values of workability, early compressive strength and durability of binders produced with the use of 5, 10 or 20% w/w of carbonated fly ash were higher than the values reported for materials produced with the respective amounts of untreated fly ash [161]. It is therefore considered that the application of fly ash in carbon dioxide mineralization process contributes to the reduction in carbon dioxide emission on the one hand and to the stabilization of fly ash properties, improving its applicability in construction industry, on the other. The present-day research targets, in terms of the carbon dioxide mineralization process with the use of fly ash cover the design and construction of installations ensuring high carbonation capacity with a reduced energy demand; integrating the utilization of various waste streams and reduction in pollutant emission, including carbon dioxide, sulfur oxides and dust; as well as the stabilization of fluidized bed fly ash properties for wider implementation in the construction industry [103].

3.5.2. Carbon Capture Potential of Fly Ash

The theoretical carbon dioxide capture potential of fly ash in carbonation reaction, mCO₂ [% w/w], is calculated according to various sources as:

$$mC{O}_2=\frac{44}{56} · CaO+\frac{44}{40}·MgO+\frac{44}{80}·S{O}_3-C{O}_2$$

(4)

where: CaO, MgO, SO₃, CO₂—mass of CaO, MgO, SO₃ and CO₂ per unit mass of untreated fly ash [163], or:

$$mC{O}_2=0.785\left(CaO-0.7 S{O}_3\right)+1.09 MgO+0.71 N{a}_{2}O+0.468 K_{2}O$$

(5)

where: CaO, MgO, Na₂O, K₂O—mass ratios of particular components in untreated fly ash [164], or:

$$mC{O}_2=0.785\left(CaO-0.5 CaC{O}_3-0.7S{O}_3\right)+1.09 MgO+0.71 N{a}_{2}O+0.468 K_{2}O$$

(6)

where: CaO, CaCO₃, MgO, Na₂O, K₂O—mass ratios of particular components in untreated fly ash [165].

The maximum carbonation efficiency, defined as a ratio of carbon dioxide bound in the process to the theoretical capture potential of fly ash [163,164,165], is estimated to be approximately 30–80% in wet carbonation method. This depends on the elemental and mineralogical composition of fly ash and carbonation process conditions (temperature, pressure, carbon dioxide flow rate, weight ratio of solid and liquid phases, mixing rate) and is achieved after approximately 40–120 min of the process duration [164,166,167]. It is estimated that the average carbonation process efficiency of 0.05 kgCO₂/kg of fly ash would enable the binding of about 0.25% of the world carbon dioxide emission from coal-fired energy production systems [99]. Higher temperatures improve reaction kinetics but at the same time decrease carbon dioxide solubility in a liquid phase. The optimum process temperature also depends on the forms of calcium and magnesium compounds present in fly ash [163]. The turbulent flow conditions and increase in the flow rate in the range of 60–250 mL/min were proven to increase the reaction rate and efficiency, resulting from better mixing and mass transfer between the gaseous and liquid reactants [166]. The maximum carbonation efficiency in wet method with the use of batch fed reactors was reported to be in the range of 0.02–0.26 kgCO₂/kg of fly ash [164,168,169,170]. The carbonation process with the use of fly ash rich in free calcium oxides or calcium hydroxides is characterized by the relatively high process efficiency at low temperatures (i.e., up to 100 °C), because of the higher reactivity of these compounds when compared to (in descending order): MgAl₂O₄ (spinel), Mg(OH)₂, CaMgSi₂O₆ (diopside) and MgO [163].

The conversion rate of calcium oxide into calcium carbonate, expressed as the weight increase in thermogravimetric analysis (TGA), and reported for carbonation of fly ash of the initial calcium oxide content of 4.1% w/w, at the temperature of 30 °C and under atmospheric pressure, was 82% [170]. The carbonation process was performed in two stages with the use of the wet carbonation method. No significant effect of the temperature in the range tested of 20–60 °C or the pressure in the range of 1–4 MPa on the carbonation process efficiency was observed in this study [170]. In the indirect carbonation of coal combustion fly ash of 43% w/w calcium and 12% w/w magnesium content with the use of a reaction gas of 33% on the basis of the molar content of carbon dioxide, under ambient temperature and pressure conditions, carbon dioxide capture of 0.017 kgCO₂/kg of fly ash and carbonation process efficiency of 56% were achieved [171]. The carbonation of a 70/30 w/w mixture of fly ash and bottom ash from the petrochemical industry, of 69% w/w calcium oxide content in the indirect method with the use of water or a 0.1/0.5 M solution of HCl in cation extraction stage at ambient temperature and pressure, and 5% w/w water solution of monoethanolamine in the carbonation stage with the use of a reaction gas of 15% vol. carbon dioxide content showed an increase in cation extraction efficiency with an increase in acid concentration, and process efficiency of 37% of the extracted cations reacted in the subsequent carbonation process [172]. In the alkaline metal cation extraction from lignite combustion fly ash with the use of ammonia salts CH₃COONH₄, NH₄NO₃ and NH₄Cl, about 35–40% of calcium ions were reported to be transferred to the liquid phase after 1 h of the extraction process [173]. Increases in the temperature from 25 to 90 °C and in salt concentration from 0.5 to 3 mole/L both were reported to improve extraction stage efficiency in the case of CH₃COONH₄ application. In the subsequent carbonation stage under ambient temperature and pressure conditions, carbon dioxide was fed into the reactor containing 500 mL of the leachate from the extraction stage performed on a 25 g fly ash sample. The carbon dioxide flow rate applied was 50 mL/min. The indirect wet carbonation process efficiency, calculated as the amount of calcium ions bound in calcium carbonate per 1 h of the process duration, was 47%, and the carbon dioxide capture ratio reported was 0.11 kgCO₂/kg of fly ash [173,174].

3.5.3. Dry and Semi-Dry Carbonation Methods with the Use of Fly Ash

Although the efficiency achievable in dry carbonation methods is considered to be in general lower than that in wet methods, the reaction rate is higher: the calcium oxide conversion rate of 50% is reached in several dozen seconds in dry methods and in several dozen minutes in wet methods [175]. Dry carbonation methods also have the advantages of technological simplicity and better cost efficiency. There is no need for solid state carbonation product recovery from the liquid, as in the cases of direct wet carbonation methods, or the recovery of chemicals, as in the cases of indirect methods, which results in lower energy demand and lower costs. The efficiency of a dry carbonation depends, as in wet carbonation, principally on chemical and mineralogical composition of the alkaline earth metals source and process conditions. Furthermore, extraction of calcium and magnesium cations in indirect wet carbonation methods may cause a counteracting effect consisting of the release of carbon dioxide from the calcium and magnesium compounds before the carbonation stage [100]. The parameters affecting the dry carbonation process efficiency most are the temperature, pressure and the addition of a small amount of water or steam.

The minimum dry carbonation process temperature recommended based on tests in a thermogravimetric analyzer with the use of a medical waste combustion fly ash of 38% w/w calcium hydroxide content resulting from a flue gas treatment process was 350 °C, and the maximum conversion rate of carbon oxide to calcium carbonate was 80%. The carbonation reaction rate was reported to increase with an increase in carbon dioxide content in a reaction gas in the range tested of 10–50% vol. [176].

Similar tests performed in a thermogravimetric analyzer on municipal combustion ash after flue gas treatment with the use of calcium hydroxide showed that the optimum temperature, enabling 66% conversion of calcium into calcium carbonates, was achieved at 400 °C, within the tested temperature range of 300–500 °C [175]. No effect of carbon dioxide content in the reaction gas, in the tested range of 5, 10, 15 or 100% vol., on the final conversion rate to calcium carbonates was observed at this temperature, which has drawn the authors to the conclusion that this process might be employed in a direct capture of carbon dioxide from flue gases. An increase in carbon dioxide content in a reaction gas had, however, a positive effect on the carbonation reaction kinetics. The maximum carbon dioxide capture efficiency achieved was 0.04 kgCO₂/kg of ash [175].

A rise in carbon dioxide pressure from 0.2 to 1 MP was reported to increase carbon dioxide capture efficiency at the temperature of 30 °C from 0.005 to 0.026 kgCO₂/kg of fly ash at the initial calcium oxide content of 6.7% w/w. The maximum calcium conversion rate reached after 1 h of process duration was 36% [167].

Wang et al. reported an increase in carbonation reaction kinetics and efficiency with addition of 8 or 15% vol. of steam in a synthetic flue gas applied as a reaction gas, irrespective of the process temperature tested in the range of 250–800 °C [177]. The process was tested in a thermogravimetric analyzer with samples of fluidized bed combustion fly ash (60/40 w/w mixture of petroleum coke and coal) and flue gas desulfurization residues of free calcium oxide content of 51% w/w.

Steam-induced intensification of the carbonation process was also reported by Patel et al. [104], who worked on fly ash samples from the combustion of petroleum coke and coal (80/20 w/w) in fluidized bed boilers with the semi-dry desulphurization of flue gas, containing 37% w/w of unreacted calcium oxide. The carbonation process was tested in a laboratory-scale fixed-bed reactor of a diameter of 2.5 cm and height of 30 cm, on 50 g samples with the use of a reaction gas composed of 30% vol. carbon dioxide in nitrogen, at the temperature range of 30–80 °C. The weight ratio of water dosed was 0.65 of calcium oxide and it was proven to enhance the process kinetics along with a temperature increase up to 50 °C. A further increase in the temperature had no effect on process improvement, which was assumed to be caused by the counteracting effect of the temperature-dependent decrease in carbon dioxide solubility [104].

The effect of water addition in the range of 25–100% w/w per fly ash mass on carbonation process was tested by Kim et al. [178]. The source of alkaline earth metals was waste combustion fly ash of calcium oxide content in oxide composition of 17% w/w. The reaction gas employed was pure carbon dioxide. An increase in calcium carbonate content in treated fly ash from 14 to 26% w/w was reported with water dose rise from 25 to 75% w/w. The dosing of water in the amount of 100% w/w per fly ash mass resulted in calcium carbonate content decrease to 22% w/w. A rise in calcium carbonate content from 15 to 20% w/w in treated fly ash was also reported with the rise in carbon dioxide concentration in a reaction gas from 10 to 50% vol. with the set value of water dosing of 20% w/w. These tests were performed in a rotating reactor on fly ash samples of 100 g. The carbon dioxide capture capacity reported was 0.10 kgCO₂/kg of fly ash.

3.5.4. Effects of Water Dosing on Carbonation with the Use of Fly Ash

The mechanism of carbon dioxide mineralization is described in the literature with the use of a shrinking core and surface coverage as well as the dissolution and precipitation models [179]. The mechanisms behind the effects of water dosing on carbonation efficiencies are still not sufficiently understood [104,177,179,180]. It is assumed however, that they consist of dissolution of carbon dioxide and calcium hydroxide in water nanoparticles on the surface of a solid phase and in a subsequent precipitation of calcium carbonate according to the reactions (7)–(8) [104,181]:

$$CaO+H_{2}O \to Ca{(OH)}_{2 }\to C{a}^{2+}+2O{H}^{-}$$

(7)

$$C{a}^{2+}+C{O}_3^{2-} \to CaC{O}_{3\left(nuclei\right)}\to CaC{O}_3$$

(8)

In total (7) + (8):

$$Ca{(OH)}_2+C{O}_2+H_{2}O \to CaC{O}_3+H_{2}O$$

(9)

Furthermore, calcium bicarbonate is created (10), reacting with calcium hydroxide (11):

$$CaC{O}_3+C{O}_2+H_{2}O \to Ca{(HC{O}_3)}_2$$

(10)

$$Ca{(HC{O}_3)}_2+Ca{(OH)}_2 \to 2CaC{O}_3+2H_{2}O$$

(11)

The effects of temperature and carbon dioxide concentration in a reaction gas and water dosage on fluidized bed fly ash carbonation were tested by Liu et al. [174]. The carbonation efficiency was reported to increase with: (i) a rise in the temperature in the tested range of 300–800 °C and pure carbon dioxide as a reaction gas, with the maximum of 47.8% after 1 h of process duration on a sample of 10 mg in a thermogravimetric analyzer, and (ii) an increase in carbon dioxide concentration in a reaction gas, of 5, 10, 15, 20 or 100% vol. at 530 °C in a fixed-bed reactor, with the maximum of 8.06%. In tests performed at the temperature of 530 °C in a fixed-bed reactor with the use of a reaction gas of 15% vol. carbon dioxide content, an increase in carbonation efficiency from 6.2% in dry conditions to 21.7% when steam was applied in the amount of 20% vol. was reported [174].

In the research study on the effects of pressure ranging from 0.1 to 1.5 MPa on carbonation of 50 g samples of coal combustion and flue gas desulphurization residues at the temperature of 25 or 45 °C an increase in carbon dioxide binding from 0.02 kgCO₂/kg under ambient pressure of 0.1 MPa (irrespectively from process temperature), to 0.14 kgCO₂/kg at 25 °C and 0.18 kgCO₂/kg at 45 °C under an elevated pressure of 1.5 MPa was demonstrated [102].

3.5.5. Effect of Carbonation on Properties of Cement Mortar and Concrete

The effects of dry carbonation of coal combustion fly ash in terms of water demand, compressive strength and volume stability of cement mortars prepared with their use were tested by Siriruang et al. [182]. The process of carbonation was proven to enhance the volume stability of cement mortars when compared to mortars produced with the use of untreated fly ash, with no effect on the remaining parameters. This was performed in a fixed-bed reactor, with a pure carbon dioxide as a reactive gas and with a process duration of 1 h at ambient temperature. The fly ash tested contained over 10% w/w of calcium oxide and over 5% w/w of sulfur oxide in oxide composition, exceeding the respective values of national standard TIS 2135 [183]. The cement mortars produced with the use of carbonated and untreated fly ash complied also with the national standards for a compressive strength of cement mortars, i.e., pozzolanic activity index of at least 75% after 7 and 28 days and at least 85% after 91 days [183]. The efficiency of carbon dioxide binding was 302–868 µmol CO₂/g of fly ash of 1.7–5% w/w free calcium oxide content. Cement mortars produced with a substitution of cement with fly ash in the amounts of 20–40% w/w were characterized by lower water demand than the ones produced with cement as a binder. The compressive strength of cement mortars with 20% w/w substitution of cement after 7 and 28 days were slightly lower (39 vs. 32 MPa and 47 vs. 45 MPa), and after 91 days higher (52 vs. 54 MPa) than the values reported for mortars with cement as a binder, because formation of calcium silicate hydrates is faster in case of mortars with pure cement as a binder [184].

The effect of the carbonation of concrete cubes of dimensions of 50 mm, produced with the use of class C and F fly ash, in deionized water with the liquid-to-solid phase weight ratio of 0.2, and after seasoning for 2 h at 45 °C and at relative humidity of 50%, was tested by Wei et al. [185]. The carbonation process was performed in a closed chamber with the use of a reactive gas of 99.5% vol. or 12% vol. content of carbon dioxide being provided to the chamber through a wet scrubber, under atmospheric pressure and at the temperature of 45, 60 or 75 °C. An increase was reported in compressive strength with the seasoning time and carbonation temperature for samples produced with the use of class C fly ash with a high content of calcium oxide (24% w/w in oxide composition). After 7 days of seasoning, an increase in compressive strength value from 15 MPa for the reference sample (not carbonated) to 35 MPa for the sample carbonated at 75 °C was reported. The efficiency of carbon dioxide binding was 0.09 kgCO₂/kg of a concrete. In case of concrete samples prepared with the use of fly ash of a lower calcium oxide content (13% w/w), the respective increase in compressive strength values caused by carbonation was less significant: from approximately 5 to 7 MPa. After 10 days of seasoning, the values of compressive strength were similar for concrete samples produced with the use of class C fly ash and carbonated with the reaction gas stream of carbon dioxide content of 99.5 or 12% vol. This implied that the energy sector flue gases may be applied in the process, without the need for an additional carbon dioxide capture installation.

The mineralization of carbon dioxide with the use of waste construction materials, like concrete or cement mortars, is also considered as a method of carbon dioxide emission mitigation from this industrial branch, which is estimated to contribute to approximately 33% of the world anthropogenic carbon dioxide release [186]. The carbonation process was proven to be a successful alternative to steam curing of concrete, enabling both the utilization of carbon dioxide and reduction in the energy demand of the process by about 80%. The compactness of concrete cured with carbon dioxide was demonstrated to be also higher [187]: the amount of pores of a diameter of up to 10 nm increased, while of those of 10–100 nm decreased, which resulted in lower permeability and improved durability of concretes [188,189], and subsequently, in an enhanced compressive strength and frost resistance of concretes and cement mortars. The carbonation process was also reported to improve the volume stability [189,190,191,192,193] and resistance to sulfate attack [194], when compared to materials cured with steam. A shift in characteristic temperatures was also observed in a thermogravimetric analysis of carbonated concrete and cement mortars components from approximately 485 °C, typical for Ca(OH)₂, to over 500 °C, characteristic for calcium carbonate [195]. The process of carbonation of recycled construction materials was reported to enhance the apparent density of materials produced with their use, decrease the average pore diameter from over 200 nm to 50–200 nm [196], increase compressive strength, durability and frost resistance and decrease shrinkage [197,198,199] in comparison with materials produced with the use of untreated recycled resources. The presence of carbonates in carbonated materials also results in the creation of calcium aluminates, stabilizing ettringite and increasing the volume of solid phase [200].

The process of carbon dioxide mineralization is also considered in terms of flue gas treatment applications. Fluidized bed fly ash was proven to give the removal efficiencies of 96, 99 and 83% for carbon dioxide, nitrogen oxides and dust, respectively, when employed in petrochemical industry flue gas treatment, and the carbonation product was applicable as a cement substitute in construction industry [201]. Other potential applications of the products of carbonation process performed with the use of industrial waste include the manufacturing of geopolymers, additives to soil and glass ceramics. Widespread industrial implementation of the process requires further studies aiming at improvements in the economic attractiveness of the solutions, including process efficiency, heat integration and process optimization. Hybridization of carbon dioxide mineralization by its integration with wastewater and flue gas treatment processes, as well as utilization of the heat of the exothermic reaction, are also important research issues. Closing the waste-to-resource cycle requires also process optimization including energy, engineering, environmental and economic aspects [202,203]. The economic aspects of ash utilization also cover the need for incentive policy in the field of ash recycling at the local and governmental level, e.g., purchase of the ash valorization products on preferential basis or tax credits for ash-recycling businesses [135].

4. Summary and Conclusions

Approximately twenty demonstration projects are at the operational or construction phase worldwide in ECBM technology or saline aquifers sequestration technology of the capacity of over 400 MgCO₂/d, while most of the remaining CCS and CCU projects have not reached the demonstration scale [121]. Two projects implemented at an industrial scale: Petra Nova, USA, and Boundary Dam, Canada, are profitable CCS examples, since they provide carbon dioxide to the enhanced hydrocarbon recovery processes on a commercial basis. Higher emission fees make the CCS projects more economically feasible, especially with cost optimization in CCS clusters of carbon dioxide capture, transport and storage, e.g., Port of Rotterdam CCUS and Norway full-chain CCS projects [122]. The need for the implementation of adequate national policies providing incentives for private investors, including the development of mechanisms for improving profitability, like e.g., tax credits, is emphasized.

The process of carbon dioxide mineralization is considered to be complementary to its utilization in enhanced hydrocarbon recovery. Unlike the remaining concepts of carbon dioxide utilization, carbon dioxide mineralization enables the permanent binding of carbon dioxide in the form of stable carbonates in materials of the applicability range covering the construction industry, road building and land remediation. The mineralization process, combined with the use of combustion products as a source of alkaline earth metals, offers additional environmental and economic benefits resulting from the valorization of large volumes of waste. Construction industry-related applications of combustion waste mainly cover conventional boilers fly ash of properties varying depending on the combustion technology employed, flue gas treatment method adopted and fuel characteristics. The application of fluidized bed fly ash in these terms is reported less often, which should be attributed to its chemical, mineralogical and structural properties differing considerably from these of conventional boilers’ fly ash. Most of the works reported in the literature on carbon dioxide utilization in mineralization process concern the determination of the process efficiency, i.e., the mass of carbon dioxide bound per unit mass of fly ash, or the percentage of alkaline earth metals extracted in the dominant, wet indirect methods, as well as the optimization of the alkaline earth metals extraction stage. Extensive studies are still lacking on the properties and performance of construction materials produced with the use of carbonated fluidized bed fly ash.

Before widespread implementation of fluidized bed fly ash as a cementitious material, the properties of construction materials produced with its use need to be proven to be comparable with these based on Portland cement as a binder, and the process of carbon dioxide mineralization as a way of stabilization of its properties needs to be mastered to be made more technologically uncomplicated and economically feasible. In the economic assessment of the process, the environmental and social benefits of utilization of carbon dioxide with parallel valorization of fly ash need to be taken into account and the relevant subsidizing and incentive mechanisms should be employed.

In terms of technological challenges, further studies are needed on application of carbonated fluidized bed fly ash of varying properties, as a cementitious material in concretes and other construction materials, including the application of properly selected chemical additives for concretes, the development of specifications of concretes and binding materials, and the use of superplasticizers in concretes’ production to reduce water demand, while not deteriorating their workability. The desired research directions also cover the effects of binding materials’ grain size, shape and porous structure, as well as of unburnt carbon content, on the inter-grain voids and sorption of the surfactants, the components of air-entraining admixtures. Methods should also be developed for the separation of chars from soot in fly ash and the determination of the effects of these forms of unburnt carbon present in fly ash on the properties of concrete [140].

The carbonation methods themselves are presently tested at a laboratory scale in wet and dry, direct and indirect options and still need improvements in terms of the process efficiency, simplicity, energy demand reduction and upscaling. The indirect wet carbonation methods are energy- and cost-intensive since they require the separation of the solid process product from the liquid phase and the recovery of chemicals used in alkaline earth metals extraction stage, while dry methods have in general lower process efficiency. The research focus, in terms of the development of economically efficient carbon dioxide mineralization methods with the use of fluidized bed fly ash, should be on the achievement of satisfactory process efficiency along with the reduction in energy and resource demand, with no chemical pre-treatment, high temperature and/or pressure needed. A reduction in the required number and costs of unit processes, as well as the level of technological complexity, is also still a research and engineering challenge on the way to wider process implementation at the demonstration and industrial scale.

Widespread utilization of fluidized bed fly ash is hindered, on the one hand, by the variations in its properties, and by a lack of relevant standards, covering its properties and new application potentials, on the other. Demonstration-scale installations and mechanisms improving the economic conditions of the process implementation are needed. Governmental support encouraging investments in the carbon dioxide utilization process, including carbon dioxide mineralization, with the use of industrial solid wastes, is being sought. There is also a strong requirement to create new markets for carbon dioxide utilization products, as well as to generate social acceptance and awareness at various levels, including decision makers at a governmental and industrial scale, in terms of the responsible management of natural resources, combustion products and waste materials. This should result in the development of relevant guidelines, legal frameworks and financial support mechanisms.