Carbon Capture Adsorbents Based on Ash Residues from the Combustion of Coal with Biomass Blended Fuels

Abstract

One of the approaches to limit the negative impact on the environment from the burning of coal in the production of heat and electricity is to limit their use by blending them with biomass. Blended fuel combustion leads to the generation of a solid ash residue differing in composition from coal ash, and opportunities for its utilization have not yet been studied. The present paper provides results on the carbon capture potential of adsorbents developed through the alkaline conversion of ash mixtures from the combustion of lignite and biomass from agricultural plants and wood. The raw materials and the obtained adsorbents were studied with respect to the following: their chemical and phase composition based on Atomic Absorption Spectroscopy with Inductively Coupled Plasma (AAS-ICP) and X-ray powder diffraction (XRD), respectively, morphology based on scanning electron spectroscopy (SEM), thermal properties based on thermal analysis (TG and DTG), surface parameters based on N₂ physisorption, and the type of metal oxides within the adsorbents based on temperature-programmed reduction (TPR) and UV-VIS spectroscopy. The adsorption capacity toward CO₂ was studied in dynamic conditions and the obtained results were compared to those of zeolite-like CO₂ adsorbents developed through the utilization of the raw coal ash. It was observed that the adsorbents based on ash of blended fuel have a comparable carbon capture potential with coal fly ash zeolites despite their lower specific surface areas due to their compositional specifics and that they could be successfully applied as adsorbents in post-combustion carbon capture systems.

1. Introduction

Replacing fossil fuels with renewable biofuels is one way to limit climate change and improve ambient air quality. Biomass is considered a renewable energy source with zero carbon emissions whose energy potential can be utilized in various technological ways, such as combustion, gasification, and biochemical and chemical processing into liquid fuels [1]. Biomass combustion is the simplest method of zero-carbon energy production, which, at a global scale, possesses the biggest share among the biomass energy conversion techniques [2]. To achieve carbon neutrality from energy production and to limit global warming, biomass is considered not only an energy source but also a major natural sink of atmospheric carbon dioxide [3]. This reveals two aspects of the role of biomass in zero-carbon scenarios—increasing its share in energy production on the one hand and preserving forest areas on the other [3]. Biomass as a fuel stabilizes the flame and the efficiency of the combustion process; however, its direct combustion as the main fuel for large-scale energy production is accompanied by difficulties, the main one of which is the accumulation of slag in combustion chambers, which greatly hinders heat exchange and causes corrosion [4]. This is due to the increased content of alkaline and alkaline earth halides, carbonates, and phosphates, as well as water-soluble components in biomass ash, which determines its relatively low softening and melting temperatures [5]. Biomass also has some unfavorable characteristics as a fuel, related to increased humidity and low bulk energy density. The requirements for the preliminary drying and dehydration of biomass make its direct combustion more expensive [4]. Another disadvantage is the increased risk of emissions of cyclic hydrocarbons, especially polychlorinated dibenzo-p-dioxins (PCDFs) and dibenzofurans (PCDDs), resulting from a severely disturbed combustion process with sub-stoichiometric oxygen, which are stable in the environment and can be transported across borders over long distances [6]. PCDFs and PCDDs are of serious global health concern because of their carcinogenic and mutagenic effects at very low concentration levels, bioaccumulation, and strong toxicity [6]. For technological feasibility and better environmental performance, the application of the pre-combustion processing of biomass to liquid or gaseous fuels is preferred, which includes gasification, pyrolysis, anaerobic digestion, or hydrothermal liquefaction, which further increases the cost of utilizing biomass for energy purposes [7]. For existing solid fuel combustion plants, a partial replacement of coal with biomass from wood or agricultural waste is applied [8]. This approach is an opportunity to use the residual operational resources of already-built combustion plants with better environmental performance and reduced carbon emissions. However, a complete transition of coal to biomass for operating Thermal Power Plants (TPPs) is not possible because of the significantly different compositional, physicochemical, and combustion characteristics of both fuels, which hinders the pulverized combustion approach for biomass [9]. In the direct co-firing of biomass and coal, which is applied in existing TPPs, biomass usually constitutes 10–20 wt.% of the fuel mixture [8]. Studies have shown that replacing even 10% of the annual global coal-based energy consumption with biomass would not only contribute to significant environmental benefits by reducing carbon emissions [8] but would also result in lower sulfur and nitrogen oxide emissions due to the low sulfur and nitrogen content of biomass. Furthermore, biomass ash would contribute to the mitigation of sulfur dioxide emissions from coal combustion due to the easy sulfation of alkali and alkaline earth oxides in the biomass ash composition [10]. Particulate matter emissions would also decrease due to the low mineral content of biomass. A recent comprehensive evaluation reveals that coal combustion TPPs with 40–50% co-combustion of biomass with installed carbon capture and storage (CCS) systems can achieve zero carbon emissions, which will enable the use of the energy potential of coal in the coming years [11].

Biomass differs significantly from coal in its macrocomponent composition, which varies widely depending on its origin [12]. It contains a higher proportion of volatile organic matter compared to coal and fewer fixed carbon and mineral fractions, while coal contains a wide variety of non-combustible minerals [10,12]. The ash content is below 1 wt.% for most types of biomasses, while for coal, it reaches 10–20 wt.%. This also leads to a significant difference in the composition of coal and biomass ash [13]. Coal fly ash (CFA) contains mainly alumina, silica, aluminosilicates, iron oxides, and alkaline earth oxides and small amounts of alkaline and transition metal oxides [14]. According to the international standard ASTM C 0618-23 [15], CFA is classified into two classes, F and C, depending on the total content of SiO₂ + Al₂O₃ + Fe₂O₃. The remaining ingredient is mostly CaO since alkaline oxides and oxides of transition metals are in small concentrations and are not decisive for the self-cementing and geopolymer-forming ability of CFA. CFA of class C contains SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 50 wt.%, while its CaO content exceeds 20 wt.%, determining self-cementing properties related to CFA utilization in construction materials. CFA class F contains SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 70 wt.% and less than 10 wt.% CaO. The chemical and mineral composition of coal ash has been studied in detail in order to rationally utilize this abundant resource, which is deposited in large amounts throughout the world and continues to be generated in many coal-fired countries [12,16]. CFA is characterized by a mixed amorphous–crystalline structure composed of amorphous aluminosilicates and crystalline phases, the main identified forms of which are quartz, mullite, anorthite, hematite, magnetite, maghemite, portlandite, and other trace phases [17,18]. The amorphous–crystalline ratio in CFA depends on the combustion temperatures and the cooling rate of the ash particles, and it is an important characteristic related to chemical processing for raw material utilization [18,19]. Unlike coal fly ash, biomass ash (BA) is enriched in alkaline and alkaline earth components, which are highly reactive and interact with the acidic oxides in the flue gases released during the combustion process and form carbonates, sulfates, and phosphates in the solid residue [12]. Another peculiarity of BA is that it is an inorganic–organic mixture with significant organic component content in addition to minerals [12,20]. A categorization of biomass ash (BA) is also proposed in terms of its chemical composition, distinguishing four main types, S (SiO₂ + Al₂O₃ + Fe₂O₃ + Na₂O + TiO₂), C (CaO + MgO + MnO), K (K₂O + P₂O₅ + SO₃ + Cl₂O), and CK (with a composition falling between types C and K), and six subclasses [13].

In recent years, coal fly ash has been intensively studied to extract valuable components from it or to convert it into zeolites, with a view to more reliable and beneficial utilization of this huge material resource [21,22,23]. The alkaline conversion of CFA to zeolites is a successfully applied approach for the development of efficient adsorbents and catalysts for different environmental applications through the utilization of coal fly ash [24,25,26]. Recent studies reveal the great potential of zeolites obtained through the alkaline conversion of CFA to adsorb CO₂ [27,28]. Coal fly ash zeolites (CFAZ) possess unique characteristics that favor their valuable application in carbon capture technologies, namely their higher hydrophobicity, a mixed micro-mesoporous structure that facilitates mass transfer processes, high surface unsaturation, and the transfer of metal oxides from the raw CFA into its zeolite products that increase adsorption potential, such as iron oxides and calcium oxide. The main benefit of CFAZ carbon dioxide adsorbents is their low-thermal regeneration, which surpasses a number of effective physical and chemical CO₂ absorbents [27]. In general, BA is used for soil revitalization, but in recent years, its potential for carbon sequestration and the possibilities for its processing into CO₂ adsorbents have also been investigated [9,29]. A number of studies also reveal the great potential of BA for the production of building materials and its application in building mixtures for obtaining concrete and cement [9,30], with the main difficulties arising from the lower proportion of active ingredients (aluminosilicates) in most types of biomass, especially that of wood, and the significant proportion of organic components in the composition of biomass ash (very high proportion of LOI compared to coal ash). BA is also characterized by weaker pozzolanic properties due to the more significant conversion of calcium oxide into carbonate. The carbon capture potential of BA is rather low, while efficient biomass-derived adsorbents are based on biomass-derived and subsequently activated carbon [31]. The benefits of carbon capture and storage (CCS) from BA are mainly related to the retention of CO₂ by alkali and alkaline earth oxides in its composition and the formation of carbonates, both through the capture of CO₂ from flue gases and its direct capture from the air during the storage of biomass ash [32,33]. The carbon capture potential of BA contributes to energy production with negative CO₂ emissions through biomass combustion, but BA adsorption potential is not recoverable due to the high decomposition temperature of the resulting carbonates. Our previous studies were focused on the utilization of coal fly ash through its alkaline conversion to zeolite X with high adsorption capacity for CO₂ and favorable thermal regeneration [27,34]. The beneficial effect of calcium and iron nanosized oxide particles and their ions occupying extra-framework positions in the zeolite structure and increasing the adsorption capacity of coal ash adsorbents has also been established [25,27]. Studies on the mechanisms of CO₂ adsorption have revealed that calcium ions in the zeolite network do not form carbonate but contribute to the electrostatic retention of more than one CO₂ molecule in the adsorption sites [27,35]. The combustion of mixed fuels leads to the production of solid waste with a different composition and characteristics than those of coal ash, which raises the question of the opportunities for the utilization of this new type of waste. The results achieved from the previous research experience to develop carbon adsorbents based on coal ash provoked our interest in the applicability of the alkaline conversion approach of ash from mixed coal and biomass fuels for the effective utilization of the resulting solid waste and to investigate the potential of the resulting products for carbon capture.

Our study aims to investigate the applicability of alkaline hydrothermal conversion to improve surface characteristics and enhance the carbon dioxide capture potential of coal and biomass ash mixtures.

2. Experiment

The coal fly ash used as raw material in this study was sampled from the dust collector systems of two large coal-fired power plants in the R. Bulgaria: coal fly ash with medium calcium content was collected from TPP Maritsa 3 Dimitrovgrad and further designated as CFA1, and coal fly ash with lower calcium content was collected from TPP AES Galabovo, further designated as CFA2. The separated coal ash with high iron content, designated as CFA3, was also used in the present study as a raw material for the synthesis of carbon dioxide adsorbents. The high-iron CFA was sampled from the hydraulic lock of the sedimentation tanks of the electrostatic precipitators of the dust collection systems at TPP AES Galabovo. Ash from mixed biomass of woody and agricultural origin burned in a Bulgarian TPP (BA1) and from wood pellets combusted in a local heating installation (BA2) were sampled and used as raw materials in this study. The alkaline conversion products were obtained at different CFA/BA ratios and constant alkalinity of the reaction solution and are designated as follows: CFA1/BA1/Z1 and CFA2/BA1/Z1 (the CFA/BA1 ratio is 5:2), CFA1/BA1/Z2 and CFA2/BA1/Z2 (the CFA/BA1 ratio is 5:4), CFA1/BA1/Z3 and CFA2/BA1/Z3 (the CFA/BA1 ratio is 5:6), and CFA1/BA2/Z3, CFA2/BA2/Z3 and CFA3/BA2/Z3 (the CFA/BA2 ratio is 5:6). When CFA is the only raw material for the synthesis, the obtained samples are further denoted as CFAZ1, CFAZ2, and CFAZ3, correspondingly. Alkaline conversion was carried out in the following sequence: The reaction mixtures were melted with the alkaline activator in nickel crucibles at 550 °C for 1 h; the resulting cooled batches were crushed and mixed with distilled water and subjected to ultrasonic homogenization for 15 min; after conditioning, the reaction suspensions were subjected to hydrothermal activation for 4 h at mild temperature conditions of 90 °C; lastly, the powdered products were removed through filtration, washed with distilled water and dried at 105 °C, and then examined.

The chemical composition of the raw coal and biomass ash was analyzed with an optical emission spectrometer with inductively coupled plasma ICP 720–OES, Agilent Technologies (USA). The samples were preliminarily dried at 105 °C. The morphology of the obtained products was examined through scanning electron microscopy (SEM) with an SEM Philips525/EDAX 9900 apparatus with 5–30 kV and maximum resolution 5 nm (at 30 kV, WD = 4.5 mm).

The phase composition of the raw ashes and their alkaline conversion products was studied using a Bruker D8Advance powder X-ray diffractometer (Bruker AXS Advanced X-ray Solutions GmbH, Karlsruhe, Germany) with CuKα radiation (40 kV, 35 mA). The diffractograms were measured in the Bragg angle 2θ range from 3 to 75° with a step size of 0.02°. The crystalline phases were identified using the following reference ICDD cards: Quartz 33-1161, Mullite 15-0776, Calcite 24-0027, Hematite 33-0664, Maghemite 39-1346, Zeolite Na-X 38-0237, Portlandite (Ca(OH)₂) 44-1481, and Anhydrite (CaSO₄) 37-1496.

The surface characteristics of alkaline conversion products were investigated with an AUTOSORB iQ-MP/AG analyzer (Anton Paar GmbH) via N₂ adsorption at a cryogenic temperature of −196 °C. The studies were carried out after the preliminary degassing of the samples under a flow of He. By means of model studies of the obtained N₂ adsorption/desorption isotherms, the specific surface area and the pore size distribution in the pressure range p/p₀ = 0.002–0.990 were determined.

The obtained adsorbents were studied through temperature-programmed reduction (TPR) using a STA449F5 Jupiter instrument by NETZSCH Gerätebau GmbH at a heating rate of 5 °C min⁻¹ in a hydrogen flow.

UV–Visible diffuse reflectance spectra were recorded using an UV–VIS spectrophotometer (V-650, JASCO) equipped with a photomultiplier tube detector. The powder sample was put into a cell and measured in the range from 200 to 800 nm at room temperature.

Thermogravimetric analysis was performed in the thermal range of 40–800°C with a TGA-4000 Perkin Elmer apparatus (Perkin Elmer Inc., Waltham, MA, USA).

The adsorption of CO₂ was studied on pre-dried samples at 150 °C for 1 h in an amount of 0.40 g. The adsorption capacity was calculated from the adsorbed amounts of CO₂ measured with a gas chromatograph NEXIS GC-2030 ATF (Shimadzu Corp., Tokyo, Japan) with a 25 m PLOT Q capillary column. The adsorption of CO₂ from the studied samples was measured under dynamic conditions at 25 °C in a flow of 3 vol.% CO₂/N₂ at a flow rate of 30 mL/min.

3. Results and Discussions

For the purposes of the performed study, samples of ash from pulverized lignite combustion with moderate (CFA1) and low (CFA2) limestone content, as well as high-iron coal ash (CFA3) separately sampled from the hydrolock of an electrostatic precipitator, were collected and investigated regarding their chemical and phase composition. A mixed biomass of woody and agricultural origin (BA1) burned in a Bulgarian TPP and biomass ash (BA2) taken from a local wood pellet heating installation were also characterized through chemical and phase composition. The results of studies on the chemical composition of ash residues used as raw materials are presented in Table 1.

Depending on the lime content in accordance with the international standard ASTM C0618 [15], coal ash, with a view to its application in construction materials, is categorized into two classes: F (low calcium oxide content CaO < 10 wt.%) and C (high calcium oxide content CaO > 20 wt.%), with class C coal ash being characterized by self-cementing properties. All studied coal ashes belong to class F according to ASTM C0618 since they contain SiO₂ + Al₂O₃ + Fe₂O₃ ≥ 70 wt.%, which suggests weak or absence of self-cementing ability. In the composition of medium-calcium ash CFA1, the CaO content is twice as high as compared to CFA2, reaching 9.36 wt.%. According to the CAN/CSA A3001-18 standard [36], CaO content of 15–20 wt.% in CFA affects its cementitious properties. Previous versions of the standard reported an influence of CaO on the cementitious ability of coal fly ash even at 8–12 wt.% CaO content [37]; therefore, an intermediate category of coal ash class Cl was introduced.

In our previous studies on the synthesis of type X zeolites from CFA from TPP AES Galabovo and TPP Maritsa 3 Dimitrovgrad and their application as carbon dioxide adsorbents, some features in the capacity and mechanism of CO₂ adsorption have been established, influenced by the different CaO content in the raw materials and hence in the zeolite products [27,35]. Both coal-fired power plants burn lignite coal, but TPP AES Galabovo is supplied by coal mined from the Maritsa East Basin, while TPP Maritsa 3 Dimitrovgrad burns blends of lignite from the Maritsa East Basin with high-carbonate coal from the nearby Maritsa Coal Basin. CFA3 has been found to have high iron oxide content, expressed as Fe₂O₃, of almost 40 wt.%. Its total content of aluminosilicate components is about 42 wt.%, which is significantly lower than that in the unseparated CFA, in which SiO₂ + Al₂O₃ exceeds 70 wt.%. A significant amount of alkaline earth components, represented as CaO and MgO, are also found in CFA3. This solid phase waste with high iron content from the hydrolocks of the electrostatic precipitators has also been studied as a starting material for the synthesis of adsorbents since a number of studies have shown an increased adsorption capacity for CO₂ of iron-containing adsorbents [25,38,39].

According to the proposed biomass classification, BA1 from mixed biomass combustion can be attributed to type CK due to the significant content of alkaline earth oxides and K-containing phosphates, while BA2 is close to type C due to the high concentration of CaO + MgO. In any case, the classification of BA1 and BA2 as CK and C classes is conditional. It has been found that woody biomass and its ash residues always have high alkaline earth component content and belong to class C, while agricultural biomass ash is often of type K [6]. Taking this into account, it is quite logical to classify mixed biomass ash as an intermediate CK type.

Standard ASTM C0618 also imposes requirements on the LOI content of CFA for its application in concrete production, which should not exceed 6 wt.% [15]. LOI in CFA is represented by the incineration of unburned carbon, the thermal decomposition of carbonates, and the dehydration of crystalline hydrates, and its higher content significantly deteriorates the characteristics of concrete and makes it difficult to utilize coal ash in building materials [40]. The LOI content in biomass ash represents a significant percentage, especially the measured values in mixed biomass ash, which exceed 50 wt.% (Table 1) and are mainly due to the organic mass in the ash composition. According to the European Standard for chemical requirements for the application of fly ash in concrete production EN 450-1, three categories of ash are distinguished regarding LOI content: A up to 5 wt.% LOI, B up to 7 wt.% LOI, and C up to 9 wt.% LOI [6]. Regarding LOI content, the investigated BA does not meet the standard requirements. In Figure 1, SEM images of the raw ash are presented. Coal ash with moderate CaO content was found to consist of individual micron particles, most of which are spherical in shape, which could be explained by CO₂ release during the decomposition of limestone in the coal (CFA1) (Figure 1a). The influence of calcium content in coal ash on morphology, phase composition, and chemical reactivity has also been discussed in other studies [41].

Fly ash from the combustion of lignite coal with low Ca content has an irregular particle shape (CFA 2), and the characteristic crystal forms are not well distinguished (Figure 1b), which is typical for ash with low-calcium content [42]. Coal ash with a high iron content consists of irregularly shaped agglomerates, but crystalline structures are also observed, which appear as surface-distributed iron microspheres (Figure 1c). The formation of iron oxide microspheres located on the surface of amorphous irregular particles has been established in studies of the morphology of iron-containing coal ash [43,44]. Mixed biomass ash (BA1) consists of agglomerates with various irregular morphologies, and layered formations are also distinguished (Figure 1d). According to some investigations, the layered micro-flake structure of BA allows for the incorporation of metal and metal oxide particles, making it suitable for catalytic applications [29].

Figure 2 presents SEM images of the alkaline conversion products of coal ash with moderate (CFA1) and low calcium (CFA2) content blended with mixed biomass ash (BA1), obtained at different CFA/BA1 ratios. In all samples synthesized from ash mixtures, individual crystallites were observed, which are most likely zeolite phase, and with increasing biomass content in the reaction mixture, the crystallinity of the samples decreases. This is most likely due to the hindered zeolitization of the reaction mixtures with the increasing proportion of alkaline earth components and LOI introduced by biomass ash and a decreasing percentage of aluminosilicates. However, the morphology of the obtained products differs significantly from that of the starting coal ash and biomass ash.

In Figure 3, SEM images of alkaline conversion products of high-iron coal ash CFA3 and wood pellet ash (BA2) are presented at the highest BA content studied in the reaction mixture (CFA3/BA2 ratio 5/6) (sample CFA3/BA2/Z3). The images do not reveal zeolite-like crystallites but rather agglomerates with a morphology close to that of the starting CFA3. In these samples, the number of aluminosilicate components in the reaction mixtures is even lower due to the high iron oxide content, which prevents the crystallization of zeolite phases.

For comparison, Figure 4 presents morphological studies of zeolitized coal fly ash without biomass addition. Submicron crystallites of uniform size can be observed, typical for a product of alkali-converted CFA with a high degree of zeolitization.

The experimental X-ray diffractograms of coal fly ash and biomass ash used as raw materials are plotted in Figure 5.

Coal fly ash is characterized by a mixed amorphous–crystalline structure, with the main detected crystalline phases being quartz, mullite, magnetite (maghemite), and hematite, with small amounts of calcite and portlandite (Figure 5a). The diversity of minerals in biomass ash is large, as the identification of 229 minerals in their composition has been reported in the literature; therefore, their unambiguous identification, especially in low concentrations, is not definitive [13]. In the studied BAs, the most intense reflexes of calcite were found, with no portlandite detected. Reflections of quartz and dolomite were also observed. This indicates a high degree of carbonization of alkaline earth components of biomass ash compared to coal ash. The X-ray diffractograms of BA also show a mixed amorphous–crystalline structure, but considering the chemical composition, the amorphous component, unlike that of CFA, is not an aluminosilicate and is most likely of organic composition, as evidenced by the large mass losses upon ignition of BA (Table 1), which exceed 50 wt.% in BA1. Figure 6 presents X-ray diffraction patterns of samples synthesized from mixtures of CFA and BA1 of plant and wood origin. For comparison, an X-ray diffraction pattern of Na-X zeolite phase synthesized from CFA alone is presented. X-ray phase analysis shows that the products of alkaline conversion of coal ash without biomass ash additives (sample CFAZ1, Figure 6a and CFAZ2, Figure 6b) predominantly contain the Na-X zeolite phase. At the same time, the samples obtained from mixtures of coal ash with biomass ash at the minimum investigated content in the reagent mixtures (sample CFA1/BA1/Z1, Figure 6a and CFA2/BA1/Z1, Figure 6b) are composed of the Na-X zeolite phase with a low degree of crystallinity, together with some calcite phase coming from the initial biomass ash. With increasing biomass ash content in the reagent mixtures, the degree of zeolitization further decreases and the reflexes of the zeolite phase are absent (CFA1/BA1/Z2, CFA1/BA1/Z3, CFA2/BA1/Z2, and CFA2/BA1/Z3), while only the calcite phase could be seen together with some quartz.

In the alkaline conversion products with the highest BA1 content (CFA1/BA1/Z3 and CFA1/BA1/Z3), only crystalline phases transferred from the raw BA1 were observed, and no reflexes of the raw CFA or any zeolite phase appeared. This observation indicates that the aluminosilicate crystalline phases of the coal ash have dissolved in the alkaline solution, but their crystallization in the zeolite phase is hindered by the BA components. This is most likely explained by an increase in the content of alkaline earth oxides (CaO and MgO) in the reaction mixtures, which are the main components of BA, which promotes the formation of alkaline earth silicates instead of the zeolite phase, thus inhibiting the zeolitization process of coal fly ash. In the alkaline conversion products, no reflexes of iron oxide phases (hematite, maghemite, and maghemite) were detected, which indicates disproportionation of iron oxides in the reaction mixture in the form of nano-oxide particles or iron ions. In all alkaline conversion products of CFA and BA mixtures, an intense calcite reflex was detected, as the calcite phase is preserved in an alkaline environment.

Figure 7 presents X-ray diffraction patterns of alkaline conversion products of all raw CFAs mixed with woody biomass ash BA2 at the maximum studied BA2 content in the reaction mixtures. The results show that for the CFA1/BA2/Z3 and CFA2/BA2/Z3 samples, only the crystalline calcite phase, characteristic of the raw BA2, was detected, while for CFA3/BA2/Z3, a finely dispersed hematite phase could also be seen together with the calcite phase. Similar to the alkaline conversion products with the highest BA1 content, the samples with the highest BA2 content possess an intense calcite reflex and lack of characteristic Na-X lines, and crystalline phases of the raw coal ashes are once again observed. In the sample CFA3/BA3/Z3 obtained from coal ash with high iron content, the transfer of iron from the raw CFA3 is mostly as Fe^2+/3+-ions and nanopartlicles of iron oxide, as the XRD reflexes typical for iron oxide phases are not intensive.

Increased content of incorporated iron in zeolites gives them better hydrophobicity, acidity and redox properties, which are important for their catalytic properties [45]. Iron ions occupy extra-framework positions, while iron oxide species are located in pores and on the external surface of the zeolite structures, reducing their specific surface area and micropore volume [45]. It is assumed that Fe³⁺-ions are predominantly octahedrally coordinated due to their larger ionic radii, and it is difficult to replace Si⁴⁺ and Al³⁺ in their oxygen tetrahedrons to build zeolite frameworks [46], therefore taking extra-framework positions. In addition, it has been reported that iron ions, especially Fe³⁺, are easily involved in the zeolite framework, compared to other transition metal ions, and could isomorphically replace Al³⁺ in tetrahedral framework positions [45,47]. The effect of iron oxides on the hydrothermal synthesis, textural characteristics, and catalytic behavior of zeolites has been the subject of many studies for the development of Fe-containing zeolites for catalytic and adsorption applications. It has been observed that iron oxide in a limited amount in reaction mixtures does not disturb zeolite synthesis; however, above critical values, it inhibits the hydrothermal crystallization of defined zeolite phases [46,47,48]. The slowing down of the crystallization rate of zeolites during hydrothermal activation in the presence of an increased concentration of iron oxides in the reaction mixtures could be attributed to the following: As the iron ions can act as a competitor in the crystallization process, occupying sites in the zeolite structure that would otherwise be occupied by aluminum or silicon, they can reduce the crystallization rate and hinder the formation of defined crystalline phases; in alkaline media applied for hydrothermal synthesis, iron oxide can react with hydroxyl ions to form complexes that inhibit the crystallization process of zeolites; and iron ions may cause reduced ion mobility, as iron can form complexes with aluminum and silicon ions, which leads to reduced mobility of these ions and hinders the crystallization process, thus slowing down the formation of the desired aluminosilicate phase. These reactions can lead to a reduced availability of free silicon and aluminum ions, which are necessary for the crystallization of the zeolite phase, which can lead to slower crystal growth and reduced crystallization efficiency.

The suppressing role of higher oxidation state ions, including Fe-ions, in zeolite crystallization has also been observed [49].

Figure 8 shows N₂ adsorption/desorption isotherms of samples obtained through the alkaline conversion of coal ash with moderate CFA1 and low calcium CFA2 content and high iron content CFA3 in mixtures with different ratios of mixed biomass ash BA1 and ash of wood biomass BA2.

The highest rate of N₂ adsorption was found in the coal ash sample without biomass ash additives CFAZ2, with an isotherm that is a combination of types I and II according to the IUPAC classification, typical of micro–macroporous materials [50]. In addition, an H4 hysteresis loop was also recorded, which can often be found at aggregated zeolite crystals and which leads to a relatively large amount of interparticle meso- and macropores (Table 2). With increasing content of mixed biomass ash in the reagent mixtures, the isotherms shift to lower values of N₂ adsorption and become only type II, typical of non-porous and macroporous solids (Figure 8).

Low adsorption in the monolayer formation zone was found for samples BA1/Z2 and BA1/Z3, prepared with raw ash CFA1 and CFA2, respectively. The samples prepared with the addition of wood ash are characterized by significantly higher N₂ adsorption and a wider hysteresis area between the adsorption and desorption isotherms, which indicates a higher proportion of meso- and macropores in the structure, especially pronounced for sample CFA3/BA2/Z3, synthesized from raw ash with high iron content.

Figure 9 presents the pore size distributions of the studied samples.

The largest proportion is occupied by pores with a diameter of 3–4 nm in all studied samples, most pronounced in those with high biomass content CFA3/BA2/Z3 and CFA2/BA1/Z3 (Figure 9). Table 2 summarizes the results of model calculations of surface characteristics, such as BET specific surface area (S_BET, m²/g), specific surface area determined only by micropores (S_micro, m²/g), total pore volume (V_total, cm³/g), micropore volume (V_micro, cm³/g), and the ratio between the volume derived from micro- and meso-macropores (V_micro/V_meso-macro). The highest S_BET was found with both samples prepared from coal ashes only (CFAZ1 and CFAZ2); however, a much higher number of micropores was recorded for CFAZ1, while CFAZ2 is characterized by the predominant presence of meso-macropores that accommodate almost 70% of the adsorbed nitrogen. The addition of biomass ash content in the reagent mixture leads to a very significant decrease in the BET specific surface area of the studied materials with the exception of CFA1/BA1/Z1, which maintains a relatively high surface area and a good amount of micropores as well (Table 2).

Moreover, higher S_BET values were established in the samples prepared with the addition of biomass ash from wood pellets. At the same time, relatively high values of the BET specific surface area were retained in the samples of high-iron coal ash CFAZ3 with the addition of wooden ash BA2.

The UV–Vis spectra of the obtained samples are depicted in Figure 10. A wide range of light absorption was observed for all samples. This phenomenon is due to the complex chemical and structural composition of the coal and biomass ashes used as raw materials for sample preparation. Typical charge transfer picks (O²⁻ → Si⁴⁺, Al³⁺) in the region of 200–300 nm were observed for all samples. For biomass BA1 (Figure 10A), the characteristic oxygen-to-metal charge-transfer bands with high intensity are located in the range of 300 to 400 nm and can be attributed to metal oxides (CaO, MgO, MnO, and Fe_xO_y) present in the samples [51,52,53,54,55]. Owing to overlapping of the signals for these species, a more detailed analysis of the spectrum is hindered. For the composite samples CFA/BA1, it can be observed that the ratio of the picks with maxima at 270 and 310 nm changes due to the presence of the highest percentage of SiO₂ and Al₂O₃ in the obtained samples compared to that of the metal oxides. A band at 480 nm appears in all composite materials and can be associated with the presence of small iron oxide particles formed during the synthesis procedure at high temperatures [56]. As expected, this band is most intense for the CFA3/BA2/Z3 sample prepared from coal ash with high iron content (Figure 10C). Based on the literature, isolated Fe³⁺ ions give rise to bands below 300 nm, while signals of small oligonuclear Fe_xO_y clusters in the pores appear between 300 and 450 nm, and Fe₂O₃ nanoparticles on the external surface of the crystal are recorded above 450 nm [57]. The band at 270 nm is assigned to charge transfer between the iron and oxygen atoms of the zeolite network, indicating the presence of octahedrally coordinated Fe³⁺ species, while the peak at around 380 nm can be attributed to the presence of an oligonuclear iron complex.

The obtained TPR graphical dependencies are presented in Figure 11. TPR is an analytical technique that studies the state of metal oxides on the surface of materials under different thermal conditions. In the different samples, three reduction zones of different intensity can be distinguished, associated with the reduction of iron oxides by a presumed two- or three-step mechanism: Fe₂O₃ → Fe₃O₄ → Fe or Fe₂O₃ → Fe₃O₄ → FeO → Fe. An intense reduction peak above 800 K was found for the adsorbent obtained from coal ash with high iron content with additives of ash of wood pellet biomass, most likely associated with the reduction of Fe^3+/2+ ions to Fe.

The samples synthesized through two-step synthesis from the CFA and BA mixtures with biomass ash content of 0 wt.%, 29 wt.%, 44 wt.%, and 55 wt.% were investigated through thermogravimetric analysis in the thermal range of 40–800 °C. Typical experimental thermograms are presented in Figure 12, and the position and assignments of thermal effects are summarized in Table 3.

The thermogravimetric analysis shows that with increasing biomass ash content in the reaction mixture, the moisture retention capacity of the samples decreases and the thermal effect of desorption shifts to lower temperatures. Samples obtained from coal fly ash have the highest moisture retention capacity, but adding biomass ash to the reaction mixture up to 45 wt.% does not significantly worsen the moisture retention capacity while at the same time significantly lowering the desorption temperature of surface water and water from the pores of the material. Apparently, carbonization is much lower at CFAZ1 with 0 wt.% biomass ash.

The breakthrough curves of CO₂ adsorption on the studied adsorbents are presented in Figure 13, and the calculated adsorption capacity values are summarized in Table 2. The CO₂ adsorption capacity reveals the role of textural parameters and the presence of metals (Fe, Ca, Mg, etc.) in different oxidation states. It was found that the adsorption capacity of alkaline-converted coal ash adsorbents with biomass ash additives was lower than that of the samples without added biomass ash, most likely mainly related to the lower specific surface area. In the sample CFA1/BA1/Z1, with the addition of 40 wt.% mixed biomass ash, however, the adsorption capacity for CO₂ increases compared to CFAZ1, associated with an increase in the content of alkaline earth oxides (CaO) in the adsorbent composition, which was previously confirmed to significantly improve the carbon dioxide capture capacity. Furthermore, for this sample, the V_micro/V_meso-macro ratio (0.21) is close to that of CFAZ2 (0.31), indicating the highest adsorption capacity for CO₂ (Table 2). It seems that the adsorption capacity is high when the adsorbent possesses predominantly meso–macropores in combination with a high surface area (S_BET), but the CO₂ adsorption can also be improved by the presence of a relatively large amount of micropores, which leads to a significant increase in the specific surface area of the material. In the case of samples CFA1/BA1/Z2 and CFA1/BA1/Z3, the adsorption capacity decreases due to a significant decrease in S_BET (Table 2) but remains relatively constant with further increases in biomass ash in the reaction mixtures.

For adsorbents derived from CFA1 with moderate CaO content, a decrease in adsorption capacity was observed, even for those with lower biomass ash content. This suggests that while CaO plays a role in CO₂ adsorption, its excessive presence may not always be beneficial, potentially due to changes in pore structure or the formation of less reactive Ca-containing phases. The comparison of samples obtained under identical preparation conditions but from different BA types further highlights the variations in adsorption performance. It should be noted that the highest adsorption capacity of 3.5 mmol/g was detected for CFA1/BA1/Z3, which was synthesized using CK type BA. In contrast, CFA1/BA2/Z3, derived from type C BA, exhibited a lower adsorption capacity despite possessing a higher specific surface area. This discrepancy suggests that surface area alone is not the determining factor for CO₂ uptake, highlighting the importance of the chemical composition and nature of the adsorption sites. As shown in Figure 13, the CO₂ adsorption curves shift towards longer retention times, indicating a more complex adsorption mechanism. This shift is not directly proportional to the BET surface area of the samples, suggesting that CO₂ capture occurs through a combination of physisorption and chemisorption. The involvement of chemisorption is further supported by the interaction of CO₂ with reactive sites on the adsorbent surface, which may include basic sites associated with metal oxides. The predominant presence of Ca in the form of calcite is identified as a key factor limiting the impact of increased Ca content on adsorption properties. While calcium-based compounds can enhance CO₂ capture through chemisorption, excessive presence of calcite may reduce the availability of more reactive adsorption sites, leading to a decrease in adsorption capacity. Among the materials tested, the highest adsorption capacity of 4.2 mmol/g was recorded for CFAZ2. This superior performance can be attributed to its high surface area and the presence of an optimal balance of Ca, Mg, and Fe ions, which act as effective adsorption sites. These metal species contribute to CO₂ capture by facilitating stronger interactions with CO₂ molecules, increasing overall adsorption efficiency.

These findings demonstrate the importance of tailoring the composition and textural properties of adsorbents to optimize their performance in CO₂ capture.

4. Conclusions

Blends of coal fly ash (CFA) and biomass ash (BA) of different types and ratios were used in an ultrasonic-assisted two-step alkaline conversion method to synthesize zeolites. It was revealed that increasing the proportion of biomass ash in the mixture significantly affects the zeolitization process, leading to a decrease in the crystallinity, surface area, and porosity of the formed zeolite. This effect is attributed to the different composition of biomass ash, in particular its higher content of CaO, K₂O, and P₂O₅, which affects the dissolution and reorganization of aluminosilicate species during synthesis. Despite the reduced yield of zeolitization, it was found that coal fly ash mixtures containing up to 29 wt.% biomass ash can still be successfully used to produce zeolite adsorbents. While these materials exhibit a lower degree of crystallinity compared to those synthesized from CFA, they retain sufficient porosity and active sites to facilitate effective CO₂ adsorption. The adsorption mechanism is predominantly chemisorption-based, with CO₂ interacting strongly with the basic sites provided by Ca- and Na-containing phases. The results obtained indicate that the presence of CaO, along with other alkaline components of biomass ash, enhances CO₂ capture by providing additional reactive adsorption sites.

Author Contributions

Conceptualization, S.B.; methodology, S.B. and M.P.; software, M.D. and I.T.; validation, B.M., A.B. and M.P.; formal analysis, S.B., M.P. and D.K.; investigation, S.B., B.M., M.P., M.D., D.K. and I.T.; resources, S.B. and M.P.; data curation, M.D., I.T., D.K., B.M. and A.B.; writing—original draft preparation, S.B. and M.P.; writing—review and editing, S.B., M.P., M.D. and D.K.; visualization, S.B., M.D., I.T. and A.B.; supervision, S.B. and M.P.; project administration S.B. and M.P.; funding acquisition, S.B. and M.P. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

This study was financed by the European Union–NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project No. BG-RRP-2.004-0005 and project BG-RRP-2.011-0021-C01.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. SEM images of raw ash residues of solid fuels in different magnifications: (a) coal ash with moderate CaO content, CFA1; (b) coal ash with low CaO content, CFA2; (c) coal ash with high iron content, CFA3; (d) ash of mixed wooden and agricultural biomass, BA1.

Figure 1. SEM images of raw ash residues of solid fuels in different magnifications: (a) coal ash with moderate CaO content, CFA1; (b) coal ash with low CaO content, CFA2; (c) coal ash with high iron content, CFA3; (d) ash of mixed wooden and agricultural biomass, BA1.

Figure 2. SEM images at different magnifications of alkaline conversion products of mixtures of CFA1 and CFA2 with BA1 at varying ratios: (a) sample CFA1/BA1/Z1, CFA1/BA1 ratio 5/2; (b) sample CFA1/BA1/Z2, CFA1/BA1 ratio 5/4; (c) sample CFA1/BA1/Z2, CFA1/BA1 ratio 5/6; (d) sample CFA2/BA1/Z1, CFA2/BA1 ratio 5/2; (e) sample CFA2/BA1/Z2, CFA2/BA1 ratio 5/4; (f) sample CFA2/BA1/Z2, CFA2/BA1 ratio 5/6.

Figure 2. SEM images at different magnifications of alkaline conversion products of mixtures of CFA1 and CFA2 with BA1 at varying ratios: (a) sample CFA1/BA1/Z1, CFA1/BA1 ratio 5/2; (b) sample CFA1/BA1/Z2, CFA1/BA1 ratio 5/4; (c) sample CFA1/BA1/Z2, CFA1/BA1 ratio 5/6; (d) sample CFA2/BA1/Z1, CFA2/BA1 ratio 5/2; (e) sample CFA2/BA1/Z2, CFA2/BA1 ratio 5/4; (f) sample CFA2/BA1/Z2, CFA2/BA1 ratio 5/6.

Figure 3. SEM images of alkaline conversion products of coal ash with high iron content, FA3, and wooden biomass ash, BA2, at different magnifications (sample CFA3/BA2/Z3): (a) image with lower magnification of tens of μm; (b,c) higher magnification images of μm surface area.

Figure 4. SEM images of coal ash alkaline conversion products of (a) CFA1 (Sample CFAZ1), CFA2 (b) (Sample CFAZ2), and (c) CFA3 (Sample CFAZ3).

Figure 5. X-ray diffraction patterns of coal fly ash (a) and biomass ash (b).

Figure 6. X-ray diffraction patterns of alkaline conversion products of coal fly ash with biomass ash of mixed origin BA1 at different ratios: (a) CFA1 with moderate calcium content and (b) CFA2 with low calcium content (b).

Figure 6. X-ray diffraction patterns of alkaline conversion products of coal fly ash with biomass ash of mixed origin BA1 at different ratios: (a) CFA1 with moderate calcium content and (b) CFA2 with low calcium content (b).

Figure 7. XRD of alkaline conversion products of BA2 with all studied CFAs.

Figure 8. Experimental N2 adsorption/desorption isotherms of samples obtained through the alkaline conversion of mixtures of coal ash and biomass ash: (a) samples obtained from moderate calcium coal ash CFA1 and (b) samples obtained from low calcium coal ash CFA2.

Figure 9. Pore size distribution in the studied samples: (a) samples obtained from moderate calcium coal ash CFA1 and (b) samples obtained from low calcium coal ash CFA2.

Figure 10. UV-VIS spectra of biomass ash and alkaline-converted samples: samples obtained from medium-calcium CFA1 and ash of mixed biomass BA1 (A); samples obtained from low-calcium CFA2 and ash of mixed biomass BA1 (B); samples obtained from low-calcium CFA2 and high-iron CFA3 and ash of wooden biomass BA2 (C).

Figure 11. TPR dependences of adsorbent samples.

Figure 12. Thermograms of alkaline-converted co-combusted coal and biomass ash residues: (a) CFAZ1; (b) CFA1/BA1/Z1; (c) CFA2/BA1/Z1; and (d) CFA3/BA2/Z3.

Figure 12. Thermograms of alkaline-converted co-combusted coal and biomass ash residues: (a) CFAZ1; (b) CFA1/BA1/Z1; (c) CFA2/BA1/Z1; and (d) CFA3/BA2/Z3.

Figure 13. Dynamic adsorption curves of CO2 on adsorbents obtained through the alkaline conversion of mixtures of coal ash and biomass ash.

Table 1

Macrochemical composition of CFA and BA.

Chemical Composition, wt.%
Component	CFA1	CFA2	CFA3	BA1	BA2
SiO2	50.80	50.20	25.98	2.31	3.84
Al2O3	21.33	23.75	15.84	1.01	1.42
CaO	9.36	4.45	8.47	21.01	34.83
MgO	0.82	2.31	2.52	9.27	6.50
Fe2O3	4.68	12.99	39.82	0.92	1.33
MnO	0.09	0.06	-	0.04	0.29
K2O	2.17	0.98	1.05	6.79	9.92
Na2O	0.37	0.78	1.26	<0.05	<0.05
P2O5	0.49	<0.05	-	4.70	3.91
SO3	0.86	1.04	3.20	1.07	3.01
TiO2	0.78	0.84	0.01	0.03	0.08
LOI	7.80	2.35	1.85	52.82	34.72

Table 2

Surface characteristic values and CO₂ dynamic adsorption capacity of alkaline-converted coal ash and biomass ash mixtures.

Sample	SBET, m2/g	Smicro, m2/g	Vtotal, cm3/g	Vmicro, cm3/g	Vmicro/Vmeso-macro	CO2 Adsorption, mmol/g
CFAZ1	486.0	334.0	0.31	0.130	0.72	2.8
CFA1/BA1/Z1	200.0	132.5	0.31	0.053	0.21	3.5
CFA1/BA1/Z2	67.6	4.8	0.22	0.002	0.01	2.3
CFA1/BA1/Z3	31.1	-	0.28	-	-	2.2
CFA1/BA2/Z3	86.9	9.5	0.37	0.004	0.01	1.9
CFAZ2	421	267	0.36	0.110	0.31	4.2
CFA2/BA1/Z1	82.3	16.4	0.22	0.007	0.03	2.4
CFA2/BA1/Z2	56.9	10.7	0.15	0.005	0.03	2.3
CFA2/BA1/Z3	75.6	5.4	0.18	0.002	0.01	2.2
CFA2/BA2/Z3	97.7	5.2	0.31	0.002	0.006	2.3
CFAZ3	257.0	-	-	-	-	3.8
CFA3/BA2/Z3	147.7	5.1	0.31	0.002	0.006	2.2

Table 3

Results from thermogravimetric analysis of alkaline-converted coal fly ash and biomass ash mixtures.

Sample	Weight Loss Due to Dehydration, wt.%	Thermal Effect Peak Position, °C	Weight Loss Due to Carbonate Decomposition,wt.%	Thermal Effect Peak Position,°C
CFAZ1	15	150	1.0	720
CFA1/BA1/Z1	12	110	3.0	750
CFA1/BA1/Z2	12	100	4.5	770
CFA1/BA1/Z3	11	60	7.0	790
CFA1/BA2/Z3	16	120	5.0	750
CFAZ2	16	70/110	4.0	760
CFA2/BA1/Z1	11	90	5.0	780
CFA2/BA1/Z2	10	60	7.0	790
CFA2/BA1/Z3	9	80	9.0	810
CFA2/BA2/Z3	10	140	4.5	750
CFA3/BA2/Z3	9	90	8.5	780

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