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Article

Distribution of Rare-Earth Elements in Ashes Produced in the Coal Combustion Process from Power Boilers

by
Zdzisław Adamczyk
*,
Joanna Komorek
,
Magdalena Kokowska-Pawłowska
and
Jacek Nowak
Faculty of Mining, Safety Engineering and Industrial Automation, Silesian University of Technology, Akademicka 2A, 44-100 Gliwice, Poland
*
Author to whom correspondence should be addressed.
Energies 2023, 16(6), 2696; https://doi.org/10.3390/en16062696
Submission received: 15 February 2023 / Revised: 9 March 2023 / Accepted: 12 March 2023 / Published: 14 March 2023
(This article belongs to the Special Issue Selected Papers from the 5th Polish Mining Congress)
Browse Figures
Versions Notes

Abstract

:
The growing demand for rare-earth elements and yttrium (REY) in modern technologies has resulted in the systematic depletion of primary ores. For this reason, research is being conducted around the world on alternative sources of rare-earth elements, e.g., on the possibilities of recovering REY from coal waste or coal combustion. The article presents the results of comprehensive tests of the fuel—hard coal, and high-temperature HTA ash, fly ash, and bottom ash. Examined samples were taken from a Polish power plant. In the tests, fuel quality parameters were determined in accordance with the standards; microscopic observations in reflected and transmitted light, as well as a scanning electron microscope (SEM/EDS), were used, and chemical and phase composition were determined using ICP-MS and XRD methods, respectively. The distribution of REY between these ashes was analyzed. Their suitability as alternative sources of REY was assessed. The obtained results showed that the process of hard coal combustion in pulverized coal boilers influenced the geochemical differentiation of REY elements in energy waste. This differentiation is manifested by higher concentrations of REY and critical elements in the fly ash than in the bottom ash. The obtained values of the Coutl prospective coefficient made it possible to classify the analyzed fly and bottom ashes as prospective REY raw materials.

1. Introduction

The growing demand for REY in modern technologies has resulted in the systematic depletion of primary REY ores. Therefore, due to their economic importance and supply risk, the European Commission included these metals on the list of critical raw materials [1,2], and the USA on the list of critical minerals [3].
The largest amounts of REY are used in the production of electrical and electronic equipment; however, despite their recycling, the recovery of REY is only 1% [4,5,6]. For this reason, research is being conducted around the world on alternative sources of rare-earth elements.
The application and reliance on rare-earth metals are ingrained in our civilization. They are abundantly utilized in metallurgy, glass, and polishing powder production, as well as state-of-the-art technologies such as electronics, chemical, automotive, defense, green energy production, aerospace, etc. [7,8,9,10,11,12,13,14].
The availability of rare-earth elements (REEs) is obvious from its title and limited to the Earth’s crust. Although many minerals contain these metals, practically only a few have mining significance, including bastnaesite, monazite, xenotime, and apatite, and less often, allanite and others. Concentrations of REE in the Earth’s crust not only have limited access but are also exhumed in trace amounts, with the average content of individual elements ranging from 0.30 to 66.5 ppm [15,16,17,18,19,20].
However, larger concentrations of REE are formed by geological processes, found in, e.g., igneous deposits (carbonatites, alkaline complexes, alkaline pegmatites, rhyolites, granites, and granitic pegmatites), hydrothermal deposits (skarn), and secondary/sedimentary deposits (heavy mineral sands, laterite, tailings, shale-hosted, and alluvial) [21,22]. The discovery of these deposits is extremely rare, although not all deposits are known. Currently, REEs are mainly mined in China, USA, Burma, and Australia. The global production of rare-earth elements steadily increased from 110,000 MT in 2012 to 280,000 MT in 2021 [23].
Additionally, rare-earth elements include lanthanides, scandium, and yttrium.
In terms of geochemistry, after taking into account yttrium in the lanthanides group (REY), the low-atomic-weight rare-earth elements (138.91–150.36) belong to the light group (LREY: lanthanum, cerium, praseodymium, neodymium, promethium, and samarium), those of high atomic weight (164.93–174. 97) belong to the heavy group (HREY: holmium, erbium, thulium, yttrium, and lutetium), and those of intermediate atomic weight (151.96–162.50 and yttrium 88.91) belong to the medium group (MREY: europium, gadolinium, terbium, dysprosium, and yttrium) [16,24,25,26,27]. In the case of industry, the Seredin and Dai [26] classification considered the forecast of supply and demand, distinguishing three groups of elements among REY: critical (Nd, Eu, Tb, Dy, Y, and Er), uncritical (La, Pr, Sm, and Gd), and excessive (Ce, Ho, Tm, Yb, and Lu).
Ocean-bottom silts are considered promising for these purposes [28], as well as some industrial waste, e.g., bauxite residue (red mud) generated during the production of alumina from bauxite (the Bayer process), phosphogypsum, sewage sludge mine water, including acidic drainage, coal waste, and byproducts of coal combustion, as well as from the recycling of technological equipment and electrical and electronic equipment waste [29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45].
Extensive research is being carried out globally on the possibilities of recovering REE from coal waste or coal combustion. Ashes of certain coal beds in the world’s basins contain up to 1% of REY [10,26]. To classify and indicate the geochemical determinants of REE concentration, the chemical and phase composition must be considered [14,36,37,38,46,47,48,49,50,51,52,53,54,55,56,57,58].
Hower et al. [55] found that the distribution of rare-earth elements (REEs) in coal-derived fly ashes can have distinctive patterns when fly ashes are produced from different coals within or between basins. However, during the combustion process, lanthanides present in the coal were only separated into fly ash and bottom ash in the coal power plant [59,60].
However, previous studies, as shown above, mainly concerned REY in fly ashes and did not take into account REY concentration in bottom ashes and fuel quality.
The article presents the results of a comprehensive study of the fuel, which was hard coal, and the byproducts of its combustion in power boilers, such as bottom ash and fly ash. The distribution of REY between these ashes was analyzed. Their suitability as alternative sources of rare-earth elements was assessed. There is currently a lack of knowledge about the behavior, transformations, and division of REY during coal combustion. This information may contribute to the development of methods for the recovery of these elements from fly ashes. This article serves to extend this knowledge.

2. Materials and Methods

A sample of the fuel—hard coal, a sample of high-temperature ash (HTA) obtained from combustion of this fuel in laboratory conditions at a temperature of 815 °C, and samples of bottom and fly ash from the combustion of this fuel in pulverized coal boilers in the L power plant were tested (Table 1). The samples were gathered from the L power plant, which was fired with hard coal without co-combustion of biomass that significantly reduced the variability of the chemical composition of ashes and the amount and type of trace elements present.
The coal sample was reduced, averaged, and ground to a fraction of φ < 1 mm; briquettes were made for microscopic examination, and then ground to a fraction of φ < 0.2 mm. The ash content Aa (PN-G-04560:1998 [61]), moisture content Wa (PN-G-04560:1998 [61]), calorific value Qsdaf (PN-ISO 1928:2020-05 [62]), and volatile matter content Vdaf (PN-G-04516:1998 [63]) were determined. During the microscopic examination, analysis of the maceral groups was performed. The percentage content of maceral groups and mineral matter were determined. Microscopic observations were conducted on grain cuts (briquettes) in accordance with ICCP recommendations while taking into account PN-ISO-7404 [64]. The measurements of the mean random vitrinite reflectance Rr according to PN-ISO 7404-5:2002 [65] were carried out. Microscopic examinations were carried out using a Zeiss reflected light microscope (Axioscop) equipped with a microphotometer. An immersion oil was used with a refractive index of no = 1.5176 at 23 °C and light wavelength λ = 546 nm.
HTA for testing was obtained during the coal combustion process in a muffle furnace at a temperature of 815 °C. The high-temperature ash (HTA) and ashes from the power plant were the research material for the determination of rare-earth elements and yttrium—REY.
The results were generated using ICP-MS method (inductively coupled plasma mass spectrometry) using the Perkin Elmer SCIEX ELAN 6000 ICP-MS spectrometer at Activation Laboratories Ltd., Ancaster, ON, Canada.
In order to assess the tested ash samples as an alternative source of REY, the prospective coefficient (Coutl) was calculated, which considered the shares of critical and excess elements according to the following formula [26]:
C outl = Nd + Eu + Tb + Dy + Er + Y Ce + Ho + Tm + Yb + Lu
The phase composition of the samples was determined by X-ray diffraction, transmitted light, and scanning electron microscopy. To determine the mineral composition of the ash samples, X-ray phase analysis was performed using a diffractometer created by Empyrean PANalytical, and CuKα radiation was employed. Diffractograms were recorded in the 2θ angle range from 5° to 70° with 0.02° step, during 2 s. Measurements were recorded without and with the use of an internal standard (zinc oxide, ZnO). The content of the identified mineral phases was determined based on quantitative diffraction analysis, which was carried out using the Rietveld method.
Microscopic observations in the transmitted light were carried out on a microscope Zeiss (Axioplan, Jena, Germany). Grain morphology observations were obtained using a scanning electron microscope and we determined the chemical composition in the micro-area (SEM SU3500 microscope by Hitachi, cooperating with an X-ray spectrometer with energy dispersion EDS UltraDry from Thermo Scientific NORAN System 7).

3. Test Results

3.1. Fuel Characterization

In the L power plant, hard coal (LWM sample) with Ad ash content of 17.31% was burned. According to the International Standard (ISO 11760:2005(E) [66]), it was determined as medium ash coal. The volatile matter content was Vdaf = 37.56%, and the calorific value was Qsdaf = 30 MJ/kg. The coal sample had a mean random vitrinite reflectance of Rr = 0.68% and a standard deviation of s = 0.07% (Table 2).
The petrographic composition of the LWM sample was dominated by macerals of the vitrinite group V = 66%, which consisted of collotelinite, colodetrinite, and telinite. The macerals content of the liptinite group was L = 6% and consisted of sporinite in the form of micro- and megaspores and resinite. The share of macerals of the inertinite group was I = 17%, and consisted of semifusinite and inertodetrinite, and, less frequently, fusinite and funginite. In the mineral matter free state, the shares of individual groups of macerals were as follows: vitrinite—Vmmf = 74%, liptinite—Lmmf = 6%, and inertinite—Immf = 20%. The mineral matter content in LWM sample was SM = 11%, and mainly consisted of carbonates, iron sulfides, and polymineral substances.
According to the International Standard (ISO 11760:2005(E) [66]), it is Medium Rank C (bituminous C), moderately high vitrinite coal.

3.2. Characteristics of Ashes

3.2.1. Phase Composition

HTA ash (LWMAS sample) was dominated by the glass phase (57.5%). Quartz (Q), hematite, and anhydrite (Ah) were also present in the sample (Figure 1). The total content of anhydrite and hematite was 3.2% and the share of quartz was 16.0%.
Feldspar (microcline) and mica (muscovite) phases were significantly present in the studied ash. The determined share was 5% and 14.9%, respectively. In all ashes obtained from the L power plant (LMOS and LZ samples), the glass phase was also dominant, and had a share above 76.0%. The ashes also contained mullite, sillimanite, and quartz (Figure 1). The mullite content was 14% and 19.5% in LMOS and LZ samples, and the quartz content was 3.1% to 4.5%, respectively. Additionally, fly ash contained 0.1% of magnetite and traces of hematite.
Microscopic observations revealed that the glass often contained sillimanite needles (Figure 2a,b). The glass grains in the LZ sample had numerous pores (Figure 3a), and the main chemical components were Si and Al with admixtures of Fe, Mg, Ca, K, and Ti (Figure 3b).
According to the Vassilev [47] phase-mineral classification system, HTA ash from the L power plant’s coal was classified as active-low pozzolanic—(A-LP). LZ bottom ash was classified as inert-medium pozzolanic—(I-MP), and LMOS fly ash was high pozzolanic (HP) type (Figure 4).

3.2.2. Main Chemical Components

In the HTA ash sample (LWMAS), SiO2 and Al2O3 were present in the largest amount; their share in total amounted to over 63%. The content of SiO2 in the analyzed sample was 42.30%, and Al2O3 was 20.79%. The other chemical components were Fe2O3, CaO, MgO, and K2O, and totaled a few percent. The remaining components, i.e., MnO, Na2O, TiO2, and P2O5, were present in amounts below 1%. LWMAS sample had a significant amount of SO3, over 21%, and had a loss on ignition (LOI) of 1.64% (Table 3).
The dominant chemical components of the LZ bottom ash were SiO2 and Al2O3, with a share of 45.61% and 21.90%, respectively. The other chemical components were Fe2O3, MgO, CaO, Na2O, and K2O, with only traces of MnO, TiO2, and P2O5 detected. The LZ sample contained much less SO3 compared to the LWMAS sample, which was 0.38%. However, the bottom ash sample showed a much higher amount of LOI, compared to the LWMAS sample, which was close to 18% (Table 3).
In the case of LMOS fly ash, similar to LWMAS and LZ, SiO2 and Al2O3 were the primary components, with a share of 39.52% and 24.48%, respectively. The other chemical components consisted of Fe2O3, MgO, CaO, Na2O, and K2O, as well as TiO2 and P2O5. Trace components, i.e., MnO and SO3, did not exceed 0.08%. Hence, the analyzed LMOS fly ash sample was characterized by a similar LOI to the LZ sample of 17.39% (Table 3).
The results derived from the chemical composition tests showed that the bottom ashes (LZ) and fly ashes (LMOS) had a similar chemical composition, whereas the HTA ash (LWMAS) was significantly different (high content of SO3 and very low share of LOI). Therefore, the tested fly ash was characterized by the share of the sum of the oxides SiO2 + Al2O3 + Fe2O3 ≥ 70% and contained CaO < 20%. These results were in accordance with the ASTM C618 (2015) standard and were classified as class F (Figure 5) [46,67].
Vassilev [48] described the chemical classification of the inorganic matter in coal ash based on the normalized content of the main oxides. It was revealed that the HTA ash obtained from coal burnt in the L power plant was classified as calsialic-low acid (CS-LA). Additionally, the LZ bottom ash belonged to the sialic-medium acid (S-MA) type, and the LMOS fly ash to the sialic-medium acid (S-MA) type. The projection points on the classification diagram of coal ashes indicate (Figure 6) that the bottom and fly ashes from the L power plant showed similarity in chemical composition, as was observed also in the case of the content of phase components.
The determined values of SiO2/Al2O3 quotient for HTA ash (LWMAS) and LZ bottom ash were similar and amounted to 2.03 and 2.08, respectively. The obtained value for LMOS fly ash was lower and amounted to 1.61. The K2O/Na2O ratio in the tested ash samples varied from 1.75 (LMOS sample) to 3.43 (LWMAS ash sample). The quotient (MgO + CaO)/(K2O + Na2O) for HTA (LWMAS) and LMOS ash samples had almost identical values of 1.44 and 1.43, respectively. In the case of bottom ash LZ, the value was lower and amounted to 1.15. CaO/MgO quotient values were different and ranged from 1.14 (LZ sample) to 2.14 (LMOS sample). The detrital/authigenic index (DAI) was determined for all analyzed samples of ashes and ranged from 2.13 (LWMAS sample) to 7.03 (LZ sample), which indicated that they were enriched in components associated with detrital minerals (Table 3) [47].

3.2.3. Rare-Earth Elements and Yttrium—REY

REY content in the analyzed ashes ranged from 307 ppm (LZ sample) to 617 ppm (LMOS sample) (Table 4, Figure 7a). The REY oxides (REO) content varied from 363 ppm to 730 ppm. In the case of HTA ash (LWMAS) and LMOS fly ash, their values were similar to or higher than the average for coal ashes from global deposits, where REY was 404 ppm and REO was 483 ppm [12,26]. The share of REY and REO oxides in the bottom ash (LZ) was lower than the average for coal ashes from global deposits.
The light elements (LREY) were characterized by the highest content among the rare-earth elements. The determined share in the analyzed samples ranged from 311 ppm (HTA sample) to 461 ppm (LMOS sample), which was from 74.7% to 77.4% of the total REY content. The lowest share, from 3.5% to 38% of the total REY, was found in the heavy elements (HREY). It should be noted that the proportions in which the particular groups of rare-earth elements occurred in the analyzed waste, e.g., LREY, MREY, and HREY, were similar (Table 4, Figure 7b).
The critical elements contained in the analyzed samples ranged from 103 ppm to 223 ppm, which constituted 33.6% to 36.1% of the total REY (Table 4, Figure 7c,d). The comparison of the critical element content in the samples revealed that REY content in the waste material changed during the hard coal combustion process. In the bottom ash (sample LZ), a lower REY content was found in relation to their share in the ash HTA (sample LWMAS). However, in the fly ash (LMOS sample), a significant increase in REY content was found in relation to the content in HTA ash.
The content of uncritical elements in the tested ashes ranged from 86 ppm to 170 ppm. The percentage share of uncritical elements in the analyzed samples ranged from 27.5% to 28.1% (Table 4, Figure 7c,d). In this case, the content of uncritical elements in the fly ash (LMOS) was significantly higher than in the bottom ash (LZ). This confirmed our earlier observation that the incineration process may cause variations in REY content in energy waste.
The content of excessive elements in the tested samples ranged from 118 ppm to 225 ppm, which is from 36.4% to 38.3% of the total REY (Table 4, Figure 7c,d). As in the case of critical and uncritical elements, excessive elements were present in higher amounts of fly ash (LMOS) compared to bottom ash (LZ).
The obtained results suggest that the hard coal combustion process in the pulverized coal boilers influenced the geochemical differentiation of REY elements in energy waste, which were present in higher concentrations in the fly ash than the bottom ash. This phenomenon was previously reported on both types of energy waste from hard coal-fired fluidized bed boilers [36].
It should be noted that in the total mass of ashes from the combustion of hard coal in the L power plant, the bottom ashes (sample LZ) accounted for 10%, and the remaining 90% were fly ashes (sample LMOS). Hence, due to fly ash’s largest share in the total mass of ashes produced, it was more attractive as a prospective source of REY than bottom ashes.
The value of the perspective coefficient Coutl for the HTA ash sample obtained under laboratory conditions was 0.81 (Table 4). Different values of the prospective coefficient for the samples of ashes taken in the L power plant were obtained. The Coutl value of the prospective coefficient for LZ bottom ash sample was 0.88, and was 0.99 for LMOS fly ash sample.
A graph was prepared to depict the relationship of the percentage share of critical elements to Coutl in order to determine the potential industrial value of the ashes [10,26]. The value of Coutl > 0.7 and share of critical elements from 30% to 51% showed that all ash samples from the L power plant were classified as prospective REY raw materials (Table 4, Figure 8).
In order to determine the degree of enrichment of the samples in rare-earth elements in relation to their content in the upper continental crust (UCC), the determined shares of REY were normalized to their shares in the UCC. Due to the distribution of REY content compared to the UCC, samples were divided into the following groups: enriched in LREY—L-type, in MREY—M-type, and in HREY—H-type.
Samples with L-type and H-type REY content distribution were distinguished by the quotient LaN/LuN > 1 and LaN/LuN < 1, respectively. Samples with an M-type distribution of REY content were distinguished by the quotients LaN/SmN < 1 and GdN/LuN > 1. Each type of normalization plot had a positive or negative anomaly depending on the different amplitudes for the various elements related to their behavior in the environment compared to other REY elements. Due to the occurrence of anomalies, subtypes and intermediate types were also distinguishable [26]. Normalization curves of the studied ashes from hard coal combustion in the L power plant were M–H-type curves (Figure 9). The content of individual rare-earth elements in the ashes was higher than in UCC. The largest shares of REY in relation to UCC, by approximately 3.25 to 5.5 times, were found in the LMOS fly ash sample.

4. Conclusions

The results presented in this paper indicate that after the coal combustion in a pulverized coal boiler, bottom ashes (active—low pozzolanic type) and fly ashes (inert—medium pozzolanic type) were produced. Mineral composition was dominated by the following phases: mullite, sillimanite, quartz, magnetite, hematite, and glass.
The chemical composition revealed that the bottom ash and fly ash belonged to the sialic-medium acid type, and the detrital/authigenic index indicated that the enrichment of the ashes in components was related to detrital minerals.
The obtained results show that the process of hard coal combustion in pulverized coal boilers influences the geochemical differentiation of REY elements in energy waste. This differentiation is manifested by higher concentrations of REY and critical elements in the fly ash than in the bottom ash.
Regardless of the type of ash, light elements (LREY) have the largest share of rare-earth elements, and heavy elements (HREY) have the smallest share.
The obtained values of the Coutl prospective coefficient made it possible to classify the analyzed fly and bottom ashes as prospective REY raw materials. Since fly ashes had the largest share in the total mass of ashes produced in the L power plant, they were considered the prospective source of REY. The tested fly ashes showed higher contents than the average for ashes from coal deposits, which only confirms their potential for recovery.

Author Contributions

Investigation, Z.A., J.K., M.K.-P. and J.N.; methodology, Z.A., J.K., M.K.-P. and J.N.; data curation, Z.A., J.K., M.K.-P. and J.N.; formal analysis, Z.A., J.K., M.K.-P. and J.N.; writing—original draft preparation, Z.A., J.K., M.K.-P. and J.N.; writing—review and editing, Z.A., J.K., M.K.-P. and J.N. All authors have read and agreed to the published version of the manuscript.

Funding

Grant 06/060/BK 18/0057 Ministry of Education and Science: Distribution of rare-earth elements in ashes produced in the coal combustion process from power boilers.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction pattern of samples (the arrows indicate the basic phase reflexes).
Figure 1. X-ray diffraction pattern of samples (the arrows indicate the basic phase reflexes).
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Figure 2. Microscope image inclusions of sillimanite rods in isotropic glass (transmitted light): (a) sample LZ—one nicol, magnification 200×; (b) sample LMOS—one nicol, magnification 400×.
Figure 2. Microscope image inclusions of sillimanite rods in isotropic glass (transmitted light): (a) sample LZ—one nicol, magnification 200×; (b) sample LMOS—one nicol, magnification 400×.
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Figure 3. SEM image of glass grain: (a) sample LZ—numerous pores in the glass grain; (b) sample LMOS—EDS spectrum of glass grain.
Figure 3. SEM image of glass grain: (a) sample LZ—numerous pores in the glass grain; (b) sample LMOS—EDS spectrum of glass grain.
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Figure 4. Positions of the tested samples in the Vassilev’s phase-mineral classification system [47].
Figure 4. Positions of the tested samples in the Vassilev’s phase-mineral classification system [47].
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Figure 5. Positions of the tested sample LMOS in the ASTM ash fly classification.
Figure 5. Positions of the tested sample LMOS in the ASTM ash fly classification.
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Figure 6. Positions of the tested samples of ash compared to Vassilev’s [48] chemical classification ash.
Figure 6. Positions of the tested samples of ash compared to Vassilev’s [48] chemical classification ash.
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Figure 7. REY distribution in the tested ashes LWMAS, LZ, and LMOS: (a) content of HREY, MREY, and LREY; (b) percentage share of HREY, MREY, and LREY; (c) content of critical, uncritical, and excessive elements; (d) percentage share of uncritical and excessive elements.
Figure 7. REY distribution in the tested ashes LWMAS, LZ, and LMOS: (a) content of HREY, MREY, and LREY; (b) percentage share of HREY, MREY, and LREY; (c) content of critical, uncritical, and excessive elements; (d) percentage share of uncritical and excessive elements.
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Figure 8. The relationship between the percentage share of critical elements in studied fly ashes and the Coutl prospective coefficient compared to the classification of coal ashes enriched in REE [26]. Source of REY: I—not prospective; II—prospective; III—highly prospective.
Figure 8. The relationship between the percentage share of critical elements in studied fly ashes and the Coutl prospective coefficient compared to the classification of coal ashes enriched in REE [26]. Source of REY: I—not prospective; II—prospective; III—highly prospective.
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Figure 9. Distribution of REY content in the tested fly ash. The share of REY was normalized to their content in the upper continental crust (UCC) [15].
Figure 9. Distribution of REY content in the tested fly ash. The share of REY was normalized to their content in the upper continental crust (UCC) [15].
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Table
Table 1. List of examined samples.
Table 1. List of examined samples.
Power PlantSample Type (Waste Type)Sample Symbol
LHard coalLWM
HTA ash from hard coal LWMLWMAS
Bottom ashLZ
Fly ashLMOS
Table
Table 2. Technological parameters and petrographic composition of a coal sample.
Table 2. Technological parameters and petrographic composition of a coal sample.
SampleWaAdVdafQsdafVLISMVmmfLmmfImmfRrs
(%)(MJ/kg)(%)(%)(%)
LWM2.7617.3137.5630.06661711746200.680.07
Explanations: Wa—moisture, Ad—ash, Vdaf—volatile matter, Qsdaf—calorific value, V—vitrinite, L—liptinite, I—inertinite, SM—mineral matter, mmf—mineral matter free state, Rr—mean random vitrinite reflectance, s—standard deviation of Rr value.
Table
Table 3. The main chemical components’ contents in the tested ash samples (in wt%).
Table 3. The main chemical components’ contents in the tested ash samples (in wt%).
SampleSiO2Al2O3Fe2O3MnOMgOCaONa2OK2OTiO2P2O5SO3LOISiO2/Al2O3(MgO + CaO)/(K2O + Na2O)CaO/MgOK2O/Na2ODAI
LWMAS42.3020.795.570.051.562.540.642.210.930.7121.031.642.031.441.633.432.13
LZ45.6121.905.610.061.782.031.002.320.930.380.3517.992.081.151.142.317.03
LMOS39.5224.486.820.061.633.491.302.271.081.860.0817.391.611.432.141.754.93
Table
Table 4. The REY content in examined ash samples.
Table 4. The REY content in examined ash samples.
SampleYLaCePrNdSmEuGdTbDyHoErTmYbLu
(ppm)
LWMAS527214416.56413.72.810.71.79.61.95.50.85.20.8
LZ405611112.1489.72.18.31.37.31.44.30.640.6
LMOS9010621024.19921.24.818.42.916.73.291.48.71.4
SampleREYLREYMREYHREYLREYMREYHREYCriticalUncriticalExcessiveCriticalUncriticalExcessiveCoutlREO
(ppm)(%)(ppm)(%)(ppm)
LWMAS4023117714.277.419.13.513611315333.928.138.00.9475
LZ3072375911.077.219.23.61038611833.628.138.30.9363
LMOS61746113323.674.721.53.822317022536.127.536.41730
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Adamczyk, Z.; Komorek, J.; Kokowska-Pawłowska, M.; Nowak, J. Distribution of Rare-Earth Elements in Ashes Produced in the Coal Combustion Process from Power Boilers. Energies 2023, 16, 2696. https://doi.org/10.3390/en16062696

AMA Style

Adamczyk Z, Komorek J, Kokowska-Pawłowska M, Nowak J. Distribution of Rare-Earth Elements in Ashes Produced in the Coal Combustion Process from Power Boilers. Energies. 2023; 16(6):2696. https://doi.org/10.3390/en16062696

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Adamczyk, Zdzisław, Joanna Komorek, Magdalena Kokowska-Pawłowska, and Jacek Nowak. 2023. "Distribution of Rare-Earth Elements in Ashes Produced in the Coal Combustion Process from Power Boilers" Energies 16, no. 6: 2696. https://doi.org/10.3390/en16062696

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