Trace Elements in Pernik Sub-Bituminous Coals and Their Combustion Products Derived from the Republika Thermal Power Station, Bulgaria

: The contents of 49 trace elements in sub-bituminous Pernik coals and their waste products from preparation and combustion processes were investigated. The studied coals have trace element contents higher than the respective Clarke values for brown coals and some of them may pose environmental concerns. The elements Li, Rb, Cs, Ba, Sc, Y, La, Ce, Nd, Sm, Eu, Er, Ga, Zr, Sn, V, Nb, Ta, W, F, Cu, Zn, In, Pb, Cr, Co, Ni, and Th in the feed coals have concentrations that exceed twice the Clarke values. Most element contents in bottom ash are enriched compared with those in feed coal. Some of the volatile elements are equal or significantly depleted including Sn, Mo, Sb, F, Bi, Cd, Ge, and Pb. Fly ash has higher contents of Ga, Zr, Hf, Sn, V, Nb, Mo, and F in comparison with bottom ash. Most elements have a significant positive correlation with ash yield, indicating their inorganic association. The mixed wastes (coal slurry, bottom ash, and fly ash) in the disposal pond are slightly depleted of most of the elements studied with the exclusion of Cl, Ba, and Br. The Pernik coals and their waste products are unpromising for the extraction of REY due to their low element contents.

Therefore, the scope of this investigation is to present additional data on the concentration, modes of occurrence, and behavior of 49 trace elements (Li, Rb, Cs, Sr, Ba, Tl, Be, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ga, Ge, Zr, Hf, Sn, V, Nb, Ta, Mo, W, F, Cl, Br, Cu, Zn, Cd, In, Pb, Bi, Sb, Cr, Mn, Co, Ni, Th, and U) in the Pernik coals and their beneficiation and waste products generated by the Pernik coal preparation plant (CPP) and Republika TPS.For the first time, the contents of the critical elements REY, Hf, Sc, Ta, and others, as well as the hazardous F are studied in the following samples: run-ofmine sub-bituminous coal, feed coal, high-grade coal, vitrain, coal slurry, associated host rocks, fly ash, bottom ash, and mixed material from the coal beneficiation and combustion processes, taken from the disposal pond.The potential environmental impacts related to hazardous elements in the aforementioned coals and products are further evaluated due to health concerns.Data on the concentrations of fluorine in Bulgarian coal are scarce.Eskenazy and Dai [20] published such data for the first time, but they were mostly for coal of noneconomic importance.It should be noted that in 2023 the European Commission updated the list of critical elements by including As, He, Mn, Cu, and Ni, and removing In [21].
The first industrial coal mining in Bulgaria started in the Pernik coal basin in 1891 [22] and, as a result, the first Bulgarian TPS was built in the Pernik town during 1895.Presently, the mining of these Late Oligocene-Miocene sub-bituminous coals and their combustion in the Republika TPS are still active.The Pernik coal preparation plant (CPP) and 105 MV Republika TPS with three units are located in Pernik town (Figure 1) surrounded by three open-pit mines and numerous waste storage facilities related to coal utilization.and combustion processes, taken from the disposal pond.The potential environmenta impacts related to hazardous elements in the aforementioned coals and products are fur ther evaluated due to health concerns.
Data on the concentrations of fluorine in Bulgarian coal are scarce.Eskenazy and Da [20] published such data for the first time, but they were mostly for coal of noneconomi importance.It should be noted that in 2023 the European Commission updated the list o critical elements by including As, He, Mn, Cu, and Ni, and removing In [21].
The first industrial coal mining in Bulgaria started in the Pernik coal basin in 189 [22] and, as a result, the first Bulgarian TPS was built in the Pernik town during 1895 Presently, the mining of these Late Oligocene-Miocene sub-bituminous coals and thei combustion in the Republika TPS are still active.The Pernik coal preparation plant (CPP and 105 MV Republika TPS with three units are located in Pernik town (Figure 1) sur rounded by three open-pit mines and numerous waste storage facilities related to coa utilization.Data about the mineralogical and chemical characterization of the Pernik coals and the solid wastes from their beneficiation and combustion have been the subject of severa investigations [23][24][25][26][27][28].The common characteristics of this fuel are 47%-60% ash content as-received basis (Aar), 6%-14% moisture, as-received basis (War), 21% volatile matter, as received basis (VMar), 74% carbon, dry ash-free basis (Cdaf), 16% oxygen, dry ash-free basi (Odaf), 5% hydrogen, as-received basis (Hdaf), 3% nitrogen, dry ash-free basis (Ndaf), 1% sulfur, dry ash-free basis (Sdaf), and 2100-2200 kcal kg −1 calorific value, dry ash-free basi (Qdaf) [27].The present work notes (1) the dominant role of inorganic matter as a sourc and concentrator of trace elements in Pernik coals; (2) the preservation of the intimat relationships of some authigenic minerals (pyrite, siderite, and clay minerals) with th organic matter [28] even after combustion process due to the formation of an oxide o aluminosilicate film on the surface of unburned organic particles; (3) the affinity of V, Mo Ni, Pb, W, and HREEs to the organic matter; and (4) the migration of Ge, Mo, Br, F, and B with flue gases.For example, it was identified that the characteristic features of the sub bituminous Pernik coal include very high ash yield; abundance of detrital quartz and feld spars and authigenic kaolinite, siderite, and pyrite; enrichment of Co, Cr, Cu, Ni, Pb, Rb and Zn; affinity to organics and authigenic minerals of Br, Cl, Co, Ni, Pb, Sr, and Zn; de trital affinity of Li and Rb; and significant occurrence of water-soluble Cl and S [27].Th results also indicate some possible environmental pollution of the air (Br, Ce, Co, Cr, Eu Data about the mineralogical and chemical characterization of the Pernik coals and the solid wastes from their beneficiation and combustion have been the subject of several investigations [23][24][25][26][27][28].The common characteristics of this fuel are 47%-60% ash content, as-received basis (A ar ), 6%-14% moisture, as-received basis (W ar ), 21% volatile matter, as-received basis (VM ar ), 74% carbon, dry ash-free basis (C daf ), 16% oxygen, dry ash-free basis (O daf ), 5% hydrogen, as-received basis (H daf ), 3% nitrogen, dry ash-free basis (N daf ), 1% sulfur, dry ash-free basis (S daf ), and 2100-2200 kcal kg −1 calorific value, dry ash-free basis (Q daf ) [27].The present work notes (1) the dominant role of inorganic matter as a source and concentrator of trace elements in Pernik coals; (2) the preservation of the intimate relationships of some authigenic minerals (pyrite, siderite, and clay minerals) with the organic matter [28] even after combustion process due to the formation of an oxide or aluminosilicate film on the surface of unburned organic particles; (3) the affinity of V, Mo, Ni, Pb, W, and HREEs to the organic matter; and (4) the migration of Ge, Mo, Br, F, and Bi with flue gases.For example, it was identified that the characteristic features of the sub-bituminous Pernik coal include very high ash yield; abundance of detrital quartz and feldspars and authigenic kaolinite, siderite, and pyrite; enrichment of Co, Cr, Cu, Ni, Pb, Rb, and Zn; affinity to organics and authigenic minerals of Br, Cl, Co, Ni, Pb, Sr, and Zn; detrital affinity of Li and Rb; and significant occurrence of water-soluble Cl and S [27].The results also indicate some possible environmental pollution of the air (Br, Ce, Co, Cr, Eu, Hf, La, Mo, S, Sc, and Zn), water (Br, Cl, F, V, and Zn), soil (Ba, Co, Cu, Hf, La, Li, Mn, Pb, and Sc), and vegetation (Ba, Bi, Cd, Pb, Sc, Sr, V, and Yb) in the areas surrounding the Republika TPS [27].

Coal Treatment and Burning
The Pernik coal preparation plant was built in 1953 for gravitational washing of the raw Pernik coals.In brief, the coals are manually separated, sieved, and briquetted.Feed coal (FC) for use in TPSs is obtained after coal sieving (≤30 mm), but this fuel still has high ash yield (50%-60%).The high-grade coal (HGC) for home heating is generated by gravitational washing of the particles sized ≥30 mm.Finely ground magnetite and water from the Struma river are used as density media.The waste products from coal beneficiation are coal slurry (CS) and associated host rock (AR).These wastes are debris materials and commonly represent the non-coal-bearing strata.They consist primarily of materials from the partings of the coal beds, as well as materials from the mine roof and floor strata in the Pernik basin, which are incorporated into the coal during mining.A portion of epigenetic coarse-grained mineralization (mostly Fe-sulfides) in coals also occurs in these waste products after coal beneficiation.The coal-fired Republika TPS was built in 1951.Since then it has been supplied with sub-bituminous Pernik coals.The fly ash cleaning equipment of this pulverized coal-fired TPS with bottom ash discharged boilers includes mechanical collectors and three electrostatic precipitator rows [28].

Sampling and Methods
Eight types of solid composite samples of the Pernik sub-bituminous coals and their products generated by Pernik CPP and Republika TPS were collected and analyzed.Each composite sample included between 15-20 individual samples and weighed around 7-10 kg.They were taken from (1) a network of points at accessible locations in DP and from CPP warehouses; (2) the moving conveyor belts of the CPP at a certain time interval; and (3) the TPS director's average control samples used in the TPS laboratory in monitoring the quality of fuel and waste products.The samples include (1) run-of-mine sub-bituminous coal (RC); (2) feed coal (FC); (3) high-grade coal (HGC); (4) coal slurry (CS); (5) associated host rock (AR); (6) fly ash (FA); (7) bottom ash (BA); and (8) mixed material from the coal beneficiation and burning processes, which were taken from the disposal pond (DP).The samples were dried at room temperature and stored in plastic bags.The samples were repeatedly quartered and some of them were pre-crushed.Samples intended for chemical analyses and X-ray diffraction were powdered in an agate mortar.A vitrain (HGC) sample was manually separated and analyzed in order to distinguish the presence of organically associated trace elements.A split of the bottom ash sample was wet-screened in four size fractions (>1 mm, 1-0.5 mm, 0.5-0.25 mm, and <0.25 mm), which were analyzed for all elements except chlorine and bromine.
The ash yield of all samples was generated at 500 • C for 12 h in an open electric furnace as the temperature was gradually raised according to ISO 1171:2010 [29] for ash determination in solid mineral fuels.Inductively coupled plasma mass spectrometry (ICP-MS) in pulse counting mode was used to determine trace elements in the samples.For ICP-MS analysis, the samples were digested using a Milestone ultraCLAVE Microwave Digestion System with high-pressure reactor (Sorisole, Italy).The basic load for the digestion tank was composed of 330 mL of distilled water, 30 mL of 30% H 2 O 2 (Metal-Oxide Semiconductor, MOS reagent), and 2 mL of 98% H 2 SO 4 (Guaranteed reagent, GR).Initial nitrogen pressure was set at 50 bars and the highest temperature was set at 240 • C for 75 min.The reagent digestion for each 50 mg coal sample included 5 mL of 65% HNO 3 and 5 mL of 40% HF, but for non-coal samples, the reagents were 2 mL of 65% HNO 3 and 5 mL of 40% HF.Multi-element standards (Inorganic Ventures: CCS-1, CCS-4, CCS-5, and CCS-6) were used for the calibration of trace element concentrations [30].Chlorine and bromine analyses were conducted by X-ray fluorescence method on pelletized coal samples [26].Fluorine was determined by pyrohydrolysis in conjunction with a fluorine ion-selective electrode following the methods described in ASTM Standard D5987-96 [31].
Mineralogical investigations were performed by semiquantitative powder X-ray diffraction (XRD) using a diffractometer with Fe-filtered Co Kα and Ni-filtered Cu Kα radiations.Char particles present in fly ash were observed and selected using a binocular microscope.Their morphology and semi-quantitative chemical composition were studied using the following scanning electron microscope: SEM-EDS by means of Jeol Superprobe 733 (Jeol Ltd., Tokyo, Japan), at accelerating voltage of the probe 15 keV, and beam current 1 nA and 6 × 10 −2 nA, for analyses and pictures, respectively.Prior to analyses, a thin carbon layer was sputter-coated on the samples.

Mineralogy of the Coals and Combustion Products
The major minerals (>3 wt%, coal basis) in the coal samples are quartz, kaolinite, and pyrite.The minor minerals (1-3 wt%) are illite, plagioclase, K-feldspar, gypsum, calcite, dolomite, and siderite.The mineral composition of the coal slurry (CS) and associated rocks (ARs) is similar to that of coal; however, the contents of quartz, kaolinite, illite, K-feldspar, plagioclase, siderite, and calcite in CS and AR are higher than in coal [28].
Fly ash (FA) and bottom ash (BA) are represented by glass, quartz, mullite, K-feldspar, plagioclase, kaolinite, magnetite, hematite, and traces of calcite, gypsum, lime, maghemite, illite, Ca 2 SiO 4 , and char (Figure 2).The common minerals for all FA and BA samples are quartz and kaolinite.bromine analyses were conducted by X-ray fluorescence method on pelletized coal samples [26].Fluorine was determined by pyrohydrolysis in conjunction with a fluorine ionselective electrode following the methods described in ASTM Standard D5987-96 [31].Mineralogical investigations were performed by semiquantitative powder X-ray diffraction (XRD) using a diffractometer with Fe-filtered Co Kα and Ni-filtered Cu Kα radiations.Char particles present in fly ash were observed and selected using a binocular microscope.Their morphology and semi-quantitative chemical composition were studied using the following scanning electron microscope: SEM-EDS by means of Jeol Superprobe 733 (Jeol Ltd., Tokyo, Japan), at accelerating voltage of the probe 15 keV, and beam current 1 nA and 6 × 10 −2 nA, for analyses and pictures, respectively.Prior to analyses, a thin carbon layer was sputter-coated on the samples.

Mineralogy of the Coals and Combustion Products
The major minerals (>3 wt%, coal basis) in the coal samples are quartz, kaolinite, and pyrite.The minor minerals (1-3 wt%) are illite, plagioclase, K-feldspar, gypsum, calcite, dolomite, and siderite.The mineral composition of the coal slurry (CS) and associated rocks (ARs) is similar to that of coal; however, the contents of quartz, kaolinite, illite, Kfeldspar, plagioclase, siderite, and calcite in CS and AR are higher than in coal [28].

Trace Elements
The samples were analyzed for 49 trace elements and the concentrations of these elements in the coals were compared with the Clarke contents (worldwide averaged values) for brown coals [1] (Tables 1-3).А concentration coefficient (CC) [32] is introduced herein, which expresses the ratio between the element content in coal to the relevant Clarke value.According to this coefficient, the concentrations of trace elements in coal samples can be presented in the following decreasing order: FC > RC > HGC > vitrain.The element contents in bottom ash were compared with those in feed coal.In order to evaluate the element redistribution, the enrichment coefficients FA/BA were calculated.The behavior of the REEs in all studied samples was coherent and the europium anomaly was very weak (Eu/Eu* = 0.69-0.79)(normalized to chondrite) [33].Most elements showed a high positive

Trace Elements
The samples were analyzed for 49 trace elements and the concentrations of these elements in the coals were compared with the Clarke contents (worldwide averaged values) for brown coals [1] (Tables 1-3).A concentration coefficient (CC) [32] is introduced herein, which expresses the ratio between the element content in coal to the relevant Clarke value.According to this coefficient, the concentrations of trace elements in coal samples can be presented in the following decreasing order: FC > RC > HGC > vitrain.The element contents in bottom ash were compared with those in feed coal.In order to evaluate the element redistribution, the enrichment coefficients FA/BA were calculated.The behavior of the REEs in all studied samples was coherent and the europium anomaly was very weak (Eu/Eu* = 0.69-0.79)(normalized to chondrite) [33].Most elements showed a high positive correlation (r > 0.90) with ash yield, indicating their inorganic association.The wastes from the disposal pond were characterized by increased contents of Cl, Ba, and Br.

Mineralogy of Combustion Products
The mineral composition of coals and associated rocks predetermines the mineralogy in the wastes from coal beneficiation and coal burning processes [34][35][36][37][38].For example, the mullite content in fly ash from the Jungar Power Plant, Inner Mongolia, China, is as high as 37.4% because of high boehmite and kaolinite contents in feed coal [34].The origin of the minerals and phases in FA and BA is discussed by a number of authors [24,34,39,40].According to Vassilev [39], the primary minerals and phases are components originally present in coals that have not undergone phase transformations during the combustion process.The secondary phases form during burning, while the tertiary minerals originate during the transportation and storage of BA and FA.
The main lithotypes in the studied coal samples are clarain and vitrain.The microlithotypes are mostly clarite, vitrite, and vitrinertite.Of the three maceral groups, vitrinite dominates [28].The unburned carbon in the fly ash from the Republika TPS is about 2% as could be presumed by the loss of ignition (LOI) values.The studied char particles are elongated porous spheroids composed of altered vitrain and clarain.Some of the particles are covered with iron oxide formed as a result of the oxidation of pyrite or siderite (Figure 2A) or aluminosilicate (Figure 2B) film.The aluminosilicate film is most probably a phase of the transformation of clay minerals because traces of Mg, Ca, Ti, Al, Si, Fe, S, and Cu were identified in this film and char matrix.Most of these elements are present in the clay minerals of Pernik coal [28].Nano-particles of Fe, Cu, and Zn included in the char matrix were observed as well.According to Hower et al. [43], vitrinite is one of the main sources for the formation of char particles in the FA from low-rank coal combustion, but without it being melted and repolymerized.On the other hand, the constant heating of Pernik coals by active fires at depth, caused by their spontaneous combustion, is probably also a factor in the type of char particles formed [22,28,44].During this process, volatile components are released [14], as well as a change in the vitrinite structure depending on the temperature and lack of oxygen [45,46].

Trace Elements
The mobility of trace elements during beneficiation and combustion depends on their mode of occurrence and concentrations in coal, as well as on the preparation and combustion technologies [35,[47][48][49].

Trace Elements in Coals and Wastes from Preparation and Disposal Pond
The Pernik sub-bituminous coals supplying the Republika TPS have a high ash yield (Table 1).As a rule, the element contents and ash yields of the raw and feed coals are similar because the initial processing of raw to feed coal is crushing and sieving.When compared with the respective world averages for brown coals [1], the elements in the raw and feed coals are more or less enriched, excluding Tl, Ge, Bi, and Sb having similar or lower contents (Table 1, Figure 3).These observations indicate the leading role of inorganic matter as a contributor and concentrator of trace elements in Pernik coals.
The high-grade coals are depleted in all elements in comparison with the feed coals (Table 1).However, compared with the element contents of the world brown coals, the HGCs are enriched in some hazardous elements such as V > Pb > Cu > Co > Ni; F > Mo > Be; Zn > Br; and Th > Cr > Cd (Figure 3).
Vitrain is the most abundant lithotype in the HGCs.The sample studied (8.8% ash yield) has the lowest trace element contents compared with the feed coal and global averages.The only strongly enriched element is V and, to a lesser extent, Mo, Ni, Pb, and W (Table 1, Figure 3).The following factors can be supposed as a source of vanadium in the coal basin: (1) the Upper Cretaceous mafic and intermediate igneous rocks, which are in contact with the eastern and north-eastern margins of the Pernik basin; their V contents are up to 535 ppm and 320 ppm, respectively [50,51], and (2) the alternated host rocks of the Breznik and Klisoura polymetallic ore deposits, where the V contents are up to 320 ppm and 200 ppm, respectively [52,53].Both deposits are located on the northern margin of the coal basin.
The associated host rocks have enhanced contents of all elements towards the Clarke concentrations, excluding Mo, Cl, and Br.
The above data show again the dominant inorganic affinity of most trace elements in the Pernik coals, which is one of the most significant factors for their behavior during combustion.
The above data show again the dominant inorganic affinity of most trace elements in the Pernik coals, which is one of the most significant factors for their behavior during combustion.

Trace Elements in Bottom Ash and Bottom Ash Size Fractions
Trace element contents in bottom ash and the respective sized fractions are listed in Table 2. Most element contents in bottom ash are enriched compared with those in feed coal.Equal or significantly depleted are some of the volatile elements including Sn, Mo, Sb, F, Bi, Cd, Ge, and Pb.The fractionation of the elements in size is assessed by the enrichment Minerals 2024, 14, 313 12 of 20 coefficient (EC), namely the element content in fraction <0.25 mm/element content in fraction >1 mm (Table 2).According to the type of distribution, the elements in the bottom ash size fractions may be grouped as follows: First group.The contents of Ba, Y, Pr, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Zr, Hf, Nb, Co, and U increase as the fraction size decreases.Their enrichment coefficient range is 1.2-2.3(Table 2 and Figure 4A).These concentrations are related to the redistribution of the fuel during the combustion process and the higher presence of unaltered clay minerals, feldspars, and plagioclases in this waste product [27].Another explanation is the occurrence of these mostly lithophile elements as proper accessory species.
probably mainly associated with specific silicates in bottom ash.The contents of Eu can be considered less affected by the occurrence of 137 Ba 16 O and 136 Ba 16 OH during analysis using quadrupole ICP-MS because the Ba/Eu is much lower than 1000 [4].
Third group.Germanium and Cd are the only elements with decreasing contents towards a fraction <0.25 mm (Figure 4C).Both elements probably have some organic affinity because the ash yield value is the minimum in the coarsest fraction.
Fourth group.Fluorine, V, and most of the sulfophile and siderophile elements (Mo, Sn, Cu, Zn, In, Pb, Sb, Cr, Mn, and Ni) show uneven partitioning with maximum contents in the fraction 1-0.5 mm (Figure 4D).Such complicated segregation may be attributed to combined factors such as the volatility of the elements and the presence of mineral and char particles containing them in this fraction.
On the whole, there is no significant redistribution of the trace elements among the bottom ash size fractions.This may be explained by the similar matrix of the fractions because they contain from 79% to 85% silicates, determined by Vassilev et al. [27], for the bottom ashes of the same boiler.For this reason, the trace element enrichment in the finest fraction (<0.25 mm) is most probably a result of more intensive adsorption processes.Second group.The contents of Li, Rb, Cs, Sr, Tl, Be, Sc, La, Ce, Nd, Eu, Ga, Ta, W, and Th increase up to the 0.5-0.25 mm fraction, and then remain the same or are slightly depleted in the smallest fraction (Figure 4B).These predominantly lithophile elements are probably mainly associated with specific silicates in bottom ash.The contents of Eu can be considered less affected by the occurrence of 137 Ba 16 O and 136 Ba 16 OH during analysis using quadrupole ICP-MS because the Ba/Eu is much lower than 1000 [4].
Third group.Germanium and Cd are the only elements with decreasing contents towards a fraction <0.25 mm (Figure 4C).Both elements probably have some organic affinity because the ash yield value is the minimum in the coarsest fraction.
Fourth group.Fluorine, V, and most of the sulfophile and siderophile elements (Mo, Sn, Cu, Zn, In, Pb, Sb, Cr, Mn, and Ni) show uneven partitioning with maximum contents in the fraction 1-0.5 mm (Figure 4D).Such complicated segregation may be attributed to combined factors such as the volatility of the elements and the presence of mineral and char particles containing them in this fraction.
On the whole, there is no significant redistribution of the trace elements among the bottom ash size fractions.This may be explained by the similar matrix of the fractions because they contain from 79% to 85% silicates, determined by Vassilev et al. [27], for the bottom ashes of the same boiler.For this reason, the trace element enrichment in the finest fraction (<0.25 mm) is most probably a result of more intensive adsorption processes.

Contents and Distribution of Trace Elements in Fly Ashes
The increased trace element contents in fly ashes produced in many coal-fired power plants are commonly due to the element condensation on the particle surface because the ratio of particle surface/volume is higher in the smallest particles [34,48,[53][54][55].Another explanation is the occurrence of these mostly lithophile elements in the finest minerals even as proper accessory species.
The trace element contents in the fly ashes from the mechanical collector and two electrostatic precipitator (ESP) rows are listed in Table 3.In order to evaluate the element redistribution, the following enrichment coefficients were calculated: m/b, e1/b, and e2/b, which represent the element contents in the mechanical collector and the 1st and 2nd ESP rows/element contents in the bottom ash, respectively (Table 3).According to these coefficients, the following trends could be distinguished: From the mechanical collector to the second electrostatic precipitator, the enrichment coefficients of Tl, Be, Ga, Ge, Zr, Hf, Tb, Sn, V, Nb, F, Zn, Cd, In, Pb, Bi, and Sb increase from 1.23 (Be and Hf) to 7.43 (Bi) as can be seen in Table 3.Similar pattern of enrichment have Li, Tb, Dy, Ho, Er, Yb, Lu, Ta, Cu, Cr, Co, and Ni; however, the enrichment coefficient is lower, from 1.05 (Ni) to 1.19 (Ta).The contents of Cs, Y, La, Ce, Pr, Nd, Sm, Gd, Mn, and Th decrease towards the second ESP row.Elements Rb, Sr, Ba, Sc, Tm, Br, and U show uneven partitioning.Chlorine is also unevenly partitioned, with the highest content in the mechanical collector, suggesting its emission with the vapor phases and condensation in this fly ash.Europium is the only element with similar contents and enrichment coefficients in all fly ashes.
Higher contents of trace elements in electrostatic precipitator ashes than in mechanically collected ash were also established for Kentucky power plants [54].Increasing contents of semi-volatile elements of fly ash with increasing distance from the boiler were found for other coal power plants [55][56][57][58].
From the mechanical collector to the ESP, the temperature of the flue gases decreases, which favors the condensation of the elements, depending on their dew point temperature.Another factor favoring element condensation is the greater adsorption/absorption capacity of the smallest particles.The dependence of trace element contents on particle size was earlier established by Davidson et al. [59] and was later confirmed for other power plants [60,61].However, the exact enrichment mechanism of trace elements of the small particles needs further study [62,63].Actually, later comparative experiments of trace element concentration on the condensation film of fly ash particles showed that the condensation processes are more complicated and depend as well on the boiling temperature of the elements [64].For example, the microscopic study of microspheres attached to the surfaces of larger fly ash spheres showed that the former particles have completely different compositions in comparison with the host spheres [65].It could be suggested that such differences also influence the accumulation of trace elements on fly ash particles.For instance, investigations on the adsorption of Hg on fly ashes reveal that "char has a leading role for the partial capture and retention behavior of Hg in fly ashes" [66].Moreover, there are differences between the capture mechanism for the fly ashes from the lignite and bituminous coals studied [66].Hence, in order to elucidate the retention mechanism of elements on fly ashes, further detailed investigations are needed.
According to Meij [56], high volatile elements such as F, Cl, and Br are almost not condensed on fly ash.However, fluorine is highly enriched in the ash of the two ESPs and evenly distributed in the bottom ash size fractions in the Republika TPS.The quantity of fluorine condensed on fly ash depends strongly on the CaO content because CaO can combine F as CaF 2 [67].Significant F contents were detected in the wastes of a Russian TPS burning brown coals [68].Additionally, in the fly ash derived from Ge-rich Wulantuga coals (northern China), F content was up to 15568 ppm on an ash basis and the major carrier of F was fluorite [69].
The main conclusion is that during combustion in the Republika TPS, no significant segregation of the trace elements occurs between the bottom ashes and fly ashes, excluding the high volatile and sulfophile elements.
In general, during the process of coal combustion, a great proportion of the trace element contents in the feed coal remains in the solid combustion residues.Such behavior is characteristic of other coal-fired power plants.For instance, similar behavior was observed for ten Turkish power plants [70] and a large Spanish power plant using sub-bituminous coal [71].The same was established for many Russian power plants where about 82% of the trace elements present in clay minerals (included in the mineral structure or adsorbed) remain in bottom ashes and fly ashes [72].

Behavior of REE
The behavior of the REEs in all samples studied is coherent.Overall, the REE chondrite and shale-normalized patterns are very similar (Figure 5A,B).However, some differences could be pointed out: Minerals 2024, 14, x FOR PEER REVIEW 13 of 19 coals (northern China), F content was up to 15568 ppm on an ash basis and the major carrier of F was fluorite [69].
The main conclusion is that during combustion in the Republika TPS, no significant segregation of the trace elements occurs between the bottom ashes and fly ashes, excluding the high volatile and sulfophile elements.
In general, during the process of coal combustion, a great proportion of the trace element contents in the feed coal remains in the solid combustion residues.Such behavior is characteristic of other coal-fired power plants.For instance, similar behavior was observed for ten Turkish power plants [70] and a large Spanish power plant using sub-bituminous coal [71].The same was established for many Russian power plants where about 82% of the trace elements present in clay minerals (included in the mineral structure or adsorbed) remain in bottom ashes and fly ashes [72].

Behavior of REE
The behavior of the REEs in all samples studied is coherent.Overall, the REE chondrite and shale-normalized patterns are very similar (Figure 5A,B).However, some differences could be pointed out: It can be seen that the ΣREE contents vary significantly (Table 4).The vitrain sample and the high-grade coals have the lowest ΣREE concentrations.Bottom ash and fly ash are enriched in REE compared with feed coal.The ratios LREE (La-Eu)/HREE (Gd-Lu) and It can be seen that the ΣREE contents vary significantly (Table 4).The vitrain sample and the high-grade coals have the lowest ΣREE concentrations.Bottom ash and fly ash are enriched in REE compared with feed coal.The ratios LREE (La-Eu)/HREE (Gd-Lu) and Ce N /Yb N are lower for vitrain and HGC, indicating a relative HREE enrichment in comparison with the rest of the studied samples.Such enrichment is characteristic of many worldwide coals [74][75][76][77] and indicates a stronger organic association of the HREE than the LREE.It may be presumed that besides the dominant proportion of aluminosilicateassociated REE in the Pernik coals, a small quantity of REE seems to be organically bound.During the combustion process, a slight differentiation occurs between the light and heavy REEs.In the fractions of bottom ash, the mean e/d coefficient for the LREEs is 1.5, while for the HREEs it is 1.3, suggesting higher adsorption/condensation on the smaller fractions (Table 4).These differences are also distinctive for the fly ash from the mechanical collector.For example, the LREE contents slightly decrease, while HREE concentrations slightly increase for fly ashes from the mechanical collector to the second ESP row.These differences may be attributed to organically associated REEs, whose mobility (particularly of the HREEs) is higher than that of aluminosilicate-associated REEs.Differences in the mobility between light and heavy REEs have been experimentally established for some Chinese and Spanish coals and their combustion wastes [69,78,79].
It was also found that the europium anomaly (Eu/Eu* = 0.69-0.79) in chondritenormalized patterns is very weak for all samples studied (Table 4 and Figure 5) and non-existent in NASC-normalized patterns (Eu/Eu* = 0.98-1.1).This observation is quite different compared with that for high-Ge coals and their combustion products from some Chinese TPSs [69].
In summary, the REEs are rather evenly distributed in the different combustion products attributed to their predominant aluminosilicate association.The REY contents of the studied samples are plotted in the unpromising area field (Figure 6) in the diagram of Dai et al. [80].During the combustion process, a slight differentiation occurs between the light and heavy REEs.In the fractions of bottom ash, the mean e/d coefficient for the LREEs is 1.5, while for the HREEs it is 1.3, suggesting higher adsorption/condensation on the smaller fractions (Table 4).These differences are also distinctive for the fly ash from the mechanical collector.For example, the LREE contents slightly decrease, while HREE concentrations slightly increase for fly ashes from the mechanical collector to the second ESP row.These differences may be attributed to organically associated REEs, whose mobility (particularly of the HREEs) is higher than that of aluminosilicate-associated REEs.Differences in the mobility between light and heavy REEs have been experimentally established for some Chinese and Spanish coals and their combustion wastes [69,78,79].
It was also found that the europium anomaly (Eu/Eu* = 0.69-0.79) in chondrite-normalized patterns is very weak for all samples studied (Table 4 and Figure 5) and nonexistent in NASC-normalized patterns (Eu/Eu* = 0.98-1.1).This observation is quite different compared with that for high-Ge coals and their combustion products from some Chinese TPSs [69].
In summary, the REEs are rather evenly distributed in the different combustion products attributed to their predominant aluminosilicate association.The REY contents of the studied samples are plotted in the unpromising area field (Figure 6) in the diagram of Dai et al. [80].

Correlations
Correlation coefficients (r) between the mean element contents of the raw coal, highgrade coal, feed coal, coal slurry, wastes from disposal pond, and associated rocks (Table 1) were calculated in order to distinguish some trends in their distribution and associations.For the six types of samples, the statistically significant correlation coefficients are ≥0.70 at a 95% confidence level.Most elements have a positive correlation (r ≥ 0.70) with ash yield, indicating their inorganic association.The correlation of F (r = 0.68), Sr (r = 0.64), Cs (r = 0.63), and Ge (r = 0.62) with ash yield is also positive, but insignificant.
Elements can have a mixed mode of occurrence, but their behavior depends on the dominant one.For example, Mo has been found as an impurity in pyrite [27], but it has the highest concentrations in the samples with low ash yield as vitrine and HGC.This implies allowing for its affinity to organic matter and the autogenous mineralization that is intimately related to it (Figure 2A).
The correlations between the trace elements confirm their behavior outlined above as follows: limited partitioning among the combustion products for most elements and low, or lack of correlation to ash or volatile elements.

Environmental Aspects
As stated by Swaine [38], "with the modern facilities, there is a reduction in the amounts of trace elements in stack emissions relative to those in bottom and fly ash".Hence, more attention must be given to the fate of trace elements in coal wastes, in particular to losses caused by leaching to nearby waters.The wastes from the Pernik preparation coal plant and Republika power station (coal slurry, bottom, and fly ashes) are transported to a disposal pond by water.
The wastes are enriched or depleted in different trace elements compared with that in feed coal.For example, the trace elements in the slurry are slightly depleted with the exclusion of Cl (3.8×), Ba (1.7×), and Br (1.3×).Most elements in bottom ash are about 1.5× higher, while some volatile elements are of equal or lower contents (Ge, Sn, Mo, Sb, Cd, Pb, and F).Fly ash is enriched (1.4×-1.8×) in all elements with the exception of the highly volatile Br only.
Compared with the feed coals, the whole mix in the disposal pond is strongly concentrated in Cl (4.3×), as the element was introduced with the water transporting the waste products (Table 1).The enrichment of most elements ranges from 1.23 to 1.75.Only some volatile elements are depleted-Ge, Mo, Br, F, and Bi, suggesting their release with the flue gases.
As the enrichment of trace elements in bottom ash and especially in fly ash particles is the result of some sorption processes on their surface, the elements are subjected to possible leaching and migration.Hence, this topic is very important from an environmental point of view and requires additional investigations.
On the other hand, the coal slurry is also a source of some mobile elements such as F, V, As, Pb, Mo, Li, Sr, Ti, Ga, Ni, Ge, Cr, and Mn (Table 1), and according to Yossifova, 2007 [27].These elements can migrate from the sludge into the water of the disposal pond.The mobility of such elements strongly depends on the concentration of their mode of occurrence, for instance, (1) proper phases (halites, sulfates, carbonates, and sulfides); (2) impurity elements in the above mobile minerals; or (3) sorbed species of mobile elements in organic matter and clay minerals [27,81].

Conclusions
The Pernik coals have trace element contents normally higher than the respective Clarke contents for brown coals worldwide and some of these elements may be of environmental concern.
It was found that some trace elements are more enriched (more than two times) in the associated rocks in comparison with coals, namely Cs, Sc, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Sn, F, Cu, Pb, Cr, Co, Th, Li, Tl, La, Ce, Pr, Ta, and Zn.The predominant inorganic association of most trace elements was confirmed by the comparison of their contents with that of a vitrain sample that is depleted in ash yield (8.8%).On the other hand, vitrain is strongly enriched in V and, to a lesser extent, Mo, Ni, Pb, Co, W, F, Nb, and Cu.The dominant inorganic affinity of the trace elements in coal has a leading role in their behavior during combustion because a great part of them is retained in solid waste products.Moreover, it was identified that most trace elements are not significantly redistributed between the different combustion wastes.In contrast, the highly volatile elements (F, Cl, and Br) and volatile elements (Bi, Ge, Zn, Sb, Cd, Mo, Sn, and Tl) are significantly partitioned between the fly ashes from the mechanical collector and the ESP.The hazardous trace elements appear to be mainly retained in the waste products and there are no appreciable emissions of them into the atmosphere with the flue gases.However, the organically bound trace elements tend to appear as vapor phases in the boiler during combustion.

Figure 1 .
Figure 1.(A) Map of Bulgaria and (B) location of the Pernik coal preparation plant, the Republik thermoelectric power station, and the Pernik coal mines.

Figure 1 .
Figure 1.(A) Map of Bulgaria and (B) location of the Pernik coal preparation plant, the Republika thermoelectric power station, and the Pernik coal mines.

Figure 4 .
Figure 4. Distribution of the elements in the bottom ash size fractions.(A) The first group is represented via Yb.(B) The second group is represented via La.(C) The third group is represented via Ge.(D) The fourth group is represented via F.

Figure 6 .
Figure 6.Evaluation of REY in the studied samples using the outlook coefficient (Coutl), established by Seredin and Dai [2], in a plot as in Dai et al. [80].Figure 6. Evaluation of REY in the studied samples using the outlook coefficient (Coutl), established by Seredin and Dai [2], in a plot as in Dai et al. [80].

Figure 6 .
Figure 6.Evaluation of REY in the studied samples using the outlook coefficient (Coutl), established by Seredin and Dai [2], in a plot as in Dai et al. [80].Figure 6. Evaluation of REY in the studied samples using the outlook coefficient (Coutl), established by Seredin and Dai [2], in a plot as in Dai et al. [80].

Table 1 .
Element contents (ppm) in coals, coal slurry, associated rocks, and mixed material from DP.

Table 2 .
Trace element contents in bottom ash and respective size fractions, ppm.
e/d-enrichment/depletion coefficient; element contents in fraction <0.25 mm/element contents in fraction >1.0 mm; and BA/FC-element contents in bottom ash/element contents in feed coals.

Table 3 .
Trace element contents (ppm) in fly ash generated from the Republika power station.

Table 4 .
Rare earth elements in coals and combustion wastes.
[74][75][76]4, 14, x FOR PEER REVIEW 14 of 19CeN/YbN are lower for vitrain and HGC, indicating a relative HREE enrichment in comparison with the rest of the studied samples.Such enrichment is characteristic of many worldwide coals[74][75][76][77]and indicates a stronger organic association of the HREE than the LREE.It may be presumed that besides the dominant proportion of aluminosilicateassociated REE in the Pernik coals, a small quantity of REE seems to be organically bound.

Table 4 .
Rare earth elements in coals and combustion wastes.