Analysis of Polycyclic Aromatic Hydrocarbon Content in Ash from Solid Fuel Combustion in Low-Power Boilers

: The production of thermal energy is one of the sources of environmental pollution, espe-cially when it uses traditional fossil fuels (in particular hard and brown coal). Burning conventional fuels contributes to air pollution because of emitting gases and producing waste after the process of burning in the form of ash. The work below was aimed at determining the indirect emission of PAHs in the form of ﬂy ash, depending on the type of fuel burned. The conducted research showed which solid fuel combustion content leaves the lowest content of PAH in the ﬂy ash. This work contains the analysis and assessment of the content of 16 PAHs (polycyclic aromatic hydrocarbons) in ﬂy ash from the combustion of ﬁve selected solid fuels in low-power boilers. The following fuels were chosen for the research: hard coal with granulation above 60 mm, coal with the grain size of 25–80 mm, coal with the grain size of 8–25 mm, pellets with the grain size of 6 mm, and mixed dry wood. The results of the research showed that the most frequent and most concentrated compounds were naphthalene and acenaphthylene from the PAH group. These hydrocarbons have the smallest number of rings in a molecule. It was also found that the content of the LMW (Low Molecular Weight Polycyclic Aromatic Hydrocarbons) fractions in the ﬂy ash from the analyzed fuels in ∑ PAHs exceeds 57% in the case of 6 mm pellets and in the case of the ash from wood and hard coal when the grain size >60 mm. The opposite dependence can be observed in the case of the HMW (High Molecular Weight Polycyclic Aromatic Hydrocarbons) fraction.


Introduction
Polycyclic aromatic hydrocarbons are classified as persistent organic pollutants (POP), which are widely spread in the environment as a result of industrialization [1,2]. PAHs are lipophile compounds consisting of two to seven aromatic benzene rings in linear, angular, or cluster arrangements. They result from the stability of the compounds in the environment, and stable PAHs tend to accumulate mainly in soil and sediments. The physicochemical properties, distribution, and behavior of the PAH in the environment differ significantly due to their molecular masses. PAH with a smaller mass (LMW PAH), containing two to three rings are mobile in the environment, whereas PAH of a medium molecular mass (MMW PAH), containing four rings, and PAH of a higher molecular mass (PAH HMW), containing five to seven rings, are relatively immobile. Together with the increase in the molecular mass, the melting point, the boiling point, and the lipophilicity of the PAH increases in relation to the log K OW (the octanol/water partition coefficient), and the solubility in water decreases, which suggests increased solubility in lipid compounds [3,4]. The US Environmental Protection Agency has recognized 16 basic PAHs as priority pollutants as they may be a dramatic threat to the environment and the wildlife [5]. Moreover, because of their properties such as carcinogenicity, teratogenicity and mutagenicity, they create a global environmental problem [6][7][8]. The scientists' interest in monitoring the PAH in ash has grown recently due to the possibility of recycling the nutrients from ash into soil, as PAHs can be absorbed and ingested by plants, and then be introduced into the food chain of animals and humans [4,9,10]. Particular PAH compounds have different toxicities, and some of them are carcinogenic, teratogenic, and mutagenic. For instance, naphthalene is described as the most toxic PAH, and benzo(a)pyrene has been classified as carcinogenic for humans [11].
The incomplete combustion of organic matter in fossil fuels is the main source of PAH [12][13][14]. PAH in ash initially emitted in the gas phase may be absorbed on the created fly ash [15]. Determining the content of the side products (such as PAH) in ashes produced during the combustion process of the most frequently used fuels, such as fossil fuels or biomass, in low-power boilers, may enable the optimization of the fuel combustion regimes. In the literature, there are no data concerning the toxicity and carcinogenicity of the ash from fuel combustion in low-power boilers, which are estimated on the basis of the content of particular compounds from the PAH group. Taking this into consideration, it was decided to conduct research that would enable estimating the content of 16 particular representatives from the PAH group in fly ash from the combustion process of: imported hard coal of the grain size over 60 mm, hard coal of the grain size of 28-80 mm, coal of the grain size of 8-25 mm, pellets of the grain size of 6 mm, and mixed firewood. The content of the ∑LMW fraction has been compared. It consists of hydrocarbons including two to four aromatic rings in its structure, in which the density of the five and six-ring hydrocarbons from the PAH group was taken into account, and the cumulated density of 16 compounds from the PAH group, whose density is recommended to be monitored in the environment by the EPA (Environment Protection Agencies), was taken into account as well. Moreover, a one-factor analysis of variance was conducted in order to verify the hypothesis assuming that the type of fuel differentiates the mean values of ∑PAH, ∑LMW, and ∑HMW in fly ash samples obtained from the solid fuels combustion process.

Materials and Methods
The combustion of solid fuels for the purpose of obtaining testing material, i.e., fly ash, was done three times in the Low-Emission Technology lab stand, which was described by Szmajda at al. [16]. In Figure 1, a pictorial diagram of obtaining the testing material and the analytic methodology of determining the PAH compounds in the fly ash samples are presented. The methodology of the chemical analysis of samples in terms of the PAH content and the working conditions of the testing equipment were described in more detail by Szatyłowicz and Skoczko [17].
A 3 g sample was taken from the fly ash and extracted twice with the hexane-acetone solvent mixture in the proportion of 20 mL/5 mL. The extraction process was carried out for 2 h. Acetone and the extracted polar compounds were removed by washing twice with 10 mL of deionized water. The remaining organic phase was dried over anhydrous sodium sulfate (anhydrous, ACS reagent, ≥99% Sigma-Aldrich form St. Louis, MO, USA). Then, the extracts were concentrated in a Turbo-Vap apparatus under an inert gas-nitrogen atmosphere to a volume of 1 mL. The prepared solutions were subjected to chromatographic analysis using a gas chromatograph coupled with an Agilent GC/MS Triple Quad 7000C form USA mass spectrometer, equipped with a split/splitless injector and an HP-5MS capillary column of the dimensions of 30 m × 0.25 mm and a film thickness of 0.25 µm [17]. Energies 2021, 14,6801 The lab stand was equipped with a Moderator Unica Vento Eko 25 kW boiler from Hajnówka Poland form with an automatic filling tray equipped with a manual and a retort grate. The fly ash was taken from the ash pan. Each time, 15 kg of each fuel were burned under identical conditions; i.e., the exhaust fumes temperature and the temperature of feedwater (on the output of the boiler), was about 70-80 • C, while the return water temperature was not lower than 50 • C. The stream of the fuel mass and the air blowing to the combustion chamber were set by the boiler controller in the Fuzzy Logic mode. The process was repeated three times in order to obtain the standard deviation and check the results correctness.
During tests, external calibration exploiting calibration curves for particular compounds were used. Calibration solutions were prepared from a standard mixture of 16 PAHs (AccuStandard, Z-014G). Figure 2 presents the 16 PAHs chromatogram.  Table 1 shows the data of the chromatographic analysis, such as the retention time of individual PAHs given in minutes, the equation of the calibration curve, and the matching factor R 2 . The limit of detection (LOD) and the limit of quantification (LOQ) were assessed from the signal-to-noise ratio (S/N). The LOD was 0.3 ng/mL (three times the noise level) and the LOQ = 1 ng/mL. The results obtained from the quantitative analysis of PAHs were converted according to Equation (1), while the result itself was expressed in mg/kg of the fly ash dry mass.
where C WWA -PAH content; A m -measured peak area of the compound in the sample extract; f -total dilution factor of the extract; V-volume of the final extract; a-slope of the calibration graph with respect to the OX axis; b-the point of intersection of the calibration graph with the OY axis; m 1 -mass of flask and sewage sample; m 2 -the mass of the empty flask.

Results
The content of ΣPAH, ΣHMW, and ΣLMW in fly ash samples was determined as follows. Table 1 contains the determined PAH in fly ash from the combustion of five selected solid fuels. In the tested samples of fly ash, all analyzed PAH hydrocarbons were found. Naphthalene and acenaphthylene were the compounds occurring most frequently-the hydrocarbons have the smallest number of rings in a molecule. Naphthalene has two rings, and acenaphthylene has three. The greatest concentration of them was noticed in the sample of fly ash from the combustion of wooden biomass in the form of mixed firewood. The content of naphthalene in this sample was 2.391 + 0.181 mg/kg DW (Dry Weight), and the concentration of acenaphthylene −1.021 ± 0.065 mg/kg DW. However, a high concentration of those aromatic hydrocarbons was also observed with the samples of hard coal of 8-25 mm combustion. The concentration of naphthalene was 1.422 ± 0.341 mg/kg DW, and the concentration of acenaphthylene was −0.447 ± 0.289 mg/kg DW. In other ashes remaining after burning hard coal of various granulation and pellets of 6 mm, the naphthalene concentration was from 0.049 ± 0.007 mg/kg DW for pellets to 1.234 ± 0.281 mg/kg DW for coal of >60 mm. The concentration of acenaphthylene in those samples was much lower than in the case of wooden biomass. Indeno(1,2,3,c,d)pyrene and benzo(g,h,i)perylene were the least frequent PAH compounds in the tested samples. Indeno(1,2,3,c,d)pyrene was found in the sample from burning pellets of 6 mm, and benzo(g,h,i)perylene was found in the sample from burning coal of >60 mm. Both compounds have six rings, which is the greatest number of rings in a molecule of all the examined compounds.
The relative uncertainty of each value of the PAH sum (Table 2 and Figures 3-7) within the conducted tests, for the purpose of this paper, during a separate combustion of particular fuels was estimated between 3% and 37.5%. This means that the combustion process has a great volatility and is nonlinear, even if the same fuel is burnt and the combustion parameters are the same in terms of the analysis of the PAH content in the fly ash.        Analyzing the content of the PAH low fraction (LMW) (Figure 9), containing hydrocarbons consisting of two and three aromatic rings, it was assumed that the fly ash from burning pellets of 6 mm and hard coal of 25-80 mm contains its lowest amount, i.e., respectively: 0.073 ± 0.013 mg/kg DW and 0.182 ± 0.509 mg/kg DW. In the case of coal, the share of this fraction was 57.03% of the PAH sum, and in the case of pellets of 6 mm and hard coal of 25-80 mm, it was 69.73%. The content of ∑LMW in the samples of fly ash from mixed firewood combustion was greater than in the case of pellets, and it equaled 3.796 ± 0.149 mg/kg DW, which was 90.57% of the PAH sum. In samples taken after burning hard coal of the grain size of 8-25 mm and coal of the grain over 60 mm, the content of ∑LMW was: 2.192 ± 0.765 mg/kg DW and 1.588 ± 0.587 mg/kg DW, respectively, which stood for 77.54% and 89.11% of their share of 16 PAH sum.
Analyzing the high PAH fraction (HMW) presented in Figure 10, in the fly ash, it was observed that its greatest fraction in samples from burning five selected solid fuels was found in hard coal of the grain size of 8-25 mm (0.635 ± 0.079 mg/kg DW), with 32.46% of the total content of 16 PAHs. In the case of the other types of coal (25-80 mm and >60 mm), the content of ∑HMW was 0.079 ± 0.013 mg/kg DW and 0.194 ± 0.115 mg/kg DW, respectively. In the group of fuels based on wooden biomass, i.e., firewood, the content of ∑HMW was 0.395 ± 0.061 mg/kg DW, and in the case of pellets of 6 mm, it was 0.055 ± 0.017 mg/kg DW.  In fly ash samples of solid fuel combustion, the percentage share of PAHs of various numbers of cycles in a molecule ( Figure 11) depended on the type of burned fuel. The greatest share in all samples was two-ring PAHs; it varied from 38.28% (in the case of pellets of 6 mm) to 69.25% (in the case of hard coal of the grain size over 60 mm). The percentage share of the hydrocarbons of medium molecule mass, i.e., having three and four rings, was biggest in the case of mixed firewood (33.52% + 6.80% = 40.32%). In the fly ash samples from burning pellets of 6 mm grain size and hard coal of 25-80 mm grain size, a greater percentage of five-ring PAH was noted than in the case of other samples. The fly ash from gasification of waste sediments was tested by the research group of Dudziak [18]. It did not contain any PAH, even though the sediments were contaminated with such hydrocarbon compounds. Peng et al. [19] analyzed 16 PAHs in ground fly ash remaining after burning municipal solid waste. It turned out that after 6 h, the total content of PAH and the TEQ (Toxic Equivalency) factor decreased by 49.3% and 41.7%, respectively, together with prolonging the time of grinding the ash in a planetary grinder. The researchers [20] found the explanation in the fact that the fly ash consisted of aliphatic and aromatic compounds. As the result of a weak binding energy of C-C, the hydrocarbon chains can be easily torn apart while grinding, and the remaining molecules evaporate, as they have the smallest molecule mass [21]. Lu et al. [22] also observed this process. They noticed that some amount of benzene appears during the ball grinding process and, when condensed, it created an amorphic, granite-like structures, which led to coloring the ground coal samples black.

ANOVA One-Way Analysis of Variance
The ANOVA variance analysis was made to confirm the hypothesis formulated on the basis of the results of the carried out tests. The hypothesis assumes that the contents of the sum of WWA, LMW, and HMW in fly ash depend on the type of the burned fuel. ANOVA is one of the most popular statistical methods that allows checking whether the considered classifying factors influence the observed measurement values of the variable that is being explained.
A simple form of ANOVA tests the influence of one classifying factor, while its expanded form enables simultaneous tests of many factors. The analysis of variance is particularly useful when trying to describe the qualitative influence of the classifying factors. In the case of one-way analysis, the hypothesis stating that the type of fuel differentiates the mean values of ∑PAH, ∑LMW, and ∑HMW in ash samples obtained from burning solid fuels was verified. It was assumed that the differences were statistically significant at the p value lower than 0.05. In one-way analysis of variance for fly ash samples, the type of fuel was the classifying factor. Table 3 contains the results of the conducted analysis of variance of the ∑PAH factor. It showed significant differences between the mean content of ∑PAH. The results in Table 4 show homogenous groups, which are the same as the results in Table 3. As a result of the tests, three homogenous groups of similar fuels were formed. It was noticed that pellets of 6 mm, hard coal of 25-80 of mm grain size, and hard coal of >60 of grain size created one homogenous group. Coal of >60 mm and coal of 8-25 grain size formed another homogenous group. Yet another homogenous group consisted of hard coal of 8-25 grain size and mixed firewood. All three groups are interconnected, which may prove they are comparable.   Table 5 contains the results of the Tukey's post hoc test for the ∑LMW factor. According to it, it was stated that there are two homogenous groups of similar samples of ash from solid fuels combustion. According to classification into homogenous groups, in Table 6, in the case of the ∑LMW content mean value, the first homogenous group consisted of pellets with a grain size of 6 mm, coal with a grain size of 25-80 mm, coal with a grain size of >60 mm, and coal with a grain size of 8-25 mm. Another group contained coal with a grain size of >60 mm, coal with a grain size of 8-25 mm, and mixed firewood.  As a result of testing, in the case of the ∑HMW factor (Table 7), three homogenous groups were formed ( Table 8). The first of them consisted of pellets with a grain size of 6 mm, coal with a grain size of 25-80 mm, and coal with a grain size of >60 mm. Another homogenous group contained coal with a grain size of >60 mm and mixed firewood. The third homogenous group was formed by mixed firewood and coal with a grain size of 8-25 mm. All the groups are interconnected; therefore, they may be compared.   Figure 12 presents a graphical interpretation of the one-factor analysis of variants results for fly ash samples in the cases of ∑PAH, ∑LMW, and ∑HMW. Identical letters "a" "b", or "c" for mean values indicate the lack of significant differences estimated with the use of the Tukey's test for p > 0.005. On the other hand, different letters, e.g., "ab", mean the occurrence of a homogeneous group between the tested solid fuels. There are statistically significant differences between homogeneous groups according to Tukey's test.

Discussion
Submitting the obtained results for discussion and comparing them with the results held by other authors, it was observed that similar dependencies were noticed in [23]. In that research, a possibility of polluting soil with PAHs included in fly ash from pulverized and fluidized fuels burned in boilers, as well as in fly ash from burned biomass, was confirmed. In a boiler analyzed in [23], both hard coal and wooden biomass were burned. In that research, naphthalene and fluorene were the most frequently occurring compounds in the fly ash (Figures 3-7). The concentration of naphthalene in the sample of the wooden biomass combustion was 49.805 mg/kg DW, whereas the concentration of fluorene was −23.258 mg/kg DW. In other ashes, remaining after burning hard coal and brown coal, those values were within <0.02 mg/kg DW and 13.815 mg/kg DW [23]. The total value of PAHs observed during the research in the fly ash from solid fuel combustion was between 0.128 ± 0.004 mg/kg DW in the case of burning pellets of 6 mm and 4.191 ± 0.091 mg/kg DW (Figure 8) in the case of mixed firewood. The content of PAH in the fly ash from hard coal of 8-25 mm was 2.827 ± 1.056 mg/kg DW, which was higher than in the samples of the fly ash from hard coal of a greater grain size. The other fly ash samples, i.e., those from burning coal of the grain size of 25-80 mm and coal of the grain size of >60 mm, contained 0.261 ± 0.064 mg/kg DW and 1.782 ± 0.356 mg/kg DW of the sum of 16 PAHs in the sample mass, respectively. According to Masto at al. [24], the total content of the PAHs sum was within 0.19-12.3 mg/kg in the fly ash from four power plants cooperating in waste combustion. The grate ash was an exception (193 mg/kg).
Similarly, Johansson and van Bavel [25] obtained similar PAH values in the fly ash and grate ash from the city solid waste incineration plant. The content of PAH in ash is probably connected with unburned coal included in the ash. According to Johansson and van Bavel's research [26], the unburned coal content in their samples was quite high for fly ash (9.72-16.9%) in comparison to the grate ash (1.33-8.12%). The content of 16 PAHs was within 40 mg/kg and 300 mg/kg in the fly ash from wood dust combustion. Re-burning of the ash decreased the PAH content to 0.24 mg/kg, and the organic coal content dropped from 40% to 5% [27]. Whereas during the hard coal combustion process, the fly ash was dominated by PAH of medium and low molecule mass, when the five-and six-ring PAH compounds dominated in the bottom ash [28]. In addition, Sahu at al. [29] observed low molecular mass PAHs in the fly ash from hard coal combustion. A great amount of PAH containing five to six rings in the bottom ash results from incomplete combustion [1]. Yet, Johansson and van Bavel [26] inform that PAHs are persistent pollutants and do not degrade easily, even though the ash is weathered. The ash from burning biomass contained a significant amount of nutrients for plants and therefore may be applied in agriculture; however, it must be thermally processed at high temperature (550 • C) or the working temperature of the plant must be increased to degrade the PAH compounds [30].
Considering the amount of waste from solid fuel combustion in households and in landfills, more extensive research should be undertaken on its reprocessing and treatment. Moreover, the development of new technologies and an increase in the research undertaken in the fields of extending innovative applications and reprocessing combustion residues from households is observed. This may be a source of increasing their rational use as well as economic and environmental effects. Table 9 presents the analysis of the use of combustion products (fly ash) from both the commercial power industry and home furnaces. Table 9. Fly ash management summaries.

Methods of Using Fly Ash Literature
• For the production of cellular concrete, the ashes act as construction aggregate and partially as a binder [31] • For the formation of road bodies, soil stabilization and improvement of the bottom layer and foundation [32] • For the production of solid bricks, checkered bricks, perforations, and hollow blocks [33] • As dyes in the production of plastics and paints, a filler for plastics [34] • As shielding of fire-endangered objects such as coal heaps, creating a sealing layer [32] • As a calcium and magnesium fertilizer [35] • As an alkalizing fertilizer for acidic soils [36,37] • For the fertilization of soil to improve their physical properties, increasing the capacity of the sorption complex, water absorption, reducing soil density [38] • Waste gas treatment [39] • The synthesis of zeolites from fly ashes and the possibility of their use in the process of carbon dioxide adsorption [32] • Dosing of fly ash to sewage sludge causes a number of exothermic reactions, as a result of which the sludge undergoes the processes of sterilization, pasteurization, and disinfection [23] • To intensify the activated sludge sedimentation process. The ashes reduce the activated sludge index and improve the quality of clarified wastewater [39] • For the adsorption of heavy metals from industrial wastewater or sludge [40,41]

Conclusions
The research results presented in the paper show that in the process of fuel combustion, polycyclic aromatic hydrocarbons are produced. They show up in fly ash, which remains after the combustion. These side products of combustion, produced in individual heat sources, do not undergo any supervision, and the PAHs included in it are emitted into the environment, which is a significant ecological problem. Additionally, PAHs, because of their toxic, mutagenic, and carcinogenic properties, are a serious threat to all living organisms. Power plants usually have such combustion waste reprocessed or, after defining its exact content, have it managed. The fly ash produced in individual heat sources are spread in the environment uncontrollably, polluting soil, water, and the air. It is often used for gardening purposes or they are stored in random locations. It is a serious threat to the atmosphere because part of the soot evaporates into the air together with the fumes. Individual heat source users usually are not aware that storing such waste or using it without defining its level of toxicity conveys a danger of polluting the soil and the water soaked up with this type of waste. It is a significant threat to the water environment because dangerous substances included in the ash are leached.
On the basis of technological research of the combustion process of five solid fuels and the fly ash obtained as a result of this process, conducted under controlled conditions of fuel combustion, the following were assumed: • As a result of the content of ∑PAH in the fly ash, the fuels may be aligned in the following way: pellets of 6 mm grain size < hard coal of 25-80 mm grain size < hard coal of 60 mm grain size < hard coal of 25 mm grain size < mixed fire wood.

•
It was also noticed that the content of the light fraction (LMM) in the fly ash of the researched fuels in ∑PAH was significantly greater than the content of soot, and it exceeded 57% in the case of the pellets of 6 mm, and in the ash from the firewood and coal of >60 mm, it was about 90%. The opposite dependence was noticed in the case of the hard fraction (HMW).
• It should be noted that a bigger percentage share of LMW in the sum of PAH proves the occurrence of easier biodegradable compounds. In order to confirm the presented hypothesis that the content of ∑PAH, ∑LMW, and ∑HMW in the samples of fly ash depends on the type of the combusted fuel, a one-factor variance analysis was conducted. It was assumed that the differences were understood as significant when the p value was lower than 0.05. In the one-factor variance analysis of the fly ash samples, the type of fuel was the qualitative classifying factor.