Introduction

Renewable energy sources played a key role in reducing greenhouse gas emissions and were expected to displace fossil fuels from the energy sector (Burg et al. 2016). The climate and energy policy in the European Union countries aimed to achieve at least 27% of renewable energy in final energy consumption by 2030 (Mirowski et al. 2018). This policy encompassed the use of various biomass raw materials and their processed substances. In the European Union, the energy demand primarily stemmed from heating and cooling buildings, with over 75% of the energy used being derived from fossil fuels and around 18% from renewable sources (Perea-Moreno et al. 2018). In Poland, approximately 12 million tonnes of hard coal were utilized annually for heating individual households furnace (GUS 2021), alongside the use of wood biomass for stoves and fireplaces.

Biomass, including agri-food biomass ashes, has emerged as a renewable alternative to conventional fossil fuels. This transition is also evident in diesel engines, where there is an increasing shift towards biofuels is observed. The environmental importance of utilizing biomass-based fuels, such as DMC, as substitutes for traditional oil-based fuels in diesel engines cannot be overstated. These fuels have the potential to address environmental concerns associated with energy consumption and emissions in diesel engines, contributing to environmental mitigation (Wei et al. 2020). The utilization of biomass-based fuels in diesel engines effectively reduces emissions, especially of particulate matter like such as soot particles. Special attention is given to the high reactivity of oxidation during combustion processes (Wei and Wang 2021) (Wei et al. 2020), as well as the formation of soot depending which depends on temperature (Llamas et al. 2022). Biomass fuels offer significant compatibility with established large-scale energy conversion technologies, notably in processes like pulverized suspension firing. Furthermore, the utilization of biomass fuels extends to biofuel production methods like entrained flow gasification, as well as to technologies aimed at mitigating CO2 emissions, such as oxy-fuel combustion (Llamas et al. 2022).

The utilization of biomass involved a wide range of physicochemical and biochemical processes (Tamelowá et al. 2021). Depending on the type of biomass used, the resulting ash exhibited variations in chemical and mineral composition (Vassilev et al. 2013). Biomass ashes could contain harmful substances that may impact their use, but they also contained valuable nutrients like calcium (Ca), magnesium (Mg), potassium (K), and phosphorus (P), which could qualify them for agricultural use as fertilizers or raw materials for fertilizer production (Wang et al. 2021; Zając et al. 2018).

In 2020, global sunflower production exceeded 50 million tonnes, with around 9.2 million tonnes produced in the European Union (US Department of Agriculture 2021). Sunflower husks were mainly used for pellets, granules, and lignocellulosic composites, which burned with minimal emission of organic compounds into the atmosphere (Sharapov 2013; Sprichez et al. 2019). The sunflower husk constituted approximately 20% to 60% of the seed weight, depending on the variety, and its utilization in energy production remained limited (Maj et al. 2017; Perea-Moreno et al. 2018). The sunflower husk typically contained around 3% ash content (Grompone 2005). Notably, sunflower husks possessed low moisture content (less than 10%), leading to reduced pre-drying time and increased pre-firing energy (Zajemska et al. 2017). Due to their high calorific value (21 MJ/kg), relatively low chlorine content, and resistance to biodegradation (attributed to high lignin content—34.17 wt.%), sunflower husk pellets were frequently employed in coal-fired power plants (Chiyanzu 2014; Kałużyński et al. 2017).

Walnut shells presented a potential source of biomass. According to various studies, global walnut shell production ranged from 2 million to 3.7 million tonnes annually (Queirós et al. 2020; Miladinović et al. 2020). Walnut shells contained cellulose (17.74%), hemicellulose (36.06%), and lignin (36.90%), constituting nearly 40% of the total weight of walnuts (Jahanban-Esfahlan et al. 2020).

Apple pomace waste accounted for 10 to 25% of the processed raw material weight. It constituted an impermanent and unstable material, characterized by high water content (up to 73% in apple pomace) that could lead to rapid growth of microbiological contamination. Worldwide, efforts were made to transform as much waste as possible into useful products, including energy production through microbial processes (Tarko et al. 2012; Nwosu et al. 2014). In Poland, pomace was predominantly processed into cattle feed or used for spirit production. It also found application in the production of diet drinks, wines, fruit teas, ice cream, lozenges, snacks, dietary supplements, meat products, instant products, and confectionery, increasing their content of polyphenolic compounds and fibre (Masiarz et al. 2019).

The diverse range of biomass available to individual customers necessitated comprehensive knowledge of its properties, impact on the firing process and heating system, and environmental implications.

The study presented results from elemental analysis of biomass, as well as the analysis of elemental and mineral composition of biomass ashes (apple pomace, walnut shell, and sunflower husk). The objective was to determine the suitability of these raw materials for combustion at temperatures up to a maximum of 400 ± 15 °C in individual household furnaces, particularly stoker-fired boilers. The ash analysis provided insights into the suitability of biomass for pellet production and firing in individual households. Additionally, attempts were made to assess the indicators of PM10 and PM2.5 dust emissions into the atmosphere resulting from the combustion of the tested biomass raw materials, aiming to determine their potential environmental impact. Given the energy crisis and the limited alternative fuels apart from wood in relation to hard coal, the search for new possibilities of firing various biomass in individual household furnaces was of utmost importance.

Materials and methods

Sample preparation

Waste from the food industry of Polish producers of sunflower husks (SH), walnut shells (WS) and apple pomace (AP) was used in the tests. Biomass samples were taken following the PN-EN ISO 18135:2017-06 (2017) standard: Solid biofuels–Sampling.

Three types of raw biomass were selected for analysis, and their elemental composition was examined through elemental analysis with each analysis repeated three times. Subsequently, 18 ashes resulting from combustion at 400 ± 15 °C were studied. The fresh biomass samples were dried at room temperature and ground using a SM 300 cutting mill (Retsch).

The biomass firing process took place at a temperature of 400 ± 15 °C. The firing temperature was adopted in such a way as to best reflect the firing technology in individual heating systems. Then, the ash samples were ground and prepared for further analyses. Samples for analysis in scanning electron microscopy were prepared in the form of polished sections.

Sample analysis

Biomass samples were subjected to elemental analysis in which total moisture, heat of firing, ash content, total sulphur, carbon, hydrogen, nitrogen and chlorine were determined. The analyses were performed at the Central Laboratory “ENERGOPOMIAR” in Gliwice (Poland). Analyses for carbon, hydrogen, and nitrogen content were performed by the PN-EN ISO 16948:2015-07 (2015) standard. An automatic IR analyser was used to determine the carbon, total sulphur, and hydrogen content. Total sulphur and chlorine were measured following the PN-EN ISO 16994:2016 (2016) standard. The nitrogen content was determined using the catharometric method of chlorine by ion chromatography (IC). The net calorific value was determined through calculation from the results of elemental and proximate analysis of individual samples. Elementary CHNSO FlashSmart series analyser (Thermo Scientific) was used for analyses of oxygen (O). Weight method in accordance with PN-EN ISO 18122:2016-06 (2016) was used to determined ash content (A). Total moisture content (W) in selected wood species was determined using the oven-drying method in accordance with PN-EN ISO 18134-2:2017-03 (2017). The content of volatile parts (Vdaf) was determined using the weight method in accordance with PN-EN ISO 18,123:2016-01 (2016), and the calorific value (Q) was calculated in accordance with PN-EN ISO 18125:2017-07 (2017).

Main elements (Si, Al, Ca, Mg, K, Na, Fe, P, Ti and S) and PTEs (As, Cr, Zn, Cd, Cu, Ni, Pb, Tl, U and Th) in ashes from combustion of biomass were determined by atomic emission spectrometry with excitation in induced plasma and mass spectrometry (ICP-OES/MS) (Bureau Veritas Laboratory, Canada). Biomass ash was analysed using STD CDV-1, STD V16, STD OREAS 45H, and STD OREAS 501D reference materials.

The mineral composition of ashes from biomass was determined by X-ray diffraction at the Institute of Earth Sciences (Faculty of Natural Sciences). The ash mineral composition was determined by X-ray diffraction using an X’Pert Pro MPD (multi-purpose diffractometer) PW3040/60 X-ray diffractometer from the PANalytical company, Almelo, Holland. Measurement conditions were lamp power of 40 kV voltage and 40 mA current, analysis range from 3 to 75° 2Θ, metre stroke of 0.01° Θ and pulse count time of 100 s. The weight fraction of minerals was estimated using the Rietveld Method module in the HighScore + software (version 4.9) coupled with the ICSD 2015 and ICDD PDF 4 + 2018 database.

The Quanta 250 scanning electron microscope with the EDS UltraDry X-ray microanalyser (Thermo Fisher Scientific) was used to observe the morphology and determine the phase composition of ash particles from biomass. Analyses were performed in a high vacuum using a voltage of 15 keV to accelerate ashes. BSE images were recorded, in which, apart from the habit and grain size, the chemical composition variation was visible both between and within the sample grains. EDS microanalysis was performed at selected sample points to determine the chemical composition of the grains present in the samples. Analyses were undertaken at the Laboratory of Scanning Microscopy at the Faculty of Natural Sciences of the University of Silesia.

Results and discussion

Elemental analysis of biomass

The results of the elemental analysis of biomass are presented in Fig. 1. In walnut shells, lower moisture content (9.4%) was determined as compared to the remaining biomass samples. Walnut shells are also characterized by the lowest ash content (0.76%) and higher calorific value (16.9 MJ/kg) compared to other biomass samples. Apple pomace is characterized by a higher content of volatile parts (80.6%) and nitrogen (1.44%) (Fig. 1), and lower chlorine content (0.026%) (Fig. 1) compared to other biomass samples tested. Sunflower husk ash has an average chlorine content of 0.051%, which is higher than other biofuels such as apple pomace and walnut shells.

Fig. 1
figure 1

Average values (dry matter) of basic parameters of biomass

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In the apple pomace biomass, the calorific value is 16.1 MJ/kg and is lower compared to the determinations made by other authors. Alves et al. (2020) determined ash and calorific value in apple pomace at the level of 1.73% and 18.72 MJ/kg, respectively, moisture content—7.88 ± 1.18%, and nitrogen content—1.56 ± 0.05%. Özyuğuran and Yaman (2017) determined ash in the amount of 2.31% and a high calorific value of 19.85 MJ/kg in the residue after processing apples. Guardia et al. (2019) determined ash at 2.3% in apple waste, while nitrogen content was 0.5%.

In walnut shells, the average calorific value is 16.9 MJ/kg and the ash content is 0.76%. The ash content in walnut shells ranges from 0.7 to 3.76% and was determined by various authors (Özyuğuran and Yaman 2017; Savage 2001; Zając et al. 2018; Queirós et al. 2020), determined the calorific value as 20.03 MJ/kg. The ash content is influenced by organic and mineral matter as well as pollutants, which are most often related to the cultivation area and the quality of the environment. European walnuts contain 51.2% C, 5.8% H, 0.10% N and 0.14% S on average (Wolfová et al. 2013).

The analysed sunflower husks contain a higher ash content (2.95%) compared to the other tested biomass samples. Authors from various research centres determined the ash content in sunflower husks in the range of 2.1–11.2% (Perea-Moreno et al. 2018; Zając et al. 2019; Isemin et al. 2020; Kienzl et al. 2021).

The chemical composition of biomass used for energy purposes is qualitatively the same as that of hard coal. The differences that occur between the tested samples are based on the different proportions of individual elements and chemical compounds they consist of. The reason for such biomass composition is the high content of volatile matter, and thus high reactivity. In the elemental chemical composition of biomass, C, O, H, N, S and Cl appear in descending order. The presence of sulphur, nitrogen, and chlorine in biomass is undesirable due to their harmful effects on the natural environment. Additionally, chlorine and sulphur corrode the furnaces in which they are burned.

Chemical composition of ashes

The chemical oxide composition of ashes from combustion of biomass differs depending on its type. Ash samples from combustion of AP biomass are characterized by a higher concentration of main elements compared to WS and HS biomass (Fig. 2), which can be ranked as follows: SiO2 > Al2O3 > K2O > CaO > Fe2O3 > P2O5 > SO3 > MgO > Na2O > TiO2.

Fig. 2
figure 2

Average concentrations of primary elements (%) in samples of ashes from biomass combustion at a temperature of 400 ± 15 °C

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Low concentrations of CaO, K2O and P2O5 in relation to AP and SH biomass ash were determined in WS biomass ash. In this ash sample, the lowest ash content was also determined–0.76% (Fig. 1). Zając et al. (2018) determined very low concentrations of Ca, Fe, K and P in samples of apple pomace, walnut shell and sunflower husk ash samples. Vassilev et al. (2014) and Hills et al. (2020) determined a high concentration of potassium in ashes from walnut shells and sunflower husks, which proves the different composition of the same type of biomass. Vassilev et al. (2014) found the highest concentration of K2O in ashes from firing of sunflower husks and walnut shells, 49.84% and 49.33%, respectively. The high aluminium content in AP and WS biomass ash samples is probably due to environmental conditions, mainly soil and atmosphere pollution. In urban agglomerations, it was found that the chemical composition of dusts includes, among others, aluminium and iron which come from firing fossil fuels and from rail transport. The very high content of SiO2 in biomass ash is partly related to contamination of the biomass via soil and emissions from combustion.

The chemical composition of biomass ashes is influenced by factors, such as the type of biomass, the use of artificial fertilizers for cultivation, the growing process and conditions, moisture content, land contamination, soil type, pH, geographical location–e.g. proximity to the sea or distance from the source of pollution (highways, industry, fuel combustion), biomass combustion (fuel preparation, firing technique and conditions), collection, transport and storage of biomass, and inorganic components present in fired biomass (Vassilev et al. 2013; 2014; Gianoncelli et al. 2013). An important factor influencing the composition of biomass ashes is the relatively low melting point (Gianoncelli et al. 2013). Despite variations in the chemical composition of biomass ashes, the fuel type remains the most critical factor for a specific combustion technique. Depending on the temperature and heating rate that the test material reaches, significant differences can be observed in the chemical composition of ashes from the agricultural and food industry (Gawlicki et al. 2018).

Ashes from biomass combustion may contain various concentrations of PTEs, e.g. As, Cd, Cr, Cu, Hg, Ni, Pb and Zn (Mirowski et al. 2018; Berra et al. 2019; Cuenca et al. 2013), depending on its kind. Figure 3 presents the results of analyses of PTE concentrations in ashes from combustion of biomass.

Fig. 3
figure 3

Average concentration of PTEs (mg/kg) in ashes from combustion of biomass

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AP and SH biomass ashes are enriched with potentially toxic elements compared to the WS biomass ash samples. Ash from AP biomass contains higher concentrations of Zn, Cu, Ni, U and Th, while ash from SH biomass—of As, Cr, Pb and Tl (Fig. 3). Higher concentrations of As (28.7–55.7 mg/kg), Ni (63.8–241 mg/kg) and Pb (186–328 mg/kg) were determined in samples of ashes from AP, WS and SH biomass, compared to the data available in the literature (Table 1) (Vassilev et al. 2010, 2012, 2013; Zając et al. 2018, 2019; Queirós et al. 2020). The Tl content cannot be compared with the literature data due to the lack of such data. If we compare the mean values of Zn, Pb, Ni, and Cu concentrations in samples of biomass ashes, we find that their order is as follows—AP and WS: Zn > Pb > Ni > Cu, SH: Zn > Pb > Cu > Cr. In the case of biomass cultivated on polluted sites, the concentration of various elements in ash (temperature of experimental ashing: 475 °C) could be very high, e.g. Zn above 1 wt%, Pb up to 2800 ppm (Michalik et al. 2013). Data regarding the chemical composition of the ashes obtained from the literature sources should be approached with caution since the chemical composition of the ashes from biomass combustion may significantly differ, even when plants of the same species are burned.

Table 1 PTE value in ashes from biomass combustion, according to various authors
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Biomass ash toxicity indicator

In the ashes of some PTEs (Cd, Cr, Cu, Pb, Ni, and Zn), toxicity indices were calculated using the modified formula of Cruz et al. (2019) (1):

$$ {\text{BATI}} = { }\frac{{{\text{BATImax}} - {\text{PTE}}}}{{{\text{BATImax}}}} $$
(1)

where BATImax–limit value for potentially toxic elements: Cd (0.7 mg/kg), Cr and Cu (70 mg/kg), Pb (45 mg/kg), Ni (25 mg/kg), Zn (200 mg/kg) (Cruz et al. 2019); PTE–amount of the potentially toxic element in the sample; Higher BATI indices were determined in the WS (~ 1) ash sample, except for Cu (0.87) (Fig. 4). In the ash samples from the SH biomass, lower indicators of the toxic elements, mainly Cu, Cd and Ni, were determined, and in the AP ash sample–arsenic. Negative values of the toxicity index indicate that it is not possible to deposit such ash in the environment.

Fig. 4
figure 4

Values of the toxicity indicator (BATI) in ashes from biomass combustion

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Emission of pollutants to the atmosphere

In the National Centre for Balancing and Management of Emissions (KOBiZE) (Poland), product indicators are developed for the emission of individual pollutants per unit of electricity production. The indicators can be used, among others, to calculate the achieved reduction of pollutant emissions or to assess the emission intensity of an installation against the average national indicator.

The total amount of emissions includes emissions reported to the National Database on emissions of greenhouse gases and other substances from fuel firing installations that in 2021 produced only electricity, or electricity and heat. All fuels, including renewable ones, reported to the National Database as used in firing processes and responsible for the emissions of the pollutants under consideration, were taken into account.

Estimated emission values (E) of total dust, PM10 and PM2.5 were calculated for the tested samples. These estimations were calculated to determine the emissions of total dust, PM10, and PM2.5, with the goal of indicating the type of biomass with the best parameters for the combustion process. The formula (2) provided by the National Centre for Atmospheric Balancing and Pollution (KOBiZE), Poland, (2021) was used. The amount of emissions was calculated per unit of energy (1GJ) on the basis of the following relationship:

$$ E = B \cdot W_{0} \cdot W\left[ {{\text{kg}}} \right] $$
(2)

where E—emission of substances [kg], B—fuel consumption [Mg], W0—fuel calorific value [MJ/kg], W—emission factor [g/GJ] according to the report by KOBiZE (2021), for small furnaces with a nominal heat output ≤ 0.5 MW (traditional furnaces with manual fuel feed)

Emission factors given by KOBiZE include parameters, such as total dust, PM10, PM2.5, CO2, CO, NOx/NO2, SOx/SO2 and benzo(a)pyrene, and relate to traditional furnaces with manual fuel feed with nominal heat output ≤ 0.5 MW.

Total dust, PM10 and PM2.5 emissions from firing of AP, WS and SH biomass are at a similar value level (Table 2). The primary emission standard for domestic boilers is the PN-EN 303-5:20,121-09 (2021) standard. This standard establishes emission classes, with Class 5 being the most stringent. Carbon monoxide, tar, dust emissions, and the minimum required efficiency are standardized. When solid biomass combustion contributes to the PM already present in the ambient air from other sources, IT can lead to adverse health effects. Inhalable PM smaller than 10 µm (PM10) is a crucial indicator of the health impacts of air pollution. There is strong evidence indicating that exposure to PM10 and PM2.5 can cause cardiopulmonary and cardiovascular diseases, ultimately resulting in higher mortality rates (Nussbaumer 2017).

Table 2 Estimated emission of dust pollutants (kg) to the atmosphere from combustion of biomass at a temperature of 400 ± 15 °C
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Mineral and phase composition of ashes from combustion of biomass

The identification of minerals by X-ray diffraction in the tested ash samples from combustion of biomass was made difficult by the high content of unburned organic matter and a very large number of peaks on diffractograms that overlap each other.

The most diverse in terms of minerals is the ash from combustion of sunflower husks (SH), in which quartz [SiO2], dolomite [MgCa(CO)3], calcite [CaCO3] containing Mg, sylvite [KCl], arcanite [K2SO4], fairchildite [(K2Ca(CO3)2] and archerite [(K,NH4)H2PO4] were determined. Amorphous substance (unburned carbonaceous and glassy substance) and quartz (sample AP) were determined in WS and AP ashes samples (Fig. 5). Minerals such as quartz, calcite, arcanite, fairchildyte, and archerite are common in the mineral composition of biomass ashes. The only difference is the amount of biomass ash present in individual samples. Differences in the mineral composition of ashes primarily occur due to the type of biomass. The inorganic amorphous phase in the ashes can lead to their greater leachability, thus limiting their economic use.

Fig. 5
figure 5

Diffractograms and mineral composition of AP, WS and SH biomass ash

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The differences in the phase composition result mainly from the type of fired biomass, which is also confirmed by the morphology of the analysed ash particles and their aggregates. Porous and fibrous forms were rarely observed, as presented in their research by Gowman et al. (2019).

Figures 6, 7 and 8 show examples of the morphology of the determined solid ash particles from combustion of biomass.

Fig. 6
figure 6

SEM image showing phase in ash from biomass apple pomace combustion: A quartz, B a mixture of archerite and iron oxides, C K and Mg phosphate (spherical form); D a spherical shape of iron oxides (bright ball) and aluminosilicates

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Fig. 7
figure 7

SEM images of particles in ash from combustion walnut shell biomass at 400 ± 15 °C

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Fig. 8
figure 8

SEM image showing phase in ash from biomass sunflower husk combustion: A a spherical form of iron oxide; B tissue structure with sylvite; C magnesium silicate; D tissue structure with lime

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In the sample of ashes from the combustion of apple pomace (AP), quartz grains, the size of which did not exceed 50 μm (Fig. 6A), aggregates consisting of archerite and iron oxides (Fig. 6B) (the percentages of potassium, phosphorus, and oxygen indicate the presence of archerite. The rest of the substance is hydrogen and nitrogen) and spherical forms of iron oxides with aluminosilicates (Fig. 6D) were observed. The K and Mg phosphate grains also exist in single forms (Fig. 6C), and their size does not exceed 50 µm.

Figure 7 shows grains of quartz (Fig. 7A) and magnesium calcite (Fig. 7B) which were most commonly observed in the walnut shell (WS) ash. The sample was difficult to analyse in scanning electron microscopy, because most of the analysed ash particles are very small (up to 1 µm).

Figure 8 presents exemplary images of mineral phases observed in ash from the combustion of sunflower husks (SH). This sample is the most diverse in terms of the morphology and phase composition of ash particles. Tissue structures were more frequently observed in this ash sample.

Figure 8A shows a spherical form of iron oxide with a diameter of about 12 µm; mainly iron oxides with sizes up to 5 µm were observed. Sylvite (Fig. 8B) has a form of crust and fills the edges of the tissue structure. Magnesium silicate (Fig. 8C) observed in the ashes from combustion of SH biomass is most often crushed and has irregular forms. There is a large amount of unburned tissue in the SH ash sample with CaO (lime) (Fig. 8D) and K2O.

Conclusion

Ashes from the combustion of biomass differ in their chemical and mineral composition. Apple pomace ash (AP) contains higher nitrogen content (1.58%). The presence of significant amounts of elements (chlorine, sulphur and alkali elements—Na, K, Mg) in biomass raw materials may pose a threat to the heating system (observations during the experiment).

The type of analysed biomass influences the content of PTEs in ash. Higher concentrations of Zn, Cu, Ni, U and Th were determined in ash from AP biomass, and As, Cr, Pb and Tl in ash from SH biomass. The analysis of PTE concentrations in ashes from agricultural and food biomass indicates that the use of such biomass may be challenging in individual home furnaces due to elevated concentrations of certain elements. These results are obtained from the laboratory production of ashes and may show some differences from those produced in home furnaces. It is recommended to include a larger sample size and survey data on exposure to ashes from agri-food biomass.

The calculated values of total dust emissions, PM10 and PM2.5, are very similar for ashes from agri-food biomass. A characteristic feature of biomass is its high content of volatiles (68.3–80.6%) and high reactivity. As a consequence, there is a higher proportion of emitted PM10 and PM2.5 dust particles. An unfavourable feature of biomass is its variable moisture content, which depends on the type of biomass (from 9.4 to 11.6%), which also affects the combustion processes.

Minerals such as quartz, calcite, arcanite, fairchildite, and archerite are common in the mineral composition of biomass ashes. The only difference is the amount of biomass ash present in individual samples. Scanning electron microscopy analyses showed differentiation of morphological forms of biomass ash particles (the largest in AP and WS biomass ash).

The variability in the chemical and mineral composition of ashes from the combustion of biomass indicates the need to continue research because the presence of some mineral phases and the content of PTEs will not always lead to a reduction in environmental burden, e.g. as a result of storing the produced ash.

It is probably possible to increase the share of biomass in the combustion process but under the condition of constant monitoring of the chemical and mineral composition of the produced ash. Low calorific value cannot determine the firing of the tested biomass raw materials in the form of pellets or granules in households, similar to the concentration of some PTEs. Further research in this area should be carried out and the possibility of obtaining biomass from ecologically clean regions should be sought in order to compare the research results. New technological solutions for the firing of such biomass in individual households should also be taken into account in order to reduce the amount of pollutants emitted.