Analysis of the Energy Potential of Hazelnut Husk Depending on the Variety

Abstract

Interest in bioenergy, in particular the use of biomass, has increased significantly in recent years due to increasing climate and economic concerns. As one of the key renewable energy sources, biomass plays an important role in the new energy framework. The aim of this research was to estimate the mass of woody husks and to check the influence of morphological features of selected hazelnut varieties on the energy parameters of waste biomass in the form of husk. Technical and elemental analyses were carried out on the husks of four varieties: ‘Kataloński’, ‘Olbrzymi z Halle’, ‘Olga’, and ‘Webba Cenny’, taking into account their weight, moisture content, heat of combustion (HHV and LHV), and pollutant emission factors (CO, CO₂, NO_x, SO₂, Dust). Research has shown significant differences between the varieties in terms of their energy potential and pollutant emissions. The varieties ‘Olbrzymi z Halle’ and ‘Olga’ were found to have higher calorific values, making them more energy efficient. On the other hand, the varieties ‘Kataloński’ and ‘Webba Cenny’ showed lower dust and NO_x emissions, which is beneficial from an environmental point of view. The analysis of the chemical and morphological composition of hazelnut husks allowed for the identification of relationships between morphological features and energy value and emission indicators. The conclusions from the conducted research suggest that hazelnut husks have significant potential as an energy raw material. The selection of an appropriate variety for energy crops should take into account both the calorific value and emission indicators, which will allow for the optimization of production processes and the promotion of sustainable development.

1. Introduction

Interest in bioenergy, in particular the use of biomass, has gained importance in recent years due to growing climate and economic problems. The growing global demand for energy, the high costs of fossil fuels, and the resulting environmental problems have prompted intensified research on renewable energy sources, including waste biomass [1,2]. Biomass, as one of the key renewable energy sources, plays an important role in the new energy framework. It can be a starting material for the production of biofuels, and one of the main benefits of its energy processing is the reduction of greenhouse gas emissions, including carbon dioxide and toxic components of exhaust gases. In the context of growing energy demand, the use of agricultural waste biomass as an alternative to fossil fuels can significantly reduce energy dependence on external sources, while contributing to promoting sustainable development. The development of biomass processing technologies, i.e., production of pellets or bio-oils, enables the more effective use of agricultural waste. Modern technologies allow for the optimal use of available resources, increasing energy efficiency and minimizing waste [3,4].

Agricultural residues are produced in significant quantities around the world, and their high energy content makes them an attractive energy raw material, not subject to temporary constraints, as is the case with solar or wind energy [5].

Biomass accounts for 10–14% of the world’s total energy demand. After forests, the agricultural sector is the largest supplier of biomass for energy production. A total of 140 billion tons of agricultural biomass is produced each year and needs to be properly managed. The most commonly used types of biomass are waste wood, straw and other crop residues, energy crops, and other organic waste.

Hazelnuts are one of the main nut crops in the world. They grow in many parts of Europe and Asia. Turkey is the undisputed leader in hazelnut production, supplying about 70% of the world’s supplies, followed by Italy and Spain, although to a much lesser extent. These three countries dominate the market mainly due to favorable climatic and soil conditions, which are ideal for growing hazel [6]. In Poland, several dozen hectares of new hazel plantings are added every year, and its growing popularity and demand contribute to the increase in hazelnut production. This is a response to the growing demand both on the domestic and European markets. Currently, the total area of hazel cultivation in Poland covers approximately 5500 ha.

Hazelnut is sold unshelled or shelled, as well as with a green cover, which provides additional aesthetic and protective value for the nuts. In addition to being directly consumed for its health-promoting properties, it has a number of uses as a raw material for various food, confectionery, or cosmetic products [7,8,9,10]. However, only the kernel is used, not its husk, which can constitute from 25% to 67% of the entire hazelnut depending on the variety. This makes it a major by-product of the nut industry. The hazelnut shell is the outer part of the shell, which binds it to the branch via the pericarp covering. It consists of (with some differences between researchers) lignin (23–25.9%), cellulose (26–15.4%), hemicellulose (30–22.4%), extractive substances (3.3–24.6%), and ash (0.9–5%) [6,11]. Therefore, during the production and processing of hazelnuts, significant amounts of waste in the form of shells are generated, and its final management generates the problem of costs and/or pollution. Uncertainty of production, due to climatic conditions or diseases, can be a great incentive to diversify crops, thinking more about the use of the whole plant, so that producers can better compensate for economic losses. This makes the issue of finding alternative uses for by-products even more important [12,13].

These varieties are widely grown in Poland, which facilitates access to research material. The popularity of the varieties also indicates their acceptance by local farmers and the food industry, which increases the practical usefulness of the research results. The selected varieties represent different morphological traits, i.e., nut size, shell thickness, and kernel content, allowing the influence of these traits on energy value and emissions to be studied.

The selection of the specific varieties ‘Kataloński’, ‘Olbrzym z Halle’, ‘Webba Cenny’, and ‘Olga’ for the study of the energy potential of hazelnut shells was justified by their morphological diversity, popularity (comparing the popular varieties such as ‘Kataloński’, ‘Olbrzym z Halle’, and ‘Webba Cenny’ with less common ones like ‘Olga’), energy potential, pollutant emissions, results of previous studies, and industrial and consumer preferences. This allows for comprehensive and practically useful results.

The choice of varieties could also have been based on previous research or literature indicating that these particular varieties have varying energy potential, which is crucial to obtain a complete picture of their suitability as energy biomass.

The aim of the research was to estimate the weight of woody shells for selected hazelnut varieties and to determine the quality parameters of the fuel by performing technical and elemental analysis, as well as determining the heat of combustion and calorific value. Moreover, the study aimed to assess emission factors, i.e., CO, CO₂, SO₂, NO_x, and dust, to determine the impact of potential bio-waste on the environment during the combustion process. An analysis of the exhaust gas composition was also carried out based on estimates from stoichiometric equations.

2. Materials and Methods

This study aimed to check the influence of morphological features of selected hazelnut varieties on the energy parameters of waste biomass in the form of woody shells obtained during the harvesting and processing of hazelnuts. Field tests were carried out in temperate climate conditions in 2023 at a private horticultural farm located in the Sandomierska Upland (50°49′20.5″ N, 21°44′35.0″ E, Zawichost commune, Świętokrzyskie Voivodeship). The experimental material consisted of ungrafted hazel bushes growing on their own roots of four varieties: ‘Kataloński’, ‘Olbrzymi z Halle’, ‘Olga’, and ‘Webba Cenny’. The bushes were planted in 2002 in a row cultivation system, with a spacing of 6 × 2.5 m, 666 pieces per hectare, on loess-based soils (class II and IIab). Samples for analysis were taken at full ripeness of fruit from 3 randomly selected bushes, 4 samples for each variety, which allowed us to determine the average value for the tested parameters. Immediately after collection, the samples were weighed with an accuracy of 0.001 kg, and then, after separating the kernel from the woody husk, their weight was determined on a PS R2 RADWAG precision scale (balance (RADWAG, Radom, Poland).

As part of the analysis, the parameters of the total yield of hazelnuts (husk and kernel) and the weight of 100 husks, one bush and per unit area of 1 ha were examined.

The energy and emission parameters of the tested materials were also assessed. Detailed research procedures are presented in

Table 1. A summary of the methods and apparatuses used for the energy and carbon analysis of the raw materials studied.

	Parameter	Standard
Proximate analysis	Higher heating value (HHV; MJ·kg−1)	EN-ISO 1928:2020; Equipment LECO AC 600 [14]
	Lower heating value (LHV; MJ·kg−1)
	Ash (A; %)	EN-ISO 18122:2022; Equipment LECO TGA 701 [15]
	Volatile matter (V; %)	EN-ISO 18123:2023; Equipment LECO TGA 701 [16]
	Moisture (M; %)	EN-ISO 18134-1:2022; Equipment LECO TGA 701 [17]
	Fixed carbon (FC; %)	FC=100-V-A-M [18]
Ultimate analysis (emission factors calculated according to studies [19])	Carbon (C; %)	EN-ISO 16948:2015-07, Equipment LECO CHNS 628 [20]
	Hydrogen (H; %)
	Nitrogen (N; %)
	Sulfur (S; %)	EN-ISO 16994:2016-10; Equipment LECO CHNS 628 [21]
	Oxygen (O; %)	O=100-A-H-C-S-N [22]
Emission factors (exhaust gas composition was calculated according to [23,24])	Carbon monoxide emission factor (Ec) of chemically pure coal (CO; kg·Mg−1)	$CO={\displaystyle \frac{28}{12}}·E_c·(C/\mathrm{CO}),$ CO—carbon monoxide emission factor (kg∙kg−1), ${\displaystyle \frac{28}{12}}$—molar mass ratio of carbon monoxide and carbon, EC—emission factor of chemically pure coal (kg∙kg−1), C/CO—part of the carbon emitted as CO (for biomass 0.06)
	Carbon dioxide emission factor (CO2; kg·Mg−1)	$C{O}_2={\displaystyle \frac{44}{12}}·\left(E_c-{\displaystyle \frac{12}{28}}·CO-{\displaystyle \frac{12}{16}}·E_{C{H}_4}-{\displaystyle \frac{26.4}{31.4}}·E_{NMVOC}\right),$ CO2—carbon dioxide emission factor (kg∙kg−1), molar mass ratio of carbon dioxide and pure coal, molar mass ratio of carbon dioxide and carbon monoxide, molar mass ratio of carbon and methane, ECH4—methane emission factor, ENMVOC—emission index of non-methane VOCs (for biomass 0.009)
	Sulphur dioxide emission factor (SO2; kg·Mg−1)	$S{O}_2={\displaystyle \frac{2S}{100}}·\left(1-r\right),$ SO2—sulfur dioxide emission factor (kg∙kg−1), 2—molar mass ratio of SO2 and sulfur, S—sulfur content in fuel (%), r—coefficient determining the part of total sulfur retained in the ash
	Emission factor was calculated from (NOX; kg·Mg−1)	$N{O}_x={\displaystyle \frac{46}{14}}·E_c·N/C·\left(N_{N{O}_x}/N\right)$, NOx—NOx emission factor (kg∙kg−1), molar mass ratio of nitrogen dioxide to nitrogen; the molar mass of nitrogen dioxide was considered due to the fact that nitrogen oxide in the air oxidizes very soon to nitrogen dioxide, N/C—nitrogen to carbon ratio in biomass, $N_{N{O}_x}$/N—part of nitrogen emitted as NOx (for biomass 0.122).
Exhaust gas composition	Theoretical oxygen demand (VO2; Nm3·kg−1)	$V_{O_2}={\displaystyle \frac{22.41}{100}}·\left({\displaystyle \frac{C}{12}}+{\displaystyle \frac{H}{4}}+{\displaystyle \frac{S-O}{32}}\right)$, C—biomass carbon content (%), H—biomass hydrogen content (%), S—biomass sulfur content (%), O—biomass oxygen content)
	The stoichiometric volume of dry air required to burn 1 kg of biomass (Voa; Nm3·kg−1)	$V_{O_a}={\displaystyle \frac{V_{O_2}}{0.21}},$ Since the oxygen content in the air is 21%, which participates in the combustion process in the boiler, the stoichiometric volume of dry air required to burn 1 kg of biomass
	Carbon dioxide content of the combustion products (VCO2; Nm3·kg−1)	$V_{C{O}_2}={\displaystyle \frac{22.41}{12}}$$· {\displaystyle \frac{C}{100}},$
	Content of sulfur dioxide (VSO2; Nm3·kg−1)	$V_{S{O}_2}={\displaystyle \frac{22.41}{32}}$$· {\displaystyle \frac{S}{100}}$,
	Water vapor content of the exhaust gas (VH2O; Nm3·kg−1)	$V_{H_{2}O}^H={\displaystyle \frac{22.41}{100}}·\left({\displaystyle \frac{H}{2}}+{\displaystyle \frac{M}{18}}\right)$ is the component of water vapor volume from the hydrogen combustion process $(V_{H_{2}O}^H$; ${\mathrm{Nm}}^3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1} \mathrm{fuel}) V_{H_{2}O}^a=1.61·x·V_{oa}$ and the volume of moisture contained in the combustion air $(V_{H_{2}O}^a$; ${\mathrm{Nm}}^3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1} \mathrm{fuel}) V_{H2O}=V_{H2O}^H+V_{H2O}^a$; M-fuel moisture content (%), air absolute humidity (kg H2O·kg−1 dry air)
	The theoretical nitrogen content in the exhaust gas $(V_{N2}$; Nm3 ·kg−1)	$V_{N2}={\displaystyle \frac{22.41}{28}}·{\displaystyle \frac{N}{100}}+0.79·V_{oa}$, Considering that the nitrogen in the exhaust comes from the fuel composition and the combustion air, and the nitrogen content in the air is 79%
	The total stoichiometric volume of dry exhaust gas $(V_{gu};$Nm3 ·kg−1)	$V_{gu}=V_{CO2}+V_{SO2}+V_{N2}$
	The total volume of exhaust gases $(V_{ga}$; Nm3 ·kg−1)	$V_{ga}=V_{gu}+V_{H2O}$ Assuming that biomass combustion is carried out under stoichiometric conditions, i.e., using the minimum amount of air required for combustion (λ = 1), a minimum exhaust gas volume will be obtained

.

3. Results and Discussion

This study analyzed the diversity of hazelnut varieties in terms of their energy properties and the productivity of waste biomass in the form of woody shells. Converting this waste into new energy raw materials is an important step towards sustainable agriculture and the growing demand for green energy sources.

Varieties, i.e., ‘Kataloński’, ‘Olbrzymi z Halle’, ‘Olga’, and ‘Webba Cenny’, were analyzed to determine the mass of woody hazelnut husks and their energy potential.

Figure 1 shows the percentage of husks in hazelnut yield for selected varieties (for 100 pcs.).

Table 2. Quantitative analysis of hazelnut shell and kernel depending on the variety tested.

Parameter		Hazelnut Variety				p-Value
		‘Kataloński’	‘Olbrzymi z Halle’	‘Olga’	‘Webba Cenny’
100 pcs. (g)	husk	209.33 ± 7.02 b *	248.00 ± 21.00 a	201.00 ± 3.46 b	203.67 ± 10.69 b	0.0054
	kernel	195.67 ± 1.53 b	207.00 ± 9.54 b	241.00 ± 6.24 a	197.00 ± 6.24 b	0.0001
1 bush (kg)	husk	2.95 ± 0.10 a	2.83 ± 0.24 ab	2.50 ± 0.04 bc	2.44 ± 0.13 a	0.0068
	kernel	2.75 ± 0.02 b	2.37 ± 0.11 c	3.00 ± 0.08 a	2.36 ± 0.07 c	0.0001
Yeld (t·ha−1)	husk	1.96 ± 0.07 a	1.89 ± 0.16 ab	1.67 ± 0.03 bc	1.62 ± 0.09 a	0.0049
	kernel	1.83 ± 0.01 b	1.58 ± 0.07 c	2.00 ± 0.05 a	1.57 ± 0.05 c	0.0001

* Significant difference a, b, c—different letters in a row indicate significant differences at α = 0.05.

shows the kernel and woody husk weight of the different hazelnut varieties analyzed at three levels for 100 units (g), 1 bush (kg), and per unit area (t/h). The results showed significant differences in husk and kernel weight in the categories evaluated between the varieties studied.

Analyzing the husk weight for 100 pieces, the values ranged from 201.00 g for the cultivar ‘Olga’ to 248.00 g for the cultivar ‘Olbrzymi z Halle’. The difference between the lowest and highest value was 47 g per 100 g, indicating a noticeable variation in husk weight between varieties. The percentage of husk to total nut weight for ‘Olga’ was 45.5 percent, for ‘Olbrzymi z Halle’ was 54.5 percent, for ‘Kataloński’ was 51.7 percent, and for ‘Webba Cenny’ was 50.8 percent (Figure 1). These figures are important when assessing the amount of waste biomass harvested for energy use. On the other hand, the kernel weight for 100 nuts oscillated from 195.67 g for ‘Kataloński’ to 241.00 g for ‘Olga’, giving a difference of 45.33 g between varieties. It is worth noting that the highest kernel weight was found in the ‘Olga’ cultivar, which may suggest its preference as a seed source with a higher edible content. At this level, the differences were statistically significant, indicating a significant effect of cultivar on these parameters.

With regard to the category of one bush, the husk weight was lowest for the cultivar ‘Webba Cenny’ at 2.44 kg, and the highest value was 2.95 kg for the cultivar ‘Kataloński’. The difference between the lowest and highest value was 0.51 kg per bush, indicating a noticeable variation in husk weight between varieties. For kernel weight per bush, values ranged from 2.36 kg for ‘Webba Cenny’ to 3.00 kg for ‘Olga’. The difference between the lowest and highest value was 0.64 kg per bush. This was a statistically significant difference, indicating a significant effect of variety on these parameters, which may influence consumer and processing preferences.

Analysis of the data at the level of husk weight per unit area shows fluctuating values, with the lowest 1.62 t/h recorded for the ‘Webba Cenny’ variety and the highest 1.96 t/h for the ‘Kataloński’ variety. The differences between the masses thus amount to 0.34 t/h. In contrast, the kernel weight ranged from 1.57 t/h also for the cultivar ‘Webba Cenny’ to 2.00 t/h for the cultivar ‘Olga’. The difference between the weights was 0.43 t/h. These values further confirm that variety has a significant influence on yield performance.

The results showed that the kernel and husk weight of hazelnuts varied significantly between varieties. The cultivar ‘Olbrzymi z Halle’ had the heaviest husk in the 100 g and one bush categories, while the cultivar ‘Olga’ had the heaviest kernels in the same categories. The lightest husks in the 100 g and one bush category were recorded for the ‘Olga’ variety and the lightest kernels for the ‘Kataloński’ variety. For yield per hectare, the ‘Kataloński’ variety had the heaviest husk, while the ‘Olga’ variety had the heaviest kernels. These differences were statistically significant, highlighting the importance of choosing the right variety to optimize hazelnut yield and quality.

Important factors modifying the quality assessment of hazelnuts may be habitat conditions [13,25,26,27,28], variety [29], and harvest date [30]. Our observations and analytical results partly confirm these opinions. In the present study, a significant effect of variety on kernel weight was shown for ‘Kataloński’, ‘Olbrzymi z Halle’, and ‘Webba Cenny’ hazelnuts. The study by Król [31] showed a significant effect of cultivar on hazelnut kernel weight. It was shown that the kernels of the cultivar ‘Webba Cenny’ were significantly heaviest among those assessed, while those of ‘Nottinghamski’ were significantly lightest. No significant effect of cultivar on hazelnut kernel weight was shown for ‘Barcelona’, ‘Cosford’, ‘Kataloński’, and ‘Olbrzymi z Halle’.

In their study, Güler and Balta (2020) [32] showed that the ratio of kernel to whole nut fruit ranged from 41.16% to 58.53% in the 35 genotypes studied, and from 47.29% to 53.70% in the ‘Delisava’, ‘Karayağlı’, and ‘Yomra’ varieties. This confirms the influence of varietal traits on nut structure. Similar conclusions were reached by Ferrão et al. (2021) [33] investigating the percentage of kernel content according to variety. The results they obtained allowed them to divide the varieties into three groups: The first group includes the varieties ‘Grada de Viseu’ (44.14 ± 6.24%), ‘Gunslebert’ (44.23 ± 9.07%), ‘Tonda de Giffoni’ (46.19 ± 4.86%), and ‘Butler’ (53.16 ± 12.39%); the second group includes ‘Segorbe’ (57.27 ± 9.17%); and the third group includes ‘Negreta’ (63.91 ± 12.05%) and ‘Longa de Espanha’ (67.68 ± 10.33%).

Comparing our own results with literature data on morphological traits, it can be observed that hazelnuts grown under Polish climatic conditions are characterized by higher nut weight, while kernel weight is quite similar to the results obtained by Özdemir and Akinci (2004) [34]. In a study by Ciemieniewska and Ratusz (2012) [35], the percentage of kernel was calculated on the basis of in-shell and shelled nut weight determinations. For the cultivars ‘Kataloński’, ‘Webba Cenny’, and ‘Cosford’ the values were, respectively, 43.41 ± 0.60, 45.18 ± 0.96, and 46.44 ± 1.26. The results obtained in the present study indicate that the kernel weight per 100 pieces for the ‘Kataloński’ variety was 195.67 g and the shelling percentage for this variety was 51.7%. In contrast, Ciemieniewska and Ratusz (2012) [35] report that the kernel percentage in the nut for this variety was 43.41 ± 0.60%. If we compare the kernel percentages, the results obtained in the present study indicate a higher kernel percentage for the ‘Kataloński’ variety (51.7%) compared to the results of Ciemieniewska and Ratusz [35] (43.41%).

The dendrogram illustrates the cluster analysis for hazelnut varieties tested for the amount of waste biomass produced. The structure of the dendrogram distinguishes two main clusters, which allows for a deeper understanding of the differences in production yields between varieties (Figure 2).

The first cluster brings together the cultivars ‘Olbrzymi z Halle’ and ‘Kataloński’. The proximity of these varieties on the dendrogram suggests that they produce similar amounts of waste biomass, which is relatively lower compared to the other varieties. Low waste production may be advantageous in situations that require waste to be minimized, such as in intensively managed growing environments or in spatially restricted areas. It is possible that these varieties will be favored in sustainable agriculture where waste reduction is important.

The second cluster is formed by ‘Olga’ and ‘Webba Cenny’, which are characterized by producing more waste biomass. The higher biomass productivity of these varieties can be used for energy purposes, where more biomass translates into better energy yields. These varieties may therefore be more desirable in the context of energy production.

The cluster analysis shown on the dendrogram provides valuable information on the differences in the amount of waste biomass produced by different hazelnut varieties. This knowledge is important for matching varieties to specific operational needs and sustainable natural resource management strategies. The use of such data can help optimize production and energy processes, offering clear directions for research on the selection and cultivation of hazelnut varieties in different application environments.

Table 3. Technical and elemental analysis of hazelnut shells depending on the variety.

Parameter	Hazelnut Variety				p-Value
	‘Kataloński’	‘Olbrzymi z Halle’	‘Olga’	‘Webba Cenny’
LHV (MJ·kg−1)	17.35 ± 0.08 a *	17.40 ± 0.03 a	17.39 ± 0.02 a	17.33 ± 0.05 a	0.3328
HHV (MJ·kg−1)	18.63 ± 0.08 a	18.68 ± 0.03 a	18.68 ± 0.02 a	18.61 ± 0.05 a	0.3235
M (%)	8.64 ± 0.04 b	8.85 ± 0.02 ab	8.9 ± 0.12 a	9.01 ± 0.12 a	0.0064
V (%)	68.28 ± 0.10 ab	68.54 ± 0.08 a	67.96 ± 0.18 b	67.2 ± 0.37 c	0.0004
A (V%)	0.73 ± 0.09 b	0.77 ± 0.02 b	0.79 ± 0.06 b	0.99 ± 0.05 a	0.0005
FC (V%)	23.85 ± 2.68 a	21.83 ± 0.06 a	22.34 ± 0.12 a	22.8 ± 0.2000 a	0.3625
C (V%)	46.23 ± 0.09 ab	47.03 ± 0.61 a	46.55 ± 0.12 ab	46.17 ± 0.06 b	0.0372
H (V%)	7.52 ± 0.02 a	7.25 ± 0.36 a	7.58 ± 0.02 a	7.5 ± 0.01 a	0.2098
N (V%)	0.30 ± 0.01 a	0.35 ± 0.08 a	0.29 ± 0.06 a	0.36 ± 0.02 a	0.3243
S (V%)	0.01 ± 0 ab	0.01 ± 0.00 b	0.01 ± 0.00 ab	0.02 ± 0.00 a	0.0421
O (V%)	45.19 ± 0.06 a	44.58 ± 0.42 a	44.77 ± 0.24 a	44.97 ± 0.10 a	0.0776
H/C	1.63 ± 0.0008 a	1.54 ± 0.0937 a	1.63 ± 0.0017 a	1.62 ± 0.0022 a	0.149
N/C	0.01 ± 0.0001 a	0.01 ± 0.0016 a	0.01 ± 0.0012 a	0.01 ± 0.0003 a	0.2986
O/C	0.73 ± 0.0021 a	0.71 ± 0.0156 a	0.72 ± 0.0057 a	0.73 ± 0.0024 a	0.0474

* Significant difference a, b, c—different letters in a row indicate significant differences at α = 0.05.

shows the results of a comparative technical and elemental analysis of woody hazelnut husks depending on the variety used in hazelnut cultivation. The parameters analyzed include lower heat of combustion (LHV), higher heat of combustion (HHV), moisture content (M), volatile content (V), ash content (A), and solid content (FC), as well as elemental composition of carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O). In addition, the analysis includes ratios, i.e., hydrogen to carbon (H/C), nitrogen to carbon (N/C), and oxygen to carbon (O/C).

The LHV value ranged from 17.33 MJ·kg⁻¹for the cultivar ‘Webba Cenny’ to 17.4 MJ·kg⁻¹ for ‘Olbrzymi z Halle’, indicating little difference in biomass energy potential between cultivars. The highest HHV value was recorded for the cultivar ‘Olga’ (18.7 MJ·kg⁻¹), suggesting its higher potential as energy biomass. The study showed that hazelnut husk has significant potential as an energy biomass. The heating values (HHV and LHV) of the different hazelnut varieties are similar to other types of woody biomass, making them an attractive energy feedstock. The results obtained are consistent with the studies presented in paper [5]. Lower calorific values for the husk were obtained in the work [36], while higher values were obtained in the research of [6]. Comparing the energy value with other types of plant biomass, it can be shown that the tested hazelnut shells had higher LHV compared to hazelnut husk [37] and hazelnut tree pellets [38], as well as lower HHV compared to hazelnut tree leaves [39]. It should be noted that the tested hazelnut shells had significantly higher energy parameters than agrobiomass [40,41,42], but they were similar to woody biomass [43,44]. Comparing the results obtained for the cultivars ‘Webba Cenny’ and ‘Katalonski’, an average of 1.2% lower HHV results were obtained than those presented for these cultivars by Hebda et. al. [45], but the same when comparing LHV. Moisture content, a key parameter affecting combustion quality, also showed differences, with the highest value of 9.01% for ‘Webba Cenny’ and the lowest 8.33% for ‘Olga’.

The volatile compound content was highest for ‘Olga’ (67.96%) and lowest for ‘Olbrzymi z Halle’ (65.4%), which may affect the ease of ignition and combustion characteristics of the biomass. In this case, the results obtained were consistent with hazelnut husks [37], as well as much lower ones such as oak tree branches, pine chips, rye straw, or miscanthus [39,46]. The ash content, which is important for assessing biomass quality in terms of the waste left after burning, varied from 0.73% for ‘Kataloński’ to 0.99% for ‘Webba Cenny’, which also affects the energy properties. Comparing the data obtained for the varieties tested with data from the literature, it was found that approximately 1.5% lower values were obtained in relation to the hazelnut husk presented in paper [39] and approximately 1% higher from paper [27]. Comparable ash content was shown for beech [37]. When comparing the data for the cultivar ‘Webba Cenny’, an ash content of 0.2% lower, and for the cultivar ‘Kataloński’, an ash content of 0.7% lower than that presented in paper [45] was obtained.

The elemental indices H/C, N/C, and O/C are important for assessing combustion properties and greenhouse gas emissions. These values are comparable between cultivars, with slight differences in scale, indicating a similar chemical structure of the biomass between cultivars.

Differences in the technical and elemental analysis parameters of woody hazelnut shells indicate the different energy potential of different varieties, which may be important when selecting a variety for energy crops. Cultivars, i.e., ‘Olga’ and ‘Olbrzymi z Halle’, show higher combustion heat values and may be preferred for energy biomass production, while differences in moisture and ash content between cultivars may influence decisions related to logistics and combustion technology.

The dendrogram (Figure 3) shows how varieties are similar or different in terms of their biomass energy potential. The cluster structure comprises two main groups. The first cluster is formed by the varieties ‘Webba Cenny’ and ‘Kataloński’, which are placed close to each other, suggesting that their woody peels have similar energy potential. The short distance between these varieties on the dendrogram indicates that they have similar HHV values, which may be beneficial in contexts where continuity of biomass energy value is desirable.

The second cluster includes the varieties ‘Olga’ and ‘Olbrzymi z Halle’, which also form a common group, but are significantly further away from the first cluster. This indicates that these varieties may have higher HHV values compared to the first cluster, making them potentially more valuable in terms of biomass energy production.

Table 4. Emission parameters for woody hazelnut husk depending on the variety used in hazelnut cultivation.

Parameter	Hazelnut Variety				p-Value
	‘Kataloński’	‘Olbrzymi z Halle’	‘Olga’	‘Webba Cenny’
CO (kg·Mg−1)	56.96 ± 0.11 ab *	57.94 ± 0.75 a	57.35 ± 0.15 ab	56.88 ± 0.07 b	0.0372
CO2 (kg·Mg−1)	1394.78 ± 2.74 ab	1418.83 ± 18.46 a	1404.48 ± 3.67 ab	1392.97 ± 1.67 b	0.0372
NOx (kg·Mg−1)	1.08 ± 0.02 a	1.23 ± 0.28 a	1.02 ± 0.20 a	1.26 ± 0.05 a	0.3243
SO2 (kg·Mg−1)	0.03 ± 0.00 ab	0.02 ± 0.00 b	0.03 ± 0.01 ab	0.03 ± 0.001 a	0.0421
Dust (kg·Mg−1)	0.93 ± 0.12 b	0.98 ± 0.03 b	1.00 ± 0.08 b	1.25 ± 0.07 a	0.0051

* Significant difference a, b, c—different letters in a row indicate significant differences at α = 0.05.

shows the results of the emissions analysis for woody hazelnut husks depending on the variety used in hazelnut cultivation. The parameters analyzed include emissions of carbon monoxide (CO), carbon dioxide (CO₂), nitrogen oxides (NO_x), sulfur dioxide (SO₂), and dust (Dust).

Carbon monoxide (CO) emissions ranged from 56.88 kg·Mg⁻¹ for the cultivar ‘Webba Cenny’ to 57.94 kg·Mg⁻¹ for ‘Olbrzymi z Halle’, with statistically significant differences between cultivars, indicating differences in biomass combustion processes. The values for carbon dioxide (CO₂) emissions were highest for the cultivar ‘Olbrzymi z Halle’ (1418.83 kg·Mg⁻¹) and lowest for ‘Webba Cenny’ (1392.97 kg·Mg⁻¹), also reflecting differences in chemical composition and biomass combustion efficiency between cultivars.

Nitrogen oxide (NO_x) emissions showed less significant differences, with the highest value of 1.26 kg·Mg⁻¹ for ‘Webba Cenny’ and the lowest value of 1.02 kg·Mg⁻¹ for ‘Olga’, suggesting little variation in the nitrogen content of the biomass.

Sulfur dioxide (SO₂) was emitted at very low levels by all cultivars, possibly indicating the low sulfur content of the shells.

Dust emissions, on the other hand, varied significantly, with the highest value of 1.25 kg·Mg⁻¹ for ‘Webba Cenny’, which may indicate a greater tendency to generate fine particles during combustion of this variety compared to the others. The lowest dust values were recorded for the ‘Kataloński’ variety (0.93 kg·Mg⁻¹).

In the case of environmental impact, it should be noted that the use of hazelnut husk can lead to a reduction in emissions compared to fossil fuels, such as hard coal [19]. In the case of CO, the use of husk as fuel leads to a reduction in emissions of this gas compared to hard coal, depending on the variety, by 69–71%; CO₂ by 70–72%; NO_x by 25–30%; SO₂ by 98–99%; and dust by 94–95%.The results obtained for emission factors indicate that the cultivars tested show similar emission levels to Eucalyptus globulus wood [1] and larch needles [19]. Furthermore, the results obtained for CO are consistent with hazel tree leaves, oak tree leaves, or maple tree leaves [47]. For CO₂, similarities were shown by oak tree leaves for ‘Olbrzymi z Halle’, and other cultivars with maple tree leaves and hazelnut tree leaves [47]. The peels of all analyzed cultivars showed lower SO₂ and NO_x emissions compared to hazelnut, oak, walnut, and maple leaves [47].

There are significant differences in emissions between hazelnut varieties, which may influence decisions on the choice of variety for energy crops, taking into account environmental aspects and combustion efficiency. The ‘Webba Cenny’ variety, despite having higher dust emissions, also shows higher NO_x emissions, which may be a factor to consider when assessing environmental impact and energy efficiency.

The analytical results presented in

Table 5. Composition of woody hazelnut husk fumes according to the variety used in hazelnut cultivation.

Parameter	Hazelnut Variety				p-Value
	‘Kataloński’	‘Olbrzymi z Halle’	‘Olga’	‘Webba Cenny’
VoO2 (Nm3·kg−1)	0.97 ± 0.003 a *	0.97 ± 0.013 a	0.98 ± 0.005 a	0.97 ± 0.002 a	0.1894
Voa (Nm3·kg−1)	4.61 ± 0.01 a	4.63 ± 0.06 a	4.67 ± 0.02 a	4.61 ± 0.01 a	0.1894
VCO2 (Nm3·kg−1)	0.86 ± 0.002 ab	0.88 ± 0.011 a	0.87 ± 0.002 ab	0.86 ± 0.001 b	0.0002
VSO2 (Nm3·kg−1)	0.00 ± 0.00 a	0.00 ± 0.00 b	0.00 ± 0.00 a	0.00 ± 0.00 a	0.0001
VH2O (Nm3·kg−1)	0.95 ± 0.002 a	0.92 ± 0.04 a	0.96 ± 0.001 a	0.95 ± 0.001 a	0.2074
VN2 (Nm3·kg−1)	3.89 ± 0.01 a	3.94 ± 0.07 a	3.92 ± 0.06 a	3.93 ± 0.02 a	0.3386
Vgu (Nm3·kg−1)	4.75 ± 0.02 a	4.81 ± 0.10 a	4.79 ± 0.07 a	4.79 ± 0.02 a	0.2082
Vga (Nm3·kg−1)	6.44 ± 0.03 a	6.48 ± 0.15 a	6.50 ± 0.08 a	6.48 ± 0.03 a	0.5286

* Significant difference a, b, c—different letters in a row indicate significant differences at α = 0.05.

show the composition of the exhaust gas of woody hazelnut husks depending on the variety used in hazelnut cultivation. The analysis includes eight key parameters related to the combustion process: V_O2, V_oa, V_CO2, V_SO2, V_H2O, V_N2, V_gu, and V_ga, expressed in normal cubic meters per kilogram (Nm³·kg⁻¹).

The analyzed parameters in Table 5 show many similarities between the tested varieties. Their values are very similar and show no statistically significant differences, suggesting uniform combustion conditions. The exception is the V_CO2 parameter.

The greatest variations in parameter values can be seen in the case of dry flue gas volume (V_gu) and wet flue gas volume (V_ga). The Vgu values for the hazelnut varieties analyzed ranged from 4.75 Nm³·kg⁻¹ for the ‘Katalońki’ variety to 4.81 Nm³·kg⁻¹ for the ‘Olbrzymi z Halle’ variety. These variations indicate some differences in the amount of dry combustion gases produced during the combustion of the different hazelnut varieties. A similar variation can be observed for the volume of wet flue gases (V_ga). The ‘Kataloński’ variety had the lowest value (6.44 Nm³·kg⁻¹), while the ‘Olga’ variety had the highest value (6.50 Nm³·kg⁻¹). These variations also indicate differences in the amount of moist flue gas generated during combustion of the different varieties. All V_SO2 values were equal to 0 or very close to zero, suggesting that sulfur dioxide emissions from these varieties are minimal or virtually non-existent.

The V_CO2 parameter shows a statistically significant difference between the varieties, which may have been due to differences in the chemical composition of the biomass. The variety ‘Olbrzymi z Halle’ had the highest value of 0.88 Nm³·kg⁻¹, while the variety ‘Webba Cenny’ had the lowest value of 0.86 Nm³·kg⁻¹. These differences can affect combustion efficiency and CO₂ emissions, which are crucial from an environmental impact assessment point of view.

The results obtained for the theoretical amount of dry flue gases were 16% higher in relation to Czech knotweed, Meadowhay, or Timothy grass [48] for the cultivar ‘Olbrzymi z Halle’ and 17% for the cultivar ‘Olga’, showing the lowest and highest emissions among the tested cultivars. Similar emissions in relation to the tested varieties were shown for Jatropha shells and were 1.46 Nm³·kg⁻¹ higher in comparison to rice husk [49]. The cultivar ‘Olga’ showed identical dry emissions to Poplar bark [50]. Thus, it can be concluded that the examined husks, regardless of variety, generated similar emissions as agrobiomass.

Despite the overall uniformity of the combustion parameters, some differences can be observed in the V_gu and V_ga values, which may be related to differences in the chemical composition and structure of the biomass of the tested cultivars. The V_gu and V_ga values show greater variation, suggesting that some varieties may generate more dry and wet combustion gases than others. These results confirm that each of the tested varieties can be effectively used as biofuel, due to minimal sulfur dioxide emissions and similar values for key combustion parameters. The uniform combustion conditions and minimal differences between varieties suggest that the selection of a particular variety can be based on other criteria, i.e., feedstock availability or processing preference.

The correlation analysis for the woody husk of selected varieties shows the influence of varietal characteristics on the variation of the combustion parameters, elements, and emission factors studied (

Table 6. Analysis of the relationship of combustion parameter values, elements, and emission factors to morphological characteristics of the woody husk for each variety assessed.

Parameter	Hazelnut Variety
	‘Kataloński’	‘Olbrzymi z Halle’	‘Olga’	‘Webba Cenny’
HHV (MJ·kg−1)	−0.35714	0.91813	−0.73876	0.86603
LHV (MJ·kg−1)	−0.35692	0.91214	−0.79127	0.86184
C (%)	−0.66684	0.44676	0.09095	−0.65616
H (%)	−0.59002	−0.8783	−0.32791	0.67284
N (%)	−0.12064	0.5402	0.20129	−0.96555
S (%)	0.3459	0.4789	0.5	0.3376
M (%)	−0.93089	−0.8417	−0.35473	−0.78611
O (%)	0.96336	0.02192	0.11973	0.96259
A (%)	0.16935	−0.43984	−0.77592	−0.89492
V (%)	0.94095	0.42984	0.18228	0.76962
FC	0.83929	−0.21254	0.48755	−0.72882
WEc	−0.66684	0.44676	0.09095	−0.65616
WEco	−0.66684	0.44676	0.09095	−0.65616
WNOx	−0.12064	0.5402	0.20129	−0.96555
WECO2	−0.66684	0.44676	0.09095	−0.65616
WESO2	0.3698	0.2874	0.5	0.3376
WEdust	0.16935	−0.43984	−0.77592	−0.89492

).

For the cultivar ‘Kataloński’, the morphological characteristics of the woody shells showed a weak negative correlation with HHV (−0.35714 MJ·kg⁻¹) and LHV (−0.35692 MJ·kg⁻¹), suggesting that the morphological characteristics of this cultivar do not have a major impact on calorific values. For carbon (C) and hydrogen (H) content, there was a moderately negative relationship, −0.66684% and −0.59002%, respectively. This indicates that the morphological characteristics of this variety are associated with a lower content of these elements, which may reduce biomass energy efficiency. A very strong negative correlation was observed for moisture content (−0.93089%), indicating that higher moisture content is strongly associated with the morphological characteristics of the variety. Higher moisture content may reduce the energy value of the biomass, as increased water content reduces the amount of energy available for combustion. There was a very strong positive correlation with oxygen content (0.96336%) and volatile matter content (0.94095%), indicating that the morphological characteristics of this variety have an effect on their higher content. A higher oxygen content may indicate a higher number of oxidizing compounds in the biomass, which may affect its combustion efficiency, as oxygen is necessary for the combustion process. Volatiles, on the other hand, are organic compounds that readily evaporate and burn at high temperatures. Their higher content can affect the ease of ignition and combustion characteristics of biomass, which is beneficial for efficient combustion. A strong positive correlation also exists for the FC content (0.83929%). A higher content of this parameter is beneficial for combustion, as a higher amount of combustible material increases the energy efficiency of biomass. A moderate negative correlation was observed for carbon (−0.66684%), carbon monoxide (−0.66684%), and carbon dioxide (−0.66684%) emission factors. These correlations suggest that the morphological characteristics of this variety may be associated with lower emissions of these pollutants, which is ecologically beneficial. Weak positive correlations were found for sulfur content (0.3459%), sulfur dioxide emission factor (0.3698%), dust emission factor (0.16935%), and ash (0.16935%). The content of these parameters can affect combustion characteristics and air pollution. On the other hand, the ‘Kataloński’ variety showed weak negative correlations in relation to N (−0.12064%) and the W_NOx index (−0.12064%). Although these correlations were weak, they suggest that the morphological characteristics of the ‘Kataloński’ variety may have a slight effect on reducing nitrogen content and nitrogen oxide emissions, which is positive for the environment.

In the case of the cultivar ‘Olbrzymi z Halle’, the morphological characteristics of the woody shells showed a very strong positive correlation with the values of HHV (0.91813 MJ·kg⁻¹) and LHV (0.91214 MJ·kg⁻¹). This means that the physical characteristics of this variety influence higher calorific values, which is beneficial from the point of view of biomass energy efficiency. For hydrogen content (H) and moisture content (M), a very strong negative correlation was shown, −0.8783% and −0.8417%, respectively. This indicates that varietal traits are associated with lower hydrogen content and moisture content. A lower hydrogen content may be beneficial as hydrogen contributes less to calorific value compared to coal. Conversely, lower moisture content means less water in the biomass, which improves its combustion efficiency as less energy is lost to water evaporation. For nitrogen (N) content and W_NOx emissions, a moderate positive correlation was observed, 0.5402% and 0.5402% respectively. These values suggest that the morphological characteristics of the cultivar are associated with higher nitrogen content and higher emissions of these pollutants. Higher nitrogen content may lead to higher NO_x emissions, which is less favorable from an environmental point of view. The moderate negative correlation for dust (−0.43984%) and ash (−0.43984%) emission rates suggests that the morphological characteristics of this variety are associated with lower dust and ash emissions. This implies less air pollution and less strain on filter systems. For WE_c, WE_co, and WE_CO2, a weak positive correlation of 0.44676% was shown for all parameters. This means that the morphological traits of the cultivar had a small but positive effect on the content of energy accumulated in biomass (WEc), energy available for combustion (WE_co), and energy released as CO₂ (WE_CO2).

In turn, the varietal characteristics of ‘Olga’ showed a strong negative correlation with the calorific values of HHV (−0.73876 MJ·kg⁻¹) and LHV (−0.79127 MJ·kg⁻¹). This means that the physical characteristics of this variety influence lower calorific values, which is unfavorable from the point of view of biomass energy efficiency. Furthermore, the varietal characteristics of ‘Olga’ showed a strong positive correlation with ash content (0.90995%). Higher ash content is disadvantageous as it increases the amount of waste after combustion. High ash content can lead to more frequent boiler cleaning and higher maintenance costs for combustion systems, which can be challenging in an operational and economic context. Additionally, the varietal characteristics of ‘Olga’ were strongly negatively correlated with dust emissions (−0.77592%). This means that the physical characteristics of this variety are associated with lower dust emissions. Lower dust emissions may lead to less air pollution, which is beneficial from an environmental point of view. Less sophisticated filtration systems may be necessary, which is associated with lower costs and easier compliance with environmental standards. A weak negative relationship was shown between H (−0.32791%) and M (−0.35473%) and varietal characteristics. A lower hydrogen content can lower the calorific value of biomass, and a lower moisture content is beneficial as it reduces the amount of energy required to evaporate water during combustion, which can improve the efficiency of the combustion process.

In contrast, the varietal traits ‘Webba Cenny’ correlated strongly positively with HHV (0.86603 MJ·kg⁻¹), LHV (0.86184 MJ·kg⁻¹), and V (0.76962%) and very strongly with O (0.96259%). The higher oxygen content may be beneficial for combustion processes, but the very high volatile content may indicate specific mineral properties of the biomass. For the hydrogen content H, a moderate positive correlation of 0.67284% was observed, suggesting that the varietal characteristics of ‘Webba Cenny’ are related to hydrogen content. Hydrogen, although less energy efficient than coal, also contributes to the calorific value of biomass. For N and W_NOx emissions, on the other hand, we observed a very strong negative correlation of −0.96555% for both parameters. This means that the physical properties of this variety are associated with lower NO_x emissions, which is beneficial from an environmental point of view. Lower NO_x emissions are desirable, due to their harmful effects on health and the environment. In addition, strong negative correlations were found for moisture content M (−0.78611%), ash content A (−0.89492%), FC content (−0.72882%), and dust emissions WE_dust (−0.89492%), indicating that the varietal characteristics of ‘Webba Cenny’ are associated with lower content of these components and lower dust emissions. The lower moisture content improves combustion efficiency, and the lower ash content reduces post-combustion waste, which is beneficial from an operational and economic point of view. Moderate negative correlations were observed for C content (−0.65616%), WE_c content (−0.65616%), WE_co content (−0.65616%), and WE_CO2 content (−0.65616%), suggesting that the varietal characteristics of ‘Webba Cenny’ are associated with the lower content of these components and emissions of these gases, which may be beneficial from an environmental point of view. A weak positive relationship of 0.3376% was shown for S and WE_SO2. This means that the morphological characteristics of this variety have a slight positive effect on the sulfur content and sulfur dioxide emission factor.

The energy efficiency of the varieties ‘Olbrzymi z Halle’ and ‘Webba Cenny’ was found to be higher due to their high calorific values, making them more energy efficient. In contrast, the varieties ‘Kataloński’ and ‘Olga’ had lower calorific values, which may affect their lower energy efficiency. The higher ash content of the ‘Kataloński’ and ‘Olga’ varieties increased the amount of waste after combustion, which can lead to higher operating costs. In contrast, lower NO_x and dust emissions in the ‘Webba Cenny’ and ‘Kataloński’ varieties are environmentally beneficial as they reduce air pollution. The varieties ‘Olbrzymi z Halle’ and ‘Olga’ emit higher amounts of NO_x, which is undesirable from an environmental point of view. The lower moisture content in the varieties ‘Webba Cenny’ and ‘Olbrzymi z Halle’ was found to improve the combustion process, as less energy is lost to evaporate water, which increases energy efficiency. The higher oxygen content of the ‘Webba Cenny’ variety may benefit combustion processes, although the very high volatile matter content may indicate the specific mineral properties of this biomass.

In conclusion, the varieties ‘Olbrzymi z Halle’ and ‘Webba Cenny’ may have higher energy efficiencies, but their potential impact on pollutant emissions needs careful monitoring. In contrast, the varieties ‘Kataloński’ and ‘Olga’, although less energy efficient, may emit lower amounts of some pollutants, which is beneficial from an environmental perspective.

4. Conclusions

• There are significant differences between hazelnut varieties in terms of lower calorific value. Varieties such as ‘Olbrzymi z Halle’ (17.40 MJ·kg⁻¹; 18.68 MJ·kg⁻¹) and ‘Olga’ (17.39 MJ·kg⁻¹; 18.68 MJ·kg⁻¹) show higher calorific values, making them more energy-efficient compared to ‘Kataloński’ (17.35 MJ·kg⁻¹; 18.63 MJ·kg⁻¹) and ‘Webba Cenny’ (17.33 MJ·kg⁻¹; 18.61 MJ·kg⁻¹).
• The morphological characteristics of the hazelnut shells influence their energy value and emission factors. The ‘Webba Cenny’ (9.01%) variety is characterized by higher moisture content, which negatively affects the biomass energy value. The ‘Kataloński’ (8.64%) variety, on the other hand, showed lower moisture content and ash content, which is beneficial from the point of view of combustion efficiency.
• The analysis of emissions during the combustion of hazelnut shells showed different results depending on the variety. The ‘Olbrzymi z Halle’ variety showed the highest CO (57.94 kg·Mg⁻¹) and CO₂ (1418.83 kg·Mg⁻¹) emissions, while the ‘Webba Cenny’ variety had higher dust and NO_x (1.26 kg·Mg⁻¹) emissions. Overall, however, emissions were comparable to other biomass types, and SO₂ content was minimal.
• Analysis of the mass of shells on an area of 1 ha, depending on the variety, showed the lowest mass for the ‘Webba Cenny’ variety (1.62 t·h⁻¹) and the highest for the ‘Catalan’ variety (1.96 t·h⁻¹).
• The selection of a suitable variety for energy crops should take into account both calorific value and emission factors. The varieties ‘Olbrzymi z Halle’ and ‘Olga’ may be preferred due to their higher calorific value, even though they may generate higher emissions. In contrast, the varieties ‘Kataloński’ and ‘Webba Cenny’ may be chosen in the context of lower emissions.

In conclusion, studies have shown that hazelnut shells have great potential as an energy feedstock. The selection of a suitable variety for energy purposes should take into account both energy efficiency and environmental impact.