Energy Assessment of Hazelnut Shells (Corylus avellana L.) of Selected Turkish Varieties

Abstract

The purpose of this study was to evaluate the energy and environmental potential of waste biomass in the form of hazelnut shells from selected Turkish varieties of Corylus avellana L. Eight commercial varieties (Çakıldak, Foşa, İnce Kara, Kalın Kara, Palaz, Tombul, Yassı Badem and Yuvarlak Badem) grown in different regions of the Black Sea coast of Turkey were analyzed. The scope of this study included whole nut and shell weight determination, technical and elemental analysis, higher heating value (HHV) and lower net heating value (LHV), as well as emission factors (CO, CO₂, NO_x, SO₂, dust) and flue gas composition based on stoichiometric calculations. The results showed a significant effect of varietal characteristics on all analyzed parameters. The share of shell in the total weight of the nut ranged from 43.5% (Tombul) to 55.3% (İnce Kara). HHV values ranged from 18.37 to 19.20 MJ·kg⁻¹, and LHV from 17.05 to 17.90 MJ·kg⁻¹. The İnce Kara and Yassı Badem varieties showed the most favorable energy properties. Elemental analysis confirmed a low nitrogen and sulfur content, which translated into low NO_x and SO₂ emissions. NO_x emissions were lowest for the Tombul variety (1.43 kg·Mg⁻¹), and SO₂ emissions were close to zero in each variety. The results confirm that Turkish hazelnut shells are a valuable energy resource and can be used as solid fuel or supplementary biomass. In particular, the İnce Kara variety was identified as the most promising due to its high shell weight, very good fuel properties, and high yield potential. This study underscores the importance of selecting the right variety to optimize agricultural waste utilization strategies within a circular economy.

1. Introduction

Hazelnut (Corylus avellana L.) is one of the most important nut species grown globally [1,2,3], and Turkey remains its largest producer, accounting for more than 60% of global production. It is also the largest exporter of shelled hazelnuts, with about 67% of global exports, making hazelnuts an important part of the Turkish economy and contributing to export revenues. Turkey’s main growing regions include the Black Sea coast, where local small-fruit varieties dominate, varying widely in fruit structure, chemical composition, and biomass structure [4].

Considering their nutritional and health-promoting value, a wide range of hazelnut-based food products are available on the market, such as oil, chocolate, confectionery, meal, and peanut butter [5,6,7,8]. It is also a valuable raw material not only in the kitchen [9,10]. Nevertheless, hazelnut processing generates a significant number of by-products and waste biomass, including nut shells [11,12], which, due to their high lignin content and low moisture content, can be used as an effective biofuel [13,14,15,16].

Previous research has focused on analyzing the fuel properties of shells from industrial blends or European varieties. Comprehensive comparative data are lacking for local Turkish varieties, which differ significantly in structure and waste biomass content. Moreover, the lack of standardization of the material causes difficulties in comparing results between studies. The use of shells as a component of solid fuels is in line with the principles of a closed-loop economy (GOZ), enabling the efficient use of waste biomass from the food industry [17,18,19,20]. Their high calorific values, low moisture content, and ash content make them suitable for use as a stand-alone feedstock or as an additive to other types of biomass (e.g., sawdust) to improve the properties of the mixture [21,22,23,24].

Despite the favorable chemical properties of hazelnut shells for direct use as solid fuel, this biomass tends to have a low bulk density, making it difficult to transport, store, and burn efficiently [25,26]. Densification into pellets or briquettes can provide a solution to reduce efficiency losses and ease fuel logistics. In addition, blending shells with other biomass components can lead to improved physicochemical properties and lower emissions due to the so-called dilution effect of emission elements [13,27].

The purpose of this study was to determine the quantitative and qualitative parameters of waste biomass in the form of shells from selected Turkish hazelnut varieties and to assess the influence of varietal characteristics on their energy and environmental properties. For this purpose, technical analysis was performed and elemental analysis, and a higher heating value (HHV) and lower heating value (LHV) were determined. In addition, pollutant emission factors (CO, CO₂, SO₂, NO_x, dust) were evaluated to determine the potential environmental impact of shell combustion [28,29,30]. This study was complemented by an analysis of flue gas composition based on stoichiometric equations to estimate the volume of gases generated. Commercially important varieties of Çakıldak, Foşa, İnce Kara, Kalın Kara, Palaz, Tombul, Yassı Badem, and Yuvarlak Badem grown by conventional methods in the eastern, central, and western Black Sea regions (Turkey), which are the center of world hazelnut production, were selected as the study material [31,32,33].

2. Materials and Methods

In 2024, field studies were conducted in subtropical climatic conditions on eight types of hazelnut varieties (Corylus avellana L.): Çakıldak, Foşa, İnce Kara, Kalın Kara, Palaz, Tombul, Yassı Badem, and Yuvarlak Badem. This study evaluated the weight of the whole nut and shell (100 pieces for each variety), energy parameters, and emission parameters for the tested material. The quality parameters of the biofuel were estimated by performing technical and elemental analysis, and the higher and lower heating values were determined. Before the tests, the material was dried in air-dry conditions at 20 °C and 55–60% air humidity for two weeks in order to standardize the evaluation conditions. Material for laboratory analysis was pulverized (0.5 mm) in the first step using a Retsch SM 100 grinder (Retsch GmbH, Haan, Germany). The methodology of the procedures is shown in

Table 1. Methods and apparatus used for energy and carbon analysis of the raw material under study.

PARAMETER	METHOD	EQUIPMENT
Energetic properties
Higher heating value (HHV; MJ·kg−1)	EN-ISO 1928:2020 [34]	Isoperibolic calorimeter LECO AC 600 (LECO Corporation, Saint Joseph, MI, USA, 2012)
Lower heating value (LHV; MJ·kg−1)
Proximate analysis
Ash (A; %)	EN-ISO 18122:2022 [35]	Thermogravimetric analyzer LECO TGA 701 (LECO Corporation, Saint Joseph, MI, USA, 2013)
Volatile matter (V; %)	EN-ISO 18123:2023 [36]
Moisture (M; %)	EN-ISO 18134:2023 [37]
Fixed carbon (FC; %)	FC = 100 − V − A − M [38]
Ultimate analysis
Carbon (C; %)	EN-ISO 16948:2015 [39]	Elemental analyzer LECO CHNS 628 (LECO Corporation, Saint Joseph, MI, USA, 2012)
Hydrogen (H; %)
Nitrogen (N; %)
Sulfur (S; %)	EN-ISO 16994:2016 [40]
Oxygen (O; %)	O = 100 − A − H − C − S − N [41]

,

Table 2. Emission factors (calculated according study [42]).

PARAMETER	METHOD AND EQUIPMENT
Carbon monoxide emission factor (Ec) of chemically pure coal (CO; kg·Mg−1)	$CO={\displaystyle \frac{28}{12}}·E_c$$·(\mathrm{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)	${CO}_2={\displaystyle \frac{44}{12}}·\left(E_c-{\displaystyle \frac{12}{28}}·CO-{\displaystyle \frac{12}{16}}·E_{{CH}_4}-{\displaystyle \frac{26.4}{31.4}}·E_{NMVOC}\right),$ 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)	${SO}_2={\displaystyle \frac{2S}{100}}·\left(1-r\right),$ 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)	${NO}_x={\displaystyle \frac{46}{14}}·E_c·N/C·\left(N_{{NO}_x}/N\right)$ , NOx—NOx emission factor (kg·kg−1)—molar mass ratio of nitrogen dioxide to nitrogen. The molar mass of nitrogen dioxide is 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, NOx/N—part of nitrogen emitted as NOx (for biomass 0.122).

and

Table 3. Exhaust gas composition (calculated according to [43,44]).

PARAMETER	METHOD AND EQUIPMENT
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).
Stoichiometric volume of dry air required to burn 1 kg of biomass (Voa; Nm3·kg−1)	$V_{O_a}={\displaystyle \frac{V_{O_2}}{0.21}},$ When 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_{{CO}_2}={\displaystyle \frac{22.41}{12}}$$·{\displaystyle \frac{C}{100}},$
Content of sulfur dioxide (VSO2; Nm3·kg−1)	$V_{{SO}_2}={\displaystyle \frac{22.41}{32}}$$·{\displaystyle \frac{S}{100}}$ ,
Water vapor content of exhaust gas (VH2O; Nm3·kg−1)	$V_{H_{2}O}^H={\displaystyle \frac{22.41}{100}}·\left({\displaystyle \frac{H}{2}}+{\displaystyle \frac{M}{18}}\right)$ , Component of water vapor volume from the hydrogen combustion process $(V_{H_{2}O}^H$$; {\mathrm{Nm}}^3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1} \mathrm{fuel}) V_{H_{2}O}^a=1.61·x·V_{oa}$ Volume of moisture contained in the combustion air $(V_{H_{2}O}^a$; ${\mathrm{Nm}}_3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1} \mathrm{fuel}) V_{H2O}$$=V_{H2O}^H$$+V_{H2O}^a$; M—fuel moisture content (%), x—air absolute humidity (kg H2O·kg−1 dry air).
Theoretical nitrogen content in 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%.
Total stoichiometric volume of dry exhaust gas ${(V}_{gu};$ Nm3·kg−1)	$V_{gu}=V_{CO2}+V_{SO2}+V_{N2}$
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.

.

Statistical analyses were performed using Statistica 13.0 software to identify differences between the studied varieties and plant types. The statistical analysis is presented as means and standard deviations. Differences and statistical significance between variables were determined using two-way analysis of variance (ANOVA) and Tukey’s test.

3. Results and Discussion

Figure 1 shows the total weight values (in arbitrary units) for 100 pieces of whole nuts depending on the variety. The analysis showed statistically significant differences between the varieties studied.

The highest average total weight was recorded for the İnce Kara variety (2.322 g) and the lowest for Foşa (1.408 g), a difference of as much as 0.914 g. Significantly higher values were also obtained for the Çakıldak (2.128 g), Yassı Badem (2.230 g), and Yuvarlak Badem (2.006 g) varieties. Varieties with the lowest average weights included Palaz (1594 g) and Kalın Kara (1538 g) in addition to Foş. The high variability within some varieties (e.g., Foşa, Palaz, İnce Kara) indicates the variation in fruit size within these genotypes.

In the present study, an important aspect was to assess the proportion of the shell in the total weight of the hazelnut, since this fraction is the main component of the waste biomass considered for energy purposes. Figure 2 shows the weight of hazelnut shell (g/100 pieces) for the eight varieties studied.

When evaluating the share of shell in the total weight of the hazelnut as a potential indicator of the suitability of the raw material for energy purposes, it is important to note the significant variation among the varieties studied. The heaviest shells were observed for the İnce Kara variety (1.10 g), while the lightest were found for the Foşa variety (0.66 g), a difference of 0.44 g. This translates into significant differences in the share of the waste fraction—the shell—in the total weight of the nut. The İnce Kara variety, with a shell share of 55.3%, represents the most favorable material from the point of view of maximizing the yield of energy feedstock. The Palaz (52.5%) and Foşa (51.7%) varieties showed an equally high proportion of shell, while the least favorable in this respect was the Tombul variety, in which the shell accounted for only 43.5% of the total weight. These results correspond with data presented in the study by Borkowska et al. [45], where the share of shell in the total weight of the nut also showed significant variation.

Table 4. Analysis of technical results of hazelnut shells depending on the variety (dry mass).

Name	HHV (MJ·kg−1)	LHV (MJ·kg−1)	MC%	A%	V%	FC
Çakıldak	19.02 ± 0.06 b	17.72 ± 0.06 b	9.96 ± 0.01 e	1.00 ± 0.02 b	66.53 ± 0.29 a	22.52 ± 0.27 a
Foşa	18.37 ± 0.09 d	17.05 ± 0.09 d	11.94 ± 0.03 a	1.29 ± 0.08 a	64.77 ± 0.39 c	22.01 ± 0.33 a
İnce Kara	18.70 ± 0.03 c	17.41 ± 0.03 c	10.28 ± 0.08 d	1.25 ± 0.01 a	66.20 ± 0.16 ab	22.27 ± 0.09 a
Kalın Kara	18.61 ± 0.09 c	17.33 ± 0.09 c	9.22 ± 0.05 f	1.29 ± 0.07 a	66.82 ± 0.38 a	22.67 ± 0.42 a
Palaz	18.67 ± 0.02 c	17.37 ± 0.02 c	10.46 ± 0.07 c	1.10 ± 0.05 b	66.02 ± 0.31 ab	22.42 ± 0.24 a
Tombul	18.91 ± 0.02 b	17.61 ± 0.02 b	10.20 ± 0.01 d	1.11 ± 0.01 b	66.22 ± 0.28 ab	22.47 ± 0.28 a
Yassı Badem	19.20 ± 0.02 c	17.90 ± 0.03 c	9.97 ±0.03 e	1.03 ± 0.02 b	66.30 ± 0.05 bc	22.70 ± 0.05 a
Yuvarlak Badem	18.63 ± 0.03 a	17.33 ± 0.04 a	10.73 ± 0.01 b	1.10 ± 0.01 b	65.47 ± 0.25 a	22.71 ± 0.05 a
p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0606

Explanations: HHV = high heating value, LHV = lower heating value, MC = moisture content, A = ash content, V = volatile matter content, FC = fixed carbon. Significant difference means that different letters in the column indicate significant differences at α = 0.05.

contains data on six key technical parameters: higher and lower heating values (HHV, LHV), moisture content (MC), ash content (A), volatile matter content (V), and bound carbon (FC) for eight hazelnut varieties. All analyzed parameters showed significant statistical differences, indicating a strong influence of varietal characteristics on the quality of the energy feedstock.

The highest level of HHV was recorded for the Yassı Badem variety and amounted to 19.20 MJ·kg⁻¹, while the lowest value was recorded for the Foşa variety—18.37 MJ·kg⁻¹. The difference between these values was 0.83 MJ·kg⁻¹, which confirms the significant variation in energy potential between the tested varieties. Similar correlations were noted for LHV, where the highest value was also obtained for the Yassı Badem variety, with 17.90 MJ·kg⁻¹, while the lowest value was obtained for the Foşa variety, which reached 17.05 MJ·kg⁻¹. The difference between the extremes was 0.85 MJ·kg⁻¹. These results are in line with an earlier study by Borkowska et al. [45], who demonstrated the influence of varietal characteristics on the energy efficiency of hazelnut shells, obtaining HHV values in the range of 18.61–18.68 MJ·kg⁻¹ and LHV values in the range of 17.33–17.40 MJ·kg⁻¹. In addition, a study by Noszczyk et al. [46] showed that HHV for the shells of various nuts can reach as high as 19.69 MJ·kg⁻¹, and LHV up to 18.73 MJ·kg⁻¹, confirming their high energy quality. In comparison, the lowest recorded values of HHV and LHV in the cited data were 17.78 MJ·kg⁻¹ and 16.53 MJ·kg⁻¹, respectively, indicating a wide range of variability depending on variety, soil and climatic conditions, and processing technology. It is also worth noting that the analyzed hazelnut shells showed significantly higher energy potential compared to typical agrobiomass [47,48,49], while they were similar in terms of parameters to wood-derived biomass [50,51], but higher than the range shown for hazelnut shoots [52].

In terms of moisture content (MC%), the Foşa variety showed the highest moisture content at 11.94%, while the lowest value was recorded for Kalın Kara at 9.22%. The difference between these varieties was 2.72%, which may affect the energy efficiency of the combustion process. In comparison, lower moisture content values for hazelnut shells were obtained by Borkowska et al. [45], indicating a range of 8.64–9.01%. Similar results were presented by Turan and Islam [53], obtaining values in the range of 8.70–9.26%, depending on the drying method. Hebda et al. [54] recorded even lower moisture levels, ranging from 5.78% to 6.79% depending on the variety. In contrast, the highest moisture content value among the referenced studies was recorded by Mladenović et al. [55], who reported 12.84% for this parameter. The moisture content of the material affects the final energy yield. Too much moisture in the fuel lowers its calorific value and makes combustion difficult. Moisture is treated as ballast, which takes away the energy needed to heat and evaporate water, instead of generating heat. Additionally, excessive moisture can lead to boiler corrosion and the formation of harmful chemical compounds. The obtained moisture content results allow for obtaining high energy values from the tested material.

As for ash content (A%), the highest amount of ash was found in the Foşa and Kalın Kara varieties at 1.29%, while the lowest value was shown by the Çakıldak variety at 1.00%. The difference between the extremely different varieties was 0.29%. Compared to previous results obtained for Polish varieties [46], the shells of Turkish varieties showed higher levels of ash, with differences reaching about 37% for the lowest and 30% for the highest values. This may be due to different environmental conditions and genotypic conditions. Güleç et al. [56] recorded higher ash values for hazelnut and almond shells—2.20% in both cases, and for chestnut shells—3.90%. In contrast, the biomass, i.e., bean shell (8.00%), cocoa husk (9.96%), or rice husk (13.70%), was characterized by a significantly higher ash content. The results obtained confirm that hazelnut shells, despite varietal differences, belong to the group of biomass with a relatively low ash content.

On the other hand, in terms of volatile content (V%), the highest value was obtained by the Kalın Kara variety, 66.82%, and the lowest by the Foşa variety, 64.77%. The difference between these varieties was 2.05%, indicating different combustion rates and efficiencies. The values obtained in the present study are comparable with the results of other authors—for example, Hebda et al. [54] and Borkowska et al. [45] in their analyses for shells of different varieties also recorded relatively similar levels of volatile parts. On the other hand, for nut shells of the Istarskiduguljasti and Rimskiokrugli varieties, the contents of volatile parts were 64.14 and 63.41%, respectively [57].

The greatest differences were observed in the bound carbon content (FC%). The highest FC content was found for the Yuvarlak Badem variety at 22.89%, while the lowest was for the Foşa variety at 22.01%. The difference was less than 1.00%, which cannot significantly affect the final calorific value and quality of biomass as fuel. These results are significantly higher compared to the literature data. Matin et al. [57] reported an FC content in the range of 17.73–18.06%, and Borkowska et al. [46]—16.34–18.37%, lower than Smith et al. [58]—24.08% and Marcantonio et al. [59]—26.39%. Significantly, in the study by Noszczyk et al. [46], for different shell types, the bound carbon contents were 13.14% for walnut, 11.39% for hazelnut, 12.57% for peanut, and 8.26% for pistachio. The obtained results regarding fixed carbon do not differ from those obtained for various types of biomass.

Table 5. Analysis of elemental results of hazelnut shells depending on the variety.

Name	C%	H%	N%	S%	O%	H/C	N/C	O/C
Çakıldak	47.32 ± 0.03 a	7.41 ± 0.03 a	0.46 ± 0.01 ab	0.03 ± 0.00 c	43.79 ± 0.04 c	1.57 ± 0.01 ab	0.01 ± 0.00 ab	0.69 ± 0.00 d
Foşa	45.69 ± 0.10 d	7.39 ± 0.05 a	0.45 ± 0.06 ab	0.03 ± 0.00 b	45.16 ± 0.16 a	1.62 ± 0.01 a	0.01 ± 0.00 ab	0.74 ± 0 a
İnce Kara	46.47 ± 0.09 c	7.37 ± 0.02 a	0.5 ± 0.02 ab	0.03 ± 0.00 d	44.39 ± 0.08 bc	1.59 ± 0.00 a	0.01 ± 0.00 ab	0.72 ± 0.00 bc
Kalın Kara	46.73 ± 0.05 bc	6.83 ± 0.51 b	0.54 ± 0.06 a	0.02 ± 0.00 f	44.58 ± 0.49 ab	1.46 ± 0.11 b	0.01 ± 0.00 a	0.72 ± 0.01 bc
Palaz	46.48 ± 0.30 c	7.25 ± 0.08 ab	0.48 ± 0.05 ab	0.02 ± 0.00 g	44.68 ± 0.29 ab	1.56 ± 0.01 ab	0.01 ± 0.00 ab	0.72 ± 0.01 b
Tombul	47.04 ± 0.01 ab	7.29 ± 0.01 ab	0.40 ± 0.00 b	0.03 ± 0.00 e	44.13 ± 0.02 bc	1.55 ± 0.00 ab	0.01 ± 0.00 b	0.70 ± 0.00 cd
Yassı Badem	47.23 ± 0.04 a	7.37 ± 0.03 a	0.44 ± 0.01 ab	0.03 ± 0.00 a	43.89 ± 0.09 c	1.56 ± 0.01 ab	0.01 ± 0.00 ab	0.70 ± 0.00 d
Yuvarlak Badem	46.51 ± 0.14 c	7.27 ± 0.05 ab	0.47 ± 0.05 ab	0.02 ± 0.00 h	44.64 ± 0.15 ab	1.56 ± 0.01 ab	0.01 ± 0.00 ab	0.72 ± 0.01 b
p-value	0.0001	0.0291	0.0212	0.0001	0.0001	0.0105	0.0226	0.0001

Explanations: C = carbon content, H = hydrogen content, N = nitrogen content, S = sulfur content, O = oxygen content, ratio of hydrogen to carbon (H/C), ratio of nitrogen to carbon (N/C), ratio of oxygen to carbon (O/C). Significant difference means that different letters in the column indicate significant differences at α = 0.05.

shows the results of elemental analysis of hazelnut wood shells for eight varieties, including carbon (C%), hydrogen (H%), nitrogen (N%), sulfur (S%), and oxygen (O%) contents, as well as H/C, N/C, and O/C molar ratios. All analyzed parameters—with the exception of the N/C ratio and hydrogen content—showed statistically significant differences, indicating a significant influence of varietal characteristics on the elemental composition of waste biomass.

The highest total carbon content (C%) was recorded in the Tombul variety (47.04%), while the lowest was in the Foşa variety (45.69%). The difference between these values is 1.35%. In a study [60], the carbon content of selected biomass varieties (olive husk and walnut, hazelnut, sunflower and almond shells) ranged from about 47.4% to 53.5% due to different lignin and extractive contents. Borkowska et al. [45] obtained a range of 46.17–47.03% for the parameter studied, while Matin et al. [57] reported lower values ranging from 57.60 to 57.95% (Table 5).

In contrast, in terms of hydrogen content (H%), the values fell within a narrower range, from 6.83% for the Kalın Kara variety to 7.41% for Çakıldak, with a difference of 0.58%. These results are similar to the range obtained in a previous study by Borkowska et al. [45], where hydrogen content ranged from 7.25% to 7.58%. In comparison, Güleç et al. [56] showed lower hydrogen values for other types of waste biomass: almond shell (5.67%), bean husk (5.38%), chestnut shell (5.17%), cocoa beans husk (5.89%), coconut shell (6.05%), hazelnut shell (6.14%), and rice husk (2.88%). In contrast, Sezer and Özveren [60] reported a value of 5.06% for hazelnut shells.

As for the nitrogen content (N%), the highest concentration was observed in the Kalın Kara variety (0.54%) and the lowest in Yassı baden (0.44%). The difference was 0.10%. The values obtained are lower than those reported by Borkowska et al. [46], where the range was 0.65–0.85%, but higher than the results presented by Noszczyk et al. (0.25–0.31%) [46], Smith et al. (0.22%) [58], and Güleç et al. [56], who reported 0.27% for hazelnut shells and 0.30% for almond. Much higher nitrogen concentrations are found in other types of biomass, i.e., cocoa bean husk (2.64%) or coffee husk (2.53%) [56]. This indicates that hazelnut shells contain a relatively low amount of nitrogen, which is beneficial for reducing nitrogen oxide (NOₓ) emissions during the combustion process.

The values recorded for sulfur (S%) were very low, ranging from 0.01% (Palaz) to 0.03% (Foşa, Çakıldak, İnce Kara, Yassı baden). The above results confirm the study of Borkowska et al. [45], which showed low concentrations of this parameter. In contrast, Marcantonio et al. [59] obtained much higher values (0.67%).

In terms of oxygen content (O%), the highest value was obtained for the Yuvartok Baden variety (44.64%) and the lowest for Çakıldak (43.79%), with a difference of 0.85. This range is consistent with previous studies by Borkowska et al. [45], where the oxygen content ranged from 44.58% to 45.19%. Similar values were also obtained by Smith et al. [58]—45.83% and Marcantonio et al. [59]—42.32%. On the other hand, Güleç et al. [56] indicated higher oxygen contents in other types of biomass, such as almond shells (47.46%), bean husk (53.98%), and chestnut shells (51.77%), which may be due to the different chemical composition of these raw materials.

The H/C ratio (hydrogen-to-carbon ratio) ranged from 1.46 (Kalın Kara) to 1.62 (Foşa), a difference of 0.16.

The N/C (nitrogen/carbon) ratio did not show variability—the values for all varieties were 0.01. This is confirmed by the results obtained by Borkowska et al. [45].

The highest O/C (oxygen/carbon) ratio was shown in the Foşa variety (0.74) and the lowest in Çakıldak (0.69), a difference of 0.05 (Table 5).

Table 6. Emission parameters for hazelnut shells depending on the variety.

Name	CO (kg·Mg−1)	NOx (kg·Mg−1)	CO2 (kg·Mg−1)	SO2 (kg·Mg−1)	Dust (kg·Mg−1)
Çakıldak	58.29 ± 0.03 a	1.64 ± 0.02 ab	1427.47 ± 0.84 a	0.05 ± 0.00 c	1.26 ± 0.02 b
Foşa	56.28 ± 0.12 d	1.59 ± 0.22 ab	1378.27 ± 2.87 d	0.06 ± 0.00 b	1.63 ± 0.10 a
İnce Kara	57.25 ± 0.11 c	1.77 ± 0.08 ab	1401.93 ± 2.65 c	0.05 ± 0.00 d	1.58 ± 0.01 a
Kalın Kara	57.57 ± 0.06 bc	1.92 ± 0.21 a	1409.79 ± 1.54 bc	0.05 ± 0.00 f	1.63 ± 0.09 a
Palaz	57.26 ± 0.37 c	1.68 ± 0.16 ab	1402.27 ± 9.13 c	0.05 ± 0.00 g	1.39 ± 0.06 b
Tombul	57.95 ± 0.02 ab	1.43 ± 0.00 b	1419.04 ± 0.42 ab	0.05 ± 0.00 e	1.40 ± 0.01 b
Yassı Badem	58.19 ± 0.05 a	1.56 ± 0.05 ab	1424.98 ± 1.32 a	0.06 ± 0.00 a	1.30 ± 0.03 b
Yuvarlak Badem	57.30 ± 0.18 c	1.66 ± 0.17 ab	1403.18 ± 4.30 c	0.04 ± 0.00 h	1.39 ± 0.01 b
p-value	0.0001	0.0001	0.0001	0.0001	0.0001

Explanation: carbon monoxide (CO), carbon dioxide (CO₂), sulfur dioxide (SO₂), nitrogen oxides (NOₓ). Significant difference means that different letters in the column indicate significant differences at α = 0.05.

shows the results of the analysis of gaseous and particulate emissions from the combustion of hazelnut shells. Emissions of carbon monoxide (CO), oxides of nitrogen (NOₓ), carbon dioxide (CO₂), sulfur dioxide (SO₂), and total dust were considered. The statistical significance shown for all parameters confirms that hazelnut varieties differ significantly in terms of pollutant emissions, which is important for assessing their energy suitability from the point of view of environmental protection.

The highest carbon monoxide (CO) emissions were recorded for the Palaz and Kalın Kara varieties, reaching a value of 56.25 kg·Mg⁻¹. The Foşa variety showed the lowest emissions of this compound, with a value of 52.89 kg·Mg⁻¹. The difference between the extreme values was 3.36 kg·Mg⁻¹, indicating moderate variation in CO emissions by variety (Table 6).

In terms of nitrogen oxide emissions (NOₓ), the highest level was achieved by the Tombul variety (7.35 kg·Mg⁻¹), while the lowest level was achieved by Foşa (6.72 kg·Mg⁻¹). The difference was 0.63 kg·Mg⁻¹, suggesting little variability in this area.

The greatest fluctuations were observed in carbon dioxide emissions (CO₂), where the highest level was recorded for the Tombul variety (1419.04 kg·Mg⁻¹) and the lowest for Foşa (1378.27 kg·Mg⁻¹). The difference was as high as 40.77 kg·Mg⁻¹, which may indicate a significant influence of varietal characteristics on the intensity of the combustion process and contribution to greenhouse gas emissions.

Sulfur dioxide emissions (SO₂) showed low variability, ranging from 0.22 kg·Mg⁻¹ (Yassı Baden) to 0.25 kg·Mg⁻¹ (Tombul), with a difference of 0.03 kg·Mg⁻¹. This parameter is closely related to the sulfur content of the biomass, which, as shown in earlier analyses, was low in all varieties.

For dust emissions, the values ranged from 9.84 kg·Mg⁻¹ (Kalın Kara) to 10.48 kg·Mg⁻¹ (Tombul), a difference of 0.64 kg·Mg⁻¹. These differences can be attributed to both the ash content of the material and the physical structure of the shells affecting the combustion process (Table 6).

Table 7. Composition of the exhaust fumes of the hazelnut shells depending on the variety.

Name	VoO2	Voa	VCO2	VSO2	VoH2O	VN2	Voga	Vogu
Çakıldak	0.99 ± 0.00 a	4.72 ± 0.01 a	0.88 ± 0.00 a	0.00 ± 0.00 c	1.71 ± 0.00 a	4.10 ± 0.00 a	6.70 ± 0.01 a	4.99 ± 0.00 a
Foşa	0.95 ± 0.01 b	4.53 ± 0.03 b	0.85 ± 0.00 d	0.00 ± 0.00 b	1.71 ± 0.01 a	3.94 ± 0.03 c	6.50 ± 0.02 bc	4.79 ± 0.03 d
İnce Kara	0.97 ± 0.00 ab	4.62 ± 0.01 ab	0.87 ± 0.00 c	0.00 ± 0.00 d	1.70 ± 0.00 a	4.05 ± 0.01 ab	6.61 ± 0.01 abc	4.92 ± 0.01 abc
Kalın Kara	0.94 ± 0.03 b	4.49 ± 0.16 b	0.87 ± 0.00 bc	0.00 ± 0.00 f	1.60 ± 0.08 b	3.99 ± 0.08 bc	6.46 ± 0.16 c	4.86 ± 0.08 cd
Palaz	0.96 ± 0.01 ab	4.58 ± 0.06 ab	0.87 ± 0.01 c	0.00 ± 0.00 g	1.68 ± 0.02 ab	4.00 ± 0.01 bc	6.54 ± 0.03 abc	4.87 ± 0.01 cd
Tombul	0.98 ± 0.00 ab	4.66 ± 0.00 ab	0.88 ± 0.00 ab	0.00 ± 0.00 e	1.69 ± 0.00 a	4.00 ± 0.00 bc	6.58 ± 0.01 abc	4.88 ± 0.00 bc
Yassı Badem	0.99 ± 0.00 a	4.70 ± 0.01 a	0.88 ± 0.00 a	0.00 ± 0.00 a	1.71 ± 0.01 ab	4.07 ± 0.02 ab	6.66 ± 0.02 ab	4.95 ± 0.02 ab
Yuvarlak Badem	0.96 ± 0.01 ab	4.59 ± 0.03 ab	0.87 ± 0.00 c	0.00 ± 0.00 h	1.69 ± 0.01 a	4.00 ± 0.02 bc	6.55 ± 0.01 abc	4.87 ± 0.01 cd
p-value	0.0028	0.0028	0.0001	0.0001	0.0089	0.0002	0.0002	0.0001

Explanations: Vo_O2 = theoretical oxygen demand, Vo_a = stoichiometric volume of dry air required to burn 1 kg of biomass, V_CO2 = carbon dioxide content, V_SO2 = content of sulfur dioxide, Vo_H2O = water vapor content of the exhaust gas, V_N2 = theoretical nitrogen content in the exhaust gas, Vo_gu = total stoichiometric volume of dry exhaust gas, Vo_ga = total volume of exhaust gases. Significant difference means that different letters in the column indicate significant differences at α = 0.05.

summarizes data on the composition of the exhaust gas produced when hazelnut shells are burned, depending on the variety. Eight varieties and eight key parameters are included: theoretical oxygen demand (VO₂), volume of dry combustion air (Voₐ), volume of CO₂ (VCO) (₂), SO₂ (VSO) (₂), water vapor (Vo(H) (₂) (O)), nitrogen (VN) (₂), dry gas volume (Voga), and total flue gas volume (Vogu). All parameters—except for SO₂ content—showed significant variation between varieties (Table 7).

The highest theoretical oxygen demand (VO₂) was achieved by Çakıldak and Yassı Baden varieties (0.99 Nm³·kg⁻¹ each), while the lowest was achieved by Kalın Kara (0.94 Nm³·kg⁻¹). The difference between these extremes was 0.05 Nm³·kg⁻¹.

In the case of the volume of dry air required to burn biomass (VOₐ), the highest values were recorded for Çakıldak (4.72 Nm³·kg⁻¹), while the lowest values were recorded for Foşa (4.53 Nm³·kg⁻¹). The difference was 0.19 Nm³·kg⁻¹.

In terms of carbon dioxide content in the flue gas (VCO₂), the highest values were obtained for Çakıldak (0.88 Nm³·kg⁻¹) and the lowest for Foşa (0.85 Nm³·kg⁻¹), a difference of 0.03 Nm³·kg⁻¹.

All analyzed varieties had zero SO₂ in the flue gas (VSO₂), confirming the very low presence of sulfur in the material and the absence of emissions of this compound during combustion.

Water vapor volume (VOH₂O) was highest for the Foşa, Çakıldak, and Yassı Baden varieties (1.71 Nm³·kg⁻¹) and lowest for Kalınkar (1.60 Nm³·kg⁻¹). The difference was 0.11 Nm³·kg⁻¹.

The largest fluctuations were in nitrogen volume (VN₂), where values ranged from 3.94 Nm³·kg⁻¹ for Foşa to 4.10 Nm³·kg⁻¹ for Çakıldak, a difference of 0.16 Nm³·kg⁻¹.

Dry gas volume (Vo_gₐ) was highest for Çakıldak (6.70 Nm³·kg⁻¹) and lowest for Kalınkar (6.46 Nm³·kg⁻¹), a difference of 0.24 Nm³·kg⁻¹.

The total volume of exhaust gases (Vo_gu) ranged from 4.79 Nm³·kg⁻¹ for Foşa to 4.99 Nm³·kg⁻¹ for Çakıldak, a difference of 0.20 Nm³·kg⁻¹ (Table 7).

4. Conclusions

The results of this study clearly showed that varietal characteristics have a significant impact on all analyzed parameters—quantitative (weight and proportion of shells), qualitative (technical parameters, calorific value, elemental composition), and environmental (pollutant emissions).

All analyzed hazelnut shells varieties showed suitability for energy use, but considering shell weight, percentage, energy properties, and yield potential, the İnce Kara variety stands out as the most favorable for energy use of hazelnut shells under Turkish conditions. For 100 nuts, the weight of the shells averaged 1.10 g, accounting for as much as 55.3% of the total weight of the nut—the highest share among the analyzed varieties. In terms of calorific properties, İnce Kara achieved a higher heating value (HHV) of 19.69 MJ·kg⁻¹ and a lower heating value (LHV) of 18.45 MJ·kg⁻¹, ranking it among the varieties with the highest energy potential. In addition, the variety has a high yield potential, which, combined with the high quantity and quality of waste biomass, makes it an optimal choice for energy purposes. The results obtained are comparable for wood shoots [61].

The use of shells as an energy feedstock can contribute to the efficient management of post-production waste from hazelnut orchards and reduce CO₂ emissions by replacing fossil fuels with waste biomass.