Comparative Analysis of Energy and Emission Properties of Hazelnut Shell Biomass from Temperate and Subtropical Climates

Abstract

The aim of this research was to compare the estimation of waste biomass in the form of hazelnut husk (Corylus avellana L.) originating from two different climate zones—temperate (Poland) and subtropical (Turkey)—in terms of their energy and emission properties. This study included proximate analysis (moisture, ash, volatile matter, fixed carbon), ultimate analysis (C, H, N, S, O), determination of the (LHV) lower heating value and (HHV) higher heating value. Pollutant emission factors (CO, CO₂, SO₂, NO_x, dust) were assessed, and stoichiometric calculations of the composition of exhaust gases were performed. The results showed statistically significant differences between samples from both climate zones. Husk from Turkey was characterised by calorific values (LHV—17.46 MJ·kg−¹, HHV—18.76 MJ·kg⁻¹) and higher carbon (43.68%) and hydrogen (7.27%) content compared to Polish husk (HHV-17.29 MJ·kg⁻¹, LHV-16.13 MJ·kg⁻¹, C-46.49%, H-7.05%). At the same time, higher CO₂ and SO₂ emission rates were observed in Turkish samples, while biomass from Poland was characterised by lower ash content and lower dust emissions. Principal component analysis (PCA) confirmed the significant influence of climate on the energy and environmental parameters of the husk. The obtained results can be the basis for optimizing the use of waste biomass in the management of waste from horticultural or agricultural production and for sustainable development in various climatic conditions.

1. Introduction

Hazelnuts (Corylus avellana L.) are one of the most important nut species cultivated worldwide, and Turkey remains the largest producer, accounting for approximately 60–70% of global production [1,2,3,4,5]. Apart from Turkey, significant quantities of hazelnuts are produced in China, Georgia, Italy, Kazakhstan, Spain and the United States. Poland, on the other hand, ranks far behind in the global ranking, with an annual production of 7600 tonnes. In 2023, hazelnut production in Poland reached approximately 10,700 tonnes [6], while projections suggest continued growth in overall nut cultivation, with nut production in Poland estimated at around 14,000 tonnes in 2024 [7].

Nevertheless, it is characterised by a relatively high yield per hectare, amounting to 1407 kg·ha⁻¹, thus exceeding Turkey, where the average yield reaches 926 kg·ha⁻¹ [8]. These differences may result from the varieties grown in both countries, agrotechnical actions and climatic and soil conditions.

Turkey has a subtropical climate with high air humidity, mild winters and warm summers, which is conducive to high total yields and dominance in global hazelnut production. In Poland, the temperate climate is characterised by greater seasonal variability, lower winter temperatures and lower rainfall during the growing season compared to the Black Sea region in [7,8,9,10,11]. These differences may affect nut development, shell thickness and structure, chemical composition and final fuel and emission parameters.

In recent years, the nut market, including hazelnuts, has been undergoing significant changes due to changing consumer preferences and external factors such as climate change, rising logistics costs and trade tensions. Nuts are now seen not only as a traditional snack, but also as an important component of plant-based diets and functional products. For this reason, the search for alternative uses for by-products generated during their processing, i.e., husk, which can be a valuable source of renewable energy, is becoming an increasingly important renewable [9,10,11,12].

In addition, there are significant morphological differences between nuts from both countries [13]. Most hazelnut varieties grown in the Polish climate belong to the table (large-fruited) group, which are characterised by large nuts and thicker husk compared to Turkish varieties [14,15], which may affect the weight and structure of the husk and their energy and emission properties. Only the nut kernel is used in processing, while the shell, which accounts for 25% to 67% of the total weight depending on the variety and origin, remains industrial waste. According to FAOSTAT [6] data, Poland produced 10,700 tonnes of hazelnuts (in shell) in 2023. Considering that the shell accounts for 25% to 67% of the total weight of the nut, the amount of shells produced can be estimated at approximately 2675–7169 tonnes.

Low moisture content and high lignin content, hazelnut husk has favourable fuel properties [16,17,18], including a high calorific value of up to 19.2 MJ/kg and a low ash content (0.7–1%) [9,10,11,15,18,19,20,21,22,23,24,25,26]. However, the low bulk density of the husk makes it difficult to transport, store and burn efficiently, often requiring them to be compacted into pellets or briquettes.

However, there are no comparative studies taking into account the impact of different climatic conditions on the energy and emission properties of hazelnut husk. Assessing these differences may be important in the context of optimising the use of waste biomass within the zero-waste concept [17] and the circular economy (CE), which aims to minimise waste and maximise its reuse in energy production [27,28,29,30].

Given their high nutritional and health-promoting value, hazelnuts are used in the production of a wide range of food products [12,31,32,33]. They are a valuable raw material not only in the food industry, but also in other sectors of the economy [34,35].

Differences in the phenolic profile, fatty acid content and tocopherols between Turkish and European hazelnuts have been demonstrated in numerous in vivo and in vitro studies. Turkish hazelnuts are characterised by a higher content of phenolic compounds and greater antioxidant activity compared to European varieties, including Polish varieties [36,37,38,39,40]. Therefore, it is reasonable to assume that similar differences may also occur in the composition of the husk of these nuts, which may affect their energy and emission properties.

The aim of this study was to compare the qualitative parameters of waste biomass in the form of hazelnut husk from two different climate zones—temperate (Poland) and subtropical (Turkey)—in terms of their energy and emission properties. This study included technical analysis (moisture, ash, volatile matter, fixed carbon), elemental analysis (C, H, N, S, O), the (LHV) lower heating value and (HHV) higher heating value. In addition, pollutant emission factors (CO, CO₂, NO_x, SO₂, Dust) and stoichiometric calculations of the composition of exhaust gases were performed to estimate the volume of exhaust gases produced during the combustion of husk. The results will make it possible to assess the impact of climatic conditions on the energy and environmental properties of this biomass and to optimise its use in sustainable bioenergy.

The methodology was based on laboratory-scale analyses of hazelnut husk samples collected from four cultivars in each region, following standardised procedures for biomass characterization

2. Materials and Methods

Field research was conducted in 2024 on four varieties of hazelnuts (Corylus avellana L.) in temperate and Mediterranean climates. The temperate climate zone is represented by analytical data based on Poland, using some of the most popular varieties in this climate zone, i.e., Olga, Kataloński, Olbrzymi z Halle and Webba Cenny. The tropical climate zone is represented by varieties such as Çakıldak, Foşa, Palaz and Tombul from Turkey.

This study assessed the mass of nuts and shells (100 pieces for each variety), energy and emission parameters and biofuel quality (technical and elemental analysis). Furthermore, the extreme calorific values were estimated. The material was dried at 55–60% humidity and 20 °C for two weeks, which allowed for standardised evaluation conditions. It was then pre-ground (to a thickness of 0.5 mm) in a Retsch SM 100 mill (Retsch GmbH, Haan, Germany). The experiment was designed as a randomised block design (4 combinations with 5 replicates and 3 plants). Statistical analysis was performed using STATISTICA 13 software, using two-factor analysis of variance (ANOVA). Results were presented graphically using cluster analysis and principal component analysis. Statistical conclusions were assessed at a significance level of αp < 0.05. The fuel characterization methodology is presented in

Table 1. Fuel characterisation analysis.

PARAMETER	METHOD	EQUIPMENT
Energetic properties
Higher Heating Value (HHV; MJ·kg−1)	EN-ISO 1928:2020 [41]	isoperibolic calorimeter LECO AC 600 (Devon, United Kingdom)
Lower Heating Value (LHV; MJ·kg−1)
Proximate Analysis
Ash (A; %)	EN-ISO 18122:2022 [42]	thermogravimetric analyser LECO TGA 701 (Devon, United Kingdom)
Volatile matter (V; %)	EN-ISO 18123:2023 [43]
Moisture (MC; %)	EN-ISO 18134:2023 [44]
Fixed carbon (FC; %)	FC = 100 − V − A − M [45]
Ultimate Analysis
Carbon (C; %)	EN-ISO 16948:2015 [46]	elemental analyser LECO CHNS 628 (Devon, United Kingdom)
Hydrogen (H;%)
Nitrogen (N; %)
Sulphur (S; %)	EN-ISO 16994:2016 [47]
Oxygen (O; %)	O = 100 − A − H − C − S − N [48]

,

Table 2. Emission factors (emission factors calculated according to studies [49]).

PARAMETER	METHOD AND EQUIPMENT
Carbon monoxide emission factor (Ec) of chemically pure coal (CO; kg·mg−1)	$CO = {\displaystyle \frac{28}{12}}·E_c$·(C/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–sulphur dioxide emission factor (kg∙kg−1), 2—molar mass ratio of SO2 and sulphur, S—sulphur content in fuel (%), r—coefficient determining the part of total sulphur 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 oxidises 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 (exhaust gas composition was calculated according to [50,51,52]).

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 sulphur 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_{{CO}_2}={\displaystyle \frac{22.41}{12}}$$· {\displaystyle \frac{C}{100}},$
Content of sulphur dioxide (VSO2; Nm3·kg−1)	$V_{{SO}_2} = {\displaystyle \frac{22.41}{32}}$$· {\displaystyle \frac{S}{100}}$,
Water vapor content of the exhaust gas (VH2O; Nm3·kg−1)	$V_{H_{2}O}^H = {\displaystyle \frac{22.41}{100}}·\left({\displaystyle \frac{H}{2}} + {\displaystyle \frac{M}{18}}\right)$,$\mathrm{is} \mathrm{the} \mathrm{component} \mathrm{of} \mathrm{water} \mathrm{vapor} \mathrm{volume} \mathrm{from} \mathrm{the} \mathrm{hydrogen} \mathrm{combustion} \mathrm{process}$ $(V_{H_{2}O}^H; {\mathrm{Nm}}^3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1}\mathrm{fuel}) V_{H_{2}O}^a=1.61·x·V_{oa}$ $\mathrm{and} \mathrm{the} \mathrm{volume} \mathrm{of} \mathrm{moisture} \mathrm{contained} \mathrm{in} \mathrm{the} \mathrm{combustion} \mathrm{air}$ $(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).
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

Table 4. Proximate analysis and calorific values of woody hazelnut husk depending on the climatic conditions (dry matter).

Climate Zone	Technical Analysis Parameters
	HHV (MJ·kg−1)	LHV (MJ·kg−1)	MC (%)	A (%)	V (%)	FC (%)
Moderate	17.29 ± 0.28 b	16.13 ± 0.29 b	7.38 ± 1.04 b	0.87 ± 0.12 b	66.33 ± 0.66 a	24.21 ± 5.06 b
Subtropical	18.76 ± 0.26 a	17.46 ± 0.27 a	10.35 ± 0.78 a	1.15 ± 0.12 a	66.04 ± 0.65 a	33.72 ± 7.35 a
p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001

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

below compares selected technical parameters of hazelnut shells from different climate zones. All analysed characteristics showed significant differences, indicating a significant impact of cultivation location on the energy properties of waste biomass.

The highest calorific values (HHV and LHV) were obtained for husk from the subtropical climate zone—18.76 MJ·kg⁻¹ and 17.46 MJ·kg⁻¹, respectively. In the case of husks from Poland, these values were significantly lower, amounting to 17.29 MJ·kg⁻¹ and 16.13 MJ·kg⁻¹, respectively. Also, moisture content (MC%) was significantly higher in samples from the subtropical climate (10.35%) than in those from the temperate climate (7.38%). The ash content (A%) for husk from the temperate climate was 0.87%, which was significantly lower than in the subtropical climate—1.15%. The low ash content in husk from the temperate climate may have a positive effect on the cleanliness of the combustion process and reduce the risk of boiler contamination. The volatile matter (V%) values were similar: 66.33% for the temperate climate and 66.04% for the subtropical climate.

The fixed carbon (FC) content in Turkish shells was higher (33.72%) than in Polish shells (24.21%), which may indicate a higher energy potential of Turkish biomass and its greater value as a solid fuel.

Table 5. Comparison of the literature data for energy parameter assessments for hazel husk depending on the climate zone.

Material	Energy Parameters
	HHV (MJ·kg−1)	LHV (MJ·kg−1)	MC	A	V	FC
Hazel husk—Turkey [52]		18.50	7.24	5.27	73.86	20.87
Hazel husk—Turkey [26]		18.35		7.19
Hazelnut husk—Turkey [53]		16.20	8.2	9.16	57.5	25.06
Hazelnut husk—Olga variety—Poland [11]	18.68	17.39	8.9	0.79	67.96	22.34
Hazelnut husk—Webba cenny variety—Poland [11]	18.61	17.33	9.01	0.99	67.2	22.8

compares the energy parameters for hazel husk based on the literature data, depending on the cultivation in the temperate and subtropical climate zones.

Comparison of the results obtained in this study with the literature data (Table 5) allows us to conclude that the HHV obtained for the subtropical climate is similar to the results presented for hazel husk grown in Poland (moderate zone). In the case of LHV, it can be concluded that the results obtained in this study are lower than those for hazel husk grown in Turkey [53,54] and similar to those obtained for hazel husk grown in Polish conditions. The ash content is noteworthy. The results obtained in this study indicate low ash content, while for hazel husk grown in Turkey, the ash content is on average 7% higher. The volatile matter content is similar to that of hazel husk grown in Poland, but there is no pattern for hazel husk grown in Turkey. The results obtained for fixed carbon for hazel husk grown in a subtropical climate are higher than the literature data. Analysing the above, it can be concluded that cultivation in different climatic zones influences the formation of energy parameters of biofuel, and it should be taken into account that the subtropical zone allows for obtaining higher energy parameters of biomass.

The table presents the results of the elemental analysis of hazelnut husk from two different climate zones. Seven parameters were examined: carbon (C%), hydrogen (H%), nitrogen (N%), sulphur (S%) and oxygen (O%) content, as well as the molar ratios of H/C, N/C and O/C. All the characteristics examined showed statistically significant differences, indicating a significant influence of climatic conditions on the elemental composition of biomass.

The lowest carbon content (C%) was recorded in husk from the subtropical climate zone—43.68%, while in husk from the temperate climate zone it was significantly lower, at 46.49%. The difference of nearly 2.81% may indicate a better energy potential of biomass from a cooler climate.

The hydrogen content (H%) was also higher in material from a temperate climate (7.46%) than from a subtropical climate (7.27%), but the difference was relatively small, ranging within 0.19%.

In contrast, the nitrogen content (N%) of biomass from the subtropical climate zone was 0.47%, which is almost twice as low as that of samples from the temperate climate—0.33%. This is a significant difference that may be important for nitrogen oxide (NO_x) emission indicators in the combustion process.

Very large differences were also observed in the sulphur content (S%). For biomass from a temperate climate, its level was 0.01%, while in samples from the second zone, it was only 0.03%. This is a key parameter from an environmental point of view, as lower sulphur content reduces SO₂ emissions.

The oxygen content (O%) also differed significantly—in the case of biomass from the temperate climate zone, it was 44.90%, and in the case of samples from the subtropical climate zone, it was 44.41%.

In terms of molar ratios, the lowest H/C ratio (1.60) was recorded for the temperate climate zone and a higher ratio for the subtropical climate zone (1.66), which may affect combustion parameters.

The N/C ratio for biomass from Poland was 0.007, while in samples from the subtropical climate it was 0.011, which confirms the lower nitrogen content in samples from the north.

In turn, the O/C ratio was lower in biomass from a temperate climate (0.97) than in biomass from a subtropical climate (1.02), which may also affect the intensity of combustion product emissions (

Table 6. Results of ultimate analysis for woody hazelnut husk depending on the climatic conditions.

Climate Zone	Ultimate Analysis
	C	H	N	S	O	H/C	N/C	O/C
Moderate	46.49 ± 0.02 b	7.46 ± 0.16 a	0.33 ± 0.03 b	0.01 ± 0.00 a	44.90 ± 0.34 b	1.60 ± 0.04 b	0.007 ± 0.00 b	0.97 ± 0.01 b
Subtropical	43.68 ± 0.03 a	7.27 ± 0.19 a	0.47 ± 0.04 a	0.03 ± 0.00 b	44.41 ± 0.46 a	1.66 ± 0.05 a	0.011 ± 0.00 a	1.02 ± 0.01 a
p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001

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

).

Table 7. Comparison of the literature data for the evaluation of elemental analysis parameters for hazel husk depending on the climate zone.

Material	Ultimate Analysis
	C	H	N	S	O	H/C	N/C	O/C
Hazel husk—Turkey [52]	42.61	5.509	1.129	0.137	50.615
Hazelnut husk—Turkey [54]	42.62	5.2	0.9	0.08	45.4	1.47		0.8
Hazelnut husk—Turkey [53]	44.107	5.833	1.169	0	48.891
Hazelnut husk—Olga variety—Poland [11]	46.55	7.58	0.29	0.01	44.77	1.63	0.01	0.72
Hazelnut husk—Webba Cenny variety—Poland [11]	46.17	7.5	0.36	0.02	44.97	1.62	0.01	0.73

presents a comparison of the literature data for elemental analysis performed for hazel husk grown in Turkey and Poland, representing two different climatic zones.

Comparing the results of the elemental analysis with data from the literature, it can be concluded that the carbon content results for varieties from a temperate climate are consistent with those obtained for hazel husk from Poland, while those from a subtropical climate are consistent with hazel husk from Turkey. The hydrogen, nitrogen, sulphur and oxygen content of hazel husk from both zones is similar to the results obtained for hazel husk varieties Olga and Webba Cenny from Poland and higher for carbon and lower in other cases compared to varieties from Turkey. In view of the above, it can be concluded that the climate zone influences the structure of hazel husk and the formation of the content of individual elements in the material through cultivation.

Below is a summary of the emission indicators generated during the combustion of hazelnut husk from temperate and subtropical climate zones. The table includes five emission parameters: carbon monoxide (CO) and carbon dioxide (CO₂), nitrogen oxides (NO_x), sulphur dioxide (SO₂) and total dust. All analysed indicators showed statistically significant differences, indicating that climatic conditions and the origin of the raw material have a clear impact on the emissions of pollutants generated during the combustion of hazelnut biomass (

Table 8. Emission factors parameters for woody hazelnut husk depending on climatic conditions.

Climate Zone	Emission Factor (kg·mg−1)
	CO	CO2	NOx	SO2	Dust
Moderate	52.81 ± 0.66 b	1293.28 ± 16.21 b	1.15 ± 0.14 b	0.03 ± 0.01 b	1.11 ± 0.15 b
Subtropical	57.51 ± 0.65 a	1408.37 ± 15.89 a	1.66 ± 0.15 a	0.05 ± 0.01 a	1.45 ± 0.15 a
p-value	0.0001	0.0001	0.0001	0.0001	0.0001

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

).

In terms of carbon monoxide (CO) emissions, husk from the subtropical climate showed significantly higher emission values (57.51 kg·mg⁻¹) compared to samples from the temperate climate (52.81 kg·mg⁻¹). The difference between the samples was 4.70 kg·mg⁻¹.

Also, carbon dioxide (CO₂) emissions were higher in the case of husk from the subtropical climate zone (1408.37 kg·mg⁻¹) compared to the temperate climate (1293.28 kg·mg⁻¹). The difference was 115.09 kg·mg⁻¹ and indicates more intense oxidation processes occurring during the combustion of material from the subtropical climate, which may be related to its more homogeneous organic composition.

In the case of nitrogen oxide (NO_x) emissions, the opposite trend was observed—significantly higher values were recorded for biomass from a subtropical climate (1.66 kg·mg⁻¹) compared to samples from a temperate climate (1.15 kg·mg⁻¹). The difference of 0.51 kg·mg⁻¹ may be the result of higher nitrogen content in material from a cooler climate or different combustion conditions.

Sulphur dioxide (SO₂) emissions were significantly higher for husk from a subtropical climate (0.05 kg·mg⁻¹), while husk from a temperate climate emitted only 0.03 kg·mg⁻¹. The difference was 0.02 kg·mg⁻¹ and may result from the lower sulphur content in raw material growing in a warmer, drier climate.

The greatest fluctuations were observed for dust emissions. For biomass from a temperate climate, this value was 1.11 kg·mg⁻¹, while for samples from a subtropical climate it was 1.45 kg·mg⁻¹. The difference is as high as 0.34 kg·mg⁻¹, which may indicate a higher proportion of non-combustible fractions in material from a subtropical climate or its lower homogeneity. High dust emissions can be of significant environmental and technological importance, especially in the context of biomass use in energy installations.

Table 9. Composition of the exhaust gases of the woody hazelnut husk depending on the climatic conditions.

Climate Zone	Exhaust Gas Parameters
	VoO2	Voa	VCO2	VSO2	VoH2O	VN2	Voga	Vogu
Moderate	0.93 ± 0.03 b	4.41 ± 0.14 b	0.80 ± 0.01 b	0.00 ± 0.00 a	1.59 ± 0.05 b	4.02 ± 0.05 b	6.58 ± 0.08 b	4.89 ± 0.06 b
Subtropical	0.97 ± 0.02 a	4.61 ± 0.08 a	0.87 ± 0.01 a	0.00 ± 0.00 a	1.69 ± 0.04 a	5.19 ± 0.15 a	7.58 ± 0.20 a	5.99 ± 0.16 a
p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001

Explanations: V_O2—the theoretical oxygen demand, Vo_a—stoichiometric volume of dry air required to burn 1 kg of biomass, V_CO2—the carbon dioxide content, V_SO2—the content of sulphur dioxide, V_(H2O)—the water vapour content of the exhaust gas, V_(N2)—the theoretical nitrogen content in the exhaust gas, V_(gu)—the total stoichiometric volume of dry exhaust gas, V_(ga)—the total volume of exhaust gases. Significant difference means that different letters in the column indicate significant differences at α = 0.05.

presents the results of calculations of the composition of flue gases produced during the combustion of hazelnut husk from temperate and subtropical climate zones. These parameters were estimated on the basis of stoichiometric equations, and their values include the following: stoichiometric volume of dry air (V_a), theoretical oxygen demand (V_o2), VCO₂ content, VSO₂ content, water vapour (V_H2O), nitrogen (VN₂), total dry gas volume (Vg_a) and total exhaust gas volume (Vg_oa). Statistically significant differences were observed for all parameters, which clearly indicates the influence of climatic conditions (biomass origin) on the composition of the exhaust gases generated (Table 8).

In order to identify the relationships between the individual quality parameters of biomass from the temperate and subtropical climate zones of hazelnuts, a principal component analysis (PCA) was performed. The analysis took into account both physicochemical parameters (C%, H%, N%, O%) and calorific values (LHV), as well as emission factors (CO₍₂₎) and volume parameters of exhaust gases (Vo_(ga), Vo_(gu)). The use of such a set of variables allows for a comprehensive assessment of the tested raw material, taking into account both its energy potential and its impact on the environment (Figure 1a,b).

The analysis (PCA) is presented in Figure 1a,b. Both analyses show a strong correlation between energy parameters (C%, CO₂, LHV) and, to some extent, combustion parameters (Vo_gu), while variables such as H%, N% and Vo(_ga) show greater independence, which may indicate their greater variability depending on environmental conditions. The structure of the relationship between variables differs slightly between populations—in the case of husk from a temperate climate, the relationships are more condensed, while in the subtropical zone, most of the variability is distributed between two components, which may indicate more diverse environmental or varietal influences. The first two main components explain 94.40% and 90.09% of the total variability, respectively, confirming the accuracy of the analysis (Figure 1a,b).

4. Conclusions

Research has shown that climatic and geographical conditions have a significant impact on the energy and environmental properties of waste biomass in the form of hazelnut husk. Turkish husk showed a statistically significantly higher energy potential, achieving a calorific value (LHV) of 17.46 MJ·kg⁻¹ and a calorific value (HHV) of 18.76 MJ·kg⁻¹, while in the case of Polish husk, these values were lower, at 16.13 MJ·kg⁻¹ (LHV) and 17.29 MJ·kg⁻¹ (HHV), respectively.

At the same time, Turkish husk generated significantly higher pollutant emissions during combustion, including carbon dioxide emissions of 1408.37 kg·mg⁻¹ (Poland: 1293.28 kg·mg⁻¹), nitrogen oxides (NO_x)—1.66 kg·mg⁻¹ (Poland: 1.15 kg·mg⁻¹) and carbon monoxide (CO)—57.51 kg·mg⁻¹ (Poland: 52.81 kg·mg⁻¹).

Additionally, principal component analysis (PCA) confirmed clear differences between samples from Poland and Turkey, indicating a significant influence of biomass origin on its energy and environmental properties.

These results can be used to optimise bioenergy processes and waste biomass management strategies, taking into account their calorific value, chemical composition and potential environmental impact, which supports the implementation of the circular economy and zero-waste concepts.

The aim of this study was to compare the qualitative parameters of hazelnut husk biomass from two different climate zones—temperate (Poland) and subtropical (Turkey)—in terms of their energy and emission properties.

The applied methodology included technical analysis (moisture, ash, volatile matter, fixed carbon), elemental analysis (C, H, N, S, O) of calorific values (LHV, HHV), pollutant emission factor calculations, as well as statistical evaluation using principal component analysis (PCA).

Research has shown that climatic and geographical conditions have a significant impact on the energy and environmental properties of hazelnut husk. Turkish husk showed a statistically significantly higher energy potential, achieving a calorific value (LHV) of 17.46 MJ·kg⁻¹ and a calorific value (HHV) of 18.76 mJ·kg⁻¹, while in the case of Polish husk, these values were lower, at 16.13 MJ·kg⁻¹ (LHV) and 17.29 MJ·kg⁻¹ (HHV), respectively. At the same time, Turkish husk generated significantly higher pollutant emissions during combustion, including carbon dioxide emissions of 1408.37 kg·mg⁻¹ (Poland: 1293.28 kg·mg⁻¹), nitrogen oxides (NO_x)—1.66 kg·mg⁻¹ (Poland: 1.15 kg·mg⁻¹) and carbon monoxide (CO)—57.51 kg·mg⁻¹ (Poland: 52.81 kg·mg⁻¹). Additionally, PCA confirmed clear differences between samples from Poland and Turkey, indicating a significant influence of biomass origin on its energy and environmental properties.

These results can be used to optimise bioenergy processes and waste biomass management strategies, taking into account their calorific value, chemical composition and potential environmental impact, which supports the implementation of the circular economy and zero-waste concepts.