Analysis of the Energy–Carbon Potential of the Pericarp Cover of Selected Hazelnut Varieties

: The research presents weight estimation and analysis of the energy and carbon potential of the pericarp cover of four hazelnut varieties. A technical and elementary biofuel analysis was carried out for the biomass studied, as well as a correlation and principal component analysis to demonstrate the influence of individual characteristics on the parameters achieved. In addition, emission factors and the composition and volume of flue gases from the combustion of the material studied were estimated based on stoichiometric equations. The research showed that the highest calorific value (LHV) was characterised by the pericarp cover of the ‘Olga’ variety (14.86 MJ · kg − 1 ) and the lowest by the ‘Katalo´nski’ variety (14.60 MJ · kg − 1 ). In the case of exhaust volume, the highest volume was obtained from the ‘Olbrzymi z Halle’ variety (250.06 Nm 3 · kg − 1 ) and the lowest from the ‘Katalo´nski’ variety (12.43 Nm 3 · kg − 1 ). The correlation analysis carried out showed that the HHV and LHV parameters in the covers showed a very strong positive correlation with sulphur content and SO 2 emissions, and a moderate correlation with nitrogen content and its associated NO x emissions, indicating their direct influence on the higher calorific value of biomass.


Introduction
The changes taking place in the era of economic development and increasing economic requirements are forcing producers and distributors of energy resources to look for new, more environmentally friendly energy options as an alternative to traditional fossil fuels [1,2].In the face of new legislative and environmental requirements, the management of the energy sector, requires them to take a fresh look and update the strategies covered [3,4].The general challenge in bioenergy production is to effectively manage the supply chain at each stage, while taking into account economic, environmental and social aspects [5][6][7].
Biomass, as a renewable resource and alternative fuel, is expected to play a key role in achieving climate and energy neutrality [8].Its role is multidimensional and is not limited to providing raw materials for economic use.It is responsible for energy diversity, carbon capture and storage, air quality, reducing greenhouse gas emissions and ensuring the continuity of ecosystems [9][10][11].Given that the new scenario envisages an increase in the share of RES to 27% in Europe, as well as a 40% reduction in greenhouse gas emissions by 2030 [12], efforts are being made to improve technology and introduce innovations for biomass conversion in order to reduce the cost of bioenergy production [13].The conversion and use of biomass are worth promoting, especially with an emphasis on less common, variable, random and often dispersed materials such as organic waste from agricultural production [14,15].Their chemical composition, quantity, availability and properties predispose them to reuse through biochemical, chemical and thermochemical processes and the creation of bioproducts, biomaterials and use through direct conversion to biofuels in gasification and pyrolysis processes, thus creating local industrial and energy value chains [15][16][17].
Hazelnut, which is the fruit of the hazelnut tree, is an excellent example of functional food due to its taste, nutritional and health-promoting qualities, and is a valuable raw material not only in the kitchen [18,19].Less well-known but equally important is the use of the waste biomass that is generated during its production.Their quantity depends on the volume of production, as well as the way it is organised.Traditionally, this waste was seen as a problem, but modern technologies and innovative approaches to waste management make it possible to turn this 'problem' into ecological and economical solutions.In an era of sustainability and closed-loop economics, even waste can become a valuable resource.Transforming them into new products for energy production aims to add value to the by-product [20,21].In addition, this approach makes it possible to reduce landfill and thus minimise the negative impact on the environment [14,15].
In the context of the use of hazelnut production residues, the pericarp cover also represents a valuable material that can be processed for energy recovery.It is formed from the fusion of three inflorescences with serrated edges.It is characterised by its green or red colour and the extent to which it covers the pericarp.When the nut ripens, the shell falls off, leaving a characteristic mark on the woody shell, known as the shield.Most of this waste decomposes on the farm.Although detailed research on pericarp casing is limited, the inclusion of this form of biomass in the energy cycle may contribute to the efficient and holistic management of waste biomass from hazelnut cultivation, regardless of its type.
The aim of the research was to estimate the weight of pericarp casings for a selected hazelnut variety and to define fuel quality parameters by performing technical and elemental analysis, as well as determining the combustion heat and calorific value.In addition, the study aimed to assess the emission factors of CO, CO 2 , SO 2 , NO x and dust, in order to determine the degree of impact of potential bio-waste generated during the combustion process.An analysis of the flue gas composition based on estimation from stoichiometric equations was also carried out.

Materials and Methods
The study tested the influence of hazelnut cultivar on the parameters of the energy potential of waste biomass in the form of pericarp cover obtained during harvesting.
The field research was conducted in 2023 in south-eastern Poland (private Horticultural Farm; Zawichost municipality; Świętokrzyskie voivodeship; 50 • 49 ′ 20.5 ′′ N, 21 • 44 ′ 35.0 ′′ E) under temperate climate conditions.The study included ungrafted hazel shrubs growing on their own roots of the varieties: 'Katalo ński', 'Olbrzymi z Halle', 'Olga' and 'Webba Cenny'.The shrubs were planted in spring 2002 on loess soil (bonitation classes II and II ab) in a row system, with a spacing of 6 × 2.5 m applied (666 pcs.•ha −1 ).The following parameters were analysed: yield based on the weight of the whole hazelnut (husk + kernel) and the weight of 100 and 1 bush of pericarp cover (waste).The obtained parameters were converted to a unit area of 1 ha.Samples for analysis were taken at full harvest maturity from 3 randomly selected bushes, 4 samples for each cultivar, which made it possible to determine the average value for the parameters studied.Immediately after harvest, samples were weighed with an accuracy of 0.001 kg, and then their weight was determined on a RADWAG PS R2 precision balance (RADWAG, Radom 26-600, Poland).
The study evaluated the energy and emission parameters for the materials tested.Fuel quality parameters were estimated by performing technical and elemental analysis, and the heat of combustion and calorific value were determined.The methodology of the procedures is shown in Table 1.
Carbon dioxide emission factor (CO 2 ; kg•Mg −1 ) 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, E CH4 -methane emission factor, E NMVOC -emission index of non-methane VOCs (for biomass 0.009).
Emission factor was calculated from (NO X ; kg•Mg −1 ) ), NO x -NO x 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, NO x /N-part of nitrogen emitted as NO x (for biomass 0.122).
The stoichiometric volume of dry air required to burn 1 kg of biomass (V oa ; Nm 3 •kg −1 ) 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 (V CO2 ; Nm  The theoretical nitrogen content in the exhaust gas (V N2 ; Nm 3 •kg −1 ) V N2 = 22.41 28 • 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 ; Nm 3 •kg −1 ) The total volume of exhaust gases (V ga ; Nm 3 •kg −1 ) 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.
Figure 1 shows the flow chart of the test rig.
Energies 2024, 17, x FOR PEER REVIEW 4 of 16 The theoretical nitrogen content in the exhaust gas $$( ; Nm 3 •kg −1 )  = .• + 0.79 •  ,$$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 $$( ; Nm 3 •kg −1 )  =  +  + The total volume of exhaust gases $$( ; Nm 3 •kg −1 )  =  +  $$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.
Figure 1 shows the flow chart of the test rig.

Results and Discussion
As part of the research, particular attention was paid to the differentiation of hazelnut varieties in terms of their energy properties and the productivity of waste biomass in the form of pericarp covers.The hazelnut, being not only a valuable source of nutrients but also a raw material for energy, is of interest in the context of sustainable agriculture and renewable energy.Cultivars, i.e., 'Kataloński', 'Olbrzymi z Halle', 'Olga' and 'Webba Cenny', were analysed to determine the mass of hazelnut pericarp covers and their energy potential.
Table 2 shows the results of the comparisons of the hazelnut yield and the pericarp

Results and Discussion
As part of the research, particular attention was paid to the differentiation of hazelnut varieties in terms of their energy properties and the productivity of waste biomass in the form of pericarp covers.The hazelnut, being not only a valuable source of nutrients but also a raw material for energy, is of interest in the context of sustainable agriculture and renewable energy.Cultivars, i.e., 'Katalo ński', 'Olbrzymi z Halle', 'Olga' and 'Webba Cenny', were analysed to determine the mass of hazelnut pericarp covers and their energy potential.
Table 2 shows the results of the comparisons of the hazelnut yield and the pericarp weight for four hazel cultivars: 'Katalo ński', 'Olbrzymi z Halle', 'Olga' and 'Webba Cenny'.The analysis was performed for the yield of whole hazelnuts and seed coats alone for 100 units (in g), per bush (in kg) and per unit area (in tons per ha).The study indicates that there are statistically significant differences between the varieties tested.±standard deviation; a, b, c, ab, bc -means with the same letter in row show no significant difference and means with different letters with significant differences at α = 0.05.
Biometric evaluation is important not only for distinguishing hazelnut varieties but also for determining their use and designing processing equipment.Analysing the data in Table 2 in the 100-unit category, 'Olbrzymi z Halle' shows the highest average weights for both whole nut and pith, 599.33 g and 144.33 g, respectively, while 'Webba Cenny' shows the lowest values for both parameters (502.33 g for whole nut and 101.67 g for pith).These differences are statistically significant, indicating a clear phenotypic diversity between varieties in the context of whole nut and pericarp weight.
A study by Król et al. [33], evaluating six hazelnut cultivars of Coryllus avellana L. produced in Poland, showed a significant effect of cultivar on nut weight.This relationship was also confirmed in the present study.The work by Ciemniewska and Ratusz [34] also showed a significant effect of cultivar on hazelnut weight, with nuts of the ' Katalo ński' cultivar being the heaviest and 'Cosford' the lightest; nuts of the 'Webba Cenny' cultivar ranked in the middle.Król et al. [33] found that indeed the 'Nottinghamski' (2.24 g) and 'Cosford' (2.41 g) varieties had the lightest nuts, while the 'Webba Cenny' variety had the heaviest (3.21 g).It was shown that the nuts of the 'Olbrzymi z Halle' variety were significantly heavier than 'Katalo ński', a relationship that was fully confirmed in the present study.The study also showed that the nut weight of the 'Webba Cenny' cultivar ranked between 'Olbrzymi z Halle' and 'Katalo ński', a finding that was not confirmed in the study by Król et al. [33], where 'Webba' produced significantly the heaviest nuts of those evaluated.Ferrão et al. [35] also observed significant differences between some hazelnut cultivars where, in terms of nut weight in the shell, Gunslebert cultivar fruit was heavier on average (3.89 ± 0.64 g), while the Negreta cultivar fruit was lighter on average (2.23 ± 0.37 g).A study by Lopes et al. [36] also found that varietal characteristics determined nut weight.
Habitat conditions [37,38], cultivar and harvest date may be important modifiers of hazelnut quality ratings [39][40][41].Our observations and analytical results confirm these opinions.Immediately after harvest, the average weight of 100 nuts was 5.09 g ('Olbrzymi z Halle'), 4.76 g ('Katalo ński') and 4.63 g ('Webba Cenny').Morphological traits are often used to identify varieties.To be useful, traits must be consistent from year to year and from tree to tree.Solar and Stampar [42] showed that the nut weight of sixteen hazelnut varieties of different origins ranged from 2.36 g to 4.30 g, the present study results also confirm this.
Nuts of small to medium size, with weights up to 3.2 g and crunchy kernels, are preferred for the processing market.In contrast, in the direct sales market for shelled nuts, larger nuts are more sought after [43].Although the hazelnut varieties studied had a higher average nut weight, the average kernel weight was quite similar to the results obtained by Ozdemir and Akinci [44], where this trait in Turkish hazelnut varieties ranged from 0.89 g to 1.33 g.Similar trends are observed when analysing hazelnut yield and seed coat weight per bush and per unit area, where 'Olbrzymi z Halle' and 'Olga' present higher weights of both whole nut and seed coat compared to the cultivars 'Katalo ński' and 'Webba Cenny'.The significant differences in the weights of whole nuts and their seed coats between the analysed varieties highlight the influence of genetics on these traits.This provides valuable information for variety selection to optimise production, where 'Olbrzymi z Halle' may be preferred for its higher hazelnut yield and seed coat weight, while 'Webba Cenny' may be less desirable in the context of commercial hazelnut production and waste biomass acquisition (Table 2).
The cluster analysis presented in the dendrogram (Figure 2) illustrates the classification of hazelnut cultivars in terms of the amount of waste seed coat biomass generated.The dendrogram distinguishes two main clusters, which allows an understanding of the differences in biomass productivity between varieties.
Energies 2024, 17, x FOR PEER REVIEW 6 of 16 Similar trends are observed when analysing hazelnut yield and seed coat weight per bush and per unit area, where 'Olbrzymi z Halle' and 'Olga' present higher weights of both whole nut and seed coat compared to the cultivars 'Kataloński' and 'Webba Cenny'.The significant differences in the weights of whole nuts and their seed coats between the analysed varieties highlight the influence of genetics on these traits.This provides valuable information for variety selection to optimise production, where 'Olbrzymi z Halle' may be preferred for its higher hazelnut yield and seed coat weight, while 'Webba Cenny' may be less desirable in the context of commercial hazelnut production and waste biomass acquisition (Table 2).
The cluster analysis presented in the dendrogram (Figure 2) illustrates the classification of hazelnut cultivars in terms of the amount of waste seed coat biomass generated.The dendrogram distinguishes two main clusters, which allows an understanding of the differences in biomass productivity between varieties.The first cluster includes the varieties 'Webba Cenny' and 'Kataloński'.The proximity of these varieties on the dendrogram suggests that they are characterised by similar, lower waste biomass production.Such characteristics may be beneficial in terms of waste management and efficiency in smaller or intensively managed growing areas.
The second cluster is formed by the varieties 'Olbrzymi z Halle' and 'Olga', which show a higher amount of waste biomass generated.The higher productivity of these varieties may be more beneficial in energy applications, where more biomass translates into better energy yield, which is important in the context of biomass energy production.
The division of varieties into two main clusters provides information on the potential use of different hazelnut varieties depending on specific production and energy needs.Knowledge of the amount of waste biomass generated enables better planning and management of biomass resources in a sustainable manner.
The results of the technical and elemental analysis of hazelnut pericarp covers show the existence of statistically significant differences between varieties in many categories, suggesting the influence of genetic diversity on biomass properties (Table 3).The first cluster includes the varieties 'Webba Cenny' and 'Katalo ński'.The proximity of these varieties on the dendrogram suggests that they are characterised by similar, lower waste biomass production.Such characteristics may be beneficial in terms of waste management and efficiency in smaller or intensively managed growing areas.
The second cluster is formed by the varieties 'Olbrzymi z Halle' and 'Olga', which show a higher amount of waste biomass generated.The higher productivity of these varieties may be more beneficial in energy applications, where more biomass translates into better energy yield, which is important in the context of biomass energy production.
The division of varieties into two main clusters provides information on the potential use of different hazelnut varieties depending on specific production and energy needs.Knowledge of the amount of waste biomass generated enables better planning and management of biomass resources in a sustainable manner.
The results of the technical and elemental analysis of hazelnut pericarp covers show the existence of statistically significant differences between varieties in many categories, suggesting the influence of genetic diversity on biomass properties (Table 3).The heat of combustion for the pericarp covers tested, depending on the cultivar used, ranged from 15.77 to 16.03 MJ•kg −1 for HHV values and from 14.6 to 14.85 MJ•kg −1 for LHV.The cultivar 'Olbrzymi z Halle' shows the highest HHV (16.02 MJ•kg −1 ) and LHV (14.85 MJ•kg −1 ) values, indicating its energy potential as the highest among the cultivars tested.'Katalo ński' presents the lowest LHV values (14.06 MJ•kg −1 ) and 'Webba Cenny' the lowest HHV (15.94 MJ•kg −1 ).The differences for HHV and LHV are statistically significant, confirming the effect of cultivars on biomass energy efficiency.The LHV and HHV results for the pericarp covers tested are lower in relation to walnut shells, hazelnut shells, peanut shells, and pistachio shells [14,45].The most similar LHV values are between the cultivars 'Olga' and 'Olbrzymi z Halle' and pistachio shells.The variety 'Katalo ński' has the same LHV values as rice husk [29].In the case of the 'Katalo ński' and 'Webba Cenny' varieties, a higher level and heat of combustion was obtained for the shells of these varieties [46].
The volatile compound (V) content ranged from 65.1% for 'Webba Cenny', which is favourable for thermal processes, to 68.01% for 'Olga'.It can be noted that for this trait the difference between the extreme results was more than 2.91%.The content of volatile parts obtained in the study is the same as the tested hazelnut shells for the cultivars Istarskiduguljasti and Rimskiokrugli [47].Comparing the hazelnut shells of the cultivars 'Webba Cenny' and 'Katalo ński' in the study of Hebda et.al. [46] an average of 4% higher content of volatile parts was obtained than in the tested pericarp shells.
On the other hand, analysis of the ash content (A) of the biomass tested showed significant differences between the varieties.The discrepancy between the extreme results was 0.36%.The pericarp covers of the 'Katalo ński' variety had the highest ash content (8.67%), while the 'Webba Cenny' variety had the lowest (8.31%).Much lower ash content was found in the hazelnut husks of the cultivars 'Webba Cenny', 'Katalo ński', 'Casina' or 'Bergera' (on average by 7%) [46] in relation to the tested pericarp covers of walnut shells (by 6%), almond shells (by 5%) or sunflower shells (by 4%) [45], or peanut shells (by 4,9%) [48].Therefore, the tested material is characterized by a much higher ash content than other types of biomass.A comparable ash content was found for hazel tree leaves [49].
The lowest content of 40.76% for carbon (C) was recorded for the pericarp covers of the cultivar 'Katalo ński', while the cultivar 'Olbrzymi z Halle' showed the highest estimated value of 41.26%.Considering the carbon content, the choice of the right variety in cultivation may result in a difference of not a whole percentage in the content of this element in the pericarp covers.A much higher (about 16%) carbon concentration was recorded in the hazelnut shells of the Istarskiduguljasti and Rimskiokrugli cultivars in relation to the tested covers [47].The results obtained for the carbon content of pericarp shells are lower by 7-10% on average in relation to hazelnut husks, walnut husks, almond husks [45] and are 3% lower for sunflower husk [50].
The atomic ratio indices, i.e., H/C, are higher for the 'Olbrzymi z Halle' and 'Olga' varieties, reaching 1.67%, indicating an increased hydrogen-to-carbon content and possibly affecting combustion properties.The lowest H/C ratio value is shown by the 'Katalo ński' variety (1.65%), indicating a lower hydrogen-to-carbon ratio compared to the two previously mentioned varieties.The differences in N/C and O/C ratios are not statistically significant.However, it is worth noting that 'Webba Cenny' has the lowest N/C value of 0.02%.In addition, 'Webba Cenny' together with 'Katalo ński' achieved the lowest O/C values of 0.79%.This indicates lower nitrogen and oxygen-to-carbon content in these varieties compared to the others.
In summary, the pericarp covers of the selected varieties differ significantly in terms of energy values, moisture, ash and carbon content, which may influence their use as biofuel (Table 3).
Figure 3 shows the result of the principal component analysis for the resulting heat of combustion for the tested shoots of the four hazelnut varieties.
relation to the tested covers [47].The results obtained for the carbon content of pericarp shells are lower by 7-10% on average in relation to hazelnut husks, walnut husks, almond husks [45] and are 3% lower for sunflower husk [50].
The atomic ratio indices, i.e., H/C, are higher for the 'Olbrzymi z Halle' and 'Olga' varieties, reaching 1.67%, indicating an increased hydrogen-to-carbon content and possibly affecting combustion properties.The lowest H/C ratio value is shown by the 'Kataloński' variety (1.65%), indicating a lower hydrogen-to-carbon ratio compared to the two previously mentioned varieties.The differences in N/C and O/C ratios are not statistically significant.However, it is worth noting that 'Webba Cenny' has the lowest N/C value of 0.02%.In addition, 'Webba Cenny' together with 'Kataloński' achieved the lowest O/C values of 0.79%.This indicates lower nitrogen and oxygen-to-carbon content in these varieties compared to the others.
In summary, the pericarp covers of the selected varieties differ significantly in terms of energy values, moisture, ash and carbon content, which may influence their use as biofuel (Table 3).
Figure 3 shows the result of the principal component analysis for the resulting heat of combustion for the tested shoots of the four hazelnut varieties.The cluster analysis on the presented dendrogram for hazelnut cultivars in terms of HHV values shows the formation of two main clusters.The first cluster is formed by the cultivars 'Olbrzymi z Halle' and 'Olga', which show higher HHV values, highlighting their greater potential for efficient use in energy production.The second cluster brings together the varieties 'Kataloński' and 'Webba Cenny', which show similar lower HHV values, which may suggest their similar energy potential and combustion properties.This analysis highlights the differences in energy potential between the different variety groups.Varieties in the first cluster may be more desirable in the context of applications where maximising the energy yield from the feedstock used is key, while varieties in the second cluster may be preferred for applications requiring lower energy biomass.With this knowledge, it is possible to better plan the use of different hazelnut varieties according to specific energy needs and processing needs (Figure 3).
Table 4 shows the estimated emission factors for the tested pericarp covers of the four hazelnut varieties.The cluster analysis on the presented dendrogram for hazelnut cultivars in terms of HHV values shows the formation of two main clusters.The first cluster is formed by the cultivars 'Olbrzymi z Halle' and 'Olga', which show higher HHV values, highlighting their greater potential for efficient use in energy production.The second cluster brings together the varieties 'Katalo ński' and 'Webba Cenny', which show similar lower HHV values, which may suggest their similar energy potential and combustion properties.This analysis highlights the differences in energy potential between the different variety groups.Varieties in the first cluster may be more desirable in the context of applications where maximising the energy yield from the feedstock used is key, while varieties in the second cluster may be preferred for applications requiring lower energy biomass.With this knowledge, it is possible to better plan the use of different hazelnut varieties according to specific energy needs and processing needs (Figure 3).
Table 4 shows the estimated emission factors for the tested pericarp covers of the four hazelnut varieties.Pollutants analysed include carbon monoxide (CO), carbon dioxide (CO 2 ), nitrogen oxides (NO x ), sulphur oxide (SO 2 ) and dust.The differences in CO and CO 2 emissions between the varieties tested are not statistically significant, suggesting similar combustion performance in terms of these pollutants for all varieties.On the other hand, statistically significant differences were found for NO x emission values, which draws attention to the variation between varieties in the context of nitrogen oxide emissions.The highest values were observed for the cultivar 'Olbrzymi z Halle' at 3.02 kg•Mg −1 , while the lowest values were recorded for 'Webba Cenny' at 2.28 kg•Mg −1 .Similar significance was shown for SO 2 emissions, where the highest emissions were recorded for 'Olbrzymi z Halle' at 0.09 kg•Mg −1 and the lowest for 'Katalo ński' at 0.06 kg•Mg −1 .The differences in dust emissions are also statistically significant, with the highest value for 'Katalo ński' 10.95 kg•Mg −1 and the lowest for 'Webba Cenny' 10.49 kg•Mg −1 .
In summary, 'Olbrzymi z Halle' has higher NO x and SO 2 emissions, which may have a negative impact on the environment, while 'Webba Cenny' shows lower values for dust emissions, which may be beneficial in terms of air quality protection.The differences in CO and CO 2 emissions are minimal which allows for greater flexibility in variety selection based on other criteria, i.e., crop performance or disease resistance.
Table 5 shows the estimated volumes of flue gas components and the volume of dry and moisture-containing flue gases.The parameters analysed include the theoretical oxygen demand (V oO2 ), dry air volume (V oa ), carbon dioxide volume (V CO2 ), sulphur oxides (V SO2 ) and nitrogen oxides (V N2 ), water vapour volume (V H2O ) and total flue gas volume (V gu , V ga ), among others.
The analysed parameters in Table 5 show many similarities between the tested varieties.In the case of V oO2 , V oa , V CO2 and V H2O , their values are very similar and do not show statistically significant differences, suggesting uniform combustion conditions.The exceptions are the parameters V CO2 , V SO2 , V N2 , V gu and V ga , which showed statistically significant differences.
The largest fluctuations in parameter values can be seen for dry flue gas volume (V gu ) and wet flue gas volume (V ga ).The V gu values for the analysed hazelnut varieties range from 4.43 Nm 3 •kg −1 for the 'Webba Cenny' variety to 4.69 Nm 3 •kg −1 for the 'Olbrzymi z Halle' variety.A similar variation can be observed for the volume of wet exhaust gas (V gu ).The lowest value is reached by the 'Katalo ński' and 'Webba Cenny' variety (5.93 Nm 3 •kg −1 ) while the highest value is reached by the 'Olbrzymi z Halle' variety (6.21 Nm 3 •kg −1 ).These variations also indicate differences in the amount of dry and wet flue gases generated during the combustion of the different varieties.All V SO2 values are very low, suggesting that sulphur dioxide emissions from these varieties are minimal.V oO2 (Nm 3 •kg −1 ) 0.84 ± 0.02 a 0.86 ± 0.02 a 0.84 ± 0.02 a 0.84 ± 0.03 a 0.5264 V oa (Nm 3 •kg −1 ) 3.98 ± 0.08 a 4.09 ± 0.08 a 4.02 ± 0.10 a 3.99 ± 0.12 a 0.5264 V CO2 (Nm 3 •kg −1 ) 0.76 ± 0.004 b 0.77 ± 0.001 a 0.76 ± 0.003 ab 0.77 ± 0.004 ab 0.0322 V SO2 (Nm 3 •kg −1 ) 0.00020 ± 0 d 0.00029 ± 0 a 0.00026 ± 0 b 0.00023 ± 0 c 0.0001 V H2O (Nm 3  ±standard deviation; a, b, c, d, ab -means with the same letter in row show no significant difference and means with different letters with significant differences at α = 0.05.
When comparing the total volume of flue gases when burning the seed coats of all hazelnut varieties, a lower volume of 1 m 3 •kg −1 was found in relation to oak and poplar [31].Comparing the dry flue gas volume, the raw materials tested showed a higher volume of 0.2 m 3 •kg −1 compared to Czech knotweed and 0.3 m 3 •kg −1 compared to meadow hay [53].In contrast, the varieties 'Webba Cenny' and 'Katalo ński' showed similar dry gas volumes to Timothy grass [53].
The results of the analyses in Table 5 show that different hazelnut varieties can differ significantly in terms of their flue gas composition, which should be taken into account when selecting them both in terms of energy efficiency and minimising negative environmental impacts.
The results in Table 6 highlight the influence of the selected parameters on the content of other compounds, combustion parameters or emission factors contained in hazelnut seed coats, regardless of cultivar.An analysis of the data shows that the parameters HHV and LHV in the covers show a very strong positive correlation with S and SO 2 , and a moderate correlation with N and NO x , indicating their direct influence on the higher calorific value of the biomass.
Correlations with carbon content (C) show its important role in increasing the energy value of biomass and CO 2 , as evident in the very strong positive correlation with CO and CO 2 .It is also moderately positively correlated with N, S, NO x and SO 2 , suggesting that higher carbon concentrations may slightly contribute to the increase in these compounds.In contrast, the negative correlation with O indicates that higher carbon content is associated with a significant decrease in the oxygen content of the biomass studied.
For hydrogen (H) content, a strong negative correlation with oxygen (O) content was observed, indicating that higher hydrogen concentration in the biomass is associated with lower oxygen content.
Nitrogen content (N) shows a strong positive correlation with sulphur content (S), oxides of nitrogen (NO x ) and sulphur dioxide (SO 2 ), suggesting that higher nitrogen concentrations may contribute to the increase in these components, which is important for exhaust emissions analysis.Additionally, N shows moderate positive correlations with values such as higher heating value (HHV), lower heating value (LHV), carbon content (C) and carbon monoxide (CO) and carbon dioxide (CO 2 ) emissions, indicating its effect on improving biomass energy properties and increasing greenhouse gas emissions.The analysis of sulphur (S) content showed a very strong and robust correlation related to SO 2 and NO x emissions, suggesting its significant impact on these compounds during combustion.The strong correlations between the HHV and LHV values indicate that sulphur may improve the energy properties of biomass.S is also strongly correlated with nitrogen (N) content, which may indicate similar accumulation pathways for these elements.In addition, moderate positive correlations with carbon (C), CO and CO 2 highlight its role in combustion processes and its impact on gas emissions.In contrast, the moderate negative correlation with moisture (M) suggests that higher concentrations of sulphur may account for the lower moisture content of biomass.
Volatile matter concentration (V) shows very strong and strong negative correlations with moisture content (M) and fixed carbon content (FC).This means that higher volatile matter levels are associated with lower moisture content and lower fixed carbon content in the material, which may affect its combustion properties and overall quality as a fuel.
Solid carbon content (FC) shows a moderately positive correlation with moisture content (M).This means that higher moisture content levels are associated with slightly higher fixed carbon content in the material, which may affect its combustion properties and quality.
Carbon monoxide (CO) shows very strong positive correlations with carbon (C) and carbon dioxide (CO 2 ) content, indicating its intensive production in processes where these components are abundant.In addition, CO shows a strong positive correlation with nitrogen (N), sulphur (S), nitrogen oxides (NO x ) and sulphur dioxide (SO 2 ) content, suggesting that its presence is closely associated with combustion processes where these compounds are present in greater quantities.
For nitrogen oxides (NO x ), there were very strong positive correlations with nitrogen (N), sulphur (S) and sulphur dioxide (SO 2 ), suggesting intensive production when these components are present.In addition, NO x moderately positively correlates with HHV, LHV, C, CO and CO 2 , indicating a link to combustion processes.In contrast, a moderate negative correlation with oxygen (O) content shows that a higher presence of oxygen can reduce NO x emissions.
Carbon dioxide (CO 2 ) is strongly correlated with carbon content (C) and carbon monoxide (CO), indicating their main influence on its production.In addition, CO 2 correlates moderately with N, S, NO x and SO 2 , showing their joint contribution to emissions during combustion.Also significant is the strong negative correlation with O, indicating a reduction in CO 2 emissions at higher oxygen levels.
Sulphur dioxide (SO 2 ), on the other hand, shows very strong positive correlations with nitrogen (N), sulphur (S) and nitrogen oxides (NO x ), highlighting their influence on its emissions.SO 2 also correlates strongly with the calorific values of HHV and LHV, and moderately with coal (C), CO and CO 2 , indicating their role in SO 2 production during combustion.In addition, the moderate negative correlation with oxygen (O) suggests that higher oxygen concentrations may reduce SO 2 emissions.
Dust content is strongly correlated with higher ash content (A) and shows a moderate negative correlation with moisture content (M), indicating that lower moisture content favours higher dust accumulation.
The correlation analysis for the seed coats of selected varieties shows the influence of varietal characteristics on the variation of these parameters (Table 7).
For the cultivar 'Katalo ński', the morphological characteristics of the seed coats are very strongly negatively correlated with the values of HHV, LHV, carbon content (C) and volatile matter (V).In addition, a strong negative correlation is observed for carbon monoxide (WE CO ) and carbon dioxide (WE CO2 ) indices.On the other hand, there is a very strong positive correlation between morphological characteristics and ash content (A), moisture content (M) and dust emission (WE Dust ).The morphological traits of the seed covers of the cultivar 'Katalo ński' tend to reduce the energy potential of this biomass, which can be an important factor in the selection of material for energy purposes.In the case of the cultivar 'Olbrzymi z Halle', its varietal traits are very strongly positively correlated with fixed carbon content (FC) and moderately positively with H.An inverse relationship was observed for M and WE Dust , where very strong negative correlations were shown.The morphological characteristics of the seed coats of the cultivar 'Olbrzymi z Halle' increase the biomass energy value through higher fixed carbon and hydrogen content, while reducing moisture and dust emission problems.These characteristics make this variety potentially more attractive for energy purposes, providing more efficient combustion and better emission Parameters.
The varietal traits of 'Olga', on the other hand, correlate very strongly positively with FC, which is beneficial for the energetic use of biomass, as carbon is the main energy component and also strongly positively with O, which can influence combustion processes by increasing the availability of oxygen necessary for efficient combustion.An inverse relationship was observed with N, A, V, WE Dust and W Nox , where a very strong negative relationship was shown and a strong negative relationship with H. Lower ash content and lower dust emissions are beneficial in terms of boiler operation and maintenance and reduced impact on air quality.
On the other hand, 'Webba Cenny' varietal traits correlate very strongly positively with HHV, LHV and O.In contrast, a very strong negative correlation was observed in relation to C, H, A, WE co , WE co2 and WE Dust .Morphological traits of the pericarp cover of the cultivar 'Webba Cenny' have a mixed effect on their energy and environmental potential.On the one hand, better energy properties indicated by higher HHV and LHV and higher oxygen content may improve combustion efficiency.On the other hand, the lower content of key energy components like carbon and hydrogen and emissions may limit the energy potential of biomass and its environmental impact.
The analysis highlights how different varietal characteristics of the seed coat affect key combustion-related parameters, which may be useful for further selection and breeding of hazelnut varieties with a view to optimising their energy use and reducing emissions of harmful components.In conclusion, different hazelnut cultivars show a variety of correlation profiles between the analysed parameters and their energy values and emissions, which is relevant for the selection of a cultivar for specific energy applications.

Figure 1 .
Figure 1.Flow chart of the test rig.

Figure 1 .
Figure 1.Flow chart of the test rig.

Figure 2 .
Figure 2. Comparative analysis of the pericarp cover of selected hazelnut cultivars in terms of the amount of waste biomass generated.

Figure 2 .
Figure 2. Comparative analysis of the pericarp cover of selected hazelnut cultivars in terms of the amount of waste biomass generated.

Figure 3 .
Figure 3. Comparative analysis of seed coats of selected hazelnut cultivars tested for energy production (HHV).

Figure 3 .
Figure 3. Comparative analysis of seed coats of selected hazelnut cultivars tested for energy production (HHV).

Table 1 .
A summary of the methods and apparatuses used for the energy and carbon analysis of the raw materials studied. 3

Table 2 .
Comparison of hazelnut yield and pericarp weight according to the cultivar used in the crop.

Table 3 .
Comparison of the results of the technical and elemental analysis of hazelnut seed coats according to the variety used in cultivation

Table 3 .
Comparison of the results of the technical and elemental analysis of hazelnut seed coats according to the variety used in cultivation.
a, b, c, d, ab -means with the same letter in row show no significant difference and means with different letters with significant differences at α = 0.05.

Table 4 .
Emission parameters for hazelnut pericarp covers depending on the variety used in cultivation.

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

Table 6 .
Analysis of significant multivariate correlations of combustion parameters, elements and emission factors independently of the hazelnut variety analysed.

Table 7 .
Analysis of the relationship of combustion parameter values, elements and emission factors to morphological characteristics of the pericarp cover for each variety evaluated.