Assessment of the Energy Parameters of Pedicels and Pomace of Selected Grapevine Varieties from the PIWI Group

Abstract

In view of the growing challenges related to energy transition and the need to implement circular economy principles, the use of waste from the wine industry as bioenergy raw materials is becoming increasingly important. The aim of the study was to assess the energy potential of biomass in the form of grape stems and pomace from four varieties (PIWI)—Hibernal, Muscaris, Regent and Seyval Blanc—grown in south-eastern Poland. The analyses included the determination of technical and elementary parameters, pollutant emission indicators and exhaust gas composition parameters. The pomace was characterised by a higher calorific value, higher carbon (C) and hydrogen (H) content and lower dust emissions compared to the stems, but with higher carbon dioxide (CO₂) emissions. Stems had a higher ash content, which may limit their energy use. The Hibernal variety achieved the highest calorific values at low moisture and low sulphur content, while Muscaris was characterised by increased nitrogen and sulphur content and higher sulphur dioxide emissions (SO₂) and dust emissions. The Regent variety showed relatively high nitrogen oxides (NO_X) emissions. Cluster analysis confirmed the diversity of varieties in terms of energy potential and waste biomass quantity. The results indicate that waste from PIWI grapevine cultivation can be a valuable local raw material for renewable energy production, contributing to waste reduction and greenhouse gas emissions in the agricultural sector, but its suitability depends on the variety and type of biomass.

1. Introduction

The international community is currently facing a profound energy crisis resulting from the rapid depletion of traditional energy resources and the growing need to urgently reduce greenhouse gas emissions [1]. It is widely recognised that climate change poses serious threats to public health, the integrity of environmental systems and social stability. Its consequences include, among others, an increase in global average temperatures and sea levels, as well as an increased frequency and intensity of extreme weather events [2]. In the face of these challenges, there is an urgent need to develop sustainable bioenergy sources that could be an integral part of the energy transition towards a low-carbon future [3]. The use of renewable energy sources in agriculture is a key element of the global strategy for decarbonising the energy sector, enabling a shift away from fossil fuels and achieving carbon neutrality by 2050 [4]. Both local and interregional cooperation play an important role in shaping sustainable bioenergy production models, promoting the efficient use of resources and the transfer of knowledge and technology [5]. A number of initiatives have now been developed to promote the energy transition; their common goal is to increase the share of renewable energy in the global energy mix and improve energy efficiency in various sectors of the economy [6,7].

By-products from agricultural activities are increasingly being used as raw materials for bioenergy production, providing an efficient and sustainable source of renewable energy [8]. Recently, there has been a noticeable increase in interest in the implementation of sustainable and environmentally friendly technologies for converting waste into energy, based on the use of agricultural biomass as a clean, inexhaustible, renewable and carbon-neutral source of energy [9]. The types of biomass used as a source of bioenergy are referred to as bioenergy raw materials. Bioenergy is used in a wide range of human activities, including heating, cooling, transport and various sectors such as industry, agriculture and communication. However, its use in the context of the biochemical use of biomass intended for food and feed purposes is excluded [10]. The production of agri-food waste is an integral part of the increasing productivity of the agricultural sector. It is predicted that even measures aimed at sustainable intensification of agricultural production, i.e., increasing yields while maintaining the integrity of ecosystems, are insufficient [11]. Plant residues and the biomass generated by agricultural activities and agriculture-related industries are often simplistically viewed as a waste problem requiring appropriate measures. A comprehensive approach to their management is essential both from an environmental perspective and to improve the economic conditions of agricultural producers [12]. Despite their wide availability, waste from the agro-industrial sector is, in many cases, insufficiently managed, resulting in both environmental degradation and inefficient use of resources. Nevertheless, these challenges present a significant opportunity: the possibility of transforming agro-industrial waste into valuable energy carriers and high value-added products [13]. Agricultural waste has been recognised as a promising source of both valuable products and renewable energy. However, its diverse and irregular chemical and physical composition poses a significant technological challenge, hindering the optimisation of processing and the full exploitation of its production potential [14].

The characteristics of food waste, including its chemical composition and physical properties, have a significant impact on the efficiency of bioenergy conversion processes. Proper waste management using the zero-waste policy can significantly contribute to the implementation of the circular bioeconomy concept, in which waste becomes a valuable resource, closing the material and energy cycle [15]. Today, sustainable agriculture is a key condition not only for achieving sustainable development goals but also for ensuring the long-term economic and environmental stability of individual farms [16]. Converting agri-food waste into bioenergy is a sustainable alternative that simultaneously reduces pollution and increases energy recovery efficiency [17]. Viticulture is one of the most profitable and longest-cultivated forms of agriculture in the world [18]. In recent years, the wine industry has shown great interest in the topic of sustainable development. It is increasingly recognised that the implementation of circular economy solutions is key to the effective achievement of the Sustainable Development Goals (SDGs) set by the United Nations [19]. It is worth noting that despite the numerous benefits of the wine industry, it also has some negative aspects, among which the generation of significant amounts of waste is particularly important [20]. Although winemaking is one of the oldest known agricultural and food production processes, the issue of waste recovery and sustainable management in this sector has only recently become a priority area of research [21]. Although the wine sector offers significant opportunities for economic benefits, the issue of its sustainable development remains a subject of ongoing debate, covering economic, social and environmental aspects [22]. By overcoming barriers and promoting sustainable development, the wine industry can support the circular economy, especially through cooperation between science and industry in the creation of innovative, resource-efficient solutions [23]. A significant proportion of global grape production is used exclusively for industrial wine production, which generates by-products. The pomace from pressing grapes is currently the main by-product, consisting of residues in the form of skins, seeds and stems. The variety of strategies developed for their management demonstrates the significant potential for the revalorisation of this raw material [24]. In view of the implementation of sustainable practices in the wine-growing and winemaking sectors, the industry must be ready to adapt immediately to the new realities, which require fundamental changes in existing operating models [25].

The growing interest in the sustainable use of biomass is due to the need to reduce greenhouse gas emissions and the growing demand for alternative energy sources. In the wine sector, waste such as grape pomace and stems remains largely untapped, despite its energy potential and biogas production possibilities. Previous studies have focused mainly on the general properties of biomass, with limited analysis of the differences between individual grape components. This study aimed to determine the energy potential of pomace and stems, examine the impact of these differences on combustion and biogas production potential, and compare the results with the literature in the context of varieties, climatic conditions and processing methods.

This work is an assessment of the energy potential derived from biomass from grape stems and pomace of PIWI grape varieties, namely ‘Seyval Blanc’, ‘Muscaris’, ‘Hibernal’ and ‘Regent’.

2. Materials and Methods

Field research was conducted in 2024 at the Experimental Vineyard of the University of Life Sciences in Lublin, located in south-eastern Poland. PIWI grapevine varieties, Hibernal, Muscaris, Regent and Seyval Blanc, were planted in spring 2020 at a spacing of 2.5 × 1.2 m (3333 plants per hectare) on loess soil. The vines were trained using a single Guyot system, with a trunk height of 40 cm, a single-shoot length of approximately 1.0 m and a single double-bud shoot. The grapes were harvested manually when they reached optimal ripeness, between 23.5° and 24.5° Brix. A total of 25 kg of grapes were harvested from each combination. The bunches were divided into stalks and berries. The stalks were dried at a controlled temperature, while the berries were crushed by hand and the juice was separated using a Lancman 55 hydropress.

The detailed methodology for measuring the parameters is discussed in

Table 1. Fuel characterization analysis.

PARAMETER	METHOD	EQUIPMENT
Energetic properties
Higher Heating Value (HHV; MJ·kg−1)	EN-ISO 1928:2020 [26]	isoperibolic calorimeter LECO AC 600
Lower Heating Value (LHV; MJ·kg−1)
Proximate Analysis
Ash (A; %)	EN-ISO 18122:2022 [27]	thermogravimetric analyser LECO TGA 701
Volatile matter (V; %)	EN-ISO 18123:2023 [28]
Moisture (MC; %)	EN-ISO 18134:2023 [29]
Fixed carbon (FC; %)	FC = 100 − V − A − M [30]
Ultimate Analysis
Carbon (C; %)	EN-ISO 16948:2015 [31]	elemental analyser LECO CHNS 628
Hydrogen (H; %)
Nitrogen (N; %)
Sulfur (S; %)	EN-ISO 16994:2016 [32]
Oxygen (O; %)	O = 100 − A − H − C − S − N [33]

,

Table 2. Emission factors (emission factors calculated according to previous studies [34]).

PARAMETER	METHOD
Carbon monoxide emission factor (Ec) of chemically pure coal (CO; kg·Mg−1)	$\mathrm{C}\mathrm{O}={\displaystyle \frac{28}{12}}·{\mathrm{E}}_{\mathrm{c}}·(\mathrm{C}/\mathrm{CO}),$ $\mathrm{CO}\text{—}\mathrm{carbon} \mathrm{monoxide} \mathrm{emission} \mathrm{factor} (\mathrm{kg}·\mathrm{kg}{-}^1), {\displaystyle \frac{28}{12}}$- 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)	${\mathrm{C}\mathrm{O}}_2={\displaystyle \frac{44}{12}}·\left({\mathrm{E}}_{\mathrm{c}}-{\displaystyle \frac{12}{28}}·\mathrm{C}\mathrm{O}-{\displaystyle \frac{12}{16}}·{\mathrm{E}}_{{\mathrm{C}\mathrm{H}}_4}-{\displaystyle \frac{26.4}{31.4}}·{\mathrm{E}}_{\mathrm{N}\mathrm{M}\mathrm{V}\mathrm{O}\mathrm{C}}\right),$ 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)	${\mathrm{S}\mathrm{O}}_2={\displaystyle \frac{2\mathrm{S}}{100}}·\left(1-\mathrm{r}\right),$ 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)	${\mathrm{N}\mathrm{O}}_{\mathrm{x}}={\displaystyle \frac{46}{14}}·{\mathrm{E}}_{\mathrm{c}}·\mathrm{N}/\mathrm{C}·\left({\mathrm{N}}_{{\mathrm{N}\mathrm{O}}_{\mathrm{x}}}/\mathrm{N}\right)$, 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 rapidly to nitrogen dioxide, N/C—nitrogen to carbon ratio in biomass, NNOX/N—part of nitrogen emitted as NOX (for biomass 0.122).

and

Table 3. Exhaust gas composition (exhaust gas composition was calculated according to [35,36]).

PARAMETER	METHOD
Theoretical oxygen demand $({\mathrm{V}}_{{\mathrm{O}}_2}$; Nm3·kg−1)	${\mathrm{V}}_{{\mathrm{O}}_2}={\displaystyle \frac{22.41}{100}}·\left({\displaystyle \frac{\mathrm{C}}{12}}+{\displaystyle \frac{\mathrm{H}}{4}}+{\displaystyle \frac{\mathrm{S}-\mathrm{O}}{32}}\right)$, 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 $({\mathrm{V}}_{{\mathrm{O}}_{\mathrm{a}}}$; Nm3·kg−1)	${\mathrm{V}}_{{\mathrm{O}}_{\mathrm{a}}}={\displaystyle \frac{{\mathrm{V}}_{{\mathrm{O}}_2}}{0.21}},$ Since the oxygen content in the air is 21%, which participates in the combustion process in the boiler, this is the stoichiometric volume of dry air required to burn 1 kg of biomass
Carbon dioxide content of the combustion products $({\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}$; Nm3·kg−1)	${\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}={\displaystyle \frac{22.41}{12}}$$·{\displaystyle \frac{\mathrm{C}}{100}},$
Content of sulphur dioxide $({\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}$; Nm3·kg−1)	${\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}={\displaystyle \frac{22.41}{32}}$$·{\displaystyle \frac{\mathrm{S}}{100}}$,
Water vapour content of the exhaust gas (${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$; Nm3·kg−1)	${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{H}}={\displaystyle \frac{22.41}{100}}·\left({\displaystyle \frac{\mathrm{H}}{2}}+{\displaystyle \frac{\mathrm{M}}{18}}\right)$, This is the component of water vapour volume from the hydrogen combustion process $({\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{H}}$$; {\mathrm{Nm}}^3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1}\mathrm{fuel}) {\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{a}}=1.61·\mathrm{x}·{\mathrm{V}}_{{\mathrm{O}}_{\mathrm{a}}}$ and the volume of moisture contained in the combustion air $({\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}^{\mathrm{a}}$$; {\mathrm{Nm}}^3{\mathrm{H}}_2\mathrm{O}·{\mathrm{kg}}^{-1}\mathrm{fuel}) {\mathrm{V}}_{\mathrm{H}2\mathrm{O}}$$={\mathrm{V}}_{\mathrm{H}2\mathrm{O}}^{\mathrm{H}}$$+{\mathrm{V}}_{\mathrm{H}2\mathrm{O}}^{\mathrm{a}}$; M-fuel moisture content (%), x-air absolute humidity (kg H2O·kg−1 dry air).
The theoretical nitrogen content in the exhaust gas (${\mathrm{V}}_{{\mathrm{N}}_2}$; Nm3·kg−1)	${\mathrm{V}}_{{\mathrm{N}}_2}={\displaystyle \frac{22.41}{28}}·{\displaystyle \frac{\mathrm{N}}{100}}+0.79·{\mathrm{V}}_{{\mathrm{O}}_{\mathrm{a}}}$, Considering that the nitrogen in the exhaust comes from the fuel composition and the combustion air and that the nitrogen content in the air is 79%.
The total stoichiometric volume of dry exhaust gas ${(\mathrm{V}}_{\mathrm{o}\mathrm{g}\mathrm{u}};$ Nm3·kg−1)	${\mathrm{V}}_{\mathrm{o}\mathrm{g}\mathrm{u}}={\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}+{\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}+{\mathrm{V}}_{{\mathrm{N}}_2}$
The total volume of exhaust gases $({\mathrm{V}}_{\mathrm{o}\mathrm{g}\mathrm{a}}$; Nm3·kg−1)	${\mathrm{V}}_{\mathrm{o}\mathrm{g}\mathrm{a}}={\mathrm{V}}_{\mathrm{o}\mathrm{g}\mathrm{u}}+{\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$ 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.

. Before testing, the material was placed in a room with a constant temperature and humidity, where it was dried naturally in air-drying conditions at a temperature of 20 °C and an air humidity of 55% to 60% for two weeks in order to standardise the assessment conditions. The material for laboratory analysis was first ground (to a thickness of 0.5 mm) using a Retsch SM 100 mill (Retsch GmbH, Haan, Germany, 2022) with a power of 1.5 kW. For the purposes of this study, 100 g of material was obtained within 2 min, which consumed 0.045 kWh. Our results represent values in an analytical state; additionally, in the case of HHV, the analytical state was converted to a dry state, without ash. The methodology of the procedures is shown in Table 1.

The detailed methodology of the measured parameters is discussed in Table 1, Table 2 and Table 3.

The experiment was conducted in a randomized block design with four combinations and five replicates, each with three plants per plot. After the experiment, the results were statistically analysed using one-way analysis of variance (ANOVA). Comparisons were made between varieties within each plant material separately, as well as comparisons of plant material for each variety separately, at a significance level of p < 0.05. Multivariate analyses were used, including cluster and principal component analyses. The yield and quality of winemaking waste, as well as the caloric value of stems and pomace, were presented using a dendrogram. The relationships between the biomass components of productive vines, regardless of the plant material, were determined separately for each variety. All analyses were performed using STATISTICA 13 software (StatSoft, Inc.; TIBCO Software; Palo Alto, CA, USA; 2015).

This article has been divided into several sections to ensure clarity and the logical flow of the research presentation. The introduction presents the objectives of the study, research questions and justification of the research gap. This is followed by a description of the materials and methods, including sample characteristics and analytical and statistical procedures. The results section presents the data in the form of tables and figures, allowing for a comparison of the properties of individual biomass components. The discussion interprets the results obtained in the context of the literature, pointing to possible reasons for the observed differences. The article concludes with conclusions that synthesize the main observations and suggest potential practical applications.

3. Results and Discussion

In this section, the results are presented systematically by dividing them into individual categories of analysed variables. The tables and figures include both mean values and standard deviations, allowing the reader to assess the distribution and variability of the data. In addition, any statistically significant differences are clearly marked, which facilitates the interpretation and comparison of individual research groups.

The table presents the results of the analysis of selected physicochemical properties of different grapevine varieties and two types of plant material, together with an assessment of the statistical significance of differences between varieties, materials and their interaction.

Among the varieties, Hibernal was characterised by the highest heating values (HHV 17.04 MJ·kg⁻¹; LHV 15.90 MJ·kg⁻¹) and the lowest moisture content (5.09%), which indicates its favourable energy potential. In contrast, Regent achieved the lowest HHV and LHV values (16.91 and 15.76 MJ·kg⁻¹, respectively).

The type of plant material significantly differentiated the parameters, except for the nitrogen and oxygen content. Pomace was characterised by a higher calorific value (HHV 17.37 MJ·kg⁻¹; LHV 15.67 MJ·kg⁻¹), higher carbon (46.76%) and hydrogen (6.75%) content, and higher volatility (67.84%) and solid fraction (20.56%) compared to stalks. In turn, stalks showed a higher ash content (8.85%), which may limit their suitability as an energy source. The sulphur content was slightly higher in pomace (0.09%) than in stalks (0.05%).

A significant interaction between variety and type of material was also found for most of the traits studied, including calorific values, nitrogen, ash, volatile matter and solid fraction, indicating that the influence of variety on these parameters varied depending on the plant material. No significant interaction was observed only for carbon, hydrogen and sulphur content (

Table 4. Technical and elemental analysis for different grapevine varieties (A), types of plant material (B) and interaction of both factors (A*B).

	Name	HHV	LHV	MC	A	V	FC
	Unit	MJ·kg−1		%
Variety (A)	Regent	16.91 ± 1.04 b	15.76 ± 1.03 b	6.05 ± 0.40 a	8.87 ± 1.22 b	65.66 ± 1.09 b	19.42 ± 0.55 b
	Seyval Blanc	17.00 ± 0.85 a	15.82 ± 0.83 ab	6.16 ± 0.66 a	6.79 ± 2.36 d	66.55 ± 2.83 ab	20.51 ± 0.23 a
	Hibernal	17.04 ± 1.36 a	15.90 ± 1.34 a	5.09 ± b 0.81 a	8.23 ± 2.47 c	67.34 ± 1.03 a	19.36 ± 3.04 b
	Muscaris	17.00 ± 0.61 a	15.86 ± 0.61 a	6.12 ± 0.11 a	10.03 ± 0.17 a	66.42 ± 1.65 ab	17.44 ± 1.8 c
	p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
Material (B)	Stem	16.11 ± 0.26 b	14.96 ± 0.25 b	6.25 ± 0.38 a	9.85 ± 0.6 a	65.14 ± 1.35 b	18.76 ± 1.48 b
	Pulp	17.87 ± 0.28 a	16.70 ± 0.28 a	5.45 ± 0.72 b	7.11 ± 2.13 b	67.84 ± 0.94 a	19.6 ± 2.43 a
	p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
A*B	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 content, V = volatile matter content, FC = fixed carbon; * Significant difference means that different letters in the column indicate significant differences at α = 0.05.

).

Table 3 and

Table 5. Comparison of LHV and HHV for shoots of selected grapevine varieties.

Material	LHV	HHV
	MJ·kg−1
Sauvignon blanc [37]	17.3	18.7
Pinot [37]	15.1	16.5
Cabernet Sauvignon [37]	16.2	17.6
Chardonnay [37]	16.2	17.6

compare energy parameters reported in the literature for selected biomass sources. Comparing the obtained HHV and LHV results with data from the literature, it can be seen that, regardless of the grape variety, grape pomace is similar to Cabernet Sauvignon and Chardonnay shoots in terms of HHV and LHV. The stems showed lower energy values than the shoots of each of the analysed grapevine varieties. Nevertheless, it can be concluded that the average HHV and LHV for each variety of both materials are lower than the corresponding values for grapevine shoots, except for the Pinot variety.

The highest nitrogen (1.82%) and sulphur (0.08%) content was found in the Muscaris variety, while the lowest values of these elements were found in Seyval Blanc (0.97% N) and Hibernal (0.07% S), respectively (

Table 6. Elemental analysis of stems and pomace depending on grape variety.

	Name	C	H	N	S	O	H/C	N/C	O/C
	Unit	%
Variety (A)	Regent	45.64 ± 1.91 a	6.77 ± 0.36 a	1.42 ± 0.06 b	0.06 ± 0.01 ab	37.24 ± 1.12 c	0.03 ± 0.00 b	11.21 ± 1.55 b	0.61 ± 0.04 b
	Seyval Blanc	45.79 ± 0.38 a	6.98 ± 0.56 a	0.97 ± 0.05 d	0.06 ± 0.03 b	39.41 ± 1.47 a	0.02 ± 0.00 d	8.57 ± 2.98 d	0.65 ± 0.02 a
	Hibernal	45.60 ± 2.02 a	6.62 ± 0.54 a	1.11 ± 0.08 c	0.07 ± 0.02 ab	38.37 ± 0.6 b	0.02 ± 0.00 c	10.39 ± 3.12 c	0.63 ± 0.03 ab
	Muscaris	45.70 ± 0.64 a	6.75 ± 0.44 a	1.82 ± 0.27 a	0.08 ± 0.03 a	35.63 ± 0.74 d	0.04 ± 0.01 a	12.67 ± 0.21 a	0.58 ± 0.02 c
	p-value	0.9963	0.6458	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
Material (B)	Stem	44.60 ± 0.86 b	6.52 ± 0.19 b	1.35 ± 0.45 a	0.05 ± 0.02 b	37.63 ± 1.09 a	0.03 ± 0.01 a	12.44 ± 0.76 a	0.63 ± 0.03 a
	Pulp	46.76 ± 0.7 a	7.04 ± 0.52 a	1.31 ± 0.24 a	0.09 ± 0.01 a	37.69 ± 2.25 a	0.03 ± 0.01 b	8.98 ± 2.69 b	0.6 ± 0.04 b
	p-value	0.0001	0.0001	0.7681	0.0001	0.9437	0.0001	0.0001	0.0001
A*B	p-value	0.0001	0.1724	0.0001	0.0816	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.

). The oxygen content was highest in Seyval Blanc (39.41%), and ash content was highest in Muscaris (10.13%). Volatility (V) reached the highest values in Hibernal (67.34%), while the fixed fraction (FC) was highest in Seyval Blanc (20.51%). No significant differences were found between the varieties in terms of carbon (C) and hydrogen (H) content. The analysis showed that the grapevine variety had a significant effect on most of the parameters studied, with the exception of carbon (C) and hydrogen (H) content. The type of material significantly differentiated all characteristics except nitrogen (N) and oxygen (O) content. A significant interaction effect of the factors was observed for most parameters, except for carbon, hydrogen and sulphur (S) content (Table 4).

The analysis of carbon content showed that, regardless of the variety, similar contents were found in both the stems and pomace. The values were similar to those in Vitis vinifera (Sabor) [38] or Pruning Vine [39], and lower than those in Grape marc [40] and Grapevine prunings [40] (

Table 7. Comparison of proximate and ultimate analysis for wine production waste.

Name	C	H	N	S	O	A	V	FC
Unit	%
Vitis vinifera (Sabor) [38]	44.2	6.1	1.0	0.07	43.0	7.2	-	-
Grape marc [40]	50.10	6.30	2.20	-	41.40	10.27	75.92	14.98
Grapevine prunings [40]	48.39	6.65	0.49	-	44.47	1.99	83.03	14.98
Pruning Vine [39]	44.62	5.77	0.70	0.05		2.6

). The hydrogen content was similar to that of Grapevine prunings [40], and the sulphur content was similar to that of Pruning Vine [39]. The nitrogen content was different for the analysed materials, and higher than for Vitis vinifera (Sabor) pruning [38] and lower than for Grape marc [40]. The ash content was high for the stalks and similar (although lower) to that for Grape marc [40] and the pomace from Vitis vinifera (Sabor) [38]. The volatile matter content in both the stems and pomace was lower than in Grape marc [40] or Grapevine prunings [40], while fixed carbon was correspondingly higher.

The table presents the results of the analysis of selected pollutant emissions and element ratios for different grapevine varieties and two types of plant material, together with an assessment of the statistical significance of differences between varieties, materials and their interaction (

Table 8. Emission indices of selected pollutants for different grapevine varieties (A), types of plant material (B) and interactions between both factors (A*B).

	Name	CO	NOX	CO2	SO2	Dust
	Unit	kg Mg−1
Variety (A)	Regent	56.22 ± 2.35 a	5.00 ± 0.21 b	1376.80 ± 57.58 a	0.13 ± 0.03 ab	1.48 ± 0.02 a
	Seyval Blanc	56.41 ± 0.47 a	3.44 ± 0.18 d	1381.37 ± 11.44 a	0.11 ± 0.06 b	1.52 ± 0.11 a
	Hibernal	56.18 ± 2.49 a	3.93 ± 0.28 c	1375.79 ± 60.92 a	± 0.14 ± 0.04 ab	1.45 ± 0.11 a
	Muscaris	56.30 ± 0.79 a	6.41 ± 0.95 a	1378.64 ± 19.34 a	0.16 ± 0.07 a	1.48 ± 0.10 a
	p-value	0.9963	0.0001	0.9963	0.0001	0.6054
Material (B)	Stem	54.95 ± 1.06 b	4.77 ± 1.6 a	1345.56 ± 25.84 b	± 0.09 ± 0.03 b	1.46 ± 0.04 a
	Pulp	57.61 ± 0.86 a	4.61 ± 0.86 a	1410.74 ± 21.17 a	0.18 ± 0.02 a	1.51 ± 0.12 a
	p-value	0.0001	0.7681	0.0001	0.0001	0.2202
A*B	p-value	0.0001	0.0001	0.0001	0.8018	0.0834

Explanation: CO—carbon monoxide emission; CO₂—carbon dioxide emission; SO₂—sulphur dioxide emission; NO_X—nitrogen oxides emission; Dust—particulate matter emission; * Significant differences are indicated by different letters within columns; significance level was set at α = 0.05.

).

The analysis showed that the grapevine variety had a significant effect on most of the parameters studied, with the exception of CO, H/C and CO₂. The type of material significantly differentiated all characteristics, except NO_X and H/C. A significant interaction effect between variety and material was also observed for most parameters, with the exception of H/C and SO₂.

Among the varieties, Regent was characterised by relatively high CO emissions (CO 56.22 kg·Mg⁻¹), the highest NO_X emissions (5.00 kg·Mg⁻¹) and high CO₂ emissions (CO₂ 1376.80 kg·Mg⁻¹). The lowest NO_X emissions were recorded in the Seyval Blanc variety (3.44 kg·Mg⁻¹), which also had the lowest SO₂ emissions (WESO₂ 0.11 kg·Mg⁻¹) and dust emissions (dust 8.57 kg·Mg⁻¹).

The Hibernal variety was distinguished by the lowest N/C ratio (0.01%), high O/C ratio (0.63%) and low dust emissions (dust 10.39 kg·Mg⁻¹). Muscaris showed the highest values for the N/C ratio (0.04%), SO₂ emissions (SO₂ 0.16 kg·Mg⁻¹) and dust emissions (dust 12.67 kg·Mg⁻¹).

No significant differences were found between varieties in terms of CO emissions (WECO) (Regent 56.22, Seyval Blanc 56.41, Hibernal 56.18, Muscaris 56.30 kg·Mg⁻¹), H/C ratio (range 1.45–1.52%) and CO₂ emissions (CO₂ in the range 1375–1381 kg·Mg⁻¹).

High NO_X emissions in the Regent variety and high SO₂ and dust emissions in the Muscaris variety may indicate less favourable environmental parameters for these biomasses in the context of their energy use.

The type of plant material significantly differentiated most parameters. Compared to stalks, pomace was characterised by higher CO emissions (CO 57.61 kg·Mg⁻¹) (54.95 kg·Mg⁻¹), higher CO₂ emissions (CO₂ 1381.37 vs. 1345.56 kg·Mg⁻¹) and lower dust emissions (dust 8.98 vs. 12.44 kg·Mg⁻¹).

The N/C ratio was slightly higher in stems (0.03%) than in pomace (0.02%), as was the O/C ratio (0.63% in stems vs. 0.60% in pomace). SO₂ (SO₂) emissions were higher in pomace (0.11 kg·Mg⁻¹) than in stems (0.09 kg·Mg⁻¹). No significant differences were observed for NO_X emissions (stems 4.77, pomace 4.16 kg·Mg⁻¹) and the H/C ratio (stems 1.46, pomace 1.52%).

The observed differences indicate that, compared to stems, pomace may be a more favourable fuel in terms of dust emissions but is characterised by higher CO₂ emissions, which should be taken into account when assessing its environmental impact.

A significant interaction between variety and material type was also found for most of the traits studied, including CO, NO_X, N/C, O/C, CO₂ and dust emissions, which means that the influence of variety on these parameters varied depending on the plant material. Only the H/C ratio and SO₂ emissions were found to have no significant interaction (Table 8).

Table 9. Comparison of emission indices for selected agrobiomass types.

Name	CO	NOX	CO2	SO2	Dust
Unit	kg Mg−1
Radiata pine wood [41]	49.85	0.41	1947.52	0.48	-
Nothofagus obliqua [41]	41.99	0.56	1801.67	0.41	-
Jackfruit seeds [33]	51.46	8.71	1232.43	0.11	-
Wheat straw [34]	50.57	1.83	1238.24	0.14	10.56

presents a comparison of emission indicators for selected biomass types. When analysing emission indicators, it can be observed that both cones and pomace, regardless of variety, exhibit higher CO emissions than plant biomass. Taking into account the heat generated from the combustion of both cones and pomace, depending on the variety, the study showed higher CO emissions. In the case of CO₂, the data looks different. Both stalks and pomace, depending on and independently of the variety, show lower emissions of this gas than Pinus radiata wood and Nothofagus obliqua, but higher emissions than non-wood biomass, i.e., Jackfruit seeds and wheat straw. Nitrogen emission indicators are quite high but lower than those for Jackfruit seeds. SO₂ emission rates were lower than those for wood biomass (Pinus radiata wood, Nothofagus obliqua). Dust emission levels are significantly lower than in agrobiomass (wheat straw) and reach a level approx. 8 kg·Mg⁻¹ lower for stalks and pomace, depending on and independently of the variety.

Among the varieties tested, Regent had the highest ${\mathrm{V}}_{{\mathrm{N}}_2}$ (4.79 Nm³·kg⁻¹), V_oga (7.22 Nm³·kg⁻¹) and V_ogu (5.64 Nm³·kg⁻¹) values. The lowest ${\mathrm{V}}_{{\mathrm{N}}_2}$ values were recorded in the Seyval Blanc variety (4.43 Nm³·kg⁻¹), which also had the lowest V_ogu (5.29 Nm³·kg⁻¹) and V_oga (6.89 Nm³·kg⁻¹) values.

The Hibernal variety was distinguished by the lowest ${\mathrm{V}}_{{\mathrm{O}}_2}$ (0.95 Nm³·kg⁻¹), ${\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}$ (0.85 Nm³·kg⁻¹) and ${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$ (0.81 Nm³·kg⁻¹) values. The highest ${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$ value was recorded in the Seyval Blanc variety (0.86 Nm³·kg⁻¹). Muscaris showed the highest ${\mathrm{V}}_{{\mathrm{N}}_2}$ (5.15 Nm³·kg⁻¹), V_oga (7.59 Nm³·kg⁻¹) and V_ogu (6.00 Nm³·kg⁻¹) values.

No significant differences were found between varieties in the case of ${\mathrm{V}}_{{\mathrm{O}}_2}$ (similar values: Regent 0.97, Seyval Blanc 0.97, Hibernal 0.95, Muscaris 0.98 Nm³·kg⁻¹) and ${\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}$ (all varieties ~0.85–0.86 Nm³·kg⁻¹). ${\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}$ values in all the analysed samples were 0.00 Nm³·kg⁻¹.

The type of plant material significantly differentiated all parameters. Pomace was characterised by higher values of ${\mathrm{V}}_{{\mathrm{O}}_2}$ (1.00 vs. 0.93 Nm³·kg⁻¹), ${\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}$ (0.87 vs. 0.83 Nm³·kg⁻¹), ${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$ (0.86 vs. 0.81 Nm³·kg⁻¹), ${\mathrm{V}}_{{\mathrm{N}}_2}$ (4.83 vs. 4.60 Nm³·kg⁻¹), V_oga (7.33 vs. 6.96 Nm³·kg⁻¹) and V_ogu (5.70 vs. 5.43 Nm³·kg⁻¹) compared to the pedicels.

A significant interaction between variety and material type was also found for most of the traits studied, including ${\mathrm{V}}_{{\mathrm{O}}_2}$, ${\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}$, ${\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}$, ${\mathrm{V}}_{{\mathrm{N}}_2}$, V_oga and V_ogu, which means that the effect of variety on these parameters varied depending on the plant material. No significant interaction was found only for ${\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}$ and ${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$.

The observed differences indicate that, compared to stalks, pomace had more favourable gas volume parameters, which may be important in assessing their combustion properties (

Table 10. Exhaust gas composition for different grapevine varieties (A), types of plant material (B) and interactions between both factors (A*B).

	Name	${\mathbf{V}}_{{\mathbf{O}}_2}$	${\mathbf{V}}_{{\mathbf{O}}_{\mathbf{a}}}$	${\mathbf{V}}_{{\mathbf{C}\mathbf{O}}_2}$	${\mathbf{V}}_{{\mathbf{S}\mathbf{O}}_2}$	${\mathbf{V}}_{{\mathbf{H}}_2\mathbf{O}}$	${\mathbf{V}}_{{\mathbf{N}}_2}$	Voga	Vogu
	Unit	Nm3 kg−1
Variation (A)	Regent	0.97 ± 0.06 a	4.62 ± 0.3 a	0.85 ± 0.04 a	0.00 ± 0.00 ab	0.83 ± 0.04 a	4.79 ± 0.28 b	7.22 ± 0.40 b	5.64 ± 0.32 b
	Seyval Blanc	0.97 ± 0.03 a	4.62 ± 0.13 a	0.86 ± 0.01 a	0.00 ± 0.00 b	0.86 ± 0.05 a	4.43 ± 0.14 c	6.89 ± 0.22 c	5.29 ± 0.15 c
	Hibernal	0.95 ± 0.06 a	4.54 ± 0.28 a	0.85 ± 0.04 a	0.00 ± 0.00 ab	0.81 ± 0.06 a	4.48 ± 0.27 c	6.87 ± 0.38 c	5.33 ± 0.30 c
	Muscaris	0.98 ± 0.03 a	4.68 ± 0.15 a	0.85 ± 0.01 a	0.00 ± 0.00a	0.83 ± 0.05 a	5.15 ± 0.22	7.59 ± 0.25 a	6.00 ± 0.22 a
	p-value	0.788	0.788	0.9963	0.0001	0.5583	0.0001	0.0001	0.0001
Material (B)	Stem	0.93 ± 0.03 b	4.45 ± 0.14 b	0.83 ± 0.02 b	0.00 ± 0.00 b	0.81 ± 0.02 b	4.60 ± 0.44 b	6.96 ± 0.48 b	5.43 ± 0.45 b
	Pulp	1.00 ± 0.03 a	4.78 ± 0.15 a	0.87 ± 0.01 a	0.00 ± 0.00a	0.86 ± 0.06 a	4.83 ± 0.24 a	7.33 ± 0.28 a	5.70 ± 0.25 a
	p-value	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001	0.0001
A*B	p-value	0.0199	0.0199	0.0001	0.0816	0.2593	0.0001	0.0001	0.0001

Explanations: ${\mathrm{V}}_{{\mathrm{O}}_2}$ = the theoretical oxygen demand, ${\mathrm{V}}_{{\mathrm{O}}_{\mathrm{a}}}$ = stoichiometric volume of dry air required to burn 1 kg of biomass, ${\mathrm{V}}_{{\mathrm{C}\mathrm{O}}_2}$ = the carbon dioxide content, ${\mathrm{V}}_{{\mathrm{S}\mathrm{O}}_2}$ = the content of sulphur dioxide, ${\mathrm{V}}_{{\mathrm{H}}_2\mathrm{O}}$ = the water vapour content of the exhaust gas, ${\mathrm{V}}_{{\mathrm{N}}_2}$ = the theoretical nitrogen content in the exhaust gas, V_ogu = the total stoichiometric volume of dry exhaust gas, V_oga = the total volume of exhaust gases. * Significant difference means that different letters in the column indicate significant differences at α = 0.05.

).

Table 10 compares the theoretical amount of air (V_oga) and the theoretical amount of dry flue gases (V_ogu) for selected biomass types. The analysis of the results for the generated exhaust gas volumes indicates that both stalks and pomace, depending on and independently of the variety, generate higher theoretical air (V_oga) and dry flue gas (V_ogu) emissions than pure white grape pomace, pure red grape pomace, Czech knotweed or Rumex. In both cases, the levels are, on average, 1 Nm3·kg⁻¹ higher (

Table 11. Comparison of the theoretical amount of air (V_oga) and the theoretical amount of dry flue gases (V_ogu) for selected biomass types.

Material	Voga	Vogu
Unit	Nm3 kg−1
Pure white grape pomace [42]	5.95	4.58
Pure red grape pomace [42]	6.22	4.79
Czech knotweed (Reynoutria × bohemica) [43]	4.20	4.13
Rumex OK 2 (Rumex patientia × Rumex tianschanicus) [43]	4.07	4.04

).

The results of our research have consistently shown that pomace has better energy properties than grape stems, which may be related to its specific biochemical composition and fibre structure. Pomace, which is richer in lipids and simple carbohydrates, has a higher calorific value, while stems have a higher lignin and cellulose content, which results in greater density and lower combustibility, as observed by Fernández-Puratich [37] and Jon et al. [40]. A comparison with data from the literature [38,39] shows that these differences are consistent across different grapevine varieties and climatic conditions, although absolute values may vary depending on soil, humidity and biomass processing methods. Furthermore, previous studies indicate that a higher lignin content in stems promotes the formation of biochar with a stable structure [40], while pomace may be more effective in combustion and energy production processes, as confirmed by studies by Fernández-Puratich and Malaťák et al. [42,43]. Thus, the observed differences in energy and thermochemical properties are the result of both the chemical composition and tissue structure of individual grape components, which should be taken into account when selecting the type of biomass for specific energy applications.

The principal component analysis (Figure 1) analyses the composition of selected energy parameters of the tested material in relation to the four grape varieties considered. The sum of PC1 and PC2 of the total variable characteristics for the stems of the grapevine varieties Hibernal, Muscaris, Regent and Seyval Blanc was 87.63% (57.00% for PC1 and 30.63% for PC2). Four clusters were observed, the first shows the relationships between N and H and V_ogu and V_oga. The second cluster shows the relationships between dust and SO₂, while the last two clusters represent a very strong relationship between C and CO₂ and LHV (Figure 1).

The principal component analysis (Figure 2) analyses the composition of selected energy parameters of the tested material in relation to the four grapevine varieties considered. The sum of PC1 and PC2 of the total variable traits for the pomace of the grapevine varieties Hibernal, Muscaris, Regent and Seyval Blanc was 84.30% (53.56% for PC1 and 30.74% for PC2). Three independent clusters were observed. The first shows the relationships between N, dust and SO₂, and V_ogu and V_oga. The second cluster shows a very strong relationship between C and CO₂ and LHV, while the last cluster represents H.

The dendrogram illustrates the cluster analysis of the tested grapevine varieties in terms of the amount of waste biomass produced in the form of stalks and the energy parameters obtained from the above biomass. The structure of the dendrogram distinguishes two main clusters, which allows for a deeper understanding of the differences between the varieties. The first cluster includes the Seyval Blanc and Muscaris varieties. The proximity of these varieties on the dendrogram suggests that they produce similar amounts of waste biomass with similar energy levels. The second cluster consists of Hibernal and Regent, which are characterised by similar waste biomass production. The dendrogram shows the differences in the amount of waste biomass produced by different grape varieties. The obtained results are important for matching varieties to the specific needs of a farmer or winemaker in a renewable resource management strategy. Obtaining this information can be helpful in optimising production and energy processes, providing clear guidelines for research on the selection and cultivation of grape varieties under various conditions of use (Figure 3).

When analysing both types of biomass, regardless of the variety, the same relationships were found between C, CO₂ and LHV in the case of stems (Figure 1) and pomace (Figure 2).

The dendrogram illustrates the cluster analysis of the tested grapevine varieties in terms of the amount of waste biomass produced in the form of stalks and the energy parameters obtained from the above biomass types. The structure of the dendrogram distinguishes two main clusters, which allows for a deeper understanding of the differences between the varieties. The first cluster includes the Seyval Blanc and Muscaris varieties. The proximity of these varieties on the dendrogram suggests that they produce similar amounts of waste biomass with similar energy levels. The second cluster consists of Hibernal and Regent, which are characterised by similar waste biomass production. The dendrogram provides valuable information on the differences in waste biomass produced by different grape varieties. This knowledge is crucial for matching varieties to specific operational needs and sustainable natural resource management strategies. Using such data can help optimise production and energy processes, providing clear guidelines for research on grape variety selection and cultivation under different conditions (Figure 3).

The dendrogram shows a cluster analysis of the studied grapevine varieties in terms of the amount of waste biomass produced in the form of pomace and the energy parameters obtained from the above biomass. The dendrogram distinguishes three main clusters, the first representing the Regent and Seyval Blanc varieties, the second representing the above varieties with Muscaris and the third one being a single cluster consisting of the Hibernal variety (Figure 4).

The results of this research have significant practical implications in the context of energy transition and the circular economy. Digestate, previously treated mainly as waste or fertiliser, can be converted into valuable solid biofuel. This means not only an increase in the share of renewable sources in the energy sector, but also the creation of a new stream of materials for reuse. The paper presents proposals for solutions that fit into a multidimensional model of the circular economy, integrating energy, environmental and economic aspects.

4. Conclusions

The study showed that both the grapevine variety and the type of biomass significantly affected the physicochemical properties, combustion parameters and pollutant emissions. Among the analysed varieties, Hibernal had the highest calorific value (HHV = 17.04 MJ·kg⁻¹; LHV = 15.90 MJ·kg⁻¹), while Regent had the lowest (HHV = 16.91 MJ·kg⁻¹; LHV = 15.76 MJ·kg⁻¹). The Muscaris variety had the highest nitrogen (1.82%) and sulphur (0.08%) content, which translated into the highest SO₂ (0.16 kg·Mg⁻¹) and dust (12.67 kg·Mg⁻¹) emissions. In contrast, Seyval Blanc proved to be the most environmentally friendly, with the lowest nitrogen content (0.97%) and the lowest emissions of NO_X (3.44 kg·Mg⁻¹), SO₂ (0.11 kg·Mg⁻¹) and dust (8.57 kg·Mg⁻¹).

The type of biomass proved to be a factor that differentiated the energy properties more strongly than the grape variety. Pomace was characterized by a higher calorific value (HHV = 17.87 MJ·kg⁻¹; LHV = 16.70 MJ·kg⁻¹), higher carbon (46.76%) and hydrogen (7.04%) content and lower ash content (7.11%) compared to stems, which had a lower calorific value (HHV = 16.11 MJ·kg⁻¹; LHV = 14.96 MJ·kg⁻¹) and a higher ash content (9.85%). Pomace generated less dust (8.98 vs. 12.44 kg·Mg⁻¹) but more CO (57.61 vs. 54.95 kg·Mg⁻¹) and CO₂ (1410.74 vs. 1345.56 kg·Mg⁻¹).

In summary, grape pomace is a more advantageous energy source than grape stalks due to its higher calorific value, better combustion parameters and lower dust emissions. Among the varieties, Hibernal stands out dues to having the highest calorific potential, while Seyval Blanc has the most favourable environmental properties. The results obtained confirm that wine waste can be a valuable source of renewable energy, supporting the development of sustainable biomass use in wine-growing regions.