1. Introduction
The wine industry is one of the largest agro-industrial sectors in the world [
1]. The International Organization of Vine and Wine estimates that in the year 2022, around 25.99 billion liters of wine were produced worldwide. In the same period, the Portuguese wine industry produced 680 million liters, producing hundreds of tons of grape pomace resulting from the crushing and pressing stages carried out in wine production [
2,
3,
4,
5].
Grape pomace is composed primarily of grape skins, seeds, and stems, each contributing uniquely to bioenergy production. The high cellulose and hemicellulose content in grape pomace provides substantial substrates for bioethanol production through fermentation, where these polysaccharides are broken down into fermentable sugars. Lignin, another significant component, enhances the biomass’s calorific value, making it suitable for bio-oil production through pyrolysis. The phenolic compounds, including flavonoids, tannins, and anthocyanins, offer antioxidant properties that can inhibit microbial activity, preserving the biomass during storage and enhancing its viability for bioenergy applications [
6,
7].
However, the utilization of grape pomace for bioenergy presents challenges. The high lignin content requires complex and energy-intensive pretreatment processes to break down the rigid plant cell walls, posing a significant disadvantage. Moreover, the variable composition of grape pomace, influenced by grape variety and winemaking processes, can lead to inconsistencies in bioenergy yield. Despite these challenges, the inherent advantages such as high energy content, waste utilization, and the antioxidant properties that aid in biomass preservation, underscore the potential of grape pomace as a valuable resource for sustainable bioenergy production [
6,
7]. There are two distinct types of grape pomace. Pomace derived from pressing grapes before fermentation begins, a method commonly used in white wine production, is called fresh pomace. The pomace obtained from pressing grapes after the fermentation process, which is typical in red wine production, is called fermented pomace [
5]. Estimates indicate that every 6 liters yields approximately 1 kg of grape pomace, which represents a global production of 10.5 to 13.1 million tons of grape pomace annually [
8]. This by-product of wine production is mainly made up of skins, stems, and seeds [
3,
9,
10,
11]. Estimates indicate that each ton of grape pomace contains approximately 430 kg of skins, 250 kg of stems, and 230 kg of seeds [
12].
Much of this by-product of wine production has been applied as an organic soil improver, as it contains significant amounts of organic matter and is rich in nutrients [
13,
14,
15]. However, due to the continuous application of agricultural by-products in agricultural fields, soils are becoming increasingly saturated, posing a significant challenge for large producers in determining the appropriate disposal methods for grape pomace [
16,
17]. This saturation can lead to nutrient imbalances, reduced soil fertility, and potential environmental contamination. As a result, producers must explore sustainable disposal and utilization options, such as composting, biogas production, or developing new markets for grape pomace-based products, to mitigate these challenges and ensure environmental compliance.
Due to technological improvements in a variety of fields, grape pomace has emerged as a valuable resource with a variety of value-added uses that can be utilized as a functional compound in the pharmaceutical and cosmetic sectors or for distillation and obtaining pomace spirit, an alcoholic beverage that has its origins in Portugal [
3,
4,
8,
18]. Pomace spirit is obtained through the distillation of grape pomace and is considered a strong drink with a high alcohol content. This potent spirit can be used as a culinary ingredient, adding depth and complexity to various dishes, particularly in sauces, marinades, and desserts. Additionally, pomace spirit is often enjoyed on its own as a digestif or incorporated into cocktails, providing a unique and robust flavor profile that enhances the overall drinking experience and stops the fermentation process in the production of fortified wines like Port wine [
17,
18]. To produce pomace spirit, the pomace is placed in a still, where it is carefully heated and distilled [
19,
20]. During this process, the liquid and volatile components of the pomace are separated from the solid by-products. The heat causes the alcohol and other volatile compounds to evaporate, which are then condensed back into liquid form, yielding the potent spirit. This method not only extracts the valuable alcoholic components but also concentrates the flavors and aromas present in the pomace, resulting in a distinctive and robust beverage. Additionally, the distillation process generates a new by-product, which includes the remaining solid by-products and any non-volatile compounds, often used in various agricultural or industrial applications [
21]. Therefore, it has become important to chemically analyze the by-products of wine and pomace brandy production with a focus on sustainable use. This analysis helps in identifying valuable components that can be repurposed, thereby contributing to a circular economy [
22,
23]. By finding innovative ways to utilize these by-products, such as converting them into biofuels, we can reduce primary energy consumption. This shift promotes renewable energy sources over fossil fuels, leading to more environmentally friendly and sustainable production practices in the wine and pomace brandy industries [
23].
Currently, most of the energy produced in Europe comes from non-renewable sources like gas, coal, and oil [
24]. Due to the increasing global energy demand, it has become crucial to establish measures and policies aimed at transitioning to a more sustainable energy system [
25], as fossil fuel-based energy production is the primary contributor to global greenhouse gas (GHG) emissions [
26]. This paradigm shift has resulted in a new appreciation for biomass, such as wood and its derivatives, traditionally reserved for cooking and heating, as possible alternative fuels to reduce GHG emissions on a global scale [
27]. Nevertheless, using such materials for energy production presents challenges. The relatively high moisture content and generally low density of these materials, compared to more regularly used fossil fuels, make the direct usage of raw, unprocessed biomass unviable [
27,
28]. To address these issues, innovative processing techniques are being developed to improve the efficiency and feasibility of biomass as a renewable energy source. These include drying and densification processes that enhance the energy content and ease of handling of biomass fuels. By investing in such technologies, we can better harness the potential of biomass, contributing to a more diversified and sustainable energy portfolio [
29,
30].
As such, transforming these biomasses into usable fuel is crucial if they are to be used in the production of energy. This is usually conducted through a pellet production process, which generally follows a series of steps, starting with a drying process followed by a series of pre-processing methods, namely a size reduction, in which the biomass is compacted through compression and impact forces or through a longer contact interaction such as attrition [
31]. Afterward, a series of treatments are usually applied to the resulting materials like torrefaction, a mild pyrolysis process where the biomass is heated in an inert atmosphere, typically between 200 °C and 300 °C. This results in the dehydration and decarboxylation of the biomass, making it more brittle and enhancing its grindability. The main advantages of torrefaction include increased energy density, improved storage properties, and reduced biological activity. However, it has the disadvantages of requiring additional energy input for the process and potential emissions of volatile organic compounds [
31,
32,
33]. Steam Explosion involves subjecting the biomass to pressurized hot steam for a period of time and then rapidly depressurizing it. This treatment disrupts the structure of the biomass, making it more accessible for further processing. The advantages of steam explosion include enhanced pellet durability and mechanical strength, as well as increased surface area for subsequent chemical reactions. The disadvantages include the complexity of the equipment and the potential loss of some volatile components [
31,
32,
33]. Hydrothermal carbonization is a process where biomass is treated with water at elevated temperatures and pressures, leading to the formation of a carbon-rich material. This method is particularly suitable for materials with high moisture content, such as wet agricultural residues. The advantages include the production of a stable, high-energy-density product, and the ability to handle wet feedstock directly. However, the disadvantages include the need for high-pressure equipment and the generation of a wastewater stream that requires treatment. Biological treatment involves exposing the biomass to microorganisms that break down the recalcitrant components of the biomass. This method can be advantageous due to its low energy requirements and the potential for bioconversion into valuable products. However, it is often a slower process and can be less predictable due to variations in microbial activity and environmental conditions [
31,
32,
33]. These advanced treatments significantly improve the efficiency and utility of biomass as a renewable energy source. By refining the raw materials through these processes, we can create biomass fuels that are more energy-dense, easier to transport and store, and capable of producing lower emissions when used. This holistic approach to biomass utilization helps address the limitations of raw biomass and maximizes its potential as a sustainable alternative to fossil fuels.
The biomass is then finally turned into pellets ranging from 6 mm up to 40 mm in size, as per literature [
32], permitting its use for energy generation [
28]. In the context of wine by-products, understanding the production of these biomass pellets is important, specifically the few nuances related to the utilization of mixed biomass pellets. These are pellets produced through the usage of biomass materials other than wood, normally by reusing biomass from agricultural wastes. If worked upon properly, these pellets can possibly achieve energy release and heating values similar to standard wood pellets [
34,
35]. However, mixed biomass pellets tend to contain problematic amounts of sulfur and chlorine, which is concerning due to the effects these chemicals have on both the equipment upon which they are used and the environment [
35]. In the case that the values exceed the commercially recommended standard set by independent pellet certification schemes it is suggested that the biomass should be mixed in with other materials in order to improve its content properties [
35]. The need to sometimes combine different materials in order to produce adequate pellets does present itself as an opportunity to reutilize biomasses such as wood shavings, sawdust and other wastes that may have been produced in a variety of industries [
28,
35]. Keeping this in mind, this study aims to set a basis of understanding for wine pomace and distillation stillage as viable biomasses for energy production in the pellet fabrication industry, and how it can contribute towards diminishing the environmental impact of the agricultural waste products resulting from the wine industry.
4. Conclusions
This study explored the potential of utilizing wine pomace and distillation stillage, by-products of the wine industry, as sustainable sources for pellets production. The findings indicate that both materials exhibit promising characteristics for energy recovery, each with distinct advantages and challenges.
The drying behavior of wine pomace and distillation stillage showed effective profiles, reaching stable states with minimal moisture content. Wine pomace exhibited a slightly higher mass loss than distillation stillage, which showed a lower moisture content (5.454%), and ash content (3.762%) compared to wine pomace, making it a preferable candidate for pellets due to the reduced residue after combustion. However, wine pomace had higher volatile matter content (62.695%), suggesting a more intense initial energy release during combustion.
Distillation stillage has higher carbon (54.928%) and hydrogen content (6.725%) than wine pomace, correlating with a higher heating value and energy output. This was confirmed by the calorimetric analysis. However, distillation stillage also had higher sulfur content (0.153%), potentially leading to more SOx emissions.
The results underscore the viability of wine pomace and distillation stillage as alternative bioenergy sources, contributing to waste valorization and sustainability in the wine industry.
Future research should focus on optimizing pretreatment processes to address the high ash content in both biomasses, improving the overall yield and consistency of bioenergy production. Investigating methods to reduce sulfur content in mixed biomass pellets can enhance their environmental compatibility and reduce potential equipment corrosion and environmental pollution. Conducting pilot-scale studies to validate laboratory findings and assess the economic feasibility of large-scale implementation of bioenergy production from these wine by-products is crucial. Furthermore, exploring additional value-added applications of grape pomace and distillation stillage in pharmaceutical, cosmetic, and agricultural sectors can promote a circular economy and reduce reliance on fossil fuels.