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Review

Sheep Wool as Biomass: Identifying the Material and Its Reclassification from Waste to Resource

by
Julita Szczecina
1,
Ewa Szczepanik
2,
Jakub Barwinek
2,
Piotr Szatkowski
2,
Marcin Niemiec
3,
Alykeev Ishenbek Zhakypbekovich
4 and
Edyta Molik
1,*
1
Department of Animal Biotechnology, Faculty of Animal Science, University of Agriculture in Krakow, Al. Mickiewicza 24/28, 31-059 Krakow, Poland
2
Department of Glass Technology and Amorphous Coatings, Faculty of Materials Science and Ceramics, AGH University of Krakow, Al. Mickiewicza 30, 30-059 Krakow, Poland
3
Department of Agricultural and Environmental Chemistry, Faculty of Agriculture and Economics, University of Agriculture in Krakow, Al. Mickiewicza 21, 31-120 Krakow, Poland
4
Department of Animal Husbandry and Aquaculture Management, Kyrgyz National Agrarian University in Biszkek, Bishkek 720005, Kyrgyzstan
*
Author to whom correspondence should be addressed.
Energies 2025, 18(19), 5185; https://doi.org/10.3390/en18195185
Submission received: 22 August 2025 / Revised: 22 September 2025 / Accepted: 25 September 2025 / Published: 29 September 2025
(This article belongs to the Special Issue Sustainable Energy Development in Liquid Waste and Biomass: 2nd Edition)
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Abstract

The growing amount of waste worldwide requires new solutions for its management. Agricultural by-products account for almost 10% of the waste generated. One of them is sheep wool, a natural fibre with beneficial physicochemical properties. Currently, sheep wool production amounts to approximately 1–2 million tonnes per year, of which 60% is used in the manufacture of clothing. Nevertheless, it poses a considerable challenge in terms of disposal due to its keratin-rich composition and slow biodegradability. This review analyses the chemical and physical properties of sheep wool and assesses its potential as biomass based on its carbon content and other elemental components. This allows us to provide a critical comparative analysis of the main technological pathways for the use of waste sheep wool as biomass, including anaerobic digestion, pyrolysis, direct combustion and gasification. The review highlights both the opportunities and limitations of these processes, comparing sheep wool in terms of energy potential and carbon footprint with other biomass. The review shows that the calorific value of sheep wool (19.5 MJ/kg) is competitive with traditional plant-based biofuels and the use of waste sheep wool as biomass source can contribute to reduction in CO2 emissions of 2.1 million tonnes per year. The use of sheep wool as biomass can not only contribute to waste reduction but also supports the goals of sustainable agriculture and climate neutrality. The selected methods may offer a new and effective way of reducing waste and allow all sheep wool produced to be introduced into the circular economy.

1. Introduction

Currently, the key environmental challenge facing the world is the increase in the global amount of waste. This is linked to rapid population growth, economic development and rapid urbanisation, which are particularly acute in developing countries. Forecasts indicate that by 2050, annual waste production could reach as much as 27 billion tonnes [1]. Therefore, it is necessary to urgently develop and implement effective technological and organisational solutions that will reduce the amount of waste generated, including the introduction of appropriate systems for waste collection, segregation, processing and recovery of raw materials. It is worth mentioning here that the lack of a proper waste management system leads to a number of serious environmental hazards, ranging from soil, water and air pollution to overfilled landfills, which take up valuable space and can pose a health risk to humans. To counteract these negative effects, strategies such as the European Green Deal are being implemented at the EU and global policy level, one of the main objectives of which is to reduce greenhouse gas emissions by 55% by 2030 and achieve climate neutrality by 2050. A key element of this strategy is to reduce waste by increasing recycling rates and implementing more sustainable waste management practices. A similar goal is pursued by Agenda 2030, which includes 17 Sustainable Development Goals (SDGs). Among them, Goal 12 places particular emphasis on a sustainable consumption and production model, encouraging the reuse of materials and recycling, using modern and environmentally friendly solutions. It is estimated that of the approximately 7–10 billion tonnes of waste generated annually worldwide, about 10% comes from the agricultural sector [2]. The scale of this phenomenon shows how important it is to seek tailor-made solutions that will enable waste reduction and effective waste management. This is an important aspect not only from the point of view of preventing adverse climate change. It is also important for maintaining a circular economy. In recent years, there has been an increase in awareness of the potential of agricultural waste and, consequently, the possibilities for its reuse, while applying modern recycling technologies [3].
Wool is a natural raw material obtained mainly from sheep [4]. Valued for centuries for its unique properties, in recent decades it has also come to be seen as a product that is difficult to utilise. Paradoxically, in an era of growing interest in natural and renewable raw materials, a significant portion of the wool obtained from sheep farms is not used in the textile industry or other industries and requires management or costly disposal [5,6]. Sheep wool belongs to the group of agricultural waste generated by traditional animal husbandry. It is a specific type of waste, as sheep shearing is a necessary procedure for both animal welfare and breeding requirements. In many regions, especially where the textile industry has stopped buying raw sheep wool, it becomes problematic waste that often has no profitable use. For this reason, instead of being processed, sheep wool is often stored or discarded, which poses environmental risks and generates additional costs for farmers. The amount of keratin waste, which includes sheep wool, was approximately 12 million tonnes in 2020 [7], while sheep wool production in Europe 10 years earlier was 260,000 tonnes [8]. The total amount of raw sheep wool produced in 2021 was less than 2 million tonnes [9]. The processing of such quantities of waste, which often takes place without permits and in violation of applicable regulations, leads to the formation of harmful gases, such as CO, CO2, sulphur oxides and nitrogen oxides [7]. The proper management of sheep wool as agricultural waste is therefore an important challenge in the context of sustainable agriculture and the circular economy [8].
Two types of wool are obtained from sheep: single-origin and mixed. Currently, sheep wool production amounts to approximately 1–2 million tonnes per year. Single-origin wool is used for clothing production, accounting for 60% of total sheep wool production [10]. The methods of its processing in the textile industry are widely known. However, the problem lies in the use of mixed wool obtained from sheep bred in mountainous areas. This is due to the high thickness of mixed wool fibres, which makes it difficult to process. Therefore, modern methods of using mixed wool are still being sought. The main technologies for using sheep wool as biomass include processes such as anaerobic fermentation, pyrolysis, direct combustion and gasification. The advantage of the above-mentioned processing methods is undoubtedly the reduction in waste and, at the same time, the lack of harm to the environment.
Considering all the difficulties mentioned above, an important direction for modern agriculture and the circular economy is to find sustainable methods of managing waste sheep wool, which inevitably arises as waste as a result of the need to shear sheep, requiring the development of effective methods of its utilisation or processing, for example, by using waste sheep wool as a raw material for the production of energy biomass.

2. Potential of Sheep Wool as Biomass

Sheep wool is mainly composed of one type of protein—keratin—which consists primarily of elements such as carbon, oxygen, nitrogen, hydrogen and sulphur, as well as trace amounts of other elements. The amounts of the main elements are shown in Table 1.
Raw sheep wool also contains dirt (26%), natural wool fats (suint) (28%) and other fats and water (13%) [13]. Some studies show a different composition, divided into 60% animal protein fibres, 15% moisture, 10% fat, 10% sheep sweat and 5% impurities [12]. The chemical composition of sheep wool varies depending on the breed, breeding location and environmental conditions. The amounts of other trace elements such as calcium and magnesium are also being studied, as their amounts vary depending on the intensity of breeding and nutrition. The studies conducted have shown the presence of elements such as calcium, potassium, magnesium, sodium, copper and iron, the amounts of which vary depending on the breeding region (Table 2). The amount of these elements is determined by environmental conditions and pollution and ranges from several hundred to several thousand ppm depending on the study [13,14,15]. Trace elements are present in small quantities and do not pose any obstacles to the processing of waste sheep wool as biomass.
Sheep wool, as a fibre, is characterised by relatively low flammability (the oxygen limit index (LOI) is approximately 25%). This is due to its unique chemical composition, as described above. In addition, sheep wool has the ability to reabsorb moisture at a level of approximately 33% [12,16,17]. The thermal decomposition of sheep wool, i.e., pyrolysis, begins at a temperature slightly above 200 °C and proceeds as an endothermic process in the range of 200–400 °C. The final oxidation of sheep wool occurs in the next stage, which begins at around 440 °C. As mentioned earlier, during thermo-oxidative decomposition, the greatest amount of heat is released in the temperature range of 450 to 600 °C, where the process becomes strongly exothermic [18]. Only at a temperature of approximately 550 °C does the fibre ignite, making it a fire-safe material [19]. These properties mean that sheep wool is increasingly being researched as an ecological and sustainable building material [20,21], which can compete with conventional, non-biodegradable polymer insulation, while offering additional advantages such as fire resistance [22,23]. Excellent thermal insulation results from the microscopic, twisted structure of the fibres, between which air is trapped, acting as a natural barrier against heat loss [4,24,25].
Due to its complex composition, sheep wool processing is costly and requires appropriate technology and facilities for cleaning, dyeing and weaving [16]. Therefore, the European textile industry prefers to import high-quality sheep wool, e.g., from merino sheep in Australia or New Zealand. This increases the need to find alternative ways of utilising sheep wool with thicker fibres [26].
With the development of waste management technologies, issues related to the life cycle assessment (LCA) of such materials and their comparison to synthetic raw materials are being raised. LCA is a globally recognised method for determining the quality of agricultural product trade and can be used to compare ways of reducing emissions caused by individual raw materials. Various methods of calculating emissions for sheep wool are being raised, and the literature states that the total CO2 equivalent emissions are 24.9 kg CO2-e per kg of raw sheep wool on a farm where the total wool production is approximately 65 tonnes per year [27]. Differences between intensive (sheepfold) and extensive (grazing) farming systems are also being studied, with values of 16.9 and 17.1 kg CO2-e, respectively [28]. In sheep farming for wool, milk or meat, primary production on the farm accounts for the largest share of gas emissions (75–90%), followed by industrial processing (2–15%) and transport [29]. A major variable in sheep farming is the grazing phase, which accounts for the largest share in climate change assessment, but a reliable assessment varies between farms and requires standardisation or the identification of similar assumptions for individual farms [9].
Given these values, it is essential to find uses for keratin waste, including sheep wool, in order to maximise its return to the economic cycle and reduce the carbon footprint of the entire breeding and processing process.

3. Processing Waste Sheep Wool as a Raw Material with Energy Potential

3.1. Sheep Wool Energy Potential and Pre-Processing

In the era of climate and energy crises, there is growing interest in the energy conversion of biomass and waste, as it combines the prospect of replacing landfill disposal with energy recovery [30]. By-products such as sheep wool can be a source of renewable energy due to their high carbon content and the amount of biomass produced annually. Sheep farming waste is characterised by a high carbon content, which can be used in the pyrolysis process. For sheep wool waste, the process can be divided into dehydration, degassing and carbonisation. Volatile products can be used as biogas [31]. In sheep wool processing, it is worth considering the use of biotechnological methods, such as enzymatic treatment and composting, in order to obtain substrates for biogas production in a sustainable manner [32,33].
Sheep wool, traditionally used in the textile industry, is becoming increasingly important as an alternative energy source in the context of the circular economy and sustainable development. In recent years, there has been growing interest in the use of sheep wool waste as biomass for the production of biogas, biochar and composite materials. According to estimates, the United Kingdom alone generates approximately 164,984 tonnes of sheep wool waste annually, which poses both an environmental problem and significant energy potential [34]. The calorific value of sheep wool is 20.67 MJ/kg for uncontaminated material, which makes it competitive with other biomass materials. This value is significantly higher than that of wood (16.2 MJ/kg) or straw (17.0 MJ/kg), although lower than that of fossil fuels (Figure 1).
The high carbon content (50.5%) and the presence of lanolin (sheep wool fat) contribute to its relatively high energy value. Studies show that sheep wool has properties comparable to other high-energy biomasses, confirming its potential as an energy source [35].
Traditional sheep wool processing methods involve a number of energy-intensive processes. Scouring is carried out at 55 °C for 45 min, using 15 L of water per kg of sheep wool. The carbonisation process, which is the most energy-intensive (8.2 MJ/kg), uses sulphuric acid (5–7%) at a temperature of 110 °C for 180 min (Figure 2) [36].
Supercritical CO2 technology eliminates the need for water, significantly reducing environmental impact. Studies have shown that treatment with biosurfactants and ‘green’ solvents improves lanolin removal efficiency by 25% compared to traditional methods [37,38]. The use of enzyme complexes in the bio-scouring process allows for treatment at lower temperatures (30–40 °C), which translates into a 40% reduction in energy consumption. The microwave-assisted keratin extraction process reduces processing time by 50% while improving efficiency (Figure 2) [36].

3.2. Sheep Wool-Energy Conversion Technologies

Wool can be used as a substrate for biogas production through anaerobic fermentation. However, due to its keratin structure, it requires appropriate pre-treatment. Research conducted by Kuzmanova and colleagues showed that raw wool has very low biodegradability, with methane production ranging from 0 to 41.3 cm3 CH4/g of dry organic matter for different sheep breeds, as shown in Figure 3 [34].
The key breakthrough was the pre-treatment with liquid nitrogen (LN2), which significantly increases biogas production efficiency by mechanically breaking down the fibre structure, increasing the contact surface with microorganisms and improving protein solubility by 14–38%. After LN2 treatment, methane yields in the range of 74.7–157.3 cm3 CH4/g vs. were obtained, representing an increase in yield of 46% to as much as 352% depending on the type of wool [34].
Pyrolysis of wool allows for the production of biochar—a solid carbon product with high energy value. The process is characterised by specific technological parameters that determine the quality of the final product. Wool biochar has a carbon content of 60–85% of dry matter and a calorific value of 25–30 MJ/kg, making it a valuable solid fuel. Additionally, its specific surface area of 50–300 m2/g and high thermal stability above 400 °C increase its application value [39].
Wool can be burned directly as a solid fuel, but this process requires control of sulphur and nitrogen emissions. It has an auto-ignition temperature of 560–600 °C and an energy efficiency of 85–90% for combustion systems. The natural fire resistance of wool results from its high nitrogen content (~16%) and high oxygen index (LOI = 25.2) [40]. Gasification of wool biomass can achieve an energy efficiency of 62–65% at process temperatures of 800–1000 °C. This process allows for the production of synthesis gas (syngas) with a calorific value of 6.53 MJ/m3 [41]. The methods are summarised in Table 3.
A comparison of different wool energy conversion technologies shows that each method has its own specific requirements and advantages. Anaerobic fermentation is characterised by the lowest process temperatures (35–40 °C) but requires special pre-treatment to increase the biodegradability of keratin [34]. Pyrolysis offers the most versatile use of end products, allowing for the production of biochar with high energy value and the possibility of use in various applications [39]. Direct combustion provides the highest energy efficiency but requires control of harmful emissions. Research on hybrid composite materials shows that wool combined with biochar creates materials with increased fire resistance, reduced smoke emissions during combustion and improved mechanical properties [45,46]. The use of native strains of microorganisms isolated from wool waste allows for the optimisation of microbial consortia, a 30–50% reduction in fermentation time and a 20–40% increase in biogas yield [47]. The study identified 52 bacterial strains and 15 fungal strains capable of degrading keratinous materials [47].
Biogas from wool can be used to produce electricity in biogas generators, for heating in biogas boilers, as a fuel for vehicles in the form of biomethane, and as a source of hydrogen through biogas reforming [48]. Biochar from wool is used as a solid fuel in co-combustion processes with coal, as a soil additive to increase carbon sequestration, as a filter material and sorbent, and as a component of organic fertilisers [48,49]. The use of wool as an energy biomass contributes to the reduction of CO2 emissions through carbon sequestration in biochar, circular economy through the management of textile waste, reduction in landfill as an alternative to disposal in landfills, and the production of fertilisers in the form of digestate from fermentation as a biofertilizer [50]. A life cycle assessment (LCA) for wool-based energy systems shows a net emission reduction of approximately 8.3 × 103 kg CO2 eq compared to conventional energy sources [49]. Systems using wool waste are particularly beneficial, as they eliminate the need for dedicated raw material production [50].
Water hydrolysis of wool is a promising technology for converting wool waste into valuable products. The process uses saturated steam under pressure at a temperature of 170–185 °C for 30–90 min without the use of chemicals. The process yield is 2–3 kg of hydrolysate per kilogram of wool processed [45]. Wool hydrolysate has a C/N ratio of 2.18–4.26, which indicates its suitability as a fast-release nitrogen fertiliser. The hydrolysis process requires energy to heat water from ambient temperature to approximately 170 °C, which for processing 1200 kg/day of water corresponds to an energy requirement of at least 35.2 kW [45].
An innovative approach combines the pyrolysis of lignocellulosic biomass with wool hydrolysis to produce sustainable fertilisers. Pyrolysis at 400–600 °C produces carbon-rich biochar, which can be mixed with nitrogen-rich wool hydrolysate. This integrated approach allows for the optimisation of the C/N ratio in the final fertiliser product [45]. The energy required for hydrolysis can be supplied through heat exchange with hot gases leaving the pyrolyser or by burning part of the pyrolysis products (bio-oil or pyrogas) (Figure 4). This approach increases the overall energy efficiency and sustainability of the process [45].
Anaerobic fermentation of wool can produce biogas containing methane, although the efficiency of this process is limited by the complex structure of keratin proteins (Figure 4). Studies have shown that hydrolysed sheep’s wool can generate 126.1–146.7 mL CH4/g dry matter in a dry anaerobic fermentation system. Anaerobic fermentation, despite its low energy efficiency (45%), offers the possibility of operating at low temperatures and producing methane [51]. Recent studies also indicate the possibility of using keratin from wool to produce triboelectric generators that can generate electricity from mechanical motion. Keratin-based devices can achieve a power density of 14.4 W/m2 and exhibit excellent cyclic stability over 8000 cycles [51].

4. Market Challenges in the Use of Waste Sheep Wool as Biomass

4.1. Challenges and Limitations in Waste Sheep Wool Processing

The low biodegradability of raw wool without pre-treatment is the main technological challenge. The keratin structure is characterised by high chemical stability and resistance to enzymatic degradation, which requires the use of costly treatment methods [52]. The high costs of pre-treatment, especially enzymatic and thermal treatment, can limit the economic viability of the processes. Liquid nitrogen treatment, although effective, requires specialised equipment and significant energy inputs [34]. Pollutant emissions during combustion, particularly NOx and SO2 due to the high nitrogen and sulphur content, require the use of advanced flue gas cleaning systems [53]. Competition with other biomass sources with higher energy efficiency may limit the adoption of wool-based technologies. The seasonality of supply related to sheep shearing periods and the costs of transport and storage are additional economic barriers [50]. The heterogeneity of the raw material depending on the breed of sheep, breeding conditions and processing methods requires an individual approach to optimise conversion processes [34].
Future research directions include optimising pre-treatment using ultrasound and microwave technology, developing keratinolytic biocatalysts to increase protein degradation efficiency, integrating with energy systems in the form of cogeneration, and scaling up processes to industrial levels [45,52]. Research on the modification of biochar through chemical and physical activation can significantly improve its sorption and catalytic properties, expanding its range of applications [46,54].
It is estimated that global wool waste could provide over 40 million tonnes of energy raw material per year, which could contribute to a 10–15% reduction in CO2 emissions in the textile industry. The development of a wool-based bioenergy sector could lead to job creation and increased energy efficiency on farms [50].
Wool as an energy biomass fits perfectly into the concept of a circular economy, where waste from one process becomes a raw material for another. Digestate from anaerobic fermentation can be used as an organic fertiliser rich in nitrogen, phosphorus and sulphur [45]. Biochar from pyrolysis serves as a carbon sequestrant in soil and a filter material in water treatment [48,49]. The development of local energy systems based on wool waste can contribute to increasing the energy self-sufficiency of rural areas and reducing the costs of transporting raw materials [55,56].
Research indicates that sheep wool has significant energy potential (20.5 MJ/kg), which exceeds that of traditional biomass, but its use as a biofuel is associated with a high production carbon footprint (14.2–52.9 kg CO2-e/kg) [57]. The combustion properties of wool are unique due to its high nitrogen and sulphur content, which give it natural fire resistance. The ignition temperature of wool is 570–600 °C, which is significantly higher than that of cotton (255 °C) or polyester (485–560 °C). During combustion, wool releases 4.9 Kcal/g of thermal energy, which is comparable to the values for other organic fibres [35].
The total energy consumption in the life cycle of wool from farm to spinning mill is 48–52 MJ/kg of processed wool (Figure 5). The contribution of individual stages to the energy balance is 52% for farm inputs, 45% for wool processing and 3% for transport. These high energy requirements for processing significantly affect the overall energy balance of wool as a potential biofuel [51].
The carbon footprint of sheep wool production from cradle to farm gate ranges from 14.158 to 52.882 kg CO2-e/kg of wool, depending on the allocation methodology used and the lifespan of the sheep. Studies conducted in the Victoria region of Australia on a sample of 20,000 sheep showed significant variability in emissions depending on the allocation method: the lowest values for mass allocation (14.2–16.7 kg CO2-e/kg) and the highest for economic allocation (49.0–52.9 kg CO2-e/kg) [57]. The main source of greenhouse gas emissions in wool production is intestinal fermentation and excretion of faeces by sheep, which account for 78.91–93.65% of the total carbon footprint (Figure 6). The second largest source of emissions is phosphorus fertilisers, followed by electricity and potassium fertilisers. Methane emissions from intestinal fermentation and nitrous oxide from manure dominate the emission structure due to the physiology of ruminants [57].
The carbon footprint of wool is strongly dependent on several key factors. Longer sheep lifespan and higher body weight lead to higher greenhouse gas emissions from intestinal fermentation and manure. Wool productivity has the opposite effect—higher wool production per sheep reduces the carbon footprint per kilogram of product [57]. The method of allocation between by-products (wool, meat, milk) significantly affects the results of carbon footprint calculations. Economic allocation gives the highest carbon footprint values for wool, while mass allocation gives the lowest. Biophysical allocation based on protein content (PMA) gives intermediate results [57].

4.2. Comparison of Wool with Other Biomass Sources

The fundamental differences between wool and cotton stem from their different chemical compositions. Wool, as a protein fibre, contains keratin rich in sulphur amino acids, while cotton consists mainly of cellulose (90%) [58]. When comparing wool and cotton fibres, significant differences in mechanical properties can be observed. Cotton has higher tensile strength (287 MPa vs. 160 MPa), but wool surpasses it in terms of elongation at break (35% vs. 7%). Wool also has better hygroscopicity (30% vs. 25%) and lower thermal conductivity (0.04 W/mK vs. 0.06 W/mK) [58]. Studies show that wool achieves 95% biodegradation in 90 days, compared to 85% for cotton. The natural lanolin content of wool provides additional protection against microorganisms, which extends the life of textile products [59]. Sheep’s wool has the best non-flammability properties among natural fibres. Studies confirm that wool has a Limiting Oxygen Index (LOI) of 24–25%, significantly higher than cotton (17–18%). The ignition temperature of wool is 570–600 °C, while cotton ignites at 210–255 °C [60].
The non-flammability of wool is due to its high nitrogen content (16.5%) and its ability to absorb moisture (up to 30% of its weight). When burned, wool forms a self-insulating charred layer that prevents the spread of flames. This process is possible thanks to the structure of keratin, which is rich in sulphur amino acids that require higher temperatures to begin thermal degradation [59]. The calorific value (Figure 7) of wool combustion (4.9 kcal/g) is higher than that of cotton (3.9 kcal/g), but thanks to its self-extinguishing properties, wool does not sustain flames after the ignition source has been removed [59].
Wool does not melt or drip when burned, eliminating the risk of burns caused by molten material sticking to the skin. Unlike synthetic fibres such as nylon (melting point 160–260 °C) and polyester (melting point 252–292 °C), wool forms a stable carbon layer [59]. Thermogravimetric analysis (TGA) of sheep’s wool shows a characteristic degradation process in several stages (Figure 8). The first stage (50–100 °C) involves the evaporation of surface moisture (8% weight loss). The main degradation of keratin occurs at a temperature of 300–500 °C, with the maximum degradation rate at 400 °C [61].
The degradation process can be divided into three main stages: dehydration (up to 200 °C), devolatilization (200–500 °C) and carbonisation (above 500 °C). The pyrolysis products of wool mainly include phenols (7.42%) and heterocyclic compounds (21.26%) [62].
Kinetic studies of wool thermal degradation show that the activation energy of wool degradation processes is significantly higher than that of cellulose materials, which explains its better non-flammability properties [63].
The global energy potential of wool is estimated at 7.3 TWh per year, with a total production of 2.5 million tonnes of wool per year (Figure 9). Approximately 15% of global wool production is suitable for energy use, which could lead to a reduction in CO2 emissions of 2.1 million tonnes per year [64]. The high sulphur content (3.7%) causes corrosion problems in power plants. Low bulk density (280 kg/m3) causes logistical difficulties in transport and storage. Further research is needed on the development of pretreatment technologies to improve the biodegradability of keratin [65].
The energy value of wool (20.5 MJ/kg) places it above typical fuels derived from lignocellulosic biomass. Compared to wood chips (12.5 MJ/kg) and wood pellets (17.0 MJ/kg), wool has a higher energy content per unit of mass (Table 4). However, it remains significantly below the energy values of fossil fuels such as hard coal (27–31 MJ/kg) or fuel oil (42.5 MJ/kg) [35].
Wool differs from other types of biomass due to its high keratin protein content [53]. Thermal degradation of keratin occurs in three stages: dehydration, volatilisation of volatile compounds and carbonisation. This process differs significantly from the pyrolysis of lignocellulosic biomass, which affects the end products and energy conversion efficiency [31]. Thermodegradation studies have shown that wool, human hair and chicken feathers have similar thermal decomposition characteristics. The main degradation products are ammonia, CO2, sulphur-containing compounds (H2S, SO2) and phenolic compounds. This unique decomposition chemistry affects the energy utilisation potential of wool [53].
The main barrier to the use of sheep wool as a biofuel is its high ignition temperature requirements (570–600 °C) compared to other organic materials. In addition, high emissions during sheep wool production (14.2–52.9 kg CO2-e/kg) can negatively affect the overall carbon balance when used as fuel (Table 3) [57]. The current low economic value of sheep wool in Europe, where farmers receive less for fleece than it costs to shear, limits the profitability of investing in energy technologies. The properties of sheep wool make it impossible to store in landfills or burn easily, leading to illegal disposal practices [45]. Recent research points to promising directions for the development of sheep wool processing technologies. Waste sheep wool is seen as an underappreciated technical resource with the potential to replace materials with a high carbon footprint, which are non-recyclable or non-recoverable. Its properties include renewability, biodegradability, antibacterial, insulating and flame-retardant properties [66]. The development of integrated processing systems combining different conversion technologies can significantly improve the energy and economic efficiency of waste sheep wool utilisation. The combination of lignocellulosic biomass pyrolysis with sheep wool hydrolysis is an example of such an approach, enabling the simultaneous processing of different types of organic waste [45].
Sheep wool acts as a temporary store for atmospheric carbon, sequestering 1.8 kg CO2-e per kilogram of sheep wool throughout the product’s lifetime (Table 3). The carbon contained in sheep wool comes from the photosynthesis of plants consumed by sheep over the last 1–2 years, making it part of a natural, renewable system. Compared to synthetic fibres, whose carbon comes from fossil fuels, sheep wool represents a much more sustainable approach [66]. Recent studies indicate that traditional life cycle assessments (LCAs) may overestimate greenhouse gas emissions from sheep wool production by not taking biogenic processes into account. An approach that takes biogenic carbon flows into account can reduce the calculated carbon footprint of sheep wool by up to 96% in cases where two-thirds of the carbon from manure is retained for reuse [66].
Several factors influence the assessment of the economic benefits of using wool as biomass. The cost of pre-processing of the waste sheep wool fibres alone is high due to their complexity and number of possible processes, i.e., cleaning, shredding and sometimes chemical or thermal treatment to improve conversion efficiency, whereas waste sheep wool storage fees range from negative values of the disposal costs to a few hundred euros per tonne [5,67]. Pilot programmes aimed at utilising wool are emerging across Europe [68], but they focus primarily on material applications rather than direct energy conversion. Economic analysis shows that wool-energy conversion requires significant infrastructure costs, similar to other advanced biomass technologies, but they also offer environmental benefits. The limited availability of wool compared to other biomass feedstocks means that energy recovery should be introduced into an integrated waste management system rather than large-scale energy production, which requires careful economic modelling taking into account the appropriate infrastructure compared to disposal costs [69].

5. Conclusions and Prospects for Development

In times of climate change and a shift towards more environmentally friendly energy sources, it is crucial to analyse all types of biomass in order to avoid unnecessary storage of valuable raw materials in landfills and fruitless utilisation. Analysis of the properties of sheep wool confirms the significant potential of this raw material as an alternative to traditional biofuels. The advantage of sheep wool over cotton in terms of non-flammability (LOI = 24–25% vs. 17–18%) and ignition temperature (570–600 °C vs. 210–255 °C) makes it safer in thermal processes.
The calorific value of sheep wool (19.5 MJ/kg) is competitive with traditional plant-based biofuels. Key challenges include the need to develop pretreatment technologies and solve problems related to high sulphur content. The energy potential of sheep wool, with a possible reduction in CO2 emissions of 2.1 million tonnes per year, represents a significant contribution to the achievement of renewable energy targets. However, the implementation of technology for the use of waste sheep wool as a biofuel requires further research into process optimisation and the development of industrial infrastructure adapted to the specific characteristics of this raw material.

Author Contributions

Conceptualization, J.S., P.S., E.M., E.S. and J.B.; methodology, P.S.; software, J.S.; validation, E.S. and J.B.; formal analysis J.S.; investigation E.M.; resources, E.S.; data curation P.S.; writing—original draft preparation, J.S., E.S. and J.B.; writing—review and editing, E.M., A.I.Z. and M.N.; visualisation, J.B.; supervision, E.M. and P.S.; project administration, E.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Energies 18 05185 g001
Figure 1. Comparison of the calorific values of various energy materials (based on [35], own modification: Jakub Barwinek).
Figure 1. Comparison of the calorific values of various energy materials (based on [35], own modification: Jakub Barwinek).
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Energies 18 05185 g002
Figure 2. Comparison of the energy consumption of different sheep wool processing methods (based on [36], own modification: Jakub Barwinek).
Figure 2. Comparison of the energy consumption of different sheep wool processing methods (based on [36], own modification: Jakub Barwinek).
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Energies 18 05185 g003
Figure 3. Methane production from different types of wool before and after treatment with liquid nitrogen (gVS—gram of biodegradable organic matter) (based on [34], own modification: Jakub Barwinek).
Figure 3. Methane production from different types of wool before and after treatment with liquid nitrogen (gVS—gram of biodegradable organic matter) (based on [34], own modification: Jakub Barwinek).
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Energies 18 05185 g004
Figure 4. Pathways for the energy use of sheep wool waste (based on [45], own modification: Jakub Barwinek).
Figure 4. Pathways for the energy use of sheep wool waste (based on [45], own modification: Jakub Barwinek).
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Energies 18 05185 g005
Figure 5. Energy and carbon balance of sheep wool at different stages of its life cycle (based on [51], own modification: Jakub Barwinek).
Figure 5. Energy and carbon balance of sheep wool at different stages of its life cycle (based on [51], own modification: Jakub Barwinek).
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Energies 18 05185 g006
Figure 6. Distribution of carbon footprint sources in sheep wool production (based on [57], own modification: Jakub Barwinek).
Figure 6. Distribution of carbon footprint sources in sheep wool production (based on [57], own modification: Jakub Barwinek).
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Energies 18 05185 g007
Figure 7. Comparison of the non-flammability properties of wool and cotton (based on [59], own modification: Jakub Barwinek).
Figure 7. Comparison of the non-flammability properties of wool and cotton (based on [59], own modification: Jakub Barwinek).
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Energies 18 05185 g008
Figure 8. Thermogravimetric analysis (TGA) of sheep wool—stages of thermal degradation (based on [61], own modification: Jakub Barwinek).
Figure 8. Thermogravimetric analysis (TGA) of sheep wool—stages of thermal degradation (based on [61], own modification: Jakub Barwinek).
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Figure 9. Global use of sheep wool production (2.5 million tonnes per year), taking into account its energy potential (based on [64], own modification: Jakub Barwinek).
Figure 9. Global use of sheep wool production (2.5 million tonnes per year), taking into account its energy potential (based on [64], own modification: Jakub Barwinek).
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Table 1. Main elements’ content in sheep wool (based on [11,12]).
Table 1. Main elements’ content in sheep wool (based on [11,12]).
ElementContent [%]
C50
N15–25
O10–25
S2–5
H6–12
Table 2. The content of additional trace elements in sheep wool [mg/kg] (based on [13,15]).
Table 2. The content of additional trace elements in sheep wool [mg/kg] (based on [13,15]).
ElementSlovakia 1PolandGreeceSyria
Ca4065 ± 2340
(729.2–4065)		1790.0 ± 392.02900.0 ± 591.01800.0 ± 351.0
K3282 ± 649.7
(2315–3282)		718.6 ± 307.7643.0 ± 312.9755.0 ± 295.3
Mg1009 ± 308.3
(163.8–1009)		120.8 ± 17.2383.5 ± 65.4590.8 ± 133.5
Na969.2 ± 344.7
(194.9–969.2)		1486.7 ± 234.22165.0 ± 573.11745.5 ± 1152.6
Cu7.54 ± 2.16
(3.79–7.54)		---
Fe71.83 ± 40.06
(17.61–71.83)		---
P-148.0 ± 32.3206.0 ± 49.2284.0 ± 59.8
1 the highest value is given, with the range for the whole country in brackets.
Table 3. Methods of sheep wool—energy conversion technologies.
Table 3. Methods of sheep wool—energy conversion technologies.
Processing TechnologyProsConsApproximate Energy YieldKey ChallengesTechnology Readiness Level (TRL)
Hydrolysis [10,42] Extracts valuable keratin and amino acidsRequires pre-treatment (chemical/physical/enzymaticModerate (e.g., considerable keratin recovery, but energy for processing)Stability of wool (disulphide bonds) requires effective hydrolysis methods
Process optimisation for scale-up		TRL 4–6 (pilot/semi-industrial scales
Pyrolysis [35,43]Produces biochar, bio-oil, and syngas
Fast process with thermal decomposition		High energy input for heating Complex product mixtureHigh (biochar yield 22–47%, bio-oil yield varies 26–60%, depends on conditions)Controlling product quality
Scaling fixed/fluidized/spouted bed reactors		TRL 6–8 (commercial in some biomass applications)
Anaerobic Fermentation [34,44] Produces biogas (methane) as renewable energy
Digestate usable as fertiliser		Wool stability reduces biodegradability without pretreatment
Long residence times needed		Moderate (Methane yield varies, example ~0.2–0.3 m3 CH4/kg volatile solidsPretreatment approaches to disrupt wool structure (chemical, enzymatic)
Process efficiency		TRL 4–7 (varies, emerging for wool biomass)
Table 4. The position of wool in the biofuel spectrum [45,57].
Table 4. The position of wool in the biofuel spectrum [45,57].
ParameterValueCategory
Calorific value of wool20.5 MJ/kgEnergy
Calorific value (comparison)1.6× wood (shavings)Energy
Calorific value (comparison)1.2× wood pelletsEnergy
Carbon footprint (min)14.2 kg CO2-e/kgEnvironment
Carbon footprint (max)52.9 kg CO2-e/kgEnvironment
Main source of emission78.9–93.7%Environment
Stored coal1.8 kg CO2-e/kgEnvironment
Processing energy48–52 MJ/kgEnergy
Hydrolysis temperature170–185 °CTechnology
Hydrolysis time30–90 minTechnology
Hydrolysis efficiency2–3 kg hydrolysate/kg of woolTechnology
Pyrolysis temperature400–600 °CTechnology
Ignition temperature570–600 °CSafety
EU wool production~240,000 tonnes/yearProduction
Market share in textiles~1.2%Market
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Szczecina, J.; Szczepanik, E.; Barwinek, J.; Szatkowski, P.; Niemiec, M.; Zhakypbekovich, A.I.; Molik, E. Sheep Wool as Biomass: Identifying the Material and Its Reclassification from Waste to Resource. Energies 2025, 18, 5185. https://doi.org/10.3390/en18195185

AMA Style

Szczecina J, Szczepanik E, Barwinek J, Szatkowski P, Niemiec M, Zhakypbekovich AI, Molik E. Sheep Wool as Biomass: Identifying the Material and Its Reclassification from Waste to Resource. Energies. 2025; 18(19):5185. https://doi.org/10.3390/en18195185

Chicago/Turabian Style

Szczecina, Julita, Ewa Szczepanik, Jakub Barwinek, Piotr Szatkowski, Marcin Niemiec, Alykeev Ishenbek Zhakypbekovich, and Edyta Molik. 2025. "Sheep Wool as Biomass: Identifying the Material and Its Reclassification from Waste to Resource" Energies 18, no. 19: 5185. https://doi.org/10.3390/en18195185

APA Style

Szczecina, J., Szczepanik, E., Barwinek, J., Szatkowski, P., Niemiec, M., Zhakypbekovich, A. I., & Molik, E. (2025). Sheep Wool as Biomass: Identifying the Material and Its Reclassification from Waste to Resource. Energies, 18(19), 5185. https://doi.org/10.3390/en18195185

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