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Article

Torrefaction of Hazelnut Shells: The Effects of Temperature and Retention Time on Energy Yield and Fuel Characteristics

by
Gökhan Devekıran
and
Hasan Sarptaş
*
Solar Energy Institute, Ege University, İzmir 35100, Türkiye
*
Author to whom correspondence should be addressed.
Energies 2025, 18(17), 4710; https://doi.org/10.3390/en18174710
Submission received: 19 July 2025 / Revised: 24 August 2025 / Accepted: 27 August 2025 / Published: 4 September 2025
(This article belongs to the Section A4: Bio-Energy)
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Abstract

Torrefaction is a key technology for upgrading biomass, which typically has high moisture content and low calorific value, into a high-quality solid biofuel. However, its effectiveness is highly dependent on the specific feedstock and operating parameters. This study investigates the torrefaction of hazelnut shell, an abundant agricultural residue in Türkiye, to examine the effects of process temperature and retention time on the torrefaction process and to determine better process conditions. Lab-scale experiments were conducted at temperatures of 260 °C, 280 °C, and 300 °C and retention times of 30 and 60 min. At the most severe condition of 300 °C for 60 min, the mass yield decreased to 59.44%, while 78.38% of the feedstock’s original energy was successfully retained in the final torrefied biomass. The Energy Densification Ratio (EDR), another significant indicator for quantifying energy concentration, consistently increased with process severity as low-energy volatile compounds were removed. A maximum EDR of 1.32 was achieved at 300 °C and 60 min. These results demonstrate that torrefaction can convert hazelnut shells into a carbon-rich, energy-dense biofuel suitable for high-energy applications.

Graphical Abstract

1. Introduction

The increasing global demand for energy, coupled with severe air and water pollution and health issues caused by fossil fuels and the urgent need to mitigate climate change, has intensified the search for sustainable and carbon-neutral alternatives to fossil fuels (i.e., renewable energy) [1,2]. Among various renewable resources, biomass is a promising option due to its abundance, attributes to generating carbon-neutral energy, and ability to provide continuous and non-intermittent power generation, unlike solar or wind energy [2,3,4,5,6,7]. However, the direct use of raw biomass as a fuel is hindered by inherent drawbacks, including high moisture content, low bulk density and low energy density, and a hygroscopic nature that complicates storage and transport of biomass [3,8].
To overcome these limitations, several pretreatment methods, including mechanical, (shredding, grinding, etc.), chemical, and thermal pretreatment, are employed to upgrade biomass into a more viable fuel [8,9,10,11]. Torrefaction, a mild pyrolysis process conducted between 200 and 300 °C in an inert atmosphere, is a particularly effective method [12,13]. This process transforms raw biomass into a high-quality solid biofuel, commonly known as charcoal. Torrefied biomass is an energy-dense, hydrophobic, and homogeneous material with significantly improved grindability and combustion characteristics, making it a suitable substitute for coal in existing power plants and industrial boilers [8,14,15,16,17,18,19,20,21].
According to the most recent available data (up to 2023), Türkiye is the world’s leading hazelnut producer with more than 60% of the global production of shelled hazelnuts (Figure 1) [22]. As reported by [23] hazelnut shells are one of the primary agro-industrial residues in the country. This abundant agricultural byproduct, often treated as low-value waste, is an ideal lignocellulosic feedstock for valorization through torrefaction [24,25,26,27].
While various studies have mainly investigated the potential of hazelnut shell for biochar production, and its applications as a soil amendment, carbon sequestration agent, and water purification material, there has been limited research on the production of biofuel by torrefaction of hazelnut shell as a lignin-rich biomass [26].
Therefore, this study aims to investigate how variations in temperature and retention time influence the energy conversion efficiency and product characteristics of torrefied biomass derived from hazelnut shell, testing whether these parameters can enhance its viability as a high-energy bioresource. By doing so, it seeks to explore the potential of hazelnut shell as a renewable biomass resource and to contribute sustainable energy production and waste valorization, offering insights into circular economy and climate change mitigation.

2. Torrefaction and Its Key Process Parameters

2.1. Torrefaction Process

Torrefaction is a thermochemical pretreatment process conducted in an inert atmosphere at temperatures between 200 and 300 °C [12,13]. Often referred to as mild pyrolysis, the process involves heating biomass for a resident time typically ranging from 10 to 60 min [24,28,29,30,31]. The principal mechanisms involve the decomposition of organic materials, primarily hemicellulose, and the removal of water and other volatiles [11,32,33]. While gas and liquid fractions are produced, the primary goal of torrefaction is to maximize the yield and quality of the solid product (i.e., biofuel) [32,33].
The torrefaction offers several advantages via converting biomass into more suitable biofuel for combustion, co-combustion, and gasification [34,35,36]. It improves the energy density of biomass and creates a more concentrated fuel source by removing moisture and volatile compounds. Torrefied biomass enhances handling and storage properties and increases durability and reduced risk of degradation. The reduction in the moisture and volatile content of biomass also results in lower GHG emissions and pollutants during combustion [8,14,15,16,17,18,19,20,21]. The torrefaction also provides several secondary benefits. It prevents the fungal growth caused by the high moisture content and hemicellulose structure in woody biomass that causes the biomass to decompose spontaneously, thus making it difficult to store the biomass [37]. Furthermore, excessive ash content in raw biomass presents significant operational challenges during thermal conversion. High concentrations of potassium, sodium, sulfur, and chlorine can lower the ash melting point. This often leads to severe issues like slagging and fouling on heat transfer surfaces, as well as bed agglomeration in fluidized bed systems. Torrefaction can also improve ash behavior during combustion. By volatilizing elements like chlorine and potassium, it can reduce the propensity for slagging and fouling, thereby preventing system blockages and decreasing maintenance costs [38]. The low bulk density, high biodegradability, and high moisture content of biomass create significant challenges for transportation, leading to increased costs for logistics. The self-decay of biomass and its limited grindability also hinder its adaptability to various systems and environments. The torrefaction process offers a solution to these factors that reduce energy conversion efficiencies in biomass energy systems [3,24].
The torrefaction provides more efficient and cleaner combustion by improving the combustion characteristics of biomass (such as increased calorific value, improved grindability, and reduced ash content). As a result, torrefied biomass can be used as a direct substitute for coal in existing power plants and industrial boilers without requiring significant infrastructure modifications, contributing to a reduction in GHG emissions and promoting a greener energy mix [8,14,15,16,17,18,19,20,21]. The adoption of torrefaction technology can create new economic opportunities, such as job creation in the biomass sector and the development of specialized torrefaction facilities [24,34].

2.2. Key Process Parameters

The quality of the final product, torrefied biomass, is governed by the torrefaction process conditions, with temperature and retention time being the most critical parameters. The cumulative effects of temperature and retention time determine the mass yield, calorific value, and other critical characteristics of the torrefied biomass [39].
Temperature is widely recognized as the dominant factor in torrefaction. Increasing the torrefaction temperature leads to more significant changes in the chemical structure of the biomass, resulting in a higher carbon content and a lower oxygen content, which enhances the calorific value [40,41,42]. Dirgantara et al., has shown that the decrease in volatile matter and the increase in fixed carbon content in both palm kernel shells and pine pellets with increasing process temperature improves the fuel quality. They observed the highest calorific values between 275 °C and 325 °C [43]. Similarly, flax shives showed the most favorable heat of combustion at 320 °C [44]. Ivanovski et al. also found that the calorific value of torrified municipal solid waste increased from 24.3 MJ/kg to 25.1 MJ/kg as the temperature rises from 200 °C to 300 °C [42]. Prasongthum et al. found that torrefied rubber wood pellets show a 35% increase in energy density at 300 °C [40]. Prasongthum et al. also stated particularly for the torrefaction of rubber wood pellets that higher temperatures improve the hydrophobicity of torrefied biomass, which is beneficial for storage and transport [40].
Retention time also plays a crucial role in conversion of biomass into torrefied biomass. While longer retention times promote more complete devolatilization and can increase fixed carbon content [43,45], they can also lead to an undesirable decrease in mass yield [42,44,46]. Dirgantara et al., found that retention time has a crucial role in the initial stages, particularly. They found that a retention time of 20–40 min significantly increases the calorific value of palm kernel shells [43]. It was reported that longer retention times can lead to a decrease in mass yield, as seen in municipal solid waste and flax shives [42,44,46]. Saadon et al. and Dirgantara et al. also observed in palm kernel shells and Napier grass that longer retention times result in the reduction in volatile content and the increase in fixed carbon and this enhances the stability and energy content of the biofuel [43,45].
The interplay between process temperature and retention time is therefore critical. For instance, an optimal condition for Napier grass was found at 260 °C for 30 min, whereas for pine pellets, a longer duration of 50–60 min at a similar temperature was proposed [41,45]. This indicates that a balanced approach is necessary to maximize the benefits of the torrefaction process for different biomass feedstocks [40,43,46]. Excessive temperatures or longer retention times can lead to undesirable mass loss and reduced energy yield. These facts indicate that a balanced approach is necessary to maximize the benefits of torrefaction process and overall contribute to the development of efficient biomass-based energy systems.

2.3. Torrefaction of Hazelnut Shells

Hazelnut shells are a promising feedstock for torrefaction due to their high lignocellulosic content and widespread availability in Türkiye [24,25,26,27]. While various studies have mainly investigated the potential of hazelnut shell for biochar production, and its applications as a soil amendment, carbon sequestration agent, and water purification material, there has been limited research on the torrefaction of hazelnut shell (production of biofuel) as a lignin-rich biomass [26]. Mahmood & Ceylan specifically examined the effects of torrefaction on high-lignin content hazelnut shells at 300 °C, revealing significant improvements in fuel properties, such as increased carbon content and decreased oxygen content, which enhance the material’s energy potential [47]. Zhao et al., explored the broader thermal decomposition behavior of hazelnut shells under different heating rates and nitrogen flow rates, providing insights into the kinetic parameters essential for designing thermochemical conversion equipment. They have found that pyrolysis temperature affects yield and characteristics of torrefied hazelnut shell; and biofuel produced at different temperatures showed varying properties [48].
Research on the torrefaction of agricultural biomass has considerably enhanced the understanding of the effects of the process on biomass fuel quality, energy yield, and chemical composition. However, several gaps remain that need further exploration to optimize torrefaction for broader commercial and environmental benefits. The primary research need on torrefaction of agricultural residues is to conduct more comprehensive research on the impact of different torrefaction atmospheres and conditions on a wider range of biomass types [34,49]. The variability in biomass composition, such as in wheat straw and corncobs, suggests that torrefaction processes might need to be tailored to specific biomass types to achieve optimal results.

3. Materials and Methods

3.1. Torrefaction System

Torrefaction studies were carried out in a muffle furnace by MSE Teknoloji, Gebze, Kocaeli, Türkiye with a temperature range of 0–1500 °C and a maximum heat flow of 10 °C/min. The muffle furnace is equipped with a stainless-steel reactor that is connected to the nitrogen line to create an inert environment in the system. Two holes with a diameter of 1 cm were drilled in the back of the reactor, and gas inlet and outlet were provided by welding two 50 cm long pipes to the places where these holes were. The furnace is heated by electricity and temperature control is provided by thermostats. Electric heaters are located inside the oven on the side surfaces. For thermal insulation, the inside of the furnace was completely covered with ceramic wool (Figure 2).

3.2. Biomass Feedstock

Hazelnut shells were supplied from a local hazelnut processing plant in Western Black Sea Region in Türkiye. As received, the feedstock was heterogeneous in particle size. After removing impurities, the shells were cracked to reduce their size, and the resulting material was prepared to obtain a uniform feedstock with a particle size of approximately 3–5 mm. Since the raw feedstock was received with a relatively low moisture content, no drying was applied to the feedstock. The prepared feedstock was stored in a dry environment until the torrefaction experiments.

3.3. Analytical Methods

The characterization of the feedstock samples and the different torrefied biomass samples was conducted through a series of standardized analyses to determine their physical and chemical properties. The analyses included proximate analysis, elemental analysis, and heating value analysis, all performed according to the relevant ASTM/ISO standards (Table 1).
LECO TGA 701, with 1000 °C maximum process temperature, was used for thermogravimetric analysis to determine the mass loss in terms of moisture, ash and volatile matter contents. For the elemental analysis of torrefied biomass, LECO TrusPec, which operates up to 1300 °C and can analyze samples weighing between 1 mg and 10 mg, was used. LECO AC-350 bomb calorimeter, which can analyze 1 g of sample in the range of 15–35 kJ/g, was used in the calorific value analysis of torrefied biomass.

3.4. Calculation of Performance Indicators

To evaluate the effectiveness of the torrefaction process, three key performance indicators were calculated for each experimental condition: Mass Yield (MY), Energy Densification Ratio (EDR), and Energy Yield (EY) [13].
Mass Yield (MY) represents the percentage of the initial feedstock mass that is retained in the final solid torrefied biomass after the torrefaction process. It was calculated using Equation (1).
MY (%) = (Mproduct/Mfeedstock) · 100
The Energy Densification Ratio (EDR) quantifies the degree to which the energy content per unit mass has been concentrated in the torrefied biomass compared to the original feedstock. It is a dimensionless value calculated as the ratio of HHV of the torrefied biomass to that of the feedstock (Equation (2)).
EDR = HHVproduct/HHVfeedstock
The Energy Yield (EY) indicates the percentage of the total energy from the initial feedstock that is preserved in the final solid product. It is calculated by combining the mass yield and the energy densification ratio, as shown in Equation (3) below. This parameter is crucial for assessing the overall energy efficiency of the conversion process.
EY (%) = MY · EDR

3.5. Selection of Experimental Parameters

The primary goals of torrefaction are to reduce the moisture content and increase the energy density of the final product, torrefied biomass. Process temperature and retention time are the key parameters that control these outcomes [28]. As temperature and retention time increase, the solid mass yield and moisture content decrease due to the thermal decomposition and devolatilization of biomass components [58]. Based on these principles, this study investigated torrefaction temperatures of 260 °C, 280 °C, and 300 °C, with retention times of 30 and 60 min.

3.6. Experimental Procedure

For each experimental run, a 100 g sample of raw hazelnut shells was placed into the reactor, and the reactor’s lid was securely sealed. To create an inert atmosphere and prevent oxidation during the process, the reactor was continuously fed with high-purity nitrogen (N2) gas at a constant flow rate of 50 mL/min.
The sealed reactor was positioned inside a furnace equipped with heaters on both sides. The experimental procedure began by heating the reactor from ambient conditions to the target temperature (260, 280, or 300 °C) at a controlled heating rate of 10 °C/min. An internal thermocouple was used to monitor and control the reactor temperature. Once the setpoint was reached, the temperature was held constant for the specified retention time (30 or 60 min).
After this isothermal period concluded, the reactor was removed from the furnace and allowed to cool to room temperature while still under the nitrogen atmosphere to prevent post-pyrolysis oxidation of the final product. To minimize variability in the feedstock, all hazelnut shells were sourced from the same harvest season. And all torrefaction experiments were conducted during the same season to maintain consistent environmental conditions. Additionally, we have replicated experiments to ensure the reliability and accuracy of the results.

4. Results and Discussions

4.1. Feedstock Characteristics

The raw hazelnut shell feedstock was characterized to determine its chemical composition and energetic properties prior to torrefaction. The key results from the proximate analysis, ultimate (elemental) analysis, and calorific value measurements are summarized in Table 2.
The feedstock exhibited characteristics typical of lignocellulosic biomass, with a high volatile matter content (74.49% on a dry basis) and a low ash content (1.51% on a dry basis). The as-received moisture content was 11.72%. The Higher Heating Value (HHV) was measured to be 20.54 MJ/kg (dry basis), confirming the material’s suitability as an energy feedstock. The Lower Heating Value (LHV) was calculated from the HHV. All characterization was performed following relevant ASTM and ISO standards given in Table 1.

4.2. Characteristics of Produced Biofuels

The torrefaction process induced significant changes in the physical structure and appearance of the torrefied biomass from hazelnut shells. As illustrated in Figure 3, the visual characteristics of the torrefied biomass varied together with the process severity. A distinct darkening of the material was observed as both torrefaction temperature and retention time increased. This progression in color—from the light brown of the raw feedstock to dark brown and finally to black at the most severe conditions (300 °C, 60 min)—is a qualitative indicator of the degree of carbonization. It visually confirms the extent of thermal decomposition and the formation of fixed-carbon structures within the torrefied biomass. In addition to the color change, the torrefied product also appeared more brittle with increasing process intensity, a common effect of the degradation of fibrous biomass components.
Beyond the visual changes, the process fundamentally altered the chemical composition and fuel properties of the material. The detailed results of the proximate analysis and the heating values for all torrefied biomass samples are presented in Table 3 and Table 4, respectively. The data shows clear trends as a function of process severity. On a dry basis, the volatile matter content decreased substantially while the fixed carbon content increased, with the most pronounced changes occurring at 300 °C for 60 min. At these conditions, the fixed carbon content increased more than two times compared to the raw feedstock (from 22.61% to 47.80%). As expected, the ash content, being non-volatile, became more concentrated in the final product (1.43% to 2.29%). Furthermore, the moisture content was significantly reduced, dropping by over 90% at the most severe conditions, which enhances the quality of torrefied biomass as a fuel.
The effectiveness of the process as a fuel upgrade is best evaluated by analyzing the relationship between mass loss and energy concentration. As shown in Table 3 and Figure 4, increasing process severity (both temperature and time) led to a progressive decrease in both mass yield (i.e., mass loss via devolatilization and decomposition) and energy yield. At the most severe condition of 300 °C for 60 min, the mass yield decreased to a minimum of 59.44%, while 78.38% of the feedstock’s original energy was successfully retained in the final product. The fact that the energy yield decreased less sharply than the mass yield confirms that the energy was effectively concentrated in the solid product.
This energy concentration is also quantified by the Energy Densification Ratio (EDR), shown in Figure 5. The EDR consistently increased with process severity as low-energy volatile compounds were removed. A maximum EDR of 1.32 was achieved at 300 °C and 60 min. This indicates that the heating value of the torrefied biomass at this condition was 32% higher than that of the original feedstock, demonstrating a significant improvement in fuel quality and its suitability for high-energy applications.

4.3. Effect of Process Parameters on Torrefied Biomass Properties

The torrefaction process parameters—temperature and retention time—had a profound and interconnected effect on the final properties of the torrefied biomass from hazelnut shells. These effects were evident in the material’s physical appearance, chemical composition, and energy content.
These physical changes correspond directly to the transformations in chemical composition detailed in Table 3. Increasing the process severity led to a systematic decrease in volatile matter and a corresponding increase in fixed carbon content. The torrefied biomass produced at 260 °C appears slightly lighter in color and less dense, this indicates a lower degree of carbonization. As compared to the higher temperatures, the structure of the hazelnut shells was more preserved. This indicates that the biomass has not been fully devolatilized in this temperature. This behavior aligns with the characteristics of a mild torrefaction process, where moisture and light volatiles are predominantly removed, while the structural integrity of cellulose and lignin remains largely intact, as reported by Wang et al. [30,59,60,61]. At 300 °C, increasing the retention time from 30 to 60 min raised the fixed carbon content substantially, achieving a maximum of 47.80% (as shown in Table 3) at the most severe condition. This represents a more than two-fold increase in the fixed carbon content compared to the raw feedstock, confirming the effectiveness of the process in creating a carbon-dense material.
The energy properties of the torrefied biomass, summarized in Table 4, improved consistently with process severity. The Higher Heating Values (HHV) increased as a result of thermal treatment. The energy densification, quantified by the Energy densification ratio (EDR) and shown in Figure 5, reached a maximum value of 1.32 under the most intense conditions (300 °C, 60 min). This confirms that while a portion of the mass is lost, the energy is effectively concentrated in the remaining solid, successfully upgrading the low-grade biomass into a high-quality solid fuel.

4.4. Discussion and Comparison with Literature

To understand the results in a broader context, the degree of carbonization in the torrefied biomass samples, produced under varying torrefaction conditions, was compared with local Turkish coals. in the Van-Krevelen diagram (Figure 6), the atomic H/C and O/C ratios of the torrefied biomass samples were plotted alongside those of Çan, Seyitömer, and Orhaneli coals. As torrefaction severity increases, the torrefied biomass samples consistently shift towards the region occupied by these low-rank coals. This shift, representing the preferential loss of oxygen and hydrogen over carbon, provides clear evidence of increasing thermal maturity. The close proximity of the torrefied biomass produced at 300 °C for 60 min to the coals highlights its potential as a viable, renewable substitute or co-firing agent in applications that traditionally use fossil fuels.
The torrefied biomass products produced under milder conditions—specifically at 260 °C and 280 °C for both 30 and 60 min—exhibited relatively higher H/C and O/C ratios when compared to the samples treated at 300 °C. This is a clear indication of a lesser degree of carbonization. This result aligns with the observed chemical composition, where higher temperatures and longer retention times promote the breakdown of hemicellulose, cellulose, and lignin, leading to the reduction in oxygen and hydrogen content relative to carbon.
As the torrefaction temperature increases from 260 °C to 300 °C, and as the retention time increases from 30 to 60 min, the torrefied biomass samples move closer to the region occupied by the local coals on the Van Krevelen diagram. This pattern provides clear evidence of the gradual thermal degradation and the increasing carbonization of the biomass under elevated torrefaction conditions. It confirms that torrefied biomass produced at 300 °C and 60 min has improved fuel characteristics, including higher energy density.
The position of 300 °C–60 min torrefied biomass as close proximity to the local coals in the Van Krevelen diagram, highlights its suitability as a viable substitute for local coals in applications requiring higher energy density, such as domestic heating or certain industrial processes.
A recent study by Klimek et al. provides an essential benchmark by analyzing the energy properties of eight different Turkish hazelnut shell varieties, allowing for a direct comparison with the feedstock investigated in this study. The highest HHV value (20.5 MJ/kg), obtained in this study, is slightly higher than the 18.37–19.20 MJ/kg range reported by Klimek et al. This suggests that the feedstock sample used in this study possesses excellent energy potential. The measured ash content of 1.51% (dry basis) is also just outside their reported range of 1.00–1.29%, which underscores the variability that can exist in this type of biomass. Furthermore, the feedstock’s low nitrogen (0.51%) and sulfur (0.07%) contents are consistent with the low values observed across the different varieties studied by Klimek et al., reinforcing the potential for low NOₓ and SO2 emissions during combustion. This comparison highlights that while varietal differences are significant, the feedstock investigated in this study is representative of high-quality hazelnut shell biomass available in Türkiye [62].
The use of torrefied hazelnut shell with coal in industrial burners offers several advantages, including improved combustion efficiency, reduced greenhouse gas emissions, enhanced fuel flexibility, and the ability to adapt to variable fuel supplies. Moreover, it reduces ash generation, thereby lowering the costs associated with ash handling and disposal. This finding demonstrates that through controlled torrefaction, biomass can be engineered to possess similar properties to fossil fuels, providing a renewable and sustainable energy source with reduced environmental impact.
The findings in this study, regarding the correlations between temperature, retention time and the resultant fuel properties, align with existing literature on torrefaction of various nut feedstocks. The highest temperature applied in this study (i.e., 300 °C) shows that the LHV and the HHV both experienced significant increases with a 60 min retention time. This trend is similar to that observed in a study focusing on almond and walnut byproducts, where torrefaction resulted in enhanced hydrophobic properties and higher calorific values as well [58]. Moreover, the findings of this study indicate significant reductions in moisture content with increased retention time. For instance, at 300 °C for 60 min, moisture content reached as low as 0.53%, consistent with lower moisture contents achieved in various studies involving nut shells [59]. Such reductions are critical for biofuel applications, as lower moisture levels enhance the combustion characteristics and storage capabilities of the resultant torrefied biomass [60].
In this study, it was observed that the fixed carbon content of torrefied hazelnut shells at 300 °C for 60 min reached 47.80%. This represents a notable higher carbon content when contrasted with the torrefied almond and walnut shells [58]. This underscores the potential of hazelnut shells as a promising feedstock for bioenergy applications.
The selection of retention time in torrefaction returns vital information about the intensity of thermal treatment. Studies have demonstrated that while mass and energy yields decrease with extended retention times, the energy densification of the final product is enhanced [63]. This study’s findings suggest a correlation between increased retention time and energy densities, indicating that optimal process conditions can enhance not just the energy characteristics but also the fuel handling qualities of nut-derived feedstocks.
It is important to note that the scope of this study was intentionally focused on the torrefaction process, which is defined by its specific temperature range of 200–300 °C. While torrefaction is a form of mild pyrolysis, a different upgrading pathway exists through low-temperature pyrolysis at higher temperatures (e.g., 300–500 °C). This more intensive thermal treatment can produce a biochar with a higher fixed-carbon content and greater thermal maturity. Such a high-grade product may be more suitable for specialized applications, for instance as a reducing agent in metallurgical processes or as a precursor for activated carbon, where maximizing carbon content is the primary objective. However, the selection of torrefaction in this work was driven by the goal of maximizing the mass and energy yield of the solid product for use as a bulk solid biofuel, such as for co-firing in power plants, where a balance between fuel quality and overall energy recovery is often the key economic driver.
In summary, this study reinforces knowledge from existing literature regarding the torrefaction of various nuts, showing that the significant increases in HHV, LHV, and fixed carbon content, along with decreased moisture, affirm the viability of hazelnut shells as a sustainable biofuel feedstock. The results obtained in the study suggest that refining torrefaction parameters for hazelnut shells could lead to superior energy outputs compared to similar nut byproducts.

5. Conclusions

This study successfully demonstrated the upgrading of Turkish hazelnut shells into a high-quality solid biofuel, torrefied biomass, through torrefaction. The investigation systematically evaluated the effects of process temperature and retention time on the final fuel characteristics. The results showed that both parameters are critical in hazelnut shell torrefaction and process temperature had a more noticeable impact on energy densification than retention time. For instance, increasing the temperature from 260 °C to 300 °C (at a 60 min retention time) boosted the energy density by 18.6%, whereas extending the retention time from 30 to 60 min (at 300 °C) resulted in a smaller, 13.6% increase.
The optimal conditions for maximizing fuel quality were identified as a torrefaction temperature of 300 °C and a retention time of 60 min. Under these conditions, the resulting torrefied biomass exhibited the most significant improvement in fuel properties, including the highest fixed carbon content (47.80%), minimal volatile matter, and a peak energy densification ratio of 1.32. Analysis using a Van Krevelen diagram confirmed that torrefied biomass produced under these severe conditions achieved a degree of carbonization comparable to local low-rank coals, highlighting its potential as a viable renewable substitute or co-firing agent in industrial applications.
The primary contribution of this work lies in providing foundational engineering data for a high-volume, regionally significant feedstock. While previous studies have characterized raw shells, this work offers the first systematic analysis of the torrefaction process itself, generating the essential quantitative data (mass/energy yields, EDR) required to design and optimize a valorization system for this agricultural residue. Furthermore, by directly comparing the produced torrefied biomass with local Turkish coals on a Van Krevelen diagram, this study provides a practical and novel benchmark for fuel substitution, addressing the specific question of how this abundant local waste can replace fossil fuels in a regional context. This work also builds upon recent findings by demonstrating how torrefaction can create a consistent, high-quality biofuel from a variable feedstock.
However, further research is needed to evaluate the scalability, economic viability, and long-term performance of torrefied biomass from hazelnut shell in real-world industrial settings. Additionally, studies on optimizing blending ratios and handling techniques for co-combustion with coal will be crucial for ensuring efficient and consistent fuel performance. While this study focused on the characterization of the upgraded solid fuel, a complete analysis of the gaseous byproducts was not conducted. Given the low nitrogen content of hazelnut shells, the concentration of resulting gases like ammonia is presumed to be minimal. A future study could perform a detailed analysis of the gas stream to quantify these minor components.
In conclusion, this research confirms that torrefaction is an effective method for converting hazelnut shell waste into an energy-dense, solid biofuel. The findings provide a strong foundation for developing sustainable energy solutions in regions with abundant agricultural residues, contributing to the transition away from fossil fuels.

Author Contributions

G.D. prepared the raw biomass feedstocks for torrefaction, operated the torrefaction system, conducted the torrefaction experiments, and wrote the literature review, methodology, and results sections of the paper. H.S. analyzed the torrefaction results, prepared the graphs for data visualization, and reviewed the manuscript. Both authors contributed to writing the conclusions of the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by TUBITAK, grant (project) number 216M348.

Data Availability Statement

The authors declare that the relevant data supporting the findings of the study are available in the article itself.

Acknowledgments

The financial support for this project by TUBITAK-ARDEB 1003 program (Project Code: 216M348) is greatly appreciated. The authors thank Hayati Olgun, from Ege University, for his valuable support to this study via the design and establishment of the torrefaction system under the research project funded by TUBITAK ARDEB 1003 and supplying feedstock materials.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Kung, C.-C.; Lee, T.-J.; Chen, L.-J. Economic Growth and Environmental Sustainability from Renewable Energy Applications. Energy Explor. Exploit. 2021, 39, 531–535. [Google Scholar] [CrossRef]
  2. Ogoshi, R.; Turano, B.; Uehara, G.; Yanagida, J.; Illukpitiya, P.; Brewbaker, J.; Carpenter, J. Evaluation of Cellulosic Feedstocks for Biofuel Production. In Bioenergy and Biofuel from Biowastes and Biomass; American Society of Civil Engineers: Washington, DC, USA, 2013; pp. 130–157. [Google Scholar] [CrossRef]
  3. Arias, B.; Pevida, C.; Fermoso, J.; Plaza, M.G.; Rubiera, F.; Pis, J.J. Influence of Torrefaction on the Grindability and Reactivity of Woody Biomass. Fuel Process. Technol. 2008, 89, 169–175. [Google Scholar] [CrossRef]
  4. Kutlu, O.; Aydin-Kandemir, F.; Sarptas, H. Assessing Primary Areas for a Sustainable Biochar Application in Soil by Using GIS-Based Multi-Criteria Evaluation. Clean Technol. Environ. Policy 2021, 23, 2443–2455. [Google Scholar] [CrossRef]
  5. Cotana, F.; Coccia, V.; Cavalaglio, G.; Barbanera, M.; Petrozzi, A. Chapter 5—Biomass-Based Systems. In Polygeneration Systems; Calise, F., Dentice D’Accadia, M., Vanoli, L., Vicidomini, M., Eds.; Academic Press: New York, NY, USA, 2022; pp. 137–192. ISBN 978-0-12-820625-6. [Google Scholar]
  6. Umakanth, A.V.; Datta, A.; Reddy, B.S.; Bardhan, S. Chapter 3—Biomass Feedstocks for Advanced Biofuels: Sustainability and Supply Chain Management. In Advanced Biofuel Technologies; Tuli, D., Kasture, S., Kuila, A., Eds.; Elsevier: Amsterdam, The Netherlands, 2022; pp. 39–72. ISBN 978-0-323-88427-3. [Google Scholar]
  7. Hanif, M.A.; Nadeem, F.; Tariq, R.; Rashid, U. Chapter 9—Renewable Energy from Biomass. In Renewable and Alternative Energy Resources; Hanif, M.A., Nadeem, F., Tariq, R., Rashid, U., Eds.; Academic Press: New York, NY, USA, 2022; pp. 555–603. ISBN 978-0-12-818150-8. [Google Scholar]
  8. Shankar Tumuluru, J.; Sokhansanj, S.; Hess, J.R.; Wright, C.T.; Boardman, R.D. A Review on Biomass Torrefaction Process and Product Properties for Energy Applications. Ind. Biotechnol. 2011, 7, 384–401. [Google Scholar] [CrossRef]
  9. Maibam, P.D.; Goyal, A. Pretreatment Methods for Overcoming Biomass Recalcitrance. In Enzymes in the Valorization of Waste; CRC Press: Boca Raton, FL, USA, 2022; ISBN 978-1-003-18771-4. [Google Scholar]
  10. Sahay, S. Impact of Pretreatment Technologies for Biomass to Biofuel Production. In Substrate Analysis for Effective Biofuels Production; Srivastava, N., Srivastava, M., Mishra, P.K., Gupta, V.K., Eds.; Springer: Singapore, 2020; pp. 173–216. ISBN 978-981-329-607-7. [Google Scholar]
  11. Shah, A.A.; Seehar, T.H.; Sharma, K.; Toor, S.S. Chapter 7—Biomass Pretreatment Technologies. In Hydrocarbon Biorefinery; Maity, S.K., Gayen, K., Bhowmick, T.K., Eds.; Elsevier: Amsterdam, The Netherlands, 2022; pp. 203–228. ISBN 978-0-12-823306-1. [Google Scholar]
  12. Javanmard, A.; Abdul Patah, M.F.; Zulhelmi, A.; Daud, W.M.A.W. A Comprehensive Overview of the Continuous Torrefaction Method: Operational Characteristics, Applications, and Challenges. J. Energy Inst. 2023, 108, 101199. [Google Scholar] [CrossRef]
  13. Keivani, B.; Gultekin, S.; Olgun, H.; Atimtay, A.T. Torrefaction of Pine Wood in a Continuous System and Optimization of Torrefaction Conditions. Int. J. Energy Res. 2018, 42, 4597–4609. [Google Scholar] [CrossRef]
  14. Cahyanti, M.N.; Doddapaneni, T.R.K.C.; Madissoo, M.; Pärn, L.; Virro, I.; Kikas, T. Torrefaction of Agricultural and Wood Waste: Comparative Analysis of Selected Fuel Characteristics. Energies 2021, 14, 2774. [Google Scholar] [CrossRef]
  15. Doddapaneni, T.R.K.C.; Pärn, L.; Kikas, T. Torrefaction of Pulp Industry Sludge to Enhance Its Fuel Characteristics. Energies 2022, 15, 6175. [Google Scholar] [CrossRef]
  16. Iroba, K.L.; Baik, O.-D.; Tabil, L.G. Torrefaction of Biomass from Municipal Solid Waste Fractions II: Grindability Characteristics, Higher Heating Value, Pelletability and Moisture Adsorption. Biomass Bioenergy 2017, 106, 8–20. [Google Scholar] [CrossRef]
  17. Lee, B.-H.; Sh, L.; Lee, D.-G.; Jeon, C.-H. Effect of Torrefaction and Ashless Process on Combustion and NOx Emission Behaviors of Woody and Herbaceous Biomass. Biomass Bioenergy 2021, 151, 106133. [Google Scholar] [CrossRef]
  18. Majamo, S.L.; Amibo, T.A. Modeling and Optimization of Chemical-Treated Torrefaction of Wheat Straw to Improve Energy Density by Response Surface Methodology. Biomass Conv. Bioref. 2023, 14, 21213–21227. [Google Scholar] [CrossRef]
  19. Niu, Y.; Lv, Y.; Lei, Y.; Liu, S.; Liang, Y.; Wang, D.; Hui, S. Biomass Torrefaction: Properties, Applications, Challenges, and Economy. Renew. Sustain. Energy Rev. 2019, 115, 109395. [Google Scholar] [CrossRef]
  20. Ramakrishna, V.; Singh, A.K.; Bayen, G.K.; Masto, R.E.; Meena, R.K.; Singh, P.K.; Arya, M.; Jyoti, S.; Usham, A.L. Torrefaction of Agro-Wastes (Palmyra Palm Shell and Redgram Stalk): Characterization of the Physicochemical Properties and Mechanical Strength of Binderless Pellets. Biomass Conv. Bioref. 2023, 13, 6953–6971. [Google Scholar] [CrossRef]
  21. Yu, Y.; Zhu, Z.; Wang, L.; Wang, G.; Bai, X. Effect of Torrefaction Treatment on Physical and Fuel Properties of Caragana (Caragana Korshinskii) Pellets. Bioenerg. Res. 2021, 14, 1277–1288. [Google Scholar] [CrossRef]
  22. FAO FAOSTAT—Crops and Livestock Products. Available online: https://www.fao.org/faostat/en/#data/QCL (accessed on 11 June 2024).
  23. TEPGE. Hazelnut Crop Report 2022; Turkish Agricultural Economic and Policy Development Institıte: Ankara, Turkey, 2022. [Google Scholar]
  24. Chen, W.-H.; Peng, J.; Bi, X.T. A State-of-the-Art Review of Biomass Torrefaction, Densification and Applications. Renew. Sustain. Energy Rev. 2015, 44, 847–866. [Google Scholar] [CrossRef]
  25. Cruz-Lopes, L.; Duarte, J.; Dulyanska, Y.; Guiné, R.P.F.; Esteves, B. Enhancing Liquefaction Efficiency: Exploring the Impact of Pre-Hydrolysis on Hazelnut Shell (Corylus avellana L.). Materials 2024, 17, 2667. [Google Scholar] [CrossRef]
  26. Haykiri-Acma, H.; Yaman, S.; Kucukbayrak, S. Effects of Torrefaction on Lignin-Rich Biomass (Hazelnut Shell): Structural Variations. J. Renew. Sustain. Energy 2017, 9, 063102. [Google Scholar] [CrossRef]
  27. Manterola-Barroso, C.; Padilla Contreras, D.; Ondrasek, G.; Horvatinec, J.; Gavilán CuiCui, G.; Meriño-Gergichevich, C. Hazelnut and Walnut Nutshell Features as Emerging Added-Value Byproducts of the Nut Industry: A Review. Plants 2024, 13, 1034. [Google Scholar] [CrossRef]
  28. Bergman, P.C.; Boersma, A.R.; Zwart, R.W.; Kiel, J.H. Torrefaction for Biomass Co-Firing in Existing Coal-Fired Power Station; Energy Research Centre of the Netherlands (ECN): Petten, The Netherlands, 2005. [Google Scholar]
  29. Devekiran, G. Investigation of Biochar Production Process and Determination of Optimum Process Parameters: Pine Shavings and Nut Shells. Ph.D. Thesis, Ege University, Bornova, Turkey, 2019. [Google Scholar]
  30. Wang, L.; Barta-Rajnai, E.; Skreiberg, Ø.; Khalil, R.; Czégény, Z.; Jakab, E.; Barta, Z.; Grønli, M. Impact of Torrefaction on Woody Biomass Properties. Energy Procedia 2017, 105, 1149–1154. [Google Scholar] [CrossRef]
  31. Yue, Y.; Singh, H.; Singh, B.; Mani, S. Torrefaction of Sorghum Biomass to Improve Fuel Properties. Bioresour. Technol. 2017, 232, 372–379. [Google Scholar] [CrossRef]
  32. Anukam, A.; Berghel, J.; Anukam, A.; Berghel, J. Biomass Pretreatment and Characterization: A Review. In Biotechnological Applications of Biomass; IntechOpen: London, UK, 2020; ISBN 978-1-83881-182-2. [Google Scholar]
  33. Pach, M.; Zanzi, R.; Björnbom, E. Torrefied Biomass a Substitute for Wood and Charcoal. In Proceedings of the 6th Asia-Pacific International Symposium on Combustion and Energy Utilization, Kuala Lumpur, Malaysia, 20–22 May 2002. [Google Scholar]
  34. Acharya, B.; Sule, I.; Dutta, A. A Review on Advances of Torrefaction Technologies for Biomass Processing. Biomass Convers. Biorefinery 2012, 2, 349–369. [Google Scholar] [CrossRef]
  35. Pentananunt, R.; Rahman, A.N.M.M.; Bhattacharya, S.C. Upgrading of Biomass by Means of Torrefaction. Energy 1990, 15, 1175–1179. [Google Scholar] [CrossRef]
  36. Phanphanich, M.; Mani, S. Impact of Torrefaction on the Grindability and Fuel Characteristics of Forest Biomass. Bioresour. Technol. 2011, 102, 1246–1253. [Google Scholar] [CrossRef]
  37. Hakkou, M.; Pétrissans, M.; Gérardin, P.; Zoulalian, A. Investigations of the Reasons for Fungal Durability of Heat-Treated Beech Wood. Polym. Degrad. Stab. 2006, 91, 393–397. [Google Scholar] [CrossRef]
  38. Kleinhans, U.; Wieland, C.; Frandsen, F.J.; Spliethoff, H. Ash Formation and Deposition in Coal and Biomass Fired Combustion Systems: Progress and Challenges in the Field of Ash Particle Sticking and Rebound Behavior. Prog. Energy Combust. Sci. 2018, 68, 65–168. [Google Scholar] [CrossRef]
  39. Waheed, A.; Naqvi, S.R.; Ali, I. Co-Torrefaction Progress of Biomass Residue/Waste Obtained for High-Value Bio-Solid Products. Energies 2022, 15, 8297. [Google Scholar] [CrossRef]
  40. Prasongthum, N.; Duangwongsa, N.; Khowattana, P.; Suemanotham, A.; Wongharn, P.; Thanmongkhon, Y.; Reubroycharoen, P.; Attanatho, L. Influence of Torrefaction on Yields and Characteristics of Densified Solid Biofuel. J. Phys. Conf. Ser. 2022, 2175, 012027. [Google Scholar] [CrossRef]
  41. Iglesias Canabal, A.; Proupín Castiñeiras, J.; Rodríguez Añón, J.A.; Eimil Fraga, C.; Rodríguez Soalleiro, R. Predicting the Energy Properties of Torrefied Debarked Pine Pellets from Torrefaction Temperature and Residence Time. Renew. Energy 2023, 218, 119346. [Google Scholar] [CrossRef]
  42. Ivanovski, M.; Goričanec, D.; Urbancl, D. The Thermochemical Conversion of Municipal Solid Waste by Torrefaction Process. Thermo 2023, 3, 277–288. [Google Scholar] [CrossRef]
  43. Dirgantara, M.; Karelius, K.; Rumbang, N. Significance of Torrefaction Effect on Energy Properties Palm Kernel Shell. JPSE (J. Phys. Sci. Eng.) 2023, 8, 119–129. [Google Scholar] [CrossRef]
  44. Molenda, J.; Zacharski, P.; Swat, M. Torrefaction of Flax Shives as a Process of Preparation Waste Vegetable Biomass for Energy Purposes. Environ. Prot. Nat. Resour. 2023, 34, 147–153. [Google Scholar] [CrossRef]
  45. Saadon, S.Z.A.H.; Osman, N.B.; Damodaran, M.; Liew, S.E. Torrefaction of Napier Grass and Oil Palm Petiole Waste Using Drop-Type Fixed-Bed Pyrolysis Reactor. Materials 2022, 15, 2890. [Google Scholar] [CrossRef]
  46. Molenda, J. Badanie właściwości energetycznych biowęgli z odpadowej biomasy roślinnej za pomocą różnicowej kalorymetrii skaningowej. Chem. Rev. 2024, 1, 95–98. [Google Scholar] [CrossRef]
  47. Mahmood, M.A.; Ceylan, S. Insights into Reaction Modeling and Product Characterization of Hazelnut Shell Pyrolysis. Bioenerg. Res. 2022, 15, 1281–1291. [Google Scholar] [CrossRef]
  48. Zhao, C.; Liu, X.; Chen, A.; Chen, J.; Lv, W.; Liu, X. Characteristics Evaluation of Bio-Char Produced by Pyrolysis from Waste Hazelnut Shell at Various Temperatures. Energy Sources Part A Recovery Util. Environ. Eff. 2020, 46, 7403–7413. [Google Scholar] [CrossRef]
  49. Yang, X.; Zhao, Y.; Zhang, L.; Wang, Z.; Zhao, Z.; Zhu, W.; Ma, J.; Shen, B. Effects of Torrefaction Pretreatment on the Structural Features and Combustion Characteristics of Biomass-Based Fuel. Molecules 2023, 28, 4732. [Google Scholar] [CrossRef]
  50. ASTM 7582-15; Standard Test Methods for Proximate Analysis of Coal and Coke by Macro Thermogravimetric Analysis. ASTM International: West Conshohocken, PA, USA, 2015.
  51. ASTM D5373-14e2; Standard Test Methods for Determination of Carbon, Hydrogen and Nitrogen in Analysis Samples of Coal and Carbon in Analysis Samples of Coal and Coke. ASTM International: West Conshohocken, PA, USA, 2016.
  52. ASTM E1755-01; Standard Test Method for Ash in Biomass. ASTM International: West Conshohocken, PA, USA, 2001.
  53. ASTM D7582-15; Standard Practice for Proximate Analysis of Coal and Coke. ASTM International: West Conshohocken, PA, USA, 2013.
  54. ASTM D4239-14e2; Standard Test Methods for Sulfur in the Analysis Sample of Coal and Coke Using High-Temperature Tube Furnace Combustion. ASTM International: West Conshohocken, PA, USA, 2014.
  55. ATSTM D3176-09; Standard Practice for Ultimate Analysis of Coal and Coke. ASTM International: West Conshohocken, PA, USA, 2009.
  56. ASTM D5865-13; Standard Test Method for Gross Calorific Value of Coal and Coke. ASTM International: West Conshohocken, PA, USA, 2013.
  57. ISO 1928:2009; Solid Mineral Fuels—Determination of Gross Calorific Value by the Bomb Calorimetric Method and Calculation of Net Calorific Value. ISO: Geneva, Switzerland, 2009.
  58. McCaffrey, Z.; Torres, L.; Chiou, B.-S.; Ferreira, S.R.; Silva, L.E.; Wood, D.F.; Orts, W.J. Torrefaction of Almond and Walnut Byproducts. Front. Energy Res. 2021, 9, 643306. [Google Scholar] [CrossRef]
  59. Noszczyk, T.; Dyjakon, A.; Koziel, J.A. Kinetic Parameters of Nut Shells Pyrolysis. Energies 2021, 14, 682. [Google Scholar] [CrossRef]
  60. Brachi, P.; Miccio, F.; Ruoppolo, G.; Miccio, M. Pressurized Steam Torrefaction of Biomass: Focus on Solid, Liquid, and Gas Phase Distributions. Ind. Eng. Chem. Res. 2017, 56, 12163–12173. [Google Scholar] [CrossRef]
  61. Zheng, A.; Jiang, L.; Zhao, Z.; Huang, Z.; Zhao, K.; Wei, G.; Wang, X.; He, F.; Li, H. Impact of Torrefaction on the Chemical Structure and Catalytic Fast Pyrolysis Behavior of Hemicellulose, Lignin, and Cellulose. Energy Fuels 2015, 29, 8027–8034. [Google Scholar] [CrossRef]
  62. Klimek, K.E.; Kordali, S.; Borkowska, A.; Yilmaz, F.; Maj, G. Energy Assessment of Hazelnut Shells (Corylus avellana L.) of Selected Turkish Varieties. Energies 2025, 18, 3612. [Google Scholar] [CrossRef]
  63. Mukherjee, A.; Okolie, J.A.; Niu, C.; Dalai, A.K. Experimental and Modeling Studies of Torrefaction of Spent Coffee Grounds and Coffee Husk: Effects on Surface Chemistry and Carbon Dioxide Capture Performance. ACS Omega 2022, 7, 638–653. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. Türkiye’s share of global hazelnut production over the years (data taken from [22]).
Figure 1. Türkiye’s share of global hazelnut production over the years (data taken from [22]).
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Figure 2. The muffle furnace and its reactor.
Figure 2. The muffle furnace and its reactor.
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Figure 3. Visual appearance of torrefied biomass samples produced at different process conditions.
Figure 3. Visual appearance of torrefied biomass samples produced at different process conditions.
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Figure 4. The change in energy yield vs. mass yield in torrefaction of hazelnut shells.
Figure 4. The change in energy yield vs. mass yield in torrefaction of hazelnut shells.
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Figure 5. Energy densification ratio by mass yield in torrefaction of hazelnut shells.
Figure 5. Energy densification ratio by mass yield in torrefaction of hazelnut shells.
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Figure 6. Van-Krevelen diagram for torrefied biomass samples and different local coals.
Figure 6. Van-Krevelen diagram for torrefied biomass samples and different local coals.
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Table 1. Standards used for analysis of raw biomass and the torrefied biomass.
Table 1. Standards used for analysis of raw biomass and the torrefied biomass.
AnalysisStandard UsedAnalysisStandard Used
Proximate analysis Ultimate analysis
 MoistureASTM 7582-15 [50] CASTM D 5373-14e2 [51]
 Volatile matterASTM 7582-15 HASTM D 5373-14e2
 AshASTM E 1755-01 [52] NASTM D 5373-14e2
 Fixed carbonASTM D 3172-13 [53] SASTM D 4239-14e2 [54]
 OASTM D 3176-09 [55]
Heating value
 Higher HV (HHV)ASTM D 5865-13 [56]
 Lower HV (LHV)ISO 1928-09 [57]
Table 2. Characteristics of the hazelnut shell feedstock.
Table 2. Characteristics of the hazelnut shell feedstock.
ParameterWeight (%)ParameterOriginal
Basis		Dry
Basis
C56.34Content (% by weight)
H5.35Moisture11.72-
N0.51Volatile Matters70.2474.49
S0.07Ash1.431.51
O36.06Fixed Carbon22.6124.00
Heating Value (MJ/kg)
LHV18.1019.34
HHV19.3620.54
Table 3. Characteristics of the torrefied biomass.
Table 3. Characteristics of the torrefied biomass.
Temp. (°C)Ret. Time (min)Moisture (%)Volatile Matter (%)Ash
(%)		Fixed
Carbon (%)
Feedstock (original)--11.7270.241.4322.61
Feedstock (dry)---74.491.5124.00
Torrefied biomass260300.7770.461.4827.29
260600.5466.361.3831.72
280300.6368.411.4029.56
280600.5458.401.6039.46
300300.5862.442.1534.83
300600.5349.382.2947.80
Table 4. Heating values of the torrefied biomass.
Table 4. Heating values of the torrefied biomass.
Temp. (°C)Ret. Time (min)LHV (MJ/kg)HHV (MJ/kg)
OriginalDryOriginalDry
Feedstock--18.1019.3419.3620.54
Torrefied biomass2603020.3920.5721.4421.60
2606021.7121.8322.7222.83
2803020.2220.4322.2522.95
2806022.8723.0023.8823.93
3003022.6422.7823.7023.84
3006026.0426.2826.8527.08
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Devekıran, G.; Sarptaş, H. Torrefaction of Hazelnut Shells: The Effects of Temperature and Retention Time on Energy Yield and Fuel Characteristics. Energies 2025, 18, 4710. https://doi.org/10.3390/en18174710

AMA Style

Devekıran G, Sarptaş H. Torrefaction of Hazelnut Shells: The Effects of Temperature and Retention Time on Energy Yield and Fuel Characteristics. Energies. 2025; 18(17):4710. https://doi.org/10.3390/en18174710

Chicago/Turabian Style

Devekıran, Gökhan, and Hasan Sarptaş. 2025. "Torrefaction of Hazelnut Shells: The Effects of Temperature and Retention Time on Energy Yield and Fuel Characteristics" Energies 18, no. 17: 4710. https://doi.org/10.3390/en18174710

APA Style

Devekıran, G., & Sarptaş, H. (2025). Torrefaction of Hazelnut Shells: The Effects of Temperature and Retention Time on Energy Yield and Fuel Characteristics. Energies, 18(17), 4710. https://doi.org/10.3390/en18174710

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