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Article

Sustainable Solutions for Energy Production from Biomass Materials

by
Penka Zlateva
1,
Angel Terziev
2,* and
Nevena Milcheva Mileva
1
1
Department of Thermal Engineering, Technical University of Varna, 9010 Varna, Bulgaria
2
Faculty of Power Engineering and Power Machines, Technical University of Sofia, 1756 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(17), 7732; https://doi.org/10.3390/su16177732
Submission received: 7 August 2024 / Revised: 28 August 2024 / Accepted: 3 September 2024 / Published: 5 September 2024
(This article belongs to the Section Environmental Sustainability and Applications)
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Abstract

:
This study reveals the possibilities of the sustainable usage of pellets produced from waste biomass based on the thermal properties of processed raw materials. For this study, a thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA) were performed to better understand the thermal properties of the pellets. This study covered two types of wood pellets with different ratios of waste material: a kind of pellet made from a combination of wood and sunflower residues, and two types of pellets made from sunflower husks. The analysis revealed that the wood pellets offered the best thermal stability and high energy values, making them the preferred choice for heating systems. Mixed pellets showed a lower thermal capacity and combustion efficiency, showing possibilities for further optimization. Sunflower-husk pellets demonstrated a high calorific value, but their application was restricted by a significantly higher ash content and worse environmental impact compared with the first two types of pellets. In addition, the outputs from this study revealed that pellets composed of wood reduced their mass the most at temperatures in the range of 310 to 323 °C. In comparison, the mass loss of sunflower husk pellets was 35.6%/°C (at a 5 °C/min temperature gradient) lower than wood pellets and about 42%/°C lower at a 10 °C/min temperature gradient. These findings highlight the importance of pellet production and technology usage to achieve better sustainability and better thermal properties of the pellets.

1. Introduction

The energy policies of both the European Union (EU) and Bulgaria are designed in line with several key objectives, including ensuring supply security, promoting sustainable development, and integrating the internal energy biomass market [1,2,3,4].
The main aspects of the EU’s energy policy are based on the security of the energy supply—reducing the necessity of external energy suppliers by diversifying sources and delivery routes and initiating an increase in the national energy production share through the integration of renewable sources and nuclear energy. The European Green Deal set targets for reducing greenhouse gas emissions by 55% by 2030 compared to 1990 levels and achieving carbon neutrality by 2050, promoting the production and use of renewable energy sources, including biomass, and the development of measures to reduce final energy consumption through various energy efficiency measures in buildings, industry, and transportation [5,6,7,8,9].
Bulgaria’s energy policy includes diversifying energy sources—developing solutions to reduce the need for external energy sources [10]. Bulgaria supports the EU’s goals for reducing greenhouse gas emissions and increasing the share of renewable energy sources in the country’s energy mix [11,12]. Bulgaria is working towards full integration with the European energy market through the development of infrastructure and regulatory frameworks compatible with European standards [13].
Forests play a key role in maintaining an ecological balance, preserving biodiversity, and mitigating climate change [14]. The state of forests in the EU is quite different as some indicators like biomass volume and productivity show improvements, while others such as defoliation and tree mortality are deteriorating [15,16,17,18,19]. In recent years, EU forests have faced extreme climate changes, which leads to weakened trees and makes them more vulnerable to pests and diseases [20]. Forests in Bulgaria cover about 37% of the country’s territory and are home to a significant number of protected plants and animal species included in the Natura 2000 ecological network, highlighting their importance for biodiversity conservation [21,22]. Key indicators to assess the condition of forests in Bulgaria include the forest area, forest thickness, biomass amount, and health of trees. In recent years, a significant portion of Bulgaria’s forests has remained healthy despite an increase in challenges such as the acidity of precipitation and nitrogen and sulfate deposition. Illegal logging and insufficient funding for flood-protection infrastructure also pose serious problems. The European Union is developing and implementing strategies for sustainable forest management [23,24]. The new EU Forest Strategy for 2030 aims to improve the quality and quantity of forests as well as their protection and restoration. The strategy includes planting at least 3 billion new trees by 2030, promoting a sustainable bioeconomy, and protecting the remaining primary forests. Bulgaria’s National Forest Sector Development Strategy (2013–2020) aims to ensure sustainable forest management and enhance the competitiveness of the forest sector. However, issues with illegal logging and the lack of funds for flood-protection infrastructure remain significant challenges [25,26,27,28].
In the EU, wood biomass is classified as one of the main sources of renewable energy [29]. Wood pellets and chips are widely used for heating homes and public buildings as well as for industrial purposes [30]. Wood biomass is also used in facilities for combined electric and thermal energy production. Technologies relating to the transformation of wood biomass into liquid biofuels such as bioethanol and biodiesel are being developed. Sunflower-waste materials such as sunflower husks and stalks offer significant potential for energy use. Sunflower-seed husks are used to produce biopellets that serve as an efficient fuel material for heating and also for industrial purposes. Sunflower residues can be utilized to produce biogas through anaerobic digestion, which can be further used for combined electricity and heat production [31,32,33].
Bulgaria has the potential to produce and export wood pellets, making them an important export commodity for EU countries [34,35]. As a significant producer of sunflowers, Bulgaria has the potential to utilize waste materials from this crop. The development of biomass facilities processing agricultural waste, including sunflower residues, can contribute to the sustainable development of rural areas and the country’s energy independence [36,37].
Wood biomass represents one of the main renewable resources for energy production and includes logs, branches, bark, leaves, and wood waste. This biomass can be utilized for energy production through direct combustion, gasification, or other biomass conversion technologies [38,39]. Wood pellets are produced by compressing wood particles, resulting in dense cylindrical pellets. One of the main physical characteristics and quality indicators of wood pellets is their high density (around 600–750 kg/m3), which provides a longer burning time and higher energy efficiency. A low moisture content (below 10%) is a key factor for the high calorific value of the pellets, with a typical energy content of around 16–18 MJ/kg. Pellets with low ash content (< 1%) are preferred because they produce fewer residues during combustion and reduce the operational and maintenance costs [40,41,42]. Biomass power plants convert wood into electricity through direct combustion or gasification, where the raw material is converted into synthetic gas that is burned to produce energy [43,44].
Sunflower plants play a crucial role in the agricultural economy of both the European Union and Bulgaria. Sunflowers are the major oilseed crop used in the production of oil, feed, biodiesel, and, in recent years, pellets [45]. The European Union is a significant producer of sunflower seeds. The major producers are Romania, Spain, Bulgaria, France, and Hungary, together accounting for over 90% of the total sunflower acreage in the EU. Climate conditions play a significant role in yields in the EU. The summer droughts and extreme weather conditions in 2022 led to a substantial reduction in grain yields, including sunflowers, in some parts of Europe [46,47,48]. However, the favorable autumn weather conditions allowed for a good harvest of summer crops [49]. Bulgaria is among the leading sunflower producers globally, ranking sixth with an annual production of approximately 2.06 million tons. The economics of sunflower production in Bulgaria depend on various factors, including the costs of seeds, fertilizers, and pesticides. The average gross profit per hectare of sunflower cultivation can reach about USD 545 when appropriate agronomic practices are used [50]. However, adverse weather conditions such as droughts and hail can cause significant damage to the crop. The EU continuously develops and implements strategies for sustainable management of sunflower crops as part of the Common Agricultural Policy, which provides financial support and a framework for the sustainable development of the sector [51,52]. This includes measures to improve the quality and quantity of sunflower crops, as well as to protect biodiversity and contest the climate change [53].
Sunflower husk pellets are a form of compressed biomass produced by pressing sunflower husks under high pressure [54]. They are an efficient energy source with specific physical characteristics and quality indicators—they have a high density (around 600–700 kg/m3), which ensures longer burning time and high energy efficiency. A low moisture content (below 12%) is a key factor for the high calorific value of the pellets, and they have an ash content of around 2–3%, which makes them suitable for combustion in various fuel systems, although they require more frequent maintenance [55,56]. Utilizing sunflower husks as a raw material for pellet production contributes to the reduction in agricultural waste in a sustainable manner. Innovations in pellet pressing and combustion technologies can enhance the efficiency and environmental value of sunflower husk pellets [57].
The subject of this study is the experimental investigation of mixed pellets, made from wood and sunflower husks, and the comparison of their combustion behavior with that of wood and sunflower pellets, using thermal analysis methods (TAMs). The combined wood and sunflower husk pellets represent an innovative and sustainable solution for energy production. They blend the advantages of both types of biomasses, offering high efficiency and environmental friendliness. With proper resource management and innovations in production technologies, combined pellets could play a significant role in future energy systems, providing a reliable and sustainable energy source.
The performed literature survey results reveal that the study of the thermal properties of different types of pellets (including a mix of more than one) has its particularities, which relate to the type of technology chosen. Table 1 presents a summary of the main challenges and limitations in the study of the thermal properties of wood and sunflower pellets, as determined through TG and DSC analyses.

2. Materials and Methods

This study focuses on five different types of pellets, produced from waste biomass, as it is specified in Table 2. Each type of pellet has been subjected to thermal analysis using thermogravimetry (TG) DSC and DTA.
All samples were taken from bags after the packaging machine. For each type, 500 g of material was collected and stored in nylon bags, which were opened immediately before the tests for sample preparation.

2.1. Materials

This study focuses on two types of wood pellet samples composed using different ratios of softwood and hardwood (SW and HW), as well as two types of sunflower pellet samples, produced with different sunflower varieties (SH). The third type of pellet is obtained by mixing softwood and sunflower husks in a 3:1 ratio.
Why is it necessary to mix different hardness types of waste biomasses? The combination of soft and hardwood in the pellet production process provides better technical characteristics and improves the energy value of the product. Softwood contributes to better bonding, which is the reason for the higher calorific value, while hardwood provides greater density and pellet stability. A balance between easy ignition and continuous burning is achieved by using both types of wood together. Sunflower husks were chosen for the mixed pellet production due to their higher calorific value and low cost, and the end-product of sunflower peeling is used for oil production. They are cost-effective and sustainable, and are classified as a preferred material in the biofuel industry. In addition, the flakes provide good structure and density to the pellets, which improves their combustion efficiency. The 3:1 mixture of wood and sunflower husks was chosen because it guarantees optimal efficiency in the combustion process and ensures high pellet quality. It also gives good density, stable binding, and balanced calorific value, resulting in a longer and more efficient combustion. In addition, the combination of the two materials also has a positive environmental effect, due to the smaller amount of ash output.
Details about these samples are shown in Table 2 and Figure 1. Pellet 1 type is composed of 60% of SW and 40% of HW. Pellet 2 type is 70% SW and 30% HW. Pellet 3 type is a mix between 70% SW and 30% SH. Pellet 4 and 5 types are composed of 100 SH different types. The producers of the discussed above pellets are using the standardized production methodology. The raw material is initially crushed to the specified size after which the crushed material is dried to the specified moisture content. Next comes the mixing process (for pellets that are composed of two types of material), followed by the pelletizing process.

2.2. Methods

Thermal analysis methods are essential for investigating the physical and chemical properties of substances and mixtures by observing their reactions in terms of changing temperature or time. These methods provide critical information on thermal stability, decomposition, and other thermal characteristics of materials. The samples of the pellets under study are subjected to controlled thermal conditions, which can be either dynamic (with continuous heating or cooling) or static (with a constant temperature, known as isothermal processes). For studying fuels such as pellets, the most commonly used methods are TG, DSC, and DTA. These methods are rarely used independently. They are most often combined, like TG and DSC, or TG and DTA being used together. The most comprehensive information about the behavior of pellets during the combustion process is obtained by performing all three methods concurrently. The results from the thermal analysis are typically presented graphically in the form of curves that show how a specific measured parameter changes with temperature or time. These graphs provide a visual representation of processes such as melting, crystallization, thermal stability, and decomposition. The data accumulated from these experiments are used to describe the thermodynamic behavior of the systems.
Contemporary experimental equipment for studying the materials uses DTA, DSC, and TGA methods within the range of the same experimental setup. These methods are widely used in the analysis of fuel pellets and other types of materials due to their high sensitivity and accuracy.
For the purposes of this study, a Linseis STA PT 1600 instrument was used [58]. The samples were prepared according to the device’s instructions. The thermal stability of each type of pellet has been tested multiple times. The validation of the results obtained is based on several experimental studies and subsequent statistical processing.
The analyses were conducted at two heating rates—5 °C/min and 10 °C/min—with heating up to 750 °C. The analyses were conducted at two heating rates—5 °C/min and 10 °C/min—with heating up to 750 °C. The lower heating rate (5 °C/min) was selected because of the lower chemical reaction time that highlighted the course of the phase transitions or decomposition processes. The higher heating rate (10 °C/min) shortens the experiment time and simulates a faster reaction process. The proposed rates offer a balance between studying thermal processes and providing enough time to observe key changes, while maintaining the duration of the experiment in a reasonable timespan. The experimental studies show that a temperature of 750 °C can be considered as an upper limit temperature, because it is high enough to cover all the main stages of thermal decomposition of the organic components and combustion of the residual material. At lower temperatures, decomposition processes may not be fully completed, while at higher temperatures, undesirable additional reactions or excessive oxidation may occur.

3. Results

Figure 2, Figure 3 and Figure 4 present the results of TGA, DSC, and DTA for the pellets made from waste materials at a heating rate of 5 °C/min up to a temperature of 750 °C. Figure 5, Figure 6 and Figure 7 show the TGA, DSC, and DTA results for the pellets made from waste materials at a heating rate of 10 °C/min up to a temperature of 750 °C.
Experimental tests were performed to determine the temperature stability for each of the five samples. Each of the samples was tested 15 times for thermal stability, and the results were processed (multifactorial experiment) as shown in the figures below.
In a thermogravimetric analysis, the initial mass of the sample at the beginning of the experiment is taken as reference value (100%). Therefore, all samples start their mass change from 0%. The temperature change during the experiment (from room temperature to 750 °C), provides a comprehensive analysis of thermal changes and decomposition of the material throughout the heating process.
Figure 2 shows that the initial mass loss is typically observed at the beginning of heating, associated with the evaporation of moisture and other volatile substances. The primary mass loss, occurring at higher temperatures, reflects the decomposition of organic materials or other components of the sample. The decomposition temperature provides valuable information about the thermal stability of the material.
The provided thermogravimetric analysis identifies the three main stages. The first stage results give an overview of the drying process (evaporation of moisture) and the release of light volatile substances, with a minor mass loss observed in this region. The second stage examines the combustion process and the release of primary volatile substances, showing a significant mass loss. In the third stage, the process of post-combustion of carbon residues occurs, as the curves show little to no mass loss. This suggests that most of the degradable material has been consumed, while the remaining material is likely more stable or has been converted into ash.
The results from Figure 3 of the differential scanning calorimetry (DSC) show that at a heating rate of 5 °C/min, Pellet 1 and Pellet 2 exhibit significant similarity in their thermal curves, suggesting that their thermal properties are identical. On the other hand, Pellet 3, Pellet 4, and Pellet 5 display a similar thermal curve behavior, but also some differences among the curves are observed.
For Pellets 1 and Pellets 2, the first exothermic peak is weakly defined and occurs at around 320 °C. The second exothermic peak is at 420 °C for Pellets 1 and 455 °C for Pellets 2. For the other three pellet types, exothermic peaks are more pronounced. For Pellets 3, the first peak is at 314 °C, and the second is at 443 °C. Pellets 4 show three peaks at 307 °C, 406 °C, and 441 °C, while Pellets 5 exhibit two peaks at 305 °C and 445 °C.
Endothermic peaks (green regions) indicate the absorption of heat during phase transitions such as melting, while exothermic peaks (red regions) reflect the release of heat, for example, during crystallization. Identifying the temperatures at these peaks is crucial for detecting phase transitions. Information on the peak ranges is necessary to determine the stability of the sample and its melting point.
The results from Figure 4 show that the peaks in the DTA curves represent the fastest rate of mass loss at specific temperatures. The maximum rate of mass loss refers to the stages of decomposition. Different stages represent various processes such as evaporation, decomposition or oxidation. Initially, minimal weight loss is observed in the tested substances. At the instants when the value of the ignition temperature is reached (fuel ignition point), the peak of maximum mass loss is also reached, and thereafter, minimal to no further mass loss is observed until the end of the experiment.
The thermogravimetric analysis presented in Figure 5 shows that at a faster heating rate, materials start the process of decomposition at a later stage. Faster heating resulted in more abrupt decomposition due to the intensified heat flux, which leads to the rapid attainment of decomposition temperatures.
The results shown in Figure 6 show that at a faster heating rate of 10 °C/min during differential scanning calorimetry, the above-discussed transitions are shifted to higher temperatures. At this rate, the curves for Pellet 2 and Pellet 3 are identical, while Pellet 5′s curve is quite similar to theirs. There is a significant difference in the shape of the curves for Pellet 1 and Pellet 4. For Pellet 1, there is no clearly defined exothermic peak. Pellet 2 shows two peaks, the first at 338 °C and the second at 468 °C. Pellet 3 and Pellet 5 also show two peaks, at 342 °C and 457 °C, and 307 °C and 454 °C, respectively. Pellet 4 has three peak values at 305 °C, 421 °C, and 460 °C. The extreme heat flux is the reason for the insufficient time available for the even distribution of thermal energy within the sample. Broad peaks are indicators for heat dispersion and reduced accuracy during the phase transition processes. This may also suggest the simultaneous running of different thermal processes.
High peak temperatures and their sharpness at a faster heating rate indicating higher intensity of decomposition are provided in Figure 7.
Table 3 shows the temperature ranges for three phases (drying, combustion and post-combustion) for each pellet type.
The results in Table 3 for Pellet 1 and Pellet 2 show very similar thermal properties for each stage of pellet processing, indicating the comparable thermal stability of the wood pellets. In contrast, Pellet 3 shows a greater mass loss in the second stage, suggesting a higher content of volatile or easily decomposable substances. Pellet 4 and Pellet 5 display different TG curves compared to the other pellet types. The very specific mass loss model highlight changes in the raw materials and the presence of thermally resistant components.
Table 4 shows the maximum rates of mass loss during the test of the pellets at the two heating rates.
The results from Table 4 show that the mass loss is the highest for Pellet 2, with values of 1.19% and 1.01% at 309 °C and 319 °C, respectively, at both heating rates. The lowest mass loss is observed for Pellet 5, with 0.65% and 0.68% at 277 °C and 286 °C, also at both heating rates. This is likely due to the properties of the waste materials from which the pellet fuels are produced.

4. Discussion

An analysis of the results from the experimental studies clearly shows that high peak temperatures, and their sharpness at a faster heating rate, are indicators for the degree of intensity of the material’s decomposition. These observations not only confirm the high reactivity during accelerated heating, but also provide a more detailed understanding of the thermal stability and combustion characteristics of the pellets. Specifically, sharp temperature peaks are a sign of quicker and more vigorous decomposition, which is a key factor in evaluating the efficiency of the combustion process, and is also confirmed by the authors in [59,60]. This information is crucial for optimizing the production and use of pellets as a sustainable energy source.
An analysis of the results from thermogravimetric analysis shows that Pellet 1 and Pellet 2 have similar behavior, with distinct stages of mass loss during drying, combustion, and post-combustion processes. This suggests that they have stable thermal characteristics, which is the main reason for their preference in heating installations, where consistency and stability of combustion are important for an efficient fuel process. Pellet 3, Pellet 4, and Pellet 5 display different thermal behavior. The analysis of the TGA results for Pellet 3 shows a higher mass loss in the range of 200–400 °C, indicating a higher content of volatile substances and lower thermal stability compared to wood pellets. This could be due to the organic compounds from sunflower husks, which decompose at lower temperatures. The analysis of the TGA results for Pellet 4 and Pellet 5 shows different thermal behavior due to the lower homogeneity of the waste material. In the context of mixed pellets, which contain both wood and sunflower husks, the results show intermediate values of thermal stability. The authors in [61] track the efficiency of wood and sunflower pellets during combustion, and their obtained data are similar to this study’s results.
An analysis of the results from differential scanning calorimetry shows that wood pellets have a higher energy density and lower ash content, compared to the sunflower pellets. Pellet 1 and Pellet 2, composed of softwood and hardwood, demonstrate higher energy values due to their higher calorific value and better thermal characteristics, as confirmed by the authors in [61,62,63]. Pellet 3 provides a balanced energy profile for biofuel, despite having lower values compared to pure wood pellets. This could be attributed to the lower energy density of sunflower husks compared to wood. The results for Pellet 4 and Pellet 5 show a high ash content, indicating a lower energy efficiency and higher maintenance requirements for combustion installations.
A global analysis of the results from the differential thermal analysis supplements the combustion profiles of the pellets, providing data on phase transitions and thermal stability. The obtained data show that mixed pellets exhibit behavior similar to those with an entirely wood composition but demonstrate diversity in thermal transitions, suggesting that the combination of different types of biomasses can trigger complex thermal reactions. Pure sunflower pellets show lower stability, which may be due to the presence of various chemical compounds and higher ash content, as noted by the authors in [63,64,65].

5. Conclusions

Experimental studies using the thermal analysis method were performed on five different types of commercial pellets, composed of different waste biomass products: wood, a combination of wood and sunflower husks, and a mix of sunflower husks. The outputs show that quite significant differences have been identified, in their thermal behavior and energy characteristics. Wood pellets show stable thermal behavior and high specific thermal capacity, identifying them as the most preferable for use in fire heating systems. Mixed pellets show intermediate values, offering a balanced energy content but with lower specific heat capacity compared to pure wood pellets. Sunflower husk pellets are identified as having a high calorific value but also a high ash content, suggesting the need for improved technologies for better cleaning and lowering the environmental impact.
The data from a thermogravimetric analysis and differential scanning calorimetry indicate that wood pellets have better thermal properties in direct firing systems. At the same time, combining wood and sunflower husks could offer sustainable solutions for utilizing agricultural waste, although optimization of thermal characteristics is necessary.
In addition, the performed analyses show that for different fuel mixes (in different ratios) the temperature peaks in the three different stages differ significantly, which also requires making specific adjustments to the combustion process. By means of the proposed methodology, the temperature profiles could be precisely identified with a view to achieving the maximum use of the energy contained in the pellets. In addition, the adjustment of the combustion process, based on the thermal properties of the pellets, leads to better combustion and lower emissions into the environment, resulting in a reduction in the harmful impact on the environment, incl. a lower amount of ash produced. In conclusion, with the proposed methodology, suggestions can be made to pellet producers for the proper mixing of different quantities of waste biomass, with the aim of achieving the maximum use of the energy contained with a minimum consumption of raw materials, which creates a certain degree of sustainability in the selection of raw materials.
Although the majority of pellets (mixed) on the market are in ratios analogous to those examined in this study, future research will be oriented at testing mixed pellets in different ratios. This will allow us to analyze more precisely the temperature stability of the pellets, and from there to make conclusions about their calorific value and the amount of ash content. Additional experimental studies using FTIR, SEM, EDS, and XRD are also planned, with the help of which it will be possible to create a methodology that prescribes the production of pellets in different ratios about their application. As a result of the reduced ash content, this will allow the more efficient use of the energy potential of the input raw materials, and have a favorable impact on the environment.

Author Contributions

Conceptualization, P.Z. and A.T.; methodology, P.Z. and N.M.M.; formal analysis, P.Z. and A.T.; investigation, N.M.M., P.Z., and A.T.; data curation, N.M.M.; writing—original draft preparation, N.M.M. and P.Z.; writing—review and editing, P.Z. and A.T.; visualization, A.T.; supervision, A.T. All authors have read and agreed to the published version of the manuscript.

Funding

This study is financed by the European Union—NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project No. BG-RRP-2.004-0005.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. View of the samples used during this study.
Figure 1. View of the samples used during this study.
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Figure 2. TGA results at a heating rate of 5 °C/min up to 750 °C.
Figure 2. TGA results at a heating rate of 5 °C/min up to 750 °C.
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Figure 3. DSC results at a heating rate of 5 °C/min up to 750 °C.
Figure 3. DSC results at a heating rate of 5 °C/min up to 750 °C.
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Figure 4. DTA results at a heating rate of 5 °C/min up to 750 °C.
Figure 4. DTA results at a heating rate of 5 °C/min up to 750 °C.
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Figure 5. TGA results at a heating rate of 10 °C/min up to 750 °C.
Figure 5. TGA results at a heating rate of 10 °C/min up to 750 °C.
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Figure 6. DSC results at a heating rate of 10 °C/min up to 750 °C.
Figure 6. DSC results at a heating rate of 10 °C/min up to 750 °C.
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Figure 7. Results from DTA at 10 °C/min up to 750 °C.
Figure 7. Results from DTA at 10 °C/min up to 750 °C.
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Table 1. Main challenges and limitations when studying the thermal properties of wood and sunflower pellets, as determined via TG and DSKC analyses.
Table 1. Main challenges and limitations when studying the thermal properties of wood and sunflower pellets, as determined via TG and DSKC analyses.
Type of StudyChallenges/LimitationsDescription
TG analysis (SW and HW)Incomplete decomposition at lower temperatures.
Complete decomposition at high temperatures.		It is necessary to reach a temperature (around 750 °C) for complete decomposition of the organic components in the pellets.
TG analysis (SH)Influence of the atmosphere (inert or oxidizing).The different atmospheric conditions change the decomposition and oxidation behavior of the samples.
DSC analysis (SW and HW)Difficulties distinguishing thermal effects.Difficulties in distinguishing exothermic and endothermic processes in multicomponent pellets.
DSC analysis (SH)Influence of heating rate.Faster heating leads to missing important thermal effects and false conclusions.
Material compositionDifferences in thermal properties between wood and sunflower pellets.Sunflower husks contain more inorganic residues, which affects the combustion behavior.
Samples homogeneityVariations in pellet structure and composition.Inhomogeneous material leads to different results in the thermal analysis and complicates the interpretation of the results.
Table 2. Types of raw materials.
Table 2. Types of raw materials.
SamplesPellet 1Pellet 2Pellet 3Pellet 4Pellet 5
typewoodwoodwood and
sunflower		Sunflower 1Sunflower 2
material60% SW + 40% HW70% SW + 30% HW70% SW + 30% SH100% SH1100% SH2
Table 3. Temperature ranges of major thermal decomposition processes, °C.
Table 3. Temperature ranges of major thermal decomposition processes, °C.
DryingCombustionPost-Combustion
Pellet 120–218 *218–400.5 *401–750 *
20–227 **227–406 **406–750 **
Pellet 220–216 *216–407 *407–750 *
20–225 **225–412 **412–750 **
Pellet 320–218 *218–406 *406–750 *
20–230 **230–413 **413–750 **
Pellet 420–215 *215–401 *401–750 *
20–220 **220–414 **414–750 **
Pellet 520–215 *215–407 *395–750 *
20–218 **218–410 **410–750 **
* 5 °C/min, ** 10 °C/min.
Table 4. Maximum rates of mass loss at 5 °C/min and 10 °C/min.
Table 4. Maximum rates of mass loss at 5 °C/min and 10 °C/min.
5 °C/min10 °C/min
Pellet 10.87%/°C at 314 °C0.79%/°C at 319 °C
Pellet 21.19%/°C at 309 °C1.01%/°C at 319 °C
Pellet 31.01%/°C at 312 °C0.86%/°C at 323 °C
Pellet 40.75%/°C at 290 °C0.88%/°C at 284 °C
Pellet 50.65%/°C at 277 °C0.68 %/°C at 286 °C
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Zlateva, P.; Terziev, A.; Mileva, N.M. Sustainable Solutions for Energy Production from Biomass Materials. Sustainability 2024, 16, 7732. https://doi.org/10.3390/su16177732

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Zlateva P, Terziev A, Mileva NM. Sustainable Solutions for Energy Production from Biomass Materials. Sustainability. 2024; 16(17):7732. https://doi.org/10.3390/su16177732

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Zlateva, Penka, Angel Terziev, and Nevena Milcheva Mileva. 2024. "Sustainable Solutions for Energy Production from Biomass Materials" Sustainability 16, no. 17: 7732. https://doi.org/10.3390/su16177732

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