Research on the Efficiency of Solid Biomass Fuels and Consumer Preferences in Bulgaria

Abstract

This study examines the qualities and potential uses of various types of biomass as fuel, focusing on wood pellets, sunflower husk pellets and mixed pellets. The primary objective is to analyze the thermal and energy properties of these pellets in order to evaluate their efficiency and acceptance by consumers in the Bulgarian market. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) are employed, revealing that the processes of drying and volatile substance release are accompanied by energy absorption, with combustion being the main stage where most heat is generated. The results show that wood pellets have 7.31% moisture, 0.72% ash and a calorific value of 18.33 kJ/kg; sunflower husk pellets have 7.62% moisture, 2.42% ash and a calorific value of 19.63 kJ/kg; and mixed pellets have 7.07% moisture, 0.69% ash and a calorific value of 18.05 kJ/kg. These findings indicate that the pellets achieve efficient combustion with minimal mass loss. The conducted marketing research reveals that Bulgarian consumers prefer wood and mixed pellets for their efficiency, although sunflower husk pellets are more affordable. Key factors influencing consumer choice include price, which is important for 51% of the respondents, and quality, prioritized by 34%. The conclusion of this study is that pellets are a promising energy source with good environmental and economic characteristics, and the results can contribute to the development of more efficient fuels adapted to the needs of the market and consumers.

1. Introduction

Nowadays, one of the key aspects related to energy resources is their contribution to energy and environmental preservation. Biomass is known as one of the most effective and sustainable energy sources, providing both renewable fuel and environmental benefits [1,2,3]. Containing a significant amount of carbon, biomass plays a crucial role in balancing greenhouse gases. It serves as an alternative to traditional energy resources [4,5]. The increasing consumption of pellets for heating and hot water production requires diversity in the raw materials used for fuels [6,7].

Forests and sunflower crops, together with the residual raw materials they generate, reveal significant potential for the development of sustainable biofuels both in Bulgaria and in the European Union (EU). Forest resources in the EU cover around 182 million hectares and generate approximately 120–130 million tons of residues per year, including branches, bark and other wood waste [8]. In Bulgaria, forests cover 4.1 million hectares, with an annual timber harvest of 7–8 million cubic meters, which creates between 2 and 2.5 million tons of residual biomass [9]. This represents a significant resource potential for the production of solid biofuels from forest waste raw materials.

Sunflower crops also represent an important source of biomass raw materials. The annual production of sunflower seeds in the EU amounts to around 10 million tons, generating between 15 and 20 million tons of residues, including stems, leaves and husks [10]. In Bulgaria, about 800,000 hectares are occupied by sunflower crops, which leads to the production of approximately 2.5–3 million tons of residual biomass per year, of which 800,000 tons are husks [11]. These raw materials represent a valuable resource for energy production, especially for pellets and other forms of solid biofuels, offering economically viable and environmentally sustainable solutions for the energy sector.

Modern technologies enable the production of renewable fuels from agricultural and industrial waste materials, offering both economic and ecological advantages. The EU and its member states aim to increase the share of energy originating from renewable sources to 34% by 2050 [12,13,14,15]. To achieve this, they promote the use of fuels derived from solid biomass waste materials. These fuels are produced from both agricultural and forestry sectors as well as industry. This is crucial for addressing waste generation issues and their negative impact on the environment. Furthermore, the reuse of these materials allows for increased revenue through the sale of secondarily generated solid biofuels, as well as reducing expenses for energy needs [16,17,18,19].

The energy needs of many people in Bulgaria, especially for heating and warming household water, are often met through the use of wood pellets as fuel. This form of fuel is produced from waste wood originating from forest clearing and the activities of the wood processing industry [20,21,22,23]. The technological steps for producing wood pellets involve the initial crushing of the products, followed by drying, fine grinding, pressing without the addition of binding agents and finally cooling the resulting pellets. The proper utilization of solid biofuel in the form of wood pellets ensures an efficient fueling process. The low bulk density of solid biofuels makes them more suitable for local use near production regions. To some extent, this can be compensated by the possibility of packaging and transporting pellets over long distances, but this increases production costs [24,25,26,27,28,29]. Global leaders in the production and use of pellets are primarily the USA and Canada, followed by Europe [29,30,31,32,33].

One of the most important oilseed crops, known to humanity for centuries, is the sunflower (Helianthus annuus) [34,35,36]. Market leaders in sunflower production are countries from Europe and Asia. After processing, large quantities of waste in the form of stalks and husks are generated following the extraction of oil from the seeds. One of the applications of sunflower husks in energy production is as a raw material for fuel in the form of pellets [37,38]. Their production is profitable and they are cheaper than wood pellets in Bulgaria. The biggest drawback of sunflower pellets is their high ash content after combustion.

Wood and sunflower pellets are fuels that can be used for heating residential buildings, industrial buildings and agricultural facilities, as well as for electricity generation through installations that utilize renewable energy sources. This diverse range of applications makes them a key component of strategies to diversify the energy mix and reduce carbon dioxide emissions. Interestingly, around the world, this type of solid biofuel often replaces coal in electricity production [39,40,41,42].

To improve the fuel parameters of produced wood and sunflower pellets, highly specialized scientific methods and technologies are used in accordance with the standards in force in EU countries. These methods involve studying key parameters such as moisture content, ash content and calorific value, as well as applying advanced analytical tools to measure their physical and chemical properties. Techniques such as mass spectroscopy, thermogravimetry, differential scanning calorimetry and others are used for this purpose [43,44]. These techniques provide detailed information about the quality of the pellets and help to better understand their physical and chemical properties, ultimately contributing to their more efficient use as an energy source.

Some authors [45,46] are engaged in the analysis of some of the main parameters of pellet fuels from different sources and their influence on the operation of pellet fuel systems around the world. They use a wide variety of data collected from different sources and studies. Currently, in scientific circles, special attention is paid to the study of important parameters such as moisture and ash content and how they affect the efficiency of pellet fuel systems, especially in our country [47]. This is essential to determine the best ways to use pellet fuels in energy systems and to reduce their environmental footprint.

An analysis of thermokinetic processes gives a clear idea of the quality of the combustion process when burning biomass material. Some authors have done similar studies, but in most cases, they use only one method and a limited number of trials.

Table 1. Overview of methods used to analyze the thermographic properties of the pellets.

No	Research Type	Authors	Temperature Range for Moisture Release, °C	Temperature Range for Volatile Compounds Release, °C	Active Oxidation Temperature Range, °C	Lignin Formation/Thermal Effects
1	TGA	[50]	100–150 °C	200–400 °C	>500 °C	250–500 °C
2	TGA	[51]	100–150 °C	200–350 °C	>450 °C	250–500 °C
3	TGA	[52]	100–150 °C	200–280 °C	280–350 °C	250–500 °C
4	DTA	[52]	100–150 °C	250–350 °C	300–500 °C	250–500 °C
5	DSC	[51]	100–150 °C	250–400 °C	250–400 °C	-
6	DSC	[53]	100–150 °C	300–400 °C	300–400 °C	-

presents a general analysis of the methods and tools used by many authors to perform thermokinetic processes [48,49,50].

In [50], five different types of biomass pellets (pine, fir, willow, pine and poplar) were analyzed, and the following ranges were obtained for the main stages of thermal decomposition: moisture release between 100 °C and 150 °C; the release of volatile substances initiated around 200 °C until up to 400 °C; and the oxidation of carbon residues occurs at over 500 °C. The multistage decomposition helps to distinguish the thermal intervals in which the specific characteristics of the process are obtained. The authors in [49] considered pure and mixed pellets of spruce and pea starch, and they showed similar temperature ranges of biomass degradation shown in Table 1. The moisture separation occurs in the same temperature range as in [48], the release of volatile substances takes place between 200 °C and 350 °C and the final combustion of the solid residues starts at 450 °C until the carbon residues are completely oxidized. In [51], the degradation of the biomass components of wheat straw, palm and olive leaves was considered in particular. Hemicellulose degrades in the 200 up to 280 °C range, cellulose at 280 up to 350 °C and lignin has a wider degradation temperature range that starts around 250 °C and continues to over 500 °C. In [50], the authors considered pure and mixed pellets of spruce and pea starch, and they showed similar temperature ranges of biomass degradation shown in Table 1. The moisture separation occurs in the same temperature range as in [50], as the release of volatile substances takes place between 200 °C and 350 °C and the final combustion of the solid residues is initiated at 450 °C until the carbon residues are completely oxidized. In [51], the degradation of the biomass components of wheat straw, palm and olive leaves was considered in particular. Hemicellulose degrades in the 200 up to 280 °C range, cellulose at 280–350 °C and lignin has a wider degradation temperature range that starts around 250 °C and continues to over 500 °C. In [51], the degradation of the biomass components of wheat straw, palm and olive leaves was specifically considered by the authors. Hemicellulose degrades in the 200 up to 280 °C range, cellulose at 280 up to 350 °C, and lignin has a wider degradation temperature range that starts around 250 °C and continues to slightly over 500 °C. The results obtained in the present study provide similar temperature ranges. Hemicellulose starts to decompose at about 180 °C and continues up to 260 °C, cellulose decomposes in the range of 280 up to 360 °C and lignin starts at 200 °C and can continue above 500 °C, due to its stable thermal decomposition. This analysis shows that the data obtained are reasonable as similar temperature ranges for the key biomass components could be observed.

Differential thermal analysis: In [51], the results obtained for DTA provide information on thermal effects during the thermal decomposition of biomass. The main temperature ranges for exothermic and endothermic reactions are as follows: release of volatile substances at temperatures between 250 °C and 350 °C (exothermic reactions) and release of moisture at 100 up to 150 °C (endothermic reactions). DTA shows that the thermal effects of cellulose and lignin degradation coincide with those observed in the TGA assay. The results of the present analysis show that the degradation of cellulose and lignin leads to the release of a significant amount of heat in the range of 300 up to 500 °C (exothermic reactions), and the evaporation of moisture and absorption of heat occurs in the range of 100–150 °C (endothermic reactions).

Differential scanning calorimetry: DSC in [49] presents data on the calorific value of pure and mixed biomass pellets, with the main processes having the following intervals: the heat absorption process associated with moisture evaporation is in the range of 100 up to 150 °C (endothermic reaction), and the process of combustion and release of energy—at 250 up to 400 °C. In [50], the authors describe reactions for sunflower pellets in the temperature range of 100 to 150 °C for moisture release (an endothermic process) and 300 to 400 °C for peak energy release (an exothermic process).

The absorption of heat, as a result of the release of moisture, occurs around 100 up to 150 °C (endothermic reaction), and the release of energy from combustion (exothermic reaction)—250 up to 450 °C.

The object of the present study is the ash content, moisture content and calorific value of different types of pellets, which are important indicators of their efficiency as biofuels. Modern methods of thermal analysis, such as thermogravimetry and differential scanning calorimetry, are used, which not only assess the quality of the fuels but also track the processes of moisture evaporation and volatile substance release. These approaches help optimize the combustion process by providing important information for improving the efficiency of pellets as biofuels.

Creating a user profile that reflects the attitudes and preferences of Bulgarian consumers regarding pellets is an important step, especially considering that in many countries people are unfamiliar with these fuels and their applications. For this purpose, marketing research was conducted to examine how consumers perceive the use and pricing of different types of pellets in the local market. By combining the results of energy analyses with the information on consumer attitudes, a clear understanding has been developed of the opportunities for improving and adapting biofuels to market needs.

2. Products Specifics

After analyzing the survey data for solid biofuels (products), two types of wood and sunflower pellet samples from different producers were selected. The third type of pellets are produced after mixing sunflower husks and coniferous waste. Research has been carried out to determine some typical parameters such as calorific value, ash content and moisture. Additionally, their mass loss and thermal properties were measured using TG and DSC.

The samples of the tested pellets (

Table 2. Types of raw materials for research.

Samples	Pellets 1	Pellets 2	Pellets 3	Pellets 4	Pellets 5
type	wood	wood	wood and sunflower	sunflower
material	70% CW + 30% DW	80% CW + 20% DW	80% CW + 20% SH	100% SH1 HE	100% SH2 HD

and in Figure 1) were produced from a variety of raw materials in order to assess the influence of different ratios on the fuel properties and qualities of the final product. The waste raw materials were combined in various proportions, including coniferous wood (CW), deciduous wood (DW) and sunflower husks (SH), as shown in Table 2 and Figure 1. Pellets 1 were produced from 70% coniferous wood and 30% deciduous wood. Pellets 2 contained 80% coniferous wood and 20% deciduous wood. Pellets 3 were made from a mix of 80% coniferous wood and 20% sunflower husks, sourced from the Hybrid Enigma CLP variety. In contrast, Pellets 4 and 5 were entirely made from 100% sunflower husks from the Hybrid Enigma CLP (HE) and Hybrid Dalena CLP (HD) varieties, respectively.

3. Methods

3.1. Energy Parameters Determination

The determination of some of the fuel’s energy parameters such as the calorific value, moisture content and ash content was conducted in accordance with European and Bulgarian standards, as per [53,54,55]. The pellets were purchased from retail stores and manufacturers. They were packaged in bags weighing 15 kg each. To obtain accurate and correct results, the bags containing the selected pellets were opened prior to this study and quantities of approximately 3 kg were prepared for examination for each type of pellets.

The moisture content (MC) of the pellets was determined by analyzing 300 g samples of each type. The samples were weighed and placed in clean, dry and pre-weighed trays and dried in a drying oven at a temperature of 100 ± 5 °C for 20 h, then weighed again. The difference between the initial weight and the weight after drying was used to calculate the moisture content. This method, according to [19], guarantees high accuracy and reliability in determining the moisture content of the pellets.

The determination of the ash content (AC) in the pellets was carried out according to established standards described in [52]. The analysis involved the use of homogenized samples of 1 g for each pellet category. The samples were placed in dry and clean crucibles with a precisely measured weight to ensure a high accuracy in the calculations. The combustion process was carried out in a laboratory furnace (see Figure 2) and occurred in two stages. Initially, the samples were gradually heated to a temperature of 250 °C for 1 h, then held at this temperature for another 1 h to ensure the removal of moisture and volatile substances. In the second stage, the temperature was increased to 550 °C and maintained for 2 h and 30 min to ensure the complete combustion of the organic components. After the thermal treatment was completed, the crucibles were cooled in a desiccator that prevents the absorption of moisture from the environment. After cooling, the crucibles with the residues were weighed using an analytical balance. The ash content was calculated as the percentage ratio between the weight of the residue after combustion and the initial mass of the sample.

The study of the calorific value (CV) was carried out using a KL-10 calorimeter (TA Instruments Ltd., New Castle, DE, USA) (see Figure 3) [54], in accordance with established standards and methodologies. The calorific value reflects the amount of energy released when burning a unit mass of solid biofuel in an oxygen environment in a calorimeter bomb under controlled conditions. Pellet samples with a mass of 1 g were burned in an oxygen environment under a pressure of 2.8–3.0 MPa. The calorimeter bomb was placed in a vessel filled with distilled water, which allows for the accurate measurement of the released heat energy. Before conducting the measurements, the device was calibrated and validated through two tests using benzoic acid as a reference. For each brand of pellets, six repeated measurements were made at a temperature of 25 °C in a darkened room, which ensured stable experimental conditions and reduced the impact of external factors on the accuracy of the results.

3.2. Thermogravimetric Analysis and Differential Scanning Calorimetry

In this study, TG and DSC were used for a detailed study of the thermal and physicochemical properties of pellets intended for combustion processes. TG monitored the change in the mass of the samples during heating, which allowed the identification of processes such as material degradation, oxidation reactions and gas evolution [56,57]. A DSC analysis was used to record the thermal changes in the samples, providing information on the temperature limits of phase transitions and thermal degradation [57,58]. The studies were carried out with a thermal analyzer STA PT 1600 (Verder Bulgaria EOOD, Sofia, Bulgaria) [57]. The device was used in a temperature range from 20 to 750 °C, and for the purposes of the analysis two heating rates of 5 °C/min and 10 °C/min were applied. The gas environment was provided by inert nitrogen with a flow of 20 to 50 mL/min to prevent oxidation reactions, or by static air to study oxidation processes. The samples were prepared as powder materials with a particle size of 0.1 mm and a weight between 65 and 85 mg, placed in stabilized corundum crucibles.

Two heating speeds were used—5 °C/min and 10 °C/min. The choice of heating rate is essential because it can affect the kinetics of the reactions that occur in the material during the analysis. The faster heating can accelerate various thermal processes, while the slower heating can allow a more complete and detailed analysis of the thermal changes.

During the heating and analysis process, changes in the mass of the samples and the difference in heat release or absorption were observed. These observations are crucial for understanding the thermal characteristics of the material and for identifying the various thermal processes that may occur under different conditions of temperature and heating rate.

The data obtained from the analysis were used to create profiles of the curves from the thermogravimetric analysis (TGA) and DSC. These profiles provide a visual representation of the thermal changes in the material as a function of temperature and heating rate, which allows the detailed study and analysis of their characteristics.

Based on the data obtained from the methods mentioned above and the marketing research of consumer preferences, an analysis can be conducted to determine whether this type of pellets is suitable for sale on the market and to explore their potential applications. First, by examining the moisture and ash content, the quality of the pellets as a fuel and their fuel efficiency can be assessed. A high moisture content can reduce their energy value and increase the amount of emitted gases and ash, which may make them less suitable for certain applications such as heating. Then, the analysis of calorific value provides insight into their energy value and potential for efficient utilization. A higher calorific value usually indicates a better fuel quality and more efficient combustion. TG and DSC help evaluate the stability of the material at different temperatures and combustion conditions. This is important for determining the suitability of the pellets for various heating purposes. Additionally, they can detect potential changes in the chemical composition of the pellets during heating, which is crucial for understanding their fuel properties and efficiency.

3.3. Marketing Research of Customer Preferences

In Bulgaria, biomass pellets, such as those made from wood and agricultural waste, are available on the market. However, wood pellets dominate due to their higher combustion efficiency and compatibility with existing heating systems, despite being more expensive. Their success is attributed to the effective exploitation of market opportunities, enabling producers to achieve competitive positions through efficient advertising and adaptation to consumer needs. In contrast, agricultural biomass pellets are cheaper but lag behind due to their lower calorific value and higher ash content and the need for specialized equipment. Exploring the market for alternative pellets is crucial, as they offer potential for waste reduction and provide a more affordable, sustainable and economically viable fuel source. However, the lack of incentives and consumer feedback limits their adoption and market potential. Conducting surveys among consumers can uncover key preferences and expectations, supporting the development of innovations and cost-effective solutions to improve the production, quality and application of these pellets.

In order to explore consumer attitudes and preferences regarding the purchase of various types of solid biomass fuels on the Bulgarian market, a specialized marketing survey was conducted. It spanned from November 2023 to February 2024, encompassing both 20% online and 80% in-store responses. The survey comprised 3 closed questions. The target demographic group included end-users (individuals and households) utilizing pellets for heating in Bulgaria.

The first question “What kind of pellets do you prefer to use?” aims to reveal consumer preferences for different types of pellets, which is essential for identifying the most sought-after products on the market.

The second question “Would the lower price of sunflower pellets make you prefer them over wood pellets?” investigates the influence of the price factor on consumer decisions and allows for an assessment of consumer price sensitivity.

The third question is “What is most important to you when choosing solid biomass fuels?”. This key question reveals the main criteria by which consumers evaluate biomass fuels, such as the price, quality or environmental friendliness. The data obtained will help producers tailor their offerings according to the needs and preferences of customers.

The survey concluded with a total of 200 respondents, providing a significant volume of information for analyzing consumer preferences and attitudes in the Bulgarian market for solid biomass fuels. More than 90% of the respondents submitted the questionnaire online.

4. Results

Based on the methods outlined above, the averaged results obtained from the research of the five types of pellets are presented.

4.1. Results of Determining the Energy Characteristics of the Pellets

Table 3. Averaged results from the examining of the energy parameters.

Indicator	Units	Pellets 1	Pellets 2	Pellets 3	Pellets 4	Pellets 5
Total moisture content	%	7.47 ± 0.20	7.14 ± 0.20	7.07 ± 0.20	7.82 ± 0.20	7.42 ± 0.20
Ash content (in dry condition)	%	0.79 ± 0.21	0.72 ± 0.21	0.74 ± 0.21	2.57 ± 0.21	2.67 ± 0.21
Ash content (in operational condition)	%	0.73 ± 0.21	0.70 ± 0.21	0.69 ± 0.21	2.37 ± 0.21	2.47 ± 0.21
Calorific value (higher, in dry condition)	kJ/kg	19.57 ± 1.80	19.96 ± 1.80	19.42 ± 1.80	21.37 ± 1.80	21.07 ± 1.80
Calorific value (higher, in operational condition)	kJ/kg	18.11 ± 1.80	18.53 ± 1.80	18.05 ± 1.80	19.70 ± 1.80	19.51 ± 1.80
Calorific value (lower, in dry condition)	kJ/kg	18.31 ± 1.80	18.69 ± 1.80	18.16 ± 1.80	20.14 ± 1.80	19.85 ± 1.80
Calorific value (lower, in operational condition)	kJ/kg	16.77 ± 1.80	17.19 ± 1.80	16.71 ± 1.80	18.39 ± 1.80	18.20 ± 1.80

presents the averaged results obtained from the study of the energy characteristics of the pellets.

The results of Table 3 show the differences in the energy properties of the different types of pellets, with each type having its own advantages and disadvantages. Mixed pellets (Pellets 3) have the lowest moisture content (7.07%), making them suitable for better combustion. However, their lower calorific value (16.71 kJ/kg in the operational state) reduces their energy efficiency, limiting them as a choice for users seeking maximum energy. Sunflower pellets (Pellets 4 and 5) show a high calorific value (up to 21.37 kJ/kg in the dry state), making them energy competitive. However, the high ash content (2.67% for Pellets 5) is a serious disadvantage, as it leads to more residues and increased maintenance costs for fuel systems. This makes them more difficult to use compared to wood pellets. Wood pellets (Pellets 1 and 2) stand out with a low ash content (0.72% for Pellets 2) and a good calorific value (up to 19.96 kJ/kg in dry state). This makes them a preferred choice for clean and efficient combustion. But they are more expensive than other types of pellets, which may limit their application for users looking for a more economical solution. Each type of pellet has a specific application: sunflower pellets are economical but require more maintenance, mixed pellets have potential for optimization and wood pellets offer high efficiency but at a higher price. Their choice depends on the balance between price, quality and energy efficiency according to the needs of users.

4.2. Results of the TG and DSC of the Pellets

Figure 4 and Figure 5 show the TG and DSC results at different heating rates of 5 °C/min and 10 °C/min up to a temperature of 730 °C. These figures illustrate the curves of TGA and DSC at different heating rates, as described above. According to the results obtained from the thermogravimetric analysis, the results can be conditionally divided into three stages. The first stage represents the drying process and the release of light volatile substances. The second stage is the main stage of the process, where the combustion process takes place and the main separation of volatile substances occurs. The third stage is when the smoldering process takes place.

The results of the thermogravimetric analysis (TGA) and DSC show a clear influence of the heating rate (5 °C/min and 10 °C/min) on the thermal behavior of the materials. The analysis reveals significant differences in the temperature of the onset of thermal processes, their kinetics and energy characteristics of the transitions. At a heating rate of 5 °C/min, the thermal events occur in a lower temperature range, which provides a better resolution and clear distinction of the individual stages of decomposition. TGA curves at this rate show a mass loss in two main stages: the initial evaporation of water or volatile components in the temperature range from 50 °C to 200 °C and subsequent decomposition of organic compounds between 300 °C and 500 °C. In this temperature range, the mass losses are smooth and well separated, which facilitates the interpretation of the processes. The DSC data demonstrate clearly pronounced endothermic and exothermic transitions, with the endothermic peaks around 50–150 °C being related to dehydration, while the exothermic events between 300 °C and 500 °C are due to thermal decomposition and possible chemical reactions. The low heating rate allows for a detailed study of the thermal properties of the materials and an accurate assessment of the temperatures and energy requirements of the thermal transitions. At a heating rate of 10 °C/min, a shift in the thermal events towards higher temperatures is observed, which is characteristic of the accelerated kinetics of the thermal processes. The TGA curves show more compressed stages of mass loss, with the onset of evaporation shifting to around 100 °C, and the decomposition of the organic components occurring in the range from 350 °C to 500 °C. In this case, the steps are less pronounced, making it difficult to separate the individual thermal events. Similarly, the DSC curves demonstrate peak overlap, which limits the accuracy in identifying specific transitions. Increased heating rates result in more energy requirements for the transitions and emphasize the influence of kinetics on the thermal stability of materials.

The comparison between the two heating rates shows that the lower rate (5 °C/min) provides a high resolution and is suitable for the detailed analysis of thermal characteristics. On the other hand, the higher rate (10 °C/min) provides valuable information on the dynamic behavior of materials under rapid temperature changes, but with a compromise in resolution. The combined use of these approaches allows for a more complete understanding of the thermal behavior, including the stability, decomposition mechanism and energy requirements of the processes.

During the drying stage, moisture evaporates from the surface of the samples and, simultaneously, volatile substances are released. The processes that occur during the examination of the samples are carried out during the absorption of energy. During the main stage of the process, combustion takes place, where, with the temperature increase, the main amount of heat is formed as a reaction to the process. The intensity of the combustion process increases the rate of the release of volatile substances, which in turn increases the amplitude of the temperature peaks in the heat load curve [59]. For each type of pellets, the observed exothermic peaks at 5 °C/min and 10 °C/min have approximately the same values.

The results in

Table 4. Values of exothermic peaks obtained in the combustion.

	Exothermic Peaks Observed
	5 °C/min	10 °C/min
Pellets 1	320 °C, 360 °C and 460 °C	330 °C, 350 °C, 380 °C and 480 °C
Pellets 2	320 °C and 460 °C	340 °C and 480 °C
Pellets 3	320 °C, 360 °C and 460 °C	340 °C, 380 °C and 480 °C
Pellets 4	310 °C, 420 °C and 460 °C	310 °C, 420 °C and 470 °C
Pellets 5	310 °C and 460 °C	320 °C and 480 °C

show that the maximum temperatures of the combustion process for all tests are observed between 400 °C and 500 °C, where significant amounts of heat are released. After reaching the temperature of about 440 °C, the combustion process is almost complete and the observed mass losses are greatly reduced. At this point, the third stage of the process begins—smoldering—which develops in the residual material. Smoldering ends when the residual heat is removed from the system.

Table 5. Stages of thermal decomposition of biofuels.

	Temperature Range, °C
	Stage 1	Stage 2	Stage 3
Pellets 1	20–115 */20–125 **	115–460 */125–480 **	460–750 */480–750 **
Pellets 2	20–111 */20–123 **	111–460 */123–480 **	450–750 */480–750 **
Pellets 3	20–123 */20–121 **	123–460 */121–470 **	460–750 */470–750 **
Pellets 4	20–117 */20–132 **	117–460 */132–470 **	460–750 */470–750 **
Pellets 5	20–129 */20–140 **	129–460 */140–480 **	460–750 */480–750 **

* Observed results at a heating rate of 5 °C/min. ** Observed results at a heating rate of 10 °C/min.

presents the temperature ranges of the thermal decomposition stages of the studied biofuels.

The burnout efficiency (ψ) of the five types of pellets was calculated according to the methodology used in [60]. The results of this study show that wood pellets (Pellets 1) have the highest burnout efficiency (ψ = 98.6%), making them the most suitable for applications requiring complete combustion and minimal residues. Pellets 2 also demonstrate a good efficiency (ψ = 93.6%), placing them close to the performance of Pellets 1. On the other hand, sunflower pellets (Pellets 5) are characterized by the lowest burnout rate (ψ = 85.8%), which is due to the higher ash content and significant solid residues after combustion. Mixed pellets (Pellets 3 and 4) show identical burnout values (ψ = 90.0%), but remain below the efficiency of wood pellets, which limits their competitiveness.

The results clearly highlight the importance of choosing the appropriate type of pellets depending on the specific application. Wood pellets stand out as the most efficient solution, while agricultural biomass pellets require optimization aimed at reducing residues and improving fuel properties. This is key to increasing their energy efficiency and competitiveness in the market.

Table 6. Mass losses during the thermal decomposition of biofuels.

	Losses from the Total Mass, %
	Stage 1	Stage 2	Stage 3
Pellets 1	7.2 */7 **	95 */94 **	1.4 */4.9 **
Pellets 2	8.1 */7.9 **	93 */92 **	6.4 */8.2 **
Pellets 3	7.6 */5.7 **	90 */84 **	10 */13.4 **
Pellets 4	9.4 */7 **	89 */90 **	10 */8.3 **
Pellets 5	9.2 */7.9 **	85 */92 **	14.2 */6.5 **

* Observed results at a heating rate of 5 °C/min. ** Observed results at a heating rate of 10 °C/min.

shows the maximum total mass losses observed during the thermal decomposition of biofuels.

The TGA results from Table 6 show that until reaching temperatures between 111–129 °C (at a rate of 5 °C/min) and 121–140 °C (at a rate of 10 °C/min), during the drying stage of samples, no major mass loss was observed. At both test rates, the mass loss varies between 7–9% depending on the type of pellets. Slightly higher mass losses (10–15%) for all pellets were observed in the range of 220–240 °C at 5 °C/min and 230–275 °C at 10 °C/min. That is where the increase in heat flow begins. By the end of the combustion process of each type of pellets, according to the thermogravimetric curves, losses between 85% and 95% can be reported. After reaching an average temperature of about 458 °C at a rate of 5 °C/min and 476 °C at a rate of 10 °C/min, the rate of mass loss is reduced until complete combustion is reached.

4.3. Results of the Marketing Research of Customer Preferences for Solid Biomass Fuels Usage in Bulgaria

The data collected from this marketing research indicate a diverse range of preferences among respondents, with coniferous pellets and mixed wood pellets being the most popular purchase choices of solid biomass fuels, respectively, 33% and 22%, followed by deciduous pellets (20%) and sunflower pellets (18%) (see Figure 6).

The second survey question (see Figure 7) aims to understand the influence of price on customers’ preferences for sunflower pellets over wood pellets. The results indicate that a slight majority (46%) would prefer sunflower pellets due to their lower price, while a significant portion (44%) would not be swayed by the lower price and still prefer wood pellets. Additionally, 10% of the respondents are uncertain about their preference, indicating some level of indecision or lack of strong preference.

The third question from the survey uncovers what factors are most important to costumers when choosing solid biomass fuels (see Figure 8). The findings reveal that the majority (51% of the respondents) prioritize price, indicating that affordability plays a crucial role in their decision-making process. Following price considerations, 34% of the respondents place importance on quality, suggesting that many customers are willing to invest more in pellets of superior quality, potentially driven by factors such as enhanced efficiency, cleanliness or environmental sustainability. Only 15% of the respondents prioritize composition, implying that while some consumers may have concerns about the specific ingredients or additives in the solid biomass fuels, it is not a significant consideration compared to price and quality for most respondents.

5. Discussion

The present study highlights the key characteristics and efficiency of different types of pellets with a focus on their application as sustainable biofuels. The results for the moisture content of Pellets 3 (7%) are consistent with data from [61], where a low moisture content is associated with a higher calorific value and better fuel efficiency. Studies from [17] also confirm that coniferous pellets provide optimal efficiency due to their low moisture content. On the other hand, the high ash content of sunflower pellets (Pellets 5) can lead to residue accumulation and the reduced efficiency of fuel systems [61]. In line with data from [25,61], the calorific values of the pellets in this study (18.05–19.70 kJ/kg) confirm their suitability for biofuel standards.

The burnout efficiency (ψ) of the pellets was calculated according to the methodology in [61], with wood pellets (Pellets 1) demonstrating the highest efficiency (98.6%), making them most suitable for applications requiring complete combustion and minimal residues. Pellets 2 also showed a high efficiency (93.6%), while sunflower pellets (Pellets 5) had the lowest efficiency (85.8%) due to the high ash and residue content after combustion. The mixed pellets (Pellets 3 and 4) achieved an average efficiency (90.0%), making them suitable for limited applications. These results are in line with [61], which emphasizes the need for the optimization of agricultural pellets. The results of the thermogravimetric analysis revealed differences in temperature peaks and heat flux, reflecting the differences in pellet composition. Studies in [62] highlight the importance of combining TG and differential scanning calorimetry (DSC) for the optimization of combustion processes. Furthermore, ref. [48] suggests that these analyses can serve as a basis for the development of new types of pellets with improved combustion properties, which is in line with the present results.

The marketing survey shows that coniferous and mixed pellets are preferred by 33% and 22% of the respondents, followed by deciduous (20%) and sunflower pellets (18%). These results are consistent with studies in [27,49], which show that consumers choose coniferous pellets for their reliability and efficiency. However, 46% of the respondents prefer the cheaper sunflower pellets, which highlights the importance of the economic factors. Only 15% of the respondents emphasize environmental aspects, suggesting the need to raise awareness of the benefits of sustainable biofuels.

This study clearly shows that the selection of the appropriate pellet type depends on the specific application and requirements. While wood pellets are established as the most efficient solution, agricultural pellets require optimization to reduce ash and solid residues. Future research should focus on developing new technologies and increasing consumer awareness of the benefits of different types of biofuels [16,19,33].

6. Conclusions

The present study provides an in-depth analysis of the characteristics, energy efficiency and applicability of different types of pellets as sustainable biofuels. The results show that wood pellets demonstrate the highest combustion efficiency and low ash content, positioning them as the preferred choice for applications requiring clean combustion and minimal residues. However, their higher price may limit their accessibility for certain users. Mixed pellets stand out with their low moisture content, making them suitable for combustion processes, but their lower calorific values limit their energy efficiency. Sunflower pellets offer competitive calorific and economic values, but their high ash content leads to increased maintenance costs and residues after combustion, which limits their competitiveness compared to wood pellets.

Thermal analyses confirm that the pellet combustion process goes through three main stages: drying, combustion and smoldering, with the temperature range from 220 °C to 275 °C being critical for the efficiency of the combustion process. The calorific values of pellets vary within the standards for bioenergy fuels, which highlights their applicability in a wide range of energy systems. These data are the basis for future optimization by controlling raw materials and production parameters.

The marketing research shows that consumers prioritize the price–quality ratio, with coniferous and mixed wood pellets remaining the preferred choice due to their reliability and stability. However, the cost-effectiveness of agricultural pellets, despite their limitations, makes them an attractive option for certain consumer groups. This requires better information on their advantages and disadvantages.

This study highlights the importance of optimizing agricultural pellets by reducing ash content and improving fuel properties, as well as developing innovative production technologies. Future research should focus on real-world tests in industrial conditions and on raising awareness about sustainable biofuels. These efforts will contribute to expanding the application of pellets by providing sustainable and energy-efficient solutions that meet modern requirements for environmental and economic efficiency.