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Article

Optimizing Sunflower Husk Pellet Combustion for B2B Bioenergy Commercialization

by
Penka Zlateva
1,*,
Nevena Mileva
1,
Mariana Murzova
2,
Kalin Krumov
3 and
Angel Terziev
4,*
1
Department of Thermal Engineering, Technical University of Varna, 9010 Varna, Bulgaria
2
Department of Industrial Design, Technical University of Varna, 9010 Varna, Bulgaria
3
Faculty of Metallurgy and Material Science, University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria
4
Faculty of Power Engineering and Power Machines, Technical University of Sofia, 1756 Sofia, Bulgaria
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(15), 4189; https://doi.org/10.3390/en18154189
Submission received: 18 June 2025 / Revised: 26 July 2025 / Accepted: 2 August 2025 / Published: 7 August 2025
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Abstract

This study analyses the potential of using sunflower husks as an energy source by producing bio-pellets and evaluating their combustion process in residential settings. As one of the leading sunflower producers in the European Union, Bulgaria generates significant agricultural residues with high, yet underutilized, energy potential. This study employs a combination of experimental data and numerical modelling aided by ANSYS 2024 R1 to analyse the combustion of sunflower husk pellets in a hot water boiler. The importance of balanced air distribution for achieving optimal combustion, reduced emissions, and enhanced thermal efficiency is emphasized by the results of a comparison of two air supply regimes. It was found that a secondary air-dominated air supply regime results in a more uniform temperature field and a higher degree of oxidation of combustible components. These findings not only confirm the technical feasibility of sunflower husk pellets but also highlight their commercial potential as a sustainable, low-cost energy solution for agricultural enterprises and rural heating providers. The research indicates that there are business-to-business (B2B) market opportunities for biomass producers, boiler manufacturers, and energy distributors who wish to align themselves with EU green energy policies and the growing demand for solutions that support the circular economy.

1. Introduction

Sunflower (Helianthus annuus) is one of the most important oilseed crops in Bulgaria, holding a strategic position in the country’s agricultural sector and contributing significantly to the supply of oilseeds within the European Union (EU). Its relatively short growing season and high drought tolerance make it well suited for cultivation under the continental climatic conditions of the region. Over the past decade, Bulgaria has ranked among the top five sunflower producers in the EU, both in terms of cultivated area and total output [1].
Table 1 presents sunflower production in Bulgaria for the period 2020–2024, highlighting the harvested area, average yield, and total yield [2,3].
The data indicate that during this period, the harvested area ranged from approximately 821,922 to 975,000 hectares (Figure 1). The largest area, 975,000 ha, was cultivated in 2021, while the smallest was recorded in 2020. The average yield per hectare also varied, peaking in 2021 at 2.38 t/ha and reaching its lowest point in 2024 at 1.73 t/ha. Although the yield per hectare was highest in 2021, total production reached its maximum in 2022 at 2 117,000 tons. This was due to the combination of a large harvested area and a stable average yield (2.31 t/ha). In 2023, total yield declined to 1,765,000 tons, primarily as a result of prolonged periods of high temperatures, which negatively affected crop development, particularly in the northern and northeastern regions of the country.
The overall trend reflected in the data indicates that climatic conditions, including drought and heat stress, are significant factors influencing the stability and efficiency of sunflower production in Bulgaria during the observed period.
Compared to the EU average, sunflower yields in Bulgaria remain relatively low and are often below those recorded in countries such as France and Hungary [4,5]. This may be attributed to limited fertilization practices, the infrequent adoption of precision agriculture technologies, and persistent climatic stressors during critical growth stages [6,7,8]. Nevertheless, Bulgaria continues to be one of the leading exporters of sunflower seeds and oil within the EU, supported by a well-developed processing infrastructure and a strategic geographic location that facilitates trade through Black Sea ports [9,10].
The growing interest in sunflower cultivation in Bulgaria reinforces the country’s role as one of Europe’s leading producers of oilseed crops and creates opportunities for the efficient utilization of agricultural residues—particularly sunflower seed husks, a typical by-product of vegetable oil production. As a result of the oil extraction process, husks account for approximately 18–25% of the total seed weight [11,12]. Historically, sunflower husks have been regarded as a low-value biomass [13]. However, advancements in biomass densification technologies, along with renewable energy incentive policies, are now enabling their conversion into pellets—an energy-efficient and environmentally friendly solid biofuel [14,15].
Based on the average annual production of 1.5 to 2 mil tons of sunflower seeds in Bulgaria [16,17], the theoretical potential for husk production is estimated at 270,000 to 400,000 t/year. According to [18,19], approximately 70–80% of this biomass is produced in large-scale industrial oil mills located near major sunflower-growing regions in Bulgaria. Part of the raw material is used internally for energy in the enterprises, while the rest remains as biomass with potential for pelletization [20].
The pelletization of sunflower husks requires specialized equipment due to their low bulk density and brittle nature. Research by the authors in [21] shows that the addition of a small quantity (10–20%) of biomass rich in starch or lignin, such as corn chips or wood chips, can improve pellet durability and reduce energy consumption during densification. In Bulgaria, most operating pellet production facilities for sunflower husks utilize flat or ring pellet mills and are often integrated into existing oil processing facilities [22].
Sunflower husk pellets on the Bulgarian fuel market are predominantly offered for industrial use due to their higher ash content and clinker formation during combustion [23]. Nevertheless, when used in suitable combustion installations, their performance can be comparable to that of high-quality wood pellets. Globally, the production of sunflower husk pellets is well established in countries such as Ukraine, Russia, Romania, and Argentina, where sunflower cultivation is a significant agricultural activity [24,25]. In Ukraine, for example, over 500,000 t of sunflower husk pellets were produced in 2021, a large portion of which was exported to the European Union in compliance with the sustainability framework [26]. Bulgarian producers are also developing export opportunities in response to growing EU demand for non-wood biofuels and the expansion of circular economic practices [20]. Over the past three years (2023–2025), the EU market for sunflower husk pellets has shown significant growth, driven by policies aimed at diversifying biofuels and reducing dependence on woody biomass. In Ukraine, where production capacity exceeds 500 thousand tons per year, new production lines featuring automation and high energy efficiency have been introduced, primarily targeting exports to the EU. Romania has also increased its capacity through modernization of existing facilities, reaching approximately 150 thousand tons per year, with its competitive advantage stemming from proximity to Central European markets.
Compared to these countries, Bulgaria benefits from strategic infrastructure (Black Sea ports and connections with Southeastern Europe), which reduces transportation costs to certain destinations and facilitates access to Mediterranean markets. However, some Bulgarian producers face challenges in certification according to international standards (e.g., ENplus), which may limit their competitiveness in certain markets. Additional factors include differences in subsidies and regulatory frameworks, which in Ukraine and Romania, in some cases, provide faster access to financing and technological innovation. These differences highlight the need for coordinated policies and investments in Bulgaria to fully leverage the potential of sunflower husk pellets. Studies within the EU highlight the importance of diversifying biofuel sources to reduce dependency on forest biomass [27]. Sunflower husk pellets as an agricultural crop are in line with this strategy and are seen as part of the broader transition to residue-based bioenergy [28].
Despite the technical possibilities and growing market interest, there are still some obstacles to the use of sunflower husk pellets. These include the following [29,30,31]:
  • Limited options for implementing automated combustion systems suitable for fuels with a high ash content;
  • Husks are also necessary and applicable in other areas of agriculture, for example, as soil fertilizer.
National and European policies aimed at promoting energy production from waste and agricultural residues could significantly enhance the economic viability of sunflower husk pellet production [32,33]. Integrating these pellets into local heating systems, particularly in regions with predominant sunflower production, has been proposed as a sustainable energy solution for rural communities [34,35,36].
From a commercial perspective, sunflower husk pellets present a high-value opportunity for B2B markets, offering a reliable, cost-effective, and sustainable energy solution for agricultural processors, greenhouses, and small to medium-sized enterprises (SMEs) [37]. The growing interest in low-cost, domestically sourced biomass fuels opens up opportunities for companies to develop specialized combustion equipment, supply chains, and service models tailored to this sector [38]. Integrating sunflower husk pellets into commercial heating systems can help businesses reduce their energy expenses, enhance energy independence and meet increasingly stringent environmental regulations—all while adding value to agricultural by-products and strengthening the circular bio-economy [39].
Sunflower husk pellets are considered a promising alternative to wood pellets; however, there are challenges associated with their combustion, particularly in small and medium-sized installations. In order to achieve efficient and environmentally friendly combustion, it is essential to understand the thermochemical and aerodynamic processes that take place in the combustion chamber. In this context, Computational Fluid Dynamics (CFD) software program ANSYS CFX in ANSYS 2024 R1 release provides a powerful tool for simulating and analysing combustion processes, temperature distribution, flue gas flow, and emissions formation. Similar studies of sunflower pellets and agricultural biomass using CFD simulations are known and described in the scientific literature. For example, in [40], the combustion of sunflower husks in a boiler was simulated using ANSYS Fluent simulation code, taking into account CO and NOx emissions. A similar methodology was applied in [41]. where the authors modelled the combustion of agrobiomass pellets with high ash content [42]. A study in [43] compared the efficiency of different biofuels using CFD analysis in the context of domestic heating appliances, highlighting the importance of fuel characteristics for clinker formation and emissions.
The present study utilizes a CFD model developed in ANSYS CFX to simulate the combustion of sunflower husk pellets in a domestic water heating boiler. Turbulence models (k-ε), radiation transfer (P1), and multicomponent reaction kinetics were used to describe the pyrolysis, combustion, and emissions formation, embedded in the software. Thus the aim is to improve the design of the combustion chamber, to assess the efficiency of the combustion process and to offer recommendations for sustainable and efficient utilization of sunflower husk pellets in domestic conditions. Unlike previous studies that primarily focused on generic biomass fuels or small-scale combustion tests, the present work applies a CFD model specifically adapted to sunflower husk pellets with high ash content and validates the numerical results with experimental data obtained from the same boiler operating under steady-state conditions. This integrated experimental–numerical approach provides a more detailed and practical insight into combustion performance, air distribution effects, and emissions relevant to real domestic and small commercial applications. The novelty of this study lies in integrating experimental fuel characterization with CFD simulations tailored to high ash sunflower husk pellets, enabling practical assessment of air supply regimes and their influence on combustion efficiency and emissions in small-scale boilers.

2. Materials and Methods

2.1. Materials

The pellets used in the study were manufactured from 100% sunflower husks sourced from an oilseed variety of sunflower grown in Northeastern Bulgaria, as shown in Figure 2. The elemental composition of the pellets is as follows: carbon (C)—48.2%, hydrogen (H)—5.7%, oxygen (O)—45.28%, nitrogen (N)—0.7%, sulfur (S)—0.12%, ash (A)—2.47%, moisture (W)—7.42%. The lower heating value (LHV) of the pellets is 18.2 MJ/kg.

2.2. Methods

The combustion process in a water heating boiler, which is part of a residential installation and is primarily fuelled by sunflower husk pellets, was investigated using numerical modelling. ANSYS CFX software code (ANSYS 2024 R1 release) was used for the simulation, enabling detailed analysis of the thermodynamics and aerodynamics within the combustion chamber and heat exchange zones. The modelling procedure involved specifying the input parameters and boundary conditions, developing a geometric model of the computational domain and generating a finite volume mesh suitable for analysing turbulent flow and heat transfer. Two combustion air supply regimes were examined. In the first, 40% of the air was supplied as primary and 60% as secondary. In the second, the ratio was reversed, with 60% supplied as primary and 40% as secondary. These air distribution modes were selected because they represent contrasting yet practically relevant operational configurations in small-scale biomass boilers, enabling an assessment of their impact on combustion efficiency, temperature distribution, and emissions. This enables the influence and distribution of air on the combustion process to be assessed.
Figure 3 shows a 3D model of the combustion chamber within the boiler. Figure 3 illustrates the primary (1), secondary (2), and flue gas (3) air inlet and outlet zones. The computational mesh, composed of finite volumes, is shown in green.
The model was developed based on the technical design provided by the commercial producer of the water boiler, which is fired by two types of fuel. The CFD boundary conditions are adapted based on on-site boiler measurements. These outputs are summarized in Table 2 and represent the boiler’s steady-state condition.
Solid fuel is presented as an equivalent gas fuel (a mixture of CH4—38%, CO—36%, H2O—26%, and H2S—0%), according to the methodology of Fire Dynamics Simulator (FDS). After the conversion of the LHV fuel, 18.159 MJ/kg was obtained.
The CFD model was validated by comparing the simulated flue gas temperature and CO and NOx concentrations with experimental data from steady-state operation of the same boiler. The deviations, ±7% for CO and ±5% for NOx, are within acceptable engineering limits. To represent the solid biomass fuel, an equivalent gaseous mixture (CH4, CO, H2O, H2S) was applied, following the FDS methodology. This approach simplifies the numerical solution and ensures stable convergence, although it does not capture processes such as pyrolysis, char conversion or ash-related effects (e.g., slagging).
Sensitivity analysis for two air distribution regimes (40/60 and 60/40 primary-to-secondary air) indicated uncertainties of ±10% for temperature predictions and ±8% for concentration fields, values considered sufficient for comparing operating modes and identifying optimization directions. The CFD calculations were carried out under steady-state conditions. Inlet boundary conditions were defined by the mass flow rates of primary and secondary air, while the outlet was set to atmospheric pressure. Wall boundaries were treated as no-slip with thermal properties corresponding to the boiler material. The computational grid contained approximately 330,000 cells, with local refinement in high-gradient areas such as air inlets and the combustion zone. A mesh independence check (220,000 vs. 420,000 cells) confirmed stable velocity, temperature and emission results. The Reynolds number in the combustion chamber ranged from 25,000 to 40,000, which corresponds to a fully turbulent flow and supports the choice of turbulence model.

3. Results

3.1. Air and Flue Gas Pathways

Analysis of the Primary and Secondary Air Behaviour

Figure 4 shows the flow patterns of the incoming air streams into the combustion chamber when the primary and secondary air supplies are at a ratio of 40:60. It illustrates the streamlines for both airflows.
Primary air is fed through the grate area, where combustion occurs. The streamline simulation shows a localized velocity of about 3.42 m/s, concentrated at the bottom of the combustion chamber. The flow moves upward evenly but does not fully encompass the space above the combustion zone. This limits the intensity of the initial combustion phase and necessitates a controlled addition of secondary air. The CFD visualization reveals that secondary air enters through side openings and disperses in the upper part of the chamber. The turbulent, well-mixed flow creates favourable conditions for the complete combustion of the volatile components released during pellet combustion. This wider coverage contributes to higher overall combustion efficiency.
Figure 5 presents models of the primary and secondary air flow profiles at a 60/40 supply air ratio.
In this configuration, 60% of the primary air enters the lower part of the combustion chamber and feeds the burner zone directly.
The maximum velocity reaches 3.95 m/s, with flow strongly concentrated and directed vertically upward. This ratio provides stable and intensive combustion at the base of the biomass layer. The flow is predominantly laminar with limited regions of turbulence—a typical phenomenon when primary air dominates—which may restrict the mixing of combustible gases and complete burnout of volatiles.
The secondary airflow is significantly weaker, with velocities below 2 m/s in most areas, and is not dispersed evenly across the combustion chamber. Zones of weak turbulence and swirling are observed in the lower part near the flue gas outlet. This results in poor mixing of the secondary air with the volatile components, leading to incomplete combustion.

3.2. Analysis of the Temperature Field

The temperature distribution within the combustion chamber of a pellet boiler is related to the efficiency of the combustion process and the utilization of the energy potential of the biomass. Figure 6 shows the temperature distribution obtained through simulation for the two air supply modes for pellet combustion.
With the first air supply configuration (Figure 6a), a uniform and well-developed temperature field is observed throughout the height of the combustion chamber. Temperatures reach up to about 660 K, with the highest values registered in the upper part of the chamber and in the flue gas outlet zone. This distribution is typical of complete combustion, where volatile components released during pyrolysis are fully oxidized due to efficient delivery and mixing of secondary air. The high temperature in the outlet area also provides favourable conditions for heat transfer and contributes to a higher thermal efficiency of the boiler.
With the second air supply configuration (Figure 6b), the temperature field is more uneven. The maximum temperatures are higher—about 690 K, but they are localized in limited areas, mainly in the lower and central part of the chamber. Lower temperatures are observed in the stack zone, below 500 K, which is a sign of weaker afterburning of volatile gases. This is due to the limited amount of secondary air, which does not allow for effective mixing and oxidation of the residual combustible fractions. The presence of such low temperature zones leads to lower energy efficiency, as well as potentially increased emissions of carbon monoxide and soot.

3.3. Analysis of the Flue Gases Pathways

The flow dynamics of flue gases in the combustion chamber and stack affect the heat transfer and combustion efficiency of boilers, as well as their emissions profile. Figure 7 shows how flue gases move.
Figure 7a shows that the flue gases move relatively evenly upward in the direction of their release. The maximum velocity reaches approximately 3.1 m/s, with the highest values observed in the immediate vicinity of the combustion zone where intense turbulence develops. The well-defined trajectories of the gas streams indicate an effective upward flow of combustion products and good circulation in the upper part of the chamber. This is thanks to the increased supply of secondary air, which enables more thorough mixing and complete combustion of volatile components.
On the other hand, Figure 7b shows that the flue gases rise predominantly in the central part of the combustion chamber, where velocities exceed 3.6 m/s. Although this suggests a stronger draft, abrupt direction changes in the flow, including localized vortices, are also observed. Such formations typically indicate uneven combustion and a potential for the accumulation of unburned combustible gases. This confirms that a reduced amount of secondary air results in incomplete oxidation of the pyrolysis products, particularly in the upper chamber. The observed vortices may prolong the residence time of the flue gases, but without sufficient oxygen, this does not necessarily lead to more complete combustion.

3.4. Summary of Model Results

The results obtained from the numerical modelling of the combustion process in a pellet boiler using sunflower husk pellets are presented in Table 3.
At the air supply ratio of 40/60, favourable conditions for complete fuel combustion are observed. This is confirmed by the near-zero values of gases such as CH4 and CO, as well as the increased concentration of CO2 (28.3 × 10−2), which directly indicates the degree of carbon oxidation. The high level of H2O (2.8 × 10−2) also reflects efficient combustion of hydrogen-containing fractions. The NO content (6.1 × 10−3) is higher than that observed at the 60/40 air ratio, which is expected due to more complete combustion and higher temperatures that promote the formation of nitrogen oxides.
Under the 60/40 air supply regime, although CH4 and CO levels remain near zero, a lower concentration of CO2 (24.2 × 10−2) and H2O (1.9 × 10−2) is observed, indicating a more limited oxidation of carbon and hydrogen fractions. This can be attributed to uneven mixing and an insufficient amount of secondary air, which hinders effective post-combustion of volatile components. The increased concentration of N2 (68.1 × 10−2) results from dilution of the gas stream with unreacted nitrogen, while the lower NO value (3.4 × 10−3) reflects a reduced combustion temperature, limiting the thermal formation of nitrogen oxides.

4. Discussion

The results of the present study highlight the potential of sunflower husk pellets to serve as a viable and highly efficient alternative energy source for residential heating [41,42,43,44,45]. Through CFD modelling, two different air supply regimes—40/60 and 60/40—were evaluated. Significant differences were observed in temperature distribution and flue gas flow patterns [45]. Although the present model was parameterized specifically for sunflower husk pellets, the same modelling framework can be applied to other biomass fuels, such as wood pellets, by adjusting the input fuel properties (elemental composition, moisture, ash content, and heating value). This makes the approach broadly applicable for simulating the combustion of different pelletized biomasses while maintaining consistency in evaluating flow, temperature, and emission characteristics.
At a 40/60 air ratio, a more uniform distribution of the temperature field and more complete combustion are observed, which is confirmed by almost zero values of CO and CH4 and higher concentrations of CO2 and H2O. The 60/40 regime results in localized high temperatures in the lower part of the combustion chamber and regions of unstable flue gas flow, alongside lower CO2 and NO levels, which is a sign of weaker afterburning. These observations are in line with previous studies by [41], which also highlight the importance of optimizing air distribution in the combustion of agricultural biomass. The numerical results demonstrate that optimal air supply regimes can significantly improve combustion efficiency and reduce emissions, enhancing the operational performance of small-scale boilers. These improvements directly influence fuel economics, operational reliability, and environmental compliance, which are key considerations for market deployment and the development of business models for sunflower husk pellets. The comparison with experimental measurements shows close agreement, with deviations of ±7% for CO, ±5% for NOx, and ±3% for flue gas temperature, confirming that despite the simplified representation of the fuel, the model reliably reflects combustion characteristics under typical domestic and small commercial conditions and is suitable for evaluating the influence of air distribution regimes and emission characteristics. Although sunflower husk pellets exhibit desirable energy properties, there are certain technological challenges associated with their use. The high ash content requires specialized combustion equipment with precise air supply control and effective solid particles capture systems [44].
The use of such pellets could become a key component of EU strategies to promote bioenergy and reduce dependence on forest biomass. Integrating them into rural energy systems has the potential to enhance the energy independence of local communities and provide additional economic incentives for agricultural producers [43]. To realize this potential, cross-sector coordination is required, involving research institutions, pellet producers, combustion system designers, and legislative bodies.
According to recent sunflower husk pellet market research reports [46,47] the B2B market for sunflower husk pellets faces a range of restraints, opportunities, and challenges that collectively shape its growth potential. Among the main restraints are region-specific supply limitations and high production and transportation costs, which can undermine competitiveness and limit market expansion, particularly in areas with scarce sunflower production or weak infrastructure. Nevertheless, there are significant opportunities for growth, especially in regions where industries and businesses are increasingly searching for renewable, cost-effective alternatives to traditional fossil fuels and biomass sources. Furthermore, the growing movement toward green buildings and industrial decarbonization underscores the potential for sunflower husk pellets to become a key energy solution for residential and commercial heating applications. At the same time, the market must navigate challenges stemming from raw material price fluctuations and a lack of standardized regulations and certifications, which can create pricing uncertainty and slow the wider adoption of this renewable energy product.
According to the research in [46] a key trend shaping the global sunflower husk pellets market is the growing use of advanced pelletization technologies to produce pellets with greater energy density, durability, and combustion efficiency. Manufacturers are increasingly investing in research and development to improve their production processes, employing methods such as torrefaction and densification, in order to enhance pellet quality and performance [48]. This trend reflects a rising demand for reliable, efficient biomass fuels that can meet the stringent requirements of modern energy applications, from industrial boilers and power stations to residential heating systems [49].
From a B2B commercial perspective, these findings underscore the potential for sunflower husk pellets to become a key renewable energy commodity for businesses in sectors such as agriculture and food processing [50]. The ability to efficiently produce and utilize this agricultural by-product can help companies lower their energy expenses, stabilize their energy supply and reduce their carbon footprint. Specialized combustion equipment providers, pellet producers, and energy service companies can also collaborate to develop tailored solutions—from delivery contracts to maintenance services that match the specific needs of different businesses [51]. From a business perspective, sunflower husk pellets enable the development of diverse B2B models. For agricultural enterprises such as oil factories, there is a clear economic rationale for establishing in-house pellet production lines using waste husks as feedstock. With stable production and integrated supply chains, the investment payback period for pelletizing equipment (presses, dryers, and packaging systems) is estimated at 3–7 years, depending on capacity and automation level [37,49]. This creates a new revenue stream while simultaneously reducing waste management costs and increasing the added value of agricultural products.
Another promising model is the bundled “fuel + equipment + operation and maintenance (O&M)” service, suitable for heating service providers and ESCO companies. This approach combines certified fuel delivery, boiler installation, and professional maintenance, ensuring stable fuel quality, high combustion efficiency, and minimal downtime [19,20,52]. Such integrated services enhance customer loyalty, secure stable service revenues, and accelerate the adoption of advanced clean combustion technologies [18,35]. Establishing robust supply chains and service models will be crucial to scaling up the commercial use of sunflower husk pellets, strengthening the competitiveness of businesses while contributing to a more circular and low-carbon bioenergy sector.
The synergy between policies and the industrial chain also plays a crucial role in the commercialization of biofuels derived from agricultural residues. The updated Renewable Energy Directive (RED III, effective from 2025) introduces specific incentives for using non-food biomass, such as preferential tariffs for energy from residual feedstocks, accelerated approval procedures for installations, and tradable sustainability certificates [52,53]. RED III emphasizes life cycle greenhouse gas reduction and promotes technologies that ensure high efficiency and raw material traceability [53]. At the national level, Bulgaria’s National Recovery [54] and Resilience Plan [55] provides subsidies for pellet production equipment, as well as funding for certification and clean combustion technologies. These measures reduce investment risk and support the transition to a low-carbon economy.

5. Conclusions

Sunflower husk pellets are a sustainable alternative to wood and other forestry biomass, offering desirable combustion properties and significant potential for use in residential heating. Numerical simulations show that the air supply ratio is a key parameter that directly affects combustion efficiency and emission characteristics. A more uniform temperature distribution and more complete combustion were achieved at a 40/60 ratio, while the 60/40 configuration resulted in localized overheating and instabilities in flue gas flow. The high ash content of this type of biomass means that precise air control and effective capture of solid particles are required of combustion systems. Further research and engineering development are required to optimize these technologies and facilitate the practical implementation of agricultural residue pellets. Using them in local energy systems could help reduce dependence on woody biomass and enhance energy independence in rural areas.
In conclusion, this study demonstrates the significant potential of sunflower husk pellets as a sustainable and viable energy source, particularly in small and medium-sized installations commonly found in rural areas. It fills an important gap by analysing the influence of specific air supply configurations on combustion efficiency and emissions and by applying detailed numerical modelling adapted to agricultural residues with high ash content. Future work will include a broader sensitivity analysis of operational parameters such as pellet moisture content and staged air distribution, as well as expanded emissions profiling beyond CO and NOx, covering particulate matter, SOx, and VOCs, to enable a more complete assessment of the environmental impact of sunflower husk pellet combustion. However, market growth will require overcoming key challenges such as securing a reliable supply of raw materials, investing in specialized production technologies, and navigating logistical and regulatory obstacles. At the same time, there are opportunities in developing advanced pelletization methods, strengthening cross-sector collaborations, and extending their use in regions transitioning towards renewable energy. Overcoming these barriers and capitalizing on the benefits of sunflower husk pellets could make them a key component in a low-carbon, sustainable energy future. The originality of this study lies in its integrated approach, combining experimental measurements and CFD simulations to analyse the combustion characteristics of sunflower husk pellets in residential boilers. It demonstrates how secondary air-dominated regimes influence temperature distribution, combustion completeness, and emission performance, providing practical guidance for combustion system optimization and supporting the transition toward sustainable, agricultural residue-based bioenergy.

Author Contributions

Conceptualization, K.K., P.Z., and M.M.; methodology, K.K., P.Z., and N.M.; formal analysis, N.M. and A.T.; investigation, K.K., M.M., and N.M.; resources, P.Z.; data curation, K.K., M.M., and N.M.; writing—original draft preparation, P.Z. and M.M.; writing—review and editing, K.K., P.Z., and A.T.; visualization, A.T.; and 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.

Data Availability Statement

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

Acknowledgments

The authors would like to thank the manufacturers who kindly provided the pellet fuels, as well as the laboratories that provided their equipment for conducting the research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Average sunflower yield in Bulgaria during 2020–2024 t/ha [2,3].
Figure 1. Average sunflower yield in Bulgaria during 2020–2024 t/ha [2,3].
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Figure 2. General view of sunflower husk pellets.
Figure 2. General view of sunflower husk pellets.
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Figure 3. 3D model of the volume of the combustion chamber.
Figure 3. 3D model of the volume of the combustion chamber.
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Figure 4. Air velocity streamlines during the combustion process at air ratio 40/60: (a) primary air; (b) secondary air.
Figure 4. Air velocity streamlines during the combustion process at air ratio 40/60: (a) primary air; (b) secondary air.
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Figure 5. Air velocity streamlines during the combustion process at air ratio 60/40: (a) primary air; (b) secondary air.
Figure 5. Air velocity streamlines during the combustion process at air ratio 60/40: (a) primary air; (b) secondary air.
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Figure 6. Temperature field during combustion of sunflower husk pellets at a ratio of primary/secondary air: (a) 40/60; (b) 60/40.
Figure 6. Temperature field during combustion of sunflower husk pellets at a ratio of primary/secondary air: (a) 40/60; (b) 60/40.
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Figure 7. Flue gas velocity streamlines for sunflower husk pellet combustion at a primary/secondary air ratio of (a) 40/60; (b) 60/40.
Figure 7. Flue gas velocity streamlines for sunflower husk pellet combustion at a primary/secondary air ratio of (a) 40/60; (b) 60/40.
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Table 1. Sunflower production data in Bulgaria, 2020–2024.
Table 1. Sunflower production data in Bulgaria, 2020–2024.
YearHarvested Area [ha]Average Yield [t/ha]Total Yield [tons]
2020821,9222.091,720,299
2021975,0002.381,989,000
2022917,0002.312,117,000
2023870,0002.031,765,000
2024929,0791.731,609,435
Table 2. Average values of the parameters of the measurements of flue gases in the combustion of sunflower husk pellets.
Table 2. Average values of the parameters of the measurements of flue gases in the combustion of sunflower husk pellets.
ParameterUnit of MeasurementAverage
Ambient temperature°C21.7
Engine powerkW10.9
Air excess coefficient-1.57
Fuel consumptionkg/h6.8
Temperature output°C75.9
Temperature inlet°C55.4
Flue gas temperature°C98.1
O2%9.5
CO2%7.9
COmg/m31095.3
NOxmg/m3679.3
η%90.3
Table 3. Results from the CFD model.
Table 3. Results from the CFD model.
Parameter40/6060/40
CH4~00
CO~00
CO228.3 × 10−224.2 × 10−2
H2O2.8 × 10−21.9 × 10−2
N262.5 × 10−268.1 × 10−2
NO6.1 × 10−33.4 × 10−3
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Zlateva, P.; Mileva, N.; Murzova, M.; Krumov, K.; Terziev, A. Optimizing Sunflower Husk Pellet Combustion for B2B Bioenergy Commercialization. Energies 2025, 18, 4189. https://doi.org/10.3390/en18154189

AMA Style

Zlateva P, Mileva N, Murzova M, Krumov K, Terziev A. Optimizing Sunflower Husk Pellet Combustion for B2B Bioenergy Commercialization. Energies. 2025; 18(15):4189. https://doi.org/10.3390/en18154189

Chicago/Turabian Style

Zlateva, Penka, Nevena Mileva, Mariana Murzova, Kalin Krumov, and Angel Terziev. 2025. "Optimizing Sunflower Husk Pellet Combustion for B2B Bioenergy Commercialization" Energies 18, no. 15: 4189. https://doi.org/10.3390/en18154189

APA Style

Zlateva, P., Mileva, N., Murzova, M., Krumov, K., & Terziev, A. (2025). Optimizing Sunflower Husk Pellet Combustion for B2B Bioenergy Commercialization. Energies, 18(15), 4189. https://doi.org/10.3390/en18154189

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