Biomass and Bioenergy

1. Introduction

Energy is a fundamental pillar of industrial progress and social well-being, driving economic growth, technological advancement, and improved living standards. However, the predominant reliance on non-renewable energy sources, such as oil, coal, and natural gas, has led to many negative environmental and health consequences.

The extraction and combustion of fossil (non-renewable) fuels are major contributors to the accumulation of greenhouse gas emissions in the atmosphere [1], leading to more frequent and severe weather events, rising sea levels, and changes in climate patterns [2,3]. Additionally, the production of energy from non-renewable fuels results in the emission of particulate matter and other pollutants that degrade air quality [4], the generation of hazardous waste, and environmental disasters such as oil spills, which contaminate soils and groundwater, causing long-lasting ecological damage. These changes disrupt natural ecosystems, threaten biodiversity, and weaken the resilience of the anthropogenic environment. Furthermore, the pollutants released by fossil fuel combustion pose significant risks to human health, particularly in urban areas where air quality is often compromised. Given the urgency of addressing these challenges, there is a growing global unanimity on the need to reduce our dependence on fossil fuels and transition towards more sustainable energy systems [5,6].

Among the renewable energy options, biomass stands out as a particularly promising resource. Biomass is a form of stored chemical energy that is derived from the sun through photosynthesis, making it abundant, renewable, and relatively cost-effective. One of its most notable attributes is its adaptability to a wide range of environmental and growth conditions, including agricultural lands, forest ecosystems, marginal lands, natural watercourses, wastewater treatment plants, and industrial facilities [7]. This versatility underscores the potential of biomass as a key component in many energy and environmental strategies. In addition, the use of biomass extends beyond energy production. Biomass is also a critical source of food and raw materials for various industries, including agriculture, forestry, and manufacturing [8].

To ensure the sustainability of biomass utilization, it is essential to find a balance between its role in energy generation and its applications in these other sectors. The overexploitation of biomass for energy could lead to resource depletion, land-use conflicts, and negative impacts on food security and biodiversity. Thus, the development of bioenergy must be guided by principles of sustainability, ensuring that the benefits of biomass as an energy source do not come at the expense of its ecological and socio-economic functions. This balanced approach is crucial for fostering a sustainable energy transition that supports both environmental protection and human well-being.

When subjected to appropriate pretreatment methods and advanced technologies, biomass can be efficiently converted into a diverse array of biofuels, including those suitable for transportation, bioheat, and bioelectricity [9,10]. These conversion processes unlock the potential of biomass as a versatile energy source that is capable of contributing to multiple sectors [11]. Currently, bioenergy ranks as the fourth largest source of primary energy globally, following oil, coal, and natural gas [12]. This significant position underscores the growing importance of biomass in the global energy mix.

The international community, through frameworks such as the Paris Agreement on climate change and the United Nations’ Sustainable Development Goals (SDGs), has increasingly recognized the critical role of bioenergy in achieving sustainable development and climate mitigation targets. The Paris Agreement, with its ambitious goals to limit global temperature rise, highlights the necessity of transitioning towards low-carbon energy sources. The European Union aims to reduce greenhouse gas emissions by 55% and to achieve 50% renewable energy consumption by 2030 [10,13], as well as achieving climate neutrality by 2050 [14,15]. In this sense, the SDGs advocate for the advancement of renewable energy technologies, positioning biomass-based energy as a cornerstone for sustainable economic and environmental progress.

As the global commitment to reducing greenhouse gas emissions intensifies, the production and utilization of bioenergy are expected to expand significantly in the near future. This anticipated growth reflects both technological advancements in biomass conversion and an increasing alignment of energy policies with sustainability objectives. Consequently, biomass and bioenergy play a pivotal role in the global energy transition, contributing not only to climate change mitigation but also to the enhancement of energy security and the promotion of rural development.

Therefore, this Special Issue, entitled “Biomass and Bioenergy”, focuses on current biomass pretreatment methods and technologies for energy recovery, the current status and technologies for obtaining biofuels from biomass (including pellets, briquettes, and tablets; pyrolysis products; syngas; biogas; biodiesel; bioethanol; biohydrogen; and biochitan); the recovery of heat from compost piles; the modeling and optimization of the technologies for energy recovery from biomass; biorefineries; and best practice models in the field of bioeconomy with an emphasis on energy recovery from biomass waste.

Ten manuscripts were published by researchers and scholars from the international scientific community in this Special Issue, comprising original scientific contributions (case studies, experiments, or systematic comparisons with existing approaches) that describe the recent progress in these topics and their related fields.

2. Review of New Advances

The clean use of coal in stoves for heating homes or cooking is a sustainable solution that contributes to the energy security of the population, reducing the negative impact on the environment and improving energy efficiency in households. Kirimi et al. (Contribution 1) evaluated the use of charcoal and its impact on indoor air pollution (IAP) in urban informal settlements, focusing on the Jikokoa stove and two types of Kenya Ceramic Jikos (KCJ). The Jikokoa stove, compared to small- and medium-sized KCJs, reduced charcoal consumption by 6.4% and 26%, respectively, while also decreasing carbon monoxide (CO) by up to 50%, fine particulate matter (PM2.5) by 77%, and carbon dioxide (CO₂) by 15.6%. Their findings highlight the Jikokoa stove as the most efficient option, reducing both fuel consumption and IAP, thus helping to mitigate environmental and health impacts. The study emphasizes the need for the scaling up of improved stoves through participatory research to enhance local cooking practices.

Biomass power plants are among the most advanced heat production systems, utilizing abundant renewable energy sources such as agricultural residues, sawdust (often in the form of pellets), and wood. They are a sustainable alternative to fossil fuel power plants and can be used in a wide range of ways, from individual homes to commercial and industrial activities. Sokrethya et al. (Contribution 2) analyzed the feasibility of using rice straw for power generation in Cambodia, identifying suitable locations for 10 MW biomass power plants using GISs. The most promising provinces were Prey Veng, Takeo, and Battambang, with a total potential of 2.13 million tons of straw per year. Economic modeling indicates a payback period of 6 to 10 years, depending on the price of straw (20–40 USD/ton). The project could generate 1251 GWh/year and avoid 1.06 million tons of CO₂ annually compared to coal-fired power plants, while also significantly reducing emissions compared to field burning. These results are essential for the design of small-scale biomass power plants in the regions studied. Furthermore, such studies can be extended to other agricultural regions where large volumes of straw are generated and can be energetically exploited in situ, thus eliminating the risk of greenhouse gas emissions and ensuring the independence of residents from fossil fuels.

Residual plant biomass from gardens and greenhouses is often burned, composted, or disposed of. However, this form of biomass has become a resource of interest for obtaining biofuel pellets and briquettes, which can be burned in stoves and biomass power plants. The study conducted by Kabaş et al. (Contribution 3) investigated the valorization of greenhouse waste into bio-briquettes and also carried out an analysis of their strength parameters. Using a mobile prototype briquetting machine with hydraulic pressures of up to 190 MPa, the researchers obtained high-quality bio-briquettes from pepper, eggplant, and tomato plant waste. Tests revealed a maximum density of 1143.52 kg/m³ and strengths of 99.24% (pepper) and 98.52% (eggplant). Bio-briquettes from tomato waste recorded the best results in compression tests (3315 N and 69.43 N/mm²). This study confirms the potential of greenhouse biomass for the production of sustainable bio-briquettes, with ecological and economic benefits for farmers, since the bio-briquettes obtained from such biomass residues can also be used for heating the greenhouses from which the biomass comes.

The release of potassium (K) during the combustion of agricultural biomass residues represents a significant advantage, given that potassium is an essential nutrient for plant growth and has a high agronomic value. Thus, the ash resulting from biomass combustion, which contains significant amounts of potassium, could be used in agriculture as a natural fertilizer, contributing to the enrichment of soils and reducing the need for chemical fertilizers. This not only supports sustainability but also optimizes the circulation of nutrients in ecosystems. A study by Zhang et al. (Contribution 4) investigated the potassium release characteristics during biomass combustion using three typical fuels in a fixed-bed reactor. Their results showed that the potassium release ratio in corn straw increases with exposure time between 700 and 900 °C, as well as at 900 °C, for 40 min; in total, 17.73% of K is released in the volatile combustion phase, compared to only 2.62% in the coal combustion phase. Also, water washing significantly reduces the ratio and amount of potassium released from corn straw. Increasing the oxygen concentration slightly improves the K release ratio in corn and wheat straw, and the effects of temperature and atmosphere on K release from the washed samples are similar to those from the raw samples.

Energy recovery from municipal solid waste represents an innovative and sustainable solution, with significant economic and environmental benefits, contributing not only to reducing the amount of waste but also to obtaining renewable energy, in accordance with the principles of circular economy and in the context of the global climate crisis. A study by Pheakdey et al. (Contribution 5) evaluated the energy potential, economic feasibility, and environmental performance of technologies for recovering gas emissions from a landfill, a waste incinerator, and a biogas plant from municipal waste that is predicted to be generated in Phnom Penh, Cambodia, between 2023 and 2042. Their results showed that the incinerator would produce the highest energy yield between 793.13 and 1625.81 GWh/year, followed by landfill gas and the biogas plant, at 115.44–271.81 GWh/year and 162.59–333.29 GWh/year, respectively. Even if the biogas plant was the most efficient in terms of environmental benefits, saving approximately 133.784 tons of CO₂-eq/year, the investments required for its construction would have the longest payback period. Thus, long-term feasibility studies are essential when choosing investments in various biomass waste-to-energy technologies, as they provide a comprehensive assessment of the economic, environmental, and operational impacts of each technology over an extended period. This allows the risks and opportunities associated with each option to be identified; long-term performance to be forecasted; and factors such as fluctuations in raw material prices, changes in environmental regulations, and energy market developments to be considered. Furthermore, a detailed study helps to optimize costs, maximize the return on investment, and ensure an efficient transition to sustainable solutions, considering the objectives of reducing CO₂ emissions and promoting a circular economy.

Biogas production is a long-standing technology for converting organic biomass into renewable energy. However, recent research continues to bring significant innovations, improving process efficiency, optimizing anaerobic digestion processes, and exploring new feedstock sources, making biogas an even more promising option for energy sustainability. Ruiz et al. (Contribution 6) presented a multi-parametric analysis for evaluating industrial waste as co-substrates in anaerobic co-digestion, considering factors like composition, C/N ratios, biochemical methane potential (BMP), production rates, and seasonality. Their findings highlight that fried corn from the snack food industry and wet fatty pomace from the olive oil industry have the highest potential due to their high BMP and their lipid and carbohydrate content, as well as suitable C/N ratios. Other types of waste, like dry olive pomace and grape pomace, also show promise. The proposed selection matrix presented in this study serves as a useful tool for decision-making in waste-to-energy applications.

Biochar production from biomass and industrial waste is a critical step toward achieving environmental and economic sustainability, with its importance being amplified by the potential to create high-value-added activated carbon, which is a solution at the forefront of sustainable innovation, with applications in water purification, air filtration, soil remediation, and carbon sequestration. In their study, Coker et al. (Contribution 7) analyzed the conversion of biomass and industrial waste into activated carbon, highlighting the importance of knowing the conversion effects of the various materials used. Among the samples investigated, piñon wood stood out as having the best results, and pyrolysis temperatures between 600 and 650 °C offered the highest yields. Methods such as slow pyrolysis and hydrothermal carbonization are recommended for the transformation of biochar into activated carbon, highlighting the potential of local low-value materials, such as agricultural and industrial residues, for environmentally sustainable applications. An integrated research approach combining the improvement of well-established pyrolysis technologies with applied research on biochar from different types of biomass waste and the use of by-products could lead to the development of more sustainable and economical processes for biomass conversion.

The recovery of waste vegetable oil is essential for reducing environmental impact. The transesterification and pyrolysis of vegetable oil are promising methods for transforming this form of waste into energy and other useful substances. Despite significant progress being made, there are still gaps in the understanding of the transesterification and pyrolysis process, such as the optimization of reaction conditions and the full recovery of the products obtained. A valuable contribution was made by Sabino et al. (Contribution 8), who investigated the pyrolysis of oleic acid and industrial vegetable oil residue using CuNiAl mixed oxide catalysts derived from layered double hydroxides. Their research reveals that copper addition to NiAl catalysts enhances oleic acid cracking, reduces aromatic and coke formation, and lowers oxygenated compounds. The CuNiAl catalyst with a Cu/Ni ratio of 0.4 showed a high catalytic activity, converting vegetable oil residues into valuable hydrocarbons in the gasoline, kerosene, and diesel range, as well as alkylbenzenes for surfactant production. These findings highlight the potential of using CuNiAl catalysts for efficient biomass conversion.

Another study, carried out by Rodrigues et al. (Contribution 9), explored the potential of a lipase-rich extract from the Amazonian endophytic fungus Endomelanconiopsis endophytica as a biocatalyst for biodiesel production from waste cooking oil. The fungus produced an enzyme activity of 11,262 U/mL after 120 h of cultivation. The lipolytic extract showed optimal catalytic activity at 40 °C and pH 5.5. Using soybean oil and frying residue, biodiesel yields of 91% and 89%, respectively, were achieved. This study highlights the feasibility and sustainability of using fungal enzymatic extracts for biodiesel production from waste cooking oil, offering an eco-friendly alternative to traditional biodiesel production.

The use of macroalgae for biohydrogen production represents an innovative frontier in the field of renewable energy, providing a sustainable and efficient source of hydrogen, which is essential for cutting-edge applications such as energy storage, zero-emission mobility, and green industrial processes. This approach promises to revolutionize the energy sector by utilizing abundant and untapped resources, with the potential to support the transition to a greener and more sustainable economy. Shankaran et al. (Contribution 10) explored a combined method involving the pretreatment of macroalgae (Chaetomorpha antennana) for biohydrogen production using sonication and alkaline liquefaction (SAL). The optimal sonication conditions (50% sonic intensity and 30 min of pretreatment) resulted in the release of 2650 mg/L of organic substances. Under SAL, the most effective pH was 11, with a liquefaction efficiency of 24.61% and a release of 3200 mg/L. Compared with the simple sonication (SL) method, SAL consumed less energy and produced more volatile acids (2160 mg/L vs. 1070 mg/L), increasing biohydrogen production by 15%, as well as saving 44.4% of energy. The results show that SAL is more energy efficient in biohydrogen production. The method proposed by the authors of this study has proven its usefulness. However, for industrial scale application, research in this field should focus on the fine adjustment of parameters such as sonic intensity, pretreatment duration, and pH in order to maximize the efficiency of the biohydrogenation process and reduce energy consumption. Extensive experiments on various types of macroalgae and biomass could help to identify the best pretreatment conditions for each material.

3. Conclusions

The results of the research studies published in this Special Issue represent a valuable contribution to the knowledge relating to the topics addressed. The use of modern technologies for biomass and organic waste valorization, as well as the efforts of researchers worldwide to innovate and to increase the scalability of such technologies, contribute significantly to advances in the field of biomass valorization for energy purposes, as well as in relation to the sustainability objectives for the years 2030–2050.