Abstract
Promoting the use of renewable energy sources has become an important policy strategy for mitigating climate change and for providing better energy security and financial sustainability. To overcome the problems generated by non-renewable energy sources, it is essential to use new energy sources. A literature review was conducted to investigate and understand the opportunities for implementing new renewable energy sources. Agricultural residues have great potential to receive significant consideration worldwide as an alternative, sustainable, and green energy source. The use of agricultural residues for bioenergy generation is a broad and favourable scenario for exploration. This review identified potential and almost unexplored research approaches with the aim of contributing and promoting researchers to deliver technological solutions for the society and industrial sectors. For example, a potentially promising technological solution would be for industries that produce machinery and agricultural implements to adapt their harvesters for different grain crops, to collect these agricultural residues simultaneously during harvest and readily perform granulation, compaction (pressing), pelletizing or briquetting directly on the property. Further studies are required to investigate the use of agricultural residues for bioenergy generation, which can contribute to the diversification of the energy matrix. Accordingly, in this review, several challenges and future research perspectives have been presented, such as suggestions for future research on how to collect, transport, process, market and use these agricultural residues to generate bioenergy, aiming at reducing the dependence on fossil fuels.
Introduction
Role of energy, its consumption and transformation
Energy is fundamental to human survival, development and growth (Coelho et al., 2018; Le et al., 2021; Vieira et al., 2021). Energy is used to produce goods and services or to meet the service requirements of the final demand sectors (Chai et al., 2009; Weiss de Abreu et al., 2021).
Energy consumption has steadily increased with the increase in population, enlargement of civilization and celerity of industrialization (Ervural et al., 2018; Pan and Wang, 2021; Zhao et al., 2019).
According to the World Energy Council, the global energy spent increased by approximately 47% between 1990 and 2010, and this consumption is estimated to double by 2050 (Frei et al., 2013; Pan and Wang, 2021). Energy is crucial for everyone, and the operation of all daily activities depends on some type of energy.
It is important to identify and procure energy sources optimally. According to the law of energy conservation, energy can neither be created nor be destroyed, but it can be transferred from one form into another. Therefore, it is necessary to transform the unutilized energy and store it for later use (Vivek et al., 2021).
The price of metals (rare metals in particular) is poised to play a more significant role in energy storage. There are growing concerns about the future supply of copper. Discussions are underway regarding the accelerated development of copper mining projects, regardless of economic, regulatory and environmental constraints.
According to estimates, copper deposits will be depleted within 25–60 years. For example, Nováková et al. (2022) evaluated the mutual price relationship of copper and zinc between 2011 and 2021 and predicted future prices until 2030. They confirmed a correlation between the price of energy, metals and grains. That is, with the depletion of copper reserves, the price of energy tends to be affected.
Another example is the research that evaluated the evolution of the price of silver from 2011 to 2021, making a safe forecast of its trend for 2022. Investment in commodities presents an interesting possibility of increasing the value of assets. This is an extremely difficult subject owing to global climate change, geopolitical tensions and turbulent economic factors globally. That is, the future prices of specific commodities show a high degree of volatility (Rowland et al., 2021).
Notably, Bartoš et al. (2022) and Vochozka et al. (2021) mention that copper and aluminium prices have long been influenced mainly by non-renewable resources and that resources are becoming increasingly scarce, with copper ore being one of the decisive factors that influence the price of this commodity; that is, the scarcity of these rare metals over time is a worrying factor that will affect several sectors.
Energy evolution ensures the availability of new energy sources, namely, natural resources from which energy can be obtained at the time of change or expansion of energy generation (Opeyemi, 2021).
Energy can be categorized in several ways, and one of them is the differentiation between primary and secondary sources. Fossil fuels, such as coal, natural gas and oil; solar energy; wind energy; and biomass are classified as primary sources. By contrast, electricity and gasoline, which are derived from the conversion of primary sources, are classified as secondary sources (Geels, 2010; Kuzemko et al., 2019; Unruh, 2000).
Main sources of energy
Based on their exhaustibility, energy sources can be categorized as renewable and non-renewable sources (Vivek et al., 2021). The renewable sources are also known as alternative energy sources (Panwar et al., 2011), clean energy (Kuzemko et al., 2019; Sovacool and Walter, 2019), sustainable energy (Chaikumbung, 2021; Pan et al., 2021), or green and modern energy (Le et al., 2020, 2021).
In relation to modern trends in renewable energy sources, the production of biohydrogen from biological waste using renewable energy sources has recently gained prominence. Biohydrogen has received substantial attention among some of the established biofuel producers as it offers numerous advantages such as low production cost (Maroušek, 2022).
According to Maroušek (2022), nanoscience maintains a promising direction that makes biohydrogen a reliable alternative to fossil fuels. The prerequisite for this is the complex refining of biological waste into a wide range of high value-added products and recovery of waste heat.
Another modern trend, according to Maroušek et al. (2022), is the use of microalgae for the production of biohydrogen and biodiesel as promising substitutes for fossil fuels.
The results of several studies on biohydrogen and biodiesel production from microalgae indicate that these new concepts have a high potential for competitiveness. However, there are still several technical–economic bottlenecks that prevent the transformation of algae biodiesel production on a commercial scale, and basically all of them are linked to algae production (Maroušek et al., 2022).
Non-renewable energy sources are also known as traditional energy (Usman and Makhdum, 2021; Yu et al., 2022), conventional energy (Adekoya et al., 2021; Panwar et al., 2011), dirty energy (Zhao et al., 2022) or fossil energy (Shao et al., 2020). Figure 1 presents the main examples of the different methods for capturing energy through renewable and non-renewable energy sources.
Main sources of energy.
This article focuses on biomass, a type of renewable energy, as shown in Figure 1.
Energy sources and their economic influences
The different energy sources play critical roles in the development of countries worldwide, significantly influencing the economy, politics, culture, society and environment (Le et al., 2020, 2021).
For example, regarding the economy, as highlighted by Vochozka et al. (2020a), the price of oil directly affects the development of many areas of the world economy, as they are affected by changes in oil prices. High volatility in oil prices can occur as a result of other factors, for example, in times of crisis (as was the case with the coronavirus pandemic) and also in cases of war (as is the recent case of the war between Russia and Ukraine).
The development of a country’s economy depends on its comprehensive economic strength and its international competitiveness, and the exchange rate is one of the important variables that affect international competitiveness. International trade in crude oil will affect the exchange rate, influencing a country’s international balance of payments or domestic oil prices (Wang et al., 2022).
Vochozka et al. (2020b) suggested that the EUR/USD exchange rate is strongly dependent on the international price of oil, and therefore, this commodity and source of energy is one of the key pillars of the world economy and plays an extremely important role in everyday life, impacting the future growth of the gross domestic product of countries and, consequently, the performance of world economies. Oil is a force that moves means of transport and also raw material for various purposes, such as the production of plastics, directly and economically affecting the entire structure of various production chains. Regarding the role of politics, Le et al. (2021) mentioned that, to implement a green growth strategy roadmap, governments must have a mechanism and policies that encourage more investments in the renewal of technological production and consumption processes and in improving energy efficiency. Governments should adopt appropriate policies and mechanisms to encourage the use of renewable energy sources such as wind, solar, tidal and bioenergy.
In the research by Valaskova et al. (2021), the results of the investigation carried out revealed that the economic sector is one of the most important determinants of earnings management, as its statistical significance was confirmed in each country analysed. In Durana et al. (2021), the growing risk of company failure is associated with the phenomenon of earnings management. Bankruptcy risk and the quality of reported earnings, along with other aspects of financial performance, vary over a company’s lifecycle.
The cost of using these types of energy sources is still high in the short term owing to high investment costs and technological innovation. Thus, governments need to provide more policy incentives, such as price subsidies and tax cuts, to reduce the burden on investors. In addition, it is essential to implement policies to mobilize resources, mainly domestic capital, to invest in the exploration and production of renewable energy according to market principles (Le et al., 2021).
It is necessary to develop a culture for using more renewable energy, for example, by disseminating the culture of self-generation of energy through solar panels in homes, businesses and small industries, to gradually replace the existing polluting energy sources. That is, individual awareness to save energy must be increased and cultural practices must be changed to increase the use of renewable energy. Resistance to the new (cultural aspects of resistance to using new technologies) often ends up hindering the development of energy worldwide.
According to Le et al. (2020), society needs to be encouraged to invest, explore and use energy sources efficiently and economically. The environment itself is another important influence, since some countries have more favourable conditions for generating energy from renewable sources, such as wind conditions for wind energy generation, high rainfall volumes for hydropower generation, or available land to be cultivated for the production of raw materials of animal and vegetable origin for the production of energy from biofuels.
Non-renewable energy sources
As a cheap and finite energy source, non-renewable energy sources, especially fossil fuels, have promoted the transformation of the human civilization from agriculture to industry (Smil, 2017). Non-renewable energy dominates global energy consumption (Opeyemi, 2021). Based on information published through the International Energy Agency (IEA), these sources constitute approximately 80% of the total global energy supply (International Energy Agency [IEA], 2016b; Mohammadi and Mehrpooya, 2018). Approximately 75% of the global energy spent can be attributed to fossil fuels: oil, coal and natural gas (Pan and Wang, 2021; Zhao et al., 2019), and will prevail as the most significant source of purveyance for the global economy for many decades in the future (Le et al., 2021).
The main sources of non-renewable energy are those derived from fossil fuels, and they cannot be replaced in nature because they were formed by the slow decomposition of dead animals and plants over thousands or millions of years (Bergmeier, 2003). Furthermore, because fossil fuel reserves are finite, these resources are expected to be depleted in the near future (Ervural et al., 2018; Kabak and Dağdeviren, 2014; Zhao et al., 2019).
Non-renewable energy reserves are limited and unevenly distributed among countries, making them vulnerable to emergencies such as wars, financial crises and technological failures. It is difficult to maintain a long-term energy supply, thereby hindering stable economic growth (Le et al., 2020; Yu et al., 2022). Previous research has extensively focused on the consumption of the main non-renewable energy sources (Apergis and Payne, 2009; Le et al., 2021); their consequences; and their adverse effects in the short, medium and long term on development (Destek and Sarkodie, 2020; Le and Quah, 2018; Wang et al., 2022a), economy (Azam et al., 2021; Chevallier et al., 2021; Christoforidis et al., 2021), politics (Guan and An, 2017; Gupta et al., 2021), culture (Gyamfi et al., 2021), society (Wang et al., 2022b) and the environment (Bian et al., 2021; Martins et al., 2021; Sonnberger et al., 2021).
There are two main challenges associated with the use of non-renewable energy. First, these sources are finite, and thus, are being increasingly depleted and can no longer be utilized in the near future (Le and Park, 2021; Le et al., 2021; Mohammadi and Mehrpooya, 2018). Second, the combustion of non-renewable energy causes many environmental problems, for example, the depletion of the ozone layer, acid rain, global warming (Mohammadi and Mehrpooya, 2018; Vivek et al., 2021), severe pollution and inadvertent, widespread, and irreversible environmental destruction (Le et al., 2021).
In addition, serious environmental pollution can occur through the emissions of greenhouse gases (GHGs) (Lee and Chang, 2018; Özkale et al., 2017; Pan and Wang, 2021), such as carbon dioxide (CO2), which contribute to global environmental changes. Other pollutants can contaminate the environment with adverse effect on our quality of life (Bilgen et al., 2008; Fontoura et al., 2015; Omer, 2008). Thus, the burning of non-renewable energy is highly polluting, generating high costs, consequences for the environment, economic impacts (Opeyemi, 2021; Yu et al., 2022) and leading to a catastrophic scenario (Elie et al., 2021; Koengkan et al., 2021; Silva et al., 2021).
Notably, CO2 is the main contributor to GHG emissions (Eisenack et al., 2021; McGlade and Ekins, 2015) and one of the main causes of global warming (Intergovernmental Panel on Climate Change [IPCC], 2018; Vivek et al., 2021); therefore, it is a potential threat to long-term sustainability (Salari et al., 2021). Historical data series and statistics reveal a strong correlation between GHG emissions and an increase in the average global atmospheric temperature (IPCC, 2018; Montoya et al., 2021; Salari et al., 2021). CO2 emissions can only be reduced if a substantial amount of fossil fuel reserves remain unutilized (Eisenack et al., 2021). This transition to a low-carbon trajectory has several challenges, including a substantial and sustainable reduction in GHG emissions through the development of renewable energy sources, improved energy efficiency and the addition of energy conservation.
Renewable energy sources
Energy security, global warming and increasing concerns about climate change have led to an increase in the use of renewable energy since 2000. The IEA estimates that the total capacity of the global renewable energy sources must increase by 50% between 2019 and 2024 (Chen et al., 2021; IEA, 2019) to effectively contribute to the mitigation of climate change.
Switching to renewable energy production has several advantages, such as promoting sustainable economic growth and creating long-term green jobs (Adekoya et al., 2021; Proenca and Fortes, 2020; Yu et al., 2022), reducing GHG emissions, improving environmental quality, increasing the energy supply, stabilizing international energy market prices, alleviating the impacts of environmental and energy conflicts on the economy (Abbas et al., 2020; Yu et al., 2022; Zhao et al., 2022), providing energy security, improving social and economic development, reducing impacts on human health and the environment (Yu, 2014), and promoting human development (Clò et al., 2013; Zhao et al., 2022).
Renewable energy development should contribute to financial development, endorse local socioeconomic expansion, improve environmental protection and complement global labours to limit global heating (Baulch et al., 2018; Le et al., 2021). Therefore, identifying and utilizing renewable energy sources is the most suitable advance to alleviate environmental pollution and the energy crisis. It is safe and contributes to an increase in domestic energy supply, diversified energy sources, low dependence on fossil energy and enhanced energy security (Le et al., 2019, 2021).
The main renewable energy sources and their forms of utilization have also been reported (Panwar et al., 2011). In addition, an ample amount of literature has inspected the result of the consumption of renewable energy sources on economic development (Chen et al., 2020, 2021; Koçak and Şarkgüneşi, 2017), including the determinants of such consumption (Baye et al., 2021; Chen et al., 2021; Ponce et al., 2020).
Environmental conferences and world agreements
To achieve a sustainable environment, formulators of public energy policies must consider the environmental, economic and social aspects (Mohsin et al., 2020; Zhao et al., 2022). The placement of renewable energy sources has been related to policy choices aimed at dealing with climate change. Reducing CO2 emissions has become a share of the world political agenda and has resulted in anxiety for an increasing amount of countries (Silva et al., 2021). Consequently, numerous conferences have been conducted and various environmental agreements have been signed by several countries (Zhao et al., 2022). Figure 2 presents the milestones of the main environmental conferences along with their main highlights.
Landmarks and highlights of the main environmental conferences.
The figure shows that, importantly, the conversion of non-renewable to renewable energy sources to reduce GHG emissions is essential for many countries to fulfil their commitments that were made in conferences and international agreements. Renewable energy sources are important factors contributing to the promotion of sustainable development. Thus, their development forms a part of the 17 Sustainable Development Goals defined by the United Nations in 2016, in which 176 countries adopted renewable energy targets (Elie et al., 2021; Murdock et al., 2018). In addition, the Conference of the Parties (COP 21), held in Paris in 2015, proposed to limit the average increase in global atmospheric temperature to a maximum of 2°C by 2100.
Thus, promoting the usage of renewable energies sources has become a significant political strategy in several nations worldwide to mitigate climate change as well as to improve energy security and economic sustainability.
Research exploring global decarbonization strategies based on renewable energies sources are suggested. Therefore, research incentive policies aimed at achieving a low-carbon world and implementing actions to mitigate global warming and climate change have received increasing attention.
One of the most promising methods to attain the sustainability goals of international agreements is to use renewable energy sources to generate energy. Identifying and adopting new renewable energy sources is an important step in providing cheap, reliable, environmentally friendly and affordable energy globally.
Research justification and agricultural residues as a power source
The residue from a productive process in agriculture can be used in several ways, including the generation of energy. Recently, many studies have been conducted to utilize agricultural waste as an energy source. These studies have shown an important effect on the diversification of the global energy matrix. However, despite preliminary research, limited studies have investigated the technical, economic, environmental and social feasibility of this waste as an alternative energy source. In general, there are few studies on using agricultural residues for bioenergy production and these have several shortcomings.
In view of this, the hypothesis presented here is that it is extremely important and necessary to propose improvements and directions for future research to contribute to advances in relation to the use of agricultural residues for the purpose of generating bioenergy to advance in research. Previous studies always focus on the calorific value of these agricultural residues and research and discussions on how to actually use them in practice are not proposed.
Therefore, methods should be determined so that the current energy requirements can be met while also maintaining resources and opportunities of the next generations. With this objective, this review aims to deliver an overview of the present scenario on the use of agricultural residues for bioenergy generation. The objective was to identify the possible unexplored approaches with the potential to encourage the growth of new studies and investigations aimed at increasing the use of agricultural residues as a source for producing green energy.
Methodology for conducting the systematic review
A systematic literature review was performed in this study based on the study and model presented in Conforto et al. (2011), with some modifications and good practices adopted by different researchers (Biolchini et al., 2007; Dybå and Dingsøyr, 2008; Levy and Ellis, 2006). Figure 3 presents a step-by-step guide of the procedure and the entire process developed in the systematic literature review of this research.
Steps of the procedure of the systematic literature review realization.
Thus, through a systematic literature review, relevant studies were reviewed for their contributions to the energy matrix based on their use of agricultural residues as a source of renewable and sustainable energy to further develop the existing research. Figure 4 presents a flowsheet diagram with the results of the keyword searches.
Flowsheet diagram of search results by keywords.
From the information present in Figure 4, it is clear that a significant number of articles were analysed to guarantee the quality of the information presented here. In addition to Sections 1 and 2 already presented, this article presents the following: Section 3 presents the relationship of population growth and economic development and how they are driving rapid growth in energy demand; Section 4 presents a overview of the energy scenario regarding the world energy scenario headquarters; Section 5 discusses the potential of agricultural residues for the production of bioenergy and some of the main research already carried out. in the generation of bioenergy from agricultural residues for demonstrate the potential aspects of its use; Section 6 presents the challenges and future perspectives related to the central theme, which include the recommendations and research opportunities that contribute to the diversification of the energy matrix and reduce the dependence on non-renewable energy sources and also shows agricultural waste as a source of energy from the perspective of the circular economy; and, finally, Section 7 explains the main considerations and conclusions of the study.
Economic development
Rapid population growth and economic development are driving a rapid growth in energy demand. Currently, there are only 15–20 countries worldwide exporting energy, mostly oil and gas, while the rest depend on imports (Le and Park, 2021).
To ensure economic development and sustainability, it is imperative to reduce the dominance of non-renewable energy sources while increasing the contribution of renewable energy sources to the energy consumption profile of economies across the world (Opeyemi, 2021).
Renewable energy is clean and recyclable, which helps reduce the high environmental costs and economic losses caused by fossil energy consumption to achieve sustainable economic development. Despite the current low contribution in energy production and energy mix, renewable energy improves economic output and plays a positive role in reducing energy intensity owing to developments in renewable energy technologies, upgrading existing industrial infrastructure and improving productivity (Koçak and Şarkgüneşi, 2017).
Reducing emissions also represents a significant opportunity to drive sustainable economic development, for example by strengthening low-carbon energy sectors to generate jobs and add value (Chen et al., 2021).
The deployment and innovation of renewable energy is important in shaping the future economic system. Renewable energy development can lead to sustainable economic development and create green jobs that contribute to growth in economic output (Chen et al., 2020).
Energy is fundamental for economic development and the quality of human life. According to an extensive literature, sustainability is based on the conjunction of three pillars: the economy, society and the environment.
Energy scenario
Globally, the energy consumption structure is prevailed by fossil fuels, with non-renewable energy representing 84% of the total energy mix in 2019. To change these scenarios, it is essential to reformulate the portion of non-renewable energy and increase the participation of renewable energy in the energy matrix of each country. Figure 5 is a graphical representation of the global energy consumption through the main energy sources from 1970 to 2020, and the data are expressed in 1000 terawatt-hours per year (TWh/y) (British Petroleum – BP, 2020).
Representation of the global energy consumption via the main sources.
Non-renewable energy sources continue to dominate energy generation, and renewable energy sources, which can generate bioenergy, always make up the last percentage (BP, 2021).
In this period, renewable energy consumption (including biofuels but excluding hydropower) increased by 2.9 exajoules at an annual growth rate of 10%, which was below the 10-year historical average; however, the absolute increase in energy was approximately consistent with that in the last 4 years and was the highest for any fuel in 2020. China was the largest contributor to the growth of renewable energy sources (1.0 exajoule), followed by the United States of America (USA) with 0.4 exajoule; they were followed by Japan, the UK, India and Germany, all with 0.1 exajoule (BP, 2021).
Wind energy had the largest contribution to the growth of electricity generation from renewable energies sources (173 TWh), closely followed by solar energy (148 TWh). The renewable generation has continuously increased between 2011 and 2021, and in 2021, solar energy understood in 27%, of the renewable energy generation, a rise from approximately 3% (BP, 2021). Figure 6 is a graphical representation of the electricity generation by type of source in 2020, wherein the data are expressed as percentages.
Regional electricity generation by type of source.
In North America, the Commonwealth of Independent States, the Middle East, and Africa, the dominant fuel used for power generation is natural gas. More than half of the power in South and Central America is from hydropower (hydroelectricity). In Asia, coal represents 57% of the energy generation mix, which is the largest share among all the regions. Electricity generation in Europe is evenly distributed among renewable energy sources, nuclear, natural gas and hydropower (hydroelectricity) (BP, 2021).
Globally, coal is the main source of power generation, but its share fell to approximately 35% in 2020, the lowest level since 1988. The share of renewable energy reached its highest level in 2020 (approximately 12%), so renewable energy and natural gas (approximately 35%) were level with coal for the first time (BP, 2021).
In Europe, part of the renewable energy generation reached approximately 24%, exceeding that of nuclear energy and making it the first region with renewable energy sources as the dominant source of energy generation. In the global energy scenario, research involving some countries has been conducted, as shown in Table 1.
Examples of national energy scenario studies.
From the examples mentioned in Table 1, it is clear that these studies mention energy from biomass origin, but its exploration is still lacking, especially when it comes to energy from agricultural residues. There are some problems associated with the main sources of renewable energy, which are practically impossible to eliminate. Hydroelectric energy can be importantly pretentious by usual situations with, for example, with rain (seasonal and dry periods, as well as water scarcity). Solar energy is unstable and intermittent, and the excellence of solar energy is highly seasonal and climate dependent (solar radiation is reduced owing to precipitation). With the addition of wind penetration, the quality of energy supply from wind is seriously challenged owing to its characteristics (Hamann and Hug, 2016; Peng et al., 2021; Zhu et al., 2017). Owing to its intermittent, irrepressible and random nature, wind energy is problematic to predict and predicting it with precision decreases over time, which can lead to financial losses (Bayon et al., 2014; Ma and Liu, 2015; Peng et al., 2021) and an inadequate source of electrical energy throughout the years (Liu and Qiu, 2016; Peng et al., 2021; Wang and Xu, 2013).
Therefore, a few studies have been recently conducted that aimed at producing bioenergy from biomass, especially agricultural residues. The generation of energy based on agricultural residues is expected to become one of the main focus areas of future research on renewable energies sources in different countries worldwide. Large amounts of agricultural waste (biomass residues) are generated from agribusiness after the harvest of the main crops, such as stalks, straw, stalks, leaves, bark, ears, roots, chips, branches, pits and pie (Avcıoğlu et al., 2019).
Potential of agricultural residues for bioenergy production
Bioenergy is the energy obtained from biomass (Pan and Wang, 2021). It can be used in the production of fuels, electricity and heat, and is considered an alternative to fossil fuels, which dominate the global energy matrix (Opeyemi, 2021). The 2017 Brazilian Bioenergy Science and Technology Conference Opening Ceremony emphasized that, by 2050, bioenergy must account for up to 30% of all energy used worldwide. In 2019, this index was only approximately 9%, according to data from the IEA.
Biomass is an energy-generating material obtained from organic substances, that is, the energy generated by biomass originates from animal and plant sources that are available in nature (Büyüközkan and Güleryüz, 2017; Cebi et al., 2016; Pan and Wang, 2021).
Biomass energy has numerous advantages, including a reduction in GHG emissions owing to a decrease in the burning of fossil fuels (Cebi et al., 2016; Pan and Wang, 2021). Most existing studies have reported that the CO2 emitted from the combustion of biomass had been absorbed by the plants during photosynthesis. In contrast, combustion gases from fossil fuels accumulate in the atmosphere, blocking the exit of hot radiation into space and reflecting this heated radiation to the Earth, causing the greenhouse effect (Shahbaz et al., 2017; Solarin and Bello, 2019). Energy production from biomass is essential to achieve the existing energy and environmental goals (Suh, 2017). The consumption of energy from biomass can rouse financial development via creating new professions while also reducing the dependence of several countries on the import of fossil fuels (Al-Mulali et al., 2016), as well as many other advantages (Bandyopadhyay et al., 2011; Kijazi and Kant, 2011; Rovere et al., 2011).
The use of biomass as energy is dependent on available resources, the energy demand and technological evolution. Data and statistics from the IEA on the current amount of energy consumption versus the amount of agricultural waste generated revealed that the potential energy supply of developing countries from agricultural waste is approximately 95% from Congo, 83% from Tanzania, 58% from Guatemala, 31% from Brazil and 20% from India. Finland (30%), Denmark (28%) and Sweden (26%), all advanced nations, previously had energy matrices with a high proportion of bioenergy. Moreover, other developed countries have even lower biomass usage energy proportions, such as Germany (10%), France (7%) and the USA (5%) (IEA, 2016a; Pena-Vergara et al., 2022).
Several types of raw materials can be used for biomass production. Figure 7 presents a classification of the main biomass sources from three main agricultural sources (non-woody vegetables, woody vegetables and residues) and some examples of raw materials.
Classification of the main sources of biomass.
The most common biomass energy sources are sugarcane bagasse, which is the source of sugar and ethanol and reforested eucalyptus, which is the main source of cellulose and charcoal (Fontoura et al., 2015).
Biomass, as a raw material, is considered the primary source. However, products derived from biomass, such as biofuel, vegetable oil, and firewood and their respective by-products, are secondary sources. Agroindustry is the stage in which primary products are transformed into by-products. The term agribusiness corresponds to the combination of several productive activities that are directly linked to the production and sub-production of products derived from agriculture and livestock. Agriculture comprises the set of primary activities, which are directly associated with the cultivation of plants (of plant origin, i.e., agriculture) and the raising of animals (of animal origin, i.e., livestock) for human consumption or for the supply of raw materials (to be sent for transformation to secondary products) in the manufacture of clothing, medicines, biofuels and beauty products.
Agriculture involves techniques used to cultivate plants to obtain food, beverages, fibre, energy, raw material for clothing, buildings, medicines and tools, or just for aesthetics (landscaping).
Figure 8 presents the main products derived from biomass according to three classifications (biofuel, vegetable oil and firewood) and some examples of their by-products are shown.
Main products and their by-products derived from biomass.
This article focuses on the products derived from biomass of type biofuel, more precisely, solid biofuel (briquette—ecological firewood and pellet), as highlighted in Figure 8. These products derived from biomass undergo some type of transformation or conversion process, which are the known routes of energy conversion of biomass, before reaching their final state of consumption including thermochemical conversion (liquefaction, pyrolysis, gasification, direct combustion, etc.) or biochemical conversion (anaerobic digestion, fermentation, etc.). These may result in different types of energy (energy purposes) such as thermal energy (heat), electrical energy (electricity) and fuels, which will be used to generate mechanical, sound and light energy.
Notably, the profitable technologies are those of Carbon Capture and Storage. According to Maroušek and Gavurová (2022), the economics of producing energy-rich gases by the fermentation of phytomass are deteriorated by the costs associated with waste management of fermentation residues. Previously, no better solution was known than to plow up fermentation residues on arable land under the claim of irrigation with an effect of improving and fertilizing the soil.
However, farmers soon realized that organic matter from fermentation residues is of little agronomic value and nutrients are at agronomically insignificant levels. As the irrigation of fermentation residues has proven to be economically irrational in many countries, the practice of separating water from fermentation residues and using the solid fraction for energy purposes (as coal) has dominated (Maroušek and Gavurová, 2022).
For Maroušek and Trakal (2022), wood biochar shows excellent results in increasing the amount of water available to plants in the soil and seems to be an excellent tool to recycle nutrients (especially in forms of phosphorus and nitrogen available to plants), that is, it is a promising material for soil improvement.
As global climate change creates a wide range of factors that damage forest cover, wood biochar consequently represents untapped potential in the field of soil, nutrients and energy management (Maroušek and Trakal, 2022). Regarding biogas (Maroušek et al., 2013), based on the metabolism of the microorganism, the proportion of carbon in nitrogen has been recognized for decades as one of the main operational indices that determine the yield and stability of biotechnologies used in the production of biofuels, such as biogas.
Maroušek et al. (2013) analysed the use of waste from greenhouses on a commercial scale, and the results implied many conclusions about the ratios of carbon to nitrogen while also highlighting the role of carbon availability in the use of phytomass.
Waste from public green areas represents a large amount of grassy phytomass. Grass is typically utilized by composting, combustion or anaerobic fermentation. Maroušek (2013) proposed a commercial-scale investigation of a new method of anaerobic fermentation of two fractions of grass residues and showed that, owing to the faster use of energy, the two-fraction technology required smaller fermenters and, therefore, the technology is approximately cheaper by one-third than the cost for conventional systems.
Biomass power generation accounts for 25% of Brazil’s primary energy and if agricultural residues also are utilized as a source of energy, biomass power generation can be increased. Considerations about the potential for the usage of agricultural residues for the produce of bioenergy are not unique to Brazil. Other nations with comparable features to Brazil, such as Oceania, Asia and Africa, have similar potentials (Cambero and Sowlati, 2014; Pena-Vergara et al., 2022). Table 2 presents some examples of agricultural residues and their lower calorific value (LCV) for bioenergy generation (based on the energy research company, Empresa de Pesquisa Energética [EPE], 2014, in Portuguese).
Availability and calorific value of the main agricultural residues.
Source: EPE (2014).
LCV: lower calorific value.
Where the productivity index is expressed in ton of dry basis per ton of grain produced (tbs/t), that is, 1 ton of harvested soybean results in 2.3 tons of soy straw residue. According to research, the availability factor for collection is indicated and the percentage of the amount of agricultural residues to be removed from the field is recommended to keep the rest for soil nutrition and also to keep agricultural residues in the field as a benefit of promoting soil cover avoiding and reducing losses due to erosion. Moreover, the LCV was considered (expressed in gigajoule per ton, GJ/t), which is the amount of internal energy of a fuel but with water in the vapor state.
Compared with other renewable energy sources, for example, wind, solar, geothermal, and biomass from woody and non-woody vegetables, biomass and waste, mainly from agricultural residues, have received little research focus in terms of their potential and opportunities as a source of bioenergy production. Existing studies involving agricultural residues as a source for bioenergy production are shown in Table 3 in chronological order.
Examples of research on agricultural residues for bioenergy production.
NA: not applicable (no specific country or type of waste mentioned).
Based on the information in Table 3, it is clear how much the subject of the potential of agricultural residues for bioenergy generation has already been explored and demonstrates how much this subject needs to advance in the face of available gaps.
Some countries considered the viability of using forestry residues to produce energy (except agricultural residues). In Sweden (Jong et al., 2017; Pena-Vergara et al., 2022), a review paper was conducted to comprehend the influence of collecting unprocessed timber residues including bark, sawdust, shavings, felling, logs and small-diameter trees, for conversion to bioenergy; in Spain (López-Rodríguez et al., 2009; Pena-Vergara et al., 2022), the bioenergy potential of forest residues was evaluated; and in Brazil (Romero et al., 2019), the accessibility and energy use of forest residues from eucalyptus and sugarcane straw were determined to analyse the potential for production of bioelectricity and bioethanol.
The following are some key aspects of the main research listed in Table 3, indicating the possible bioenergy that could be generated via agricultural residues. In Pakistan, biomass from agricultural waste is a possible source of renewable energy that can contribute to a greener economy in the country; this indicates that biomass is the main source of energy conversion and is more suitable than solar, wind and geothermal energy (Zhao et al., 2022). In Bangladesh, 88% of the total national energy demand can be met by agricultural residues (Szulczyk et al., 2021).
In Greece, the energy potential of biomass from agricultural residues is promising. On a small scale, the thermochemical energy conversion route of gasification can be used to crop electricity and thermal energy (Alatzas et al., 2019). Bolivia demonstrates a vast energy potential to replace fossil fuels with biomass from agricultural residues in energy production. Several municipalities were identified as large producers of agricultural residues with energy potential for annual electricity production, providing energy to 58% of the population (Morato et al., 2019).
In Nigeria, the big bulk of accessible agricultural residues in Nasarawa State can be resourcefully used for bioenergy production when converted into briquettes. Among all the research analysed, this study (Kpalo and Zainuddin, 2020) was the most significant in terms of briquetting, which is a compaction technique that adapts waste with a small heating value per element capacity into high-density fuels and energy concentrates. In Mexico, the calculated availability of agricultural residues demonstrates that the country has great potential for bioenergy production (Molina-Guerrero et al., 2020). Iran also presented a high potential for energy (for electricity and heat production) obtained from agricultural residues by gasification technology (Samadi et al., 2020).
Pakistan produces large amounts of biomass from agricultural residues, which are left to rot in the fields or burned, and a considerable portion of the national energy demand can be met if these agricultural residues are properly managed and utilized by the energy sector and converted to bioenergy on a large scale (Abdullah et al., 2021).
Surveys by Rocha-Meneses et al. (2019) and Abdullah et al. (2021) were found to be the most comprehensive. Rocha-Meneses et al. (2019) ranked 294 countries worldwide with the greatest potential for everything, from agricultural grain waste to bioenergy production. Abdullah et al. (2021) examined Pakistan’s potential to use agricultural residues as a renewable energy source for bioenergy production to address energy-related challenges, which would also help address economic and environmental issues.
Other research by Szulczyk et al. (2021) in Malaysia also emphasized that the use of agricultural residues for bioenergy produce must be further explored in. In addition, studies have shown that this source can generate electricity for the USA (Jones, 2014; Solarin and Bello, 2019; Suh, 2016); however, further related studies are required.
Some studies have reported that, in terms of the diversification of the energy matrix, few authors have incorporated biomass in their analysis, especially from agricultural residues, because most authors have mainly focused on traditional fuels (bioethanol and biofuel), while the biomass from agricultural waste remains unexplored (Solarin and Bello, 2019; Stern, 2012). This clarifies the reputation of the contribution of the agricultural segment in the situation of bioenergy production through the reuse of agricultural residues (Lamas and Giacaglia, 2013).
Several studies have provided an imprecise estimation of the energy that can possibly be derived from agricultural residues, re-demonstrating the necessity for further research on this topic. The use of agricultural residues for bioenergy produce is an effective waste disposal system to evade environmental damage and provide renewable energy. Numerous papers have attempted to estimate the energy potential of biomass. However, they did not consider the features of diverse categories of agricultural residues in their studies because of the accessibility of limited information. Hence, the valuation of the potential energy generated from important agricultural residues is essential for the substitution of fossil energy to renewable energy.
Discussion
Challenges and future perspectives
To use agricultural residues for bioenergy production and to contribute to the diversification of the global energy matrix, certain aspects must be addressed. The following inspiring insights are presented with the aim of contributing to deliver solutions for society and industrial sectors through suggestions for future research. These are not necessarily directed only toward academia but are also for companies to develop and generate new technologies for the benefit of society.
1) It is essential to examine the development of bioenergy consumption from agricultural residues and its importance and contribution to reducing GHGs to mitigate global warming. Thus, an analysis of the contributions of the use of agricultural residues to bioenergy production is required to identify the contributions to the agreements signed at environmental conferences.
2) An assessment is necessary to determine whether the environmental policies are being formulated and, if required, identify methods for their formulation and implementation. This would promote the use of agricultural residues for bioenergy produce and contribute to the variation of the local and international energy matrix in the short, intermediate and long term. Moreover, it is essential to establish a correct structure of policy, regulations, subsidies, benefits and support so that all regions of all countries significantly attract investment opportunities. Therefore, a study should be organized that can act as a reference for policymakers and decision-makers in the development of energy policies, as all countries continue to strive to increase the participation of new energy sources in the energy matrix.
3) A survey should be conducted considering the main agricultural residues that can be used in each country and region to analyse whether this opportunity can be implemented for everyone or concentrated in some isolated areas.
4) It is necessary to analyse the possibilities of income generation and growth for small producers with the use of these agricultural residues to add value to their properties and rural activities, and studies should aim at promoting these practices.
5) Research estimating the amount of agricultural residue of a specific country, region or continent; relating the type of harvest with the harvest residues (stalks, straws, stalks or culms, leaves, bark, ears, roots, chips, twigs, lumps and pie) and investigating existing residues for their potential for bioenergy generation are indispensable.
6) It is essential to present the advantages and disadvantages of using agricultural residues for the production of bioenergy, as well as its impacts on the removal of these residues, and to carefully study the limitations in their percentage removal for each type of crop. This would ensure that the supply of nutrients and organic matter from these residues to fertilize soil (avoiding its degradation, erosion and reduction of soil moisture) is not compromised and does not impact the productivity of the succeeding crops.
7) The interaction of bioenergy production using agricultural residues on a small scale or family farming and its effects on the production of small farmers must be explored to verify whether it is advantageous to collect these agricultural residues for bioenergy production.
8) It is essential to identify the potential for transforming agricultural residues into pellets or briquettes to store bioenergy, which can be used at certain times of the year when an industry receives a lower amount of some type of raw material from seasonal biomass. These can be used for direct combustion to generate steam in its processes. In addition, these pellets or briquettes should be analysed for advantages in terms of their storage and transport because of their shapes and sizes.
9) Studying biomass from agricultural residues in the form of pellets or briquettes is crucial as they are used for different purposes including domestic (for heating fireplaces and in wood stoves in kitchens, which can be related to the use and consumption of liquefied petroleum gas), commercial (barbecues and wood ovens in restaurants and pizzerias) and industrial (steam boilers that are used in turbines to move equipment, heat products, and generate electricity; wood ovens, food industries, ceramic and brick factories, blast furnaces as an aid to charcoal) purposes. Biomass is also used in agriculture, in ovens for drying stored grains, temperature control in cold regions or even in aviaries for thermal control of chicken on farms, that is, as solid fuel (pellets or briquettes) for bioenergy production.
10) The calorific value of various agricultural residues should be analysed for bioenergy production, relating them to density and moisture. Moreover, each part of the residue (stalks, straws, stalks or culms, leaves, bark, ears, roots, chips, branches, seeds and pies) should be analysed to identify any specific parts with higher opportunities for bioenergy production.
11) It is necessary to conduct studies on the production process of pellets or briquettes from agricultural residues, ranging from the choice of suitable inputs to the analyses of the concluding product to verify its durability and burning characteristics, the calorific value of pellets or briquettes and the residual product after combustion to avoid environmental pollution.
12) A comparison should also be conducted between the two technological routes for obtaining pellets or briquettes from agricultural waste: pelletizing or briquetting–carbonization and carbonization–briquetting or pelletizing. In the first method, the biomass is compacted to obtain a briquette, which is then carbonized to produce a charcoal briquette. In the second method, the biomass was first carbonized and crushed to obtain charcoal, which is then briquetted. The two routes can be studied in terms of their results and yields, calorific value, ecological effects and suitability for small-, medium- and large-scale production.
13) It is necessary to investigate agricultural residues from crops with high potential for use as biomass in the production of bioenergy, such as rice husk, cashew nut husk, coconut husk, palm oil residues, buriti, babassu, andiroba, cocoa, barley, wheat, soy, coffee, potato, apple, onion, tomatoes, vegetables, dried fruits, beetroot, rye, oats, tobacco/smoke, sorghum, tea, millet, canola, beans, sunflower, crambe, radish, turnip, and carrot.
14) An analysis of the possibilities of bioenergy production from agricultural residues is required to mitigate the problems of electricity supply in isolated communities. Furthermore, it is of extreme relevance to analyse a sample of the amount of agricultural residue available at the national, regional or even global level and to determine the amount of electricity that could be produced with this waste, relating it to the capacity to supply a city with a certain number of inhabitants for a specific number of hours.
15) An investigation of the amount of planted area associated with the amount of dry matter obtained per hectare and the energy potential of this dry matter for each type of crop is necessary. This analysis can be compared for different countries in terms of their potential for agricultural production and, consequently, for the generation of these agricultural residues to identify the countries that have more potential in using this alternative. The findings may also be related to certain regions and climates and reveal the possibilities of some countries that could participate more in agricultural practices, to issue carbon credits, which is a current and promising market.
16) It is feasible to propose an environmental and economic model for the production of pellets or briquettes from agricultural residues for the production of bioenergy to verify the possibility of implementing a production system in rural areas (densifying agricultural residues in the field) to be stocked, marketed and distributed.
17) It is advantageous to conduct a study on the importance of pelletizing or briquetting a densification technology that converts a unit volume of low-calorie waste into high-density fuel and energy concentrates (additional energy efficacy because of an addition in the calorific value at the compaction pressure). Studies could demonstrate that pelletizing or briquetting can positively influence the physicochemical properties of waste, offering advantages such as addition of waste density, improving waste storage, preventing its possible scarcity in the off-season periods and improving the conditions for transporting pellets or briquettes, allowing the residues to arrive more easily at the power generation sites. The analysis should help determine whether pelletizing or briquetting is more suitable for handling these specific agricultural residues.
18) It is necessary to analyse the technical, economic, environmental and social feasibility of producing pellets or briquettes from agricultural waste directly in the field of rural properties, immediately after harvesting and present a viable production layout (detailing all necessary investments and equipment to perform the pelletizing or briquetting), for the entire production process, whether the output is for the producer’s consumption or for commercial purposes. Spot prices for pellets or briquettes and the net revenues that can be generated in a layout for small-, medium- and large-scale production should be presented, proposing different physical arrangements and layouts founded on the number of agricultural residues available in the place or region. An analysis of the return on investment, operational expenditure, capital expenditure, and variable costs for each proposed physical arrangement and proposed layout is suggested.
19) Presenting and proposing improvements in production processes in certain sectors and industries (private sector) is important because it aims to promote efforts to reduce their GHG emissions, highlights opportunities for adaptation or introduction of new innovative technologies and boosts the circular economy using agricultural residues to produce bioenergy in these industrial units. In addition, proper utilization of the agricultural waste will integrate agricultural sector with a circular economy.
20) A fundamental practical study should be conducted to propose a set of flowcharts as an optimization scheme that can assist in the choice-making process based on the type of agricultural residues (crop), available quantities of these residues and depending on whether it is a small-, medium- or large-scale production. The flowcharts should identify the best energy conversion routes that are the most appropriate for each type of agricultural waste in footings of their calorific value, the location and distribution of these final products (pellets or briquettes) to potential consumers, and optimization based on the supply chain and distribution logistics, as well as their most appropriate applications using the processes and equipment available. The best storage method can be proposed for these agricultural residues in natural, pellet or briquette form to reduce losses. A simulation of the best decision routes proposed for some studied scenarios can also be performed.
21) It is extremely relevant for businesses and the industry to identify the amount of energy that can be generated from these agricultural residues (annual crop production vs generated residues vs potential energy to be generated), and to propose a related key performance indicator on that basis.
22) A lifecycle analysis of pellets or briquettes produced from agricultural residues used for bioenergy production, as well as an analysis of the factors that affect their strength and durability, is suggested.
23) It is also necessary to analyse the ash content of agricultural waste, and the ash content generated from each of its parts during the conversion route through direct combustion (transformation of heat from chemical into thermal energy, that can later be transformed into mechanical and even electrical energy). Opportunities should also be identified for the final use of these ashes (for fertilizing vegetable gardens, in the cement industry, etc.) in addition to adding value to this generated by-product.
24) A study should be conducted on the transport, logistics and distribution chain of agricultural residues collected on rural properties, analysing the advantages and disadvantages of the biomass when it is baled, densified or loose. In addition, the final delivery strategies must be analysed in terms of their distances (the maximum distance from which it is more economically convenient to collect agricultural residues for bioenergy production).
25) It is extremely important to present the costs of collecting and transporting agricultural residues to analyse whether the distance and the harvesting system used (agricultural implements and machinery) influence costs, and whether they must be evaluated logically to prevent rotating a likely profitable business into a detrimental one.
26) Methods should be proposed to stimulate the development and improvement of agricultural implements, such as tools, machinery and equipment used during harvests, which can contribute to the development of technologies to collect these residues simultaneously during harvest and already crush and perform granulation, compaction (pressing), pelletizing or briquetting in location.
27) The potential and requirements for adding energy substances or binders to pellets or briquettes should be analysed, or a study should be conducted on composite pellets or briquettes, which are produced from more than one type of agricultural waste (combination and mixing among agricultural residues), which may be implemented in small properties, such as in a homestead or family farming, permaculture system, culture consortium or also in large properties that cultivate more than one type of culture simultaneously.
28) It is also crucial to perform an optimization of the calorific worth of pellet fuel or agricultural waste briquettes, to not damage the equipment in which they are to be used. Moreover, the relationship between the type of pellets or briquettes (according to the origin of the raw material and its calorific value) and identifying the ideal equipment for its use (according to the purpose of their use) should be explored.
29) Furthermore, as in any field of research, it is essential to propose a study presenting the limitations and possible disadvantages of using agricultural residues to produce bioenergy.
30) Notably, no study using biomass and agricultural residues from the roots was found; practical difficulties in their use were considered and there was a lack of information on their use for the production of bioenergy, which might be owing to the difficulties in collecting, accumulating and separating them from the soil. However, they are generally sufficient to contribute to the soil quality, thus ensuring the removal of other residues without interfering in the obtainability of nutrients in the ground; thus, this is another opportunity that can be explored in future studies.
To overcome the energy crisis, the capacity to generate energy from alternative sources must be developed to diversify the sources of energy consumption to the highest extent. Thus, studying and understanding the temporal evolution of energy sources and demands is essential to estimate and prepare for future scenarios to ensure the availability of sufficient energy for socioeconomic progress.
In general, the greatest challenges are related to the collection, transportation and distribution of these agricultural residues for use as raw materials in the production of pellets or briquettes. Thus, numerous solutions and opportunities are presented, as mentioned in the various directions of future research that have been cited in this section.
Conclusions have been drawn from the discussions on these recommendations and research opportunities, and the problems, proposed solutions, new ideas and innovative technologies that can contribute to the scientific community, the private sector, and policymakers and decision-makers have been presented. These conclusions can sustain the evolution to a low-carbon future and achieve the global emission targets as established in international conferences and agreements and, consequently, ensure economic and sustainable development in the short, intermediate and long term. Notably, all the listed opportunities can be studied individually or as a combination of the various gaps identified.
Agricultural residues as a source of energy from the perspective of the circular economy
The use of agricultural waste must follow the principles of the circular economy, with the aim of transforming the bottom line into active resources (Stahel, 2016). These resources have favourable attributes to be reincorporated into the economy, returning to the supply chain as raw material for bioenergy generation (Morales et al., 2016).
Agricultural production is one of the most important human socioeconomic activities, since it aims at the production of grains, food, fibre and bioenergy. More recently, agriculture has also started to provide various raw materials for the production of new bioproducts and bioinputs, in what is conventionally called the circular economy or bioeconomy.
This was only possible owing to the constant scientific–technological advances that allowed the development of new biomasses as well as new industrial processes for the use of dedicated crops and also of what was once called waste.
To obtain an idea of the dimension of such activity, recent data estimate that world agricultural production is approximately 7.26 gigatons (Gt) and that the volume of dry residues of plant biomass reaches the equivalent of 140 Gt. This huge amount of waste can become a serious environmental problem.
Fortunately, with adequate strategies and technologies, we can now remedy possible environmental liabilities that could arise from not using agricultural residues at the same time that we add and generate value from them.
Agricultural biomass (dedicated or residual) is currently used as raw material for the production of new products such as biofuels, bioenergy, biopolymers, biomaterials, chemicals, pharmaceuticals, cosmetics and hygiene products, in addition to agrochemicals such as biofertilizers and biopesticides, under the perspective of the circular economy, generating numerous socioeconomic and environmental benefits to our society.
Thus, this work presented a view of the production of current agricultural biomass and its residues, also addressing some opportunities for its sustainable use and the benefits generated by incorporating them into the modern circular economy. For example, producing briquettes and pellets from agricultural waste is a key point in boosting the circular economy.
This enhances the circular economy, with closed-loop strategies, in which agricultural waste is transformed into raw materials for the generation of steam, heat, and energy in industries, businesses and homes, as well as on the rural properties themselves, to generate energy to be used, for example, in irrigation systems and grain dryers.
In Brazil, the installations of large corn-based ethanol production plants, close to the grain producing regions, allow a closed cycle strategy, transforming agricultural residues into inputs for these large plants (as is the case with the use of rice husks, corn straw and cob, and cotton leaves) for combustion in boilers and the generation of thermal, mechanical and electrical energy.
Thus, the proper use of agricultural residues to generate bioenergy aiming to diversify the world’s energy matrix can encourage the valorization of these residues and, consequently, will provide an integrated alternative to enhance the circular economy in this sector.
Conclusion
A large amount of biomass from agricultural residues come from production and agribusiness activities and are partially left in the field after harvesting. Post-harvest removal of excess agricultural waste can increase farmers’ incomes, contribute to the addition of value, and provide inputs that can be worn for industrial and energy purposes. Moreover, using agricultural residues to produce bioenergy leads to a more sustainable end-use than simple disposal.
There are numerous major problems associated with the use of hydro, wind and solar energy sources (cited as unstable renewable energy sources). Thus, the usage of agricultural residues for bioenergy production can contribute to the diversification of the energy matrix, mainly aiming at increasing the part of renewable energy sources from biomass, which leads to a sustainable energy matrix.
Further studies are essential to explore the connection between the usage of agricultural residues for bioenergy production and renewable energy sources to contribute to an increasingly diversified energy matrix.
It is well known that the use of renewable energy, especially biomass from agricultural waste, has a positive impact on environmental sustainability. It reduces GHG emissions, environmental pollution and energy dependence on fossil fuels, while mitigating global climate change.
The main objectives of this study were achieved, as it presented an overview of the current energy scenario with the possibilities of conducting research involving the replacement of fossil fuels with renewable and sustainable energy and suggested numerous research opportunities for the potential usage of agricultural residues for the produce of bioenergy.
Furthermore, the challenges mentioned in Section 4 portray aspects that must be developed so that agricultural residues can become a representative renewable energy source in the global energy matrix.
Despite the substantial existing investigations on the topic of renewable energy sources, research involving agricultural residues for bioenergy production remains a promising topic with numerous future opportunities, as it includes several themes that have not yet been explored. Thus, the usage of agricultural residues to produce bioenergy is a broad and favourable scenario with potential for further exploration in the future.
Therefore, this research recommends the promotion of a circular economy from agricultural waste, for example, producing briquettes and pellets from this waste is a key point to boost the sustainable circular economy in the agriculture sector. That is, instead of keeping all these agricultural residues in the field, the indicated percentage can be withdrawn (according to the availability factor for collection), and for this, it is suggested to develop robust public policies to promote a circular economy based on pillars such as green growth, diversification of the energy matrix and economic development.
In view of this, it is recommended to promote a sustainable circular economy with a focus on ecological modernization that enables industries that produce briquettes and pellets from forest residues to engage in the sustainable manufacturing of these products from agricultural residues.
Finally, it can also be concluded that the hypothesis initially presented is confirmed, since from the systematic literature review, in which a total of 455 articles were analysed, it was possible to identify that research involving the use of agricultural residues for bioenergy generation has always been carried out in relation to the calorific value of these residues. Possible ways of how to collect these agricultural residues in the field; transport them to an industrial production unit of briquettes and pellets; and process them for use in industry, commerce and domestic purposes are lacking. That is, the current research is only the result of a laboratory-scale analysis.
Thus, as a way of contributing to these advances and with the aim to more precisely direct and identify the possible pathways that are currently almost unexplored that have real potential of using agricultural residues to generate bioenergy, a total of 30 potential solutions were presented.
Moreover, it is necessary to carry out a study involving a detailed drawing of the equipment (or adapter) that is used to collect agricultural residues in the field, as well as a detailed project (layout) of an industrial unit for the production of briquettes and pellets from the raw materials collected in the field.
Footnotes
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work had financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), rograma de Demanda Social (DS).
