Potentials and prospects of solid biowaste resources for biofuel production in Ethiopia: a systematic review of the evidence

Abstract

Biowaste is becoming a significant category in the global energy mix to mitigate the negative impacts of burning fossil fuels. The aim of this review paper was to investigate the potential, conversion mechanisms, benefits, and policy gaps related to the utilization of solid biowaste resources as renewable, clean, and affordable energy sources. Thus, a systematic review approach was employed to undertake a comprehensive analysis of the studies that dealt with solid biowaste resources for energy recovery. This review paper was conducted from November 2022 to June 2023. The relevant literature was searched using databases from scholarly journal publishers, online search engines, and websites. A total of 82 studies were determined to be eligible from 659 records. Ethiopia has a huge potential for biowaste resources, with an annual generation potential of 18,446.4 MJ per year. The multifaceted advantages associated with biowaste-to-energy conversion such as clean energy production, waste management, forest conservation, greenhouse gas emission reduction, and maintaining soil fertility using the digestate left after anaerobic digestion were mentioned. This review highlights various conversion technologies for converting solid biowastes into valuable forms of energy, such as thermochemical, biochemical, and physico-mechanical techniques. It also investigated the value-added products of the Solid Biowastes-to-Energy (SBWtoE) process, including bio-oil, syngas, bioethanol, biodiesel, biomethane, bio-briquettes, and pellets, with applications ranging from transportation to power generation. Furthermore, this review addresses the multifaceted challenges associated with implementing a circular economy, emphasizing the need to overcome policy, technological, financial, and institutional barriers. These efforts are crucial for harnessing the growing biowaste resources in Ethiopia, ultimately promoting sustainable and cost-effective energy production while advancing the nation's environmental objectives.

1 Introduction

The world’s energy demand is rising dramatically, and fossil fuels account for about 88% of the total energy demand [1, 2]. This is a cause for the accumulation of GHG pollutants, particularly CO₂, which is the principal cause of climate change despite having a lower global warming potential as compared to N₂O and CH₄. However, [3] claimed that by cutting CO₂ emissions by 70% by 2050, the weather and global warming might be stabilized. Thus, reducing human-induced CO₂ emissions is viewed as a crucial step in mitigating climate change and limiting the rise in the world average earth’s temperature to less than 2 °C [4, 5].

In actuality, the world's low-income nations were unable to take advantage of the cheap fossil fuel regime and the related contemporary energy services. A little over 45% of people without access to modern energy services reside in Sub-Saharan African nations, where traditional biomass accounts for more than 90% of all energy consumption [2]. In Ethiopia, the amount of energy generated from biomass accounted for 20 MWh per household per year [6]. Most rural households in this area cook foods using firewood, crop residues, tree leaves, and cow manure. Imported petroleum oil covers only 7% of the national energy demand, while electric energy generated from hydropower accounts for only 2% of the overall energy consumption. The country’s power system only benefits towns and villages that are accessible to a grid system. Most rural homes are not linked to the grid and are therefore unable to take advantage of the electrical distribution system [6]. According to the statistics published in 2011, 83% of the Ethiopian population residing in rural areas are not connected to the grid system [6]. It is less likely that they will be soon connected to the grid because of their sparse distribution and undulating landscape.

The continued use of fossil fuels and their detrimental effects on the environment has spurred research toward the creation of alternative fuels from bioresources. Thus, localizing energy production and distributing it to consumers can be accomplished through the diversity of energy sources [7]. Renewable energy sources must be used to lessen reliance on oil and reduce CO₂ levels to minimize the impact of climate change [8]. One of the new categories among the world's energy mixes is the conversion of biowaste resources to energy as a tactical way to reverse the negative effects of using fossil fuels.

A separate collection of waste is thought to be an effective way to improve waste management systems and harness clean energy from biowaste resources in both developed and developing countries. Biowaste is compostable garden and park waste, food and kitchen trash from homes, restaurants, caterers, and retail establishments, as well as equivalent waste from food processing facilities [9]. The sustainability of renewable urban energy systems will be increased by using urban biowaste rather than biomass directly derived from the primary output (crops and wood) [10]. According to a UNEP report, municipal solid waste (MSW) generated in Africa is valued at $8 billion USD per year, although these prospects are still mostly untapped [11]. In Ethiopia, the annual solid waste generated in 2015 was estimated to be 6 million tons [13]; it is predicted to rise to 10 million tons annually by 2030 and 18 million tons annually by 2050 [12]. Based on the other study undertaken by Hirpe et al. [13] Ethiopia will generate roughly 0.65 kg of MSW per person per day (19,690 tons per day, or 7.18 million tons annually) by 2025.

The growing need for clean energy has made biomass fuel an increasingly appealing alternative to fossil fuels in recent years [14]. The most popular methods of converting biowastes into usable forms of energy are the biochemical, thermochemical, and mechanical densification processes [15]. These processes use a variety of biowastes as feedstock, including sewage sludge, the organic fraction of MSW, and agricultural and livestock residues [16]. The popular biowaste conversion technique that can handle 90% of the moisture content is anaerobic digestion (AD) [17]. Besides, the alternative method for converting biomass into high energy is the densification of raw materials into fuel briquettes. This review paper extensively examines the byproducts of bioenergy conversion, including bio-oil, syngas (a combustible gas), bioethanol, biodiesel, biogas, bio-briquettes, and pellets, along with a comprehensive discussion of their respective applications. In Ethiopia, the focus was placed on other renewable energy sources that are more expensive than energy recovered from biowaste, despite the massive volume of biowaste resources available for biofuel production. On top of that, no well-compiled and comprehensive studies have been done so far showing the potential and prospect of biowastes at a national scale. Therefore, the current review sought to demonstrate Ethiopia's potential for recovering renewable energy from solid biowaste through thermos-chemical, biochemical, and physico-mechanical conversion mechanisms. Thus, this paper specifically addresses the following issues: (i) Reviewing the national potential of biowaste resources for biofuel energy production, ii) Assessing the major solid biowaste resources readily available for biofuel production, (iii) Exploring solid biowaste to energy conversion technologies. (iv) Reviewing the impact of deploying biowastes for biofuel in the existing energy mix, waste management, deforestation reduction, and GHG emissions, and (V) Analysing the policy gaps and prospects of utilizing solid biowaste resources and its challenge in the implementation of circular economy in Ethiopia.

1.1 Purpose of the review

Numerous empirical investigations have been undertaken to examine the Ethiopian energy infrastructure. Those studies have assessed the capacity, outlook, and sustainable utilization of renewable energy sources for fulfilling the country's energy demands. Among these energy sources, hydroelectric, wind, solar, geothermal, and biomass resources have been extensively scrutinized in terms of their potential for renewable energy production [6, 18]. Nevertheless, the exploration of the feasibility, obstacles, and potential for utilizing solid biowaste resources as a clean and sustainable energy source has thus far remained relatively overlooked.

Ethiopia finds itself in a pressing need to expand its energy infrastructure, spurred by the ongoing transformations in industry, urbanization, population growth, and evolving lifestyles. The frequent power outages experienced across residential, business, industrial, and institutional sectors stand as stark reminders of the energy poverty prevalent in the nation. Additionally, a lack of adequate facilities for recovering waste energy has led to the unregulated dumping of millions of tons of solid biowaste into the natural environment, exacerbating the country’s waste management challenges.

Fortunately, there is a positive aspect that a substantial portion of the waste generated within the nation is organic in nature, rendering it amenable to conversion into valuable energy through various biochemical, thermochemical, and physico-mechanical processes. Notably, municipal solid waste (MSW) and agricultural residues exhibit year-round availability and can serve as a dependable energy reservoir. Furthermore, MSW streams originating from urbanized areas are not only locally accessible but also possess high degradability, making them a promising source of bioenergy [19]. By establishing a linkage between the accessibility of urban solid biowaste and the local energy demand, Ethiopia can take significant strides toward achieving equilibrium within its electricity system. Additionally, the adoption of waste-to-energy conversion practices in developing nations like Ethiopia circumvents the contentious trade-off between food production and fuel resources, thereby offering a sustainable and multifaceted solution [19, 20].

Biomass stands as the primary energy source for heating and cooking in Ethiopia, yet the existing methods of biomass utilization have been exacting a toll on the environment. These practices have resulted in ecological consequences, including biodiversity loss, forest resource depletion, land degradation, and the emission of greenhouse gases. While most of the off-grid power generation is concentrated in rural regions; however, a substantial reservoir of biowaste resources resides in urban areas. Leveraging these urban resources presents a unique opportunity to allocate significant financial resources toward rural electrification efforts.

This review undertakes a comprehensive analysis of the solid biowaste resources generated within the burgeoning urban areas of Ethiopia. A notable gap in the existing literature has looked at the potential inherent in municipal solid waste as a source of clean energy. Previous research in the Ethiopian context has predominantly emphasized the utilization of biomass sources such as agricultural residues, byproducts from the sugar industry, and animal manure for biofuel production, with a primary focus on rural applications. In contrast, this review delves into the untapped potential, available technologies, associated benefits, and policy shortcomings concerning the deployment of abundant solid biowaste resources in urban regions of Ethiopia. The overarching goal is to facilitate the production of clean, dependable, and cost-effective biofuels, thus addressing the energy needs of urban areas while contributing to a more sustainable future.

1.2 The concept of systematic review

A systematic review is described as a review of a clearly specified question that employs systematic and explicit methods to find, select, and critically evaluate relevant research, and to gather, analyze, and synthesize data from the included studies [21]. It is an accurate and thorough review methodology that involves gathering, analyzing, and synthesizing all quantitative and/or qualitative data to produce a solid, empirically derived response to a specific research issue [22, 23]. Thus, the main goal of a systematic review is to provide a thorough account of all primary research that has been conducted to address a specific research issue.

1.3 Points of the review

In this review, six major issues were addressed; these include: -

• 1. The national potential of biowaste resources to produce biofuel energy.

• 2. The major biowaste resources, which are readily available for biofuel production.

• 3. The available biowaste-to-energy conversion technologies

• 4. The impact of deploying biowastes for biofuel in the existing energy mix, waste management, and reduction of deforestation and GHG emissions.

• 5. Challenges of waste-to-energy conversion in achieving the implementation of the circular economy.

• 6. The policy gaps and prospects of utilizing solid biowaste resources as renewable reliable and affordable sources of energy in Ethiopia.

1.4 Search strategy and data sources

It is essential to read the current review articles to evaluate the appropriate literature. When the review papers are analyzed, the work in biowaste and energy recovery is highlighted. This helps to identify any gaps and supports the need for additional review research. It was compiled and analyzed from empirical studies that evaluate the potential and prospects of using solid biowaste as a source of renewable energy to provide evidence for the use of biowaste as a resource for clean energy production, as well as the environmental effects (deforestation, biodiversity loss and greenhouse gas emissions) of biowaste utilization. To address the research questions, it was necessary to synthesize the potential, sustainability, and prospects of biowaste resources as medium- and small-scale renewable energy production in Ethiopia. This was done by reviewing government reports on energy, energy policy, and several empirical studies.

Relevant kinds of literature were searched using databases provided by scholarly journal publishers, online search engines, and academic and development organizations' websites. The search spanned from November 2022 to June 2023 and included scientific literature were accessed from research databases. Key e-journal publishers and platforms such as Scopus, Web of Science, Google scholar, and ResearchGate were utilized for this purpose.

In preparation for the current review, an exhaustive literature search was conducted, resulting in the identification of a total of 659 potentially eligible records. Subsequently, these records underwent a rigorous screening process. Initially, titles were scrutinized to identify and eliminate duplicates and unrelated items, resulting in the exclusion of 344 entries. In the subsequent phase, executive summaries, and abstracts of the remaining 315 records were meticulously reviewed and assessed against predefined inclusion and exclusion criteria. Consequently, 184 out of the 315 records were excluded from consideration as they did not align with the specific objectives of this review. The remaining 131 records underwent a thorough examination of their full texts, leading to the exclusion of an additional 49 records due to their lack of relevance, suboptimal quality, or unreliable data. In the final stage of the process, a carefully curated selection of 82 studies was made for inclusion in the systematic review. Figure 1 provides a visual representation of the data extraction and search process.

Fig. 1

Article selection process [21]

2 Biowaste resource potentials of Ethiopia

Following Nigeria, Ethiopia is the second-largest population in Africa. It contributes to 0.35% of the global Municipal Solid Waste (MSW) production while accommodating 1.43% of the world's population [24]. According to UN data from 2021, Ethiopia's total population stood at 117,876,227, with approximately 81% residing in rural areas. The nation's population is projected to surge to 232.99 million by 2060, nearly doubling the current figure [25]. This growth is expected to lead to a twofold increase in MSW production, from approximately 7 million tonnes in 2018 to a projected 13,431,250 tonnes by 2060, representing 0.35% of the global MSW output. This substantial volume of MSW necessitates careful management. Presently, Ethiopia primarily employs open dumping and landfilling as a disposal method for MSW, which has adverse environmental and financial consequences. Consequently, recycling and waste-to-energy solutions are unquestionably viable alternatives.

In 2012, cities worldwide collectively produced a staggering 1.3 billion tonnes of solid waste annually, and by 2025, it is projected that this figure will surge to 2.2 billion tonnes [26]. In Ethiopia, the daily per capita generation of waste varies from 0.28 to 0.83 kg [26], with reports indicating an annual production of 140 Gt of biowaste in the country [27]. Studies conducted in Ethiopian urban areas have revealed that households generate between 0.23 to 2.03 kg of solid waste per person daily, highlighting Ethiopia's significant contribution to solid waste generation compared to many other nations [28].

Table 1 below summarizes the volume of biowaste generated and energy consumed in Ethiopia in 2017, in comparison to five other African countries [12]. The data in Table 1 also underscores the connection between waste generation and energy consumption with population size. Countries with larger populations tend to generate more waste and consume higher levels of energy. Scholars have noted that an increase in population correlates with increased energy consumption due to the energy demands of various human activities [29, 30]. This trend is particularly prominent in emerging countries, and while a smaller population may lead to reduced energy consumption, it can also impact the economic growth of a given population.

Table 1 Estimated energy potentials from wastes of crop production, forest residues, animals, and MSW in selected African countries; the sum of the energy potentials is compared with the total energy consumed in each country

Biomass currently dominates Ethiopia’s primary energy supply, comprising a substantial 92%, with biomass waste resources contributing a modest 3% to this total [31]. Importantly, these primary energy sources are often used without undergoing any conversion process. In terms of energy utilization, domestic use within Ethiopia constitutes 38.27% of the total energy consumption. Industries and commercial activities consume 36.03% and 24.68% of the total energy output, respectively. Energy allocation for street lighting and other miscellaneous purposes remains relatively small, at 0.78% and 0.24%, respectively.

To effectively address waste management and harness sustainable renewable energy sources (excluding fossil fuels), converting Municipal Solid Waste (MSW) into energy aligns with the principles of the circular economy and emerges as a pivotal option. This approach has the potential to reduce waste volume significantly, ranging from 50 to 90%. Moreover, it yields a net energy potential of 0.13 to 0.38 tonnes of oil equivalent per ton [32].

2.1 Solid waste resource composition in Ethiopia

Assessing a country’s potential for generating renewable energy from biowaste resources hinges on understanding the composition and production rate of its waste. Biowaste sources encompass various origins, such as public green areas, households, gardens, supermarkets, food processing facilities, canteen leftovers, restaurants, hotels, business establishments (e.g., bakeries, distilleries, breweries, sugar refineries, meat processing plants), animal farms (including manure, liquid manure, and litter), domestic waste, and sewage sludge. These waste streams play a crucial role in harnessing renewable energy in the form of biofuels like biogas and bio-briquettes [33].

Waste composition in developing-world cities typically comprises 43% food and green waste, 8.6% plastics, 10% paper and cardboard, 5% metal, 3% glass, less than 1% wood, and 30% other waste [12, 34]. Remarkably, over 70% of Ethiopia's waste resources consist of organic materials [12, 34]. However, these resources are often inadequately managed, giving rise to numerous environmental concerns. Calculating the energy content of Municipal Solid Waste (MSW) in Ethiopia involves considering the composition and calorific value of MSW materials on a dry basis. Table 2 presents the energy content of MSW in kilowatt-hours per kilogram (kWh/kg).

Table 2 The composition and energy content of solid waste fraction [35]

2.2 Waste generation and composition in selected cities of Ethiopia

The increasing rate of population migration to urban areas in Ethiopia is contributing to higher levels of trash generation [25]. This, in turn, is a driving factor behind the escalating pace of energy consumption. It is important to note that the composition of Municipal Solid Waste (MSW) can vary from one city or town to another across different regions of Ethiopia. Table 3 show the most up-to-date data on the MSW composition of several cities in Ethiopia.

Table 3 Waste composition in various cities of Ethiopia [25, 34, 36]

Ethiopia generates between 0.26 and 0.56 kg of waste per inhabitant every day, with an average of 0.39 kg. Table 4 lists the major cities of Ethiopia's average per capita per day MSW generation rate. The cities are representative of the solid waste generation situations as it comprises the capital of the country and of the regional states. They are also more prominent in terms of population, commercial, and industries.

Table 4 Waste generation rate of major cities of Ethiopia

2.3 Status of municipal solid waste management (MSWM) in Ethiopia

Ethiopia is situated among the world's least developed nations, characterized by a substantial population, rapid urbanization, and a burgeoning economy. The country experiences notable rates of population growth, with the total population expanding at a rate of approximately 2.5%, while urban areas exhibit an even higher growth rate of 4.7% [42]. As previously elucidated in earlier sections of this review paper, the average daily per capita municipal solid waste (MSW) generation rate stands at 0.32 kg, with variations observed in different regions, ranging from 0.17 to 0.48 kg in urban zones and 0.11 to 0.35 kg in rural areas. Notably, these MSW streams are comprised of approximately 70% organic components, and the waste volume escalates annually at a rate of 5% [34].

The management of municipal solid waste (MSWM) stands as a pressing issue with far-reaching consequences, impacting the environment, human well-being, and economic activities [43]. In Ethiopia, the focus of MSWM efforts has predominantly revolved around waste collection at its source and its subsequent transportation to disposal sites. However, a substantial portion, estimated to be between 20 and 50% of the waste generated in numerous urban areas, remains uncollected at its point of origin [44]. This unattended waste can accumulate on the land’s surface and is susceptible to being carried away by runoff water, especially during rainy seasons. Consequently, this leads to the build-up of solid waste in water bodies and drainage systems, directly resulting in blocked drains and urban flooding.

Furthermore, inadequate MSWM practices have given rise to environmental issues, including groundwater, land, and air pollution, as well as public health concerns, such as respiratory ailments, growth impediments, skin disorders, and waterborne diarrhea problems [45]. These multifaceted challenges stem primarily from a range of factors, including deficient institutional capacity, limited financial resources, insufficient knowledge and awareness, a lack of comprehensive solid waste baseline data, minimal collaboration among stakeholders, feeble political commitment and prioritization, and an absence of effective planning and implementation measures [43].

A study conducted by Kaza et al. [46] underscores the direct influence of a country's financial capacity on global waste treatment practices. The data reveals that waste disposal methods vary significantly worldwide, with approximately 36.6% of municipal solid waste (MSW) being landfilled, 33% composted, 13.5% subjected to materials recovery via recycling, 11.1% incinerated, 5.5% openly dumped, and 0.3% treated through other means. Importantly, the choice of waste treatment exhibits substantial disparities based on a nation's income level (see Fig. 2). The high-income countries exhibit notably higher rates of material recycling, landfilling, and incineration in comparison to average or low-income countries. Moreover, high-income nations engage in minimal open waste dumping, accounting for approximately 2% of their waste disposal practices, while average or low-income countries can have up to 93% of their waste openly dumped [51].

Fig. 2

Waste treatment methods of countries based on their income

Regarding waste recycling and reuse, reports indicate that approximately 70–80% of municipal solid waste generated in Africa can be recycled, yet only a mere 4% is presently being recycled by the private sector [47]. In Ethiopia, urban solid waste recycling efforts are currently insufficient, with only around 5% of the generated solid waste being recycled [48]. Notably, the Koshe dumpsite in Addis Ababa houses the sole waste recycling plant commissioned in 2018, designed to incinerate up to 1400 tons of waste daily, equivalent to roughly 80% of the city’s waste. This facility aims to cater to 30% of the city’s household electricity demands, generating approximately 50 MW of electric power [13]. However, the plant faces operational challenges due to a lack of raw materials and financial resources.

In summary, Ethiopia lacks robust sustainable municipal solid waste management (MSWM) strategies, including waste prevention, reduction, reuse, recycling, and waste-to-energy practices. Therefore, it is imperative for the country to revise its existing MSW policy and strategies while bolstering its technical, institutional, and financial capabilities to attain sustainable MSWM.

3 Energy recovery pathways

Ethiopia has faced challenges in effectively harnessing the energy potential of solid biowastes, primarily due to constraints in funding and technology. Consequently, these biowastes have continued to accumulate over time. Historically, the predominant waste disposal methods in the country have been haphazard landfill disposal and unregulated combustion [52]. However, there is promising potential for utilizing these accumulated biowastes to generate energy and produce essential bioproducts by adopting various conversion technologies. The primary pathways for converting biowastes into valuable forms of energy include thermochemical, biochemical, and physico-mechanical conversion techniques [16]. The subsequent sections investigate the principal biowaste conversion processes and technologies.

3.1 Thermochemical conversion

The thermochemical conversion process is employed to transform biomasses at elevated temperatures and with a higher conversion rate. It is particularly effective when dealing with feedstock that has lower moisture content and often yields a broader spectrum of products [53]. This method is well-suited for wastes containing a substantial amount of organic, non-biodegradable components but minimal moisture. Notable thermochemical conversion techniques include pyrolysis, gasification, hydrothermal gasification, and incineration/combustion of biomasses [54].

3.1.1 Pyrolysis

Pyrolysis is a type of heat degradation that occurs between 400 and 900 °C in the absence of oxygen. When heated to a high temperature, the polymeric components of macromolecular structures are broken down into smaller molecules and produce hydrocarbons as final products. These final products include solid coke (char), pyrolysis oil (bio-oil or tar), and pyrolysis gas (syngas), which contains different concentrations of CO₂, CO, CH₄, H₂, C₂H_6, and C₂H₄. The ideal application for this technique is the synthesis of liquid fuel from organic wastes [49, 50], the diagrammatic representation of pyrolysis processes illustrated in Fig. 3.

Fig. 3

A schematic representation of MSW (Municipal Solid Waste) pyrolysis [52, 53]

To avoid some of the negative effects associated with incineration, pyrolysis of bio-waste has been suggested as an alternative to incineration. Biochar is often produced via the traditional pyrolysis process, which is an irreversible and slow breakdown of organic material (heating rate of 0.1–1 K/s, residence period of 450–550 s, and temperatures ranging between 550 and 950 K). The material is subjected to fast pyrolysis (heating rate of 10–200 K/s, residence time of 0.5–10 s, and temperatures between 850 and 1250 K) or flash pyrolysis (heating rate above 1000 K/s, residence time of 0.5 s, and temperatures between 1050 and 1300 K) to achieve high productivity of liquid products. The liquid may contain a range of substances, including acids, phenols, ketones, esters, alcohols, and aldehydes [51].

Pyrolysis reactors

In the context of the pyrolysis process, the choice of reactor where all the chemical reactions occur holds utmost significance [54]. In contemporary research, a diverse array of reactor types has been developed, primarily aimed at maximizing the production of bio-oil, the primary byproduct of pyrolysis. There are two main systems into which reactors can be classified: batch systems and continuous systems. In a batch system, biomass is processed in discrete batches, whereas in a continuous system, biomass flows continuously, allowing for the continuous collection of the resulting products [55]. Among the most prevalent pyrolytic reactors used today are fluidized bed reactors, which include both bubbling and circulating configurations. Additionally, there are fixed bed reactors, jet beds, rotary cylinders, cyclonic reactors, and rotary cones employed for this purpose. Furthermore, there exist other types of reactors, such as micro pyrolyzers and pyroprobes, which are widely utilized as small-scale reactors for the study of fundamental aspects of fast pyrolysis involving biomass and solid waste. It is worth noting that while these reactors serve a crucial role in research, they have not been commercialized for industrial applications yet (Fig. 4).

Fig. 4

Diagram of commercial gasification of MSW for integrated energy system [70]

Fixed bed reactor

A typical pyrolysis reactor for converting biomass into useful biofuels and valuable goods is the fixed-bed reactor. It is an effective pyrolysis system with a straightforward layout that can be utilized with biomass that has a range of uniform sizes and little particles. It is constructed of steel or firebrick reactors, each of which has a feeding unit, cooling system, ash cleaning unit, and gas exit (Fig. 5). Long residence durations for the biomass, good carbon conservation, less ash entrainment, and low sweeping gas velocity are some of the general operational characteristics of fixed bed reactors. They are typically employed in small-scale energy-producing systems. The removal of tar from fixed-bed reactors is difficult because it affects the product gas [56, 57].

Fig. 5

Schematic diagram of a typical fixed-bed pyrolysis reactor [56]

Fluidized bed reactor

Fluidized bed reactors are the most suitable and commonly used reactors due to their rapid heat transfer rates, enhanced velocity, increased surface area contact, and the ability to regulate the residence time of the vapor during the pyrolysis reaction. In a fluidized bed reactor, the biomass is brought into contact with sand particles through mixing, which are then heated to a higher temperature. This mixing of biomass and sand promotes improved heat and mass transfer within the reactor. The reactor bed is externally heated, and the heat can be transferred either directly or indirectly [57, 58]. There are three main configurations of fluidized bed reactors: the bubbling fluidized bed reactor, the entrained fluidized bed reactor, and the circulating fluidized bed reactor, as illustrated in Fig. 6.

Fig. 6

Bubbling fluidized bed reactor (a) and circulating fluidized bed reactor (b) (International Energy Agency (IEA) [59]

Rotary kiln reactor

The rotary kiln is a type of pyrolysis reactor that stands out for its exceptional productivity, particularly in its ability to efficiently heat the feedstock. This efficiency is achieved through the inclined ceramic-lined cylinder and slow rotation (as shown in Fig. 7), which promotes the effective mixing of Municipal Solid Waste (MSW). The rotary kiln reactor boasts a wide range of applications. However, it's important to note that it operates under a slow pyrolysis process, indicating a relatively low heating rate.

Fig. 7

Schematic diagram of rotary kiln pyrolysis reactor [61]

Nonetheless, the rotary kiln reactor has gained prominence in the field of MSW pyrolysis due to several distinct advantages, as highlighted in previous research [60]. These advantages encompass efficient mixing of MSW, adaptability in adjusting residence times, the capability to handle heterogeneous materials with ease, no prerequisite for MSW pre-treatment, and straightforward maintenance procedures.

3.1.2 Gasification

Gasification is a popular thermochemical method for treating biowaste, which converts solid biomass into liquid or gaseous biofuel by oxidizing it with a finite amount of oxygen, carbon dioxide, steam, or a combination of both [62]. Gasifier is another name for the gasification reactor [63, 64]. In the gasification process, the gasifying agents and organic feedstock are heated together to create syngas. In the process of gasification, organic wastes are transformed into synthesis gas, also known as syngas, at a high temperature (500–1800 °C) and with a controlled proportion of oxygen. Oxygen, carbon dioxide, and/or steam are used as gasification agents in a gasification process. Using steam has also helped to solve the problems with tar and char production.

Gasification is associated with less harmful emissions and better feedstock flexibility, in addition to selectively producing H₂ in comparison to other thermal conversion processes [65]. In comparison to combustion, gasification is more effective at converting biomass to biofuel. As a result, biofuel produced through the gasification process is a promising technology that offers greater energy-generating efficiency. The power output of biomass gasification plants can range from a few kilowatts to many megawatts [66]. Furthermore, depending on the technology employed and the feedstock, the gasification plant's efficiency can range from 70 to 80% [66].

The gasification agent applied during the gasification of biomass affects the syngas’ composition. The most common gasifying agents include carbon dioxide, oxygen, steam, air, a mixture of air and steam, and streams of varying ratios of carbon dioxide and air. The air is one of several gasifying agents, but due to its widespread availability and lack of a purchase price, it is often used [63]. Other biomass source materials including the dry organic component of MSW can also be converted into energy using gasifiers [67].

The application of gasification for MSW treatment presents several important benefits. Notably, maintaining controlled oxygen input into the reactor is crucial to minimize the production of dioxins in the exhaust gases, including nitrogen oxides (NOx) and sulfur oxides (SOx). The diagrammatic representation of the gasification process is presented in Fig. 4 for a visual illustration of the process. Compared to incineration and pyrolysis methods, gasification yields a higher average net energy of 36–63 kgoe/t MSW, with the possibility of further enhancement to 63–81 kgoe/t MSW when intensified with plasma [68]. Throughout the gasification process, an effective volume reduction of MSW, reaching up to 80–90%, can be achieved concurrently with syngas production [69]. This syngas proves valuable for electricity generation by integrating gas turbine or fuel cell modules.

Gasification reactors

The gasification process makes use of a variety of reactor types, these include fixed bed, fluidized bed, Entrained Flow, and plasma reactors; fixed-bed reactors are the most common type [71, 72]. Fixed bed gasifiers can also be divided into updraft (concurrent) and downdraft (counter-current) types. In an updraft gasifier, the gasifying agents like air or steam are delivered at the bottom of the grate, causing the resultant gas to rise upward as the biomass feed is introduced from the top and moves downhill as illustrated in Fig. 8. The product gas departs from the top of the gasifier at a lower temperature (about 500 °C), while combustion takes place at the hottest region of the gasifier, which is the bottom of the bed. Because of the lower departure temperature, the resultant gas frequently has high levels of tar. However, in a downdraft gasifier, both the input feed and the resulting gas flow downwards, and the gas produced exits from the bottom at a significantly higher temperature of around 800 °C. In this case, a substantial portion of the tars is consumed as the gas passes through a high-temperature region. However, it is necessary to recover heat from the high-temperature product gas to enhance energy efficiency. Historically, these two types of gasifiers, updraft, and downdraft, have been extensively used [73].

Fig. 8

Schematic diagram of the gasification process in a fixed bed reactor (a) downdraft, (b) updraft [74]

The feed is provided at the bottom of the fluidized bed gasifier, where it is fluidized with air, nitrogen, and/or steam before the product gas is released and flows upward (Fig. 9). This gasifier produces gas with higher particle levels [75]. Fluidizing the bed promotes heat transport to the biomass particle, increasing reaction rates and conversion efficiencies. Fluidized beds can also handle a wide range of fuel kinds and their characteristics.

Fig. 9

Schematic diagram of the gasification process in a fluidized bed reactor [74]

To produce high-quality and stable syngas that can deliver exceptional power generation performance, the selection of an appropriate gasifier is of paramount importance. This choice should be made based on the specific requirements of the feedstock in use and the downstream equipment. The key attributes of commonly used gasification reactors are summarized in Table 5 for reference and consideration.

Table 5 Characteristics of commercially available gasifiers for biomass and municipal solid waste conversion [76, 77]

Industrial application

In recent times, there has been a significant surge in interest regarding the utilization of pyrolysis and gasification processes for the conversion of waste into energy. These methods are more environmentally friendly and efficient when compared to direct incineration. Despite being relatively novel concepts, industrial applications of these technologies are now becoming more organized. According to reports, there are approximately 100 operational plants worldwide [78].

For instance, the world's inaugural gasification plant, located in Vaasa City, Finland, focuses on the utilization of solid waste resources known as SRF (Solid Recovered Fuel). This plant operates based on a circulating fluidized bed system and can produce both electricity (50 MW) and district heat (90 MW). Its commercial operations commenced in 2013, with an annual handling capacity of approximately 250,000 tons of SRF [79]. In Güssing, Austria, there exists the world's largest gasification plant designed for combined heat and power generation, utilizing steam gasification. This facility employs a dual fluidized bed gasifier and boasts an energy-generating capacity of 8 MW [80].

3.1.3 Incineration/combustion

The most popular thermo-chemical conversion technology is the incineration of bio-wastes-waste [81]. It entails a regulated combustion process in which the heat produced is frequently used to drive steam turbines that produce electricity and/or heat exchangers that provide either industrial or district heating. Utilizing incinerators, it is possible to reduce the volume of solid wastes to be disposed of by up to 80 to 85% by volume while also eradicating potential pathogens [82]. In addition to energy recovery, this technology enables to increase in the chance of reduction of over-reliance on fossil-based energy resources. The production of ash that is high in metals and the flue gas from incineration could be harmful if no measures are taken to reduce the harmful components like heavy metals, CO, acids, SO₂, dioxin, furan, and NO_x [83] As a result, certain nations do not fully support incineration technology.

The incineration method is favored for its high specific energy output while requiring a relatively small installation area for full operation [84]. A schematic diagram depicting a typical MSW-incineration plant is presented in Fig. 10, offering insights into its functioning. Initial capital investment and compliance expenses are anticipated to fall within the medium-to-high range, primarily due to the substantial costs associated with heavy machinery (e.g., furnace) and skilled labor [85]. Regarding energy yield, various factors, such as waste feedstock density, composition, moisture percentage, and inert compounds, play pivotal roles. Optimizing these parameters within a controlled combustion environment is essential for achieving maximum waste disposal and heat recovery. In comparison to other available alternatives, incineration proves to be a more economically appealing option, albeit requiring integrated measures for managing the co-generation of ashes, flue gas, dioxins, and acidic gases (NOx, SOx, and HCl) [86].

Fig. 10

Incineration process for MSW-based energy conversion [87]

3.1.4 Hydrothermal carbonization (HTC)

A thermochemical procedure called hydrothermal carbonization (HTC) is used to pre-treat biomass with high moisture content so that it will be used in a variety of applications. The biomass is heated under pressure (2–6 MPa) for 5–240 min while being submerged in water during HTC, which is carried out at a temperature range of 180–350 °C [88]. The primary output of HTC is a substance called hydrochar. By-products include liquid (aqueous soluble) and gas (mostly CO₂) [89]. HTC is a process that shows promise for utilizing biomass’s potential for cleaner manufacturing. However, more study is needed on the chemistry of the HTC, its kinetics, and heat transport, as well as the technical and financial aspects and the impact of operational factors and catalysts. The reactor is the most important part of a hypothetical industrial HTC plant. Most reactors used in the literature are batch reactors; however, an industrial facility requires a continuous reactor that can operate at high temperatures and pressures [90]. The high degree of feedstock flexibility offered by HTC is one of the main advantages. One possible large category of potential feedstocks for HTC is lignocellulosic biomass. Due to the structural variations of the feedstock materials, the HTC process will behave differently, producing various yields, energy, and porous qualities.

3.2 Biochemical conversion

Biochemical conversion encompasses the utilization of yeast and/or specialized bacteria yeast to convert biomass or waste into useful energy. The traditional process choices for producing various biofuels include anaerobic digestion, alcoholic fermentation, and transesterification methods [91].

3.2.1 Fermentation

Several bacteria convert carbohydrates like starch and sugar into ethanol through a process called fermentation. After the biomass is broken down, enzymes change the starch into sugars, and yeast subsequently turns the sugars into ethanol. The most frequent microorganisms utilised in the process are Saccharomyces cerevisiae, and the feedstock used for this kind of process is broken down into three categories: sugars, starches, and lignocellulosic substrates [92, 93]. Wastes that are high in carbohydrates could be used in the process to produce hydrogen. The primary process by which ethanol is fermented, known as glycolysis, converts one glucose molecule into two pyruvate molecules. Under anaerobic circumstances, pyruvate is further converted to ethanol and CO₂ [94].

Consequently, the overall reaction yields two molecules of ethanol and two molecules of CO₂ per glucose unit. However, the metabolic pathways have the potential to be redirected toward alternative products, as shown in Fig. 11. For instance, the pyruvic acid intermediate can undergo dehydrogenation to produce lactic acid, which serves as a fundamental component for the Cargilpolylactictate polymer [95].

Fig. 11

Conversion of glucose via pyruvic and acetaldehyde to ethanol in biomass fermentation [96]

A well-known fuel product, ethanol produced from first-generation crops like corn and wheat or from sugar feedstock, accounting for more than 13 billion US gallons of annual output worldwide in 2007 [97]. The rivalry to use agricultural food crops to produce fuel has recently aroused concerns that this is raising food prices, yet these raw materials are not sufficient to meet the growing demand for fuels. Indirect environmental problems including biodiversity loss, climate change, and the destruction of tropical rainforests to make way for additional cropland may also be brought on by the usage of food crops to produce fuel. As an alternative, lignocellulosic materials can also be utilized to produce ethanol, including special energy crops, municipal solid waste, wood, paper, and yard trash [98].

Ethiopia produces about 8 million liters of bioethanol per year using molasses as feedstock. Additionally, the nation wants to add 5% ethanol to its petrol supply. The local manufacturing and use of ethanol have already begun as a petrol (B5) mix that has since been improved too (B10). According to data from the Ministry of Water and Energy, the nation produced and used roughly 5.55 Tera Joule (TJ) and 4.8 TJ in the years 2008/09 and 2009/10, respectively. By blending about 38.2 million liters of bioethanol with petrol, the government has saved nearly $30.9 million on oil imports since 2008 [99]. Ethiopia's government has also established a detailed plan to increase bioethanol production from molasses biowaste generated from sugar factories.

3.2.2 Transesterification

Triglycerides and alcohol react during the transesterification process at a predetermined temperature and pace while being mixed with catalyst-producing fatty acids, such as biodiesel and crude glycerol [100]. It is made up of a series of reversible reactions that take place in order. Triglycerides progressively change into diglycerides, monoglycerides, and then glycerol and biodiesel. The primary alcohols employed in transesterification reactions are methanol, ethanol, or butanol [101]. It also refers to the process in which the organic group (alkyl) of alcohol is substituted with the organic group of a triglyceride, either without or with the aid of an enzyme, base, or acid catalyst as shown in Fig. 12. There are four different forms of transesterification reactions: enzymatically catalyzed, non-catalyzed, acid-catalyzed, and base-catalyzed [102]. Base-catalyzed transesterification is the most prevalent form because it has the lowest cost, the lowest corrosive potential, the highest yield, and the mildest reaction conditions [103].

Fig. 12

Shows the reaction of glyceride transesterification [101]

A useful by-product, glycerol can be utilized to make heat or as a raw material in the cosmetics sector, while the main product, biodiesel can potentially substitute petrol diesel. Biodiesel has distinctive qualities that are high in biodegradability, low in toxicity, and nearly non-existent in emissions.

Ethiopia has a significant amount of potential for biodiesel production and has prioritized the production of three feedstocks: castor beans, Jatropha curcas, and palm trees. Considering this, the nation has allotted adjacent land (23.3 million ha) for the development of feedstocks. However, a variety of criteria (such as the cost of feedstock, the cost of other fuels, the accessibility of enough land, etc.) determine whether biofuel can compete with other types of energy [104, 105].

The introduction of biodiesel into transportation fuel has not yet started, despite Ethiopia's recent announcement of the Climate Resilient Green Economy (CRGE) program, which calls for a 5% biodiesel blend in transportation petrol by 2030 [106]. With 25 projects already approved, private investment (both domestic and foreign) has helped Ethiopia's biodiesel industry grow [107]. However, just one castor bean production firm has started cultivating and exporting feedstock. This suggests that even though biodiesel projects have been approved since 2006, the nation currently lacks biodiesel manufacturing, apart from a castor bean cultivation plant that has started exporting castor beans.

3.2.3 Anaerobic digestion

The anaerobic digestion (AD) process converts complex organic waste into biogas and highly concentrated sludge (digestate) through hydrolysis, acidogenesis, acetogenesis, and methanogenesis [17]. This process is suitable for organic wastes with high biodegradable organic matter and moisture content. It enables the conversion of organic wastes into biofertilizers and high-methane-content (55–70%) gases, recovering nutrients and energy [108].

Biogas, a product of anaerobic digestion, consists of methane, carbon dioxide, hydrogen sulfide, and other trace compounds. The composition of biogas typically includes CH₄ (55–80%), CO₂ (20–45%), and H₂ (10–12%), with small quantities of H₂S and other contaminants [109]. Among these components, CH₄ and H₂ are the combustible elements of biogas, while other gases are considered worthless, poisonous, or hazardous [109].

The increasing adoption of renewable energy sources and growing environmental awareness have sparked interest in biogas technology [73]. In Ethiopia, where there is a significant livestock population, the promotion of biogas technology becomes a realistic solution. The use of animal waste as fuel has depleted soil fertility, as organic matter from animal manure is directly burned for fuel [110]. With approximately 35.4 million cattle in Ethiopia, there is potential for utilizing 10.6 to 14.2 million m³ of biogas for cooking and lighting in homes. Furthermore, the maximum amount of slurry that can be produced after generating usable biogas energy is estimated at 78,000 m³ [75].

The National Biogas Programme of Ethiopia (NBPE), established in 2008, has successfully built over 18,000 bio-digesters in two phases. The program involves various actors working to promote biogas technology within the country's developing biogas sector [111]. The Netherlands Development Organization (SNV) provides technical support in management, credit and finance methods, private sector development, and the utilization of bio-slurry for food security. The digesters used in the initiative have capacities of 4, 6, 8, and 10 m3 [111].

3.2.4 Feedstocks suitable for anaerobic digestion

The feedstock and co-substrate types could significantly affect the composition and yield of biogas. Even though fats degrade more slowly than carbohydrates and proteins, it is said to produce a larger yield of biogas [112]. However, to avoid process failures, biowastes that are high in cellulose, hemicellulose, and lignin need to be pre-treated. Pre-treatment procedures promote substrate degradation, increasing the efficiency of the process and the amount of biogas produced. Although the decomposition process can be accelerated by chemical, thermal, mechanical, or enzymatic methods, those methods do not always result in a higher biogas output [113]. For instance, per kilogram of waste, solid organic waste typically produces 0.36 to 0.53 m³ of methane [114].

The biochemical methane potential (BMP) of 54 samples of fruit and vegetable waste varied between 0.18 and 0.732 L/g VS and 0.19 to 0.4 L/g VS, respectively [115], while the biogas output and electricity production capacity of the chosen feedstock materials are listed in Table 6 [112].

Table 6 Comparison of biogas yield and electricity production potential of different substrates in European energy industries [112, 113]

According to the data shown in Table 6, substrates like fat and maize silage produced more biogas than other substrates while producing less electricity. This might be caused by variations in the content of the biogas they create and how well it is converted to power. Due to their high organic content and anaerobic digestibility, these substrates may produce more biogas, but they may also have higher levels of CO₂ or other inert gases.

3.2.5 Characteristics of selected substrates for anaerobic digestion

Food waste

Food waste (FW), which makes up a significant portion of MSW, has a high nutritional content, making it an appropriate feedstock for anaerobic digestion. Since 70–80% of FW is water, it is extremely biodegradable. Anaerobic digestion has been a widely used treatment method for it because of its high biodegradability and organic content [116, 117]. However, important factors influencing the AD process include temperature, volatile fatty acids (VFA), pH, ammonia, nutrition, and trace elements. A steady habitat, together with a suitable nutrient and trace element balance, is necessary for proper microbial growth. For the long-term operation of AD [118], it is crucial to maintain the right range of critical parameters.

The AD of FW can affect methane (CH₄) production and the process stability due to acid pH, the lack of bicarbonate alkalinity, the accumulation of volatile fatty acids (VFAs), and the deficiency of nutrients always an imbalance in the anaerobic digester: while, e.g., trace elements (Zn, Fe, Mo, etc.) are insufficient, macronutrients (Na, K, etc.) are excessive [84] and the C/N ratio of FW was outside of the optimum reported in the literature [119]. Furthermore, the lipid concentration in FW is always higher than the maximum concentration, which results in inhibition [119]. Anaerobic co-digestion (AcoD), which combines FW with other organic substrates such as cow manure, wastewater, sewage sludge, and green waste that have complementary properties, is one method used to address these inadequacies. Additionally, a scientific literature study found that pre-treated food waste that had been thermally and chemically yielded more methane than untreated food waste [120].

Fruit and vegetable waste (FVW)

An estimated 2.16 million tonnes of fruits, vegetables, and root crops are produced annually in Ethiopia. The country’s fruit, vegetable, root, and tuber crops are typically grown in all regions with varying densities over an area of 0.55 million hectares with a harvest of 60.78 tonnes [121]. According to Tesema (2010), Addis Ababa, the nation’s capital, produces 23.1 tonnes of garbage from fruits and vegetables per day. These numbers can rise as the horticultural sector expands year after year. Most organic wastes were discovered to be comprised of fresh fruits, vegetables, and salads.

Fruit and vegetable wastes (FVW) are well suited for energy recovery by anaerobic digestion because they are defined by a high percentage of moisture (> 80%), high organic content (volatile solids > 95% of total solids), and rapid biodegradation [122]. A review report by Gunaseelan [115] showed that anaerobic digestion of various kinds of fruit and vegetable waste resulted in high specific methane output as compared to other municipal solid wastes. Fruit and vegetable waste typically contains 75% sugars and hemicelluloses, 9% cellulose, and 5% lignin in its organic portion. However, in FVW, particularly in single-stage anaerobic digesters, the presence of simple cellulose in large quantities may lead to acidification and ultimately limit the synthesis of methane [123]. For sustained digestion, a proper carbon-to-nitrogen ratio must be also maintained in the range of 25 to 30 ratios.

Studies indicate that the co-digestion of these fruit wastes with other substrates can enhance process performance in the case of feedstocks with an unfavorable C/N ratio [124]. Vegetable waste could not be employed as the only substrate for anaerobic digestion without the addition of sufficient trace elements [125], After roughly three HRT, digesters stopped working because of a buildup of volatile fatty acids and a drop in their pH. Therefore, unless the digester's pH is maintained between 6.5 and 7.5, extra VFA generation causes the digestion to become acidic, which lowers the pH below 6. For effective methanogenic activities, the VFA/alkalinity ratios should be between 0.3 and 0.4 [126].

Animal manure

Ethiopia has a large number of dairy and beef cattle which has the potential to have a lot of extra manure that may be used in biogas facilities to provide renewable energy. The country has about 60.39 million cattle, 31.3 million sheep, 32.74 million goats, 11.32 million horses, and 1.42 million camels are thought to be in the country’s livestock population [127]. Ethiopia’s rural households have a cow population of over 77%, making them eligible for small-scale biogas production. Thus, animal manure has been used in Ethiopia’s biogas program to produce the fuel, with cow dung serving as the main supply of substrate for home biogas digesters. The ordinary farmer can create enough cow manure and produce enough biogas to cover their daily basic energy needs by keeping at least three heads of stall-fed cattle [128].

Animal manure is growing in popularity as a waste to produce renewable energy due to its ability to improve soil fertility and substitute industrial fertilizer [129]. Manure does have some drawbacks, though it has a poor C/N ratio, little volatile solids (VS), and a lot of components that are difficult to degrade, like recalcitrant lignocellulosic materials [130]. These restrictions result from the use of pasture wastes as animal feed, which contain considerable amounts of lignocellulosic components.

By making animal dung more susceptible to hydrolysis and subsequent anaerobic digestion, the physical, chemical, physicochemical, and biological treatments could boost enzyme accessibility and solve the issues mentioned above. Pre-treatments could increase the methane concentration by 74%. More specifically, chemical pre-treatments, thermal pre-treatments and physical pre-treatments done on livestock waste (cow, pig, and poultry manure) have enhanced methane production by 45%, 41%, and 30%, respectively [131]. Lignin, which forms protective barriers that hinder microbial activity and the growth of hydrolysis, is the primary barrier preventing increased methane production from cattle manure [131].

3.3 Physico-mechanical conversion

3.3.1 Biomass densification

Using binding chemicals or rarely mechanical techniques to compact biomass, biomass densification produces uniform briquettes or pellets with consistent shapes and sizes and bulk densities between 450 and 700 kg/m³ [132]. It is suitable for lignocellulosic biomass residues, which have a wide range of physical forms, significant dust content, a subpar heating value per volume, and a low bulk density [133]. Increasing bulk density also reduces storage and transportation expenses, improves handling, and makes it easier to manage. The primary direct-burning bio-briquette and pellet products generated utilizing such methods are those intended for briquettes [134].

Drying, grinding, pelletizing, briquette, cooling, screening, bagging, storage, and transportation are all processes in a typical biomass densification process [132]. Because it may be used in a variety of sizes, from simple, low-cost production to complex, high-throughput systems, biomass densification technology is practical [135].

Briquetting

Solid wastes can be compressed into a highly dense and long-lasting fuel through the efficient alternative of briquetting, which uses biomass as a fuel source. By compression into useful feedstock, it can be made with or without binders into various shapes or forms, such as square and cylindrical [136]. Because it is cleaner, devoid of incombustible, uniform in size, affordable, environmentally friendly, and has lower ash and moisture levels than coal and wood, it has an advantage over those fuels [132, 134].

Even though locally produced briquettes are a desirable energy source for individual consumers around the world, particularly in low- and middle-income environments, briquetting technology has not yet established itself in these nations due to technical barriers and a lack of knowledge about how to modify the technology to suit local conditions [137]. Studies of the biomass briquette industries and their perspectives in other countries, such as Nigeria, Kenya, Uganda, and Brazil, show that there is significant interest in this technology and the accompanying sector [138, 139].

Even though bio-briquettes are more efficient, smokeless, and renewable than other fuels, less effort has been made in Ethiopia to increase briquette manufacturing to the needed level. The lack of an effective institutional structure, poor planning, and a lack of focus on the utilization of municipal and agricultural solid organic wastes are few of the causes.

Pelletizing

Biomass that has been pelletized has better and more consistent qualities than biomass that has not been pelletized, such as low moisture content, high energy content, and uniform shape and size [140]. Compression of the feedstock (biomass) is how the pelletizing process is described. The final product, pellets, maintain their shape and density due to bonding that occurs between the particles at high pressure inside press channels [141]; this process is carried out by the pellet mill. Friction between the biomass and the press channel generates a force that results in the compression of the biomass. Ring or flat die pellet mills are typically used by large-scale companies, with ring die mills being the most prevalent.

There are chemicals used in the pelletizing process that help the particles clump together. Water, natural adhesives known as “extractives,” proteins, waxes, and lignin, for example, can be of natural origin and be found in biomass. Under high pressure, these compounds are ejected from within towards the walls, promoting the union and stabilizing of particles. Other binders, such as glycerol, can help glue particles together [142] and, depending on the biomass's energy content, can also increase the pellets’ energy density.

3.3.2 Feedstock materials of solid biofuel/briquetting and pelleting

Recently, more attention has been paid to second-generation fuels that use waste residues for the production of waste-based solid biofuels [143]. The moisture level of the waste should be as low as possible, often between 10 and 15% for the production briquette or pellet.

Two types of lignocellulosic biowastes that can be used as input materials for densification are crop residues and agro-industrial wastes. Crop wastes include things like paddy straw, bean straw, soy straw, maize straw, and wheat straw that are left over after crops have been harvested. On the other hand, agro-industrial waste is created when crops or logs are processed, and it includes rice, coffee, soybean husks, bagasse, sawdust, and other items used in the wood processing sector. Other lignocellulosic wastes have also been researched as potential feedstocks for briquette and pellet manufacture, including groundnut shells, mustard stalks, cotton stalks, coconut fibres, palm fruit, grass, and wastepaper [144].

4 Value-added products of the SBWtoE process and their applications

Bioenergy encompasses a diverse array of sustainable energy carriers derived from biowaste resources, each offering unique advantages and applications. These carriers include bio-oil, syngas/combustible gas, bioethanol, biodiesel, and biomethane. These biowaste-derived energy carriers play an important role in advancing clean and renewable energy solutions. Below, the details and applications of these bioenergy products are discussed.

4.1 Bio-oil

Bio-oil, the primary output of the pyrolysis process, is a dark, brownish-black liquid characterized by a smoky odor. Its elemental composition closely resembles that of the biomass source. This substance goes by various names, including pyrolysis oil, crude bio-oil, pyrolytic tar, or wood oil. Bio-oil is a complex mixture, containing oxygenated compounds and a significant water content, originating from the moisture present in the biomass and the chemical reactions involved [55]. Bio-oil can undergo refinement processes to make it suitable for use as a transportation fuel or as a chemical feedstock. It has applications in turbines, electric power generation engines, and boilers for heat generation. Currently, there is a growing demand for converting biomass into liquid fuels to replace conventional petrol and diesel in ships, trains, and aircraft [145]. Additionally, bio-oil can be co-fired with fossil fuels, enhancing efficiency, and reducing investment costs when compared to establishing entirely biomass-based power plants [146].

4.2 Syngas/combustible gas

Syngas is a product resulting from gasification and pyrolysis processes, comprising elements like hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO₂), nitrogen (N₂), methane (CH₄), ethene (C₂H₄), ethane (C₂H₆), and more. The specific composition of these gas components varies depending on the type of biomass utilized and the operational conditions employed during gasification and pyrolysis [57]. Syngas is a combustible gas that finds applications in combustion engines and gas turbines to generate power. It serves as an alternative to traditional cooking gas. Moreover, it plays a crucial role in the production of various high-volume chemicals, including ammonia, methanol, formaldehyde, OXO alcohols, and aldehydes. Syngas can also be used to synthesize hydrocarbons [147]. Additionally, the hydrogen fuel present in syngas has potential applications in heating and transportation processes, further enhancing its versatility and utility [50].

4.3 Bioethanol

Bioethanol, an alcohol produced through microbial fermentation, primarily utilizes carbohydrates found in sugar- or starch-rich plants such as corn, sugarcane, sweet sorghum, or lignocellulosic biomass [148]. It has emerged as the predominant biofuel for global transportation, positioned as a leading candidate to substitute a significant portion of liquid fuels derived from oil sources. One of bioethanol’s key advantages is its compatibility with existing vehicles, behaving much like conventional fuels. It can be blended with gasoline up to 10% without requiring engine modifications [149]. Furthermore, specially designed engines can run on 100% bioethanol. Recent developments highlight bioethanol as a promising raw material for the chemical industry, expanding its potential beyond transportation fuel [149]. Additionally, bioethanol boasts a high-octane rating, enabling higher engine compression ratios that enhance engine efficiency and performance [150].

4.4 Biodiesel

Biodiesel, derived mainly from renewable lipid feedstocks, stands out as an environmentally friendly and renewable energy source [151]. It has recently garnered substantial attention as one of the most promising alternatives to fossil fuels across various applications, including transportation and internal combustion engines. The beauty of biodiesel lies in its compatibility, as it can be seamlessly integrated without requiring extensive retrofitting. Furthermore, its adoption appears to enhance economic prospects and environmental preservation in rural areas [91, 152].Notably, biodiesel plays a significant role in reducing engine emissions. It contributes to substantial reductions in various pollutants, including unburned hydrocarbons (68%), particulates (40%), carbon monoxide (44%), sulfur oxide (100%), and polycyclic aromatic hydrocarbons (PAHs) (80–90%) [153].

4.5 Biomethane

Biomethane, a biofuel, is garnering increasing interest due to its reputation as a sustainable and environmentally friendly energy source. It is being explored as a potential substitute for natural gas and is generated through the “power to gas” production method. Under specific purity standards, biomethane can be injected into the gas network and can also serve as a resource for producing “green hydrogen” [154]. Moreover, biogas with up to 70% biomethane content can be used for domestic cooking since it burns in a controlled manner with adjustable air supply. However, for applications such as transportation and gas grid injection, a higher methane content is required [155]. On a global scale, the biomethane market was valued at USD 0.62 billion in 2017, and it is projected to reach a market size of $4.96 billion by 2026, with an annual growth rate of 26%. Several countries have set ambitious targets for biomethane adoption as a replacement for natural gas in household consumption [156].

4.6 Bio-briquette and pellet

Bio-briquettes serve as a versatile fuel source with a wide array of applications, including cooking, heating water, space heating, grilling, and even the generation of both heat and steam. These applications have diverse utility across various sectors, spanning domestic, commercial, and institutional environments, such as catering, hospitality, and food processing [157]. While their use is extensive for household heating and power generation in regions like Europe, America, and some parts of Asia, their adoption in certain developing countries, like those in Sub-Saharan Africa, has been notably limited [158]. Nevertheless, efforts have been undertaken as development interventions to substitute traditional fuels such as firewood, charcoal, or other solid fuels. This initiative has gained momentum due to the current scarcities and escalating prices of conventional fuel sources, compelling consumers to seek more affordable alternatives [159].

Pellets are also eco-friendly, high-quality, granular biomass materials that typically have spherical or cylindrical shapes and measure a few centimeters in size [160]. These fuel pellets find use in electricity and power generation, as well as for residential and district heating purposes [161]. Various types of fuel pellets, sourced from materials such as wood, agricultural residues, and bio-wastes, are produced through different processing methods, including torrefaction [161]. It's important to note that fuel pellets and their components are not hazardous since they primarily consist of organic matter and lack harmful chemicals. Moreover, they serve to manage biogenic waste in an environmentally friendly manner. Fuel pellets offer calorific values like wood, have low sulfur content and moisture levels, and result in minimal ash production [160].

4.6.1 Solid biowaste to energy (SBWtoE) for sound waste management

More than 1.6 million tonnes of organic solid waste and more than 715 million m³ of municipal wastewater are created daily [162] by people who live in cities. Only high-income nations collect and treat urban waste streams. More than 700 million urban dwellers still lack access to better sanitary facilities in low- and middle-income countries. Additionally, they depend on impure techniques for handling solid waste, such as burning and open dumping. As a result, a sizable amount of urban waste is dumped outside, which is detrimental to both ecosystems and human health. Therefore, in the future decades, technical development and investment will be necessary to address the numerous issues related to poor waste management and disposal.

Because organic waste streams contain resources like nutrients, energy, and water that can be recovered after treatment, resource recovery from them is essential. This can reduce the demand on limited resources, improve the security of the electrical supply in and around cities, and reduce the risks of diseases and environmental harm brought on by poor sanitary conditions and waste management [163]. Investments in enhanced sanitation services may be stimulated by the financial rewards from waste recovery [164]. Electricity recovery from trash can assist MSW have as little of a negative environmental impact as possible while also having the added benefit of offering a local source of electricity. The best estimate places the heating value of municipal waste between 12 EJ in 2010 and 20 EJ in 2025, with a range of 6 to 14 MJ/kg.

As in many other Sub-Saharan African countries, the Ethiopia’s waste disposal is characterized by uncontrolled dumping and open burning, with limited disposal in sanitary landfills, or the diversion of waste away from landfills towards reuse, recycling, and recovery [165]. Consequently, waste control is becoming the main public challenge; studies conducted in different parts of Ethiopia have shown poor waste control practices [166]. and many factors are associated with the waste control practices. Conventional ways of transportation, trash disposal in water bodies, irregular rubbish collection programmes, infrastructure and budgetary limitations, a shortage of qualified human resources, and unregulated landfills are the main causes [167].

The majority of the Ethiopia’s cities and towns, including the capital Addis Ababa, are densely inhabited. The total solid waste generation in rural and urban areas of Ethiopia is estimated from 0.6 to 1.8 and 2.2–7 million tons/year, respectively [34]. In 2018, Ethiopia built the Repi waste-to-energy plant in Addis Ababa at the Koshe dumpsite, which is anticipated to incinerate up to 1000 tons of waste per day (roughly 80% of Addis Ababa's waste). The incineration plant was designed to produce 25 MW of electricity that can cover 30% of the city's household electricity demand. The project was constructed at a cost of US$95 million [168]. However, due to an insufficient funding the factory is not functioning in full of its capacity. As a result, waste management in Ethiopia has become a serious issue, and technologies applied for waste reduction, reuse, recycling, and converting into useful products are at an infant stage Wilson & Rodic [169], claimed that sustainable MSW management strategy systems in low-income countries are weak due to budgetary difficulties.

5 Environmental implications of SBWtoE

5.1 Influence of SBWtoE on deforestation reduction

Most people in the world—more than 43% of them [170]—use firewood for cooking. To produce electricity, roughly 60% of the world’s forests have been removed. More than 64% of the world’s forest has reportedly been destroyed for human use in developing nations [171]. The most frequent source of energy in Ethiopia is fuelwood, which accounts for 78% of all energy use. This is followed by animal dung (12%) and agricultural residue (9%). In both rural and urban Ethiopia, fuelwood is utilised for cooking, heating, and lighting [171]. As a result of deforestation and a decrease in related ecosystem services, such as carbon emissions, air pollution that fuels climate change is produced [172].

In Ethiopia, the amount of wood and charcoal produced and used in the household sector far surpasses the amount used for other purposes. At a per-capita consumption of some 0.7 tons of fuel wood and charcoal, the aggregate amount of wood consumed annually is estimated to be 55 million tons [105]. Geissler et al. [173] reported that the total household consumption of charcoal and fuelwood is estimated to be 91.2 million tons/year. Approximately 14% of homes in rural areas and 0.4% of these in urban areas use biomass for illumination. According to [174], biomass consumed in Ethiopia per person and household ranged between 0.7–1.0 ton/year and 3.5–5.0 ton/year, respectively. Moreover, the annual consumption of residues and dung as an energy source is estimated to be between 19.3–20.7 million tons. Thus, the higher reliance on traditional biomasses has resulted in land degradation and massive deforestation [175, 176].

For instance, about 230,000 tons/year of charcoal is used for Ethiopia’s domestic purposes [177]. Following this, biogas technology is one of the promising solutions to the diverse the environmental problems associated with the use of traditional biomass fuels. According to Arthur et al. [178], the rampant exhaustion of wood-fuel supplies, predicted increase in wood-fuel demand in the future and the resulting social and environmental effects urge the need to look for alternative sources of cooking fuel in developing countries. Consequently, biogas technology has been identified as one of the promising options to reverse the problem of deforestation and related land degradation problems [179].

To reverse the challenges of deforestation a biogas technology is an option that has the potential to provide low-cost energy without the need to harvest wood [180] It is a renewable fuel with wide applications in the world [181]. Over 60.8 billion m³ of biogas are produced annually in the world [182]. The global biogas production has increased 3.7 times, i.e., from 0.28 EJ to 1.31 EJ in between 2000 to 2016. Almost 54% of biogas is produced in the Europe. Africa accounts for only 0.03% of the annual global biogas production [183], despite massive volume of biowastes for biogas production are reported.‬ According to the estimation of IEA [2], estimation, biogas produced using 1m³ digester is comparable to between 21 and 37.5 MJ of energy, 2.04 kWh of electricity and 5.5 kg of firewood.‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬‬

In 2008 about 98 household biogas digesters were installed in Ethiopia for demonstration purposes. The total biodigester installed in between 2009–2013 was reported to be 8063. The nation is currently carrying out its second phase, and the total number of installed biodigester plants across the country has reached 10,109. A study undertaken by [176] revealed that the total built biodigester across Ethiopia was 10,678, which can potentially save around 8732 tons of charcoal, 27,162 tons of fuel wood, and 5336 hectares. A total of 485 tons of fossil fuel and 66,463 tons of biomass were also replaced by the implemented plants. Also, the recent study conducted by Wassie & Adaramola [184], showed that the built biogas plant could save about 0.071 million tonnes of wood and 0.13 million tonnes of CO₂e emission per year.

5.2 Influence of SBWtoE on GHG emission reduction

Emissions of greenhouse gases (GHG) will increase from 150 to 400 Mt CO₂e (2010–2030) under a standard development path. As a result, renewable energy sources have attracted a lot of attention [185]. The production of methane-rich biogas from biowaste can greatly reduce greenhouse gas (GHG) emissions. Because it can lessen reliance on fossil fuels, which are a substantial source of GHG emissions, biogas has a larger potential to reduce GHG emissions [186]. According to Arthur et al. [178], biogas contributes to the international effort to combat climate change by lowering the usage of fossil fuels and lowering GHG emissions. Some have estimated home biogas plant GHG emission reductions per unit ranging from 1.3 t to 9.7 t of CO₂ equivalents (CO₂e) [119]. According to studies, households using biogas were able to lower their annual greenhouse gas (GHG) emissions by an average of 1.9 t of CO₂ equivalents per digester by switching from traditional biomass fuels and kerosene to biogas energy [187]. Anaerobic digestion and biomass densification of biowaste resources are therefore more environmentally friendly than open dumping, landfilling, and direct burning of agricultural waste and livestock manure.

5.3 Potentials of replacing the use of chemical fertilizer

Serious environmental issues have been brought about by the usage of artificial fertilizers in agriculture and direct burning of agricultural residue and animal manure, which severely degrades soil quality and heavily pollutes the air and water [188]. The agricultural yield is reduced in Ethiopia due to the lack of nutrients in the soil [189]. The agricultural wastes and sundried livestock manure is mostly used as an alternative energy source rather than retaining on the farm. Meanwhile, dung cakes have a low heat conversion efficiency (8%) [190], and inhibit the soil fertilisation that would have occurred if livestock manure would not be combusted [191]. As a result of these problems, an increasing number of scientific scholars have expressed their concerns about the replacement of chemical fertilizers with fertilizers made from renewable resources, which are more effective for agricultural output and better for the environment. Hence, the digestate left after anaerobic digestion is a high-grade organic fertilizer and thus increases productivity and reduces the need to expand croplands to forest areas [192].

6 Sustainability and prospects of SBWtoE

Recent research by Muscat et al. [193], indicates that biomass is a limited resource used for food, feed, and fuel. To avoid the argument between fuel and food/feed, it is crucial to create techniques and ideas on how to use biomass fractions like biowaste more effectively. Thus, more intensive use of these wastes as a biomass resource could help to reduce the cost of waste disposal [194]. Beyond the basic issue of safe disposal, sustainability initiatives and the circular economy, which sees it as a resource looking for appealing economic and environmental conversion paths, are receiving more attention.

The cost associated with urban biowaste generated from cities, towns, and business centres, as well as costs related to sorting these biowastes according to their type, is very costly [195]. It takes into consideration food waste (FW) residues from both residential and commercial sources, as well as municipal residues such sweepings and leftover garden trash that are frequently referred to as “green garbage” (GW). Using biowaste for bioenergy production could create job opportunities and minimize waste management costs [196]. This might motivate those who work in these fields to produce more, which would lead to more biowaste and, ultimately, better economic conditions. Utilizing biowaste as energy source will also improve environmental protection because it will prevent unrestrained burning and dumping.

The ability of a system to meet user wants while having no negative effects on the environment, natural resources, or the needs of future generations is how much a technology contributes to social, economic, and environmental balance [197]. Therefore, it is crucial to consider the systems' outputs, such as by-products, co-products, and emissions, throughout the whole life cycle of the technology, from development to unit disposal. The primary resources needed to produce the technology should not contribute to the depletion or scarcity of natural resources. There are no anticipated negative effects on the user or the environment from the emissions produced by the systems. Therefore, the need for efficient biowaste conversion is critical given that population growth increases energy consumption and biowaste pollution. Possessing efficient conversion technologies that deliver necessary energy at the lowest possible cost is a crucial concern. Technologies should be user-friendly and adaptable so that any household may use them. For sustainable energy and the environment, emphasis should be placed on the research and development of relevant technologies [198].

6.1 SBWtoE conversion and challenges of the circular economy implementation

A circular economy is defined as the closed-loop flow of materials and the efficient utilization of raw materials and energy across multiple stages [199]. It operates on a “spiral-loop system” that minimizes resource usage, energy flow, and environmental degradation without impeding economic growth or social and technological advancement. Researchers have recently placed significant emphasis on the circular economy due to its dual benefits of environmental preservation and societal well-being [200]. The transition from a linear economy to a circular one is imperative to preempt the escalating demands for natural resources that exert stress on the environment. Among the primary objectives of the circular economy is the reduction of natural resource consumption, minimization of waste generation, lowering greenhouse gas emissions and hazardous substance usage, and the transition to renewable and sustainable energy sources, thereby alleviating pressure on resource suppliers [201].

To establish a sustainable biobased economy, it is imperative to efficiently convert organic waste generated from both commercial and residential activities into energy and other valuable materials. This objective harmonizes with the fundamental principles of a circular economy, which advocates for the reuse and recycling of waste materials [202]. Nevertheless, the implementation of a circular economy confronts a multitude of challenges. As outlined by Henrysson and Nuur [203], prevailing studies consistently identify institutional and regulatory hurdles as the most significant obstacles, along with financial and technological impediments to the adoption of circular economy strategies. Notably, a study conducted in various European countries, including Belgium, Germany, the Netherlands, Portugal, Sweden, and the UK, has identified key barriers to circular economy implementation encompassing technological, cultural, market-related, and regulatory factors. Table 7 encapsulates the findings of an interview-based investigation conducted by Kirchherr et al. [204], featuring insights from businesses, academics, and policymakers as subjects of the study.

Table 7 Challenges/barriers of circular economy [204]

6.2 Technical challenges

In the pursuit of implementing a circular economy, the prevailing challenge, as highlighted in existing literature, is the presence of technical bottlenecks. Remarkably, 35% of relevant studies identify these bottlenecks as the foremost hurdle, surpassing other barrier categories [205]. It is worth noting that the potential benefits of embracing a circular economy are vast, contingent upon overcoming these technical obstacles [206].To shift from conventional practices, there is a necessity for the integration of new sustainable production and consumption technologies, particularly in the domains of eco-design, clean production, and life cycle assessment. Moreover, the management of these technologies requires skilled professionals. However, a persistent issue lies in the relatively low demand for environmentally friendly technologies, coupled with inadequate technical capabilities [207]. Additionally, the underinvestment in technologies geared toward circular product designs (eco-design) and operational processes, as well as the absence of advanced resource efficiency technologies, exacerbates the situation. These challenges are further compounded by the prevailing low pricing signals for raw materials [208], collectively posing significant barriers to the widespread adoption of circular economy approaches.

6.3 Economic challenges

Transitioning to a circular economy necessitates strong economic viability for companies engaged in circular practices. Nevertheless, the insufficiency of capital remains a significant but often overlooked obstacle in the path toward circularity. This insufficiency encompasses a lack of initial capital, limited financial prospects, and a dearth of alternatives to traditional bank funding or private investments [209]. This challenge persists due to current legal and financial frameworks that were established in a historical context heavily influenced by linear economic thinking. Consequently, embracing a circular economy demands substantial time and financial investments. Factors such as upfront expenses, indirect costs (in terms of time and human resources), and the projected payback period are of particular significance during this transition [209].

Scholars have pinpointed the market as a key impediment to advancing toward a circular economy, citing both low prices for virgin materials and the costs associated with circular practices. For instance, Mont et al. [210] argue that the “low prices of many virgin materials” hinder circular economy products from competing effectively with their linear counterparts. Similarly, some experts contend that the recycling of certain materials remains uneconomical compared to virgin material production [206]. Consequently, circular economy initiatives frequently require financial subsidies to ensure their economic viability [211].

6.4 Policy and regulatory challenges

In the realm of environmental investments, the absence of government support and effective legislation, encompassing funding opportunities, training, taxation policies, laws, and regulations, is widely acknowledged as a significant impediment [212]. According to de Jesus and Mendonça [205], regulatory hurdles emerge as the second most prominent barrier, cited in 23% of the reviewed literature. The absence of a comprehensive, cohesive, and stringent legal framework often hinders the successful integration of green solutions into developmental endeavors. An illustrative example is the lack of legislation pertaining to waste materials, which lacks clear definitions and classifications to distinguish waste from byproduct materials utilized in recycling processes. This deficiency results in constraints on cross-border waste transportation [208]. Additionally, the dearth of appropriate market signals, such as low prices for raw materials, fails to incentivize efficient resource utilization and the transition toward a circular economy [213].

6.5 Social challenges

Securing public acceptance of the principles underlying a circular economy is a pivotal prerequisite for successfully transitioning to such a model. Presently, awareness and a sense of urgency among consumers, producers, and logistics companies fall short of the necessary threshold to instigate widespread adoption of circular practices. A study conducted in Tianjin, China, a pilot city for circular economy initiatives, illustrates this by highlighting limited public awareness and a poor understanding of circular economy programs [214]. Similarly, research encompassing 3,000 European and 151 Chinese firms underscores a significant disparity between a firm’s awareness of circular systems and its actual implementation, influenced by various contextual and cultural factors [215].

A predominant challenge, both within society and among participating firms in circular economy implementation, is the lack of information regarding the benefits of circular practices and environmental regulations [216]. Addressing this challenge necessitates the dissemination of accurate information, knowledge sharing, and the promotion of best practices. For instance, the public often lacks an understanding of the value that waste can hold and remains insufficiently aware of the implications of the growing popularity of online shopping and home delivery for urban distribution. People tend to embrace circular behaviors when they perceive that significant individuals in their lives expect them to do so [217].

6.6 Policy gaps and future perspectives

Ethiopia has undertaken a proactive approach to ensure the realization of the Sustainable Development Goals (SDGs) by unveiling a series of policy and strategy documents. Among these, notable ones include the Biomass Energy Strategy, Ethiopia's National Energy Policy, and the Climate Resilient Green Economy Strategy (CRGE).

The Climate Resilient Green Economy Strategy (CRGE) stands out as an important instrument in Ethiopia’s commitment to sustainability. This strategy places a strong emphasis on programs designed to foster the sustainable management of forestry resources and reduce the demand for fuelwood. It achieves this through various means, including the distribution and adoption of fuel-efficient stoves and the promotion of alternative fuel cooking and baking methods such as liquefied petroleum gas (LPG), electric stoves, or biogas stoves. These initiatives not only address the pressing issue of forest management but also contribute significantly to the enhancement of carbon sequestration, thus aiding in the reduction deforestation. By prioritizing these programs, Ethiopia aligns itself with the broader global agenda of sustainable development and environmental preservation, as encapsulated in the SDGs [173].

Energy policy significantly shapes a nation's path toward efficient energy resource utilization for socio-economic growth. However, having an energy policy alone does not guarantee responsible resource management [218]. Ethiopia boasts abundant renewable energy potential, including hydroelectric, solar, biomass, geothermal, and wind sources. Biomass is predominantly utilized fuels in both urban and rural areas. Unfortunately, the country’s policy focus has not prioritized efficient conversion technologies, particularly overlooking the conversion and use of cost-effective solid biowaste as a sustainable energy source.

Ethiopia has yet to fully embrace the integration of solid biowaste into its energy portfolio. The challenges stem from a combination of factors, including the absence of suitable technology, insufficient funding, and inadequate policies. Furthermore, a primary concern is the dearth of capable organizations equipped with well-defined mandates and long-term action plans, contributing to a deficiency in institutional coordination. In addition, both the government and financial institutions have exhibited limited enthusiasm for supporting innovative endeavors or investing in and employing appropriate solid biowaste conversion technologies [198]. While a policy framework outlined by the Ethiopian Ministry of Water, Irrigation, and Energy (MoWIE) in 2013 addresses the development of advanced bio-energy conversion technologies, with a specific focus on utilizing agro-industrial waste for thermal and electric power applications, notable attention is primarily directed towards wood resources, sewage, and animal and poultry manures [105]. Unfortunately, a substantial portion of the solid biowaste resources generated within the rapidly expanding urban areas of the country remains overlooked in the policy formulation process.

To promote the utilization of solid biowastes in the energy sector, several key initiatives are imperative. These initiatives should encompass increased expenditure, heightened public awareness, and the adoption of state-of-the-art conversion technologies. To facilitate the integration of solid biowastes into the energy mix, it is crucial for the government to enact well-crafted policies that actively encourage their utilization. These policies should include amplified incentives, the phasing out of fossil fuel subsidies, the advancement of net-zero emissions strategies, and essential research and development efforts [12]. Furthermore, as part of a broader strategy for fostering scientific progress, governments should actively facilitate collaboration among academia, industry, and governmental entities. This can be achieved through the establishment of effective governance structures and the implementation of sustainable policies aimed at fostering cooperation among these diverse groups [219].

7 Conclusion

According to the empirical data examined in this research, Ethiopia produces a sizable amount of garbage from a variety of sources. Municipal solid waste (MSW) production in the nation was 7 million tonnes in 2018 and predicted to increase to 13,431,250 tonnes by 2060, or 0.35% of the world’s total MSW production. Effective methods are required for the proper management of this waste potential to reduce the harmful effects of waste products on the environment. By converting biowaste, one tries to produce material, get rid of leftovers, and recover energy. Many different conversion technologies have been created and are in use all around the world. The three primary biowaste conversion processes discussed in this work are thermochemical (pyrolysis, gasification, combustion, and hydrothermal carbonization), biochemical (fermentation, transesterification, and anaerobic digestion), and physico-mechanical (briquetting and pelletizing).

Anaerobic digestion has been proven to be a trustworthy and effective method for treating both solid and liquid organic wastes, according to published articles. Roughly, 1m³ of biogas produced by anaerobic digestion is comparable to between 21 and 37.5 MJ of energy, 2.04 kWh of electricity, and roughly 5.5 kg of firewood, according to the research. However, out of the 14,000 household biogas plants envisioned for the period during the NBP's first phase (2009–2013), Ethiopia only built 8063 (57.6%) biogas digesters.

Numerous practical investigations have confirmed the value of using biowaste resources to produce renewable energy, particularly through anaerobic digestion and mechanical densification. The management of waste materials that might otherwise be hazardous to human health and the environment is one of the many implications of energy recovery from biowaste resources. By lowering the use of fossil fuels, preventing deforestation, and minimising the deterioration of forest resources, it also helps the worldwide effort to combat climate change. The by-product or bio sludge of anaerobic digestion also plays a significant role in improving soil fertility, which raises agricultural production.

Finally, Ethiopia’s energy policy has highlighted the possibility of producing renewable energy from waste resources, but the government has not yet identified the sector-supporting mechanisms necessary to utilize the constantly growing biowaste resources in the country’s cities. To use the nation’s accumulated biowaste resource as a source of renewable energy, it should be essential to design practical policies that address technological, financial, and non-financial incentives. Governments should encourage collaboration between the relevant key players and the government itself as part of the system for technological advancement.