Bioenergy potential from crop residue biomass resources in Ethiopia

Abstract

Using crop statistics (2020–21), publicly accessible data, standard procedures and literature, this study estimates the bioenergy potential of crop biomass residues in all regions of Ethiopia. The assessment considered 44 different types of residues from 30 different crops grown in Ethiopia. The country generates 69 569–105 522 kt y⁻¹ gross crop residue biomass, of which 42 621–72 194 kt y⁻¹ (or 61–68% of gross) are estimated as recoverable for bioenergy production. Amongst all the eleven regions, Oromia produces the highest amount of recoverable crop residue (45%) at region level. Cereals produce the highest recoverable crop residue (80%), followed by fruit crops (8%). Maize (36%) and sorghum (29%) are the two crops that produce the highest recoverable residue amongst all the crops. The estimated 559–1144 PJ y⁻¹ bioenergy potential for Ethiopia from recoverable crop residue varies by region and ranges from 0,15 to 0,37 PJ y⁻¹ (Afar) to 254–521 PJ y⁻¹ (Oromia). Decentralized energy planning using crop residues in Ethiopian regions is expected to benefit from the produced data, which will help the country's overall growth in renewable energy. Biological and thermo-chemical conversion systems that are now in various stages of demonstration, commercialization, development, and research can convert biomass into energy. Performing research and development, establishing a database for local biomass resources, and developing a unified bioenergy unit and policy with all stakeholder's participation and engagement are all critical aspects of Ethiopia's sustainable bioenergy sector. Furthermore, value chain analysis of biomass feedstock, capacity building and awareness creation, and decentralized models development are all important for the country's bioenergy development.

1. Introduction

Biomass currently makes up around 10% of the primary energy used globally, making it a significant renewable energy source [1]. Biomass needs to be utilized more efficiently to meet the growing demand for access to modern energy sources [2]. Ethiopian energy supplies and consumption rates per capita is the lowest in the world, along with one of the lowest rates of access to modern energy services [3]. According to the World Energy Trilemma Index 2021 report [4], Ethiopia is ranked 88^th out of 101 countries in terms of its capacity to provide sustainable energy. In Ethiopia, approximately 56% of the total population does not have access to electricity [5]. To meet household energy needs, an estimated 95% of Ethiopia's population uses traditional biomass fuels such as wood, crop residues, dung, and charcoal [6]. The proportion of the population who has access to clean cooking fuels is only around 7.80% in 2020 [7]. Even by African standards, Ethiopia has one of the least diverse energy systems [8]. Accordingly, the country is one of the most biomass-dependent in the world. Ethiopian energy trends and characteristics show that a flexible, modern, reliable, and cost-effective energy system capable of meeting the country's rapidly increasing energy demand is limited in Ethiopia. This is corresponding to the rapid growth of population, urbanization, and industrialization [3]. It is detrimental to society's social, economic, and environmental well-being that so many people still rely heavily on conventional energy sources for lighting, heating and cooking [9]. Most households in the country frequently use three-stone fires for cooking as their primary energy source, which causes indoor air pollution and detrimental health effects. Over 50 000 people are killed by indoor air pollution each year, which contributes to nearly 5% of Ethiopia's disease burden [6]. Indoor air pollution is brought on by the harmful smoke that solid biomass fuels emit when they are burned. Additionally, it has a detrimental effect on agricultural households' capacity for production while arousing worries about significant CO₂ emissions, local deforestation, and land degradation [9].

Traditional fuels can be substituted by biomass if appropriate technology is used properly, positively contributing to rural development and alleviating energy poverty and emissions of greenhouse gas [10]. Renewable energy resources are abundant in Ethiopia. However, much of this abundant potential remains untapped. Energy poverty, inefficiency, and insecurity remain major concerns for the country's energy system [3]. Biomass resources generated during agricultural production (crop residue after harvesting of main crops) are good bioenergy sources and can significantly contribute to bioenergy production [11]. Several studies on the potential of biomass resources have been conducted in various countries worldwide [[12], [13], [14], [15], [16], [17]]. For the generation of gaseous and liquid transportation fuel, electricity, heat, etc, modern biomass utilization benefit from modern biomass conversion technologies (such as anaerobic digestion, fermentation, combustion, gasification, and pyrolysis) [18].

Modern and renewable biomass energy planning based on agricultural residue biomass is critical for Ethiopia because the agricultural sector is the country's primary source of biomass resources and bioenergy programs. Agriculture is considered as Ethiopian economy's backbone. Because of the country's agricultural strength, a large amount of agricultural residue is produced. However, due to the diversity of agro-climatic conditions and cropping practices in Ethiopia, the residue availability, distribution, and characteristics are very spatiotemporal in nature. Furthermore, agricultural residues' competing uses differ geographically. As a result, for decentralized bioenergy programs, local biomass databases are critical. However, both at the national and regional levels, Ethiopia's agricultural residue biomass potential estimate and database are limited. For local or region-based bioenergy planning, national estimates may not always be appropriate as most biomass-based bioenergy projects are small-scale and intended for decentralized use. With this knowledge, the current analysis for Ethiopia's bioenergy resources fills data gaps regarding data availability on the quantity of each available feedstock.

There hasn't been enough information reported on the potential availability of crop biomass residue resources in Ethiopian agricultural systems for the production of bioenergy. Therefore, the goal of this paper is to assess whether Ethiopia's agricultural systems' crop residues are adequate to generate sufficient amounts of modern energy and make bioenergy generation sustainable and viable. Quantifying the gross and recoverable crop residue biomass and bioenergy potential that is available for use in bioenergy production is the study's main objective. Overall, the study estimates the potential of crop residue biomass and then bioenergy potential in Ethiopia's regions. The assessment considered 44 different residue types generated by 30 different crops grown throughout Ethiopia.

The estimates in this study applied standard procedures and took into account a variety of global and regional bioenergy sources, as well as differences in local implementation. In order to develop renewable energy strategies and plans based on biomass residues it is critical to understand the potential of biomass sources. For determining the best sites for potential renewable biomass energy plants to be constructed, as well as the sustainability of energy sources knowing where crops are potentially farmed, as well as their residue types and qualities, and energy capacities are critical.

Furthermore, the study solves data gaps for bioenergy resources in Ethiopia, such as the availability and amount of each feedstock and its regional distribution. To establish a comparable baseline and quantify the bioenergy production potential from locally available biomass resources in other parts of the country and elsewhere, the established baseline for estimating biomass residues at the regional level can be used. It also serves as the foundation for future research into how (social, environmental, economic, and technical factors) this bioenergy potential might be realized, as well as providing guidelines for future research. Since they will be able to access, refer to, and use the data and recommendations for policy and strategy formation and other uses, bioenergy practitioners, analysts, academics, and policymakers will greatly benefit.

2. Methodology

2.1. Data sources and data preparation

Crop statistics from the Ethiopian Statistics Service (ESS) are a valuable source of information for agricultural residue biomass availability estimation [19]. For estimating crop production, cropping area and yield, the Central Statistical Agency (CSA) of Ethiopia has implemented a dependable and scientifically sound method. Crop statistics for the year 2020/21 was obtained from the Ethiopian Central Statistical Agency of ESS and used throughout the estimation, see Table 1.

Table 1.

Region wise total crop production of different crop types in Ethiopia in kiloton per year, kt.y⁻¹ [19].

Type of Crop	Cereals	Pulses	Oil Seeds	Vegetables	Root Crops	Fruit Crops	Coffee	Sugar Cane
Tigray	1860,53	27,61	87,67	11,01	2,93	1,41	–	–
Afar	6,93	0,00	0,01	–	–	–	–	–
Amhara	9607,47	529,55	215,27	144,45	400,98	27,06	4,37	66,77
Oromia	15242,15	603,15	191,62	201,17	1530,32	253,34	395,12	307,69
Somale	189,09	0,00	0,41	0,87	–	5,04	–	–
Benishangul-Gumuz	569,87	64,36	60,68	2,74	19,83	14,29	0,39	–
SNNP	2496,09	159,83	4,80	50,21	3062,51	676,71	144,25	505,68
Sidama	184,98	7,80	–	2,01	96,42	112,48	40,60	464,94
Gambela	16,31	0,02	0,11	–	–	–	–	–
Harari	19,98	0,00	1,95	–	1,42	5,46	0,03	–
Dire Dawa	12,02	0,00	0,14	0,01	–	–	0,03	–
National	30205,43	1392,33	562,65	412,49	5114,41	1095,78	584,79	1345,08

As was previously mentioned, a total of 44 residues from 30 different crops were considered for crops selection and classified into different categories listed. Cereals include teff, wheat, maize, sorghum, finger millet, oats/aja, and rice. Faba beans, lentils, and soybeans were included among the pulses. Linseed, groundnut, safflower, sesame, and rapeseed are all oil seeds. Lettuce, tomatoes, green peppers, and red peppers are among the vegetables. Potatoes, yam/boye, taro/godere, and sweet potatoes are root crops. Fruit crops include bananas, lemons, mangos, and oranges. Coffee and sugarcane are also considered as other crops. Table 2 shows the residue types (stalk, straw, husk, peelings, shell, pod, pruning, leaves, and so on) and their respective residue-to-product ratios (RPR), recoverable fraction (RF) and lower heating values (LHV) for all the crop residues considered. In relation to the studied area, the estimation of RPR, RF, and LHV should be as specific as possible. Nevertheless, these statistics are available rarely at the local and global levels. Consequently, to reflect more climatic and agricultural conditions this paper has covered numerous explored specific areas for crop residues in different countries around the World. As detailed below, a new approach was also defined and applied to estimate crop residue biomass energy potential in Ethiopia.

Table 2.

RPR, RF and lower heating values of crop residues.

Crop group	Crop type	Residue type	Ratio of Product Residue (RPR), Dimensionless		Recoverability Fraction (RF), Dimensionless		Lower Heating Value (LHV), MJ/kg
			Range	Average	Range	Average	Range	Average
Cereals	Teff	Straw	2,30	2,30 [20]	0,30	0,30 [20]	15,0	15,00 [20]
	Barley	Straws	1,20–1,30	1,26 [16,18,20,21]	0,28	0,28 [21]	13,6–18,2	15,88 [18,21]
	Wheat	Straw	1,20–1,50	1,30 [[14], [15], [16],18,21]	0,29	0,29 [14]	15,6–17,2	16,10 [14,15,18]
		Husks	0,23–0,30	0,27 [14,18]	0,29	0,29 [14]	12,9–17,4	15,15 [14,18]
	Maize	Stalk	1,15–2,00	1,72 [[13], [14], [15], [16],22,23]	0,80	0,80 [2,14,17,23]	12,6–16,7	14,66 [14,15,18]
		Husk	0,20–0,23	0,21 [2,14,15,22,23]	1,00	1,00 [2,14,23]	12,6–15,6	14,08 [14,15]
		Cob	0,27–0,57	0,40 [2,[14], [15], [16],22,23]	1,00	1,00 [2,14,17]	12,6–17,4	15,43 [14,15,18]
	Sorghum	Straw	1,25–2,00	1,75 [2,15,24]	0,80	0,80 [2]	12,4	12,38 [15]
		Stalk	1,40–4,75	2,85 [13,15,22,24]	0,80	0,80 [2]	15,0–17,0	16,00 [15,16]
	Finger millet	Straw	1,30–1,80	1,53 [20,25,26]	0,80	0,55 [2,20]	12,4–15,5	14,46 [13,15,16]
	Oats/Aja	Straw	1,00–2,00	1,40 [21,27]	0,28	0,28 [21]	13,7	13,70 [21]
	Rice	Straw	1,10–1,66	1,46 [2,[14], [15], [16],18,23]	0,25–0,72	0,49 [2,23]	15,5–16,0	15,78 [15,18]
		Husk	0,20–0,36	0,27 [2,15,16,18,23]	0,62–1,00	0,81 [2,23]	15,5–19,3	17,44 [15,18]
Pulses	Faba beans	Stems-leaves	1,45	1,45 [16]	0,15	0,15 [16]	12,4–18,5	15,19 [15,16]
	Lentils	Straw	1,67–1,80	1,74 [18,28]	0,20	0,20 [18]	14,7	14,65 [18]
	Soybeans	Straw	2,00–2,66	2,32 [[14], [15], [16],21]	0,60–0,80	0,70 [2,16]	12,4–17,2	15,16 [15,16]
		Pods	1	1,00 [14,15]	1,00	1,00 [2]	12,4	12,38 [15]
Oil Seeds	Linseed	Stalk	1,47	1,47 [18]	0,50	0,50 [18,28]	14,4	14,35 [18]
	Groundnut	Shells	0,35–0,48	0,40 [[14], [15], [16],18,22]	0,80–1,00	0,93 [2,14,16]	14,1–25	17,56 [[14], [15], [16],18]
		Straw	2,00–2,30	2,20 [[14], [15], [16],18]	0,80–1,00	0,93 [2,14,16]	14.4–17,6	15,59 [[14], [15], [16],18]
	Safflower	Stalk	3	3,00 [18]	0,83	0,83 [18]	13,9	13,90 [18]
	Sesame	Stalk	1,20–1,80	1,50 [16,18]	0,13	0,13 [18]	14,4	14,35 [16,18]
	Rapeseed	Straw	1,10–1,80	1,58 [16,18,21,27]	0,15–0.30	0,23 [16,21]	12.0–17,1	14,55 [16,21]
Vegetables	Lettuce	Waste	1,2	1,20 [21]	0,50	0,50 [21]	12,8	12,80 [21]
	Tomatoes	Stem	0,3	0,30 [24]	0,50	0,50 [21]	13,7	13,70 [21]
		Leaves	0,3	0,30 [24]	0,50	0,50 [21]	13,7	13,70 [21]
	Green Peppers	Residues	0,45	0,45 [24]	0,50	0,50 [21]	12,0	12,00 [21]
	Red Peppers	Residues	0,45	0,45 [24]	0,50	0,50 [21]	12,0	12,00 [21]
Root Crops	Potatoes	Peelings	0,75	0,75 [15]	1,00	1,00 [14]	10,6	10,61 [15]
		Leaves	0,76	0,76 [14]	0,80	0,80 [14]	16,0	16,00 [14]
	Yam/Boye	Peelings	0,20–0,25	0,23 [15,24]	1,00	1,00 [14]	10,6	10,61 [15,16]
		Straw	0,5	0,50 [2]	0,80	0,80 [2]	10,6	10,61 [15]
	Taro/Godere	Peelings	0,20–0,36	0,28 [15,24]	1,00	1,00 [2,14]	10,6	10,61 [15]
		Straw	0,5	0,50 [23]	0,80	0,80 [14]	10,6	10,61 [15]
	Sweet Potatoes	Leaves & peels	0,40–0,60	0,50 [14,15]	0,80	0,80 [2,14]	10,6	13,31 [[14], [15], [16]]
Fruit Crops	Bananas	Leaves	0,35	0,35 [15]	0,80	0,80 [16]	11,4	11,37 [15]
		Stem	5,6	5,60 [15]	0,80	0,80 [16]	11,7–13,1	12,38 [15,16]
		Peels	0,25	0,25 [15]	0,80	0,80 [16]	13,1–17,4	15,83 [15,16,18]
	Lemons	Prunings	0,28–0,30	0,29 [16,28]	0,80	0,80 [16]	17,6	17,60 [16]
	Mangoes	prunings	1,8	1,80 [16,24]	0,80	0,80 [16]	17,5	17,50 [16]
	Oranges	prunings	0,22–0,35	0,29 [16,24]	0,80	0,80 [16]	18,1	18,10 [16]
Others	Coffee	Husk	2,1	2,10 [13,24]	1,00	1,00 [2,14]	12,6–15,9	14,23 [13]
	Sugar Cane	Baggase	0,10–0,33	0,23 [2,14,18,25]	0,21–1,00	0,74 [2,14,29]	17,9–20,0	18,95 [14,18]
		Topes/Leaves	0,05–0,32	0,22 [14,18,25]	0,80–0,99	0,86 [2,14,29]	15,8–20,0	17,90 [14,18]

2.2. Estimation of crop residue potential

Crop residue is a byproduct of the crop production process during agricultural activities. Gross crop residue potential refers to the total amount of residue produced, whereas recoverable crop residue potential refers to the residue that is left over after competing uses (such as organic fertilizer, heating and cooking fuel, animal bedding, cattle feed, etc.). The recoverable or available fraction can be used for the production of bioenergy. Standard procedures are used to estimate the gross and recoverable potentials; these procedures are covered in more detail below [12,23]. The energy potential of the recoverable residue biomass resources was estimated in this study according to the steps illustrated in Fig. 1 [17].

Fig. 1.

Process flow diagram for determining the energy potential of crop residues.

2.2.1. Gross crop residue potential

Gross residue potential of a specific crop is determined by the area cultivated, crop produce, and RPR. Crop residue yields vary even more than crop yields from crop to crop, making them difficult to account for because they depend on plant type and variety, location, farming methods, weather patterns, and other variables [27]. In order to estimate residue, the values of RPR for various crops at the crop level found in the relevant literature were compiled and used, as shown in Table 2. Eq. (1) is used to calculate the potential of gross crop residue:

$$GCRP=\sum _{i=1}^t{CA}_{\left(i\right)}\mathrm{x}{CY}_{\left(i\right)}\mathrm{x}{PRP}_{\left(i\right)}$$ (1)

where GCRP is the potential of produced gross crop residue from “t” numbers of crops, tons (t); CY_(i) is the yield of the i^th crop, ton per hectare (t.ha⁻¹); CA_(i) is crop area under the i^th crop, hectare (ha); and RPR_(i) is the residue-to-product ratio of the i^th crop.

2.2.2. Recoverable residue biomass potential

This study made the assumption that not all crop residue biomass would be available and suitable (because of variations in nature, competitive uses and technical limitations) for the production of bioenergy. The quantity of field-based residue that can be reasonably collected is calculated using the crop residue biomass recoverability fraction (RF) [14,15]. The RF is the portion of residues that can realistically be used to generate bioenergy after some of it has been used elsewhere [2,14,30]. For each crop residue type, the minimum, average, and maximum RF value compiled from relevant literature was used to estimate the recoverable residue potential because there was a lack of data specifically for Ethiopia. This information is provided in (Table 2). Eq. (2) is used to calculate the recoverable residue potential:

$$RCRP=\sum _{i=1}^{t}GCR{P}_{\left(i\right)}\mathrm{x}R{F}_{\left(i\right)}$$ (2)

where, RCRP is the potential of generated recoverable crop residue from “t” number of crops, tons (t); GCRP_(i) is the generated gross crop residue potential from the i^th crop, tons (t); and RF_(i) is recoverability factor of the i^th crop.

2.2.3. Bioenergy potential estimation

The following expression Eq. (3) is used to calculate the bioenergy potential of crop residue biomass:

$${EP}_{\left(j\right)}=\sum _{i=1}^{t}RCR{P}_{\left(i,j\right)}\mathrm{x}{LHV}_{\left(i,j\right)}$$ (3)

where, EP_(j) is the potential of bioenergy from t crops at the j^th region, mega joule (MJ); RCRP_(i,j) is the recoverable crop residue of the i^th crop at the j^th region, t; LHV_(i,j) is lower heating value of i^th crop at the j^th region, MJ kg⁻¹. The lower heating values of the residues considered in this study were obtained from literature, as presented in Table 2.

3. Results and discussion

3.1. Crop residue biomass potential

The availability of crop residue biomass resources in all Ethiopian regions were investigated in this study. Fig. 2 shows that cereals, pulses, oilseeds, vegetables, root crops, fruit crops, coffee, and sugarcane were the most common crop categories cultivated in Ethiopia, but some farmers also grew chat, hops/gesho, enset, and other crops. Ethiopia's gross and recoverable residue biomass potential, as well as corresponding bioenergy potential, were estimated at the national and regional levels. Ethiopian crop production statistics were used to estimate crop residue biomass. The production data used for the estimation only includes crops harvested in the Meher (main) season (2020–2021). Crops utilized for grazing as well as those harvested for hay or green for food, feed, or silage are not included.

Fig. 2.

Ethiopia's crop production (kt.y⁻¹) by crop category during the year 2020/21.

3.1.1. Gross crop residue biomass potential

The primary crop residues in Ethiopia's agricultural system taken into account for this study during the 2020–21 production year were from the aforementioned 30 crops divided into eight crop groups. After harvesting and/or processing, crop residues from these crops that are applicable to bioenergy production include stalk, straw, cobs, husks, leaves, peels, pruning, and shells/pods. Based on crop production data and RPR, the gross residues in the farming system are estimated on annual basis.

Based on the 44 crop residues produced by the 30 crop types, Ethiopia's average total gross crop residue potential is estimated to be around 88 041 kt in 2020/21. The gross residue potential is determined by taking into account the minimum and maximum values of the same parameters. The results are 69 569 kt y⁻¹ and 105 522 kt y⁻¹, respectively. Cereals, pulses, and oilseed crops contribute an average of about 75 781 kt y⁻¹; vegetables, root crops, and fruit crops contribute 10 420 kt y⁻¹; and others (such as sugarcane and coffee) contribute 1840 kt y⁻¹. Cereal contributes the highest (about 72 264 kt y⁻¹) (82%) to crop group, followed by fruit crop (about 5855 kt y⁻¹) (7%). At the crop level, maize contributes the highest (28% gross residue), followed by sorghum (24%) and teff (14%). In both Oromia and Amhara regions, maize is a widely grown crop. Due to its importance as the second-most important food in the country after teff (an indigenous crop), maize accounts for a large portion of the residue. Table 3 shows region level gross residue potential from the studied crops.

Table 3.

Region wise gross or theoretical crop residue biomass potential in kt.y⁻¹ in Ethiopia.

Type of Crop		Tigray	Afar	Amhara	Oromia	Somale	B/Gumuz	SNNP	Sidama	Gambela	Harari	Dire Dawa
Cereals	Min.	3768	15	17705	27166	387	1030	4294	285	30	50	31
	Ave.	5394	23	23090	35460	615	1508	5605	385	47	85	54
	Max.	7140	31	28420	43157	836	1937	6700	455	61	122	79
Pulses	Min.	42	–	970	920	<1	191	233	11	<1	<1	<1
	Ave.	43	–	1014	930	<1	211	233	11	<1	<1	<1
	Max.	43	–	1059	940	<1	233	233	11	<1	<1	<1
Oil Seeds	Min.	109	<1	313	363	1	137	11	–	<1	5	<1
	Ave.	134	<1	372	399	1	152	12	–	<1	5	<1
	Max.	159	<1	426	424	1	163	12	–	<1	5	<1
Vegetables	Min.	6	–	66	95	1	1	23	1	–	–	<1
	Ave.	6	–	66	95	1	1	23	1	–	–	<1
	Max.	6	–	66	95	1	1	23	1	–	–	<1
Root Crops	Min.	4	–	599	1217	–	22	2134	49	–	1	–
	Ave.	4	–	600	1316	–	23	2371	57	–	1	–
	Max.	4	–	600	1416	–	23	2609	66	–	1	–
Fruit Crops	Min.	2	–	53	1219	3	47	3829	687	–	14	–
	Ave.	2	–	53	1220	3	47	3830	687	–	14	–
	Max.	2	–	53	1221	3	47	3830	687	–	14	–
Coffee, green	Min.	–	–	9	830	–	1	303	85	–	<1	<1
	Ave.	–	–	9	830	–	1	303	85	–	<1	<1
	Max.	–	–	9	830	–	1	303	85	–	<1	<1
Sugar Cane	Min.	–	–	10	46	–	–	76	70	–	–	–
	Ave.	–	–	30	140	–	–	230	212	–	–	–
	Max.	–	–	43	200	–	–	329	302	–	–	–

Between regions of Ethiopia, there are significant differences in the potential of crop residue. Afar has the lowest gross potential, between 15 and 31 kt y⁻¹, and Oromia has the highest, between 31 856 and 28 284 kt y⁻¹ (Fig. 3). Oromia is one of Ethiopia's most agriculturally advanced regions. Maize (34%) is the most widely grown crop in the region, followed by sorghum (21%), and teff (15%). More than 70% of the region's total gross residue production comes from these three crops. Amhara, which produces between 19 725 and 30 677 kt y⁻¹ gross residues, is another agriculturally advanced region of Ethiopia.

Fig. 3.

Region wise comparison of the calculated total gross residue biomass in Ethiopia.

3.1.2. Recoverable crop residue biomass potential

Table 4 shows the recoverable residue potential of the studied crops. The average national recoverable residue potential is approximately 57 744 kt y⁻¹ on an annual basis, indicating that 66% of the gross residue is available for recovery after accounting for the recoverable residue portions from the chosen crops. The recoverable residue potential results are approximately 42 621 kt y⁻¹ (61% of gross residue) and approximately 72 194 kt y⁻¹ (68% of gross residue), respectively, when the minimum and maximum values of the same parameters are taken into account. Cereals contribute the highest recoverable residue (46 004 kt y⁻¹), followed by fruit crops (4684 kt y⁻¹), root crops (3802 kt y⁻¹), and others (vegetables, coffee, pulses, and oilseeds) (3254 kt y⁻¹) on an average basis. Maize contributes the most recoverable residue (36%), followed by sorghum (29%), teff (7%) and wheat (5%). The majority of crops yield the most gross residue but little recoverable residue. The reason for this is that some residues (mostly husk and straw) have more competing uses than others, including animal feed, fuel for heating and cooking, and packing material. In Ethiopia, the potential for recovering residue from banana, coffee, and potatoes is also significant.

Table 4.

Region wise recoverable crop residue biomass potential in Ethiopia in kt.y⁻¹.

Type of Crop		Tigray	Afar	Amhara	Oromia	Somale	B/Gumuz	SNNP	Sidama	Gambela	Harari	Dire Dawa
Cereals	Min.	2275	11	9672	15830	318	743	2674	208	25	40	25
	Ave.	3619	17	14108	22429	503	1156	3731	290	38	68	43
	Max.	5079	23	18555	28497	684	1551	4617	349	50	98	64
Pulses	Min.	7	<1	364	182	<1	139	36	2	<1	<1	<1
	Ave.	7	<1	417	192	<1	166	36	2	<1	<1	<1
	Max.	8	<1	479	204	<1	197	36	2	<1	<1	<1
Oil Seeds	Min.	20	<1	117	242	1	105	8	–	<1	4	<1
	Ave.	24	<1	146	299	1	136	10	–	<1	5	<1
	Max.	28	<1	167	337	1	155	10	–	<1	5	<1
Vegetables	Min.	3	–	33	48	<1	1	11	<1	–	–	<1
	Ave.	3	–	33	48	<1	1	11	<1	–	–	<1
	Max.	3	–	33	48	<1	1	11	<1	–	–	<1
Root Crops	Min.	4	–	539	1055	–	19	1829	40	–	<1	–
	Ave.	4	–	539	1137	–	20	2054	47	–	1	–
	Max.	4	–	540	1219	–	20	2278	55	–	1	–
Fruit Crops	Min.	2	–	42	975	2	37	3063	549	–	11	–
	Ave.	2	–	42	976	2	37	3064	549	–	11	–
	Max.	2	–	43	977	3	37	3064	549	–	11	–
Coffee, green	Min.	–	–	9	830	–	1	303	85	–	<1	<1
	Ave.	–	–	9	830	–	1	303	85	–	<1	<1
	Max.	–	–	9	830	–	1	303	85	–	<1	<1
Sugar Cane	Min.	–	–	4	19	–	–	31	28	–	–	–
	Ave.	–	–	24	112	–	–	184	169	–	–	–
	Max.	–	–	43	199	–	–	326	300	–	–	–

The highest amount of recoverable residues is produced by Oromia (19 180–32 310 kt y⁻¹), followed by Amhara (10 780–19 869 kt y⁻¹) and SSNP (7955–10 647 kt y⁻¹), while Afar and Gambela produce the least amount of recoverable residues out of the 11 regions. Fig. 4 depicts the recoverable residue potential by crop group for all Ethiopian regions.

Fig. 4.

Region wise comparison of the calculated total recoverable residue biomass in Ethiopia.

3.2. Bioenergy potential of crop residue biomass

The country's recoverable residue has a bioenergy potential of 559–1144 PJ per year (PJ.y⁻¹). On an average basis, cereals account for 682 PJ y⁻¹, followed by fruit crops (60 PJ y⁻¹), root crops (46 PJ y⁻¹), coffee (17 PJ y⁻¹), pulses (12 PJ y⁻¹), oil seeds (10 PJ y⁻¹), sugarcane (9 PJ y⁻¹) and vegetables (1 PJ y⁻¹). Maize residues (308 PJ y⁻¹), sorghum residues (243 PJ y⁻¹), teff residue (57 PJ y⁻¹), and wheat residues (42 PJ y⁻¹) make the highest contribution at the crop level. From other crop groups bananas residues and coffee husk is also important in Ethiopia.

Regional differences in the annual bioenergy potential from recoverable residue range from 0.15 to 0.37 PJ y⁻¹ in Afar to 25 to 521 PJ y⁻¹ in Oromia (Table 5 and Fig. 5). Cropping pattern, cropped area and yield, and recoverable fraction of residue are some of the main factors affecting variation in recoverable residue potential and consequently related bioenergy potential across regions. Ethiopia's major recoverable residue potential regions are Oromia (382 PJ y⁻¹), Amhara (226 PJ y⁻¹), SNNP (126 PJ y⁻¹), Tigray (54 PJ y⁻¹), Benishangul-Gumuz (22 PJ y⁻¹), and Sidama (16 PJ y⁻¹) on average basis. Only, the first four regions (Oromia, Amhara, SNNP, and Tigray) account for 94% of Ethiopia's recoverable residue generation.

Table 5.

Region wise crop residue bioenergy potential in petajoules (PJ.y⁻¹) in Ethiopia.

Type of Crop		Tigray	Afar	Amhara	Oromia	Somale	B/Gumuz	SNNP	Sidama	Gambela	Harari	Dire Dawa
Cereals	Min.	31,18	0,15	131,56	212,34	4,20	9,76	35,24	2,65	0,32	0,55	0,35
	Ave.	53,25	0,25	209,05	332,69	7,39	17,06	55,35	4,30	0,56	1,00	0,64
	Max.	80,20	0,37	298,19	463,66	10,99	25,14	76,00	5,86	0,82	1,55	1,00
Pulses	Min.	0,10	0,00	4,56	2,28	0,00	1,72	0,45	0,02	0,00	0,00	0,00
	Ave.	0,11	0,00	5,98	2,84	0,00	2,34	0,55	0,03	0,00	0,00	0,00
	Max.	0,13	0,00	7,69	3,51	0,00	3,09	0,67	0,03	0,00	0,00	0,00
Oil Seeds	Min.	0,28	0,00	1,68	3,46	0,01	1,51	0,11	–	0,00	0,05	0,00
	Ave.	0,35	0,00	2,25	4,67	0,02	2,16	0,15	–	0,00	0,07	0,01
	Max.	0,42	0,00	2,92	6,11	0,02	2,92	0,18	–	0,00	0,10	0,01
Vegetables	Min.	0,04	–	0,40	0,59	0,00	0,01	0,14	0,01	–	–	0,00
	Ave.	0,04	–	0,40	0,59	0,00	0,01	0,14	0,01	–	–	0,00
	Max.	0,04	–	0,40	0,59	0,00	0,01	0,14	0,01	–	–	0,00
Root Crops	Min.	0,05	–	7,01	12,84	–	0,24	20,13	0,45	–	0,00	–
	Ave.	0,05	–	7,02	14,65	–	0,26	23,20	0,61	–	0,01	–
	Max.	0,05	–	7,04	17,09	–	0,28	26,72	0,83	–	0,01	–
Fruit Crops	Min.	0,02	–	0,61	11,90	0,03	0,51	36,40	6,45	–	0,17	–
	Ave.	0,03	–	0,64	12,60	0,04	0,53	38,67	6,86	–	0,17	–
	Max.	0,03	–	0,66	13,25	0,04	0,55	40,80	7,25	–	0,17	–
Coffee, green	Min.	–	–	0,12	10,42	–	0,01	3,80	1,07	–	0,00	0,00
	Ave.	–	–	0,13	11,81	–	0,01	4,31	1,21	–	0,00	0,00
	Max.	–	–	0,15	13,19	–	0,01	4,82	1,36	–	0,00	0,00
Sugar Cane	Min.	–	–	0,07	0,31	–	–	0,51	0,47	–	–	–
	Ave.	–	–	0,45	2,06	–	–	3,38	3,11	–	–	–
	Max.	–	–	0,86	3,97	–	–	6,53	6,00	–	–	–

Fig. 5.

Region wise comparison of the calculated bioenergy potential in Ethiopia.

The total energy potential from nationally recoverable crop residues has a minimum of 559 PJ y⁻¹, an average of 836 PJ y⁻¹, and a maximum of 1144 PJ y⁻¹ (Table 6). According to a source [31], Ethiopia uses about 173 PJ y⁻¹ of energy annually. The conventional biomass utilization method accounts for about 80% of this energy. On average, 682 PJ y⁻¹ of the total 836 PJ y⁻¹ of crop residues come from the cereal group, which accounts for nearly 82% of the total recoverable bioenergy potential from crop residues. Fruit crop residues have a bioenergy potential of 60 PJ y⁻¹, while root crop residues have a potential of 46 PJ y⁻¹. The remaining 48 PJ y⁻¹ potential comes from other crop groups. As can be seen from Fig. 5, biomass is primarily generated in the country's regional states of Oromia, Amhara, and SNNP, contributing approximately 46%, 27%, and 15%, respectively. Other parts of the country generated the remaining 15%. The bioenergy potential is very high in these areas similarly, whereas it is lower in the regional states of Somale, Harari, Dire Dawa, Gambela, and Afar, respectively. Crop residues are primarily produced in three regions. This is because these three regions (Oromia, Amhara, and SNNP) cover larger and more fertile areas and produce the majority of crops and crop residues. This demonstrates that these regions of the country have a tremendous amount of bioenergy potential, to the point where they can provide massive amounts of bioenergy to other regions.

Table 6.

Summary of total crop residue biomass resource and bioenergy potential in Ethiopia.

	Unit	Min.	Ave.	Max.
Gross residue	kt.y−1	69569	88042	105522
Recoverable residue	kt.y−1	42621	57744	72194
Percentage recoverable	%	61	66	68
Bioenergy potential	PJ.y−1	559	836	1144

The current study findings appear to be slightly higher than previous findings. Given that the current study took into account more crops and crop groups, the higher potential was expected from the start. For example, Gabisa and Gheewala [31] calculated that the gross bioenergy potential of crop residues was around 550 PJ y⁻¹, but that the recoverable bioenergy potential was only about 250 PJ y⁻¹. In their study, they only considered 16 residue types from 10 different crops, including coffee, millet, rice, sorghum, cane, sweet potatoes, wheat, and barley. The study also stated that the majority of the country's biomass was produced in the regional states of Oromia, Amhara, and SNNP, with maize accounting for roughly 45% of the total biomass, which is consistent with the current study. A study on Ethiopia's biomass resources potential for biofuels, with a focus on renewable energy sourcing strategies, was also conducted by Guta [32]. The study investigated the biomass resources (crop residue) potential found in the country's various regions. However, the sources of each biomass residue (such as whether the crop residue is from sorghum or wheat or maize and for other residues) were not specified in the study. Estimation differences could be attributed to differences in crop production year and types, RPR, RF and HVs.

The majority (above 80%) of people in rural Ethiopia live completely off the grid and consume nearly 55 million tons of biomass in traditional ways [31]. Connecting those areas to the grid is difficult and they lack access to modern energy. As a result, the only solution is to introduce decentralized and modern technologies into rural communities. Even if they do not go collect biomass from forests, making efficient use of crop residues from their farmland will substantially enhance their lifestyle while saving them money and time. However, social, financial, technological and institutional factors prevent Ethiopia from deploying bioenergy technology [33]. There is, for example, no current clear and focused advanced institutional and legal bioenergy framework for the bioenergy sector. Furthermore, the sector lacks a proper funding mechanism and is poorly organized. The low fossil fuel prices are impeding new technologies development on the other hand, which will unavoidablly be more expensive while they are not in mass production. Another significant obstacle to the development and use of large-scale energy economy models is the absence of a national standard for collecting energy statistics [3]. This problem could be solved by developing and maintaining an extensive national energy database. Sector-specific efforts could be unified and coordinated as a starting point.

In order to produce clean cooking fuels and/or electricity using residue biomass could have a meaningful impact on both country's development and people's living standards. In rural areas with limited access to electricity and clean cooking fuels, biomass can significantly help to alleviate energy poverty and provide modern energy to citizens. As a clean source of energy, it can avoid the GHG emissions brought on by conventional fuels burning. Furthermore, if properly developed bioenergy industry in Ethiopia, rural areas will reap enormous benefits. Here are a few illustrations: Increased school enrollment and skill acquisition, productivity growth, poverty reduction, job creation, infrastructure development, and market expansion in rural areas [15].

Based on the study's findings, it is possible to draw the conclusion that the residue biomass present in Ethiopia's regions can be used as a substitute energy source to augment traditional power sources, both for heating and electricity generation. Future research must include more in-depth analyses of the community's heat or power consumption patterns, as well as technical and financial analyses of potential replacement technologies for converting residual biomass into energy. To entirely comprehend the biomass energy availability potential, a comprehensive analysis of feedstock value chains for bioenergy and food security is also suggested. Moreover, how much bioenergy will be generated must be quantified, as well as which types of feedstock are more productive and where they should be grown in the country. To ensure food security while also alleviating energy poverty this should be investigated. National and regional energy and agriculture bureaus should also maintain a centralized database of existing bioenergy resources and bioenergy. For researchers in various locations, it should be easily accessible and updated on a regular basis.

Regarding crop residue bioenergy use, there are numerous vital issues, namely: collection, handling, and storage practices, soil health, and suitable conversion technology that enhance fuel characteristics, and fuel replacement economics, which are anticipated to differ across Ethiopian regions. To user facilities, the generated crop residue must be made available in nearby. Feedstock costs have an impact on the residue supply chain that entails gathering, storing, and transporting residue from the field to the bioenergy production facility. Crop residues are difficult to transport, store, handle, and convert due to their low energy density or bulky nature. Planning bioenergy production from crop residue includes significant costs for harvesting and transportation. The viability of crop residue-based energy projects can be influenced by the potential for introducing mechanical harvesting methods versus harvesting practices in place. The cost of biomass transportation is also determined by the available biomass amount in a region and the distance traveled [34]. As a result, ensuring the availability of sufficient crop residue near the location of the energy plant is preferable. The characteristics of crop residue fuel, including moisture content would impact the suitable conversion method choice [35].

Crop residue biomass are only available for a few months during the crop harvesting season. As a result, seasonal variability can also impede their continuous use. This can be the most significant constraint for most rural regions that rely on a rain-fed production system that produces crops only once a year. Otherwise, households must store their residues to ensure a steady supply of energy sources. The use of some modern technology might be necessitating a steady supply of biomass for consistent production. However, the low density of the residue makes storage difficult [36]. Therefore, the seasonal availability of residues must be considered in future studies.

The aforementioned variable dynamics would need to be taken into consideration while considering potential estimates from the current study. Additionally, according to this study, biomass-based energy technology should be created or modified in response to regional circumstances (including biomass type, nature, and availability). This could be done to investigate possible ways to increase biomass potential and make better use of available resources (e.g., analysis on whether the energy from different residue biomass convert is uniform and feasible in local situations). Further research on the utilization of residue material composition of different crops as an energy source in a local context is also advised. As this study relied solely on a desktop analysis to draw significant results, conclusions, and recommendations more practical work on the feasibility and challenges of the conversion process is required. This could be done to ensure that the assumptions used in this study correspond to actual operational data (practicality).

3.3. Overview of conversion technologies for biomass to energy

As shown in Fig. 6, two main conversion routes can be used to convert biomass into fuels, energy, and other products: thermochemical conversion and biochemical conversion. A variety of factors influenced appropriate biomass conversion technologies, including biomass feedstock type and amount, and desired energy form (requirements of end-use, project-specific factors, economic considerations, and environmental restrictions) [35]. Additionally, the use, particle size and shape, material, reactor type, and gas flow all have an effect on the effectiveness of biomass conversion [37].

Fig. 6.

Conversion technologies for biomass [38].

3.3.1. Thermochemical conversion route

The three basic methods in the thermochemical conversion route are (i) direct combustion, (ii) pyrolysis, and (iii) gasification. The process of combustion is an exothermic chemical reaction in which a fuel substance is combined with an oxidizer to produce combustion products (chemical compounds). Almost all chemical reactions involve the addition of heat to the process (endothermic) or the production of heat as a byproduct of the reaction (exothermic) [38]. The oxidizer in combustion processes is commonly air, but it might be pure oxygen (O₂). In biomass, the chemical energy stored is converted into heat, electricity, or mechanical power during combustion, which is typically used for biomass with <50% moisture content unless it is pre-dried [35,39]. From household types (for very small use) to industrial plants (large-scale) with capacities ranging from 100 to 3000 MW, there are many different sizes of combustion plants [35]. Small-scale biomass combustion is used for cooking, particularly in developing countries where cook stoves are utilized. Particularly for woody fuels, direct combustion is usually the most affordable biomass-to-energy technology to build and operate. MSW, dry manure, and Agro residues are just a few of the biomass fuels that can be burned in almost any type of combustion system [38].

Pyrolysis is the fundamental thermochemical process by which biomass is converted into more valuable or convenient products. Pyrolysis is explained as the thermal destruction (or an irreversible chemical change) of fuel materials brought on by heat when air (or an adequate supply of air) is not present. This typically occurs at temperatures between 300 and 500 °C [38]. Materials go through a series of changes during pyrolysis, producing solid, liquid, and gas phase products, with quantities and compositions determined by feedstock qualities, process parameters and environmental conditions [15,38]. Many energy products are produced by a range of feedstock types, temperatures, heating rates, and residence periods, including fuel gas (low temperature, flash pyrolysis), bio-oil (low temperature, flash pyrolysis), and charcoal/biochar (slow pyrolysis, carbonization) [35,40]. Subsequent use of the oil and problems with the conversion process, such as its corrosive properties and low thermal stability, must be addressed [35].

Gasification is a biomass conversion process that uses a limited oxygen amount to transform any material containing carbon into a combustible gas. Only 30% of the oxygen required for complete combustion can be present during normal gasification conditions [15,38]. Gas produced from biomass feedstock during the gasification process is also known as syngas or wood gas. Carbon monoxide (CO) and hydrogen (H₂) as fuels are primarily compounds during this process, along with a trace of carbon dioxide (CO₂) and methane (CH₄). Depending on the chemical composition of the fuel, it may also comprise additional compounds for example sulfur (S) and nitrogen (N). Gasification gas composition is highly reliant on the gasification temperature, gasification technique type, and gasification agent [38]. For example, low-temperature gasification process (<1000 °C) produces producer gas that comprises carbon monoxide, hydrogen, methane and other C_xH_y, aliphatic hydrocarbons, toluene, benzene, and tars. Catalytic gasification or >1200 °C high temperature is used to produce syngas and the biomass is completely transformed into hydrogen and carbon monoxide under these conditions. In all applications, syngas can be used in place of natural gas as it is chemically similar to natural gas.

3.3.2. Biological or biochemical conversion technology

Biological conversion is the process of converting biomass to fuel by exposing it to specific microbes. As a result of microorganism metabolic activity, the second fuels are generated. The two basic processes in the biochemical conversion route are (i) anaerobic digestion and (ii) fermentation. The main and common end products of these processes are biogas and ethanol, respectively. In the absence of oxygen, bacteria convert organic biomass through a process called anaerobic digestion (AD) into a mixture of gases known as biogas [35,41,42]. Microorganisms break down biomass (approximately 85%–90% moisture content) into biogas without oxygen the during AD process, producing a mixture of methane (CH₄), carbon dioxide (CO₂), and trace amounts of other gases like hydrogen sulfide (H₂S) [15]. While a natural process called fermentation started by microorganisms (a commonly type of Saccharomyces, which is identical to popular yeast cultures in anaerobic circumstances) [38]. As metabolic waste products, pentose or hexose sugars (fructose, sucrose, and glucose) are transformed into ethanol and CO₂ in the subsequent fermentation process. Commonly for bioethanol production, biomass materials such as sugars, starches, and cellulose are the three primary types of raw materials utilized in the fermentation process. Any sugar-containing substance can generate Ethanol. Currently, ethanol is widely employed as an alternate source of liquid fuels for the transportation sector. Ethanol as a fuel has various advantages, including a higher-octane number (99) than petrol (80–100), lower thermal energy content, and low emissions [38].

4. Conclusion

The study estimated the biomass resource and bioenergy potential of crop residues using 44 residues produced by 30 crops across Ethiopia's eleven regions. The country generates 69 569–105 522 kt y⁻¹ gross crop residue biomass, of which 61–68% of gross is estimated as recoverable. Oromia produces the highest recoverable residue in the country (45%), followed by Amhara (26%), and SSNP (16%). At the national level, the bioenergy potential from recoverable residue is roughly 559–1144 PJ y⁻¹, and it varies by region, ranging from 0,15 to 0,37 PJ y⁻¹ (Afar) to 254–521 PJ y⁻¹ (Oromia). Estimated bioenergy potential at the national, regional, and crop levels in this study is expected to aid policy decisions and regional bioenergy planning in the country. Furthermore, the comprehensive literature survey of the residue product ratio, recoverability fraction and lower heating values of crop residues provides practitioners in the field a firm starting point for conducting similar studies in different contexts. The developed approach can aid in decision-making and planning for the implementation of bioenergy technology and is particularly helpful for preliminary biomass feasibility screening tests.

Author contribution statement

Amsalu Tolessa: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data included in article/supp. material/referenced in article.

Declaration of interest’s statement

The authors declare no competing interests.