Keywords

1 Introduction

Senegalese agriculture mainly relies on subsistence crops. This agriculture has long been seasonal, but recent initiatives brought in mechanization and harvesting periods out of traditional seasons, as well as crop diversification.

In its Intended Nationally Determined Contribution (INDC), Senegal ambitions to produce 4,000,000 Gigajoules (685,000 MWh) of biofuels annually, recovered from waste (Ministere en charge de l'Environnement, 2015). The recovery process would avoid 457,000 tonnes of CO2 per year. This objective constitutes the framework of current initiatives using crop residues to produce fuels for industrial applications. Our study specifically targets the use of these residues for producing pellets.

The requirements of the pelleting process are specific to each crop residue and depend on its characteristics. The parameters that we consider meaningful in the process are the physical characteristics of the biomass (e.g. moisture content), the energy output required (e.g. gas or electricity), and the economic conditions throughout the value chain from the collection of residues in fields, followed by transport and preparation in situ of raw materials (e.g drying and grinding) to the production of pellets. The chapter provides valorization schemes that convert five crop residues into pellets. Our methodology combines in-laboratory experiments where the characteristics of crop residues are measured, and different hybridization schemes are tested, followed by documentation of results from the experiments. We confront these results with lessons learned from previous studies on pelleting and the use of pellets. Before, we start by a review of the literature on the state-of-the art of crop residues production in Senegal.

2 Potential of Crop Residues in Senegal

The crop residues in our study sample are residues that have a potential for energy recovery and are available in significant quantities in different regions of the country. These include groundnut shells, palm nutshells, corn cob, rice husk, and bulrush. Figure 6.1 shows these residues, with quantities available per year and harvest areas.

Fig. 6.1
figure 1

Crop residues potential and location in Senegal

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Groundnuts (Arachis hypogaea) are harvested mainly in the so-called groundnut basin (centre of Senegal). The potential of groundnut shells available in this area is estimated at 142,000 tonnes (ANSD, 2019). The groundnut shell, despite its light weight, has a density of 270 kg/m3. Its low calorific value (LCV) is 16,704 kJ/kg (4.64 kWh/kg) (BRISK-2-Project, 2019).

Palm nuts (Elaeis guineensis) are produced mainly in the Lower Casamance region (south of Senegal). The potential of palm kernel shells available in this area is estimated at 50,000 tonnes (ANSD, 2019). The palm kernel shell features both the highest density and calorific value of the five crop residues considered, with 630 kg/m3 and 25,095.6 kJ/kg equivalent to 6.971 kWh/kg (BRISK-2-Project, 2019).

Maize (Zea mays) is harvested mainly in the centre, and south-east regions of Senegal. The corn cob potential of this area is estimated at 348,000 tonnes (ANSD, 2019). The corn cob has a density of 270 kg/m3 (BRISK-2-Project, 2019) and a low calorific value (LCV) of 24,760.8 kJ/kg (6.878 kWh/kg).

Rice (Oryza glaberrima) is traditionally cultivated during the rainy season in southern Senegal (Casamance) and now out-of-season in northern Senegal. The rice husk potential from both regions is estimated at 260,000 tonnes (ANSD, 2019). The rice husk has a low density of 120 kg/m3 (BRISK-2-Project, 2019) and a low calorific value (LCV) of 14,004 kJ/kg (3.89 kWh/kg).

Bulrush (Typha australis) is an aquatic plant that grows along the Senegal River. Bulrush cannot be directly linked to a food crop, unlike the other residues cited previously, but was included in this study considering its availability and its energy potential. The quantities of bulrush produced annually are estimated at 520,000 tonnes of dry material (ANSD, 2019). The bulrush has a significant energy potential, with a low calorific value (LCV) of 17,388 kJ/kg (4.83 kWh/kg) and a density of 120 kg/m3 (BRISK-2-Project, 2019). Table 6.1 summarizes the quantities of crops and their residues in the study sample.

Table 6.1 Estimates of crops production and residues in Senegal in 2018 (ANSD)
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3 Pellets Processing

The processing of crop residues through pelleting provides a wide range of energy recovery schemes. The pelleting protocol remains the same for the five crops’ residues in our sample. A preliminary drying phase is necessary, followed by mechanic grinding. After grinding, the residues are compacted and transformed into pellets. The major challenge in pelleting is the moisture content before compaction. The challenge is more important in the hybridization scenarios, where we combine biomass materials with different properties. The experimental pelleting process we conducted required a hammer mill and a pelletizer with a power of 7.5 kW each.

The experiment returned high-quality and high-energy pellets. The physical characteristics of pellets produced were tested in the laboratory. Table 6.2 shows the physical characteristics of the pellets, which are produced through mixing different crop residues.

Table 6.2 Characteristics of experimental pellets
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The physical properties of the pellets in Table 6.2 make them suitable for use in the thermal processes of various industrial applications; besides the energy potential, they have low humidity and low ash rates. The combination groundnut shell and palm kernel shell returns the highest calorific value, because the two residues have a high percentage of fixed carbon, which made the mix easy to homogenize.

4 Technology Solutions for Valorization of Pellets

Pellets previously produced can be used in both thermal and electrical applications. In thermal applications, the pellets are used directly as fuel in industrial boilers and ovens to generate. In the electrical applications, the process requires to convert the pellet energy content into steam that powers electricity production.

4.1 Combustion

The combustion of biomass pellets produces hot gases at temperatures around 800–1000 °C (McKendry, 2002a, b). In our first experiment, the pellets that result from the combination of groundnut shells and palm kernel shells were tested in a bakery oven as fuel to produce the heat needed for baking bread. This experiment showed that the pellets were more efficient in terms of quantities and energy generated than the conventional fuel (charcoal) used in the bakery.

The five crop residues cited in this article, with the exception of risk husks, could also be burned directly in waste-to-energy plants without any chemical processing to produce steam for generating electricity. In this case, fluidized bed combustion would be the best combustion technology (Saidur et al., 2011). Fluidized bed combustion has emerged as a viable alternative to conventional combustion systems and offers multiple benefits such as compact boiler design, fuel flexibility, higher combustion efficiency, and reduced emission of noxious pollutants. The fluidized bed boilers have a wide range of capacities, from 0.5 T/h (tonnes/hour) to over 100 T/h (Saidur et al., 2011).

4.2 Gasification

The gasification process converts pellets into a combustible gas mixture (syngas) by partial oxidation of the pellet at high temperatures (800–1500 °C) (McKendry, 2002a, b).

The gasification process can be used for direct gas production, but also for electricity production (conversion of the gaseous fuel to electricity through gas turbine). The quantities of rice husk, groundnut shell, palm kernel shell, bulrush, and corn cob have an energy potential of 7400 GWh/year. This potential can fuel large power plants in the areas where the crop residues are produced, which operate according to a proven protocol: gasification of the residues and production of electricity with an alternator coupled to a combustion turbine. The average efficiency of such systems is estimated at 35% (McKendry, Energy production from biomass (part 3): gasification technologies, 2002a, b). The option is particularly interesting to consider given areas in Fig. 6.1 where the residues are produced are yet to achieve universal access to electricity.

Our second experiment consisted of using the pellets in a cooking stove. The results showed that the stove with pellets was as efficient as liquefied petroleum gas (LPG) cooking stoves in terms of cooking time. Our estimates showed that by using the pellets in Table 6.2, a household in Senegal could save up to 50% in its cooking energy bill with a better energy service.

4.3 Anaerobic Digestion

The anaerobic digestion converts the crop residues into biogas, which is a mixture of methane and carbon dioxide, with small quantities of other gases such as hydrogen sulphide (McKendry, 2002a, b). The biogas produced has an energy content equivalent to about 20–40% of the lower heating value of the feedstock. Anaerobic digestion and biochemical conversion, in general, are usually preferred for biomass with high moisture content such as manure.

In Senegal, the National Domestic Biogas Programme (PNB-SN) was developed in order to promote this technology in rural areas, with cow and donkey dungs as fuels. Recent developments in technology opened the possibility of using paddy rice husks and other crop residues in biogas technologies with the solid-state anaerobic digestion (SS-AD) digesters. Indeed, SS-AD operates with biomass fuels that have a proportion of solid content higher than 15%, making it particularly suitable for digesting the lignocellulose content of rice husks (Matin & Hadiyanto, 2018). The rice husk potential (260,000 tonnes per year) makes SS-AD relevant for biogas production in northern and southern Senegal, therefore providing an eco-friendly solution to the management of waste, which has so far been difficult to value due to material composition.

5 Conclusion

Senegal has a considerable potential for generating electricity from crop residues, which is not fully valued as of today. In this chapter, groundnut shells, palm kernel shells, corn cobs, rice husks and bulrush were exemplified as residues with high energy potential and high material quantities.

The research experiments build from previous studies that demonstrate the energy potential of these crops taken individually and introduces hybridization schemes that benefit from the complementary characteristics of two or more crops combined into pellets. Our results open new perspectives on the design of pellet mixtures that suit different energy recovery techniques. Our results show that the most promising options for energy recovery from pellets are combustion and gasification. Our experiments also demonstrate the potential of pellets’ use in small-scale businesses such as bakeries and restaurants. Besides, the pellets could also replace conventional fuels in ovens and boilers in industrial applications. In fact, our pellets feature the physical characteristics of high calorific values and low moisture content and ash rate, which make them reliable combustion fuels.

Future studies could delve into heat generation with pellets in a combustion technology and test the potential of various types of pellet mixtures to substitute fossil fuels in industrial boilers. It would also be interesting to explore the economic potential of pellets for electricity generation in small- and medium-scale transformation companies that could add value to crops initially harvested.