1. Introduction
The need for new forms of energy has given rise to a rapid development of energy technologies from all renewable sources [
1]. Those developments led to major technological breakthroughs, which are reflected in greater energy production capacity and cost reductions [
2]. The latter is likely the most important factor considered in the choice of a particular type of energy to be implemented. Decision makers tend to opt either for choosing the cheapest forms or, at the very least, for energy profiles that required less investment vs. production capacity, while allowing permanent cash flow, thus, justifying a return on investment that is acceptable to investors [
3].
Energy production, however, is an issue that arguably should be addressed through a strategy-setting approach at the national levels. In the case of European Union countries, the following strategy has, however, been mainly addressed from a purely economic perspective, wherein financial models determine the priority agenda of investments, often disregarding environmental and decentralized variables related to energy production [
4]. In reality, energy production is one main topic that should be based on a thorough strategic discussion, since, for highly energy-dependent countries, a greater renewable energy production capacity should lead to significant reductions in the balance of transactions as well as a higher level of sustainability, related to economic decarbonization [
5].
Currently, with the paradigm shift caused by environmental issues raised by climate change anticipated adverse effects, there has been a gradual mindset change regarding the issue of managing the energy matrix [
6]. Policy makers increasingly show a greater willingness to give priority to alternative renewable energy sources at the expense of traditional sources of fossil origin, such as oil or coal. This new principle is especially valid when it is possible to use renewable sources in direct substitution for fossil sources [
7] if coal consumption can be avoided because the installed wind capacity is able to satisfy the needs of the electricity grid, or if the annual hydrological regime is able to keep the hydroelectric power plant system operational [
8]. However, this direct relationship between these renewable sources and meteorological factors leads to intermittent production and it is, therefore, not possible to have a constant supply for the power grid without extensive energy storage capacity [
9]. For this reason, it is necessary to have dispatchable production points, which are able to ensure this supply when seasonal or daily weather conditions make it necessary [
10].
From this perspective, biomass, being a renewable source of energy, can be a viable alternative fuel. At the same time, biomass presents a global availability in practically all latitudes of the globe, except for the poles and some desertic regions, but is widely distributed among the most inhabited regions where energy consumption is more intense [
11]. However, present production of usable and available biomass that can be used as a source of energy is mostly carried out far from centers of consumption [
12]. For example, residual forms of biomass of agricultural origin are produced far from large urban centers, where energy needs are more pressing, forcing the transport of energy products [
13]. This transport, and the subsequent set of associated logistic operations, such as handling or storage, entails a set of costs that can make the operation and the sustainability of the process unfeasible [
14].
In the above context, biomass shows distinct advantages, such as its widespread availability, the decentralized ability to be used in gas, liquid, or solid forms the energy potential and the capacity to serve as a counterpoint to the intermittency of other sources. However, biomass shows challenges, which include its low heating value, high moisture content, and low density, coupled with geographical dispersion, all reflected, for higher scale utilizations, in higher transport costs [
15]. The sum of these issues makes the use of biomass for energy production often difficult to implement, since the costs associated with all the ancillary operations make the process unfeasible, making it imperative to study variables associated with the logistics process, in addition to variables associated with energy recovery processes [
16].
There are several forms of biomass resulting from commercial agroforestry businesses, such as the production of coconut, sugarcane, cashew nuts, or palm oil, which deliver a steady supply of waste that, in the absence of use, can turn into environmental problems through the emission of greenhouse gases (GHGs), either by their combustion to eliminate the materials, or by rotting due to lack of destination or form of recovery [
17]. Thus, the possibility of transporting these materials to a destination where they can be valorized should be a very interesting possibility, were it not for the previously mentioned problems related to the logistical costs associated with these materials [
18]. The scale of food processing facilities is generally not large enough to serve as an “end use point” for utility scale production, but it is often of sufficient scale to serve as a location for preprocessing prior to shipment to a larger, centralized conversion facility.
The possibility of materials being subjected to energy densification processes, such as thermochemical conversion processes, eliminating moisture and volatile compounds with low heating value, is associated with a densification process, such as pelletization or briquetting, which can transform waste with little value into readily transportable energy products, capable of functioning as an alternative to coal. This is mainly of the most common types with widespread commercial use, such as sub-bituminous and bituminous coals [
19]. Another advantage is that those energy value-added products can be feedstock used directly in coal-fired power plants, without the need for major changes, since they have similar physico-chemical properties with regard to heating value and grindability [
20].
In the above context, the aim of the present study was to characterize the properties of four widely used residual biomass feedstocks of agroforestry origin, namely coconut shells, sugarcane bagasse, palm kernel shells (PKS), and cashew nutshells, which are abundant in tropical and subtropical regions, and constitute a reserve with the potential to supply countries that are currently replacing coal as a way to reduce greenhouse gases (GHGs) emissions. Based on the results obtained, it was intended to demonstrate that the energy densification of these materials allows the reduction of logistical costs, and, thus, their transport to places of final use away from the points of production.
4. Results
4.1. Elemental Analysis
The results obtained in the elementary analysis are shown in
Table 2, including
p-values for two-way ANOVA with n = 2 for the effect of feedstock type and thermal treatment type. This information was added as well in Tables 3–5.
The choice of feedstock impacts elemental composition for all measured elements, with cashew nutshells and PKS having higher carbon, lower nitrogen, and lower oxygen contents. Hydrogen content is highest in bagasse and cashew nutshells. Thermal treatment impacts carbon content and oxygen content in all samples, even though there is not a consistent trend when comparing the 300 °C and 400 °C treatment. This is likely due to preferential volatilization of oxygen-rich compounds from the biomass, which is not enhanced by temperatures above 300 °C. For example, in the case of sugarcane bagasse, wherein, after a rise in carbon content from 47.30% to 78.60% for the test carried out at 300 °C, a slight reduction was observed at 400 °C. A similar situation is observed for the PKS, where, after a rise from 52.00% to 58.50%, there is a decrease to 56.00%. In the case of cashew nutshells and coconut shells, values of consecutive rise of 53.50%, 69.50%, and 76.70% and 46.70%, 70.10%, and 74.10%, respectively, were obtained. There is a tendency similar to carbon for the hydrogen contents in sugarcane bagasse with the values evolving from 6.56% to 6.46% and 7.59%. In the remaining materials analyzed, there was a downward trend in the levels of hydrogen content. Nitrogen has a tendency of content increase with temperature for cashew nutshells and PKS samples, while there is a downward trend in the content in the case of sugarcane bagasse. In coconut shells, there was an increase from 0.869% to 1.120% followed by a decrease to 1.030%. The oxygen content shows a downward trend in cashew and coconut shells, from 39.68% and 47.08% without heating, to 24.35% and 25.18% at 300 °C, and to 18.75% and 21.59% at 400 °C, respectively.
4.2. Proximate Analysis
The results obtained in the proximate analysis are shown in
Table 3.
Moisture content does not show a statistically significant variation with respect to either feedstock or thermal treatment. Moisture presents a generalized downward trend in all materials, except for sugarcane bagasse, wherein the values are, respectively, 3.31%, 3.79%, and 3.67%. In the case of cashew nutshells, there was a decrease from 5.85% to 3.55%, but followed by an increase to 5.24% for the test carried out at 400 °C, likely related to the fact that the samples were not stabilized in the desiccator after being removed from the muffle. The volatile content shows a downward trend in all samples. The ash content shows an upward trend in all samples, related to the concentration of non-volatile materials and directly proportional to the mass losses which were, for 300 °C and 400 °C, and for each of the materials in the order of
Table 1, of 60.16% and 73.63%; 37.77% and 76.67%; 35.79% and 75.46%; and 59.46% and 64.72%, respectively. The fixed carbon content shows an upward trend in all analyzed samples.
4.3. Determination of Heating Value
The results calculated for HHV and LHV are shown in
Table 4.
HHV and LHV are both impacted by feedstock and thermal treatment (at a 95% level of confidence). A generalized upward trend was observed in the values of HHV and LHV with increasing test temperature in all materials. The maximum values for HHV and LHV were related with sugarcane bagasse samples at 400 °C with 34.37 MJ/kg and 31.36 MJ/kg, respectively. PKS samples showed the lower values of HHV and LHV for 300 °C and 400 °C with 20.42 MJ/kg, 21.38 MJ/kg, 19.01 MJ/kg, and 20.23 MJ/kg, respectively.
4.4. Material Grindability
The results of grindability, calculated for the HGI, are shown in
Table 5.
HGI is not impacted by feedstock choice but is affected by thermal treatment (p < 0.05). The calculated values were all greater than 50, with the exception of the value obtained for cashew nutshells at 300 °C and for PKS at 300 °C. In the remaining results, the values were distributed between 51, for coconut shells at 300 °C, and 60, for coconut shells at 400 °C.
4.5. Specific Density and Energy Density
The results calculated for EDR, MY, and EY are shown in
Table 6.
The results obtained for EDR with the tests carried out at 300 °C between 1.06 for coconut shells and 1.08 for sugarcane bagasse and cashew shells. For the tests carried out at 400 °C, the values ranged between 1.05 for cashew and coconut shells, and 1.10 for sugarcane bagasse. MY values, except for sugarcane bagasse, showed a downward trend, in line with the mass losses presented. EY values followed the same trend as MY, rising for all materials, except for sugarcane bagasse.
5. Discussion
The main objectives of the characterization of the selected biomasses were the evaluation of their potential for replacing fossil fuels such as coal and possible logistical gains due a densification of these feedstocks, allowing for a financial feasibility of the transportation to more distant locations from the origin. Many studies are available on processes of physical densification of biomass, namely on the form of pellets and briquettes, such as the works of Bhattacharya et al. (1989), wherein an analysis of the pioneering state-of-the-art is presented, of Li and Hiu (2000), which address the issue of high-pressure densification of wood residues, of Tumuluru et al. (2010), presenting a technical review on biomass processing, with emphasis on the optimization of the densification process, of Panwar et al. (2011), presenting a characterization of briquettes made from residual biomass, of Obidziñski (2014), addressing the pelletization of waste biomass, or the most recent works of Bajwa et al. (2018), Zhang et al. (2020), or Nunes (2020), where priority is given to balancing between the technological properties of densified biomasses vs. their potential to replace coal in energy production [
11,
137,
138,
139,
140,
141,
142].
It was concluded that physical densification alone was not capable of solving all logistical problems of biomass conversion, especially these related with properties such as density and heating value, which allow the materials to become transportable over long distances [
143,
144,
145,
146,
147,
148]. Moreover, it was found that, with some residual biomasses, transport and use away from the origin would be possible only in addition to the physical densification, an energy densification through thermochemical conversion technologies such as torrefaction would be carried out (e.g., Uslu et al. (2008), Van der Stelt et al. (2011), and Chen et al. (2015) [
19,
149,
150]. Thermochemical conversion technologies show interesting advantages from the point of view of improved logistical properties because they promote energy densification, as can be easily proven from the results obtained in this study. As can be seen in
Figure 3, the thermo-chemical conversion processes of biomass promote energy densification, since their products show increases between 28% and 70% of the available energy per unit of mass.
From a perspective of cost analysis associated with long-distance maritime transport, of the transatlantic type, where the vessels used can transport around 60,000 tons of biomass pellets, it is possible to quantify a reduction in transport costs per unit of energy, which can vary between 35% and 52%. In the case of smaller boats that carry pellets within the European space, with capacities of around 7500 tons, the cost reductions are smaller. However, they are still quite interesting, ranging from 20% to 40%. In other words, thermochemical conversion technologies present themselves as enhancing the competitive logistical advantages of biomass products, associated with energy transport. In addition, the torrefied biomass products show other advantages, presented in the works of Ciolkosz et al. (2011), Kambo and Dutta (2014), Chen et al. (2018), or Zhang et al. (2020). These advantages are related to the hydrophobicity of materials, which allow their storage in less demanding situations, which can be arranged outdoors, similarly to coal. Moreover, their storage period can even be prolonged indefinitely, since these products do not react to biological activity [
151,
152,
153,
154]. This is an advantage, especially for short distance supply chains, since torrefied or carbonized biomass can also be used as a raw material for other processes, namely for the production of hydrogen by gasification [
155,
156,
157].
From the point of view of the aptitude to combustion of the biomasses, there is some agreement of the results obtained in this work with the results from other previous works. In the case of sugar cane bagasse, the results of this work are concordant with those by Nunes et al. (2020), wherein the feasibility of using sugarcane bagasse subjected to carbonization of biomass at different temperatures was demonstrated, for applications of energy recovery from biomass products [
21].
There are several references regarding torrefaction and pyrolysis of coconut shell, e.g., Chen and Kuo (2010), which concludes that severe torrefaction is not recommended to pretreat biomass due to a high percentage of mass loss and due to the possible difficulty of densifying torrefied products posteriori [
79]. In the work presented by Nasution and Limbong (2017), it is shown that the average yield is 38.20% for a process temperature of 348 °C, which agrees with the results obtained in the present work, where, for 300 °C, a 41% mass yield was obtained, and, for 400 °C, a 35% mass yield was obtained [
158]. PKS, perhaps the most studied product, of all those analyzed here, is the product that also presents the most interesting results, mainly for tests carried out at 300 °C, resulting in products presenting a MY of 64% and a 72% EY.
Regarding cashew nut shells, there are currently no studies available that can serve as a point of comparison. However, the results obtained were good, especially mass yield, which is 62%, and EY, which is 170% relative to tests carried out at 300 °C. These values showed a significant potential technological upgrading of the feedstock with thermochemical conversion. In
Table 7, the LHV of commercial coals, used for the production of electricity, is shown with values ranging between 16.1 MJ/kg and 33.4 MJ/kg. This LHV range is similar to the LHV range between 19.01 MJ/kg and 31.36 MJ/kg found for the biomass samples analyzed in this work.
Another important characteristic of biomass torrefied products is grindability evaluated in this work through the HGI calculation. This characteristic, which defines the greater or lesser difficulty in grinding a fuel, and, consequently, the amount of energy spent in the fuel grinding, which must be injected into the furnace. This is one of the stages of the energy production process wherein the constraints for replacing coal by biomass products normally occur. Most commercial coals show HGI values ≥ 50 [
161], whereas fuels such as non-thermally processed biomass have HGI values < 50 [
162], indicating that the amount of energy needed to pulverize the products from torrefaction or pyrolysis is similar to that of coals. This quality upgrading delivered through the thermochemical conversion turns biomass, in its different forms, as an alternative to the use of coal for energy production. This was previously shown by authors such as Nunes et al. (2014), Proskurina et al. (2017), Nunes (2020), or Sher et al. (2020) [
11,
163,
164,
165].
However, despite the evident advantages found in the use of forms of biomass, especially those that fit into the residual forms, due to its enormous availability and low cost, the logistical disadvantages remain, associated with its low density and with the significant distances between the origin to the point of consumption. These challenges can still be overcome through a physical densification process such as palletization added to energetic densification by torrefaction, carbonization, or pyrolysis [
166]. Economic issues related to the transportation of biomass take on an often-decisive role with regard to the use and recovery of these materials, especially those that are considered residual. The costs associated with transport vary widely from case to case, namely due to the issues related to the density, inherent to the type of material, but also to the distances to which the materials have to be transported. It is, at this point, that the energetic densification of materials assumes a decisive role, since it can enhance their transport over greater distances, especially intercontinental transport, creating a true value chain for waste materials, in a perspective of circular economy. This value chain, if possible, will allow products that, until now, have no use, or at least do not have a use that values waste completely, to be incorporated into a global supply chain, serving as alternative, traditional fossil fuels. This path presents itself as a true tool for mitigating climate change, by contributing to the reduction of GHG emissions.
6. Conclusions
The use of residual biomass as an energy alternative to the use of fossil fuels presents itself as a possibility that assumes increasing importance. In fact, the availability of these materials, combined with their dispersion, makes them very viable alternatives. However, despite these apparent competitive advantages, in the vast majority of cases, it is necessary to use technologies that promote energy densification, while improving other properties, such as grindability, the reduction of humidity, or hydrophobicity. Thermochemical conversion technologies play an important role in this context, since they allow obtaining products with optimized combustible properties from residual biomasses while improving the perspectives of logistics involved in the transport process between the locations of production and consumption in which the latter is associated with energy production. Overall, the results were very positive with EY values ranging from 24.5%, for samples of cashew nutshells processed at 400 °C, to 72%, for PKS samples processed at 300 °C. At the same time, there is an increase in the values calculated for the HGI, which, in all situations, approach, or exceed, the value of 50, considered to be the most common value found in commercial coals. These results clearly showed the existence of a significant increase in the energy density of the products from thermochemical conversion processes such as torrefaction or pyrolysis. An additional optimization of the logistic transport processes is possible if the thermochemical conversion is associated with physical densification processes, such as pelletization or briquetting. This possibility allows the creation of a value-added chain for waste materials in a real perspective of a circular economy, at the same time that it contributes to the creation of an alternative to fossil fuels. However, further studies are needed, mainly related to the combustion of residual biomasses in their different states of thermochemical conversion, to verify the potential for the occurrence of corrosive, fouling, and slagging phenomena, as well as the combustion stability when used in co-firing processes with coal.