Briquetting of black soldier fly frass as a post-harvest stabilization strategy

Sustainable energy sources are a pre-requisite for successful socioeconomic stability and industrial transformation. Energy has impacts on all development aspects, including economic, social, environmental, agriculture, population, and livelihood (Kalak 2023). The rising energy demand is a global challenge due to the upsurge in human population, and increased industrial and commercial activities. Petroleum products are the major source of energy (Pop 2018) and face acute peaking problems due to the ever-increasing demand in the midst of the declining petroleum reserves. According to Kpalo et al. (2020), consumption of petroleum products has raised the levels of pollution and increased negative effects associated with climate change from resulting greenhouse gas (GHG) emissions. Thus, focus is currently shifting to green and renewable energy sources to combat these challenges and subsequently reduce the associated negative impacts.

Bioenergy from organic waste is a promising sustainable source that is vital in minimizing the increasing economic challenges (Guman & Wegner-Kozlova 2020) and environmental issues concerning fossil fuels depletion (Pop 2018) and waste disposal. Use of biomass energy has a high profitability, large potential, and different environmental and social benefits (Shelke & Mahanta 2016). Thus, energy recovery from waste streams is of increasing interest owing to availability, environmental benefits, low resource cost, and hence a transition to a circular waste-based bioeconomy (Cong et al. 2023).

Globally, bioenergy derived from biomass materials contributes 14% out of 18% of the global energy supply from renewables and further contributes 10% of global energy consumption (World Energy Council 2016). In Sub-Saharan Africa, wood fuel accounts for higher than 80% of the primary energy supply (Iiyama et al. 2014). Therefore, the waste-to-energy nexus based on the reduce, reuse, recycle, recover, and restore (5R) principle is important in supplementing the escalating energy demands in Kenya and Africa at large.

Thermochemical technology is gaining much interest among researchers due to the flexibility of the feedstock application, product upgrading, and product distribution (Chan et al. 2019). Thermochemical conversion technologies such as combustion, gasification, pyrolysis, and liquefaction are suitable for agro-waste valorization (Shahbaz et al. 2021). The biomass is broken down into smaller hydrocarbon chains through chemical reactions or controlled heating resulting in production of biofuel in solid, liquid, or gaseous forms (Jha et al. 2022). The composition of the biomass greatly influences the reaction rate, process parameters, biochar yields and quality. Researchers have employed animal dung (Mainkaew et al. 2023), human faeces (Kizito et al. 2022), agricultural residues (Okot 2019), and industrial residues (Onukak et al. 2017) to produce fuel briquettes. However, research on insect frass for energy production is limited in the literature.

Insect frass is a residual material of the insect rearing process and comprises feed remains and insect excrement (Wedwitschka et al. 2023). The use of the black soldier fly frass (BSFF) for various purposes stimulates a transition towards a circular economy. BSFF has been used as a biofertilizer (Siddiqui et al. 2022), growing media in agriculture, and feed ingredient for hybrid tilapia (Lopes et al. 2022). Mishra & Suthar (2023) analyzed the BSFF for its suitability as biomanure for agronomical application immediately after BSF larval cultivation. The study reported a poor carbon-to-nitrogen ratio in the collected frass samples, which limits using ready frass as biomanure. According to Van Looveren et al. (2022), BSFF contains Enterobacteriaceae and vegetative forms of Clostridium perfringens, which can be reduced below the detectable limits by heat treatment, particularly for using as a biofertilizer. A study by Mishra & Suthar (2023) reported that the obtained frass failed to fulfil the minimal criteria required for biosolids for agronomic application, thus further stabilization was necessary.

Moreover, the BSFF is the main pool of nitrogen after the bioconversion (Parodi et al. 2021) and its emissions are a source of GHGs in the larval bioconversion system (Mertenat et al. 2019). Previous studies have suggested further stabilization of the biologically unstable BSFF through anaerobic digestion (Wedwitschka et al. 2023) and vermicomposting (Bortolini et al. 2020). Thus, without proper post-treatment of the harvested frass, the GHG reductions obtained through larval assimilations could be offset. Therefore, the full potential of frass should be thoroughly investigated so as to fill the existing knowledge gaps and promote the adoption of BSF bioconversion. Besides, the waste–energy (W–E) nexus is capable of tackling waste management, energy generation, and environmental pollution simultaneously. But the W–E nexus has not been adequately explored for BSFF to complete the value chain.

Based on the ‘made-to-be-made-again’ policy, this study investigated the calorific value, and proximate and emission properties, of briquettes produced from carbonized BSFF (from different substrates) densified using cassava starch gel as a binder. This study adds knowledge on employing BSFF briquetting as a post-harvest stabilization strategy and the practical application of the biofuel.

The experimental setup was at Jomo Kenyatta University of Agriculture and Technology, Kenya, in a greenhouse tunnel where BSF were reared on three substrates, namely, faecal matter, pig manure, and poultry manure. The resultant BSFF from the three waste streams was collected and dried under a greenhouse for seven days so as to attain a constant moisture content. The dried BSFF was then pyrolyzed at 350 °C for 2 h in an electric muffle furnace (Model FM-36) so as to liberate the light volatiles thus increasing the carbon content and calorific value. The samples were cooled in a desiccator. The resulting char was grounded and screened through 0.5 mm sieve for homogeneity.

Charcoal dust (AC) was purchased from a local vendor, ground, and screened through a 0.5 mm sieve. The grounded BSFF (from faecal matter, pig manure, and poultry manure) and charcoal char were blended at a mix ratio of 50:50 weight % (wt%) ratio and stored in air-tight plastic containers till further processing of the briquettes.

Cassava gel was prepared from raw cassava sourced from a local farmer. The raw cassava was dried, crushed, and sieved through a 0.5-mm mesh sieve. A sample of 30 g of crushed cassava starch was dissolved in a bowl containing 100 ml of cold water and initially mixed to produce cassava paste. Precisely 400 ml of boiling water was added to the cassava paste and mixed properly with a stirrer to form a smooth homogeneous gelatinized starch gel. An electronic weighing balance was used to weigh 450 g of both BSFF char and charcoal char and then mixed homogeneously in 1:1 weight ratio. Subsequently, 150 g of the cassava starch was added to the charred samples and mixed using a stirring stick till a thick, black dough was formed.

The briquette mass was estimated using a digital weighing balance (Model: FA 214) and the volume calculated by taking the linear dimensions (diameter and height) of the briquettes using a digital vernier calliper (Model: 30-412-1). Relaxation ratio was computed as a ratio of compressed density to relaxed density of the briquettes.

The gross calorific values for the briquettes were determined using a bomb calorimeter (Model: Yoshinda 1013-J) by burning a unit mass of the various briquettes, each in triplicate.

The proximate characteristics were determined as detailed in the standard test method of wood charcoal (ASTM 2007) and described as follows:

• (a) Moisture content

In the equations, VM, A, FC, and MC indicate the mass percentages (%) of volatile matter, ash, fixed carbon, and moisture content obtained from proximate analysis of the briquettes, respectively.

The correlation of the calculated and experimental higher heating value (HHV) was tested using mean absolute percentage error (MAPE) and root mean square error (RMSE). MAPE quantifies the proximity of the calculated HHV to the experimental value with the lower MAPE indicating a higher accuracy since the correlation error is smaller.

The raw data were analyzed using SPSS for mean and standard deviation. One-way ANOVA was used for determining statistically significant differences among briquettes for calorific values, and ultimate and proximate parameters.

Carbon, hydrogen, and oxygen are the main constituents of biofuels as observed in the ultimate analysis results presented in Table 1.

From Table 1, the carbon, oxygen, and hydrogen content ranged from 56.6 to 61.4%, 32.6 to 38.8%, and 3.1 to 5.0%, respectively. From One-way ANOVA, a statistical significance was identified in C, N, H, and O (p < 0.05). However, there was no statistical significance in S across the different briquettes produced (p = 0.203). The H/C content was highest in CMF biochar and lowest in PMF biochar.

The BSFF was pyrolyzed in order to minimize the emission of CO₂ and smoke through the reduction of the oxygen containing functional groups and the increase of aromatic carbon content. The H/C from this study is within the range of 0.05–0.15 reported for light yard waste biochar (Xu et al. 2021) but lower than 0.52–0.23 for rice straw and husk biochar (Vendra Singh et al. 2020). According to Xu et al. (2021), a low H/C ratio indicates high aromaticity of the biochar, which represents a more stable carbon content in the biochar. Thus, the stability and aromaticity of the biochar used in this study were affected by the H/C and O/C ratios.

The concentration of carbon and hydrogen immensely impact the combustibility of the biofuels. Compared with other biofuels, the carbon and oxygen contents are within the ranges of 30–60% (Ofori & Akoto 2020) and 30–40% (Akowuah et al. 2012). An acceptable level of oxygen is an important parameter in biofuels since oxygen binds carbon and hydrogen, and lowers the calorific value of the biofuel. In addition, oxygen causes flame elongation by diluting the hydrocarbons that are separated, resulting in reduced char quantities in furnaces (Xu et al. 2021). However, the hydrogen content is reasonably comparable to the 3.5 and 5–6% reported by Senchi & Kofa (2020) and Chaney (2010). High carbon content in briquettes indicates that the briquette burns faster since high carbon content aids combustion (García et al. 2012).

Sulphur and nitrogen are significant in the formation of harmful gases and influence the reactions forming ash (Ofori & Akoto 2020). Analysis of the obtained data shows that the sulphur content in all the briquettes ranged from 0.07 to 0.12%. Compared with previous studies, these results are within the acceptable range of 0.14–0.22% and 0.17–0.46% reported by Kpalo et al. (2021) and García et al. (2012). The concentrations of sulphur and nitrogen should be low since they influence the emission of harmful gases (NO_x and SO_x) during combustion. The nitrogen content in the frass could be significantly reduced by increasing the carbonization temperature, which would consequently degrade the protein since changes in elemental composition are dependent on the carbonization temperature (Ward et al. 2014). The low nitrogen and sulphur content in the frass-derived briquettes is a welcome development since there will be a lower risk of emission of nitrogen and sulphur oxides during combustion. This is an indication that burning of the briquettes under this study will contribute less in polluting the environment when compared with briquettes from other substrates.

Table 2 shows the obtained density, proximate analysis results, and the calorific values for the briquettes.

• Density

The average compressed density varied from 980.45 ± 7.8 kg/m³ to 1184.6 ± 33.7 kg/m³. The values obtained for all the briquette types are comparable to the compressed densities from 650 to 950 kg/m³ recommended for briquettes by Mani et al. (2006), and within the range from 952 to 1437 kg/m³ reported by Aransiola et al. (2019). The high density in the different briquettes is an indication of higher energy content in the biofuel. Relaxed densities of the briquettes ranged between 584 and 763 kg/m³, which is in agreement with studies by Aransiola et al. (2019) and Oladeji & Enweremadu (2012). The high values of relaxed densities obtained are an indication of the good quality of the briquettes.

According to Onukak et al. (2017), high-quality fuel should have a higher density and strength, which implies a higher energy-to-volume ratio and longer burning time. However, Okot (2019) reported that increase in briquette density leads to a decrease in porosity, thus limiting air movement and lowering the combustion rate.

The relaxation ratios for the produced briquettes PMF, FMF, and CMF were found to be 2.1, 1.5, and 1.5, respectively (Table 2). These values compare favourably well with the findings by Oladeji & Enweremadu (2012), who reported the maximum and minimum relaxation ratios of 1.33 and 2.86 for briquettes produced from corncob, and Ajimotokan et al. (2019a; b), who reported a relaxation ratio ranging from 2.21 to 2.94 for composite briquettes from corncob and rice husk. Aransiola et al. (2019) further reported relaxation ratios of 1.31 and 1.46 from carbonized corncob briquettes, which is lower when compared to the findings of this study. The high relaxation ratio in the PMF briquette implies presence of more voids in the compressed materials and the small relaxation ratios for FMF and CMF indicated more volume replacement, which according to Ajimotokan et al. (2019a; b) is a crucial aspect for packaging, storage, and transportation. The relaxation ratio indicates the relative stability of the briquettes with a lower relaxation ratio indicating a more stable briquette and vice versa. Thus, the FMF and CMF briquettes are more stable during transportation than the PMF briquette.

• Proximate analysis

The proximate characteristics of the produced briquettes were further analyzed and are presented in Figure 4.

• Moisture content

In this study, the statistical analysis showed that there was a significant difference (p < 0.05) in the MC of the briquettes among the substrates (Table 3). The mean MC for the briquettes was 2.95 ± 0.13, 4.04 ± 0.04, and 5.08 ± 0.04% for the CMF, FMF, and PMF briquettes. These results are favourably comparable to the 3.6–5.1% MC of faecal sludge–derived briquettes reported by Kizito et al. (2022). However, Kebede et al. (2022) reported a range of 8–12% MC for biomass briquettes, which is more than the results of the current study. The MC of briquettes influence the quantity of energy produced during combustion. According to Onukak et al. (2017), briquettes with low moisture content have a higher calorific value since less energy is required to remove any moisture during combustion. In addition, biofuels with lower moisture content have larger pore spaces, which allows easy oxygen diffusion during combustion (Carnaje et al. 2018) thus releasing more heat. Reduced biofuel moisture increases the heat of combustion (Magnago et al. 2020), ignites easily with no slanginess during burning (Katimbo et al. 2014), and burns with minimal smoke emission (Bonsu et al. 2020). Moisture content is an important parameter in defining briquette quality and durability during ignition, transportation, and storage.

• Ash content

Ash content is the concentration of non-combustible mineral material in the biomass. Ash influences the diffusion of oxygen to the fuel surface during combustion and heat transfer to the surface of the fuel (Katimbo et al. 2014). There was a statistical significance (p < 0.05) between the means of the ash content from the three substrates as presented in Table 2. In this study, the ash content ranged from 22.01 to 37.2% with CMF briquettes producing the highest ash content. This is attributed to the presence of sand and other inorganic debris in the chicken manure. The high ash content lowered the calorific values since ash causes slagging, which minimizes diffusion of oxygen to the briquette surface and heat transfer to the interior parts of the fuel during combustion (Demeke et al. 2023). Frass-derived briquettes in this study resulted in higher ash content compared to charcoal (4.2%) reported by Otieno et al. (2022). According to the published literature, excellent non-woody briquettes should have ash content ranging from 6 to 10% (ISO 17225-7 2021). However, the ash content of the frass-derived briquettes was out of this range probably due to the high inorganic composition in the insect frass used in this study. Besides, the ash content results were well comparable to 9.49–55.35% from faecal sludge blended with food market waste (Kizito et al. 2022) and 44–53% from faecal sludge char briquettes (Mwamlima et al. 2023). High ash content affects combustion efficiency and results in particulate matter emissions, hence air pollution. According to Vasileiadou et al. (2023), a biofuel with a high calorific value and high ash content has a lower environmental footprint impact compared to a low HHV and high ash content fuel. Thus, frass briquettes are recommendable for use in a properly ventilated environment.

• Volatile matter

The One-way ANOVA revealed that there are significant differences in the volatile matter values of the briquettes (p < 0.05). Volatile matter in biofuel includes the light hydrocarbons, CO₂, CO, moisture, hydrogen gas, and tars. As presented in Table 3, the highest percentage of volatile matter of the produced briquettes was observed in PMF briquettes (32.59 ± 0.80), while the lowest was in CMF (29.26 ± 0.52%). The volatile matter content observed in this investigation was higher than the findings of Otieno et al. (2022) and Sathiyabarathi et al. (2022), which reported that the produced faecal char–sawdust char briquettes and cowdung briquettes contained 16.9 and 2.17–3.38% volatile matter content, respectively. By contrast, its VM was lower than elephant dung green fuel briquettes (Mainkaew et al. 2023) as well as the sawdust briquettes (Ramírez-Ramírez et al. 2022).

PMF and FMF briquettes had high volatile matter of 32.59 and 31.39%; this indicates that more energy will be needed to burn off the volatile matter prior to the release of its heat energy. According to Onukak et al. (2017), the briquette with the lowest volatile matter should have the highest energy content. However, this was contrary to the findings in this study for CMF briquettes. This can be attributed to the low fixed carbon and high ash content in the CMF briquettes. Nevertheless, a higher volatile matter content implies a higher quantity of emissions and high compatibility with low ash content during burning. In addition, higher volatile matter in biofuels results in easy ignition, improved carbon burn out, reduced nitrogen oxide emission, and better flame stability (Sathiyabarathi et al. 2022).

• Fixed carbon

Table 2 shows that as the volatile matter and ash content decreased, the fixed carbon content increased, which is an indication of HHV of the briquettes. The FC of the briquettes ranged from 29.66 to 42.01%, which was higher than that for elephant dung green fuel briquette (17.80%) (Mainkaew et al. 2023); industrial sludge and textile solid wastes (14.11–31.91%) (Demeke et al. 2023); and corncob and oil palm trunk bark briquettes (21.66–23.12%) (Kpalo et al. 2021). However, the results are lower than the faecal char–sawdust char briquette produced by Otieno et al. (2022) but favourably compare with results of a study by Kizito et al. (2022). Thus, the fixed carbon values obtained in this study can sufficiently enhance briquette combustion. A high fixed carbon indicates an increase in the calorific value. During combustion, fixed carbon serves as the primary heat generator and it's not strictly regulated by any standard since it is primarily dependent on the ash and volatile matter content.

• Calorific value

From the One-way ANOVA subjected to the means of the HHV of the various briquettes, significant (p < 0.01) differences among the HHV of the briquettes were determined. Compared to other biofuel materials, the results obtained for PMF and FMF briquettes were higher than 19.8 MJ/kg (Otieno et al. 2022) and 17.00 MJ/kg (Mainkaew et al. 2023). However, Ramírez-Ramírez et al. (2022) highlighted a calorific value ranging from 19.2 to 21.2 MJ/kg, which is similar to the results of PMF and FMF briquettes. The high HHV of PMF and FMF briquettes is related to the high percentage of volatiles and low moisture content. Despite CMF briquettes having higher ash content and lower fixed carbon, the reported value of 16.26 MJ/kg is comparable to 14.61–16.45 MJ/kg (Sathiyabarathi et al. 2022) and also exceeded the minimum requirement of 14.5 MJ/kg recommended for non-woody briquettes under ISO 17225-7 (2021) standards. The analysis of the results presented in Table 2 shows that carbon content of the briquettes was substantial and influenced the calorific value. Kpalo et al. (2021) elucidated that the higher the fixed carbon (FC), the higher the calorific value, which was also observed in this study. However, the briquettes in this study had lower calorific values than charcoal; 25.7 and 25.6 MJ/kg (Ward et al. 2014; Otieno et al. 2022). A comparison of the HHV of the frass-derived briquettes in this study with other biowaste-derived briquettes is presented in Table 3.

The calorific values of the frass briquettes produced in this study are adequate to sustain combustion and produce adequate heat to cook, thus providing an energy source for low-income communities. In addition, their utilization can reduce the environmental challenges associated with post-treatment of the BSF residues.

The empirical models showed some variations from Model (6) resulting in lower MAPE ranging from 2.12 to 4.47% for the three briquette types as summarized in Table 4.

At 95% confidence level, there is a strong correlation between the empirically determined and experimental calorific values for PMF and FMF briquettes (R² > 0.9). However, this was not the case for the CMF briquette since the R² ranged between 0.58 and 0.636, which indicated a poor correlation. This is attributed to the high ash content and low fixed carbon in the CMF briquettes since the models are based on the proximate analysis characteristics of the briquettes.

Energy system models focus on bioenergy processes, technologies, and feedstocks (Welfle et al. 2020). Using the models provides essential quantitative insights through assessment of biomass resources and bioenergy processes, which is crucial for energy policy decision makers. The use of multiple models in parallel is important so as to build strong overall conclusions.

The lump charcoal had higher emissions compared to the briquettes. The CO and CO₂ varied significantly (p < 0.05) across the treatments, with the AC having the lowest concentration of CO₂ and the highest concentration of CO. CO₂ results from complete combustion while CO is produced due to incomplete combustion of fuels and is harmful to human health. The emissions of CO from the combustion stove are attributed to the low combustion temperatures and short residence time of the combustion gases in the combustion area (Tissari et al. 2009). In addition, the amount and distribution of air in the combustion chamber influence CO production. A large amount of air entering in the flame zone (from the top) of the stove lowers the emission of CO (Petrocelli & Lezzi 2014), which is applicable in a ventilated environment.

The CO emissions obtained in this study are comparable to those of a study by Pałaszyńska et al. (2021), which reported a range of 606.67 to 4,623.76 ppm from combustion of agricultural biomass pellets. Mainkaew et al. (2023) reported 2,070 ppm CO emissions from the combustion of elephant dung green fuel briquettes, which is within the range reported in the current study.

Depending on the thermal capacity of the briquettes, their concentrations of NOx and N₂O varied significantly (p = 0.000). The study has resulted in low concentrations of NOx and N₂O due to the low concentrations of nitrogen compound in the briquettes. The AC produced minimal NOx and N₂O compared to the produced briquettes. The literature shows that NOx formation during combustion is due to three reasons: formed from atmospheric nitrogen, above 1,300 °C (thermal NOx), formed at the flame front (the prompt NOx), and formed from elemental nitrogen contents of the fuel (the fuel-NOx) (Habib et al. 2008). For small domestic stoves, only fuel-NOx is expected since the temperatures are below 1,300 °C (De Soete 1991) and NOx emissions are due to NO formation. The NOx and N₂O emissions were as a result of the oxidation of elemental nitrogen content in the biofuel. The NOx formed is the sum of NO and NO₂, but only the presence of NO was significant in this study. At temperatures above 950 °C, N₂O rapidly decomposes (Nordin 1993), thus N₂O formation was due to low combustion temperatures during combustion. Results of NOx from this study are comparable to 212.59 ppm obtained during combustion of coffee husk (Werther et al. 2000), 100.45–277.95 ppm for combustion of agricultural biomass pellets (Pałaszyńska et al. 2021); but higher than 110 ppm was obtained during the combustion of elephant dung green fuel briquettes (Mainkaew et al. 2023).

From this study, it was determined that reduced oxygen concentration led to incomplete combustion, hence high CO and lower NOx emissions. Reduced CO emissions can be achieved through increasing the ratio between total and secondary air mass flow rate. Thus, burning of the briquettes should be in a well-ventilated area especially for indoor applications (Otieno et al. 2022) so that carbon oxidation is enhanced. Therefore, valorization of frass into briquettes not only stabilizes insect frass but also provides a biofuel and reduces adverse environmental effects, which improves the quality of life in respect to environmental health.

Under the waste-to-wealth approaches, these findings proved that BSFF can be valorized into briquettes for energy production. The low nitrogen and sulphur content in the frass-derived briquettes is a welcome development since there is a lower risk of emission of the resultant toxic gases (NO_x, SO_x) during pyrolysis. The calorific value obtained (16.25–20.70 MJ/kg) indicates remarkable energy potential of the briquettes when compared to established lignocellulose bioenergy feedstocks. From the findings, it can be concluded that BSFF is a promising candidate for solid biofuel applications, which will significantly impact the global bioenergy carbon capture, utilization, and sequestration. However, there is a need for further testing of the fuel characteristics of uncharred frass briquettes produced under pressure and heat or blending with other biomass materials.

The authors greatly appreciate Jomo Kenyatta University of Agriculture and Technology for the provision of the facilities for the experimental setup and all the support provided during the academic journey. Also, we appreciate the European Action Program for University Staff Mobility (ERASMUS), for their financial support that enabled the exchange programme in the University of Jyvaskyla, Finland, and the Meru University of Science and Technology, for their support. The authors also greatly appreciate the Kenya Forestry Research Institute, Karura, for the facilities used to conduct the physicochemical characterization of the briquettes. Special thanks goes to Kenya Marine and Fisheries Research Institute, Mombasa, for helping in carrying out the ultimate analysis in this study. Prof. Anthony Gachanja is acknowledged for the provision of the Sauermann Combustion Gas Analyser, model SiCa-230 (Serial no.1D2404000096) used in this study.

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

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.