Introduction

With increasing energy costs and environmental pollution, opportunities are being sought to use renewable energy resources in all industrial sectors, including transport, which contributes significantly to environmental pollution. Already at the end of the twentieth century, biofuels, which are obtained from natural resources, began to be produced and used. Currently, most EU countries use biodiesel, which is obtained from vegetable oil or suitable fatty waste by transesterification of triglycerides with short-chain alcohol, usually methanol1. New raw materials for biofuel production and production technologies with lower energy and material costs and having less negative impact on the environment are being investigated.

The possibilities of using lignocellulosic biomass for biofuel synthesis are being studied, as well as innovative biodiesel synthesis technologies, and the production of new types of biofuels is being researched. It is proposed to replace the conventional transesterification of vegetable oil with an interesterification process, which does not produce glycerol. In this process, a mixture of conventional biodiesel (fatty acid alkyl esters) and glycerol derivatives soluble in biodiesel is produced. As acyl receptors in this process, carboxylate esters of low molecular weight are used, such as methyl acetate, methyl formate, methyl butyrate, methyl isobutyrate, and methyl propionate2,3. From an environmental point of view, it is appropriate to apply the enzymatic catalysis method as a catalyst using enzymatic preparations4. Enzymatic processes occur at low temperatures without the use of chemical reagents.

In order to reduce energy and material costs in biodiesel synthesis, the possibility of directly using oilseeds instead of vegetable oil in its production is being investigated, i.e., applying the simultaneous oil extraction and interesterification process, so-called in situ. In the enzymatic in-situ process, a mixture of oilseed cake and catalyst is formed as a waste product, which is separated from the liquid fuel. It is impossible to separate and reuse the enzymatic catalyst from the waste generated in the process, so it is necessary to look for possibilities of effective utilisation of waste product from biodiesel production. One such option is the production of fuel pellets or biogas.

The production of solid fuel from wood is well studied and applied in practice. In addition, biomass sources such as fast-growing energy crops, cereal crop residues, and woody grasses are increasingly being studied5. Pelleting technology is often used in the production of solid fuel6. It helps to mitigate or avoid the problem of low density, which is particularly common in herbaceous and large-stemmed herbaceous plants. The density of a straw, elephant grass biomass sample is usually less than 150 kg/m37. Even when crushed to a length of 23.2 ± 2.89 mm, the density of the Artemisia dubia only reaches 226.0 ± 6.1 kg/m38. This low-density biomass feedstock, compared to coal or oil, which have densities of 850 kg/m3 and 1100–1500 kg/m3 respectively, is uneconomical to transport beyond 200 km9, and its storage requires a lot of space. Pelletizing can increase the density of straw samples to 600–800 kg/m310. Also, granulated biomass is characterized by better bulkiness. Compared to sawn and chopped wood, the free fall angle of wood pellets is about 1.32 times smaller, 26–30 degrees11: it is easier to mechanize and automate its loading operations. An equally significant advantage is that solid biofuel pellets are characterized by greater homogeneity than the primary raw material. The morphological structure of some plants, as well as the properties of their biomass (moisture, chemical, and fractional composition), are characterized by a large degree of heterogeneity12. The biomass of fibre hemp, which after harvesting the seeds remains more than 10 tons of dry matter per hectare, consists of fundamentally different parts: stems, leaves, hurds and fibres, as well as husks after dehulling the seeds. Crushing and mixing—operations that are applied in the production of pellets or briquettes—eliminate these differences. Therefore, pelletizing creates the prerequisites for more efficient use of agricultural production waste and other biomass waste, implementation of waste-free production technologies and saving resources13. In addition, it is stated that biomass pelletizing leads to a higher quality combustion process, and at the same time, lower pollution and ash content14. Thus, pelletizing is an effective way to produce solid biofuels using both various types of biomass and organic waste.

Although it has been known since ancient times, recently there has been a significant increase in interest in the cultivation of hemp (Cannabis sativa L.) and the possibilities of using its biomass15. The global industrial hemp market is expected to grow from 4.6 billion US dollars in 2019 to 26.6 billion US dollars by 202516. Hemp is most widely cultivated in China, Europe, and Canada. It can grow in loamy and marshy soils, improves soil aggregate structure, and is suitable for rotation with cereals. Hemp is characterized by high yield, is used in various industries, including pharmaceutical, food industry, and production of plant protection products16. Hemp stems contain 60–70% cellulose and are promising in production for biohydrogen, bioethanol, biogas, and solid fuels. Hemp seeds are edible; they contain oil, which can be used for food and biodiesel production. Hemp is mainly grown for its seeds. Hemp seed yields vary widely across European Union countries, from less than 1 ton per hectare to 7.81 tons, according to 2021 results17.

Before the oil is extracted from the seeds, they are subjected to mechanical processing to obtain hemp husks, which are most often used in feed production. Some authors have studied the possibilities of using husks for the production of polymer composites18. The possibilities of using hemp husks for wastewater treatment from heavy metals and petroleum products have also been analysed19. The husks contain over 54% cellulose, over 16% hemicellulose, and about 4% lignin. The protein content is about 12%. The heating value of hemp husks is higher than the heating value of wood and is about 20 MJ/kg20. Considering the development of hemp cultivation in the world and the heating value of the husks, it can be stated that hemp husks, as a by-product of seed processing, are promising for the production of solid fuel.

Another possibility of application of interesterification waste is biogas production. Edible and non-edible oilseed cakes have been found to have good potential for biogas production21. For the production of biogas, using olive cake together with animal manure (pigeon and rabbit waste), at a temperature of 30 °C, biogas can be obtained sustainably in 40 days, and the higher the amount of olive cake in the mixture, the higher the biogas yield22. However, when producing biogas from pure sunflower oil cake, the biogas yield obtained was not very high and reached only 186 to 215 mL CH4/g of volatile solids. This is explained by the fact that the cellulose polymer complex, consisting of cellulose, hemicellulose, and lignin, is difficult to decompose by microorganisms23. Therefore, researchers apply various physicochemical pre-treatment methods in order to overcome the low accessibility of biodegradable organic fraction in lignocellulosic biomass. The researchers found that after processing sunflower oil cake at 170 °C with 1% H2SO4, a yield of 302 ± 10 mL CH4/g of volatile solid (VS) was obtained, i.e., as much as 35% more than from the untreated sample23. The results of another study showed that after treating sunflower oil cake with lime at 75 °C, a 25% higher methane yield was obtained24. It was aimed to increase the yield of biogas from sunflower oil cake by pre-treating at temperatures of 25, 100, 150, and 200 °C. The authors noted that methane yield was only 6.5% higher after treatment at 100 °C compared to pre-treatment at 25 °C25. When studying the production of biogas from a mixture of activated sludge and 1–5% of rapeseed cake, it was found that microwave-assisted pre-treatment of substrates yields twice as much biogas and 10–14% more methane than from pure activated sludge26. After 23 days, 78 ml of methane was obtained from 1 g of cotton oil cake27.

The methane content of biogas is influenced by the carbon-to-nitrogen (C/N) ratio. It is observed that at a high C/N ratio, the reproduction of microorganisms is limited, and at the same time, the yield of biogas decreases. At a lower C/N ratio, a high amount of ammonia accumulates, which is toxic to methanogens, resulting in less biogas production28. Researchers have found that in order to maximize efficiency, it is recommended to adjust the C/N ratio29. After investigating biogas production from various feedstocks with C/N ratios ranging from 10:1 to 82:1, the optimal carbon to nitrogen ratio was found to be between 20 and 3030. However, published research results are quite contradictory. Guarino et al., who studied the anaerobic digestion of water and buffalo manure mixture with a C/N ratio of 9 to 20 for 3 years, found that the C/N ratio of buffalo manure does not affect the bio-methane yield31. Some researchers claim that a lower C/N ratio (10–20) can give good results32. Halmaciu et al. studied biogas production from animal manure (pig, cattle, chicken, ostrich, horse) when the C/N ratio varied from 6.7 (in a medium-sized chicken farm) to 30.2 (in a medium-sized cattle farm), and the highest methane yield was obtained in a small in a pig farm, where the C/N ratio of raw materials is 13.833.

The review of research conducted by scientists shows that there are different opinions on what should be the optimal C/N ratio in raw materials in order to obtain the highest yield of biogas and the amount of methane in it.

Our work aimed to investigate the possibilities of utilising the waste of in situ interesterification with methyl formate when rapeseed is used for the production of biodiesel directly, without oil extraction. In this process, crushed rapeseed is mixed with methyl formate and an enzyme preparation. During the process liquid phase—biodiesel is produced, and solid residues of the rapeseed cake mixture with the catalyst are formed. Some studies were conducted to implement interesterification in a mineral diesel medium to obtain a mixture of 10% biodiesel and mineral diesel as a result. The interesterification in situ using methyl formate has been investigated for the first time under laboratory conditions, and an industrial process has not yet been implemented. However, to comprehensively assess the possibilities for implementing such a process, it was necessary to evaluate the potential uses of the waste generated during this process. In the case of conventional biodiesel production, the oil is already extracted from oilseeds, and the oilseed residues (cake or meal) are used for animal feed. In the case of enzymatic interesterification in situ, a mixture of rapeseed cake and catalyst is produced, the use of which for feedstock production is debatable, especially when the process takes place in a mixture with diesel fuel. Therefore, it was necessary to examine the possibilities of utilizing the waste from the interesterification process for purposes other than feedstock production. The opportunities to use them in energy production—for solid fuel and biogas production—were explored. Optimal composition of the raw materials for producing the highest quality of pellets and biogas was determined, and properties of the produced solid fuel pellets and biogas were evaluated.

Materials and methods

Materials

Two types of biodiesel production waste were used for biogas production, which mainly consisted of a mixture of rapeseed cake and catalyst. In the first case, the waste W was obtained by in situ interesterification of rapeseed oil with methyl formate using the biocatalyst Lipozyme TL TIM (Novozyme) and separation of the liquid phase. In the second case, waste WD was generated by in situ interesterification with methyl formate in mineral diesel medium using the biocatalyst Lipozyme TL TM and separation of the liquid phase. Interesterification was performed in the laboratory under previously described optimal conditions34,35.

Winter rapeseed of variety Cult, included in the National list of plant varieties, was cultivated in the Experimental Farm of Vytautas Magnus University. Biodiesel was filtered from the enzymatic catalyst and rapeseed residues (cake). The solid fraction remaining after filtration was used for the production of pellets. The moisture and volatile matter content in the waste and hemp husks were analysed by the drying to constant weight method.

In order to determine the amount of liquid fraction (oil) in the waste, extraction was performed from the dried waste using hexane as the solvent and 24 h extraction at 60 °C for 24 h in a Soxlet apparatus. The solvent was removed from the extracted liquid phase using a vacuum rotary evaporator “IKA Werke RV06-ML” at 60 °C, and the amount of liquid fraction was determined according to the requirements of the ISO 659 standard. The oil content in waste W reached 7.5%. Waste WD contained a residual amount of mineral diesel; therefore, the total oil and mineral diesel content was determined to be 13.5%.

Hemp husks (H) were used as an additional biomass component for the production of fuel pellets. They were obtained from a hemp processing company that buys hemp from Lithuanian farmers, growing hemp of variety Finola, included in the National list of plant varieties, and by applying the provisions of the Law on Fiber Hemp Cultivation of the Republic of Lithuania.

Biogas production studies were performed using W and WD mixture with sewage sludge from Kaunas wastewater treatment plant JSC Kauno vandenys.

Methods

Production of pellets and evaluation of properties

Pelleting For pelleting, crushed hemp seed hulls with a moisture content of 11% were used. For the research, the hulls were ground in a Retsch SM300 mill (Retsch GmbH, Germany) at a speed of 2400 rpm, using a three-blade rotor and a metal mesh with a mesh size of 2 × 2 mm. The crushed hulls were thoroughly mixed with 5–25% of waste obtained during in situ interesterification (W or WD). For both cases, 6 different samples were formed: crushed hemp seed hulls with 5%, 10%, 15%, 20% and 25% of biodiesel production waste by mass. The compaction of the crushed mass was carried out using a hydraulic automatic press, ATLAS Specac 40 T, and a 20 mm diameter pressing matrix. Each time, 4 ± 0.02 g of the prepared raw material was added to the pressing matrix. Its compaction was carried out using 5, 10, 15, 20, and 25 tons of pressing force and 0.2 min. holding time after reaching the maximum compression force. In order to achieve greater accuracy, 10 pellets of the same composition and pressed with the same force were pressed from each sample.

Evaluation of pellet properties

Determination of calorific value The calorimeter IKA Werke C2000 was used to determine the calorific value of fuel pellets. The samples were weighed on a KERN ABJ balance (accuracy of readings 0.001 g). The test was repeated five times, using different pellets from the same sample. The result is based on the average of the numerical values and the confidence interval.

Determination of ash content The ash content of raw materials and produced pellets was determined following the requirements of standard EN 14775 by weighing and comparing the mass of the burned ash and the initial mass of the materials. The samples were dried in a drying chamber MEMMERT SFP 600 and burned in a laboratory chamber furnace at a temperature of 550 ± 10 °C for 120 min. After cooling, the obtained ash was weighed on an analytical balance KERN ABJ. By assessing the moisture content of the samples under study, their ash content was calculated. The ash content in the dry mass of biomass or their mixtures (AC) was calculated as a percentage according to Eq. 136:

$$AC = \left( {\frac{{m_{3} - m_{1} }}{{m_{2} - m_{1} }}} \right) \cdot 100 \cdot \left( {\frac{100}{{100 - m}}} \right)$$
(1)

where AC—ash content in the sample (%); m1—mass of the empty container (g); m2—mass of the container with the sample before drying (g); m3—mass of the container with ash after drying (g); m—moisture content in the sample (%).

Determination of durability The granules were placed in the DELTA equipment for durability testing according to the requirements of the standard EN 15,210–1:2010. A rectangular parallelepiped-shaped container was rotated 500 times at a speed of 50 rpm. After that, the pellets were sieved through a sieve with holes of 3.15 mm in diameter and weighed on a KERN ABJ balance. Their mechanical durability was assessed by calculating the fraction of the mass of the pellets that remained on the sieve with holes of 3.15 mm in diameter compared to their initial mass before the durability test. The mechanical durability test for each pellet variant was carried out in triplicate. The durability of the pellets was calculated by Formula 237.

$${\text{D}} = \frac{{m_{a} }}{{m_{e} }} \times 100$$
(2)

where mE—pellet weight before the durability testing (g); mA—pellet weight after durability testing (g).

Determination of deformation The deformation of produced pellets was measured according to the requirements of EN ISO 17829:2016 standard. An electric calliper BMI (0–150 mm) was used to determine the height and diameter of the pellets. The height and diameter of the pellet were always measured at the same marked location. The measurements were performed immediately after compression and after 15 min, 60 min, and 3 days after the pellet was removed from the matrix. According to the obtained data, changes in the height and diameter of the formed pellets were analysed.

Biogas production and evaluation of properties

Two types of biodiesel production waste were used for biogas production W and WD.

The amount of moisture and volatile matter in the biogas production substrate was determined by the weight method by drying the sample at a temperature of 105 °C to a constant mass. Carbon and nitrogen content were determined using a CHNS-O analyser (Perkin Elmer Series II CHNS/O 2400). The principle of operation of the analyser is based on the conversion of the measured elements into simple gases (CO2, N2, H2O, and SO2) by burning the sample. The gas was identified by a thermal conductivity detector (TCD). To determine the amount of carbon, hydrogen, nitrogen, and oxygen, the sample was weighed (~ 2 mg), placed in an oven, burned in pure oxygen atmosphere at a temperature of 975 °C, the gas was analysed and calculated as a percentage of C and N. EA2400 Data Manager program is used for data collection and processing.

Biogas production studies were performed in a laboratory anaerobic bioreactor with controlled environmental conditions. Biogas was produced at a mesophilic temperature of 37 ± 1 OC. 300 ml of spent substrate after biogas production (JSC Kauno Vandenys) was used as bacterial substrate. Test samples were mixed with substrate; the amount of the test samples was calculated so that they contained 2.5 g of dry matter. Biogas production studies were performed using: W—catalyst and rapeseed meal mixture after interesterification with methyl formate, WD—a mixture of catalyst and rapeseed meal after transesterification with methyl formate in mineral diesel medium. Sewage sludge (WS) mixtures with 10, 20, or 30% W or WD were also investigated. A sewage sludge sample (WS) without biodiesel production waste was used as a control.

The biogas production process was performed (depending on the amount of biogas produced) for about 30 days by measuring the volume of the produced gas via a milli gas counter (Ritter Apparatebau GmbH, Bochum, Germany) and analysing the gas composition on a Clarus 580 GC gas chromatograph (Perkin Elmer) with a thermal conductivity (TCD) detector. Carrier gases helium and nitrogen (purity > 99.999%). Biogas samples are injected manually with the help of a chromatographic loop. All experiments were performed in three replicates. The result is the arithmetic mean of three measurements.

Statistical analysis

The characteristics of pellets and the results of the anaerobic digestion process were compared statistically. The analyses were performed at least 4 times, and the arithmetic average was calculated. The standard deviation was determined using STATISTICA 6.0. The characteristics of pellets and biogas yield were compared statistically using ANOVA with the level of significance set at 0.05.

Results and discussions

Properties of fuel pellets containing interesterification waste

The moisture and volatile matter content of the raw materials used for the production of pellets were assessed. The moisture content of crushed hemp husks (H) was 8.24%. In the waste from biodiesel production, when biodiesel synthesis was carried out in situ interesterification with methyl formate without mineral diesel (W), the moisture content was 7.38%, and in the waste from biodiesel synthesis in situ in diesel fuel media (WD), the moisture content was 12.31%. Before producing the pellets, the raw material was moistened to 11%. The optimal value of the raw material depends primarily on its type38: for elephant grass, fescue, willows—8–10%39, for Artemisia dubia—from 11.0 to 9.4%, depending on the applied compression force40.

Mechanical durability of pellets is an important property that determines the quality and usability of pellets. One of the biggest problems related to durability occurs during transportation and loading, and unloading work. It is necessary to ensure that the pellets are sufficiently durable. The problem of pellet durability arises due to moisture absorption and pellet falling41. Considering biofuel requirements, pellet durability should be no higher than 98% and no lower than 97.5%, but usually this indicator is higher than 95% for most biomass sources42. The results of the mechanical durability of the pellets are presented in Fig. 1. It was found that pellets containing 100% hemp husks are completely unstable. Lignin in the raw material acts as a natural binder43, but in husks, it is only about 4%. Meanwhile, in wheat straw, it is 15–18%, and in softwood, up to 30%.

Fig. 1
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Mechanical durability (%) of pellets obtained using interesterification waste W.

By adding a larger amount of biodiesel production waste without mineral diesel (W), the mechanical durability of the pellets increases. When the composition of the pellets is 95% hemp husks and 5% of W, the mechanical durability reaches 28.2%, and when the waste content increases to 25%, the mechanical durability of the pellets reaches 91%. However, the durability of the pellets was still lower than that of pellets made from other types of raw materials. The mechanical durability could be increased by increasing the waste amount in the pellets, but for now, one cannot expect that the waste amount can be large, as the in situ interesterification process is still only at the laboratory research stage. Another way to enhance durability is the use of various additives, among which glycerol has good properties44.

Pellets produced using biodiesel production waste with mineral diesel (WD) were unstable. This indicates that mineral diesel residues in the waste negatively affect pellet durability at any tested waste content in the pellet feedstock.

Ash is the material remaining after incomplete combustion of fuel or combustible materials. It consists of non-combustible and non-volatile mineral substances. According to the ISO 17225-2:2021 standard (PN-EN ISO 17225-2:2021-10E; 2021), the ash content of biofuels cannot exceed 2%. When using fuels with a higher content, ash must be removed continuously, which increases energy consumption and process costs. The lowest ash content of 2.44% was found in pellets made from pure hemp husks (Fig. 2). The ash content increases with the addition of biodiesel production waste: with 5% waste W in mixture with hemp husks, the pellets’ ash content reaches 3.04%, and with the waste content increasing to 25%, the ash content of pellets increases to 5.89%.

Fig. 2
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Ash content (%) of pellets produced from a hemp husk mixture with interesterification waste without mineral diesel W.

When using biodiesel waste with mineral diesel WD for pellet production, the ash content was slightly lower (Fig. 3). Using 5% of waste WD for pellet production, the ash content of pellets reaches 2.83%, and using 25% of waste in a mixture with hemp husks, the ash content is higher and reaches 4.31%.

Fig. 3
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Ash content (%) of pellets produced from a hemp husk mixture with interesterification waste with mineral diesel WD.

Other researchers have studied properties of pellets made from 100% hemp stalks, which, like hemp seed husks, are waste37. In that case, the ash content was 3.53%, i.e., 1.4 times higher than the ash content determined in our study using pure hemp husks. When examining pellets made from hemp waste, the ash content determined was 2.59%, while in pellets made from 50% hemp waste, 25% lignin, and 25% oak sawdust ash content was higher than 3%45. When comparing the ash content obtained in this study with that obtained by other researchers, it was observed that the ash content obtained in our study was slightly higher. These differences may be due to the different parts of hemp used and differences in pellet preparation methods.

The calorific value of pellets directly reflects their energetic and economic properties. This study determined the calorific value of 100% hemp husks, biodiesel production waste without mineral diesel, biodiesel production waste with mineral diesel, and pellets produced using different amounts of biodiesel production waste.

The calorific value of pellets made from 100% hemp husks was 19.47 MJ/kg, while pellets produced from biodiesel production waste without mineral diesel W had a calorific value of 18.02 MJ/kg. When the pellets contained from 5 to 25% of this waste, the calorific value ranged from 18.89 to 19.13 MJ/kg (Fig. 4). Meanwhile, the calorific value of pellets produced using 100% biodiesel production waste WD was higher and reached 28.39 MJ/kg. When the pellets contained from 5 to 25% of this waste, the calorific value ranged from 19.55 to 21.49 MJ/kg (Fig. 5). Stulpinaitė et al. determined the calorific value of pellets produced from hemp waste, which was 16.8 MJ/kg37. This is less than in this study; this difference may be due to the use of different parts of the hemp plant for pellets. It can be concluded that hemp husks are a more valuable waste for pellet production than other parts of the hemp plant. This was also confirmed by other research: the calorific value of the husks reached as much as 22.2 MJ/kg and was 1.15 times higher than the calorific value of hemp stems17.

Fig. 4
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Calorific value of pellets produced from a hemp husk mixture with interesterification waste without mineral diesel W.

Fig. 5
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Calorific value (MJ/kg) of pellets produced from a hemp husk mixture with interesterification waste with mineral diesel WD.

Determination of the longitudinal and transverse deformation of the pellets is important for assessing the ability of the pellets to maintain their shape after compression. The length and height of the pellets were measured immediately after compression and after 15 min., 60 min. and 3 days, while they were stored at room temperature. When evaluating the transverse deformation of pellets produced using waste W, it is clear that 15 min. after compression, the transverse deformation of the pellets ranged from 0.003 to 0.006%, and after 3 days from 0.0055 to almost 0.008% (Fig. 6). The longitudinal deformation of these pellets, regardless of the composition of the pellets, varies slightly (Fig. 7). As in the previously mentioned case of transverse deformation, the greatest deformation occurs during the first 15 min. after compression of the pellets, after which the deformation of the pellets occurs less intensively. This variation in pellet deformation is a natural process that depends on various properties, such as moisture content or particle characteristics of the starting material. For example, pellets made from Artemisia dubia raw material with a moisture content ranging from 4.3 to 21.5% showed a density change of more than 20%40.

Fig. 6
Fig. 6The alternative text for this image may have been generated using AI.
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Transverse deformation of pellets produced from a hemp husk mixture with interesterification waste without mineral diesel W.

Fig. 7
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Longitudinal deformation of pellets produced from a hemp husk mixture with interesterification waste without mineral diesel W.

When evaluating the transverse deformation of pellets produced using WD, it can be seen that after compression, after 15 min. and 60 min. the deformation was found to be insignificant, and after 3 days this deformation was negative in most cases, which means that the pellets were not durable and they fell off (Fig. 8). When determining the longitudinal deformation of pellets produced using WD, the trends were similar to those in pellets using W, when this deformation differs slightly, regardless of the composition of the pellets (Fig. 9).

Fig. 8
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Transverse deformation of pellets produced from a hemp husk mixture with interesterification waste with mineral diesel WD.

Fig. 9
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Longitudinal deformation of pellets produced from a hemp husk mixture with interesterification waste with mineral diesel WD.

Biogas yield and properties

The use of sewage sludge for biogas production in combination with other organic raw materials has been studied by many scientists. It is emphasised that this provides several benefits for improving biogas quality and quantity. Gorzna et al. indicate that additional organic raw materials increase microbial activity and C: N ratio46.

The moisture and volatile matter content of the substrate used in biogas production varied significantly: sewage sludge (WS) contained 95% moisture and volatile matter, waste of in situ interesterification of methyl formate (W) contained 7.38% moisture and volatile matter, and waste of in situ interesterification of methyl formate in mineral diesel medium (WD) contained 12.31% moisture and volatile matter.

The dry weight (DM) of the sewage sludge was 5.0%, the dry weight of in situ interesterification with methyl formate waste was 92.62%, and the dry weight of the waste of interesterification in diesel fuel media was 87.69%.

The amounts of carbon and nitrogen in the raw materials used for biogas production were determined, and the C/N ratio was calculated (Table 1). The lowest C/N ratio of 3.2 was determined in the sewage sludge. The ratio of carbon to nitrogen in the waste of interesterification with methyl formate was 7.6, and the C/N of waste from interesterification in diesel fuel media was 17.07.

Table 1 C/N ratio in the substrate used for biogas production.
Full size table

The obtained research results showed that the ratio of carbon and nitrogen in all investigated biodiesel production wastes is too low compared to the optimal ratio determined by scientists studying the biogas production process from various raw materials and wastes47,48. They indicate that the highest biogas yield and methane concentration are obtained at a ratio of carbon to nitrogen content of 10 to 20.

Figure 10 shows the biogas yield using waste of interesterification W. For comparison, the given gas yield was obtained using pure sewage sludge or its mixtures, which contained from 10 to 30% of the waste.

Fig. 10
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Biogas yield from biodiesel production waste W and its mixtures with sewage sludge.

It can be seen from the presented data that the highest amount of biogas is obtained from biodiesel production waste W, which reaches 871.6 ml/g DM after 30 days. Meanwhile, sewage sludge had the lowest biogas yield. Within 30 days, the yield of biogas from sewage sludge was two times lower—488 ml/g DM, compared to the yield obtained from biodiesel production waste W. The yield of biogas is increased when waste is added to the sewage sludge. Biogas yield increases most intensively in the first 10 days. When using sewage sludge mixtures with 10% of W, after 9 days, 468.74 ml/g DM of biogas is formed, after 19 days, 537.27 ml/g DM of biogas, and after 30 days, 561.4 ml/g DM of biogas is produced. The amount of gas increased with a higher amount of waste. Using a mixture containing 10% of W in a mixture with sewage sludge 13% higher biogas yield was determined. The higher biogas yield 677.5 ml/g DM was obtained from 30% interesterification waste W in a mixture with sewage sludge. Using this mixture for biogas production after 5 days 485.3 ml/g DM of biogas was produced, i.e., almost as much as from pure sewage sludge after 30 days. The higher yield of biogas from biodiesel production waste W can be explained by the composition of this waste.

The higher yield of biogas from biodiesel production waste W can be explained by the composition of this waste. It was found that when applying the in situ interesterification process, part of the oil contained in rapeseed remains in the cake, and the oil content in the waste was 7.54%. It is known that oil in the raw material increases the biogas yield. In the biogas production process, fats are hydrolysed into fatty acids, which are subsequently converted into short-chain fatty acids, hydrogen, acetone, and methane. Oil or grease in the substrate for biogas production has a high chemical oxygen demand and high volatile solids content, so it is well digestible49, and the use of such raw materials produces a higher biogas amount when comparing raw materials with a higher protein or carbohydrate content50. Petrovic et al. investigated the dependence of biogas yield on the use of different oilseed meals and processing waste and found a similar biogas yield to ours, which ranged from 750 to 1400 mL/g DM51. Similar results were obtained by Grübel et al. using a mixture of rapeseed meal (1–5%) with activated sludge waste and found that twice as much biogas yield was obtained in 20–22 days as using only activated sludge52. Jha et al. using mango peels and de-oiled seed kernel mixture in a mass ratio of 2:1, the biogas yield of 700 ml/g DM was obtained. Using their mixture in a mass ratio of 1:1, the 740 ml/g DM of biogas was produced, and by mixing mango peels with de-oiled seed kernel in a ratio of 1:4, the 930 ml/g DM was produced54. However, the results obtained by some researchers are not so optimistic. It was determined that Jatropha curcas de-oiled seed kernel produces 490 ml/g DM, mango peels 370 ml/g DM53. The yield of the obtained biogas is lower than in the cases obtained by us.

The fact that oil residues increase the yield of biogas is also confirmed by the authors who studied the possibilities of using non-oily seeds. It was found that using wild tree species, namely Acacia nilotica, Prosopis juliflora, Albizia lebbeck, and Leucaena leucocephala, the yield of biogas obtained under anaerobic conditions is relatively very low, respectively, 208, 227, 219, and 210 ml/g DM53. The data published by Jha et al. are somewhat different. Although rice does not have a high oil content, the use of de-oiled rice bran in biogas production resulted in a relatively high yield of biogas, which reached 547 ml/g DM in 90 days54, a similar yield to our study using WS + WD20% (545.23 mL/g DM). Other authors stated that a significant increase is observed only when the fat content in the feedstock is high. Most studies have found that only a 50:50 fat to sewage sludge ratio results in significantly higher methane content55. In our case, the residual oil in biodiesel production waste content was not so high, so it did not have a very significant impact on the quantity of biogas.

Figure 11 shows the biogas yield using waste from in situ interesterification in diesel fuel media WD and mixtures with sewage sludge. Mixtures containing 10% biodiesel production waste WD gave a biogas yield of 521.27 ml/g SM, which was 6% higher than that of pure sewage sludge. The 545.23 ml/g DM of biogas was obtained from sewage sludge mixtures containing 20% biodiesel production waste WD, and a 572 ml/g DM yield was obtained when the mixtures containing 30% of WD were used. Although not significant, the waste from biodiesel production increased the yield of biogas.

Fig. 11
Fig. 11The alternative text for this image may have been generated using AI.
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Biogas yield from biodiesel production waste WD and its mixtures with sewage sludge.

Although the C/N ratio of WD is 17.1, the biogas yield obtained using pure WD was only slightly higher than that obtained from pure sewage sludge with C/N ratio 3.2. Therefore, it can be assumed that the biogas production process is hindered by the mineral diesel contained in the waste, which negatively affects the bacteria and slows down the biogas production process. This is partially confirmed by the large difference in the yield of biogas when it is produced from pure biodiesel production waste without mineral diesel W. As can be seen from the obtained results, the yield of biogas using W reached 871.6 ml/g DM after 30 days, while the yield using WD was about 40% lower and reached 513.17 ml/g DM.

Another important indicator for evaluating the biogas production process is the quality of the biogas, which is evaluated by the amount of methane in it. The composition of the produced biogas is presented in Table 2. In biogas produced from pure sewage sludge, the methane concentration reached 66.65%. The methane concentration in biogas produced from W was 66.45% and from WD—5.08%. The differences are not significant; a slightly lower concentration is obtained when waste WD was used as substrate. When using mixtures of interesterification waste W and WD with sewage sludge, the concentration of methane varied from 66.06% to 67.7%, and the concentration of carbon dioxide varied between 24 and 28%. From the obtained results, it can be stated that the composition of the obtained biogas is almost unchanged when different wastes or their mixtures with sewage sludge are used; the amount of methane obtained is higher than 65%.

Table 2 Biogas composition depends on the substrate used.
Full size table

Scientists studying the production of biogas from mango peels, de-oiled seed kernel, and their mixtures found that biogas from mango peels contained 67% methane, and from de-oiled seed kernel 70%. The highest methane concentration of 71% as well as biogas yield was obtained in biogas production using a mixture of mango peels and de-oiled seed kernel in a mass ratio of 1:456. A lower concentration of methane was obtained in biogas production using de-oiled rice bran: the biogas contained 59.8% methane39. When producing biogas from non-food seeds—Acacia nilotica, Prosopis juliflora, Albizia lebbeck, and Leucaena leucocephala, it was determined that the methane content in biogas is about 52%57. For biogas production using de-oiled cake and non-edible de-oiled cakes, the highest methane yield of 68% was obtained using groundnut/ jatropha58. The results of our research showed that the amount of methane in biogas is similar to that obtained by other researchers. The positive influence of rapeseed on methane concentration in biogas was observed by Grübel et al., who found that methane yield increases by 10–14% when 1–5% rapeseed meal was is added to activated sludge waste52, but in our case, such a clear effect of rapeseed meal was not observed. Also, no significant negative effect of mineral diesel on the composition of biogas was observed, since the difference in methane content of biogas produced from waste W and WD was not significant. The biogas produced using waste of in situ interesterification with methyl formate W contained a lower amount of hydrogen sulphide than the biogas produced using waste of in situ interesterification in diesel fuel media (WD). Correspondingly, the content of hydrogen sulphide in biogas obtained using mixtures of W with sewage sludge was also lower than in biogas obtained using mixtures WD.

In summary, it can be stated that the waste obtained after the biodiesel synthesis process by applying enzymatic in situ interesterification with methyl formate can be used for biogas production, because both the yield and composition of biogas are similar to those obtained for biogas production using conventional raw materials—sewage sludge. Both pure biodiesel production waste and its mixtures with conventional raw materials are suitable materials for the biogas industry.

Conclusions

Waste of enzymatic interesterification with methyl formate together with hemp husks can be used for the production of solid biofuel. The mechanical durability of pellets increases proportionally when the amount of waste from interesterification with methyl formate without mineral diesel is increased to 25% and reaches 91%. Pellets produced using biodiesel production waste with mineral diesel are unstable. Increasing the amount of interesterification waste in the mixture with hemp husks proportionally increases the ash content of pellets. It reaches 5.89% when producing pellets from a mixture with hemp husks containing 25% interesterification waste without mineral diesel, using the same amount of waste from interesterification in diesel fuel media, the ash content of pellets is lower, 4.31%. Interesterification waste containing residues of mineral diesel positively affects the calorific value of pellets, which increases significantly with increasing waste content in the pellet production feedstock. The highest calorific value was obtained for pellets produced from a mixture containing 25% of interesterification waste in diesel fuel media, and it reached 28.39 MJ/kg, while the calorific value of hemp husk pellets was only 19.79 MJ/kg. The addition of waste without mineral diesel to hemp husks had the opposite effect, with its content increasing to 25%, the calorific value decreased to 18.02%. The transverse deformation of pellets produced using interesterification waste without mineral diesel increased with increasing duration, but the longitudinal deformation after 5 min, 24 h and 3 days changed insignificantly, while in the production of pellets using interesterification was in diesel fuel media after 15 min. and 60 min. the deformation was found to be insignificant, and after 3 days, in most cases, this deformation is negative, which means that the pellets were not durable and fell off. The longitudinal deformation differs insignificantly, regardless of the composition of the pellets.

Interesterification waste can be used for biogas production directly or by mixing it with sewage sludge up to 30%. By using pure waste, a biogas yield of 871.6 ml/g SM was obtained in 30 days. It was almost double the biogas yield obtained from sewage sludge at 488 ml/g DM. In the case when using waste containing diesel residues, the yield of biogas was slightly lower—513.17 ml/g DM. It is likely that the biogas production process is hindered by the diesel contained in the waste, which has a negative effect on the bacteria. When using biodiesel synthesis waste mixtures with sewage sludge, the biogas yield was higher with higher amounts of waste in the mixtures. The difference was observed to be less significant when the waste used for biogas production was obtained during interesterification in mineral diesel medium. In biogas production using pure sewage sludge, the methane concentration reached 66.65%, and when using pure biodiesel production waste W and WD, it was 66.45% and 65.08% respectively. When using mixtures of waste with sewage sludge, the concentration of methane varied from 66.06 to 67.7%, and the concentration of carbon dioxide varied between 24 and 28%. Both the yield and the composition of biogas are similar to those obtained using conventional raw materials for biogas production—sewage sludge, so both pure biodiesel production waste and its mixtures with conventional raw materials can be used for biogas production.