Residues from the Oil Pressing Process as a Substrate for the Production of Alternative Biochar Materials

Abstract

The purpose of this study was to evaluate the feasibility of using residues from cooking oil production to produce alternative biochar fuels along with optimizing the pyrolysis process. The work consisted of carrying out the pyrolysis process at varying temperatures and holding times at the final temperature, and then evaluating the energy potential of the materials studied. Taking into account aspects of environmental emissions, the content of selected oxides in the flue gases generated during the combustion of cakes and the biochar obtained from them was evaluated. Plant biomass derived from a variety of oilseeds, i.e., fennel flower (Nigella sativa L.), rapeseed (Brassica napus L. var. Napus), flax (Linum usitatissimum L.), evening primrose (Oenothera biennis L.), milk thistle (Silybum marianum L. Gaertn.) and hemp (Cannabis sativa L.), was used to produce biochar. The experimental data have shown that the obtained biochar can have a calorific value of nearly 27 MJ kg⁻¹. The use of pyrolysis allowed for a maximum increase in the calorific value of nearly 41% compared to non-thermally processed cakes and a several-fold decrease in carbon monoxide, nitrogen oxides and sulfur dioxide emissions. According to these results, it can be concluded that the pyrolysis process can be an attractive method for using residues from the production of various cooking oils to produce alternative biofuels, developing the potential of the circular economy.

1. Introduction

In the face of increasing climate change, environmental degradation and depletion of fossil fuels, humanity faces a major challenge. However, understanding these problems allows you to approach them as an opportunity to make beneficial changes. Countries are focusing on intensifying research to reduce greenhouse gas emissions. A key area of this research is the development of zero-emission technologies. In the context of growing interest in alternative energy sources and the need to reduce greenhouse gas emissions, unconventional fuels with high energy efficiency are being sought. Between 2004 and 2016, energy consumption in the EU fell by 10%. How much countries invest in renewable energy in many cases depends on environmental and geographic factors. To harness hydropower, we need many rivers and significant land gradients. Austrians, Slovenians or Scandinavian countries are eager to use such solutions. If one wants to invest in wind energy, open spaces are necessary and favorable wind conditions, such as in Ireland or the Netherlands. Solar energy requires many sunny days a year, so countries such as Spain, Cyprus and Malta have favorable conditions for this type of energy [1].

Through the diversity of its sources, and its widespread availability around the world, biomass represents a promising, sustainable and renewable energy source. It is one of the three most important fuel sources for electric and thermal power generation, as up to 50% of the world’s population relies on biomass [2].

An industry sector capable of supplying large quantities of raw material for biochar production is proving to be the oil market, whose continued growth is caused by increasing consumerism among people and growing demand for food. Converting cakes through the pyrolysis process, based on the controlled thermal decomposition of biomass, can provide an alternative form of energy generation and positively influence the development of more sustainable and efficient energy systems. With the above in mind, the optimization of the pyrolysis process of waste biomass, i.e., cakes produced in the process of pressing oil from the seeds of six different types of oil crops, was chosen as the goal of this work. This work consisted of carrying out the pyrolysis process at varying temperatures and holding times at the final temperature, and then evaluating the energy potential of the materials studied. Taking into account aspects of environmental emissions, the content of selected oxides in the flue gases generated during the combustion of cakes and the biochar obtained from them was evaluated.

2. Literature Review

Biomass is a renewable source of energy, but just as importantly, it is available virtually anywhere in the world and in large quantities. The development of technology toward biomass as a renewable source of heat makes it possible to reduce emissions, which has a direct impact on the natural environment. Biomass energy can be in the form of biogas, liquid and solid biofuels, enabling its wider use in place of fossil fuels in the energy and transportation sectors. How biomass is converted can generate different types of energy. Among other things, the diversity of biomass transformation methods is related to the variety of morphology and physical characteristics of biomass. It can be in wet or dry form, thick or fluffy, with high or low ash content, small or large in shape, being homogeneous or heterogeneous. This complexity means that the use of biomass fuels in most cases requires some form of feedstock processing. Energy production mainly uses biomass from low-quality wood residues, straw and other agricultural production wastes, animal manure, sewage sludge, organic wastes such as beet pulp, corn stalks, grasses, vegetable oils and animal fats [3,4,5,6].

The oil market both in Poland and around the world is developing very rapidly. This is mainly due to the fact that there is an increasing demand for food, for the production of which oil is needed; another factor is the increasing population. Cold-pressed oils are also gaining more popularity due to consumer interest driven by their increasing concern for health. The main raw material in Poland used for oil production is rapeseed, which accounted for between 9 and 97% of the total crop area as of 2021. Soybeans, sunflower or flax are less important in the oil industry. Rapeseed oil sales in 2021 were 0.3 million tons. Other popular oils in Poland are sunflower oil at 28,900 tons and soybean oil at 55 tons [7]. The oil pressing process produces pomace (cakes), a byproduct of the process. Cakes are treated as waste and used mainly as a feed additive for livestock. The pomace can be a high-value feed product, such as rapeseed cakes, which has a higher protein content than whole seeds [8]. Flax pomace, on the other hand, can be used in the production of cookies as a substitute for oat, which is high in fat [9]. Cakes from various materials can also be used for food production, such as protein isolates. The amount of protein, carbohydrates or fiber in cakes depends on the raw material from which they were obtained and the method of pressing the oil. Rapeseed pomace, which is mainly used for animal feed, can also be used as a natural fertilizer due to its rich mineral composition. Rapeseed cakes can also be used as an ingredient in food production, for example, to make crisps [10].

Despite the various possibilities for the use of cakes, there is still a need to manage a large portion of these residues. A possible solution can be provided by the energy sector. There are several types of biomass used for energy purposes. One of these is combustion or co-combustion. The co-combustion process can be carried out indirectly, directly or in parallel. Another method is gasification, which is a thermochemical process that aims to convert organic matter or carbonized materials into a combustible gas, which mainly contains methane, carbon dioxide and hydrogen. The gasification medium is air [11,12]. Another method is hydrothermal liquefaction, which is the process of converting wet or dried lignocellulosic raw materials into liquid products that can be used as fuels after valorization [13,14,15,16]. Torrefaction, on the other hand, involves roasting biomass in the temperature range of 250–350 °C at atmospheric pressure and without oxygen, resulting in moisture-free solid products that, unlike raw biomass, have hydrophobic properties. Moreover, they are more homogeneous and more durable, which is important when using waste from the agricultural sector. Biochar is characterized by properties similar to brittle substances, which means it can be mixed with coal in amounts as high as 30% [17].

Pyrolysis is a thermochemical process of biomass transformation. It occurs under anaerobic conditions, or in a small amount of oxygen sufficient for combustion. This process involves the thermal decomposition of a substance rich in organic carbon. The pyrolysis process is classified by many factors, one of which is the temperature at which the process is carried out. In this case, a distinction is made between low-temperature pyrolysis, which is carried out in the temperature range from 450 °C to 700 °C, and high-temperature pyrolysis, or so-called coking, in the temperature range from 900 °C to 1200 °C. Another division of the pyrolysis process concerns its speed. In this case, a distinction is made between slow and fast pyrolysis. To distinguish between them, two basic parameters are used, namely temperature and time. Most often, the process is carried out at a temperature of 400 °C to 1000 °C, and the time the raw material spends in the reactor ranges from a few minutes for slow pyrolysis to up to 0.5 s for very fast pyrolysis. Running the process at a low temperature, as well as slowly heating the raw material, are more effective in terms of solid product formation, while to obtain more liquid raw material, higher temperatures are used. Using very fast pyrolysis at 900 °C, we can obtain gaseous products. The conditions under which the pyrolysis process is carried out are chosen depending on the type of raw material; in this way, one can obtain the desired product. Pyrolysis with a slow increase in temperature was used for thousands of years, and the main product was charcoal. During the slow pyrolysis process, the biomass is heated to 500 °C, and the time it stays in the reactor is between 5 and 30 min. The residence of the particles in the chamber for a long time causes the additional transformation of the originally formed products. The components of the gas phase react with each other to form a solid phase in the form of carbon and a liquid phase. The material processed by pyrolysis can be kept at a constant temperature or gradually heated, and the emitted gases can be regularly removed during the process. Fast pyrolysis is a process carried out at high temperatures. The biomass is heated at a high rate and there is no access to air. The products of this process are gases, aerosols and a charred residue. If the gas phase and aerosols condense, a brown liquid is obtained, which has half of the heating value of traditionally obtained diesel fuel. The advantage of fast pyrolysis is that it generates no waste. Bio-oil and charred products can be used as fuel, and gases can be recycled back into the process. In this process, an important aspect is the rate of heat transfer, which is facilitated by the finely ground material. A feature of fast pyrolysis is also the residence time of the vapors in the reactor, which is very short (less than 2 s). Rapid cooling followed by the condensation of vapors and aerosols helps to produce bio-oil. During the process of rapid pyrolysis, i.e., at temperatures between 500 °C and 700 °C and a short residence time of primary products in the conversion zone, the content of individual products is as follows: liquid fraction—75%, biochar—12% and gas—13%. Comparing this to slow pyrolysis, carried out at temperatures below 550 °C and a long residence time of primary products in the conversion zone, the content of each product is distributed differently: liquid fraction—30%, biochar—35% and gas—35%. Knowing what material is to be subjected to the pyrolysis process and what products are to be obtained, the type of pyrolysis to be carried out should be selected accordingly. Thermal conversion of biomass opens up prospects for creating a more sustainable and environmentally friendly energy system. The move toward zero-carbon energy sources and the efficient use of biomass as a renewable resource is not only a technological challenge, but also a strategic step toward a more sustainable future [18,19,20,21,22,23,24,25,26].

The steadily growing global economy and increasing energy intensity of global industry is contributing to excess CO₂ emissions into the atmosphere. Carbon balance issues are an important problem for modern industry and the subject of ongoing research work. Measures are therefore needed to balance carbon in the atmosphere using carbon capture and storage, for example, in the soil. One solution to this problem may be the use of biochar produced from different types of biomass [27,28].

Biochar is produced from organic matter, by thermal treatment without oxygen. As reported in the scientific literature, the wide range of applications for biocarbon continues to grow and, as it stands, mainly concerns industry, agriculture, cosmetics production, pharmaceuticals and environmental aspects. It can be introduced into soils as a natural additive, added to feedstuffs and silage and also used for water purification [29,30,31]. Biochar can also be used for greenhouse gas reduction, carbon sequestration, soil contaminant removal, wastewater treatment and as a supporting material in composting as well as methane fermentation [32,33,34,35,36,37]. It should also be emphasized that biochar products can be versatile materials used in the field of modern energy storage and conversion. It seems desirable to develop new, efficient, complementary and fully safe methods for the production of carbonaceous materials from renewable resources, which have high efficiency and limited environmental impact [38]. The energetic use of plant biomass residues as a substrate for the production of biochar can also be a viable alternative for the waste management sector. The structure and properties of the carbonate strictly depend on the raw material used in the pyrolysis process. Biochar can be produced from materials of different origins, and these can include energy crops and forestry waste as well as agricultural biomass [39,40,41,42]. Waste from agri-food processing, e.g., fermenting oats, chicken manure and cattle manure, can also be used to produce pyrolysates [43,44].

3. Materials and Methods

3.1. Research Object

Waste plant biomass in the form of fennel flower (Nigella sativa L.), rapeseed (Brassica napus L. var. Napus), flax (Linum usitatissimum L.), evening primrose (Oenothera biennis L.), milk thistle (Silybum marianum L. Gaertn.) and hemp (Cannabis sativa L.) cakes was used to conduct a study on the possibility of using residues from the oil pressing process to produce biochar (Figure 1). The material intended for testing was dried to a moisture level of less than 10% and then crushed to a fraction of less than 10 mm.

Materials for this study were obtained from an oil mill located in Podkarpackie Province, Poland. The post-process biomass created during oil extrusion (cakes) at this plant is intended as an additive for animal feed, but its full use for this purpose is difficult to realize due to the developing oil market in Poland.

3.2. Pyrolysis Process

A retort furnace FCF 2 R designed for heat treatment in the atmosphere of inert gas and equipped with a post-process gas cooler with water well (CZYLOK, Jastrzębie-Zdrój, Poland) was used to carry out the pyrolysis process.

Pyrolysis tests of the test samples were performed in nitrogen atmosphere of 99.99% purity with a gas flow of 10 L/min at temperatures of 400, 450 and 500 °C, respectively, and in times of 5, 10 and 15 min. The obtained pyrolysates were then sifted through a sieve with a diameter of holes equal to 1 mm. The samples were rinsed several times with distilled water and then dried for 12 h (at 80 °C) in order to remove potential contaminants.

3.3. Combustion Process

The process of combustion of post-production residues from the oil pressing process and the biochar produced from them was carried out using a TGA 701 from LECO (LECO Corporation, Saint Joseph, MI, USA). This test consisted of burning 100 g of the sample under controlled conditions at 800 °C. The process took 20 min until the selected material was completely burned.

3.4. Analysis of Samples

The materials were analyzed with the aim of determining the basic physical and chemical parameters, e.g., the total content of carbon, ash, nitrogen, hydrogen, volatile substances and calorific value. The thermogravimetric method, with use of TGA 701 apparatus from LECO (LECO Corporation, Saint Joseph, MI, USA), was used to analyze the contents of ash and volatile substances in the samples. The TrueSpec CHN analyzer from LECO (LECO Corporation, Saint Joseph, MI, USA) was used to test the contents of the total carbon, hydrogen and nitrogen. The AC500 calorimeter from LECO (LECO Corporation, Saint Joseph, MI, USA) was used to determine the calorific value of the materials analyzed. Analysis of the content of carbon monoxide (CO), nitrogen oxides (NOx) and sulfur oxide (SO₂) was carried out using an ULTRAMAT 23 gas analyzer (SIEMENS AG, Munich, Germany). An algorithm that takes into account the density of each gas was used to calculate the content of the individual gases.

Table 1. Algorithm for calculating gas quantities (example for CO).

Calculated Parameters	Method of Calculation
The amount of carbon monoxide (CO)	SCO = 0.5 × (|Xn−1 − Xn|) × (tn−1 − tn)
Amount of gas mixture taken	Sgases = tc × 100
Percentage of carbon dioxide (CO) in the analyzed gas mixture	%CO = SCO/Sgases
Amount of gas mixture taken during the test	AL. = tc × 0.025 [L/s]
Amount of carbon monoxide (CO) taken during the test	ACO = AL. × %CO
Carbon monoxide (CO) density at room temperature (20 °C) [mg L−1]	1250
Amount of carbon monoxide (CO) taken during measurement [mg]	ACO × 1250

presents an example of the calculations for CO.

where

SCO—the calculated amount of CO;

S_gases—the amount of gas mixture taken;

X_n−1 − X_n—CO concentration results recorded by the analyzer during the intervals;

t_n−1 − t_n—measured time intervals;

t_c × 100—100-percentage sum of absorbed gases;

AL.—the amount of gas mixture taken during the test;

ACO—the amount of CO taken during the test;

0.5—the complement of the curve integrating the measurement result;

0.025—the amount of air suctioned per unit time [L/s].

Table 2. The densities of the analyzed gases at room temperature (20 °C).

Gases Analyzed	Gas Density [mg L−1]
Carbon monoxide (CO)	1250
Nitrogen oxides (NOx)	1340
Sulfur dioxide (SO2)	2830

summarizes the density parameters of the analyzed gases, which were used in the calculations.

Samples of biomass and biochars were subjected to laboratory analyses using current analytical standards (

Table 3. Parameters analyzed with research methods.

Parameter	Research Method
Content of carbon, nitrogen and hydrogen	PN-EN ISO 16948:2015-07 [45]	Solid biofuels—determination of total carbon, hydrogen and nitrogen content
Ash content	PN-EN ISO 18122:2023-05 [46]	Solid biofuels—determination of ash content
Volatile matter	PN-EN ISO 18123:2023 [47]	Solid biofuels—determination of ash content
Calorific value	PN-EN ISO 18125:2017-07 [48]	Solid biofuels—determination of ash content

).

3.5. Names of Tests

Biomass samples were described with the use of symbols depending on the type of material, temperature and duration of the pyrolysis process and used for further identification (

Table 4. Names of tests.

Number	Symbol	Type of Material	Pyrolysis Temperature [°C]	Duration of the Pyrolysis [min.]
1.	NC	fennel flower cakes	-	-
2.	NC400/5		400	5
3.	NC400/10			10
4.	NC400/15			15
5.	NC450/5		450	5
6.	NC450/10			10
7.	NC450/15			15
8.	NC500/5		500	5
9.	NC500/10			10
10.	NC500/15			15
11.	RC	rapeseed cakes	-	-
12.	RC400/5		400	5
13.	RC400/10			10
14.	RC400/15			15
15.	RC450/5		450	5
16.	RC450/10			10
17.	RC450/15			15
18.	RC500/5		500	5
19.	RC500/10			10
20.	RC500/15			15
21.	FC	flax cakes	-	-
22.	FC400/5		400	5
23.	FC400/10			10
24.	FC400/15			15
25.	FC450/5		450	5
26.	FC450/10			10
27.	FC450/15			15
28.	FC500/5		500	5
29.	FC500/10			10
30.	FC500/15			15
31.	EPC	evening primrose cakes	-	-
32.	EPC400/5		400	5
33.	EPC400/10			10
34.	EPC400/15			15
35.	EPC450/5		450	5
36.	EPC450/10			10
37.	EPC450/15			15
38.	EPC500/5		500	5
39.	EPC500/10			10
40.	EPC500/15			15
41.	MTC	milk thistle cakes	-	-
42.	MTC400/5		400	5
43.	MTC400/10			10
44.	MTC400/15			15
45.	MTC450/5		450	5
46.	MTC450/10			10
47.	MTC450/15			15
48.	MTC500/5		500	5
49.	MTC500/10			10
50.	MTC500/15			15
51.	HC	hemp cakes	-	-
52.	HC400/5		400	5
53.	HC400/10			10
54.	HC400/15			15
55.	HC450/5		450	5
56.	HC450/10			10
57.	HC450/15			15
58.	HC500/5		500	5
59.	HC500/10			10
60.	HC500/15			15

).

For instance, RC-non-heat-treated rapeseed cakes, HC500/10–hemp cakes after pyrolysis at 500 ℃ and a time of 10 min (Figure 2).

3.6. Statistical Analysis

The effects of experimental factors reflected by the relevant parameters and the relationships between them were examined by Analysis of Variance (ANOVA) with the use of Duncan’s test. In order to compute the statistical analyses, STATISTICA version 12.0 (StatSoft Inc., Tulsa, OK, USA) was applied. A significance threshold of ≤0.05 was set for all analyses. The data were analyzed separately for each type of materials.

4. Results and Discussion

4.1. Elemental Analysis

There are many ways and processes to improve the energy properties of materials. One of them is pyrolysis, the main purpose of which is to increase the calorific value. This parameter is one of the most important when it comes to determining fuel efficiency.

In this study, the pyrolysis process was carried out at temperatures of 400, 450 and 500 °C and holding times of 5, 10 and 15 min at these temperatures. The test material consisted of six types of residues from the oil pressing process in the form of cakes. The biochar obtained was statistically significantly different from the control sample (non-thermally processed biomass) in terms of the content of basic biogenic elements. In contrast to the typical lignocellulosic plant biomass used in the production of biochar, the cakes produced after oil pressing were characterized by a relatively high content of total nitrogen. Increasing the time and temperature of the pyrolysis process further affected the concentration of this element. In the case of fennel flower cakes, the value changed statistically significantly against the initial value (4.77%) from the moment of the pyrolysis process carried out at 400 °C and holding time of 15 min. The highest content of total nitrogen, i.e., 7.68%, was recorded for the pyrolysis process carried out at 450 °C and a temperature holding time of 15 min. In the case of rapeseed cakes, the nitrogen content of the control sample was at the level of 4.34%, and the highest value, i.e., 6.50%, was measured for biochar made from this material by pyrolysis at 500 °C and a temperature holding time of 15 min. The same parameters of the pyrolysis process yielded the highest nitrogen content, i.e., 6.92%, in the biochar prepared on the basis of cakes formed from the production of flax seed oil. The content of total nitrogen in the residue from the production of evening primrose, milk thistle and hemp seed oil was 3.49, 3.12 and 4.7%, respectively. Each time, the pyrolysis process increased the concentration of this element in the materials obtained. In the case of pyrolysates from evening primrose, the content of total nitrogen was in the range of 3.60–4.32%, milk thistle of 3.25–3.77% and hemp of 4.88–5.74%. Pyrolysates prepared at 500 °C and in 15 min had the highest values, recording statistically significant changes relative to the control sample. In the case of research by Ferens (2017), the author notes that both for biochar from coffee brewing waste as well as from sunflower shells, the highest nitrogen value was measured for the pyrolysis process conducted at 450 °C. These values were 4.83% for coffee brewing waste and 3.57% for sunflower shells [49]. In contrast, Hmid et al. (2014) report the nitrogen content of biochar from olive pomace being waste after oil production to be less than 1%. These values were obtained after using a pyrolysis process at 430–530 °C [50]. The study by Martínez-Gómez et al. again indicates a significantly lower concentration of nitrogen in biochar than in our own study. The authors report that in the biochar obtained from grape pomace (pyrolysis 350 and 700 °C), the nitrogen content was in the range of 1.8–2.24% [51]. On the other hand, results coinciding with those obtained in this study are reported by Mazurek et al. (2024). The authors determined nitrogen content in the range of 4.5 to 5.6% for biochar from rapeseed cakes prepared at 700 °C and chemically modified [52]. In a study by Sładeczek and Głodek-Bucyk (2017) on biochar from rye straw, the authors obtained a total nitrogen content result of 1.25%, which was more than four times lower than the maximum total nitrogen content of the biochar analyzed in our own work. In a study by the same authors on biochar obtained from chicken manure, the content of total nitrogen was equal to 3.6%, which was within the range of the content in our own study [53].

The percentage of hydrogen decreased in the obtained biochar as the temperature and time of the anaerobic thermal treatment increased. The hydrogen content changed statistically significantly with respect to the values for the control samples, i.e., fennel flower cakes—7.43%, rapeseed cakes—7.25%, flax cakes—7.16%, evening primrose cakes—5.71%, thistle cakes—6.9% and flax cakes—6.34%. The lowest hydrogen content was recorded each time for pyrolysis carried out at 500 °C and a holding time of 15 min, respectively, for biochar from fennel flower, rapeseed, flax, evening primrose, milk thistle and hemp at 3.93, 3.81, 3.94, 3.66, 3.87 and 3.67%. There was an inverse relationship related to the change in temperature and pyrolysis process time for this parameter than for the nitrogen content. Similar correlations were noted by Ferens (2017) when it came to biochar from coffee brewing waste and sunflower shells. For biochar obtained at the highest temperature, the values were 1.28% for coffee brewing waste and 1.05% for biochar from sunflower shells [49]. In the results reported by Li et al. (2023), the percentage of hydrogen increased with the increase in the temperature of performing the pyrolysis process. In the case of biochar from wheat straw, the increase was more than five times, with a final value of 2.73%. As for corn straw in this case, the difference was even greater, more than 20 times, and the final value was 3.68% [54]. In a study by Crombie et al. (2013) on rice husk biochar, the authors obtained a hydrogen content of 1.97% in the biochar, which was lower than in the analyses conducted in our own study [55]. Mazurek et al. (2024) determined the hydrogen content of rapeseed cake biochar in the range of 1.5 to 2.6% [52].

All pyrolysates obtained were statistically significantly different from the control sample with respect to the total carbon content. The average content of total carbon in non-heat-treated cakes of fennel flower, rapeseed, flax, evening primrose, milk thistle and hemp was at the level of 48.79%. The highest content of total carbon in the pyrolysate from fennel flower, i.e., 64.06%, was recorded at a pyrolysis temperature of 500 °C and a holding time of 15 min. Pyrolysates prepared from rapeseed cakes had the highest total carbon content at 66.43%. Thermal treatment of flax cakes made it possible to obtain a material characterized by a total carbon content of a maximum of 65.71%, again obtained under temperature parameters of 500 °C and a time of 15 min. Pyrolysates prepared from evening primrose cakes had carbon concentrations in the range of 58.45–64.47%, from milk thistle of 62.5–71.67% and from hemp of 59.29–66.83%. Similar relationships are reported by Li et al. (2023) on the pyrolysis process in the 300–650 °C temperature range for materials such as wheat and corn straw. The authors report that the percentage of carbon increases with the increase in the temperature at which the pyrolysis process is carried out [54]. Convergent results were also reported by Ferens (2017) when performing tests on sunflower shells and coffee brewing waste. The biochar was obtained by pyrolysis in the temperature range of 250–850 °C with a heat-up rate of 10 °C/min. The author determined the highest total carbon content for biochar obtained at the highest temperature. The increase in content for coffee brewing waste was 34%, while for sunflower shells, it was 21% [49]. In contrast, in a study by Yao Y. et al. (2012), the total carbon content of bamboo biochar was 76.9%, which is higher than in our own study [56]. Ferreira et al. (2021), in a study on the characterization of biochars from grape pomace, determined the carbon content of the unprocessed material to be 51.1%. The biochar obtained from this material was characterized in turn by a concentration of this element in the range of 67–81% (pyrolysis at 300–800 °C) [57]. Angın et al. (2014), when producing biochar from safflower seed cakes at 500 °C, achieved a total carbon content of 62.45%. Subjecting this material to chemical activation with the use of KOH and a temperature of 800 °C increased the content of this element to 72.09% [58]. Sun et al. (2021), in a study of the pyrolysis of flax seed residues as solid waste generated during flex seed oil production, reported an increase in the total carbon content from 44.9% for unprocessed material to less than 55% for the biochars obtained [59]. Similar correlations as in our own study are reported by Mukhambet et al. (2022), noting an increase in the carbon content with the increasing flax straw pyrolysis temperatures at 400, 500 and 600 ℃ [60].

The percentage of ash increased as the temperature and time of the pyrolysis process increased. After processing the biomass into biochar, the ash content was statistically significantly different from the control sample (fennel flower—6.15%, rapeseed—5.32%, flax—6.35%, evening primrose—12%, milk thistle—99% and hemp—6.55%). After thermal processing, the ash content of the pyrolysates was statistically significantly different from the control sample and was in the following ranges: fennel flower 12.86–23.22%, rapeseed 11.21–22.09%, flax 11.06–21.91%, evening primrose 16.79–19.53%, milk thistle 15.75–17.22% and hemp 20.39–22.41%. The highest percentage of ash among all analyzed biochars, i.e., 23.22%, was recorded for fennel flower pyrolysates prepared at 500 °C and a temperature holding time of 15 min. Tag et al. (2016), in a study on biochar produced from corn kernels, report a percentage content of ash at 18.7%, which meant a result within the range of ash content variations in the analyzed biochars from the cakes in our own study [61]. On the other hand, Mulimani and Navindgi (2017) report that biochar made from mahua de-oiled seed cakes has an ash content of 16.4% [62]. An inverse relationship was observed for the volatile matter, which decreased as the temperature of the pyrolysis process increased. Changes from an average content for the control samples from 73.57 to 16.36% were recorded for biochars obtained at the maximum pyrolysis parameters tested (500℃, 15 min). The lowest volatile matter content (13.93%) of all analyzed materials was in the biochar prepared from flax cakes (

Table 5. Average content of total nitrogen, total carbon, hydrogen, ash and volatile substances for cakes of fennel flower nigella (Nigella sativa L.), rapeseed (Brassica napus L. var. Napus), flax (Linum usitatissimum L.), evening primrose (Oenothera biennis L.), milk thistle (Silybum marianum L. Gaertn.), hemp (Cannabis sativa L.) and prepared biochars.

Research Material	General Nitrogen	Total Carbon	Hydrogen	Ash	Volatile Substances
	%
NC	4.77 a ± 0.04	49.55 a ± 0.11	7.43 d ± 0.02	6.15 a ± 0.02	73.41 f ± 0.11
NC400/5	4.84 a ± 0.02	57.66 b ± 0.10	4.94 c ± 0.01	12.86 b ± 0.06	27.13 e ± 0.05
NC400/10	4.90 a ± 0.03	61.61 bc ± 0.07	4.91 c ± 0.01	13.96 b ± 0.11	25.19 e ± 0.07
NC400/15	5.57 b ± 0.07	61.76 bc ± 0.08	4.92 c ± 0.02	15.62 c ± 0.09	24.48 de ± 0.08
NC450/5	6.64 c ± 0.07	61.49 bc ± 0.13	4.53 b ± 0.01	16.81 c ± 0.10	23.25 cd ± 0.07
NC450/10	7.40 d ± 0.05	62.01 bc ± 0.07	4.53 b ± 0.01	18.92 d ± 0.11	22.41 cd ± 0.06
NC450/15	7.68 d ± 0.05	62.53 bc ± 0.09	4.53 b ± 0.01	20.09 de ± 0.10	21.63 c ± 0.10
NC500/5	6.84 cd ± 0.04	62.95 bc ± 0.13	4.10 a ± 0.01	21.24 de ± 0.16	19.44 b ± 0.10
NC500/10	6.96 cd ± 0.01	63.57 c ± 0.08	4.09 a ± 0.01	22.77 e ± 0.08	17.66 ab ± 0.04
NC500/15	7.30 d ± 0.03	64.06 c ± 0.09	3.93 a ± 0.00	23.22 e ± 0.09	16.50 a ± 0.06
RC	4.34 a ± 0.06	51.97 a ± 0.06	7.25 c ± 0.00	5.32 a ± 0.02	79.33 f ± 0.07
RC400/5	4.65 ab ± 0.05	60.33 b ± 0.16	4.74 b ± 0.00	11.21 b ± 0.06	28.16 e ± 0.09
RC400/10	4.89 b ± 0.04	62.32 b ± 0.06	4.66 b ± 0.00	12.48 b ± 0.05	27.14 de ± 0.11
RC400/15	5.40 c ± 0.04	62.80 b ± 0.03	4.66 b ± 0.01	13.89 c ± 0.05	24.84 d ± 0.02
RC450/5	5.60 c ± 0.05	63.65 b ± 0.10	4.34 b ± 0.01	14.72 c ± 0.05	21.46 c ± 0.09
RC450/10	6.01 d ± 0.02	64.32 b ± 0.06	4.26 ab ± 0.04	17.03 d ± 0.14	20.28 c ± 0.10
RC450/15	6.22 d ± 0.05	65.06 b ± 0.05	4.25 ab ± 0.01	18.57 d ± 0.07	18.62 bc ± 0.07
RC500/5	5.77 cd ± 0.04	65.89 b ± 0.08	4.06 ab ± 0.15	19.86 de ± 0.06	17.53 ab ± 0.05
RC500/10	5.95 cd ± 0.02	66.45 b ± 0.05	3.89 a ± 0.05	21.13 e ± 0.03	17.07 ab ± 0.05
RC500/15	6.50 d ± 0.02	66.43 b ± 0.14	3.81 a ± 0.00	22.09 e ± 0.02	16.33 a ± 0.08
FC	4.92 a ± 0.03	49.29 a ± 0.05	7.16 c ± 0.01	6.35 a ± 0.02	75.69 f ± 0.08
FC400/5	5.37 ab ± 0.06	59.47 b ± 0.05	4.89 b ± 0.01	11.06 b ± 0.05	27.36 e ± 0.02
FC400/10	5.75 b ± 0.06	62.43 b ± 0.05	4.84 b ± 0.01	12.25 b ± 0.08	25.87 de ± 0.08
FC400/15	6.10 b ± 0.03	62.75 b ± 0.05	4.84 b ± 0.01	13.80 c ± 0.07	24.37 d ± 0.03
FC450/5	5.91 b ± 0.05	63.04 b ± 0.11	4.49 ab ± 0.01	14.80 c ± 0.04	21.07 c ± 0.02
FC450/10	5.96 b ± 0.02	63.63 b ± 0.04	4.44 ab ± 0.02	17.01 d ± 0.12	20.05 bc ± 0.06
FC450/15	6.49 cd ± 0.04	64.27 b ± 0.03	4.44 ab ± 0.01	18.36 d ± 0.08	18.84 b ± 0.08
FC500/5	6.43 cd ± 0.04	64.89 b ± 0.08	4.11 a ± 0.09	19.59 de ± 0.05	15.50 a ± 0.04
FC500/10	6.63 cd ± 0.03	65.48 b ± 0.07	4.01 a ± 0.05	20.98 e ± 0.04	14.38 a ± 0.04
FC500/15	6.92 d ± 0.01	65.71 b ± 0.04	3.94 a ± 0.03	21.91 e ± 0.03	13.93 a ± 0.05
EPC	3.49 a ± 0.01	49.23 a ± 0.05	5.71 d ± 0.02	7.12 a ± 0.02	70.32 e ± 0.04
EPC400/5	3.60 ab ± 0.02	58.45 b ± 0.37	4.81 c ± 0.02	16.79 b ± 0.02	30.15 d ± 0.07
EPC400/10	3.65 ab ± 0.03	60.97 b ± 0.04	4.64 c ± 0.02	16.85 b ± 0.03	29.22 d ± 0.08
EPC400/15	3.76 ab ± 0.03	61.06 b ± 0.07	4.75 c ± 0.01	17.52 b ± 0.07	28.45 d ± 0.10
EPC450/5	3.90 bc ± 0.05	58.69 b ± 0.03	4.31 bc ± 0.02	18.6 bc ± 0.05	27.92 cd ± 0.19
EPC450/10	4.49 c ± 0.05	61.64 b ± 0.05	4.3 bc ± 0.01	19.37 bc ± 0.06	25.74 c ± 0.07
EPC450/15	4.77 c ± 0.05	62.22 b ± 0.10	4.25 bc ± 0.01	19.63 bc ± 0.06	24.17 b ± 0.12
EPC500/5	3.95 bc± 0.03	58.63 b ± 0.04	3.98 ab ± 0.01	18.79 bc ± 0.04	23.99 b ± 0.10
EPC500/10	4.05 bc ± 0.01	64.28 b ± 0.05	3.94 ab ± 0.01	18.85 bc ± 0.03	22.47 a ± 0.02
EPC500/15	4.32 c ± 0.02	64.47 b ± 0.07	3.66 a ± 0.04	19.53 c ± 0.03	21.45 a ± 0.02
MTC	3.12 a ± 0.06	47.80 a ± 0.07	6.90 d ± 0.01	5.99 a ± 0.02	69.54 g ± 0.12
MTC400/5	3.25 ab ± 0.02	62.50 b ± 0.04	5.41 c ± 0.02	15.75 b ± 0.06	25.80 f ± 0.12
MTC400/10	3.36 ab ± 0.01	65.40 bc ± 0.08	5.24 c ± 0.01	16.14 b ± 0.07	25.80 f ± 0.09
MTC400/15	3.39 ab ± 0.01	66.83 bc ± 0.06	5.08 c ± 0.03	16.49 bc ± 0.04	24.20 e ± 0.13
MTC450/5	3.42 ab ± 0.01	67.03 bc ± 0.07	4.53 bc ± 0.01	16.29 bc ± 0.05	22.95 de ± 0.09
MTC450/10	3.43 ab ± 0.01	67.44 bc ± 0.08	4.51 bc ± 0.03	16.34 bc ± 0.02	21.70 d± 0.11
MTC450/15	3.47 ab ± 0.02	69.56 bc ± 0.06	4.34 bc ± 0.01	16.87 bc ± 0.04	19.33 c ± 0.06
MTC500/5	3.54 ab ± 0.02	68.44 bc ± 0.06	4.16 ab ± 0.01	16.81 bc ± 0.04	17.47 b ± 0.06
MTC500/10	3.65 b ± 0.02	70.82 c ± 0.03	4.13 ab ± 0.01	16.92 bc ± 0.05	16.71 ab ± 0.02
MTC500/15	3.77 b ± 0.06	71.67 c ± 0.07	3.87 a ± 0.01	17.22 c ± 0.02	15.67 a ± 0.12
HC	4.70 a ± 0.05	47.89 a ± 0.03	6.34 d ± 0.02	6.55 a ± 0.05	73.12 f ± 0.04
HC400/5	4.88 a ± 0.03	59.29 b ± 0.12	4.86 c ± 0.04	20.39 b ± 0.02	23.92 e ± 0.07
HC400/10	5.09 a ± 0.01	62.06 bc ± 0.05	4.74 c ± 0.02	20.50 b ± 0.04	23.46 de ± 0.08
HC400/15	5.19 a ± 0.02	62.74 bc ± 0.07	4.68 c ± 0.01	21.12 b ± 0.03	22.26 de ± 0.10
HC450/5	5.23 ab ± 0.03	61.69 bc ± 0.05	4.19 b ± 0.004	21.54 b ± 0.04	21.35 d± 0.19
HC450/10	5.32 ab ± 0.02	63.39 bc ± 0.06	4.16 b ± 0.02	21.93 b ± 0.02	19.65 c ± 0.07
HC450/15	5.42 ab ± 0.01	64.75 bc ± 0.03	4.07 ab ± 0.01	22.36 b ± 0.03	17.71 b ± 0.12
HC500/5	5.46 ab ± 0.02	62.40 bc ± 0.04	3.85 ab ± 0.03	21.89 b ± 0.05	16.63 b ± 0.10
HC500/10	5.63 ab ± 0.02	66.42 bc ± 0.02	3.81 ab ± 0.01	22.00 b ± 0.04	15.51 ab ± 0.02
HC500/15	5.74 b ± 0.02	66.83 c ± 0.04	3.67 a ± 0.01	22.41 b ± 0.02	14.32 a ± 0.02

Differences between average values marked with the same letters are not statistically significant at the level of p ≤ 0.05 according to the Duncan test. The data were analyzed separately for each type of materials.

). The statistically significant value has already changed for the biochar with the smallest transformation (400 °C/5 min). These values were practically three times lower than the initial value for raw biomass. Similar conclusions were reached by Sładeczek and Głodek-Bucyk (2017), where the proportion of volatile matter also decreased during the conversion of biomass to biochar [53]. In a study by Kazimiersi et al. (2022) on biochar from pistachio nut shells, it was reported that the volatile matter content was equal to 30.62%, giving a similar but slightly higher volatile matter content compared to the analyses in our own work [63]. In a research paper by Wu et al. (2012) on rice straw, a content of volatile matter ranging from 40.2 to 13.1% was reported. The authors noted a significant decrease in the content of volatile matter as the pyrolysis temperature increased from 300 to 700 °C [64].

According to the Duncan test results, statistically significant differences were noted in the content of carbon, hydrogen, ash and volatile substances between the control sample and biochars for all the analyzed materials. In the case of the total nitrogen content, smaller discrepancies in the results were noted and only some samples differed statistically significantly. It was found that the time and temperature of the pyrolysis process affected statistically significant changes in the values of the parameters tested.

4.2. Calorific Value

One of the main parameters defining the energy usefulness of a substance is its calorific value, which is why the post-production residues from the oil pressing process in the form of cakes from fennel flower nigella (Nigella sativa L.), rapeseed (Brassica napus L. var. Napus), flax (Linum usitatissimum L.), evening primrose (Oenothera biennis L.), milk thistle (Silybum marianum L. Gaertn.) and hemp (Cannabis sativa L.), as well as biochar prepared from them, were subjected to calorific value analysis. There was an upward trend, i.e., as the temperature and time of the pyrolysis process increased, the calorific value also increased, and the maximum increase in this parameter was recorded each time for a temperature of 500 °C and a process time of 15 min. The lowest calorific value at 18.88 MJ kg⁻¹, among the six types of cakes analyzed, was characterized by milk thistle seed residues, while the highest, i.e., 22.03 MJ kg⁻¹, was that of rapeseed residues. On the other hand, the calorific value of the prepared pyrolysates was in the following ranges: fennel flower 21.77–24.33 MJ kg⁻¹; rapeseed 22.57–25.22 MJ kg⁻¹; flax 21.46–24.67 MJ kg⁻¹; evening primrose 21.06–24.22 MJ kg⁻¹; milk thistle 23.93–26.59 MJ kg⁻¹ and hemp 22.2–25.13 MJ kg⁻¹. The highest increase in the calorific value after applying anaerobic thermal modification was obtained for the biochar from the milk thistle seed residues produced by pyrolysis at 500 °C and a temperature holding time of 15 min (Figure 3). In a study conducted by Hmid et al. (2014), the authors prepared biochars from the waste of olive processing at temperatures of 430, 480 and 530 °C. The highest calorific value for biochar made from olive cakes was obtained for pyrolysis carried out at 430 °C, and the value obtained was 31 MJ kg ⁻¹ [50]. On the other hand, in Mulimani and Navindgi’s (2017) study of biochar from butter tree seed cakes, a calorific value of 26.43 M/kg was obtained, which was almost identical to the results of biochar from milk thistle cakes obtained in our own study [62]. According to Martínez-Gómez et al. (2023), the calorific value of the grape pomace tested was at 20.4 MJ kg⁻¹. The pyrolysis process of this material increased the calorific value to 27.5 MJ kg⁻¹ after pyrolysis at 700 °C [51]. Ozcimen and Karaosmanoglu (2004), in their study of the liquid fraction obtained from the pyrolysis of rapeseed cake waste, showed, on the other hand, that the bio-oil obtained in this fraction is a fuel with a high combustion value of 36 MJ kg⁻¹ [is]. In contrast, the calorific value of biochar from rapeseed cake obtained at 500 °C was 25.3 MJ kg⁻¹ [65]. In a study by Sun et al. (2021) on the pyrolysis of flax seed residues, the authors identify the calorific value of this material at 25 MJ kg⁻¹. They also state that this parameter decreased after the pyrolysis process. The calorific value of the obtained biochars decreased to 19.5 MJ kg⁻¹ after pyrolysis at 650 °C [59]. The literature also reports the production of biochar from mahua de-oiled cakes. The calorific value of such biomass was at the level of 19.97 MJ kg⁻¹ and increased to the value of 26.43 MJ kg⁻¹ after pyrolysis under temperature conditions of 400–600 °C [62]. According to the results of Durak and Genel (2020), the energy values of liquid products from cakes of fennel flower, after the hydrothermal liquefaction method, ranged from 21.87–31.78 MJ kg⁻¹, and the energy values of solid products (biochar) ranged from 16.22 to 18.28 MJ kg⁻¹ [66]. The literature reports that hemp biochar obtained by pyrolysis at 400–600 °C can be treated as a material with high potential for applications in the solid biofuels sector. The above thesis is correlated with high energy density, associated with high calorific values, i.e., reaching nearly 31 MJ kg⁻¹ [67,68,69,70,71,72,73].

According to the Duncan test results, statistically significant differences in the calorific value were noted between the control sample and biochars for evening primrose cakes, milk thistle cakes and hemp cakes. In the case of the remaining three types of cakes, statistically significant changes were noted between the control sample and biochars obtained at the highest temperature and the longest time. In most cases, the change in the temperature and time of pyrolysis did not affect a statistically significant change in the calorific value between pyrolysates.

4.3. Emissions of CO, NOx and SO₂

Analyses of carbon monoxide (CO) emissions have indicated high emissions of this gas during the combustion of oil pressing residues. The highest value at 5712.3 g kg⁻¹ among the analyzed cakes was recorded for flax residues, while the lowest value was recorded for milk thistle residues (2523 g kg⁻¹). Biochar materials, on the other hand, were characterized by significantly lower carbon monoxide emissions. Each time, the lowest values were recorded for pyrolysates obtained at the maximum parameters of the pyrolysis process. Analyzing all the biochars obtained, the emission levels during their combustion were approximated in the range of 35.33–2821.67 g kg⁻¹. The lowest value characterized the pyrolysates obtained from fennel flower cakes. The content of nitrogen oxides in the cakes subjected to combustion was in the range of 78.96–116.33 g kg⁻¹. The highest emission value was recorded during the combustion of evening primrose seed residues, while the lowest was recorded for rapeseed cake. In the case of the pyrolysates obtained, inconclusive results regarding nitrogen oxide emissions during their combustion were obtained. In the case of cakes from fennel flower, rapeseed, evening primrose and milk thistle, significantly lower values were recorded than in the control sample. In turn, the pyrolysates obtained from flax and hemp had higher NOx emissions than the control samples. The lowest NOx emissions were determined in the case of the combustion of pyrolysates of fennel flower, with an average value of 17.14 g kg⁻¹ and a minimum value of 2.67 g kg⁻¹. Subjecting the obtained cakes to the combustion process made it possible to determine the emission of sulfur dioxide in the range of decreasing values, i.e., 291.33, 142.33, 123.32, 112.67, 87.66 and 94.31 g kg⁻¹ for rapeseed, hemp, fennel flower, evening primrose, flax and milk thistle, respectively. Pyrolysis of cakes at 400 °C resulted in a several-fold decrease in SO₂ in the pyrolysates. Pyrolysates obtained at temperatures of 450 and 500 °C showed no sulfur dioxide emissivity in most of the variants analyzed. Only pyrolysates obtained from rapeseed were found to emit SO₂ during their combustion. The values ranged from 103.67 (pyrolysates obtained at 400 °C and in 5 min) to 13.65 g kg⁻¹ (pyrolysates obtained at 500 °C and in 15 min) (Figure 4). According to Wei et al. (2012), CO emissions during the combustion of firewood and crop residues were 47.8 and 52 g kg⁻¹, respectively. In contrast, the combustion of pine and corn pellets generated 4.4 and 17.9 g kg⁻¹ CO, respectively [74]. Jelonek et al. (2021), in their study on the emission of carbon monoxide, total hydrocarbons and particulate matter during wood combustion in an operating stove, report CO emissions in the range of 795.2 to 1983.1 ppm [75]. In contrast, in a study by Vicente et al. (2021) on the emission of carbon monoxide, total hydrocarbons and particulate matter during wood combustion in a stove operating under distinct conditions, CO emissions did not exceed 73.9 g kg⁻¹ for the materials analyzed, i.e., pine (Pinus pinaster) and beech (Fagus sylvatica) [76]. According to Martínez-Gómez et al. (2023), biochar from grape pomace (pyrolysis in 350 °C) heated to 900 °C generates 2155 μmol g⁻¹ CO. In contrast, an increase in the pyrolysis temperature (biochar 700 °C) resulted in a decrease in emissions to a level of 1154 μmol g⁻¹ [51]. According to Sher et al. (2020), NOx emissions depend on a number of factors including the temperature, combustion time, volatile matter and burner configuration. The authors report that NOx emissions were over 700 ppm when coal was burned. Beech wood generated significantly less NOx in comparison to coal. On the other hand, the smallest values in the range of 251–269 ppm were obtained for torrefied beech wood. The authors noted a similar trend for the emissions of SO₂, which amounted, respectively, to 593, 13.47 and 1.07–1.98 ppm for coal, beech wood and beech biochar. The authors correlate this relationship with the lower content of siraka in biomass that has undergone torrefaction [77]. Lasek et al. (2017) found similar results in their study; they indicate the emissions of SO₂ during the combustion of coal and torrefied willow at 1184 and 2 ppm, respectively [78]. Numerous authors indicate, in turn, that burning mixtures of “raw” fuels such as biomass or coal with torrefied biomass can significantly reduce CO₂, SOx and NOx emissions into the atmosphere [79,80,81,82].

The obtained results allowed for the comparison of the energy potential of a wide group of production residues from the edible oil production sector. Additionally, the parameters of the pyrolysis process of waste biomass generated from six different types of oilseeds were optimized. The results achieved complement the existing knowledge on the impact of the pyrolysis process on gas emissions during the combustion of a wide group of biochars generated from oil cakes.

5. Conclusions

In recent years, increasing emphasis has been placed on research related to the use of biochar as a substitute for fossil fuels. Biochar is treated as a carbon-neutral fuel. When it is burned, 15× less sulfur and 10× less chlorine are released into the atmosphere compared to coal, while maintaining similar heating values. Biochar is undoubtedly a material with high potential for application in the fight against climate change. However, it should be noted that the use of biochar on a global scale involves huge costs in terms of technology, the search for new substrates and the adaptation of production facilities to modern solutions. The obtained results indicate directions for the management of biomass residues from the production of edible oils together with the determination of processing parameters for biocarbon fuels. Appropriate use of the knowledge obtained could allow for the development of the food production and energy production sectors along with technological development and employment growth.

Promoting closed-loop production and the use of a variety of post-production residues seems to be extremely important in times of energy crisis and large-scale energy transition in many countries. Given the above, it seems important to deepen knowledge and conduct further research in the field of the management of waste biomass being a residue of oil production from a variety of oilseeds. Due to possible differences in the composition of the residue of oilseeds depending on its origin, the conducted research requires further replication. Such material heterogeneity could have a significant, uncontrolled impact on the energy parameters of the obtained biocarbon fuels. Additionally, in the next stages, an analysis of the durability of the produced biochar fuels and safety tests during storage and processing of biomass should be carried out. Future research directions should include assessment of dust explosion hazard and assessment of biomass material stability during storage. The possibility of producing biochars from oilseed cakes in conditions close to real ones, i.e., on an industrial scale, should also be assessed. Taking into account the development of energy policy, it would be necessary to develop a detailed scheme of procedure for the management of post-processing residues from the edible oil production sector for use as energy carriers.

The conducted tests allowed us to conclude that the use of the pyrolysis process increased the calorific value of the analyzed materials, and the highest calorific value was characterized by biochar produced in the pyrolysis process conducted at a temperature of 500 °C and a holding time of 15 min. This translated into an increase against the “raw” material of 14.4; 14.5; 19.9; 26.7; 40.8 and 27.3%, respectively, for the cakes of fennel flower, rapeseed, flax, evening primrose, milk thistle and hemp. Among all the prepared pyrolysates, the highest calorific value was the biochar based on milk thistle seed residue, i.e., 26.59 MJ kg⁻¹. It is worth noting that the use of the pyrolysis process of cakes resulted in a several-fold decrease in CO and NOx emissions during their combustion. In the case of sulfur dioxide, there was a total reduction in emissions for biochar produced at higher temperatures.

Based on the analysis performed, it can be concluded that the production of biochar from cakes made from the pressing of various oilseeds offers the prospect of producing alternative biofuels, which can indirectly have a positive impact on the natural environment and the fight against progressive climate change. The use of residues from oil production to make biochar can also be crucial in sustainable development and production and fit in with closed-loop ideas. However, it should be noted that the production of biochar from oilseed residues depends on many factors. Maintaining the quality of the feedstock, reducing costs and energy inputs, and producing high-quality biofuel involves, among other things, properly storing oilseed waste, developing appropriate storage techniques, ensuring optimal logistics processes or optimizing the pyrolysis process on a large scale. The establishment of appropriate production practices, standards and procedures based on modern technologies appears to be essential to realize the energy potential of alternative biochar fuels based on seed residues from cooking oil production.