## 4 Results and Discussion  
### 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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)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  
Figure 2: Selected biochar (500 \({}^{\circ}\)C/10 min) from fennel flower (**a**), rapeseed (**b**), flax (**c**), evening primrose (**d**), milk thistle (**e**) and hemp (**f**) cakes. 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 \({}^{\circ}\)C and in 15 min had the highest values, recording statistically significant changes relative to the control sample. In the case of research by [PERSON] (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 \({}^{\circ}\)C. These values were 4.83% for coffee brewing waste and 3.57% for sunflower shells [49]. In contrast, [PERSON] 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 \({}^{\circ}\)C [50]. The study by [PERSON] 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 \({}^{\circ}\)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 [PERSON] 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 \({}^{\circ}\)C and chemically modified [52]. In a study by [PERSON] and [PERSON] (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.