## 4 Results and Discussion
### Elemental Analysis
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 \({}^{\circ}\)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 [PERSON] (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 [PERSON] 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 [PERSON] 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]. [PERSON] 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 \({}^{\circ}\)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 \({}^{\circ}\)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 [PERSON] et al. (2023) on the pyrolysis process in the 300-650 \({}^{\circ}\)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 [PERSON] (2017) when performing tests on sunflower shells and coffee brewing waste. The biochar was obtained by pyrolysis in the temperature range of 250-850 \({}^{\circ}\)C with a heat-up rate of 10 \({}^{\circ}\)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 [PERSON] et al. (2012), the total carbon content of bamboo biochar was 76.9%, which is higher than in our own study [56]. [PERSON] 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%.