## 4 Results and Discussion
### 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 usitatsissimum_ L.), evening primrose (_Oenothera biennis_ L.), whilstistle (_Silybumarianium_ L. _Gaertn._) and hemp (_Cantabis 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 \({}^{\circ}\)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 \({}^{\circ}\)C and a temperature holding time of 15 min (Figure 3). In a study conducted by [PERSON] et al. (2014), the authors prepared biochars from the waste of olive processing at temperatures of 430, 480 and 530 \({}^{\circ}\)C. The highest calorific value for biochar made from  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Research Material** & **General Nitrogen** & **Total Carbon** & **Hydrogen** & **Ash** & **Volatile Substances** \\ \cline{2-6}  & & & **\%** & & \\ \hline MTC & 3.12 \({}^{\mathrm{a}}\)\(\pm\) 0.06 & 47.80 \({}^{\mathrm{a}}\)\(\pm\) 0.07 & 6.90 \({}^{\mathrm{d}}\)\(\pm\) 0.01 & 5.99 \({}^{\mathrm{a}}\)\(\pm\) 0.02 & 69.54 \(  
olive cakes was obtained for pyrolysis carried out at 430 \({}^{\circ}\)C, and the value obtained was 31 MJ kg \({}^{-1}\)[50]. On the other hand, in [PERSON] and [PERSON]'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 [PERSON] 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 \({}^{\circ}\)C [51]. [PERSON] and [PERSON] (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 \({}^{\circ}\)C was 25.3 MJ kg\({}^{-1}\)[65]. In a study by [PERSON] 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 \({}^{\circ}\)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 \({}^{\circ}\)C [62].