## 3 Results and Discussion
### Statistical Analysis  
After an extensive analysis of the gorse and broom feedstock and the biochars, some interactions were assessed in order to understand the synergic effect of the pyrolysis conditions and the feedstock on the properties of the particular biochars. The Pearson product-moment correlation analysis (Supporting Information, Table S3) and PCA (Supporting Information, Figure S1) were performed using elemental composition parameters, HHV, and proximate analysis parameters. The results showed that the amount of C was the key variable determining the HHV and the related energy indices. The HHV positively correlated (r = 0.86) with C, while it negatively correlated with H, O, H/C, O/C, and (O + H)/C. Moreover, HHV was positively related to FC and ash content and negatively related to VM.  
The first and second PCs explained 78.8% and 10.9% of the variation, respectively, while these two PCs accounted for 86.7% of the total variation. The distributions of feedstocks and biochars produced at 300 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, resulting from the plot of PC1 versus PC2 scores, are shown in Figure S2 in the Supporting Information. The figure provides a precise visualization of the separation between the studied datasets. Consistent with previous results, we found that pyrolysis temperature was the main factor influencing sample distribution, and not feedstock type or residence time.  
Table 7 shows the results of the ANOVA. The elemental analysis, proximate analysis, energy properties, and C\({}_{\text{oxi}}\) did not show an interactions between the residence time and the feedstock. For the abovementioned properties, the major interactions were found between pyrolysis temperature and residence time. No interactions were registered between feedstock: temperature: residence time, except for the ED, pH, EC, and WEOC (Table 7). Properties like %H, %S, ash, HHV average, EEF, and CVI did not show any interactions.  
The temperature was a key factor affecting the biochar properties, and the residence time was less important. Variation in the feedstock did not seem to have an effect; this result could have been due to the nature of the species: gorse and broom are species of the same family with similar characteristics, and both of them have a high fuel value.  
The analyses of the feedstock and biochars indicate a suitable vaporization strategy. Galicia and north Portugal are areas well known for several fires each year [34]. Species like broom and gorse represent a fire risk in this area due to (i) covering vast areas of scrubland and forest [55], (ii) spreading rapidly, (iii) their susceptibility to fire [27], and (iv) increasing the frequency, intensity, and severity of fires [95].  
Figure 2: PAH contents (ng g\({}^{-1}\)) for broom, gorse, and their respective biochars.  
Gorse has been used as animal bedding for livestock, compost to fertilize fields, and waste [30], but, at the present time, these shrulands have been mostly abandoned [53], generating unused biomass. Broom has been less studied and used in Galicia than gorse, but some investigations in North America indicated Scotch broom as an important competitor for commercial crop trees [19].  
Broom and gorse shrulands have been mostly abandoned, and this biomass remains unemployed. As such, the biochars derived from these feedstocks are suitable as an energy source. From extensive characterization, broom and gorse biochars presented appropriate properties for use as fuels for energy production. From an environmental point of view, they pose no risk related to PAH content. However, future investigations are needed to better understand the use of broom and gorse biochars in energy production; as these biochars are rich in carbon, low in ash, they may be used as carbon-reducing materials in different processes based on carbothermic reduction reactions.