## 3 Results  
### Biochars Characterization  
A summary of the biochars physiochemical properties of interest in the present research is reported in Table 2.  
Figure 1: Polymerization process of epoxy resin with secondary amine based curing agent. Using the same HTT and similar residence times, the differences in the properties of biochars are due to the differences of the feedstocks [20] as clearly evidenced by TGA, ultimate and proximate analysis [14]. Rice husk (RH) showed a remarkably high ash content, up to 47.93 wt.%, with the ash mainly composed by silica [21]. Oil seed rape (OSR) ash content was particularly high (19.50 wt.%) according to [PERSON] et al. [22], while _Miscanthus_ straw (MSP) and wheat straw (WSP) ash content close to 10 wt.%. Mixed softwood (SWP) ash content was the lowest: 1.25 wt.%. Carbon percentages of WSP and OSR were similar (68.26 wt.% and 68.86 wt.%, respectively), while higher carbon contents were found for MSP and SWP (75.41 wt.% and 85.53 wt.%, respectively). In agreement with its high ash content, RH showed a low carbon content: 48.49 wt.%. Nitrogen content was generally between up to 1 wt.% with the exception of MSP and SWP that showed very low values (0.78 wt.% and 0.10 wt.%, respectively). Specific surface area values of RH, WSP, and RH were all close of up to 20 m\({}^{2}\)/g while MSP and OSR had higher and lower values, respectively (33.6 and 7.3 m\({}^{2}\)/g, respectively). The trend of surface area values can be understood looking at the FE-SEM captures shown in Figure 2.  
According to Figure 2c,d, OSR showed the presence of pores and quasi-collapsed channels filled with small carbon particles (average size below 1 \(\upmu\)m) that sensibly reduced the overall available area. MSP showed a similar behavior but, as clearly reported in Figure 2b, the 3 to 4 \(\upmu\)m wide channels were generally empty and filled with particles only after damage occurred. Open and diffuse channels were present in WSP together with small round shaped inter-channels pores. Inter-channels pores represented a proper wood structure and they became more relevant after pyrolytic treatments [9; 23]. According to its softwood original structure [24], SWP showed a very clean surface with isolated pores having an average diameter of 2 \(\upmu\)m (Figure 2g). RH showed a very interesting morphology with the presence of an external core covered by spikes (Figure 2e) enveloping a honeycomb structure (Figure 2f) with an average size around 1 \(\upmu\)m and wall thickness around 700 nm. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Proximate Analysis (wt.\%)**} & \multicolumn{4}{c}{**Ultimate Analysis (\%)**} & **Specific Surface** \\ \hline
**ID** & **Moisture** & **Ash** & **VOCs** & **Fixed Carbon a** & **C** & **\(H\)** & **\(N\)** & **\(O\) b** & **Area (m\({}^{2}\)/g)** \\ \hline MSP & 1.83 & 12.15 & 11.62 & 74.40 & 75.41 & 2.42 & 0.78 & 9.24 & 33.6 \\ \hline WSP & 1.68 & 10.55 & 21.25 & 66.52 & 68.26 & 2.10 & 1.39 & 6.92 & 26.4 \\ \hline RH & 1.54 & 47.93 & 7.48 & 43.05 & 48.49 & 1.24 & 1.04 & 2.47 & 20.1 \\ \hline SWP & 1.52 & 1.25 & 14.20 & 83.03 & 85.53 & 2.77 & 0.10 & 10.36 & 26.4 \\ \hline OSR & 2.61 & 19.50 & 16.38 & 61.51 & 68.85 & 1.82 & 1.59 & 8.91 & 7.3 \\ \hline \hline \end{tabular}  
* Calculated as difference, b calculated as difference considering the ash content.  
\end{table} 
Table 2: Main chemical and physical properties of biochar samples. Figure 2: FE-SEM captures of (**a,b**) _Miscanthus_ straw (MSP), (**c,d**) oil seed rape (OSR), (**e,f**) rice husk (RH), (**g,h**) mixed softwood (SWP), and (**i,j**) wheat straw (WSP). Biochars were also analyzed by FT-IR in ATR mode and spectra were reported in Figure 3.  
\(\
u_{\text{O-H}}\) (3000-3500 cm\({}^{-1}\)) and \(\
u_{\text{C-H}}\) (2850-2900 cm\({}^{-1}\)) bands were not detected in the samples in agreement with the low amount of hydrogen and oxygen (Table 2).