## 3 Results
### Heat Treatment in HRS in N\({}_{2}\) and CO\({}_{2}\)
#### 3.2.2 Char Analysis  
The ash content of chars is reported in Table 2. The mass balance on ash content allows for the estimation that the volatile loss occurred in the HSR ranged between 80 and 96%. Comparison of such values with the proximate analysis of raw biomasses confirms the completion of pyrolysis in all the heated strip treatments.  
Figure 5 reports differential thermogravimetric curves obtained from combustion tests of the char samples. The patterns of most samples result from the superposition of multiple peaks. Within a first order approximation, each peak identified one component, and the temperature of the peak provided an indication of its combustion reactivity. The peak temperatures are reported in Table 2. Notably, they were all higher than the peak temperatures obtained in the differential thermogravimetric curves of the raw biomasses, confirming that no residual undevolatilized matter was included in the char samples.  
Figure 4: Analysis of heavy tar samples: (**a**) Size exclusion chromatography (SEC) chromatograms; (**b**) UV-Vis spectra.  
Figure 5: TGA combustion tests of char samples: (**a**) WS, (**b**) PW; (**c**) ST.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Raw** & **HSR** & **HSR** & **HSR** & **HSR** \\  & **Biomass** & **1573 K N2** & **1573 K CO2** & **2073 K N2** & **2073 K CO2** \\ \hline \multicolumn{5}{c}{\(\mathbf{Ash_{dry}\) (\%)}} \\ \hline WS & 0.4 (\(\pm\)0.1) & 5.8 (\(\pm\)0.6) & 8.8 (\(\pm\)1) & 5.4 (\(\pm\)0.6) & 17 (\(\pm\)2) \\ PW & 0.1 (\(\pm\)0.3) & 3.9 (\(\pm\)0.4) & 4.3 (\(\pm\)0.4) & 4.4 (\(\pm\)0.4) & 4.9 (\(\pm\)0.5) \\ ST & 6.1 (\(\pm\)0.5) & 29.8 (\(\pm\)3) & 32.2 (\(\pm\)3) & 36.8 (\(\pm\)4) & 42.8 (\(\pm\)4) \\ \hline \multicolumn{5}{c}{**DTG peaks (K, \(\pm\)5)**} \\ \hline WS & 520; 560; 690 & 720 & 650; 710 & 820 & 760 \\ PW & 590; 720 & 750 & 750 (\(\pm\)5) & 630; 720 *; 760 & 630; 720; 810 \\ ST & 530; 680 & 690; 710 * & 690; 710 & 710; 810 & 810 \\ \hline \hline \end{tabular}  
\end{table}
Table 2: Results of combustion tests in TGA on raw biomass and Heated Strip Reactor (HSR) chars.  
In the char produced from PW at 1573 K, only one component with a peak at 750 K was identified. After heat treatment of PW at 2073 K in N\({}_{2}\), three char components were observed with peaks at 630, 720 and 810 K. After heat treatment of PW at 2073 K in CO\({}_{2}\), two main components were observed: the first at 630 and the second with two contributions at 720 K and 760 K. The question whether the early components appeared at 630 K after severe heat treatment could be due to tar deposited on the char surface was addressed by extraction in ethanol of the char, according to the procedure already described in [25], as a means to separate char particles from soot or lighter species eventually mixed with the sample collected from the strip. The char after extraction was again analyzed by TGA, and all the peaks remained unchanged, indicating that they cannot be ascribed to undvolatilized or condensed phases.  
Chars produced from ST at 1573 K in N\({}_{2}\) and CO\({}_{2}\) exhibited two partly overlapped peaks at 690 and 710 K. Increasing the temperature of heat treatment in CO\({}_{2}\), the first component decreased, while a new one appeared with a peak around 810 K. The phenomenon was more remarkable in CO\({}_{2}\).  
WS chars produced in N\({}_{2}\) at 1573 K had a main component with a peak at 710-720 K. In the char produced in CO\({}_{2}\) at 1573 K, a more reactive component below 650 K was also present. Increasing the temperature resulted in chars with much lower reactivity: the WS 2073 N\({}_{2}\) char had the lowest reactivity, with a pronounced component peaking at 820 K. The char produced in CO\({}_{2}\) at 2073 K also exhibited a single component peaking at 760 K.