## 3 Results
### Heat Treatment in HRS in N\({}_{2}\) and CO\({}_{2}\)  
#### 3.2.1 Tar Analysis  
Figures 3 and 4 report the results of the analyses carried out on the fraction of tars soluble in acetone (light tar) and on the fraction of tar, which was insoluble in acetone and required further treatment with NMP (heavy tar).  
Five classes of chemical species were identified in the light tar: aliphatics (alkanes and alkenes, aliphatic acid), oxo-aromatics (aromatic aldehydes and alcohols), anhydrous monosaccharides and PAHs with C! 16 (light PAHs) and C! 16 (heavy PAHs).  
[PERSON] et al. [22] report that anhydrous monosaccharides are by far the largest component in light tar of cellulose and hemicellulose, while oxo-aromatics are the most abundant species in lignin tar, followed by anhydrous monosaccharides and light PAHs. In Figure 3, it can be observed that in the light tar produced at 1573 K from both WS and PW, oxo-aromatics and anhydrous monosaccharides were present in comparable amounts, followed by aliphatics and small PAHs present in much smaller amounts. In the light tar produced from ST at 1573 K, instead, the oxo-aromatics definitely prevailed, followed by aliphatics and light PAHs in a comparable amount. When the heat treatment temperature increased from 1573 to 2073 K, anhydrous monosaccharides were completely lost and PAHs, in particular the heavy PAHs, largely increased. The latter were more abundant when pyrolysis was carried out in N\({}_{2}\) than in CO\({}_{2}\), in the case of WS, whereas the opposite occured in the case of ST. PAHs concentration (both light and heavy) was independent from the atmosphere for PW light tar. A particularly relevant concentration of large PAHs was found in tar produced from WS at 2073 K in N\({}_{2}\).  
Figure 3: \(Cont.\)  
Figure 2: Derivative thermogravimetric (DTG) curves of biomasses in: (**a**) N\({}_{2}\), (**b**) air.  
Figure 4a reports the chromatograms obtained by SEC for heavy tar fractions. The heavy tar fraction was present only after the pyrolysis tests at 2073 K. The column was calibrated by using polystyrene and carbonaceous particles in order to correlate retention time with MW. The MW distribution of all the samples were spread in the 10\({}^{4}\) (-7.5 min)-10\({}^{7}\) (-6.5 min) u range. For ST samples in CO\({}_{2}\), a third peak can be observed at very high MW values (at about 4 \(\times\) 10\({}^{8}\) u, which corresponds to dimensions of about 100 nm). The peak at 4 \(\times\) 10\({}^{8}\) u was observed for both ST and WS samples in N\({}_{2}\). This peak was already found in [22] in the heavy tar from pyrolysis at 2073 K of cellulose in both N\({}_{2}\) and CO\({}_{2}\) and of lignin only in N\({}_{2}\). Indeed, it was not observed when lignin was pyrolyzed in CO\({}_{2}\), where, however, a not negligible amount of soot (5 w%) was collected. In the present work, high MW species are also more abundant when pyrolysis is carried out in N\({}_{2}\) than in CO\({}_{2}\), even though soot was not obtained.  
The normalized UV-Vis spectra features of the heavy tar fraction, reported in Figure 4b, are consistent with SEC results; indeed, they show that the highest absorption in the visible part of the spectrum (\(\lambda\) > 400 nm) was registered for the tar produced from ST pyrolysis, which means that it is composed of species with a higher aromatic character, i.e., characterized by a larger aromatic extension. Tars produced in N\({}_{2}\) are for all samples more absorbing than tars produced in CO\({}_{2}\). This effect is particularly evident in WS tar.  
Figure 3: Gas Chromatography Mass Spectrometry (GC–MS) analysis on light tar samples of: (**a**) walnut shells (WS); (**b**) pinewood (PW); (**c**) straw (ST).