## 3 Results and Discussion  
### Hydrochar Solid Fraction  
#### 3.1.1 Hydrochar Yields and Chemical-Structural Characteristics  
In order to analyze separately the effect of reaction temperature and time on hydrochar yields, a series of experiments at fixed solid to water ratio were carried out [30]. After 60 min of reaction, the hydrochar yield rose with the reaction temperature from 18.8% at 180 \({}^{\circ}\)C up to a maximum of 30.4% at 300 \({}^{\circ}\)C (Figure 1). The reaction time provided less decomposition of biomass than the temperature. It can be assumed that for relatively short reaction times of 60 min and for low reaction temperatures, the conversion of the initial biomass is still at a germinal stage, as evidenced by the SEM analysis reported in Figure 1. On the other hand, after only 60 min of reaction, as the temperature increases the conversion process becomes significant, favoring the increase in hydrochar yield. After 180 min of reaction, the yield increases with the temperature from 45.4% at 180 \({}^{\circ}\)C up to the highest value of 54.1% at 210 \({}^{\circ}\)C. Such behavior is in line with the generally accepted reaction mechanism of the HTC process of lignocellulosic materials, where the cellulose degradation starts at a reaction temperature higher than 200 \({}^{\circ}\)C while, for a higher reaction temperature, a clear decrease in hydrochar yield is observed as a consequence of gas phase products formation. Accordingly, due to the more relevant volatilization processes under drastic conditions, a notable yield decrease is evident at longer reaction  
Figure 1: Hydrochar yield as a function of reaction temperature at a fixed reaction time. times (300 min) already at 180 \({}^{\circ}\)C. Such findings, taking into account data obtained by other authors [30], confirm that process parameters (the reaction time and the reaction temperature) play a fundamental role on the hydrochar yields and cannot be analyzed separately. The reported results differ from those reported by [PERSON] et al. [23] where a correlation between the reaction time and the hydrochar yield, in the HTC of the orange pomace in the T range 175-260 \({}^{\circ}\)C, was not found. This substantial difference is probably due to the distinctive content of cellulose and hemicellulose and their weight ratio, in the orange peel and in the pomace [31], that probably affects the global carbonization process with a consequent different amount of solid obtained over time, making the results between the two materials difficult to compare. The simultaneous effect of temperature and reaction time, expressed in terms of logR\({}_{0}\), on hydrochar yields is reported in Table 1. A substantial difference was observed in hydrochar yield produced at different logR\({}_{0}\). Experimental data, also summarized in Table 1, indicate that a rise in the logR\({}_{0}\) exerts a comparable promoting effect on the hydrochar production, resulting in a peculiar increasing trend with the severity coefficient and attaining a plateau of ca. 54.05 wt% at log R\({}_{0}\) equal to 5.49 (210 \({}^{\circ}\)C at 180 min). A further increase of the logR\({}_{0}\) causes a hydrochar yield decrease, with the lowest value of 19.69% found at the highest severity of 8.36 (300 \({}^{\circ}\)C at 300 min). A similar trend in the simultaneous effect of temperature and reaction time upon energy densification and mass yield have been reported by other researchers using different biomass raw materials [32; 33; 34], supporting that higher temperature and reaction time lead to an enrichment in fixed carbon and a consequent higher heat production but lower solids recovery, probably due to a higher conversion of cellulose, hemicellulose and lignin. This allows for a series of deoxygenating processes (e.g., dehydration, decarboxylation), resulting in the formation of organic matter and a decrease in oxygen and hydrogen content [31].