## 2 Materials and Methods
### HTC Experimental Procedure  
The HTC experimental apparatus used in this study, including a 50 mL internal volume stainless-steel (AISI 316) batch reactor, consisted of an adaptation of reaction system configuration already reported in the literature [51]. A schematic diagram of the experimental system and few pictures of the apparatus are shown in the Supplementary Materials (Figures S1 and S2, respectively).  
Figure 1 shows the temperature trend during HTC trials at the different operating conditions. The apparatus took between 21 and 34 min to reach the set temperature and 33 to 37 min to cool down from set temperature to room temperature, once the HTC run was stopped.  
It can be observed that the temperature of the reactor rose after about two minutes after the beginning of the heating phase. This can be explained by the time delay due to heat transfer between the thermal input of the band heater and the temperature measured by the thermocouple.  
Experimental trials were carried out at three different temperatures (180, 220, and 250 \({}^{\circ}\)C) keeping a fixed residence time of 1 h. In the first step (HTC_R0), a mixture of 5.0 \(\pm\) 0.1 g of dried biomass and 25.0 \(\pm\) 0.1 g of deionized water (DI) was loaded into the reactor, in order to obtain a biomass to water ratio on a dry basis (B/W) of 0.2. The reactor was then sealed and N\({}_{2}\) was fluxed through the system to purge the reactor from the air. After purging, excess nitrogen pressure was released and all the needle valves were closed, before starting the heating phase. The residence time was measured starting from the instant the controller indicated the desired value of the temperature set point.  
At the end of the residence time, the reactor was cooled down by a two-step procedure. Firstly, a massive stainless-steel disc at \(-10\)\({}^{\circ}\)C was positioned at the bottom of the reactor; secondly, once the reactor reached the temperature of about 130 \({}^{\circ}\)C, quenching to room temperature was achieved by using a stainless-steel radiator with flowing tap water. The radiator was specifically designed for the scope, to be easily inserted into the reactor body. To evaluate the gas yield, the excess of pressure was released by opening the reactor valve and the gas left flowing inside the graduated cylinder.  
Once the volume was measured, the reactor was opened and the reaction mixture (slurry) was filtered to separate solid phase from aqueous phase by vacuum filtration.  
The solid phase was washed with 30 mL of DI and dried in a ventilated oven at 105 \({}^{\circ}\)C for 24 h. The filtrate was referred as process water (PW) and stored at \(-10\)\({}^{\circ}\)C.  
For the recirculating steps 1-2 (HTC_R1 and HTC_R2), PW of the previous step was used as solvent and mixed with biomass at BW of 0.2. Each run was repeated at least three times, to evaluate the deviation of the data and ensure reproducibility.  
After drying the solid yield was then computed as mass of dry hydrochar collected to the mass of initial dry feedstock (\(\mathrm{g_{hydrochar}}/\mathrm{g_{dry\, feedstock}}\)). The gas yield was computed  
Figure 1: Typical trends of temperature during hydrothermal carbonization (HTC) runs performed at 180, 220, and 250 \({}^{\circ}\)C.  
with the ideal gas law, assuming atmospheric pressure, temperature of 30 \({}^{\circ}\)C and CO\({}_{2}\) as the sole gaseous product [51]. The liquid yield was computed by difference.  
Hydrochar (HC) and PW obtained from the HTC process with DI were designated as R0. The products obtained from the two recirculating steps were defined as R1 and R2, as it can also be seen in Figure 2.