## 2 Materials and Methods
### HTC Experimental Procedure  
In a typical run, a mixture of wet OPW and deionized water at the desired biomass to water ratio was ultrasonically agitated for 15 min at room temperature (20 \({}^{\circ}\)C) and then transferred into a 300 mL stainless steel autoclave (series 4540 Parr Instrument Company, IL, USA) for HTC. Since the water to solid ratio has a significant effect on the reaction products, a series of experiments setting the L\(\cdot\)S ration to 4:1, 6:1, 12:1 and 24:1 (\(w/w\)) were performed. For all the experiments, a fixed amount of water was used. Prior to reaction, residual air was removed from the sealed reactor by repeatedly pressurizing with nitrogen and venting to atmosphere. In a typical HTC test, the reaction mixture was heated under autogenous pressure up to the reaction temperature (150-300 \({}^{\circ}\)C), at a heating rate of 5 \({}^{\circ}\)C min\({}^{-1}\) continuously monitored through a thermocouple placed into the autoclave and connected to the reactor controller within the whole. The residence time, after reaching the reaction temperature, was set at 30 min, 60 min, 180 min and 300 min, at a stirring speed of 600 rpm. After the HTC reaction, the autoclave was rapidly cooled at room temperature. Then, the HTC solid and liquid products were separated by vacuum filtration, with a Buchner funnel and filter paper. Afterwards, 40 mL of the obtained liquid aqueous product was extracted by 40 mL of diethyl ether in a separation funnel for carrying out the GC-MS analysis. A rotary evaporator was used for removing diethyl ether at 40 \({}^{\circ}\)C. Each extraction was performed twice (150 mL of diethyl ether). The remaining liquid was defined as light bio-oil in this study. Anhydrorous sodium sulfate was used as a drying agent of the light bio-oil after the extraction. The obtained light bio-oil samples were hereafter named as L-HC T-t, where T denotes the reaction temperature (\({}^{\circ}\)C) and t represents the time (min) of the HTC experiments. The product yield was calculated using the following equation:  
\[\text{product yield }(\%)=\frac{\text{mass of desired product }(\text{g})}{\text{mass of initial wet OPW }(\text{g})}\ \times\ 100 \tag{1}\]  
The solid hydrochar was sequentially washed with warm distilled water and finally dried overnight under vacuum at 100 \({}^{\circ}\)C. The obtained hydrochar samples are hereafter named as S-HC T-t, where T denotes the reaction temperature (\({}^{\circ}\)C) and t represents the time (min) of the HTC experiments. The hydrochar samples, obtained in the experiments performed at different initial pH values at 180 \({}^{\circ}\)C and 60 min, were designated as SA-HC1, SA-HC2, AA-HC1, AA-HC2 and AA-HC3, where the number represents the pH value theoretically calculated,and the prefixes SA and AA refer respectively to sulfuric acid and acetic acid used as acidifying agents. The mass yield of HC (MY) was calculated using the following equation:  
\[\text{mass yield, wt }\%_{\text{o}}=\frac{\text{M}_{Hydrochar, g(\text{db})}}{\text{M}_{\text{Feedstock, g(\text{db})}}}\times 100 \tag{2}\]  
It is well known that the relationship between temperature and time, defined and quantified by [PERSON] and [PERSON] using the severity coefficient (R\({}_{0}\)) based on the assumptions of first-order kinetics and Arrhenius temperature behavior of the aqueous pre-hydrolysis of Kraft pulping, greatly influences the physicochemical properties of the lignocellulosic substrates during subcritical and supercritical water treatments [26].