2.2.2. Hydrothermal Carbonization (HTC)

HTC was carried out in the 300 mL Parr reactor (Moline, IL, USA) for 30 min at three different temperatures 200, 230, and 260 ◦C (an image of the HTC reactor system is shown in the supplementary section (Figure S2)). In a typical HTC run, 20 g of dry CS (HAF or ORG CS) was mixed with 180 mL of deionized (DI) water inside the reactor vessel. The reactor was stirred at 150 ± 5 rpm throughout the HTC experiment. The temperature of the reactor was controlled using a proportional integral derivative (PID) controller. The heating rate was maintained at 10 ◦C min−1. The pressure was not controlled but monitored throughout the HTC, which was found to be 210 ± 20 psi, 380 ± 20 psi, and 620 ± 20 psi for the 200 ◦C, 230 ◦C and 260 ◦C operating temperatures, respectively. When the reactor reached the desired HTC temperature, it was kept isothermal for 30 min. At the end of the HTC reaction time, the heater was turned off and the reactor was quenched by an ice water bath. It took 15 ± 5 min to cool down the reactor below 50 ◦C. The gas produced during HTC was not the focus of this work and was vented in a fume hood. The slurry was then filtered using Whatman 41 filter paper in a vacuum filtration system. The hydrochar was first washed with approx. 200 mL deionized (DI) water to remove the HTC liquid products that adhered to the hydrochar, and then dried at 105 ◦C for 24 h in an oven. The dried hydrochar was stored in a ziplock bag for further analysis. The solid mass yield (MY) was calculated using Equation (1). Each hydrochar referred to in this work is

labeled according to the HTC temperature. For instance, CS ORG HTC200 represents the hydrochar produced from ORG CS at 200 ◦C.

$$\text{MY} (\text{wt. } \%) = \frac{\text{Mass of dried post} (\text{g}) - \text{process solid} (\text{g})}{\text{Mass of untreated dry feedstock} (\text{g})} \times 100\% \tag{1}$$
