# Hydrothermal Carbonization of Spent Coffee Grounds
## 2 Materials and Methods
### Experimental Methods  
In hydrothermal reactions, the fuel and material characteristics of the reaction products vary depending on the experimental conditions, such as the sample type, reaction temperature, pressure, and water content [8]. Therefore, in this study, the reaction temperature was increased by 10 \({}^{\circ}\)C in the 180-250 \({}^{\circ}\)C range, in which hydrothermal reactions are generally known to exhibit high efficiency. For the water content, experiments were performed with only the raw SCGs (40 g) with a water content of 60% (mass ratio between the dry sample and water = 1:1.5) and 40 g of additional water, to sufficiently submerge the sample (mass ratio between the dry sample and water = 1:4). For the reaction pressure, the internal pressure of the high-pressure reactor was measured as the temperature varied. In addition, to ensure sufficient stirring and reaction time, a residence time of 30 min was provided after each target temperature was reached, and the stirring speed was set to 180 rpm. Each experiment was repeated three times to ensure the reliability of the experi \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multicolumn{6}{c}{**Element Analysis [wt.\%]**} & \multicolumn{6}{c}{**Proximate Analysis [wt.\%]**} & **Higher Heating Value [MJ/kg]**} \\ \hline
**C** & **H** & **N** & **O** & **S** & **Others** & **VM** & **FC** & **Ash** & \\ \hline
50.8 & 6.9 & 2.5 & 35.5 & - & 4.3 & 80.4 & 18.1 & 1.5 & \\ (0.35) & (0.06) & (0.07) & (0.30) & - & (0.33) & (1.86) & (1.71) & (0.13) & 22.6 (0.22) \\ \hline \hline \end{tabular}  
* VM: volatile matter; FC: fixed carbon; HHV: higher heating value; moisture of raw SCGs: 62%; (): standard deviation.  
\end{table} 
Table 1: Characteristics of dry spent coffee grounds used in this study. Figure 2: Schematic diagram of the hydrothermal carbonization experimental apparatus used in this study. mental results. Upon completion of each experiment, the pressure was released through the shut-off valve, and for the reactor was cooled for ten min using the water tank to prevent further reactions. The reaction products were then collected to investigate the changes in the characteristics of SCGs under each experimental conditions. These products were dried at 105 \({}^{\circ}\)C for 24 h before elemental analysis, proximate analysis, heating value analysis, TGA, DTG, COD, and FT-IR analysis was conducted. For the heating value analysis, the HHV was measured using a bomb calorimeter (6100 Compensated Jacket Calorimeter, Parr Instrument Co., Moline, IL, USA). Proximate analysis was conducted using an electric furnace in accordance with ASTM D 3175-89 and ASTM 3174-89 [18]. For elemental analysis, C, H, N, O, and S were analyzed using an elemental analyzer (Flash 2000, Thermo Fisher Scientific K.K, Waltham, MA, USA). IR analysis was conducted at room temperature in the 500-4000 cm\({}^{-1}\) range at 2 cm\({}^{-1}\) intervals using an FT-IR analyzer (Spectrum 100, PerkinElmer, Buckinghamshire, UK) to investigate changes in the chemical structure of the reaction products under each experimental condition. In addition, to investigate the thermal characteristics the thermal characteristics of the reaction products under each experimental condition, TGA/DTG analysis was then conducted with a thermogravimetric analyzer (Pyris 1 TGA, PerkinElmer, Buckinghamshire, UK). The reactor was heated from room temperature to 700 \({}^{\circ}\)C at a rate of 5 \({}^{\circ}\)C/min in a nitrogen atmosphere, with a flow rate of 20 mL/min. To investigate changes in the storage characteristics of the SCGs' reaction products under each hydrothermal carbonization condition, the reaction products generated under each experimental condition were steeped in water for approximately 10 min, after which the resulting solution was collected and subjected to COD analysis.