# Hydrothermal Carbonization of Spent Coffee Grounds
## 3 Results and Discussion
### TGA/DTG, FT-IR, and COD Analysis  
To investigate how hydrothermal reactions change the thermal characteristics of SCGs reaction products, TGA and DTG analysis was performed on the raw material and hydrothermally carbonized reaction products. The results are shown in Figure 10. The TGA results in Figure 10a,b show an increase in the amount of residue after analysis with increasing reaction temperature. This appears to be because the components that decompose at high temperatures became concentrated, as the components that decompose at low temperatures were decomposed and removed by subcritical water. The DTG results in Figure 10a,b, as well as TGA/DTG analysis of lignocellulosic biomass in previous  
Figure 9: Correlations between carbon and hydrogen contents and the HHV of hydrothermally carbonized SCGs produced with different reaction temperatures and water contents: (**a**) SCG: water = 1:1.5; (**b**) 1:4.  
studies, indicate that at both water content conditions, the DTG cellulose peak between 300 and 350 \({}^{\circ}\)C decreased as the temperature increased, while the hemicellulose peak near 200 \({}^{\circ}\)C tended to increase [23; 24]. Therefore, it was confirmed that as the reaction temperature increased, the proportion of the hemicellulose component increased as the cellulose component was decomposed. Figure 11 shows the FT-IR spectra of the raw materials and the hydrothermally carbonized reaction products. In Figure 11a,b, the peak in the 3500-3200 cm\({}^{-1}\) range that represents O-H bond stretching inside cellulose [25] tended to decrease as the reaction temperature increased. This appears to be because the OH-containing components inside the SCGs were decomposed and removed by subcritical water as the reaction temperature increased. In particular, the peak representing OH was more clearly reduced when the SCG  
Figure 10: Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) curves of raw and hydrothermally carbonized SCGs produced with different reaction temperatures and water contents. **(a)** SCG: water = 1:1.5; **(b)** 1:4.  
water ratio was 1:1.5, probably because the decomposition of OH-containing components increased as the maximum reaction temperature was significantly higher with the reduced water content, due to the process insufficient reactor temperature control, as mentioned in Section 2.1. The peaks in the 2900 and 2800 cm\({}^{-1}\) regions, which represent the stretching of aliphatic C-H bonds in cellulose and hemicellulose, and the peaks in the 1700 and 1600 cm\({}^{-1}\) regions, which represent the C=C and C=O stretching of lignin series, hardly changed. This is because the hemicellulose content, which is the main component of SCGs, was relatively increased due to the pyrolysis of cellulose, and the hydrothermal carbonization temperature set in this study was too low for the sufficient pyrolysis of the lignin component [4; 26; 27]. In this study, COD analysis was performed to investigate changes in the storage characteristics of the hydrothermally carbonized reaction products, and the results are shown in Figure 12. Samples of the solution were recovered after steeping the reaction products in water. With an SCG-water ratio of 1:1.5, this solution exhibited very low COD compared  
Figure 11: Fourier transform infrared spectroscopy (FT-IR) spectra of raw and hydrothermally carbonized SCGs produced with different reaction temperatures and water contents: (**a**) SCG: water = 1:1.5; (**b**) 1:4.  
to the solution obtained with a ratio of 1:4. Thus, the reaction products hydrothermally carbonized with the 1:1.5 ratio were less affected by the reaction temperature than those produced with a ratio of 1:4 in terms of storage characteristics. From Figure 11, it was confirmed that the 1:1.5 condition resulted in a larger reduction in the OH component, which is closely related to reactions involving water, than the 1:4 condition. In addition, the COD decreased as the reaction temperature increased under the 1:4 condition.