# Hydrothermal Carbonization of Spent Coffee Grounds
## 1 Introduction  
The production and consumption of coffee is increasing worldwide. Since the 2000s, the popularization of coffee-drinking culture has contributed to the significant growth of the coffee shop market, and there has been a growing demand for various types of coffee products. As shown in Figure 1, the annual coffee imports of South Korea have tended to increase each year, with a total of more than 700,000 tons imported over the last five years [1]. Accordingly, the amount of spent coffee grounds (SCGs), the by-products generated after coffee extraction, has also continuously increased. Although some SCGs are used as compost, most are disposed of through incineration and landfills [2]. Therefore, there has been a growing interest in technologies that use SCGs as a renewable energy source [3].  
In particular, the high lipid content and heating value of SCGs have attracted attention in the field of pyrolysis. Thus, studies have been conducted on converting SCGs into fuel, through biodiesel extraction using transesterification, as well as torrefraction, hydrothermal carbonization, and pelletizing [4; 5; 6]. Among them, the biodiesel extraction process requires various pretreatment steps, such as sample homogenization, drying, and extraction, and the typical carbonization method has a low yield of high-efficiency fuel because of the time and energy required for the drying process. To overcome these shortcomings, there is growing interest in wet processes, that do not require a separate pre-drying process, and hydrothermal carbonization, which is performed at lower temperatures than the typical carbonization process [7; 8; 9]. Hydrothermal reactions use the strong dissolution property of subcritical water in a high-pressure reactor to produce finer particles by converting organic polymers into low-molecular-weight compounds [10; 11]. Subcritical water, in particular,has stronger dissolution characteristics than typical solvents due to the increased ionization of H\({}^{+}\) and OH\({}^{-}\)[12]. In addition, to enhance fuel characteristics, the reaction in which the physical properties of subcritical water depolymerize organic polymers and refine particles by breaking the bonds of elements other than carbon is referred to as a hydrothermal carbonization [8]. The experimental conditions that determine the reaction mechanism include the sample type, reaction temperature, and water content. In particular, the reaction temperature determines the main reaction atmosphere, and hydrothermal reactions are known to be the most active at reaction temperatures between 180 and 250 \({}^{\circ}\)C [8; 13]. The water content is known to affect the determination of the main reaction among various hydrothermal reactions, such as decomposition and polycondensation reactions, but it is not exactly known which reaction mechanisms are formed by quantitative factors [14; 15; 16]. According to previous studies, however, sufficient water to submerge samples must be added for the uniform hydrothermal reaction of biomass inside a reactor [8]. As such, in this study, experiments were performed in the 180-250 \({}^{\circ}\)C temperature range with only the undried raw material and sufficient water to submerge the sample added to the reactor. These experiments were conducted to compare and investigate the fuel and material characteristics of the hydrothermal carbonization reaction products of SCGs, and their variation with temperature and water content. In addition, elemental analysis, proximate analysis, heating value analysis, thermogravimetric analysis (TGA), derivative thermogravimetry (DTG), and Fourier transform infrared spectroscopy (FT-IR) were conducted. To investigate their storage characteristics, the reaction products were steeped in water for a period of time. The resulting solution was sampled and subjected to chemical oxygen demand (COD) analysis.