Process simulation of hydrothermal carbonization of digestate from energetic perspectives in Aspen Plus

Digestate, a nutrient-rich substance, is a potential resource of income in biogas plants. It can be utilized as a soil amendment and solid biofuel due to containing inorganic and organic compounds. However, concerning the environmental regulations, there is a requirement for further processes, including hydrothermal carbonization (HTC). Regarding the selection of optimum conditions from the experimental data, this study proposes the energetic yield as the performance indicator from the techno-economic viewpoint, taking solid load and relative equipment size comparison into account as well as the heating value of hydrochar. The energetic yield is the energy content in hydrochar as the heating value per unit mass of reactor inlet, MJ/kg reactor inlet. Among the investigated data for various digestates, the optimum feedstock and conditions were HTC agricultural residue (with 5.02 Energetic yield MJ/kg reactor inlet) at 200 °C and the residence time of 1 h with 30 % solid load based on the energetic yields, i.e. selected for process simulation. This study also investigates required experimental data for enabling mass balance and simulation models. In addition to yield and proximate analysis of feedstock and hydrochar, the characterization of process water is important for representing the dissolved organics. The available data influences the accuracy and closure of elemental mass balances for process simulation. In addition, this study also investigates simulation aspects for producing hydrochar with 20 % moisture content as well: the impact of property method on energy balance, the heat of reaction compared to the literature values, and heat integration concepts. SRK, PSRK, NRTL, and IDEAL methods reported the same value, −122 kW, as the heat duty of the reactor. This value corresponds to −1.46 MJ/kg dry solid and −1.74 MJ/kg dry-ash-free solid inlet as the heat of reaction.