Agro-industrial waste to solid biofuel through hydrothermal carbonization

• HTC of grape marc at T: 180, 220, 250 °C and residence time (RT): 1, 3, 8 h.
• Hydrochar yield in the range 0.56 (T: 250 °C; RT: 8 h)–0.76 (T: 180 °C; RT: 1 h).
• Gas yield in the range 0.02 (T: 180 °C; RT: 1 h)–0.09 (T: 250 °C; RT: 8 h).
• Molar fractions in the gas: CO2 from 94% to 99%, CO from 0.7% to 6%, H2 + CH4 < 1%.
• Increase in HHV due to HTC from 24% (T: 180 °C; RT: 1 h) to 54% (T: 250 °C; RT: 8 h).

In this paper, the use of grape marc for energy purposes was investigated. Grape marc is a residual lignocellulosic by-product from the winery industry, which is present in every world region where vine-making is addressed. Among the others, hydrothermal carbonization was chosen as a promising alternative thermochemical process, suitable for the treatment of this high moisture substrate. Through a 50 mL experimental apparatus, hydrothermal carbonization tests were performed at several temperatures (namely: 180, 220 and 250 °C) and residence times (1, 3, 8 h). Analyses on both the solid and the gaseous phases obtained downstream of the process were performed. In particular, solid and gas yields versus the process operational conditions were studied and the obtained hydrochar was evaluated in terms of calorific value, elemental analysis, and thermal stability. Data testify that hydrochar form grape marc presents interesting values of HHV (in the range 19.8–24.1 MJ/kg) and physical–chemical characteristics which make hydrochar exploitable as a solid biofuel. In the meanwhile, the amount of gases produced is very small, if compared to other thermochemical processes. This represents an interesting result when considering environmental issues. Statistical analysis of data allows to affirm that, in the chosen range of operational conditions, the process is influenced more by temperature than residence time. These preliminary results support the option of upgrading grape marc toward its energetic valorisation through hydrothermal carbonization.