Hydrothermal carbonization of biomass: design of a batch reactor and preliminary experimental results

Hydrothermal carbonization (HTC) is a thermochemical conversion process through which it is possible to directly treat a wet organic substrate with reduced process efforts. The main interesting product obtainable by HTC is the so-called “hydrochar”, that is a solid enriched in carbon and with chemical characteristics comparable to those of fossil coals. The interest aroused by HTC stems from the fact that it represents an effective alternative to the common treatments for wet biomass, i.e. composting and anaerobic digestion. In fact, if compared to these conventional processes, HTC has several advantages: it is faster, it is not affected by inhibiting or toxic substances and, finally, the process produces a solid fraction exploitable in different fields: energy production, soil improvement, raw material for high added value applications. Thus, a batch reactor (Vreactor: 50 mL; Pdesign: 140 bar; Tdesign: 300 °C) was designed and constructed to investigate the HTC process. At first, the scheme of the equipment (P&I) and its main characteristics are detailed. Then, preliminary experimental tests and relevant analyses are addressed. HTC experimental runs were performed with two different substrates: a model mixture of water and sugar and a mixture of water and grape seeds, which represent a by-product of the wine-making industry. For the first time in the literature, HTC data on these two kinds of substrates are reported. The substrates and the hydrochar were analyzed (chemical and thermal characterization). Preliminary data on the effect of process conditions (temperature and residence time) on process yields (kghydrochar/kgdry feed) and on hydrochar carbon content (kgcarbon/kghydrochar) were obtained. Moreover, based on the analytical data, a van Krevelen diagram was constructed for the HTC of both sugar and grape seeds.