Hydrothermal carbonization of biomass: Design of a bench-Scale reactor for evaluating the heat of reaction

HydroThermal Carbonization (HTC) is a thermochemical process capable of converting wet biomass into a carbon-enriched solid, commonly referred to as hydrochar. Hydrochar finds application as bio-fuel, soil improver and for the production of carbon-advanced materials. In recent years, interest in HTC technology has grown significantly, in terms of both scientific research and industrial development. The HTC process consists of several reactions occurring both in series and in parallel: hydrolysis, dehydration, decarboxylation, condensation, aromatization, and others. Some reactions are known to be exothermic, while others are endothermic. Knowing the enthalpy of the “whole” HTC reaction would be beneficial in terms of both process design and energy calculations, in particular to evaluate the process heat duty. Unfortunately, such kind of information is barely available in the literature: some data have been obtained at the “micro-scale” using differential scanning calorimetry (DSC), with the limits of using a few milligrams of (usually heterogeneous) biomass per trial, while punctual data at larger scale are actually missing. In order to fill this gap, we designed and constructed in-house a 2 L batch reactor equipped with four thermocouples - placed at different heights inside the reactor - and capable to withstand pressures up to 140 bar and temperatures up to 300 °C. The reactor, controlled in temperature, is heated by four electrical resistances (1 kW each) and thermally insulated. An electric power meter allows monitoring and recording the electrical consumption during HTC trials. Thermal trials were performed with the bench-scale reactor fed with only water to provide a baseline for calculations. HTC trials were then performed using biomasses, namely organic fraction of municipal solid waste and agave pulp. At the different HTC operating conditions investigated (residence time: 3 h; reactor filling degree: 67 %; temperatures: 180, 220, and 250 °C; dry biomass to water ratio: 0.10 and 0.15), our data testify that the “whole” HTC reaction is exothermic, and the heat released by the reaction increases with temperature. The design of such a reactor and the data obtained so far encourage an in-depth analysis of the enthalpy of the HTC reaction for different biomasses and at various operating conditions.