Theoretical framework for estimating design reactor pressure for water-based hydrothermal carbonization (HTC) systems

Abstract

Hydrothermal carbonization (HTC) has been shown to be a valuable system component in sustainable management strategies for wet organic residues from agriculture, industries and municipalities. While the reaction temperatures in HTC are much lower than those in alternative thermochemical processes, the pressures reached in HTC are much higher, rising with temperature as the autogenic pressure of water rises, and as reaction gas is produced from hydrothermal reactions. An important basis for designing cost-effective equipment for the HTC systems is understanding the safety aspects and costs associated with the reactor pressure. This paper presents a theoretical framework to predict the expected HTC reactor pressure for hydrothermal reactions with biomass that produce CO2. A model was developed that uses the thermodynamic properties of CO2-water mixtures at HTC reaction conditions and was validated using well-defined experiments with CO2-H2O. Comparison of the pressures predicted by the theoretical model to actual pressures in HTC reactions with real biomass (bark mulch, sugar beet pulp) showed relative errors ranging from −18.5% to 7.3%. A simple design procedure was suggested to predict HTC reactor pressure and demonstrated on a further case. The results of a sensitivity analysis showed that the pressure estimation is most affected by the parameters related to the amount of CO2 formed during the HTC reaction. The easy-to-follow methodology developed in this study will help researchers, design engineers, and manufacturers to estimate the pressure reached in the HTC reactor based on desired design goals and promote the widespread use of HTC for converting wet wastes into value added hydrochar which can improve soil health and reduce environmental pollution.