Innovative mathematical approach for hydrothermal carbonization process using an inverse method: Experimental analysis, rheology behavior, and numerical comparative investigation

Hydrothermal carbonization (HTC) shows promise for converting biomass into carbon solids, but simulating it at high biomass-to-water ratios is difficult due to limited models. Existing numerical methods have not been sufficiently tested and validated under such conditions. An experimental approach and a novel non-Newtonian mixture model (NNM) were developed to improve simulation accuracy. This model incorporated rheological parameters using COMSOL Multiphysics software and an inverse method, especially the Levenberg-Marquardt algorithm. The results show that the proposed design achieved desired HTC conditions within 55–70 min, depending on the ratio. Besides, the innovative NNM model proved more effective for predicting thermal behavior and exhibited significantly reduced computation time than other CFD models in the literature, with a maximum 5 % relative deviation, indicating lower computational costs. Infrared camera measurements validated the heat behavior predicted by the NNM. Moreover, during HTC, the mixture behaves as a Newtonian fluid at a 1/6 ratio, and as a shear-thinning fluid at 1/3 and 1/2 ratios, with viscosity decreasing with increasing shear rate and temperature. This study presents innovative findings in the field of HTC, offering a reliable NNM model for simulating HTC under challenging conditions, addressing the mixture's rheology, and proposing a novel design.