Hydrochar-derived fuels from waste walnut shell through hydrothermal carbonization: characterization and effect of processing parameters

Abstract

The raw walnut shell was converted to hydrochar fuel through hydrothermal carbonization. The structural, morphological, and compositional analyses were carried out using Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetric and basic elemental analyses. The effect of the processing parameters including reaction temperature, residence time, and water:biomass ratio on the hydrothermal treatment was studied. The selected hydrochar has been produced at the optimized condition of T = 250 °C, t = 5 h, and water:biomass ratio = 6.0 mL/g. The H:C and O:C atomic ratios decreased from 1.51 and 0.73 to 0.91 and 0.26 for walnut shell and the selected hydrochar, respectively. The elemental reduction occurred through the carbonization process, which mainly includes dehydration, decarboxylation, and depolymerization reactions in liquid phase. The higher heating value increased from 18.85 for walnut shell to 27.95 for the selected hydrochar. The obtained walnut shell–based hydrochar presents a relatively homogenized structure consisting of microspherical particles with a mean diameter of 6 μm. This lignin-like material with high carbon content and low ash content shows high thermal stability. These promising results are relevant to the development of hydrochar products from waste biomass for fuel applications.