Single-pot upgrading of run-of-mine coal and rice straw via Taguchi-optimized hydrothermal treatment: Fuel properties and synergistic effects

Highlights: • ROM coal and RS upgraded via co-HTC in oxidative solvents. • Optimization of co-HTC conditions performed using Taguchi method. • Sulfur contents significantly reduced. • Synergistic effects led to reduced ash contents. • Desirable hydrochar fuel properties effectively maximized. Blended fuels of run-of-mine (ROM) coal and untreated rice straw (RS) have not yet been used for fuel applications due to their poor fuel performance and the release of pollutants during combustion. Combining co-hydrothermal carbonization (co-HTC) with oxidative acid/alkaline solvents could be useful as a single-pot approach for effectively upgrading ROM coal/RS blended fuels. However, the determination of optimal co-HTC conditions is critical to achieve maximal performance of ROM coal/RS hydrochars. In this study, the Taguchi method was used to investigate optimal co-HTC conditions for the production of hydrochars with desired fuel properties. Characterization of produced hydrochars was performed by elemental analysis, proximate analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and thermogravimetric analysis. Based on calculations of S/N ratios, hydrochar with the best combustibility was produced using the lowest reaction temperature (180 °C) and residence time (4 h), hence minimizing energy consumption. Hydrochars produced under the optimal conditions for the best fuel ratio (FR) and proximate analysis (PA) indices had superior high heating values (HHVs) of 29.4 and 29.0 MJ kg⁻¹, respectively. In addition, synergistic effects from the optimal conditions resulted in lower contents of sulfur and post-combustion ash of the hydrochars.