| Summary: | One of the by-products of a mechanical-biological waste treatment plant is the under-sieve fraction, which requires separation prior to further processing of municipal mixed waste. This stream usually follows the fate of landfilling. Instead, it could be introduced to the hydrothermal carbonization (HTC) process to improve its fuel properties and become solid recovered fuel. The organic fraction and high moisture content (approximately 26%) of under-sieve fraction are favorable properties for the HTC process. In this study, hydrochars, the solid product of HTC, were produced at 200 and 220 °C with residence times of 1, 4, and 8 h. The main aim of this investigation was to establish the influence of different process parameters on hydrochars’ fuel properties. Thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed in the analyses. The results confirmed the positive effects of hydrothermal carbonization on the under-sieve fraction of municipal mixed waste properties. The ignition temperature increased from 247 °C to 288 °C and burnout temperature decreased to 443 °C from 489 °C after hydrothermal carbonization, causing a shorter combustion process. The determined key combustion parameters were: <i>S</i> = 12.4 × 10<sup>−8</sup>%·min<sup>−2</sup>·°C<sup>−3</sup>, <i>H<sub>f</sub></i> = 1174.9 °C and <i>Di</i> = 0.0075%·min<sup>−3</sup>, which in comparison to USF decreased by 44%, increased by 33%, and decreased by 29%, respectively, and became closer to those of coal. Furthermore, the identified structural changes indicate that hydrochars could be successfully used in energy production. The most promising results were found for hydrochar produced at 220 °C for 1 h, leading to a better combustion performance and providing a more stable and a less violent flame.
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