Coalification of Expired Oatmeal: Hydrothermal Carbonization for Solid Biofuel Production

Hydrothermal carbonization (HTC) of food waste is challenging because food-waste feedstocks are heterogeneous and compositionally variable, and their major components—carbohydrates, proteins, and lipids—undergo different transformations during HTC. A well-characterized feedstock can therefore provide a more controlled basis for assessing HTC behavior and product properties. Here, expired oatmeal (EO), containing 68.66 wt % carbohydrates, 12.99 wt % protein, and 7.25 wt % oil, was used as a model food-waste feedstock. HTC experiments were conducted at 200–250 °C, with solid-to-water ratios of 12.5–50 g EO per 150 mL water and residence times of 1.5–12 h. The resulting hydrochars were characterized using thermogravimetry–mass spectrometry (TG-MS), pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS), and conventional analytical techniques. Compared with raw EO (HHV: 18.1 MJ kg–1), all hydrochars showed markedly improved fuel properties, with HHVs of 29.3–32.3 MJ kg–1 and ash contents of 0.14–0.48 wt %, compared with 1.62 wt % for the raw feedstock. HTC also increased fixed carbon and decreased volatile matter, while the H/C and O/C atomic ratios decreased to 0.91–1.14 and 0.12–0.21, respectively, placing the hydrochars in or near the bituminous-coal region of the Van Krevelen diagram. The hydrochars exhibited high heating values and low ash contents, supporting their potential as solid biofuel candidates; the use of a well-characterized feedstock provides a reliable basis for evaluating HTC behavior and supports the design of waste-to-energy conversion systems.