Temperature and Agitation Are Highly Influential on Yield and Monodispersity of Self-Generated Carbon (SGC) Formed in Hydrothermal Carbonization Filtrate

Hydrothermal carbonization (HTC) offers significant potential for converting residual waste streams into advanced carbon materials with diverse applications. However, a key challenge in scaling up HTC is managing the large volumes of organic-rich filtrate produced during the process. Through a resting process, the filtrate can be repurposed to produce self-generated carbon (SGC). The spontaneously formed SGC exhibited a spherical morphology and low ash content, even when derived from complex, ash-rich precursors such as anaerobic digestate. SGC production from HTC filtrate may open up a new valorization route for industrial and municipal side-streams. In this study, we investigate how temperature, time, and agitation influence SGC yield, morphology, and particle size distribution. The cumulative yield was measured at intervals (days 2, 5, 7, 9, 26). The average cumulative yield after 26 days increased by 102 % at 50 °C compared to 20 °C, but decreased by 42 % at 4 °C. Agitated samples had the highest yield, increasing by over 260 % at 20 °C. The products showed variations in morphology and size distribution, with agitated samples producing more uniform and smaller particles. SEM imaging indicated a distinct product at 4 °C, with no visible spherical material being generated. Our results imply that changes in temperature and agitation are highly influential in the formation of SGC and may be used in optimizing product yield, sphere size and uniformity. The consistent formation rate over the 26-day period suggests that extending the experimental duration could further increase material yield. This is supported by mass balance calculations.