Hydrothermal carbonization of Colombian plantain peels: Physicochemical properties, thermal behavior, and bioactive potential

Hydrothermal carbonization (HTC) is a promising technology for the conversion of biomass into high-added-value products, offering an effective treatment for organic waste with high moisture content. In this study, the impact of reaction conditions on the distribution of products generated from Dominico Harton plantain peel, an abundant agro-industrial waste in Colombia, was investigated. The HTC of plantain peels was investigated at various temperatures (150–230 °C), biomass moisture content (50–85%), and residence times (2–4 h). The yield of hydrochar decreased with increasing temperature, ranging from 41.59 to 82.30% at 85% moisture content, 25.19–60.29% at 65% moisture content, and 26.29–49.07% at 50% moisture content. The results indicated that the energy and mass performance of hydrochar are influenced mainly by the moisture content of the biomass and the reaction temperature. At 230 °C and 85% moisture content, the energy densification (ED) ratio and the higher calorific value (HHV) of hydrochar reached maximum values of 1.69 and 27.70 MJ kg⁻¹, respectively. The relationships among the elemental compositions (H/C and O/C) observed in the Van Krevelen diagram suggest that the carbonization process was influenced mainly by dehydration reactions. Thermogravimetric analysis confirmed the increased thermal stability of hydrochar compared with that of fresh raw material. HTC process water analysis revealed acidic pH values (3.76–5.00), high chemical oxygen demand (24447–92963 mg O₂ L⁻¹), and increased electrical conductivity (11.45–18.55 mS cm⁻¹) with increasing temperature. The concentration of 5-hydroxymethylfurfural (5-HMF) reached maxima of 980.9 and 1418.3 μg mL⁻¹ at 190 °C for 2 h and 4 h, respectively. The total phenolic content and antioxidant activity were highest at 170–190 °C, suggesting the potential for valuable bioactive compounds in HTC process water. Principal component analysis (PCA) indicated that the physicochemical properties of the hydrochar remained stable within certain temperature ranges, particularly between 210 and 230 °C, where improved energy characteristics were noted. This suggests that operating within this range is both energy efficient and cost effective for process optimization. These findings highlight the effectiveness of HTC in managing plantain residues in Colombia, producing hydrochar with desirable fuel properties, and generating process water for potential applications in health, fine chemicals, and agriculture.