ABSTRACT
Research around hydrothermal carbonization (HTC) has seen a huge development in recent years, materializing in the first pilot and industrial plants. Even though HTC reactions are slightly exothermic, the overall process entails energy consumption to both reach operating conditions and tackle heat losses. To face this issue and to develop a zero-energy process, this work proposes an innovative solution: the coupling of an HTC reactor with a solar concentrator, designed to fully cover the HTC energy needs. A 300 ml stainless steel HTC reactor was constructed and positioned on the focus of a parabolic dish concentrator (PDC), consisting of one parabolic mirror of 0.8 m2. To maximize the light absorption, the illuminated side of the HTC reactor was coated with a thin layer of nanostructured copper oxide, realized via electron beam deposition. Then, the effectiveness of the hybrid solar-HTC solution was demonstrated by carrying out an experimental campaign on a residual agro-biomass (grape seeds), which was treated at 180, 220, and 250 °C for 2 h. The coating confers excellent absorbing performances to the system, exhibiting an absorptance of up to 95.6% (at 300 nm wavelength). Heating times, yields, composition, and energy properties of "solar hydrochars" resemble those of studies performed in traditional HTC systems. This research work proves the feasibility of the solar-HTC prototype apparatus and opens the way to the development of a zero-energy solar-HTC technology.
Subject(s)
Carbon , Biomass , TemperatureABSTRACT
Bioplastics possess the potential to foster a sustainable circular plastic economy, but their end-of-life is still challenging. To sustainably overcome this problem, this work proposes the hydrothermal carbonization (HTC) of residual bioplastics as an alternative green path. The focus is on cellulose acetate - a bioplastic used for eyewear, cigarette filters and other applications - showing the proof of concept and the chemistry behind the conversion, including a reaction kinetics model. HTC of pure and commercial cellulose acetates was assessed under various operating conditions (180-250 °C and 0-6 h), with analyses on the solid and liquid products. Results show the peculiar behavior of these substrates under HTC. At 190-210 °C, the materials almost completely dissolve into the liquid phase forming 5hydroxymethylfurfural and organic acids. Above 220 °C, intermediates repolymerize into carbon-rich microspheres (secondary char), achieving solid yields up to 23 %, while itaconic and citric acid form. A comparison with pure substrates and additives demonstrates that the amounts of acetyl groups and derivatives of the plasticizers are crucial in catalyzing HTC reactions, creating a unique environment capable of leading to a total rearrangement of cellulose acetates. HTC can thus represent a cornerstone in establishing a biorefinery for residual cellulose acetate.
ABSTRACT
Several limitations hinder the industrial-scale implementation of hydrothermal carbonization (HTC) of biomass, especially the quality of as-carbonized hydrochar. This work investigates solvent extraction of hydrochars to enhance their potential applications. Hydrochars were produced at several HTC temperatures (190, 220, 250 °C) from cellulose and extracted using combinations of green polar solvents (ethyl acetate, acetone, and methanol). Results show that the composition of the extractable fraction resembles that of the HTC liquor, rich in carboxylic acids and furan derivatives, while the non-extractable solid phase shows improved thermal profiles devoid of highly volatile compounds. Carbon microspheres (non-dissolvable secondary char) are unaffected by extraction. The organics adsorbed on the hydrochar surface comprise highly volatile species and solvent washing effectively removes them.