Hydrothermal carbonization (HTC) has emerged as a pivotal technology in the battle against climate change and fosters circular economies. Operating within a unique reaction environment characterized by water as a solvent and moderate temperatures at self-generated pressures, HTC efficiently converts biomass residues into valuable bio-based products. Despite HTC’s potential—from the management of challenging biomass wastes to the synthesis of advanced carbons and the implementation of biorefineries—it encounters hurdles transitioning from academic exploration to industrial implementation. Gaps persist, from a general comprehension of reaction intricacies to the difficulty of large-scale integration with wastewater treatments, to the management of process water, to the absence of standardized assessment techniques for HTC products. Addressing these challenges demands collaboration to bridge the many scientific sectors touched by HTC. Thus, this article reviews the current state of some hot topics considered crucial for HTC development: It emphasizes the role of HTC as a cornerstone for waste management and biorefineries, highlighting potentialities and challenges for its development. In particular, it surveys fundamental research aspects, delving into reaction pathways, predictive models, analytical techniques, and HTC modifications while exploring HTC’s crucial technological applications and challenges, with a peculiar focus on combined HTC, wastewater integration, and plant energy efficiency.

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 5-hydroxymethylfurfural 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.

The transition from mild to harsh hydrothermal conditions involves the passage from progressive hydrothermal carbonization (HTC) to liquefaction (HTL) of biomass, shifting products from solid to oily phases. Understanding the reaction pathways biomass undergoes during hydrothermal processing can help researchers tune operating conditions according to the desired products. This work investigates the transition from HTC to HTL of two model and one heterogeneous biomass: cellulose, lignin, and wood chips. The experimental method approaches a "real-time" study by sampling the reaction slurry during hydrothermal operation. Analyses were performed on the solid and liquid (aqueous and oily) products obtained from processing over a range of temperatures (220-320(degrees)C) and residence times (0-2 h). Hydrochars' thermal stability and carbon content increase with process severity (time and temperature). For all the substrates, thermogravimetric analyses show that volatile, thermally unstable compounds form at moderate temperatures. Samples drawn during the reaction with immediate liquid analysis enabled us to follow the evolution of organics, mainly carboxylic acids, furans, and phenol derivatives, providing an insight into hydrothermal reaction pathways. Overall, as temperature increases, organic acids and furans progressively transform to ketones and phenol-derivatives, while 5-hydroxymethylfurfural yield reaches a maximum at 30 min at 250 and 280 C-degrees.

Bioplastics are increasingly replacing traditional plastics in many sectors, but the legislative and operative frameworks for their disposal remain unclear: they should be collected and treated together with the organic fraction of municipal solid waste (OFMSW), but often do not biodegrade satisfactorily in the plants that treat OFMSW. This work focuses on a type of cellulose diacetate employed in the eyewear industry to analyse hydrothermal carbonization (HTC) as a pre-treatment before anaerobic digestion (AD). The results show that HTC can hydrolyse this bioplastic even at moderate temperatures, reaching an almost total dissolution in the liquid phase at 210 °C and, at higher temperatures, producing hydrochar. When the HTC slurry obtained at 210 °C is fed to mesophilic or thermophilic AD, both the amount and the production rate of biogas are enhanced compared to the raw bioplastic. In particular, in thermophilic conditions, the amount of produced biogas undergoes at least a threefold increase compared to the untreated cellulose diacetate. Thus, this work confirms that a prior HTC step may be a suitable approach to enhance the disposal and energy recovery of bioplastics through AD.

During laser ablation a mechanical impulse is generated on the target material. This allows to generate thrust on far objects, becoming an attractive space propulsion technique for space debris removal and nanosatellite propulsion. Polymers showed promising results as fuel materials for laser ablation propulsion (LAP) for nanosatellites, and among the commercial ones poly(vinyl chloride) (PVC) gave the best performances. This polymer, however, is transparent for wavelengths > 200 nm, so that strategies have been tried to increase its absorption and enable laser ablation, among which the loading of carbon nanoparticles (CNPs) as absorbers. Despite the growing number of reports, the role of CNPs size and concentration is still not clear. In this work, different sizes and number density of CNPs are employed as absorbers in order to investigate their role in the impulse generation process. Samples are characterized from the structural, optical and thermodynamical point of view, and laser generated impulse is measured for fluences < 7 J/cm(2) by using a specifically designed ballistic pendulum. Our results show that the main parameter is CNPs number density. For a fixed total mass concentration, a reduced CNP size increases optical absorption and lowers the fluence threshold of impulse generation (F-th) because of a better heat distribution in the sample. A lower limit was also found for F-th at high absorption. Additionally, a considerable enhancement in impulse generation was observed by confining laser ablation with a transparent PVC layer.

called upon to develop strategies to provide market-ready biomass technologies and bio-based products for industrial implementation.A list of current, promising conversion processes follows.

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 degrees 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.

This study examines the kinetics of pyrolysis and oxidation of hydrochars through thermal analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques were used to investigate the decomposition profiles and develop two distributed activation energy models (DAEM) of hydrochars derived from the hydrothermal carbonization of grape seeds produced at different temperatures (180, 220, and 250 °C). Data were collected at 1, 3, and 10 °C/min between 30 and 700 °C. TGA data highlighted a decomposition profile similar to that of the raw biomass for hydrochars obtained at 180 and 220 °C (with a clear distinction between oil, cellulosic, hemicellulosic, and lignin-like compounds), while presenting a more stable profile for the 250 °C hydrochar. DSC showed a certain exothermic behavior during pyrolysis of hydrochars, an aspect also investigated through thermodynamic simulations in Aspen Plus. Regarding the DAEM, according to a Gaussian model, the severity of the treatment slightly affects kinetic parameters, with average activation energies between 193 and 220 kJ/mol. Meanwhile, the Miura–Maki model highlights the distributions of the activation energy and the pre-exponential factor during the decomposition.

Hydrothermal carbonization (HTC) is a thermochemical process that can reduce the environmental burdens of wet, heterogeneous biomasses such as the organic fraction of municipal solid waste (OFMSW). Whilst the effect of processing parameters on hydrochar properties is well known, post-treatments to valorize hydrochars are infrequently investigated. Moreover, more severely carbonized hydrochars have a reactive species present on their surface that may limit hydrochars' use as a solid fuel or soil amendment/environmental adsorbent. To address these potential limitations, a low-temperature (180 degrees C) thermal treatment and a chemical extraction (1:4 methanol: dichloromethane) were performed on OFMSW hydrochars. The thermal extraction removed up to 12% of this reactive volatile matter, comprised of alkanes, furans, ketones, and fatty acids. Chemical extraction removed up to 61% of the hydrochar, and the extract comprised mostly fatty acids, suggesting a potential pathway for recovery of fatty acids and condensation of fuel molecules in the solid hydrochar. The higher heating values of the extracts were much greater than the non-extractable solid hydrochar. The non-extractable primary char showed similar oxidative and pyrolytic behavior to a standard bituminous coal. The results indicate that HTC could valorize OFMSW by converting this wet waste into a dry solid fuel, soil amendment or environmental adsorbent while simultaneously extracting valuable biodiesel and biofuel precursors. Given increasing legislative pressure to divert OFMSW from landfills, this new pathway offers an alternative to traditional anaerobic digestion management strategies that produce only methane as a green energy product. (C) 2021 Elsevier Ltd. All rights reserved.

Hydrothermal processes enable an effective conversion of waste biomasses into fuels and carbonaceous materials. Covering the heat requirements with concentrated solar energy is a clever strategy to increase the plant efficiency and pursue the principles of circular economy. With the purpose of producing liquid and solid biofuels through zero-energy routes, this work presents two conceptual designs for integrating a concentration solar system (CSS) with a hydrothermal liquefaction (HTL) and a hydrothermal carbonization (HTC) plant. The solar configuration used to cover hydrothermal heat requirements consists of a set of parabolic trough collectors operating with molten salts, which are used as both thermal carrier fluid and thermal energy storage medium. Two different scenarios were modelled to continuously process wood and organic wastes. In the first scenario, the CSS is coupled with a continuous HTL reactor (operating at 400 °C and 300 bar) followed by thermal cracking and hydroprocessing for upgrading bio-crude to a saleable liquid biofuel. The second scenario considers the CSS operating with a continuous HTC reactor (working at 220 °C and 24 bar) for the conversion of organic wastes into a solid fuel (hydrochar). The CSS and both the hydrothermal plants were modelled based on experimental data. Energy consumptions and techno-economic aspects were investigated.

Hydrothermal carbonization (HTC) is an emerging path to give a new life to organic waste and residual biomass. Fulfilling the principles of the circular economy, through HTC “unpleasant” organics can be transformed into useful materials and possibly energy carriers. The potential applications of HTC are tremendous and the recent literature is full of investigations. In this context, models capable to predict, simulate and optimize the HTC process, reactors, and plants are engineering tools that can significantly shift HTC research towards innovation by boosting the development of novel enterprises based on HTC technology. This review paper addresses such key-issue: where do we stand regarding the development of these tools? The literature presents many and simplified models to describe the reaction kinetics, some dealing with the process simulation, while few focused on the heart of an HTC system, the reactor. Statistical investigations and some life cycle assessment analyses also appear in the current state of the art. This work examines and analyzes these predicting tools, highlighting their potentialities and limits. Overall, the current models suffer from many aspects, from the lack of data to the intrinsic complexity of HTC reactions and HTC systems. Therefore, the emphasis is given to what is still necessary to make the HTC process duly simulated and therefore implementable on an industrial scale with sufficient predictive margins.

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.