Hydrothermal Carbonization

Over the past decade, hydrothermal carbonization (HTC) has emerged as a promising thermochemical pathway for treating and converting wet wastes into fuel, materials, and chemicals. Many of the earlier studies have been carried out to understand HTC reaction mechanisms and reaction kinetics using model compounds [1,2]. More recently, HTC studies have shifted from model compounds to researching lignocellulosic biomasses [3,4,5]; however, these works remain focused on solid biofuel production in the form of hydrochar. Perhaps the real potential for HTC lies in wet wastes (such as sewage sludge, agricultural wastes, animal wastes, etc.) and their conversion to fertilizer, high value materials, and sustainable chemicals [1]. However, the kinetics and reaction mechanism of the real wet wastes often deviate from those of the model compounds [6]. Therefore, studying the reaction kinetics of real wet waste is an important step towards revealing the viability of HTC in the commercial process.

This Special Issue titled, “Hydrothermal Carbonization” has invited researchers from around the World to contribute to these issues in the field of HTC research. Including the editorial, ten papers are listed in this Special Issue. Researchers from eight different countries have contributed their research to this Special Issue. The highlights of the Special Issue are as follows:

• Maniscalco et al. [7] reviewed the recent progress made towards using HTC as a valuable tool for energy and environmental applications. The review identified that pristine hydrochar from organic wastes such as food waste are suitable for combustion or soil amendment based on feedstock. However, the review also reported recent progresses with respect to further activating hydrochar for energy storage and pollutant adsorption applications.
• Three articles are reported aspects of HTC related to sewage sludge. Merzari et al. [8] have evaluated HTC systems at various points in wastewater treatment plants to treat various type of sludges (e.g., primary, secondary, and digestate sludge) for different applications. Meanwhile, Luhmann and With have optimized the dewaterability and phosphorus release from sewage sludge during HTC [9]. Sewage sludge is one of the major sources of phosphorus and recovering phosphorus effectively from sewage sludge may enhance the success of HTC. Finally, Gerner et al. [10] have reported the nutrient recovery and fuel production potentials from sewage sludge. The study reveals that the HTC process could be much cheaper than the current practice of sewage sludge treatment in Switzerland; therefore, fuel production and nutrient recovery from sewage sludge are economically viable.
• Ro et al. [11] have compared the HTC process with vapothermal carbonization (VTC). When feedstock does not have an adequate volume of water to create subcritical water conditions, the thermochemical conversion is called VTC. There are distinct product formations for HTC and VTC [12]. This study reveals the minimum amount of water required in the feedstock to be considered conducive to HTC. This fundamental study will clarify future research directions and ensure the practice of HTC.
• Islam et al. [13] reported an approach integrating air classification with HTC. In the study, air classification separated high-ash-fraction (waste) corn stover from the light ash fraction type [14]. This paper reported that the HTC of waste products can be performed and the hydrochar can be used to enhance the pelletization of clean corn stover.
• Gonnella et al. [15] have studied the thermal analysis and kinetics modelling of both the pyrolysis and oxidation of hydrochar. Although hydrochar has been reported as solid fuel, few sources have reported the combustion of hydrochar.
• Delahaye et al. [16] evaluated the cationic heavy metal adsorption of hydrochar both experimentally and computationally. Confirmed via density functional theory (DFT) simulation, this fundamental study reveals that hydrochar possesses superior binding sites for copper (II) ions compared to ion exchange resins.
• Finally, Hasan et al. [17] reported how various precursors affect the carbon quantum dot (CQD) properties via HTC. CQDs are very high-value materials with a variety of applications, such as bioimaging, photocatalysis, and energy storage. This paper demonstrates the possibility of separating CQDs from HTC process liquid. The utilization of HTC process liquid has always been a concern for HTC commercialization and this paper reveals a potential application for this very liquid.

Overall, HTC has shown tremendous potential toward commercialization. We hope that readers will find this Special Issue informative and inspiring. The editor is thankful to the authors for submitting their HTC research. The Energies editors and the reviewers are also acknowledged for reviewing the manuscripts, which have made significant improvements to this Special Issue.