**1. Introduction**

The global increase in energy demand, driven mainly by the growing global population, urbanization, and economic growth, is depleting the world's reserves of fossil fuels, already strained by their high consumption rate, heightening environmental concerns, resource depletion, and rising costs. To face the problem, different renewable energy sources have been investigated, like wind, solar, hydropower, geothermal, and biomass. Among the different biomasses, those deriving from waste or agricultural residues, agro-industrial, horticulture, and wood processing, are cheap and convenient [1,2]. Besides, the use and valorizing of waste biomass are part of a circular economy context, where residues become the input for a new process [3]. At present, most of the waste produced in Europe is disposed of without any further treatment resulting in wastefulness of energy and resources and in serious environmental impact. A high-efficient use of waste not only increases its economic value but also results in environmental benefits [4].

In the last decades, biomass waste treatment and management have seen an increasing interest from the research community. Currently, biomass waste has characteristics that limit its use, including a low heating value, low fixed carbon content, high moisture content, low grindability, and non-uniformity. Studies on the conversion of biomass into high-quality biofuels have been performed to avoid the problems caused by direct combustion and find more effective methods of utilization, such as pyrolysis, gasification, torrefaction, and hydrothermal carbonization [5,6].

**Citation:** Gonnella, G.; Ischia, G.; Fambri, L.; Fiori, L. Thermal Analysis and Kinetic Modeling of Pyrolysis and Oxidation of Hydrochars. *Energies* **2022**, *15*, 950. https:// doi.org/10.3390/en15030950

Academic Editors: M. Toufiq Reza and Javier Fermoso

Received: 30 December 2021 Accepted: 24 January 2022 Published: 27 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Despite pyrolysis, gasification, and torrefaction processes reducing moisture content and increasing fixed carbon content and heating value, all of them have a problem concerning high energy consumption and high pollutant emissions [6,7].

Hydrothermal carbonization (HTC) is a promising technique that can turn lignocellulose biomass into carbon-rich, solid, lignite-like fuel.

HTC is a thermochemical treatment able to recover energy from biomass wastes and eliminate organic contaminants and pollutants [8]. Similar to the natural coalification process, HTC lowers both the oxygen and hydrogen content of the feedstock, obtaining a final coal-like product referred to as hydrochar [9]. The process consists of subjecting the feedstock to mild temperatures (180–250 ◦C) and autogenous pressure in liquid water, ensuring a wide range of applicability for substrates with high moisture, up to 95 wt.% [10]. Hydrochar is endowed with higher calorific value and higher fixed carbon content compared to its precursor biomass [11]. Due to its potential, HTC is a mindful tool to treat biomass waste, recover materials, and provide an energy source. Hydrochar can be used as solid fuel and integrating HTC with other processes (like gasification, pyrolysis, anaerobic digestion [12], and composting [13]) can face shortcomings related to the single-stage, and improve the overall process efficiency. For example, HTC as a pre-treatment can affect the pyrolysis performance [1]. This last factor is important to control because it is the primary stage during the combustion and gasification processes (in view of using hydrochar as a fuel) and because it is a standalone process to produce biofuels (i.e., char, oil, syngas). In this framework, HTC can be used for the pre-conditioning of high moisture content biomass, making it hydrophobic (and thus allowing for a much cheaper dewatering/drying [2]), and then the biomass can undergo the dry pyrolysis treatment [14]. How much HTC could affect the pyrolysis rate depends on the type of feedstock involved [15,16].

In this context, understanding the kinetics of pyrolysis and oxidation of hydrochar could help in designing a thermochemical conversion process.

Even though the application of hydrochar is important in boilers and burners, the kinetics and combustion performance of hydrochar have been poorly studied. Sharma et al. [17] showed that adding hydrochar to pulverized coal helps to decrease pollutant gas emissions. Moreover, the study of process kinetics and the modeling of its control in the industrial field is weak. Different models (e.g., the Kissinger–Akahira–Sunose method and the Coats–Redfern method) have also been adopted to investigate the combustion kinetics of hydrochar [18,19], but the results obtained did not lead to quantitative data that fit well with the experimental values. Due to the importance of the kinetic behavior of hydrochar combustion, additional studies are needed. Actually, the distributed activation energy model (DAEM) is the most comprehensive model to investigate the thermal reaction kinetics of biomass. Bach et al. [20] employed DAEM to study the kinetic performance of combustion of wet-towered forest residues, while Chen et al. [6] applied DAEM to study the kinetic performance of pyrolysis of five lignocellulosic biomasses. Still, Zhang et al. [21] used a multi-Gaussian DAEM to investigate the decomposition characteristics of cellulose, hemicellulose, and lignin in different atmospheres. Therefore, the DAEM offers a higher quality of fit to the experimental data and can provide more information about the kinetics of biomass pyrolysis. DAEM was proposed originally by Vand [22] to model coal pyrolysis and is based on the hypothesis that the decomposition occurs through an infinite number of parallel reactions characterized by a certain distribution of activation energy. The energy distribution is often modeled through a continuous function, like the Gaussian, Weibull, or Boltzmann function [23]. Despite the diffusion of DAEMs to predict kinetics parameters of biomass and coal pyrolysis and oxidation [24–28], studies on the hydrochars are still very rare in the literature [20]. Therefore, this study used thermogravimetric analysis (TGA) to investigate the pyrolytic and oxidative behavior of hydrochars derived from an agro-industrial residue (grape seeds [29]).

In particular, TGA is a technique commonly adopted to analyze and examine biomass degradation in oxidation or pyrolysis patterns, and can be used to deduce kinetics parameters and mechanisms [30,31]. TGA data under inert and oxidative atmosphere were

collected and used to determine kinetic parameters through two DAEMs (Gaussian and Miura–Maki DAEM). To do this, TGA data at different heating rates were collected, and activation energies and pre-exponential factors were computed. Analyses were also performed on the raw biomass and the oil contained inside the seeds. To investigate more in-depth the decomposition, derivative conversion curves were deduced, and the heat involved during the pyrolysis or oxidation was determined through differential scanning calorimetry (DSC). A simple thermodynamic model was also developed in Aspen Plus to compare experimental outputs with thermodynamic predictions. Through the software, pyrolysis and oxidation of the hydrochars were simulated and the outputs were compared with experimental measures from DSC to understand if this modeling approach can be used as a preliminary study of the heat of reactions.

#### **2. Materials and Methods**
