Biomass Hydrochar: A Critical Review of Process Chemistry, Synthesis Methodology, and Applications

Abstract

Hydrothermal carbonization (HTC) is a novel thermochemical process that turns biomass into hydrochar, a substance rich in carbon that has potential uses in advanced material synthesis, energy production, and environmental remediation. With an emphasis on important chemical pathways, such as dehydration, decarboxylation, and polymerization, that control the conversion of lignocellulosic biomass into useful hydrochar, this review critically investigates the fundamental chemistry of HTC. A detailed analysis is conducted on the effects of process variables on the physicochemical characteristics of hydrochar, including temperature, pressure, biomass composition, water ratio, and residence time. Particular focus is placed on new developments in HTC technology that improve sustainability and efficiency, like recirculating process water and microwave-assisted co-hydrothermal carbonization. Furthermore, the improvement of adsorption capacity for organic contaminants and heavy metals is explored in relation to the functionalization and chemical activation of hydrochar, namely through surface modification and KOH treatment. The performance of hydrochar and biochar in adsorption, catalysis, and energy storage is compared, emphasizing the unique benefits and difficulties of each substance. Although hydrochar has a comparatively high higher heating value (HHV) and can be a good substitute for coal, issues with reactor design, process scalability, and secondary waste management continue to limit its widespread use. In order to maximize HTC as a sustainable and profitable avenue for biomass valorization, this study addresses critical research gaps and future initiatives.

1. Introduction

The advancement of human activities has depleted natural resources unsustainably. This process uses less sustainable raw materials and generates heavily contaminated effluents [1,2]. These contaminants are largely released into the water and consequently into the soil [3]. Toxic gases released into the air settle on soil, vegetation, and water via rain or ascend into the atmosphere, exacerbating global warming [4]. Therefore, the generation of waste from industrial sectors has become a problem worldwide [5]. One possibility is the use of these residues in other products of high economic value [6]. Based on this, research using hydrothermal carbonization to generate profitable materials applicable in different areas of society is intensifying [7,8].

In hydrothermal carbonization, biomass is added to water undergoing chemical reactions (dehydration, hydrolysis, and decarboxylation), leading to a solid product with an appearance similar to coal which is generally called hydrochar [9,10,11]. The temperatures used ranged from 453 to 533 K, under a pressure of 4 MPa [12]. Hydrothermal carbonization is advantageous due to lower toxic gas emissions, reduced energy use, suitability for high-humidity raw materials, simple operation, high solid yield, and low ash content [13,14]. Its most widespread use is in the energy sector; however, in recent years, other areas have been applying hydrochars, such as in the remediation of contaminants present in water, in nanotechnology, in the development of sensors, in biocatalysis, etc., [15]. The chemical and physical properties of the raw material, together with the operational conditions of the thermochemical treatment, directly influence the final characteristics of the hydrochar [16]. With this, hydrothermal carbonization produces hydrochar with tailored structural and chemical properties [17,18]. As described in Figure 1, hydrothermal carbonization and pyrolysis are described as basic routes to obtain hydrochar synthesis through thermochemical conversion [19,20].

The impact of important process variables, including reaction temperature, residence time, biomass-to-water ratio, and pH, on the yield, structure, and functionality of hydrochar have been thoroughly examined in the literature on hydrochar preparation. A thorough mechanistic knowledge of the HTC process has been made possible by numerous investigations, which have identified important reaction pathways and how they affect the composition of hydrochar. Additionally, improvements in hydrochar modification methods, such as chemical activation (KOH, H₃PO₄) and functionalization (carboxylation, nitrogen doping), have been extensively studied and show improved adsorption properties for wastewater treatment applications [21,22]. Co-hydrothermal carbonization (co-HTC) and microwave-assisted HTC are two novel techniques that have been emphasized in recent studies. These techniques have been demonstrated to increase hydrochar characteristics, decrease reaction time, and improve process efficiency. Furthermore, studies comparing hydrochar and biochar have shed light on their respective benefits. Hydrochar is especially useful for environmental remediation because it frequently has a higher oxygen-to-carbon ratio, a wider variety of functional groups, and superior hydrophilicity.

Notwithstanding the advancements, there are still several important gaps in the literature on the preparation of hydrochar. The inconsistent reported results resulting from variations in reactor design, feedstock composition, and experimental settings constitute a significant constraint. It is challenging to create broadly applicable reaction models for HTC reactions due to their complexity, which includes simultaneous hydrolysis, condensation, and polymerization [23,24]. Although certain research offers kinetic insights into the production of hydrochar, the reproducibility of results is limited by the absence of standardized methodology and predictive models. Furthermore, the majority of research is conducted at the laboratory scale using batch reactors, leaving continuous or semi-continuous HTC systems relatively unexplored, and scale-up issues are still largely unresolved.

This study describes current advances in different synthesis methodologies in hydrochar production. The main focus is on hydrothermal conversion technology, describing the existing gaps regarding economic viability, the quality of the final product generated, and the scaling of the process. Operational parameters, such as temperature, biomass feedstock, residence time, and pH, are carefully analyzed. In this aspect, the reactions that can occur during the process and how much this can affect the final properties of the hydrochar are described. Finally, the application of hydrochar in different areas of society is analyzed. This analysis is essential to provide a new vision to researchers in the area about the challenges and the paths that should be followed.

2. Hydrothermal Carbonization

The hydrothermal carbonization process generates a solid material as the final product and the product generated from the liquid phase (bio-oil) is also observed alongside a small fraction of gases (H₂, CH₄, and CO₂) [25]. The experimental parameters and biomass properties determine the composition of the final products and their percentages [26]. The liquid portion is composed of organic and volatile fatty acids, compounds of phenols, esters, amino acids, small fractions of lignin and sugars, and nutrients, such as phosphorus and nitrogen [27]. Initially, these fractions were classified as residual parts, which increased the operational cost due to the treatment applied for decontamination. However, recent research has highlighted a more positivist view of aggregate resource potential [28]. Among the advantages are its use as a raw material in gasification, its use to obtain other products that have added value (anaerobic digestion and the separation of sugars), application in the culture medium with algae and other microorganisms, and bioelectrochemical systems [27].

The varied definitions of hydrochar in the literature have generated a bit of confusion; this is corroborated by some studies that define hydrochar as the solid phase generated after the thermochemical step [29]. In this case, the product is related to the slow pyrolysis process. Both the hydrothermal carbonization step and the pyrolysis step enable the thermochemical conversion of the biomass of organic origin, generating the solid compound as the final product. This process is represented in Figure 2 through an adaptation of the van Krevelen diagram. It is worth noting that the formation mechanisms, experimental conditions, and chemical reagents used are highly variable in the literature. Because of this, each study generates a solid as a final product with different chemical and physical properties [30]. The use of wet roasting has also been highlighted as a controversial process when using this terminology to describe hydrothermal carbonization, which is often also called hydrothermal treatment or conversion [26]. Although roasting also uses water, both terminologies are not equivalents, since they present differences in the final products as well as in the experimental parameters used in the process (Figure 2). Hydrothermal carbonization varies from other procedures in addition to the temperatures employed. When gasification occurs, a solid or liquid fuel is transformed into a flammable gas by heat decomposition. Several processes are involved in the process, such as pyrolysis (heating biomass to 200–300 °C without or almost without oxygen) and drying (heating solid biomass to eliminate moisture) [31]. Combustion (the volatile compounds and some of the charcoal combine with oxygen to form carbon dioxide and carbon monoxide) and volatiles are released during this process. This process generates heat for the gasification reactions, cracking (which is the process of exposing large complex molecules, like tar, to heat and breaking them down into lighter gases), and reduction (which is the process of removing oxygen atoms from the combustion products of hydrocarbon (HC) molecules in order to form molecules that can burn again) [32]. Pyrolysis is a thermal process that uses high temperatures and no oxygen to break down organic compounds into various molecules. Biomass, polymers, and other resources can be transformed into valuable goods using this technique. The first phase in the three-step process involves heating the organic material to a high temperature, typically 400–1000 °C. In the second, the material is thermally decomposed into different molecules, and in the third, the products undergo separation into different forms, such as solid, liquid, or gas. Among its final products are syngas, biochar, and bio-oil. On the other hand, organic matter is transformed into a solid, carbon-rich substance by the thermochemical process of torrefaction. The biomass is heated to temperatures between 200 and 320 °C in a low-oxygen atmosphere. It involves three steps: condensation (organic matter condenses into a solid substance rich in carbon), decarboxylation (carbon bonds break, releasing lipophilic chemicals), and dehydration (water is released from the biomass) [33]. Torrefied biomass, or the solid, dry, and blackened material left over after the procedure, and torgas are the end products of this process. Lastly, liquefaction, which is also depicted in Figure 2, is the process by which a solid, gas, or sediment turns into a liquid. It can be produced artificially or organically. In contrast to the other processes, this one employs higher pressures and lower temperatures to produce mostly bio-oil with trace amounts of coal and gasses.

The primary distinctions between hydrochar and biochar are outlined in

Table 1. Comparison of physicochemical properties of hydrochar and biochar.

Properties	Biochar	Hydrochar
Aromaticity	Contains aromatic groups	Contains alkyl moieties
Specific surface area and porosity	Porous and, depending on the reaction temperature, could exhibit higher specific surface area (>400 m2 g−1)	Non-porous, low specific surface area
pH	Mostly alkaline	Mostly acidic
Morphology	Graphite-like layers	Spherical shape
O/C molar ratio	>0.7	>1.7
Total carbon content (wt%)	60–80	58–64
H/C molar ratio	>1.5	>2.3

. The physicochemical characteristics of biochar and hydrochar differ, which has a big impact on their possible uses. Under various reaction circumstances, the biomass feedstock experiences diverse chemical processes, such as degradation, dehydration, and repolymerization, revealing distinct chemical compositions and porosity properties (temperature, time, heating rate, and pressure). As a result, they are easily differentiated. Reaction circumstances have a substantial impact on the yield and properties of biochar and hydrochar [34]. Because it affects which reaction mechanism predominates, the reaction temperature has a major effect on the physicochemical characteristics as well as the yield of hydrochar and biochar. Because hot water is available during the reaction, hydrothermal carbonization is carried out at a lower temperature than the pyrolysis process used to produce biochar. In terms of a solid product’s yield, temperature, carbon conversion, and yield are all related. A greater heating value (HHV), less carbon conversion, and a high solid product yield are the results of this process when the temperature and residence time are lowered [35]. The lower temperature of the HTC process leads to a lower carbon conversion than pyrolysis, and consequently higher atomic H/C and O/C ratios. As a result, hydrochar has higher atomic ratios of hydrogen to carbon and oxygen to carbon than biochar [36]. Hydrochar, therefore, contains larger atomic ratios of hydrogen to carbon and oxygen to carbon than biochar. In terms of the aromaticity of hydrochar and biochar, hydrochar from HTC produced at a lower temperature (200–250 °C) has more alkyl moieties, while biochar from pyrolysis produced at a higher temperature (500–600 °C) has aromatic groups. Furthermore, because pyrolysis takes place at a higher temperature, biochar has a lower H/C ratio because of its high carbon conversion and graphite-like layers, which include particles of various sizes, whereas hydrochar samples have spherical particles with more uniform particle sizes on their surface [37]. Because hydrochar has more oxygenated functional groups than biochar, it is somewhat more acidic. However, biochar is alkaline because pyrolysis results in the loss of carboxyl and hydroxyl groups. Furthermore, metal and inorganic substances like calcium and magnesium are thought to be responsible for the alkaline pH. Hydrochar’s pH would become acidic as a result of part of the inorganics being washed away in water media during HTC [38]. Reaction temperature, heating rate, reaction time, and biomass characteristics all affect the characteristics of biochar [39]. Hydrochar typically exhibits very low porosity and specific surface area. A high temperature and heating rate during pyrolysis can clog the pores and damage the porous structure, which would reduce the specific surface area. Therefore, raising the temperature may increase the specific surface area of the biochars produced by pyrolysis, and once it reaches its maximum, it may decrease as a result of clogged pores [40].

2.1. Solid Yield and Chemical Composition Characterization of Biochar and Hydrochar

One of the main obstacles to biomass’s use in the energy sector is its high alkali and alkaline earth metal content. During the combustion of biomass, these metal compositions display unfavorable and well-known behaviors, such as slag, scale, clinker formation, corrosion, etc., [41]. The proportion of ash in the initial feedstock has a direct bearing on the percentage composition of these metallic species. When employed for energy generation, the decrease in the ash composition of hydrochar or biochar would be very beneficial. The total amount of ash in the solid product is decreased by the HTC process because it is conducted with water present, which offers a chance to demineralize these inorganic components from the biomass to the liquid product stream. The effective use of wastewater generated by the HTC process is still a problem for industrial HTC implementation, though. As an alternative, the issue might be resolved by recirculating the HTC process water. In comparison to the raw biomass feedstock, the hydrochar samples generated by the HTC process described in this publication exhibit a significant decrease in ash content. When compared to the raw biomass, the biochar produced by slow pyrolysis had a higher ash percentage than the hydrochar samples [42]. The primary distinction between these two procedures is that the HTC process uses compressed liquid, which aids in demineralizing the biomass’s ash composition and lowering its ash concentration.

It should be mentioned that the quantity (% of mass yield) and quality (percentage of carbon and HHV) of the finished solid product are constantly traded off. Low-temperature processes exhibit low carbon conversion efficiency and high mass yield, whereas high-temperature processes exhibit the exact opposite characteristic. Generally speaking, a feedstock with a high hemicellulose-to-lignin ratio yields more volatiles than solids [43]. Variations in the degradation of the biomass polymers are the primary cause of the variance in the mass loss of the feedstock between the two distinct thermal pretreatments. The reaction media used during the procedure has a major impact on the degree of breakdown of the biomass polymers [7]. A substantial mass loss of the solid product results from the partial transformation of biomass polymers, primarily hemicellulose, into an aqueous phase when subcritical water is present during hydrothermal conditions.

Using proximate analysis (ash and fixed carbon), elemental analysis (O/C–H/C ratios), and HHV, biochar and hydrochar are chemically characterized. As the temperature and reaction time increase, the energy density rises while the solid yield and H/C–O/C ratio fall in both procedures. The most crucial factor influencing the reaction mechanism and the end product’s physicochemical characteristics is the maximum reaction temperature (peak) attained during the procedure. Since lignin has a far greater HHV than hemicellulose, eliminating hemicellulose from biomass would result in a product with a higher energy density and lignin concentration [44]. Because subcritical water is present, hemicellulose breaks down more quickly in the HTC process than in slow pyrolysis. As a result, hydrochar samples have a higher energy density than biochar samples made by slow pyrolysis [45]. The production of high-quality intermediate compounds like 2,5-HMF, which have HHVs comparable to lignin’s, is likewise linked to the high HHV seen for the hydrochar samples. The total HHV (carbon content) of the hydrochar would rise if such compounds precipitated in its porous structure. The VanKrevelen diagram was used to illustrate the atomic ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C) of raw and pretreated samples in order to compare the differences in elemental compositions [46]. The Van Krevelen diagram gives broad details regarding the kind and quality of fuel. The reduction in smoke, water vapor, and energy losses during burning makes a fuel with a low atomic ratio of O/C–H/C extremely advantageous [47]. The H/C–O/C ratios of lignite and bituminous coal were also shown on the same graphic for comparison [46]. The H/C–O/C ratios of both coals are lower than those of the raw feedstock. However, compared to the biochar produced by slow pyrolysis, the H/C–O/C ratios for the hydrochar samples were found to be greater and more akin to those of natural coal. This implies that HTC has a higher ratio of decarboxylation to dehydration reaction rates than the sluggish pyrolysis process [46].

2.2. Hydrothermal Carbonization: Water

Hydrothermal carbonization compresses the water used in the process at low temperatures compared to the pyrolysis stage, and the pressure is maintained autogenously (Figure 2). In the end, the hydrochar yield is around 40 to 80% [48]. Subcritical or compressed water encompasses the portion that remains in a liquid state when subjected to pressure (not exceeding 221 atm; critical water pressure) and at a temperature (critical temperature 374 °C and boiling point 100 °C). Under these conditions, the liquid acts as a reagent for various reactions and also as a solvent for the organic compounds in the raw material. The ionization increases with the temperature and pressure while the dielectric constant tends to decrease. Being related to the molecule which is dissociated into hydroxyl ions (OH⁻) and hydronium ions (H₃O⁺), the change in these ions corroborates the occurrence of reactions that can be catalyzed by bases and acids; this occurs when there is no presence of other chemical compounds [49].

Using temperatures above 260 °C indicates a process called hydrothermal liquefaction, in this case, the highest yield is the liquid fraction [48]. Values above critical (temperature = 374 °C; pressure = 221 atm) cannot prevent boiling; in this case, water is formed in its supercritical state. At this point, hydrothermal gasification or supercritical gasification of water can occur, generating synthesis gas as the final product (CO₂, H₂, and CH₄). Lower temperatures enable greater solid yield in gasification and hydrothermal liquefaction processes [50]. In this review article, greater emphasis will be placed on the analysis and study of the parameters and functionalities of hydrothermal conversion to generate hydrochar as a final product.

The temperature and pressure conditions also control the scale formation inside the reactor and connecting tubulations. In addition to that, the biomass species and origin may lead to differences in the calcium, magnesium, sodium, potassium, and phosphorous content, major components that may generate scaling [51]. In the case of supercritical conditions, the inorganic salts are quickly precipitated, which prevents scale formation, and is the basis of the supercritical water desalination (SCWD) process, commonly employed in desalinization [52]. On the other hand, subcritical conditions can lead to scale formation, since the diluted salts can form nuclei and adhere to the surface of the reactor and tubes. The acidic material created or present during the carbonization process is what largely controls the fouling process. The composition of the biomass and the reactor’s operating conditions in turn determine the acidic content. Generally speaking, the system’s oxidation and heating cause fouling to occur as a result of different ions and salts. Although other oxides can also affect fouling production, potassium and sodium oxides are typically the primary fouling agents [53]. The diluted ions present during hydrochar carbonization go through oxidation and crystallization, as seen in Figure 3. Furthermore, throughout the process, magnesium oxide, calcium oxide, and iron oxide may also be produced. Because of their extreme reactivity, sodium and potassium oxides are frequently regarded as the main fouling agents.

Physically speaking, these oxides are deposited onto the equipment’s surface (Figure 3). Because of the system’s internal shear pressure, continuous deposition eventually results in the creation of compacted layers. Sintering (effects of high temperatures), chemical processes (involving carbonates and silicates), or hydration–dehydration (producing changes in the crystalline structure) can all lead to hardening eventually [54]. Scale formation is, therefore, a significant problem in the carbonization process since it can shorten the life of materials by resulting in corrosion, clogging, and breaking inside the system [55]. The fouling index is frequently used to assess fouling, even though it depends on the feedstock and operating conditions. According to [56], the fouling index depends on the base acid ratio present and the sodium and potassium oxides; Equation (1).

$$\mathrm{Fu}=\left[{\displaystyle \frac{{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CaO}+\mathrm{MgO}+{\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}}{\mathrm{Si}{\mathrm{O}}_2+{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{TiO}}_2}}\right]\times \left({\mathrm{Na}}_2\mathrm{O}+{\mathrm{K}}_2\mathrm{O}\right)$$

(1)

where Fe₂O₃ is the iron oxide mass fraction, CaO is the calcium oxide mass fraction, MgO is the magnesium oxide mass fraction, Na₂O is the sodium oxide mass fraction, K₂O is the potassium oxide mass fraction, SiO₂ is the silicon dioxide mass fraction, Al₂O₃ aluminum oxide mass fraction, TiO₂ is the titanium oxide mass fraction, Fu is the fouling index, where Fu < 0.6 indicates low fouling, 0.6 to 40 indicates high fouling, and above 40 is extremely high fouling. A study by Lachman et al. [57] shows that softwood chips have a low fouling index while hardwood chips and amaranth have a high fouling index. It was discovered that straw, hay, and sunflower pellets had low fouling levels. Regarding grains, rye and sunflower have a high fouling index. The following prevention/control methods are based on the technology employed in other processes since the hydrochar articles lack the investigation on this field. According to Cywar et al. [58] there are three different methods to control the scale formation: (i) altering the feed water characteristics; (ii) optimization of the operational parameters; and (iii) addition of an anti-scaling agent. Altering the feed characteristics involves a pre-treatment of water to remove the salt. The main methods employed are coagulation, ionic exchange, reverse osmosis, and acidic change. It should be noted that this method can only remove the salts from the water before the conversion. This means that any other salt present in the subcritical water that was transferred from the biomass needs further treatment after the process. Also, one should take into consideration that batch and continuous processes will change the salt removal. The second option is the optimization of the operational parameters, and it requires knowledge of the kinetics of scale formation. Through this kinetic understanding, it is possible to predict the minimum scale formation and exchange the operational conditions to retain the same crystal formation level. Although this method may be efficient, it alters the overall performance of the system, meaning that fewer quantities of hydrochar are attained [59]. Also, if the scale formation kinetics are not known, it will require time and experiments to obtain a predictive model and later, optimize the operational conditions. The last option is the addition of anti-scalant to prevent the formation, which retards the scale formation. This method tends to be well received, being specially applied to the boilers. However, in this case, the anti-scalant may not be applied to the water feed, since it can interfere with the hydrochar carbonization. In addition to that, anti-scalants are generally designed according to the system, meaning that further research and development are needed to ensure maximum efficiency [60]. Overall, it is possible to conclude that the scale prevention/control for hydrochar carbonization is still to be explored and detailed in the case of subcritical water. Figure 4 shows the application of the prevention/control methods which could prevent the scaling formation. Anti-scaling and pre-treatment could be a solution to prevent scale formation and remove the salt. However, it should be taken into consideration that the biomass could act as a salt source, which cannot be easily removed from the feedstock. Thus, it suggests implementing a salt removal present in water after the separation of the hydrochar.

2.3. Chemistry of Hydrothermal Carbonization

For the comprehension of the process, it is essential to understand that the raw materials are composed of lignin, cellulose, carbohydrates, and hemicellulose. This variety of polymers enables several chemical reactions, and from the compounds in the feedstock, different routes are proposed until the formation of the final product. Figure 5 describes the reactions sequentially; however, not all compounds necessarily occur in this order as most occur simultaneously [61]. The reactions that occur during the hydrothermal carbonization process are influenced by temperature. At the temperature of 180 to 200 °C, the cellulose and hemicellulose start to degrade. The pyrolytic process requires a temperature of 250 to 350 °C, whereas lignin only stabilizes after 360 °C, much higher than hydrothermal carbonization, which starts at 200 °C [50]. The lower energy consumption required in hydrothermal carbonization compared to pyrolysis is due to the greater pressure applied to the latter. At lower temperatures, the activation energy of lignin and cellulose is reduced, generating the destabilization of their chemical products [25].

Under subcritical conditions, the water molecule dissociates, generating H₃O⁺, which contributes to the hydrolysis of carbohydrates, such as starch or sucrose, into fructose and glucose. In sequence, the depolymerization and degradation reactions of cellulose and hemicellulose occur, generating oligomers, monomers, and glucose that are soluble in water. Lignin partially hydrolyzes generating phenols [12]. The intermediate complexes formed: erythrose, furfural, or 2,5-hydroxymethyl furfuraldehyde are formed by the decarboxylation and dehydration of sugars; this step is crucial in the formation of hydrochar [50]. In the initial hydrothermal carbonization stages, acids are generated, such as levulinic, lactic, formic, and acetic acids, as well as other chemical compounds. Generally, these chemical elements increase the acidity of the medium, contributing to the dehydration and hydrolysis process. The elements generated in these steps, such as 2,5-hydroxymethyl furfuraldehyde, generate bio-crude resulting from the polycondensation, condensation, and aromatization reactions. Then the bio-crude (with or without self-nucleation) is converted into a solid (aromatization and polymerization) called hydrochar [12].

Carbohydrates and proteins are the main components of non-lignocellulosic biomass. Through hydrolysis, carbohydrates are transformed into fructose or glucose, which can be transformed into 2,5-hydroxymethyl furfuraldehyde through dehydration. Proteins, through hydrolysis, convert into amino acids that, when in contact with sugars, form chemical elements containing a ring with N (Maillard reaction) [62,63]. As illustrated in Figure 4, a second route more similar to pyrolysis encompasses the raw material that has not been dissolved. In this case, the S-S (solid–solid) reactions depend on gravity and the experimental conditions used. In this case, the hydrochar produced has more lignin and consequently, greater aromaticity [48]. The hydrochar morphology tends to be amorphous, with a spherical geometry with furan condensed through the surface [64]. In the literature, it is possible to observe studies that describe at least two pathways that can generate hydrochar from lignin and cellulose [48]; however, the reaction time and the mechanism that forms carbon still need to be better clarified [49]. One of the factors is the limitation in the analytical techniques used, which limits the formation of hydrochar and reduces studies [64].

Through the growth of carbon spheres resulting from oligomers, the first report of hydrochar was observed. Through nucleation, the oligomers form spherical hydrophobic cores (hydrophilic shells) [65]. More recent studies have reported the possibility of growth through hydrophobic maturation (three stages) [66]. In this study, hydrochar was formed from the hydrophobic core generated after condensation and through the coalescence of furan oligomers [66]. With the possibility of applying analytical analyses (X-ray spectromicroscopy), a four-stage growth model was established that also occurs through hydrophobic maturation [64]. The difference is the core-shell structure that uses La Mer concepts. In this case, the structures make up a different chemistry because the dominant reactions that occur on the surface (aldol condensation) and in the core (dehydration) are not the same.

2.4. Chemical Route for Biomass Conversion

The hydrochar chemical path is complex and depends on the percentage composition of lignin, cellulose, and hemicellulose. Despite that, the comprehension of the chemical mechanism is important to understand the material developed and to further predict the [67] final material proprieties and composition. In general, hydrochar carbonization can be separated into three different stages. Being related to the break of the lignin, cellulose, and hemicellulose, this occurs in the form of hydrolysis, dehydration, and or fragmentation (Figure 6) [68]. Around 200 °C, the lignin dissolution process starts through hydrolyzation and fragmentation. The hydrolysis occurs due to the hydroxyl molecules and the hydronium ions, which attack the polymer bonds; this process leads to the fragmentation of the polymeric chains [69]. Following that or in parallel, occurs the dehydration mechanism, where the hydroxyl group (OH) is removed leading to the formation of furfural-like compounds [70]. It should be taken into consideration, that lignin is harder to dissolve in comparison to cellulose and hemicellulose. Thus, part of it will not be dissolved in the medium, leading to the formation of polyaromatic hydrochar. This mechanism is achieved through a solid–solid interaction due to the high pressure and temperature. The polyaromatic phase plays a fundamental role since it works as a support for the polymerized hydrochar as further detailed [71]. In addition to the lignin mechanism, there is the cellulose and hemicellulose conversion. Both follow a similar route as previously described for the lignin route, where the cellulose and hemicellulose are fragmented due the hydrolyzation and converted into smaller components due to dehydration, as depicted in Figure 7 [72]. After that, polymerization and condensation take place; the first mechanism is due to double bonds being broken and forming new single bonds; the latter occurs when the hydroxyl and hydrogen from different molecules are eliminated. Following that, other mechanisms, such as intramolecular dehydration (hydroxyl elimination and double bond formation) and tautomerism, change the molecule for aromatic formation [73]. Figure 8 shows the aromatic formation and cluster, which leads to the polymerized hydrochar. Last, the polyaromatic hydrochar group serves as a base for the polymerized hydrochar, forming the final material as depicted in Figure 9 [71]. For the creation of the chemical routes (Figure 6, Figure 7, Figure 8 and Figure 9), Marvin was used for drawing, designing, and displaying the structures, substructures, and reaction pathways. Marvin was used for drawing, displaying, and characterizing chemical structures, substructures, and reactions; Marvin 17.21.0 (Chemaxon, Budapest, Hungary).

3. Experimental Conditions Influencing the Thermal Carbonization Process

3.1. Chemical and Physical Composition of Biomass

The characteristics and properties of hydrochar are highly influenced by biomass chemistry [74]. Due to its complexity, its composition is the most difficult variable to analyze. Firstly, when modifying the source, the chemical composition can change qualitatively and quantitatively in different ways. Therefore, when comparing different hydrochars obtained in various studies, the nature of the biomass, as well as the experimental conditions used, must be carefully observed. In this case, it is recommended to use multivariable statistical models that analyze the generated effects on final chemical composition. Another possibility to be considered is to employ experimental data from different studies, aiming to investigate the effect of a single variable. In this field of science, some studies have reached conclusions [75,76], showing that the properties of hydrochar are sensitive to the original composition biomass (non-lignocellulosic and lignocellulosic). Figure 10 exemplifies the properties of biomass that can influence the properties of the final product. In this case, different statistical models were applied, so whether the upper limit to the level of sensitivity to a specific property of biomass is classified as significant may differ between samples [76].

Biomasses that have a high carbon content enable a final product with a higher yield, which corroborates the levels of hydrophobicity and aromaticity [74]. Another point is that solubility also influences yield, due to its ability to alter hydrolysis. The yield is also dependent on the amount of lignin, as it is not completely degraded. The H⁺ content affects the dehydration reaction, reducing the yield. Concerning lignocellulosic biomass, cellulose and lignin also affected the amount of carbon present in the final product. During the thermal carbonization process, lignin is changed the least, so the carbon remains in the material. Hemicellulose has an amorphous structure with a less complex carbonization process than cellulose, directly influencing the carbon in the final product. Another aspect involves the interaction of hemicellulose, cellulose, and lignin, which can cause different influences on the structure and properties of the biomass, altering the conversion process. Finally, knowing the final application of hydrochar can support the synthesis and choice of biomass with favorable properties (and this involves the operational designer) for its final application. In this specific case, the reactions that occur with the raw material can generate a hydrochar similar to coal, anthracite, peat, or lignite with great energy potential. In this case, the levels of ash, hydrogen, and oxygen are fundamental in the greater heating power (Figure 10).

According to

Table 2. Hydrothermal carbonization process to produce hydrochar from different raw materials.

Raw Material	Temperature (°C)	Time (h)	Hydrochar Yield (wt%)	Carbon Content (wt%)	H/C Atomic Ratio	Higher Calorific Value (MJ/kg)	Reference
Rice husk	200	6	66	4.8	1.27	15.7	[77]
Pinewood chips	250	6	74	53.3	-	-	[78]
Pinewood meal	265	20	60	74.2	5.6	-	[79]
Wood meal	260	0.5	68	67.4	5.8	-	[80]
Rubberwood fiber	260	9	91	66.9	6.4	-	[81]
Eucalyptus wood	240	3	72	-	-	-	[82]
Pinewood	240	1	79	65.4	6.6	-	[83]
Sweet potato peels	300	2	64	46.9	6.2	-	[84]
Sugarcane bagasse	300	0.5	88	70	0.6	-	[85]
Microalgae	222	0.58	15.4	55.6	0.12	-	[86]
Sewage sludge	220	0.5	56	7.9	0.16	3.6	[87]
Microalgae	272	0.16	12.1	54.3	0.12	-	[88]
Wastewater-grown algae	350	0.5	-	27	0.06	20.3	[89]
Microalgae	250	1	22.7	27.33	0.12	-	[90]
Sugarcane bagasse and vinasse	230	13	27	62.7	5.4	-	[91]
Moso bamboo	220	1	51.8	-	-	19.8	[92]
Green waste	190	1	80	48.8	1.2	19.2	[93]
Cotton stalk	240	2	57	69	0.91	-	[94]
Cotton stalk	180	4	60	51.2	1.2	-	[94]
Corn cob residue	250	0.55	46.6	61.7	0.08	24.3	[95]
Bamboo shoot bark	210	0.5	56.4	51.3	-	-	[96]
Grape pomace	190	-	-	55.7	0.1	-	[97]
Apple pulp with chips	190	-	-	55.9	0.13	-	[97]
rotten apple	190	-	-	62.5	0.09	-	[97]
Apple juice pulp	190	-	-	53.9	0.11	-	[97]
Chinese fan palm	210	0.5	60	57.3	1.62	24.9	[98]
Chinese fan palm	180	1	61	55.9	1.64	24.2	[98]

, it can be seen that hydrothermal carbonization has successfully produced various hydrochars because the yield is high when compared to the other two technologies that can also be used in the synthesis of the solid. In addition, the data presented in the table show that the yield, the upper heating value, and the carbon content are strongly influenced by the origin of the raw material and also by the operational conditions used during the process. By increasing the temperature and the process time, a decrease in the yield of the generated solid is observed. On the other hand, by increasing the temperature and the process time, a greater removal of volatile compounds is observed. In this case, the carbon content increases and the oxygen content decreases. It was also observed that the temperature had a high influence on the yield values as well as on the characteristics, with this influence being greater than the variation in time.

Co-Hydrothermal Carbonization

The biomass used generally comes from industrial processes or agricultural processes and is considered waste. It can be lignocellulosic or non-lignocellulosic, and generally has a high carbon content; a classic example is forestry waste. In the case of sewage sludge, food waste, and animal manure, they have a high ash content and low carbon content. However, it is observed that the joint treatment of the biomass of plant and non-plant origin has a synergistic effect, which corroborates the final production of hydrochar with favorable chemical and physical properties [99,100]. The lignin and cellulose content favors dehydration, increasing coalification, fixed carbon, and energy yield in the process (forest residues + swine manure) [101]. The same behavior was observed by adding lignite to sewage sludge [102]. Studies also report that a reduction in activation energy can occur during the pyrolysis reaction. In addition, hemicellulose undergoes hydrolysis, generating organic acids, which can lead to the decomposition of functional groups. These behaviors were observed in the hydrothermal carbonization of sewage sludge with rice straw [103]. By adding a protein-rich species of microalgae (Chlorella pyrenoidosa), the authors reported that the functional groups present on the hydrochar surface were changed in the hydrothermal carbonization process with rice waste [104]. This led to the anchoring of N-containing groups, mainly improving the adsorption properties of the material against contamination with heavy metals.

In the literature, studies using biomass mixtures allow for expanding the range of applications, corroborating the problem of solid waste management. For example, the hydrothermal conversion of waste containing polyvinyl chloride generates the formation of chlorinated dioxin or hydrogen chloride, both of which are highly toxic to the environment. Therefore, replacing chlorine with OH− groups has a synergistic effect in the process of dechlorinating polyvinyl chloride in the co-hydrothermal carbonization stage using vegetable residues [105]. Another example is the application of bamboo which can accelerate dechlorination at a temperature of 200 °C. The temperature maintained during the process also influences chlorine removal when using water hyacinth [106] and bamboo [107] plant biomasses. It is also worth noting that the quantities in the mixture of biomass and polyvinyl chloride also influence the dechlorination process [108]. Therefore, the application of co-hydrothermal carbonization can be a way to fill the gap that exists between solid waste management and the generation of renewable fuels.

3.2. pH and Temperature

The reactions that occur in the hydrothermal carbonization process are temperature dependent; as this magnitude increases, the yields of solid content decrease. However, the yields of liquids and gases increase. Regarding the yield of hydrochar from forest residues, when the temperature increases to 300 °C, the yield is reduced by 10%, using the same residence time [109]. The influence of the degree of dehydration and hydrolysis raises the process temperature, increasing the possibility of biomass dissolution, generation of intermediates, and volatilization. This corroborates carbonization using cassava rhizome where elevation to 200 °C led to decarboxylation and dehydration, increasing the carbon content and reducing hydrogen and oxygen. The carbon content also influences the calorific value; however, it reduces the yield of the final product [8,110]. When analyzing the pathways of materials containing lignin, hemicellulose, and cellulose, it was observed that hemicellulose is the most sensitive to temperature [111,112]. When carbonizing cellulose at different temperatures, the authors highlight that the solubilization of the compound is high at the highest temperature, and the final solid product is composed of aromatic, furanic (sp² carbon), and alkyl groups [113]. When analyzing penicillin, tea, and sewage mycelial residues as biomass, it was observed that the yield of all biomasses was reduced with increasing temperature; however, the percentages varied [63]. The sludge hydrochar had a higher yield due to the ash content. The penicillin mycelial hydrochar had a low yield due to the presence of polysaccharides and proteins (easy degradation). Hydrogen and oxygen contents are reduced with increasing temperature (devolatilization) [63]. Temperature increases result in a morphology change in the surface and this can be proven with SEM scanning microscopy analyses [114,115,116]. It was observed in the pores on the surface of hydrocarbons (160 °C); this results from the vaporization and degradation of organic elements present in the gas and liquid phase. The carbon obtained from glucose had its diameter affected by temperature, where at values above 180 °C, the fibrous network is broken, forming smaller cellulose fragments (micro/nano); these form spherical envelopes reducing contact with water circulating and reducing the probability of glycosidic bonds undergoing hydrolysis.

The pH is influenced by water and consequently affects the decomposition reactions; in this aspect cellulose and hemicellulose are highly reactive in an acidic environment, which reduces reactivity in an alkaline environment [117]. The pH present in the water generated chemical and physical changes in the plant biomass hydrochar (corn straw), where the final product was acidic regardless of the values used in the feed water [117]. The dissociation of the H₂O molecule forms H₃O⁺, reducing the pH and contributing to hydrolysis [99]. The authors also highlighted that in acidic conditions, hydrochar showed excellent textural properties (surface area and pores), compared to hydrochar prepared in acidic conditions [118]. The effect of pH can also influence heavy metals [119]. In acidic conditions, chromium and copper are unstable; in basic media, they present better mobility, while zinc, cadmium, nickel, and lead are better mobilized at pH = 7 [120]. Therefore, pH harms the stability of the metals present on the hydrochar surface [7].

3.3. Residence Time

The reaction time changes the quantity and quality of the final product, and increasing this variable increases the intensity of the reactions. In the literature, it is reported to go from 30 min to 12 h [121]. When analyzing the process on a large scale, shorter times are preferable with a view to continuous production [122]. The residence time depends on the source of raw material used [123]. Studies analyzed the carbon content and yield by varying times using a constant temperature, where raw materials, such as chitosan, glucose, and wood residues, at a temperature of 200 °C obtained different yields [124]. The main differences are in decarboxylation and dehydration, leading to a reduction in the oxygen/carbon and hydrogen/carbon content. When the residence time is around 12 h, it was reported that the carbon and the oxygen increased. After this point, the carbon/oxygen ratio will not change anymore; however, the morphology changes due to polymerization and hydrolysis [125], corroborating an improvement in textural properties at longer times. There is a greater likelihood of compounds being solubilized in shorter times [113]. Therefore, despite the reduction in yield, carbon and energy increase in longer processes [126]. It is important to highlight that the time is not defined uniformly; therefore, the reactor heating time may or may not be added to the residence time [8].

3.4. Water and Biomass Ratios Used

Studies highlight that this relationship presents small changes to the yield of the final product. When hydrothermally carbonizing olive pits, the authors highlighted that the water and biomass ratio led to a small reduction in solid yield [127]. This may be related to the greater potential present in the liquid phase to maintain and dissolve chemical molecules [128]. Therefore, this relationship can affect residence time and temperature [127]. The water present at the end of the process may contain stable end products and reactive organics, so the final residual volume must be treated before disposal, which generates higher costs in the process. One possibility is its reuse in the hydrothermal carbonization process, thereby achieving a reduction in total carbon, reducing operational costs as it increases the acidity of the water, and reducing expenditure on external heating by recovering the heat present in the water that is compressed [129]. The various effects that water has on hydrochar are described in the literature. In one study, agricultural residue and microalgae were used, and the behavior was similar with an increase of around 6% in product yield and around 12% in energy recovery [109]. Another aspect is related to the moisture content of the raw material, which changes the amount of water recirculated. An example was the study that used tree biomass with 25% moisture content, where it was observed that only 75% of the water was recirculated, corroborating the dilution effect [129]. When using biomass with a low moisture content, it is necessary to recycle practically all the water present in the process [130]. Studies highlight greater yield efficiency in the first recirculation, being more stable in subsequent cycles [109]. Analyzing water recirculation is fundamental as it affects reactions under different experimental conditions and thus, results in a material with different morphological properties. Therefore, at a temperature of 220 °C, a greater quantity of soluble compounds was detected compared to a temperature of 200 °C (possibly due to the incomplete hydrolysis of cellulose). Microsphere formation is also more common at 220 °C due to polymerization reactions. It should be noted that water recycling at 200 °C does not affect the hydrochar surface.

3.5. Pyrolysis and Gasification

The literature describes in detail that plant biomass can be converted through thermochemical processes, such as pyrolysis and gasification, and through biochemical processes, which include fermentation [131]. In the case of the transformation of plant biomass, five pathways must be followed. The first is direct combustion, generating energy and heat. The second involves co-combustion; in this process, coal generates energy with the use of pellets (thermal conversion). The third is hydrothermal liquefaction or carbonization. The fourth is fast pyrolysis, used in biofuels and also in slow pyrolysis to produce biochar/coal. Finally, the fifth stage involves gasification to generate gas or hydrogen. There is also the possibility of achieving poly-generation conversion using two or even more processes. An example is the use of forest biomass to achieve the co-production of bio-oil and synthesis gas using coal gasification and fast pyrolysis. This combination (pyrolysis + gasification) is carried out through the thermal conversion of coal, which has made it possible to maximize the conversion of coal into gaseous and liquid fuels [132]. This is performed to fully release the volatile raw material and fixed carbon, making the most of the potential of the raw materials. Due to high energy and fixed carbon content values (generally above 15%), it is highly important to study the performance of biomass during both the coal gasification and pyrolysis processes. The process of combining these two technologies can enable the conversion of volatiles into bio-oil and non-condensable gas through pyrolysis. In the case of fixed carbon, it can be transformed into synthesis gas through gasification. The combination of the two processes is still limited; the factors that generate influence, such as the composition of the raw material, the gasification agent, and the temperature, still need to be studied in more detail.

In the case of the gasifying agent, it is important because it influences the reactivity of the reaction and the composition of the product. Sansaniwal et al. [133] analyzed steam, oxygen, and carbon dioxide as gasification agents. The results showed that the concentration of hydrogen in the product was inhibited, and carbon monoxide improved with the use of oxygen. Other studies have shown that gasification with carbon dioxide can generate the Boudouard reaction during the formation of CO and the H₂/CO ratio in the synthesis gas can be adjusted to a higher value, which is favorable in the process [134]. The use of carbon dioxide in the gasification process can also reduce it from the atmosphere and the development of bioenergy with carbon capture and storage technology [135].

Kinetic analyses are also important in the biomass conversion stage; these parameters can corroborate the reactor design as well as the evaluation of the process viability, which includes numerical simulation and fluid simulation [136]. The thermochemical conversion process parameters must also comply with the required kinetic standards, as well as mass and heat transfer [137]. In these cases, two methods of kinetic calculation are frequently used, the model fitting method and the isoconversional (model-free) method. Finding an ideal kinetic mechanism is difficult for both gasification and pyrolysis because almost different mechanism functions can be well matched with the experimental data, but the cost is the occurrence of large differences in values in the kinetic parameters [138]. In the case of isothermal coal gasification kinetics, most investigations use model adjustment methods, including the volumetric homogeneity, the random pore model, the shrunken core model, or the modified random pore model [129,130,131,132,133,134,135,136,137,138,139,140,141]. In general, there is no conclusion about the function of the kinetic mechanism of isothermal coal gasification. Many studies place the random pore model as the most ideal since it takes into account the change in the gas–solid reaction interface and the collapse of micropores [142]. Conversely, the study of Xiu et al. [143] showed that the unreacted shrunken core model was more appropriate because the continuous accumulation of the ash layer indicated in the model is ideal for describing ash inhibition during the gasification process. In another study, the authors state that the random pore model is not ideal if it does not consider the reduction in the coal surface area or the mixing of gases [144]. Other factors affect the gasification process besides ash and porosity, such as gas–solid contact, reaction temperature, and diffusion control. Therefore, using isoconversional methods can corroborate the process, avoiding generating model adjustment uncertainties when calculating kinetic parameters.

Despite various studies on biomass pyrolysis and gasification, comparing the two processes is very difficult mainly due to the differences in the experimental methods, such as the materials used, the instruments, and the calculation methods. For this comparison, it is essential to select a rigorous control variable method to finally reach a clear conclusion about the differences in the thermal conversion behavior of different biomasses. In this sense, the study by Wang et al. [145] selected nine different forest biomass species and used them in studies of pyrolysis and CO₂ coal gasification. Firstly, the properties of the biomasses, such as ash composition, and proximate and ultimate analysis were performed. A thermogravimetric analyzer was used for the biomass pyrolysis and coal gasification experiments. Finally, the apparent activation energy of coal gasification was calculated by isoconversional methods. The biomass pyrolysis and biochar gasification performance of different forest biomasses show a similar trend, i.e., the reactivity of pyrolysis and char gasification in the following order as Salicaceae > Pinaceae. Forest biomass contains more volatile matter fixed carbon and less ash, so it is very suitable as a feedstock for the poly production process of biomass pyrolysis combined with char gasification. Salicaceae biomass is more suitable as a feedstock for the above process due to its higher pyrolysis activity and lower E of the gasification reaction. The reasons for the differences in the thermal conversion behavior of different biomasses can be attributed to the physicochemical properties, including chemical composition and ash composition, especially the content of alkali metals and alkaline earth metals. The presence of alkali metals and alkaline earth metals can simultaneously increase the reactivity of the biomass pyrolysis and char gasification process.

4. Large-Scale Hydrothermal Carbonization Reactors

The application of the hydrothermal carbonization process on a full scale has different purposes. It generally encompasses the conversion of large volumes of waste through thermal conversion into a smaller volume by-product (hydrochar). However, this is still applied less; generally, most studies are on batch processes, using a small laboratory-scale reactor (3 L) [146]. Larger-diameter cylindrical reactors are employed because they can dissipate more heat energy from the top and bottom of the reactor. In this instance, it takes longer for the temperature to reach the appropriate level. A reactor design must always take into account the cost, simple design, easily obtainable materials, material availability at the level of poorer countries, easy handling, safety and durability, and modular construction. In addition to this, it is also desired that the project has a continuous filling system for feed water and biomass, is highly automated (water and hydraulic coal discharge), and has the possibility of exchanging energy through an exothermic process (pre-heat new biomass). A large-scale technique can apply semi-continuous treatment to about three kilograms of biomass. The authors discuss the manufacturing procedures in detail [146]. The tubular reactor operates under pressures of up to 1300 psi. In an optimized way, the drills carry the biomass and hot water in a pressurized manner; in the end, the hydrochar is allocated to the container. The authors highlight that the results of the products generated (solid and liquid) are in agreement with the results obtained on a small scale. Another research study used forest residue biomass in a reactive twin-screw extrusion system using continuous hydrothermal carbonization [147]. The reaction time was reduced to less than one minute with the use of the device, increasing the yield when analyzed concerning discontinuity. Life cycle studies also concluded that electricity formed by waste had a higher concentration of greenhouse gases, with a higher energy use compared to that obtained by electricity from coal [148].

To minimize environmental impacts and reduce costs, reusing water produced in the hydrothermal carbonization process is always viable. In this aspect, studies observe the advantage of applying an autoclave with an operating volume of 20 L for 15% dry matter content [149]. The water can be reused in the biogas plant, increasing the energy efficiency of the entire process. To overcome the limitations of the combined stage, carbonization can be combined with other technologies, such as microwave-assisted hydrothermal carbonization, which has the advantage of a higher dehydration rate, a relatively fast process compared to the conventional one, and a lower consumption cost of energy [93]. Studies highlight an improvement in the calorific properties of a hydrochar obtained by microwave-assisted hydrothermal carbonization [150]. The other study analyzed both technologies in the conversion of urban waste, where the difference in the heating source interfered with particle size, energy consumption, and dehydration speed; however, both final hydrochars presented similar energy capacities [151].

5. Hydrochar Manufacturing Methods

Hydrochar production methods can be divided into three types: hydrothermal carbonization, hydrothermal liquefaction, and hydrothermal gasification (Figure 11). Each process varies due to the operating conditions employed and the end products generated. The hydrothermal carbonization method generates mainly solid hydrochar, while hydrothermal liquefaction produces crude bio-oil together with hydrochar, and finally, hydrothermal gasification produces gaseous fuels, such as hydrogen and methane [152]. The choice of process depends on the nature of the biomass and the desired end products. As already described, hydrothermal carbonization uses lower temperatures through subcritical water pressures, making it particularly energy-efficient for the processing of wet biomass. The advantage is that this process eliminates the need for drying, which is necessary for other conversion processes. This results in lower energy consumption and an increase in the final yield. Despite this, the problem is that the process generates a final product with a lower carbon content and also a smaller surface area, especially when compared to pyrolysis biochar, which limits its usefulness in applications, such as catalysis and pollutant adsorption [153]. Liquefaction uses more moderate temperatures, generating a variety of end products, such as hydrochar and bio-petroleum. Due to the variety of products generated, this process is considered economically viable, however, it requires more energy due to the high temperatures used. This process generally has a lower yield when compared to the hydrothermal method, and also involves more complex chemical reactions, making optimization more challenging [37]. In the case of hydrothermal gasification, the conversion occurs at high pressure and temperature values above the supercritical point of water. It is recommended for biomasses with high water content and does not require a drying step. Hydrothermal gasification produces valuable gases, such as methane and hydrogen, which can be used as raw materials and fuels. In this process, the hydrochar generated has a low yield and is often unviable for generating solid coal. In addition, this method has severe operating conditions, leading to a high energy input rate and sophisticated equipment [47]. When choosing the hydrochar production method, several factors must be considered, such as the desired end products, biomass moisture content, and, most importantly, energy efficiency. When it comes to high yields and low energy consumption, hydrothermal carbonization is recommended, while the other two methods offer the advantage of producing additional products, such as bio-crude oil and gaseous fuels. In certain industrial contexts, these products have high added value. Therefore, each method has its strengths and weaknesses, which make them suitable for different applications based on specific needs.

6. Possible Areas of Application of Hydrochar

6.1. Energy

In the area of energy production, the use of hydrochar as a solid fuel has direct wide application. This indicates a switch from the use of conventional coal to hydrochar. However, some requirements must be analyzed, such as the possibility of grinding, close energy density, stability, hydrophobic properties, and combustion potential [148]. The variety and quantity of waste is a possibility; however, they often present limitations as they do not have all the necessary characteristics (humid content, hydrophilic and fibrous nature, and high bulk volume) [154]. One possibility is thermochemical treatment, which can improve the properties of the material. Hydrothermal carbonization can increase mobility [154], higher heating value [155], hydrophobicity [156], the probability of a hydrochar with low ash content [151], and easy pelletization [157]. With these properties, the added value of hydrochar increases [99], as explained in Figure 12.

It is often described that conventional coal has more favorable properties than hydrochar [24]; however, it is worth noting that this can be highly variable as it depends on the origin of the biomass and the optimization of the parameters used in hydrothermal carbonization [99]. Temperature is the most important and influenceable parameter, as it affects the hydrogen and carbon, and oxygen and carbon rations of the final product [122]. In general, the greater the rigor of the reactions, the greater the possibilities of generating a hydrochar with high energy density. The limitation is that when using higher temperatures, it is necessary to have a more complex and costly operational design, which can also lead to a lower final product yield; therefore, moderate temperatures are recommended [48]. This results in a solid through more sustainable means; however, there are still challenges to be overcome, and more investigations must be carried out to overcome the limitations using the full potential of the product as a fuel. Microwave-assisted hydrothermal carbonization, traditional process operational improvement, and co-hydrothermal carbonization are all viable options.

In the energy area, one of the main aspects is the potential application of hydrochar as fuel [158]. This depends directly on the combustion proprieties, such as a higher heating value (HHV), ignition characteristics, flame speed, flame blowout, and extinction characteristics [159]. One of the main properties of the fuel is the HHV, which in the case of the hydrochar, will change according to the species of the biomass and the type. For example, hydrochar from wood will present a higher HHV, reaching 34.60 MJ kg⁻¹ (white pine); on the other hand, hydrochar from mushrooms has an HHV of 18.81 MJ kg⁻¹ [160]. In addition to the biomass type and species, the operational conditions will influence the final HHV. In the case of sycamore leaves, the increase in the system temperature from 550 °C to 650 °C also increases the HHV from 18.81 to 25.54 MJ kg⁻¹. Regarding other common fuels, it is normal to compare the hydrochar with coal, which ranges from 21.41 to 23.14 MJ kg⁻¹ [161,162]. This means that the hydrochar can have higher HHV than coal, meaning that it can be a better resource or a complement to be mixed with. Another fuel concurrent is the oil from vegetables and biodiesels. Similar to the hydrochar, it will depend on the plant species and the processing, reaction medium, etc. However, in this case, the hydrochar is no longer a major concurrent since the biodiesels present higher HHV, e.g., caproic acid, the fat acid with the lowest carbon count, in its pure form, can reach an HHV of 35.87 MJ kg⁻¹ [163]. This means that an acid with a carbon count will have a higher HHV, which further diminishes the hydrochar as a possible alternative. From this point on, diesel oil also has a high HHV of 45.38 MJ kg⁻¹ [164]. In general, it is possible to confirm that with the current methods of hydrochar fabrication, it cannot replace biodiesel or diesel. The ignition proprieties were studied by Nguyen et al. [165] for hydrochar obtained from grape marc. They found that the hydrochar produced at 260 °C had an ignition time delay of 0.28 s with an ignition time of 0.2 s. In addition to that, the best hydrochar evaluated had the lowest ignition temperature of 179 °C. According to the authors, these results are justified based on the thar formation that occurred at 260 °C. Ignition time was also evaluated by An et al. [166] through thermogravimetric analysis. The hydrochar was obtained from corncob produced at a temperature of 300 °C; the authors reported an ignition temperature of 272 °C, being attributed to the usage of the water during the HTC process, which diminished the dehydration and decarboxylation process. Similar results were also reported by Poomsawat & Poomsawat [167] from hydrochar from cattail at the temperature of 220 °C and with an ignition temperature of 275 °C. Even though the ignition time and delay are reported by these works, some other factors are lacking, such as water content and particle diameter, which control the combustion process and the ignition times. Furthermore, a fair comparison with other fuels (coal, biodiesel, petroleum liquified gas, or diesel) is limited since the combustion proprieties are correlated with the combustion engine [168]. Other proprieties are still to be reported in the literature, including special flame speed, flame blowout, and extinction characteristics.

6.2. Application as an Adsorbent in Water Decontamination and Its Differences to Biochar

Firstly, due to certain confusions present in the literature, it is important to delimit the differences between biochar and hydrochar (Figure 13). The chemical and physical properties are different, and this defines the possibility of final use. The percentage of chemical components varies, as well as the morphological and porous characteristics, which corroborates the different textural properties (pore volume and surface area) [169,170,171]. It is also related to the fact that biomass undergoes different chemical reactions (repolymerization, degradation, and dehydration) through different operational parameters (heating rate, pressure, temperature, and time). Added to this, differences in reactions directly affect the yield of the final product [25].

Hydrochar presents a greater heterogeneity of functional groups on its surface, which may corroborate its application as an adsorbent of undesirable contaminants present in water (

Table 3. Use of different biomasses for the production of hydrochar and its application as an adsorbent in water remediation.

Biomass	Hydrothermal Carbonization	Adsorbate	Kinetic Model	Isotherm/Qmax (mg g−1)	Reuse (Cycles)	Ref.
Sawdust	190 °C for 12 h (50 mL of 0.1 M N-cyclohexyl sulfamic acid + 10 g)	Benzotriazole	Pseudo-second order	Langmuir/160	7	[172]
		Cu (II)	-	Temkin/299	-
Orange peels	190 °C for 24 h, (110 mL of distilled water + 15 g), (30 mL 70% of HNO3 with 1 g)	Methylene Blue	Pseudo-second order	Langmuir/107	-	[173]
Corn cobs	300 °C for 30 min (240 mL of ultrapure water + 40 g)	Cr (IV)	-	Langmuir/34	-	[174]
		Ni (II)	-	Freundlich/29	-
Bamboo	200 °C for 24 H (160 mL of HCl + 40 g)	Methylene Blue	Pseudo-second order	Langmuir/141	-	[175]
		Cu (II)	-	1239	-
The mixture of walnut and peanut tree residues	200 °C total of 6 h, with 2 h being under CO2 flux of 150 mL/min)	Acetone	Pseudo-second order	39.4	5	[176]
Bamboo	200 °C, HCL (1 M) with 40 g (1:2 w/w C4H2O3, maleic anhydride, +200 mL of NaHCO3, sodium bicarbonate at 140 °C for 20 min)	Methylene Blue	Pseudo-second order	Langmuir/621	4	[177]
		Cd (II)	-	49	-
The mixture of walnut and peanut tree residues	200 °C for 6 h of 1:1 (w/w) with activation with KOH (50%) or H3PO4 (85%) for 1 h at 600 °C	Acetone	Pseudo-second order	50.5	5	[178]
		Cyclohexane	-	159.66	-
Corn stover	240 °C for 12 (H3PO4)	Pb (II)	Pseudo-second order	Langmuir/354	-	[172]
	Polyethyleneimine (240 °C for 12 h)			Langmuir/214	-
Sugarcane bagasse	200 °C 3 h (100 mL, 10% of H2O2 + 5 g)	Cd (II)	Pseudo-second order	Langmuir/323	-	[49]
		Pb (II)	-	357	-
Sucrose	800 °C process for 12 h with a mixture of 0.75 mol/L of sucrose 1 to 3 (w/w) with KOH with N2 flux of 100 mL/min	Acetone	-	226	4	[179]
		Toluene	-	251	-
		Acetic ether	-	241	-
Rice straw	200 °C for 70 with 0.1 g per mL using a microwave reactor	Dye	-	Langmuir/221	-	[180]
		Berberine	-	Freundlich/174	-
Coffee residue (grounds)	160 °C for a period of 2 to 12 h with a mixture of 1:10	Sulfadiazine	Pseudo-second order	Langmuir/0.08	-	[181]
Sawdust	220 °C for 60 days using a 150 L anaerobic fermenter.	Cd (II)	Pseudo-second order	Langmuir/20	-	[182]

). Therefore, despite its low surface area and few pores, it still has great potential for use as a more sustainable adsorbent in the remediation of effluents and the removal of volatile organic compounds. It is worth noting that this field still needs to be better investigated; however, several studies already provide significant results on removal capacity, mainly for dyes and heavy metals, as described in Table 3.

The removal potential is directly linked to the source of the biomass and the conditions and reagents used during the hydrothermal carbonization process. One possibility is the use of activation (chemical/physical), which allows for a final product with better textural properties, increasing the removal capacity [183,184]. There are differences in the reagent used and the moment of activation, which can occur after two-step carbonization, or together with one-step carbonization [12]. The improvement in applicability after activation is related to the increase in surface area and greater porosity and change in surface chemistry (more homogeneous surface) [7,178,185]. Studies reveal the strong dependence of the improvement of adsorption capacity on the change in surface chemistry, mainly for heavy metals (presence of functional groups with charges opposite to the adsorbate) [174,186].

Another possibility would be biological activation; the use of microorganisms in an anaerobic fermenter for 60 days increased the acidity of the medium (more negative charges), the surface area, and the ability to remove cadmium metal [182]. The mechanisms involving interactions between the pollutant and the adsorbent surface are complex and variable [115,116,187]. Generally, the adsorption of heavy metals is related to electrostatic interactions [49]. The interaction of methylene blue dye molecules with the hydrochar surface involves hydrogen bonds and electrostatic interactions [177]; similar behavior was also observed in the removal of methylene blue in hydrochar synthesized with polyaminocarboxylated [175]. Therefore, the mechanisms are related to the nature of the biomass and the operational conditions used in the carbonization process. As mentioned, interactions of a physical nature involve most processes; however, it is possible to observe a smaller number of publications that involve chemisorption through ion exchange and coordination bonds [172]. In this aspect, it is worth highlighting that for such an interpretation, it is necessary to analyze the thermodynamic parameters of adsorption [187,188]. Table 3 also describes that not all studies adjusted the experimental data to kinetic and equilibrium isotherm models; however, those that did estimate observed that the isothermal data are in agreement with the Langmuir monolayer model [189] and the kinetic curves with the model pseudo-second order [190].

As given in Table 3 the adsorption capacity depends on the system, adsorbate, and hydrochar (species and operational condition). However, the same analogy is taken for other commonly employed materials in the adsorption field. One of the most employed adsorbents is activated carbon, e.g., the hydrochar from sawdust (Table 3) has an adsorption capacity of 299 mg g⁻¹ while traditional activated carbon from sawdust obtained 208 mg g⁻¹ for the removal of Cu (II) [191]. Orange peels are another common biomass employed in the adsorption; the hydrochar achieved 107 mg g⁻¹, while the activated carbon is reported an adsorption capacity of 38 mg g⁻¹ [192]. Another type of common adsorbent is the zeolite; the LTA zeolite has an adsorption capacity of 223.5 mg g⁻¹, which is inferior in comparison to the hydrochar. In addition to the activated carbon and zeolites, there are biosorbents which are biomass materials with none or with chemical/physical modifications. In some cases, it can lead to noteworthy results, e.g., methylene blue adsorbed by bamboo treated with NaOH, reaching an adsorption capacity of 606 mg g⁻¹ [193]. One should take into consideration that the adsorption capacity may not be the best indicator if one adsorbent is better than the other, since the adsorption capacity is the function of the adsorbent dosage (g L⁻¹), solution pH, and textural proprieties [194]. Lastly, the same conclusion can be drawn to the magnetic hydrochar, being the major difference in the addition of magnetic metals that facilitate the adsorbent separation in non-fixed bed applications.

Finally, one of the great advantages of adsorption is the possibility of reusing the material corroborated with interactions of a physical nature [195]. In this case, it is expected that the material can be used for several cycles and that its removal remains viable [184]. In this aspect, some studies carried out desorption/reuse experiments, where materials can be viable for 7.5 and 4 cycles. Despite this, further investigation is recommended to optimize for a more sustainable strategy. More studies are also needed to enable the application of these adsorbents to a wider range of pollutants, such as pharmaceuticals, pesticides, endocrine disruptors, and microplastics [196,197,198]. Analysis that evaluates the ecotoxicological potential and economic viability of the adsorbent must also be conducted; only then will it be possible to advance it to large-scale applications.

Generally speaking, the physicochemical features of the hydrochars, the characteristics of the contaminants, and the experimental setup all have a significant impact on the removal efficiency. Specifically, raising the doses of hydrochar results in more functional sites that are accessible to react with the target contaminants, which raises the removal efficiency. Furthermore, the hydrochar surface area (pHzpc) and the pH of the solution regulate how well loaded contaminants are removed. Additionally, hydrochars’ adsorption efficiency often rises with the temperature of the solution. In comparison to other materials, like activated carbons that are discussed in the literature, the dye adsorption efficiencies of hydrochars are medium to low [199,200,201]. Using distilled water as a neutral feed solution and a raw material does not greatly enhance the physicochemical characteristics of the hydrochars that are formed. Additionally, the availability of different oxygen-containing surface functional groups is typically the primary factor influencing the adsorption effectiveness of dyes by hydrochars [175]. When compared to raw biomasses, the HTC procedure tends to lower the concentration of these groups on the hydrochars’ surface [202]. This is because long carbonization times or relatively high temperatures may trigger a decarboxylation pathway. Consequently, pre- and post-treatment procedures have been put in place to enhance their physicochemical characteristics and, consequently, their capacity to retain dye [203]. Higher temperatures have been shown to enhance the fixed carbon content through hydrothermal carbonization, which subsequently reduces the oxygen and hydrogen concentration through a dehydration mechanism [204].

Therefore, utilizing a variety of chemical activators, chemical post-treatment has been widely used to increase the adsorption capacity of hydrochars and restore the concentration of oxygen functional groups. For instance, scientists have proposed that covalent bonding was the most important of multiple mechanisms involved in the retention of copper ions and methylene blue [175]. While hydrogen bonds probably played a role in the preservation of these contaminants, FTIR spectra indicate that the dye formed a strong binding with the carboxylic (1600–1400 cm⁻¹) and hydroxyl (3600–3200 cm⁻¹) groups. However, the physical characteristics of the hydrochar and the extent to which its porous structure has developed are directly related to this adsorption mechanism. Since carbonization in humidified conditions and the conversion process at low to moderate temperatures (as opposed to pyrolysis) reduce the likelihood of producing considerable porosity, hydrochars typically do not have significant surface areas. Consequently, significant adsorption capability may be present in modified hydrochars with a high specific surface area. The majority of the hydrochars’ surface is made up of the tiniest porosities (less than 2 nm). As a result, the significant presence of these holes will improve adsorption, particularly for organic molecules that can be propelled by pore filling and hydrogen bonding mechanisms. In fact, the surface acidity is increased by the amount of oxygen-containing surface functional groups (such as lactonic, phenolic, and carboxylic). Consequently, a large number of polar groups are typically found on the hydrochar surface, which aids in the sorption of water and mostly molecules that are electronegatively charged. The adsorption capabilities of hydrochars can be greatly impacted by the hydrothermal carbonization of the feedstock and its effect on the elemental composition of hydrochars. Moreover, the removal of volatile materials is responsible for the drop in the atomic ratio (O + N)/C, which raises the availability of microporosity [7].

6.3. Magnetic Hydrochar

Hydrochars are good carbon-based adsorbents, but the difficulty of separating/desorbing pollutants and rapid solid–liquid separation after adsorption can be a challenge. Therefore, recent research is focused on the production of hydrochars that can be easily separated from their solution after the adsorption process, while retaining their high separation efficiency with little or no operational stress [205]. Magnetic hydrochar characterized by a magnetic source supported on/in the surface of the hydrochar materials comes to mind as a good alternative to alleviate this challenge. According to Franzreb et al. [206], magnetic adsorbents possess good affinity in the separation of specific pollutants from an aqueous media and their magnetic properties give them a lift over other adsorbents in different industrial applications [206]. The basic advantage of these adsorbents over others is their ease of separation/recovery from the liquid by applying the principle of magnetism; here, the adsorbent is retained in the magnetic field. Hence, this technique offers new alternatives in the area of water treatment/purification through the introduction of intensified equipment/process (magnetism) [207].

In the preparation of magnetic hydrochars, the most commonly used metallic nanoparticles/materials are iron, nickel, and cobalt [208,209], with iron being the most preferred over others. This is attributed to its low cost, good magnetic properties, good support modifier at low temperatures, low toxicity, and great stability [209]. Some of the commonly used iron solutions are ferric oxides (Fe₂O₃), ferrous sulfate (Fe₂SO₄), ferric nitrite (Fe₂(NO₃)₃), ferrous ferric oxide (Fe₂O₄), ferric chloride (Fe₂Cl₃), etc., [207,209]. Each synthesis pathway includes more than two stages before the final material is obtained. Again, the conditions for each production vary according to the carbon-based source (hydrochar), but all approaches generally involve the impregnation of the carbon source with iron salts and the chemical treatment of the material to obtain magnetic hydrochar [207].

Currently, the synthesis of magnetic hydrochar for the adsorption of pollutants from aqueous media has been reported by several researchers as seen in

Table 4. Utilization of magnetic hydrochar from aqueous phase adsorption.

Biomass	Magnetization Technique Used	Fe Source	Adsorbate	Isotherm Model/Qmax (mg/g)	Kinetic Model	Ref.
Vinasse and red mud	Co-hydrothermal treatment	Fe2O3	Pb(11)	Freundlich/223.144	Pseudo-second order	[212]
Watermelon	Co-precipitation	FeCl3.6H2O	Cadmium	Freundlich/347.2	Pseudo-second order	[213]
Waste lignin	Hydrothermal treatment	FeCl3.6H2O	Thallium	Langmuir/278.9	Pseudo-second order	[210]
Raffia fibers	Hydrothermal treatment	Fe3O4	Cadmium	Temkin/16.34	Elovich	[211]
Coffee husk	Co-precipitation	Fe3O4	Methylene blue dye	Freundlich/78	Pseudo-second order	[214]
Phytolacca acinose	Hydrothermal treatment	Fe3O4	Cadmium	Langmuir/246.6	Pseudo-second order	[215]
Wheat straw	Pyrolysis	Fe solution	Tetracycline and Mercury	Langmuir/268.3/127.4	-	[205]

. The hydrothermal carbonization technique is widely applied in the production of hydrochars because of its favorable conditions when compared with the others. This includes the generation of surface area rich in functional groups; it requires less energy consumption, and as a result, it is a low-cost process. On the other hand, the impregnation with iron salts is obtained by mixing the hydrochar and the magnetic salt prepared through sol-gel, co-precipitation, etc., in an aqueous solution [207]. Wang et al. [210] investigated the magnetization of waste lignin-derived hydrochar through hydrothermal treatment for the removal of thallium from wastewater [210]. The characterization results indicate the successful incorporation of iron species (metallic solution) in the produced magnetic hydrochar. The high adsorption capacity recorded for the magnetic hydrochar is an indication that the use of the adsorbent (magnetic hydrochar) in the removal of thallium from wastewater was very effective. In another study, Staron et al. [211] reported the possibility of obtaining hydrochar biocomposites with magnetic properties via hydrothermal treatment for the removal of cadmium ions from aqueous solutions. From the research conducted, new data on the use of biocomposites obtained from biomass, micro-organisms, and iron oxide were provided for the bioremediation of heavy metals from an aqueous media. The data obtained showed an effective removal of pollutants, suggesting subsequent use of biocomposites in real environmental situations due to their easy recovery from contaminated solutions.

As observed from Table 4, different carbon-based feedstock, together with a metal compound (iron solution), can be effectively used to prepare magnetic hydrochar, hence, taking advantage of their outstanding properties to obtain magnetic adsorbents with special characteristics for the removal of pollutants in water treatment/purification and other industrial applications. The adsorption capacities in Table 4 vary for different pollutants studied and for the different types of hydrochar used. The adsorptive properties of the final material produced could depend on the preparation conditions, such as the source of the precursor (carbon), temperature, and impregnation ratio [207]. Finally, the adsorbent can be recovered through the introduction of a magnetic field between the adsorbent and a bar magnet.

6.4. Sensors

Hydrochar has been recently used to develop high-performance chemical sensors by harnessing its electrical and electrochemical performances [216]. Many gases, such as H₂, NO₂, NH₃, etc., can easily be detected by these sensors (hydrochar) through the interactions between the gas species and the carbon surface of the hydrochar in their structural arrangements, which could either be unidimensional, bidimensional, or a 3D structure [217,218]. Babeker & Chen [219] and Torrinha et al. [220] discovered from their research work that carbon nanostructures possess outstanding electrochemical properties. This gave them the highest lead as the main option in terms of electrode materials to be considered in the determination of organic pollutants through electrochemical analysis. Electroanalysis has been reported severally to profound better analytical solutions when compared with conventional detectors/sensors (gas chromatography and mass spectrometry), owing to their high sensitivity and specification, analyst detection at a lower concentration, low cost, less time requirement, and finally, portable instrument size [221]. Nevertheless, the use of hydrochar as chemical sensors has not been widely researched, notwithstanding the growing production and applications of hydrochar [216]. However, Espro et al. [216] utilized orange peel-derived hydrochar in the conductometric and electrochemical detection of NO₂ and dopamine at part per million (ppm) and nanomolar concentration, respectively. It was reported that the electrical and electrochemical properties of the hydrochar depend on the hydrothermal treatment temperature, with the conductometric-based NO₂ sensor having a sensitivity of about 50 ppm of NO₂ and the electrochemical dopamine sensor having good performances for dopamine determination.

Hydrochar is characterized by low surface area and porosity despite its high electrochemical properties. However, it is good to note that the carbon surface of hydrochar can be potentially modified/functionalized through several modification techniques by enhancing its surface reactivity to improve its selectivity towards the proposed gas under consideration [158,222,223]. Modifiers, such as metallic nanoparticles (silver, gold, iron, copper, etc.), have shown promising results in enhancing the surface properties/chemistry of hydrochar for the effective electrochemical detection of pollutants. This could be attributed to their high surface area, good conductivity, high chemical stability, and improved mass transport [221]. So, Barretto et al. [221] employed copper nanoparticles and spent coffee grounds hydrochar to potentially modify glassy carbon electrodes for the effective detection of hydroxychloroquine and bisphenol A in natural water.

Electrodes play major roles in the electrochemical sensing of both organic and inorganic pollutants. These electrodes are made of metals (mercury, gold, platinum, silver, etc.), which makes them possess different properties according to the composition of their constituent metal [224]. Hydrochars can be used to modify electrodes to introduce one or more desired properties into the based electrode. Several techniques, such as chemical adsorption, electrochemical deposition, and maceration [224,225], have been employed in the modification of electrodes using biomass. The technique to be used depends on the type of electrode under consideration and the nature of the modifying agents. In this context, the choice of the modifying agent to be used depends on the new property that is to be introduced into the electrode to enhance its performance towards the targeted pollutant. Some of the likely properties to be introduced include selectivity, sensitivity, surface area, electrical stability, etc., and these properties are mostly found in hydrochar, thus, making its application here very necessary. Nevertheless, the use of hydrochar as an electrode modifier for electrochemical sensors has not been widely researched, notwithstanding its growing production and applications [216]. However, Espro et al. [216] modified a screen-printed carbon electrode using hydrochar obtained from orange peels via hydrothermal treatment. The electrochemical sensor produced was first used as a conductometric sensor for NO₂ detection at a limit of detection of 50 ppb and secondly, for the detection of dopamine. It was reported that the modified electrode produced at a temperature of 300 °C showed greater sensitivity for NO₂ and dopamine. Again, increasing the temperature of hydrochar increases the electrical conductivity of the material, hence, accelerating the transfer of electrons in the electrodes, and thereby creating an efficient response in the system. Durai et al. [226], through the drop casting technique, modified glassy carbon electrodes using hydrochar generated from acorn shells. The hydrochar was first treated with H₂SO₄ to obtain sulfonated hydrochar microspheres and nanosheets (SCMN). The modified electrode was used to determine glutathione, and it showed good sensitivity for glutathione. This was attributed to the mesoporous nature of the electrode as well as the large surface area and the high conductivity of the modifier material. Ma et al. [227] also modified glassy carbon electrodes using hydrochar obtained from paper waste pulp. The electrochemical sensor produced was utilized in detecting clenbuterol (CLB), a very strong chemical; when found in the human body, it causes a series of harm. From the results obtained in this study, the modified sensor was very effective in the detection of CLB. It recorded a good sensitivity of 3.26 µA µM⁻¹cm⁻² as against 0.751 µA µM⁻¹cm⁻² for the unmodified electrode. This high sensitivity recorded was a result of the good electrical conductivity, electrocatalytic activity, high electric flux, large specific surface area, and anti-spinel structure of the nickel–iron bimetal hydrochar used. Additionally, Ferlazzo et al. [228] reported on the use of hydrochar obtained from orange peel waste in the modification of commercial screen-printed carbon electrodes for the determination of nitrites and sulfites. From the electrochemical analysis carried out, it was observed that the modified sensor exhibits better electrochemical properties than the unmodified sensor for the determination of sulfites and nitrites in water.

Hydrothermally synthesized carbon can also be used for the colorimetric and fluorescence detection of ascorbic acid [229]. The sensor used fluorescence “on–off–on” mode for fluorescence quenching and a peroxidase-like activity for converting colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxTMB (Figure 14).

6.5. Biocatalysts

The use of hydrochar as a protein or enzyme carrier needs to be further investigated. Studies showed that hydrochar enabled the immobilization of the lectin (non-covalent) through the interactions of the COO⁻ groups (hydrochar) and the NH₃⁺ and NH₂ groups (protein) [230]. These interactions did not modify the protein or its binding action. More studies are still needed as there are a variety of these macromolecules that can immobilize hydrochar, leading to a more effective and low-cost non-covalent method. Immobilization can also occur via covalent bonding, where they are generated by the interaction of the hydrochar groups with the enzyme groups together with the cross-linking agent. The agent glutaraldehyde is used in immobilization, which reacts with amine groups of enzymes. Studies describe that laccase and pepsin formed covalent bonds on the hydrochar derived from manure activated with glutaraldehyde, highlighting the immobilization potential [231,232]. Through the use of residues from olive oil production and cellulose hydrocarbons as carriers, the authors highlighted the high potential for cellulose immobilization through covalent bonding [233]. The administration of pentaethylenehexamine increased the quantity of amino groups on the surface of the enzyme, which encouraged cross-linking. Despite the favorable properties in which hydrochar can be a profitable carrier, the textural properties result in greater enzymatic activity and immobilization. Therefore, enzymatic immobilization and hydrochar synthesis can reduce the costs of biocatalysts through a more sustainable process.

7. Future Perspectives

One of the biggest challenges is the application of hydrochars on an industrial scale, so the analysis of production costs, energy consumption, and operational design must be carefully sized and further investigated. Compared to biochar, hydrochar production requires less energy consumption, making it a less costly process. In this case, the residence time of the process, the relationship between the amount of biomass and water, and the reaction temperature must be analyzed [234]. The study by [235] used residual plant biomass for the production of hydrochar where the useful life was fifteen years for a cost of 158.72 USD/t, which is similar to those obtained for the production of hydrochar from the waste of wood, 218 USD/t [236]. When analyzing the production costs of conventional coal, it is observed that these values are not competitive, making this a new challenge to overcome. Another issue is the issue of atmospheric pollutants (CO₂). An alternative is the use of circular economy processes, supporting a cleaner process. Using a carbon dioxide-neutral hydrochar reduces the generation of pollutants, leading to C₂ compensation, and consequently reducing costs.

Another aspect involves the use of hydrothermal carbonization with other coupled technologies. The use of anaerobic digestion with hydrothermal carbonization provides an alternative for obtaining energy from sewage sludge (Figure 15) [237]. In this aspect, the viability of the process depends on the cost of transporting the residual biomass [236]. Another limitation of large-scale use involves the high volume of products retained in the liquid phase (greater than 20% biomass carbon and odorless compounds), which influences the water footprint when using the technology. More techniques are needed to help increase the value of the product, thus increasing global energy recovery and reducing the negative impact. One possibility is supercritical water gasification, microalgae cultivation, anaerobic fermentation or digestion, and bioelectrochemical systems [109]. The recommendation is to separate products with added market value. Therefore, future investigations in the area of hydrothermal carbonization must be concerned with obtaining a process with greater efficiency (less costs and greater removal of contaminants). The properties of the final product must be improved; an alternative are treatments, such as activation, enabling an increase in its added value in the areas of biocatalysis, energy, environmental remediation, etc.

In the area of water treatment and purification, the use of magnetic hydrochars, hydrochar-based sensors, and nanomaterials has gained significant attention due to their attractive features, such as their selectivity and great performances in the removal and detection of a wide range of both organic and inorganic pollutants from/in their aqueous media. To date, great effort has been made to discover novel and competitive magnetic hydrochars, hydrochar sensors, and nanoparticles through the best possible route. Hence, increasing the applications of these materials by creating more room for further research/findings. Due to the limited number of these materials and possibly, the few methods of their production, which are evident in the number of publications and patency in these fields, there is a need for the further research and development of improved versions of these materials/methods to foster the commercialization of their products for possible real-life applications. Finally, researchers should focus their studies on answering the following hypotheses: How can selectivity be increased by optimizing the properties of hydrochar to obtain high removal from real effluents? How can hydrochar be made economically viable and environmentally sustainable? How can regeneration be prevented from producing secondary pollution? Is it feasible to combine hydrochar with advanced treatment technologies? And if possible, what are the limitations and operating conditions applied? How does hydrochar behave when faced with competitive adsorption?

8. Conclusions

Hydrothermal carbonization (HTC) represents a groundbreaking approach to address the challenges of waste management by converting biogenic raw materials into valuable resources with enhanced sustainability. By optimizing processes, such as hydrochar synthesis, through systematic studies on the influence of the physical and chemical properties of biomass and temperature, HTC has demonstrated its potential to become more economically viable while aligning with global sustainability goals. This method not only reduces waste production but also improves process profitability, making it a compelling solution for integrating waste materials into valuable resources across various industries. The integration of innovative techniques, such as water recirculation with post-treatment synthesis changes and microwave-assisted co-hydrothermal carbonization, has further expanded the applicability of HTC. These advancements address existing limitations in the field, such as the narrow scope of application and incomplete treatment of complex contaminants. For instance, while most current studies focus on removing a limited range of dyes, like methylene blue, future research should expand to include a broader spectrum of contaminants, including metals and toxic gases, such as carbon dioxide, methane, and hydrogen sulfide. This expanded approach would more closely mirror real-world conditions encountered in wastewater treatment systems. Life cycle assessments (LCAs) and technical-economic evaluations (TVEs) have been instrumental in demonstrating the environmental and economic feasibility of HTC for mass production and industrial application. By focusing on circular economy principles, such as nutrient recovery from hydrochar—phosphorus, nitrogen, and others—it becomes possible to enhance soil fertility, improve water retention, and reduce contaminant leaching. The ability of hydrochar to act as a carbon sink after release into the soil also contributes to achieving carbon neutrality, further underscoring its potential for sustainable resource management. The versatility of HTC extends beyond chemical synthesis to include applications in energy production, biocatalysis, wastewater treatment, and agriculture. For example, the effluent from HTC or co-hydrothermal carbonization can be repurposed as a valuable fuel source for microalgae cultivation, anaerobic digestion systems, or even as a medium for hydrochar recycling within another co- or hydrothermal cycle. The latter option is particularly noteworthy due to its potential to lower water demand and catalyze industrial processes through its acidic pH. Additionally, the use of effluent for biological processes enhances resource efficiency while minimizing the need for external water sources. The growing interest in HTC technology, evidenced by an increasing number of patents and expanding industry applications, highlights its potential to become a cornerstone of future biorefineries. Its ability to integrate with other advanced technologies, such as nanotechnology or bioinformatics, could further enhance its performance and applicability across diverse sectors. Furthermore, the alignment of HTC with global climate goals, particularly in reducing greenhouse gas emissions, reinforces its role as a sustainable solution for the chemical industry. In conclusion, hydrothermal carbonization offers a comprehensive and versatile framework for transforming waste materials into valuable resources while promoting sustainability and resource efficiency. By addressing gaps in current practices through innovative synthesis routes and expanded applications, HTC positions itself as a viable and impactful technology for achieving circular economy objectives across various industries. Its ability to adapt to evolving scientific and industrial needs ensures that it will remain at the forefront of sustainable chemical innovation for years to come.