**5. Hydrochar for Biorefinery**

HTC products were recently proposed, from lab scale up to pilot plant unit, as precursor for the production of many organic compounds that were traditionally synthetized from fossil fuel-derived hydrocarbons [173–176]. Most of the efforts are focused on the production of ethanol, furfural compounds and glucose. During HTC, biomass endures significant modifications leading to the production of a vast amount of different chemical components, both in solid or liquid forms. Ethanol was successfully produced from the cellulose extracted from agave and sugarcane bagasse hydrochar in [177] and [178], achieving a production up to 145 L of ethanol from 1 ton of raw sugarcane bagasse. In order to maximize cellulose production, low temperature HTC (180–190 ◦C) is preferred. The obtained hydrochar is further treated trough enzymatic hydrolysis and fermentation to obtain ethanol. Through the enzymatic hydrolysis of both liquid and solid products of the HTC, glucose was produced from palm oil waste by Zakaria et al. [179].

Furfural and 5-hydroxymethylfurfural (HMF) were also produced from Miscanthus, corncob wastes and hemp [180–183]. HMF was successfully generated from both hydrolyzed cellulose or hemicellulose contained in the original biomass. In both cases, acid treatment through H2SO4 or catalytic conversion were necessary to produce HMF.

#### **6. Conclusions**

HTC represents a valid pre-treatment technology to convert waste biomass into new valuable products which could find applications in a wide range of fields, from energy production to environment remediation and soil conditioning. Since HTC operates in aqueous medium, it is perfectly suitable to convert wet biomass, without any drying step. The most common waste biomass materials include those produced by agro-industries, municipalities (organic fraction of municipal solid waste and sewage sludge) but also from forestry and paper mill industries. The removal of part of the volatile compounds and ashes increases the combustion properties of the hydrochar, making them suitable for direct combustion in energy production applications. Chemical and physical activation, by using KOH or CO2, respectively, were successfully proposed to increase the surface area and internal porosity of the material. By modifying the morphology and chemical surface, activated hydrochar can be successfully used for the production of energy storage devices for the immobilization of pollutants in gaseous or liquid streams. Pores development as well as elemental and surface composition can be tailored by changing the carbonization and activation parameters. Finally, the addition in soil can induce important benefits on pant growth, increasing water retention and reducing at the same time possible leaching of contaminants into ground water.

Recently, some researchers focused their attention on how to mitigate the energetic impact of the hydrothermal carbonization process by coupling it with renewable energy production systems. Indeed, biochar can represent a valuable product to increase the efficiency of co-digestion systems, with respect to its pristine composition. HC produced from coffee spent, rice straw or microalgae, as well as the liquid phase produced from the hydrothermal carbonization, induced higher biogas

production as well as an increased methane content [184–188]. In such a way, the energy content of the produced biogas can be used to run the HTC reactor, ensuring cleaner combustion with respect to the hydrochar itself. Another option that has recently been studied is to couple the HTC reactor with a solar concentration system which can satisfy the thermal load. Linear parabolic and parabolic disc concentrators ensured a carbonization temperature up to 200 and 250 ◦C respectively, ensuring heating conditions of traditional systems [189,190].

Nevertheless, there are still some mechanisms in the HTC process that need further study to be completely understood, in order to gain a stronger control over the process parameters, their influence on the final product properties and thus their possible applications. Depending on the HTC operating conditions and the nature of the feedstock, the technology still presents some limitations for its full development. Between them, the correct process liquid treatment and management often present challenges due to the presence of toxic hydrocarbons (such as polycyclic aromatic hydrocarbons (PHAs) and high heavy metals concentrations. The presence of toxic compounds severely limits the possibility to use such residue (as for example in agriculture as a possible source of nutrients) increasing the overall costs of the HTC process. Investigations related to toxic hydrocarbons removal and heavy metals recovery represent a necessary step for further development of the HTC technology.

**Author Contributions:** Conceptualization, M.P.M., M.V. and A.M.; Writing-Original Draft Preparation, M.P.M.; Writing-Review & Editing, M.P.M., M.V. and A.M.; Supervision & final review, A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
