**1. Introduction**

The increasing need of finding new renewable energy alternatives to fossil fuels together with the need to safely dispose of organic waste has pushed, in the last few years, the investigation for more efficient and reliable technologies for waste biomass energy exploitation and conversion towards valuable materials. Indeed, the constant rise in the global energy consumption together with the parallel decrease in the fossil fuel reservoirs, has driven the scientific community toward the research of more sustainable sources [1]. Waste biomass, and in particular the organic fraction of municipal solid waste (OFMSW) and industrial and sewage sludges are among the most studied residual feedstocks to produce energy and valuable carbonaceous materials [2–6].

One of the major drawbacks of choosing waste biomass for energy production, is the need for one or more pre-treatments before use [7]. The low energy density due to the high moisture content of waste biomass, makes it often not suitable for a direct energy conversion by combustion and/or gasification [8–10]. The high moisture in residual biomass raises significantly the operational cost of transportation and energy consumption for drying [11]. However, due to its great abundance, globally availability, carbon neutrality and the necessity to find efficient and valuable technologies to treat and convert it into sustainable energy sources and exploitable materials, waste biomasses has gained increasing attention and investigation [12].

Between different valuable alternative treatments for waste biomass, wet thermochemical technologies for their flexibility and, principle, their low investment and operating costs are receiving increasing attention in the last years [13,14]. In particular, hydrothermal carbonization (HTC), also known as wet pyrolysis, is a thermochemical process performed in the presence of sub-critical water at temperature usually between 180 and 280 ◦C and autogenous saturated vapor conditions (10–80 bars). Residence time is varied from minutes up to several hours [3,15,16]. At high temperature and pressure, even at subcritical conditions, water undergoes a dramatic properties change acting more as an organic solvent and its increased ion product favors reactions that are typically catalyzed by acids or bases, promoting biomass decomposition through hydrolysis, dehydration and decarboxylation reactions [17,18]. By means of these reactions, it is possible to increase the carbon content of the initial feedstock, by removing most of the more volatile oxygenated compounds (furans and low molecular fatty acids) that are typically moved to the aqueous phase [19]. It is also well documented in the literature that, during HTC, part of the inorganics are moved to the liquid phase [19–21] thus, on the one hand, reducing in the solid residue (hydrochar), minerals and ash formation during combustion, and on the other hand, increasing the high heating values (HHV) [17,22]. This behavior leads to a reduced risk of slugging and fouling phenomena during combustion of hydrochars in boilers if compared to the use of pyrochars [2,23,24]. Hydrochar can be used in numerous applications, as a bio fuel for energy production [1,13,25], as a source of carbon and nutrients in soil application [19,26,27] or even as starting material for further advanced utilization in supercapacitors or a porous matrix for the adsorption of pollutants [28,29]. Moreover, the possible scalability of the process up to the industrial level [30], as well as relatively mild process conditions, make HTC a valid solution for the near future for the transformation of a wide range of raw biomass into valuable materials [13,30]. In Figure 1 is reported a flowchart of hydrochar production from waste biomass and its possible energetic and environmental applications.

Since HTC is performed under wet conditions, this technology can find its best application for biomasses that present high initial moisture content. Indeed, when dealing with direct combustion, a detrimental overall efficiency was shown when the starting biomass presents high moisture content [10]. The main reaction pathways involved in the hydrothermal carbonization process are still under discussion among the scientific community [31]. One possible route was discussed by Jatzwauck and Schumpe [32], according to which the HTC process is essentially divided into three steps. The first one is mainly related to the hydrolyzation of cellulose, hemicellulose and part of the lignin. During the second step, the generated intermediates endure further re-arrangement resulting in the generation of gaseous products. As an alternative to the formation of the gas phase, intermediate products can conglomerate to form secondary char, during the third step. The other mostly discussed route, regarding the evolution of HTC reactions, involves two main pathways: a solid–solid transformation, according to which hydrochar is directly generated from the dehydration of the initial biomass, and a vapor–solid reaction in which the dissolved organics present in the solution, back polymerize to form again a solid material, often named coke or secondary char [33–35].

The present work attempted to review the most relevant research on HTC of the last 5 years, in order to derive the best operative conditions for the production of valuable hydrochar materials. Moreover, whenever possible, the present review was integrated with a further pyrolysis or gasification process in order to evaluate how HTC behaves as a thermo-chemical pre-treatment for the production of porous material or precursor for energy application purposes.

**Figure 1.** Waste biomass hydrochar potential applications.
