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Hydrothermal carbonization (HTC) is an environmentally friendly thermochemical process for biomass conversion. This process operates at temperatures of 180–300 °C under the pressure of 8–20 bar in water [59]. The thermal processing at high pressure in water destructs the biomass structure and the resulting carbon material is called a hydrochar. The HTC is proposed as a productive process for fine biomass particles because it presents a higher yield of the bio‐based carbon product compared to regular carbonization [60]. Moreover, carbonization under high vapor pressure of water can produce hydrochar with uniform particles.

According to the hydrothermal reaction of the lignocellulosic material, the cellulose and hemicellulose are hydrolyzed to obtain CO₂ as a by‐product according to Eq. (2.11).

(2.11)

Carbon atoms do not appear in Eq. (2.11), which differs from the ordinary carbonization process shown in Eq. (2.1), as the transformation of biomass into hydrochar is essentially associated with three reactions: hydrolysis, dehydration, and decarboxylation [61]. However, the operating conditions of the HTC process are not able to destroy the lignin structure. Fourier‐transform infrared (FTIR) spectra showed the amount of lignin that was still present in the carbon material produced from the HTC process [62]. The HTC process is thus not appropriate for the conversion of lignin‐rich biomasses. Interestingly, the hydrothermal conversion of glucose (C₆H₁₂O₆) or other monosaccharides as seen in Eq. (2.12) could directly affect the carbon yield at 200 °C and 20 bar [27].

(2.12)

Even though hydrochars usually have a lower surface area than biochars, hydrochars possess a 1–4 times higher oxygen content than the carbon materials derived from carbonization and pyrolysis processes [33]. The hydrochars prepared from pinewood, peanut shells, or bamboo showed several oxygen‐containing groups such as C–O, –OH, and C=O, which were absent in the biochar [30]. The superior oxygen content in hydrochar makes it advantageous for many industrial sections such as catalysts or catalyst support. Nevertheless, an increase in hydrothermal temperature decreased the amount of oxygen and, conversely, the carbon content increased [11, 38]. Hydrochar is frequently used as a starting material for the production of activated carbon. The oxidizing agent can react with the oxygenated groups on the hydrochar surface resulting in its excellent catalytic behavior [63]. Moreover, the surface of hydrochar could be modified for specific applications. For example, hydrochar treated with sulfonic groups was effective in the production of biofuels [64].