A thermochemical process called hydrothermal carbonization (HTC) has been increasingly investigated and employed as a means of converting raw biomass into a higher-value solid fuel. Hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG) can be considered more severe forms of HTC. All three forms of hydrothermal treatment are conducted in hot, pressurized, liquid water, but with the objective of providing different products. A significant advantage of HTC is its ability to utilize wet feedstocks and avoid the complexity, cost, and energy requirements of pre-drying. Due to its aromatic structure— and hence, higher C/O ratio, lignin has a higher energy content than cellulose or hemicellulose. In utilizing HTC to create an energy-dense solid fuel, the product of greatest interest is hydrochar. The overall energy balance of the HTC process is an important consideration in evaluating the commercial viability of solid biofuels produced by the process. The kinetics of biomass degradation by HTC treatment is complex and difficult to ascertain.