Hydrothermal carbonization of industrial kraft lignin: Assessment of operational parameters

Lignin, one of the main constituents in wood and the second most abundant natural polymer, has generated great interest as a source for generating bio-based materials and fuels. The properties of lignin can be affected by various methods, for example, hydrothermal carbonization (HTC). The thermal treatment could thus be used to produce more suitable raw materials for added-value products. This work studies the relationship between the properties of the solid product (hydrochar) from HTC treatment and the choice of reaction conditions for typical HTC parameters: temperature – 200, 220, and 240 °C; residence time – 3, 6, 12, and 24 h; and water recirculation– 40, 60 and 80 vol% of process water (PW). The chemical composition of the hydrochar (HC) generated from industrial kraft lignin is evaluated along with their thermal degradation behavior under inert and oxidizing conditions by thermogravimetry. Compositional variations led to changes in characteristics such as volatile, fixed carbon, and ash content, varying to a greater or lesser extent according to the operational parameters. Although an increase in the temperature of the process leads to an increase in the caloric value of the hydrochar, this temperature change also leads to a slight decrease in the thermal stability of HC in oxidizing atmospheres. Additionally, the integration of a lignin HTC treatment process in a large modern Nordic pulp mill is presented, indicating minor disturbances when adding an HTC unit to the plant.