Conversion of Lavandula Straw into High-Quality Solid Fuel: Effect of Hydrothermal Carbonization Conditions on Fuel Characteristics

Hydrochars obtained by hydrothermal carbonization (HTC) of Lavandula straws have been studied as an eco-friendly and economical valorization route of residual biomass from cosmetic industry into solid fuels. HTC process has been performed in a temperature range from 180 to 260 °C and retention time ranging from 1 to 20 h. The two key parameters (time and temperature) were normalized to severity factor (SF) ranging from 4.74 to 7.79, in order to compare the fuel characteristics of hydrochars according to the thermal treatment conditions. Proximate and ultimate analysis and microscopy and thermal analysis were used to characterize hydrochar surface, combustion behavior, and kinetics as a function of SF, and to compare them with those of the raw lavandin sample. Results showed that after HTC, hydrochar properties were close to sub-bituminous coal and lignite, and SF was a relevant optimization parameter for solid fuel application. In the case of Lavandula , SF = 6 was the optimal value for the HTC process. Nevertheless, in addition to SF, it is important to consider that HTC-temperature is a more influencing parameter than HTC-retention time for the fuel properties of the resulting hydrochars.