Study of variables in energy densification of olive stone by hydrothermal carbonization

The influence of the variables biomass/water ratio (1.1-12.3%), temperature (150-250 °C) and residence time (3.2-36.8 h) on the hydrothermal carbonization of olive stone has been studied. The implementation of a design of experiments - response surface methodology approach allowed the process to be optimized in terms of reactivity (solid yield) and energy densification (increase in higher heating value), and the importance of each variable in the process to be identified. Solid yield ranged between 30.95% and 55.75% and HHV from 22.2 to 29.59 MJ kg−1. Interactions between the different variables involved were investigated to provide more precise control of the overall process so as to satisfy any given energy densification requirement of the final hydrochar.