Hydrothermal carbonization of olive wastes to produce renewable, binder-free pellets for use as metallurgical reducing agents

Abstract

Torrefaction or hydrothermal carbonization processes were compared for conversion of olive pulp into metallurgical reducing agent. The dependence of yield, CO2 reactivity, and mechanical properties to reaction time and heat treatment temperature was investigated. Hydrochar yield increased with increasing residence time and the maximum solid yield was observed for a residence time of 15 h. On the other hand, CO2 reactivity slightly decreased with increasing heat treatment temperature at a residence time of 2 h. Notably, the CO2 reactivity of hydrochar was less than that of olive pulp char produced by torrefaction, approximating that of carbon-based reducing agents derived from non-renewable resources. An additional heat treatment improved hydrochar pellet durability to greater than 95%, whereas stable torrefied char pellets could not be produced under any set of conditions. Hydrothermal carbonization is superior to torrefaction for production of renewable reducing agents with reactivity and mechanical properties comparable to those afforded by reducing agents from non-renewable sources.