Oxidation Kinetics of Eucalyptus Chars Produced at Low and High Heating Rates

Abstract

An experimental and kinetic study of the oxidation of eucalyptus chars obtained by pyrolysis at 900 °C with low and high heating rates (LHR and HHR, respectively) has been performed. Oxidation experiments were carried out in a quartz reactor with an inlet oxygen concentration ranging from 100 to 1000 ppmv in a nitrogen flow, a temperature of 900 °C, and a flow rate of 1000 mL min−1 (STP). In order to analyze the temperature influence on the process and to obtain the process activation energy, char oxidation tests were also performed in the 650–900 °C temperature range, for a given oxygen concentration of 500 ppmv. Kinetic parameters were derived from the oxidation reaction data, which fitted properly the macroscopic model of type I in regime II. The reaction orders with respect to the oxygen concentration were found to be 0.58 and 0.66 for LHR and HHR chars, respectively. In both cases, it was observed a “break” in the Arrhenius plot. At low temperatures (650–800 °C), the activation energy was similar for the oxidation of both chars, in the 35–45 kJ mol−1 range. However, in the high temperature range (800–900 °C), the activation energy was 100 kJ mol−1 for the LHR char oxidation and 60 kJ mol−1 for the HHR char oxidation. The differences between the oxidative reactivities of the LHR and HHR chars are further discussed in terms of the chemical and physical char properties.