After undergoing thermal pretreatment, the chemical formulas for OPF in N2 and air at the temperature of 350 °C were transformed to CH0.050O0.223N0.027S0.002 and CH0.040O0.326N0.082S0.004, respectively, whereas they are CH0.048O0.281 and CH0.016O0.317 for eucalyptus in N2 and air, respectively