This fact can be explained by increased reactivity of the biomass caused by availability of air, which oxidized biomass volatiles during the second and third degradation stages [22,23]. At temperatures of 370–490 8C (i.e., at the beginning of the fourth stage), the diminishing of the sample mass was generally due to the lignin volatilization. Within this temperature range, heat released from the volatile combustion was not high enough to ignite the char and sustain its combustion. From Fig. 3,