• Multiple DTG peaks correspond to hemicellulose, cellulose, young char, less reactive char, lignin and annealed char. • Torrefaction (hemicellulose removal) increases thermal annealing tendency. • Heat treatment in N 2 and CO 2 yields similar char reactivity; γ and δ peaks are sharper and earlier in CO 2 . • Particle size and morphology changes are similar in CO 2 and N 2 atmospheres. Pyrolysed biomass char particles may have different morphologies, as various types of biomass are prone to shrinking/swelling, spheroidisation and thermal annealing during pyrolysis in flow reactors. These features have important implications on burning mode, conversion rate and combustion efficiency. However, addressing these effects is difficult due to the inherent complexity of biomass structure. In fact, both thermal annealing and changes in particle morphology during heat treatment have often been neglected and underestimated. In this work, experiments were carried out to measure the effects of biomass lignocellulosic compositions on char reactivity and morphology. Torrefaction converts raw biomass into a solid fuel with reduced moisture and increased calorific value, mainly by degrading hemicellulose. Natural biomasses with different lignocellulosic compositions and pre-torrefied samples are compared. The experimental campaign included experiments in a drop tube reactor and a heated strip reactor, with fast heating rates of 10 3 °C/s, maximum temperatures up to 1800 °C, and residence times ranging from 0.2 s to 2 s. Combustion reactivity, particle shape and size are analysed to discuss the role of holocellulose/lignin in the propensity of biomass to undergo thermal annealing and plastic deformation. Hemicellulose appears to play a significant role, limiting the effects of heat treatment in terms of thermal annealing and the tendency to particle shrinkage and spheroidisation. This is attributed to its capacity to link with other structural parts of the biomass, such as lignin, making the biomass structure more rigid and to the formation of a plastic melt, which hinders free rearrangement of the biomass structural units.
Freisewinkel et al. (Sat,) studied this question.