Xylan fast pyrolysis: An experimental and modelling study of particle changes and volatiles release

Biomass char particles produced by pyrolysis may have different morphologies, which has important implications on burning mode, conversion rate and boiler efficiency. These features are difficult to address due to the complexity of biomass structure and pyrolysis reaction models. The present work reports preliminary results on the morphological changes and volatile release that solid particles of Xylan experience upon fast heating in a Drop Tube Reactor (DTR) and in a Heated Strip Reactor (HSR) in a range of temperature between 1100 and 1573 K under inert atmosphere with heating rate in the order of 103 K/s. Two different Xylan samples were chosen as representative of Hemicellulose in angiosperm biomass: xylooligosaccharide extracted from Beechwood (hardwood biomass) and from Corn Cob (herbaceous biomass). During the heatup phase, Xylan particles do not retain the original shape and morphology, neither in DTR nor in HSR experiments. In the early pyrolysis stage, Xylan particles melt and form viscous droplets. As devolatilization proceeds these droplets in the DTR evolve into highly viscous sponge-like particles with spherical shape, which swell and eventually shrink, ultimately producing highly porous spherical char particles. The experimental results are compared with the predictions obtained by the Bio-CPD Xylan pyrolysis model. The model is fairly able to calculate the pyrolysis yields in the DTR, but predicts unexpectedly low extent of metaplast formation. To explain the remarkable melting phenomena and morphological changes observed in the experiments, tuning of some CPD parameters and inclusion of mass transfer and pressure build-up within the particles might be necessary in future work.