Experimental and kinetic modelling investigation on the effects of crystallinity on cellulose pyrolysis

The present work studies the effects of the crystallinity level of cellulose on the primary reactions during the slow and fast pyrolysis conducted in a wire-mesh reactor at 40 °C/min and 6000 °C/min, respectively. The sugars present in the water-soluble intermediates were analysed in a chromatogram equipped with a Q-exactive high-resolution mass spectrometer. The products in the washed primary tars were quantified through GC/MS. Slow pyrolysis experiments were also performed in a thermogravimetric analyser at four different heating rates (5, 10, 20 and 40 °C/min). All experimental data was used to develop a modified kinetic model that considers the effects of crystallinity on cellulose pyrolysis, which is based on the cellulose sub-mechanism of the Bio-PoliMi kinetic model. The kinetic parameters of each reaction in the modified model were calibrated using a two-step fitting procedure. The experimental results show that low crystallinity promotes the dehydration reactions in the cellulose solid base and the formation of oligosaccharides in the water-soluble reaction intermediates, increasing the formation of furans and decreasing that of levoglucosan in the final primary tar. The effect of crystallinity on the pyrolysis characteristics of cellulose is weaker during fast pyrolysis than during slow pyrolysis. The isoconversional kinetic approach of cellulose pyrolysis presents a low initial activation energy of the pyrolysis of amorphous cellulose, which is close to the activation energy of cellulose hydrolysis. Assuming that the amorphous region of cellulose decomposition path prefers depolymerization through hydrolysis accompanied by the dehydration of nearby hydroxyl group, the modified kinetic model succeeds to predict the effect of crystallinity on the thermal stability and the yields of char, levoglucosan and furans during cellulose pyrolysis.