Investigation of primary and secondary char formation during pyrolysis of torrefied cellulose

Biomass preparation by torrefaction could be a suitable option to improve the quality of biomass feedstocks for a subsequent thermal conversion process. Char produced from the pyrolysis of torrefied biomass is critical to downstream process development. This study explored the primary and secondary char formation of the torrefied cellulose, with the extent of secondary reaction manipulated by the sample dimension, pyrolysis pressure and pyrolysis heating rate. Cellulose samples were torrefied at 200 °C, 260 °C, and 320 °C, and subsequently pyrolyzed in a hot-plate reactor at 1000 °C, under heating rates of 10–200 °C/min with a pressure of 0.1 or 1000 mbar, using either thin-film or thick-layer samples. The results indicated that the structural transformation induced by torrefaction and the secondary reactions promoted by the pyrolysis conditions led to significant variations in the char yield, ranging from 1.6 wt% to 57.8 wt%. Torrefaction enhanced char formation during the primary reactions in two ways: by reducing the degree of depolymerization of the carbohydrate structure and by promoting the formation of crosslinked structures. The sensitivity of secondary char formation to the pyrolysis conditions varied with torrefaction severity, with the maximum increase in char yield from promoted secondary reactions decreasing as torrefaction severity intensified, ranging from 3.9 to 1.1 times. Raman spectra revealed that the secondary reactions enhanced the condensation of aromatic systems of the char. These findings provide insight into the complex mechanisms underlying primary and secondary char formation during the pyrolysis of torrefied biomass.