Biomass behaviour upon Fast Pyrolysis in CO2-rich Atmospheres: role of Lignin, Hemicellulose and Cellulose

The present work focuses on the quality of char and primary tar produced from fast pyrolysis of lignocellulosic biomasses in both nitrogen (N2) and carbon dioxide (CO2) atmosphere. Three biomasses are investigated: Walnut Shells, the richest in Lignin, Straw, the richest in Hemicellulose, Pinewood, the richest in Cellulose. Heat treatment has been carried out in a heated strip reactor (HSR) at 1573 and 2073 K with holding times of 3 s and heating rate of 104 K/s. The equipment allows quenching the volatiles as soon as they are emitted from the particles and collecting them for further chemical analyses. The char samples are also collected and analysed by thermogravimetric analysis in air. Primary tar produced from pyrolysis of Walnut shells results to be the richest in monosaccharides among the samples investigated. Surprisingly monosaccharides are scarce in the tar produced from Straw, despite its high content of Hemicelluloses. PAHs and high molecular weight tar are present in the primary products of all the biomasses investigated, but particularly in tar from Walnut Shells, the biomass with the largest lignin fraction. The chars of the three investigated biomasses appear to be all constituted by multiple components. After heat treatment in N2 at 1573 K, the most reactive char is the one obtained from Straw, and the least reactive is the Walnut Shells char. More severe heat treatment and the presence of CO2 in the atmosphere generate additional char components with both higher and lower reactivity. Altogether the quality of the pyrolysis products does not correlate with the content of Cellulosde/Hemicellulose/Lignin of the original biomasses.