Biomass & Bioenergy, Vol.104, 17-35, 2017
Chemical and morphological evaluation of chars produced from primary biomass constituents: Cellulose, xylan, and lignin
The effect of pyrolysis temperature on the morphology, bulk and surface chemistry of cellulose, xylan and lignin chars has been examined. Chars were produced between 300 and 700 degrees C. Raman, X-ray photoelectron, nuclear magnetic resonance spectroscopic techniques, scanning electron microscopy and surface area analysis were used to characterize each sample. Formation of polyaromatic structures were found to be prevalent at 400 degrees C for both cellulose and lignin chars, and showed development at 300 degrees C for Xylan. The low A bands identified in the lignin char produced at 400 degrees C indicate that crosslinking at low temperatures preferentially forms hexagonal or larger rings. Aromatic condensation increased until 500 degrees C, and remained stable up to 700 degrees C. The sharp drop in oxygen content as temperature is increased up to 500 degrees C and the formation of ether (C-O-C) groups indicates that oxygen is a primary reaction component for crosslinking and polycondensation. Despite the considerable increase in ring size for char produced at 700 degrees C compared to 500 degrees C, only very mild loss of oxygen is identified, suggesting that C-C bonds are forming with minimal oxygen mediation The increased broadness of the NMR peak and the Raman G band suggest that at these temperatures the increase in ring size occurs largely in a 3 dimensional manner rather than planar with contributions from non-hexagonal rings and out-of-plane distortion. By comparing the NMR dephasing data to the I(D)/I(G) ratio an approximate relation between this Raman data and cluster size has also been derived. (C) 2017 Elsevier Ltd. All rights reserved.