Pyrolysis-GC-MS analysis of the formation and degradation stages of charred residues from lignocellulosic biomass.
The structural transformations undergone by lignocellulosic biomass (freeze-dried rye grass, Lolium rigidum) subjected to progressive isothermal heating (burning at 350 degrees C under oxidizing conditions for 30, 45, 60, 75, and 90 s) have been monitored by Curie-point pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). The pyrograms suggest that even charred residues after severe heating (carbon loss ca. 50%) still contain substantial concentrations of some resistant plant structural components. Several trends were observed when monitoring the relative concentrations of the different groups of pyrolysis compounds released during successive charring stages: (i) the tetrapyrrole moiety of chlorophylls is rapidly destroyed as indicated by the decreasing yields of pyrroles and pyrrolines, whereas the phytol backbone is comparatively more resistant, leading to phytadienes after dehydration and reduction; (ii) the increasing yields of imidazoles from progressively heated samples (maximum at 45 s stage) suggest accumulation of newly formed nitrogen-containing compounds that may survive natural fires; (iii) the lignin backbone shows a relative resistance, the yields of aromatic products pointing to progressive demethoxylation; and, (iv) a selective accumulation of recalcitrant alkyl material occurred, which is interpreted as the result of thermal condensation of hydrocarbons and fatty acids into macromolecular materials in the charred residue. In terms of the intensity of the isothermal heating, the yields of the different classes of alkyl compounds follow the order phytadienes < fatty acids < alkanes < wax esters < sterols.