Molecular characterization of Ulex europaeus biochar obtained from laboratory heat treatment experiments – A pyrolysis–GC/MS study

Gorse species (Ulex sp.) are ubiquitous in the shrublands of NW Spain and have the potential to become key players in an integral biofuel/biochar program in NW Spain. Here we present molecular characterization (using pyrolysis–GC/MS) of a biochar “thermosequence” obtained by laboratory heating of Ulex europaeus wood in a muffle furnace between 200 and 600 °C (TCHAR). Low temperature chars (TCHAR ≤ 350 °C) produced significant amounts of pyrolysis products of which the precursor biopolymer could be recognized, while high-temperature chars (TCHAR ≥ 400 °C) produced mainly phenols and monocyclic and polycyclic aromatic hydrocarbons, which are not specific for any biopolymer. Carbohydrate could hardly be recognized at TCHAR ≥ 350 °C. The thermal rearrangement of polyphenols, mainly lignin, was reflected in more detail (1) C3-side chain shortening and probably depolymerization (TCHAR 200–350 °C), (2) demethoxylation of syringyl and probably also some guaiacyl lignin (TCHAR 300–400 °C), (3) elimination of virtually all remaining methoxyl groups (TCHAR 350–400 °C), through dehydroxylation and demethoxylation, (4) almost complete dehydroxylation of lignin and other biopolymers (TCHAR 400–500 °C), (5) progressive condensation into polyaromatic structures (TCHAR 300–500 °C) and (6) partial elimination of alkyl bridges between (poly)aromatic moieties (TCHAR 450–500 °C). These results were supported by Fourier transform infrared spectroscopy (FTIR) of the same samples. We conclude that pyrolysis–GC/MS can be used as a rapid molecular screening method of gorse-derived biochar. Molecular properties elucidation is an essential part of predicting the stability and agronomical behavior of gorse-derived biochar after future implementation in soils.

► Molecular properties of Ulex-derived biochar created at 200–600 °C assessed by pyrolysis–GC/MS.
► Lignin thermal degradation recognized as propanoid side chain shortening, demethoxylation, dehydroxylation and finally condensation.
► Pyrolysis fingerprints can be used to estimate the degree of thermal alteration of lignin in Ulex wood.