Rapid molecular screening of black carbon (biochar) thermosequences obtained from chestnut wood and rice straw: A pyrolysis-GC/MS study

Abstract

Rice straw and chestnut wood were heated between 200 and 1000 °C (T CHAR ) to produce Black C 'thermosequences'. The molecular properties of the charred residues were assessed by pyrolysis-GC/MS to investigate the relation between charring intensity and pyrolysis fingerprint. Samples obtained at T CHAR > 500 ºC (wood) or > 700 ºC (straw) gave low quality pyrograms and poor reproducibility because of high thermal stability, but pyrolysis-GC/MS allowed to track the thermal degradation of the main biocomponents (polysaccharides, lignin, methylene chain-based aliphatics, triterpenoids, chlorophyll and proteins) in the lower temperature range, mostly occurring between T CHAR 250 and 500 ºC. With increasing T CHAR , the charred residues of these biocomponents lose characteristic functional groups, aromatise and finally condense into non-pyrolysable biomass. The proportions of the pyrolysis products of unspecific origin (benzene, toluene, PAHs, etc.), increase with charring intensity, while the ratios that reflect the abundance of alkyl cross-linkages between aromatic moieties (e.g. benzene/toluene, naphthalene/alkylnaphthalene) decrease. These results provide the guidelines to using pyrolysis-GC/MS for the molecular characterisation of different components in Black C and biochar, which is an important parameter for predicting Black C/biochar behaviour in soil. Results are consistent with earlier studies of these samples using the BPCA (benzenepolycarboxylic acid) method and the ring current-induced 13 Cbenzene chemical shift NMR (Nuclear Magnetic Resonance) approach. Pyrolysis-GC/MS provides more information on molecular structures in the low temperature range (T CHAR ≤ 500 ºC) while the BPCA and NMR ring current methods provide more reliable estimations of charring intensity, especially at higher temperatures (T CHAR ≥ 500 ºC).3

Influence of feedstock properties and pyrolysis conditions on biochar carbon stability as determined by hydrogen pyrolysis

“…6) which can be explained by volatilization into gaseous form [22], in addition to the loss of N functional groups with carbonization [37,40]. However in the mid-pyrolysis temperature range (500 Ce600 C) it is clear that there is little additional loss of N. Instead it is observed that N is retained and progressively incorporated into recalcitrant structures within the biochar.…”

The carbon isotope composition of semi-labile and stable pyrogenic carbon in a thermosequence of C3 and C4 derived char

“…Lipids were found to always be a minor component, but were more important in the rice char. When char formed > 700°C, the dominant component was SPAC, which led to low quality pyrograms and poor reproducibility (Kaal et al, 2012).…”

“…levoglucosan) into larger PAH compounds. Kaal et al (2012) investigated the pyrolysis products from a thermosequence of rice and chestnut wood using flash pyrolysis gas chromatography-mass spectrometry (GC-MS); the components investigated were largely PyC SL . Low temperature PyC components were dominated by carbohydrate and lignin, with lignin-indicative pyrograms more important at mid-range formation temperature.…”

Molecular characterization of the thermally labile fraction of biochar by hydropyrolysis and pyrolysis-GC/MS

“…We can presume that a dominant non-BCHyPy fraction was formed at 340-400 °C by unimolecular cyclization, dehydrogenation, dealkylation, and aromatization reactions of wood constituents (lignin, hemicellulose, and cellulose as well as lipids such as resins) [43]. At 600 °C this fraction was condensed by pyrosynthesis into larger aromatic structures increasing the BCHyPy [13,23,44]. The highly polycondensed aromatic moieties of BCHyPy cannot be cracked into volatile compounds analyzable by GC-MS.…”