Understanding the Fast Pyrolysis of Lignin

Abstract

In the present study, pyrolysis of corn stover lignin was investigated by using a micro-pyrolyzer coupled with a GC-MS/FID (FID=flame ionization detector). The system has pyrolysis-vapor residence times of 15-20 ms, thus providing a regime of minimal secondary reactions. The primary pyrolysis product distribution obtained from lignin is reported. Over 84 % mass balance and almost complete closure on carbon balance is achieved. In another set of experiments, the pyrolysis vapors emerging from the micro-pyrolyzer are condensed to obtain lignin-derived bio-oil. The chemical composition of the bio-oil is analyzed by using GC-MS and gel permeation chromatography techniques. The comparison between results of two sets of experiments indicates that monomeric compounds are the primary pyrolysis products of lignin, which recombine after primary pyrolysis to produce oligomeric compounds. Further, the effect of minerals (NaCl, KCl, MgCl(2), and CaCl(2)) and temperature on the primary pyrolysis product distribution is investigated. The study provides insights into the fundamental mechanisms of lignin pyrolysis and a basis for developing more descriptive models of biomass pyrolysis.

Quantitative Insights into the Fast Pyrolysis of Extracted Cellulose, Hemicelluloses, and Lignin

“…In particular, the high concentrations of short‐chain acids (e.g., acetic and propionic acids levels of 4.8 and 3.8 wt % detected for hemicelluloses‐derived liquids vs. 1.8 and 2.1 wt % for the lignin‐derived liquids) have a catalytic effect on the oligomerization of phenolic compounds. This has been demonstrated experimentally in the presence of acetic acid,7 whereas the role of propionic acid is still unknown. Notably, only the lignin‐derived liquids contained a monomeric phenol fraction derived from the hydroxy‐ and methoxy‐substituted phenylpropane units 40…”

“…Indeed, the detection capability of GC techniques is limited by the volatility of the products and the heaviest compounds, such as oligomers, are not analyzed. To solve this technical issue, alternative chromatographic conditions (e.g., different columns) or techniques (e.g., gel‐permeation chromatography; GPC) must be used 7. In this study, we exploit the performance of liquid‐state NMR spectroscopy to assess the chemical composition of the whole bio‐oil, which is certainly less specific in terms of organics identification but more representative of its chemical composition.…”

“…The primary condensates are composed mainly of monomeric phenolic compounds 7. This was confirmed by the detection of a range of phenols (e.g., 0.18 wt % of phenol, 0.60 wt % of 4‐vinylphenol), methoxyphenols (e.g., 0.17 wt % of guaiacol, 0.52 wt % of 4‐vinylguaiacol, 0.14 wt % of vanillin) and dimethoxyphenols (e.g., 0.17 wt % of syringol, 0.13 wt % of 4‐vinylsyringol, 0.14 wt % of acetosyringol).…”

“…For several decades, researchers have tried to elucidate the main degradation pathways for biomass fast pyrolysis modelling 6, 7, 8, 9, 10, 11. As a result, the overall degradation scheme of biomass fast pyrolysis is seen as an interplay between physical and chemical events, which are often impossible to separate 12.…”

“…Indeed, the chemical aspect of biomass fast pyrolysis can be described as a combination of parallel and successive reactions of a nonionic and ionic nature 13. If radical mechanisms are invoked and are predominant in coal pyrolysis,14 recent experimental evidence6, 7, 15 and theoretical calculations16, 17 for biomass fast pyrolysis suggest the predominance of nonionic reactions during the primary pyrolysis stage. This ongoing discussion on the importance and predominance of the ionic and/or nonionic character of fast pyrolysis reactions13 has provided important clues that have not yet been used to rationalize the degradation modes.…”