Gas- and solid/liquid-phase reactions during pyrolysis of softwood and hardwood lignins

Pyrolytic reactions of Japanese cedar (Cryptomeria japonica, a softwood) and Japanese beech (Fagus crenata, a hardwood) milled wood lignins (MWLs) were studied with thermogravimetry (TG) and by pyrolysis in a closed ampoule reactor (N2/600 °C). The data were compared with those of guaiacol/syringol as simple lignin model aromatic nuclei. Several DTG peaks were observed around 300–350, 450, 590 and 650 °C. The first DTG peak temperature (326 °C) of beech was lower than that (353 °C) of cedar. This indicates that the volatile formation from cedar MWL is slightly delayed in heating at 600 °C. The gas-phase reactions via GC/MS-detectable low MW products were explainable with the temperature-dependent reactions observed for guaiacol/syringol in our previous paper. The methoxyl groups became reactive at ∼450 °C, giving O–CH3 homolysis products (catechols/pyrogallols) and OCH3 rearrangement products (cresols/xylenols). The former homolysis products were effectively converted into gaseous products (mainly CO) at >550–600 °C. However, the GC/MS-detectable tar yields, especially syringyl unit-characteristic products, were much lower than those from guaiacol/syringol. Thus, contributions of higher MW intermediates and solid/liquid-phase reactions are more important in lignin pyrolysis. From the results of stepwise pyrolysis of char + coke fractions at 450 and 600 °C, the methoxyl group-related reactions (450 °C) and intermediates gasification (600 °C) were suggested to occur also in the solid/liquid phase. This was consistent with the DTG peaks observed around these temperatures. These solid/liquid-phase reactions reduced the tar formation, especially catechols/pyrogallols and PAHs. Different features observed between these two MWLs are also focused.

► Lignin pyrolysis proceeds in the gas and solid/liquid phases in heating at 600 °C.
► The gas- and solid/liquid-phase reactions are explainable with the temperature-dependent reactions, that is, devolatilization (200–400 °C), methoxyl group-related reactions (>450 °C) and intermediates gasification (>550–600 °C).
► The O–CH3 bond homolysis and rearrangement occurring at >450 °C give catechols/pyrogllols and cresols/xylenols, respectively, in the gas phase.
► The solid/liquid-phase reactions give significant amounts of noncondensable gases, while reduces the formation of tar, particularly catechols/pyrogallols and PAHs.
► Hardwood lignin forms more volatile products at a lower temperature than softwood lignin.