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

摘要

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.